COAL, OIL SHALE, NATURAL BITUMEN, HEAVY OIL, AND PEAT Jinsheng Gao College of Resource and Environment Engineering, East China University of Science and Technology, Shanghai, China Keywords : fossil fuel, coal, oil shale, natural bitumen, tar sand, heavy oil, peat, lignite, anthracite, bituminous coal, resource, reserve, origin, formation, geochemistry, geology, mining, exploitation, processing, utilization, technology, preparation, mineral matter, maceral, clean coal technology, environmental pollution, sustainable development, desulfurization, combustion, power generation, carbonization, gasification, liquefaction, pyrolysis, non-fuel use, shale oil, tar, coke, gas, ecological system, acid rain, primary energy, industrial feedstock, crude oil, biomass, organic rock, coalification, coal rank, humic acid, plant debris, surface mining, underground mining, coal basin, seam, petrology, coking property, coking behavior, hard coal, vitrain, clarain, durain, fusain, vitrinite, exinite, inertinite, fusinite, ash, volatile matter, heating value, moisture content, metamorphism, fixed carbon, sulfur, nitrogen, trace element, heavy metal, pyrite, swamp, peatland, coal structure, crush, grind, washed coal, beneficiation, heavy medium, jig, fluidized bed, sorbent, wet scrubber, atmospheric pollutant, coal conversion, iron-making, regeneration, synthesis, catalyst, alternative, organic, inorganic, matter, kerogen, solvent extraction, retorting, chemicals. Deposit, recoverable, reservoir, enhanced oil recovery, mire, cut away, conservation, transportation, biodiversity, bog, humification, np-grading, refinery, retort, processing technology, char, asphalt Contents 1. Introduction 2. Coal Geology and Geochemistry 3. Coal Technology 1 4. Coal Technology 2 5. Oil Shale 6. Natural Bitumen (Tar Sands) and Heavy Oil 7. Peat 8. Conclusion Acknowledgments Related Chapters Glossary Bibliography Biographical Sketch Summary Coal, oil shale, natural bitumen (tar sand), and heavy oil all belong to the group of fossil fuels. Peat is usually not classified as a coal, but can be seen as its precursor. Coal is the end product of a sequence of biological, geochemical, and geological processes (or a "coalification" process) originating from plant debris. It has been used as a major source of fuel by humankind for thousands of years. Known global reserves of coal are much greater than that of any other fossil fuels. Currently, coal is widely used for power generation, heat supply, coke making, the production of gaseous and liquid fuels, and so on. As a complex, heterogeneous fuel, it is composed of organic and inorganic matters, and contains a very large
number of elements. It is difficult to burn or process without serious environmental implications and therefore substantial worldwide attention is being focused on the more efficient and clean use of coal. The future of coal is to a great extent dependent on the development and availability of new processes. Natural bitumen, found in tar sand and heavy oil from various reservoirs, belongs to a subclass of petroleum. Oil shale is also a sedimentary rock, containing kerogen as its main organic constituent, and to a lesser extent, bitumen; both embedded in an inorganic matrix. Generally speaking, the latter are more closely similar to conventional crude oil in their elemental composition of organic matter than they are to coal. Naturally, it is much easier to convert them into liquid fuels. Peat, as an important biomass resource, is an acidic mixture of dead and decomposed vegetable matter that forms in boggy areas. Although it is a low-quality fuel, it has a number of special applications and is important for the conservation of global ecological systems and environments. 1. Introduction Coal, oil shale, natural bitumen (tar sand), and heavy oil are all fossil fuels. Strictly speaking, peat is not yet a real organic rock and can be regarded as the precursor of coal. In global primary energy consumption, crude oil is still a major contender, supplying 40 percent of fuel, while coal supplies 30 percent and natural gas 20 percent. This means that fossil fuels currently supply 90 percent of our energy, which gives some idea of how much fossil fuel today’s society consumes and how dependent upon those fuels the world has become. Fossil fuels are essentially the stored solar energy of several hundred million years. They are non-recyclable, exhaustible natural resources that will one day no longer be available. Over the past 150 years, we have already used up one third of the proven amount of oil reserves, or about 700 billion barrels (1 barrel = 159 liters), which leaves only 1.5 trillion barrels remaining. The estimated remaining reserves of coal, calculated in terms of oil equivalent, are thought to be 9.1 trillion barrels; those of natural gas, 1.3 trillion barrels; and those of oil shale and natural bitumen together, 2.1 trillion barrels. The data suggests that coal is the most abundant fossil fuel yet discovered. If one adds to it the other low quality fossil fuels, such as oil shale, tar sand, and heavy oil, together they can certainly meet the global energy demand for at least several hundred years, thus acting as a bridge between oil and new energy resources in the future. In addition, these fuels are valuable resources of organic carbon and can be converted into a variety of industrial feedstock and materials. By comparison with crude oil and natural gas, the above-mentioned fuels have some intrinsic drawbacks: first, they are relatively deficient in hydrogen and rich in carbon, particularly in the case of coal; second, they have a higher content of impurities, such as mineral matter and sulfur, leading to more environmental problems and more difficulties in processing; and third, they are not convenient for transportation and handling and so on. The world requires sustainable development, involving clean energy and its reliable supply. Therefore, more attention should be focused on the development of more advanced technologies for the conversion of low-quality fossil fuels into synthetic crude oil and natural gas to compensate for the depletion of both. Coal has had a glorious history and we are convinced, with good reason, of its continued glorious future.
2. Coal Geology and Geochemistry 2.1. Origin of Coal and its Reserves of the World Coal, an organic rock, was formed from partially decomposed and decomposing plant debris that collected in regions where waterlogged swampy conditions prevailed. These conditions prevented complete decay of the debris to carbon dioxide and water as it accumulated and the formation of peat gradually occurred. In general terms, the organic debris consisted of trees, ferns, rushes, lycopods, and several thousand plant species, the remnants of which have been identified in coal beds. Initially, most of the plant material making up the peat was biochemically broken down. Most of the cellulose was digested away by bacteria, and lignin was transformed into humic acid and humins. Partial combustion or biochemical charring also thermally altered some plant material. Still more plant material, such as spores and pollen, survived the diagenesis stage without much change. Strictly speaking, peat is not yet a real coal, and normally is not included in the coal series, but it is nevertheless believed to be the precursor of coal. During the first stage of coalification, the various biological-bacterial processes might have predominated over any other potential processes. When peat was buried underneath sedimentary cover, the biochemical stage was terminated, and a variety of physicochemical and chemical processes (metamorphic), determined by temperature and pressure, subsequently occurred. Then peat was transformed through brown coal (lignite) and bituminous coal, finally into anthracite. (Shown in Figure 1.)
Figure 1. Schematic representation of the coalification process Although coal seams are found in rocks of all geologic ages since the Devonian, the age distribution is not even. Major coal deposits of Carboniferous age occur in eastern and central North America, in the British Isles, and on the European continent. Major deposits of Permian age occur in South Africa, India, South America, and Antarctica. In Jurassic times the major coal accumulation was in Australia, New England, and parts of Russia and China. The last great period of coal deposition was at the end of the Cretaceous period and the beginning of the Tertiary period. Coals originating at this time are found in the Rocky Mountains of North America, Japan, Australia, and in parts of Europe and Africa. The distribution of coal seams throughout the world is also not uniform; most of the world’s coal is located in only three countries: the United States, Russia, and China. Although the figures vary from source to source, each of these countries has about 25 percent of the total coal resources, while the rest of the world shares the remaining 25 percent. The world’s estimated reserves of coal are about 6.9 Tt of hard coal and 6.5 Tt of brown coal. With current technology only about 7–8 percent of the total coal reserves are economically recoverable. However, this is a much higher amount than any other fossil fuel. The world coal production for 1991 was ca. 5.1 billion tonnes (short) and the major coal producing countries are China, United States, Russia, Germany, India, Australia, and Poland. 2.2. Coal Exploration and Mining
The purpose of coal exploration is to determine the nature, location, and extent of the resources available in a particular situation, and delineate the features that may affect their economic extraction. A program of geological exploration for coal usually has one of two possible objectives: first, to find an area from which a given amount of coal of a specific quality may be successfully recovered, or second, to determine the amount and quality of coal that can be economically extracted from a given area. Like other exploration activities, the evaluation of coal deposits involves the following operations. One must:
obtain legal title to explore the area evaluate the geological information already available carry out surface exploration carry out subsurface exploration collect and analyze samples estimate the coal resources and the significance communicate the results with other members of the project team.
Geophysical methods now play a critical role in many coalfield investigations. The techniques used at an early stage in the exploration program are normally those that give broad-scale information on a large area at relatively little cost. These include air-borne magnetometer investigation, regional gravity surveys, and broad-scale seismic studies, used to delineate the sedimentary and structural framework of the area involved. They may be followed by ground magnetic, electrical resistivity and more detailed seismic investigations that give a higher resolution of individual features, but at a significantly greater expense. Further information on the depth, thickness, and quality of coal at any point across the area, and of the strata with which the coal is associated, requires the effective use of exploratory drilling techniques such as core drilling and non-core drilling. Because of the wide range of information that the both drillings can yield, core samples of coal seam and the adjacent non-core strata should be examined in as much detail as possible. Especially with coal seams, but also overburden or mine roof and floor rocks, the collection of a complete core, with as little disturbance as possible, is vital to the success of this stage. Coal mining techniques can be divided into two categories: surface mining (open cast or open cut mining) and underground mining. Surface mining has a number of advantages over underground mining, including greater recovery, greater safety for personnel, and, in most cases, a greater level of overall productivity. The chief disadvantage, however, lies in the often-unfavorable impact, at least in the short term, on the surrounding environment. Surface mining involves two main methods. These are: first, strip mining, where the material above the seam (known as overburden) is emplaced directly from the digging equipment used to remove it from the ground, into an area immediately adjacent to the working place; and second, open-pit mining, where the overburden is moved from the face to an emplacement site some distance away, by an intermediate haulage or transposition process. However, most coal deposits so far have been extracted by underground mining due to thick overburden and the depth of coal seams. Underground mining includes room and pillar mining, short-wall mining, and long-wall mining. In conventional mining the operations are performed in a cyclical pattern as follows:
a slot is cut in the coal face to make controlled blasting possible a pattern of holes is drilled into the face around this slot the holes are charged with an approved explosive and fired
the broken coal is picked up from the floor of the opening and transferred to the haulage system the roof of the extended opening is supported as required.
In continuous mining, a single machine using a cutting head equipped with hardened metal picks breaks up the solid coal at the face without the need for any blasting opening. The continuous miner provides a greater rate of output than a cyclical conventional unit and requires fewer operating personnel. In order to meet the need of the modern coal industry, more advanced techniques for exploration and mining are in development. 2.3. Coal Geology Coal is the end product of a sequence of biological, geochemical, and geological processes. Coal geology, as an important field in coal science, deals with the formation, distribution, composition, and character of coals, as well as with the exploration, extraction, and utilization of coal resources. Paleobotanical, paleogeographic, and paleotectonic factors affected the evolution of coal formation; therefore, the various characteristics of coal from different basins are closely related to the different periods of geological history. Coal geologists both study this and attend to the relationship between the utility of coal and the development of human society. With industrial development and scientific progress, coal requirements have greatly increased and the study of coal geology has thus made great progress. Achievements in the fields of coal petrology, coal-bearing formations, coal basins, coal accumulating environments, coal metamorphism, the distribution of trace elements in coal, and the utilities of coal have been made, and this has greatly extended the content of the study of coal geology. Generally speaking, the formation of a coal seam represents the evolutionary process of a peat swamp. The formation of a thick or a thin coal seam often depends on crustal movement, which not only influences the thickness of coal seams, but also controls the split and thin away of coal seams. Studies on global and regional coal-accumulating laws show that there were several important coal-forming periods in geohistory, and the distribution of chief coal fields in each coal-forming period are all regularly zonal. The migrations of coalaccumulating zones and coal-accumulating centers are closely related to paleostructure and paleoclimate. Coal basin analyses play an important role in the understanding of regional coal-accumulating laws and thus offer a capacity for seeking both areas abundant in coals and areas within which are distributed excellent coals. For example, low-sulfur and low-ash coals were separated from high-sulfur and high-ash coals using the analysis of Carboniferous coal basins in Appalachia, in the east of the United States. Coal metamorphism—one of the important factors influencing coal ranks and qualities—can be divided into the following types according to its genesis and characteristics:
Geothermal or deep burial metamorphism. The degree of metamorphism of this kind increases markedly with increasing depth of burial. Telemagmatic metamorphism, resulting from the geothermal abnormality produced by magma actions. Contact metamorphism, caused by direct contact with magma intrusion. Hydrothermal metamorphism, resulting from the gas and liquid thermals generated by the actions of magma and groundwater.
Tectonic metamorphism, caused by the intensive compression and shear of strata due to crustal movement.
Many researchers in the world have studied the potentials and the geochemical characteristics of coal and coal measures with regard to producing other fuels. The discovery and exploitation of some industrial oil-gas fields confirmed the potentials of coal and coal measures in the United States, Australia, Indonesia, and Canada. Statistics suggest that about 70 to 80 percent of the large gas fields and their natural gas reserves are from coal measures. In the mid-1980s, it was found that the oil-generating ability of humic coal is related to the desmocollinite content of vitrinite. Recent studies of the repressed desmocollinite thus give us a new way to research coal-formed oil. 2.4. Classification of Coal The nature of a classification system will depend upon the particular application for which the system is to be employed. Classification of coal may be subdivided into scientific or genetic categories, and technical or commercial ones. In order to classify coals by means of numerical parameters, different kinds of tests are necessary, such as:
chemical analysis, including proximate and ultimate and so on technological assay, simulating coals in their behavior on heating petrographic analysis.
In addition, a number of supplementary tests have been developed, which in some special cases may be extremely useful, for example: the friability (grindability) test for combustion coals; the fusibility test of coal ash for combustion and gasification of coals; and the plasticity test for coking coals. Some of the newer analytical techniques that have been used to characterize coal are also being tested, for example, FTIR, NMR, DTG, Py-MS, GPC and so on. These techniques may provide parameters that are more reproducible than some of the conventional empirical tests. Coal rank is an important concept in all classification of coal. The rank of coal is the degree or stage that the coal has reached during its coalification process; that is, its degree of metamorphism or geochemical maturity. The parameters conventionally used to characterize coal rank are first, fixed carbon, carbon content, reflectance, and heating value (only for low rank coal), and second, volatile matter, and hydrogen content (only for anthracite). The other parameters used to characterize the technological properties or grade are caking and coking parameters, ash content, and sulfur content and so on. Every country with a coal industry has tended to develop its own criteria in order to classify its domestic coals, often for a particular application. The international system of coal classification came into being just after the Second World War as a result of the greatly increased volume of trade between the various coal-producing and coal-consuming nations and still finds limited use in Europe. It divides coals into two major types: hard coal, which is defined as any coal with a calorific value greater than 23.9 MJ/kg (5700 kcal/kg) on a moist but ash-free basis, and brown coal, defined as coal with a calorific value less than 23.9 MJ/Kg. In this system, the hard coals are firstly divided into classes according to their volatile matter, and then coals of the same class are subdivided into groups and subgroups according to their caking and coking properties, respectively. A three-digit code number is then employed to identify the coal. The first figure indicates coal class, the second figure indicates
the group into which the coal falls, and the third figure is the subgroup. In 1998 the International Classification of Medium and High Rank Coals was published to assist in characterizing coals involved in the international coal trade. The new system published involved eight parameters and a fourteen-digit code. The international system (ISO 2950, 1974) categorized brown coal on the basis of the total moisture content (ash-free) and the yield of tar produced from a dry ash-free sample. The moisture content is indicated by the class number (10–15), and the tar yield is described by the group number (00–40). A fourdigit code number is finally derived from the combination of the class and group number categories. 2.5. Geochemistry of Coal Coal geochemistry—a marginal and interdisciplinary subject—is rapidly developing to meet the need for the sustainable development of coal resources. It intensively examines the origin, migration, enrichment, and distribution of elements using the perspective and methodology of chemistry. It also studies the formation, composition, content, and distributing characteristics of minerals in coals, and it studies too the interactive and combinative relationship between organic and inorganic macerals to offer suggestions for the effective and clean utilization of coal. Coal is an organic rock, whose petrographic constituents were classified macroscopically by Stopes into four lithotypes: vitrain, clarain, durain, and fusain, and microscopically by Thiessem into anthraxylon, attritus (translucent and opaque), and fusain. The latter classification was based on visual characteristics of the constituents in thin section under transmitted light. It has since been replaced by measuring the reflectance on a polished surface because of the experimental convenience of the latter technique. European scientists define the micro constituents of coal as macerals, which are grouped into vitrinite, exinite, and inertinite according to the International Classification of Macerals of Hard Coals. Vitrinite and exinite are respectively equivalent to anthraxylon and translucent attritus in US nomenclature. Inertinite is frequently grouped into micrinite and fusinite, which are equivalent to opaque attritus and fusain, respectively. Macerals are derived from the particular plant tissues commonly preserved in peat swamps. Vitrinite is derived from variously decomposed woody tissues under watery and reduction conditions. It is the most frequent and important maceral group occurring in bituminous coals. Sometimes, cellular structures are visible in vitrinite under a microscope. Of the three maceral groups, vitrinite has medium reflectance, carbon, and hydrogen content-qualities, which vary according to coal rank. Exinite is derived from special plant tissues, such as spore and pollen coats, culticules, resins, and other fatty secretions, hence it is hydrogen-rich and has the highest volatile matter yield. Inertinites are derived from the partial carbonization by fire of various plant tissues in the peat swamp stage, and also from woody tissues by a special microbiological process under more or less oxidative condition for a short time; therefore, they are rich in carbon and chemically inert. The characteristic optical property of inertinites is their high reflectance. In the organic matter of coals there is a special group of organic substances known as biomarkers. Their content is very low, but is of great significance in geochemistry. They are derived from different organisms that underwent geological evolution and are still preserved in sediments. In coal extracts, some biomarkers have been found which can be used to analyze
the sources, formation environment, and the evolution of coals. The biomarkers in coal are usually alkanes, propylene/ phellogen, terpenoids, and steroids. Since some oil reservoirs were found to originate from, or relate to, coals or coal measure, the hydrocarbons derived from coal have become the main subject of the organic geochemistry of coal. Hydrocarbons derived from coal can be divided into two types: gaseous hydrocarbon (predominantly methane) and liquid hydrocarbon, which itself can be further classified into three types: light liquid oil, paraffin-poor crude oil, and paraffin-rich oil. Research has pointed out that the resinite-rich coals can produce immature light crude oil at low rank. 2.6. Mineral Matter in Coal Mineral (inorganic) matter in coal includes minerals and inorganic materials in or associated with macerals. The mineral matter content in mined coals ranges widely from about 5 percent to nearly 50 percent. Mineral matter in coal can be classified into five groups:
crystalline mineral particles and aggregates non-crystalline mineral detritus and aggregates inorganic elements and compounds associated with the organic molecules of macerals inorganic elements and compounds in the pore water and surface water in coal inorganic constituents in coal-bed gas.
Of all the above, minerals are the most important, since other inorganic elements are small in quantity, although they are numerous in species. Those elements with content below 1,000 ppm in most dry coal are called trace elements. Mineral matter in coal comes from a number of sources:
mineral matter contained in the coal-forming plant solid particles washed or blown into peat swamp water solution that has flowed into peat swamp ground water and hydrothermal solution that has flowed into seam cleats, fissures, and cavities gas trapped in coal seams volcanic eruptive products fallen into peat swamp.
Although the mineral species in coal may number more than a hundred, the common minerals are mainly the following:
clay minerals, such as kaolinite and illite-sericite oxide and hydroxide minerals, such as quartz or rutile sulfide minerals, such as pyrites carbonate minerals, such as calcite and siderite others such as feldspar, zircon, apatite, phosphorite, harite, and gypsum.
Apart from the minerals mentioned above, some very special minerals are found in a few coalfields. For example, in Germany and the UK, there is a kind of coal rich in saline, called "salt coal." In the Donetskiy Coal Basin in Russia, the coal contains cinnabar. In some
Mesozoic and Cenozoic coalfields in China, coal containing some uranium-bearing minerals is found. With the development of analytical techniques, more and more trace elements have been identified, and therefore greater attention has been focused on those elements, even if their content is below 1,000 ppm in most dried coals. Some trace elements such as uranium, germanium, and gallium etc. may have exploitable value as a resource. However, it is important to note that some of the trace elements in coal may pollute the environment during mining and processing. During the last twenty years, seventy of the trace elements have been analyzed and studied by scholars in various countries, during the course of which twenty-four elements (As, Cd, Cr, Hg, Se, B, and so on) have been found to have a potential influence on the environment. Ash is the residue derived from mineral matter when coal is burned off. The constituents of coal ash may be classified into acidic and basic. The acidic constituents are silica, alumina, and titania; the basic constituents are iron, calcium, magnesium, and alkaline oxides. The ratio of acidic to basic constituents dictates the ash-softening temperature and slag viscosity. During the incineration of mineral matter to ash, various weight changes take place due to loss of hydration water from minerals, loss of carbon dioxide from carbonates, oxidation of pyrite to iron oxides, and fixation of sulfur oxides by calcium and magnesium oxides. The ash content, therefore, is usually less than the corresponding percentage of mineral matter originally present in coal. Research on the mineral matter in coal is thus very significant for coal geology, exploitation, storage, preparation, combustion, gasification, and liquefaction, as well as other uses of coal, such as marketing and so on. For example, mineral matter is an excellent geochemical indicator in the study of the depositional environment of coal and that of other problems in coal geology. Also, the particle size and association modes of minerals with coal macerals are important factors in coal preparation, while slagging and fusion behaviors of coal ash are important factors in the evaluation of boiler coal and so on. 3. Coal Technology 1 This section is a discussion of coal structure, properties, and related environmental problems. 3.1. Coal Structure and Properties Coal is a very complex heterogeneous solid that varies widely in its physical and chemical properties. Viewed in terms of physical entities (macerals, minerals, and pores), macerals, molecules, and structural units, coal is a heterogeneous matter on different levels. Therefore, there is no universal molecular structure available for various kinds of coals. Since the beginning of the last century, coal structure research has been a hot issue in the field of coal chemistry. Research on the illustration of coal structure has, in the last twenty years, made substantial breakthroughs. This has been achieved using a series of physical and chemical approaches such as NMR, FTIR, Py-GC-MS, solvent extraction and swelling, flash pyrolysis, and different depolymerization or degradation reactions etc. Computer-aided molecular design (CAMD) has also been especially useful, as have newly developed coal- processing technologies. It is well-known that all coals consist of organic and mineral matters. The former is composed of carbon, oxygen, hydrogen, nitrogen, and sulfur. The carbon content ranges from 65 percent
to 95 percent, and increases during the coalification process, with a simultaneous decrease in the percentage of oxygen and hydrogen which typically range between 2 percent and 30 percent, and 2 percent and 7 percent, respectively. Nitrogen and sulfur contents are independent of the coal rank, and are normally less than 2 percent. The basic structure of coal is a graphite-like aromatic/ hydroaromatic system. The average number of condensed aromatic rings is two to four, with the exception of anthracite; the non-aromatic part of the molecules consists mostly of a cycloparaffinic and hydroaromatic ring system, and there are few alkyl and oxygen-containing functional groups. The aromaticity of coal varies with rank from about 0.66 for lignite, over 0.75–0.88 for bituminous coal, to above 0.90 for anthracite. There are mainly two kinds of coal structure models: the "two phase" (immobile and mobile phase) model and the "monophase" model. The "two phase" model currently has widespread acceptance and considers that coal consists of covalently cross-linked, three-dimensional networks, similar to a cross-linked polymer but without repeating monomeric units, and containing small amounts of low molecular weight substances trapped in the network. This model can explain a lot of coal properties and behaviors. Recently it was found that an Nmethyl-2-pyrrolidinone/carbon disulfide mixed solvent can extract up to 60 to 79 percent by weight of some bituminous coals at room temperature. This means that the covalent crosslinking only makes a small contribution to the coal structure and the co-operative nonvalent interactions, such as hydrogen bonds, p-interactions, and charge transfers can form rather stronger associations. Thus a new monophase model has been proposed: coal consists of molecules with a continuous distribution of molecular weight from giant aggregates to smaller molecules associated through the interactions mentioned earlier. In this area, further studies are still needed. Important physical properties of coal relating to its processing and utilization are specific gravity (density), magnetic and optical properties, surface and thermal properties, grindability or friability, and so on. Differences in these properties permit separation of mineral matter from coal substance, the concentration of coal maceral groups in different fractions, the manufacture of coal-water or coal oil slurries, and the characterization of different kinds of coals. Important chemical properties of coal also relating to its processing and utilization are pyrolysis (thermal decomposition, or carbonization), hydrogenation, oxidation, depolymerization, and other organochemical reactions. Because all of the major coal processing technologies (such as coke making, gasification, hydroliquefaction, and combustion) essentially belong to a thermolchemical process, the pyrolysis behaviors of coal have long been seen as their most important characteristic. Coal pyrolysis is a destructive heating of coal in the absence of air with the production of a solid, porous, carbonaceous residue called coke or char, and the evolution of volatile gas, tar, and light oil products. The product yield and composition vary with coal rank and pyrolysis conditions such as temperature, heating rate, residence time, and the presence of hydrogen. The behaviors of coal mineral or ash at high temperatures are also a subject of pyrolysis study. 3.2. Coal Preparation Coal preparation is a pre-treatment process to crush and grind coal, and then to separate undesirable mineral-rich parts. A physical approach called physical cleaning or beneficiation is commercially used to control the heating value and physical characteristics of coal. It is a
more efficient utilization of coal in both combustion and conversion (removing potential pollutants such as sulfur-containing minerals prior to combustion) that has become an important alternative means of meeting air quality standards. Obviously, it is of great significance for clean coal technology. Basic unit operations in current preparation plants consist of crushing and washing. Crushing is needed to liberate the heavier minerals or mineral-rich parts from the desirable but relatively lighter components of coal for enhancing the separation efficiency. After having been liberated by crushing, the undesirable parts of raw coal are removed by washing, or other separation methods. The number of processes needed in a complete washing circuit is dependent on the washing ability of crushed coal, and the desired quality of washed coal. The most commonly used coarse-coal washers are the heavy-medium vessel and the Baum jig types, while particulate-coal washers include the heavy-medium cyclone, hydrocyclone, Batac jig, concentrating table, and Humphreys Spiral Concentrator types. Generally speaking, the heavy-medium vessel is superior to Baum jigs for cleaning coal larger than 6.35mm. The recovery efficiency (> 98 percent versus 89 to 94 percent) and sharpness of separation (error area of 20 to 25 versus 75) are better for heavy-medium vessels than for Baum jigs. However, the capacity of the Baum jig to clean a wide size range (from 15.24 cm to 200 mesh) is desirable from the standpoint of capital investment and operating cost. The heavy-medium cyclone and Batac jigs are used for particulate-coal (1.27 cm to 28 mesh) cleaning; the former is a commercial particulate-coal washer capable of the near-duplication of laboratory float/sink separations, while the latter’s performance characteristics are similar to those of Batac jigs in coarse-coal cleaning. When coal is crushed in a preparation plant, a certain portion of the coal is reduced to a fine consistency. The fine size coal is often disposed of in the wastewater pond because conventional fine-coal washers are unable to recover the 200 mesh fraction with reasonable efficiency. As coal prices and concern for environmental protection increase, however, recovery of fine-coal from the wastewater pond becomes more and more attractive. The physical methods for fine-coal cleaning include froth flotation, oil agglomeration, magnetic separation, and dry separation processes. The flotation method is based on the differences in surface property. Coal (organic matter) is considered to be naturally hydrophobic, while most coal minerals are hydrophilic, and pyrites are intrinsically neither strongly hydrophobic nor hydrophilic. The optimum coal particle sizes used in flotation range from 48 to 150 mesh. However, very fine coal of 200 to 325 mesh has also been processed. As a matter of fact, flotation is the only method commercially available for the cleaning of coal of more than 200 mesh. Much of the pyrite content in fine coal can be rejected together with the high-ash refuse during coal flotation by optimizing process variables, such as frother quantity, solid concentration in slurry, and aeration. However, a portion of the pyrites cannot be removed under these conditions because their particles are too fine to entrap in the froth and thus attach to the floatable coal. The need to further reduce the pyrite content that remains in the singlestage clean coal concentrate has led to the development of a two-stage flotation process. As regards oil agglomeration, it is more effective than froth flotation in ash reduction, but less effective in sulfur reduction. Magnetic separation, especially high gradient magnetic separation (HGMS), offers a useful means to remove 60 to 84 percent pyrite sulfur at a coal recovery rate of 85 to 93 percent. 3.3. Clean Coal Technology
The term "clean coal technology" is a quite new conception first put forward by American and Canadian coal scientists in the 1980s. Soon afterwards, the United States took the lead in carrying out clean coal technology programs, focusing on the elimination of sulfur oxide pollution derived from high-sulfur coal combustion. Since the 1992 UN conference on Environment and Development in Brazil, more and more countries have paid close attention to the production of clean energy and its effective utilization. As mentioned before, coal emits sulfur oxides, nitrogen oxides, and particulate and solid wastes when coal is burned in boilers or at power plants. In addition to such pollutants, coal emits a large amount of carbon dioxide, which may be responsible for global warming through its greenhouse effect. Nowadays, the term clean coal technology is used worldwide to represent a series of new coal utilization technologies that are environmentally clean, highly efficient, and economically acceptable. Clean coal technology can reduce emissions of sulfur oxides, nitrogen oxides, and other pollutants at various points of coal use, from a mine to a power plant or factory. They offer the potential for the cleaner use of coal, thereby having a direct effect on the environment, and contributing to the solution of problems related to acid rain and global climate change. They also promote the use of coal and thus offer some degree of energy security to those countries that are not oil importers but have plentiful supplies of coal. The current clean coal technologies can be summarized as follows:
Clean and effective combustion techniques involving the development of new combustion technology with high efficiency and low pollution such as PFBC, IGCC and CFBC; the development of new technology in fuel gas cleaning, and the increase in the technology level of newly-built power stations etc. In coal preparation, the development of new technology in coal washing and separation to substantially reduce the ash and sulfur content of coal. The introduction of large-scale coal gasification technology for power generation, town gas, and synthetic fuel. The development of coal briquetting and coal-water slurry technologies. The development of long-term clean coal technologies; for instance, those of fuel cell, coal liquefaction, underground coal gasification, and magnetohydrodynamic (MHD) techniques.
In order to solve the conflict between energy supply and environmental protection, it is vital to actively develop clean coal technologies. In this area, international co-operation must also be promoted. 3.4. Desulfurization of Coal The total sulfur content of coal varies within a single deposit as well as between deposits. It ranges from below 0.5 percent to roughly 10 percent, but mostly between 0.5 and 1.5 percent. Sulfur present in coal exists in either inorganic or organic forms. The inorganic form is mostly pyritic sulfur (FeS2), although smaller amounts of sulfate and elemental sulfur are also observed. The forms of organic sulfur are less well established. In summary, the majority of the organic sulfur in high rank coal is in thiophenic forms, and the proportion of nonthiophenic groups including sulfide forms is higher in low rank coal, but the presence of thiol forms in coal is unclear. Increasingly stringent regulation for SOx emissions during coal combustion has encouraged the technical development of coal cleaning for sulfur removal.
Three methods for the desulfurization of coal before, during, and after combustion have thus appeared. Coal desulfurization techniques before combustion are usually classified into three methods: physical, chemical, and biological sulfur removal. Physical cleaning techniques to remove inorganic sulfur (mainly pyrite sulfur) are well established and widely used. On the other hand, chemical and biological cleaning have not yet been applied commercially. As mentioned above, during the physical cleaning of coal, the pyrite sulfur in coal is normally removed with the simultaneous decrease of coal mineral content. The main chemical cleaning processes involve oxidative leaching, chlorinolysis, leaching with an alkaline solution or melt, and microwave irradiation. In many cases, the desulfurization efficiency of pyrite sulfur is over 80 percent, whereas the extent of organic sulfur removal is less than 50 percent. Biological desulfurization of coal is potentially attractive because the process can operate usually at approximately ambient temperature and thus removes the sulfur selectively without carbon loss. In general, the biological desulfurization of coal can remove up to 90 percent of both sulfur forms. However, there remain some problems and limitations; for example, very low reaction rate, difficulty for microorganism culture, and maintaining the necessary reaction conditions. In fluidized bed combustion and gasification, coal particles and calcium-based materials are mixed and fluidized by gas jets at temperature below 1273K, and the sulfur involved in the conversion process is immediately captured in beds as CaSO4 or CaS. Limestone and dolomite are well-proven in-bed desulfurization sorbents. Fluidized bed combustion (FBC) processes usually operate with a Ca/S ratio of 2 to 4 for sulfur removal up to more than 90 percent. In contrast with SO2 capture, the removal of H2S evolved in coal gasification (a reducing gas atmosphere is thermodynamically limited) an additional desulfurization step may be needed to meet with gas turbine and emission requirements. To reduce the use of this method, the minimization of the costs for sorbent and residue management is required. Flue gas desulfurization (FGD) is a post-combustion method to remove SO2 from the flue gas of coal-fired plants and is the most widely used technology, so far, for controlling SO2 emissions. At the end of 1996, there were 590 FGD installations in operation with a total capacity of 230 GWe, which corresponded to 22 percent of all existing coal-fired capacity (922 GWe). Currently, 93 percent of FGD installations are in OECD countries, in particular in Japan, Germany, and the United States. Commercial FGD technologies can be grouped into five categories: wet scrubbers, spray dry scrubbers, sorbent injection, regenerable processes, and combined SO2/NOx processes. Wet scrubbers are the commonly used FGD technology, having the largest market share-75 percent or 86 percent of the total installation unit or capacity, respectively followed by spray dry scrubbers and sorbent injection processes. The majority of the wet scrubbers are of the wet lime or limestone/gypsum variety, in which CaCO3 reacts with the SO2 in water finally to produce CaSO4·2H2O. An SO2 removal efficiency of more than 90 percent can be achieved with an almost stoichiometric sorbent consumption. 3.5. Environmental Problems Arising from Coal Handling and Processing Coal is an "unclean" fuel, which contains numerous impurities, mainly mineral matter, sulfur, nitrogen, and trace elements. All impurities are of little help in coal utilization and cause a number of problems regarding environmental sustainability. Coal holds less hydrogen and more carbon and oxygen than oil and natural gas, which is also environmentally unfriendly. Atmospheric pollutants arising from coal systems mainly include particulate matter: SO2, NOx, hydrocarbons, and trace elements. The particulate matter of most concern in the coal
system is coal dust, fly ash, smoke or soot, and acidic aerosols. Particles less than or equal to 10 µm in diameter are called PM10, whose concentration in the air is a target monitored and regulated conventionally. In 1997, the US Environmental Protection Agency (EPA) instituted a new air quality standard to control PM2.5 (particles of a diameter less than or equal to 2.5µm) for the first time. Sulfur oxides are the other main pollutants. In spite of the adoption of desulfurization processes for coal-fired plants in most industrial countries, the global emissions of sulfur oxides from human activities are still large:-at the present time around 70 to 80 Mt of sulfur per year. High-temperature combustion of coal produces nitrogen oxides, primarily NO and NO2, known jointly as NOx, whose amount is closely related to the combustion temperature, air-fuel ratio, burner type, and nitrogen content of coal. Fluidized bed combustion is generally operated at low temperatures between 1073K and 1173K, and typically it only has NOx concentrations in the fuel gas of 100 to 150 ppmv. The greenhouse gases arising from coal production and utilization are carbon dioxide, methane, and nitrogen oxides. Carbon dioxide is emitted in very large quantities from coal combustion. Global emissions of carbon dioxide from coal amount to about 2.39 Gt carbon per year, in contrast to 2.54 Gt carbon per annum from oil, and 1.00 Gt carbon per annum from natural gas. In addition to atmospheric pollutants, most coal processing plants, like coal preparation, carbonization, gasification, liquefaction, and combustion, yield large quantities of wastewater. In wet scrubbers, used for gas cleaning and most technologies for physical coal beneficiation, process water is required to contact directly with coal or related products, and many pollutants are thus transferred to it, forming wastewater. The pollutants are generally suspended solid particles, inorganic cations and anions, phenols, oils, and the like. Ammonia liquor occurring in coke-making plants is a typical wastewater, contaminated by coal processing. Another typical water pollution is acidic mining drainage caused by coal mining. A perceivable deterioration of the environment has increased the emphasis on pollution control. In the coal system, rapid progress has been made over the last decades on the standard regulation of atmospheric pollutant emissions for coal-fired plants. On the whole, the emission standards have become more and more stringent. Taxation policy is sometimes used to orient coal-fired plants towards upgrading with regard to pollutant emission control. Most countries limit the particulate emissions from the stack of coal-fired power plants. The current particulate emission standards for new coal-fired plants set by the EU are 100 mg m–3 for plant sizes of 50 to 500 MWt, and 50 mg m –3 for plant sizes of 500 MWt or more. In the United States, the Acid Rain Program of the 1990 Clean Air Act Amendments (CAAA) aimed at cutting annual emissions by 10 million tonnes, to about 60 percent of 1980 levels by 2010. Control of NOx emissions from coal-fired plants has just recently been considered in most countries. Japan is the country with the earliest limits on NOx emissions having had leglislation in place since the 1970s. More stringent standards with 515 mg m–3 for 32 to 560 MWt plants and 410 mg m–3 for more than 560 MWt plants were introduced in 1989. Australia is one of the few countries that have set specific trace element limits for coal-fired plants. Australia’s limits on fuel gas are 2.0 mg m–3 for Cr, Pb, and Zn (together), 0.5 mg m–3 for As, Co, and Ni (together), and 0.05 mg m–3 for Cd and Hg (separately). But, at present, there are no CO2 emission standards for coal-fired plants. However, several countries have national programs proposed in the 1992 UN Framework Convention on Climate Change to stabilize greenhouse gas concentration in the atmosphere. 4. Coal Technology 2
This section refers to all the main technologies for coal utilization and processing, including coal combustion, thermal decomposition, carbonization, gasification, and liquefaction. In all thermal-chemical processing of coal, the thermal decomposition of organic matter in coal is involved as a primary step. 4.1. Coal Combustion Nowadays, direct coal combustion is extensively utilized for industrial and domestic purposes because of the large-scale reserves and low cost of coal. Most of the world’s coal is burned in boilers of power plants, industrial boilers, and kilns. Space heating and domestic use also consume a large amount of coal every year. The statistical data shows that nearly 47 percent of global electricity comes from coal-fired plants. The coal characteristics affecting combustion include:
heating value volatile matter yield or coal rank ash content and fusion characteristics sulfur content moisture content coking properties and so on.
They make significant impacts on process efficiency, boiler design, combustion operation, and environmental pollution. It is known that the principal combustion process of coal involves three basic stages:
release of volatile matter resulting from the thermal decomposition of coal, or coal devolatilization ignition and burning of the released volatile matter ignition and burning of the remaining char.
Depending upon various combustion conditions, the burning process of volatile matter and coal char may take place simultaneously, sequentially, or with some overlapping. Depending upon the size and rank of coal, and heating conditions, coal devolatilization takes from a few milliseconds to several minutes or more to complete. The volatile matter reacts with oxygen in the vicinity of coal particles and forms bright diffusion flames. The reaction between char and oxygen is a gas–solid heterogeneous reaction, being often much slower than the devolatilization and volatile matter burning process. The lowest temperature at which coal can be ignited is referred to as ignition temperature. In general, coal containing a higher volatile matter has a lower initial volatile-releasing temperature and is easier to ignite. Compare lignite, with an ignition temperature of 523K to 723K, to anthracite, which burns at 973K to 1073K. On the basis of particle size and feeding methods of burning coal, coal combustion technology can be classified into three kinds: fixed-bed combustion, suspending combustion, and fluidized-bed combustion. At one time, the fixed-bed combustion of coal was the only known way of burning it. Here, the coal bed is supported on a grate, being fixed or movable, and the air needed for combustion generally passes upward through the coal bed either by chimney or by fan. It is
obvious that this combustion model is particularly sensitive to coal properties and its steam output is limited. Suspending combustion, also called pulverized coal combustion, was first used as a means of firing cement kilns. From 1930 onwards, nearly all coal-fired power plants and large industrial boilers have been fired by pulverized coal. In practice, the fineness ranges from 60 percent through 200 mesh for lignite and bituminous coal, to 85 percent for anthracite. Not more than about 20 percent should be retained on 50 mesh. The pulverized coal, injected into the combustion chamber by a conveying air stream, forms a jet at the burner outlet. On entering the chamber, the fine coal cloud is heated by the fire and high temperature furnace wall, then ignites and burns. The coal fluidized-bed combustion was developed in the 1960s. It is widely applicable to various coal combustion facilities, from small furnaces to large power generating boilers. Its principal advantages, as in the last section mentioned, are the following:
it achieves desulfurization in combustion it depresses the formation of NOx it simplifies coal feeding and ash removal.
The main categories of coal fluidized-bed combustion are the atmospheric pressure fluidized bed combustor (AFBC), the circulating fluidized bed combustor (CFBC), and the pressurized fluidized bed combustor (PFBC). The CFBC and PFBC types, both of which are associated with clean coal technology, have been under speedy development. 4.2. Thermal Decomposition of Coal In the absence of air, coal will produce gas, coal tar, and char. This process is termed as thermal decomposition, or the pyrolysis of coal. The terms "thermal decomposition," "pyrolysis," and "carbonization" are often interchangeable. However, carbonization is more correctly applied to the process of the production of char or coke when the coal is heated at temperatures in excess of 773K. Thermal decomposition of coal is especially important since it is the initial step in most coal conversion processes and is the step that is strongly dependent on the coal properties. Coal will undergo a variety of physical and chemical changes when heated to a temperature at which thermal decomposition occurs. The overall decomposition process is generally composed of three successive stages:
The release of water, absorbed methane, and carbon dioxide from coal at temperatures below 473K. Active decomposition, leading to the generation and discharge of the bulk of volatile matter, between the initial decomposition temperature of coal and about 820K. The secondary degasification and condensation of char, resulting in the formation of gases with hydrogen as their main component and coke, at temperatures from 820K to 1273K.
During the second stage, typical bituminous coals will undergo a series of interesting changes in appearance. These coals may soften, melt, fuse, swell, and form "plastic mass," releasing
simultaneously volatile matter. With the further increase of temperature, the "plastic mass" will re-solidify to form char. The gases derived from the thermal decomposition of coal are largely composed of hydrogen, gaseous hydrocarbons with methane as main component, carbon monoxide, ammonia, and hydrogen sulfide. Their yield and composition are strongly dependent on coal properties and heating conditions, especially the final temperature. In fact, the carbonization gas has a heating value of about 18.6 MJ per cubic meter and is therefore a medium-Btu gas. The tar and light oil products from coal pyrolysis contain a number of valuable chemicals, such as benzene and its derivatives, phenol and its derivatives, polycyclic aromatics, and heterocyclic aromatics, and are therefore excellent sources of chemical feedstock. Char and coke are solid products of coal pyrolysis. The former is usually used as a clean and smokeless fuel and the latter is widely used in the iron-making industry. The most important factors affecting the thermal decomposition of coal are coal rank, maceral composition, particle size, temperature, heating rate, atmosphere, pressure, and reactor configuration. The rapid heating technologies in coal pyrolysis give substantially larger yields of volatile matter than that are obtained by the slow heating of coal in conventional packedbed carbonization retorts or standard volatile matter crucibles. Pyrolysis of coal in a hydrogen atmosphere, called hydropyrolysis, results in larger weight losses and a significantly different product distribution. The main part of the increase is in the methane content produced from the reaction of hydrogen with the active carbon atom. Both flash pyrolysis and flash hydropyrolysis have come under consideration as possible advanced processes for the conversion of coal to gaseous and liquid fuels on a commercial basis. Some of the techniques that have been employed to study the pyrolysis of coal are:
laser microwave flash tube plasma electric arc shock tube electric current entraining gas methods.
Most of these techniques can be classified only as research tools useful as characterization techniques, but the entraining gas and plasma techniques are under consideration for commercialization. Low temperature (less than 1073K) carbonization is an old art process, including retort, entrained-bed, and fluidized-bed carbonization. The retort processes range from batchwise to continuous, from vertical to horizontal, and from complicated mechanical devices to relatively simple ones. The typical industrial processes of low temperature carbonization are:
the FMC (Food Machinery Corporation) coke process the Char Oil Energy Development (COED) process the US Steel Clean Coke process the Lurgi-Ruhrgas process, and so on.
4.3. Carbonization of Coal High temperature carbonization of coal, with a global coal consumption of about 450 million tonnes per year, represents one of the major uses of coal, and is an essential process for the production of metallurgical coke, high temperature coal tar, and gas. The term "high temperature carbonization" is usually called carbonization for short and is also known as coke making. The commercial importance of coke as an effective reducing agent in iron making can be traced back to 1707. In the last 300 years, coking technology has experienced great changes, from the oldest pile ovens and beehive ovens, through the so-called flame ovens and initial heat regeneration and by-product recovery ovens, to the current advanced large-scale ovens. In 1992, the Kaiserstuehl coking plant, with the most advanced technology in the world and almost perfect environmental protection measures, was built in Germany. Its oven chamber has a height of 7.63m, a length of 18m, and a width of 0.61m on average, and has the largest effective capacity (78.84 cubic meters) in the world. Production of a good metallurgical coke depends primarily on two factors: first, the coking or plastic properties of coal, which are essential for the formation of a coherent structure with proper qualities, and, second, the grade of coal, determined generally by the ash and sulfur content, which affects the grade of coke produced. Apart from the deficient supply of coking coal for the rapid development of the coking industry, some other factors, such as the low yield of chemicals from low volatile matters in coal and its high swelling pressure against the oven wall, have urged people to find some way out by using a blend of various kinds of coal. Blending bituminous coals of different volatility in appropriate proportions can meet the technological specifications of coke making. Also, a greater yield of chemicals and a longer operation life for the coke oven can hence be realized. In the coking chamber, heat is supplied from either side of the chamber walls to the coal charge, and consequently, different layers coexist in the feedstock, owing to the temperature distribution. After eight hours of the coal being fed to the chamber, it has been found that directly adjacent to the chamber wall a coke layer already exists. Next to it is a semi-coke layer, followed by a plastic mass layer, then a layer of heated coal, and finally, in the center, there is a layer of coal with comparatively low temperatures. In an oven chamber, the two plastic layers move slowly from the opposite walls toward each other and finally meet at the center. The junction of the layers appears as a vertical crack running lengthwise through the oven at the center of the charge. When an oven chamber is pushed, the coke cake divides vertically at this crack. Advanced iron-making technologies require more and more stringent coke quality control. Its specifications are normally as follows: volatile matter must be less than 1.5 percent; ash content less than 10 percent; and the sulfur content less than 1.0 percent— preferably 0.5 percent. In addition, the post-reaction strength must not be less than 58 percent; the mechanical strength (the crushing strength parameter of coke, called M40) must be more than 85 percent; and the abrasive strength parameter of coke (called M10) must be less than 7 percent. Coke ovens are the specialized equipment for coke making, usually composed of a coking chamber, a combustion chamber, regenerators, slits, top, foundation, waste gas channel, and so on. The horizontal slot-type coke oven is the most modern and economic, and is of paramount importance for coking bituminous coal to produce high-quality blast furnace or foundry coke. In order to conserve heat and space, modern coke ovens are built in batteries. A battery may contain from 20 to over 120 ovens. Coke ovens vary greatly in capacity and range
from small ovens capable of coking about four tonnes of coal per charge, up to ovens with a capacity of sixty tonnes or more. According to the method of charging coal to the oven, the coke oven types are classified as the ovens of charging coal on top and of charging stamped coal cake in the side direction. Some coke-making processes under active development include further increasing the capacity of the coking chamber (for instance, the Jumbo coking reactor) and developing a noby-product recovery coking process, as well as developing a formed coking process. 4.4. Coal Gasification Coal gasification generally refers to the reaction of coal with air, oxygen, steam, carbon dioxide, or a mixture of these gases, to yield a gaseous product; coal gas. Gasification of coal under hydrogen atmosphere is called hydrogasification. Coal gasification has been developed and abandoned periodically during the past two centuries. The first oil crisis in 1973, for example, altered the price and availability relationship between oil and coal and provided a strong incentive for coal gasification technology. Since then, many coal gasification processes (also called second or third generation technologies for coal gasification) have been developed or are under development. Coal gas is a mixture of such gases as carbon monoxide, hydrogen, carbon dioxide, nitrogen, and methane. According to heating value, coal gas can be classified into three kinds: lowheat-content (also termed low-Btu) gas with a heating value below 7 MJ m–3; medium-heatcontent (or medium Btu) gas with a heating value between 7 and 17 MJ m–3; and high-heatcontent (or high Btu) gas with a heating value about 37 MJ m–3, consisting mainly of methane. For the production of each specific gas, coal is first crushed and sometimes dried, then fed into a gasifier in which coal reacts with steam and either air or oxygen. The gasification reaction usually occurs at high temperatures from 1073K to 1873K and high pressure up to 10 MPa. When coal is burned with less than a stoichiometric quantity of air, with or without steam, the product is low-heat-content gas, which after purification can be used as fuel gas. Utilizing oxygen in place of air produces medium-heat-content gas. The gas produced is used as synthetic gas and some of the CO in the gas must be reacted with steam (known as shift reaction) to get additional hydrogen. High-heat-content gas is a synthetic natural gas (SNG), which is usually produced from the medium-heat-content gas by a catalytic methanation process. Coal gasification, similar to coal combustion, involves two distinct stages, they are devolatilization and char gasification. Although the initial stage is completed within seconds, or even less at elevated temperatures, the subsequent gasification of coal char produced at the devolatilization stage is much slower, requiring minutes or hours to obtain significant conversion under practical conditions. Since reactor designs for commercial gasifiers are strongly dependent on the coal-char reactivity, the kinetics of coal-char gasification systems have been extensively investigated. It is generally considered that high reaction temperature, high reaction pressure, high reactivity of coal char, and fine particulate coal feed, all favor the gasification process. There are a number of gasification processes with variations in reactor type, gasification agent, heat supply mode, state of feed, and ash removal. According to the type of gasifiers, coal gasification processes can be divided into four groups:
moving bed or fixed bed process, such as Lurgi and BGC-L (British Gas Corporation-Lurgi) gasifier fluidized bed process, such as Winkler, High-Temperature Winkler, and Ugas gasifier entrained bed process, such as Koppers-Totzek, Shell-Koppers, Texaco, and Dow gasifier molten bath bed process, such as Iron Bath gasifier.
The coal gas produced in coal gasification is widely used in a number of ways:
as fuel gas for direct combustion and for production of SNG in the combined cycle gas/steam turbine power station for hydrogen production for the production of reduction gas as synthetic gas for the production of ammonia, methanol, oxoalcohols, and liquid hydrocarbons.
As an advanced power generation system, the Integrated Coal Gasification Combined Cycle (IGCC) offers an attractive approach to producing electrical power at high efficiency and has shown the greatest potential for meeting stringent emission control requirements. Therefore coal gasification is a very important clean coal technology from the viewpoint of sustainable development. 4.5. Coal Liquefaction Coal is a solid carbonaceous material with H/C atom ratio of about 0.7, substantially lower than that of petroleum (1.5 to 2.0), and with much higher molecular weight than petroleum. Therefore coal conversion into liquid hydrocarbons requires hydrogen addition and chemical degradation. It may be conducted directly through hydro liquefaction and indirectly through Fischer-Tropsch synthesis. Direct coal liquefaction involves hydrogenation of coal in a solvent slurry with addition of catalyst at an elevated temperature of 643K to 753K and an elevated pressure of 15 to 30 MPa under hydrogen atmosphere. So long as coal is heated up to its initial decomposing temperature, the weak bonds connecting structural units of coal begin to break up to yield radical fragments, which will be stabilized by the abstraction of active hydrogen to form preasphaltene and asphaltene. The cracking rate is accelerated with increasing temperature. At higher temperatures, this step proceeds very fast, needing only less hydrogen and is not dependent on a catalyst. On the other hand, the conversion of the primary products mentioned above into oils is a slower process and needs favorable hydrogen-donor condition, a higher consumption of hydrogen, and active catalysts to obtain a desirable oil yield. For guaranteeing the hydrogen-donor ability of the recycling solvent (oil), a prehydrogenation process is usually conducted in a special reactor, such as in the Exxon Donor Solvent (EDS) process and the New Energy Development Organization (NEDO) process. The main products of coal hydroliquefaction are liquids and gases along with solid residue, including coal minerals, catalysts, and insoluble organic matter. The liquids are subsequently separated from the solid residue by flash distillation or by filtration. The liquid products, with a yield of distilled oil of about 55 percent by weight on coal, daf basis, will be further processed to motor oils and chemicals. The solid residue can be gasified to produce hydrogen required in the process and/or carbonized to recover additional oils.
Indirect coal liquefaction, as the term suggests, is a process in which the feed coal is first gasified to a synthesis gas (CO+H2), and subsequently the gas is fed into synthesis converter to form liquid hydrocarbons and oxygenates. A well-known technology, Fischer-Tropsch (FT) Synthesis, has already been commercialized for several decades and now is used in Sasol, South Africa, with a total production capacity of about 5 million tonnes per annum. Catalysts used in F-T Synthesis include iron, cobalt, nickel, and ruthenium. Iron-based catalysts especially have been widely used commercially. When synthesis is operated under medium pressures, and reaction temperatures accordingly lie in the range of 493K–513K, the iron-based catalysts display a good activity in F-T Synthesis. Reactors used in the F-T Synthesis process can be classified into three types: fixed bed, entrained bed, and slurry bed. Generally speaking, both coal liquefaction technologies need high investment and also have a high operating cost. When the oil price is still relatively low, economics perhaps is the crucial obstacle to the commercialization of coal liquefaction technologies, although demonstration plants in the United States, Germany, and Japan have already justified technical feasibility. It is a single exception that the F-T Synthesis plants in South Africa have been established and operated up until now. Nevertheless, oil prices will sooner or later rise because of decreasing reserves of crude oils, and, with the increasing need for motor oils, coal-derived oil as synthetic oil will become one of the important alternatives. 5. Oil Shale 5.1. Introduction Oil shale is a kind of solid fossil fuel. It is defined as a sedimentary rock containing combustible organic matter, commonly called kerogen, as well as a much smaller bitumen content, both embedded in an inorganic mineral matrix. As a potential source of energy, oil shale can be used for producing shale oil and combustible gas by retorting, or for producing steam and electricity by direct combustion. Although shale oil in today’s world market is not competitive with petroleum, natural gas, or coal, it is still used in several countries that possess easily exploited deposits of oil shale but lack other fossil fuel resources. Oil shale deposits occur widely throughout the world. Earlier attempts to determine the size of world oil shale resources were based on few facts, and estimating the grade and quantity of many oil shale deposits were, at best, a guess. The situation today is not much better. However, it is reported, as a solid fossil fuel, the organic matter of the world’s oil shale deposits is second only to coal in abundance. As a source of liquid fuel, the world’s shale oil reserves calculated from known oil shale deposits are higher than the world’s known crude oil reserves. Therefore it is seen as a potential alternative to crude oil in the future. The world’s top ten countries in terms of plentiful, known shale oil resources expressed in billions of tonnes are as follows:
United States Brazil Russia China
280 112 48 26 (proven, 1.8)
Australia Zaire Canada Jordan Estonia Monaco
25 (recoverable, 4) 14 12 5 4(recoverable, 1) 3.4
5.2. Origin and Formation Oil shale deposits range from Cambrian to Tertiary in age and occur in quantities ranging from minor occurrences of no economic value to deposits occupying thousands of square kilometers, and of a thickness of more than 700 meters. Most geologists have recognized that petroleum and oil shale may originate from the same source materials. The main original materials of oil shale are planktonic organisms such as algae, as well as unicellular protozoa and other animals such as worms and mollusks, while some higher aquatic plants are also present as source materials. After the perished organisms had settled at the bottom of the water body, the organic matter underwent a long period of biological action in an oxygen-deficient, still water environment and turned into jelly-like algal ooze, which changed gradually into sapropel under the action of anaerobic bacteria. In addition to the organic matter, large amounts of inorganic matter, such as clay, mud, and sand, were also carried by flowing water and deposited along with the dead organisms. Sapropel underwent chemical changes under oxygen-free conditions at increased burial depths, and, along with the inorganic matter, formed sapropelic rock. It is usually called oil shale when the inorganic matter content is high. Oil shales were deposited in a wide variety of depositional environments, including small or large fresh waters and highly saline lakes, epicontinental marine basins and related subtidal shelves, and in association with coal deposits that were deposited in lentic and coastal swamps. Because of this wide diversity of depositional environments, it is not surprising that oil shales range widely in their organic and mineralogical content and composition. 5.3. Characteristics and Mining Oil shale has the following characteristics:
It is usually a fine-grained, non-porous solid, frequently showing bedding, with a laminated structure. Its color ranges from light gray to dark brown. It normally contains fewer organic and more inorganic substances than various types of coal. The organic content of oil shale often accounts for less than 35 percent of the total mass. The organic matter of oil shale is predominantly kerogen, insoluble in ordinary organic solvents. Bitumen, soluble in organic solvents, is the minor component, generally not exceeding 1.2 percent by weight. When oil shale is heated in the absence of air or oxygen to 673K to 773K, known as the retorting of oil shale, the kerogen is pyrolyzed to form shale oil, gaseous products, solid carbonaceous residue, and small quantities of decomposition water. The carbonaceous residue is obtained in a mixture with inorganic materials of the original oil shale.
The molar ratio of hydrogen to carbon of the kerogen in the oil shale is higher than that of the organic component in coal, and the retorting of oil shale normally yields a higher amount of oil than coal pyrolysis, based on the equivalent amount of organic material. Oil shale is quite different from tar sand. The organic substance of tar sand is bitumen, which can be extracted and separated from tar sand by using hot alkaline aqueous solution, while the kerogen in oil shale cannot be extracted by the same solution, due to the macromolecular polymerized three-dimensional structure of kerogen.
If oil shale is to be used in surface retorting plants or in power units, it must be mined and transported to these users. Just like coal, there are two mining technologies: aboveground mining and underground mining. Generally speaking, oil shale with gentle dipping and thin overburden is suitable for aboveground mining, and the opposite cases are suitable for underground mining. An important factor determining the technical and economic feasibility of aboveground mining is the stripping ratio, that is the overburden to be excavated in cubic meters for every tonne of oil shale exploited. In general, above-ground mining has the following advantages over underground mining: shorter construction time, lower production cost, less oil shale loss, higher efficiency, more safety in operation, and easier production of associated minerals. Its disadvantages are: vulnerability to climate changes, the occupation of too much land, and large capital construction in the case of thick overburden. The most important aboveground methods are open-pit and stripping mining, while the main underground mining method is room and pillar, suitable for conveniently accessible deposits of great thickness. Longwall mining is an older method, used in a few underground mines. Scotland, Spain, Sweden, Australia, and South Africa are some of the countries that had sizable oil shale industries in the past, but have since shut down operations owing to the availability of cheaper supplies of petroleum. Oil shale is still being mined today in Estonia, China, Brazil, and Russia. 5.4. Chemical Composition and Pyrolysis As noted above, oil shale is composed of organic and inorganic matter. Its organic matter content and composition may differ greatly with different deposits. In general, the organic content varies from 15 to 50 percent: in Kukersite, Estonia, the figure is 29 to 48 percent; in Green River, USA, 14 percent; in Irati, Brazil, 18 percent; in Australia, 19 percent; and in Fushun, China, it is 21 percent. The organic matter of oil shale consists of kerogen and bitumen. Kerogen is insoluble in water, acid, alkali, and ordinary organic solvents. It is generally considered that kerogen is a kind of macromolecular polymer with a threedimensional structure consisting of aromatic, polycylic, and heterogeneous condensed rings with aliphatic side chains of different lengths; these side chains are cross-linked with each other via oxygen or sulfur atoms. The bitumen content in the organic matter of oil shale is very low. Ordinary organic solvents such as benzene and ethanol can extract it, but the extraction yield varies with the extraction solvents and conditions. The elemental analysis and infrared spectra of bitumen are similar to those of kerogen, so bitumen is considered as being fragments or homologues of kerogen. The mineral fraction of some oil shales is composed of carbonate minerals such as calcite and dolomite, with lesser amounts of aluminosilicate minerals, whereas for some other oil shales, the reverse is true: quartz, feldspars, and clay minerals are dominant.
The grade of oil shale can be determined by many methods. Conventional methods, including proximate analysis, Fischer Assay, and heating value can give a general account of oil shale quality and its potential for commercial uses. Proximate analysis involves the determination of moisture, ash, volatile matter, and mineral carbon dioxide. The procedure follows the standard for proximate analysis for coals. Fischer Assay is also a standard method used for coal and oil shale. This method is to measure the quantity of oil that an oil shale can produce by destructive distillation. The third method is determining the heating value of the oil shale with a calorimeter, which is useful for evaluating the quality of an oil shale that is burned directly in a power plant to produce electricity. The gross heating value of oil shale on a dry basis is in the range of 2,090 to 16,720 kJ/kg of rock. By comparison, the heating value of lignite is in range of 14,630 to 19,270 kJ/kg on a dry mineral-free basis. The analytical results of some oil shales are given in Table 1. Table 1. Analytical results of some oil shales Fischer Assay Proximate analysis, wt percent Shale oil Volatile Moisture matter Ash (dry CO2 (dry yield basis) basis) Wt percent
Heating value kJ/kg
Kukersite (Estonia)
12.0
21.5
50.0
21.0
23.6
13,000
Fushun (China)
3.5
17.5
75.4
3.3
6.7
5,000
Green Rivar (USA)
no data
no data
67.7
18.5
10.3
6,500
Irati (Brazil)
no data
no data
79.8
2.6
7.2
5,200
Table 1. Analytical results of some oil shales The pyrolysis of oil shale refers to its thermal processing by heating without the presence of air or oxygen to a temperature of 723K to 873K. This process generally consists of three stages:
The first stage is the heating of oil shale, that is, heat transfer from heat carrier to oil shale surface, and then from surface to its interior. The second stage is the pyrolysis of organic matter; the kerogen is thermally decomposed to produce shale oil vapor, non-condensable gases, pyrolysis water, and a carbonaceous residue; some of the mineral matter such as clay minerals release structural water and some carbonate minerals decompose to give CO2 with heat absorption. The third stage is the diffusion and flow of shale oil vapor, noncondensable gas, and water vapor from the internal voids and capillaries of the shale matrix to the surface. They move subsequently to the exterior spaces, and finally escape from the retorting vessel.
Extensive studies on the pyrolysis mechanism of the organic matter in oil shale have found that the pyrolysis of kerogen in oil shale undergoes two stages: first, the formation of
pyrobitumen from kerogen, and second, the further conversion of pyrobitumen into shale oil, gas, and carbonaceous residue. A lot of research on the pyrolysis kinetics of oil shales indicates that, overall, a first order reaction model is quite fit for modeling the pyrolysis of oil shale with sapropelic type kerogen and its apparent activation energy is in a very narrow range of 160 to 170 kJ/mol. For Chinese Fushun and Maoming pulverized oil shale, the time required for pyrolysis at the temperature 723K to 773K is about two to three minutes. 5.5. Retorting Technology The oil shale excavated from mines varies greatly in size, from several millimeters to hundreds of millimeters, and even larger than 1,000 millimeters. Pre-treatment by crushing and screening is necessary to meet the demands of retorting operations. Usually, shale fraction is divided into lump shale (greater than 10mm in size) and particulate shale (less than 10mm in size). Both can be separately fed into different retorts. According to the heating manner, the oil shale retorts can be classified in two types: internal heating and external heating. Because of the small unit capacity, expensive heating transfer, and low thermal efficiency for external heating retorts, nowadays almost all of the commercial retorts and the retorts in development are internal heating retorts. In general, for lump shale, internal hot gas, produced by burning a part of the pyrolysis gas, is usually used for supplying heat. For particulate shale, an internal hot solid carrier derived from the combustion of carbonaceous residue is usually employed. Due to the low heat conductivity coefficient of oil shale, it takes several hours for retorting lump shale; for particulate shale, the heating rate is high, and the time required for retorting is much shorter: only several minutes or little more than ten minutes. The typical lump oil shale retorts are as follows:
Kiviter Retort, Estonia: daily processing capacity, 1,000 tonnes of oil shale; oil yield 85 percent of Fischer Assay. Petrosix Retort, Brazil: daily processing capacity, 2,000 tonnes of oil shale; 140 tonnes of product oil per day. Fushun Retort, China: 200 tonnes of oil shale per day; oil yield about 55 percent of Fischer Assay. Union Oil Rock Pump Type Retort, United States: 10,000 tonnes of oil shale per day; 1,000 tonnes of product oil per day.
Solid heat carrier retorts for particulate oil shale in development include:
Galoter Retort, Estonia: processing capacity 3750 tonnes of oil shale per day; oil yield 90 percent of Fischer Assay, with hot shale ash as solid heat carrier. Tosco Retort, United States: processing capacity 900 tonnes of oil shale pe day, with heated ceramic balls as heat carrier.
Besides these aboveground retorts, underground retorting, also called in-situ retorting, is already in development in the United States. 5.6. Shale Oil and Shale Ash Utilization Shale oil is a kind of synthetic crude oil produced by retorting oil shale. Usually it is a brownish paste at room temperature with a pungent odor. Shale oil is similar to petroleum,
with a high content of paraffinic, naphthenic, and aromatic hydrocarbons, but is usually rich in unsaturated organic compounds. Shale oil with relatively high H/C mole ratio is suitable for upgrading to produce light liquid fuels. Distillation of shale oil, the thermal cracking of middle oil, and the coking of residue oil have for example, commercially processed Chinese Fushun and Maoming shale oils. The light fractions obtained are treated by acid-alkali washing, or hydro-refining, thus gasoline and diesel oil are produced. In addition, the medium distillates are deeply cooled for producing white paraffin wax. Kukersite shale oils produced in Estonia and Russia contain 50 percent oxygen compounds, including 20 percent phenols and diphenols. From this oil shale, fifty to sixty kinds of chemicals have been commercially produced, such as phenol resins, tanning agent, anti-corrosion agent, organic solvents, adhesive agent, insecticides, and regenerative rubber softener. Shale ash, derived from oil shale combustion or the retorting process, accounts for about 70 to 80 percent of raw oil shale. Therefore, a large amount of shale ash or spent shale has to be handled and utilized, a matter that must be considered not only from the viewpoint of economics, but also from that of environmental protection. Estonia Kukersite shale ash has been used as blending material for producing high quality cement, and as a soil improver. In the United States, Green River retorted shale, covered with soil, has been used for surface reclamation of the open pit mined land, and the reclaimed land has been planted with various types of vegetation. 5.7. Perspective Although oil shale occurs abundantly in many areas of the world, developing an oil shale industry to produce synthetic fuels is expensive. The costs of land acquisition and environmental constraints, mining, crushing, and retorting operations, as well as refining shale oil into marketable products are very high. It is also doubtful that using oil shale as a fuel for power generation, considering the unresolved problems of air and water pollution associated with open-pit and underground mining, is economically viable in the long term. However, in some countries, where the local liquid fuel costs are higher, or where the shale oil is used for producing high value chemicals, then a shale oil industry may be viable. Some experts predict that after several decades, when the crude oil price rises, the world shale oil industry may develop. Perhaps the brightest future for oil shale lies in the manufacture of petrochemicals and construction materials, such as building blocks, glass, cement, and roofing materials that can be substituted for wood products. 6. Natural Bitumen (Tar Sands) and Heavy Oil 6.1. Introduction In addition to conventional petroleum, there are two other subclasses of petroleum that can offer some compensation for the potential shortfalls in the supply of liquid oils and other products. These subclasses are natural bitumen, which is found in tar sand (also known as oil sand and bitumen sand) deposits, and heavy oil, found in various reservoirs. Bitumen and heavy oil are not uniform materials. On a molecular basis, they are similar to conventional petroleum and both are complex mixtures of hydrocarbons with small amounts of organic compounds containing sulfur, oxygen, and nitrogen, as well as compounds containing metallic constituents, particularly vanadium, nickel, iron, and copper. The typical
properties of natural bitumen, heavy oil, and conventional crude oil are listed in Table 2. In fact, in order to classify petroleum, heavy oil, and bitumen together, the use of a single parameter such as API gravity is not enough. Other properties such as viscosity, elemental analysis, fractional composition, and the properties of the fluid in the reservoir, as well as the method of recovery, need to be acknowledged. Because both bitumen and heavy oil have a lower mobility or full immobility under reservoir conditions, it is necessary to apply enhanced recovery processes or mining techniques to recover them. Table 2. Properties of conventional crude oil, heavy oil, and bitumen Lloydminster
Conventional
Cold Lake heavy oil
heavy oil
crude oil
8
12
14
35
Centipoise @100 °F (38°C)
500,000
2,000
500
10
Centipoise @210 °F (99°C)
1,700
no data
no data
no data
SUS @100 °F(38 °C)
35,000
no data
no data
30
SUS @210 °F(99 °C)
500
no data
no data
no data
Pour point, °F
50
no data
5
0
83
84
83
86
Hydrogen
10.6
11
12
13.5
Sulfur
4.8
4.4
3.6
0.1
Nitrogen
0.4
0.4
0.4
0.2
Oxygen
1
0.2
1
0.2
12
12
5
Property
Gravity, °API
Bitumen
Viscosity
Elemental analysis (percent by weight) Carbon
Fractional composition, (percent by weight) Asphaltenes
19
Resins
32
28
17
10
Aromatics
30
35
24
25
Saturates
19
25
47
60
Vanadium
250
190
100
10
nickel
100
70
40
5
(percent by weight)
14
11
10
5
Heating value kJ/kg
40,770
41,940
42,400
45,400
Metals (parts per million)
Carbon residue
Table 2. Properties of conventional crude oil, heavy oil, and bitumen 6.2. Natural Bitumen (Tar Sands) Natural bitumen mostly occurs in tar sand deposits impregnated with dense, viscous petroleum-like material that is usually immobile under reservoir conditions. Tar sand deposits are found throughout the world, often in the same geographical areas as conventional petroleum. The largest tar sand deposits in the world are in Alberta, Canada, and East Central Venezuela. In addition, there are smaller tar sand deposits in the United States, Peru, Trinidad, Madagascar, the former Soviet Union, the Balkan states, the Philippines, and China. The potential reserves of bitumen that occur in tar sand deposits have been variously estimated on a world basis to be in excess of three trillion barrels. Assuming that only 10 to 15 percent of this resource is recoverable, the bitumen reserves range from 300 to 450 billion barrels: a potential petroleum resource for the future. The tar sand deposits of the world have been described as belonging to two types: first, in situ deposits resulting from breaching and exposure of an existing geologic trap; and second, migrated deposits resulting from accumulation of migrating material at outcrop. However, there are inevitable gradations and combinations of these two types of deposit. The origin of bitumen is much more speculative. There are dominant theories relating to the origin of Canadian bitumen that might well apply to the bitumen in other tar sand deposits. One theory is that the bitumen was formed locally and has neither migrated a great distance, nor been subjected to large overburden pressures. Given these conditions, the bitumen cannot have been subjected to thermal degradation, is geologically young, and is therefore dense and viscous. Another theory assumes a remote origin for the bitumen resulting from the migration of a bitumen precursor like conventional crude oil into the sand. As its light portions were evaporated, a dense viscous residue was left behind.
Tar sand is a mixture of sand, water, and bitumen with the sand component occurring predominantly as quartz. The arrangement of sand, water, and bitumen has been assumed to be an arrangement whereby each particle of the sand is water-wet, and a film of bitumen surrounds the water-wet grains. The balance of the void volume is filled with bitumen, coherent water, or gas; fine material, such as clay, occurs within the water envelope. The physical properties of tar sands that are of general interest are porosity, bitumen saturation, permeability, and bulk density. Porosity is the ratio of the aggregate volume of the interstices between the particles, to the total volume expressed as a percentage. High-grade tar sand usually has porosity in the range of 30 to 35 percent, higher than the porosity (5 to 25 percent) of most reservoir sandstone. Bitumen saturation (SO) is expressed in the percentage of pore volume or percentage by weight of tar sands. The optimum bitumen saturation is likely to be in the range of 50–70 percent of pore volume for the application of in situ recovery processes. Permeability is a measure of the ability of a sediment or rock to transmit fluids. It is, to a major extent, determined by the size and shape of the pores, as well as of the channels between the pores; the smaller the channel, the more difficult it is to transmit fluids. The bitumen content of tar sands varies from zero percent to as much as 22 percent by weight, but for Canadian tar sands, bitumen contents from 8 to 14 percent by weight may be considered as normal or average. The properties of bitumen extracted from tar sands are listed in Table 3. It is a high boiling material with little, if any, material boiling below 623K and its boiling range is approximately equivalent to the boiling range of an atmospheric residue, therefore the bitumen is also classified as extra-heavy oil. A very important property of Athabasca bitumen (separated by a hot water process) is the variation of bitumen density with temperature. Over the temperature range of 303K to 403K, the bitumen is lighter than water; hence the floating of the bitumen with aeration on water is facilitated. Table 3. Properties of synthetic crude oil from Athabasca bitumen Property
Bitumen
Synthetic crude oil
8
32
Sulfur, percent by weight
4.8
0.2
Nitrogen, percent by weight
0.4
0.1
Gravity, °API
Viscosity 500,000 Centipoise @ 100 °F
10
Distillation profile, per cent by weight °C
°F
0
30
0
5
30
85
0
30
220
430
1
60
345
650
17
90
550
1,020
45
100
Table 3. Properties of synthetic crude oil from Athabasca bitumen On a commercial basis, tar sand currently is recovered by open pit mining, after which it is transported to a processing plant where the bitumen is extracted and the sand is discharged. The Athabasca Tar Sands deposit in Canada is the site of the only commercial tar sands mining operation. The Suncor mining and processing plant in Alberta started production in 1967 and the Suncrude Canada mining and processing plant (located 8km away from the former plant) began production in 1978. The mining operation is currently carried out using 8000 tonnes per hour bucket-wheel excavators and 80 cubic-yard capacity draglines as the primary mining equipment. To recover bitumen from mined tar sands, the hot water process (used on the linear and nonlinear variation of bitumen and water density, respectively) is applied with temperatures as mentioned above. It is the only successful commercial process. The essentials of this process involve conditioning, separation, and scavenging components. For converting tar sand bitumen that is hydrogen deficient into synthetic crude oil, an upgrading process is used to improve the H/C ratio by carbon removal or hydrogen addition. Currently, processes-delayed coking and fluid coking methods that employ the concept of carbon removal are determined to be the most effective and economical. In general, to produce synthetic crude oil from tar sands, it is necessary to combine three operations, each of which contributes significantly to the cost of the venture. These are:
a mining operation capable of handling two million tonnes or more of tar sands per day an extraction process to release the bitumen from the sand an upgrading plant to convert the bitumen to a synthetic crude oil.
6.3. Heavy Oil Heavy oils are other types of petroleum that are different to conventional crude oil insofar as they have a higher density with an API gravity of less than 20°, lying between the conventional petroleum and tar sand bitumen. They have a lower mobility in the reservoir, therefore application of a secondary or enhanced recovery method is necessary to bring the oil to the surface. The term "heavy oil" has also been arbitrarily used to describe both heavy oil in the sense mentioned above and the tar sand bitumen often classified as "extra heavy oil." Heavy oil reservoirs are widely distributed in many countries throughout the world. They can be seen as reservoirs that are similar in geological character to petroleum reservoirs. On the basis of API gravity, the United States has an estimated eighty billion barrels of the oil originally in large
(more than twenty billion barrels of oil) reservoirs where the oil has an API gravity between 10° and 20°. Another 20 billion barrels of such oil exist in smaller reservoirs representing a total resource of approximately 100 billion barrels. Heavy oil reservoirs in the rest of the world are less well delineated because of the criteria used to define heavy oil and are therefore difficult to enumerate. The recovery of heavy oil differs markedly from the recovery of conventional crude oil. For example, primary oil production, as the term suggests, is the first method of producing conventional oil from a well, depending upon natural reservoir energy; therefore no pumping equipment is required. If the reservoir energy is not sufficient to force the oil to the surface, then the well must be pumped. The secondary recovery operation usually involves the application of a pumping operation or of the injection of material into a well to encourage movement and recover the remaining petroleum. When water is used, the process is called a water flood; with gas, it is called gas flood. Separate wells are usually used for injection and production. The injected fluids maintain reservoir pressure, or re-pressure the reservoir after primary depletion, and displace a portion of the remaining crude oil to production wells. When the heavy oils are recovered from a reservoir, its viscosity is an important factor that must be taken into account. In fact, certain reservoir types, such as those with very viscous crude oils and some low permeability carbonate reservoirs, respond poorly to conventional secondary recovery technique. In this case, it is desirable to initiate enhanced oil recovery (EOR) operations as early as possible. Enhanced oil recovery is defined as recovery of the incremental ultimate oil over the oil that can be economically recovered by conventional primary and secondary methods. To reduce or eliminate the viscous properties of heavy oil and improve its mobility, the enhanced oil recovery processes involve chemical, fluid phase, and thermal behavior effects. Chemical methods include polymer flooding, surfactant flooding, and alkaline flooding. Polymer flooding is water flooding in which organic polymers are injected with the water to improve area and vertical sweep efficiency. This process is conceptually simple and inexpensive, and its commercial use is increasing despite relatively small potential incremental oil production. Thermal methods for enhanced oil recovery are most useful when the oil in the reservoir has a high viscosity. For example, most heavy oils are viscous with a viscosity ranging from about one hundred centipoise to a few million centipoise at the reservoir conditions. Thermal enhanced oil recovery processes add heat to the reservoir to reduce oil viscosity or to vaporize the oil. In both instances, the heavy oil is made more mobile so that it can be more effectively driven to producing wells. Thermal recovery methods include cyclic steam injection; steam flooding, and in situ combustion. The steam processes are the most advanced of all enhanced oil recovery methods in terms of field experience and thus involve the least uncertainty in estimating performance, provided that a good reservoir description is available. Since the early 1960s, it has been commercially applied in reservoirs containing viscous oils. In situ combustion has been field tested under a wide variety of reservoir conditions, but few projects have proven economical and advanced enough for use on a commercial scale. The essential step required of refineries in the upgrading of heavy oil, being similar to bitumen, is the conversion of the low value feedstock to high value products such as liquid fuels. Heavy oil has fewer components distilling at atmospheric pressure and under vacuum than does conventional petroleum. Nevertheless, some heavy oils still pass through the
distillation stage of a refinery before further processing is undertaken. Technologies for upgrading heavy crude oils such as heavy oil, bitumen, and residue can be broadly divided into carbon rejection—visbreaking, steam breaking, catalytic cracking, coking, and flash pyrolysis—and hydrogen addition—hydro-cracking, hydro-visbreaking, hydro-pyrolysis, and donor solvent—processes. Currently, visbreaking and coking are the conventional processes for the conversion of heavy oil as a primary upgrading step. Hydrocracking has been proposed for increasing the yield of liquid oil. It includes the H-Oil process, the LC-Fining process, the Veba-Combi Cracking process, and the Chiyoda process. 7. Peat 7.1. Introduction Geologically, peat (an unconsolidated rock) is the youngest and least altered member of the group of combustible rocks. It is a unique product of mire vegetation, consisting of undecomposed dead plant debris from peatland vegetation growing in a waterlogged, humid, and poorly aerated environment. The conversion of vegetable matter to peat is a long process, the continuation of which leads to the formation of lignite, bituminous coal, and anthracite; so peat can be seen as the precursor of coal. Peat has been used as fuel for centuries. In the twentieth century, however, it found increasing applications in horticulture and agriculture, and now it is also of value in the chemical industry and manufacturing of medical products. In recent years, those of nature conservation and the preservation of biological species have joined the various disciplines interested in this aspect of the natural environment. In some places, the discussion of peat utilization has become very controversial. However, at the same time, co-operation among the individual disciplines has increased. National and international bodies like the International Peat Society have promoted such interdisciplinary co-operation. 7.2. Peat Sources and Distribution About 3 percent of the Earth’s land area is covered by peatland, whose area totals 3,985,000km2 (398.5 Mha). The average thickness of the world’s peatlands has been estimated to be only about 1.5m. On this basis, the total global peat resources amount to 5,000 to 6,000Gm3. Peatlands are of great importance as a store of carbon, because the amount of stored carbon is at least 234 to 252Gt, constituting 15 percent or more of all the carbon in the global soil pool. The distribution of peat resources is uneven. Peatlands are mainly distributed through North America, Asia, and Europe, making up more than 95 percent of the total global peatland area, whereas the three continents of the Southern Hemisphere contain less than five percent (see Table 4). Table 4. Distribution of peatland in the world Area North America
Peatland /km2 1,735,000
Asia
1,119,000
Europe
957,000
Africa
58,000
Central and South America Australia and Oceania Total
102,000 14,000 3,985,000
Table 4. Distribution of peatland in the world Western Siberia in Russia, with a total peatland area of 760×103 km2 and a peat reserve of 113 Gt is the peat accumulation center of the world. The thickest known peat deposit in the world is Philippi Peatland in Greece. Here, the 55km2 large mire is almost 190m deep and the accumulation of peat started during the Cromerian stage, around 700,000 years ago, whereas, for the main part, global mires are less than 10,000 years old. 7.3. Peat Formation and Classification Peat deposits are found in mires that form where there is a constantly high water level, at or very near the surface. Most peatland vegetation contains Sphagnum moss, and in many places this genus constitutes the majority of the peat forming material. Because of the continual excess of water, and poor aeration, dead plant debris is not completely decomposed. It therefore accumulates and simultaneously undergoes different kinds of biochemical and chemical reactions to form peat. The basic factors controlling peat formation and development are the climatic, geological, geomorphological, and hydrological conditions. Temperature and humidity together influence the growth rate and growing quantity of vegetation, as well as the reproduction and activity of soil microbes. The accumulative quantity of peat in cold-humid or mild-humid coniferous forest and broadleaf forest zones is the greatest. The annual growth rate of the raised bog peat is only about 1mm. Geological and geomorphic factors control the spatial area of peat formation. As the mire vegetation and accumulating peat protects the mineral surface beneath, erosion is minimal and the accumulating plant material gradually raises the surface of the vegetation. Hydrological factors, including rainfall and groundwater, are also major determinants of peat formation and development, as the vegetation will change if the surface dries out for part of the year. A constantly high water level is essential. On the basis of differences in the geomorphological situation and water supply, and consequently in plant composition, peatlands are usually divided into three types: low (groundwater bogs), high (raised or rainwater bog), and transition. In regions with very high rainfall, such as the north of Scotland, there are large areas of blanket bog: a mosaic of peatland and pools on gently sloping ground. As the water supply is entirely from rainfall, the concentrations of minerals and nutrients are very low, as is the pH. On mountainsides, where
ombrogenous peatlands receive drainage water from higher ground, the mineral content can be higher and this is reflected in the plant community. 7.4. Peat Composition and Properties Peat is a heterogeneous mixture of solid, liquid, and gaseous matter. The solid matter includes undecomposed vegetable debris, black humus formed by decomposed vegetable debris, and mineral matter. The water content in peat is very high, ranging from 50 to 95 percent. It is normally considered that the content of organic matter in peat must be above 50 percent, because organic matter is clearly the most important part of peat, but there is another opinion that the organic content can be as low as 30 percent. The organic matter of peat can be analytically classified into the following groups:
bitumen, which can be extracted from peat using organic solvent, and which includes plant resins and waxes water-soluble matter, mainly composed of saccharides, which can be extracted by cold or hot water easily hydrolyzed matter mainly composed of hemi-cellulose, which can be hydrolyzed with dilute inorganic acid cellulose, which can be hydrolyzed with dense acid humic acid, which can be extracted with dilute alkaline solution then reprecipitated with acid. This contains important and complicated organic compounds formed during the humification or peat forming process non-hydrolyzed matter, mainly consisting of lignin, therefore also called lignin.
The relative abundance of these components is closely related to peat type and decomposition. The decomposition degree of peat is the relative content of the non-setting matter that loses cell structure due to the decomposition of plant debris. It mainly depends upon the accumulative environment. Under natural conditions, the decomposition degree of peat ranges from 1 to 70 percent. The higher the rate of decomposition, the lower the cellulose and lignin content. The elements in the organic matter of peat are the same as those of other fossil fuels. A typical elemental analysis (in wt. percent) of raised-bog peat—the exact proportions of which depend on the decomposition degree—is: carbon 50 to 60; oxygen 30 to 45; hydrogen 5 to 6.5; nitrogen 0.5 to 2; and sulfur 0.1 to 0.5. The presence of humic acid gives peat a lot of valuable properties, such as water absorbency and retainment, cation ion exchange ability, a lower water surface tension, and greater physiological activity. 7.5. Peat Extraction and Processing There are around twenty countries in Europe and more than ten in other continents with peat extraction and processing industries. For instance, in EU countries, the peat industry had in 1998 around 15,000 employees. This branch of the fossil fuel industry turns over between 2 billion and 3 billion euros annually.
In peat extraction, the first stage after the surveying and assessment of the deposit is ditching and drainage of the production area. After this, the surface vegetation must be removed and the production field leveled. The majority of the peat is now produced by the milling method in which the topmost layer of the peat deposit (typically 1 to 2 cm thick) is cut into small pieces by special harvesting machines and spread over the surface for drying and collection. Milling breaks the capillary link between the cut material and the rest of the peat layer. The next operation is the turning over of the peat to expose a new surface to the air and sun for drying. The procedure is repeated several times until the average moisture content has dropped below 50 percent. During the production season, the average number of harvesting cycles in milled peat production is ten to fifteen, and annual yield per hectare is 300 to 600 m3 of peat with 40 percent moisture content. In the sod peat method, the peat mass is excavated from a vertical section of about 0.5m, working through the peat layer with a cutting disk, bucket elevator, or digging machine. After the cutting, the peat mass is macerated, mixed, and pressed out through nozzles into sods of different size and shape. The optimum sod diameter lies between 55 and 120mm depending on the climate and market. The initial moisture content of the sod peat varies usually between 81 and 84 percent. After two to four weeks drying, sods of 30 to 40 percent moisture content are collected into stockpiles. During the past few years, wave-like sod peat technology has been developed. The wave-like sods have less surface contact with the peat layer and hence dry more quickly. Peat processing technologies are as follows:
thermo-chemical processing-pyrolysis or coking to peat coke, peat tar, and peat gas; gasification to fuel gas or synthetic gas; liquefaction to liquid fuels extraction of humic acid and bitumen, using alkali solution and organic solution, respectively acidic or enzymatic hydrolysis to monosaccharides and their subsequent fermentation to alcohol or acid anaerobic fermentation to methane and carbon dioxide.
7.6. Utilization of Peat About 50 percent of the peat produced is used as fuel, and the rest is used as a growing medium, a soil conditioner, and in various specialized environmental applications such as wastewater cleaning and purification of aerial emissions. 7.6.1. Fuel In the use of peat as a fuel, a major problem is the wide variation of properties (moisture content, bulk density, heating value, ash content, and composition) in the harvested material. In terms of energy content, one cubic meter of peat is equivalent to 0.08 to 0.15 tonnes of oil. On a worldwide scale, peat is a minor contributor to energy production. Peat contributes only about one thousandth of global energy. The total energy use of peat accounts for 70 to 80 Mm3 annually. Belarus, Finland, Ireland, Latvia, Lithuania, Russia, Sweden, and Ukraine are the main countries using peat for energy production. The energy use of peat is relatively most important in Ireland, where milled peat is used for power generation. In Finland, where the heating of buildings consumes 25 percent of primary energy consumption, the use of energy peat accounted in 1998 for 7 percent of the total energy production. The largest peat power
station currently operating, in Haapavesi, Finland, has an electrical output of about 20 MWel and requires about 2.5×106 m3 of milled peat per year as feedstock. Using briquetting methods, milled peat can be processed to a more usable fuel form with a lower moisture content, higher density, and specific heating value. The use of peat briquettes has a long history in many countries. The above-mentioned processing technologies of peat can convert peat into more convenient and valuable fuel products, such as fuel gas, oils, alcohol, peat coke, and so on. 7.6.2. Non-Fuel Uses Horticultural and Agricultural Uses: Peat, especially raised bog peat, is a favorite constituent of growing media because it combines more favorable physical, chemical, and biological properties than other materials. The cellular structure of raised bog (sphagnum) peat ensures an excellent and stable structure and hence provides good water-holding ability and good aeration. The global amount of peat used as a growing medium reaches 70 Mm3 annually. Russia is the largest producer of horticultural peat; the next two are Germany and Canada. Environment Protection: With high cation exchange capacity and other properties in metal ion-containing wastewater treatment, there is widespread use of peat as a filter medium for wastewater cleaning, exhaust air purification, oil absorption, and so on. In addition, peat coke can be processed to become activated carbon. Medical Purposes: More than 500,000 m3 of fresh raw peat is used annually as mud baths and mudpacks in balneology in Germany. These mud baths and mudpacks are prescribed for such things as illnesses of the rheumatism group. Peat-based bath additives, pastes, and ointments are supplied for external application or drink cures. Other Uses: Peat can be processed by chemical methods into organic and organo-mineral fertilizers, biostimulators, and plant growth promoters, as well as drilling-mud conditioners, special anticorrosive additives, and dyestuff for wood etc. In summary, the global peat resources are valuable but limited, so non-fuel uses of peat should be given precedence over fuel uses to postpone the exhaustion of peat resources. 7.7. Peatland and Ecological Systems The conservation of ecological systems and the environment is a relatively new and rapidly growing factor in peat exploitation and utilization. Peatland, as a unique natural ecosystem is very important for the conservation of biodiversity, as it supports many specifically adapted plant and invertebrate species, which could not survive elsewhere. The habitat is also locally important for the conservation of birds such as wildfowl, waders, and raptors. Peat exploitation alters the original environment and leads to a series of undesirable changes. First, bog drainage turns peatland from a hydrochemical accumulation area to a hydrochemical releasing area, and increases the leaching of nutrients. After drainage and extraction of the surface layer, peatland loses its water-holding capacity and other special properties, and the local ecosystem is substantially destroyed. The movement of drifting peat particles, during
peat extraction and drying, into nearby water bodies is also harmful, leading to pollution and siltation. Due to their high reactivity, some peats present an explosion or fire hazard. Also, peat extraction, processing, and combustion cause more or less the same environmental problems as the other fossil fuels. The life cycle of a peat production site usually requires a period of fifteen to twenty years. The most common uses of cut-away peatlands in the North Temperate Zone are either forestry or agriculture. In recent years, not only peatland conservation, but also the regeneration of cut-away peatland has been practiced more and more widely. When the water level is restored, mire vegetation quickly takes over the site and a rapid carbon sequestering process, a typical feature of young mires, begins. Much attention has been paid to this kind of after-use of cutaway peatland areas in Ireland, Finland, the UK, the Netherlands, and Germany. In Germany, more than half of the areas currently used by the peat industry (about 180 km2) will be "restored" in the next few decades. Farmers who favor organic growing methods are also interested in growing herbs and medical plants here, since there are no traces of pesticides or fertilizers in the subsoil of cut-away areas. In addition, experiments have been carried out with fast-growing energy plants, such as canary grass and certain willow species. Some cutaway peatlands have been transformed into bird sanctuaries, fishponds, or flood controlling basins. The Netherlands and France, as large-scale users of peat for horticultural purposes, have stopped peat harvesting for lack of suitable deposits or because of ecological pressure. In Russia, Finland, and Ireland, peat resources will last for more than thirty years because of the enormous peat reserves in these countries. In the United States and Canada, peat consumption is very small and almost insignificant in relation to their great wealth of native peat resources. 8. Conclusion Coal, oil shale, natural bitumen (tar sand), and peat all belong to the group of fossil fuels. Coal is the end product of a sequence of biological, geochemical, and geological processes, originating from plant debris. Known global reserves of coal are much greater than any other fossil fuels. Currently, coal is widely used for power generation, heat supply, coke making, and the production of gaseous and liquid fuels. Its future is to a great extent dependent on the development and availability of clean coal technologies. Natural bitumen, found in tar sand, and heavy oil belong to subclasses of petroleum, while oil shale is a sedimentary rock containing kerogen as its main organic constituent and, to a lesser extent, bitumen, both embedded in an inorganic matrix. Because natural bitumen, heavy oil, and oil shale are more closely similar to conventional crude oil in the elemental composition of their organic matter than is coal, naturally it is much easier to convert them into liquid fuels to make up for crude oil shortage. Peat is a mixture of dead and decomposed vegetable matter that forms in boggy areas. Although it is a low-quality fuel, it has a number of special applications. Although the peat resources of the northern hemisphere are still very large, production of peat is likely to decline. Extraction of peat from currently undisturbed peatland areas is undesirable on environmental grounds, both on account of biodiversity conservation and atmospheric carbon dioxide concentration. Natural peat deposits still represent a very important global storehouse of carbon.
Acknowledgments The author wishes to express thanks to all authors of this theme, Coal, Oil Shale, Natural Bitumen, Heavy Oil, and Peat, for their close co-operation. Related Chapters Click Here To View The Related Chapters Glossary Anthracite
: The highest rank of coal, very high in fixed carbon and low in volatile matter, hydrogen, and oxygen. Asphalt : A dark brown to black cementitious solid or semisolid in consistency, in which the predominant constituents are bitumens, which occur in nature as residue in the refining of petroleum. Petroleum asphalts are produced from crude oils by a variety of manufacturing methods. Clean coal : A quite new conception, this term is used worldwide to represent a series technology of new coal utilization technologies, which are environmentally clean, highly efficient, and economically acceptable. It can reduce emissions of sulfur oxides, nitrogen oxides, and other pollutants at various points of coal use from a mine to a power plant or factory. Coal : One of the fossil fuels, originating from peat deposits formed in prehistoric swamps through the accumulation of plant remains and the end product of a sequence of biological, geochemical, and geological processes. It can be described as a complex heterogeneous mixture of amorphous organic material with inorganic matter interspersed. Enhanced oil : Defined as recovery of the incremental ultimate oil over the oil that can be recovery economically recovered by conventional primary and secondary methods. To reduce or eliminate the viscous properties of heavy oil and improve its mobility, the enhanced oil recovery processes involve chemical, fluid phase, and thermal behavior effects. Heavy oil : One type of petroleum that is different from conventional crude oil. It has a higher density with an API gravity of less than 200, lying between the conventional petroleum and tar sand bitumen. Lignite : The lowest-rank coal, similar to brown coal. It is lower in fixed carbon, and higher in volatile matter and moisture content, than bituminous coal. Natural bitumen : Mostly occurring in tar sand (bitumen sand) deposits that are impregnated (tar sand) with dense, viscous petroleum-like material that is usually immobile under reservoir conditions. Oil shale : A sedimentary rock containing combustible organic matter, commonly called kerogen, along with a much smaller portion of bitumen, both embedded in an inorganic mineral matrix. Peat : An acidic mixture of dead and decomposed vegetable matter, which forms in boggy areas. Compared with other fossil fuels, peat is characterized by the lowest carbon content and the highest oxygen content.
Bibliography Lappalainen, E. 1996. Global Peat Resources, Jyskae, Finland, International Peat Society. 359 pp. [This work deals with peat resources, production and utilization.] Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire R. J. 1994. The Structure and Reaction Process of Coal. New York, Plenum. 471 pp. [Coal is the focus of this book, which attempts to document and integrate current understanding of the structure of coal and its reaction processes.] Speight, J. G. 1994. The Chemistry and Technology of Coal, 2nd edn. New York, Marcel Dekker. 642 pp. [This book introduces the reader to the science of coal, beginning with the coal formation through the various chemical and analytical aspects to the coal processing technologies.] ––––. 1999. The Chemistry and Technology of Petroleum, 3rd edn. New York, Marcel Dekker. pp. 14–644. [This book is a sister work of the book mentioned above.] Weiss, H. J. 1991. Oil Shale. In: B. Elvers, Ullmann’s Encyclopedia of Industrial Chemistry, 5th edn. Vol. A18, Basel, Switzerland, VCH. pp. 101–25. [This describes the world’s oil shale activities.]
Biographical Sketch Dr. Jinsheng Gao was born on January 8 1939. He graduated in Fuel Chemical Technology in 1961 at the East China University of Chemical Technology (renamed in 1993 as East China University of Science and Technology (ECUST)), and received his Dr.Ing. in 1981 at the Technical University Clausthal, Germany. He was appointed professor of ECUST in 1990. He has worked from 1961 to the present day teaching and researching at ECUST. For over thirty years he was in charge of research projects in the field of coal chemistry and processing. He has published 160 scientific papers and nine books, mostly in Chinese, and obtained four (Scientific and Technological Process) awards from ministries in China. The author has been actively involved in national and international co-operation projects, and acts as member or chairman in several scientific and technical societies and editorial boards of journals.
To cite this chapter Jinsheng Gao, (2004), COAL, OIL SHALE, NATURAL BITUMEN, HEAVY OIL, AND PEAT, in Coal, Oil Shale, Natural Bitumen, Heavy Oil and Peat, [Ed. Gao Jinsheng], in Encyclopedia of Life Support Systems (EOLSS), Developed under the Auspices of the UNESCO, Eolss Publishers, Oxford ,UK, [http://www.eolss.net] [Retrieved April 12, 2007]
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