Insights into Biomass Pyrolysis Process Pyrolys Pyrolysis is is the thermal thermal decom decompos position ition of bioma biomass ss occurr occurring ing in the absence absence of oxygen oxygen.. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide. Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil. Pyrolysis can be performed at relatively small scale and at remote locations which enhance energy density density of the biomass biomass resource resource and reduce transport transport and handling handling costs. Heat transfer transfer is a critical area in pyrolysis as the pyrolysis process is endothermic and sufficient heat transfer surface has to be provided to meet process heat needs. Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transported liquid, which can be successfully used for the production of heat, power and chemicals.
A wide range range of biomass feedstocks feedstocks can be used used in pyrolysis pyrolysis processes. processes. The pyrolysis pyrolysis process process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only
produces produces dust instead instead of oil. High-moisture High-moisture waste streams, such as sludge sludge and meat processing processing wastes, require drying before subjecting to pyrolysis. Biomass pyrolysis has been attracting much attention due to its high efficiency and good environmental performance characteristics. It also provides an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into clean energy. In addition, biochar sequestration could make a big difference in the fossil fuel emissions worldwide and act as a major player in the global carbon market with its robust, clean and simple production technology.
On Biochar and Bio-oil
Bioc Biocha harr
sequ seques estr trat atio ion n
is
cons consid ider ered ed
carb carbon on
negative as it results in a net decrease in atmospheric carbon dioxide over centuries or millennia time scales. Instead of allowing the organic matter to decompose and emit CO 2, pyrolysis can be used to sequester sequester the carbon and remove remove circulating circulating carbon dioxide from the atmosphe atmosphere re and stores stores it in virtually permanent soil carbon pools, making it a carbon-negative carbon-negative process. According According to Johannes Johannes Lehmann Lehmann of Cornell Cornell University, University, biochar biochar sequestra sequestration tion could could make a big difference in the fossil fuel emissions worldwide and act as a major player in the global carbon market with its robust, robust, clean clean and simple simple produ productio ction n techno technolog logy. y. The The use of pyrolys pyrolysis is also also provid provides es an opportunity for the processing of agricultural residues, wood wastes and municipal solid waste into useful useful clean energy. energy. Although some organic organic matter is necessary for agricultural agricultural soil to maintain its productivity, much of the agricultural waste can be turned directly into biochar, bio-oil, and syngas. Pyrolysis transforms organic material such as agricultural residues and wood chips into three main components: syngas, bio-oil and biochar (which contain about 60 per cent of the carbon contained in the biomass. The two main methods for biochar production are fast pyrolysis and slow pyrolysis. The biochar yield is more than 50% in slow pyrolysis but it takes hours to complete. On the other hand, fast pyrolysis yields 20% biochar and takes seconds for complete pyrolysis. In addition, fast pyrolysis gives 60% bio-oil and 20% syngas. The essential features of a fast pyrolysis process are:
1. Very high heating heating and heat transfer rates, which often require require a finely ground biomass biomass feed
2. Carefully controlled reaction temperature of around 500 C in the vapour phase and residence o
time of pyrolysis vapours in the reactor less l ess than 1 s 3. Quenching (rapid cooling) of the pyrolysis vapours vapours to give give the bio-oil product. product. Bio-oil is a dark brown liquid and has a similar composition to biomass. It is composed of a complex mixture of oxygenated hydrocarbons with an Bio-oil has a much higher density than woody materials (three to six times, depending on form), which reduces storage and transport costs. Bio-oil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store. Since the oil has a density of about 1200 kg m -3, it can be conveniently transported over long distances. Current end-use possibilities are as a boiler fuel for stand-alone heat or in combined heat and power (CHP) using the steam cycle after either diesel or gas turbine electricity generation. The majority of these options have been found to be technically feasible. In addition, bio-oil is also a vital source for a wide range of organic compounds compounds and speciality chemicals.
Introduction to Biomass Pyrolysis April 13, 2012 6:09 am Leave a Comment Salman Zafar ⋅
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Pyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. The products of biomass pyrolysis include biochar, bio-oi bio-oill and gases gases includ including ing metha methane, ne, hydro hydrogen gen,, carbo carbon n monox monoxide ide,, and and carbon carbon dioxide. Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases g ases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.
Pyrolysis can be performed at relatively small scale and at remote locations which enhance energy density of the biomass resource and reduce transport and handling costs. Pyrolysis offers a flexible and attractive way of converting solid biomass into an easily stored and transported liquid, which can be successfully used for the production of heat, power and chemicals. A wide range of biomass feedstocks can be used in pyrolysis processes. The pyrolysis process is very dependent on the moisture content of the feedstock, which should be around 10%. At higher moisture contents, high levels of water are produced and at lower levels there is a risk that the process only produces dust instead of oil. High-moisture waste streams, such as sludge and meat processing wastes, require drying before subjecting to pyrolysis. The efficiency and nature of the pyrolysis process is dependent on the particle size of feedstocks. Most of the pyrolysis technologies can only process small particles to a maximum of 2 mm keeping in view the need for rapid heat transfer through the particle. The demand for small particle size means that the feedstock has to be sizereduced before being used for pyrolysis.
Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis. Fast pyrolysis is currently the most widely used pyrolysis system. Slow pyrolysis takes several hours to complete and results in biochar as the main product. On the other hand, fast pyrolysis yields 60% bio-oil and takes seconds for complete pyrolysis. In addition, it gives 20% biochar and 20% syngas.
Bio-oil is a dark brown liquid and has a similar composition to biomass. It has a much higher density than woody materials which reduces storage and transport costs. Biooil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store. Bio-oil can offer major advantages over solid biomass and gasification due to the ease of handling, storage and combustion in an existing power station when special start-up procedures are not necessary. In addition, bio-oil is also a vital source for a wide range of organic compounds and speciality chemicals.
Primary Biomass Conversion Technologies – Thermochemical April 21, 2009 12:47 am Leave a Comment Salman Zafar ⋅
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A wide range of technologies exists to convert the energy stored in biomass to more useful forms of energy. These technologies can be classified according to the principal energy carrier produced in the conversion process. Carriers are in the form of heat, gas, liquid and/or solid products, depending on the extent to which oxygen is admitted to the conversion process (usually as air). The three principal methods of thermochemical conversion corresponding to each of these energy carriers are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. Conventional combustion technologies raise steam through the combustion of biomass. This steam may then be expanded through a conventional turbo-alternator to produce electricity. A number of combustion technology variants have been developed. Underfeed stokers are suitable for small scale boilers up to 6 MWth. Grate type boilers are widely deployed. They have relatively low investment costs, low operating costs and good operation at partial loads. However, they can have higher NOx emissions and decreased efficiencies due to the requirement of excess air, and they have lower efficiencies. Fluidized bed combustors (FBC), which use a bed of hot inert material such as sand, are a more recent development. Bubbling FBCs are generally used at 10-30 MWth capacity, while Circulating FBCs are more applicable at larger scales. Advantages of FBCs are that they can tolerate a wider range of poor quality fuel, while emitting lower NOx levels. Gasification of biomass takes place in a restricted supply of oxygen and occurs through initial devolatilization of the biomass, combustion of the volatile material and
char, and further reduction to produce a fuel gas rich in carbon monoxide and hydrogen. This combustible gas has a lower calorific value than natural gas but can still be used as fuel for boilers, for engines, and potentially for combustion turbines after cleaning the gas stream of tars and particulates. If gasifiers are ‘air blown’, atmospheric nitrogen dilutes the fuel gas to a level of 10-14 percent that of the calorific value of natural gas. Oxygen and steam blown gasifiers produce a gas with a somewhat higher calorific value. Pressurized gasifiers are under development to reduce the physical size of major equipment items. A variety of gasification reactors have been developed over several decades. These include the smaller scale fixed bed updraft, downdraft and cross flow gasifiers, as well as fluidized bed gasifiers for larger applications. At the small scale, downdraft gasifiers are noted for their relatively low tar production, but are not suitable for fuels with low ash melting point (such as straw). They also require fuel moisture levels to be controlled within narrow levels. Pyrolysis is the term given to the thermal degradation of wood in the absence of oxygen. It enables biomass to be converted to a combination of solid char, gas and a liquid bio-oil. Pyrolysis technologies are generally categorized as “fast” or “slow” according to the time taken for processing the feed into pyrolysis products. These products are generated in roughly equal proportions with slow pyrolysis. Using fast pyrolysis, bio-oil yield can be as high as 80 percent of the product on a dry fuel basis. Bio-oil can act as a liquid fuel or as a feedstock for chemical production. A range of bio-oil production processes are under development, including fluid bed reactors, ablative pyrolysis, entrained flow reactors, rotating cone reactors, and vacuum pyrolysis
Pyrolysis is the thermal decomposition of organic fuels (e.g., biomass resources, coal, plastics) into volatile compounds (e.g., gases and bio-oil) and solids (chars) in the absence of oxygen and usually water. Pyrolysis types are differentiated by the temperature, pressure, and residence (processing) time of the fuel which determines the types of reactions that dominate the process and the mix of products produced. Slow (conventional) pyrolysis is characterized by slow heating rates (0.1 to 2oC per second), low prevailing temperatures (around 500oC), and lengthy gas (> 5 seconds) and solids (minutes to days) residence times. Flash pyrolysis is characterized by moderate temperatures (400600oC), rapid heating rates (> 2°C per second), and short gas residence times (< 2 seconds). Fast pyrolysis(thermolysis) involves rapid heating rates (200 to 105°C per second), prevailing temperatures usually in excess of 550oC, and short residence times. Currently, most of the interest in pyrolysis focuses on fast pyrolysis because the products formed are more similar to fossil fuels currently used. Of particular interest is the production of bio-oil which can be used for heating and to produce transportation fuels and organic chemicals. Pyrolysis of Biomass Resources
All biomass resources are composed primarily of cellulose (typically 30 to 40 percent of dry weight), hemicellulose (25 to 30 percent of dry weight), and lignin (12 to 30 percent of dry weight), but the percent of each compound differs significantly among biomass resources. This heterogeneity creates variability in the yields of pyrolysis products. Cellulose is a straight and stiff molecule with a polymerization degree of approximately 10.000 glucose unite (C6 sugar) Hemicellulose are polymers built C5, C6 sugars with a polymerisation degree of about 200 sugar units. Both cellulose and hemicellulose can be vapored with negligible char formation at temperatures above 500 "C. Lignin is a three dimensional branched polymer composed phenolic units. Due to the aromatic content of lignin, it degrades slowly on heating and contributes to a major fraction of the char formation. In addition to the major cell wall composition like cellulose, hemicellulose and lignin, biomass often contains varying amounts
of species called "extractives". These extractives, which are soluble in polar or no polar solvents, consists of terpenes, fatty acids, aromatic compounds and volatile oil. The composition of various biomass materials is present. Cellulose is converted to char and gases (CO, CO2, H2O) at low temperatures (< 300oC), and to volatile compounds (tar and organic liquids, predominantly levoglucosan) at high temperatures (> 300oC) (Funakuzuri, 1986). The yield of light hydrocarbons (i.e., C1 - C4) is negligible below 500°C but increases substantially at high temperatures (Scott et al., 1988). At temperatures above 600°C, tar yields drop, gas yields increase, and the pyrolysis of cellulose is complete (Hajaligol, 1982; Bradbury, 1979; Funazukuri, 1986; and Piskorz, 1986). Hemicellulose is the most reactive component of biomass and decomposes between 200 and 260oC (Koufopanos, 1989). The decomposition of hemicellulose is postulated to occur in two steps—the breakdown of the polymer into water soluble fragments followed by conversion to monomeric units and decomposition into volatile compounds (Soltes and Elder, 1981). Hemicelluloses produce more gases and less tar than cellulose, and no levoglucosan. They also produce more methanol and acetic acid than cellulose. Lignin is a highly linked, amorphous, high molecular weight phenolic compound which serves as cement between plant cells and is the least reactive component of biomass. The time required to pyrolyze biomass resources is controlled by the rate of pyrolysis of lignin under operating conditions. Decomposition of lignin occurs between 280°C and 500°C, although some physical and/or chemical changes (e.g., depolymerization, loss of some methanol) may occur at lower temperatures (Koufopanos, 1989). At slow heating rates, lignin loses only about half of its weight at temperatures below 800°C (Wenzel, 1970). Pyrolysis of lignin yields more char and tar than cellulose (Soltes and Elder, 1981). For wood, the decomposition of the major components occurs separately and sequentially with the hemicellulose decomposing first and the lignin last. Up to 200°C, moisture is removed, volatile products such as acetic acid and formic acid are released, and non-condensable gases such as CO and CO2 are produced. Between 200 and 280°C, further decomposition of the char and wood occur resulting in the release of pyroligneous acids, water and non-condensable gases. Separation of tar occurs. Between 280 and 500°C, release of combustible volatile products (CO, CH4, H2, formaldehyde, formic acid, methanol, and acetic acid) occurs. Char formation decreases and the carbon content of the char increases. Condensable tar is released. Above 500°C, carbonization is complete. Secondary reactions begin if the materials are not removed from the reaction zone as quickly as they form. When cooled, some volatile compounds produced during the pyrolysis of biomass resources condense to form a liquid called bio-oil. Bio-oil consists of 20-25% water, 25-30% pyrolytic lignin, 5-12% organic
acids, 5-10% non-polar hydrocarbons, 5-10% anhydrosugars, and 10-25% other oxygenated compounds. Due to large amounts of oxygenated compounds, bio-oil is polar and does not mix readily with hydrocarbons (such as petroleum-derived fuels). It contains less nitrogen than petroleum, and almost no metal or sulfur. Bio-oil is acidic (pH of 2 to 4) due to the creation of organic acids (e.g., formic and acetic acid) when biomass degrades and is corrosive to most metals except stainless steel. Hydrophilic bio-oils contain 15 to 35 percent water by weight which cannot be removed by conventional methods like distillation. High water content decreases its viscosity which aids in transport, pumping and atomization, improves stability, and lowers the combustion temperature which reduces NOx emissions. Some additional water can be added, but only up to a point before phase separation occurs which prevents bio-oils from being dissolved in water. Bio-oil is relatively unstable compared to fossil fuels due to the presence of more polymeric compounds. Table 1 summarizes select properties of bio-oil derived from the pyrolysis of wood.
A number of studies have examined factors that affect the kinetics of biomass pyrolysis reactions. Studies that have examined temperature and heat rate interactions include Scott, 1988 (maple wood); Aarsen, 1985 (wood); Ayll’on, 2006 (meat and bone meal); Koufapanos, 1989 (sawdust); Nunn, 1985 (wood and cellulose); Utioh, 1989 (wheat grain); Sadakata, 1987. These studies indicate that temperature is more important than rate of heating with respect to the mix of products, and that at any given temperature and heat rate, bio-oil and char are the dominant products. Bio-oil yields increase up to temperatures between 550°C and 680°C and then decline. As temperatures increase, char production decreases (to a steady level above 650°C) and the carbon content of the char increases. Hydrocarbon gas yields (e.g., C2H6, C3H6) increase up to about 660°C and then decline, probably due to thermal
cracking. The time required to obtain a given conversion level decreases with increasing temperature. Biomass weight loss is higher at lower pressures (Ward and Braslaw, 1985). At any given temperature, char residues increase pressure. Cellulose displays the strongest pressure dependency and lignin the lowest--the pressure effect is observable at temperatures above 350°C. The higher pressure increases the residence time of the volatile compounds resulting in higher yields of low molecular weight gases and lower yields of tar and liquid products (Blackadder and Rensfelt, 1985). The presence of inorganic materials (either as additives or as the natural ash content of the biomass resource) affects the mix of pyrolysis products. The impacts are measured using thermogavimetry (TG), thermal evolution analysis (TEA), and differential thermal analysis (DTA). Alkaline compounds have a more pronounced effect than do acidic compounds. Alkaline catalysts increase gas yields and char production and decrease tar yields; reduce the decomposition temperature; increase weight loss; and increase reaction rates (Utioh, 1989; Roberts, 1970; Tsuchiya and Sumi, 1970). Acid catalysts cause transglycosylation reactions in small quantities, and dehydration of the anhydrosugars in larger quantities. Acidic catalysts enhance the condensation of intermediate compounds and affect char oxidation. Inorganic salts reduce CO, H2, and hydrocarbon gases, but increase CO2; decrease tar; increase water yields; and increase char yields (Nasser, 1986). The presence of catalysts are most significant for wood and cellulose pyrolysis but negligible for lignin pyrolysis(Nassar and MacKay, 1986). Pyrolysis Reactions
The sequence and rate at which pyrolysis reactions occur and the factors that influence the rate are described by the kinetics of the reaction. The kinetics of fast pyrolysis reactions are characterized by Equation 1,
(Equation 1)
where Wt is the particle weight after reaction time (in grams), t is the pyrolysis time (in seconds), K o is the frequency factor (in seconds),W∞ is the ultimate particle weight (in grams), R is the universal gas constant (in Joule per grams Kelvin), E is the activation energy (in Joule per grams), and T is the temperature (in degrees Kelvin). The reported value of E varies substantially (ranging from 40 to 250 kJ/mole) depending on the operating conditions and the type of material used. Factors that affect the kinetics of pyrolysis reactions include the heat rate (length of heating and intensity), the prevailing temperature, pressure,
the presence of ambient atmosphere, the existence of catalysts, and the chemical composition of the fuel (e.g., the biomass resource). Pyrolysis reactions occur over a range of temperatures, and products formed earlier in the process tend to undergo further transformations in a series of consecutive reactions. Control of these factors determines the yield and mix of products formed. Figure 1 presents a schematic of pyrolysis reactions. During pyrolysis, two main types of reactions occur—dehydration reactions and fragmentation reactions.
Dehydration reactions occur under conditions of slow heat rates, low temperatures (< 310°C), and long residence times. During these reactions, the molecular weight of the fuel is reduced (in part due to the elimination of water) and char and water vapor are formed. As the heat rate and temperature increase, free radicals and low molecular weight (< 105) volatile compounds such as hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2), are formed. Increasing temperatures reduce char formation and alter the chemical composition of the char. Conversion of non-aromatic hydrocarbons to aromatic hydrocarbons (i.e., carbon compounds that are unsaturated (contain few hydrogen compounds) and that show low reactivity) occurs at temperatures between 300 and 400°C. Dehydration reactions are typical of slow pyrolysis.
Fragmentation reactions occur at > 310°C. During these reactions, the fuel is de-polymerized to form levoglucosan (an anhydrosugar derived from cellulose) and tar. The tars undergo secondary reactions depending
on heat rate, temperature, and pressure which affects the residence time of compounds. Under conditions of medium temperatures (200 to 600°C), high pressure, and long residence times, the volatile compounds and light tars are recombined to form stable secondary tars. Under conditions of rapid heat rates, high temperature, and low pressure, tars vaporize and produce transient oxygenated fragments which are further cracked to yield olefins (alkenes—organic chemicals characterized by double bonds between carbon atoms), CO, N2, and other hydrocarbons such as acetol, furfural, and unsaturated aldehydes. If high temperatures are maintained for an extended period of time (long residence times), the olefins are converted to permanent hydrocarbon gases (e.g., C2H6, C3H6), condensable aromatic vapors (e.g., benzenoid and non-benzenoid hydrocarbons), and carbon black (mixture of partially burned hydrocarbons). Rapid quenching of intermediate products (i.e., short residence times) is needed to recover the ethylene-rich gases (olefins) used to produce alcohols, gasoline, and bio-oil. Fragmentation reactions are typical of fast pyrolysis.
Ambient atmosphere affects the heat rate and the nature of the secondary reactions and may be a vacuum, an inert surrounding, or a reactive surrounding. In a vacuum, primary products are rapidly removed in the gas phase and are unavailable for further reactions. Water or steam speeds up the breakdown of molecules (hydrothermolysis) and may be catalyzed by acid or alkali reagents. The presence of inorganic salts and acid catalysts can lower the process temperature, increase char formation, and alter char properties.
The chemical and physical properties of the fuel are key variables in the pyrolysis kinetics and thus significantly affect the yields and product mix. The heat rate is a function of the fuel size and type of pyrolysis equipment. Heat rates are lower for large particle sizes which favors the formation of char and higher for small particles which favors the formation of tars and liquids. Pyrolysis Technology Variant Tech.
Residence Heating Temperatur Products time rate e °C Carbonation days very low 400 charcoal Conventiona 5-30 min low 600 oil, gas, l char Fast 0.5-5s very high 650 bio-oil Flash-liquid <1s high < 650 bio-oil Flash-gas <1s high < 650 chemicals, gas
Ultra
< 0.5
very high
1000
Vacuum Hydro-pyro. Methanopyro.
2-30s < 10s < 10s
medium high high
400 < 500 > 700
chemicals, gas bio-oil bio-oil chemicals
Characteristics of Pyrolysis Technologies
Feedstocks Feedsize Moisture Parameters Temp °C Pressure, bar Max. input, t/h Product Gas, % wt dry MJ/Nm3 Liquid % MJ/Kg Solid % MJ/Kg
Flash low T
Flash high T
small
small
v.low
v. low
Slow Carbonizat ion modera te low
large low
450-600 650-900 500-600 1 0.1- 1 1
450-600 1
0.05
0.02
5
10
< 30
< 70
< 40
< 40
10-20 < 80 23 < 15 30
10-20 < 20 23 < 20 30
5-10 < 30 23 < 30 30
2-4 < 20 10-20 < 35 30
Comparison of Pyrolysis Process Technologies: ranking according to the desired products Technology
Organization
Capacity (kg/h)
Fixed bed
Bio-Alternative
2000
Fluid bed
THEE
500
Radiation Furnace Conventional
Univ. Zaragoza
100
Alten (KTI+ Itaenergy)
500
Desired T (°C) Gas/Tar/char produc (Wt%) t Char 55/15/3 500-800 0 Gas 80/10/1 6500 1000 Gas 90/8/2 10002000 Tar
Circulation fluid bed Fast entrained flow Vacuum
Ensyn Engineering
30
Tar
Georgia Tech Research Ins. Laval University
50
Tar
30
Tar
Solar Energy research Ins. low temperature Tubingen University Flash fluid bed Waterloo University
30
Tar
10 3
Tar
Rotation cone reactor
10
Tar
Vortex reactor
Univ. Twente
25/65/1 0 30/60/1 0 15/65/2 0 35/55/1 0
450-800 400-550 250-450 475-725
20/70/1 425-625 0 20/70/1 500-700 0
Energy and Density Characteristics Feed Straw Woodchips pyro-oil Charcoal char-water slurry (50/50) char-oil slurry (20/80)
Bulk density Kg/M3) 100 400 1200 300 1000
Heating value dry basis (GJ/T) 20 20 25 30 15
Energy density (GJ/M3) 2 8 30 9 15
1150
23
26
Products and characteristics The primary products can be gas, liquid and solid depending on the process employed. Most of the projects interest in the liquid products due to their high energy density and potential for oil substitution. The liquid, when formed, approximates to biomass in elemental composition with a slight higher heating value of 20-25 MJ/Kg, and is composed of a very complex mixture of oxygenated hydrocarbons. The complexity arise from the degradation of lignin, and the broad spectrum of phenolic compounds. The liquid is often called oil, but is more like tar. This also can be degraded to liquid hydrocarbon fuels. The crude pyrolysis liquid is a thick black tarry fluid with up to 20 % wt water and viscosity as heavy oil. The solid products from pyrolysis process is char, which has limited application in developed countries for metallurgical and leisure use. An alternative approach to liquid products lies in grinding the car and slurry it
with water with a stabilizer. Stable and mobile concentration of up to 60 % wt has been reported. The slurry can also be made from the bio-oil and char, but the maximum solid concentration appears to be 30 %. The gas product from pyrolysis usually a MHV fuel gas around 15 -22 MJ/NM3. or a LHV fuel gas of around 4-8 MJ/Nm3 from partial gasification depending on feed and processing parameters. Bio-oil Crude pyrolysis liquid or bio-oil is dark brown and approximates to biomass in elemental composition. It is composed of a very complex mixture of oxygenated hydrocarbons with an appreciable proportion of water from both the original moisture and reaction product. Solid char may also be present. The liquid is formed by rapidly quenching and thus freezing the intermediate products of flash degradation of hemicellulose, cellulose and lignin. The liquid thus contains many reactive species, which contribute to its unusual attributes. Bio-oil can be considered a micro-emulsion in which the continuous phase is an aqueous solution of holocellulose decomposition products, that stabilizes the discontinuous phase of pyrolytic lignin macromolecules through mechanisms such as hydrogen bonding. Aging or instability is believed to result from a breakdown in this emulsion. In some ways it is analogous to asphaltenes found in petroleum. Fast pyrolysis liquid has a higher heating value of about 17 MJ/kg as produced with about 25% wt. water that cannot readily be separated.It will not mix with any hydrocarbon liquids. It is composed of a complex mixture of oxygenated compounds that provide both the potential and challenge for utilisation The liquid has a distinctive odour - an acrid smoky smell, which can irritate the eyes if exposed for a prolonged period to the liquids. The cause of this smell is due to the low molecular weight aldehydes and acids. The liquid contains several hundred different chemicals in widely varying proportions, ranging from formaldehyde and acetic acid to complex high molecular weight phenols, anhydrosugars and other oligosaccharides. The liquid contains varying quantities of water, which forms a stable single phase mixture, ranging from about 15 wt% to an upper limit of about 3050wt% water, depending on how it was produced and subsequently collected. Pyrolysis liquids can tolerate the addition of some water, but there is a limit to the amount of water, which can be added to the liquid before phase separation occurs, in other words the liquid cannot be dissolved in water. It is miscible with polar solvents such as methanol, acetone, etc. but totally immiscible with petroleum-derived fuels. The density of the liquid is very high at around 1.2 kg/litre compared to light fuel oil at around 0.85 kg/litre. This means that the liquid has about 42% of the energy content of fuel oil on a weight basis, but 61% on a
volumetric basis. This has implications on the design and specification of equipment such as pumps and atomisers in boilers and engines. The viscosity of the bio-oil as produced can vary from as low as 25 cSt to as high as 1000 cSt (measured at 40?C) or more depending on the feedstock, the water content of the oil, the amount of light ends that have been collected and the extent to which the oil has aged. Viscosity is important in many fuel applications (Diebold et al. 1997). Pyrolysis liquids cannot be completely vaporised once they have been recovered from the vapour phase. If the liquid is heated to 100?C or more to try to remove water or distil off lighter fractions, it rapidly reacts and eventually produces a solid residue of around 50wt% of the original liquid and some distillate containing volatile organic compounds and water. While bio-oil has been successfully stored for several years in normal storage conditions in steel and plastic drums without any deterioration that would prevent its use in any of the applications tested to date, it does change slowly with time, most noticeably there is a gradual increase in viscosity. Recent samples that have been distributed for testing have shown substantial improvements in consistency and stability. It’s a complex mixture of chemicals resulting from the thermal decomposition of biomass. The types and amounts of each chemical may vary widely depending on the thermal process used. Pyrolysis oil is also known as biocrude, bio-oil or bioleum. There are a number of technologies that convert biomass into pyrolysis oil and all of them involve high temperature (between 400 and 650 degrees Celsius). Pyrolysis oil has the potential to become a viable alternative to fossil fuel. The U.S. DOE estimates that there are more than 1.3 billion tons of recoverable biomass today. Using current technologies, this biomass could be converted to 130 billion gallons per year of fuel per year, or 65 percent of the total U.S. consumption. Some energy crops such as switchgrass and Miscanthus, as well as algae, can also be used as feedstock for pyrolysis oil, which can then be further processed into transportation fuels. Pyrolysis oil is highly acidic and corrosive. As a result, it is difficult to store and transport, and can damage engines, boilers and refinery processing equipment. Upgrading pyrolysis oil in a cost-effective manner to remove the high acid content is a challenge that must be overcome. Technologies like hydrogenation, hydro-deoxygenation and other similar conventional processes address the high acidity problem; however, they require large-scale plants and capital, and exhibit substantial yield losses (up to 50 percent sometimes) mainly due to the use of hydrogen. Some next-generation technologies attempt to use existing infrastructure and eliminate the use of hydrogen to reduce the high capital and operating
costs of upgrading pyrolysis oil. My company, the Houston-based Enhanced Biofuels, has developed such a technology. We use a proprietary reactor system that bolts onto existing facilities, to share infrastructure, and it uses a readily available and relatively inexpensive alternative to hydrogen for reducing or eliminating the acidity of pyrolysis oil in a cost-effective manner. Once upgraded, the pyrolysis oil can be used many different ways. It could be fed directly into a conventional refinery to make green transportation fuels while using existing infrastructure, becoming a domestic green feedstock for the U.S. refining infrastructure. It could also be a blend-stock for bunker or marine fuel. Bio-oil has lower heating value than bunker fuel, but it also has lower viscosity, essentially no sulfur, and high oxygen content. These properties will likely make a bunker fuel/pyrolysis oil mix easier to handle, and will improve the burning characteristics of bunker fuel. Pyrolysis oil is also a domestic, green low-cost alternative to heating oil and boiler fuel, which are fossil-fuel based. Upgraded bio-oil can also be used for power generation in diesel and sterling engines. This application supports distributed green power efforts, because the fuel’s easier to transport and handle, and has higher energy density than biomass. Finally it can be a chemical feedstock. Pyrolysis oil contains carboxylic acids, and phenolic and nitrogen compounds, which can be converted into resins, adhesives, solvents, fertilizers, flavors and more. In addition to these valuable building blocks, pyrolysis oil can be processed into traditional petrochemicals and plastic intermediates through refining and processing. Pyrolysis oil has the potential to be available in large amounts and competitively priced. Upgrading bio-oil in a cost-effective fashion to reduce its high acid content is essential. Next-generation technologies that attempt to address this challenge will give pyrolysis oil great promise as a domestically grown, green feedstock for production of fuels and chemicals using the current refining and transportation infrastructure. Uses of Bio Oil Fuel Oil Given its instability, it is not possible to distil bio oil, but it may be used directly as an alternative to diesel and other fuel oils. The heating value of bio oil is about 50% that of conventional petroleum-derived fuel oil, requiring adjustments to the rate of feed of the fuel. Chemical Feedstock As it cannot be distilled, other separation methods must be used, including making use of the fact that part of the oil is water miscible, part is not.
Although the range of substances present is wide, several tend to predominate, and these are potential candidates for chemical stock. • •
•
Bio oil can also be used as a chemical resource without separation: Reaction with ammonia and amines results in a cocktail of non-toxic compounds suitable for use as a slow-release fertiliser. Reaction of the carbonyl and carboxyl groups with ethanol (which itself can be produced from renewable resources)gives a mixture that is more stable than raw bio oil. As a fuel, this has a higher thermal output, and a significant proportion can now be distilled to obtain many other useful products.
Control of conditions can result in oil that is particularly high in compounds of phenol, and this can be used directly to make phenolic resins for wood products like plywood and MDF.
Biomass Pyrolysis Reactors
Substantial differences in the temperature of the biomass resource and the reactor temperature may affect the heat rate. A number of different kinds of pyrolysis reactors are available. Pyrolysis is a precursor to gasification and combustion, and the same reactors used for gasification(i.e., fixed bed and fluidized bed reactors) can be used for pyrolysis. Bubbling fluidized bed reactors are simpler to design and construct than other reactor designs, and have good gas to solids contact, good heat transfer, good temperature control, and a large heat storage capacity. High liquid yields (60 to 75 percent weight of wood on a dry basis) can be typically achieved. Small fuel particle sizes are needed (< 2-3 mm) to ensure high heat rates. The rate of particle heating is the rate limiting factor. Figure 2 shows a fluidized bed pyrolysis reactor developed by Daugaard (2003).
In the BioTherm reactor (Figure 3), fluidized sand in a zero-oxygen environment quickly heats the fuel to 450oC where the fuel is decomposed into solid char, gas, vapors and aerosols. After exiting the reactor zone, these products pass through two sequential cyclones where most of the solid char particles are removed and collected. The scrubbed gases, vapors and aerosols enter a direct quenching system where they are rapidly cooled (< 50oC) with a liquid immiscible in bio-oil. The bio-oil is condensed and collected and the quench liquid is recovered (in a heat exchanger) and recycled. Non-condensable gas and residual bio-oil aerosol droplets enter a precipitator that electrostatically removes particulates and aerosols. The clean, inert gas is then recycled back to the bubbling fluidized bed reactor. The excess non-condensable gas (a medium Btu gas) is combusted to provide heat to the reactor sand.
Circulating fluidized bed pyrolysis reactors are similar to bubbling fluidized bed reactors but have shorter residence times for chars and vapors which results in higher gas velocities, faster vapor and char escape, and higher char content in the bio-oil. They have higher processing capacity, better gas-solid contact, and improved ability to handle solids that are difficult to fluidize than bubbling fluidized bed reactors, but are less commonly used. The heat supply typically comes from a secondary char combustor. Ablative pyrolysis reactors function on the premise that, while under pressure, heat transferred from a hot reactor wall will soften feedstockin contact with it allowing the pyrolysis reaction to move through the biomass in one direction. The feedstock is mechanically pushed through the reactor. High rates of pressure significantly affect the rate of the reaction and the velocity of the feedstock on the heat exchange surface. Rather than limited by the rate of heat transfer through the biomass particle, the reaction rate is limited by the rate of heat supply to the reactor and thus larger particles can be pyrolyzed. Inert gases are not required resulting in smaller processing equipment and more intense reactions. However, the process is dependent on surface area so scaling to larger facilities is costly and the use of mechanical drivers is more complex. In a rotating cone pyrolysis reactor, room temperature biomass particles and hot sand are introduced near the bottom of the cone, mixed, and transported upwards by the rotation of the cone. Pressures are slightly above atmospheric levels. Rapid heating and short gas phase residence times can be achieved. Figure 4 presents a schematic of a rotating cone pyrolysis reactor.
Under flash (or fast) pyrolysis, the feedstock is ground to very fine particles, in order to facilitate rapid heat transfer, and moderate temperatures of 450-700oC are employed for a very short residences time (in the order of seconds). The gases evolved are rapidly quenched in order to prevent their depolymerisation to non-condensable gases. The end result is a high yield of bio-oil, which may be up to 70% by mass of the feedstock, depending on the starting material. This bio-oil is a complex mixture of water, carboxylic acids and lignin- and carbohydrate-derived products that can deteriorate rapidly over time, with polymerisation reactions forming additional water and causing phase separations. Dynamotive, a company that has commercial facilities producing electricity via the combustion of bio-oil, claim that the bio-oil produced from sugarcane baggasse processed in their flash pyrolysis reactor contained an array of products [135] including water (20.8%), levoglucosan (3.0%), hydroxyacetaldehyde (10.2%), acetic acid (6.6%), and formaldehyde (3.4%).Ensyn is another company with several commercialscale bio-oil to energy facilities. Due to the presence of water and abundance of oxygen, the heating value of the bio oil is low compared to fossil oils and is only suitable as a heating fuel. It can be used as a starting point towards a transport fuel if catalytic upgrading mechanisms are employed. Alternatively, there may (according to the chemical composition of the starting feedstock) be valuable chemicals that can be extracted from the bio-oil. Ultimately, it is hoped that, either through catalytic upgrading after pyrolysis or through modifications to the pyrolysis process itself, an oil can be produced that is suitable as a transport fuel or is compatible with existing petrochemical processing facilities. A production of a bio-oil that is a viable substrate for biochemical processing is another option. This can occur via acid hydrolysis of the levoglucosan to glucose, but there also microorganisms that can directly ferment levoglucosan. Where lower temperatures (300-5500C) are employed and the biomass is less finely comminuted, residence times tend to be longer (slow pyrolysis) and a higher yield of biochar results at the expense of significant bio-oil formation. These temperatures facilitate the depolymerisation of polysaccharides along with their slow dehydration to unsaturated species that can further react, via an array of potential pathways, to unsaturated polymers and char. The biochar that is produced is of value, both as a fuel due to its high energy content (~30 GJ/t [117]) and as a soil amender due to the porosity properties conferred by particular pore-sizes, and the ultrastructure that result from the pyrolysis process. For more information on bochar please refer to the appropriate webpage on Carbolea.
In contrast to slow pyrolysis, the higher temperatures of fast pyrolysis result in the depolymerised polysaccharides being made volatile and forming complex tars and oils. When the conditions are so severe (over 700oC) that these tars are extensively broken to non-condensable gases, a potential fuel gas, containing hydrogen and methane as well as carbon oxides, is produced. As with gasification technologies, this gas has potential as a feedstock for chemical synthesis via various catalytic pathways. A detailed consideration of the feedstock of interest is also often necessary when predicting how it will perform under pyrolysis, and what end products will result. For example, ash tends to promote char formation during pyrolysis and the bio-oils that are generated from high-ash feedstocks may contain high levels of chlorine and alkali metals such as potassium that may be damaging to turbines if the oil is to be utilised for power production. Here at Carbolea we will shortly receive a stainless steel reactor (1.5 dm3), designed and built to our specifications and illustrated below, to conduct pyrolysis of biomass in moderate pressure (up to 30 atm). Bio-oil in a vapour form will be transported to the catalytic reactor and cracking experiments will be performed. Characterisation of product bio-oil will include CHNOS elemental analysis, H2O by Karl Fisher, Calorific vale, Acid, ester, iodine value, density, viscosity, pH, etc. We will also shortly have a larger gasificaiton system that can also run in the pyrolysis mode. For more information on this apparatus please consult the appropriate webpage.
Specific technologies for bio-oil production. Bio-oil production is maximal at medium process temperatures (450-650) and short vapor residence times in the reactor. Useful criteria for selecting pyrolysis technologies for bio-oil production are: i) the bio-oil yield per unit
of mass of wood which should be as high as possible, ii) the reactor capacity of the process should be as large enough to limit the number of scale-up steps to full plant capacity. Pyrolysis technologies include in the following survey are selected on the basis of these criteria. Accordingly, it was decided to consider only processes with bio-oil yield larger than 50 weight percent on dry wood basis and a plant capacity of more than 10kg/h. A schematic arrangement of four notable technologies is present in Fig.7.1; their specific features are given in table 5 together with those of the "Twente rotation cone process".
Fast Pyrolysis Analytical techniques that were used for studying the pyrolysis are TGA at small scales Fourier transorm infra red spectroscopy Mass spectrometry Gas and Liquid chromatography Simulation models Essential feature for fast pyrolysisHigh heating and heat transfer rate. Size of the biomass feed in pyrolysis Carefully controlled reaction with very short residence time(<1) Quenching technology used for rapid cooling
Fast pyrolysis (Flash pyrolysis) takes place in less than two seconds with temperatures between 300 and 550 degrees Celsius. Char accumulates quickly in fast pyrolysis and must be removed frequently. Fast Pyrolsis can be further categorized into the following: Ablative Fast Pyrolysis - pressure is applied to biomass to increase speed of decomposition through use of centrifugal or mechanical force. Larger particles of biomass can be used in this process. Cyclonic Fast Pyrolysis - also called vortex fast pyrolysis, separates the solids from the non-condensible gases and returns them to the mixer.
Rotating Cone Fast Pyrolysis - uses a compact high intensity reactor in which biomass of ambient temperature is mixed with hot sand. Upon mixing with the hot sand, the biomass decomposes into 70% condensible gases with 15% non-condensible gases and 15% char
a. Entrained flow reactor Biomass pyrolysis in an entrained flow reactor has been studied by Gorton et al (1990) at the Georgia Institute of Technology, Atlanta, GA, U.S.A. A flow sheet of their process is given in Fig.7.1a. The vertical reactor tube has a length of 6.4m and an internal diameter of 0.15m. Air and propane are introduced stoichiometrically and combustion in the bottom section of their reactor. The produced hot flue gas flows upwards through the tube while passing the biomass fees point. In this way the thermal energy of the combustion gas is used to heat the biomass particles and, if necessary, provide the heat of the pyrolysis reaction. Typical operation condition are ratio of carrier-gas mass flow over the pyrolysis mass flow of about 4, a reactor inlet temperature of 900 °C, an atmospheric reactor pressure and a reactor throughput of 500 kg.h. The disadvantage is that it needs large amount of carrier gas (nitrogen).
b. Circulating fluid bed reactor.
An upflow circulating fluid reactor has been operated by Ensyn in Ottawa, Canada (Graham, 1988). Fig.7.1b show that the biomass particles and preheated sand are fed together in the bottom section of the circulating fluid reactor. Unfortunately there is no literature available reporting the dimension and the flow rates of the preheated carrier gas and sand for this process. Typical operation of this reactor are a temperature 600 ° C and a biomass throughput of 100 kg/h. It is claimed 60% bio-oil can be achieved with poplar wood as the feed stocks. The use of the sand as a heat carrier offers the advantage of a compact construction because of the high heat transfer rate from the sand to biomass particles. Another advantage is the short residence time of gas, by which secondary tar cracking is suppressed. When this reactor becomes scales-up, special attention should be pay to the rapid mixing of biomass particles with solid heat carrier. Again the requirement of carrier gas is a disadvantage. c. Vacuum furnace reactor The vacuum pyrolysis of aspen polar in a multiple hearth reactor has been studied by Roy et al (1992, 1993) at the University of Laval, Quebec, Canada. Six heated hearths with a diameter of 0.7 m are staked on top of a total height of 2 m as part of the reactor given in Fig.7.1c. Wood is fed into the top compartment of reactor and transported downwards by gravity and by the action of scrapers which at present in each compartment. If the biomedicine is converted completely, the bottom compartment contains only charcoal which can be easily removed from the reactor. The temperature of top hearth is about 200 °C and increases towards the bottom the reactor where it reaches 400 ° C to achieve a maximum bio-oil products. A vacuum pump is used to keep the reactor pressure at a value of 1 KPa. A difficulty in scaling -up the reactor is necessary installing a large capacity vacuum pump which is sensitive to fouling and also it is very expensive. d. Vortex reactor A vortex reactor has been constructed by Diebold and Power (1988) at solar Energy Research Institute, Golden, Co. U.S.A. This reactor has a tube diameter of 0.13 m and a length of 0.7 m For proper operation the reactor, biomass particles should be entrained in a nitrogen flow with velocity of 400 m/s and enter the reactor tube tangentially (see Fig.7.1d). For such condition the biomass particles experience high centrifugal forces which induce high particle ablation rates on the heated reactor wall (625 °C). The ablating particles leave a liquid film of bio-oil on the wall which evaporates rapidly. If the wood particles are not converted completely they may be recycled with a special solids recycle loop. In their paper, Dieblod and Power (1988) estimate the number of cycles required to achieve completely conversion of the biomass particles to be about 15, which is considered to be quit high. However, 80 weight percent bio-oil on dry wood basis has been achieved up to now.
Pyrolysis experiment: Sample of Gmelina arborea wood was pyrolyzed using a bench scale screw reactor (Fig. 1) at temperature of 450°C. A total of 605 g of the sample was metered into the reaction chambers externally heated by 3 pairs of band heaters at 400, 430 and 450°C forming 3 reaction zones, respectively. Nitrogen gas was used as sweep gas to purge out oxygen in the system before and during the reaction. The hot vapour products produced were cooled to room temperature by a refrigeration system using ethyl glycol solution at -24°C. The cooling (condensation) chambers are made of two vertical copper tubes. The first condenser uses water from melting ice flowing in counter direction with the hot vapour while the second condenser runs on the ethyl glycol. Most of the liquid products condensed at the first condenser while at much lower temperature further liquid product was collected in the second condenser. The noncondensable gases were let out of the chamber by gas delivery tube and collected into gas trap bags for analysis with gas analyzers. When tested for combustion, the flame was blue implying the presence of flammable gases confirmed to be H2, C2H4, CH4 by analysis of the gas. The pyrolysis products were analyzed using elemental, Karl-Fisher, thermal and spectroscopic methods to determine the potential of the products to be sources of renewable fuels and industrial raw materials. Structural carbohydrates and lignin: The composition helps in understanding its thermal behavior since, the cellulose and hemicellulose decompose at much lower temperature than the lignin. High lignin will lead to oil with much lignin derived water insoluble compounds which density is expected to be high also. These parameters were determined by two-step acid hydrolysis method as provided by NREL/MRI laboratory analytical procedure NREL/TP-510-42618 (Sluiter et al., 2008). The sample was analyzed on as received basis using 72% sulphuric acid to breakdown all structural carbohydrates. The sugars in the hydrolysate were analyzed by high-performance liquid chromatography with refractive index detection (HPLC/RI) using D(+)glucose, D(+)xylose, D-cellobiose, L(+)arabinose, D(+)galactose and D(+)mannose as calibration standards for determination of cellulose and hemicellulose components of the sample. The insoluble lignin was filtered using filtration crucibles and later ashed at 575°C. The acid soluble lignin was determined using Shimadzu UV spectrophotometer 1800 at a wavelength of 320 nm. Physical characterization: The results are discussed in terms of the physical and chemical properties of the solid raw sample, the pyrolysis products and comparison to the products of other wood and agricultural residues previously used for bio-oil production. Table 1shows the physical characterization of the Gmelina sample used in the analysis. It was air dried to a moisture content of 5.25% typical of that required for thermochemical analysis. The sample is a low ash material, with only about 1.37 wt.% on dry basis implying that the rest was available for conversion to energy. Table 1: Physical and chemical
properties of theGmelina wood
ar: as received, db: dry basis and daf: dry and ash free. *Source Hossain (1999) This possibly accounts for the high volatile matter content 84.36% on dry basis. Similarly the low ash content implies low alkali metal content in the biomass. Biomass materials of high ash content do not make good sources of pyrolysis oil since, ash plays catalytic role in cracking of the liquid products to form gases (Nik-Azar et al., 1997; Pattiya et al., 2007) because of presence of alkali metal. The HHV and LHV of the sample are typical of wood samples used for pyrolysis. The values are lower than those of coal and higher than those of straw ( Tsai et al., 2006). The lower values are as a result of high oxygen content of the raw sample (Table 1). This is responsible for the oxygenated pyrolysis liquid product (Table 2) whose heating values are lower than those of fossil fuels. The sample is a high carbon material as revealed by the elemental analysis while the low Nitrogen and Sulphur values position the sample as a source of bio-oil of low pollutant effect. The values of nitrogen and sulphur are, respectively lower than values for coal which contain 1.4 wt.% Nitrogen and 1.7 wt.% sulphur (Pattiya et al., 2007). Hence, the biomass feedstock and its products are environmentally friendly source of bio-fuel. The structural carbohydrate composition of the raw sample (Fig. 2) shows that the sample is a low lignin material hence, the bio-oil is expected to contain less water insoluble heavy compounds but posses good thermal behaviour. The pyrolysis temperature should not be too high to avoid secondary cracking of the products. The sample composed of about 60 wt. % of cellulose and hemicelluloses which decompose at much lower temperature than the lignin (Mohan et al., 2006). The result of the bio-oil characterization is presented in Table 2. The crude bio-oil is red-brown and has pungent barbecue-like odour characteristic of
bio-oil from woody biomass ( Tsai et al., 2006; Bridgwater et al., 1999). The pyrolysis liquid product is composed of 26.42 wt.% moisture. Table 2:
Gmelina arborea bio-oil
Fig. 2: Result of the compositional analysis of the wood sample
Fig. 3:
Pyrolysis product yield percentage composition
This could be due to inherent moisture in the feedstock and water of dehydration reaction. The gross calorific values on wet and dry bases are
lower than those of fossil fuels due to the oxygen content as explained above as well as for its high moisture content. These values however, are higher than those of pretreated and untreated pine wood reported by Hassan et al. (2009) even though its moisture content is higher. It is not certain if the location of the feedstock contributed in the quality of the bio-oil as regards its heating values. This may suggest that Gmelina may produce bio-oils of higher calorific values if produced from feedstock of very low water content. The LHV is higher than one half of that from fossil fuel. The liquid product ash content is typical of bio-oil from biomass materials.
Fig. 4: Bio-oil chromatogram showing some major chemicals identified: a: Acetol, b: Dioxanediol, c: Acetic acid, d: Acetol, e: Ethyl pyruvate, f: 1H-Pyrazole, 3,5dimethyl-, g: 6-Oxa-bicyclo[3.1.0]hexan-3-one, h: 1,2-Cyclopentanedione, i: Guaiacol, j: Phenol, or, 2-methoxy-4-methyl-, k: Phenol, 2,6-dimethoxy-, l: 1,2,4-Trimethoxybenzene, m: Phenol, 2,6-dimethoxy-4-(2-propenyl), n: 1,6Anhydro-.beta.-D-glucopyranose (levoglucosan), o: Phenol, 2,6-dimethoxy-4(2-propenyl)-, p:Benzaldehyde, 4-hydroxy-3,5-dimethoxy The % product yield result of the pyrolysis experiment shown in Fig. 3 presents the major classes of products formed which include 13 wt.% of biochar, 71 wt.% of pyrolysis crude oil and 16 wt.% non-condensable flammable gas. The result shows that under reactor and feedstock optimized conditions, higher yield of liquid product can be anticipated. The characterization of the solid product shows that the biochar HHV is 37 MJ kg-1 with 4 wt.% of ash on as received basis. Even though the percentage bio-char yield is relatively low, its energy content is higher than most coal species indicating that the product is a valuable source of energy. The biochar can be utilized as source of energy for the heating of the reactor chamber.
The range of pH of typical wood-derived bio-oil is 2.5-3.4 (Ingram et al., 2008). The pH of the pyrolytic liquid crude from Gmelina is 2.55 which falls within the range presented above from other woody biomass samples used in fast pyrolysis. This is because of the presence of organic acids - formic acid, carboxylic acids, acetic acids etc. (Tsai et al., 2006; Czernik and Bridgwater, 2004; Bridgwater et al., 1999) and alcohols as confirmed by Fig. 4. This implies that for engine and boiler applications, the bio-oil from Gmelina arborea should undergo upgrading to increase the pH to neutral level in order to prevent or reduce corrosive effects of bio-oil. Chemical characterization of the bio-oil: The chemical characterization of the whole bio-oil from the bench scale reactor was analysed by dissolving 2 g of the oil in 10 mL methanol, after which the solution was analyzed using a Shimadzu QP-5050A GC/MS. The GC used a Restek Rtx-1701 column, 60 mx25 μ m with a 0.25 μ m film thickness. A split ratio of 1:100 was used to reduce the sample amount migrating through the column. Helium was used as the carrier gas at a column flow of 0.5 mL min-1. The GC oven program was set to start at 40°C for 1 min and a heating rate of 4°C min-1 to 260°C. The injector and detector temperatures were set at 270°C. The mass spectrometer was operated in the Electron Ionization (EI) mode at ionization energy of 80 eV, m/z of 40-400 and a sampling interval of 0.34 sec. Identification of chromatographic peaks (Fig. 4) representing some of the chemicals was carried out by comparing the mass ions (m/z) of each peak with the 2005 NIST mass spectral database. Figure 4 shows the chromatogram of the bio-oil when analyzed using GC/MS. Most of the chemicals have been identified by other scholars working with other biomass species (Pattiya et al., 2007; Sipila et al., 1998; Hassan et al., 2009). Char characterization: The solid product of the fast pyrolysis was characterized by determining its heating value and ash content. The Higher heating value was found to be 37 MJ kg-1 while its ash content is 4 wt.%. These values show that the bio-char can supplement coal in co-generation power plant for heat and electricity generation. It can also be a very good fuel in pyrolysis heating chambers. MATERIALS AND METHODS Feedstock: The feedstock was obtained and prepared at National Centre for Energy Research and Development, University of Nigeria, Nsukka. The wood sample was cut from mature Gmelina tree although its age was not known. It was air dried and ground to a particle size less than 1 mm passing mesh 40 (425 μm) using Wiley milling machine.
Feedstock and product characterization: The thermochemical characterization of the feedstock for proximate analysis (moisture,
volatile and ash contents) were determined according to ASTM D3173-03, ASTM D3175-07, ASTM D3174-07, respectively while Fixed carbon was calculated as difference according to the relation: Fixed C = 100 - Ash (ar)water content-volatiles (ar). The values were converted to dry and ash free (daf) basis using values of moisture content and ash. The elemental analysis for determination of Carbon, Hydrogen, Nitrogen, Oxygen and Sulphur content of the solid sample and liquid product was carried out at Galbraith Labs Inc, Knoxville, Tennessee USA. Oxygen was determined using Thermo Finnigan Flash EA Elemental Analyzer. Sulphur was determined according to ASTM D4239-83, using LECO SC-432DR (Trace E16-2A) while Carbon, Hydrogen and Nitrogen of the feedstock material were determined by the LECO CHN 2000 analyzer. The ASTM D5373-02, Standard Test Methods for Instrumental Determination of Carbon, Hydrogen and Nitrogen, Laboratory Samples of Coal and Coke was used for the analysis. The bio-oil was analyzed for carbon and hydrogen content according to ASTM D5291-02, Standard Test Methods for Instrument Determination of Carbon and Hydrogen in Petroleum Products and Lubricants. While its moisture (water) content was determined by Karl Fischer (KF) titration method according to ASTM E203, Standard Test Method for Gross Calorific Value of Coal and Coke. Heating values: The heating value is important parameter for measuring the energy value of samples. The heating value for the analysis sample was determined according to ASTM D5865, Standard Test Method for Gross Calorific Value of Coal and Coke on dry basis using the oven dried samples. The PAAR 1341 oxygen bomb calorimeter, standardized using benzoic acid pellets was used to determine the gross caloric value (HHV) while the net calorific value (LHV) was calculated using wt.% of hydrogen resulting from elemental analysis of the sample. The samples were pelletized using the Paar pellet press 2811. Ignition inside the bomb was in excess of oxygen at a pressure of 30 atm using 45 C10 fuse wire while the temperature of 2 kg of water surrounding the bomb was measured using the mercury thermometer.
Fig. 1:
Schematic diagram of the reactor used in the work. 1: Motor, 2: Biomass feed hopper, 3: Screw shaft, 4: Reactor barrel, 5: Feed cooling system, 6: Band heaters, 7: Char trap, 8: Bio-oil condensers and 9: Bio-oil containers Upgrading of Bio-Oil
The bio-oil obtained from the fast pyrolysis of biomass has a high oxygen content. Ketones and aldehydes, carboxylic acids and esters, aliphatic and aromatic alcohols, and ethers have been detected in significant quantities. Because of the reactivity of oxygenated groups, the main problems of the oil are instability. Therefore study of the deoxygenation of bio-oil is needed. In the present work the mechanism of hydrodeoxygenation (HDO) of bio-oil in the presence of a cobalt molybdate catalyst was studied. Particularly, the effects of reaction time, temperature, and hydrogen pressure on the HDO activity were examined. On the experimental results, a kinetic model for HDO of bio-oil was proposed Many researchers have investigated the possibility of directly using pyrolysis oils and usually concluded that it requires adaptations of engines or gas turbine to suite fuel characteristics. Further investigations are necessary to fully validate a technology at demonstration scale.On the other hand the bio-oil can be modified. The kind and degree of modifications depends on the final product usage e.g. fuel for turbine, boiler, combustion engine, chemical recovery of diesel miscible product. Upgrading can be physical (e.g. separation, extraction, solvent addition, etc.) or chemical. Catalytic upgrading, hydrotreatment, esterification, cracking and blending with other liquid fuels are some upgrading techniques employed in improving the quality of the bio-oil. For instance, Oasmaa et al. (2004), improved the homogeneity and physical dilution of pyrolysis liquids by
addition of alcohol. This enhanced the solubility of the hydrophobic compounds which were high molecular mass lignin and extractives. Upgrading by Means of Esterification-Bio-oil was mixed with ethanol, at a ratio of 2:1. This mixture was then brought to total reflux (320-330K) for 30min. After the 30 min period any excess ethanol present was removed by distillation. The primary reactions which occur during this process are between carboxylic acids and ethanol to yield esters.
In the original oil water is the most abundant compound present. At this concentration water is miscible with many of the oiligimeric lignin derived components, this is due to the solubilising effect of hydrophilic compounds. There is also an azeotrope system formed between the carboxylic acids and the water, which means that the water cannot be removed by distillation. It was possible to convert low molecular weight acids into their corresponding esters, which have a lower boiling point. This allowed a large quantity of the water to be easily removed by distillation. Physical Properties of Upgraded Bio-Oil Miscanthus Bio-oil from pellets miscanthus (feedstock) pellets Water content(wt %)
6-8
Bio-oil from miscanthus pellets after esterification
21-22
5-8
2.5
4.2
0.2
0.1
Solid(wt%)
0.2-1
0.2-1
HHV(MJ/Kg)
19-21
24-25
Viscosity(@25oC) (Cp)
6.69
5.88
pH Ash(wt%)
3.8
Elemental Analysis Both the miscanthus feedstock and the bio oil were analysed for the elemental composition. This analysis was carried out using an Elementar Vario el Cube analyser. This was used to determine the wt% of C,H,N,O and S present in the samples.
Elemental Composition (wt. %)
Miscanthus Bio-oil from pellets miscanthus (feedstock) pellets
Bio-oil from miscanthus pellets after esterification
C
56.62
57.09
63.29
H
6.357
4.61
5.45
N
0.41
0.41
0.31
S
0.34
0.38
0.39
O
33.24
27.86
23.61
Chemical Composition of the Bio-Oil Bio oil is produced by using a very short vapour residence time, this means rapidly cooling and quenching the reaction. Bio oils are very complex mixtures of organic compounds. The reactions which are responsible for the formation of bio-oil are depolymerisation and fragmentation of cellulose, hemicelluloses and lignin, resulting in the formation of an excess of 200 different compounds, many of which can be identified using Gas Chromatography/Mass Spectroscopy. The types of compounds which have being identified include, carboxylic acids, ketones, phenols, aldehydes, alcohols and esters.
Functional Group
Crude BioOil (wt. %)
Upgraded BioOil (wt. %)
Alcohol
2.57
6.93
Aromatic Hydrocarbon
3.10
11.5
Ester
6.68
17.5
Alkylphenol
10.0
10.6
Oxyphenol
8.67
6.47
Furan
6.64
15.8
Carboxylic Acid
10.8
5.92
Ketone
10.6
3.92
Aldehydes
3.29
3.34
The analytical results can be used to evaluate the fuel quality of the bio-oil. In comparison with a standard diesel fuel the bio-oil has many undesirable properties, such as high water, oxygen, ash, solid contents, along with a low pH and a relatively low heating value. These unfavourable properties will cause problems if the bio-oil is to be utilized directly by thermal devices. However several techniques can be employed to upgrade the bio-oil to a higher quality fuel. This poster describes how esterification was carried out to upgrade the oil. It can clearly be seen from the results, that this method has improved many of these undesirable properties. There is a significant reduction in the water and oxygen content and also an increase in the pH and the heating value of the bio-oil. In spite of the improved properties of the bio-oil much more work is required on the stabilization and upgrading of bio-oils before implementing them for power generation. If used as a combined process, which produces other useful co-products, biomass can be an economically feasible renewable resource for efficient energy production.
A paper, entitled "Pressurised pyrolysis of Miscanthus using a frixed bed reactor" has been published. Abstract: Miscanthus x giganteus was pyrolysed, in a fixed bed reactor in a constant flow of dinitrogen gas, at a rate of 13 °C/min from ambient to 550 °C, then held for 25 min at this temperature. The pressures employed ranged from atmospheric to 26 bar. The major compounds identified in the bio-oil were water, phenol, and phenol derivatives. The water contents impact on the usefulness of the bio-oil as a fuel. However, the phenols could provide useful platform chemicals and products.
