SASOL’S ACHIEVEMENTS IN THE 20 th CENTURY AS A BUILDING BLOCK FOR THE 21 st
Presentation to the 2000 Gasification Technologies Council October 8-11, 2000, San Francisco, California, USA
P van Nierop*, HB Erasmus and JW van van Zyl, Sasol Technology, Republic of South Africa
* Corresponding Author: E-mail piete r.van r.vannier nierop@sa
[email protected] sol.com om P O Box 1 Sasolburg 1947 Republic of South Africa +27 16 960 3301 (Tel) +27 16 960 4382 (Fax)
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SASOL’S ACHIEVEMENTS IN st BUILDING BLOCK FOR THE 21
THE
th
20 CENTURY
AS
A
P VAN NIEROP, HB ERASMUS and JW VAN ZYL SASOL TECHNOLOGY REPUBLIC OF SOUTH AFRICA
A.
SASOL - THE PAST 50 YEARS 1.
INTRODUCTION
SASOL was established on 26 September 1950 with the prime objective to convert low grade coal into petroleum products and chemical feedstocks. Sasol One was built in Sasolburg and produced its first liquid product in 1955. In 1969 the Natref crude oil refinery was commissioned and in 1980 and 1982 Sasol Two and Sasol Three respectively began production in Secunda. Today, 50 years after the initial announcement, Sasol produces the equivalent of 150 000 barrels per day of fuels and petrochemicals from coal via its indirect liquefaction process. The process produces in excess of 40% of South Africa’s liquid fuels requirements. The company is listed on the Johannesburg Stock Exchange (SOL) with a market capitalisation of $4,6 billion and on Nasdaq (SASOY). Sasol manufactures more than 200 fuel and chemical products at its plants in Sasolburg and Secunda in South Africa as well as at several global locations. Its products are exported to more than 70 countries around the world. From the relative humble beginning as an liquid fuels producer Sasol has grown into a company with a turnover of $3,7 billion from which more than 50% is generated from chemicals. This is proof of the success of the decision in the mid 1980’s to diversify into higher value chemicals from Sasol’s treasure chest of intermediate feedstocks. During the 1990’s Sasol has intensified its growth and diversification programme, primarily focussing on three strategic initiatives: v v v
The formation of synergistic alliances and joint venture companies; The acquisition of strategically beneficial businesses; The expansion and improvement of current operations, through a continuous capital investment program.
The history of Sasol revolves around the Group’s impressive track record in innovating new or improved products and processes. Notable breakthroughs have been achieved in the fields of geology, mining production systems, coal
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preparation and gasification systems, mining explosives, fertilizers, biotechnology, environmental engineering and gas to liquids technology.
2.
THE SASOL PROCESS
The Sasol process starts with the Gasification plant where coal under pressure and at high temperature in the presence of steam and oxygen is converted into crude gas. Condensates from subsequent cooling of the gas yield co-products such as tar and oils. Other co-products such as nitrogenous compounds, sulfur and phenolic compounds are recovered as ammonia, sulfur, cresols and phenols respectively. The purified synthesis feed gas is made available for conversion through the Sasol Advanced Synthol (SAS) or Sasol Slurry Phase Distillate (SSPD) proprietary technology to high value products (Figure 1).
FIGURE 1 : THE SASOL PROCESS
Crude CrudeOil Oil Imported crude oil
Liquid fuels
Natref refinery Sasolburg
COAL MINING - 49 million ton/a
Synthetic Syntheticfuels fuels Coal
- 6 underground mines
Synthesis Coal gasification gas
Synfuels production
Liquid fuels
Secunda
- 2 strip mines
Chemical feedstocks
- 3.1million ton export
Chemicals Chemicals Coal
Synthesis Coal gasification gas
Sasolburg& Secunda
Chemicals production
200 chemical products
Sasolburg& Secunda
2.1
H igh-temperature F ischer-Tropsch conversion After purification, synthesis feed gas is sent to the SAS reactors where the hydrogen and carbon monoxide react under pressure in the presence of a fluidised, iron-based catalyst at moderate temperature to yield a broad spectrum of hydrocarbons in the C1 – C15 range. This process is primarily used to produce liquid fuels.
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Oxygenates in the aqueous stream from the SAS process are separated and purified in the chemical work-up plant to produce alcohols, acetic acid and ketones including acetone, methyl ethyl ketone (MEK) and methyl iso butyl ketone (MIBK).
2.2
Low-temperature F ischer-Tropsch conversion An alternative use for the synthesis feed gas from gasification or reformed natural gas, is the Sasol Technology developed low-temperature Sasol Slurry Phase Distillate (SSPD) process. In this process, which Sasol uses at Sasol Chemical Industries (Sasol One) at Sasolburg, the synthesis feed gas is reacted at a lower temperature than is the case in the SAS reactors, in a SSPD reactor producing linear-chained hydrocarbon waxes and paraffins. Apart from hard wax, candle wax and specialty Fischer Tropsch (FT) waxes, high-quality diesel can also be produced in this process. Residual gas from the SSPD process is sold as pipeline gas, while lighter hydrocarbons are hydro-treated to produce pure kerosene or paraffin fractions. Ammonia is also produced and is sold as such or is utilized downstream to produce explosives and fertilizers.
2.3
The Sasol/Lurgi Gasifi cation Process At both Sasolburg and Secunda, all synthesis gas is produced from coal using the Sasol/Lurgi fixed bed dry bottom (FBDB) gasifiers. High ash content, high ash melting point coal is used to produce a high H2 /CO syngas to satisfy the high demand for hydrogen in Fischer-Tropsch synthesis.
FIGURE 2:
MAJOR DIMENSIONS OF GASIFIERS
Scale Up Of Gasifiers At Sasol Ton Coal \ h
m 3n/h / gasifier
90000 75
n o i t c u d o r P s a G e d u r C
65000
54
34000 28
Mark III
Mark IV
Mark V
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83
1
5
Sasol One was originally equiped with ten Mark III gasifiers with an internal diameter of 3,66 m. Three gasifiers of similar design were added in 1966. In 1978 three Mark IV gasifiers, scaled up 55% above original design, were installed and a Mark V (114% scaled up above original) in 1980 (Figure 2). Sasol currently operates 83 Mark IV gasifiers of which Sasol Two and Three each have 40 units. These units can truly be seen as the “work horses” of syngas production from coal at Sasol, as well as in the world. The demand for synthesis gas at Sasol, consisting volumetrically of roughly 58% H2 , 29% CO, 11% CH4 , 1% CO2 and virtually no sulphur, increased steadily over the years, resulting in continuous pressure to increase production rates of individual units. During the last 3 financial years (1997, 1998 and 1999), average gas production rates for these gasifiers have been roughly 20% above design capacity (Table 1).
TABLE 1:
2.4
AVERAGE SYNGAS PRODUCTION RATE/ GASIFIER
Syngas production
% above design
Design syngas load (Secunda)
38 500 m3 n/h
-
Actual 1997
45 600 m3 n/h
(18%)
Actual 1998
45 600 m3 n/h
(18%)
Actual 1999
46 500 m3 n/h
(21%)
Operational H ighlights
2.4.1 Reliability The reliability with which syngas is being produced at Sasol has always been regarded as one of the corner stones of many of the successes achieved. Some are illustrated below. For Sasol West, (Sasol Two), 36 out of 40 gasifiers had at least one run of 100 consecutive days or more on line during the last 30 months. Ten of these gasifiers achieved more than 200 consecutive days on line, including four gasifiers doing more than 300 days (two cases of 343 days were recorded). The longest continuous off-line period for a gasifier is 30 days, which is required for general overall and sometimes jacket replacement. Average time between two
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of these occurrences (per gasifier) is currently about 3 years and actions are being taken to increase this to 4 years.
2.4.2 Availability Continuous improvements of mechanical components, but also to operational and managerial systems and procedures, resulted in a higher than 90% on-line availability over the last 30 months. That translates to more than 72 of Secunda’s 80 gasifiers continuously on-line. This could be achieved in spite of constantly increasing individual gas loads (Table 1). Overall plant shut down for maintenance to common systems have been reduced from the original 560 gasifier days/annum to 400 out of a maximum possible of 29 200 gasifier days, thus a 30 % reduction.
2.4.3 Gasification plants It is important to note that the significant achievements in reliability and increase in availability have been achieved while continuously increasing the gasloads. Each of the Secunda plants has originally been designed to produce 1 100 x 103 m3 n/h syngas with 36 gasifiers. (Four have later been added.) Actual production rates for the two plants for the last 3 financial years can be seen in table 2. TABLE 2: AVERAGE ANNUAL SYNGAS PRODUCTION Year
Syngas Production of m3n/h
1997
1 520 x 103
1998
1 520 x 103
1999
1 551 x 103
This has been achieved without the addition of major process units – only optimizing and de-bottlenecking of existing systems.
2.5
Gasifi cation Technology Development at Sasol The conversion of coal to synthesis gas and co-products is a core Sasol technology. Sasol has over the past fifty years developed the process extensively by improving on the original facilities commissioned at Sasolburg in the early 1950's and by progressively implementing improvements to the operating plants. The main processes associated with the conversion of coal to synthesis gas and co-products covers the following (Figure 3):
7 v v v v v v v v v v
Coal preparation Coal distribution to the top of the gasifiers Gasifiers Ash removal Gas quench and initial cooling Tar/oil/condensate cooling and separation Gas purification Sulfur recovery Ammonia Recovery Phenols recovery
FIGURE 3: THE SASOL/LURGI GASIFICATION SYSTEM
Coal Bunker Feeder Coal lock
Crude gas to gas cooling Quench liquor Steam Quench cooler
Waste heat boiler
Boiler feed water Rotating grate
Steam & oxygen
Ash lock Gas liquor Ash to sluiceway
These processes are all linked together in an overall process scheme. The most important achievement of Sasol in this field is that gas from coal is produced on a mega scale, continuously improving on the output of the plants and achieving high mechanical availability of equipment. This could only be achieved through the technical break-throughs in each of the component plants in the overall flow scheme. Highlights of the specific achievements in these plants can be summarized as follows:
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2.5.1 Typical Developmental H ighlig hts Coal R & D Enabling the coal preparation plants to produce coal for the gasifiers according to specific requirements was a steady process stretching over many years. This entailed implementing projects to minimize coal degradation, improving homogenization and efficiency in coal screening. The development of an understanding of the effects of the various properties of the coal on the gasification process and the quantification of these effects have been an ongoing activity. Much knowledge has been gained and modifications implemented, resulting in higher gasifier throughputs, improved carbon utilization and ability of the process to handle variation in coal properties. Sasol has gained significant expertise in evaluation of different coal resources for suitability in the Sasol/Lurgi coal gasification process and have tested and evaluated numerous coal types from all over the world. Highly specific analytical laboratory techniques were developed to characterize coals for compatibility with the gasification process and commercial scale testing of large batches has also been done on numerous occasions. During the period 1998 - 2000, renewed efforts were made to characterize the effects of coal properties on gasifier performance more accurately. A single gasifier at Secunda was isolated and a separate coal feed system was provided including additional measurement facilities. The so-called Test gasifier was extensively used to conduct a number of commercial scale tests.
2.6
Turning co-products into value In addition to the technical breakthroughs, the economic implications of gasification by-products optimization has been significant. Some examples are given below:
2.6.1 Gas Liquor The aqueous stream, resulting from cooling of crude gas, contains valuable co products such as tars, phenols and ammonia. After recovery of these, the remainder (stripped gas liquor or SGL) can directly be used as cooling water in the plant. This results in a reduction of raw water intake to the synfuels complex and also savings on bio-treatment of effluent streams. It is envisaged that this system will be further optimized to include FT effluents as water make-up.
2.6.2 Tar and Pi tch The Sasol/Lurgi gasification process produces tar in the devolitalization area of the gasifier as a co-product. The tar is classified as medium temperature tar, as the temperature of formation is lower than that of coke oven tar. The bulk of the
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tar is converted to liquid fuels in Secunda with the resulting pitch fraction being processed by Sasol Carbo-Tar. Sasol Carbo-Tar, a division of Sasol Chemical Industries, was established in 1995 and produces and markets a wide range of carbon and tar products including calcined anthracite, recarburisers, creosotes and disinfectants. 80% of the division’s sales are exported to North America, Europe, South East Asia and Australia. Currently the feasibility of extracting high value chemicals from the tar distillates as well as the production of high value binder pitch for use in the aluminum industry, are being evaluated.
2.6.3 Phenol and Cr esol Phenol is a very highly valued co-product from the FBDB gasifier . Crude phenol is recovered from the aqueous stream by solvent extraction and refined to very high purity phenol, cresols, etc. These products are marketed worldwide by Merisol RSA.
2.6.4 Methane The relatively high percentage of methane formed in the Sasol/Lurgi gasification process, (which takes place at a temperature of less than 1 000 C), is fully utilized. Part of it is used as blend stock to produce hydrogen rich and methane rich pipeline gas. As there are virtually no natural gas resources within South Africa, Sasol supplies over 29 million gigajoules of pipeline gas to more than 700 customers via its own pipeline system extending over 1 400 kilometers. The remainder of the methane is converted to syngas by a steam oxygen-reforming process. °
2.6.5 The H ydrogen Advantage of FBDB Hydrogen is an essential element required for the production of fuels, chemicals and ammonium based fertilizers from coal. For example: hydrogen plays a key role in the mechanism of converting syngas to hydrocarbon products via FT synthesis. The specifications for a typical FT syngas requires a hydrogen to carbon monoxide ratio 1.8-2 to 1. Ammonia synthesis feedgas requires a hydrogen to nitrogen ratio of 3.0 to 1. Hydrogen is also a key component of Sasol’s hydrogen rich pipeline gas (48% by volume). 3
SUMMARY OF THE PAST 50 YEARS
After 50 years, the Sasol/Lurgi FBDB gasification technology is today a reliable and comparatively risk free process. Sasol believes that further optimization
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potential exists beyond the current levels of maturity and is actively exploiting these opportunities. It is the only viable process for the conversion of low-grade high ash content, high melting point ash coals to high value products. Such coals are abundant in countries like China and India. As these countries develop, energy demand growth will require that the low-grade coal resources be exploited. Dedicated operations development of the process over the past 50 years have enabled Sasol to potentially become a major contributor in the establishment of such facilities. The escalating number of inquiries being received by Sasol emphasizes a growing demand for this expertise. What does the future hold for gasification technology in general? Is this the end of a successful era or a solid base for the future? We have attempted to give a perspective on these questions in the section to follow.
B
THE FUTURE
1.
INTRODUCTION
The successes achieved over the past 50 years and the continuous development of Coal to syngas (CTS) technology combined with Sasol’s proprietary Gas to Liquids (GTL) technology put Sasol in a unique and strong position for the future scenarios on syngas production and utilization. Sasol’s commitment to the future of CTS technology can be highlighted by our vision to be globally respected and acknowledged as the leader in the field of the competitive conversion of coal and related feedstocks to syngas. It is important to notice that the vision is not restrictive on the type of gasification technology to be utilised or the feedstock to be converted, but rather on utilizing Sasol’s strengths to grow into the future.
2.
2.1
THE FUTURE OF GASIFICATION TECHNOLOGY
I ntroduction Numerous publications and presentations have recently seen the light on this subject and it is not our intention to evaluate the pros and cons of each scenario, but rather to highlight future scenarios where Sasol can contribute through the application of its experience gained over the last 50 years.
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2.1.1 The Sasol Secunda F actories The focus in the near future will be on further de-bottlenecking of the current facilities by implementing results from the Test gasifier. Studies are currently underway on feed preparation, optimizing process control, modifications on the internals of the gasifier, pyrolyses and optimization of carbon conversion to valuable products. Increasing the current production rate of individual gasifiers may also be considered if required by the market demand.
2.1.2 Coal to Liquids Preliminary studies have indicated that there are selected geographical regions in the world where the economic conversion of coal to liquid fuel and chemicals may be feasible. In such cases coal should be relatively inexpensive, significant gas and oil reserves should be absent and markets for the total spectrum of products should be available. Sasol, with its experience in gasification technology combined with GTL and other technologies can play a leading role in such projects.
2.1.3 Petroleum Coke, Refinery bottoms and other unutilized carbonaceous products The integration of gasification technology combined with GTL technology may in future provide a solution to the challenge faced by crude oil refiners, namely processing increasingly lower quality feed stocks. Conversion of the resulting bottoms to high quality fuel, while complying to more stringent environmental requirements, may become a future objective.
2.1.4 F ine Coal as feed stock The impact of discarded fine coal on the environment is another issue becoming increasingly important in many parts of the world. The application of gasification technology to solve this issue is currently being investigated. A study, partly funded by the US DOE evaluates the Texaco gasifier in combination with Sasol’s FT conversion technology for a potential coal to liquids plant in Pennsylvania, USA.
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THE IMPORTANCE OF BASIC SCIENTIFIC KNOWLEDGE FOR THE FUTURE OF GASIFICATION
Other potential applications for gasification technology may also arise in the near future. For development and evaluation of such opportunities Sasol has developed a strong base of basic scientific knowledge. Some examples are given below. Apart from the developments over the past 50 years and the subsequent increase in knowledge on gasification technology, the modeling capabilities and focus on
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long term R & D are also regarded as particular strengths of Sasol which may in future contribute to the strengthening of its global competitive position.
3.1
Gasifi er Modeling At present Sasol is in possession of 5 different gasifier models each having a specific purpose. These models are: v v v v v
A thermo-dynamic model; A one dimensional pseudo-homogeneous model; A one dimensional heterogeneous model; A three dimensional pseudo-homogeneous model; A stochastic model.
These models each have unique strengths and applicability in the gasification environment. This paper will not provide the detail of gasifier modeling, but it is worth mentioning that the Test gasifier has been of significant importance in the evaluation of various modeled results and subsequent further development of the various models. Continuous model development and verification of models utilizing Test gasifier results enjoy a high priority. Using modeled results as a tool to predict actual plant performance has led to substantial savings in development resources.
3.2
R& D The focus of Gasification R & D has shifted from short term optimization to the development of long term solutions ensuring that the vision of global respect for the technology is achieved. Research and Development is fully integrated with the rest of the Coal to Syngas team.
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TEST GASIFIER
Two years ago Sasol embarked upon a unique project to optimize the performance of the Sasol/Lurgi fixed bed coal gasification process. It entailed the isolation of one of the 80 gasifiers in Secunda, and instrumenting it with accurate measuring equipment to measure all process variables. Provision was also made for a dedicated coal feeding and coal preparation system. The Test gasifier is described in detail in the presentation by E L Koper : “Perspectives on Long Term Practical Coal Research for Gasification”- GTC 2000 conference.
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CONCLUSION
It is the view of the authors that gasification technology has a bright future in the areas mentioned above and that the role of Sasol in the future of gasification technology globally is unique. Not only does the company have valuable and unique operating experience gained over 50 years of successful operations, but has also been at the forefront of technological breakthroughs in optimizing and streamlining the technology. Sasol is essentially, however, a company focussing on the successful operation and optimization of technology and is not primarily focussing on new technology development. The unique skills of Sasol may however be complementary to those of other parties who share our view on the future of gasification and related technologies.
