SASOL PERSPECTIVES ON LONG TERM PRACTICAL COAL RESEARCH FOR GASIFICATION.

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1 SASOL PERSPECTIVES ON LONG TERM PRACTICAL COAL RESEARCH FOR GASIFICATION. Presentation to the Gasification Technology Conference October 8-11, 2000, San Francisco, California, USA E L Koper, Chief Process Engineer, Sasol Technology, Secunda, South Africa. SYNOPSIS The Sasol coal-to-synthetic fuel and chemicals processes in Sasolburg and Secunda, South Africa, are world renowned as the largest commercial plants converting low grade coal into high value added products. From the relatively small beginnings in Sasolburg in the early 1950 s, Sasol has grown to the current size where 97 Fixed Bed Dry Bottom gasifiers convert approximately 30 million ton per annum of low grade coal into synthesis gas (another approximately 15 million ton per annum is produced for steam generation and export purposes). The Secunda Sasol Synthetic Fuel (SSF) complex in Secunda currently operates at a raw gas (crude gas ex the gasifiers) production level of approximately 4,5 million m 3 n/h compared to the design basis of 3,3 million m 3 n/h. This increase in raw gas production was made possible through step-by-step debottlenecking of the gas production units, the down-stream and utility units as well as an increase in the understanding of the gasification process. Further increases in gas production are envisaged, and these increases need to be accompanied not only with the necessary equipment debottlenecking but also with further optimization of the gasifier s performance. To enable the Sasol design engineers and production teams to further optimize the gasification process and to identify to what extent certain key factors play a role in the processes, Sasol decided to have one of the SSF West production gasifiers allocated as a test gasifier. A R45 million (± US$ 7 million) project was launched in 1998 to establish coal preparation, coal feed and gasifier test facilities, including additional instrumentation, sampling and analysis equipment. The opportunity now exists to evaluate different gasifier feed stocks under controlled conditions, in a well-instrumented gasifier coupled to a sampling system to allow mass and energy balances. Process data, lab analyses and process observations are used to evaluate the performance and products of the gasifier and to link this performance to the characteristic parameters of the feed material used.

2 2 The test gasifier team has to date completed some 36 tests, and obtained more than 4000 hours of gasifier performance data under well-defined feed and operating conditions. This project confirms the commitment in Sasol to determine the contributing factors to improving the gas production business units. TEST GASIFIER BACKGROUND The Sasol Synthetic Fuels complex in Secunda, South Africa, has over the years been debottlenecked to such an extent, that the current complex gas production rate is approximately 40 % higher than the original design. After the initial start-up period, four additional gasifiers were added per site. From then on, gas production per gasifier has increased in line with the total complex gas rate increase, albeit at a slightly lower rate since gasifier availability has also been increased. It is most likely, that further increases in gasifier load would require a more controlled feed stock quality. The complex has been in operation since 1979 and has consumed a vast amount of coal. The underground mining activities have moved further away from the original shafts and have entered coal areas of coal qualities different from the original design. Future mining activities will be in areas of different grade, composition and mining conditions. Coal purchases could be considered to supplement the coal mined by Sasol Coal. This coal will most probably have different characteristics, which may or may not affect the gasifier. The combination of increasing demands on the gasifiers and the potential changes and/or deterioration of the coal fed to the gasifiers make it necessary that the boundaries of gasifier performance and the influence of coal quality on those boundaries be investigated. The benefits of better understanding of the contributing factors in coal quality on the performance of the gasifiers and the unit cost of gas produced can be enormous. As an example, the test programme may identify that the cut-off point for the inherent ash content of the coal (the ash in the seam, the lenses, but not the floor, sills, dykes etc) could be increased without significantly affecting the economics and performance of the gasifiers. This could result in a significant increase in the percentage of coal that could be extracted from the Sasol reserves (mining conditions may however prevent the full utilization of that potential). Conservatively estimated, the replacement value of a million ton of coal inclusive of transportation cost, is more than R30 million. With the large volume of coal consumed by SSF Secunda annually, it is clear that the potential increase in utilization of own reserves resulting from the test gasifier programmes could eventually be worth many hundreds of million Rand in postponed or avoided replacement costs. On the shorter term, business optimization through reduction of cost per volume gas produced could be achieved through better coal quality control, tuning and tweaking the gasifier operating conditions for the coal parameters available and reduction of carbon losses as the result of better monitoring and controlling of the processes. Small-scale tests, laboratory bench or pilot scale, have been found to be not fully representative of the commercial size operation. The effect of particle size, particle degradation, caking Page 2 of 8 pages

3 3 tendencies etc. in the smaller scale gasifiers is not necessarily the same in the larger Mark 4 Lurgi Fixed Bed Dry Bottom gasifiers of SSF. In 1998, Sasol decided to convert one of the SSF West factory s 40 gasifiers into a commercial size test gasifier. The proposal included a dedicated coal preparation plant, coal feed system, sampling and analysis equipment, gas conditioning units and samplers for the product streams. All process parameters were to be measured with the most modern instruments, and a comprehensive data acquisition system was proposed to capture the massive stream of process data. The selected gasifier will be used for testing during certain periods only. Outside these test periods, the gasifier is used by SSF Gas Production as a normal production gasifier. The Sasol boards have allocated R45 million (± US$ 7 million) to this project for hardware and testing. This shows, that Sasol takes the longer-term investigation into gasifier development and coal quality impacts seriously indeed. TEST GASIFIER TEST DESIGN The coal parameters selected for the first series of tests are the top size of the particles, the bottom size and the percentage material heavier than RD 1,95. The objective is to identify the potential benefits of de-stoning on the operation and unit cost of gas produced by the gasifier, as well as the effect of different particle size ranges on efficiency, maximum gas throughput and overall performance in a commercial, large scale gasifier. A statistically acceptable test programme using these three parameters has been developed (Benkin box approach) with a full and a half-factorial design approach. The basis for the proposed tests is to vary the top and bottom sizes for three values each to simulate the effect of PSD, and to add 5 or 10 % by mass of the removed + RD 1,95 material back into the clean coal to simulate the effect of adventitious mineral matter (called stone inhouse by the test team for convenience). Included are tests with the clean coal (float at RD 1,95) too, without any addition of heavier material. The combinations of sizes and stone content are all in the Benkin box, hence statistical interpretation of the data obtained should render correlations with high enough confidence intervals to predict the behaviour of the gasifier regarding these three parameters, for the values in-between the ones tested. An initial series of 22 tests was scheduled to take place within approximately 20 months. This included repeat tests to confirm earlier findings and to calibrate the process. Base cases on normal, non-prepared coal were identified as necessary at a later stage, to provide a comparison between prepared coal/controlled conditions and normal operation with normal SSF coal. Each test consists of ten blocks of 12 hours each. Each block is defined by a particular gas load and gas composition. Operating conditions are closely monitored by dedicated, more experienced operating personnel in close co-operation with the engineering team. Performance deviations as result of the test conditions are included in the evaluations. Those resulting from mechanical or instrument failures are not. Page 3 of 8 pages

4 4 PLANT AND EQUIPMENT Significant time has been spent by the test team to convert the test designs into practical, executable programmes of logical steps to cover the approximately 120 hours that each test period would last. The test team of Sasol Technology engineers, scientists, Sasol Coal miners, SSF production personnel and SSF laboratory staff understood from the beginning that for the tests to be successful, the utmost attention needs to be given to establishing constant coal feed characteristics. The characteristics should not only be constant during the particular test periods within one test, but also between the different tests. The first period of testing was initially envisaged to be as long as two years! To ensure constant coal characteristics, the 5 mines of Sasol Coal Secunda were evaluated. The characteristics evaluated included inherent ash, caking tendencies and petrographic composition. It was found that Middelbult mine had sections which would be more or less constant over a two year period, and could be spared from normal operation enough to last the full test period. Blocks of up to ton coal were identified for the individual tests. During routine interruptions in the normal mine production process, this block of coal was produced, homogenized and stacked on a dedicated stockpile in the Sasol Coal Supply stockyard. The coal was then trucked over to the Pilot Plant at the Twistdraai Mine. This Pilot Plant was the original test plant upon which the design of Sasol s Twistdraai mine export coal preparation plant was based. Since the facilities are still available, with suitable equipment of sufficient capacity, it was decided to modify the Pilot Plant to suit the project s need and utilize it to prepare coal for the tests. ( Picture 1). The ROM coal is stacked as homogeneously as is possible with the equipment available and screened to remove the undersize fraction (size determined by the particular test). Size reduction of the larger coal to suit the top size requirements of particular tests can also be done. A dense medium wash section, based on cyclones, removes the plus RD 1,95 material from the feed. Discard material (RD 1,95 sink) is stacked separately for future back-blending, product is stacked on a half-moon stacker for later reclaiming. Depending on the test, the stone content of the final product is adjusted by back-blending RD 1,95 sinks material on a mass percentage basis into the washed coal. This is done in-line, on the product conveyor and controlled by the ratio of the on-line mass measurement readings of stone and clean material. The final product per test amounts to approximately 8000 ton. (Pictures 2 and 3). The final feed material is transported by trucks into the SSF West factory. Close to 270 truckloads of 30 ton each are required to transport the prepared coal from the preparation area to the plant. Special logistics arrangements are made to make the transport into the factory as quick and smooth as possible to ensure uninterrupted feed to the gasifier. The gasifier on SSF West has been equipped with its own feeding system. The coal arrives by truck from the preparation plant and is fed into a Buffalo feeding system. From there, the coal is transported by conveyor into a surge hopper. The hopper is above a double deck wet screen. This screen is used to remove the fine material (below test-cutpoint) that might have been generated during the handling and transport. The overflow of the screen is fed onto a high angle (90 degrees) sandwich conveyor, routing the coal to the top of the Gasification structure into the bunker of the test gasifier. (Pictures 4 and 5). Page 4 of 8 pages

5 5 A coal sampling system is installed on this trajectory. Representative samples are taken every 30 minutes for analysis of size, physical and chemical properties. The gasifier itself has been completely re-instrumented with smart transmitters for temperature, pressure and flow measurement. Some of the more difficult-to-measure systems required extra temperature and pressure compensation to ensure proper measurement. Coriolis flow measurement is used for the dusty gas liquor flow from the gasifier to the gas liquor treatment plant. On-line analyzers for hydrogen, carbon monoxide, carbon dioxide, methane and hydrogen sulfide are used to monitor the gas quality during the tests. All the data from the instruments is captured by a Moore SCADA / data logging system with Wonderware and IN-Sequel data management software, and stored on optical drives and CD- ROM for later evaluation. The Lurgi Fixed Bed Dry Bottom gasifier receives coal via a coal lock on the top of the gasifier. High pressure steam and oxygen are introduced at the bottom of the gasifier. Coal and the agent (steam/oxygen mixture) move in counter flow through the gasifier. The reactions, which take place at approximately 1300 C, convert the coal and agent into carbon dioxide, carbon monoxide, hydrogen and methane as the main products. Unreacted steam leaves the gasifier with the raw gas and condenses in subsequent scrubbing and cooling steps. Ash from the coal is extracted at the bottom of the gasifier through an ashlock. To enable the test team to perform mass and energy balances, samples are taken from the coal fed into the gasifier, the gas and gas liquor leaving the gasifier and the ash leaving the gasifier ash lock at the bottom. A wide range of analyses is performed on the feed coal : particle size distribution, petrography, mineralogy, proximate an ultimate analysis, ash composition and melting temperatures, float and sink tests, caking tendencies, thermal stability, CV etc.. All in all, more than 7 ton of coal samples are taken per test at the gasifier feed system alone to characterize the coal. Additional samples are taken throughout the coal preparation system to ensure homogeneity and adherence to coal quality specs for the specific test during the preparation process. For balance purposes, all the product streams are analyzed for carbon content, and where needed for hydrogen and oxygen containing components too. Product-specific analyses are done to identify possible changes in product spectrum and/or composition due to the conditions in the gasifier. Data evaluation focuses on the carbon balances, the carbon-split to the various products, the consumption of utilities to produce the products and physical well-being of the gasifier : temperature and pressure profiles, pressure drop, stability. As can be imagined, each test produces an enormous amount of data. Although each test can be seen as standing alone, the full spectrum of tests is required to develop correlations and models between the test parameters. The amount of data to be handled grows exponentially with the number of tests to be included in the evaluations. Statisticians from Sasol Technology convert the mass of data into more understandable forms, for evaluation by the test team against logic, gasifier understanding and experience. Page 5 of 8 pages

6 6 OBSERVATIONS FROM TESTS To date, the team has performed some 36 tests on the test gasifier, based on Middelbult coal with different top and bottom size material, with and without certain percentages adventitious mineral matter. A first set of commercial tests has also been done. These tests were to evaluate a feedstock source from an alternative coal supplier, as part of a programme to evaluate the technical and economical suitability of this coal in the Sasol Secunda system. Several base cases on normal (standard SSF Wet Screening coal) have been performed to benchmark against the prepared coal runs. Interesting observations have been made with respect to the effect of blending and homogenization of the coal on the performance of the gasifier and the improved tolerance towards stone. The effect of material in the feed smaller than the prescribed test minimum size was also noted, and more detailed quantification of the effects is scheduled in further tests. Opportunities have been identified to change to the feedstocks to enable improved operation of the downstream gas treatment units and increase the carbon-to-useful gas ratios of the gasification process. These changes include the separation of coal particle size ranges for separate gasifier clusters, and different degrees of de-stoning/coal preparation on selected size ranges of the factory feed coal. The test gasifier has been able to operate stable and delivering product on-specification at gas production loads of more than 160 % of original design load, indicating the potential for even further factory load increases. As the database increases and models improve, the team will get into a better position to predict the behaviour of the gasifier for a particular type and grade of coal. To this extent, the team has proposed further tests with variations in other coal parameters like inherent ash content, caking tendencies, petrography, and tests to evaluate the effect of homogenization in more detail. Since the tests have been performed on Middelbult coal mainly, and the SSF complex receives coal from all five mines in Secunda, the team has proposed to test different blends of coal from the different areas, to identify whether the behaviour of the coal is influenced by the origin of the coal. Also, an indication will then be obtained whether the behaviour of a blend of coals is equivalent to the sum of behaviours of the individual components, or whether there is a degree of synergy. The consumption of utilities, volumes of product made and the composition of the products are currently being processed in plant performance and operation-economics models for the identification of operation sweet spots and economic optimization opportunities. A spin-off from the test work is the identification of process control optimization opportunities resulting from the high level of attention and minute-to-minute monitoring of the gasifier operation. This close attention by a number of experienced persons has already lead to the identification of improvements to be made to the ash extraction control programme. During the tests, there is also an increased level of knowledge sharing and skills improvement between the test team and the operating staff. As an education tool the test gasifier is ideal, since the gasifier behaviour observed is now that due to the operating conditions imposed, and not rather due to the variations in the quality of the coal fed to the gasifier (for a particular test). Page 6 of 8 pages

7 7 FUTURE ACTIVITIES The test gasifier and accompanying equipment has put Sasol in a position where other feedstocks can also be tested in the gasifier. After completion of the test programmes for the Sasol Secunda coal field, requests will be handled from the colleagues in Sasolburg. The system also lends itself for the testing of various process discard materials and other carbon containing feedstocks. The investigations into these areas can open a complete new perspective on gasification and the applicability thereof in Sasol, and even outside South Africa. Sasol has identified various modifications to the coal feed system, the ash extraction system and other mechanical design changes. Some of these changes have been incorporated in the tests performed until now, and others will be subjected to stand-alone testing later during the programme. Combinations of the various identified changes will also be tested, to evaluate the possible synergistic effects between the modifications. The team envisages a few more years of test work, as the potential of the test gasifier to evaluate parameters on a commercial scale becomes more appreciated. We anticipate that there will be requests in the near future to test feedstocks other than coal in the test gasifier. Sasol Technology, Sasol Coal and SSF have been spending significant numbers of man-hours, resources and effort into this project. The commitments shown by the various teams in the Divisions indicate the enthusiasm and drive that exists to better understand the technologies used and business Sasol is in. The main driver behind the project is the wish to improve the unit cost of gas produced even further, and to maximize the utilization of coal reserves. The identification of the process control handles and control mechanisms for these handles will allow Sasol to use the maximum of the gasifier s flexibility with respect to feedstocks, operating conditions and utility consumption. The improved knowledge will allow more stable operation of the gasifiers, nearer to the limits, and eventually their performance can be predicted from the characteristics of the coal to be used for optimal economic performance. The test team looks forward to continue the test work and to further explore the economic potential of the Lurgi Fixed Bed Dry Bottom gasifiers. Page 7 of 8 pages

8 8 Picture 1 : Feed system of test gasifier coal preparation plant Picture 2 : Discard and product coal Picture 3 : Product coal blend and stacker Picture 4 : Gasifier feed system Picture 5 : Hopper and final screen Page 8 of 8 pages