SUITABILITY OF FEEDSTOCKS FOR THE SASOL-LURGI FIXED BED DRY BOTTOM GASIFICATION PROCESS

Transcription

1 SUITABILITY OF FEEDSTOCKS FOR THE SASOL-LURGI FIXED BED DRY BOTTOM GASIFICATION PROCESS Gasification Technologies 2001, San Francisco, California, October 7-10, 2001 JC van Dyk, MJ Keyser, JW van Zyl Sasol Technology, R&D Division, Deartment of Carbon to Syngas Research, P. O. Box 1, Sasolburg, 1947, South Africa Abstract Coal is a crucial feedstock for South Africa s unique synfuels and etrochemicals industry and it is used by Sasol as a feedstock to roduce synthesis gas (CO and H 2 ) via the Sasol-Lurgi fixed bed dry bottom gasification rocess. A detailed knowledge of coal characteristics is essential to redict gasification behavior when a secific coal source is to be gasified. The rincile aim of this aer is to highlight and discuss those secific coal characteristics, which affect gasification behavior and stability. In order to determine the suitability of a coal source for gasification uroses, the coal is characterized and the results comared with historical data. Benchmark data, obtained when the gasifiers are oerating without roblems and with relatively high stability, is used as a reference. Coal from sources with extreme roerties (e.g. ash content <10% to as high as 35% or brown coal with moisture content of aroximately 30%) can be gasified in a Sasol-Lurgi fixed bed dry bottom gasifier rovided that certain oerational changes are imlemented. Other roerties, like high caking roensity for examle, require blending to accetable levels and /or mechanical modifications. The coal characteristics discussed in this aer are not the only roerties affecting gasifier stability, but are those roerties which are easily measurable on laboratory scale and can be related to gasifier erformance. Interretation of coal characterization data gives an indication of exected gasifier erformance, and the suitability of a secific coal source for Sasol/Lurgi Fixed Bed Gasification. Data on a number of coal sources will be discussed. Keywords: Sasol-Lurgi fixed bed dry bottom gasification, coal characteristics, South African coal sources, other feedstocks. 1. INTRODUCTION Coal is used as main feedstock for South Africa s unique synfuels and etrochemicals industry and it is used by Sasol as a feedstock to roduce synthesis gas (CO + H 2 ) via the Sasol-Lurgi fixed bed gasification rocess. South Africa, as well as many other countries in the world, will for many years to come rely on its abundant coal resources for energy and etrochemical roducts. The Sasol lants located in Secunda and Sasolburg (South Africa) gasify aroximately 30 million tons of bituminous coal to synthesis gas, which is converted to fuels and chemicals via the Fischer-Trosch rocess. A total of 97 fixed bed dry bottom gasifiers, 17 at Sasolburg and 80 at Secunda, have a combined roduction caacity of aroximately 5.1 x 10 6 m 3 n/h dry crude gas, which is equivalent to aroximately 3.6 x 10 6 m 3 n/h ure synthesis gas. These roduction rates are well in excess of the design caacity and were achieved by continuous debottlenecking and otimization. Corresonding author. Tel.: ; fax.: ; johan.vandyk@sasol.com

2 The coal from the sources used by Sasol vary substantially in terms of chemical and hysical roerties and this directly relates to gasifier behaviour. The ability of fixed bed gasifiers to handle a variety of different feedstocks is seen as a significant advantage over other gasification technologies. At the Schwarze Pume site in the former East Germany, 7 fixed bed dry bottom Lurgi gasifiers are utilized for treatment of solid wastes, such as lastics, sewage sludge, rubber, contaminated wood, aint residues and household wastes [10]. Other distinct characteristics of fixed bed gasifiers are the following: It uses lum coal and limited grinding is required. Coal used for fixed bed gasification is mined, crushed down to <70mm and screened at a bottom size of 5-8mm. Coal with a high ash content can be gasified without severe losses in thermal efficiency, since the ash is not extracted in the molten state. High cold gas thermal efficiency is achieved through counter-current oeration, which allows the gas and solid roduct streams to exit at relatively low temeratures. Low oxidant requirements due to the high thermal efficiency. Valuable co-roducts like tars, itches, oils and chemicals are roduced. A H 2 /CO ratio of 1.7 to 2.0 is roduced directly which is suitable for Fischer-Trosch synthesis without the need for additional water-gas shift conversion to adjust the H 2 /CO ratio. The Sasol-Lurgi gasifiers has some limitations, e.g.: Limited ability to handle excessive fine coal or coal with a high caking roensity. Broad article size distributions can lead to excessive coal segregation, which in turn may cause channel burning and unstable gasifier oeration. Pressure dro can limit gas throughut in certain instances. Relatively high steam consumtion. The rincile aim of this aer is to highlight and discuss those secific coal or feedstock characteristics, which affect gasification behavior and stability. Due to the large variation in coal roerties from various sources, detail coal and feedstock characteristics are essential to redict gasification erformance when a secific coal source is to be gasified. 2. COAL MINING AT SASOL The coal used by Sasol for Sasol-Lurgi fixed bed dry bottom gasification in South African has a low rank, is inertinite rich, and has roerties which may vary significantly from one mine to the next. Sasol Mining (Pty) Ltd. is resonsible for coal mining in the Sasolburg and Secunda regions and sulies coal to Sasol s synthetic fuels and chemical lants. The division oerates regional oerations comrising the Sigma Colliery and Wonderwater stri mining oerations at Sasolburg and the Secunda Collieries, which consist of six underground oerations and a stri mine near Trichardt. The combined run-of-mine outut from the Sasolburg and Secunda oerations increased from a total roduction of ±20 million tons in the ten year eriod , to almost 51 million tons 2

3 er annum in Sulies to the Sasol Chemical Industries (Sasol 1) lant in Sasolburg are sulemented with external urchases of 0.9 million tons in During the 2000 financial year, the comany sulied 46.7 million tons of saleable coal from the 50.9 million tons of coal extracted to the oerations of Sasol Synthetic Fuels (SSF) at Secunda and the Sasol Chemical Industries (SCI) at Sasolburg. The comany commenced its coal exort oerations at Secunda during the 1997 financial year and roduced 3.2 million tons of exort quality coal, which was exorted mainly to Euroe, during the 2000 financial year [8]. TABLE 1 SASOL MINING (Pty) Ltd. PRODUCTION HIGHLIGHTS [8] Production (millions of tons) Total roduction Sigma Colliery including Wonderwater Secunda Collieries: Bosjessruit Colliery Brandsruit Colliery Middelbult Colliery Twistdraai Colliery Twistdraai Exort Colliery Syferfontein (underground and stri) Colliery Saleable roduction from all mines External coal urchases from other mines Total sales excluding exorts Sales to SCI (Sasol 1), Sasolburg Sales to SSF, Secunda International sales 3. COAL CHARACTERISTICS AND THE EFFECT ON GASIFIER PERFORMANCE New coal sources and areas under exloration for utilization in Sasol-Lurgi fixed bed dry bottom gasification are characterized in detail and the results comared with historical data in order to determine the suitability of a coal source for gasification uroses. Benchmark data, obtained when it was ossible to oerate the gasifier without roblems and with relatively high stability, is used as a reference. The following tests are conducted on coal sources to determine suitability for gasification uroses: Proximate analysis Ultimate analysis CO 2 gasification reactivity Particle size distribution Ash melting roerties and ash comosition Caking roerties under 26 bar ressure Thermal fragmentation (atmosheric ressure) Mechanical fragmentation Fischer Assay 50,9 5,1 7,4 8,7 9,0 5,6 6,0 9,1 49,4 0,9 49,9 6,2 40,5 3,2 49,0 5,5 6,5 8,6 8,8 5,9 5,1 8,6 47,0 0,7 49,0 6,5 39,4 3,1 3

4 Total sulhur Heating value Maceral analysis and rank Data obtained on a number of coal will be discussed. 3.1 Proximate analysis Ash content gives an indication of the amount of inorganic material in the coal from a source and includes mineral matter inherent in the coal structure, as well as out-of-seam inorganic contamination. The measurement of ash content is used by Sasol Mining as an oerating tool to reare blends with an ash content within the agreed limits and with a relatively small variation in ash content over time. The budgeted ash content of the coal blend requested by gasification at SSF in Secunda is ±28 % (air dry basis). Gasification at the Sasolburg lant oerates on a coal blend with an ash content exceeding 32 % during certain eriods. % Ash (average - air dried basis) FIGURE 1 VARIATION IN ASH CONTENT (air dried basis) Secunda Sasolburg Non South- African Biological sludge A B C D E F I II a b c d e 3.2 CO 2 reactivity CO 2 reactivity is determined in order to get an indication of the exected rate of the gasification reactions. Reactivity is exressed as a mass loss er time at 50% burn-off under a CO 2 atmoshere. The reactivity of coal from the various sources used by Sasol vary between 2-5 hr -1, although coal sources with lower and higher reactivities have been gasified in the ast. Some of the non South-African coal sources tested for gasification uroses have showed reactivities of as low as 0.5 hr -1 and also as high as 9 hr -1. However, it is uncertain what the lower limit for reactivity is below which the gasification reactions will become too slow for comlete conversion. 4

5 3.3 Particle size distribution Particle size distributions of coal blends are determined in order to estimate or redict which size distributions are more likely to cause unstable oeration due to ressure dro effects. Pressure dro roblems manifest themselves in a variety of ways, and include grate traction loss (due to bed fluidization), channel burning (leading to unaccetable gas outlet temeratures) and solids elutriation (carry-over). Probably the best known estimation method for ressure dro is the Ergun equation, which gives ressure dro as a function of bed voidage ε, viscosity µ, fluid density ρ, suerficial velocity U s and article diameter d [2]: P L = ( 1 ε ) µ U ( 1 ε ) 3 ε d 2 s U 3 ε d 2 s (1) When dealing with article size distributions instead of uniformly sized articles, the article size d has to be relaced by φ d, where φ is the article shericity and d the average article size reflecting the mean surface area (also referred to as the Sauter mean diameter). The Sauter diameter of a coal samle with a secific article size distribution is calculated as follows: 1 d = (2) xi i d, i where i = screen number x i = fraction (mass %) on screen i d,i = diameter (mm) of screen i Exerience has shown that d is a useful arameter for redicting which PSD s are more likely to result in gasifier instability. Extensive research on the effect of coal tyes and PSD on ressure dro and how the data fit the Ergun equation have been conducted, but will not be reorted here. The value of d is extremely sensitive to the smaller article sizes, or the so-called tail of the PSD. As illustrated in Table 2, a 10% change to the coarser side resulted in a change of only 3% in the Sauter diameter, while a 10% change in article size to the finer fraction resulted in a 7% change. Extensive oerating exerience has shown that size distributions with a Sauter diameter below a secific value can result in unstable gasifier oeration. Inefficient screening due to screen overload causes mislacement of fine coal, which can easily reduce the Sauter diameter to unaccetably low values resulting in highly unstable gasifier oeration. 5

6 TABLE 2 EFFECT OF CHANGE IN PARTICLE SIZE ON SAUTER DIAMETER Fraction (mm) Standard comosition Coarser fraction Finer fraction % Change in Sauter diameter Thermal fragmentation It is known that when lum coal from certain origins (e.g. South-African low rank inertinite rich coals) is exosed to high temeratures (700 o C), it will tend to undergo fragmentation (rimary and secondary fragmentation)[1]. Primary fragmentation occurs during devolatilization, while secondary fragmentation occurs during combustion of the char by burnout of carbon bridges connecting arts of the article. In the case of fixed bed gasification, fine material formed in the gasifier may lead to the same kind of hydrodynamic roblems as was described reviously. Thermal fragmentation of coal is measured by lacing a samle with a secific redetermined size distribution into a re-heated muffle oven at 100 o C under atmosheric ressure [9]. The coal is then heated to 700 o C (final temerature) at a rate of ±12 o C/min. The exeriments are conducted under nitrogen with a reaction time of 60 minutes at the final temerature. After the samle is cooled under nitrogen and screened again, the change in size distribution is calculated. The ercentage thermal fragmentation of coal is given as a ercentage decrease in Sauter diameter. The smaller the ercentage decrease, the better the thermal stability. Thermal fragmentation is defined as: d before test after test % Thermal fragmentation = x100 (3) before test d As illustrated in Figure 2, weathering / oxidation and moisture content affect the thermal fragmentation of coal sources. An extensive study revealed that the effect of moisture contributes to ±75% of the thermal fragmentation of coal [1]. This is not only surface moisture, but a combination of surface moisture and inherent moisture catured within the ores and the coal structure. Although moisture contributes significantly towards fragmentation, it is also affected by a comlex interaction with other factors. d 6

7 FIGURE 2 THERMAL FRAGMENTATION OF COAL SOURCES (effect of moisture and weathering) % Thermal fragmentation Secunda Sasolburg % Thermal fragmentation (dry) % Thermal fragmentation (w et) 0 A B C D E F1 F2 F3 I & II 3.5 Caking Caking of coal articles can be described as the softening or lasticity roerty of coal, which cause articles to melt or sinter together to form larger articles when heated. Caking of coal within the gasifier can cause ressure dro fluctuations and channel burning, resulting in unstable gasifier oeration. In severe cases oxygen break-through can occur, which can be a safety hazard due to the ossibility of downstream exlosions. The caking roensity of coal is determined by yrolizing a coal samle with a secific redetermined size distribution in an argon atmoshere at the tyical gasifier ressure, i.e. ±26 bar. The samle is screened afterwards and the increase (if any) in article size determined. This test is unique and was develoed in-house by Sasol for characterizing coal under conditions similar to those revail within the gasifier. Pressure significantly influences the caking roensity of coal. Coal with a medium to low caking roensity shows no caking at atmosheric conditions, and a highly caking coal will have a much lower caking roensity at atmosheric conditions than at 26 bar. Atmoshere (i.e. nitrogen, CO 2, etc.) does not have a significant effect on the caking roensity of coal. According to revious exerience, coal from sources with a relatively high caking roensity resulted in unstable gasifier oeration and are therefore mixed with other coal sources having a low caking roensity to obtain an accetable blend. Normal blends used for gasification at Secunda have a caking roerty of ±20 % and coal blends used in the Sasolburg lant have no or a very little caking. The variation in caking roerties between the different coal sources used for gasification in Secunda and Sasolburg, as well as non-south African coal sources tested, are given in Figure 4. 7

8 FIGURE 3 CAKING PROPERTIES OF SASOL S COAL COURCES 100 Secunda Sasolburg Non South- African High risk % Caking A B C D E F1 F2 F3 I & II a b d Uncertain area Safe oerating region 3.6 Ash fusion temeratures and ash comosition The ash fusion temerature (AFT) of a coal source gives an indication to what extent ash agglomeration and ash clinkering is likely to occur within the gasifier. Ash clinkering inside the gasifier can cause channel burning, ressure dro roblems and unstable gasifier oeration. The results of an AFT analysis consist of four temeratures, namely the initial deformation temerature, softening temerature, hemisherical temerature and flow temerature. Ideal gasifier oeration is to oerate at a temerature above the initial deformation temerature in order to obtain enough agglomeration to imrove bed ermeability, but to oerate below the ash melting temerature to revent excessive clinkering. Ideal coal sources will thus have a big difference between the initial deformation temerature and the melting temerature. Secunda and Sasolburg coal sources currently used for gasification have an ash melting temerature > 1350 o C and an initial deformation temerature of >1300 o C. Non South-African coal sources tested (China, India and Ethioia) have ash fusion temeratures similar to the Sasol coal sources. This does not imly that coal sources with lower ash fusion temeratures are not suitable for the Sasol-Lurgi gasification rocess, rovided that gasifier oeration is adoted accordingly. The ash comosition, secifically the Ca and Fe content in the coal, gives a fair indication of the exected ash fusion behaviour. A Ca and/or Fe rich coal source normally has a low ash fusion temerature due to the fluxing roerties of the Ca and Fe minerals. Although the standard AFT analysis is currently used as the only rediction tool for ash fusion temeratures of coal, literature studies have showed that this may not reresent the actual flow temerature of certain minerals and mineral hases [6]. Fe, for examle, in a secific hase can slag at temeratures as low as 700 o C and then solidify again. This is not reflected by a standard AFT analysis. Sasol Technology, R&D Division, is currently investigating this issue. Visual investigation of actual ash roduced from a fixed bed gasifier showed that the coarse and fine ash is sometimes extracted from the gasifier with the imortant middle fraction being absent. This can ossibly be exlained by the fact that some minerals already slag and clinker at low temeratures. 8

9 4. MAXIMUM THEORETICAL PURE GAS YIELD In order to comare the maximum theoretical ure gas yield of different coal sources, a detailed exerimental evaluation, together with thermodynamic modeling, is conducted on the coal to simulate the gasification rocess. The theoretical ure gas yield redictions are subject to certain assumtions: The thermodynamic model calculates the theoretical ure gas yield of the remaining fixed C after yrolysis. Exerimental data for yrolysis gas roduction and comosition is added afterwards to obtain the maximum theoretical ure gas yield. Inuts in model: Coal comosition from roximate analysis. Pyrolysis roduct yields. Comosition of yrolysis gas. Assumtions made in model: Percentage unconverted C reorting in the ash is 3% of total amount of C in the coal feed, but can be adjusted according to exerimental data. CH 4 and water-gas shift reactions are at chemical equilibrium. CH 4 formation and water-gas shift aroach to equilibrium can be adjusted to match actual CH 4 roduction figures and H 2 /CO ratios. No kinetic, hydrodynamic or article segregation effects are taken into account. This means that the model is not affected by load, article size, coal reactivity, diffusion effects, heat and mass transfers. All of these factors may have an effect on the actual carbon conversion and yield redictions. Examles of theoretical PG yield redictions for a few South African coal sources and one non South-African source are shown in Figure 4. Exact agreement between redictions and lant data are normally not obtained, but exerience showed that the redicted trends are valid (e.g. if blend comosition changes and an increase in yield is redicted, then an increase is measured on the lant). 9

10 FIGURE 4 COMPARISON OF MAXIMUM THEORETICAL PURE GAS YIELD BETWEEN COAL SOURCES 1700 Pure Gas Yield (Nm 3 /t DAF) I & II X I & II lus 15% X Y Non South African 5. CONCLUSIONS The coal characteristics discussed in this aer are not the only roerties affecting gasifier erformance and stability, but are those roerties that are measurable on laboratory scale and are easily related to gasifier erformance. Interretation of these results gives an indication of exected gasifier erformance, and also the suitability of a secific coal source for Sasol-Lurgi fixed bed gasification. Interretation of standard coal analyses and uniquely develoed laboratory tests, together with revious exerience gained by Sasol over the ast 50 years, has ut Sasol in a osition to identify suitable coal sources for a Sasol-Lurgi fixed bed dry bottom gasification rocess. Sasol is furthermore in the unique osition not only to have the ability to characterize and test coal from various sources on laboratory scale, but also to test new and current coal sources on an isolated commercial scale Sasol-Lurgi MK IV test gasifier at the Secunda site. These full scale tests require aroximately 4000 tons of test coal, which allows a 6 day test run on a MK IV Sasol-Lurgi fixed bed dry bottom gasifier at a coal feed rate of ± 50 tons/h. Full scale test results are suortive to laboratory scale coal characterization data, since ast exerience showed that gasifier erformance is usually not determined by one or two coal characteristics, but is deendent on the combined effect of all roerties due to the large degree of interaction between them. This facility is used and will assist in future to further otimize roduct yields and to increase throughut by means of a thorough understanding of those coal characteristics that influence gasifier erformance [7]. It is strongly recommended that full scale test work be conducted to confirm the design basis for new lants with unfamiliar coal sources. Sasol has exerience and a roven track record on commercial scale test runs in Sasol Two and Sasol Three with South-African coal sources as well as with American coal sources, (e.g. DGC, EL Paso, CarteOil, Tristate, Phillis-Petroleum). 10

11 REFERENCES 1. Van Dyk, J.C., Thermal friability of coal sources used by Sasol Chemical Industries (SCI) for gasification Quantification and statistical evaluation, M.Sc. Thesis, University of the Witwatersrand, Johannesburg, Kunii, D. and Levensiel, O., Fluidization Engineering, Robert E. Krieger Publishing Comany, Huntington, New York, Keyser, M.J., Van Dyk, J.C. Full Scale Sasol/Lurgi Fixed Bed Test Gasifier Project: Exerimental Design and Test Results, Paer resented at the 17 th Annual International Pittsburgh Coal Conference, Pittsburgh, USA, Setember Adding value to life, Sasol Annual reort Stubington, J.F., Linjewile, T.M., The effects of fragmentation on devolatilization of large coal articles, Fuel, Vol. 68, 1989, Alern, B., Nahuys, J., Martinez, L., Mineral matter in ashy and non-washable coals Its influence on chemical roerties, Comun. Serv. Geol. Portugal, 1984, t. 70, fasc. 2, Keyser, M.J., Findings on the statistical evaluation of tests 14 to 23 on the test gasifier with a standard grate and standard grate/uniflo grate comarison, 28 January Sasol Annual Reort Van Dyk, J.C., Develoment of an alternative laboratory method to determine thermal fragmentation of coal sources during yrolysis in the gasification rocess, Fuel 80 (2001), Hirschfelder, H., Buttker, B., Steiner, G., Concet and realisation of the Schwarze Pume, FRG Waste to Energy and chemicals centre, IChemE Conference Gasification in Practice, Assolombarda, Milan, Italy, February ACKNOWLEDGEMENTS The author gratefully acknowledges suort and inuts received from the co-authors MJ Keyser, JW van Zyl as well as P van Niero and also wishes to thank B Ashton and S du Plessis for their work contribution to the successful comletion of this aer. 11