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Preventing Ash Agglomeration during Gasification of High-Sodium Lignite Robert S. Dahlin,*,† Johnny R. Dorminey,‡ WanWang Peng,‡ Roxann F. Leonard,‡ and Pannalal Vimalchand‡ Southern Research Institute and Southern Company SerVices, Power Systems DeVelopment Facility, PostOffice Box 1069, WilsonVille, Alabama 35186 ReceiVed July 17, 2008. ReVised Manuscript ReceiVed NoVember 5, 2008
Various additives were evaluated to assess their ability to prevent ash agglomeration during the gasification of high-sodium lignite. Additives that showed promise in simple muffle furnace tests included meta-kaolin, vermiculite, two types of silica fume, and one type of bauxite. Additives that were tested and rejected included dolomite, calcite, sand flour, kaolinite, fine kaolin, and calcined bauxite. Based on the muffle furnace test results, the meta-kaolin was selected for a follow-on demonstration in a pilot-scale coal gasifier. Pilot-scale testing showed that the addition of coarse (minus 14-mesh, 920-µm mean size) meta-kaolin at a feed rate roughly equivalent to the ash content of the lignite (∼10 wt %) successfully prevented agglomeration and deposition problems during gasification of high-sodium lignite at a maximum operating temperature of 927 °C (1700 °F).
1. Background This paper discusses gasification tests and laboratory studies conducted at the Power Systems Development Facility (PSDF) in order to understand the cause of ash agglomeration problems and develop methods of preventing ash agglomeration during the gasification of high-sodium North Dakota lignite. Significant strides have been made in dealing with this type of ash agglomeration in fluidized-bed gasifiers. Bartels et al.1 recently published a review of the potential agglomeration mechanisms and methods of detecting and preventing agglomeration in fluidized beds. There have been numerous attempts to deal with agglomeration problems in coal gasifiers by the use of various additives. In particular, van Dyk and Waanders2 recently reported that the addition of alumina successfully increased the ash fusion temperature and prevented agglomeration in the fixedbed gasifier at Sasol. In a study that was more relevant to fluidized-bed gasifiers, Vamvuka et al.3 reported that the alkalirelated agglomeration problems occurring with biomass gasification could be successfully addressed by water leaching the alkali from the fuel or by the use of additives. Previous gasification studies with high-sodium lignites have shown that sodium vapor released from high-sodium lignites can react with silica to form sticky sodium silicates.4-6 These sticky sodium silicate compounds tend to glue particles together, * Corresponding author. † Southern Research Institute. ‡ Southern Company Services. (1) Bartels, M.; Lin, W.; Nijenhuis, J.; Kapteijn, F.; van Ommen, J. R. Prog. Energy Combust. Sci. 2008, 34 (5), 633–666. (2) van Dyk, J. C.; Waanders, F. B. Fuel 2007, 86 (17-18), 2728– 2735. (3) Vamvuka, D.; Zografos, D.; Alevizos, G. Bioresour. Technol. 2008, 99 (9), 3534–3544. (4) Bhattacharya, S. P.; Hartigg, M. Energy Fuels 2003, 17 (4), 1014– 1021. (5) Vuthaluru, H. B.; Linjewile, T. M.; Manzoori, A. R.; Zhang, D. K. Fuel 1999, 78, 419–425. (6) Manzoori, A. R.; Agrawal, P. K. Fuel 1993, 72, 1069–1076.
Figure 1. Locations of deposits in the gasifier system.
leading to formation of agglomerated deposits in the gasifier. Laboratory studies reported in a previous paper suggested that the formation of sticky sodium silicates was responsible for the agglomeration and deposition problems encountered during the use of high-sodium lignite in a previous test campaign, TC13, at the PSDF.7 Locations where deposits have been found in the PSDF gasification system are illustrated in Figure 1. The syngas temperature at these locations is typically in the range of 800-1000 °C (or 1450-1850 °F). The gasification system is a Kellogg, Brown, and Root (KBR) transport gasifier that operates in the high-velocity, entrained(7) Dahlin, R. S.; Peng, W. W.; Nelson, M.; Vimalchand, P.; Liu, G. Energy Fuels 2006, 20 (6), 2465–2470.
10.1021/ef800568z CCC: $40.75 2009 American Chemical Society Published on Web 12/16/2008
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flow (or transport) regime. The gasifier operating temperatures are typically in the range of 800-1000 °C (1450-1850 °F), well below the operating temperatures of the slagging-type, entrained-flow gasifiers. Because of the relatively low operating temperature range of the transport gasifier, ash agglomeration problems are avoided with most coals, refractory life is significantly extended, and the need for expansion joints is eliminated. Details of the KBR transport gasifier operations have been described in previous papers.8-11 Four separate gasification test campaigns (TC13, TC16, TC21, and TC23) have been performed with the high-sodium North Dakota lignite. An analysis of the TC13 and TC16 results was published in our previous paper7 and will not be duplicated here The laboratory work described in this paper focuses on the subsequent tests (TC21 and TC23) and the analysis of those samples. Section 2 describes the nature of the agglomeration problems in TC21 and the analysis of the TC21 deposits. Section 3 describes various laboratory studies that were done to understand the cause of TC21 agglomeration and to determine the potential effectiveness of various additives that could be used to prevent or minimize agglomeration. Section 4 discusses gasifier operations in the various test campaigns with emphasis on the results of TC23, where we successfully demonstrated the additive that was identified as most promising from the laboratory studies. The conclusions drawn from the laboratory studies and gasification test campaigns are discussed in section 5. Previous laboratory studies suggested that the agglomeration occurring in TC13 could be minimized by eliminating the startup sand bed, reducing the gasifier operating temperature, and introducing an inert particulate material to reduce the contact between sticky particles.7 To verify these observations, a subsequent test campaign, TC16, was performed with the sand bed material eliminated, the gasifier operating temperature limited to about 930 °C (1700 °F), and dolomite added into the gasifier solids recycle loop. With these remedial steps in place, the high-sodium lignite was successfully gasified without any signs of agglomeration or deposition problems.7
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Figure 2. Chunks of agglomerated deposit removed from lower mixing zone of gasifier after TC21.
Figure 3. SEM photograph of TC21 deposit.
2. Agglomeration Problems in TC21 After successful gasification operations in TC16, another test campaign, TC21, was attempted with high-sodium lignite to evaluate performance after modifications were made to the gasifier. In TC21, the sodium content of the lignite was somewhat higher (about 8-9 wt % as Na2O in the ash versus about 6-7 wt % in TC16). With the higher sodium content, the problems with agglomeration and deposition recurred. Extensive deposition was found in the gasifier lower mixing zone (Figure 2). (8) Leonard, R.; Pinkston, T.; Rogers, L.; Rush, R.; Wheeldon, J. The PSDF - Commercial Readiness for Coal Power Revisited. In Proceedings of the 19th Annual Pittsburgh Coal Conference, Pittsburgh, PA, September 23-27, 2002. (9) Smith, P. V.; Vimalchand, P.; Pinkston, T.; Henningsen, G.; Longanbach, J. L. Transport Reactor Combustor and Gasifier Operations. In Proceedings of the 8th International Energy Forum (Energex 2000), Las Vegas, NV, July 23-28, 2000. (10) Vimalchand, P.; Leonard, R. F.; Pinkston, T. E. Power Systems Development Facility: Operation of a Transport Reactor System with a Westinghouse Candle Filter. Presented at the Advanced Coal-Based Power and Environmental Systems ’98 Conference, Morgantown, WV, July 2123, 1998. (11) Peng, W. W.; Nelson, M.; Liu, G.; Vimalchand, R.; Dahlin, R. S. High-Sodium Lignite Gasification with the PSDF Transport Gasifier. In Proceedings of the 22nd Annual Pittsburgh Coal Conference, Pittsburgh, PA, September 12-15, 2005.
Figure 4. EDX analysis of TC21 deposit.
Examination of the deposits by scanning electron microscopy (SEM) revealed that the particles in the deposit were bonded together (Figure 3). The predominant elements detected by energy-dispersive X-ray (EDX) analysis were Mg, Ca, Al, Si, Fe, and Na (Figure 4). The high concentrations of Mg and Ca were attributed to dolomite addition, while the other elements were expected components of lignite ash. 3. Laboratory Studies with Gasifier Deposits from the Lower Mixing Zone 3.1. Experiments with Heated Deposit Samples. To better understand the nature of deposit formation in TC21, samples of deposits taken from the lower mixing zone of the gasifier after TC21 were subjected to various laboratory tests. First, the deposit chunks shown previously in Figure 2 were pulverized
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Figure 7. Effect of dolomite addition on reconsolidation of TC21 deposit.
Figure 5. Reconsolidation of pulverized TC21 deposit at relatively low temperature.
Figure 6. Comparison of reconsolidation tendencies of TC21 and TC13 deposits.
and heated to various temperatures to determine the minimum temperature required for reconsolidation. Surprisingly, these tests showed that the deposits reconsolidated even at temperatures as low as 540 °C (1000 °F), as shown in Figure 5. Moreover, these tests showed that the TC21 deposit had a greater tendency to reconsolidate than the TC13 deposit that was studied previously (see Figure 6). The differences in behavior of the TC13 and TC21 deposits could be related to differences in the particle-size distributions and chemical compositions of the two samples. The TC13 deposits, which were taken from the loop seal of the recycle cyclone, were mostly composed of very large sand particles that were glued together by the sticky sodium silicates. The mean particle size of the TC13 deposits was in the range 250-350 µm. The TC21 deposits, which were taken from the lower mixing zone of the gasifier, contained a substantial amount of dolomite and very little sand (since the sand was replaced with ash prior to the test). Because of these differences, the mean particle sizes of the TC21 deposits were significantly smaller than those of the TC13 deposits (50-70 µm for TC21 versus 250-350 µm for TC13). This difference in particle size could be related to gasifier solids recycle loop modifications that were made between TC19 and TC20, which enabled the recycle of finer material to increase carbon conversion. The difference in particle size may partly explain why the TC21 deposits tended to agglomerate at a lower temperature than did the TC13 deposits. In addition to the differences in particle size, differ-
ences in surface characteristics of the ash may also be contributing to the observed difference in agglomeration tendencies. 3.2. Experiments with Various Additives to Prevent Agglomeration. Given the relatively low temperatures at which reconsolidation was observed, it was clear that reduction of the gasifier operating temperature alone would not be a viable means of preventing the agglomeration seen in TC21. Moreover, because of the relatively large reconsolidation tendency of the TC21 deposit, it was clear that any additives used to prevent agglomeration would need to be more effective than the dolomite used in TC16. With these factors in mind, a series of experiments was conducted to evaluate various additives that might help minimize agglomeration and deposition problems. The experiments were done by mixing the selected additive with a sample of the pulverized deposit, placing the mixture in a crucible, and heating the mixture in a muffle furnace. Tests were conducted with various weight percentages of additive and at various temperatures as described below. In some cases, the additive was used as a top layer over the deposit material to examine the effect of sodium vapor released from the underlying deposit. These procedures were described in more detail in our prevous paper.7 The additives tested included amorphous silica fume, bauxite, vermiculite, kaolinite, waste-derived kaolinite, and meta-kaolin. The dolomite that was used in TC16 (and later in TC21) was also tested for comparison even though it did not prevent agglomeration in TC21. During these laboratory tests, the amount of sodium actually captured by the additive was not quantified but was examined qualitatively by energy-dispersive X-ray (EDX) analysis of the products. When the most promising additive was selected and tested in the gasifier, a sodium mass balance was used to obtain a more quantitative measurement of the sodium capture in the gasifier as discussed in section 4. In the past, testing has shown that dolomite and calcite act mainly as inert materials to prevent contact between particles that are covered with sticky sodium silicates.7 Unlike dolomite and calcite, the other additives listed above will react with sodium and form sodium silicates, aluminates, and aluminosilicates. While the sodium silicate alone would be quite sticky, it was postulated that the presence of the aluminates and aluminosilicates in these materials would increase the softening temperature sufficiently so that the other additives would not be sticky at gasifier temperatures. Dolomite. Previous studies have shown that dolomite addition at 20 wt % coal feed was sufficient to completely eliminate any consolidation in the TC13 deposit.7 Dolomite was tested again with the TC21 deposit for comparison with the TC13 results. On a weight basis, the composition of the dolomite was 56% CaCO3, 41% MgCO3, 2% SiO2, and 1% Al2O3. As shown in Figure 7, addition of even 50 wt % dolomite did not completely prevent reconsolidation of the TC21 deposit. This result was not surprising given the greater reconsolidation tendency of the TC21 deposits.
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Figure 8. (Left) Norchem silica fume over deposit; (right) sand over Norchem silica fume over deposit.
As observed previously with the TC13 deposits, dolomite particles can physically block agglomeration by limiting the contact between particles that are covered with sticky sodium silicates. However, in the case of the TC21 deposits, the amount of dolomite required to achieve the desired effect is too large. In view of this result, it seems doubtful that agglomeration problems can be successfully addressed by an additive that physically blocks the contact between sticky particles. With this in mind, additional testing was focused on other additives that could scavenge sodium vapor released from the lignite (amorphous silica fume, bauxite, vermiculite, kaolinite, waste-derived kaolinite, and meta-kaolin). Amorphous Silica Fume. It is well-known that silica will react very readily with sodium vapor, forming sticky sodium silicates.4-7 If the sticky sodium silicates are retained in the recycle loop of the gasifier, the result will be serious problems with agglomeration and deposition. However, if the silica is in a very fine form (e.g., a submicrometer fume), it may be able to avoid collection in the recycle cyclones so that it is not retained in the recycle loop. By virtue of their large surface area, the very fine silica particles should also be very effective in scavenging sodium vapor. The ability of the silica fume to capture sodium vapor was tested by placing a layer of silica fume over the pulverized deposit and heating the two layers at 927 °C (1700 °F). This allowed sodium vapor released from the deposit to pass through the top layer of silica fume, providing ample opportunity for reaction. This type of test was conducted on silica fume samples obtained from three different suppliers. Two samples of densified silica fume were obtained from Norchem and BASF. Both of these materials were byproducts of silicon and ferrosilicon production and contained >85% SiO2 (amorphous),
