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Gasifiers
CHAPTER
4
Gasifiers Contents Overview Moving Bed Gasifiers: The Lurgi Gasifier BGL Gasifier Fluidized Bed Gasifiers: The Winkler Gasifier High Temperature Winkler Gasifier U-Gas Gasifier Foster-Wheeler Partial Gasifier KBR Transport Gasifier Entrained Flow Gasifiers: The GE Gasifier ConocoPhillips E-Gas Gasifier Shell Gasifier Siemens Gasifier Mitsubishi Heavy Industries (MHI) Gasifier Pratt and Whitney Rocketdyne (PWR) Gasifier Less Conventional Gasifiers: The Alter NRG Plasma Gasification System References
73 73 75 77 79 79 79 80 83 88 90 91 92 93 96 98
OVERVIEW As discussed in the previous chapter, gasifiers are generally classified according to the fluidization regime in the gasifier; moving bed, fluidized bed, and entrained flow. Examples of each type of gasifier will be given in this chapter. Following this is a description of less conventional gasifiers. Commercial gasification has a long history, and a great number of gasifier designs have been developed. This chapter gives examples of gasifiers, but it is not a comprehensive description of all gasifier technologies.
MOVING BED GASIFIERS: THE LURGI GASIFIER The Lurgi gasifier is the oldest gasifier technology that is still widely used in commercial practice. In the early 1950s, Sasol, a South African firm, acquired the rights to use this gasifier from Lurgi, a German firm. Sasol still uses this gasifier to produce synthetic liquid fuels from coal. Because Sasol has made a number of improvements in the gasifier over the years, the Lurgi gasifier is often called the Sasol-Lurgi gasifier. In 2007, the Sasol-Lurgi gasifier was the most widely used gasifier in the world.1 The Sasol-Lurgi gasifier has been used with a full spectrum of coals, ranging from anthracite to lignite.2 Coal Gasification and Its Applications. ISBN B978-0-8155-2049-8.10004-X, doi:10.1016/B978-0-8155-2049-8.10004-X
Ó 2011 Elsevier Inc. All rights reserved.
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Figure 4.1 shows an illustration of the Sasol-Lurgi gasifier. The gasifier design solves the non-trivial problem of how to feed solid coal into a pressurized vessel, and how to remove solid ash from a pressurized vessel. Coal is fed to an atmospheric pressure bunker above the gasifier. When the bunker is full, a valve on the bottom opens, allowing the coal to drop into a coal lock. The valve then closes, and the coal lock is pressurized until it reaches the gasifier pressure, typically 2.4 to 3.5 MPa. The valve on the bottom of the coal lock opens, and the coal drops into the gasifier. Typically, each batch of coal weighs about 8 tons. Coal is held in the gasifier vessel for about an hour3 while oxygen and steam flow through the grate and into the coal bed. Figure 4.2 shows how the coal and gasses move counter-currently. This makes the Sasol-Lurgi design an especially energy efficient gasifier technology. The highest bed temperature, 615 to 760 C, occurs just above the grate, where the char is gasified. The hot syngas then rises and contacts cooler coal above the gasification zone. In the middle of the bed, rising hot gasses pyrolyze the coal, producing coal tar and char. In the top of the bed, the coolest region, coal is preheated and dried. Syngas and coal tar leave the gasifier at about 370 to 590 C. Compared to other gasification technologies, the operating temperatures in the Sasol-Lurgi gasifier are relatively low. Because of these low temperatures, a refractory lining is not required.
Figure 4.1 The Sasol-Lurgi gasifier. Reprinted by permission from Sasol.
Gasifiers
coal
syngas + coal tar
pre-heating
pyrolysis
gasification (highest temperature)
grate
ash
O2, steam
Figure 4.2 Counter-current gas/solid flow in the Sasol-Lurgi gasifier. The bed is hottest at the bottom, and coolest at the top.
Because the bed must be free-flowing, only non-caking coals can be used. Fine coal cannot be used, because it will plug the interstitial spaces between the large coal particles. The feed coal is sized to about 3 to 30 mm.4 At the Great Plains Synfuels plant in Beulah, North Dakota, fine coal from the grinding circuit is sent to an adjacent pulverized coal plant. The Sasol-Lurgi gasifier produces a considerable quantity of tar, which complicates operations. Hot gasses leaving the reactor are quenched with a recycled water stream. The quench liquid is decanted to produce an organic liquid, which contains the bulk of the tar, and an aqueous layer. The aqueous layer contains water-soluble tar compounds, including phenol and cresylic acid (mixed isomers of methyl phenol). At the Great Plains Synfuels plant, tar is burned to produce steam for plant utilities, and the phenol and cresylic acids are recovered for sale. Newer, higher temperature gasification technologies often compare themselves to the Sasol-Lurgi gasifier and because of their higher operating temperature, little or no tar is produced. This is taken to be an advantage. Before crude oil became inexpensive in the 1950s and 1960s, coal tar was widely processed to make organic chemicals and liquid fuels. With increasing crude oil prices, coal tar may regain its former role as a liquid feedstock, and the tar yield from gasification may be regarded as an advantage, rather than as a disadvantage.
BGL GASIFIER The BGL (British Gas Lurgi) gasifier is a slagging version of the Lurgi gasifier. As shown in Figure 4.3, instead of a grate at the bottom of the gasifier, oxygen and steam are
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Figure 4.3 The BGL gasifier. This is a slagging version of the Lurgi gasifier, in which oxygen is injected into the slag layer.
injected into the slag layer using tuyeres. A conventional Lurgi coal feed system is used. Un-ground, as-received coal is fed to the gasifier. A stirrer near the top of the bed allows the use of caking coals. Because of the high temperature slag layer, greater than 99% carbon conversion is claimed. A refractory lining is required to withstand the high slag temperatures. In a process design by Bartone and White5, Illinois No. 6 bituminous coal is gasified in a BGL gasifier at about 2.7 MPa. Gas leaves the gasifier at 540 C, and is quenched by direct contact with water to 165 C. The condensed tar stream is about 8 wt.% of the coal feed. This tar stream is recycled back to the gasifier by injecting the tar, along with oxygen and steam, through the tuyeres and into the slag layer. Bartone and White also examined PRB subbituminous coal, and estimated about the same condensed tar yield. Table 4.1 shows the syngas composition estimated by Bartone and White, after quenching and further cooling. Note that the syngas contains 6 to 7 mole% hydrocarbons, primarily methane, which is the uncondensed tar fraction.
Gasifiers
Table 4.1 Estimated syngas compositions for the BGL gasifier after a direct quench and further cooling.5 Coal component Illinois No. 6 mole% Powder River Basin mole%
Ar Benzene, toluene, xylene CH4 C2H4 C2H6 C3H6 C3H8 C4H8 C4H10 CO CO2 COS HCN H2 H2O H2 S N2 Total hydrocarbons
0.0141 0.1033 5.9139 0.0471 0.2343 0.0190 0.0991 0.0147 0.0405 54.2938 4.2867 0.0619 0.0288 29.7464 0.2389 1.1521 3.7083 6.4719
0 0.1064 6.1009 0.1321 0.2415 0.0198 0.1019 0.0148 0.0396 55.9980 4.4245 0.0638 0.0297 30.7551 0.2390 1.1851 0.6398 6.7571
FLUIDIZED BED GASIFIERS: THE WINKLER GASIFIER The Winkler gasifier, commercialized in 1926, was the first industrial application of fluidized bed technology.6 The Winkler gasifier operates near atmospheric pressure, in the bubbling fluidized bed regime. Coal is ground to 0e8 mm. As shown in Figure 4.4, coal is fed to a bunker, and a screw feeder withdraws coal from the bunker and injects it into the bubbling fluidized bed. Since the bed is nearly at atmospheric pressure, a relatively simple coal feeding system can be used. The gas feed to the gasifier, consisting of steam and either air or oxygen, is split into two streams. Most of the gas is fed underneath the grate. This gas fluidizes and reacts with the solid bed. As the coal particles react, they become smaller and less dense. About 30% of the ash falls through the grate and is produced as bottom ash. The remaining 70% of the ash is entrained by the fluidizing gas and is carried into the head space. A major issue with this gasifier is that the entrained ash contains a significant quantity of unreacted carbon. The bubbling bed operates at about 1,000 C, a little below the ash softening temperature. The bed is operated near this upper operating temperature limit to maximize carbon conversion.
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syngas + fly ash hydrocyclone
steam coal bunker
water freeboard
screw feeder
fines return bubbling fluidized bed
coal steam + air or O2
rotating grate bottom ash
Figure 4.4 The Winkler gasifier.
The remainder of the gas feed is injected into the freeboard. Oxygen reacting with the gas/solid mixture boosts the temperature to about 1,200 C, which partially melts the ash. The increase in temperature further increases the conversion of carbon in the entrained ash. Heat is removed from the top of the gasifier to re-solidify the ash before it leaves the gasifier. A hydrocyclone returns some of the entrained ash to the fluidized bed to further convert residual carbon in the ash. The Winkler gasifier was once widely used, but few, if any, commercial Winkler gasifiers continue to operate. The persistent issue of low carbon conversion appears to be reason for the demise of this gasifier technology.
filter bunker charge bin
gasifier
lock hoppers
compressor
Figure 4.5 Pressurized feed system for the High Temperature Winkler Gasifier.8
Gasifiers
HIGH TEMPERATURE WINKLER GASIFIER The High Temperature Winkler gasifier was developed in the 1970s and 1980s. The primary change to the original design appears to be pressurized operation, at about 1 MPa.7 Pressurized operation increases gasification rates, which should improve carbon conversion. Pressurized operation required a change in the coal feed system, shown in Figure 4.5. Coal is loaded into an atmospheric pressure bunker. One of the two lock hoppers is then depressurized, and coal from the bunker falls into the lock hopper. The lock hopper inlet closes, and the lock hopper is pressurized. The bottom of the lock hopper opens, and coal falls into a line where it is pneumatically conveyed to the filter. Gas from the filter is recycled to the compressor, and coal falls into the charge bin. A solids metering valve on the bottom of the charge bin, which is pressurized, controls the flow of coal into the gasifier. The use of two lock hoppers allows pressurized coal to be continuously fed to the gasifier.
U-GAS GASIFIER The U-Gas gasifier, like the Winkler gasifier, is a bubbling fluidized bed gasifier. Significant differences include pressurized operation, and a conical screen instead of a rotating grate. The simplified flowsheets shown in the open literature do not show oxygen injection into the freeboard space. The gasifier operates at 0.3 to 3 MPa and 840 to 1,100 C. The U-Gas gasifier was developed by the Gas Technology Institute, and commercial licensing rights were acquired by Synthesis Energy Systems.9 At the time this book was written, three commercial U-Gas gasifiers were either operating (Hai Hua in Zaozhuang City) or under construction (Golden Concord in Inner Mongolia and YIMA in Henan Province) in China.10 A 100 ton/day bagasse, sugar cane waste, gasifier was built in Hawaii, and a 150 ton/day wood gasifier was built in Denmark11. Capital construction costs for the U-Gas gasifier are claimed to be lower than most competing gasifier technologies. This is an important consideration because the economic feasibility of most plants based on coal gasification is more strongly influenced by capital costs than by operating costs.
FOSTER-WHEELER PARTIAL GASIFIER The Foster-Wheeler Partial Gasifier,12,13 shown in Figure 4.6, is a fairly straightforward implementation of a circulating fluidized bed reactor. The goals for this gasifier were rather modest. Coal was to be partially gasified, and the remaining char would then be burned in a pulverized coal power plant. Carbon conversions between 45 and 80% were obtained for several bituminous coals. With sub-bituminous Powder River Basin coal, carbon conversions were 80 to 90% due to the higher reactivity of low grade coals.
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recycle cyclone
syngas cooler
syngas
gasifier body candle filter pre-cleaner cyclone
solids standpipe
char
char
coal, air, steam
Figure 4.6 The Foster-Wheeler Partial Gasifier.
The gasifier body consists of a vertical pipe. Coal, air, and steam are injected into the bottom of the pipe, and the gas velocity is sufficient to carry the solids up. Near the top of the gasifier, the hot gas/solid mixture is sent to the recycle cyclone where coarse solids return to the bottom of the gasifier via a solids standpipe. The gasifier body, recycle cyclone, and solids standpipe are all refractory lined. Recycling the solids in this manner gives unreacted carbon in the coarse solids an additional opportunity to react. The inert fraction of the recycled solids has a thermal inertia role, dampening temperature variations in the gasifier. Gasses and fine solids leaving the recycle cyclone are cooled in the syngas cooler, and then sent to the pre-cleaner cyclone which removes a portion of the solid char. The remaining char is separated from the syngas in a candle filter. The gasifier operates at 0.7 to 0.9 MPa. Bituminous coals were gasified at 995 C to 1,065 C, and the subbituminous PRB coal was gasified at 945 to 995 C. A pilot scale gasifier, 12 m high by 18 cm I.D., was built and tested. No commercial units were built.
KBR TRANSPORT GASIFIER The Transport gasifier,14e19 shown in Figure 4.7, is a circulating fluidized bed gasifier. The main body of the gasifier has two sections, a larger-diameter mixing zone, on the bottom, and a smaller-diameter riser section, on the top. This differs from the FosterWheeler Partial Gasifier, shown in Figure 4.6, which has a constant diameter. The larger
Gasifiers
solids separation unit primary gas cooler riser
syngas
particulate control device
coal from lock hoppers air, O2 ,steam
mixing zone standpipe
Startup burner
ash depressurization
recycle syngas fly ash air, O2 ,steam ash depressurization
bottom ash
Figure 4.7 The Transport gasifier.
diameter of the mixing zone lowers gas velocity, which allows more solids back-mixing and increases solids retention time. The gasifier is preheated with a gas-fired startup burner. The coal is dried sufficiently to remove moisture of the surface of the coal particles, so that the coal will flow readily through the feed system. Low grade coals often contain substantial moisture, much of it absorbed in the interior of coal particles, and this interior moisture does not need to be removed. The coal is ground to 250e600 mm average diameter,17 and fed to the gasifier using a pressurized lock hopper system. Coal is fed near the top of the mixing zone. Gas and reacting solids flow up through the riser. Coarse solids are removed by solids separation unit, and recycled via a standpipe with a J-leg seal on the bottom. Solids in the J-leg are fluidized by a recycle syngas stream.
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The original Transport gasifier design included two solids recycle loops. The first removed coarse solids using a dis-engager, and returned the solids via a standpipe and a J-leg seal. Medium solids were then removed with a cyclone and a short standpipe and loop seal that fed into the dis-engager standpipe. The design was later simplified to a single solids recycle loop. The solids separation unit in the new design appears to be a cyclone. Air or oxygen and steam enter the bottom of the reactor, just below where the recycle solids enter. The concept behind this configuration is that heat for the reactor would be primarily provided by burning carbon in the recycle solids. Analysis of the standpipes solids,14 however shows that the recycle solids generally contain less than 1 wt.% carbon. Temperatures within a gasifier tend to rise rapidly wherever oxygen is injected, because reactions with oxygen are fast and exothermic. As the flowing mixture moves away from the oxygen injection point, the reaction temperature tends to drop because of the endothermic steam and CO2 gasification reactions. A second air or O2 and steam injection point, just below the coal injection point, helps to level the temperature profile in the gasifier. Most gasification tests were air-blown. As shown in Table 4.2, syngas produced during O2-blown tests still contained large quantities of nitrogen due to the nitrogen used in the feed system and in purging instrumentation ports. Gas and fine solids leaving the solids separation unit pass through the primary gas cooler and into the particulate control device, which is a combined cyclone and candle filter. Gas enters the particulate control device tangentially, which throws some of the solids to the vessel wall, where they slide to the bottom. The remaining Table 4.2 Gas feed and product compositions for a Transport gasifier fed PRB coal.14 Gas feed Air-blown, mole% Oxygen-enriched, mole%
Air O2 Steam N2 Riser exit temp, C Pressure, MPa Syngas Ar CH4 C2H6 CO CO2 H2 H2 O N2
58.0 0 7.0 35.0 916 1.55
9.9 13.2 36.4 40.5 907 1.26
0.5 1.1 0.0 7.5 8.5 6.7 10.3 65.4
0.1 1.9 0 7.6 12.8 12.6 27.5 37.4
Gasifiers
solids are removed by banks of sintered metal tube filters in the center of the vessel. Solid filter cakes on the exterior of the tubes are periodically dislodged by backpressure pulses. Like other fluidized bed gasifiers, carbon conversion is generally higher for low grade coals than high grade coals. During air-blown operation, carbon conversion averaged 84% for an Illinois bituminous coal, 90% for Hiawatha bituminous coal, 95% for both PRB subbituminous coal and Freedom lignite, and 97% for Falkirk lignite18. Also like most fluidized bed gasifiers, much of the unconverted carbon can be attributed to fine particles that are quickly swept out of the gasifier. The bottom ash, which is essentially carbon-free, has a mean diameter of about 100 mm.17 The fly ash, on the other hand, has a mean diameter of about 10 mm. During gasification of PRB coal, the fly ash contained 20e40 wt.% unconverted carbon. Table 4.2 shows measured gas compositions from a process development unit14 for a Transport gasifier fed PRB coal operating in both air-blown and oxygen-enriched modes. Levels of C2H6 were not measurable, which shows that very little tar formed at these conditions. Methane, on the other hand, constituted a significant fraction of the combustible gas product. A considerable amount of nitrogen was fed to the gasifier through the coal feed system and by blanketing instrumentation taps. In a full-scale gasifier, the fraction of nitrogen fed to the gasifier would be reduced. In the oxygenenriched mode shown in Table 4.2, the O2/N2 ratio is closer to air than the air-blown mode, because of the effect of the N2 diluent. In the oxygen-enriched mode, a high steam feed rate was used. Due to the relatively low temperature, compared to entrained flow gasifiers, the water gas shift reaction favored conversion of CO and steam to H2 and CO2. The CO2/CO ratio was a relatively high 1.67, and the H2/CO ratio was a relatively high 1.65. A pilot scale Transport gasifier was built at the University of North Dakota. A larger process development facility was built and operated by the Southern Company in Wilsonville, Alabama. A previously-announced commercial IGCC plant19 based on a Transport gasifier was cancelled in 2009. This plant was to be an air-blown unit and fed PRB coal. No carbon capture and sequestration were included. The plant was cancelled due to uncertainties about greenhouse gas regulations. A commercial IGCC plant in Kemper County, Mississippi, using local lignite, will start operations in 2014. A 65% carbon capture and sequestration level is planned for this plant.
ENTRAINED FLOW GASIFIERS: THE GE GASIFIER The GE gasifier, shown in Figure 4.8, was originally developed by Texaco, an oil company that is now part of Chevron. This gasifier is known as the Texaco gasifier in older literature. The first commercial application was as an oil gasifier in 1956. The first commercial Texaco coal gasifier started operation in 1983.
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Figure 4.8 The GE (Texaco) gasifier showing radiant heat recovery system.
The standard method of feeding coal to a high pressure GE gasifier is to finely grind the coal and then mix it with water to form a pumpable slurry. This slurry resembles the heavy oil fed to the first Texaco gasifier. The slurry and oxygen are injected into the top of the gasifier, and the gas/solid/slag mixture flows downward. There are two basic modes of operation. The first is the quench mode, illustrated in Figure 4.9. The hot gas/slag mixture is bubbled through a water bath, which solidifies the slag. The gas is cooled, and boiling water increases the steam content of the syngas. The slurry leaves the gasifier as a slag/water slurry. Note that solids are both fed into the pressurized gasifier, and are removed from the gasifier, as a water slurry. The gasifier is refractory lined. Hot slag slowly attacks the refractory liner, so the liner must be periodically replaced.20e22 This is a common problem in most slagging gasifiers. Figures 4.8 and 4.10 show the radiant heat recovery mode. A longer gasifier body is used, and steam tubes are embedded in the walls in the lower part of the gasifier body.
Gasifiers
O2
coal/water slurry
syngas quench water
slag/water slurry
Figure 4.9 The GE gasifier in quench mode.
coal/water slurry
O2
steam
steam
water
water syngas
quench water
slag/water slurry
Figure 4.10 The GE gasifier in radiant heat recovery mode.
The estimated operating temperatures from Woods et al.23 are a 1,316 C syngas temperature, which is cooled to 593 C by the steam tubes, and further cooled to 210 C in the water quench. The estimated operating pressure is 5.6 MPa, which is a higher pressure than used by most other gasifiers. The radiant heat recovery mode has greater heat recovery, but the quench mode has a much lower capital cost. Frequently, gasification is followed by a water gas shift reactor
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to increase the H2/CO ratio. Steam must be fed to the water gas shift reactor, and the quench mode increases the steam content of the syngas. Table 4.3 shows syngas compositions estimated by Woods et al.23 for the radiant heat recovery version of the GE gasifier when fed Illinois No. 6 coal. Two stream compositions are shown. The “before quench” stream illustrates the effect of gasification conditions on syngas composition. The “after quench” stream shows the gas product from the gasifier, since the quench is an integral part of the gasifier. Argon constitutes about 0.9 mole% of air, and it is a common impurity in oxygen produced by cryogenic air distillation units, so nearly all syngas contains small quantities of argon. At the high gasification temperature, essentially no tar is produced and only a small quantity of methane is produced. Refer to Figure 3.3, which shows the effect of temperature and pressure on the methane steam reforming reaction equilibrium constant. The high gasification temperature also does not favor the conversion of CO and H2O to CO2 and H2 via the water gas shift reaction (see Figure 3.2), so the CO2/CO ratio is 0.44, a relatively low level. The quench, in addition to lowering the syngas temperature, also boosts the H2O syngas concentration from 14 to 27 mole%, which dilutes the other gas components. The coal/water slurry feed technique works well with bituminous coals, but with lower grade coals. The water/coal ratio is often far in excess of optimum due to the high intrinsic moisture in lower grade coals. To illustrate this point, the Wyoming State government commissioned a conceptual design of a 10,000 barrel/day PRB coal to liquids plant from a company called Rentech in 2005. They evaluated four different gasifiers, GE, Shell, ConocoPhillips and Future Energy (now Seimens), to incorporate into the design. Table 4.4 shows the results of the analysis for the four gasifiers. The GE gasifier required the highest coal feed rate, 13,600 ton/day for the GE gasifier compared to only 7650 ton/day for the Future Energy gasifier, because the higher water content of the slurry feed had to be vaporized by coal pyrolysis.
Table 4.3 Estimated syngas compositions23 for the GE gasifier in radiant heat recovery mode when fed Illinois No. 6 coal. Component Before quench, mole% After quench, mole%
Ar CH4 CO CO2 COS H2 H2 O H2 S N2 NH3
0.79 0.10 34.42 15.11 0.02 33.49 14.29 0.73 0.89 0.17
0.67 0.08 29.22 12.76 0.02 28.49 27.26 0.61 0.76 0.14
Gasifiers
Table 4.4 Screening results for a 10,000 barrel/day Fischer-Tropsch coal to liquids plant using Powder River basin sub-bituminous coal. Screening Results - Process Performance & Cost Once-Through FT Operation Producing 10,000 BPD Fuels GE ConocoPhillips Future Energy Shell (“Texaco”) (E-Gas) (GSP) (SCGP)
Coal Feed AR (STPD) Water in Coal AF (wgt.%) Oxygen Contained (STPD) Water Balance (GPM) Raw SG H2/CO ratio (molar) Net Power Export (MW) FT Products (BPD) EPC-CAPEX rating
13,600 55% 11,300 -185 1.29 64 10,000 4
9,150 48% 5,750 -200 0.88 100 10,000 2
7,650 10% 4,250 110 0.40 94 10,000 1
7,850 2% 4,450 200 0.42 81 10,000 3
To address this problem, GE acquired the Stamet pump technology.24,25 The Stamet pump, shown in Figure 4.11, is designed to pump dry solids against a pressure gradient. To do this, the pump uses a solids lock-up mechanism. If a large pressure gradient is applied to an unconfined bed of solids, the solids will fluidize and move towards lower pressure. If those same solids are pressed between two surfaces, then the solids are not free to fluidize, and the solids are in a lock-up state. The coal particles are compressed between pump surfaces and rotated from an atmospheric pressure hopper to a pressurized gasifier. Using this technology, the GE gasifier can be fed a dry coal feed obviating the need for the high water slurry fed operation previously used. GE expects that this technology will overcome its deficiency with respect to low rank, high moisture coals.
Figure 4.11 The Stamet pump is designed to pump dry solids against a pressure gradient.
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steam generator
syngas candle filter char
2nd stage coal/water slurry
1st stage coal/water slurry, O2
1st stage coal/water slurry, O2 quench water slag/water slurry
Figure 4.12 ConocoPhillips E-Gas gasifier.
At the time this book was written, a pilot scale gasifier was under construction in Cheyenne, Wyoming.26 This pilot unit, jointly funded by GE and the State of Wyoming, is primarily intended to demonstrate the gasification of low grade coals using a GE gasifier equipped with a Stamet pump. In 2007, the GE gasifier was the second most commonly installed commercial gasifier1, the most common is the Sasol-Lurgi.
CONOCOPHILLIPS E-GAS GASIFIER The ConocoPhillips E-Gas gasifier was originally developed in the 1970s and 1980s by Dow Chemical.23 At that time, it was known as the Destec gasifier. As discussed in Chapter 3, a gasifier is normally fed a gas mixture consisting of steam and oxygen or air. Although rarely done, CO2 may substitute for all or part of the steam. The basic idea is to feed enough oxygen and steam to gasify all of the coal, and to adjust the oxygen rate to achieve the desired gasification temperature. An increase in the oxygen/steam ratio will increase the gasification temperature, but the syngas will have a lower heat of combustion. Entrained flow gasifiers operate at relatively high temperatures, which require relatively high oxygen/steam ratios. The basic concept of the E-gas gasifier is to use the high temperature syngas generated at slagging conditions to gasify coal injected at a second point. This more efficiently uses the heat generated by reactions with O2. As shown in Figure 4.12, a coal/water slurry and oxygen are injected into the high temperature first stage, operating at 1,316 to 1,427 C and 4.2 MPa.23 Slag formed at these conditions drops to
Gasifiers
the bottom of the gasifier, where it is quenched with water and withdrawn as a slag/water slurry. Hot syngas rises, passing through a restriction and into the second stage of the gasifier. A second coal/water slurry stream is injected at the inlet of the second stage; and the hot syngas provides heat for the endothermic pyrolysis, steam gasification, and CO2 gasification reactions. Gas and solids leave the second stage at about 1,010 C, which is below the ash softening temperature.23 Gasification of the second stage coal feed is incomplete, so the gas is cooled in a steam generator, the solid char is removed by a candle filter; and the char is returned to the first stage. The gasifier is refractory lined. The cold gas efficiency of the gasifier increases with increasing second stage slurry feed, and oxygen demand declines.27 At the Wabash River Coal Gasification Repowering Project,28 a commercial scale IGCC demonstration plant, the typical coal slurry feed split is 85% to the first stage and 15% to the second stage.27 Rutkowski et al.29 compared the cost of producing hydrogen from Pittsburgh No. 8 bituminous coal to PRB sub-bituminous coal. They chose an E-Gas gasifier, because this is one of the few gasifiers that has been demonstrated using both bituminous and sub-bituminous coals. A key assumption in their study was that both cases would have the same coal feed rate on a dry basis. They concluded that Pittsburgh No. 8 was the preferred feed, even though this coal is considerably more expensive than PRB. The cost of hydrogen was dominated by capital costs is the primary reason for the expense. In both cases, the capital cost was about the same. Pittsburgh No. 8, with its higher heating value, yielded more hydrogen product, which diluted the capital cost per unit of hydrogen product. Herbanek et al.,30 however, stated that lower grade coals, due to their higher reactivity, could be fed at higher rates to the E-Gas gasifier. They compared the costs of electricity from an IGCC plant and found that sub-bituminous PRB coal had a lower production cost than three bituminous coals. Table 4.5 shows syngas compositions estimated by Rutkowski et al. for Pittsburgh No. 8 and PRB coals. The relatively low temperature second stage boosts the methane concentration compared to the GE gasifier (see Table 4.3); although it is not obvious why the methane content differs for Pittsburgh No. 8 and PRB coals. The moisture content of the PRB coal, 26.6 wt.%, is much higher than the moisture content of the Pittsburgh No. 8 coal, 6.0%. This leads to higher water content in the PRB-derived syngas, which dilutes the other components. The higher water content in the PRBderived syngas and the relatively low temperature second stage also favors the conversion of CO and H2O to CO2 and H2 via the water gas shift reaction, so the PRB-derived syngas has a higher CO2/CO ratio than the Pittsburgh No. 8 derived syngas. The sulfur content of Pittsburgh No. 8 is much higher than PRB, 3.07 versus 0.82 wt.%, both dry basis, and this causes higher levels of H2S and COS in the syngas. In 2007, the ConocoPhillips E-Gas gasifier was the fourth most commonly installed gasifier.1
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Table 4.5 Estimated syngas compositions29 for the ConocoPhillips E-Gas gasifier. Pittsburgh No. 8 Powder River Basin Coal component (bituminous), mole% (sub-bituminous), mole%
Ar CH4 CO CO2 COS H2 H2O H2 S N2 NH3
0.82 0.42 41.95 9.75 0.04 33.20 12.19 0.78 0.57 0.28
0.75 0.17 26.61 16.04 0.01 28.34 27.17 0.19 0.52 0.21
SHELL GASIFIER Shell developed oil gasifiers in the 1950s. These units differ substantially from Shell’s coal gasifier, which was developed jointly23 with Krupp-Koppers from 1974 to 1981. After 1981, Krupp-Koppers offered a similar gasifier known as Prenflo.31 recycle compressor
quench gas gasifier
coal
fly ash
steam
steam syngas cooler
N2
grinding, drying, & feeding
boiler feed water boiler feed water
O2 slag
dry solids removal
fly ash (recycled)
Figure 4.13 The Shell coal gasifier.
syngas
Gasifiers
The Shell coal gasifier23,32 is shown in Figure 4.13. The gasifier features a dry feed system. Coal is ground and dried, and N2 is used to pressurize the feed hoppers to the gasifier operating pressure, about 4.2 MPa. Coal is fed to opposite sides of the gasifier, near the bottom. Oxygen is fed to the gasifier below the coal injection points. Slag flows out the bottom. The hot gas/solid/slag mixture flows up through the gasifier at about 1,600 C. Near the top of the gasifier, a relatively cold recycle syngas stream, at about 200 C, is injected into the gasifier. This is shown as quench gas in Figure 4.13. The quench gas drops the syngas mixture to about 900 C. The steam boiler tubes in the syngas cooler cannot withstand the 1,600 C temperature of the unquenched syngas, so quenching drops the temperature to a tolerable level. The syngas cooler is taller than the gasifier. Shell also developed a water quench system to replace the syngas cooler.33 The fly ash contains unconverted carbon. This is recycled to the coal feed system. The Shell gasifier features a membrane wall, shown in Figure 4.14, rather than a refractory lining. The gasifier wall is cooled by boiling water to generate steam. A layer of solidified slag forms on the boiler tubes, and this solidified slag layer effectively functions like a refractory layer. Since the solid slag layer is the same composition as the liquid slag, the liquid slag does not attack the solid layer. Table 4.6 shows an estimated syngas composition for the Shell gasifier fed Illinois No. 6 bituminous coal. Because of the high gasification temperature and the lack of water fed to the gasifier, the syngas has very little methane and a high CO/CO2 ratio. In 2007, the Shell coal gasifier was the third most commonly installed gasifier, and the most popular gasifier for newly planned installations.1
SIEMENS GASIFIER Development of the Siemens gasifier started in the 1970s, and the first commercial plant was built in 1984. In 2006, Siemens acquired the gasifier technology from Future Energy.34,35 steam tube gasifier wall
atmosphere
solidified slag molten slag
hot syngas
Figure 4.14 Gasifier membrane wall. Solidified slag replaces refractory lining.
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Table 4.6 Estimated syngas composition23 for the Shell gasifier fed Illinois No. 6 coal. Component mole%
Ar CH4 CO CO2 COS H2 H2 O H2 S N2 NH3
0.97 0.04 57.16 2.11 0.07 29.01 3.64 0.81 5.85 0.33
Siemens, like GE, offers gasifiers, gas turbines, and steam turbines, all important components of an IGCC plant.36 The gasifier, shown in Figure 4.15, is also roughly similar to a GE gasifier in quench mode, shown in Figure 4.9. There are, however, significant differences. The Siemens gasifier features a dry feed system, using a lock hopper arraignment similar to that shown in Figure 4.5.37 The hoppers are pressurized with N2 or CO2 to the gasifier pressure, about 2.8 MPa.34,36 A gas fuel is used to preheat the gasifier. Coal and O2 are fed to the top of the gasifier, which operates at about 1,400 C. The gasifier is lined with a membrane wall instead of a refractory. Siemens also offers a refractory-lined version of their gasifier for low ash fuels. A wide variety of fuels have been gasified.38 Like the GE gasifier, the Siemens gasifier uses a water quench to solidify the slag, and the slag is removed as a slag/water slurry. The quench arraignment differs between the two gasifiers. In the GE gasifier, the syngas/slag mixture is blown into a water bath. In the Siemens gasifier, a water spray quenches the syngas/slag mixture.
MITSUBISHI HEAVY INDUSTRIES (MHI) GASIFIER Development of the MHI gasifier started in the 1980s.39 A 1,700 T/D demonstration plant in Nakoso, Japan started operations in 2007. A wide range of coals, including both bituminous and sub-bituminous coals, have been successfully gasified. The basic concept of the MHI gasifier is similar to the E-gas gasifier. The gasifier, shown in Figure 4.16, uses a split coal feed. Coal and air are fed to the bottom stage; and the rising, hot syngas is used to gasify a second coal feed stream. In the E-gas gasifier, these stages are known as the first and second stages. The equivalent nomenclatures in the MHI gasifier are the combustor and reductor stages.40 Endothermic reactions in the reductor section lower the temperature from the slagging condition to about 700 C, well below the ash softening temperature. Char is separated from syngas leaving the reductor stage and returned to the combustor stage.
Gasifiers
Figure 4.15 Siemens gasifier.
The gasifier is air-blown. Most gasification plants that employ carbon capture and sequestration are oxygen-blown. Mitsubishi claims that the capital cost of their air-blown system is nearly the same as an oxygen-blown system, and that their efficiency is a little higher.41 The syngas contains about 30% CO and 10% H2. An oxygen-blown version of the MHI gasifier is being developed.42 The MHI gasifier uses a dry feed system and a membrane wall. Slag is removed from the bottom of the gasifier as a slag/water slurry. A very similar gasifier was developed by the Thermal Power Research Institute of China.43
PRATT AND WHITNEY ROCKETDYNE (PWR) GASIFIER The Pratt and Whitney Rocketdyne (PWR) compact gasifier, shown in Figure 4.17, was inspired by rocket engine technology. This gasifier was undergoing pilot scale development44 at the time this book was written. Commercial licenses are marketed by Zero Emissions Energy Plants (ZEEP).45
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syngas
reductor
char coal
combustor
air
slag water
slag/water slurry
Figure 4.16 Mitsubishi Heavy Industries (MHI) gasifier.
Coal, O2
rapid mix injector
cooled membrane wall
rapid spray quench
syngas/slag
Figure 4.17 The Pratt and Whitney Rocketdyne Compact gasifier.
Gasifiers
Pulverized coal and oxygen are split into multiple streams and fed to a rapid mix injector, which is then cooled. Tested injector pressures range from 1.5 to 6.7 MPa.46 Flame temperatures can reach 2,760 C, an extremely high temperature.47 The gasifier wall is cooled to keep the metal temperature below 540 C. Figure 4.18 shows a more complete PWR gasifier system. The feeding system features a dry solids pump,48 shown in Figure 4.19, similar in concept to the Stamet pump shown in Figure 4.11. The PWR solids pump features two opposed rotating belts. Coal particles are squeezed between the belts to achieve solids lock-up, allowing the solids to be pushed against a pressure gradient. Gas and solid slag leaving the gasifier first enter a hydrocyclone and then a bank of tube filters to remove the solid slag. A peer review49 of the PWR gasifier technology in 2006 identified the dry solids pump and the rapid mix injector as key items that require further technical development. Matuszewski et al.50 compared the PWR gasifier to the GE and Shell gasifiers and concluded that the PWR gasifier is both more efficient and less expensive. These results should be greeted with cautious skepticism, as Matuszewski et al. based their estimates on projected results from Pratt and Whiney for technology that had yet to be demonstrated. Still, such results encourage further development. Pratt and Whitney claims that their gasifier is only about 10% of the size of comparable commercial gasifiers, which should lessen capital costs. The high efficiency claims for the PWR gasifier are a little surprising when one considers the very high temperature and rapid gasification conditions. Normally, high low pressure hopper
quench
syngas hydrocylcone tube filter bank
dry solids pump gasifier
high pressure hopper
coarse slag
fine slag
Figure 4.18 Pratt and Whitney Rocketdyne (PWR) Compact Gasifier System.
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Gasifiers
Figure 4.19 The PWR dry solids feed pump.48 Coal is fed from a low pressure hopper above the pump, and discharged to a high pressure hopper below the pump.
efficiency processes require relatively low temperatures and a close approach to equilibrium, which lowers the driving forces, and leads to slow reactions. The high efficiency of the PWR gasifier may be due to the quench. The high temperature gasses produced by the upper part of the gasifier drive steam gasification reactions in the lower part of the gasifier; and the steam/solid slag stream leaves the gasifier at a relatively low temperature.
LESS CONVENTIONAL GASIFIERS: THE ALTER NRG PLASMA GASIFICATION SYSTEM The Alter NRG plasma gasifier51,52 uses a plasma torch to gasify a solid feedstock. This gasifier can be used to gasify coal, but it is especially attractive for difficult-to-gasify feedstocks, such as municipal solid waste (MSW). The largest current installation of the gasifier is the Eco-Valley Waste-to-Energy Facility in Utashinai, Japan, which gasifies 180 T/day of MSW and automotive shredder waste. At the time this book was written, a 120 MW coal power plant retrofit in Somerset, Massachusetts was planned,53 as well as a 40,000 BBL/day coal-to-liquids plant in Fox Creek, Alberta. Coskata54 built a semicommercial facility that gasifies wood chips using the Alter NRG plasma gasifier. The syngas is converted to ethanol in a bioreactor.
Gasifiers
Figure 4.20 Westinghouse Plasma Corporation torch used in the Alter NRG Plasma gasifier.55
The heart of the process is a Westinghouse Plasma Corporation55 torch shown in Figure 4.20. Westinghouse Plasma Corporation is a subsidiary of Alter NRG. A plasma is an ionized gas and, in theory, a plasma can be made from any gas. Air appears to be the plasma feed gas for most gasifier applications.
Figure 4.21 Alter NRG Plasma gasifier.51
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Gasifiers
The plasma gasifier is shown in Figure 4.21. Solid feed enters through the side of the gasifier, and plasma torches are directed towards a moving bed of reacting solids. Near the plasma torch, temperatures can be as high as 3,000 C. Molten metal and slag leave the bottom of the reactor. Slagging operation is attractive for an MSW application, because toxic metals in the slag are essentially unleachable. Gases rise through the gasifier and enter an expanded freeboard zone, which allows coarser solids to fall back into the bed. The gas leaving the gasifier is about 900 to 1,000 C, which is hot enough to avoid tar. The heat of combustion of the syngas is about 80% of the feed.53 The syngas product can be burned to generate electricity. Electric power for the plasma torches is only about 2 to 5% of the feedstock energy input.
REFERENCES 1. Gasification World Database 2007: Current industry status. U.S. Dept. of Energy, Office of Fossil Energy, National Energy Technology Laboratory; 2007. 2. van de Venter E. Sasol-Lurgi coal gasification and low rank coal, Gasification Technologies Council 2005 Annual Conference. 3. Howard-Smith I, Werner GJ. Coal Conversion Technology. Noyes Data Corp; 1976. 4. Krichko AA. Theoretical basis of coal gasification. In: Oil and Gases from Coal. Pergamon Press for the United Nations; 1980. p. 89-124. 5. Bartone LM Jr, White J. Industrial size gasification for syngas, substitute natural gas, and power production, DOE/NETL-401/040607,; 2007. 6. Squires AM. Clean fuels from coal gasification. Science 1974;184:340-346. 7. Rezenbrink W, Wischnewski R, Engelhard J, Mittelsta¨dt A. High Temperature Winkler (HTW) coal gasification: A Fully-Developed for Methanol and Electricity Production, Gasification Technologies Council 1998 Annual Conference. 8. Bockelie M, Denison M, Swenson D, Senior C, Sarofim A. Modeling entrained flow gasifiers, 26th Ann Int Pittsburgh Coal Conf. Sept. 20e23, 2009. 9. Vail T. SES deployment & commercialization of U-GAS gasification technology Gasification Technologies Council 2007 Annual Conference. 10. Synthesis Energy Systems, Cleanly unlocking the value of coal: Our projects. . 11. Lau F. Commercial development of the SES U-GAS gasification technology, Gasification Technologies 2009 Annual Conference. 12. Robertson A. Development of Foster Wheeler’s Vision 21 partial gasification module, Vision 21 Program Review Meeting, Morgantown, West Virginia, Nov. 6e7, 2001. 13. Engstro¨m F. Overview of power generation from biomass, Gasification Technologies Council 1999 Annual Conference. 14. Southern Company Services, Inc., Power Systems Development Facility Topical Report, Test Campaign TC 16, July 14eAugust 24, 2004, U.S. D.O.E. contract DE-FC21-90MC25140, . 15. Shadle LJ, Monazam ER, Swanson ML. Coal gasification in a transport reactor. Ind Eng Chem Res. 2001;40:2782-2792. 16. Mann MD, Knutson RZ, Erjavec J, Jacobsen JP. Modeling reaction kinetics of steam gasification for a transport gasifier. Fuel. 2004;83:1,643-1,650. 17. Leonard R, Lambrecht RC, Vimalchand P, Yongue RA. Update on gasification testing at the power systems development facility, 32nd International Technical Conference on Coal Utilization & Fuel
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18. 19. 20. 21. 22. 23.
24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
Systems, June 10e15, 2007,. Southern Company Services, Inc. Power Systems Development Facility Topical Report, Test Campaign TC 17, October 26, 2004 e November 18, 2004, U.S. D.O.E. contract DE-FC21-90MC25140, . Pinkston T. Orlando Gasification Project: Demonstration of a 285 MW coal-based transport gasifier, Gasification Technologies Council 2006 Annual Conference. Johnson KI, Williford RE, Matyas J, Pilli SP, Sundaram SK, Korolev VN. Modeling slag penetration and refractory degradation using the finite element method, 25th Annual International Pittsburgh Coal Conference; 2008. Bennett JP, Kwong K-Y, Petty AV, Thomas H, Krabbe R. Interactions between slag and high chrome oxide refractory liners in air cooled slagging gasifiers, 25th Annual International Pittsburgh Coal Conference; 2008. Matyas J, Sundaram SK, Rodriguez CP, Edmunson AB, Arrogoni BM. Slag penetration into refractory lining of slagging coal gasifier, 25th Annual International Pittsburgh Coal Conference; 2008. Woods MC, Capicotto PJ, Haslbeck JL, Kuehn NJ, Matuszewski M, Pinkerton LL, et al. Cost and performance baseline for fossil energy plants, volume 1: Bituminous coal and natural gas to electricity, DOE/NETL-2007/1281, ; 2007. Project Fact Sheet: Continuous pressure injection of solid fuels into advanced combustion system pressures. U.S. Dept. of Energy, Office of Fossil Energy, National Energy Technology Laboratory, ; 2006. Crew J. High efficiency feed system for PRB and other high-moisture feedstocks, Gasification Technologies 2008 Annual Conference. High Plains Gasification-Advanced Technology Center, ; 2000. Rutkowski MD, Buchanan TL, Klett MG, Schoff RL. Capital and operating cost of hydrogen from coal gasification, U.S. D.O.E. contract DE-AM26e99FT40465, ; 2003. Herbanek R, Shah J, Gadde S, White J. Feedstock impact on an IGCC plant with CO2 capture, Gasification Technologies Council 2008 Annual Conference. Radtke K, Heinritz-Adrian M, Hooper M, Richards B. PRENFLO PSG and PDQ: Latest developments based on 10 years operating experience at Elcogas IGCC, Puertollano, Spain, Gasification Technologies Council 2008 Annual Conference. Zuideveld PL. Shell coal gasification process for power and hydrogen/chemicals, Gasification Technologies Council 2004 Annual Conference. Fournier G, Harteveld W, Prins M, Von Kossak T, Van den Berg R. A water quench for the Shell coal gasification process, 25th Annual International Pittsburgh Coal Conference; 2008. Volkman D. Future Energy Gmbh: An update on technology and projects, Gasification Technologies Council 2004 Annual Conference. Zwirn R. Gasification: Journey to commercialization, Gasification Technologies Council 2006 Annual Conference. Morehead H. Siemens IGCC and gasification activities: North America & China, Gasification Technologies 2009 Annual Conference. Klemmer K-D. The Siemens gasification process and its application to the Chinese Market, Gasification Technologies Council 2006 Annual Conference. Hannemann F, Schingnitz M, Lamp J, Wu B. Applications of Siemens fuel gasification technology for different types of coal, 25th Annual International Pittsburgh Coal Conference; 2008.
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39. Sakamoto K. Commercialization of IGCC/gasification technology for the US Market, Gasification Technologies Council 2008 Annual Conference. 40. Sakamoto K. Development of Mitsubishi air-blown IGCC technology with carbon capture, Gasification Technologies Council 2008 Annual Conference. 41. Fujii T. Deployment of IGCC technology with carbon capture, Sixth Annual Conference on Carbon Capture and Sequestration; 2007. 42. Sakamoto K. Mitsubishi IGCC project updates, Gasification Technologies Council 2009 Annual Conference. 43. Shisen X. GreenGendnear zero emission coal based power demonstration project in China, Gasification Technologies Council 2009 Annual Conference. 44. Darby A, Hartung J. Compact gasification system development status, Gasification Technologies Council 2009 Annual Conference. 45. Bernard B. Gasification for the next generation: Zero emissions energy plants, Gasification Technologies Council 2009 Annual Conference. 46. Hartung J, Darby A. Pratt and Whitney Rocketdyne (PWR) compact gasification system, Gasification Technologies Council 2007 Annual Conference. 47. Darby A. Status of the Pratt & Whitney Rocketdyne/DOE advanced single-stage gasifier development program, Gasification Technologies Council 2005 Annual Conference. 48. Sprouse KM, Matthews DR, Saunders T, Weber GF. PWR dry feed development status, 2008 International Pittsburgh Coal Conference. 49. Clayton S, Powell C, Rath L, Keairns D, Gray D, Geertsema A. et al. PWR gasifier peer review,; 2006. 50. Matuszewski M, Rutkowski MD, Schoff RL. Comparison of Pratt and Whitney Rocketdyne IGCC and commercial IGCC performance, DOE/NETL-401/062006, ; 2006. 51. Alter NRG, . 52. van Nierop P. Alter NRG Plasma gasification system for waste and biomass gasification, Gasification Technologies Council 2009 Annual Conference. 53. Bower R. Somerset plant refueling through plasma gasification, Gasification Technologies Council 2008 Annual Conference. 54. .
4
Gasifiers Contents Overview Moving Bed Gasifiers: The Lurgi Gasifier BGL Gasifier Fluidized Bed Gasifiers: The Winkler Gasifier High Temperature Winkler Gasifier U-Gas Gasifier Foster-Wheeler Partial Gasifier KBR Transport Gasifier Entrained Flow Gasifiers: The GE Gasifier ConocoPhillips E-Gas Gasifier Shell Gasifier Siemens Gasifier Mitsubishi Heavy Industries (MHI) Gasifier Pratt and Whitney Rocketdyne (PWR) Gasifier Less Conventional Gasifiers: The Alter NRG Plasma Gasification System References
73 73 75 77 79 79 79 80 83 88 90 91 92 93 96 98
OVERVIEW As discussed in the previous chapter, gasifiers are generally classified according to the fluidization regime in the gasifier; moving bed, fluidized bed, and entrained flow. Examples of each type of gasifier will be given in this chapter. Following this is a description of less conventional gasifiers. Commercial gasification has a long history, and a great number of gasifier designs have been developed. This chapter gives examples of gasifiers, but it is not a comprehensive description of all gasifier technologies.
MOVING BED GASIFIERS: THE LURGI GASIFIER The Lurgi gasifier is the oldest gasifier technology that is still widely used in commercial practice. In the early 1950s, Sasol, a South African firm, acquired the rights to use this gasifier from Lurgi, a German firm. Sasol still uses this gasifier to produce synthetic liquid fuels from coal. Because Sasol has made a number of improvements in the gasifier over the years, the Lurgi gasifier is often called the Sasol-Lurgi gasifier. In 2007, the Sasol-Lurgi gasifier was the most widely used gasifier in the world.1 The Sasol-Lurgi gasifier has been used with a full spectrum of coals, ranging from anthracite to lignite.2 Coal Gasification and Its Applications. ISBN B978-0-8155-2049-8.10004-X, doi:10.1016/B978-0-8155-2049-8.10004-X
Ó 2011 Elsevier Inc. All rights reserved.
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Figure 4.1 shows an illustration of the Sasol-Lurgi gasifier. The gasifier design solves the non-trivial problem of how to feed solid coal into a pressurized vessel, and how to remove solid ash from a pressurized vessel. Coal is fed to an atmospheric pressure bunker above the gasifier. When the bunker is full, a valve on the bottom opens, allowing the coal to drop into a coal lock. The valve then closes, and the coal lock is pressurized until it reaches the gasifier pressure, typically 2.4 to 3.5 MPa. The valve on the bottom of the coal lock opens, and the coal drops into the gasifier. Typically, each batch of coal weighs about 8 tons. Coal is held in the gasifier vessel for about an hour3 while oxygen and steam flow through the grate and into the coal bed. Figure 4.2 shows how the coal and gasses move counter-currently. This makes the Sasol-Lurgi design an especially energy efficient gasifier technology. The highest bed temperature, 615 to 760 C, occurs just above the grate, where the char is gasified. The hot syngas then rises and contacts cooler coal above the gasification zone. In the middle of the bed, rising hot gasses pyrolyze the coal, producing coal tar and char. In the top of the bed, the coolest region, coal is preheated and dried. Syngas and coal tar leave the gasifier at about 370 to 590 C. Compared to other gasification technologies, the operating temperatures in the Sasol-Lurgi gasifier are relatively low. Because of these low temperatures, a refractory lining is not required.
Figure 4.1 The Sasol-Lurgi gasifier. Reprinted by permission from Sasol.
Gasifiers
coal
syngas + coal tar
pre-heating
pyrolysis
gasification (highest temperature)
grate
ash
O2, steam
Figure 4.2 Counter-current gas/solid flow in the Sasol-Lurgi gasifier. The bed is hottest at the bottom, and coolest at the top.
Because the bed must be free-flowing, only non-caking coals can be used. Fine coal cannot be used, because it will plug the interstitial spaces between the large coal particles. The feed coal is sized to about 3 to 30 mm.4 At the Great Plains Synfuels plant in Beulah, North Dakota, fine coal from the grinding circuit is sent to an adjacent pulverized coal plant. The Sasol-Lurgi gasifier produces a considerable quantity of tar, which complicates operations. Hot gasses leaving the reactor are quenched with a recycled water stream. The quench liquid is decanted to produce an organic liquid, which contains the bulk of the tar, and an aqueous layer. The aqueous layer contains water-soluble tar compounds, including phenol and cresylic acid (mixed isomers of methyl phenol). At the Great Plains Synfuels plant, tar is burned to produce steam for plant utilities, and the phenol and cresylic acids are recovered for sale. Newer, higher temperature gasification technologies often compare themselves to the Sasol-Lurgi gasifier and because of their higher operating temperature, little or no tar is produced. This is taken to be an advantage. Before crude oil became inexpensive in the 1950s and 1960s, coal tar was widely processed to make organic chemicals and liquid fuels. With increasing crude oil prices, coal tar may regain its former role as a liquid feedstock, and the tar yield from gasification may be regarded as an advantage, rather than as a disadvantage.
BGL GASIFIER The BGL (British Gas Lurgi) gasifier is a slagging version of the Lurgi gasifier. As shown in Figure 4.3, instead of a grate at the bottom of the gasifier, oxygen and steam are
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Gasifiers
Figure 4.3 The BGL gasifier. This is a slagging version of the Lurgi gasifier, in which oxygen is injected into the slag layer.
injected into the slag layer using tuyeres. A conventional Lurgi coal feed system is used. Un-ground, as-received coal is fed to the gasifier. A stirrer near the top of the bed allows the use of caking coals. Because of the high temperature slag layer, greater than 99% carbon conversion is claimed. A refractory lining is required to withstand the high slag temperatures. In a process design by Bartone and White5, Illinois No. 6 bituminous coal is gasified in a BGL gasifier at about 2.7 MPa. Gas leaves the gasifier at 540 C, and is quenched by direct contact with water to 165 C. The condensed tar stream is about 8 wt.% of the coal feed. This tar stream is recycled back to the gasifier by injecting the tar, along with oxygen and steam, through the tuyeres and into the slag layer. Bartone and White also examined PRB subbituminous coal, and estimated about the same condensed tar yield. Table 4.1 shows the syngas composition estimated by Bartone and White, after quenching and further cooling. Note that the syngas contains 6 to 7 mole% hydrocarbons, primarily methane, which is the uncondensed tar fraction.
Gasifiers
Table 4.1 Estimated syngas compositions for the BGL gasifier after a direct quench and further cooling.5 Coal component Illinois No. 6 mole% Powder River Basin mole%
Ar Benzene, toluene, xylene CH4 C2H4 C2H6 C3H6 C3H8 C4H8 C4H10 CO CO2 COS HCN H2 H2O H2 S N2 Total hydrocarbons
0.0141 0.1033 5.9139 0.0471 0.2343 0.0190 0.0991 0.0147 0.0405 54.2938 4.2867 0.0619 0.0288 29.7464 0.2389 1.1521 3.7083 6.4719
0 0.1064 6.1009 0.1321 0.2415 0.0198 0.1019 0.0148 0.0396 55.9980 4.4245 0.0638 0.0297 30.7551 0.2390 1.1851 0.6398 6.7571
FLUIDIZED BED GASIFIERS: THE WINKLER GASIFIER The Winkler gasifier, commercialized in 1926, was the first industrial application of fluidized bed technology.6 The Winkler gasifier operates near atmospheric pressure, in the bubbling fluidized bed regime. Coal is ground to 0e8 mm. As shown in Figure 4.4, coal is fed to a bunker, and a screw feeder withdraws coal from the bunker and injects it into the bubbling fluidized bed. Since the bed is nearly at atmospheric pressure, a relatively simple coal feeding system can be used. The gas feed to the gasifier, consisting of steam and either air or oxygen, is split into two streams. Most of the gas is fed underneath the grate. This gas fluidizes and reacts with the solid bed. As the coal particles react, they become smaller and less dense. About 30% of the ash falls through the grate and is produced as bottom ash. The remaining 70% of the ash is entrained by the fluidizing gas and is carried into the head space. A major issue with this gasifier is that the entrained ash contains a significant quantity of unreacted carbon. The bubbling bed operates at about 1,000 C, a little below the ash softening temperature. The bed is operated near this upper operating temperature limit to maximize carbon conversion.
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Gasifiers
syngas + fly ash hydrocyclone
steam coal bunker
water freeboard
screw feeder
fines return bubbling fluidized bed
coal steam + air or O2
rotating grate bottom ash
Figure 4.4 The Winkler gasifier.
The remainder of the gas feed is injected into the freeboard. Oxygen reacting with the gas/solid mixture boosts the temperature to about 1,200 C, which partially melts the ash. The increase in temperature further increases the conversion of carbon in the entrained ash. Heat is removed from the top of the gasifier to re-solidify the ash before it leaves the gasifier. A hydrocyclone returns some of the entrained ash to the fluidized bed to further convert residual carbon in the ash. The Winkler gasifier was once widely used, but few, if any, commercial Winkler gasifiers continue to operate. The persistent issue of low carbon conversion appears to be reason for the demise of this gasifier technology.
filter bunker charge bin
gasifier
lock hoppers
compressor
Figure 4.5 Pressurized feed system for the High Temperature Winkler Gasifier.8
Gasifiers
HIGH TEMPERATURE WINKLER GASIFIER The High Temperature Winkler gasifier was developed in the 1970s and 1980s. The primary change to the original design appears to be pressurized operation, at about 1 MPa.7 Pressurized operation increases gasification rates, which should improve carbon conversion. Pressurized operation required a change in the coal feed system, shown in Figure 4.5. Coal is loaded into an atmospheric pressure bunker. One of the two lock hoppers is then depressurized, and coal from the bunker falls into the lock hopper. The lock hopper inlet closes, and the lock hopper is pressurized. The bottom of the lock hopper opens, and coal falls into a line where it is pneumatically conveyed to the filter. Gas from the filter is recycled to the compressor, and coal falls into the charge bin. A solids metering valve on the bottom of the charge bin, which is pressurized, controls the flow of coal into the gasifier. The use of two lock hoppers allows pressurized coal to be continuously fed to the gasifier.
U-GAS GASIFIER The U-Gas gasifier, like the Winkler gasifier, is a bubbling fluidized bed gasifier. Significant differences include pressurized operation, and a conical screen instead of a rotating grate. The simplified flowsheets shown in the open literature do not show oxygen injection into the freeboard space. The gasifier operates at 0.3 to 3 MPa and 840 to 1,100 C. The U-Gas gasifier was developed by the Gas Technology Institute, and commercial licensing rights were acquired by Synthesis Energy Systems.9 At the time this book was written, three commercial U-Gas gasifiers were either operating (Hai Hua in Zaozhuang City) or under construction (Golden Concord in Inner Mongolia and YIMA in Henan Province) in China.10 A 100 ton/day bagasse, sugar cane waste, gasifier was built in Hawaii, and a 150 ton/day wood gasifier was built in Denmark11. Capital construction costs for the U-Gas gasifier are claimed to be lower than most competing gasifier technologies. This is an important consideration because the economic feasibility of most plants based on coal gasification is more strongly influenced by capital costs than by operating costs.
FOSTER-WHEELER PARTIAL GASIFIER The Foster-Wheeler Partial Gasifier,12,13 shown in Figure 4.6, is a fairly straightforward implementation of a circulating fluidized bed reactor. The goals for this gasifier were rather modest. Coal was to be partially gasified, and the remaining char would then be burned in a pulverized coal power plant. Carbon conversions between 45 and 80% were obtained for several bituminous coals. With sub-bituminous Powder River Basin coal, carbon conversions were 80 to 90% due to the higher reactivity of low grade coals.
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Gasifiers
recycle cyclone
syngas cooler
syngas
gasifier body candle filter pre-cleaner cyclone
solids standpipe
char
char
coal, air, steam
Figure 4.6 The Foster-Wheeler Partial Gasifier.
The gasifier body consists of a vertical pipe. Coal, air, and steam are injected into the bottom of the pipe, and the gas velocity is sufficient to carry the solids up. Near the top of the gasifier, the hot gas/solid mixture is sent to the recycle cyclone where coarse solids return to the bottom of the gasifier via a solids standpipe. The gasifier body, recycle cyclone, and solids standpipe are all refractory lined. Recycling the solids in this manner gives unreacted carbon in the coarse solids an additional opportunity to react. The inert fraction of the recycled solids has a thermal inertia role, dampening temperature variations in the gasifier. Gasses and fine solids leaving the recycle cyclone are cooled in the syngas cooler, and then sent to the pre-cleaner cyclone which removes a portion of the solid char. The remaining char is separated from the syngas in a candle filter. The gasifier operates at 0.7 to 0.9 MPa. Bituminous coals were gasified at 995 C to 1,065 C, and the subbituminous PRB coal was gasified at 945 to 995 C. A pilot scale gasifier, 12 m high by 18 cm I.D., was built and tested. No commercial units were built.
KBR TRANSPORT GASIFIER The Transport gasifier,14e19 shown in Figure 4.7, is a circulating fluidized bed gasifier. The main body of the gasifier has two sections, a larger-diameter mixing zone, on the bottom, and a smaller-diameter riser section, on the top. This differs from the FosterWheeler Partial Gasifier, shown in Figure 4.6, which has a constant diameter. The larger
Gasifiers
solids separation unit primary gas cooler riser
syngas
particulate control device
coal from lock hoppers air, O2 ,steam
mixing zone standpipe
Startup burner
ash depressurization
recycle syngas fly ash air, O2 ,steam ash depressurization
bottom ash
Figure 4.7 The Transport gasifier.
diameter of the mixing zone lowers gas velocity, which allows more solids back-mixing and increases solids retention time. The gasifier is preheated with a gas-fired startup burner. The coal is dried sufficiently to remove moisture of the surface of the coal particles, so that the coal will flow readily through the feed system. Low grade coals often contain substantial moisture, much of it absorbed in the interior of coal particles, and this interior moisture does not need to be removed. The coal is ground to 250e600 mm average diameter,17 and fed to the gasifier using a pressurized lock hopper system. Coal is fed near the top of the mixing zone. Gas and reacting solids flow up through the riser. Coarse solids are removed by solids separation unit, and recycled via a standpipe with a J-leg seal on the bottom. Solids in the J-leg are fluidized by a recycle syngas stream.
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The original Transport gasifier design included two solids recycle loops. The first removed coarse solids using a dis-engager, and returned the solids via a standpipe and a J-leg seal. Medium solids were then removed with a cyclone and a short standpipe and loop seal that fed into the dis-engager standpipe. The design was later simplified to a single solids recycle loop. The solids separation unit in the new design appears to be a cyclone. Air or oxygen and steam enter the bottom of the reactor, just below where the recycle solids enter. The concept behind this configuration is that heat for the reactor would be primarily provided by burning carbon in the recycle solids. Analysis of the standpipes solids,14 however shows that the recycle solids generally contain less than 1 wt.% carbon. Temperatures within a gasifier tend to rise rapidly wherever oxygen is injected, because reactions with oxygen are fast and exothermic. As the flowing mixture moves away from the oxygen injection point, the reaction temperature tends to drop because of the endothermic steam and CO2 gasification reactions. A second air or O2 and steam injection point, just below the coal injection point, helps to level the temperature profile in the gasifier. Most gasification tests were air-blown. As shown in Table 4.2, syngas produced during O2-blown tests still contained large quantities of nitrogen due to the nitrogen used in the feed system and in purging instrumentation ports. Gas and fine solids leaving the solids separation unit pass through the primary gas cooler and into the particulate control device, which is a combined cyclone and candle filter. Gas enters the particulate control device tangentially, which throws some of the solids to the vessel wall, where they slide to the bottom. The remaining Table 4.2 Gas feed and product compositions for a Transport gasifier fed PRB coal.14 Gas feed Air-blown, mole% Oxygen-enriched, mole%
Air O2 Steam N2 Riser exit temp, C Pressure, MPa Syngas Ar CH4 C2H6 CO CO2 H2 H2 O N2
58.0 0 7.0 35.0 916 1.55
9.9 13.2 36.4 40.5 907 1.26
0.5 1.1 0.0 7.5 8.5 6.7 10.3 65.4
0.1 1.9 0 7.6 12.8 12.6 27.5 37.4
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solids are removed by banks of sintered metal tube filters in the center of the vessel. Solid filter cakes on the exterior of the tubes are periodically dislodged by backpressure pulses. Like other fluidized bed gasifiers, carbon conversion is generally higher for low grade coals than high grade coals. During air-blown operation, carbon conversion averaged 84% for an Illinois bituminous coal, 90% for Hiawatha bituminous coal, 95% for both PRB subbituminous coal and Freedom lignite, and 97% for Falkirk lignite18. Also like most fluidized bed gasifiers, much of the unconverted carbon can be attributed to fine particles that are quickly swept out of the gasifier. The bottom ash, which is essentially carbon-free, has a mean diameter of about 100 mm.17 The fly ash, on the other hand, has a mean diameter of about 10 mm. During gasification of PRB coal, the fly ash contained 20e40 wt.% unconverted carbon. Table 4.2 shows measured gas compositions from a process development unit14 for a Transport gasifier fed PRB coal operating in both air-blown and oxygen-enriched modes. Levels of C2H6 were not measurable, which shows that very little tar formed at these conditions. Methane, on the other hand, constituted a significant fraction of the combustible gas product. A considerable amount of nitrogen was fed to the gasifier through the coal feed system and by blanketing instrumentation taps. In a full-scale gasifier, the fraction of nitrogen fed to the gasifier would be reduced. In the oxygenenriched mode shown in Table 4.2, the O2/N2 ratio is closer to air than the air-blown mode, because of the effect of the N2 diluent. In the oxygen-enriched mode, a high steam feed rate was used. Due to the relatively low temperature, compared to entrained flow gasifiers, the water gas shift reaction favored conversion of CO and steam to H2 and CO2. The CO2/CO ratio was a relatively high 1.67, and the H2/CO ratio was a relatively high 1.65. A pilot scale Transport gasifier was built at the University of North Dakota. A larger process development facility was built and operated by the Southern Company in Wilsonville, Alabama. A previously-announced commercial IGCC plant19 based on a Transport gasifier was cancelled in 2009. This plant was to be an air-blown unit and fed PRB coal. No carbon capture and sequestration were included. The plant was cancelled due to uncertainties about greenhouse gas regulations. A commercial IGCC plant in Kemper County, Mississippi, using local lignite, will start operations in 2014. A 65% carbon capture and sequestration level is planned for this plant.
ENTRAINED FLOW GASIFIERS: THE GE GASIFIER The GE gasifier, shown in Figure 4.8, was originally developed by Texaco, an oil company that is now part of Chevron. This gasifier is known as the Texaco gasifier in older literature. The first commercial application was as an oil gasifier in 1956. The first commercial Texaco coal gasifier started operation in 1983.
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Figure 4.8 The GE (Texaco) gasifier showing radiant heat recovery system.
The standard method of feeding coal to a high pressure GE gasifier is to finely grind the coal and then mix it with water to form a pumpable slurry. This slurry resembles the heavy oil fed to the first Texaco gasifier. The slurry and oxygen are injected into the top of the gasifier, and the gas/solid/slag mixture flows downward. There are two basic modes of operation. The first is the quench mode, illustrated in Figure 4.9. The hot gas/slag mixture is bubbled through a water bath, which solidifies the slag. The gas is cooled, and boiling water increases the steam content of the syngas. The slurry leaves the gasifier as a slag/water slurry. Note that solids are both fed into the pressurized gasifier, and are removed from the gasifier, as a water slurry. The gasifier is refractory lined. Hot slag slowly attacks the refractory liner, so the liner must be periodically replaced.20e22 This is a common problem in most slagging gasifiers. Figures 4.8 and 4.10 show the radiant heat recovery mode. A longer gasifier body is used, and steam tubes are embedded in the walls in the lower part of the gasifier body.
Gasifiers
O2
coal/water slurry
syngas quench water
slag/water slurry
Figure 4.9 The GE gasifier in quench mode.
coal/water slurry
O2
steam
steam
water
water syngas
quench water
slag/water slurry
Figure 4.10 The GE gasifier in radiant heat recovery mode.
The estimated operating temperatures from Woods et al.23 are a 1,316 C syngas temperature, which is cooled to 593 C by the steam tubes, and further cooled to 210 C in the water quench. The estimated operating pressure is 5.6 MPa, which is a higher pressure than used by most other gasifiers. The radiant heat recovery mode has greater heat recovery, but the quench mode has a much lower capital cost. Frequently, gasification is followed by a water gas shift reactor
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to increase the H2/CO ratio. Steam must be fed to the water gas shift reactor, and the quench mode increases the steam content of the syngas. Table 4.3 shows syngas compositions estimated by Woods et al.23 for the radiant heat recovery version of the GE gasifier when fed Illinois No. 6 coal. Two stream compositions are shown. The “before quench” stream illustrates the effect of gasification conditions on syngas composition. The “after quench” stream shows the gas product from the gasifier, since the quench is an integral part of the gasifier. Argon constitutes about 0.9 mole% of air, and it is a common impurity in oxygen produced by cryogenic air distillation units, so nearly all syngas contains small quantities of argon. At the high gasification temperature, essentially no tar is produced and only a small quantity of methane is produced. Refer to Figure 3.3, which shows the effect of temperature and pressure on the methane steam reforming reaction equilibrium constant. The high gasification temperature also does not favor the conversion of CO and H2O to CO2 and H2 via the water gas shift reaction (see Figure 3.2), so the CO2/CO ratio is 0.44, a relatively low level. The quench, in addition to lowering the syngas temperature, also boosts the H2O syngas concentration from 14 to 27 mole%, which dilutes the other gas components. The coal/water slurry feed technique works well with bituminous coals, but with lower grade coals. The water/coal ratio is often far in excess of optimum due to the high intrinsic moisture in lower grade coals. To illustrate this point, the Wyoming State government commissioned a conceptual design of a 10,000 barrel/day PRB coal to liquids plant from a company called Rentech in 2005. They evaluated four different gasifiers, GE, Shell, ConocoPhillips and Future Energy (now Seimens), to incorporate into the design. Table 4.4 shows the results of the analysis for the four gasifiers. The GE gasifier required the highest coal feed rate, 13,600 ton/day for the GE gasifier compared to only 7650 ton/day for the Future Energy gasifier, because the higher water content of the slurry feed had to be vaporized by coal pyrolysis.
Table 4.3 Estimated syngas compositions23 for the GE gasifier in radiant heat recovery mode when fed Illinois No. 6 coal. Component Before quench, mole% After quench, mole%
Ar CH4 CO CO2 COS H2 H2 O H2 S N2 NH3
0.79 0.10 34.42 15.11 0.02 33.49 14.29 0.73 0.89 0.17
0.67 0.08 29.22 12.76 0.02 28.49 27.26 0.61 0.76 0.14
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Table 4.4 Screening results for a 10,000 barrel/day Fischer-Tropsch coal to liquids plant using Powder River basin sub-bituminous coal. Screening Results - Process Performance & Cost Once-Through FT Operation Producing 10,000 BPD Fuels GE ConocoPhillips Future Energy Shell (“Texaco”) (E-Gas) (GSP) (SCGP)
Coal Feed AR (STPD) Water in Coal AF (wgt.%) Oxygen Contained (STPD) Water Balance (GPM) Raw SG H2/CO ratio (molar) Net Power Export (MW) FT Products (BPD) EPC-CAPEX rating
13,600 55% 11,300 -185 1.29 64 10,000 4
9,150 48% 5,750 -200 0.88 100 10,000 2
7,650 10% 4,250 110 0.40 94 10,000 1
7,850 2% 4,450 200 0.42 81 10,000 3
To address this problem, GE acquired the Stamet pump technology.24,25 The Stamet pump, shown in Figure 4.11, is designed to pump dry solids against a pressure gradient. To do this, the pump uses a solids lock-up mechanism. If a large pressure gradient is applied to an unconfined bed of solids, the solids will fluidize and move towards lower pressure. If those same solids are pressed between two surfaces, then the solids are not free to fluidize, and the solids are in a lock-up state. The coal particles are compressed between pump surfaces and rotated from an atmospheric pressure hopper to a pressurized gasifier. Using this technology, the GE gasifier can be fed a dry coal feed obviating the need for the high water slurry fed operation previously used. GE expects that this technology will overcome its deficiency with respect to low rank, high moisture coals.
Figure 4.11 The Stamet pump is designed to pump dry solids against a pressure gradient.
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steam generator
syngas candle filter char
2nd stage coal/water slurry
1st stage coal/water slurry, O2
1st stage coal/water slurry, O2 quench water slag/water slurry
Figure 4.12 ConocoPhillips E-Gas gasifier.
At the time this book was written, a pilot scale gasifier was under construction in Cheyenne, Wyoming.26 This pilot unit, jointly funded by GE and the State of Wyoming, is primarily intended to demonstrate the gasification of low grade coals using a GE gasifier equipped with a Stamet pump. In 2007, the GE gasifier was the second most commonly installed commercial gasifier1, the most common is the Sasol-Lurgi.
CONOCOPHILLIPS E-GAS GASIFIER The ConocoPhillips E-Gas gasifier was originally developed in the 1970s and 1980s by Dow Chemical.23 At that time, it was known as the Destec gasifier. As discussed in Chapter 3, a gasifier is normally fed a gas mixture consisting of steam and oxygen or air. Although rarely done, CO2 may substitute for all or part of the steam. The basic idea is to feed enough oxygen and steam to gasify all of the coal, and to adjust the oxygen rate to achieve the desired gasification temperature. An increase in the oxygen/steam ratio will increase the gasification temperature, but the syngas will have a lower heat of combustion. Entrained flow gasifiers operate at relatively high temperatures, which require relatively high oxygen/steam ratios. The basic concept of the E-gas gasifier is to use the high temperature syngas generated at slagging conditions to gasify coal injected at a second point. This more efficiently uses the heat generated by reactions with O2. As shown in Figure 4.12, a coal/water slurry and oxygen are injected into the high temperature first stage, operating at 1,316 to 1,427 C and 4.2 MPa.23 Slag formed at these conditions drops to
Gasifiers
the bottom of the gasifier, where it is quenched with water and withdrawn as a slag/water slurry. Hot syngas rises, passing through a restriction and into the second stage of the gasifier. A second coal/water slurry stream is injected at the inlet of the second stage; and the hot syngas provides heat for the endothermic pyrolysis, steam gasification, and CO2 gasification reactions. Gas and solids leave the second stage at about 1,010 C, which is below the ash softening temperature.23 Gasification of the second stage coal feed is incomplete, so the gas is cooled in a steam generator, the solid char is removed by a candle filter; and the char is returned to the first stage. The gasifier is refractory lined. The cold gas efficiency of the gasifier increases with increasing second stage slurry feed, and oxygen demand declines.27 At the Wabash River Coal Gasification Repowering Project,28 a commercial scale IGCC demonstration plant, the typical coal slurry feed split is 85% to the first stage and 15% to the second stage.27 Rutkowski et al.29 compared the cost of producing hydrogen from Pittsburgh No. 8 bituminous coal to PRB sub-bituminous coal. They chose an E-Gas gasifier, because this is one of the few gasifiers that has been demonstrated using both bituminous and sub-bituminous coals. A key assumption in their study was that both cases would have the same coal feed rate on a dry basis. They concluded that Pittsburgh No. 8 was the preferred feed, even though this coal is considerably more expensive than PRB. The cost of hydrogen was dominated by capital costs is the primary reason for the expense. In both cases, the capital cost was about the same. Pittsburgh No. 8, with its higher heating value, yielded more hydrogen product, which diluted the capital cost per unit of hydrogen product. Herbanek et al.,30 however, stated that lower grade coals, due to their higher reactivity, could be fed at higher rates to the E-Gas gasifier. They compared the costs of electricity from an IGCC plant and found that sub-bituminous PRB coal had a lower production cost than three bituminous coals. Table 4.5 shows syngas compositions estimated by Rutkowski et al. for Pittsburgh No. 8 and PRB coals. The relatively low temperature second stage boosts the methane concentration compared to the GE gasifier (see Table 4.3); although it is not obvious why the methane content differs for Pittsburgh No. 8 and PRB coals. The moisture content of the PRB coal, 26.6 wt.%, is much higher than the moisture content of the Pittsburgh No. 8 coal, 6.0%. This leads to higher water content in the PRB-derived syngas, which dilutes the other components. The higher water content in the PRBderived syngas and the relatively low temperature second stage also favors the conversion of CO and H2O to CO2 and H2 via the water gas shift reaction, so the PRB-derived syngas has a higher CO2/CO ratio than the Pittsburgh No. 8 derived syngas. The sulfur content of Pittsburgh No. 8 is much higher than PRB, 3.07 versus 0.82 wt.%, both dry basis, and this causes higher levels of H2S and COS in the syngas. In 2007, the ConocoPhillips E-Gas gasifier was the fourth most commonly installed gasifier.1
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Table 4.5 Estimated syngas compositions29 for the ConocoPhillips E-Gas gasifier. Pittsburgh No. 8 Powder River Basin Coal component (bituminous), mole% (sub-bituminous), mole%
Ar CH4 CO CO2 COS H2 H2O H2 S N2 NH3
0.82 0.42 41.95 9.75 0.04 33.20 12.19 0.78 0.57 0.28
0.75 0.17 26.61 16.04 0.01 28.34 27.17 0.19 0.52 0.21
SHELL GASIFIER Shell developed oil gasifiers in the 1950s. These units differ substantially from Shell’s coal gasifier, which was developed jointly23 with Krupp-Koppers from 1974 to 1981. After 1981, Krupp-Koppers offered a similar gasifier known as Prenflo.31 recycle compressor
quench gas gasifier
coal
fly ash
steam
steam syngas cooler
N2
grinding, drying, & feeding
boiler feed water boiler feed water
O2 slag
dry solids removal
fly ash (recycled)
Figure 4.13 The Shell coal gasifier.
syngas
Gasifiers
The Shell coal gasifier23,32 is shown in Figure 4.13. The gasifier features a dry feed system. Coal is ground and dried, and N2 is used to pressurize the feed hoppers to the gasifier operating pressure, about 4.2 MPa. Coal is fed to opposite sides of the gasifier, near the bottom. Oxygen is fed to the gasifier below the coal injection points. Slag flows out the bottom. The hot gas/solid/slag mixture flows up through the gasifier at about 1,600 C. Near the top of the gasifier, a relatively cold recycle syngas stream, at about 200 C, is injected into the gasifier. This is shown as quench gas in Figure 4.13. The quench gas drops the syngas mixture to about 900 C. The steam boiler tubes in the syngas cooler cannot withstand the 1,600 C temperature of the unquenched syngas, so quenching drops the temperature to a tolerable level. The syngas cooler is taller than the gasifier. Shell also developed a water quench system to replace the syngas cooler.33 The fly ash contains unconverted carbon. This is recycled to the coal feed system. The Shell gasifier features a membrane wall, shown in Figure 4.14, rather than a refractory lining. The gasifier wall is cooled by boiling water to generate steam. A layer of solidified slag forms on the boiler tubes, and this solidified slag layer effectively functions like a refractory layer. Since the solid slag layer is the same composition as the liquid slag, the liquid slag does not attack the solid layer. Table 4.6 shows an estimated syngas composition for the Shell gasifier fed Illinois No. 6 bituminous coal. Because of the high gasification temperature and the lack of water fed to the gasifier, the syngas has very little methane and a high CO/CO2 ratio. In 2007, the Shell coal gasifier was the third most commonly installed gasifier, and the most popular gasifier for newly planned installations.1
SIEMENS GASIFIER Development of the Siemens gasifier started in the 1970s, and the first commercial plant was built in 1984. In 2006, Siemens acquired the gasifier technology from Future Energy.34,35 steam tube gasifier wall
atmosphere
solidified slag molten slag
hot syngas
Figure 4.14 Gasifier membrane wall. Solidified slag replaces refractory lining.
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Table 4.6 Estimated syngas composition23 for the Shell gasifier fed Illinois No. 6 coal. Component mole%
Ar CH4 CO CO2 COS H2 H2 O H2 S N2 NH3
0.97 0.04 57.16 2.11 0.07 29.01 3.64 0.81 5.85 0.33
Siemens, like GE, offers gasifiers, gas turbines, and steam turbines, all important components of an IGCC plant.36 The gasifier, shown in Figure 4.15, is also roughly similar to a GE gasifier in quench mode, shown in Figure 4.9. There are, however, significant differences. The Siemens gasifier features a dry feed system, using a lock hopper arraignment similar to that shown in Figure 4.5.37 The hoppers are pressurized with N2 or CO2 to the gasifier pressure, about 2.8 MPa.34,36 A gas fuel is used to preheat the gasifier. Coal and O2 are fed to the top of the gasifier, which operates at about 1,400 C. The gasifier is lined with a membrane wall instead of a refractory. Siemens also offers a refractory-lined version of their gasifier for low ash fuels. A wide variety of fuels have been gasified.38 Like the GE gasifier, the Siemens gasifier uses a water quench to solidify the slag, and the slag is removed as a slag/water slurry. The quench arraignment differs between the two gasifiers. In the GE gasifier, the syngas/slag mixture is blown into a water bath. In the Siemens gasifier, a water spray quenches the syngas/slag mixture.
MITSUBISHI HEAVY INDUSTRIES (MHI) GASIFIER Development of the MHI gasifier started in the 1980s.39 A 1,700 T/D demonstration plant in Nakoso, Japan started operations in 2007. A wide range of coals, including both bituminous and sub-bituminous coals, have been successfully gasified. The basic concept of the MHI gasifier is similar to the E-gas gasifier. The gasifier, shown in Figure 4.16, uses a split coal feed. Coal and air are fed to the bottom stage; and the rising, hot syngas is used to gasify a second coal feed stream. In the E-gas gasifier, these stages are known as the first and second stages. The equivalent nomenclatures in the MHI gasifier are the combustor and reductor stages.40 Endothermic reactions in the reductor section lower the temperature from the slagging condition to about 700 C, well below the ash softening temperature. Char is separated from syngas leaving the reductor stage and returned to the combustor stage.
Gasifiers
Figure 4.15 Siemens gasifier.
The gasifier is air-blown. Most gasification plants that employ carbon capture and sequestration are oxygen-blown. Mitsubishi claims that the capital cost of their air-blown system is nearly the same as an oxygen-blown system, and that their efficiency is a little higher.41 The syngas contains about 30% CO and 10% H2. An oxygen-blown version of the MHI gasifier is being developed.42 The MHI gasifier uses a dry feed system and a membrane wall. Slag is removed from the bottom of the gasifier as a slag/water slurry. A very similar gasifier was developed by the Thermal Power Research Institute of China.43
PRATT AND WHITNEY ROCKETDYNE (PWR) GASIFIER The Pratt and Whitney Rocketdyne (PWR) compact gasifier, shown in Figure 4.17, was inspired by rocket engine technology. This gasifier was undergoing pilot scale development44 at the time this book was written. Commercial licenses are marketed by Zero Emissions Energy Plants (ZEEP).45
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syngas
reductor
char coal
combustor
air
slag water
slag/water slurry
Figure 4.16 Mitsubishi Heavy Industries (MHI) gasifier.
Coal, O2
rapid mix injector
cooled membrane wall
rapid spray quench
syngas/slag
Figure 4.17 The Pratt and Whitney Rocketdyne Compact gasifier.
Gasifiers
Pulverized coal and oxygen are split into multiple streams and fed to a rapid mix injector, which is then cooled. Tested injector pressures range from 1.5 to 6.7 MPa.46 Flame temperatures can reach 2,760 C, an extremely high temperature.47 The gasifier wall is cooled to keep the metal temperature below 540 C. Figure 4.18 shows a more complete PWR gasifier system. The feeding system features a dry solids pump,48 shown in Figure 4.19, similar in concept to the Stamet pump shown in Figure 4.11. The PWR solids pump features two opposed rotating belts. Coal particles are squeezed between the belts to achieve solids lock-up, allowing the solids to be pushed against a pressure gradient. Gas and solid slag leaving the gasifier first enter a hydrocyclone and then a bank of tube filters to remove the solid slag. A peer review49 of the PWR gasifier technology in 2006 identified the dry solids pump and the rapid mix injector as key items that require further technical development. Matuszewski et al.50 compared the PWR gasifier to the GE and Shell gasifiers and concluded that the PWR gasifier is both more efficient and less expensive. These results should be greeted with cautious skepticism, as Matuszewski et al. based their estimates on projected results from Pratt and Whiney for technology that had yet to be demonstrated. Still, such results encourage further development. Pratt and Whitney claims that their gasifier is only about 10% of the size of comparable commercial gasifiers, which should lessen capital costs. The high efficiency claims for the PWR gasifier are a little surprising when one considers the very high temperature and rapid gasification conditions. Normally, high low pressure hopper
quench
syngas hydrocylcone tube filter bank
dry solids pump gasifier
high pressure hopper
coarse slag
fine slag
Figure 4.18 Pratt and Whitney Rocketdyne (PWR) Compact Gasifier System.
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Figure 4.19 The PWR dry solids feed pump.48 Coal is fed from a low pressure hopper above the pump, and discharged to a high pressure hopper below the pump.
efficiency processes require relatively low temperatures and a close approach to equilibrium, which lowers the driving forces, and leads to slow reactions. The high efficiency of the PWR gasifier may be due to the quench. The high temperature gasses produced by the upper part of the gasifier drive steam gasification reactions in the lower part of the gasifier; and the steam/solid slag stream leaves the gasifier at a relatively low temperature.
LESS CONVENTIONAL GASIFIERS: THE ALTER NRG PLASMA GASIFICATION SYSTEM The Alter NRG plasma gasifier51,52 uses a plasma torch to gasify a solid feedstock. This gasifier can be used to gasify coal, but it is especially attractive for difficult-to-gasify feedstocks, such as municipal solid waste (MSW). The largest current installation of the gasifier is the Eco-Valley Waste-to-Energy Facility in Utashinai, Japan, which gasifies 180 T/day of MSW and automotive shredder waste. At the time this book was written, a 120 MW coal power plant retrofit in Somerset, Massachusetts was planned,53 as well as a 40,000 BBL/day coal-to-liquids plant in Fox Creek, Alberta. Coskata54 built a semicommercial facility that gasifies wood chips using the Alter NRG plasma gasifier. The syngas is converted to ethanol in a bioreactor.
Gasifiers
Figure 4.20 Westinghouse Plasma Corporation torch used in the Alter NRG Plasma gasifier.55
The heart of the process is a Westinghouse Plasma Corporation55 torch shown in Figure 4.20. Westinghouse Plasma Corporation is a subsidiary of Alter NRG. A plasma is an ionized gas and, in theory, a plasma can be made from any gas. Air appears to be the plasma feed gas for most gasifier applications.
Figure 4.21 Alter NRG Plasma gasifier.51
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The plasma gasifier is shown in Figure 4.21. Solid feed enters through the side of the gasifier, and plasma torches are directed towards a moving bed of reacting solids. Near the plasma torch, temperatures can be as high as 3,000 C. Molten metal and slag leave the bottom of the reactor. Slagging operation is attractive for an MSW application, because toxic metals in the slag are essentially unleachable. Gases rise through the gasifier and enter an expanded freeboard zone, which allows coarser solids to fall back into the bed. The gas leaving the gasifier is about 900 to 1,000 C, which is hot enough to avoid tar. The heat of combustion of the syngas is about 80% of the feed.53 The syngas product can be burned to generate electricity. Electric power for the plasma torches is only about 2 to 5% of the feedstock energy input.
REFERENCES 1. Gasification World Database 2007: Current industry status. U.S. Dept. of Energy, Office of Fossil Energy, National Energy Technology Laboratory; 2007. 2. van de Venter E. Sasol-Lurgi coal gasification and low rank coal, Gasification Technologies Council 2005 Annual Conference. 3. Howard-Smith I, Werner GJ. Coal Conversion Technology. Noyes Data Corp; 1976. 4. Krichko AA. Theoretical basis of coal gasification. In: Oil and Gases from Coal. Pergamon Press for the United Nations; 1980. p. 89-124. 5. Bartone LM Jr, White J. Industrial size gasification for syngas, substitute natural gas, and power production, DOE/NETL-401/040607,
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18. 19. 20. 21. 22. 23.
24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
Systems, June 10e15, 2007,
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39. Sakamoto K. Commercialization of IGCC/gasification technology for the US Market, Gasification Technologies Council 2008 Annual Conference. 40. Sakamoto K. Development of Mitsubishi air-blown IGCC technology with carbon capture, Gasification Technologies Council 2008 Annual Conference. 41. Fujii T. Deployment of IGCC technology with carbon capture, Sixth Annual Conference on Carbon Capture and Sequestration; 2007. 42. Sakamoto K. Mitsubishi IGCC project updates, Gasification Technologies Council 2009 Annual Conference. 43. Shisen X. GreenGendnear zero emission coal based power demonstration project in China, Gasification Technologies Council 2009 Annual Conference. 44. Darby A, Hartung J. Compact gasification system development status, Gasification Technologies Council 2009 Annual Conference. 45. Bernard B. Gasification for the next generation: Zero emissions energy plants, Gasification Technologies Council 2009 Annual Conference. 46. Hartung J, Darby A. Pratt and Whitney Rocketdyne (PWR) compact gasification system, Gasification Technologies Council 2007 Annual Conference. 47. Darby A. Status of the Pratt & Whitney Rocketdyne/DOE advanced single-stage gasifier development program, Gasification Technologies Council 2005 Annual Conference. 48. Sprouse KM, Matthews DR, Saunders T, Weber GF. PWR dry feed development status, 2008 International Pittsburgh Coal Conference. 49. Clayton S, Powell C, Rath L, Keairns D, Gray D, Geertsema A. et al. PWR gasifier peer review,