1984

Vacuum residue/ammonia

Note: F-T, Fischer-Tropsch synthesis; SNG, synthetic natural gas.

Source: SFA Pacific, Inc., and U.S. DOE, Gasification: Worldwide Use and Acceptance, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., January 2000.

developed a slagging version of the gasifier, referred to as the BGL gasifier, in which the temperature at the bottom of the gasifier is sufficient for the ash to melt.

Lurgi Gasifier The most successful fixed-bed gasifier is the Lurgi gasifier, which was developed in Germany during the 1930s as a means to produce town gas. The first commercial plant was built in 1936. It was initially used for lignites, but process developments in the 1950s allowed for the use of bituminous coals as well. The Lurgi gasification process has been used extensively worldwide.

The Lurgi dry-ash gasifier, shown schematically in Figure 5-28, is a pressurized gasifier typically operating at 30 to 35 atm. Sized coal enters the top of the gasifier through a lock hopper and moves down through the bed. Steam and oxygen enter at the bottom and react with the coal as the gases move up the bed. Ash is removed at the bottom of the gasifier by a rotating grate and lock hopper and is kept in a dry state through the injection of steam

Distributor Drive

Distributor Drive

Grate

Tar and Water

Grate

Water Jacket

FIGURE 5-28. Schematic diagram of a modern Lurgi dry-ash gasifier. (From Berkowitz, N., An Introduction to Coal Technology, Academic Press, New York, 1979. With permission.)

Tar and Water

Grate Drive

Water Jacket

Steam and Oxygen

FIGURE 5-28. Schematic diagram of a modern Lurgi dry-ash gasifier. (From Berkowitz, N., An Introduction to Coal Technology, Academic Press, New York, 1979. With permission.)

to cool the bed below the ash fusion point. As the coal moves down the gasi-fier it goes through sequential stages of drying and devolatilization with the resultant char undergoing gasification and combustion. The countercurrent operation results in a temperature drop in the gasifier. Gas temperatures are approximately 500 to 1000° F in the drying and devolatilization zone, 1800° F in the gasification zone, and 2000°F in the combustion zone [22,44]. The raw syngas, a mixture of carbon monoxide and hydrogen, which also contains tar, exits the gasifier at 570 to 930°F.

BGL Gasifier The BGL fixed-bed gasifier was developed in the 1970s to provide a syngas with a high methane content in order to provide an efficient means of manufacturing SNG from coal. The BGL fixed-bed gasifier, shown schematically in Figure 5-29, is a dry-feed, pressurized, slagging gasi-fier. The operational concept is similar to the Lurgi dry-ash gasifier with two notable differences. The BGL fixed-bed gasifier is more fuel flexible in that it can use run-of-mine coal (rather than sized coal), and the gasifier is operated at temperatures above the ash fusion point to form a liquid slag.

Coal or Hydrocarbon

Coal or Hydrocarbon

Oxygen

Oils/Tars Liquors

Slag Quench Chamber

Lock Hopper

Oxygen

Oils/Tars Liquors

Slag Quench Chamber

Lock Hopper

FIGURE 5-29. Schematic diagram of the BGL fixed-bed gasifier. (Adapted from Ratafia-Brown, J. et al. [44].)

Slag is withdrawn from the slag pool through an opening in the grate. The slag flows into a quench chamber and lock hopper in series. Syngas exits the gasifier at ~1040°F and passes into a water quench vessel and a boiler feedwater preheater designed to lower the gas temperature to approximately 300°F. Soluble hydrocarbons, such as tars, oils, and naphtha, are recovered from the aqueous liquor in a gas-liquor separation unit and recycled to the gasifier [44].

Fluidized-Bed Gasifiers

Fluidized-bed gasifiers may differ in ash conditions, dry or agglomerating, and in design configurations for improving char use. Commercial versions of this type of gasifier include the high-temperature Winkler (HTW) and Kellogg-Rust-Westinghouse (KRW) designs.

HTW Gasifier The high-temperature Winkler (HTW) gasifier, shown schematically in Figure 5-30, is a dry-feed, pressurized, fluidized-bed, dry-ash gasifier. The HTW process was developed by Rheinbraun in Germany during the 1920s to utilize coal with a small particle size and too friable for use in existing fixed-bed gasifiers. The HTW technology is capable of gasifying a variety of feedstocks, including reactive low-rank coals with a higher

Feed

Feed

FIGURE 5-30. Schematic diagram of an HTW fluidized-bed gasifier. (From Ratafia-Brown, J. et al., Major Environmental Aspects of Gasification-Based Power Generation Technologies, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., December 2002.)

ash-softening temperature and reactive caking and noncaking bituminous coals. Coal with a particle size less 1/8 in. is fed into the gasifier using a screw feeder. The upward flow of the gasifying medium, air or oxygen, keeps the particles of coal, ash, and semi-coke/char in a fluidized state. Gas and elutriated solids flow up the gasifier, and additional air or oxygen is added in this region to complete the gasification reactions. Fine ash particles and char that are entrained in the gas are removed in a cyclone and recycled to the gasifier. Ash is removed from the base of the gasifier by means of an ash screw. The syngas exiting the gasifier is at a high temperature so it does not contain any high-molecular-weight hydrocarbons such as tars, phenols, and other substituted aromatic compounds [44]. The gasifier fluid bed is operated at about 1470 to 1650°F, and the temperature is controlled to ensure that it does not exceed the ash-softening point. The temperature in the freeboard can be significantly higher, up to 2000°F. The operating pressure can vary between 145 psig for syngas manufacture and 360 to 435 psig for an IGCC application [44].

KRW Gasifier The Kellogg-Rust-Westinghouse gasifier, shown schematically in Figure 5-31, is a pressurized, dry feed, fluidized-bed, slagging gasifier

Synthesis Gas

Coal & Limestone "I. Feedstock_J

FIGURE 5-31. Schematic diagram of the KRW gasifier. (From Ratafia-Brown, J. et al., Major Environmental Aspects of Gasification-Based Power Generation Technologies, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., December 2002.)

Coal & Limestone "I. Feedstock_J

FIGURE 5-31. Schematic diagram of the KRW gasifier. (From Ratafia-Brown, J. et al., Major Environmental Aspects of Gasification-Based Power Generation Technologies, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., December 2002.)

developed by the M.W. Kellogg Company. The KRW gasifier is capable of gasifying all types of coals, including high-sulfur, high-ash, low-rank, and high-swelling coals. Coal and limestone, crushed to less than 1/4 in., are fed into the bottom of the gasifier, and air or oxygen enters through concentric, high-velocity jets [44]. This process ensures thorough mixing of the fuel and air or oxygen. The coal immediately releases its volatile matter upon entering the gasifier, and it oxidizes rapidly to produce the heat for the gasification reactions. An internal recirculation zone is established, with the coal/char moving down the sides of the gasifier and back into the central jet. Steam that is introduced with the air or oxygen and through jets in the side of the gasifier reacts with the char to form the syngas. Fine ash particles that are carried out of the bed are captured in a high-efficiency cyclone and reinjected into the gasifier. The internal recycling of the larger char particles results in the char becoming enriched in ash, and the low-melting components of the ash cause the ash particles to agglomerate. As the ash particles become larger, they begin to migrate toward the bottom of the gasifier where they are removed along with spent sorbent (i.e., limestone that has reacted with sulfur to form calcium sulfide (CaS)) and some unreacted char. The ash, char, and spent sorbent flow into a fluidized-bed sulfator, where the char and calcium sulfide are oxidized. The calcium sulfide forms calcium sulfate, CaSO4, which is chemically stable and can be disposed of in a landfill.

Entrained-Flow Gasifiers

Differences among entrained-flow gasifiers include the coal feed systems (coal-water slurry or dry coal), internal designs to handle the very hot reaction mixture, and heat-recovery configurations. Entrained-flow gasifiers have been selected for nearly all the coal-based IGCCs currently in operation or under construction [44]. The major commercial entrained-flow gasifiers include the ChevronTexaco, Shell, Prenflo, and E-Gas gasifiers. Of these, the ChevronTexaco gasifier and the Shell gasifier technologies are in use in over 100 units worldwide [44].

ChevronTexaco Gasifier The ChevronTexaco gasifier, shown schematically in Figure 5-32, is a single-stage, down-fired, entrained-flow gasifier [44]. A fuel-water slurry (e.g., 60-70% coal) and 95% pure oxygen are fed to the pressurized gasifier. The coal and oxygen react exothermally at a temperature ranging from 2200 to 2700°F and a pressure greater than 20 atm to produce syngas and molten ash. Operation at the high pressures eliminates the production of hydrocarbon gases and liquids in the syngas. The hot gases are cooled using either a radiant syngas cooler located inside the gasifier to produce high pressure steam or an exit gas quench. Slag drops into the water pool at the bottom of the gasifier, is quenched and separated from the blackwater, and is removed through a lockhopper. The ChevronTexaco technology has operated commercially for over 40 years with feedstocks such as natural gas,

Coal Slurry Oxygen

Feed Water

Water

Steam

Synthesis Gas .

Slag

FIGURE 5-32. Schematic diagram of the ChevronTexaco gasifier. (From Ratafia-Brown, J. et al., Major Environmental Aspects of Gasification-Based Power Generation Technologies, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., December 2002.)

heavy oil, coal, and petroleum coke. Currently, 60 commercial plants are in operation, with 12 using coke and coal, 28 using oil, and 20 using a gas feedstock [44].

Shell Gasifier The Shell gasifier, the successor to the Koppers-Totzek process and then the Shell-Koppers process, is shown schematically in Figure 5-33 and is a dry-feed, pressurized, entrained-flow, slagging gasifier that can operate on a wide variety of feedstocks. Pulverized coal is pressurized in lock hoppers and fed to the gasifier by dense-phase conveying with transport gas, which can be nitrogen or syngas [44]. Preheated oxygen is mixed with steam and used as a temperature moderator prior to feeding to the fuel injector. Temperatures and pressures in the gasifier are 2700 to 2900°F and 350 to 650 psig, respectively. A syngas is produced that contains mainly hydrogen and carbon monoxide with little carbon dioxide. Elevated temperatures eliminate the production of hydrocarbon gases and liquids in the product gas. The high temperature converts the ash into molten slag, which runs down the refractory walls into a water bath, where it is quenched and the ash/water slurry is removed through a lock hopper. The raw gas leaving the gasifier at 2500 to 3000° F contains a small quantity of char and about half of the molten ash. The hot gas is partially cooled to temperatures below

Quench

Quench

FIGURE 5-33. Schematic diagram of the Shell gasifier. (From Ratafia-Brown, J. et al., Major Environmental Aspects of Gasification-Based Power Generation Technologies, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C., December 2002.)

the ash fusion point by quenching it after it leaves the gasifier. The syngas undergoes further cooling before the particles are removed in a wet scrubber. The first Shell gasification process units were commissioned in the 1950s. Shell started development work with coal in 1972.

Prenflo Gasifier The Prenflo gasifier, developed by Uhde (formerly Krupp Uhde) and shown schematically in Figure 5-34, is a pressurized, dry-feed, entrained-flow, slagging gasifier [44]. Coal, ground to ~100 ^m, is pneumatically conveyed by nitrogen to the gasifier. The coal is fed through injectors located in the lower part of the gasifier with oxygen and steam. Syngas, produced at temperatures of ~2900°F, is quenched with recycled cleaned syngas to reduce its temperature to ~1500°F in an internal syngas cooler. The syngas is further cooled to ~700°F through evaporator stages before exiting the gasifier. The molten slag flows down the walls into a water bath, where it is quenched and granulated before removal through a lock hopper system.

E-Gas Gasifier The E-Gas gasifier, shown schematically in Figure 5-35, is a slurry-feed, pressurized, entrained-flow gasifier. It is an upward flow gasifier with two-stage operation. The coal is slurried via wet crushing, with coal concentrations ranging from 50 to 70 wt.%, and about 75% of the total slurry feed is fed to the first stage of the gasifier, which operates at 2600°F and

Steam

0 0

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