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Cleaning up coal's act.

Destec Energy Inc. (Houston) has developed a coal gasification process that it claims will reduce dependence on imported fuels and meet the tougher standards of the new Clean Air Act to boot.

This assertion is based on the performance of the world's only commercial-scale coal gasification plant, operated by Destec's subsidiary, Louisiana Gasification Technology Inc. (Plaquemine, La.). (Destec itself is a wholly owned subsidiary of Dow Chemical Co. of Midland, Mich.)

The plant consumes 2400 tons of coal daily to produce 161 megawatts of electricity for Dow's 1400-acre chemical manufacturing complex at Plaquemine. Destec's coal gasification combined-cycle process involves putting coal and water into a rod mill that will crush the coal and form a slurry for continuous feeding. This eliminates the need for drying processes and lockhoppers prone to malfunction.

About 80 percent of the slurry is mixed with oxygen in burner nozzles and injected into the first stage of a two-stage gasifier. A temperature of 2600 [degrees] F in the first stage melts the coal ash so that it flows out a taphole in the gasifier's bottom. Quenching with water turns the ash into slag. After the slag is removed, crushed, and dried, it is used by Dow for road and railroad bed construction. The state of Louisiana is also testing the slag for use in road resurfacing.

The carbon in the coal reacts to form a synthesis gas, or syngas, made of hydrogen sulfide and carbonyl sulfide. Hot syngas flows into the second stage of the gasifier, where the remaining 20 percent of the slurry is injected to cool the gases by forming more syngas, making the process more efficient.

The syngas is piped to a heat recovery unit or boiler that yields superheated 750 [degrees] F steam fed to turbines to create additional electricity. A wet particulate scrubber removes particles for recycling in the gasifier. Water containing ammonia, carbon dioxide, carbon monoxide, and other dissolved gases is collected and treated while the syngas passes through a series of heat exchangers that cool it down to 120 [degrees] F.

An acid gas stripper removes the hydrogen sulfide from the syngas, leaving a "sweentened" syngas that can be combusted in gas turbines to generate electricity.

The stripped hydrogen sulfide is converted to 99.7 percent pure elemental sulfur via catalytic oxidation. it is sold to manufacturers who use sulfuric acid in applications such as agricultural fertilizers.

While these waste disposal advantages are assets, Destec stresses that emission reductions are the major selling point for the process. Under phase I of the acid rain provisions of the act, sodium dioxide emissions from coal-fired power plants will be capped at no more than 2.5 pounds per million British thermal units by 1995. Phase II calls for reductions to 1.2 pounds per million Btus by the year 2000. The new Louisiana plant produces only 0.08 pounds per million Btus of [SO.sub.2].

Candidates for future use of the coal gasification process, which can be adapted for use in existing coal-fired plants, may come from the approximately 110 power plants in the United States that will be affected by phase I of the Clean Air Act.

On the economics front, Destec is citing a study published by the Electric Power Research Institute (Palo Alto, Calif.) in 1989, stating that the Plaquemine plant required $1201 of capital investment to produce a kilowatt of electricity, while competing coal-burning technologies using pulverized coal and wet limestone required $1281; pulverized coal with advanced flue gas desulfurization required $1537; and atmospheric fluidized bed processes required $1603.
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Title Annotation:Tech News; new coal gasification process
Publication:Mechanical Engineering-CIME
Date:Apr 1, 1991
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