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NFCRC Tutorial: Gasification

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Gasification or partial oxidation in power generation applications consists of converting a fuel that is often "dirty" (such as coal, refinery residues and biomass) and cannot be directly used in an engine or a fuel cell, to a clean gaseous fuel which meets the engine or fuel cell specifications as well as the environmental emissions standards.

The partial oxidation reaction for carbon is: C + 1/2 O2 = CO. This reaction is exothermic and thus, water is introduced into the gasifier in the from of steam or liquid water to moderate the temperature of the reaction by the endothermic reaction: C + H2O = H2 + CO. Other reactions that occur within a gasifier are the shift reaction: CO + H2O = H2 + CO2 and the hydrogasification reaction: C + 2H2 = CH4. The sulfur present in the feed is evolved mostly as H2S with some as COS. CS2 and mercaptans are insignificant when the gasification reaction occurs at high temperature (>2000 deg F or > 1090 deg C).

There are three major types of gasifiers: The moving bed gasifier as exemplified by the Lurgi gasifier consists of introducing coarse solids at the top of the gasifier while the oxidant and steam are introduced at the bottom of the gasifier. The gasifier may be divided into four distinct zones: (1) the top being the drying/preheating zone followed by (2) the devolatilization zone followed by (3) the gasification zone and at the bottom (4) the combustion zone. The gases that leave at the top of the gasifier contain H2, CO, CO2, H2O, CH4 and other hydrocarbons including oils and tars as well as other organic compounds, sulfur compounds such as H2S, COS, some CS2 and mercaptans, nitrogen compounds such as NH3 and HCN. The tars also contain some sulfur and nitrogen. The thermal efficiency of the gasifier is high but since it produces the tars and oils, the gas clean up is more complex, and the gasifier cannot handle fines in the feed. The British Gas Lurgi gasifier which is similar to the original Lurgi in many respects, however, recycles the tars and oils separated from the gas to the gasifier by introducing these components in to the bottom section of the gasifier along with fines. The fluidized bed gasifier as exemplified by the Kellog Rust Westinghouse gasifier consists of introducing dried feed that is typically less than 1/4 inch in size, at the bottom of the gasifier and accomplishing devolatilization and gasification in the bed fluidized by the oxidant and steam. Recycle synthesis gas may also be used to maintain the required gas velocity for fluidization of the bed material. The bottom of the gasifier where oxidizing conditions prevail, the temperature is high enough to fuse the ash particle together to form agglomerates while in the top section of the gasifier where gasification reactions predominate, the temperature is in the neighborhood of 1800 to 1900 deg F or 980 to 1040 deg C. The gases leaving the gasifier are essentially free of hydrocarbons heavier than CH4. The entrained bed gasifier as exemplified by the Noell, Texaco and Shell gasifiers consists of introducing finely ground feed in the case of solids, or liquid feed into the gasifier along with the oxidant and steam or liquid water as the moderator. In the case of the Texaco gasifier, the solids are introduced in the form of a water slurry while in the case of the Noell and Shell gasifiers, the solid feed is fed dry and requires drying prior to grinding the solids and steam as the temperature moderator. The gasification occurs at temperatures typically in excess of 2200 deg F or 1200 deg C with the ash forming a slag while the gases leaving the gasifier are free of any hydrocarbons, especially those heavier than CH4.

The raw gas leaving a gasifier is cooled either by a heat exchanger while generating steam or is directly quenched with water if the gas contains particluates that are in the molten or semi-molten state. The cooled gas is purified by further treatment to remove the particulates, alkalis, chlorides as well as the nitrogen and sulfur compounds. An advantage with gasification is the sulfur may be recovered as salable elemental sulfur.

Technologies are currently (1997) in the demonstration phase to clean gas without completely cooling it. The gas is kept above the dew point of water which eliminates the extensive waste water treatment typically required by gasification using "cold gas cleanup."

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