Coal gasifier designs are almost as numerous as the many different types and ranks of coal. The basic configurations, hardware, and operations that have been considered are described here because several of them are applicable to biomass gasification (cf. National Academy of Engineering, 1973, and accompa­nying references).

Modern coal gasification processes consist of a sequence of operations: coal crushing, grinding, drying, and pretreatment, if necessary; feeding the coal into the gasifier; contacting the coal with the reacting gases for the required time in the gasifier at the required temperature and pressure; removing and separating the solid, liquid, and gaseous products; and treating the products downstream to upgrade them and to stabilize and dispose of solid and liquid wastes, dust, fines, and emissions. A large number of solids-feeding devices have been developed for low-pressure, atmospheric gasifiers. These include

screws and star valves. For gasifiers operating at elevated pressures, lockhop — pers and slurry pumping are the two leading solids-feeding devices. Lockhop — pers are operated in an intermittent fashion so that coal fills the hopper vessel at atmospheric pressure. The vessel is pressurized with gas; the coal then flows to the gasifier at elevated pressure, and the lockhopper is restored to atmospheric pressure. If one lockhopper is used, the flow of coal to the gasifier is intermittent; two or more can be used for continuous feeding. The ash is

withdrawn from the gasifiers as a slurry or by lockhopper. If the ash is molten as in slagging gasifiers, it is ordinarily quenched in water to solidify and break it up before disposal.

Gasifier operating temperatures range from 500 to 1650°C and pressures range from atmospheric to 7.6 MPa. The feedstocks are lump coal or pulverized coal. Processes using moving beds of lump coal can operate at temperatures up to about 980°C if the ash is recovered as a dry solid. Higher temperatures are possible if the ash is removed in a molten state. The methods of contacting

the solid coal feed with reactant gases include reactors that contain a descending bed of solids with upflowing gas, a fluidized bed of solids, entrained flow of solids in gas, or molten baths of gasifying media. Modern processes generally utilize fixed-bed reactors operated under nonslagging or slagging conditions, circulating or bubbling fluid-bed reactors with ash recovered from the bed in either a dry or agglomerated form, entrained-flow reactors with pulverized coal suspended in the gas stream wherein gasification is completed before the gas containing the ash leaves the gasifier, or molten bath reactors.

Fixed-bed gasifiers, which are also called moving-bed gasifiers, are usually counterflow systems. Coal is fed at the top of the gasifier and air or oxygen along with steam is generally injected near the bottom. The maximum temperature, normally 930 to 1430°C, occurs at the bottom, and the residence time in the gasifier is 1 to 2 h. The fixed-bed gasifier involves countercurrent flow in which large particles of coal move slowly down the bed and react with gases moving up through the bed. Various processes occur in different zones of the reactor. At the top of the gasifier, the coal is heated and dried while cooling the exiting product gas. The temperatures of the exit gas range from 315°C for high-moisture lignites to 550°C for bituminous coal. As the coal descends through the gasifier, sequential heating, drying, devolatilization, carbonization, and gasification take place. Fixed-bed coal gasifiers are characterized by lower gasification and product gas temperatures, lower oxygen requirements, lower tar and oil production, higher methane content in the product gas, and limited ability to handle caking coals and coal fines.

Fluid-bed gasifiers generally require coal in the 10- to 100-mesh size range, and the maximum bed temperature is determined by the fusion point of the ash, which is usually 815 to 1040°C. Operation below the fusion temperature avoids formation of sticky, molten slag. Fresh coal feed is well mixed with the particles of coal and char already undergoing gasification. Steam and oxygen or air is usually injected near the bottom of the bed. Some unreacted coal and char particles are reduced in size during gasification and are entrained in the hot exit gas. This material is recovered for recycling. The ash is removed at the bottom of the bed and is cooled by heating the incoming feed gas and recycle gas. Fluid-bed gasifiers generally utilize significant recycle of flyash, operate at moderate and constant temperatures, and are limited in their ability to convert high rank coals. Agglomerated ash operation, which can be achieved by incorporation of a hot-ash agglomerating zone in the bottom of the reactor so that the ash particles stick together and grow in size until separated from the unreacted char, improves the ability of the process to gasify high rank and caking coals.

Entrained-flow gasifiers use pulverized coal, about 70% of which is smaller than 200 mesh, and have high feedstock flexibility. The coal particles are entrained in the steam-oxygen feed and the recycled gas stream and gasified at residence times of a few seconds, after which the product gas is separated from the ash. The lower residence times can offer potentially higher through­puts at elevated pressures. Entrained-flow gasifiers can be operated at lower temperatures to maintain the ash as a dry solid, or at temperatures well above the ash fusion point in the slagging mode so that the ash is removed as a molten liquid. Operation at higher temperature results in little or no tars and oils in the product gas.

In molten bath processes, crushed coal is passed with reacting gases into the liquid bath, where gasification occurs. The ash can become part of the liquid bath or can be separated. The media include liquid iron and liquid sodium carbonate.

Low-, medium-, and high-energy gases can be produced in coal gasification processes. The important parameters are essentially the same as those for

biomass gasification systems. The higher heating values of the combustible gases commonly formed in coal-derived gases are listed in Table 9.2. As in the case of biomass gasification, the primary combustible components in low — energy product gases are carbon monoxide and hydrogen. In gasifiers where the coal particles are in direct contact with the oxygen-containing gas, nitrogen is a major component in the product gas if air is used as a coreactant instead of oxygen. Medium-energy gases are usually formed with oxygen and contain a higher percentage of combustibles, in addition to hydrogen and carbon monoxide, such as methane. High-energy gases approaching heating values of 39.3 MJ/m3 (n) (1000 Btu/SCF), the approximate higher heating value of pure methane, are produced at lower temperature conditions with oxygen instead of air, to maximize methane concentration. Further processing is necessary to methanate residual carbon monoxide and to separate noncombustible gases to provide a high-energy gas.