20150

Lignite, reactive bituminous coal, anthracite, petcokes < 25% preferred <2372

<725 High Low

Up to 700 Large amount of sensible heat energy in the hot raw gas Raw gas cooling aFixed-bed gasifiers operating on low-rank coals have exit temperatures lower than 800°F.

Source: 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

Coal

Product gas

Drying and devolatilization zone

Drying and

Product gas devolatilization zone

Reduction zone

Combustion zone

Steam & oxygen

Combustion zone

FIGURE 5-23. Reaction zones in a fixed-bed gasifier. (From Elliot, M. A., Editor, Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

removed from the bottom of the gasifier either in dry form if the temperature in the gasifier is controlled with excess steam to maintain the temperature below the ash fusion point or as liquid slag. Both the ash and the product gas leave at modest temperature as a result of heat exchange with the entering gasifying medium and fuel, respectively. Fixed-bed gasifiers have the following characteristics [44]:

• Low oxidant requirements;

• Design modifications required for handling caking coal;

• High cold-gas thermal efficiency when the heating value of the hydrocarbon liquids is included;

• Limited ability to handle fines.

Fluidized-Bed Gasifier

In a fluidized-bed gasifier, coal crushed to less than 1/8 to 1/4 in. in size enters the side of the reactor and is kept suspended by the gasifying medium. Similar to a fluidized-bed combustor, mixing and heat transfer are rapid, resulting in uniform composition and temperature throughout the bed. The temperature is sustained below the ash fusion temperature, which avoids clinker formation and possible slumping (i.e., de-fluidization of the bed). Some char particles are entrained in the product gas as it leaves the gasifier, but they are recovered and recycled back into the gasifier via a cyclone. The ash is discharged with the char, and the product gas and char temperatures are high, with some heat transfer occurring with the incoming steam and recycled gas. Fluidized-bed gasifiers have the following characteristics [44]:

• Acceptance of a wide range of solid feedstock (including solid waste, wood, and high ash content coals);

• Uniform temperature;

• Moderate oxygen and steam requirements;

• Extensive char recycling.

Entrained-Flow Gasifier

In the entrained-flow gasifier, pulverized coal (<0.005 in.) is entrained with the gasifying medium to react in co-current flow in a high-temperature flame. Residence time in this type of gasifier is very short. Entrained-flow gasifiers generally use oxygen as the oxidant and operate at high temperatures, well above ash-slagging conditions, to ensure high carbon conversion. The ash exits the system as a slag. The product gas and slag exit close to the reaction temperature. Entrained-flow gasifiers have the following characteristics [44]:

• Ability to gasify all coals regardless of coal rank, caking characteristics, or amount of coal fines, although feedstocks with lower ash contents are favored;

• Uniform temperatures;

• Very short fuel residence times in the gasifier;

• Very finely sized and homogenous solid fuel required;

• Relatively large oxidant requirements;

• Large amount of sensible heat in the raw gas;

• High-temperature slagging operation;

• Entrainment of some molten slag in the raw gas.

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