Solid Waste Incineration

The destruction of solid waste by fluidized-bed incineration has two thermal processes. The first is combustion, in which organic materials are burned in the presence of oxygen; the second is pyrolysis, in which the decomposition of solids takes place in the presence of an inert gas at a high temperature. The products of the combustion reaction are totally oxidized, whereas the pyrolysis reaction yields hydrogen, methane, and carbon monoxide, which are unoxidized products. The reactions of each are as follows:

Solid waste + Oxygen°mb»si*oCO2 + H2O + Ash + Heat

(methane)

Since these reactions take place in fluidized beds at about 1400°F, the heat produced from the combustion reaction helps maintain the bed temperature, while pyroly-sis requires heat.

The advantages of using fluidized beds for the destruction of solid waste materials include the following:

1. The heated particles store large quantities of readily available heat.

2. Particle movement throughout the bed prevents the formation of hot spots or temperature zones.

3. High heat-transfer rates result in rapid combustion.

4. Bed agitation prevents solids stratification.

5. Unfavorable gases and products can undergo total combustion, eliminating the need for expensive air pollution control equipment.

6. Few moving parts are located within the bed, reducing maintenance.

7. Temperature variations are minimal (less than 5° to 10°F) throughout the bed.

The disadvantages of using fluidized beds for solid waste incineration are as follows:

1. The maximum temperatures cannot exceed 2000°F when sand is used as a bed medium because sand softens at this temperature.

2. The power costs are high.

3. Equipment is necessary to recover fine solids because the catalyst media become entrained.

4. Auxiliary fuel is usually necessary because the composition and heating value of solid waste vary.

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