For smaller incinerators, chimneys provide sufficient draft to discharge flue gases at a high enough point where no nuisance is caused by the emissions. A fully loaded chemistry should provide at least 0.25 in of water draft (—62 Pa). Table 10.11.5 lists the diameters and heights of chimneys according to the weight rate of waste burned in a continuously charged multiple-chamber incinerator. The table assumes that the incinerator uses no dilution air and

FIG. 10.11.4 Incinerator with automatic charging system.

that the breechings between the furnace and chimney are of minimum length. The lining thicknesses shown are for outdoor chimneys; chimneys inside buildings need additional insulation.

For proper incinerator operation, the cold air supply to the furnace should not be restricted. In most designs, the furnace receives its air supply from the incinerator room. The air supply should be sized for about 15 lb of air per lb of MSW burned. If the air supply is insufficient, the mechanical ventilation system of the building can cause smoking due to downdrafts. When a chimney's natural draft is insufficient, fans are installed to generate the required draft. In onsite incinerators, the forced-draft air is usually introduced underfire. Introducing overfire air to improve combustion efficiency is not widely used in onsite units.

When the waste is wet or its heating value is low, auxiliary fuels are needed to support combustion. In continuously charged incinerators, the primary burner is sized for 1500 Btu per lb of type 3 waste or for 3000 Btu per lb of type 4 waste (see Table 10.11.2). The heat capacity of the secondary burner is also 3000 Btu per lb of waste. When the incinerator is fully loaded, the secondary burner runs for only short periods at a time.

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