## Calculating Heat Generation

Calculating the amount of heat generated through the incineration of MSW is necessary to determine how much auxiliary fuel is needed for combustion. The moisture content of MSW ranges from 20 to 50% by weight, and the combustible content is 25 to 70% by weight. The heating value of MSW depends on its composition.

Assuming that the average heating value of the combustible is 8500 Btu/lb and the moisture and inert concentration of the MSW is known, environmental engineers can estimate the heat content of MSW using Figure 10.9.10. If the heating value of the combustibles is less than 8500 Btu/lb, the number in Figure 10.9.10 must be multiplied by the ratio of the actual heating value divided by 8500.

Table 10.9.4 gives a material balance of burning 100 lb of MSW. The table assumes the MSW to have a heat content of 5000 Btu/lb, a moisture content of 22.4%, and a noncombustible content of 19% and that it contains 28

FIG. 10.9.9 Incinerator for continuous overbed feed of waste. Schematic represents solid bed, zone I, with overbed, zone II. For overbed feed, zone I has subzones, including I(A), the combustion and gasification section on the grate, and I(B), the drying and pyrolysis above I(A). Zone II (overbed combustion) has a backmix (stirred) region called subzone II(A), followed by a plug flow burnout region, subzone II(B). With underfeed, zone I is inverted, with drying and pyrolysis below the combustion subzone and the reaction front moving down instead of up as shown.

FIG. 10.9.9 Incinerator for continuous overbed feed of waste. Schematic represents solid bed, zone I, with overbed, zone II. For overbed feed, zone I has subzones, including I(A), the combustion and gasification section on the grate, and I(B), the drying and pyrolysis above I(A). Zone II (overbed combustion) has a backmix (stirred) region called subzone II(A), followed by a plug flow burnout region, subzone II(B). With underfeed, zone I is inverted, with drying and pyrolysis below the combustion subzone and the reaction front moving down instead of up as shown.

FIG. 10.9.10 Moisture-heat content relation with 8500 Btu/lb combustible material. (Reprinted, with permission, from Velzy and Hechlinger 1987.)

lb of carbon and 0.6 lb of hydrogen. It also assumes that 1-3 lb of combustibles escape unburned, and 140% excess air is needed to cool the refractories. Therefore, the total air required is 2.4 times the stoichiometric requirement, or 8.24 lb of air per pound of MSW.

TABLE 10.9.4 MATERIAL BALANCE FOR FURNACE (IN LB/100 LB OF REFUSE)

Input:

### Refuse

Combustible material Cellulose Oils, fats, etc. Moisture Noncombustible Total air, at 140% excess air Oxygen Nitrogen Moisture in air Residue quench water Total

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