Requirements And Combustion Product Yields

Elemental

Waste

Component

Stoichiometric

Oxygen

Requirement

Combustion Product Yield

C H2 O2 Cl2

Br2 I2 S P

Air N2

Stoichiometric air requirement

-O.25 lb H2O/lb Cl2 1.05 lb/HF/lb F2 -O.47 lb H2O/lb F2 1.0 lb Br2/lb Br2 1.0 lb I2/lb I2 2.0 lb SO2/lb S 2.29 lb P2O2/lb P

2(stoich)

a = the inert and ash fraction of the MSW

M = the moisture fraction of the MSW

S = the cubic feet of stoichiometric cold air required per Btu of heat release e = the excess air fraction

In cases where metals are not burned and combustibles are predominantly organic, the value of S is approximately 0.01 (i.e., 1 cu ft of cold air per 100 Btu). This approximation is valid, generally to within 10 to 20%, for a wide range of organic fuel. Consequently, variations between wastes depend largely on their noncombustible content (a + M), particularly as the DIF calorific values lie within the narrow range of 8000 to 10,000 Btu/lb. The stoichio-metric air requirements of most DIF waste are therefore 80 to 100 cu ft/lb or 6.4 to 8 lb of air per pound of waste. If the waste contains 50% ash and moisture, the calorific values drop to 4000 to 5000 Btu/lb. If this waste is fired at 150% excess, the air requirements are 8 to 10 lb of air per pound of waste as fired.

In modern, mechanical, grate furnace chambers, the un-derfire and overfire air are usually provided by separate blower systems. Underfire air is admitted to the furnace under the grates and through the fuel bed. It supplies primary air for the combustion process and also cools the grates. Underfire air is usually more than half of the total air (50 to 70%). Particulate emissions from incinerators tend to increase with heat release and underfire air flow, while they tend to decrease with increasing fuel particle size (see Figure 10.9.8).

Overfire air can be introduced at two levels:

Immediately above the fuel bed to promote turbulence and mixing and to complete the combustion of volatile gases driven off the bed of burning solid waste. From rows of nozzles placed high on the furnace wall. These nozzles allow secondary overfire air to be introduced into the furnace to promote additional turbulence of gases and control temperature. The number, size, and location of the overfire inlet ports determine the amount of turbulence and backmixing in the stirred reaction region above the burning waste. See Figure 10.9.6. For good combustion, the overfire air system must have broad flexibility to accommodate changes in fuel moisture, ash content, and Btu value.

Operators control flue temperature and smoking by modulating the total air flow and the underfire-to-overfire air ratio. For most U.S. grate designs, the required underfire air pressure is about 3 in of water. The overfire air pressure is adjusted so that entrance velocities at the nozzle are high enough to guarantee high turbulence without impinging on the opposite wall and residence times are long enough to assure complete combustion.

Influent air is usually at an ambient temperature, normally 27°C (80°F). It can get as high as 1650°C (2100 to 2500°F) in the immediate proximity of the flame. When the gas leaves the combustion chamber, the temperature

COMBUSTION RATE (mass flow rate of fuel per unit area of grate)

FIG. 10.9.8 Effects of combustion rate, underfire air, and fuel particle size on particulate emissions generated by combustion of wood waste. (Reprinted, with permission, from K.L. Tuttle, 1986, Combustion generated particulate emissions, National Waste Processing Conference, Denver, 1986 [ASME].)

COMBUSTION RATE (mass flow rate of fuel per unit area of grate)

FIG. 10.9.8 Effects of combustion rate, underfire air, and fuel particle size on particulate emissions generated by combustion of wood waste. (Reprinted, with permission, from K.L. Tuttle, 1986, Combustion generated particulate emissions, National Waste Processing Conference, Denver, 1986 [ASME].)

should be reduced to between 760 and 1000°C (1400 to 1800°F). If air pollution control devices are installed, induced draft fans must be installed, and the temperature should probably not exceed 260 to 370°C (500 to 700°F).

The mathematical modeling of the incinerator presented by Essenhigh (1974) provides a better understanding of the combustion processes taking place in incinerators. Figure 10.9.9 describes the gas-phase (II) and solid-phase (I) zones in a top-charged incinerator (overbed feed).

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