Control of the Incineration Process

The poor image that incineration has in the eyes of many people is due largely to the failure to control the operation, with resultant destruction of the equipment and air pollution. A properly designed and operated incinerator requires control instrumentation for (1) temperature, (2) draft pressures, (3) smoke emission, (4) weights of solid wastes coming in and leaving the plant, and (5) air pollution control equipment. Competent well-trained operators are also essential.

Temperature Temperature monitoring is necessary for control purposes to monitor the incoming air and gases leaving the combustion chamber at the settling chamber outlet, the cooling chamber outlet, the dust collector inlet and outlet, and the stack temperature. Furnace temperature can be controlled by adjusting the amount of overfire or underfire air. The temperature of the gases leaving the furnace is reduced by spraying with water (causes a white stack plume unless the flue gas is reheated before discharge), dilution with cool air (high equipment cost to handle large volumes of diluted gases), or passing through heat exchangers (ready market for heat, steam, electricity, or high-temperature water needed). Gas scrubbers using water sprays can be used to cool effluent gas so that an induced-draft fan can be used to reduce the chimney height; large particulates can also be removed.

Draft Pressure Draft pressure measurements are needed to control the induced-draft fan and the stack draft. Measurements should be made at the underfire air duct, overfire air duct, stoker compartment, sidewall air duct, sidewall low-furnace outlet, dust collector inlet and outlet, and induced fan inlet. Control of underfire air can provide more complete combustion with less fly-ash carryover up the stack.

Smoke Density The smoke emission can be controlled by continuous measurement of the particulate density in the exhaust gas. A photoelectric pickup of light across the gas duct is used, preferably located between the particulate collector and the induced fan duct.

Weigh Station Platform scales to weigh and record the incoming solid waste and outgoing incinerator residue, fly ash, siftings, and other materials are generally required.

Instrumentation Devices should include those to keep record of overfire and underfire air flow rates; temperature and pressure in the furnace, along gas passages, in the particulate collectors, and in the stack; electrical power and water use; and grate speed.

Odor Odor control requires complete combustion of hydrocarbons—that is, excess air and a retention time of 1 sec at 1500 °F (816 °C) [above 1400 °F (760 °C) at the exit of the furnace]. Adequate dilution of gases leaving the stack by an effective stack height (actual stack height plus plume rise) is another possible method for odor control, but its effectiveness is related to meteorological conditions and persistence of the odors. Wet scrubbers can also be used to absorb odors while removing particulates.

Gaseous Emissions The principal gaseous emissions from the combustion of mixed wastes are: carbon dioxide, water vapor, sulfur oxides, nitrogen oxides, carbon monoxide, and hydrogen chloride. Hydrogen chloride and other acids can cause corrosion of air pollution control equipment. A lime spray dry scrubber followed by a baghouse (fabric) filter is effective in greatly reducing sulfur dioxide and hydrogen chloride gases, metals, dioxins, furans, and organic emissions, as well as fly ash. There is some evidence that the lower the temperature of flue gases [below about 300 °F (149 °C)] entering the pollution control devices, the greater the amount of phenols, benzenes, dioxins, and other organics condensed and collected on the particulates. Typical gaseous emission guidelines are presented in Table 3.25.

Particulate Emissions These can be controlled by settling chambers, wetted baffle spray system, cyclones, wet scrubbers, electrostatic precipitators, and fabric filters. Apparently, only wet scrubbers, electrostatic precipitators, and bag filters can meet air pollution code requirements. Cyclones in combination with other devices might approach the standard. Typical particulate emission design and operating guidelines are presented in Table 3.25.

Continue reading here: Residue Management

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