Incineration, or complete combustion, is a rapid exothermic oxidation of combustible elements in fuels. The use of incineration for sludge disposal is the result of tightening limits for land disposal and/or sea disposal of sludge by regulatory agencies. The use of combustion to reduce waste-water sludges into inert ash is an effective but costly process unless there is a cheap fuel resource available. The ash from incineration operations is usually disposed in a landfill. The fuel requirement for incinerating sludges depends on two parameters: the amount of water in the sludges and fuel value of the contents of the sludges. For example, FOG scum from preliminary treatment or primary treatment processes of food and agricultural wastewater can be readily burned in the incinerator, which reduces the fuel requirement and lowers the cost of the operation. Raw primary and undigested secondary sludges will have fuel values ranging from about 14,000 to 28,000 kJ/kg (6,000 to 12,000 Btu/lb) dry solids (Metcalf and Eddy, Inc., 1991). If sludges are managed to dewater to about 25% solids, incineration will be self-sustaining.
Oxygen requirements for incineration of sludge may be determined from knowledge of its constituents, assuming C and H are oxidized to the ultimate end product (Equation 7.2):
CaObHcNd + (a + 0.25c - 0.5b)02 b a CO2 + 0.5cH20 + 0.5dN2
The theoretical quantity of air will be 4.23 times the calculated quantity from Equation 7.2 (02 is about 23% of air by weight). Excessive amount of air (about 50% of the theoretical quantity) will be used to ensure the complete combustion of the sludge.
The heat requirement for incineration operations is the following (Equation 7.3):
Cp = specific heat for each constituent of substances in ash and in flue gases m = mass of each substance
T1, T2 = initial and final temperatures mw = mass of water in sludge
\ = latent heat of evaporation of water per unit mass
It is obvious that any reduction in moisture in sludge will lower the fuel requirements; thus, moisture content determines whether additional fuel will be required in incinerating a particular sludge.
There are two popular incinerators used for sludge incineration: multiple-hearth type and fluidized bed type. The first multiple-hearth incinerator for sludge incineration was built in 1935 in Dearborn, Michigan. From that time on through the late 1960s, the multiple-hearth type was the choice for sludge incineration. At present, there are still some 150-175 multiple-hearth incinerators in operation in North America.
A multiple-hearth incinerator is a vertical, cylindrical, refractory-lined, steel-shell furnace. It consists of 6 to 12 horizontal hearths and a rotating center shaft with rabble arms. Cooling air is introduced into the shaft, which extends above the hearths. The sludge enters the top hearth and flows downward while combustion air flows from the bottom to the top. The rabble arms are shaped to sweep the sludge in a spiral motion, alter nating in direction from the outside in, to the inside out, between hearths. The effect of the rabble motion is to break up solid material to allow better surface contact with heat and oxygen. Depending on the shaft speed and on the number of hearths, the retention time of the sludge in the incinerator ranges from 0.5-3 hr.
Ambient air is first pumped through the central shaft and its associated rabble arms. A large portion of this air is then taken from the top of the shaft and recirculated back to the lowermost hearth as preheated combustion air and mixed with additional ambient combustion air. The temperature of the mixed air is limited because the lower hearths serve as an ash-cooling zone. Sludge burns in the center hearths, where it is the hottest, and releases heat and combustion gas. The combustion gas flows upward through the drop holes in the hearths, countercurrent to the flow of the sludge, before being exhausted from the top hearth.
The flue gases rising through the furnace are cooled in the upper hearths by the evaporation of sludge moisture, which degrades or stops combustion on the top hearths. In this drying zone, some volatiles are released from the sludge and exit the furnace without exposure to the full combustion temperatures. The feed sludge must contain more than 15% solids due to limitation of evaporating capacity of the incinerator. Average loading rates range from 5 to 15 lb/ft2-h (25 to 75 kg/m2-h). Auxiliary fuel is often required when the feed sludge has a solids content ranging from 15-30% (Metcalf and Eddy, Inc., 1991). Additional ash-handling, such as wet or dry scrubbing, is needed to minimize the air pollution.
In the1970s, fluid bed incinerators became the preferred choice for incinerating sludge mainly due to tighter emission regulations and to increasing cost of auxiliary fuel. The advantages of the fluid bed are lower emission, reduced auxiliary fuel use, and reduced operating and maintenance costs. The fluidized bed incinerator is a vertical, cylindrically shaped, refractory-lined steel shell that contains a sand bed called the combustion zone and the refractory arch containing alloy tuyeres or nozzles that allow hot air to be distributed homogeneously throughout the bed in order to produce and sustain combustion (Fig. 7.14). The air from the refractory distributor causes the bed of sand to fluidize to a height of 5 ft. Sludge and auxiliary fuel, if required, are fed into fluidizing sand bed through lateral feed ports. The intensive mixing of the solid and combustion air in the fluidized state yields a high heat transfer rate resulting in rapid combustion of the sludge fed into the furnace. In the lower section, the area below the refractory arch distributor of the furnace is called the
windbox; it acts as a plenum in which the air is received. The refractory arch distributor and the refractory-lined windbox are designed to allow combustion air to be preheated up to 1,250°F. The section above the bed is called the freeboard or disengagement zone. It is typically 15 ft high and usually is expanded laterally along its height to maximize residence time and to reduce sand usage. The freeboard typically provides 6 to7 seconds of gas residence-time, which completes the combustion of any volatile hydrocarbons escaping from the bed. The freeboard thus acts as an integral afterburner and normally operates at 50 to 100°F higher than the bed, ensuring complete combustion of the volatiles. Combustion exhaust and ash leave the bed and are transported through the freeboard area to the gas outlet through the top of the furnace. Because of the fine size of the ash, there is no ash exit from the bottom of the incinerator. The entrained ash is scrubbed with a venturi scrubber as part of an air pollution control system.
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