A variety of incinerator types have been used for the combustion of solid waste, including (1) mass-fired combustors, (2) refuse-derived fuel- (RDF) fired com-bustors, (3) modular combustion units, and (4) on-site commercial and industrial incinerators.
Mass-Fired Combustors In a mass-fired combustor, minimal processing is given to solid waste before it is placed in the hopper used to feed the combus-tor. The crane operator in charge of loading the charging hopper can manually reject obviously unsuitable items. However, it must be assumed that anything in the MSW stream may ultimately enter the combustor, including bulky oversize noncombustible objects (e.g., broken tricycles) and even potentially hazardous wastes deliberately or inadvertently delivered to the system. For these reasons, the combustor must be designed to handle these objectionable wastes without damage to equipment or injury to operational personnel. The energy content of mass-fired waste can be extremely variable, dependent on the climate, season, and source of waste. In spite of these potential disadvantages, mass-fired com-bustors have become the technology of choice for most existing and planned incineration facilities.25
A typical mass-burn incinerator schematic showing steam and electricity production is illustrated in Figure 3.42. Types of furnaces used are the rectangular refractory lined, the rotary kiln, and the rectangular furnace with waterwalls. In the rectangular furnace, two or more grates are arranged in tiers. The rotary kiln furnace incorporates a drying grate ahead of a rotary drum or kiln where burning is completed. Waterwall furnaces substitute water-cooled tubes for the exposed furnace walls and arches. Other types of furnaces are also available.
All furnaces should be designed for continuous feed. Reciprocating or moving and traveling grates are the most common. Mass-burn incinerators usually burn raw solid wastes in a refractory-lined rotary kiln after drying and combustion, with underfire and overfire air and a tube boiler to generate steam, hot water, or electricity. In a cogeneration incinerator, steam and electricity are produced.
Modern furnace walls are usually lined with tile or have waterwalls. With tile refractories, repairs can be readily made without the need for expensive and time-consuming rebuilding of entire solid brick walls found in old plants. Special plastic or precast refractories can be used for major or minor repairs. Waterwalls in a furnace actually consist of water-cooled tubes that also serve as heat exchangers, thereby reducing the outlet gas temperature and simplifying dust collection. The tubes also cover and protect exposed furnace walls and arches. Less air is required: 100 to 200 percent excess air for refractory walls compared to less than 80 percent for waterwalls. External pitting of the water-cooled tubes may occur if the water temperature drops below 300 °F (149 °C) due to condensation of the corrosive gases. Internal tube corrosion must also be prevented by recirculation of conditioned water.
RDF-Fired Combustors Compared to the uncontrolled nature of unprocessed commingled MSW, RDF can be produced from the organic fraction of MSW with fair consistency to meet specifications for energy content, moisture, and ash content. The RDF can be produced in shredded or fluff form or as densified pellets or cubes. Densified RDF (d-RDF) is more costly to produce but easier to transport and store. Either form can be burned by itself or mixed with coal and combusted in a waterwall furnace (see Figure 3.13) equipped with a traveling gate for ash management.
Because of the higher energy content of RDF compared to unprocessed MSW, RDF combustion systems can be physically smaller than comparatively rated mass-fired systems. However, more space will be required if the front-end processing system needed to prepare the RDF is to be located adjacent to the combustor. A RDF-fired system can also be controlled more effectively than a mass-fired system because of the more homogeneous nature of RDF, allowing for better combustion control and better performance of air pollution control devices. Additionally, a properly designed system for the preprocessing of MSW can effect the removal of significant portions of metals, plastics, and other materials that may contribute to harmful air emissions.15
Modular Combustion Units Modular combustion units are available for capacities of less than 700 lb/hr to 250 tons/day and include a secondary combustion chamber. These units may be used for the batch incineration of municipal, hospital, commercial, and industrial wastes. Volume reduction of 80 to 90 percent and energy recovery of about 55 percent are claimed. Emission control (scrubber and/ or baghouse) is needed and skilled operation is required.
On-site Commercial and Industrial Incinerators When possible, a large municipal incinerator should be used in preference to a small on-site incinerator. Better operation at lower cost with less air pollution can usually be expected. Based on past experience, conventional mass-fired incinerators generally are not economically feasible for communities with a population of less than 50,000 to 100,000, but modular controlled air units incorporating heat recovery are suitable for smaller volumes of waste. However, on-site incinerators are used in hospitals, schools, and commercial and industrial establishments. Their continued use is being severely limited by air pollution control requirements. Many of the units now in use need to be replaced or redesigned to meet modern air-pollution control standards. The controlled-air incinerator with a waste-heat boiler for energy recovery can overcome many, if not all, of the deficiencies.
Incinerator Capacity and Stack Heights Incinerators are rated in terms of tons of burnable or incinerable waste per day. For example, an incinerator having a furnace capacity of 600 tons/day can theoretically handle 600 tons in 24 hours with three-shift operation, 400 tons in 16 hours with two-shift operation, and 200 tons in 8 hours with one-shift operation. Hence, if 400 tons of incinerable wastes collected per day are to be incinerated in 8 hours, an incinerator with a rated capacity of 1,200 tons per day will be required plus a 15 percent downtime allowance for repairs. In determining design capacity, consideration must also be given to daily and seasonal variations, which will range from 85 to 115 percent of the median.
High stacks (chimneys) 150 to 200 feet above ground level are usually constructed to provide natural draft and air supply for combustion. Stack heights of 300 to 600 feet are not uncommon. Discharge of gases at these heights also facilitates dilution and dispersal of the gases. In some designs, short stacks are used for aesthetic reasons, and the equivalent effective stack height is obtained by induced draft. Meteorological conditions, topography, adjacent land use, air pollution standards, and effective stack height should govern.
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