Types of Incinerators

Types of Incinerators

Three standards and two less common types of incinerators are used in North America. Each can be operated, with some modifications, to produce energy.

The mass-burn incinerator is the most common type and is similar to a coal-fired steam boiler. A schematic cross section of massburn incinerators  is shown in Figure 9.1, and an aerial view of an actual incinerators is that the waste requires minimal processing. Mixed garbage, from which only the largest items such as appliances and logs are removed, is brought to the plant and placed in a large waste storage pit. An overhead crane mixes the refuse to provide a relatively uniform fuel and then loads it into hoppers which carry the waste into grates in the furnace. Fans in the furnace floor and walls provide air for the oxidation ( i.e., combustion ) process. The waste is burned at an optimal temperature of about 1100 and remains on the grate for 45 to 70 minutes to ensure complete combustion. The gases that form are heated by supplemental fuel injection for an additional second or two to ensure complete destruction of resistant chemicals. The hot gasses are then cooled by water in boiler tubes that generates steam for electricity, heating, or other purposes. Then the gases are sent to pollution control device, which may include ammonia injection for NO ( nitrogen oxides ) control, a dry scrubber for SO₂ and acid gas control, carbon injection to remove mercury and dioxin, and a baghouse to remove particulate matter.

The ash that accumulates at the bottom of the furnace is removed through a water-quenched conveyor and emptied into a storage area from which it is periodically removed and transported to a landfill. Some plants remove and recycle the larger pieces of iron and other metals that have not burned. Fly ash is collected from a dry scrubber and baghouse and taken to a landfill. Mass-burn incinerators can have capacities of 90 to 2,700 tonnes of garbage per day. A case history of a mass-burn incinerator is presented in chapter 11.

A modular incinerator is similar to mass-burn incinerator but typically has a smaller capacity, in the range of 14 to 365 tonnes of waste per day. It is modular in design and can be built in units at the factory and then shipped to the facility site.

A refuse-derived fuel (RDF)incinerator burns garbage that has been processed before being burned. Although processing is required, the prepared fuel will be consistence and will meet specifications for energy content, moisture, and ash content. A significant advantage is that recyclable materials such as iron, aluminum, and glass can be removed during the processing. The RDF can be produced in shredded or fluff form, or it can be compacted into a denser fuel such as pellets or cubes. Densified RDF is more costly to produce, but it has the advantage of being easier to transport and store. This fuel works more effectively in specially designed boilers, but it can also be used in coal-fired boilers. RDF has an energy value comparable to that of coal and can be used either alone in mixed with coal. Because of the higher energy content and more uniform nature of the fuel, RDF incinerators are smaller and can be more effectively controlled than mass-burn units of similar capacity. By the end of 1992, RDF facilities accounted for about 20% of the waste-to-energy plants in the United States.

The fluidized-bed incinerator is a relatively new technology in North America for garbage, although it has been used to burn sludges. This incinerator injects refuse-derived fuel into loose, moving bed of limestone and sand, which is suspended above the furnace floor, like a fluid, by an upward flow of air. The “fluidized” bed of sand and limestone helps to distribute the heat evenly throughout the burn, resulting in more complete combustion efficiency, results in lower emissions of nitrogen oxides, sulphur dioxide, and dioxins than occurs from the other types of incinerators.

Because fluidized-bed incinerators require preprocessing of waste, they fit well with materials recycling. These incinerators, being much smaller than mass-burn incinerators, may be more appropriate for smaller communities.

Rotary kiln furnaces  similar to those used in cement industry can be used for incinerating wastes. The kilns are large, gently sloped cylinders lined with refractory (heat-resistance) materials that rotate slowly while they are heated to very high temperatures. A supplementary fuel such as oil or gas is generally used. The kiln system is very flexible and can be handle a wide variety of waste types and sizes. Kilns 2.5 or 3.0 meters in diameter are common and can handle large waste pieces, including drums. The kilns are slightly inclined so that waste moves down the slope. The length of the kiln and the amount of incline control the time of exposure of the waste to high temperatures, and these features can be designed to provide the required destruction level. Some kilns are designed to maintain a layer of melted glasslike slag on the inside of the drum; this prospects the lining, or refractory, of the furnace from the high temperatures and prolongs its life; it also produces a more leach-resistant vitrified ash residue and helps to capture fine combustion particles. Gas scrubber and dust removal systems are easily attached.

In all incinerators, the hot gases produced by incineration must be cooled to stop chemical reactions and to protect the downstream pollution-control equipment. Cooling is usually done by quenching the hot gases with large volumes of water. The water and condensate are sent to a wastewater treatment plant, which forms a necessary part of a modern incinerator.