Incineration of waste products

DEFINITION: Burning of waste materials under controlled conditions

Incineration has several benefits as a method of waste disposal: It reduces the volume of waste by about 95 percent while producing useful amounts of heat, and it can be used to sterilize medical waste and to neutralize dangerous chemicals. Waste incineration can also produce environmentally harmful by-products if it is not conducted carefully, and for this reason the practice has not achieved widespread acceptance in the United States.

The incinerators used in burning products vary in type depending on the kinds and amounts of wastes to be processed. Solid usually generates a lot of ash; if large amounts of such waste are incinerated, must be continuously extracted. This may be done with a moving grate incinerator, in which the grate is a conveyor belt. Waste is dumped onto the grate’s front end, and ash and clinkers (unburned solids) are removed at the back end. Air is forced up through the grate to cool the grate and to aid combustion. If necessary, the grate can also be water-cooled. Air is also injected above the grate to ensure complete combustion of the gases. European law concerning waste requires that the gases reach at least 850 degrees Celsius (1,560 degrees Fahrenheit) for at least 2 seconds to guarantee the breakdown of toxic material. If the gases are not hot enough, an oil burner is used, so wastes with relatively low fuel value can be treated in a moving grate incinerator.

If heat from the incinerator is to be used, the combustion gases pass through a heat exchanger. The heated working fluid from the exchanger may be used to produce steam that powers a turbine and produces electricity, or the hot fluid may carry energy elsewhere. Among the nations that have found ways to use the heat created by incinerators is Denmark; in 2005 waste incinerators produced 4.8 percent of the electricity and 13.7 percent of the space heating used in that country. A typical moving grate incinerator can handle 38.5 tons of municipal waste per hour, and 1.1 tons of such waste may produce 0.67 megawatt-hours of electricity and 2 megawatt-hours of space heating. However, as of 2023, more than 60 percent of waste in the United States is dumped in landfills. The only US states that rely heavily on incineration were Massachusetts and Hawaii, and this was because of space limitations.

Pollutants not destroyed by heat or burning during incineration may be confined to the ash, which must be disposed of properly. Pollutants in the must be neutralized. Ash mixed with flue gas, called fly ash, must be captured. The cheapest capture method works best on larger particles: Flue gas is made to swirl in a cyclone chamber so that centrifugal force drives dust particles toward the outer chamber wall, from which they then drop into a hopper. In electrostatic precipitation, dust particles are given a negative charge and then the flue gas is passed between large, highly positively charged plates. The charged dust particles migrate to the plates and stick to them. Periodically shaking the plates allows the dust to fall down into a hopper for later removal. An excellent can remove 99.9 percent of fly ash.

Another capture method involves forcing flue gas through filter bags in a bag room. A final method involves spraying water droplets into the gas and letting the drops gather the dust as they fall. If there are pollutants in the flue gas, chemicals can be added to the water spray, or they may be blown in as a dry powder. Acids are neutralized with sodium bicarbonate, and bases and chlorides are treated with sodium hydroxide.

Other Types of Incinerators

Medical wastes must be incinerated at more than 1,000 degrees Celsius (1,832 degrees Fahrenheit) to ensure the destruction of all pathogens. Such wastes are burned in a rotary kiln incinerator, which is completely lined with firebricks and has no moving parts in the heated region, so it can withstand such temperatures. The incinerator consists of a rotating horizontal cylinder with one end higher than the other. Waste is put in at the high end, and the burning automatically migrates to the lower end for ash removal.

When the waste particles and their composition are relatively homogeneous, as they are in liquid municipal sludge waste, a fluidized bed incinerator is used. The bed is a layer of sand about 1 meter (3.3 feet) thick. Air blown from beneath the sand bed lifts the sand particles and keeps them suspended. Fuel is blown in and ignited at start-up, then waste is sprayed onto the fluidized bed. Fluid waste droplets are quickly reduced to particles that are burned. They are given sufficient oxygen and uniformly heated from all sides so that burning is complete. Flue gases pass through a heat exchanger, preheating the air blown from under the sand bed. The gases are treated as necessary before they are vented to the atmosphere.

In a plasma arc incinerator, a plasma arc (which looks like a very big spark) is created by the passing of a high-voltage, high-amperage current between two electrodes. A gas passing between the electrodes is heated to 13,900 degrees Celsius (25,000 degrees Fahrenheit). The gas temperature drops to 2,800 to 4,400 degrees Celsius (5,100 to 8,000 degrees Fahrenheit) as it circulates into the incinerator’s waste-containing chamber. The temperature is hot enough to break chemical compounds into their constituent atoms, so with proper flue gas treatment a plasma arc incinerator can handle difficult wastes such as batteries and asbestos.

Incineration provides one of the best methods for destroying dangerous chemicals such as those used in chemical weapons. After the United Nations Convention on the Prohibition of the Development, Production, Stockpiling, and Use of Chemical Weapons and on Their Destruction went into effect in 1997, the United States began using incineration to destroy its stockpile of some 33,000 tons of nerve gas and mustard gas. The poison chemicals are broken down by extremely high temperatures; they are heated in an incinerator to 1,500 degrees Celsius (2,700 degrees Fahrenheit), and their former containers are heated to 900 degrees Celsius (1,650 degrees Fahrenheit) for 2.5 hours. By 2012 the United States had destroyed 90 percent of its chemical weapons stockpile and closed seven of its nine disposal facilities.

Bibliography

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