Science Fair Project Encyclopedia
Incineration is the process of destroying something through fire.
The method of burning while sitting on the ground or in a pit, has been replaced by specialized devices and structures that have been developed and used. The ancient Hebrews burned their waste in a pit called Gehenna.
Incinerators (particularly large ones) can be used for generating electricity or provide energy in other ways such as generating steam for heat. Such a use is known as waste to energy . However, a significant amount of energy is lost due to "scrubbers ", and other methods used to clean up the exhaust. The small amounts of the leftover ash are often sent to a landfill. The poor efficiency of production of energy by incinerators may not present a major disadvantage, since it is destruction of harmful chemicals that remains their main purpose and benefit.
Use of incinerators for waste management is controversial, and most Americans passionately oppose it. This controversy roots from the understandable conflict between short-term concerns and long-term ones, in this case between burning the wastes now, or postponing this problem by passing the waste burden to future generations.
Most of the general public in the United States abhor the prospect of polluting the air by combustion gases, and their politicians feel bound by that unified will. On the other hand, people and organizations professionally involved in this issue such as EPA and AQMD are concerned by the fact that the overall harm produced by incineration is generally millions times lower than the long-term harm caused by the original materials buried in the landfills.
These conflicts of opinions can be illustrated by two examples:
- Gasoline contains e.g 20% of benzene, a non-malodorous chemical that kills blood cells and causes cancer, if benzene gets inside human body. But it does not get there, because the vehicle's engine incinerates the benzene so completely, that even traces of the aromatic smell of benzene are absent from the California air, even though 2 million vehicles are moving in the streets of the Los Angeles conurbation at any moment, incinerating about 3,000 tons of benzene every hour. Conversely, if the same amount of benzene were equally efficiently incinerated in an industrial facility, people (especially local) would be outraged about "that horrible incinerator".
- The chemical warfare agent sarin is so deadly that a smallest droplet quickly kills a person; when incinerated properly, all it leaves are slightly acidic gases: carbon dioxide, phosphorus- and halogene- containing gases, and minimal amounts of dioxins. Dioxins are certainly very harmful, but their toxicity is incomparable with that of the un-incinerated sarin; also, only a tiny fraction of the sarin is converted to dioxins by incineration. USA has thousands of tons of sarin stockpiled during the Cold War era, as a countermeasure against even larger stockpiles of the Cold War enemies. Having signed the international ban, US Government has been trying its best to destroy that sarin by the best method, that is by incineration, but the destruction is hampered by the environmental concerns about incineration.
This is an extreme example of what seems ot be a general misconception about incineration. It seems natural for people, when facing the choice between an immediate and certain albeit small harm, and a larger but much slower harm, to choose the latter.
As a result, millions of tons of harmful wastes are dumped into landfills where they will remain forever, contaminating the soil, water and air, for all generations to come and inherit it; rather than burning them smokelessly to produce relatively smaller amounts of relatively much less harmful products. Experts in the United States Environmental Protection Agency are fully aware that incineration is a much "lesser evil" than all its alternatives, but they can not overcome the political pressure due to public opinion, and especially the "Not in my backyard !" thinking.
The use of incinerators has been on the decline in the United States. If a laboratory or factory has a small flask of hazardous waste, to dispose of it legally $1000 must be paid to someone who will transport it to one of the few remaining incinerators. There were 98 such plants in 2002 and 89 in 2004, so it is often cheaper to take waste to a landfill, with obvious harm to the Earth. The difficulties and costs of disposing of harmful chemical wastes in America are so excessive and prohibitive due to the unpopularity of incineration, that chemistry teachers of public schools often dump their chemicals into ground.
Since the general public in the United States is taught little chemistry or physics outside of colleges, naive science often prevails over facts. A result of this is to separate the concepts of chemicals and natural substances, and consequently industrial wastes are seen less favorably than those generated by individuals. The fact is that all chemical molecules of the same compound are completely identical, regardless whether they have been produced by a flower, by a factory, by incineration, by a car, or by a baby. When a cow produces methane, the methane molecules contribute to the global warming, and much more so than many carbon-dioxide-producing cars. The same principle applies to incineration: the tiny traces of residual by-products of industrial incineration are neither better nor worse than those produced by our cars, but the former are abhorred.
A versatile type of incinerator is the cement kiln , whose main product is Portland cement, but can also be used for "no-byproduct" incineration. A cement kiln is a rotating cylinder, of the length of a football field, almost horizontal but slightly inclined, with the upper end continuously fed with a mixture of clay and lime or limestone, and the lower end fed with burning fuel. The temperature of thousands of degrees causes the lime and clay react chemically, and a continuous stream of the white-hot molten portland cement flows from the lower end of the cylinder.
Cement kilns benefit from this fact in their auxiliary role of incinerators, since by their nature, they combine incineration with scrubbing those inorganic gases. Kilns convert and lock those gases and ashes into mineral products. This is done by the molten cement and lime covering the entire walls of the rotating kiln. The alkaline properties of the hot, molten mixture neutralize those gases. A conventional incinerator would produce sulfur dioxide, or when equipped with a scrubber, harmless but cumbersome by-products. A cement kiln converts that sulfur dioxide into the mineral of gypsum, later locked in the portland concrete. The chlorine becomes less toxic calcium chloride, the phosphorus becomes the mineral of apatite, fluorine becomes the mineral of spat, and other similar reactions occur. The only remaining gaseous contaminants that leave a cement kiln are small amounts of nitrogen oxide, and very small residues of dioxins.
All organic chemicals, without exception, contain carbon and hydrogen, so practically any organic substance, no matter how harmful, toxic or deadly, when exposed to high-enough temperature in presence of oxygen, will burn, producing water vapor and carbon dioxide. If chemical elements other than carbon and hydrogen (i.e. chlorine, sulfur, phosphorus, nitrogen etc.) are also present, they become liberated and act according to their own nature, which (as a rule) is sometimes harmful, but much less harmful than that of their organic original. Any type of an incinerator is fully capable of conducting incineration up to this point, which solves 99% of the problem of waste destruction, but there remains the other 1%, which causes most of the environmental uproar: those liberated elements of chlorine, nitrogen, sulfur etc., and their inorganic compounds.
A process that is hoped to end up supplanting incineration of plastics (if proved to be efficient) is thermal depolymerisation. The reality however is, that (unlike the high-temperature incineration), the intermediate-temperature thermal depolymerisation produces almost equal mass of hundreds of chemical compounds, for which there is no application, and many of them are hazardous and harmful.
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