User:HLHJ/Thermal breakdown of organic matter






 * At low temperature, volatiles, including some water, evaporate; some changes, such as denaturation (unfolding of proteins) occur below boiling point, but many organic compounds are stable
 * Not until all the water has boiled off can the temperature rise above the boiling point. This consumes a lot of energy.
 * At sufficiently high temperatures, the organic molecules thermally degrade, releasing chemically-altered organic molecules and more volatiles, which leave as smoke. Generally, this process also absorbs energy. Carbon is left behind, and the material becomes black and charred.
 * As temperatures rise further, pyrolysis continues, but more reactions become possible. Carbon can now combine with oxygen, a highly exothermic reaction. The released heat allows more carbon to combine with oxygen, and the reaction can more easily become self-sustaining. Most organic matter contains a few percent nitrogen, which can oxidize as nitrogen oxides, NO2 and NO3.
 * Finally, the material is reduced to a whitish ash. This has only a fraction of the volume and weight of the original organic material, as the water, carbon, and minor constituents have largely left in the smoke (as gas, droplets, and particulate matter).

Other terms
The word "combustion" is taken by some to cover only oxidative combustion, or only self-sustaining exothermic reactions. The word "burned" is commonly used to refer to things that are only charred, as well as those that are oxidatively combusting, such as burned food and a burning log.
 * "Complete combustion" refers to material being reduced to non-flammable ash
 * "Oxidative combustion" is combustion involving oxidation
 * "Pyrolysis", literally "fire-breaking", refers, in organic materials, to the breaking-apart of large organic molecules under heat. Pyrolysis products may be smaller organic molecules or inorganic molecules.
 * "Charring" is the blackening of carbonization; carbonized material is called "char". "Charcoal" is lumps of char, usually used as a fuel.
 * "Distillation" is a chemist's term for separating materials with different boiling points by boiling only one of them, then taking the vapour somewhere eise to cool.
 * "Destructive distillation" is a parallel term for separating out a material by pyrolysing the mixture, then taking the smoke elsewhere. In some cases (i.e., separating sugar and glass) then one material might already have been present, but it can also be used to split the desired molecule out of a larger one.