Fire, and the end product of a perfect combustion – fine, smooth ash.
Perfect – Imperfect
Wood can burn either perfectly or imperfectly. In order to get all energy from the wood, the burning needs to be clean and perfect.
A good example of imperfect combustion is the flame of match. After the flame has burned, an ashy stick is what is left. In a process like this, 67 % of the heat is not gained, because the burning match does not reach the combustion temperature of carbon, 800 Celsius degrees.
Wood can burn either slowly when it is rotting in a forest, or fast in high temperature.
Perfect Combustion of Carbon in a Two-Phase Process
Perfect combustion of carbon always takes place in two phases:
In the first phase carbon and oxygen in the air combine producing carbon monoxide, also called coal gas. Coal gas is invisible, odourless and very toxic combustion gas. The process requires 800 Celsius degrees combustion temperature, in which carbon ignites. After this first phase, 28 % of the carbon’s total energy has become heat energy.
In the second phase, higher combustion temperature is needed: 850 Celsius degrees. By adding oxygen, carbon monoxide, or coal gas is burned and transformed into carbon dioxide. Now carbon has given out the rest of its energy content, 72 %. What is left of the wood is only clean, gray ash.