Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 203

Direct combustion of biomass is a thermochemical technique in which the biomass is burned (combusted) in open air or in the presence of excess air. In this process, the photosynthetically stored chemical energy of the biomass will be converted into gases, unless char is also produced.

There are five thermal approaches that are commonly used to convert biomass into a renewable fuel: direct combustion, gasification, liquefaction, pyrolysis, and partial oxidation.

When biomass is heated under oxygen-deficient conditions, it generates synthesis gas that consists primarily of hydrogen and carbon monoxide. This syngas can be directly burned or further processed for other gaseous or liquid products. In this sense, thermal or chemical conversion of biomass is similar to that of coal.

Indoor direct combustion of biomass fuels in unvented cooking and heating spaces has caused considerable health problems to the direct users, primarily the women and children of developing countries. Biomass fuels, when used improperly in this manner, release considerable amounts of toxic or hazardous gases into the unvented area. These gases are typically: carbon monoxide (CO), nitrogen oxides (NO_x), hydrocarbon derivatives, organics, aldehydes, and trace amounts of aromatics and ketones. As the moisture content of the wood increases, and as other biomass fuels of lower energy content (such as animal and crop waste) are used, the emissions increase.

When the direct combustion of biomass is conducted in a well-ventilated area, biomass burning used for domestic stoves and boilers can be a sound substitute for combustion of conventional fossil fuel.

Most electrical power generation systems are relatively inefficient, due to the loss of a significant portion of energy, as much as half to two-thirds, in a form of waste heat. If this heat is used efficiently for industrial manufacture, space heating, district heating, or other purposes, the overall efficiency can be greatly enhanced. Therefore, smaller biopower systems are more suitable for cogeneration type of processes than much larger counterparts.

See also: Biomass, Biomass-Chemistry, Biomass – Combustion, Biomass – Gasification, Biomass – Liquefaction, Biomass – Pyrolysis.