<P> Thermal conversion processes use heat as the dominant mechanism to convert biomass into another chemical form . Also known as thermal oil heating, it is a type of indirect heating in which a liquid phase heat transfer medium is heated and circulated to one or more heat energy users within a closed loop system . The basic alternatives of combustion (torrefaction, pyrolysis, and gasification) are separated principally by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature). </P> <P> Energy created by burning biomass is particularly suited for countries where the fuel wood grows more rapidly, e.g. tropical countries . There are a number of other less common, more experimental or proprietary thermal processes that may offer benefits such as hydrothermal upgrading (HTU) and hydroprocessing . Some have been developed for use on high moisture content biomass, including aqueous slurries, and allow them to be converted into more convenient forms . Some of the applications of thermal conversion are combined heat and power (CHP) and co-firing . In a typical dedicated biomass power plant, efficiencies range from 20--27% (higher heating value basis). Biomass cofiring with coal, by contrast, typically occurs at efficiencies near those of the coal combustor (30--40%, higher heating value basis). </P> <P> A range of chemical processes may be used to convert biomass into other forms, such as to produce a fuel that is more conveniently used, transported or stored, or to exploit some property of the process itself . Many of these processes are based in large part on similar coal - based processes, such as Fischer - Tropsch synthesis, methanol production, olefins (ethylene and propylene), and similar chemical or fuel feedstocks . In most cases, the first step involves gasification, which step generally is the most expensive and involves the greatest technical risk . Biomass is more difficult to feed into a pressure vessel than coal or any liquid . Therefore, biomass gasification is frequently done at atmospheric pressure and causes combustion of biomass to produce a combustible gas consisting of carbon monoxide, hydrogen, and traces of methane . This gas mixture, called a producer gas, can provide fuel for various vital processes, such as internal combustion engines, as well as substitute for furnace oil in direct heat applications . Because any biomass material can undergo gasification, this process is far more attractive than ethanol or biomass production, where only particular biomass materials can be used to produce a fuel . In addition, biomass gasification is a desirable process due to the ease at which it can convert solid waste (such as wastes available on a farm) into producer gas, which is a very usable fuel . </P> <P> Conversion of biomass to biofuel can also be achieved via selective conversion of individual components of biomass . For example, cellulose can be converted to intermediate platform chemical such a sorbitol, glucose, hydroxymethylfurfural etc . These chemical are then further reacted to produce hydrogen or hydrocarbon fuels . </P>

Land conversion through burning of biomass releases which gas