The base of thermochemical conversion is the pyrolysis process in most cases. The products of conversion include water, charcoal (carbonaceous solid), biocrude, tars, and permanent gases including methane, hydrogen, carbon monoxide, and carbon dioxide depending upon the reaction parameters such as environment, reactors used, final temperature, rate of heating, and source of heat.

2.1 Combustion

Combustion is the sequence of exothermic chemical reactions between a fuel and an oxi­dant accompanied by the production of heat and conversion of chemical species. During the combustion of lignocellulosic biomass, the heat is generated due to oxidation reaction, where carbon, hydrogen, oxygen, combustible sulfur, and nitrogen contained in biomass react with air or oxygen. By far the most common means of converting biomass to usable heat energy is through straightforward combustion, and this account for around 90% of all energy attained from biomass (http://www. esru. strath. ac. uk/EandE/Web_sites/06-07/Biomass/HTML/ combustion_technology. htm). It contributes over 97% of bioenergy production in the world. Combustion is a proven low-cost process, highly reliable technology, relatively well under­stood and commercially available. There are three main stages that occur during biomass combustion: drying, pyrolysis and reduction, and combustion of volatile gases and solid char.

Typically, the biomass contains high moisture and high oxygen content, which causes to have low heating values for biomass. The high moisture content is one of the most significant disadvantage features. Although the combustion reactions are exothermic, the evaporation of water is endothermic. As the moisture content increases, both the higher heating value (HHV) and lower heating value (LHV) decrease. HHV and LHV are used to describe the heat pro­duction of a unit quantity of fuel during its complete combustion. In determining the HHV and LHV values of a fuel, the liquid and vapor phases of water are selected as the reference states, respectively. The negative linear relationship exists between the moisture content and the heating value. Fouling (alkali and other elements) and corrosion (alkali, sulfur, chlorine, etc.) of the combustor are typical issues associated with biomass combustion. These are con­sidered to be detrimental because of the resulting reduction in heat transfer in the combustor.

There are a number of combustion methods/technologies/reactors available for biomass combustion and the main ones can be categorized under two headings: Fixed-bed combustion systems and fluidized-bed combustion systems.