Chemistry of gasification

Gasification is a quite complex thermo-chemical process. Conceptually zones of the gasifier may be divided into different stages viz., drying, pyrolysis, oxidation and reduction. The reactions occur at the same time in different parts of gasifier. The generation of gas occurs in two significant steps. The first step involves exothermic reactions of oxygen in air with the pyrolysis gas under fuel-rich conditions. The second step involves the endothermic reaction of these gases largely CO2 and H2O with hot char leading to product gases namely, CO, H2 and CH4.
Drying
Biomass fuel consists of moisture ranging from 5 to 35% and at the temperature above 100C, the water is removed and converted into steam. In the drying stage, fuels do not experience any kind of decomposition.
Pyrolysis
Pyrolysis is the thermal decomposition of biomass fuel in the absence of oxygen. Pyrolysis involves release of three kinds of products: solid, liquid and gases. The ratio of products is influenced by the chemical composition of biomass fuels and the operating conditions. The heating value of gas produced during the pyrolysis process is low (3.5 - 8.9 MJ/m 3). It is noted that no matter how gasifier is built, there will always be a low temperature zone, where pyrolysis takes place, generating condensable hydrocarbon.
Oxidation
Air is introduced in the oxidation zone contains oxygen, water vapours and inert gases such as nitrogen and argon. These inert gases are considered to be non-reactive with the fuel constituents. Oxidation takes place at 700-2000C temperature. Heterogeneous reaction takes place between oxygen in the air and solid carbonised fuel, producing carbon monoxide. Positive and negative symbol indicate the release and supply of heat energy during the process respectively.
C + O 2 → CO 2 + 406 MJ/kmol
Hydrogen in fuel reacts with oxygen in the air blast, producing steam.
H 2 + ½ O 2 → H 2 O + 242 MJ/kmol
Reduction
In the reduction zone, a number of high temperature chemical reactions take place in the absence of oxygen. The principal reactions that take place in reduction are mentioned below:
Boudouard reaction: CO 2 + C → 2CO - 172.6 MJ/kmol
Water-gas reaction: C + H2 O → CO + H 2 - 131.4 MJ/kmol
Water shift reaction:CO 2 + H 2 → CO + H 2 O + 41.2 MJ/kmol
Methane production reaction:C + 2H 2 → CH 4 + 75 MJ/kmol
Main reactions show that heat is required during the reduction process and hence, the temperature of gas goes down during this stage. If complete gasification takes place, all the carbon is burned or reduced to carbon monoxide, a combustible gas and some other mineral matter is vaporized. The remains are ash and some char (unburned carbon).