Biomass gasification consists in the decomposition of a solid, for example wood, by a reactive gas, for example air, to obtain a gaseous fuel. During this process, biomass is subjected to four successive thermochemical phenomena : drying, pyrolysis, oxidation and reduction.
The fuel moisture is removed by evaporation. This is an endothermic process, it occurs at a temperature between 100 °C and 160 °C.
As the temperature of the solid increases, gases are generated by dry biomass from 250 °C. These gases are constituted of non-condensable species (methane, hydrogen, carbon monoxide, carbon dioxide,…) and condensable vapours (tar). The solid residue from this operation is called coke and contains carbon and mineral materials.
Oxidation occurs in the presence of a reactive gas (air, steam, pure oxygen, hydrogen) which condition the calorific value of the gas leaving the gasifier. The use of air as reactive gas is the more common. Oxidation is the phase that provides heat for the three phases of the gasification process.
The coke reacts with water vapour and carbon dioxide, thereby forming hydrogen and carbon monoxide, principal constituents of the combustible gas produced.
In an updraft gasifier, the biomass is supplied through the top of the reactor and the air is injected through the bottom of the unit through a gate. Coke undergoes partial oxidation that provides thermal energy needed for the various process steps. The gas passes through the areas of reduction and pyrolysis and is cooled down drying the biomass. This type of reactor does not allow for tar cracking, thus the syngas produced may contain a high concentration of tar.
In a downdraft gasifier, biomass and oxidizing agent flow in the same direction. The product gas passes through the hot zone which is able to crack tars formed during the reaction of pyrolysis. The product gas thus leaves the reactor at a high temperature, around 700 °C, with a minimum content of tar.
The solid fuel is fluidized by the addition of air at high velocity into the bed which is made of small diameter particles such as sand or alumina to improve the fluidization. The solid thus behaves as a fluid for a good homogenization of temperature and reactants.
The fuel is finely pulverized in the jet of gasifying agent (air, pure oxygen or air / water vapor) and the reaction proceeds at high temperature and high pressure, thereby preventing the formation of tar and methane.
