Gasification is a thermochemical conversion process that transforms a solid com­bustible into a gaseous combustible which shows ease of complete combustion without the need for excess air, ease of turning on and ease of transport and clean­ing of the combustion. The disadvantage is the energy expense that is required for the gasification process. The gas obtained from the gasification of biomasses is called producer gas; it is composed of a mixture of carbon monoxide, hydro­gen, carbon dioxide, methane, hydrocarbons (ethylene, ethane), vapour, nitrogen (in air gasification); it also contains several pollutants such as cinder and char particles (agglomerates of a complex nature which are mainly composed of carbon), tar (complex mixture of condensable hydrocarbons) and oils. Producer gas can be obtained through partial combustion of the biomass (using air or oxygen) or through pyrolytic gasification (using vapour). Gasification in air results in a low calorific power gas (5.5-7.5 MJ/N m3), whereas by oxygen and vapour gasification a medium calorific power gas is obtained (11 MJ/N m3 and 10 MJ/N m3, respectively). For pyrolytic gasification (or indirect heating), an external heat supply is necessary.

All gasification processes involve, with different modalities depending on the technology applied, the following four steps: drying, pyrolysis, oxidation and reduction.

Drying involves the evaporation of the water content, introduced in the reactor, in the biomass. Pyrolysis is the decomposition of the biomass at high temperatures without exposure to oxygen: the products of pyrolysis are gas (containing tar in the vapour state, in addition to substances such as methane, hydrogen, carbon monoxide, carbon dioxide and hydrocarbons with a few carbon atoms) and char. In the oxidation phase, the exothermic reactions, which provide the heat required for the reduction reactions (endothermic), take place from which the constituents of the producer gas originate. There are multiple technologies by which it is possible to realize the gasification and which are mainly distinguished by the manner in which the bio­mass is brought into contact with the gasifying agent. We can distinguish two main reactor categories: fixed bed (updraft, downdraft, crossdraft) and fluid bed (boiling and circulating). Most of the gasificators in use are of the downdraft type [2, 46].