Thermochemistry groups several processes bases on cracking biomolecules under the effect of heat. The organic matter decomposed by the heat produces different solid compounds, liquids or gases depending on the operating conditions (temperature, pressure, oxidising agent concentration and moisture).The main thermochemical processes are:
- Excessive combustion of Oxygen, an old, well-known process, that produces heat directly from complete oxidation of the biomass. Noncombustible gases and ashes are formed. The heat given off can be converted into electricity in cogeneration units.
- Pyrolysis, which breaks down the organic matter under the effect of heat without oxydising agents. Three phases are recovered through this process: A gaseous, non-condensable fraction, a liquid fraction otherwise known as pyroligneous liquor (tars, alcohols and acids), and a solid fraction (Carbon). The proportions of these fractions vary according to operating conditions.
- Gasification is oxidation under pressure (air, O2, steam, CO2) that produces fuel gases (CO, H2) and ashes. To favour the reactions involved, it is generally necessary to start the process by pyrolysis as well as homogeneous and heterogeneous oxidation procedures to manufacture coal and the necessary reactants (CO2 and H2O). The calorific value of synthetic gas obtained varies between 3.5 and 6 MJ.Nm3. There are several technologies that can be used to gasify biomass. There are either of the fixed bed type (counter-current, co-current), or of the fluidized bed type (dense, circulating, pressurised, entrained).
- Liquefaction, which produces a bio-oil with a high oxygen and water content, which reduces their calorific potential to less than half of that of petroleum. Bio-oils, however, have low viscosity properties and burn easily in burners, furnaces, turbines and diesel engines. Direct liquefaction is a slow process at high pressure and moderate temperature. We use a catalyser and often a deoxidising agent to produce a heavy and thick bio-oil that will thus have a low water and Oxygen content. We also call this flash pyrolysis.