We call biomass all organic materials, mainly of vegetable origin, natural or cultivated, land or sea, produced by chlorophyll conversion of solar energy, fossil fuels excluded.
 

Biomass is mainly composed of lignin (C40H44O6) (25%) and carbohydrate Cn(H2O)m (cellulose C6H10O5 and hemicellulose) (75%).
 

Biomass can be converted in energy by three main categories of processes:

• biochemical conversion: digestion, hydrolysis and fermentation;
• chemical conversion (esterification);
• thermochemical conversion: combustion, co-combustion, pyrolysis and gasification.

 Biochemical conversion comprises two main types: anaerobic digestion and aerobic digestion.

In the first case, which occurs in the absence of oxygen, there is production of a “biogas”, consisting mainly of methane (50–65%), CO2 (30–35%) and other gases. Reactions take place at temperatures between 20 and 70 °C. The basic reaction is: CH3COOH -> CH4 +CO2
 

In the second case, there is production of more CO2 and H2O.

 

Thermochemical conversion is divided into combustion and co-combustion (excess air), gasification (lack of air) and pyrolysis (in the absence of air).
 

Combustion is the oldest and probably most used conversion mode for both domestic and industrial uses. Its effi ciency is good insofar as the fuel is rich in structured carbohydrates (cellulose and lignin), and especially dry enough (humidity below 35%).
 

Co-combustion is to simultaneously burn a fossil fuel, usually coal, and biomass (up to 15%) to reduce, in an existing boiler, the amount of initial fuel.

We call the C/N ratio the ratio of the amounts of carbon and nitrogen in biomass. It varies from 10 to about 100. Pyrolysis allows conversion of relatively dry biomass (humidity below 10%) of C/N atio greater than 30, in various high LHV fuels, storable, gaseous, liquid and solid (charcoal). It takes place at temperatures between 400 and 800 °C, and can be done in several modes: slow pyrolysis or char (solid product only), conventional pyrolysis at moderate temperature (600 °C) (1/3 gas, 1/3 liquid, 1/3 solid), fast pyrolysis at 500–600 °C, which takes place in about one second (70-80% of liquid), flash pyrolysis at over 700 °C and in less than one second (more than 80% of liquid).
 

Gasification of biomass is obtained by performing combustion with lack of air in schematically two main stages: pyrolysis producing gas, liquid and solid phases, followed by the gasification itself of the last two phases.
 

In a gasifier, the fuel begins by being dried and is then pyrolyzed, these two steps being endothermic. The gaseous products are then burned at high temperature, releasing heat, a part of which is used by the previous two steps. The exhaust gases are then put into contact with the solid phase after pyrolysis and with water from the drying, which causes a reduction reaction which leads to the formation of a synthesis gas rich in CO and H2, whose LHV is approximately 70 to 75% of that of the original biomass.

Several gasification processes exist:

• fixed bed systems, which consist of two main technologies, the parallel-flow gasifiers (downdraft), and the counter-flow (updraft);
• fluidized bed systems, which consist of three categories depending on the fluidization speed, dense, circulating and entrained fluidized beds.
  

The first match small or medium facilities, and the latter large ones. 

Modeling thermochemical conversion