Photosynthesis is the property possessed by plants, through the action of chlorophyll, to convert electromagnetic radiation into chemical energy, stored mainly in plants as carbohydrate, i.e. as sugar, and used for their growth and reproduction.

Under the action of sunlight, carbon dioxide from the air and water from the soil are processed as carbohydrates from which all the components of the organic matter then derive.

The maximum efficiency of the bioconversion does not exceed 6%, but considering the area covered by plants, this represents considerable amounts of energy.

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. The latter, as we have seen, are the byproduct of fossil plant materials corresponding to a form of biomass stored for millions of years.

Biomass produces three types of resources: forestry, agriculture and aquatic. It is mainly composed of lignin (C40H44O6) (25%) and carbohydrate Cn(H2O)m (cellulose C6H10O5 and hemicellulose) (75%).

Biomass energy can be used in 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.

Biochemical conversion uses wet products. They involve microbiological processes that have the effect of degrading the plant material:

• methane fermentation to produce biogas, a mixture of carbon dioxide (30-35%) and methane (50-65%), good quality fuel. The reactions take place at temperatures between 20 and 70 °C;

• alcoholic fermentation transforms carbohydrates into ethanol;

• acetono-butyl fermentation produces, under the action of certain bacteria, a mixture of butanol, acetone and ethanol.

Thermochemical conversion is divided into combustion and co-combustion (excess air), gasification (lack of air) and pyrolysis (in the absence of air). Thermochemical processes are suitable for dry materials such as wood and straw.

Combustion (T ~ 1900 °C) is the oldest and probably most used conversion mode for both domestic and industrial uses. Its efficiency is good insofar as the fuel is rich in structured carbohydrates (cellulose and lignin), and especially dry enough (humidity below 35%).

Co-combustion consists in simultaneously burning a fossil fuel, usually coal, and some 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 to convert relatively dry biomass (humidity below 10%) of C/N ratio greater than 30, in various high LHV fuels, storable, gaseous, liquid and solid (charcoal). It takes place at temperatures between 400 and 800 °C.

Gasification of biomass (T ~ 800 °C-1000 °C) 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 for the last two phases. It produces a gas called "poor" because of its low heating value (1 kWh/m3 against 10 kWh/m3 for methane). By replacing air with oxygen, a synthesis gas (CO + H2) is obtained and used for the manufacture of methanol.

In a gasifier, the fuel is first dried and 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 the pyrolysis and with water from the drying, which causes a reduction reaction that 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 co-current gasifiers (downdraft), and the counter-current (updraft);

• fluidized bed systems, which consist of three categories depending on the fluidization speed: dense, circulating and entrained fluidized beds.

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

If you want to deepen the study of biomass combustion, refer to its thematic page: