Heat generation, commonly referred as the combustion process, is the fastest way to recover energy from solid biomass. In the presence of heat and an excess of oxygen, the biomass decomposes completely into gas which ignites by releasing a large amount of energy in the form of heat.
The combustion of biomass is an oxidation reaction. With an initial supply of heat, oxygen (O2) in the air combines with carbon and hydrogen in the biomass to form carbon dioxide (CO2) and water (H2O) while releasing much energy in the form of heat.
The combustion process takes place in several phases:
- drying: thanks to heat, the water contained in the biomass evaporates;
- pyrolysis: under the action of heat, the constituents of the biomass are decomposed into gas and fine droplets of tar which vaporise. The majority of these gaseous compounds are combustible. This decomposition leaves a carbonaceous residue;
- combustion of gases: as soon as they escape from the piece of wood, the decomposition gases combine with oxygen and burn, generating the flame;
- combustion of the carbonaceous residue: when the gases have cleared, the carbonaceous residue burns (incandescence of the embers).
When biomass burns, these four steps overlap permanently. As long as gases are released from the fuel, the temperature of the latter remains close to 800 °C while the flames are around 1,300 °C. When the gases have cleared, the carbonaceous residue burns at about 1000 °C.
A great diversity of fuel sources
Fuels used to produce heat can be of fossil origin, i.e. non-renewable, such as natural gas or fuel oil, or of natural origin and renewable, such as wood (wood chips, logs or pellets), biogas, sewage sludge, etc. Almost any biomass can be recovered in heat under conditions of being previously dried.
The fast development of new solid biomass fuels has pushed stove and boiler manufacturers to adapt their equipment to reap the benefit of these upgraded materials (woodchips, wood pellets, briquettes). Far from the traditional fireplace, a great deal of technical innovation has occurred over the past decades, improving installations’ thermal efficiency, grip, reliability and maintenance. Faced with such a great diversity of appliances, it can be sometimes difficult for an individual or a company to find its way.
In general, one can differentiate two main types of appliances: stoves and boilers. A stove is a local space heater made for generating heat in the room it is located. In some cases, a hot air circulation system can be installed for heating up several rooms. Modern stoves are now semi or fully automatised and new domotic systems are now reaching their commercialisation phase to simplify the maintenance of the operation of installations.
A boiler is made for heating up the water of the central heating system. Boilers can be fully automatised with large capacity storage allowing at least one year of range. Wood pellet boilers for central heating exist for every power requirement – ranging from a few kW for low-energy houses to over 1000 kW for larger buildings, district heating and industrial processes.
The efficiency of various heating systems varies from 10-15% for open stoves, to more than 95% for modern stoves and boilers. It should be noted that the best performance, both from the energy and environmental point of view (atmospheric emissions), comes from the modern pellet-fired stove and boiler.