Nowadays, the combustion of biomass in boilers is a widely-adopted technology, used for the production of domestic hot water, hot water for heating or even steam to be utilized in industrial thermal processes. It is noteworthy that in countries like Sweden, biomass combustion systems supply hot water to the district heating networks.

The fuels used in biomass-fired boilers are often pellets, briquettes, wood chips or other wood remnants (e.g. in wood-processing units). During the last few years the technology of biomass-fired boilers has advanced significantly, which has resulted in the reduction of emissions and the improvement of boiler efficiencies. In addition, biomass-fired boilers have appeared in the market which possess sophisticated automation systems for the fuel feeding, the control of the flow of the combustion air and the cleaning of the heat exchangers’ surfaces.

Prior to the investment in a biomass-fired heating system, it is recommended that a systematic calculation of the thermal energy needs of the building is performed, so that the boiler and the buffer tank can be properly dimensioned. In addition, the regular maintenance of the system after the installation is of high importance. Finally, the owner of the system should purchase and use pellets or woody biomass which complies with the recommendations of the boiler’s manufacturer.

The technologies which are applied for the cogeneration of electrical energy and heat from biomass are the following:

• Biomass combustion in a boiler for the production of steam which will drive a steam turbine generator to produce electrical energy. The boilers used for the combustion of biomass are distinguished between fixed- bed boilers and fluidized-bed boilers. In small-scale projects, hot water or thermal oil boilers (instead of steam generating boilers) are used in combination with ORC systems for the generation of electrical energy.

• Biomass gasification and utilization of the produced gas in an internal combustion engine (ICE) or gas turbine in order to generate electrical energy. Gasification is a process that converts carbonaceous materials into gaseous mixtures by reacting raw materials at a high temperature with a limited amount of oxygen and/or steam. The resulting gas mixture consisting of various energy-rich gas products, such as CO, H2 and CH4 is called syngas. The syngas undergoes a cleaning process so that it can be utilized in an internal combustion engine or a gas turbine. Biomass gasifiers are generally distinguished in three main types: fixed-bed, fluidized-bed and entrained-flow systems.

The financial feasibility of an investment in a biomass CHP installation is greatly influenced by the selection of the appropriate technology and the security of long-term supply of the biomass. The scale of the project and the characteristics of the available biomass play an important role in the selection of the appropriate technology for a biomass CHP project.