Wood is a very competitive source of renewable energy, particularly for heating purposes. In the energy sector, forest biomass is used:
• in biomass boilers, gasifiers and pyrolysis units;
• by biomass pellet, densified wood or biofuel briquette manufacturers;
• by liquid or gaseous biofuel producers.
Each biomass conversion technology has its own characteristic energy performance. Technology choice is based on:
• energy requirements;
• availability, proximity and quality of the biological feedstock;
• transformation and distribution costs;
• environmental context; and
• availability of technology for using the end product.
Biomass must be processed before use. Clean biomass is shredded. If it is contaminated (sand, earth, rocks), it is put through a grinder. The moisture content of the feedstock determine the fuel’s energy content and its energy efficiency ratio can be increased by 20 to 30% through drying. Drying also conserves feedstock quality during storage.
There are several technologies for forest biomass exploitation: densification, combustion, gasification, carbonization, torrefaction, pyrolysis and cogeneration.
Ground, dried biomass is compressed into pellets, logs or briquettes used for fuel. These may be used instead of heating oil.
Wood pellets generally are free of synthetic additives and their manufacturing is controlled. Ease of handling and storage make them the ideal fuel for completely automated heating systems.
Logs and briquettes are used in fireplaces, in log boilers, wood stoves and large automatic boilers. Some are made of sawdust and wood chips to which paraffin or vegetable oil has been added and others are untreated compressed sawdust and wood chips.
Combustion technologies use biomass to produce heat locally. Burning pretreated biomass can produce heat, generate electricity or a combination of both, which is called cogeneration. The burning of biomass produces heat, ashes and combustion gasses that must be cleaned before being released in the atmosphere. There are several types of equipment designed to get rid of most of suspended particulate matter.
Pyrolysis is an unavoidable stage in the process of gasification. Syngas produced by forest biomass gasification systems contains hydrogen, carbon monoxide and tars, among others. Gasification also produces syngas and ashes. There are several types of fireplaces, heat exchangers and equipment that clean out these gases.
Carbonisation densifies wood heat capacity, producing biochar used to replace metallurgical coke used by the steel industry. Easily transported and stored, biochar can be used alone for fuel or can be mixed with petroleum products such as heating gas-oil. Biomass carbonisation produces biochar and tar.
Torrefaction is a thermal decontamination process. It is a stage between drying and carbonisation. In 2011, several torrefaction technologies were being developed to produce a high energy density biofuel similar to that of the coal used in electric power generation. Replacing fossil coal with torrefied pellets would reduce CO2. Biomass torrefaction produces torrefied biomass and fuel gas.
Pyrolysis is a thermal decomposition of forest biomass; it produces pyrolysis oil used in biofuels, high value added chemical and industrial products and soil improvement. Canada is the world leader in the field. This technology increases wood heat capacity in a transportable and storable liquid that can be used in the transportation sector. Biomass pyrolysis produces biochar, pyrolysis oil and pyrolysis gasses.
Cogeneration is achieved by pairing a biomass boiler with a steam turbine. Producers are increasingly using polygeneration strategies, combining electric power generation, heat and biofuel production and pellet manufacturing.
This text is a translation of web content written with the help of engineers conversant with biomass energy. It is featured on the English website of the Association québécoise de la production d’énergie renouvelable.