How is biomass energy used?
One of the advantages of biomass is its versatility. It can be burned directly, or converted into a gas or oil, to generate electricity (biopower) and heat. It can also be converted into liquid fuels (biofuels) for our transportation needs.
There are several ways of turning biomass into electricity and heat, including direct combustion, co-firing, gasification, pyrolysis and anaerobic digestion.
The simplest way, and oldest way, of generating electricity from biomass is to burn it. This is called direct combustion. Direct combustion (or “direct-fired”) systems burn biomass in boilers to produce high pressure steam. This steam turns a turbine connected to a generator. As the steam causes the turbine to rotate, the generator turns and electricity is produced. Most of the world’s biomass power plants use direct combustion. In some cases, the steam from the plants is also captured to heat water and buildings. These are known as cogeneration facilities.
Although this technology is dependable and proven, its efficiency is limited. Direct combustion systems typically have thermal efficiencies around 20 per cent. These efficiencies can be increased through cogeneration.
Co-firing involves burning biomass, along with coal, in traditional power plant boilers. This is considered to be one of the most economic ways to produce electricity from biomass, because existing power plant equipment can be used without major modifications. Some coal-fired power plants in North America use this technology to help reduce the use of coal and, thereby, lower emissions of carbon dioxide, sulphur dioxide and nitrogen oxides. Co-firing also allows biomass to be converted to electricity at a higher thermal efficiency in the 33 per cent to 37 per cent range.
New technologies have been developed to gasify biomass into a useful energy source. These operate by heating solid biomass at high temperatures in an oxygen-deprived environment to produce a fuel gas, which contains between one fifth and one half the heat content of natural gas. This gas can be used to drive highly efficient combined cycle systems to generate electricity. Gasification offers some advantages over direct combustion: for example, gasifying biomass to produce electricity is twice as efficient as burning it directly and results in lower emissions of particulate matter and greenhouse gases. Gasification systems can also be combined with fuel cell systems, which convert hydrogen gas to electricity and heat.
In pyrolysis, biomass is heated to high temperatures in an oxygen-free environment to produce a gas rich in hydrocarbons. This gas is quickly cooled to create an oil-like liquid (bio-oil) and the remaining solid is a charcoal-like residue (called “char”). The bio-oil can then be burned like petroleum to generate electricity while the char can be used for heating.
Certain kinds of bacteria break down or “digest” organic material in the absence of oxygen and produce biogas as a waste product. This biological process is called anaerobic digestion. This process occurs naturally in city dumps or landfill sites containing organic wastes. When these materials are buried, they are digested by bacteria, resulting in biogas (“landfill gas”) rich in methane. This gas can be collected and used to heat buildings, run engines or generate electricity. Biogas can also be produced by mixing plant and animal wastes with water in oxygen-free tanks.
previous | next