U.S. Department of Energy - Energy Efficiency and Renewable Energy
Bioenergy Technologies Office
Biomass power (biopower) technologies convert renewable biomass fuels into heat and electricity using processes similar to that used with fossil fuels. Next to hydropower, more electricity is generated from biomass than any other renewable energy resource in the United States. A key attribute of biomass is its availability upon demand—the energy is stored within the biomass until it is needed; whereas, other forms of renewable energy are dependent on variable environmental conditions, such as wind speed or sunlight intensity.
Until several decades ago in the United States, biomass was primarily used to provide heat for cooking and comfort. Technologies that can generate electricity from the energy in biomass fuels have since been developed. The scale is small enough to be used on a farm or in remote villages or large enough to provide power for a small city.
Electricity from Biomass/Technologies at Work
There are four primary classes of biopower systems: direct-fired, co-fired, gasification, and modular systems.
Direct-fired systems make up most of today's biopower plants and are similar to most fossil-fuel-fired power plants. The biomass fuel is burned in a boiler to produce high-pressure steam. This steam is introduced into a steam turbine, where it flows over a series of aerodynamic turbine blades, causing the turbine to rotate. The turbine is connected to an electric generator, so as the steam flow causes the turbine to rotate, the electric generator turns and electricity is produced.
While steam generation technology is very dependable and proven, its efficiency is limited. Biomass power boilers are typically in the 20–50 MW range, compared to coal-fired plants in the 100–1500 MW range. The small capacity plants tend to be lower in efficiency because of economic trade-offs; efficiency-enhancing equipment cannot pay for itself in small plants. Although techniques exist to push biomass steam generation efficiency to more than 40%, actual plant efficiencies are in the low 20% range.
Co-firing involves substituting biomass for a portion of coal in an existing power plant furnace. It is the most economic near-term option for introducing new biomass power generation. Because much of the existing power plant equipment can be used without major modifications, cofiring is far less expensive than building a new biopower plant. Compared to the coal it replaces, biomass reduces sulphur dioxide (SO2), nitrogen oxides (NOx), and other air emissions. After "tuning" the boiler for peak performance, there is little or no loss in efficiency from adding biomass. This allows the energy in biomass to be converted to electricity with the high efficiency (in the 33%–37% range) of a modern coal-fired power plant.
Biomass gasifiers operate by heating biomass in an environment where the solid biomass breaks down to form a flammable gas. This process offers several advantages over directly burning the biomass:
- The biogas can be cleaned and filtered to remove problem chemical compounds.
- The gas can be used in more efficient power generation systems called combined-cycles, which combine gas turbines and steam turbines to produce electricity.
- The efficiency of these systems can reach 60%.
Gasification systems will be coupled with fuel cell systems for future applications. Fuel cells convert hydrogen gas to electricity (and heat) using an electrochemical process—there are very little air emissions and the primary exhaust is water vapor. As the costs of fuel cells and biomass gasifiers come down, these systems will proliferate.
Modular systems employ some of the same technologies mentioned above, but on a smaller scale that is more applicable to villages, farms, and small industry. These systems are now under development and could be most useful in remote areas where biomass is abundant and electricity is scarce. There are many opportunities for these systems in developing countries.