Bioenergy is energy made or generated from biomass, which consists of recently living (but now dead) organisms, mainly plants. Types of biomass commonly used for bioenergy include wood, food crops such as corn, energy crops and waste from forests, yards, or farms.
The IPCC (Intergovernmental Panel on Climate Change) defines bioenergy as a renewable form of energy. Bioenergy can either mitigate (i.e. reduce) or increase greenhouse gas emissions. There is also agreement that local environmental impacts can be problematic.
Terminology
Since biomass can be used as a fuel directly (e.g. wood logs), the terms biomass and biofuel have sometimes been used interchangeably. However, the word biomass usually denotes the biological raw material the fuel is made of. The terms biofuel or biogas are generally reserved for liquid or gaseous fuels respectively.
Input materials
Wood and wood residues is the largest biomass energy source today. Wood can be used as a fuel directly or processed into pellet fuel or other forms of fuels. Other plants can also be used as fuel, for instance maize, switchgrass, miscanthus and bamboo. The main waste feedstocks are wood waste, agricultural waste, municipal solid waste, and manufacturing waste. Upgrading raw biomass to higher grade fuels can be achieved by different methods, broadly classified as thermal, chemical, or biochemical:
Thermal conversion processes use heat as the dominant mechanism to upgrade biomass into a better and more practical fuel. The basic alternatives are torrefaction, pyrolysis, and gasification, these are separated mainly by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature).
Many chemical conversions are based on established coal-based processes, such as the Fischer-Tropsch synthesis. Like coal, biomass can be converted into multiple commodity chemicals.
Biochemical processes have developed in nature to break down the molecules of which biomass is composed, and many of these can be harnessed. In most cases, microorganisms are used to perform the conversion. The processes are called anaerobic digestion, fermentation, and composting.
Applications
Biomass for heating
Biofuel for transportation
Based on the source of biomass, biofuels are classified broadly into two major categories, depending if food crops are used or not:
First-generation (or "conventional") biofuels are made from food sources grown on arable lands, such as sugarcane and maize. Sugars present in this biomass are fermented to produce bioethanol, an alcohol fuel which serves as an additive to gasoline, or in a fuel cell to produce electricity. Bioethanol is made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as corn, sugarcane, or sweet sorghum. Bioethanol is widely used in the United States and in Brazil. Biodiesel is produced from the oils in for instance rapeseed or sugar beets and is the most common biofuel in Europe.
Second-generation biofuels (also called "advanced biofuels") utilize non-food-based biomass sources such as perennial energy crops and agricultural residues/waste. The feedstock used to make the fuels either grow on arable land but are byproducts of the main crop, or they are grown on marginal land. Waste from industry, agriculture, forestry and households can also be used for second-generation biofuels, using e.g. anaerobic digestion to produce biogas, gasification to produce syngas or by direct combustion. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is being developed as a feedstock for ethanol production, and biodiesel can be produced from left-over food products like vegetable oils and animal fats.
Production of liquid fuels
Biomass to liquid
Bioconversion of biomass to mixed alcohol fuels
Comparison with other renewable energy types
Land requirement
The surface power production densities of a crop will determine how much land is required for production. The average lifecycle surface power densities for biomass, wind, hydro and solar power production are 0.30 W/m2, 1 W/m2, 3 W/m2 and 5 W/m2, respectively (power in the form of heat for biomass, and electricity for wind, hydro and solar). Lifecycle surface power density includes land used by all supporting infrastructure, manufacturing, mining/harvesting and decommissioning.
Another estimate puts the values at 0.08 W/m2 for biomass, 0.14 W/m2 for hydro, 1.84 W/m2 for wind, and 6.63 W/m2 for solar (median values, with none of the renewable sources exceeding 10 W/m2).
