What is Bioenergy?

Bioenergy is renewable energy derived from recently living biological material, or biomass.  Fossil carbon sources of energy, such as coal and petroleum, are not sources of bioenergy since these materials are the result of geological processes that transformed plants living  many thousands of years ago.  Bioenergy is a form of renewable energy because the energy contained in biomass is energy from the sun captured through natural processes of photosynthesis, and so long as the quantity of biomass used is equal to or less than the amount that can be regrown it is potentially renewable indefinitely.  Bioenergy includes power and fuels derived from biomass.  Biopower, for example, is electricity generated from combustion of biomass.  Heat and steam, or a combination of both, may also be produced through combustion of biomass, and may be produced in co-generation with electricity.  Biofuel is commonly used to refer to biomass-derived liquid fuels and gases most typically used in transportation.

The majority of biomass for bioenergy comes from three sources: forests, agriculture, and waste (see figure 1.1).  Algaculture is a fourth source currently in development with much promise.  Forest-based biomass includes merchantable stem wood [wood in the stems of trees greater than 5” diameter at breast height (dbh)], tops and branches of harvested trees, and understory trees (less than 5” dbh).  Agriculture-based biomass includes crops grown specifically for bioenergy production, or dedicated bioenergy crops, and plant residues collected after harvest of crops grown for food or feed.  Dedicated bioenergy crops include annual crops grown for their sugars or starches, such as sugarcane and grains, as well as perennial herbaceous and woody crops grown for their cellulose.  Cellulosic bioenergy crops include grasses like switchgrass and woody plants like hybrid poplar trees.  Waste-based biomass includes organic materials leftover from industrial processes such as mill and pulp production, municipal solid wastes, construction wastes, and landfill gas.  Biomass from algaculture involves production of microalgae – organisms less than 0.4 mm in diameter and capable of photosynthesis.  A more detailed overview of specific biomass types is provided in Unit 2.1 of this module.

To make use of the energy available in biomass it is necessary to utilize technology to either release the energy directly, as in burning of biomass materials for heat, or to transform it into other forms such as solid or liquid fuel.  There are three types of conversion technologies currently available, each appropriate for specific biomass types and specific energy products: thermal, chemical and biochemical (see figure 1.2).  As implied by its name, thermal conversion processes use predominantly heat to convert biomass into other forms.  Thermal conversions processes include combustion, torrefaction, pyrolysis and gasification.  Chemical conversion involves use of chemical agents to convert biomass into liquid fuels.  Biochemical conversion involves use of enzymes of bacteria or other microorganisms to break down biomass through processes of anaerobic digestion, fermentation or composting.

Figure 1.1 Current major biomass materials and feedstocks.  (C.L. Williams after Baye, 2010)

Although relevant technologies exist (and continue to be developed), some are not yet cost-effective, particularly for large-scale conversion of cellulosic biomass.  For additional information on conversion technologies see the reference resources at the end this module.

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