Energy Capture and Conversion

Biomass by Clyde Stanley

Definition Biomass is defined as matter derived from photosynthesis or other biological processes that is suitable for conversion to energy. Biomass can be burned directly or converted into fuels made of carbon, hydrogen, and oxygen. The remaining elements are nutrients that can be returned to the soil.

Energy Capture and Conversion Biomass captures its energy from the sun through photosynthesis and stores it in the form of cellulose, sugars, and starches within the plant. The energy is readily released through direct combustion. Alternately, the energy may be converted to alcohol (methanol or ethanol) by fermenting these components and distilling the resultant liquid. Fermentation can also yield methane gas. Some algae can produce hydrogen under the proper conditions. If the plant is high in oils, they may be extracted chemically or mechanically to produce biodiesel fuels. Sewage releases methane during the digestion process used in a treatment plant.

Sources Biomass is obtained from a wide variety of organic sources. The most common source is grains, particularly corn, grown for fermentation into alcohol. In tropical areas, sugar cane is grown for the same purpose. Native plants such as switchgrass are being investigated as biomass energy sources because they require fewer resources than cultivated plants, increasing the energy efficiency of the process and reducing the impact on the food supply.

Plant wastes, such as tree trimmings and crop by-products, can be collected, shredded, and fermented to produce alcohol, or dried for burning. Municipal wastes contain a variety of organic materials that can be separated into components suitable for burning, fermentation, or composting.

Wood waste, sewage, and algae are sources of biomass that can be fermented or, in the case of algae, even used to generate hydrogen during photosynthesis under controlled conditions.

Methods of Energy Conversion Most biomass materials can be dried and shredded into a form that is suitable for combustion in a furnace. While biomass combustion is generally cleaner than fossil fuel combustion, it may generate significant particulate pollution. Pollution may be reduced, and storage efficiency increased, by fermenting the biomass into alcohol (methanol or ethanol). Enzymes are used to break down the sugars and starches so they may be fermented. The alcohol is recovered by distillation.

If the plant contains oils, they may be extracted mechanically or with solvents and processed into biodiesel, a cleaner substitute for diesel fuel.

Plants and sewage can be digested with anaerobic bacteria (which grow in the absence of air) to generate methane. This method has the advantage of working with wet biomass, saving the energy that would be required for drying.

Advantages Biomass has the following advantages over other energy sources:  Its energy is obtained directly from the sun via photosynthesis.  The carbon dioxide released by burning was sequestered from the atmosphere by the growing plants, so it recycles atmospheric carbon.  The burning of biomass or biomass-derived fuels releases carbon dioxide, which is a less potent greenhouse gas (by a factor of 27) than the methane that would be released if the biomass were allowed to naturally decompose.  It comes from a wide variety of sources, available in some form throughout the world, so it is widely available in most nations.  Can use any material that will decompose or burn (grass clippings, animal waste, corn stalks, acorns, tree limbs).  It can be obtained from existing food crops, using the 90 percent of the plant that is discarded as waste.

Disadvantages Biomass has the following disadvantages when compared to other energy sources:  Use of cultivated crops for biomass competes for land, water, and fertilizer needed for food, fiber, and timber crops, increasing the prices of these commodities.  Use of crop ‘wastes’ for biomass fuel prevents these materials from being used for animal feed or returned to the soil, resulting in long-term depletion of the soil.  It can contribute to global warming and particulate pollution if directly burned.  Small scale production may result in a net loss of energy (more energy expended to grow and harvest than recovered from end product).  Collection can be labor and energy intensive.  It requires costly equipment for collection, processing, and storage.  Biomass material is subject to degradation during storage.  It requires a large storage volume compared with fossil fuels for the same energy output.  Many of the biomass crops (especially grains) are seasonal.  Production is subject to weather cycles and pest infestations. Conclusion Biomass can supplement the energy supply economically where local conditions are favorable to production and use of a particular fuel source. However, it is unlikely to replace existing fossil fuels on a large scale due to the large volume of material required and competition with agriculture for already strained natural resources.