Biomass Energy

Biomass energy is produced by extracting volatile gases, primarily methane, from organic materials such as wood, switch grass, corn stalks, or municipal solid waste. The variety of methods and levels of efficiency for extracting the gas are extensive. Biomass energy production, either for direct combustion of the gas or electric power generation, has been utilized widely for decades, but recently, large-scale production of biomass energy as a substitute or enhancement for traditional carbon fuel sources has received a great deal of investment and experimentation. Traditionally, a winter fireplace produces biomass energy from wood.

What the new biomass energy industry adds is mass production of energy for use in the economy as well as residential comfort. Gas from biomass sources can be conditioned (cleaned) and injected into existing or new gas lines. Its more frequent use is in combined heat and power (CHP) installations, particularly at landfill sites, for generating electricity. These more recent applications fill a double purpose, in the same vein as the much more recognized solar and wind generation, by both producing energy and reducing greenhouse gas (GHG) emissions into the atmosphere.

Methane, the primary biomass energy source, is rated by the International Panel on Climate Change (IPCC) as 21 times as warming in the atmosphere as carbon dioxide (CO2), the most abundant GHG. When a biomass energy project can extract and combust methane, the potential warming affect is significantly lower than if the methane escapes into the air—even though burning methane produces CO2 along with water:

CH4 + 2O2 = CO2 + 2H2O

Feedstocks and Production Technology Biomass developments must pair feedstock with technology. Applying the wrong technology or process to a biomass source will be unproductive and waste money. Some technology is still being tested, but several are proven and in common use. The most common, reliable, and profitable technology for extracting methane energy is anaerobic (without oxygen) digestion. Retention of a liquid slurry of organic biomass in a storage container for a period, usually, 21–30 days, will allow naturally occurring bacteria to digest the hydrocarbons in the material and produce methane. The bacteria work best in a moderately warm environment, around 98–100 degrees F (36–37 degrees C), and are self-generating. With care, digesters will produce a sustainable, predictable stream of gas for years.

There are many types of organic feedstocks for digesters. The content of the feedstock determines the quantity and quality of biogas production. For centuries, small farms in central Asia have deposited manure from their animals into pits and captured the methane for heating and cooking. Today, thousands of operational digesters produce commercial quantities of gas from dairy and hog manure, waste from farm operations, and even commercial fields of convertible feedstock such as grasses and fibrous plants.

Wastewater (sewage) treatment plants in developed countries employ digesters for reducing pollutants in water before returning it to the environment. The digesters flare excess digester gas that is not used for heating the digesters. This flare gas has become a credible source of bioenergy and can either generate electricity for the power grid or be conditioned for injection to the gas grid. Digesters are attractive technology because they produce a purified dry cake of nitrogen-rich fertilizer, as well as the remaining liquid fertilizer after the biogenesis process is completed.

European enterprises have built pilot plants using dry biomass technology in which large quantities of industrial, commercial, and municipal solid waste can be ground into fine particles and processed to produce the proper moisture content for combustion in boilers or gasifiers.

Often, the best use for such dry biomass is in the form of pellets for ease of storage and shipment. Dry biomass has some of the advantages in efficiency and reduced pollution of biogas, but it is still in its infancy for large production for the energy market. It is an especially attractive power source in densely populated urban areas because it is reduced to a much smaller quantity of ash and debris that can be disposed of in a landfill.

Much effort is being invested in improving pyrolysis of organic feedstock. Pyrolysis occurs when organic materials are heated in the range of about 662–932 degrees F (350–500 degrees C) in a closed container that prevents oxidation. At high temperatures and depending on the fuel, light oils can be produced, as well as biochar, a soil enhancement much sought after in the Amazon region; charcoal; activated charcoal for filters; and other valuable products.

Pyrolysis has the advantage of eliminating infectious (pathogenic) organisms in the biomass, as does anaerobic digestion, producing a clean gas for use in power generation. However, the multiple stages in the process of producing biogas through pyrolysis have so far been uneconomical. Very large pyrolytic plants are in use for enhancing the power production of coal-fired generators, and in northern Europe, pyrolytic plants provide power from wood. Currently, the capital investment required for large, complex engineered plants for pyrolysis reduces a number of economical pyrolysis applications.