James G. Speight - Encyclopedia of Renewable Energy

Biochemical methods of biomass breakdown involve the use of bacteria, yeasts, and enzymes, which also break down carbohydrates. Fermentation, the process used to make wine, changes biomass liquids into alcohol, a combustible fuel. A similar process is used to turn corn into grain alcohol or ethanol, which is mixed with gasoline to make gasohol. Also, when bacteria break down biomass, methane and carbon dioxide are produced. This methane can be captured, in sewage treatment plants and landfills, for example, and burned for heat and power.

Biomass oils, like soybean and canola oil, can be chemically converted into a liquid fuel similar to diesel fuel, and into gasoline additives. Cooking oil from restaurants, for example, has been used as a source to make biodiesel for trucks.

See also: Biofuels, Biochemical Platform, Biomass Conversion, Thermochemical Platform.

The biomass-to-liquid (BTL) process concept is a process for converting various types of biomass to liquid fuels and uses a variety of processes ( Table B-23).

Table B-23Conversion of biomass to fuels.

There are two types of biomass feedstock that can be employed to produce BTL fuels – woody and herbaceous. Woody feedstock comprises wood chips, wood powder and sawdust, obtained from ordinary forestry (wood logs), short-rotation forestry, various wood residues, and wood waste. Herbaceous feedstock includes chaffed dedicated energy crops and straw.

Owing to biomass composition, woody feedstock is better suited for energy applications than herbaceous feedstock. Woody biomass also has a larger production potential for energy (including BTL) application; however, the production potential of herbaceous biomass is currently under-explored as the path to useful products is open, for example ( Table B-24):

Table B-24Routes to liquid fuels from biomass.

Biomass pyrolysis is a process by which a biomass feedstock is thermally degraded in the absence of air/oxygen. It is used for the production of solid (charcoal), liquid (tar and other organics), and gaseous products. These products are of interest as they are possible renewable sources of energy. The study of pyrolysis is gaining increasing importance, as it is not only an independent process but it is also a first step in the gasification or combustion process, and has many advantages over other renewable and conventional energy sources. The actual reaction scheme of pyrolysis of biomass is extremely complex because of the formation of over a hundred intermediate products.

Plasma pyrolysis provides high temperature and high energy for reaction as the reaction sample is rapidly heated up to a high temperature. This review also covers the experimental and modeling study status of plasma-assisted pyrolysis.

Biofuels produced via gasification routes include direct gasoline and diesel substitutes made from gas-to-liquid processes (i.e., the Fischer-Tropsch process), methanol, ethanol, mixed alcohols, and hydrogen. Gas-to-liquids technologies are utilized commercially using natural gas or stranded natural gas as feedstock. Coal was used extensively by Germany in WWII and is still used in the Sasol (South Africa) facilities for gasoline and diesel fuel synthesis along with a wide variety of other products.

In a biomass-to-liquids process, the feedstock undergoes a pretreatment or selection (sizing, drying, and sorting) and is then gasified in a reactor. The gas product (carbon monoxide, hydrogen, low-boiling hydrocarbon derivatives, tars, and particulate material) undergoes extensive cleanup to remove catalyst poisons and other undesirable components. This is followed by gas processing/reforming where the hydrogen/carbon monoxide ratio is adjusted before entering the (Fischer-Tropsch) synthesis reactor. The liquid synthesis reactor contains catalyst material and operates at elevated pressure and temperature forming hydrocarbon compounds or alcohols from the synthesis gas. The liquids can be further refined to the desired end product.

See also: Biomass – Gasification, Biomass to Syngas, Synthesis Gas.

Biomass is the material derived from plants that use sunlight to grow which include plant and animal material such as wood from forests, material left over from agricultural and forestry processes, and organic industrial, human, and animal wastes. Biomass comes from a variety of sources which include (alphabetically and not by preference or use):

Agricultural residues such as straw, stover, cane trash and green agricultural wastes

Agro-industrial wastes, such as sugarcane bagasse and rice husk

Animal wastes

Food processing wastes

Forestry plantations

Forestry residues

Municipal solid wastes (MSW)

Industrial wastes, such as black liquor from paper manufacturing

Sewage

Wood from natural forests and woodlands

The energy contained in biomass originally came from the sun. Through photosynthesis, carbon dioxide in the air is transformed into other carbon-containing molecules (e.g., sugars, starches, and cellulose) in plants. The chemical energy that is stored in plants and animals (animals eat plants or other animals) or in their waste is called bio-energy.

When biomass is burned, it releases its energy, generally in the form of heat. The biomass carbon reacts with oxygen in the air to form carbon dioxide. If fully combusted, the amount of carbon dioxide produced is equal to the amount which was absorbed from the air while the plant was growing.

In nature, if biomass is left lying around on the ground, it will break down over a long period of time, releasing carbon dioxide and its store of energy slowly. By burning biomass, its store of energy is released quickly and often in a useful way. So converting biomass into useful energy imitates the natural processes but at a faster rate.

Biomass wastes can be transformed into clean energy and/or fuels by a variety of technologies, ranging from conventional combustion process to state-of-the-art thermal depolymerization technology. Besides recovery of substantial energy, these technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safe disposal in a controlled manner while meeting the pollution control standards.