James G. Speight - Encyclopedia of Renewable Energy

Biomass feedstocks generally produce a biogas rich in methane. This medium-to-high Btu (heat content) gas can, in some instances, be upgraded to a substitute natural gas (SNG). However, depending on the feedstock, non-negligible amounts of sulfur are also produced ( Table B-13).

Table B-13Comparison of different biogas feedstocks.

See also: Anaerobic Digestion – Gas Production.

Biological hydrogen (biohydrogen) is hydrogen produced biologically. The main reactions involve fermentation of sugar derivatives, such as glucose:

A related reaction produces formic acid (leading to the potential for the production of formate derivatives (esters of formic acid) instead of carbon dioxide:

See also: Biohydrogen.

Biomass refers to biological material derived from living or recently living organisms, such as plants or plant-derived materials. As an energy source, biomass can either be used directly via combustion to produce heat, or indirectly after converting it to various forms of biofuel. Conversion of biomass to biofuel can be achieved by different methods, which are broadly classified into: (i) thermal, (ii) chemical, and (iii) biochemical methods. This biomass conversion can result in fuel in gas, liquid, or solid form.

Biomass is a renewable energy source unlike other resources such as crude oil, natural gas, tar sand, coal, and oil shale. Agricultural products specifically grown for biofuel production include crops such as corn, soybeans, rapeseed, wheat, sugar beet, sugar cane, palm oil, Jatropha, as well as wood ( Table B-14).

Biomass is generally produced in a sustainable manner from water and carbon dioxide by photosynthesis. There are three main categories of biomass—primary, secondary, and tertiary.

Table B-14Major categories of biomass feedstocks.

Biomass is biological organic matter but is more often used to refer to (i) energy crops grown specifically to be used as fuel, such as fast-growing trees or switch grass, (ii) agricultural residues and by-products, such as straw, sugarcane fiber, and rice hulls, and (iii) residues from forestry, construction, and other wood-processing industries.

Many different types of biomass can be grown for the express purpose of energy production ( Table B-15).

Table B-15Sources, processing options, and uses of biomass-derived products *.

Crops that have been used for energy include sugar cane, corn, sugar beets, grains, elephant grass, kelp (seaweed), and many others. There are two main factors which determine whether a crop is suitable for energy use. Good energy crops have a high yield of dry material per unit of land (dry tonnes/hectare). A high yield reduces land requirements and lowers the cost of producing energy from biomass. Similarly, the amount of energy which can be produced from a biomass crop must be less than the amount of energy required to grow the crop. In some circumstances like the heavily mechanized corn farms in the U.S. Midwest, the amount of ethanol which can be recovered from the corn is barely larger than the fuel required for tractors, fertilizers, and processing.

The components of biomass include triglycerides, sterols, alkaloids, resins, terpenes, terpenoids, and waxes. This includes everything from primary sources of crops and residues harvested/collected directly from the land to secondary sources such as sawmill residuals, to tertiary sources of post-consumer residuals that often end up in landfills. A fourth source, although not usually categorized as such, includes the gases that result from anaerobic digestion of animal manures or organic materials in landfills.

Primary biomass is produced directly by photosynthesis and includes all terrestrial plants now used for food, feed, fiber, and fuel wood. All plants in natural and conservation areas (as well as algae and other aquatic plants growing in ponds, lakes, oceans, or artificial ponds and bioreactors) are also considered primary biomass. However, only a small portion of the primary biomass produced will ever be harvested as feedstock material for the production of bioenergy and by-products.

Biomass feedstocks and fuels exhibit a wide range of physical and chemical properties ( Table B-16).

Table B-16Selected properties of representative biomass materials.

For example, there are many types of coal and the gross heating value of these types varies from 8,600-12,900 Btu/ lb. However, nearly all kinds of biomass feedstocks destined for combustion range from 6,450 to 8,200 Btu/lb. For most agricultural residues, the heating values are even more uniform – approximately 6,450 to 7,300 Btu/lb; the values for most woody materials are 7,750 to 8,200 Btu/lb. Moisture content is probably the most important determinant of heating value. Air-dried biomass typically has approximately 15 to 20% moisture, whereas the moisture content for oven-dried biomass is around 0%. Moisture content is also an important characteristic of coals, varying in the range of 2 to 30%. However, the bulk density (and hence energy density) of most biomass feedstocks is generally low, even after densification, approximately 10 and 40% of the bulk density of most fossil fuels. Liquid biofuels have comparable bulk densities to fossil fuels.