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-16 Selected properties of representative biomass materials.
| Mass %, dry | Wood | Grain* | Municipal Solid Waste** | Animal Wastes (Manure) |
|---|---|---|---|---|
| Carbon | 50-53 | 45.0 | 47.6 | 35.1 |
| Hydrogen | 5.8-7.0 | 5.8 | 6.0 | 5.3 |
| Nitrogen | 0-0.3 | 2.4 | 1.2 | 2.5 |
| Sulfur | 0-0.1 | 0 | 0.3 | 0.4 |
| Oxygen | 38-44 | 42.5 | 32.9 | 38.7 |
| Volatile matter | 77-87 | 70-80 | 77 | 76.5 |
| Fixed carbon | 13-21 | — | 11 | 0 |
| Ash | 0.1-2.0 | 4.0 | 12 | 23.5 |
| H/C atom ratio | 1.4-1.6 | 1.5 | 1.5 | 1.8 |
| GCV, MJ/kg (dry) | 19.8-21.0 | 16.8 | 19 | 13.4 |
| Moisture, % | 25-60 | 16 | 20 | 7-35 |
| * Red corn cob (corn stover) contains approximately 25% cellulose, 10% lignin, and 15% moisture. ** Combustible portion; may contain 9% metals and 12% glass/ceramics on an as-received basis. | ||||
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.
Combustion offers the most direct route for energy recovery from biomass, and is an effective means of utilizing the total energy content of whole wood and other biomass. Technology for small-scale combustion is certainly well developed. Larger-scale operation for electricity generation at the 5 to 50 MW level should be feasible, although it is not expected to be economically competitive with crude oil- or natural-gas-fired systems. Due to the large land area over which wood must be harvested, the seasonal nature of the supply, and the large volume of material to be transported and stored, the reliable provision of wood to a large plant can present considerable problems.
Biofuels are derived from biomass and have the potential to produce fuels that are more environmentally benign than crude oil-based fuels. In addition, ethanol, a crop-based fuel alcohol, adds oxygen to gasoline thereby helping to improve vehicle performance and reduce air pollution. Biodiesel, an alternative or additive to crude oil diesel, is a nontoxic, renewable resource created from soybean or other oil crops.
Direct biofuels are biofuels that can be used in existing unmodified crude oil engines. Because engine technology changes all the time, direct biofuel can be hard to define; a fuel that works well in one unmodified engine may not work in another. In general, newer engines are more sensitive to fuel than older engines, but new engines are also likely to be designed with some amount of biofuel in mind.
See also: Biofuels.
Biomass Ash
The mineral matter in biomass is reflected in the yield of mineral ash that is produced during combustion of the biomass. Biomass ash is naturally alkaline. The mineral matter content of wood is small wood but can be as high as 20% w/w