rate or the vacuum relief device fails.
Gas leaving the digester is almost saturated with water vapor. As the gas cools, the water vapor condenses causing problems, which are more severe when digesters are heated. It is essential to remove as much of the moisture as possible before the gas comes into contact with the gas system devices. For this reason, water traps should be located as close to the digester as possible. All piping should be sloped a minimum of 1% toward the water trap, which should be situated at a low point in the gas line.
Flame traps are emergency devices installed in gas lines to prevent flames traveling back up the gas line (flashback) and reaching the digester. The flame trap generally consists of a box filled with stone or a metal grid. If a flame develops in the gas line, the temperature of the flame is reduced below the ignition point as it passes through the trap and the flame is extinguished. Many anaerobic digestion waste treatment plants have a means of storing excess gas, either in the form of a floating roof on the digester or a separate gasholder.
A mixture of biogas and air can be explosive. Methane gas in concentrations of between 5% and 15% in air by volume is explosive and all piping and equipment must be sealed properly to prevent gas from escaping to the outside. In addition, all electrical installations, including light switches, and torches must be of the explosion-proof type, as the smallest spark could ignite escaped gases.
See also: Aerobic Digestion, Anaerobic Digestion.
Animal Waste
Animal wastes, or manure, as a source of biomass has the advantage that it is not competitive with other uses for this material. In the broader senses, animal waste can also include, in addition to animal manure, related animal waste (such as bedding and feed), dead animals, and waste from slaughter and meat processing. All such wastes generally fall in to a sub-class of biomass. Animal waste must of necessity come from confined operations such as dairy farms, cattle feedlots, and slaughter houses as well as meat processing plants.
Animal waste from farms and livestock/poultry and dairy production operations can severely threaten water quality if not managed properly and has the potential to contribute excess nutrients, pathogens, organic matter, solids, and odorous compounds to the environment. This pollution can cause eutrophication of surface waters, degradation of ground water quality, and threats to human health.
Historically, manure generated by livestock has been returned to the soil to improve its tilth and fertility. Recently, the use of animal waste to produce an renewable fuel (biogas) has become of interest. Anaerobic digestion is a renewable solution to livestock waste management that offers economic and environmental benefits.
Anaerobic digestion is a process to decompose organic material in the absence of oxygen. Biogas is produced as a waste product of digestion. In the first stage, the volatile solids in manure are converted into fatty acids by anaerobic bacteria (acid formers). In the second stage, these acids are further converted into biogas by more specialized bacteria (methane formers). With proper planning and design, this anaerobic-digestion process can be managed to convert the waste-stream from a farm into an asset and is, currently, the most appropriate option for farms and small-scale operation. The conversion of animal waste to gas by this process could result in animal husbandry operations being self-sufficient in energy.
Biogas produced in an anaerobic digester typically contains methane (60 to 70% v/v), carbon dioxide (30 to 40% v/v), various toxic gases (including hydrogen sulfide, ammonia, and sulfur-derived mercaptans), and 1 to 2% v/v water vapor.
See also: Agricultural Waste, Alternate Fuels, Anaerobic Digestion, Biogas.
Annual Removal
The annual removal is the net volume of growing stock trees removed from the inventory during a specified year by harvesting, cultural operations, such as, timber stand improvement, land clearing, and removal of trees killed or damaged by natural causes (natural losses), e.g. fire, wind blow, insects and diseases.
An example is clipping (i.e., harvesting aboveground plant biomass) which is common in agriculture and for bioenergy production.
Forest ecosystems contain large amounts of nutrients in woody biomass that may exist either as standing material, on the soil surface, or within the soil profile. Logs removed during timber harvesting remove considerable amounts of nutrients, and the disturbance caused by the process may sometime later, if not immediately, affect the amount of nutrients left on the site due to increased soil erosion, mineralization, and leaching. Thus, intensive harvesting, which removes a greater proportion of the forest biomass than conventional harvesting and the associated nutrients, may cause a decline in forest productivity.
Separately, warming and clipping alter soil and plant properties in either similar or contrasting fashions. For example, in grassland areas, both warming and clipping were observed to increase soil temperature and decrease soil moisture. In contrast, warming increased net primary productivity and plant carbon input to soil, but clipping reduced both. Moreover, warming led to an extended growing season length, while clipping caused compensatory root growth and stimulated exudation of carbon. The interactive effects of warming and clipping on these soil and plant properties rely on the mechanisms governing each single factor and the degree to which these factors interact. The responses of soil microbial communities to two or more factors are even less predictable than those of soil and plant properties, owing to their extremely high diversity.
See also: Biomass.
Apparent Density
The apparent density (bulk density) is the weight per unit volume of a material, such as coal, which includes the voids that exist in the tested material (Table A-22).
Table A-22 Measurement of the apparent density of materials.
| Method | Use | Test |
|---|---|---|
| A | For fine granules and powders that can be poured through a small funnel. | Test is performed by pouring the material through a funnel into a cylinder of known volume. The apparent density is calculated by dividing the weight of the material in the cylinder by the volume of the cylinder. |
| B | For coarse, granular materials that either cannot be poured or that pour with difficulty through the funnel from Method A. | Test is performed by pouring the material through a funnel into a cylinder of known volume. The apparent density is calculated by dividing the weight of the material in the cylinder by the volume of the cylinder. |
| C | For coarse flakes, chips, cut fibers, or strands that cannot be tested with Methods A or B. | Test is performed by pouring the material into a graduated cylinder and allowing a 2300-g plunger to pack the material for 1 minute. The apparent density is taken as the mass of the material divided by the settled volume. |
The bulk factor is the ratio of the density of a material after molding to the density of the raw material and provides a measure of the volume change that can be expected during processing.
