Another term – agrifuel – also referred to a fuel produced directly from crops and other agricultural sources. The term is also more generally applied to organic matter, converted to transportation fuels that reduce the use of imported oil and are more environmentally friendly.
Biodiesel is also an agrifuel that is blended with or replaces crude oil diesel. With biodiesel, any diesel engine can become a renewable fuel engine. Biodiesel is made in the USA from vegetable oils, animal fats, and used restaurant grease. Biodiesel is safer to use, handle, and store than crude oil diesel and is less environmentally damaging to produce, cleaner at the tailpipe (lower emissions), and less damaging if it spills on the ground or in water.
In addition, agricultural biodiesel (agri-biodiesel, agribiodiesel) is biodiesel produced from virgin vegetable oils derived from corn, soybeans, sunflower seeds, canola, cottonseeds, rapeseeds, safflowers, flaxseeds, rice bran, and mustard seeds, as well as from animal fats.
See also: Agrofuel, Biodiesel.
Air Emissions
Air emissions can arise from any one of several sources in any energy production plant, including: (i) combustion emissions associated with the burning of fuels, including fuels used in the generation of electricity, (ii) equipment leak emissions, which are also referred to (fugitive emissions) released through leaking valves, flanges, pumps, or other process devices, (iii) process vent emissions, which are also referred to as (point source emissions) released from process vents during manufacturing, (iv) storage tank emissions released when products are transferred to and from storage tanks, and (v) wastewater system emissions from tanks, ponds, and sewer system drains.
The numerous process heaters used in refineries to heat process streams or to generate steam (boilers) for heating or steam stripping, can be potential sources of sulfur oxides (SOx), nitrogen oxides (NOx,) carbon oxides (CO, CO2), particulate matter, and volatile organic compounds, VOCs). When operating properly and when burning cleaner fuels such as natural gas, fuel gas, fuel oil, and process gas, these emissions are relatively low.
The three main greenhouse gases that are products of the various processes are carbon dioxide, nitrous oxide, and methane. Carbon dioxide is the main contributor to climate change. Methane is generally not as abundant as carbon dioxide but is produced during refining and, if emitted into the atmosphere, is a powerful greenhouse gas and more effective at trapping heat. However, gaseous emissions associated with biomass conversion to fuels typically include process gases, volatile organic compounds (VOCs), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), particulates, ammonia (NH3), and hydrogen sulfide (H2S). These effluents may be discharged as air emissions and must be treated. However, gaseous emissions are more difficult to capture than wastewater or solid waste and, thus, are the largest source of untreated wastes released to the environment.
In addition to the corrosion of equipment by acid gases, the escape into the atmosphere of sulfur-containing gases can eventually lead to the formation of the constituents of acid rain, i.e., the oxides of sulfur (SO2 and SO3). Similarly, the nitrogen-containing gases can also lead to nitrous and nitric acids (through the formation of the oxides NOx, where x = 1 or 2) which are the other major contributors to acid rain. The release of carbon dioxide and hydrocarbons as constituents of refinery effluents can also influence the behavior and integrity of the ozone layer.
Emissions from the sulfur recovery unit typically contain some hydrogen sulfide (H2S), sulfur oxides, and nitrogen oxides. Other emissions sources from refinery processes arise from periodic regeneration of catalysts. These processes generate streams that may contain relatively high levels of carbon monoxide, particulates, and volatile organic compounds (VOCs). Before being discharged to the atmosphere, such off-gas streams may be treated first through a carbon monoxide boiler to burn carbon monoxide and any volatile organic compounds, and then through an electrostatic precipitator or cyclone separator to remove particulates.
The processes that have been developed to accomplish gas purification vary from a simple once-through wash operation to complex multi-step recycling systems. In many cases, the process complexities arise because of the need for recovery of the materials used to remove the contaminants or even recovery of the contaminants in the original, or altered, form.
See also: Acid Rain, Air Emissions, Pollutant.
Air Emissions – Biomass Plant
Biomass power plant emissions will be different from emissions form a coal-based power plant because biomass is lower in sulfur than most US coals. Typical biomass contains 0.05 to 0.20% w/w sulfur on a dry basis. In comparison, coal has 2 to 3% w/w sulfur on a dry basis. The biomass sulfur content translates to approximately 0.12 to 0.50 lb SO2/MMBtu. Because of this, burning biomass to generate power typically produces less SO2 emissions than using coal.
NOx emissions should usually be lower for biomass than for coal, due to lower fuel nitrogen (N) content and due to the higher volatile fraction of biomass versus coal. However, this difference may not have much influence on the selection of the technology (i.e., coal or biomass) because the compliance costs are relatively insignificant given the small difference in NOx emissions.
Biomass power production can result in large emissions of carbon dioxide (CO2). Sometimes, biomass combustion results in larger emission values than for fossil fuels because of the lower combustion efficiencies. However, because the carbon dioxide released by combustion was removed from the atmosphere in the recent past through photosynthesis and new plant growth will continue to remove carbon dioxide from the atmosphere after biomass is harvested, it is sometimes argued that biomass is carbon dioxide neutral.
However, in practice, the issue is more complicated since other carbon flows are involved, including carbon dioxide emissions associated with fossil fuel used in harvesting, processing, and transportation operations. Although it is certain that the net amount of carbon dioxide emitted from a biomass power plant is less than a fossil power plant, it must be recognized that under current production practices, biomass power is not a net zero carbon dioxide process.
Biomass combustion device emissions are estimated on a site-specific basis to determine whether a particular location is appropriate for developing a facility. In many cases, data specific to a particular wood-burning appliance are not readily available. However, the United States Environmental Protection Agency provides emission factors that can be used to estimate emissions from wood-fired boilers as part of efforts to estimate the effects of specific combustion sources and determine the applicability of relevant permitting programs.
See also: Acid Rain, Air Emissions.
Air Flotation Processes
Air flotation processes
