Encyclopedia of Renewable Energy. James G. Speight

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      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 (SO_x), nitrogen oxides (NO_x), 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.