Pollution Control Technologies Applied to Fossil Fuel Combustion

Burning of fossil fuels (hydrocarbons) is the major source of energy in today's global economy with over one-third of the world's power generation derived from coal combustion and over one-half in the U.S. In addition to power production, fossil fuel combustion is used for heating and transportation.

Concerns of Burning Fossil Fuels
Although fossil fuels are abundantly available, burning these fuels presents many environmental problems, for animal and plant life as well as for human health.

Four major concerns arise from fossil fuel combustion: release of sulfur dioxide; formation and release of nitrogen oxides; release of particulate matter (ash); and release of mercury. Although not considered a pollutant due to its natural presence in the environment, carbon dioxide is a growing concern as it relates to global warming.

Sulfur dioxide (SO2) causes problems in the environment through the formation of sulfuric acid, resulting in acid rain. This has been linked to damage in many areas, including the northeastern United States (e.g., Adirondacks) and eastern Canada. SO2 has been identified as a contributor to regional haze and fine particulate matter formation. Sulfur dioxide is formed by burning sulfur-containing fuels such as coal and heavy fuel oil.

Oxides of nitrogen (NOx) cause two significant problems in the environment. As with sulfur dioxide, nitrogen oxides contribute to acid rain by forming nitric acid. More significantly, nitrogen oxides are key in the creation of ground level ozone, contributing to smog and causing or aggravating human respiratory problems. Additionally NOx is a precursor to ozone transport and, to some degree, fine particulate matter formation. NOx compounds are formed from nitrogen in the air used to burn fuel and from nitrogen contained in hydrocarbon fuel. For this reason, nitrogen oxides are produced in the combustion of almost all types of fuel.

Particulate matter is commonly referred to as PM10 or PM2.5. PM10 refers to particulate matter that is 10 microns in diameter or smaller while PM2.5 refers to fine particulate matter that is 2.5 microns in diameter or smaller. Approximately 28 microns equal the width of a human hair. Current standards limit PM10 emissions. Studies report that very fine particles can lodge in human lungs, resulting in aggravated asthma and decreased lung function. Fine particle release is associated most closely with coal combustion because of coal's ash content, but it is also released by the burning of heavy fuel oil.

Mercury, when released to the air, is deposited on water and land, potentially bioaccumulating in fish and animal tissue. High exposure to mercury, generally through consuming large amounts of contaminated fish, may cause damage to the brain, kidneys, and developing fetuses. Mercury emissions from coal-fired generating plants are the largest source of man-made mercury emissions to the air. In March of 2005, the U.S. Environmental Protection Agency issued the Clean Air Mercury Rule to cap and reduce mercury emissions from coal-fired power plants.

A potential problem of emerging significance in the combustion of hydrocarbon fuels is the formation and release of carbon dioxide (CO2), which may play a role in the reported warming of the atmosphere. This poses a completely different problem from those created by the release of SO2, NOx, particulate matter, and mercury. Unlike these emissions, carbon dioxide is the preferred product of combustion, with its formation resulting in much of the energy released in the burning process. It is also produced in much greater concentrations than the other pollutants, generally making up from 5% to 20% of the combustion exhaust gas. Emissions of carbon dioxide are presently unregulated at the federal level, but some states have established CO2 limits.

Current Pollution Control Technologies
Current pollution control technologies for combustion exhaust gas generally treat the release of regulated pollutants—sulfur dioxide, nitrogen oxides and particulate matter—as three separate problems, instead of as parts of one problem.

Sulfur dioxide control technology, applied principally to coal-fired electricity generating stations, is referred to as Flue Gas Desulfurization (FGD), or Wet Scrubbing. The technology consists of capturing SO2 following combustion in a slurry or on a solid material. Calcium compounds (most commonly lime and limestone) are used to form calcium sulfate, which must be separated from the exhaust gas and discarded. In some instances the discarded calcium sulfate (gypsum) is utilized to manufacture wallboard.

Removing sulfur from coal before combustion is also done on a limited scale. Its effectiveness is limited because much of the sulfur is bound to carbon in coal and cannot be effectively removed. Alternatively, coal-fired power generators switch to fuels containing lower sulfur content to control SO2 emissions. Modifications to the boiler and electrostatic precipitator are generally required to adapt to the lower sulfur fuel.

NOx control technologies are categorized as pre- and post-combustion techniques. Low NOx burners and other combustion modifications provide pre-combustion NOx control by reducing the temperature of combustion, which reduces NOx formation. While this is appealing, drawbacks exist. The efficiency of converting heat to useful energy (work) is sacrificed, requiring more fuel (and greater carbon dioxide emissions) for the same power output. A second issue is that low NOx burners are difficult to adapt to existing installations, often requiring new boilers or furnaces for a guaranteed reduction in NOx emissions.

Post-combustion NOx control is primarily accomplished by reacting ammonia with nitrogen oxides, forming nitrogen and water vapor. Two variations exist, using thermal energy (high temperature) or a catalyst. The thermal technique (selective non-catalytic reduction, or SNCR) can achieve about 50% removal at maximum but is difficult to control due to a narrow temperature window for operation. Temperatures above the maximum convert ammonia to NOx, while operating at too low a temperature results in no reaction, with both ammonia and NOx released to the atmosphere. SNCR's advantages include ease of installation and low equipment costs relative to the alternative, SCR. Using a catalyst (selective catalytic reduction, or SCR) broadens the operating temperature window of the process while also lowering it. Higher conversion of NOx (~80–90%) in the gas can be achieved with the catalyst. However, catalysts can be made inactive by ash in the gas stream and are expensive to install or replace. If it cannot be regenerated, the catalyst must be disposed of as a hazardous waste.

Particulate matter control in the combustion of fossil fuels is primarily accomplished with electrostatic precipitators (ESPs). An ESP is a post-combustion device which uses electrical forces to move fine particles out of the flue gas stream onto collecting plates where the particles can be removed. ESP collection efficiencies range from 99.5% to 99.9%.

Baghouses, or fabric filters, are employed to a lesser extent in the power industry although their use is increasing. Baghouses are large filter bags, which are effective at capturing very fine particulate matter that may escape an ESP.

A number of different mercury control technology options are now commercially available, including controls that specifically target mercury (such as activated carbon injection systems) as well as those that enhance the removal of mercury from currently installed control equipment, including flue gas desulfurization and selective catalytic reduction systems.

Carbon dioxide capture technologies are commercially available for use on coal-fired power plants, however, they have substantial capital and operating costs.