Pollutant Emissions
Sulfur Oxides
The main sources of sulfur compounds in air are anthropogenic, with sulfur dioxide (SO2) the predominant form. Emissions of sulfur oxides are mainly generated during combustion as the sulfur contained in the fuels is oxidized; therefore, sulfur oxides emissions are almost exclusively dependent on the sulfur content of the fuel and not on boiler size, burner design, or fuel grade. In combustion systems, approximately 95% of the sulfur present in the fuel is converted to sulfur dioxide (SO2), between 1 to 5% is further oxidized to sulfur trioxide (SO3), and 1 to 3% is emitted as sulfate particulates. As SO2 is a colorless, corrosive gas, it has a very harmful effect on plants, animals and humans and even on the physical environment. In air it can be further oxidized to SO3, which reacts with water vapor to form sulfuric acid (H2SO4), one major component of acid rain. Also, the sulfate anion (SO4=) can be inhaled by humans into the lungs where it is very detrimental. In addition, sulfate particles contribute to impairment of visibility and affect the Earth's albedo, or global radiation balance, which in turn, has an effect on climate [46–48].
Mercury
Mercury is a known persistent, bioaccumulative and toxic (PBT) trace metal that occurs naturally in coal at very low concentrations. The combustion of coal is considered the major anthropogenic source of this pollutant to the atmosphere. According to data from the UNEP [49], combustion of coal in power plants, industrial boilers, residential boilers, heaters and stoves contributed with around 888 tonnes (46%) to the total global anthropogenic emissions in 2005. Coal-fired power plants are one of the most important sources of mercury due to the large quantity of coal used for electricity generation. For example, about one-half of the anthropogenic mercury emissions in 2005 in the United States came from these plants, approximately 52.4 t/year [52]. Most of the mercury in the atmosphere is in the form of elemental mercury vapor; however, in water, soil, sediments, or biota it is found in both organic and inorganic forms.
Elemental mercury vapor is relatively insoluble and nonreactive, which is why it can remain aloft, carried by air currents over vast distances for very long periods—up to a couple of years—before it is finally deposited on land or in surface waters. Once mercury is deposited, microbes can convert it into an organic form (methylmercury) that can be absorbed by other organisms and accumulated as it passes through food webs. Mercury has a variety of important ecological and human health impacts. For example, mercury pollution is the most common cause of impairment of rivers and lakes in the United States, and many US states have issued warnings about eating fish from those water bodies.
Ingestion of mercury from eating contaminated fish can lead to impaired neurological development in fetuses, infants and children. In adults it can cause neurological damage [53]. The United States National Institutes of Health (NIH) estimates that one woman in 12 in the United States has more mercury in her blood than the amount considered safe by the Environmental Protection Agency. According to NIH estimates, health impairment due to mercury cause nearly US$9 billion annually in higher medical costs and lost productivity in the workforce [47, 52].
Particulate Matter
Particulate matter (PM) consists of a wide range of materials in solid or liquid phase that range in size from less than 1 nanometer up to one hundred micrometers and can have complex chemical composition. Some of the components include nitrates, sulfates, metals, organic compounds, soil, pollen, soot, etc. Particulate matter (PM) is measured using a variety of size metrics, of which the most common are PM10 and PM2.5. Both are measures of the mass of particles with an aerodynamic diameter of less than 10 or 2.5 micrometers, respectively. There are a large number of sources of particulate matter, with stationary combustion (e.g., non-mobile sources, such as power plants) being one of the major contributors, along with road transportation. Among the stationary, fuel combustion sources are industrial sector activities like iron and steel manufacturing, the residential sector heating and power stations.
Emissions of PM from the combustion of solid fuels (like coal) are, in general, larger in diameter than those originating from the combustion of liquid fuels, and the latter are coarser than particles generated from gas combustion. But in general, particles produced by combustion are smaller in diameter than 1 micrometer. Some environmental impacts of particulate matter emissions are reduction in visibility, acid rain, and damage and stain on materials (statues and monuments). Deposition of particulate matter can also contribute to the acidification of lakes and rivers, change the nutrient balance of water bodies and soil, and affect forests and farm crops [53]. Particulate matter can cause severe health problems in humans, especially the particles whose diameter is smaller than 10 micrometers, since they can be inhaled deeply into the lungs and even be absorbed into the bloodstream. The most common health effects of particulate matter are respiratory symptoms such irritation of the air passages, coughing, difficulty in breathing, decreased lung function, asthma, chronic bronchitis and premature death [53].
Greenhouse Gases
Greenhouse gases (GHGs) are those gases that trap heat in the atmosphere; this allows an average temperature on Earth of approximately 15°C. Without this natural "greenhouse effect," the average ambient temperature would be about 33°C lower than it is now, making most life as we know it today impossible. But since the Industrial Revolution, human activities have added significant amounts of greenhouse gases to the atmosphere, enhancing the natural greenhouse effect. This is causing an increase in the global average temperature, resulting in severe effects on the climate. Some of these gases, such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), are emitted to the atmosphere from both natural and anthropogenic processes, while other greenhouse gases, such as chlorofluorocarbons (usually known as CFCs), stem exclusively from industrial activities.
The principal greenhouse gases released from human activities, particularly from fossil fuel combustion, are carbon dioxide, methane, and nitrous oxide. GHG emissions, regardless of the gas concerned, are commonly reported in terms of equivalent emissions of carbon dioxide. This measure is used to compare the ability of each GHG to trap heat (Global Warming Potential, or GWP) in the atmosphere relative to that of CO2, which is taken as a reference gas. The carbon dioxide equivalent for a gas is derived by multiplying the amount of gas emitted by its GWP.
Brief descriptions of the principal GHGs are given below:
Carbon dioxide (CO2) is a nontoxic and innocuous gas. The steady increase in CO2 concentration in the atmosphere that is of concern for its effects on climate change is mostly due to human activities. It has been estimated that global atmospheric concentrations of CO2 in 2005 were 35% higher than the values observed before the Industrial Revolution. The main source of this gas is the burning of fossil fuels, (of which electric power sources contribute between 17 and 40% of total CO2 emissions); other sources are forest and grass fires, and combustion processes in producing material for cement [48, 56–58].
Methane (CH4) remains in the atmosphere for 9 to 15 years and is 21 times more effective in trapping heat in the atmosphere than carbon dioxide. Like carbon dioxide, methane is emitted to the atmosphere from diverse natural and anthropogenic sources. Natural sources include wetlands, termites, oceans, wildfires, etc., while anthropogenic sources are mainly combustion of fossil fuels, enteric fermentation, landfills, natural gas systems, fossil fuel production, rice cultivation, biomass burning, and waste handling. It is estimated that natural sources contribute approximately 37% of the total methane emitted into the air every year; therefore, anthropogenic sources are the principal sources of its release to the atmosphere. Nitrous oxide (N2O) is a colorless gas with a slightly sweet odor and it is about 310 times more effective in trapping heat in the atmosphere than carbon dioxide. As with carbon dioxide and methane, nitrous oxide is also emitted from natural and human-related sources, but contrary to the situation with the two other gases, natural sources of this gas contribute approximately 64% of the total inputs to the atmosphere.
Nitrogen Oxides
The most important anthropogenic sources of nitrogen oxides are combustion processes. Nitrogen oxides can be formed in combustion processes from the nitrogen contained in the fuel or from the nitrogen that is part of the air. In most of the external fossil fuel combustion systems, around 95% of the nitrogen oxides emitted are in the form of nitrogen monoxide (NO), whereas the remaining 5% is in the form of nitrogen dioxide (NO2). The NO emitted oxidizes further in the atmosphere to NO2. The term NOX refers to the sum of nitric oxide (NO) and nitrogen dioxide (NO2), expressed as NO2.
NO2 is a highly reactive gas whose color gives the peculiar tone of reddish-brown to photochemical smog. Also, these oxides react with water to produce nitric acid (HNO3), which, together with sulfuric acid, results in acid rain. Another harmful effect of nitrogen oxides is the atmospheric deposition of nitrogen as nitrates and nitrites derived from NOX, which leads to eutrophication of inland waters and coastal seas [46–48].