3.1
POLLUTION
Purpose
This chapter is an introduction to pollution caused by the combustion of fossil fuels.
Summary
There are a number of different types of pollution. The first general category of pollution is caused when impurities or small particles trapped in the fuel are released into the air when the fuel is burned. The second type of pollution is an intrinsic part of combustion—the creation of greenhouse gases. Greenhouse gases don’t present a health hazard and are generally safe. However, when large quantities of greenhouse gases are released into the atmosphere, they have the potential to change the world’s climate.
Key Topics
• Some types of pollution can be prevented by using fuel that contains fewer impurities.
• Other types of pollution, like carbon dioxide, are an unavoidable part of combustion.
The burning of fossil fuels (combustion) produces a variety of unwanted byproducts that are collectively termed pollution. Some types of pollution are poisonous to humans, while other types have the potential to harm the world around us. Combustion is an oxidation process. Carbon-based fuels are combined with oxygen to produce heat. Some types of pollution result from the release of particles trapped inside the fuel as it burns. Other types of pollution are a result of the chemical reaction that produces heat.
The pollutants that form a health risk are usually caused by impurities trapped within fuel. Some of these impurities are supersmall particles of sand or rock called fly ash. These rocks are not poisonous, but their small size means that it is easy to breath into lungs. Similar to ash, some other impurities, like sulfur and nitrogen, aren’t poisonous. However, these substances can form acid rain when in the presence of water. Still other impurities are directly harmful. These include heavy metals, like mercury and lead, which are toxic to humans. Any amount of metals released into the air presents a health hazard.
In addition, every fossil fuel produces some unavoidable byproducts during combustion. Combustion of any hydrocarbon fuel (which covers almost all fuels) will produce water (H2O) and carbon dioxide (CO2). It is the creation of these products that produces energy. It is not possible to create energy without producing these byproducts. Neither byproduct poses an immediate health threat—these are the same byproducts caused by breathing and vigorous exercise.
There is no single approach that works equally well for every type of pollution. For example, the carbon dioxide emissions from any single power plant aren’t particularly important. Carbon dioxide is a nontoxic gas that makes up a large portion of the atmosphere around us. Plants rely on carbon dioxide to grow. However, the aggregate carbon emissions from every power plant can be significant enough to affect the Earth’s climate. Greenhouse gases are a global problem.
Acid rain is another type of pollution that becomes important in aggregate. A little bit of acid rain isn’t dangerous. Water is commonly slightly acidic. However, while water evaporates, most acids do not. Given enough time, sufficient acid can accumulate in a single area that it starts to present a health threat to wildlife and plants. Acid rain is generally a regional problem. It declines in intensity the farther it goes from its source. Like greenhouse gases, acid rain is a cumulative problem. The output from a single plant may not be particularly dangerous. But, when combined with the output from several plants, the cumulative sum of the acid rain might be extremely harmful.
Combustion is an oxidation process. Fuels are combined with oxygen to produce heat. That heat can then be used for a variety of useful work—everything from powering an electrical turbine to powering the motor in a car. There are five main types of pollution produced by power plants—carbon dioxide, nitrogen oxides, sulfur oxides, heavy metals, and fly ash/soot. The first three are a result of the oxidation process that produces power. The remaining two are generally due to particles trapped inside the fuel.
Combustion Pollution
The first three types of pollution (carbon dioxide, nitrogen oxides, and sulfur oxides) are byproducts of combustion. Combustion, also called burning, is an oxidation process where some type of chemical bond (which will vary by the product being burned) is broken and replaced with an oxygen bond. Replacing almost any type of chemical bond with an oxygen bond is an exothermic reaction that releases heat. For example, when coal is burned, carbon-carbon bonds are broken and replaced with carbon-oxygen bonds. The source of the fire’s heat is the breaking and the creation of chemical bonds. Carbon dioxide is an unavoidable part of combustion.
Different fuels have different types of bonds that can be broken to provide energy. For example, the primary component of natural gas, methane, has the chemical formula CH4. There are four carbon-hydrogen bonds that can be broken to create energy. When methane combines with oxygen, it will produce a chemical reaction that creates heat, carbon dioxide, and water (Figure 3.1.1).
Figure 3.1.1 Combustion
When methane is burned the carbon-hydrogen bonds are broken, and the carbon and hydrogen combine with oxygen to produce heat. To be combustible, a substance needs some bonds that can be replaced with oxygen bonds. For example, it is impossible to burn water in oxygen. The chemical formula for water is H2O—there are two hydrogen-oxygen bonds in a water molecule. There is no net energy gain that results from breaking one hydrogen-oxygen bond to create another. Therefore, water is not combustible in oxygen regardless of its temperature.
Most types of combustion require some heat, like a spark, to get started. For example, for methane to combust, something must provide the energy to break the first carbon-hydrogen bond and the first oxygen-oxygen bond so that the methane and oxygen can combine. Upon combining, heat will be produced. If the heat that is produced is sufficient to cause additional chemical bonds to break, the reaction will be self-sustaining until there is nothing left to burn.
Carbon Oxides
The most common byproducts of burning hydrocarbon fuels are carbon oxides (either carbon dioxide or carbon monoxide) and hydrogen oxides (primarily water). Since water is not considered a pollutant, the primary concern is the production of carbon oxides. Carbon dioxide is a colorless, odorless, nontoxic gas that wouldn’t be considered a pollutant except for concerns over its effect on the global climate. In contrast, carbon monoxide is highly toxic. Carbon monoxide is formed when there is an insufficient amount of oxygen available for complete combustion. Power plants try to completely eliminate carbon monoxide emissions.
However, it is impossible to eliminate carbon dioxide creation when combusting fossil fuels. Any type of carbon-based fuel will produce carbon dioxide when burned. The relative quantities of carbon dioxide and water that are produced by combustion vary by fuel. Methane has the highest proportion of hydrogen to carbon (a four to one ratio). Longer chain hydrocarbons, like coal, contain more carbon. As a result, compared to burning methane, coal will produce a much higher portion of carbon dioxide relative to the amount of water produced. Restrictions on carbon dioxide emissions are commonly seen as a global issue.
Nitrogen Oxides
Nitrogen is a colorless, odorless gas that makes up 78 percent of Earth’s atmosphere. Sufficiently high temperatures will force atmospheric nitrogen to combine with oxygen. Since most combustion uses air to provide oxygen, nitrogen will also be present. It is possible for either nitrogen oxide (NO) or nitrogen dioxide (NO2) to form. This has led to the generic term NOx to refer to any nitrogen oxide. In sunlight, nitrogen oxides will often break apart and cause ozone to form (O3). Ozone is a major source of smog and can cause shortness of breath, chest pain, asthma, and coughing attacks. It is also a source of acid rain. NOx pollution is generally a localized problem addressed on a regional level.
Sulfur Oxides
Fossil fuels often contain sulfur distributed throughout the fuel. When this sulfur is burned, it creates sulfur dioxide (SO2). The generic abbreviation for a sulfur oxide is SOx. In the presence of a catalyst, like nitrogen dioxide (NO2), sulfur dioxide converts into sulfuric acid, a major component of acid rain. A little bit of acid rain generally isn’t dangerous, but it can build up over time. When enough acidity builds up, the plants and animals exposed to the acid will die. Like NOx emissions, SOx emissions are usually addressed on a regional level.
Particulate Pollution
Heavy Metals
Coal-burning power plants are one of the main sources of atmospheric heavy metal pollution. For coal-fired power plants, the most dangerous of these pollutants is mercury. In many regions, coal-fired power plants can account for 90 percent of the mercury released into the atmosphere. Mercury is a powerful neurotoxin that has been associated with a host of medical problems ranging from autism and sensory impairments to death. While the magnitude of the public health risk due to mercury emissions can vary dramatically between regions, it is such a powerful toxin that cap and trade solutions are impractical. Unlike greenhouse emissions and acid rain restrictions, any level of mercury emissions is dangerous. Lead and plutonium are examples of other heavy metals that can be released through combustion.
Fly Ash/Soot
A large number of small particles can be released by combustion or formed as a result of SOx and NOx emissions. These particles are a major cause of breathing problems and can lead to heart attacks and strokes. Some of these particles are so small that they can easily be breathed into the lungs and enter blood vessels. Once in the body, they can cause substantial damage to the respiratory system. Children, people with asthma, and the elderly are the most at risk. Particulate pollution also reduces visibility and is a major cause of urban haze.
CO2 Emissions and Fuel Type
Longer chain hydrocarbons that are primarily of carbon-carbon bonds, like coal, produce more CO2 emissions than natural gas because they contain more carbon. At two extremes, comparing natural gas and coal, the difference in chemical structure is obvious. Natural gas is composed of small, self-contained molecules while coal is composed of large sheets of carbon atoms (Figure 3.1.2).
Figure 3.1.2 Natural gas and coal
Toxic Pollution
After it is produced, there are only a couple of options for dealing with toxic chemicals. The first option is to combine the toxin with something that makes the substance nontoxic. Another approach is to dilute the toxic substance until it is no longer a major health risk. A third approach is to concentrate the toxin into something that is extremely deadly and then try to store it. If it can be done, the first approach is always the best approach. For example, sulfur dioxide (an acid) can be combined with lime (a base) to produce a harmless substance (gypsum). At that point, even if placed in a landfill, the toxic substance isn’t a major health risk.
If it isn’t possible to make the pollution safe, it’s necessary to choose between two bad alternatives. For example, nuclear waste can’t be made nonradioactive. Diluting a toxic substance means spreading out—basically polluting everything a little bit. The alternative isn’t a lot better—to concentrate the toxin and attempt to store it somewhere it can’t escape. A leak of a concentrated toxin is usually catastrophic.