Concerns about air pollution from coal began with widespread coal use. The initial and still relevant focus was on the visible effects of spewing soot (particulates) from coal. Coal has a substantial but variable particle content. At best, the content is much greater than that of oil or gas.

Coal contains a variety of other contaminants. The most notable is sulfur. Sulfur contents of coals are also widely variable, and the lowest sulfur coals may have considerably less sulfur than many crude oils. During combustion, nitrogen oxides (NOx), carbon dioxide (CO2), and carbon monoxide form. In the 20th century, the problem of largely invisible pollu­tion from SOx and NOx emerged, as did concern regarding smaller, invisible particulates. Recently, attention has turned to CO2. It is the main green­house gas suspected of contributing to increased temperatures in the atmosphere (global warming).

Pollution control can be performed in several ways. Naturally less polluting fuels can be used, precombustion cleanup can be performed, and the pollutants can be captured after combustion. How­ever, precombustion treatment is not an economically feasible option with coal. Available coal washing and sorting technologies can only modestly reduce parti­cle and sulfur content.

More radical transformation is technically possi­ble but economically unattractive. Prior to the development of a natural gas distribution network, coal gasification provided the input for local gas distribution systems. Other technologically estab­lished possibilities are to produce a higher quality gas than obtained from the earlier gasifiers, synthesize petroleum products, or simply produce a liquid that would compete successfully with oil and gas. All these processes would have the further benefit of facilitating the removal of pollutants. To date, the costs have proven too great to permit competition with crude oil and natural gas. Thus, emphasis is on changing the type of fuel and postcombustion cleanup. Changing to oil and gas lessens particulate and SOx emissions. Another approach to SOx is to use lower sulfur coal.

An alternative is to trap the particles and sulfur before discharge into the air and deposit the trapped wastes on nearby lands. The latter approach is widely used. Particulate control is invariably effected by postcombustion methods. Two standard methods are available—a mechanical filter known as a bag house or devices called electrostatic precipitators that trap the particles by electrical charges. Particle control techniques are long extant and have low control costs. Thus, visible discharges are largely eliminated. Little coal burning occurs in facilities too small to control particulates. Even the best particu­late control devices, however, cannot capture all the smallest particles, and many pre-1960 regulations did not require the greatest control that was physically possible.

Sulfur oxide control proved more expensive and more difficult to perfect than expected when control rules were imposed. The wastes from these cleanup facilities can cause disposal problems. The reduction in sulfur and particulate emission associated with a switch to oil or gas is associated with increases in nitrogen oxides and unburnt hydrocarbons.

Similarly, the lowest cost, low-sulfur coal in the United States is subbituminous from Wyoming, which has a higher ash content than eastern coal. Thus, lowering sulfur by changing fuel may result in increased particulate emissions. Some observers argue that the close association between particulate and SOx emissions precludes definitive assignment of impacts. SOx may be blamed for effects that are actually caused by the accompanying particulates, and coal-source shifting to fuel lower in SOx may be harmful.

Complex air pollution control programs have been developed. United States air pollution laws have resulted in multitiered regulations that are periodi­cally tightened. The practice involves increasing the number of regulated pollutants. In the major cases, a two-phased approach prevails. One set of rules determines air-quality objectives, and another con­trols emissions from different sources.

The overall goals specify the allowable concentra­tions in the atmosphere of the pollutants with which the law is concerned. States must establish imple­mentations plans (SIPs) to ensure that they meet these goals. Failure to do so results in a region being labeled a nonattainment area.

In addition, a federal court ordered that the preamble to the law that called for maintaining air quality require “prevention of significant deteriora­tion’’ (PSD) in areas in compliance with the rules. Widespread failure to meet SIP goals and the vagueness of the PSD mandate were treated in the 1977 Clean Air Act amendments. The country was separated into nonattainment areas in which actions must be taken to comply and PSD regions in which increases in pollution were to be limited to amounts specified by the law. Continued nonattainment resulted in further remedial legislation in 1990. This effectively divided the country into nonattainment areas legally obligated to come into compliance with the goals and PSD areas. Attaining compliance is difficult, and the major amendments in 1977 and 1990 to the Clean Air Act contained extensive provisions to stimulate compliance further.

Similarly, ever more complex rules have been developed to determine how major polluters should limit pollution. Newer facilities have long been subjected to more severe new source performance standards. The new source rules originally simply imposed stricter emission controls on new facilities. This was justified by the fact that the cost of incorporating controls into a newly designed plant was less than that of adding controls to an old plant. The disincentives to adding and operating new plants were ignored. (An additional factor that became controversial was precisely defining how much refurbishment of an old facility was consistent with maintaining its status as an existing plant.)

The initial rules resulted in considerable changes in the type of fuel used. Western coal was heavily used in such producing states as Illinois and Indiana and in states such as Minnesota that previously relied on Illinois coal. Mainly in response to this change, the rules were changed in the 1977 Clean Air Act amendments to restrict how emissions were reduced.

The amendments required use of best available control technology for preventing the discharge of pollution. The available option was employing scrubbers, a cleanup technique that emphasizes capture of pollutants between burning and discharge of waste up the smokestacks. Since some cleanup was necessary, the advantage of shifting to a cleaner fuel was diminished.

Under the 1990 Clean Air Act amendments, a new program targeted the reduction of emissions in the most heavily polluting existing electric power plants. Reflecting a long-standing debate about ‘‘acid rain,’’ the 1990 amendments instituted a complex program for reducing sulfur and nitrogen oxide emissions from existing sources.

This two-phased program initially imposed roll­backs on the units (specifically named in the law) at 111 power plants that exceeded both a critical size and level of sulfur oxide emissions. By January 1, 2000, electric power plants were required to under­take a 10 million ton reduction (from 1980 levels) of sulfur dioxide and a 2 million ton reduction in nitrogen oxide emissions. The named plants had to undertake the reductions in the first phase of the program. The law encouraged cheaper abatement by allowing the polluters opportunities to buy offsets to their activities. The provisions thus involve a paradox. Congress first extended the command and control approach by specifying limits on specific plants. Then a concession to market forces was added.

The compliance choice for existing plants, in principle, is limited by the constraints on the plants that must make changes and the supply of lower sulfur coals. Thus, boiler design may prevent the use of some types of coal and result in loss of capacity when other coals are used. At the time of enactment, those modeling the impacts debated the availability and usability of low-sulfur coal from different states. Factors such as proximity to coal fields, railroads, and waterways affect the ease of procurement from alternative suppliers. The nature of available receiv­ing facilities is another influence. Some plants were built near coal mines to which they are connected by conveyer belts. Others are served by private rail lines and in one case a slurry pipeline. Some lack land to add large new facilities.

In practice, implementation involved fuel changes rather than increased scrubber use. Wyoming coal was shipped farther than expected (e. g., Alabama and Georgia). Emissions trading proved effective in lowering compliance costs.

Global warming, an increase in temperature produced by discharge of carbon dioxide from fossil fuel burning, became prominent in the late 1980s. It is contended that emission of carbon dioxide into the atmosphere will inevitably and undesirably raise atmospheric temperatures.

There is an enormous amount of literature on the effect of global warning and what it might mean. First, questions arise about the exact extent of warming that will occur. Second, uncertainties prevail about the physical effects of this warming. Third, numerous estimates have been made about the economic effects. Should the United States commit to massive reductions in greenhouse gas emissions, it may result in severe problems for the coal industry. Coal burning produces more greenhouse gas than the use of other fuels.

Updated: December 15, 2015 — 6:50 am