Written by Ryan McGuine //
Many sources of energy generate air pollution when consumed. Air pollution is a concern because it has harmful effects on both human health and natural ecosystems. Thus, air pollution generates what are referred to as “negative externalities.” In the case of air pollution, that can be anything from healthcare for respiratory diseases among those who live near coal-fired power plants, repairs to buildings necessitated by acid rain, or declining crop yields due to changing weather patterns. Air pollution can be broken into a number of different sub-categories: natural vs. anthropogenic (man-made), household vs. ambient, and local vs. global. This post focuses on anthropogenic air pollution, since that is the one most associated with energy consumption. The further divisions will be covered later, but first: an introduction to significant energy-related air pollutants.
Sulfur-oxygen compounds, generically referred to as SOx (the x indicates any number in the oxygen’s subscript), are produced by the combustion of fossil fuels, as sulfur that is chemically bonded to other elements within the fuels is released. SOx can have acute health effects if inhaled and is one of the main causes of acid rain, which harms man-made structures and organic matter alike. SOx emissions can be successfully reduced to safe levels through a variety of control techniques. The best technique is to eliminate its production outright by chemically removing sulfur from fuel prior to its use, or by using fuels that are naturally lower in sulfur content. If it is not possible to eliminate it prior to combustion, sulfur can be removed from flue gas using a sorbent. The sorbent-sulfur slurry can then be either disposed of responsibly or reused in another application.
Nitrogen-oxygen compounds (NOx) are produced by the high-temperature combination of nitrogen gas and oxygen gas (this is not limited to the combustion of fossil fuels, and in fact is produced when anything burns). NOx is the other main cause of acid rain, and reacts with other compounds in the atmosphere to produce ozone, the main constituent of “smog,” which can both trigger new health problems and aggravate existing ones when inhaled. While SOx can be almost completely eliminated thanks to the combination of fuel switching and post-combustion control technologies, NOx can only be reduced by optimizing combustion conditions, or post-combustion using a process called selective catalytic reduction. Modern pollution control technology makes it possible to significantly reduce NOx from individual sources, but where rapid population growth, rapid economic growth, and relatively low incomes exist concurrently (read: developing countries), vehicle emissions outweigh power generation emissions, and the the sheer magnitude of car ownership growth, paired with the fact that many of those cars are older models, means that NOx emissions are still on the rise there.
Particulate matter (PM) — colloquially: dust, soot, and smoke — is produced by the combustion of solid matter, as well as natural sources as simple as wind blowing dust into the air. PM can cause a wide variety of respiratory illnesses when inhaled. Particles less than 10 micrometers in diameter are especially dangerous, since they are small enough to get deep into the lungs, and may even be absorbed into the bloodstream. Fortunately, PM is fairly easy to eliminate. As is the case for SOx, it is possible for many applications to switch to fuels that produce less PM. Further, many technologies exist to remove PM from flue gas for industrial processes.
Finally, greenhouse gases (GHGs) are numerous and produced by many common industrial processes. Those GHGs most related to energy are carbon dioxide, which is produced by the combustion of fossil fuels, and methane, which is emitted during the production and transport of hydrocarbon fuels, as well as from livestock and the decay of organic waste in landfills. Methane is of little concern if it is burned (it emits roughly half the carbon dioxide as coal when combusted using equivalent technology, and modern natural gas-fired combined cycle technology is even more efficient than conventional coal- or natural gas-fired boilers), but of serious concern if allowed to enter the atmosphere unburned. While GHGs are not directly hazardous to humans or the environment, they trap heat in the atmosphere and gradually cause the planet to warm, disrupting weather patterns that life has grown accustomed to. From a control standpoint, removing carbon dioxide at the source is theoretically possible, but expensive and challenging, and removing it directly from the atmosphere is even more so. Methane can be mitigated by preventing leakage in pipelines and flaring any excess gas at generation sources, but many worry that this is not currently being done effectively enough.
The first important distinction within anthropogenic air pollution is household vs. ambient air pollution. Household air pollution refers to air pollution that is generated, and mostly remains, indoors. The main contributor to household air pollution is PM from burning biomass like animal dung, wood, and charcoal for cooking or heating and without proper ventilation. While household air pollution has been eliminated in the developed West, it remains one of leading causes of disease and premature death in developing countries. According to the World Health Organization (WHO), over 3 billion people worldwide rely on heavily-polluting sources of energy for cooking, and approximately 3.8 billion deaths can be attributed to household air pollution ever year. Women and children are more at risk for diseases related to household air pollution, since they tend to spend more time indoors in developing countries. Recently, there has been much attention paid to this issue by the global community, and there are a number of initiatives to replace heavily-polluting sources of energy for indoor applications.
Ambient air pollution refers to outdoor air pollution, and can itself be separated into local air pollution and global air pollution. Local ambient air pollution refers to ambient air pollution whose effects are limited to the vicinity of where they were generated. In contrast, global ambient air pollution refers to ambient air pollution whose effects are not limited by geography. The WHO estimates that 91% of the world’s population lives in places that exceed the its guidelines (no doubt aggravated by accelerating urbanization), and that 4.2mn deaths can be attributed to ambient air pollution every year.
Local ambient air pollution includes SOx, NOx, and PM, as well as some others not detailed above like carbon monoxide, lead, and mercury. To illustrate local air pollution’s distinction, consider smog. Smog in a given city may well affect an expansive geographic space, including area outside the city limits, but smog in LA has no effect on London. In addition to its localized effects, because local ambient air pollution flows both into and out of the atmosphere, the severity of its effects has to do with the rate that it flows into and out of the air. As long as it leaves the air more rapidly than it enters it, the severity of effects will remain low. Its flow into the atmosphere is due to human and natural actions to emit it, while its flow out of the atmosphere depends on weather conditions. Even for days with an identical quantity of emissions from identical sources, smog will be worse on a warmer, sunnier day.
Global ambient air pollution exclusively refers to GHGs. The effect of global air pollution is not limited to any geographic location — a ton of carbon dioxide emitted in LA has the same effect on the climate as a ton of carbon dioxide emitted in London. Further, global air pollutants remain in the atmosphere after they are emitted and accumulate over time. Whereas local ambient air pollution will be wiped out with a good rain, carbon dioxide emitted today can remain in the atmosphere for up to 100 years, and methane, which is 25 times more effective at trapping heat than carbon dioxide, remains in the atmosphere for about 12 years.
In many ways, air quality control systems merely convert air pollution into solid or liquid pollution. For example, coal-fired power plants generate hazardous solid and liquid wastes as byproducts of air pollution reduction like lime-sulfur slurry, selective catalytic converter catalyst, and captured PM. Additionally, the manufacturing and recycling of batteries used in electric vehicles present a myriad of environmental challenges, even while electric vehicles result in less ambient air pollution than internal combustion vehicles. The solid or liquid pollution tends to be easier to deal with in an environmentally-friendly way, but it is important to note that there are tradeoffs between the different forms.