Coal pollution mitigation (clean coal)

As a relatively abundant and inexpensive fossil fuel, coal has maintained its status as a major energy source since the earliest stages of the Industrial Revolution (ca. 1760–ca. 1840). Despite offering multiple advantages, coal has fallen out of favor in many countries as a consequence of its negative environmental impacts. Beyond emitting large quantities of the greenhouse gases that scientists blame for global climate change, coal also produces toxic and carcinogenic byproducts that cause pollution and pose significant health risks to people who come in regular contact with it.

To address these environmental and health concerns, scientists and engineers began developing coal pollution mitigation technologies in the late twentieth century. These systems seek to manage the harmful waste and toxic byproducts that coal produces. Commonly referred to as “clean coal technologies” (CCTs), these techniques include carbon capture and storage (CCS) and numerous other methods that primarily focus on neutralizing the carbon emissions and pollutants coal releases when burned for fuel.

Background

Coal is a hardened sedimentary deposit with an appearance often likened to a black, shiny rock. The conditions that led to its formation date to approximately 300 million years ago, during a time known in geology as the Carboniferous Period. At that time, much of the Earth’s surface was covered by large, shallow bodies of water and thick forest cover. When the plants populating these forests died, their remains sank and became submerged under water and soil. Under this pressurized weight, and through the higher temperatures the plant remains encountered as they sank deeper under the Earth’s surface, these organic materials slowly changed into coal over millions of years.

Human populations have been using coal as a source of heat and fuel for thousands of years. Archaeological evidence suggests coal was used for heating in Roman Britain during the second and third centuries, and coal was also exploited for fuel and other purposes by later civilizations including the Aztec Empire. Coal’s widespread global use began during the Industrial Revolution, when it powered the manufacturing equipment used to mass-produce goods. Early industrialists favored coal over wood fuel as an energy source because it was less expensive to produce and generated more energy when used.

By the early nineteenth century, coal was also powering early steam locomotives as rail networks began connecting cities in industrialized areas. During the second half of the nineteenth century, coal found new industrial applications in steel manufacturing and electricity generation. Limited awareness and concern for coal’s environmental impacts led to coal becoming the dominant energy source used in electricity generation in the United States by the mid-twentieth century.

According to the United States Department of Energy (DOE), about 90 percent of the coal mined in the United States is used for electricity generation. Coal-fired power plants activate the energy stored in coal by burning it to create electricity. The International Energy Agency (IEA) reported that, in 2021, electricity production accounted for 59 percent of all coal used globally. Coal is an abundant, readily available, and inexpensive energy source. Its favorable cost profile remains the main driver of its continued widespread use. Despite increased awareness of coal’s negative environmental and health impacts, the IEA reports that global coal consumption reached an all-time high in 2022 by surpassing 8 billion tonnes (8.82 billion tons).

Environmental groups often describe coal as the “dirtiest” of all fossil fuels, as it releases large quantities of atmospheric pollutants including lead, mercury, nitrogen, and sulfur, in addition to dense concentrations of carbon. Coal ranks among the leading drivers of climate change, and it also contributes to the acidification of rainfall. It additionally causes air, ground, and water pollution and contamination, which tends to be most densely concentrated around areas where coal-fired power plants are located. The Union of Concerned Scientists (UCS) cites coal as having the potential to cause asthma and other respiratory disorders along with cancer, heart disease, brain and neurological disorders, and lifespan reductions. During the late twentieth century, these charges caused increasing concern in the scientific community, and innovators began actively developing technologies designed to mitigate the pollution and contamination caused by coal’s use in energy production.

Overview

The World Coal Association (WCA) divides coal pollution mitigation efforts into three main subgroups: high-efficiency low-emission (HELE) technologies, CCS technologies, and systems that reduce the emission of toxic particulates. Coal producers have also introduced multiple new mining strategies and techniques engineered to reduce the environmental impacts of coal recovery operations.

HELE technologies primarily focus on improving the operational efficiency of coal-fired power plants to reduce carbon emissions and the release of other toxic byproducts. High-efficiency systems enable engineers to extract greater amounts of energy output from the same amount of coal input, with the WCA stating that a 1 percent efficiency improvement translates into a 2–3 percent reduction in carbon emissions.

Specific examples of HELE technologies include fluidized bed combustion (FBC), integrated gasification combined cycle (IGCC), and supercritical technologies. FBC is a specialized electricity production technique that significantly reduces nitrogen and sulfur emissions from coal-fired power plants. IGCC produces a synthesis gas (syngas) through reactions between coal, steam, and oxygen. The resultant syngas, which primarily consists of carbon monoxide and hydrogen, is treated to remove impurities and is then used in combustion turbines to create steam and produce electricity. IGCC vastly improves energy output relative to energy input, which significantly reduces carbon byproducts.

Supercritical technologies generate efficiency improvements and reduce carbon emissions by producing steam at hotter temperatures and higher pressures. This family of techniques includes supercritical, ultra-supercritical, and advanced ultra-supercritical systems, with each successive level marking greater efficiency improvements and lower carbon emissions.

Carbon capture and storage technologies, also known as carbon capture, use, and storage (CCUS), work to prevent carbon byproducts from ever being released into the atmosphere when coal is burned to generate electricity. CCS uses three main techniques—pre-combustion capture, post-combustion capture, and oxyfuel combustion—to separate carbon gases from the other gases released via coal combustion. These captured carbon gases are then collected and transported for disposal or safe usage in other industrial applications.

Disposal primarily uses underground rock formations in a technique known as carbon sequestration or geological storage, in which captured carbon is pumped deep below the surface of the Earth. There, the carbon gas becomes pressurized and changes into a liquefied form, allowing it to be stored safely in rock reservoirs for hundreds of years. Though CCS creates what is known as an “energy penalty,” which essentially describes a loss in energy output potential relative to coal input, it is nonetheless considered vitally important to ongoing efforts to mitigate carbon emissions from coal and other fossil fuels including oil.

Coal cleaning, also known as coal washing, has been used in industrialized countries for decades to treat coal prior to combustion. The technique removes surface residues from coal, reducing sulfur and ash emissions when it is burned.

A process known as coal liquefaction can convert coal from solid to liquefied forms. Liquefied coal burns much cleaner than conventional coal, and also contains significantly less sulfur. Thus, it has a comparatively favorable usage profile with respect to pollution reduction and mitigation efforts.

Additional particulate emissions reductions technologies include fabric and hot gas filtration systems, wet scrubbers, and electrostatic precipitators (ESPs). These techniques trap particulates to prevent their release into the atmosphere while further reducing sulfur emissions.

Legal jurisdictions increasingly require coal producers to adhere to elevated standards and best practices for studying the potential environmental impacts of proposed mining operations when evaluating permit applications. These best practices include detailed studies of the likely and potential impacts of coal mining on soils, ground-level and subsurface water deposits, local plant and animal populations, and future land-use profiles. Jurisdictions in many countries are increasingly moving toward denying permits to applicants whose proposed operations are likely to result in negative impacts. For instance, in 2024, the US Environmental Protection Agency (EPA) announced stricter emission guidelines for coal power plants that called for a 90 percent reduction in greenhouse gas emissions countrywide by 2039.

Contemporary coal mines are also designed and engineered to optimize their configurations and proportions in the interests of controlling a phenomenon known as surface subsidence. Subsidence describes the gradual sinking of land at surface level that can occur when subsurface structures are altered or disturbed.

The primary advantage of clean coal technologies is that they facilitate the continued use of coal as an inexpensive, stable, and readily available energy source while reducing the environmental damage it causes. Many coal pollution mitigation strategies and techniques are compatible with existing energy generation infrastructure, which reduces adoption costs and capital investment needs. Clean coal is also capable of generating more energy over a longer period than conventional coal.

However, even when used alongside the most effective pollution mitigation technologies, coal still produces many harmful byproducts. CCS technologies in particular carry a complex set of drawbacks: they carry high costs, result in efficiency losses, and still require the safe transport of dangerous and toxic byproducts to disposal sites. Underground carbon sequestration capacity also remains unknown to scientists, introducing uncertainty over the long-term feasibility of CCS as a fossil fuel pollution mitigation strategy.

Bibliography

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