Clean coal is a term used to describe processes or approaches for mitigating emissions generated by the burning of coal for electrical power generation (Figure 1). Historically, the term has been used to refer to technologies for reducing emissions of nitrous oxide (NOx - a major component of acid rain that became an environmental issue in the 1970s), sulfur dioxide (SO2 - yellow haze as well as a component of acid rain), particulate matter (smoke and soot), and heavy metals such as mercury. However, all these emissions are controllable using modern emissions control technology that must be built into any new coal-fired power plants and can be retrofitted to older plants to meet EPA limits. Such technologies include flue-gas desulfurization (FGD) to remove SO2, fluidized-bed combustion, integrated gasification combined cycle (IGCC), low nitrogen oxide burners, and selective catalytic reduction (SCR) to control NOx, electrostatic precipitators and baghouses to capture particulate matter, and processes such as activated carbon injection for mercury removal. See OWOE Topic: How are air emissions from burning coal to generate electricity controlled?
for more information about these pollution control systems.
Such emissions control equipment is quite expensive to add to an existing power plant. A different approach to reducing NOx emissions, the use of so-called refined coal, was encouraged by the American Jobs Creation Act of 2004. Refined coal is produced by a pre-combustion treatment that uses chemicals in conjunction with heat and pressure to remove approximately 30 percent of the moisture from soft coal such as lignite and or sub-bituminous coal (see OWOE topic: What is coal?
). The process can raise the coal's thermal content from about 8,500 Btu per pound to approximately 11,000 Btu per pound. The theoretical result is less pollutants per Kwh of electricity generated. Plants that achieve a specified reduction in NOx emissions are entitled to production tax credits. Unfortunately, recent studies
indicate that refined coal has been more hype than results at the cost of billions of dollars in tax subsidies. Apparently, the laboratory testing
required to demonstrate NOx reductions does not scale well to real-world operations, and the majority of plants burning refined coal that collect the tax credits have not met the specified reduction levels. In fact, 22 of the 56 plants that were burning refined coal in 2017 showed higher NOx emissions than when they were burning raw coal in 2009.
Given that equipment and techniques exist to address the traditional emissions from burning coal, the term clean coal is now being associated with efforts to reduce emissions of carbon dioxide (CO2) and other greenhouse gases that are responsible for global warming and climate change. According to the EPA, in 2015 burning of coal contributed about 70% of all CO2 emissions from the electrical power sector and about 30% from all sources. In its new context, clean coal refers to the process of stripping the CO2 out of the coal, either before or after it is burned, and then capturing, or sequestering, it. It can then be used for industrial purposes, or it can be pressurized into a liquid and injected underground where it is intended to remain indefinitely. The overall process is called carbon capture and storage (CCS). See OWOE Topic: What is carbon capture and storage (CCS)?
for more information about CCS.