Burning coal for power generation gives rise to a variety of wastes that must be controlled or at least accounted for. So-called ‘clean coal’ technologies are a variety of evolving responses to late 20th-century environmental concerns, including that of global warming due to carbon dioxide releases into the atmosphere. Coal contains impurities, that, when burned, escape into the air as harmful air pollutants, including acid-rain-causing chemicals such as sulfur dioxide and nitrogen oxides.
There are many ways coal-burning by-products are dealt with. Some are:
The EPA formally designated carbon dioxide as an air pollutant in 2009, in recognition of the harm the gas poses “to public health and welfare” in its role in global warming. The main goal for coal researchers is a means of removing CO2 from the flue gas stream. At present, there are some promising technologies that may be used but none are ready for production yet. Some of these methods impose operating costs and energy efficiency loss without concomitant benefit to the operator, though external costs will almost certainly be increasingly factored in through carbon taxes or similar which will change the economics of burning coal.
The best technology cannot make coal completely pollution-free or carbon-neutral, but it can mitigate a lot of the negative environmental impacts. One technology, known as carbon capture and sequestration (or storage), or CCS, is a process in which CO2 is trapped and kept out of the atmosphere, usually by forcing the gas into below-ground formations. But the technology is expensive and is not yet widespread.
There are three main ways to do carbon capture, which differ in when and how the CO2 is removed:
Other “clean coal” methods also address carbon dioxide emissions, including novel ways of burning coal more efficiently. In a standard coal-fired power plant, coal is burned to heat a boiler, which creates steam that turns a turbine, which powers a generator to create electricity. Increasing the temperature and pressure of the steam, as is done in so-called supercritical and ultra-supercritical plants, makes this process more efficient. Because less coal needs to be burned to create the same amount of energy, the plants have smaller carbon footprints. These improvements, however, usually yield only modest reductions in carbon dioxide, such that they still do not match the lower emissions of modern natural gas plants.
Because of these technologies, coal is cleaner than ever before. The National Energy Technology Laboratory (NETL) states that a new coal plant with pollution controls reduces NOx emissions by 83%, SO2 by 98%, and particulate matter by 99.8%1.