An industrial power plant at sunset

Can Coal be Clean?

Burning Coal

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.

Coal Burning By-Products

There are many ways coal-burning by-products are dealt with.  Some are:

  • Electrostatic Precipitators – remove fly ash from the flue gas stream
  • Scrubbers – remove unwanted SO2 and convert it into gypsum.   SO2 is a constituent of acid rain
  • Low NOx Burners – used to limit the amount of NOx produced.  NOx can also produce acid rain


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.

“Clean” Coal Technology

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.

Carbon Capture

There are three main ways to do carbon capture, which differ in when and how the CO2 is removed:

  • Post-combustion: CO2 is captured after the coal has been burned. This is the only capture method in commercial operations. Essentially, before the flue gases are sent up the chimney, the CO2 is pulled out with chemicals, usually nitrogen-rich amines. This technique can be applied to existing coal plants as a retrofit — a huge advantage over the other methods. But it still takes quite a bit of energy to run the system mainly because the CO2-absorbing chemicals need to be stripped of CO2 so they can be regenerated and used again.
  • Pre-combustion: CO2 is captured from a coal-sourced gas before being burned. The carbon capture part is less expensive than post-combustion capture, but the overall cost of gasification plants is high. Just two American power plants use gasification at all. One plant, in Kemper County, Mississippi, was designed to do coal gasification with carbon capture, but they had to scrap those plans after cost overruns and delays. It now runs on natural gas.
  • Oxy-fuel: Coal is burned in oxygen rather than air. This makes it easier to collect the CO2 but running an oxygen plant takes a lot of energy, and the economics may or may not make sense compared with post-combustion capture.

Other Methods

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.