What is "clean coal"?

"Clean coal" is not a particular type of coal – it is a set of techniques that help reduce emissions of pollutants such as sulfur and nitrogen oxides and particles and make it possible to produce more energy with less coal, which in turn cuts down on emissions of carbon dioxide (CO2). Capturing CO2 as it comes out of a power plant and storing it would make the coal even "cleaner" but this procedure will be long and costly to implement.

Qu'est-ce que le charbon propre ?
Boundary Dam in Canada is the world's only coal-fired power plant with a carbon capture system. ©Sask power

Coal-fired powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... plants are thermal power plants in which coalCoal is ranked by its degree of transformation or maturity, increasing in carbon content from... is burned to generate steam that activates a turbine to produce electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... . Intense research has been undertaken in recent years both in China and in western countries to improve the performance of coal-fired power plants.

Eliminating local pollutants

The first improvements were achieved in the 1980s, thanks to the removal of sulfur dioxide (SO2) from stack gases. SO2 had caused acid rain in North America and northern Europe, destroying forests and marking public opinion. Growing environmental concerns then led to the treatment of nitrogen oxides (NOx) – also found in car exhaust –, soot and non-combustible ashes, i.e., particles of between 1 and 100 microns. These are eliminated using electrostatic precipitators, cyclone separators, fabric filters and other systems. Research is being conducted on particles of less than one micron. TreatingTreating is all of the refining processes intended to remove unwanted compounds (contaminants)... sulfur, nitrogen and particles can increase production cost by more than 20%, and therefore the final cost of electricity.

From 33% to 55%: the increase in efficiency at coal-fired power plants thanks to technological advances

Improving power plant efficiency

A second path has been to improve power plant efficiency. By producing more energy with the same amount of coal, a plant improves its financial profitability, while emitting fewer pollutants and CO2See Carbon Dioxid . A traditional power plant has an efficiency of 33%, which means that 67% of the carbon goes into the atmosphere in the form of CO2.

New supercritical or ultra-supercritical power plant technologies lift efficiency to 45-46%. This is done by increasing the temperature to 620°C and pressure to 280/300 bars; the heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... is recovered to produce additional electricity in a process known as cogenerationThe simultaneous production of both heat (thermal energy) and power... .

Advanced ultra-supercritical power plants are expected to be running by 2020, with a temperature of between 700 and 760°C and pressure at 350-400 bars driving efficiency up to 50%. This type of power plant will have a capacity of up to 1100 MW, which is more than a traditional nuclear unit.

Gasifying coal

Another, more efficient method is to gasify coal before using it in this form in a gas-powered plant.  Here, the efficiency approaches 55%. Gasifying the coal also serves to clean it at the same time. But the cost is high since it essentially requires two adjoining power plants.

Lastly, there is the future of the future in which coal will be gasified and sent to a fuel cellA device that produces electricity by oxidizing a reducing agent (fuel) in one electrode (the anode) and reducing an oxidizing agent in another... rather than a gas turbine.  The resulting efficiency is anticipated at more than 60%, but there is not yet an industrial application for this procedure.

Capturing and storing

Capturing CO2 (see Feature Report "Carbone Capture and Storage") could provide a way forward if it were applied systematically to coal-fired power plants. Doing so runs up against two major obstacles, however:

  • Capturing CO2 requires large amounts of energy. At the world's only thermal power plant equipped for carbon capture, the Boundary Dam in Canada's Saskatchewan province, one third of the plant's energy is used to capture CO2, decreasing output from 165 MW to 110 MW.
  • The only use for captured CO2 at present is to re-inject it into oil deposits to facilitate the extraction of preservation (hydrocarbons)The final phase in petroleum system formation, after a deposit has accumulated... . The United States has developed an industry of this type. But storing CO2 on a larger scale below ground would only be economically feasible if a price is put on carbon – and that doesn’t even take the transportation difficulties into account.

Gas combustion emits half as much CO2 as coal combustion.

Moving from coal to gas

As efficient and promising as these techniques are, the related CO2 emissions do not measure up to replacing coal with gas. Natural gas emits half as much CO2 as coal, and substituting gas for coal brings greater advances in the fight against global warmingGlobal warming, also called planetary warming or climate change... .

This is the case in the United States where the surge in shale gasShale gas is found in deeply buried clayey sedimentary rock that is both the source rock and the reservoir for the gas... production has helped scale back the use of coal in electricity generation and enabled the country to reduce its emissions. But dependence on coal is still strong in large nations like China and India, where it is accessible at a lower cost and where coal reserves are substantial.