Clean Coal Technologies (CCTs)

CCTs are technologies that lower the environmental impact of burning coal. CCTs are designed to meet environmental regulations covering emissions, effluents, and residues. In some situations, CCTs offer the possibility of satisfying even more stringent standards, at an economically viable cost.

A basic approach to the cleaner use of coal is to reduce emissions by reducing the formation of pollutants such as NOx and/or cleaning the flue gases after combustion. A parallel approach is to develop more thermally efficient systems so that less coal is used to generate the same amount of power, together with improved techniques for flue gas cleaning, for effluent treatment and for residues use or disposal. Thermal efficiency may be increased by using a higher grade coal.

Various methods for coal-fired power generation are well established and widely used, and different steps are being taken to ensure that environmental requirements are satisfied. The methods include:

  • Pulverized Coal Combusion (PCC) with subcritical steam driving a steam turbine, with various levels of flue gas cleaning to meet local requirements
  • Cyclone Fired Wet Bottom Boilers with subcritical steam driving a steam turbine, again with various levels of flue gas cleaning to meet local requirements
  • Stoker Boilers for small applications, with subcritical steam, possibly using a low sulphur content coal.

Various new technologies are undergoing development in order to provide an environmentally satisfactory method of using coal as a basic fuel for power production in new plants. Some are now commercially available, backed by large-scale operating experience in a number of countries. Others are still at the demonstration stage. These technologies include:

  • PCC with supercritical steam driving a steam turbine, together with flue gas cleaning units
  • Atmospheric Pressure Fluidized Bed Combustion (FBC) in both bubbling (BFBC) and circulating (CFBC) beds, mainly with subcritical steam turbines, together with sorbent injection for SO2 reduction and particulates removal from flue gases
  • Pressurized Fluidized Bed Combustion (PFBC) currently uses bubbling bed boilers, in combined cycle with both a gas and steam turbine. Sorbent injection is used for SO2 reduction and particulates removal from flue gases
  • Integrated Gasification Combined Cycle (IGCC) using different types of gasifier, and in combined cycle with both a gas and steam turbine. The syngas stream is cleaned of H2S and particulates, before combustion and expansion of the combustion products through the turbine. Various levels of integration are used
  • Combined Heat and Power (CHP) applications where the (subcritical) steam turbine is designed to produce both power and useful heat for process or district heating.

Carbon Capture and Sequestration

Carbon Capture and Sequestration (CCS) is a way to reduce greenhouse gas emissions by capturing CO2 emissions at the emissions source (e.g., coal-fired power plants) and subsequently storing it instead of releasing it into the atmosphere. CCS stores carbon dioxide by injecting the gas into non-atmospheric reservoirs like depleted oil and gas reservoirs, unmineable coal seams, deep saline formations, or the deep ocean. Many of these geological structures have already held hydrocarbons or liquids for many millions of years.

Source: IEA GHG R&D Programme

Coal to Liquids

Coal to Liquid (CTL) technology uses coal as a raw material or feedstock for producing liquid fuels. Indirect and direct liquefaction are primary methods used in CTL production. In indirect coal liquefaction, coal is subjected to intense heat and pressure to create a synthesis gas comprised of hydrogen and carbon monoxide. The synthetic gas is treated to remove impurities and unwanted compounds such as mercury and sulfur. This clean gas enters a second stage Fisher-Tropsch process which converts the synthesis gas into clean liquid fuels and other chemical products.

Source: Futurecoalfuels.org

Diesel fuel produced by Fischer-Tropsch synthesis — nearly sulfur-free with low aromatics and a high cetane value — is cleaner than conventional diesel. It burns more completely and emissions are significantly lower than low-sulfur diesel, as tested by the Department of Defense in 6.5 liter diesel engines. Most of the CO2 is already concentrated and ready for capture and possible sequestration or for use in enhanced oil or gas recovery. In direct coal liquefaction, coal is pulverized and mixed with oil and hydrogen in a pressurized environment. This process converts the coal into a synthetic crude oil that can then be refined into a variety of fuel products. The direct coal liquefaction technology has been demonstrated in the United States and is now being commercially deployed in China and other countries.