Clean Coal Technologies
Clean Coal Technologies - Examples
Clean coal technologies are a family of new technological innovations that are environmentally superior to the technologies in common use today.
Clean coal technologies can be new combustion processes - like fluidized bed combustion and low-NOx burners - that remove pollutants, or prevent them from forming, while the coal burns.
Clean coal technologies can be new pollution control devices - like advanced scrubbers - that clean pollutants from flue gases before they exit a plant's smokestack.
Still other clean coal technologies can convert coal into fuel forms that can be cleaned before being burned. For example, a clean coal plant may convert coal into a gas that has the same environmental characteristics as clean-burning natural gas.
Examples of clean coal technologies currently in operation or under development around the world include:
Stack Gas Treatment - applied to gaseous emissions from Pulverised Fuel (PF) Combustion
Pulverised Fuel (PF) combustion is the most widely used method for burning coal for power generation. In PF combustion, coal is milled to a powder and blown into the boiler with air. As a powder, the coal has a large surface area and is easily combusted in burners. This provides the heat which is used to produce superheated steam to drive turbines and hence generate electricity. At present, nearly all of the world's coal-fired electricity is produced using PF combustion systems.
Typical coal-fired power station with Flue Gas Desulphurisation
Emissions from PF combustion can be reduced by post-combustion CCTs. Electrostatic Precipitators and/or fabric filters can remove more than 99% of fly ash from flue gases. Flue Gas Desulphurisation (FGD) methods can remove 90-97% of the oxides of sulphur (SOx) from flue gases, and can convert it into gypsum for use in the building trade.
Among the key CCTs for PF combustion that reduce emissions of nitrogen oxides (NOx) are low-NOx burners, which modify emissions by up to 40%, and reburning techniques. Together these modify the combustion process to reduce NOx emissions by up to 70%, and are being widely adopted as they can be installed into existing plant. Selective catalytic NOx reduction, a post-combustion technique, can achieve reductions of 80-90%.
Among the key CCTs for PF combustion that reduce emissions of nitrogen oxides (NOx) are low-NOx burners, which modify emissions by up to 40%, and reburning techniques. Together these modify the combustion process to reduce NOx emissions by up to 70%, and are being widely adopted as they can be installed into existing plant. Selective catalytic NOx reduction, a post-combustion technique, can achieve reductions of 80-90%.
Advanced Pulverised Fuel (PF) Combustion
Industry has continuously striven to increase efficiencies of conventional plant; for example, the average thermal efficiency of US power stations has increased from 5% in 1900, to around 35% currently. In China, most power plants are relatively small, average efficiency is about 28% compared to an OECD average of 38%. New conventional PF power plants achieve above 40% efficiency.
Advanced modern plants use specially developed high strength alloy steels, which enable the use of supercritical and ultra-supercritical steam (pressures >248 bar and temperatures >566°C) and can achieve, depending on location, close to 45% efficiency.
Application of new advanced materials to PF power plant should enable efficiencies of 55% to be achieved in the future. This results in corresponding reductions in CO2 emissions as less fuel is used per unit of electricity generated.
Advanced modern plants use specially developed high strength alloy steels, which enable the use of supercritical and ultra-supercritical steam (pressures >248 bar and temperatures >566°C) and can achieve, depending on location, close to 45% efficiency.
Application of new advanced materials to PF power plant should enable efficiencies of 55% to be achieved in the future. This results in corresponding reductions in CO2 emissions as less fuel is used per unit of electricity generated.
Fluidised Bed Combustion (FBC)
Fluidised bed combustion is a method of burning coal in a bed of heated particles suspended in a gas flow. At sufficient flow rates, the bed acts as a fluid resulting in rapid mixing of the particles. Coal is added to the bed and the continuous mixing encourages complete combustion and a lower temperature than that of PF combustion.
The advantages of fluidised beds are they produce less NOx in the outlet gas, because of lower combustion temperatures, and they produce less SOx when limestone is continuously added with the coal. They can also use a wider range of fuels than PF combustion.
Atmospheric-pressure fluidised beds are commercially available now as two types, bubbling-bed (known as Atmospheric Fluidised Bed Combustion - AFBCs) and circulating-bed (CFBCs). The efficiency of most fluidised beds used for power generation is similar to that of conventional plant. However, use of this technology has been stimulated by its better environmental performance when utilising lower grade fuels.
Pressurised fluidised beds, which can achieve efficiencies of 45%, are now in commercial operation. As with PF plants, employing higher steam conditions would further boost efficiency.
Atmospheric-pressure fluidised beds are commercially available now as two types, bubbling-bed (known as Atmospheric Fluidised Bed Combustion - AFBCs) and circulating-bed (CFBCs). The efficiency of most fluidised beds used for power generation is similar to that of conventional plant. However, use of this technology has been stimulated by its better environmental performance when utilising lower grade fuels.
Pressurised fluidised beds, which can achieve efficiencies of 45%, are now in commercial operation. As with PF plants, employing higher steam conditions would further boost efficiency.
Gasification and Integrated Coal Gasification Combined Cycle (IGCC)
An alternative to coal combustion is coal gasification. When coal is brought into contact with steam and oxygen, thermochemical reactions produce a fuel gas, largely carbon monoxide and hydrogen, which when combusted can be used to power gas turbines.
Integrated Coal Gasification Combined Cycle Unit
Integrated Coal Gasification Combined Cycle (IGCC) power generating systems are presently being developed and operated in Europe and the USA. These systems give increased efficiencies by using waste heat from the product gas to produce steam to drive a steam turbine, in addition to a gas turbine.
Existing commercial systems achieve efficiencies close to 45%. With recent advances in gas turbine technologies these systems are now capable of reaching above 50%. IGCC systems additionally produce less solid waste and lower emissions of SOx, NOx and CO2. Over 99% of the sulphur present in the coal can be recovered for sale as chemically pure sulphur.
Existing commercial systems achieve efficiencies close to 45%. With recent advances in gas turbine technologies these systems are now capable of reaching above 50%. IGCC systems additionally produce less solid waste and lower emissions of SOx, NOx and CO2. Over 99% of the sulphur present in the coal can be recovered for sale as chemically pure sulphur.
Hybrid and Advanced Systems
Hybrid combined cycles are also under development. These combine the best features of both gasification and combustion technologies, using coal in a two-stage process. The first stage gasifies the majority of the coal and runs a gas turbine, the second stage combusts the residual 'char' to produce steam. Efficiencies greater than 50% are possible.
In addition to these CCTs, a development which can apply to all of the generating systems is the co-firing with coal of biomass or wastes. This involves burning or gasifying such materials together with coal. Benefits can include reductions in CO2, SOx and NOx emissions relative to coal-only fired plants, and recovery of useful energy from biomass and wastes at high efficiencies can be achieved, without the need for building dedicated plant. Hence, the coal-fired power industry can support the renewable energy and waste industries.
In addition to these CCTs, a development which can apply to all of the generating systems is the co-firing with coal of biomass or wastes. This involves burning or gasifying such materials together with coal. Benefits can include reductions in CO2, SOx and NOx emissions relative to coal-only fired plants, and recovery of useful energy from biomass and wastes at high efficiencies can be achieved, without the need for building dedicated plant. Hence, the coal-fired power industry can support the renewable energy and waste industries.
(The description of clean coal technologies above has been drawn from the World Coal Institute publication - Coal, Power for Progress.
Fuel Cell Technologies
Fuel cells and magnetohydrodynamics (MHD) are two technologies still in the early development stage. Fuel cells allow hydrogen from natural gas, methanol or coal gas to react electrochemically with oxygen from the air to generate electricity.
Fuel cells have the potential for very high power generation efficiency and low carbon dioxide emissions. ln a coal-fired magnetohydrodynamics system, coal is burned to form an extremely hot gas or plasma. This is given an electric charge by adding a seed compound like potassium salt.
When the charged gas is passed through a strong magnetic field, electricity is produced. Heat from the combustion gases is also used to produce electricity using a conventional steam turbine.
Fuel cells have the potential for very high power generation efficiency and low carbon dioxide emissions. ln a coal-fired magnetohydrodynamics system, coal is burned to form an extremely hot gas or plasma. This is given an electric charge by adding a seed compound like potassium salt.
When the charged gas is passed through a strong magnetic field, electricity is produced. Heat from the combustion gases is also used to produce electricity using a conventional steam turbine.
The use of fuel cells has been demonstrated at the 2 MWe size and plans are underway to use hydrogen from coal gasification in this and other technologies.
Together with sequestration of CO2 in isolation this clean coal technology provides a nil CO2 option. However, lower cost equipment and more particularly markets for hydrogen need to be developed.
Together with sequestration of CO2 in isolation this clean coal technology provides a nil CO2 option. However, lower cost equipment and more particularly markets for hydrogen need to be developed.
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