COAL TECHNOLOGIES
Technologies are continuously being developed to improve the efficiency of coal, meet environmental challenges, and increase the ways in which coal can be utilised.
The technologies employed and being developed to meet coal's environmental challenges are collectively referred to as clean coal technologies. Different technologies are being developed to suit different coal types, different environmental issues and different levels of economic development.
Coal's technical response to the environmental challenge is ongoing - with three core elements:
• Eliminating emissions of pollutants such as particulates, oxides of sulphur and nitrogen
• Improving combustion technologies to increase efficiency and to reduce carbon dioxide and other emissions
• Reducing carbon dioxide emissions with the development of carbon capture and storage
Through liquefaction and gasification coal can also provide low cost, secure alternatives to oil and natural gas for use in electricity generation, transport and domestically.
Coal can also be used to generate hydrogen for completely clean future energy systems.
Pollution Control Technologies
Coal-fired electricity generation today normally uses conventional pulverised coal combustion (PCC).
PCC is a well-developed technology, with thousands of units in operation around the world - making up over 90% of coal-fired capacity. Conventional PCC is still the technology of choice in many places around the world.
There are a number of pollution control technologies that can be applied to both existing and new build coal-fired power stations to reduce emissions of the major coal related pollutants, including:
• NOx
• SOx
• Particulates
If left unabated, these pollutants can cause local environmental issues including ground level ozone, acid rain, smog and respiratory problems. However, with today’s pollution control technologies – such as coal cleaning, flue gas desulphurisation, fabric filters and selective catalytic reduction – emissions of all these pollutants can be significantly reduced.
Coal Cleaning
Coal cleaning – also known as coal preparation, beneficiation or coal washing – is the cleaning process in which mineral matter is removed from mined coal to produce a cleaner product.
Mined coal varies in quality and contains substances such as clay, sand and carbonates.
Coal preparation has some important environmental benefits, it can:
• reduce the ash content of coal by over 50%, leading to much lower particulate emissions, reducing SO2 emissions and improving efficiency (which in turn leads to lower emissions of carbon dioxide).
• increase the heating value and the quality of the coal, by lowering the level of sulphur and mineral constituents.
The coal preparation process involves characterisation, liberation, separation and disposition.
• Characterisation - identifies the composition of the different raw coal particles.
• Liberation - involves crushing the mined coal and reducing it to very fine particles.
• Separation - is the partitioning of the individual particles into their appropriate size groupings and separating the mineral matter particles from the coal.
• Disposition - involves the dewatering and storage of the cleaned coal and the disposal of the mineral matter.
While coal cleaning is standard in many countries, there is significant scope for using it more widely in developing countries.
Particulate Emissions
Particulate emissions, such as ash from coal combustion, can affect people's respiratory systems, impact local visibility and cause dust problems.
There are a number of technologies that can be applied to existing and new plant that can reduce particulate emissions by as much as 99.5%, these include:
• Electrostatic Precipitators (ESPs)
• Fabric Filters
• Hot Gas Filtration Systems
• Wet Particle Scrubbers
Electrostatic Precipitators (ESPs)
Electrostatic precipitators are the most widely used particulate emissions control technology in coal-fired power generating facilities. Particulate/dust laden flue gases are passed horizontally between collecting plates, where an electrical field creates a charge on the particles. The particles are then attracted towards the collecting plates, where they accumulate.
The Lethabo Power Station in South Africa, for example, uses ESPs to control emissions, removing 99.8% of fly ash.
Fabric Filters
Fabric filters, also known as baghouses, collect particulates from the flue gas on a tightly woven fabric by sieving and other mechanisms. The choice between electrostatic separation and fabric filtration depends on coal type, plant size, and boiler type and configuration. Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitators.
Hot Gas Filtration Systems
Hot gas filtration systems operate at higher temperatures (500-1000ºC) and pressures (1-2 MPa) than conventional particulate removal technologies, eliminating the need for cooling of the gas. A range of technologies have been under development for many years with some in the demonstration stage but further development is needed to enable commercial exploitation.
Wet Particle Scrubbers
Wet particle scrubbers for particulate control are used in a limited number of coal-fired plants to capture fly ash in addition to sulphur dioxide (SO2). Water is injected into the flue gas stream to form droplets. The fly ash particles impact with the droplets forming a wet by-product, which then requires disposal. Wet particle scrubbers have a removal efficiency of 90-99.9%. Most installations using wet particle scrubbers are located in the USA.
SOx and NOx
Nitrogen Oxides (NOx) and Sulphur Oxides (SOx) are formed during the coal combustion process.
NOx can contribute to smog, ground level ozone, acid rain and greenhouse gas emissions. SOx – mainly sulphur dioxide – is produced from the combustion of elemental sulphur, present in many coals, and can lead to acid rain and acidic aerosols.
Technologies are available to minimise SOx emissions by removing the gas from the waste stream or by using advanced power generation methods. Emissions can be reduced by over 90%.
NOx emissions can be cut by the use of specialised burners, advanced combustion methods, catalysts and 'selective non-catalytic reduction'. Over 90% of NOx emissions can be removed using existing techniques.
Flue Gas Desulphurisation
Flue gas desulphurisation (FGD) technologies are used to remove sulphur emissions post-combustion. FGD technologies can be classified into six main categories:
• wet scrubbers
• spray dry scrubbers
• sorbent injection processes
• dry scrubbers
• regenerable processes
• combined SO2/NOx removal processes
Wet scrubbers tend to dominate the global FGD market. The technology uses alkaline sorbent slurry, predominantly lime or limestone based. A 'scrubbing vessel' or scrubber is located downstream of the boiler and flue gas cleaning plant, in which the sulphur dioxide in the flue gases reacts with the limestone sludge, forming gypsum.
The use of FGD in the USA, for example, has had a major impact on sulphur dioxide emissions, and costs have declined dramatically.
Low NOx Burners
Low NOx burners control the way that coal and air mixes at each burner within a power station in order to reduce the maximum flame temperature. This in-turn limits the formation of NOx and improves the efficiency of the burner. Low NOx burners can reduce NOx emissions by 30-55%. Currently there are over 370 coal-fired units (125 GWe) worldwide that use low NOx burners.
Selective Catalytic and Non-Catalytic Reduction (SCR & SNCR)
Techniques such as SCR and SNCR lower NOx emissions by treating the NOx post-combustion in the flue gas. SCR can achieve reductions of 80-90% in NOx emissions.
In selective catalytic reduction systems, ammonia vapour is used as the reducing agent and is injected into the flue gas stream, passing over a catalyst. The reducing agent reacts with NOx in the flue gas to form water and nitrogen. The key difference between SCR and SNCR is the presence of a catalyst - which accelerates the chemical reactions in the SCR systems. The catalyst is needed because SCR systems operate at much lower temperatures than SNCR (typical temperatures for SNCR are 870-1200ºC).
Combustion Technologies
A range of advanced coal combustion technologies have been developed to improve the efficiency of coal-fired power generation.
Improving efficiency levels increases the amount of energy that can be extracted from a single unit of coal. Already huge improvements have been made - the thermal efficiency of electricity from coal improved eightfold during the 20th century.
These improvements are continuing with technologies such as:
• Fluidised Bed Combustion
• Supercritical & Ultrasupercritical Boilers
• Integrated Gasification Combined Cycle
New, more efficient coal-fired combustion technologies reduce emissions of carbon dioxide, as well as pollutants such as NOx, SOx and particulates.
The technologies employed and being developed to meet coal's environmental challenges are collectively referred to as clean coal technologies. Different technologies are being developed to suit different coal types, different environmental issues and different levels of economic development.
Coal's technical response to the environmental challenge is ongoing - with three core elements:
• Eliminating emissions of pollutants such as particulates, oxides of sulphur and nitrogen
• Improving combustion technologies to increase efficiency and to reduce carbon dioxide and other emissions
• Reducing carbon dioxide emissions with the development of carbon capture and storage
Through liquefaction and gasification coal can also provide low cost, secure alternatives to oil and natural gas for use in electricity generation, transport and domestically.
Coal can also be used to generate hydrogen for completely clean future energy systems.
Pollution Control Technologies
Coal-fired electricity generation today normally uses conventional pulverised coal combustion (PCC).
PCC is a well-developed technology, with thousands of units in operation around the world - making up over 90% of coal-fired capacity. Conventional PCC is still the technology of choice in many places around the world.
There are a number of pollution control technologies that can be applied to both existing and new build coal-fired power stations to reduce emissions of the major coal related pollutants, including:
• NOx
• SOx
• Particulates
If left unabated, these pollutants can cause local environmental issues including ground level ozone, acid rain, smog and respiratory problems. However, with today’s pollution control technologies – such as coal cleaning, flue gas desulphurisation, fabric filters and selective catalytic reduction – emissions of all these pollutants can be significantly reduced.
Coal Cleaning
Coal cleaning – also known as coal preparation, beneficiation or coal washing – is the cleaning process in which mineral matter is removed from mined coal to produce a cleaner product.
Mined coal varies in quality and contains substances such as clay, sand and carbonates.
Coal preparation has some important environmental benefits, it can:
• reduce the ash content of coal by over 50%, leading to much lower particulate emissions, reducing SO2 emissions and improving efficiency (which in turn leads to lower emissions of carbon dioxide).
• increase the heating value and the quality of the coal, by lowering the level of sulphur and mineral constituents.
The coal preparation process involves characterisation, liberation, separation and disposition.
• Characterisation - identifies the composition of the different raw coal particles.
• Liberation - involves crushing the mined coal and reducing it to very fine particles.
• Separation - is the partitioning of the individual particles into their appropriate size groupings and separating the mineral matter particles from the coal.
• Disposition - involves the dewatering and storage of the cleaned coal and the disposal of the mineral matter.
While coal cleaning is standard in many countries, there is significant scope for using it more widely in developing countries.
Particulate Emissions
Particulate emissions, such as ash from coal combustion, can affect people's respiratory systems, impact local visibility and cause dust problems.
There are a number of technologies that can be applied to existing and new plant that can reduce particulate emissions by as much as 99.5%, these include:
• Electrostatic Precipitators (ESPs)
• Fabric Filters
• Hot Gas Filtration Systems
• Wet Particle Scrubbers
Electrostatic Precipitators (ESPs)
Electrostatic precipitators are the most widely used particulate emissions control technology in coal-fired power generating facilities. Particulate/dust laden flue gases are passed horizontally between collecting plates, where an electrical field creates a charge on the particles. The particles are then attracted towards the collecting plates, where they accumulate.
The Lethabo Power Station in South Africa, for example, uses ESPs to control emissions, removing 99.8% of fly ash.
Fabric Filters
Fabric filters, also known as baghouses, collect particulates from the flue gas on a tightly woven fabric by sieving and other mechanisms. The choice between electrostatic separation and fabric filtration depends on coal type, plant size, and boiler type and configuration. Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitators.
Hot Gas Filtration Systems
Hot gas filtration systems operate at higher temperatures (500-1000ºC) and pressures (1-2 MPa) than conventional particulate removal technologies, eliminating the need for cooling of the gas. A range of technologies have been under development for many years with some in the demonstration stage but further development is needed to enable commercial exploitation.
Wet Particle Scrubbers
Wet particle scrubbers for particulate control are used in a limited number of coal-fired plants to capture fly ash in addition to sulphur dioxide (SO2). Water is injected into the flue gas stream to form droplets. The fly ash particles impact with the droplets forming a wet by-product, which then requires disposal. Wet particle scrubbers have a removal efficiency of 90-99.9%. Most installations using wet particle scrubbers are located in the USA.
SOx and NOx
Nitrogen Oxides (NOx) and Sulphur Oxides (SOx) are formed during the coal combustion process.
NOx can contribute to smog, ground level ozone, acid rain and greenhouse gas emissions. SOx – mainly sulphur dioxide – is produced from the combustion of elemental sulphur, present in many coals, and can lead to acid rain and acidic aerosols.
Technologies are available to minimise SOx emissions by removing the gas from the waste stream or by using advanced power generation methods. Emissions can be reduced by over 90%.
NOx emissions can be cut by the use of specialised burners, advanced combustion methods, catalysts and 'selective non-catalytic reduction'. Over 90% of NOx emissions can be removed using existing techniques.
Flue Gas Desulphurisation
Flue gas desulphurisation (FGD) technologies are used to remove sulphur emissions post-combustion. FGD technologies can be classified into six main categories:
• wet scrubbers
• spray dry scrubbers
• sorbent injection processes
• dry scrubbers
• regenerable processes
• combined SO2/NOx removal processes
Wet scrubbers tend to dominate the global FGD market. The technology uses alkaline sorbent slurry, predominantly lime or limestone based. A 'scrubbing vessel' or scrubber is located downstream of the boiler and flue gas cleaning plant, in which the sulphur dioxide in the flue gases reacts with the limestone sludge, forming gypsum.
The use of FGD in the USA, for example, has had a major impact on sulphur dioxide emissions, and costs have declined dramatically.
Low NOx Burners
Low NOx burners control the way that coal and air mixes at each burner within a power station in order to reduce the maximum flame temperature. This in-turn limits the formation of NOx and improves the efficiency of the burner. Low NOx burners can reduce NOx emissions by 30-55%. Currently there are over 370 coal-fired units (125 GWe) worldwide that use low NOx burners.
Selective Catalytic and Non-Catalytic Reduction (SCR & SNCR)
Techniques such as SCR and SNCR lower NOx emissions by treating the NOx post-combustion in the flue gas. SCR can achieve reductions of 80-90% in NOx emissions.
In selective catalytic reduction systems, ammonia vapour is used as the reducing agent and is injected into the flue gas stream, passing over a catalyst. The reducing agent reacts with NOx in the flue gas to form water and nitrogen. The key difference between SCR and SNCR is the presence of a catalyst - which accelerates the chemical reactions in the SCR systems. The catalyst is needed because SCR systems operate at much lower temperatures than SNCR (typical temperatures for SNCR are 870-1200ºC).
Combustion Technologies
A range of advanced coal combustion technologies have been developed to improve the efficiency of coal-fired power generation.
Improving efficiency levels increases the amount of energy that can be extracted from a single unit of coal. Already huge improvements have been made - the thermal efficiency of electricity from coal improved eightfold during the 20th century.
These improvements are continuing with technologies such as:
• Fluidised Bed Combustion
• Supercritical & Ultrasupercritical Boilers
• Integrated Gasification Combined Cycle
New, more efficient coal-fired combustion technologies reduce emissions of carbon dioxide, as well as pollutants such as NOx, SOx and particulates.
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