Gencos are increasingly looking at technologies and solutions to reduce the environmental impact of coal-based power generation. From emission control equipment to systems for ash handling and disposal, gencos are looking for solutions that offer maximum efficiency and entail minimal use of natural resources. In coal-based power plants, ash is the primary combustion residue and needs proper collection and disposal so that it does not end up contaminating the environment and can rather be utilised for gainful purposes.
In a typical coal-based power plant, ash from the combustion process is collected at different locations. Of the total ash generated by a thermal power plant (TPP), 75-80 per cent is fly ash, which is collected in electrostatic precipitators (ESPs) or fabric filters. The bottom ash accounts for 15-20 per cent of the total ash and is collected below the boiler. The remaining (about 5 per cent of the total) is collected below the economiser and the air preheater. Different systems are installed for the disposal of bottom ash and fly ash, which collect and transport the bottom ash and fly ash from the boiler to the respective storage points.
Power Line presents an overview of various ash handling systems and technologies…
Bottom ash handling system
In conventional ash handling plants (AHPs), the bottom ash is collected and crushed to produce fine powdered ash. The powdered ash is then mixed with water to produce slurry, which is disposed of in an open pond called the ash dyke. However, this results in significant water consumption. It is estimated that over 40 per cent of a TPP’s water requirement is accounted for by the AHP.
Alternative solutions such as mechanical drag systems and dry bottom ash systems have been developed by the industry to overcome this issue. Mechanical drag systems are used to convey bottom ash to the dewatering storage bin because they use less water and usually have a lower initial cost. The system can handle ash, dropping into the conveyance system at temperatures as high as 2,400° F, requiring each system to have a quenching volume of water at the initial collection point (bottom ash hopper).
Another recent innovation is the development of a completely dry bottom ash conveyor system, which operates without the required volume of water and delivers a dry, lower carbon content material to the discharge end of the conveyor. Rejects from the pulveriser are typically transported via the bottom ash conveyance system. The dry bottom ash system comprises a submerged scrapper conveyor for cooling and transmitting the hot bottom ash to the crusher. The conveyor is enclosed in an airtight housing to prevent uncontrolled air infiltration. The fine ash is collected by a spill chain located at the bottom of the housing. The ash so collected is then cooled by the controlled flow of ambient air. Since there is no requirement of water for bottom ash cooling and conveying, the need for wastewater treatment does not arise. In addition, the costs associated with pumps, piping, dewatering bins and corrosion damage are also avoided. The operations and maintenance (O&M) costs of the dry bottom ash system are considerably lower than these of the wet system. Further, being a zero-discharge system, it can be easily integrated with the power plant’s environmental management system.
Fly ash handling system
Most of the ash from a coal-fired boiler is carried through the boiler and air heater by flue gas. Of the ash generated by the combustion of pulverised coal, 50-70 per cent is removed from the flue gas by fabric filters or ESPs, which have rows of collection hoppers that are emptied regularly by the fly ash transport system.
Fly ash can be transported through hydro sluicing or pneumatic conveying. The former method is not finding many takers because of its significant water requirement as well as the need for storage area (ash pond). Further, since fly ash consists of fine particles with low density, they cannot be easily separated from water at the end of the transport process. Also, hydro sluicing renders the ash unsuitable for utilisation by industries such as cement and brick manufacturing. Therefore, fly ash is generally transported pneumatically. Once the economiser ash is removed from the high temperature gas stream, it can be combined with the pneumatic fly ash transport system. Pneumatic fly ash conveying systems can be categorised into four types – vacuum, pressure, combination and dense phase systems.
In a vacuum-based system, hydraulic exhausters or vacuum pumps are used to create vacuum and fly ash from various hoppers is evacuated at a high velocity in a lean phase, up to the immediate silo. It is suitable for short distance conveying. In a pressure conveying system, ash is conveyed in suspension in pipes through the use of positive displacement blowers and compressors. This system operates at a high velocity and is suitable for long distance conveying. Meanwhile, the dense phase pneumatic system pushes the ash along an enclosed pipe, in a plug form, using small amounts of gas, at a low velocity but high pressure. The low velocity of dense phase conveying allows for less wear and tear on the system, thereby minimising maintenance requirement and cost.
Issues and the way forward
Gencos often experience O&M issues in AHPs. In dry ash disposal, one of the major issues is the blockage of lines due to low transportation capacity. This generally happens when the AHP is designed on the basis of assumptions, rather than the actual particle size distribution. Hence, it becomes important to analyse ash particles prior to the design of the AHP. Other issues include the need for appropriate sizing of the air compressor and possibilities of choking of the wetting head and air washer nozzles and corrosion of the water pump.
The conservation of water usage in wet AHPs is a key area for gencos deploying conventional conveying technologies. In such systems, optimising the ash to water ratio can help in reducing water consumption. In most plants, the ash to water ratio is found to be 1:20. Restricting this to 1:5 for fly ash and 1:8 for bottom ash can significantly help in conserving water. For every percentage reduction in the ash to water ratio, there is a saving potential of 60 cubic metres per hour of water. After initiating water conservation measures, many TPPs have brought down their ash-water ratios to a reasonable range of 1:10 to 1:12. Another alternative is the use of the high concentration slurry disposal (HCSD) method, which involves pumping high concentration slurry with over 60 per cent solids by weight through positive displacement pumps as compared to the lean slurry transportation at 25-30 per cent concentration. Owing to the benefits of cost optimisation, lower water consumption and less maintenance, the HCSD method is preferred to wet disposal. Since the ash concentration is high in HCSD, water consumption is reduced significantly and minimum water is released in the disposal area.
The installation of an ash water recirculation system is another measure being taken for reducing the water requirement in ash handling. In plants using ash water recirculation, typically, 70 per cent of the ash pond water can be recovered and reused in the AHP. In this condition, the available cooling tower blowdown is sufficient to meet the make-up water requirement of the AHP and no additional water is required to be drawn from raw water sources. As such, additional water from raw water sources will be required only till the ash water recirculation system becomes operational. Therefore, the net water to be supplied for ash disposal is reduced to about 30 per cent of the total requirement of the AHP.
In addition, changes in the internal design of the centrifugal pumps can also lower the water requirement. Centrifugal pumps with improved design can take the slurry up to 50-55 per cent concentration. However, distance is a constraint for these pumps, as they can carry slurry up to only 2-3 km. For longer distances, positive displacement pumps are ideal as they can carry slurry concentration of up to 65-70 per cent for a distance of 8-10 km. High rating positive displacement pumps can go up to a longer distance. Typically, pumps capable of transporting slurry of medium and high concentration are often made to operate on low concentration slurry, which increases the water requirement for ash management at a plant. Further, gencos are deploying online ash analysers that use gamma rays to measure the ash content and calorific value in coal. The analyser measures the total ash content in coal on a belt conveyor and reports the results to the plant operator in real time, where it is used for informed decision-making and process control.
Net, net, new technologies in ash disposal and conveying are centred on minimising water usage and eliminating wastewater generation, which augurs well for the environment. Given the criticism attracted by coal-based power plants owing to their adverse impact on the environment as well as the tightening of environmental regulations, these technological advancements are the need of the hour for thermal gencos.