Indian coal has a high ash content of 30-45 per cent. As per industry estimates, coal-based units ranging from 250 MW to 800 MW produce 100-320 million tonnes of ash per hour. High ash content increases system throughput and erosion, and reduces the life of the boilers and ash handling systems. It also leads to slagging of furnace walls and fouling of convection passes. Excessive slagging blocks the convection passes and plugs air preheaters. Further, the ash may contain heavy metals and other hazardous components and thus, it should be disposed of properly.
The effective collection and management of ash is fast becoming a key priority as well as a huge challenge for power generators given the growing environmental pressures due to regulatory requirements. Thus, concerted efforts are being made to ensure better ash management, which includes the deployment of new advanced technologies such as high concentration slurry disposal systems.
Power Line takes a look at some of the recent technologies being used for ash management and utilisation…
Coal-based generation technologies and ash properties
The behaviour of ash during the combustion process and the management of coal ash are the major issues in boiler design. The high combustion temperatures and high gas temperatures in boilers result in the creation of ash. This ash is then deposited on various heat transfer surfaces of the boiler. The resulting slagging and fouling create operational as well as performance problems, such as reduction in heat transfer, handling and burning capacity, efficiency and power generation, and increase in gas temperatures and maintenance requirements (slag falls, etc.). This can significantly impact the overall economics of the power plant and therefore there is a need to take into account the properties of coal and its ash while developing the design of a boiler.
Although the dominant technology for coal-fired power generation remains pulverised coal combustion, newer, cleaner coal technologies such as fluidised bed combustion and gasification (usually as part of an integrated cycle) have been developed and are increasingly finding application. The ash produced through these advanced technologies is often different in character from conventional coal ash and requires special consideration.
Fluidised bed combustors generate two major ash streams – fly ash, which is elutriated from the fluidised bed and collected from the flue gas stream in a bag filter or an electrostatic precipitator, and bottom ash. In both cases, the ash contains a mixture of fuel ash, unburned carbon residues, calcium sulphate and sulphite and unreacted lime or limestone, if the latter has been added for sulphur capture.
The properties of the two ash streams are considerably different from the ash produced from pulverised coal firing since in fluidised bed combustion the coal is not pulverised prior to combustion and thus the fly ash particles are much larger than the fly ash from pulverised coal combustion. Owing to their significant lime content, these residues are frequently classified as hazardous wastes with high disposal costs. These high costs make it important to identify ash utilisation options for the economic operation of plants.
In terms of electricity generation, integrated gasification combined cycle (IGCC) plants are more efficient than conventional thermal power plants, but they produce a high quantity of wastes in the form of slag and fly ash. Most of the residue from IGCC systems is produced as slag or agglomerated ash from the bed, while some is collected as fly ash.
Meanwhile, there is now a considerable interest in the utilisation of biomass and municipal/industrial wastes within the existing coal-fired plants. Some of the technologies in this regard are stokers, bubbling fluidised bed combustion, circulating fluidised bed combustion, cyclone combustors, pulverised coal plants, advanced coal gasification plants and carbonisation plants. In most situations, the biomass materials are co-fired at relatively low co-firing ratios, with the result that the overall chemistry of the mixed ash material is dominated by coal ash.
Wet fly ash disposal techniques
Of the total ash generated, only about 20 per cent takes the form of bottom ash and the remaining is fly ash. Fly ash is collected using electrostatic precipitators (ESPs) that operate by charging the particulate matter entrained in the flue gas stream through high voltage and then electrostatically influencing those particles towards oppositely charged electrodes. ESPs are installed near the flue gas chimney. The fly ash particles in the flue gas stick to the screen of the ESP and the remaining flue gas escapes through the chimney. The stuck ash is then scraped from the screen using an automated scraper and collected in a hopper. The collected fly ash is then transported either hydraulically or pneumatically. In the pneumatic method, fly ash is transported to silos using pressurised air in order to deliver it to potential buyers and not disposed of like bottom ash.
Typically, utilities use the low concentration slurry disposal method, in which the concentration of ash is 10-15 per cent by weight. However, this method suffers from a number of disadvantages. It is uneconomical, uses excessive water, and results in wear and tear of pipelines.
A new technological development is the high concentration slurry disposal (HCSD) method, which is more environment-friendly. The ash concentration is 60 per cent or above by weight, thus significantly reducing the water consumption. Under this method, fly ash from silos is fed into a mixing tank/agitator retention tank through a weighting unit, a rotary feeder and an ash conditioner. The conditioned fly ash is wetted further by adding water in the mixing tank. Then the ash is blended using a mixer to form uniform slurry and the quantities of ash and water are controlled in the mixing tank to reach the desired characteristics. The slurry so prepared is then regulated through a control loop, a water control system and a material feed rate control system. The control loop continuously monitors the density of the slurry and manages the addition of water, if required. Thereafter, the slurry is transferred into a positive displacement type high concentration ash slurry disposal pump. The HCSD pump discharges the concentrated fly ash slurry into the ash dyke through seamless pipelines.
Dry fly ash disposal system
Dry fly ash disposal systems are gaining popularity owing to their economic benefits to plant owners. This is because the ash collected using these systems can be used as a raw material for cement manufacturing. In this method, fly ash from ESP hoppers is collected in ash vessels. To ensure a free flow of ash into the ash vessels from the hopper, the lower portions of the hoppers are deployed with electric heaters. These heaters maintain the temperature well above the ash fusion temperature to prevent the formation of clusters of ash and ensure the smooth functioning of the conveying system. The ash is then transported to fly ash silos with the help of compressed air and then further carried to sealed vessel trucks. The bottom of each fly ash silo is also equipped with an additional discharging channel, which is connected with a wet mixer as a backup for the dry ash disposal system. A key limitation of this system is that it is capital intensive as compared to the wet disposal system.
Dry bottom ash handling techniques
The dry bottom ash system comprises a submerged scraper conveyor for cooling and transmitting the hot bottom ash to the crusher. The conveyor is enclosed in a tight housing to prevent uncontrolled air infiltration. The fine ash is collected through a spill chain located at the bottom of the housing and is cooled with the controlled flow of ambient air. Some of the advantages of dry bottom ash handling techniques are increased thermal efficiency, reduced unburned carbon and improved ash quality.
Management of disposed ash
The management of ash disposal through wet disposal involves lagooning within the allotted land, continuous inspection, monitoring and maintenance, and commitment towards safe disposal. In order to minimise the risk of failure, preventive measures such as recirculation of ash water, operations and maintenance, emergency action plan and preparedness, control of fugitive dust, and control of pollution of groundwater can be undertaken for managing ash disposal.
With regulatory changes and technology developments, ash management has evolved significantly over the years and continues to improve. Technologies such as HCSD and dry ash disposal systems improve the overall efficiency of plants and provide monetary benefits to operators. They also help operators conform to emission regulations and standards.