According to the power ministry’s estimates, 1,800 million tonnes (mt) of coal is used every year and 600 mt of fly ash will be generated by 2031-32. Of this, about 40 per cent of the ash generated from coal-based power plants is not utilised and ends up in landfill sites as a waste product, which is a serious threat to the environment. Therefore, it is imperative for coal-based generators to install adequate ash conveying, handling and disposal systems at their power plants.
Fly ash and bottom ash generated in the plant have traditionally been disposed of into ash pond in the form of wet slurry. However, this results in significant water consumption. It is estimated that over 40 per cent of a thermal power plant’s (TPP) water requirement is accounted for by the ash handling plant. Keeping in view the paucity of natural resources such as water, steps need to be taken expeditiously to reduce water consumption in wet ash disposal and also deploy adequate systems for dry disposal of ash. NTPC Limited has taken significant steps in this direction by setting up dry fly ash evacuation and safe storage systems at its coal-based stations. At its Rihand TPP, the facility for loading fly ash into rail wagons has been provided so that fly ash can be transported in bulk through the railway network.
Power Line presents a round-up of the key technologies for ash handling…
Technologies for ash disposal
Dry fly ash disposal system
Ash collected by using dry fly ash disposal systems can be used as a raw material for cement manufacturing, thereby providing economic benefits to the plant owner. In the dry fly ash disposal system, fly ash from electrostatic precipitator hoppers is collected in ash vessels. To ensure free flow of ash into the ash vessels from the hopper, the lower portions of the hoppers are provided with electric heaters. The heaters maintain the temperature well above the ash fusion temperature to prevent the formation of clusters and ensure smooth functioning of the conveying system. The ash is then transported to fly ash silos with the help of compressed air. The moisture from the compressed air is removed by using an adsorbent air dryer or refrigeration air dryer. From the ash silos, the ash is finally transported to sealed vessel trucks.
Dry bottom ash system
This environment-friendly technique offers increased thermal efficiency, reduced unburned carbon and improved ash quality. 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 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.
In the dry bottom ash system, there is no requirement of water for bottom ash cooling and conveying. As a result, the need for waste water 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 also considerably lower than those of the wet system. Further, being a zero-discharge system, it can be easily integrated with the environmental management system.
Wet disposal systems
These systems use wet disposal of slurry and high concentration slurry disposal (HCSD) methods. In recently developed plants, wet disposal of ash has been adopted with a slurry concentration of 30 per cent for fly ash and 25 per cent for bottom ash. For wet/semi-wet disposal of ash, water is secured from the cooling tower blowdown and any additional requirement is met from the raw water source.
HCSD involves the process of pumping high concentration slurry with over 60 per cent solids by weight employing positive displacement pumps as compared to 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 minimal water is released in the disposal area. Fly ash from silos is fed into the mixing tank/agitator retention tank through the weighting unit, rotary feeder and ash conditioner. The conditioned fly ash is wetted further by adding water in the mixing tank, following which the entire ash is blended using a mixer to form uniform slurry. The quantities of ash and water are controlled in the mixing tank to achieve the desired characteristics. The mixer tank is equipped with an agitator to create smooth and homogeneous slurry that is lump-free. The slurry so prepared is then regulated through a control loop, water control system and material feed rate control system. The control loop continuously monitors the density of the slurry and automatically signals for the addition of water if required. Thereafter, the slurry is transferred to 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.
Water conservation in ash handling
In wet ash handling power plants, optimising the ash to water ratio can help in reducing water consumption. In most plants, the ash to water ratio is found out 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 per cent 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.
Similarly, recycling of clear ash water, after the ash gets settled in the ash dyke, is another method to conserve water. This can be done by the installation of ash water recirculation/recovery systems. The water from dykes is decanted and pumped into the stilling chamber of the ash water recovery system. It is then transferred to the flash mixer, where certain chemicals are added for its treatment. The water then flows to the clariflocculator to separate it from the sludge, following which clear water is pumped back for reuse. Tube settlers can also be used to separate ash particles from ash water. Through ash water recirculation, typically 70 per cent of the ash pond water can be recovered and reused in the ash handling plant. Thus, the net water to be supplied for ash disposal gets reduced to about 30 per cent of the plant requirement.
Going forward, an important factor to be considered by gencos in ash management is the new environment norms. In response to the new environment regulations, dry sorbent injection (DSI) systems are being considered for acid gas control (hydrogen chloride and sulphur dioxide [SO2]) and mercury capture. Since most utilities are just beginning their efforts to install DSI systems, there is limited experience in the implementation of these systems. DSI can have a significant impact on the fly ash handling system. Injection of sorbents before the particulate control device will increase the quantities of ash sent to the fly ash handling system. In most cases, increasing the size of fly ash hoppers is not practical because these hoppers are installed under an existing particulate control device. However, it is typically more feasible to modify the fly ash system, if needed, to convey higher quantities of ash. Alternatively, since most fly ash systems are run intermittently, the utility may want to consider operating the existing fly ash handling systems more frequently. Another approach to limiting the impact on the fly ash system is to reduce the amount of pollutants in the coal. When DSI systems are being used for SO2 control, the utility may want to consider a more stringent limit on the fuel sulphur content.
Overall, technologies for dry ash disposal systems benefit operators by improving the overall efficiency of plants as well as providing economically valuable material – ash that can be used as a raw material in concrete and cement or as a filling material in stabilisation projects and road beds. An added benefit of using these technologies is that they will help operators in conforming to emission regulations and standards. Thus, while these technologies may entail a high initial investment cost, in the long run they will prove to be cost effective and environment friendly.