Almost 40 per cent of ash generated from coal-based power plants is not utilised and ends up in landfill sites as a waste product. Fly ash, which accounts for 80 per cent of the total ash generated from a thermal power plant (TPP) while the rest is bottom ash, poses a serious threat to the environment. It may consist of silicon dioxide, aluminium oxide, calcium oxide and heavy metals, depending on the source and composition of coal. Groundwater contamination, fugitive dust emissions and land pollution are some of the common issues associated with ash disposal, which severely impact the local population around ash lagoons. Ash disposal through traditional methods results in significant water consumption. Ash is transported from TPPs to cement units, brick kilns, mine filling sites and road construction sites. This requires extreme caution as ash lost in transit will cause environment degradation.
With increasing policy focus on environment protection, power plant operators need to deploy state-of-the-art ash handling technologies, which will help reduce pollution and save natural resources such as water. In recent years, several technological advancements have taken place in ash conveying and handling. Earlier, bottom and fly ash were disposed of in the ash pond in a wet slurry form. Now, dry conveying and dense slurry systems are being installed for their disposal. Besides, techniques to recover and treat the wastewater from ash ponds are gaining traction.
Ash handling and conveying systems
The fly ash generated from coal-based power plants is captured by electrostatic precipitators (ESPs) or baghouse filters before the flue gases reach the stack. The fly ash later falls on the pyramidal hoppers located at the bottom of the ESPs. The collected fly ash is transported to storage silos through wet or dry conveying systems.
Wet conveying systems
These systems use the conventional slurry disposal (CSD) and high concentration slurry disposal (HCSD) methods. In CSD, the content of ash in water is 10-35 per cent while in HCSD, it is 55-65 per cent. The transportation of ash at a low concentration is highly un-economical and leads to higher water consumption. CSD also leads to excessive wear and tear of pipelines carrying the slurry as it is transported at a high velocity. Further, owing to excessive water usage, both ground and surface water sources become highly polluted. Therefore, the HCSD method is preferred to CSD as the former offers the benefits of cost optimisation, lower water consumption, and less maintenance.
Under HCSD, fly ash from silos is fed into a mixing tank/agitator retention tank through a weighing unit, a rotary feeder and an ash conditioner. The conditioned fly ash is wett 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. 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 conveying systems
Dry conveying systems use mechanical and pneumatic methods. The mechanical method deploys conveyors and belts whereas pressurised air is used in the pneumatic method to transport fly ash to silos. The pneumatic method can be further classified into two types lean phase systems and dense phase systems.
In lean phase systems, the oldest method of pneumatic transport, fly ash is carried at high conveying velocities of over 20 metres per second. In this method, the ash to air ratio is relatively low and the method is only used for short distance transportation of fly ash.
Meanwhile, in the dense phase pneumatic system, the ash flows through pipelines in a plug form as the ash to air ratio is high, and at low conveying velocities of 3-8 metres per second. It is suitable for a wide range of applications over short to long distances and entails low operations and maintenance (O&M) costs.
Bottom ash collection systems
Of late, ash pond failures are forcing TPP operators to re-evaluate their wet ash handling practices. Therefore, dry bottom ash handling techniques have been developed which offer increased efficiency, reduction of unburned carbon and an improvement in ash quality.
A dry bottom system comprises a conveyor enclosed in a tight housing to prevent uncontrolled air infiltration. A sturdy, field-proven steel belt, referred to as a superbelt, conveys the hot ash to a crusher. The fine ash is collected by a spill chain located at the bottom of the housing. The ash so collected is cooled by the controlled flow of ambient air.
One of the biggest advantages of the dry bottom system is that it is environment friendly. There is no requirement of water for bottom ash cooling and conveying. As a result, the need for wastewater treatment also does not arise. Moreover, no extra costs are incurred for pumps, piping and dewatering bins, and corrosion damages. The O&M costs of a dry bottom system are considerably lower. Further, being a zero-discharge system, it can easily be integrated with the environmental management system.
Ash water recovery systems
Although dry ash handling and conveying techniques are improving, a large number of plants still use wet ash handling systems. Such plants also need to install ash water recovery or recirculation systems in order to minimise water wastage. These systems can recover up to 70-80 per cent of water from ash dykes. The water from the 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. These are more compact and have a high settling efficiency.
Several O&M challenges are faced by power plant operators in the functioning of ash handling systems. It is generally observed that ash evacuation from ESP hoppers slows down with the increase in the coal firing rate and ash generation. This progressively leads to a buildup of ash inside hoppers, which are designed for handling higher-than-expected ash quantities. These issues arise because ash particle distribution is not analysed prior to the design stage. The plant is generally designed with assumptions on particle size distribution. However, owing to the deterioration of coal quality in recent years, the ash particle size is much bigger than the design consideration.
Another common concern is the incomplete charging of ESP hoppers, which hampers the even collection of fly ash particles. Proper charging of ESP hoppers is crucial for the complete collection of fly ash particles and their evacuation through pneumatic systems. The abrasive nature of fly ash makes the internal surface of ESP hoppers rough, which in turn weakens their structure. To avoid this, the surfaces should be smoothened by coating with stainless steel claddings. Besides, jamming of ash conveying pipelines is a common occurrence, which requires the flushing out of fly ash resulting in air pollution and the loss of ash. This can be avoided by using an air dryer to remove moisture from the conveying medium, that is, air. Further, ash from silos is loaded on tankers using telescopic-type unloading spouts, which leads to considerable spillage. In order to mitigate the ash loss level, switch-based unloading spouts should be used with proper sealing at the tanker hatch.
TPP operators can significantly benefit from new ash management and disposal technologies, which are more efficient and environment friendly. With its range of benefits and better design features, new ash handling technologies will prove to be cost effective in the long run, even if their initial cost is higher. Further, these technologies would help alleviate the pressure on TPP operators to comply with the emission norms.