Ash handling technologies help in collecting, conveying, storing and loading different types of residual ash generated by coal-based power plants. The most common types of ash, resulting from the combustion of coal, lignite, wood and other solid fuels (like biomass and refuse-derived fuel [RDF]) are bottom ash and fly ash. While the coarser ash particles, referred to as bottom ash, fall to the bottom of the combustion chamber, the lighter fine ash particles or fly ash remain suspended in the flue gas and are removed by particulate emission control devices such as electrostatic precipitators (ESPs) or filter fabric baghouses.
Ash handling processes
In ash handling systems, different processes are used – hydraulic, pneumatic and mechanical. In a hydraulic system, typically used in large thermal power plants, ash from the furnace grate falls into a system of water, travelling at high pressure, and is then carried to sumps. Meanwhile, in a mechanical system, cooled ash falls on the belt conveyed and is carried continuously to the bunker. The ash is then moved to the dumping site from the ash bunker. In a pneumatic system, ash from the boiler ash outlet falls into a blow tank provided below the ash hopper. It is then pushed into pipelines and conveyed by the air stream to the point of delivery. The air leaving the ash separator is passed through a filter to remove dust and other particles and release clean air into the atmosphere.
Of late, pneumatic conveying systems have gained uptake among gencos for handling of fly ash in dry conditions since most applications entail usage of dry fly ash. A key benefit of handling ash in dry conditions with pneumatic conveying systems is that the ash can be collected from various fields separately, thus taking the required grade of dry ash to the users for specific applications like blending with cement for which a finer grade of ash is preferred.
Dry fly ash disposal system
The ash collected by dry fly ash disposal systems can be utilised as a raw material in cement production, resulting in cost savings for the plant owner. Fly ash from ESP hoppers is collected in ash containers in the dry fly ash disposal system. The bottom sections of the hoppers are equipped with electric heaters to ensure that ash flows freely into the ash containers from the hopper. To avoid the formation of clusters and ensure the smooth operation of the conveying system, the heaters keep the temperature far above the ash fusion temperature. With the help of compressed air, the ash is transported to fly ash silos. An adsorbent air dryer or a refrigerated air dryer is used to remove moisture from compressed air. The ash is ultimately transferred to sealed vessel trucks from the ash silos.
Dry bottom ash system
This environmentally friendly method improves ash quality while increasing thermal efficiency and reducing unburned carbon. The dry bottom ash system comprises a submerged scraper conveyor for cooling and transmitting the hot bottom ash to the crusher. To avoid uncontrolled air penetration, the conveyor is encased in an airtight container. A spill chain positioned at the bottom of the housing collects the fine ash. After that, the ash is cooled by a regulated flow of ambient air. There is no requirement of water for bottom ash cooling and conveying in the dry bottom ash system. Therefore, there is no requirement of wastewater treatment. This helps avoid the expenditure on pumps, piping, dewatering bins and corrosion damage. The dry bottom ash system also has significantly lower operations and maintenance expenses as compared to wet systems. Further, it can be readily integrated with the environmental management system because it is a zero-discharge technology.
NTPC Limited, the country’s largest power generator, has taken initiatives to minimise its water consumption. These include adopting a dry bottom ash handling system instead of a conventional wet bottom ash handling system for its upcoming coal-based power plants – Patratu, Singrauli III and Lara II – and for the renovation and modernisation project at the Rihand supercritical TPP. As per the company, the dry bottom ash handling system will facilitate the extraction of bottom ash in dry form, thereby eliminating the water requirement. Only a small quantity of water will be required for conditioning and dust suppression. The system not only reduces water consumption for the disposal of bottom ash in wet form, but also reduces power consumption for bottom ash disposal and facilitates the separation of bottom ash and fly ash, resulting in better utilisation of ash.
Wet disposal systems
These systems use wet disposal of slurry and high concentration slurry disposal (HCSD) methods. Wet ash disposal has been adopted in recently developed plants, 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 requirement, the HCSD method is preferred to wet disposal. Since 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 results. The mixer tank is equipped with an agitator to create a smooth and homogeneous slurry that is lump-free. 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 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.
Ash bagging systems
Some gencos also deploy fly ash bagging systems to mechanically fill fly ash into bags for transportation to the site of application (such as cement or brick manufacturing plants). Typically, bagging systems are installed in the silo. This system is capable of filling dry fly ash into bags of variable capacity by adjusting the quantity and number of dumps to be filled in each bag. It is thus possible to fill varying capacity bags from the same system. NTPC is also planning to implement a fly ash classification system and a bagging plant at its upcoming coal-based TPPs – Singrauli III and Lara II. The feasibility report has been prepared for implementation of the classification and bagging system at NTPC’s Simhadri project.
Ash handling in RDF-based plants
RDF is a category of alternative fuel produced as by-products of the process of waste management. It can be used in heat and power production. RDF being a low density fuel generates more fly ash during combustion. Fly ash acts as a catalyst for denovo synthesis (at 200-450 °Celsius) for the production of dioxins and furans. In order to reduce the formation of dioxins and furans, it is imperative that maximum fly ash is removed before the gases cool to the range of 200-450 °Celsius. This requires multiple passes in the radiative section of the boiler. The flue gases produced in the boilers have to be treated by an elaborate air pollution control system. The resultant ash from the incineration of solid waste can be used as construction material after the necessary processing, while the residue can be safely disposed of in a landfill. To cater to the increase in the use of alternative fuels like RDF in industrial processes such as captive power generation, specialised ash handling systems for waste incinerators, upgraded ash or material management and conveying systems are required.
Conclusion
Net, net, dry ash disposal methods help operators by increasing plant efficiency and providing economically valuable material. The ash produced can be utilised as a raw material in concrete and cement, as well as a filler material in stabilisation projects and road beds. Using this technology can help operators adhere to emission regulations and standards. While these technologies may have a high initial investment cost, they will prove to be cost effective and environmentally friendly in the long term.