To lower the pollution from coal-based thermal plants, the Central Electricity Authority (CEA) has come out with guidelines to dispose of various types of ash from thermal plants. The CEA guidelines suggest that fly ash can be disposed of through a high-concentration slurry disposal (HCSD) system.
“The generated ash is disposed of in well-designed, constructed, and maintained ash ponds generally in wet slurry form. At a specific location, compared to a wet ash disposal system, a dry ash disposal system may also be suitable. In new projects, due to the inherent benefit of the HCSD system, is being preferred to reduce land and water requirements as well as to prevent contamination of groundwater,” said the CEA report.
Basically, thermal power projects (TPP) are to be provided with systems for 100% dry ash extraction and storage and supply of ash to various entrepreneurs for promoting ash utilisation.
“As per MOEF&CC notification, each TPP shall install dedicated dry fly ash silos for the storage of at least 16 hours of ash-- based on the installed capacity having separate access roads so as to ease the delivery of ash,” said the CEA guidelines.
Wet disposal in high-concentration slurry form is an advanced system of wet disposal with a fly ash concentration of 60% to 70% of ash by weight. Due to the high concentration of ash, it is pumped through high-pressure slurry pumps to the disposal area and needs steel pipes for conveying the slurry. Flexible pipes are used at the disposal area. Centrifugal pumps have also been implemented for conveying high-concentration slurry to long distances in one plant and have since then been reported to be working satisfactorily.
The dry ash disposal system is entirely different from the wet disposal system. In the dry ash disposal system, furnace bottom ash (FBA) and pulverised fly ash (PFA) are transported in moistened form from hydro bins and silos, respectively, to ash mound sites on fixed belt conveyors in enclosed gantries. In the ash mound area, ash is disposed of by various types of equipment like fixed, extendable, shiftable, and mobile belt conveyors, a crawler-mounted boom spreader, a crawler-mounted bucket wheel reclaimer, and a variety of wheeled and crawler-mounted mobile equipment. At present, it is being used at only one station – National Capital Thermal Power Station at Dadri.
The CEA has recommended that all new plants will use a high-concentration slurry disposal system for ash ponds.
The existing plants will carry out a feasibility study to switch over to a high-concentration slurry disposal system and submit the report and time-bound action plan for construction to SPCBs, said the CEA guidelines.
In the case of ash mounds, a dry ash disposal system, which is a requirement of the process, will be used. Ash ponds are engineered dam and dyke facilities used for the storage of bottom ash and Pulverised Fly Ash (PFA) generated at Thermal Power Stations.
“Ash ponds are also used to enable water to separate from the fly ash slurry. Water from the ash ponds is recycled, reducing the use of fresh water. Ash ponds use gravity to settle out large particulates (measured as total suspended solids) from the thermal power plant,” said the guidelines.
“The combustion of coal in Thermal Power Plant (TPP) produces coal combustion residues (CCRs) which is a collective term referring to fly ash, bottom ash, boiler slag, and fluidised bed combustion ash,” said the CEA report.
Most of the fine dust entrained by the flue gases leaving the boiler and collected by fabric filter or electrostatic precipitator is known as precipitated fly ash (PFA), which results in 80% of the total coal combustion.
The rest of the 20% worth of particles, including unburned carbon settle to the bottom of the boiler called bottom ash (BA). Because of economic viability, thermal power stations most widely dispose of both precipitated fly ash and bottom ash together as a slurry to the pond in which it is stored for a longer period.
“As the reuse potential of ash has been increasing during recent years, segregated storage of fly ash and bottom ash is likely to gain popularity among power plants considering better economical returns from the sale of fly ash,” said the report.
Presently in India, more than 40,000 hectares of land are occupied for storage of ash. Over a period of time, fly ash disposal can cause problems like large surface setting lagoons for storage, infiltration of transport of water from deposit to the soil, dust carryover in wind from dried lagoons, and leads to ecological and environmental imbalances if proper safeguards are not taken in their design, construction, operation, and maintenance.
In India, about 73% of the total electrical energy is generated from coal-based sources. Annually about 271 million tonnes of ash as solid waste/by-product are released during the process of generation of electricity by combustion of pulverised bituminous, sub-bituminous, and lignite coal.
Indian coal has low calorific value (3500 Kcal/kg), and results in 30-60% of ash content. India’s major source of power, even in the near future, is going to remain coal-based thermal power plants hence, ash disposal would continue to be a subject of a priority since environmental issues hold greater importance in this century.
“Though coal ash has proven to be a resource material for various uses such as earth material, ingredient for cement manufacture, raw material for the manufacture of bricks, tiles, and aggregates, the demand for ash may not at all time match with the supply of ash, which is produced 24x7, as the power plant operates,” said the CEA report.
Recently a state-level analysis by the Centre for Science and Environment (CSE) on compliance with sulfur dioxide emission norms by thermal plants said only 5% of the capacity is meeting the norms currently. Plants in all the eastern states are non-compliant. Very few plants in the remaining regions are meeting the norms, says the analysis.
The CSE analysis is based on the updated Flue Gas Desulfurisation (FGD) status released by the CEA. “The Ministry of Environment, Forest and Climate Change (MoEFCC) issued a notification specifying the emission norms for coal-based power plants in December 2015. Since then, the norms have been diluted for several parameters and deadlines delayed,” said Nivit Yadav, programme director, of the industrial pollution unit, CSE.
As per the CSE analysis, 5% of plants that have so far installed FGDs for controlling SO2 emissions include 9,280 MW that has been reported to have commissioned FGDs and another 1,430 MW that ‘claim to be SO2 compliant’.
“We have found that despite five to eight years of extensions in deadlines, 43% of the capacity (Category A, which includes plants within 10 km radius of Delhi-NCR or cities with million-plus population); 11% of the capacity (Category B -- within 10 km radius of critically polluted areas); and 1% of the remaining capacity (Category C) are unlikely to meet the norms by the latest deadlines of 2024, 2025 and 2026, respectively,” said CSE programme officer, industrial pollution unit Anubha Aggarwal.
