Coal-based power plants are the largest producers of pollutants such as sulphur oxide (SOx) and nitrogen oxide (NOx) emissions. As per industry experts, the power sector in India contributed about 50 per cent of the annual SOx, about 30 per cent of NOx, and 8 per cent of the particulate matter emissions. To reduce SOx emissions, coal-based power plants are required to install flue gas desulphurisation (FGD) systems, based on feasibility studies. The major FGD technologies available in the market today are dry sorbent injection solutions/systems, semi-dry FGDs (including spray dryer absorbers and circulating dry absorbers) and wet FGDs (limestone, seawater and ammonia based).
Water is required for various processes in an FGD plant including the absorber system, the mist eliminator wash system, the limestone grinding and slurry preparation system, and the gypsum de-watering system. Process water is required to be pumped from storage tanks to cater to the water requirement of the entire FGD system. Also, de-mineralised water is required for cooling FGD plant equipment.
Water usage in FGD systems
For a 500 MW coal-based power project, FGD plants are expected to use about 110-130 kilolitre (kl) per hour of fresh water for the desulphurisation process. This translates into about 0.2–0.3 kl per MWh of water consumption per MWh of generation. Further, the FGD plant is expected to discharge about 20-25 kl per hour as wastewater, which has to be treated to achieve zero liquid discharge. Complying with the revised SOx emission norms through the installation of FGD systems is set to increase the water requirement of plants. This, in turn, necessitates a revision in the norms for specific water consumption by plants. Apart from the available sources of water like river water for inland coal-based plants and seawater for coastal coal-based power plants, FGD systems can also make use of process water, such as from cooling tower blowdown for processes like grinding limestone. Thus, developers have to take into account the overall cap prescribed by the Ministry of Environment, Forests and Climate Change on water consumption by thermal power plants (TPPs) while managing the additional water requirement for FGD systems.
By far, the most commonly used FGD technology is wet scrubber. In a wet scrubber, a reagent such as limestone or lime in slurry form, perhaps with additives, reacts in a spray tower with sulphur oxides to form calcium sulphite, which is oxidised to form calcium sulphate or gypsum. This technology, however, accounts for around 10–15 per cent of the water losses in power plants with water cooling systems. This figure is considerably higher when dry/air cooling systems are employed. The evaporative water losses can be reduced by about 40–50 per cent when the flue gas is cooled before it enters the wet scrubber. Wet FGD units require water to clean the demisters, and together with lime, create a suspension suitable for SOX treatment. Much of this water is evaporated and exits the stack, while the remaining water is further treated in a wastewater treatment plant.
Typically, the requirements of water quality for the demisters are that the salt content should not exceed that of the seawater; it should be free of particles and the calcium content should not be too high to prevent scaling on the demisters. The amount of water needed for lime in the FGD is relatively lower – around 20 m3 per hour. At high chloride content, clotting can occur, which leads to a reduction of calcium reactivity, resulting in poor gypsum quality. Benchmark studies reveal that the amount of make-up water required varies per site and is subject to its quality. To abide by water conservation policies, make-up water can be a mixture of other water sources like surface run-off water, seawater or fresh surface water. However, the maximum amount of seawater that can be added is restricted by the allowable chloride content in gypsum.
Water usage can be reduced by using semi-dry scrubbers, such as spray dry scrubbers (SDSs) or circulating dry scrubbers (CDSs), or dry scrubbing technologies. Dry FGD processes use about 60 per cent less water, but significant water is consumed in this process. Another advantage of SDS is that there is no production of wastewater, as all the water added to the scrubber is evaporated. Thus, no wastewater treatment system is required. However, as there is no saleable by-product for dry FGD, the opex costs can be significantly higher and consequently, most FGD units give preference to wet technology over dry units. The main drawback of SDSs compared to wet scrubbers is their lower SO2 removal efficiency (90–97 per cent). State-of-the-art CDSs can remove over 98 per cent, approaching the efficiency of wet scrubbers. SDSs are typically used at power plants burning low-to-medium sulphur coals, while CDSs are applied to units burning low-to-high sulphur coals.
Sorbent injection processes have the lowest water consumption of the various FGD systems, consuming no water, or a minimal amount if the sorbent needs hydrating or the flue gas is humidified to improve performance. They are simple to install and operate, easy to retrofit with their small space requirements, and produce no wastewater. An accompanying benefit is the capture of some of the hydrochloric acid, hydrofluoric acid and mercury in the flue gas. The capital cost and energy consumption of the sorbent injection systems are considerably lower than the semi-dry and wet FGD processes. Nevertheless, operating costs are generally higher, mainly due to the high consumption of the sorbent. The calcium-based sorbents are cheaper than the sodium-based ones.
FGD wastewater treatment
To maintain the required conditions in the scrubber, a constant purge stream is discharged from the scrubber system. The FGD purge stream contains pollutants from coal, limestone and make-up water. It is acidic and supersaturated with gypsum, with high total dissolved solids (TDSs), total suspended solids (TSSs), heavy metals, chlorides and dissolved organic compounds. The scrubber purge stream can be treated in a dedicated wastewater facility rather than any existing treatment system. The power plant’s existing wastewater treatment facility may not have adequate capacity or if the construction materials of the treatment facility most likely are unsuitable for receiving a high chloride stream. FGD wastewater composition can vary significantly from plant to plant. The quantity and quality is affected by the rated capacity of the boiler, scrubber chloride concentration, efficiency of fly ash removal, type and efficiency of the dewatering system, type of FGD process used, and the composition of coal, limestone and make-up water.
A well-designed, integrated FGD wastewater treatment facility typically includes a number of technologies, such as lime neutralisation, gypsum desaturation, heavy metal removal, clarification, filtration, biological treatment, and sludge thickening and dewatering. One of the options of treating the wastewater is to have a dedicated physical-chemical treatment, consisting of precipitation, coagulation, clarification and filtration. This is the most common approach for FGD wastewater treatment. It lowers the potential for gypsum scaling and removes heavy metals and TSS to low levels. It requires specific reaction-train, clarifier, thickener and filter designs, with specialised materials of construction.
Net, net, the selection of the appropriate FGD technology by developers that can not only deliver optimum efficiency in SOx removal but also reduce the water footprint of TPPs is becoming the need of the hour as water consumption regulations tighten.