With the tightening of emission control regulations, a flue gas desulphurisation (FGD) systems have become a must-have technology to capture sulphur oxide (SOx) emissions. 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 dewatering system. With the majority of the thermal power plants adopting FGD systems to comply with the new air emission norms, their water requirement is expected to increase.
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 0.2-0.3 kl per MWh of water consumption per MWh of generation. With the installation of FGD systems, a plant’s specific water consumption increases as the system requires water for scrubbing sulphur dioxide (SO2) out from flue gas. According to the Central Electricity Authority, the operation of FGD systems requires additional water of about 0.3 m3 per MWh. In June 2018, the Ministry of Environment, Forest and Climate Change issued an amendment to its 2015 notification, in which the water consumption limit of 2.5 m3 per MWh for new plants (installed after January 1, 2017) was revised to accommodate the water requirement of FGD systems. Therefore, the revised water norms mandate only freshwater-based plants to meet the norm of 3.5 or 3 m3 per MWh. All freshwater-based once-through cooling plants are additionally required to install cooling towers and subsequently achieve the norm of 3.5 m3 per MWh.
The most commonly used FGD technology is wet scrubber. In wet FGD processes, the flue gas is brought into contact with the sorbent in a separate absorber unit (wet scrubber). In a wet scrubber, a reagent such as limestone or lime in a 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. Wet scrubbers are responsible for around 10-15 per cent of the water losses in power plants with water cooling systems. 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.
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. These systems consume about 60 per cent less water than wet scrubbers. In SDS, no wastewater is produced as all the water added to the scrubber is evaporated. The SO2 removal efficiency of SDSs compared to wet scrubbers is lower at 90-97 per cent. On the other hand, sorbent injection processes have the lowest water consumption of the various FGD systems, consuming no water, or a minimal amount if at all, and produce no wastewater. 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. Calcium-based sorbents are cheaper than sodium-based ones.
One of the ways to reduce evaporative water loss in wet FGD systems by 40-50 per cent is by cooling the flue gas before it enters the wet scrubber. Cooling the flue gas from a typical approximately 140 ºC to 90-100 ºC prior to its entry into the wet scrubber reduces the scrubber’s evaporative water losses. The flue gas exits the scrubber at a temperature of approximately 50 ºC and is then reheated in plants where the cooling tower is used as the flue gas stack. Thus, capturing flue gas vapour could substantially lower the power plant’s water consumption. Also, membranes that are highly selective for water vapour can recover at least 40 per cent and up to 90 per cent of the water vapour in the flue gas that exits the wet scrubber. A membrane condenser system also recovers the latent heat in the flue gas. The water recovered is mineral-free and is ready for direct utilisation within the power plant, by industry, or for public consumption purposes.
With regard to wastewater generation, an FGD plant is expected to discharge 20-25 kl per hour as wastewater, which has to be treated to achieve zero liquid discharge. Wastewater from plants with wet FGD systems contains highly soluble salts such as calcium and ammonium chlorides, and certain heavy metal salts. Overall, a well-designed, integrated FGD wastewater treatment facility consists of a number of technologies such as lime neutralisation, gypsum desaturation, heavy metal removal, clarification, filtration, biological treatment, and sludge thickening and dewatering.
To conclude, in order to obtain superior operational performance of an FGD system and optimal resource utilisation, it is paramount to select the best suited technology through a detailed cost-benefit analysis. This would also ensure efficient water consumption and prevent injudicious water usage.