Wet flue gas desulphurisation (FGD) systems have been the most popular solution for SOX and NOX emission control, and have been adopted by many thermal power plants (TPPs). More than 90 per cent of TPPs are installing wet limestone-based FGD systems as they are more economical. Apart from sources such as plant drains, filter backwash, RO reject water, and ash sluice water, plants with FGD systems also produce wastewater from FGD processes, particularly wet FGD process. In wet FGD systems with scrubbers, many contaminants end up in the circulating water in the scrubber. Owing to the harsh composition of FGD wastewater, its discharge and recycling is limited.
To maintain appropriate operating conditions, a constant purge stream is discharged from the scrubber system. The purge stream contains contaminants from coal, limestone and makeup water. Prior to discharge, the wastewater requires treatment for the reduction of key contaminants. Wastewater composition can vary significantly from plant to plant depending on coal selection, scrubber chloride concentrations, efficiency of the fly ash removal process, type of gypsum dewatering system, and type of FGD process used. To meet the regulatory requirements, plants need to manage wastewater from FGD systems in an appropriate manner.
Features of wastewater from FGDs and the impact of impurities
FGD wastewater contains high volumes of suspended solids such as gypsum, monox, metal hydroxide and fly ash, salts like soluble chloride, calcium sulphate, magnesium sulphate, and heavy metals such as cadmium, mercury, chromium, arsenic, lead, nickel, copper and zinc.
The water volume is unstable and the pH value lies between 4 and 6, making it acidic. Water with such heavy pollutants needs a separate treatment plant with several processes at work. This is because the power plant’s existing wastewater treatment facility may not have adequate capacity or be able to receive a high chloride wastewater stream, or comply with very strict FGD wastewater discharge limits.
Many impurities filter down into the system and these are mainly heavy metals and chloride salts. Heavy metals and other salts find their way into the system with the coal used in the process of electricity generation. The water used for lime slurry preparation, or the fly ash in the flue gas often contains chlorides. A higher concentration of salts such as chlorides and magnesium can have a negative impact on the FGD system. They can inhibit the dissolution of limestone, which can reduce the efficiency of SO2 removal. These salts can also corrode the system equipment and stress the waste water management system by adding to the sludge, which needs to be filtered out. Hence, for more reasons than one, it is absolutely necessary to manage wastewater from the FGD system.
Best practices for managing wastewater from FGDs
The negative impact of impurities can be reduced to a large extent with the following measures that can be taken at the time of operating the plant.
Controlling at source: Controlling coal quality and checking for impurities and heavy metals is a very important step to ensure that fewer impurities enter the wastewater. Monitoring the quality of water taken up for plant processes is an effective way to reduce the inflow of chlorides. The plant can take additional measures to include high quality limestone in the processes to block the chlorides and sulphates out of the system.
Process and operational control: At the operational level, many steps can be taken to reduce the impact of impurities. Optimising wastewater system operation to not overburden the system filters can improve the efficiency of filtration. Plants should stick to the prescribed level of flue gas dust content to reduce filter waste. With the help of OEM practices, the start-up and stoppage of FGD can be regulated to improve equipment health. Installing a gas heater can reduce water evaporation and consequently waste concentration build-up in the slurry.
Technology options
Many new technologies have come up to manage effluents from FGD processes.
Forward osmosis (FO): This process uses the driving force of osmotic pressure differences across the FO membrane. It treats fouling waste streams with high salinity and suspended solids by separating the fluids and solids via osmotic separation. It uses a draw solution as an osmotic agent to pull clean water through a semi-permeable membrane. The recovered draw solution is reconcentrated by an HBCR system.
FO is considered an alternative to RO, which is widely used for desalting wastewater with high total dissolved solids (TDSs). Since FO utilises the osmotic pressure difference as a driving force between feed and draw solutions, it can achieve low energy consumption as well as high fouling reversibility thorough non-pressurised operation. As a result, FO can be beneficial in controlling membrane fouling.
It, however, requires partial softening of water prior to membrane concentration processes to optimise operation. This system leads to zero liquid discharge and can operate on lower flow, making it a good fit for plants in semi-dry areas.
Metal ion precipitation process: It is a process that removes heavy metals and other pollutants from the water. The end result is clear non-turbid water that contains only chlorides and solids. The chlorides concentration is then adjusted in the absorber system by adding fresh water.
The challenges in this process are the disposal of solid waste such as sludge cakes and maintenance of the absorber concentration level within the limits during low loading of the absorber. Often the absorber chokes on impurities such as heavy metals that were originally brought in with the coal, chlorides that come in from the lime slurry and fly ash content in flue gas. The impurities often disrupt the functioning process of the absorber.
Seawater-based plants
The wastewater from seawater-based plants needs a special treatment system that uses lime milk to neutralise the water and improve the pH, TMT 15 and ferrous chloride for oxidation reduction, and precipitating heavy metal pollutants and polyelectrolyte to flocculate the suspended particles. The products of this process like the waste sludge can be utilised along with the gypsum. The treated water can be used in cooling towers and help plants achieve zero liquid discharge, making it an ideal option for the plants.
Outlook
Despite several obstacles, plants are actively opting for FGD and trying their best to achieve the ZLD targets. Developers and generation companies, however, must not overlook the installation of supplementary systems that are needed in order to operate FGD systems in a greener way. Water management is difficult with regard to the additional input requirements of FGDs and the wastewater generated by FGD. As the norms for discharging wastewater are becoming more stringent, more treatment methods may be required to reduce the concentration of some chemicals such as mercury and selenium to the part per trillion levels. Despite the roadblocks, TPPs have great potential and can be aided by newer technologies to become environmentally safe.