With the tightening of emission control regulations, flue gas desulphurisation (FGD) systems will become a must have for the thermal power industry over the next few years. Recently, the Ministry of Environment, Forest and Climate Change (MoEFCC) extended the timelines for complying with the new emission norms by three to five years, based on plant location.
Limestone is a key raw material in most FGD systems, and high-quality limestone (CaCO3 > 90 per cent) with minimal impurities is desirable for the optimum performance of FGDs. Meanwhile, gypsum is the key by-product of FGD systems. Utilisation of FGD gypsum has been increasing across the world. It has replaced the use of mineral gypsum in many places and comprises a significant share of total gypsum consumption in the world.
As of February 2021, FGDs have been commissioned in six thermal units totalling 2,160 MW of capacity.
Various processes are used for the removal of sulphur oxides (SOx) from thermal power plants (TPPs), such as wet scrubbers, spray dry scrubbers, sorbent injection processes, dry scrubbers, and regenerable processes. The raw materials required and by-products generated include limestone, hydrated lime, calcium oxide, dolomite and ammonia. The quality of the raw materials can have a significant influence on the performance of a process, its operating costs, and the quality of the by-products.
Limestone wet scrubbers are the most widely used technology in FGD systems. The calcium carbonate present in limestone is the main source of alkalinity for the neutralisation of sulphur dioxide in this technology. Magnesium carbonate is also effective in the neutralisation of sulphur dioxide. It is more reactive than calcium carbonate, and is found in variable amounts in many sources of limestone.
Finally, ammonia can also be used for FGD applications as a water-solution at atmospheric pressure or in pure anhydrous form, kept under high-pressure and in refrigeration.
Various residues are produced as a result of SOx removal through FGD, such as gypsum and/or large quantities of fly ash. The composition of these residues depends on factors such as the source of the parent coal and the type of FGD system used. Older techniques usually produced residues that were contaminated with fly ash, but as FGD systems become more advanced, sulphur removal is becoming more efficient, and the residue produced is of a higher purity.
With increasing implementation of FGD by TPPs, many avenues are being explored for the potential utilisation of FGD residues, such as using them as soil amendment. Gypsum-containing materials are of great benefit in the improvement of the physical qualities of soil, such as water holding capacity and subsurface acidity. Adding FGD residue to soil has shown an increase in the dry matter production of cotton, corn, soybean and radish. Furthermore, FGD gypsum can be used for manufacturing wallboards, plaster products and cement additives, and in mining mortars, floor screens, road bases, and artificial reefs. Some construction-based avenues for re-use are also being explored, as gypsum is suitable for dry compaction using conventional land-filling methods. Another by-product of the FGD process is ammonium sulphate, which is used as a fertiliser.
FGD installations can facilitate cleaner air, while being beneficial in other significant ways. One of the main by-products, gypsum, is a scarce resource in India. India’s own gypsum production has been declining, and it has been relying on imported gypsum. FGD systems can help solve this problem. However, a few issues need to be addressed to achieve this. If the quality of limestone to be used in FGDs is specified by the Bureau of Indian Standards, there would be an assurance of good quality FGD gypsum. Additionally, MoEFCC guidelines for the handling and storage of limestone and gypsum in power plants, and directions to the power plants to utilise 100 per cent of FGD gypsum, could be of great help in resolving challenges. Since the agricultural sector is the primary user of gypsum, research and field testing by agricultural research organisations, such as the Indian Council of Agricultural Research, would ensure the applicability of gypsum produced from Indian power plants.