Curbing Emissions

Efficacy of NOX and SOX control technologies in the Indian context

The implementation of emission norms is one of the key priorities of power plant developers. In order to comply with the tightened norms, identifying and adopting suitable technology is paramount. Various pilot projects are underway to study the efficacy of emission control technologies in the Indian context. For NOx control, combustion modification, selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are useful, whereas SOx control could be achieved through flue gas desulphurisation (FGD) systems and dry sorbent injection. Identifying the best-suited technology depending on the age, location and size of the plant is paramount.

NOx control technologies

One of the primary solutions for NOx emission control is to limit NOx production through combustion modification or optimisation. This involves the use of low NOx burners and secondary over fire air. These technologies do not generate any by-product or wastewater and entail minimum investment. However, combustion modification affects specific coal consumption at the power plant.

A key technology for NOx control is SCR. Its NOx removal efficiency is 70-85 per cent, and it generates N2 and wastewater as by-products. Further, catalysts such as titanium oxide, vanadium, molybdenum and tungsten are required for operating SCR. Although this technology offers high removal rate, its performance was not satisfactory with high ash coal. Pilot studies are underway to study the performance of SCR. These are being undertaken at the Vindyachal, Rihand, Korba, Simhadri, Ramagundam, Sipat and Kahalgaon thermal power plants (TPPs). While the catalyst being used at the Simhadri TPP is plate and honeycomb type, at other TPPs a play-type catalyst is being used.

One of the key issues faced in the implementation of SCR is that it is not possible to retrofit an SCR due to space constraints considering the size and weight of the SCR system. Apart from this, with SCR, the 100 mg per Nm3 limit cannot be achieved on a continuous basis. Its operation is heavily dependent on the inlet flue gas temperature (300-380 °Celsius), which cannot be achieved below the 70-80 per cent load of a unit. Apart from this, SCR pilot tests have failed to achieve the mechanical life of the catalyst (around 8,000-16,000 hours as against 60,000 hours achieved internationally). Further, a huge amount of catalysts will have to be disposed of due to the low mechanical life of Indian coal. There is also the risk of contamination of fly ash due to ammonia and other components. Besides this, anhydrous ammonia is classified as a hazardous material and requires special permits. Apart from this, the analyser changeover valve often gets damaged after 3,000 hours of operation due to the accumulation of ash inside it.

Another key NOx control technology is SNCR, which involves the reaction of ammonia or urea at high temperatures without using a catalyst. It generates N2 and wastewater and is not prone to plugging. Pilot studies for SCNR technology are being undertaken at the Rihand and Vindhyachal TPPs. At these plants, four soot blowers were temporarily replaced with SNCR injection lances (two per boiler sidewall). For SCNR technology, a high temperature of 850-1,150 °Celsius is to be maintained, and more NOx should be generated if the temperature is above 1,150 °Celsius. The technology offers a 20-30 per cent reduction in emissions. The limit of 300 mg per Nm3 cannot be achieved on a continuous basis. Also, SNCR decreases the heat rate and adversely impacts boiler efficiency. Further, ammonia slip can cause fly ash contamination and affect the downstream equipment.

SOx control technologies

One of the key SOx control technologies is wet limestone FGD, which is suitable for power plant units ranging from 60 MW to 800 MW of capacity. It involves a high capex and a low opex. Besides, there is a strong supply chain for inputs and by-products in case of wet limestone FGDs. Some of the other popular FGDs are seawater-based FGDs, which are suitable for coastal power plants, and ammonia-based FGDs. Another SOx control technology is semi-dry FGD, which is suitable for power plant units of 300 MW and below, and can be used on the existing chimney. However, it increases the auxiliary power consumption (APC) of the power plant. Dry sorbent injection (DSI) is another SOx control technology that is suitable for older units. It involves a low capex and a high opex.

With regard to the demerits of SOx control technologies, the wet lime desulphurisation process removes one mole of SO2 and in turn, produces one mole of CO2, which is a greenhouse gas responsible for global warming and climate change. The same is applicable to other SO2 control technologies such as DSI, semi-dry FGD and seawater FGD. Apart from this, the coal consumption is expected to increase by up to 1 per cent (due to increased auxiliary power consumption), depending on the FGD technology implemented, and greenhouse gases will be released due to increased coal consumption. Besides, the increased APC reduces the efficiency of power plants.

With regard to the generation of by-products, post the implementation of wet lime FGD in approximately 214 GW of power plants (about 90 per cent of the total capacity installing wet Lime FGD), gypsum production is excepted in the range of 14-21 million metric tonnes per annum, depending on the PLF range of 55-80 per cent and the sulphur content of of coal.

The FGD market in India is currently evolving and there is a limited availability of vendors. The overbooking of suppliers has resulted in an increase in manufacturing time for FGD equipment. There is a dependency on imports from neighbouring countries, and certain import restrictions have also been imposed. With regard to equipment availability, the domestic manufacturing is not sufficient for equipment such as booster fans, slurry RC pumps/gas cooling pump, oxidation blowers, wet limestone grinding mills, slurry pumps, agitators, gypsum de-watering system, mist eliminatorss and spray nozzles. Meanwhile, domestic manufacturing is not available for equipment such as borosilicate glass blocks lining with adhesive and primer for wet stack, spray nozzles for some bidders, rubber lining for absorber tanks, and auxiliary absorbent tanks and slurry tanks.

On the financing front, the installation of FGD systems is likely to entail a huge capital expenditure. The FGD systems are to be installed at 170 GW of existing capacity, 10 GW of capacity commissioned after a phasing plan is prepared by the Central Electricity Authority (CEA) and around 58 GW of under-construction capacity. Considering the average price of Rs 5.50 million per MW, the FGD installations are estimated to entail a total capital expenditure of around Rs 131 trillion. The funding issues are exacerbated in the case of private power plants. Apart from this, the outbreak of Covid-19 has added to the challenges being faced in FGD equipment manufacturing and installation.

The way forward

The CEA has recently recommended a phasing approach for implementing the emission norms, as it will help in understanding the impact and effectiveness of emission control equipment while providing time for future course corrections. The plan proposes the immediate installation of FGD at TPPs located in critically polluted areas, while the other areas could be taken up for FGD installation later. Since the aim is to ensure uniform good ambient air quality across the country and not the uniform emission norms for TPPs, implementing uniform emission norms may result in different air quality at different locations. Simultaneously, a phased manufacturing programme for FGD units may be initiated. The graded implementation of norms will provide sufficient time for developing an indigenous manufacturing facility, and thus help avoid outsourcing of new technology, skilled manpower and equipment. In addition, it will provide sufficient time to test the suitability and efficacy of various technologies in Indian conditions.

Net, net, although there are various emission control technologies available in the market, understanding their efficacy in the Indian context is critical. Besides, addressing issues pertaining to the availability of equipment, trained manpower, and funding could accelerate compliance with emission norms by TPPs.

Based on a presentation by B.C. Mallick, Chief Engineer, CEA, at a recent Power Line conference

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