A significant proportion of the emissions produced by the industrial sector in the country are attributable to the coal-based thermal power industry. The combustion of coal converts the nitrogen bound in it to products such as nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O), creating pollution. To curb NOx emissions from thermal power plants, plant operators are increasingly installing emission control technologies such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) to meet regulatory guidelines.
To recall, in December 2015, the Ministry of Environment, Forest and Climate Change (MoEFCC) had notified revised emissions standards. With regard to limitations on the amount of permissible NOx emissions, power plants commissioned between 2003 and 2016 were required to cap their NOx emissions at 300 milligram per cubic metre (mg per Nm3). This limit was later raised to 450 mg per Nm3. For plants commissioned 2017 onwards, the limit for NOx emissions was set at 100 mg per Nm3. For older plants commissioned prior to December 2003, the NOx limit was set at 600 mg per Nm3.
Primary and secondary technologies are the two different categories of NOx reduction techniques needed to meet these norms. Secondary methods are used to reduce the NOx generated by TPPs whereas primary methods are used to prevent the generation of NOx in the first place. Some of the primary measures for NOx control in coal-fired power plants are the use of low NOx burners (LNBs), overfire air (OFAs) systems and combustion optimisation. Some of the secondary NOx control technologies are SCR, SNCR and multi-pollutant control systems.
Primary technology options
The initial fuel combustion in LNBs takes place in the fuel-rich and oxygen-deficient zone. The air necessary to complete the coal combustion is introduced after this combustion. This lowers the peak flame temperature and delays the formation of NOx. LNBs can contribute to a 30-50 per cent reduction in NOx. They also have a proven track record and are simple to install. LNBs have been in use for over 30 years in countries with similar emission control standards. Over the years, the LNB design has advanced. Modern ultra-low NOx boilers utilise cutting-edge techniques to lower emissions. They are designed to regulate the air and fuel blending at each burner, resulting in more branched and bigger flames. Moreover, they lower the peak flame temperature, which reduces NOx production. The enhanced flame structure increases the effectiveness of the burner while simultaneously reducing the quantity of oxygen present in the hottest section of the flame.
By regulating the oxygen availability close to the burner, an OFA system reduces the production of NOx. It diverts a portion of the combustion air away from the primary combustion zone. Of the required combustion air, 70-90 per cent is provided near the burners, which creates an oxygen-deficient fuel-rich zone, leading to partial combustion of fuel. Subsequently, the balance combustion air is injected above the burner elevation through OFA nozzles into the furnace, where combustion is completed.
The comparatively low temperature of the secondary stage restricts the production of NOx. Another benefit of the OFA system is that higher plant efficiency is achieved by reducing the amount of excess air in the system. In India, standalone OFA systems are present in the majority of boilers; however, they are ineffective. If used properly, OFA technology can cut NOx creation by 20-45 per cent. LNBs and OFA systems should be used in tandem for optimum NOx reduction.
Another method to reduce NOx emissions is combustion optimisation. When boilers are subject to frequent load changes and changes in coal quality, there are localised hotspots or temporary periods of incomplete combustion. Together with various other emissions and adverse effects, this results in an increase in NOx emissions. Combustion optimisation is crucial for managing this. The majority of boilers in India are tangentially fired and emit far less NOx than wall-fired boilers. Moreover, tangentially fuelled boilers contain mechanisms that allow the burner to be tilted along a horizontal arc between -30 degrees and +30 degrees. Controlling NOx emissions can also be accomplished by optimising the burner’s angle. Hence, NOx emissions can be significantly lowered by adjusting the boiler’s operational parameters (such as burner tilt, extra air, and coal mill operations).
Secondary measures
SCR, SNCR and a combination of both are examples of secondary NOx control measures. Both technologies neutralise NOx into nitrogen and water, either with the presence of a catalyst (SCR) or without (SNCR). Usually, secondary measures are applied to plants that need to reduce NOx limits beyond the capability of primary measures.
SCR technology produces N2 and wastewater as byproducts and removes NOx with 70-85 per cent efficiency. SCR operation also requires the use of catalysts such as titanium oxide, vanadium, molybdenum and tungsten. Although this technology has a high removal rate, it may not work well with high-ash coal. Pilot studies are being conducted to examine the performance of SCR technology. They are being carried out at the TPPs at Vindyachal, Rihand, Korba, Simhadri, Ramagundam, Sipat and Kahalgaon. While the catalyst being used at the Simhadri TPP is of plate and honeycomb type, at other TPPs, a play-type catalyst is being used.
The inability to retrofit an SCR owing to space limitations, given the size and weight of the SCR system, is one of the major problems encountered in the installation of SCR systems. In addition, the 100 mg per Nm3 limit cannot be continuously reached with SCR. 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. In addition, SCR pilot experiments have not been able to extend the catalyst’s mechanical life (8,000-16,000 hours as against 60,000 hours achieved internationally). A significant number of catalysts would also need to be disposed of since Indian coal has a short mechanical life. There is also the risk of contamination of fly ash due to ammonia and other components. Moreover, 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.
SNCR, which employs the reaction of ammonia or urea at high temperatures without the need for a catalyst, is another important NOx control technology. It produces wastewater and N2 and is not prone to plugging. At the Rihand and Vindhyachal TPPs, pilot tests for SCNR technology have been conducted. Four soot blowers were temporarily swapped out for SNCR injection lances (two per boiler sidewall) at these sites. With SCNR technology, a high temperature of between 850 °Celsius and 1,150 °Celsius must be maintained. If the temperature rises over 1,150 °Celsius, more NOx is produced. The technique promises a 20-30 per cent reduction in emissions. The restriction of 300 mg per Nm3 cannot be met continuously. Moreover, SNCR lowers the heat rate and has a negative effect on boiler efficiency. Further, ammonia slip can cause fly ash contamination and affect downstream equipment.
Progress so far
NTPC Limited has awarded NOx combustion modification tenders for more than 20 GW of capacity. Additionally, combustion modification work is at various stages of commissioning for 29 units with an aggregate capacity of 8.2 GW. Furthermore, combustion modification in 28 units aggregating about 13 GW, including units located in the NCR (two units of Dadri and three units of Jhajjar), has already been completed. NTPC has also conducted pilot-based studies on SNCR and SCR technologies at seven stations by various SCR system suppliers and assessed the technically viable emission limit after taking into consideration Indian coal. The results of the tests have been submitted to the Supreme Court.
Conclusion
Appropriate solutions need to be chosen on a case-by-case basis to ensure effective installation of NOx reduction technologies at TPPs. To achieve the stringent NOx limit of 100 mg per Nm3, gencos should take decisive action to finalise their NOx control strategies and begin execution as soon as possible.