The environmental norms notified by the Ministry of Environment, Forest and Climate Change in December 2015 have prescribed the nitrogen oxides (NOx) emission norms for thermal power plants (TPPs) in the country. Prior to this, there were no stipulated limiting values for NOx emissions. As per the new norms, the limiting value for NOx emission for TPPs installed before December 31, 2003 is 600 mg per Nm3; for TPPs installed after January 1, 2004, up to December 31, 2016 is 300 mg per Nm3; and for TPPs installed from January 2017 is 100 mg per Nm3. The NOx emissions could be limited to 600 mg-300 mg per Nm3 by using one or more primary methods broadly involving combustion modification techniques that restrict NOx generation. However, advance control techniques such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are required to achieve 100mg-300 mg per Nm3 NOx emission levels. Although globally SNCR and SCR systems have delivered the desired outcome, their efficacy for the Indian coal containing high ash content is yet to be ascertained. To this end, NTPC is undertaking various pilot projects to study the performance of these systems at power plants in India.
Broadly, NOx emission control measures can be categorised into primary and secondary methods. The primary methods, wherein the formation of NOx is curtailed include combustion optimisation, boiler tuning, use of low-NOx burners (LNBs) and separate over fire air (OFA) system. These technologies are already being used across the power plants in the country and are economical from capex and opex points of view. On the other hand, secondary NOx emission control (post-combustion process) includes SNCR, SCR and hybrid-SCR. While the primary methods would be used to be meet the norm of 600 mg-300 mg/Nm3, the use of secondary methods is necessary for achieving emission levels of 300 mg-100 mg per Nm3.
Primary methods
LNBs reduce the formation of NOx by altering the air-fuel mix. In such burners, the initial fuel combustion occurs in a zone with more fuel and less oxygen to minimise NOx formation. This is the most basic and cost-effective mechanism and can help achieve NOx reduction of 30-50 per cent. However, LNBs reduce combustion efficiency, which leads to an increase in carbon monoxide emissions. This can be managed by undertaking certain design modifications.
Another primary NOx control method is the use of OFA systems wherein the availability of oxygen near the burner area is controlled to minimise NOx formation. Initially, 70-80 per cent of the oxygen is provided near burners, leading to partial combustion of the fuel. The remaining oxygen is injected through OFA nozzles above the burner where combustion is completed. The relatively low temperature of the secondary stage limits the production of NOx. Although most of the existing boilers in the country have stand-alone OFA systems, they are not operated properly. OFA technology can reduce NOx formation by 20-45 per cent. Apart from this, combustion optimisation using tangentially fired boilers is another primary NOx control method. The tilting of burners can be used to control steam temperature inside boilers.
Secondary methods
SCR and SNCR are two main secondary methods of NOx control. These systems can reduce NOx emissions by 80-90 per cent. SCR utilises ammonia vapour as the reducing agent, which is injected into the flue gas stream after passing over a catalyst. The optimum temperature for the process should be maintained between 300 °C and 400 °C. In SCR, a catalyst is used to facilitate reduction reactions of NOx to form nitrogen and water. On the other hand, SNCR systems involve the injection of a reagent (ammonia or urea) into flue gas in the furnace within a temperature window of 900 °C-1,100 °C (depending on the reagent and condition of operation). At an appropriate temperature, NOx and reagent react to form nitrogen and water. SNCR systems have been used commercially in oil- and gas-fired power plants since the 1970s. These systems are used in cement, waste incinerators as well as biomass and conventional fuel-based boilers. SNCR can help reduce NOx emissions by 30-50 per cent, which is much less than that of SCR systems. However, SNCR entails lower cost and installation time. The main components of an SNCR system include reagent storage, multilevel reagent injection equipment and the associated control instrumentation. Although SNCR reagent storage and handling systems are like those of SCR systems, the reagent requirement in the former is more.
NTPC’s roadmap for NOx compliance
NTPC is undertaking combustion modification and other related work across 56 units (aggregating 17 GW), installed before 2003, which are required to meet the emission norm of 600 mg per Nm3. As per the company’s annual report 2018-19, work contract for combustion modification has been placed for 33 units aggregating 17 GW of capacity. Besides this, combustion modification in one unit of Dadri TPP has already been completed. In April 2019, NTPC had awarded a Rs 1.42 billion contract to GE for the supply and installation of low NOx combustion systems for 10 GW of TPP capacity. This was the first project awarded on such a large scale by NTPC to install low NOx combustion technology at its TPP fleet.
Broadly, for units that need to meet the emission norms of sub-600 mg per Nm3 levels, complying with NOx standards would require the installation of SCR or SNCR. To this end, NTPC is undertaking pilot study based on SCR/SNCR technology at 11 locations to identify the optimal DeNOx system for coal. These projects are at the final stage of completion. Based on the result of these pilots, the company would draw its plan for executing SCR and SNCR at its power plants.
Conclusion
According to the Centre for Science and Environment, of the 197 GW of installed coal capacity in the country, about 187 GW must meet the NOx emission norms of either 600 mg per Nm3 or 300 mg per Nm3, based on the plant commissioning dates. These plants do not need to implement secondary methods for NOx emission control as compliance with the norms can be achieved through primary methods only. As per many equipment suppliers, a combination of LNBs and OFA can help achieve NOx emission levels well below 300 mg per Nm3. Many TPPs have also started installing primary control systems for NOx reduction as much as 50 per cent of the power plants have reported that they already meet the relevant NOx norms (300 mg per Nm3 or 600 mg per Nm3).
To conclude, primary De-NOx methods such as LNBs and OFA systems are witnessing wider uptake by developers as the very first step for NOx emission curtailment. This achieves only a basic level of NOx reduction, often not enough to comply with the emission norms. Therefore, the plants falling under stricter NOx norms categories need to necessarily set up SCR and SNCR. The efficacy of these systems is yet to be ascertained for the Indian coal with high ash content that can adversely affect the catalyst. The entire power industry is looking forward to the outcome of the pilot test being undertaken by NTPC to assess the viability and efficacy of these technologies. Besides, several other challenges are likely to be encountered in the implementation of these technologies such as reduction in boiler efficiency, high reagent combustion and additional carbon dioxide emissions. Further, the issue of availability, transportation, handling and storage of a large quantity of ammonia required in these processes need to be addressed by the developers.
Overall, for the successful implementation of NOxx reduction technologies at the TPPs, it is necessary to select suitable technology solution on a case-to-case basis, taking into consideration various factors such as age of the plant, availability of raw material and the boiler make.