Coal-fired thermal power plants (TPPs) account for a significant amount of the country’s greenhouse gas emissions, contributing to pollution and poor air quality. The combustion process in TPPs converts some of the nitrogen bound in coal to oxides of nitrogen (NOx) such as nitric oxide, nitrogen dioxide and nitrous oxide, which are collectively referred to as NOx. In December 2015, the Ministry of Environment, Forest and Climate Change (MoEFCC) notified the environmental standards, which specified the NOx emission levels for TPPs in the country. The NOx outflow limit for TPPs installed before December 31, 2003 was set at 600 mg per Nm3; for TPPs installed after January 1, 2004 and up to December 31, 2016, at 300 mg per Nm3; and for TPPs installed from January 2017 onwards at 100 mg per Nm3. Later, in October 2020, the MoEFCC relaxed the NOx emission norms for power plants commissioned between January 1, 2004 and December 31, 2016 from 300 mg per Nm3 to 450 mg per Nm3, following a request from power developers, who stated that it was not possible to meet the 300 mg per Nm3 standard at all loads. More recently, in April 2021, the MoEFCC extended the timelines for coal-based TPPs to comply with the emission norms by three to four years, depending on the location of the plant.
On the technological front, NOx mitigation strategies can be divided into two categories, primary and secondary technologies. While the primary methods are used to prevent the formation of NOx in the first place, the secondary methods are used to reduce the NOx produced by TPPs. The use of low NOx burners (LNBs), overfire air (OFAs) systems and combustion optimisation are some of the primary measures for NOx control in coal-fired power plants. Secondary NOx control systems include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and multi-pollutant control systems. Power Line explores various technology trends for controlling NOx emission…
Primary technologies
The primary technologies include combustion measures that promote the conversion of NOx from the combustion zone to nitrogen, hence limiting the formation of NOx from the combustion zone. OFA systems and LNBs are some of the technologies that are less expensive and can be implemented quickly in comparison to advanced control methods. Most TPPs commissioned after 2000 feature some type of in-combustion NOx control.
The initial fuel combustion in LNBs takes place in the fuel-rich and oxygen-deficient zone. Following this combustion, the air required to complete the coal combustion is added. This lowers peak flame temperatures and delays the formation of NOx. LNBs can help achieve a 30-50 per cent NOx reduction. Plus, they are simple to install and have a proven track record. The design of LNBs has improved over time, and today’s ultra-low NOx boilers utilise advanced technologies to decrease emissions. They are designed to handle the mixing of air and fuel at each burner, resulting in bigger and more branched flames. They also lessen the peak flame temperature, resulting in reduced NOx formation in the process. The enhanced flame structure decreases the quantity of oxygen available in the hottest part of the flame, thus increasing the burner’s efficiency.
An OFA system controls the availability of oxygen near the burner area, reducing the generation of fuel NOx. It redirects some of the combustion air away from the primary combustion zone. Of the required combustion air, 70-90 per cent is provided near burners, which creates an oxygen-deficient, fuel-rich zone, leading to partial combustion of fuel. The comparatively low temperature of the secondary stage restricts the production of NOx. Another advantage of the OFA system is that it improves plant efficiency by minimising the quantity of extra air in the system. The majority of existing boilers in India have standalone OFA systems, but they do not operate properly. When used properly, OFA technology can minimise NOx formation by 20-45 per cent. LNBs and OFA systems should be used in combination for optimal NOx reduction.
Another strategy for reducing NOx emissions is combustion optimisation. Localised hotspots or temporary periods of incomplete combustion are common when boilers are subjected to frequent load changes and changes in coal quality. This raises NOx emissions, as well as that of several other pollutants and adverse consequences. Therefore, the importance of combustion optimisation in controlling this cannot be overstated. In India, most boilers are tangentially fired and emit less NOx than wall-fired boilers. Moreover, tangentially fired boilers have devices that can tilt the burner through an arc range of -30 to +30 degrees horizontally. Optimising the angle of this burner can also aid in controlling NOx emissions. Thus, NOx emissions can be significantly reduced by controlling boiler operating parameters (such as burner tilt, excess air and coal mill operations).
Secondary technologies
Secondary or post-combustion NOx control methods neutralise the NOx in the flue gas into nitrogen through chemical reactions. These technologies offer a greater potential for NOx reduction, but their capital and operational costs are significantly higher. SCR and SNCR are the two most extensively utilised secondary methods in NOx control systems.
In SCR technology, ammonia is used as a denitrification agent. A reagent such as aqueous ammonia, anhydrous ammonia or urea is injected into the exhaust stream, which is maintained at a specific temperature (depending on the catalyst used). The heat vaporises the ammonia. Nitrogen gas and NOx present in the flue gas stream react with vaporised ammonia in the presence of a catalyst (such as titanium oxide, vanadium, molybdenum and tungsten) and form nitrogen, water and trace amounts of carbon dioxide, which are expelled from the exhaust pipe. The reagent is optimised by maintaining a near-equal ratio with the NOx, to be removed from the flue gas stream. This technology is suitable for NOx reduction of about 90 per cent. However, a disadvantage is that some unreacted ammonia might slip through the catalyst due to over-injection. Moreover, the catalysts used in the process are prone to contamination by compounds in the combustion gas.
SNCR is an alternative method for the reduction of NOx emissions. Under this, the injection of ammonia or urea into the flue gas is carried out without the use of a catalyst. The reagent is injected into specific temperature zones in the upper furnace or convective pass to break down NOx into nitrogen and water.
Overall, SCR reduces emissions more effectively, but it is also substantially more expensive owing to the requirement of purchasing and maintaining the catalyst. A key issue with these systems in India is that TPPs have yet to solve the availability, transportation, handling and storage of such enormous volumes of ammonia. The reduction in heat rate/ boiler efficiency is one of the issues with SNCR technology.
NTPC’s roadmap for NOx compliance
NOx control at NTPC’s coal-fired facilities is accomplished through the use of optimal combustion methods (primarily through excess air and combustion temperature optimisation). NTPC has awar-ded combustion modification for over 20 GW of capacity to reduce NOx emissions to optimal levels. According to the company’s Annual Report 2020-21, NTPC has completed combustion modification in 15 units totalling around 7 GW, including two units of the Dadri power plant and three units of the Jhajjar power plant situated in the NCR. Furthermore, combustion modification for 34 units totalling around 14 GW has already been awarded.
NTPC is exploring the possibility of SCR systems for units installed after 2017 in order to meet the NOx emission limit of 100 mg per Nm3. Although SCR is a proven technology for low ash coal, it has yet to be proven for abrasive Indian coal with high ash content. In light of this, various SCR system providers performed pilot tests at seven NTPC facilities to investigate the viability of SCR technology for Indian coal. These were conducted at the Vindhyachal, Rihand, Korba, Simhadri, Ramagundam, Sipat and Kahalgaon TPPs. While the catalyst being used at the Simhadri TPP is the plate and honeycomb type, plate-type catalysts are used at other TPPs. Further, 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).
Outlook
To conclude, TPPs should prioritise the reduction of harmful NOx emissions. To reduce emissions, simple procedures such as modifying boiler dimensions, installing auxiliary equipment such as LNBs or OFA systems, and improving fuel quality can be implemented. Meanwhile, for advanced NOx control (such as installation of SCR or SNCR systems), undertaking pilot studies to assess the performance of the technology is useful.
Net, net, for the successful implementation of NOx reduction technologies at TPPs, appropriate technical solutions must be selected on a case-by-case basis.