While burning coal in the boiler, coal-based thermal power plants (TPPs) convert a large portion of nitrogen in the coal to various forms of nitrogen oxide (NOx) such as nitric oxide, nitrogen dioxide and nitrous oxide. The formation of NOx during coal combustion is influenced by combustion conditions and the high nitrogen content in coal.
According to studies, around 30 per cent of NOx emissions in India are traceable to TPPs, making it vital to ensure proper treatment prior to emission. The government is, therefore, putting in place regulations to ensure that TPPs implement technological solutions that reduce NOx emissions to a minimum.
In 2015, the Ministry of Environment, Forest and Climate Change (MoEFCC) notified environment standards with specific limitations on the amount of permissible NOx emissions. Under the norms, 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 has been set at 100 mg per Nm3. For older plants commissioned prior to December 2003, the NOx limit has been set at 600 mg per Nm3.
As per the MoEFCC’s notification in March 2021, the NOx standards are to be met by the specified timelines, by December 2022, December 2023 and December 2024 as per Category, A, B and C projects respectively. Any deviation in the above norms beyond the timeline would be liable for the levy of emission compensation (EC).
NOX reduction technologies
Primary NOx reduction technologies include modification in combustion processes so as to limit the production of NOx at the points of high combustion. In addition, some of the primary technologies ensure its expeditious/rapid conversion to nitrogen. Over-fire-air (OFA) systems and low NOx burners (LNBs) are the two primary technologies that are less expensive and can be executed much faster versus advanced control procedures. Most TPPs commissioned after 2000 already have some form of in-house combustion-based NOx control.
Low NOx burners are designed to control fuel and air mixing at each burner in order to create larger and more branched flames. Peak flame temperature is thereby reduced, resulting in less NOx formation. The improved flame structure also reduces the amount of oxygen available in the hottest part of the flame, thus improving burner efficiency. LNBs can be combined with other primary measures such as OFA, reburning or flue gas recirculation. Plant experience shows that a combination of LNBs and other primary measures can help achieve up to 74 per cent NOx removal efficiency.
Similarly, OFA and flue gas recirculation (OAFGR) systems can optimise NOx generation in a cost-effective manner. In this system, flue gas is taken from downstream of the boiler and combined with combustion air from a draught fan. In certain configurations, the flue gas may also be mixed with OFA. In essence, it replaces the excess air that the stoker usually uses with recirculated flue gas. This is advantageous because flue gas has a higher heat capacity than simple air, which means it can wick away more heat from the fuel bed, thus dramatically reducing peak temperatures in the unit. It accomplishes reduction in NOx in a cost-effective way as it only requires minimal retrofitting of the boiler while simultaneously increasing plant efficiency by reducing excess air in the system. The reduction in excess air helps in directly reducing the fuel necessary for combustion of coal as well a cooler fuel bed that generates less NOx. Also, a cooler fuel bed contributes to lower NOx emissions as it leads to lower fuel-bound nitrogen conversion to NOx. According to studies, the OAFGR process results in reductions in excess air by as much as 60 per cent and fuel savings as high as 7 per cent or more.
Combustion optimisation is another method to control NOx emissions. Often when boilers are subject to frequent load changes and coal quality changes, there are localised hotspots or temporary periods of incomplete combustion. This increases NOx emissions along with some other emissions and undesirable effects. To control this, combustion optimisation is important. In India, most boilers are tangentially fired and have lower NOx emissions than wall-fired boilers. Moreover, tangentially fired boilers have devices that can tilt the burner. Optimising the angle of this burner can also help in controlling NOx emissions. Thus, by controlling boiler operating parameters (such as burner tilt, excess air and coal mill operations), NOx emissions can be significantly reduced.
Secondary technologies for NOx reduction
Secondary technologies limit the production of NOx more substantially; however, they cost more and necessitate the addition of specific NOx control units in contrast to primary technologies that only need some retrofitting. Secondary NOx reduction technologies comprise selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) units. These technologies reduce NOx production to 100-300 mg per Nm3 as against the primary NOx reduction technologies that can only bring NOx emissions down to 300-600 mg per Nm3.
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. The 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. It 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. Also, the catalysts used in the process are prone to contamination by compounds in the combustion gas.
SNCR is an alternative method for reducing NOx emissions. Under this, the injection of ammonia or urea into the flue gas is done without the use of a catalyst. The reagent is injected into specific temperature zones in the upper furnace or convective pass to reduce NOx to nitrogen and water.
Overall, SCR is more efficient in reducing emissions, but it is also significantly more expensive due to the need to purchase and maintain the catalyst. In India, a major problem with these systems is that the arrangement for availability, transportation, handling and storage of such large quantities of ammonia is yet to be addressed by TPPs. Another issue in SNCR technology is that TPPs have to maintain a sufficiently high temperature in the boiler for effective disposal of NOx. Essentially, maintaining a high temperature is not a problem when the plant is operating at high plant load factors; however, it becomes expensive and suboptimal to maintain the boiler temperature when plant utilisation is low.
Recent developments
NTPC Limited has awarded NOx combustion modification tenders for more than 20 GW of capacity. Additionally, co0mbustion 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 adding up to around 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.
The way forward
While there are a range of NOx reduction technologies available, proper cost assessment and planning will be useful to identify the constraints and challenges involved in implementing them. It is vital for TPPs, regulators and equipment suppliers to collaborate on the installation of NOx treatment units based on SCR and SNCR technologies, coupled with NOx modification units, so as to bring down NOx emissions further and meet the regulatory requirements.