The environmental norms notified by the Ministry of Environment, Forest and Climate Change (MoEFCC) in December 2015 prescribed the nitrogen oxide (NOx) emission levels for thermal power plants (TPPs) in the country. Prior to this, there were no stipulated limitations on NOx emissions. As per the norms, the NOx emission 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 the emission limit was 100 mg per Nm3. In a recent development, the Supreme Court has allowed power stations commissioned between December 2003 and 2016 to emit 450 mg per Nm3, following the in-principle approval given by the MoEFCC in 2019 to revise the norms from 300 mg per Nm3 to 450 mg per Nm3.
In terms of technologies, NOx emissions could be limited to 300-600 mg per Nm3 by using one or more primary methods involving combustion modification techniques that restrict NOx generation. However, to achieve 100-300 mg per Nm3 emission levels, advanced control techniques such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are required.
Under the guidance of the Ministry of Power, NTPC set out to test SCR/SNCR technologies at some of the NTPC units on a pilot basis to assess the viability of these technologies for Indian coal, which contains a high amount of ash. The installation of SCR at these units has reportedly been completed.
Technologies for NOx control
According to a study done by CSE India, primary combustion measures control and limit the production of NOx from the combustion zone by promoting its reduction to nitrogen. These are relatively low-cost NOx control technologies and can be implemented quickly. Equipment manufacturers point out that plants commissioned after 2000 already have some form of in-combustion NOx control. The following are some of the most widely used technologies for NOx control that have been identified by technology suppliers as suitable for high ash Indian coal:
- LNBs: In low-NOx burners (LNBs), the initial fuel combustion occurs in a fuel-rich, oxygen-deficient zone. After the primary combustion, the air required to complete the combustion of coal is added. This staging reduces peak flame temperatures, thus slowing down NOx formation. LNBs typically achieve 30-50 per cent NOx reduction on their own and are relatively easy to install. It is a well-proven, mature technology that has been in use for over 30 years in countries with similar control standards. In India, new boilers are equipped with LNBs.
- OFA systems: An over fire air (OFA) system controls the availability of oxygen near the burner area, minimising the formation of fuel NOx. Of the required combustion air, 70-90 per cent is provided near burners, creating an oxygen-deficient, fuel-rich zone, leading to partial combustion of fuel. The balance combustion air is then injected above the burner elevation, through the OFA nozzles into the furnace, where combustion is completed. The relatively low temperature of the secondary stage limits the production of thermal NOx. Although the majority of existing boilers in India have stand-alone OFA systems, they are not operated properly. OFA technology can reduce NOx formation by 20-45 per cent. LNB and OFA systems should be used in combination to achieve optimum NOx reduction.
- Combustion optimisation: Boilers are subject to frequent load changes as well as changes in the quality of coal. Hence, there are localised hotspots or temporary periods of incomplete combustion. This increases NOx, CO, unburnt carbon emissions as well as the exit furnace temperature, leading to other undesired effects such as slagging (molten ash and incombustible by-products that can stick to furnace components following coal combustion). In India, the majority of boilers are tangentially fired and have lower NOx emissions compared to wall-fired boilers. Moreover, tangentially fired boilers incorporate devices that can tilt the burner through an arc range of -30 to +30 degrees horizontally. Optimising this burner tilt angle can help in controlling NOx emissions. Thus, by controlling the existing boiler operating parameters (like burner tilt, excess air, coal mill operations, etc.), NOx emissions can be significantly reduced.
Plants with OFA and LNBs are designed to provide optimum NOx reduction at a given load on a particular fuel. However, the unit’s NOx performance decreases whenever a variable changes (operating profile, load, fuel quality, etc.). To maintain NOx performance, combustion optimisation systems that monitor key combustion parameters such as NOx, O2, CO, unburnt carbon and boiler efficiency should be integrated into boiler control systems. All these measurements, if performed accurately, can be used to control both excess air and coal flow to the individual burners resulting in optimised combustion conditions. Combustion optimisation incurs minimal cost and requires very little time for implementation (about five months). Its NOx reduction potential is 15-35 per cent, depending on the fuel type, boiler dimensions, existing burners, OFA technology and existing coal mill performance. Several manufacturers have stressed the fact that the majority of plants commissioned between 2003 and 2016 are already equipped with LNB and OFA. These units can achieve compliance by simply carrying out combustion optimisation.
- Post-combustion NOx control: Post-combustion control methods can reduce NOx emissions by neutralising the NOx in the flue gas into nitrogen via chemical reactions with or without the use of a catalyst. These technologies have a higher NOx reduction potential, but they also require higher capital and operating costs. The following are the two most widely used post-combustion NOx control technologies:
- SNCR: SNCR is used to reduce NOx to N2 by injecting either ammonia or urea into the boiler furnace at locations where the flue gas temperature is 900 °C-1,100 °C. SNCR is a simple post-combustion control system, which can help achieve NOx reductions ranging from 25 per cent to 50 per cent and can be installed within a regular plant outage schedule.
- SCR: SCR is the most effective and well-established NOx removal technology, in use since the early 1970s. It can be applied as a stand-alone control technology, or in combination with other technologies, including SNCR, combustion optimisation and combustion controls such as LNB and OFA. Typically, an SCR is installed at a power plant that requires a much higher level of NOx reduction compared to the reduction achievable through primary measures and SNCR. Only those units that are required to meet the strictest NOx emission standards of less than 100 mg per Nm3 should consider opting for this technology.
The way forward
NTPC has done pilots to assess the efficacy of various NOx reduction technologies and methods. In the trials, it has been found that NOx reduction achievable from combustion modification is in the range of 450-750 mg per Nm3. For SNCR technology, NOx reduction is in the range of 20-30 per cent only. Further, it was found that while NOx reduction of around 80 per cent is achievable, it is not consistent due to part load operation of power plants.
Under NTPC’s de-NOx action plan, combustion modification has already been implemented at five units aggregating 2,480 MW. Combustion modification contracts have been awarded and are under execution at 42 units with 18,080 MW of capacity. Further, de-NOx contracts for three units totalling 1,000 MW are currently under tendering.
To meet the stringent NOx norms of 100 mg per Nm3, Indian power plants will need to install SCR/SNCR technology. Although plants have been given a deadline of 2022, none of them have installed this technology yet. A major concern facing the industry is the technological limitation in implementing de-NOx systems. Going further, gencos should take firm steps to finalise their NOx control plans and start implementation at the earliest.