Combustion of coal generates large volumes of nitrogen oxides (NOx), including nitric oxide, nitrogen dioxide and nitrous oxide. The amount of NOx formed depends on combustion conditions and the nitrogen content of the coal. The Ministry of Environment, Forest and Climate Change (MoEF&CC) has established a tiered regulatory framework to control NOx emissions from TPPs in India. TPPs commissioned between 2003 and 2016 are currently subject to a less stringent NOx emission limit of 450 mg per cubic metre (Nm3), compared to the stricter 100 mg per Nm3 limit applicable to plants commissioned after 2017. Older plants, built before December 2003, are allowed a maximum emission level of 600 mg per Nm3. These regulations are to be implemented in a phased manner, with progressively stricter deadlines for compliance.
NTPC Limited has implemented combustion modifications in 46 units of 20 GW capacity. It is under implementation for the remaining capacity. The installation of overfired dampers has enabled regulation of the flow and direction of the secondary air injected into the furnace by the OFA system. By controlling the amount and distribution of this secondary air, OFA dampers help optimise the staged combustion process for efficient fuel conversion and maximum NOx reduction.
In-combustion control
In-combustion technology regulates temperature and fuel-air mixing during the combustion process to reduce thermal NOx formation. These are relatively low-cost technologies that are also easy to install.
One of the most widely used technologies for in-combustion control is low NOx burners (LNBs). In LNBs, fuel is initially burned in a limited oxygen zone, creating an environment that allows hydrocarbons (unburnt fuel) to react with already formed NOx and convert it into nitrogen gas. By controlling the initial combustion stage, LNBs achieve lower peak flame temperatures, further reducing NOx formation. A proven technology, they are relatively easier to install in existing boilers. A potential concern with LNBs is a slight decrease in combustion efficiency, which could lead to more unburnt carbon and carbon monoxide emissions. However, proper design and operation can minimise these issues.
Overfire air (OFA) systems are another approach to reducing NOx emissions in coal power plants. OFA systems control the availability of oxygen near the burners, creating a fuel-rich zone that limits the formation of NOx. Only a portion of the combustion air is initially supplied, and the rest is injected higher up in the furnace at a lower temperature, minimising NOx formation. While standalone OFA systems can achieve a 20-45 per cent reduction in NOx emissions, proper operation is crucial for effectiveness. Several variations of OFA systems exist, offering potentially higher reduction rates.
LNBs and OFA systems represent a synergistic approach to in-combustion NOx reduction. LNBs establish a fuel-rich initial stage, suppressing NOx formation. OFA then introduces additional air higher up in the furnace, completing combustion at a lower temperature and further minimising NOx generation.
Combustion optimisation is a technique for actively controlling the burning process in boilers. This focuses on achieving the best possible outcome between complete combustion (100 per cent efficiency) and acceptable emission levels. For coal-fired boilers, even larger reductions in efficiency (20-60 per cent) are necessary to achieve similar results. Active control measures allow manual adjustment of boiler settings to find the optimal balance between efficiency and emissions. It is a cost-effective way for coal plants to reduce NOx emissions by 15-35 per cent. This approach is particularly suitable for boilers commissioned between 2003 and 2016, which might already have LNBs and OFA systems installed.
Flue gas recirculation, or FGR, is another technique for reducing NOx formation during combustion. In this process, a portion of the hot exhaust gas (flue gas) is diverted back into the furnace. This recycled gas lowers the combustion temperature and reduces oxygen availability. The cooler flue gas acts as a coolant, reducing the overall peak temperature within the furnace, which suppresses the formation of NOx. Furthermore, by reintroducing flue gas (which already contains some burned oxygen), the overall oxygen concentration in the furnace is slightly diluted. This creates a more oxygen-limited environment, further limiting the formation of fuel NOx. FGR can also be used as a carrier for injecting fuel into a separate “reburn zone” within the furnace. This promotes better mixing of the fuel with the hot gases, improving combustion efficiency.
Post-combustion control
Post-combustion control systems capture or chemically convert NOx formed during combustion into harmless products such as nitrogen gas (N2) after the flue gas has exited the boiler with or without a catalyst. However, these technologies require higher capital and operating costs.
Selective non-catalytic reduction (SNCR) is one of the post combustion methods used to reduce NOx to nitrogen gas through the injection of ammonia or urea into the boiler furnace at locations where the flue gas temperature is 900-1,100 °Celsius. For successful SNCR operation, it is essential that the injected reducing agent (such as ammonia or urea) spends enough time within the ideal temperature zone, and that there is uniform distribution and proper mixing of the agent across the entire furnace cross-section. Plasma-assisted SNCR combines traditional SNCR with plasma technology. The plasma creates a more uniform temperature distribution and enhances the reaction between ammonia and NOx, potentially leading to lower ammonia consumption and improved NOx reduction efficiency compared to traditional SNCR.
Selective catalytic reduction (SCR) is a process where ammonia is injected into the hot exhaust gas (flue gas) after it exits the economiser (a heat exchanger), but before it leaves the air preheater (another heat exchanger). This injection creates a chemical reaction between the ammonia and NOx, converting harmful NOx into harmless nitrogen gas and water vapor. The reaction between ammonia and NOx is most efficient when the flue gas is between 300 °Celsius and °Celsius, which is why the SCR system is placed strategically in the exhaust stream. The SCR system uses a catalyst to speed up the chemical reaction.
Recently, Bharat Heavy Electricals Limited (BHEL) achieved a significant milestone with the production of India’s inaugural batch of SCR catalysts, which are crucial for reducing NOx emissions in TPPs. Moving away from its prior reliance on imports, this achievement aligns with the Indian government’s Make in India initiative. BHEL has established a cutting-edge SCR catalyst manufacturing facility within its solar business division unit, dedicated to addressing NOx emissions in TPPs. Responding to the MoEF&CC’s directives and recognising the long-term environmental impacts of NOx emissions, various entities have placed orders for SCR systems: Telangana State Power Generation Corporation Limited for the 5×800 MW Yadadri TPS, Maharashtra State Power Generation Company Limited for the 1×660 MW Bhusawal TPS, West Bengal Power Development Corporation Limited for the 1×660 MW Sagardighi TPS, and National Aluminium Company Limited for the 1×18.5 MW Damanjodi TPS. In another development, in September 2023, GE Power India Limited secured a contract from Vedanta Limited to implement NOx reduction measures at the Lanjigarh Combined Gas Power Plant unit. The contract, valued at Rs 250 million, focuses on the 90 MW (3 x 30 MW) unit.
The Way forward
The current regulations for NOx emissions from coal-fired power plants in India prioritise primary control methods, such as LNBs and OFA systems, for certain plant categories. These technologies can achieve compliance with the applicable standard (e.g., 450 mg/Nm3) without needing more expensive post-combustion controls. In fact, advancements such as third-generation LNB and OFA combinations show promise, with the potential to reduce NOx emissions to even below the 300 mg/Nm3 mark. Recognising this, power plants nationwide are actively implementing these primary control technologies to meet the emission standards.
While current primary control technologies offer a path toward compliance with existing emission standards, continued efforts are necessary for optimal NOx mitigation. Fostering collaboration between government agencies, research institutions and power plant operators can accelerate the development and deployment of emerging technologies with even greater NOx reduction potential. Furthermore, maintaining strict emission standards and conducting regular inspections of thermal power plants at established intervals are crucial for ensuring long-term adherence to regulations and achieving sustained reduction in atmospheric NOx levels.