Coal- and lignite-fired power plants are among the largest sources of nitrogen oxide (NOx) emissions. The combustion of coal at high temperatures converts the elemental nitrogen in the air and fuel to NOx. With increasing reliance on coal for power generation, the release of NOx into the environment has more than doubled over the past century, leading to ozone pollution, acid rain and other environmental challenges.
Coal-based power plants in India have remained exempt from NOx emission controls for decades. It was only in December 2015 that the Ministry of Environment, Forests and Climate Change imposed NOx control standards on power plants, irrespective of their capacity. As per the new norms, the NOx standards have been fixed at 600 milligram per normal cubic metre (mg per Nm3) for thermal power plants (TPPs) older than 2003. For units installed after 2003 and up to December 31, 2016, the NOx standard has been fixed at 300 mg per Nm3. Meanwhile, for TPPs that come online after January 1, 2017, the limit has been restricted to 100 mg per Nm3.
Ill-effects of NOx
NOx and its by-products cause significant harm to the environment and human health. On reacting with volatile organic compounds in the presence of heat and sunlight, NOx forms ground-level ozone or smog, which is a strong oxidising agent. This can lead to respiratory distress such as asthma in humans as well as have harmful effects on vegetation and fragile ecosystems. NOx also forms small nitrate particles that are associated with serious health impacts and result in hazy skylines. Further, nitrate particles can form nitric acid in the atmosphere, contributing to acid rain and overloading the ecosystems with nitrogen.
Methods to control NOx emissions
Different fuels have different combustion rates and, therefore, require different NOx reduction and control techniques. In the case of TPPs, different types of coals have a varying content of volatile ingredients, including nitrogen, sulphur, lead, mercury and other contaminants. NOx formation depends on the temperature and residence time of gases in the combustion chamber at that particular temperature, and increases with an increase in boiler capacity because larger boilers have more intense combustion with higher combustion temperatures and longer residence time for flue gases.
NOx formation can be reduced by installing low NOx burners, or using ultra-low nitrogen content fuels. The design of the boiler, internal combustion engine, or gas turbine also has a major effect on the NOx reduction system. In addition, power plants can deploy NOx reduction systems based on selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR), which are discussed in the subsequent sections…
SCR
SCR systems have been commercially deployed in power plants using low-to-medium sulphur coal in Japan since 1980 and in Germany since 1986. With the introduction of stringent limits to regulate NOx emissions, the use of SCR systems expanded to other countries, including the US. SCR systems are capable of reducing 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. This is usually the temperature of the flue gas at the economiser outlet.
In SCR, a catalyst is used to facilitate the reduction reactions of NOx with the selected reducing agent (anhydrous ammonia, aqueous ammonia solution or urea solution) to form nitrogen and water, thereby limiting side reactions. Care must be taken to choose a high quality reducing agent as pollutant content or catalyst poisons must be minimised. The catalyst used in the SCR process can be based on titanium oxide, zeolite, iron oxide or activated carbon. Typically, coal-fired plants use catalysts composed of a mixture of vanadium (active catalyst) and titanium (support material). However, the final catalyst composition can consist of many active metals and support materials to meet the specific requirements of each SCR system.
In coal-fired power plants, SCR systems can be configured in three different ways. The most widely used SCR configuration is the high dust position, which does not require particulate emissions control prior to the de-nitrification process. This configuration is especially used in plants with dry bottom boilers. Another configuration, the low dust position, is somewhat costly as it requires electrostatic precipitators but offers the benefit of lesser degradation of the catalyst by fly ash. Meanwhile, in the tail-end position, SCR is typically used with wet bottom boilers with ash re-circulation to prevent catalyst degradation caused by arsenic poisoning.
SNCR
SNCR systems have been used commercially in oil- and gas-fired power plants in Japan since the 1970s. By the end of the 1980s, many countries in Western Europe started deploying SNCR systems in coal-fired power plants. Over the next decade, the commercial deployment of SNCR systems spread to other countries, including the US. Today, SNCR technology is widely used in cement, waste incinerators as well as biomass and conventional fuel-based boilers. The SNCR process can help reduce NOx emission by 30-50 per cent, which is significantly less than that of SCR systems, but the former scores over SCR owing to its lower cost and installation time.
SNCR systems involve the injection of a reagent (ammonia or urea) into the flue gas in the furnace within a temperature window of 900 °C to 1,100 °C (depending on the reagent and condition of operation). At an appropriate temperature, the NOx and reagent react to form nitrogen and water. The main components of an SNCR system include reagent storage, multi-level reagent-injection equipment and associated control instrumentation. Although the SNCR reagent storage and handling systems are similar to those for SCR systems, the reagent requirement in the former is more. In SNCR, both ammonia and urea require three or four times as much reagent as SCR systems to achieve similar NOx reductions because of higher stoichiometric ratios.
When the reaction temperature increases over 1,000 °C, there is a decline in the NOx removal rate due to the thermal decomposition of ammonia. Meanwhile, at temperatures below 1,000°C, the NOx reduction rate declines and the ammonia slip may increase. The optimum temperature typically occurs in the steam generator and convective heat transfer areas. As a thumb rule, the longer the reagent is in the optimum temperature window, the better the NOx reduction.
The leakage of ammonia from SNCR systems can occur either due to injection at low temperatures or over-injection of the reagent, leading to uneven distribution. An adequate reagent injection system is a must for the effectiveness of the SNCR process. An injection system with few injection control points or one that injects a uniform amount of ammonia across the entire section of the boiler can lead to a poor distribution ratio and high ammonia slippage. An inadequate distribution ratio is especially an issue in large coal-fired boilers because of the long injection distance required in the boiler. In order to overcome this, multiple layers of reagent injection and individual injection zones are commonly used in boilers.
A potential disadvantage of the SNCR process is that it can produce nitrous oxide, which contributes to the greenhouse effect. The formation of nitrous oxide depends on the reagent used, the amount of reagent injected and the injection temperature. Also, occasionally, unreacted ammonia can react with sulphur trioxide to form ammonium bisulphate, which precipitates at air heater-operating temperatures and can lead to fouling and plugging.
Overall, the effectiveness of an SCNR system depends on the design and performance of injectors, optimisation and programming of the controller, the location of the injection points, the temperature window at the injection point, the choice of reagent and the possible residence time.
The way forward
Research and development (R&D) efforts are required for generating more effective emission control solutions at reasonable costs. In recent years, a combination of SCR and SNCR technologies has been developed for NOx reduction. These hybrid systems reduce NOx in the flue gas with a hydrocarbon treatment in two stages – SNCR followed by SCR. These hybrid systems entail a lower capex and opex than individual systems as such systems are compact in size and lower in volume. Such systems are used when a significant reduction in NOx emissions is required along with flexibility of operation, or there is space restriction for the installation of the catalyst, or when increasing the pressure drop is critical.