NOx Control

Before the new environment norms were put in place in December 2015, there were no rules to control nitrogen oxide (NOx) emissions at thermal power plants (TPPs) in India, despite the fact that coal-based power plants account for a significant share of the annual industrial NOx emissions. These emissions were as high as 30 per cent in 2014-15. The current standards require TPPs installed before December 31, 2003 to limit their NOx emissions at 600 milligrams per cubic metre (mg/Nm3). The standards are more stringent for newer plants, with the levels set at 300 mg/Nm3 for plants installed between December 31, 2003 and December 31, 2016; and 100 mg/Nm3 for those installed after December 31, 2016. The new standards are in line with the global standards: NOx emission limits in China, Japan and the US are 50 mg/Nm3, 200 mg/Nm3 and 110 mg/Nm3 respectively.

Power Line presents an overview of the control measures that need to be adopted by TPPs in light of the new norms as well as issues and concerns in their adoption…

Control measures

There are various factors at work in TPPs that influence the level of NOx emitted by a plant. For example, the heat release rates in the furnace affect the levels of thermal NOx as greater levels of NOx are produced at higher temperatures in the furnace. Furnace size and geometry is another factor that affects the NOx emission levels. Excess air in the unit leads to higher NOx emissions because of the excess availability of O2 content in the combustible air. Residence time and burner design and spacing are other factors that affect NOx emissions. For example, in a low-NOx burner, initial fuel combustion occurs in a zone with more fuel and less oxygen, thus minimising NOx formation.

At the new TPPs, in order to meet the 100 mg/Nm3 standard, plants will have to utilise selective catalytic reduction (SCR), a technology that injects ammonia into flue gas to reduce NOx in the presence of a catalyst. SCR has been proven as an effective method to reduce NOx emissions from coal-fired power plants globally. However, older plants aiming to achieve the 300 mg/Nm3 standard can potentially attain these levels through a combination of the in-furnace, combustion control and SNCR technologies. SNCR is a simpler post-combustion control system, which can help achieve reliable NOx reductions ranging from 25 to 50 per cent, and can be installed within a regular plant outage schedule. SNCR systems do not require a catalyst; however, their effectiveness is dependent upon sufficient reaction time within a narrow flue gas temperature window and adequate mixing of the reagent with the flue gas. Other non-catalytic technologies such as low NOx burners and over-fire air systems too are available.

The effective temperature for reduction of NOx through an SCR catalyst ranges from 200 °C-400 °C; for SNCR to be effective, ammonia injection and reduction needs to be in the range of 900 °C to 1,100 °C. Thus, what helps to reduce NOx will drive up carbon monoxide (CO) and vice versa. Settings optimising NOx and CO levels may lead to a reduction in boiler efficiency. Thus, new plants will need to plan for optimisation right from the setting-up stage by taking into consideration the characteristics, types and models of the generating units available; and the regulatory and statutory compliances, including environmental standards.

Issues and concerns

One of the key limitations of these control measures is the high upfront investment. SNCR or SCR is estimated to cost Rs 1 million-Rs 1.5 million per MW. While investments and high capital costs are no longer a concern, given that the government has allowed the pass-through of costs, another key industry concern is that the SCR system required for the reduction in NOx emission has not been proven for high ash Indian coal. Also, the availability of space would be a constraining factor for installing this system. Moreover, ammonia would be required for operating the SCR system, involving challenges in transportation and storage due to its toxic nature.

A recent technical paper by NTPC points out that SCR systems are sensitive to contamination and plugging during both normal and abnormal operations. Certain pollutants in flue gas can render the system ineffective for NOx reduction, or cause oxidation of the ammonia present (forming more NOx). SCR systems have operational difficulty with binding of the catalyst by fly ash. Because of these issues, SCR catalysts have a limited operational lifetime of up to 40,000 hours in coal-fired power plants, depending on the flue gas composition. According to Ministry of Environment, Forests and Climate Change deadlines, all TPPs are required to meet the new NOx emission standards by 2022. That said, as much as 50 per cent of the power plants have reported that they already meet the relevant NOx norms (300 mg/Nm3/or 600 mg/Nm3). However, there is a technology challenge for meeting the 100 mg/NM3, which is required for plants established after January 2017. A demonstration study was recently undertaken by NTPC for its Vindhyachal thermal project for evaluating the currently available DeNOx technologies (SCR/SNCR).

Going forward, choosing the right emission reduction technology could help power plants to not only earn profits but also improve power generation for the entire system.

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