NOx control, high ash coal and the new Indian emissions standards18 July 2018
A new IEA Clean Coal Centre report*, summarised below, looks at NOx control for high ash coals, currently a particular challenge for India, which has introduced new emissions standards.
The combination of new emission standards and the high ash content of Indian coals has created concerns that some commonly deployed NOx control technologies may not be adequate in the Indian context.
The new standards in India are broadly similar to those in the EU and the USA. As well as NOx, they regulate particulate matter, sulphur dioxide, and mercury emissions, as well as water consumption.
For NOx, they stipulate that plants installed before the end of 2003 emit no more than 600 mg NOx/m3, plants installed after 2003 and before the end of 2016 emit no more than 300 mg/m3 NOx, and plants installed from 1 January 2017 limit their emissions to 100 mg/m3. New norms were obligatory from 1 January 2017 for new plants. Existing units have two years in which to comply. Revisions of the norms and delays in implementation are expected.
As with all retrofits and the introduction of technologies to new markets, there are challenges. In India, these include, as well as the high ash, highly erosive coals, technical implementation difficulties including space constraints, financial issues as it is costly to introduce multiple technologies at the same time, lack of local suppliers and a subsequent need to import technologies as well as all materials and reagents, lack of local skills and expertise, and lack of utility experience with continuous emissions monitoring.
NOx control options
NOx control technologies can be broadly divided into primary and secondary measures. Most controls can be used alone or in combination depending on the rate of NOx removal required.
Primary measures include: low NOx burners (LNBs); overfire air (OFA); fuel biasing; low excess air; fuel reburning; flue gas recirculation; and combustion optimisation. In general, these controls reduce NOx by controlling coal combustion. Many boilers in India have OFA and deploy fuel biasing, but only a few have LNBs. Several power plants will have to install LNBs and/or OFA. These are relatively quick to install but can adversely impact the boiler’s operation leading to increased NOx and CO emissions, carbon in ash and changes in the burner flame geometry. Therefore, careful design and the measurement and control of various combustion parameters are vital. The latter can be achieved by using advanced sensor and control systems.
Retrofits can take place during planned outages, and many were scheduled to take place during 2017 and 2018.
Secondary NOx control measures include: selective catalytic reduction (SCR); selective non-catalytic reduction (SNCR); and combinations of the two. Generally, they are applied to plants which need to reduce NOx limits beyond the capability of primary measures.
SCR achieves the highest NOx removal rates of all NOx controls at 80–90%, but it is also the most expensive option. In India, over 300 units may require SCR. In coal- fired power plants an SCR unit is generally installed between the economiser and the air heater, where the temperature of the flue gas is optimal for an SCR reaction. However, in this configuration, known as hot-side, high dust, the SCR is exposed to the fly ash and chemical components of the flue gas, which can cause excessive wear on: the ductwork; large-particle ash screens; ammonia injection grid (AIG) nozzles; flow distribution devices; and the SCR catalyst. The ash also leads to poor distribution of velocity into the catalyst, accelerates its deactivation, and increases the costs of catalyst management. These issues can be mitigated by appropriate design measures, such as the use of abrasion resistant coatings, erosion resistant wear plate, and wear shields on AIG lances, proper reactor sizing, and modifications to catalyst module shape and pitch size. Pilot tests are currently underway (2017-18) on NTPC units to find the best solutions.
On its own, SNCR reduces NOx by 30– 50%, but SCR levels of performance can be achieved if it is applied in conjunction with other combustion controls. Historically, the effectiveness of SNCR has been limited, especially in utility-scale boilers, due to a lack of accurate real-time tools to measure the temperature and CO profile within the boiler – parameters which are important for an effective NOx reduction reaction. However, recent developments in measurement systems allow the effective use of SNCR even on large furnaces (>400 MW). In India, an SNCR would have to cover a greater area and not all types of spraying nozzles would be applicable; this can be verified with CFD modelling and field tests. In Indian applications it would be preferable to use urea as the reagent rather than ammonia, as urea is non-toxic and its use would avoid various environmental and health hazards. Also, urea is much more effective than ammonia on large furnaces, according to some experts. Current tests on NTPC units will ascertain the applicability of SNCR.
As India is introducing emission standards for more pollutants, and pollution controls are expensive and time consuming to install, which disrupts power generation, it would make sense to co-ordinate installation of pollution control systems and to focus on multi-pollutant control systems. There are a few multi-pollutant controls which can remove NOx. Some of them, such as ReACTTM have been used in coal- fired power plants for several years. Some are deployed in non-coal applications but have the potential to be applied to coal-fired plants and are in various stages of testing and demonstration.
Waiting for NTPC test results
Choosing appropriate methods of NOx control for a power plant requires a site- specific strategy which considers cost, performance and safety, and also water requirements.
NOx controls for high ash coals are broadly the same as for ‘normal’ boilers, but they must be customised to local requirements as has already happened with low NOx burners.
There is a lack of NOx reduction commercial installations in India, of either the SCR or SNCR type. However, the experience from high ash lignite-fired plants and from high dust industries such as cement kilns indicates their potential, and vendors have confidence their systems will be applicable to Indian plants. But utilities are not expected to make decisions about SNCR and SCR until the test results from NTPC units are clear.
*NOx control for high-ash-coal-fired plant, by Malgorzata Wiatros-Motyka and Herminé Nalbandian-Sugden, CCC/285, ISBN 978-92-9029-608-9, 87 pp, April 2018. WWW.IEA-COAL.ORG, tel +44 (0)20 3905 3870, [email protected]