Selective catalytic reduction (SCR) for NOx control

In SCR systems, ammonia vapour is used as the reducing agent and is injected into the flue gas stream, passing over a catalyst. NOx emission reductions over 80-90% are achieved. The optimum temperature is usually between 300°C and 400°C. This is normally the flue gas temperature at the economiser outlet.

There are three typical layout arrangements of SCR systems applied to coal-fired power stations. High dust position is the most widely used SCR configuration, especially with dry bottom boilers, because it does not require particulate emissions control prior to the denitrification process. Low dust positioning has the advantage of less catalyst degradation caused by fly ash erosion, but requires a more costly hot-side ESP. Tail end position SCR has been used primarily with wet bottom boilers with ash recirculation to avoid catalyst degradation caused by arsenic poisoning. The configuration is also favoured with retrofit installations (due to SCR space requirements) between the economiser outlet and ESP.

The efficiency of SCR process reactions (reagent stoichiometry and utilisation nearly 1.0) allows very close and effective reagent injection-control based on feedback, of measured NOx concentrations in the flue gas at the economiser outlet. The temperature of the flue gas in the SCR reactor is controlled by mixing the flue gas exiting the economiser with the flue gas from the economiser bypass. The ammonia injection grid is located in the ductwork leading to the SCR catalyst, far enough upstream to ensure optimum gas and reagent distribution across the catalyst cross-section.

The catalysts can have different compositions: based on titanium oxide, zeolite, iron oxide or activated carbon. Most catalysts in use in coal-fired plants consist of vanadium (active catalyst) and titanium (used to disperse and support the vanadium) mixture. However, the final catalyst composition can consist of many active metals and support materials to meet specific requirements in each SCR installation. Catalyst geometry may typically be a flat plate or honeycomb. A moving bed is used for granular activated carbon. German experience shows that plate types generally have a higher resistance to deposition and erosion than honeycombs. In this case, catalytic converters are used in an air preheater.

SCR technology has been used commercially in Japan since 1980 and in Germany since 1986 on power stations burning mainly low-sulphur coal and in some cases medium-sulphur coal. There are now about 15 GWe of coal-fired SCR capacity in Japan and nearly 30 GWe in Germany, out of a total of about 53 GWe worldwide. During the 1990s SCR demonstration and full-scale systems have been installed in US coal-fired power plants burning high-sulphur coal. Their commercial use has followed the introduction of stringent limits to regulate NOx emissions in each country.

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