Selective Non-Catalytic Reduction (SNCR)

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Selective Non-Catalytic Reduction (SNCR) is a method to reduce nitrogen oxide emissions in conventional power plants that burn biomass, waste and coal. The process involves injecting either ammonia or urea into the firebox of the boiler at a location where the flue gas is between 760 and 1,093 °C to react with the nitrogen oxides formed in the combustion process.


SNCR technologies came into commercial use on oil- or gas-fired power plants in Japan in the middle of the 1970s. In Western Europe, SNCR systems have been used commercially on coal-fired power plants since the end of the 1980s. In the USA, SNCR systems have been used commercially on coal-fired power plants since the early 1990s.

In the EU specifically this measure falls under the remit of the Large Combustion Plant Directive (2001/80/EC), the purpose of which is to limit the amount of sulphur dioxide, nitrogen oxides and dust emitted from large combustion plants each year.


The SNCR process can be highly effective but needs to be carefully managed. It utilises the injection of urea or ammonia reagent into either the upper furnace or convective pass of the boiler. The reagent reacts selectively with the NOXin the flue gas, but these reactions are highly temperature dependent and only occur within a certain temperature window (i.e. 760 – 1,093 °C). The resulting product of the chemical reaction is elemental nitrogen (N2), carbon dioxide (CO2), and water (H2O), which are formed without the need for a catalyst. However, at temperatures outside of this range the NO and ammonia won’t react. Ammonia that hasn't reacted is called ammonia slip and is undesirable, as the ammonia can react with other combustion species, such as sulphur trioxide (SO3), to form ammonium salts. At temperatures above 1093 °C ammonia decomposes. In that case NO is formed instead of reduced. Thus the reaction needs a specific temperature window to be efficient. The reaction also needs sufficient reaction time in that temperature window.

A further complication is mixing. Generally more NO will form in the centre and less near the walls, as the walls are cooler than the centre. Thus optimally more ammonia must find its way to the centre and less near the walls, otherwise NO in the centre meets insufficient ammonia for reduction and excess ammonia near the walls slips through.

Though in theory SCNR can achieve the same efficiency of about 90% as SCR, these practical constraints of temperature, time, and mixing often lead to worse results in practice.

Costs & Benefits

SNCR has the lowest reduction efficiency but also the lowest capital cost of all of the post combustion NOXcontrol technologies. SNCR has an economical advantage over SCR as the cost of the catalyst isn't there. Thus it offers lower capital equipment cost and is extremely versatile for medium levels of NOX control. Despite the relatively modest reduction efficiency, it still has an important role to play in the reduction of NOX emissions, particularly in combination with combustion modifications or where large reductions are not required. Indeed it can be combined with other upstream and downstream NOX control technologies. SNCR is applied in many industries including, Power, Steel, Pulp and Paper, Petrochemical, Waste to Energy, Glass and others.

Evidence & Reference

Modelling this Measure

As with SCR in combustion plants, the uptake and share of the SNCR control is guided by the Large Combustion Plant Directive (2001/80/EC). The uptake of this measure can be modelled by accounting for the influence of removal efficiency of the technology over the given activity.

Site Entry Created by Policy Measures Admin on Oct 08, 2010

Reference This Source (2019). Selective Non-Catalytic Reduction (SNCR). Available: Last accessed: 19th February 2019

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