Secondary reduction techniques reduce emissions by treating pollutants that have already been released into flue gases, unlike primary techniques, which reduce emissions at source.

Secondary techniques for reducing emissions involve injecting a reagent (mainly urea or ammonia) into the flue gases to promote NHi formation. The reducing reactions may take place in the presence of a catalyst or not.



Selective Catalytic Reduction (SCR) is a process that reduces emissions in the presence of a catalyst. An NHi precursor such as urea or ammonia is injected into the flue gas upstream from the SCR reactor containing the catalyst. The reducing agent is vaporised before being mixed with air and injected into the flue gas. The are then broken down by the injected reagent into molecular nitrogen (N2) and water (H2O). The chemical reactions at the catalyst surface are written as follows.

With ammonia used as a reducing agent:

4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O

8 NH3 + 6 NO2 → 7 N2 + 12 H2O

With urea used as a reducing agent:

2 (NH2)2CO + 4 NO + O2 + 2 H2O → 4 N2 + 6 H2O + CO2

4 (NH2)2CO + 6 NO2 + 2 H2O → 7 N2 + 12 H2O + 4 CO2

SCR can significantly reduce emissions, by up to 90% under optimum conditions. Its average efficiency in existing installations is 70%.

The efficiency of the technique depends on the following parameters:

  • the temperature of the catalytic beds;
  • the NH3/NOx ratio;
  • the quality of the reducing agent/flue gas mixture;
  • the type of catalyst used.


For optimum operation, the temperature should be in the range of 250°C to 380°C, to promote the reaction that breaks down into N2 and H2O. The temperature varies depending on the catalysts used.


NH3/NOx Ratio

The amount of reducing agent injected depends on the amount of to be broken down. The higher the N2/H2O ratio, the more effective the emissions treatment process will be, but a high ratio can also increase the amount of NH3 emissions that have not reacted (NH3 leakage). The NH3/NOx ratio must be close to 1 to ensure reduction while also limiting NH3 leakage.


Reducing agent/flue gas mixture

The injection of the reducing agent must ensure a homogenous mixture of the agent and the flue gas. A homogeneous mixture will reduce emissions more effectively while also limiting NH3 leakage. The design of the injection nozzles therefore plays an important part in reducing emissions.


The catalyst

The most commonly used catalysts are mixtures of metal oxides, iron oxides, crystalline zeolites and activated carbon. The catalyst in most general use is a mixture of metal oxides comprising titanium oxide (TiO2) and vanadium oxide (V2O5).

The different catalysts have different specific surfaces and operate at different temperatures for different volumes of flue gases. The operating temperature for precious metal catalysts (platinum, rhodium) is in the range of 175 – 190°C, while metal oxide based catalysts operate at a higher temperature range of 260 to 450°C. The operating temperature for zeolites is even more important.

The catalysts are confined in a heat-insulated metallic catalytic reactor that can contain up to several dozen m2 of catalyst in up to four layers.

Catalysts can be in different geometrical shapes. The most common are:

    • Plate catalysts: mainly used for flue gases containing large quantities of particulate matter (refineries, coal combustion, etc.). These catalysts are less affected by erosion and dust deposition.
    • Honeycomb catalysts: used for processes that release less particulate matter. These catalysts are less resistant to dust deposition, which considerably reduces their performance and lifetime, but the honeycomb design greatly increases their specific surface area.
    • Catalysts in pellet form: mainly activated carbon.

The design and choice of catalyst especially depends on the particulate matter content of flue gases, its characteristics and the acceptable drop in pressure as the gases pass through the reducer. To ensure optimum performance, particulate deposition must be prevented as far as possible and pressure drop must be kept to a minimum.

Heavy metals and ammonium bisulphate, a sticky compound formed when sulphur oxides react with ammonia (NH3) will deteriorate and eventually deactivate the catalyst.

The lifetime of a catalyst does not depend on its composition alone, but also on the fuel used, the design characteristics of the combustion plant, the NOx concentrations to be treated and the NH3/NOx ratio.

A catalyst’s lifetime, as estimated in the reference document on Best Available Techniques (BAT) on large combustion plants, ranges from six to 10 years when used to clean flue gases from coal combustion, and eight to 12 years for flue gases from gas or fuel oil combustion. Regeneration and activation processes can lengthen or renew the lifetime of a catalyst, and delaying their replacement.

To reduce nitrogen oxides formed during combustion, an SCR reactor must be fitted into the flue gas circuit, upstream from the chimneys. This is usually done in one of three ways:

    • « high dust » configuration: the reactor is fitted upstream of all flue gas treatment systems. At this point, the gases passing through the catalyst contain sulphur oxides and particulate matter. Plate catalysers are the most appropriate in this case as they are less sensitive to dust deposition and abrasion. This configuration is mainly used for coal combustion.
    • « low dust » configuration: The reactor is fitted downstream of the dust extractor and upstream of the other flue gas treatment systems. Prior dust extraction increases the lifetime of the catalyst as it reduces abrasion and dust deposition. However, the flue gases still contain sulphur oxides that can form ammonium sulphide, which deteriorates the catalyst. As dust extraction takes place at high temperatures, this configuration is more costly than the « high dust » solution.
    • « tail gas » configuration: the reactor is fitted downstream of all other flue gas treatment systems. This configuration limits deterioration of the catalyst by dust deposition and sulphur compounds and allows the catalyst to take up less space. The disadvantage of the “tail gas” configuration is that the flue gases have to be heated upstream of the reactor to the catalyst operating temperature. This usually entails additional energy consumption.

SCR is considered to be a costly process in both existing and new facilities. This is because the cost of the catalyst is very high in proportion to the total cost of the reduction measure, including maintenance and replacement. The cost depends on the quantity of NOx to be reduced, the size of the plant and the configuration chosen to install the SCR reactor.



Selective non catalytic reduction is a secondary measure for reducing nitrogen oxides formed during combustion. The emission reduction principle is similar to SCR but does not involve a catalyst. The technique is based on injecting a reducing agent at the exit of the combustion chamber. The most commonly used agents are urea and ammonia. The reaction, which takes place at high temperatures (850 – 1100°C), reduces previously formed nitrogen oxides into molecular nitrogen and water:

Main reaction (reduction) : 4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O (a)

Secondary reaction, to be avoided (oxidation) : 4 NH3 + 5 O2 → 4 NO + 6 H2O (b)

The process requires two interconnected units: a storage unit for the reducing agent, and an injection unit usually comprising several injection nozzles. SNCR can reduce nitrogen oxides emissions by 40 to 65%.

The efficiency of the technique depends on the following parameters:

    • the temperature;
    • the injection system;
    • the type of reducing agent used;
    • the length of time the reducing agent remains where the reaction takes place;
    • the NH3/NOx ratio.



Temperature greatly influences the process. For optimum nitrogen oxides reduction, the temperature must be in the range of 850 to 1 100°C. At lower temperatures, less reduction takes place because the formation of NHi radicals is less active; higher temperatures produce other reactions, including oxidation of the reducing agent in particular, causing additional nitrogen oxides to form (reaction b).

Because temperatures in the installation fluctuate, the reducing agent must be injected at different points to ensure that it reacts at the optimum temperature.


Injection system

The design of the injection system (pressure, flow rate, nozzles) significantly influences the SNCR performance. The mixture must be homogeneous to limit NH3 leakage, and the spray droplets must comply with certain characteristics. If the droplets are too small, they may vaporise too quickly and react at temperatures that are too high; if they are too large, they will react at lower temperatures and increase ammonia leakage.


Reducing agent

The choice of reducing agent influences not only the effectiveness of emission reduction, but also the formation of nitrous oxide, a powerful greenhouse gas. Different reagents may be used:

    • ammonia gas;
    • dissolved ammonium;
    • urea pellets;
    • dissolved urea.

Ammonia gas is highly toxic and explosive, and precautions must be taken for its use. There are fewer regulatory constraints on its use in dissolved form.

Dissolved urea is the simplest reagent to use from the regulatory point of view, but it is often more expensive than ammonia.

Using solid urea reduces energy consumption as it is already dehydrated, but storage conditions are more restrictive than for liquid urea. It is also more difficult to ensure an even distribution of urea pellets in the flue gases than with liquid urea.


The length of time the reducing agent remains where the reaction takes place

The time required to ensure optimum reduction of nitrogen oxides is in the range of 0.2 – 0.5 s. Because it depends on the size of the droplets injected, the optimum time is not easy to achieve.


NH3 / NOx Ratio

To ensure maximum contact between the nitrogen oxides and the reducing agent, the NH3/NOx ratio must be higher than 1. The optimum ratio, i.e. ensuring a high level of performance (40 to 60% reduction of NOx ) while limiting NH3 leakage, is in the range of 1.5 to 2.5.

The average efficiency of SNCR in reducing nitrogen oxides is 40 to 65%. To maximise NOx reduction, it is therefore usually combined with primary NOx reduction techniques, such as low-NOx burners, staged air combustion or flue gas recirculation.

Implementing SNCR may increase emissions of N2O, CO and NH3.

SNCR is considered to be a cost-effective process for reducing nitrogen oxides. Installation costs differ slightly between new and existing facilities but mainly depend on the type of fuel used, the nitrogen oxides concentrations to be treated and the reducing agent.