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Flue Gas Denitrification Systems


Our Solutions Emissions Con

Flexible solutions tailored to customer needs

Getting clean energy from combustion plants involves smart emissions control. Using fuel like coal, oil products, natural gas, municipal and industrial waste unavoidably produces harmful pollutants like nitrogen oxides (NOx). These emissions can be reduced by turning NOx into nitrogen (N2) and water (H2O).

There are two main options that involve injecting reduction agents into the flue gas stream. These processes, Selective Non-catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR), can be used separately or in combination.

SNCR is based on the high-temperature reaction of gaseous ammonia or urea with nitrogen oxides without the use of a catalyst. SCR uses the reaction of nitrogen oxides with ammonia in the presence of a catalyst (active compound) within an appropriate temperature window.


Achieving the right conditions for SNCR is extremely important. The optimum temperature is between 800°C and 1100°C, depending on the reagent. The reduction achieved depends on the temperature, the ratio of reagent to NOx, proper distribution of the reagent and how long the particles are in the reaction zone. Large particles may evaporate too slowly, leaving a high unreacted amount of the reagent (NH3 slip). Small particles may evaporate too quickly if the temperature is too high, limiting NOx reduction.

It's often necessary to retrofit the boiler by introducing or optimizing techniques used to:

  • Lower the flame temperature and reduce the formation of thermal nitrogen oxides (low-emission combustion).
  • Equalize and stabilize flue gas temperature.
  • Increase the residence time of the reactant molecule in the reaction zone.

With SCR, the amount of ammonia injected depends on the input NOx concentration, the exhaust gas volume and the required removal rate. The process takes place at temperatures between 270°C and 400°C.

There are two potential options for positioning the SCR deNOx reactor in the plant layout:

  • Tail end: boiler / dust filter / deNOx / stack
  • High dust: boiler / deNOx / dust filter / stack.

There are pros and cons with each configuration relating to the space needed, downtime required for installation and connection, and the type of environment.


  • SNCR and SCR systems can be set up to suit individual customer conditions and needs. Parameters like dustiness, sulfation and flue gas temperature and space available determine the right choice.
  • A variety of catalysts are available. Plate catalysts are primarily used for boilers fired with lignite and coal with a high ash content. Honeycomb catalysts are better for gas boilers, oil boilers, waste boilers and most coal-fired boilers.

Flexible and efficient

  • Expansion, or just changing the management of catalysts allows for further significant reductions in NOx emissions.
  • SNCR and SCR can be used in concert. The SNCR system allows an initial reduction of NOx. SCR reduces NOx further as well as the NH3 remaining after the non-catalytic process. This makes it possible to keep emissions at the required level and to meet any further legal restrictions.
  • All SCR catalysts change the oxidation degree of mercury from Hg0 to Hg2+, significantly contributing to the increase of mercury removal efficiency in the downstream flue gas equipment.

Anyone operating a combustion plant needs to remove NOx from their emissions. Denitrification systems are installed in many facilities, both in the power sector (power plants, combined heat and power plants) and in industrial plants (cement works, paper mills, glassworks, etc.). They are used in:

  • Plants for thermal utilization of all kinds of waste
  • Boilers fired with alternative fuel such as RDF
  • Boilers fired with coal, gas, biomass (if required, depending on the composition of the biomass).

Our SNCR and SCR denitrification systems provide the flexibility to stay compliant for the long term.