Primary techniques are applied to reduce emissions at source, unlike secondary techniques, which treat pollutants after they are released into flue gases.

Nitrogen oxides are formed during combustion. Three distinct mechanisms are involved, known as “thermal NO”, “prompt NO” and “fuel NO”.

Primary techniques for reducing NOx emissions are designed to inhibit these mechanisms.

 

Low excess air combustion

Low excess air (LEA) combustion is a simple and low-cost method for reducing NOx emissions. The principle is to reduce the amount of available oxygen in order to reduce oxidation of the nitrogen in combustion air. Reducing the amount of O2 to the theoretical (stochiometric) minimum required for complete combustion significantly reduces oxidation of the fuel’s nitrogen content and, to a lesser extent, the formation of thermal NO.

This method does not require any additional energy input, but it does require an efficient control and regulation system. This is because controls must be finely tuned to ensure that combustion is complete and will not release any unburned compounds.

 

Staged air combustion

Staged air injection during combustion creates two distinct combustion zones, one with low oxygen concentrations and therefore excess fuel, the other with excess oxygen. The first (substoichiometric) zone produces reducing conditions (characterised by relatively high concentrations of flue gases, suspended particulate matter and sulphur dioxide). The combustion process is completed in the second zone.

Staged air combustion is achieved in boilers or multiple-burner shaft furnaces by fitting pressure-jet burners, burners or by injecting more air.

The main drawbacks of this method are:

    • formation of large carbon monoxide concentrations if secondary air intakes are not correctly positioned;
    • increased quantities of unburned carbon when the technique is implemented in existing boilers, as doing so reduces available space between the combustion zone and the heat exchanger.

 

Flue gas recirculation

Flue gases are recirculated by reinjecting them into the combustion chamber, either with combustion air or with the fuel. Thus reinjected and diluted, the flue gases reduce the flame temperature and oxygen concentrations, which in turn reduces the formation of thermal and prompt NOx. 20% to 30% of flue gases are reinjected, by either internal or external recirculation.

 

Staged fuel combustion

Also known as reburning, the principle here is to create different combustion zones to reduce NOx that have already formed.

Three combustion zones are created: a primary zone, a secondary zone and a post-combustion zone.

80 to 85% of the fuel is injected into the primary combustion zone. Combustion takes place under classic conditions of excess air, which causes thermal NOx to form.

The remaining fuel (15-20%) is injected into the secondary combustion or “reburning” zone with the flue gases released from the primary combustion zone. This reducing atmosphere reduces the NOx formed previously, as the hydrocarbon radicals (CHi) produced in this atmosphere reduce thermal NOx into N2.

The post-combustion zone is where air is injected to complete the combustion process and oxidise unburned matter from the reburning zone.

The efficiency of the system depends on the following parameters:

  • the temperature in the reburning zone;
  • the stoichometry of the combustion taking place;
  • the type of fuel used;
  • how long the fuel remains in the combustion zone;
  • the excess air in the primary combustion zone.

 

Low-NOx burners

Low-NOx burners differ from conventional burners in the fuel/air mix and injection system. They operate on three principles that reduce NOx formation:

  • Delaying the air/fuel mix,
  • Reducing the flame temperature,
  • Reducing the O2 content.

There are three types of low-NOx burners, all based on the techniques described above:

  • staged air injection burners,
  • burners recirculating flue gases,
  • staged fuel injection burners.

The design of most new generation low-NOx burners combines all three NOx reduction techniques.

They can achieve significant reductions in NOx emissions, ranging from 25 to 60% compared to conventional burners. Their performance mainly depends on the type of fuel used. Reductions of 25 to 40% can be expected with liquid fuel, and up to 60% in gas-fired boilers. Performance also varies between newly installed and existing facilities.