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Flue gas recirculation

NOx emissions – basics

NOx, and therefore nitrogen oxides, are a component of emissions that harm the environment and are therefore subject to legal regulation.


Chemical formation process
Nitrogen oxides is the collective term for gaseous oxides of nitrogen (nitric oxide NO and nitrogen dioxide NO2). Due to the many oxidation states of nitrogen, quite a few compounds of nitrogen and oxygen can form. These are abbreviated as NOx. The chemical formation process is illustrated in the figure below:

The chemical formation process of NOx

The chemical formation process of NOx

The unit in which NOx is expressed is mg/Nm3. It is important to be familiar with the defined reference value. The reference value or the defined reference quantity for the specified NOx value in milligrams per standard cubic metre is generally 3 % oxygen.


Influencing factors on NOx formation
The following factors influence the formation of NOx.

  • Fuel
    • Fuel moisture
    • Nitrogen content
    • Oxygen content
  • Flame tube geometry
    • Length
    • Diameter
    • Design (undulating, smooth)
  • Combustion process
    • Burner system
    • Air quantity/excess air
  • Temperature
    • Flame core temperature
    • Combustion air temperature
    • Boiler water temperature
  • Time
    • Dwell time in the combustion zone

Types of NOx

Thermal NOx
Thermal NOx is expected to form at combustion temperatures of around 1,000 °C or higher, with a sharp increase observed from 1,400 °C onwards. The formation rate increases exponentially with the temperature. The thermal NOx emission formation occurs in the hottest part of the flame (flame core). The formation of thermal NOx can be predicted using the Zeldovich mechanism. The reaction rates are low at low temperatures, but increase exponentially.

Fuel NOx
The source of this NOx type is the bound nitrogen content of the fuel and is therefore fuel-dependent. These occur from temperatures around approximately 800 °C, predominantly in flame fronts of combustion systems.

Prompt NOx
Instead of producing a conversion to N2, the reaction of hydrocarbon radicals (CHn) with N2 may lead to the formation of NOx again. Prompt NOx occurs in very rapid formation reactions, in comparatively low amounts and, by comparison with thermal NOx, has practically no relationship with temperature.

The key finding is this: The hotter the flame, the higher the NOx content.

Regulations
There is no uniform global standardisation regarding NOx limits. Regulations may therefore vary at national, regional or even local level. It is important to comply with the legislation and standards that specifically apply to the installation site.

NOx emissions – approaches to optimisation

The relationship between excess air and NOx emissions

The air ratio is the mass ratio of air to fuel in combustion. Specifically, this indicates the ratio of available air mass to the air mass required as a minimum for complete combustion. An air ratio of λ = 1.2 means that 20 % more air is involved in the combustion process than would be required for complete combustion. This means that there is 20 % excess air. This implies the underlying connection. An increasing excess of air therefore has a positive effect on NOx emissions. At the same time, however, an increasing excess air has a negative impact on efficiency.

NOx emissions as a function of the air-fuel ratio λ

NOx emissions as a function of the air-fuel ratio λ


Relationship between flame temperature and NOx emissions

The figure “NOx emissions as a function of the flame temperature” shows that the proportion of NOx emissions increases as the flame temperature rises. This finding is relevant for alternative fuels. For example, the flame temperatures of hydrogen, which can reach up to 2000 °C, are significantly higher than those of natural gas.

There are various technical options available for meeting the NOx emissions targets that have been set. The following section examines internal and external flue gas recirculation, as well as surface burners.

NOx emissions as a function of the flame temperature

NOx emissions as a function of the flame temperature

Internal flue gas recirculation

The aim of the internal flue gas recirculation is to reduce NOx emissions. This is achieved by lowering the core flame temperature, thereby reducing the formation of thermal NOx. To achieve this, flue gases are directed back from the combustion chamber to the burner mixing unit and into the flame. In the hot reaction zone during the combustion process, internal flue gas recirculation reduces the oxygen concentration and the supply of oxygen. In addition, the flow speed is increased, which reduces the dwell time in the hot combustion zone. In terms of the factors involved, internal flue gas recirculation works in much the same way as external flue gas recirculation, although it is less efficient.

External flue gas recirculation

External flue gas recirculation also aims to reduce NOx emissions during combustion. It works on the same principle as internal flue gas recirculation, but is more efficient.

External flue gas recirculation from the front reversing chamber
Here, a portion of the flue gases is extracted from the boiler after the first flue tubing and mixed with the combustion air. The flue gas mass flow rate for flue gas recirculation is set via a flue gas control valve during commissioning. The flue gas recirculation system is designed for a maximum flue gas temperature of 550 °C. The flue gas control valve can be operated manually or by motor.

External flue gas recirculation from the flue system
The flue gases, which are extracted at the end of the boiler for recirculation, should remain within a temperature range of at least 95 °C (to prevent flue gas condensate in the recirculation pipe) and a maximum of 300 °C across the entire load range of the boiler. Depending on the system configuration, the flue gases must be extracted either from the flue gas line or from the flue gas collection chamber. If the flue gas temperature in the recirculation pipe falls below the dew point, care must be taken to ensure that corrosion-resistant materials are used. In addition, drainage and insulation should generally be provided within the pipe. The flue gas control valve can be operated manually or by motor.

Surface burner

A surface burner operates on the principle of premixing fuel and air to enable clean and efficient combustion directly on the surface of a porous material.

The surface burner has separate gas and air supply lines. The micro-mesh made of a metal alloy ensures a uniform flame pattern and prevents a single large flame from forming. Controlled premixing and surface combustion result in lower thermal stress, which leads to lower levels of nitrogen oxides (NOₓ) in the combustion process.

Information about flue gas temperature or flue gas loss

Surface burner (Image: Weishaupt)

Surface burner (Image: Weishaupt)