Incineration units for wastes containing fixed nitrogen
The chemical NOX (also called “fuel-NOX”) is decisive for the formation of NOX during the combustion of wastes containing organic nitrogen or ammonia. Depending on type and concentration of the nitrogen compound, up to 85% of the fixed nitrogen will form NOX during a single-stage combustion process. The selection of the appropriate technology for the NOX treatment is depending on the nitrogen concentration in the wastes and on the plant setup downstream the combustion stage.
In case of lower nitrogen content in the waste streams and no dioxin formation out of the combustion of chlorine containing wastes is expected, a SNCR (selective non catalytic reduction) process can be used. After the combustion step where the organic parts are oxidised at high temperature, the flue gas will be cooled to a defined temperature. This cooling stage can be either part of the radiation section of a steam boiler or a separate cooling/reaction chamber. Ammonia will be injected into the flue gas and reacts with the NOX to N2 and H2O.
If the waste streams contain higher concentration of bound nitrogen, the efficiency of a SNCR system is too low for meeting the emission regulations. In this case, the formation of chemical NOX can be minimised by two-stage combustion. With sufficient residence time and reducing conditions in the first combustion step (lack of oxygen), the chemical nitrogen in the waste is converted into molecular N2. In the second, the oxidising step, the reducing components (CO and H2) are incinerated with secondary combustion air. The heat of combustion can be utilised with high efficiency for the generation of steam or for heating of thermal oil or water.
The third option to treat NOX from the combustion of nitrogen containing wastes is the SCR (selective catalytic reduction) process. This technology is typically used if the resulting NOX content in the flue gas is not too high and if it also contains dioxins from chlorine combustions. At a temperature level between 200°C – 300°C, ammonia is injected into the fluegas stream. On the catalyst surface, the NOX is reduced to N2 while H2O is formed. The catalyst used for the NOX reduction is also able to destroy dioxins which are typically formed if a chlorine containing flue gas passes a steam boiler.
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