Division 1.7.1.2 NOx
United Kingdom
| Technique | Description |
|---|---|
| Reduction of air/fuel ratio | The technique is mainly based on the following features:
|
| Optimised combustion and combustion control | Based on permanent monitoring of appropriate combustion parameters (e.g. O2, CO content, fuel/air ratio, un-burnt components), this technique uses control technology for achieving the best combustion conditions. NOx formation and emissions can be decreased by adjusting the running parameters, the air distribution, excess oxygen, flame shaping and the temperature profile. |
| Staged incineration | Staged incineration is based on the use of two burning zones, with controlled air ratios and temperatures in a first chamber. The first burning zone operates at sub-stoichiometric conditions to convert ammonia compounds into elementary nitrogen at high temperature. In the second zone, additional air feed completes combustion at a lower temperature. After the two-stage incineration, the flue-gas flows to a second chamber to recover the heat from the gases, producing steam to the process. |
| Fuel selection/low-N fuel | The use of fuels with a low nitrogen content reduces the amount of NOx emissions from the oxidation of nitrogen contained in the fuel during combustion. The combustion of CNCG or biomass-based fuels increases NOx emissions compared to oil and natural gas, as CNCG and all wood-derived fuels contain more nitrogen than oil and natural gas. Due to higher combustion temperatures, gas firing leads to higher NOx levels than oil firing. |
| Low-NOx burner | Low-NOx burners are based on the principles of reducing peak flame temperatures, delaying but completing the combustion and increasing the heat transfer (increased emissivity of the flame). It may be associated with a modified design of the furnace combustion chamber. |
| Staged injection of spent liquor | The injection of spent sulphite liquor into the boiler at various vertically staged levels prevents the formation of NOx, and provides for complete combustion. |
| Selective non-catalytic reduction (SNCR) | The technique is based on the reduction of NOx to nitrogen by reaction with ammonia or urea at a high temperature. Ammonia water (up to 25 % NH3), ammonia precursor compounds or urea solution is injected into the combustion gas to reduce NO to N2. The reaction has an optimum effect in a temperature window of about 830 °C to 1 050 °C, and sufficient retention time must be provided for the injected agents to react with NO. Dosing rates of ammonia or urea have to be controlled to keep NH3 slip at low levels. |