If you are to have combustors that emit NOx reduction, you will need to have some modifications made to the design of the conventional combustion. Some of the changes may include the following:
- Different methods of wall cooling
- Fuel injection techniques that are more sophisticated
- Changes to the liner’s geometry
- Changes in how air is distributed
As you make these changes, you can leave the configuration and size of the combustor intact, which is a great advantage. The changes listed above are also within the boundaries of currently existing technology. The disadvantage you will face is that as you improve on the emissions, you may have to compromise on some other characteristics of the combustor’s performance conventional combustion.
Here are some approaches to look into when considering the lowering of NOx emissions:
- Improving the homogeneity of combustion – You want to improve the rate of mixing before going into combustion so as to reduce the temperature of the flame. When you combine this with a lean fuel or air ratio, you will find a significant NOx reduction. If you use an average air/fuel ration, then the NOx will increase instead.
- Re-circulating exhaust gas – Since exhaust gas is plentiful, cool it first and then re-introduce it into the primary zone. You may experience increased CO emissions as NOx reduces so you will need to keep an eye on that. Recirculating exhaust gas also requires an increase in the complexity, weight and size of the equipment used for combustion.
- The primary zone must be lean – This was commonly done in the past because lean mixtures would reduce NOx emissions as well as the size of the combustor. The downside is that as the flame temperature reduces, the UHC and CO emissions increase. This causes limited NOx reduction.
- Inject water – Temperature is key for NOx formation. Injecting water can therefore reduce NOx considerably. The challenge is usually in storing, pumping and handling the copious amounts of water needed.
- Primary zone that is rich – In this approach, you need excess air and fuel so as to reduce the temperature of the flame leading to lower NOx emissions.
- Decrease resident time – This is done by ensuring that the amount of time spent by the gases in high combustion temperatures is reduced. This in turn reduces the emission of NOx. Unfortunately, the effect on UHC emissions and CO is negative because the time needed for their proper oxidation is insufficient.
NOx reduction is important in lowering the levels of NOx being released into the atmosphere during conventional combustion. When a form of NOx reduction is used, it is important to have accurate measurements. We provide resident engineers to support installation & commissioning, operation & maintenance activities aiming to improve plant reliability, availability and maintainability.