Part of the answer to NOx control lies in the selection of the fuel source. I must say that we are talking about commercial power generation systems here. There are two NOx control systems currently in use — one is catalytic, and the other is liquid.

The liquid source relies principally on urea solutions to react with the NOx from the combustion process, and reduces it to nitrogen gas and ammonia. The catalytic sources rely on coated ceramics — and I'm guessing here — where the coating is a Vanadium or zinc, or other coating on the ceramic. The ceramic is used in a packed bed. However, this is exactly where the problem comes in. If you have a coal-fired boiler — you have particulates. Particulates tend to blind a packed bed very quickly, resulting in maintenance headaches and frequent changeouts.

The ceramics like to operate at temperatures well above the normal operating range for Nomex fibers, which are most commonly used in baghouses. Nomex will operate at temperatures up to 550°F, but 450°F is much better. The catalysts like higher temperatures of between 700°F and 1,000°F. If you have a coal-fired boiler, you may have to cool the exhaust stack by injecting water or dumping in a lot of dilution air in order to clean the particulates from the gas stream and then reheat the air in order to get the gas to the effective range for the catalyst to clean the NOx.

Under those circumstances it appears that the urea system may be a better choice. If however, you have a clean — at least particulate-free gas stream — the catalytic system may be an excellent choice. I also want to advise that there are low NOx burners that are available and, due to the way they atomize the fuel, seem to generate less NOx than other commercially available burners.