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SNCR systems offer many benefits for NOX reduction. Their installation is relatively simple and effective for all boilers and fuels. However, continued operating costs due to ammonia consumption need to be considered. Additional controls and instrumentation are required to operate ammonia forwarding and injection systems. SNCR systems are not 100 percent effective at converting all ammonia injected into the furnace into nitrogen and water. This inefficiency is referred to as ammonia slip. Air permit modifications might be required to account for the additional slip being discharged into the atmosphere. Also, high ammonia slip can cause ammonia to be entrained in boiler flyash and affect its salability. High ammonia slip also can cause other harmful compounds, such as ammonia sulfate and ammonia bisulfate, to form in the flue gas stream. These compounds will result in air heater corrosion and can cause clogging of pulse jet fabric filters. Economic analyses should be performed to determine if urea or ammonia is more cost effective as the reagent stored for NOX
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Gas reburn
Natural gas reburning is another technique used to chemically reduce NOX that has already formed from combustion. Hydrocarbon fragments contained in the natural gas will contain free radicals that will reduce NOX to molecular nitrogen, while over-fire air will complete the combustion reaction.
One side effect to using natural gas reburning is the interim formation of hydrogen cyanide, although the final stages of combustion will tend to reduce this chemical.
Some of the benefits to a natural gas reburn system, are the simple boiler installation and effectiveness for all boiler styles and fuels. However, several potential negative impacts can result from the installation of this system. As much as 25 percent of the heat input to the boiler might come from the addition of natural gas. This results in significant increases to the operating cost to maintain steam loading. Due to the amount of high furnace combustion, a significant increase in furnace exit gas temperature will be encountered. This will result in metallurgical issues with superheater/reheater elements, higher attemperation rates, higher economizer outlet temperature, and can reduce the effectiveness of ESPs. Additional controls and instrumentations will be required to control the amount and distribution of gas into the furnace. Lastly, with the amount of natural gas piping required and the introduction of gas into the furnace for combustion, NFPA requirements will most likely require additional safety modifications to the boiler and surrounding areas.
Flue gas recirculation
Flue gas recirculation systems have previously been installed on pulverized coal boilers as a means of reducing NOX formation. The theory is that combustion neutral cooled flue gas will be introduced into the combustion zone to cool peak temperatures and reduce the generation of NOX. It also will create more boiler turbulence, resulting in a better air to fuel distribution. These systems might already be installed on an existing unit. If it is not installed, then additional foundations, support structures, auxiliary loading, controls & instrumentation, and duct runs would be required for installation. All these components also will require a large amount of maintenance, with most of it being dedicated to upkeep of the FGR fans. Unfortunately, FGR systems have proven in the past to not be an effective NOX reduction technique in pulverized coal boilers.
