Thermal NOx accounts for the majority of NOx formed (typically in the range of 80 to 95%) when firing gaseous fuels. The majority of NOx reduction control technologies are directed toward lowering thermal NOx. The use of steam injection to reduce peak flame temperatures has been used for years; however, the drawback to this process is the cost to produce the steam, not being able to use it in the plant and the effect on the boiler’s overall efficiency.

It has been found that alternate methods of thermal NOx control, such as staged combustion and flue gas recirculation, could provide the required NOx reductions with a much lower capital and operating cost impact. Cleanup technologies such as Selective Non Catalytic Reduction (SNCR) systems can lower NOx by 40-45%; however in cases where NOx reduction has to exceed 90%, these techniques alone are not always adequate. The use of flue gas cleanup technologies, such as Selective Catalytic Reduction (SCR), has demonstrated the ability to achieve NOx reductions in excess of 90%. The drawbacks to these systems are the high capital cost of implementing them and their continued operational costs.

Moss designs NOx emissions reduction technology on solid fuel boilers through the use of a combination of combustion techniques such as flue gas recirculation (FGR), fuel-air staging and/or steam injection into our thermal reactor (overfire air system) providing a method to achieve lower NOx emission levels without the cost and complexity of flue gas cleanup systems.

Flue gas recirculation mixes flue gases, generally from the discharge of the heat exchanger device (i.e. boiler or dryer) with combustion air for the Moss thermal reactor. This modified mixture of air has lower oxygen and higher moisture concentrations than ambient air. With such a mixture, the flame temperature is reduced, which causes a decrease in the level of (thermal) NOx produced. The Moss thermal reactor low excess air ignition system provides turbulent multi-stage mixing of syngas and combustion air; therefore, allowing the implementation of these techniques without runaway CO emissions. In severe cases where these techniques will not lower the emissions to acceptable regulatory levels, SNCR and SCR equipment will be required for the application.