External selective catalytic reduction of nitrogen oxides is showing promise to efficiently reduce NOx in fired facilities such as industrial boilers and process heaters. Paul Kuten and William Harkins of Atkins Americas and Doug Sullivan of Colt Technologies describe a unique design for an emission control system intended for reducing NOx emissions from existing fired equipment.
NOx in the combustion process can come from two sources. Nitrogen from atmospheric air can form nitrogen oxides at elevated temperature. This product is referred to as “thermal NOx.” NOx may also form upon the combustion of organically bound nitrogen components found in the fuel and is termed “fuel NOx.” The addition of a second catalyst for the simultaneous reduction of VOCs and CO is also possible.
Most common designs for NOx control are based on the reduction of thermal NOx. For example, light oil with zero content of organically bound nitrogen fired in a boiler produces NOx emissions of between 100 and 132 ppm. Through air staging and flue gas recirculation, thermal NOx may be reduced by approximately 30 percent to a new NOx emission level of between 70 and 106 ppm. In areas where these levels are not acceptable, and for equipment fired with fuels with organically bound nitrogen components, these techniques will not be entirely effective and installation of a NOx catalyst is required to meet regulations. Since the optimum operating temperature for both the NOx and CO/VOC catalyst is between 650 F and 800 F, both catalysts may be installed in the same housing. The installation of conventional SCR systems within an existing boiler or heater may in many cases be expensive and require long shutdown periods.
External SCR technology solves the long downtime problem by installing a NOx catalyst system as an add-on system outside the boiler or heater. The external SCR can reduce total NOx emission by 80-95 percent and can be designed to minimize cost and downtime for the user. The important component of the system is ceramic media-based heat recovery beds that are capable of recovering 85-95 percent of the heat required to raise the temperature of the gas to the activation temperature of the catalysts. Such a device could be shop-fabricated, thereby minimizing downtime, and should enable candidate boilers to achieve low NOx levels without converting to natural gas fuel. An opportunity to market NOx offsets is also available.