A multi-functional, mixed fuel containing natural gas for NO reduction, and lignite ash for reducing the reburning intermediate, HCN, has demonstrated remarkably high efficiency in reburning (AIChE J., 47, 2781 (2001); Fuel, 85, 1781 (2006)). While the price of natural gas has increased and fluctuated significantly in the last five years, there is an incentive to find substitutes for natural gas. In this study, five waste materials, labeled A through E in our study, are selected as natural gas substitutes. Selection of HCN reducing agents is based on our critical review of literature data. The major experimental apparatus is a two-stage furnace in series that allows us to simulate reburning and burnout zones, independently or simultaneously. In two-stage tests, the reburning furnace was set at 1150 or 1250 °C and the burnout furnace at 1150 °C. The simulated flue gas had a composition of 16.8% CO2, 1.95% O2, and 0.05% NO in a helium base. It was found that HCN conversion to N2 by a newly identified material (labeled F) in the reburning zone is enhanced by the presence of water vapor. 6.35% water vapor was introduced into the reburning feed stream by passing helium through from the bottom of a water bottle maintained at a desired temperature. Results show that the combination of material A and material F forms an excellent substitute for natural gas. Material A can achieve up to 88% nominal NO reduction when 4000 ppm of material F is added into the reactor at a SR2 = 0.9 and SR3 = 1.2. This feed rate is equivalent to 185 metric tons of material F per day for a 172 MW bituminous coal-fired boiler, which is much more efficient than lignite ash. Material F does not cause known problems in boilers, such as fouling or slagging on the heat transfer surfaces. For the other fuels, char-N conversion to NO in the burnout stage is limiting the overall NO reduction efficiency. This reburning technology will meet the US Environmental Protection Agency's regulation of removing 85%, or up to 0.15 lb/MillionBtu, of NOX in boilers. This technology utilizes widely available industrial byproducts and its cost makes it competitive to the currently preferred technologies such as SCR. An invention disclosure is prepared for patent consideration.
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