The effect of reaction temperature, Normalized Stoichiometric Ratio (NSR; 0.5*[urea]/[NO]), gas additives (CO, $C_3H_8$), liquid additives ($CH_3OH$, $C_2H_5OH$, PEO), and the presence of $SO_2$ gas on the reduction of NOx with urea solution have been determined in a pilot scale combustor.
The maximum conversions of NOx reduction by urea and ammonia exhibit at 970℃ and 950℃, respectively. Conversion of NOx reduction increases with increasing NSR up to 2.0 and then remains constant. The optimum temperature and maximum conversion of NOx reduction decrease with increasing concentrations of alcohol additives ($CH_3OH$, $C_2H_5OH$). With an addition of polyethylene oxide (PEO; M.W. = 600,000) into urea solution, power consumption to carry urea solution can be reduced. With an addition of PEO to urea solution, the efficiency of NOx reduction increases at lower temperatures since the rate of radical formation is faster than that of radical termination reaction and the mixing effect is promoted by increasing penetration length of droplets from the nozzle.
Temperature window which covers the range of NOx reduction is widened and the optimum temperature decreases with an increase in the concentration of gas additives (CO, $C_3H_8$). The amounts of CO was varied from 1.0 to 4.0 of the relative molar ratio to NO and the molar ratio of urea to NO is 2.0. It has been found that CO additive provides wider temperature window for the optimum $NO$ reduction than that of urea solution only. The optimum temperature window shifted to lower reaction temperatures with increasing the molar ratio of CO/NO. Oxidation of CO in the presence of $H_2O$ promotes the supply of OH and O at lower temperatures. The availability of OH and O radicals may provide an environment of both NO reduction and $NH_2$ oxidation reactions to occur at lower temperatures. The presence of $SO_2$ gas in flue gas slightly reduces NOx reduction and ammonium salt is formed from the reaction between $SO_2$ and ammonia from the decomposition of urea.
Among the tested liquid and gas additives, the enhanced urea solution ${[CH_3OH]/[NO]=0.5+PEO=50ppm}$ with ${[CO]/[NO]=1.0}$ gas additive is found to be the most effective one for NO reduction with wider effective temperature window. Since the additives can be ignited at lower temperatures than urea solution only, urea is more readily converted to nitrogenous intermediates by radicals formed from the additives in combustion.
