New approach into NOx removal from flue gas by carbohydrazide

Selective non-catalytic reduction (SNCR) is a proven technique to reduce nitrogen oxide (NO_x) emissions from stationary sources. The process utilizes NH₃ or urea as a reducing agent, which is only effective in a narrow and high temperature range with the maximum NO_x abatement falling with the increase of the O₂ content. In large industrial boilers, the effectiveness of the deNO_x process may be decreased with boiler load variations and excess O₂. This study investigated carbohydrazide as a reagent to improve SNCR performance. NO reduction by carbohydrazide was experimentally investigated in a pilot-scale flow reactor. The effect of reaction temperature, molar ratio of NH₂ to NO (normalized stoichiometric ratio, NSR) on the NO reduction was determined. Moreover, the promotion effect of carbohydrazide on the reactivity of NH₃ and urea was also investigated. The experimental results indicated that NO reduction by carbohydrazide occurred in a relatively low temperature range of 650–850 °C, and the optimum reaction temperature was about 730 °C at NSR of 2.0, the O₂ content of 11.5%–16.6% and the initial NO_x concentration of 635 mg/m³. After adding a small amount of carbohydrazide to an NH₃ or urea solution, the data showed that carbohydrazide could shift the temperature window of NH₃ to the left, but had no positive effect on urea deNO_x and the NO conversion efficiencies of NH₃ deNO_x. Consequently, carbohydrazide is a potential reducing agent for NO_x, but was not adapted to improve the deNO_x performance with NH₃ and urea.