Experimental Investigation of Nitrogen Species Distribution in Wood Combustion and Their Influence on NO_x Reduction by Combining Air Staging and Ammonia Injection

The formation of the nitrogen species HCN, NH₃ (N-intermediates), and NO out of fuel-bound nitrogen has a major influence on NO chemistry. Experiments have been carried out on an entrained flow reactor with pulverized wood as fuel. Staged combustion establishes a fuel-rich primary zone, where both N-intermediates and NO exist. The introduction of NRP as the ratio of the N-intermediates to NO offers a parameter that describes the nitrogen distribution in the primary zone, whereas TFN describes the overall amount of nitrogen. Air staging is an effective method for NO_x reduction; the main controlling parameter is the primary air ratio, which defines both NRP and TFN. In fuel-rich conditions, NRP exceeds 1; with increased oxygen availability and temperature, the N-intermediates are depleted and NO is formed (NRP < 1). Thus, the NRP can be increased by adding NH₃. Conventional SNCR is strongly temperature-dependent; hence, with increased temperatures, the best operation point shifts to lower air ratios. A combination of air staging and ammonia injection directly in the primary zone furthers NO_x reduction, as long as it is realized in almost stoichiometric conditions. Since the reduction efficiency increases at high temperatures, the technology is called selective high temperature reduction.