Experimental Studies on Reburning of Biomasses for Reducing NOx in a Drop Tube Furnace

Processes to reduce nitric oxide (NO) by reburning were studied in a drop tube furnace (DTF). The NO removal efficiencies of two biofuels, corn straw (CS) and rice husk (RH), and bituminous pulverized coal (BF) were compared. The influences of fuel type, stoichiometric ratio (SR), and reburning distance on efficiency of removing NO were studied by experiments. Flue gas analysis was per-formed to measure O-2, CO, CxHy, HCN, and NH3. Conversion ratios of C, N, and H in fuels were also measured and analyzed to understand the reburning capacity of the above fuels. Experimental results indicate that the biofuels corn straw and rice husk have a better ability to reduce NO compared with tested BF, which is mainly caused by the higher content of volatiles and high levels of CH4 concentration in gas produced by rapid pyrolysis. The high specific surface area and pore volume of biofuels also benefit the NO heterogeneous reducing rate. A stronger reducing atmosphere is established by biomass fuels, which is characterized by high CO and CxHy concentration and low 02 concentration. Under this atmosphere NO predominantly reacts with a large amount of reducing species not O-2 and converts to N-2. Highly efficient NO reduction was obtained with a high carbon conversion ratio in the reburning zone for biofuels at a reburning ratio of 15%-20% and SR of 0.8-0.9. For tested fuels, a relative higher NH3 concentration was observed at low SR compared with HCN. The mean ratio of NH3/HCN was roughly 1.6-2.5 at SR = 0.8 and rebuerning ratio = 20%. Total fixed nitrogen (sum of NO, NH3, and HCN) concentration dropped fast in the middle of the reaction zone for a high reduction rate of NO. Conversion ratio measurements of nitrogen and carbon indicated that N and C were released from reburning biofuels synchronously and their releasing paths are mainly via volatiles pyrolysis. The conversion ratios of C and N for biomass fuels were higher than for pulverized coal during reburning. Conversion ratios of H reached above 90% in the initial stage of reburning. Fast releasing hydrogen from fuels mainly produces H-2, H radicals, and other hydrogen-contained radicals, which helps to establish strong reducing atmosphere, and H containing radials are active reactants in NO reduction reactions.