| Low NO_x reburning combustion is suitable for energy structure dominated by coal in our country, since Low NO_x reburning is a cost-effective NO_x control technology. The range of reburning fuel can be widened by using biomass as the reburning fuel which is a reproducible energy. Biomass reburing which is a combination of biomass and coal combustion displaces the supply of fossil fuels and reduces the release of CO2 and SO2, while reduces the NO_x emission during the combustion, therefore, biomass reburning will be a promising combination combustion technology in future applications.A drop tube furnace which could well simulate biomass reburning was built in this study and a comprehensive investigation on NO_x reduction by biomass reburning was conducted.The pyrolysis characteristic of reburning fuel is the key to the effect of NO_x reducing. The release behavior of gas components of straw,rice husk,sawdust and bituminous coal was investigated in a drop tube deposition furnace at fast pyrolysis temperature of 900,1000,1100℃. The volatile, oxygen and H/C of biomass components were higher than that of bituminous coal, which determines a higher gas yield in the fast pyrolysis. The gas yield of sawdust was 92.06 %, that of straw was 86.66 %, that of rice husk was 78.33 % and that of coal was 25.91 %. The main pyrolysis gas component of coal is H2, while other components such as CO, CH4, CO2 are relatively less. While the main pyrolysis gas component of biomass is CO, and H2 is the second, which is opposite to the pyrolysis results of pulverized coal. The total amount of pyrolysis gas and the amount of CH4,CO and H2 of biomass is obviously higher that of pulverized coal, which is the reason of higher level NO_x reducing of biomass. Also, the pyrolysis kinetic model was built and the calculated kinetic parameters (activation energy and pre-exponent factor) of straw,rice husk,sawdust were the same and that of pulverized coal was relatively less.Absorption isotherm of nitrogen at low temperature was obtained by using auto-absorption analyzer (AUTOSORB-1-C, Quantachiome), and the specific surface area, pore volume and fractal dimension of char were calculated. With the increase of prolysis temperature, the specific surface area and pore volume of chars were all increased and the specific surface area and pore volume of biomass char were obviously higher than that of pulverized coal char. The fractal dimension of pulverized coal char is much lower than that of biomass char, which indicates that the porosity is lower and surface is smoother for pulverized coal char in comparison with biomass char. In a word, the higher specific area of biomass char supplies necessary contact surface for gas-solid reaction and increases more active sites on char surface during the reburning process, which creates good condition for NO_x reducing by heterogeneous reaction. The pore structure parameters and analysis results obtained by fast pyrolysis experiments of pulverized coal and biomass supply basic data of NO_x formation and reducing mechanism of biomass.The Nitric oxide (NO) reducing processes by biomass and pulverized coal reburning were studied in a drop tube furnace. The influences of fuel type, stoichiometric ratio (SR), reburning temperature, residence time and reburning ratio on efficiency of removing NO were studied by experiments, while burnout of reburning fuel and conversion of C, N, H were also analyzed. Under the same reburning condition, the NO reducing ability of biomass is obviously higher than that of pulverized coal. For example, when reburning ratio was 25% and SR was 0.7, the NO reduction efficiency of sawdust could reach 87.99%, while the reduction efficiency of pluverzied coal was only 5.82%. A great amount of reducing gas are released by biomass due to the higher volatile and H/C, which leads to a high NO reducing efficiency. The reduction efficiency of biomass is decreased by the increase of SR in reburning zone. Biomass can complete the reducing process in 0.6 s, which is close to reburning of nature gas. Biomass can obtain reasonable reducing efficiency when the reburing ratio is 15~20% and SR is 0.8~0.9. The combustion thermal loss can be kept in a reasonable range during the reburning of biomass. Through the analysis of the conversion of NH3 and HCN in the biomass reburning atmosphere, HCN is mostly released from the reaction between NO and CHi released by reburning fuel; the formation of NH3 is favored by reducing atmosphere and the reaction between NO and NH3 is the main reaction of NO reduction. The release of H element is the fastest. At the initial phase of reburning, H element of biomass and pulverized coal almost is released completely and C and N are released simultaneously. The consumed ratio of C and N are evidently higher than that of pulverized coal. The high consumed ratio of C and N in reburning zone of biomass firstly decreases the formation of NO from char-N in burnout zone and decreases the emission of NO, secondly the burnout condition in burnout zone is improved due to the less amount of chars. This study can provide theories and technical references for the application of the technology of biomass reburning.
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