Nitrogen Conversion Mechanism During Char Combustion And Develepment Of Low Nox Technology

Coal is the most important energy resource in China.50%of the total coal consumption is used for coal-fired power plant evergy year. During combustion a large part of nitrogen in coal evolves into nitrogen oxide, a well-known pollutant causing environmental problems. That investigation of nitrogen conversion mechanisms and development of new low NOx combustion technology is of great significance to NOx control in power plant and environmental improvement. Compared with homogeneous mechanisms of nitrogen conversion, heterogeneous mechanisms of nitrogen conversion during char combustion are less well understood. Heterogeneous formation and reduction mechanisms of NOx were investigated theoretically and experimentally in this work. A new low NOx combustion technology was developed and a pilot experiment was carried out in a2.11MW tangentially fired boiler.It is found by XPS that the nitrogen is retained in char primarily as pyridine. Reasonably simplified char models and nitrogen-containing char models with zigzag and armchair configuration were used for quantum chemistry simulation. Mechanisms of heterogeneous oxidation of char nitrogen to NO, heterogeneous reduction of NO on the surface of char and reaction between NO and nitrogen-containing char were studied on molecular level. NO was formed through heterogeneous oxidation of char nitrogen by O2. Chemisorption of O2on char is a strong exothermic reaction.430kJ-mol"1was released during NO desorption from oxidation of char nitrogen with zigzag configuration. There were two different pathways for NO desorption from oxidation of char nitrogen with armchair configuration, which were97.2kJ/mol and241kJ/mol exothermic.The non-selective oxidation of carbon and nitrogen took place during char combustion.Heterogeneous reduction of NO on the surface of char could take place through two different mechanisms. Mechanism1is by reaction between C(N) formed by pre-adsorbed NO molecule and another NO molecule in surrounding gas. Mechanism2is by combination of two neighboring C(N) active sites. The highest energy barrier in mechanism1was lower than that in mechanism2. Predicted rate constants of rate-limiting steps of mechanism1by transition state theory was much higher than that of mechanism2. Calculation results suggest that mechinism1 was more favorable than mechanism2. N2O was another product during NO reduction, which could decompose quickly on the surface of char.NO could react with pyridinic nitrogen incorporated in char structure. When N-N bond was formed with NO chemisorption on nitrogen-containing char in side-on mode, N2was released to gas phase through exothermic reactions. When N-O bond was formed, reactions similar to nitrogen exchange took place and NO was released. N2O desorption would take place if NO chemisorbed on nitrogen-containing char in N-down mode to form N-N bond. The highest energy barrier for N2O desorption was418kJ/mol.Mechanisms of NO formation and reduction during char combustion were studied on a horizontal tube furnace. The conversion rate of char nitrogen to NO increased with temperature if it is below1000℃; and decreased with increasing temperature when it is higher than1000℃. The lowest conversion rate was obtained when O2concentration was3%. The conversion rate increased with increasing coal rank. Heterogeneous reduction rate increased with increasing temperature, and decreased with increasing oxygen concentration. The smaller the pulverized char, the higher the reduction rate during char combustion.An experimental study of NO heterogeneous reduction on the surface of char was carried out on a temperature-programmed system. In the absence of O2, the reduction ability of char to NO was not found below600℃. The reduction rate increased with increasing temperature. CO and N2were the main products of reduction reaction.850-900℃was a suitable temperature range for CO2formation. HCN was detected during the reaction between NO and acitivate carbon. Addition of CO would not influence the initial reaction temperature. CO enhanced NO reduction rate on char surface and depressed the formation of HCN.Below fire air was proposed and the new developed technology based on combination of BFA with air staging or fuel staging was tested in a2.11MW tangentially fired boiler. Increase of OFA was benefit for NOx control, but it resulted in decrease of furnace exit gas temperature and increase of carbon content in fly ash. The use of BFA decreased temperature in lower furnace. There existed an optimized BFA ratio to obtain the lowest NOx emission concentration. Carbon content in fly ash decreased with increasing BFA ratio.