Biomass Advanced Reburning Denitrification Test And NO Reduction Chemical Kinetics Simulation

Nitrogen oxides (NOX) are one of the major pollutants emitted from coal-fired power plants. Fuel reburning has been considered as a very promising NOX emission control technology. According to whether adding reducing agents (aqueous ammonia, aqueous urea, etc) in the reburning process, fuel reburning is divided into basic reburing (BR) and advanced reburning (AR). Because of biomass behaving the characteristics of low nitrogen,sulfur content, high volatiles, relatively low ash content, high char activity, and zero net CO2emissions, the research of adopting biomass reburning denitration has been pay more attention from the researchers in recent years. In this paper, experiments of selective non-catalytic reduction (SNCR), biomass reburning and advanced reburning were conducted in an entrained flow reactor (EFR) with rice husk (RH), saw dust (SD), biomass char (BC) and biomass briquette (BB) as reburning fuels. The simulation of chemical dynamics of SNCR process using Chemkin software was also carried out. The main conclusions are presented as follows:1) The effects of reductant species, reaction temperature, residence time, ammonia nitrogen molar ratio (NSR), the flue gas composition, flyash and alkali metal additives on NO reduction during SNCR process were investigated. The results indicate that:NO removal efficiency of SNCR shows a tendency of increase first, decrease later with increasing the reaction temperature. Within the scope of the NSR=0.7-2.0, NO removal efficiency increases with the increasing of NSR, however the tendency of increase is stable gradually while NSR is larger than1.0. NO removal efficiency shows a tendency of increase first, decrease later and being stable gradually with increasing water vapor content (0-15%) in the simulated flue gas, and the best NO removal efficiency is obtained at4%water vapor content, which is73.0%. Alkali metal additives (sodium, potassium) have significant promoting effects on NO reduction of SNCR, whereas the tlyash of coal has a certain inhibitory effect on NO reduction during SNCR process. NO removal efficiency through SNCR can reach84.9%with the addition of Na2CO3(125μmol/mol). By the coupling effect of water vapor, CO, Na2CO3and fly ash, NO removal efficiency of SNCR still reaches72.1%. The above results suggest that water vapor and alkali metal additives can promote the homogeneous reduction reactions between ammonia reductant and NO during SNCR process.2) The effects of biomass species, reaction temperature in the reburning-zone (T2), stoichiometric ratio, residence time, the initial NO concentration, water vapor and alkali metal additives etc, on NO reduction during biomass reburning were investigated. The results indicated that:NO removal efficiency of biomass reburning increases with increasing the reaction temperature in the reburning-zone at T2=850-1150℃. NO removal efficiency shows a tendency of increase first, decrease later and being stable with increasing water vapor content, and the best NO removal efficiency is79.1%while water vapor content is about4%and temperature is about1100℃. Alkali metal additives have certain promoting effects on NO reduction, and the improvement of NaCl, Fe2O3on NO reduction is the most significant, following by NaOH and Na2CO3, Ca(OH)2and KC1get the worst NO removal efficiency. The concentration of additives (50-150μmol/mol) has no significant effect on NO reduction, and the coupling effect of water vapor and Fe2O3can promote the NO removal efficiency significantly.3) The effects of biomass species, reaction temperature in the reburning-zone, water vapor content in the simulated gas, the composition and concentration of additives etc, were mainly investigated during biomass advanced reburning, and the homogeneous and heterogeneous NO reduction mechanism during biomass advanced reburning were discussed. The results show that:NO removal efficiency of biomass advanced reburning presents a tendency of increase first and decrease later with increasing reaction temperature in the range of850～1150℃. When T2=1000℃, rich husk has the best NO reduction efficiency, followed by saw dust, biomass char and biomass briquette get the worst performance of NO reduction. NO removal efficiency shows a tendency of increase first, decrease later and being stable with increasing water vapor content, and moisture in the flue gas not only improves the NO removal efficiency, but also broadens the window temperature; the best NO removal efficiency is obtained while water vapor content in the simulated flue gas is about4%. Additives have better promoting effects on NO reduction, in which Fe2O3gets the most significant effect on NO reduction, followed by NaOH, NaCl and Na2CO3, KC1and Ca(OH)2get the worst effects on NO reduction. The concentration of additives has no significant effect on NO reduction when it is above50μmol/mol, whereas the coupling effect of water vapor and additives has the significant effect on NO removal efficiency during biomass advanced reburning. Under the coupling role of4%water vapor and100μmol/mol NaCl or Na2CO3, NO removel efficiency on biomass advanced reburning can reach about85.5%or84.4%, respectively, which increased by12.5and11.4percentage compared with no additives. When T2>900℃, the release rate of chlorine and potassium during biomass advanced reburning with rice husk can be more than95.0%and59.8%, respectively.4) Based on Chemkin software and chemical kinetics models of denitration, chemical dynamics simulation during the SNCR process was carried out. The results show that:under the different initial NO concentration, NO removal efficiency of SNCR shows the tendency of increase first, and being stable with increasing residence time. Longer retention time is needed for getting the maximal denitration efficiency with the greater initial NO concentration. NO removal efficiency increases with the increasing NSR, and the tendency of increase is stable gradually while NSR>1.5. At the same condition, the simulation results of the SNCR results agree well with the test results. The results of sensitivity analysis and rate of production analysis on NO removal efficiency show that:On one hand, the reactions of RI-25and RI-26have the largest sensitivity coefficient, which means that the NH2and NNH groups have greater influence on the NO reduction; On the other hand, the reactions of RI-25, RI-26and RI-56have the negative value of production coefficient, which means that the reactions of RI-25, RI-26and RI-56have greater contribution on NO reduction.