Optimizing Experiments And Mechanism Study On Advanced Reburning For NO_x Reduction

Coal resource is comparative rich in China, and coal plays an important role in energy consumption. Electricity generated by coal-fired power plants keeps the dominant in the electricity frame; coal burned by thermal power plants occupies half total amount of raw coal consumption approximately. NO_x emission from coal-fired boilers is one of main substances polluted atmosphere and environment. In recent 10 years, total amount of NO_x emission from coal-fired plant increases year by year, pollution becomes severe gradually. Coal burned by thermal power plant covers many types, quality of which is diversified. The generating unit capacity of coal-fired plant is huge and types of boilers are various. The regulation of NOx emission control is diverse for different plants built in different epochs. The NO_x removal efficiency of a single low NO_x techonology is rather low, which can not satisfy the regulation of NO_x control for many plants; the investment and operating cost of post-combustion NO_x control technology is too expensive; effect of ash on the SCR catalyst and additional SO₃ emission caused by catalyst bring many unthinkable risk and challenges. Presently, our country has few mature NO_x control technologies with self-dominated intellectual property right, facing the urgent post-combustion NO_x control for coal-fired plant. It is required to develop the low cost and high efficiency NO_x control technology. All kinds of NO_x reduction technologies have been investigated widely in our country. Advanced reburning integrated reburning with selective non-catalytic reduction (SNCR) has some advantages of low cost and high efficiency, which may become one of the optional NO_x control technologies adapted to the situation of coal-fired plants.The NO_x reduction characteristics of advanced reburning have been systemically studied by theory analysis, chemical kinetics modeling and many experiments.(1) NO_x reduction characteristics of methane reburning were studied by detail chemical kinetics modeling, in the same way reburning characteristics of C₂, C₃, petroleum gas, volatile of typical bituminous coal, gas from biomass pyrolysis and natural gas are researched. Reactions subsystem of sodium carbonate (Na₂CO₃) and iron pentacarbonyl [Fe(CO)₅] were collected and founded, the effect of Na₂CO₃ and Fe(CO)₅ on the NOx reduction of methane reburning were modeling, and the promotion mechanism of additives was analyzed. It was found that the effect of additives is similar to the result of reducing the stoichiometric ratio of reburning. The chain transformations of sodium-containing species and iron-containing species consume the radical H, at the same time produce OH radical, which restrain the reaction H+02=0H+0, decreasing the concentrations of H, O, OH radicals. As a result, the reaction of CH_i and O is prohibited, and the reaction of CH_i and NO is inspired.(2) On the base of modeling performances of ammonia and urea for SNCR process, the effects of sodium carbonate (Na₂CO₃) and ethanol (C₂H₅OH) on the NO_x reduction were modeled, and the promotion mechanisms of additives were analyzed. The modeling results show that Na₂CO₃ can broaden the temperature window, lower the optimal temperature, and not change the reaction under high temperatures. Na₂CO₃ can reduce the production of CO and N₂O. Ethanol shifts the active temperature window towards lower temperatures, obviously enhances the NO_x reduction efficiency under lower temperatures, and decreases the efficiency under high temperatures. N₂O production increases with enhancing the amount of ethanol. Ethanol under lower temperatures can lead additional CO emission. The chain transformation of sodium-containing species and pyrolysis / oxidation of ethanol can increase the concentration of radicals, especially OH, further promoting the conversion of NH3 to NH2, so the performances of N-agents are improved.(3) The reburning NO_x reduction characteristics of three types of typical fuel gas were studied by experiments. Effects of operating parameters and additives on the NOx reduction were researched, which validated the modeling of reburning process. The experimental results show that the optimal temperature of liquefied petroleum gas(LPG) is 1273 K～1373 K, 100 K～150 K lower than that of natural gas(NG). The optimal stoichiometric ratio of LPG and NG reburning is 0.7～0.8. The NO_x removal efficiency of LPG and NG reburning is 73%⁷5%under typical operating conditions. The efficiencies of LPG and NG increase with enhancing the reburn heat input of gas. The optimal reburn ratio of compressed natural gas(CNG) is 20% because it contains some organic compound with amidogen. The NO_x removal efficiency of reburning can be enhanced by prolonging the reactive time under certain lower temperatures.Calcium acetate, sodium acetate and sodium carbonate can promote the NO_x reduction of gas reburning. The calcium acetate is effective for gas reburning than sodium acetate and sodium carbonate. The additives show selective promotion for temperatures, and the optimal temperature is about 1273 K. It was found that sodium chloride and iron sulfate have no effect on the NO_x reduction of reburning.(4) The combustion property and NO_x reduction characteristics of four types of biomass were studied by experiments. The optimum operating parameters for biomass reburning were researched and found, i.e. the reburning temperature is 1223 K～1323 K, the reburning ratio is 15%, the stoichiometric ratio is 0.6～0.8, and the resident time is about 1 s. The NO_x removal efficiencies of cornstalk, straw, peanut shell and poplar under typical operating conditions are 67.4%, 62.3%, 56.4% and 50.9%, respectively. The higher initial NO_x concentration is, the bigger the efficiency is. The relativity of reburning characteristics and combustion property shows that if the ignition temperature of one type of biomass is lower, its combustion velocity is bigger under comparative lower temperature, and its volatile combustion give out much more heat, then the reburning temperature of this biomass is lower, owning good reduction performance; on the contrary, the reburning temperature of the biomass is higher, and it presents inferior reduction performance.(5) Based on the study on NO_x reduction characteristics of SNCR by experiments, the effects of operating conditions and four types of additives on the performances of ammonia and urea were researched. The modeling of SNCR process is validated by the experimental results. The results suggest that the optimal temperatures of ammonia and urea are 1273 K, and the temperature windows of both are 1223K～1340K and 1240K～1330K, respectively. The optimal molar ratio of NH₃ to NO (NSR) is about 1.5, and optimum resident time is 1s～1.5s. The efficiency decreases with lowering the initial NO_x concentration at the same NSR. The effect of oxygen concentration on the performance of urea is obvious and lower efficiency results from higher oxygen concentration. With increase of oxygen concentration, the performance of ammonia weakens gently.Sodium carbonate can extend the temperature window of SNCR, and shift it towards the lower temperatures. When the Na₂CO₃ concentration in flue gas is 200×10^-6, the active temperatures of ammonia and urea are 1200K～1373K and 1200K～1340K respectively. Organic additives including ethanol, glycerol, and methyl acetate can activate the NO_x reduction under lower temperatures, and decrease the efficiency under window temperatures. As concluded from modeling sodium carbonate can reduce the additional CO emission caused by injection of N-agents solution. And organic additives can bring additional CO emission diversely.(6) The effect of SO₂ on the NO_x reduction, and interaction of SO₂ with additives in the reburning and SNCR process are investigated. The experimental results indicate that the SO2 promotes gently the NO_x reduction of reburning, and weakens the promotion of additives in the reburning process. The presence of SO₂ reduces the efficiency of N-agents in SNCR process, and evidently restrains the promotion of additives including sodium carbonate, calcium acetate etc. Sodium carbonate, calcium acetate and sodium acetate not only promote the NO_x reduction but also capture the SO₂ in the reburning and SNCR process clearly.(7) The NO_x reduction characteristics of different gases and biomass advanced reburning were studied by experiments, including the effects of styles of AR, reburning ratio, temperature, additives and N-agent on the efficiency. From the experimental results, the efficiency of AR-lean is 82%～89%, the AR-lean promoted by sodium carbonate 83%～90%; the efficiency of AR-rich is 75%⁸0%, the AR-rich promoted by sodium acetate 77%～86%. The maximum efficiency of AR-lean+SNCR and AR-rich+SNCR is above 90%. The efficiency of typical biomass AR-lean achieves 60%～81%, the AR-lean promoted by sodium carbonate reaches 71%～85%, and that of AR-lean+SNCR is 81%～92%.(8) Based on the comprehensive experimental studies on the NO_x reduction of AR, the Synergistic reaction characteristics are analyzed and concluded. For AR-rich, the performance of ammonia is better than that of urea, the optimal injecting temperature of ammonia is 1200 K, and that of urea is 1273 K. The optimal effective temperature of additives is 1273 K. The efficiency is higher if the injection of N-agents and additives are delayed longer after injecting overfire air. The active temperatures of N-agents accord with the temperature window of SNCR process. The promotion of sodium carbonate in reburning agrees with that in SNCR process, but not as great as in the SNCR process. The promotion of additives in the thermal DeNOx process is better than that in NOxOUT process. Ethanol as additive weakens the performance of ammonia and urea, and leads additional CO emission diversely.(9) The efficiency distribution characteristics of AR process is summarized. The efficiency of basic reburning affects the performance of downstream N-agents noticeably, and the higher efficiency of basic reburning the lower NO reduction of SNCR. A higher efficiency is obtained by multiple injections of N-agents. Addition of sodium promoter does not significantly impact performance of N-agents in multiple injections process. The performance of ammonia injected in the overfire area is significant in comparison with that of urea. Synergistic effect exists between the reburning and the SNCR process. The higher efficiency of basic reburning results in the lower NO_x reduction of N-agents. The additives and ash of biomass exhibit a greater promotional effect on NO_x reduction of N-agents, so the AR process achieves up to 90% NO_x reduction.(10) A detailed reaction mechanism was developed to model the AR chemical processes. The SNCR, basic reburning and advanced reburning process under typical operating conditions were modeling adopting physical model of PFR and its series connection. The simulated and experimental results were analyzed. SNCR modeling may help optimize experimental study and practice operating parameters. The mixing of N-agents and main flue gas affects the NO_x reduction significantly. The active temperatures of NO_x reduction with N-agents solution are higher than the window temperatures obtained by chemical kinetics modeling and simulating-flue-gas experiments. Promotion of additives by modeling accords with that of experiments. Temperature and mixing of reburn fuel, overfire air and main flue gas have a great effect on the NO_x reduction of reburning. The practical reburning process including the intense mixing process and complex reactions can not be well modeled by the simple PFR model. The resident time, stoichiometric ratio and promotion of additives can be predicted well by the modeling excluding the influence of temperature and mixing. The modeling of typical AR process suggested that the efficiency can get more than 90% by optimizing Synergistic reactive conditions involved reburning, SNCR and additives. Sodium carbonate has a greater promotion for ammonia than urea, which agrees with experimental results.This study would provide theory foundation for optimizing advanced reburning process, and guidance for modeling and engineering application of advanced reburning.