Experiments And Mechanism Study Of Advanced Reburning And Selective Non-catalytic Reduction On NO Removal

As the largest coal producer and consumer in the world, electricity generated by coal-fired power plants keeps the dominant in China. Coal consumption by power plants accounts for nearly 50% of the total. The combustion of coal releases abundance of poisonous gas. In recent 10 years, total amount of NO_x emission from coal-fired plant increases year by year, pollution becomes severe gradually. The acid rain pollution of China is converting to mixed type of sulfuric acid and nitric acid from the past sulfur-based acid rain. Although some low-NO_x technology such as low-NO_x burners, over fire air has been used to control the emission of NO_x, the limited ability of NO removal is difficult to meet the increasingly stringent environmental standards. At present, our country owns few mature NO_x control technologies with self-dominated intellectual property rights. Facing the urgent task of NO_x emission control of coal-fired plant, it is required to develop the low cost and high efficiency NO_x control technology. Advanced reburning(AR) and selective non-catalytic reduction(SNCR) technology gain some advantages of low cost, high efficiency, easy to be accepted by old type boiler which may become dominating NO_x control technologies of coal-fired plants.In this paper, a multi-purpose equipment and a simulation atmosphere reactor were established to study the effect of basic conditions parameters on NO removal of SNCR, focusing on performances of the 5 kinds of sodium additives, 3 kinds of oxygen-containing organic compounds and 4 kinds of gas additives on SNCR process. A mechanism model is set up to simulate the SNCR process which considers not only ammonia and urea reaction but also additives reaction. The results of model simulation and experiment are compared to verify the rationality of mechanism model. The basic reburning process of 3 kinds of gas fuel, 4 kinds of biomass, 3 kinds of pulverized coal and the mixture of biomass and coal were studied on multi-purpose equipment. The denitrification characteristics are compared for different reburn fuel. The performances of N-agents and additives co-reburning process were studied meticulously. The features of basic reburning, advanced reburning and second generation advanced reburning were analyzed to optimize the advanced reburning technology. Based on GRI-Mech 3.0 mechanism, NO_x reduction mechanism by ammonia and pyrolysis and hydrolysis reaction of urea is remedied and also the effect of HCO, HNCO and NCO. Besides, additive mechanism of sodium carbonate and iron pentacarbonyl are also taken into consideration. All of the research aims to optimize experimental and modeling study of advanced reburning and SNCR process.(1) The NO reduction performances of SNCR process have been systemically studied by theory analysis, chemical kinetics modeling and experiments. The NO removal characteristics of SNCR process are clearly summarized. The SNCR process with ammonia as agent achieves maximum NO removal efficiency about 89.2% at 1000℃accompanying the temperature windows within 950℃to 1067℃. While, the top efficiency of urea is 1000℃and corresponding maximum efficiency about 90.1% with temperature windows for 967℃to 1057℃. The NO removal efficiency of ammonia and urea is similar at optimum reaction temperature. However, below the reaction temperature windows, the efficiency of urea is obviously lower than that of ammonia for the decomposition limitation of urea at low temperature. The experiments and modeling simulation both verify the best NSR of ammonia and urea is 1.5. The modeling simulation achieves a little short time for reaction balance than that of experiments. The efficiency becomes lower with increasing the oxygen concentration of flue gas.(2) The NO removal efficiency of SNCR is improved drastically at 700℃to 900℃with sodium carbonate additive. Sodium carbonate is decomposed instantly since it is injected into furnace and will be translated into the steady phase sodium hydroxide under the effect of moisture. Based on the sensitive analysis of sodium compound on NO reduction, it is found that sodium hydroxide goes through chain reactions via NaOH→NaO₂→Na→NaO→NaOH, the result of net reaction equal to convert H₂O and HO₂ to active OH. The increasing of active OH excitated the NO reduction at low temperature. Within the optimal temperature zone, concentration of active OH generated by sodium carbonate is small compared to N-H-O system self, so the promotion of sodium carbonate is feeble at high temperature. 5 kinds of sodimu compounds additives were chosen to investigate the promotion of additives on SNCR process at 900℃. It is observed that all of the sodium compounds promote the SNCR process. Among of additives, sodium hydroxide, sodium formate and sodium acetate present best performances and the efficiency is improved by 23%. The following is sodium carbonate which improves the efficiency byl9.6%. While sodium chloride acts the poorest result accompanying a 10.8% enhancement. There is a peak concentration value of 400×10^-6 for sodium carbonate, and exceeds this concentration a little prohibitive performance was observed. In addition, carbon monoxide concentration is reduced obviously by adding sodium additives mainly for the reactions: NaO+CO→Na+CO₂, NaO₂+CO→NaO+CO₂.(3) Investigating reaction mechanism of cooperating ethanol with SNCR process founds that active radicals like CH_i,OH are produced by the reaction of ethanol and oxygen. NO reduction is enhanced evidently from 750℃to 850℃and the optical temperature is removed from 1000℃to 850℃. However, the NO removal efficiency is compromised at high temperature. As a result, adding ethanol shifts the temperature window towards low temperature by 100℃, but the range of temperature window broadens hardly. NO reduction will be strengthened as the ethanol concentration increasing under 850℃, but it presents opposite when the reaction temperature exceeds 850℃. Ethanol, glycerol and methyl acetate were selected to investigate the effect of oxygen-containing organic additives on SNCR process. It is observed that all of the organic compounds could improve NO reduction evidently from 800℃to 900℃and then weak the process when reaction temperature exceeds 950℃. It is also found that carbon monoxide concentration of the flue gas is increased at the present of the additives especially when temperature lowers than 950℃.It is caused by the incomplete oxidation under low temperature . So more attention should be paid to the rationality of oxygen-containing organic compounds used as SNCR additive for generation of carbon monoxide generation. (4) Experiments were also conducted using carbon monoxide, natural gas(NG), liquefied petroleum gas(LPG) and sulfur dioxide as additives respectively to investigate the effect on SNCR process. Carbon monoxide could produce H, OH etc, via oxidation reaction at the present of vapor which removes the temperature window towards low temperature by 100℃. The optical temperature shifts from 1000℃to 850℃and 900℃respectively for ammonia and urea. However, carbon monoxide does not broaden the temperature windows and maximum NO removal efficiency changes feebly. The NO reduction characteristic depend on temperature of SNCR is changed as injecting NG and LPG into reaction atmosphere. The more content of gas fuel, the more independent of temperature of SNCR was observed by experiments. But carbon monoxide emission increases obviously when the reaction temperature is lower than 850℃and the mole ratio of fuel gas to nitrous monoxide exceeds 1.0. The present of sulfur dioxide in the flue gas shifts the temperature window towards higher temperature and reduces the maximum efficiency around the optical temperature. It is also found that the promotion of sodium carbonate and calcium acetic on SNCR process is weakened by the effect of sulfur dioxide.(5) Operating parameters influencing the NO reduction including reburn temperature, reburn stoichiometric(SR₂), reburn fuel fraction(Rff) and residence time, et al, were investigated by experimental system and the improved GRI-Mech 3.0 model. There exists best SR₂ value 0.8 of reburn zone for LPG and NG. Prolonging the residence time of reburn zone is favorable to for NO reduction, especially when the reburn temperature under 1100℃. The indispensable residence time of NG reburning is short which is about 0.68s and values of LPG and compressed NG should be exceed 1.0s for its abundant in hydrocarbon-macromolecules. The NO reduction performance of experimental gas fuel become accordantly by prolonging the residence time of reburn zone. The modeling simulation shows that heightening pressure favors in NO reduction and reducing the quantity of TFN(TFN=NO_x+HCN +NH₃) at exit of the reburning zone. The NO removal efficiency of gas fuel which contains unsaturated hydrocarbon is obviously higher than that of which mainly containing saturated hydrocarbon for chemical bond energy of the unsaturated hydrocarbon is low and easily to be ruptured.(6) Injecting ammonia, urea and HCN could promote the denitration process of reburning evidently. NO removal efficiency of advanced reburning is higher than basic reburning by 20%-38% at the representative reburning temperature from 1000℃to 1300℃. Consideration emissions of TFN and the economically of N-agents, ammonia and urea are more suited as N-agents of advanced reburning than hydrogen cyanide. The promotion of urea is lower than ammonia under 1100℃, because urea must be decomposed to NH₃ and HNCO which can react with NO directly, but the low temperature limit the decomposition. NO removal efficiency of urea is invariable when the reburn temperature exceeds 1100℃, but the promotion of ammonia falls. So ammonia should be selected as N-agents when the reburn temperature lower than 1100℃, otherwise urea would be chosen. Both of modeling simulation and experiments proved that the raise of NSR benefits the NO reduction process. Consideration of the TFN emission and NO reduction, the value of NSR about 1.5-2.0 is suggested.(7) The performance of sodium carbonate and pentacarbonyl iron on rebuning process is dominated by reburn temperature and the concentration of additives. The higher concentration of additives in the flue gas, the more prohibitive of NO reduction was observed at low temperature. The optimum reburning temperature moves to high temperature with the effect of additives. When the additives injected with N-agents together, sodium carbonate promotes advanced reburning process effectively at the whole reburn temperature especially after 1100℃. Pentacarbonyl iron restricts the NO reduction below 1100℃, but acts opposite contrarily when reburn temperature exceed 1100℃. The concentration of pentacarbonyl iron affects NO reduction feebly after 1150℃.(8) Ethanol is unfitted additive for advanced reburning for its imperfect NO reduction of ammonia and urea, and increases the carbon monoxide emission. Sulfur dioxide hardly affects the reburning process, but it can weaken the promotion of the additive for the reaction of sulfur dioxide with additive which generates steady sulfide. NO removal efficiency of advanced reburning combining with SNCR than advanced reburning by 10% and 5% respectively. Sodium carbonate hardly promotes the secondary ammonia injection.(9) The NO reduction characteristics of reburning and advanced reburning with biomass, pulverized coal and the mixture as reburn fuel were studied by experiments. The experimental results indicate that NO removal efficiency increases rapidly with the reburning temperature at 700℃-900℃and NO removal efficiency declines slightly after 900℃. The efficiency of pulverized coal reburning increases with the raise of temperature, but it is much lower than that of biomass at the same conditions. The mixture of coal and biomass in different ratio can promote the NO reduction of pulverized coal distinctly from 900℃to 1000℃. To achieve high denitration efficiency and large eligible reburning temperature range, the mixing ratio (λ) should be greater than 1.0. There is an optimum SR₂ value of 0.8 and 0.6-0.8 for biomass and the mixture. All experimental solid reburning fuel achieves optimal efficiency when the reburn fuel fraction is 20%.(10) Both ammonia and urea could promote the NO removal efficiency of biomass reburnng and the efficiency of AR-lean is higher than that of AR-rich. Adding sodium carbonate to reburning zone can urge active hydrogen atom and hydroxyl radical integrate to water and the net reaction is H+OH(?)H₂O. The present of sodium carbonate could weaken the reaction of hydroxide radical with hydrogen atom and hydroxyl radical, and raise the reaction probability with NO. As a result, NO removal efficiency is promoted. The stimulative mechanism of injecting sodium carbonate into burnout zone is adverse. It is suggested that the effect of sodium carbonate on NO reduction for can be explained by the ability of additive to increase hydroxyl radical by reverse reaction. Sodium carbonate co-injection with the reburning fuel is more effective than injecting it into the burnout zone. Because the additive promotes NO reduction both of reburn zone and burnout zone, and in the latter case the time of the additive evaporates and mixes with flue gas is relative short which is another reason.