A Study On Dual Reduction Of SO₂ And No By Calcium Based Organic Compounds Through Experimental Investigation And Mechanism Analysis

Sulfur dioxide (SO2) and nitric oxide (NO), which are relased from coal fired power plants, have brought about serious environmental problems and it is urgent to explore economical technologies to reduce SO2 and NO simultaneously. Previous studies proved that the calcium based carboxylic materials could be used to abate these two pollutant gases together. However, the mechanisms for thermal decomposition, desulfurization and denitrification of calcium based organic compounds have not been well acknowledged.The calcium propionate (CP), modified calcium hydroxide by propionic acid (MCP) and modified calcium hydroxide and magnesium oxide by propionic acid (MCMP) are mainly mentioned in this study and their reaction characteristics and mechanisms on SO2 and NO dual reduction are investigated from experiments, kinetic calculations and mechanism analysis.Thermal decomposition characteristics are investigated through thermogravimetric (TG) and the kinetic parameters are calculated through model-free and model-fitting approach. Different from calcium carbonate (CC), there are two mass loss segments, which are the release of the organic gas and carbon dioxide (CO2), during the thermal decomposition process of CP, MCP and MCMP. Enriching oxygen concentration or reducing temperature heating rate, thermal events of the calcium based organic compounds are prompted towards lower temperature zone. At the same time, O2/CO2 atmosphere, which is compared with O2/N2 atmosphre, leads the CO2 release segment moving higher temperature zone. The released organic gas favors reburning for NO reduction and the solid residual product could capture SO2. Meanwhile, the scanning electron microscrope analysis shows that the grain diameter of CP and MCMP is obviously smaller than the one of CaCO3 after calcinations at 1173K. Also, the whole structure of these two calcium based organic compounds is more dispersed and the pole is more magnified.The values of apparent activation energy, which are calculated through model-free approach of Ozawa-Flynn-Wall method and Vyazovkin method, are close to each other. The values for MCP are 146-735kJ/mol and 138-761 kJ/mol and for MCMP are 370-474kJ/mol and 375-490kJ/mol. The reaction orders, calculated through Avrami theory, are 0.050-0.386 and 0.090-0.649, respectively, for MCP and MCMP. Based on Ozawa-Flynn-Wall method, values of apparent activation energy for CP and CC under O2/N2 atmosphere are 83-346kJ/mol and 193-202kJ/mol and their reaction orders are 0.061-0.608 and 1.647-2.084 from Avrami theory. Kinetic calculations through model-fitting approach of Coats-Redfern method show that reaction model of the fourth order of chemical reaction (C4) properly describes the reaction mechanism of the second mass loss segment for both CP and MCP under either O2/N2 or O2/CO2 atmosphere and for the third mass loss segment of CP and MCP, one-way transport of diffusion mechanism (D1) and Ginstling-Brounshtein equation of diffusion mechanism (D4) are fit for the O2/N2 atmosphere and O2/CO2 atmosphere, respectively. At the same time, the apparent activation energy values achieved under O2/CO2 atmosphere are much higher than the ones achieved under O2/N2 atmosphere.Performances of CP, MCP and MCMP on desulfurization during coal combustion are labeled through the fast intelligent sulfur fixing experimental system. At 1323K, the desulfurization efficiency of 69.80% and 57.08% for brown coal (BC) could be achieved by CP at Ca/S equaling to 1 and 1.5, respectively. At 1223K and 1323K, desulfurization efficiency of 73.46% and 65.40% could be achieved for lean coal (LC) if MCP is added at Ca/S of 2. At the same time, the inorganic compound of CC behaves poorly and at 1323K, the desulfurization efficiency of only 34.08% and 40.07% could be achieved, respectively, for BC and LC, if CC is added at Ca/S equaling to 2.Characteristics of calcium based organic compounds on sulfur fixing are investigated through analyzing the CaO conversion of CP and MCP on thermogravimetric analyzer. At 1323K, the values are 44.32% and 54.95% for CP and MCP, which are 5.49 times and 6.80 times of CC. The modified grain model is applied to deal with the data of both the surface reaction segment of Gfp(χ)～t and the product layer diffusion reaction segment of Pfp(χ)～t and the satisfying linear relationship is achieved through the linear regression process. In higher temperature zone, the slope of the regression line is increased and the sulfation progress gets strength.Investigations on NO reduction through CP, MCP and MCMP reburning are conducted on the drop tube experimental system. At 1323K, NO reduction efficiency of 79.65%,76.36% and 72.65% could be achieved through basic reburning (BR) of CP, MCP and MCMP, respectively. These values are comparable with the ones of biomass and obviously higher than the ones of coal powder. At the same time, for satisfying outcome, the reburnign fuel fraction should be kept about 25%, oxygen concentration is normally lower than 4% and the residence time of 0.65s is acquired. There is a distinct "temperature window" for both ammonia based and urea based selective non-catalytic reduction (SNCR). At 1273K, ammonia performs best and the NO reduction efficiency is 85.34% and 79.32%, respectively, at mole ratio of N-reducing agent to NO of 1.75 and 1.25. At 1223K, urea achieves the best performances and the efficiency is 78.89% and 70.19%, respectively, at mole ratio of N-reducing agent to NO of 2 and 1.5. Taking NO reduction efficiency and ammonia reagent utilization into consideration, mole ratio of N-reducing agent to NO is normally 1.5-2. Increaing O2 concentration, the NO reduction is weakened for both ammonia and urea. Also, residence time is required about 0.60s. NO reduction achieved during CP and MCMP advanced reburning (AR) is obviously high than the one in SNCR and BR. At reburning fuel fraction of 19.83% and mole ratio of N-reducing agent to NO of 0.8, CP and MCMP achieve the best performances at 1273K and their values are 93.37% and 91.74%, respectively. The "temperature window" is obviously broadened and as oxygen concentration is increased from 2% to 6% in AR, NO reduction gets a lower depression compared with BR. At the same time, quantity of ammonia reagent injection, which is required at mole ratio of N-reducing agent to NO of 0.8 in AR, could show satisfying outcome.Characteristics of CP, MCP and MCMP on SO2 and NO dual reduction during BC and LC combustion process are investigated on fixed bed experimental system. From 1073K to 1373K, CP, MCP and MCMP perform well on desulfurization. At mole ratio of calcium compounds to sulfur of 2. CP acquires the highest SO2 reduction efficiency of 66.01% and 71.12%. The values for MCP are 67.20% and 69.85% and for MCMP are 70.72% and 67.06%. And all these values are higher than the ones of CaO. NO reduction of calcium based organic compounds exists in temperature zone higher than 1173K. At mole ratio of calcium compounds to sulfur of 2.5, the highest efficiency of CP for BC and LC is 49.38% and 50.15%. The values of MCP are 47.57% and 56.44% and of MCMP are 46.19% and 56.67%. With addition calcium based organic compound, the ignition temperature, the maximum mass loss rate temperature and the conversion curve are shifted towards lower temperature zone. Also, the maximum mass loss rate is reduced and the half peak width is broadened. Kinetic calculations through model-fitting approach shows that calcium based organic compounds decrease the apparent activation energy to make the coal combustion more easily occur.Performances of CP on SO2 and NO dual reduction in post combustion flue gas are conducted on the drop tube furnace experimental system. Under the effect of 1500×10-6 of SO2, the CP basic reburning performance is improved. Calcium sulfuration needs oxygen participation and as oxygen concentration is increased from 2% to 6%, desulfurization of CP is continuously reinforced. Like basic reburning, SO2 intensifies NO reduction of advanced reburning. As AR itself could gain high efficiency, promotion of 1500×10-6 SO2 on NO reduction at 1273K is only 1.96% and 2.03%, respectively, for oxygen concentration of 4% and 6%. However, addition of ammonia makes little sense on SO2 reduction. At 1273K, improvement of SO2 reduction during advanced reburning over basic reburning is just 1.19%,0.67% and 0.53% for oxygen concentration of 2%, 4% and 6%, respectively.With the combination of 3-pentanone combustion model, small molecule hydrocarbon compounds combustion model and reaction model between hydrocarbon compounds and NO, the kinetic model, which includes 453 elementary reactions and 110 species, is established to describe the NO reduction reaction mechanism by propionic acid based compounds reburning.Elementary reaction H+O2=O+OH makes the greatest contribution to NO reduction and it could supply plenty of active radical OH through chain reaction. As a result, the NO reductions through HCO, CH3, CH2, CH2CO, CH2O, CH2OH, CH3O, HOCHO get strength. The key parameter for NO reduction of the chain branching ratio is 0.29 in this investigation and this value could guarantee the process self-sustaining. At the same time, elementary reactions of NH2+NO=NNH+OH and NH2+NO=N2+H2O play the most important role in NO reduction. Under the combined effect of reburning fuel and ammonia, sensitivity coefficient of elementary reaction H+O2=O+OH gains the largest value. Also, NH2+NO=NNH+OH and NH2+NO=N2+H2O make a greater contribution to NO in comparison with SNCR. Meanwhile, C1 hydrocarbon compounds show a stronger ability in NO reduction than C2 hydrocarbon compounds.Under the effect of SO2, not only the existent elementary reactions of H+O2=O+OH and C2H2+O=HCCO+H get strength, but also new elementary reactions of H+SO2=HOSO are generated. More active radical, like OH and O, are produced and the NO reduction is subsequently intensified. At the same time, the sulfur-containing intermediate products are participated in NO reduction, triggering a series of new reactions, like, SN+NO=N2+SO, and CH2 (S)+NO=HCN+OH. All of these factors make contributions to improvement in NO reduction with SO2 addition during basic reburning and advanced reburning.