The Study On Transfer Behaviors Of Sulfur And Nitrogen Of Coal In Heat Field In O₂/CO₂ Atmosphere

The pollutants from coal combustion are harmful to environment.Among them,CO₂ is responsible for greenhouse effect,SO₂ is the cause of acid rain and NOx can form optical chemical fog.O₂/CO₂ combustion technology using CO₂ instead of N₂ for coal combustion with flue gas recirculation,which enriches CO₂ concentration up to 95%for easily CO₂ capture,can also reduce the emission of SO₂ and NO_x.So it is a promising advanced coal combustion technology which can simultaneously control CO₂,SO₂ and NO_x and friendly to environment.The main researches related to this technology focused on the coal combustion behavior and the economic evaluation.However,few papers have been reported on the coal's component transfer process under this technology.The investigation on the transfer behavior of some pollutant components such as sulfur and nitrogen is very important for clarity the mechanism of SO₂ and NO_x reduction under this technology.In this work,the reaction characteristics and mechanism of calcium carbonate as desulphurization agent and the transfer characteristics of sulfur and nitrogen of coal in O₂/CO₂ mixtures were studied and the chemical kinetic simulation by CHEMKIN was also obtained.The main contents are as follows:The effect of temperature on the characteristic of the reaction of CaCO₃ and SO₂ under O₂/CO₂ atmosphere was invesitigated.It was found that the atmosphere influenced the calcium conversion ratio when temperature was above 500℃.For instance,the calcium conversion ratio in O₂/CO₂ mixture was lower than that in air at 900℃and higher than that in air at 1200℃.The difference of calcium conversion ratio between O₂/CO₂ mixtures and air decreased with the temperature increasing.It indicated that high temperature was beneficial for SO₂ capture in O₂/CO₂ mixtures.The results also suggested that the calcium conversion ratio increased with increasing of SO₂ concentration for high SO₂ concentration contributed to the equilibrium of reaction of CaCO₃ - SO₂ and prevented the decomposition of CaSO₄.The enrichment of SO₂ in combustion atmosphere by flue gas recirculation is one of the reasons for higher desulphurization ratio under O₂/CO₂ combustion technology than that under air. The CO₂ concentration influenced the microstructure of CaO in O₂/CO₂ atmosphere. The low CO₂ concentration catalyzed the sintering of CaO while the high concentration prevented the decomposition of CaCO₃.The kinetic and thermodynamics analysis suggested that high CO₂ concentration increased the decomposition temperature of CaCO₃ and decreased the CaCO₃ decomposition velocity.The results also indicated that CO₂ concentration had pronounced influence on the microstructure of product of sulfation reaction It has an optimum microstructure in calcium sorbent;in this case,the desulphurization ratio was the highest because the velocity of decomposition reaction and the sulfation reaction was approximately the same.The results also suggest that the conversion ratio can be increased by increasing the concentration of SO₂,which causeed the inhibition of CaSO₄ decomposition and shifting of the reaction equilibrium toward the products.XRD analysis of the product showed that the reaction mechanism was different at varied temperatures,i.e.CaCO₃ directly reacted with SO₂ at low temperatures and CaO from CaCO₃ decomposition reacted with SO₂ at higher temperatures.The static sulfur transfer behaviors of three different coals(heshan,suanpanshan and duanpiqiao) under O₂/CO₂ atmosphere were obtained using fixed bed apparatus.From the results,the SO₂ emission was lower than ordinary air combustion and the SO₂ emission difference between O₂/CO₂ and air varied with the temperature,the maximum difference occurred at 700℃for the CO reduction effect.The coal property such as sulfur and volatile content influenced the SO₂ emission,but this effect was weaker under O₂/CO₂ compared with in air.When using CaCO₃ as SO₂ sorbent,the desulphurization ratio under O₂/CO₂ was higher than that in.air when the temperature above 1000℃and the enhancement extent of desulphurization ratio with temperature was also higher than that under air.The dynamic sulfur transfer characteristics of Heshan high sulfur coal was carried out in a vertical tube electrical heating reactor at temperature range of 800℃-1300℃under O₂/CO₂.The effect of the CO₂ concentration and the temperature on the emission of SO₂ was investigated and the mechanism of the emission of SO₂ under O₂/CO₂ was revealed by FT-IR and elemental analysis.It was found that CO₂ in atmosphere decreased the emission of SO₂ and the lowest SO₂ emission occurred under 80%CO₂.The elemental analysis results showed that the sulfur in bottom ash was higher also suggested the lower transfer ratio from sulfur to SO₂ under O₂/CO₂.COS in flue gas suggested sulphur in coal could be transferred in different species under O₂/CO₂ and SO₂ is only one of them.The optimum temperature of limestone for desulphurization was affected by CO₂ concentration;it was 900℃under 40%CO₂ and 1000℃under 80%CO₂.The sulfur transfer chemical kinetics simulation which was performed with software CHEMKIN revealed that the final SO₂ concentration was affected by both temperature and the concentration of CO₂.The quantity of COS was increase with increasing the concentration of CO₂.The calculational results are consistent with the experimental data.The static nitrogen transfer characteristics of different coals under O₂/CO₂ atmosphere were studied in a fixed bed tube-fumace.The results showed NO_x emission was affected markedly by temperature.High CO₂ concentration significantly decreased NO_x emission during coal combustion compared with conventional air condition due to the reduction of CO in O₂/CO₂.The effect of calcium carbonate on the NO_x peak value also related to the temperature.The NO_x emission was also influenced by the coal property and Ca/S molar. O₂/CO₂ atmosphere was beneficial for the decrease of NO_x emission of the coal with high volatility.The effect of CO₂ on the reduction process of NO was conducted in bench-scale laminar CH₄ flames.NO reduction ratio related to the property of atmosphere.CO₂ accelerated the reduction of NO under oxidizing atmosphere,and the higher CO₂ concentration resulted in higher reduction ratio.However,reducing atmosphere is more promotional for the reduction process of NO.The chemical kinetics simulation results by CHEMKIN were consisted with that of the experiments.