Study On The Effect Of Reductive Atmosphere On Sulfur Fixation Characteristics In Circulating Fluidized Bed Furnace

NOx and SO2 are the main atmospheric pollutants produced during the combustion of coal,and they are also the main causes of environmental problems such as smog.Increasingly stringent emission standards of coal-fired flue gas in China have prompted scholars to carry out a lot of work on the coordinated control of NOx and SO2 emissions in CFB boilers.Lean-oxygen combustion combined with post-combustion can not only achieve low emissions,but also ensure combustion efficiency.However,the technology changed the original oxidizing atmosphere into reducing atmosphere in the furnace,and the mechanism of desulfurization in the furnace is no longer applicable.Therefore,in order to explore the feasibility of the technology,the mechanism of desulfurization in reducing atmosphere and the influencing factors were systematically studied by means of TG-MS&ECSA(thermogravimetry-mass spectrometry with equivalent characteristic spectrum analysis),tubular furnace experiment and CFB real holier test.In the paper,TG-MS&ECSA was used to quantify the real-time escaped gases during each process.Reactions of graphite or Shenmu char and CaSO4,the oxidation reaction of CaS in the atmosphere of O2/Ar or CO2/Ar,the solid-solid reaction of CaS and CaSO4 were carried out from the aspect of gas evolution.In addition,experiments were carried out on the tubular furnace in order to verify the conclusion from the aspect of solid products after the reaction.In addition,the paper also directly studied the influence of temperature and CO concentration on the sulfur-fixing characteristics of the CFB furnace through the vertical tube furnace experiment.Finally,a simple verification experiment was carried out on a 100kW CFB test platform.The mechanism of sulfur fixation in reducing atmosphere in a CFB furnace was comprehensively studied.In the experiment of TG-MS,the results were as follows.The addition of graphite and Shenmu char can greatly advance the temperature at which CaSO4 began to decompose.But their mechanisms were different.In the reduction of CaSO4 by graphite,the main reaction was the formation of CaO,SO2 and CO2.And it started at about 900℃.In the reduction of CaSO4 by Shenmu char,the desulfurization rate of CaSO4 decreased sharply with the increase of Shenmu char.CaSO4 and Shenmu char can react directly to form CaS and CO2.And it started at 770 ℃.But CaO in the reaction products and SO2 escaped during the process were not produced by the direct reaction between CaSO4 and Shenmu char.They were produced by the reacton of CaSO4 and CaS.And it started at about 850 ℃.The reaction of CaS and O2 to produce CaSO4 started at 617℃ and ended at 1015 ℃.The reaction of CaS with O2 to produce CaO and SO2 started at 997 ℃.Besides,CaS can react with CO2 to produce CaO,SO2 and CO,and it started at 900 ℃.CaS can react with CaSO4 to produce CaO and SO2,It started at 850 ℃ and the reaction rate was much higher than the self-decomposition rate of CaSO4.In the paper,the fluidization state and reaction atmosphere in CFB furance were simulated in a vertical tubular furnace.The results were as follws.When the concentration of CO was less than 0.6%,the mass fraction of CaS in the reaction product was strongly dependent on temperature,and increased significantly with the increase of temperature.But when the concentration of CO was more than 1%,the dependence of the mass fraction of CaS on temperature was weak and the overall trend of change was negatively correlated with the temperature,but the change was not significant.When the concentration of CO was less than 1%,the mass fraction of CaS depended strongly on the concentration of CO,and increased greatly with the increase of CO concentration.However,when the CO concentration was more than 1%,the mass fraction of CaS had a weak dependence on CO concentration.With the increase of CO concentration,the mass fraction of CaS basically maintained at a certain value.In addition,as the reaction proceedd,a dense CaS film would be formed on the surface of the sample,which hinders the further reaction.Verification experiments were carried out on a 100kW CFB test paltform.The results showed that only CaSO4 was the sulfur-fixing product in oxidizing atmosphere at 950～980 ℃.But CaS started to appeare in weak reducing atmosphere at the same temperature.The results provides an important basis for optimizing the technology of lean-oxygen combustion combined with post-combustion in circulating fluidized bed furnace and making it a combined desulfurization and denitrification technology.