Study On The Migration And Transformation Law And Removal Mechanism Of Sulfur During Coal Chemical-looping Combustion Based On Copper And Nickel-based Oxygen Cerrier

Energy consumption is an indispensable element for the economic development,but the"Greenhouse Effect"caused by the massive burning of fossil energy is becoming more and more serious.At present,coal occupies a dominant position in China’s energy structure,and there is an urgent need to vigorously develop efficient and clean coal-burning technologies.Chemical-looping combustion（CLC）has a great potential as result of its inherent separation CO₂ without costing any extra energy.Although CLC has obvious advantages in capturing CO₂ and inhibiting the generation of NOx,it cannot avoid the generation and emission of SOx.When coal is used as fuel,the sulfur in coal will not only affect the purity of CO₂ capture in CLC,but also affect the activity of oxygen carrier and the operation of reaction system.Therefore,it is difficult to explore the transformation and evolution mechanism of sulfur in coal during CLC process to make efficient use of medium sulfur coal or even high sulfur coal.In this paper,the thermodynamics simulation and experimental study on the sulfur transformation and removal mechanism of Beisu coal and Xinzhou coal under two kinds of oxygen carriers were carried out.Based on thermodynamics analysis,HSC Chemistry 5.0 was used to simulate the distribution of sulfur-containing compounds in the CLC process of two coals under Cu O/Si O₂ and Ni O/Al₂O₃respectively.The simulation results showed that at lower temperatures and low excess coefficients of oxygen carriers,the sulfurous gases were more likely to react with oxygen carriers to form metal sulfides,and the production of SO₂ was significantly promoted with an increase in temperature and oxygen excess coefficient of oxygen carrier.Moreover,part of sulfur was captured and fixed in the forms of alkali/alkaline earth metal sulfates due to the high amount of alkali/alkaline earth metal oxides in the coal ash or/and oxygen carrier.The sulfur precipitation and conversion rules of Beisu coal and Xinzhou coal during the CLC process using two oxygen carriers（Cu O/Si O₂ and Ni O/Al₂O₃）were investigated respectively.The experimental results showed that sulfur in coal mainly released in the forms of SO₂ and H₂S during the reduction process while SO₂ is the only sulfur-containing gas in the oxidation process.The fractions of sulfur converted to sulfur-containing gas(X_S,g,red)of Beisu coal were 48.20%and 54.48%when using Cu O/Si O₂ and Ni O/Al₂O₃ respectively,while the corresponding values of Xinzhou coal were 62.34%and 61.25%respectively.In the oxidation process,The fractions of sulfur converted to sulfur-containing gas(X_S,g,oxd)of Beisu coal were 7.64%and 7.92%respectively,while the corresponding values of Xinzhou coal were 6.25%and 7.44%respectively.It was also found that the X_S,g,red of Xinzhou coal under Cu O/Si O₂ and Ni O/Al₂O₃ was higher than those of Beisu coal.This is mainly because Xinzhou coal has higher organic sulfur content and it is easier to precipitate sulfur out of coal at higher temperatures.During the five cyclic process,it was found that the two oxygen carriers maintained high stability,and there was no sintering phenomenon.Through XRD and SEM-EDS analyses,Cu₂S was found the Cu O/Si O₂,and Ni₃S₂ and Ni S were also detected in Ni O/Al₂O₃ after the reduction process.The two oxygen carriers both showed good regeneration performance.In addition,the residue and accumulation of coal ash on the surfaces of two oxygen carriers were all detected after the five cyclic processes.To realize the effective control of sulfurous gases during CLC process,the effect of adding calcium based desulfurizer on sulfur migration and transformation in coal was investigated.Through thermodynamic analysis,it was found that calcium sulfide would be generated at lower temperature and decomposed easily at higher temperatures.The sulfur-containing gases were easily absorbed by desulfurizer to generate calcium sulfide at high oxygen carrier excess coefficient.In addition,the desulfurization performance with addition of Ca O were studied by taking copper-based oxygen carrier as an example.The experimental results showed that the concentration of sulfur-containing gases at the exit of the reduction reactor decreased obviously after adding Ca O.When the Ca/S molar ratio was 2.0 and the temperature is 800 ^oC,the desulfurization efficiency of Beisu coal reached 38.80%,while the corresponding values of Xinzhou coal was 44.33%.Based on SEM-EDS analysis,it was found that the content of Ca element and S element increased obviously,and the existence of calcium sulfide was also found in the solid phase after reduction process,which indicating that Ca O can capture sulfur and control the release of sulfur-containing gases to a certain extent.