Study On The Migration And Transformation Law And Removal Mechanism Of Sulfur In Coal During Chemical-looping Combustion Process

The direct combustion of fossil fuels,especially coal,releases a large amount of CO₂,which results in a severer situation of greenhouse effect.Environmental problems such as acid rain and smog caused by coal burning have received wide attention all around the world.Chemical-looping combustion?CLC?is a new,clean,environmentally friendly and efficient combustion technology with inherent separation of CO₂.This technology can effectively suppress the emission of NO_x and capture CO₂,but it can not avoid the generation and emission of SO_x.Affected by the geographical distribution and sedimentary environment,the range of sulfur content in coal in China varies widely.Therefore,the effective use of medium or even high-sulfur coal will alleviate the pressure of China's energy demand to some extent.In this paper,the transformation and removal mechanism of sulfur in coal in CLC process was studied.In this paper,HSC chemistry 5.0 was used to simulate the CLC process of two high-sulfur coals based on the thermodynamic balance.It was found that when the temperature is lower than950?,the sulfur in the coal will react with CaO to form stable CaSO₄.When the temperature is higher than 950?,CaSO₄ will be decomposed to form SO₂;when the fuel excess coefficient is low,sulfur is more likely to react with alkali metal?K₂O and Na₂O?and alkaline earth metal?CaO?oxides in the oxygen carrier to form K₂SO₄,Na₂SO₄ and CaSO₄ respectively.As the fuel excess coefficient increases,the sulfur in the coal will also be precipitated and converted into SO₂.When the temperature is low,the Ca/S molar ratio has no obvious effect on the removal efficiency,but at high temperature,the desulfurization efficiency decreases significantly due to the thermal decomposition of CaSO₄.The more decrease the Ca/S molar ratio,the lower the desulfurization efficiency.The CLC process of two kinds of typical coal was carried out in a batch fluidized bed reactor using iron ore as an oxygen carrier.The experiments including coal gasification process and multicycle tests were also performed.The results showed that the sulfur in the coal was mainly released in the form of H₂S during the gasification process.The concentration of H₂S reached the highest value within 2 minutes,and the conversion rate of sulfur in Xinzhou coal reached 86.8%.In the reduction process of CLC,the carbon conversion rate of Beisu coal was79.32%,the relative concentration of CO₂ was 78.87%.SO₂ was the main sulfur-containing gaseous compound,and the corresponding conversion rate was 51.05%.When it comes to the Xinzhou coal,the carbon conversion rate was 80.19%,and the relative concentration of CO₂ was76.05%.However,whilethe conversion rate of sulfur to sulfur-containing gaseous compound reached 64.27%.This maybe due to the high content of thiol sulfide and sulfone sulfur,which is easier to release into the gaseous compounds.During the 5 cyclic processes at 950?,the iron ore particles maintained high stability.Analysis of the solid phase material by XRD and SEM-EDS revealed that the presence of S was observed in the oxygen carrier after one and 5cyclic reaction,and the deposition of coal ash was also observed on the surface of the oxygen carrier.The mechanism of sulfur removal in two high sulfur coals during CLC was studied.The results showed that the use of CaO as a desulfurizer effectively reduced the concentration of sulfur-containing gaseous substances at 950?,and the X_S,g,red,g,red of Beisu coal and Xinzhou coal reduced to 43.78%and 40.74%,respectively.When the Ca/S ratio was higher than 2.0,the desulfurization efficiency of Xinzhou coal could reach 50.72%,but there was a moderate increase when continued to increase the Ca/S ratio.Considering the economical and reaction performance of the calcium-based desulfurizing agent,Ca/S of 2.0 was selected as the optimum ratio.The XRD and SEM-EDS analysis of the solid phase materials added to the desulfurizer showed that the ratio of Ca and S in the iron ore after the reaction increased significantly,indicating that CaO has obvious sulfur-fixing effect on the high-sulfur coal of CLC process.