Migration And Conversion Of Sulfur And Nitrogen In Chemical Looping Combustion Of Coal Using Fe-Based Oxygen Carrier

Low-carbon and clean utilization of fossil fuels are always hot issues,especially for the emission of CO₂,NO_x and SO_x.In China,coal is still main primary energy,so it is urgent to need novel technology to realize the low-carbon and clean utilization of coal.Chemical Looping Combustion?CLC?is one of the most promising technologies for CO₂ capture.During the CLC process,the sulfur and nitrogen from coal are mainly released in the form of gas phases.On the one side,these sulfur-containing gases may react with the oxygen carrier to form the metal sulfide or sulfate in the fuel reactor,which results in the attenuation of oxygen capacity in oxygen carrier particles.On the other side,the migration and conversion path of sulfur and nitrogen in coal is essential to attain the high-efficiency removal or control of these sulfur&nitrogen-containing pollutants in chemical looping combustion of coal.This study will provide useful information for understanding the migration and conversion behaviors of sulfur and nitrogen elements in chemical looping combustion of coal.Firstly,the thermodynamic and phase equilibrium between coal gasification products?containing sulfur and nitrogen gases?and Fe-based oxygen carrier were analyzed by the HSC software.It was found that the H₂S and COS could be oxidized by the oxygen carrier to form SO₂ at 950?,and various reduced states of Fe-based compound were to be appeared.Also,the sulfide could be formed as the oxygen to fuel ratio less than 1.0,nevertheless,most of the sulfur-containing gases were converted into SO₂ as the oxygen carrier to fuel ratio higher than 1.0.For chemical looping combustion process,the nitrogen from coal was mostly converted to N₂ rather than NO_x.However,higher oxygen carrier to fuel ratio has positive effect on increasing the NO release.The migration path and distribution characteristics of sulfur in chemical looping combustion was investigated in a tailor-made two-stage fluidized bed reactor,and the lignite was used as fuel and the Fe₂O₃/Al₂O₃ prepared by sol-gel method was used as oxygen carrier.The effects of temperature,oxygen to fuel ratio and coal type on the sulfur distribution were evaluated in this work.And there was no sulfide detected from the used oxygen carrier at all experiments.The SO₂ and H₂S were the main sulfurous gases during the chemical looping combustion of volatile.However,SO₂ was the main sulfurous gas during the chemical looping combustion of char gasification products,which was partly derived from the decomposition of CaSO₄ in ash.Partial sulfur in coal can be fixed in coal ash as sulfate by the alkali metal.Moreover,the release amount of SO₂ will be promoted at higher temperature and oxygen to fuel ratio.The types of sulfur gas released from different coal are similar,mainly SO₂,though the amount of sulfur gas released is obviously different.The conversion of the nitrogen in coal was comparably investigated using the Fe-based OC.At the typical temperature in the CLC,the precursors of nitrogen were released as HCN and NO at the fast pyrolysis stage.The Fe-based OC showed approximately full oxidization of HCN and the product was NO.And the released NO per gram coal was lowest at the oxygen to fuel ratio 2.0.Moreover,the temperature showed little impact on the NO release amount.