Investigation On Chemical-looping Combustion Of Multi-Rank Coal Char Using Iron Ore Of Different Grades As Oxygen Carriers

Chemical Looping Combustion(CLC)is a potential way of fossil fuels ultilization with low carbon emission.The cornerstone of CLC are oxygen carriers,but synthetic oxygen carriers usually have a high cost and long production period.Therefore,it is infeasible to use synthetic oxygen carriers in industrial-scale CLC units.Natural iron ores,which have abundant storage in the crust and moderate oxygen transport capacity,are suitable to be used as oxygen carriers.In this work,a series of chemical looping combustion experiments were performed on a batch fluidized bed,using different ranks of coal char as fuel.Iron ores of different grades were used as oxygen carriers to investigate the relationship between coal ranks,ore grades and operation conditions.Considering that the reduction reaction was controlled by the char gasification process,4 types of coal chars were used as fuels to investigate the optimal operation conditions of gasification process.By changing the temperature of the reactor and the percentage of vapor contained in the fluidized gas,the effects of temperature and vapor content on char gasification process were investigated.The results showed that as the temperature increased,the carbon conversion rate and gasification rate of char increased.The gasification temperature should not be too high for medium and low-rank coal chars because of the high CO yield at high temperatures which could cause negative contribution to CO₂ capture efficiency and combustion efficiency.However,high-rank coal chars were more suitable to be gasified at high temperature.The increasement of vapor content in the fluidizing agent could accelerate the carbon conversion rate and gasification rate,as well as improving CO₂ yield.But compared with temperature,vapor content had less effect on the entire gasification process.Based on the optimal operation conditions of different chars,different grades of natural hematites(where Fe₂O₃ content is approximately 50%,60%,70%,80%,and90%)were used as oxygen carriers to investigate the relationship between coal ranks and ore grades.The experimental results indicated that the natural hematite had excellent performance in chemical looping combustion of coal char.The carbon conversion rate was accelerated with the increasement of ore grades,but limited at grades around 90%.Hematite of 90%grade was not suitable for high-rank coals in chemical looping combustion.In general,high-rank coals had stronger selectivity and smaller selection range to ore grades than low-rank coals.As a result,hematites of ore grades from 70%to 80%had excellent performance in almost all ranks of coal char.