| Zhundong coal reserves in China are abundant,but high sodium characteristics in Zhundong coal have brought many problems to the use of coal,such as slagging and corrosion of heating surface.The clean and efficient use of coal is one of the widely concerned issues in our country.Chemical looping combustion(CLC)of coal based on cycle oxygen carrier is a new combustion technology.Coal in the two reactors(fuel reactor and air reactor)realizes the cascade transformation,reducing exergy loss and realizing high efficiency and clean coal utilization.The conversion of sodium which is included in the Zhundong coal happens in the fuel reactor,effectively avoiding heating surface corrosion in the air reactor.It is a potential technology to solve the problems about slagging and heating surface corrosion,resulted by the combustion of Zhundong coal.This article based on the high activity of Cu-Fe bimetallic oxygen carrier carried out experimental study on the combustion characteristics about chemical looping combustion of Zhundong coal,studying the key factors which had effect on Zhundong coal combustion,and exploring the effect of coal ash and sodium in Zhundong coal on the process in CLC.Cu-Fe bimetallic oxygen carriers prepared by impregnation method were applied in the CLC of Zhundong coal,and the experiments were conducted in a laboratory fluidized bed reactor.The effect of Cu/Fe ratio,temperature,medium water and cyclic characteristics were researched.The study found that Cu-Fe bimetallic oxygen carriers had high reaction activity,promoted the gasfication of Zhundong coal and improved the rate of carbon conversion.With the increase of the mass ratio of CuO in the bimetallic oxygen carrier,the conversion rate of coal was accelerated,and the rate was stable when the mass ratio of CuO was 10wt.%.The coal conversion rate was significantly accelerated with the increase of reaction temperature or water vapor concentration based on 10wt.%CuO composite oxygen carrier.When the reaction temperature was 900 ? and the amount of water was 0.5 g/min,Zhundong coal conversion reached 93.4%.In the 10 cycles,the results showed that the reaction property of oxygen carrier in the first 5 cycles had slight fluctuations.And the property tended to be stable as the number of cycle increased.After 10 cycles,the grain size of oxygen carrier became larger.The surface became porous,and there was no obvious sintering phenomenon.X ray diffraction(XRD)analysis showed that Cu in the bimetallic oxygen carrier was in the form of CuFe2O4 with high reaction activity,which was mainly caused by the high temperature reaction of CuO and Fe2O3.At the same time,there is no the cluster of large particles in the bed material.XRD characterization analysis of the oxygen carrier by magnetic separation showed that part of Na was deposited on the oxygen carrier during chemical looping combustion of Zhundong coal.The oxygen carriers in oxidation state and reduction state both can be detected with Na2O·Al2O3·6SiO2 which had high melting point,indicating that part of Na in Zhundong coal migrated to the oxygen carrier to inhibite slagging.According to the results of current study,water soluble sodium is the main speciation of sodium in zhundong coal.Acid soluble sodium,including ammonium acetate and hydrochloric acid soluble sodium and insoluble sodium are another two forms.The effect of sodium in the Zhundong coal on chemical looping combustion characteristic was researched.The comparison of different washing time(0 h,2 h,12 h and 24 h)of Zhundong coal in CLC showed that carbon conversation rate of Zhundong coal gradually dwindled and carbon conversation gradually lowered with the increase of washing time,indicating that water soluble sodium could catalyze coal gasification rate and the catalytic property had relationship with the content of water soluble sodium.XRD characterization analysis of Cu-Fe bimetallic oxygen carriers before and after reaction for 10 cycles showed that water soluble sodium were easy to volatilize and migrate to the copper and iron composite oxygen carrier and effectively inhibitedthe formation of CuAl2O4,ensuring high oxygen carrier activity.Zhundong coal after washing by ammonium acetate and hydrochloric acid was applied in CLC.The results showed that there was no obvious influence of ammonium acetate soluble sodium and acid soluble sodium on CLC characteristicsZhundong coal ash modified Cu-Fe bimetallic oxygen carriers and hematite oxygen carriers were prepared by dry high temperature calcination method.The effect of ash amount and cyclic characteristics on the reaction activity of oxygen carriers were researched during the CLC based on a small fluidized bed.The results indicated that the Zhundong coal ash modified oxygen carriers showed better reaction activity.The ash modified Cu-Fe bimetallic oxygen carriers with 5wt.%,10wt.%and 15wt.%load amount before and after reaction were respectively analyzed by scanning electron microscopy(SEM).It was found that with the load amount increased,the surface pores of oxygen carriers decreased and the diffusion resistance of coal gasification product to oxygen carrier activity center increased,reducing the reaction activity of oxygen carrier,which showed that the suitable ash loading amount was 5wt.%.During the CLC of Zhundong coal based on ash modified Cu-Fe bimetallic oxygen carrier with 5wt.%loading amount in 850?,the coal conversion was up to 97.7%.And in the 10 cycles,the results showed that the carbon conversion was relatively stable,but with the increase of cycle number,the carbon conversion rate showed a slight downward trend.In order to reveal the reasons,XRD characterization analysis of ash modified Cu-Fe bimetallic oxygen carriers with 5wt.%loading amount before and after reaction for 10 cycles showed that the fresh oxygen carrier was mainly composed by Fe2O3,CuFe2O4 and CaSO4.The active constituent of CaSO4 and Fe2O3 in coal ash had oxygen carrying property.As the function of oxygen carrier in CLC process of coal,coal ash and oxygen carrier synergistically promoted the conversion of coal.After 10 reduction reactions,part of Fe2O3 was converted to Fe3O4,while CuFe2O4 was reduced to CuFeO2.Part of CaSO4 in the ash was reduced to CaS,and the other part was transformed into Ca0.15Fe2.85O4 and Ca(Al,Si)2O4,reducing the content of CaSO4 and the reaction performance of oxygen carriers,resulting that the coal conversion efficiency decreased slightly after multiple repeated experiments. |
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