Effect Of Biomass Ash On The Performance Of Fe-Based Oxygen Carrier In Chemical Looping Combustion

Chemical-looping combustion (CLC) is a novel process with inherent separation of CO2 without energy penalty. In recent years, the study of chemical-looping combustion mainly focused on solid fuel (such as coal, biomass and sewage sludge). In a CLC process of solid fuels, the existence of ash is a major challenge. At present, the research of fuel ash in CLC was concentrated on the coal ash. However, the effect of biomass ash in the CLC system has received little attention in literature. In this paper, interaction between Fe-based oxygen carrier and biomass ash was preliminary studied. The influence mechanism of alkali metal K in biomass ash was the focus of study. And the feasibility of ehnhanced reduction performace of Fe-based oxygen carrier modified by biomass ash was investigated.Experiment on chemical looping combustion was conducted in a fixed bed reactor with iron ore as an oxygen carrier and CO as fuel. The effect of biomass ash addition on the performance of the iron ore oxygen carrier was carried out. Several key factors were discussed, including the biomass ash type (corn stalk ash, rape stalk ash, straw ash), the ash ratio and the alkali metal in the ash. The results indicate that the effect of the biomass ash on the reactivity of iron ore depended on the chemical composition of the ash. The abundant alkali metal K in corn ash and rape ash was release in gaseous state and captured by iron ore through complicated reactions. The new formation of K3FeO4 enhanced the reaction between CO and iron ore, and thus the reduction reactivity of the iron ore. However, the high content of Si in the straw ash would react with K and FeO to form low melting point compounds which caused serious sintering on the surface of the iron ore. The decreased contact surface caused the decrease in the cumulative CO conversion.Chemical looping combustion of gaseous fuel was performed in a fluidized bed reactor with Fe2O3 oxygen carrier modified with biomass ash. The feasibility of ehnhanced reduction performace of Fe2O3 oxygen carrier modified by biomass ash was investigated, and the effect of ash type and chemical composition of the ash were disscussed. The results indicated that the addition of biomass ash into Fe2O3 oxygen carrier could efficiently enhance the reduction reactivity of the Fe2O3 oxygen carrier, depending on the content of K and Si in ash. The elevated K content in the ash enhanced the reactivity during the whole reduction stage, including Fe2O3/Fe3O4 and Fe3O4/FeO. But the suppression effect of alkali metal silicates with a low melt point on the oxygen carrier reduction was also observed in the stage of Fe3O4/FeO. The 10-cycle redox experiments indicate that the modified Fe2O3 oxygen carrier maintained the enhanced reactivity although with a reactivity decrease. There was a loss of K in the modified Fe-based oxygen carrier during the cycle, but the alkali metal silicates could reduce the loss of K.Furthermore, the feasibility of the biomass ash-modified iron ore as oxygen carrier was analysed in the 1 kW interconnected fluidized bed during the fuel reactor temperature range of 800℃-930℃.The results indicated that the biomass ash-modified iron ore showed high reactivity in the 1 kW interconnected fluidized bed and the reactivity of oxygen carrier increased with the fuel reactor temperature. The concentration of CO and CH4 leaving the fuel reactor was significant reduced and the conversion of H2 with both oxygen carriers reached nearly 100%. Meanwhile, the CO2 capture efficiency was significant increased. The slight sintering was observed on the biomass-modified iron ore after reacted, but the porous structure of particle was still maintained. The EDS results showed that the content of alkali metal K in the biomass ash-modified iron ore could efficiency maintained.