Feasibility Study Of The Magnetic Material In Incineration Bottom Ash As Oxygen Carrier For Biomass Chemical Looping Gasification

Biomass chemical looping gasification（BCLG）is an efficient combustion process with redox active metal oxides as the reaction medium,while eliminates the need for additional CO₂separation via avoiding direct contact between biomass and air,which is known as an innovation breakthrough to solve the conflicts between energy enquiry and the environment.The choice of oxygen carrier（OCs）is one of the most concerns in chemical looping combustion processes.Magnetic fractions of municipal waste incineration bottom ash（IMFs）has a high portion of iron oxides accompanied with a small amount of other transition metals（such as nickel,copper,manganese and cobalt）,which is inert and stable,promising to replace the traditional iron based oxygen carrier.Moreover,the potential synergistic effect of various transition metal oxides in BCLG can improve the disadvantages of low reactivity and low oxygen transfer capacity of traditional iron-based OCs.In brief,the aim of this paper is to study a new type of low cost and widely available oxygen carrier:the magnetic fractions of bottom ash.It can be rationalization of solid waste disposal and recycling use,greatly reduce the cost of OCs and avoid the disadvantages of the traditional solid waste disposal to some extent,promoting the sustainable development strategic target."Waste-waste combination"of bottom ash and biomass also promotes the development strategy of green circular economy and carbon neutral.In this research,IMFs as the OCs in BCLG were studied with pine sawdust as the fuel.The IMFs in municipal waste incineration ash were separated by magnetic separation（named IMF-2～IMF-6,respectively）,and the iron-based OC with similar iron content（syn-75）was prepared by coprecipitation method for comparison study so that the above two types of materials as OCs for the biomass chemical looping processing（BCLP）.Thermogravimetric analyzer（TGA）and CO chemical looping combustion experiments were performed on OCs,and dynamic analysis of thermogravimetric test data was carried out.Then the biomass chemical looping gasification was studied in a fixed bed.Characterization methods（XRD,XPS,SEM and BET）were used to analyze the structure and physicochemical properties of OCs before and after the experiments.The following conclusions were drawn:（1）The main crystalline phases in IMFs include Fe₂O₃（55.10%～73.26%）and a small amount of Al₂O₃,Si O₂ and Ca Mg Si₂O₆.Syn-75 mainly consists of Fe₂O₃ and Si O₂.A small amount of Fe²⁺except Fe³⁺was detected by XPS characterization.SEM characterization results showed that the crystal phase of IMFs was different in shape,disorderly and irregular in arrangement,and the porosity was high.In contrast,the crystal phase of the syn-75 was denser and flatter.The surface area of syn-75（0.83 m²/g）（which was obtained by BET characterization）was significantly higher than that of IMFs（0.12～0.26 m²/g）.（2）Thermogravimetric analysis showed that IMFs had good redox capacity and actual oxygen transfer capacity（OTC）.IMF-4,IMF-5 and IMF-6 had a large mass loss in reduction stage（18.87%,17.10%and 19.52%,respectively）.Compared with the theoretical oxygen transfer capacity（22.18%,22.13%and 20.64%）,IMFs were reduced to Fe or Fe O at 900℃,rather than Fe₃O₄.And the reduction rate follows IMF-4>IMF-5>IMF-6>IMF-3>IMF-2,however,is not consistent with the order of their iron contents,which confirmed that other active metal oxides except for iron were also lattice oxygen providers or accessory ingredients in redox process,participating in the chemical looping processing（CLP）.The reduction of IMF-4 in CO atmosphere can be divided into three stages.The reduction process in the first and second stages（Fe₂O₃-Fe₃O₄-Fe O）conforms to the nucleation and growth model.The reduction process in the third stage（Fe O-Fe）conforms to the shrinking core model.（3）The performance of IMFs and syn-75 in CLC of CO was investigated and the stability of OCs with different particle sizes in 20 redox cycles was tested by calculating CO combustion efficiency.The combustion efficiency of IMFs and syn-75 was basically stable at 83～92%,among which IMF-4 was the best（88～92%）,Syn-75 has higher iron content and theoretical oxygen transfer capacity than IMF-4 and IMF-5,but CO combustion efficiency in CLC was not as high as IMF-4 and IMF-5 due to the synergistic effect of other active metal oxides in IMFs with iron.The characteristic peak of Fe in IMF-4 after air oxidation is basically Fe₃O₄,but no Fe₂O₃,indicating that IMFs had the ability to resist complete oxidation during the cycles.After IMF-4was reduced by CO（20th redox cycle）,Fe₃O₄and Fe O coexist,while for syn-75,Fe₂O₃ only reduced to Fe₃O₄,and Fe O crystal phase is not observed.（4）The optimal component IMF-4 was selected to study the chemical looping gasification process of pine sawdust as fuel.The effect of gasification temperature,the steam flow rate and OC mass on gasification process was investigated.and the gas yield and carbon conversion efficiency were calculated.It was found that when the mass of pine sawdust is 0.5g,the optimal result that the temperature is 900℃,the OC mass is 0.5 g and the steam flow rate is 0.03 m L/min was achieved,with the relative composition of 10%CH₄,53%H₂,12%CO and 25%CO₂,and the gas yield and carbon conversion efficiency being 1.18 L/g and 62.18%,respectively.IMF-4performs better than syn-75 in terms of both the maximum instantaneous gas yield and total gas yield.The gas yield of CH₄ and CO reached the maximum instantaneous yield at 6～9 min,in which the gas yield of CO is the highest,and the gas yield of H₂ is larger in the whole reduction process.The effect of OC mass on the composition of tar was studied.The results showed that the peak position was basically the same,and the tar content was 1.8 and 1.5 g/Nm³ respectively（The amount of OCs was 0.5 g and 0.6 g,respectively）.The main compounds were aromatic compounds such as phenaphthalene(C₉-C₁₆)and a certain amount of straight chain hydrocarbons,such as pentadecane,and the increase of oxygen carrier mass significantly reduced the formation of naphthalene.IMF-4 and syn-75 were reduced to Fe₃O₄,and Fe O and Fe were not detected.IMF-4 recovered to Fe₂O₃ after air oxidation,and Fe₃C was not observed in both two OCs,which reflected good oxidation conditions of the system.