Chemical looping combustion of coal in interconnected fluidized beds

Science in China Series E: Technological Sciences02007Science in China PressSpringer-VerlagChemical looping combustion of coal ininterconnected fluidized bedsSHEN LaiHong t, ZHENG Min, XIAO Jun, ZHANG Hui XIAO RuiKey Laboratory of Clean Coal Power Generation and Combustion Technology of Ministry of Educationoutheast University, Nanjing 210096, ChinaChemical looping combustion is the indirect combustion by use of oxygen carrier.It can be used for CO, capture in power generating processes. In this paper,chemical looping combustion of coal in interconnected fluidized beds with inherentseparation of Co2 is proposed. It consists of a high velocity fluidized bed as an airreactor in which oxygen carrier is oxidized, a cyclone, and a bubbling fluidized bedas a fuel reactor in which oxygen carrier is reduced by direct and indirect reactionswith coal. The air reactor is connected to the fuel reactor through the cyclone. toraise the high carbon conversion efficiency and separate oxygen carrier particlefrom ash, coal slurry instead of coal particle is introduced into the bottom of thebubbling fluidized bed Coal gasification and the reduction of oxygen carrier withthe water gas take place simultaneously in the fuel reactor. The flue gas from thefuel reactor is Co2 and water. Almost pure Co2 could be obtained after the condensation of water. The reduced oxygen carrier is then returned back to the airreactor, where it is oxidized with air. thermodyanmics analysis indicates thatNiO/i oxygen carrier is the optimal one for chemical looping combustion of coal.Simulation of the processes for chemical looping combustion of coal, includingcoal gasification and reduction of oxygen carrier, is carried out with Aspen Plussoftware. the effects of air reactor temperature fuel reactor temperature, and ratioof water to coal on the composition of fuel gas, recirculation of oxygen carrier par-ticles, etc. are discussed. Some useful results are achieved the suitable tem-perature of air reactor should be between 1050--1150Cand the optimal temperatureof the fuel reactor be between 900 -950Cchemical looping combustion, coal, interconnected fluidized beds, CO2 separationEfforts to reduce emissions of greenhouse gases have been growing steadily. CO2 is a kind ofgreenhouse gas that affects the climate of the earth. Fossil fuel combustion is a major source ofReceived December 20, 2005; accepted December 11, 2006doi:10.1007/s11431007-0019-zTHe中国煤化工Supported by the National Natural Science Foundation of China( Grants NosNMHNational Basic Research Program of China( Grant No. 2006CB20030201)and the High-Tech Research and Development Program of ChinaGrant No. 2006AA05Z318)www.scichina.comwww.springerlink.comSci China Ser E-Tech Sci l April 2007 I vol. 50 I no. 2 1 230-240CO2 emission. One option for reducing emissions of CO2 to the atmosphere and the use of fossilfuels as an energy source is to separate and dispose the co2 from combustion. In a conventionalcombustion system, fossil fuel is directly mixed with air and burnt. The concentration of cO2 inthe flue gas, diluted by the nitrogen of air, is merely 10%-14%. when disposing the diluted coa large amount of energy will be consumed in CO2 separation and compressionChemical looping combustion(CLC)is the indirect combustion by use of oxygen carrierRichter and Knochel firstly proposed it as an alternative to conventional combustion techniquein 1983. The chemical looping combustion consists of two separate reactors: an air reactor and afuel reactor, as shown in Figure 1. Metal oxide, whichcirculates between the two reactors, acts as oxygenCO.HOcarrier, and transfers oxygen from air to fuel. In thisMe oway, the nitrogen from the air leaves the system fromthe air reactor, whereas the flue gas from the fuel re-actor consists of only CO2 and water. Water can beremoved via condensation, and pure COz is obtainedreactorand then compressed into liquid for storageMe oIn addition, the fuel and air are never mixed duringthe process of chemical looping combustion, and no,formation can be avoided. Thus, chemical loopingAcombustion is a novel and clean combustion tech- Figure 1 Chemical looping combustion. Me,0, andnology with energy-free separation of CO2 from the Me, Oy-I denote oxidized and reduced oxygen carriersflue gas, in which emission of other pollutants, suchas nitrogen oxide, can be eliminatedIn the process of chemical looping combustion, metal oxide Me,Oy is reduced by gaseous fuelto its reduced form Me Or- in the fuel reactor.(2n+m)Me,Oy+CnH,m=(2n+ m)Me,Ox-+mH2o+ nCo2+eThe reduced metal oxide, Me, Oy-I, is oxidized by air to its oxidized form, Me, Oy, in the airreactor2Me,Om-1+O2=2Me Ov -22Depending upon metal oxide, reaction 1 is often endothermic, while reaction 2 is exothermic withsubsequent heat releaseConsidering the two reactors as a whole, the net reaction and the energy balance over reactors1 and 2 in the process of chemical looping combustion are presented as follows:(n+im)O2+C,Hzm=mH20+nCO2-Q33=2Therefore the total amount of heat evolved from reactions 1 and 2 is the same as that for nor-mal combustion where the oxygen is in direct contact with the fuel, as shown in reaction 3. Itshould be pointed out that a full conversion from metal oxide to metal and back to metal oxidenot necessarily obtained in a real system. The difference in conversion of oxygen carrier betweenthe two reactors may be small. The main advantageYHa中国煤化工ombustion is thatCO2 is not diluted with N2, and obtained in almost pureCO2 separation.CNMHGeTgy needed forSHEN LaiHong et al. Sci China Ser E-Tech Sci l April 2007 I vol 50 I no. 2 1 230-240The property of oxygen carrier is the most important for the practice ofMetalxide with its reduced oxide or metal, which is used as oxygen carriers in CLC, should have astrong affinity for reaction with the gaseous fuel as well as a high rate of oxidation by air. In ad-dition, it is also an advantage if the metal oxide is cheap and environmentally sound. a number ofdifferent metals, such as Fe Ni, Co Cu, Mn and Cd, as well as some metal blends have beenntensively carried out in either a thermogravimetric analysis or lab-scale fixed bed using gaseousfuels such as methane, hydrogen and syngas from coal gasification process 2-3)The process of chemical looping combustion will require a good contact between gas andoxygen carrier, as well as an exchange of oxygen carrier between the air reactor and the fuel re-actor. Two interconnected fluidized beds should have advantages over alternative designs. Theconfiguration possesses a good gas-solid contacting efficiency and reactivity as well as a highmixing rate of solids in order to organize the CLC process %, IO)Since chemical looping combustion was firstly introduced in 1983, the majority of studies andprocess development have been with gaseous fuels such as methane and natural gas. However,the methane and the natural gas supply cannot fully support the energy demands for the longterm. Coal has been seldom used in the concept of chemical looping combustion because of tech-ical problems. Although the adaptation of ClC to coal combustion faces many challenges, thetechnique is very attractive because of rich coal depositsIn this paper, the concept of chemical looping combustion of coal in interconnected fluidizedbeds with inherent separation of CO2 is presented. The technical approach for CLC of coal,chemical equilibriums of coal gasification and the reduction of oxygen carrier are evaluatedSimulation of the processes is carried out with Aspen Plus software. The reaction mechanism andthermodynamic characteristics in the process of chemical looping combustion of coal are inves-tigated1 Technical approach for CLC of coalThere are two approaches to chemical looping combustion to coal combustion. The first approachis to gasify coal in a separate gasifier with pure oxygen to produce a syngas of co, H2 and CH4without nitrThe syngas is supplied for the ClC system, as in the clc process using anatural gas. However, the production of pure oxygen and the fabrication of an additional gasifierwill be required, thus dramatically increasing the cost of the CLC systemThe second approach, which is proposed for CLC of coal in this study, is to directly supplyoal into the fuel reactor in the CLC system. It has economic advantages, but has also severaltechnical problems. There are two reaction paths in the fuel reactor, direct reduction of oxygencarrier by coal and indirect reduction by the syngas of coal gasification in the fuel reactor. Thereis actually a mixed mechanism of these two reaction paths. For the first path, the primary technical concern is the lower reactivity between coal and oxygen carrier due to the low efficiency ofsolid-solid contact. For the second path, the technical concern is the coal gasification rate lowerthan its combustion rate at the same temperature Such issues might prolong the residence time ofcoal inside the fuel reactor to fulfill the higher carbon conversion efficiencyOther important criteria for the development of cher中国煤化工-a of coal includ(i)the separation of oxygen carrier from ash, (ii)thebon particles intothe air reactor, (ii) the sufficient energy transferred frdCNMH Gel reactor in thepresence of coal gasification, and (iv) the prevention of gas leakage between the fuel reactor and232SHEN LaiHong et al. Sci China Ser E-Tech Scil April 2007 I vol 50 I no. 2 1 230-240the air reactorTo overcome the technical difficulties mentioned above in an economical way, we propose interconnected fluidized beds for chemical looping combustion of coal, as shown in Figure 2. It isin a loop with an end-to-end configuration composed of a high velocity fluidized bed as an airreactor in which oxygen carrier is oxidized, a cyclone, and a bubbling fluidized bed as a fuel re-actor in which oxygen carrier is reduced. The air reactor is connected to the fuel reactor throughthe cyclone. The air reactor is constructed as a high velocity fluidized bed with oxygen carrierparticles being transported together with air stream to the top of the air reactor, and then transferred to the fuel reactor using the cyclone. In order to prevent gas leakage between the tworeactors, conventional loop seals can be usedIn the air reactor, air is supplied as the fluidizingagent to oxidize metal or reduced metal oxide to itsN2+O2oxidized form where oxygen is transferred from airCycloneto oxygen carrier. After being separated by the cyAir reactorclone, the oxidized oxygen carrier returns to thefuel reactor. The temperature in the air reactor iskept in a range of 1050--1150C. a heat ex-CO+HOchanger can be installed in the air reactor to pro-duce steam for power generationonly a small amount of steam is used as theeration or transport medium and gasificationagent in the fuel reactor. Metal oxide is reduced tometal or reduced metal oxide by direct or indirectSteamreaction with coal in the fuel reactor, depending onthe properties and reaction mechanisms of coal andoxygen caner.Coal slurryTo fulfill the high carbon conversion efficiencyand separate oxygen carmer particle from ash, coal Figure 2 Configuration for chemical looping combus-tion of coal in interconnected fluidized bedsslurry instead of coal particle is introduced into thebottom of the bubbling fluidized bed. Then an exquisite contact between coal slurry and hotmetal oxide occurs, followed by the intense exchange of heat and mass. The fresh coal slurry isimmediately heated up to the bed temperature(900-950C), and thereby the volatilization andpyrolysis of coal as well as coal gasification occurs. Simultaneously, the reduction of metal oxidewith the syngas accelerates the process of coal gasificationThe process of coal gasification in the bubbling fluidized bed is composed of water-gas reacion and water-gas shift reaction. The syngas contains a small amount of methane as a consequence of the volatilization and pyrolysis of coal and a series of methanation. The major reactionsfor coal gasification under atmospheric pressure are as followsC+h,o=co+h+131.5 MJ/kmolCO+H2O=CO 2+H2-410 MJ/kmol(6)The reduced oxygen carrier will return to the air real中国煤化工 Figure2).Inaddition, fresh oxygen carrier has probably to be added tqCNMHA oxygen carrier.Ash is much smaller and lighter than oxygen carrier. lus,caulIcI auu ash can be sepaSHEN LaiHong et al. Sci China Ser E-Tech Sci l April 2007 I vol 50 I no. 2 1 230-240rated on the basis of weight difference By taking advantage of the fluidizing velocity, ash can beentrained out from the bubbling fluidized bed. The flue gas exhausting from the bubbling fluidized bed is a nitrogen-free gas mixture with CO2, H2O, and possibly CO and H2, which dependson the properties of oxygen carrier.2 Thermodynamics analysisChemical reaction thermodynamics is important for the understanding of the reaction mechanism,the product composition as well as the design of technical parameters in chemical-looping com-bustion of coal. With the standard Gibbs free energy changes, the equilibrium constants can becalculated for various reactions of metal oxide reduction and coal gasification in a wide range ofoperating temperatures. The relationship between the equilibrium constant Kp and temperature Tis expressed as△HeRIn K+△STwhere AHr and ASr are the standard formation heat, the standard formation entropy at thecorresponding reaction temperature, respectively, and R is the gas constant.With related thermodynamic parameters, the standard formation heat AHT and the equi-librium constant kp can be calculated. Figure 3 shows the equilibrium constants, Kp =Pco, /Pcoas a function of temperature for the reduction of metal oxides with Co. with an increase in theequilibrium constant, the affinity of the reduction of metal oxide with Co also is enhancedCo O/Mn. oCuO/Cu,oFe O/Fe oCuo/cuMoNo甲 NiO/Ni-t. C+HQO=CO+H2▲Fe,O/FeOg9:600700800900中国煤化工TemperCNMHGFigure 3 Variation of equilibrium constant K, for reduction of metal oxide by CO as a function of temperature.234SHEN LaiHong et al. Sci China Ser E-Tech Sci l April 2007 I vol. 50 I no. 2 1 230-240ompared with the process of oxygen carrier oxidation in the air reactor, the reaction processes in the fuel reactor are very complicated. The reactions of coal gasification and oxygen carrier reduction in the fuel reactor are governed by both the prevailing chemical thermodynamicsand reaction kineticsThe equilibrium constants of metal oxide reduction with o have a monotonously decreasingtrend with the increasing temperature, which is in accordance with the exothermic process of thereductions of metal oxide with CO( Figure 3). To make comparison with the metal oxide reduc-tions, the equilibrium constants of major reactions of coal gasification in the fuel reactor underatmospheric pressure are also presented in Figure 3. The equilibrium constants of reactions 5 and6 are much lower than that of the metal oxide reduction with CO. The reaction rates of coal gasification are slower than that of metal oxide reductions with the syngas of Co and H. To raise ahigh-level efficiency of carbon conversion, the diameter of coal particle in coal slurry should besmall in the process of chemical looping combustion of coal. Thus the rate of coal gasificationcould be high enough to match the rate of oxygen carrier reduction. It is the reason why coalslurry instead of coal particle is utilized in the proposed process for chemical looping combustionAs far as the reaction rate of coal gasification, as well as the reduction conversion of oxygencarrier with the syngas of CO and H2 is concerned, a reasonable temperature of the fuel reactor isvery important. For chemical looping combustion of coal, the temperature of the bubbling fluid-ized bed should be kept high enough to maintain the process of coal gasification and the reduction of metal oxide. Based on the above discussions, the temperature should be recommended at900-950℃As shown in Figure 3, the metal oxides based on Ni, Fe, Co, Cu and Mn have a good affinitywith CO, and are thermodynamically feasible as oxygen carrier. However, Mn2O3, Co3O4 andCuo will decompose at a temperature above 820, 890 and 1030C, respectively. And Cu is meltedat 1084'C. These oxides are unsuitable as oxygen carriers in the process of chemical loopingcombustion of coal at a high temperature.For Ni-based and Fe-based oxygen carrier, the oxygen transfer capabilities per mass forFe2O3/Fe3O4, Fe3O4/FeO and NiO/Ni, are 0.03, 0.07 and 0.21, respectively. The larger the oxygentransfer capability of metal oxide, the more the oxygen can be transferred from the air reactor tothe fuel reactor, and the lower the energy consumed by the recirculation of oxygen carrier ininterconnected fluidized beds for chemical looping combustion of coalGenerally, Ni and its corresponding oxide show higher oxidation and reduction rates than Fe,and NiO/Ni has a good capacity of oxygen carrying. Thus, NiO/Ni is the suitable oxygen carrierfor chemical looping combustion of coal3 Results and discussionUsing Aspen Plus software, the thermodynamic equilibrium of chemical looping combustion ofcoal is analyzed. With the use of NiO/Ni as oxygen carrier, the effects of fuel reactor temperature,air reactor temperature, and ratio of water to coal on the composition of flue gas, recirculation ofoxygen carrier particles are discussed. The following as中国煤化工 d on the application of Aspen Plus software:CNMHG(i) The bubbling fluidized bed as the fuel reactor is in a steady stateSHEN LaiHong et a. Sci China Ser E-Tech Scil April 2007 I vol. 50 I no. 2 1 230-240(ii) Except the heat carried away by the flue gas, there is no heat output from the bubbling fluidized bed(iii) More than fourteen gas components including COz, H2O, CO, H2, CH4, N2, SO2, H2S, NO,NH3, COS, etc, are taken into account.Based on mass balance, chemical equilibrium and energy balance between the fuel reactor andthe air reactor, a mathematic model for chemical looping combustion of coal in interconnectedfluidized beds is set up. The Gibbs free energy of the process is minimal when the chemical equilibrium for clc of coal is achievedIn the air reactor, there is a full conversion of Ni into Nio. simulation is performed withXuzhou bituminite at atmospheric pressure. The proximate analysis and the ultimate analysis results of the coal are illustrated in Table 1Table 1 Proximate analysis and ultimate analysis results of coalroximate analysisUltimate analysisLHVCHN(kJ/kg)2.8028.14458223.2459753.798831.140.4528023.24232203.1 Effect of fuel reactor temperatureAt air reactor temperature of 1100 C, and the ratio of water to coal of 0. 3, the gas concentrationsfrom the fuel reactor are shown as a function of the fuel reactor temperature(Figure 4). The totaconcentration of CO2 and H2O is more than 99%, which means that almost pure COz can be ob-tained after H,o is condensed. with an increase in the fuel reactor temperature co in the fluegas increases remarkably, while H2O increases slightly, and co2 decreases correspondinglyThe major reaction of coal gasification, C+H,O CO+H2, is an intensive endothermicprocess, while the water-gas shift reaction, CO+H2O+ CO2+H2, is an exothermic reactionas well as the reduction of NiO with CO and H2. with an increase in the fuel reactor temperature,the equilibrium process of the endothermic reaction goes toward the positive direction, and thatof the exothermic reaction goes towards the reverse one(Figure 3). As a result, it leads to a significant increase in CO and a slight decrease in CO20150.10490030606307007508083090900905506006507008008509009501000Fuel reactor temperature (c)lel reactor temperatre(c)中国煤化工Figure 4 Effect of fuel reactor temperature on the flue gas Air reactorCNMH Gof water to coal is 0.3Sulfur contained in coal is converted into SO2 and H2s in the process of chemical loopingSHEN LaiHong et al. Sci China Ser E-Tech Scil April 2007 I vol 50 I no. 2 1 230-240combustion of coal. Their concentration curves, which intersect at a temperature of 678C, present symmetry and contrary variation. with an increase in the fuel reactor temperature, SO2 in-creases gradually while H2s decreasesAt the condition without oxidizing agent, sulfur contained in coal is converted to inorganiculfur(H2S)and organic sulfur( COS and CS2) in the process of coal gasification. In the presenceof steam, organic sulfuralmost converted to Hs at a high temperature(COS+H2O=CO2+HS, CS2+2H20=2H-S+CO2), so that sulfur in coal is mainly turnedinto H2s in the syngas. As there is an excess of Nio in the fuel reactor, which acts as oxygen carrier, NiO also reacts with h2s to form SO2 and Ho as followsH2S+3NiO=H2O+SO2+3NiIn order to explain the result that SO2 increases with the fuel reactor temperature while H2sdecreases as shown in Figure 4(b), the relationship between the equilibrium constant kp of reac-tion 8 and the temperature is illustrated in Figure 5. The constant Kp is equal to 1.0 at 678C. Asthe temperature is above 678, the equilibrium constant increases exponentially with the tem-perature, thus impelling the process of reaction 8 towards the positive direction and leading toSO2 formation, and H2S abatement, as shown in Figure 4(b)H S+3Nio-H. O+SO+3Ni500600900000l1001200Figure 5 Variation of equilibrium constant Kp for reaction of Nio-HnS as a function of temperaturCoal gasification in the bubbling fluidized bed is an intensive endothermic process. Metal oxde serves not only as oxygen carrier, but also as heat carrier, transferring heat from the air reactorto the fuel reactor. To prevent a larger temperature drop in the bubbling fluidized bed, the recir-culation of oxygen carrier particles must be sufficient enough to transfer heat from the air reactorto the fuel reactor. The recirculation of oxygen carrier is defined as the mass ratio of oxygen carrier circulation to coal feed. The required recirculation(Re) of oxygen carrier is the minimalamount of oxygen carrier that provides the heat to maintain the fuel reactor temperature, whereasthe theoretical recirculation (Ro) of oxygen carrier is the minimal amount of oxygen carrier thattransfers the oxygen amount for sufficient combustion of fuel in the fuel reactor.The effect of fuel reactor temperature on the中国煤化工 eoretical recirculation of oxygen carrier are shown in Figure 6. The reqCNMHGes exponentiallywith the fuel reactor temperature, whereas the theoretical recirculation keeps almost constant.SHEN LaiHong et al. Sci China Ser E-Tech Scil April 2007 I vol. 50 I no. 2 1 230-240Generally, the recirculation of bed material in a conventional circulating fluidized bed is about15-20. At a temperature of the bubbling fluidized bed of 900-950C, the required recirculationof NiO/Ni oxygen carrier should be between 40-60 for chemical looping combustion of coal, asshown in Figure 6. The required recirculation of oxygen carrier can be easily obtained by the par-ticle size of oxygen carrier and the fluidizing velocity of the air reactorRequired recirculationTheoretical recirculation5005506006507007508008509009501000Fuel reFigure 6 Effect of fuel reactor temperature on recirculation of oxygen carier. Air reactor temperature is 1100C, and ratio ofwater to coal is 0.33.2 Effect of ratio of water to coalIn the case where the air reactor temperature is 1100C and the fuel reactor temperature is 900Cthe effect of the ratio of water to coal on the flue gas from the fuel reactor is shown in Figure 7.(a)(b)H0.10.20.3040.50.60.70.809100.0.20.3040.5060.70.8091.0Figure 7 Effect of ratio of water to coal on the flue gas. Air reactor temperature is 1100C, and fuel reactor temperature is900℃The figure clearly demonstrates that the total concentration of CO2 and H2o sums up to 99% inthe flue gas from the fuel reactor, and the CO2 and H2O concentrations from the fuel reactor present almost symmetry and contrary variation. with an increase in the ratio of water to coal, theconcentrations of N2, so2 and co fall down gradually while H2 increases. It is the reason that theincrease in water impels the process of water-shift r v凵中国煤化工)2+H2 towardsthe positive directionCNMHGThe heat required for coal gasification in the fuel reactor goes up with the increasing ratio ofSHEN LaiHong et al. Sci China Ser E-Tech Scil April 2007 I vol. 50 I no. 2 1 230-240water to coal. To keep the temperature of the fuel reactor, more heat should be transferred fromthe air reactor to the fuel reactor by the recirculation of oxygen carrier. The effect of the ratio ofwater to coal on the required recirculation of oxygen carrier particles is shown in Figure 8. Therequired recirculation increases linearly with an increase in the ratio of water to coal6058420640.3040.5060.70.8091.0Ratio of water to coal (kg/kg coalFigure8 Effect of ratio of water to coal on required recirculation of oxygen carrier. Air reactor temperature is 1100C, andfuel reactor temperature is 900C3.3 Effect of air reactor temperatureFor the fuel reactor temperature 900C, and the ratio of water to coal 0.3, the effect of the air re-actor temperature on the required recirculation of oxygen carrier particles is studied. The higherthe air reactor temperature, the larger the heat capacity of oxygen carrier particles. For the sameamount of heat to transfer from the air reactor to the fuel reactor, the required recirculation ofoxygen carrier deceases with an increase in the air reactor temperature. The required recirculationof oxygen carrier particles decreases exponentially with the increase of the air reactor tempera-ture, as shown in Figure 970中国煤化工Air reactor temFigure9 Effect of air reactor temperature on required recirculation ofCNMHGis900℃,andratio of water to coal is 0.3SHEN LaiHong et al. Sci China Ser E-Tech Sci I April 2007 I vol 50 I no. 2 1 230-240 239When the temperature of the air reactor is higher than 1200C, the nitrogen of air could reactwith the oxygen of air to form NOr. The larger the required recirculation of oxygen carrier in theinterconnected fluidized beds, the more the energy consumption to provide the driving force forthe circulation of particles between the two reactors. Also, the large recirculation of oxygen carrier results in the breakage and attrition of oxygen carrier particles in the air reactor. From theabove discussion, it can be concluded that the temperature of air reactor should be between1050-1150C, and the temperature for the fuel reactor should be between 900--950C4 ConclusionsChemical looping combustion of coal in interconnected fluidized beds with inherent separation ofCO2 is proposed in this paper To fulfill a high-level efficiency of carbon conversion and separateoxygen carrier particle from ash, coal slurry instead of coal particle is introduced into the bottomof the bubbling fluidized bed. Simulation of the processes for chemical looping combustion ofcoal, including coal gasification and reduction of oxygen carrier is carried out with Aspen Plussoftware. The effects of air reactor temperature, fuel reactor temperature, and ratio of water tocoal on the composition of fuel gas, recirculation of oxygen carrier particles, etc, are discussedSome useful results are achieved as follows:(i) NiO/Ni is more suitable as oxygen carrier than other metal oxides in chemical loopingcombustion of coal(ii) The suitable temperature of air reactor should be between 1050--1150C, and the tem-perature of the fuel reactor should be between 900--950C(iii)with the increase of fuel reactor temperature, H2O in the flue gas from the fuel reactor in-reases appreciably and CO2 decreases, while Co increases rapidly.iv) Sulfur contained in coal is converted into So, and H2s in the fuel reactor with the in-creasing fuel reactor temperature, SO2 increases while H2s decreases. They present symmetryand contrary variation with the fuel reactor temperature(v)To maintain the fuel reactor temperature, the recirculation of oxygen carrier particles in-creases linearly with an increase in the ratio of water to coal, and decreases exponentially with anincrease in the air reactor temperature.I Richter H, Knoche K Reversibility of combustion processes. ACS Symp Ser, 1983, 235: 71-862 Mattisson T, Lyngfelt A, Cho P. The use of iron oxide as an oxygen carrier in chemical-looping combustion of methanewith inherent separation of COz. FueL, 2001, 80: 1953-19623 Lyngfelt A, Leckner B, Mattisson T. A fuidized-bed combustion process with inherent COz separation: Application ofchemical-looping combustion. Chem Eng Sci, 2001, 56: 3101-31134 Jin H, Okamoto T, Ishida M. Development of a novel chemical-looping combustion: Synthesis of a looping material with adouble metal oxide of Coo-NiO. Energy Fuels, 1998, 12: 1272-12775 Jin H, Okamoto T, Ishida M. Development of a novel chemical-looping combustion: Synthesis of a solid looping materialof NiO/NiAl2O4 Ind Eng Chem Res, 1999, 3: 126-1326 Jin H, Ishida M. Reactivity study on a novel hydrogen fueled chemical-looping combustion Hydr Energy, 2001, 26: 8898947 Cho P, Mattisson T, Lyngfelt A. Comparison of iron, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion Fuel, 2004, 83: 1215-12258 de Diego L F, Garcia-Labiano F, Adanez J et al. Development of Cu-based oxygen carriers for chemical-looping combus-tion.Fuel,2004,83:1749-17579 Johansson E, Lyngfelt A, Mattisson T, et al. Gas leakage measurements in a cold model for an interconnected fluidized bedfor chemical looping combustion. Powder Tech, 2003, 134: 210中国煤化工10 Shen L H, Xiao J Chemical Looping Combustion of Coal withChinese).CN P03152977.1,2003-0908CNMHG11 Ye D L, Hu J H. Thermochemical Properties of Inorganic Substances (n chinese). Being: Metalurgy Industrial Press,2002SHEN LaiHong et a. Sci China Ser E-Tech Scil April 2007 I vol 50 I no 21 230-240