Chemical Looping Gasification of Charcoal with Iron-Based Oxygen Carriers in an Annular Dual-Tube Moving Bed Reactor

Charcoal used as solid fuel for chemical looping gasification (CLG) with Fe2O3/Al2O3 oxygen carriers was investigated in an annular dual-tube moving bed reactor (ADMBR). Pure CO2 and steam/CO2 mixture were used to gasify the charcoal in a fixed bed reactor. More combustible gases, CO and H2, were observed in the outlet gas streams for charcoal gasification conducted with steam/CO2 mixture than those with only CO2. The yields of fuel gases were determined to modify the equation of oxygen carrier-to-fuel ratio (φ) for moving bed operation. More CO and H2 were generated for charcoal gasification conducted in the moving bed reactor operated with higher oxygen carrier-to-fuel ratios. The heat demand for experiment conducted in the ADMBR was calculated to be 60% of output processing capacity due to the heat consumptions of fuel gasification. In sum, ADMBR is technically feasible to be a fuel reactor for partial oxidization of charcoal by CLG to achieve syngas production under appropriate operating conditions without auxiliary fuel.


INTRODUCTION
Syngas (CO/H 2 ) is an important raw material that can be further converted to a number of products, for instance hydrogen, liquid fuels, chemicals, and electricity (Bhattacharya, 2013).Syngas is mainly generated from natural gas reforming through steam methane reforming reaction, or from solid fuel gasification through water gas and Boudouard reactions.Solid fuels are more carbon intensive than methane; however, are much difficult to convert.In order to enhance the gasification of solid fuels, several gases, such as CO 2 and steam, are employed to facilitate gasification and improve the quality of syngas.However, the gasification of solid fuels with CO 2 or steam is endothermic and requires external heat supply to proceed.Hence, air (or O 2 ) is mixed with CO 2 or steam, is introduced to initiate the partial oxidation of solid fuels for providing heat for gasification (Wang et al., 2008).
Chemical looping process (CLP) for fuel combustion has been actively studied and is considered to be a unique alternative to achieve efficient energy generation as well as inherent CO 2 capture (Richter et al., 1983;Fan and Li, 2010;Figureoa et al., 2008).Chemical looping gasification (CLG) is an innovative application of chemical looping process to generate syngas through the partial oxidation of fuels with deficient oxygen carriers (Fan et al., 2008).The reduced oxygen carrier is then oxidized and regenerated by air for heat generation to maintain heat balance of the process.
Metal oxides (called oxygen carriers) providing oxygen for fuel combustion by chemical looping process is rapidly developed recently.Cao et al. (2014) indicated that the agglomeration of copper-based oxygen carrier was avoided by using Al 2 O 3 as support and promoted by lanthanum.In addition, the formation of a crystal phase (CuAl 2 O 4 ), crosslinking CuO and γ-Al 2 O 3 , was supposedly to contribute to the stability of copper-based oxygen carrier, whereas the lanthanum contributed speeding the formation of the crosslinking between CuO and γ-Al 2 O 3 .Zhou et al. (2014) demonstrated that high CO 2 selectivity and CH 4 conversion for CLC operated in the batch fluidized bed reactor with NiO, and also reported that the operation time of approximately 6 min was achieved for 95% of NiO conversion by CLC operated in the fixed bed reactor, while the operation time of 95% of NiO conversion by CLC operated in the fluidized bed reactor reached lower than 3 min, indicating that the longer reaction times of the gas inside the reactor lead to faster conversions of the oxygen carrier.Wang et al. (2016) studied that the kinetic study of Mn-based oxygen carrier for oxygen release by chemical looping air separation (CLAS), and also reported that the first-order chemical reaction model (C1) and Avrami-Erofe'ev random nucleation and subsequence growth model (A2) are appropriate for the O 2 release of MnO 2 and Mn 2 O 3 , respectively.In addition, the activation energies of MnO 2 and Mn 2 O 3 for oxygen release were determined to be 130 and 163 kJ mol -1 , respectively.Chiu et al. (2014) reported that Fe 2 O 3 /Al 2 O 3 oxygen carriers containing 60wt% Fe 2 O 3 sintered at 1300°C demonstrated reasonable oxygen conversion and delivered high CO 2 yields for chemical looping combustion of methane and syngas (CO/H 2 ) in a fixed bed reactor.Most Fe-based oxygen carriers demonstrate higher melting point, better mechanical strength, and lower cost than others.Therefore, various Febased carriers were developed and extensively employed for CLP in previous studies.Kuo et al. (2015) reported that the spinel structure of NiFeAlO 4 oxygen carriers presented high CO conversion and H 2 generation has been proved in fixed bed reactor for chemical looping hydrogen generation process (CLHG), because the ability of self-supported and agglomeration resistance were significantly enhanced.Chiu and Ku (2012) reported that the gasification of solid fuel in the fuel reactor is the rate limiting step for direct solid fuel combustion.The addition of gasification reactants, such as CO 2 , H 2 O and O 2 might enhance the gasification rate of the fixed carbons contained in these solid fuels.
Coal and petroleum coke are major fuels for power generation; however, combustion of coal and petroleum coke is also a major CO 2 emission source because of their high carbon contents (IPCC, 2007).Combustion of coal by chemical looping process is developed and considered to be an efficient alternative for power generation (Li and Fan, 2008).For direct coal combustion in the fuel reactor, a two-step mechanism is proposed, including gasification and combustion.Gasification of coal is a partial oxidation process that fixed carbon in the coal is partially oxidized by gasification reactants, such as O 2 , H 2 O and CO 2 to form CO and/or H 2 as gaseous fuels.The fixed carbon of coal is gasified to CO and H 2 (as H 2 O employed to be the gasification reactant) by gasification reactants in the fuel reactor.The CO and H 2 from gasification process is immediately combusted by oxygen carriers in the same manner of gaseous combustion to yield H 2 O and CO 2 .However, gasification is a ratelimiting step in the fuel reactor to reduce the overall fuel combustion efficiency (Chiu and Ku, 2012).Therefore, retrofit the design of fuel reactor to extend the residence time of solid fuel in the fuel reactor is an alternative way to improve the limitation of low gasification rate for direct coal combustion by chemical looping process.The counter-flow current pattern of solid fuel and oxygen carriers was proposed as one of the engineering designs for better CO 2 yield and fuel conversion using Fe-based oxygen carriers (Li et al., 2010).
Steam or CO 2 as gasification agent to the chemical looping process is widely studied by previous researchers employed with fluidized bed reactor and moving bed reactor (Berguerand et al., 2010;Bayham et al., 2013).However, application of steam/CO 2 mixture for charcoal gasification and combustion is rarely investigated.Hence, charcoal was selected to study its gasification with various gasification agents in a fixed bed.The performance of ADMBR using Fe-based oxygen carriers was also discussed in this study.In this study, pulverized charcoal was employed for chemical looping gasification with Fe-based oxygen carriers in an annular dual-tube moving bed reactor (ADMBR).The compositions of flue gas were determined for experiments carried out at 900°C with varied oxygen carrier feeding rate.The fuel conversion and oxygen carrier conversion for charcoal gasification by CLG with 2.5% steam/CO 2 mixture and Fe 2 O 3 /Al 2 O 3 oxygen carriers were evaluated by ADMBR.Heat analysis for CLG gasification of charcoal was employed to evaluate the heat balance of input and output processing capacities.

Preparation and Characterization of Fe 2 O 3 /Al 2 O 3 Oxygen Carriers
The oxygen carriers used in this study were composed of hematite (99.9% Fe 2 O 3 , China Steel) and alumina (99% Al 2 O 3 , Chin Jung).The Fe 2 O 3 /Al 2 O 3 oxygen carriers were prepared with hematite and alumina at weight percentage of 60/40.Hematite and alumina particles of roughly 1 µm in diameter were mixed with deionized water at room temperature.The well-mixed slurry was then dried at 80°C for 6 hours and was subsequently pulverized and screened for sizes between 1.2 and 1.4 mm.The particles were later sintered at 1300°C for 2 hours in a muffle furnace.The crush strength of prepared Fe 2 O 3 /Al 2 O 3 oxygen carriers ranged of 1.2 to 1.4 mm was determined to be approximately 22N by a texture machine (TA.XT plus) according to ASTM D4179-01.The reactivity and recyclability of prepared Fe 2 O 3 /Al 2 O 3 oxygen carriers was analyzed by thermogravimetric analyzer (TGA), the phase transformation of prepared Fe 2 O 3 /Al 2 O 3 oxygen carriers during redox cycle was characterized by X-ray diffraction (XRD), and surface morphology and interfacial behaviors of Fe 2 O 3 /Al 2 O 3 oxygen carriers were examined by a field emission scanning electron microscope (FESEM) have been clearly described in previous publication.(Chiu et al., 2014)

Characterization of Charcoal
Charcoal was pulverized and sieved for sizes between 1.2 and 2 mm before it was applied as solid fuels in this study.Proximate analysis of charcoal was conducted by a Netzsch STA 449F3 thermogravimetric analyzer (TGA) to determine the contents of moisture, volatile matters, fixed carbon and ash.Charcoal was heated in a TGA chamber for pyrolysis in nitrogen atmosphere, the temperature of TGA chamber was elevated with a ramping rate of 20°C min -1 from room temperature to 700°C.Based on the TGA results, the contents of moisture and volatile matters were determined.Air was then introduced to the TGA chamber to combust the residues to determine the fixed carbon and ash contents of charcoal.Ultimate analysis of charcoal was analyzed by an elemental analyzer (Elementar vario EL III) to determine the elemental contents of C, H, O, N, and S.

Establishment of Fixed Bed Reactor System
The fixed bed reactor system employed in this study, demonstrated in Fig. 1, is composed of a 60 mm ID stainless steel tubular reactor and with a PID-controlled heating element covering 200 mm of reactor height.The thermocouple connected to the center of the top cover of the fixed bed reactor was composed of a 6.35 mm ID SS310 tube.Two 6.35 mm ID SS310 tubes were connected at the top cover of the fixed bed reactor for gas inlet and flue gas outlet, respectively.75 g of Charcoals were gasified at 900°C by introducing gasification agents of various compositions.The outlet stream from the fixed bed reactor was passed through a cold trap to condense steam, and was consequently analyzed by a gas chromatography equipped with a thermal conductivity detector (GC-TCD, China Chromatography 2000) to detect carbon dioxide, carbon monoxide, methane, hydrogen, and oxygen.The detailed test conditions were listed in the Table 1.

Establishment of Moving Bed Reactor System
The schematic diagram of an annular dual-tube moving bed reactor (ADMBR) for charcoal gasification and combustion employed in this study is shown in Fig. 2. The ADMBR was composed of a 76.20 mm ID SS310S outer tube covered with an electric heating element to provide isothermally atmosphere, and a 25.40 mm ID SS310S inner gasification tube.For empty bed operations, 17.17 g min -1 of charcoal was introduced into the gasification tube by a screw conveyor.Various flow rate of 2.5 mol.% steam/CO 2 mixture was also fed into the gasification tube to serve as gasification agent.The gaseous products generated by charcoal gasification along with steam and CO 2 were then left the gasification tube, and flew through the fuel reactor.The temperature for empty bed reactor system was controlled at 900°C, and the total flow rate of the 2.5 mol.% steam/CO 2 mixture was operated between 41.14 and 205.80 mmol min -1 .The char, residues and ash were collectively removed out of the gasification tube by a screw conveyor.
For continuously moving bed operations, roughly 2.0 kg of prepared Fe 2 O 3 /Al 2 O 3 oxygen carriers was initially packed in the spacing between inner and outer tubes of the ADMBR.Fe 2 O 3 /Al 2 O 3 oxygen carriers were then continuously fed into the packed reactor by a screw conveyer at flow rate operated between 11.48 and 29.67 g min -1 .The charcoal particles were fed into gasification tube through the screw conveyor, and 2.5 mol.% steam/CO 2 mixture was also introduced into the gasification tube at a molar flow rate of 164.66 mmol min -1 .All the operating flow rates were limited by the experimental equipment, such as syringe pump, mass flow controller and screw conveyer.The fuel gas generated by charcoal gasification was then passing through the bottom of gasification tube for consequent combustion with Fe 2 O 3 /Al 2 O 3 oxygen carriers fed into and moved through the moving bed reactor.The reduced oxygen carrier was collectively removed out of the  fuel reactor by a screw conveyor.The outlet stream from the moving bed reactor was cooled by a cold trap to condense and was afterward analyzed by a gas chromatography equipped with a thermal conductivity detector (GC-TCD, China Chromatography 2000) to detect carbon dioxide, carbon monoxide, methane, hydrogen, and oxygen.The detailed test conditions were listed in the Tables 2 and 3.

Charcoal Gasification with Various Gasification Agents
The results of proximate and ultimate analyses for charcoal studied in this research are listed in Table 4.The content of volatile materials in charcoal was determined to be 21.14 wt%; while fixed carbon and ash contained in charcoal was 71.54 and 4.14 wt%, respectively.Based on the result of ultimate analysis, carbon and oxygen were the major elements in the charcoals.As also listed in Table 4, the higher heating value (HHV) of the charcoal was calculated to be 34.31MJ kg -1 , demonstrating similar results reported by previous researchers (Bayham et al., 2013).
Gasification is a rate-limiting step in the fuel reactor to reduce the overall fuel combustion efficiency.Hence, the performance of the charcoal gasification with various gasification agents was investigated in a fixed bed.In this section, charcoal was gasified by CO 2 /N 2 mixtures containing 2.5 to 20 mol% CO 2 in a fixed bed reactor at 900°C, to carry out the following inversed Bondouard reaction: The outlet streams from the fixed bed reactor were composed mainly of CO, with minimum amounts of CO 2 , as illustrated in Fig. 3. Based on the carbon balance, the carbon gasification rate (F c,gasification ) is defined as: where F c,gasification is the gasification rate of carbon of charcoal; F CO 2 ,in is the input molar flow rate of CO 2 which gasified with charcoal; F CO 2 ,out and F CO,out are the output molar flow rate of CO 2 and CO, respectively.The concentrations of CO and carbon gasification rates were increased with increasing the inlet concentration of CO 2 , possibly because of the Le Chatelier's principle.
For gasification with steam, charcoal is gasified by 2.5 to 20 mol% steam based on N 2 through water gas and water gas shift reactions, as described by reactions (3) and (4).CO and H 2 are the major fuel gases generated during charcoal gasification with steam at 900°C, as illustrated in Fig. 4. CO and H 2 concentrations of the outlet gas stream for charcoal gasification was found to be increased for experiments carried out with greater the inlet concentration of steam, whereas CO 2 concentrations decreased.The lower carbon gasification rate was observed for charcoal gasified by 2.5 to 15 mol.% steam than those by 2.5 to 15 mol.%CO 2 , while higher carbon gasification rates was reached for experiments with 20 mol.% steam than those with 20 mol.% CO 2 , as shown in the Figs. 3 and 4. As described by reaction (3), the gasification of charcoal at 2.5 to 15 mol.% steam may form CO and H 2 , whereas the carbon gasification rate was increased for charcoal gasified by 20 mol.% steam possibly due to the more CO 2 was generated by water gas shift reaction for further charcoal gasification, similar results were reported by previous researchers (Berguerand et al., 2010).
Charcoal gasification with steam/CO 2 mixture was also investigated in a fixed bed reactor.Outlet gas stream containing about 82% CO and 18% CO 2 were achieved for charcoal gasification using pure CO 2 .More combustible gas were observed in the outlet gas stream for experiments conducted with 2.5% steam/CO 2 mixture than those with only CO 2 , possibly due to the occurrence of Boudouard and water gas reactions.However, CO concentration of the outlet gas stream was reduced to about 50 vol.%for experiments conducted with steam/CO 2 mixture containing more than 2.5 mol.% steam, as shown in Fig. 5.More CO 2 and H 2 were generated due to the occurrence of water gas shift reaction, as depicted by reaction (4), similar results were reported by Berguerand et al. (2010).

Charcoal Gasification with Steam/CO 2 Mixture
According to the results of the section 3.1, 2.5% steam/CO 2 mixture was employed as gasification agent for consequent gasification of charcoal were conducted in the empty bed reactor and chemical looping gasification of charcoal were operated in an ADMBR with iron-based oxygen carrier.In this section, effect of 2.5% steam/CO 2 mixture flow rate on the combustible gas composition after charcoal gasification were investigated in an empty bed reactor.The gasification of charcoal were conducted in the empty bed reactor at 900°C by feeding charcoal particles at flow rate of 17.17 g min -1 and steam/CO 2 mixture containing 2.5 mol.% steam and 97.5 mol.%CO 2 at flow rate from 41.14 to 205.80 mmol min -1 .The main components of outlet streams for charcoal gasification with steam/CO 2 mixture were determined to be CO, H 2 , and CO 2 , as illustrated in Fig. 6.The molar flow rate of outlet stream was increased with experiments conducted with increasing steam/CO 2 mixture flow rate.The yields of CO, H 2 , and CO 2 in the outlet stream, Y CO , Y H 2 , and Y CO 2 , were determined to be 1.63, 0.16, and 0.04, respectively, indicating CO 2 was majorly converted to CO.
The H 2 /CO ratio was 1:1 through water gas shift reaction described by reaction (4); however, more CO was generated by the inversely Bondouard reaction described by reaction (1), whereas the amount of CO 2 was decreased.Therefore, the amount of CO was consequently reached approximately 10 times greater than H 2 based on CO and H 2 gas yields.The molar flow rate of fuel gas, F Fuel gas , is described by Eq. ( 5), which is sum of F CO and F H 2 for using charcoal as solid fuel. The

Charcoal Gasification in a Moving Bed Reactor
Chemical looping gasification of charcoal was conducted by feeding deficient Fe 2 O 3 /Al 2 O 3 oxygen carriers into the ADMBR to provide lattice oxygen for production of syngas generated form charcoal.Effect of the oxygen carrier-to-fuel ratio on the combustible gas composition, fuel conversion and oxygen carrier conversion were investigated in an ADMBR.Chemical looping gasification of charcoal was conducted in the ADMBR at 900°C by feeding 17.17 g min -1 charcoal particles and 164.66 mmol min -1 gasification agent.For moving bed operations, 2.0 kg of Fe 2 O 3 /Al 2 O 3 oxygen carriers were packed with 220 mm of bed height.The flow rate of Fe 2 O 3 /Al 2 O 3 oxygen carriers feed into the moving bed reactor was operated between 11.48 and 29.67 g min -1 .In this study, the oxygen carrier-to-fuel ratio (ϕ) is the ratio of oxygen provision by Fe 2 O 3 /Al 2 O 3 oxygen carrier and oxygen demand by fuel gas in the fuel reactor, as described by Eq. ( 6).where F Fe 2 O 3 is the molar flow rate of Fe 2 O 3 /Al 2 O 3 ; F Fuel is the molar flow rate of fuel gas; b is the stoichiometric coefficient of fuel gas combusted with Fe 2 O 3 .Based on the result for charcoal gasification in an empty bed reactor, CO and H 2 are the major fuel gases generated; the stoichiometric coefficients for CO and H 2 combustion with Fe 2 O 3 are both unity, as expressed by Eqs. ( 7) and (8).
The oxygen carrier-to-fuel ratio is rearranged and rewritten as Eq. ( 9) for charcoal combustion with Fe 2 O 3 /Al 2 O 3 oxygen carriers.Y g is 1.79 based on the results of empty bed experiments.ϕ is determined to be in the range of 0.15 to 0.38.
where ṁ OC is the mass flow rate of Fe 2 O 3 /Al 2 O 3 oxygen carriers; x Fe 2 O 3 is the fraction of Fe 2 O 3 contained in the oxygen carrier; M Fe 2 O 3 is the molecular weight of Fe 2 O 3 .CO and H 2 concentrations of the outlet gas stream from charcoal gasification were found to be increased with increasing ϕ, whereas CO 2 concentration was decreased, as demonstrated in Fig. 7. Hautman et al. (1981) reported that the oxidation of hydrocarbons was two-step reactions, as described in reactions ( 10) and (11), while the reaction of CO oxidized to CO 2 was slight slower.Hence, CO and H 2 might be formed from the charcoal gasified by CLG with Fe 2 O 3 /Al 2 O 3 oxygen carriers and steam/CO 2 mixtures at ϕ lower than 1.Ge et al. (2015Ge et al. ( , 2016) ) reported that the maximum syngas (CO and H 2 ) yield of rice straw gasification reached about 0.33-0.74Nm 3 kg -1 for experiments conducted at 750°C.In this study, the maximum syngas yield of around 0.28 Nm 3 kg -1 was achieved for the chemical looping gasification of charcoal with Fe 2 O 3 /Al 2 O 3 flow rate of 29.7 g min -1 , much lower than those reported for rice straw and rice husk gasification, because fewer oxygen and volatile material were present in the charcoal.
Outlet gas stream comprised of 27.8 mol.%CO, 0.9 mol.%H 2 and 71.3 mol.%CO 2 were achieved for experiments conducted with the oxygen carrier-to-fuel ratio of 0.15.More CO and H 2 were generated for charcoal gasification conducted with higher oxygen carrier-to-fuel ratios because more oxygen was provided.Similar to the result reported by previous study (Cuadrat et al., 2012) that coal gasification and combustion efficiencies were decreased for experiments conducted with the oxygen carrier-to-fuel ratio lower than 1.
The fuel conversion (X Fuel ) and oxygen carrier conversion (X OC ) for charcoal gasification by chemical looping were subsequently calculated by the following equations, respectively: , , , , , , are the outlet molar flow rates of CO 2 , H 2 O and CO generated, respectively.Fuel reactor design for longer residence time of Fe-based oxygen carriers should be considered due to their low reactivity.However, as shown in Fig. 8, X Fuel for charcoal gasification was slightly decreased for experiments conducted with decreasing oxygen carrier-tofuel ratio, indicating insufficient oxygen was provided, similar to the results reported by previous study (Luo et al., 2014).The conversion for Fe 2 O 3 /Al 2 O 3 oxygen carriers was around 33.33% that corresponded to the amount of oxygen utilization from Fe 2 O 3 to FeO.Approximately, 38% of the oxygen carrier conversions were maintained for charcoal gasification with oxygen carrier-to-fuel ratio from 0.15 to 0.38, demonstrating the iron-based oxygen carriers operated in a moving bed reactor was majorly reduced to FeO.

Heat Analysis
Heat balance and processing efficiency are analyzed for the charcoal gasification during CLG operation in this study.The input processing capacity (Q in ) is calculated as: where ṁ and ΔH Fuel are the mass flow rate (g s -1 ) and the higher heating value of charcoal (MJ kg -1 ), respectively.Thus, the Q in is determined to be 9,818 W for experiments conducted with 17.17 g min -1 charcoal.The heat balance analysis is determined by the following equation: where Q g,out and Q C,out are the output processing capacity (W) for charcoal gasification and CLG, respectively; Q unC,out is the unburned processing capacity for charcoal, CO and H 2 .The output processing capacity for CLG and unburned was subsequently calculated by the following equation: where F i,out,g and F i,out,C are the molar flow rates of species i generated by charcoal gasification and CLG, respectively; ΔH rxn,i is the enthalpy of species i; i is denoted as CO, H 2 , O 2 and C. The compositions in the outlet gas stream for charcoal gasification and charcoal combustion conducted with ϕ of 0.38 for moving bed operation at 900°C were listed in Table 5, respectively.Hence, the Q C,out , Q unC,out and Q g,out were determined to be 497 W, 9,602 W and -281 W, respectively, as calculated by Eqs. ( 15)-( 17).Q C,out for chemical looping gasification of charcoal was slightly decreased for experiments conducted with higher ϕ, and reached to about 500 W, as shown in Fig. 9.However, Q g,out increased slightly with increasing ϕ, and enhanced to around -300W, as shown in Fig. 9, indicating that above 60% of Q C,out generated was consumed for charcoal gasification in the ADMBR.Less heat was demanded for CLG of charcoal is possibly caused by the fuel gases combusted with Fe 2 O 3 /Al 2 O 3 oxygen carrier, and more heat was generated for experiments carried out with greater ϕ.Therefore, ADMBR is technically feasible to be a fuel reactor for partial oxidization of charcoal by CLG    to achieve syngas production under appropriate operating conditions without auxiliary fuel.

CONCLUSIONS
CO is the major fuel gases generated during charcoal gasification with CO 2 at 900°C.The carbon gasification rate of charcoal reached 12.27 mmol min -1 for experiment conducted with the CO 2 at concentration of 20 mol.%.CO and H 2 are the major fuel gases generated from charcoal gasification with steam in fixed bed at 900°C.13.25 mmol min -1 of the carbon gasification rate of charcoal was achieved as the steam concentration reached 20 mol.% for charcoal gasification in fixed bed.By comparing with the CO 2 gasification of charcoal, the lower carbon gasification rate was observed for charcoal gasified by 2.5 to 15 mol.% steam than those by 2.5 to 15 mol.%CO 2 , while higher carbon gasification rates was reached for experiments with 20 mol.% steam than those with 20 mol.% CO 2 .Pure CO 2 was used to gasify the charcoal in fixed bed.Nearly 82% of CO concentration of charcoal gasification was achieved for experiments carried out with pure CO 2 , while the CO 2 concentration was reached to about 18%.More combustible gas and fewer CO 2 were observed in the outlet gas streams for experiments conducted with steam/CO 2 mixture than those with only CO 2 .However, the CO concentration of the outlet gas stream for experiment conducted with the inlet concentration of steam greater than 2.5 mol.%, and decreased to about 50 vol.%.For empty bed operation, CO and H 2 are the major fuel gases generated during charcoal gasification prior to combustion with oxygen carrier at 900°C.The yields in the outlet stream were linearly increased with increasing steam/CO 2 mixture flow rate.Thus, the amounts of CO and H 2 generated by charcoal were estimated to modify the equation of oxygen carrier-to-fuel ratio (ϕ) for moving bed operation.CO and H 2 are the major components in the outlet gas stream generated from charcoal gasification with Fe 2 O 3 / Al 2 O 3 oxygen carriers in the annular dual-tube moving bed reactor (ADMBR).Generation of CO and H 2 were enhanced for charcoal gasification conducted with higher oxygen carrier-to-fuel ratios.Output processing capacity estimated for CLG by employment of ADMBR as fuel reactor was slightly decreased for experiments conducted with higher ϕ, and reached to about 500W.About 300 W was required for charcoal gasification in the ADMBR, indicating that 60% of output processing capacity generated for charcoal gasification in the ADMBR.In sum, ADMBR is technically feasible to be a fuel reactor for partial oxidization of charcoal by CLG to achieve syngas production under appropriate operating conditions without auxiliary fuel.authors appreciated China Steel Corp. for providing hematite powders for the preparation of oxygen carriers.

Fig. 1 .
Fig. 1.Schematic diagram of the fixed bed reactor used in this study for charcoal gasification.

Fig. 2 .
Fig. 2. Schematic diagram of the moving bed reactor system used in this study for charcoal combustion.

Fig. 3 .
Fig. 3. Concentration of outlet gas stream obtained from charcoal gasification in fixed bed reactor under various inlet concentration of CO 2 based on nitrogen as carrier gas.

Fig. 4 .Fig. 5 .
Fig. 4. Concentration of outlet gas stream obtained from charcoal gasification in fixed bed reactor under various inlet concentration of H 2 O based on nitrogen as carrier gas.

Fig. 6 .
Fig. 6.Effect of gasification agent flow rate on the gasification of charcoal in empty bed.

Fig. 8 .
Fig.8.Effect of oxygen carrier-to-fuel ratio on fuel and oxygen carrier conversions for CLG of charcoal in moving bed.

Fig. 9 .
Fig.9.Effect of oxygen carrier-to-fuel ratio on output processing capacity for CLG of charcoal in moving bed.

Table 1 .
The operating conditions of charcoal gasification with various gasification agents in the fixed bed reactor.

Table 2 .
The operating conditions of charcoal gasification with 2.5 mol.% steam/CO 2 mixture in the empty bed reactor.

Table 3 .
The operating conditions of charcoal gasified for CLG operation in the moving bed reactor.

Table 4 .
Indexes for evaluation of oxygen carriers.
fuel gas yield, Y g , which is sum of Y CO and Y H 2 , is Effect of oxygen carrier-to-fuel ratio on the CLG of charcoal in moving bed. ,in and F H 2 O,in are the inlet molar flow rates of H 2 O and CO 2 for charcoal gasification; F CO 2 ,out,C , F H 2 O,out,C and FCO,out,C

Table 5 .
Molar flow rates of gas component in the outlet streams from the decomposition and combustion of charcoal.