Chemical Looping Process-A Novel Technology for Inherent CO 2 Capture

Chemical looping is a novel energy technology that undertakes inherent CO2 capture and has better energy efficiency than existing CO2 capture technologies. Metal oxides (also known as oxygen carriers) are applied for the chemical looping process in place of air to offer oxygen for fuel combustion, thus enhancing CO2 purity in the effluent stream. This article introduces the major aspects of chemical looping technology, including the oxygen carrier, reactor and fuel. Fe-, Niand Cu-based metal oxides were found to be suitable oxygen carriers for the chemical looping process based on the evaluation indexes of oxygen capacity, cost, reactivity, and mechanical strength. The modification of oxygen carriers by supporting materials is the typical research regime for enhancing mechanical strength and reactivity. The reactor systems currently used for the chemical looping process include fluidized and moving bed reactors. Fluidized bed reactors are well developed for gaseous fuel combustion applications, while moving bed reactors operated with a counter-flow mode between the solid fuel and oxygen carriers would enhance combustion efficiency, and reduce the operating solid inventory of oxygen carriers. Gaseous fuels, including natural gas and syngas gasified from coal, could achieve complete fuel conversion and high CO2 yield for different combinations of oxygen carrier and reactor system. Gasification of solid fuels, including coal, petroleum coke and biomass in the fuel reactor, is a critical step that might reduce overall combustion efficiency. To improve solid fuel combustion and CO2 yield, modification of the reactor design or operational conditions are necessary for the combustion of solid fuels by a chemical looping process.

chemical intermediates, and is considered a promising technology for efficient power generation as well as inherent CO 2 capture for possible carbon reuse or storage (Figureoa et al., 2008).For chemical looping process, a fuel reactor and an air reactor are utilized for fuel combustion and heat generation, respectively, as illustrated in Fig. 1.Metal oxide (Me x O y ) is reduced by carbonaceous fuel to generate CO 2 and/or H 2 O in the fuel reactor as described as reaction (1).The reduced metal (Me x O y-1 ) is then oxidized to Me x O y by oxygen in the air reactor as described as reaction (2), which is an exothermic reaction that provides heat to external facilities.
For practical operation of chemical looping system, oxygen carrier particles are transported within and between the fuel reactor and the air reactor.Currently, fluidized bed and moving bed reactor systems are developed for the application of chemical looping process, depending on the Development of chemical looping technology is extremely rapid in last decade while tests of pilot scale reactors for thousands hour operation were proceeded in many countries around the world with various fuels, such as syngas, methane, natural gas, coal, petroleum coke and biomass (Mattisson et al., 2001; Johansson et al., 2006;  Leion et al., 2009; Shen et al., 2009a; Lyngfelt, 2011).The chemical looping reactors are suggested to substitute the conventional boilers for power generation, as illustrated in Fig. 1; high concentrations of CO 2 can be captured in the flue gas emitted from the fuel reactor while the air reactor is connected with steam engine for electricity generation.Besides, hydrogen can be an alternative product by oxidation of reduced oxygen carriers in the H 2 generator as illustrated in Fig. 1.The H 2 can be generated by employment of Febased oxygen carriers through chemical looping operation, and transported to external units, such as solid oxides fuel cell (SOFC), storage and chemical processes (Li and Fan, 2008).Efficiency of electricity generation for syngas (CO/H 2 ) combustion by chemical looping process was evaluated to be 43.2%(on LHV basis) (Xiang and Wang, 2008), much higher than those for pulverized coal power plants with amine absorption for CO 2 capture, estimated to be about 21 to 35% (Li and Fan, 2010).
The development of chemical looping technology can be divided by three categories, including oxygen carriers, reactors and fuels.The oxygen carrier is the core of chemical looping technology, the performance of reactivity, recyclability and mechanical strength is crucial for long-term and continuously operation in the chemical looping reactor system.In addition, cost of oxygen carrier is considered as one of the most important issues to commercialize chemical looping technology.Recently, low cost Fe-based oxygen carriers are trying to employ for solid fuel combustion; however, low reactivity of Fe-based oxygen carriers should be resolved through modification of oxygen carriers and retrofit of the reactor design to increase fuel conversion efficiency.Chemical looping combustion of gas fuels, such as syngas and natural gas have been widely validated to provide with high combustion efficiency by chemical looping process.However, for solid fuel combustion, gasification is a rate-limiting step in the fuel reactor to reduce the overall combustion efficiency.Therefore, how to improve the solid fuel combustion efficiency by enhancing gasification rate or prolonging residence time by reactor operation is a challenge to achieve solid fuel chemical looping combustion with high fuel conversion efficiency.

OXYGEN CARRIERS FOR CHEMICAL LOOPING PROCESS
Oxygen carriers applied for chemical looping process should provide excessive oxygen carrying capacity, high reaction rate, great mechanical strength, and long-term recyclability (Hossain and de Lasa, 2008).Ni-, Fe-, Cu-, Mn-and Co-based metal oxides are the typical materials to be employed as oxygen carrier for chemical looping process (Mattisson et al., 2003).CaSO 4 was also used for transferring oxygen for methane combustion in fluidized bed reactors (Song et al., 2009).However, sintering and attrition of these oxygen carriers during chemical looping operation are considered to be the main concerns to reduce their reactivity and recyclability.Therefore, the coupling of oxygen carriers with support materials, such as Al 2 O 3 , TiO 2 , ZrO 2 , NiAl 2 O 4 , MgO, SiO 2 , Y 2 O 3 + ZrO 2 (YSZ), sepiolite and MgAl 2 O 4 , is employed for improving the life of the oxygen carriers for long-term operation (Adanez et  al., 2004).Fabrication techniques of composite oxygen carriers with suitable particle size and mechanical strength include mechanical mixing, co-precipitation, dissolution, freeze-granulation, impregnation and spray drying (de Deigo et al., 2005;Gayan et al., 2008;Hossain and de Lasa, 2008).
The redox analysis by TGA is a typical preliminary procedure to evaluate the reactivity and recyclability of oxygen carriers employed for chemical looping process (Adanez et al., 2004; Mattisson et al., 2003; Li et al., 2009).The degree of conversion for a specific oxygen carrier is the fraction of oxygen in the oxygen carrier that reduced by fuel, and defined by Eq. ( 3): where m o is the weight of fully oxidized oxygen carrier; m r is the weight of fully reduced oxygen carrier; m(t) is the weight of oxygen carrier at a specific operation time, t.For instance, TiO 2 supported Fe 2 O 3 illustrated stable degree of conversion for 100 redox cycles conducted with 10% CO and 10% H 2 for reduction and air for oxidation processes at 900°C in TGA as shown in Fig. 2 (Liu, 2011).The preliminary test by TGA with stable reactivity for continuous redox cycling as illustrated in Fig. 2 can be selected to examine in a bench scale chemical looping reactor to assess its fuel conversion efficiency as well as degree of agglomeration after long-term operation.
For the application of oxygen carriers in chemical looping process, the partial pressure of CO 2 and H 2 O should be as high as possible to approach 100% of CO 2 yield.Therefore, the analysis of equilibrium phase diagram between metal oxide and fuel is a feasible tool for selecting appropriate oxygen carriers.Gupta et al. (2007) analyzed the equilibrium phase diagrams of numerous Ni-, Cd-, Cu-Co-, Mn-, Snand Fe-based metal oxides as reacted with CO and H 2 that would be fully oxidized by these metal oxides.Cao and Pan (2006) compared the phase diagrams of metal oxides with regards to syngas indicated that Cu-and Fe-based metal oxides exhibit highest ratios of p CO2 /p CO and p H2O /p H2 , both are approximately 10 5 .Hence, through the analysis of thermodynamic phase diagrams, high CO 2 yields can be achieved without additional separation unit by selecting proper metal oxides for chemical looping process operated at suitable temperatures.
The properties of Fe-, Ni-, Cu-, Mn-and Co-based oxygen carriers are listed in Table 1.The properties for selection of an oxygen carrier for chemical looping process are including oxygen capacity, cost, reactivity, conversion efficiency of fuel, melting point, mechanical strength and impacts on environment and health (Fan, 2010).Beside of these properties, the use of support materials for better performance on reactivity and mechanical strength are summarized in the last column of Table 1 for reference (Adanez et al., 2012).Most Fe-based oxygen carriers demonstrate higher melting point, better mechanical strength, lower environmental impact and lower cost than the others.Therefore, various Fe-based oxygen carriers were developed and extensively employed for chemical looping process in previous studies (Mattisson et al., 2001; Johansson et  2004; Berguerand and Lyngfelt, 2009;Li et al., 2009;Azis et al., 2010).Moreover, the reduced Fe-based oxygen carriers (Fe or FeO) were reported to be thermodynamically feasible being oxidized by steam to generate hydrogen (Li and Fan, 2008).However, reactivity of Fe-based oxygen carriers is lower comparing to that of Ni-, Mn-and Cubased oxygen carriers, causing lower combustion efficiency with solid fuels (Mattisson et al., 2004; Adánez et al., 2010;  Cuadrat et al., 2011).Ni-and Mn-based oxygen carriers demonstrate high oxygen capacity and reactivity; therefore, are suitable for applications in fluidized bed reactors for chemical looping combustion.However, cost, toxicity and carbon deposition are major drawbacks have to be resolved for Ni-and Mn-based oxygen carriers (Mattisson et al.,  2003; Zafar et al., 2005; Johansson et al., 2006).Co-based oxygen carriers provide with high melting point to reduce sintering during chemical looping operation; however, the application of Co-based oxygen carrier is limited to their lower fuel conversion efficiency and inherent high toxicity (Zafar et al., 2006).Cu-based oxygen carriers present similar oxygen capacity and reactivity to Ni-and Mn-based oxygen carriers at lower cost.However, low melting points of Cubased oxygen carriers limit their long-term operation at high temperatures.Besides, the reduction of Cu-based oxygen carriers is exothermic while other oxygen carriers are reduced endothermically.Therefore, the design of fuel reactor system with regard to Cu-based oxygen carriers should pay special attention on heat transfer concerns (Cao and Pan, 2006).

REACTOR SYSTEMS OF CHEMICAL LOOPING PROCESS
Depending on the mode of oxygen carrier transportation in the fuel reactor, fluidized bed and moving bed reactor systems are developed for the application of chemical looping process.A fluidized bed chemical looping system usually composes of a fuel reactor, an air reactor, an air compressor, loop seals, cyclones and a cold trap as shown in Fig. 3.The oxygen carrier particles are mixed with fuel by fluidizing gas for gas/solid fuel combustion in the fuel reactor.The effluent stream from the fuel reactor consists of mainly CO 2 and H 2 O, and high purity CO 2 is yielded after separation of H 2 O by the cold trap.The reduced oxygen carrier particles are flown to the inert gas-pressurized loop seal, which avoid gaseous fuel flow into the air reactor, and are transported to the air reactor.The reduced oxygen carrier particles are oxidized by pressurized air in the air reactor for heat generation, and are rose to the cyclone.Small particles are separated from the air reactor by the cyclone, and then the remaining oxygen carrier particles flow back to the fuel reactor through a loop seal for next operation cycle.
The fluidized bed chemical looping systems in different designs, such as circulating fluidized bed, dual circulating fluidized bed and spouted fluidized bed, were operated and studied extensively (Kronberger et al., 2004; de Diego et  al., 2007; Berguerand and Lyngfelt, 2008; Gao et al.,  2008; Ryu et al., 2008; Lyngfelt, 2011).The fluidized bed systems have been validated to carry out chemical looping combustion at 120 kW level of heat capacity with gaseous fuels and Ni-based oxygen carriers (Bolhàr-Nordenkampf et  al., 2009).The particle circulation rates of oxygen carriers for a 1,000 MW chemical looping combustion using methane were estimated to be from 3,000 to 10,000 kg/s, indicating that the circulation of oxygen carrier particles would be a major energy penalty to decrease overall energy efficiency (Fan, 2010).For coal combustion, the consumption of oxygen carriers would be significant because the separation of ash and oxygen carrier particles is difficult; hence, the design of cyclone as carbon stripper is significant for particle separation (Berguerand and Lyngfelt, 2008).Therefore, several low cost oxygen carriers, such as ilmenite, were employed recently for the application of chemical looping process using fluidized bed systems because high consumption of oxygen carrier resulted in high operation cost on oxygen carrier (Leion et al., 2008;  Proll et al., 2009a; Adanez et al., 2010)  The schematic diagram for moving bed chemical looping system is shown in Fig. 4 that are composed of a fuel reactor, a hydrogen generator, an air reactor (riser) and loop seals/valves, an air compressor and a cold trap (Sridhar et  al., 2012).The moving bed chemical looping system was designed for application of Fe-based oxygen carriers moved counter-currently with fuels according to the thermodynamic analysis (Gupta et al., 2007; Li et al., 2010a).The crystalline phase distribution of Fe-based oxygen carriers are Fe 2 O 3 , Fe 3 O 4 , FeO and Fe from the top to the bottom in the counter-flow moving bed fuel reactor.The gaseous fuel passed through the fuel reactor from the bottom to the top; therefore, high p CO2/ p CO and p H2O/ p H2 ratios are achieved in the effluent stream (Sridhar et al., 2012).Moreover, higher oxygen capacity in oxygen carriers can be utilized in the moving bed system than that in the fluidized bed system (Li and Fan, 2008).The reduced oxygen carriers are moved by loop seal or mechanical valves purged by inert gas between the fuel reactor and the hydrogen generator.Mechanical valves are high cost and less reliable for a MW level moving bed reactor; therefore, application of gas valves, such as L-valve, J-valve and H-valve, is considered suitable for large scale operation (Fan, 2010).The reduced Fe-based oxygen carriers, FeO/Fe, are moved to hydrogen generator oxidizing by H 2 O to generate H 2 , and the H 2 in effluent stream can provide to external energy unit or H 2 demanded process after steam separation (Fan et al., 2009).However, the Fe 3 O 4 is the final crystalline phase that can not be oxidized by H 2 O in the hydrogen generator; hence, the Fe 3 O 4 oxygen carrier particles are moved and oxidized by pressurized air in the air reactor accompanied with heat generation.The Fe 2 O 3 oxygen carrier particles flow back to the fuel reactor through a loop seal or mechanical valves for next operation cycle.

FUELS FOR CHEMICAL LOOPING PROCESS
For chemical looping process, combustion of gaseous and solid fuels with oxygen carriers exhibited different mechanisms in the fuel reactor.Fuel combustion in chemical looping process is to fully oxidize the fuels to be CO 2 and H 2 O by oxygen carriers.The mechanism of gaseous fuel combustion is illustrated in Fig. 5, the gaseous fuel reacted with the oxygen carrier, and the effluent components are expected to be mainly CO 2 and H 2 O with fully combustion of fuel.For solid fuel combustion in the fuel reactor, a twostep mechanism is proposed, including gasification and combustion, as illustrated in Fig. 5. Gasification of solid fuel is a partial oxidation process that fixed carbon in the solid fuel 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 solid fuels 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 .
Oxygen carrier is reduced in the fuel reactor to provide oxygen for gaseous fuel combustion.Natural gas is one of the primary fuels composed with approximately 90% of methane; hence, methane combustion by chemical looping process is commonly studied for bench scale reactors by previous researchers (Ishida and Jin, 1994; Mattisson et  al., 2001; Ryu et al., 2003), and described by reaction (4) with H 2 O and CO 2 to be the effluent components.
The thermodynamic analysis for CH 4 as fuel to react with NiO illustrated that CH 4 conversion was between 97.7 and 99.8% for experiments carried out between 700 and 1200°C (Mattisson et al., 2006).However, carbon deposition and incomplete combustion were practically taken place in the fuel reactor (Jin and Ishida, 2002).Two possible pathways have been proposed for carbon deposition, methane decomposition and Boundouard reaction, listed as reactions ( 5) and ( 6), respectively (Cho et al., 2005).

Methane decomposition reaction: CH
Methane decomposition was considered to be the dominant pathway of carbon deposition for chemical looping combustion carried out at higher than 600°C (Claridge et  al., 1993).In order to achieve high CO 2 yield as well as high methane combustion efficiency, addition of steam gas can trigger a series of chain reactions, including methanereforming, carbon gasification, and water gas shift reactions as described by reactions ( 7), ( 8) and ( 9) respectively.Methane would be reformed to be CO and H 2 before it is combusted with oxygen carriers, avoiding carbon deposition by methane decomposition described by reaction (5).

Methane reforming reaction: CH
As steam introducing at the ratio of H 2 O/CH 4 = 2.0, no carbon deposition was found on the Ni-based oxygen carriers (Jin and Ishida, 2002).For chemical looping combustion conducted in fluidized bed reactors, insufficient available oxygen in oxygen carrier might also result in carbon deposition (Ryu et al., 2003).For experiments conducted with Ni-based oxygen carriers in the temperature range of 750 and 950°C, carbon was found to be deposited rapidly on oxygen carriers (Cho et al., 2005).For natural gas combustion with Fe-based oxygen carriers in a fluidized bed reactor operated at 950°C, the loading of oxygen carriers was estimated to be 1200 kg/MW in order to achieve fully conversion (Abad et al., 2007a).For methane combustion, the oxygen carrier-to-fuel ratio, , is proposed as a significant parameter for evaluating available oxygen provided by oxygen carriers on combustion efficiency of methane described in Eq. ( 10): where F OC is the molar flow rate of oxygen carrier; F CH4,in is molar flow rate of inlet methane.For Ni-based oxygen carrier with regard to methane combustion in a 500 W fluidized bed reactor operated between 800° and 880°C, has to be higher than 3 for 99% of CH 4 conversion (Adanez et al., 2009).For Cu-based oxygen carriers tested in a 10 kW fluidized bed reactor, full conversion of CH 4 to CO 2 and H 2 O was achieved at 800°C as  above 1.5 (de Diego et al., 2007).CO 2 yield was steadily maintained 98% for a 50 kW th fluidized bed reactor system for natural gas combustion with Ni-based oxygen carriers (Ryu et al., 2004).A 140 kW th pilot plant was designed for combustion of natural gas with Ni-based oxygen carriers operated at 950°C, fuel conversion and CO 2 yield were 99 and 94%, respectively (Kolbitsch et al., 2009a).From the literatures with regard to natural gas combustion by chemical looping process, over 90% of CO 2 yield can be easily achieved.
Coal and petroleum coke are composed of high contents of carbon element by ultimate analysis and heat values are estimated to be from 25.1 to 30.9 MJ/kg (25.1 MJ/kg for Indonesian coal; 27.9 MJ/kg for Colombian coal; 30.9 MJ/kg for Mexican petroleum coke) (Leion et al., 2009).Hence, combustion of coal and petroleum coke is the typical power generation means to offer over 40% of electricity demands in the world (IEA, 2010).However, coal and petroleum coke combustions are also a major CO 2 emission source (IPCC, 2007).For improving energy efficiency and reducing CO 2 emissions, integrated gasification combined cycle (IGCC) for coal gasification and combustion is proposed as a promising alternative for electricity generation (Tsatsaronis et al., 1994).Syngas (CO/H 2 ) is generated through the coal gasification process by the oxidation of coal with O 2 or H 2 O.The mechanism of coal gasification to form syngas is usually through Boundouard reaction and carbon gasification reactions described by reactions ( 6) and ( 8), respectively.The gasified syngas from coal is combusted by oxygen carriers in the fuel reactor, the syngas combustion is described as reaction ( 11), H 2 O and CO 2 are the expected effluent components.
However, IGCC system as a post-combustion technology requires separating CO 2 with additional recovery system that energy penalty is estimated 17.5% resulted in declining of electricity generation efficiency from 46 to 39.3% HHV (Amelio et al., 2007).Chemical looping is proposed to be combined with IGCC for syngas combustion to reduce energy penalty for CO 2 capture, and the exergetic power efficiency was estimated as high as 48.7% by employment of Fe 2 O 3 as oxygen carrier in chemical looping combustion (Anheden and Svedberg, 1998).The simulation for coal direct chemical looping combustion was estimated around 50% for electricity generation with over 90% CO 2 capture (Li and Fan, 2008).
Reaction kinetics of oxygen carriers for syngas combustion was analyzed by TGA indicating that reactivity of Cu-based oxygen carriers was higher than Fe-and Nibased oxygen carriers (Abad et al., 2007b).Based on the results analyzed by TGA, the inventory of oxygen carriers which represented the amount of oxygen carriers for chemical looping operation was estimated to be 19 to 34 kg/MW varied by types of oxygen carriers for power generation with syngas (45% CO and 30% H 2 ) combustion in fluidized bed systems.Composited oxygen carriers, NiO/MgAl 2 O 4 and Fe 2 O 3 /Al 2 O 3 , have been applied to syngas (50% CO and 50% H 2 ) combustion in a 300 W fluidized bed reactor with over 99% of conversion that closed to theoretical thermodynamic equilibrium upon the Ni-and Fe-based oxygen carriers (Johansson et al., 2006;Abad et al., 2007b).Moreover, NiO/Al 2 O 3 composited oxygen carrier was tested to carry out syngas combustion in a fluidized bed reactor for 50 hours, and maintained 99% of combustion efficiency (Dueso et al., 2009).For syngas combustion, the oxygen carrier-to-fuel ratio, , is proposed as a significant parameter for evaluating available oxygen provided by oxygen carriers on combustion efficiency of syngas described in Eq. ( 12):

(
) where F OC is molar flow rate of oxygen carrier; F syngas,in is molar flow rate of inlet syngas; x H2 and x CO are molar fraction of inlet H 2 and CO, respectively.For CuO/Al 2 O 3 oxygen carrier fabricated by impregnated method, over 1.5 of oxygen carrier-to-fuel ratio should be maintained for complete syngas combustion in a 500 W fluidized bed reactor (Forero et al., 2009).Therefore, oxygen carrier-tofuel ratio is a significant parameter for process operation that determines syngas combustion efficiency.Syngas combustion in a moving bed reactor system was investigated by Fe-based oxygen carrier with 99.5% of syngas (44% CO and 29% H 2 ) conversion was achieved, which is closed to the values simulated by ASPEN PLUS (Li et al., 2009).Degree of conversion with regards to Fe-based oxygen carriers by moving bed operation was approximately 50% that much greater than by fluidized bed operation with around 10% under the same condition (Li et al., 2010a).Thus, moving bed reactors with Fe-based oxygen carrier could be designed for smaller size, comparing to the fluidized bed reactors with similar heat capacity (Li et al., 2009).Directly combustion of coal by chemical looping process in the fuel reactor is developed for more efficient power generation (Li and Fan, 2008).Steam in the inlet fluidizing stream can sufficiently gasify solid fuels for direct combustion to achieve over 95% of fuel conversion (Gao et al., 2008;Leion et al., 2007).Inlet fluidizing gas containing 100% steam was employed in a fluidized bed reactor system for coal combustion by NiO/NiAl 2 O 4 oxygen carrier, and 95% of CO 2 yield was achieved for operating temperatures above 900°C (Gao et al., 2008;Shen et al., 2009a).Since Ni-based oxygen carriers demonstrate great performance in reactivity, sulfur contained in solid fuels may react with nickel to form Ni 3 S 2 causing deactivation and bed agglomeration (Leion et al., 2009).Fe-based oxygen carriers, such as Fe 2 O 3 /MgAl 2 O 4 and natural mineral ilmenite, were used for coal and petroleum coke combustions in fluidized bed reactors to achieved 95% fuel conversion at 950°C, and the solid inventory was estimated to be less than 2,000 kg/MW for residence time less than 5 minutes (Leion et al., 2007).For high combustion efficiency by Febased oxygen carriers, the appropriate operating temperature in fuel reactor was suggested higher than 950°C, because both solid fuel gasification and oxygen carrier reduction are endothermic reactions (Leion et al., 2008(Leion et al., , 2009)).For fluidized bed reactors, the solid inventories of oxygen carriers for solid combustion would much higher than those for gaseous fuel combustion, because part of gasification of solid fuels taken place near the gaseous outlet of the reactor may not be completed (Berguerand andLyngfelt, 2008, 2009).To achieve fully fuel conversions, high solid inventory was estimated to employ in existed fluidized bed reactors (Leion et al., 2007(Leion et al., , 2008)).In addition, 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 and Fan, 2010).For coal and petroleum combustion by chemical looping process, operating temperature, steam fraction of inlet stream and particle separation are significant design and operational factors.Deactivation of oxygen carrier is also needed to be concerned, especially for solid fuels with reactive species like sulfur.
Biomass combustion by chemical looping process is aimed to eliminate biomass wastes and to generate heat simultaneously.The content of biomass such as wood, rice husk, and bagasse are mainly volatile compounds, such as CO, CO 2 , CH 4 , H 2 and C n H m whereas coal and petroleum coke are majorly fixed carbon (Shen et al., 2009a, b).The literatures reported by Shen et al. indicated that the volatile compounds are decomposed at high temperatures in the fuel reactor, and the biomass combustion by chemical looping process is supposed to achieve higher CO 2 yield with less solid inventory than coal combustion.The combustion of hybrid poplar with Fe-based oxygen carriers in a moving bed reactor was simulated by ASPEN PLUS, the operating temperature in the fuel reactor should be above 900°C to achieve fully biomass conversion and nearly 100% of CO 2 yield (Li et al., 2010b).Pine sawdust contained 14.8% of fixed carbon and 75.8% of volatile compounds was combusted by chemical looping process with Fe-based oxygen carriers in a 10 kW th fluidized bed reactors, and achieved 95% of CO 2 yield at 920°C (Shen et al., 2009a).
The comprehensive summary for chemical looping process in practical operation is listed in Table 2.The major groups experienced on running chemical looping reactors are selected in this table.The processing capacity of reactors are ranged from 0.01 to 3 MW th indicated that chemical looping technology for energy generation and CO 2 capture is under the level of sub-pilot scale.Gas fuels combustion are generally higher than 95% in CO 2 yield in the effluent stream, thus CO 2 can be sequestrated or utilized without separation process.However, CO 2 yields by solid fuels combustion are not as high as gas fuels combustion, especially using low cost Fe-based oxygen carriers.Hence, solid fuels combustion is aimed to improve fuel combustion efficiency and CO 2 yield in the subsequent works.However, the reactivity has to be improved to enhance fuel combustion efficiency.The fabrication of oxygen carrier is one of the major costs on preparation of oxygen carriers; therefore, modification of an oxygen carrier through complex procedure is not to recommend for increasing the reactivity of oxygen carrier.To retrofit the design and operation of chemical looping reactors should be considered for application of Fe-based oxygen carriers.The fuel reactor design for longer residence time should be considered due to low reactivity of Fe-based oxygen carriers.Besides, longer residence time also improves the limitation of low gasification rate for direct coal combustion by chemical looping process.The moving bed reactor system with counter-flow pattern between solid fuel and oxygen carrier in the fuel reactor is proposed as one of the alternative designs to increase the residence time.The particle size of oxygen carriers for moving bed operation is from 1.5 to 5.0 mm which is much larger than the particle size of 0.2 mm for fluidized bed reactors system.The movement of oxygen carriers is a significant engineering issue for moving bed system; therefore, the design of sealed valves, such as mechanical valves and loop seals is essential to fluently move oxygen carriers as well as to avoid malfunction on the chemical looping system.The combustion of gas fuels achieved both high fuel conversion and high CO 2 yield by many combinations of oxygen carriers and reactor systems.However, as for solid fuels combustion, the gasification of solid fuel in the fuel reactor is the rate limiting step comparing with combustion of the gasified fuels and oxygen carriers.The addition of gasification reactants, such as CO 2 , H 2 O and O 2 can enhance the gasification rate of the fixed carbons in these solid fuels.In addition, reactor design for longer residence time is an alternative way to increase the solid fuel conversion efficiency, which mentioned previously.Beside of addition of gasification reactant and retrofit reactor design, chemical looping with oxygen uncoupling (CLOU) is proposed for resolving the problem of gasification rate that oxygen carrier release oxygen gas at specific temperature under oxygen-free atmosphere, thus oxygen gas combusted with solid fuel to avoid gasification step.The problem of low gasification rate is possibly resolved by different mechanism of oxygen releasing by CLOU.

Fig. 5 .
Fig. 5. Mechanisms of gaseous and solid fuels combustion in the fuel reactor.
combines with material science, mechanical engineering and chemical engineering, and still under development to be a new energy and environmental technology.The widely developed oxygen carriers are including Ni-, Cu-and Fe-based metal oxides and their composited particles.The development of Niand Cu-based oxygen carriers is limited in high cost for plenty of fabrication in spite of their great reactivity for chemical looping process.Fe-based oxygen carriers are the candidates for commercialization of chemical looping process due to economic feasibility.The use of natural mineral like ilmenite as oxygen carrier is a developing research direction for reducing cost on production of oxygen carriers.

Table 1 .
Indexes for evaluation of oxygen carriers.

Table 2 .
Summary of chemical looping reactor systems.