CO2 Capture with Chemical Looping Combustion of Gaseous Fuels

Review pubs.acs.org/EF

CO2 Capture with Chemical Looping Combustion of Gaseous Fuels: An Overview Jing Li,†,‡ Hedong Zhang,† Zuopeng Gao,† Jie Fu,† Wenya Ao,† and Jianjun Dai*,† †

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College of Chemical Engineering, Beijing University of Chemical Technology 15 Beisanhua East Road, Chaoyang District, Beijing 100029, China ‡ GreenGenTech Energy Inc., 18 Gardengate Way, Ottawa, Ontario K2H 5Z2, Canada ABSTRACT: The world-wide consumption of natural gas (NG) and other fossil fuels (e.g., coal and crude oil) is ever increasing. However, most CO2 resulting from either NG combustion or other processes is released into the atmosphere without capture. Chemical looping combustion (CLC) is a two-step combustion technology for power and heat generation with inherent CO2 capture, using either gaseous fuels or solid and liquid fuels. A previous review focused on CLC of solid fuels or CLC of all types of fuels but did not give an in-depth and specific discussion of gaseous fuel CLC systems. China is one of the largest consumers of NG, coal, and crude oil in the world, and it is essential to develop an alternative technology to take the place of gaseous fuel (e.g., NG) combustion. This Review summarizes recent research and development work on CLC using gaseous fuels, including a technological and economic assessment, types of oxygen carriers (OCs), reactor types, coke formation and OCs poisoning, efficiency and exergy analyses, and model development based on a literature survey. The plant efficiency of NG-CLC can be up to 52−60% (LHV), including CO2 compression, based on calculations and simulations, which is about 3−5% more efficient than a NG combined cycle with CO2 capture. Ni-based materials have been widely developed and applied for NG-CLC because of its fast kinetics for methane conversion. CuO−Cu2O/Cu, Mn3O4−MnO, and Fe2O3−Fe3O4 are typical OCs with high selectivity toward CO2 and H2O. The operating conditions are closely dependent on reactor configurations, hydrodynamics, mass and heat balances, and characteristics of the OCs in the system. CLC and other CO2 capture technologies are also compared in the present Review, which has rarely been investigated previously. From simulation and process analysis, a conceptual design of a NG-CLC power plant of thermal input 655 MWth is conducted to clarify its technological advantages and economic benefits compared to other power generation processes. The air reactor, fuel reactor, and OCs do not impose significant economic barriers for scale-up and commercialization of CLC.

1. INTRODUCTION Emission of greenhouse gases (in particular CO2, CH4, and N2O) is the main contributor to global warming, with CO2 being the most prevalent of these gases. CO2 emissions resulting from human activity have led to an increase in atmospheric CO2 concentration from a pre-industrial level of 280 to 360−396 ppmv. Release of CO2 from fossil fuel combustion is the most important source of these emissions.1−4 Post-combustion capture, pre-combustion decarbonization, and oxy-fuel combustion are three major routes to limit CO2 emissions (see Table 1). However, commercially available CO2 capture technologies are not cost-effective and considerably increase the cost of electricity (COE). Chemical looping combustion (CLC), initially proposed by Ritcher and Knoche,5 is an indirect two-step combustion technology with inherent separation of CO2 from N2 (see Figure 1). CLC is a precommercialization technology, but experimental tests and simulation studies on its integration with power plants indicate that it has the potential to be more efficient than all other CO2 capture technologies.6−27 CLC usually consists of two reactors, a fuel reactor (FR) and an air reactor (AR), with the oxygen carrier (OC) transferring oxygen from the air to the fuel. Its main advantage is the ability to inherently separate both CO2 and H2O from the N2 gas stream, while maintaining high efficiency of heat and power generation. After condensation of water, pure CO2, free of © 2017 American Chemical Society

Figure 1. Schematic representation of the chemical looping combustion (CLC) process.

nitrogen, can be compressed for sequestration or enhanced oil recovery (EOR).28−33 Moreover, CLC minimizes NOx formation since both the AR and the FR operate at temperatures 90%), high-pressure gases various types of membrane materials are available including polymers, metals, and rubber composites

adsorption (e.g., PSA and TSA) chemical looping cryogenics

3478

membrane

amisol

chilled ammonia mixture of DIPA or MDEA, water, and tetrahydrothiophene (DIPAM) or diethylamine mixture of methanol and MEA, DEA, and diisopropylamine (DIPAM) or diethylamine

chilled ammonia sulfinol

K2CO3 + catalysts Lurgi and Catarcab with arsenic trioxide

low-temperature methanol

solvent/sorbent name

Rectisol

CO2 capture technology

Table 2. CO2 Capture Technologies Used2

5/40 °C, >1 MPa (mixed physical and chemical solvent)

refs 2, 473−476

refs 28, 49 refs 2, 469−472

refs 2, 465−468

Lurgi, Germany

∼2−10 °C, ambient−intermediate pressures >0.5 MPa (mixed physical and chemical solvent)

Others ∼ambient temperature, 10−30 bar; based on a physical binding of gas molecules to adsorbent material; high adsorption efficiency achievable (>85%) ∼600−1000 °C, ambient−intermediate pressure low-temperature operation, refrigeration energy consumption is large; water has to be removed before the gas stream is cooled varying temperature and pressure; low gas throughputs requiring multistage operation or stream recycling

Lurgi, Germany; Eickmeyer and Associates, USA; Giammarco Vetrocoke, Italy Alstrom Shell, The Netherlands

70−120 °C, 2.2−7 MPa

Chemical Solvents ∼40 °C, ambient−intermediate pressures, ∼40 °C, ambient−intermediate pressures

Exxon, USA; MHI, Japan

Dow Chemicals, USA Union Carbide, USA

−40 °C, 2−3 MPa −20/+40 °C, >2 MPa below ambient temperature, 3.1−6.9 MPa, recovery efficiency over 90%

developer/licensor/authors Lurgi and Linde, Germany; Lotepro Corp., USA Union Carbide, USA; UOP Lurgi, Germany Fluor, El Paso, USA

Physical Solvents −10/−70 °C, >2 MPa

process conditions and descriptions

Energy & Fuels Review

DOI: 10.1021/acs.energyfuels.6b03204 Energy Fuels 2017, 31, 3475−3524

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2. CLC CONCEPT AND COMPETING CO2 CAPTURE TECHNOLOGIES Ritcher and Knoche initially proposed CLC concept in 1980s,5 which is based on the transport of oxygen from air to the fuel by means of an OC. One of the most important aspects for CLC concept is inherent CO2 capture. Hence, the tradional CO2 capture technologies are briefly introduced in this section for comparison with CLC, showing advantages of the CLC system over other technologies. Various CO2 capture methods have been used to remove CO2 from impure NG, power plants, food processing, and chemical industries (Table 1 and Table 2). A variety of methods are used to separate CO2 from gas mixtures during production of hydrogen for petroleum refining and ammonia production and in other industries.2,31,81−87 Selection of a CO2 capture technology depends on many factors, e.g., partial pressure of CO2, extent of CO2 recovery required, impurities in the gas stream (e.g., particulate and acid gases), purity requirements of the CO2 product, capital and operating costs, and cost of additives to overcome fouling and corrosion. Nowadays the most commonly used CO2 capture methods include absorption by physical and/or chemical solvents, adsorption (e.g., pressure swing adsorption (PSA)), membrane, cryogenic, and cyclic calcium looping technologies. Conventional CO2 capture technologies for large-scale power plants (e.g., post-combustion, pre-combustion, and oxy-fuel combustion) are energy intensive resulting in a significant decrease of the overall efficiency, resulting in a price increase of the produced electricity. Both pre- and post-combustion capture are based on separation by membrane or other costly conventional separation technologies of high energy consumptions. Pre-combustion produces a CO2-rich gas stream, leading to easier separation of CO2 and a slightly lower operation cost. However, its cost is still high. Oxy-combustion would seem a good alternative, given that high-concentration CO2 is obtained in the gases. However, the oxygen required for the process must be produced in an air-separation unit, also highly energy-demanding.88 CLC reaction occurs with a lower irreversibility compared to a conventional combustion, leading to a somewhat higher overall thermal efficiency.88 It is estimated that CO2 capture may lead to an energy efficiency penalty that can reach 10−15% points depending on the technology adopted.89−91 Romano92 and Hawthorne et al.93 have done thermodynamic analysis and simulation using the carbonator for CO2 capture and the calciner for sorbent (i.e., CaO) regeneration; a net efficiency of 37.4% (LHV) was predicted for the selected reference case, with 97% CO2 capture from Romano’s simulation.92 Dean et al.82 reported the current status of development for cyclic calcium looping for CO2 capture from power generation, cement manufacture, and hydrogen production. The sorbents in this case are derived from cheap, abundant, and environmentally benign limestone and dolomite, and the process imposes a relatively small efficiency penalty on the power/industrial process (i.e., estimated to be 6−8%, compared to 9.5−12.5% from aminebased post-combustion capture). CLC was proposed to have a relatively high energetic efficiency in integrated power generation with inherent CO2 separation.94−96 Alvaro et al.88 carried out simulations to evaluate the energetic efficiency of the CLC-based power plant under diverse working conditions. A comparison of a conventional integrated gasification power plant with pre-combustion capture of CO2 has been made. Two

different synthesis gas compositions have been tried to check its influence on the results. The energy saved in carbon capture is found to be significant and even notable, inducing an improvement of the overall power plant thermal efficiency of around 7% in some cases. For CLC at atmospheric pressure, a steam cycle could achieve about 40−42% (LHV) efficiency when using gaseous or solid fuels including energy consumptions for CO 2 compression before transport and sequestration.97,98 CLC combined cycle is another option, but CLC reactor should operate at higher pressure and temperature to increase energy efficiency. A pressure in a CLC system above 1.3 MPa is not recommended because theoretical calculations show little effect beyond this pressure on the overall efficiency.73,99 The CO2 turbine downstream the reduction reactor was reported no substantial impact on the energetic efficiency of the process, mainly due to the higher compression work needed to repressurize the CO2 stream to sequestration conditions after decompression by the CO2 turbine. However, there is controversy in CO2 turbine application in CLC processes, the compression of CO2 was reported to reduce the efficiency by about 2% based on previous study.73 CO2 as a commodity can be used in firefighting, food industry, fish farms, agricultural greenhouse, and other chemical industries. However, the overall demand for CO2 is very small compared to the total CO2 emitted annually, and sequestration of CO2 is therefore essential, with considerable efforts being focused on enhance oil recovery (EOR), sequestration in depleted oil and gas reservoirs, mineral carbonation, and saline aquifers sequestration.32,100−102 For EOR, the market price of CO2 varies widely, depending on both the price of crude oil and the amount of CO2 required to produce a barrel (Bbl) of oil. The cost of CO2 avoided ($/tonne) is calculated from the following equation (see Table 3): costavoided =

COEcapture − COEreference (MCO2)reference − (MCO2)capture

(1)

Table 3. Comparison of CO2 Capture Costs in a 500 MWe Planta,101 net plant capacity (MWe) net plant capacity (kWh/yr) CO2 emission rate (g CO2/kWh) SO2 emission rate (g SO2/kWh) NOx emission rate (g Nox/kWh) CO2 sequestered (tonne/yr) CO2 sequestered (g/s) COE ($/MWh) CO2 cost ($/tonne CO2 avoided)

reference plant

with CO2 capture

462 3.33 × 109 941 2.45 0.45 − − 49.2 −

326 2.35 × 109 133.0 0.0003 0.58 2.58 × 106 1.12 × 105 97 59.2

a

Based on coal-fired power plant with and without MEA-based CO2 capture.

where COE is cost of electricity ($/MWh). The cost of CO2 avoided can be used to compare the ecomonics for different CO2 capture technologies. More information is provided in section 6. For CLC, thermo-gravimetric analyzers (TGA), packed/fixed beds, moving beds, and FBs have all been investigated for different OCs, determining reaction kinetics, conversion, yield, 3479


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Energy & Fuels and longevity (see section 1). Reaction kinetics models have been developed for both reduction and oxidation. Solids circulation rate, bed inventory, and hydrodynamic performance of the CLC system have been studied for FB configurations. Reactor design, modeling, and hydrodynamics still require investigation. CLC has the potential for achieving efficient and low-cost CO2 capture. Scale-up to commercial CLC systems is promising and mainly depends on the properties of the OCs prepared by different methods.

3. PROCESS DESCRIPTIONS AND CHARACTERISTICS OF CLC WITH GASEOUS FUELS 3.1. Process Description. Previous works provided the basic foundations of CLC and its potential application to gas turbine (GT) or combined cycle (CC) power generation. Further developments consider the use of alternative fuels for the chemical looping combustor.103 A coal-direct chemical looping (CDCL) process has been recently patented. Such a system converts pulverized coal (PC) feedstock to fuel in one integrated system, without additional gasification, and allows both electricity and/or hydrogen production.22,104,105 The chemical looping concept has been extended to consider reforming (chemical looping reforming, CLR) and gasification (chemical looping gasification, CLG), where complete oxidation of the fuel is prevented by using low air to fuel ratio.106 Further, the simple CLC configuration has been extended to include a third reactor that favors the simultaneous generation of hydrogen.107 The oxidative coupling of methane (OCM) is an attractive alternative concept for ethylene production from methane-based feedstocks using chemical looping concept.108 The basic principle of OCM is the activation of methane by a catalyst material (e.g., Na2WO4/ Mn/SiO2) which leads to methyl radical formation by C−H bond cleavage. The methyl radicals can couple to ethane in the gas phase, close to the catalyst surface. More information is available in.109 In a typical CLC system, the OC particles circulate between two reactors present in the process and provide two-step combustion of the fuel. The process of CLC avoids direct contact between fuel and air and achieves inherent separation of CO2 from N2110,111 (see Figure 2). As mentioned above, CLC primarily consists of two main reactors: an air reactor (AR) and a fuel reactor (FR). In the AR, OC particles are oxidized by oxygen from air as shown below in reactions 2−6. The oxidized form of the OC is then transported to the FR, transferring heat and releasing oxygen in the FR without any direct efficiency loss. If the fuel is in gaseous form, no additional steam and/or CO2 are needed for the FR. The gaseous fuel then reacts with oxygen available in the OC particles. In the FR, the OC is reduced. The reduced metal oxide particles are returned to the AR, where they are again oxidized by air. In CLC, the total heat evolved from the combined oxidation and reduction remain the same as in conventional combustion which is in accordance with Hess’s law, i.e., the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. The product gas from FR and the flue gas from AR can both be used for heat and/or power generation, with almost pure CO2 stream produced after condensation of water. CLC realizes inherent separation of both CO2 and H2O from flue gases; therefore, the energy penalty for the capture of CO2 is minimized. Furthermore, very low NOx emissions have been reported due to the low combustion temperature and flameless

Figure 2. Schematic diagram of the proposed CLC reactor (cold flow):68 1, air reactor; 2, primary cyclone; 3, secondary cyclone; 4, dipleg; 5, upper downcomer; 6, primary downcomer; 7, internal cyclone; 8, lower downcomer; 9, fuel reactor; 10, hole opened on the lower downcomer; 11, seal at the bottom of the lower downcomer; 12, loop-seal 1; 13, loop-seal 2.

combustion.112 NOx formation usually occurs well above 1200 °C, which is hardly the highest temperature attained in CLC.7 The main reactions in the FR (with CH4 and H2 as fuels) and AR are (2n + m)MexOy + CnH 2m → (2n + m)MexOy − 1 + nCO2 + mH 2O MexOy − 1 +

1 O2 → MexOy 2

(2)

(3)

where MexOy denotes a metal oxide and MexOy−1 represents the reduced form of the metal oxide. The oxidation of OC is exothermic, whereas the reaction in the FR may be either endothermic or exothermic, depending on the OC and the fuel types. The total amount of heat evolved from these reactions is the same as for normal combustion of the fuel. The following reactions occur in the AR and FR for CLC with NG as fuel and Ni-based metal oxide as the OC: (O2 + 3.76N2) + 2(Ni + Al 2O3) → 2(NiO + Al 2O3) + 3.76N2 3480

(4) DOI: 10.1021/acs.energyfuels.6b03204 Energy Fuels 2017, 31, 3475−3524

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3.2. CLC of Gaseous Fuels vs CLC of Solid and Liquid Fuels. CLC can use fuels in the gas, liquid or solid forms to achieve indirect combustion. There were extensive investigations for different types of fuels using different types of reactors and OCs in the past decades. However, previous study mainly focused on specific reactor types and configurations, specific OC types, and properties. Hence, these results are not comparable and may not be applicable to large-scale commercial CLC processes. A 300 W CLC pilot plant of continuous operation with NG and syngas as fuel was reported.27,113 It showed that unburned methane was detected, and CO and H2 were present at low concentrations in the exit flue gases when NG was used as the fuel. However, the combustion of syngas was complete for all experimental conditions with no CO or H2 present. GarciaLabiano et al.30 used syngas from an IGCC power plant as fuel for the CLC study. Tian et al.31 employed the simulated syngas as fuel for the CLC research, which showed stable reactivity for production of CO2 from fuel gas at 800 and 900 °C and full consumption of hydrogen during the reaction. Forero et al.35 used CH4 and H2S as the fuel. The influence of H2S concentration on the product gas distribution, combustion efficiency, sulfur splitting between the FR and the AR, and agglomeration tendency was investigated. Iliuta et al.36 used CH4 as fuel in two different reactors: one was a fixed-bed microreactor, and the other one was a FB reactor. In the FB reactor, complete conversion of the fuel was achieved within several minutes. However, in the fixed-bed microreactor, the fuel conversion tends to decrease after some time due to imperfect gas−solid contact and slow reaction rate of fuel with the OCs. Johansson et al.37 demonstrated CLC of methane as gaseous fuel with Ni-based and Fe-based OCs. Kolbitsch et al.38 used NG, which mainly consisted of CH4 with only traces of other gas species (C2H6, other CxHy, CO2, and NO2), as fuel in their CLC study. The CLC experiments were carried out mostly with gaseous fuels. However, with advances in time, the growing interest in CLC of solid and liquid fuels has increased. Lots of different OCs have been developed using different metal oxides, support materials, and production methods. Four different OCs were tested with focus on reactor temperature, total solids inventory, specific FR inventory, solids circulation rate, and air-to-fuel ratio to investigate the interchangeability of OCs in a 120 kW th pilot plant. 15 Different reactor configurations and potentially limiting factors for each type of OC were identified. Changing OCs in an existing chemical looping plant seems to be possible without big changes of the reactor configuration. Previous studies of low-cost OCs for use with solid fuels covered iron ore,114−116 manganese ore,117 ilmenite, and industrial waste materials,41,118 and comparisons of materials of different sources were also investigated.119,120 Aspen Plus simulation indicated that the energy efficiency of the CDCL process could exceed 80% (HHV) for H2 production and >50% for electricity generation with 100% of the CO2 captured.81 Tests for a CDCL process have been carried out in a 2.5 kWth bench-scale moving-bed unit at The Ohio State University. Different feedstocks (e.g., simulated coal volatiles, lignite coal char, bituminous coal char, and anthracite coal) have been tested. It was found that coal/coal char conversion as high as 95.5% has been obtained. The CO2 concentration in the flue gas was >97% (dry basis) in all cases. In addition, the reactivity of the OC was maintained after three redox cycles.55 OC research for solid fuels has focused mainly on oxides of Ni, Fe,

2(NiO + Al 2O3) + 0.5CH4 → 0.5CO2 + H 2O + 2(Ni + Al 2O3)

(5)

The net reaction combining these two reactions is (O2 + 3.76N2) + 0.5CH4 → 0.5CO2 + H 2O + 3.76N2 (6)

A makeup flow (F) of new OCs is required to compensate for the natural decay of activity and solid losses due to attrition/ fragmentation during operation. For simplicity, the makeup flow rate can be neglected since it should be small relative to the circulation flow of solids. The oxygen balance in the AR and FR is then FO2ΔXO2 = dFf ΔX f

(7)

The circulation rate, expressed as mass flow of completely oxidized OCs, mainly depends on the type of OC and the fuels used, as well as on the solids conversion difference in the AR and FR: m̊ OC = 0.001brMNiOFf ΔX f /(x NiOΔXs,FR )

(8)

Although eq 8 gives the circulation rate necessary to satisfy the oxygen mass balance in the CLC system, it is also necessary to consider other aspects related to the hydrodynamic behavior and heat balance in the system, addressed in the next sections. The flow rates and diameters of the two reactors must be such that the reduction of the oxidized metal oxides (e.g., NiO) balances the oxidation of reduced metal oxides or metal (e.g., Ni). This will occur if the flow rates correspond to 2 mol of O2 for every 1 mol of CH4 per unit time. As a result, assuming ideal gas behavior, we can have 0.21(πD12 /4)(U1/T1) = 2(πD2 2 /4)(U2/T2)

(9)

where D denotes reactor diameter, U superficial velocity, and T temperature, while the subscripts 1 and 2 refer to the air and fuel reactors, respectively.The corresponding diameter ratio is D2 /D1 =

0.21U1T2/2U2T1

(10)

For oxidation of Ni, the heat release is 17% greater than for conventional combustion of NG. This leads to a drop in temperature in the FR since the total heat of reaction in the AR and FR is the same as for conventional combustion. Most OCs give such a temperature fall. For a CLC process, heat input is essential for start-up of the AR because the heat release only occurs when the oxidation occurs. The temperature in the AR and FR is related to the fuel type, OC type, and operating conditions (e.g., gas velocity, solids circulation rate, and bed mass), as well as the process control strategy. Both reaction heat and preheating of air and CH4 need to be considered when carrying out the energy balance calculations. In general, the energy balance for the AR and FR can be written

∑ Finh + Q OX = ∑ input

∑ Finh = ∑ input

Fouth + Q remove,AR

output

output

(11)

Fouth + Q re + Q remove,FR (12)

More information about mass and heat balance of CLC is available in a previous study.68 3481


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fuels involves char gasification, as the rate-limiting step in the solid fuels conversion, different from CLC of gaseous fuels.143,144 The feeding systems of liquid and solid fuels to the FR is also completely different from CLC of gaseous fuels. CLC of gaseous fuels are applicable for almost any type of reactor. However, OC properties and reactor performance still need further research. CO2 concentration in the flue gas from NG combustion is in the range of 3−8v%, lower than that from coal combustion (∼15 vol%), which indicates that post-combustion CO2 capture for NG combustion is even more difficult compared to coal combustion processes. Hence, CLC of NG is very promising as an alternative to NG combustion with inherent CO2 capture. Previous studies and reviews discussed CLC of different types of fuels together and ignored the specific characteristics for CLC of different fuels. The present study overcame this problem and mainly focused on CLC of gaseous fuels to try to clarify the important aspects for CLC burning combustible gas. However, experience in CLC of solid and liquid fuels is useful for development of CLC with gaseous fuels with proper modification and caution. 3.3. Reactor Types and Characteristics. Various reactor configurations were investigated for CLC systems (e.g., interconnected moving beds and interconnected fluidized beds), such as coated monolith reactors,145 packed beds,146 and fixed beds.147 Interconnected FBs are usually considered the most promising CLC concept, especially for CLC with gaseous fuels. However, other reactor types have also been tested and analyzed. Hydrodynamic study provides some key information for the design of reactors, especially for selection of the flow regime, system configuration, and operating conditions of FB reactors. For FB applications, minimum fluidization velocity, minimum bubbling velocity, minimum slugging velocity, and terminal velocity should been calculated and analyzed on the basis of the properties of OCs (e.g., particle density, size) and gases. There are several equations used to calculate minimum fluidization velocity. One of the commonly used equations is

Mn, and Cu. Nickel oxide is expensive and easily deactivated by sulfur, being less suitable for the solid fuels than iron and manganese oxides. Metal oxides may also be combined forming new compounds with new properties although these materials have not yet been tested successfully in operation with exception of calcium manganates.121,122 Ilmenite (FeTiO3), a naturally occurring mineral and combined oxide, is often used with solid fuels due to its low cost, reasonably high reactivity toward syngas, and good fluidization behavior.123−127 Mixed oxides build on synergies of mixing OC materials with different properties, for example, addition of limestone to ilmenite in CLC using solid fuels.128,129 CDCL does not need any gasifier, and there is no corresponding oxygen requirement. The solid fuel is mixed with the OC in the FR. The OC reacts with the in situ gasification products formed inside the FR. The FR is fluidized by H2O and/or CO2. Chemical looping with oxygen uncoupling (CLOU) combustion systems have the potential to assist in the capture of CO2 from power plants and have been investigated in previous studies.8,55,130−136 CLOU processes require OC materials to be able to release oxygen in the FR and to regenerate by reoxidation in oxygen-rich atmosphere in the AR at elevated temperature. Hence, the OC is segregated to liberate gaseous oxygen in the FR to promote the combustion of the fuel.136,137 This process has recently received great interests to overcome the slow char gasification step in the CDCL process. Copper oxide is a CLOU material although it has a higher cost than iron and manganese oxides. The largest experimental setup (1 MWth) of CDCL has been tested with ilmenite as the OC. However, the plant did not demonstrate autothermal operations. The oxygen transfer capacity of ilmenite is low, which is the principle impediment to commercializing a process with this ore (huge solid inventory).138 CLC of liquid fuels used non-sulfurous and sulfurous kerosene, as well as heavy fuel oils and heavy vacuum residues generally produced during refining of crude oil, as the fuel.139 Bitumen and asphalt, domestic fuel oil, and waste cooking oil were reported in the past years for CLC study, using either packed-bed or FB reactor.140−142 A continuously operating reactor of capacity 300 W has been designed, constructed, and successfully operated using non-sulfurous and sulfurous kerosene as fuel. The conversion of fuel carbon to CO2 was as high as 99%.139 Very limited studies have been reported so far on CLC of liquid fuels. CLC of gaseous fuels means the primary fuel is in the gas form (e.g., NG, syngas, and refinery gas, etc.), which reacts directly with the OCs, and the fundamental chemistry behind it is the same as discussed above. If the gasification of solid fuels (e.g., coal, petcoke, solid wastes, or biomass) is first carried out, and the produced syngas is introduced in the CLCit is usually termed the Syngas-CLC. In the CLC of gaseous fuels, the reaction rate is dependent on gas−solid contact, interphase mass and heat transfer, OC particle properties, and reaction kinetics. For the fluidzed-bed CLC system, hydrodynamics is crucial for both oxidation and reduction reactions. The solid phase in the CLC of gaseous fuels is mainly OC particles. Whereas the solids in the CLC of solid fuels include OC particles, ash, and some carbonaceous particles (e.g., char), there are also complicated interactions between gas/vapor and solids, which add complexity to the system analysis. For CLC of gaseous fuels, the gaseous fuel is not only the fuel combusted, but also a fluidization gas for FB configurations. There is no ash in the reactor, different from the case of CDCL. CLC of solid

Remf =

C12 + C2 Arr − C1

(13)

where C1 = 27.2 and C2 = 0.0408 based on the report by Grace.148 Terminal velocity can be calculated from equations recommended by DallaValle.149 The gas velocity in the AR provides the driving force for the circulation of particles.28 As mentioned above, the flow regimes in the CLC system could be chosen as fast fluidization for the AR and bubbling fluidization for the FR in order to obtain a reasonable reactor size and performance. Hydrodynamic models developed for fast FBs, bubbling fluidized beds (BFBs), and other fluidization regimes148,150−153 can be used to simulate CLC performance. For a CLC system, the superficial gas velocity in the AR is normally in the 5−20Ut range, whereas the superficial gas velocity in the FR is in the range of 5−30Umf, and the loop-seal and downcomer superficial gas velocities are of order of magnitude ∼Umf. For a packed bed of particles, if the fluidization velocity is less than the minimum fluidization velocity, the particles remain fixed. Pressure profiles and relationships between superficial gas velocities, total solids inventory, and net solids circulation rate for FBs have been reported.57,67,154−157 However, most hydrodynamic studies have been conducted in a narrow range of operating conditions in cold-flow models consisting 3482


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Energy & Fuels of relatively small-scale fluidization columns. A detailed operating mapping of a CLC system consisting of one AR operating in the fast fluidization regime and one FR in the bubbling fluidization regime has been prepared at UBC, including superficial gas velocities, solids circulation flux, pressure profile, solids hold-up, gas leakage, aeration gas velocity, and their relationships68 (see Figure 2). Gas leakage between reactors can adversely affect CO2 capture efficiency and CO2 purity. The pressure in the two reactors should be approximately equal in order to minimize gas leakage between them. Johansson et al.,66 Kronberger et al.,57,67 Min,68 and Saayman et al.158 have measured gas leakage in cold-flow models and investigated the effect of operating conditions. Gas leakage is generally not a major problem during the operation of systems with loop-seals since it can be avoided by using steam or other gases. CFD modeling for CLC study is presented in section 4. One of the important features that an OC must possess is to have high resistance to attrition, especially for two interconnected FB CLC systems. In a continuously operated CLC unit for gaseous fuels, several particle characteristics related to attrition resistance were analyzed on 23 natural and synthetic OCs, prepared by different methods based on different metal oxides.159 The OCs tested were based on Mn, Ni, Cu, and Fe, which were mainly prepared by the incipient impregnation methods. However, the Mn-based OCs were prepared by spraydrying, and there is also a natural Fe-based ore as the OC. The main supports are γ-Al2O3, α-Al2O3, CaAl2O4, MgAl2O4, and ZrO2. The particle size used was 100−500 μm, depending on different OCs. Particle crushing strength and air jet index were determined for the fresh materials, as well as the attrition rate and the corresponding particle lifetime during multicycle redox reactions. A comparison was made of the different methods used to evaluate attrition behavior. Schwebel et al.85 tested a parallel arrangement of a movingbed FR and a fixed-bed AR. This arrangement reduced the power requirement for fluidization, avoided fuel segregation with less char at the reactor exit. Thon et al. reported a 25 kW CLC system of coupled FBs with solid fuels and ilmentile as the OC.86 The CLC system consisted of a circulating fluidized bed (CFB) AR coupled with a two-stage BFB FR. Carbon capture efficiency of ∼90% was achieved in this system. However, the presence of unconverted combustible gas in the FR was reported. Penthor et al.87 reported a coupled operation of dual fluidized bed (DFB) pilot plants.The first one is a 100 kW steam gasifier, the syngas produced was introduced to a 120 kW CLC of continuous operation. The coupling of the two was done by a hot product gas fan. Chiu et al.57 used an annular dual-tube moving-bed reactor to study the CLC process with polyurethane and polypropylene as the fuel and Fe-based metal oxides as the OC. Strohle et al. reported a pilot plant CLC of capacity 1 MWth based on two interconnected CFB reactors with hard coal as the fuel and ilmentite as the OC.88 Abad et al.89 developed an interconnected CFB CLC unit where they achieved a carbon capture of 88% at 991 °C and a total oxygen demand of 8.5%, with a solid inventory in the FR of 470 kg/ MWth. FBs can be employed for CLC with either gaseous fuels or solid and liquid fuels due to the fuel flexibility and excellent gas and solid contacting. An alternative to interconnected FB CLC configurations is packed-bed design.160−163 The packed-bed reactor shows high tunability in heat management and reactor configuration, because the AR and FR are completely independent.164 In a

packed-bed reactor configuration for CLC, the OC is statically contained and alternately exposed to oxidizing and reducing gases. Solid circulation is not required for this case. Hence, high-pressure operation can be achieved without much difficulty. The packed-bed reactor is compact and offers potential for better utilization of the OC and low capital cost. Moreover, two packed beds in series could help the CLC system to obtain the desired temperature rise (450−1200 °C) for hot air and avoid fuel slip, making the OC selection become less critical.165,166 The packed-bed CLC system has been tested at up to 7 bar using syngas as fuel and ilmenite as OC.167 It has been found that wet syngas has to be used as reducing fuel to avoid excessive carbon deposition that reduces the CO2 capture efficiency of the system. A maximum temperature rise of 340 °C has been obtained in the reactor, which is in good agreement with the theoretical prediction when accounting for the heat losses. The main disadvantages of the packed-bed reactor are its dynamic and batch operation, and the necessity of high temperature valves. It has been shown that the maximum temperature is independent of the gas mass flow rate or oxidation kinetics of the OC in a packed-bed reactor CLC system, which results in high flexibility to changes in production capacity, with little disturbance due to changes in reaction kinetics.146 However, the overall process efficiency of FB CLC and packed-bed CLC systems using Ni-based oxides as OC and syngas as fuel is similar.168 Dahl et al.169,170 proposed a rotating reactor system, with the OC materials being rotated between different gas streams flowing radially outward through the metal oxide bed, avoiding the periodic switching valves operating with gases.171,172 However, pressure drop and temperature control when the processes are scaled up will be challenging because of the large fraction of OCs in the fixed-bed reactors (i.e., low bed void fraction) and the high-pressure conditions.173,174 Although an inert gas is introduced to avoid mixing of the two reacting gases, gas interchange between the AR and FR is a challenge and may be unavoidable based on previous research. At IFP-France, a 10 kWth unit with three interconnected bubbling beds (one FR and two AR) has been designed and constructed. Pneumatic L-valves were used to control the solid circulation rates independently of the gas flow and solids inventory in each reactor.7,175 Ryu et al.176 reported a 50 kWth CLC unit using solid injection nozzles inside each reactor to control the solids flow. Aeration gas fed to the two loop seals was used to adjust the solids circulation rate for a CLC unit consisting of two interconnected fluidized beds at The University of British Columbia (UBC),68 with the AR operating in the fast fluidization flow regime, while the FR is a BFB). A prototype countercurrent moving-bed bench-scale unit was initially studied using iron oxide to convert syngas and methane.35,177 Zeng et al. employed a moving-bed reducer with iron oxide as the OCs and NG as the fuel. A bench-scale countercurrent moving-bed demonstration unit was built and tested, and the reducer operating conditions were studied using thermodynamic models. The thermodynamic equilibrium model also established a system baseline performance. An optimized set of operating criteria were determined from the Ellingham Diagram analysis. A thermodynamic criterion for selecting iron-oxide-based OC material and designing the reaction system was developed. Solids analysis determined the iron oxide conversion and verified the lack of carbon deposition or iron carbide formation. An optimal set of operating 3483


Review

Energy & Fuels conditions (e.g., solids-to-gas ratio and temperature) is identified for further testing on a larger scale.177 The Korean Institute of Energy Research has also considered using a moving bed for the three-reactor chemical looping combustion (TRCLC) system.178 Countercurrent moving beds have also been studied by Wu et al.179 and Chiu180 at The National Taiwan University of Science and Technology to determine the proper conditions for complete methane conversion in a moving bed consisting of iron and aluminum oxides. They have tested gasified polyurethane and polypropylene and have shown the feasibility of treating plastic wastes in a moving-bed reducer.177,180 The moving-bed reactor with a counter-flow pattern can be used to enhance the utilization of iron-based OCs and implement the co-production of electricity and hydrogen via step oxidation of reduced OCs.165,166,181−184 In general, smaller OC particles lead to larger pressure drop, but larger particles may lower the utilization of the OCs due to the intra particle diffusion limitation. Moreover, a highly exothermic reaction (e.g., the oxidation of the reduced oxides) in the fixed-bed system may result in hot spots and thermal runaway, which is detrimental for the stability of the OC and for the safety of the operation.165,166,181−184 The size and geometry of OCs are key factors to determine the efficiency of a large-scale CLC system in fixed-bed reactors, because they strongly affect the dynamic conditions of the gas− solid reactions (e.g., the intraparticle mass-transfer limitation for reactants, the pressure drop, and the flow distribution).164 The monolithic OCs (4.5 cm long, 6.0 cm in diameter, square cell size of 2.0 mm, and wall thickness of 0.9 mm) used in the CLC of methane show high activity in a high gas hourly space velocity (GHSV, 6000/h). This can be attributed to the special geometric structure and layered microstructure.164 For largescale utilization, multiple points of the gas feed along the reactor may be employed for the monolith to improve the gas− solid reaction. The powder and monolithic OCs show similar reduction behaviors either in hydrogen or in methane atmosphere.164 3.4. Equilibrium Analysis for Oxygen Carriers. CLC with gaseous fuels, e.g., NG, has been widely investigated,49,185 and it has been shown that high gas conversion can be achieved with acceptable OC properties. The main differences among Cu-, Fe-, and Ni-based OCs in relation with the design of a CLC system are available from previous studies.43 Metal oxides with the capacity to transfer oxygen in a CLC system are essential for successful CLC operation. The metal oxides should show a favorable tendency toward high conversion of fuel gas to CO2 and H2O, or in the case of chemical looping reforming (CLR), to CO and H2. Jerndal et al.186 identified oxides of Cu, Ni, Co, Fe, and Mn with favorable thermodynamics for CH4 and syngas conversion. At temperatures and pressures relevant to CLC, CH 4 is not thermodynamically stable in the FR, and variable amounts of CO2, H2O, CO, and H2 appear, depending on the operating conditions. A higher equilibrium constant means a higher conversion of the reducing gas. CuO-Cu2O/Cu, Mn3O4-MnO, and Fe2O3-Fe3O4 are typical OCs with high selectivity toward CO2 and H2O (see Table 4 and Table 5). For Ni-based OCs, equilibrium conversions of 99−99.5% for H2 and 98−99.4% for CO can be obtained at 800−1000 °C. When Al2O3 is used as a supporting material, the formation of NiAl2O4 is favorable at CLC conditions, leading to a lower conversion of these gases (93−98%). For a CoO/Co system, the maximum conversion at equilibrium conditions is 95−97%

Table 4. Chemical Looping Combustion Reaction Schemes (Ni-Based OC) oxidation reaction

Ni oxidation

2Ni + O2(g) → 2NiO

ΔH1173K = −469.4 kJ/ mol

OC reduction reactions

CH4 oxidation

CH4(g) + 4NiO → 4Ni + CO2(g) + 2H2O(g) CH4(g) + 2NiO → 2Ni + CO2(g) + 2H2(g) CH4(g) + NiO → Ni + CO(g) + 2H2(g) H2(g) + NiO → Ni + H2O(g) CO(g) + NiO → Ni + CO2(g) Ni + H2O(g) → NiO + H2(g)

ΔH1073K = 139 kJ/mol ΔH1073K = 165 kJ/mol ΔH1073K = 212 kJ/mol ΔH1073K = −13 kJ/mol ΔH1073K = −47 kJ/mol ΔH1073K = 13 kJ/mol

CH4(g) + H2O(g) → CO(g) + 3H2(g) CO(g) + H2O(g) → CO2(g) + H2(g) CH4(g) + CO2(g) → 2CO(g) + 2H2(g) CH4(g) → C + 2H2(g)

ΔH1073K = 225 kJ/mol ΔH1073K = −34 kJ/mol ΔH1073K = 259 kJ/mol ΔH1073K = 89 kJ/mol ΔH1073K = 136 kJ/mol

partial CH4 oxidation partial CH4 oxidation H2 oxidation CO oxidation water splitting

reactions catalyzed by Ni

steam reforming water−gas shift dry reforming methane decomposition carbon gasification by steam carbon gasification by CO2 methanation reaction methanation reaction

combustion reaction

CH4 combustion

C + H2O(g) → CO(g) + H2(g) C + CO2(g) → 2CO(g)

ΔH1073K = 170 kJ/mol

CO(g) + 3H2(g) → CH4(g) + H2O(g) CO2(g) + 4H2(g) → CH4(g) + 2H2O(g)

ΔH1073K = −225 kJ/mol ΔH1073K = −191 kJ/mol

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

ΔH1073K = −802 kJ/mol

for H2 and 92−97% for CO at 800−1000 °C. Full conversion could be obtained in the redox system Co3O4−CoO, but oxidation to Co3O4 is not favored in air at temperatures above 880 °C.7 The CaSO4-CaS system for CLC application has also been reported,7,187−189 with thermodynamic limitations similar to NiO for maximum conversion of H2 and CO (see Table 5). Fe3O4 usually appears as an intermediate product in Fe2O3 reduction, with further reduction of Fe3O4 to FeO being a slow reaction. Formation of Fe(II)Al2O4 or Fe(II)TiO3 changes the thermodynamics when Al2O3 or TiO2 is used as the support, the reduction of Fe(III) to Fe(II) provides more oxygen to the fuel, leading to almost complete conversion of CO and H2 to CO2 and H2O, respectively. The equilibrium constant of Fe3O4FeO with H2 and CO is 1.5−2.5 and even 1300 °C with stable redox cyclic performance. NiO loading on yttria-stabilized zirconium (YSZ) provides high solid diffusivity for the nickel oxide ion and helps to improve the composite material reactivity, as well as regenerability. 49 NiO on bentonite also has been investigated, showing promising activity and stability under repeated redox cycles.248,260−262 However, the oxidation curve showed that the oxidation reaction rate was quite small. Moreover, the thermal stability of NiO/bentonite OCs is limited at higher temperature. TiO2 is also a common support material for catalysts, which has been tested for OC development. It was found that NiO was more prone to interact with TiO2, forming NiTiO3, which is less reducible than NiO. The loaded nickel is completely converted to NiTiO3 after several reduction/oxidation cycles, leading to lower reactivity of the OC. Moreover, coke formation was greater than for NiO on other common support materials.263−265 NiO supported on SiO2 and ZrO2 has also been investigated. It was shown that the reactivity of both NiO/ SiO2 and NiO/ZrO2 decreased with repeated reduction− oxidation cycles above 900 °C. Formation of nickel complexes is the main reason for the decreasing reactivity over time.240,244,245 Different preparation methods, such as spray drying, incipient impregnation, and mechanical mixing, have been used to improve performance of the OCs and reduce the production cost. OC properties and quality (e.g., reactivity and crushing strength) rely heavily on the preparation methods.266 High reactivity and low NiAl2O4 formation were found in some cases to result from mechanical mixing or impregnation methods.245,266−269 Ni-based OCs prepared by impregnation on α-Al2O3 had very high reactivity, with low attrition rates and low agglomeration during FBs operation.268 Dueso et al.270 concluded that about 80% of the Ni reduced in the FR was oxidized to free NiO, while the remaining Ni oxidized to NiAl2O4, indicating that the formation of the spinel compound (i.e., NiAl2O4) cannot be completely avoided. Despite these limitations, NiO on Al2O3‑based support holds significant promise as a potential OC for large-scale CLC applications.173,212,270 Crushing strength and attrition resistance are important properties for OCs, especially for uses in moving-bed and/or fluidized-bed processes. After about 200 cycles in the temperature range of 500−800 °C, there is a small decrease in the solid conversion of Ni-CaAl2O4 with H2, CO, and CH4 as reducing agents in the TGA and fixed-bed tests, mainly due to agglomeration of the NiO grains.271 Nevertheless, the redox kinetics is still sufficiently fast for low temperature applications if the OC is preactivated. The kinetics rates for the gas−solid reactions and gas-phase catalytic reactions have been determined, which can be used to predict the performance of the activated NiO/CaAl2O4 OC for low-temperature CLC

OCs may be prepared by co-precipitation, impregnation, or freeze-granulation. In most cases, part of the metal oxides used in the preparation reacts with the support to give aluminates not active for the intended reactions. The work on finding suitable OCs is still limited. It is likely that these materials can be improved considering the many choices available for making these particles. Note that Ni-derived compounds tend to be carcinogenic while Co emissions also involve significant health and safety concerns. On the other hand, Fe-, Mn-, and Cubased OCs are considered to be nontoxic or nonhazardous materials for CLC applications. Table 6 shows the oxygen Table 6. Oxygen Ratio (%) for Different Oxygen Carriers MeO (wt%)

CaSO4/ CaS

NiO/ Ni

CuO/ Cu

Fe2O3/ Fe3O4

Fe2O3/ FeO

Mn3O4/ MnO

100 80 60 40 20 10

47.01 37.61 28.21 18.80 9.40 4.70

21.42 17.13 12.85 8.57 4.28 2.14

20.11 16.09 12.07 8.05 4.02 2.01

3.34 2.67 2.00 1.34 0.67 0.33

10.02 8.02 6.01 4.01 2.00 1.00

6.99 5.59 4.20 2.80 1.40 0.70

transport capacity for different OCs with different active metal oxide contents. The metal oxides used in previous study, as well as their main properties and reaction rates, are listed in Table 7 and Table 8. 4.1. Ni-Based Oxygen Carriers. Nickel-oxide-based materials have been widely developed and applied for methane conversion because of their fast kinetics for methane conversion.253,254 Ni-based OCs have shown high reactivity and good performance at high temperatures (900−1100 °C). Nearly complete CH4 conversion was obtained, although there are thermodynamic restrictions resulting in small amounts of CO and H2 at the outlet of the FR. In terms of previous research, NiO/Ni is a very good candidate due to its chemical and physical properties.173,176,212,251 However, these materials are very expensive and also harmful to the environment upon disposal. Unsupported NiO is more likely to have agglomeration problems at high temperatures, making it unsuitable for CLC applications compared to supported NiO.103 Among available support materials, Al2O3-based compounds have received the greatest attention given its high reactivity, favorable fluidization properties, thermal stability without agglomeration, low attrition rate, and limited carbon deposition during operation in FBs at CLC conditions.79,251,255 Nickel is mainly present as a dispersed nickel oxide phase, less prone to sintering and agglomeration than other metals (e.g., Cu), although there is always the likelihood of nickel aluminate (NiAl2O4) formation, a phase hard to reduce below 1000°C. The formation of the spinel structure (e.g., NiAl2O4) depends on the crystalline nature of the support, e.g., γ-Al2O3 leads to formation of substantial amounts of NiAl2O4. Oxygen in NiAl2O4 is not available for reduction temperatures < 1000 °C. Interactions between metal oxides, support materials, and promoters/additives affect the reactivity of the OC. As NiAl2O4 formation continues during the redox cycles, the reactivity of the OC decreases with time.256 Cho et al.257 and Mattisson et al.235 suggested using excess nickel to compensate for the loss of nickel aluminate. Other researchers51,220,258 proposed NiAl2O4 as the support instead of Al2O3. Although NiAl2O4 needs more Ni, which is expensive, as a support compared to 3489


reactor: TGA gases: CH4 (5.04%) (reduction); air (oxidation) gas flows: 100 mL/min (reduction); 100 mL/min (oxidation) temperature: 650−1000 °C pressure: atmospheric reactor: fixed bed gases: 13% CH4 (reduction); 8.6% O2 (oxidation) gas flows: 2.3 L/min (reduction); 2.2 L/min (oxidation) temperature: 500−1000 °C pressure: atmospheric bed materials: 40 g reactor: fluidized/fixed-bed reactor gas: H2 temperature: 850−560 °C pressure: atmospheric activity run under constant temperature gases: 50 vol% CH4 + He (reduction); 50 vol% CH4 + H2O (reduction); 20 vol% O2 + He (oxidation) gas flow: 20 cm3/min (reduction) temperature: 800 °C pressure: atmospheric bed mass: 2 g reactor: fixed bed gases: 10% CH4 + 10% H2O + 5% CO2 + 75% N2 (reduction); 10% O2 + N2 (oxidation) flow rate: 80 NmL/min temperature: 750−950 °C pressure: atmospheric bed mass: 100 mg

NiO (59%) + bentonite (Al3SiO2) T sin = 900 °C ρp = 4038 kg/m3

NiO(59%) + bentonite T sin: 900 °C ρp = 4080/m3

NiO + NiAl2O4 dp = 300−500, 600−1000, 1200−1700, 2000−3150 μm

Ni-Al-O (Ni/Al = 0.5−2.25) = NiO (6−100%), Ni-Mg-Al-O (Ni/Mg = 1, (Ni+Mg)/Al = 1)

3490

NiO (33%) + Al2O3, Mn2O3 (29.4%) + Al2O3,

Co3O4 (34.8%) + Al2O3, CuO (33.8%) + Al2O3 Tsin: 550 °C ω = 0.25−0.29 mL/g

Tsin: 1000 °C

ρb = 1407 kg/m3 ω = 0.59− 0.714 dp = 400 μm

ρb = 1319 kg/m3 ω = 0.69; dp = 91 μm

reactor: TGA gases: H2 (5.6%) + Ar (reduction); air (oxidation)

NiO (20%, 30%, 40%) + hexaaluminate (NiO + LaAl11O18) (NiO + NiAl2O4) T sin = 1000 °C

experimental conditions reactor: TGA reduction: H2, 550−950 °C oxidation: air, 1000 °C pressure: atmospheric

NiO, NiO (60%, 80%) + YSZ, Fe2O3 (60%) + YSZ T sin = 1300 °C dp = 1.3, 2.0, 2.8 mm

oxygen carrier particles

remarks

ref

235

249

Addition of Mg was found to stabilize Ni2+ in the cubic oxide and spinel phase; increasing the reduction temperature markedly improves regenerability. CH4/H2O (1:1) could avoid coke formation in Ni-Mg-Al-O. Ni-based systems are poorly selective to H2O and CO2, preferring CO and H2 if feeding CH4.

Ni- and Cu-based OCs showed high reactivity, with reduction rate up to 100%/min for CuO and 45%/min for Ni, and oxidation rate up to 25%/min for them. Mn and Co are not suitable; 560−620 kg/MW OC would be needed. Solids circulation rate is 1−8 kg/(MW·s).

479

The mass transfer mechanisms, i.e., particle-external and particle-internal diffusion, control the overall rate of reduction reaction.

380

248

Carbon deposition, reduction kinetics, and regenerative ability were examined; 900 °C is the appropriate for reduction and avoiding carbon deposition.

Carbon deposition, sintering, and lump of NiO + bentonite take place at 1000 °C; 900 °C is the most appropriate temperature for NiO + bentonite.

478

477

Showed good reduction and oxidation properties.

NiO/YSZ is a suitable material, and the reaction temperature is the strongest factor in the reduction.

Table 7. Summary of Investigation on Oxygen Carriers (Metal Oxides) for Chemical Looping Combustion



3491

ρp = 1400−3500 kg/m3

Fe2O3 (40−80%) + MgAl2O3 Tsin = 950−1300 °C

Tsin = 1300 °C size: pellet (d = 4.0 mm, h = 1.5 mm)

NiO (60%) + NiAl2O4, CoO-NiO (60%) + YSZ

dp = 1800, 2100 μm Tsin = 1300 °C

NiO + Al2O3, NiO + TiO2, NiO + MgO; CoO + Al2O3, CoO + TiO2, CoO + MgO; Fe2O3 + Al2O3, Fe2O3 + TiO2, Fe2O3 + MgO; NiO + Al2O3, NiO + NiAl2O4, NiO + YSZ reactant:binder = 60%:40%

reactor: FB reactor of quartz gases: 50% CH4 + 50% H2O (reduction); 5% O2(oxidation) velocity: 2−8Umf(reduction); 5−11Umf (oxidation)

reactor: fixed bed gases: 33−40 vol% CO, 17−20 vol% H2, 20−33 vol% H2O, 0−10 vol% CO2, Ar (reduction); 100 vol% air (oxidation) gas flow: 500−1200 mL/min temperature: 600−1000 °C pressure: 1−9 atm bed mass: 50 g

reduction: 600 °C, H2 100 mL/min (STP); 700 ° C, CH4 + H2O (1:2) oxidation: 1000 °C air pressure: 1−9 atm

reactor: TGA

reactor: TGA gases: CH4 (5.04%) (reduction); air (oxidation) gas flows: 100 mL/min (reduction); 100 mL/min (oxidation) temperature: 650−1000 °C pressure: atmospheric

(1) NiO (26.0−78.4%) + bentonite (Al3SiO2) Tsin: 900 °C ρp = 3562−3731 kg/m3

ρb = 1020−1530 kg/m3 ω = 0.59−0.714 (2) Ni (21.6−74.0%) + bentonite (Al3SiO2) Tsin = 1050 °C ρp = 2655−7374 kg/m3 ρb = 847−1515 kg/m3 ω = 0.681−0.795 dp = 80 μm

(1) reactor: TGA gases: H2 (reduction); air (oxidation) temperature: 900 °C pressure: atmospheric bed mass: 10 mg (2) reactor: fluidized bed gases: 67% H2 + 33% Ar (4.7 cm/s, STP) (reduction); air (1.7 cm/s, STP) (oxidation) temperature: 600, 900, 1200 °C pressure: atmospheric

experimental conditions

NiO (60%) + NiAl2O4 Tsin: 1300 °C dp = 97 μm


Table 7. continued

60% Fe2O3 on 40% MgAl2O3 is the most suitable (sintered at 1100 °C). Bed mass needed in FR was in the order of 150 kg/MWth.

Identify the reaction kinetics of coal gas-fueled chemical looping combustion.

NiO + NiAl2O4 will provide an outstanding performance for CLC. H2O/CH4 = 2.0 could avoid the carbon deposition.

Oxidation reaction rate increases as mass content of Ni and temperature increase; reduction rate shows maximum point with increase of temperature and mass ratio of NiO. Oxidation is controlled by product layer diffusion; reduction is controlled by chemical reaction rate.

NiO (60%) + NiAl2O4 has good circulation properties, high reactivity, and high mechanical strength. It could be used in the CLC circulation.

remarks

296

103

258

415

51

ref



3492

reactor: TGA gases: CH4 + H2 + H2O (30 wt%), CO + H2 + H2O (30 wt%) (reduction); air (oxidation) gas flow: 25 nL/h temperature: 800 °C pressure: atmospheric bed mass: 20−40 mg reactor: FB reactor of quartz gases: 100% CH4 (reduction); 5% O2 (oxidation) gas flows: 250, 500 NmL/min (reduction); 1000, 1000 NmL/min (oxidation) temperature: 950 °C pressure: atmospheric reactor: fixed bed, TGA

CuO (40%, 60%, 80%) with Al2O3, sepiolite, SiO2, TiO2, ZrO as inert

Fe2O3 (40−80%) + Al2O3 Tsin = 1300 °C dp = 125−250 μm

NiO (42.6%) + YSZ,

T sin = 500, 1100 °C dp = 200−400 μm

Tsin = 950−1300 °C ρp = 1400−5000 kg/m3 ω = 0.1−0.77

reactor: TGA gases: 70% CH4 + 30% H2O (reduction); air (oxidation) gas flow: 25 nL/h temperature: 800−950 °C bed mass: 20−100 mg

reactor: FB reactor of quartz gases: 50% CH4 + 50% H2O (reduction); 5% O2 (oxidation) velocity: 5−10Umf (reduction), 10− 20Umf(oxidation) temperature: 950, 850 °C pressure: atmospheric bed mass: 10 g

40−80% Cu, Fe, Mn, Ni oxides with Al2O3, sepiolite, SiO2, TiO2, ZrO as inert

Tsin = 1300 °C dp = 125−180 μm

Fe2O3 (60%) + Al2O3, Al2O3(32%) + kaolin (8%), NiO (60%) + NiAl2O4, CuO (60%) + CuAl2O4, Mn3O4 (60%) + MnAl2O4

dp = 125−180 μm ω = 0.22−0.76

ρp = 1100−4200 kg/m3

gases: 50% CH4 + 50% H2O (reduction); 5% O2(oxidation) velocity: 2−8Umf (reduction); 5−12Umf (oxidation) temperature: ∼950 °C pressure: atmospheric

reactor: FB reactor of quartz

Fe2O3 (60%) + Al2O3, Al2O3 (32%) + kaolin (8%), MgAl2O4, ZrO2, TiO2 Tsin = 950−1400 °C

experimental conditions temperature: 650−950 °C pressure: atmospheric bed mass: 10 g, 15 g

dp = 90−250 μm ω = 0.23−0.59


Table 7. continued

417

323 NiO/NiAl2O4 and CoO-NiO/YSZ are good candidates. Carbon deposition could be avoided completely by CoO-NiO/YSZ without addition of water and by NiO/NiAl2O4 at a H2O-to-CH4 ratio of 2.0.

267

267

256

250

ref

The feasibility of using iron oxide as an OC was investigated.

CuO-based carriers prepared by wet impregnation using titania and silica as supports have good chemical and mechanical properties for CLC.

SiO2 and TiO2 are best for Cu-based OC; Al2O3 and ZrO2 are best for Fe-based OC; ZrO2 is best for Mn-based OC; TiO2 is best for Ni-based carriers.

OCs based on Fe, Ni, and Cu showed high reactivity; those based on CuO and Fe2O3 on Al2O3 showed agglomeration. NiO has limited strength. Bed mass needed is 80−330 kg/MWth, and recirculation needed is 4−8 kg/(s·MWth).

Fe2O3 + MgAl2O4 sintered at 950 °C, Fe2O3 + ZrO2 sintered at 1100 °C, and Fe2O3 + Al2O3 sintered at 1300 °C are good OCs.

remarks



experimental conditions

gases: 100% CH4 (reduction), air (oxidation) gas flows: 300 mL/min (reduction); 900 mL/min (oxidation) temperature: 950 °C pressure: atmospheric bed mass: 20 g, 60 g, 90 g reactor: fixed bed gases: 50 mL/min CH4 (reduction); 1000 mL/min air (oxidation) temperature: 850−950 °C

Tsin = 1300 °C dp = 120−500 μm

CaSO4 bulk density = 1500 kg/m3specific density = 2900 kg/m3

3493

CaMn0.875Ti0.125O3 freeze granulation and freeze-drying

MnO2/Mn3O4 and MgO with optional addition of Ca(OH)2 or TiO2 freeze granulation dp = 125−180 μm

reactor: CFB gases: 0.15−0.75 Ln/min NG and 3−10 Ln/min air

gases: 450 mLn/min pure CH4 temperatures: 810, 850, 900, and 950 °C

reactor: quartz reactor

reactor: TGA gases: 1.5 L/min of 10% H2 + 90% N2 (reduction) temperature: 950 °C

hematite oxygen carrier BET surface area = 3.98 m2/g porosity = 0.03 cm3/g average aperture size = 22.22 nm dp = 0.1−0.5 mm

X-ray diffraction showed that the particles underwent changes in their phase composition at 950 °C.

Addition of Ca(OH)2 facilitates oxygen release and combustion of CH4, whereas addition of TiO2 does not have a significant effect on either oxygen uncoupling or reactivity of the OC.

Relatively stable structure was seen in the region rich in SiO2 or Al2O3 contents.

The performance in the cyclic experiments was tested. The oxygen carrier conversion after the reduction reaction decreased gradually in the cyclic test.

reactor: fluidized bed gases: 600 mL/min of 50% H2 + 25% CO + 25% CO2 at 7.12−8.15Umf (reduction); 1200 mL/min of 5% O2 + N2 at 15.6Umf (oxidation) temperature: 950 °C

CaSO4 bulk density = 1500 kg/m3 specific density = 2900 kg/m3

dp = 0.15−0.2 mm

Kinetic parameters of the decomposition reaction were achieved.

reactor: TGA gases: 40% CO2 + 40% N2 + 20% CO (reduction); 21% O2 + 79% N2 (oxidation) temperature: increase from 25 to 1355 °C at different heating rates

Mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO4 oxygen carrier.

( ddXt )red. = 3−23%/min ( ddXt )ox. = 20−90%/min, considerably fast

remarks

CaSO4 dp = 8.9 μm

dp: 0.15−0.2 mm

reactor: fixed-bed reactor of quartz

gases: CH4/H2O = 1:2 (reduction); 10% O2 + N2 (oxidation) flow rate: 300 mL/min (0.2m/s) temperature: 600−700 °C pressure: 1−3 atm bed materials: 50 g

Fe2O3 (58−100%) + Al2O3

CoO-NiO(42.7%)/YSZ Tsin = 1300 °C size: pellet (d = 4.0 mm, h = 1.5 mm)

NiO (48.7%) + NiAl2O4,


Table 7. continued

121

136

359

189

481

188

480

ref



Review

Energy & Fuels

gases: 10% H2 or 10% CH4 in He atmosphere, 300 mL/min temperature: 900 °C

reactor: TGA

perovskite-structured Ca0.8Sr0.2Ti0.8Ni0.2O3, fluorite-structured CeO2, and spinel-structured MgAl2O4 solid-state reaction (SSR), co-precipitation (CP), and citric acid (CA) methods dp < 150 μm

dp = 425−500 μm

reactor: TGA and FB gases: 5% H2 in N2 and 10% CO in N2 (50 mL/ min for TGA and 1.8 L/min for FB) temperature: 850−950 °C Fe2O3-CaO granulation and followed by impregnation

Assessment: 800 MWth (NG, Sweden), 655 MWth (NG, Canada); 50 kWth (Ni-Co, NG, and syngas, South Korea)

304 Activity of MgAl2O4-supported oxygen carrier is found to increase during redox cycles in methane. The activity increase is a consequence of surface area increase caused by filamentous carbon formation and oxygen carrier fragmentation.

314 Addition of CaO to Fe2O3 leads to the formation of CaFe2O4; the role of this phase during the redox reactions needs further investigation.

ref remarks experimental conditions

temperature: 720−950 °C


sintered for 3 h at 1200 °C dp = 90−212 μm bulk density = 1100 kg/m3 crushing strength = 1.25 N

Table 7. continued

applications. More information of Ni-based OCs is available in previous literatures.253,272 4.2. Cu-Based Oxygen Carriers. Cu-based OCs have several advantages relative to other OCs: (1) they are highly reactive in both reduction and oxidation; (2) reduction and oxidation reactions are both exothermic; (3) CuO reduction is favored thermodynamically to reach complete conversion using gaseous fuels such as methane; (4) higher oxygen transport capacity than Fe; (5) less costly than Ni; and (6) low toxicity. However, CuO has not received significant attention due to its tendency to undergo agglomeration and decomposition at relatively high temperatures.273−276 Different support materials (e.g., Al2O3, bentonite, CuAl2O4, MgO, MgAl2O4, SiO2, TiO2, and ZrO2) and different preparation methods (impregnation, co-precipitation, spray drying, freeze granulation, and mechanical mixing) have been used to make Cu-based OCs. Impregnation on α-Al2O3, γAl2O3, MgAl2O4, or NiAl2O4-Al2O3 has been found to be the optimum preparation method to avoid agglomeration and improve performance of Cu-based OCs.275,276 Impregnation on SiO2, TiO2, or γ-Al2O3235,275−277 or co-precipitation with Al2O3278 produced Cu-based OCs with excellent chemical stability and good mechanical strength. If Al2O3 is used, CuAl2O4 is formed, becoming dominant in the OC after a few cycles. However, CuAl2O4 is highly reducible, showing very high reduction reaction rates, similar to those of CuO.235,276−278 SiO2 is quite inert to Cu, even at high temperatures, and did not form any Cu-SiO2 complex.276,278 Particle reactivity and stability were reasonable, with reactivity inferior to that of Ni supported on SiO2. However, the copperbased OC suffered from CuO decomposition to Cu2O.240,267,273−275,279,280 TiO2, as a Cu support, displays limitations for CLC due to its tendency to form CuTiO4.276,278,279 It was shown that composite particles consisting of CuO (28−37 wt%) and inert support materials such as ZrO2, YSZ, CeO2, and MgAl2O4 provided full conversion of CH4 at 900−925 °C, and were also found to release gas-phase O2 into inert atmosphere at these temperatures when fluidized with N2, whereas OCs using semiactive support such as Fe2O3, Mn2O3, and Al2O3 formed combined spinel structures with CuO. Materials with semiactive support had less reactivity with CH4.281 The rate of CuO oxygen uncoupling becomes significantly higher when reaction temperature reaches 900 °C, whereas the increase in the reaction rates between CuO and H2/CO/CH4 is less profound.282 Low CuO content (e.g., 50 cycles, segregation of CuO from Al2O3 in the CuAl2O4 was observed during gaseous fuels combustion, which produced more available oxygen for CLOU than the initial material. 3494


Review

Energy & Fuels Table 8. Reaction Rate of Typical Oxygen Carriers oxidation

reduction NiO/YSZ

NiO/YSZ NiO/Al2O3 NiO/Al2O3 NiO/Al2O3 NiO/Al2O3 α-Fe2O3 α-Fe2O3 Fe2O3/Al2O3 Fe2O3/ Al2O3 CuO/SiO2 CuO/SiO2 CuO/TiO2 CuO/TiO2 CuO/ZrO2 CuO/ZrO2

conversion range

average reaction rate (%/min)

gases used

T (°C)

dp(mm)

ref

1.0−0.23 0.3−1.0 1−0.2 0.05−1 0.8−0.2 0.3−0.8 1.0−0.88 0.72−0.99 1−0.98 0.97−0.99 1.0−0.0 0.0−1.0 1.0−0.1 0.0−0.8 1.0−0.05 0.0−0.70

60 20 16 20 210 90 1−8 50 11 3.7 100 50 90 40 95 70

CH4 air 1 /3 CH4 + 2/3 H2O air H2 air CH4 air 1 /2 CH4 + 1/2 H2O 5% O2 + N2 CH4 air CH4 air CH4 air

600 1000 700 1000 900 900 950 950 950 950

1.8 1.8 2.1 2.1 0.07 0.07 0.180−0.250 0.180−0.250 0.125−0.18 0.125−0.18 0.2−0.4 0.2−0.4 0.2−0.4 0.2−0.4 0.2−0.4 0.2−0.4

298 298 258 258 263 263 360 360 256 256 285 285 285 285 285 285

decomposition.256,279 Previous study indicated that agglomeration occurred at 950 °C for the Cu-based OCs, mainly depending on CuO content and preparation methods.279 4.3. Fe-Based Oxygen Carriers. Fe-based OCs are considered to be an attractive option for CLC applications due to their low cost and toxicity.43,292 However, they have low oxygen transport capacity, weak redox characteristics, and low methane conversion. Different oxidation states (Fe3O4, FeO, or Fe) can be found when hematite (Fe2O3) is reduced. Only the transformation from hematite to magnetite (Fe2O3-Fe3O4) may be applicable for industrial CLC systems due to thermodynamic limitations. Further reduction to wustite (FeO) or Fe is slow and would produce more CO and H2, reducing the purity of CO2 produced by the FR.186 Preparation methods included mechanical mixing, freeze granulation, impregnation, and co-precipitation. The active metal oxide content has ranged from 20 to 100 wt%, and most studies have included metal contents exceeding 60 wt% due to the low oxygen transport capability of the Fe-based OC. Some of the works have used pure Fe2O3. The original hematite (Fe2O3) is a nonporous and smooth textural material of low specific surface area. It changes to a coarser texture with cracks and fissures when exposed to alternating reduction and oxidation cycles. The oxidation rates of these natural ores are adequate in CLC. Some researchers256,293 have found agglomeration problems in the bed associated with phase change from wustite to magnetite when oxidized in air. Abad et al.43 noted that Fe2O3 metal contents below 10 wt% are unlikely to be suitable for CLC operation due the limited solid circulation rates between interconnected FB reactors. To improve reactivity and/or overcome agglomeration, a variety of materials have been used as supports for Fe-based OCs, e.g., Al2O3, MgAl2O4, SiO2, TiO2, and ZrO2, with alumina being the most common. The solid-state reaction between Fe and Al2O3 is considered to be the main cause of the loss in particle reactivity.49 However, with Al2O3 or TiO2 as the support, iron aluminate (FeAl2O4) or iron titanate (FeTiO3) can be formed as the reduced compounds corresponding to Fe(II). The reduction of Fe(III) in Fe2O3 to Fe(II) in FeAl2O4 and FeTiO3 increases the oxygen transport capacity from the AR to FR compared with hematite (Fe2O3) being reduced and transformed into Fe+2.67 in magnetite (Fe3O4). This is beneficial to fully convert the fuel gas to CO2 and H2O.124,294,295 Note

It was reported that ZrO2 support could increase the chemical stability, mechanical strength, and reactivity of Cubased OC.284,285 Wang et al.284 showed that the reduction and oxidation rates of Cu-based OCs with different inert supports increased in the order of ZrO2 > TiO2 > SiO2. Gayán et al.285 suggested that the attrition rate of the Cu-based OCs supported on ZrO2 was low and stable (0.045%/h). However, the reaction mechanisms between CuO supported on ZrO2 and reaction gas (CO, O2) have not been investigated, and the detailed electronic properties and the sintering inhibition mechanism of CuO/ZrO2 are still not well understood.286 Adánez et al.61 and de Diego et al.81,274 tested 15 wt% CuO impregnated on γ-Al2O3 in 500 Wth and 10 kWth CLC units with syngas and CH4 as fuels. A total of 120 h of operation was achieved in the continuous 10 kWth CLC unit, and the effects of the operating conditions on fuel conversion and performance of the OC were tested. The OC-to-fuel ratio, ϕ, was found to be the most important parameter affecting fuel conversion. For a FR temperature of 800 °C and ϕ > 1.4, CH4 could be 100% converted to CO2 and H2O without CO and H2 emissions, with no observable agglomeration and carbon deposition, and with at attrition rate almost constant and as low as 0.04 wt%/h during 50 h of operation. Forero et al.287 analyzed the behavior of CuO impregnated on γ-Al2O3 and found that only 60 h stable operation could be reached for the FR at 800 °C and AR at 900 °C due to agglomeration. Gayán et al.276 reported that Cu-based OCs with γ-Al2O3 modified with 3 wt% NiO as support achieved more than 67 h of stable operation with FR and AR temperatures of 900 and 950 °C, respectively. In addition, Forero et al.288 discovered that sulfur impurities present in the feed gas did not affect the reactivity of the OC and that full CH4 conversion was reached in the FR. Penthor et al.289 tested a 120 kW chemical looping pilot plant with NG as the fuel and Cu-based oxides as the OC. The Cu-based OC showed good performance regarding conversion of CO and H2 (∼100%). However, only moderate conversion (up to 80%) was achieved for CH4. Cao et al.290,291 studied a FB CLC process at 600 °C using solid fuels and Cu-based OCs. No particle agglomeration was observed. However, after the test, Cu2O was found, attributed to the decomposition of CuO. It was shown that CuO decomposition reactions occurred in the AR at a low oxygen concentration (e.g., 4 during 50 h of operation with no carbon deposition, agglomeration, or defluidization problems. Combined oxides of iron, manganese, and silicon have been used as OCs for CLC combustion in a FB reactor with continuous circulation of solids designed for a thermal power of 300 W,303 and full conversion of syngas and >95% conversion of NG above 900 °C have been achieved. The study showed that it was possible to achieve very high fuel conversion with combined oxides of iron, manganese, and silicon as the OC. However, the mechanical stability of the particles is rather poor and needs to be improved. Galinsky et al.304 investigated the effect of support on the cyclic redox performance of iron oxides 3496


Review

Energy & Fuels FB reactor.312 The presence of MgO on the hematite OC significantly improved the oxygen utilization of hematite for methane CLC. Cyclic CLC tests conducted in the FB with MgO-promoted hematite showed better performance than that with hematite. The CuO−Fe2O3/ Al2O3 OC exhited excellent performance during the 25-cycle FB CLC tests conducted at 800 °C with methane and air. Full conversion of CH4 to CO2 and stable oxygen transfer capacity were observed during all the cycles. The fluidization of the OC was good, with no particle agglomeration as is traditionally observed with CuO-containing materials. Attrition resistance of both OCs with particle size of 100−150 μm was better than that of standard FB cracking catalysts.312 Reactivity and stability of the MgO-promoted hematite were also tested in the TGA and bench-scale reactors. The incorporation of 5 wt% MgO led to an increased reaction rate and an increase in oxygen utilized as compared to those with the pure hematite OC. These studies revealed that the best performing OC was the 5 wt% MgO/Fe2O3, which exhibited no observed degradation in the kinetics and conversion performance in the methane step over 15 reduction and oxidation cycles. The Mg-promoted OC also showed reduced coke formation as compared to that with the pure hematite carrier.313 OCs were made from Fe2O3 powder by granulation and impregnation with Ca(NO3)2·4H2O.314 The reactivity and cyclic stability of the OCs were tested by alternating reducing (using H2 or CO) and oxidizing environments (using air and CO2) in a TGA and a FB reactor at 850−950 °C. It was found that the one containing 20 mol% CaO showed optimal performances. However, others showed improvements in oxygen carrying capacity, cyclic stability, and reactivity over unmodified iron oxide but to a lesser extent.314 4.4. Mn-Based Oxygen Carriers. Mn-based materials are considered nontoxic and inexpensive. Moreover, their oxygen transport capacity is higher than for iron compounds. However, relatively few studies have dealt with Mn-based materials as the OCs for CLC. The highest oxidized manganese compound, MnO 2 , decomposes at 500 °C. Mn2O3 is more thermodynamically stable than MnO2 in air at relatively high temperatures.315 However, only Mn3O4 is present at temperatures above 800 °C.259 Therefore, only the transformation between Mn3O4 and MnO is considered for CLC applications. Pure manganese oxides (e.g., Mn2O3, Mn3O4) have shown low reactivity with CH4 and coal.316,317 Manganese oxides react with inert materials, e.g., SiO2, TiO2, Al2O3, or MgAl2O4, to form highly irreversible and unreactive phases which reduce the reactivity of OCs.235,244,256,267 The mechanical strength of the Mn-based OC was low when sepiolite (a complex magnesium silicate) was the support.267 Manganese oxides with bentonite (aluminum phyllosilicate) as binder has shown good reactivity to a mixture of H2 and CO. However, the reactivity was very sensitive to the presence of H2S in the gas mixture.318 OCs with ZrO2 as the support showed good reactivity and stability through consecutive redox cycles.185 During heat treatment and reactivity testing, Mn-based OCs with ZrO2 showed agglomeration or underwent a phase transformation, producing cracks in the structure.194,319,320 To avoid these problems, new OCs were prepared with ZrO2 stabilized by addition of MgO, CaO, or CeO2.194 Mn-based OCs supported on ZrO2 stabilized with MgO showed good reactivity with syngas,321 but lower reactivity was found for CH4.185 These particles were better suited for syngas than for methane combustion in terms of

reactivity, which was demonstrated in a continuously operated 300Wth CLC unit.28 Very high efficiencies (>99.9%) were obtained at temperatures of 800−950 °C for syngas combustion. For NG combustion, some methane was detected in the gas outlet from the FR, and combustion efficiencies ranged from 88 to 99%. The perovskite-based OCs were synthesized. Ca(OH)2 and MnCO3 powders with an average particle size of ∼46 μm (325 mesh) were mixed in a certain mass ratio with or without La2O3, Fe2O3, ZrO2, and SrCO3 to make the OC particles. This new modified calcium manganese perovskite structures applicable in CLC were investigated.322 All prepared samples showed a porous surface, and the perovskites phase formation was confirmed by XRD results. Reactivity and oxygen uncoupling behaviors of the prepared OCs were also evaluated using a FB CLC reactor using methane as the fuel at four different temperatures (800, 850, 900, and 950 °C). All OCs showed acceptable quantity. Oxygen uncoupling properties and reactivities for methane combustion of 12 OC particles, produced from mixtures of Mn and Mg oxides with optional addition of TiO2 or Ca(OH)2, were investigated in a quartz batch reactor at 810, 850, 900, and 950 °C. The addition of Ca(OH)2 facilitated oxygen release and combustion of methane. However, addition of TiO2 did not have a considerable effect on either oxygen uncoupling or reactivity of the OC. OCs with greater extent of oxygen release generally have better methane combustion properties.136 4.5. Co-Based Oxygen Carriers. The cobalt oxide has been considered as a possible OC because of its high oxygen transport capacity, but subject to high cost and environmental concerns. Several oxidation states can be involved in the redox reactions cycles with cobalt, Co3O4 is unstable above 900 °C, being converted into CoO. Hence, only the transformation between CoO and Co is considered for CLC applications, although it is less thermodynamically favorable, with maximum conversion of 95−97% for H2 and 87−97% for CO in the temperature range of 800−1200 °C.7 With Al2O3, TiO2, and MgO as inert supports, the metal oxide and support materials suffered strong interactions, forming unreactive compounds such as CoAl2O4, CoTiO3, and Mg0.4Co0.6O. Mattisson et al.235 reached a similar conclusion with an OC prepared by impregnation using Al2O3 as support, indicating that these materials were not suitable for CLC. Jin et al.258,298 observed that CoO/YSZ OCs exhibited good reactivity and low carbon deposition in their TGA studies. Ryu et al.176 reported 25 h of continuous operation in a 50 kWth CLC unit with a Co-based material supported on CoAl2O4. They reported a 99.6% of CH4 conversion, although the attrition resistance of the OC required improvement to accomplish long-term CLC operation. 4.6. Mixed Oxide Oxygen Carriers. Mixed metal oxides sometimes provide better features than individual metal oxides. However, only limited studies have focused on mixed metal OCs for CLC application. The investigations have been performed either by mixing different active metal oxides in the same particle or combining different OCs each composed of single metal-oxides. The purpose is to increase reactivity, thermal stability, and mechanical strength, improve reaction rate, conversion, and efficiency, decrease carbon deposition and poisoning of the OC, and minimize cost and toxicity. 4.6.1. Cu-Ni. Adanez et al.47 and Johansson et al.297 reported a stable bimetallic Cu−Ni/Al2O3 OC. It was claimed that Cu and Ni stabilized each other, providing improved performance 3497


Review

Energy & Fuels

this spinel material increased both its oxygen capacity and the reactivity of the resulting material. However, addition of CuO on the spinel led to agglomeration and defluidization of the bed during the reduction and oxidation. The CuO content should be O2.56 Nevertheless, an important dispersion in the values for each type of OCs was observed. The interaction between the metal oxides and the support affects the activation energy. Reducing the affinity of the metal oxide with the support reduces the activation energy and makes reduction of the metal oxide easier.50,77,259 Thus, it can be concluded that the kinetic parameters for every OC should be determined specifically. Extrapolation of kinetic models to various OCs is not yet viable.412 The solid-state kinetics of the reduction and oxidation are mainly developed using the nucleation and nuclei growth model (NNGM) and unreacted shrinking core model (USCM).412 Ryu et al.262,415 assumed a pseudo-first-order reaction for the reduction of NiO by CH4. The rate constant, k, for a NiO/ bentonite particle based on the Arrhenius equation is as follows: ⎛ 16349 ⎞ ⎟ k re = 73.63 exp⎜ − ⎝ RT ⎠

(14)

The reaction rate of OCs is the most important property of particles, because the solid circulation rate between AR and FR, and the amount of bed materials necessary in the two reactors, are inversely proportional to the reaction rate of the OC.114 The reaction rate varies widely depending on the particle size, temperature and pressure, the composition and type of the metal oxides and the support, and gas composition.279,298,416 In order to describe the reaction rate, the degree of oxidation, X, called solid conversion, is introduced: X=

ma − mred. mox. − mred.

(15)

The reaction rate could then be presented by dx/dt. The conversion rates of OCs can be increased further by reducing the particle size or by increasing the reaction temperature. The OC must have enough reactivity to fully convert the fuel gas in the FR and to be regenerated in the AR. Many studies of CLC focus on the OC development. Different OCs were testified in the TGA, fixed beds, FBs, and the particle properties including reactivity were measured, as mentioned in above sections. However, the reactivity data are usually obtained for one single operation condition, and limited information can be extracted for design and optimization purposes. Reaction rates under different operating conditions (e.g., different temperature, pressure, and gas concentrations) should be determined.49,50,251,252,255,256,412,417 The reactions with the OC in the ARs and FRs can be considered as noncatalytic gas−solid reactions. The most frequently used models for predicting the time dependence of the solids conversion and the effect of operating conditions on the reaction rate are changing grain size model (CGSM), shrinking core model (SCM), and nucleation and nuclei growth model (NNGM).7,68,310,391 Gas−solid reaction kinetics becomes more complicated considering supported metals and metal oxides with added promoters and additives into the OC. Preparation methods of the OC may also affect the physical properties, chemical composition, and structure of the OC, which are closely related 3503


Review

Energy & Fuels

for the CLC application; a diffusion coefficient dependent on solid conversion was used.56,239,424 The rate-controlling step can change in the course of the reaction.420,424 If we ignore particle size and mass transport resistance and assume the solids conversion is uniform throughout the solid, the model corresponds to a SCM in the grains considering only the chemical reaction term, which was used to calculate the kinetic parameters of the reduction and oxidation reactions for Cu-, Ni-, and Fe-based OCs.56,189,294,424 Particle properties (e.g., particle size, porosity and specific surface area, pore size and structure), active metal oxide content, support materials and promoters/additives, and gas composition are all related to resistance to the mass or heat transfer, as well as to reaction rates. 5.2.2. Shrinking Core Model (SCM). When the resistance to gas diffusion in the unreacted particle is very high, the SCM in the particle should be considered. Thus, a layer of the solid product is formed around an unreacted core inside the particle. The time required to reach a certain conversion is calculated in a similar way as that in the grain model, but replacing the radius of the grain by the radius of the particle7 (see Figure 6). Xiao and Song425 investigated the kinetics of CaSO4 reacting with CO in more detail using the SCM, kinetic parameters with high precision were obtained.

to the reaction mechanism and chemistry. The rate-controlling step of the gas−solid reactions in CLC process is defined by observing the reaction rates at different set of experimental conditions as well as applying theoretical calculations. Kinetics parameters are mainly assessed using TGA, TPR, and TPO and validated employing reaction data obtained from experimental study (e.g., fixed beds and FBs). Reaction kinetics is crucial for design, optimization, and scale-up of the CLC process. 5.2.1. Changing Grain Size Model (CGSM). The CGSM418,419 takes most of steps involved in gas−solid reactions into account: gas film transfer, diffusion through the interstices among the grains (i.e., pores), diffusion through the product layer around the grain, and chemical reaction on the interface in the grain. The grain size changes and the unreacted core size shrinks as reaction proceeds (see Figure 5).420

Figure 5. Scheme of changing grain size mode. Adapted from ref 326.

The local reaction rate in terms of the gas concentration in the pores can be expressed as −rg =

S0(rc/r0)2 1+

ksrc Ds

(

1−

rc r1

)

ksCg (16)

Effectiveness factor was considered to take into account the gas diffusion in the pores for the reaction rate, which is referred to as the diffusion-reaction model.420 Szekely et al.421 used specific conversion function for each time accounting for the specific resistance followed by summing the time of each one. Erri and Varma422,423 observed that the reduction reaction of Ni-based OC particles supported on NiAl2O4 (40 wt% NiO, particle size up to 425 μm) were not limited by diffusion effects. The rate of diffusion through the film, pores, and product layer presents a negligible resistance based on most of studies related to OC particles, especially at high temperature (e.g., >800 °C). The resistance to heat and mass transfer in the gas film and inside the particle together with the chemical reaction on the particle surfaces were also considered by some researchers for both oxidation and reduction reactions with different fuel gases (CH4, H2 and CO) and metal oxides (Ni, Cu, Fe, Mn, and Co)

Figure 6. Scheme of shrinking core model. Adapted from ref 326.

In general, the unreacted shrinking-core models are classified as (1) the grain model, which describes the solid reactant phase as a juxtaposition of dense objects, and (2) the pore model, which considers the porous solid as a collection of hollow objects.50,426 The models of the above classification are different in calculating the surface area of the active sites. However, they are not totally independent, and each of these models can possibly be derived from a more generalized mathematical form depending upon the appropriate structure of the solid material.50 3504


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Energy & Fuels

Figure 7. Scheme of nucleation and nuclei growth model. Adapted from ref 326.

is a dynamic process and practically initiates the reaction. There is an induction period for activating the solid phase to form nuclei before nucleation, the reaction rate increases as the number of nuclei increases during the first moment of reaction. The length of the induction period primarily depends on the gas−solid system and reaction temperature. The nuclei growth occurs due to the overlap of the nuclei and/or ingestion of a nuclei site. After the induction period, the reaction will occur uniformly over the solid surface, and the reaction front advances uniformly into the inner part of the grain. Thus, the conversion vs time curves are characterized by a sigmoid behavior, often described by the Avramie−Erofeev model (AEM).326 For a particular gas−solid reaction, the nuclei growth rate is constant at a given temperature and composition of the gas phase. The relative rate of nucleation, nuclei growth, and the concentration of the potential nucleus-forming sites (for generation of metallic nuclei) determine the overall conversion of the reaction. Figure 7 shows the possible steps during a gas−solid reaction following the NNGM. Nucleation effects are often significant in systems such as reduction of metallic oxides. These models have been widely used in reduction of Ni-based catalyst at low temperatures. The nucleation process is accelerated as the temperature increases, for example, the induction period was imperceptible at > 340 °C.421 Sedor et al.269 found that the reaction starts immediately at temperatures above 600 °C without an induction period. It was believed that the reduction and oxidation of metal oxide proceed through nucleation and crystal growth, and NNGM was applied to describe the kinetics of OC based on the reaction-rate-controlling AEM.28,429 A reaction rate equation can be expressed as following:326,430,431

The USCM incorporates particle size and pore structure of the solid reactant particles. According to this model, as the reaction progresses the metal−metal oxide interface moves toward the center of the grain, leaving behind a porous metallic/metal oxide product layer through which gaseous reactants and products diffuse.418,427 The SCM has been used to calculate the kinetic parameters for reduction with CH4 and H2 and oxidation of millimetersized Ni-based particles. The particles are considered as a matrix of nonporous individual grains of uniform size.43,239,263,294,415,428 The conversion vs time curves indicate that reduction was controlled by chemical reaction, but oxidation was in the intermediate regime between chemical reaction control and product layer diffusion control.415 According to the SCM, the reaction rate is first order in Rp for chemical reaction control, second order for product layer diffusion control, and in the interval 1.5−2.5 order in Rp for gas film diffusion. This fact was properly analyzed by Ishida et al.263 for particle sizes from 1 to 3.2 mm. The SCM has been able to predict experimental data for smaller particles.260,415 However, uncertainty arises about the suitability of the SCM in these cases because of the high porosity and small size of the particles, in the order of 100 μm. Assuming a hypothetical first-order gas−solid reaction shown by eq 17, A(g) + b B(s) → cC(g) + d D(s)

(17)

the unreacted-core shrinking model gives the following change of the core radius, rp: −

drp dt

=

cCA /ρB rp2 R p2kg

+

(R p − rc)rc R pDe

+

1 ks

dX s = ks′(T )Cg nf (Xs) dt

(18)

where the solids conversion, Xs, is given by ⎛ rp ⎞3 1 − Xs = ⎜⎜ ⎟⎟ ⎝ Rp ⎠

(20)

The general equation for the function of the solids conversion is f (Xs) = ν(1 − Xs)[−ln(1 − Xs)](ν − 1)/ ν

(19)

The three terms in the denominator of eq 18 represent the external gas film diffusion, the product layer diffusion, and the chemical reaction, respectively. 5.2.3. Nucleation and Nuclei Growth Model (NNGM). Many gas−solid reactions with formation of a solid product proceed by the nuclei formation and nucleation process, which

(21)

The AEM has been applied to the reduction and oxidation of Ni-based oxygen-carriers.326 Temperature-programmed reduction (TPR) or oxidation (TPO) was performed to obtain the kinetic parameters, using a heating rate of 10 °C/min. The temperature was 200−500 °C for the oxidation reaction, reduction reaction proceeds from 300 to 600 °C, above which 3505


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Energy & Fuels the reduction is complete. The best fit was obtained from the random nucleation model (RNM) with ν = 1 (see eq 21). When ν = 1, an induction period is not present. An expression equivalent to that for the RNM can be obtained from the power law model (PLM) or a modified volumetric model (MVM). These models have been used when the reaction occurs uniformly all through the particle, i.e., no diffusion resistance exists. Son and Kim260 used the same time−conversion dependence for the MVM to obtain kinetic parameters for the reduction with CH4 of Ni- and Fe-based OCs. In a CLC system, higher temperatures in the range 900− 1200 °C should be necessary to get high electrical efficiency, although temperatures of about 600−800 °C could be sufficient for industrial process.65,73 For the temperature range involved in CLC system, the nucleation process could be relatively fast and of low relevance regarding the conversion of the bulk solids. When the nucleation occurs rapidly over the entire solid surface, the models which deal with interfacial chemical reactions can be applied (e.g., SCM).421 Actually, both the SCM and RNM have been shown to fit the same experimental data reasonably well.269 The nucleation model only emphasizes on the chemical mechanism and kinetics of the gas−solid reactions, but it does not consider the morphological factors, which may be equally important in determining the kinetics. It has been shown that the reaction rate of the gas−solid reactions can be influenced by the grain size for a particle diameter greater than 10 μm.432 Specifically for the porous particles, the effect of particle size and its state during the reaction is very important in determining reaction rates. To take into account the various aspects discussed above, literature studies investigated both the NNGM and USCM, in order to describe the gas−solid reaction kinetics during both the reduction and oxidation cycles of the CLC processes. 5.3. Process Simulations and Mathematical Models for CLC Systems. Aside from the need of technological improvement of the CLC system, there are some other important issues that have to be addressed for commercial implementation of an integrated CLC plant with CO2 capture. Mass and energy balance, thermodynamic analysis, process synthesis, and simulation have been conducted in order to optimize operating parameters and process integration and to minimize cost.49 Most of these studies consider methane, syngas (CO and H2), or some carbonaceous solids as possible fuels with Ni-, Fe-, and Cu-based OCs due to their favorable cost, reactivity, and stability. 5.3.1. CLC of syngas. Syngas is from gasification of fossil fuels (e.g., coal and petroleum coke) or biomass. After cleaning, syngas of certain gas composition and impurities passes through the FR, where syngas reacts with metal oxides to form CO2 and H2O. This configuration is called integrated gasification chemical looping combustion (IG-CLC).433,434 There are several process options for this plant configuration: (1) IGCLC with gas turbine system and heat generation (IG-CLCGT); (2) IG-CLC with steam turbine system (IG-CLC-ST); and (3) IG-CLC with combined cycle (IG-CLC-CC). In refineries, refinery gas may be used instead of syngas. A simplified scheme of an integrated CLC-based power plant is shown by Brandvoll and Bolland.99 In this configuration, the outlet gas stream of the AR drives the GT before it is routed to heat recovery steam generator (HRSG) to produce steam, which is used in the ST to generate extra power, whereas the exhaust of the FR drives the CO2 turbine. After circulating

through the CO2 turbine, the gas is further cooled to nearly ambient temperature in order to condense the water, leaving almost pure CO2 (greater than 90%). Finally the concentrated CO2 stream is compressed for transportation and sequestration. Spallina et al.91 simulated different cycle strategies using a numerical model for the heat management in a dynamically operated packed-bed reactor with syngas as the fuel for CLC. Different layouts have been compared in order to discuss the effects on the axial solid temperature and solid conversion profiles, the fuel conversion, and the reactor outlet conditions. Several studies analyzed a CLC system of two reactors using syngas from coal gasification for electricity and hydrogen coproduction. The authors already conducted an exergy analysis for the design.435 Xiang et al.436 analyzed an IGCC design with electricity and hydrogen co-production using a Fe-based OC in a TRCLC system or using a combination of Ni- and subsequent Fe-based OCs.437 Both studies used the oxidizer (steam reactor) for the production of hydrogen. To apply fluidization within the steam and air reactor, the feed gas streams are injected at a pressure of 40 bar. Experimental investigations concerning the CLC design mostly use steam or nitrogen to seal the interconnections among the reactors. Sorgenfrei and Tsatsaronis438 have done design and thermodynamic evaluation of an IGCC process using syngas chemical looping combustion (IG-CLC-CC) of three reactors (multistage moving bed for reducer, and FBs for oxidizer and combustor) for generating electricity, as well as for CO2 capture and H2 production. The syngas from coal gasification is cleaned using high-temperature gas desulfurization (HGD), and the Fe-based OC is selected. The GT downstream combustor is expected to exhibit low NOx emissions due to the high ratio of water in the combustion chamber. BGL slagging gasifier and the Shell entrained flow gasifier were employed to evaluate the net efficiency. The option of using a CO2 turbine after the FR was also investigated. It was found that the best net efficiency of 43% (based on HHV) can be obtained using a BGL gasifier without a CO2 turbine at an AR temperature of 1000 °C, including CO2 compression for transport and storage. A sensitivity analysis on the AR outlet temperature was compared on the basis of the power output and the net efficiency. Please note that both the HGD and CLC systems are not under commercial operation so far. Energetic and exergetic analyses of CLC GTs fueled with syngas were conducted in previous study. Second-law analysis of syngas-CLC-CC and simulation results of syngas-CLC with different OCs were provided.439 However, energy savings in the capture of CO2 were not quantified. There is also a potential of integrating CLC with combined cycles (syngas-CLC-CC), like the analysis of the trigeneration.440 Combining CLC and IGCC (IG-CLC-CC) in particular could achieve highly efficient power generation together with nearly zero greenhouse gas emissions. An accurate thermodynamic modeling is required for the optimization of several design parameters. Simulations to evaluate the energetic efficiency of this CLC-based power plant under diverse working conditions have been carried out, and a comparison of a conventional integrated gasification power plant with pre-combustion capture of carbon dioxide has been made. Two different syngas compositions have been tried to check its influence on the results. The energy saved in carbon capture and storage by the IG-CLC-CC process is found to be significant, inducing an improvement of the overall power plant thermal efficiency of around 7% in some cases.88 3506


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Energy & Fuels 5.3.2. CLC of NG. NG-CLC is similar to syngas CLC systems except replacing syngas with NG. There are also several process options for this plant configuration: (1) NG-CLC with gas turbine system and heat generation (NG-CLC-GT); (2) NGCLC with steam turbine system (NG-CLC-ST); and (3) NGCLC with combined cycle (NG-CLC-CC).97,395,433 Consonni et al.207 analyzed a methane-fueled combined cycle of GTs and STs with CLC configurations (NG-CLC-CC). Song et al.189 studied the viability of nonconventional OC for CLC of methane. Peltola et al.441 provided a model-based evaluation at precommercial stage of a combined cycle, and Zhang et al.442 made a theoretical exploration of chemical looping hydrogen (CLH) generation with methane as fuel, which is a CLC variation.88 Kolbitsch et al.398 introduced a model to simulate the interconnected FB CLC system using Ni-based OCs with ARs and FRs in fast and turbulent fluidization regimes, respectively. One of the assumptions was that the reacting gas was only in contact with a defined fraction of well-mixed solids in each reactor. The hydrodynamic profile of the reactor was described only by the prescribed solids concentration profile along the reactor height. Energy and solids were balanced globally across the whole reactor, and plug flow was assumed for the gases. Abad et al.395 built a model for the FR using a Ni-based OC and CH4 as the fuel, including the conversion of the OC and the gas composition at the reactor exit, the axial profiles of gas concentrations, the fluid dynamic structure of the reactor, and combustion efficiency. Although the model was validated in a 120 kWth CLC unit, no systematic process simulation was available. An analysis using Fe-based OCs in a three-reactor configuration focused on a NG fueled CLC integrated in a combined cycle (NG-CLC-CC).443 Wolf and Yan444 analyzed a NG-fueled TRCLC system using Nibased OCs in a steam-injected gas turbine (STIG). Naqvi et al.97 presents thermodynamic cycle analysis of a NG-CLC power plant for combined cycle and steam cycle: a steady-state model was developed for the gas−solid reactions occurring in the reactor systems, with energy consumption of CO2 capture being taken into account. Effects of exhaust recirculation for preventing coking formation and incomplete fuel conversion were also investigated. The results show that an optimum efficiency of 49.7% (LHV) can be achieved under given conditions with a NG-CLC-CC at zero emissions level. The NG-CLC-ST is capable of achieving 40.1% efficiency (LHV) with zero emissions. Fernandez and Alarcon69 presented a process scheme based on fixed-bed reactors for carrying out the CLC of NG at high pressure with ilmenite as the OC. High-pressure and hightemperature operations can achieve highly efficient power cycles at at the price of complex heat management, and switching valves able to function at very high temperatures are required. The authours have done a preliminary conceptual design of 500 MWth capacity with NG as fuel and ilmenite as OC. The design has shown that a minimum of five reactors, 10 m long, with an inner diameter of 6.7 m, would be required to fulfill the overall process, assuming cycles of 10 min with maximum pressure drops per stage of less than 6%. These results demonstrate the potential of the CLC technology for power generation in combination with CO2 capture. The design and operation strategies of dynamically operated packedbed reactors of a CLC system were reported.70 This CLC system was included in an IGCC for electric power generation. The CLC reactors employed ilmenite as the OC and operated sequentially across the following phases: oxidation, purge,

reduction, and heat removal. The results indicated that 14−16 units with 5.5 m of internal diameter and 11 m of length are required for continuous operation of a 350−400 MWe coalfired power plant. Penthor et al.289 tested the Cu-based OC in a 120 kW chemical looping pilot plant with NG as the fuel, variations of several process parameters like temperature, fuel power, solids inventory, and solids circulation rate have been performed. The copper particles showed good performance regarding conversion of CO and H2 (almost full conversion), but only moderate conversion of CH4 (up to 80%) was achieved. Continuous analysis of the OCs revealed an initial decay of active CuO content caused by attrition on the external surface of the particles. The CuO content stabilized after 30 h of operation at around 9 wt%, and no further decrease was observed. Kallen et al.303 carried out CLC experimetnts in a FB reactor with continuous circulation of solids designed for a thermal power of 300 W, using syngas and NG as the fuels. The OCs used were combined oxides of iron, manganese, and silicon, with varying composition of these three materials. It was shown that full conversion of syngas and >95% conversion of NG above 900 °C have been achieved. Zeng et al. modeled a countercurrent gas−solid flow pattern using Aspen Plus software to provide insight into the operating conditions of a CLC system with NG as the fuel. The sensitivity analysis shows that, at 900 °C and 1 atm, an Fe2O3-to-CH4 molar ratio of greater than 2.6 is essential for complete conversion of methane in a countercurrent moving-bed reactor, as any values lower than 2.6 will result in unconverted fuel exiting with the reducer flue gas stream. The model was validated experimentally using a bench-scale countercurrent moving-bed demonstration unit. However, the experimental gas analysis results show a higher CH4 content and lower CO and H2 concentrations compared with the equilibrium composition from the simulation results, indicating kinetic limitations. Further kinetic simulation is needed to address the discrepancy between the multistage equilibrium model and experimental study.177 5.3.3. Chemical Looping Combustion with Solid or Liquid Fuels. Solid fuels mainly refer to coal and biomass,277,445−448 whereas liquid fuels may be various carbonaceous fuels from petroleum and refinery plants (e.g., kerosene, heavy oils, crude oil).449 For CLC with solid and liquid fuels, there are also interation between gases and solids in the AR and FR, similar to NG and syngas CLC systems from this point. However, the CLC process with solid and liquid fuels is generally more complex than that using gaseous fuels in terms of issues related to char and pollutants. A model based on semiempirical correlations was established to simulate the performance of the 1 MWth CLC rig with coal as the fuel and FR in the fast fluidization regime, using ilmenite as OC. The model considered the reactor fluid dynamics, the coal conversion, and the reaction of the OC with evolved gases from coal. The efficiency of a carbon separation system (i.e., two cyclones and one carbon stripper) was analyzed for the FR performance.450 A carbon stripper between the AR and FR increased carbon conversion for a CDCL combustion system.134,450, The effects of the carbon separation efficiency, the solids inventory and temperature in the FR, ratio of OC to fuel, OC reactivity, and coal properties (e.g., size, reactivity) on the carbon capture and combustion efficiency were studied. For CDCL, the FR temperature and the efficiency of the carbon separation system (i.e., two cyclones and one carbon stripper) are key parameters to improve the performance of the process. The WGS reaction 3507


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Energy & Fuels Table 10. Comparison of COE and CO2 Cost for Different CO2 Capture Technologiesa,89,200 supercritical PC performance thermal power, MWth net power, MWe generating efficiency (HHV) CO2 capture efficiency CO2 capture rate (tonne/year) CO2 emitted (g/kWhe) levelized cost of electricity (COE) capital charges (cents/kWhe) O&M (cents/kWhe) fuel costs (cents/kWhe) COE (cents/kWhe) CO2 cost ($/tonne CO2 avoided) cost year(s)

SubC PCOXY

IGCC

NGCC

CLC

w/o capture

w/capture

w/capture

w/o capture

w/capture

w/o capture

w/capture

w/capture

1298.7 500 0.385

1706.5 500 0.293 0.9 3653469 109.0

1634.0 500 0.306 0.9 3498257 104.0

1302.1 500 0.384

1602.6 500 0.312 0.9 3430983 102.0

654.7 379.1 0.579 (LHV)

655.1 326.9 0.499 (LHV) 0.90 872230 39.9

655.1 340.66 0.52 (LHV) 0.98 946337 7.6

4.34 1.75 1.60 7.69 40.36 2000−2004

3.85 1.67 1.45 6.97 30.17 2000−2004

1.82 0.60 2.41 4.83 44.82 2002

1.36 0.67 2.09 4.12 18.55 2002

830.0 2.70 1.33 0.75 4.78 2000−2004

832.0 2.90 1.33 0.90 5.13 2000−2004

3.83 1.64 1.05 6.52 19.04 2000−2004

343.4 1.03 0.36 2.09 3.47 2002

a

Note: 365 day/year, capacity factor = 0.85, plant life = 20 years, capital charge rate (CCR) = 0.151. Ni-based oxygen carrier is assumed to be CAD $6/kg (2002−2004). Selexol is used for integrated gasification combine cycle (IGCC), and amine-based solvents are used for pulverized coal (PC) and natural gas combined cycle (NGCC) plants. SubC PC-OXY denotes subcritical PC oxy-fuel combustion. CO2 compression, transportation, and storage are not included.

Figure 8. Schematic of a CLC process for power generation and CO2 capture.

was also evaluated.134 It was shown that the carbon capture efficiency of 98.6% could be reached if the FR temperature of 1100 °C, a solid inventory of 1000 kg/MWth, and a carbon separation efficiency of 98% were met.134 Aspen Plus simulations and mathematical modeling of a CLC process consisting of three reactors (i.e., fuel reactor, oxidizer, and combustor) indicated that the incorporation of a small amount of copper in the Fe-based OC led to increased hydrogen yield and process efficiency.307 Li et al.445 simulated and analyzed biomass direct chemical looping (BDCL) based on Aspen Plus process simulation. The process also consists of three chemical looping reactors (i.e., the reducer, oxidizer, and combustor) for CO2 capture, H2 production, and power generation, with a Fe-based OC circulating among the reactors. The four oxidation states of iron and their distinct thermodynamic properties give rise to a large degree of freedom in the process design and operating parameters. From

the process simulation, the suitable reactor design, operating conditions, and process configuration for the BDCL process were determined. Integration of CLC system with the existing plant and process equipment is very important to achieve high overall efficiency and low cost for power generation and CO2 capture.433,434 Fan et al.452 simulated three chemical looping processes, i.e., Syngas-CLC process, CDCL process, and CLP, utilizing simple reaction schemes to convert carbonaceous fuels into products such as hydrogen, electricity, and synthetic fuels based on Aspen Plus simulator. As operation with liquid fuels in CLC is still in its infancy, experience from the feeding process is limited.449,453,454 Mixing the liquid fuels with superheated steam at the moment of injection is a viable option.455 The steam will evaporate the liquid, resulting in a mixture of steam and gaseous fuel that enters the bed together. For the solid and liquid fuels CLC system, experimental studies of the reaction mechanism and 3508


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Energy & Fuels

FR. Depending on flue gas temperature and process requirements, there may be only HRSG for steam generation downstream the CO2 expansion turbine in Figure 8. Recall that in fluid catalytic cracking (FCC) in refinery, after gas−solid separation using multiple two-stage cyclones and swirl tubes, the exhaust gas (∼700 °C, 2−3 barg) from the catalyst regenerator is expanded through an expansion turbine to provide power for the process, then expanded flue gas is routed through a steam-generating boiler (or CO boiler) where CO is burned as fuel to provide heat needed. The same principle applies for FR flue gas. The levelized cost of electricity (COE) is comprised of three components: capital charge, operation and maintenance costs (O&M, fixed and variable), and fuel costs. Capital cost is generally the largest component of COE for coal power plants, fuel price is significant for a NGCC system because high price of NG. The electrical efficiency, COE, and cost of CO2 avoided are compared for different CO2 capture technologies in order to identify the potential application of the CLC process for power generation and CO2 capture. Most research and development work is based on operation at atmospheric pressure and reactor temperature

CO2 Capture with Chemical Looping Combustion of Gaseous Fuels

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