6.1 OCAC with Pressurized Oxy-Fuel Combustion

6.1.1 Pressurized Oxy-Fuel Combustion

Pressurized oxy-fuel combustion (POFC) can be deemed as a second generation oxy-fuel combustion technology, the schematic diagram of POFC is shown in Fig. 6.1. In the conventional oxy-fuel combustion, the ASU, combustor, and CO2 purification unit (CPU) are operated at around 0.6 MPa, 0.1 MPa and 8.0 MPa, separately [1, 2]. Compared to oxy-fuel combustion, POFC can dramatically reduce the energy loss caused by the pressure swing of the system. In addition, operation at elevated pressure reduces the boiler size [3], avoids air ingress, reduce the cost of CPU [4], improve the combustion efficiency [5, 6], lower the emission of pollutants [7,8,9,10], enhance heat transfer and gas–solid mixing characteristics [11, 12], improve the latent heat recovery of steam from flue gas [13], and so on. Many results from process simulation show that the net efficiency of power generation by the oxy-fuel combustion system can be improved by 1~5% with the increase of pressure [1, 14,15,16,17]. Gopan et al. [18] analyzed the fractional pressurized oxygen combustion system, and the results showed that the system efficiency loss caused by carbon capture could be reduced by 6% of the energy supplied (the comparison of net plant efficiencies for various cases is shown in Fig. 6.2). Therefore, POFC is expected to be a competitive CO2 capture technology for coal-fired power plants.

Fig. 6.1
A schematic diagram of pressurized oxy-fuel combustion. The primary parts include an air separator unit, heat recovery, steam generator, oxy-coal combustor, and power island. It also consists of a C O 2 purification unit to release carbon dioxide.

The schematic diagram of pressurized oxy-fuel combustion

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Fig. 6.2
A bar graph represents the net plant efficiency percentage with respect to the current U S average, supercritical without C C S, oxy S C, S P O C P R B, and S P O C Illinois.

The comparison of net plant efficiencies for various cases, SC: supercritical; SPOC: staged pressurized oxy-combustion; Both SPOC cases are supercritical (reprinted from [18] with permission of Elsevier)

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Two modes for POFC have been proposed: oxy-fuel pressurized pulverized-coal combustion (oxy-PPCC) and oxy-fuel pressurized fluidized-bed combustion (oxy-PFBC). To date, the PFBC technology has been considered as a more mature choice, which has been widely used in the energy and chemical industries [19]. Furthermore, employing FBC technology in the POFC system can combine all the advantages of the two technologies.

Despite its advantages, the application of POFC technology is still facing many challenges. Compared with atmospheric oxy-fuel combustion, pressurized combustion systems place higher requirements on equipment and operation. For example, although the mixing and mass transfer characteristics of a pressurized FB boiler are better than those of the atmospheric one [11, 20, 21], the locally uneven mixing of fuel and oxygen is still inevitable, although the smaller size of a pressurized reactor makes mixing between fuel and oxygen easier than at atmospheric pressure. This will bring a series of problems: (1) Unburnt combustible gases (CO, hydrocarbons, H2S and NH3, etc.) may damage the oxide layer (for corrosion resistance) on the surface of the heat exchanger [22] and may also cause an explosion of the downstream equipment [23]. Since pressurization can accelerate the corrosion rate and increase the deflagration/explosion hazard, these potential risks in a pressurized system are significantly higher than in an atmospheric pressure system; (2) Incomplete combustion not only reduces the boiler efficiency but also significantly increases the power consumption of CPU [24]; (3) To eliminate local oxygen deficiency, large combustion equipment need to increase the number of injection points of fuel and oxygen, which not only increases the equipment costs and system complexity, but also negatively affect the cost and reliability of operation [25, 26]. Therefore, there is a trade-off between technical and economic performance based on the ability of the system to transfer sufficient oxygen to complete combustion throughout the combustion region. In combustion technology, the sudden increase and decrease (or even interruption) of oxidant or fuel supplies cause the fuel to burn in a region with insufficient O2 or excessive O2, which brings great safety risks. When anoxic combustion occurs, a sufficient proportion of combustible gases in the flue gas may explode in downstream equipment, causing catastrophic damage to the system [27]. However, if the fuel burns at excessive oxygen concentration, a high temperature (even up to 2000 °C) can be generated. This not only brings the risk of equipment damage (such as high-temperature tube bursting) but also increases the pollutant emissions (such as NOx) and increases the cost of deoxygenation in the CPU module. The above-mentioned drawbacks will be more significant in a pressurized atmosphere. Therefore, the improvement of the controllability of the oxygen-fuel ratio in the combustion system is of great significance to optimize the pressurized oxy-fuel combustion system.

6.1.2 Oxygen-Carrier-Aided Oxy-PFBC Process

A potential configuration of an oxy-PFBC system incorporating OCAC technology was proposed by the researchers from CanmetENERGY [25, 28,29,30], as shown in Fig. 6.3. The main innovation is to replace the inert bed material with OC material partially or completely. The OC not only improves oxygen uniformity in the boiler but it also provides a property of oxygen buffering to improve the operation safety, which will effectively alleviate the problems associated with fluidized bed POFC technology described in Sect. 6.1.1.

Fig. 6.3
A schematic diagram represents the flow of processes between the gas technology institutes and Linde consisting of various compressors, reactors, storage units, and coolers. It indicates the purified C O 2 and process condensate at the end.

Oxygen carrier assisted Oxy-PFBC process flow diagram [28, 29]

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An important difference between an OC aided POFC and an OC aided oxy-fuel combustion system is the operating pressure. At atmospheric pressure, the reaction rate (for reduction and possibly oxidation) for many OCs with fuels is relatively low [31]. Many studies [32,33,34,35,36] have proven that the reaction rate is a function of the partial pressure of the reactants. Increasing the partial pressure of reactants can improve the reaction rate of an OC. Taking ilmenite ore as an example, Fig. 6.4 shows its reduction rate measured by a pressurized TGA at different pressures. Regardless of whether CH4 or CO was used as the reduction gas, the reduction rate of the OC increases significantly with the increase of the operating pressure. Therefore, it is expected that the introduction of OCAC in POFC can achieve more significant auxiliary effects than OC-assisted atmospheric oxy-fuel combustion. This may lead to higher oxygen utilization efficiency, which also means lower cost of oxygen production (in the ASU) and deoxidation (in the flue gas).

Fig. 6.4
A set of 2 curve graphs represents the plots for conversion ratio versus time. Graph a plots 4 curves and graph b plots 3 curves with an increasing trend.

Reduction of ilmenite ore at various pressures: a CH4 partial pressure; b CO partial pressure (reprinted from [34] with permission of American Chemical Society)

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In POFC, fuel and desulfurization adsorbent are usually carried by the high-pressure gas and injected into the bottom of the bed, where the drying, devolatilization and char combustion processes occur in sequence. The conveying gas velocity, location and quantity of injection points should be determined according to the load and hydrodynamic characteristics of the boiler. The conveying gas is typically purified CO2 that is generated from the CPU. The high-pressure O2 is mixed with the recycled flue gas (mainly composed of CO2 and H2O) and then injected into the wind box and subsequently into the combustion chamber through the distributor. The premixing of pure O2 and recycled flue gas is to avoid local hot points caused by locally high O2 concentration, thereby avoiding agglomeration of bed material and damage of the equipment.

The feed system of the pressurized combustion/gasification unit usually adopts pneumatic conveying or pump conveying [3], which inevitably causes fluctuations in the fuel feed. The unstable ratio of fuel to oxygen affects the combustion efficiency and operating safety. In general, to ensure combustion efficiency, the oxygen supply will be increased, which increases the cost of oxygen production and deoxidization [1, 36,37,38]. Moreover, in extreme conditions, if fuel or even oxygen interruption occurs during the operation, it will lead to disastrous results. The introduction of OCAC may greatly alleviate the threat of this problem because OC not only improves oxygen uniformity but it also provides oxygen buffering [39]. When the fluctuation of fuel and oxygen occurs, OCs can maintain the low combustible content and low oxygen concentration in the flue gas at through oxygen absorption and release. Take ilmenite ore (60.9 wt% hematite) as an example, the maximum theoretical oxygen transfer capacity is 9.9 wt%, which means that there is potentially 60.8 g of oxygen available in every kg of bed that can be provided to fuel combustion. Even in extreme cases of fuel and oxygen disruption, the buffering capacity of OC providing operators with valuable operating time, which can greatly improve the economy and safety of boiler operations. This exciting feature of OCAC has been demonstrated in atmospheric combustion [39] and can be also expected applicable in a POFC system. The elevated pressure only affects the OC’s redox rate while maintaining its ability in oxygen transportation [34].

The OC aided oxy-fuel combustion can significantly improve the desulfurization efficiency [40], and the POFC technology can reduce SO2 emission [10]. Hence, the combination of OC aided oxy-PFBC technology is expected to achieve a further reduction of the SO2 emission compared to the former two technologies. It should be noted that the increase of operating pressure will enhance the partial pressure of CO2, thus strongly inhibiting the decomposition of CaCO3 and weakening the indirect desulfurization reaction. Meanwhile, the direct desulfurization reaction will be significantly improved, since the increase of partial pressure of SO2 facilitates the diffusion of SO2 into the pores of desulfurized particles and promotes the desulfurization reaction. However, these conclusions were obtained when inert bed material was used, and the SOx emission under OCAC conditions need to be further studied. In addition, the presence of Fe2O3 at high pressure may also promote the oxidation of SO2 [41, 42], and the produced SO3 may intensify the corrosion of equipment. Therefore, the mechanism of sulfur conversion under the OC aided POFC conditions needs to be further studied.

Although the OCAC technology applied in oxy-fuel combustion increases the NOx emission [40], it has been confirmed that the POFC can effectively reduce the NOx emission [7, 8, 10, 43]. So, OC aided POFC is expected to achieve lower NOx emission than OC aided oxygen-fuel combustion while gaining all the advantages of OCAC technology. In addition, both OCAC technology and pressurized oxy-fuel combustion can reduce unburnt matter in the flue gas [9, 44], which lowers the emission of soot related PM [45, 46]. Therefore, the combination of the two technologies has the potential to yield a significant advantage in PM control.

6.1.3 The Potential Advantages of OC Aided POFC

Although research on the new route—OC aided oxy-PFBC technology has not been carried out yet, according to the summary and analysis of the research progress related to pressurized oxy-fuel combustion and OCAC in air and oxy-fuel combustion, the new technology route can be summarized in the following potential advantages:

  1. (1)

    The existence of OCs in the bed materials can not only improve the combustion efficiency but also improves the stability and safety of operation, which is particularly valuable for pressurized combustion systems;

  2. (2)

    An appropriate OC as the bed material may absorb alkali metals and alkaline earth metals (such as K and Ca). This can slow down the ash deposition process on heat-transfer surfaces and it can reduce PM emissions;

  3. (3)

    This technology leads to the reduction of SO2 emissions and the possible increase of desulfurization efficiency due to the more uniform oxygen distribution;

  4. (4)

    The pressurized boiler has a high volumetric heat load and heat-transfer coefficient, thus, the boiler size and heat-transfer surface can be greatly reduced compared to those in an atmospheric boiler, which may further reduce the capital cost of the facility;

  5. (5)

    The combustion process can be conducted at a stoichiometric ratio closer to 1, which can significantly improve the utilization ratio of oxygen and reduce the energy consumption of ASU and CPU modules, and improve the system efficiency;

  6. (6)

    The pressurized system can not only avoid the significant increase in CPU energy consumption caused by air ingress, but it can also mitigate the pressure swing of the system;

  7. (7)

    The inherently low NOx emission characteristics in pressurized oxy-fuel combustion may be equally effective for OC-aided POFC;

  8. (8)

    High pressure raises the dew point of steam and facilitates the recovery of the latent heat of steam from flue gas.

6.2 OCAC Coupled with Staged Conversion of Fuel

6.2.1 Staged Conversion of Fuel

NOX is one of the important culprits causing acid rain, smog, and other environmental problems. The main source of NOx emission is the combustion of various fuels, where the contribution from power plants and industrial boilers is essential. Therefore, in recent decades, the NOx emission has attracted a great attention with strict restriction measures because of emission limitations introduced in various countries. The CFB boilers, which are usually operated at 800–900 °C mainly generate fuel-type NOx with no thermal NOx produced due to the low combustion temperature. Thus, they show low NOx emission compared to pulverized-coal boilers [47, 48]. Traditional CFB boilers can achieve an NOx concentration as low as ~200 mg/Nm3, which is still not low enough to meet the increasingly stringent environmental protection requirements (below 50 mg/Nm3 in China) [49].

At present, researchers are devoted to developing technologies for further reduction of the NOx emission. The factors that affect the NOx emissions from CFB boilers are the combustion temperature, excess air coefficient, the ratio of primary and secondary air, air staging, flue-gas recycling, Ca/S molar ratio, etc. [50,51,52,53,54]. Optimizing these parameters controls NOx emissions to a certain extent, but it is normally not enough to make the emission reach ultra-low levels. Therefore, most CFB boilers have introduced SNCR or even SCR accessory equipment to control NOx emissions [55, 56], thereby increasing the cost of investment and the complexity of operation. Besides, ammonia slip from SNCR/SCR equipment is almost inevitable, bringing a series of problems including the blockage of air pre-heater and secondary pollution. Therefore, it is highly desired to establish a safe, stable, and efficient process to lower the NOx emission, designed for a CFB boiler.

Decoupling the combustion technology of solid fuel (a kind of staged conversion of fuel) has been proven as a high-efficiency combustion technology with low NOx emissions [57, 58]. In this process, the solid fuel is firstly pyrolyzed or gasified to produce a highly reducing product gas and coal char, which are then burned in a separate combustion boiler. Since the NOx generated from the devolatilization stage can be mostly reduced to N2 by the product gas, this technology will greatly reduce NOx emissions during the whole fuel conversion process. For some fuels with high volatile content, the NOx originating from volatiles in a conventional combustion process is high, so this technology has a significant advantage in reducing NOx for high volatile fuels.

6.2.2 OCAC Coupled with Staged Conversion of Fuel

The semi-industrial and industrial scale OCAC tests show that the addition of OC increases the NOx emission, resulting in the injection of an excess of ammonia into the deNOx equipment [59,60,61]. It can be assumed that the presence of OC makes the oxygen distribution more uniform, and as a result the reductive zone and the concentration of reductive gases (CO, H2, and CiHj) are significantly reduced. Therefore, the NOx reduction in the boiler is weakened. By combining OCAC with staged combustion of fuel, the advantages of the two technologies can be achieved, resulting in high efficiency and lower NOx.

Figure 6.5 is a schematic diagram of a proposed process (coal combustion is taken as an example). Both the gasification and the combustion reactor are CFBs. The coal is first added to the gasifier for air gasification. A portion of the char particles separated by the cyclone is recycled to the gasifier to continue the gasification, and the remainder is sent to the dense phase of the combustor for combustion. The product gas of the gasification is injected into different locations of the combustor. The flue gas of the combustor is partially circulated back to the wind box and mixed with air as secondary air. The combustion boiler uses OC as bed materials. This process has the following advantages: (1) The strong reduction of product gas reduces the NOx to N2 in the gasifier, which will greatly reduce the conversion ratio of fuel N to NOx; (2) The introduction of circulating flue gas can further reduce the emission of NOx. On the one hand, the introduction of circulating flue gas can reduces the oxygen concentration of the secondary air, thereby reducing the temperature of the char particles [62] and reducing the conversion of char-N to NOx; on the other hand, when the circulating flue gas passes through the dense phase, the NOx from flue gas can be reduced on the surface of char particles; (3) The introduction of OC as bed material obtains all the technical advantages of OCAC technology, as described in Sects.  6.2 and 6.3.

Fig. 6.5
A schematic diagram represents the flow of processes between the C F B gasifier, C F B combustor, purification system, dust remover, stack, cyclones, and loop seals.

The schematic diagram of the OCAC coupling with staged combustion

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The use of staged combustion of fuel and flue-gas circulation should significantly reduce NOx emissions without SNCR and SCR, so the new process—OCAC coupled with staged combustion is expected to become a competitive, efficient, and low-pollution combustion process. In addition, if the primary and secondary air in Fig. 6.5 are adjusted to O2/CO2 or O2/H2O, it also applies to oxy-fuel combustion, which simultaneously achieves efficient CO2 capture and low NOx emission. However, the process in Fig. 6.5 is just an example, OCAC technology can also control NOx emissions through a combination of staged air combustion, optimizing air distribution, separators, and adjusting the fluidization state, and so on.

6.3 OCAC Application Beyond Fluidized Bed

To date, the existing applications of OCAC technology are all carried out in FB reactors. It is expected that the unique ability of OCAC technology of oxygen transfer and buffering can also applied in other kind of reactors in complementing the poor gas–solid mixing, such as rotary kilns, grate furnaces, and fixed-bed reactors. In the following part, the possible assistance provided by OCAC for rotary-kiln combustion will be introduced.

Rotary-kiln incinerators have been widely used for the incineration of various gaseous, liquid, or solid wastes, such as hazardous wastes, sludge and MSW [63,64,65]. The schematic diagram of typical rotary kiln incinerator system is shown in Fig. 6.6 The benefits lie in the drastic volume reduction of wastes and the substantial heat energy recovery from the exhaust gas. The harmless treatment of hazardous wastes is an important principle of this process, that is, the emission control of residual substances (such as CO and dioxins) in the exhaust gas from combustion is very important. The complete destruction of hazardous waste depends on a variety of factors [66, 67]: (1) the residence time of wastes in the incinerator and secondary combustion chamber (SCC), i.e. time; (2) the temperature distribution in the reactor, i.e. temperature; (3) the mixing of air and wastes, i.e. turbulence; (4) the oxygen concentration of the flue gas at the outlet; and so on. Factors 1–3 are called “The 3T”, considered decisive for a combustion process [66]. The residence time of materials in the reactor affects the combustion efficiency, the burning rate, and the decomposition of dioxin. A longer residence time of materials can bring a higher combustion efficiency, and a more thorough decomposition of precursors of dioxin. The combustion temperature is important for a rotary-kiln incinerator. Generally, low combustion temperature can cause insufficient combustion. In addition, from the perspective of the decomposition of the precursors to dioxines, the temperature should not be lower than 850 °C. With the increase of temperature, the reaction rate of the fuel increases, and this shortens the residence time of the fuel in the furnace, thereby reducing the equipment volume and investment. However, the high temperature not only accelerates the corrosion rate of the furnace structure and promotes ash melting and agglomeration in the furnace but also increases the formation of NOx [68]. The turbulence inside the kiln is an appropriate index for the mixing of fuel and oxygen. Enough turbulence ensures the effective utilization of oxygen and the burnout of fuel. If the turbulence is low, more air should be introduced to the chamber to ensure burnout. However, excessive air brings more violent disturbance and leads to more dust in the flue gas, resulting in a high heat loss and a low combustion efficiency [69].

Fig. 6.6
A schematic diagram of the rotary kiln incinerator. It indicates the shredded solid waste, main, S C C, and sludge burners along with the secondary combustion chamber, rotary kiln, and air sump.

The schematic diagram of the rotary kiln incinerator system for solid waste [70]

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The rotary-kiln incinerator has the advantages of simple equipment, reliable operation, wide fuel applicability, and low investment, etc. But it cannot be ignored that it also has the characteristics of poor gas–solid mixing and insufficient heat and mass transfer, which affects the system efficiency and pollutant emissions. In addition, the type of wastes is complex and mostly appears in the form of mixtures. It is difficult to characterize the physical, chemical, and thermal properties of the feed materials. This leads to very complex material transportation, transfer, and chemical reactions in the kiln [68]. Therefore, large fluctuations usually occur in the incineration process, such as fluctuations in oxygen and CO concentration, and temperature distribution. The combustion fluctuations in the incinerator are difficult to control, and that not only causes problems, such as low combustion efficiency, high CO emission, and slagging at the end of the kiln, but it also causes higher emissions (NOx, SOx and dioxin) [66].

A potential design of a rotary-kiln incinerator coupled with OCAC is shown in Fig. 6.7. The mixture of OC and fuel is fed into the rotary-kiln incinerator. When the OC moves to the surface of the bed material, the OC fully contacts with O2 and oxidizes. When the OC moves inside the material, the surrounding is hypoxic, and the oxygen from the OC will be consumed by the fuel. This process achieves the oxygen transfer from an oxidizing area to an anoxic one, which may improve the uniformity of oxygen and temperature in the incinerator. The OC is finally discharged from the kiln tail together with ash and slag. After the solid mixture passes through the separation unit, OC will be recovered and recycled. With the above design, the following advantages are expected: (1) A good mixing of oxygen and fuel in time and space can be obtained in the main combustion chamber of the rotary kiln, which improves the utilization of oxygen, ensures the full combustion of fuel, and enhances the combustion efficiency; (2) The addition of OC can improve the uniformity of oxygen and temperature distribution in the main combustion chamber, resulting in lower emissions of CO, NOx, SOx and hydrocarbons. It can alleviate the problem that the emission of flue gas is unstable and frequently exceeds the emission standard in the existing rotary-kiln incineration. As a result, flue-gas treatment becomes easier and the equipment investment and operating cost are reduced; (3) A more uniform temperature also reduces the corrosion of the chamber structure and extends the service life; (4) This process has the characteristics of a simple system, compact structure, and easy scale-up, and the application prospect is broad. The OCAC technology not only can be directly applied to the existing rotary kiln device without changing the system, but also can be used in the design of new rotary-kiln incinerator.

Fig. 6.7
A diagram illustrates the input of a mixture of O C and fuel to the rotary kiln incinerator produces slag that goes through the slag cooler followed by a separator and again to the mixture of solid and fuel. It releases the flue gas to the S C C.

The schematic of a potential rotary kiln incinerator coupling with OCAC

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6.4 Multi-functional OCAC

OCAC technology excels in transporting the oxygen species from oxygen-enriched locations to the oxygen-deficient areas within a fluidized-bed combustor, resulting in an even oxygen distribution throughout the dense phase. The oxygen transportation performed by the OCs may reduce the local reducing areas in a furnace, and further reduce the reactive species that are essential for NOx reduction. In such a situation, the denitration reaction rate should be fast enough to achieve an ideal performance. It is widely accepted that various transition metals can catalyze the denitration reaction, and almost all the OC materials are composed of transition metal oxides. Therefore, it provides an opportunity to introduce the process of catalytic denitration inside a fluidized bed combustor. If the OC materials are appropriately selected and designed, they are expected to have multi-functions, i.e. the functions of catalyst and OC, to achieve a more stable combustion with lower NOx emission.

Currently, the most intensively studied OCs for OCAC are Fe- and Mn-based materials. Ilmenite is the most widely employed Fe-based OC. Due to its thermodynamic properties, ilmenite can be reduced by the fuels, mainly the gaseous fuels or the volatiles released from solid fuels, via the gas–solid reactions in the oxygen-deficient areas. This is the same case for Mn-based OCs at high temperatures, such as the typical temperature range of 800–1000 °C for a fluidized-bed combustor. In chemical looping technologies, certain types of OCs exhibit the ability to release molecular O2 in the oxygen-deficient reactor and to replenish the OCs with oxygen in an oxygen-enriched reactor. If the OC, which can release molecular O2 (i.e. gas-phase oxygen), is introduced into the OCAC process, the fuel combustion in the oxygen-deficient areas of the combustor can be achieved through not only gas–solid but also gas–gas reactions with a faster reaction rate. Therefore, it is expected that the promotional effect of OCAC can be further enhanced by employing OCs with the ability of reversible O2 release. To date, the Mn- and Cu-based materials are the well-known OCs with the ability of O2 release learned from the experience of chemical looping technologies. The drawback of the Mn-based OC is that it can release O2 at a high temperature but it can hardly be re-oxidized at such a temperature with a slightly lower oxygen pressure \(P_{O_2 }\) [71]. This is due to the thermodynamic limit of manganese oxides, which has been widely studied by many researchers [72, 73]. However, such a drawback can be addressed by combining the ilmenite with Mn as guest species resulting in an OC of Mn-modified ilmenite [71]. The Mn-modified ilmenite contains an oxide pair between (Fe1−xMnx)2O3 and (Fe1−xMnx)3O4, which endows the OC an ability of O2 release, at the same time, overcomes the thermodynamic limit of pure manganese oxide. On the contrary, the Cu-based OC materials have a favourable thermodynamic property for a reversible O2 release under OCAC-relevant conditions. However, it may impose a risk of bed agglomeration under the reaction condition of fluidized-bed combustor due to the low Tammann temperature of Cu/CuOx. In general, it should be highly promising for employing OC materials with the ability of reversible O2 release in OCAC processes, and the design and searching for an OC with a suitable thermodynamic properties and sufficient physical strength under OCAC-relevant conditions is crucial.