Reactivity of a CaSO<Subscript>4</Subscript>-oxygen carrier in chemical-looping combustion of methane in a fixed bed reactor

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. A reactivity study of CaSO₄ oxygen carrier in CLC of methane was conducted in a laboratory scale fixed bed reactor. The oxygen carrier particles were exposed in six cycles of alternating reduction methane and oxidation air. A majority of CH₄ reacted with CaSO₄ to form CO₂ and H₂O. The oxidation was incomplete, possibly due to the CaSO₄ product layer. The reactivity of CaSO₄ oxygen carrier increased for the initial cycles but slightly decreased after four cycles. The product gas yields of CO₂, CH₄, and CO with cycles were analyzed. Carbon deposition during the reduction period was confirmed with the combustible gas (CO+H₂) in the product gas and slight CO₂ formed during the early stage of oxidation. The mechanism of carbon deposition and effect was also discussed. SO₂ release behavior during reduction and oxidation was investigated, and the possible formation mechanism and mitigation method was discussed. The oxygen carrier conversion after the reduction decreased gradually in the cyclic test while it could not restore its oxygen capacity after the oxidation. The mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO₄ oxygen carrier. XRD analysis illustrated the phase change of CaSO₄ oxygen carrier. CaS was the main reduction product, while a slight amount of CaO also formed in the cyclic test. ESEM analysis demonstrated the surface change of particles during the cyclic test. The reacted particles tested in the fixed bed reactor were not uniform in porosity. EDS analysis demonstrated the transfer of oxygen from CaSO₄ to fuel gas while leaving CaS as the dominant reduced product. The results show that CaSO₄ oxygen carrier may be an interesting candidate for oxygen carrier in CLC. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, China, June 26–28, 2008.     

Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers

Chemical-looping combustion (CLC) is a novel technology that can be used to meet demands on energy production without CO₂ emissions. The CLC-process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N₂ from the air with CO₂ from the combustion. The combustion gases consist almost entirely of CO₂ and H₂O. Therefore, the technique reduces the energy penalty that normally arises from the separation of CO₂ from other flue gases, hence, CLC may make capture of CO₂ cheaper.Iron ore and oxide scale from steel production were tested as oxygen carriers in CLC batch experiments with solid fuels. Petroleum coke, charcoal, lignite and two bituminous coals were used as fuels.The experiments were carried out in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction phases. The exhaust gases were led to an analyzer where the contents of CO₂, CO, CH₄ and O₂ were measured. Gas samples collected in bags were used to analyze the content of hydrogen in a gas chromatograph.The results showed that both the iron ore and the oxide scale worked well as oxygen carrier and both oxygen carriers increased their reactivity with time.     

Integrating desulfurization with CO2-capture in chemical-looping combustion

Chemical looping combustion (CLC) is an emerging technology for clean combustion. We have previously demonstrated that the embedding of metal nanoparticles into a nanostructured ceramic matrix can result in unusually active and sinter-resistant nanocomposite oxygen carrier materials for CLC which maintain high reactivity and high-temperature stability even when sulfur contaminated fuels are used in CLC. Here, we propose a novel process scheme for in situ desulfurization of syngas with simultaneous CO₂-capture in chemical looping combustion by using these robust nanocomposite oxygen carriers simultaneously as sulfur-capture materials. We found that a nanocomposite Cu-BHA carrier can indeed strongly reduce the H₂S concentration in the fuel reactor effluent. However, during the process the support matrix is also sulfidized and takes part in the redox process of CLC. This results in SO₂ production during the reduction of the oxygen carrier and thus limits the degree of desulfurization attainable with this kind of carrier. Nevertheless, the results suggest that simultaneous desulfurization and CO₂ capture in CLC is feasible with Cu as oxygen carrier as long as appropriate carrier support materials are chosen, and could result in a novel, strongly intensified process for low-emission, high efficiency combustion of sulfur contaminated fuel streams.     

Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion

For combustion with CO₂ capture, chemical-looping combustion (CLC) with inherent separation of CO₂ is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125-180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80-330 kg/MW_th and a needed recirculation flow of oxygen carrier of 4-8 kg/s, MW_th.     

Multiphase CFD Modeling for a Chemical Looping Combustion Process (Fuel Reactor)

There are growing concerns about increasing emissions of greenhouse gases and a looming global warming crisis. CO₂ is a greenhouse gas that affects the climate of the earth. Fossil fuel consumption is the major source of anthropogenic CO₂ emissions. Chemical looping combustion (CLC) has been suggested as an energy‐efficient method for the capture of carbon dioxide from combustion. A chemical‐looping combustion system consists of a fuel reactor and an air reactor. The air reactor consists of a conventional circulating fluidized bed and the fuel reactor is a bubbling fluidized bed. The basic principle involves avoiding direct contact of air and fuel during the combustion. The oxygen is transferred by the oxygen carrier from the air to the fuel. The water in combustion products can be easily removed by condensation and pure carbon dioxide is obtained without any loss of energy for separation. With the improvement of numerical methods and more advanced hardware technology, the time required to run CFD (computational fluid dynamic) codes is decreasing. Hence, multiphase CFD‐based models for dealing with complex gas‐solid hydrodynamics and chemical reactions are becoming more accessible. To date, there are no reports in the literature concerning mathematical modeling of chemical‐looping combustion using FLUENT. In this work, the reaction kinetics models of the (CaSO₄ + H₂) fuel reactor is developed by means of the commercial code FLUENT. The effects of particle diameter, gas flow rate and bed temperature on chemical looping combustion performance are also studied. The results show that the high bed temperature, low gas flow rate and small particle size could enhance the CLC performance.     

On the evaluation of synthetic and natural ilmenite using syngas as fuel in chemical-looping combustion (CLC)

Chemical-looping combustion (CLC) is a combustion technique where the CO₂ produced is inherently separated from the rest of the flue gases with a considerably low energy penalty. For this reason, CLC has emerged as one of the more attractive options to capture CO₂ from fossil fuel combustion. When applying CLC with solid fuels, the use of a low cost oxygen carrier is highly important, and one such low cost oxygen carrier is the mineral ilmenite. The current work investigates the reactivity of several ilmenites, some which are synthetically produced by freeze granulation and two natural minerals, one Norwegian ilmenite and one South African ilmenite.     

Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier

Chemical-looping combustion (CLC) is a method for the combustion of fuel gas with inherent separation of carbon dioxide. This technique involves the use of two interconnected reactors. A solid oxygen carrier reacts with the oxygen in air in the air reactor and is then transferred to the fuel reactor, where the fuel gas is oxidized to carbon dioxide and water by the oxygen carrier. Fuel gas and air are never mixed and pure CO₂ can easily be obtained from the flue gas exit. The oxygen carrier is recycled between both reactors in a regenerative process. This paper presents the results from a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. The feasibility of the use of a manganese-based oxygen carrier supported on magnesium stabilized zirconia was tested in this work. Natural gas or syngas was used as fuel in the fuel reactor. Fuel flow and air flow was varied, the thermal power was between 100 and 300 W, and the air ratio was between 1.1 and 5.0. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated at all conditions with no signs of agglomeration or deactivation of the oxygen carrier. The same particles were used during 70 h of combustion and the mass loss was 0.038% per hour, although the main quantity was lost in the first hour of operation. In the combustion tests with natural gas, methane was detected in the exit flue gases, while CO and H₂ were maintained at low concentrations. Higher temperature or lower fuel flows increases the combustion efficiency, which ranged from 0.88 to 0.99. On the other hand, the combustion of syngas was complete for all experimental conditions, with no CO or H₂ present in the gas from the fuel reactor.     

Interaction of mineral matter of coal with oxygen carriers in chemical-looping combustion (CLC)

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are novel solutions for efficient combustion with direct separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor, where the fuel reacts with the solid oxygen carrier. When utilizing coal in CLC, the oxygen carrier particles could be affected through interaction with the ash-forming mineral matter found in coal, causing deactivation and/or agglomeration. In this work, possible interactions between minerals commonly encountered in coal and several promising oxygen carriers that are currently under investigation for their use in CLC are studied by both experiment and thermodynamic equilibrium calculations. Possible interaction was studied for both highly reducing and oxidizing conditions at 900 °C. Under highly reducing conditions pyrite was found to have by far the most deteriorating effect on the oxygen carrier particles, as the sulfur in the pyrite reacted with the oxygen carrier to form sulfides. Quartz and clay minerals were found to have a rather low influence on the oxygen carriers. Out of the oxygen carriers investigated, CuO/MgAl₂O₄ and the Mn₃O₄/ZrO₂ oxygen carriers tended to be quite reactive towards mineral matter whereas ilmenite has been shown to be the most robust oxygen carrier. Although sulfur can clearly deactivate Ni, Cu and Mn based oxygen carriers under sub-stoichiometric conditions, when the fuel is converted fully to CO₂ and H₂O, sulfides are only expected for Ni-based oxygen carriers.     

Solid fuels in chemical-looping combustion using a NiO-based oxygen carrier

The feasibility of using three different solid fuels in chemical-looping combustion (CLC) has been investigated using NiO as oxygen carrier. A laboratory fluidized-bed reactor system for solid fuel was used, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. In each reducing phase 0.2 g of fuel was added to the reactor containing 20 g oxygen carrier. The experiments were performed at 970 °C. Compared to previously published results with other oxygen carriers the reactivity of the used Ni-particles was considerably lower for the high-sulphur fuel and higher for the low-sulphur fuel. Much more unconverted CO was released and the fuel conversion was much slower for high-sulphur fuel such as petroleum coke, suggesting that the nickel-based oxygen carrier was deactivated by the presence of sulphur. The NiO particles also showed good reactivity with methane and a syngas mixture of 50% H₂ and 50% CO. For all experiments the oxygen carrier showed good fluidizing properties without any signs of agglomeration.     

Effect of solid residence time on CO<Subscript>2</Subscript> selectivity in a semi-continuous chemical looping combustor

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO₂ capture and sequestration, which is able to mitigate greenhouse gases (GHGs) emission. In this study, to design a 0.5MW_th pressurized chemical looping combustor for natural gas and syngas the effects of solid residences time on CO₂ selectivity were investigated in a novel semi-continuous CLC reactor using Ni-based oxygen carrier particle. The semi-continuous chemical looping combustor was designed to simulate the fuel reactor of the continuous chemical looping combustor. It consists of an upper hopper, a screw conveyor, a fluidized bed reactor, and a lower hopper. Solid circulation rate (G _s ) was controlled by adjusting the rotational speed of the screw conveyor. The measured solid circulation rate increased linearly as the rotational speed of the screw increased and showed almost the same values regardless of temperature and fluidization velocity up to 800°C and 4 U _mf , respectively. The solid circulation rate required to achieve 100% CH₄ conversion was varied to change G _s -fuel ratio (oxygen carrier feeding rate/fuel feeding rate, kg/Nm³). The measured CO₂ selectivity was greater than 98% when the Gs-fuel ratio was higher than 78 kg/Nm³.     

Thermodynamic study of combining chemical looping combustion and combined reforming of propane

Existing energy generation technologies emit CO₂ gas and are posing a serious problem of global warming and climate change. The thermodynamic feasibility of a new process scheme combining chemical looping combustion (CLC) and combined reforming (CR) of propane (LPG) is studied in this paper. The study of CLC of propane with CaSO₄ as oxygen carrier shows thermodynamic feasibility in temperature range (400-782.95 °C) at 1 bar pressure. The CO₂ generated in the CLC can be used for combined reforming of propane in an autothermal way within the temperature range (400-1000 °C) at 1 bar pressure to generate syngas of ratio 3.0 (above 600 °C) which is extremely desirable for petrochemical manufacture. The process scheme generates (a) huge thermal energy in CLC that can be used for various processes, (b) pure N₂ and syngas rich streams can be used for petrochemical manufacture and (c) takes care of the expensive CO₂ separation from flue gas stream and CO₂ sequestration. The thermoneutral temperature (TNP) of 702.12 °C yielding maximum syngas of 5.98 mol per mole propane fed, of syngas ratio 1.73 with negligible methane and carbon formation was identified as the best condition for the CR reactor operation. The process can be used for different fuels and oxygen carriers.     

Comparison of natural ilmenites as oxygen carriers in chemical-looping combustion and influence of water gas shift reaction on gas composition

Chemical-looping combustion (CLC) is a promising technology for CO₂-capture for storage or reuse as a method to mitigate CO₂ emissions from the use of fossil fuels. In a CLC system the oxygen carrier is of great importance. Environmentally sound and low cost materials seem to be preferable especially for CLC of solid fuels. The natural occurring ore ilmenite has already been the target of different studies in order to work out its feasibility as oxygen carrier for different fuels. The initial part of this work is a screening of five commercial available ilmenite minerals as oxygen carrier, crushed and sieved to 125–180 μm. The screening includes an examination of the sulfur released during the first heat up and the activation of the oxygen carrier, indicated by the fuel conversion using alternating reduction (syngas 50 vol.% CO in H₂) and oxidation conditions (10 vol.% O₂ in N₂). The five first cycles were carried out at 850 °C to avoid initial agglomeration whereas the main activation cycles have been performed at 950 °C in a tubular quartz reactor under fluidized bed conditions. From these experiments it is concluded that rock ilmenites are preferable as oxygen carriers since they revealed an improved fuel conversion, although offering a higher sulfur content, which is released during the initial heat up.     

Effect of fuel particle size on reaction rate in chemical looping combustion

Chemical looping combustion (CLC) uses an oxygen carrier circulating between an air and a fuel reactor to replace direct burning of fuels in air. The very low energy penalty for CO₂ separation in CLC gives it the potential to become an important technology on the way to a CO₂ neutral energy supply. In this work, the influence of the particle size of coal on the rate of reaction of the coal was investigated in a bed of oxygen carrier. In order to do this, a method to quench the reaction of coal with oxygen carriers at a specified time and measure the particle size distribution of the remaining coal was developed. Three size fractions of coal were used in the experiments: 90–125, 180–212 and 250–355 μm. Particle size distributions of the fuel show a decrease in particle size with time. The influence of devolatilisation of the coal on the coal particle size was measured, showing that coal particles do not break in the fluidized bed reactor used for the experiments. Reaction rates based on measurements of gas phase concentrations of CO₂, CO and CH₄ showed that the reaction rate is independent of the particle size. These results are in line with literature findings, as studies have shown that carbon gasification is size-independent at conditions similar to those in the performed CLC experiments.     

Syngas combustion in a 500 Wth Chemical-Looping Combustion system using an impregnated Cu-based oxygen carrier

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. For its use with coal as fuel in power plants, a process integrated by coal gasification and CLC would have important advantages for CO₂ capture. This paper presents the combustion results obtained with a Cu-based oxygen carrier in a continuous operation CLC plant (500 Wth) using syngas as fuel. For comparison purposes pure H₂ and CO were also used. Tests were performed at two temperatures (1073 and 1153 K), different solid circulation rates and power inputs. Full syngas combustion was reached at 1073 K working at f higher than 1.5. The syngas composition had small effect on the combustion efficiency. This result seems to indicate that the water gas shift reaction acts as an intermediate step in the global combustion reaction of the syngas. The results obtained after 40 h of operation showed that the copper-based oxygen carrier prepared by impregnation could be used in a CLC plant for syngas combustion without operational problems such as carbon deposition, attrition, or agglomeration.     

Development of Cu-based oxygen carriers for chemical-looping combustion

In a chemical-looping combustion (CLC) process, gas (natural gas, syngas, etc.) is burnt in two reactors. In the first one, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO₂ and steam the reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO₂ can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing fuels preventing the CO₂ emissions to the atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use.In this paper the behaviour of CuO as an oxygen carrier for a CLC process has been analysed in a thermogravimetric analyser. The effects of carrier composition and preparation method used have been investigated to develop Cu-based carriers exhibiting high reduction and oxidation rates without substantial changes in the chemical, structural and mechanical properties for a high number of oxidation-reduction cycles. It has been observed that the carriers prepared by mechanical mixing or by coprecipitation showed an excellent chemical stability in multicycle tests in thermobalance, however, the mechanical properties of these carriers were highly degraded to unacceptable levels. On the other hand, the carriers prepared by impregnation exhibited excellent chemical stability without substantial decay of the mechanical strength in multicycle testing. These results suggest that copper based carriers prepared by impregnation are good candidates for CLC process.     

Multiphase CFD-based models for chemical looping combustion process: Fuel reactor modeling

Chemical looping combustion (CLC) is a flameless two-step fuel combustion that produces a pure CO₂ stream, ready for compression and sequestration. The process is composed of two interconnected fluidized bed reactors. The air reactor which is a conventional circulating fluidized bed and the fuel reactor which is a bubbling fluidized bed. The basic principle is to avoid the direct contact of air and fuel during the combustion by introducing a highly-reactive metal particle, referred to as oxygen carrier, to transport oxygen from the air to the fuel. In the process, the products from combustion are kept separated from the rest of the flue gases namely nitrogen and excess oxygen. This process eliminates the energy intensive step to separate the CO₂ from nitrogen-rich flue gas that reduce the thermal efficiency.Fundamental knowledge of multiphase reactive fluid dynamic behavior of the gas-solid flow is essential for the optimization and operation of a chemical looping combustor.Our recent thorough literature review shows that multiphase CFD-based models have not been adapted to chemical looping combustion processes in the open literature. In this study, we have developed the reaction kinetics model of the fuel reactor and implemented the kinetic model into a multiphase hydrodynamic model, MFIX, developed earlier at the National Energy Technology Laboratory. Simulated fuel reactor flows revealed high weight fraction of unburned methane fuel in the flue gas along with CO₂ and H₂O. This behavior implies high fuel loss at the exit of the reactor and indicates the necessity to increase the residence time, say by decreasing the fuel flow rate, or to recirculate the unburned methane after condensing and removing CO₂.     

Carbon capture and utilization via chemical looping dry reforming

Chemical looping combustion (CLC) is a clean energy technology for CO₂ capture that uses periodic oxidation and reduction of an oxygen carrier with air and a fuel, respectively, to achieve flameless combustion and yield sequestration-ready CO₂ streams. While CLC allows for highly efficient CO₂ capture, it does not, however, provide a solution for CO₂ sequestration.Here, we propose chemical looping dry reforming (CLDR) as an alternative to CLC by replacing air with CO₂ as the oxidant. CLDR extends the chemical looping principle to achieve CO₂ reduction to CO, which opens a pathway to CO₂ utilization as an alternative to sequestration. The feasibility of CLDR is studied through thermodynamic screening calculations for oxygen carrier selection, synthesis and kinetic experiments of nanostructured carriers using cyclic thermogravimetric analysis (TGA) and fixed-bed reactor studies, and a brief model-based analysis of the thermal aspects of a fixed-bed CLDR process.Overall, our results indicate that it is indeed possible to reduce CO₂ to CO with high reaction rates through the use of appropriately designed nanostructured carriers, and to integrate this reaction into a cyclic redox (“looping”) process.     

Experimental investigation of some metal oxides for chemical looping combustion in a fluidized bed reactor

Chemical looping combustion (CLC) is the process in which metal oxides, rather than air or pure oxygen, supply the oxygen required for combustion. In this process, different gaseous fuels can be burnt with the inherent separation of CO₂. The feasibility of the CLC system depends greatly on the selection of appropriate metal oxides as oxygen carriers (OC). In this study, NiO-NiAl₂O₄, Cu_0.95Fe_1.05AlO₄, and CuO-Cu_0.95Fe_1.05AlO₄ were tested experimentally in a fluidized bed reactor as a function of oxidation-reduction cycles, temperature, bed inventory and superficial gas velocity. The results showed that flue gases with a CO₂ concentration as high as 97% can be obtained. The flue gases should be suitable for transport and storage after clean-up and purification. With an increase in the bed inventory or a decrease in superficial gas velocity, the flue gas characteristics improved i.e. more CO₂ and fewer secondary components or less unreacted fuel were obtained. Carbon formation could occur during the reduction phase but it decreased with an increase in temperature and inventory and could be completely avoided by mixing steam with the fuel. The reactivity of NiO/NiAl₂O₄ was higher than the Cu- and Fe-based oxygen carriers. Increasing the CuO fraction in the oxygen carrier led to defluidization of the bed during the reduction and oxidation phases.     

化学链燃烧钙基载氧体CaSO4与CO在不同温度下的反应行为

基于热重和红外联用进行等温实验,探讨了化学链燃烧载氧体CaSO₄在CO气氛下的还原反应特性。研究发现：温度对CaSO₄还原反应历程和速率有显著的影响,在10％CO气氛下,温度低于900℃时,发生单一反应,CaSO₄的还原产物只是CaS,气相产物为CO₂;当温度高于950℃后,发生平行反应和连串反应组合成的多重反应,固体产物为CaS和CaO,而产物气中除了有CO₂,还存在SO₂和COS,且气相硫化物的析出以COS为主;随着反应温度的升高,CaSO₄与CO反应速率显著增加,而目标产物CaS在固体产物中所占的摩尔分数呈下降趋势;基于钙基载氧体化学链燃烧中燃料反应器温度不宜高于950℃。     

Investigation into sulfur release in reductive decomposition of calcium sulfate oxygen carrier by hydrogen and carbon monoxide

Chemical looping combustion (CLC) is a promising technology with the inherent property of separating CO₂ from flue gas. For calcium sulfate (CaSO₄) oxygen carrier, the inhibition of the produced sulfurous gases in the reduction of CaSO₄, including sulfur dioxide (SO₂), hydrogen sulfide (H₂S) and carbonyl sulfide (COS), is the key for a CLC system. In this paper, the sensitivities of reacting temperature, oxygen ratio number (defined in this paper) and the mole fraction of both carbon monoxide (CO) and hydrogen(H₂) in the syngas to the sum of the amounts of released SO₂, H₂S and COS are discussed respectively. Thermo-gravimetric analysis (TGA) tests demonstrated that the amount of the produced sulfurous gases is greatly dependent on the partial pressure of H₂ or CO in the reduction of CaSO₄. When the partial pressure of H₂ or CO is higher than 40 kPa, the production of sulfurous gases, indicating the deterioration of the recyclability of CaSO₄, can be prevented completely even if the reacting temperature is as high as 1000 °C. A new kind of CaSO₄/CaCO₃ oxygen carrier is prepared using a mechanical mixing method. The addition of CaCO₃ nanoparticles largely improves the recyclability of the oxygen carrier in comparison with the fresh CaSO₄ oxygen carrier, without CaCO₃ nanoparticles, in a multi-cycle TGA test.