Chemical Looping With Oxygen Uncoupling Process Research Articles

CuxMg1-xFe2O4-type spinels as potential oxygen carriers for waste wooden biomass combustion

Waste wood biomass is considered a renewable energy source. Combining biomass combustion with emerging clean combustion technologies such as chemical looping combustion (CLC) can yield effective and affordable carbon capture and, consequently, lead to negative net emissions of greenhouse gases. Oxygen carrier (OC) is a crucial material in CLC technology that must exhibit certain properties, such as high durability, good chemical stability during numerous red-ox cycles and, important for the combustion of solid fuels, the capability of spontaneously releasing oxygen in a process referred to as chemical looping with oxygen uncoupling (CLOU). In this work, a series of nine CuxMg1-xFe2O4 spinel-based materials were synthetized and evaluated for the first time as potential OCs for a waste biomass combustion. Their properties, such as oxygen transport capacity and reactivity with biomass (wood chips) as a fuel, were evaluated in a function of temperature (900–1000 °C). Tested oxygen carriers were characterized with an excellent oxygen transport capacity in CLOU process (up to 2.78 wt%) and good reaction rates with the fuel (up to 1.19 wt. %/min), and regeneration rates (up to 3.8 wt. %/min). High conversion of the waste biomass was also achieved (98.9 %). Moreover, new findings revealed a strong positive effect of magnesium addition on mechanical strength (crushing strength > 4 N for samples with Mg content above 0.5).

Novel BECCS implementation integrating chemical looping combustion with oxygen uncoupling and a kraft pulp mill cogeneration plant

Bioenergy with CO2 capture and storage (BECCS) is a promising negative emission technology (NET). When using sustainably produced biomass as fuel, BECCS allows the production of power and heat with negative CO2 emissions. The main technical challenges hindering the deployment of BECCS technologies include energy penalties associated with the capture process. This work evaluates the performance of an advanced CO2 capture technology, chemical looping with oxygen uncoupling (CLOU), replacing a conventional fluidized bed boiler in the power boiler role in a large, modern integrated pulp and paper mill. Results from a MATLAB/Simulink reactor model were incorporated in a plant and integration model developed in a commercial process simulation software to quantify the performance of the CLOU-integrated cogeneration plant. The results show that in this specific application, the typically already low efficiency penalty of CLOU-based carbon capture and storage (CCS) systems could be eliminated entirely, and actually even a very small efficiency gain could be obtained. The highly efficient operation is possible due to the high moisture and hydrogen contents of the biomass and the separation of combustion products and excess air streams in the CLOU process; this provides an opportunity to recover a significant amount of heat by flue gas condensation at a higher temperature level than what is possible in a conventional boiler. Together with abundant low-temperature heat sinks available at the pulp and paper application allows freeing a considerable amount of low-pressure steam for expansion in the condensing turbine. The resulting increase in gross generator output proved enough to not only match, but very slightly exceed the approximately 18 MW parasitic load introduced by the CLOU system in comparison to the conventional boiler.

Development of new Mn-based oxygen carriers using MgO and SiO2 as supports for Chemical Looping with Oxygen Uncoupling (CLOU)

Chemical Looping with Oxygen Uncoupling (CLOU) is a technological adaptation of CLC, most applicable for the combustion of solid fuels. In the CLOU process, an oxygen carrier in the fuel reactor, avoiding the direct contact of the fuel with the air, releases the oxygen needed for the fuel combustion. The oxygen carrier is regenerated with air in the interconnected air reactor. The present work explores the behavior of the system Mn/Mg/Si as oxygen carriers for chemical-looping with oxygen uncoupling (CLOU). Six different mixed oxides of the system Mn/Mg/Si were investigated for the CLC/CLOU process. Materials were prepared by spray drying with different metal ratios used in the investigation. The properties of interest for the viability of these materials are the lattice oxygen supply for CLC and the gaseous oxygen release for CLOU, properties that were explored in a TGA. Further, the fluidization behavior and the mechanical resistance were investigated in a batch fluidized bed reactor. In the TGA it was observed that the most reactive oxygen carriers for the CLOU process were materials without Si in the structure, more specifically M24Mg76 and M48Mg51 which had a molar ratio of Mn/Mg of 0.17 and 0.51 respectively. It was also observed that for the oxygen carriers with Si in the composition, the regeneration was very poor. Oxygen carriers M24Mg76 and M48Mg51 were selected for batch fluidized bed reactor testing showing good behavior with respect to the CLOU reactivity, and mechanical stability. One of the materials, the M24Mg76 showed activation during the experiments in the batch fluidized bed reactor experiments, increasing the oxygen transport capacity by 20 % during the experiment. However, 10 vol% of O2 was needed to regenerate both oxygen carriers at 850 °C. No agglomeration tendencies were seen, and the attrition rate was low, obtaining high-extrapolated lifetime values. The fact that highly reactive oxygen carriers can be made with cheap and highly available metals oxides, i.e. Mn and Mg, makes this system very promising and a possible alternative to benchmark Cu-based CLOU materials.

Developing magnetic, durable, agglomeration resistant and reactive copper-based oxygen carrier particles by promoting a kaolin-reinforced, manganese‑iron mixed oxide support

Chemical looping with oxygen uncoupling (CLOU) process, involves the use of two interconnected fluidized bed reactors, where oxygen carrier particles are able to release O2 at the fuel reactor operation conditions. Cu-based oxygen carriers containing a kaolin-reinforced MnFe mixed oxide as magnetic support, were prepared by granulation to improve their oxygen carrier properties by means of a simplified preparation method. A study was performed into the effect of kaolin concentration and calcination temperature on magnetic permeability, crushing strength, attrition, agglomeration resistance, and reactivity. Two oxygen carriers containing 28.5 wt% CuO-5 wt% kaolin and 30 wt% CuO-7.5 wt% kaolin, respectively, and both calcined at 1100 °C, were selected because they maintained their magnetic properties and reactivity after kaolin addition and showed improvement in their mechanical resistance. They were tested in a batch fluidized bed reactor (BFBR) to evaluate their fluidization behaviour. Low attrition rate without any agglomeration phenomena was observed for both of oxygen carriers up to temperature of 950 °C. The material prepared with 7.5 wt% kaolin demonstrated improved mechanical resistance, very low attrition rate and higher CuO utilization for the CLOU process after use in the BFBR, while maintaining reactivity during redox cycles and magnetic properties without agglomeration problems.

Coal and biomass combustion with CO2 capture by CLOU process using a magnetic Fe-Mn-supported CuO oxygen carrier

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a derivative technology of the Chemical Looping Combustion (CLC) process for the combustion of both renewable and non-renewable solid fuels. The present work studies the combustion and CO2 capture efficiency of burning coal or biomass using a single Cu-based oxygen carrier with magnetic properties as an oxygen carrier. The combustion of two different coals and three different biomasses was performed in a 1.5 kWth CLOU continuously operated unit for 36 h. The magnetic properties of the oxygen carrier were also studied after combustion and its separation from the ashes. Complete combustion of the fuel to CO2 and H2O was achieved in all cases. For both coals, CO2 capture efficiency increased with fuel reactor temperature, with both reaching values of 97% at 900 °C. The same increase was obtained with the biomasses. The highest CO2 capture efficiency for a biomass was obtained with pine sawdust, which achieved a value of 93%, followed by almond shell (86%) and olive stone (80%) at 900 °C. Oxygen carrier reactivity remained constant after use in the continuous unit with one small increase in the attrition jet index (AJI) value. The used particles were found to still be as magnetic as the fresh particles, and the fines were also highly magnetic. Values higher than 99% were achieved with separation from the fines reached. It was therefore proven that the elutriated oxygen carrier from the plant was still magnetic and could be easily separated from the elutriated ash.

Investigations on the chemical looping with oxygen uncoupling process using Indian coal and copper oxide oxygen carrier

In this study, the effectiveness of CuO as an oxygen carrier (OC) in chemical looping with oxygen uncoupling (CLOU) process was investigated using three Indian coals with varying origin and nature. The thermogravimetric analysis (TGA) experiments were performed with coal to OC mass-ratio of 1:20, 1:30, and 1:40 at varying temperatures of 800, 850, 900, 950 °C. The mass loss profiles of coal/CuO indicated that the CLOU combustion reaction was the most suitable with a mass ratio of 1:30 and at 850 °C. Multicyclic combustion-reoxidation performance (for 45 h) also showed sufficiently good repeatability with minor agglomeration. Besides, the kinetic triplet, such as apparent activation energy (Ea), pre-exponential factor (A), and kinetic model function, g(x), were estimated for reactions in the pyrolysis and combustion zones applying the Coats and Redfern model on the TGA mass loss data. From mathematical model fitting, it was found that the three-dimensional Jander-diffusion model provides an excellent fit to the experimental data for the selected temperature zones, and the combustion reaction kinetics is likely to be a diffusion dominated process. The estimated apparent activation energies for three coals studied in this work were in the range of 38.3–60.1 kJ/mol and 102.6–129.0 kJ/mol in the temperature zone of 400–600 °C and 600–950 °C, respectively. The findings of the present study suggest first-time that the Indian coal-based CLOU process using CuO OC is promising, and the derived kinetic information will be useful for CLOU reactor design using Indian coal and CuO OC.

Extremely Stable and Durable Mixed Fe–Mn Oxides Supported on ZrO2 for Practical Utilization in CLOU and CLC Processes

This paper contains the results of research on a promising combustion technology known as chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU). The remarkable advantages of CLC are, among others, that concentrated CO2 stream can be obtained after water condensation without any energy penalty for its separation or significant decrease of NOx emissions. The objective of this work was to prepare a novel bi-metallic Fe–Mn supported on ZrO2 oxygen carriers. Performance of these carriers for the CLOU and CLC process with nitrogen/air and hard coal/air was evaluated. One-cycle CLC tests were conducted with supported Fe–Mn oxygen carriers in thermogravimetric analyzer utilizing hard coal as a fuel. The effects of the oxygen carrier chemical composition and process temperature on the reaction rates were determined. Our study proved that for CLOU, properties formation of bixbyite and spinel forms are responsible. Among iron ferrites, we concluded that iron-rich compounds such as Fe2MnO4 over FeMn2O4 spinel type oxides are more effective for CLOU applications.

Development of a magnetic Cu-based oxygen carrier for the chemical looping with oxygen uncoupling (CLOU) process

Chemical looping with oxygen uncoupling (CLOU) is an efficient process for solid fuel combustion with CO2 capture. This process requires a material that is capable of releasing gaseous O2 within a high temperature window. Screening was performed of reactive CuO-based oxygen carriers prepared for the CLOU process with magnetic properties for easy separation from solid fuel ashes. The effects of the CuO fraction, method of preparation (with prior preparation of the magnetic Fe-Mn support or by direct mixing of Cu, Fe and Mn oxides) and calcination conditions on oxygen carrier performance were studied. Three oxygen carriers were chosen owing to their good Cu activity, reactivity and magnetism, and were prepared by fluidized-bed granulation for evaluation in a batch fluidized bed reactor. Two of these, both with a 30% CuO content, showed similar performance throughout the process. One was prepared by physically mixing components with a Mn/Fe ratio of 1:1, and the other was prepared using a previously calcined Fe-Mn support, mixed with CuO. Both oxygen carriers showed high reactivity for gaseous oxygen release, sustained reactivity and magnetism with redox cycles, in addition to low attrition rates and no agglomeration problems.

Co-precipitated Cu-Mn mixed metal oxides as oxygen carriers for chemical looping processes

Chemical looping with oxygen uncoupling (CLOU) and chemical looping air separation (CLAS) are novel and potentially promising processes for the combustion of solid fuels (e.g. biomass) for power generation with inherent CO2 capture. Redox-experiments at 850–950 °C confirmed that copper manganese spinel oxides are promising oxygen carriers for these processes, as they combine a relatively high O2 release capacity and fast O2 release kinetics. Furthermore, this work presents a novel method to calculate the O2 partial pressure equilibrium and the heat of O2 release from observed rates of reaction. To demonstrate this method, oxygen carriers were prepared via mechanical mixing and co-precipitation with varying molar Cu:Mn ratios and synthesis conditions, thereby tuning material properties and the pore structure. The precursors and calcined materials were characterised, and the crystalline phases were determined using X-ray diffraction. The insights from the post cycling analysis of the oxygen carriers and the experimentally obtained O2 release capacities were combined to elucidate the redox-reactions relevant for the two processes. It was found that the presence of a higher partial pressure of O2 during the O2 release results in the formation of different (perovskite-like) phases than those occurring during the decomposition in an O2-free environment. The oxygen carriers demonstrated excellent stability at CLOU and CLAS process conditions during extended redox cycling (100 cycles in a thermo-gravimetric analyser and 50 cycles in a fluidised bed reactor), showing no significant loss of reactivity or O2 release capacity and a high resistance towards attrition and agglomeration. The degree of degradation after 100 cycles was in the order: temperature swing (CLAS) < O2 partial pressure swing (CLOU) < reduction with CH4 (chemical looping combustion).

Chemical Looping with Oxygen Uncoupling of the Lignocellulosic Biomass Main Model Compound: Product Distribution and Kinetic Analysis on Lignin

Product distribution and kinetic analysis of lignin were investigated during the chemical looping with oxygen uncoupling (CLOU) process. Thermodynamic simulation of the CLOU process was performed o...

High-Efficiency Bioenergy Carbon Capture Integrating Chemical Looping Combustion with Oxygen Uncoupling and a Large Cogeneration Plant

Bioenergy with CO2 capture and storage (BECCS) is a promising negative emission technology (NET). When using sustainably produced biomass as fuel, BECCS allows the production of power and heat with negative CO2 emissions. The main technical challenges hindering the deployment of BECCS technologies include energy penalties associated with the capture process. This work evaluates the performance of an advanced CO2 capture technology, chemical looping with oxygen uncoupling (CLOU), in conjunction with biomass-fired combined heat and power (CHP) generation. Results from a MATLAB/Simulink reactor model were incorporated in a plant and integration model developed in a commercial process simulation software to quantify the key performance indicators of the CLOU-integrated CHP plant. Both energy and exergy analysis were conducted. The results show a remarkably low efficiency penalty of 0.7% compared to a conventional reference plant, and a high carbon capture efficiency of 97%. The low efficiency penalty is due to the high moisture and hydrogen contents of the biomass, and the separation of combustion products and excess air streams in the CLOU process; these together provide an opportunity to recover a significant amount of heat by flue gas condensation at a higher temperature level than what is possible in a conventional boiler. The condensing heat recovery yields an 18 MW generator power increase (3 MW loss in net power output) for the CLOU plant; in the reference plant with conventional boiler, the same scheme could achieve an increase of 9 MW (generator) and a decrease of 8 MW (net).

Chemical looping with oxygen uncoupling: an advanced biomass combustion technology to avoid CO2 emissions

Bioenergy with carbon dioxide (CO2) capture and storage (BECCS) technologies represent an interesting option to reach negative carbon emissions, which implies the removal of CO2 already emitted to the atmosphere. Chemical looping combustion (CLC) with biomass can be considered as a promising BECCS technology since CLC has low cost and energy penalty. In CLC, the oxygen needed for combustion is supplied by a solid oxygen carrier circulating between the fuel and air reactors. In the fuel reactor, the fuel is oxidized producing a CO2-concentrated stream while the oxygen carrier is reduced. In the air reactor, the oxygen carrier is regenerated with air. Chemical looping with oxygen uncoupling (CLOU) is a CLC technology that allows the combustion of solid fuels as in common combustion with air by means of an oxygen carrier that release gaseous oxygen in the fuel reactor. In the last years, several Cu-based, Mn-based, and mixed oxide oxygen (O2) carriers have been tested showing good CLOU properties. Among them, copper (Cu)-based and Cu-Manganese (Mn) mixed oxides showed high reactivity, O2 release rate, and high O2 equilibrium concentration. The aim of this work is to study the viability of biomass combustion by CLOU process. The combustion of three types of biomass (pine sawdust, olive stone, and almond shell) were studied in a continuous 1.5 kWth CLC unit. Two O2 carriers were tested: a Cu-based oxygen carrier with Magnesium, Aluminum, Oxgen (MgAl2O4) as an inert prepared by spray drying (Cu60MgAl) and a mixed Cu-Mn oxide prepared by spray granulation (Cu34Mn66). These materials are capable of releasing gaseous oxygen when they are reduced in a different range of temperatures. CO2 capture and combustion efficiency were evaluated. Two fuel reactor operation temperatures were used: 775–850 °C for Cu34Mn66 and 900–935 °C for Cu60MgAl. High CO2 capture efficiencies and 100% combustion efficiency were reached with both oxygen carriers and with all the biomasses tested. Therefore, the CLOU technology with the Cu- and Cu-Mn-based oxygen carriers allowed avoiding CO2 emissions maintaining high combustion efficiencies. Results obtained demonstrate that this innovative biomass combustion technology combined with carbon storage lets an efficient BECCS process implementation.

Evaluation of Mn-Fe mixed oxide doped with TiO2 for the combustion with CO2 capture by Chemical Looping assisted by Oxygen Uncoupling

Bimetallic manganese-iron materials have been identified as suitable oxygen carriers for Chemical Looping Combustion (CLC) and Chemical Looping with Oxygen Uncoupling (CLOU) processes. These materials allow the combustion of a fuel with inherent CO2 capture according to two parallel mechanisms: reaction with lattice oxygen and oxygen uncoupling. This work is focused on the evaluation of the reactivity and physicochemical characterization of oxygen carrier particles consisting of a manganese-iron mixed oxide with manganese to iron molar ratio of 66:34 and doped with titanium (7 wt% TiO2) with the objective of determining suitable conditions to be used in CLC and CLOU processes. Particles were prepared by spray drying and the sintering procedure was optimized in order to achieve particles with high reactivity and mechanical strength. In addition, suitable magnetic properties were also sought in order to allow oxygen carrier recover and reuse in a chemical looping unit burning solid fuels. Optimum operating conditions for the fuel combustion and regeneration stages were determined in order to promote the oxygen uncoupling mechanism. Thus, temperature during the fuel combustion must be as high as possible to enhance the oxygen transference; but conditions for oxygen carrier regeneration by air must be carefully selected in order to take advantage of the oxygen uncoupling capability of this material. An oxidizing temperature interval of 1123–1173 K maximized the regeneration, while an air excess higher than 20% would be recommended in order to guarantee oxygen uncoupling capability.

Process Simulation and Energy Analysis of Chemical Looping Combustion and Chemical Looping with Oxygen Uncoupling for Sawdust Biomass

AbstractThe integral part of chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) for solid fuels such as biomass consists of an interconnected fluidized air and fuel reactor. The heat recovery/loss in both reactors needs to be analyzed to commercialize these novel fuels based on CLC and CLOU processes. Here, complete process level modelling of CLC and CLOU process is attempted and explored using the ASPEN simulator with sawdust as the biomass. Initially, a benchmarking study of CLC is performed to validate the CO2, CO, and CH4 concentration in the fuel reactor and O2 and CO2 concentrations in the air reactor using 81.1 % Fe2O3 (oxygen carrier) as the reactive component. Thereafter energy analysis is carried out with sawdust biomass using the catalyst combination of 60 % Fe2O3−Al2O3 and 40 % CuO−ZrO2 for CLC and CLOU, respectively. In biomass, for CLOU a 51 % conversion is realized in the fuel reactor and 48 % is realized in the air reactor. The energy analysis gave a difference in energy output between CLC and CLOU of 43 Watts i.e., 201 Watts in CLOU and 158 Watts in CLC.

CLOU process performance with a Cu-Mn oxygen carrier in the combustion of different types of coal with CO2 capture

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a Chemical Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2 separation by using oxygen carriers based on metal oxides. This technology has a low energy penalty and thus low CO2 capture costs. The oxygen carrier used in the CLOU process must be able to release gaseous oxygen, an aspect that limits the availability of metal oxides for this process. This work investigated the suitability of an oxygen carrier containing 34 wt% CuO and 66 wt% Mn3O4 (active phase Cu1.5Mn1.5O4) prepared by granulation regarding the CO2 capture, combustion efficiency and lifetime of the particles. The effect of the different types of coal (two sub-bituminous and a lignite) on combustion and CO2 capture efficiencies by CLOU was studied at different oxygen carrier to coal ratios in a continuous 1.5 kWth rig. It was found that full combustion could be reached regardless of the coal used. However, CO2 capture efficiencies were highly determined by coal rank. Finally, it was found that working with oxygen carrier to coal ratios higher than ϕ = 4, which corresponded to values of the variation of the oxygen carrier conversion lower than ΔXoc = 0.25, decreased the effect of chemical stress on the attrition rate. Therefore, it is clearly beneficial for the lifetime of oxygen carrier particles to operate with low variations of the oxygen carrier conversion (ΔXoc) between fuel and air reactors.

Coal combustion with a spray granulated Cu-Mn mixed oxide for the Chemical Looping with Oxygen Uncoupling (CLOU) process

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a form of Chemical Looping Combustion (CLC) technology that enables the combustion of solid fuels by means of oxygen carriers that release gaseous oxygen in the fuel reactor, i.e. with oxygen uncoupling capability. In recent years, tests have found several Cu-based, Mn-based and mixed oxide oxygen carriers with suitable properties for the CLOU process. Among them, Cu-Mn mixed oxides show high reactivity and high O2 equilibrium concentration at temperatures of interest for coal combustion. In this work, proof of concept was demonstrated by burning coal with Cu-Mn mixed oxides in a 1.5kWth CLOU unit for the first time. The effect of fuel reactor temperature, solids circulation flow, fluidization agent in the fuel reactor (inert N2 or steam as gasifying agent), and excess air in the air reactor on combustion efficiency and CO2 capture rate was analysed. The results showed that high combustion efficiency and CO2 capture are feasible using this material at relatively low fuel reactor operating temperatures. Therefore, the developed Cu-Mn mixed oxide was a suitable material for use as an oxygen carrier for the CLOU combustion of solid fuels. Optimum operating conditions were determined for this oxygen carrier with regard to the oxygen circulation rate and air reactor conditions for the regeneration of the oxygen carrier.

Spray granulated Cu-Mn oxygen carrier for chemical looping with oxygen uncoupling (CLOU) process

One form of Chemical Looping Combustion (CLC) with CO2 capture as an alternative to the burning of solid fuels is the Chemical Looping with Oxygen Uncoupling (CLOU) process. The CLOU process requires metal oxides with O2 release capacity at temperatures that are suitable for solid fuel combustion, e.g. copper and manganese oxides. A Cu-Mn mixed oxide comprising 34wt% CuO and 66wt% Mn3O4 and prepared by fluidized bed granulation was analysed for use as an oxygen carrier for the CLOU process. TGA and batch fluidized bed reactors were used to investigate oxygen generation rates, agglomeration behaviour and oxygen carrier attrition. A medium volatile coal from South Africa and its char were used as fuel in a batch fluidized bed reactor in a temperature range of 800–930°C. The Cu-Mn oxygen carrier showed good release O2 rate and a high rate of oxygen transfer by direct gas-solid reaction. The presence of CO as a combustion product appeared only at low oxygen carrier to coal ratios. This material has the capacity to generate O2 at lower temperatures than needed for Cu-based oxygen carriers, which suggests that this material is suitable for work at lower temperatures in a fuel reactor with complete combustion of the fuel. The low attrition rate and attrition index, together with the stable crushing strength of the particles with redox cycles, make this Cu-Mn oxygen carrier suitable for operation in a continuous CLOU unit.

Titanium substituted manganese-ferrite as an oxygen carrier with permanent magnetic properties for chemical looping combustion of solid fuels

Mixed oxides of Mn-Fe have been identified as suitable materials for Chemical Looping Combustion (CLC) with solid fuels both via in-situ Gasification Chemical Looping Combustion (iG-CLC) and Chemical Looping with Oxygen Uncoupling (CLOU) processes. These materials show the property of react with gaseous fuels as well as release oxygen under given conditions, while cheap metals are used. In addition, these materials can show magnetic properties that can be used for an easy separation from ash in CLC with solid fuels. Thus, losses of oxygen carrier material in the ash drain stream would be reduced. Different cations have been proposed for improving the magnetic properties of manganese ferrites, including Ti4+. In this context, the present work accomplishes a screening of (MnxFe1−x)2O3 doped with 7wt.% TiO2, with x ranging from 0 to 1. The influence of Mn:Fe ratio on their physical and chemical properties was evaluated. In general, particles with high crushing strength values (>4N) were obtained, and magnetic characteristics were highlighted when x⩽0.66. The oxygen uncoupling capability depended on the Mn:Fe ratio and the oxidation conditions, i.e. temperature and oxygen partial pressure. Broader oxidation conditions to take advantage of the oxygen uncoupling capability were found for materials with low Mn content. On contrary, the reactivity with fuel gases (CH4, H2 and CO) increased with the Mn content. Thus, oxygen carriers with Mn/(Mn+Fe) molar ratio in the 0.5–0.9 interval showed interesting properties at suitable temperatures for the iG-CLC and CLOU processes (i.e. 850–980°C). The material with Mn/(Mn+Fe)=0.55 was preferred considering a trade-off between reactivity and magnetic properties.

Chemical looping with oxygen uncoupling of high-sulfur coal using copper ore as oxygen carrier

Chemical looping with oxygen uncoupling (CLOU) has been viewed as a promising candidate for solid fuel combustion with inherent CO2 separation at considerably low energy penalty. This work aimed to investigate the sulfur evolution behavior and performance of copper ore oxygen carrier (OC) within the CLOU process using a typical high-sulfur content coal in Sichuan (SC), China as fuel. Experiments at different temperatures and oxygen to fuel ratios were conducted in a laboratory-scale fluidized bed reactor and comparisons have been made between SC coal and Gaoping (GP) anthracite with low sulfur content. It was found that both the increase of temperature and decrease of oxygen to fuel ratio can enhance the generation of sulfurous gases. There were two SO2 peaks during the CLOU process of both SC and GP coals, due to the two relatively independent stages of coal combustion process: volatiles combustion and coal char combustion. The second peak was generally more sensitive to temperature, and a higher temperature would contribute to much more SO2 generation. Cyclic redox experimental results of SC coal in fluidized bed reactor showed stable sulfur evolution trend and moderate reactivity of copper ore OC. Moreover, calcium oxide (CaO) was introduced as desulfurizing agent to remove the sulfurous gases generated in the CLOU process of SC coal and the desulfurization efficiency was achieved as high as 98%. XRD and XPS analyses showed that no metallic sulfide was detected on the surface of the reduced OC samples and ESEM images indicated that no serious sintering problem occurred to the used particles.

Chemical Looping for Energy Technology: A Special Issue

Chemical Looping for Energy Technology: A Special Issue