Chemical Looping Combustion Operation Research Articles

Interaction of ilmenite oxygen carrier with wheat straw ash during chemical looping combustion: Mechanisms and performance variation

Chemical looping combustion (CLC) offers a promising approach for biomass utilization. The utilization of biomass as the primary fuel in CLC (bio-CLC) is a way to generate negative CO2 emissions and thus interesting for climate change mitigation. However, the presence of ash in biomass raises concerns about the performance variation of the oxygen carrier during CLC. This study investigated the impact of adding wheat straw ash (WSA) to the ilmenite oxygen carrier throughout the CLC process. The effects of WSA addition ratio (0–15 %) and cycle number on the performance of oxygen carrier were evaluated. The results showed a progressive enhancement followed by a subsequent reduction in the performance of ilmenite with the increase of WSA addition ratio, and the peak reactivity of ilmenite was attained with 10 % WSA addition. Addition of 15 % WSA resulted in severe particle sintering, causing a slight diminishment in ilmenite reactivity. Furthermore, the mechanisms of the performance variation of ilmenite with WSA addition were explored. The superior redox reactivity and oxygen release capability of the ilmenite with 10 % WSA addition were evidenced by X-ray photoelectron spectroscopy and hydrogen temperature-programmed reduction analysis, respectively. Notably, X-ray diffraction and thermodynamic calculation confirmed the formation of KFeO2, which significantly enhanced the reactivity of ilmenite. The specific surface area of the ilmenite was greatly enlarged with 10 % WSA addition, and a porous structure was evidenced by scanning electron microscope. The findings suggest the interaction between the WAS and ilmenite enhances the reactivity of the ilmenite with an optimal WAS addition ratio of 10 %, providing a guidance for the long-term operation of CLC.

Investigation of LD-slag as oxygen carrier for CLC in a 10 kW unit using high-volatile biomasses

A steel slag from the Linz-Donawitz process, called LD-slag, having significant calcium and iron-fractions, was investigated as an oxygen carrier in a recently developed 10 kWth chemical-looping combustor with three high-volatile biomass fuels. In order to improve operability, the LD-slag was found to require heat-treatment at high temperatures before being used in the unit. In total, operation with the biomasses was conducted for more than 26 h at temperatures of 870–980 °C. The fuel thermal power was in the range of 3.4–10 kWth. The operation involved chemical looping combustion (CLC), chemical looping gasification (CLG) and oxygen carrier aided combustion (OCAC). Around 12 h was in CLC operation, 13.3 h was conducted in CLG-conditions, while the remaining 0.7 h was OCAC. Here, the results obtained during the CLC part of the campaign is reported. Increased temperature in the fuel reactor and higher airflows to the air reactor both lead to better combustion performance. Steam concentration in the fuel reactor has little effect on the performance. The LD-slag showed higher oxygen demand (31.0%) than that with ilmenite (21.5%) and a manganese ore (19.5%) with the same fuel and normal solids circulation. However, with the LD-slag, there is possibility to achieve a lower oxygen demand (15.2%) with high solids circulation.

Outstanding performance of a Cu-based oxygen carrier impregnated on alumina in chemical looping combustion

Chemical looping combustion (CLC) allows the combustion of a fuel with inherent CO2 capture by using an oxygen carrier to transfer the oxygen from the air to the fuel. Previously to the CLC scale-up, tests should be done in lab-scale CLC units to determine the combustion performance of promising oxygen carriers and, at the same time, to evaluate the durability of these materials. For that, the effect of operating conditions on the oxygen carrier ageing should be considered, which is often disregarded. In this work, tests were done burning a simulated biogas stream in a continuous 500 Wth CLC unit with a promising Cu-based oxygen carrier (Cu14Al_ICB). Operating conditions were carefully selected to maximize the durability of this oxygen carrier while complete CH4 combustion was achieved. These operating conditions were based on limiting both the variation of solids conversion by choosing suitable solids circulation rates and the oxidation degree of the particles in the air reactor by minimizing the air excess. Thus, the Cu14Al_ICB material exhibited an excellent mechanical resistance, estimating a particle lifetime of 20,000 h at 900 °C. In addition, copper migration to the surface of the particles and loss of copper were not relevant. This is the first time that under certain operating conditions, a Cu-based oxygen carrier can exhibit outstanding mechanical properties during continuous CLC operation at 900 °C.

Semi-continuous Operation of Chemical Looping Combustion of Coal Using a Low-Cost Composite Oxygen Carrier

To reach the carbon peak by 2030 and achieve carbon neutrality by 2060, the implementation of low-carbon combustion approaches for fossil fuels (especially coal, a high-carbon fuel) is urgent in China. Chemical looping combustion (CLC) has been considered as one of the most promising technologies for low-carbon, efficient, and clean utilization of coal. However, a low-cost and high-performance oxygen carrier (OC) holds the bottleneck that constrains the industrial application of coal-fueled CLC. In this paper, a low-cost composite OC named Cu13.0Red87.0@C (with the mixing mass ratio of copper ore to red mud being 13.0:87.0, except for a 20 wt % cement bonder) was first prepared by the hydroforming method and then systematically evaluated by designing CLC tests in a semi-continuous fluidized bed using lignite as fuel. The results showed that this composite OC exhibited superior combustion performance in comparison to pure red mud. Moreover, it was found that increasing the temperature could significantly improve the average carbon capture efficiency but had a relatively small positive effect on the average CO2 yield. Additionally, with the elevation of the oxygen/fuel ratio, the resulting average carbon capture efficiency and average CO2 yield both tended to increase. Generally, the semi-continuous unit can simulate the continuously operated fuel reactor with an adjustable bed inventory and solid circulation rate, and it is easy to attain smooth operation of ca. 30 min, achieving both carbon capture efficiency and a CO2 yield of ca. 90%.

Effects of Temperature, Operation Mode, and Steam Concentration on Alkali Release in Chemical Looping Conversion of Biomass-Experimental Investigation in a 10 kWth Pilot.

Alkali release was studied in a 10 kWth chemical looping pilot operated with a Linz–Donawitz (LD) slag oxygen carrier (OC) and three biomass fuels. Experiments were performed at three temperatures and in three operation modes: chemical looping combustion (CLC), chemical looping gasification (CLG), and oxygen-carrier-aided combustion (OCAC). Gas-phase alkali release was measured with a surface ionization detector (SID). Fuel reactor (FR) gas-phase alkali emissions increased with the temperature. This occurred as a result of increased evaporation of KCl and enhanced decomposition of alkali salts during char conversion. Air reactor (AR) alkali emissions were lower than in the FR and independent of the operating temperature. In comparison of operating modes, CLC and CLG modes resulted in similar gas-phase alkali emissions due to the similar extent of char conversion. In contrast, operation of the reactor system in OCAC mode resulted in significantly lower levels of gas-phase alkalis. The difference in alkali emission was attributed to the steam-rich atmosphere of CLC. The effect of steam was further investigated in CLC and OCAC tests. Lowering steam concentrations in CLC operation resulted in lower gas-phase alkali emissions, while introducing steam to the FR during OCAC operation resulted in higher alkali emissions. It was concluded that steam likely enhances gas-phase K release through a reaction of K2CO3 within the fuel char with steam to produce KOH(g). Solid sampling and analysis for K content was used along with SID measurements to develop a K mass balance for the reactor system. Mass balance results for the straw pellet fuel tests showed that LD slag OC absorbs approximately 15–51% of fuel K, 2.2% of fuel K is released to the gas phase, and up to 3.4% of fuel K is captured in the AR fly ash. The residual 40–80% of fuel K was determined to leave the FR as K-rich fly ash.

Achieving Adequate Circulation in Chemical Looping Combustion─Design Proposal for a 200 MWth Chemical Looping Combustion Circulating Fluidized Bed Boiler

Chemical looping combustion (CLC) has unique potential for avoiding the large costs and energy penalties of existing CO2 capture technologies. Oxygen is transferred to the fuel using an oxygen carrier, thus avoiding contact between air and fuel. Consequently, the combustion products, CO2 and H2O, come in a separate stream, and more or less pure CO2 is obtained after condensation of H2O. CLC is normally conceived as a dual fluidized bed process, with high gas velocities in an air reactor driving the circulation, similar to circulating fluidized beds (CFBs), except that the material is led to a fuel reactor before being returned to the air reactor. Crucial for the process is the properties of the oxygen carrier and that circulation is sufficient to transfer needed oxygen and heat to the fuel reactor. Comprehensive literature shows successful use of many oxygen carriers in sustained pilot operation. In contrast, the need for reaching adequate circulation in an industrial-scale system has been given little consideration. Normally, a system similar to CFB boilers is assumed to give sufficient circulation. However, literature data indicate that circulation in CFB boilers is 5–50% of what is needed. Measures to provide sufficient circulation may cause difficulties, such as erosion or bed material loss in the cyclone. Here, a circulation system based on collection of the downflow of particles along the walls is proposed, and a design of a 200 MWth combined CLC–CFB boiler based on this principle is presented. Further, operational strategies and the need for flexibility are discussed. The design is focused on making an industrial-scale demonstration boiler, which can be used in CLC operation with different oxygen carriers and different fuels and that can explore different operational strategies to find optimal conditions. It is recommended that the upscaling of the technology aims directly at the industrial scale.

Performance of electronic waste based mixed metal oxide as novel oxygen carriers for chemical looping co-combustion of high ash coal and rice straw.

Electronic waste (e-waste) is one of the major pollutants accumulated due to its huge demand and short lifespan. Hence, it is essential to reuse and extract the value added components from e-waste. In this context, firstly, a printed circuit board (PCB) is used to produce calorific valuable gases by pyrolysis and gasification reactions. Secondly, the resultant residue of PCB is combusted to extract metals such as iron, copper, nickel etc. as oxygen carriers for the chemical looping combustion (CLC) process. CLC is an emerging and appealing technology for producing rich CO2 that can be directly sent for sequestration. In the present study, a detailed investigation is performed to ensure the reactivity of the e-waste based metal oxide with high ash coal, rice straw and their blends in the CLC process. CO2 yield, gas conversion, and char conversion are evaluated to assess the performance of the co-combustion based CLC process. It is found that 90.9% CO2 yield, 94.1% gas conversion and 93.2% char conversion can be obtained using the blends of coal and rice straw in the first cycle of the CLC operation. Further, a reduction of 5% to 7% of these parameters is evaluated at the end of the third consecutive cycle of CLC operations. The interaction between coal and rice straw is further studied by evaluating their synergistic effects, char-oxygen carrier interaction and kinetic parameters using a thermogravimetric analyzer under N2 and CO2 atmosphere. The co-combustion process has reduced the activation energy by 13.4% at 800-1000°C under CO2 atmosphere.

Air jet attrition measurements at hot conditions of oxygen carriers for chemical looping combustion

The attrition behaviour of several promising oxygen carriers tested in Chemical Looping Combustion (CLC) units has been evaluated using an air-jet apparatus based on ASTM D5757 at 20 and 800 °C. Results were evaluated to determine a general testing procedure that allows estimating the adequacy at working temperature of a potential oxygen carrier. For both fresh and used oxygen carriers, attrition was lower when the temperature of the Air Jet attrition Index (AJI) test was increased. Thus, tests with potential oxygen carriers must be performed close to the temperature they are planned to be used to obtain reliable information. Results indicated that used oxygen carriers with AJI value at hot conditions between 1 and 2% showed long lifetimes under continuous CLC operation.

Preventing Agglomeration of CuO-Based Oxygen Carriers for Chemical Looping Applications

Chemical looping combustion (CLC) is a promising alternative to the conventional combustion-based, fossil fuel conversion processes. In CLC, a solid oxygen carrier is used to transfer oxygen from air to a carbonaceous fuel. This indirect combustion route allows for effective CO2 capture because a sequestrable stream of CO2 is inherently produced without any need for energy-intensive CO2 separation. From a thermodynamic point of view, CuO is arguably one of the most promising oxygen carrier candidates for CLC. However, the main challenge associated with the use of CuO for CLC is its structural instability at the typical operating temperatures of chemical looping processes, leading to severe thermal sintering and agglomeration. To minimize the irreversible microstructural changes during CLC operation, CuO is commonly stabilized by a high Tammann temperature ceramic, for example, Al2O3, MgAl2O4, and so forth. However, it has been observed that a high Tammann temperature support does not always provide a high resistance to agglomeration. This work aims at identifying the descriptors that can be used to characterize accurately the agglomeration tendency of CuO-based oxygen carriers. CuO-based oxygen carriers supported on different metal oxides were synthesized using the Pechini method. The cyclic redox stability and agglomeration tendency of the synthesized materials were evaluated using both a thermo-gravimetric analyzer and a lab-scale fluidized bed reactor at 900 °C using 10 vol % H2 in N2 as the fuel and air for re-oxidation. In order to study the diffusion of Cu(O) during redox reactions, well-defined model surfaces comprising thin films of Cu/CuO and two different supports, namely, ZrO2 and MgO, were prepared via magnetron sputtering. Energy-dispersive X-ray spectroscopy on focused ion beam-cut cross-sections of the thin films revealed that Cu atoms have a tendency to diffuse outward through the support material under redox conditions. The support that inhibits the outward movement of Cu(O), that is, avoiding the presence of low melting Cu on the oxygen carrier surface, is found to provide the highest agglomeration resistance. The support MgO was found to possess such diffusion characteristics.

Investigation of biomass alkali release in a dual circulating fluidized bed chemical looping combustion system

Chemical looping combustion (CLC) of biomass is a promising technology for power generation with integrated carbon capture. In CLC, alkali content of biomass poses potential issues of bed agglomeration, as well as heat exchanger fouling and corrosion. The fate of biomass alkalis was investigated in a dual-interconnected circulating fluidized bed CLC system. Experiments were conducted in oxygen carrier aided combustion (OCAC) and CLC modes. Ilmenite and braunite oxygen carriers and three biomass fuels (wood pellets, wood char, straw pellets) were tested. Flue gas alkali emissions in the air reactor (AR) and fuel reactor (FR) were measured with a surface ionization detector (SID). Results showed that CLC operation yields gas-phase alkali emissions that are up to 15 times higher than in comparable OCAC operation. Results analysis concluded that increased alkali emissions in CLC arise from the steam atmosphere in the FR, whereby steam accelerates the decomposition of alkali compounds in the biomass. Retention of alkalis in the condensed phase was found to be >97% for ilmenite and >92% for braunite CLC operation. Up to 60–80% of the retention was attributed to fuel ash formation. The residual retention was attributed to absorption of alkalis by the oxygen carriers. Absorption likely occurred mainly through formation of alkali manganates and silicates in braunite, and formation of alkali silicates, aluminosilicates, manganates, and titanates in ilmenite. Gas-phase alkali emissions in the AR, although less than in the FR, were found to occur due to combustion of unconverted fuel carried over from the FR to the AR.

Techno-economic evaluation of electronic waste based oxygen carriers for co-chemical looping combustion of coal and biomass integrated combined cycle power generating systems

Chemical looping combustion (CLC) is an emerging technology to capture CO2 in a cost-effective way. Metal particles such as Fe, Ni, and Cu can separate oxygen from air by forming their oxides and they could thereby act as oxygen carriers for the combustion of fuels producing nitrogen-free CO2 enriched flue gas. To commercialize the CLC based carbon capture technology, low cost oxygen carriers are essential for economical operation. Electronic waste contains enormous metal components, which can be a potential source for the production of oxygen carriers. In the present study, mixed oxygen carrier particles obtained from a printed circuit board (PCB) based electronic wastes are proposed to use in the CLC process. A combustion experiment is conducted to extract metals from the PCB and their composition is identified using an XRF analyzer. These metals are found with 21% Fe2O3, 22.8% CuO and 3% NiO, 9.6% CaO and 10% Al2O3. Using these oxidized printed circuit board metals (OPCB) as oxygen carriers, Aspen plus simulations are performed by integrating a CLC system with a combined cycle power plant having a net capacity of 150 MW for electricity generation. High ash coal (33% ash) and low ash coal (2.1% ash) and rice straw are used in a co-combustion mode for the CLC operation. The effect of co-firing on the net thermal efficiency of the power plants is analyzed. Further economic analyses are carried out for the proposed system and the levelized cost of electricity (LCOE) is estimated for the power plants using various fuels. It is found that the use of electronic waste based oxygen carriers achieved almost an equivalent net thermal efficiency of 42.4% based on the combined cycle power plants, compared to pure Fe2O3 based power plants. Further the economic analysis has shown that the levelized cost of electricity (LCOE) of the CLC integrated combined cycle power system employed with electronic waste based oxygen carriers is 85.9 $/MWh, which is the lowest among other power systems. Metal oxide interaction with fuel ash is investigated experimentally using a thermogravimetric analyzer (TGA). The X-ray diffraction (XRD) analyses of the residue confirmed that there is no formation of complex components due to the ash interaction with OPCB.

Characterisation of Australian ilmenite oxygen carrier during chemical looping combustion of Victorian brown coal

Chemical looping combustion (CLC) offers a low emission route for fossil fuel-based power generation. In this study, a compact fully looped chemical looping reactor was developed in-house, and operated continuously with a Victorian brown coal as fuel and an Australian ilmenite as oxygen carrier at 940–980 °C and atmospheric pressure for 36 h. The study found that the CLC performance increased with increasing operation time. The CO2/CO molar ratio, used to evaluate the CLC reaction degree, increased from 2.3 to 8.9 over the 36 h of operation. Specifically, this paper provides a comprehensive study and evaluation of the ilmenite characteristics on morphology changes and ash interaction, due to the continuous operation. No noticeable effect by ash interaction on the CLC performance or operation was observed during the continuous operation. The characterisation results revealed a clear segregation of iron species from the iron‑titanium matrix, its migration to and enrichment over the particle surfaces. Hematite (Fe2O3) formation became more pronounced while pores developed internal and external of the ilmenite particles. These changes are believed to improve the carrier reactivity, and so the CLC performance, but also to weaken the mechanical strength of the carrier, promoting particle attrition and leading to its loss by elutriation.

Iron ore modified with alkaline earth metals for the chemical looping combustion of municipal solid waste derived syngas

The redox performances of iron ore (IO) modified with different alkaline earth metals (AEM) as oxygen carriers (OCs) were evaluated with thermogravimetric analysis (TGA) and fluidized bed reactor experiments for the chemical looping combustion (CLC) of municipal solid waste (MSW) derived syngas. AEM oxides (BaO, CaO, and MgO) and AEM aluminates (BaAl2O4, CaAl2O4, and MgAl2O4) were systematically studied for their ability to promote the redox cycles performance of IO. Acid digestion, FESEM, and XRD analyses were conducted for the characterization of the OCs before and after redox reactions to investigate the promoting mechanism. It was found that the BaO-modified IO demonstrated the best reactivity with the reduction rate of 2.04%/min, which was twofold that of the pristine IO. AEM oxides presented superior redox performance than AEM aluminates owing to their strong interaction with Fe2O3 with the formation of AEM ferrites. The formed CaFe2O4 and MgFe2O4 with spinel structure and BaFe12O19 with hexagonal structure, as a result of the interaction, facilitated the lattice oxygen mobility and improved the reactivity of the modified IO. This interaction also increased the sintering resistance, while the morphology of the IO became more porous after redox cycles, which was also responsible for the improved performance during the long term CLC operation.

Mapping the operating performance of a novel internally circulating fluidized bed reactor applied to chemical looping combustion

Chemical looping combustion is a promising technology for minimizing the energy penalty of CO2 capture. To accelerate the scale-up and commercialization of this technology for pressurized operation, the internally circulating reactor (ICR) was recently proposed. ICR integrates the two reactors, cyclones, loop seals and solids transport lines of the conventional chemical looping configuration into a single unit that simplifies design and operation. This work reports the ICR operating performance over a range of operating parameters applied to chemical looping combustion (CLC). The concept proved relatively simple to operate, allowing the oxygen-carrier circulation rate to be controlled over a wide range by varying the bed loading and the air reactor feed rate. Fully autothermal CLC operation was demonstrated as an illustration of the ease of ICR operation. Gas leakage between the two reactor chambers decreased strongly with decreasing solids loading, resulting in CO2 capture and purity up to 94% for the lowest bed loading. The data showed that significant room for further optimization of the solids transport ports in the reactor exists, which will further increase the CO2 separation performance. These results demonstrate the promise of ICR concept and provide valuable insights for the design of larger-scale units in future work.

Avoiding CO2 capture effort and cost for negative CO2 emissions using industrial waste in chemical-looping combustion/gasification of biomass

Chemical-looping combustion (CLC) is a combustion process with inherent separation of carbon dioxide (CO2), which is achieved by oxidizing the fuel with a solid oxygen carrier rather than with air. As fuel and combustion air are never mixed, no gas separation is necessary and, consequently, there is no direct cost or energy penalty for the separation of gases. The most common form of design of chemical-looping combustion systems uses circulating fluidized beds, which is an established and widely spread technology. Experiments were conducted in two different laboratory-scale CLC reactors with continuous fuel feeding and nominal fuel inputs of 300 Wth and 10 kWth, respectively. As an oxygen carrier material, ground steel converter slag from the Linz–Donawitz process was used. This material is the second largest flow in an integrated steel mill and it is available in huge quantities, for which there is currently limited demand. Steel converter slag consists mainly of oxides of calcium (Ca), magnesium (Mg), iron (Fe), silicon (Si), and manganese (Mn). In the 300 W unit, chemical-looping combustion experiments were conducted with model fuels syngas (50 vol% hydrogen (H2) in carbon monoxide (CO)) and methane (CH4) at varied reactor temperature, fuel input, and oxygen-carrier circulation. Further, the ability of the oxygen-carrier material to release oxygen to the gas phase was investigated. In the 10 kW unit, the fuels used for combustion tests were steam-exploded pellets and wood char. The purpose of these experiments was to study more realistic biomass fuels and to assess the lifetime of the slag when employed as oxygen carrier. In addition, chemical-looping gasification was investigated in the 10 kW unit using both steam-exploded pellets and regular wood pellets as fuels. In the 300 W unit, up to 99.9% of syngas conversion was achieved at 280 kg/MWth and 900 °C, while the highest conversion achieved with methane was 60% at 280 kg/MWth and 950 °C. The material’s ability to release oxygen to the gas phase, i.e., CLOU property, was developed during the initial hours with fuel operation and the activated material released 1–2 vol% of O2 into a flow of argon between 850 and 950 °C. The material’s initial low density decreased somewhat during CLC operation. In the 10 kW, CO2 yields of 75–82% were achieved with all three fuels tested in CLC conditions, while carbon leakage was very low in most cases, i.e., below 1%. With wood char as fuel, at a fuel input of 1.8 kWth, a CO2 yield of 92% could be achieved. The carbon fraction of C2-species was usually below 2.5% and no C3-species were detected. During chemical-looping gasification investigation a raw gas was produced that contained mostly H2. The oxygen carrier lifetime was estimated to be about 110–170 h. However, due to its high availability and potentially low cost, this type of slag could be suitable for large-scale operation. The study also includes a discussion on the potential advantages of this technology over other technologies available for Bio-Energy Carbon Capture and Storage, BECCS. Furthermore, the paper calls for the use of adequate policy instruments to foster the development of this kind of technologies, with great potential for cost reduction but presently without commercial application because of lack of incentives.

Optimal design of combined cycle power plants with fixed‐bed chemical‐looping combustion reactors

Process intensification options are explored for near‐carbon‐neutral, natural‐gas‐fueled combined cycle (CC) power plants, wherein the conventional combustor is replaced by a series of chemical‐looping combustion (CLC) reactors. Dynamic modeling and optimization are deployed to design CLC‐CC power plants with optimal configuration and performance. The overall plant efficiency is improved by optimizing the CLC reactor design and operation, and modifying the CC plant configuration and design. The optimal CLC‐CC power plant has a time‐averaged efficiency of 52.52% and CO2 capture efficiency of 96%. The main factor that limits CLC‐CC power plant efficiency is the reactor temperature, which is constrained by the oxygen carrier material. CLC exhaust gas temperature during heat removal and gas compressor to gas turbine pressure ratio are the most important operating variables and if properly tuned, CLC‐CC power plants can reach high thermodynamic efficiencies. © 2018 American Institute of Chemical Engineers AIChE J, 65: e16516 2019

Pressurised chemical-looping combustion of an iron-based oxygen carrier: Reduction kinetic measurements and modelling

Chemical-looping combustion (CLC) is a novel combustion techology offering the potential to provide uninterrupted and reliable heat and power production from fossil or bio-derived fuels with integrated, intrinsic CO2 capture and minimal energy penalty. Operation of CLC at elevated pressures provides the potential for integration with a combined cycle, which makes the use of solid fuels significantly more feasible. To date, only a few experimental studies investigating CLC processes and oxygen carrier performance under pressurised conditions have been reported in the open literature. This article reports findings from investigations into the effect of pressure, temperature and CO concentration on the intrinsic reaction kinetics of an Al2O3-supported Fe-based oxygen carrier. Our study employed an innovative pressurised fluidised-bed reactor, designed for operation at temperatures up to 1273K and pressures up to 20bara, to simulate ex-situ gasification of solid fuels at elevated pressures. An intrinsic reaction model was developed and pseudo-intrinsic rate constants were derived. Differences in the activation energies and pre-exponential factors of the Al2O3-supported Fe2O3 and a pure Fe2O3 oxygen carriers were observed, indicating a change in reaction mechanism when Al2O3 was present. Subsequently, an adapted random pore model was developed to describe the variation of reaction rate with solid conversion. The good agreement between the adapted random pore model and empirical measurements indicated that the change in mechanism was due to a significantly higher product layer diffusivity for the Al2O3-supported Fe2O3 oxygen carrier compared with the pure Fe2O3 material. When pressurised, the observed reaction order with respect to CO was slightly lower than 1. The model developed using atmospheric pressure measurements was successfully applied to predict reaction kinetics at elevated pressures up to 5bara providing further validation of the model.

Chemical looping combustion of hard coal and torrefied biomass in a 1 MWth pilot plant

Chemical looping combustion (CLC) is an emerging CO2 capture technology utilizing two interconnected circulating fluidized beds (CFB). An oxygen carrier is oxidized in the air reactor and reduced in the fuel reactor. In comparison to a classic oxyfuel process, no air separation unit is required to provide the oxygen needed to burn the fuel. The solid fuel, such as coal or biomass, is gasified in the fuel reactor, and the products from gasification are oxidized by the oxygen carrier. This work presents results of 110h autothermal and partial autothermal CLC operation with ilmenite and iron ore as oxygen carriers. Hard coal and a fuel blend of hard coal and torrefied biomass were used as fuels. The results show that the oxygen demand of the fuel reactor required for a complete conversion of unconverted gases was in the range of 27%. At the same time, the carbon capture efficiency was low in the present configuration of the 1 MWth pilot. This means that unconverted char left the fuel reactor and burned in the air reactor. The reason for this is that no carbon stripper unit was used during these investigations. Some possible modifications like an oxygen carrier with oxygen uncoupling, a lower fuel feeding point or a secondary fluidization of the fuel reactor could significantly enhance the carbon capture efficiency.

Unique phase identification of trimetallic copper iron manganese oxygen carrier using simultaneous differential scanning calorimetry/thermogravimetric analysis during chemical looping combustion reactions with methane

Chemical looping combustion (CLC) is a promising combustion technology that generates heat and sequestration-ready carbon dioxide that is undiluted by nitrogen from the combustion of carbonaceous fuels with an oxygen carrier, or metal oxide. This process is highly dependent on the reactivity and stability of the oxygen carrier. Thus, the development of oxygen carriers remains one of the major barriers for commercialization of CLC. Synthetic oxygen carriers, consisting of multiple metal components, have demonstrated enhanced performance and improved CLC operation compared to single metal oxides. However, identification of the complex mixed metal oxide phases that form after calcination or during CLC reactions has been challenging. Without an understanding of the dominant metal oxide phase, it is difficult to determine reaction parameters and the oxygen carrier reduction pathway, which are necessary for CLC reactor design. This is particularly challenging for complex multi-component oxygen carriers such as copper iron manganese oxide (CuFeMnO4).This study aims to differentiate the unique phase formation of a highly reactive, complex trimetallic oxygen carrier, CuFeMnO4, from its single and bimetallic counterparts using thermochemical and reaction data obtained from simultaneous differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) during temperature programmed reductions (TPR) with methane. DSC/TGA experiments during TPR with methane provides heat flow data and corresponding reaction rate data that can be used to determine reaction routes and mechanisms during methane reduction. Furthermore, non-isothermal TPR data provides the advantage of distinguishing reactions that may not be observable in isothermal analysis.The detailed thermochemical and reaction data, obtained during TPR with methane, distinguished a unique reduction pathway for CuFeMnO4 that differed from its single and bimetallic counterparts. This is remarkable since X-ray diffraction (XRD) data alone could not be used to distinguish the reactive trimetallic oxide phase due to overlapping peaks from various single and mixed metal oxides. The unique reduction pathway of CuFeMnO4 was further characterized in this study using in-situ XRD TPR with methane to determine changes in the dominant trimetallic phase that influenced the thermochemical and reaction rate data.

Pressurized chemical looping combustion with CO: Reduction reactivity and oxygen-transport capacity of ilmenite ore

Ilmenite ore has shown promise as an oxygen-carrier in chemical looping combustion operation. Since operating at higher pressures has been proven to hold important benefits for chemical looping combustion, this study aims to investigate the performance of ilmenite ore under pressurized conditions. The reduction reactivity and oxygen-transport capacity of Canadian ilmenite ore in pressurized chemical looping combustion during redox cycles were investigated in a pressurized thermogravimetric analyzer, using carbon monoxide and air as reductant and oxidant respectively. While increasing the CO partial pressure from 0.32 to 0.80MPa (at a constant total pressure) led to higher rates of reaction and higher oxygen-transport capacity, increasing the total pressure up to 2.4MPa (at a constant CO partial pressure while varying the pCO2) had the reverse effect. Furthermore, increasing the temperature from 1123 to 1323K resulted in a pronounced increase in the reduction rate. Although increasing the particle size from 75–125μm to 300–425μm resulted in a slower rate of reduction, the effect was minor. A kinetic grain model with a changing grain size was fitted to reduction rate data obtained at 1.6MPa. The order of reaction with respect to the CO partial pressure was calculated as 0.67, while the activation energy was determined to be 115kJ/mol.