Chemical Looping Combustion Cycles Research Articles

An improved method for feeding ash model compounds to a bubbling fluidized bed – CLC experiments with ilmenite, methane, and K2CO3

AbstractBiomass conversion with carbon capture and storage (Bio‐Energy CCS; BECCS) is one of the options considered for mitigating climate change. In this paper, the carbon capture technology chemical‐looping combustion (CLC) is examined in which the CO2 is produced in a stream separate from the combustion air. A central research topic for CLC is oxygen carriers; solid metal oxides that provide oxygen for the conversion process. Biomass and waste‐derived fuels contain reactive ash compounds, such as potassium, and interactions between the oxygen carrier and the ash species are critical for the lifetime and performance of the oxygen carrier. This work develops and demonstrates an improved method for studying the interactions between ash species and oxygen carriers. The method uses a lab‐scale reactor operating under fluidized conditions, simulating CLC batch‐wise by switching between feed gas. The novelty of the setup is the integrated system for feeding solid particles of ash model compounds, enabling the simulation of ash species accumulating in the bed. Ilmenite is a benchmark oxygen carrier for solid fuel conversion and was used in this study to evaluate the method using K2CO3 as a model ash compound. Experiments were done at 850 and 950°C. Methane conversion in CLC cycles and fluidization was evaluated with gas analysis and pressure drop measurements. Scanning electron microscopy and energy dispersive X‐ray spectroscopy (SEM‐EDS) and X‐ray diffraction (XRD) analysis of bed particles were done after the experiments to establish changes in the morphology and composition of the ilmenite. The method for feeding the ash model compound was concluded to be satisfactory. At 950°C, K accumulated in the particles forming K‐titanates and agglomeration was enhanced with K2CO3 addition. The agglomeration mechanism was solid‐state sintering between the Fe‐oxides forming on the particle surfaces. The bed defluidized at 950°C, but no such effect was seen at 850°C. The method is suitable for studying the Fe‐Ti‐K system with ilmenite and potassium without the influence of other ash species. © 2023 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Techno-economic implications of time-flexible operation for iron-based chemical looping combustion cycle with energy storage capability

The renewable energy is predicted to be further expanded to reduce the fossil energy and associated CO2 emissions with the aim of achieving climate neutrality. One of the main issues in large-scale deployment of wind and solar power applications is the time-variability of these renewable sources. To tackle this issue, the modern energy conversion systems must have an inherent time-flexibility. The chemical looping combustion (CLC) is an innovative energy-efficient system with inherent CO2 capture. This work evaluates a time-flexible natural gas-based CLC system for efficient power generation (250 MW net electricity production) and very high decarbonization rate (>99%). The iron-based CLC cycle is fitted with Oxygen Carrier (OC) storage facilities (for both oxidized and reduced stages) to enhance the thermo-chemical energy storage capability. Two illustrative process layouts were assessed: one conventional base-load system and one with energy storage capability for flexible time operation. Various relevant process engineering tools were employed: process modelling and simulation, validation, energy integration, techno-economic assessment. The integrated evaluation of main techno-economic indicators shows that the time-flexible design has improved performance indicators such as lower specific investment costs (down to about 3%), reduced operational and power generation costs (down to about 2%) as well as lower CO2 capture costs (down to 8%).

Oxygen carrier derived from ferric sludge for chemical looping combustion of MSW syngas: Waste derived material performance and carbon footprint assessment

The utilization of water treatment sludge (i.e., ferric sludge) was explored as the oxygen carrier (OC) for chemical looping combustion (CLC) of municipal solid waste (MSW) syngas. The performance of ferric sludge (FS) in CLC and simultaneous HCl removal was evaluated in a bench-scale fluidized bed, and was compared with iron ore (IO) as a benchmarked OC. The results show that FS (92 %) performed better than IO (82 %) in terms of syngas combustion efficiency and FS was able to remove HCl (77 % removal efficiency) while IO was unable to remove any HCl. Comparison of reaction temperature reveals that FS performed better at lower CLC temperature between 800 °C and 850 °C. Agglomeration was observed when FS was used in extended CLC cycles. Thus, investigation of using inert α-Al2O3 support together with FS for extended CLC cycles was investigated. Agglomeration and sintering were drastically reduced and good combustion efficiency of 88 % for H2 and 74 % for CO was attained. Carbon footprint assessment illustrated that FS was superior as a greener material based on global warming potential of 20 and 100 years. Conversion of FS as an OC were characterized with 104.23 and 108.0 kgCO2/t lower than IO for GWP-20 and GWP-100, exhibiting the competitiveness in the usage of waste-derived material in CLC. Hence, FS can be considered as a novel, cost-effective, and greener OC for CLC. The co-utilization of waste-derived material and waste-derived syngas through CLC would be an integrated solution for improved circularity and sustainability.

Energy and exergy analysis of combined power, methanol, and light olefin generation system fed with shale gas

Environmental problems and limitations of fossil fuel resources, especially crude oil, have intensified the importance of using cleaner and cheaper fuels besides enhancing energy conversion processes. Therefore, a novel power, methanol, and light olefin multi-generation system is designed and modeled in this paper. Chemical looping reforming, chemical looping combustion cycles, and Rankine power system are combined with methanol and light olefin production processes. The input fuel of the system is shale gas. The mass, energy, and exergy balance equations are applied for each system unit as a steady-state control volume to assess its thermodynamic operation. Then, the effects on the system performance of critical parameters are studied comprehensively. The results show that the necessary syngas can be supplied when 71.5% of the inlet shale gas is used in the steam reforming reactor of the chemical looping reforming cycle, and the steam to fuel ratio and carbon dioxide to fuel ratio are 0.61. Furthermore, if 31% of the produced methanol is consumed in the olefin production unit, the system energy and exergy efficiencies are achieved at 67.3% and 71.5%, respectively. In this case, the carbon dioxide flow rate is 800 kmol/hr, separated and stored in the chemical looping combustion cycle, leading to a clean thermodynamic system.

Migration and emissions of selenium during chemical looping combustion of coal

AbstractSelenium is one of the most volatile toxic elements in coal, and its emissions must be strictly controlled. Chemical looping combustion (CLC) is a clean and efficient technology for coal. Herein, the iron‐based oxygen carrier (OC) was used as an adsorbent to study the migration and emissions of selenium during the CLC of coal. Due to the oxidation and adsorption of selenium by iron‐based OC, most of the selenium was retained in OC or distributed in the CO2 stream. The proportion of gaseous selenium released into the atmosphere was less than 10%—significantly lower than that from the traditional combustion process of coal, which had a value of 91.79%. The presence of OC increased the distribution phase of selenium, promoted the conversion of gaseous selenium to solid selenium, and reduced selenium emissions in flue gas. During CLC of coal, the fuel reactor (FR) temperature and the number of OC re‐oxidation cycles played an important role in the emissions and retention of selenium. The increasing FR temperature increased the gaseous selenium in the CO2 stream, reduced the particulate selenium absorbed by OC, and reduced the selenium emissions in the atmosphere. After 10 continuous CLC cycles, the selenium concentration in OC increased from 0.889 to 8.20 mg kg−1. The continuous cycling of CLC could realize the enrichment of selenium from coal to OC. Furthermore, the migration and transformation mechanism of selenium during CLC was deduced by experiments and thermodynamic simulation. This research provides a suitable reference for reducing selenium emissions and developing CLC technology.

A study on the chemical looping combustion of sewage sludge: The emission of NOx and its precursors

Chemical looping combustion (CLC) is a promising low-NOx combustion technology for sewage sludge disposal. Herein, the emissions of NOx and its precursors (NH3 and HCN) during the pyrolysis and CLC processes are investigated on a fixed bed reactor. In particular, the effects of reaction (gas) temperature, sludge ash accumulation, and reaction intermediates (CO2 and H2O) are studied. When temperature increased from 800 to 900 °C, the content of gas-phase carbon is shown to increase from 57.9 to 79.7%. With ash accumulation, the NH3 generation rate increases from 10.8 to 11.7%, and the HCN generation rate decreases from 9.68 to 8.25%. Sludge ash accumulation is shown to promote the release of NH3, but inhibit the release of HCN and NOx. After 8 consecutive CLC cycles, the average removal efficiencies of NH3, HCN, and total N-containing pollutants are ~61%, ≥92%, and ~ 60%, respectively. The presence of CO2 and H2O in the reaction atmosphere is shown to enhance the oxidizability, thereby promoting the formation of NOx. In addition, char gasification and the reforming reaction of N-containing pollutants are promoted, thus leading to the formation of NOx precursors. Furthermore, NO is shown to be the predominant form of NOx in the CLC of sewage sludge.

Spinel ferrite-contained industrial materials as oxygen carriers in chemical looping combustion

Spinel ferrites (MxFe3-xO4) as oxygen carriers (OC) in chemical looping combustion (CLC) process have drawn significant interest owing to unique lattice structure and oxygen transfer properties. However, their employment in practice is likely cost prohibitive due to the complicated synthesis processes compared to natural occurring materials. In the work, several of the low-cost industrial materials naturally containing ferrite spinel structure (MxFe3-xO4, M = Cr and Mn) were, for the first time, investigated to explore the possibility as an alternative of synthetic OCs. The reactivity and their cyclic performance during the chemical looping combustion were examined by lab-scale fluidized bed and TGA. It was demonstrated that all the tested materials contain spinel structures, especially after CLC cycles. They exhibited almost the same reactivity and stability, and less agglomeration occurred when compared to the synthetic materials contain the same kind of spinel structure. But the tested materials presented lower oxygen transport capacity than the synthetic ones. Specifically, the FM1 material containing MnFe2O4 showed best reactivity towards CH4 and syngas conversion, which may attribute to the oxygen uncoupling ability of Mn based species and the formation of spinel ferromanganese structure. But the stability was not good enough. It may because of the cracked particles attributing to the shrinking during the reduction of Mn2O3 to Mn3O4. The Fe-Cr based sample showed more superior stability and improved performance due to the formation of (Fe,Mg)(Cr,Fe)2O4 spinel structure. The Fe-Cr based samples exhibit poor performance for complete combustion of fuel; however, it appears to convert more CH4 to CO which may be desirable for hydrogenation, gasification, and cracking processes. It is worth noting that these industrial materials did not show significant difference in reactivity and stability when compared to the same kind of synthetic materials, presenting a possibility of potential substitution, especially taking its low cost into account.

Integration of LNG Regasification Process in Natural Gas-Fired Power System with Oxy-Fuel Combustion

Oxy-fuel combustion power systems can utilize the cold energy released during the liquefied natural gas (LNG) regasification to reduce the power consumption of CO2 capture, but the specific LNG cold energy consumption of CO2 capture is still too large. To recover more CO2 with the limited LNG cold energy at a low energy cost, a novel natural gas-fired oxy-fuel power system with the cascade utilization of LNG cold energy is proposed in this work, where the LNG cold energy could be sequentially utilized in the air separation unit and the CO2 recovery process. The new system is evaluated with the Aspen Plus software. The results show that the net electrical efficiency and the specific primary energy consumption for CO2 avoided (SPECCA) of the new system are comparable to those of the chemical looping combustion cycle, and superior to those of the conventional O2/CO2 cycles. Moreover, the specific LNG needed for CO2 avoided (SLNCC) of the new system is more than 67.2% lower than the existing oxy-fuel power systems utilizing the LNG cold energy. Furthermore, it is found that the O2 purity of 97.0 mol.% and the CO2 capture ratio of 97.0% are optimal conditions, because the SPECCA, the specific exergy consumption for CO2 avoided (SECCA) and the SLNCC are at the minimum of 1.87 $${\rm{G}}{{\rm{J}}_{LHV}} \cdot {{\rm{t}}_{{\rm{C}}{{\rm{O}}_2}}}^{- 1},2.60\,\,{\rm{GJ}} \cdot {{\rm{t}}_{{\rm{C}}{{\rm{O}}_2}}}^{- 1}$$ and $$1.88\,\,{{\rm{t}}_{{\rm{LNG}}}} \cdot {{\rm{t}}_{{\rm{C}}{{\rm{O}}_2}}}^{- 1}$$ , respectively. Meanwhile, the net electrical efficiency and the exergy efficiency of the new system reach 51.51% and 49.23%, respectively.

Evaluation of performance of copper converter slag as oxygen carrier in chemical-looping combustion (CLC)

In this study, copper converter slag sample was utilized as oxygen carrier (OC) for Chemical Looping Combustion (CLC) of synthetic syngas. The CLC cycles were substantiated in a bench-scale fluidized bed reactor by altering the inlet atmosphere between fuel and air consecutively. The tests were conducted at 4 times the minimum fluidization velocity, and 1123 K (850 °C) reduction half-cycle temperature (TRHC) for 50 cycles. The mean fuel gas conversion values obtained during 30 s of RHC time (tRHC) were as; 78% for 1:1 CO in nitrogen, 84–89% for syngas, and 97% for 1:1 H2 in nitrogen. Copper converter slag sample was stable and sufficiently reactive throughout the tests, moreover, the reactivity increased substantially by the consecutive CLC cycles. The increase of the porosity and the formation of the copper ferrite during the CLC cycles were found to be the main contributors to the activation of the copper converter slag. It is concluded that the evaluation of copper converter slag in the CLC process is both an economical option for OC supply (especially in coal-fueled CLC processes where some OC loss is inevitable), and an environmentally favorable alternative for recycling copper converter slag unless wasting the precious metals in it.

Zeolite-Supported Iron Oxides as Durable and Selective Oxygen Carriers for Chemical Looping Combustion

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO2 separation from flue gases, circumventing high cost for CO2 capture and NOx elimination as in conventional combustion processes. Metal oxides are commonly used as oxygen carriers (OCs) in CLC. However, sintering and coking of OCs and the consequent degradation in their activity and durability during multiple cycles inhibit the practical applications of CLC technology. In the present study, we employed a silicalite-1 zeolite support to achieve OCs with high resistance against sintering and coking in CLC. Iron oxides (Fe2O3) with methane fuel were employed to demonstrate the approach and to quantify the influence of silicalite-1 support on conversion efficiency, durability, and selectivity of these OCs in CLC cycles. Two iron oxide–zeolite geometrical structures, a core–shell Fe2O3@silicalite-1 and a Fe2O3-impregnated silicalite-1 (Fe2O3/silicalite-1), were created to improve Fe2O3 stability. The CLC tes...

CuO supported on olivine as an oxygen carrier in chemical looping processes with pine sawdust used as fuel

Chemical looping is a promising low-cost technology for CO2 capture with a low energy penalty. In this study, CuO supported on olivine (CuO/olivine) was investigated as an oxygen carrier (OC) in chemical looping combustion (CLC) and in chemical looping with oxygen uncoupling (CLOU). The reactivity and oxygen uncoupling behavior were evaluated by thermogravimetric analysis (TGA). CLC and CLOU experiments using pine sawdust (PS) as fuel were conducted in a fluidized-bed reactor. The CLC cyclic performance and the oxygen release and uptake behaviors of CuO/olivine were explored. In CLC experiments, both carbon conversion (χc) and carbon capture efficiency (ηc) increased with increasing temperature and with increases in OC-to-fuel ratio (Φ); the maximum value of χc and ηc—96.81% and 95.54%, respectively—were achieved at 900°C with Φ=1.50. In CLC cyclic tests, the CuO/olivine displayed stable reactivity, and the values of χc and ηc were 94.64% and 92.59% at the 20th cycle, respectively. In the CLOU tests, oxygen transport capacity (Ro) of 3.55% was obtained via TGA. Complete conversion of pine PS and ηc=99.29% were also achieved at 950°C in a fluidized bed reactor. MgCu2O4 was found in the fresh CuO/olivine sample and it disappeared immediately during the first redox cycle; most of CuO accumulated on the olivine surface and then gradually spread and covered the olivine upon additional cycles. After 20 CLC cycles. CuO content of the CuO/olivine OC decreased slightly from 38.81% to 35.22%; a crushing strength of 4.8N was maintained, and no significant agglomeration was observed. The CuO/olivine has an excellent cyclic performance and it can be used as a good OC candidate in chemical looping processes.

The redox reaction kinetics of Sinai ore for chemical looping combustion applications

The objective of this work was to study the use of Sinai ore, a Fe–Mn-based ore from Egypt, as a low-cost oxygen carrier (OC) in Chemical Looping Combustion (CLC). The Sinai ore was selected because it possesses relatively high amounts of iron and manganese oxides. Furthermore, those oxides have low cost, very favorable environmental and thermodynamic properties for the CLC process. The performance of the Sinai ore as an OC in CLC was compared to that of ilmenite (Norway Tellnes mine), the most extensively studied naturally occurring Fe-based mineral.The kinetics of the reduction and oxidation reactions with the two minerals were studied using a thermogravimetric analyzer (TGA). Experiments were conducted under isothermal conditions, with multiple redox cycles, at temperatures between 750 and 950°C. For the reduction and oxidation reactions, different concentrations of CH4 (10–25vol.%) and O2 (5–20vol.%) were applied, respectively. The kinetic parameters, such as the activation energy (Ea), pre-exponential factor (A0), and reaction order (n), were determined for the redox reactions. Furthermore, models of the redox reactions were selected by means of a model-fitting method. For the Sinai ore, the D3 model (3-dimensional diffusion) was suitable for modeling reduction reaction kinetics. The calculated Ea was 35.3kJ/mole, and the reaction order was determined to be approximately 0.76. The best fit for the oxidation reaction was obtained for the R3 model (shrinking core). The oxidation (regeneration) reaction Ea was equal to 16.7kJ/mole, and the determined reaction order was approximately 0.72.The crystalline phases present, as well as the morphology and inhomogeneities in elemental composition were studied for both materials, fresh as well as after multiple redox cycles, by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) combined with X-ray Microanalysis - Energy Dispersive Spectroscopy (EDS). Structural and morphological changes were detected and correlated to the reaction temperature as well as the reactant compositions and thus the stability of the ores in repetitive CLC cycles was determined.

Investigations into the effects of volatile biomass tar on the performance of Fe-based CLC oxygen carrier materials

In this study we present findings from investigations into interactions between biomass tar and two iron based oxygen carrier materials (OCMs) designed for chemical-looping applications: a 100% Fe2O3 (100Fe) OCM and a 60 wt% Fe2O3/40 wt% Al2O3 (60Fe40Al) OCM. A novel 6 kWe two-stage, fixed-bed reactor was designed and constructed to simulate a chemical-looping combustion (CLC) process with ex situ gasification of biomass. Beech wood was pyrolysed in the first stage of the reactor at 773 K to produce a tar-containing fuel gas that was used to reduce the OCM loaded into the 2nd stage at 973 K. The presence of either OCM was found to significantly reduce the amount of biomass tars exiting the reactor by up to 71 wt% compared with analogous experiments in which the biomass tar compounds were exposed to an inert bed of sand. The tar cracking effect of the 60Fe40Al OCM was slightly greater than the 100Fe OCM although the reduction in the tar yield was roughly equivalent to the increase in carbon deposition observed for the 60Fe40Al OCM compared with the 100Fe OCM. In both cases, the tar cracking effect of the OCMs appeared to be independent of the oxidation state in which the OCM was exposed to the volatile biomass pyrolysis products (i.e. Fe2O3 or Fe3O4). Exposing the pyrolysis vapours to the OCMs in their oxidised (Fe2O3) form favoured the production of CO2. The production of CO was favoured when the OCMs were in their reduced (Fe3O4) form. Carbon deposition was removed in the subsequent oxidation phase with no obvious deleterious effects on the reactivity in subsequent CLC cycles with reduction by 3 mol% CO.

Characterization of an ilmenite ore for pressurized chemical looping combustion

This work presents the characterization results of a Canadian ilmenite ore tested in pressurized chemical looping combustion cycles. The ilmenite ore was isothermally cycled in a pressurized thermogravimetric analyzer using carbon monoxide as a reducing fuel and air as an oxidizing gas. All samples were calcined prior to submission to cycles. X-ray diffraction (XRD) comparing raw and calcined ore samples indicate the disappearance of ilmenite crystals and the formation of rutile and ferric pseudobrookite. Cycled ilmenite ore surface morphology was found to be insensitive to the total pressure (up to 51bar) and CO partial pressure (3.2–8.0bar). These findings are in agreement with thermodynamic equilibrium predictions using FactSage. SEM images reveal the development of cracks in the oxidized particles after 4 redox cycles at 950°C and 16bar, which was correlated to the original lamellar structure of the raw ilmenite ore. Increasing the number of redox cycles from 3.5 to 19.5 resulted in large cracks near iron-rich lamellae. Furthermore, the average grain size was smaller after 19.5 cycles. Increasing the reaction temperature from 850°C to 1050°C resulted in similar surface morphologies, but increased grain size. Despite being below melting temperatures, the sample cycled at 1050°C did agglomerate. Local temperature excursions during sample oxidation may have led to melting. This study provides insights into the phase transformations and morphological changes accompanying the use of this type of ilmenite ore in a pressurized chemical looping combustion process, highlighting the need for proper adjustments to the design and operation of the process.

Ca–Cu looping process for CO2 capture from a power plant and its comparison with Ca-looping, oxy-combustion and amine-based CO2 capture processes

This study reports the technical performance of an advanced configuration of the calcium looping (Ca-looping) process, called Ca–Cu looping, for the reduction of CO2 emissions from a subcritical CFB power plant with a net output of 250MWe. In this new process concept, the thermal power requirement in the calciner is satisfied using a chemical looping combustion cycle. Thus, the power consumption associated with the air separation unit that is required in the conventional Ca-looping process scheme can be eliminated. The reference power plant has also been modified to integrate oxy-combustion, Ca-looping and amine scrubbing technologies for comparison. The fully integrated process flowsheets have been simulated using Honeywell's UniSim Design Suit R400. For a more accurate prediction of the CO2 capture efficiency in the carbonator of the Ca–Cu looping process, the experimentally obtained cyclic activity of a novel copper-functionalised, calcium-based composite sorbent has been implemented yielding a rigorous carbonator model. The process simulations also include a comprehensive analysis of the CO2 compression and purification unit as well as a heat exchanger network optimized using pinch analysis for the recovery of excess heat from the high temperature gas and solid streams available in the Ca-looping and Ca–Cu looping processes. Compared to the energy penalty of 10.5 percentage points for a conventional amine scrubbing configuration, the Ca–Cu looping process evaluated in this study achieves the same overall CO2 capture efficiency with a significantly reduced energy penalty of 3.5 percentage points; the lowest value for all CO2 capture architectures assessed here.

Studies on the redox reaction kinetics of selected, naturally occurring oxygen carrier

This paper presents the results of a chemical-looping combustion (CLC) study. The CLC technology is believed to be one of the most promising combustion technologies. The production of a concentrated CO2 stream that is obtained after the water condensation without any loss of energy in its separation is one of the most crucial advantages of this technology. The objective of this work was to study the kinetics of both the reduction and oxidation reactions for naturally occurring oxygen carriers that show promising reactivity in CLC reactions, and might therefore be utilized as oxygen carrier materials. Kryvbas, a Fe-based ore, was selected for this analysis because it possessed sufficient concentrations of the active metal oxides (Fe oxide above 80 % and traces of Mn oxide) and a high melting temperature that was above 1500 °C. Experiments were conducted under isothermal conditions within the temperature range of 750–950 °C with multiple redox cycles using a thermogravimetric analyzer (TG). For the reduction and oxidation reactions, CH4 (at different concentrations) and air were used, respectively. The sample showed promising results where a sufficient reactivity was observed with the fuel, and these results were reproducible. Both fresh material and samples that were used in multiple redox cycles were characterized by X–ray Diffraction (XRD) and Scanning Electron Microscopy combined with X–ray Microanalysis (SEM–EDS) in order to detect any structural or morphological changes as well as to determine the stability of the ore in repetitive CLC cycles. Kinetic parameters, such as the activation energy, the preexponential factor, and the reaction model, were determined for the redox reactions. Models of the redox reactions were selected by using a model fitting method. The F1 model (volumetric) was a suitable model for the modeling of the Kryvbas ore reduction reaction kinetics. The calculated E a was equal to 42.00 kJ mol−1, while the reaction order was determined to be equal to 1.98. The best fit for the oxidation reaction was obtained for the R3 model (shrinking core model). The oxidation (regeneration) reaction activation energy was equal to 16.70 kJ mol−1, and the reaction order was determined to be equal to approximately 0.49.

Efficient cycles for carbon capture CLC power plants based on thermally balanced redox reactors

The rotary reactor differs from most alternative chemical looping combustion (CLC) reactor designs because it maintains near-thermal equilibrium between the two stages of the redox process by thermally coupling channels undergoing oxidation and reduction. An earlier study showed that this thermal coupling between the oxidation and reduction reactors increases the efficiency by up to 2% points when implemented in a regenerative Brayton cycle. The present study extends this analysis to alternative CLC cycles with the objective of identifying optimal configurations and design tradeoffs. Results show that the increased efficiency from reactor thermal coupling applies only to cycles that are capable of exploiting the increased availability in the reduction reactor exhaust. Thus, in addition to the regenerative cycle, the combined CLC cycle and the combined-regenerative CLC cycle are suitable for integration with the rotary reactor. Parametric studies are used to compare the sensitivity of the different cycle efficiencies to parameters like pressure ratio, turbine inlet temperature, carrier-gas fraction and purge steam generation. One of the key conclusions from this analysis is that while the optimal efficiency for regenerative CLC cycle was the highest of the three (56% at 3bars, 1200°C), the combined-regenerative cycle offers a trade-off that combines a reasonably high efficiency (about 54% at 12bars, 1200°C) with much lower gas volumetric flow rate and consequently, smaller reactor size. Unlike the other two cycles, the optimal compressor pressure ratio for the regenerative cycle is weakly dependent on the design turbine inlet temperature. For the regenerative and combined regenerative cycles, steam production in the regenerator below 2× fuel flow rate improves exhaust recovery and consequently, the overall system efficiency. Also, given that the fuel side regenerator flow is unbalanced, it is more efficient to generate steam from the fuel side regenerator than from the air side regenerator.

Combined calcium looping and chemical looping combustion cycles with CaO–CuO pellets in a fixed bed reactor

The feasibility of using integrated CaO/CuO-based pellets in combined calcium and chemical looping cycles in a fixed bed was investigated with emphasis on CO2 capture performance. Three types of pellets were tested, namely; integrated core-in-shell CaO/CuO-based pellets; integrated homogeneous CaO/CuO-based pellets; and mixed CaO-based pellets and CuO-based pellets. Redox cycles consisted of oxidation/carbonation at 650°C in 15% CO2; 4% O2, and N2 balance, and reduction/calcination at 850°C in a CO/H2 mixture. The results showed that mixed pellets exhibited the highest CO2 uptakes of 0.11g CO2/g bed in the first cycle and 0.07g/g after 11 cycles (corresponding to CaO conversions of 32.1% and 19%, respectively). This enhanced performance was attributed to the significantly higher pore volume and surface area in the calcined CaO-based pellets, which were superior to those of integrated CaO/CuO-based pellets. The reduction of CuO to Cu2O was found to be partially irreversible, with ∼45% of copper oxide in the pellets in the form of Cu2O despite the excess O2 in the oxidizing environment. It was also noted that cycled core-in-shell pellets were the most susceptible to fragmentation, indicating the low mechanical strength of these pellets. It was concluded that combined calcium and chemical looping cycles in a fixed bed system is a feasible process providing suitable pellets are used. Incorporating CuO in CaO-based pellets reduces the porosity of the pellets, thus compromises CO2 uptake. Among the three groups of pellets, mixed pellets are the most promising for CaL–CLC processes due to their better performance, easier manufacturing, and better quality control than integrated pellets.

Analysis of thermally coupled chemical looping combustion-based power plants with carbon capture

A number of CO2 capture-enabled power generation technologies have been proposed to address the negative environmental impact of CO2 emission. One important barrier to adopting these technologies is the associated energy penalty. Chemical-looping Combustion (CLC) is an oxy-combustion technology that can significantly lower this penalty. It utilizes an oxygen carrier to transfer oxygen from air/oxidizing stream in an oxidation reactor to the fuel in a reduction reactor. Conventional CLC reactor designs employ two separate reactors, with metal/metal oxide particles circulating pneumatically in-between. One of the key limitations of these designs is the entropy generation due to reactor temperature difference, which lowers the cycle efficiency. Zhao et al. (Zhao et al., 2014; Zhao and Ghoniem, 2014) proposed a new CLC rotary reactor design, which overcomes this limitation. This reactor consists of a single rotating wheel with micro-channels designed to maintain thermal equilibrium between the fuel and air sides. This study uses three thermodynamic models of increasing fidelity to demonstrate that the internal thermal coupling in the rotary CLC reactor creates the potential for improved cycle efficiency. A theoretical availability model and an ideal thermodynamic cycle model are used to define the efficiency limits of CLC systems, illustrate the impact of reactor thermal coupling and discuss relevant criteria. An Aspen Plus® model of a regenerative CLC cycle is then used to show that this thermal coupling raises the cycle efficiency by up to 2% points. A parametric study shows that efficiency varies inversely with pressure, with a maximum of 51% at 3bar, 1000C and 60% at 4bar, 1400C. The efficiency increases with CO2 fraction at high pressure ratios but exhibits a slight inverse dependence at low pressure ratios. The parametric study shows that for low purge steam demand, steam generation improves exhaust heat recovery and increases efficiency when an appropriate steam production strategy is adopted.

The performance of Fe2O3-CaO Oxygen Carriers and the Interaction of Iron Oxides with CaO during Chemical Looping Combustion and H2 production

Chemical-looping combustion (CLC) has emerged as a promising and low cost carbon capture process which produces a pure stream of CO2. In CLC, oxygen from a solid oxygen carrier (OC) is used to combust fuels. Iron oxide is a promising material, being relatively cheap and environmentally benign. The thermodynamics of the Fe-O system allows reduced iron oxides to be also used for partial oxidation or hydrogen production. However, pure Fe2O3 has the problem of degradation in reactivity over repeated CLC cycles. This work is concerned with inclusion of a suitable CaO support material within the Fe-based OCs to increase its strength and cycle ability. OCs were made from Fe2O3 powder by granulation and followed by impregnation with Ca(NO3)2.4H2O. The reactivity and cyclic stability of the OCs were tested by being subjected to alternating reducing (using H2 or CO) and oxidizing environments (using air and / CO2) for many cycles, in a thermogravimetric analyser and a fluidized bed reactor at 850-950°C. It was found that, for samples with a range of loading of CaO, from 2 to 30 mol%, the one containing 20 mol% CaO showed optimal performances, whereas others showed improvements in oxygen carrying capacity, cyclic stability and reactivity over unmodified iron oxide but to a lesser extent.