Pressurized Chemical Looping Combustion Research Articles

Dimensioning Air Reactor and Fuel Reactor of a Pressurized CLC Plant to Be Coupled to a Gas Turbine: Part 2, the Fuel Reactor

Bioenergy with Carbon Capture and Storage (BECCS) technologies are fundamental to reach negative CO2 emissions by removing it from the atmosphere and storing it underground. A promising solution to implement BECCS is pressurized Chemical Looping Combustion (CLC), which involves coupling a pressurized CLC reactor system to a turboexpander. The typical configuration chosen is to have an air reactor and a fuel reactor based on coupled circulating fluidized beds. The fluidization regime in both reactors is preferred to be fast fluidization to enhance gas particle contact and solids circulation among reactors. To design the two reactors, Aspen Plus software was used, given that the new version has a module for fluidized bed modeling. At first, the oxygen carrier was designed ex novo, but given that it is a composite compound mainly made by nickel oxide freeze-granulated on alumina (Ni40Al-FG), the molecular structure has been inserted in Aspen Plus. Then, based on the power of the gas turbine, the power output per kg of evolving fluid (in this case, depleted air) is calculated using Aspen Plus. Once the nitrogen content in the depleted air is known, the total air at the inlet of the air reactor is calculated. The fuel reactor is modeled by inserting the reduction reactions for nickel-based oxygen carriers. The paper presents a useful methodology for developing pressurized Chemical Looping Combustors to be coupled to gas turbines for power generation. The provided data will be cross-validated with 0D-models and experimental results.

Exploring design options for pressurized chemical looping combustion of natural gas

Exploring design options for pressurized chemical looping combustion of natural gas

Comparative exergoeconomic analysis of atmosphere and pressurized CLC power plants coupled with supercritical CO2 cycle

Recently, the incorporation of chemical looping combustion (CLC) and sCO2 Brayton cycle has been regarded as a promising technology for clean and efficient power generation. The intrinsic characteristics of the CLC reactors (interconnected circulating fluidized bed or fixed bed) allow them to operate at different pressures, leading to different technological directions. Therefore, a pair of “CLC coupled sCO2 cycle” systems was proposed, namely the atmosphere CLC (A-CLC-sCO2) and pressurized CLC (P-CLC-sCO2). The purpose of this paper is to determine which configuration holds more benefits from a combined exergetic and economic perspective. The operational boundary conditions for thermodynamic advantage of P-CLC-sCO2 over the optimized A-CLC-sCO2 was firstly investigated. Under respective optimal conditions, the exergy efficiency of P-CLC-sCO2 (51.09%) was found to be 1.16% higher than that of A-CLC-sCO2 (49.91%). Besides, the P-CLC-sCO2 exhibited a lower levelized cost of electricity (79.59 $/MWh) than A-CLC-sCO2 (81.31 $/MWh) when using a nickel-based oxygen carrier (OC), but the A-CLC-sCO2 could further reduce LCOE to 78.93 $/MWh by adopting ilmenite-based OC. In addition, the exergy and exergoeconomic analysis of the main components was carried out to find the system improvement strategies.

Exploring design options for pressurized chemical looping combustion of natural gas

Chemical Looping Combustion (CLC) is a low-cost CO2 capture technology highly competitive for industrial and energy applications. Up to date, CLC has been mostly developed at atmospheric pressure. Pressurized CLC may bring additional advantages due to the intensification of the process. The main objective of this work was to determine whether an increase in the operating pressure could improve the fuel conversion in CLC systems of gaseous fuels when ilmenite was used as oxygen carrier. Fluid dynamics considerations were evaluated to determine a window of suitable design parameters at pressurized conditions. A theoretical model previously validated at atmospheric pressure was modified to consider fluid dynamics and reaction kinetics at pressurized conditions, both in the bubbling and turbulent regimes, using ilmenite as oxygen carrier. The conversion of natural gas was estimated as a function of several design and operating parameters, such as the gas velocity, particle size and reacting temperature. Uncompleted combustion of natural gas was predicted at atmospheric conditions, and a limited improvement was estimated at pressurized conditions. A discussion about the required reactivity of the oxygen carrier to achieve complete combustion was performed. Furthermore, an energy analysis of a CLC system operating at atmospheric or pressurized (0.5 MPa) conditions with fuel reactor designed under the bubbling or turbulent regimes was carried out using the Aspen HYSYS process simulation software for two different applications of heat generation: production of domestic hot water and process steam. From this energy balance, it could be concluded that the energy efficiency achieved at pressurized conditions was slightly higher than that obtained at atmospheric pressure, essentially because at pressurized conditions it was possible to use the latent heat of condensation of the water generated in the fuel reactor. In addition to the results obtained in this work, a thorough assessment of technical risks and a techno-economic analysis would be required in order to make a final decision about the feasibility of operating at pressurized conditions in CLC systems.

Effects of pressure on the chemical looping combustion of coal with CuFe2O4 combined oxygen carrier

Chemical looping combustion (CLC) has fast developed as an attractive technology for clean utilization of coal as well as inherent CO2 separation at the low energy penalty. Increase of the system pressure has been acknowledged as one of the effective measures in CLC for sufficient conversion of coal along with affiliated benefits for CO2 sequestration, but the complex consequences as incurred should be well considered. In this research, reaction of a typical Chinese bituminous coal of high sulfur content (designated as LZ) with CuFe2O4 combined oxygen carrier (OC) within 0.1–3.0 MPa in the CO2 atmosphere was systematically investigated, with focus on the effect of the system pressure to the conversion of LZ coal, transfer of the oxygen involved in CuFe2O4, sulfur evolution and redistribution. Sequential reactions of CuFe2O4 with the volatiles emitted from pyrolysis of LZ coal and further reaction with the released gasification products and residual char were accomplished, which was beneficial to overcome the mass transfer limitation of the reducing gases evolved from LZ coal to OC under the pressurized conditions and alleviate the inhibition effect resulting from accumulation of the transferred reactive gases around the surroundings of OC. And thus, under the pressurized conditions, improved conversion of LZ coal was reached by oxidization of more intractable C-C/C-H groups to oxygen bound carbon groups. Meanwhile, deep reduction of the added CuFe2O4 occurred with deficient Fe3O4 at the lower valence being formed, which would interact with the silicon minerals present in LZ coal to form various iron silicates. Furthermore, sulfur evolution from coal was found to directionally interact with the reduced CuFe2O4 to mainly form solid Cu2S due to their high combination potential. And deeper reduction of Cu2S was instigated especially at 3.0 MPa with Cu1.8S formed, and the total solid sulfur fraction was increased with the elevated system pressure. Overall, comprehensive information on the interaction of coal of high sulfur content with CuFe2O4 under the extended pressurized conditions around 0.1–3.0 MPa was provided, which would be meaningful for the realistic pressurized CLC in the future.

A modified correlation to calculate the transport velocity for pressurized chemical looping combustion

Chemical looping combustion (CLC) is a next-generation combustion technology that uses an oxygen carrier in a metal oxide particulate form to supply oxygen that is necessary for combustion. This study analyzed the effect of pressure on the transport velocity with the aim of designing a pressurized CLC. The emptying time method was used to determine the transport velocity with a two interconnected pressurized fluidized bed system. This investigation also verified the feasibility of the existing correlations to predict the effect of pressure on the transport velocity in the gas fluidized bed. A modified correlation that considers the pressure and the particle properties was then proposed. This study demonstrated that the modified correlation led to a smaller difference between the measured transport velocity and calculated values under ambient and pressurized conditions so that the prediction of transport velocity in a pressurized circulating fluidized bed can be expected to be more accurate.

Experimental screening of oxygen carrier for a pressurized chemical looping combustion

Chemical looping combustion (CLC), an oxy-combustion CO2 capture process, is a promising technology with great potential to reduce the CO2 separation cost and increase the power generation efficiency. The overall efficiency can be determined by the appropriate employment of oxygen carriers, playing a significant role in the economic penalties. In this study, a suitable oxygen carrier for a pressurized CLC was successively and systematically identified from seven types of oxygen carriers. OCN703 and N002-R1, with an adequate solid conversion rate but low durability, were excluded from the candidates by the thermogravimetric analysis results and attrition indices. From the atmospheric fluidized bed, the fuel conversion and CO2 selectivity of LOC023, LOC031, and LOC032 were found to be lower than that of OCN717 and N016-R4. From the semi-continuous fluidized bed, it was inferred that N016-R4 can achieve high CO2 selectivity with a low solid circulation rate when compared to OCN717. To conclude, N016-R4 was identified as the optimal oxygen carrier for a pressurized CLC. The experimental screening process and the criteria to identify the optimal oxygen carrier used in this study are expected to be applicable to new oxygen carriers that will be developed in the future.

Experimental demonstration of pressurized chemical looping combustion in an internally circulating reactor for power production with integrated CO2 capture

This study presents an experimental demonstration of pressurized chemical looping combustion (CLC) in an internally circulating reactor (ICR). The ICR concept is a novel alternative to the conventional interconnected fluidized bed CLC configuration as it eliminates all cyclones, loop seals and solids transport lines, and it can be pressurized in a single pressure shell. Stable operation with high fuel conversion was established for about 40 h of operation at pressures up to 6 bar, achieving reasonable CO2 purity and capture efficiency (up to 97%). The solids circulation rate was found to increase with increasing the operating pressure at a constant fluidization velocity with no effect on CO2 capture and purity. The experimental campaign also examined the effects of solids inventory and fluidization velocities in the air and fuel reactors. The CO2 purity and capture efficiency were most sensitive to the solids inventory, whereas the solids circulation rate was most sensitive to the air reactor fluidization velocity and the solids inventory. A correlation for solids circulation rate was derived from the collected experimental data, thus providing a robust tool for designing an ICR system for pressurized operation. This correlation can assist in further scale-up and demonstration of the ICR concept in commercial scale.

Economic analysis of pressurized chemical looping combustion for steam assisted gravity drainage applications

A great effort is still pending in terms of thorough bottom-up economic assessments in the particular case of Chemical Looping Combustion (CLC) and Pressurized Chemical Looping Combustion (PCLC) processes. This work tries to reduce this knowledge gap through a comprehensive techno-economic analysis of PCLC technology focused on the replacement of a Once-through Steam Generator (OTSG) for the production of steam in a Steam Assisted Gravity Drainage (SAGD) facility in Alberta (Canada) by a PCLC system that burns natural gas using ilmenite as oxygen carrier.The evaluation of capital expenditure (CAPEX) for the different CLC cases analyzed revealed that the main contribution to costs stemmed from the rotating equipment. The economic performance of the CLC processes was evaluated through the Levelized Cost of Steam (LCOS) and the cost of CO2 captured. These parameters ranged between 30.45 and 39.93 CAD/t of steam (23.61–30.95 USD/t) and between 44.39 and 107.05 CAD/t of CO2 (34.41–82.98 USD/t), respectively. In the particular case of the CLC unit that operated at atmospheric pressure, the results were 34.92 CAD/t of steam (27.07 USD/t) and 75.46 CAD/t of CO2 (58.50 USD/t), which highlighted the economic benefits resulting from the use of CLC technology under pressurized conditions.Finally, comparing the economic results obtained in this framework with those found in the literature using other Carbon Capture and Storage (CCS) technologies, it can be concluded that PCLC technology has the potential to be a competitive CO2 capture option in the future for use in industrial SAGD facilities.

Ilmenite oxidation kinetics for pressurized chemical looping combustion of natural gas

Pressurized Chemical Looping Combustion (PCLC) is a promising technology for mitigating CO2 emission from fossil fuel combustion. This work investigates the oxidation kinetics of a natural ilmenite ore to assess the PCLC of natural gas since process economics are largely dependent on the air reactor performance, and data at pressurized conditions are scarce. Thermogravimetric experiments were conducted varying the temperature from 800 to 1050 °C and the oxygen partial pressure from 0.2 to 1.7 bar at a total pressure of 8 bar. The total pressure was also varied from 1 to 16 bar at 900 °C with air. The total pressure was found to hinder the rate of the oxidation reaction when maintaining a constant O2 partial pressure and enhance the rate of the oxidation reaction when maintaining a constant O2 volume fraction, up to a threshold. The oxidation kinetics were modelled using a dual-step mechanism; random nucleation and growth followed by solid-state diffusion. The activation energy and reaction order with respect to oxygen were found to respectively be 16.6 kJ/mol and 0.34 for the nucleation and growth, and 48.7 kJ/mol and 1.26 for the solid-state diffusion regime. Finally, considering the design and economics of PCLC, it was shown that elevated O2 partial pressures negatively affect the ilmenite particle structural integrity subjecting it to fragmentation, whereas lower O2 partial pressures favour the migration of iron to the particle surface where inherent attrition loses would result in an enriched titanium ore as spent material.

Systems analysis of pressurized chemical looping combustion for SAGD applications

Chemical looping combustion is a promising option for the conversion of fossil fuels with inherent separation of CO2 and efficient use of energy. Nevertheless, key process challenges such as oxygen carrier reaction rates, costs, and environmental concerns still exist. We expected that these challenges would be overcome by operating chemical looping combustors at elevated pressures with ilmenite ore as the oxygen carrier. This work describes a design study that was performed to examine the potential for replacement of once-through steam generators (OTSGs) at steam assisted gravity drainage (SAGD) facilities for bitumen production with near zero CO2 emission pressurized chemical looping combustors. In order to minimize capital costs for the systems, design was constrained to ensure that all major components could be shop built. Heat and material balances for many different process configurations were generated using Aspen HYSYS process simulation software allowing for a range of options to be evaluated. We determined that operating pressures should be restricted to between 4 and 7 bar(g) to achieve very high steam generation efficiencies, while avoiding high power demand. Energy integration of pressurized chemical looping combustion with a Canada’s Oil Sands Innovation Alliance (COSIA) typical SAGD plant using Pinch analysis was performed resulting in a significant decrease in natural gas and boiler feed water (BFW) make-up requirements, while at the same time reducing CO2 emissions by over 95% down to 2.85 kg CO2/bbl bitumen. Having established that process performance is attractive, an economic study to establish the most economical design for pressurized chemical looping combustion for steam generation is now required.

Potential and limitations of power generation via chemical looping combustion of gaseous fuels

Pressurized chemical looping combustion (CLC) was studied with regard to its potential for power generation from gaseous fuels, such as natural gas. A process simulation model was set up for a simplified gas turbine combined cycle (GTCC) around a pressurized CLC reactor system and studied with respect to process parameters influencing electric efficiency. The process model is based on typical large scale GTCC arrangements with a gas turbine topping cycle and a heat recovery steam generator unit (HRSG). The results are compared to conventional GTCC process with similar arrangement and process parameters. It was found that the CLC process comes along with considerable technological limitations for the efficiency of the combined cycle: (i) turbine inlet temperature is limited by the oxygen carrier material, (ii) pressure drop of CLC AR path increases the required air compression work, and (iii) the requirement for low pressure steam for gas-sealing between air reactor and fuel reactor reduces the efficiency of the steam cycle. These effects limit the achievable net electric efficiency to values below 45%, which is similar to what could be reached with atmospheric pressure CLC in a conventional steam cycle power plant arrangement (e.g. Benson-type steam generator). The gas turbine inlet temperature (TIT) was identified as the greatest limitation to the process, the pressure ratio has to be reduced accordingly to maintain sufficient exhaust gas temperatures for the HRSG, which limits the efficiency potential of the gas turbine. As a conclusion, when it comes to power generation from gaseous fuels, these limitations will need to be resolved to make CLC technology competitive to conventional GTCC power plants combined with post combustion CO2 capture technologies.

Experimental evaluations of solid-fueled pressurized chemical looping combustion – The effects of pressure, solid fuel and iron-based oxygen carriers

Coal-based Pressurized Chemical Looping Combustion Combined Cycle (PCLC-CC) is the second generation of coal-fueled CLC plant, which possesses much higher plant efficiency and lower-CO2 capture cost compared to the first generation - Coal-based CLC combined solely with steam cycle. PCLC-CC has a similar plant configuration to the Pressurized Fluidized Bed Combined Cycle (PFBC), and is composed of a PCLC Island, gas turbine, Heat Recovery Steam Generator (HRSG) and steam cycle. In the fuel reactor of PCLC Island, the metal-based oxygen carrier (OC) supplies oxygen for coal combustion and in-situ CO2 capture. The air reactor of PCLC Island, where the OC is re-oxidized by air, serves as a combustion reactor to produce oxygen-depleted air of high temperature and high pressure to drive the gas turbine and the following steam cycle for large-scale power generation. This research provides an initial understanding of the complex reactions in the fuel reactor of the solid-fueled PCLC Island in the pressures range of 1–6bars. Experiments conducted in the TGA apparatus and the fixed- and fluidized-bed reactors demonstrated the effects of operational pressure, coal char reactivity and different iron-based OCs behavior on the performance of PCLC.

Fluidized bed reactor design study for pressurized chemical looping combustion of natural gas

Chemical looping combustion (CLC) is considered a viable option for efficient power production with inherent carbon sequestration through an unmixed combustion process. Pressurized reactor systems promise the potential for increased efficiency compared to atmospheric processes because gas turbine technology can be used in a combined cycle to achieve high electric efficiency, comparable to GTCC plants with up to 60%el. A design study was conducted to investigate the potential of pressurized chemical looping combustion of natural gas for power generation, as well as its limitations. Basic design calculations have been carried out based on fluidization engineering methods and a practical process configuration has been evaluated based on mass- and energy balances. It turns out that a high gas turbine single cycle efficiency (>14%el) can only be reached if the CLC air reactor is operated at high temperature levels (>1000°C). An optimal range of operating conditions was identified for operation of a pressurized CLC plant and design considerations for a Dual Circulating Fluidized Bed reactor system are reported. Accordingly, also the fluidization gas (steam) demand for loop seals turns out to be relatively increased for pressurized systems. Based on the results of the present work, a net efficiency of up to 40.34%el can be expected for power generation from pressurized CLC, which is low compared to standard GTCC technology with and without CO2 capture measures implemented. The reasons are the limitation of reactor/turbine inlet temperature, higher relative pressure drop of CLC systems compared to conventional turbine combustion chambers, and the required loop seal fluidization steam.

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.

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.

Comparative study between fluidized-bed and fixed-bed operation modes in pressurized chemical looping combustion of coal

The performance differences between the fluidized-bed and the fixed-bed operation modes were comparatively studied in a pressurized coal-fuelled chemical looping combustion (CLC) process. Pressurized thermogravimetric (TG) analysis was applied to elucidate the findings observed in pressurized fluidized-bed and fixed-bed experiments. The results showed that the performance of the reaction between coal and oxygen carrier was strongly dependent on the operating pressure and the employed operation mode. Elevated pressure lower than 0.5MPa promoted the conversion of coal gasification products to CO2 and H2O but pressure beyond 0.5MPa resulted in a negative effect for both operation modes. TG and characterization analyses confirmed that the negative effect of pressure had little to do with oxygen carrier property but was mainly ascribed to the suppression of coal pyrolysis at high pressure which leads to a decrease in char reactivity. Fixed-bed mode showed superiority over fluidized-bed mode in enhancing the CO2 concentration and the carbon conversion, but was unfavourable for the pore structural properties of oxygen carrier, and thereby undesirable for long-term stable operation.

Pressurized chemical-looping combustion of coal using an iron ore as oxygen carrier in a pilot-scale unit

Chemical looping combustion (CLC) of coal is established as a new concept for inherent CO2 separation with the advantages of low cost and high conversion efficiency. A pilot-scale unit consisting of two fluidized bed reactors was constructed and operated to study the performance of pressurized chemical-looping combustion (PCLC). Experiments were conducted using Shenhua bituminous coal as fuel and MAC iron ore as oxygen carrier at three different operation pressures (0.1, 0.3 and 0.5MPa) and the unit has been operated for totally 19h with steady coal-feeding, about 13.5h of which were realized under stable operation. The results showed that PCLC of coal demonstrated many remarkable advantages over the conventional CLC of coal, in terms of the improvement of carbon conversion in the fuel reactor, CO2 concentration of exhaust gas and combustion efficiency. At the operating pressure of 0.5MPa, the CO2 concentration, carbon conversion and combustion efficiency reached very high values of 97.2, 84.7 and 95.5% respectively. The potential of low-cost iron ore as oxygen carrier for commercial coal-fueled CLC unit was also examined. The loss of oxygen carrier due to fine particles leaving the reactors increased with the increase of operation pressure and no agglomeration of oxygen carrier particles occurred during these tests. The oxygen carrier used in PCLC tests were also characterized by SEM-EDX and BET analysis to further emphasis on the effects of operation pressures and runtime.

Pressurized chemical-looping combustion of coal with an iron ore-based oxygen carrier

Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO2. Most of the previous investigations on CLC of solid fuels were conducted under atmospheric pressure. A pressurized CLC combined cycle (PCLC-CC) system is proposed as a promising coal combustion technology with potential higher system efficiency, higher fuel conversion, and lower cost for CO2 sequestration. In this study pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor. CVRD iron ore particles were exposed alternately to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87.2% steam/N2 mixture and oxidation with 5% O2 in N2 at 970 °C. The operating pressure was varied between 0.1 MPa and 0.6 MPa. First, control experiments of steam coal gasification over quartz sand were performed. H2 and CO2 are the major components of the gasification products, and the operating pressure influences the gas composition. Higher concentrations of CO2 and lower fractions of CO, CH4, and H2 during the reduction process with CVRD iron ore was achieved under higher pressures. The effects of pressure on the coal gasification rate in the presence of the oxygen carrier were different for pyrolysis and char gasification. The pressurized condition suppresses the initial coal pyrolysis process while it also enhances coal char gasification and reduction with iron ore in steam, and thus improves the overall reaction rate of CLC. The oxidation rates and variation of oxygen carrier conversion are higher at elevated pressures reflecting higher reduction level in the previous reduction period. Scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM–EDX) analyses show that particles become porous after experiments but maintain structure and size after several cycles. Agglomeration was not observed in this study. An EDX analysis demonstrates that there is very little coal ash deposited on the oxygen carrier particles but no appreciable crystalline phases change as verified by X-ray diffraction (XRD) analysis. Overall, the limited pressurized CLC experiments carried out in the present work suggest that PCLC of coal is promising and further investigations are necessary.

Effect of Pressure on the Behavior of Copper-, Iron-, and Nickel-Based Oxygen Carriers for Chemical-Looping Combustion

The combustion process integrated by coal gasification and chemical-looping combustion (CLC) could be used in power plants with a low energy penalty for CO2 capture. This work analyzes the main characteristics related to the CLC process necessary to use the syngas obtained in an integrated gasification combined cycle (IGCC) power plant. The kinetics of reduction with H2 and CO and oxidation with O2 of three high-reactivity oxygen carriers used in the CLC system have been determined in a thermogravimetric analyzer at atmospheric pressure. The iron- and nickel-based oxygen carriers were prepared by freeze-granulation, and the copper-based oxygen carrier was prepared by impregnation. The changing grain size model (CGSM) was used for the kinetic determination, assuming spherical grains for the freeze-granulated particles containing iron and nickel and a platelike geometry for the reacting surface of the copper-based impregnated particles. The dependence of the reaction rates on temperature was low, with the activation energy values varying from 14 to 33 kJ mol-1 for the reduction and 7 to 15 kJ mol-1 for the oxidation. The reaction order depended on the reacting gas and oxygen carrier, with values ranging from 0.25 to 1. However, an increase in the operating pressure for the IGCC + CLC system increases the thermal efficiency of the process, and the CO2 is recovered as a high pressure gas, decreasing the energy demand for further compression. The effect of pressure on the behavior of the oxygen carriers has been analyzed in a pressurized thermogravimetric analyzer at 1073 K and pressures up to 30 atm. It has been found that an increase in total pressure has a negative effect on the reaction rates of all the oxygen carriers. Moreover, the use of the CGSM with the kinetic parameters obtained at atmospheric pressure predicted higher reaction rates than those experimentally obtained at higher pressures, and therefore, the kinetic parameters necessary to design pressurized CLC plants must be determined at the operating pressure.