Cu-based Oxygen Carrier Research Articles

In situ gasification and CO 2 separation using chemical looping with a Cu-based oxygen carrier: Performance with bituminous coals

This paper is concerned with the chemical looping combustion of coal in a technique whereby the fuel is gasified in situ using CO 2 in the presence of a batch of supported copper oxide (the “oxygen carrier”) in a single reactor. As the metal oxide becomes depleted, the feed of fuel is discontinued, the inventory of fuel is reduced by further gasification and then the contents are re-oxidised by the admission of air to the reactor, to begin the cycle again. A catalyst support, impregnated with a saturated solution of copper and aluminium nitrates, acted as a durable oxygen carrier over numerous cycles of reduction and oxidation, using air as the oxidant. Two bituminous coals (Taldinskaya, Russia, and Illinois No. 5, USA) were investigated and compared with a lignite (Hambach, Germany). The lignite was highly reactive and was gasified completely by 15 mol% CO 2 in N 2 at 1203 K and 1 bar, so that there was no build up of char in the bed. The bituminous coals produced chars much less reactive than the lignite char, so that there was a steady accumulation of char in the bed with number of cycles, with the degree of accumulation being dependent on the reactivity of the char. Since the kinetics of gasification by CO 2 of the chars from either bituminous coal were slow, their rates were controlled by intrinsic chemical kinetics and were not affected by the ability of the oxygen carrier to alter the rates of external mass transfer when gasification is rapid. However, it is likely that rates of gasification in the presence of the carrier are still larger than in its absence, owing to the overall lower [CO] present in the bulk of the fluidised bed during chemical looping. At the temperature used, the carrier was cycling between Cu and Cu 2O, since CuO is only stable if the partial pressure of O 2 exceeds 0.03 bar at 1203 K. The CuO decomposes to Cu 2O and O 2 relatively rapidly at these temperatures, once the oxygen concentration is effectively zero. It was impossible to ascertain in our experiments whether the oxygen so generated, after the switching of the air for nitrogen before the start of the succeeding cycle of gasification, made any substantial difference to the reactivity of the char present in the bed. The rate of oxidation of the carrier was found to be much more rapid than the rate of oxidation of the inventory of char. This allows a preferential oxidation of the carrier and most likely accounts for why progressively less CO and CO 2 is produced during successive cycles with short periods of oxidation: the increasingly reduced carrier reacts more rapidly than the char. There was no obvious impact from the sulphur contained in the fuels, but longer-term testing is needed. No agglomeration between the carrier particles and the ash was observed, despite the high temperatures during oxidation.

Modeling of the chemical-looping combustion of methane using a Cu-based oxygen-carrier

A mathematical model for a bubbling fluidized bed has been developed to simulate the performance of the fuel-reactor in chemical-looping combustion (CLC) systems. This model considers both the fluid dynamic of the fluidized bed and freeboard and the kinetics of reduction of the oxygen-carrier, here CuO impregnated on alumina. The main outputs of the model are the conversion of the carrier and the gas composition at the reactor exit, the axial profiles of gas concentrations and the fluid dynamical structure of the reactor. The model was validated using measurements when burning CH 4 in a 10 kW th prototype using a Cu-based oxygen-carrier. The influence of the circulation rate of solids, the load of fuel gas, the reactor temperature and size of the oxygen-carrier particles were analyzed. Combustion efficiencies predicted by the model showed a good agreement with measurements. Having validated the model, the implications for designing and optimizing a fuel-reactor were as follows. The inventory of solids for a high conversion of the fuel was sensitive to the reactor’s temperature, the solids’ circulation rate and the extent to which the solids entering to the reactor had been regenerated. The optimal ratio of oxygen-carrier to fuel was found to be 1.7–4 for the Cu-based oxygen-carrier used here. In this range, the inventory of solids to obtain a combustion efficiency of 99.9% at 1073 K was less than 130 kg/MW th. In addition, the model’s results were very sensitive to the resistance to gas diffusing between the emulsion and bubble phases in the bed, to the decay of solids’ concentration in the freeboard and to the efficiency contact between gas and solids in the freeboard. Thus, a simplified model, ignoring any restriction to gas and solids contacting each other, will under-predict the inventory of solids by a factor of 2–10.

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of light hydrocarbons

Chemical-Looping Combustion (CLC) is an emerging technology for CO 2 capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of light hydrocarbons. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH 4 and light hydrocarbons (LHC) (C 2H 6 and C 3H 8) as fuel. The effect on combustion efficiency of the fuel reactor temperature, solid circulation flow rate and gas composition was studied in a continuous CLC plant (500 W th). Full combustions were reached at 1073 and 1153 K working at oxygen to fuel ratios, ϕ higher than 1.5 and 1.2 respectively. Unburnt hydrocarbons were never detected at any experimental conditions at the fuel reactor outlet. Carbon formation can be avoided working at 1153 K or at ϕ values higher than 1.5 at 1073 K. After 30 h of continuous operation, the oxygen carrier exhibited an adequate behavior regarding attrition and agglomeration. It can be concluded that no special measures should be taken in a CLC process with Cu-based OC with respect to the presence of LHC in the fuel gas.

In situ gasification of a lignite coal and CO2 separation using chemical looping with a Cu-based oxygen carrier

Abstract Chemical looping combustion (CLC) has the inherent property of separating CO 2 from flue gases. This paper is concerned with the application of chemical looping to the combustion of a solid fossil fuel (a lignite and its char) in a technique whereby the fuel is gasified in situ using CO 2 in the presence of a batch of supported copper oxide (the “oxygen carrier”) in a single reactor. As the metal oxide becomes depleted, the feed of fuel is discontinued, the inventory of fuel is reduced by further gasification and then the contents are re-oxidised by the admission of air to the reactor, to begin the cycle again. The choice of oxides is restricted because it requires an oxide which is exothermic during reduction to balance the endothermic gasification reactions. Copper has such oxides, but a key question is whether or not it can withstand temperatures at which gasification rates are significant (∼1173 K), particularly from the point of view of avoiding sintering and deactivation of the carrier in its reduced form. It was found that an impregnated carrier, made by impregnating a θ -alumina catalyst support (BET area 157 m 2 /g) with a saturated solution of copper and aluminium nitrates, acted as a durable carrier over 20 cycles of reduction and oxidation, using both Hambach lignite coal, and its char, and with air as the oxidising agent. During the course of the experiments, the BET surface area of the support fell from ∼60 m 2 /g, just after preparation, to around 6 m 2 /g after 20 cycles. However, this fall did not appear to affect the overall capacity of the oxygen carrier to react with fuels and its effect on the kinetics of the reaction with CO did not influence the outcome of the experiments, since the overall performance of the looping scheme is dominated by the much slower kinetics of the gasification reaction. The apparent kinetics of the gasification are faster in the presence of the looping agent: this is because the bulk concentration of CO in the presence of the looping agent is lower, and partly because the destruction of CO in the vicinity of a gasifying particle enhances the rate of removal of CO by mass transfer (and increases the local concentration of CO 2 ). There was little evidence to suggest a direct reaction between carbonaceous and carrier solids, other than via a gaseous intermediate. However, the observation of finite rates of conversion in a bed of active carrier, fluidised by nitrogen, is a scientific curiosity, which we have not been able to explain satisfactorily. At 1173 K, as used here, rates of gasification of Hambach lignite, and its char, are significant. The CuO in the carrier decomposes at 1173 K to produce gas-phase O 2 and Cu 2 O: both can react with CO produced by gasification, whilst the O 2 can react directly with the char.

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

Chemical-Looping Combustion (CLC) is an emerging technology for CO 2 capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. For its use with coal as fuel in power plants, a process integrated by coal gasification and CLC would have important advantages for CO 2 capture. This paper presents the combustion results obtained with a Cu-based oxygen carrier in a continuous operation CLC plant (500 Wth) using syngas as fuel. For comparison purposes pure H 2 and CO were also used. Tests were performed at two temperatures (1073 and 1153 K), different solid circulation rates and power inputs. Full syngas combustion was reached at 1073 K working at f higher than 1.5. The syngas composition had small effect on the combustion efficiency. This result seems to indicate that the water gas shift reaction acts as an intermediate step in the global combustion reaction of the syngas. The results obtained after 40 h of operation showed that the copper-based oxygen carrier prepared by impregnation could be used in a CLC plant for syngas combustion without operational problems such as carbon deposition, attrition, or agglomeration.

Using chemical-looping with oxygen uncoupling (CLOU) for combustion of six different solid fuels

Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn solid fuels in gas-phase oxygen without the need for an energy intensive air separation unit. This paper presents batch laboratory fluidized bed CLOU tests where six different solid fuels are used with a Cu-based oxygen carrier. The results show that CLOU results in a factor 3 to 15 faster fuel conversions than conversional chemical-looping combustion.

Modeling of the chemical-looping combustion of methane using a Cu-based oxygen carrier

A mathematical model for a bubbling fluidized bed has been developed to simulate the performance of the fuel reactor (FR) in chemical looping combustion systems. This model considers both the hydrodynamic of the fluidized bed (dense bed and freeboard) and the reduction kinetics of the oxygen carrier (OC). The main outputs of the model are solid conversion and gas composition at the reactor exit, the axial profiles of gas composition, and the fluidynamics structure of the reactor. The FR model was validated using the experimental data obtained from the CH4 combustion in a 10 kWth prototype built at the “Instituto de Carboquímica” (ICB-CSIC, Spain) using a Cu-based OC. The influence of OC circulation rate, fuel gas load, reactor temperature and OC particle size were analyzed. Combustion efficiency predicted by the model showed a good agreement with experimental results. Once the FR model was validated, important consequences for the design and optimization of the FR in a CLC system were obtained.

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

Chemical-looping combustion (CLC) has the inherent property of separating the product CO 2 from flue gases. Instead of air, it uses an oxygen carrier, usually in the form of a metal oxide, to provide oxygen for combustion. This paper focuses on the development and performance of a suitable Cu-based oxygen carrier for burning solid fuels using CLC. Carriers were made from CuO and Al 2O 3 (as a support) in three different ways: mechanical mixing, wet impregnation, and co-precipitation. The reactivity of these solids was assessed by measuring their ability to oxidize CO, when in a hot bed of sand fluidized by a mixture of CO and N 2. After that, the Cu in the carrier was oxidized back to CuO by fluidizing the hot bed with air. These oxygen carriers were tested over many such cycles of reduction and oxidation. This work confirms that supporting CuO on Al 2O 3 enhances the ability of the resulting particles to withstand mechanical and thermal stresses in a fluidized bed. Also, only co-precipitation produces particles that have a high loading of copper and do not agglomerate at 800–900 °C. The performance of co-precipitated particles of CuO/Al 2O 3 at oxidizing CO to CO 2 was significantly affected by the pH of the solution in which precipitation occurred: a high pH (9.7) gave particles that reacted completely and rapidly. After 18 cycles, such a co-precipitated carrier with 82.5 wt% CuO yielded all its oxygen when oxidizing CO. X-ray analysis showed that when heated, CuO reacted with Al 2O 3 to form CuAl 2O 4, which was fully reducible, so CuO experienced no loss in extent of reaction after forming this mixed oxide. An increase in operating temperature from 800 to 900 °C led to the CuO providing slightly less oxygen; this was because a little of the CuO decomposed to Cu 2O between its reduction and oxidation, when the bed was fluidized by pure N 2.

Impregnated CuO/Al2O3 Oxygen Carriers for Chemical-Looping Combustion: Avoiding Fluidized Bed Agglomeration

Chemical-looping combustion (CLC) is a combustion technology with inherent separation of the greenhouse gas CO2 that involves the use of an oxygen carrier, which transfers oxygen from air to the fuel avoiding the direct contact between them. An oxygen carrier in a CLC power plant must show high reaction rates and sufficient durability in successive cycle reactions. Furthermore, the oxygen carrier particles must not agglomerate. To produce materials with these characteristics, different Cu-based oxygen carriers with a range of CuO content between 10 and 26 wt % were prepared by impregnation using alumina as support. The particles were calcined at different temperatures in the range of 550−950 °C. The oxygen carriers were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray, and thermogravimetry. The samples were evaluated in a fluidized bed facility in order to determine their behavior in multicycle reduction−oxidation tests. A number of 100 cycles was done when the carrier did not agglomerate. The effects of metal content, calcination temperature, and method of preparation on the oxygen carriers reactivity, gas product distribution, attrition rate, and agglomeration were analyzed for the different oxygen carriers. It was observed that the CuO content in the oxygen carrier, the calcination temperature used in the preparation, and the conversion reached by the oxygen carrier during the reduction period affected the agglomeration process. In this work, the preparation conditions to produce Cu-based oxygen carriers with high reactivities, and small attrition rates were found. Moreover, the agglomeration of these oxygen carriers in the fluidized reactor, which is the main reason adduced in the literature to reject the Cu-based oxygen carriers for the CLC process, was avoided.

Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier Prepared by Impregnation for Chemical-Looping Combustion

The kinetics of reduction with CH4, H2, and CO and oxidation with O2 of a Cu-based oxygen carrier prepared by impregnation on alumina to be used in a chemical-looping combustion (CLC) system have been determined in a thermogravimetric analyzer. The oxygen carrier exhibited high reactivity in both reduction and oxidation with times for complete conversion lower than 40 s at 1073 K and 5−70 vol % of the fuel gas and 5−21 vol % of O2. The analysis of the sample carried out by scanning electron microscopy using energy-dispersive X-ray and chemisorption showed that the CuO was well dispersed in the porous surface of the alumina matrix and a uniform thin layer on the porous surface was considered. The shrinking-core model for platelike geometry of the reacting surface was used for the kinetic determination, in which the chemical reaction controlled the global reaction rate. No effect of the gas products (H2O and CO2) on the reaction rate was detected. The reaction order depended on the fuel gas, and values of 0...

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

In a chemical-looping combustion (CLC) process, gas (natural gas, syngas, etc.) is burnt in two reactors. In the first one, a metallic oxide that is used as oxygen source is reduced by the feeding gas to a lower oxidation state, being CO 2 and steam the reaction products. In the second reactor, the reduced solid is regenerated with air to the fresh oxide, and the process can be repeated for many successive cycles. CO 2 can be easily recovered from the outlet gas coming from the first reactor by simple steam condensation. Consequently, CLC is a clean process for the combustion of carbon containing fuels preventing the CO 2 emissions to the atmosphere. The main drawback of the overall process is that the carriers are subjected to strong chemical and thermal stresses in every cycle and the performance and mechanical strength can decay down to unacceptable levels after enough number of cycles in use. In this paper the behaviour of CuO as an oxygen carrier for a CLC process has been analysed in a thermogravimetric analyser. The effects of carrier composition and preparation method used have been investigated to develop Cu-based carriers exhibiting high reduction and oxidation rates without substantial changes in the chemical, structural and mechanical properties for a high number of oxidation–reduction cycles. It has been observed that the carriers prepared by mechanical mixing or by coprecipitation showed an excellent chemical stability in multicycle tests in thermobalance, however, the mechanical properties of these carriers were highly degraded to unacceptable levels. On the other hand, the carriers prepared by impregnation exhibited excellent chemical stability without substantial decay of the mechanical strength in multicycle testing. These results suggest that copper based carriers prepared by impregnation are good candidates for CLC process.

04/00428 Clean coal fluidized-bed technology in Poland: Nowak, W. Applied Energy, 2003, 74, (3–4), 405–413

A mathematical model for a bubbling fluidized bed has been developed to simulate the performance of the fuel reactor (FR) in chemical looping combustion systems. This model considers both the hydrodynamic of the fluidized bed (dense bed and freeboard) and the reduction kinetics of the oxygen carrier (OC). The main outputs of the model are solid conversion and gas composition at the reactor exit, the axial profiles of gas composition, and the fluidynamics structure of the reactor. The FR model was validated using the experimental data obtained from the CH4 combustion in a 10 kWth prototype built at the “Instituto de Carboquímica” (ICB-CSIC, Spain) using a Cu-based OC. The influence of OC circulation rate, fuel gas load, reactor temperature and OC particle size were analyzed. Combustion efficiency predicted by the model showed a good agreement with experimental results. Once the FR model was validated, important consequences for the design and optimization of the FR in a CLC system were obtained.