Comparison of Different Concepts for Solar Reforming of Natural Gas

Solar redox reforming of methane is investigated and compared to heterogeneously catalyzed solar reforming. An analysis of potential redox materials based on material prices and thermodynamic calculations yields iron oxide and ceria as promising candidates for redox reforming. Steam and dry methane reforming were studied for both materials, by process simulations in Aspen Custom Modeler. Models for heterogeneously catalyzed steam and dry methane reforming were used as reference processes. In total, six models were developed and used for the simulations. The impact of heat recovery between the redox material in the oxidized and the reduced state is studied. With a solar-to-fuel efficiency of 49.7 % at an oxidation temperature of 823 K, a reduction temperature of 1190 K and a corresponding pressure of 30 atm, redox steam methane reforming with iron oxide is the only redox cycle that is competitive with the reference process, in terms of efficiency. Redox steam methane reforming with iron oxide and redox dry methane reforming with ceria can produce high purity H2 and CO, respectively. No carbon formation was observed at those points of operation that lead to high solar-to-fuel efficiencie

Oxygen Crossover in Solid-Solid Heat Exchangers for Solar Water and Carbon Dioxide Splitting: A Thermodynamic Analysis

In solar thermochemical redox cycles for H2O/CO2-splitting, a large portion of the overall energy demand of the system is associated with heating the redox material from the oxidation temperature to the reduction temperature. Hence, an important measure to improve the efficiency is recuperation of sensible heat stored in the redox material. A solid-solid heat exchanger can be subject to undesirable oxygen crossover, which decreases the oxygen uptake capacity of the redox material and consequently the system efficiency. We investigate the extent of this crossover in ceria based cycles, to identify, under which conditions a heat exchanger that allows oxygen crossover can improve the system efficiency. In a thermodynamic analysis we calculate the amount of transferred oxygen as a function of the heat exchanger efficiency and show the system efficiency of such a concept. A second law analysis is applied to the model to check the feasibility of calculated points of operation. For the investigated parameter set the heat exchanger design improves the system efficiency by a factor of up to 2.1

Performance Assessment of a Heat Recovery System for Monolithic Receiver-Reactors

The most advanced solar thermochemical cycles in terms of demonstrated reactor efficiencies are based on temperature swing operated receiver-reactors with open porous ceria foams as a redox material. The demonstrated efficiencies are encouraging but especially for cycles based on ceria as the redox material, studies have pointed out the importance of high solid heat recovery rates to reach competitive process efficiencies. Different concepts for solid heat recovery have been proposed mainly for other types of reactors, and demonstration campaigns have shown first advances. Still, solid heat recovery remains an unsolved challenge. In this study, chances and limitations for solid heat recovery using a thermal storage unit with gas as heat transfer fluid are assessed. A numerical model for the reactor is presented and used to analyze the performance of a storage unit coupled to the reactor. The results show that such a concept could decrease the solar energy demand by up to 40% and should be further investigated

Study of a new receiver-reactor cavity system with multiple mobile redox units for solar thermochemical water splitting

Solar thermochemical redox cycles could be a path to efficient, large-scale renewable hydrogen production. A new receiver-reactor concept is presented that combines characteristics of the most successful receiver-reactor systems to date, with features of concepts showing the highest efficiency potential. The key features of the system are movable reactive structures in combination with linear transportation systems and dedicated oxidation reactors. The application of several of these units allows the continuous operation of the receiver-reactor and permits the implementation of a solid–solid heat recovery system. Both of these characteristics are important to increase the system efficiency beyond the current state of technology. A numerical model is developed to simulate a basic cubic design version of the new concept and to analyse its performance. Parameter variations are studied amongst others for different cavity sizes, solar concentration factors and numbers of movable reactive structures. By avoiding the cyclic heating of the inert reactor vessel, the model predicts high efficiency values above 14% even for non-optimized designs. Furthermore, the basic concept of the heat recovery system is modelled with heat recovery rates of up to 20% in its most simple implementation. The new concept has the features required for highly performant systems and opens up a large parameter space for reactor design and operation optimization

Study of a new receiver-reactor cavity system with multiple mobile redox units for solar thermochemical water splitting

Solar thermochemical redox cycles could be a path to efficient, large-scale renewable hydrogen production. A new receiver-reactor concept is presented that combines characteristics of the most successful receiver-reactor systems to date, with features of concepts showing the highest efficiency potential. The key features of the system are movable reactive structures in combination with linear transportation systems and dedicated oxidation reactors. The application of several of these units allows the continuous operation of the receiver-reactor and permits the implementation of a solid–solid heat recovery system. Both of these characteristics are important to increase the system efficiency beyond the current state of technology. A numerical model is developed to simulate a basic cubic design version of the new concept and to analyse its performance. Parameter variations are studied amongst others for different cavity sizes, solar concentration factors and numbers of movable reactive structures. By avoiding the cyclic heating of the inert reactor vessel, the model predicts high efficiency values above 14% even for non-optimized designs. Furthermore, the basic concept of the heat recovery system is modelled with heat recovery rates of up to 20% in its most simple implementation. The new concept has the features required for highly performant systems and opens up a large parameter space for reactor design and operation optimization

Efficiency assessment of solar redox reforming in comparison to conventional reforming

Solar redox reforming is a process that uses solar radiation to drive the production of syngas from natural gas. This approach caught attention in recent years, because of substantially lower reduction temperatures compared to other redox cycles. However, a detailed and profound comparison to conventional solar reforming has yet to be performed. We investigate a two-step redox cycle with iron oxide and ceria as candidates for redox materials. Process simulations were performed to study both steam and dry methane reforming. Conventional solar reforming of methane without a redox cycle, i.e. on an established catalyst was used as reference. We found the highest efficiency of a redox cycle to be that of steam methane reforming with iron oxide. Here the solar-to-fuel efficiency is 43.5% at an oxidation temperature of 873 K, a reduction temperature of 1190 K, a pressure of 3 MPa and a solar heat flux of 1000 kW/m2. In terms of efficiency, this process appears to be competitive with the reference process. In addition, production of high purity H2 or CO is a benefit, which redox reforming has over the conventional approach