A strategy for optimizing efficiencies of solar thermochemical fuel production based on nonstoichiometric oxides

Thermochemical cycling (TC) is a promising means of harvesting solar energy. Two-step TC with a redox active metal oxide (e.g., ceria, a benchmark material) serving as a reaction intermediate for dissociating steam or carbon dioxide, has attracted much attention recently. However, further improving the energy conversion efficiency of this process remains a major challenge. In this work, we propose an innovative modification to the heat recovery approach as a means of enhancing efficiency. Specifically, a variable amount of oxidant (e.g., steam) is injected to actively assist the cooling of thermally reduced metal oxide, achieving both in-situ heat recovery and potentially faster cooling rates than conventional approaches. Our analysis, based on a thermochemical heat engine model, shows that the solar-to-fuel efficiency using ceria under typical solar TC operating conditions could be significantly improved (the efficiency of the new strategy can reach 24.36% without further gas or solid heat recovery when the reduction temperature is 1600 °C) whilst temperature swing be reduced simultaneously compared with conventional methods. Exergy efficiency is also analyzed for thermochemical splitting of water and CO₂. This new strategy contributes significantly to the simplification of solar reactor design and to potential enhancement in both fuel productivity and energy conversion efficiency on a temporal basis.     

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H₂. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H₂ via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).     

Enhanced coal gasification heated by unmixed combustion integrated with an hybrid system of SOFC/GT

For clean utilization of coal, enhanced gasification by in situ CO₂ capture has the advantage that hydrogen production efficiency is increased while no energy is required for CO₂ separation. The unmixed fuel process uses a sorbent material as CO₂ carrier and consists of three coupled reactors: a coal gasifier where CO₂ is captured generating a H₂-rich gas that can be utilized in fuel cells, a sorbent regenerator where CO₂ is released by sorbent calcination and it is ready for capture and a reactor to oxidize the oxygen transfer material which produces a high temperature/pressure vitiated air. This technology has the potential to eliminate the need for the air separation unit using an oxygen transfer material. Reactors' temperatures range from 750 °C to 1550 °C and the process operates at pressure around 7.0 bar. This paper presents a global thermodynamic model of the fuel processing concept for hydrogen production and CO₂ capture combined with fuel and residual heat usage. Hydrogen is directly fed to a solid oxide fuel cell and exhaust streams are used in a gas turbine expander and in a heat recovery steam generator. This paper analyzes the influence of steam to carbon ratio in gasifier and regeneration reactor, pressure of the system, temperature for oxygen transfer material oxidation, purge percentage in calciner, average sorbent activity and oxidant utilization in fuel cell. Electrical efficiency up to 73% is reached under optimal conditions and CO₂ capture efficiencies near 96% ensure a good performance for GHG's climate change mitigation targets.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.     

Reactivity investigation on biomass chemical looping conversion for syngas production

Chemical looping gasification (CLG) is regarded as an innovative and promising technology for producing syngas. In this work, CLG of straw was conducted in a fixed bed reactor with Fe₂O₃ as the oxygen carrier, whose results led to conclusions that Fe₂O₃, the oxygen carrier, proved advantageous to the secondary gasification reaction and the formation of CO and CO₂. It was also found that CO was further oxidized to CO₂ at high Fe₂O₃/C molar ratio, which resulted in a decreased gasification efficiency and low heat value of syngas. Therefore, a conclusion was drawn that the most optimized Fe₂O₃/C molar ratio was 0.2. In addition, the alkali metals in the biomass evaporated as chlorine salts into gas phase and retained as alkali metal oxide at high temperature, resulting in coking, slagging and heating surface corrosion. In the mean time, the oxygen carrier mainly converted to Fe and sintering phenomenon was serious at high temperature despite the fact that high temperature promoted gas yield, carbon conversion efficiency and gasification efficiency. Therefore, the most optimized temperature was set to 800 °C in order to maximize gas yield and gasification efficiency.     

Hydrogen production using fusion energy and thermochemical cycles

Thermochemical cycles for the production of synthetic fuels would be especially suited for operation in conjunction with controlled thermonuclear fusion reactors because of very high temperature energy which such reactors could supply. Furthermore, fusion energy when developed is considered to be an inexhaustible supply of energy. Several high-temperature, two step thermochemical cycles for the production of hydrogen are examined. A thermodynamic analysis of the Fe₃O₄-FeO, CrCl₃-CrCl₂, and UCl₄-UCl₃ pairs reveals the feasibility of the processes. A more detailed process analysis is given for the Fe₃O₄-FeO system using steam as the heat transfer medium for decomposing the higher valent metal oxide for oxygen production, and hydrolyzing the lower oxide for hydrogen production. The steam could be heated to high temperatures by refractory materials absorbing the 14 MeV neutrons in the blanket region of a fusion reactor. Process heat transfer and recovery could be accomplished by regenerative reactors. Proposed operating conditions, the energy balance, and the energy efficiency of water decomposition process are presented. With a fusion blanket temperature of 2500 K, thermal efficiencies for hydrogen production (HHV) of 74.4% may be obtained.     

Thermodynamic analysis of ethanol processors for fuel cell applications

A thermodynamic analysis of hydrogen production from ethanol has been carried out with respect to solid polymer fuel cell applications. Ethanol processors incorporating either a steam reformer or a partial oxidation reactor connected to water gas shift and CO oxidation reactors were considered and the effect of operating parameters on hydrogen yield has been examined. Employment of feeds with high H₂O/EtOH ratio results in reduced energy efficiency of the system. When hydrogen, non-converted in the fuel cell, is used to supply heat in the steam reformer, the effective hydrogen yield is essentially independent of the temperature of the reformer and the water gas shift reactor. Optimal operating conditions of partial oxidation processors have been determined assuming an upper limit for the preheat temperature of the feed. Results are discussed along with other practical considerations in view of actual applications.     

A review of catalytic sulfur (VI) oxide decomposition experiments

Sulfur (VI) oxide, also known as sulfur trioxide or SO₃, decomposition is an oxygen-generating decomposition reaction that proceeds in the gaseous system SO₃/SO₂/O₂/H₂O at temperatures above 500 K. Maximum decomposition yield of SO₃ to SO₂ and O₂ is best achieved at temperatures of over 1000 K with an appropriate catalyst. According to the literature, noble metals and some transition metal oxides are highly effective catalysts in the laboratory environment. Sulfur (VI) oxide decomposition is the energetic and temperature limiting step of several endothermic hydrogen generating chemical process heat plants. In particular, the General Atomics Sulfur Iodine cycle and the Westinghouse Hybrid Sulfur cycle are candidates for thermal coupling to a high temperature nuclear reactor. Therefore the sulfur (VI) oxide decomposition reaction is a potential heat sink for a high temperature nuclear reactor. Thus, optimization of catalyst selection is required, both for operational efficiency and safety. In this paper, reaction mechanisms and catalyst composition for sulfur (VI) oxide decomposition are reviewed. Chemical kinetics data from previous sulfur (VI) oxide decomposition experiments are extracted from archival journal papers or other open literature. The available experimental database suggests that Pt-based catalysts have the highest stable activity among the noble metals and Fe₂O₃-based catalysts have the highest stable activity among the transition metal oxides. The decomposition temperature of the corresponding metal sulfate dictates the catalytic activity of a given transition metal oxide.     

A rotating fluidized bed reactor for rapid temperature ramping in two-step thermochemical water splitting

Two-step thermochemical water splitting (TWS) is a promising carbon-free/low-carbon technology for producing hydrogen in a mass production scale, in which water is dissociated in the presence of metal oxide-based catalysts via redox cycles driven by thermal energy. While active research is underway to develop high-performance metal oxide catalysts, less attention has been paid to reactor design and relevant system analysis, which are essential for constructing an actual system. The gas and solid flow configuration is one of the key design parameters in reactor design that determines thermodynamic and kinetic characteristics of the entire two-step TWS system. In this study, we propose a rotating fluidized bed reactor design wherein the rotating current flow configuration allows a much larger relative velocity between sweep gas and redox particles compared to conventional flow configurations. The rotating current flow configuration significantly improves the temperature ramp rate of redox particles via enhanced heat transfer between particles and sweep gas. Through thermodynamic and kinetics analysis of the reactor system, we show that the large temperature ramp rate of the proposed reactor results in a considerable improvement in the hydrogen yield per hour. This work adds a new dimension to the reactor design for two-step TWS.     

Thermochemical H2 production via solar driven hybrid SrO/SrSO4 water splitting cycle

This article reports the thermodynamic efficiency analysis of the strontium oxide – strontium sulfate (SrO-SrS) water splitting cycle by applying the principles of the second law of thermodynamics and by utilizing the commercially available HSC Chemistry software. Initially, the thermodynamic equilibrium compositions allied with a) the thermal reduction of SrSO₄, b) H₂ production via water splitting reaction (through SrO re-oxidation) are recognized. Moreover, the temperatures desirable for performing the thermal reduction and the water splitting steps are determined. The consequence of the molar flow rate of Ar on the thermal reduction of SrSO₄ is also examined in detail. The effect of the thermal reduction and water splitting temperatures on the total solar energy input mandatory to run the cycle, re-radiation shortfalls from the cycle, heat energy emitted by the coolers and the water splitting reactor, and the cycle and the solar-to-fuel energy conversion efficiency (with heat recuperation) is scrutinized in detail. The attained outcomes specify that the cycle and the solar-to-fuel energy conversion efficiency up to 18.9 and 22.8% can be accomplished if the thermal reduction and the water splitting steps are conducted at 2380 and 1400 K (with 30% heat recuperation).     

Experimental investigation on heat recovery from condensation of thermal power plant exhaust steam by a CO2 vapor compression cycle

In this paper, a method that utilizes CO₂ vapor compression thermodynamic cycle to recover low‐temperature heat from exhausted water steam of fossil fuel thermal power plants is reported. Experimental investigation was carried out to study the characteristics of low‐temperature heat recovery by liquid CO₂ evaporation process from vacuum exhausted steam condensation occurring at the turbine exit. Furthermore, measured heat recovery performances over one whole year are presented and discussed. Experimental results show that the present heat recovery process by CO₂ vapor compression cycle is able to operate stably. The yearly averaged water temperature at the CO₂ condenser outlet was measured at 87.5 °C with a COP value above 5.0. This high energy efficiency ratio is found to be mainly due to two factors: the transcritical CO₂ vapor compression and steam condensation phase change occurring on the CO₂ evaporator. The findings from this paper provide helpful guidelines for low‐temperature heat recovery system design and improving fossil fuel utilization efficiency. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling a counter-current moving bed for fuel and steam reactors in the TRCL process

A mathematical model for the moving bed is developed to simulate the fuel and steam reactor in the TRCL (Three-Reactor Chemical-Looping) process. An ideal plug flow of the solid and gas is assumed in modeling the fuel and steam reactor in the TRCL process. The model considered the mass, heat balances, equilibrium, physical properties, such as the heat capacity and viscosity, and kinetics. From this model, the temperature, gas conversion and solid conversion profiles can be predicted for fuel and steam reactors. The oxygen carrier inventory (the mass of the oxygen carrier) in the fuel and steam reactor was calculated with variation of the solid inlet temperature, solid conversion, Fe₂O₃ content and steam feed rate. The temperature of the oxygen carrier to the reactor was the most sensitive parameter for determining the required inventory of the oxygen carrier. An increase in the solid inlet temperature was predicted to decrease the required inventory of the oxygen carrier. In the steam reactor, a solid inlet temperature increase over 1150 K will cause an increase in the inventory of the oxygen carrier due to the equilibrium conversion. An excessively low or high active material content will require a larger inventory of the oxygen carrier in the fuel reactor. In this study, approximately 20 wt.% of the Fe₂O₃ content was suitable for reducing the inventory of the oxygen carrier while achieving a solid conversion of 0.9 in the fuel reactor.     

Natural gas steam reforming for hydrogen production over metal monolith catalyst with efficient heat-transfer

Heat transfer performance of the natural gas (NG) steam reforming in a reactor bed with metal monolith catalyst has been evaluated in comparison with that in the conventional packed bed with pellet catalysts. 2%Ru/Al₂O₃ catalyst with high intrinsic activity has been wash-coated on metal monolith substrates or used as it was for the packed bed application. The prepared metal monolith catalyst has been applied for NG steam reforming to increase heat-transfer efficiency. Under the same degree of temperature gradient from the furnace wall to the catalyst bed, the heat flux obtained in the monolithic bed reactor was about twice higher than that in the packed bed reactor. Maximum heat transfer coefficient achieved in this study for the former was 0.65 kW/m² K, while that for the latter was 0.3 kW/m² K. This is mainly due to enhanced heat-transfer via metal monolith catalyst.     

Modeling of fixed bed downdraft biomass gasification: Application on lab‐scale and industrial reactors

This study aimed at presenting a model to simulate downdraft biomass gasification under steady‐state or unsteady‐state conditions. The model takes into account several processes that are relevant to the transformation of solid biomass into fuel gas, such as drying; devolatilization; oxidation; CO₂, H₂O, and H₂ reduction with char, pressure losses, solid and gas temperature, particle diameter, and bed void fraction evolution; and heat transfer by several mechanisms such as solid–gas convection, bed–wall convection, and radiation in the solid phase. Model validation is carried out by performing experiments in two lab‐scale downdraft fixed bed reactors (unsteady‐state conditions) and in a novel industrial pilot plant of 400 kW_th–100 kW_e (steady‐state conditions). The capability of the model to predict the effect of several factors (reactor diameter, air superficial velocity, and particle size and biomass moisture) on key response variables (temperature field, maximum temperature inside the bed, flame front velocity, biomass consumption rate, and composition and calorific value of the producer gas) is evaluated. For most response variables, a good agreement between experimental and estimated values is attained, and the model is able to reproduce the trend of variation of the experimental results. In general terms, the process performance improves with higher reactor diameter and lesser air superficial velocity, particle size, and moisture content of biomass. The steady‐state simulation appears to be a versatile tool for simulating different reactor configurations (preheating systems, variable geometry, and different materials). Copyright © 2013 John Wiley & Sons, Ltd.     

Assessment of integration of methane-reduced ceria chemical looping CO2/H2O splitting cycle to an oxy-fired power plant

In this paper, we investigated the effect of reaction kinetics and moving bed reactors for chemical looping (CO₂/H₂O) splitting unit (CL) that produces syngas and fed back to the power plant to gain the efficiency loss due to carbon capture. The reduction reactor (RED) produces methane is partially oxidized to make syngas and reducing the non-stoichiometric ceria which is transported to oxidation reactor (OXI) where the flue gases (CO₂ and H₂O) split to produce syngas. We developed the kinetics for methane reduced ceria and CO₂/H₂O splitting in a tubular reactor for an operating temperature range of (900–1100 °C) for different methane concentration which yielded to Avrami-Erofeev (AE3) model fits well for both redox reaction with different reaction constants. A moving bed reactors system is developed representing RED and OXI reactors of CL unit with kinetics hooked to the model in Aspen Plus with FORTRAN code. The effect of thermodynamics and the kinetics of redox reaction was investigated in the proposed integrated plant. The CL unit efficiency obtained is 42.8% for kinetic-based CL unit compares to 64% for thermodynamic based CL unit. However, the maximum available efficiency of the proposed layout lowered as 50.9% for kinetic-based CL unit plant compare to than 61.5% for thermodynamic based CL unit. However, the proposed plant shows an improvement in the energy efficiency penalty from 11.3% to 3.8% after CCS.     

A strategy of mid-temperature natural gas based chemical looping reforming for hydrogen production

Steam methane reforming (SMR) needs the reaction heat at a temperature above 800 °C provided by the combustion of natural gas and suffers from adverse environmental impact and the hydrogen separated from other chemicals needs extra energy penalty. In order to avoid the expensive cost and high power consumption caused by capturing CO₂ after combustion in SMR, natural gas Chemical Looping Reforming (CLR) is proposed, where the chemical looping combustion of metal oxides replaced the direct combustion of NG to convert natural gas to hydrogen and carbon dioxide. Although CO₂ can be separated with less energy penalty when combustion, CLR still require higher temperature heat for the hydrogen production and cause the poor sintering of oxygen carriers (OC). Here, we report a high-rate hydrogen production and low-energy penalty of strategy by natural gas chemical-looping process with both metallic oxide reduction and metal oxidation coupled with steam. Fe₃O₄ is employed as an oxygen carrier. Different from the common chemical looping reforming, the double side reactions of both the reduction and oxidization enable to provide the hydrogen in the range of 500–600 °C under the atmospheric pressure. Furthermore, the CO₂ is absorbed and captured with reduction reaction simultaneously.Through the thermodynamic analysis and irreversibility analysis of hydrogen production by natural gas via chemical looping reforming at atmospheric pressure, we provide a possibility of hydrogen production from methane at moderate temperature. The reported results in this paper should be viewed as optimistic due to several idealized assumptions: Considering that the chemical looping reaction is carried out at the equilibrium temperature of 500 °C, and complete CO₂ capture can be achieved. It is assumed that the unreacted methane and hydrogen are completely separated by physical adsorption. This paper may have the potential of saving the natural gas consumption required to produce 1 m³ H₂ and reducing the cost of hydrogen production.     

Hydrogen production with integrated CO2 capture in a novel gas switching reforming reactor: Proof-of-concept

This paper reports an experimental investigation on a novel reactor concept for steam-methane reforming with integrated CO₂ capture: the gas switching reforming (GSR). This concept uses a cluster of fluidized bed reactors which are dynamically operated between an oxidation stage (feeding air) and a reduction/reforming stage (feeding a fuel). Both oxygen carrier reduction and methane reforming take place during the reduction stage. This novel reactor configuration offers a simpler design compared with interconnected reactors and facilitates operation under pressurized conditions for improved process efficiency.The performance of the bubbling fluidized bed reforming reactor (GSR) is evaluated and compared with thermodynamic equilibrium. Results showed that thermodynamic equilibrium is achieved under steam-methane reforming conditions. First, a two-stage GSR configuration was tested, where CH₄ and steam were fed during the entire reduction stage after the oxygen carrier was fully oxidized during the oxidation stage. In this configuration a large amount of CH₄ slippage was observed during the reduction stage. Therefore, a three-stage GSR configuration was proposed to maximize fuel conversion, where the reduction stage is completed with another fuel gas with better reactivity with the oxygen carrier, e.g. PSA-off gases, after a separate reforming stage with CH₄ and steam feeds. A high GSR performance was achieved when H₂ was used in the reduction stage. A sensitivity analysis of the GSR process performance on the oxygen carrier utilization and target working temperature was carried out and discussed.     

Numerical study on unsteady heat transfer and fluid flow in a closed cylinder of reciprocating liquid hydrogen pumps

Modeling and optimization of liquid hydrogen (LH₂) pumps require accurate in-cylinder heat transfer correlations. However, the applicability of existing correlations based on gas mediums to LH₂ remains to be verified. In this paper, the unsteady heat transfer and fluid flow in a closed LH₂ pump cylinder are numerically studied by adopting the gas spring model. The phase shifts and temperature distribution in the closed pump cylinder are investigated. LH₂ is less affected by in-cylinder heat transfer and has a more uniform temperature distribution compared to nitrogen gas, while a low-temperature zone appears near the piston face at 120 rpm. Finally, the validity of Lekic's correlation in predicting the heat flux of the LH₂ compression process in the closed pump cylinder is verified, and the efficiency decrement versus rotational speed is analyzed based on the correlation. This work would be useful for selecting a proper in-cylinder heat transfer model for predicting the thermodynamic process in reciprocating LH₂ pumps.     

Reactivity study on a novel hydrogen fueled chemical-looping combustion

In this paper the reactivity study on hydrogen fueled chemical-looping combustion, which is capable of making breakthrough in simultaneous contribution to the efficient use of energy and being environmentally benign, has been carried out by a thermogravimetric analyzer (TGA) and a fixed bed reactor. The hydrogen fueled chemical-looping combustion in the new gas turbine cycle consists of two successive reactions: hydrogen fuel is reacted with metal oxide (reduction of metal oxide), instead of air or pure oxygen, and then the reduced metal is successively oxidized by air. Here, we have developed looping materials based on the integration of NiO, as solid reactants, with a composite metal oxide of NiAl₂O₄, as a binder, leading to a significant role in improving reaction rate, conversion, and regenerability in cyclic reaction in this combustor, compared with the other materials. These promising results indicated that this novel hydrogen fueled chemical-looping combustion is expected to be an effective use of hydrogen energy in power generation.     

Multiobjective optimization for exergoeconomic analysis of an integrated cogeneration system

In this paper, a novel cogeneration system integrating Kalina cycle, CO₂ chemical absorption, process, and flash‐binary cycle is proposed to remove acid gases in the exhaust gas of solid oxide fuel cell (SOFC) system, improve the waste heat utilization, and reduce the cold energy consumed during CO₂ capture. In the CO₂ chemical absorption process, the methyldiethanolamine (MDEA) aqueous solution is utilized as a solvent, and feed temperature and absorber pressure are optimized via Aspen Plus software. The single‐objective and multiobjective optimization are carried out for the flash‐binary cycle subsystem. Results show that when the multiobjective optimization is applied to identify the exergoeconomic condition, the cogeneration system can simultaneously satisfy the high thermodynamic cycle efficiency and also the low product unit cost. The optimal results of the exergy efficiency, product unit cost, and normalized CO₂ emissions obtained by Pareto chart were 75.84%, 3.248 $/GJ, and 13.14 kg/MWhr, respectively.