Chemical Looping Hydrogen Research Articles

The influence of hydrogen sulfide contaminations on hydrogen production in chemical looping processes

Chemical looping with iron-based oxygen carriers enables the production of hydrogen from various fossil and biogenic primary energy sources. In applications with real producer gases, such as biogas or gasified biomass, hydrogensulfide represents one of the most challenging contaminants. The impact of H2S on the reactivity of a Fe2O3/Al2O3 oxygen carrier material in chemical looping hydrogen production was investigated in the present work. First, potential sulfur deactivation mechanisms are discussed in detail on the basis of thermodynamic data. Afterwards, an experimental study in a fixed-bed reactor system gave experimental evidence on the fate of sulfur in chemical looping hydrogen systems. The chemisorption of hydrogen sulfide (H2S) was identified as the main cause for the accumulative adsorption of H2S in the reduction phase and was confirmed by ex-situ ICP-EOS analysis. In the subsequent steam oxidation step, significant quantities of H2S were released resulting in an undesirable contamination of the hydrogen product gas. The reason was found as weakened sulfur bonds through increasing reactor temperatures caused by the exothermic oxidation reactions. In additional air oxidation steps no further contaminants as sulfur dioxide were identified. A profound interpretation was achieved through the fulfillment of the overall sulfur mass balance within a mean deviation of 3.7%. Quantitative investigations showed that the hydrogen consumption decreased by 12% throughout the reduction phase in the event of 100 ppm H2S in the feed gas.

Energy-saving and environmentally-benign integrated ammonia production system

In this work, an integrated conversion system of coal to ammonia (NH3) is developed with the objectives of total energy efficiency maximization and process simplification. The system integrates drying, chemical looping hydrogen production, cryogenic nitrogen separation, NH3 synthesis, and power generation. Due to this integration, the total exergy destruction from each individual process, as well as the whole system, can be reduced significantly, leading to high energy efficiency. Several operating parameters, including mass fraction of oxygen carrier, reduction temperature, oxidation temperature, operating pressure, and conversion per pass during NH3 synthesis are evaluated. Low-rank coal with a flowrate of 100 t h−1 (moisture content of 65.5 wt% wb) is used as the feedstock, while Fe2O3 is adopted as an oxygen carrier. From the results, the system shows high total energy efficiency (NH3 production and total energy efficiencies of 58% and 60%, respectively) and significant system simplification. Higher Fe2O3 mass fraction lowers the total flowrate of circulated material in the chemical looping module. Moreover, lower reduction temperature and higher combustion temperature are preferred due to lower amount of circulated material in the chemical looping module. Conversion per pass of 40% during NH3 synthesis leads to largest NH3 production amount, which is 40 t h−1.

Ni, Co and Cu-promoted iron-based oxygen carriers in methane-fueled chemical looping hydrogen generation process

Oxygen carrier (OC) is essential in methane based chemical looping hydrogen generation (CLHG) process. Fe-based oxygen carriers have attracted lots of attention and attempts have been made to improve the CH4 conversion and hydrogen productivity. However, the influence of different promoters has yet to be compared. In this work, the interaction of promoters (Ni, Co and Cu) with Fe and the feasibility of tiny doping were investigated. Results showed that the oxygen activity and CH4 conversion increased after the addition of Ni, Co and Cu. Ni-promoted oxygen carriers presented the highest CH4 conversion and H2 yield. Co dopant benefitted the oxygen capacity, but Cu dopant restricted the deep reduction of Fe-based oxygen carriers. H2-TPR results revealed the oxygen activity was improved significantly at the intermediate temperature. A low concentration of Ni addition worked on hydrogen productivity as Mg-Fe-Al-O spinel with 1.7 wt% of Ni produced 1.3 mmol/gOC comparing with 1 mmol/gOC for Mg-Fe-Al-O. The promoters also facilitated the Fe segregation from the spinel phase. And the surface sintering of the promoted OCs was more serious, possibly resulting from the impregnation method. These findings could provide guidance for selection of promoters in chemical looping hydrogen generation process fueled by CH4.

Thermodynamic performance study of a new SOFC–CCHP system with diesel reforming by CLHG to produce hydrogen as fuel

In this paper, a combined cooling, heating and power system based on diesel-fueled chemical looping hydrogen generation and solid oxide fuel cell, and with CO2 capture was established. The system can achieve efficient power generation while separating CO2 without energy consumption, and effectively avoid the carbon deposition problem. Aspen software is used to simulate the process of the system, and FORTRAN program is used to calculate, through the thermodynamic analysis model, the unique performance change law of the new system using diesel is obtained, and thermodynamic analysis is performed on the system at last. The results show that the power efficiency of the new system is 54.1%, while the exergy efficiency and fuel energy saving ratio can reach 53%. At the same time, the influence of fuel flow and fuel utilization factor on the system performance does not continue to increase or decrease, and the system performance parameters will have a peak value with the change of pressure. The results not only provide a theoretical basis for the future construction of new diesel-fueled energy supply system, but also provide a new idea for the optimization scheme of energy-saving system and carbon recovery.

Study on Hydrogen Production of ZhunDong Coal Using Chemical Looping with Cu-Fe as Oxygen Carrier

ABSTRACT The main problem of coal direct chemical looping hydrogen generation (CLHG) is that coal ash and oxygen carrier (OC) cannot be effectively separated, which makes the OC activity is reduced after multiple cycles. In this study, a double plate-fixed bed reactor is adopted. The gasification of coal and the reduction reaction of oxygen carrier were integrated in the same furnace, and the hydrogen production characteristics of ZhunDong coal by chemical looping were studied. Cu-Fe composite oxygen carrier was prepared by chemical synthesis method. The results show that the higher the copper oxide content and the smaller the coal particle size, the better the effect of hydrogen yield. The potassium can promote the rate of coal gasification and increase oxygen carrier reduction rate, but faster reduction reaction rate will aggravate the formation of carbon deposition. The oxygen consumed in reduction of OC is preferentially from CuO, while Fe/FeO reacts with H2O prior to Cu/Cu2O in the hydrogen production stage. Compared with Cu-Fe OC, the implantation of MgAl2O3 plays an important role in stabilizing the redox ability and mechanical properties of OC.

Computational discovery of metal oxides for chemical looping hydrogen production

Summary Chemical looping hydrogen (CLH) production is a promising pathway that can offer both use of renewable resources and efficient CO2 capture capabilities. Here, we use the CALculation of PHase Diagrams (CALPHAD) thermodynamic database to study the water conversion capability of metal oxides (MOx) for CLH. We report the discovery of iron-based oxides with theoretical hydrogen yields up to 8 times higher than those of state-of-the-art oxides (e.g., ceria and ferrites). More specifically, Fe0.4Co0.6Ox is found to have a theoretical conversion efficiency capability > 50% at 700°C. Experimental results are presented, and a technoeconomic model quantifies the importance of MOx oxygen capacity and water conversion in this process. This reflects the potential of CLH production with a hydrogen cost of $1.25 ± $0.38/kg at a scale of 50 tons per day. This is comparable to steam methane reforming but with the added benefit of producing a stream of pure CO2.

Reaction mechanism of methane conversion over Ca2Fe2O5 oxygen carrier in chemical looping hydrogen production

Density functional theory (DFT) calculations were employed to explore the reaction mechanism of methane conversion over Ca2Fe2O5 oxygen carrier in chemical looping hydrogen production. The elementary reaction steps, including CH4 activation, CO and CO2 formation, H2 and H2O generation, and oxygen diffusion, were investigated to propose the reaction network. It was found that the H2 generation is very difficult on the surface of Ca2Fe2O5 oxygen carrier and the H atoms tend to react with surface oxygen to form H2O molecule. The CO formation is prone to occur and the generated CO molecule is very easily oxidized by surface oxygen to form the CO2 molecule. The surface oxygen of Ca2Fe2O5 oxygen carrier could exhibit the excellent reactivity. The oxygen diffusion needs to overcome a high energy barrier and it is an inherent high-temperature process. For the whole reaction of methane conversion over Ca2Fe2O5 oxygen carrier, the CH4 activation process is the dominant rate-determining step. In order to increase the CH4 reactivity of Ca2Fe2O5 oxygen carrier, it is necessary to find promoters that could decrease the energy barrier of CH4 activation and enhance the oxygen diffusion ability.

Effect of support on hydrogen generation over iron oxides in the chemical looping process.

Fe2O3 is recognized as an excellent oxygen carrier for its low cost and high oxygen capacity. However, pure Fe2O3 must be deposited on supports to ensure high reactivity and durability. Here, we proposed several Fe2O3-based oxygen carriers using MgAl2O4, Ce0.8Gd0.2O1.9, and Zr0.8Y0.2O1.9 as supports and investigated their performance for chemical looping hydrogen generation. The support effect on chemical looping hydrogen generation performance was evaluated, and the fundamental insights were investigated in depth. Fe2O3/Ce0.8Gd0.2O1.9 exhibited a superior performance regarding high hydrogen yield and stable trend over 20 cycles at 750 °C. However, hydrogen yield of Fe2O3/Zr0.8Y0.2O1.9 exceeded that of Fe2O3/Ce0.8Gd0.2O1.9 at higher temperatures (850 °C). Characterizations show that Ce0.8Gd0.2O1.9 exhibits the highest oxygen vacancy concentration, which significantly improves the reduction and reoxidation reactions of Fe2O3, thus leading to an enhanced hydrogen yield. However, the interaction between Fe2O3 and Ce0.8Gd0.2O1.9 contributed to the increase in Fe2+ concentration, thus decreasing the oxygen capacity during the redox cycle and contributing to the declined hydrogen yield at higher temperatures. This work highlights the potential of Ce0.8Gd0.2O1.9 to be used as an effective support for Fe2O3 at mid-temperatures. We hope that the support effect in this work can be extended to design and select more active and durable oxygen carriers.

Fabrication of Iron-based Oxygen Carriers on Various Supports for Chemical Looping Hydrogen Generation

Fe2O3/Al2O3 and Fe2O3/TiO2/Al2O3 oxygen carriers were fabricated and evaluated for chemical looping hydrogen generation (CLHG) using a TGA system and a fixed-bed reactor. Oxygen carriers were converted to around 33% in a fixed-bed reactor to ensure uniform reduction. The reduced oxygen carrier was tested for steam generation, where in all cases Fe2O3/Al2O3 displayed a better conversion than Fe2O3/TiO2/Al2O3. It was found that increasing the reaction temperature from 800 to 850 °C had little effect for either oxygen carrier, but a further increase to 900°C resulted in an increased steam conversion. A higher steam flow rate caused a lower overall steam conversion but a higher H2 production. A higher feed rate of reduced oxygen carrier let to a higher steam conversion up to a rate of 18.4 and 14.9 g min–1 for Fe2O3/Al2O3 and Fe2O3/TiO2/Al2O3, respectively. A final comparison was performed with up to 50 redox cycles, where Fe2O3/Al2O3 showed superior reactivity in the first cycles but ended at a conversion of 54.6% with Fe2O3/TiO2/Al2O3 ending at a conversion of 64.6%.

Exergy Analysis of a Novel Chemical Looping Hydrogen Generation System Integrated with SOFC

In this paper, a novel system integrating chemical-looping hydrogen generation (CLH) system and solid oxide fuel cell (SOFC) has been proposed. This new methane-fuelled energy system was investigated with energy balance and exergy analysis. CLH produces the hydrogen as the fuel of the SOFC, and the FeO and Fe3O4 are selected as the looping material. Waste heat from the SOFC is absorbed by the CLH and converted to chemical energy through the reduction reaction of CLH. Owing to the cascade utilization of the fuel between the CLH and SOFC, the net efficiency of this novel system can achieve 62.8% considering CO2 separation, more than 10 percentage points higher than a methane reforming fuelled SOFC system. Meanwhile, by using the CLH to produce the hydrogen, the CO2 can be recovered without an energy penalty. Through the analysis of the graphical exergy, the cascade utilization of waste heat and the high-efficiency hydrogen production is the main reason of high performance. This novel system also has the advantage of CO2 capture without energy penalty, so this combined system is an advantaged method to accomplish the efficient utilization of methane.

Redox Performance of Cu-Doped Fe2O3/Al2O3 as Oxygen Carriers for Chemical Looping Hydrogen Production

Chemical looping hydrogen (CLH) production, or hydrogen production combined with CO2 capture, is becoming an emerging alternative technology that has attracted much attention. Iron-based oxygen car...

Energy and economic analysis of a hydrogen and ammonia co-generation system based on double chemical looping

In this work, a model of hydrogen production by double chemical looping is introduced. The efficiency benefit obtained was investigated. The chemical looping hydrogen generation unit is connected in series to the downstream of a chemical looping gasification unit as an additional system for 100 MW·h coal gasification, with the function of supplementary combustion to produce hydrogen. Using Aspen Plus software for process simulation, the production of H2 and N2 in the series system is higher than that in the independent Chemical looping gasification and Chemical looping hydrogen generation systems, and the production of hydrogen is approximately 25.63% and 12.90% higher, respectively; The study found that when the gasification temperature is 900 °C, steam-carbon ratio is 0.84 and oxygen-carbon ratio is 1.5, the hydrogen production rate of the system was the maximum. At the same time, through heat exchange between logistics, high-pressure steam at 8.010 × 104 kg·h−1 and medium-pressure steam at 1.101 × 104 kg·h−1 are generated, and utility consumption is reduced by 61.58%, with utility costs decreasing by 48.69%. An economic estimation study found that the production cost of ammonia is 108.66 USD·(t NH3)−1. Finally, cost of equipment is the main factors influencing ammonia production cost were proposed by sensitivity analysis.

Energy quality factor and exergy destruction processes analysis for chemical looping hydrogen generation by coal

SummaryChemical looping hydrogen generation (CLHG) is a hydrogen production process with broad prospects for its advantages of not requiring H2 purification process and direct CO2 capture. Presently, the research on the integrated system of CLHG focuses on the impact of integrated processes and operating variables on the overall energy efficiency and exergy efficiency of the system. However, the research on the matching relationship between quantity and quality in the process of energy conversion is insufficient, and the flow direction of exergy and the specific exergy destruction process for equipment are not clear. The integrated system of coal gasification (CG) and three reactors CLHG was established in this paper with a hydrogen production efficiency of 42.55% and system exergy efficiency of 52.98%. This paper studied the exergy flow direction and efficiency of the equipment in the system, providing the improved direction for the amelioration of the system; a detailed description was present on the change process of energy quality in each equipment with energy quality factor, and the concept of energy quality factor ratio was proposed as an evaluation tool for matching the quantity and quality of energy in the system. It is calculated that the ratio of energy quality factor of the integrated system of CG and CLHG is 0.944. By comparing with the energy quality ratio of the traditional CG hydrogen production system (0.896), it can be concluded that the integrated system of CG and CLHG has a better energy matching relationship.

Research of coal‐direct chemical looping hydrogen generation with iron‐based oxygen carrier modified by NaAlO2

AbstractThis study reports a high‐performance modified oxygen carrier (OC) in a three‐cycle chemical looping process in which coal is used as the raw feedstock to produce hydrogen. A series of experiments of coal‐direct chemical looping hydrogen generation (CLHG) were conducted in a batch fluidized bed reactor, in the absence or presence of the NaAlO2. The experimental results indicated that the carbon conversion and hydrogen yield reached 87.0% and 1.30 L · g−1, respectively, when the coal/oxygen carrier (Fe4Al6) mass ratio was 1:20. However, FeO was the main product of the reduction process with Fe4Al6, which limits the production of hydrogen during steam oxidation. Therefore, NaAlO2 was used to improve the reaction performance of the OC. The carbon conversion and hydrogen production of NaAlO2‐loaded oxygen carriers were 10.7% and 9.6% higher than Fe4Al6, respectively. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer‐Emmett‐Teller (BET), and temperature‐programmed reduction (TPR) techniques were used to characterize the oxygen carriers. According to the XRD analysis results, the addition of 5 wt% NaAlO2 can generate NaFeO2. The unique layer structure of NaFeO2 can loosen the structure between Fe2O3 and Al2O3. Thus, the surface morphology and pore structure of the oxygen carrier were improved, which are responsible for the enhancement in reactivity rather than the catalytic effects of the alkali metal. The cycle performance of the Na0.5Fe4Al6 remained stable after multi‐redox cycles, and no serious sintering phenomena were observed; in addition, the carbon conversion and hydrogen yield remained above 86% and 1.40 L · g−1, respectively.

Modified CeO2 as active support for iron oxides to enhance chemical looping hydrogen generation performance

Chemical looping hydrogen generation (CLG) is a promising pathway that can offer both the high purity hydrogen as well as the efficient CO2 capture capability. However, this process was significantly hindered by the lack of active oxygen carriers at relatively low temperatures. Mixed ionic-electronic (MIEC) supported iron oxides exhibit desirable redox performance for the improved oxygen-ion conductivity. In this work, we prepared several AxCe1-xO2-δ (A = Gd, La; x = 0, 0.1, 0.3) supported Fe2O3 for hydrogen production at 750 °C. It was shown that Fe2O3/Gd0.3Ce0.7O2-δ shows the highest hydrogen generation performance and stability over 50 redox cycles. The reactivity follows the sequence of: Fe2O3/Gd0.3Ce0.7O2-δ > Fe2O3/La0.1Ce0.9O2-δ > Fe2O3/Gd0.1Ce0.9O2-δ > Fe2O3/La0.3Ce0.7O2-δ. The fundamental investigation shows that the doping of rare earth (Gd, La) into CeO2 contributes to the formation of oxygen vacancies, thus improving the lattice oxygen diffusion. The enhanced hydrogen generation performance attributes to the high lattice oxygen diffusion to improve the reactivity and inhibiting outward diffusion of Fe. The roughly linear relation between the oxygen vacancy concentration and chemical looping performance can be extended to predict the performance of oxygen carriers for other chemical looping applications, methane reforming, combustion, and ethane dehydrogenation, etc.

Thermodynamic performance study of the CLHG/SOFC combined cycle system with CO2 recovery

In this paper, the chemical looping hydrogen generation (CLHG) and solid oxide fuel cell (SOFC) are combined to achieve efficient power generation and CO2 separation and capture without energy consumption. The system can realize the utilization of H2 produced by CLHG in SOFC to avoid carbon deposition in it. The anode of the SOFC is completely refluxed into the steam reactor (SR) of CLHG without the need to install an afterburner, which avoids the direct combustion of fuel and greatly reduces the generation of NOx. High temperature air at SOFC cathode outlet and high temperature gas at fuel reactor (FR) outlet enter GT to do work. The transcritical carbon dioxide cycle (TRCC) is used to recover waste heat, making the system compact and efficient. The new CLHG/SOFC combined cycle system proposed in this paper has high performance, which can recover CO2 and prevent NOx generation, and also prevent carbon deposition in SOFC. In addition, the anode of SOFC is completely refluxed to SR to further improve the system performance. This paper discusses the effects of fuel flow, fuel utilization, SOFC operating temperature, system operating pressure, and CO2 turbine inlet pressure on system performance. The calculation results show that the total efficiency of the system can reach more than 80%, the power generation efficiency can reach more than 65%, and the exergy efficiency is more than 60%.

Reaction mechanism of Ca2Fe2O5 oxygen carrier with CO in chemical looping hydrogen production

The CO adsorption behavior, CO oxidation and oxygen anion diffusion of Ca2Fe2O5(010) surface were systematically investigated to deeply understand the reaction mechanism of Ca2Fe2O5 oxygen carrier with CO based on density functional theory (DFT) calculations. The results indicate that the activity of CO adsorption on four terminations of Ca2Fe2O5(010) surface follows the order: FeO2- terminated > FeO-terminated > CaO-terminated > OCa-terminated. Four reaction pathways for CO oxidation were proposed. It was found that CO oxidation reaction is very easy to proceed, and that both types of surface oxygen (O1 and O2) could exhibit high activity. The oxygen diffusion in Ca2Fe2O5 is inherently a high-temperature process. In addition, the energy barrier of oxygen diffusion is much higher than that of CO oxidation, implying that oxygen diffusion is the rate-limiting step for the entire reaction between Ca2Fe2O5 oxygen carrier and CO. The superior reactivity of Ca2Fe2O5 oxygen carrier is attributed to its excellent surface oxygen activity and good oxygen diffusion ability.

Comparative exergy and exergoeconomic analysis between liquid fuels production through chemical looping hydrogen generation and methane reforming with CO2

Recently, the concept of carbon utilization has been widely investigated to reduce greenhouse gas emissions and mitigate dependence on fossil fuels. For this purpose, two different carbon capture and utilization processes to produce liquid fuels, carbon looping cyclic reforming (CCR) and carbon dioxide hydrogenation based on chemical looping hydrogen generation (CH-CLHG), are introduced. This article aims to identify which utilization method holds more potential for future technical development. To achieve it, comprehensive thermodynamic, economic and exergoeconomic assessments are implemented. The results indicate that the exergy efficiency of CH-CLHG system is 3.84% more than that of CCR system, and the cost of liquid fuels production is 36.64% less. It is also suggested that CCR system is superior to CH-CLHG system in terms of the liquid fuels output. When the electricity price is below 0.068 $/kWh, the cost of liquid fuels of CCR system is more competitive than that of CH-CLHG system. Since the higher fuel cost and larger exergy destruction, CCR system’s exergy destruction cost is more significant. In addition, the exergy destruction and exergy efficiency as well as exergoeconomic performance of each main component are presented in detail to reveal the cost formation process, and to find out the measure that would improve system efficiency and cost-effectiveness.

Exergy analysis on the process for three reactors chemical looping hydrogen generation

Three reactors chemical looping (TRCL) is a promising hydrogen production process without needing H2 purification and capturing CO2 directly. Based on the deficient research on the energy conversion mechanism in TRCL and the role of external heat sources in the TRCL process, this paper studied the thermodynamic characteristics of energy conversion in methane-TRCL and syngas-TRCL and water-gas shift (WGS) systems, and established the mathematical model of the related parameters through the concept of energy grade. Meanwhile, in this system the energy grade effects of input heat, heat quantity, hydrogen concentration in syngas and steam quantity on the thermodynamic performance are studied. The results show that the TRCL system of methane and syngas has the highest exergy efficiency with 89% and 88.3% respectively, while the energy grade of external input heat is 0.694. Under mentioned above, methane-TRCL is suitable for the case where the heat input value is greater than 310.52 kJ/mol, and the syngas-TRCL system is more efficient than WGS system with 4.8%.

Computational Discovery of Metal Oxides for Chemical Looping Hydrogen Production

Chemical looping hydrogen production (CLH) that uses methane and water as feedstock is a promising pathway that can offer both the use of renewable resources as well as efficient CO2 capture capabilities. Here, we utilized CALculation of PHAse Diagrams (CALPHAD) thermodynamic database to study the water conversion efficiency of metal oxides (MOx) for CLH. We report the discovery of new oxides with theoretical hydrogen yields up to 8 times higher compared to state-of-the-art oxides such as ceria and ferrites. Results show that oxygen capacity in these oxides is a non-monotonic function of iron concentration, a prediction that has been experimentally verified and yielded an optimal concentration. More specifically, Fe0.40Co0.60Ox was found to be the optimal stoichiometry with the largest water conversion efficiency capability at >50% at 700°C. A technoeconomic model quantified the importance of MOx oxygen capacity and water conversion efficiency in this process, but also shed light into the great potential of chemical looping hydrogen production with a hydrogen cost of $1.32 ± 0.40 per kg at a scale of 90 tons per day. This is comparable to steam methane reforming (SMR), but with the added benefit of producing pure CO2, and thereby eliminating the cost of CO2 separation prevalent in SMR. Sensitivity analysis on key variables such as cycle time, oxygen capacity and oxide lifetime allowed quantification of their effect in the final unit cost of H2.