Effects of supports on hydrogen production and carbon deposition of Fe-based oxygen carriers in chemical looping hydrogen generation

Chemical looping hydrogen generation (CLHG) can produce high purity hydrogen from fuel gases with inherent separation of CO₂. However, the performance of oxygen carrier in CLHG varies with the support materials. In this paper, the reactivity, carbon deposition, redox stability, hydrogen yield and purity, and sintering behavior of the Fe-based oxygen carriers were analyzed to investigate the effects of supports, i.e. Al₂O₃, SiO₂, MgAl₂O₄, ZrO₂ and YSZ (yttrium-stabilized zirconia). The results showed that the properties of the oxygen carriers, e.g. carbon deposition, reactivity and stability, mainly depended on the support and its interaction with iron oxides. The reactivity and hydrogen yield for the oxygen carriers investigated followed the order: Fe₂O₃/MgAl₂O₄ > Fe₂O₃/ZrO₂ > Fe₂O₃/YSZ > Fe₂O₃/Al₂O₃ > Fe₂O₃/SiO₂, and the order of hydrogen purity was identical with that of hydrogen yield as a result of carbon deposition. Furthermore, the hydrogen purity of the Fe-based oxygen carriers supported by MgAl₂O₄, ZrO₂, or YSZ could reach above 99.5% and Fe₂O₃/YSZ showed the lowest carbon deposition. The oxygen carriers, Fe₂O₃/MgAl₂O₄ and Fe₂O₃/SiO₂, were selected to be characterized by SEM images and XRD patterns before and after the redox cycles.     

Solar-driven novel methane reforming with carbon looping for hydrogen production

As a renewable source, solar energy plays an important role in meeting energy demand for human beings and in relieving global warming. In this paper, the concentrated solar heat is utilized to drive the high-endothermic methane reforming with carbon looping. In so doing, the process increases the utilization of CO₂ and reduces the carbon emissions as well as saves the extra fuel consumption for combustion, leading to high efficiency of energy utilization. By optimizing this proposed system, the energy efficiency can reach approximate 67.13% with simultaneously reducing CO₂ emissions by 34.98% compared with SMR process. Exergy analysis is used to assess the location of irreversibility within process. The maximal part of exergy destroyer was localized in reformer with a contribution of 68%. In addition, the effects of hourly variation of direct normal irradiation on thermodynamic performance and methane conversion on the four typical days (spring equinox, summer solstice, autumn equinox, winter solstice) were analyzed in this work. The current work might be insightful for solar-hydrogen production field.     

Performance of hydrogen and power co-generation system based on chemical looping hydrogen generation of coal

An integrated hydrogen and power co-generation system based on slurry-feed coal gasification and chemical looping hydrogen generation (CLH) was proposed with Shenhua coal as fuel and Fe₂O₃/MgAl₂O₄ as an oxygen carrier. The sensitivity analyses of the main units of the system were carried out respectively to optimize the parameters. The syngas can be converted completely in the fuel reactor, and both of the fuel reactor and steam reactor can maintain heat balance. The purity of hydrogen produced after water condensation is 100%. The energy and exergy analyses of the proposed system were studied. Pinch technology was adopted to get a reasonable design of the heat transfer network, and it is found pinch point appears at the hot side temperature of 224.7 °C. At the given status of the proposed system, the hydrogen yield is 1040.11 kg·h⁻¹ and the CO₂ capture rate is 94.56%. At the same time, its energy and exergy efficiencies are 46.21% and 47.22%, respectively. According to exergy analysis, the degree of exergy destruction is ranked. The gasifier unit has the most serious exergy destruction, followed by chemical looping hydrogen generation unit and the heat recovery steam generator unit.     

Evaluation of activated carbons based on olive stones as catalysts during hydrogen production by thermocatalytic decomposition of methane

Catalytic decomposition of methane over carbon materials has been intensively studied as an environmental approach for CO₂-free hydrogen production without further by-products except hydrogen and valuable carbon. In this work, we will investigate the catalytic activity of activated carbons based on olive stones prepared by two different processes. Additionally, the effect of three major operational parameters: temperature, weight of catalyst and flow rate of methane, was determined. Therefore, a series of experiments were conducted in a horizontal-flow fixed bed reactor. The outflow gases were analysed using a mass spectrometer. The textural, structural and surface chemistry properties of both fresh and used activated carbons were determined respectively by N₂ gas adsorption, X-Ray Diffraction and Raman and Temperature Programmed Desorption. The results reveal that methane decomposition rate increases with temperature and methane flow however it decreases with catalyst weight. The two carbon samples exhibit a high initial activity followed by a rapid decay. Textural characterization of the deactivated carbon presents a dramatic drop of surface area, pore and micropore volumes against an increase of average pore diameter confirming that methane decomposition occurs mainly in micropores. XRD characterization shows a turbostratic structure of fresh samples with more graphitization in deposed carbon explaining the lowest activity at the end of reaction. Raman spectra reveal the domination of the two bands G and D which varying intensities affirm that the different carbons tend to organise in aromatic rings. Finally the surface chemistry qualitatively changes greatly after methane dissociation for CAGOC unlike CAGOP but quantitatively a small difference is observed which indicates that these functionalities may have a role in this heterogeneous reaction but cannot be totally responsible. Among the two catalysts tested, CAGOC has the highest initial methane decomposition rate but CAGOP is the most stable one.     

Hydrogen production from fossil fuels with carbon capture and storage based on chemical looping systems

This paper analyzes innovative processes for producing hydrogen from fossil fuels conversion (natural gas, coal, lignite) based on chemical looping techniques, allowing intrinsic CO₂ capture. This paper evaluates in details the iron-based chemical looping system used for hydrogen production in conjunction with natural gas and syngas produced from coal and lignite gasification. The paper assesses the potential applications of natural gas and syngas chemical looping combustion systems to generate hydrogen. Investigated plant concepts with natural gas and syngas-based chemical looping method produce 500 MW hydrogen (based on lower heating value) covering ancillary power consumption with an almost total decarbonisation rate of the fossil fuels used.The paper presents in details the plant concepts and the methodology used to evaluate the performances using critical design factors like: gasifier feeding system (various fuel transport gases), heat and power integration analysis, potential ways to increase the overall energy efficiency (e.g. steam integration of chemical looping unit into the combined cycle), hydrogen and carbon dioxide quality specifications considering the use of hydrogen in transport (fuel cells) and carbon dioxide storage in geological formation or used for EOR.     

Synergistic effects of binary oxygen carriers during chemical looping hydrogen production

The use of binary oxygen carrier allows for the materials of enhanced activity or stability during chemical looping process. However, the lack of mechanical understanding of the origin of the improvements hindered the rational design and control of the doping process in the oxygen carrier production. Here, we synthesized a series of M_0.6Fe_2.4O_y (M = Ni, Cu, Co, Mn) binary spinel materials and carried out various characterization techniques to study how the dopants influenced the material phase change, the oxygen transfer as well as the chemical looping performance. The results showed the chemical looping reactivity can be related to the oxygen transformation between lattice oxygen and oxygen vacancy, which was determined by the redox properties of both dopants and iron. The metal in tetrahedral site for Cu, Mn, Ni-doped sample were relatively stable, limiting oxygen transformation ability. In comparison, Co dopant promoted the reducibility of iron in tetrahedral site as well as metals in other sites, making almost all lattice oxygen rapidly transformed to oxygen vacancy during reduction. This was the main cause for the subsequent high hydrogen production rate (average ∼0.02 mmol. g⁻¹.s⁻¹) and yield (∼15.9 mmol.g⁻¹). Upon cycling, the phase separation of single oxides from Co_0.6Fe_2.4O_y and Mn_0.6Fe_2.4O_y spinels led to the decreased ability of oxygen transformation. However, the performance was extremely stable for Cu_0.6Fe_2.4O_y with reversible phase change between spinel and (Fe, Cu) wusitite by the Cu-Fe interaction. Based on the current results, this work points to a promising Cu-Co co-doping material with both good reactivity and stability.     

High-pressure hydrogen production with inherent sequestration of a pure carbon dioxide stream via fixed bed chemical looping

The proof of concept for the production of pure pressurized hydrogen from hydrocarbons in combination with the sequestration of a pure stream of carbon dioxide with the reformer steam iron cycle is presented. The iron oxide based oxygen carrier (95% Fe₂O₃, 5% Al₂O₃) is reduced with syngas and oxidized with steam at 1023 K. The carbon dioxide separation is achieved via partial reduction of the oxygen carrier from Fe₂O₃ to Fe₃O₄ yielding thermodynamically to a product gas only containing CO₂ and H₂O. By the subsequent condensation of steam, pure CO₂ is sequestrated. After each steam oxidation phase, an air oxidation was applied to restore the oxygen carrier to hematite level. Product gas pressures of up to 30.1 bar and hydrogen purities exceeding 99% were achieved via steam oxidations. The main impurities in the product gas are carbon monoxide and carbon dioxide, which originate from solid carbon depositions or from stored carbonaceous molecules inside the pores of the contact mass. The oxygen carrier samples were characterized using elemental analysis, BET surface area measurement, XRD powder diffraction, SEM and light microscopy. The maximum pressure of 95 bar was demonstrated for hydrogen production in the steam oxidation phase after the full oxygen carrier reduction, significantly reducing the energy demand for compressors in mobility applications.     

Joint-use of activated carbon and carbon black to enhance catalytic stability during chemical looping methane decomposition process

Chemical-looping methane decomposition using activated carbon as a catalyst has been considered a potentially promising approach for high-purity H₂ production with low cost and low CO₂ emission. However, activated carbon is known to deactivate fast despite its high initial catalytic activity. Oppositely, carbon black has shown stable catalytic methane decomposition that even increases slowly with time of reaction. Considering these two different activity trends, activated carbon and carbon black are jointly used to prepare catalysts and then test for the decomposition of the methane via chemical looping in this study that aimed to examine catalyst and reaction properties which may combine the high initial activity of activated carbon with the steady-and-increasing activity of carbon black. These mixed catalysts are examined using X-ray diffraction analysis (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), Brunauer-Emmett-Teller (BET) and high-resolution transmission electron microscopy (HRTEM) before and after reaction testing to reveal chemical and physical constituents which contributed to their reactivities, and the mechanism of long catalytic activity has been discussed. The results point to insights and potential directions for modifying carbonaceous catalysts for chemical looping thermo-catalytic decomposition of methane.     

Chemical looping glycerol reforming for hydrogen production by Ni@ZrO2 nanocomposite oxygen carriers

The research describes the synthesis of nanocomposite Ni@ZrO₂ oxygen carriers (OCs) and lanthanide doping effect on maintaining the platelet-structure of the nanocomposite OCs. The prepared OCs were tested in chemical looping reforming of glycerol (CLR) process and sorption enhanced chemical looping reforming of glycerol (SE-CLR) process. A series of characterization techniques including N₂ adsorption-desorption, X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), high resolution transmission electron microscopy (HRTEM), H₂ temperature-programmed reduction (H₂-TPR), H₂ pulse chemisorption and O₂ temperature-programmed desorption (O₂-TPD) were used to investigate the physical properties of the fresh and used OCs. The results show that the platelet-stack structure of nanocomposite OCs could significantly improve the metal support interaction (MSI), thus enhancing the sintering resistance. The effect of lanthanide promotion on maintaining this platelet-stack structure increased with the lanthanide radius, namely, La³⁺ > Ce³⁺ > Pr³⁺ > Yb³⁺. Additionally, the oxygen mobility was also enhanced because of the coordination of oxygen transfer channel size by doping small radius lanthanide ions. The CeNi@ZrO₂ showed a moderate ‘dead time’ of 220 s, a high H₂ selectivity of 94% and a nearly complete glycerol conversion throughout a 50-cycle CLR test. In a 50-cycle SE-CLR stability test, the CeNi@ZrO₂CaO showed high H₂ purity of 96.3%, and an average CaCO₃ decomposition percentage of 53% without external heating was achieved.     

Microwave-assisted catalytic decomposition of methane over activated carbon for -free hydrogen production

The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production of CO₂-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to . However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced. The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise with time. An increase in the VHSV has a negative effect on CH₄ conversion, and even more so in the case of microwave heating. Nevertheless, the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments is also reported.     

Evaluation of iron based chemical looping for hydrogen and electricity co-production by gasification process with carbon capture and storage

Integrated Gasification Combined Cycle (IGCC) is one of power generation technologies having the highest potential for carbon capture with low penalties in efficiency and cost. Syngas produced by gasification can be decarbonised using chemical looping methods in which an oxygen carrier (usually a metallic oxide) is recycled between the syngas oxidation reactor (fuel reactor) and the chemical agent oxidation reactor (steam reactor). In this way, the resulted carbon dioxide is inherently separated from the other products of combustion and the syngas energy is transferred to an almost pure hydrogen stream suitable to be used not only for power generation but also for transport sector (PEM fuel cells).     

Biomass pyrolysis oils for hydrogen production using chemical looping reforming

This study considers the feasibility of using highly oxygenated and volatile pyrolysis oils from biomass wastes as sustainable liquid fuels for conversion to a hydrogen-rich syngas using the chemical looping reforming process in a packed bed. Pine oil and palm empty fruit bunches oil- ‘EFB’- were investigated with a Ni/Al₂O₃ catalyst doubling as oxygen transfer material (OTM). The effect of molar steam to carbon ratio (S/C) and weight hourly space velocity were investigated at 600 °C and atmospheric pressure on the fuel and steam conversion, the H₂ yield and the H- and C-products distribution. With a downward fuel feed configuration and using a H₂-reduced catalyst, maximum averaged fuel conversions of ∼97% for pine oil and 89% for EFB oil were achieved at S/C ratios of 2.3 and 2.6 respectively (on a water-free oil basis). This produced H₂ with a yield efficiency of approximately 60% for pine oil and 80% for EFB oil notwithstanding equilibrium limitations, and with little CH₄ by-product. Both oils exhibited very similar outputs with varying S/C. Upon a short number of cycles, i.e. starting from an oil-reduced catalyst, the fuel conversion dropped slightly but the steam conversion was constant, resulting in a slow decrease in H₂ yield. Despite their high level of oxygen content, the pyrolysis oils were shown to maintain close to 90% reduction of the oxidised catalyst upon repeated cycles, but the rate of reduction decreased with cycling.     

Techno-economical evaluations of decarbonized hydrogen production based on direct biogas conversion using thermo-chemical looping cycles

Hydrogen production by biogas conversion represent a promising solution for reduction of fossil CO₂ emissions. In this work, a detailed techno-economic analysis was performed for decarbonized hydrogen production based on biogas conversion using calcium and chemical looping cycles. All evaluated concepts generate 100,000 Nm³/h high purity hydrogen. As reference cases, the biogas steam reforming design without decarbonization and with CO₂ capture by gas-liquid chemical absorption were also considered. The results show that iron-based chemical looping design has higher energy efficiency compared with the gas-liquid absorption case by 2.3 net percentage points as well as a superior carbon capture rate (99% vs. 65%). The calcium looping case shows a lower efficiency than chemical scrubbing, with about 2.5 net percentage points, but the carbon capture rate is higher (95% vs. 65%). The hydrogen production cost increases with decarbonization, the calcium looping shows the most favourable situation (37.14 €/MWh) compared to the non-capture steam reforming case (33 €/MWh) and MDEA and iron looping cases (about 42 €/MWh). The calcium looping case has the lowest CO₂ avoidance cost (10 €/t) followed by iron looping (20 €/t) and MDEA (31 €/t) cases.     

Improved iron oxide oxygen carriers for chemical looping hydrogen generation using colloidal crystal templated method

Iron oxide has been widely studied in chemical looping hydrogen generation (CLHG) process as an oxygen carrier, but fast decline of its activity in redox cycles due to sintering and agglomeration is one of the main drawbacks. In this work, the colloidal crystal templated (CCT) method was applied to synthesize Fe₂O₃/CeO₂ oxygen carrier and the mole ratio of Fe/Ce was 8:2, aiming to inhibit adjacent grains from agglomerating and improve the contact between the fuel gas and the oxygen carrier. The redox performances were evaluated with CO as fuel in a batch fixed bed reactor for 20 redox cycles, with oxygen carriers prepared by co-precipitation (CP) and sol-gel (SG) methods as references. X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), and H₂-temperature programmed reduction (H₂-TPR) were used for characterization. The results showed that the calcination temperature lower than 750 °C was suitable for the CCT. The redox experiments showed that the H₂ yield and the redox stability for the oxygen carrier prepared by CCT were higher than those by co-precipitation and sol-gel methods. The H₂ yield of CCT oxygen carrier kept stable from the 3rd cycle and was 8.5 mmol/gOC in the 20th cycle. The pore structures resulting from CCT were different from another two oxygen carriers before and after the cycles, but maintained well through SEM images, leading to high activity and stability during redox cycles. The crystallite sizes of Fe₂O₃ and CeO₂ before and after redox cycles were the smallest for the CCT oxygen carrier from XRD patterns. In addition, H₂-TPR demonstrated that CCT oxygen carrier exhibited the highest reactivity.     

Nickel-iron modified natural ore oxygen carriers for chemical looping steam methane reforming to produce hydrogen

Chemical looping steam methane reforming (CL-SMR) is a promising and efficient method to produce hydrogen and syngas. However, oxygen carrier (OC) prepared by synthesis are complex, expensive and poor mechanical performance, while natural ore OCs are low activity and poor selectivity. In order to avoid these problems, Ni/Fe modification of natural ores were proposed to improve the reactivity and stability of OC to CL-SMR. The results indicated that the modified calcite recombined and improved the structural phase during the reaction, enhancing performance and inhibiting agglomeration. Moreover, high ratio of iron to nickel was easy to sinter and decline the OC performance. In addition, with the increase of steam flow, both CH₄ conversion and carbon deposition decreased. Thereinto, the highest H₂ concentration, CH₄ conversion efficiency and H₂ yield were obtained when the ratio of steam to OC was 0.05. Furthermore, CH₄ flow rate had a great impact on CL-SMR performance. When the ratio of CH₄ to OC was 0.04, it achieved the highest CH₄ conversion efficiency of 98.96%, the highest H₂ concentration of 98.83% and the lowest carbon deposition of 3.23%. However, the carbon deposition increased with the increase of CH₄ flow rate. After a long-time chemical looping process, the Ni/Fe modified calcite showed a consistently stable performance with average H₂ concentration of 93.08%, CH₄ conversion efficiency of 88.03%, and carbon deposition of 2.15%.     

Deep regeneration of activated carbon catalyst and autothermal analysis for chemical looping methane thermo-catalytic decomposition process

The application of a chemical looping process to methane thermo-catalytic decomposition using activated carbon (AC) as a catalyst has been recognized as a promising technology for continuous high-purity H₂ production in a carbon constrained world. However, it usually needs an external heat supply for the endothermic decomposition reactions. By taking advantage of the chemical looping combustion (CLC) technology, this study proposed a deep regeneration approach using H₂O and O₂ as regeneration agents to overcome the issues with maintaining catalytic activity and producing the heat needed for the endothermic reactions of H₂ production from methane. TG-DTA and bench scale fluidized bed experimental results indicate that a deep regeneration degree of 30% or above could completely reactivate the spent AC catalyst and simultaneously generate sufficient heat than required in the methane decomposition reaction. Characterization study implies that the deep regenerated AC catalyst could maintain its physical properties within a certain number of cycles. Based on the experimental results, the chemical looping methane thermo-catalytic decomposition process was further optimized and assessed by Aspen Plus^® thermodynamic simulation. The results indicate that heat and mass balances could be attained, and the circulation of the AC catalyst with a temperature difference of 262 °C between the decomposer and the regenerator enabling the process to run autothermally.     

Natural sand as a non-conventional catalyst for hydrogen production by methane thermo-catalytic decomposition

The thermo-catalytic decomposition of methane is considered a promising process for H₂ production in the carbon constrained world. A durable and cost-effective catalyst is required for practical methane decomposition processes within industrial applications; unfortunately, most catalysts suffer from extensive deactivation because of carbon deposition. To address this issue, this study assessed a low-cost, widely-available material - natural sand - as a non-conventional catalyst with the realization that it contained impurities such as iron oxides which may impart reaction activity. Its interesting performance in the methane decomposition reaction is reported herein and assessed relative to a potential cause of increasing catalytic activity with longer reaction times. One result of possible significance is the development of tubular carbon structures on the sand's surface that grew significantly in diameter and length with longer reaction times. High Resolution Transmission Electron Microscopy (HRTEM) imaging showed that this tubular carbon contained extensive humps on the external surface of the tube walls which grew in prominence with longer reaction times. The humps did not contain iron particles, in contrast to the heads of the tubes, and consisted of highly disordered graphitic layers. Previous research has pointed to the existence of free radicals or unsaturated bonding in these types of disordered layers, which can provide sites for catalytic reactions. Hence, it is proposed that the increasing prominence of the humps as the reaction time was increased, and by extension an increasing number of surface free radicals, was a possible cause for an increasing catalytic activity after the iron particles on the sand surface were covered with carbon and tube growth was initiated. These data are seen as potentially useful for devising alternative approaches to diminish catalytic deactivation during methane conversion to H₂.     

Evaluation of syngas-based chemical looping applications for hydrogen and power co-generation with CCS

This paper evaluates hydrogen and power co-generation based on coal-gasification fitted with an iron-based chemical looping system for carbon capture and storage (CCS). The paper assess in details the whole hydrogen and power co-production chain based on coal gasification. Investigated plant concepts of syngas-based chemical looping generate about 350–450 MW net electricity with a flexible output of 0–200 MW_th hydrogen (based on lower heating value) with an almost total decarbonisation rate of the coal used.     

Thermodynamic investigation on hydrogen production via self-sufficient chemical looping reforming of glycerol (CLRG) using metal oxide oxygen carriers

The self-sufficient chemical looping reforming of glycerol (CLRG) utilizes the same basic principles as chemical looping combustion (CLC), the main difference being that the desired product in CLRG is not heat but H₂. Therefore, in the CLR process the O/C ratio is kept low to prevent the complete oxidation of glycerol to H₂O. A systematic thermodynamic study of CLRG using metal oxide oxygen carriers (NiO, CuO, CoO, Co₃O₄, Mn₃O₄, Mn₂O₃ and Fe₂O₃) is performed to analyze the product yield, carbon deposition and energy requirements at different temperatures and pressures. The calculation results show higher temperatures promote, but higher pressures inhibit H₂ production. Favorable conditions (800 °C and 1 atm) are obtained for H₂ manufacture from CLRG process. CuO is the best performing oxygen carrier followed by Mn-based oxygen carriers, while Fe₂O₃ is the least preferred oxygen carrier for CLRG. These results obtained in this theoretical study can offer helpful information for CLRG experimental tests.     

An energy-efficient plasma methane pyrolysis process for high yields of carbon black and hydrogen

A novel thermal plasma process was developed, which enables economically viable commercial-scale hydrogen and carbon black production. Key aspects of this process are detailed in this work. Selectivity and yield of both solid, high-value carbon and gaseous hydrogen are given particular attention. For the first time, technical viability is demonstrated through lab scale reactor data which indicate methane feedstock conversions of >99%, hydrogen selectivity of >95%, solid recovery of >90%, and the ability to produce carbon particles of varying crystallinity having the potential to replace traditional furnace carbon black. The energy intensity of this process was established based on real-time operation data from the first commercial plant utilizing this process. In its current stage, this technology uses around 25 kWh per kg of H2 produced, much less than water electrolysis which requires approximately 60 kWh per kg of H2 produced. This energy intensity is expected to be reduced to 18–20 kWh per kg of hydrogen with improved heat recovery and energy optimization.