The role of surface reactions on the active and selective catalyst design for bioethanol steam reforming

In order to study the role of surface reactions involved in bioethanol steam reforming mechanism, a very active and selective catalyst for hydrogen production was analysed. The highest activity was obtained at 700 °C, temperature at which the catalyst achieved an ethanol conversion of 100% and a selectivity to hydrogen close to 70%. It also exhibited a very high hydrogen production efficiency, higher than 4.5 mol H₂ per mol of EtOH fed. The catalyst was operated at a steam to carbon ratio (S/C) of 4.8, at 700 °C and atmospheric pressure. No by-products, such as ethylene or acetaldehyde were observed. In order to consider a further application in an ethanol processor, a long-term stability test was performed under the conditions previously reported. After 750 h, the catalyst still exhibited a high stability and selectivity to hydrogen production. Based on the intermediate products detected by temperature programmed desorption and reaction (TPD and TPR) experiments, a reaction pathway was proposed. Firstly, the adsorbed ethanol is dehydrogenated to acetaldehyde producing hydrogen. Secondly, the adsorbed acetaldehyde is transformed into acetone via acetic acid formation. Finally, acetone is reformed to produce hydrogen and carbon dioxide, which were the final reaction products. The promotion of such reaction sequence is the key to develop an active, selective and stable catalyst, which is the technical barrier for hydrogen production by ethanol reforming.     

Thermodynamic analysis of hydrogen production for fuel cell via oxidative steam reforming of propane

A thermodynamic analysis of hydrogen production from propane by oxidative steam reforming (OSR) is performed with a Gibbs free energy minimization method. Addition of oxygen reduces the enthalpy of the system and facilitates the heat supply. Equilibrium compositions of OSR as a function of temperature (300, 500, 700 and 900 °C), H₂O/C₃H₈ ratio (1.0–20.0) and O₂/C₃H₈ ratio (0.0–2.0) under oxidative and thermo-neutral (TN) conditions are evaluated. The results for oxidative conditions demonstrate that at 700 °C with H₂O/C₃H₈ ratios above 7.0 and/or O₂/C₃H₈ ratios higher than 1.3 are beneficial for hydrogen production which facilitates superior hydrogen yield, i.e. close to 9.0 mol/mol propane, with coke and methane formation reactions being suppressed effectively. For TN condition, autothermal temperature and equilibrium composition have a stronger dependence on O₂/C₃H₈ ratio than on H₂O/C₃H₈ ratio. Further calculations show that the condition at 700 °C with an appropriate H₂O/C₃H₈ ratio between 7.0 and 13.0 is favorable for achieving a high hydrogen yield and a low carbon monoxide yield. Therefore, a favorable operational range is proposed to ensure the most optimized product yield.     

Zirconia supported catalysts for bioethanol steam reforming: Effect of active phase and zirconia structure

Three new catalysts have been prepared in order to study the active phase influence in ethanol steam reforming reaction. Nickel, cobalt and copper were the active phases selected and were supported on zirconia with monoclinic and tetragonal structure, respectively. To characterize the behaviour of the catalysts in reaction conditions a study of catalytic activity with temperature was performed. The highest activity values were obtained at 973 K where nickel and cobalt based catalysts achieved an ethanol conversion of 100% and a selectivity to hydrogen close to 70%. Nickel supported on tetragonal zirconia exhibited the highest hydrogen production efficiency, higher than 4.5 mol H₂/mol EtOH fed. The influence of steam/carbon (S/C) ratio on product distribution was another parameter studied between the range 3.2–6.5. Nickel supported on tetragonal zirconia at S/C = 3.2 operated at 973 K without by-product production such as ethylene or acetaldehyde. In order to consider a further application in an ethanol processor, a long-term reaction experiment was performed at 973 K, S/C = 3.2 and atmospheric pressure. After 60 h, nickel supported on tetragonal zirconia exhibited high stability and selectivity to hydrogen production.     

Hydrogen production by sorption enhanced steam reforming of propane: A thermodynamic investigation

Thermodynamic features of hydrogen production by sorption enhanced steam reforming (SESR) of propane have been studied with the method of Gibbs free energy minimization and contrasted with propane steam reforming (SR). The effects of pressure (1-5 atm), temperature (700-1100 K) and water to propane ratio (WPR, 1-18) on equilibrium compositions and carbon formation are investigated. The results suggest that atmospheric pressure and a WPR of 12 are suitable for hydrogen production from both SR and SESR of propane. High WPR is favourable to inhibit carbon formation. The minimum WPR required to eliminate carbon production is 6 in both SR and SESR. The most favourable temperature for propane SR is approximately 950 K at which 1 mol of propane has the capacity to produce 9.1 mol of hydrogen. The optimum temperature for SESR is approximately 825 K, which is over 100 K lower than that for SR. Other key benefits include enhanced hydrogen production of nearly 10 mol (stoichiometric value) of hydrogen per mole of propane at 700 K, increased hydrogen purity (99% compared with 74% in SR) and no CO₂ or CO production with the only impurity being CH₄, all indicating a great potential of SESR of propane for hydrogen production.     

Nickel and cobalt as active phase on supported zirconia catalysts for bio-ethanol reforming: Influence of the reaction mechanism on catalysts performance

Steam reforming of ethanol for hydrogen production was investigated on Co/ZrO₂ and Ni/ZrO₂ catalysts promoted with lanthana. Catalysts were prepared by impregnation method and characterized by XRD and TPR. TPD-R experiments were also carried out to determine the role of active phase on reaction mechanism. The results suggest that adsorbed ethanol is dehydrogenated to acetaldehyde producing hydrogen. Then, the adsorbed acetaldehyde may evolve by different mechanisms, depending on the nature of active phase. On one hand, in cobalt-based catalyst, acetaldehyde could be reformed directly. By acetaldehyde thermal decomposition, methyl and formaldehyde groups are obtained. By coupling of methyl groups, ethane can be obtained. At medium temperature range, WGS reaction contribution is noteworthy.     

Impact of bioethanol impurities on steam reforming for hydrogen production: A review

Hydrogen fuel cells (H₂–FCs) are promising devices for pollution-free and efficient power production. Renewable H₂ from biomass is often produced through catalytic ethanol steam reforming (ESR), which requires a steam/ethanol molar ratio of at least three. The bioethanol obtained by biomass fermentation contains large amounts of water and can be directly subjected to ESR without complex purification steps. However, a wide spectrum of impurities is present in such bioethanol samples, thus complicating the ESR process. Acetic acid, fusel alcohols, ethyl acetate, and sulfur components have been reported as important bioethanol impurities, and also as the main precursors of carbon deposits on the ESR catalyst. On the other hand, amines, methanol, and aldehydes, which are minor bioethanol impurities, have been reported to enhance the H₂ production. This review seeks to define alternatives to reduce the above negative impurities and increase the positive ones during biomass pretreatment and fermentation. Additionally, ESR catalysts are reviewed to identify the features that make them more resistant to deactivation. The combination of strategies to control the impurities during biomass pretreatment, fermentation, purification and the development of highly resistant catalysts may allow processes to produce H₂ from biomass with a low carbon footprint, rendering H₂–FCs an environmentally friendly technology for power production.     

Thermodynamic analysis of propane dry and steam reforming for synthesis gas or hydrogen production

Thermodynamics was applied to investigate propane dry reforming (DR) and steam reforming (SR). Equilibrium calculations employing the Gibbs free energy minimization were performed upon a wide range of pressure (1–5 atm), temperature (700–1100 K), carbon dioxide to propane ratio (CPR, 1–12) and water to propane ratio (WPR, 1–18). From a thermodynamic perspective, it is demonstrated that DR is promising for production of synthesis gas with low hydrogen content, as opposite to SR which favours generation of synthesis gas with high hydrogen content. Complete conversion of propane was obtained for the range of pressure, temperature, CPR and WPR considered in this study. Atmospheric pressure is shown to be preferable for both DR and SR. Approximately 10 mol of synthesis gas can be produced per mole of propane at a temperature greater than 1000 K from DR when CPR is higher than 6. The optimum conditions for synthesis gas production from DR are found to be 975 K (CPR = 3) for a H₂/CO ratio of 1 and 1100 K (CPR = 1) for a H₂/CO ratio of 2. The greatest CO₂ conversion (95%) can be obtained also at 1100 K and CPR = 1. Preferential conditions for hydrogen production from SR are achieved with the temperatures between 925 and 975 K and WPRs of 12–18. The maximum number of moles of hydrogen produced is 9.1 (925 K and WPR = 18). Under conditions that favour hydrogen production, methane and carbon formation can be eliminated to negligible level.     

Reaction/separation coupled equilibrium modeling of steam methane reforming in fluidized bed membrane reactors

An equilibrium model of steam methane reforming coupled with in-situ membrane separation for hydrogen production was developed. The model employed Sievert’s Law for membrane separation and minimum Gibbs energy model for reactions. The reforming and separation processes were coupled by the mass balance. The model assumed a continuously stirred tank reactor for the fluidized bed hydrodynamics. The model predictions for a typical case were compared with those from the model of Ye et al. [15] which assumed a plug flow for bed hydrodynamics. The model predictions show satisfactory agreement with experimental data in the literatures. The influences of reactor pressure, temperature, steam to carbon ratio, and permeate side hydrogen partial pressure on solid carbon, NH_x and NO_x formation were studied using the model.     

Hydrogen production by ethanol steam reforming: A study of Co- and Ce-based catalysts over FeCrAlloy monoliths

Co- and Ce-based structured catalysts deposited on FeCrAlloy monoliths have been prepared. A new two-step strategy for coating the monolith is used: (i) first, a MgAl₂O₄ spinel layer is generated on the FeCrAlloy substrate, and (ii) then, Co and Ce are incorporated in two different molar ratios by the conventional wet impregnation method. The spinel layer is formed from a solution of colloidal alumina and Mg(NO₃)₂, with an apparent viscosity of around 3300 mPa s. The results indicate that a homogeneous spinel coating with excellent adherence is obtained after two immersions and a calcination at 700 °C. Both structured catalysts are active in the steam reforming of ethanol at 650 °C. The system with a Co/Ce molar ratio of 3.7 exhibits the best performance with a high stability. A complete ethanol conversion and a hydrogen selectivity of around 95% are obtained in two reaction cycles of 36 h each with intermediate regeneration.     

Development of biogas reforming Ni-La-Al catalysts for fuel cells

In this work, the results obtained for Ni-La-Al catalysts developed in our laboratory for biogas reforming are presented. The catalyst 5% Ni/5% La₂O₃-γ-Al₂O₃ has operated under kinetic control conditions for more than 40 h at 700 °C and feeding CH₄/CO₂ ratio 1/1, similar to the composition presented in biogas streams, being observed a stable behaviour.     

Thermodynamic equilibrium analysis of combined dry and steam reforming of propane for thermochemical waste-heat recuperation

The thermochemical waste-heat recuperation is one for perspective way of increasing the energy efficiency of the fuel-consuming equipment. In this paper, the thermochemical waste-heat recuperation (TCR) by combined steam-dry propane reforming is described. To understand the influence of technological parameter such as temperature and composition of inlet gas mixture on TCR efficiency, thermodynamic equilibrium analysis of combined steam-dry propane reforming was investigated by Gibbs free energy minimization method upon a wide range of temperature (600–1200 K) and different feed compositions at atmospheric pressure. The carbon and methane formation was also calculated and shown. From a thermodynamic perspective, the TCR can be used for increasing energy efficiency at temperatures above 950 K because in this range the maximum conversion rate is reached (from 1.22 to 1.30 for the different feed composition). Approximately 10 mol of synthesis gas can be generated per mole of propane at the temperatures greater than 1000 K. Furthermore, the propane conversion rate and yield of hydrogen are increased with the addition of extra steam to the feed stock. Also, undesirable carbon formation can be eliminated by adding steam to the feed. The thermodynamic equilibrium analysis was accomplished by IVTANTHERMO which is a process simulator for thermodynamic modeling of complex chemically reacting systems and several results were checked by Aspen-HYSYS.     

Effect of acidity on the performance of a Ni-based catalyst for hydrogen production through propane steam reforming: K-AlSixOy support with different Si/Al ratios

Propane steam reforming (PSR) for the production of H₂ was catalyzed by a NiO/K-AlSi_xO_y catalyst synthesized with various Si/Al ratios (Si/Al = 0, 0.3, 0.5, 0.7, and 1.0). The effect of the Si/Al ratio on the acidity of the NiO/K-AlSi_xO_y catalyst for PSR was investigated. NiO/K-AlSi_xO_y gave a higher H₂ selectivity and stability during PSR than NiO/K-SiO₂ and NiO/K-Al₂O₃. The NH₃-TPD results showed that the acid quantity and strength of NiO/K-AlSi_xO_y changed significantly depending on the Si/Al ratio. With an increased Si/Al ratio, the densities of both weak and strong acid sites increased. The C₃H₈- and CO-TPD results indicated that desorption amounts increased significantly in all NiO/K-AlSi_xO_y catalysts relative to those of NiO/K-SiO₂ and NiO/K-Al₂O₃, and the adsorption amount increased with the Si/Al ratio. PSR results showed that the NiO/K-AlSi_xO_y catalyst exhibited much better stability than the NiO/K-SiO₂ and NiO/K-Al₂O₃ catalysts. This study confirms the following facts: when the acidity is appropriately adjusted for the catalyst, adsorption of the reaction gas increases, which eventually increases the reaction rate and also inhibits strong sintering between the nickel and the Al₂O₃ support. As a result, deterioration of the catalyst can be reduced.     

Comparative study of propane steam reforming in vanadium based catalytic membrane reactor with nickel-based catalysts

The performance of catalytic membrane reactor with Pd-coated V membrane was examined for steam reforming of propane. The long term reforming experiment confirmed the stability of the V membrane with high hydrogen selectivity and permeability. The effect of types of hydrogen permeable membranes on the performance of the catalytic membrane reactor was studied by comparing Pd-coated V, Pd–23Ag, and Pd–10Ag membranes. The types of hydrogen separation membranes (i.e. hydrogen removal rates) did not have a marked effect on the propane conversion rates, while the product compositions were largely influenced by the hydrogen removal rate. Varying metal oxide supports of Ni-catalysts resulted in significant differences in the product compositions. Further, the evaluation of various catalyst-support systems (9wt%Ni–1wt%M/CeO₂, M = Co, Pt, Ag, Ru) revealed that hydrogen yield was the highest when 1wt%Ag was added to Ni/CeO₂. However, it was also found that excessive secondary metal additions can have negative impact on the catalytic behaviour of parent catalysts.     

On-board reforming of biodiesel and bioethanol for high temperature PEM fuel cells: Comparison of autothermal reforming and steam reforming

In the 21st century biofuels will play an important role as alternative fuels in the transportation sector. In this paper different reforming options (steam reforming (SR) and autothermal reforming (ATR)) for the on-board conversion of bioethanol and biodiesel into a hydrogen-rich gas suitable for high temperature PEM (HTPEM) fuel cells are investigated using the simulation tool Aspen Plus. Special emphasis is placed on thermal heat integration. Methyl-oleate (C₁₉H₃₆O₂) is chosen as reference substance for biodiesel. Bioethanol is represented by ethanol (C₂H₅OH). For the steam reforming concept with heat integration a maximum fuel processing efficiency of 75.6% (76.3%) is obtained for biodiesel (bioethanol) at S/C = 3. For the autothermal reforming concept with heat integration a maximum fuel processing efficiency of 74.1% (75.1%) is obtained for biodiesel (bioethanol) at S/C = 2 and λ = 0.36 (0.35). Taking into account the better dynamic behaviour and lower system complexity of the reforming concept based on ATR, autothermal reforming in combination with a water gas shift reactor is considered as the preferred option for on-board reforming of biodiesel and bioethanol. Based on the simulation results optimum operating conditions for a novel 5 kW biofuel processor are derived.     

Catalytic steam reforming of bio-oil model compounds for hydrogen production over coal ash supported Ni catalyst

The development of a high performance and low cost catalyst is an important contribution to clean hydrogen production via the catalytic steam reforming of renewable bio-oil. Solid waste coal ash, which contains SiO₂, Al₂O₃, Fe₂O₃ and many alkali and alkaline earth metal oxides, was selected as a superior support for a Ni-based catalyst. The chemical composition and textural structures of the ash and the Ni/Ash catalysts were systematically characterized. Acetic acid and phenol were selected as two typical bio-oil model compounds to test the catalyst activity and stability. The conversion of acetic acid and phenol reached as much as 98.4% and 83.5%, respectively, at 700 °C. It is shown that the performance of the Ni/Ash catalyst was comparable with other commercial Ni-based steam reforming catalysts.     

Experimental evaluation of methanol steam reforming reactor heated by catalyst combustion for kW-class SOFC

The distributed power generation of methanol steam reforming reactor combined with solid oxide fuel cell (SOFC) has the characteristics of outstanding economic advantages. In this paper, a methanol steam reforming reactor was designed which integrates catalyst combustion, vaporization and reforming. By catalyst combustion, it can achieve stable operation to supply fuel for kW-class SOFC in real time without additional heating equipment. The optimal operating condition of the reforming reactor is 523–553 K, and the steam to carbon ratio (S/C) is 1.2. To study the reforming performance, methanol steam reforming (MSR), methanol decomposition (MD), water-gas shift (WGS) were considered. Operating temperature is the greatest factor affecting reforming performance. The higher the reaction temperature, the lower the H₂ and CO₂, the higher the CO and the methanol conversion rate. The methanol conversion rate is up to 95.03%. The higher the liquid space velocity (LHSV), the lower the methanol conversion rate, the lowest is 90.7%. The temperature changes of the reforming reactor caused by the load change of stack takes about 30 min to reach new balance. Local hotspots within the reforming reactor lead to an excessive local temperature to test a small amount of CH₄ in the reforming gas. The methanation reaction cannot be ignored at the operating temperature. The reforming gas contains 70–75% H₂, 3–8% CO, 18–22% CO₂ and 0.0004–0.3% CH₄. Trace amounts of C₂H₆ and C₂H₄ are also found in some experiments. The reforming reactor can stably supply the fuel for up to 1125 W SOFC.     

Biogas upgrade to syn-gas (H2-CO) via dry and oxidative reforming

Fuel reforming processes are primarily used to generate hydrogen for fuel cells and in automotive internal combustion engines to improve combustion characteristics and emissions. In this study, biogas is used as the fuel source for the reforming process as it has desirable properties of being both renewable and clean. Two reforming processes (dry reforming and combined dry/oxidative reforming) are studied. Both processes are affected by the gas stream temperature and reactor space velocity with the second process being affected by O₂/CH₄ ratio as well. Our results imply that oxidative reforming is the dominant process at low exhaust temperatures. This provides heat for the dry reforming of biogas and the overall reforming is exothermic. Increase in O₂/CH₄ ratio at low temperature promotes hydrogen production. At high exhaust temperatures (>600 °C), dry reforming of biogas is dominant and the overall reaction is net endothermic.     

Biogas as hydrogen source for fuel cell applications

In the last decade, production of biogas from biomass degradation has attracted the attention of several research groups. The interest on this hydrogen source is focused on the potential use of this gas as raw material to supply high temperature fuel cells (HTFC). This paper reports a wide research investigation carried out at CNR-ITAE on biogas reforming processes (steam reforming, autothermal reforming and partial oxidation). A mathematical model was developed, in Aspen Plus, and an experimental validation was made in order to confirm model results. Simulations were performed to determine the reformed gas composition and the system energy balance as a function of process temperature and pressure. The value of Gas Hour Space Velocity (GHSV) was selected for calculating compositions at full equilibrium, as it is expected in operative large scale plant. To obtain a realistic evaluation of the reforming processes efficiency, the energy balance for each examined process was developed as available energy of outlet syngas on inlet required energy ratio. The comparison between values of efficiency process gives useful indication about their reliability to be integrate with fuel cell systems.     

Hydrogen production from formaldehyde steam reforming using recyclable NiO/NaF catalyst

Harmless disposal of formaldehyde for hydrogen production is attractive in energy and environmental science. Herein, hydrogen production from formaldehyde steam reforming (FSR) using recyclable NiO/NaF was investigated, and the optimized 4 wt% catalyst exhibited 100% formaldehyde conversion, 104.9% H₂ selectivity and excellent stability at 400 °C. The used catalysts can be easily recycled by water washing and pickling, and the activity of the regenerated catalyst is comparable to that of fresh one. We confirmed that FSR was a rapid surface reaction process, and the special contribution of NaF ensured the high activity of NiO/NaF with low specific surface areas. F ions in the carrier could activate the surface OH, which promoted adsorption and oxidation to reactants. Furthermore, the in situ DRIFTs study revealed that formate species (HCOO) was the key intermediate in formaldehyde steam reforming. Formaldehyde was first oxidized to HCOO and then decomposed into CO₂ and H.     

Glycerol steam reforming on supported Ru-based catalysts for hydrogen production for fuel cells

Glycerol steam reforming on Ru and Ru–Me (Me = Fe, Co, Ni, and Mo) catalysts supported on yttria, ceria–zirconia, and γ-alumina is studied at high temperatures for the production of hydrogen for fuel cell applications. The nature of the support notably affects the catalytic properties of these catalysts resulting in significant enhancements of the H₂ production turnover rate and product selectivity on the reducible yttria and ceria–zirconia supported Ru-based catalysts via facilitation of the water–gas shift reaction. The acidic γ-alumina supported Ru-based catalysts demonstrate a low H₂ production turnover rate with a high CO product selectivity and also favor the formation of C₁–C₂ hydrocarbons. Differently, the promotion effects due to Fe, Co, Ni, and Mo on the bimetallic Ru–Me catalysts are limited with only small increases in the glycerol conversion turnover rate for the Ru–Ni, Ru–Mo, and Ru–Co catalysts. The alumina supported Ru-based catalysts are deactivated by a significant extent with increasing on-stream time due to coking. The carbon deposition is insignificant on the yttria and ceria-zirconia supported catalysts, but moderate deactivation occurs due to sintering of the dispersed metal clusters. Influenced by the surface MoO_x species that hamper sintering of the surface metal clusters and by the Y₂O₃ support that prevents coking on the catalyst, the Ru–Mo/Y₂O₃ catalysts exhibit superior catalytic stability against deactivation.