Steam reforming of technical bioethanol for hydrogen production

Essentially all work on ethanol steam reforming so far has been carried out using simulated bioethanol feedstocks, which means pure ethanol mixed with water. However, technical bioethanol consists of a lot of different components including sugars, which cannot be easily vaporized and steam reformed. For ethanol steam reforming to be of practical interest, it is important to avoid the energy-intensive purification steps to fuel grade ethanol. Therefore, it is imperative to analyze how technical bioethanol, with the relevant impurities, reacts during the steam reforming process. We show how three different distillation fractions of technical 2nd generation bioethanol, produced in a pilot plant, influence the performance of nickel- and ruthenium-based catalysts during steam reforming, and we discuss what is required to obtain high activity and long catalyst lifetime. We conclude that the use of technical bioethanol will result in a faster catalyst deactivation than what is observed when using pure ethanol–water mixtures because of contaminants remaining in the feed. However, the initial activity of the catalysts are not affected by this, hence it is important to not only focus on catalyst activity but rather on the lifetime of the catalyst.     

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.     

Renewable hydrogen production concepts from bioethanol reforming with carbon capture

This paper is assessing the hydrogen production from bioethanol at industrial scale (100000 Nm³/h hydrogen equivalent to 300 MW thermal) with carbon capture. Three carbon capture designs were investigated, one based on pre-combustion capture using chemical gas–liquid absorption and two based on chemical looping (one based on syngas and one using direct bioethanol looping). The carbon capture options were compared with the similar designs without carbon capture. The designs were simulated to produce mass and energy balances for quantification of key performance indicators. A particular accent is put on assessment of reforming technologies (steam and oxygen-blown autothermal reforming) and chemical looping units, process integration issues of carbon capture step within the plant, modelling and simulation of whole plant, thermal and power integration of various plant sub-systems by pinch analysis. The results for chemical looping designs (either syngas-based or direct bioethanol) show promising energy efficiency coupled with total carbon capture rate.     

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.     

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.     

Hydrogen production via the solar thermal decarbonization of fossil fuels

Three hybrid solar/fossil-fuel endothermic processes, in which fossil fuels are used exclusively as the chemical source for H₂ production, and concentrated solar radiation as the energy source of high-temperature process heat, are considered: (1) the thermal decomposition; (2) the steam-reforming; and (3) the steam-gasification. A second-law analysis is performed for establishing their maximum exergy efficiency and CO₂ mitigation potential vis-à-vis the conventional combustion-based power generation. These hybrid solar thermochemical processes offer viable and efficient routes for fossil fuel decarbonization and CO₂ avoidance, and further create a transition path towards solar hydrogen.     

Industrial scale steam reforming of bioethanol: A conceptual study

This study investigates the technical feasibility of using existing steam reforming and hydrogen separation technologies to produce hydrogen from bioethanol on an industrial scale (100,000 Nm³/h of hydrogen). Two separate reaction schemes for producing hydrogen from bioethanol are compared. One scheme utilises both a prereformer and steam reformer, and the other requires a steam reformer only.     

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.     

Hydrogen production by steam reforming of ethanol over Ni-based catalysts promoted with noble metals

The catalytic activity of Ni/La₂O₃-Al₂O₃ catalysts modified with noble metals (Pt and Pd) was investigated in the steam reforming of ethanol. The catalysts were characterized by ICP, S_BET, X-ray diffraction, temperature-programmed reduction, UV–vis diffuse reflectance spectroscopy and X-ray absorption fine structure (XANES). The results showed that the formation of inactive nickel aluminate was prevented by the presence of La₂O₃ dispersed on the alumina. The promoting effect of noble metals included a marked decrease in the reduction temperatures of NiO species interacting with the support, due to the hydrogen spillover effect, facilitating greatly the reduction of the promoted catalysts. It was seen that the addition of noble metal stabilized the Ni sites in the reduced state throughout the reaction, increasing ethanol conversion and decreasing coke formation, irrespective of the nature or loading of the noble metal.     

Simulation of a hydrogen production and purification system for a PEM fuel-cell using bioethanol as raw material

A process to produce “fuel-cell grade” hydrogen from ethanol steam reforming is analyzed from a thermodynamic point of view. The hydrogen purification process consists of WGS and COPROX reactors. Equations to evaluate the efficiency of the system, including the fuel cell, are presented. A heat exchange network is proposed in order to improve the exploitation of the available power. The effect of key variables such as the reformer temperature and the ethanol/water molar feed ratio on the fuel-cell efficiency is discussed. Results show that it is feasible to carry out the energy integration of the hydrogen catalytic production and purification—PEM fuel-cell system, using ethanol as raw material. The technology of “fuel-cell grade” hydrogen production using ethanol as raw material is a very attractive alternative to those technologies based in fossil fuels.     

Hydrogen production by methanol reforming in a non-thermal atmospheric pressure microplasma reactor

On board reforming of hydrocarbons for fuel cell feed has become an attractive research topic due to the low energy densities of batteries. The implementation of a microplasma as a means for reforming the liquid fuel methanol is explored in this work. Hydrocarbon reforming is commonly accomplished through catalysis, but catalysts have a number of limitations such as poisoning, coking, coarsening, long start-up times and excessive costs. Published studies have shown the viability of plasma reforming but none have succeeded in achieving suitable system efficiencies for portable applications. Non-thermal microplasmas are particularly attractive for reforming due to their extremely high electron and power densities and the scale of microplasma devices make them well suited for portable applications. This study describes experimental microplasma reactors reforming methanol. The reactors are based on the microhollow cathode discharge (MHCD) structure fabricated with microelectromechanical systems (MEMS) fabrication techniques. Through modeling the reaction for all five experiments, conversions within the microchannel were found to be nearly 100%. Despite the variations in the five experiments due to input electrical power, flow rate and concentration, the model was validated in each test. The experiments discussed in this work show the promise of a portable, non-thermal microplasma reformer that generates hydrogen for fuel cells for portable power.     

Hydrogen production from solar energy powered supercritical cycle using carbon dioxide

A hydrogen production method is proposed, which utilizes solar energy powered thermodynamic cycle using supercritical carbon dioxide (CO₂) as working fluid for the combined production of hydrogen and thermal energy. The proposed system consists of evacuated solar collectors, power generating turbine, water electrolysis, heat recovery system, and feed pump. In the present study, an experimental prototype has been designed and constructed. The performance of the cycle is tested experimentally under different weather conditions. CO₂ is efficiently converted into supercritical state in the collector, the CO₂ temperature reaches about 190 °C in summer days, and even in winter days it can reach about 80 °C. Such a high-temperature realizes the combined production of electricity and thermal energy. Different from the electrochemical hydrogen production via solar battery-based water splitting on hand, which requires the use of solar batteries with high energy requirements, the generated electricity in the supercritical cycle can be directly used to produce hydrogen gas from water. The amount of hydrogen gas produced by using the electricity generated in the supercritical cycle is about 1035 g per day using an evacuated solar collector of 100.0 m² for per family house in summer conditions, and it is about 568.0 g even in winter days. Additionally, the estimated heat recovery efficiency is about 0.62. Such a high efficiency is sufficient to illustrate the cycle performance.     

Hydrogen production via steam-methane reforming in a SOMBRERO fusion breeder with ceramic fuel particles

In the present study, hydrogen production potential of SOlid Moving BREeder ReactOr (SOMBRERO) fusion reactor and heat recovery of this system is investigated. The original SOMBRERO fusion reactor has a 1000 MWe KrF laser-driven IFE power plant. This reactors fusion power is 2677 MW and total thermal power is 2891 MW. The blanket is divided into three breeding zone and these breeding zones have different C, Li₂O and ceramic fuel particles. One-dimensional neutronic calculations of SOMBRERO fusion reactor have been performed by using XSDRNPM/SCALE4.4a neutron transport code. Steam Methane Reforming (SMR) method is used for large-scale hydrogen production and heat recovery of waste heat is analyzed. The numerical results show that the considered SOMBRERO fusion reactor has a good neutronic performance as well as the high hydrogen production potential with heat recovery of SMR process.     

Hydrogen production by steam reforming of bio-oil/bio-ethanol mixtures in a continuous thermal-catalytic process

The feasibility of the steam reforming of bio-oil aqueous fraction and bio-ethanol mixtures has been studied in a continuous process with two in-line steps: thermal step at 300 °C (for the controlled deposition of pyrolytic lignin during the heating of the bio-oil/bio-ethanol feed) followed by steam reforming in a fluidized bed reactor on a Ni/α-Al₂O₃ catalyst. The effect of bio-ethanol content in the feed has been analyzed in both the thermal and reforming steps, and the suitable range of operating conditions (temperature and space-time) has been determined for obtaining a high and steady hydrogen yield. Higher ethanol content in the mixture feed improves the reaction indices and reduces coke deposition. Operating conditions of 700 °C and space-times higher than 0.23 g_catalyst h (g_bio-oil+EtOH)⁻¹ are suitable for attaining almost fully conversion of oxygenates (bio-oil and ethanol) and hydrogen yields above 93%, with low catalyst deactivation.     

Effects of particle sizes on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system

The discrete element geometric model of the horizontally buried-pipe steam generator was set up. The effects of particle size (20 mm–80 mm) on performances of the horizontally buried-pipe steam generator using waste heat in a bioethanol steam reforming hydrogen production system was studied. When the particle size increases, the particle layer flatness decreases, the particle layer flow ununiformity increases. The volatility of the particle residence time distribution increases with the particle size increases, and the standard deviation of the particle residence time increases. When the particle size increases, the voidage of the particle system increases. So the particle thermal resistance in the steam generator increases with the particle size increases, the steam production of the generator decreases, and the system hydrogen production of decreases.     

Hydrogen production from methanol reforming in microwave “tornado”-type plasma

A microwave (2.45 GHz) “tornado”-type plasma with a high-speed tangential gas injection (swirl) at atmospheric pressure conditions has been applied for methanol reforming. The vortex gas flow “detaches” the hot plasma core from the wall and stable operation of the plasma source has been achieved. The hydrogen production rate dependence on the partial methanol flux has been investigated both in Ar and Ar + water plasma environments. Hydrogen, carbon oxide and carbon dioxide are the main decomposition products. Mass and FT-IR spectroscopy have been used to detect the species in the outlet gas stream. It has been found that the hydrogen production rate increases by nearly a factor of 1.5 when water is added into the plasma. Higher energetic hydrogen mass yield is achieved when compared with the results obtained under laminar gas flow conditions. Practically 100% methanol conversion rate has been achieved. Moreover, optical emission spectroscopy has been applied to determine the gas temperature, the electron density and the radiative species present in the plasma. A theoretical model based on a set of equations describing the chemical kinetics and the gas thermal balance equation has been developed. The theoretical results on the decomposition products agree well with the experimental ones and confirm that microwave plasma decomposition of methanol is a temperature dependent process. The results clearly show that this type of plasma is an efficient tool for hydrogen production.     

Hydrogen production by reforming of liquid hydrocarbons in a membrane reactor for portable power generation—Experimental studies

One of the most promising technologies for lightweight, compact, portable power generation is proton exchange membrane (PEM) fuel cells. PEM fuel cells, however, require a source of pure hydrogen. Steam reforming of hydrocarbons in an integrated membrane reactor has potential to provide pure hydrogen in a compact system. Continuous separation of product hydrogen from the reforming gas mixture is expected to increase the yield of hydrogen significantly as predicted by model simulations. In the laboratory-scale experimental studies reported here steam reforming of liquid hydrocarbon fuels, butane, methanol and Clearlite^® was conducted to produce pure hydrogen in a single step membrane reformer using commercially available Pd–Ag foil membranes and reforming/WGS catalysts. All of the experimental results demonstrated increase in hydrocarbon conversion due to hydrogen separation when compared with the hydrocarbon conversion without any hydrogen separation. Increase in hydrogen recovery was also shown to result in corresponding increase in hydrocarbon conversion in these studies demonstrating the basic concept. The experiments also provided insight into the effect of individual variables such as pressure, temperature, gas space velocity, and steam to carbon ratio. Steam reforming of butane was found to be limited by reaction kinetics for the experimental conditions used: catalysts used, average gas space velocity, and the reactor characteristics of surface area to volume ratio. Steam reforming of methanol in the presence of only WGS catalyst on the other hand indicated that the membrane reactor performance was limited by membrane permeation, especially at lower temperatures and lower feed pressures due to slower reconstitution of CO and H₂ into methane thus maintaining high hydrogen partial pressures in the reacting gas mixture. The limited amount of data collected with steam reforming of Clearlite^® indicated very good match between theoretical predictions and experimental results indicating that the underlying assumption of the simple model of conversion of hydrocarbons to CO and H₂ followed by equilibrium reconstitution to methane appears to be reasonable one.     

Hydrogen production by reforming of liquid hydrocarbons in a membrane reactor for portable power generation—Model simulations

One of the most promising technologies for lightweight, compact, portable power generation is proton exchange membrane (PEM) fuel cells. PEM fuel cells, however, require a source of pure hydrogen. Steam reforming of hydrocarbons in an integrated membrane reactor has potential to provide pure hydrogen in a compact system. In a membrane reactor process, the thermal energy needed for the endothermic hydrocarbon reforming may be provided by combustion of the membrane reject gas. The energy efficiency of the overall hydrogen generation is maximized by controlling the hydrogen product yield such that the heat value of the membrane reject gas is sufficient to provide all of the heat necessary for the integrated process. Optimization of the system temperature, pressure and operating parameters such as net hydrogen recovery is necessary to realize an efficient integrated membrane reformer suitable for compact portable hydrogen generation. This paper presents results of theoretical model simulations of the integrated membrane reformer concept elucidating the effect of operating parameters on the extent of fuel conversion to hydrogen and hydrogen product yield. Model simulations indicate that the net possible hydrogen product yield is strongly influenced by the efficiency of heat recovery from the combustion of membrane reject gas and from the hot exhaust gases. When butane is used as a fuel, a net hydrogen recovery of 68% of that stoichiometrically possible may be achieved with membrane reformer operation at 600 °C (873 K) temperature and 100 psig (0.791 MPa) pressure provided 90% of available combustion and exhaust gas heat is recovered. Operation at a greater pressure or temperature provides a marginal improvement in the performance whereas operation at a significantly lower temperature or pressure will not be able to achieve the optimal hydrogen yield. Slightly higher, up to 76%, net hydrogen recovery is possible when methanol is used as a fuel due to the lower heat requirement for methanol reforming reaction, with membrane reformer operation at 600 °C (873 K) temperature and 150 psig (1.136 MPa) pressure provided 90% of available combustion and exhaust gas heat is recovered.     

Hydrogen production with integrated CO2 capture in a membrane assisted gas switching reforming reactor: Proof-of-Concept

This paper presents a new membrane reactor concept for ultra-pure hydrogen production with integrated CO₂ capture: the membrane-assisted gas switching reforming (MA-GSR). This concept integrates alternating exothermic and endothermic redox reaction stages in a single fluidized bed consisting of catalytically active oxygen-carrier particles, by switching the feed between air and methane/steam, where the produced hydrogen is selectively removed via Pd-based membranes. This concept results in overall autothermal conditions and allows easier operation at high pressure compared to alternative novel technologies. In this work, the MA-GSR concept is demonstrated at lab scale using four metallic supported membranes (Pd–Ag based) immersed into a fluidized bed consisting of a Ni-based oxygen carrier. The performance of the reactor has been tested under different experimental operating conditions and high methane conversions (>50%) have been obtained, well above the thermodynamic equilibrium conversion of a conventional fluidized bed as a result of the selective H₂ extraction, with (ultra-pure) H₂ recoveries above 20% at relatively low temperatures (<550 °C). These results could be further improved by working at elevated pressures or by integrating more membranes. Even though the concept has been successfully demonstrated, further research is required to develop suitable membranes since post-mortem membrane characterization has revealed defects in the membrane selective layer as a consequence of the frequent exposure to thermal cycles with alternating oxidative and reducing atmospheres.     

Hydrogen production by coupling pressurized high temperature electrolyser with solar tower technology

Solar hydrogen production by coupling of pressurized high temperature electrolyser with concentrated solar tower technology is studied. As the high temperature electrolyser requires constant temperature conditions, the focus is made on a molten salt solar tower due to its high storage capacity. A flowsheet was developed and simulations were carried out with Aspen Plus 8.4 software for MW-scale hydrogen production plants. The solar part was laid out with HFLCAL software. Two different scenarios were considered: the first concerns the production of 400 kg/d hydrogen corresponding to mobility use (fuel station). The second scenario deals with the production of 4000 kg/d hydrogen for industrial use. The process was analyzed from a thermodynamic point of view by calculating the overall process efficiency and determining the annual production. It was assumed that a fixed hydrogen demand exists in the two cases and it was assessed to which extent this can be supplied by the solar high temperature electrolysis process including thermal storage as well as hydrogen storage. For time periods with a potential over supply of hydrogen, it was considered that the excess energy is sold as electricity to the grid. For time periods where the hydrogen demand cannot be fully supplied, electricity consumption from the grid was considered. It was assessed which solar multiple is appropriate to achieve low consumption of grid electricity and low excess energy. It is shown that the consumption of grid electricity is reduced for increasing solar multiple but the efficiency is also reduced. At a solar multiple of 3.0 an annual solar-to-H₂ efficiency greater than 14% is achieved at grid electricity production below 5% for the industrial case (4000 kg/d). In a sensitivity study the paramount importance of electrolyser performance, i.e. efficiency and conversion, is shown.