Thermodynamic analysis of hydrogen production via autothermal steam reforming of selected components of aqueous bio-oil fraction

From a technical and economic point of view, autothermal steam reforming offers many advantages, as it minimizes heat load demand in the reformer. Bio-oil, the liquid product of biomass pyrolysis, can be effectively converted to a hydrogen-rich stream. Autothermal steam reforming of selected compounds of bio-oil was investigated using thermodynamic analysis. Equilibrium calculations employing Gibbs free energy minimization were performed for acetic acid, acetone and ethylene glycol in a broad range of temperature (400–1300 K), steam to fuel ratio (1–9) and pressure (1–20 atm) values. The optimal O₂/fuel ratio to achieve thermoneutral conditions was calculated under all operating conditions. Hydrogen-rich gas is produced at temperatures higher than 700 K with the maximum yield attained at 900 K. The ratio of steam to fuel and the pressure determine to a great extent the equilibrium hydrogen concentration. The heat demand of the reformer, as expressed by the required amount of oxygen, varies with temperature, steam to fuel ratio and pressure, as well as the type of oxygenate compound used. When the required oxygen enters the system at the reforming temperature, autothermal steam reforming results in hydrogen yield around 20% lower than the yield by steam reforming because part of the organic feed is consumed in the combustion reaction. Autothermicity was also calculated for the whole cycle, including preheating of the organic feed to the reactor temperature and the reforming reaction itself. The oxygen demand in such a case is much higher, while the amount of hydrogen produced is drastically reduced.     

Comparison of compact reformer configurations for on-board fuel processing

Two compact reformer configurations in the context of production of hydrogen in a fuel processing system for use in a Proton Exchange Membrane Fuel Cell (PEMFC) based auxiliary power unit in the 2–3 kW range are compared using computer-based modeling techniques. Hydrogen is produced via catalytic steam reforming of n-heptane, the surrogate for petroleum naphtha. Heat required for this endothermic reaction is supplied via catalytic combustion of methane, the model compound for natural gas. The combination of steam reforming and catalytic combustion is modeled for a microchannel reactor configuration in which reactions and heat transfer take place in parallel, micro-sized flow paths with wall-coated catalysts and for a cascade reactor configuration in which reactions occur in a series of adiabatic packed-beds, heat exchange in interconnecting microchannel heat exchangers being used to maintain the desired temperature. Size and efficiency of the fuel processor consisting of the reformer, hydrogen clean-up units and heat exchange peripherals are estimated for either case of using a microchannel and a cascade configuration in the reforming step. The respective sizes of fuel processors with microchannel and cascade configurations are 1.53 × 10⁻³ and 1.71 × 10⁻³ m³. The overall efficiency of the fuel processor, defined as the ratio of the lower heating value of the hydrogen produced to the lower heating value of the fuel consumed, is 68.2% with the microchannel reactor and 73.5% with the cascade reactor mainly due to 30% lower consumption of n-heptane in the latter. The cascade system also offers advanced temperature control over the reactions and ease of catalyst replacement.     

Experimental and computational investigations of a compact steam reformer for fuel oil and diesel fuel

The present work describes the optimisation of a compact steam reformer for light fuel oil and diesel fuel. The reformer is based upon a catalytically coated micro heat exchanger that thermally couples the reforming reaction with a catalytic combustion. Since the reforming process is sensitive to reaction temperatures and internal flow patterns, the reformer was modelled using a commercial CFD code in order to optimise its geometry. Fluid flow, heat transfer and chemical reactions were considered on both sides of the heat exchanger. The model was successfully validated with experimental data from reformer tests with 4 kW, 6 kW and 10 kW thermal inputs of light fuel oil. In further simulations the model was applied to investigate parallel flow, counter flow and cross flow conditions along with inlet geometry variations for the reformer. The experimental results show that the reformer design allows inlet temperatures below 773 K because of its internal superheating capability. The simulation results indicate that two parallel flow configurations provide fast superheating and high fuel conversion rates. The temperature increase inside the reactor is influenced by the inlet geometry on the combustion side.     

Combustion characteristics of mixture of anode off gas and LNG in reformer

Fuel processing system which converts hydrocarbon fuel into hydrogen rich gas (by stream reforming, partial oxidation, auto-thermal reforming) needs high temperature environment (600-1000 °C). Generally, anode off gas or mixture of anode off gas and LNG are used as input gas for a fuel reformer. In order to constitute efficient and low emission burner system for fuel reformer, it is necessary to elucidate the combustion and emission characteristics of fuel reformer burner. In this study, lean flat flame using the ceramic porous burner was analyzed numerically and experimentally. Burning velocity of anode off gas calculated by CHEMKIN simulation was 51.8 cm, which was faster than that of LNG having 40.63 cm/s at the stoichiometric ratio because of high composition of hydrogen in anode off gas. As composition of LNG in mixture of anode off gas + LNG is increased, the burning velocity decreases and in the other hand the adiabatic temperature increases. CO, NO_x were measured below 50 ppm in operating load range of the reformer. Blue flame pattern was found as stable flame region for design of fuel reformer and anode off gas flame was maintained in blue flame pattern at equivalence ratio 0.55-0.62 under 1-5 kW power range.     

Externally reformed solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)

Solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid power plants integrate a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200–500 kW). The SOFC–MGT systems currently developed are fueled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and di-methyl-ether (DME) (200–350 °C) is significantly lower than that of natural gas (700–900 °C), the reformer can be sited outside the stack. External reforming in SOFC–MGT plants fueled by methanol and DME enhances efficiency due to improved exhaust heat recovery and higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio (SCR)) must be carefully chosen to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fueled hybrid plant is approximately 240 °C, giving efficiencies of about 67–68% with a SOFC temperature of 900 °C (the efficiency is about 72–73% at 1000 °C). Similarly, for DME the optimum reforming temperature is approximately 280 °C with efficiencies of 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around one for methanol and 1.5–2 for DME.     

Metal membrane-type 25-kW methanol fuel processor for fuel-cell hybrid vehicle

A 25-kW on-board methanol fuel processor has been developed. It consists of a methanol steam reformer, which converts methanol to hydrogen-rich gas mixture, and two metal membrane modules, which clean-up the gas mixture to high-purity hydrogen. It produces hydrogen at rates up to 25 N m³/h and the purity of the product hydrogen is over 99.9995% with a CO content of less than 1 ppm. In this fuel processor, the operating condition of the reformer and the metal membrane modules is nearly the same, so that operation is simple and the overall system construction is compact by eliminating the extensive temperature control of the intermediate gas streams. The recovery of hydrogen in the metal membrane units is maintained at 70–75% by the control of the pressure in the system, and the remaining 25–30% hydrogen is recycled to a catalytic combustion zone to supply heat for the methanol steam-reforming reaction. The thermal efficiency of the fuel processor is about 75% and the inlet air pressure is as low as 4 psi. The fuel processor is currently being integrated with 25-kW polymer electrolyte membrane fuel-cell (PEMFC) stack developed by the Hyundai Motor Company. The stack exhibits the same performance as those with pure hydrogen, which proves that the maximum power output as well as the minimum stack degradation is possible with this fuel processor. This fuel-cell ‘engine’ is to be installed in a hybrid passenger vehicle for road testing.     

Micro methanol reformer combined with a catalytic combustor for a PEM fuel cell

This paper presents the development of a micro methanol reformer for portable fuel cell applications. The micro reformer consists of a methanol steam reforming reactor, catalytic combustor, and heat exchanger in-between. Cu/ZnO was selected as a catalyst for a methanol steam reforming and Pt for a catalytic combustion of hydrogen with air. Porous ceramic material was used as a catalyst support due to the large surface area and thermal stability. Photosensitive glass wafer was selected as a structural material because of its thermal and chemical stabilities. Performance of the reformer was measured at various test conditions and the results showed a good agreement with the three-dimensional analysis of the reacting flow. Considering the energy balance of the reformer/combustor model, the off-gas of fuel cell can be recycled as a feed of the combustor. The catalytic combustor generated the sufficient amount of heat to sustain the steam reforming of methanol. The conversion of methanol was 95.7% and the hydrogen flow of 53.7 ml/min was produced including 1.24% carbon monoxide. The generated hydrogen was the sufficient amount to operate 4.5 W polymer electrolyte membrane fuel cells.     

Performance analysis of a membrane‐based reformer‐combustor reactor for hydrogen generation

Hydrogen is mostly produced in conventional steam methane reforming plants. In this work, we proposed a membrane‐based reformer‐combustor reactor (MRCR) for hydrogen generation in order to improve heat recovery and overall thermal efficiency. The proposed configuration will also reduce the complexity in existing steam methane reforming (SMR) plants. The proposed MRCR comprises combustion zone, hydrogen permeate zone, and SMR zone. A computational fluid dynamics model was developed using ANSYS‐Fluent software to simulate and analyze the performance of the proposed MRCR. Results show that high hydrogen yields were observed at high reformer pressures (RPs) and low gas hourly space velocities (GHSVs). Furthermore, by increasing the steam to methane ratio and addition of excess air in the combustion side, the hydrogen yield from the MRCR decreases. This is attributed to the reduction in the effective temperature of the hydrogen membrane. High RP, low GHSV, and low steam to methane ratio that increased the hydrogen yield also decreased carbon monoxide (CO) emissions. For an increased RP from 1 to 10 bar, the CO emission decreased by about 99%. The reduction in CO emission at high RP would be attributed to the effect of water gas shift reaction in the MRCR. Results of the extensive parametric study presented in this work can be used to determine the operating conditions based on tradeoffs between hydrogen yield (mole H₂/mole CH₄), hydrogen production rate (kg of H₂/h), allowable CO emissions, and exhaust gas temperature for other applications such as gas turbine.     

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.     

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.     

A highly efficient six-stroke internal combustion engine cycle with water injection for in-cylinder exhaust heat recovery

A concept adding two strokes to the Otto or Diesel engine cycle to increase fuel efficiency is presented here. It can be thought of as a four-stroke Otto or Diesel cycle followed by a two-stroke heat recovery steam cycle. A partial exhaust event coupled with water injection adds an additional power stroke. Waste heat from two sources is effectively converted into usable work: engine coolant and exhaust gas. An ideal thermodynamics model of the exhaust gas compression, water injection and expansion was used to investigate this modification. By changing the exhaust valve closing timing during the exhaust stroke, the optimum amount of exhaust can be recompressed, maximizing the net mean effective pressure of the steam expansion stroke (MEP_steam). The valve closing timing for maximum MEP_steam is limited by either 1 bar or the dew point temperature of the expansion gas/moisture mixture when the exhaust valve opens. The range of MEP_steam calculated for the geometry of a conventional gasoline engine and is from 0.75 to 2.5 bars. Typical combustion mean effective pressures (MEP_combustion) of naturally aspirated gasoline engines are up to 10 bar, thus this concept has the potential to significantly increase the engine efficiency and fuel economy.     

On-board generation of hydrogen to improve in-cylinder combustion and after-treatment efficiency and emissions performance of a hybrid hydrogen–gasoline engine

Hydrogen on-board fuel reforming has been identified as a waste energy recovery technology with potential to improve Internal combustion engines (ICE) efficiency. Additionally, can help to reduce CO₂, NO_x and particulate matter (PM) emissions. As this thermochemical energy is recovered from the hot exhaust stream and used in an efficient way by endothermic catalytic reforming of petrol mixed with a fraction of the engine exhaust gas. The hydrogen-rich reformate has higher enthalpy than the petrol fed to the reformer and is recirculated to the intake manifold, which will be called reformed exhaust gas recirculation (rEGR).The rEGR system has been simulated by supplying hydrogen (H₂) and carbon monoxide (CO) into a conventional Exhaust Gas Recirculation (EGR) system. The hydrogen and CO concentrations in the rEGR stream were selected to be achievable in practice at typical gasoline exhaust temperatures (temperatures between 300 and 600 °C). A special attention has been paid on comparing rEGR to the baseline ICE, and to conventional EGR. The results demonstrate the potential of rEGR to simultaneously increase thermal efficiency, reduce gaseous emissions and decrease PM formation.Complete fuel reformation can increase the calorific value of the fuel by 28%. This energy can be provided by the waste heat in the exhaust and so it is ideal for combination with a gasoline engine with its high engine-out exhaust temperatures.The aim of this work is to demonstrate that exhaust gas fuel reforming on an engine is possible and is commercially viable. Also, this paper demonstrates how the combustion of reformate in a direct injection gasoline engine via reformed Exhaust Gas Recirculation (rEGR) can be beneficial to engine performance and emissions.     

Two novel oxy-fuel power cycles integrated with natural gas reforming and CO2 capture

Two novel system configurations were proposed for oxy-fuel natural gas turbine systems with integrated steam reforming and CO₂ capture and separation. The steam reforming heat is obtained from the available turbine exhaust heat, and the produced syngas is used as fuel with oxygen as the oxidizer. Internal combustion is used, which allows a very high heat input temperature. Moreover, the turbine working fluid can expand down to a vacuum, producing an overall high-pressure ratio. Particular attention was focused on the integration of the turbine exhaust heat recovery with both reforming and steam generation processes, in ways that reduce the heat transfer-related exergy destruction. The systems were thermodynamically simulated, predicting a net energy efficiency of 50–52% (with consideration of the energy needed for oxygen separation), which is higher than the Graz cycle energy efficiency by more than 2 percentage points. The improvement is attributed primarily to a decrease of the exergy change in the combustion and steam generation processes that these novel systems offer. The systems can attain a nearly 100% CO₂ capture.     

An onboard hydrogen generator for hydrogen enhanced combustion with internal combustion engine

Hydrogen enhanced combustion (HEC) for internal combustion engine is known to be a simple mean for improving engine efficiency in fuel saving and cleaner exhaust. An onboard compact and high efficient methanol steam reformer is made and installed in the tailpipe of a vehicle to produce hydrogen continuously onboard by using the waste heat of the engine for heating up the reformer; this provides a practical device for the HEC to become a reality. This use of waste heat from engine enables an extremely high process efficiency of 113% to convert methanol (8.68 MJ) for 1.0 NM of hydrogen (9.83 MJ) and low cost of using hydrogen as an enhancer or as a fuel itself. The test results of HEC from the onboard hydrogen production are presented with 2 gasoline engine vehicles and 2 diesel engines; the results indicate a hike of engine efficiency in 15–25% fuel saving and a 40–50% pollutants reduction including 70% reduction of exhaust smoke. The use of hydrogen as an enhancer brings about 2–3 fold of net reductions in energy, carbon dioxide emission and fuel cost expense over the input of methanol feed for hydrogen production.     

Hydrogen production from ethanol over Co/ZnO catalyst in a multi-layered reformer

A Co/ZnO catalyst was prepared by coprecipitation method, and was applied for ethanol steam reforming. The effect of reaction conditions on the ethanol steam reforming performance was studied in the temperature ranges from 400 °C to 600 °C and the space velocity ranges from 10,000 h⁻¹ to 120,000 h⁻¹ in a fixed bed reactor. The Co/ZnO showed high activity with an ethanol conversion of 97% and a H₂ concentration of 73% at a gas hourly space velocity of 40,000 h⁻¹ and a moderately low temperature of 450 °C. EXAFS analysis for fresh and spent samples confirms that Co phase maintains during reaction. The catalyst was then loaded into a multi-layered reformer of which the design concept allows for integrating endothermic steam reforming, exothermic combustion and evaporation in a reactor. The performance of the compact reformer demonstrated that the hydrogen production rate satisfy a PEMFC stack power level of 540 W suitable for portable power supplies.     

CFD simulation of Pd-based membrane reformer when thermally coupled within a fuel cell micro-CHP system

In this work, a bi-dimensional CFD simulation investigates a fuel processor for hydrogen production from natural gas or biogas composed by a steam methane reformer coupled with a palladium-based hydrogen permeable membrane, the so-called “membrane reformer” (MREF). The heat required for the endothermic reforming reaction taking place on the MREF is supplied by a stream of hot gas coming from an external source, typically represented by a combustor burning the unconverted fuel and the unpermeated hydrogen. The resulting fuel processor arrangement, which has already been simulated by the point of view of energy and mass balances, may achieve a very high efficiency and is particularly suited for integration with fuel cells. The interest on this configuration relies on the possibility to implement this technology within a PEMFC-based micro-cogenerator (also micro-Combined Heat and Power, or m-CHP) with a net electrical power output in a range of 1–2 kW. In particular, the work focuses on the temperature profiles along the membrane, which should be kept as close as possible to 600 °C to favourite permeation and avoid any damages, and examines the advantages of hot gas on co-current direction vs. counter-current with respect to the reformer flux direction.     

Study on a compact methanol reformer for a miniature fuel cell

A compact methanol reformer for hydrogen production has been successfully fabricated, which integrated one reforming chamber, one water gas shift reaction chamber, two preheating chambers and two combustion chambers. It can be started-up at room temperature by the combustion of liquid methanol in the combustion chamber within 7 min without any external heating. The cold start response of the methanol reformer has been investigated using different parameters including methanol and air supply rate, and the experiments revealed that the optimum methanol and air flow rate were 0.55 mL/min and 3 L/min respectively. The results indicated that this methanol reformer can provide a high concentration of hydrogen (more than 73%) and the system efficiency is always maintained above 74%. It is further demonstrated in more than 1600 h continuous performance that the reformer could be operated autothermally and exhibited good test stability.     

Hydrogen production from glycerol steam reforming for low- and high-temperature PEMFCs

This paper presents a thermodynamic study of a glycerol steam reforming process, with the aim of determining the optimal hydrogen production conditions for low- and high-temperature proton exchange membrane fuel cells (LT-PEMFCs and HT-PEMFCs). The results show that for LT-PEMFCs, the optimal temperature and steam to glycerol molar ratio of the glycerol reforming process (consisting of a steam reformer and a water gas shift reactor) are 1000 K and 6, respectively; under these conditions, the maximum hydrogen yield was obtained. Increasing the steam to glycerol ratio over its optimal value insignificantly enhanced the performance of the fuel processor. For HT-PEMFCs, to keep the CO content of the reformate gas within a desired range, the steam reformer can be operated at lower temperatures; however, a high steam to glycerol ratio is required. This requirement results in an increase in the energy consumption for steam generation. To determine the optimal conditions of glycerol steam reforming for HT-PEMFC, both the hydrogen yield and energy requirements were taken into consideration. The operational boundary of the glycerol steam reformer was also explored as a basic tool to design the reforming process for HT-PEMFC.     

Thermochemical waste‐heat recuperation by steam methane reforming with flue gas addition

The process flow schematic of fuel‐consuming equipment with thermochemical waste‐heat recuperation by steam methane reforming with an addition of flue gas to the reaction mixture is suggested. The advantages of such a thermochemical recuperation (TCR) system compared with the TCR system by steam methane reforming are shown and justified. Based on the first law energy analysis, the heat inputs and outputs of the TCR system were determined. To determine the exhaust gases heat transformed into chemical energy of a new synthetic fuel, the thermodynamic analysis by minimizing Gibbs energy via Aspen HYSYS was performed. It was found that with an increase in the mole fraction of combustion products in the reaction mixture, the enthalpy of the methane reforming reaction increases, especially noticeable at the temperature range above 1000 K. Based on the heat, balance of the TCR system was established that the addition of combustion products to the reaction mixture has the following effects: reducing the heat input for steam production in a steam generator; reduction of the steam generator size because of the need to produce a smaller amount of steam in comparison with TCR by pure steam methane reforming; and reducing the amount of heat transferred through the wall of the reformer and, as a consequence, reduction in size of the reformer.