Analysis of radial flow reformer for fuel cell applications

In this paper, the performance of a radial flow reformer is analyzed. Fundamental principles of reaction engineering are utilized to design this reactor where methane is reformed to produce sufficient hydrogen to generate 20 W of power in a fuel cell. It is shown that the radial flow geometry leads to modest pressure drop. The reactor operates at a pressure of 150 kPa, a steam to methane ratio of 3 and an inlet temperature of 848 K and is able to generate sufficient hydrogen for 20 W of power. The heat duty required for the reformer is approximately 43% of the power generated.     

Performance Research on a Methane Compact Reformer Integrated with Catalytic Combustion

A multichannel reformer integrated with catalytic combustion was investigated for methane steam reforming to produce hydrogen. In this system, the main portion of the required heat was supplied by methane oxidation in the catalytic combustor located on two sides of the reformer. In the compact multichannel reactor, the methane conversion rate is high enough compared to the equilibrium values at different temperatures. The performance of the multichannel reformer was investigated under various operating conditions, such as the reformer temperature and the feed stream ratios in both the reformer and the catalytic combustor. The best feed flow rate ratio of reforming to combustion ranged from 1.3 to 1.5, with > 95 % methane conversion. It is anticipated that this multichannel reformer can generate enough hydrogen for a 30‐W fuel cell system, due to its small volume.     

Burning velocity of methane/diluent mixture with reformer gas addition

The motivation of this study is to explore the feasibility of extending the EGR (exhaust gas recirculation) diluent tolerance for methane/air mixtures with reformer gas (CO and H₂). A preheated cylindrical combustion chamber was used to measure the laminar burning velocity of methane/air mixture with variations of EGR diluent, reformer gas, temperature and pressure. The experiments were carried out at the range of initial temperature from 298 K to 498 K and initial pressure from 1 atm to 5 atm. The maximum EGR fraction is 40%. Reformer gas was introduced to raise the burning velocity of methane/EGR mixture to the undiluted level. Results indicate that the reformer gas has potential to improve the burning velocity while reducing the nitric oxide emission.     

Hydrogen production from methane in a dielectric barrier discharge using oxide zinc and chromium as catalyst

The hydrogen fuel cell is a promising option as a future energy resource; however, the nature of the gas is such that the conversion process of other fuels to hydrogen on board is necessary. Among the raw fuel resources, methane could be the best candidate as it is plentiful. In this experiment, the possibility of producing hydrogen with less carbon formation from methane by a dielectric barrier discharge (DBD) was investigated. Without the addition of a catalyst, the formation of hydrogen reached between 30% and 35% at methane residence time of 0.22 min and supplied powers in the range of 60-130 W. The hydrogen selectivity increased at higher supplied power, but the process efficiency, defined as a ratio of the produced hydrogen to the supplied power, decreased slightly. In order to boost the hydrogen production with less carbon formation, a mixed oxide catalyst of zinc and chromium was added to the reactor. It was shown that the production of hydrogen was ca. 40% higher than the non-catalytic plasma process.     

Model‐based optimization of hydrogen generation by methane steam reforming in autothermal packed‐bed membrane reformer

An autothermal membrane reformer comprising two separated compartments, a methane oxidation catalytic bed and a methane steam reforming bed, which hosts hydrogen separation membranes, is optimized for hydrogen production by steam reforming of methane to power a polymer electrolyte membrane fuel cell (PEMFC) stack. Capitalizing on recent experimental demonstrations of hydrogen production in such a reactor, we develop here an appropriate model, validate it with experimental data and then use it for the hydrogen generation optimization in terms of the reformer efficiency and power output. The optimized reformer, with adequate hydrogen separation area, optimized exothermic‐to‐endothermic feed ratio and reduced heat losses, is shown to be capable to fuel kW‐range PEMFC stacks, with a methane‐to‐hydrogen conversion efficiency of up to 0.8. This is expected to provide an overall methane‐to‐electric power efficiency of a combined reformer‐fuel cell unit of ～0.5. Recycling of steam reforming effluent to the oxidation bed for combustion of unreacted and unseparated compounds is expected to provide an additional efficiency gain. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Hydrogen production from oxidative steam reforming of ethanol in a palladium-silver alloy composite membrane reactor

In this investigation, we studied the oxidative steam reforming reaction of ethanol in a Pd-Ag/PSS membrane reactor for the production of high purity hydrogen. Palladium and silver were deposited on porous stainless steel (PSS) tube via the sequential electroless plating procedure with an overall film thickness of 20 μm and Pd/Ag weight ratio of 78/22. An ethanol-water mixture (n_water/n_ethanol = 1 or 3) and oxygen (n_oxygen/n_ethanol = 0.2, 0.7 or 1.0) were fed concurrently into the membrane reactor packed with Zn-Cu commercial catalyst (MDC-3). The reaction temperatures were set at 593-723 K and the pressures at 3-10 atm. The hydrogen flux in the permeation side increased proportionately with increasing pressure; however, it reduced slightly when increasing oxygen input. This is probably due to the fast oxidation reaction that consumes hydrogen before the onset of the steam reforming reaction. The effect of oxygen plays a vital role on the ethanol oxidation steam reforming reaction, especially for a Pd-Ag membrane reactor in which a higher flux of hydrogen is required. The selectivity of CO₂ increased with increasing flow rate of oxygen, while the selectivity of CO remained almost the same.     

Upgrading of a syngas mixture for pure hydrogen production in a Pd-Ag membrane reactor

Water gas shift (WGS) is a thermodynamics limited reaction and CO equilibrium conversion of a traditional reactor is furthermore reduced owing to the presence of H₂ (ca. 50%) in the feed stream coming from a reformer.The upgrading of a simulated reformate stream was experimentally investigated as a function of temperature (280-320 °C), feed pressure (up to 600 kPa), gas hourly space velocity (GHSV), etc. using a Pd-alloy membrane reactor (MR) packed with a commercial catalyst CuO/CeO₂/Al₂O₃; no sweep gas was used. The MR performance was also evaluated using new parameters such as conversion index, H₂ recovery and extraction index, etc., which evidence the advantages with respect to a traditional reactor.A Pd-based MR operated successfully overcoming the thermodynamic constraints of a traditional reactor and, specifically, the drawback introduced by the hydrogen presence. In fact, a CO conversion of 90% significantly exceeded (three times) the thermodynamics upper limit (<36%) of a traditional reactor owing to ca. 80% of hydrogen permeated through the membrane.The overall process performance was significantly improved by the presence of the Pd-based membrane and, thus, by the high reaction pressure which allowed and drove the hydrogen permeation.     

Poly-dimethylsiloxane (PDMS) based micro-reactors for steam reforming of methanol

A miniaturized methanol steam reformer with a serpentine type of micro-channels was developed based on poly-dimethylsiloxane (PDMS) material. This way of fabricating micro-hydrogen generator is very simple and inexpensive. The volume of a PDMS micro-reformer is less than 10 cm³. The catalyst used was a commercial Cu/ZnO/Al₂O₃ reforming catalyst from Johnson Matthey. The Cu/ZnO/Al₂O₃ reforming catalyst particles of mean diameter 50-70 μm was packed into the micro-channels by injecting water based suspension of catalyst particles at the inlet point. The miniaturized PDMS micro-reformer was operated successfully in the operating temperatures of 180-240 °C and 15%-75% molar methanol conversion was achieved in this temperature range for WHSV of 2.1-4.2 h⁻¹. It was not possible to operate the micro-reformer made by pure PDMS at temperature beyond 240 °C. Hybrid type of micro-reformer was fabricated by mixing PDMS and silica powder which allowed the operating temperature around 300 °C. The complete conversion (99.5%) of methanol was achieved at 280 °C in this case. The maximum reformate gas flow rate was 30 ml/min which can produce 1 W power at 0.6 V assuming hydrogen utilization of 60%.     

Simultaneous growth of diamond thin films and carbon nanotubes at temperatures ?550 °C

Diamond thin films (on silicon wafers) and carbon nanotubes (CNTs) (on Inconel plates) were simultaneously synthesized at temperatures ?550 °C without any additional catalyst. The synthesis was achieved in a microwave plasma enhanced chemical vapor deposition (CVD) reactor with graphite etching in a gas mixture of hydrogen and methane. The substrate stage consisted of an Inconel 600 plate and a stainless steel plate separated by a 53 mm long quartz tube. Silicon wafers were placed on the stainless steel plate located at the upper part of the substrate stage, while Inconel plates were placed at the lower part of the substrate stage. During the deposition, the substrates were heated only by the plasma and the substrate temperature was controlled by the applied microwave power, which ranged from 350 W to 950 W. The backside temperatures of Si wafers ranged from 290 °C to 550 °C, higher than the corresponding temperatures of Inconel 600, which ranged from 220 °C to 350 °C. The Raman spectroscopic and electron microscopic results show that the thin films deposited on Si consist of well faceted polycrystalline diamond, and that the black soot deposited on Inconel plates is composed of multiwall carbon nanotubes as long as one millimeter.     

Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa

Using gas hydrates as materials for storage and transportation of natural gas have attracted much attention in recent years. However, there are two barriers in industrializing this new method. Firstly, methane hydrate induction time is relatively high. On the other hand the amount of gas trapped in methane hydrate crystals is too low. In this survey, silver nanoparticles were synthesized using a chemical reduction method and introduced to the hydrate reactor. Experiments were conducted at initial reactor pressures of 4.7 MPa and 5.7 MPa. At each pressure three independent experiments were performed. According to the results, in the presence of silver nanoparticles, methane hydrate induction time decreased by 85% and 73.9%, and the amount of methane trapped in hydrate crystals increased by 33.7% and 7.4% at the pressures of 4.7 MPa and 5.7 MPa respectively.     

Hydrogen production from heavy oil in the presence of calcium hydroxide

A new hydrogen production method, the HyPr-RING process was applied to a vacuum residue of Arabian light crude oil to clarify the effects of added water, calcium hydroxide, which absorbs carbon dioxide, and the reaction temperature. It was determined that when a sufficient amount of calcium hydroxide was present, it provided enough water to produce hydrogen and additional water was not necessary. To consume all of the carbon dioxide in 1 mol of carbon from the feedstock, 25 mol% of calcium hydroxide was needed and hydrogen production was saturated at 50 mol%. Carbon conversion was dependent mainly on the temperature and was slightly dependent on water and pressure. The reaction pressure was as low as 4.2 MPa. Thermal decomposition of the feedstock was the dominant reaction below 600 °C, which produced methane.     

Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry

The methane hydrate heat of decomposition was directly measured up to 20 MPa and 292 K using a high pressure differential scanning calorimeter (DSC). The methane hydrate sample was formed ex-situ using granular ice particles and subsequently transferred into the DSC cell under liquid nitrogen. The ice and water impurities in the hydrate sample were reduced by converting any dissociated hydrate into methane hydrate inside the DSC cell before performing the thermal properties measurements. The methane hydrate sample was dissociated by raising the temperature (0.5-1.0 K/min) above the hydrate equilibrium temperature at a constant pressure. The measured methane hydrate heat of dissociation (H→W+G), ΔH_d, remained constant at 54.44±1.45 kJ/mol gas (504.07±13.48 J/gm water or 438.54± 13.78 J/gm hydrate) for pressures up to 20 MPa. The measured ΔH_d is in agreement with the Clapeyron equation predictions at high pressures; however, the Clausius-Clapeyron equation predictions do not agree with the heat of dissociation data at high pressures. In conclusion, it is recommended that the Clapeyron equation should be used for hydrate heat of dissociation estimations at high pressures.     

Fluidised bed membrane reactor for ultrapure hydrogen production via methane steam reforming: Experimental demonstration and model validation

Hydrogen is emerging as a future alternative for mobile and stationary energy carriers in addition to its use in chemical and petrochemical applications. A novel multifunctional reactor concept has been developed for the production of ultrapure hydrogen from light hydrocarbons such as methane for online use in downstream polymer electrolyte membrane fuel cells. A high degree of process intensification can be achieved by integrating perm-selective hydrogen membranes for selective hydrogen removal to shift the methane steam reforming and water-gas-shift equilibriums in the favourable direction and perm-selective oxygen membranes for selective oxygen addition to supply the required reaction energy via partial oxidation of part of the methane feed and enable pure CO₂ capture without costly post-treatment. This can be achieved in a proposed novel multifunctional bi-membrane bi-section fluidised bed reactor [Patil, C.S., van Sint Annaland, M., Kuipers, J.A.M., 2005. Design of a novel autothermal membrane assisted fluidized bed reactor for the production of ultrapure hydrogen from methane. Industrial and Engineering Chemistry Research 44, 9502-9512]. In this paper, an experimental proof of principle for the steam reforming/water-gas-shift section of the proposed novel fluidised bed membrane reactor is presented. A fluidised bed membrane reactor for steam reforming of methane/water-gas-shift on a commercial noble metal-based catalyst has been designed and constructed using 10 H₂ perm-selective Pd membranes for a fuel cell power output in the range of 50-100 W. It has been experimentally demonstrated that by the insertion of the membranes in the fluidised bed, the thermodynamic equilibrium constraints can indeed be overcome, i.e., increased CH₄ conversion, decreased CO selectivity and higher product yield (H₂ produced/CH₄ reacted). Experiments at different superficial gas velocities and also at different temperatures and pressures (carried out in the regime without kinetic limitations) revealed enhanced reactor performance at higher temperatures and pressures (3-4 bar). With a phenomenological two-phase reactor model for the fluidised bed membrane reactor, incorporating a separately developed lumped flux expression for the H₂ permeation rate through the used Pd-based membranes, the measured data from the fluidised bed membrane reactor could be well described, provided that axial gas back-mixing in the membrane-assisted fluidised bed reactor is negligible. This indicates that the membrane reactor behaviour approached that of an ideal isothermal plug flow reactor with maximum H₂ permeation.     

Biomass gasification using capacitively coupled RF plasma technology

A laboratory-scale capacitively coupled radio frequency (RF) plasma pyrolysis reactor working in reduced pressure has been developed. Experiments have been performed to examine the characteristics of this RF plasma reactor and the products of biomass gasification. It was found that the electrode geometry, input power and reactor pressure were the key parameters affecting the plasma characteristics such as plasma length, temperature, and energy transfer efficiency. Biomass gasification using input power 1600-2000 W and reactor pressure 3000-8000 Pa produced a combustible gas consisted of H₂, CO, CH₄, CO₂ and light hydrocarbons as well as a pyrolytic char. On average, the gas yield can reach 66 wt% of the biomass feed. An energy balance analysis on the RF plasma pyrolysis system was also given.     

Upgrading biomass pyrolysis gas by conversion of methane at high temperature: Experiments and modelling

Biomass gasification at temperatures below 1273 K produces gas which contains methane and too much tar for Fischer-Tropsch synthesis. The aim of this study is to investigate methane conversion at high temperature. Experimental tests were performed between 1273 and 1773 K, with a mixture of gas representative of wood pyrolysis at 1100 K (main components only: CO, CO₂, CH₄, H₂, H₂O). Two different kinetic schemes were used to predict the gas composition, and PAH molecules formation. For a residence time of 2 s in the reactor, the gas must be heated to at least 1650 K to reach a methane conversion rate of 90%. A parametric study was performed at 1453 K, by varying the initial methane, steam and hydrogen contents, so as to find out which components are the most influent on methane conversion and soot production.     

Using sonochemical reactor for degradation of LAS from effluent of wastewater treatment plant

Linear alkylbenzene sulfonates (LAS) are anionic surfactants, which are found in relatively high amounts in domestic and industrial wastewaters. The effectiveness of using sonochemical reactor for the degradation of LAS from effluent of wastewater treatment plant has been investigated. In this study, experiments of LAS solution were performed using methylene blue active substances (MBAS) method. The effectiveness of sonochemical reactor for LAS degradation is evaluated with emphasis on the effect of sonication time and initial LAS concentration. Experiments were carried out at initial concentrations of 0.2 mg/L, 0.5 mg/L, 0.8 mg/L and 1 mg/L, frequency of 130 kHz, acoustic power value of 400 W, temperature of 18-20 °C and pH value of 6.8-7. This study showed that LAS degradation was found to increase with increasing sonication time. In addition, as the concentration is increased, the LAS degradation rate decreases in the sonochemical reactor.     

Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter

Methane hydrate equilibrium has been studied upon continuous heating of the water-hydrate-gas system within the temperature range of 275-300 K. This temperature range corresponds to equilibrium pressures of 3.15-55 MPa. The hydrate formation/dissociation experiments were carried out in a high-pressure reactor under isochoric conditions and with no agitation. A small amount of surfactant (0.02 wt% sodium dodecyl sulfate, SDS) was added to water to promote hydrate formation. It was demonstrated that SDS did not have any influence on the gas hydrate equilibrium, but increased drastically both the hydrate formation rate and the amount of water converted into hydrate, when compared with the experiments without surfactant. To understand and clarify the influence of SDS on hydrate formation, macroscopic observations of hydrate growth were carried out using gas propane as hydrate former in a fully transparent reactor. We observed that 10^-3 wt% SDS (230 times less than the Critical Micellar Concentration of SDS) were sufficient to prevent hydrate particles from agglomerating and forming a rigid hydrate film at the liquid-gas interface. In the presence of SDS, hydrates grew mainly on the reactor walls as a porous structure, which sucked the solution due to capillary forces. Hydrates grew with a high rate until about 97 wt% of the water present in the reactor was transformed into hydrate.Our data on methane hydrate equilibrium both confirm already published literature data and complement them within the pressure range of 20-55 MPa.     

Hydrogen production from bioethanol reforming in supercritical water

Hydrogen production by reforming and oxidative reforming of ethanol in supercritical water (SCW) at the intermediate temperature range of 500-600 °C and pressure of 25 MPa were investigated at different ethanol concentrations or water to ethanol ratios (3, 20 and 30), with the absence and the presence of oxygen (oxygen to ethanol ratio between 0 and 0.156). Hydrogen was the main product accompanied with relatively low amounts of carbon dioxide, methane and carbon monoxide. Some liquid products, such as acetaldehyde and, occasionally, methanol were present. The ethanol conversion and hydrogen yield and selectivity increased substantially as the water to ethanol ratio and the reaction temperature increased. Ethanol was almost completely reformed and mainly converted to hydrogen giving a H₂/CO ratio of 2.6 at 550 °C and water to ethanol ratio of 30 without carbon formation. Coke deposition was favored at low water to ethanol ratio, especially at high temperatures (≥550 °C). The hydrogen yield improved as the ethanol was partially oxidized by the oxygen added into the feed at oxygen to ethanol ratios <0.071. It was evidenced that the metal components in Inconel 625 reactor wall reduced by a hydrogen stream acted as a catalyst promoting hydrocarbon reforming as well as water-gas-shift reactions while dehydrogenation of ethanol forming acetaldehyde can proceed homogeneously under the SCW condition. However, at high oxygen to ethanol ratio, the reactor wall was gradually deactivated after being exposed to the oxidant in the feed. The loss of the catalytic activity of the reactor surface was mainly due to the metal oxide formation resulting in reduction of catalytic activity of the reactor wall and reforming of carbon species was no longer promoted.     

Novel compact steam reformer for fuel cells with heat generation by catalytic combustion augmented by induction heating

The overall objective of the project is to investigate, design and test an improved steam reformer for fuel cell power plants.

Catalytic combustion is used to minimise the disadvantages of conventional external reforming. By integrating various technologies, smaller-scale reformer, lower emission levels and reduced start-up time along with improved temperature control can be achieved.

Catalytic combustion is used to generate the heat required for the reforming reaction, thus avoiding NO_x formation. The heat can be generated locally and made available for the reforming reaction using a thermally conductive substrate.

The first results of the catalytic test reactor show good conversion rates for both the combustion and the reforming reaction. Based on the hydrogen production rate, the initial objectives for the volume of the reformer have proven to be feasible.

The results are used to build a 20 kW integrated test reactor.     

Comparative study of homoepitaxial single crystal diamond growth at continuous and pulsed mode of MPACVD reactor operation

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in pulsed regime of a 2.45 GHz MPACVD reactor operation at pulse repetition rates of 150 and 250 Hz was investigated. The high quality CVD diamond layers were deposited in the H₂-CH₄ gas mixture containing 4% and 8% of methane, gas pressures of 250 and 260 Torr and substrate temperature of 900 °C without any nitrogen addition. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. At pulse repetition rate 150 Hz the high quality single crystal diamond was grown with growth rate of 22 μm/h. The comparison of the single crystal diamond growth rates in CW and pulsed wave regimes of MPACVD reactor operation at microwave power density 200 W/cm³ was made. It was found that at equal power density, the growth rate in pulsed wave regime was higher than in CW regime. Differences in single crystal diamond growth for two sets of experiments (with continuous and pulsed wave regimes) were explained.