The optimization via response surface method for micro hydrogen gas actuator

Development of an innovative sensor for detection of hydrogen gas is essential for new applications and devices. In current article, inclusive parametric analysis has been performed to disclose the chief operative term on the performance of the micro sensor of MIKRA for the detection of the hydrogen in the mixture. The main mechanism of this micro actuator highly relies on the value of the exerted Knudsen force which occurs owing to the temperature gradient in the low-pressure region. The response surface methodology (RSM) is applied to obtain an optimized formula for the evaluation of sensor performance. Besides, analysis of variance (ANOVA) is employed to analyze the influence of individual factors on sensor formulation. This work tries to estimate the effect of major parameters such as a gap of the arm, the pressure of domain, mass fraction and temperature difference on the value of Knudsen force. Moreover, reliable correlations for the estimation of the Knudsen force are presented to determine the efficiency of the micro gas actuator in the various operating conditions. Our findings confirm that the precision of the sensor enhances as the temperature difference of the cold and hot arms as well as the hydrogen mass fractions augment in our actuator.     

Planar solid-state amperometric hydrogen gas sensor based on Nafion®/Pt/nano-structured polyaniline/Au/Al2O3 electrode

The effects of Nafion^® film thickness and charges passed for the preparation of Pt and nano-structured polyaniline (nsPANi) on the sensing properties of a planar solid-state amperometric hydrogen gas sensor are investigated. The surface morphology, Pt loading and electroactive surface area (ESA) are analyzed by FESEM, inductively coupled plasma (ICP) and cyclic voltammetry (CV), respectively. The specific sensitivity of the hydrogen gas sensor can be effectively promoted by decreasing the thickness of the Nafion^® film and the charge passed for the electrodeposition of Pt and PANi (from 30 to 10 mC), respectively. The very low Pt loading of the sensing composite electrode is due to the use of the nanofibrous PANi as support, which remarkably promotes Pt utilization. The specific sensitivity and the response time of the hydrogen gas sensor based on the Nafion^® (5.7 μm)/Pt/nsPANi/Au/Al₂O₃ electrode with a Pt loading of 1.87 μg are found to be 338.50 μA ppm^?1 g^?1 and 100–250 s, respectively, for measuring 10–10,000 ppm H₂.     

Effect of sweep gas on hydrogen permeation of supported Pd membranes: Experimental and modeling

Membrane reactor processes are being increasingly proposed as an attractive solution for pure hydrogen production due to the possibility to integrate production and separation inside a single reactor vessel. High hydrogen purity can be obtained through dense metallic membranes, especially palladium and its alloys, which are highly selective to hydrogen. The use of thin membranes seems to be a good industrial solution in order to increase the hydrogen flux while reducing the cost of materials. Typically, the diffusion through the membrane layer is the rate-limiting step and the hydrogen permeation through the membrane can be described by the Sieverts’ law but, when the membrane becomes thinner, the diffusion through the membrane bulk becomes less determinant and other mass transfer limitations might limit the permeation rate. Another way to increase the hydrogen flux at a given feed pressure, is to increase the driving force of the process by feeding a sweep gas in the permeate side. This effect can however be significantly reduced if mass transfer limitations in the permeate side exist. The aim of this work is to study the mass transfer limitation that occurs in the permeate side in presence of sweep gas. A complete model for the hydrogen permeation through PdAg membranes has been developed, adding the effects of concentration polarization in retentate and permeate side and the presence of the porous support using the dusty gas model equation, which combines Knudsen diffusion, viscous flow and binary diffusion. By studying the influence of the sweep gas it has been observed that the reduction of the driving force is due to the stagnant sweep gas in the support pores while the concentration polarization in the permeate is negligible.     

Effect of the microchannel obstacles on the pressure performance and flow behaviors of the hydrogen Knudsen compressor

The hydrogen Knudsen compressor has potential applications on the hydrogen transmission for the microdevices and systems. In this paper, the numerical model of the hydrogen Knudsen compressor was established, combining the NS continuity equations with the slip boundary conditions. The effect of structures on the performance of the hydrogen Knudsen compressor is studied by generating different obstacles in the microchannels. This paper is mainly concerned on the rectangular and the triangular obstacles, and the influence of the obstacles length and height are investigated, respectively. The Knudsen number distribution and the rarefaction of the hydrogen gas flow are analyzed. Also, the characteristic of the pressure increase for the compressor under different parameters are investigated and discussed. The effect of the structure parameters on the flow velocity distributions are detailed described, as well as the velocity contour and the vortex distributions. Moreover, the variation of the Knudsen layers of the hydrogen gas flow in the hydrogen Knudsen compressor is presented, and the key factor of the Knudsen layers is analyzed and discussed. The results is significantly beneficial for the applications and designs of hydrogen Knudsen compressor.     

Application of catalytic membrane reactor for pure hydrogen production by flare gas recovery as a novel approach

In the present study, application of catalytic membrane reactor as a novel approach for the flare gas recovery is proposed. A comprehensive two-dimensional non-isothermal model has been constructed to evaluate the performance of flare gas recovery process in the membrane reactor. The model is developed by taking into accounts the main chemical kinetics, heat and mass transfer phenomena and hydrogen permeation in the radial direction across a Pd–Ag membrane. The model predictions are validated based on different experimental results reported in literature. The impact of reactor operating conditions on the recovery process such as temperature and pressure, feed molar ratio and sweep gas ratio are investigated and discussed. The modeling results confirm that the flare gas conversion and hydrogen recovery improves with increasing the operating temperature, pressure and sweep ratio as a consequence of increasing the driving force for H₂ permeation through membrane. The environmental consideration revealed that by application of catalytic membrane reactor for the flare gas recovery of Asalouyeh gas processing plant (Iran), not only the equivalent mass of greenhouse gases emission reduces from 2179 kg/s to 36 kg/s, but also, 12.7 kg/s pure hydrogen will be produced by flare gas recovery at 750 K, 5 bar, sweep ratio of 5 and feed molar ratio of 4.     

Enhancement of the room-temperature hydrogen sensing performance of MoO3 nanoribbons annealed in a reducing gas

Uniform-sized orthorhombic MoO₃ nanoribbons were synthesized by a simple hydrothermal method at 240 °C. The nanoribbons grew along the [001] orientation, with average length, width and thickness of approximately 20 μm, 270 nm and 90 nm, respectively. The obtained nanoribbons were further annealed in a hydrogen atmosphere at different temperatures to modify their surface states. The treatment of the nanoribbons at 300 °C significantly elevated the concentration of non-stoichiometric Mo⁵⁺ to 24.7%, much larger than the original concentration (∼14.8%). A positive relationship was found between the non-stoichiometric Mo⁵⁺, chemisorbed oxygen ion and sensor response. The sensor based on the MoO₃ nanoribbons treated at 300 °C exhibited a faster response time of approximately 10.9 s, and a higher sensor response of 17.3 towards 1000 ppm H₂, compared with the results of original tests (∼21 s and ∼5.7, respectively), indicating the significantly improved gas sensing performance of the treated MoO₃. Meanwhile, the sensor also exhibited excellent repeatability and selectivity toward hydrogen gas. The enhancement of the hydrogen gas sensing performance of treated MoO₃ nanoribbons was attributed to the more effective adjustment of the width of the depletion region on the nanoribbon surface and the height of the potential barrier at the junctions, induced by the interaction between hydrogen molecules and higher-concentration oxygen ions. Our research implied that the gas sensing performance of nanostructured metal oxides could be successfully enhanced through annealing in the reducing gas.     

Room temperature hydrogen gas sensing properties of mono dispersed platinum nanoparticles on graphene-like carbon-wrapped carbon nanotubes

We present platinum nanoparticles dispersed wrinkled graphene-like carbon-wrapped carbon nanotubes (Pt/GCNTs) as a room temperature chemiresistive hydrogen gas sensor. Pt nanoparticles are decorated over GCNTs surface using poly (sodium 4-styrene sulfonate) (PSS) functionalization, followed by ethylene glycol reduction method. The highly defective wrinkled graphene-like surface of GCNTs provides large surface area and PSS functionalization provides stable immobilization of mono dispersed Pt nanoparticles on the carbon surface. A simple and inexpensive drop cast technique is used to fabricate the thick film sensor of the material. Hydrogen resistive gas sensing properties of Pt/GCNTs are studied at different gas concentrations, temperatures and Pt wt. % loadings. Pt/GCNTs sensor shows optimal sensitivity at room temperature with stable and reproducible response towards hydrogen. The sensor with 2 wt. % of Pt showed maximum sensitivity that is three fold higher than Pt decorated carbon nanotubes (Pt/CNTs) with the same Pt wt. % loading. The present study shows potential to explore novel H2 sensors^.     

Investigations on performance and emission characteristics of an industrial low swirl burner while burning natural gas,methane, hydrogen-enriched natural gas and hydrogen as fuels

Although many detailed chemical reaction mechanisms, skeletal mechanisms and reduced mechanisms are available in the literature to modeling the natural gas, they are computational expensive, required high power computing especially for three dimensional complex geometries with intense meshes. For example, though the DRM19 reduced mechanism does not include NO and NO₂ species, it includes 19 species and 84 reactions. On the other hand, Eddy Dissipation combustion model in which the overall rate of reaction is mainly controlled by turbulent mixing can be utilized as a practical approach for fast burning and fast reaction fuels such as natural gas. Unlike fossil fuels, hydrogen is a renewable energy and quite clean in terms of carbon monoxide and carbon dioxide emissions. However, numerical and experimental studies on hydrogen combustion in burners are very restricted. In this study, the combustion of natural gas in an industrial low swirl burner–boiler system has been experimentally investigated. The results obtained from the experimental setup have been utilized as boundary conditions for CFD simulations. With the use of Eddy Dissipation method, methane-air-2-step reaction mechanism is used for modeling of natural gas as methane gas and the reaction mechanism has been modified for natural gas considering the natural gas properties to reveal the similarities and differences of both fuels in modeling. In addition, the combustion performances of natural gas with the use of full and periodic models, which are geometric models of the burner–boiler pair, are compared. Moreover, in order to reveal the effect of the hydrogen-enriched natural gas and pure hydrogen on the performance of low swirl burner–boiler considering the combustion emissions, four various gas contents (thermal load ratio: 75%NG + 25%H₂, 50%NG + 50%H₂, 25%NG + 75%H₂, 100%H₂) at the same thermal load have been investigated. The turbulent flames of the industrial low swirl burner have been studied numerically using ANSYS Fluent 16.0 for the solution of governing equations. The results obtained in this study show that with the utilizing Eddy Dissipation method, natural gas can be modeled as methane gas with well-known methane-air-2step reaction mechanism or as natural gas with modified methane-air-2step reaction mechanism with approximate results. Additionally, the use of periodic boundary condition, which enables studying with 1/4 of geometric model, gives satisfactory results with less number of meshes when compared to the full model. Furthermore, in the case of using hydrogen-enriched natural gas or pure hydrogen instead of natural gas as the fuel, the combustion emissions of the burner–boiler such as CO and CO₂ are remarkably decreasing compared to the natural gas. However, the NOx emissions are significantly increasing especially due to thermal NO.     

Non-equilibrium evolution and characteristics of the serrated microchannel hydrogen knudsen compressor

The hydrogen Knudsen compressor has great potential to transport hydrogen and provide the required pressure in MEMS and microfluidic systems. The microchannel composed of cold and hot serrated surfaces is beneficial to the temperature control of the multistage Knudsen compressor. In the present study, a serrated hydrogen Knudsen compressor model is established initially, and the non-equilibrium evolution is numerically studied by using the method of N–S equations with the slip boundary. The key factors affecting the non-equilibrium evolution are comprehensively analyzed. The flow behaviors and performance of the serrated hydrogen Knudsen compressor in different times are studied. It is found that the main factors affecting the non-equilibrium evolution are the thermal expansion flow, thermal transpiration flow, and Poiseuille flow. Meanwhile, the serrated structure affects the local flow in the serrated microchannel at different times. Under the interaction of the thermal transpiration flow and the Poiseuille flow, the pressure difference between the two containers first increases rapidly and then decreases slowly, and finally approaches 1886 Pa. The research reveals the flow mechanisms of the hydrogen Knudsen compressor in the non-equilibrium evolution, which provides theoretical support for the safety and reliability of the hydrogen Knudsen compressor.     

Investigation of PdCuSi metallic glass film for hysteresis-free and fast response capacitive MEMS hydrogen sensors

In this study, we investigated PdCuSi metallic glass (MG) as a sensing material for capacitive MEMS hydrogen sensors. We first confirmed by film analysis that the fabricated PdCuSi film was MG and that it had a trigonal prism cluster. The measured pressure-composition-temperature curve of PdCuSi MG exhibited no hysteresis during hydrogen absorption and desorption. The response time was found to become faster by two orders of magnitudes compared with that of Pd polycrystal. These properties were attributed to the trigonal prism clusters. Strain was evaluated in the low hydrogen concentration regime of 0.05 vol% to 4.0 vol%, and the strain of PdCuSi MG was found to follow Sieverts' law well, indicating that hydrogen is present in the MG in a diffuse state. The hydrogen-concentration dependence of a capacitive MEMS hydrogen sensor was measured and hysteresis-free characteristics were obtained, implying advantages in hydrogen leak detection applications.     

Detecting hydrogen concentrations during admixing hydrogen in natural gas grids

The first applications of hydrogen in a natural gas grid will be the admixing of low concentrations in an existing distribution grid. For easy quality and process control, it is essential to monitor the hydrogen concentration in real time, preferably using cost effective monitoring solutions. In this paper, we introduce the use of a platinum based hydrogen sensor that can accurately (at 0.1 vol%) and reversibly monitor the concentration of hydrogen in a carrier gas. This carrier gas, that can be nitrogen, methane or natural gas, has no influence on the accuracy of the hydrogen detection. The hydrogen sensor consists of an interdigitated electrode on a chip coated with a platinum nanocomposite layer that interacts with the gas. This chip can be easily added to a gas sensor for natural gas and biogas that was already developed in previous research. Just by the addition of an extra chip, we extended the applicability of the natural gas sensor to hydrogen admixing. The feasibility of the sensor was demonstrated in our own (TNO) laboratory, and at a field test location of the HyDeploy program at Keele University in the U.K.     

Nano-bitter gourd like structured CuO for enhanced hydrogen gas sensor application

Hydrogen gas sensing were investigated using nano-bitter gourd like structured CuO material synthesized via a chemical route. Morphology of CuO was revealed using FE-SEM and TEM image analysis. CuO phase confirmation and molecular structural fingerprint were verified by XRD and Raman analysis respectively. Elemental composition and atomic states of elements were investigated by EDS and XPS techniques respectively. A remarkable high gas response of 175% was recorded by CuO sensor towards 100 ppm hydrogen (H₂) at the operating temperature 200 °C with response time 150 s. The lowest detection of H₂ was observed at 2 ppm concentration with the gas response of 5%. The gas response was studied as functions of different operating temperatures and concentrations. Transient gas response and stability were also confirmed for CuO sensor. Hydrogen sensing mechanism of CuO sensor was elucidated.     

Graphene decorated Pd-Ag nanoparticles for H2 sensing

Hydrogen sensor based on graphene nano-composite with Pd-Ag nanoparticles was fabricated by MEMS process. Structural and morphological properties of the sensing film were studied by an energy dispersive spectroscopy (EDS) and field emission scanning electron microscopy (FESEM), respectively. The H₂ sensing properties of as-formed sensor were investigated by measuring the resistance changes at different H₂ concentrations. The maximum gas response was 16.2% at 1000 ppm of H₂ gas. The gas sensitivity of the as-formed H₂ sensor showed linear behavior with the hydrogen concentration. Experimental results showed that the coupling of graphene with Pd/Ag alloy enhanced significantly hydrogen sensing performance.     

High pressure membrane separator for hydrogen purification of gas from hydrothermal treatment of biomass

For hydrogen purification and green hydrogen production in the context of biomass hydrothermal gasification, a palladium membrane system with microchannels on feed and permeate side was studied. The high pressure in the product gas of the hydrothermal process could potentially be used to generate pressurized pure hydrogen on the permeate side. Stabilizing the membrane by an additional porous metal support, experimental verification of the concept was done at feed pressure up to 50 bar and permeate pressure up to 20 bar. The temperatures were varied between 370 °C and 425 °C. The device was found to be highly selective and efficient for pure hydrogen separation. The membrane was characterized regarding the hydrogen flux and a deviation of the permeation from Sievert's law above 30 bars feed pressure was found. Generally, the microchannels on the feed side minimized concentration polarization effects, leading to high hydrogen fluxes with hydrogen feed mixtures and with real gas samples from hydrothermal gasification.     

Fluid Dynamics and Heat Transfer Analysis of Three Dimensional Microchannel Flows with Microstructures

The subsonic gas flows through straight rectangular cross-sectional microchannel with patterned microstructures was simulated using the direct simulation Monte Carlo (DSMC) method. An implicit treatment for low-speed inflow and outflow boundaries for the DSMC of the flows in microelectromechanical systems (MEMS) is employed. The 3-D microchannel flows are simulated with the cross-section aspect ratio ranging between 1 and 5. The comparison between 3-D cases and 2-D case shows that when the aspect ratio < 3, the two extra side-walls in the 3-D case have significant effects on the heat transfer and flow properties. When the aspect ratio increases, the flow pattern and heat transfer characteristics tend to approach those of 2-D results. The 2-D simplification is found to be reasonable when the cross-section aspect ratio is larger than 5. The microchannel flows with microstructures are also calculated with three different Knudsen numbers regime cases, and each case is calculated with three different microstructure temperatures, 273 K, 323 K, and 373 K. One Knudsen numbers regime ranges between 0.72 and 1.8, another regime ranges between 0.24 and 0.6 and the other regime ranges between 0.08 and 0.2. The computations show that the cooling and heating effects of the microstructure temperature on flow properties are enhanced with decreasing Knudsen number, and the higher microstructure temperature accelerates the velocity of the flow at the locations above the microstructures.     

Efficient room temperature hydrogen gas sensing based on graphene oxide and decorated porous silicon

In this study, a triple system for a hydrogen gas sensor was fabricated using graphene oxide, palladium nanoparticles, and porous silicon as a substrate. The fabricated sample was investigated by energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, and Raman spectroscopy. Field emission scanning electron microscopy images displayed a relatively uniform distribution of Palladium nanoparticles over porous silicon. In addition, it was observed that the graphene oxide nanosheets accumulated over the Palladium nanoparticles. Hydrogen-sensing measurements demonstrated that the fabricated system can even detect hydrogen at 200 ppm and 15 °C. The formation of palladium hydride was the main mechanism for detection. In fact, this structure caused a change in resistance through the creation of new electron pathways. Furthermore, the H₂ concentration showed a linear function to the reciprocal of the response time; this suggests that the sensing kinetics of the sample depends on the atomization of hydrogen molecules, which occurs via Pd nanoparticles. Moreover, the fabricated sample displayed significant selectivity for hydrogen gas compared to other examined gases.     

Hydrogen gas production from food wastes by electrohydrolysis using a statical design approach

Hydrogen gas production was investigated by electrohydrolysis of food waste due to its high organic content. Different voltages generated by DC power supply were applied to food waste in order to produce hydrogen gas. Effects of the DC voltage, reaction time and initial solid content on cumulative hydrogen gas production, hydrogen gas content in the gas phase and total organic carbon (TOC) removal were investigated by using a Box-Behnken statistical experiment design approach. The most suitable voltage/reaction time/solid content values resulting in the highest hydrogen gas content (99%), the highest cumulative hydrogen gas formation (7000 mL) and total organic carbon removal (33%) were determined as 5 V/75 h/20%. The results indicated that food wastes constitute a good source for H₂ gas production by electrohydrolysis. Hydrogen gas produced by electrohydrolysis of food waste can be directly used in fuel cells due to its high putrity.     

Fabrication & performance study of a palladium on alumina supported membrane reactor: Natural gas steam reforming,a case study

In this work, a synthetic mixture of natural gas is considered in a steam reforming process for generating hydrogen by using a membrane reactor housing a composite membrane constituted of a Pd-layer (13 μm) supported on alumina. The Pd/Al₂O₃ membrane separates part of the produced hydrogen through its selective permeation, although it shows a relatively low H₂/N₂ ideal selectivity (>200 at 0.5 bar of trans-membrane pressure and T = 425 °C).The steam reforming reaction is performed at 420 °C, by varying the gas hourly space velocity between 4400 h^?1 and 6900 h^?1 and by using two different mixtures containing some common impurities found within natural gas pipeline. Specifically, the effect of N₂ and CO₂ as impurities in the feed line is analyzed. The reaction pressure and steam-to-carbon ratio (S/C) are kept constant at 3.0 bar (abs.) and 3.5/1, respectively.The best performance of the Pd-based membrane reactor is obtained at 420 °C, 3.0 bar and 100 mL/min of sweep-gas, yielding a methane conversion of 55% and hydrogen recovery >90%.     

Application of Reproducing the Kernel Particle Method on the Simulation of the Membrane of a Capacitive Micromachined Microphone in Viscothermal Air

Simulation of the microphone membrane determines whether highest sensitivity is attainable when it comes to the field of microelectromechanical system (MEMS) microphone design. The reproducing kernel particle method is introduced to simulate a MEMS microphone membrane in viscothermal fluid. The result from a numerical axisymmetry model of 1 mm radius and 10 μm thickness membrane with a fixed boundary condition upon 1 mm thickness viscothermal air is identical to that from the theoretical model. Finally, a MEMS axisymmetry model of a 180 μm radius and 10 μm thickness membrane upon 10 μm thickness viscothermal air is simulated here.     

Development of a hydrogen dual sensor for fuel cell applications

The development and application of a hydrogen dual sensor (HDS) for the application in the fuel cell (FC) field, is reported. The dual sensing device is based on a ceramic platform head with a semiconducting metal oxide layer (MOx) printed on Pt interdigitated contacts on one side and a Pt serpentine resistance on the back side. MOx layer acts as a conductometric (resistive) gas sensor, allowing to detect low H₂ concentrations in air with high sensitivity and fast response, making it suitable as a leak hydrogen sensor. The proposed Co-doped SnO₂ layer shows high sensitivity to hydrogen (R₀/R = 90, for 2000 ppm of H₂) at 250 °C in air, and with fast response (<3 s). Pt resistance serves as a thermal conductivity sensor, and can used to monitor the whole range of hydrogen concentration (0–100%) in the fuel cell feed line with short response-recovery times, lower than 13 s and 14 s, respectively. The effect of the main functional parameters on the sensor response have been evaluated by bench tests. The results demonstrate that the dual sensor, in spite of its simplicity and cheapness, is promising for application in safety and efficiency control systems for FC power source.