Modeling and simulation of methane steam reforming in a thermally coupled membrane reactor

A distributed mathematical model for thermally coupled membrane reactor that is composed of three channels is developed for methane steam reforming. Methane combustion takes place in the first channel on a Pt/δ–Al₂O₃$\text{Pt}/\delta \text{–}{\text{Al}}_2{\text{O}}_3$ catalyst layer that supplies the necessary heat for the endothermic steam reforming reaction. In the second channel, catalytic steam reforming reactions take place in the presence of Ni/MgO–Al₂O₃ catalyst. The combustion catalyst forms a thin layer next to the reactor wall to minimize the heat transfer resistance. Selective permeation of hydrogen through the palladium membrane is achieved either by co-current or counter-current flow of sweep gas through the third channel. The burner is modeled as a monolith reactor and the reformer is assumed to behave as a pseudo-homogenous reactor. The mass and energy balance equations for the thermally coupled membrane reactor form a set of 22 coupled ordinary differential equations. With the application of appropriate boundary conditions, the distributed reactor model for steady-state operation is solved as a boundary value problem. The model equations are discretized using spline collocation on finite elements. The discretized nonlinear modeling equations, along with the boundary conditions, form a system of algebraic equations that are solved using the trust region dogleg method. The performance of the reactor is numerically investigated for various key operating variables such as inlet fuel concentration, inlet steam/methane ratio, inlet reformer gas temperature and inlet reformer gas velocity. Simulations for both the co-current and the countercurrent flow modes are also performed using different sweep gas flow rates. For each case, the reactor performance is analyzed based on methane conversion and hydrogen recovery yield.     

Surface regeneration of sulfur-poisoned Ni surfaces under SOFC operation conditions predicted by first-principles-based thermodynamic calculations

The surface regeneration or de-sulfurization process of a sulfur-poisoned (i.e. sulfur-covered) nickel surface by O₂ and H₂O has been studied using first-principles calculations with proper thermodynamic corrections. While O₂ is more effective than H₂O in removing the sulfur atoms adsorbed on nickel surface, it readily reacts with the regenerated Ni surface, leading to over-oxidization of Ni. Thus, H₂O appears to be a better choice for the surface regeneration process. In reality, however, both O₂ and H₂O may be present under fuel cell operating conditions. Accordingly, the effects of the partial pressures of O₂ [pO₂$p_{{\text{O}}_2}$] and H₂O [pH₂O$p_{{\text{H}}_2\text{O}}$] as well as the ratio of pO₂/pH₂O$p_{{\text{O}}_2}/p_{{\text{H}}_2\text{O}}$ on the regeneration of a sulfur-covered Ni surface without over-oxidization at different temperatures are systematically examined to identify the best conditions for regeneration of Ni-based SOFC anodes under practical conditions.     

Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure

Auto-ignition characteristics of methane/hydrogen mixtures with hydrogen mole fraction varying from 0 to 100% were experimentally studied using a shock tube. Test pressure is kept 1.8 MPa and temperatures behind reflected shock waves are in the range of 900–1750 K and equivalence ratios from 0.5 to 2.0. Three ignition regimes are identified according to hydrogen fraction. They are, methane chemistry dominating ignition (XH₂≤40%)$\left(X_{{\text{H}}_2}\le 40\mathrm{\%}\right)$, combined chemistry of methane and hydrogen dominating ignition (XH₂=60%)$\left(X_{{\text{H}}_2}=60\mathrm{\%}\right)$, and hydrogen chemistry dominating ignition (XH₂≥80%)$\left(X_{{\text{H}}_2}\ge 80\mathrm{\%}\right)$. Simulated ignition delays using four models including USC Mech 2.0, GRI Mech 3.0, UBC Mech 2.1 and NUI Galway Mech were compared to the experimental data. Results show that USC Mech 2.0 gives the best prediction on ignition delays and it was selected to conduct sensitivity analysis for three typical methane/hydrogen mixtures at different temperatures. The results suggest that at high temperature, ignition delay mainly is governed by chain branching reaction H + O₂ ⇔ OH + O, and thus increasing equivalence ratio inhibits ignition of methane/hydrogen mixtures. At middle-low temperature, contribution of equivalence ratio on ignition delay of methane/hydrogen mixtures is mainly due to chemistries of HO₂ and H₂O₂ radicals.     

Catalytic performance of Pt-promoted cobalt-based catalysts for the steam reforming of ethanol

In this study, Pt-promoted Co_x/ZrO₂ catalysts (including 1.5 wt% Pt and Co loading of 0.75, 1.5, 3.0, 6.0, 9.0, 12, 24 and 48 wt%, respectively) were prepared by the incipient wet impregnation method for hydrogen production by steam reforming of ethanol (SRE). Evaluation of catalytic activities and products distribution toward the SRE reaction were carried out in a fixed-bed reactor with 100 mg of catalyst under an H₂O/EtOH molar ratio of 13.0 and gas hourly space velocity (GHSV) of 22,000 h⁻¹. The results revealed that the optimal loading of cobalt could enhance the capacities of catalytic performance and C–C scission, further raised the yield of hydrogen (YH₂)$\left({\text{Y}}_{{\text{H}}_2}\right)$ and lowered the deposited carbon, CH₄ and CO side-products due to the combination effect of cobalt and platinum. The Pt_1.5Co₁₂/ZrO₂ catalyst exhibited superiority in the SRE reaction, where complete conversion of ethanol was achieved at 275 °C. The (YH₂)$\left({\text{Y}}_{{\text{H}}_2}\right)$ approached 4.95, and only minor CO (<2%), CH₄ (<4%), and carbon deposition (<0.5%) were detected at 500 °C. In addition, the ethanol conversion still remained complete, and the selectivity of hydrogen exceeded 70% during a 116 h time-on-stream test under the same condition.     

Catalytic steam reforming of acetic acid for hydrogen production

A variety of supported metal catalysts were tested under conditions of steam reforming of acetic acid (HAc), which was selected as a model compound for pyrolysis oil. The influence of several parameters on catalytic activity and selectivity were examined, including catalyst composition, i.e. nature of the metal and the carrier, reaction temperature and time on stream. The metallic phase of such catalysts was comprised of various metals, such as Pt, Pd, Rh, Ru and Ni, which were supported on metal oxides carriers, such as _Al2O3${\mathrm{Al}}_2{\mathrm{O}}_3$, _La2O3/_Al2O3${\mathrm{La}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$, MgO/_Al2O3$\mathrm{MgO}/{\mathrm{Al}}_2{\mathrm{O}}_3$ and _CeO2/_Al2O3${\mathrm{CeO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$. It was found that Ni-based and Ru-based catalysts present high activity and selectivity toward hydrogen production. Ru catalysts supported on _La2O3/_Al2O3${\mathrm{La}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$ and MgO/_Al2O3$\mathrm{MgO}/{\mathrm{Al}}_2{\mathrm{O}}_3$ carriers, showed good long-term stability as a function of time on stream. However, Ni catalysts were not as stable as Ru catalysts. The amount of carbon deposited on each catalyst was estimated, and it was found that it depends strongly on the nature of the catalyst.     

Selective CO oxidation with real methanol reformate over monolithic Pt group catalysts: PEMFC applications

Alumina supported Pt group metal monolithic catalysts were investigated for selective oxidation of CO in hydrogen-rich methanol reforming gas for proton exchange membrane fuel cell (PEMFC) applications. The results are described and discussed in the present paper and show that Pt/γ_Al2O3$\mathrm{Pt}/\gamma {\mathrm{Al}}_2{\mathrm{O}}_3$ was the most promising candidate to selectively oxidize CO from an amount of about 1 vol% to less than 100 ppm. We have investigated the effect of the O₂ to CO feed ratio, the feed concentration of CO, the presence of H₂O and/or CO₂, and the space velocity on the activity, selectivity and stability of Pt/Al₂O₃ monolithic catalysts. Afterwards, the Pt/Al₂O₃ catalyst was scaled up and applied in 5 kW hydrogen producing systems based on methanol steam reforming and autothermal reforming. The hydrogen produced was then used as fuel for an integrated PEMFC.     

Optimization of H2 production with CO2 capture by steam reforming of methane integrated with a chemical-looping combustion system

Methane steam reforming (SR) integrated with a chemical-looping combustion (CLC) system is a new process for producing hydrogen from natural gas, allowing carbon dioxide capture with a low energy penalty. In this study, mass and enthalpy balances of an SR-CLC system were carried out to determine the autothermal operating conditions for optimal H₂ production. The evaluation was conducted using iron-based oxygen carriers. Two configurations were analysed, firstly with the reformer tubes inside the fuel reactor and, secondly, with the reformer tubes inside the air reactor. This paper analyses the effect of two parameters affecting the SR process, namely the conversion of methane in the reformer (XCH₄$X_{{\text{CH}}_4}$) and the efficiency of the hydrogen separation of a pressure swing adsorption (PSA) unit (η_PSA), as well as two parameters affecting the CLC system, namely the Fe₂O₃ content in the oxygen carrier and its conversion variation (ΔX_OC), on the H₂ yields. Moreover, it also analyses the reduction of Fe₂O₃ to Fe₃O₄ or to FeAl₂O₄. The results shown that a H₂ yield value of 2.45 mol H₂ per mol of CH₄ can be obtained with the reformer tubes located inside the air reactor and with Fe₂O₃ being reduced to Fe₃O₄. This corresponds to a CH₄ to H₂ conversion of 74.2%, which is similar to state-of-the-art H₂ production technologies, but with inherent CO₂ capture in the SR-CLC process.     

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20 atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55 mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1 atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO₂${\text{CO}}_2$ concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO₂${\text{CO}}_2$ dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.     

Measurement of hydrogen and oxygen chemical potential in yttria doped barium cerate (BCY) electrolyte of anode-supported protonic ceramic fuel cells

Yttria doped barium cerate (BCY) electrolyte, Ni + BCY anode supported protonic ceramic fuel cells were fabricated with Pt reference electrodes embedded in a thin (∼40 microns) electrolyte layer. The embedded electrodes function as selective probes exchanging only electrons with the BCY so that the voltage measurements (ΔV) using the embedded probes through the electrolyte correspond to a change in the reduced negative electrochemical potential of electrons (Δφ  ). Using this method, the corresponding change in hydrogen and oxygen chemical potential (ΔμH₂$\Delta {\mu }_{{\text{H}}_2}$, ΔμO₂$\Delta {\mu }_{{\text{O}}_2}$) or partial pressure of hydrogen and oxygen (ΔpH₂$\Delta p_{{\text{H}}_2}$, ΔpO₂$\Delta p_{{\text{O}}_2}$) were determined on the basis of the local equilibrium assumption, allowing us to investigate ionic and electronic transport properties through the BCY electrolyte. The results indicate that the pH₂$p_{{\text{H}}_2}$ and pO₂$p_{{\text{O}}_2}$ change mainly occurs across the middle electrolyte region while the electrolyte regions close to the anode and the cathode showed very small variation. The present work revealed that the BCY electrolyte consists of three major parts with different transport properties; 1) mixed ionic-electronic conduction in the electrolyte close to the anode side (reducing atmosphere), 2) predominantly ionic conduction in the middle region, 3) mixed ionic-hole conduction in the electrolyte close to the cathode side (oxidizing atmosphere).     

Reducing the idle speed of a spark-ignited gasoline engine with hydrogen addition

Reducing idle speed is an effective way for decreasing engine idle fuel consumption. Unfortunately, due to the increased residual dilution and dropped combustion temperature, spark-ignited (SI) gasoline engines are prone to suffer high cyclic variation and even stall at low idle speeds. This paper investigated the effect of hydrogen addition on the performance of an SI gasoline engine at reduced idle speeds of 600, 700 and 800 rpm. The test results shows that cyclic variation was raised with the decrease of idle speed but reduced obviously with the increase of hydrogen energy fraction (βH₂)$\left({\text{β}}_{{\text{H}}_2}\right)$. Decreasing idle speed and adding hydrogen were effective for reducing engine idle fuel consumption. The total fuel energy flow rate was effectively dropped from 30.8 MJ/h at 800 rpm and βH₂${\text{β}}_{{\text{H}}_2}$ = 0% to 17.6 MJ/h at 600 rpm and βH₂${\text{β}}_{{\text{H}}_2}$ = 19.9%. Because of the dropped fuel energy flow rate causing the reduced combustion temperature, both cooling and exhaust losses were markedly reduced after decreasing idle speed and adding hydrogen. HC and CO emissions were dropped with the increase of βH₂${\text{β}}_{{\text{H}}_2}$, but increased after reducing idle speed. However, NOx emissions were decreased after reducing idle speed and adding hydrogen, due to the dropped peak cylinder temperature.     

Study on laminar flame speed and flame structure of syngas with varied compositions using OH-PLIF and spectrograph

Various Bunsen flame information of premixed syngas/air mixtures was systematically collected. A CCD camera was used to capture the flame images. The OH-PLIF technique was applied to obtain the flame OH distribution and overall flame radiation spectra were measured with a spectrograph. Experiments were conducted on a temperature un-controlled burner and syngas over a wide range of H₂/CO ratios (from 0.25 to 4) and equivalence ratios (from 0.5 to 1.2). Results show that increasing hydrogen fraction (XH₂$X_{{\text{H}}_2}$) extends the blow-off limit significantly. The measured laminar flame speed using cone-angle method based on CCD flame imaging and OH-PLIF images increases remarkably with the increase of XH₂$X_{{\text{H}}_2}$, and these measurements agrees well with kinetic modeling predictions through Li's mechanism when the temperature for computation is corrected. Kinetic study shows that as XH₂$X_{{\text{H}}_2}$ increases, the production of H and OH radicals is accelerated. Additionally, the main H radical production reaction (or OH radical consumption reactions) changes from R29 (CO + OH = CO₂ + H) to R3 (H₂ + OH = H₂O + H) as XH₂$X_{{\text{H}}_2}$ increases. Sensitivity analysis was conducted to access the dominant reactions when XH₂$X_{{\text{H}}_2}$ increases. The difference on flame color for different XH₂$X_{{\text{H}}_2}$ mixtures is due to their difference in radiation spectrum of the intermediate radicals produced in combustion.     

H2 processes with CO2 mitigation: Thermo-economic modeling and process integration

Within the challenge of greenhouse gas reduction, hydrogen is regarded as a promising decarbonized energy vector. The hydrogen production by natural gas reforming and lignocellulosic biomass gasification are systematically analyzed by developing thermo-economic models. Taking into account thermodynamic, economic and environmental factors, process options with CO₂ mitigation are compared and optimized by combining flowsheeting with process integration, economic analysis and life cycle assessment in a multi-objective optimization framework. The systems performance is improved by introducing process integration maximizing the heat recovery and valorizing the waste heat. Energy efficiencies up to 80% and production costs of 12.5–42 $/GJH₂${\text{GJ}}_{{\text{H}}_2}$ are computed for natural gas H₂ processes compared to 60% and 29–61 $/GJH₂${\text{GJ}}_{{\text{H}}_2}$ for biomass processes. Compared to processes without CO₂ mitigation, the CO₂ avoidance costs are in the range of 14–306 $/tCO_2,avoided${\text{t}}_{{\text{CO}}_2,\text{avoided}}$. The study shows that the thermo-chemical H₂ production has to be analyzed as a polygeneration unit producing hydrogen, captured CO₂, heat and electricity.     

New inorganic–organic proton conducting membranes based on Nafion and [(ZrO2)·(SiO2)0.67] nanoparticles: Synthesis vibrational studies and conductivity

In this report is described the preparation of six nanocomposite membranes of formula {Nafion/_[(ZrO_2)⋅(SiO_2)0.67]ΨZrO₂}$\{\text{Nafion}/{[(\text{Zr}{\text{O}}_2)\cdot {(\text{Si}{\text{O}}_2)}_{0.67}]}_{{\Psi }_{\text{Zr}{\text{O}}_2}}\}$ with ΨZrO₂${\Psi }_{\text{Zr}{\text{O}}_2}$ ranging from 0 to 1.79 based on Nafion^® and [(ZrO₂)·(SiO₂)_0.67] nanofiller. Morphology investigations carried out by SEM measurements indicate that the composition of membranes is asymmetric. Indeed, with respect to the direction of the films after casting procedure, the top side (A-side) and bottom side (B-side) present a different nanofiller concentration. The concentration of nanofiller increases gradually from A to B side. The membranes present thicknesses ranging from 170 to 350 nm and are studied by FT-IR ATR and micro-Raman measurements.     

Hydrogen production from biogas using hot slag

Possibility of hydrogen production from biogas using hot slag has been studied, in which decomposition rate of _CO2${\mathrm{CO}}_2$–_CH4${\mathrm{CH}}_4$ in a packed bed of granulated slag was measured at constant flow-rate and pressure. The molten slag, discharged at high temperature over 1700 K from smelting industries such as steelmaking or municipal waste incineration. It has enough potential for replacing energy required for hydrogen production due to the catalytic steam reforming or carbon decomposition of hydrocarbon. However, heat recovery of hot slag has never been established. Therefore, the objective of this work is to generate hydrogen from methane using heated slag particles as catalyst, in which the effect of temperature on the hydrogen generation was mainly investigated at range from 973 to 1273 K. In the experiments a mixed gas of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ was continuously introduced into the packed bed of hot slag at constant flow-rate and atmospheric pressure and then the outlet gas was monitored by gas chromatography. The results indicate that slag acted as not only thermal media but also good catalyst, for promoting decomposition. The product gases were mainly hydrogen and carbon monoxide with/without solid carbon deposition on the surface of slag, depending on the reaction temperature. Increasing temperature led to large hydrogen generation with decreasing un-reacted methane in the outlet gas, at when the largest methane conversion was about 96%. The results suggested a new energy-saving process of hydrogen production, in which the waste heat from molten slag can replace the energy required for hydrogen production, reducing carbon dioxide emission.     

Time-Resolved Particle Image Velocimetry of dynamic interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures

Time-Resolved Particle Image Velocimetry was used to study transient interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures. Lean and stoichiometric mixtures with hydrogen mole fraction in the fuel (hydrogen plus   methane), xH₂$x_{{\text{H}}_2}$, varying in the range of 0–0.5 were investigated.     

An experimental and kinetic modeling study of cyclopentadiene pyrolysis: First growth of polycyclic aromatic hydrocarbons

The importance of 1,3-cyclopentadiene (CPD) and cyclopentadienyl (CPDyl) moieties in the growth of polycyclic aromatic hydrocarbons (PAHs) was studied using new experimental data and ab initio calculations. The experimental investigation was performed in a tubular continuous flow pyrolysis reactor under both high (24molN₂/molCPD)$(24{\text{mol}}_{{\text{N}}_2}/{\text{mol}}_{\text{CPD}})$ and low (5molN₂/molCPD)$(5{\text{mol}}_{{\text{N}}_2}/{\text{mol}}_{\text{CPD}})$ nitrogen dilutions, covering a temperature range of 873–1123 K, at a fixed pressure of 1.7 bara. At the most severe conditions up to 84% of CPD is converted, and the amount of PAHs is more than 65 wt%. Major products observed during CPD pyrolysis were benzene, indene, methyl-indenes and naphthalene, in line with previous studies. On-line GC × GC-FID/(TOF-MS) also allowed to quantify minor species (methane, toluene, styrene, phenanthrene, anthracene, etc.), never reported before at this level of accuracy. The new experimental data have been used to further analyze the role of the successive interactions of CPD, indene, and naphthalene as well as the recombination and addition reactions of their resonantly stabilized radicals and refine their kinetics. The results of the modeling study are in good agreement with existing and new experimental observations.     

Auto-ignition and flame stabilization of hydrogen/natural gas/nitrogen jets in a vitiated cross-flow at elevated pressure

The influence of natural gas (NG) on the auto-ignition behavior of hydrogen (H₂)/nitrogen (N₂) fuel jets injected into a vitiated cross-flow was studied at conditions relevant for practical combustion systems (p = 15 bar, T_cross-flow = 1173 K). In addition, the flame stabilization process following auto-ignition was investigated by means of high-speed luminosity and shadowgraph imaging. The experiments were carried out in an optically accessible jet in cross-flow (JICF) test section. In a H₂/NG/N₂ fuel mixture, the fraction of H₂ was stepwise increased while keeping the N₂ fraction approximately constant. Two different jet penetration depths, represented by two N₂ fraction levels, were investigated. The results reveal that auto-ignition kernels occurred even for the lowest tested H₂ fuel fraction (XH_2/NG=XH₂/(XH₂+XNG)=80%)$\left(X_{{\text{H}}_2/\text{NG}}=X_{{\text{H}}_2}/\left(X_{{\text{H}}_2}+X_{\text{NG}}\right)=80\%\right)$, but did not initiate a stable flame in the duct. Increasing XH_2/NG$X_{{\text{H}}_2/\text{NG}}$ decreased the distance between the initial position of the auto-ignition kernels and the fuel injector, finally leading to flame stabilization. The H₂ fraction for which flame stabilization was initiated depended on jet penetration; flame stabilization occurred at lower H₂ fractions for the higher jet penetration depth (XH_2/NG$X_{{\text{H}}_2/\text{NG}}$ = 91% compared to 96%), revealing the influence of different flow fields and mixing characteristics on the flame stabilization process. It is hypothesized that the flame stabilization process is related to kernels extending over the duct height and thus altering the upstream conditions due to considerable heat release. This enabled subsequent kernels to occur close to the fuel injector until they could finally stabilize in the recirculation zone of the jet lee.