Measurements and kinetic study on ignition delay times of propane/hydrogen in argon diluted oxygen

Measurements on ignition delay times of propane/hydrogen mixtures in argon diluted oxygen were conducted for hydrogen fractions in the fuel mixtures (XH₂)$\left(X_{{\text{H}}_2}\right)$ from 0 to 100%, pressures of 1.2, 4.0 and 10 atm, and temperatures from 1000 to 1600 K using the shock-tube. Results show that for XH₂$X_{{\text{H}}_2}$ less than 70%, ignition delay time shows a strong Arrhenius temperature dependence and it decreases with the increase of pressure, while for XH₂$X_{{\text{H}}_2}$ larger than 90%, there is a crossover pressure dependence of the ignition delay time with increasing temperature. Numerical studies were made using the selected kinetic mechanisms and results show that the predicted ignition delay time gives a reasonable agreement with the measurements. Both measurements and predictions show that for XH₂$X_{{\text{H}}_2}$ less than 70%, the ignition delay time is only moderately decreased with the increase of XH₂$X_{{\text{H}}_2}$, indicating that hydrogen addition has weak effect on ignition enhancement. Sensitivity analysis reveals the key reactions that control the simulation of ignition delay time. Kinetic study is made to interpret the ignition delay time dependence on pressure and XH₂$X_{{\text{H}}_2}$.     

Effect of pressure and equivalence ratio on the ignition characteristics of dimethyl ether-hydrogen mixtures

Experimental and numerical study on the effect of pressure and equivalence ratio on the ignition delay times of the DME/H₂/O₂ mixtures diluted in argon were conducted using a shock tube and CHEMKIN II package at equivalence ratios of 0.5–2.0, pressures of 1.2–10 atm and hydrogen fractions of 0–100%. It was found that the measured ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For the DME/H₂ mixture at XH₂$X_{{\text{H}}_2}$ ≤80%, the ignition is controlled by the DME chemistry and ignition delay times present a typical Arrhenius pressure dependence and weak equivalence ratio dependence. For the DME/H₂ mixture at 80% < XH₂$X_{{\text{H}}_2}$ < 98%, the ignition is controlled by the combined chemistries of DME and hydrogen, and the ignition delay times give higher ignition activation energy at higher pressures and a typical Arrhenius equivalence ratio dependence. However, for the DME/H₂ mixture at XH₂$X_{{\text{H}}_2}$≥98%, the ignition is controlled by the hydrogen chemistry and ignition delay time shows complex pressure dependence and weak equivalence ratio dependence. Comparison of the measurements of neat DME and neat hydrogen with the calculations using three generally accepted mechanisms, NUIG Aramco Mech 1.3 [1], LLNL DME Mech 2, 3 and 4 and Princeton-Zhao Mech [5], shows that NUIG Aramco Mech 1.3 gives the best predictions and can well capture the pressure and equivalence ratio dependence at various hydrogen fractions. The sensitivity and normalized H-radicals consumption analysis were performed using NUIG Aramco Mech 1.3 and the key reactions that control the ignition characteristics of DME/H₂ mixtures were revealed. Further chemical kinetic analysis was made to interpret the ignition delay time dependence on pressure and equivalence ratio at varied hydrogen fractions.     

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.     

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.     

Hydrogen production via decomposition of methane over activated carbons as catalysts: Full factorial design

The thermo-catalytic decomposition of methane is proposed as an alternative for producing hydrogen without CO₂ emissions. The present study was divided into three parts. First, a screening study of the rate of methane decomposition (RCH₄)$\left(R_{{\text{CH}}_4}\right)$ was performed using two types of activated carbons as catalysts with progressive time of methane decomposition at four different temperatures. The catalysts differed in textural properties. A full factorial design consisting of 20 experimental points for each catalyst was applied in the second part. Quadratic RCH₄$R_{{\text{CH}}_4}$ models as functions of the relative time of catalyst deactivation and decomposition temperature were developed by regression analysis of variance. The results of the RCH₄$R_{{\text{CH}}_4}$ models showed that the relative time had twice as much influence as temperature. Finally, a general RCH₄$R_{{\text{CH}}_4}$ model was then developed representing both catalysts regardless of their textural properties. All the empirical models were consistent with experimental results and were adequate for designing the methane decomposition process.     

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.     

Effect of medium composition on biohydrogen production by the extreme thermophilic bacterium Caldicellulosiruptor saccharolyticus

Up to now, the analysis of the effects of medium composition on biohydrogen production of Caldicellulosiruptor saccharolyticus was focused mainly on salt concentrations and complex compounds. Within this work we studied the effects of the presence of organic and/or inorganic nitrogen in the medium composition aiming to induce metabolic changes in C. saccharolyticus   to improve its hydrogen evolution rate (HER) and hydrogen specific productivity (qH₂)$\left(q_{{\text{H}}_2}\right)$. Biohydrogen productivities and hydrogen to substrate yield (Y₍H_2/s₎)$\left(Y_{\left({\text{H}}_2/\text{s}\right)}\right)$ of C. saccharolyticus   on xylose in batch mode were higher working in a complex medium than in a defined one; but no significant difference could be settled according to hydrogen to carbon dioxide yields (Y₍H_2/CO₂₎)$\left(Y_{\left({\text{H}}_2/{\text{CO}}_2\right)}\right)$. The specific growth rate of C. saccharolyticus on complex medium was settled at 0.1 h⁻¹ operating in chemostat mode to achieve the highest H₂-productivities under stable conditions. In chemostat mode on xylose, a reduction of the ammonium feed concentration in a defined medium until N-limiting conditions involved higher qH₂$q_{{\text{H}}_2}$ comparing with a straight C-limiting growth.     

CO2 reforming of CH4 by combination of thermal plasma and catalyst

Experiments on synthesis gas preparation from dry reforming of methane by carbon dioxide with thermal plasma only and cooperation of thermal plasma with commercial catalysts have been performed. In all experiments, nitrogen gas was used as the plasma gas to form a high-temperature jet injected into a tube reactor. A mixture of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ was fed vertically into the jet. Both kinds of experiments were conducted in the same conditions, such as total flux of feed gases, the molar ratio of _CH4/_CO2${\mathrm{CH}}_4/{\mathrm{CO}}_2$, and the plasma power except with or without catalysts in the tube reactor. Higher conversion of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$, higher selectivity of _H2${\mathrm{H}}_2$ and CO, and higher specific energy of the process were achieved by thermal plasma with catalysts. For example, the conversions of _CH4${\mathrm{CH}}_4$ and _CO2${\mathrm{CO}}_2$ were high to 96.33% and 84.63%, and the selectivies of CO and _H2${\mathrm{H}}_2$ were also high to 91.99% and 74.23%, respectively. Both were 10–20%$10–20\%$ higher than those by thermal plasma only.     

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).     

Phototrophic hydrogen production in photobioreactors coupled with solar-energy-excited optical fibers

A novel solar-energy-excited optical fiber (SEEOF) photobioreactor (PBR) was developed to enhance the phototrophic H₂ production by Rhodopseudomonas palustris WP3-5 using acetate (HAc) as the sole carbon source. The PBR was illuminated by combinative light sources, including an internal illumination with optical fiber excited by solar energy (OF(sunlight)) as well as external irradiation of tungsten filament lamp (TL). The photo-H₂ producing performance of the SEEOF photobioreactor was further improved by using an innovative light dependent resistor (LDR) system, which could maintain sufficient and continual light supply. The results show that combination of OF(sunlight)/TL was more effective than the TL/TL illumination system, leading to a 138% and 136% increase in cumulative H₂ production (VH₂)$\left(V_{{\text{H}}_2}\right)$ and H₂ yield (YH₂)$\left(Y_{{\text{H}}_2}\right)$, respectively. The LDR-coupled SEEOF photobioreactor was able to solve the problems of diurnal variation in solar light intensity, enabling the control of a constant total light irradiation intensity on the PBR surface. Combining OF(sunlight)/TL with LDR, the VH₂$V_{{\text{H}}_2}$ and YH₂$Y_{{\text{H}}_2}$ were nearly 27% higher than without LDR. For bioreactor scale up from 50 to 1800 ml working volume, the LDR-coupled SEEOF photobioreactor worked well during daytime, leading to a marked improvement in phototrophic H₂ production with a VH₂$V_{{\text{H}}_2}$ and YH₂$Y_{{\text{H}}_2}$ of 3606 ml and 2.45 mol H₂/mol HAc, respectively. Moreover, continuous cultures operated at a hydraulic retention time (HRT) of 48 h show a high hydrogen production rate of 32.4 ml/l/h with stable operation for over 15 days. This optimal performance of LDR-coupled SEEOF photobioreactor is superior to most reported results and is a favorable choice of electricity-saving PBR strategy to improve photo-H₂ production efficiency.     

Hydrogen storage and induced properties in non-metallic catalytic materials

Particular active sites, ^xM${}^x\mathrm{M}$–^yM^′${}^y{\mathrm{M}}^{\prime }$ (where x$x$ and y$y$ are the number of unsaturations, i.e. anionic vacancies, on each cation M and M^′${\mathrm{M}}^{\prime }$) involving reactive hydrogen are created during the activation of non-metallic catalytic materials. The anionic vacancies created in bulk and at the surface of the solid, by the loss of _H2O${\mathrm{H}}_2\mathrm{O}$ or _H2S${\mathrm{H}}_2\mathrm{S}$, are able to receive hydrogen in a hydridic form according to a heterolytic dissociation (X^2-Mⁿ⁺□+_H2→XH^-Mⁿ⁺H^-${\mathrm{X}}^{2-}{\mathrm{M}}^{n+}\square +{\mathrm{H}}_2\to {\mathrm{XH}}^{-}{\mathrm{M}}^{n+}{\mathrm{H}}^{-}$ with X=O$\mathrm{X}=\mathrm{O}$ or S). The non-metallic catalytic materials become catalytic hydrogen reservoirs. Besides a high reactivity, the hydrogen species, stored in the solid, present marked diffusion properties leading to a dynamic behavior of the solid and active sites.     

Kinetic study on nonisothermal dehydrogenation of TiH2 powders

The nonisothermal dehydrogenation of TiH₂ powders was studied using thermogravimetry and differential scanning calorimetry. The reaction model was established by estimating the activation energy. The results show the nonisothermal dehydrogenation occurred in a four-step process. The hydrogen released from the TiH_1.5∼2$\text{Ti}{\mathrm{H}}_{1.5\sim 2}$ phase in the first step, which led to the decrease of activation energy. The second step was derived from the formation of βH${\beta }_H$ in δ$\delta$ phase and the reaction model was Phase boundary reaction. In the third step, the hydrogen started to release from the βH${\beta }_H$ phase, and then the βH→αH${\beta }_{\mathrm{H}}\to {\alpha }_{\mathrm{H}}$ phase transformation happened. So the activation energy Eα$E_{\alpha }$ underwent a decrease followed by a quick increase. The fourth step corresponded to the formation of αH${\alpha }_H$ in βH${\beta }_H$ phase, and the slight oxidation resulted in the small fluctuation of activation energy.     

Development of a galvanostatic analysis technique as an in-situ diagnostic tool for PEMFC single cells and stacks

A new galvanostatic analysis technique was developed for PEMFC single cells and stacks, while conventional potentiodynamic techniques, such as cyclic voltammetry for an electrochemical active surface area (EAS) and linear sweep voltammetry for a crossover current (iH₂)$\left(i_{{\text{H}}_2}\right)$, cannot be directly utilized for stacks. Using a developed relationship for double-layer charging region, the iH₂$i_{{\text{H}}_2}$ and C_dl (double-layer capacitance) of a PEMFC single cell could be determined from the galvanostatic data under an atmosphere of nitrogen (cathodes) and hydrogen (anodes). Then, simply from the elapsed time in hydrogen adsorption/desorption region, EAS or roughness factors could be analyzed for a PEMFC single cell. For a 5-cell PEMFC stack, it was experimentally confirmed that the same analysis technique can be applied to analyze performance distribution in PEMFC stacks. As the characteristics of catalyst layers (EAS and C_dl) and polymer electrolyte membranes (iH₂)$\left(i_{{\text{H}}_2}\right)$ of individual cells can be analyzed without stack disassembly, the developed galvanostatic technique is expected to be utilized for the degradation study and performance monitoring of practical PEMFC stacks.     

Hydrogen production by Escherichia coli depends on glucose concentration and its combination with glycerol at different pHs

Escherichia coli produces molecular hydrogen (H₂) during glucose or mixed carbon (glucose and glycerol) fermentation. Dependence of H₂ production rate (VH₂)$\left(V_{{\text{H}}_2}\right)$ on glucose at different pHs was studied in a concentration dependent manner. During growth of wild-type on glucose, increasing glucose concentration from 0.05% to 0.2% resulted in the marked inhibition of VH₂$V_{{\text{H}}_2}$. Inhibitory effect of glucose was shown at pH 7.5 and 6.5 but not pH 5.5. However, glycerol added in the growth medium with 0.1% glucose significantly increased VH₂$V_{{\text{H}}_2}$ but different effects at different pHs were established upon glucose or glycerol assays. The results indicate that H₂ production is inhibited by glucose in a concentration dependent manner during glucose fermentation but glucose in combination with glycerol might enhance H₂ production during mixed carbon fermentation.     

Effects of hydrogen addition and cylinder cutoff on combustion and emissions performance of a spark-ignited gasoline engine under a low operating condition

Because of the low combustion temperature and high throttling loss, SI (spark-ignited) engines always encounter dropped performance at low load conditions. This paper experimentally investigated the co-effect of cylinder cutoff and hydrogen addition on improving the performance of a gasoline-fueled SI engine. The experiment was conducted on a modified four-cylinder SI engine equipped with an electronically controlled hydrogen injection system and a hybrid electronic control unit. The engine was run at 1400 rpm, 34.5 Nm and two cylinder cutoff modes in which one cylinder and two cylinders were closed, respectively. For each cylinder closing strategy, the hydrogen energy fraction in the total fuel (βH₂)$\left({\beta }_{{\text{H}}_2}\right)$ was increased from 0% to approximately 20%. The test results demonstrated that engine indicated thermal efficiency was effectively improved after cylinder cutoff and hydrogen addition, which rose from 34.6% of the original engine to 40.34% of the engine operating at two-cylinder cutoff mode and βH₂=20.41%${\beta }_{{\text{H}}_2}=20.41\%$. Flame development and propagation periods were shortened with the increase of the number of closed cylinders and hydrogen blending ratio. The total cooling loss for all working cylinders, and tailpipe HC (hydrocarbons), CO (carbon monoxide) and CO₂ (carbon dioxide) emissions were reduced whereas tailpipe NO_x (nitrogen oxide) emissions were increased after hydrogen addition and cylinder closing.     

Catalytic hydrogen storage in cerium nickel and zirconium (or aluminium) mixed oxides

Interaction of hydrogen with a series of cerium nickel and zirconium (or aluminium) mixed oxides _CeM0.5NixOy${\mathrm{CeM}}_{0.5}{\mathrm{Ni}}_x{\mathrm{O}}_y$ (M=Zr$\mathrm{M}=\mathrm{Zr}$ or Al, 0?x?3$0?x?3$) has been studied in the 50–800 °C temperature range. Hydrogenation of 2-methyl-1,3-diene (isoprene) under helium flow in the absence of gaseous hydrogen is used to reveal and titrate reactive hydrogen species present in the solid previously treated under _H2${\mathrm{H}}_2$ at various temperatures. The _CeM0.5NixOy${\mathrm{CeM}}_{0.5}{\mathrm{Ni}}_x{\mathrm{O}}_y$ mixed oxides are large catalytic hydrogen reservoirs and among the solids studied, the highest amount of hydrogen (about 10 wt%, 540 g/L) is stored in _CeZr0.5Ni1Oy${\mathrm{CeZr}}_{0.5}{\mathrm{Ni}}_1{\mathrm{O}}_y$ pretreated in _H2${\mathrm{H}}_2$ at about 200 °C. Compared to the binary mixed oxides _CeNixOy${\mathrm{CeNi}}_x{\mathrm{O}}_y$, the presence of M allows to increase the hydrogen storage and give a better stability to the system, in particular, with temperature. Different physico-chemical techniques (TPR, TGA …) have been used to characterize the solids studied.