The instability of laminar methane/hydrogen/air flames: Correlation between small and large-scale explosions

Darrieus–Landau (D-L) instability can cause significant acceleration in freely expanding spherical flames, which can lead to accidental large-scale gas explosions. To evaluate the potential of using high-pressure lab-scale experiments to predict the onset of cellular instabilities in large-scale atmospheric explosions, experimental measurements of the cellular instabilities for hydrogen and methane mixtures are conducted, in laboratory spherical explosions at elevated pressures. These measurements are compared with those from several large-scale atmospheric experiments. Comprehensive correlations of the pressure effect on a critical Karlovitz number, $K_{cl}$, together with those of strain rate Markstein number, $M{a}_{sr}$, are developed for hydrogen/air mixtures. The regime of stability reduces for all mixtures, as $M{a}_{sr}$ becomes negative. Values derived from large-scale experiments closely follow the same correlation of $K_{cl}$ with $M{a}_{sr}$. As a result, the extent of the regime where the laminar explosion flames become unstable can be predicted as a function of $M{a}_{sr}$ and pressure.     

Effect of hydrogen addition on the detonation performances of methane/oxygen at different equivalence ratios

Detonation performances of methane/hydrogen/oxygen (CH₄/H₂/O₂) mixtures were investigated experimentally in a 3000 mm long tube with an inner diameter of 30 mm at different initial pressures $p_0$ (ranging from 10 kPa to 50.5 kPa). Mixtures with different proportions of H₂ in the total fuel $\alpha$ (0%, 14.29% and 25%) and different equivalence ratios $\Phi$ (0.8, 1.0 and 1.2) were tested. Signals of flame front and pressure were obtained by ion probes and high frequency pressure transducers, respectively. Results showed that with the increase of $p_0$, $\alpha$and $\Phi$, the average velocity of steady detonation $V_{ave}$ increased. For mixtures with the given $\alpha$, when $\Phi$ increased by 0.2, $V_{ave}$ increased by 100 m/s. In the present study, velocity deficits were found to be within 5%, and when $p_0$ was higher than 20 kPa, the velocity deficits were within 2%. The average peak pressure of steady detonation $p_{ave}$ was close to the von Neumann pressure $p_{vN}$. Both the increase of $p_0$ and $\Phi$ led to the increase of the $p_{ave}$. But the addition of H₂ led to the decrease of $p_{ave}$, and $p_{ave}$ decreased with the increased of $\alpha$.     

Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures

Measurements on ignition delay times of n-butane/hydrogen/oxygen mixtures diluted by argon were conducted using the shock tube at pressures of 2, 10 and 20 atm, temperatures from 1000 to 1600 K and hydrogen fractions (${\mathrm{X}}_{{\mathrm{H}}_2}$) from 0 to 98%. It is found that hydrogen addition has a non-linear promoting effect on ignition delay of n-butane. Results also show that for ${\mathrm{X}}_{{\mathrm{H}}_2}$ less than 95%, ignition delay time shows an Arrhenius type dependence and the increase of pressure and temperature lead to shorter ignition delay times. However, for ${\mathrm{X}}_{{\mathrm{H}}_2}$ = 98% and 100% mixtures, non-monotonic pressure dependence of ignition delay time were observed. The performances of the Aramco2.0 model, San Diego 2016 model and USC2.0 model were evaluated against the experimental data. Only the Aramco2.0 model gives a reasonable agreement with all the measurements, which was conducted in this study to interpret the effect of pressure and hydrogen addition on the ignition chemistry of n-butane.     

A novel three-step GeO2/GeO thermochemical water splitting cycle for solar hydrogen production

This investigation reports the thermodynamic exploration of a novel three-step GeO₂/GeO water splitting (WS) cycle. The thermodynamic computations were performed by using the data obtained from HSC Chemistry thermodynamic software. Numerous process parameters allied with the GeO₂/GeO WS cycle were estimated by drifting the thermal reduction ($T_H$) and water splitting temperature ($T_L$). The entire analysis was divided into two section: a) equilibrium analysis and b) efficiency analysis. The equilibrium analysis was useful to determine the $T_H$ and $T_L$ required for the initiation of the thermal reduction (TR) of GeO₂ and re-oxidation of GeO via WS reaction. Furthermore, the influence of $P_{O_2}$ on the $T_H$ required for the comprehensive dissociation of GeO₂ into GeO and O₂ was also studied. The efficiency analysis was conducted by drifting the $T_H$ and $T_L$ in the range of 2080 to 1280 K and 500–1000 K, respectively. Obtained results indicate that the minimum ${\dot{Q}}_{solar-cycle}=624.3\text{kW}$ and maximum ${\eta }_{solar-to-fuel}=45.7\text{\%}$ in case of the GeO₂/GeO WS cycle can be attained when the TR of GeO₂ was carried out at 1280 K and the WS reaction was performed at 1000 K. This ${\eta }_{solar-to-fuel}=45.7\text{\%}$ was observed to be higher than the SnO₂/SnO WS cycle (39.3%) and lower than the ZnO/Zn WS cycle (49.3%). The ${\dot{Q}}_{solar-cycle}$ can be further decreased to 463.9 kW and the ${\eta }_{solar-to-fuel}$ can be upsurged up to 61.5% by applying 50% heat recuperation.     

Effect of burner diameter and diluents on the extinction strain rate of syngas-air non-premixed Tsuji-type flames

The present study focuses on the experimental determination of the global extinction strain rate ($a_g$) for different syngas-air combinations using the Tsuji type configuration. To study the effect of porous burner diameter (D), $a_g$ values were obtained for four values of D at atmospheric pressure. The experimentally obtained $a_g$ for a given fuel-oxidizer combination decreases with an increase in burner diameter (D). This trend is consistent with the limited data available in the literature for hydrocarbon fuels. Other geometric and flow-field effects namely, (1) plug flow, (2) flow-field blocking by the burner, and (3) heat loss by the flame to sidewalls that can affect $a_g$ were also experimentally quantified. The results from this study show that the plug flow boundary condition is always satisfied for oxidizer inlet distance > 2 times the largest porous burner diameter. Burner diameter less than 1/4 times side wall length (as is the case for all burners used in this study) does not significantly modify the flow. Hence, these two flow-field modifications do not affect $a_g$. However, heat loss from the flame to the ambient through the side walls can cause a 4–9 % decrease in $a_g$. Experiments showed that, CO/H₂ mixtures diluted with N₂ yield 1.6–2.25 times higher $a_g$ in comparison to CO/H₂ mixtures diluted with CO₂. Increasing H₂ from 1 to 5 % leads to 2.5–3.8 times increase in $a_g$, compared to 5 to 10 % increase in H₂ which leads to only 1.3–1.7 times increase in $a_g$ for 70 % of N₂ (v/v) in fuel mixture. Global extinction strain rate ($a_g$) increases by 1.5–2.4 times with 10 % increase in CO for fuel mixtures consisting of H₂ (1 and 5 % by v/v), CO₂ (50, 60 and 70 % by v/v) and N₂ (50, 60, 70 and 80 % by v/v). The change in overall reactivity (${\omega }_o$) due to different diluents is used to quantitatively explain the variation of $a_g$ for different fuel compositions. These effects are also qualitatively explained using OH radical concentration change with H₂ % in the fuel mixtures.     

Hybrid solar-driven hydrogen generation by sorption enhanced–chemical looping and hydrocarbon reforming coupled with carbon capture and Rankine cycle

Hydrogen ($H_2$) production from fossil fuels using Hydrocarbon Reforming Methods (HRM) accounts for nearly 95% of Global $H_2$ production. Unlike hybrid Chemical Looping Steam Reforming (CL-SR) systems, the Integrated Solar-Driven Sorption Enhanced–Chemical Looping of Hydrocarbon Reforming (SE-CL-HR) utilises solar thermal energy from the Concentrating Solar Power (CSP) system to drive the endothermic decomposition of feedstocks. Furthermore, the simulated hybrid systems utilise recovered heat to generate electricity, reuse of by-product ${CO}_2$ for more syngas production and finaly, ${CO}_2$ capture by reaction of $CaO$ to form ${CaCO}_3$. This work focused on simulating hybrid CSP systems and SE-CL-HR plants with Heat Transfer Fluid (HTF) output temperatures between 750 and 1050 °C. In this study, System Advisor Model (SAM) and MATLAB software are used to develop the CSP system. While the CSP result saved in the MATLAB workspace gets exported to Simulink to feed SE-CL-SMR, SE-CL-POX and SE-CL-ATR Aspen plus models. The integrated system was fed with ${CH}_4$ as the working fluid of the solar furnace. Stoichiometric and Gibbs free-energy minimisation were employed to investigate the effect of operating parameters. The output of the integrated system shows ≥9.5% exergy efficiency in comparison to conventional HRM. In addition, ${CO}_2$ capture by $CaO$ and high-pH water ($Ca$, $Mg$, $N{a}^{+}$, $O_2$, $O{H}^{-}$ and $C{l}^{-}$) to produce ${CaCO}_3$, ${MgCO}_3$ and other valuable products was also investigated in a process simulation. The research results revealed that for 8.1 $tons/hr$ of ${CH}_4$ and 277.1 $tons/hr$ $H_{2}O$ (steam) flowrates, 62 $tons/hr$ of $H_2$ can be generated and 338.5 $tons/hr$ of ${CO}_2$ emission can be reused and captured by the adoption of these new innovative technologies.