Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2–LiBH4–MgH2 nanoparticles

In situ Raman spectroscopy was used to monitor the dehydrogenation of ball-milled mixtures of LiNH₂–LiBH₄–MgH₂ nanoparticles. The as-milled powders were found to contain a mixture of Li₄BN₃H₁₀ and Mg(NH₂)₂, with no evidence of residual LiNH₂ or LiBH₄. It was observed that the dehydrogenation of both of Li₄BN₃H₁₀ and Mg(NH₂)₂ begins at 353 K. The Mg(NH₂)₂ was completely consumed by 415 K, while Li₄BN₃H₁₀ persisted and continued to release hydrogen up to 453 K. At higher temperatures Li₄BN₃H₁₀ melts and reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen gas. Cycling studies of the ball-milled mixture at 423 K and 8 MPa (80 bar) found that during rehydrogenation of Li₄BN₃H₁₀ Raman spectral modes reappear, indicating partial reversal of the Li₄BN₃H₁₀ to Li₂Mg(NH)₂ transformation.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Thermophysical properties and devitrification of SrO–La2O3–Al2O3–B2O3–SiO2-based glass sealant for solid oxide fuel/electrolyzer cells

Thermal behaviors and stability of glass/glass–ceramic-based sealant materials are critical issues for high temperature solid oxide fuel/electrolyzer cells. To understand the thermophysical properties and devitrification behavior of SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ system, glasses were synthesized by quenching (25 − X)SrO–20La₂O₃–(7 + X)Al₂O₃–40B₂O₃–8SiO₂ oxides, where X was varied from 0.0 mol% to 10.0 mol% at 2.5 mol% interval. Thermal properties were characterized by dilatometry and differential scanning calorimetry (DSC). Microstructural studies were performed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). All the compositions have a glass transition temperature greater than 620 °C and a crystallization temperature greater than 826 °C. Also, all the glasses have a coefficient of thermal expansion (CTE) between 9.0 × 10⁻⁶ K⁻¹ and 14.5 × 10⁶ K⁻¹ after the first thermal cycle. La₂O₃ and B₂O₃ contribute to glass devitrification by forming crystalline LaBO₃. Al₂O₃ stabilizes the glasses by suppressing devitrification. Significant improvement in devitrification resistance is observed as X increases from 0.0 mol% to 10.0 mol%.     

Synthesis and electrochemistry of cubic rocksalt Li–Ni–Ti–O compounds in the phase diagram of LiNiO2–LiTiO2–Li[Li1/3Ti2/3]O2

On the basis of extreme similarity between the triangle phase diagrams of LiNiO₂–LiTiO₂–Li[Li_1/3Ti_2/3]O₂ and LiNiO₂–LiMnO₂–Li[Li_1/3Mn_2/3]O₂, new Li–Ni–Ti–O series with a nominal composition of Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ (0 ≤ z ≤ 0.5) was designed and attempted to prepare via a spray-drying method. XRD identified that new Li–Ni–Ti–O compounds had cubic rocksalt structure, in which Li, Ni and Ti were evenly distributed on the octahedral sites in cubic closely packed lattice of oxygen ions. They can be considered as the solid solution between cubic LiNi_1/2Ti_1/2O₂ and Li[Li_1/3Ti_2/3]O₂ (high temperature form). Charge–discharge tests showed that Li–Ni–Ti–O compounds with appropriate compositions could display a considerable capacity (more than 80 mAh g⁻¹ for 0.2 ≤ z ≤ 0.27) at room temperature in the voltage range of 4.5–2.5 V and good electrochemical properties within respect to capacity (more than 150 mAh g⁻¹ for 0 ≤ z ≤ 0.27), cycleability and rate capability at an elevated temperature of 50 °C. These suggest that the disordered cubic structure in some cases may function as a good host structure for intercalation/deintercalation of Li⁺. A preliminary electrochemical comparison between Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ (0 ≤ z ≤ 0.5) and Li_6/5Ni_2/5Ti_2/5O₂ indicated that charge–discharge mechanism based on Ni redox at the voltage of >3.0 V behaved somewhat differently, that is, Ni could be reduced to +2 in Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ while +3 in Li_6/5Ni_2/5Ti_2/5O₂. Reduction of Ti⁴⁺ at a plateau of around 2.3 V could be clearly detected in Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ with 0.27 ≤ z ≤ 0.5 at 50 °C after a deep charge associated with charge compensation from oxygen ion during initial cycle.     

Rh promoted and ZrO2/Al2O3 supported Ni/Co based catalysts: High activity for CO2 reforming,steam–CO2 reforming and oxy–CO2 reforming of CH4

Ni (2.5 wt%) and Co (2.5 wt%) supported over ZrO₂/Al₂O₃ were prepared by following a hydrolytic co-precipitation method. The synthesized catalysts were further promoted by Rh incorporation (0.01–1.00 wt%) and tested for their catalytic performance for dry CO₂ reforming, combined steam–CO₂ reforming and oxy–CO₂ reforming of methane for production of syngas. The catalysts were characterized by using N₂ physical adsorption, XRD, H₂–TPR, SEM, CO₂–TPD, NH₃–TPD, TEM and TGA. The results revealed that ZrO₂ phase was in crystalline form in the catalysts along with amorphous Al oxides. Ni and Co were confirmed to be in their respective spinel phases that were reducible to metallic form at 800 °C under H₂. Ni and Co were well dispersed with their nano-crystalline nature. The catalyst with 0.2% loading of Rh showed superior performance in the studied reactions for reforming of methane. This catalyst also showed good coke resistance ability for dry CO₂ reforming reaction with 3.8 wt% of carbon formation during the reaction as compared to 11.6 wt% carbon formation over the catalyst without Rh. The catalyst performance was stable throughout the reaction time for CH₄ conversions, irrespective of carbon formation with slight decline (～1%) in CO₂ conversion. For dry CO₂ reforming reaction, this catalyst showed good conversion for both CH₄ and CO₂ (67.6% and 71.8% respectively) with a H₂/CO ratio of 0.84, while for the Oxy-CO₂ reforming reaction, the activity was superior with CH₄ and CO₂ conversions (73.7% and 83.8% respectively) and H₂/CO ratio of 1.05.     

Nanoconfined 2LiBH4–MgH2–TiCl3 in carbon aerogel scaffold for reversible hydrogen storage

Nanoconfinement of 2LiBH₄–MgH₂–TiCl₃ in resorcinol–formaldehyde carbon aerogel scaffold (RF–CAS) for reversible hydrogen storage applications is proposed. RF–CAS is encapsulated with approximately 1.6 wt. % TiCl₃ by solution impregnation technique, and it is further nanoconfined with bulk 2LiBH₄–MgH₂ via melt infiltration. Faster dehydrogenation kinetics is obtained after TiCl₃ impregnation, for example, nanoconfined 2LiBH₄–MgH₂–TiCl₃ requires ∼1 and 4.5 h, respectively, to release 95% of the total hydrogen content during the 1st and 2nd cycles, while nanoconfined 2LiBH₄–MgH₂ (∼2.5 and 7 h, respectively) and bulk material (∼23 and 22 h, respectively) take considerably longer. Moreover, 95–98.6% of the theoretical H₂ storage capacity (3.6–3.75 wt. % H₂) is reproduced after four hydrogen release and uptake cycles of the nanoconfined 2LiBH₄–MgH₂–TiCl₃. The reversibility of this hydrogen storage material is confirmed by the formation of LiBH₄ and MgH₂ after rehydrogenation using FTIR and SR-PXD techniques, respectively.     

H2 and CO production over a stable Ni–MgO–Ce0.8Zr0.2O2 catalyst from CO2 reforming of CH4

Ni–Ce_0.8Zr_0.2O₂ and Ni–MgO–Ce_0.8Zr_0.2O₂ catalysts were investigated for H₂ production from CO₂ reforming of CH₄ reaction at a very high gas hourly space velocity of 480,000 h⁻¹. Ni–MgO–Ce_0.8Zr_0.2O₂ exhibited higher catalytic activity and stability (CH₄ conversion >95% at 800 °C for 200 h). The outstanding catalytic performance is mainly due to the basic nature of MgO and an intimate interaction between Ni and MgO.     

Releasing 9.6 wt% of H2 from Mg(NH2)2–3LiH–NH3BH3 through mechanochemical reaction

Ball milling the mixture of Mg(NH₂)₂, LiH and NH₃BH₃ in a molar ratio of 1:3:1 results in the direct liberation of 9.6 wt% H₂ (11 equiv. H), which is superior to binary systems such as LiH–AB (6 equiv. H), AB–Mg(NH₂)₂ (No H₂ release) and LiH–Mg(NH₂)₂ (4 equiv. H), respectively. The overall dehydrogenation is a three-step process in which LiH firstly reacts with AB to yield LiNH₂BH₃ and LiNH₂BH₃ further reacts with Mg(NH₂)₂ to form LiMgBN₃H₃. LiMgBN₃H₃ subsequently interacts with additional 2 equivalents of LiH to form Li₃BN₂ and MgNH as well as hydrogen.     

Cu/MnOx–CeO2 and Ni/MnOx–CeO2 catalysts for the water–gas shift reaction: Metal–support interaction

Monometallic copper and nickel catalysts supported on cerium-manganese mixed oxides are prepared, characterized and evaluated for the Water–Gas Shift (WGS) reaction. Active metal loading of 2.5 wt% and 7.5 wt% are used to impregnate MnO_x–CeO₂ supports with 30% and 50% Mn:Ce molar ratio. The structure of the samples strongly depends on both the active metal employed and the manganese content in the mixed support. For both Cu and Ni samples, the best catalytic behavior is found in samples supported on the MnO_x–CeO₂ oxides with 30% Mn:Ce molar ratio, as a result of the presence of Cu_xMn_yO₄ spinel-type phases in the case of copper catalysts and the presence of a NiMnO₃ mixed oxide with defect ilmenite structure in the case of nickel catalysts.     

Reversible hydrogenation/dehydrogenation performances of the Na2LiAlH6–Mg(NH2)2 system

Complex hydrides and Metal–N–H-based materials have attracted considerable attention due to their high hydrogen content. In this paper, a novel amide–hydride combined system was prepared by ball milling a mixture of Na₂LiAlH₆–Mg(NH₂)₂ in a molar ratio of 1:1.5. The hydrogen storage performances of the Na₂LiAlH₆–1.5Mg(NH₂)₂ system were systematically investigated by a series of dehydrogenation/hydrogenation evaluation and structural analyses. It was found that a total of ∼5.08 wt% of hydrogen, equivalent to 8.65 moles of H atoms, was desorbed from the Na₂LiAlH₆–1.5Mg(NH₂)₂ combined system. In-depth investigations revealed that the variable milling treatments resulted in the different dehydrogenation reaction pathways due to the combination of Al and N caused by the energetic milling. Hydrogen uptake experiment indicated that only ∼4 moles of H atoms could be reversibly stored in the Na₂LiAlH₆–1.5Mg(NH₂)₂ system perhaps due to the formation of AlN and Mg₃N₂ after dehydrogenation.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Improved hydrogen storage performance of the LiNH2–MgH2–LiBH4 system by addition of ZrCo hydride

Significant improvements in the hydrogen absorption/desorption properties of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ composite have been achieved by adding 3wt% ZrCo hydride. The composite can absorb 5.3wt% hydrogen under 7.0 MPa hydrogen pressure in 10 min and desorb 3.75wt% hydrogen under 0.1 MPa H₂ pressure in 60 min at 150 °C, compared with 2.75wt% and 1.67wt% hydrogen under the same hydrogenation/dehydrogenation conditions without the ZrCo hydride addition, respectively. TPD measurements showed that the dehydrogenation temperature of the ZrCo hydride-doped sample was decreased about 10 °C compared to that of the pristine sample. It is concluded that both the homogeneous distribution of ZrCo particles in the matrix observed by SEM and EDS and the destabilized N–H bonds detected by IR spectrum are the main reasons for the improvement of H-cycling kinetics of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ system.     

High-performance biofilm photobioreactor based on a GeO2–SiO2–chitosan-medium-coated hollow optical fiber

To improve hydrogen production performance, this paper describes a novel approach for fabricating a biofilm photobioreactor by adsorption of the photosynthetic bacteria (PSB) Rhodopseudomonas palustris CQK 01 on a hollow optical fiber (HOF) with a GeO₂–SiO₂–chitosan medium (GSCM) coating. The composition of the coating is analyzed using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The biocompatibility of the GSCM-coated HOF and the PSB in the hungry condition are examined. We also quantitatively investigate the biofilm dry weight; protein, polysaccharide, bacteriochlorophyll, carotenoid, and ATP contents of the biofilm cell; and average H₂ production rates. The GSCM-coated HOF exhibits enhanced the biofilm biomass, improved the biofilm activity, and an increased H₂ production rate. The proposed photobioreactor yielded fairly stable long-term performance with a hydrogen production rate of 2.65 mmol/L/h, which is 1.56 and 1.51 times higher than those of photobioreactors with an uncoated HOF and with a fiber having a roughened surface obtained by wrapping it in wire mesh, respectively.     

Effective nanoconfinement of 2LiBH4–MgH2 via simply MgH2 premilling for reversible hydrogen storages

To improve nanoconfinement of LiBH₄ and MgH₂ in carbon aerogel scaffold (CAS), particle size reduction of MgH₂ by premilling technique before melt infiltration is proposed. MgH₂ is premilled for 5 h prior to milling with LiBH₄ and nanoconfinement in CAS to obtained nanoconfined 2LiBH₄–premilled MgH₂. Significant confinement of both LiBH₄ and MgH₂ in CAS, confirmed by SEM–EDS–mapping results, is achieved due to MgH₂ premilling. Due to effective nanoconfinement, enhancement of CAS:hydride composite weight ratio to 1:1, resulting in increase of hydrogen storage capacity, is possible. Nanoconfined 2LiBH₄–premilled MgH₂ reveals a single–step dehydrogenation at 345 °C with no B₂H₆ release, while dehydrogenation of nanoconfined sample without MgH₂ premilling performs in multiple steps at elevated temperatures (up to 430 °C) together with considerable amount of B₂H₆ release. Activation energy (E_A) for the main dehydrogenation of nanoconfined 2LiBH₄–premilled MgH₂ is considerably lower than those of LiBH₄ and MgH₂ of bulk 2LiBH₄–MgH₂ (ΔE_A = 31.9 and 55.8 kJ/mol with respect to LiBH₄ and MgH₂, respectively). Approximately twice faster dehydrogenation rate are accomplished after MgH₂ premilling. Three hydrogen release (T = 320 °C, P(H₂) = 3–4 bar) and uptake (T = 320–325 °C, P(H₂) = 84 bar) cycles of nanoconfined 2LiBH₄–premilled MgH₂ reveal up to 4.96 wt. % H₂ (10 wt. % H₂ with respect to hydride composite content), while the 1st desorption of nanoconfined sample without MgH₂ premilling gives 4.30 wt. % of combined B₂H₆ and H₂ gases. It should be remarked that not only kinetic improvement and B₂H₆ suppression are obtained by MgH₂ premilling, but also the lowest dehydrogenation temperature (T = 320 °C) among other modified 2LiBH₄–MgH₂ systems is acquired.     

Material properties and empirical rate equations for hydrogen sorption reactions in 2 LiNH2–1.1 MgH2–0.1 LiBH4–3 wt.% ZrCoH3

2 LiNH₂–1.1 MgH₂–0.1 LiBH₄–3 wt.% ZrCoH₃ is a solid state hydrogen storage material with a hydrogen storage capacity of up to 5.3 wt.%. As the material shows sufficiently high desorption rates at temperatures below 200 °C, it is used for a prototype solid state hydrogen storage tank with a hydrogen capacity of 2 kWh_el that is coupled to a high temperature proton exchange membrane fuel cell. In order to design an appropriate prototype reactor, model equations for the rate of hydrogen sorption reactions are required. Therefore in the present study, several material properties, like bulk density and thermodynamic data, are measured. Furthermore, isothermal absorption and desorption experiments are performed in a temperature and pressure range that is in the focus of the coupling system. Using experimental data, two-step model equations have been fitted for the hydrogen absorption and desorption reactions. These empirical model equations are able to capture the experimentally measured reaction rates and can be used for model validation of the design simulations.