A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Catalytic partial oxidation of CH4–H2 mixtures over Ni foams modified with Rh and Pt

The catalytic partial oxidation (CPO) of methane–hydrogen mixtures in air, intended for the first stage of hybrid radiant catalytic burners, was investigated under self-sustained short contact time conditions on commercial Ni foam catalysts eventually modified with Rh and Pt. The modified catalysts were prepared by a simple novel method based on the spontaneous deposition of noble metals via metal exchange reactions onto those Ni foam substrates. SEM-EDS, electrochemical methods and H₂-TPR analysis were integrated to characterize morphology, surface area of metal deposits and reducibility of foam catalysts before and after exposure to severe conditions in the CPO reactor. In particular Rh forms finely dispersed deposits that retain their high specific surface area at temperatures up ca. 1100 °C. Modification with noble metals enhances stability and reducibility of the Ni foam whereas the overall CPO performance is not significantly improved. Safe operation of the CPO reactor with up to 70% vol. H₂ in the fuel mixture has been achieved by properly increasing the feed equivalence ratio to avoid catalyst overheating, while guaranteeing high methane conversions and a persistent net hydrogen production.     

The retardation kinetics of magnetite reduction using H2 and H2–H2O mixtures

Cylindrical compacts of magnetite were isothermally reduced at 773–1273 K with pure H₂ or H₂–H₂O mixtures. The initial reduction rates increased with temperature and partial pressures of H₂ in the H₂–H₂O mixtures. However, with progressing reduction, a dense iron layer formed around the wüstite grains and the rate significantly reduced. In this regime, solid state oxygen diffusion through the dense iron layer was rate limiting. This retardation of reduction occurred at degrees of reduction of 51–89%, depending on the temperature and H₂ partial pressure, which has a linear relationship with the dimensionless kinetic parameter, k₁^mixed/k₂^mixed, (k₁^mixed, k₂^mixed: contribution of gaseous mass transport (GMT) and interfacial chemical reaction (ICR) to the reduction rate, respectively) in the reaction-regime controlled by a combination of both mechanisms. However, under certain conditions (H₂, H₂–10%H₂O, 773 K//H₂–10, 20%H₂O, 873 K//H₂–20%H₂O, 973 K) the retardation was absent because of the formation of a microporous iron layer product.     

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.     

Effects of alumina powder characteristics on H2 and H2O2 production yields in γ-radiolysis of water and 0.4 M H2SO4 aqueous solution

The effects of powder characteristics on H₂ and H₂O₂ productions in ⁶⁰Co γ-radiolysis were studied in pure water and in 0.4 M H₂SO₄ aqueous solutions containing alumina powders. In 0.4 M H₂SO₄ solution, the H₂ yields strongly depended on alumina structures and decreased in the order of α > θ > γ-alumina, although the specific surface areas increased as α < θ < γ. The yields increased with increasing specific surface area when compared among α-alumina. In pure water, similar dependence was observed but not as strong as that for 0.4 M H₂SO₄ solution. The H₂O₂ yields were strongly decreased by adding the alumina powders in both water and 0.4 M H₂SO₄ aqueous solution, although the amounts of decrease were almost neither correlated with specific surface areas nor structures. The enhancing H₂ production was discussed in terms of the electron supply from alumina to aqueous solution as well as the adsorption of OH radicals on alumina surfaces.     

H2 and CH4 oxidation on Gd0.2Ce0.8O1.9 infiltrated SrMoO3–yttria-stabilized zirconia anode for solid oxide fuel cells

Strontium molybdate (SrMoO₃) as an electronic conductor was incorporated with yttria-stabilized zirconia (YSZ) to form an anode scaffold for solid oxide fuel cells. Gd_0.2Ce_0.8O_1.9 (GDC) nanoparticles were introduced by wet impregnation to complete the Ni-free GDC infiltrated SrMoO₃–YSZ anode fabrication. The effects of SrMoO₃ on the electrode conductivity and GDC infiltration on the catalytic activity were examined. A pronounced performance improvement was observed both on wet H₂ and CH₄ oxidation for the 56 wt.% GDC infiltrated SrMoO₃–YSZ. In particular, the polarization resistance decreased from 8 Ω cm² to 0.5 Ω cm² under wet H₂ (3% H₂O) at 800 °C with the introduction of GDC. Under wet CH₄ at 900 °C, a maximum power density of 160 mW cm⁻² was obtained and no carbon deposition was observed on the anode. It was found that the addition of H₂O in the anode caused an increase of electrode ohmic resistance and a decrease of open circuit voltage. Redox cycling stability was investigated and only a slight drop in cell performance was observed after 5 cycles. These results suggest that GDC infiltrated SrMoO₃–YSZ is a promising anode material for solid oxide fuel cells.     

Numerical simulations of non-premixed turbulent combustion of CH4–H2 mixtures using the PDF approach

The present work is devoted to the study of non-premixed turbulent combustion with the PDF approach using three turbulence models: k-? model, modified k-? model and RSM model. A detailed kinetic mechanism is used in the numerical simulations. The three turbulence models are compared and evaluated with the experimental data and the numerical results of the literature. The evaluation concludes that the modified k-? is the most appropriate for simulating this kind of flame. A study of the effect of hydrogen addition on methane combustion is performed. Hydrogen addition causes the elevation of combustion temperature, the decreasing of CO and CO₂ mass fractions but leads to the increase of NO mass fraction.     

Detailed investigation of NO mechanism in non-premixed oxy-fuel jet flames with CH4/H2 fuel blends

This study systematically investigates the detailed mechanism of nitrogen oxides (NO_x) in CH₄ and CH₄/H₂ jet flames with O₂/CO₂ hot coflow. After comprehensive validation of the modeling by experiments of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]; the effects of CO₂ replacement of N₂, mass fraction of oxygen in the coflow (Y_O2), and mass fraction of hydrogen in the fuel jet (Y_H2) on NO formation and destruction are investigated in detail. For methane oxy-fuel combustion, the NNH route is found to control the NO formation at Y_O2 ≤ 3%, while both NNH and N₂O-intermediate routes dominate the NO production at 3% < Y_O2 < 10%. When Y_O2 ≥ 10%, NO is obtained mainly from thermal mechanism. Moreover, in the oxy-combustion of methane and hydrogen fuel blends with Y_O2 = 3%, with hydrogen addition the contribution of the NNH and prompt routes increases, while that of the N₂O-intermediate route decreases. Furthermore, the chemical effect of CO₂ is significant in reducing NO in both oxy-combustion of methane with Y_O2 ≤ 3% and combustion of methane and hydrogen fuel blends with Y_H2 ≤ 10%.     

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.     

Hydrogen addition effect on laminar burning velocity,flame temperature and flame stability of a planar and a curved CH4–H2–air premixed flame

The effects of hydrogen fraction on laminar burning velocity, flame stability (Markstein number) and flame temperature of methane–hydrogen–air flame at global equivalence ratios of 0.7, 1.0 and 1.2 have been investigated numerically based on the full chemistry and the detailed molecular species transport. The effect of stretch rate on combustion characteristics is examined using an opposed-flow planar flame model, while the effect of flame curvature is identified by comparing a tubular flame to the opposed-flow planar flame. The difference in response on hydrogen fraction between the planar and curved flames has been observed. The results show when hydrogen fraction increases, the flame temperature and laminar burning velocity increases, and this effect is more significant at a large stretch rate; while Markstein length decreases. At a fixed stretch rate of 400 s⁻¹, under which the flame approaches extinction limit, the flame temperature of the tubular flame is considerably higher than that of the planar opposed flow flame, which results most likely from the contribution of the positive flame curvature to the first Damkohler number.     

CO2 reforming of CH4 by combination of cold plasma jet and Ni/γ-Al2O3 catalyst

CO₂ reforming of CH₄ to synthesis gas was investigated by cold plasma jet (CPJ) only and combination of cold plasma jet with Ni/γ-Al₂O₃ catalyst at atmospheric pressure. The higher selectivity of H₂ and CO, and higher energy efficiency was obtained by this novel process. The optimum experimental conditions are: CH₄ = 3.33 Nl/min, CO₂ = 5.00 Nl/min, N₂ = 8.33 Nl/min, and the input power at 770 W. The results showed that, for the plasma only, the conversions of CH₄ and CO₂ were 46% and 34%, the selectivities of CO and H₂ were 85% and 78%, the energy efficiency was 2.9 mmol/kJ, respectively; for the combination of cold plasma jet with Ni/γ-Al₂O₃ catalyst, the conversions of CH₄ and CO₂ were increased by 14% and 6%, the yield of H₂ and CO increased by 18% and 11%, the energy efficiency reached at 3.7 mmol/kJ, respectively. And the catalyst hasn't accessorial heating. The CPJ method has the advantage of simple processing and is easy to be industrialized.     

Syngas production from CH4 dry reforming over Co–Ni/Al2O3 catalyst: Coupled reaction-deactivation kinetic analysis and the effect of O2 co-feeding on H2:CO ratio

The dry and oxidative dry reforming of CH₄ over alumina-supported Co–Ni catalysts were investigated over 72-h longevity experiments. The deactivation behaviour at low CO₂:CH₄ ratio (≤2) suggests that carbon deposition proceeds via a rapid dehydropolymerisation step resulting in the blockage of active sites and loss in CO₂ consumption. In particular, at high temperatures of 923 K and 973 K, a ‘breakthrough’ point was observed in which deactivation that was previously slow suddenly accelerated, indicating rapid polymerisation of deposited carbon. Only with feed CO₂:CH₄ > 2 or with O₂ co-feeding was coke-induced deactivation eliminated. In particular, O₂ co-feeding gave improved carbon removal, product H₂:CO ratios more suitable for downstream GTL processing and stable catalytic performance. Conversion-time data were adequately fitted to the generalised Levenspiel reaction-deactivation model. Activation energy estimate (66–129 kJ mol⁻¹) was dependent on the CO₂:CH₄ ratio but representative of other hydrocarbon reforming reactions on Ni-based catalysts.     

Revision of the NaBO2–H2O phase diagram for optimized yield in the H2 generation through NaBH4 hydrolysis

The binary phase diagram NaBO₂–H₂O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH₄ hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO₂·4H₂O (Na[B(OH)₄]·2H₂O), NaBO₂·2H₂O (Na[B(OH)₄]), NaBO₂·2/3H₂O (Na₃[B₃O₄(OH)₄]), NaBO₂·1/3H₂O (Na₃[B₃O₅(OH)₂]) and NaBO₂ (Na₃[B₃O₆]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from −10 to 80 °C have been determined, confirming literature data at low temperature (20–50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO₂·4H₂O, occurring at −7 °C, has been determined by DSC. The Liquid–Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO₂·2/3H₂O has been estimated at 131.6 °C. This knowledge is paramount in the field of hydrogen storage through NaBH₄ hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO₂–H₂O binary phase diagrams and its consequence related to hydrogen storage through NaBH₄ hydrolysis.     

Thermodynamic and kinetic studies of NaBH4 regeneration by NaBO2–Mg–H2 ternary system at isothermal condition

NaBH₄ is a candidate for H₂ storage in solid phase. NaBH₄ hydrolysis readily produces H₂ gas and NaBO₂ which can regenerate NaBH₄ with pressurized hydrogen and the aid of a reducing agent like Magnesium above 500 °C. This paper deals with the NaBH₄ thermochemical regeneration from the NaBO₂–Mg–H₂ ternary system at isothermal temperatures between 558 and 634 °C and H₂ pressure in the range 2–31 bar. A simplified Langmuir adsorption model has been applied for the interpretation of the in-situ H₂ pressure variations. The applied model is zero-dimensional but provides a reasonable approach to identify the rate determining step and acquire relevant thermodynamic and kinetic parameters such as equilibrium constant (K_eq), Gibbs free energy (Δ_rG⁰) and reaction rate coefficients (k). The study provides an apparent activation energy and Gibbs free energy of this process of 29.2 kJ/mol and −76.9 kJ/mol, respectively.     

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.     

Start-up behaviors of a H2SO4–H2O distillation column for the 50 NL H2/H sulfur–iodine cycle

The sulfur–iodine (SI) cycle to produce hydrogen from water requires a multistage distillation column to concentrate a sulfuric acid solution. To design a concentration process of a sulfuric acid solution that can be applied to the cycle, its static and dynamic simulation is essentially demanded. A 50 NL H₂/h scale SI test facility to be operated under a pressurized environment has been constructed in Korea. This study focuses on the sulfuric acid multi-stage distillation column (SAMDC-50L) for the 50 NL H₂/h SI test facility. The SAMDC-50L was designed and installed in 2012. Based on the design specifications and operation method, a start-up behavior of the SAMDC-50L has been analyzed using the simulation code “KAERI-DySCo”. As a result of the start-up dynamic simulation, it is confirmed that the SAMDC-50L will approach to the steady state value within 30,000 s to fulfill the hydrogen production rate of 50 NL H₂/h. On the other hand, it is expected that the operation time approaching a steady state decreases with an increase in the set point of the condenser temperature until a dew point of the top vapor product and the time required for the transition to the complete steady state is increased with an increasing reflux ratio and reboiler hold-up.     

HI extraction by H3PO4 in the Sulfur–Iodine thermochemical water splitting cycle: Composition optimization of the HI/H2O/H3PO4/I2 biphasic quaternary system

Iodine excess separation from hydriodic acid (HI) is one of the most challenging steps of the Sulfur–Iodine thermochemical water splitting cycle. One promising method is the extraction of HI by using phosphoric acid (H₃PO₄), with the subsequent separation of gaseous hydriodic acid from water and H₃PO₄ by a distillation step.