Synergistic effects of isobutene and carbon dioxide on suppressing hydrogen-air explosions

The effects of isobutene (i-C₄H₈) and carbon dioxide (CO₂) on hydrogen explosions have been explored in a 14 L spherical chamber at 100 kPa and 298 K. The results show that compared with CO₂, the cooperation of i-C₄H₈ and CO₂ can more effectively suppress the explosion pressure, pressure rise rate, and deflagration index. The minimum suppression concentration of CO₂ with 1.5% i-C₄H₈ is much lower than that of CO₂ alone. Numerical simulation reveals the mechanism of the suppression of i-C₄H₈ and CO₂. I–C₄H₈ changes the reaction paths, and the addition of CO₂ further lowers the adiabatic flame temperature and the active radical concentration. The excellent performance of the cooperation of i-C₄H₈ and CO₂ in hydrogen explosions will provide a new idea for the prevention and control of hydrogen explosions.     

A review of the effects of hydrogen,carbon dioxide,and water vapor addition on soot formation in hydrocarbon flames

Addition of reactive or inert substances is one of the most effective and practical ways to control soot formation in combustion of hydrocarbon fuels. In this paper, the research progress on the effects of hydrogen, carbon dioxide, and water vapor addition on soot formation in hydrocarbon flames in the last few decades is systematically summarized. The summary shows that the number of studies on the effects of these three common diluents has increased dramatically in the last five years. Although the overall effects of all these three common diluents suppress soot formation, there is inconsistency with regard to the role of their chemical effects. The chemical effect of hydrogen (CE-H₂) mainly acts on the soot nucleation process, followed by the soot surface growth and finally the soot oxidation process. CE-H₂ seems significantly affected by the fuel type, oxygen concentration, and the ambient pressure. The chemical effect of carbon dioxide (CE-CO₂) affects soot formation indirectly mainly through the reaction CO + OH ↔ CO₂ + H. Some studies believe that CE-CO₂ suppresses soot production by increasing the hydroxyl radical (OH) concentration, while other studies believe that it is primarily attributed to the decrease of the hydrogen radical (H) concentration. The reaction H₂O + H ↔ H₂ + OH plays a vital role in the chemical effect of water vapor (CE-H₂O) addition on inhibiting soot formation. Most studies support the view that the chemical effect of water vapor mainly increases the OH concentration and suppresses soot formation by weakening the soot nucleation process. Moreover, we believe that reaction H₂O + O ↔ OH + OH and phenylacetylene also play an essential effect on the CE-H₂O.     

Efficiently generating COx-free hydrogen by mechanochemical reaction between alkali hydrides and carbon dioxide

This article aims to investigate the mechanochemical hydrogen desorption reactions of alkali hydrides (XH: X = Li, Na, or K) with carbon dioxide. The result of this investigation shows that CO₂ can be efficiently reduced by XH (X = Na or K), generating CO_x-free hydrogen under room-temperature mechanical ball milling condition in the absence of a catalyst. The mole percentage and production rate of hydrogen in the gaseous product, which can reach 98.72% and 95.37%, respectively, depend on the milling time, rotation rate of milling and the mole ratio of XH/CO₂. During the mechanochemical reactions, carbon dioxide is fast and wholly consumed by XH, producing element carbon, alkali carbonates, and H₂. This work establishes a new, simple and efficient means for the room-temperature preparation of CO_x-free hydrogen and the elimination of CO_x contaminant in hydrogen.     

Intrinsic affinity of acid-activated bentonite towards hydrogen and carbon dioxide

The effect of acid activation on bentonite affinity toward carbon dioxide (CO₂) and hydrogen (H₂) was investigated at ambient conditions. Characterization through X-ray diffraction and fluorescence, thermal gravimetric analysis, nitrogen adsorption-desorption isotherms, Fourier Transform Infrared spectroscopy and differential scanning calorimetry allowed correlating newly induced textural and structural features with adsorptive properties. Optimum acid treatment improved the specific surface area and porosity. The resulting decrease in dehydration temperature indicates decay in hydrophilic character. The affinity improvement towards hydrogen was due to Brønsted acidity suppression and surface basicity attenuation, which are essential requirements for adsorption on aluminosilicates (AS) via weak Lewis Acid-Base interactions, but excessive acid attack was detrimental. Low Si/Al surfaces should be suitable for CO₂ capture, while moderately acid-treated clays should be interesting candidates as hydrogen adsorbents. This allows envisaging promising prospects for low-cost AS-based materials intended for selective CO₂-free capture and storage of hydrogen without energy and safety constraints.     

Conversion of ethane to ethylene and hydrogen by utilizing carbon dioxide: Screening catalysts

Owing to the necessity of carbon dioxide conversion and exploring new routes for ethylene and hydrogen production, carbon-dioxide-assisted dehydrogenation over alumina-supported catalysts is evaluated in the present contribution. In this regard, the experimental results of a wide range of catalysts at 700 °C and the GHSV of 3600 L_reactants/kg_catalyst.hr are presented and compared. The utilized catalysts showed activity toward the conversion of ethane. However, a few of them showed good selectivity for the production of either ethylene or hydrogen. The catalysts made up of the oxides of cobalt and molybdenum showed very good conversion, selectivity, and yield for ethylene production. Investigating the effect of time on steam on the catalyst performance indicated that these catalysts would be suitable choices in the course of ethylene production. 58% conversion of ethane and 21.9% ethylene yield are the achievements of utilizing the molybdenum-cobalt oxide catalyst. By utilizing rhenium-platinum nickel-potassium catalysts, 100% ethane conversion in tandem with more than 260.0% hydrogen yield was also obtained.     

Investigation on unconfined hydrogen cloud explosion with external turbulence

Fully understanding the coupling mechanism between the enhancement of explosion overpressure and flame acceleration is a prerequisite for assessing hydrogen cloud explosion overpressure. In this research, unconfined fan-stirred hydrogen explosion experiments were performed to study the effects of flame instability and external turbulence on flame propagation and overpressure characteristics. The results showed that the combination of the external turbulence and the flame instability could result in great flame acceleration and explosion overpressure enhancement. For the intensity of external turbulence considered in this study, the combustion regimes were all in the flamelet zone. With the increase of the external turbulence intensity, the flame gradually got accelerated, and the explosion overpressure got enhanced. A theoretical prediction model for the upper and lower limit of the maximum overpressure was proposed, which fully accounted for flame instability, external turbulence, and flame-induced turbulence. It provides a conservative evaluation for hydrogen cloud explosion.     

Experimental investigation on unconfined hydrogen explosion with different ignition height

Experimental research is performed to investigate the effects of ignition height on explosion characteristics in a 27 m³ hydrogen/air cloud. With the ignition height decreasing, the flame propagation velocity increases gradually. The flame travels in oscillating mode and the average oscillating frequency lies between 145Hz and 155Hz. An original parameter τ, which involves flame scale and flame propagation velocity, is proposed to measure the effect of buoyancy. The higher the value of τ, the more obvious the buoyancy effect. As the ignition height increases, the critical flame scale for flame deceleration increases. The middle ignition height in the gas cloud causes the highest overpressure peak, overpressure impulse, overpressure rising and decreasing rate. As the ignition point approaches the initial gas boundary, the explosion intensity would decrease gradually. For the open space outside the flame, overpressure peak for the lower space is higher, while, the middle space experiences higher overpressure impulse.     

Carbon-neutral economy with fossil fuel-based hydrogen energy and carbon materials

Hydrocarbon fossil fuels can be considered as hydrogen ores for CO₂-free energy, and carbon ores for carbonaceous construction materials. Hydrogen fuel can be extracted from fossil fuels by decarbonization, and used as an energy resource. The carbon byproduct can be used as a versatile construction material. Carbon materials would sequester carbon, and replace CO₂-generating steel and concrete. Approximate comparison of the global consumption of energy and construction materials suggests a rough mass balance of energy and materials markets. The cost of foregoing the carbon energy content as a fuel can be easily offset by the value of the carbon-based construction material. The nature and properties of carbon materials and conventional infrastructural materials are compared.     

Study on hydrogen storage properties of Mg nanoparticles confined in carbon aerogels

Magnesium nanoparticles confined in carbon aerogels were successfully synthesized through hydrogenation of infiltrated dibutyl-magnesium followed by hydrogen desorption at 623 K. The average crystallite size of Mg nanoparticle is calculated to be 19.3 nm based on X-ray diffraction analyses. TEM observations showed that the size of MgH₂ particle is mainly distributed in the range from 5.0 to 20.0 nm, with a majority portion smaller than 10.0 nm. The hydrogenation and dehydrogenation enthalpies of the confined Mg are determined to be −65.1 ± 1.56 kJ/mol H₂ and 68.8 ± 1.03 kJ/mol H₂ by Pressure–composition–temperature tests, respectively, slightly lower than the corresponding enthalpies for pure Mg. In addition, the apparent activation energy for hydrogen absorption is determined to be 29.4 kJ/mol H₂, much lower than that of the micro-size Mg particles. These results indicate that the thermodynamic and absorption kinetic properties of confined Mg nanoparticles can be significantly improved due to the ‘nanosize effect’.     

A study of numerical hazard prediction method of gas explosion

A 3-dimensional computational fluid dynamics (CFD) simulation of a premixed hydrogen/air explosion in a large-scale domain is performed. The main feature of the numerical model is the solution of a transport equation for the reaction progress variable using a function for turbulent burning velocity that characterizes the turbulent regime of propagation of free flames derived by introducing the fractal theory. The model enables the calculation of premixed gaseous explosion without using fine mesh of the order of micrometer, which would be necessary to resolve the details of all instability mechanisms. The value of the empirical constant contained in the function for turbulent burning velocity is evaluated by analyzing the experimental data of hydrogen/air premixed explosion. The comparison of flame behavior between the experimental result and numerical simulation shows good agreement. The effect of mesh size on simulated flame propagation velocity is also tested, showing that the numerical result agrees reasonably well with experiment when the mesh size is less than about 20 cm.     

Hydrogen production from methane hydrate with sequestering of carbon dioxide

Methane hydrate exists in large amounts in certain locations, in sea sediments and the geological structures below them and below artic regions permafrost, at low temperature and high pressure. It has recently been shown that there are suitable methods for producing methane, perhaps on a floating platform. There it could be reformed in an endothermic process to produce hydrogen and carbon dioxide. Some of the methane could be used to provide heat energy for a power plant on the platform to provide all needed power and support for the reforming process. After separation, hydrogen is the valuable and transportable product. All carbon dioxide produced on the platform could be separated from other gases and then sequestered, in one of several possible forms. In this way, hydrogen could be made available without the release of carbon dioxide to the atmosphere and the hydrogen could be an enabling step toward a world hydrogen economy, free of particles and carbon dioxide pollution.     

Tech-eco Efficiency Evaluation of hydrogen production industry under carbon dioxide Emissions regulation in China

This study introduces the concept of non-separable variables, and conducts a super-efficiency slack-based measure (SBM) model considered undesirable output variable to evaluate the economic and technical efficiencies of 15 hydrogen production methods in China. The results show that the average technical efficiency and scale efficiency values of hydrogen production industry are 0.438 and 0.352, respectively, which are relevantly low. The average pure technical efficiency value is 1.090. When the strong carbon constraint is introduced, the average technical efficiency and pure technical efficiency values of hydrogen production industry are increased by about 12.3% and 22.2%, respectively, but the average scale efficiency is reduced by 1.7%. The coal-to-hydrogen production with carbon capture, utilization and storage (CCUS) technology produces the highest technical efficiency, and the technical efficiency value and pure technical efficiency value are 1.435 and 1.679, respectively. The scale efficiency value of hydrogen production from chlor-alkali is 0.951, which is the most effective scale among all measured hydrogen production processes. The methanol-to-hydrogen production also performs great. However, hydrogen production by water electrolysis does not have the advantages of economic nor technical efficiencies. The results of the input-output slack of 15 hydrogen production methods show that the excessive production inputs and carbon dioxide emission are the main reasons for the poor efficiency evaluation results of most hydrogen production methods. The CO₂ emission regulation reduces the redundancy of the cost, comprehensive energy consumption and CO₂ emissions of 13 hydrogen production processes by more than four times, which affected the technical efficiency and the scale efficiency of hydrogen production industry. It indicates that CO₂ emission regulation has improved the economic and technical efficiency of hydrogen production processes in China.     

Hydrogen production from bio-oil aqueous fraction with in situ carbon dioxide capture

This paper presents the results of the investigation on steam reforming bio-oil aqueous fraction coupled with in situ carbon dioxide capture for hydrogen production. Experiments were carried out in a bench-scale fixed-bed reactor with calcined dolomite as the sorbent. The effects of temperature and water to bio-oil ratio on hydrogen production are reported. In the presence of calcined dolomite, maximum hydrogen yield of 75% was obtained among without sorbent, with CaO and with calcined dolomite at 600 °C, whereas hydrogen content was 83%, a little lower than that of 85% when CaO was used. Hydrogen content varies little at different water to bio-oil ratios and hydrogen yield was the greatest at the water to bio-oil ratio of 1:1. After regeneration of the sorbent, hydrogen content was back to the initial level but the hydrogen yield dropped.     

A simulation for the piston effect in supercritical carbon dioxide with the non-flow model

A simulation for piston effect in supercritical carbon dioxide by employing a simple model is conducted.In the first place,the thermal properties of carbon dioxide near its liquid-vapor critical point are discussed.It is calculated that the heat capacity ratio and isobaric expansion coefficient of supercritical fluids are extremely high.Furthermore,the simulation for piston effect in supercritical carbon dioxide between two infinite vertical walls is presented.The numerical results prove that piston effect has a much faster speed of heat transfer than thermal conduction under microgravity conditions.Moreover,the piston effect turns out to be stronger when closer to the critical point.     

Features of low-temperature oxidation of hydrogen in the medium of nitrogen,carbon dioxide,and water vapor at elevated pressures

The article discusses novel research results on combustion features of high-density Н₂/О₂ mixtures (ρ_H2 = 0.70–1.89 mol/dm³, ρ_O2 = 0.32–0.81 mol/dm³) diluted with nitrogen, carbon dioxide, or water vapor (from 46 to 76% mol.) at the uniform heating (1 K/min) of tubular reactor. Based on time dependencies of temperature increment in the reaction mixtures caused by the heat release during oxidation of H₂, it is found that the self-ignition temperature of Н₂/О₂/N₂ and Н₂/О₂/H₂O mixtures is by ≈ 30 K lower than that of the Н₂/О₂/СО₂ mixture. Unlike combustion of H₂ in the N₂ medium, in the CO₂ and H₂O media a chain-thermal explosion is observed at a certain concentration of reagents. The influencing mechanisms of diluents on the H₂ oxidation dynamics, as well as the contribution of homogeneous and heterogeneous reactions in the heat release are revealed. It is established that high heat capacity of H₂/O₂/CO₂ mixture, chemical interaction between its components, and presence of CO₂ molecules adsorbed on the reactor inner surface, are the factors determining the H₂ oxidation dynamics in CO₂ medium. At oxidation of H₂ in the H₂O medium, the process takes place against the background of water evaporation and, as a consequence, is characterized by increased heat capacity and thermal conductivity of the H₂/O₂/H₂O reaction mixture.     

Experimental research on combined effect of obstacle and local spraying water fog on hydrogen/air premixed explosion

In order to reveal the mechanism of water fog explosion suppression and research the combined effect of water fog and obstacle on hydrogen/air deflagration, multiple sets of experiments were set up. The results show that the instability of thermal diffusion under lean combustion conditions is the main influencing factor of hydrogen/air flame surface instability, and the existence of water fog will aggravate the hydrogen/air flame surface instability. When obstacle is not considered, 8 μm, 15 μm, 30 μm water fog can significantly reduce the flame velocity and explosion overpressure of hydrogen/air, 45 μm fine water fog plays the opposite role. When considering the relative position of the water fog release position and the obstacle, the 8 μm, 15 μm, 30 μm water fog has almost no suppression effect when released near the obstacle, but a significant suppression effect occur, when using the 45 μm water fog. In the field of theoretical research, the research results not only provide an experimental basis for the fine water fog to reduce the consequences of hydrogen explosion accidents, and the optimal diameter range used by the water fog, but also provide experimental reference for the numerical simulation of hydrogen/air explosion suppression in semi-open space, and promote the development of hydrogen explosion suppression theory. In terms of engineering applications, this study can provide a theoretical basis for the layout of fire fighting equipment in the engine room of nuclear power plants or hydrogen-powered ships.     

Modeling of hydrogen explosion on a pressure swing adsorption facility

Computational fluid dynamic simulations have been performed in order to study the consequences of a hydrogen release from a pressure swing adsorption installation operating at 30 barg. The simulations were performed using FLACS-Hydrogen software from GexCon. The impact of obstruction, partial confinement, leak orientation and wind on the explosive cloud formation (size and explosive mass) and on explosion consequences is investigated. Overpressures resulting from ignition are calculated as a function of the time to ignition.     

Effects of supports on hydrogen production and carbon deposition of Fe-based oxygen carriers in chemical looping hydrogen generation

Chemical looping hydrogen generation (CLHG) can produce high purity hydrogen from fuel gases with inherent separation of CO₂. However, the performance of oxygen carrier in CLHG varies with the support materials. In this paper, the reactivity, carbon deposition, redox stability, hydrogen yield and purity, and sintering behavior of the Fe-based oxygen carriers were analyzed to investigate the effects of supports, i.e. Al₂O₃, SiO₂, MgAl₂O₄, ZrO₂ and YSZ (yttrium-stabilized zirconia). The results showed that the properties of the oxygen carriers, e.g. carbon deposition, reactivity and stability, mainly depended on the support and its interaction with iron oxides. The reactivity and hydrogen yield for the oxygen carriers investigated followed the order: Fe₂O₃/MgAl₂O₄ > Fe₂O₃/ZrO₂ > Fe₂O₃/YSZ > Fe₂O₃/Al₂O₃ > Fe₂O₃/SiO₂, and the order of hydrogen purity was identical with that of hydrogen yield as a result of carbon deposition. Furthermore, the hydrogen purity of the Fe-based oxygen carriers supported by MgAl₂O₄, ZrO₂, or YSZ could reach above 99.5% and Fe₂O₃/YSZ showed the lowest carbon deposition. The oxygen carriers, Fe₂O₃/MgAl₂O₄ and Fe₂O₃/SiO₂, were selected to be characterized by SEM images and XRD patterns before and after the redox cycles.     

Performance modelling of a carbon dioxide removal system for power plants

In this paper, a carbon dioxide removal and liquefaction system, which separates carbon dioxide from the flue gases of conventional power plants, was modelled. The system is based on an amine chemical absorption stripping system, followed by a liquefaction unit to treat the removed CO₂ for transportation and storage. The effect of the main parameters on the absorption and stripping columns is presented. The main constraints set for the model are a capture efficiency of 90% and the use of an aqueous solution with a maximum 30% amine content by weight. The goal of this study is to remove the CO₂ with minimum energy requirements for the process when it is integrated in a fossil fuel fired power plant. Results of the simulation are compared to experimental and literature data from feasibility studies and existing plants.

The power plant to which the removal system is connected is a 320 MW steam power plant with steam reheat and 8 feedwater heaters. Two different fossil fuels were considered: coal and natural gas. The effect of the modifications necessary to integrate the CO₂ removal system in the power plant is also studied.

The capital cost of the removal and liquefaction system is estimated, and its influence on the cost of generated electricity is calculated.     

Synergistic effect of CNT bridge formation and spillover mechanism on enhanced hydrogen storage by iron doped carbon aerogel

In the present, enhanced hydrogen sorption over activated iron-doped carbon aerogel (CA) through the spillover effect was taken up. Iron doped carbon aerogel was prepared through sol-gel polymerization of resorcinol-formaldehyde (R–F) with sodium carbonate as a catalyst. Iron doping was made at 1, 5, and 10 wt % levels. It has been further inferred that the presence of nano iron caused carbon nanotube (CNT) formation in the carbon matrix during the carbonization and activation processes. Fe doped CAs were characterized by XRD, SEM, BET, FTIR, and TEM. Hydrogen sorption by 1, 5, and 10% Fe doped CAs at liquid nitrogen temperature (77K) and up to 25 atm. had storage capacities as 1.47 wt%, 1.38 wt%, and 1.28 wt%, respectively. Activation of CAs brought a significant increase in the storage capacity (∼3.80 wt%), which could have been driven by the presence of CNT in the matrix and an increase in the microporosity on activation.