Experimental study on multi-layered type of gas-to-gas heat exchanger using porous media

Based on an effective energy conversion method between flowing gas enthalpy and thermal radiation, a multi-layered type of gas-to-gas heat exchanger using porous media has been proposed. A series of experiments have been conducted for the inlet temperature of high temperature gas 300-700 °C, the optical thickness of porous media 0-15.4, the number of layers 2-5 and two types of walls (bare or finned) placed in the system. As a result, a heat recovery section is shown to play an important role in lowering an outer wall temperature of the system and at the same time in increasing the total heat recovery rate H_tot,N. In addition, it is clarified that the optical thickness of about 8 is enough to obtain sufficient H_tot,N, and the finned walls are quite effective to promote H_tot,N under the present experimental conditions.     

Tailoring thermal resistance of porous materials with void filling for improved hydrogen adsorption

Porous materials such as 5A molecular sieve (5A) display huge thermal resistance due to high porosity and lots of voids between grains that is negative to hydrogen isotope separation engineering. Generally, introducing thermal conductive fillers contributes to reducing thermal resistance while results in decreasing volume ratio of porous materials and then certainly causes declined hydrogen adsorption capacity. Here, a liquid polydimethylsiloxane (PDMS) is pressed into the voids between 5A grains, which is further developed into a silicon, oxygen and carbon (SiOC) structure suffering from 350 °C sintering. 5A/SiOC composite at 10 wt% polydimethylsiloxane (5A/SiOC10) displays 0.74 W/mK of thermal conductivity, which is about 300% higher than that of neat 5A. More importantly, enhanced rather than reduced hydrogen adsorption capacities at a fixed volume of the composite are determined. 5A/SiOC10 shows adsorption capacities of H₂ (175.4 mL/cm³) and D₂ (188.4 mL/cm³) while neat 5A shows that of H₂ (171.6 mL/cm³) and D₂ (165.8 mmol/cm³) at 77 K with 1 bar. Besides, enhanced thermal conductivity of porous materials shortens the cycle time of hydrogen isotope separation that contributes to reducing energy consumption. This work proposes a novel strategy on void filling to tailor thermal resistance of porous materials, which open a window to improve hydrogen isotope separation with thermal management materials.     

The use of inert porous media based reactors for hydrogen production

The present review paper examines the production of hydrogen in inert porous media based reformer by thermal partial oxidation. Here we consider, specifically, the rich combustion of hydrocarbon fuels and the conversion of H₂S to hydrogen. The different technologies to produce hydrogen beside the experimental and numerical work done in this field are presented. The effect of different operating conditions, such as the equivalence ratio, the mass flow rate and the reactant feed temperature are explained. Additionally, design parameters, including the reactor geometry and porous material specifications, are discussed.     

Reactivation of limestone sorbents in FBC for SO2 capture

Fluidized bed combustion (FBC) has the considerable advantage of being capable of burning high-sulphur fuels while achieving in situ sulphur capture by means of limestone addition. Unfortunately the efficiency of this process is limited, and limestone utilization in the range of 30–45% is not uncommon. In consequence, improving limestone utilization has long been an aim of FBC research. The principal directions this research has taken are the use of water (as liquid or vapour) to reactivate the spent sorbent, or mixing of chemical additives with the limestone to improve its utilization. Despite research stretching over the entire history of FBC combustion, there are still no working commercial applications of reactivation technology noted in the open literature. It is the aim of this paper to present some of the more important research undertaken in this field and to explore the major knowledge gaps that still exist in the area of sorbent reactivation.     

Investigation of the process of thermal runaway in nickel–cadmium accumulators

It has been established experimentally that during thermal runaway a great amount of hydrogen is given off from nickel–cadmium accumulator KSX-25. Through the thermal decomposition of nickel–cadmium accumulator electrodes, it has been shown that hydrogen had been present in the electrodes before the onset of thermal runaway. Through an analysis of the energy balance of thermal runaway, it has been shown that it is not an external source of current that causes thermal runaway, but rather a powerful internal exothermic reaction.     

Numerical study of methane TPOX within a small scale Inert Porous Media based reformer

Methane Thermal Partial Oxidation (TPOX) within a small scale Inert Porous Media (IPM) based reactor was investigated numerically in order to explore the operating conditions and possible procedures for maximizing the reforming efficiency and minimizing the soot formation. A quasi-1D model of the TPOX reactor was validated and further used to study the process. The model considers detailed chemistry and solves the energy balances for both gas and solid phases, including radiative heat transfer in the solid phase. The parametric results of the reactor operation show that the optimal air–fuel ratio is a compromise between soot formation and reforming efficiency. Moreover, a high preheating temperature of the reactants is found to be always beneficial for the process, and the effect of power input is negligible for the reforming efficiency. The numerical investigations also suggest that shorting the IPM length, as well as mixing small amounts of water vapor with the reactants, appear to be effective procedures for improving the operation performance of the TPOX reactor.     

Enhanced thermal stratification in a liquid storage tank with a porous manifold

Experiments were conducted to investigate the effectiveness of a porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank. A thermal storage tank with a capacity of 315 L and a height-to-radius ratio of 4 was used for the experiment. The porous manifold used was made from rolling up a nylon screen into the shape of a tube. Stratification was observed at a Richardson number as low as 0.615. Flow visualization was also performed to confirm the effectiveness of the porous manifold in the promotion and maintenance of stable thermal stratification. From the results of flow visualization, one can conclude that a porous manifold is able to reduce the shear-induced mixing between fluids of different temperature, and thus is able to promote and maintain a stable stratification.     

Investigation of hydrogen storage behavior of silicon nanoparticles

Porous Silicon (PS) freestanding film is a derivative of single crystal Si wafer. PS films obtained on electrochemical etching of p-type silicon (Si) wafer were used to synthesize Si nanoparticles by ultrasonication. 12 μm thick and 29% porous freestanding PS films were sonicated for 4 h in 120 W ultrasonication bath at 42 kHz. HRTEM image shows Si nanoparticles in the range of 8–20 nm in size. In this paper we present results of hydrogen absorption experiments conducted on Si nanoparticles. Standard Seivert’s type apparatus was used to carry out hydrogen absorption pressure composition isotherm measurements in the pressure range of 1–10 bar and in the temperature range of 29 °C–150 °C. Theoretically SiH_x system has 3.44, 6.66 and 9.67 wt% of hydrogen for x = 1, 2, 3 respectively. Experimental results show maximum hydrogen uptake of 2.25 wt% at the temperature of 120 °C and at 9.76 bar pressure. Hydrogenation of Si nanoparticles exhibits frequency downshifts from 510.7 to 507.3 cm⁻¹ in Raman spectra. Raman peaks were de-convoluted in two bands to study effect of hydrogenation on FWHM, crystallanity and elastic strain of the nanoparticles. Bonding between Si, O and H atoms were investigated using Fourier transform infrared spectroscopy(FTIR) spectroscopy. UV–Vis spectra and Tauc plots were used to discuss the relation between hydrogenation and optical band gap of the Si nanoparticles. Optical band gap was found to increase from 1.6 to 2.25 eV on subjecting Si nanoparticles to hydrogenation.     

High density hydrogen storage in nanocavities: Role of the electrostatic interaction

High pressure H₂ adsorption isotherms at N₂ liquid temperature were recorded for the series of cubic nitroprussides, Ni_1−xCo_x[Fe(CN)₅NO] with x = 0, 0.5, 0.7, 1. The obtained data were interpreted according to the effective polarizing power for the metal found at the surface of the cavity. The cavity volume where the hydrogen molecules are accumulated was estimated from the amount of water molecules that are occupying that available space in the as-synthesized solids considering a water density of 1 g/cm³. The calculated cavity volume was then used to obtain the density of H₂ storage in the cavity. For the Ni-containing material the highest storage density was obtained, in a cavity volume of 448.5 Å³ up to 10.4 hydrogen molecules are accumulated, for a local density of 77.6 g/L, above the density value corresponding to liquid hydrogen (71 g/L). Such high value of local density was interpreted as related to the electrostatic contribution to the adsorption potential for the hydrogen molecule within the cavity.     

柴油机排气管隔热技术现状

柴油机排气管隔热技术的应用己有几十年的历史,至今大多数柴油机的隔热技术仍与用户的需求有较大的差距。本文考察了柴油机排气管隔热所使用的材料、技术及隔热效果等问题,特别是侧重对所应用的材料做了进一步的分析,并对今后隔热稳定性方面的工作进行了初步的讨论。     

Facile one-step synthesis of porous graphene-like g-C3N4 rich in nitrogen vacancies for enhanced H2 production from photocatalytic aqueous-phase reforming of methanol

Exfoliation of bulk graphitic carbon nitride (g-C₃N₄) to single- or few-layered structures is an effective way to improve the photocatalytic performance. However, the synthesis methods for few-layer g-C₃N₄ are relatively complicated and time-consuming, with the bandgap of g-C₃N₄ increasing through quantum size effects, thus hampering effective utilization of visible light. To effectively exfoliate the bulk g-C₃N₄ to single or few-layered structures in a facile way without losing its visible light absorption ability is still a challenge. Herein, porous graphene-like g-C₃N₄ nanosheets with abundant nitrogen vacancies were prepared by facile thermal polymerization of melamine using graphene oxide (GO) as a sacrificial template. The two-dimensional (2D) layer morphology and nitrogen defect structure were proved using AFM, SEM, TEM, EA, XPS and EPR techniques. Compared with the bulk g-C₃N₄, the as-prepared g-C₃N₄ nanosheet possesses a high specific surface area, enhanced absorption ability of visible light, and elevated charge carrier generation and separation efficiency because of the unique structural features. The in situ DRIFT spectrum indicates that the surface nitrogen vacancies also serve as excellent locations for methanol adsorption and activation. Consequently, an excellent photocatalytic activity of hydrogen production from methanol aqueous-phase reforming is obtained, which is about 14 times more productive than the bulk g-C₃N₄.