Hydrolysis of sodium borohydride solutions both in the presence of Ni–B catalyst and in the case of microwave application

In this study, the nickel boron (Ni–B) catalyst was studied in the microwave environment for hydrogen production from the hydrolysis of a sodium borohydride solution to release H₂. The catalytic activity of the Ni–B catalyst was measured by hydrogen production from the hydrolysis of sodium borohydride. The catalytic properties of the Ni–B catalyst in the microwave medium were examined by considering parameters such as NaOH concentration, NaBH₄ concentration, catalyst amount, temperature, and microwave power. Thus, the results obtained from the experiments carried out with Ni–B catalyst both in non-microwave and microwave media were compared. In the experiments, under microwave irradiation, the best result was the release of hydrogen gas from the Ni–B catalyst by applying 100 W of microwave energy at 40 °C. Activation energy values were calculated using the reaction rate constants obtained at different temperatures in the nth order kinetic model and the Langmuir - Hinshelwood model.     

Energy conversion performances during biomass air gasification process under microwave irradiation

Biomass gasification technology under microwave irradiation is a new and novel method, and the energy conversion performances during the process play a guiding role in improving the energy conversion efficiencies and developing the gasification simulation models. In order to improve the energy utilization efficiency of microwave biomass gasification system, this study investigated and presented the energy conversion performances during biomass gasification process under microwave irradiation, and these were materialized through detailing (a) the energy conversion performance in the microwave heating stage, and (b) the energy conversion performance in the microwave assisted biomass gasification stage. Different forms of energies in the biomass microwave gasification process were calculated by the method given in this study based on the experimental data. The results showed that the useful energy (energy in silicon carbide (SiC), 18.73 kJ) accounted for 31.22% of the total energy input (electrical energy, 60.00 kJ) in the heating stage, and the useful energy (energy in the products, 758.55 kJ) accounted for 63.41% of the total energy input (electrical and biomass energy, 1196.28 kJ) in the gasification stage. During the whole biomass gasification process under microwave irradiation, the useful energy output (energy in the products, 758.55 kJ) accounted for 60.38% of the total energy input (electrical and biomass energy, 1256.28 kJ), and the energy in the gas (523.40 kJ) product played a dominate role in product energy (758.55 kJ). The energy loss mainly included the heat loss in the gas flow (89.20 kJ), magnetron loss (191.80 kJ) and microwave dissipation loss (198.00 kJ), which accounted for 7.10%, 15.27% and 15.76% of the total energy, respectively. The contents detailed in this study not only presented the energy conversion performances during microwave assisted gasification process but also supplied important data for developing gasification simulation models.     

Microwave-assisted heating of cementitious materials: Relative dielectric properties,mechanical property,and experimental and numerical heat transfer characteristics

The characteristics of cementitious materials subjected to microwave energy are presented. First, the dielectric properties of cementitious materials during a 24-hour first-hydration period were measured at a frequency of 2.45 GHz. Second, the characteristics of hardened cement paste as subjected to heating for a short period (without loss of moisture) by microwave energy with a single-mode rectangular wave guide, with specific attention to temperature rise, compressive strength, and the use of the maturity function was investigated experimentally and theoretically. The obtained results show that dielectric properties are relatively high and remain constant during the dormant period. After this period, the hydration reaction resumes and dielectric properties decrease rapidly. With the use of microwave heating, early-age strength increases during the first 14 days; however, during the next 14 days, early-age strength decreases slightly, until it reaches its lowest at the 28-day mark. The temperature rise as actually recorded at the center of the sample during microwave heating in our experiment consistently agreed with figures calculated by a mathematical model.     

Computational energy analysis of an innovative isothermal chamber for testing of the special equipment used in the transport of perishable products

This paper describes an improved numerical simulation study of an isothermal chamber recently constructed at Zografou Campus of the National Technical University of Athens (NTUA) for the testing of special equipment used for transporting perishable foodstuffs in accordance with the United Nations ATP agreement. Using a transient finite difference model, a simulation is developed for a modern ATP test chamber and a typical specimen refrigerated vehicle to be tested. The simulation results are compared to experimental measurements taken under real conditions by a data acquisition system and a refrigerated semi‐trailer as specimen. Proportional–integral control is employed for the regulation of the cooling and heating system. The impact of various parameters on the time required to reach the set‐point temperature (t_set) is investigated and the energy consumption is simulated for a period of 22 h. In particular, the impact of specimen insulation thickness and the thickness of the chamber insulation floor are considered in detail. The total energy consumption increases by approximately 16% when the concrete floor layer thickness is increased from 8 to 16 cm for typical initial conditions and desired chamber and specimen temperatures of 32.5 and 7.5°C, respectively. Using a floor insulation of 6 cm extruded heavy strain‐resistant polystyrene reduces the energy consumption by at least 13%. Specimen insulation thickness increase from U‐value of 0.35 W m^?2 K to 0.75 W m^?2 K result to an increase in energy consumption by a percentage of 28%. Thermal capacity, temperature of car body and specimen dimensions are also treated as variables that affect the total duration of an ATP test and its total energy consumption. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermite powder ignition by localized microwaves

This paper presents a new method to ignite pure thermite powder by low-power microwaves (～100 W). In this method, the microwave energy is supplied locally to the powder. It creates a confined hotspot, and initiates a self-propagating combustion in the entire powder volume. The coupled thermal–electromagnetic interaction evolved within the powder prior to its ignition is simulated theoretically, taking into account the powder’s temperature-dependent parameters. The simulation results show a thermal-runaway instability and localized heating within a confined hotspot, induced mostly by the microwave’s electric-field component. The experimental setup employs accordingly an open-end applicator implemented by a miniature solid-state microwave-drill device inserted into the thermite powder as a local igniter. The experimental results show ignition within ～3 s at 2.1-GHz, 100-W microwave injection, in agreement with the theoretical model. The dependence of the minimal microwave power on the exposure time required to reach combustion is identified. Practical aspects and potential applications of this mechanism, such as rust conversion, energy production, and propulsion are indicated.     

Comparison of oil extraction methods,energy analysis and biodiesel production from flax seeds

In recent years, the commercial potential of oil extraction and biodiesel production derived from vegetable seed is being realized. The process energy input requirements are important factors in oil extraction and biodiesel production. This research work investigated oil extraction from flax seeds and compared extraction yield with the energy load. The effect of moisture content on the oil yield was compared between a mechanical oil expeller, organic solvent extraction, organic solvent and microwave assisted, organic solvent and ultrasonic assisted, and combined microwave and ultrasonic with organic solvent. The maximum oil yields % wt/wt from these techniques was 22.6%, 36.3%, 10.0%, 42.0% and 27.8%, respectively. The moisture content had a significant effect on oil yield with the mechanical oil expeller, organic solvent method and ultrasonic assisted extraction, whereas no or little effect was found on microwave‐assisted extraction. The microwave‐assisted extraction showed better results compared with the ultrasonic‐assisted and combined treatment methods. The relative energy consumption of these processes was experimentally investigated; energy ratios were calculated based on the amount of energy recovered to the amount of energy supplied to the flax seed for oil extraction. The net energy ratios showed that microwave‐assisted extraction had the highest (25.21%), followed by organic solvent method (14.04%), ultrasonic method (6.33%) and lowest was with combined ultrasonic and microwave assisted treatment (5.73%). These results showed that flax seed oil can be extracted using microwave‐assisted methods efficiently and in an energy feasible manner. In situ ultrasonic transesterification was applied to powdered samples with 4%, 8% and 12% moisture content (on % dry basis) within an ultrasonic bath having an intensity of 0.124 W/cm². The flax seed biodiesel produced showed a highest conversion yield of 93%, and the effect of different moisture content on the yield showed that 4% moisture content sample produced the greatest biodiesel yield. Copyright © 2013 John Wiley & Sons, Ltd.     

A feasibility study into joining of engineering thermoplastics utilising concentrated beam solar radiation

Most manufacturing technologies and facilities today are being developed for efficient and environment-friendly production. However, regardless of what type of material processing technology we use, we need energy. Solar energy, although often called alternative or even new energy source, is probably the oldest energy source available on earth. In this paper, the attempts made by the authors to explore the feasibility of utilising concentrated solar beam radiation for joining engineering thermoplastics such as acrylonitrile/butadiene/styrene (ABS), polycarbonate (PC) and polymethylmethacrylate (PMMA) are presented. In addition, to study the joining of the materials, necessary experimentation with applying primer was performed. Tensile tests were conducted to determine the bond strength achieved at the specimen joint interface. Microscopic examinations of the fractured joints were performed in order to analyse the overall bond quality. Finally, the results in terms of bond strength achieved at the joint interface and energy consumed in the process were compared with those obtained with similar thermoplastic joining technique utilising microwave energy. In conclusion, some advantages and limitations were outlined and necessary improvements of the joining technique were recommended.     

Numerical analysis of microwave melting of ice-saturated porous medium filled in a rectangular waveguide with resonator using a combined transfinite interpolation and PDE methods

A numerical study is performed for the melting process of ice-saturated porous medium filled in a rectangular waveguide with a resonator subjected to electromagnetic energy. A microwave system supplies a monochromatic wave in a fundamental mode (TE₁₀ mode) with operating frequency of 2.45 GHz. Focus is placed on establishing a computationally efficient approach for solving moving boundary heat transfer problem in a two-dimensional structured grid. Numerically, preliminary grids are first generated by an algebraic method, based on a transfinite interpolation method, with subsequent refinement using a PDE mapping method. A preliminary case study indicates successful implementation of the numerical procedure. The predicted results from two-dimensional melting model are then validated against available experimental results and subsequently used as a tool for efficient computational prototyping. Based on the numerical results are performed illustrating the influence of resonator and layered configuration, in case of the installed resonator has strongly affected on the microwave power absorbed, temperature distribution, and the melting front during microwave melting process.     

Theoretical and experimental investigation of microwave thawing of frozen layer using a microwave oven (effects of layered configurations and layer thickness)

The thawing of layered sample due to microwave energy (2.45 GHz) in oven has been investigated theoretically and experimentally. Based on a model combining of the electromagnetic field and thermal model, it is shown that the variation of layered configurations and unfrozen layer thickness changes the degree of temperature level within layered sample as well as the thawing rate. This is due to the changing of characteristics of dielectric properties of phase change materials during microwave thawing process. The simulated results are in agreement with the experimental results for the microwave thawing process.     

Simulation study of parameters influencing microwave heating of biomass

Experimental investigation of microwave-assisted pyrolysis was successfully done using a specially modified domestic microwave oven. At conditions where no parameterised experimental study was conducted (except temperature measurement), changing other parameters that influence the microwave heating can be investigated qualitatively in detail by means of simulation work. Therefore, a computer based simulation using Comsol Multiphysics software was applied not only to predict how the electromagnetic field distributes within the cavity but also to study different parameters that influence heating distribution inside the microwave oven. The simulation work was initially performed with verification between experimental and simulation temperature profiles at temperature settings of 500 °C and 800 °C and an agreement was achieved in terms of the temperature profile and heating behaviour of the biomass. The simulation work has proved that the inhomogeneity of temperature of the biomass is reflected by the local occurrence of hot spots and cold spots. The effect of different positions of the waveguide is remarkable where the bottom-fed microwave energy oven was shown to have a poor electric field distribution. However, when simulation was done on combining the effect of having the microwave energy fed from the bottom and the presence of the mode stirrer, the electric field was greatly improved with the heating distribution of the biomass resembling that obtained from the side-fed microwaves energy oven (usually refers to a common home microwave oven). The effect of having a mode stirrer rotating inside the microwave oven is also pronounced where the mode stirrer acts to stir the electric field strength within the cavity so that a more uniform heating within the biomass can be achieved. Interestingly from the simulation, for a specified microwave cavity, an optimum bed size of biomass was found at 50 mm height where maximum microwaves energy absorption takes place. In this sense, more microwaves energy can be converted into heat thereby ultimately helping the biomass to reach the desired pyrolysis temperature in shorter time. The COMSOL modelling on microwave heating therefore has shown to be simple and practical for use as a framework in predicting temperature profile of the biomass and intensity of the electric field.     

Hydrogen production by direct injection of ethanol microdroplets into nitrogen microwave plasma flame

Liquid ethanol introduced as microdroplets into the tip of microwave nitrogen plasma, operating at 2.45 GHz under atmospheric pressure, has been investigated. Injection of ethanol outside the region of plasma generation eliminated a problem of soot formation at that region, which was responsible for short reactor lifetime. Using liquid ethanol allows to save energy needed for vaporization. Hydrogen, carbon monoxide and solid carbon were the main outlet products. Other products detected with gas chromatography were CH₄, C₂H₄ and C₂H₂. The best results concerning hydrogen production were as follows: concentration in the outlet gas up to 28%, production rate up to 1043 L/h, energy yield up to 209 L per kWh of microwave power, and were obtained for liquid C₂H₅OH flow rate of 3.7 L/h. A numerical 0D model was used to determine contributions of chemical reactions in formation of measured gaseous products. Simplified model involving only radical reactions without any ions and electrons predicts final concentrations of main compounds quite well for microwave power up to 4 kW.     

Optimization of wet microalgal FAME production from Nannochloropsis sp. under the synergistic microwave and ultrasound effect

The synergistic effect of microwave and ultrasound irradiations was evaluated for biodiesel production from microalgae biomass (Nannochloropsis sp.) as raw material. A response surface methodology technique based on central composite design was used to understand the process parametric interdependence and optimize the process reaction variables. Reaction kinetics of algal fatty acid methyl ester (FAME) production was also studied. The optimum reaction conditions were determined as wet algal biomass to methanol ratio of 20 g to 30 mL, 1 wt% catalyst concentration, and 7‐minute reaction time at 140 W of microwave power and 140 W of ultrasound power. The estimated activation energy was 17,298 J/mol^?1 K^?1 for a first‐order reaction kinetics. This study revealed that microwave energy dissipation at a low rate of 140 W combined with 140 W of ultrasound intensity is adequate to produce FAMEs at a maximum yield of 48.2%. Results from this optimization study suggest that a more detailed and mechanistic energy optimization study is critical to increase the FAME yield and maximize energy benefits.     

热水器污垢实时监测及其节能潜力估算

热水器的实测结果表明了热水器的污垢不可忽视.水质和垢样分析结果则指出了垢的成因,为了消除垢的不良影响,建立了热水器污垢(热阻)实时监测模型用以监测污垢的积聚状态,并进而据以估算热水器污垢对策的潜在节能价值.     

Application of atmospheric pressure microwave plasma source for hydrogen production from ethanol

In contrast to conventional technologies of hydrogen production like water electrolysis or coal gasification we propose a method based on the atmospheric pressure microwave plasma. In this paper we present results of the experimental investigations of the hydrogen production from ethanol in the atmospheric pressure plasma generated in waveguide-supplied cylindrical type nozzleless microwave (915 MHz and 2.45 GHz) plasma source (MPS). Argon, nitrogen and carbon dioxide were used as a working gas. All experimental tests were performed with the working gas flow rate Q ranged from 1500 to 3900 NL/h and absorbed microwave power P_A up to 6 kW. Ethanol was introduced into the plasma as vapours carried with the working gas. The process resulted in the ethanol conversion rate greater than 99%. The hydrogen production rate was up to 210 NL[H₂]/h and the energy efficiency was 77 NL[H₂] per kWh of absorbed microwave energy.     

微波热风技术的研究及应用

微波热风技术是以微波为热源,利用吸波材料吸收微波能转换为热能,与冷空气接触实现热交换得到热风,并加以利用的一种新兴技术,具有加热速度快、温度稳定、能耗低、热效率高、热风清洁无污染等优点。主要阐述了目前高温热风技术的研究及应用,并对微波腔体内强化换热机理与结构进行了分析与设计。     

Mathematical modeling of continuous flow microwave heating of liquids (effects of dielectric properties and design parameters)

A detailed numerical model is presented to study heat transfer in liquids as they flow continuously in a circular duct that is subjected to microwave heating. Three types of food liquids are investigated: apple sauce, skim milk, and tomato sauce. The transient Maxwell's equations are solved by the finite difference time domain (FDTD) method to describe the electromagnetic field in the microwave cavity and the waveguide. The temperature field inside the applicator duct is determined by the solution of the momentum, energy, and Maxwell's equations. Simulations aid in understanding the effects of dielectric properties of the fluid, the applicator diameter and its location, as well as the geometry of the microwave cavity on the heating process. Numerical results show that the heating pattern strongly depends on the dielectric properties of the fluid in the duct and the geometry of the microwave heating system.     

Influence of microstructure-driven hydrogen distribution on environmental hydrogen embrittlement of an Al–Cu–Mg alloy

The hydrogen trap sites and corresponding hydrogen binding energies in an Al–Cu–Mg alloy with the different microstructures were investigated to unravel the environmental hydrogen embrittlement (HE) behavior of the alloy. The results showed that hydrogen can reside at interstitial lattices, dislocations, S′-phase, and vacancies. In the aged specimen with the highest hydrogen content, it was firstly reported that hydrogen resided at S′-phase particles with relatively high binding energy, which is a determinant factor on HE resistance of the alloy. In the cold-rolled specimen, high content of hydrogen trapped at dislocations with a reversible nature leads to intergranular hydrogen-assisted cracking. In the solution-treated specimen, hydrogen migration to the surface due to low trap density results in low hydrogen content and prevents the GBs from reaching critical hydrogen concentration. The obtained results clearly reveal that trap site density, and the nature of trap sites can determine environmental HE susceptibility of the alloy.     

Evaluating the direct and indirect rebound effects in household energy consumption behavior: A case study of Beijing

This paper examines whether increases in energy efficiency of major household items cause additional short-run utilization of these end uses and other end uses for households in Beijing. An integrated model is first developed by combining a Logit model and a resource allocation model, where the former represents the choice of end-use ownership and the latter describes the end-use usage. The rebound effects are finally obtained from calculating the own- and cross-elasticities based on the prediction. The empirical results show that for refrigerators, electric fans, gas showers, TVs, and PCs, no evident rebound occurs; while for air conditioners, clothes washers, microwave ovens, and cars, either a direct rebound effect or an indirect rebound effect exists significantly. The respective average upper bound of direct rebound effects for them are 60.76%, 106.81%, 100.79%, and 33.61%, suggesting a possibility of backfire for the clothes washers and microwave ovens, while the respective upper bound of total rebound effects are 88.95%, 100.36%, 626.58%, and 31.61%. Furthermore, increasing the efficiency of air conditioners and cars can definitely reduce the total household energy consumption during the use phase.     

Mathematical modeling of the primary and secondary vacuum freeze drying of random solids at microwave heating

This paper presents a complex model of the primary and secondary vacuum freeze-drying stages at microwave heating. The simulation of the process was performed for typical adsorbents which were chosen as ideal representatives of random solids having particle and bed porosity. One-dimensional two-region model of the primary freeze-drying at microwave heating was formulated and then solved numerically using the finite-difference MacCormack method. Varying during the process sublimation front temperature T_s(t) was taken into account. Simulated drying curves were compared with experimental results giving fairly good agreement. A mathematical model of the secondary freeze-drying at microwave heating was developed and solved using the numerical method of lines. Pressure drop in the material was taken into account and calculated using Ergun equation. As a result of the model solution, the moisture content and the temperature distributions in drying material were obtained. In both mathematical models steady internal heat source capacity was calculated as a function of material temperature.     

Energy analysis in fluidized‐bed drying of large wet particles

Energy analysis of a fluidized‐bed drying system is undertaken to optimize the fluidized‐bed drying conditions for large wet particles (Group D) using energy models. Three critical factors; the inlet air temperature, the fluidization velocity, and the initial moisture contents of the material (e.g., wheat) are studied to determine their effects on the overall energy efficiency to optimize the fluidized bed drying process. In order to verify the model, different experimental data sets for wheat material taken from the literature are used. The results show that the energy efficiencies of the fluidized‐bed dryer decrease with increasing drying time and become the lowest at the end of the drying process. It is observed that the inlet air temperature has an important effect on energy efficiency for the material where the diffusion coefficient depends on both the temperature and the moisture content of the particle. Furthermore, the energy efficiencies showed higher values for particles with high initial moisture content while the effect of gas velocity varied depending on the material properties. A good agreement is achieved between the model predictions and the available experimental results. Copyright © 2002 John Wiley & Sons, Ltd.