Condensation in a vertical tube bundle passive condenser – Part 1: Through flow condensation

An experimental study and a boundary layer analysis were performed for the steam condensation in a vertical tube bundle passive condenser operating in a through flow mode. Four condenser tubes were submerged in a water pool and the heat from the condenser tube was removed through boiling. Experimental data were obtained for various system pressures (100–170 kPa), inlet steam flow rates (15–47 g/s) and non-condensable gas concentration (0–15%). The experimental results showed substantial deterioration in condensation when non-condensable gas was present. With increase in steam flow rate and system pressure the condensate rate increased. The boundary layer thickness and non-condensable gas concentration increased along the condenser tube length.     

“Preliminary results of hydrogen production from water vapor decomposition using DBD plasma in a PMCR reactor”

The water decomposition is considered one of the most attractive chemical processes for the production of hydrogen. The present work describes the preliminary results obtained in the experimental study of the water vapor dissociation into hydrogen and oxygen species using Dielectric-Barrier Discharge (DBD) plasma in a plate micro-channel reactor (PMCR). The water vapor molecules are injected without using carrier gas into the PMCR reactor at pressure of 100 kPa and temperature of 573 K. The applied high voltage of the plasma was within range of 14–18 kV and different steam flow rates have been analyzed within range of 100–200 ml/h. The product gases have been separated in ice trap which it was connected directly to the PMCR reactor to prevent the recombination of hydrogen and oxygen species. The concentration of the outlet species has been measured in a gas phase chromatography (GC) instrument. The PMCR reactor heating temperature effect on the water vapor decomposition has been analyzed. It was found that the water vapor is dissociated into their constituent molecular elements of hydrogen and oxygen gas using plasma. The maximum obtained mole fraction, hydrogen flow rate and conversion rate were 2.3%, 9.42 g/h, 42.51% respectively, at steam temperature of 573 K, pressure 100 kPa, PMCR heating temperature 403 K, steam flow rate of 200 ml/h and the plasma discharge high voltage of 18 kV. It was observed that the amount of evolved hydrogen concentration increased with the increase of the PMCR reactor heating temperature. Also, the thermal efficiencies versus the heat supplied have been calculated and the maximum obtained efficiency was 49.32%. Consequently, the evolved hydrogen flow rate appears to depend mainly on the plasma voltage, PMCR reactor heating temperature and the separating temperature of outlet hydrogen and oxygen species. The steam dissociation experiment will be extended to separate hydrogen and oxygen species elements at high temperature conditions.     

Steam gasification of biochars derived from pruned apple branch with various pyrolysis temperatures

In this study, the gas production behavior from the steam gasification of the biochar derived from the pruned apple brunch was investigated using a fixed-bed reactor. The optimal biochar obtained at the pyrolysis temperature of 550 °C was gasified under different operating conditions for the hydrogen rich gas production. The experimental results indicated that high reaction temperature and high water flow rate were both beneficial to the hydrogen gas yield, but excess steam had a negative impact contrarily. Besides, the small size particles (0.5–1.0 mm) showed better performance in the hydrogen gas production at the low water flow rates (0.05–0.20 g/min); while the large size particles (1.0–2.8 mm) showed better performance at the high water flow rates (0.25–0.30 g/min). The suitable operating conditions for the gasification of the biochar were determined as the reaction temperature of 850 °C, water flow rate of 0.25 g/min, and particle size of 1.0–2.8 mm.     

Performance of anodic recirculation by a variable flow ejector for a solid oxide fuel cell system under partial loads

An anode gas recycle (AGR) system driven by a variable flow rate ejector was developed for use in small-scale solid oxide fuel cell (SOFC) systems. The partial load conditions were simulated through recycling power generation experiments to clarify the fundamental characteristics of the variable flow ejector by using actual 1 kW-class SOFC equipment at the steady state. We achieved power generation in a range of recirculation ratios under partial load conditions of 62.5%–80% by controlling the recirculation characteristics with the developed ejector by using a needle. Results showed that the recirculation ratio can be controlled in the range of 0.595–0.694 by adjusting the driving energy with the ejector even at a partial load where the fuel gas flow rate of the ejector changes. Furthermore, the effect of the recirculation ratio on SOFC output was discussed based on the results of gas analyses and temperature measurements. As the recirculation ratio increased, the fuel concentration at the SOFC inlet decreased and the water vapor concentration increased. However, the effect of the recirculation ratio on the stack temperature and output power was proposed to be small. In addition, it was confirmed that the operation was performed under safe conditions where no carbon deposition occurred by circulating the steam generated inside the SOFC without an external water supply. Ejector characteristics during power generation experiments were lower than those at room temperature, which indicates that an ejector upstream pressure of approximately 20–170 kPa gauge pressure was required. Variations in the fluid properties of the driver gas in the ejector motive nozzle heated by the hot suction gas were found to degrade the performance of the ejector installed in the SOFC system, as compared with the results of simulation experiments at room temperature. Nevertheless, the recirculation ratio range required for operation could be satisfied by adjusting the flow velocity of the driving gas through needle control.     

不凝气对池内亚音速蒸汽直接接触凝结影响

利用VOF多相流模型和修正的热相变凝结模型对含不凝气蒸汽亚音速射入池内的直接接触凝结过程进行了数值模拟。主要研究了不同不凝气含量对蒸汽直接接触凝结过程中气羽形态、温度和压力分布的影响。研究结果表明：随着凝结的进行不凝气在气液界面处集聚成为一层不凝气层,随着不凝气含量的增加,不凝气层的厚度也增加,气羽不再呈现周期性的变化;不凝气的存在使得池内温度高温区域增大,温度分布相对均一;同时随着不凝气含量的升高,压力振荡的强度减弱,凝结形成的负压值升高。     

Reduction of tars by dolomite cracking during two-stage gasification of olive cake

The effective implementation of biomass gasification has to overcome some difficulties such as the minimization of tars. On the other hand, with a proper design of experimental conditions, biomass gasification can be directed towards the production of hydrogen. The aim of the present study was to investigate the use of dolomite as catalyst to improve tar removal and hydrogen production by a two-stage steam gasification process, using olive cake as raw material. Fixing the olive cake gasification conditions on the first reactor (900 °C, steam flow rate of 190 mg min⁻¹, O₂ flow rate of 7.5 cm³ min⁻¹), the cracking of tars was prompted by: a) steam gasification (steam flow rate in the range 40-190 mg min⁻¹) at 1000 °C, b) catalytic gasification, using dolomite (5% wt.). It was found that increasing steam flow rate up to 110 mg min⁻¹ involves an increase in hydrogen fraction due to the enhancement of water gas and water gas shift reactions. Also, the influence of dolomite was studied at 800 and 900 °C in a second reactor, finding better results at 800 °C, which gave an hydrogen fraction of 0.51.     

Phase change characteristics and their effect on the performance of hydrogen recirculation ejectors for PEMFC systems

The working fluid of the hydrogen recirculation ejector in proton exchange membrane fuel cell (PEMFC) systems is humid hydrogen containing water vapour. However, previous studies on the hydrogen recirculation ejector using computational fluid dynamics (CFD) were based on the single-phase flow model without considering the phase change of water vapour. In this study, the characteristics of the phase change and its effect on the ejector performance are analysed according to a two-phase CFD model. The model is established based on a non-equilibrium condensation phase change. The results show that the average deviation of the entrainment ratio predicted by a single-phase flow model is 25.8% compared with experiments involving a hydrogen recirculation ejector, which is higher than the 15.1% predicted by the two-phase flow model. It can be determined that droplet nucleation occurs at the junction of the primary and secondary flow, with the maximum nucleation rate reaching 4.0 × 10²⁰ m^?3s^?1 at a primary flow pressure of 5.0 bar. The higher temperature, lower velocity, and higher pressure of the gas phase can be found in the mixing region due to condensation, resulting in a lower entrainment performance. The nucleation rate, droplet number, and liquid mass fraction increase remarkably with an increasing primary flow pressure. This study provides a meaningful reference for understanding phase change characteristics and two-phase flow behaviour in hydrogen recirculation ejectors for PEMFC systems.     

Combustion characteristics of anode off-gas on the steam reforming performance

The combination of steam reforming and HT-PEMFC has been considered as a proper set up for the efficient hydrogen production. Recycling anode off-gas is energy-saving strategy, which leads to enhance the overall efficiency of the HT-PEMFC. Thus, the recycling effect of anode off-gas on steam-reforming performance needs to be further studied. This paper, therefore, investigated that the combustion of anode off-gas recycled impacts on the steam reformer, which consists of premixed-flame burner, steam reforming and water-gas shift reactors. The temperature rising of internal catalyst was affected by lower heating value of fuels when the distance between catalyst and burner is relatively short, while by the flow rate of fuels and the steam to carbon ratio when its distance is long. The concentration of carbon monoxide was the lowest at 180 °C of LTS temperature, while NG and AOG modes showed the highest thermal efficiency at LTS temperature of 220–300 °C and 270–350 °C, respectively. The optimum condition of thermal efficiency to maximize hydrogen production was determined by steam reforming rather than water gas shift reaction. It was confirmed that the condition to obtain the highest thermal efficiency is about 650 °C of steam reforming temperature, regardless of combustion fuel and carbon monoxide reduction. The difference of hydrogen yield between upper and lower values is up to 1.5 kW as electric energy with a variation of thermal efficiency. Hydrogen yield showed the linear proportion to the thermal efficiency of steam reformer, which needs to be further increased through proper thermal management.     

Steam-gasification of biomass with CaO as catalyst for hydrogen-rich syngas production

Biomass is extensively considered as a feed-stock for bio-chemicals and bio-fuels production. Among all options for the utilization of biomass, gasification process is more popular because of its environmental advantages. In this study, biomass gasification with CO₂ removal by CaO sorbent was simulated by using a commercial simulator. The model accuracy was validated with reported results from steam only gasification of biomass in presence of CaO. The system was evaluated through tar yield, carbon conversion, gas quality and H₂ yield by varying the reaction temperature, steam flow rate and CaO flow rate. The hydrogen yield enhanced slightly from 187.32 ml/g to 198.49 ml/g with the increase of CaO/B from 1.0 to 1.5. However, a further enhancement in CaO/B from 1.5 to 2 sharply enhanced the hydrogen yield approximately 1.55 times (from 198.49 ml/g to 308.54 ml/g).     

Oxidative steam reforming of ethanol over a Pt/Al2O3 catalyst in a Pd-based membrane reactor

In this study, the ability of a Pd-Ag membrane reactor of producing ultrapure hydrogen via oxidative steam reforming of ethanol has been evaluated. A self supported Pd-Ag tube of wall thickness 60 μm has been filled with a commercial Pt-based catalyst and assembled into a membrane module in a finger-like configuration. In order to evaluate the hydrogen yield behavior under different operating conditions, experimental tests have been performed at temperatures of 400 and 450 °C and pressures of 150 and 200 kPa. The oxidative steam reforming of ethanol has been carried out by feeding the membrane reactor with a gas stream containing a dilute water-ethanol mixture and air. Different water/ethanol feed flow rates (5, 10, 15 g h⁻¹), several water/ethanol (4, 10, 13) and oxygen/ethanol (0.3, 0.5, 0.7) feed molar ratios have been tested. The results pointed out that the highest hydrogen yield (moles of permeated hydrogen per mole of ethanol fed) corresponding to almost 4.1 has been attained at 450 °C and 200 kPa of lumen pressure by using a water/ethanol/oxygen feed molar ratio of 10/1/0.5.The results of these tests have been compared with those reported for the ethanol steam reforming in a Pd-Ag membrane reactor filled with the same Pt-based catalyst. This comparison has shown a positive effect on the hydrogen yield of small oxygen addition in the feed stream.     

A comparative study on hydrogen production from steam-glycerol reforming: thermodynamics and experimental

A detailed comparative study on thermodynamic and experimental analyses of glycerol reforming for hydrogen production has been conducted in terms of the effects of temperature, pressure, water to glycerol feed ratio, feeding reactants to inert gas ratio and feeding gas flow rate (residence time). The thermodynamic analysis was conducted by using a non-stoichiometric methodology based on the minimisation of Gibbs free energy. And the experiments were carried out with a pilot scale set-up. The results show that the thermodynamic and experimental data agree fairly well with each other. The measured hydrogen production is slightly lower than that predicted by the thermodynamic analysis, which is mainly because the conversion of steam is incomplete. High temperature, low pressure, low feeding reactants to inert gas ratio and low gas flow rate are favourable for steam reforming of glycerol for hydrogen production. There is an optimal water to glycerol feed ratio for steam reforming of glycerol for hydrogen production which is about 9.0. The glycerol conversion is a strong function of water to glycerol ratio, whereas a weak function of other parameters over the conditions of this work. A novel adsorption enhanced reaction process incorporating water and heat recovery is proposed for further optimisation of hydrogen production from steam reforming of glycerol.     

Experimental analysis of an ejector for anode recirculation in a 10 kW polymer electrolyte membrane fuel cell system

For analyzing ejector's performance in the system, an ejector for a 10 kW polymer electrolyte membrane fuel cell (PEMFC) system was first designed, manufactured, and a 10 kW PEMFC system bench was built up. A proportional valve and PI pressure feedback control method were adopted to control the hydrogen supply and anode inlet pressure. During the test, performances between dead-ended anode (DEA) mode and ejector mode were compared. Ejector's performances in the system, i.e., volume flow recirculated ratio, difference pressure, dynamic responses of primary pressure, anode inlet pressure, and recirculated gas flow rate during the purge process and current variation condition, were investigated. The results show that pressure adjustment is accurate, continuous, and fast using the proportional valve and PI pressure feedback control method. The hydrogen consumption rate in the ejector mode can reduce from 5% to 10% compared with the rate in the DEA mode except for the stack current 5 A and 10 A conditions. For better water removal out of the anode channel in ejector mode, the maximum stack power increases from 5.11 kW (DEA mode) to 9.56 kW (ejector mode). Anode pressure surge caused by the purge valve switching enhances the ejector's recirculated performance significantly.     

Investigation of hydrogen production using wood pellets gasification with steam at high temperature over 800 °C to 1435 °C

A fixed-bed gasifier was developed to study the effects of steam flow rate and temperature on the hydrogen production during biomass gasification at high temperature over 800 °C to 1435 °C. An optimum steam flow rate for peak of hydrogen yield was found. As temperature increases, amount of hydrogen increases first, subsequently decreases and then increases again with a maximum peak of hydrogen yield at 917 °C. In the temperatures of 1018 °C through 1435 °C post the peak hydrogen production increases with temperature. The maximum volume fraction of hydrogen and hydrogen production ratio are 60% and 76%, respectively. Chemical equilibrium calculation was also done using ASPEN software, which demonstrates that the more the steam flow rate, the lower the temperature for maximum hydrogen yield; the higher the temperature, the lower the effect of steam flow rate. The results are expected to develop high temperature gasification technology.     

Improvement of hydrogen production with Rhodopseudomonas palustris CQK-01 by Ar gas sparging

For improving photo-biohydrogen production, a novel gas bubble column photobioreactor with Ar gas sparging was developed for biohydrogen production by purple non-sulfur phototrophic bacteria, Rhodoseudomonas palustris CQK-01. The dissolved hydrogen concentration was in-situ measured by a hydrogen microsensor. Experimental results demonstrated that Ar gas sparging dramatically decreased the dissolved hydrogen concentration, resulting in an improvement in the photo-biohydrogen production performance. Furthermore, effects of the gas flow rate and the time interval of gas sparging were investigated. The results showed that with an increase in the gas flow rate, the hydrogen production performance increased initially due to the reduced dissolved hydrogen concentration and enhanced mass transport, and then it decreased as a result of an increased shear stress. Meanwhile, the short sparging time interval resulted in a low accumulation of dissolved hydrogen in the bioreactor, hence high hydrogen production performance. The optimal hydrogen production rate (5.86 mmol/l/h) and hydrogen yield (3.38 mol H₂/mol glucose) were obtained at the gas flow rate of 10 ml/min, respectively.     

Design and optimization of reforming hydrogen production reaction system for automobile fuel cell

Mathematical modeling and simulation analysis of the dimethyl ether steam reforming reaction system were carried out in the study. The numerical results of simulation and experiment were consistent. The effects of reaction conditions on the conversion of dimethyl ether and hydrogen production were analyzed. The internal structure of the reforming reactor was adjusted to obtain higher hydrogen production efficiency. The study established the reforming hydrogen production industry system, and analyzed the thermal efficiency of the system. The results show that when the temperature of the conversion bed is 673 K, the inlet flow rate of the mixture gas is 0.5 ms^?1 and the ratio of water to ether is 3, the dimethyl ether steam reforming reaction system could obtain the dimethyl ether conversion rate of 90%, the hydrogen production rate of 88% and system thermal efficiency of 74%.     

Combined Hydrogen,Heat and Power (CHHP) pilot plant design

The Combined Hydrogen, Heat and Power (CHHP) system consists of a molten carbonate fuel cell, DFC300. DFC300 consumes biogas, and produces electricity and hydrogen. The high temperature flue gas can be recovered for useful purposes. During the hydrogen recovery process, the anode exhaust gas (37.1% H₂O, 45.9% CO₂, 5.7% CO, and 11.2% H₂) is sent through a water gas shift (WGS) reactor to increase the hydrogen and carbon dioxide composition, and then water is removed in a vapor–liquid separator. The remaining hydrogen and carbon dioxide mixture gas is separated using a 2-adsorber pressure swing adsorption unit under 1379 kPa. Resulting hydrogen can achieve 99.99% purity, and it can be stored in composite hydrogen storage tanks pressurized at 34,474 kPa. Hydrogen is produced at a rate of 2.58 kg/h. The produced hydrogen is filled into transportable hydrogen cylinders and trucked to a residential community 7.5 km away from the CHHP site. The community is powered by fuel cells to supply electricity to approximately 51 apartments. A heat recovery unit to produce steam and hot water recovers hot air exhaust from the DFC300, having a total heating value of 405 MJ/h. The greenhouse employs a two-phase steam heating system. Hot water supply is mainly needed for the CHHP education center. DFC300 produces electricity at a maximum capacity of 280 kW. A substation is built to set up the interconnections. Power poles and power lines are built to distribute electricity to the CHHP system, the education center, and the greenhouse. The overall electricity consumption of the CHHP system is 86 kW, and the greenhouse consumes 40 kW. Therefore, an aggregate of 154 kW of power can be used to provide power to the UC Davis campus.     

Potential to retrofit existing small-scale gasifiers through steam gasification of biomass residues for hydrogen and biofuels production

This work investigates the opportunity of retrofitting existing small-scale gasifiers shifting from combined heat and power (CHP) to hydrogen and biofuels production, using steam and biomass residues (woodchips, vineyard pruning and bark). The experiments were carried out in a batch reactor at 700 °C and 800 °C and at different steam flow (SF) rates (0.04 g/min and 0.20 g/min). The composition of the producer gas is in the range of 46–70 % H₂, 9–29 % CO, 12–27 % CO₂, and 2–6 % CH₄. A producer gas specific production factor of approx. 10 NL_pg/g_char can be achieved when the lower SFs are used, which allows to provide 80 % of the hydrogen concentration required for biomethanation and MeOH synthesis. As for FT synthesis, an optimal H₂/CO ratio of approx. 2 can be achieved. The results of this work provide further evidence towards the feasibility of hydrogen and biofuels generation from residual biomass through steam gasification.     

Gasification of biomass to hydrogen-rich gas in fluidized beds using porous medium as bed material

Experiments were carried out to study the characteristics of biomass gasification in a fluidized bed using industrial sand and porous medium as bed materials. Analysis was conducted to investigate the effects of different operation parameters, including bed material, gasification temperature (600 °C–900 °C), oxygen enrichment in the gasifying agent (21 vol.% to 50 vol.%), and steam flow rate (1.08 kg/h to 2.10 kg/h), on product yields and gas composition. The results of gas chromatography show that the main generated gas species were H₂, CO, CO₂, CH₄, and C₂H₄. Compared with industrial sand as bed material, porous medium as bed material was more suitable for gasifying biomass to hydrogen-rich gas. The physical characteristics of porous structure are more favorable to heat transfer, producing the secondary crack of heavy hydrocarbons and generating more hydrogen and other permanent gases. The product yields of hydrogen-rich gas increased with increasing gasification temperature. The hydrogen concentration improved from 22.52 vol.% to 36.06 vol.%, but the CO concentration decreased from 37.53 vol.% to 28.37 vol.% with increasing temperature from 600 °C to 900 °C under the operation parameters of porous bed material at a steam flow rate of 1.56 kg/h. With increasing oxygen concentration, H₂ concentration increased from 12.36% to 20.21%. Over the ranges of the examined experimental conditions, the actual steam flux value (e.g., 1.56 kg/h) was found to be the optimum value for gasification.     

Production of hydrogen via an Iron/Iron oxide looping cycle: Thermodynamic modeling and experimental validation

An incremental thermodynamic equilibrium model has been developed for the chemical reactions driving a clean, hydrogen producing iron/iron oxide looping cycle. The model approximates a well-mixed reactor with continuous reactant gas flow through a stationary solid matrix, where the gas residence time is long compared to time constants associated with chemical kinetics and species transport. The model, which computes the theoretical limit for steam-to-hydrogen conversion, has been experimentally validated for the oxidation reaction using an externally heated, 21 mm inner diameter, tubular fluidized bed reactor. Experiments were carried out at 660 and 960 °C with steam flow rates ranging from 0.9 to 3.5 g/min. For small flow rates, i.e., for long residence times, the experimentally observed cumulative steam-to-hydrogen conversion approaches the theoretically predicted conversion. At a 960 °C operating temperature, the measured hydrogen yield approaches the theoretical limit (experimental yields are always within 50% of the theoretical limit), and the yield is insensitive to variations in the steam flow rate. In contrast, the measured hydrogen yield deviates significantly from the theoretical limit at a 660 °C operating temperature, and strong variations in hydrogen yield are observed with variations in steam flow rate. This observation suggests that the reaction kinetics are significantly slower at lower temperature, and the model assumption is not satisfied.     

Enhancing hydrogen production from steam electrolysis in molten hydroxides via selection of non-precious metal electrodes

There are still gaps in the field of reference electrode that is needed to assist electrolysis in high temperature electrolytes (e.g. molten hydroxides) for H₂ gas production. This research aims to fill the gaps by preparing Ni/Ni(OH)₂ reference electrode and more importantly testing its effectiveness against important performance factors including; ion conducting membrane (e.g. mullite tubes), internal electrolyte composition, working temperature and electrochemical control (e.g. potential scan rate). Then, this reference electrode was used to study the electrocatalytic activity various cheaper working electrode materials including; stainless steel (St.st), Ni, Mo and Ag in comparison with Pt by the means of chronoamperometry and voltammetry. The effect of introducing steam into electrolyte (eutectic mixture of NaOH and KOH) on the electrocatalytic activity of these working electrodes was also studied. It was observed that the potential of hydrogen evolution with different working electrodes followed an order as; Pt > Ni > St. st > Ag > Mo (positive to negative). The performance of each working electrode was confirmed through chronoamperometry for hydrogen evolution at a constant potential of −0.7 V. It was also found in cyclic voltammetry and confirmed by chronoamperometry that the introduction of steam was apparent as increasing the current density at cathodic limit for hydrogen evolution. This study could help to develop non-precious metal electrodes for the production of hydrogen fuel. In future, there will be a potential in the threshold concentration of steam for H₂ gas production.