A modified micro reactor fueled with hydrogen for reducing entropy generation

In order to reduce the entropy generation of premixed hydrogen/air flame in the micro combustion chamber, the combustion chamber of the old micro reactor is modified by gradually varying the diameter of the combustion chamber. Extensive numerical investigations about the entropy generation of premixed hydrogen/air flame in old and modified micro reactors are conducted under various hydrogen mass flow rates, hydrogen/air equivalence ratios, solid materials and inlet/outlet diameter ratios. Results suggest that the modified micro reactor has lower total entropy generation than that of the old micro reactor. This is attributed to that the temperature gradient of the burned gas in the modified micro reactor is lower than that in the old micro reactor after chemical reactions. Finally, the largest descent percentages of total entropy generation are achieved under various conditions, which provide significant reference values for hydrogen energy usage under micro scale combustion.     

微热光电系统中的微燃烧研究

描述了一种新颖的MEMS动力源概念，即微热光电(TPV)系统。该系统将使用氢气作为燃料，每立方厘米体积能够发出1～10W的电力。微燃烧室是该系统中最重要的元件之一，为了获得较高的电能输出，燃烧室壁面的温度分布要求高而且均匀。由于微燃烧室面容比大，热损失显增加；着火困难并使火焰窒熄。为了测试微燃烧室内燃烧的可行性和确定影响燃烧的有关因素，进行了实验和数值模拟。结果表明燃烧室壁面能够得到要求的高温，且温度分布均匀。     

Microscale combustion research for microthermophotovoltaic systems

A novel power MEMS concept, a micro thermophotovoltaic (TPV) system, is described in this work, which would use hydrogen as fuel and would be capable of delivering 1–10 W electrical power in a package less than 1 cubic centimeter in volume. A micro combustor is one of the most important components of a micro TPV system. A high and uniform temperature distribution along the wall of a micro combustor is required to get a high electrical power output. However, sustaining combustion in a MEMS‐sized combustor will be largely affected by the increased heat losses due to the high surface‐to‐volume ratio, which tends to suppress ignition and quench the reaction. In order to test the feasibility of combustion in micro devices and determine relevant factors affecting micro combustion, numerical and experimental work was carried out. Results indicated that a high and uniform temperature could be achieved along the wall of a flame tube. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 369–379, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20078     

Micro methanol reformer combined with a catalytic combustor for a PEM fuel cell

This paper presents the development of a micro methanol reformer for portable fuel cell applications. The micro reformer consists of a methanol steam reforming reactor, catalytic combustor, and heat exchanger in-between. Cu/ZnO was selected as a catalyst for a methanol steam reforming and Pt for a catalytic combustion of hydrogen with air. Porous ceramic material was used as a catalyst support due to the large surface area and thermal stability. Photosensitive glass wafer was selected as a structural material because of its thermal and chemical stabilities. Performance of the reformer was measured at various test conditions and the results showed a good agreement with the three-dimensional analysis of the reacting flow. Considering the energy balance of the reformer/combustor model, the off-gas of fuel cell can be recycled as a feed of the combustor. The catalytic combustor generated the sufficient amount of heat to sustain the steam reforming of methanol. The conversion of methanol was 95.7% and the hydrogen flow of 53.7 ml/min was produced including 1.24% carbon monoxide. The generated hydrogen was the sufficient amount to operate 4.5 W polymer electrolyte membrane fuel cells.     

Performance of low bandgap thermophotovoltaic cells in a small cogeneration system

In recent years there has been significant progress in fabrication of low bandgap thermophotovoltaic (TPV) devices, such as InGaAsSb, InGaAs and GaSb cells. However, only limited data are available in the literature with respect to the performance of these TPV cells in combustion-driven radiant sources. In this study, power generation using InGaAsSb TPV cells has been investigated in a gas-fired home heating furnace. The radiant power density and radiant efficiency of a gas-heated radiator were determined at different degrees of exhaust heat recuperation. Heat recuperation is shown to have a certain effect on combustion operation and radiant power output. The electric output characteristics of the InGaAsSb TPV devices were investigated under various operating conditions. An electric power density of 5.4×10³ W m⁻² was produced at a radiator temperature of 1463 K for the small cogeneration system. The cell short circuit density was observed to be greater than 1×10⁴ A m⁻² at a radiator temperature of 1203 K. Furthermore, the design aspects of combustion-driven TPV systems have been discussed. It is shown that development of a special combustion device with high conversion level of fuel chemical energy to useful radiant energy is required, to improve further the system efficiency.     

Investigation of coal gasification hydrogen and electricity co-production plant with three-reactors chemical looping process

This paper analyzes a novel process for producing hydrogen and electricity from coal, based on chemical looping combustion (CLC) and gas turbine combined cycle, allowing for intrinsic capture of carbon dioxide. The core of the process consists of a three-reactors CLC system, where iron oxide particles are circulated to: (i) oxidize syngas in the fuel reactor (FR) providing a CO₂ stream ready for sequestration after cooling and steam vapor condensation, (ii) reduce steam in the steam reactor (SR) to produce hydrogen, (iii) consume oxygen in the air reactor (AR) from air releasing heat to sustain the thermal balance of the CLC system and to generate electricity. A compacted fluidized bed, composed of two fuel reactors, is proposed here for full conversion of fuel gases in FR. The gasification CLC combined cycle plant for hydrogen and electricity cogeneration with Fe₂O₃/FeAl₂O₄ oxygen carriers was simulated using ASPEN^® PLUS software. The plant consists of a supplementary firing reactor operating up to 1350 °C and three-reactors SR at 815 °C, FR at 900 °C and AR at 1000 °C. The results show that the electricity and hydrogen efficiencies are 14.46% and 36.93%, respectively, including hydrogen compression to 60 bar, CO₂ compression to 121 bar, The CO₂ capture efficiency is 89.62% with a CO₂ emission of 238.9 g/kWh. The system has an electricity efficiency of 10.13% and a hydrogen efficiency of 41.51% without CO₂ emission when supplementary firing is not used. The plant performance is attractive because of high energy conversion efficiency and low CO₂ emission. Key parameters that affect the system performance are also discussed, including the conversion of steam to hydrogen in SR, supplementary firing temperature of the oxygen depleted air from AR, AR operation temperature, the flow of oxygen carriers, and the addition of inert support material to the oxygen carrier.     

Enhancing efficiency of hydrocarbons to synthesis gas conversion in a counterflow moving bed filtration combustion reactor

An improvement is considered for the partial oxidation conversion of hydrocarbon gases to synthesis gas in a continuous non-premixed filtration combustion reactor with inert solid granular material flowing countercurrently to the gas flow. The reactor is supplemented with an additional heat exchanger, wherein the second reactant gas is preheated prior to supply to the middle part of the reactor. The composition of the gaseous products self-consistent with the temperature of combustion are assessed using approximation of established thermodynamic equilibrium in the products. The parametric domain for major control parameters, namely oxygen-to-fuel supply ratio, granular solid flowrate, and steam supply rate providing highly efficient conversion is determined. Calculations for the POX conversion of methane and a model biogas composition (50% methane, 40% carbon dioxide, 10% nitrogen) with air and steam are provided as examples. The calculations show that the process gives a possibility to substantially improve energy efficiency and provides a flexibility to control hydrogen yield through steam supply. The process provides a high chemical efficiency of conversion even with air used as an oxidant for conversion of low-caloric gases.     

Mini CHP based on the electrochemical generator and impeded fluidized bed reactor for methane steam reforming

The paper presents a configuration of mini CHP with the methane reformer and planar solid oxide fuel cell (SOFC) stacks. This mini CHP may produce electricity and superheated steam as well as preheat air and methane for the reformer along with cathode air used in the SOFC stack as an oxidant. Moreover, the mathematical model for this power plant has been created. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) considered as the basis for the synthesis gas (syngas) production to fuel SOFC stacks has been studied experimentally as well. A fraction of conversion products has been oxidized by the air fed to the upper region of the impeded fluidized bed in order to carry out the endothermic methane steam reforming in a 1:3 ratio as well as to preheat products of these reactions. Studies have shown that syngas containing 55% of hydrogen could be produced by this reactor. Basic dimensions of the reactor as well as flow rates of air, water and methane for the conversion of methane have been adjusted through mathematical modelling.The paper provides heat balances for the reformer, SOFC stack and waste heat boiler (WHB) intended for generating superheated water steam along with preheating air and methane for the reformer as well as the preheated cathode air. The balances have formed the basis for calculating the following values: the useful product fraction in the reformer; fraction of hydrogen oxidized at SOFC anode; gross electric efficiency; anode temperature; exothermic effect of syngas hydrogen oxidation by air oxygen; excess entropy along with the Gibbs free energy change at standard conditions; electromotive force (EMF) of the fuel cell; specific flow rate of the equivalent fuel for producing electric and heat energy. Calculations have shown that the temperature of hydrogen oxidation products at SOFC anode is 850 °C; gross electric efficiency is 61.0%; EMF of one fuel cell is 0.985 V; fraction of hydrogen oxidized at SOFC anode is 64.6%; specific flow rate of the equivalent fuel for producing electric energy is 0.16 kg of eq.f./(kW·h) while that for heat generation amounts to 44.7 kg of eq.f./(GJ). All specific parameters are in agreement with the results of other studies.     

Simulation of gas exothermic chemical reaction in porous media reactor with lattice Boltzmann method

Exothermic reactor is the main part in a chemical heat pump. It involves complex multi-component exothermal chemical reaction in catalyst-filled porous media. The lattice Boltzmann method (LBM) is developed to simulate the characteristics of fluid flow, heat and mass transfer coupling chemical reaction in the exothermic reactor of the isopropanol/acetone/hydrogen chemical heat pump system. Fractal theory is used to structure a porous medium model in the reactor. The simulation results show that LBM is suitable for the simulation and the conversion has an optimal value with different inlet velocities.     

Characteristics of hydrogen-assisted catalytic ignition of n-butane/air mixtures

External heating and hydrogen-assisted catalytic ignition characteristics of n-butane (n-C₄H₁₀) were studied experimentally in a Pt-coated monolith catalytic reactor. Special attention was paid to the chemical effect of hydrogen on hydrogen-assisted ignition. A comparison of the ignition temperature for these two ignition methods shows hydrogen can lower ignition temperature. Furthermore, the ignition experiment at low hydrogen mole fraction (1.5%) shows that hydrogen has a positive chemical effect on hydrogen-assisted ignition. At constant n-butane/air flow and within certain limits of hydrogen mole fraction of mixtures, the ignition temperature changes little, whereas the time required for ignition and the cumulative amount of hydrogen decrease substantially. Consequently, high hydrogen mole fraction is favorable to hydrogen-assisted ignition. Two startup methods and thermal insulation are discussed. The co-feed method (n-butane/air/hydrogen mixtures are fed into reactor) and thermal insulation were found to be beneficial to hydrogen-assisted ignition.     

Thermal analysis of a magnesium oxide/water chemical heat pump for cogeneration

A chemical heat pump is examined experimentally as a chemical heat storage system in order to evaluate the contribution of the chemical heat pump to decentralised cogeneration. A new system that combines cogeneration with a chemical heat pump that uses a magnesium oxide/water reaction is proposed, and the feasibility of the combined system is discussed. A packed bed reactor of a magnesium oxide/water chemical heat pump was examined experimentally under various operation conditions. Thermal performance of the heat pump was analysed using the experimental results. The heat pump containing the reactor is expected to enhance the energy utilisation efficiency of the cogeneration system by storing and utilising surplus exhaust heat generated by the cogeneration system.     

Continuous hydrogen production by biomass gasification in supercritical water heated by molten salt flow: System development and reactor assessment

Hydrogen production by biomass gasification using solar energy is a promising approach for overcoming the drawbacks of fossil fuel utilization, but the storage of discontinuous solar flux is a critical issue for continuous solar hydrogen production. A continuous hydrogen production system by biomass gasification in supercritical water using molten-salts-stored solar energy was proposed and constructed. A novel double tube helical heat exchanger was designed to be molten salts reactor for hydrogen production. Model compounds (glycerol/glucose) and real biomass (corn cob) were successfully gasified in this molten salts reactor for producing hydrogen-rich gas. The unique temperature profiles of biomass slurry in the reactor were observed and compared with that of conventional electrical heating and direct solar heating approaches. Product gases yield, gasification efficiency and exergy conversion efficiency of the reactor were analyzed. The results showed that the performances of reactor were determined by feedstock style, biomass concentration, residence time and biomass slurry temperature profiles.     

Stabilized three-stage oxidation of DME/air mixture in a micro flow reactor with a controlled temperature profile

Ignition and combustion characteristics of a stoichiometric dimethyl ether (DME)/air mixture in a micro flow reactor with a controlled temperature profile which was smoothly ramped from room temperature to ignition temperature were investigated. Special attention was paid to the multi-stage oxidation in low temperature condition.Normal stable flames in a mixture flow in the high velocity region, and non-stationary pulsating flames and/or repetitive extinction and ignition (FREI) in the medium velocity region were experimentally confirmed as expected from our previous study on a methane/air mixture. In addition, stable double weak flames were observed in the low velocity region for the present DME/air mixture case. It is the first observation of stable double flames by the present methodology. Gas sampling was conducted to obtain major species distributions in the flow reactor. The results indicated that existence of low-temperature oxidation was conjectured by the production of CH₂O occured in the upstream side of the experimental first luminous flame, while no chemiluminescence from it was seen.One-dimensional computation with detailed chemistry and transport was conducted. At low mixture velocities, three-stage oxidation was confirmed from profiles of the heat release rate and major chemical species, which was broadly in agreement with the experimental results.Since the present micro flow reactor with a controlled temperature profile successfully presented the multi-stage oxidations as spatially separated flames, it is shown that this flow reactor can be utilized as a methodology to separate sets of reactions, even for other practical fuels, at different temperature.     

A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor: Design and modelling

Currently, combustion-based micro power devices encounter the problem of low conversion efficiency. A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor is proposed, because thermophotovoltaic (TPV) cells and thermoelectric (TE) modules work at different temperature levels. The system consists of a planar micro-combustor with a bended extension at the exit, two GaSb TPV modules to convert high temperature thermal radiation and two Bi–Te based TE modules attached to the bended extension to harness medium temperature thermal energy. The mathematical modelling approach to quantify the power output and conversion efficiency is systematically presented. The modelling results show that the integration of the TE modules could significantly improve the system efficiency. When burning the H₂/air mixture, the overall system efficiency could reach 2.5% under the flow condition of U₀ = 3 m/s and Φ = 1.0. Finally, measures for better thermal management to further enhance the conversion efficiency are discussed.     

An annulus-type micro reforming system integrated with a two-staged micro-combustor

A new configuration of a micro reforming system integrated with a micro-combustor is studied experimentally and computationally. The micro-combustor as a heat source is a simple cylinder, which is easy to fabricate, but is two-staged (expanding downstream) to control ignition and stable burning. A micro-evaporator to vaporize methanol–water mixtures and a micro-reformer to convert the vaporized methanol–water mixtures to hydrogen are annuli, which are effective to transfer heat from the first and second-stage micro-combustors, respectively. The annulus-type micro reforming system is designed to produce 1–10 W (based on lower heating value, LHV) of hydrogen using the steam reforming method. The molar ratio of water to methanol, the feed rate of water–methanol mixtures, the micro-combustor inlet velocity of fuel–air mixtures and the micro-combustor materials substantially affect the performance of the designed micro reforming system. Under optimized design and operating conditions, the micro reforming system produces 6.9 W (based on LHV) of hydrogen with a conversion rate of 97.5%, an overall system efficiency of 39.7% and a carbon monoxide concentration of 6.7 ppm. Thus, the present configuration can be applied to practical micro reforming systems for use with fuel cells.     

Model-based investigation of a CO preferential oxidation reactor for polymer electrolyte fuel cell systems

This paper deals with a two-dimensional model of a preferential oxidation (PROX) reactor to be used in a beta 5 kWe hydrogen generator, named HYGen II, to integrate with polymer electrolyte fuel cells (PEFCs) for residential applications. The reactor geometrical configuration developed is a single-stage multi-tube configuration, in which a cocurrent air flow in the interspace is aimed at improving heat transfer and consequently controlling the temperature of the reactor. The aim of the model is to investigate the process performance of the reactor in order to enhance optimization and control of the PROX unit. The model concerns chemical kinetics and heat and mass transfer phenomena in the reactor. The model plays a key role in overcoming the issues of system design, by evaluating the temperature and the gas concentration profiles in the reactor. The CO removal from simulated reformate was examined with varying inlet temperature. Simulation results showed the strong dependence of the overall performance upon the reactor geometrical configuration.     

A compact integrated fuel-processing system for proton exchange membrane fuel cells

A compact integrated fuel-processing system consisting of a plate-fin reformer (PFR) and a multi-stage preferential oxidation reactor is designed in this paper. The PFR, which was based on a plate-fin heat exchanger, is very compact, and reactant vaporization, methanol steam reforming and combustion are all integrated in it. Both internal plate-fins and external catalytic combustion were used to enhance heat transfer of the reformer, which offers both high methanol conversion ratio and low CO concentration, so that the water–gas shift reactor, which provides primary CO cleanup, is not necessary in this fuel-processing system. This will result in simplification of the fuel-processing system design and capital cost reduction. The performance of the main components in the fuel-processing system has been investigated. The axial temperatures of the different chambers in PFR were uniform, and the temperatures at the inlet and outlet of the PROX reactors were controlled strictly by plate-fin exchangers so that it could minimize parasitic hydrogen oxidation. In addition, the results indicated that this fuel-processing system can provide a high concentration of hydrogen and the system efficiency is always maintained above 75%. It is further demonstrated that the fuel-processing system could be operated autothermally and exhibited good test stability.     

Hydrogen storage using liquid organic carriers: Equilibrium simulation and dehydrogenation reactor design

Liquid organic hydrogen carriers (LOHC) are unsaturated organic compounds used for chemical hydrogen storage. Using an equilibrium model of the LOHC N-ethylcarbazole, we discuss potential efficiency increases of hydrogen storage systems based on N-ethylcarbazole by the integration of low-temperature waste heat. N-ethylcarbazole is well suited for pressure swing operation with heat exchange between hydrogenation and dehydrogenation. We present and discuss kinetic data of the dehydrogenation reaction gathered in a tubular reactor that was mounted in different orientations and flow configurations. Similar maximum values of power density are reached in vertical and in horizontal orientation. Vertical orientation allows the favorable operation with counter-flow of the liquid carrier and the evolved hydrogen gas and radial heat transfer is significantly better than in horizontal orientation. In vertical reactor configurations, catalyst efficiency and operational stability are impaired at high void fractions. This issue can be reduced by dehydrogenation at elevated pressure and intermediate gas separation from the catalyst bed.     

微小空腔内气体的预混燃烧

采用Longwell良搅拌反应器模型和详细化学反应机理对微尺度空腔内气体预混燃烧过程进行了零维数值模拟,从微腔体内稳定燃烧的临界半径、点火极限以及稳定流率范围着手,分析了不同预混气成分、不同当量比和不同环境对流换热系数等外部条件对微尺度燃烧点火与熄火特性的影响.稳定燃烧时,大腔体可对应较大的上极限流率,低预混气流率可对应较小的下临界半径;腔体越小(或流率越大),系统启动所需的温度和压力越高.     

Performance simulation and experimental confirmation of a mini-channel metal hydrides reactor

A theoretical and experimental study about a proposed mini-channel reactor was carried out to enhance heat transfer performance for metal hydrides applications, such as hydrogen storage, hydrogen compression and chemical heat pumps. The configuration of the reactor and working principles are described in detail. The predicted hydride bed temperature profiles in the reactor are compared with the experimental data from the performance test system, and a reasonable agreement is observed. The simulation of the hydrogen adsorption and desorption processes in a mini-channel reactor packed with LaNi₅ is conducted, and the influences of some important parameters, e.g. the bed thickness, the number of the mini-channels, hydrogen supply and discharge pressure are analyzed. Comparing with the traditional reactors, such as tubular reactor and disc reactor, the mini-channel reactor has some obvious advantages, therefore can be recommended for applications.