Enhancement of thermal performance by converging-diverging channel in a micro tube combustor fueled by premixed hydrogen/air

As the core component of the micro thermophotovoltaic (MTPV) system, the micro combustor with a high and uniform wall temperature distribution is beneficial to improve the energy conversion efficiency. In this paper, a micro tube combustor with converging-diverging channel is proposed and the thermal performance is numerically investigated, compared with that of the micro combustor with cylindrical channel. The effects of inlet velocity of H₂/air mixture, dimensionless position and diameter of throat, and solid material on the thermal performance are widely analyzed. Results show that the outer wall temperature and emitter efficiency of the micro combustor with converging-diverging channel are higher than that of the micro combustor with cylindrical channel, and the converging-diverging channel has more uniform temperature distribution. The converging-diverging micro combustor with dimensionless throat position l = 0.375 and dimensionless throat diameter β = 0.4 is more suitable for the application of MTPV system. When H₂/air inlet velocity is 11 m/s and H₂/air equivalence ratio is 1.0, the mean wall temperature is increased by 82.39 K and the emitter efficiency is increased by 6.59%, while the normalized temperature standard deviation is reduced by 65.85%. Additionally, the use of SiC as wall material can improve the thermal performance of the micro combustor. It is worth noting that this work will offer us significant guidelines for the optimized work of micro tube combustor.     

Multi-factor impact mechanism on combustion efficiency of a hydrogen-fueled micro-cylindrical combustor

As it is important to achieve higher combustion efficiency for applications of micro-cylindrical combustor, the multi-factor impact mechanism on the combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor is investigated in this work. Firstly, six factors such as hydrogen/air equivalence ratio, inlet velocity, inlet temperature, wall thermal conductivity, wall emissivity and convective heat transfer coefficient of outer wall and five levels of each factor are determined. Orthogonal design table L₂₅(5⁶) is introduced to arrange cases. Secondly, grey relational analysis is adopted to investigate the effects of the six factors on combustion efficiency. Finally, the results of grey relational analysis are validated by analysis of variance. Based on grey relational analysis and analysis of variance, it is determined that the impact ranking from the largest to the smallest is hydrogen/air equivalence ratio, inlet velocity and inlet temperature, followed by the other three factors. The impact of wall thermal conductivity, convective heat transfer coefficient of outer wall and wall emissivity is considered to be equal due to their difference of impact on combustion efficiency is very small. This work provides us significant reference for optimizing combustion efficiency of a hydrogen-fuelled micro-cylindrical combustor.     

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NO_x profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.     

Thermodynamics analysis of a hybrid system based on a combination of hydrogen fueled compressed air energy storage system and water electrolysis hydrogen generator

In front of the opportunity of the rapid development of renewable energy power generation, energy storage is playing a more important role in improving its utilization efficiency. In this paper, a hybrid energy system based on combination of hydrogen fueled compressed air energy storage system and water electrolysis hydrogen generator is proposed. The superfluous renewable energy power is charged by compressing the air and/or producing hydrogen through water electrolysis. A hydrogen combustor is introduced to raise the air temperature in the discharging process. A thermodynamic model of the proposed system is built. Energy and exergy analysis found that under the design condition, the proposed system can achieve a round trip efficiency of 65.11%, an exergy efficiency of 79.23%, and an energy storage density of 5.85 kWh/m³. The exergy loss of water electrolysis hydrogen generator and hydrogen combustor rank in the top two of all components. Sensitivity analysis indicates that the outlet temperature of hydrogen combustor and specific energy consumption of water electrolysis hydrogen generator are the crucial influencing factor of system performance.     

Numerical study on premixed hydrogen/air combustion characteristics in micro–combustor with slits on both sides of the bluff body

An approach to improve premixed hydrogen/air combustion in micro combustor is numerically studied in this paper. The micro–combustor with slits on both sides of the bluff body shows better combustion efficiency and uniformity of temperature distribution. The effects of the controllable flow ratio (γ) and the angle of bluff body (θ) on combustion characteristics are investigated by using a two–dimensional model with the H₂/O₂ reaction mechanism. The results show that the increase of controllable flow ratio and angle of bluff body can improve combustion efficiency and decrease velocity extinction limit. However, at higher θ, increasing γ do not play an important role in improving combustion efficiency. In addition, at higher inlet velocity, combustion efficiency do not increase dramatically with the increase of θ. Moreover, at high inlet velocity, a special phenomenon of temperature ‘waist’ is observed in the micro–combustor with slits on both sides of the bluff body, which has a huge impact on combustion characteristics. Therefore, controllable flow ratio and angle of bluff body should be reasonably chosen to improve combustion characteristics.     

Effect of inserted baffles on hydrogen heterogeneous reaction in planar catalytic micro combustor

Heterogeneous reaction characteristics of premixed H₂/Air mixture are numerically investigated in micro combustor with coating platinum (Pt) catalyst on the inner wall. The well-designed combustor with the inserted baffle is committed to improving the transport of bulk species on the catalytic surface, therefore enhancing the fuel conversion ratio. A two-dimensional numerical model with detailed heterogeneous reaction mechanism is developed and verified. In this work, the numerical results reveal that the heterogeneous reaction rate and fuel conversion ratio are significantly improved in the combustor with inserted baffle. The velocity of gaseous mixture swiftly increases between the baffle and catalytic surface, resulting in the increasing adsorption-desorption of bulk species on the catalytic surface. The main influencing factors of inserted baffle body comprise of slit width between two ribs in the same row (W), rib length (L), distance between adjacent baffles row (D), number of baffle row (n) which are studied to obtain a set of optimized parameters of baffle in the well-designed combustor. The optimized parameters of the baffle are W = 0 mm, L = 0.4 mm, D = 4 mm and n = 4, which are employed in the combustor to obtain a high hydrogen conversion ratio. Moreover, the effect of different inlet velocities and equivalent ratios of the mixture on the heterogeneous reaction characteristics are carried out in the designed combustor. As the designed combustor performs well, and the improvement effect of the optimized baffle gradually increases together with the inlet velocity, particularly for high inlet velocity (7 m/s). The promotion effects of baffle in the designed combustor are presented when the rich and lean fuel are adopted at inlet velocity of 7 m/s. Finally, the relative parameters of this work can serve as a reference and guidance for the design of micro scale combustor.     

Nonlinear analysis of a pulse combustor model with exhaust decoupler and vent pipe

A nonlinear pulse combustor model with an exhaust decoupler and vent pipe was solved using the Poincaré–Lindstedt perturbation analysis method. The solutions included expressions for the pressure in the combustion chamber and the exhaust decoupler respectively. Experiments were made to validate the theoretical analysis; results showed that an exhaust decoupler and a vent pipe can affect the frequency of the pulse combustor and the pressure in the exhaust decoupler (both amplitude and phase). There are five main dimensionless parameters: dimensionless exhaust decoupler volume v, dimensionless vent pipe length l, dimensionless vent pipe area s and dimensionless enthalpy h_t0, h_d0. Following the values of these five parameters, the working domain of the pulse combustor was divided into three parts: the inphase zone, critical interval and antiphase zone. In the critical interval domain, the pulse combustor cannot work stable. To make a pulse combustor work at the antiphase zone (recommended in engineering applications), a large exhaust decoupler volume, and fairly long vent pipe with a proper cross-section area are required.     

Influence of controllable slit width and angle of controllable flow on hydrogen/air premixed combustion characteristics in micro combustor with both sides–slitted bluff body

To improve the combustion stability of micro combustor, both sides-slitted bluff body is applied to the micro combustor. The H₂/O₂ reaction mechanism is used to study the influence of controllable slit width (d) and angle of controllable flow (β) on combustion characteristics of the micro-combustor. The result shows that the reduction of d can significantly improve the combustion efficiency. Under the same controllable flow ratio (γ), the reduction of d can effectively expand the recirculation region. However, the blow-off limit will be reduced if d is too large or too small. With the same γ, the recirculation region and low-velocity zone expand when β decreases, which can entrain more high-temperature gas and increase the residence time of fuel in the low-velocity zone, thus improving the combustion efficiency. The residence time of fuel in the combustor will be reduced when β is too large or small, resulting in lower combustion efficiency and blow-off limit. Therefore, it is significant to choose the parameter of d and β.     

Optimizing thermal performances and NOx emission in a premixed ammonia-hydrogen blended meso-scale combustor for thermophotovoltaic applications

As a carbon-free energy carrier, ammonia has attracted significant interest in the combustion field as a potential substitute for fossil fuels. However, the focus has been given to the application at meso-scale conditions, particularly with regard to thermal performance and NO_x emissions. Therefore, the present study numerically investigates a 3-dimensional time-domain premixed ammonia/oxygen meso-scale combustor to optimize its' thermal performance and NO_x emission for power generation applications. The numerical model is firstly validated by using experimental data available in the literature. Then, the effects of 1) the inlet pressure (P_in), 2) the equivalence ratio, and 3) the hydrogen blended ratio on the temperature uniformity, the combustor outer wall mean temperature (OWMT), NO emission, and exergy efficiency are examined. The results indicate that increasing P_in intensifies the mixing process of the mixture gases, thus reducing the residence time for the high-temperature flame in the combustion chamber. The optimized OWMT and NO emissions are up to 26% and 40.3% respectively, with only 9% compensation of the standard deviation achieved, when the inlet velocity is set to 0.5 m/s and P_in is 3.0 bar. Furthermore, varying the equivalence ratio in the range of 0.95–1.1 has a minor influence on improving thermal performances, but a significant impact on mitigating the NO_x emission performance. Additionally, blending less than 15% hydrogen has a significant reduction in the maximum NO_x emission (up to 53%); however, the influence on the OWMT can be neglected. Further exergy analysis reveals that elevating P_in results in a decrease in the exergy efficiency due to the increased inlet exergy. In general, this work provides a preliminary method for improving the thermal performance and NO_x emission of an ammonia/hydrogen-oxygen-fueled meso-scale combustor for power generation purpose.     

Performance assessment of an electrochemical hydrogen production and storage system for solar hydrogen refueling station

This paper investigates the performance of a hydrogen refueling system that consists of a polymer electrolyte membrane electrolyzer integrated with photovoltaic arrays, and an electrochemical compressor to increase the hydrogen pressure. The energetic and exergetic performance of the hydrogen refueling station is analyzed at different working conditions. The exergy cost of hydrogen production is studied in three different case scenarios; that consist of i) off-grid station with the photovoltaic system and a battery bank to supply the required electric power, ii) on-grid station but the required power is supplied by the electric grid only when solar energy is not available and iii) on-grid station without energy storage. The efficiency of the station significantly increases when the electric grid empowers the system. The maximum energy and exergy efficiencies of the photovoltaic system at solar irradiation of 850 W m-2 are 13.57% and 14.51%, respectively. The exergy cost of hydrogen production in the on-grid station with energy storage is almost 30% higher than the off-grid station. Moreover, the exergy cost of hydrogen in the on-grid station without energy storage is almost 4 times higher than the off-grid station and the energy and exergy efficiencies are considerably higher.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

A methanol autothermal reforming system for the enhanced hydrogen production: Process simulation and thermodynamic optimization

Methanol autothermal reforming is a potential way to produce hydrogen that can be used for vehicle power batteries like PEMFC. Combining a reformer with a combustor to produce substantial hydrogen is promising, but the challenge of heat transfer efficiency between the reformer and combustor must be considered. Furthermore, the complexity of the system structure is not conducive to its large-scale operation level. In this paper, a novel methanol autothermal reforming hydrogen production system without catalytic combustion was built and developed, aiming to produce hydrogen-rich gas with low CO concentration. Process simulation and thermodynamic optimization on the target system were detailedly performed using Aspen Plus software and parameter sensitivity analysis methods. In addition, a methanol autothermal reforming hydrogen production system using catalytic combustion was taken as the reference system. The results indicated that the novel system could achieve a self-sustaining operation by the coupled methanol partial oxidation and steam reforming. And the product gas contained very low CO concentration (<10 ppm) due to the combined effects of water-gas shifting and CO preferential oxidation reactions. It was verified that under the maximal exergy efficiency condition, the exergy efficiency of the novel system is not significantly improved compared with the reference system, but the hydrogen yield is increased by about 27.65%, the thermal efficiency is increased by about 17.51%, and the exergy loss when generating unit molar H₂ is reduced by 20.53 kJ/mol; Under the condition of maximum hydrogen yield, the indicators of the novel system also perform better. Notably, the reformer is the main exergy loss source in the novel system, which provides a theoretical basis for further optimization of parameter configuration. This work will be beneficial to researchers who study the miniaturization design of the integrated system of methanol hydrogen production coupled vehicle power battery.     

Investigation of the effect of chemistry models on the numerical predictions of the supersonic combustion of hydrogen

In this numerical study, the influence of chemistry models on the predictions of supersonic combustion in a model combustor is investigated. To this end, 3D, compressible, turbulent, reacting flow calculations with a detailed chemistry model (with 37 reactions and 9 species) and the Spalart-Allmaras turbulence model have been carried out. These results are compared with earlier results obtained using single step chemistry. Hydrogen is used as the fuel and three fuel injection schemes, namely, strut, staged (i.e., strut and wall) and wall injection, are considered to evaluate the impact of the chemistry models on the flow field predictions. Predictions of the mass fractions of major species, minor species, dimensionless stagnation temperature, dimensionless static pressure rise and thrust percentage along the combustor length are presented and discussed. Overall performance metrics such as mixing efficiency and combustion efficiency are used to draw inferences on the nature (whether mixing- or kinetic-controlled) and the completeness of the combustion process. The predicted values of the dimensionless wall static pressure are compared with experimental data reported in the literature. The calculations show that multi step chemistry predicts higher and more wide spread heat release than what is predicted by single step chemistry. In addition, it is also shown that multi step chemistry predicts intricate details of the combustion process such as the ignition distance and induction distance.     

Numerical investigations on thermal performance and flame stability of hydrogen-fueled micro tube combustor with injector for thermophotovoltaic applications

In order to design a micro tube combustor with good thermal performance and flame stability for thermophotovoltaic applications, in this work, thermal performance and flame stability of hydrogen-fueled micro tube combustors with backward facing step and with injector are compared. It is found that the decrease of diameter ratio d₂/d₁ leading to expansion of the symmetrical recirculation zone, which is helpful for fluid and heat circulation, and higher flame locations, which is not helpful for flame stabilization. Furthermore, effects of diameter ratio d₂/d₁ on thermal performance and flame stability are analyzed and discussed. Results suggest that when the diameter ratio d₂/d₁ is decreased from 0.9 to 0.8, positive effects of injector on thermal performance are enhanced and flame stability is improved under lower hydrogen/air equivalence ratio. Finally, the applications conditions of the micro tube combustor with injector are achieved. This work will provides us significant reference for designing micro tube combustor with injector.     

Exergy analysis and CO2 emission evaluation for steam methane reforming

This paper investigates the industrial production of hydrogen through steam methane reforming (SMR) from both exergy efficiency and CO₂ emission aspects. An SMR model is constructed based on a practical flow diagram including desulfurizer, furnace, separation unit and heat exchangers. The influence of reformer temperature (Tr) and steam to carbon (S/C) ratio is analyzed to optimize exergy efficiency and CO₂ emission. A clear correlation is obtained between exergy efficiency and CO₂ emission. Results also show optimal S/C ratio decreases with Tr. An exergy load distribution analysis which evaluates interactions between the system and its subsystems with parameter variations is employed to find promising directions for efficiency improvement. Results show that the greatest improvement lies in increasing efficiency of furnace without increasing its relative exergy load. Integration of oxygen-enriched combustion (OEC) with SMR is also evaluated. The integration of OEC can increase the system efficiency greatly when the reformer operates above critical point, while in other cases the system efficiency may decrease.     

Characterization of wall temperature and radiation power through cylindrical dump micro-combustors

The micro-combustor (emitter) is a key component of the micro-thermophotovoltaic (TPV) system. An experimental study on the wall temperature and radiation power through the wall of a series of cylindrical dump micro-combustors was carried out. The effects of combustor diameter (d), combustor length (L), step height (s), flow velocity (u₀) and fuel-air equivalence ratio (Φ) on the wall temperature distribution were investigated. ‘Emitter efficiency’ was defined and quantified based on the measured wall temperature. It was demonstrated that for this particular configuration, that is, a cylindrical micro-combustor with a backward-facing step, the two dimensionless ratios - L/d and s/d, are sufficient to determine the emitter efficiency, provided the flow velocity (U_e) and Φ are known. Based on this result, the effects of the dimensionless step height (s/d) on the emitter efficiency were examined. It was shown that such a sudden flow expansion in the dump combustors does not favor the radiation through the outer wall. Finally, the position of the highest wall temperature and its variation with the flow Reynolds number were discussed. It was noted that the Reynolds number and the relative flow expansion (s/d) alone are inadequate to determine the relative position of the highest wall temperature.     

Exergetic efficiency and options for improving sewage sludge gasification in supercritical water

The present article deals with an exergy analysis of a process under development for the gasification of biomass in supercritical water (supercritical water gasification, SCWG). This process is aimed at generating hydrogen out of the biogenic feedstock sewage sludge. The principle of the process is based on making use of the modifications of specific physical and chemical properties of water above the critical point (T=374°C, p=221 bar). These properties allow for a nearly complete conversion of the organic substance contained in the feed material into energy-rich fuel gases, containing hydrogen, carbon dioxide and methane. Based on a steady-state model of the process, exergy flow rates are calculated for all components and a detailed exergy analysis is performed. From the exergetic variables, options to improve the individual plant components as well as the overall plant are derived. The components with the highest proportion of exergy destruction in the complete process are identified and possibilities of improving them and the complete system in order to increase the overall efficiency are demonstrated. The combustion chamber necessary for heat supply is found to be the component with the highest proportion of exergy destruction of the complete plant. Moreover, the components of air preheater, reactor contribute significantly to the exergy destruction of the complete system. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical analysis of the combustion characteristics of graphitic hydrogen-chlorine synthesis combustor with different intake strategies

This study investigates the effect of intake strategies on the combustion and flows characteristics of hydrogen-chlorine synthesis combustors via numerical methods. A crucial issue of hydrogen-chlorine synthesis combustor is to have a sufficiently low flame height and high conversion efficiency. In this study, the combustion performance of combustors equipped with the annular tube, plum nozzle, and porous-bullet nozzle has been thoroughly analyzed. The temperature distribution and gas flow are analyzed using the method of fluid-solid coupling, which indicates that the combustor with porous-bullet nozzle had the best gas distribution, the maximum HCl mole fraction at outlet is 97.24%, and the lowest flame height is 3.4 m, which is 27.15% lower than the combustor with the annular tube. Furthermore, the nozzle structure has a great influence on the fluid velocity in the recirculation zone of the combustor. Finally, the effect of hydrogen/chlorine equivalence ratio (?) and inlet volume flow rate were analyzed, and it can be concluded that with the increase of inlet volume flow, the high-temperature area inside the combustor gradually increases. As the equivalent ratio increases, the combustor outlet's mole fraction changes with a normal distribution trend. It is the most appropriate when the chlorine gas flow rate is 1,100 m³/h and ? = 1.05. The research can be applied to the field of high-purity hydrogen chlorine production, providing researchers with some solutions.     

Influence of Pt decoration on the hydrogen storage performance of cup-stacked carbon nanotubes: A DFT study

The hydrogen adsorption behaviour of cup-stacked carbon nanotubes (CSCNTs) decorated with the platinum atom at four positions of the conical graphene layer (CGL) is investigated using density functional theory. The optimization shows that the inside lower edge position (IL) results have the best hydrogen adsorption parameters among the four positions. The Pt–H₂ distance is 1.54 Å, the H–H bond length (l_H-H) is 1.942 Å, and the hydrogen adsorption energy (E_ads) is 1.51 eV. The hydrogen adsorption of CSCNTs decorated by Pt at the IL position also has larger E_ads and l_H-H than the Pt-doped planar graphene, Pt-doped single-wall carbon nanotubes and Pt-doped carbon nanocones. The Pt atom at the IL position has a more significant polarization effect on the adsorbed H₂, it has trends to convert H₂ into two separate H atoms. While the hydrogen adsorption behaviour at other positions belongs to the Kubas coordination, the l_H-H and the E_ads increased not significantly.     

Exergy analysis of hydrogen production from steam gasification of biomass: A review

Exergy analysis of hydrogen production from steam gasification of biomass was reviewed in this study. The effects of the main parameters (biomass characteristics, particle size, gasification temperature, steam/biomass ratio, steam flow rate, reaction catalyst, and residence time) on the exergy efficiency were presented and discussed. The results show that the exergy efficiency of hydrogen production from steam gasification of biomass is mainly determined by the H₂ yield and the chemical exergy of biomass. Increases in gasification temperatures improve the exergy efficiency whereas increases in particle sizes generally decrease the exergy efficiency. Generally, both steam/biomass ratio and steam flow rate initially increases and finally decreases the exergy efficiency. A reaction catalyst may have positive, negative or negligible effect on the exergy efficiency, whereas residence time generally has slight effect on the exergy efficiency.