Effect of heat loss on the syngas production by fuel-rich combustion in a divergent two-layer burner

The effect of heat loss on the syngas production from partial combustion of fuel-rich in a divergent two-layer burner is numerically studied using two-dimensional model with detailed kinetics GRI-Mech 1.2. Both the radiation and wall heat losses to the surrounding are considered in the computations. It is shown that two types heat losses have different effects on the syngas production. The radiation heat loss has significant effect on the syngas temperature and the syngas temperature is dropped as radiation heat loss is increased, but it has neglected effect on the reforming efficiency and methane conversion efficiency. The wall heat loss has a comprehensive effect on the syngas production. The wall heat loss not only reduces the conversion efficiency, but also significantly decreases the syngas temperature. The effect of wall heat loss becomes weak as the equivalence is increased. The reforming efficiency drops from 0.440 to 0.424 for equivalence ratio of 2 and mixture velocity of 0.17 m/s for the predictions between adiabatic wall and non-adiabatic conditions.     

Four different configurations of a 5 kW class shell-and-tube methane steam reformer with a low-temperature heat source

The methane steam reforming reaction is an extremely high endothermic reaction that needs a high temperature heat source. Various fuel cell hybrid systems have been developed to improve the thermal efficiency of the entire system. This paper presents a low temperature steam reformer for those hybrid systems to maximize the utilization of energy from a low temperature waste heat source. In this study, the steam reformer has a shell and tube configuration that is divided into the following zones: the inlet heat exchanging zone, the reforming zone and the exit heat exchanging zone. Four different configurations for methane steam reformers are developed to examine the effect of heat transfer on the methane conversion performance of the low temperature steam reformer. The experimental results show that the overall heat transfer area is a critical parameter in achieving a high methane conversion rate. When the heat transfer area increases about 30%, the results showed elevated dry mole fractions of hydrogen about 3% with about 30 °C rise of reformer outlet temperature.     

有序堆积床内预混气体燃烧特性实验研究

为研究预混气体在多孔介质燃烧器中的火焰燃烧特性,设计了一种新型多孔介质燃烧器,其中多孔介质区域由氧化铝圆柱体有序堆积而成。分别研究了当量比和入口速度对甲烷/空气预混气体在多孔介质燃烧器中的火焰温度分布、火焰最高温度以及火焰传播速度的影响。结果表明:在当量比0.162~0.324、入口速度0.287~0.860 m/s的实验工况下火焰均可以稳定向前传播,并且都发生了超绝热燃烧;当量比越大,入口速度越大,火焰最高温度越高;当入口速度为0.430 m/s时,贫可燃极限的当量比可以扩展到0.162;火焰传播速度随着入口速度的增加和当量比的减小而增大,其数量级为0.100 mm/s,属于一种十分典型的低速过滤燃烧。     

Modeling hydrogen production in a catalytic-inert packed bed reactor by rich combustion of heavy fuel oil

This work presents simulation results for the production of hydrogen by the rich combustion of heavy fuel oil in a dual zone packed bed reactor. The first zone provides catalytic-thermal cracking of the fuel and is followed by a second zone for partial oxidation reforming of the cracked products. The kinetic model for the heavy fuel oil reactions in the catalytic zone uses decalin as a model compound. The partial oxidation reforming zone uses model compounds for the product groups formed from decalin cracking, and uncracked decalin. The hybrid reactor model is compared to results from a model of an inert (non-catalytic) porous media reactor. The work considers equivalence ratios from 1 to 2, filtration velocities between 15.0 and 65.5 cm/s, heat loss from 10 to 108% and particle diameter between 3 and 7 mm, and evaluates their effect on conversion. The simulations with the hybrid reactor model, in slightly rich conditions (equivalence ratio = 1.3) and constant filtration velocity of 19.3 cm/s deliver maximum hydrogen production for an optimal length of the intermediate zone. Considering this optimization: the total energy conversion efficiencies improve with the increase of the equivalence ratio due to the presence of hydrocarbon species generated by the cracking process. It is observed that the hybrid reactor model makes a better use of vaporized fuel, compared to a model for an inert packed bed reactor, when the deposits of carbonaceous material in the latter exceed 7.4%.     

Development and CFD analysis for determining the optimal operating conditions of 250 kg/day hydrogen generation for an on-site hydrogen refueling station (HRS) using steam methane reforming

The objective of this study to develop and undertake a comprehensive CFD analysis of an effective state-of-the-art 250 kg/day hydrogen generation unit for an on-site hydrogen refueling station (HRS), an essential part of the infrastructure required for fuel cell vehicles and various aspects of hydrogen mobility. This design consists of twelve reforming tubes and one newly designed metal fiber burner to ensure superior emission standards and performance. Experimental and computational modeling steps are conducted to investigate the effects of various operating conditions, the excess air ratio (EAR) at the burner, the gas hourly space velocity (GHSV), the process gas inlet temperature, and the operating pressure on the hydrogen production rate and thermal efficiency. The results indicate that the performance of the steam methane reforming reactor increased significantly by improving the combustion characteristics and preventing local peak temperatures along the reforming tube. It is shown that EAR should be chosen appropriately to maximize the hydrogen production rate and lifetime operation of the reformer tube. It is found that high inlet process gas temperatures and low operating pressure are beneficial, but these parameters have to be chosen carefully to ensure proper efficiency. Also, a high GHSV shortens the residence time and provides unfavorable heat transfer in the bed, leading to decreased conversion efficiency. Thus, a moderate GHSV should be used. It is shown that heat transfer is an essential factor for obtaining increased hydrogen production. This study addresses the pressing need for the HRS to adopt such a compact system, whose processes can ensure greater hydrogen production rates as well as better durability, reliability, and convenience.     

Conversion of jet fuel and butanol to syngas by filtration combustion

Replacing batteries with fuel cells is a promising approach for powering portable devices; however, hydrogen fuel generation and storage are challenges to the acceptance of this technology. A potential solution to this problem is on-site fuel reforming, in which a rich fuel/air mixture is converted to a hydrogen-rich syngas. In this paper, we present experimental results of the conversion of jet fuel (Jet-A) and butanol to syngas by non-catalytic filtration combustion in a porous media reactor operating over a wide range of equivalence ratios and inlet velocities. Since the focus of this study is the production of syngas, our primary results are the hydrogen yield, the carbon monoxide yield, and the energy conversion efficiency. In addition, the production of soot that occurred during testing is discussed for both fuels. Finally, an analysis of the potential for these fuels and others to be converted to syngas based on the present experiments and data available in the literature is presented. This study is intended to increase the understanding of filtration combustion for syngas production and to illuminate the potential of these fuels for conversion to syngas by non-catalytic methods.     

Experimental investigation on the partial oxidation of methane with the effect of shrunk structure

Hydrogen energy is an ideal clean energy to solve the expanding energy demand and environmental problems caused by fossil fuels. In order to produce hydrogen, a double-layer porous media burner with shrunk structure was designed to explore the partial oxidation (POX) of methane. And the combustion temperature, species concentration and reforming efficiency were studied under different shrunk parameters and operating conditions. The results indicated that the shrunk structure greatly influenced the flame position and temperature distribution. The flame moved to the downstream section with the decreasing of the inner shrunk diameter and the increasing of the shrunk height. When the diameter of the filled Al₂O₃ pellets was 8 mm, the hydrogen yield reached the highest value of 43.8%. With the increasing of equivalence ratio, the reforming efficiency increased first and then decreased, and the maximum value of 53.0% was reached at φ = 1.5. However, the reforming efficiency and axial temperature kept increasing when the inlet velocity increased from 10 to 18 cm/s. The corresponding results provided theoretical reference for the control of flame position and species production by the design of shrunk structure in porous media burner.     

Thermal management and catalytic combustion stability characteristics of premixed methane/air in heat recirculation meso‐combustors

In order to illuminate heat recirculation effect on catalytic combustion stability and further improve energy conversion efficiency in meso‐combustor, the catalytic combustion characteristics of the combustor with/without preheating channels are numerically studied at steady conditions. It is found that methane conversion rate and combustion efficiency increases by 2% to 3% and approximately 9% in the heat recirculation meso‐combustor, indicating that heat recirculation effect facilitates more complete combustion of methane and medium components. Preheating channels show positive effects on improving combustion stability in the heat recirculation meso‐combustor. On one hand, preheating channels facilitate heat recirculation effect, and heat recirculation rate exceeds 10% for all cases and reaches 31.8% with an inlet velocity of 0.5 m/s, leading to significant increment of methane‐specific enthalpy at the preheating channel outlet. On the other hand, Rh(s)/O(s) ratios of catalytic surface and catalytic surface temperature in main reaction zone are enlarged by the preheating channels, facilitating methane adsorption at catalytic surface. Specially, most of fuels are consumed in a shorter distance with higher methane conversion speed, which brings benefits to promote combustion efficiency and may be helpful to inhibit the combustion instability in heat recirculation meso‐combustors.     

A flame fuel cell stack powered by a porous media combustor

A flame fuel cell stack is successfully assembled and operated in this paper. A micro-tubular solid oxide fuel cell (mT-SOFC) stack was directly operated in and powered by a fuel-rich methane flame in a porous media combustor. The combustor consists of a combustion region and a hot zone. The combustion region and the hot zone are connected by an expansion region, which is designed to match the combustion kinetics and the electrochemical kinetics, thereby increasing the fuel utilization efficiency of the stack. With a 36-Al₂O₃-tube stack located in the hot zone, the temperature field and composition distribution were found to be suitable for the operation of high-temperature SOFCs with traditional materials. Four mT-SOFCs are arranged in a parallel configuration and placed in the center of the 36-tube stack, with the power reached 3.6 W at 0.6 V when the fuel-rich methane flame was operated at an equivalence ratio of 1.6. The maximum electrical efficiency was 6% with a fuel utilization efficiency of 23%. The present configuration demonstrated a promising technology for a self-sustained combined heat and power (CHP) system.     

Parameter study of a porous solar-based propane steam reformer using computational fluid dynamics and response surface methodology

Solar-driven steam reforming of fossil fuels is a promising renewable method for hydrogen production that reduces emissions compared with traditional approaches such as combustion-based technologies. In the present study, a steady-state computational fluid dynamic (CFD) model is developed to investigate a porous solar propane steam reformer (PSR). P1 approximation for radiation heat transfer is coupled with the CFD model, employing User-Defined Functions (UDFs). Innovative propane steam reformers have received less attention in terms of optimization and sensitivity analysis to improve their performance and efficiency. Hence, the effects of porosity, pore diameter, inlet velocity, solar irradiation flux, inlet temperature, and foam thermal conductivity on the propane conversion, hydrogen production rate, and pressure drop are studied using response surface methodology (RSM). The inlet velocity, solar irradiation flux, and pore diameter are found to be the most influential parameters, among those mentioned, on propane conversion, hydrogen productivity, and pressure drop, respectively. Furthermore, optimization is carried out in order to minimize pressure drop and maximize hydrogen production. The reformer with the 70% propane conversion provides the lowest pressure drop maintaining the same hydrogen productivity compared with 80% and 90% propane conversions.     

Performance analysis of a membrane‐based reformer‐combustor reactor for hydrogen generation

Hydrogen is mostly produced in conventional steam methane reforming plants. In this work, we proposed a membrane‐based reformer‐combustor reactor (MRCR) for hydrogen generation in order to improve heat recovery and overall thermal efficiency. The proposed configuration will also reduce the complexity in existing steam methane reforming (SMR) plants. The proposed MRCR comprises combustion zone, hydrogen permeate zone, and SMR zone. A computational fluid dynamics model was developed using ANSYS‐Fluent software to simulate and analyze the performance of the proposed MRCR. Results show that high hydrogen yields were observed at high reformer pressures (RPs) and low gas hourly space velocities (GHSVs). Furthermore, by increasing the steam to methane ratio and addition of excess air in the combustion side, the hydrogen yield from the MRCR decreases. This is attributed to the reduction in the effective temperature of the hydrogen membrane. High RP, low GHSV, and low steam to methane ratio that increased the hydrogen yield also decreased carbon monoxide (CO) emissions. For an increased RP from 1 to 10 bar, the CO emission decreased by about 99%. The reduction in CO emission at high RP would be attributed to the effect of water gas shift reaction in the MRCR. Results of the extensive parametric study presented in this work can be used to determine the operating conditions based on tradeoffs between hydrogen yield (mole H₂/mole CH₄), hydrogen production rate (kg of H₂/h), allowable CO emissions, and exhaust gas temperature for other applications such as gas turbine.     

稀混合气低速过滤燃烧的数值模拟与理论分析

应用一维数值模拟方法,研究了多孔介质引起的热回流效应、多孔介质的导热系数、系统的热损失、混合气当量比对燃烧波波速和反应区最高温度的影响.分别将燃烧区简化为无限薄和极其狭窄的区域,从理论上得到了低速过滤燃烧波波速和反应区的最高温度两个关系式,构成了两者的封闭解.利用数值模拟和理论分析求得的燃烧波波速和反应区的最高燃烧温度,取得了与实验结果相同的趋势.     

Syngas production by methane-rich combustion in a divergent burner of porous media

The superadiabatic combustion in porous media contributes to the efficient conversion of methane to syngas. In this paper, a divergent packed bed burner of two-layer was proposed to obtain the characteristics of methane partial oxidation. The divergent angle, interface location and pellet diameter were used to study the temperature and species distributions. Results indicate that the upper limit of velocity gradually decreased as the equivalence ratio increased and the limit of the divergent burner is obviously higher than that of the cylindrical one. The increasing of the divergent angle within a certain range enhances the methane conversion and the 15° shows the best among the selected five angles. The mole fractions of H₂ and CO gradually decrease when the interface locations move from the cylindrical region to the divergent one. As the equivalence ratio increased from 1.3 to 3.5, the yields of H₂ and CO and the energy conversion efficiency of syngas increase first and then decrease, and the maximum efficiency of 45.9% appears at the equivalence ratio of 2.0. The divergent region weakens the influence of inlet velocities and contributes to the stability of reforming reactions.     

Simulation of catalytic oxidation of lean hydrogen–methane mixtures

The combustion of preheated lean homogeneous mixtures of hydrogen with methane in air in a catalytic packed-bed reactor was modeled at atmospheric pressure. The non-equilibrium, one-dimensional model developed employs multi-step surface and gas-phase reactions and accounts for the three modes of heat transfer within the bed as well as for heat loss from the bed. The catalyst considered was platinum. It was demonstrated that the model could predict the effects of changes in operational conditions such as inlet mixture temperature, fuel composition and mixture equivalence ratio on the methane and hydrogen conversions, as well as species concentrations and gas temperature profiles along the bed. It was shown that the hydrogen is consumed completely within the early part of the reactor length in all the cases considered for simulations. It was also shown that the improving effect of hydrogen on methane conversion is particularly evident at relatively low inlet temperatures and for very lean mixtures. However, this effect diminishes significantly with increasing inlet temperature and equivalence ratio. It was also shown that the positive effect of hydrogen addition which is more pronounced at its low concentrations in the fuel mixture, decreases somewhat with a further increase of the hydrogen content. The displayed trends were in good agreement with the corresponding experimentally observed.     

Optimization of operating parameters for methane steam reforming thermochemical process using Response Surface Methodology

The methane steam reforming to produce hydrogen under concentrated irradiation was proposed and the novel thermodynamic analysis interaction effects on the conversion of methane via Design Expert software was utilized in this paper. The four parameters (material porosity, inlet gas temperature, steam-to-methane ratio, inlet gas velocity) and three levels (low, center and high) were designed via the Box-Behnken Design in Response Surface Methodology. The response model was established to optimize and analyze the condition parameters, which showed effects on the methane conversion under concentrated irradiation. The results showed that the value of material porosity, inlet gas temperature and S/C had a positive effect on the methane conversion rate, while the value of inlet gas velocity had a negative effect on it. The analysis of variance of the methane conversion in the Response Surface Methodology was 0.9914. When the material porosity, gas inlet temperature, S/C and gas inlet velocity were 0.770, 579.925 K, 2.996 and 0.031, respectively, methane conversion was 94.03%; Among the above four factors, material porosity had the most significant effect on the reaction of methane steam reforming to produce hydrogen under concentrated irradiation. These results provided theoretical guidance for application of methane steam reforming under concentrated irradiation.     

多孔介质中预混式燃烧的数值研究

本文考虑向燃烧室中插入高孔隙率的多孔介质的燃烧过程,根据气固两相局部非热平衡假设,建立了混合气体在惰性多孔介质中预混燃烧的一维数学模型,模拟了不同条件下甲烷-空气的预混合气在多孔介质中燃烧时的温度分布及气体流速、当量比和吸收系数对燃烧室气体温度峰值的影响.结果表明,多孔介质的存在明显改善了燃烧室的换热性能,强化了对新鲜混合气的预热,加速了燃烧反应的进行,燃烧室利用率提高.     

Numerical studies on heat coupling and configuration optimization in an industrial hydrogen production reformer

Steam methane reforming furnaces are the most important devices in the hydrogen production industry. The highly endothermic reaction system requires reaction tubes in the furnace to have a large heat transfer area and to be operated under high temperature and pressure conditions. In order to enhance heat transfer efficiency and protect reaction tubes, the controlling and optimization of the furnace structure have increasingly received more and more research attention. As known from the furnace structure, it is essential to couple the exothermic combustion with the endothermic reforming reactions due to the highly interactive nature of the two processes. Thus, in this paper, the combustion process in the furnace was numerically studied by using computational fluid dynamics (CFD) to model the combustion chamber, coupled with methane steam reforming reaction inside the reaction tubes, defined by a plug flow model. A set of combustion models were compared for the furnace chamber and a plug flow reaction model was employed for reforming reaction tubes, and then a heat coupling process was established. The predicted flue gas temperature distribution showed that the heat transfer in the furnace was not uniform, resulting in hot spots and heat losses on the tube wall. Therefore, structure optimization schemes were proposed. Optimization on arrangements of the tubes and the nozzles promoted the uniform distribution of flue-gas temperature and then improved heat transfer efficiency, thereby enhancing performance of the steam reforming process.     

Effect of methane reforming before combustion on emission and calorimetric characteristics of its combustion process

In this study, combustion and emission characteristics of methane mixed with steam (CH₄/H₂O) and the products of methane reforming with steam (CO/H₂/H₂O) were compared. Four fuel compositions were analysed: CH₄+H₂O, CH₄+2H₂O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NO_x formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NO_x comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NO_x emissions in all cases considered.     

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.     

Millisecond methane steam reforming for hydrogen production: A computational fluid dynamics study

The potential of methane steam reforming to produce hydrogen in thermally integrated micro-chemical systems at short contact times was theoretically explored. Methane steam reforming coupled with methane catalytic combustion in microchannel reactors for hydrogen production was studied numerically. A two-dimensional computational fluid dynamics model with detailed chemistry and transport was developed. To provide guidelines for optimal design, reactor behavior was studied, and the effect of design parameters such as catalyst loading, channel height, and flow arrangement was evaluated. To understand how steam reforming can happen at millisecond contact times, the relevant process time scales were analyzed, and a heat and mass transfer analysis was performed. The importance of energy management was also discussed in order to obtain a better understanding of the mechanism responsible for efficient heat exchange between highly exothermic and endothermic reactions. The results demonstrated the feasibility of the design of millisecond reforming systems, but only under certain conditions. To achieve this goal, process intensification through miniaturization and the improvement in catalyst performance is very important, but not sufficient; very careful design and implementation of the system is also necessary to enable high thermal integration. The channel height plays an important role in determining the efficiency of heat exchange. A proper balance of the flow rates of the combustible and reforming streams is an important design criterion. Reactor performance is significantly affected by flow arrangement, and co-current operation is recommended to achieve a good energy balance within the system. The catalyst loading must be carefully designed to avoid insufficient reactant conversion or hot spots. Finally, operating windows were identified, and engineering maps for designing devices with desired power were constructed.