In recent decades, there has been a continuous effort to reduce global environmental pollution and fossil oil consumption [1
]. According to Akande et al. [2
] the demand for hydrogen has increased recently due to progress in fuel cell technologies. Fuel cells are electrochemical devices described as continuously operating batteries and are considered as a clean source of electric energy, containing high energy efficiency, and its resulting emission is just water [3
]. It can be produced from different kinds of renewable feedstocks, such as ethanol. The use of ethanol as a raw material presents several advantages because it is easy to transport, biodegradable, has low toxicity, contains high hydrogen content, and easy to store and handle [4
]. Furthermore, ethanol is economically, environmentally and strategically attractive as an energy source. Ethanol can be a hydrogen source for countries that lack fossil fuel resources, but have significant agricultural economy. This is feasible, because virtually any biomass can now be converted into ethanol as a result of recent advances in biotechnology [5
]. These attributes have made H2
obtained from ethanol reforming a very good energy vector, especially in fuel cells applications. Hydrogen production from ethanol has advantages compared to other H2
production techniques, including steam reforming of methanol and hydrocarbons. Unlike hydrocarbons, ethanol is easier to reform and is also free of sulfur, which is a catalyst poison in the reforming of hydrocarbons [6
]. Also, unlike methanol, which is produced from hydrocarbons and has a relatively high toxicity, ethanol is completely biomass-based and has low toxicity and as such it provides less risk to the population [7
]. The fact that methanol is derived from fossil fuel resources also renders it an unreliable energy source in the long run. Conclusively, amongst the various processes and primary fuels that have been proposed for hydrogen production in fuel cell applications, steam reforming of ethanol is very attractive. Ethanol steam reforming (ESR) proceeds at temperatures in the range of 300–600 °C, which is significantly lower than those required for CH4
or gasoline reforming. This is an important consideration for the improved heat integration of fuel cell vehicles.
The main objective of the computational fluid dynamics (CFD) modeling in this work is to analyze the structural integrity and performance of an industrial ESR system with considerable modeling accuracy that can help evaluate various modeling assumptions usually employed.
Extensive work has been performed on the mathematical modeling the development of first-principles reformer models. With the dramatic increase of computing power, CFD modeling has become an increasingly important platform for reformer modeling and design, combining physical and chemical models with detailed representation of the reformer geometry. When compared with first-principles modeling, CFD is a modeling technique with powerful visualization and computational capabilities to deal with various geometry characteristics, transport equations and boundary conditions.
A 3D CFD simulation pioneering study of the ESR in microreactors with square channels has been carried out by Uriz et al. [8
]. A phenomenological kinetic model based on simple power-law rate equations and considering the following reactions—ethanol dehydrogenation to acetaldehyde, acetaldehyde steam reforming to H2
, ethanol decomposition to CO, CH4
and the water-gas shift—describes satisfactorily the ESR over a Co3
-ZnO catalyst. The CFD computational study shows that high reforming temperatures (above 625 °C) should be avoided. This is because the decomposition of ethanol competes effectively with the dehydrogenation of the alcohol to acetaldehyde, which is the key intermediate of the ESR process, and results in a reduced hydrogen yield and an increased content of CO in the reformate stream. Their work shows that micro reactors can help to overcome these difficulties by increasing the surface area-to-volume ratio and the catalyst loading. By using these micro reactors, the operating temperature can be reduced while increasing the selectivity and maintaining the level of ethanol conversion [8
Uriz et al. [9
] showed that CFD is a very useful tool for advancing in the development and optimization of these technologies. The interest of CFD for assisting in the understanding of the behavior of hydrogen technologies is also important. Nevertheless, there are also limitations regarding the description of chemical transformations and the physics of some complex phenomena such as in multi-physics systems.
Lao et al. [10
] developed a CFD model of an industrial scale steam methane reforming reactor (reforming tube) used to produce hydrogen. The CFD model of an industrial-scale reforming tube has been developed in ANSYS Fluent (version 15.0) with realistic geometry characteristics to simulate the transport and chemical reaction phenomena with approximate representation of the catalyst packing.
In this work a CFD simulation study of the ESR system has been performed. The proposed computational work is composed of two phases—simulation of ethanol burner by using fire dynamics simulator (FDS) software and a multiphysics simulation of steam reforming process occurring inside the reformer. COMSOL multi-physics software has been applied in this work. It solved simultaneously the fluid flow, heat transfer, diffusion with chemical reaction kinetics equations and structural analysis. It is probably the first time that FDS software has been applied in order to simulate the combustion processes of the burner inside the gas heated heat exchange reformer (GHHR). The FDS software is described in detail in Section 2.1
and Section 2.2
. The multiphysics reformer model is described in Section 2.3
. As far as I know, this work is the first coupled CFD and structural analysis study on the ESR system. The structural analysis of the reformer steel tube is essential in order to verify that the structural integrity of the tube under the operating conditions of the reformer is maintained. Steel tube rupture and cracks may cause release of the hydrogen gas to the reformer facility. Hydrogen has wide flammability limits and very low ignition energy [11
]. Therefore, hydrogen present safety concerns at limited ventilation conditions because of the danger of explosive mixture formation that may cause severe damage [12
]. Diéguez et al. [12
] showed examples of application of CFD to safety issues such as hydrogen leakages, hydrogen flames, detonation and application of the simulation results to evaluate possible physiological injuries. They performed a CFD for simulating the Hydrogen leakage.
1.1. Mechanism of Steam Ethanol Reforming Process
The main reactions that occur during the ESR process on Ni catalyst are [15
• Ethanol steam reforming:
C2H5OH + 3H2O → 2CO2 + 6H2
• Ethanol decomposition to methane:
• Ethanol dehydrogenation:
• Ethanol decomposition to acetone:
2C2H5OH → CH3COCH3 + CO + 3H2
• Water-gas shift reaction:
1.2. Ethanol Burner Steam Reformer
The reforming system described in this work is composed from ethanol burner and a catalytic bed reactor. The liquid ethanol is burned inside the firebox. The heat from the flue gases is transferred to the catalytic bed reactor (the theoretical model of the reformer is described in Section 2.3
) for transforming the ethanol steam mixture to hydrogen and carbon dioxide. The catalyst is made from Ni/Al2
. The schematic of the system is described in [16
Burner Heat Transfer
The heat produced from ethanol burning from the combustion products is transferred to catalytic bed walls in by three modes (see Figure 1
Heat is transferred through fluids in motion and between a fluid and solid surface in relative motion. When the motion is produced by forces other than gravity, the term forced convection is used. In engines the fluid motions are turbulent. Heat is transferred by forced convection between the in-cylinder gases and the cylinder head, valves, cylinder walls, and piston during induction, compression, expansion and exhaust processes [18
1.2.3 Radiative Heat Transfer
There are two sources of radiative heat transfer within the burner: The high temperature burned gases and the soot particles in the ethanol flames. In the burner, most if the fuel burns in turbulent diffusion flame as fuel and air mix together. The flame is highly luminous, and soot particles are formed at an intermediate step in the combustion process [18
1.2.4 Conduction Heat Transfer
Heat is transferred by molecular motion, through solids and through fluids at rest, due to temperature gradient. It is transferred by conduction through the reformer walls [18
Hydrogen could be a promising fuel, and if often considered as a clean energy carrier as it only emits water during combustion. It can be produced from different kinds of renewable feedstocks, such as ethanol. The use of ethanol as raw material presents several advantages because it is a renewable feedstock, easy to transport, biodegradable, has low toxicity, contains high hydrogen content, and easy to store and handle. Ethanol steam reformation occurs at relatively lower temperatures compared with other hydrocarbon fuels and has been widely studied due to the high yield provided for the formation of hydrogen. In this reformation reaction, water and ethanol react over a catalyst bed to produce a mixture of hydrogen rich gas. A new tool has been developed in this work in order to analyze the reformer operation. The proposed computational work is composed of two phases—simulation of ethanol burner by using FDS software and multiphysics simulation of steam reforming process occurring inside the reformer. This software calculates the temperature, density, pressure, velocity, and chemical composition within each numerical grid cell at each discrete time step. It computes the temperature, heat flux, and mass loss rate. There are three major components of the model: Hydrodynamic, combustion, and radiation models. FDS code is formulated based on CFD of fire-driven fluid flow. The FDS numerical solution can be carried out using either a DNS method LES. FDS uses a mixture fraction combustion model. The mixture fraction is a conserved scalar quantity that is defined as the fraction of gas at a given point in the flow field that originated as fuel. The model assumes that combustion is mixing controlled, and that the reaction of fuel and oxygen is infinitely fast. Radiative heat transfer is included in the model via the solution of the radiation transport equation for a non-scattering gray gas. The radiation equation is solved using a technique similar to a finite volume method for convective transport, thus the name given to it is the FVM. One advantage of FDS simulation is that it can provide much detailed information on the fire, including the local and transient gas velocity, gas temperature, species concentration, solid wall temperature, composite burning rate, radiation heat transfer, convection heat transfer and HRR. It has been found out that the maximal temperature at t = 77.5 s approaches to 815 °C. The magnitude of the radiation heat flux is 95,300 (w/m2).
The second part deals with numerical analysis of the reformer. The reformation chemical reactions occur in a porous catalytic bed where the heat is supplied through the Ethanol burner to drive the endothermal reaction system. The reactor is enclosed with a HK-40 alloy steel tube. Ethanol and steam are mixed in stoichiometric amounts and enter through the inlet of the catalytic reactor. The COMSOL multi-physics software has been applied in this work. It solved simultaneously the fluid flow, Heat transfer, diffusion with chemical reaction kinetics equations and structural mechanics. The diffusion equations values are obtained from the Chapman-Enskog theory. As far as I know, this work is the first CFD structural simulation study of ESR system. It is probably the first time that FDS software has been applied to simulate the ethanol burner.
A parametric study has been performed in order to analyze the influence of the heat flux on the reformer performance and its structural integrity. The numerical results were obtained for heat fluxes of 95,300 (W/m2) and 200,000 (W/m2). It is shown that the HRR produced by the ethanol burner, can provide the necessary heat flux required for maintaining the reforming process.
It has been found out that the mass fractions of the ethanol and steam decays along the reformer axis. The hydrogen and carbon dioxide mass fraction are increased along the reformer axis. The hydrogen mass fraction increases with enhancing the radiation heat flux. For the first case (heat flux magnitude of 95,300 (W/m2)) it was found out that the ethanol conversion is 80.3%. The ethanol and the steam are decaying at the same rate. Similar values have been reported in the literature. A sensitivity test has been carried out in order to analyze the influence of inlet temperature on ethanol conversion. The results show that for inlet temperature of 500 °C, the ethanol conversion is 70.0%. For inlet temperature of 400 °C, the ethanol conversion is 43.8%. Therefore, the ethanol conversion increases with the inlet temperature. There is a considerable increasing of Hydrogen mass fraction along the reformer axis. The increase in hydrogen mass fraction is 89.4%. The sum of hydrogen, ethanol, carbon dioxide and steam mass fraction at the reformer output is 1, as expected according the mass conservation law.
Similar physical behavior has been observed in the second case. The ethanol conversion is about 85.4%. The increase in hydrogen mass fraction along the reformer is 89.4%. The thermal stresses which are developed inside the steel tube are higher for the case of heat flux of 200,000 (W/m2). The increase of the heat flux enhances a little the ethanol conversion, but also increases the thermal stresses of the steel tube. This effect can cause to mechanical failure in the steel tube. In order to maintain the structural integrity of the tube, HK-40 alloy has been applied in this work. HK alloy, known as HK 40, is an austenitic Fe-Cr-Ni alloy that has been a standard heat resistant material with moderately high temperature strength, and oxidation resistance. This alloy is used in a wide variety of industrial applications such as: Ammonia, methanol, and hydrogen reformers; ethylene pyrolysis coils and fittings; and steam super heater tubes and fittings.
It has been found out that by increasing the heat flux of the reformer, the decrease in the tensile yield strength of HK-40 alloy is less than 10% (the temperature of the steel is about 600 °C or 1112 °F). The effect of increasing the heat fluxes increases the temperature inside the steel tube and enhances the thermal stresses (produced by the thermal expansion). Since ethanol steam reforming occurs at relatively lower temperatures compared with other hydrocarbon fuels, the decrease in the strength of the steel tube is low. Thus, the structural integrity of the HK-40 steel tube is kept.
The numerical results for clearly indicates that under normal conditions of the ethanol reforming (the temperature of the steel tube is about 600 °C or 1112 °F), the rupture time increases considerably. For this case the rupture time is greater than 100,000 h (more than 11.4 years). The described algorithm described in this work may be applied other reformer system (diesel, methanol, or methane).