CFD Simulation of Ethanol Steam Reforming System for ... - MDPI

chemengineering Article

CFD Simulation of Ethanol Steam Reforming System for Hydrogen Production Alon Davidy

ID

Israel Military Industries Ltd., Ramat Hasharon 47100, Israel; [email protected]; Tel.: +972-03-904-9118 Received: 26 May 2018; Accepted: 30 July 2018; Published: 2 August 2018

Abstract: Hydrogen could be a promising source fuel, and is often considered as a clean energy carrier as it can be produced by ethanol. The use of ethanol 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. Reforming ethanol steam 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. A new computational fluid dynamics (CFD) simulation model of the ethanol steam reforming (ESR) has been developed in this work. The reforming system model is composed from an ethanol burner and a catalytic bed reactor. The liquid ethanol is burned inside the firebox, then the radiative heat flux from burner is transferred to the catalytic bed reactor for transforming the ethanol steam mixture to hydrogen and carbon dioxide. The proposed computational model is composed of two phases—Simulation of ethanol burner by using Fire Dynamics Simulator software (FDS) (version 5.0) and a multi-physics simulation of the steam reforming process occurring inside the reformer. COMSOL multi-physics software (version 4.3b) has been applied in this work. It solves simultaneously the fluid flow, heat transfer, diffusion with chemical reaction kinetics equations, and structural analysis. It is shown that the heat release rate 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 hydrogen and carbon dioxide mass fraction are increased along the reformer axis. The hydrogen mass fraction increases with enhancing the radiation heat flux. It was shown that Von Mises stresses increases with heat fluxes. Safety issues concerning the structural integrity of the steel jacket are also addressed. This work clearly shows that by using ethanol which has low temperature conversion, the decrease in structural strength of the steel tube is low. The numerical results clearly indicate that under normal conditions of the ethanol reforming (The temperature of the steel is about 600 ◦ C or 1112 ◦ F), the rupture time of the HK-40 steel alloy increases considerably. For this case the rupture time is greater than 100,000 h (more than 11.4 years). Keywords: ethanol burner; ethanol steam reformer; catalytic bed reactor; hydrogen production; fuel cell; CFD; Fire Dynamic Simulation software (FDS); Large Eddy Simulation (LES); radiative heat flux; convective heat flux; Multi-physics Simulation; Darcy Flow Equation; chemical kinetics; arrhenius coefficients; diffusion equation; heat transfer; thermal expansion; structural analysis; steel tube; HK-40 alloy; ultimate tensile strength; rupture time

1. Introduction 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

ChemEngineering 2018, 2, 34; doi:10.3390/chemengineering2030034

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ChemEngineering 2018, 2, 34

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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 and CO2 , ethanol decomposition to CO, CH4 and H2, and the water-gas shift—describes satisfactorily the ESR over a Co3 O4 -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


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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 Sections 2.1 and 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 [12–14]. 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: C2 H5 OH + 3H2 O → 2CO2 + 6H2

• Ethanol decomposition to methane: C2 H5 OH → CO + H2 + CH4

• Ethanol dehydration: C2 H5 OH → C2 H4 + H2 O

• Ethanol dehydrogenation: C2 H5 OH → CH3 CHO + H2

• Ethanol decomposition to acetone: 2C2 H5 OH → CH3 COCH3 + CO + 3H2

• Water-gas shift reaction: CO + H2 O → CO2 + H2 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 O3 . 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):


(1) (2) (3)

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Convection Radiation (2) Radiation Conduction

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(3) Conduction

Figure Heattransfer transfermechanisms mechanisms in [17]. Figure 1. 1. Heat insteam steamreformer reformer [17].

Convection 1.2.1.1.2.2 Convection Heat is transferred through fluids in motion and between a fluid and solid surface in relative

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 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 used.the In in-cylinder engines the fluidand motions are turbulent. Heatcylinder is transferred by piston forcedduring convection between gases the cylinder head, valves, walls, and induction, the in-cylinder gases and the cylinder head, valves, cylinder walls, and piston during induction, compression, expansion and exhaust processes [18]. compression, expansion and exhaust processes [18]. 1.2.3 Radiative Heat Transfer

1.2.2. 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

There are two sources of radiative heat transfer within the burner: The high temperature burned turbulent diffusion flame as fuel and air mix together. The flame is highly luminous, and soot gasesparticles and theare soot particles the ethanol flames. In the burner, most if the fuel burns in turbulent formed at an in intermediate step in the combustion process [18]. diffusion flame as fuel and air mix together. The flame is highly luminous, and soot particles are 1.2.4 Conduction Heat Transfer formed at an intermediate step in the combustion process [18]. Heat is transferred by molecular motion, through solids and through fluids at rest, due to

1.2.3.temperature Conductiongradient. Heat Transfer It is transferred by conduction through the reformer walls [18]. Heat is transferred by molecular motion, through solids and through fluids at rest, due to 2. Materials and Methods temperature gradient. It is transferred by conduction through the reformer walls [18]. 2.1. Fire Dynamic Simulation Modeling of the Burner

2. Materials and Methods

The FDS has been developed at the Building and Fire Research Laboratory (BFRL) at the National Institutes of Standards andofTechnology 2.1. Fire Dynamic Simulation Modeling the Burner (NIST), e.g., McGrattan et al. [19,20]. This software calculates simultaneously the temperature, density, pressure, velocity, and chemical composition The FDS hasnumerical been developed Building Research Laboratory at the heat National within each grid cellatatthe each discreteand timeFire step. It also calculates the(BFRL) temperature, Institutes of Standards Technology (NIST), e.g., McGrattan [19,20]. calculates flux, and mass loss and rate of the enclosed solid surfaces. The latteretisal.used in theThis casesoftware where the fire heat release rate is unknown. The major components of this software are: simultaneously the temperature, density, pressure, velocity, and chemical composition within each Model—FDS code step. is formulated based on the CFDtemperature, of fire-drivenheat fluidflux, flow.and Themass numericalHydrodynamic grid cell at each discrete time It also calculates FDS numerical solution can be carried out using either a Direct Numerical Simulation (DNS) method loss rate of the enclosed solid surfaces. The latter is used in the case where the fire heat release rate is or Large Eddy Simulation (LES). The latter is relatively low Reynolds numbers and is not severely

unknown. The major components of this software are:


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Hydrodynamic Model—FDS code is formulated based on CFD of fire-driven fluid flow. The FDS numerical solution can be carried out using either a Direct Numerical Simulation (DNS) method or Large Eddy Simulation (LES). The latter is relatively low Reynolds numbers and is not severely limited in grid size and time step as the DNS method. In addition to the classical conservation equations considered in FDS, including mass species momentum and energy, thermodynamics-based state equation of a perfect gas is adopted along with chemical combustion reaction for a library of different fuel sources. Combustion Model—For most applications, 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. The mass fractions of all of the major reactants and products can be derived from the mixture fraction by means of “state relations”, empirical expressions arrived at by a combination of simplified analysis and measurement [21]. Radiation Transport—Radiative heat transfer is included in the model via the solution of the radiation transport equation for a non-scattering gray gas. In a limited number of cases, a wide band model can be used in place of the gray gas model. 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 Finite Volume Method (FVM) [19]. FDS also has a visual post-processing image simulation program named “smoke-view”. 2.2. Governing Equations of FDS Software This section introduces the basic conservation equations for mass, momentum and energy for a Newtonian fluid. These are the same equations that can be found in almost any textbook on fluid dynamics or CFD. 2.2.1. Mass and Species Transport Mass conservation can be expressed either in terms of the density, ρ [19]: . ∂ρ + ∇·ρu = m000 b ∂t

(1)

where ρ is the density (kg/m3 ), and u is the velocity field (m/s). In terms of the individual gaseous specie, Yα : . . ∂ (2) (ρYα ) + ∇·ρYα u = ∇·ρDα ∇Yα + m000 α + m000 b,α ∂t Dα is the diffusion coefficient of α component of the mixture (m2 /s). 2.2.2. Momentum Transport The momentum equation in conservative form is written [19]: ∂ (ρu) + ∇·ρuu + ∇ p = ρg + fb + ∇·τij ∂t

(3)

where fb is the force term (Pa/m). The stress tensor, τij (Pa), is defined [19]:

τij

2 = µ 2Sij − δij (∇·u) 3

(

; δij =

1 0

1 i = j ; Sij = 2 i 6= j

∂u j ∂ui + ∂x j ∂xi

! i, j = 1, 2, 3

(4)

The term Sij is the symmetric rate-of-strain tensor, written using conventional tensor notation. The symbol µ is the dynamic viscosity of the fluid. The overall computation can either be treated as a DNS, in which the dissipative terms are computed directly, or as a LES, in which the large-scale


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eddies are computed directly and the subgrid-scale dissipative processes are modeled. The numerical algorithm is designed so that LES becomes DNS as the grid is refined. Most applications of FDS are LES. For example, in simulating the flow of smoke through a large, multi-room enclosure, it is not possible to resolve the combustion and transport processes directly. However, for small-scale combustion experiments, it is possible to compute the transport and combustion processes directly. 2.2.3. LES The most distinguishing feature of any CFD model is its treatment of turbulence. Of the three main techniques of simulating turbulence, FDS contains only LES and DNS. There is no Reynolds-averaged Navier-Stokes (RANS) capability in FDS. LES is a technique used to model the dissipative processes (viscosity, thermal conductivity, material diffusivity) that occur at length scales smaller than those that are explicitly resolved on the numerical grid. This means that the parameters µ, k and D in the equations above cannot be used directly in most practical simulations. They must be replaced by surrogate expressions that “model” their impact on the approximate form of the governing equations. This section contains a simple explanation of how these terms are modeled in FDS. There is a small term in the energy equation known as the dissipation rate, ε (Pa/s) the rate at which kinetic energy is converted to thermal energy by viscosity [19]:

ε = τij ·∇u = µ 2Sij Sij −

∂v ∂x

+

∂u ∂y

2

+

∂w ∂y

+

∂v ∂z

2

2 2 3 (∇· u )

+

∂u ∂z

+

∂w ∂x

2 2 2 ∂v ∂w = µ 2 ∂u + 2 + 2 + ∂x ∂y ∂z 2

−

2 3

∂u ∂x

+

∂v ∂y

+

∂w ∂z

2

(5)

Following the analysis of Smagorinsky, the viscosity µ is modeled: µ LES = ρ(Cs ∆)

2

2 2Sij : Sij − (∇·u)2 3

1 2

(6)

where Cs is an empirical constant and ∆ is a length on the order of the size of a grid cell. The bar above the various quantities denotes that these are the resolved values, meaning that they are computed from the numerical solution sampled on a coarse grid (relative to DNS). The other diffusive parameters, the thermal conductivity and material diffusivity, are related to the turbulent viscosity by [16]: k LES =

µ LES c p µ ; (ρD )t,LES = LES Prt Sct

(7)

The turbulent Prandtl number Prt and the turbulent Schmidt number Sct are assumed to be constant for a given scenario. The model for the viscosity, µ LES , serves two roles: Firstly, it provides a stabilizing effect in the numerical algorithm, damping out numerical instabilities as they arise in the flow field, especially where vorticity is generated. Second, it has the appropriate mathematical form to describe the dissipation of kinetic energy from the flow. 2.2.4. Energy Transport The energy conservation equation is written in terms of the sensible enthalpy, hs (J/kg) [19]: . . . ∂ Dp + q000 − q000 b − ∇·q00 + ε (ρhs ) + ∇·(ρhs u) = ∂t Dt

(8)

The sensible enthalpy (J/kg) is a function of the temperature [19]: hs =

∑ Yα hs,α α

(9)


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where sensible heat of each component in the mixture is calculated by: where sensible heat of each component in the mixture is calculated by: ZT T

T 0  dT 0 hs,h s,α T(T) =c p, cTp,α  dT T0

(10)

T0

.

(10)

is theper HRR unit volume from a chemical (W/m is the energy qb energy q  HRR q000 is the q000 b ),is the wherewhere unitper volume from a chemical reactionreaction (W/m3 ), transferred . 00 3 3  represents ) and qthe theand conductive and radiative to the evaporating to thetransferred evaporating liquid (W/m ) liquid and q (W/m represents conductive radiative heat fluxes heat (W/m2 ): fluxes (W/m2):

.

.

s,a a a q  = kT -  h∑ a  Da Ya + qr s,a

q00 = k∇ T −

h ρD ∇Y + q00 r

a

3

(11) (11)

2.2.5. Equation of State 2.2.5. Equation of State

The pressure is calculated by using the ideal gas equation of state (the burning occurs at Thepressure pressure which is calculated byless using thethe ideal gas pressure equation of stateair): (the burning occurs at atmospheric is much than critical of the atmospheric pressure which is much less than the critical pressure of the air):

ρRT p =RT W p=

W

(12)

(12)

where p is the pressure in (Pa), T is the temperature in (K), R is the universal gas constant and W is the where p is the pressure in (Pa), T is the temperature in (K), R is the universal gas constant and W molecular mass of the gaseous mixture in (J/mol). is the molecular mass of the gaseous mixture in (J/mol).

2.2.6. FDS Modelling of the Ethanol Burner

2.2.6. FDS Modelling of the Ethanol Burner

The geometry of the ethanol burner inFigure Figure2.2. The geometry of the ethanol burnermodel modelis is shown shown in

Figure Thegeometry geometry of Figure 2.2.The of the theethanol ethanolburner. burner.

At bottom the bottom of the burner, ethanolisisinjected injected and and ignited. ethanol burning in still air isair is At the of the burner, ethanol ignited.The The ethanol burning in still controlled by buoyancy (see Figure 3) [22]. controlled by buoyancy (see Figure 3) [22].


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Figure 3. The physics of liquid fuel burning [22].

Figure 3. The physics of liquid fuel burning [22].

It was assumed that the heat of combustion of ethanol is 26,780 (kJ/kg) [23]. The thermal

It was assumed that the heat of combustion of ethanol is 26,780 (kJ/kg) [23]. The thermal conductivity of the liquid ethanol is 0.17 (W/(m·K)). The specific heat of the liquid ethanol is 2.45 conductivity of the liquid ethanol is 0.17 (W/(m·K)). The specific heat of the liquid ethanol is (kJ/(kg·K)) and its density is 787 (kg/m3) [24]. There are three reasons that lead to justify this 3 2.45 (kJ/(kg · K)) assumption: and its density is 787 (kg/m ) [24]. There are three reasons that lead to justify this assumption: (1)

(2) (3)

(1) The thermal diffusivity of the ethanol is low (the ratio between the thermal conductivity and the capacity multiplied by the density). Therefore, thebetween heat frontthe penetrates a short and The heat thermal diffusivity of the ethanol is low (the ratio thermalwithin conductivity distance insidemultiplied the ethanol by pool. the heat capacity the density). Therefore, the heat front penetrates within a short (2) The radiative heat flux is pool. attenuated inside the liquid phase. distance inside the ethanol (3) The internal flow inside the liquid can be neglected (natural convection). Therefore, the The radiative heat flux is attenuated inside the liquid phase. convective heat transfer inside the ethanol liquid pool can be neglected.

The internal flow inside the liquid can be neglected (natural convection). Therefore, the convective The FDS simulation is based on the lower heating values of the ethanol. This model is flexible. heat transfer inside the ethanol liquid pool can be neglected.

There is an option to increase the heat release rate (HRR) by using fuel with higher heating value (suchFDS as gasoline or diesel). The simulation is based on the lower heating values of the ethanol. This model is flexible.

There is an option to increase the heat release rate (HRR) by using fuel with higher heating value (such 2.3. Multiphysics Analysis of the Reformer Model as gasoline or diesel). The second part deals with numerical analysis of the reformer. Figure 4 shows the geometry of

2.3. Multiphysics this system. Analysis of the Reformer Model The second part deals with numerical analysis of the reformer. Figure 4 shows the geometry of this system.


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Steel Tube

Catalytic bed– H2O + C2H5OH

Figure 4.of3D of the reformer. Figure 4. 3D plot theplot reformer.

TheThe reformer is made of catalyst bed bed material and steel tube. tube. It hasItbeen assumed that the reformer is made of catalyst material and steel has been assumed that the radius radius of the reformer is 0.2 m. The height of the reformer is 0.5 m. The thickness of the steel (HK-40 of the reformer is 0.2 m. The height of the reformer is 0.5 m. The thickness of the steel (HK-40 alloy) alloy) tube is 0.01 m (10 mm). The reformationchemical chemical reactions reactions occur bed tube is 0.01 m (10 mm). The reformation occurininaaporous porouscatalytic catalytic bed where the where the heat is supplied through the ethanol burner to drive the endothermal reaction system. The heat is supplied through the ethanol burner to drive the endothermal reaction system. The reactor is reactor is enclosed with steel tube. Ethanol and steam are mixed in stoichiometric amounts and enter enclosed with steel tube. Ethanol and steam are mixed in stoichiometric amounts and enter through through the inlet of the catalytic reactor. COMSOL multi-physics software (version 4.3b) has been the inlet of the catalytic reactor. COMSOL multi-physics software (version 4.3b) has been applied applied in this work. It solved simultaneously the fluid flow, heat transfer, diffusion with chemical in thiskinetics work.equations It solvedand simultaneously the fluid flow, heat transfer, diffusion with chemical reaction reaction structural analysis.

kinetics equations and structural analysis. 2.3.1. Model KineticsReformer Bed

2.3.1. Model Kinetics—Reformer Bed

Inside the reformer, steam and ethanol react to form hydrogen and carbon dioxide:

Inside the reformer, steam and ethanol react to form hydrogen and carbon dioxide:

C2 H5OH + 3H 2O  2CO2 + 6H 2

(13)

C H OH + 3H O → 2CO + 6H

(13)

2 2 to [8]: The rate constant of the ESR reaction2 is 5temperature 2dependent according

The rate constant of the ESR reaction dependent according to [8]:  isEtemperature 

k =Aexp     Rg T  E k = A exp −

(14)

(14)

Rg T

where A (frequency factor) is 7 × 10 (1/s) and E (activation energy) is 65.8 (kJ/mol) [15]. 5

where A (frequency factor) is 7 × 105 (1/s) and E (activation energy) is 65.8 (kJ/mol) [15].

2.3.2. Fluid FlowReformer Bed

The flow gaseous species through 2.3.2. FluidofFlow—Reformer Bed the reformer bed is described by Darcy’s Law:

The flow of gaseous species through bed is described by Darcy’s Law:    the reformer

      psr   =0      κ

∇· ρ − ∇ psr η

(15)

= 0

(15)


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Here, ρ denotes the gas density (kg/m3 ), η is the viscosity (Pa·s) and κ is the permeability of the porous medium (m2 ) and psr is the pressure inside the reformer bed (Pa). All other boundaries are impervious, corresponding to the condition: k − ∇ psr ·n = 0 η

(16)

2.3.3. Energy Transport—Reformer Bed The energy transport equation is used to describe the average temperature distribution in the porous bed: ∂Tsr ρC p + ∇·(−k sr ∇ Tsr ) + ρC p u·∇ Tsr = Q (17) ∂t The volumetric heat capacity of the bed is given by: ρC p

t

= ε ρC p

f

+ (1 − ε) ρC p

s

(18)

In the above equations, the indices “f” and “s” denote fluid and solid phases, respectively, and ε is the volume fraction of the fluid phase. Furthermore, Tsr is the temperature (K) and k sr is the thermal conductivity of the reformer bed w/(m·K). Q represents a thermal source (w/m3 ), and u is the fluid velocity field (m/s). Assuming that the porous medium is homogeneous and isotropic, the steady-state equation becomes: ∇·(−k sr ∇ Tsr ) + ρC p u·∇ Tsr = Q (19) The volumetric heat source due to the reaction is calculated: Q = ∆Hr ·r

(20)

where r represents the reaction rate. The steam reformation of ethanol is endothermic, with an enthalpy of reaction of ∆Hr = 1.74 × 105 J/mol. Equation (19) also accounts for the conductive heat transfer in the steel tube. As no reactions occur in this domain, the description reduces to:

∇·(−k steel ∇ Tsr ) = 0

(21)

where k steel is the thermal conductivity of the steel. The temperature of the gas is 700 K at the inlet. At the outlet, it is assumed that convective heat transport is dominant: n·(−k sr ∇ Tsr ) = 0 The thermal properties of the reformer are presented at Table 1 [9]. Table 1. Thermal properties of the reformer [10]. Material Property

Value

rho Cp k

3960 (kg/m3 ) 880 (J/(kg·◦ C)) 33 (w/(m·◦ C))

(22)


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2.3.4. Mass Transport—Reformer Bed The mass-balance equations for the model are the Maxwell-Stefan diffusion and convection equations at steady state: n

∇· ρωi u − ρωi

∑

j=1

Dij

∇p ∇xj + xj − ωj p

−

∇T DiT T

!

= Ri

(23)

Here ρ denotes the density (kg/m3 ), ωi is the mass fraction of species i, x j is the molar fraction of species j, Dij is ij component of the multicomponent Fick diffusivity (m2 /s), DiT denotes the generalized thermal diffusion coefficient (kg/(m·s)), T is the temperature (K), and Ri is the reaction rate (kg/(m3 ·s)). 2.3.5. Calculation of the Binary Diffusion Coefficients—Reformer Bed The diffusion coefficients of the components in the mixture, Dij are obtained from the Chapman-Enskog theory [25]: r T3 Dij = 5.9543·10−4

1 Mi

+

1 Mj

(24)

pσij2 Ωij

In which σij and Ωij are the Lennard-Jones collision diameter and collision integral between one molecule of i and one molecule of j respectively. The collision diameter is obtained from σij = 1/2 σi + σj and Ωij is taken from [26]: Ωij =

1.03587 1.76474 1.06036 0.19300 + + + 2 exp 0.47635τij exp 1.52996τij exp 3.89411τij τij

(25)

In which τij = kT/ε ij is the dimensionless temperature, where k is Boltzmann constant and √ ε ij = ε i ε j is the maximum attractive energy between one molecule of i and one molecule of k. The values of σi and ε i for the substances used in this work are given in Table 2 [27]. Table 2. Properties of the gaseous species [27]. Lennard-Jones Collision Diameter

σi (Angstroms)

Ethanol H2 O CH4 CO CO2 H2

4.455 2.655 3.882 3.590 3.996 2.968

Maximum Attractive Energy between Two molecules

σi /k B (K)

Ethanol H2 O CH4 CO CO2 H2

391.0 363.0 136.5 110.3 190.0 33.3

The diffusion coefficients as a function of the temperature are shown in Figure 5.


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The diffusion coefficients as a function of the temperature are shown in Figure 5.

Figure 5. The binary diffusion coefficients of the main species as a function of the temperature.

Figure 5. The binary diffusion coefficients of the main species as a function of the temperature.

This figure clearly shows that the spreading (diffusion) gaseous species inside the reformer

This figure clearly shows that the spreading (diffusion) gaseous species inside the reformer increases with the temperature. increases with the temperature. 2.3.6. Structural Analysis—Reformer Bed and Steel Tube

2.3.6. Structural Analysis—Reformer Bed and Steel Tube It was assumed that the tube is made of Steel HK-40. HK alloy, known as HK 40, is an austenitic

It was assumed that tube is made heat of Steel HK-40. HK alloy, as HK 40,temperature is an austenitic Fe-Cr-Ni alloy that hasthe been a standard resistant material. Withknown moderately high Fe-Cr-Ni alloy that has been a standard heat resistant material. With moderately high strength, oxidation resistance, the alloy is used in a wide variety of industrial applications temperature such as: strength, oxidation resistance, the alloyreformers; is used in a widepyrolysis variety of industrial applications such as: Ammonia, methanol and hydrogen ethylene coils and fittings; steam super heater methanol tubes and fittings [28]. Thereformers; thermo-physical and pyrolysis thermomechanical of the steel are Ammonia, and hydrogen ethylene coils andproperties fittings; steam super heater in Table[28]. 3 [10,29]. tubeslisted and fittings The thermo-physical and thermomechanical properties of the steel are listed in Table 3 [10,29]. Table 3. Thermo-physical and thermomechanical properties of steel HK-40 [10,27].

Table 3. Thermo-physical and thermomechanical properties of steel HK-40 [10,27]. Material Property Value E 186 × 109 (Pa) Material Property Value nu 0.3 3) rho 7720 E 186 ×(kg/m 109 (Pa) −6 (1/°C) alpha 17.64 × 10 nu 0.3 Cp 502 (J/(kg·°C)) rho 7720 (kg/m3 ) k 29.58× (w/(m·°C)) alpha 17.64 10−6 (1/◦ C) Cp 502 (J/(kg·◦ C)) k 29.58 (w/(m·◦ C)) 3. Results

This section divided into two parts. In Section 3.1 the thermal results of Fire Dynamics

3. Results Simulation (FDS) software are shown. In Section 3.2 the COMSOL multi-physics results for the reformer are presented.

This section divided into two parts. In Section 3.1 the thermal results of Fire Dynamics Simulation (FDS)3.1. software are shown. Section 3.2 the COMSOL multi-physics results for the reformer Fire Dynamics Simulator In Software Results are presented. One advantage of FDS simulation is that it can provide much detailed information on the ethanol burner, including the Results local and transient gas velocity, gas temperature, species 3.1. Fire Dynamics Simulator Software

One advantage of FDS simulation is that it can provide much detailed information on the ethanol burner, including the local and transient gas velocity, gas temperature, species concentration, solid wall temperature, fuel burning rate, radiative heat flux, convective heat flux and HRR. The temperature field at t = 77.5 s is shown in Figure 6.

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concentration, ChemEngineering 2018, solid 2, 34 wall temperature, fuel burning rate, radiative heat flux, convective heat flux and13 of 24 concentration, solid wall temperature, fuel burning rate, radiative heat flux, convective heat flux and HRR. The temperature field at t = 77.5 s is shown in Figure 6. HRR. The temperature field at t = 77.5 s is shown in Figure 6.

◦ C) inside Figure Temperaturefield field ((°C) at tat= t77.5 s. s. Figure 6. 6. Temperature insidethe theburner burner = 77.5 Figure 6. Temperature field (°C) inside the burner at t = 77.5 s.

◦ The The maximal temperature at t = 77.5 ss approaches approaches to 815 °C. velocity field of flue gases is The The maximal temperature 815 The velocity of gases flue gases is maximal temperatureatatt t==77.5 77.5 s approaches toto815 °C. C. The velocity fieldfield of flue is shown in Figure 7. shown in Figure 7. 7. shown in Figure

Figure 7. Velocity field (m/s) of the flue gases at t = 77.5 s.

Figure Velocityfield field(m/s) (m/s) of gases at tat= t77.5 s. s. Figure 7. 7.Velocity ofthe theflue flue gases = 77.5

Figure 8 shows the radiation heat flux emitted by the burner.

Figure 8 shows radiationheat heatflux flux emitted emitted by Figure 8 shows thethe radiation bythe theburner. burner.


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Figure 8. Radiation heat flux produced by the ethanol burner.

Figure 8. Radiation heat flux produced by the ethanol burner.

Figure 8 shows that the maximal radiation heat flux is 100,000 (W/m2). It should be noted that

8. Radiation heat fluxheat produced ethanol burner.2 ). It should be noted that Figure 8 shows that Figure the maximal radiation flux by is the 100,000 (W/m the FDS simulation is based on the lower heating values of the ethanol. This model is flexible. There the FDS is basedthe onHRR the by lower heating values of heating the ethanol. This model is flexible. There is 2). It as is ansimulation option 8toshows increase using fuel with higher value (such gasoline or diesel). Figure that the maximal radiation heat flux is 100,000 (W/m should be noted that an option to simulation increase the HRR bythe using fuel with higherofheating (such asisgasoline orflux diesel). As see later, HHR the ethanol canvalue provide the necessary heatThere the we FDSshall is the based on produced lower by heating values burner, the ethanol. This model flexible. As we shall see later, the HHR produced by the ethanol burner, can provide the necessary heat required for maintaining the reforming process. is an option to increase the HRR by using fuel with higher heating value (such as gasoline or diesel). flux required forshall maintaining theHHR reforming process. As we see later, the produced by the ethanol burner, can provide the necessary heat flux

3.2. Multi-Physics Analysisthe of the Operation of the Ethanol Steam Reformer required for maintaining reforming process.

3.2. Multi-Physics Analysis of the Operation of the Ethanol Steam Reformer

A parametric study has been performed in order to analyze the influence of the heat flux on the 3.2. Multi-Physics Analysis of the Operation of the Ethanol Steam Reformer

reformer performance andbeen its structural integrity. numerical for heat flux of flux 95,300 A parametric study has performed in orderThe to analyze theresults influence of the heat on the 2) are presented in Section 3.2.1. The numerical results for heat flux of 200,000 (W/m2) are (W/m 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 for heat flux of 95,300 (W/m2 ) presented in Section 3.2.2. reformer performance and The its structural Theheat numerical heat 2flux 95,300 in are presented in Section 3.2.1. numericalintegrity. results for flux of results 200,000for (W/m ) areofpresented 2) are presented in Section 3.2.1. The numerical results for heat flux of 200,000 (W/m2) are (W/m Section 3.2.2. 3.2.1. Multiphysics Results for Low Heat Flux Input presented in Section 3.2.2.

Figure 9 shows the 3D field Input inside the reformer. 3.2.1. Multiphysics Results fortemperature Low Heat Flux 3.2.1. Multiphysics Results for Low Heat Flux Input

Figure 9 shows the 3D temperature field inside the reformer. Figure 9 shows the 3D temperature field inside the reformer.

Figure 9. 3D plot of the reformer temperature field. Figure 9. 3D plot of the reformer temperature field.

Figure 9. 3D plot of the reformer temperature field.

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As can be seen Figure 9, the temperature at the bottom section of reformer is lower ChemEngineering 2018,from 2,from x FOR PEER REVIEW 15 of than 24 than As can be seen Figure 9, the temperature at the bottom section of reformer is lower the temperature at the upper side. The temperature of the steel is much higher than the temperature the temperature at the upper side. The temperature of the steel is much higher than the temperature As can be seen from Figure 9, thereasons: temperature at thethe bottom section of reactions reformer is lower than of of thethe catalyst. because two Firstly, endothermic theand heat, catalyst.This This is is because ofoftwo reasons: Firstly, the endothermic reactions absorbabsorb the heat, the temperature at the upper side. The temperature of the steel is much higher than the temperature and secondly, the catalyst is much thicker than the steel tube. Figure 10 shows the mass fractions of secondly, the catalyst is much thicker than the steel tube. Figure 10 shows the mass fractions of thethe of the catalyst. This is because of two reasons: Firstly, the endothermic reactions absorb the ◦ heat, and species (ethanol, CO H22O) reformer axis axisfor forinlet inlettemperature temperatureofof600 600 2 and species (ethanol, CO 2 and H O) along along the the reformer °C.C. secondly, the catalyst is much thicker than the steel tube. Figure 10 shows the mass fractions of the species (ethanol, CO2 and H2O) along the reformer axis for inlet temperature of 600 °C.

Figure Mass fractions the species (ethanol, CO and 2O) along the reformer axis. Figure 10. Mass fractions thespecies species(ethanol, (ethanol, CO 2O) along thethe reformer axis.axis. Figure 10.10. Mass fractions ofofof the CO222and andHHH along reformer 2 O)

The mass fractions ofofethanol the and decay along the reformer reformer axis. Theethanol ethanol The mass fractions the ethanol ethanol and steam steam along the axis. The The mass fractions of the and steam decay decay along the reformer axis. The ethanol conversion conversion is 80.3%. The ethanol and the steam decays at the same slope, and according to Equation conversion is 80.3%. The ethanol and the steam decays at the same slope, and according to Equation is 80.3%. The ethanol and the steam decays at the same slope, and according to Equation (13) the (13)(13) thethe of and steam to each each other. Similar values have been in amounts ofethanol ethanol andthe theproportional steam are are proportional proportional to other. Similar values have been amounts ofamounts ethanol and the steam are to each other. Similar values have been reported reported in in Reference seen that that the theHRR HRRproduced producedbybythethe ethanol reported Reference[4]. [4].From Fromthis this figure figure itit can can be seen ethanol Reference [4]. From this figure it can be seen that the HRR produced by the ethanol burner, can provide burner, providethe therequired requiredheat heatflux flux for for maintaining maintaining the 1111 shows burner, cancan provide the reforming reformingprocess. process.Figure Figure shows the required heat flux for maintaining the reforming process. Figure 11 shows the mass fractions of the mass fractions theethanol ethanolalong alongthe thereformer reformer axis for temperatures of 400 °C, 500 °C and thethe mass fractions ofofthe for inlet inlet temperatures of 400 °C, 500 °C and ethanol along the reformer axis for inlet temperatures of 400 ◦ C, 500 ◦ C and 600 ◦ C. 600600 °C.°C.

Figure 11. Mass fractions of the ethanol along the reformer axis for various inlet temperatures.

Figure 11. Mass fractions of the ethanol along the reformer axis for various inlet temperatures. Figure 11. Mass fractions of the ethanol along the reformer axis for various inlet temperatures.


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this figure it can be seen that for inlet temperature of 500 ethanol conversion is FromFrom this figure it can be seen that for inlet temperature of 500 ◦ C,°C, thethe ethanol conversion is 70.0%. 70.0%. For inlet temperature of 400 °C, the ethanol conversion is 43.8%. Thus, the ethanol conversion ◦ From this figure it can that conversion for inlet temperature 500 the °C, the ethanol conversion is For inlet temperature of 400 C, be theseen ethanol is 43.8%. of Thus, ethanol conversion increases increases with the inlet temperature. Figure 12 shows the mass fractions of hydrogen along the 70.0%. For inlet temperature of 400 °C, the ethanol conversion is 43.8%. Thus, the ethanol conversion with the inlet temperature. Figure 12 shows the mass fractions of hydrogen along the reformer axis. reformer axis. increases with the inlet temperature. Figure 12 shows the mass fractions of hydrogen along the reformer axis.

Figure 12. Mass fractions of the H2 along reformer axis.

Figure 12. Mass fractions of the H2 along reformer axis. Figure 12. Mass fractions of the H2 along reformer axis.

This figure clearly shows a considerable increasing of hydrogen mass fraction along the

This figure clearly shows a in considerable increasing mass along the reformer reformer axis. The increase Hydrogen mass fractionofishydrogen 89.4%. The sumfraction of hydrogen, ethanol, This figure clearly shows a considerable increasing of hydrogen mass fraction along the axis. carbon The increase inand Hydrogen mass fraction is 89.4%. The sum is of1hydrogen, carbon dioxide, dioxide, steam mass fraction at the reformer output as expectedethanol, according the mass reformer axis. The increase in Hydrogen mass fraction is 89.4%. The sum of hydrogen, ethanol, and steam mass fraction reformerstress output is 1 as expected according mass conservation law. conservation law. Theat 3Dthe Von-Mises distribution of reformer is shownthe in Figure 13. carbon dioxide, and steam mass fraction at the reformer output is 1 as expected according the mass The 3D Von-Mises stress distribution of reformer is shown in Figure 13. conservation law. The 3D Von-Mises stress distribution of reformer is shown in Figure 13.

Figure 13. 3D plot of the reformer Von-Mises stress field. Figure 13. 3D plot of the reformer Von-Mises stress field.

As expected, the Figure maximal is located at the reformer head corner. The Von-Mises thermal 13.stress 3D plot of the reformer Von-Mises stress field. stresses which is developed inside the steel are much higher than the Von-Mises stresses which are As expected, the maximal stress is located at the reformer head corner. The Von-Mises thermal developed inside the catalyst material. This is because the temperatures inside the catalyst material stresses which the is developed steel areat much higher than the Von-Mises stresses which thermal are As expected, maximalinside stressthe is located the reformer head corner. The Von-Mises developed the catalyst material. Thisare is because the temperatures inside the catalyst material stresses which inside is developed inside the steel much higher than the Von-Mises stresses which are

developed inside the catalyst material. This is because the temperatures inside the catalyst material are much lower than the temperature in the steel tube (Since the reforming reaction is endothermic, it


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are more much heat lowerand thancools the temperature in thematerial). steel tube (Since the reforming reaction is maximal endothermic, absorbs of the reformer According to this figure, the stress in ChemEngineering 2018, 2, x FOR PEER REVIEW 17 of 24 it absorbs more heattoand cools ofvalue the reformer material). According to this figure, the maximal stress the steel tube reached maximal of 1.3 GPa. in the steel tube reached to maximal value of 1.3 GPa. are much lower than the temperature in the steel tube (Since the reforming reaction is endothermic,

3.2.2.itMultiphysics Results High Heat Flux material). Input absorbs more heat and for cools of the reformer According to this figure, the maximal stress 3.2.2. Multiphysics Results for High Heat Flux Input

in thenumerical steel tube reached value 1.3 GPa. The results to formaximal heat flux of of 200,000 (W/m2 ) are presented in this section. Figure 14 The numerical results for heat flux of 200,000 (W/m2) are presented in this section. Figure 14 shows the 3D temperature field inside the reformer. 3.2.2. Results field for High Heat Input showsMultiphysics the 3D temperature inside theFlux reformer.

The numerical results for heat flux of 200,000 (W/m2) are presented in this section. Figure 14 shows the 3D temperature field inside the reformer.



The maximal temperature obtained in the steel jacket for the case of 200,000 (W/m2) is about 652 Figure 14. 3D plot of the reformer temperature field. °C. Figure 15 shows the mass fractions ofin thethe species COthe 2 and H2O) the reformer The maximal temperature obtained steel(ethanol, jacket for case ofalong 200,000 (W/m2 )axis. is about

652 ◦ C. Figure 15 shows the mass fractions of the species CO2(W/m and2)H O) along The maximal temperature obtained in the steel jacket for the(ethanol, case of 200,000 is2about 652 the °C. Figure reformer axis. 15 shows the mass fractions of the species (ethanol, CO2 and H2O) along the reformer axis.

Figure 15. Mass fractions of the species (ethanol, CO2 and H2O) along the reformer axis.

It can be seen from Figures 10 and 15 that the mass fractions of ethanol and steam decay with Figure 15. Mass fractions of the species (ethanol, CO2 and H2O) along the reformer axis. increasing heat ethanol about 85.4%. Figure shows mass fractions of Figure 15.flux. MassThe fractions of conversion the species is (ethanol, CO2 and H2 O)16along thethe reformer axis. hydrogen along the reformer axis. It can be seen from Figures 10 and 15 that the mass fractions of ethanol and steam decay with It can be seen Figures 10 conversion and 15 that mass fractions and steam decay increasing heat from flux. The ethanol is the about 85.4%. Figure of 16 ethanol shows the mass fractions of with increasing heat flux.the The ethanol conversion is about 85.4%. Figure 16 shows the mass fractions of hydrogen along reformer axis.

hydrogen along the reformer axis.


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Figure 16. Mass fractions of the H2 along the reformer axis. Figure 16. Mass fractions of the H2 along the reformer axis.

Figure 16. Mass fractions of the H2 along the reformer axis.

Figure 16 indicates that the increase in hydrogen mass fraction is 89.9%. The hydrogen mass

Figure 16 indicates that thetheincrease in hydrogen massfraction fractionis is 89.9%. The hydrogen Figure 16 indicates that increase hydrogen distribution mass 89.9%. massmass fraction is increased. Figure 17 shows the in temperature along the z The axis hydrogen of the reformer fraction is increased. Figure 17 shows the temperature distribution along the z axis of the reformer fraction is increased. Figure 17 shows the temperature distribution along the z axis of the reformer for for the two cases. the two cases. for the two cases.

Figure 17. Temperature distribution along the z axis of the reformer. Figure 17. Temperature distribution along the z axis of the reformer. Figure 17. Temperature distribution along the z axis of the reformer. It can be seen from Figure 17 that the temperature increases with the increasing heat flux. As a It can bethe seen from Figure 17fraction that theincreases temperature increases with the increasing flux. As a result hydrogen mass (the reforming increases). The heat 3D Von-Mises It can of bethat seen from Figure 17 that the temperature increasesrate with the increasing heat flux. As a result of that the hydrogen mass fraction increases (the reforming rate increases). The 3D Von-Mises distribution of reformer shown in Figure 18. resultstress of that the hydrogen massisfraction increases (the reforming rate increases). The 3D Von-Mises stress distribution of reformer is shown in Figure 18.

stress distribution of reformer is shown in Figure 18.


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Figure 18. 3D plot of the reformer Von-Mises stress field.

Figure 18.18.3D Von-Misesstress stress field. Figure 3Dplot plotof of the the reformer reformer Von-Mises field.

Figure 19 shows the Von-Mises distribution along the z axis of the steel tube for the two cases.

Figure 19 shows thethe Von-Mises distribution the zzaxis axisofofthe thesteel steel tube cases. Figure 19 shows Von-Mises distributionalong along the tube forfor the the twotwo cases.

Figure 19. Von-Mises distribution along the z axis of the steel tube. Figure 19. Von-Mises distribution along the z axis of the steel tube.

Figure 19. Von-Mises distribution along the z axis of the steel tube. Figure 19 indicates that Von-Mises stresses increases with the heat flux. The effect of increasing Figure 19 indicates that Von-Mises stresses increases withand heat flux.the The effect of increasing the heat fluxes increases the temperature inside the steel tube enhances thermal stresses. The Figure 19 indicates that Von-Mises stresses increases withthe the heat flux. The effect of increasing the heat fluxes increases the temperature inside the steel tube and enhances the thermal stresses. The thermal stresses which are developed inside the steel tube are much higher than those which were the heat fluxes increases the temperature inside the steel tube and enhances the thermal stresses. thermal are developed inside the steel tubeheat are much higher than were obtainedstresses in the which previous section. The increase of the flux enhances a those little which the ethanol The thermal stresses which are developed inside the steel tube are much higher than those which obtained in but the also previous section. The increase the heat inside flux enhances a little theeffect ethanol conversion, increases the thermal stressesofdeveloped the steel tube. This can wereconversion, obtained in previous Thestresses increase of the heat flux little the can ethanol butthe also increasessection. the thermal developed inside theenhances steel tube. aThis effect

conversion, but also increases the thermal stresses developed inside 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


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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 has standard heat resistant materialmaterial with moderately high temperature strength, and oxidation thatbeen has abeen a standard heat resistant with moderately high temperature strength, and resistance. Figure 20 shows the effect of temperature increasing on the tensile ultimate strength of HK oxidation resistance. Figure 20 shows the effect of temperature increasing on the tensile ultimate 40 alloy and 31040 stainless steels strength of HK alloy and 310[30]. stainless steels [30]. According maximal stress in the steel tubetube reached to maximal valuevalue of 482ofMPa AccordingtotoFigure Figure20, 20,the the maximal stress in the steel reached to maximal 482 (70 ksi). From Figures 19 and 20 it can be seen that the thermal stresses developed for low heat flux are MPa (70 ksi). From Figures 19 and 20 it can be seen that the thermal stresses developed for low heat much lower than thethan ultimate tensile strength of the HK-40 to Figure 20, the 20, Vonthe Mises flux are much lower the ultimate tensile strength of the (According HK-40 (According to Figure Von stresses developed in the middle section of the tube are less than 450 MPa). Mises stresses developed in the middle section of the tube are less than 450 MPa). From From Figure Figure 20, 20, itit can can be be also also seen seen that that the the decrease decrease in inthe theultimate ultimate tensile tensile strength strength is is less less than than ◦ C or 1112 ◦ F). Since ethanol steam reforming occurs at 10% 10%(The (Thetemperature temperatureofofthe thesteel steelisisabout about600 600 °C or 1112 °F). Since ethanol steam reforming occurs relatively lower temperatures compared with other fuels, thefuels, decrease the strength of at relatively lower temperatures compared withhydrocarbon other hydrocarbon the indecrease in the the Steel tube is low. theTherefore, structural integrity of the integrity HK-40 steel tubeHK-40 is kept.steel Figure 21isshows strength of the SteelTherefore, tube is low. the structural of the tube kept. the effect temperature stress rupture of HK 40-2 steel alloy [30]. Figure 21of shows the effect of temperature stress rupture of HK 40-2 steel alloy [30]. Figure indicates that under normalnormal conditions of the ethanol reforming temperature Figure2121clearly clearly indicates that under conditions of the ethanol(the reforming (the ◦ ◦ of the steel tube is about 600 is C about or 1112 increases considerably. For this case the temperature of the steel tube 600F), °Cthe or rupture 1112 °F),time the rupture time increases considerably. For rupture time is greater than 100,000 h (more than 11.4 years). this case the rupture time is greater than 100,000 h (more than 11.4 years).

Figure20. 20.The Theeffects effectsofofthe thetemperature temperatureon onthe theultimate ultimate tensile tensile strength strength of Figure of HK HK 40 40 and and 310 310 steels steels [28]. [28].

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Figure 21. The effects of the temperature on the rupture time of HK 40-2 alloy [30]. Figure 21. The effects of the temperature on the rupture time of HK 40-2 alloy [30].

4. Discussion 4. Discussion Hydrogen could be a promising fuel, and if often considered as a clean energy carrier as it only

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, emitssuch water during combustion. It canasberaw produced different of renewable as ethanol. The use of ethanol materialfrom presents severalkinds advantages because feedstocks, it is a such renewable as ethanol. The use of to ethanol as raw material presents several contains advantages feedstock, easy transport, biodegradable, has low toxicity, highbecause hydrogenit is a renewable feedstock, transport, biodegradable, has low toxicity, contains high hydrogen content, content, and easyeasy to to store and handle. Ethanol steam reformation occurs at relatively lower and easy to storecompared and handle. steam reformation occurs at relatively temperatures with Ethanol other hydrocarbon fuels and has been widely studiedlower due totemperatures the high yield provided forhydrocarbon the formation fuels of hydrogen. this reformation reaction, ethanol compared with other and hasInbeen widely studied due water to theand high yield react provided over a catalystof bed to produce mixture of hydrogen rich gas. A new has been developed in for the formation hydrogen. Inathis reformation reaction, water andtool ethanol react over a catalyst this work in order to analyze the reformer operation. The proposed computational work is bed to produce a mixture of hydrogen rich gas. A new tool has been developed in this work in composed of two phasessimulation of ethanol burner by using FDS software and multiphysics order to analyze the reformer operation. The proposed computational work is composed of two simulation of steam reforming process occurring inside the reformer. This software calculates the phases—simulation of ethanol burner by using FDS software and multiphysics simulation of steam temperature, density, pressure, velocity, and chemical composition within each numerical grid cell reforming the the reformer. This heat software calculates the temperature, density, at eachprocess discrete occurring time step. Itinside computes temperature, flux, and mass loss rate. There are three pressure, velocity, and chemical composition within each numerical grid cell at each discrete major components of the model: Hydrodynamic, combustion, and radiation models. FDS code is time step. formulated It computes the on temperature, heat flux, mass Theresolution are three components based CFD of fire-driven fluidand flow. Theloss FDSrate. numerical canmajor be carried out of theusing model: Hydrodynamic, anda radiation models. FDS codemodel. is formulated based on either a DNS methodcombustion, LES. FDS uses mixture fraction combustion The mixture is a conserved scalarThe quantity is defined as the fraction gas at out a given point in the CFD fraction of fire-driven fluid flow. FDS that numerical solution can be of carried using either a DNS flow field that originated as fuel. The model assumes that combustion is mixing controlled, and that method LES. FDS uses a mixture fraction combustion model. The mixture fraction is a conserved reactionthat of fuel and oxygen is infinitely Radiative heat transfer included in thethat model via scalarthe quantity is defined as the fractionfast. of gas at a given point inisthe flow field originated the solution of the radiation transport equation for a non-scattering gray gas. The radiation equation as fuel. The model assumes that combustion is mixing controlled, and that the reaction of fuel and is solved using a technique similar to a finite volume method for convective transport, thus the name oxygen is infinitely fast. Radiative heat transfer is included in the model via the solution of the given to it is the FVM. One advantage of FDS simulation is that it can provide much detailed radiation transport equation for a non-scattering gas.gas Thevelocity, radiation is solved information on the fire, including the local and gray transient gasequation temperature, speciesusing a technique similarsolid to awall finite volume method for convective thustransfer, the name given to it is concentration, temperature, composite burning rate,transport, radiation heat convection the FVM. One advantage FDSbeen simulation is that thatthe it can provide much detailed information on the heat transfer and HRR. of It has found out maximal temperature at t = 77.5 s approaches to 815 °C. The of the radiation heat fluxgas is 95,300 (w/m2). species concentration, solid wall fire, including the magnitude local and transient gas velocity, temperature, The composite second partburning deals with of theconvection reformer. The chemical temperature, rate,numerical radiation analysis heat transfer, heatreformation transfer and HRR. It has ◦ reactions occur in a porous catalytic bed where the heat is supplied through the Ethanol burner been found out that the maximal temperature at t = 77.5 s approaches to 815 C. The magnitudetoof the driveheat the endothermal radiation flux is 95,300reaction (w/m2 ).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 The second part deals with numerical analysis of the reformer. The reformation chemical reactions reactor. The COMSOL multi-physics software has been applied in this work. It solved occur in a porous catalytic bed where the heat is supplied through the Ethanol burner to drive the simultaneously the fluid flow, Heat transfer, diffusion with chemical reaction kinetics equations and 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


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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). 5. Conclusions A new tool has been developed in this work in order to analyze the burner and steam reformer operation. This model may be implemented for simulating other reforming systems (such as methanol, and diesel). Future work will focus on performing coupled heat transfer, chemical reactions, and creep analyses on the reformer. Funding: This research received no external funding. Conflicts of Interest: The author declares no conflict of interest. No funding sponsors had any role in the numerical analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.


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References 1.

2.

3. 4. 5. 6. 7.

8. 9.

10. 11. 12.

13. 14. 15.

16.

17. 18. 19.

20. 21. 22.

Poran, A.; Tartakovsky, L. Performance and emissions of a direct injection internal combustion engine devised for joint operation with a high-pressure thermochemical recuperation system. Energy 2017, 124, 214–226. [CrossRef] Akande, A.; Aboudheir, A.; Idem, R.; Dalai, A. Kinetic modeling of hydrogen production by the catalytic reforming of crude ethanol over a co-precipitated Ni-Al2 O3 catalyst in a packed bed tubular reactor. Int. J. Hydrogen Energy 2006, 31, 1707–1715. [CrossRef] Marino, F.; Boveri, M.; Baronetti, G.; Laborde, M. Hydrogen production from steam reforming of bioethanol using Cu/Ni/K/γ-Al2 O3 catalysts. Effect of Ni. Int. J. Hydrogen Energy 2001, 26, 665–668. [CrossRef] Bineli, A.R.; Tasi´c, M.B. Catalytic Steam Reforming of Ethanol for Hydrogen Production: Brief Status. Chem. Ind. Chem. Eng. Q. 2016, 22, 327–332. [CrossRef] Akande, A.J. Production of Hydrogen by Reforming of Crude Ethanol. Master’s Thesis, University of Saskatchewan, Saskatoon, SK, Canada, 2005. Athanasios, N.F.; Kondaridesm, D.I. Production of hydrogen for fuel cells by reformation of biomass-derived ethanol. Catal. Today 2002, 75, 145–155. Klouz, V.; Fierro, V.; Denton, P.; Katz, H.; Lisse, J.P.; Bouvot-mauduit, S.; Mirodatos, C. Ethanol reforming for hydrogen production in a hybrid electric vehicle: Process optimization. J. Power Sources 2002, 105, 26–34. [CrossRef] Uriz, I.; Arzamendi, G.; López, E.; Llorca, J.; Gandía, L.M. Computational fluid dynamics simulation of ethanol steam reforming in catalytic wall micro channels. Chem. Eng. J. 2011, 167, 603–609. [CrossRef] Uriz, I.; Arzamendi, G.; Diéguez, P.M.; Gandía, L.M. Computational fluid dynamics as a tool for designing hydrogen energy technologies. In Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety; Elsevier: Oxford, UK, 2013; Chapter 17; pp. 401–435. Lao, L.; Aguirre, A.; Tran, A.; Wu, Z.; Durand, H.; Christofides, P.D. CFD modeling and control of a steam methane reforming reactor. Chem. Eng. Sci. 2016, 148, 78–92. [CrossRef] Lewis, B.; Von Elbe, G. Combustion Flames and Explosion of Gases, 2nd ed.; Academic Press Inc.: New York, NY, USA; London, UK, 1961. Diéguez, P.M.; López-San, M.J.; Idareta, I.; Uriz, I.; Arzamendi, G.G.; Luis, M. Hydrogen Hazards and Risks Analysis through CFD Simulations. In Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety; Elsevier: Oxford, UK, 2013; Chapter 18; pp. 437–452. Tartakosky, L.; Sheintoch, M. Fuel Reforming in Internal Combustion Engines. Prog. Energy Combust. Sci. 2018, 67, 88–114. [CrossRef] European Industrial Gases Association AISBL. Combustion Safety for Steam Reformer Operation; IGC Doc 172/12/E; European Industrial Gases Association AISBL: Brussels, Belgium, 2012. Olafadehan, O.A.; Ayoola, A.A.; Akintunde, O.O.; Adeniyi, V.O. Mechanistic Kinetic Models for Steam Reforming of Concentrated Crude Ethanol on Ni/Al2O3 Catalyst. J. Eng. Sci. Technol. 2015, 10, 633–653. [CrossRef] Loock, S.; Ernst, W.S.; Thomsen, S.G.; Jensen, M.F. Improving Carbon Efficiency in an Auto-Thermal Methane Reforming Plant with Gas Heated Heat Exchange Reforming technology. In Proceedings of the 7th World Congress of Chemical Engineering, Glasgow, UK, 10–14 July 2005. Carlsson, M. Carbon Formation in Steam Reforming and Effect of Potassium Promotion. Johnson Matthey Technol. Rev. 2015, 59, 313–318. [CrossRef] Heywood, J.B. Internal Combustion Engine Fundamentals; McGraw-Hill Book Company: New York, NY, USA, 1988. McGrattan, K. Fire Dynamics Simulator (Version 5)—Technical Reference Guide Volume 1: Mathematical Model; NIST Special Publication 1018; National Institute of Standards and Technology U.S. Department of Commerce: Gaithersburg, MD, USA, 2010. McGrattan, K.; Forney, G.P. Fire Dynamics Simulator (Version 5)—User’s Guide; NIST Special Publication 1019; National Institute of Standards and Technology U.S. Department of Commerce: Gaithersburg, MD, USA, 2010. McGrattan, K. Numerical Simulation of the Caldecott Tunnel Fire, April 1982; NISTIR 7231; National Institute of Standards and Technology U.S. Department of Commerce: Gaithersburg, MD, USA, 2005. Hu, L. A review of physics and correlations of pool fire behavior in wind and future challenges. Fire Saf. J. 2017, 91, 41–55. [CrossRef]


23. 24. 25. 26. 27. 28. 29. 30.

24 of 24

Hamins, A.; Kashiwagi, T.; Bruch, R.R.; Toten, G.E. Fire Resistance of Industrial Fluids, ASTM STP 1284; Toten, G.E., Reichel, J.E., Eds.; American Society for Testing and Materials: Philadelphia, PA, USA, 1995. Thermo-Physical Properties of Ethanol. Available online: https://www.thermalfluidscentral.org/ encyclopedia/index.php/Thermophysical_Properties:_Ethanol (accessed on 8 June 2018). De-Souza, Z.G.M.; Moraes, F.F. Parametric Study of Hydrogen Production from Ethanol Steam Reforming in Membrane Micro-reactor. Braz. J. Chem. Eng. 2013, 30, 355–367. [CrossRef] Neufeld, P.D.; Jansen, A.R.; Aziz, R.A. Empirical equations to calculate 16 of the transport collision integrals Ω(l,s)* for the Lennard-Jones (1216–) potential. J. Chem. Phys. 1972, 57, 1100–1102. [CrossRef] Perry, R.H.; Green, D.W.; Maloney, J.O. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw Hill: New York, NY, USA, 1997. Kubota Metal Corporation. Fahramet Division, Alloy Data Sheet, Heat Resistant Alloy HK40; Kubota Metal Corporation: Orillia, ON, Canada, April 1991. Steel Founders’ Society of America. Steel Casting Handbook; Supplement 9, High Alloy Data Sheets Heat Series; Steel Founders’ Society of America: Crystal Lake, IL, USA, 2004. Van Echo, J.A.; Roach, D.B.; Hall, A.M. Short Time Tensile and Long-Time Creep Rupture Properties of the HK-40 Alloy and Type 310 Stainless Steel at Temperature to 2000 ◦ C. ASME J. Basic Eng. Trans. 1967, 89, 465–478. [CrossRef] © 2018 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).