CFD SIMULATION OF ETHANOL STEAM REFORMING FOR MARINE STIRLING ENGINE

1. INTRODUCTION 

External combustion engines, like the Stirling engine, have benefits over their internal combustion counterparts due to their ability to remain completely sealed from the elements and their nearly silent operation. Stirling engines are inherently robust, quiet, and only require an external heat source for operation. These characteristics make these engines ideal candidates for military and commercial applications where clean, quiet power is desired [1]. The demand for hydrogen has increased recently due to progress in fuel cell technologies. Fuel cells are electro-chemical devices described as continuously operating batteries and are considered as a additional clean source of electric energy for marine vessel, containing high energy efficiency, and its resulting emission is just water [2]. Hydrogen can be produced by steam reforming of Ethanol. The use of ethanol presents several advantages, because it is a renewable feed-stock, easy to transport, biodegradable, has low toxicity, contains high hydrogen content, and easy to store and handle. Ethanol Steam Reforming (ESR) 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 burner. The radiative heat flux from burner is transferred to the catalytic bed reactor for transforming the ethanol steam mixture to hydrogen and carbon dioxide (see Figure 1).

Fig. 1: Heat transfer mechanisms in steam reformer [3].

Inside the reformer, steam and ethanol react form Hydrogen and Carbon Dioxide:

C2H5OH + 3H2O → 2CO2 + 6H2

The rate constant of the ESR reaction is temperature dependent (Arrhenius equation):

2. METHDOLOGY

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. The FDS has been developed at the Building and Fire Research Laboratory (BFRL) at the National Institutes of Standards and Technology (NIST), e.g., McGrattan et al. [4,5]. This software calculates simultaneously the temperature, density, pressure, velocity, and chemical composition within each numerical grid cell at each discrete time step. It also calculates the temperature, heat flux, and mass loss rate of the enclosed solid surfaces. The latter is used in the case where the fire heat release rate is unknown. The major components of this software are:

Hydrodynamic Model—FDS code is formulated based on CFD of turbulent 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. The coherent structures are described in [6]. The LES method is described in detail in [7]

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 [8].

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) [4]. FDS also has a visual post-processing image simulation program named “smoke-view”.

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.

3. RESULTS

This section divided into two parts. In Section 3.1 the thermal results of Fire Dynamics Simulation (FDS) software are shown. In Section 3.2 the COMSOL multi-physics results for the reformer are presented.

3.1 FDS Results

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 2.

Fig. 2: Temperature field (°C) inside the burner at t = 77.5 s.

3.2 COMSOL Results

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

Fig. 3: 3D plot of the reformer temperature field.

As can be seen from Figure 2, the temperature of the steel is much higher than the temperature of the catalyst. This is because of two reasons: Firstly, the endothermic reactions absorb the heat, and secondly, the catalyst is much thicker than the steel tube. 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. Figure 4 shows the mass fractions of the species (ethanol, CO2 and H2O) along the reformer axis for inlet temperature of 600°C.

Fig. 4: Mass fractions of the species (ethanol, CO2 and H2O) along the reformer axis.

The mass fractions of the ethanol and steam decay along the reformer axis. The ethanol conversion is 80.3%. The ethanol and the steam decays at the same slope. Similar values have been reported in Reference [9]. Figure 5 shows the mass fractions of hydrogen along the reformer axis.

Fig. 5: Mass fractions of the H2 along reformer axis.

This figure clearly shows 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. Safety issues concerning the structural integrity of the steel tubes are also addressed. 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 [10]. Therefore, hydrogen present safety concerns at limited ventilation conditions because of the danger of explosive mixture formation that may cause severe damage [11–13].

4. 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 for marine vessels (such as methanol, and diesel). Future work will focus on performing coupled heat transfer, chemical reactions, and creep analyses on the reformer.

5. FUTURE WORK

There are three major factors influencing the structural integrity of Steam Reforming tube. Three failures modes are discussed in this paper. These are: Thermal creep, High Temperature Hydrogen Attack (HTHA) and Hydrogen embrittlement.

REFERENCES

[1] PCI, https://www.precision-combustion.com/products/burners-oxidizers/stirling-engine-burner.

[2]    Marino, F.; Boveri, M.; Baronetti, G.; Laborde, M. (2001) Hydrogen production from steam reforming of bioethanol using Cu/Ni/K/γ-Al2O3 catalysts. Effect of Ni. Int. J. Hydrogen Energy 26, 665–668, doi:10.1016/S0360-3199(01)00002-7.

[3]  Carlsson, M. (2015) Carbon Formation in Steam Reforming and Effect of Potassium Promotion. Johnson Matthey Technol. Rev, 59, 313–318.

[4] McGrattan, K. Fire Dynamics Simulator (Version 5)—Technical Reference Guide Volume 1:Mathematical Model; NIST Special Publication 1018; 2010, National Institute of Standards and Technology U.S. Department of Commerce.

[5] McGrattan, K.; Forney, G.P. Fire Dynamics Simulator (Version 5)—User’s Guide; NIST Special Publication 1019; 2010, National Institute of Standards and Technology U.S. Department of Commerce.

[6] Kaganovich, D. The role of coherent turbulence structures in self sustained turbulence, Ph.d. dissertation submitted to Stanford university, 2017.

[7] Tomer A., Let’s LES, Tomer’s Blog – all about CFD, https://allaboutcfd-tomersblog.com/2017/12/12/lets-les-iv/ (accessed in 12/4/2020).

[8] McGrattan, K. Numerical Simulation of the Caldecott Tunnel Fire, April 1982; NISTIR 7231; 2005, National Institute of Standards and Technology U.S. Department of Commerce.

[9]  Bineli, A.R.; Tasić, MB. (2016) Catalytic Steam Reforming of Ethanol for Hydrogen Production: Brief Status. Chem. Ind. Chem. Eng. Q., 22, 327–332, doi:10.2298/ciceq160216017b.

[10]  Lewis, B.; Von Elbe, G. Combustion Flames and Explosion of Gases, 2nd ed.; Academic Press Inc.: New York, NY. USA; London, UK, 1961.

[11] Diéguez, P.M.; López-San, M.J.; Idareta, I.; Uriz, I.; Arzamendi, G.G.; Luis, M. (2013) Hydrogen Hazards and Risks Analysis through CFD Simulations. In Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety; Elsevier: Oxford, UK; Chapter 18, pp. 437–452.

[12]  Tartakosky, L.; Sheintoch, M. (2018) Fuel Reforming in Internal Combustion Engines. Prog. Energy Combust. Sci. 67, 88–114, doi:10.1016/j.pecs.2018.02.003.

[13]   European Industrial Gases Association AISBL. (2012) Combustion Safety for Steam Reformer Operation; IGC Doc 172/12/E, Brussels.

 

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