2D heat and mass transfer modeling of methane steam reforming for hydrogen production in a compact reformer

TL;DR: In this paper, a 2D heat and mass transfer model is developed to investigate the fundamental transport phenomenon and chemical reaction kinetics in a compact reformer for hydrogen production by methane steam reforming (MSR).

About: This article is published in Energy Conversion and Management.The article was published on 2013-01-01 and is currently open access. It has received 51 citations till now. The article focuses on the topics: Methane reformer & Steam reforming.

Summary

1. Introduction

• Hydrogen is an ideal energy carrier to support sustainable energy development [1].
• In the long term, hydrogen can be produced in a clean way by solar thermochemical water splitting, photocatalytic water splitting or water electrolysis driven by solar cells/wind turbines [2,3].
• In MSR reaction (Eq.1), methane molecules react with steam molecules to produce hydrogen and carbon monoxide in the catalyst layer of reformers.
• It’s still not very clear how the change in inlet temperature and rate of heat supply can influence the coupled transport and reaction kinetics in the reformer, which are important for optimization of the reformer operation conditions.
• As the present study do not consider the carbon deposition behavior in the reformer, a constant SCR of 2.0 is adopted.

2. Model development

• Heat from the combustion duct is supplied to the Ni-based (i.e. [10]) catalyst layer via the solid thin film layer and it is specified as a boundary condition [6].
• Without considering the 3D effect, the coupled transport and chemical reaction phenomena in the computational domain can be shown in Figure 2, including the solid plate, the reforming duct, and the porous catalyst layer.
• The chemical model is developed to calculate the rates of chemical reactions and corresponding reaction heats.
• The CFD model is used to simulate the heat and mass transfer phenomena in the CR.

2.1 Chemical model

• In operation, methane-containing gas mixture (CH4: 33%; H2O: 67%) is supplied to the reforming duct.
• The gas species are then transported from the gas duct into the porous catalyst layer, where MSR reaction (Eq. 1) and WGSR (Eq. 2) take place.
• The formulas proposed by Haberman and Young [11] have been widely used for simulating the rates (mol.m-3.s-1) of MSR ( MSRR ) and WGSR ( WGSRR ), thus is adopted in the present study.
• The amount of heat generation from WGSR and heat consumption by MSR reaction can be calculated using corresponding enthalpy changes [12].
• Assuming linear dependence on operating temperature between 600K and 1200K, the reaction heats (J.mol-1) for MSR reaction and WGSR can be calculated as [13].

2.2. Computational Fluid Dynamics (CFD) model

• Assuming local thermal equilibrium in the porous catalyst layer, the governing equations for mass conservation, momentum conservation, and energy conservation for the whole computational domain are summarized below [14].
• The Darcy’s law (Eq.26 and 27) is used as source terms in momentum equations (Eqs. (13) and (14)), so that the momentum equations are applicable for both the gas channels and the porous catalyst layers.
• The source term in energy equation (Eq. (15)) represents reaction heat from the chemical reactions can be calculated by Eq. (28).
• Detailed descriptions of the source terms can be found in the previous publications [17].

2.3 Numerical scheme

• The governing equations in the CFD model are solved with the finite volume method (FVM) [14].
• As a real reformer stack consists of many identical single compact reformers, it is assumed that heat is supplied from the combustion channel (Fig. 1) and there is no heat transfer between compact reformers through the upper boundary (y=yM).
• The convection terms and diffusion terms are treated with the upwind difference scheme and central difference scheme, respectively.
• The velocity and pressure are linked with the SIMPLEC algorithm.
• Computation is repeated until convergence is achieved.

3. Results and discussions

• The chemical model and CFD model have been validated in the previous publications by comparing the modeling results with data from the literature [17].
• The dimensions and typical simulation parameters are summarized in Table 2.
• The following sections focus on parametric simulations to analyze the effects of operating and structural parameters on the coupled transport and reaction kinetics in CR.

3.1 Coupled transport and reaction in a compact reformer for hydrogen production

• Figure 3 shows the distributions of MSR reaction rates, WGSR rates, temperature, velocity, gas composition (CH4 and H2 as examples) in the compact reformer at an inlet temperature of 1073K, inlet gas velocity of 3m.s-1, and heat supply rate (from the solid plate) of 1kW.m-2.
• The reaction rates for MSR and WGSR are the highest (25.4 and 14 mol.m-3.s-1 respectively) at the inlet and decrease considerably in the downstream of the reformer (Fig. 3a and 3b).
• In addition, the temperature is the highest at the inlet (Fig. 3c).
• A locally low molar fraction of CH4 is also observed near the inlet in the catalyst layer (Fig. 3e).
• For comparison, the molar fraction of H2 increases along the CR gas flow stream (Fig. 3f).

3.2. Effect of inlet temperature

• The reaction rates of MSR and WGSR are found to decrease along the main flow stream (Fig. 4a and 4b), but their values are significantly higher than those at 1073K (Fig. 3a and 3b).
• In addition, the reaction rates decrease more rapidly in the reformer than at 1073K.
• The high reaction rate of MSR causes the temperature to decrease rapidly along the main flow stream from 1173K at the inlet to about 1040K at the outlet (Fig. 4c).
• As the reaction rates of MSR and WGSR are higher at 1173K than at 1073K, more CH4 is consumed and more H2 is produced, leading to larger gas composition variation in the reformer (Fig. 4d and 4e).
• In a word, increasing the inlet temperature increases the reaction rates, temperature gradient, and gas composition variation.

3.4. Effect of inlet gas velocity and microstructure of the catalyst layer

• It’s found that the reaction rates of MSR and WGSR are the highest at the inlet but decrease considerably along the reformer, due to large temperature drop along the main flow stream.
• Three-dimensional simulation of chemically reacting gas flows in the porous support structure of an integrated-planar solid oxide fuel cell, Int. J. Heat Mass Transfer 47(2004) 3617-3629. [12].
• Parameters used in calculating the effective diffusion coefficients [16].

Figures

Table 1. Parameters used in calculating the effective diffusion coefficients [16]

Table 2 Parameters used in simulation.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "2d heat and mass transfer modeling of methane steam reforming for hydrogen production in a compact reformer" ?

In this study, a 2D heat and mass transfer model is developed to investigate the fundamental transport phenomenon and chemical reaction kinetics in a CR for hydrogen production by methane steam reforming ( MSR ). 

Q2. What are the future works in "2d heat and mass transfer modeling of methane steam reforming for hydrogen production in a compact reformer" ?

The effects of SCR and the catalyst nature on CR performance are not included but will be considered in future works.