How to Model Fuel Reformers with Simulation

Addressing the Evolving Needs of Powertrain Engineering Through Simulation

As the powertrain market begins to pivot from traditional diesel and gasoline engines towards hydrogen engines and fuel cells, there is the open question of how to provide hydrogen to these new powertrains. In the short term, it seems that converting an available hydrocarbon fuel to hydrogen will be needed. For mobile applications, methanol and ethanol as well as compressed natural gas are logical options.  For stationary applications, using the natural gas supply infrastructure makes sense.

Gamma Technologies has created three example models of fuel reformers for methanol, ethanol, and methane in GT-SUITE activated with the GT-xCHEM product license.  These example models help support research and development of H₂ combustion engines and fuel cells, as well as expansion into general chemical processing. The results of each of these new fuel reformer example models are summarized in this blog.

Note that in the methanol and methane reformer sections, the X axis of the figures is the catalyst material load divided by the molar flow rate of the key reactant, W/Fm, which is sometimes referred to as the contact time.  A low W/Fm value represents high flow (short residence time), and a high W/Fm represents low flow (long residence time).

Methanol Reformer Model

The first model we’re simulating demonstrates a methanol steam reformer reactor.  Methanol (CH₃OH) and water (H₂O) react over a CuO/ZnO/Al₂O₃ catalyst to form H₂ and CO₂. The reaction mechanism, input data, and measurement data for the reformer are from the reference Purnama et al¹.

In this specific reaction mechanism, the methanol steam reforming reaction (reaction 1) is modeled in the forward direction only. The water gas shift (WGS) and reversible WGS are modeled as two separate reactions (reactions 2 and 3).  At high temperature, the H₂ and CO₂ can react through the reverse WGS reaction to form CO and H₂O.

Reaction 1: CH₃OH + H₂O → CO₂ + 3H₂

Reaction 2: H₂ + CO₂ → CO + H₂O

Reaction 3: CO + H₂O → H₂ + CO₂

This example model is designed to recreate several figures (Figures 1 and 2) from Purnama et al¹.  Methanol and water are supplied to a packed bed reactor at a 1:1 molar ratio. Four temperatures: 230, 250, 270, and 300°C are simulated, and the catalyst load to molar flow ratio W/Fm is varied from 0.0001 to 0.03 kg_cat-s/mmol_CH3OH. The result is a good correlation for both the overall methanol conversion efficiency and the prediction of the products including hydrogen as shown in the figures below.

Figure 1. Simulation results of methanol conversion efficiency vs. W/Fm for four temperatures: 230, 250, 270, 300°C

Figure 2. Simulation results of product concentrations vs. W/Fm at 230°C

Ethanol Reformer Model

The next model to simulate is the ethanol reformer.   Three reactions were used to model the ethanol steam reforming process to produce H₂ over an Rh-Pd/CeO₂ catalyst.  Reactions 2 and 3 are modeled as reversible reactions.

Reaction 1: C₂H₅OH → CH₄ + H₂ + CO

Reaction 2: CO + H₂O ↔ CO₂ + H₂

Reaction 3: CH₄ + H₂O ↔ 3H₂ + CO₂

Simulations were run with an operating pressure of 4.5 bar, a steam-to-carbon ratio of 3, and an operating temperature of 500 to 1000 K. The results are shown in the figures below along with the measured data from Lopez et al².

Figure 3. Ethanol conversion and H2 yield vs. temperature & product species molar flow rate vs. temperature

In Figure 3 above shows that between 500 and 700 K the ethanol conversion rises steadily from near zero to 100%.  However, not all ethanol is converted directly into H₂.  As a result, the H₂ yield does not follow the same pattern as the ethanol.  Reaction 3 is the steam methane reforming (SMR) reaction, which begins after 700 K and causes a distinct slope shift in H₂ production as more H₂ is produced from methane.

Regarding the species molar flow rate, the bottom figure shows that the CH₄ exhibits a unimodal-shaped curve with regard to operating temperature, culminating at 700 K, when ethanol breakdown reaches 100%.  Increased temperature maintains CH₄ generation via ethanol decomposition and activates SMR to produce additional H₂.  This results in a drop in CH₄ molar flow rate.  The SMR reaction also accounts for increased CO generation after 700 K, resulting in a bimodal-shaped CO curve, with the first mode being from the ethanol breakdown process combined with the water gas shift reaction (WGS).  WGS causes an increase in the molar flow rate of CO₂ following the first CO peak.  All of these patterns are well captured by the model.

Methane Reformer Model

The methane reformer uses a chemical process called steam methane reforming (SMR) to convert methane into hydrogen gas.  The methane reformer model can be used to study the effect of temperature and pressure on the efficiency of the reformer.  The reaction mechanism used in the model is shown below.  All three reactions are modeled as reversible in this reaction mechanism.

Reaction 1: CH₄ + H₂O ↔ CO + 3H₂

Reaction 2: CO + H₂O ↔ CO₂ + H₂

Reaction 3: CH₄ + 2H₂O ↔ CO₂ + 4H₂

At higher temperatures (700-1100 K), these reactions proceed in the forward direction resulting in conversion of methane (CH₄) and water (H₂O) into carbon dioxide (CO₂) and hydrogen (H₂).  If the operating temperature is reduced (450 – 650 K), the mechanism runs in the backward direction producing CH₄ and H₂O from the reaction of CO₂ and H₂, also known as the methanation process.

The SMR model is made from information found in the reference Xu and Froment³ for a Ni/MgAl₂O­_4­ catalyst in a packed bed reactor.  In the model, the inlet feed contains H₂O, CH₄, and H₂ in the molar ratio 3:1.25:1, and the temperature is varied from 773 K to 848 K. For each temperature case, the methane contact time (W/F_CH4) is varied from 0.01 to 0.425 g_cat-hr/mol_CH4.

In Figure 4, shown below, the GT-xCHEM simulation results of the conversion of CH₄ and production of CO₂ and H₂ are plotted along with the experimental results reported by Xu and Froment³.  The GT-xCHEM simulation results correlate well with the experimental data as the conversion efficiency of the reformer increases with increasing contact time and increasing operating temperature.

Figure 4. Comparison of simulation results of conversion of CH4 and production of CO2 and H2 with experimental data

Learn More About Our Chemical Systems Modeling Solutions

In this blog we presented three fuel reforming example models available in GT-xCHEM.  These example models help support research and development in H₂ combustion engines and fuel cells, as well as expansion into general chemical processing.  Gamma Technologies will continue to add to the library of ready-to-use catalyst and reactor models available in the installation directory of GT-SUITE activated with the GT-xCHEM product license.  You may need to get the newest build update to see them, or if you have an older version or build then you can request the models from [email protected].  If you have any questions and would like more information about fuel reforming modeling with GT-SUITE please contact us here.

References

1. “CO formation/selectivity for steam reforming of methanol with a commercial CuO/ZnO/Al2O3 catalyst,” Purnama, H., Ressler, T., Jentoft, R.E., Soerijanto, H., Schlögl, R., Schomäcker, R., 2004, Applied Catalysis A: General, v259, 83-94. https://doi.org/10.1016/j.apcata.2003.09.013
2. “Ethanol steam reforming for hydrogen generation over structured catalysts,” López, E., Divins, N. J., Anzola, A., Schbib, S., Borio, D., & Llorca, J, 2013, International Journal of Hydrogen Energy, 38(11), 4418–4428. https://doi.org/10.1016/j.ijhydene.2013.01.174
3. “Methane Steam Reforming, Methanation and Water-Gas Shift: I. Intrinsic Kinetics,” J. Xu, G.F. Froment, 1989, AIChE J., 35 (1), 88-96. https://doi.org/10.1002/aic.690350109