1. Introduction

Energy efficiency improvement is one of society's most important instruments for mitigating climate change. Transportation is responsible for a large part of the energy consumption worldwide. According to International Energy Agency (IEA) data, about 26% of all energy-related CO₂ emissions were caused by transportation[1]. It is likely to account for a higher share in the future, unless special measures are taken. To limit the long-term global heating down to acceptable levels, United Nations Intergovernmental Panel on Climate Change (IPCC) recommended that annual global greenhouse gas (GHG) emissions must be reduced by 50 - 85% by year 2050 in comparison with the emissions level in 2000[2]. According to the IEA's BLUE map scenario[1], above 80% of the projected GHG emission mitigation may be achieved by improvement of vehicle efficiency, introduction of alternative fuels and electricity decarbonization.

Results of recent studies suggest that there is a big potential for improvement of internal combustion engine (ICE) technologies[3-6]. It is well known that about 30% of fuel energy introduced to ICE is wasted with engine exhaust gases[3]. Even its partial utilization could lead to a significant improvement of ICE energy efficiency.

One of the ways to recover an engine's waste heat is by using the energy of the exhaust gases to promote endothermic reactions of alcohol steam reforming – ASR[7, 8]. In principle, any renewable fuel may be used, not only alcohol. ICE is fed by the gaseous products of ASR, mainly mixture of hydrogen and carbon monoxide, frequently called syngas. The latter has, as a rule, greater heating value than the primary liquid fuel and may be more efficiently burned in the engine compared with the original fuel. This approach, called thermo-chemical recuperation (TCR), is considered nowadays as one of the possible methods of powertrain efficiency improvement and emissions reduction[7].

Many studies published on TCR are focused on gas turbine applications[9-11]. Ivanic et al.[12] studied a partial fuel reforming for ICE. Computational analysis of a spark ignition (SI) engine performance was carried outby Galloni and Minutillo[13] for partial gasoline replacement by a reformate gas. In their work the reformate gas was produced by exothermic partial oxidation of gasoline rather than bio-fuel.

Alcohol reforming to a syngas is widely investigated today as a promising hydrogen source for propulsion systems based on fuel cells[14-16]. The thermodynamics of methanol reforming has been extensively discussed in the literature[17-19]. The methanol steam reforming process may be described by three main reactions, namely, methanol decomposition (1), water gas shift -WGS (2) and direct methanol steam reforming (3):

(1)

(2)

(3)

Reactions (1) and (3) are endothermic, whereas WGS is exothermic. In overall, the process is endothermic; therefore, heat has to be supplied from an external source. Methanol steam reforming may be effectively performed over the commercially available methanol synthesis catalyst, alumina-supported Cu/ZnO, the kinetics of which is already well-studied. The Cu/ZnO catalyst has a good activity at moderate temperatures of 250-300 ^oC. More important, the Cu catalyst is not active for the methanation reaction (reverse reforming reaction) and a methane-free gas can be produced at low temperatures and at high pressures to yield full conversion to CO₂, CO and hydrogen. The optimum choice of operating conditions of Cu/Zn/Al catalysts[20], mainly employed in small hydrogen plants, is close to water-to-methanol ratio of 1.5 and temperature range of the 250- 300°C. The equilibrium conversion is above 99%. Other catalysts may be also considered. As copper catalysts are sensitive to air exposure during start-up of on-board reformers, there had been an interest in noble metal catalysts for the methanol reforming process. Noble metal catalysts also show better thermal stability and lower deactivation rates[21]. Research & development of reformers for methanol steam reforming benefit from the interest in fuel cells for automotive applications[22, 23]. Ethanol steam reforming is not as simple as the conversion of methanol, because ethanol is easily dehydrates to ethylene, which is a coke precursor. In addition, optimal conditions for ethanol steam reforming appear at high temperatures – around 1000-1100K[24], thus making problematic utilization of theengine's exhaust gases heat. On the other hand, ethanol ismuch less toxic compared with methanol.

Some efforts have been made to feed ICE with methanol reforming products[25-26]. The obtained results exposed a number of serious problems, mainly caused by the multi-regime nature of the ICE operation in a motor vehicle (cold startability, need to address momentary change of load etc.). The catalytic reformer, if used in the motor vehicle, should operate efficiently in a wide range of fuel flow rates and exhaust gas temperatures because of engine load and speed changes. Low initial temperatures during the engine cold start and warm-up result in a non-efficient operation of the reformer. The requirement of addressing any momentary change in engine load leads to a serious complication of the fuel supply and engine control systems. In order to enable satisfactory engine operation in the whole range of working regimes, different amounts of liquid methanol were added to the methanol reforming products in the work[25]. This brought up the problems typical for methanol fuel: higher aldehyde emissions, increased wear, poor cold startability etc. These problems remain unsolved, thus precluding further development of the reformer-ICE concept. We propose considering a hybrid propulsion system (Figure 1) that has an additional energy source and thus provides a basis for overcoming these drawbacks.

Figure 1. Hybrid propulsion system with a reformer-ICE

2. Methodology

In fuel cell applications dealing with alcohol steam reforming an effort is focused on achievement of maximal possible hydrogen outcome together with prevention of CO formation, which is a poison for the fuel cell catalyst. For these purposes, the overall reaction (3) is preferable, thus implying that the WGS reaction extent is very high. In contrast with the strict requirement of high-purity hydrogen typical for fuel cells application, ICE is much more flexible and can effectively burn different mixtures of hydrogen, carbon monoxide and other gases. This feature greatly reduces the cost of energy obtained from renewable fuels. In this case methanolreforming to CO₂ and H₂ is undesirable, because CO₂ is a diluent gas and does not carry energy. Therefore, for ICE fuelling, realization of the reaction (1) would be preferable with negligible WGS reaction extent. In ICE the exhaust heat is used to promote on-board reforming of methanol into a mixture of hydrogen and carbon monoxide with small amounts of diluents and contaminants (carbon dioxide, unreacted water, etc.). Because this gas has a greater heating value and may be more efficiently burned than the primary fuel, an improvement in engine fuel economy can be expected along with a sensible emissions reduction. CO formed in reaction (1) does not constitute an environmental hazard because it is further oxidized to CO₂ during the combustion in the engine. Therefore, for ICE application methanol reforming process has to be optimized for the maximal yield of hydrogen and CO together with prevention of contaminants formation.

One important parameter that should be used in reformer optimization is the energy efficiency of the reforming process[24] defined as a ratio of the added enthalpy of combustion (H_out - H_in) to the heat duty, H_d (the sum of the ASR reactions heat, latent heat of methanol vaporization and sensible heat):

(4)

where H_out and H_in are enthalpy of combustion of the reforming products and primary reactants, respectively. Calculations performed for the example of steam reforming of methanol (SRM) at H₂O/CH₃OH ratio = 1.3 and T=570K derives the following result: H_out = 22.3 MJ/kg, whereas H_in= 19.9 MJ/kg[34].

The lack of detailed kinetic data (reaction rates, residence time, known intermediate species, etc.) has determined a modelling of the steam reforming by using the equilibrium reactor and Gibbs reactor models of the CHEMCAD software package. We simulated a chemical reactor by solving the heat and mass balances and minimizing the free energy of the components that can be produced during a reforming process. The minimization of total Gibbs free energy is a suitable method to calculate the equilibrium compositions of any reacting system. The method of minimizing the Gibbs free energy is normally preferred in the fuel reforming analysis[27]. The total Gibbs free energy of a system is given by the sum of i species:

(5)

where:G_t is the total Gibbs free energy, the G⁰_i - standard

Gibbs free energy of species i, R - the molar gas constant, T - the temperature of the system, f_i - the fugacity in the system, f ⁰_i - the standard-state fugacity, and n_i - the mole number of species i. For reaction equilibrium in gas phase are assumed, where ∆G⁰_fi is the standard Gibbs free energy of formation of species i.

The reaction paths including reactions (1) – (3) as well as carbonation, dehydrogenation, olefine and formaldehyde synthesis, methanation and other reactionswere considered. For the analysis simplification the following main assumptions have been made:

• Chemical equilibrium in the reactor;

• Ideal heat exchanger;

• All gases are ideal.

The products of the reforming were calculated for a reactor with minimum Gibbs free energy, when all the possible products were taken into account under the following working conditions of the reactor: atmospheric pressure, water-to-methanol molar ratio 0.6-3 and temperature range of 150-950°C.

3. Results and Discussion

3.1. Equilibrium Simulation Results

The examples of the calculation resultsobtained with the assumption of chemical equilibrium are shown in Figure 2.

Figure 2. Methanol reforming products – equilibrium assumption

The obtained results showed 100% methanol conversion efficiency at any reforming temperature and steam to methanol ratios. The reforming products obtained from the calculations in sensible yields are hydrogen, carbon monoxide, carbon dioxide, methane, carbon and unreacted water. The higher the reforming temperature – the less water is remained and more hydrogen is yielded. The carbon (coke) yield decreases with temperature rise or with increase of thewater-to-methanol ratio.

3.2. Comparison with Available Experimental Data

The available experimental results[28-31] show that the actually measured methanol reforming products are usually composed of: hydrogen, carbon monoxide, water and carbon dioxide (Figure 3). Methane normally does not present between the reforming products. The experimental results also show that the methanol conversion is less than 100% under the temperatures lower than 300 ^oC (Figure 3). The methanol conversion increases with the temperature rise until full conversion is reached. Full conversion of methanol is usually observed at temperatures of 300-320 ^oC.

Figure 3. Methanol reforming products – experimental data.Reprinted from: Journal of Power Sources, Vol. 106, Agrell, J., Birgersson, H., Boutonnet, M., Steam reforming of methanol over a Cu/ZnO/Al2O3 catalyst: a kinetic analysis and strategies for suppression of CO formation, 249-257, Copyright (2002); Applied Catalysis A: General, Vol. 253, Agrell, J., Boutonnet, M., Fierro, J.L.G., Production of hydrogen from methanol overbinary Cu/ZnO catalysts. Part II: catalytic activity and reaction pathways, 213–223, Copyright (2003) with permission from Elsevier

The results obtained with the equilibrium model do not agree with the experimental data because this model considers an equilibrium state within an infinite reactor with full methanol conversion. The experimentally tested reformers are not infinite, and they usually do not reach equilibrium state. The measured data are normally obtained for catalytically supported reactions, whereas in the equilibrium theoretical calculations the presence of catalyst cannot be taken into account.

The theoretical analysis suggests that the methane appears in the reforming products at all steam-methanol ratios. Its yield decreases to zero with increasing the temperature. In the experiments, even at low temperatures, the methane does not appear among the reforming products. One possible explanation of this phenomenon may be in the fact that the real reformer does not reach the steady state as assumed in the theoretical model. This hypothesis is supported by the results of Jones et al[32], who showed that in the methanol steam reforming the appearance of methane in the reforming products is a function of time. Its yield was found to be increasing with a time. The available experimental results show that there is an influence of the catalyst material on the appearance of methane in the final reforming products. With a catalyst based on Cu the formation of methane is normally suppressed, whereas with other catalytic materials, such as Pt, Rh, Ru and Pd, formation of methane is possible.It is important to note that also carbon was not found in the reforming products with most of the catalysts, which somewhat simplifies the methanol reforming analysis and reactors development.

3.3. Empiric Correction of Equilibrium Predictions

To improve accuracy of the equilibrium predictions and account for the non-equilibrium reforming behaviour, empiric correction of the simulation results was applied. The empiric correction functions were developed using available experimental results for copper-based catalysts[28, 29]. These functions are suggested for the all products appearing in the experiments (CO, CO₂, H₂ and CH₃OH). Carbon and methane, which normally are not observed in the reforming products, wereexcluded from consideration. The proposed correction functions are shown in Table 1. Here indexes "cor" and "eq" relate to corrected and equilibrium molar fractions, respectively; CH₃OH^conv – methanol conversion ratio; T – reforming temperature, ^oC.

Table 1. Empiric Correction Functions

The empiric correction should be applied, if methanol conversion is below 100%. When full methanol conversion is reached, the equilibriummodel can be used without correction,because under these conditions it provides acceptable prediction accuracy.

To validate the suggested empiric correction functions, a comparison of the corrected simulation results with the experimental data[30, 33], which were not used in the correction functions development, was performed. An example of the comparison between the empirically corrected predictions and experimentally measured results is shown in Figure 4. As can be seen, reasonable agreement between the predictions and the experimentaldata was obtained. The developed correlations were not validated for catalyst types other than copper-based.

Figure 4. Comparison of the corrected prediction with experimental data from[30].- theoreticalprediction; ·······- experimental data H2O/CH3OH ratio = 1.3

Figures 5-8 show the prediction results after applying the empiric correction to the equilibrium simulation data. As can be seen, the predicted dependence of the methanol steam reforming products on steam-to-methanol ratio and reforming temperature is qualitatively similar to the available experimental data[30].

Figure 5. Methanol reforming products - corrected prediction. Water tomethanol ratio – 0.5:1

Figure 6. Methanol reforming products - corrected prediction. Water to methanol ratio – 1:1

Figure 7. Methanol reforming products - corrected prediction. Watertomethanol ratio – 1.3:1

Figure 8. Methanol reforming products - corrected prediction. Water tomethanol ratio – 3:1

3.4. Analysis of Simulation Results

Simulation results show that increase of the steam to methanol ratio leads to reduction of the H₂ and CO yields with an appropriate increase of CO₂ formation. Starting from a temperature of circa 300-350°C the methanol is totally converted. As the temperature further increases, the yields of H₂ and CO₂ reduced. This phenomenon is linked to the reverse water gas shift reaction. The reaction uses hydrogen to produce H₂O, and is thermodynamically dominant above 300-350 °C.

The prediction results show that maximum hydrogen production can be achieved at a relatively low temperature of approximately 300^oC. At the same temperatures full methanol conversion is taking place, as well. The moderate temperature of methanol reforming makes possible utilization of the ICE exhaust gases heat to sustain the endothermic reactions in the reformer.

Figure 9 shows the maximal energy efficiency values of methanol steam reforming as a function of the water/alcohol ratio. The required values of the reforming process temperature are shown near corresponding points of the plot. Simulation results showed that the energy efficiency of methanol steam reforming approached maximal value of 0.66 at water to methanol ratio of 1.3 and the reaction temperature of approximately 300^oC. For the all considered water to methanolratios (0.5 – 3.0) maximal energy efficiency of the reforming process was observed in the narrow range of temperatures 297-307^oC.

Comparison of energy efficiency values achievable for methanol steam reforming with those of ethanol[34] showsthat the difference between the maximal values of energy efficiency for these two fuels is not significant. However, for methanol it can be achieved under much lower temperature: 570K compared with 1100K for ethanol. This is a significant benefit of methanol over ethanol in the view of the TCR concept realization.A potential of ICE efficiency improvement and emissions reduction by application of the TCR concept is analysed in[34].

Figure 9. Energy efficiency of methanol reformingcalculated by eq.(4)

4. Conclusions

Modelling and analysis of the methanol steam reforming for the purpose of ICE feeding was performed. The results obtained with the equilibrium simulation model do not agree with the experimental data because this model considers an equilibrium state within an infinite reactor with full methanol conversion.An empiric correlationderived on the base of available experimental datais suggested to bring the simulation results closer to the actual findings. This approach can be useful for analysis of methanol steam reforming when detailed kinetic data are unavailable.

The simulation results show that the maximum hydrogen production can be achieved underthe reformingtemperature of approximately 300oC, when full methanol conversion takes place. The moderatetemperature of methanol reforming makes it possible to utilize the ICE exhaust gases heatfor sustaining the endothermic reactions in the reformer.

The difference between the maximal achievable values of energy efficiency for methanol and ethanol steam reforming is not significant (0.66 for methanol compared with 0.59 for ethanol). However, for methanol it can be achieved under much lower temperature. This is a significant benefit of methanol over ethanol in the view of the TCR concept realization. This concept can be realized most efficiently in a hybrid propulsion system, where cold start and transient operation problems can be successfully overcome due to availability of additional energy source.

ACKNOWLEDGEMENTS

Financial support of the Israel Science Foundation, Ministries of Energy & Water Resources and Environmental Protection is highly appreciated. Authors are grateful to Ms. Ron Karasenti and Mr.TamirBengio for their valuable help in calculations carrying out.