Thermodynamic Analysis of Hydrogen Generation from Methanol

Aug 22, 2013 - CH4 formation are limited, the thermodynamic data may be more agreeable with the ... Thermodynamic analysis of the system can help to...
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Thermodynamic Analysis of Hydrogen Generation from Methanol− Formic Acid−Steam Autothermal System Zebao Rui and Hongbing Ji* Department of Chemical Engineering, School of Chemistry and Chemical Engineering, The Key Lab of Low-Carbon Chem and Energy Conservation of Guangdong Province, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ABSTRACT: This work examines the H2 production from methanol (MeOH)−formic acid (FA)−steam (H2O) system (FSRM) by thermodynamic analysis using the Gibbs free energy minimization method. The compounds considered in FSRM are CH3OH, HCOOH, H2O, CO2, CO, H2, HCOOCH3, HCHO, and CH3OCH3 and together with or without CH4 and C (graphite). The addition of FA lowers the enthalpy of the system and favors the heat recycle. Thermal-neutral (TN) conditions are obtained, at which the heat released from exothermic reactions makes up exactly for the requirement of the endothermic reactions. For the case with consideration of CH4 and C formation, C and CH4 formation is thermodynamically dominated at a low temperature ( 400 °C with a concentration under ppm level. The yields of HCOOCH3 and CH3OCH3 are suppressed at essentially zero (mole fraction lower than 1.00 × 10−14). Thus, although HCOOCH3, HCHO, and CH3OCH3 are observed in some experimental reports,38−40 they are likely to exist as the intermediates and will not be studied in detail in the following parts. 3.2. Hydrogen Yield. The effects of H2O/MeOH and FA/ MeOH ratios on H2 yield at four temperatures (200, 400, 600, 800 °C) are plotted in Figure 2. At fixed H2O/MeOH and FA/ MeOH ratios, the yield of hydrogen increases with increasing temperature. For all the four temperatures, the hydrogen fraction decreases monotonically with the increase in FA/ MeOH ratio at a fixed H2O/MeOH ratio. At FA/MeOH = 0, the H2 concentration decreases with increasing H2O/MeOH ratio, and the decrement increases with increasing temperature. At FA/MeOH = 1−9 for 200 °C and 400 °C and FA/MeOH = 2−9 for 600 °C, the H2 concentration increases with increasing H2O/MeOH ratio at a fixed FA/MeOH ratio. While at 800 °C, the H2 concentration decreases with increasing H2O/MeOH ratio at a fixed FA/MeOH ratio. 3.3. Carbon Monoxide Yield. As is known, carbon monoxide is not a favored product in the hydrogen production for fuel cells, as it is a poison to the platinum cathode in the low-temperature proton exchange membrane fuel cell (PEMFC).28 Figure 3 reveals the equilibrium yield of carbon monoxide in CO mole fraction according to FA/MeOH and H2O/MeOH ratios at four different temperatures: 200, 400, 600, and 800 °C. The CO yield is close to 0.0 at 200 °C and increases quickly with the rise of the temperature at fixed FA/ MeOH and H2O/MeOH ratios. The CO mole fraction decreases with an increase in the H2O/MeOH ratio at a fixed FA/MeOH ratio. The influence of the FA/MeOH ratio on CO concentration is complex. At 200, 400, and 600 °C, the CO mole fraction increases with an increase in the FA/MeOH ratio

Figure 1. Plots of the composites equilibrium mole fraction as a function of temperature at H2O/MeOH ratio = 1 and FA/MeOH ratio = 1.

Besides the basic species set (CH3OH, HCOOH, H2O, CO2, CO, H2, CH4, and C), the products HCOOCH3, HCHO, and CH3OCH3 are also involved in this work. The equilibrium mole

Figure 2. Plots of thermodynamic equilibrium H2 yield as a function of H2O/MeOH and HCOOH/MeOH ratios at different temperatures: (a) 200 °C, (b) 400 °C, (c) 600 °C, and (d) 800 °C. 5451

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Figure 3. Plots of thermodynamic equilibrium CO yield as a function of H2O/MeOH and HCOOH/MeOH ratios at different temperatures: (a) 200 °C, (b) 400 °C, (c) 600 °C, and (d) 800 °C.

Figure 4. Plots of thermodynamic equilibrium CH4 yield as a function of H2O/MeOH and HCOOH/MeOH ratios at different temperatures: (a) 200 °C, (b) 400 °C, (c) 600 °C, and (d) 800 °C.

at a fixed H2O/MeOH ratio. At 800 °C, the equilibrium CO concentration increases with increasing the FA/MeOH ratio at H2O/MeOH = 1−3. While at 800 °C and H2O/MeOH = 0 or 0.5, the equilibrium CO mole fraction first increases with increasing FA/MeOH ratio, and then decreases slowly with a further increase in FA/MeOH ratio. 3.4. Methane Yield. Methane is also an unwanted product in this system, since its formation deprives hydrogen of H atoms, which will inevitably lower the hydrogen yield. The yield of methane

in CH4 mole fraction as a function of H2O/MeOH and FA/MeOH ratios at four different temperatures (200, 400, 600, 800 °C) is shown in Figure 4. It is clear that methane yield reduces quickly as the temperature increases at fixed H2O/MeOH and FA/MeOH ratios. The CH4 yield decreases with increasing FA/MeOH ratio at a fixed H2O/MeOH ratio, and decreases with increasing H2O/MeOH ratio at a fixed FA/MeOH ratio especially at a high temperature. 3.5. Coke Formation. Carbon deposition has been recognized as one of the major reasons for the rapid 5452

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Figure 5. Plots of thermodynamic equilibrium coke yield as a function of H2O/MeOH and HCOOH/MeOH ratios at different temperatures: (a) 200 °C, (b) 400 °C, (c) 600 °C, and (d) 800 °C.

Figure 6. Plots of the composites equilibrium mole yield or fraction without consideration of CH4 and carbon formation as a function of temperature at different feeding compositions (a) FA, (b) MeOH, and (c, d) H2O/MeOH/FA = 1.

and 200 °C, the coke mole fraction decreases with increasing FA/MeOH ratio at H2O/MeOH = 0, 0.5, 1 or 1.5, while increases with increasing FA/MeOH ratio at H2O/MeOH = 2 or 3. At FA/MeOH = 1−9 and 400, 600, and 800 °C, the coke mole fraction decreases with increasing FA/MeOH ratio at H2O/MeOH = 0 or 1, while increases with increasing FA/ MeOH ratio at H2O/MeOH = 1.5, 2, or 3. 3.6. Effect of Preassumed Composition. The calculated results aforementioned are consistent with those reported

deactivation of the reforming catalyst.43 Figure 5 depicts the potential of coke formation. It can be seen that carbon yield decreases with increasing temperature at fixed H2O/MeOH and FA/MeOH ratios, and H2O/MeOH ratio at a fixed FA/ MeOH ratio for all the four temperatures. The coke formation suppression by the increase in H2O/MeOH ratio is more significant at a low FA/MeOH ratio. Except for the point at FA/ MeOH = 0, the carbon yield decreases with an increase in FA/ MeOH ratio for all the four temperatures. At FA/MeOH = 1−9 5453

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the reaction, and actually for realizing the TN condition, reaction temperature is a function of the feed composition. Figure 7 plots the relationships between the reaction temperature

thermodynamic calculations for H2 production from MeOH that the formation of C and CH4 is more favorable at a low temperature.26−28 It is well-known that carbon deposition has been recognized as one of the major reasons for the rapid deactivation of organics decomposition or reforming catalyst, and CH4 is also an unwanted product in this system since its formation deprives hydrogen of H atoms, which will inevitably lower the hydrogen yield. Thus, those calculation results indicate that the favorable operation window for H2 production is at temperatures higher than 600 °C. However, most experimental research on the generation of H2 from FA decomposition has been conducted at temperature lower than 250 °C,12,18−20 and most experimental research on the generation of H2 from MeOH decomposition or SRM has been conducted at temperature lower than 400 °C.6,10,11 Since the inclusion of C and CH4 in the system led to little useful information concerning the practical H2 production especially at a low temperature, the calculation results without involving C and CH4 are presented in Figure 6. As shown in Figure 6a for the thermodynamic calculation results for FA decomposing system, the yield of H2 is almost the same with that of CO2, and it decreases with an increase in temperature. While the yield of CO is equal to that of H2O, which increases with increasing temperature over the temperature range 200− 400 °C. These calculated results are consistent with the reported experimental results.18,19 Tedsree et al.18 reported that Ag nanoparticles coated with a thin layer of Pd atoms could significantly enhance the production of H2 from FA at ambient temperature. A H2/CO2 ratio close to unit was obtained and no CO was detected when the reaction temperature is lower than 50 °C. While at elevated temperatures (>50 °C), CO generated from the FA decomposition.18,19 Figure 6b is the thermodynamic calculation results for MeOH decomposing system. As shown, the yields of H2 and CO increase with an increase in temperature with a H2/CO mole ratio close to 2. While the yields of H2O and CO2 are close to zero over the temperature range studied. These calculated results are also consistent with the reported experimental results. Imamura et al.10 reported that Pt/CeO2 decomposed MeOH completely at 230 °C with 99.2% and 94.6% of selectivities to H2 and CO, respectively. Considering the equilibrium calculation in this work and the reported experimental results together, we can see that the carbon and CH4 formation should be kinetically limited at a low temperature. The thermodynamic prediction without considering the carbon and CH4 formation are more agreeable with the experimental results for FA or MeOH decomposing system, and such a point may also be applicable to the FSRM process. Thus, the thermodynamic calculation for FSRM process without considering the carbon and CH4 formation is processed and presented in Figure 6c and d. A shown, H2, CO2, H2O, and CO are the dominant products. The H2 and CO2 yields decrease while the CO and H2O yields increase with increasing temperature during the temperature range 200−400 °C. The yields of HCOOCH3, HCHO, and CH3OCH3 are suppressed at a much lower yield in comparison with the main products (Figure 6d), indicating that they are likely to exist as the intermediates. 3.7. Autothermal Conditions. The FA decomposing reaction A is exothermic, while the MeOH decomposing reaction D and the reforming reaction E are endothermic. Thus, it can be expected that by properly adjusting the reactive conditions, the FSRM process for H2 production can be run autothermally. Supposing a TN condition, that is, ΔH = 0, for

Figure 7. TN temperature under 1 bar plotted as a function of FA/ MeOH ratio for H2O/MeOH = 0, 1, 2 and 3, (a) with consideration of CH4 and C formation and (b) without consideration of CH4 and C formation.

and the feeding FA/MeOH and H2O/MeOH ratios under the TN conditions for the case (a) with consideration of CH4 and C formation and the case (b) without consideration of CH4 and C formation. For both cases, at a fixed FA/MeOH ratio, the equilibrium temperature increases with the decrease in H2O/ MeOH ratio, and at a fixed H2O/MeOH ratio, the equilibrium temperature increases with the increase in FA/MeOH ratio. With a restraint that FA/MeOH = 0−9, there are some limits for reaching the TN condition. For case a, no TN condition exists in the condition range examined at reaction temperature higher than 570 °C, and for case b, no TN condition exists in the condition range examined at reaction temperature higher than 170 °C. It is noted that the preassumed compositions have a significant effect on the adiabatic temperature; that is, the adiabatic temperatures for case a are much higher than the corresponding temperatures for case b even under the same feeding FA/MeOH and H2O/MeOH ratios. The equilibrium compositions corresponding to the conditions of Figure 7 are illustrated in Figure 8. Figure 8a presents the equilibrium mole fraction of H2, CO, CH4, and C for case a, which are most concerned in the H2 production from fossil system. The mole fractions of CO2, H2O, HCOOH, HCOOCH3, HCHO, CH3OCH3, and CH3OH are not shown in the figure for simplicity. As shown, the H2, CO, and C mole fractions decrease while CH4 concentration increases with increasing H2O/MeOH molar ratio at the FA/MeOH molar 5454

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concentration first increases with increasing H2O/MeOH ratio (=0, 1), then decreases with further increasing H2O/MeOH ratio (=2, 3) at a fixed FA/MeOH ratio. At a fixed H2O/MeOH ratio, the equilibrium H2 fraction shows little dependence on the FA/MeOH ratio. It is noted that only the TN temperatures higher than 100 °C in Figure 7b have been considered in Figure 8b considering that the boiling points of FA and H2O are around 100 °C under 1 bar.

4. DISCUSSION The emphases of this work is the examination of the potential of H2 production from FSRM process, which combines the exothermic FA decomposing and endothermic SRM and makes up a thermal neutral (TN) or nearly TN reaction system, both under TN and non-TN conditions. The effects of H2O/MeOH (0.0−3.0) and FA/MeOH ratios (0.0−9.0) are examined. The compound basis involved in this calculation are CH3OH, H2O, CO 2 , CO, H 2 , CH 4 , HCOOH, HCOOCH 3 , HCHO, CH3OCH3, and C (graphite). The following discussion is mainly based on such an expanded compound basis unless otherwise specified. 4.1. Nonthermal Neutral Conditions. In general, high temperature is favorable for H2 production and can effectively inhibit CH4 and carbon formation but leads to high CO yield. The temperature dependence is an indication of the competition between the endothermic and exothermic reactions in the system. Because methanation is a strongly exothermic reaction, the amounts of methane decrease substantially with the increase in temperature. Due to the exothermic nature of the water−gas shift reaction, which is a main reaction process for CO removal, the high temperature leads to a high equilibrium CO yield in the FSRM system. It has been long recognized that three reactions are the most probable ones that lead to carbon formation22,44 2CO = CO2 + C

(F)

CH4 = 2H 2 + C

(G)

CO + H 2 = H 2O + C

(H)

The Boudouard reaction F, and reaction H are the major pathways for carbon formation at low temperatures, while the methane decomposition reaction G dominates the carbon formation at high temperatures.22 Meanwhile, as the reaction temperature and the H2O/MeOH and FA/MeOH ratios vary, the concentration of the participation molecules for carbon formation and the transition conditions from the noncarbon formation region to the carbon formation region change. Thus, carbon formation can be effectively suppressed at a high temperature due to the small CH4 mole fraction. The increase in the FA/MeOH ratio leads to low equilibrium H2 mole fraction and high CO concentration due to the lower containing of hydrogen in FA than that in MeOH. Meanwhile, a high FA/MeOH ratio suppresses CH4 formation and coke formation at a low H2O/MeOH ratio, but promotes high coke mole fraction at a high H2O/MeOH ratio. Although the stoichiometry of SRM suggests that one mole of water is consumed for converting per mole of MeOH, a higher H2O/ MeOH ratio can effectively suppress CO, CH4, and carbon formation. In addition, high H2O/MeOH ratio can also improve H2 mole fraction at 200, 400, and 600 °C. However, more H2O leads to more heat supply and larger reactor volume, and an appropriate H2O/MeOH ratio is necessary. According to the thermodynamic analysis aforementioned, SRM (FA/MeOH = 0)

Figure 8. Equilibrium mole fraction of H2, CO, CH4, and C formed for the case with consideration of CH4 and C formation (a) and equilibrium mole fraction of H2 and CO for the case without consideration of CH4 and C formation (b) plotted as a function of FA/MeOH and H2O/MeOH ratios under 1 bar and TN conditions.

ratio of 0−9 and H2O/MeOH molar ratio of 0−3. Moreover, the effect of H2O/MeOH ratio on the H2 and C yields weakens while such an effect on the CO yield strengthens with increasing FA/MeOH ratio. At a fixed H2O/MeOH ratio, the CO concentration increases while the CH4 concentration decreases with increasing FA/MeOH ratio. At a low H2O/ MeOH ratio (=0 or 1), the H2 molar fraction decreases with an increase in H2O/MeOH ratio, while the H2 concentration increases slowly with increasing FA/MeOH ratio at a high H2O/MeOH ratio (=2 or 3). At a fixed H2O/MeOH ratio and except for the point at FA/MeOH = 0, the C molar fraction decreases with increasing FA/MeOH ratio at the FA/MeOH ratio of 1−9. Figure 8b plots equilibrium mole fraction of H2 and CO for the case b as a function of FA/MeOH and H2O/MeOH ratios under TN conditions. The equilibrium H2 5455

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temperature higher than 100 °C. Since the TN temperatures are located in the low temperature range, the thermodynamic data without consideration of C and CH4 formation are more agreeable with the reported experimental results at a low temperature, as aforementioned in Section 3.6. Under the TN conditions, the H2 mole fraction can be around 0.51 with a CO mole fraction as low as∼ 0.001 for H2O/MeOH = 2 and FA/ MeOH = 5 and 6. As is known, catalyst systems can have great impact on the selectivity of different products. The results of the calculation for the simplified case are not directly applicable to situations in which byproducts are selective other than H2 and CO2 are formed. For example, some catalysts such as Ni-based ones that are active in methanation reaction show high CH4 selectivity in the process of H2 production from MeOH.11 As a result, such catalysts should be designed carefully for production of pure H2 from FSRM. Thus, the development of the catalyst with high selectivity, good activity, and good stability should be an important topic for H2 production from FSRM process in the future study.

seems to be the best way to produce H2-rich gas for fuel cell, that is, a high H2 yield with low C and CO formations. However, the FSRM autothermal process is also meaningful for H2 production in practical applications as the combination of exothermic FA decomposing reaction with endothermic MeOH decomposing or SRM reaction lowering the energy demand of the system. More important is that, unlike the reported fuel−O2−H2O (or CO2) autothermal process,21,22,45,46 it can reduce the system energy input at no cost of the burning of fuel and H2. 4.2. Autothermal Conditions. The FSRM process provides a thermal efficient approach for heat integration in the system. With a TN restraint, the equilibrium temperature of the system is determined by the feeding H2O/MeOH and FA/ MeOH ratios. The data presented in Figure 7a show that the TN conditions can be realized at a relatively high temperature (>348 °C) for a wide range of H2O/MeOH and FA/MeOH ratios. Further calculation shows that the demand of heat increases with the increase in the reaction temperatures; that is, a high FA/MeOH ratio is required. The good point is that, unlike the reported fuel-O2−H2O autothermal process, which has a upper limit of the interested range of O2/fuel ratio,21,22 the increase in FA/MeOH can reduce the system energy input at no cost of the burning of fuel and H2. For example, as shown in Figure 8a, the equilibrium H2 mole fraction is around 0.16 at H2O/MeOH = 2 and a wide range of FA/MeOH = 2−9. However, although a fairly wide condition range exists for TN, carbon formation can not be effectively suppressed by either increasing H2O/MeOH or FA/MeOH ratios, as shown in Figure 8a. In all, the FSRM reaction is a complicated system. It is difficult to find a thermodynamics perfect operation region, that is, high yield of H2, no risk of carbon formation and low selectivity of methane and CO, under both TN and non-TN conditions at such an expanded set of products. 4.3. Compound Basis Set. The expanded compound basis set is CH 3OH, H2 O, CO 2, CO, H 2, CH4 , HCOOH, HCOOCH3, HCHO, CH3OCH3, and C (graphite) in this work. Thermodynamically, MeOH and FA can be converted completely at any H2O/MeOH and FA/MeOH ratios in the condition range considered. Even if those species such as CH3CHO, CH3COOH, HCHO, and CH3OCH3 are likely to be important intermediates in the actual process, their equilibrium concentrations were close to zero. While the thermodynamic study both in this work and in the literature26−28 show that the byproducts CH4 and C (graphite) are important products coexisting with H2O, CO2, CO, and H2, especially at a low temperature. In practice, however, carbon formation was not detected by most kinetic investigators. This may be due to several reasons including rate control rather than equilibrium control of the process so that ultimate equilibrium is never reached.45 A similar reason as rate controlling can be given to the absence of CH4 from reaction products in practice.47,48 Thus, a simplified system without consideration of C and CH4 formation (other species considered are the same with the expanded set) was examined for the low temperature H2 production. The calculated results showed a consentaneous trend with the experimental measurements for both the FA decomposing and MeOH decomposing cases.10,18 For the case without consideration of C and CH4 formation, the TN conditions can be realized at the low temperatures range (