Key Factors on the Pressurized Tri-Reforming of Methane over Ni

Chapter 7

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Key Factors on the Pressurized Tri-Reforming of Methane over Ni-SiO2 Hua-Ping Ren, Yong-Hong Song, Zhao-Tie Liu, and Zhong-Wen Liu* Key Laboratory of Applied Surface and Colloid Chemistry (MOE), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China *Tel: +86-29-8153-0801. Fax: +86-29-8153-0727. E-mail: [email protected].

The effect of operating conditions, i.e., space velocity, temperature, pressure, and feed CH4/CO2/H2O/O2 molar ratios, on the reforming of methane with CO2, H2O, and O2 (tri-reforming, TRM) was systematically investigated in a fixed-bed reactor over the Ni-SiO2 synthesized by the complex-decomposition method by using citric acid as the complexing agent. Under different temperatures, pressures, and molar ratios of reactants, the thermodynamics equilibrium compositions of the TRM were calculated by using HSC chemistry 5.0. Irrespective of the operating conditions applied, results indicate that the Ni-SiO2 was highly active for the TRM, in which the conversions of CH4 and CO2 and H2/CO molar ratios in the products nearly reach equilibrium values. Moreover, the changes for the conversions of CH4 and CO2 and H2/CO molar ratios in the products induced by varying the space velocity, temperature, pressure, and feed CH4/CO2/H2O/O2 molar ratios were discussed and compared with the results of the thermodynamics calculations. By simply changing the molar ratios of the feeds, the syngas with adjustable H2/CO molar ratios between 1.0 and 3.0 was successfully obtained. Importantly, under much severe conditions of T = 750 °C, P © 2015 American Chemical Society In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

= 10.0 atm, CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, and GHSV = 26600 mL(CH4)·g-1·h-1, the almost identical conversions of CH4, CO2, and the H2/CO molar ratios to those of the thermodynamics equilibrium were kept for over 50 h. Thus, the Ni-SiO2 is a highly active and stable catalyst for the TRM.


Introduction The production of syngas (H2+CO) from natural gas (methane), which is the key technology for the gas-to-liquid process, has drawn quantitative attention in recent years (1–3). Generally, there are three primary processes via reforming reactions to convert natural gas to syngas, i.e., steam reforming of methane (SRM, eq. 1), carbon dioxide reforming of methane (CDR, eq. 2), and catalytic partial oxidation of methane (CPOM, eq. 3) (4).

Although the SRM has successfully applied in many industrial processes, the obvious disadvantage of this process is the high energy demand due to the strong endothermic nature of the reaction (eq. 1). Moreover, the syngas with a high H2/CO ratio of greater than 3 from the SRM process is only suitable to the synthesis of ammonia and hydrogen (5, 6). In contrast to SRM, CPOM is an exothermic reaction, and has long been received considerable attention since it produces the syngas with a H2/CO ratio of about 2.0, which well meets the requirement for the synthesis of methanol, dimethyl ether, and hydrocarbons via Fischer-Tropsch (FT) route (7–9). However, besides the potential explosive risk of the CPOM process, the issue of the hot spot in the catalytic bed is still a barrier for its industrialization (9, 10). In the case of CDR, the principal merit is that it converts the two potent greenhouse gases (CO2 and CH4) into the syngas with a H2/CO ratio of near to 1.0, which can be used as the feedstock for synthesizing valuable oxygenates and long-chain hydrocarbons (3, 11–13). However, the CO2 capture and its separation, purification, and transportation are economic barriers for the efficient utilization of CO2 (14, 15). In the catalytic aspects, the severe coke deposition and sintering of the industrially important Ni-based catalyst are still main challenges for the commercialization of CDR (16–18). Combining the advantages of the three primary processes, a mixed route for the production of syngas from the reforming of methane, i.e., tri-reforming of methane (TRM), has been proposed in recent years (3, 19). For the TRM, the reactions of the endothermic SRM (eq. 1) and CDR (eq. 2), and the exothermic CPOM (eq. 3) synergistically occur in one reactor. As a result, an autothermal operation characterizing the neutral energy demand may be accomplished during 156 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


the TRM. Moreover, the flue gas from the fired power plant is composed of CO2, O2, and H2O, which is a potential candidate of the feedstock for the TRM. Indeed, it is well known that the composition of the flue gas varies depending on the operational modes of the power plant. For example, the typical volumetric composition of the flue gas from the natural gas-fired power plants is 8-10, 18-20, 2-3, and 67-72% for CO2, H2O, O2, and N2, respectively, which is different from that of the coal-fired plant, i.e., 12-14, 8-10, 3-5, and 72-77% for CO2, H2O, O2, and N2, respectively (20). However, the syngas with a desired H2/CO may be achieved via adjusting the molar ratios of CO2, H2O, and O2 by co-feeding the flue gas with either H2O or O2. When the catalytic process is concerned, the presence of H2O and/or O2 significantly reduces the coke deposition over the catalyst (21, 22). More importantly, the TRM may largely eliminate the separation of the flue gas, which has a great economic benefit. Moreover, the outlet temperature of the flue gas is around 1000 °C, which well matches the high-temperature reaction of the tri-reforming process (23). Thus, the TRM by using the flue gas as a feedstock is of significantly important and promising (23–26). Recently, a few studies have been concentrated on the TRM by co-feeding of H2O, CO2, and O2 with different molar ratios (8, 9, 27). Considering the two facts that methane is preserved and transported under elevated pressures and the process for converting the syngas to chemicals and fuels such as FT synthesis is kinetically favored under elevated pressures, the TRM under pressurized conditions is industrially more desirable in terms of higher process efficiency (3, 28). Thus, the TRM under pressurized conditions is more practical from an economical viewpoint. However, to the best of knowledge, very few works is reported in recent years (3). In our previous works (29, 30), the complex-decomposition method was applied to develop Ni-based catalysts for the pressurized CDR, and a highly active and stable Ni-SiO2 was obtained by optimizing the complex agents and fuels. In this work, the Ni-SiO2 catalyst optimized for CDR was quantitatively evaluated for the TRM under different feed CH4/CO2/H2O/O2 molar ratios, space velocities, reaction temperatures, and pressures. The effect of operating conditions on conversions and product compositions was revealed and compared with the calculated equilibrium data. As expected, the syngas with adjustable H2/CO molar ratios was obtained. Importantly, the optimal Ni-SiO2 catalyst developed for the CDR was still highly active and stable for the pressurized TRM, and near the equilibrium conversion of methane was kept for a time on stream (TOS) of 50 h without an observable decrease.

Experimental Catalyst Preparation The Ni-SiO2 catalysts with a metallic nickel content of 10 wt.% was prepared by the complex-decomposition method by using nickel nitrate and tetraethoxysilane (TEOS) as the precursors of NiO and SiO2, respectively. Firstly, the desired amount of nickel nitrate and TEOS were dissolved in ethanol. Then, the aqueous solution of citric acid as a complexing agent was added to the ethanol 157 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

solution until the molar ratio of citric acid to nickel plus silicon was 1.0. After this, the solvent was evaporated at 60 °C until the formation of a viscous gel, and a foam-like grey solid was obtained by burning the gel. Finally, the sample was calcined at 700 °C for 4 h leading to the Ni-SiO2 catalyst. The more detailed procedure was described in our previous work (29).


Reaction Procedure The TRM experiments were performed in a stainless steel fixed-bed reactor (i. d. = 8 mm). Prior to the reaction, the catalyst (0.15 g, 40-60 mesh) diluted with quartz sands (the volumetric ratio of quartz sands to the catalyst = 3.0) was reduced under the conditions of P = 1.0 atm, T = 700 °C, t = 2.5 h, and 50 mL·min-1 of 10% H2/N2. After purging with N2, the feed gases with a desired molar ratio of CH4 to CO2, H2O, and O2 were switched, and the reaction was started under the conditions of T = 650 ~ 750 °C, P = 1.0 ~ 10.0 atm, GHSV = 15792 ~ 36840 mL(CH4)·g-1·h-1, and CH4/CO2/H2O/O2 molar ratios between 1.0/0.5~0.1/0.5~0.1/ 0~0.3. The effluent products after condensing water with an ice-water trap were analysed by an on-line GC (GC-9560, Shanghai Huaai chromatographic analysis Co., Ltd) equipped with 5A and PQ capillary columns and a TCD detector.

Results and Discussion Characteristics of Thermodynamics Equilibrium Pressure Effects The equilibrium conversions of CH4, CO2, H2O, O2, and H2/CO molar ratios were calculated with HSC chemistry 5.0 under the conditions of T = 750 °C, CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, and the results under different pressures are provided in Figure 1. As shown in Figure 1, the O2 conversion was kept at 100% with increasing the pressures from 1.0 to 10.0 atm, indicating the negligible effect of pressures on the O2 conversion under the given temperature and system composition. In contrast, the conversions of CH4, CO2, and H2O were obviously decreased with increasing the pressure from 1.0 to 10.0 atm. Moreover, the conversions of CH4, CO2, and H2O were linearly decreased until the pressure of about 4.0 atm, and were leveled off when the pressure was further increased. Importantly, the conversion of CH4 was obviously higher than those of CO2 and H2O. In the case of H2/CO molar ratio, its decrease was very limited with increasing the pressure from 1.0 to 10.0 atm, indicating the pressure plays a less important role in determining the H2/CO molar ratios.

158 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 1. Effect of the pressure on equilibrium conversions and H2/CO molar ratios under the conditions of T = 750 °C and CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2.

Temperature Effects The impact of temperature on the equilibrium conversions was calculated under the conditions of P = 1.0 atm and CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, and the results are provided in Figure 2. The conversions of CH4, CO2, and H2O were rapidly increased with increasing the temperature from 400 to 800 °C. A further increasing of the temperature leaded to slowly increased conversions, and near 100% conversions were obtained at 1000 °C. As indicated in eqs.1 and 2, both the SRM and CDR reactions are strong endothermic, leading to the increased conversions of CH4, CO2, and H2O at higher temperatures. These are agreeable with the reported results (19, 27). In the case of the H2/CO molar ratio, it was sharply decreased from 11.8 to 2.1 with increasing the temperature from 400 to 600 °C, and it was around 1.8 when the temperature was further increased to 1000 °C. This indicates an important effect of the reaction temperature on the H2/CO molar ratios at lower temperatures. It is well known that the reverse water-gas shift reaction (RWGS, H2 + CO2 → CO + H2O, ΔH°298K = 41 kJ/mol) is endothermic (26). Thus, the RWGS reaction is thermodynamically favorable at higher temperatures, leading to the decrease of H2/CO molar ratios with increasing reaction temperatures.



Figure 2. Effect of the temperature on equilibrium conversions and H2/CO molar ratios under the conditions of P = 1.0 atm and CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2.

Effect of System Compositions The equilibrium conversions of CH4, CO2, H2O, and H2/CO molar ratios were calculated under the conditions of CH4/CO2/H2O/O2 = 1.0/0.0~0.5/ 0.0~0.5/0.5~0.0, T = 400~1000 °C, and P = 1.0 atm. To reveal the effect of system compositions, the molar ratio of CH4 to (CO2+H2O+O2) was kept at the stoichiometric ratio of 1.0. Moreover, the CO2 to H2O molar ratio was kept at 1.0 to obtain a syngas with the H2/CO molar ratio of 2.0. As shown in Figure 3a, the CH4 conversion was clearly increased with either increasing the O2 content or decreasing the CO2 and/or H2O content, the extent of which is dependent on the temperature. Furthermore, the effect was negligible when the temperature was above 800 °C. In the case of the CO2 conversion, irrespective of the feed CH4/CO2/H2O/O2 molar ratios, it was slightly decreased with increasing the reaction temperatures from 400 to 500 °C while it was significantly increased with the further increasing of the reaction temperatures until 900 °C. The complete conversion of CO2 was observed at 1000 °C (Figure 3b). With increasing the temperature from 400 to 800 °C, the conversion of H2O was almost linearly increased, and 100% conversion was also obtained at 1000 °C (Figure 3c). This can be explained as follows (31, 32): (1) methane was completely combusted to CO2/H2O; (2) The H2O was also produced via RWCS reaction by consuming the CO2; (3) The CDR and SRM reactions are favorable at higher temperatures while the RWGS reaction is advantageous at lower temperatures. In the case of H2/CO molar ratios, as shown in Figure 3d, it was sharply decreased with increasing the reaction temperature from 400 to 600 °C irrespective 160 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


of the CH4/CO2/H2O/O2 molar ratios. Moreover, the further increase of the reaction temperature until 1000 °C leaded to an almost constant H2/CO molar ratio, of which is clearly dependent on the composition of CH4, CO2, H2O, and O2.

Figure 3. Equilibrium conversions of CH4 (a), CO2 (b), H2O (c), and H2/CO molar ratios (d) under P = 1.0 atm.

TRM Results over Ni-SiO2 Effect of Space Velocity The effect of space velocity on the TRM performance over Ni-SiO2 was investigated under the operating conditions of T = 750 °C, P = 5.0 atm, and CH4/CO2/H2O/O2 = 1.0/0.4/0.4/0.1. From the results given in Figure 4, the conversions of CH4 and CO2 and H2/CO molar ratio at steady state were about 62%, 43%, and 1.80 at a GHSV of 15792 mL(CH4)·g-1·h-1, respectively. With increasing the GHSV from 15792 to 36840 mL(CH4)·g-1·h-1, the steady-state conversions of CH4 and CO2 and H2/CO molar ratio were kept almost unchanged. 161 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


This indicates that the Ni-SiO2 is still highly active for TRM, which is consistent to its high activity towards CDR as reported in our previous works (29, 30). Moreover, the experimental results were nearly identical to those of the calculated equilibrium data (Figure 1). Thus, the Ni-SiO2 prepared by the complex-decomposition method is really highly active for the reforming reactions of methane even under much severe conditions of T = 750 °C and P = 5.0 atm. In the case of the catalytic stability, there was no observable decrease of the conversions within 20 h test at GHSV of 15792 mL(CH4)·g-1·h-1. With the further increase of GHSV from 26600 to 36840 mL(CH4)·g-1·h-1, the steady-state conversions and H2/CO molar ratio were still unchanged. This indicates that the Ni-SiO2 was also stable for TRM under much severe operating conditions. Thus, the Ni-SiO2 was both active and stable for TRM.

Figure 4. Effect of the space velocity on CH4 conversion (a), CO2 conversion (b), and H2/CO molar ratio in the products (c) of TRM over Ni-SiO2 under the conditions of T = 750 °C, CH4/CO2/H2O/O2 = 1.0/0.4/0.4/0.1, and P = 5.0 atm.

Effect of Reaction Pressure To reveal the impact of the reaction pressures on the performance of Ni-SiO2 for TRM, the reaction was carried out under the conditions of T = 750 °C, 162 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, and GHSV = 26600 mL(CH4)·g-1·h-1, and the results are given in Figure 5. As shown in Figure 5a, the steady-state CH4 conversion was about 82, 61, and 50% at the pressure of 1.0, 5.0, and 10.0 atm, respectively, which is quite similar to those of the thermodynamics equilibrium (Figure 1). Thus, irrespective of the pressures, near equilibrium CH4 conversions were obtained over Ni-SiO2. Moreover, the steady-state CH4 conversion was kept over 20 h without observable decrease, indicating its high stability even at elevated pressures of 10.0 atm.

Figure 5. The time-on-stream CH4 conversion (a), CO2 conversion (b), and the H2/CO molar ratio in the products (c) for TRM under the conditions of T = 750 °C, CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, GHSV = 26600 mL(CH4)·g-1·h-1. When the CO2 conversions were concerned (Figure 5b), the most obvious observation was the fluctuation at the initial reaction stage, the extent and the time to the steady state of which are increased with increasing the pressure from 1.0 to 10.0 atm. Thus, the time required to approach the steady-state of CO2 conversion was longer at higher pressures, indicating the fluctuated compositions of the catalytic bed at the initial stage of the reaction. In the case of the steady-state CO2 conversion, the changing trend with increasing the pressure from 1.0 to 10.0 atm was the same to that of CH4 conversion. However, the extent was more significant, i.e., sharply decreased from about 70% at P = 1.0 atm to about 10% at P = 10.0 atm. Moreover, the CO2 conversion of TRM at elevated pressures 163 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

was significantly lower than that of the pressurized CDR (29, 30). These results indicate that the reforming of CO2 with CH4 at a higher pressure was inhibited under the presence of O2 and/or H2O, which is consistent to the slightly higher steady-state H2/CO molar ratio at 1 atm (~ 1.85) than those at 5 and 10 atm (~ 1.95, Figure 5c). This can be well explained based on the weaker oxidation ability of CO2.


Effect of Reaction Temperature The influence of the reaction temperature on the conversions and H2/CO molar ratio under the conditions of CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, P = 5.0 atm, and GHSV = 26600 mL(CH4)·g-1·h-1 is presented in Figure 6. The steady-state conversions of CH4 and CO2 and the H2/CO molar ratio were quite stable at all of the temperatures applied. Moreover, it is clear that the conversions of CH4 and CO2 were obviously decreased with decreasing the reaction temperature. In contrast, the H2/CO molar ratio was slightly increased with decreasing reaction temperatures. This suggests that the endothermic RWGS occurred very limitedly at lower temperatures, which is consistent to the equilibrium results shown in Figure 2. Importantly, after a recycle of the temperature from 750 to 650 °C, both the conversions of CH4 and CO2 and the H2/CO molar ratio were fully restored, indicating the high stability of the catalyst for TRM.

Figure 6. Effect of reaction temperature on the performance of Ni-SiO2 for TRM under the conditions of P = 5.0 atm, CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, and GHSV = 26600 mL(CH4)·g-1·h-1.

Effect of Feed Molar Ratios The performance of Ni-SiO2 for TRM was also carried out by changing the feed CH4/CO2/H2O/O2 molar ratios between 1.0/0.5~0.2/0.5~0.2/0.0~0.3, and the results are presented in Figure 7. It was observed that the variation of the CH4 conversion was very limited irrespective of the feed compositions, which is nearly 164 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


identical to the equilibrium values under the conditions of T = 750 °C and P = 5.0 atm (Figure 3). Specifically, the CH4 conversion was slightly increased while the CO2 conversion was quickly decreased with increasing the O2 content. These results are well agreeable with those of the equilibrium (Figure 3) and the reported results (33). Thus, simultaneous CDR and CPOM under the presence of CO2 and O2 in the feed occurred, and the CPOM was preferred than CDR. Considering the stoichiometric H2/CO molar ratios of CDR (1.0) and SRM (3.0), the expected H2/CO molar ratio between 1.0 and 3.0 can be conveniently adjusted by simply changing the feed CO2/O2/H2O ratios. To illustrate this, the feed with a molar ratio of CO2 to H2O at 1.0 was applied for producing a syngas with the H2/CO molar ratio of 2.0. Indeed, results in Figure 7 indicate that the H2/CO molar ratio of about 2.0 was obtained under varied feed compositions. When the results in Figure 7c were carefully examined, the H2/CO molar ratios were slightly varied between 1.8 ~ 2.0, which can be explained as the different extents of the RWGS under varied feed compositions.

Figure 7. Effect of the feed CH4/CO2/H2O/O2 molar ratios on CH4 conversion (a), CO2 conversion (b), and H2/CO molar ratio in the products (c) for TRM under the conditions of T = 750 °C, P = 5.0 atm, and GHSV = 26600 mL(CH4)·g-1·h-1.



Long-Term Stability To further confirm the stability of the Ni-SiO2 for TRM, much severe operating conditions of T = 750 °C, P = 10.0 atm, CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, and GHSV = 26600 mL(CH4)·g-1·h-1 were applied for a longer TOS, and the results are given in Figure 8. Consistent to the results in Figure 5, the steady state was approached at a TOS of about 10 h. Moreover, the steady conversions of CH4 and CO2 and H2/CO molar ratio were about 52%, 18%, and 2.0, respectively, which are nearly identical to the equilibrium values as shown in Figure 1. Within the TOS of 50 h, no observable decrease in conversions of CH4 and CO2 and H2/CO molar ratio were found. Thus, the Ni-SiO2 is highly active and stable for the TRM under much severe conditions with elevated pressures.

Figure 8. Long-term results of TRM over Ni-SiO2 under the conditions of T = 750 °C, CH4/CO2/H2O/O2 = 1.0/0.3/0.3/0.2, GHSV = 26600 mL(CH4)·g-1·h-1, and P = 10.0 atm.

Conclusions In summary, the thermodynamics equilibrium and the kinetic behavior of the TRM over Ni-SiO2 were systematically investigated. Irrespective of the operating conditions applied, the conversions of CH4 and CO2 and H2/CO molar ratios in the products for TRM over Ni-SiO2 almost reached the thermodynamics equilibrium values. Moreover, the syngas with a desired H2/CO molar ratio between 1.0 and 3.0 was successfully obtained by simply adjusting the molar ratios of the feeds. Under the much severe operating conditions of T = 750 °C, P = 10.0 atm, CH4/CO2/H2O/ O2 = 1.0/0.3/0.3/0.2, GHSV = 26600 mL(CH4)·g-1·h-1, no observable deactivation of the Ni-SiO2 was found for 50 h test. Thus, the highly efficient Ni-SiO2 is very promising for the TRM. 166 In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Acknowledgments The authors gratefully acknowledge the financial supports of Shaanxi Innovative Team of Key Science and Technology (2013KCT-17), the Changjiang Scholars and Innovative Research Team in University (IRT-14R33), Natural Science Foundation of Shaanxi Province (2012JZ2001), and the Fundamental Research Funds for the Central Universities (GK201305012).


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Key Factors on the Pressurized Tri-Reforming of Methane over Ni

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