CO hydrogenation combined with water-gas-shift reaction for synthetic natural gas production: a thermodynamic and experimental study
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The hydrogenation of CO to synthetic natural gas (SNG) needs a high molar ratio of H2/CO (usually large than 3.0 in industry), which consumes a large abundant of hydrogen. The reverse dry reforming reaction (RDR, 2H2 + 2CO ↔ CH4 + CO2), combining CO methanation with water-gas-shift reaction, can significantly decrease the H2/CO molar ratio to 1 for SNG production. A detailed thermodynamic analysis of RDR reaction was carried out based on the Gibbs free energy minimization method. The effect of temperature, pressure, H2/CO ratio and the addition of H2O, CH4, CO2, O2 and C2H4 into the feed gas on CO conversion, CH4 and CO2 selectivity, as well as CH4 and carbon yield, are discussed. Experimental results obtained on homemade impregnated Ni/Al2O3 catalyst are compared with the calculations. The results demonstrate that low temperature (200–500 °C), high pressure (1–5 MPa) and high H2/CO ratio (at least 1) promote CO conversion and CH4 selectivity and decrease carbon yield. Steam and CO2 in the feed gas decrease the CH4 selectivity and carbon yield, and enhance the CO2 content. Extra CH4 elevates the CH4 content in the products, but leads to more carbon formation at high temperatures. O2 significantly decreases the CH4 selectivity and C2H4 results in the generation of carbon.
KeywordsSynthetic natural gas Reverse dry reforming of methane Gibbs free energy minimization Experimental study CO conversion
List of symbols
Total mass of k element in the feed
Standard-state fugacity of species i (Pa)
Fugacity of species i (Pa)
Gibbs free energy of species i (J/mol)
Standard Gibbs free energy of species i (J/mol)
Standard-state Gibbs free energy of formation of species i (J/mol)
Partial molar Gibbs free energy of gas carbon (J/mol)
Partial molar Gibbs free energy of solid carbon (J/mol)
Standard-state Gibbs function of formation of solid carbon (J/mol)
Standard-state reaction enthalpy change (J/mol)
Standard-state equilibrium constant
Mole of species I (mol)
Mole of carbon (mol)
Number of components
System pressure (Pa)
Pressure of the standard state (Pa)
Molar gas constant (J/(mol K)
Mole fraction of species i
Number of atoms of the k element present in each molecule of species i
Chemical potential of species i (J/mol)
Fugacity coefficient of species i
Natural gas is a highly efficient and clean fossil fuel due to its high calorific value, low sooting tendency and slag free products, leading to its increasing consumption year by year (Gao et al. 2015; Meng et al. 2015a; Rönsch et al. 2016). In 2014, the consumption of natural gas in China increased to 197.3 billion cubic meters, with a growth rate of 30.9% every year in the last decade (BP 2016). Recently, the consumption of natural gas has raised a serious concern regarding its depletion because of its limited reserves (Kopyscinski et al. 2010; Huo et al. 2013), in comparison, coal is considered as a much more abundant energy resource in many countries. The production of synthetic natural gas (SNG) from coal has been developed to be a potential route to circumvent the limited supply of natural gas, especially in China (Li et al. 2014a, b; Lu et al. 2014).
Among the coal-to-SNG production processes, SNG is produced through the four major steps, i.e., coal gasification, water-gas-shift (WGS) reaction (CO + H2O ↔ H2 + CO2), gas cleaning and CO methanation (3H2 + CO ↔ CH4 + H2O) (Shinde and Madras 2014; Wang et al. 2015). The CO methanation reaction is a key process for increasing SNG production (Meng et al. 2015b; Götz et al. 2016; Gao et al. 2016). If one mole of CO is converted to methane, three moles of H2 are stoichiometrically required. However, the content of carbon in coal is usually more than 60 wt% (up to more than 90 wt% in bituminous coal) whereas that of hydrogen is <5 wt% (Martelli et al. 2011; Shen et al. 2016). The high content of carbon in coal results in low H2/CO molar ratios, usually less than one, of produced gas from coal gasification (Messerle et al. 2016). For example, the produced gas of the British Gas-Lurgi (BGL) coal gasification process is composed of 60%–70% CO, 27%–30% H2, 0%–7% CH4, 1%–4% CO2, and trace amounts of O2 and light hydrocarbons (Yu and Wang 2010). To increase the H2/CO ratio, the WGS reaction should be well controlled. It is worthwhile to mention, in order to enhance the CO conversion and CH4 yield during industry processes, an even higher H2/CO ratio is usually used. For instance, the H2/CO ratio of the Lurgi process for methanation was optimized at about 3.2, and that of the Topsøe Recycle Energy Efficient Methanation (TREMP) process reached about 3.5 (Kopyscinski et al. 2010). More amount of CO needs to be converted to produce H2 by WGS reaction in order to get a high H2/CO ratio, which results in the high operating cost and energy consumption.
The reverse dry reforming (RDR) reaction (2CO + 2H2 ↔ CH4 + CO2), which is the combination of CO methanation with WGS reaction, can be used to produce SNG. Recently, many studies have focused on the RDR reaction with the H2/CO ratio of one. Yan et al. (2013) found that the catalyst preparation methodologies significantly affected the activity and stability of Ni/SiO2 catalysts. Jiang et al. (2013, 2014) investigated the stepwise sulfidation and sulfidation temperature on the catalytic activity of MoO3/CeO2–Al2O3. It is because there are many advantages of this reaction. First, the feed gas has a low H2/CO ratio of one, which needs less hydrogen; Secondly, the water-free products can diminish the damage of steam on methanation catalyst, and the by-product CO2 can be easily removed by employing low-temperature methanol purification process. In other words, less H2 is needed from the gas of coal gasification, which simplifies the SNG production process and reduces the cost.
In literatures, data are available for the thermodynamic analysis of methanation reactions. Miguel et al. (2015) and Sahebdelfar et al. (2015) conducted a thermodynamic calculation of CO2 methanation based on the method of Gibbs free energy minimization and compared with the experimental data. Gao et al. (2012) analyzed the thermodynamic properties of several reactions during the complete methanation of CO and CO2. However, these thermodynamic studies were carried out based on the complete methanation reactions. To our knowledge, there is little information on the thermodynamic analysis of the RDR reaction that occurs at low temperatures. Therefore, it needs to perform the calculations based on the Gibbs free energy minimization method and validate the data through experimental means.
It is well known that the produced gas from coal contains many impurities, such as steam, CO2, CH4, O2 and light hydrocarbons of C2H4 and C2H6. In order to increase the production of SNG and optimize the H2/CO ratio of the produced gas, effects of these substances on the catalytic performance of the RDR reaction have to be investigated. Moreover, the yield of solid carbon should be taken into account during the thermodynamic analysis.
The objective of this work is to elucidate, through a thermodynamic study supported by experimental data, the effects of temperature, pressure and the other factors affecting the RDR reaction, such as the H2/CO molar ratio and addition of H2O, CH4, CO2, O2, and C2H4 in feed gas on the catalytic activity and selectivity and the yield. For this purpose, this study does not take into account of reaction kinetics, practical heat and mass transfer processes. It is expected to produce necessary thermochemical data to describe the effectiveness of the RDR reaction and to provide useful guidance to chemical engineers for optimizing the individual processes.
2.1 Thermodynamic analysis software
The HSC Chemistry software 6.0 allows simulating chemical reactions and processing on the thermochemical data basis. In this study, the modules of reaction equations and equilibrium compositions were utilized to calculate the effects of various substances in conversion, selectivity and yield. The calculations were performed based on an extensive thermochemical database, which contains enthalpy (H), entropy (S) and heat capacity (C p) data of more than 17000 chemical compounds (Roine 2010; Kumar et al. 2016).
2.2 Thermodynamic analysis method
The equilibrium products at different temperatures and pressures were calculated using the Gibbs free energy minimization method, which has been widely applied for thermodynamic calculations (Adhikari et al. 2007; Nahar and Madhani 2010; López Ortiz et al. 2015). The detailed interpretation of this theory can be referenced by Wang et al. (Wang and Cao 2012; Wang et al. 2014).
In the HSC Chemistry software 6.0, the reaction system needs to be specified, in terms of its phases and species, and the amount of the reactants. The program calculates the amount of products at equilibrium in isothermal or isobaric condition for a heterogeneous system. At the equilibrium state, the free energy of the system is minimized.
The relevant reactions in the reverse dry reforming reaction
2CO + 2H2 ↔ CH4 + CO2
Reverse dry reforming reaction
CO + 3H2 ↔ CH4 + H2O
CO2 + 4H2 ↔ CH4 + 2H2O
CO + H2O ↔ H2 + CO2
2CO ↔ C + CO2
CH4 ↔ 2H2 + C
CO + H2 ↔ C + H2O
CO2 + 2H2 ↔ C + 2H2O
Here, i indicates all carbon containing species (CO, CO2, CH4 and C2H4) at inlet, and N i indicates the number of carbon atom of i-th species.
2.3 Experimental study
The alumina (191 m2/g, Shandong Aluminum Co., China) supported homemade Ni-based catalyst was prepared by the co-impregnation method, as described in Meng’s works (Meng et al. 2017). The Ni-based catalyst, with the Ni loading of 20 wt% and La loading of 4 wt%, showed the specific surface area of 128 m2/g and pore size of 5.1 nm, and the catalyst was denoted as ExCat. To validate the thermodynamic calculations, the RDR reaction was carried out in a stainless steel, high-pressure fixed-bed tube reactor (10 mm × 2 mm × 500 mm) within the temperature range of 300–550 °C. 300 mg of Ni/Al2O3 catalyst (20–40 mesh) was placed in the reactor. Prior to the RDR reaction, the catalyst was reduced at 550 °C in a H2 (99.99%, purchased from Taiyuan Iron & Steel (Group) Co., Ltd., China) flow diluted with 25% N2 (99.995%, purchased from Taiyuan Iron & Steel (Group) Co., Ltd., China) for 6 h. A mixed feed gases of H2/CO = 1 (the gas of CO with a purity of 99.9% was purchased from Taiyuan Iron & Steel (Group) Co., Ltd., China) were introduced and controlled with the mass flow controller (MFC), preheat treatment was finished at 200 °C in first oven at a space velocity of 20000 mL/(g h)−1. In the second oven, two thermocouples are employed for the reaction. One is placed closely to the reactor, in the middle of the oven to control the oven temperature. The other one is placed inside of the catalyst bed for the measurement of reaction temperature of catalyst bed. The outlet gas steam was cooled by condenser (2 °C) and quantitatively analyzed by an online gas chromatography (GC, Agilent 7890A) using helium (99.999%, purchased from Taiyuan Iron & Steel (Group) Co., Ltd., China) as the carrier gas. The GC equipped with a flame ionization detector (FID) with an HP-AL/S column was employed to analyze CH4, and a thermal conductivity detector (TCD) equipped with a Porapak-Q column, HP-PLOT/Q column, and HP-MOLESIEVE column was employed to analyze CO2, CO, and N2.
3 Results and discussion
3.1 Equilibrium analysis of the reactions
It can be seen in Fig. 1, as the temperature increases, all the K values decrease except that of R6, which agrees with the Le Chatelier’s principle. R1, R2, R3, R5, and R7 play important roles in the RDR reaction system. When the temperature is lower than 500 °C, the equilibrium constant K reduces in the order of R1 > R2 > R5 > R3 > R7 > R8 > R4 > R6. Among all these reactions, R1 and R2 show elative high K values at low temperatures, which will lead to the high conversions of CO. CO2 could be converted via reactions of R3 and R8; however, the CO2 cannot be fully converted, which is due to that the reactions of R1, R4, and R5 generate CO2. Moreover, the solid carbon generated from the reaction of R5 to R8, and the Boudouard reaction (R5) acts a dominant role due to its largest K value. Importantly, all these reactions may occur simultaneously in the system, resulting in a balanced composition of the products.
3.2 Equilibrium compositions
3.3 Effect of temperature and pressure
The variation of carbon yield is presented in Fig. 3d. All these carbon yield curves exhibit a volcano characteristic, with less yield of carbon at high pressures. The solid carbon results from many reactions, including R5, R6, R7, and R8 (as shown in Table 1), from which have different K values. Since the K value of R6 is negative at 200–550 °C and the value of R5 is higher than that of R7 and R8 at 200–800 °C (Fig. 1), so R5 is the main reason for the deposition of carbon. At the point of 0.1 MPa and around 575 °C, the carbon yield reaches the maximum (23%). Accordingly, at this condition, the occurrence of R6 triggered a higher production of carbon. However, further increase the temperature results in the decrease of carbon yield, possibly because the reverse reactions of R5, R7 and R8 consumes a comparable amount of solid carbon.
3.4 Effect of H2/CO ratio
3.5 Effect of H2O content
Steam controls the H2/CO ratio via WGS reaction (R4), which is mostly used in methanation and ammonia synthesis industrial process. Moreover, it can be also used for eliminating the carbon deposition to some extent via reverse R7 and R8.
3.6 Effect of CH4 content
3.7 Effect of CO2 content
3.8 Effect of O2 content
3.9 Effect of C2H4 content
3.10 Comparison between thermodynamic calculations and experimental results
Figure 10b shows the CH4 selectivity at various temperatures and pressures. The calculated results shows that increasing temperature decreases the CH4 selectivity, whereas the increasing pressure enhances the CH4 selectivity. At low temperature and pressure, the experimental results are slightly higher than the calculation ones, while at high temperature, the experimental CH4 selectivity is much higher than the calculated one. Figure 10c shows the comparison of CO2 selectivity. The calculation results exhibit that the selectivity of CO2 is constant at various temperatures and pressures, and the experimental results are slightly lower than the calculation ones. The above comparison show that more amount of CO2 converted to CH4 during the reaction. Figure 10d shows the comparison of carbon yields. The calculated carbon yields rose as the temperature increased, and decreased as the pressure increased, which means a large amount of CO is converted to carbon. However, the experimental results show that the yields of carbon were nearly zero, probably due to the catalyst inhibit the formation of carbon. The above discussion show that the experimental results are generally in accordance with the calculated ones at different temperatures and pressures. The result also indicates that the Gibbs free energy minimization method is an ideal tool for thermodynamic analysis of the RDR process.
A detailed thermodynamic equilibrium analysis of reverse dry reforming (RDR) reaction by minimizing the Gibbs free energy method in the range of 200–800 °C and 0.1–3 MPa, and an experimental results in the range of 300–550 °C and 0.1–3 MPa are studied. The calculation results demonstrate that low temperature and high pressure are beneficial for the CO conversion and CH4 yield, and high H2/CO ratio (at least 1) promotes CH4 yield and decreases carbon yield. In the range of 200–500 °C and 1–5 MPa, the CO conversion and CH4 yield reach 95%–100% and 43%–50%, respectively. Steam in the feed gas enhances the CO2 selectivity and inhibits the generation of carbon, almost no carbon formed at the H2/CO/H2O ratio of 1/1/0.4, when the temperature is below 600 °C at 3 MPa. CH4 contained in the recycling product gas elevates the CH4 content in the products, but also leads to more solid carbon at 500–800 °C, especially at 0.1 MPa. CO2 has a negative effect on CH4 selectivity, but it could result in a slightly decrease of carbon yield at the temperature higher than 500 °C. O2 is not preferable for increasing CH4 selectivity and decreasing the CO2 selectivity although it decreases the carbon yield. C2H4 is prone to crack, creating a high carbon yield. As impurities, O2 and C2H4 should be completely removed to get a high CH4 yield. The experimental data are consistent with the calculation ones, indicating that minimizing the Gibbs free energy is effective to analyze the RDR reaction thermodynamically. This work is expected to provide a valuable suggestion in the process optimization for SNG production by combining CO methanation with WGS reaction.
This work was supported by Youth Foundation of Shanxi Province (No. 2013021007-4) and National Basic Research Program of China (No. 2012CB723105).
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