Investigation of Alternative Substances for Replacing Hydrogen in Methanation

Abstract

Currently, a power-to-gas technology that obtains electrolytic hydrogen from renewable energy sources, synthesizes it with carbon dioxide, and converts it to methane has received a great deal of attention. It is called methanation, but there are few studies examining alternative substances to replace the raw material of hydrogen. Since hydrogen does not exist naturally, it is important to find other substances that react with carbon dioxide. We focus on flammable gases formed in oil refineries and petrochemical plants. In this study, based on chemical equilibrium calculations of the so-called NASA-CEA, we tested several gases including flammable and nonflammable gases by reacting them with carbon dioxide. Some of them are included in flare stacks. The reactants in the present gas conversion were H₂O, CH₃OH, C₂H₅OH, NH₃, CH₃CN, CH₃N₂CH₃, C₃H₈O (1-propanol), C₃H₈O (2-propanol), C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₃H₄ (allene), C₃H₄ (propine), C₆H₅OH, (CH₃COOH)₂, HCOOH, HF, HCl, HBr, H₂S, HNO₃, and SiH₄. The results show that substances with more hydrogen atoms per mol of reactant, such as C₃H₈, CH₃N₂CH₃, and SiH₄, can produce more synthetic methane. One more finding is that graphite due to coking increases proportionately to the number of carbon atoms in the chemical formula.

1. Introduction

Flare stacks are used in oil refineries and petrochemical plants [1]. The gas burned in flare stacks is not excess fuel but rather the gas produced when manufacturing from raw materials. Flare stacks are generally applied for two purposes: first, they are supposed to release gas from pressure vessels for safety in the event of a fire or leak in refinery and petrochemical plant facilities; second, they would temporarily release gas during the operation to render the gas harmless. In most cases, the gases are treated safely by the recovery facility for effective use. Still, if the recovery facility malfunctions or the recovery equipment is stopped, the unrecovered harmful or flammable gases are released from the flare stack. However, the gases burned in flare stacks are often combustible and can be used as fuel. Especially in vast oil fields, there are huge amounts of gases coming from the extracted crude oil (associated gas), so it is desirable to make effective use of these gases. In this study, we tried gas reforming where synthetic methane is produced by reacting these gases with carbon dioxide.

It is known that methanation synthesizes methane through the Sabatier reaction, in which carbon dioxide (CO₂) reacts with hydrogen [2]. Hydrogen is produced by electricity generated from sources of renewable energy. Since hydrogen never emits CO₂ when it burns, it can be used as an alternative fuel, instead of fossil fuels [2,3,4]. As another way to reduce carbon dioxide emissions, some attempts are being made to use a mixed fuel of hydrogen and fossil fuels in thermal power plants [5,6]. Recently, more research related to co-combustion of hydrogen and hydrocarbon has been conducted [7,8,9,10,11]. By utilizing the existing infrastructure such as pipelines [12,13], the mixing of natural gas and hydrogen has been tried [12,13]. The mixed fuel can be applied to gas turbines [14].

Here, we describe more about methanation. It uses surplus energy from renewable sources by using a water electrolyzer to produce green hydrogen, which is then synthesized with CO₂ for methane production. It is called a PtG (power-to-gas) technology [15]. In methanation, we need carbon dioxide emitted from sources such as power plants and factories. Then, the synthetic fuel is carbon-neutral because collected carbon dioxide is offset by the amount produced during its combustion. In addition, methane is known to be a major component of natural gas, which is the raw material for city gas. Then, we can utilize existing infrastructure and facilities such as city gas pipelines and gas storage facilities. That means natural gas is directly replaced by synthetic methane, making it an economically efficient and cost-saving way to promote decarbonization.

Currently, many demonstration tests of a PtG system for producing methane are being conducted, mainly in Europe [15,16]. In this process, the key is the catalytic activity for methanation [17,18,19]. Methanation reactors are being developed around the world. However, in the methanation process, only the Sabatier reaction is considered, where methane is synthesized from H₂ and CO₂. There are few studies examining the alternative substances that can replace hydrogen. Hydrogen does not exist naturally, so having an alternative that can react with carbon dioxide to produce methane opens up more options.

In this study, based on chemical equilibrium calculations of the so-called NASA-CEA, we investigated alternative chemical species to hydrogen. In total, 23 flammable and nonflammable gases were selected from the thermodynamic database of NASA-CEA. Some of them are included in gases in flare stacks. The results were compared with that of hydrogen in the conventional Sabatier reaction.

2. Numerical Method

2.1. Chemical Equilibrium Calculation by NASA-CEA

NASA-CEA is a chemical equilibrium program code, which calculates the equilibrium concentrations of products based on the thermodynamics of reactants and products from an arbitrary composition of reactants [20]. It has been used in many applications including hybrid rockets [21,22], a catalytic reactor [23], fuel cells [24], and a fire assessment [25]. In calculations, two thermodynamic states, as well as the chemical composition, are initially given, and the concentration and temperature at equilibrium are determined for the following six modes of the two independent variables as:

• TP mode (temperature and pressure);
• HP mode (enthalpy and pressure);
• SP mode (entropy and pressure);
• TV mode (temperature and volume);
• UV mode (internal energy and volume);
• SV mode (entropy and volume).

In this paper, all equilibrium calculations were performed under constant temperature and pressure conditions (TP mode). For specific usage, please refer to Ref. [20].

To investigate alternative chemical species to hydrogen, we reacted those gases with carbon dioxide. We selected 23 chemical species from the database of NASA-CEA. They reacted with carbon dioxide at a molar ratio of 1:1. We set the temperature range from 300 to 800 K, and the pressure was mostly 1 atm. The reactants selected here were H₂O, CH₃OH, C₂H₅OH, NH₃, CH₃CN, CH₃N₂CH₃, C₃H₈O (1-propanol), C₃H₈O (2-propanol), C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₃H₄ (allene), C₃H₄ (propine), C₆H₅OH, (CH₃COOH)₂, HCOOH, HF, HCl, HBr, H₂S, HNO₃, and SiH₄. By comparing the reaction of CO₂ with hydrogen, we made clear which is the most promising candidate for hydrogen alternatives in methanation.

2.2. Definition of Methane Yield

As we explained, one of the major methanation technologies is based on the Sabatier reaction [4]. Its process is the reaction between CO₂ and H₂ [15], which is expressed as follows:

CO₂ + 4H₂ = CH₄ + 2H₂O

(1)

There are important features in this reaction. First, it is an exothermic reaction. Second, we recognize that, before the reaction, there are 5 moles, whereas after the reaction the number of moles decreases to 3 moles. Therefore, the total number of moles is reduced in the Sabatier reaction. Due to these characteristics, it is expected that, given Le Chatelier’s principle, more methane is produced at lower temperature or higher pressure.

Before investigating the proper hydrogen alternatives in the methanation process, it is better to use the methane yield [26]. This can be calculated by the initial molar numbers of the reactants. In the Sabatier reaction, they are the molar numbers of CO₂ and H₂, N_CO2,0 and N_H2,0, which are the numerical parameters to determine the final component under chemical equilibrium. It is important to consider which reactants are in excess. This is because a deficient reactant can control the mass of methane production. Previously, we used the following two equations [27]. The methane yield, P_CH4, is given by:

$$P_{\mathrm{C}\mathrm{H}4}={\displaystyle \frac{N_{\mathrm{C}\mathrm{H}4}}{{\left(N_{\mathrm{C}\mathrm{H}4}\right)}_{max}}}={\displaystyle \frac{N_{\mathrm{C}\mathrm{H}4}}{N_{\mathrm{C}\mathrm{O}2,0}}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}$$

(2)

$$P_{\mathrm{C}\mathrm{H}4}={\displaystyle \frac{N_{\mathrm{C}\mathrm{H}4}}{{\left(N_{\mathrm{C}\mathrm{H}4}\right)}_{max}}}={\displaystyle \frac{N_{\mathrm{C}\mathrm{H}4}}{{(N}_{\mathrm{H}2,0})/4}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{i}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}$$

(3)

It is the content of methane in the products divided by the maximum amount of methane that can be produced. Here, N_CH4 is the molar quantity of methane. Instead, the mass fractions of reactants can be used, because they are the input variables in the code of NASA-CEA. The methane yield is also given by the following equation:

$$P_{\mathrm{C}\mathrm{H}4}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{{\left(Y_{\mathrm{C}\mathrm{H}4}\right)}_{max}/M_{\mathrm{C}\mathrm{H}4}}}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{Y_{\mathrm{C}\mathrm{O}2,0}/M_{\mathrm{C}\mathrm{O}2}}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}$$

(4)

$$P_{\mathrm{C}\mathrm{H}4}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{{\left(Y_{\mathrm{C}\mathrm{H}4}\right)}_{max}/M_{\mathrm{C}\mathrm{H}4}}}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{Y_{\mathrm{H}2,0}/M_{\mathrm{H}2}/4}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{i}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}$$

(5)

where Y_CH4 is the methane mass fraction in the equilibrium composition, Y_CO2,0 or Y_H2,0 is the initial mass fraction of CO₂ and H₂, and M_CO2 and M_H2 are the molecular weights of these reactants, respectively.

When other species react with CO₂, the following reaction can be considered for producing methane:

CO₂ + a X = b CH₄ + c H₂O

(6)

Here, X is the hydrogen alternative, and a, b, and c are the stoichiometric coefficients. In this case, the methane yield, P_CH4, is determined by:

$$P_{\mathrm{C}\mathrm{H}4}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{{\left(Y_{\mathrm{C}\mathrm{H}4}\right)}_{max}/M_{\mathrm{C}\mathrm{H}4}}}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{Y_{\mathrm{C}\mathrm{O}2,0}/M_{\mathrm{C}\mathrm{O}2}}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{d}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{i}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}$$

(7)

$$P_{\mathrm{C}\mathrm{H}4}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{{\left(Y_{\mathrm{C}\mathrm{H}4}\right)}_{max}/M_{\mathrm{C}\mathrm{H}4}}}={\displaystyle \frac{Y_{\mathrm{C}\mathrm{H}4}/M_{\mathrm{C}\mathrm{H}4}}{Y_{\mathrm{X},0}/M_{\mathrm{X}}/a}} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{i}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}$$

(8)

where Y_X is the mass fraction, and M_X is the molecular weight of the species, X.

3. Results

3.1. Effect of Temperature for Each Hydrogen Alternative

First, the results of the Sabatier reaction are explained. In the calculation, the molar ratio of n_CO2:n_H2 = 1:4 was used as the condition for the stoichiometric ratio in the methanation reaction. The methane mole fraction in the products was varied by the values of pressure and temperature of the reactants. Figure 1 shows the results when the pressure was 1 atm. We changed the temperature from 300 to 800 K. The mole fractions of carbon dioxide, hydrogen, methane, and water vapor are shown. At the relatively low temperature, the methane mole fraction was about 0.3, and it decreased at higher temperatures. These results are exactly the same as in the other references obtained by another solver [26,28], and we could confirm the validity of the present calculations. To see the dependence of the pressure, we set the pressures to be 5 and 10 atm. The results are shown in Figure 2. When the pressure was higher, the total number of moles of methane increased. By considering Le Chatelier’s principle, it is quite reasonable because the total molar number in Equation (1) is reduced before and after the reaction.

Next, we changed the initial composition before the reaction. The ratio of carbon dioxide to hydrogen, n_CO2:n_H2, was 1:1. This was mainly to narrow down the candidate substances that could replace hydrogen. It corresponds to the case where the amount of hydrogen is less than the stoichiometric condition. Figure 3 shows the results when the pressure was 1 atm. Although the amount of methane increased somewhat as the temperature was enlarged, the mole fraction of methane was less than 0.1. It is derived that a significant amount of CO₂ remains unreacted. Compared to Figure 1, graphite of carbon is found to precipitate more than the methane produced. Similar to Figure 2, we conducted the calculations for pressures of 5 and 10 atm. Although the amount of methane produced increased when the pressure was higher, the mole fraction of methane produced was still less than 0.1.

In the methanation process, it is reported that graphite of solid carbon is formed. This is called coking [29,30]. It is known that there are two mechanisms:

CH₄ = C(solid) + 2H₂

(9)

2CO = C(solid) + CO₂

(10)

Briefly, Equation (8) is the pyrolysis reaction of methane. On the other hand, Equation (9) is the so-called Boudouard reaction, which appears when the gasification of coal and other substances with excess carbon occurs [31]. In Figure 3, as the temperature increased, the amount of graphite precipitation decreased, and methane production increased. It is noted that graphite precipitation is not a favorable situation because it increases the pressure drop in the flow path of the methanation reactor and may lead to catalyst deactivation.

From now on, we searched for a chemical species that could potentially serve as a hydrogen alternative in methanation described in Equation (1). As explained, we checked both flammable and nonflammable gases. The initial compositions before the reaction were at a molar ratio of 1:1 under the pressure of 1 atm. The results are compared with those in Figure 3 to provide a quantitative discussion of which substances can be used for hydrogen alternatives.

Here, we explain the results. First, it was found that H₂O, HF, HCl, HBr, H₂S, and HNO₃ could not produce any methane when they were reacted with carbon dioxide. These species are chemically stable, even though they contain hydrogen atoms. Resultantly, they did not react with carbon dioxide to any significant extent. Additionally, HCOOH produced little methane, with a maximum methane mole fraction of 0.02. Therefore, these seven species were not candidates for replacing hydrogen.

Then, the results for C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₃H₄ (allene), and C₃H₄ (propine), whose chemical formulas consist only of C and H, were discussed. These are shown in Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8, except for that of C₃H₄ (propine) because its profile was almost the same as that of C₃H₄ (allene). The pressure was 1 atm. As seen in Figure 4, each mole fraction changed largely when the temperature was higher. We obtained the following three findings. First, when these six species were reacted with carbon dioxide, the amount of methane in the products was larger than that of hydrogen. Interestingly, as the number of hydrogen atoms in the chemical formula increased, more methane was produced. Second, the amount of graphite was larger when the number of carbon atoms in the chemical formula increased. Third, as mentioned before, the results for C₃H₄ (allene) and C₃H₄ (propine) were almost identical, indicating that it was not necessary to distinguish between the structural isomers of the reactants in the chemical equilibrium, since the amount of product after the reaction was the same regardless of the structural isomerism.

Next, CH₃OH (methanol), C₃H₈O (1-propanol), and C₆H₅OH (phenol), whose chemical formulas consist of C, H, and O, were shown. Only the mole fractions of methane were compared. These results are shown in Figure 9, together with the result of hydrogen reacting with CO₂. In the case of CH₃OH and C₃H₈O, the methane production was promoted in comparison with that of hydrogen. By examining the results of C₃H₈O (1-propanol) and C₃H₈O (2-propanol), we derived the fact that there was no need to distinguish between structural isomers in chemical equilibrium. We also found that the produced methane from the reaction of C₂H₅OH was less than that of C₃H₈O (1-propanol).

Finally, we examined ammonia and SiH₄, which do not contain any carbons in their chemical formulas but have a high number of hydrogen elements. The results are shown in Figure 10 and Figure 11. It should be noted that there was no ammonia or SiH₄ in the products, indicating that all of the ammonia and SiH₄ had reacted with carbon dioxide. In the case of NH₃, less methane was produced even when the temperature was changed. In the case of SiH₄ in particular, the amount of methane was roughly more than five times larger than that of hydrogen in the Sabatier reaction, although less methane was produced as the temperature was higher.

Here, we compared all results simultaneously. These are shown in Figure 12. In this comparison, carbon dioxide and hydrogen-substituting species were reacted at a molar ratio of 1:1. The abscissa shows the temperature, and the ordinate shows the methane mole fraction in the equilibrium composition. The largest amount of methane was the case of SiH₄, showing that the lower the temperature, the larger the amount of methane produced. It was also found that the second most abundant methane producer was C₃H₈. It could be derived that when the number of hydrogen atoms in the chemical formula is larger, more methane is produced.

3.2. Effect of Molar Ratio of Reactants on Methane Production

For further investigation, four promising reactants out of 23 species were selected, which were CH₃OH, C₃H₈, NH₃, and SiH₄. Then, more calculations were performed at different molar ratios for the reaction with carbon dioxide. The temperature was set at 600 K, which is the typical reaction temperature of the Sabatier process [32]. To compare the amount of methane production, only mole fractions of methane are shown in Figure 13. Moreover, the amount of graphite is shown in Figure 14. This is because graphite is more or less related to methane production. Here, X is the reactant with CO₂. The results of hydrogen were also plotted. It should be noted that when CO₂ reacts with H₂, the mole fraction of methane after the reaction reaches the maximum when the ratio of CO₂/H₂ is 0.25. This condition is the stoichiometric in the Sabatier reaction. As seen in Figure 13, when CH₃OH reacted with carbon dioxide, the amount of methane produced increased as the CO₂/CH₃OH ratio decreased, i.e., as the amount of CH₃OH was increased. At the same time, the amount of graphite precipitation in Figure 14 also decreased, indicating that more methane was produced when the amount of CH₃OH increased. The results for C₃H₈ show that, as with CH₃OH, the amount of methane produced increased as the amount of C₃H₈ that reacted with carbon dioxide increased. However, the amount of graphite formed by coking was about two times larger than that of CH₃OH.

Next, the results of the reaction of NH₃ with carbon dioxide were examined. The mole fraction of methane produced after the reaction was at its maximum when the CO₂/NH₃ ratio was 0.375. At the stoichiometry, the reaction is as follows:

$${\mathrm{C}\mathrm{O}}_2+{\displaystyle \frac{8}{3}}{\mathrm{N}\mathrm{H}}_3={\mathrm{C}\mathrm{H}}_4+{\displaystyle \frac{4}{3}}{\mathrm{N}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(11)

It is an exothermic reaction. The molar ratio in the above reaction is 8/3 mole of CO₂ to 1 mole of NH₃ (CO₂/NH₃ = 0.375), indicating that the maximum amount of methane is produced at this stoichiometric ratio. On the other hand, the results for SiH₄ showed that the maximum amount of methane was observed when the CO₂/SiH₄ ratio was 1.0. Since the molar ratio of methane to SiO₂ in the product was almost 1:1, the reaction between CO₂ and SiH₄ could be as follows:

$${\mathrm{C}\mathrm{O}}_2+\mathrm{S}\mathrm{i}{\mathrm{H}}_4={\mathrm{C}\mathrm{H}}_4+{\mathrm{S}\mathrm{i}\mathrm{O}}_2$$

(12)

However, if the molar ratio of CO₂ to SiH₄ differs from the stoichiometric ratio of 1:1, the composition of the products would change significantly, and the reaction in Equation (11) is not valid. In particular, as seen in Figure 14, graphite was formed instead of methane when CO₂/SiH₄ < 0.5.

Finally, the methane yield was calculated, assuming that X = hydrogen, NH₃, and SiH₄. Here, we could not determine the methane yield of CH₃OH or C₃H₈, because more methane was produced as more CH₃OH or C₃H₈ was reacted. Then, it was impossible to consider their stoichiometry conditions. The methane yield was determined by the reactions with carbon dioxide described in Equations (1) and (6), respectively. The calculated results are plotted in Figure 15. We set the abscissa to be the ratio to CO₂. The methane yield was the maximum around the stoichiometric compositions of hydrogen and ammonia, and the methane yield was close to 100% under the condition with less carbon dioxide than the stoichiometric ratio (oxidant deficient condition). However, under the condition of more carbon dioxide content (oxidant excess condition), ammonia showed a higher methane yield than that of hydrogen. On the other hand, SiH₄ showed a nearly 100% methane yield under stoichiometric conditions, but the methane yield decreased drastically as the condition was away from the stoichiometry.

4. Discussion

As for the reactants of carbon dioxide and hydrogen in the Sabatier reaction, it is easier to find data. Based on the chemical equilibrium calculations, carbon oxide methanation has been investigated by changing temperature and the initial components of the reactants [33]. For example, at the stoichiometric condition of the molar ratio of CO₂:H₂ = 1:4 mixture, the methane mole fraction at 400 K obtained by the kinetic model of CO₂ methanation is 0.33 [32], which is close to the equilibrium value in Figure 2. However, we could not find any studies that examined methanation by substituting hydrogen with other substances. Our unique attempt to find hydrogen alternatives in methanation would be worthwhile. Here, we discussed the suitable chemical species that could replace hydrogen in methanation to produce synthetic methane. Methane consists of one carbon atom and four hydrogen atoms, while carbon dioxide is a chemical species containing one carbon atom and two oxygen atoms. Therefore, to substitute hydrogen for another chemical species, it is easily expected that the alternative species must include some hydrogen atoms. Thus, in the present study, we selected the following chemical species: H₂O, CH₃OH, C₂H₅OH, NH₃, CH₃CN, CH₃N₂CH₃, C₃H₈O (1-propanol), C₃H₈O (2-propanol), C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₃H₄ (allene), C₃H₄ (propine), C₆H₅OH, (CH₃COOH)₂, HCOOH, HF, HCl, HBr, H₂S, HNO₃, and SiH₄. The amount of synthesized methane may increase if the chemical species contain more hydrogen atoms. Thus, the obtained results in Figure 12 were re-examined based on the number of H atoms. The results are shown in Figure 16. In this figure, results whose temperature was 600 K are shown.

It is seen that there was no tendency for more methane to be produced as the number of hydrogen atoms in the chemical formula increased. Surprisingly, the amount of methane produced was rather smaller even for species containing more hydrogen atoms than hydrogen. SiH₄ was the only species that produced more methane than hydrogen, and, except for hydrogen and SiH₄, none of the other species could produce more methane than hydrogen. It is noted that SiH₄ contains four hydrogen atoms, twice as many as hydrogen, but the amount of methane was not twice as much in comparison with hydrogen.

Here, we discussed why other species except for SiH₄ could not produce more methane even though they had more hydrogen atoms in the chemical formula. As shown in Equations (8) and (9), the possible explanation may be coking, which reduces methane production due to the precipitation of graphite. Therefore, we examined the correlation between the number of C in each chemical species and the amount of graphite contained in the product. All results are plotted in Figure 17. It is seen that SiH₄, which produced more methane than hydrogen in Figure 16, was not shown in this figure. This is because it does not contain any carbon atoms. According to the results, the amount of graphite in the product tended to increase as the number of carbons in the chemical formula increased. However, when the reactants contained oxygen atoms, coking was suppressed, and less graphite was produced.

Therefore, species that contain more hydrogen atoms than hydrogen could not produce methane, because graphite was produced due to coking. The only exception was SiH₄. SiH₄ is not affected by coking because it does not contain carbon atoms, and it has more hydrogen atoms than hydrogen, resulting in more methane production.

5. Conclusions and Future Work

In this study, equilibrium calculations of NASA-CEA were performed to investigate alternative species to hydrogen in the methanation process, compared with the results of the Sabatier reaction. The reactants were H₂O, CH₃OH, C₂H₅OH, NH₃, CH₃CN, CH₃N₂CH₃, C₃H₈O (1-propanol), C₃H₈O (2-propanol), C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₃H₄ (allene), C₃H₄ (propine), C₆H₅OH, (CH₃COOH)₂, HCOOH, HF, HCl, HBr, H₂S, HNO₃, and SiH₄. Resultantly, we obtained the following findings.

It is not necessary to distinguish between structural isomers in chemical equilibrium. The six reactants of H₂O, HF, HCl, HBr, H₂S, and HNO₃ do not produce methane. HCOOH produces little methane, and these seven species are unsuitable for the hydrogen alternative. On the other hand, substances with more hydrogen atoms in their chemical formula, such as C₃H₈, CH₃N₂CH₃, and SiH₄, produce more methane, but the amount of graphite formed by coking increases with the number of carbons in chemical formula as well. In the present calculation, the maximum amount of methane is produced by SiH₄. However, under conditions of high carbon dioxide content (oxidant excess conditions), ammonia also shows a higher methane yield than hydrogen, suggesting that SiH₄ and ammonia could be promising candidates for replacing hydrogen.

It is well noticed that, even today, a wide range of CO₂ utilization is being considered in a variety of fields [2]. Our present approach for reactions between CO₂ and hydrogen alternatives could be one of the new ways to utilize CO₂. In the first step of the present paper, we tried to find candidate alternatives using the equilibrium calculations. In the next step, we will perform more calculations to validate them, as well as experiments. Moreover, using detailed kinetic models, it is necessary to seek promising reactions for methane production. A technical and economic assessment is also effective in identifying the most appropriate substances. This work could be a stepping-stone to provide a variety of options using non-hydrogen reactants for future power-to-gas technology.