With the aim of directly producing a high-quality syngas with a ratio of H2/CO = 2, bi-reforming of CH4 with the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2 is carried out in a gliding arc-based warm plasma catalytic reactor. The gliding arc plasma is a typical warm plasma (WP), which provides favorable conditions for CO2 activation, and it is found that the highest conversions are obtained in the case of reaction using the WP alone. A comparison of reactions using the WP alone (the WP case), the conventional catalyst alone (the CC case), and the WP plus catalyst (the WPC case) reveals that the WPC case can overcome the disadvantages of both the WP and CC cases. In the WPC case, CH4, CO2, and H2O react at the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2. In addition, higher reactant conversions and energy efficiencies are obtained in the WPC case than in the WP case. A high-quality syngas with H2/CO = 2 is obtained, with similar conversions of (89 ± 1)% for all of CH4, CO2, and H2O and an energy efficiency of 71%.

Syngas, composed of CO and H2, is one of the important feedstocks for the Fischer–Tropsch process and chemical synthesis. In particular, syngas with a ratio H2/CO = 2 has been used for methanol synthesis as a basis of the “methanol economy.”1 CH4 is the preferred source for syngas production, owing to its plentiful supply and its highest H/C atomic ratio. Steam reforming of CH4 according to the following reaction is the most commonly used commercial reaction route for syngas production:2,
CH4+H2O(g)=CO+3H2,ΔH2980=206kJ/mol,
(R1)
This is a strongly endothermic reaction, producing syngas with a high H2/CO molar ratio of ∼3. Dry reforming of CH4 according to
CH4+CO2=2CO+2H2,ΔH2980=247kJ/mol,
(R2)
is another promising reaction route for CH4 to syngas, but it produces syngas with a low H2/CO molar ratio of ∼1.3 Therefore, additional steps are required for both reactions (R1) and (R2) to adjust the H2/CO molar ratio for practical application. By combining reactions (R1) and (R2) in a CH4/CO2/H2O mixture, the H2/CO molar ratio of the produced syngas can be adjusted by varying the combination coefficient α in single step with an overall reaction of (R1) + (1 − α)(R2). Theoretically, a high-quality syngas with H2/CO = 2 can be obtained at α = 2/3 with the following reaction, called bi-reforming of CH4:
3CH4+CO2+2H2O(g)=4CO+8H2,ΔH2980=659kJ/mol.
(R3)
However, this is considered difficult,4 owing to the water gas shift reaction that occurs as a side-reaction:
CO+H2O(g)=CO2+H2,ΔH2980=41kJ/mol.
(R4)

In this way, a syngas with H2/CO = 2 has been achieved with excess CO2 and H2O feeding, as reported previously.4–7 There have been no reports of direct production of a syngas with H2/CO = 2 at the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2.

In nonthermal plasmas, electrons can be accelerated selectively in such a way that almost all the plasma energy goes into the formation of a thermal nonequilibrium state.8 Impacts between energetic electrons and molecules produces reactive species that can induce reactions. Plasmas are commonly generated by applying electrical power to a gas, which can be quickly switched on/off. Hence, it is feasible to power a plasma with fluctuating renewable electricity and use this as a form of fuel storage.9 To date, there have been no published reports about bi-reforming of CH4 in plasmas, but various plasma types have been studied for dry, steam, and oxidative reforming of CH4 to syngas.10–14 Of these, warm plasmas (WPs), with intermediate gas and electron temperature between those of cold and thermal plasmas and formed by a gliding arc (GA), are believed to be among the most promising types,13,15 especially for the conversion of CH4 and CO2, because they operate at atmospheric pressure and exhibits a high energy efficiency.9,13,15 In addition, WPs have electron temperatures of 1–2 eV, and therefore provide a favorable environment for CO2 conversion via efficient vibrational excitation.15,16 However, previous works have concluded that plasma reforming suffers from low energy efficiency and poor conversion.17,18 For practical application of plasma-based power to fuels, it would be of great interest to combine plasmas with catalysts to improve conversion, product selectivity, and energy efficiency.19,20 WP reactors have already been used in combination with catalysts for CH4 reforming, but the reactor designs were not optimized to supply the required activation temperature for the catalysts (∼800 °C), and thus low energy efficiencies (<50%) were obtained.20,21 In our previous work, we have reported an efficient WP catalytic (WPC) reforming reactor combining a WP and a Ni/CeO2/Al2O3 catalyst. This WPC reactor has shown excellent performance (high energy at high conversion) for oxidative and dry reforming of CH4.22–24 In addition, a novel route for power to fuel synthesis in this WPC reactor has been proposed.22 In the present work, we use the same reactor to investigate bi-reforming of CH4 and demonstrate the production of a high-quality syngas with H2/CO = 2 at the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2.

The WPC reactor is the same as that used in our previous work.22,24 Briefly, the reactor is composed of a vortex gas flow GA reactor and a downstream tubular catalytic reactor, as illustrated in Fig. 1. To avoid carbon deposition in the plasma reactor, the reactants are fed from two inlets (F1 and F2). One flow (F1), which is a mixture of CH4/CO2/H2O = 1.5/x/3 − x (x = 0–3), is used as a discharge gas to form a vortex flow in the GA reactor with a flow rate of 2.2 standard liters per minute (SLM). The other flow (F2), which is pure CH4 with a flow rate of 0.7 SLM, is fed after the GA reactor but before the catalyst bed to adjust the total molar ratio of CH4/CO2/H2O = 3/x/3 − x. The GA plasma is powered with a direct current power supply with a maximum voltage of 10 kV and a current of 100 mA. The discharge voltage, current, and power Pd are recorded by an oscilloscope (Tektronix DPO 4104B) via a voltage probe (Tektronix P6015A) and a 51 Ω sampling resistor, respectively.

FIG. 1.

Schematic of plasma catalytic reactor.

FIG. 1.

Schematic of plasma catalytic reactor.

Close modal

The temperature of the catalytic reactor is kept by a homemade furnace at 1123 K, whose power Ph is monitored by a power meter. The Ni/CeO2/Al2O3 catalyst pellets (∼2 mm diameter) with weight percentages of 11% nickel and 8% cerium, prepared by sequential incipient wetness impregnation as described in detail in Ref. 23, are packed into the catalytic reactor. This Ni-based catalyst is auto-reduced by the off gas from the GA plasma, and thus there is no pre-reduction in this process. The reactor can run in three modes: with a WP only (henceforth referred to as the WP case), with conventional catalysis (the CC case), and with a WP plus catalyst (the WPC case). The gas hourly space velocity (GHSV) in the CC and WPC cases is changed by varying the packing amount of catalyst.

The temperature profile inside the catalyst bed is measured with a movable K-type thermocouple along the central axis of the bed. The gaseous products are analyzed by two online gas chromatographs (Agilent 1790 and 6890 N), using N2 as an internal standard for CO, CH4, CO2, and hydrocarbons (C2H6, C2H4, and C2H2), and helium as an internal standard for H2. The method for analysis of the products is described in detail in the supplementary material. Briefly, the outlet flow rates of reactants and products are measured in terms of the flow rate FIS and concentration CISout of the internal standard gas as follows:
Fiout=FISCioutCISout,
(1)
where Fiout and Ciout are the outlet flow rate and concentration of i (= CH4, CO2, CO, hydrocarbons, and H2). This allows us to calculate the conversion as
Xi=FiinFioutFiin×100%,
(2)
where Fiin is the inlet flow rate of reactant i.
The selectivities of CO and C2 hydrocarbons (C2H2 ≫ C2H4 > C2H6) are defined on a carbon basis:
SCO=FCOoutFCH4inXCH4+FCO2inXCO2×100%,
(3)
SC2=2FC2outFCH4inXCH4+FCO2inXCO2×100%.
(4)
The selectivity of H2 is defined on a hydrogen basis:
SH2=FH2out2FCH4inXCH4+FH2OinXH2O×100%.
(5)
In addition, the specific energy input SEI and energy efficiency η are given by
SEI=PdF1,
(6)
η=FH2outLHVH2+FCOoutHVCOPd+Ph+FCH4inXCH4LHVCH4×100%,
(7)
where LHVH2, LHVCH4, and HVCO are the lower heating values of H2 and CH4 and the heating value of CO.

In plasma reforming, especially in the case of endothermic reactions, the reactions are induced by electrons and reactive species produced by electron impact. Hence, the reactant conversion and product selectivity depend strongly on the plasma and electrical parameters. We measured the waveforms of discharge voltage, current and power at various SEI values (80–110 kJ/mol), gas flow rates, and CH4/CO2/H2O ratios. The results show that similar trends are obtained under all the experimental conditions. Hence only the waveforms at an F1 molar ratio of CH4/CO2/H2O = 1.5/1/2, a flow rate of 2.2 SLM, and an SEI of 110 kJ/mol is illustrated in Fig. 2. It can be seen that the GA discharge possesses the classic “ignition–gliding–extinguishment” pattern,25,26 with an average cycle time of ∼1.6 ms. The arc channel is ignited at a voltage of ∼3.5 kV. Once the arc ignites, the discharge voltage drops to ∼2.6 kV at a current of ∼69 mA, and then the voltage and current respectively increase and decrease gradually to extinguishment/re-ignition with arc gliding. Balanced by the opposite trends of voltage and current, the discharge power fluctuates around 180 ± 6 W. Under the assumption that the largest arc length is equal to the distance from the high-voltage electrode to the edge of the GA reactor outlet (∼29 mm), the intensity of the electric field E along the arc channel is estimated to lie in the range of 90–120 V/mm during the arc evolution. This implies that the plasma generated in this GA reactor always remains in a nonthermal condition, but not undergo a transition from thermal to nonthermal.27 In addition, the cycle time of this GA discharge (∼1.6 ms) is much shorter than that of the traditional 2D GA reactor (tens of ms), which can significantly enhance the interaction between plasma and reactants to increase reactant conversions.27,28

FIG. 2.

Waveforms of voltage, current, and power in a discharge gas mixture with F1: CH4/CO2/H2O = 1.5/1/2 and an SEI of 110 kJ/mol.

FIG. 2.

Waveforms of voltage, current, and power in a discharge gas mixture with F1: CH4/CO2/H2O = 1.5/1/2 and an SEI of 110 kJ/mol.

Close modal

In a plasma, dissociation of reactants through direct electron impact and stepwise vibrational excitation is the major contribution to the initiation of reactions.29–31 This means that in a gas mixture of CH4/CO2/H2O, plasma reforming should mainly start with the three reactions e + CH4e + CH3 + H, e + CO2e + CO + O, and e + H2O → e + H + OH. The H, O, and OH radicals thereby produced then induce chain reactions to form the final products. As mentioned above, the plasma parameters are insensitive to SEI, flow rate, and gas composition in this GA-based warm plasma, and thus the reaction pathway is only weakly dependent on SEI and flow rate at a given CH4/CO2/H2O molar ratio. Changes in SEI and flow rates of reactants have strong effects on conversions, but only weak effects on product selectivities. This trend has been found previously,32–34 and therefore we only present the effects of CO2/(CO2 + H2O) molar ratio on bi-reforming of CH4 in the WP case. As illustrated in Fig. 3, with increasing CO2/(CO2 + H2O) molar ratio from 0 (steam reforming) to 1 (dry reforming) at an F1 flow rate of 2.2 SLM, an F2 flow rate of 0.7 SLM, and an SEI of 110 kJ/mol, XCO2 and XCH4 remain around 36% and 26%, respectively. However, XH2O significantly decreases with increasing CO2/(CO2 + H2O) molar ratio, even falling to negative values when the CO2/(CO2 + H2O) molar ratio exceeds 0.5. XCO2 is the highest among those three reactants, which can be attributed to the plasma parameters determined above. This GA-based warm plasma can achieve efficient vibrational excitation and provide favorable conditions for CO2 activation.15,35 In addition, CO2 can also be reduced by H atoms produced as a result of CH4 dissociation in a WP.26,36 XH2O is the lowest among the reactants because the cross section of H2O for electron impact dissociation is lower than those of the other two molecules.37,38 Moreover, the produced radicals are easily converted back to H2O via their reactions. Figure 3(b) presents the effect of CO2/(CO2 + H2O) molar ratio on selectivities of CO, C2, and H2. CO and H2 are the main gaseous products, which indicates that steam and dry reforming are the main reaction routes in a WP. The by-products of C2 hydrocarbons are generated by dehydrogenation of CH4. As mentioned above, plasma parameters are insensitive to CO2/(CO2 + H2O) molar ratio, which implies that the contributions of electrons to the conversion of CH4, CO2, and H2O should be similar at various CO2/(CO2 + H2O) molar ratios. Hence, variations in conversions and product selectivities are induced by changes in the radical distributions at various CO2/(CO2 + H2O) molar ratios. When the initial reactions induced by electrons are considered, an increased CO2/(CO2 + H2O) molar ratio results in more O atoms and CO being produced in the plasma. O atoms easily react with CH4 and H to form OH radicals, and reduction of CO2 by H atoms also produces OH radicals. These two pathways are more efficient than dissociation of H2O. Obviously, this reduces H2 selectivity and finally produces more H2O to decrease its conversion.

FIG. 3.

Effect of CO2/(CO2 + H2O) molar ratio on (a) conversions of CH4, CO2, and H2O and outlet temperature of GA plasma reactor and (b) selectivities of H2, CO, and C2 hydrocarbons at an SEI of 110 kJ/mol.

FIG. 3.

Effect of CO2/(CO2 + H2O) molar ratio on (a) conversions of CH4, CO2, and H2O and outlet temperature of GA plasma reactor and (b) selectivities of H2, CO, and C2 hydrocarbons at an SEI of 110 kJ/mol.

Close modal

As shown in Fig. 4, the H2/CO molar ratio decreases monotonically with CO2/(CO2 + H2O) molar ratio owing to the competition between dry and steam reforming of CH4 in a WP. The ideal ratio H2/CO = 2 is obtained at CO2/(CO2 + H2O) = 1/3. The energy efficiency remains around (61 ± 1)% in the test range of CO2/(CO2 + H2O) molar ratio.

FIG. 4.

Effect of CO2/(CO2 + H2O) molar ratio on energy efficiency and H2/CO molar ratio at an SEI of 110 kJ/mol.

FIG. 4.

Effect of CO2/(CO2 + H2O) molar ratio on energy efficiency and H2/CO molar ratio at an SEI of 110 kJ/mol.

Close modal

Although H2/CO = 2 is achieved at the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2 in a WP, the conversions and energy efficiency in a WP are still too low for practical application. To produce a high-quality syngas at a high conversion and a high energy efficiency, 11%Ni/8%CeO2/Al2O3 catalyst is packed downstream of the GA-based warm plasma in the same way as in our previous work.22 The average gas temperature post GA reactor is measured by a thermocouple as ∼850 K at a SEI of 110 kJ/mol. This temperature is much lower than the required catalyst temperature for bi-reforming, and thus almost no reaction occurs over the catalyst bed if we simply pack the catalyst after the plasma, and the same reactant conversions and product selectivities are obtained as in the WP case. To activate the catalyst, a tubular furnace is used to maintain the catalyst bed temperature at 1123 K. By varying the amount of packed catalyst from 10 to 30 g, the effect of gas hourly space velocity (GHSV) on conversions and product selectivities was studied for the CC and WPC cases at the ideal CO2/(CO2 + H2O) = 1/3 in the GHSV range of 5800–17 400 ml h−1 g−1. As illustrated in Fig. 5, all the conversions decrease with increasing GHSV. In the CC case, XH2O becomes the highest and XCO2 the lowest with increasing GHSV, because of the water gas shift reaction.4 These opposite trends in the CC and WP cases imply that CO2 conversion is kinetically favored in the WP case and H2O conversion in the CC case. In the WPC case, similar values of XCH4, XCO2, and XH2O are achieved in the test range of GHSV, because WPC provides favorable conversion environments for both CO2 and H2O. In addition, the WPC case gives higher conversions than both the CC and WP cases, because it is a combination of these cases. In particular, the conversions of CO2 (66%) and H2O (64%) in the WPC case are higher than the sums of their respective values in the WP (XCO2 = 37% and XH2O=11%) and CC (XCO2 = 26% and XH2O=47%) cases at a GHSV of 17 400 ml h−1 g−1. By combining WP and catalyst, portions of the reactants, especially CO2, are converted and produce active species in the plasma, and it is then possible for energy and reactive species with long lifetime carried by the plasma to reach the catalyst and enhance reactions. This higher reactant conversion also leads to the high syngas concentration in WPC case, which reaches a range of 85%–96% under the test GHSV conditions. By contrast, the syngas concentration in the CC case is only in the range of 67%–90%. This trend is also revealed by the temperature distribution of the catalyst bed in the CC and WPC cases. Figure 6 illustrates the temperature profiles of the catalytic reactor wall and catalyst bed at a GHSV of 17 400 ml h−1 g−1. Since the wall temperature of the catalytic reactor is controlled by the furnace in both the CC and WPC cases, it is similar in the two cases and is maintained at ∼1123 K along the catalyst bed. The temperature profiles measured along the central axis in the CC and WPC cases are much lower than the wall temperature, owing to the strong endothermic reaction over the catalyst. In addition, the temperature of the catalyst in the WPC case is higher than that in the CC case, which can be attributed to the WP supplying heat and active species in the WPC case.

FIG. 5.

Effect of GHSV on conversions of CH4, CO2, and H2O at an SEI of 110 kJ/mol and a furnace temperature of 1123 K.

FIG. 5.

Effect of GHSV on conversions of CH4, CO2, and H2O at an SEI of 110 kJ/mol and a furnace temperature of 1123 K.

Close modal
FIG. 6.

Temperature distributions of catalytic reactor wall and catalyst bed for CC and WPC cases at a GHSV of 17 400 ml h−1 g−1.

FIG. 6.

Temperature distributions of catalytic reactor wall and catalyst bed for CC and WPC cases at a GHSV of 17 400 ml h−1 g−1.

Close modal

The reactions of CH4, CO2, and H2O are fed with the ideal stoichiometric ratio of bi-reforming (CH4/CO2/H2O = 3/1/2). Theoretically, if all the reactants convert in equivalent amounts, the consumption rates of CH4, CO2, and H2O should meet the values of RCO2/RCH4 = 1/3 and RH2O/RCH4=2/3. With the WPC case, it is possible to overcome the respective disadvantages of the WP and CC cases. Both RCO2/RCH4 = 1/3 and RH2O/RCH4=2/3 are achieved in the test range of GHSV. This means that all the reactants are reformed with the ideal stoichiometric ratio for bi-reforming of CH4 in the WPC case. This is a result of coupling the advantages of WP and CC for conversions of CO2 and H2O, respectively. In addition, with catalyst packing, only CO and H2 are found with selectivities close to 100%, and almost all the C2 hydrocarbons produced in the WP are converted to syngas. Therefore, the desired H2/CO ratio of 2 is also obtained in the WPC case (see Fig. 7). With monitoring of the heating power of the furnace, the energy efficiencies for the CC and WPC cases can be calculated using Eq. (7). As the GHSV increases from 5800 to 17 400 ml h−1 g−1, the energy efficiency of bi-reforming of CH4 in the WPC case decreases from 72% to 65%. In addition, to further compare CC and WPC cases, we characterized the used catalyst in the WPC case by x-ray diffraction and transmission electron microscopy. The results shows that the catalyst characteristics are similar to those found in previous work.23 The nickel particle size is distributed in the range of 15–30 nm, with an average of ∼22 nm.

FIG. 7.

Effect of GHSV on energy efficiency and H2/CO ratio in CC and WPC cases at an SEI of 110 kJ/mol and a furnace temperature of 1123 K.

FIG. 7.

Effect of GHSV on energy efficiency and H2/CO ratio in CC and WPC cases at an SEI of 110 kJ/mol and a furnace temperature of 1123 K.

Close modal

Bi-reforming in a warm plasma catalytic reactor using a warm plasma (WP) and a Ni/CeO2/Al2O3 catalyst with the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2 has been investigated. A high-quality syngas with a H2/CO = 2 is obtained at similar conversions of (89 ± 1)% for all of CH4, CO2, and H2O. Electrical diagnostics show that the gliding arc plasma in a CH4/CO2/H2O gas mixture is a warm one and can provide favorable conditions for CO2 activation, as a consequence of which the highest conversion is achieved with a WP alone. A comparison of the results obtained with the WP alone (the WP case), a conventional catalyst alone (the CC case), and the WP plus catalyst (the WPC case) reveals that the WPC case can overcome the disadvantages of both the WP and CC cases. In the WPC case, CH4, CO2, and H2O react with the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2 in the tested GHSV range. In addition, higher conversions and energy efficiencies are obtained in the WPC case than in the WP case.

See the supplementary material for the detailed products analytical method.

This work was supported by the National Natural Science Foundation of China (Grant No. 12375247).

The authors have no conflicts to disclose.

Jing-Lin Liu: Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Writing – original draft (equal). Ai-Min Zhu: Conceptualization (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.

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Supplementary Material