Effect of Nitrogen/Oxygen Substances on the Pyrolysis of Alkane-Rich Gases to Acetylene by Thermal Plasma

: It is important to convert alkane-rich gases, such as coke oven gas, to value-added chemicals rather than direct emission or combustion. Abundant nitrogen/oxygen substances are present in the actual alkane-rich gases. However, the research about how they inﬂuence the conversion in the pyrolysis process is missing. In this work, a systematic investigation on the effect of various nitrogen/oxygen-containing substances, including N 2 , CO, and CO 2 ,on the pyrolysis of CH 4 to C 2 H 2 was performed by a self-made 50 kW rotating arc thermal plasma reactor, and the pyrolysis of a simulated coke oven gas as a model of alkane-rich mixing gas was conducted as well. It was found that the presence of N 2 and CO 2 was not conducive to the main reaction of alkane pyrolysis for C 2 H 2 , while CO, as a stable equilibrium product, had little effect on the cracking reaction. Consequently, it is suggested that a pretreatment process of removing N 2 and CO 2 should be present before pyrolysis. Both input power and feed rate had considerable effect on the pyrolysis of the simulated coke oven gas, and a C 2 H 2 selectivity of 91.2% and a yield of 68.3% could be obtained at an input power of 17.9 kW.


Introduction
As the essential material basis for human survival and development, petroleum, coal, natural gas, and other fossil fuels support the development of human civilization and economy. The utilization process of fossil fuels produce a great deal of alkane-rich gases as their main product or byproduct, such as coke oven gas, refinery gas, natural gas and unconventional natural gas (coalbed methane, shale gas, and tight sandstone gas). For example, the coke-making process produces coke oven gas (COG) as a byproduct, and typically 1.25-1.65 tons of coal produce a single ton of coke, along with approximately 300-360 m 3 of COG (6-8 GJ/t coke) [1]. Although there are reports that using those alkane-rich gases as feedstocks of chemicals, most of them are still utilized as fuel by combustion (ca. 70-80%) or directly discharged into the atmosphere without treatment or as a torch, not only causing significant waste of resources, but also producing large amounts of greenhouse gases and hazardous pollutants which jeopardize the environment. Therefore, it is of great importance to develop more ways to convert alkane-rich gases to value-added chemicals through effective reactions.
Acetylene, known as the "mother of organic chemical products", plays a very important role not only in the field of metal processing, welding and cutting [2], but also in the production of poly(vinyl chloride) (PVC), trichloroethylene, vinyl acetate, acrylonitrile, poly-acrylonitrile [3], etc. Its abundant downstream products attract the growing pursuit in the chemical industry for alternative production methods of acetylene, because the two conventional methods-calcium carbide hydrolysis and

Reactor Setup
A 50 kW rotating arc plasma reaction device system was used in this experiment, as shown in Figure 1a, mainly consisting of four parts: a constant current power supply, plasma reactor, cooling and quenching system, and a data acquisition and control system. Figure 1b shows the structure of the 50 kW rotating arc plasma reactor, mainly including a cathode, cathode flange, anode, anode upper flange, anode bottom flange, and the excitation coil. The excitation coil is sheathed outside the anode sleeve generating a magnetic field under the action of impressed current, to produce the Lorenz force in the reactor, which results in a high-speed rotation of the arc.

Gas Analysis
The components of pyrolysis gas are complex, possibly including CH4, C2H2, C2H4, C2H6, CO, CO2, and N2, and it is convenient to adopt gas chromatography for analysis with GC 1690 provided by Hangzhou Kexiao Chemical Equipment Co, Ltd. (Hangzhou, China). The chromatographic columns are a PLOT 5A zeolite-packed column provided by Hangzhou Kexiao Chemical Equipment Co, Ltd. (Hangzhou, China) and a PLOT Q capillary column from Agilent Technologies Co, Ltd. (Hangzhou, China). As a possible product in the nitrogen reaction, HCN is highly toxic and must be carefully handled. The relatively safe and simple AgNO3 titrimetric method was adopted, using a NaCl standard solution (0.0100 mol/L) to calibrate the standard solution of AgNO3 (0.0100 mol/L), then determining the concentration of cyanide ions in the collection solution with a calibrated AgNO3 solution. The cyanide ion collecting solution is obtained by slowly accessing the sample bag gas (about 2 L) to 0.1 mol/L NaOH solution to be absorbed.

Gas Analysis
The components of pyrolysis gas are complex, possibly including CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , CO, CO 2 , and N 2 , and it is convenient to adopt gas chromatography for analysis with GC 1690 provided by Hangzhou Kexiao Chemical Equipment Co, Ltd. (Hangzhou, China). The chromatographic columns are a PLOT 5A zeolite-packed column provided by Hangzhou Kexiao Chemical Equipment Co, Ltd. (Hangzhou, China) and a PLOT Q capillary column from Agilent Technologies Co, Ltd. (Hangzhou, China). As a possible product in the nitrogen reaction, HCN is highly toxic and must be carefully handled. The relatively safe and simple AgNO 3 titrimetric method was adopted, using a NaCl standard solution (0.0100 mol/L) to calibrate the standard solution of AgNO 3 (0.0100 mol/L), then determining the concentration of cyanide ions in the collection solution with a calibrated AgNO 3 solution. The cyanide ion collecting solution is obtained by slowly accessing the sample bag gas (about 2 L) to 0.1 mol/L NaOH solution to be absorbed.

Data Processing Method
Carbon yield (Y) of acetylene in the product is generally defined as: Carbon conversion efficiency (CE) of the feed gas is generally defined as: Selectivity (S) of acetylene is defined as: When pyrolyzed, the siimulated coke oven gas is defined as: Specific energy requirement (SER) of acetylene is defined as: where Q 0 CH 4 , Q 0 C 2 H 6 represent the volume flow of CH 4 and C 2 H 6 in the feedstock, respectively, and Q C 2 H 2 , Q CH 4 , Q C 2 H 6 are the volume flow of C 2 H 2 , CH 4 , and C 2 H 6 in the product.

Effect of Nitrogen/Oxygen Substances on Methane Pyrolysis
It has been acknowledged that the thermal energy is the critical point for pyrolysis of hydrocarbons to acetylene by thermal plasma, but the mass and heat transfer mechanism in the reaction system is difficult to describe quantitatively. Up to now, the accredited mechanism and sequence of chemical transformations during the pyrolysis of saturated hydrocarbons is described below [14]: • • CH 2 · · · · · · · · · · · · · · · · · · C 4 H 2 → C 6 However, because CO 2 , N 2 , and CO may also decompose in the atmosphere and react with the intermediate product of alkane pyrolysis in Equation (8), their presence in alkane-rich feed gas is expected to influence the pyrolysis products. To clarify the effect of each of them, CO 2 , N 2 , and CO were respectively mixed into methane (as a representative alkane) to serve as a binary feed gas for pyrolysis. The experimental conditions are as follows: the molar ratio of input H 2 /CH 4 was 1.5/1, the total feed rate of H 2 and CH 4 was 7.5 Nm 3 /h, the magnetic induction intensity was 0.077 T, and the input power was controlled at 15 ± 0.5 kW. By adding different amount of CO 2 in methane, the CO 2 volume fraction changed in the range of 2-14%. The plasma-enhanced process of oxidation conversion may be explained as the reaction below in Equation (9) along with reactions in Equation (8): Figure 2 shows the effect of CO 2 content on the kinds of products, including mainly hydrocarbons, CO 2 , and CO, at an input power of 15 kW. As can be seen from the figure, with the increase in CO 2 content, the mole fraction of C 2 H 2 reduced from 8.4% to 6.3% and C 2 H 4 concentration reduced slightly from 0.61% to 0.31%. This is mainly ascribed to a competitive relationship between the oxygen-free pyrolysis reactions in Equation (8) and oxidation reaction in Equation (9), and the oxidation reaction hindered the dominant pyrolysis reactions that produce C 2 H 2 . This is also supported by the result that increasing CO 2 content in the feed gas improved the CO content in the product. On the contrary, CH 4 content in the product ascended from 10.1% to 15.2% although the CH 4 content in the feed gas was slightly reduced when adding CO 2 . This is probably because when the CO 2 content increased, the pyrolysis of CO 2 to CO and O· radicals consume a great deal of input energy, which restricted the conversion of CH 4 by either Equation (8) or Equation (9).
However, because CO2, N2, and CO may also decompose in the atmosphere and react with the intermediate product of alkane pyrolysis in Equation (8), their presence in alkane-rich feed gas is expected to influence the pyrolysis products. To clarify the effect of each of them, CO2, N2, and CO were respectively mixed into methane (as a representative alkane) to serve as a binary feed gas for pyrolysis. The experimental conditions are as follows: the molar ratio of input H2/CH4 was 1.5/1, the total feed rate of H2 and CH4 was 7.5 Nm 3 /h, the magnetic induction intensity was 0.077 T, and the input power was controlled at 15 ± 0.5 kW.

Effect of CO2 on Methane Pyrolysis
By adding different amount of CO2 in methane, the CO2 volume fraction changed in the range of 2-14%. The plasma-enhanced process of oxidation conversion may be explained as the reaction below in Equation (9) along with reactions in Equation (8): Figure 2 shows the effect of CO2 content on the kinds of products, including mainly hydrocarbons, CO2, and CO, at an input power of 15 kW. As can be seen from the figure, with the increase in CO2 content, the mole fraction of C2H2 reduced from 8.4% to 6.3% and C2H4 concentration reduced slightly from 0.61% to 0.31%. This is mainly ascribed to a competitive relationship between the oxygen-free pyrolysis reactions in Equation (8) and oxidation reaction in Equation (9), and the oxidation reaction hindered the dominant pyrolysis reactions that produce C2H2. This is also supported by the result that increasing CO2 content in the feed gas improved the CO content in the product. On the contrary, CH4 content in the product ascended from 10.1% to 15.2% although the CH4 content in the feed gas was slightly reduced when adding CO2. This is probably because when the CO2 content increased, the pyrolysis of CO2 to CO and O· radicals consume a great deal of input energy, which restricted the conversion of CH4 by either Equation (8) or Equation (9).  Figure 3 illustrates the effect of CO2 content on the yield (Y) of C2H2 and CH4 conversion efficiency (CE) at 15 kW. With the increase of CO2 content, the yield of C2H2 and CH4 conversion efficiency showed a synchronous decreasing trend. This is consistent to the results of Figure 2, suggesting that the increase of CO2 content was not conducive to the pyrolysis reaction based on the fact that more input energy was taken away by both the pyrolysis of CO2 and the endothermic reaction Equation (9). The effect of different CO2 content on the SER of C2H2 was illustrated in Figure  4. With the increase of CO2 content, the SER of C2H2 was rising. This directly revealed that the higher the proportion of CO2 in the reaction system, the more energy was consumed by side-reactions. From  Figure 3 illustrates the effect of CO 2 content on the yield (Y) of C 2 H 2 and CH 4 conversion efficiency (CE) at 15 kW. With the increase of CO 2 content, the yield of C 2 H 2 and CH 4 conversion efficiency showed a synchronous decreasing trend. This is consistent to the results of Figure 2, suggesting that the increase of CO 2 content was not conducive to the pyrolysis reaction based on the fact that more input energy was taken away by both the pyrolysis of CO 2 and the endothermic reaction Equation (9). The effect of different CO 2 content on the SER of C 2 H 2 was illustrated in Figure 4. With the increase of CO 2 content, the SER of C 2 H 2 was rising. This directly revealed that the higher the proportion of CO 2 in the reaction system, the more energy was consumed by side-reactions. From the viewpoint of product composition and energy utilization, the presence of CO 2 was not conducive to the pyrolysis production of C 2 H 2 and it should be removed from alkane-rich feed gas before the reaction. the viewpoint of product composition and energy utilization, the presence of CO2 was not conducive to the pyrolysis production of C2H2 and it should be removed from alkane-rich feed gas before the reaction.

Effect of CO on Methane Pyrolysis
The content of CO in several alkane-rich gases such as coke oven gas cannot be ignored, so it is significant to explore the effect of CO on the pyrolysis of alkanes. Figure 5 showed the effect of CO content on the concentration of major gaseous compounds, with CO volume fraction changing in the range of 2-14%. As seen from the diagram, there was basically no change in the molar fraction of major hydrocarbon products, with CH4 maintained 13.1-14.1%, C2H2 10.4-11.1%, and C2H4 0.59-0.68%. As for the changes of CO and CO2 content in the pyrolysis products, only a very small amount of CO2 was detected with a linear increase of the CO content in the product, and the CO content in the product was very close to that in the feed gas, indicating that CO was almost not involved in the reactions. This is ascribed to the relatively inert character of CO due to the very large energy of carbon-oxygen triple bond in CO molecule (1072 kJ/mol). the viewpoint of product composition and energy utilization, the presence of CO2 was not conducive to the pyrolysis production of C2H2 and it should be removed from alkane-rich feed gas before the reaction.

Effect of CO on Methane Pyrolysis
The content of CO in several alkane-rich gases such as coke oven gas cannot be ignored, so it is significant to explore the effect of CO on the pyrolysis of alkanes. Figure 5 showed the effect of CO content on the concentration of major gaseous compounds, with CO volume fraction changing in the range of 2-14%. As seen from the diagram, there was basically no change in the molar fraction of major hydrocarbon products, with CH4 maintained 13.1-14.1%, C2H2 10.4-11.1%, and C2H4 0.59-0.68%. As for the changes of CO and CO2 content in the pyrolysis products, only a very small amount of CO2 was detected with a linear increase of the CO content in the product, and the CO content in the product was very close to that in the feed gas, indicating that CO was almost not involved in the reactions. This is ascribed to the relatively inert character of CO due to the very large energy of carbon-oxygen triple bond in CO molecule (1072 kJ/mol).

Effect of CO on Methane Pyrolysis
The content of CO in several alkane-rich gases such as coke oven gas cannot be ignored, so it is significant to explore the effect of CO on the pyrolysis of alkanes. Figure 5 showed the effect of CO content on the concentration of major gaseous compounds, with CO volume fraction changing in the range of 2-14%. As seen from the diagram, there was basically no change in the molar fraction of major hydrocarbon products, with CH 4 maintained 13.1-14.1%, C 2 H 2 10.4-11.1%, and C 2 H 4 0.59-0.68%. As for the changes of CO and CO 2 content in the pyrolysis products, only a very small amount of CO 2 was detected with a linear increase of the CO content in the product, and the CO content in the product was very close to that in the feed gas, indicating that CO was almost not involved in the reactions. This is ascribed to the relatively inert character of CO due to the very large energy of carbon-oxygen triple bond in CO molecule (1072 kJ/mol). Figure 6 shows the effect of CO content on the yield of C 2 H 2 and CH 4 conversion efficiency in the pyrolysis products at 15 kW. Agreeing with the above analysis, both the yield of C 2 H 2 and CH 4 conversion efficiency changed only slightly with the CO content in the feed gas. This is also the case for the influence of CO content on the SER of C 2 H 2 (Figure 7). This revealed that within the scope of the experiment, the CO content in the reactant had little influence on the pyrolysis of alkanes. Therefore, the separation of CO from alkane-rich gas is not always necessary before the pyrolysis process, and it can be conducted after pyrolysis because the separation of CO from C 2 H 2 will be easier than from alkanes. The C 2 H 2 molecule is a notable hydrogen-bond donor due to the weak hydrogen-bonding acidity of its C-H bond, thus, it can be readily dissolved/adsorbed from non-acidic CO and alkanes by dipolar aprotic solvent, ionic liquids, or anion-pillared microporous frameworks [15,16].  Figure 6 shows the effect of CO content on the yield of C2H2 and CH4 conversion efficiency in the pyrolysis products at 15 kW. Agreeing with the above analysis, both the yield of C2H2 and CH4 conversion efficiency changed only slightly with the CO content in the feed gas. This is also the case for the influence of CO content on the SER of C2H2 (Figure 7). This revealed that within the scope of the experiment, the CO content in the reactant had little influence on the pyrolysis of alkanes. Therefore, the separation of CO from alkane-rich gas is not always necessary before the pyrolysis process, and it can be conducted after pyrolysis because the separation of CO from C2H2 will be easier than from alkanes. The C2H2 molecule is a notable hydrogen-bond donor due to the weak hydrogenbonding acidity of its C-H bond, thus, it can be readily dissolved/adsorbed from non-acidic CO and alkanes by dipolar aprotic solvent, ionic liquids, or anion-pillared microporous frameworks [15,16].    Figure 6 shows the effect of CO content on the yield of C2H2 and CH4 conversion efficiency in the pyrolysis products at 15 kW. Agreeing with the above analysis, both the yield of C2H2 and CH4 conversion efficiency changed only slightly with the CO content in the feed gas. This is also the case for the influence of CO content on the SER of C2H2 (Figure 7). This revealed that within the scope of the experiment, the CO content in the reactant had little influence on the pyrolysis of alkanes. Therefore, the separation of CO from alkane-rich gas is not always necessary before the pyrolysis process, and it can be conducted after pyrolysis because the separation of CO from C2H2 will be easier than from alkanes. The C2H2 molecule is a notable hydrogen-bond donor due to the weak hydrogenbonding acidity of its C-H bond, thus, it can be readily dissolved/adsorbed from non-acidic CO and alkanes by dipolar aprotic solvent, ionic liquids, or anion-pillared microporous frameworks [15,16].

Effect of N2 on Methane Pyrolysis
By adding different volume fractions of N2 (2-14%) to the alkane-rich gas, its effect was investigated in this part, as shown in Figure 8. Along with the N2 concentration increasing, the changes of hydrocarbons content in the product are obvious. The molar fraction of CH4 increased

Effect of N 2 on Methane Pyrolysis
By adding different volume fractions of N 2 (2-14%) to the alkane-rich gas, its effect was investigated in this part, as shown in Figure 8. Along with the N 2 concentration increasing, the changes of hydrocarbons content in the product are obvious. The molar fraction of CH 4 increased from 9.4% to 14.1%, while the C 2 H 2 decreased from 12% to 8.7%, and the C 2 H 4 concentration dropped from 0.82% to 0.49%. As for the N 2 and HCN, the content of N 2 in the product ascended significantly while the HCN content increased slightly. The N 2 fraction in the product was notably lower than that in the feed gas, indicating that a part of N 2 was decomposed in the plasma environment and generated HCN, as shown in Equation (10), although the energy to break up the N≡N bond in nitrogen was relatively large (946 KJ/mol). The process of breaking N≡N bonds consumed the input energy that was originally intended to pyrolyze CH 4 , thus, both the conversion efficiency of CH 4 and the yield of C 2 H 2 decreased (Figure 9), and the CH 4 fraction in the product gas increased, while C 2 H 2 and C 2 H 4 decreased ( Figure 8). Consistently, the SER of C 2 H 2 ascended with the increase of the N 2 concentration in feed gas ( Figure 10). Therefore, similar to the case of CO 2 , it is advisable to remove N 2 from alkane-rich gas before pyrolysis, because the presence of N 2 not only jeopardizes the production of C 2 H 2 , but also generates highly-toxic HCN.    Figure 9. The effects of N2 content on yield of C2H2 and conversion efficiency of CH4 at 15 kW (total gas flow rate of methane and hydrogen: 7.5 Nm 3 /h; magnetic flux intensity: 0.077 T).

Pyrolysis of Simulated Coke Oven Gas
Coke oven gas is a typical alkane-rich mixed gas containing many nitrogen/oxygen compounds, thus, the simulated coke oven gas shown in Table 1 was used for further investigation on the pyrolysis process of alkane-rich gas by a rotating arc thermal plasma reactor. The influences of input power and feed rate on the pyrolysis process were investigated as they are two important operation parameters in pyrolysis.

Influence of Input Power on the Simulated COG Pyrolysis Process
The investigated experimental conditions were listed as follows: feed rate of 7.5 Nm 3 /h, magnetic induction intensity of 0.077 T, and input power of 13-26 kW. Figure 11 presents the influence of the input power on the molar fraction of main products. It can be seen that the content of CH 4 , CO 2 , and N 2 decreased gradually, while CO increased from 6.2% to 7.1% and HCN increased from 0.43% to 0.76% in the whole power range. This is because the increase of input power promoted the transformation of alkanes through not only the desired reactions (toward C 2 H 2 ), but also side reactions. This is also reflected in Figure 12, where the conversion efficiency of alkanes increased with the rise in power. The presence of CO and HCN in the product gas was a result of the pyrolysis of CO 2 and N 2 in the feed gas, as demonstrated by the previous sections. As for the hydrocarbon products, C 2 H 6 content did not change obviously, only a small amount, because of its small content in the feed and similar pyrolysis properties to methane. C 2 H 4 maintained a low content at first, and then increased from 0.64% to 3.3%. However, the change of C 2 H 2 content was complex, showing an increasing trend at first, and then decreasing from 8.0% to 4.4%. Correspondingly, as Figure 13 displays, when the power increased from 12.9 kW to 21.6 kW, the C 2 H 2 yield rose from 54.8% to 68.3%, and then it dropped to 42.7% with a continual increase of power. As for the C 2 H 2 selectivity, it maintained above 90.3% when the power was between 12.9 kW to 17.9 kW, and then it dropped to 51.0%. The SER of C 2 H 2 also exhibited similar change ( Figure 12). When the power ascended from 12.9 kW to 21.6 kW, the SER of C 2 H 2 increased slowly from 16.7 kWh/kg C 2 H 2 to 18.8 kWh/kg C 2 H 2 , but then increased rapidly from 18.8 kWh/kg C 2 H 2 to 41 kWh/kg C 2 H 2 when the power continued to increase from 21.6 kW to 25.6 kW.
increasing trend at first, and then decreasing from 8.0% to 4.4%. Correspondingly, as Figure 13 displays, when the power increased from 12.9 kW to 21.6 kW, the C2H2 yield rose from 54.8% to 68.3%, and then it dropped to 42.7% with a continual increase of power. As for the C2H2 selectivity, it maintained above 90.3% when the power was between 12.9 kW to 17.9 kW, and then it dropped to 51.0%. The SER of C2H2 also exhibited similar change (Figure 12). When the power ascended from 12.9 kW to 21.6 kW, the SER of C2H2 increased slowly from 16.7 kWh/kg C2H2 to 18.8 kWh/kg C2H2, but then increased rapidly from 18.8 kWh/kg C2H2 to 41 kWh/kg C2H2 when the power continued to increase from 21.6 kW to 25.6 kW. 12 Figure 11. The effect of input power on the molar fraction of major products in the pyrolysis of simulated coke oven gas (total gas flow rate of simulated coke oven gas: 7.5 Nm 3 /h; magnetic flux intensity: 0.077 T). This could be attributed to the dual effect of the input power on the pyrolysis. On one hand, increasing input power can provide more energy to convert the components in feed gas and, thus, produce more C2H2. On the other hand, increasing input power will also elevate the temperature in the reactor, and once the temperature is too high, the generated metastable C2H2 decomposes into solid carbon and H2. Therefore, there should be an optimum input power for the pyrolysis of alkanerich gases to produce the C2H2-rich product gas. In the current experiment, taking the various factors into consideration, an input power of approximately 17.9 kW was a desired condition for pyrolysis because it combined high alkane conversion (75.4%), high C2H2 yield (68.3%) and selectivity (91.2%), and low SER (18.8 kWh/kgC2H2). This could be attributed to the dual effect of the input power on the pyrolysis. On one hand, increasing input power can provide more energy to convert the components in feed gas and, thus, produce more C 2 H 2 . On the other hand, increasing input power will also elevate the temperature in the reactor, and once the temperature is too high, the generated metastable C 2 H 2 decomposes into solid carbon and H 2 . Therefore, there should be an optimum input power for the pyrolysis of alkane-rich gases to produce the C 2 H 2 -rich product gas. In the current experiment, taking the various factors into consideration, an input power of approximately 17.9 kW was a desired condition for pyrolysis because it combined high alkane conversion (75.4%), high C 2 H 2 yield (68.3%) and selectivity (91.2%), and low SER (18.8 kWh/kgC 2 H 2 ).
increasing input power can provide more energy to convert the components in feed gas and, thus, produce more C2H2. On the other hand, increasing input power will also elevate the temperature in the reactor, and once the temperature is too high, the generated metastable C2H2 decomposes into solid carbon and H2. Therefore, there should be an optimum input power for the pyrolysis of alkanerich gases to produce the C2H2-rich product gas. In the current experiment, taking the various factors into consideration, an input power of approximately 17.9 kW was a desired condition for pyrolysis because it combined high alkane conversion (75.4%), high C2H2 yield (68.3%) and selectivity (91.2%), and low SER (18.8 kWh/kgC2H2).

Influence of Feed Rate on the Simulated COG Pyrolysis Process
Experimental conditions in investigating the influence of feed rate are listed as follows: input power of 15 ± 0.5 kW, magnetic induction intensity of 0.077 T, and feed rate of 3.6-10 Nm 3 /h. Figure  14 exhibits the effect of the feed rate on the major products' content. With the increase of the feed rate, the CO fraction showed a decreasing trend in the pyrolysis product from 7.5% to 6.2%, and the CO2 fraction had a sensible growth from 0.042% to 0.79%, implying that the reaction in Equation (9) Figure 13. The effects of input power on the yield and selectivity of C 2 H 2 in the pyrolysis of simulated coke oven gas (total gas flow rate of simulated coke oven gas: 7.5 Nm 3 /h; magnetic flux intensity: 0.077 T).

Influence of Feed Rate on the Simulated COG Pyrolysis Process
Experimental conditions in investigating the influence of feed rate are listed as follows: input power of 15 ± 0.5 kW, magnetic induction intensity of 0.077 T, and feed rate of 3.6-10 Nm 3 /h. Figure 14 exhibits the effect of the feed rate on the major products' content. With the increase of the feed rate, the CO fraction showed a decreasing trend in the pyrolysis product from 7.5% to 6.2%, and the CO 2 fraction had a sensible growth from 0.042% to 0.79%, implying that the reaction in Equation (9) was weakened to some extent. At the same time, the side reaction of N 2 in Equation (10) also became slightly weakened, as indicated by the results that with the increase of the feeding rate, the N 2 content in the product increased slightly overall, while the content of HCN slightly decreased. The content of C 2 H 4 and C 2 H 6 hardly changed, as could be seen as the molar fraction of C 2 H 4 was between 0.63% and 0.85%, and that of C 2 H 6 was below 0.26%. The most obvious and important changes were the levels of CH 4 and C 2 H 2 . With the increase of the feed rate, the molar fraction of CH 4 in the product increased sharply from 0.86% to 10.4%, but that of C 2 H 2 dropped from 9.5% to 6.2%. This suggested that more CH 4 was not cracked as the feeding rate increased, resulting in an increase in the CH 4 mole fraction and a dilution of C 2 H 2 . In fact, as shown in Figures 15 and 16, the alkanes' conversion efficiency decreased rapidly from 96.1% to 56%, but the C 2 H 2 selectivity remained at a high level, between 88.1% and 90%. This indicated that when the alkane conversion efficiency decreased, the side reactions were not enhanced, and the conversion of alkanes was still dominated by the formation of C 2 H 2 . As the conversion of alkanes decreased, and C 2 H 2 selectivity remained unchanged, the yield of C 2 H 2 decreased with the increase of the feed rate, from 84.7% to 50.3%. Moreover, the SER of C 2 H 2 was reduced from 26.4 kWh/kg C 2 H 2 to 15.8 kWh/kg C 2 H 2 . The phenomenon above was probably the result of the shorter gas residence time and lower temperature in the reactor caused by the increase of the feed rate, which made it difficult to convert the increased alkanes into C 2 H 2 in time.
In the entire range of the flow rates investigated, the C 2 H 2 selectivity was always maintained at a high level, but the conversion of alkanes and the yield of C 2 H 2 decreased with the flow rate. This indicated that the processing capacity of alkanes was limited under this input power, and the feed rate could be appropriately increased to obtain optimum technical and economic performance. of C2H2. As the conversion of alkanes decreased, and C2H2 selectivity remained unchanged, the yield of C2H2 decreased with the increase of the feed rate, from 84.7% to 50.3%. Moreover, the SER of C2H2 was reduced from 26.4 kWh/kg C2H2 to 15.8 kWh/kg C2H2. The phenomenon above was probably the result of the shorter gas residence time and lower temperature in the reactor caused by the increase of the feed rate, which made it difficult to convert the increased alkanes into C2H2 in time.   Figure 14. The effects of feeding flow rate on the molar fraction of major products in the pyrolysis of siimulated coke oven gas (input power: 15 ± 0.5 kW; magnetic flux intensity: 0.077 T).  was reduced from 26.4 kWh/kg C2H2 to 15.8 kWh/kg C2H2. The phenomenon above was probably the result of the shorter gas residence time and lower temperature in the reactor caused by the increase of the feed rate, which made it difficult to convert the increased alkanes into C2H2 in time.   Figure 14. The effects of feeding flow rate on the molar fraction of major products in the pyrolysis of siimulated coke oven gas (input power: 15 ± 0.5 kW; magnetic flux intensity: 0.077 T).  In the entire range of the flow rates investigated, the C2H2 selectivity was always maintained at a high level, but the conversion of alkanes and the yield of C2H2 decreased with the flow rate. This indicated that the processing capacity of alkanes was limited under this input power, and the feed rate could be appropriately increased to obtain optimum technical and economic performance.
In this work, the effect of various nitrogen/oxygen-containing substances including N2, CO, and CO2 on the pyrolysis of CH4 to C2H2 was conducted by thermal plasma, as well as pyrolysis of simulated coke oven gas as a model of alkane-rich gas. The above experimental results indicated that In this work, the effect of various nitrogen/oxygen-containing substances including N 2 , CO, and CO 2 on the pyrolysis of CH 4 to C 2 H 2 was conducted by thermal plasma, as well as pyrolysis of Energies 2018, 11, 351 13 of 14 simulated coke oven gas as a model of alkane-rich gas. The above experimental results indicated that nitrogen/oxygen-containing substances CO 2 and N 2 had palpable and negative effects on the pyrolysis process, whereas CO exhibited little effect. Therefore, the following technical flow is advisable for the pyrolysis process: separate CO 2 and N 2 from the alkane-rich feed gas at first, and then pyrolyze the feed gas with a rotary thermal plasma reactor to produce C 2 H 2 -rich pyrolysis gas, and, finally, separate C 2 H 2 from the other components, including CO, in the pyrolysis gas.

Conclusions
The influence of nitrogen/oxygen substances CO 2 , N 2 , and CO for the pyrolysis of alkane-rich mixing gases to produce C 2 H 2 had been illustrated for the first time, using a 50 kW self-made rotary thermal plasma reactor. In the mole fraction range of 2-14%, N 2 and CO had negative effects on the main reaction that converted CH 4 to C 2 H 2 , while CO had little effect. Considering this difference, along with the toxicity of byproducts and the level of difficulty in separating different gas mixtures, it is advisable to remove CO 2 and N 2 from the feed gas before pyrolysis, and to separate the product gas from CO after pyrolysis. The pyrolysis of simulated coke oven gas, an important alkane-rich mixing gas containing CO 2 , N 2 , and CO, was also conducted in this work, and the effect of the input power and feeding rate on pyrolysis were investigated. Considering the non-monotonic change of C 2 H 2 selectivity and yield, along with the input power, the selectivity of C 2 H 2 up to 91.2% and a C 2 H 2 yield of 68.3% were obtained at the input power of 17.9 kW. Higher C 2 H 2 selectivity and a lower C 2 H 2 -specific energy requirement could be obtained when the feed rate was increased, although at the expense of lower alkane conversion and C 2 H 2 yield. Overall, the experimental results revealed that it was a promising method to convert alkane-rich gases to value-added chemicals, and provided indispensable information for creating a technical flow for a whole process integrating pretreatment, pyrolysis, and posttreatment.