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

energies

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

Wei Huang, Junkui Jin, Guangdong Wen, Qiwei Yang * ID , Baogen Su * and Qilong Ren

Key Laboratory of Biomass Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China; [email protected] (W.H.); [email protected] (J.J.); [email protected] (G.W.); [email protected] (Q.R.) * Correspondence: [email protected] (Q.Y.); [email protected] (B.S.); Tel.: +86-571-87951125 (Q.Y.)

Received: 23 December 2017; Accepted: 29 January 2018; Published: 2 February 2018

Abstract: 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 N2, CO, and CO2,on the pyrolysis of CH4 to C2H2 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 N2 and CO2 was not conducive to the main reaction of alkane pyrolysis for C2H2, while CO, as a stable equilibrium product, had little effect on the cracking reaction. Consequently, it is suggested that a pretreatment process of removing N2 and CO2 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 C2H2 selectivity of 91.2% and a yield of 68.3% could be obtained at an input power of 17.9 kW.

Keywords: thermal plasma; pyrolysis; alkane-rich gas; acetylene; coke oven gas

1. 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 m3 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

Energies 2018, 11, 351; doi:10.3390/en11020351 www.mdpi.com/journal/energies Energies 2018, 11, 351 2 of 14 partial oxidation of methane—have shown obvious shortcomings in energy cost, water consumption, and pollution [4]. In this context, increasing attention has been paid to studies on thermal plasma pyrolysis of hydrocarbons to acetylene. Acetylene is the main long-lived metastable intermediate product of pyrolysis in a hydrogen atmosphere at high temperatures (T > 1800 K); therefore, it can be obtained as the main hydrocarbon product as long as the product gas is rapidly cooled after pyrolysis to prevent further conversion to carbon black and hydrogen [5]. Due to the ultrahigh reactivity of thermal plasma, this method can effectively convert various hydrocarbons to acetylene-rich gaseous products within milliseconds, along with very little water consumption and CO2 discharge. In addition to solid and liquid hydrocarbon-rich feedstocks, such as coal, polyethylene, and aromatic compounds [6–10], gaseous alkanes can also be converted to acetylene-rich products with high acetylene yield and selectivity by this method [11,12], which creates a new method for the utilization of alkane-rich gas. However, one significant problem in the current studies on plasma pyrolysis of alkanes or alkane-rich gases is that little attention has been paid to the effect of nitrogen/oxygen-containing substances on the pyrolysis. Actually, there are always various nitrogen/oxygen-containing substances, such as N2, CO, CO2, and O2, in practical alkane-rich gasses, and their concentrations are highly inconstant within a range of approximately 0.1–15%. In addition to the alkanes, these compounds may also be strongly activated by the thermal plasma, leading to different gaseous products from the ideal pure-alkane case, thus, it is imperative to explore how the variety and concentration of nitrogen/oxygen-containing substances would influence the gas conversion and the product composition. It is not only essential to evaluate the intrinsic performance of the pyrolysis process, but also indispensable to establish an optimum separation-pyrolysis-separation integral process. That is, whether a pre-separation of any nitrogen/oxygen-containing compound from alkanes is needed before pyrolysis, and what kinds of purification procedures are needed after pyrolysis, strongly depend on the effect of those compounds on pyrolysis. In this work, we systematically investigated the effect of various nitrogen/oxygen-containing substances, including N2, CO, and CO2 on the pyrolysis of methane to acetylene by thermal plasma, and conducted the pyrolysis of simulated coke oven gas as a model of alkane-rich mixing gas. A self-made rotating arc plasma reactor was employed to pyrolyze those alkane-rich gases for the first time, because it was proved to have notable advantages in feedstock-plasma mixing and temperature homogenization over conventional thermal plasma reactors. The experimental results indicated that CO2 and N2 had palpable and negative effects on the pyrolysis process, thus, it is advisable to remove them from the mixing gas before pyrolysis. CO has little effect on the pyrolysis, thus, it can be removed after pyrolysis for an easier separation. Additionally, the operating conditions, including input power and feed rate, significantly affected not only the yield and selectivity of the products, but also the conversion efficiency of alkanes and the specific energy requirement of acetylene.

2. Materials and Methods

2.1. Materials

CH4,C2H6,H2, CO, CO2,N2, and Ar (all gases had a purity of 99.99%) were purchased from Hangzhou Jingong Special Gas Co, Ltd. (Hangzhou, China). Sodium hydroxide was obtained from Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China). Sodium chloride, potassium chromate, silver nitrate, and rose red reagent were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). The composition of the simulated coke oven gas is presented in Table1, which was produced according to the main components of actual coke oven gas [13].

2.2. Reactor Setup A 50 kW rotating arc plasma reaction device system was used in this experiment, as shown in Figure1a, mainly consisting of four parts: a constant current power supply, plasma reactor, cooling and quenching system, and a data acquisition and control system. Figure1b shows the structure of the Energies 2018, 11, x 3 of 14

Table 1. The basic composition of practical coke oven gas (a) and simulated coke oven gas (b) in this work.

Energies 2018, 11,Main 351 3 of 14 H2 CH4 CmHn CO CO2 O2 N2 Component Vol % 55–60 23–27 2–4 5–8 1.5–3 0.3–0.8 3–7 (a) 50 kW rotating arc plasma reactor, mainly including a cathode, cathodeorganic flange, anode, anode upper Impurity NH3 benzene HCN H2S naphthalene Tar flange, anode bottom flange, and the excitation coil. The excitationsulfur coil is sheathed outside the anode sleeve generatingg/Nm a3 magnetic≤0.05 field under≤2 the action≤0.3 of impressed≤0.02 current,≤0.05 to produce≤0.1 the Lorenz≤0.05 force Main in the reactor, which resultsH in2 a high-speedCH4 rotationC2H6 of theCO arc. CO2 N2 Ar (b) Component Table 1. TheVol basic% composition56.00 of27.00 practical coke 3.01 oven gas6.09 (a) and2.02 simulated coke 4.88 oven gas (b) 1.00 in this work. 2.2. Reactor Setup Main H CH C H CO CO O N A 50 kWComponent rotating arc plasma2 reaction4 devicem n system was used 2in this experiment,2 as shown2 in Figure 1a, mainly consisting of four parts: a constant current power supply, plasma reactor, cooling (a) Vol % 55–60 23–27 2–4 5–8 1.5–3 0.3–0.8 3–7 and quenching system, and a data acquisition and control system. Figure 1b shows the structure of Impurity NH3 benzene HCN H2S organic sulfur naphthalene Tar the 50 kW rotating arc plasma reactor, mainly including a cathode, cathode flange, anode, anode g/Nm3 ≤0.05 ≤2 ≤0.3 ≤0.02 ≤0.05 ≤0.1 ≤0.05 upper flange, anode bottom flange, and the excitation coil. The excitation coil is sheathed outside the Main H2 CH4 C2H6 CO CO2 N2 Ar anode (b)sleeveComponent generating a magnetic field under the action of impressed current, to produce the Lorenz force inVol the % reactor,56.00 which results 27.00 in a 3.01high-speed 6.09 rotation 2.02of the arc. 4.88 1.00

FigureFigure 1. 1. TheThe 50 50 kW kW rotating rotating arc arc plasma plasma reactor. reactor. ( a)) Reaction Reaction device; device; and ( b) the reactor structure.

2.3. Gas Gas Analysis Analysis

The components of pyrolysispyrolysis gasgas areare complex,complex, possiblypossibly includingincluding CH CH4,C4, C22HH22,C, C22H4,,C C2HH66, ,CO, CO, CO2,, and and N 2,, and and it is convenientconvenient toto adoptadopt gas gas chromatography chromatography for for analysis analysis with with GC GC 1690 1690 provided provided by byHangzhou Hangzhou Kexiao Kexiao Chemical Chemical Equipment Equipment Co, Ltd. Co, (Hangzhou, Ltd. (Hangzhou, China). China). The chromatographic The chromatographic columns columnsare a PLOT are 5Aa PLOT zeolite-packed 5A zeolite-packed column providedcolumn provided by Hangzhou by Hangzhou Kexiao ChemicalKexiao Chemical Equipment Equipment Co, Ltd. Co,(Hangzhou, Ltd. (Hangzhou, China) China) and a and PLOT a PLOT Q capillary Q capillary column column from from Agilent Agilent Technologies Technologies Co,Co, Ltd. Ltd. (Hangzhou, China). As As a a possible possible product product in in the the nitr nitrogenogen reaction, HCN is highly toxic and must be carefully handled. The The relatively relatively safe and simple AgNO 3 titrimetrictitrimetric method method was was adopted, adopted, using using a NaCl standard solution (0.0100 mol/L)mol/L) to to calibrate the standard solution ofof AgNOAgNO33 (0.0100(0.0100 mol/L),mol/L), then determining determining the concentration of cyanide ions in the collection solution with a calibrated AgNO3 solution. The The cyanide cyanide ion ion collecting collecting solution solution is is obtained obtained by by slowly slowly accessing accessing the the sample sample bag gas (about 2 L) to 0.1 mol/L NaOH NaOH solution solution to to be be absorbed. absorbed.

2.4. Data Processing Method Carbon yield (Y) of acetylene in the product is generally deﬁned as:

2 × QC2H2 YC H = × 100% (1) 2 2 Q0 CH4 Energies 2018, 11, 351 4 of 14

Carbon conversion efﬁciency (CE) of the feed gas is generally deﬁned as:

QCH4 CECH = (1 − ) × 100% (2) 4 Q0 CH4

Selectivity (S) of acetylene is deﬁned as:

Y C2 H2 SC2H2 = × 100% (3) CECH4

When pyrolyzed, the siimulated coke oven gas is deﬁned as:

2 × QC2H2 YC H = × 100% (4) 2 2 Q0 + 2 × Q0 CH4 C2H6 + × QCH4 2 QC2H6 CECH +C H = (1 − ) × 100% (5) 4 2 6 Q0 + 2 × Q0 CH4 C2H6

YC2H2 SC2H2 = × 100% (6) CECH4 + C2H6 Speciﬁc energy requirement (SER) of acetylene is deﬁned as:

Input power(kW) SER(kWh/kg·C2H2) = (7) Mass flow rate of C2H2(kW/h) where Q0 , Q0 represent the volume flow of CH and C H in the feedstock, respectively, CH4 C2H6 4 2 6 and QC2H2 , QCH4 , QC2H6 are the volume flow of C2H2, CH4, and C2H6 in the product.

3. Results and Discussion

3.1. 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]:

• •CH2 ·················· C4H2 → C6H2 → PY → Polymer ↑ ↓ ↑ ↓ • CH4 → CH3 → C2H6 → C2H4 → C2H2(+H2) ↔ C3H4 C2nHm(m ≤ n)(CB) + H2 ↑ ↓ ↑ (8) • CnH2n+2 → Ri + Rk → · · · → CmH2m •C4H4 → C8H8(l) → Polymer ↓ -

C6H6(l) → B → T → X → PAH → PCA

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, 3 the total feed rate of H2 and CH4 was 7.5 Nm /h, the magnetic induction intensity was 0.077 T, and the input power was controlled at 15 ± 0.5 kW. Energies 2018, 11, x 5 of 14

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 Energies 2018, 11, 351 5 of 14 total feed rate of H2 and CH4 was 7.5 Nm3/h, the magnetic induction intensity was 0.077 T, and the input power was controlled at 15 ± 0.5 kW.

3.1.1. Effect of CO2 on Methane Pyrolysis 3.1.1. 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%.By Theadding plasma-enhanced different amount process of CO of2oxidation in methane, conversion the CO2 mayvolume be explained fraction changed as the reaction in the belowrange ofin Equation2–14%. The (9) plasma-enhanced along with reactions process in Equation of oxidation (8): conversion may be explained as the reaction below in Equation (9) along with reactions in Equation (8):

CH4 + CO+→+2 → 2CO + 2H2 ∆H =+= +247 kJ/mol (9) CH42 CO 2CO 2H 2 ΔH 247 kJ/mol (9)

Figure2 2shows shows the the effect effect of CO of2 contentCO2 content on the kindson the of products,kinds of including products, mainly including hydrocarbons, mainly hydrocarbons,CO2, and CO, atCO an2, inputand CO, power at an of input 15 kW. power As can of be15 seenkW. fromAs can the be ﬁgure, seen from with the the figure, increase with in CO the2 increasecontent, thein CO mole2 content, fraction the of mole C2H2 fractionreduced of from C2H 8.4%2 reduced to 6.3% from and 8.4% C2H to4 concentration 6.3% and C2H reduced4 concentration slightly reducedfrom 0.61% slightly to 0.31%. from This0.61% is to mainly 0.31%. ascribed This is mainly to a competitive ascribed to relationship a competitive between relationship the oxygen-free between thepyrolysis oxygen-free reactions pyrolysis in Equation reactions (8) and in Equation oxidation (8) reaction and oxidation in Equation reaction (9), and in theEquation oxidation (9), reactionand the oxidationhindered thereaction dominant hindered pyrolysis the reactionsdominant that pyrolysis produce reactions C2H2. This that is produce also supported C2H2. byThis the is result also supportedthat increasing by the CO result2 content that inincreasing the feed gasCO improved2 content in the the CO feed content gas improved in the product. the CO On content the contrary, in the product.CH4 content On the in the contrary, product CH ascended4 content from in the 10.1% product to 15.2% ascended although from the 10.1% CH4 contentto 15.2% in although the feed gasthe CHwas4 slightlycontent reducedin the feed when gas addingwas slightly CO2. reduced This is probably when adding because CO when2. This theis probably CO2 content because increased, when the CO pyrolysis2 content of COincreased,2 to CO the and pyrolysis O· radicals of consumeCO2 to CO a greatand O· deal radicals of input consume energy, a which great deal restricted of input the energy,conversion which of CHrestricted4 by either the conversion Equation (8) of or CH Equation4 by either (9). Equation (8) or Equation (9).

20 CH 18 4 C2H4 16 C H 2 2 14 CO

12 CO2 10 8

Mole Fraction / % Fraction Mole 4

2 0 2468101214 CO Concentration Vol/% 2

Figure 2. The effect of CO 2 concentrationconcentration on on the the molar molar fraction fraction of of major major products products at 15 kW (total gas flowﬂow rate of methane and hydrogen: 7.5 Nm 3/h;/h; magnetic magnetic flux ﬂux intensity: intensity: 0.077 0.077 T). T).

Figure 3 illustrates the effect of CO2 content on the yield (Y) of C2H2 and CH4 conversion Figure3 illustrates the effect of CO 2 content on the yield (Y) of C2H2 and CH4 conversion efﬁciency efficiency (CE) at 15 kW. With the increase of CO2 content, the yield of C2H2 and CH4 conversion (CE) at 15 kW. With the increase of CO2 content, the yield of C2H2 and CH4 conversion efﬁciency efficiencyshowed a synchronousshowed a synchronous decreasing trend.decreasing This istrend. consistent This tois theconsistent results ofto Figurethe results2, suggesting of Figure that 2, suggesting that the increase of CO2 content was not conducive to the pyrolysis reaction based on the the increase of CO2 content was not conducive to the pyrolysis reaction based on the fact that more fact that more input energy was taken away by both the pyrolysis of CO2 and the endothermic input energy was taken away by both the pyrolysis of CO2 and the endothermic reaction Equation (9). reaction Equation (9). The effect of different CO2 content on the SER of C2H2 was illustrated in Figure The effect of different CO2 content on the SER of C2H2 was illustrated in Figure4. With the increase 4. With the increase of CO2 content, the SER of C2H2 was rising. This directly revealed that the higher of CO2 content, the SER of C2H2 was rising. This directly revealed that the higher the proportion of the proportion of CO2 in the reaction system, the more energy was consumed by side-reactions. From CO2 in the reaction system, the more energy was consumed by side-reactions. From 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. Energies 2018, 11, x 6 of 14

Energies 2018, 11, x 6 of 14 the viewpoint of product composition and energy utilization, the presence of CO2 was not conducive Energiesto2018 the, 11pyrolysis, 351 production of C2H2 and it should be removed from alkane-rich feed gas before the 6 of 14 the viewpoint of product composition and energy utilization, the presence of CO2 was not conducive reaction. to the pyrolysis production of C2H2 and it should be removed from alkane-rich feed gas before the reaction. 80 80

80 80 70 70

70 70 60 60

60 60 50 50 / %

4 / % 2 CH 50 50 H 2 / %

4 C / % 2 Y CH CE 40 40 H 2 C Y CE 40 40 30 30 30 30

20 20 20 246810121420 2468101214 CO Concentration Vol/% 2 CO Concentration Vol/% 2 FigureFigure 3. The 3. effect The effect of CO of COconcentration2 concentration on on the the yieldyield of of C C2HH2, conversion, conversion efficiency efficiency of CH of4 at CH 15 kWat 15 kW Figure 3. The effect of2 CO2 concentration on the yield of C2H22, conversion2 efficiency of CH4 at 15 kW4 (total gas flow rate of methane and hydrogen: 7.5 Nm33/h; magnetic flux intensity: 0.077 T). (total gas(total flow gas rate flow of rate methane of methane and and hydrogen: hydrogen: 7.5 7.5 NmNm3/h;/h; magnetic magnetic flux fluxintensity: intensity: 0.077 T). 0.077 T).

22 22 21 21 20 20 19 19

) 18 2

) 18 2 H

2 17 H

2 17 1616 1515 1414 /(kWh/kg C 2 /(kWh/kg C 2

H 13 2 H 13 2 C 12C 12 SER SER 1111 1010 9 9 8 8 22 4 4 6 6 8 8 101214 101214 COCO concentrationconcentration Vol/%Vol/% 22

Figure 4. The effect of CO2 concentration on SER of C2H2 at 15 kW (total gas flow rate of methane and FigureFigure 4. The 4. effect The effect of CO of 2COconcentration2 concentration on onSER SER of C C22HH2 2atat 1515 kW kW (total (total gas gasflowﬂow rate of rate methane of methane and and hydrogen: 7.5 Nm3/h; magnetic flux intensity: 0.077 T). hydrogen:hydrogen: 7.5 Nm 7.53 Nm/h;3/h; magnetic magnetic ﬂux flux intensity: intensity: 0.077 0.077 T). T).

3.1.2.3.1.2. Effect Effect of of CO CO on on Methane Methane Pyrolysis Pyrolysis 3.1.2. 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 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 Thesignificant content to of explore CO in the several effect alkane-richof CO on the gasespyrolysis such of alkanes. as coke Figure oven gas5 showed cannot the be effect ignored, of CO so it is content on the concentration of major gaseous compounds, with CO volume fraction changing in the signiﬁcantcontent to on explore the concentration the effect of major CO on gaseous the pyrolysis compounds, of alkanes.with CO volume Figure fraction5 showed changing the effect in the of CO range of 2–14%. As seen from the diagram, there was basically no change in the molar fraction of range of 2–14%. As seen from the diagram, there was basically no change in the molar fraction of content onmajor the hydrocarbon concentration products, of major with gaseousCH4 maintained compounds, 13.1–14.1%, with C CO2H2 10.4–11.1%, volume fraction and C2H changing4 0.59– in the major hydrocarbon products, with CH4 maintained 13.1–14.1%, C2H2 10.4–11.1%, and C2H4 0.59– range of0.68%. 2–14%. As As for seenthe changes from theof CO diagram, and CO2 therecontent was in the basically pyrolysis no products, change only inthe a very molar small fraction amount of major 0.68%. As for the changes of CO and CO2 content in the pyrolysis products, only a very small amount hydrocarbonof CO2 products, was detected with with CH a linear4 maintained increase of the 13.1–14.1%, CO content Cin2 Hthe2 product,10.4–11.1%, and the and CO Ccontent2H4 0.59–0.68%. in of theCO product2 was detected was very with close a linearto that increasein the feed of gas,the COindicating content that in COthe wasproduct, almost and not the involved CO content in the in As for the changes of CO and CO2 content in the pyrolysis products, only a very small amount of CO2 thereactions. product wasThis veryis ascribed close to to that the inrelatively the feed iner gas,t indicatingcharacter of that CO CO due was to almostthe very not large involved energy in of the was detected with a linear increase of the CO content in the product, and the CO content in the product reactions.carbon-oxygen This is triple ascribed bond to in theCO relativelymolecule (1072 inert kJ/mol). character of CO due to the very large energy of was verycarbon-oxygen close to that triple in bond the feed in CO gas, molecule indicating (1072 kJ/mol). 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).

Figure6 shows the effect of CO content on the yield of C 2H2 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 (Figure7). 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 hydrogen-bonding Energies 2018, 11, x 7 of 14

8 CH4 Energies 2018, 11, 351 C H 7 of 14 6 2 2

C2H4 4 Mole Fraction /% Fraction Mole CO acidity of its C-H bond, thus, it can be readily dissolved/adsorbed from non-acidic CO CO and alkanes by 2 2 dipolarEnergies aprotic 2018 solvent,, 11, x ionic liquids, or anion-pillared microporous frameworks [15,16]. 7 of 14 0 16 468101214 14 CO Concentration Vol /% 12 Figure 5. The effect of CO concentration on the molar fraction of major products at 15 kW (total gas flow rate of methane and10 hydrogen: 7.5 Nm3/h; magnetic flux intensity: 0.077 T).

8 CH4 Figure 6 shows the effect of CO content on the yield of C2H2 and CH4 conversion efficiency in C H 6 2 2 the pyrolysis products at 15 kW. Agreeing with the above analysis, both the yield of C2H2 and CH4 C2H4 conversion efficiency changed4 only slightly with the CO content in the feed gas. This is also the case Mole Fraction /% Fraction Mole CO for the influence of CO content on the SER of C2H2 (Figure 7). This revealed CO that within the scope of 2 2 the experiment, the CO content in the reactant had little influence on the pyrolysis of alkanes. Therefore, the separation of0 CO from alkane-rich gas is not always necessary before the pyrolysis process, and it can be conducted468101214 after pyrolysis because the separation of CO from C2H2 will be easier CO Concentration Vol /% than from alkanes. The C2H2 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 Figure 5.FigureThe 5. effect The effect of CO of concentrationCO concentration on on the the molarmolar fr fractionaction of ofmajor major products products at 15 kW at 15 (total kW gas (total gas alkanes by dipolar aprotic solvent, ionic liquids,3 or anion-pillared microporous frameworks [15,16]. ﬂow rateflow of rate methane of methane and hydrogen:and hydrogen: 7.5 7.5 Nm Nm3/h;/h; magnetic magnetic flux ﬂux intensity: intensity: 0.0770.077 T). T).

Figure 6 shows the 75effect of CO content on the yield of C2H2 and CH754 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 70 70 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. 65 65 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

/% 60 60 4 /% 2 than from alkanes. The C2H2 molecule is a notable hydrogen-bond donor due to the weak hydrogen- H CH 2 bonding acidity of its C-H bond, thus, it can be readily dissolved/adsorbed fromC non-acidic CO and Y CE alkanes by dipolar aprotic55 solvent, ionic liquids, or anion-pillared microporous55 frameworks [15,16].

5075 7550

4570 7045 468101214 CO Concentration Vol/% 65 65 Figure 6. The effect of CO concentration on the yield of C H , conversion efﬁciency of CH at 15 kW Figure 6. The effect of CO concentration on the yield of C22H2,2 conversion efficiency of CH4 at 154 kW

/% 60 60 4 /%

3 2 Energies 2018, 11, x 3 8 of 14

(total gas ﬂow rate of methane and hydrogen: 7.5 Nm /h; magnetic ﬂux intensity:H 0.077 T). CH (total gas flow rate of methane and hydrogen: 7.5 Nm /h; magnetic flux intensity:2 0.077 T). C Y CE 55 18 55

17 50 50 16

45 15 45 )

2 468101214 H 2 14 CO Concentration Vol/%

Figure 6. The effect of CO concentration on the yield of C2H2, conversion efficiency of CH4 at 15 kW

(kWh/kg C (kWh/kg 12 2 3

(total gas flow rate of methaneH and hydrogen: 7.5 Nm /h; magnetic flux intensity: 0.077 T). 2 C 11 SER 10

8 2468101214 CO concentration Vol/%

Figure 7. The effect of CO concentration on SER of C H at 15 kW (total gas ﬂow rate of methane and Figure 7. The effect of CO concentration on SER of C2 2H22 at 15 kW (total gas flow rate of methane and 3 hydrogen:hydrogen: 7.5 Nm 7.5 Nm/h;3 magnetic/h; magnetic ﬂux flux intensity: intensity: 0.0770.077 T).

3.1.3. 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 from 9.4% to 14.1%, while the C2H2 decreased from 12% to 8.7%, and the C2H4 concentration dropped from 0.82% to 0.49%. As for the N2 and HCN, the content of N2 in the product ascended significantly while the HCN content increased slightly. The N2 fraction in the product was notably lower than that in the feed gas, indicating that a part of N2 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 CH4, thus, both the conversion efficiency of CH4 and the yield of C2H2 decreased (Figure 9), and the CH4 fraction in the product gas increased, while C2H2 and C2H4 decreased (Figure 8). Consistently, the SER of C2H2 ascended with the increase of the N2 concentration in feed gas (Figure 10). Therefore, similar to the case of CO2, it is advisable to remove N2 from alkane-rich gas before pyrolysis, because the presence of N2 not only jeopardizes the production of C2H2, but also generates highly-toxic HCN.

+→∗∗ ++ +→ ++ CH43 e CH H e, CH 32 e CH H e  +→∗∗ ++ +→++ CH2 e CH H e, CH e C H e  +→++∗ Ne2 NNe (10) (a) CH+→ N HCN, (b) C +→ H CH  (c) HCN+→ e∗ CN ++ H e (d) CH + CH → CH = CH  e* (high-energy electron)

Energies 2018, 11, 351 8 of 14

3.1.3. 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 Figure8. Along with the N 2 concentration increasing, the changes of hydrocarbons content in the product are obvious. The molar fraction of CH4 increased from 9.4% to 14.1%, while the C2H2 decreased from 12% to 8.7%, and the C2H4 concentration dropped from 0.82% to 0.49%. As for the N2 and HCN, the content of N2 in the product ascended signiﬁcantly while the HCN content increased slightly. The N2 fraction in the product was notably lower than that in the feed gas, indicating that a part of N2 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 CH4, thus, both the conversion efﬁciency of CH4 and the yield of C2H2 decreased (Figure9), and the CH 4 fraction in the product gas increased, while C2H2 and C2H4 decreased (Figure8). Consistently, the SER of C2H2 ascended with the increase of the N2 concentration in feed gas (Figure 10). Therefore, similar to the case of CO2, it is advisable to remove N2 Energies 2018, 11, x 9 of 14 from alkane-rich gas before pyrolysis, because the presence of N2 not only jeopardizes the production Energies 2018, 11, x 9 of 14 of C2H2, but also generates highly-toxic HCN.

18 CH C H C H 18 4 2 2 2 4 16 CH C H C H N2 4 HCN2 2 2 4 16 N HCN 14 2 14 12 12 10 10 8 8 6 6 Mole Fraction /% Fraction Mole 4 Mole Fraction /% Fraction Mole 4 2 2 0 0 2468101214 2468101214 N Concentration Vol/% N2 Concentration Vol/% 2

Figure 8. The effects of N2 concentration on the molar fraction of major products at 15 kW (total gas FigureFigure 8. The 8. The effects effects of Nof2 Nconcentration2 concentration onon thethe molar fraction fraction of of major major products products at 15 at kW 15 kW(total (total gas gas flow rate of methane and hydrogen: 7.5 Nm33/h; magnetic flux intensity: 0.077 T). ﬂowflow rate ofrate methane of methane and and hydrogen: hydrogen: 7.5 7.5 Nm Nm3/h;/h; magnetic magnetic flux ﬂux intensity: intensity: 0.077 0.077 T). T).

80 80 80 80

75 75 75 75

70 70 70 70

65 65 65 65 /% 4 /% /% 2

4 /% 2

H CH 2 H

CH 60 60 2 60 60 C C Y CE Y CE 5555 5555

5050 5050

4545 4545 2468101214 2468101214 N ConcentrationConcentration Vol/% 22 Figure 9. The effects of N content on yield of C H and conversion efficiency of CH at 15 kW (total gas FigureFigure 9. 9. The The effects effects of of2 NN22 contentcontent onon yieldyield ofof2 C2H2 2 and conversionconversion efficiencyefficiency ofof CH CH44 4at at 15 15 kW kW (total (total 3 flowgasgas rate flow flow of methanerate rate of of meth meth andaneane hydrogen: andand hydrogen:hydrogen: 7.5 Nm 7.5 Nm/h;3 magnetic/h; magnetic flux fluxflux intensity: intensity:intensity: 0.077 0.0770.077 T). T).

2020

1919

1818

) 17

) 17 2 2 H H 2 2 1616

1515

(kWh/kg C (kWh/kg 14 2

(kWh/kg C (kWh/kg 14 2 H 2 H 2 C C 1313 SER SER 1212

1111 10 10 2 4 6 8 10 12 14 2 4 6 8 10 12 14 N concentration Vol/% N2 concentration Vol/% 2

Energies 2018, 11, x 9 of 14

18 CH C H C H 4 2 2 2 4 16 N2 HCN 14

Mole Fraction /% Fraction Mole 4

0 2468101214 N Concentration Vol/% 2

Figure 8. The effects of N2 concentration on the molar fraction of major products at 15 kW (total gas flow rate of methane and hydrogen: 7.5 Nm3/h; magnetic flux intensity: 0.077 T).

80 80

75 75

70 70

65 65 /% 4 /% 2

H CH 60 60 2 C Y CE 55 55

50 50

45 45 2468101214 N Concentration Vol/% 2

Figure 9. The effects of N2 content on yield of C2H2 and conversion efficiency of CH4 at 15 kW (total Energies 2018, 11, 351 9 of 14 gas flow rate of methane and hydrogen: 7.5 Nm3/h; magnetic flux intensity: 0.077 T).

) 17 2 H 2 16

(kWh/kg C (kWh/kg 14 2 H 2 C 13 SER 12

10 2 4 6 8 10 12 14 N concentration Vol/% 2

Figure 10. The effects of N2 content on SER of C2H2 at 15 kW (total gas ﬂow rate of methane and hydrogen: 7.5 Nm3/h; magnetic ﬂux intensity: 0.077 T).

 ∗ ∗  CH4 + e → CH3 + H + e, CH3 + e → CH2 + H + e  ∗ ∗  CH2 + e → CH + H + e, CH + e → C + H + e  ∗ N2 + e → N + N + e  (10)  (a) CH + N → HCN, (b) C + H → CH   (c) HCN + e∗ → CN + H + e (d) CH + CH → CH = CH e ∗ (high − energy electron)

3.2. 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 Table1 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.

3.2.1. Influence of Input Power on the Simulated COG Pyrolysis Process The investigated experimental conditions were listed as follows: feed rate of 7.5 Nm3/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 CH4, CO2, and N2 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 C2H2), 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 CO2 and N2 in the feed gas, as demonstrated by the previous sections. As for the hydrocarbon products, C2H6 content did not change obviously, only a small amount, because of its small content in the feed and similar pyrolysis properties to methane. C2H4 maintained a low content at first, and then increased from 0.64% to 3.3%. However, the change of C2H2 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 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 Energies 2018, 11, x 10 of 14

Figure 10. The effects of N2 content on SER of C2H2 at 15 kW (total gas flow rate of methane and hydrogen: 7.5 Nm3/h; magnetic flux intensity: 0.077 T).

3.2. 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.

3.2.1. Influence of Input Power on the Simulated COG Pyrolysis Process

The investigated experimental conditions were listed as follows: feed rate of 7.5 Nm3/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 CH4, CO2, and N2 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 C2H2), 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 CO2 and N2 in the feed gas, as demonstrated by the previous sections. As for the hydrocarbon products, C2H6 content did not change obviously, only a small amount, because of its small content in the feed and similar pyrolysis properties to methane. C2H4 maintained a low content at first, and then increased from 0.64% to 3.3%. However, the change of C2H2 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 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%.Energies The2018 SER, 11, of 351 C2H2 also exhibited similar change (Figure 12). When the power ascended10 of 14 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 slowly from 16.7 kWh/kg C H to 18.8 kWh/kg C H , but then increased rapidly from 18.8 kWh/kg increase from 21.6 kW to 25.62 kW.2 2 2 C2H2 to 41 kWh/kg C2H2 when the power continued to increase from 21.6 kW to 25.6 kW.

CH4 C2H 2 C2H 4 C 2H 6 CO CO N HCN 10 2 2

4 Mole Fraction /% Fraction Mole

12 14 16 18 20 22 24 26 P /kW FigureFigure 11.11. TheThe effect effect of of input input power power on thethe molarmolar fraction fraction of of major major products products in the in pyrolysisthe pyrolysis of of simulated coke oven gas (total gas ﬂow rate of simulated coke oven gas: 7.5 Nm3/h; magnetic ﬂux simulated coke oven gas (total gas flow rate of simulated coke oven gas: 7.5 Nm3/h; magnetic flux Energies 2018intensity:, 11, x 0.077 T). 11 of 14 intensity: 0.077 T). 100 45

90 40 -1 ) 80 35 2 H 2 /% 6 H 2 70 30

+C 4 CH /kWh*(kg C /kWh*(kg 2 CE 60 25

H 2 C

50 20 SER

40 15 12 14 16 18 20 22 24 26 P /kW

FigureFigure 12. The 12. The effects effects of ofinput input power power on on the conversionconversion of of alkanes alkanes and andSER SERof C2 ofH2 Cin2H the2 in pyrolysis the pyrolysis of of simulatedsimulated coke coke oven oven gas gas (total gasgas ﬂowflow rate rate of of simulated simulated coke coke oven oven gas: gas: 7.5 Nm7.5 3Nm/h;3 magnetic/h; magnetic ﬂux flux intensity:intensity: 0.077 0.077 T). T).

ThisThis could could be attributed be attributed to tothe the dual dual effect effect of of the inputinput power power on on the the pyrolysis. pyrolysis. On On one one hand, hand, increasingincreasing input input power power can can provide provide more more energy energy to to convertconvert the the components components in in feed feed gas gas and, and, thus, thus, produce more C H . On the other hand, increasing input power will also elevate the temperature in produce more C2H22. On2 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 the reactor, and once the temperature is too high, the generated metastable C2H2 decomposes into carbon and H2. Therefore, there should be an optimum input power for the pyrolysis of alkane-rich solid carbon and H2. Therefore, there should be an optimum input power for the pyrolysis of alkane- gases to produce the C2H2-rich product gas. In the current experiment, taking the various factors into rich gasesconsideration, to produce an input the C power2H2-rich of approximately product gas. 17.9In the kW current was a desired experiment, condition taking for pyrolysis the various because 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).

100 100

90 90

80 80

70 70 /% /% 2 2

H H 2 2

60 60 C C S Y

50 50

40 40

30 30 12 14 16 18 20 22 24 26 P /kW

Figure 13. The effects of input power on the yield and selectivity of C2H2 in the pyrolysis of simulated coke oven gas (total gas flow rate of simulated coke oven gas: 7.5 Nm3/h; magnetic flux intensity: 0.077 T).

3.2.2. 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 Nm3/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)

Energies 2018, 11, x 11 of 14

100 45

90 40 -1 ) 80 35 2 H 2 /% 6 H 2 70 30

+C 4 CH /kWh*(kg C /kWh*(kg 2 CE 60 25

H 2 C

50 20 SER

40 15 12 14 16 18 20 22 24 26 P /kW

Figure 12. The effects of input power on the conversion of alkanes and SER of C2H2 in the pyrolysis of simulated coke oven gas (total gas flow rate of simulated coke oven gas: 7.5 Nm3/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 alkane- rich gases to produce the C2H2-rich product gas. In the current experiment, taking the various factors Energies 2018, 11, 351 11 of 14 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%), andit combined low SER high (18.8 alkane kWh/kgC conversion2H2). (75.4%), high C2H2 yield (68.3%) and selectivity (91.2%), and low SER (18.8 kWh/kgC2H2).

100 100

90 90

80 80

70 70 /% /% 2 2

H H 2 2

60 60 C C S Y

50 50

40 40

30 30 12 14 16 18 20 22 24 26 P /kW

Figure 13. The effects of input power on the yield and selectivity of C2H2 in the pyrolysis of simulated Figurecoke oven 13. The gas effects (total of gas input ﬂow power rate ofon simulatedthe yield and coke selectivity oven gas: of C 7.52H2Nm in the3/h; pyrolysis magnetic of simulated ﬂux intensity: coke 3 oven0.077 gas T). (total gas flow rate of simulated coke oven gas: 7.5 Nm /h; magnetic flux intensity: 0.077 T).

3.2.2.3.2.2. Influence Influence of of Feed Feed Rate Rate on on the the Simulated Simulated COG COG Pyrolysis Process ExperimentalExperimental conditions conditions in in investigating investigating the the influence influence of of feed feed rate rate are listed as follows: input 3 powerpower of 15 ±± 0.50.5 kW, magneticmagnetic inductioninduction intensity intensity of of 0.077 0.077 T, T, and and feed feed rate rate of 3.6–10of 3.6–10 Nm Nm3/h./h. Figure Figure 14 14exhibits exhibits the the effect effect of theof the feed feed rate rate on theon the major majo products’r products’ content. content. With With the increasethe increase of the of feedthe feed rate, rate, the CO fraction showed a decreasing trend in the pyrolysis product from 7.5% to 6.2%, and the the CO fraction showed a decreasing trend in the pyrolysis product from 7.5% to 6.2%, and the CO2 COfraction2 fraction had had a sensible a sensible growth growth from from 0.042% 0.042% to 0.79%, to 0.79%, implying implying that that the reactionthe reaction in Equation in Equation (9) was (9) weakened to some extent. At the same time, the side reaction of N2 in Equation (10) also became slightly weakened, as indicated by the results that with the increase of the feeding rate, the N2 content in the product increased slightly overall, while the content of HCN slightly decreased. The content of C2H4 and C2H6 hardly changed, as could be seen as the molar fraction of C2H4 was between 0.63% and 0.85%, and that of C2H6 was below 0.26%. The most obvious and important changes were the levels of CH4 and C2H2. With the increase of the feed rate, the molar fraction of CH4 in the product increased sharply from 0.86% to 10.4%, but that of C2H2 dropped from 9.5% to 6.2%. This suggested that more CH4 was not cracked as the feeding rate increased, resulting in an increase in the CH4 mole fraction and a dilution of C2H2. In fact, as shown in Figures 15 and 16, the alkanes’ conversion efficiency decreased rapidly from 96.1% to 56%, but the C2H2 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 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. 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. Energies 2018, 11, x 12 of 14 wasEnergies weakened 2018, 11, xto some extent. At the same time, the side reaction of N2 in Equation (10) also became12 of 14 slightly weakened, as indicated by the results that with the increase of the feeding rate, the N2 content inwas the weakened product increased to some extent.slightly At overall, the same while time, the the content side reactionof HCN ofslightly N2 in decreased.Equation (10) The also content became of Cslightly2H4 and weakened, C2H6 hardly as indicatedchanged, byas thecould results be seen that aswith the the molar increase fraction of the of feeding C2H4 was rate, between the N2 content 0.63% andin the 0.85%, product and increased that of C slightly2H6 was overall,below 0.26%. while theThe content most obvious of HCN and slightly important decreased. changes The contentwere the of levelsC2H4 andof CH C24H and6 hardly C2H2 . changed,With the asincrease could ofbe theseen feed as therate, molar the molar fraction fraction of C2 Hof4 CHwas4 inbetween the product 0.63% increasedand 0.85%, sharply and that from of 0.86% C2H6 wasto 10.4%, below but 0.26%. that ofThe C 2Hmost2 dropped obvious from and 9.5% important to 6.2%. changes This suggested were the thatlevels more of CHCH44 andwas Cnot2H cracked2. With the as theincrease feeding of ratethe feedincreased, rate, the resulting molar infraction an increase of CH in4 inthe the CH product4 mole fractionincreased and sharply a dilution from of0.86% C2H to2. 10.4%,In fact, but as thatshown of C in2H Figures2 dropped 15 fromand 9.5%16, the to 6.2%.alkanes’ This conversion suggested efficiencythat more decreased CH4 was not rapidly cracked from as the96.1% feeding to 56%, rate but increased, the C2H resulting2 selectivity in an remained increase atin athe high CH 4level, mole betweenfraction 88.1%and a anddilution 90%. ofThis C2 Hindicated2. In fact, that as shownwhen the in alkaneFigures conversion 15 and 16, efficiency the alkanes’ decreased, conversion the sideefficiency reactions decreased were not rapidly enhanced, from and 96.1% the conversionto 56%, but of the alkanes C2H2 wasselectivity still dominated remained by at the a formationhigh level, ofbetween C2H2. As 88.1% the conversion and 90%. Thisof alkanes indicated decreased, that when and Cthe2H alkane2 selectivity conversion remained efficiency unchanged, decreased, the yield the ofside C2H reactions2 decreased were with not theenhanced, increase and of thethe feedconversion rate, from of alkanes 84.7% to was 50.3%. still dominatedMoreover, theby the SER formation of C2H2 wasof C reduced2H2. As the from conversion 26.4 kWh/kg of alkanes C2H2 to decreased, 15.8 kWh/kg and C C2H2H2. 2The selectivity phenomenon remained above unchanged, was probably the yield the resultof C2H of2 decreasedthe shorter with gas theresidence increase time of theand feed lower rate, temperature from 84.7% in to the 50.3%. reactor Moreover, caused bythe the SER increase of C2H 2 ofwas the reduced feed rate, from which 26.4 made kWh/kg it difficult C2H2 to to 15.8 convert kWh/kg the C increased2H2. The phenomenonalkanes into C above2H2 in was time. probably the result of the shorter gas residence time and lower temperature in the reactor caused by the increase ofEnergies the feed2018 rate,, 11, 351 which made it difficult to convert the increased alkanes into C2H2 in time. 12 of 14 C H CH C H C H 12 2 2 4 2 6 2 4 CO CO N HCN 2 2 10 C H CH C H C H 12 2 2 4 2 6 2 4 CO CO N HCN 2 2 8 10

6 8

4 6 Mole Fraction /% Mole Fraction

2 4 Mole Fraction /% Mole Fraction

0 2

345678910 3 0 Q(Nm/h) 345678910 3 Figure 14. The effects of feeding flow rate on theQ molar(Nm/h) fraction of major products in the pyrolysis of siimulated coke oven gas (input power: 15 ± 0.5 kW; magnetic flux intensity: 0.077 T). Figure 14. The effects of feeding ﬂow rate on the molar fraction of major products in the pyrolysis of 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). siimulated coke oven gas (input power: 15 ± 0.5 kW; magnetic flux intensity: 0.077 T).

90 94 85 92 8090 9094 7585 8892 7080 8690 6575 /% /% 2 2

H 88 H

2 84 2 C 6070 C S Y 8286 65 /%

55 /% 2 2 H H

2 84 80 2 C 60 C S Y 50 7882 4555 7680 4050 34567891078 45 Q (Nm3/h) 76 40 345678910

Figure 15. The effects of feed rate on the yield and selectivity3 of C H in the pyrolysis of simulated Q (Nm /h) 2 2 Figure 15. The effects of feed rate on the yield and selectivity of C2H2 in the pyrolysis of simulated Energiescoke 2018 oven, 11, x gas (input power: 15 ± 0.5 kW; magnetic ﬂux intensity: 0.077 T). 13 of 14 coke oven gas (input power: 15 ± 0.5 kW; magnetic flux intensity: 0.077 T). 100 30 Figure 15. The effects of feed rate on the yield and selectivity of C2H2 in the pyrolysis of simulated coke oven gas (input power: 15 ± 0.5 kW; magnetic flux intensity: 0.077 T). 90 27 ) 2 H 80 24 2 /% 6 H 2

+C 4 CH 70 21 (kWh/kgC 2 H 2 CE C

60 18 SER

50 15 34567891011

3 Q (Nm /h)

Figure 16. The effects of the feed rate on the conversionconversion efﬁciencyefficiency ofof alkanesalkanes andand SERSER ofof CC22H22 in pyrolysis of simulated coke oven gas (input power:power: 15 ± 0.50.5 kW; kW; magnetic magnetic flux ﬂux intensity: intensity: 0.077 0.077 T). T).

In the entire range of the flow rates investigated, the C2H2 selectivity was always maintained at In this work, the effect of various nitrogen/oxygen-containing substances including N2, CO, a high level, but the conversion of alkanes and the yield of C2H2 decreased with the flow rate. This and CO2 on the pyrolysis of CH4 to C2H2 was conducted by thermal plasma, as well as pyrolysis of 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 nitrogen/oxygen-containing substances CO2 and N2 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 CO2 and N2 from the alkane-rich feed gas at first, and then pyrolyze the feed gas with a rotary thermal plasma reactor to produce C2H2-rich pyrolysis gas, and, finally, separate C2H2 from the other components, including CO, in the pyrolysis gas.

4. Conclusions

The influence of nitrogen/oxygen substances CO2, N2, and CO for the pyrolysis of alkane-rich mixing gases to produce C2H2 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%, N2 and CO had negative effects on the main reaction that converted CH4 to C2H2, 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 CO2 and N2 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 CO2, N2, 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 C2H2 selectivity and yield, along with the input power, the selectivity of C2H2 up to 91.2% and a C2H2 yield of 68.3% were obtained at the input power of 17.9 kW. Higher C2H2 selectivity and a lower C2H2- specific energy requirement could be obtained when the feed rate was increased, although at the expense of lower alkane conversion and C2H2 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.

Acknowledgment: This work was supported by the National Key Research and Development Program of China (2016YFB0301800).

Author Contributions: Baogen Su and Qiwei Yang conceived and designed the experiments. Junkui Jin and Wei Huang performed the experiments, and all the authors analyzed the data. Wei Huang wrote the paper.

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 CO2 and N2 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 CO2 and N2 from the alkane-rich feed gas at first, and then pyrolyze the feed gas with a rotary thermal plasma reactor to produce C2H2-rich pyrolysis gas, and, finally, separate C2H2 from the other components, including CO, in the pyrolysis gas.

4. Conclusions

The influence of nitrogen/oxygen substances CO2,N2, and CO for the pyrolysis of alkane-rich mixing gases to produce C2H2 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%, N2 and CO had negative effects on the main reaction that converted CH4 to C2H2, 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 CO2 and N2 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 CO2,N2, 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 C2H2 selectivity and yield, along with the input power, the selectivity of C2H2 up to 91.2% and a C2H2 yield of 68.3% were obtained at the input power of 17.9 kW. Higher C2H2 selectivity and a lower C2H2-specific energy requirement could be obtained when the feed rate was increased, although at the expense of lower alkane conversion and C2H2 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.

Acknowledgments: This work was supported by the National Key Research and Development Program of China (2016YFB0301800). Author Contributions: Baogen Su and Qiwei Yang conceived and designed the experiments. Junkui Jin and Wei Huang performed the experiments, and all the authors analyzed the data. Wei Huang wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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