アービド アブドゥラA. キナーナ

佐藤 慎太郎

山﨑 亮

有谷 博文

柳井 尚輝

菅原 利史

Saitama Institute of Technology Academic Collections

Nonequilibrium Plasma Reforming of Greenhouse Gases  Under Atmospheric Pressure by Using a  Conventional ReactorRyo YAMAZAKI＊, Kinanah ABED ABDULLAH  A.＊＊, Naoki YANAI＊＊,  Shintaro SATO＊＊＊, Toshifumi SUGAWARA＊＊＊ and Hirofumi ARITANI＊＊†＊　　  Advanced Science Research Laboratory, Saitama Institute of Technology＊＊　  Department of Life Science & Green Chemistry, Faculty of Engineering, Saitama Institute of Technology＊＊＊  Division of Life Science & Green Chemistry, Graduate School of Engineering, Saitama Institute of Technology†aritani＠sit.ac.jp簡便な非平衡プラズマ反応器による 温室効果ガスの大気圧下低エネルギー改質山﨑　　亮＊，キナーナ アービド アブドゥラ A.＊＊，柳井　尚輝＊＊， 佐藤慎太郎＊＊＊，菅原　利史＊＊＊，有谷　博文＊＊＊　　  埼玉工業大学先端科学研究所＊＊　  埼玉工業大学工学部生命環境化学科＊＊＊  埼玉工業大学大学院工学研究科生命環境化学専攻Abstract　　　By using a conventional tubular reactor, nonequilibrium plasma reforming of greenhouse gases was carried out in low energy emission (below 12.5 W) under atmospheric pressure.   Conversion of CO2 increased to produce CO and O2 according to increasing the output.   By mixing CO2 and CH4 as a reactant feed, useful compounds such as ethane, ethylene (by way of methane coupling), and their partially oxidized compounds (ethanol, acetaldehyde, etc.).   It is suggested the distance among electrodes, which is due to the thickness of the reactor wall, is very important to affect both the conversion of greenhouse gases (CO2 and CH4) and the selectivity of product the useful chemicals.Key Words:  Conversion of greenhouse gases, Nonequilibrium plasma process, Conventional tubular reactor, From CO2 and CH4 to useful compounds1. Introduction　　　As a critical global issue, increasing greenhouse gases deepens concerns about the pace of global warming1).   For sustainable society by significantly reducing greenhouse gases, reduction of carbon dioxide (CO2) population in the atmosphere to account for enormous discharges is urgent2-3) because the greenhouse gases in the atmosphere ―　　―15Nonequilibrium Plasma Reforming of Greenhouse Gases  Under Atmospheric Pressure by Using a  Conventional Reactor（YAMAZAKI, ABED, YANAI, SATO, SUGAWARA and ARITANI）have certain property of absorbing infrared radiation. for.   In 2010, the government of Japan approved the new CO2 emission target, promising to reduce emissions by 25％ by 2020 compared to the 1990 level.   However, the reduction plan of the CO2 failed virtually by a serious accident of Fukushima Daiichi Nuclear Power Station (F1NPS) due to the Great East Japan Earthquake in May 2011.   As a result, the quantity of carbon dioxide emission increased after 2011 because of re-operation of the thermal power station4).   Even in Japan, it becomes essential to participate in a global approach for the reduction of the CO2 emission immediately.　　　To provide a noble method for converting or immobilizing CO2, new chemical approach which can be accelerated with low energy is called for. But it has been energetically and economically difficult.   Since CO2 is chemically so stable that converting CO2 needs considerable energy, high energy processes such as high temperature and/or pressure cannot be avoided for converting CO2 to useful compounds (methane, methanol, syngas, etc.) even in catalytically major processes.   On the other hand, the use of CO2 feedstock can lead to the development of a new generation of useful and innovative chemical products with superior technical performance or unique properties in specific cases. If conventional decomposition of CO2 (based on a fragmentation of the C=O bond) under low energy, a desired chemical process producing chemicals and fuels from CO2 as a renewable carbon source will be acquired.   It is one of an area to watch for one or more revolutions of the chemical industry in near future.　　　In the present study, we introduced a nonequilibrium plasma reforming of greenhouse gases (CO2 and CH4).   By applying atmospheric pressure glow discharge, the greenhouse gases can activate to the radical intermediates under low energy field.   The nonequilibrium plasma reactors employed in the present study are made easily by hand,  and  very  simple  and  inexpensive.   As reported in a previous paper5), a conventional surface discharge tubular reactor can be applied for direct conversion of greenhouse gases even under low energy emission below 20 W.   By using this type of reactor, CO2 can easily decompose according to the output under atmospheric pressure.   Conversion of methane also proceeds more  easily.   At  the  same  time,  combination between nonequilibrium plasma process and catalytic one (plasma-catalysis), which can be achieved by filling the catalytic materials into the reactor, will be applied widely for converting greenhouse  gases  to  useful  chemicals6).   When  a spherical material pellet is filled into the reactor, another type of reaction as a dielectric barrier discharge Plasma (DBD) reaction can be proceed.　　　As a subject, it is difficult to control the selectivity of products over the nonequilibrium plasma  reaction.   For  example,  CO  formation  is dominant in nonequilibrium plasma reaction because decomposition of CO2 (to CO＋1/2 O2) and partially oxidation of CH4 (by way of reacting O2 from CO2) tend to proceed at the same time. To produce the useful chemicals such as ethane, ethylene (by way of oxidative coupling of methane) and partially oxidized compounds (alcohols, carboxylic acids, ketones, etc.), it is very important to achieve the selectivity control of the products by controlling the parameters on the nonequilibrium plasma reaction.   In particular, the condition plasma-irradiated field onto the reactor strongly relates to the reactivity for converting.   In this study, correlation between the reactor parameters and reactivity of CO2 and CH4 conversion to produce useful chemicals is studied by using several types of nonequilibrium plasma reactor.2. Experimental　　　A surface discharge reactor, which consists of PYREX glass tube with branched pipes, a spiral stainless wire coil (as an inner electrode into the reactor tube, 0.35 mm of wire thickness), stainless foil tape (as an outer electrode), and terminal rubber cap, was used for the nonequilibrium plasma reaction.   A spiral wire coil was set in contact with the inner wall of a glass tube as a 埼玉工業大学工学部紀要　第 30号　2020―　　―16barrier, and stainless foil tape was set on the outer side of the tube as the ground electrode.   Two electrodes are connected a high-voltage amplifier, which used a M-5 (100 V) inverter-type neon transformer (Lecip) via a power meter (3333 Power HiTESTER, Hioki) and variable voltage type transformer.   Fig. 1 shows the outline of the nonequilibrium plasma reaction system in the present study.　　　Several types of tubular reactor were used in this  study  as  summarized  in  Table 1.   The parameters; (A) indicates the length of plasma filed, (B) indicates the internal diameter of tubular reactor, (C) indicates the thickness of the reactor wall (due to the distance among electrodes), and (D) indicates the number of turns in an inner electrode.   As a dielectric barrier discharge Plasma (DBD) reaction, spherical filler with ca.  2 mm diameter was filled into each reactor.   The fillers are; silica-gel (Fujifilm-Wako), alumina for catalyst (Sumitomo Chemical), and water absorptive alumina (Union Showa).　　　The reactant gas mixture is; [i] CO2 (15 mL/min)＋Ar (30 mL/min), [ii] CO2 (15 mL/min)＋CH4 (15 mL/min)＋Ar (15 mL/min), or [iii] CO2 (15 mL/min)＋CH4 (30 mL/min).   For each case, the reactant gas mixture was fed into the reactor at a total flow rate of 45 mL/min under atmospheric pressure, and a high-voltage ac was applied across the electrodes at ambient temperature (without heating).   Under stepwise increasing power output below 15.0 W (with 2.5 W step), nonequilibrium plasma reaction was performed for 20 min in each output step.   In case of 15.0 W output for 20 min, temperature in center of the reactor was 53.0 degrees Celsius measured by thermography.   After the plasma reaction, the product gas was analyzed by two TCD gas chromatographs (GC-8A, Shimadzu) equipped a Shincarbon-ST (3 m) column or Porapac-Q (2 m)＋Gaschropack54 (3 m) connected columns.  From the analytical results, Carbon-based conversion and selectivity were obtained.3. Results and Discussion　　　Fig. 2 shows the reactivity of nonequilibrium plasma reforming of CO2 and CH4 by using (I)-(III) reactors.   For CO2＋Ar reactant (i), CO and O2 was detected in all the reactors below 12.5 W output.   In the case of type (II) reactor, highest CO2 conversion is shown among (I)-(III) reactors. This result indicates that the reactivity for converting CO2 is not depend on the turn number of coil (as an inner electrode) or a volume of plasma-irradiating field.   For CO2＋CH4 reactant Fig. 1 Outline of the non-equilibrium plasma reactor (for surface discharge plasma reaction under atmo-spheric pressure).Table 1 Parameters of the plasma filed in tubular reactor for surface discharge plasma reaction.Reactor(A) Length/ mm(B) Diameter/ mm(C) Thickness/ mm(D) Turn number of coil Volume / mLI 177  7.0 1.5 30  6.8II 184  9.0 1.5 40 11.7III 198 12.0 2.0 45 22.4―　　―17Nonequilibrium Plasma Reforming of Greenhouse Gases Under Atmospheric Pressure by Using a Conventional Reactor  （YAMAZAKI, ABED, YANAI, SATO, SUGAWARA and ARITANI）(ii and iii), it is definite that type (II) reactor shows high conversion for CO2＋CH4.   This result suggests that the factors of maximum reactivity for CO2＋CH4 conversion are possibly include appropriate surface discharge condition, which is based on the distance between the electrodes and an effective volume of the reaction field.   In fact, the turn number of coils as an internal electrode reflects  the  spread  of  plasma  flame.   But  it  is difficult for the plasma flame to reach the center (due to the core of cross-section circle in the reactor) of the reactor.   Thus, the wide diameter of the reactor is not always effective for surface discharge plasma reactor even in large volume of plasma irradiation.   In addition, the distance of the electrodes due to thickness of wall in the reactor is very important because of spread of plasma flame. When CO2 and CH4 react, production of H2O cannot  be  avoided.   The  produced  H2O  surely inhibits the reactivity under a weak plasma flame. As a result, it is suggested that the factor of appropriate surface discharge condition includes the moderate distance of electrodes and spread of  plasma  flame  at  the  same  time.   It  may  be estimated that the reason of maximum reactivity over type (II) reactor is due to the comfortable irradiation of surface discharged plasma flame.　　　Table 2 and 3 shows the product selectivity Fig. 2 Conversion of CO2 and CH4 by glow discharge vs. input power using (I)-(III) reactors without fillers.Table 2 Selectivity of the products by glow discharge vs. input power over (ii) CO2＋CH4＋Ar flow reaction using the (II) reactor without fillers.Output(W)Total Conv. (%)Selectivity (%)CO CH3OH HCHO HCOOH C2H2 C2H4 C2H6 C2H5OH CH3CHO C3H82.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 100.0 0.0 0.0 0.05.0 23.9 48.2 0.5 0.0 0.0 0.0 1.6 21.3 2.6 0.9 24.97.5 31.8 55.1 0.4 0.0 0.0 0.0 1.0 17.7 1.9 0.3 23.510.0 36.7 69.6 0.3 0.0 0.0 0.0 0.6 11.7 1.1 0.5 16.112.5 37.0 73.9 0.3 0.0 0.0 0.0 0.4 10.0 0.9 0.4 14.2Table 3 Selectivity of the products by glow discharge vs. input power over (iii) CO2＋2CH4 flow reaction using the (II) reactor without fillers.Output (W)Total Conv. (%)Selectivity (%)CO CH3OH HCHO HCOOH C2H2 C2H4 C2H6 C2H5OH CH3CHO C3H82.5 3.4 26.1 0.8 0.0 0.0 0.0 8.4 25.9 5.0 2.1 31.65.0 16.5 26.8 0.6 0.0 0.0 0.0 3.5 21.7 3.4 1.2 42.87.5 30.7 23.7 0.5 0.0 0.0 0.0 2.6 21.6 3.1 1.1 47.310.0 30.8 36.7 0.7 0.0 0.0 0.0 1.6 15.4 2.3 1.0 42.312.5 32.8 38.1 0.5 0.0 0.0 0.0 1.3 14.8 2.2 0.9 42.2埼玉工業大学工学部紀要　第 30号　2020―　　―18against input power over (ii) CO2＋CH4＋Ar and (ii) CO2＋2CH4 flow reaction, respectively, using the (II) reactor without fillers.   For the (ii) CO2＋CH4＋Ar reactant as shown in Table 2, formation of C2H6 and C3H8 is remarkable along with the CO formation in the low output region (2.5-7.5 W).   It is presumed that radicalization of CH4 occurs in an initial step, and dehydrogenated CHx convert C2H6 and C3H8 by way of methyl radical species.   In parallel, hydrogen react O species, which can be formed by CO2 decomposition, to form H2O. Even in a low selectivity, C2H5OH and CH3CHO are formed as partially oxidized products from C2H6.   They can possibly be formed by reaction between O- or OH-type radicals and C2H6.   With increasing the output below 12.5 W, the CO selectivity increased while the selectivity of C2H6 and C3H8 decreased.   It suggests the proceeds of oxidation form CH4, C2H6, and C3H8 to form CO under high energy output region.   For the (iii) CO2＋2CH4 reactant as shown in Table 3, formation of C2H6 and C3H8 is definite with high selectivity in whole output region.   It can be proposed that the rate of CH4 promotion (dehydrogenation to CHx radicals) is higher than that of CO formation (by way of both CO2 decomposition and oxidation of CHx species), and thus, in CH4-rich reactant under low output below 7.5 W.   Fig. 3 shows a simplified conceptual diagram of initial plasma reaction process  via  radical  reactions.   As  an  activation to plasma-induced species, activation of CH4 to CHx species is relatively faster than CO2 decomposition to CO＋1/2 O2.　　Formed H2 (from CH4 dehydrogenation) can easily react with O2 to produce H2O, and react with hydrocarbon radical (CHx and/or C2Hx) species to produce C2H5OH and  CH3CHO  as  minor  produces.   CH3OH  was hardly  formed  on  the  plasma  reaction.   It  is suggested from the results of selectivity that a direct reaction between an ethyl radical and an oxygen radical occurs much easier than between methane and oxygen (or a methyl radical and a hydroxyl radical).　　　The results of product distribution shown in Table 3 can be expected to be extremely attractive to approach the purpose of effective use of greenhouse gases at low energy.   To enhance the yield of useful chemical compounds, several ceramic fillers were attempted to insert into the tubular reactor for DBD-like reaction.   The filler with ca.  2 mm diameter used in this study is silica-gel, alumina for catalyst, and water absorptive alumina.   The  total  conversion  due  to  CO2  and Fig. 3 Simplified conceptual diagram of plasma reaction process via radical-induced reactions.―　　―19Nonequilibrium Plasma Reforming of Greenhouse Gases Under Atmospheric Pressure by Using a Conventional Reactor  （YAMAZAKI, ABED, YANAI, SATO, SUGAWARA and ARITANI）CH4 by glow discharge vs. input power using the (II) reactor inserted each filler is shown in Fig. 4. For total conversion, inserted reactor with filler becomes quite lower than the reactor without filler.   The  major  reason  is  a  residence  time  of plasma irradiation: The volume of plasma field in the case of without filler (empty) and inserted with filler is 11.3 and 3.0 mL, respectively.   Therefore, contact time in the plasma field is quite different from each other.   Among the filler inserted into the reactor, water absorptive alumina shows relatively high conversion.   In this case, unique selectivity is exhibited.   Table 4 and 5 shows the product selectivity against input power over (ii) CO2＋CH4＋Ar and (ii) CO2＋2CH4 flow reaction, respectively, using the (II) reactor inserted water absorptive alumina fillers in the reactor.   It is noted that high selectivity of C2H2, C2H4, and C2H6 is definite.   Because the surface discharge is enhanced, it is suggested that the insertion of the filler promotes the methane conversion  to  methyl  radicals.   On  this  plasma reaction condition, a subject is the enhancement of total conversion as well as high selectivity due to hydrocarbons and partially oxidized ones.References 1) T.R. Anderson, E. Hawkins, P.D. Jonescd, Endeavour, 40(3), 178‒187 (2016). 2) S. Rao, K. Riahi, The Energy Journal, International Association for Energy Economics, 0(Sp. I), 177‒ 200 (2006). 3) J.E. Szulejko, P. Kumar, A. Deep, K-H. Kim, Atmospheric Pollution Research, 8(1), 136‒140 (2017). 4) P.A. Kharecha, M. Sato, Energy Policy, 132, 647‒653 (2019). 5) A. Ogata, K. Saito, H.-H. Kim, M. Sugasawa, H. Aritani, H. Einaga, Plasma Chem. Plasma Process, 30, 33‒42 (2010). 6) H.-H. Kim, Y. Teramoto, A. Ogata, H. Takagi, T. Nanba, Plasma Chem. Plasma Process, 36, 45‒72 (2016).Fig. 4 Conversion of CO2 and CH4 by glow discharge vs. input power using (I) reactors inserted a various ceramic fillers.Table 4 Selectivity of the products by glow discharge vs. input power over (ii) CO2＋CH4＋Ar flow reaction using the (II) reactor inserted water absorptive alumina fillers in the reactor.Output(W)TotalConv.(%)Selectivity (%)CO CH3OH HCHO HCOOH C2H2 C2H4 C2H6 C2H5OH CH3CHO C3H82.5 0.5 47.9 0.0 0.0 0.0 0.0 19.0 33.1 0.0 0.0 0.05.0 2.6 39.9 0.7 0.0 0.0 7.1 10.8 29.6 4.6 2.6 4.87.5 4.6 41.2 0.5 0.0 0.0 5.9 8.7 29.6 4.7 2.2 7.210.0 7.0 42.0 0.5 0.0 0.0 4.8 6.9 29.5 4.5 2.3 9.412.5 9.3 40.7 0.5 0.0 0.0 3.9 5.7 32.0 5.0 1.8 10.3Table 5 Selectivity of the products by glow discharge vs. input power over (iii) CO2＋2CH4 flow reaction using the (II) reac-tor inserted water absorptive alumina fillers in the reactor.Output(W)TotalConv.(%)Selectivity (%)CO CH3OH HCHO HCOOH C2H2 C2H4 C2H6 C2H5OH CH3CHO C3H82.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.05.0 1.5 27.6 0.7 0.0 0.0 8.8 13.5 30.6 4.9 2.4 11.57.5 2.8 29.8 0.7 0.0 0.0 7.2 11.6 32.4 5.2 2.4 10.810.0 5.1 28.1 0.6 0.0 0.0 6.0 10.0 34.5 7.4 3.4 10.012.5 6.9 25.8 0.5 0.0 0.0 5.0 8.3 34.5 11.1 4.2 10.7埼玉工業大学工学部紀要　第 30号　2020―　　―20

Nonequilibrium Plasma Reforming of Greenhouse Gases Under Atmospheric Pressure by Using a Conventional Reactor

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Nonequilibrium Plasma Reforming of Greenhouse Gases Under Atmospheric Pressure by Using a Conventional Reactor

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