Manuscript1-with Full Ref

Decomposition of green house gasses by plasma: a short review Jae-Wook Choi. Hwaung Lee, Hyung Keun Song Korea Institute of Science and Technology, PO BOX 131, Cheongryang, Seoul, Korea Email: [email protected] The topic on decomposition and reduction of green house gases were important issues in tackling the global warming effect. Several technologies including plasma were proposed to improve the process for this purpose. In this review paper, the application of plasma to reduce the emission of greenhouse gases was brief summarized. 1. Introduction The emission from various industrial areas into ambient environments causes problems on environmental [10]. It usually contains greenhouse gases, such as carbon dioxide (CO2), methane (CH4), and chlorinated volatile organic compounds (CVOCs), e.g. methylene chloride (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4). Among all greenhouse gases, however, CO2 with CH4 contributes the most of man-made greenhouse effect. Numerous amounts of CO2 was released into the environment, estimated to be around 2 x 1015 g per annum, and industrial sector has been suspected as the main contributor (Kiani et al., 2004; Yabe, 2004). Chlorinated VOC emission, resulted by degradation chlorine-containing chemicals, could be the source of chloric acid (HCl) and suspected to contribute in the acid rain problem [11]. Some studies reported that these compounds, in stratosphere, could produce highly active chlorine radical due to solar radiation reactions and it has possibility to react and reduce the ozone molecules [12]. However, the most significant problem related to the emission of chlorinated VOCs is remained on the high toxicity and carcinogenic [13]. Concerning this situation, Kyoto protocol obligates industrialized countries to cut their greenhouse gases emissions by an average 5.2% between 2008 and 2012. This will deliver a strong message to us for finding effective methods for destructing and eliminating these industrial wastes [14]. In this discussion, we would like to address the issue of the use of plasma technology as the alternative way to reduce or decompose gaseous emissions. The topic will not too much on the physical-mathematical terms but in the application of plasma for environmental purposes. 2. Plasma in General The discovery of plasma and its application have been known for more than a century

[1]. Electric - created plasma – assisted chemical process has been intensively studied in the way of finding some potential applications for new chemical synthesis or other purposes. Moreover, worldwide application and industrial large-scale process using plasma are getting famous, e.g. ozone production and surface treatment [2-3]. One of the main advantages of the plasma method is that plasma is able to produce very high density of discharge species compared to other methods, e.g. electrochemical [4] and combustion. In general, based on the flame temperature, plasma can be divided into two major parts, thermal plasma and cold (non-thermal) plasma. Thermal plasma, having temperature more than 10,000 K, is widely used to decompose (strong bond) toxic chemicals and solid particle synthesis. This type of plasma has very high-energy species, able to destruct stable or strong chemical-bond of the compounds. Some of them, e.g. thermal-arc, torch plasma or supersonic plasma, can handle high input flow rates with very short reaction time. The second type of plasma is called non-thermal plasma or non-equilibrium plasma. Although it is not as strong as thermal plasma, cold (non-thermal) plasma is much favorable for the application in gaseous chemical synthesis type reactions, such as methane conversion for synthesis gas production. Non-thermal plasma is referring to the temperature of the bulk gas which usually as lower as room temperature. The examples are quite many and some of them are already applied in industrial scale [1], such as corona, dielectric barrier discharge (DBD), microwave plasma, and radio frequency (RF) plasma. Table 1. General comparison between thermal and non-thermal plasma C h a ra c te ristic

T h e rm a l P la sm a

C o ld P la sm a

> 1 0 ,0 0 0

5 ,0 0 0 - 1 0 ,0 0 0

D isc h a rg e d v o lu m e

h ig h

lo w

R e a c tio n c o n v e r s io n

h ig h

lo w

I n p u t flo w r a te

h ig h

lo w

d iffic u lt

easy

r e la tiv e d a n g e r o u s

re la tiv e s a fe

e x p e n siv e

ch eap

T e m p e r a tu r e (K )

D iffic u lty to h a n d le S a fe ty I n s ta lla tio n c o s t

Instead of their physical characteristics, the consideration to choose the plasma used for the application is also depending on other aspects. Table 1 shows the general comparison between thermal and cold plasma in some non-physical factors. This table clearly shows some limitations of each division which could be a barrier to be applied in industrial scale. Thermal

plasma consumes high energy and the installation and/or the operating cost is expensive. On the other hand, cold plasma has a main problem related to the low conversion of the reactants, especially for toxic gas decomposition case when the concentration is higher than ppm level. But, the maintenance and operation cost are relatively easy and cheap. Some optimizations are necessary in order to obtain better performance with lower cost. Currently, the remaining key point for plasmachemical methods to be acceptable as a chemical-advanced process or method is how to optimize the condition to produce profitable reaction. Plasma for chemical synthesis offers a wide spectrum of possible application. In particular, depending on the physical characteristics of the plasma which produced by different ionization systems, three types of processes can be classified: (1) Destruction of toxic/harmful materials; (2) Modification of existing materials, e.g. surface treatment for catalyst; (3) Creation of new materials [5]. For the first point, many researches have been conducted for decomposing many industrial emissions, such as H2S [6], N2O [7], CHCl3 and CCl4 [8,9]. High percentage of destruction efficiency has been claimed using this the plasma method. However, in this discussion, we would like to focus on the three type industrial gaseous: chlorinated VOCs, methane (CH4) and carbon dioxide (CO2) 3. Decomposition of Chlorinated VOC compounds The most widely adopted or common technique for the treatment of chlorinated CVOC emission was thermal combustion or incineration [15]. This method was mostly used due to its easiness and simplicity by reacting directly with air under high temperature condition c.a. 8001,100oC. It was reported that when the combustion did not occur perfectly (incomplete combustion), the reaction tend to produce a large amount of complex chlorinated products [16] which could be more toxic that the reactants itself. Catalytic oxidation which is employing metal and metal oxide catalyst has been investigated also. It has been reported that using this method, the conversion and the product selectivity were very good, but the catalyst become easily deactivated by impurities and solid product [17]. Also, this method requires temperature heater to achieve the activated catalyst temperature and reaction rate. Other limitation to use this method in industrial plant is the input flow rate which was very small. Many studies were carried out on the application plasma-assisted technology as the alternative method which could overcome above problems. Plasma-assisted technology, such as Radio Frequency (RF) plasma [18], surface discharge reactors [19], dielectric barrier discharge reactors [20], pulsed discharge reactors [21], and capillary-tube type discharge reactors [22] have been studied and develop. Lee et al. [18] has tried to decompose 1,1 dichloroetylene

(DCE) using RF plasma. It showed that below 10% concentration of DCE (diluted on oxygen) at gas flow rate 15 sccm, the DCE was perfectly removed. The products were dominated by CO and CO2 which the concentration reached 50% at high oxygen feeds. This method has a major drawback that at higher O2 injection, instead of increasing CO2 concentration, the concentration of COCl2 (phosgene) was also rising. This phenomenon could be happen due to the reaction between cloride molecules with oxygen CHC12 + O = COC12 + H CHC13 + O = COCl2 + HCl CC13 + O = COCl2 + OCl CC13 + O = C12 + Cl Using TiO2-series catalysts, Oda et al. has tried to remove trichloroetylene (TCE) using dielectric barrier discharge (DBD). The catalyst acted very well regarding the energy saving [19]. The decomposition of CCl4 was initiated by Tonkyn et al. [20] using DBD with introducing ZnO2 catalyst. The observed that the reduction mechanism was likely a synergistic effect that involves changes in the plasma density, scavenging of low energy secondary electrons, and possible surface passivation. This point, which could be the advantage of plasmaassisted process, carried an important role on the decomposition process and the energy density supplied from electrical generator should be measured in order to control the strength of the energy. Yamamoto et al. [21] has performed the decomposition of various VOCs, including DCE and trichlorotrifluoroethane (known as CFC-113), and able to remove those compounds up to c.a. 67%. They concluded that the plasma process was strongly depend on the energy strength of the electron and in the presence of halogen species, it can help to increase the performance by colliding with carbon. Kohno et al. [22] was also conducted experiment to remove some VOCs using capillary column plasma. The author claimed that they able to remove the VOCs as high as 90% under a short residence time (3.8 ms) with a destruction energy efficiency of up to 95 g (VOC)/kWh. This promising result could be applied for the the semiconductor clean-room environment. Currently, a series of experiment to decompose VOCs has been conducted by Indarto et al. []. The experiments were performed using gliding arc plasma, another type of non-thermal plasma which produced arcs from two or more electrode blades. Compare to the other non-thermal plasmas, gliding arc plasma produced higher flame temperature, stronger power, and able to handle higher input flow rates. This type of plasma is currently getting famous and better possibility to be utilized for industrial chemical applications [3]. Study about destruction of carbon tetrachloride (chlorinated methane compounds) under gliding plasma has been studied [8,9], but no explanation about reaction mechanism. That is important to make evaluation and optimization of its technology.

Kinetic model of chlorinated methane compounds decomposition in dry air under electron beam has been investigated also [23-25]. The theoretical model suggested that the main product could be Cl2, COCl2, CO, CO2, ClNO3 and ClO3. Decomposition of chlorinated methane compounds, it self, consists of two kinds of reaction mechanism [23]. First mechanism took place when CCl4 collides with dissociated species, such as ground state atomic oxygen O(3P) and excited atomic oxygen O(1D), O(3P) + CCl4 = ClO + CCl3

(2)

1

O( D) + CCl4 = CCl3 + ClO2

(3)

Another mechanism was through the secondary electrons. The secondary electrons could dissociate CCl4 to produce CCl3 and negative ion Cl‾, e + CCl4 = CCl3 + Cl-

(4)

However, radical ClO and CCl3 were the most important intermediate species in the formation of final product of the decomposition process and the amount O and Cl have most responsibility on radical reaction both initiation and termination reaction. CO, Cl2 and CO2 could be formed by dissociative process with electron and CCl2O (phosgene) according [25]: e + COCl2 = CO + Cl2

k = 10-7 cm3/mol

(5)

and CO could react continuously into CO2: CO + ClO = CO2 + Cl

(6)

4. Methane conversion Instead becoming an energy source, methane could be the potential source of green house glasses. The last one is mostly due to man-made activities, e.g. gas flaring in mining sites, petrochemical industry, and the decomposition of organic compounds. Conversion of methane into usable energy and/or higher-price compounds, such as hydrogen, synthesis gas, acetylene, and other higher hydrocarbon or black carbon is still becoming a challenge [26]. Together with carbon dioxide (CO2), many studies have been done intensively for several decades especially for direct methane conversion. The main problem that we are facing so far so far is the inertness of the molecule to any kinds of reaction which makes it very stable. Many research groups used thermal-catalytic method to overcome the above problem and reported some interesting results. The activation of methane on the surface of catalyst is the key point of the process. But it was still not free from further problems. Carbon solid deposition on surface, produced by fragmentation of C-H bond, reduced the catalyst performance. Moreover, the catalyst required a specific temperature operation range which is usually c.a. 100200oC higher than room temperature. Small flow of injected raw gas is one another barrier to apply this process in industrial scale.

Nowadays, plasmas, both thermal and non-thermal plasmas, have been studied for methane conversion. One of the interesting points of using plasma technology is that the different plasmas and operation conditions, used in the process, could produce different products distribution. This characteristic made it suitable for chemical synthesis selection. Methane utilization using glow discharge [27-29], Dielectric Barrier Discharge (DBD) [30-36], Corona [28,37], Spark [28], arc plasma-jet [38], RF plasma [39,40], thermal plasma [41,42] have been investigated. Another plasma variables effect on CH4 plasma reaction such as plasma power generator [43,44], catalyst process-assisted [45,46], water vapor injection [47] were also experimentally investigated. The conversion of CH4 could produce higher hydrocarbons (HCs), especially when non-thermal plasmas were used. Thermal plasma will convert CH 4 mainly into carbon (C) and hydrogen (H2) [] CH4 + E  C + H2 Higher HCs could be formed by coupling reactions of methyl radical [] or carbon with radical species [27]. CH2 + CH3  C2H6 C + CH2  C2H2 Coupling reactions also occurs in temperature-based reactions []. In some particular cases, the reaction between carbon and radical shows important role, especially in the case when the concentration of acetylene is the highest product [27]. Legrand et al [28-29] has initiated to investigate the methane conversion using non thermal plasma. Using IR spectroscopic to analyze the excitation of N2, they proposed that the reactions were not initiated by electron but due to the existence of excited nitrogen compounds [27,28]. This result is quite interesting and arguable since in the absence of N2, the conversion of methane can be obtained in a higher rate by increasing the supplied power. In another word, increasing the power means increasing the population of electron inside the reactor. In the (movement) speed comparison, electron is much faster than other species such as ion, radical, or excited molecule. In the case of non-thermal plasmas, numerous research papers have been published which showing very diverse results. Diamy at al. found that the acetylene was the majority of the products when corona-like discharge was used. Other groups [] resulted ethane (C 2H6) as the major products although the distribution could be different with some process manipulation or optimization. An interesting result from Kim et al., supported by Indarto et al. and also majority of papers, is that the coupling reactions will be followed by dehydrogenation reactions. C2H6



C2H4



C2H2

From the transition region between non-thermal and thermal plasma, e.g. gliding arc, the reaction was dominated by total fragmentation of CH4 into carbon and hydrogen (H2). As CH4

can be converted into various products, the important factors as the consideration of the choosing the best method is the energy efficiency and instrumentation. Some power supplies, due to the different of wave power, have different energy efficiencies. 5. Decomposition of CO2 Emission of this gas chemical into the atmosphere from various industries caused environmental problems. Usually, CO2 initiated and participated in ozone-depleting reactions [50,51]. The increasing amounts of CO2 released into the environment, estimated to be around 2 x 1015 g per annum, have increased the demand for finding effective methods to reduce its concentration. That is the reason why the conversion of carbon dioxide (CO2) into more valuable gases is still becoming a challenge now [49,49]. The main problem to reduce the concentration of CO2 from industrial gas waste is the bond energy of CO2 which is chemically strongly-bounded. The thermodynamic calculation of CO2 decomposition mentioned homolitic-cracking of C-O bond starts at 1500oC. It delivers a strong message that high energy must be supplied to the system to reach the required temperature. The proposed way to avoid above problem is by introducing ionic system which allows ionic dissociation mechanism. Plasma which is full of ionic species could be a good solution. In recent years, some studies are carried out on the application of new technologies to reduce the emission of CO2. Plasma-assisted method, such as RF plasma [52], corona [53,54], dielectric barrier discharge [55], glow discharge [56,57], and thermal plasma [58], have been develop. Currently, Indarto et al. tried to convert CO2 into CO and O2 []. Wen & Jiang [53] showed that CO2 was able to be destructed using corona discharge which known as the lowest electrons density of non-thermal plasmas. At 24 ml/min, the conversion of CO 2 was below 10% and significantly increasing up to 16% when the reactor was packed with Al2O3. The yield of CO was between 15 and 23%. Very low conversion of CO2, supported by Maezono et al. [54], was due to the lack of electron and energy to enhance the conversion of CO2. The presence of Al2O3 would help to increase the conversion of CO2 by adsorbing CO2 on the surface of solid material which this phenomenon is exactly similar to the thermal-adsorption process. Moreover, instead of CO and O2 as the main products, ozone (O3) was also found in few concentrations. Another experiment by Maezono & Chang [54] using another type of corona discharge, socalled corona torch, showed that the conversion rate could be increased significantly by addition of halogen gases, such as argon (Ar) [53], helium (He) [56]. Light and lower excitation energy of halogen gases will enable plasma to produce numerous amounts of active species which are useful to break the C-O bond. N2 was actually act the same phenomena, however, the presence

of oxygen will result side products, NO2, NO, or N2O [68,69]. Li et al. [55] investigated the effect of dielectric materials of DBD on the conversion of CO2 to CO and O2. They mentioned that choosing the correct dielectric material is very important to increase the conversion of CO2. A comprehensive experimental to study the plasma parameters for CO2 conversion by using DBD was done by Wang et al. [56]. The conversion of CO 2 will increase by increment of frequency, residence time, supplied voltages, and the amounts of halogen gases. In the absence of the catalyst, the process was far from efficient. Not all additive gases work well to enhance the better conversion of CO2. Buser and Sullovan [57] found that H 2 reduced the dissociation of CO2 in the DBD. Bi-functional behavior of O2 was detected as oxygen can oxidize the CO2 [68] CO2 +  CO + O2 CO2 +  CO + O3 In the beginning of the reaction, but further it can oxidize C and CO back to CO2 C +  CO2 CO + O  CO2 By thoroughly investigate on the different rates of two isotopic carbon (12C16O2 and 13

C16O2) diluted in nitrogen (N2), Savinov et al, concluded that the decomposition of CO2 was

due to the vv-transfer of energy (transfer of vibrational energy between molecules) within the asymmetric mode of CO2 vibration [71]. N2 + E = N2* (due to plasma energy) CO2 + N2* = CO + 0.5 O2 + N2 (energy transfer into asymmetric vibrational mode) As the energy different by 1st excitation of N2 due to plasma was almost similar to asymmetric vibrational mode of CO2, the C-O breaking could be occurred naturally. In the absence of nitrogen, Indarto et al. [68,69] successfully decomposed CO2 into O2 and CO in relatively high conversion rate. Later on by simulating the kinetic model, they [70] mentioned the importance of fast electron in collision with CO2. CO2 + e  CO + O + e CO2 + e  C+ + O2 + 2e Once the population of radical oxygen (1O), ionic oxygen (O2+), and excited oxygen increased, these species could significantly fastening the decomposition rate of CO2 by recombination reaction O2+ + CO2 + e  CO2 + O2 Furthermore, by variation of the O2 to N2 ratios of the input, they showed that the effect of N2 to the CO2 conversion is less than that by electron. Moreover, the behavior of N 2 itself is almost similar to halogen gases. The application of thermal plasma is initiated by Kobayashi et al [58]. High thermal efficiency was achieved above 80% by using high-energy torch. In very strong supplied power, the decomposition of CO2 could break the two C-O bonds and produce solid

carbon and oxygen. CO2 + E  C + O2 Some researchers took advantage of this process to produce nano-sized carbon or black carbon. However, the energy efficiency of thermal plasma was usually very low compared to nonthermal plasma []. 6. Conclusion The application of plasma to decompose toxic compounds has just been studied for a few years. This area is attracting more and more researchers’ attention and is flourishing. However, there are still a lot of problems needed to be solved. The low conversion of reactant and the utilization cost which usually very expensive have to be overcome. Most experiments on plasma methane conversion to methanol are based on trial and error method and the detailed mechanism on decomposition kinetics has not been significantly addressed. Only a few mechanisms were assumed in the experiments, but they are still needed to be improved. However, it showed that the plasma application is visible and such combination, e.g. with appropriate catalyst, may be a good choice for increasing the performance.

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