Microwave Air Plasma Applied to Naphthalene Thermal Conversion

Subscriber access provided by MAHIDOL UNIVERSITY (UniNet)

Article

MICROWAVE AIR PLASMA APPLIED TO NAPHTHALENE THERMAL CONVERSION Henrique S Medeiros, Aliaksandr Pilatau, Olena Sergeevna Nozhenko, Argemiro S da Silva Sorbino, and Gilberto Petraconi Filho Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02451 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

MICROWAVE AIR PLASMA APPLIED TO NAPHTHALENE THERMAL CONVERSION

H. S. Medeiros, A. Pilatau*, O. S. Nozhenko, A. S. da Silva Sobrinho, G. Petraconi Filho Technological Institute of Aeronautics (ITA), Praça Mal. Eduardo Gomes, 50, Sao Jose dos Campos, Sao Paulo, 12228900, Brazil [email protected] KEYWORDS Naphthalene reforming; Microwave plasma jet; Conversion efficiency; Energy efficiency.

ABSTRACT: In this paper a naphthalene (C10H8) thermal cracking model is presented. The model is based on a simple model which takes into account microwave (MW) plasma thermal influence on naphthalene cracking and accompanying its steam reforming reactions. It was established the temperature level of 1573 K for complete C10H8 cracking at 1.75 kW plasma power. High conversion efficiency of C10H8 is achieved varying the air flow rate in the range (0.6..1.2) m3/h. The model approximates the characteristics of the considered MW plasma to thermal plasma in Local Thermal Equilibrium (LTE). Experimental data have good agreement with calculated data at the cited region of air flow rate and power. Conversion efficiency up to 99.36% was obtained.

ACS Paragon Plus Environment

1

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

1. INTRODUCTION Gasification has been gaining attention as a route for alternative energy production, providing the reduction of carbon dioxide emission. As well as, it is environmentally friendly and economically competitive. However, thermo-chemical conversion of fuel involves different processes like combustion, gasification, and pyrolysis. Fuels which are produced in gasification process can be used for internal combustion engine. However, they usually contain unacceptable levels of detrimental impurities such as tar and particulates [1, 2]. Tars can cause operational problems in downstream processes by blocking gas coolers, filter elements, and engine suction channels which can lead to a seriously damages in the units. Therefore, before the gas can be used, a tar removal is required. Thus, tar removal is appointed as a key challenge for a successful commercial application of gasification technologies. Several plasma processes has been studied, including both hydrocarbons reforming and tars destruction,which can be summarized as [3]: plasma torch for isooctane reforming[4]; corona discharge for hydrogen production from hydrocarbon fuels [5], and from natural gas [6]; gliding arc for heavy hydrocarbon reforming [7, 8], destruction of biomass tars [9] and naphthalene destruction [10], as well as methane partial oxidation [11]; and microwave (MW) plasma for n-hexane reforming [12], for using in promising bioconversion technique [13], microwave assisted pyrolysis of coal and biomass for fuel production [14]. Although not all of the cited reforming hydrocarbons can be considered as tars, the physical and chemical processes of hydrocarbons cracking and oxidation are relevant for applying to naphthalene reforming. Investigations have proved that plasma torch provides good conversion capabilities with average value of conversion rate about 96% under average specific energy requirement of about 300 kJ/mol and average value of efficiency about 45% [3, 15]. Plasma torch is also


2

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

used in order to achieve a larger reactive volume in auto-thermal reforming of isooctane at atmospheric pressure [4]. Petitpas et al. noticed that microwave plasma technologies used as gas reformer has the specific energy requirement, the conversion rate, and reforming efficiency slightly below to the average level [3]. However, as showed in references [12, 13], microwave plasma has some additional advantages. According to the literature, average efficiency of non-thermal plasma processes is about 50%, specific energy requirement of the processes is slightly above 100 kJ/mol, and conversion rate of the processes is about 80% (in some cases it is up to 90%). There are some possible ways to chemical reaction enhancement. It is based on microwave irradiation technique [16], including thermal effects (the influence of a high reaction temperature) [17], specific microwave effects (the unique nature of the microwave irradiation heating mechanism) [18], and non-thermal effects (acceleration of chemical transformation) [19]. According to the considered effects of MW plasma impact in naphthalene (C10H8) decomposition, it can be possible to determine three main influences. They are based on mechanisms of: chemical active molecules in carrier gas (such as O2.), plasma ionization and generation of radicals and ions (such as OH, O, O+, O- i.e.), and MW plasma thermal impact. At the same time, MW plasma contribution to naphthalene decomposition can be considered as complex due to the interconnected impact of different factors. The purpose of this paper is to describe the naphthalene thermal cracking based on a simple model of naphthalene which is based on kinetic scheme [22-24] with additional hydrogen generation reactions [23,24]. The aim also included the following tasks: determination of necessary gas temperature for achieving high conversion of C10H8 (90%); establishing a range of air flow at the


3

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

predetermined power of MW plasma (1.75 kW) which could provide the same level of conversion efficiency in comparison with previous researches [20]. 2. MODEL COMPONENTS AND REACTION PATHWAY C10H8 is a PAH (Polycyclic Aromatic Hydrocarbon). Tertiary aromatics (like naphthalene) are the most common tar species founded in gasification process [20], they have the highest formation temperature (1073-1273)K.They are the most stable and difficult to crack catalytically in comparison to other tars. Also it often appears as the main species which is found in tertiary tars (in gasification, tertiary aromatics are predominant). For these reasons, the naphthalene has been selected as a model compound representative of the tertiary class tars. The naphthalene thermal cracking and its accompanying pathway in thermal plasma including cracking and steam reforming scheme, are described in [20]. Based on negative Standard Gibb’s Energy of these reactions, they are represented as: Cracking: -

pC n H x → qC n H y + rH 2 ;

Steam reforming: - C n H x + nH 2 O → (n + x 2)H 2 + nCO . where p, q, r, n, x, and y are the balance coefficients. According to general scheme and established reactions of naphthalene decomposition [22], it has been chosen the simple reactions scheme showed in Table 1. The scheme has been chosen considering the MW plasma as thermal plasma taking into account the influence of additional hydrogen generation reaction.


4

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1. Reactions and reaction rates used in the thermal cracking model (concentration (kmol.m-3), rate (kmol m-3 s-1), M is concentration of a molecule such as N2, H2O). № Reaction 1 C10H8→10C +4H2

Reactionrates k1=5.56×1015×exp(-3.6×105/RT)

References [20]

2

C10H8 +4H2O→

r1=k1[C10H8]2[H2]-0.7 k2=1.58×1012×exp(-3.24×105/RT)

[20]

3

→C6H6+4CO+5H2 H2O+e-→ H+OH+e-

r2=k2[C10H8]2[H2]0.4 k6=2.66 ×10-10×exp(-57491/T)

[23]

4

2H+M→H2+M

r6=k6[H2O]nek7=5.4×109 T-1.3

[21]

H2+M→2H+M

r7=k7[H]2[M] K8=2.23×1011×exp(48306/T)

[22]

5

r8=k8[H2][M]

Based on the reactions rates (Table 1), the evolution of remaining naphthalene concentration has been calculated for further determination of the conversion parameters, such as conversion efficiency [7]. The temperature, involved in the equations (R.1-R8) of the kinetic scheme (Table 1), has been considered as constant and calculated as average temperature of MW plasma torch volume which was closing to the temperature of local thermodynamic equilibrium (LTE). According to the assumption of LTE existing, the electron density can be described based on Timofeev’s equations [25]

(

)

5.91×1015 × exp(− 14.42 T − 1.74 ,T > 1.74 av av ne =  0, Tav ≤ 1.74

(1)

where - electron density in cm-3; Tav – the temperature at LTE, measured in 103 K. The temperature Tav corresponds to an average thermodynamic level of MW plasma torch heating which provides necessary energy level for thermal cracking (R.1) and steam reforming (R.2) and can be assessed based on equation of distribution of released distribution


5

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

of electromagnetic field heat between a gas volume heating and a heating for ionization of the gas QMW ⋅ η =Q heat +Qion ,

(2)

where QMW - power of electromagnetic field which is absorbed in gas volume, W. QMW = 1750 W was used in this study; η - thermal efficiency of MW plasma jet. The value can be chosen according to the values presented in [27] and is about 50% of all dissipated power which is transformed into gas heating; Q heat − the part of QMW power which provides thermal heating of gas volume, W; Qion - the part of QMW power which provides directly ionization of gas molecules in the volume, W. The power Qheat of gas volume heating can be written as

( )(

)

Qheat = G ⋅ c p Tav ⋅ Tav − To ,

(3)

where G- flow of carrier gas, kg/s; c p − heat capacity of the gas, kJ/kg K; To − initial gas temperature, K. The power Qion of ionization gas can be determined as  3  Qion =  ∑ es ⋅ ns ⋅10 − 19  × V ,  s =1 

(4)

where es -threshold energy of species (1 is nitrogen, 2 is oxygen, 3 is naphthalene) ionization ,eV; ns - density of electrons which is released after the species ionization, m-3; V – volume flow of air through the MW jet, m3/s. Substituting (3) and (4) in (2) we have equations for gas temperature Tav calculation. Based on the scheme (Table 1), the microwave plasma provides hydrogen atoms generation under steam decomposition. Hydrogen atoms generated in the plasma react with assisting molecules of air (such as nitrogen and oxygen). Hydrogen molecules inhibit cracking process


6

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

[24], but an increased H2 concentration leads to a greater rate constant for naphtalene steam reforming, as can be seen in table 1. Oxygen takes part in oxidization of secondary product (i.e., the benzene which is generated from naphthalene under the steam reforming). 3. MATERIALS AND METHODS The experimental setup for the tar reforming includes a carrier gas (air) preheating system, a chamber for the naphthalene heating, gas mixing and a MW plasma jet. The preheated gas temperature in both carrier gas system and naphthalene chamber are measured by thermocouples and controlled by an electrical panel in order to keep them constant. The gas line is also heated and the temperature is carefully controlled to avoid tar condensation. The carrier gas and gas line temperature was kept at 473 K. The naphthalene temperature inside the chamber remains constant at 343 K. Figure 1 and Figure 2 shows a schematic diagram and a photography of the microwave plasma system, respectively. The experiments were carried out with a fixed microwave power at 1.75 kW and the carrier gas flow rate in the range of 0.6..1.8 m3/h. Incoming gas composition is presented (% in vol) as N2= 79.11%; O2=19.4%; Ar = 0.43%; H2O=0.3%; CO2=0.2%; C10H8= 0.56% (~27 g/m3). The 2.45 GHz microwave plasma jet reformer utilized in this work has a maximum power of 3 kW and source voltage of 5500 V, a rectangular waveguide, short circuit movable piston, the stubs, and the circulator from SAIREM Company. A 10 mm inner diameter quartz tube was used as a reactor where the microwave is absorbed by the gas coming from the feeding system, on swirl way, ionizing a fraction of molecules in this gas and then generating the plasma jet.


7

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

Figure 1. Schematic diagram of the microwave plasma jet apparatus

Photography of the plasma jet is presented in Figure3. A MAX300-LG Laboratory Gas Analyzer from Extrel was used to measure the gas concentration before and after the plasma reactor. The gas temperature in plasma core was measured by optical spectroscopy technique based on Boltzmann Plot method considering a LTE MW plasma jet (Figure 3). The spectrometer used was an Ocean Optics spectrometer (model LIBS 2500). For assessment of tar removal under plasma technology, basically, it is used conversion efficiency which can be evaluated according to [7]:

ηd =

[C10 H8 ]in − [C10 H8 ]out × 100% [C10 H8 ]in

(5)

where, [C 10 H 8 ]in − input naphthalene concentration, g/m3; [C 10 H 8 ]out − output naphthalene concentration, g/m3.


8

Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 2. Experimental setup of microwave plasma jet

Figure 3. Microwave plasma jet ignited. (P=1.75 kW) 4. RESULTS AND DISCUSSION 4.1. Model validation According to the aim of the study, it was considered the MW plasma as thermal plasma (existing at least a LTE) characterizing by the level of temperature. The temperature was involved in the suggested kinetic scheme (Table 1). Validation of the assumption was carried out by comparison (Figure 4) of accuracy of prediction conversion efficiency, calculated by suggested cracking scheme, with previous results which was obtained by CHEMKIN


9

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

computation model [7]. The calculating was carried out in a special software (NaphTermCrack 1.0 ) developed by authors for naphthalene evolution of chemical concentration at predetermined different stationary temperature that was involved in the kinetic scheme (Table 1).

Figure 4. Comparison between the approved [7] and calculated conversion efficiency (conv. effic.)

During all computations, the MW power was kept as constant with value of 1.75 kW. The value of absorbed power [25] of microwave plasma torch (MPT) can be expressed as

∞ e 2n e E 2 f (ε)dε , Pabs = ∫ eff m ν 0 e m

(6)


10

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

where e is electron charge ; me – electron mass; ε − the electron energy; f (ε ) − the electron energy distribution function (EEDF); Eeff − effective amplitude of microwave field; ne − electron density; ν m − momentum transfer collision frequency [26]. The equation (6) was used for electron density validation. After determination of the electron density according to equation (1), the value has been substituted to the equation (6) tacking into account current momentum transfer collision frequency which can be calculated by [26]. The validation of the obtained electron density was carried out through comparison calculated absorbed power with predetermined initially as a power of MW plasma torch (1.75kW). The electron energy distributed function (EEDF) was determined according to [25] for air MW plasma torch. It should be also noted that each value of electron density corresponds to the value of average gas temperature Tav of MPT. The validation of assumption about existing of the LTE with a characterized temperature has been done after comparison between the calculated and experimental results. It was established a region of air flow (0.6 ..1.2 )m3/h, where the experimental results have good agreement with calculation and , therefore, the considered model of naphthalene MW plasma decomposition can be described through the LTE, temperature Tav, involved in the kinetic scheme (Table 1). During the calculations, the temperature of the LTE was stationary, but it was varied in air flow range, according to equations (2-4). Reaction time has been changed in the range of (0.18..2.50) ×10-3sec. according to the air flow variation of (0.6..1.8) m3/h.

4.2. Sensitivity analysis


11

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

For sensitivity analysis, two influences on conversion efficiency have been studied: the gas temperature, as a temperature of the LTE, in MW reactor and flow rate of incoming heated air. 4.2.1 Influence of gas temperature in the MW plasma reactor

The temperature is one of the most important parameter that influences on the naphthalene cracking in MW plasma. At once, it should be noted that the temperature field of MW plasma particles can be sufficient varied. High internal energy which can be characterized by temperature, provides high level of ionization with high concentration of radicals, ions, and so forth. Herewith, each type of the particles, generated through electron-neutral molecules/atoms collisions is characterized by different energy level and corresponding its temperature (electron temperature Te , ion temperature Ti, neutral gas temperature Tg). Besides the temperature distribution between the plasma particles, MW plasma has plasma core [27] which contains plasma particles generated with charge and excitation. The plasma core is flowed around neutral carrier gas (in this study it is air with naphthalene) and a ratio of the plasma core diameter and size of the carrier gas, flowing around the plasma core, depends on operating parameters (MW power and carrier gas flow rate) [27]. For determination of plasma core temperature it is used a Bolzman plot method [27] through a optical spectrometer. The results of data treatment is presented in Figures 5 and 6. The Figures present the N2+ rotational spectrum corresponding to the Ʃ , = 0 →

∏, → ′ = 0 transition and the Boltzmann Plot, respectively.


12

Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 5. N2+ rotational spectrum corresponding to the Ʃ , = 0 → ∏, → ′ = 0 transition According to the Figure 6 is possible to estimate the plasma gas temperature. Bolzman plot gives the typical rotational temperature for nitrogen/air plasma [27] TR and it is in the range of 4500-5000K. In the study [27], the temperature is considered as a gas temperature in plasma core because the rotational temperature can be nearly equal to the neutral gas temperature due to very fast rotational states redistribution (in order of ps [28]).


13

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

Figure 6. Boltzmann Plot illustration

In the Figure 6, B is rotation constant [27] for the rigid diatomic molecule. A is the parameter which can be evaluated according to [27] as A=

λ4 , Sj

(7)

where λ − a length of rotation line which is determined frequency of the rotation line [27]; Sj – a line strength which is determined according to [27].

Hence, it was mentioned, the plasma core is only a part of whole gas volume in MW plasma reactor. The other part is a passing flow of neutral carrier gas which blows around the core. For assessment of thermal plasma impact in microwave plasma torch (MPT) heating of the whole volume, we suggest a thermodynamic level of the temperature Tav which can be calculated through the equations (2)-(4). The temperature Tav characterizes an average level of heating of whole volume of MW plasma torch through dissipated power which is transformed into the heating of whole gas volume per time, passing through MW plasma reactor, taking into account its thermal efficiency of MPT (about 50%). The temperature Tav is compared


14

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

with temperature which was estimated below (Figure 7) as necessary for completely naphthalene decomposition according to kinetic of cracking and reforming reaction (Table 1). On the other side, we can assume that if the average gas temperature Tav is close to the measured rotational temperature TR in plasma core we can consider that higher number of points in the MPT can have conditions, closing the plasma to a thermodynamic equilibrium, and probability of LTE existence has been increasing in the MW plasma. The conditions of LTE were described in many researches, such as [27]. In the case of the LTE, temperatures of species in gas can approach each other. So, the temperature, involved in the considered kinetic scheme, can mean, conventionally, a common temperature of all species in MW plasma jet as its level of heating. We have detected two influences of temperature in the naphthalene thermal cracking. There were influences on chemical kinetic of naphthalene decomposition and on ratio of volume gas heating to ionization heating.

a)

Influence on chemical kinetic

Results of modeling of naphthalene concentration at the air flow rate 1.2 m3/h are presented in the Figure 7.


15

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

Figure 7. Evolution of naphthalene concentration at different temperatures at 1.2 m3/h Based on analysis of showed data (Figure 7), it is observed that the minimal gas temperature in MW plasma jet should be higher than 1573K. It is also the temperature necessary for achievement of high efficiency of naphthalene removal in the gas. b)

Influence on ratio of gas volume heating to ionization heat

Based on ratio of obtained equation (3)-(4) the ratio of power of gas volume heating to power of ionization

(

)

G ⋅ c p (T ) ⋅ Tav − To Q heat = Qion  3   ∑ e ⋅ n ⋅10 − 19  × V s s   s =1 

(8)

Due to temperature increasing, flow rate V can be roughly estimated by [27].

T V = av × Vcold , To

(9)

where Tav- gas temperature, K; To – initial temperature, K; Vcold – initial flow rate, m3/h.


16

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Substituting (9) in (8) and taking into account the equation for heat capacity we can consider the equation (8) as a temperature function

( )

Q heat = f T av Qion

(10)

The function of the ratio (10) has been drawn and presented on the Figure 8.

Figure 8. Ratio of volume gas heating power to ionization heating by variation of air flow

Based on the obtained data (Figure 8), it can be concluded that ionization heat overrides Joule heating only in the short region of temperature (500-550) K. The equilibrium between the parts of ratio (8) and (10) can be shifted through two approaches: 1) increasing of electron density in MW plasma jet; 2) decreasing of initial cold gas density. c) Influence of the flow rate of incoming heated air In order to investigate the influence of the flow rate of incoming heated air on the model, computations have also been performed with values of the flow rate of incoming air at the


17

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

range of 0.6 – 1.8 m3/h, which is the flow rate range that the MW plasma discharge has a stable operation. Figure 9 shows the influence of this flow rate on conversion efficiency of naphthalene in the MW plasma discharge. Based on the equations of electromagnetic heat distribution (2), it can be concluded that the average gas temperature Tav is inversely proportional to incoming air flow. Increasing of air flow causes decreasing of the temperature while the rotational temperature TR in plasma core is almost flat (with difference up to 5.5%) Figure 10. Conditions of LTE can be observed only in the region nearby 0.6 m3/h at low air flow where the average temperature Tav is closing to rotational gas temperature TR. Then, after further air flow increasing the convective cooling by higher flow rate has intensified. Simultaneously, a total plasma diameter should decrease according to our and earlier carried out observation [27]. The fact means that number of naphthalene particles which were caught in the plasma core (its probability), became fewer than that promoted by lower air flow. Moreover, the average temperature Tav level of plasma presented the same behavior. Therefore, the mentioned factors have negative impact in naphthalene conversion, although rotational temperature TR in plasma core was to near as the same as by low air flow of carrier gas. According to our calculations of conversion efficiency which was based on the presented equations (2)-(4), it can be showed (Figure 9) that naphthalene conversion was down if the average temperature Tav become values lower than necessary level (1500K) of earlier estimated temperature for completely naphthalene cracking (Fig.6). Hence, the actual conversion efficiency is much higher than its calculation. It can be explained through existing of plasma core with almost constant gas temperature TR≈4500K and capture of naphthalene inside the core through convectional processes in MPT. Secondary, the process of convectional cooling of plasma core by high air flow can transfer a heat from plasma core to


18

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

other MPT regions heating the passing air and increases its average temperature. The previously discussion haven’t been taken into account in calculations (2)-(4) of the average temperature Tav. Analysis of the obtained data (Figure 9) showed that high conversion efficiency can be achieved even if the temperature Tav is higher than 1573K. Further its reduction leads to recession of conversion efficiency due to increase of incoming air flow. The temperature decreasing when air flow rate is greater than 1.2 m3/h, which is pointed on loss of the LTE in the MW plasma, as well as inapplicability of the considered concept of similarity of thermal and the MW plasmas.

Figure 9. Dependence of the air flow on conversion efficiency in the MW plasma discharge


19

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

Figure 10. Gas temperature variation in plasma core (rotational temperature) with the air flow rate

4.3. Comparison with previous literature Naphthalene conversion of (90.7.. 99.36) % was achieved at air flow range of 0.6-1.2 m3/h (low values of conversion efficiency relate to bigger air flow), at the 1.75 kW power of MW plasma jet. These values are acceptable when compared with the conversion efficiency found in previously research (96.7%) [20]. 5. CONCLUSIONS The thermal cracking of naphthalene was studied in the MW plasma. A reaction pathway and additional kinetic of derived steam hydrogen generation has been presented according to accepted assumption of similarity of the considered MW plasma with a thermal plasma at the range of (0.6..1.2) m3/h air flow.


20

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Based on accepted kinetic scheme (Table 1) was established the minimal level of temperature (1573K), (Figure 4, Figure 7) which has been sufficient for almost completely naphthalene conversion (greater than 90%). That is possible due to the electromagnetic field energy takes a part in gas volume heating providing, therefore, sufficient energy for naphthalene atomic bonds breaking. For determination of applicability of thermal plasma concept for studying of the considered MW plasma, the influence of air flow operating parameters has been investigated. The air flow was in the range of (0.6 .. 1.8) m3/h at the 1.75 kW MW plasma power. Analysis of the obtained data (Figure 9) showed that the accepted kinetic scheme (Table 1) has good agreement with observed experimental data. Therefore, we can conclude that the considered MW plasma is in LTE. Hence, after achievement of the air flow of 1.2 m3/h, it is observed a significant dissimilarity (70% of actual conversion efficiency vs. 7% of calculated results) in conversion efficiency, which was calculated based on accepted kinetic scheme (Table 1) and actual conversion efficiency, measured experimentally. It can be explained through higher probability of shift from the LTE due to decreasing of heavy particles and neutral atoms temperatures in gas. For achievement of the same level in conversion efficiency (90.78 .. 99.36)% over previous value of (96.8%) research in naphthalene plasma reformer, is necessary to provide the conditions for thermal plasma existing at air flow of (0.6 .. 1.2) m3/h at 1.75 kW power of MW plasma. AUTHOR INFORMATION

Corresponding Author *AliaksandrPilatau, Technological Institute of Aeronautics (ITA), Praça Mal. Eduardo Gomes, 50, Sao Jose dos Campos, Sao Paulo, 12228900, Brazil [email protected]


21

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

ACKNOWLEDGMENT The authors would like to acknowledge the financial support of the Brazilian Research Agencies FAPESP Project no 2012/14568-4, CAPES no 88887.060497/2014-00 and CNPq. REFERENCES

(1)Boloy,R.A.M.;Silvera,J.L.; Tuna,Coronado, C.E.; C.R.; Antunes, J.S.; Renew. Sust.Energ.Rev., 2011, 15, 5194-5202 (2)Martinez, J.D.;Mahkamov, Andrade, K.;R.V.; SilvaL.,E.E.;Renew.Energ.,2012 38,1-9 (3)Petitpas, G.;Rollier, J.D.; Darmon, A.; Gonzalez-Aguilar, J.;Metkemeijer,R.; Fulcheri,L.;Int. J. HydrogenEnerg., 2007, 32, 2848-2867, (4)Rollier,J.D.; Theoretical and experimental studies of non-thermal plasmaassisted reforming of gasoline. PhD thesis, Center for Energy andProcesses, EcoleNationaleSupérieure Mines de Paris, 2006 (5)Sobacchi,M.G.;Saveliev,A.V.;Fridman,A.A.; Kennedy, L.A.; Ahmed, S.; Krause, T.; Int. J. Hydrogen Energ., 2002, 27, 635-642 (6)Mutaf-Yardimci, O.;Saveliev, A.V.;Fridman, A.A.;Kennedy,L.A.; Employing plasma as catalyst in hydrogen production. Int. J. Hydrogen Energ.,1998, 23,1109-1111 (7)Nunnally, T.;Tsangaris, A.;Rabinovich, A.;Nirenberg, G.; I. Chernets, A. Fridman, Int. J. Hydrogen Energ., 2014, 39,11976-11989 (8) Gallagher, M. J.; Geiger, R.;Polevich, A.;Rabinovich, A.;Gutsol, A.;Fridman, A.; Fuel,

2010, 89,1187-1192


22

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(9)Nunnally, T.;Gutsol, K.;Rabinovich, A.;Fridman, A.;Gutsol,A.;Kemoun, A.; J. Appl.Phys (2011), doi:10.1088/0022-3727/44/27/274009 (10)Nunnally, T.;Gutsol, K.;Rabinovich, A.;Fridman, A.;Starikovsky, A.;Gutsol, A.;Potter, R.W.;Int. J. Hydrogen Energ.,2009, 7618-7625, 34 (11)Kalra, C. S.;Gutsol, A. F.;Fridman, A.; IEEE T. Plasma Sci.,2005, 33, 32-41 (12)Sekiguchi, H.;Mori, Y.; Thin Solid Films, 2003, 435, 44-48 (13)Motasemi, T.;Afzal, M.T.; Renew. Sust.Energ.Rev.,2013, 28, 317-330 (14)Mushtaq, F.;Mat, R.;Ani, F.N.; Renew. Sust.Energ.Rev.,2014, 39, 555-574 (15) Paulmier, T.;Fulcheri, L.; Use of non-thermal plasma for hydrocarbon reforming. Chem. Eng. J.,2005, 106, 59-71 (16) Kape, C.O.;Controlled microwave heating in modern organic synthesis,Angew. Chem. Int. Edit.,2004, 43, 6250-6284 (17) Kuhnert, N.; Microwave-assisted reaction in organic synthesis – are there any nonthermal microwave effects? Angew. Chem. Int. Edit., 2002, 41, 1863-1866 (18)Chemat, F.;Esveld, E.; Chem. Eng. Technol., 2001, 24, 735-744 (19)Shibata, С.;Kashima, T.;Ohuchi, K.;Jpn. Japanese Journal of Applied Physics., Part 1: Regular Papers and short Notes and Review Papers, 1996, 35, 316-319 (20)Fourcault, A.;Marias, F.; U. Michon, Biomass & Bioenergy.,2010, 34,1363-1374 (21) Brioukov, M. G.;Park, J.;Lin,M.C.; Int. J. Chem.Kinet., 1999, 31,577-582 (22)Frassoldati, A.;Faravelli, T.;Ranzi, E.; Int. J. Hydrogen Energy. 2007, 32, 3471-3485


23

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

(23) Shin, D.H.; Hong, Y. C.; Lee, S. J.; Kim, Y. J.; Cho, C. H.; Ma, S. H.; Chun, S. M.; Lee, B. J.;Uhm, H. S.; Surface and Coating Technology, 2013, 228, 520-523 (24)Jess, A.; Fuel, 1996, 75, 1441-1448 (25) Nam, S.K.; Venboncoeur; ComputPhysCommun.,2009, 180, 628-635 (26) Kortshagent, U.; Shivarova, A.; Tatarova, E.; Zamfirov, D.; J. Phys. D: Appl. Phys.,

1994, 27, 301-311 (27) Su, L.; Kumar R.; Ogungbesan B.; Sassi M.; Energ. Convers. Manage, 2014, 78, 695703 (28) Optical emission spectroscopy of plasma-Internet source.- Access: http://www.fch.vutbr.cz/~krcma/vyuka/plazma/3-oes-en.pdf, data of access:18.12.2015


24

Microwave Air Plasma Applied to Naphthalene Thermal Conversion

Recommend Documents