Gliding arc plasma oxidative steam reforming of a simulated syngas containing naphthalene and toluene

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Gliding arc plasma oxidative steam reforming of a simulated syngas containing naphthalene and toluene T. Nunnally b,*, A. Tsangaris b, A. Rabinovich a, G. Nirenberg a, I. Chernets a, A. Fridman a a b

Drexel University, 3141 Chestnut Street, Philadelphia, PA 19128, USA Plasco Energy Group, Inc, 1000 Innovation Drive, Kanata, ON K2K 3E7, Canada

article info

abstract

Article history:

Conversion of a simulated syngas containing vaporized toluene and naphthalene was

Received 21 November 2013

studied in a non-equilibrium gliding arc plasma reformer. The reformer was designed for

Received in revised form

efficient reforming of high temperature syngas (greater than 650
C) containing heavy

24 April 2014

hydrocarbons, air, and water vapor. The reactor utilized forward vortex flow, where a

Accepted 1 June 2014

preheated simulated syngas containing vaporized naphthalene and toluene tar surrogate

Available online 30 June 2014

was injected tangentially in the flow to ensure effective mixing and reforming of all components. At low tar concentration (30 g/m3), over 90% naphthalene and toluene con-

Keywords:

version was achieved at the benchmark specific energy input of 0.1 kWh/m3 and energy

Plasma

efficiencies of 62.5 g/kWh for naphthalene and 215 g/kWh for toluene. At higher tar con-

Gliding arc

centration (75 g/m3), over 70% naphthalene and toluene conversion was achieved at the

Partial oxidation

benchmark specific energy input of 0.1 kWh/m3 and energy efficiencies of 93.6 g/kWh for

Plasma assisted combustion

naphthalene and 369 g/kWh for toluene. Explanations for the results include effective gas

Non-equilibrium

mixing and plasma chemistry, such as the very fast reaction kinetics from ions, radicals

Tar

and active species, specifically hydroxyl. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The product gas formed from MSW gasification contains the major components CO, H2, CO2, CH4, H2O, and N2. In addition, organic and inorganic (H2S, HCl, NH3, alkali metals) impurities and particulates are present. A wide range of organic molecules are present including low molecular weight hydrocarbons (acetylene, ethylene, etc.) to high molecular weight

polynuclear aromatic hydrocarbons (naphthalene, pyrene, etc.). The lower molecular weight hydrocarbons can be used as fuel in gas turbine or engine application. However, the high molecular weight hydrocarbons are generally referred to as “tars,” and can be defined as the downstream condensable hydrocarbon component in the product gas. Tars are problematic to downstream equipment and require more sophisticated methods of conversion/removal than lower molecular weight hydrocarbons. Consequently, tar removal, conversion,

* Corresponding author. Advanced Cooling Technologies, Inc., 1046 New Holland Avenue, Lancaster, PA 17601-5688, USA. Tel.: þ1 717 295 6127; fax: þ1 717 208 2612. E-mail addresses: [email protected], [email protected] (T. Nunnally). http://dx.doi.org/10.1016/j.ijhydene.2014.06.005 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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and/or destruction can be considered as one of the greatest technical challenges that must be overcome for the successful development of commercial advanced gasification technologies. MSW gasification can be divided into three main stages: moisture evaporation; pyrolysis; and pyrolysis product reforming and gasification. During the first stage, moisture evaporates at around 100
C. This moisture vaporization lowers the temperature of combustion (absorption of latent and sensible heat) and as a result, slows the combustion process. During the second stage, MSW pyrolysis, the thermal breakdown of long chains of cellulose, hemicellulose and lignin occur, with the main pyrolysis products being H2, CO, CO2, gaseous hydrocarbons, tar, volatile hydrocarbons and char. The process conditions determine the relative composition of these main products. For example, low temperature decreases the syngas yield (H2 and CO) and increases tar content. The third stage is reforming and gasification. Reforming is the breakdown of heavy and volatile hydrocarbons into lighter hydrocarbons and CO, H2, and CO2. In addition, char, which consists of carbon and a small amount of hydrogen and oxygen and ash, mainly yields CO, H2 and CO2. The predominant location for tar formation is the pyrolysis (thermal) stage of MSW gasification, which occurs in the range of 800e1000
C. This stage occurs at the optimum temperature for PAH production, where the primary tar components are Tertiary-akyl and Tertiary-PNA tars which include benzene, toluene, naphthalene, pyrene and indene [1]. Furthermore, at 900
C, naphthalene has been identified as the major single component of tar content in the produced gas [1]. These tars are problematic because they result in clogging and fouling of downstream equipment. As a result, the reduction of these tars has become a focal point of the gasification industry. Unfortunately, once tars are formed, they can be quite difficult to decompose. The thermal requirement varies depending upon process parameters and the individual molecule. Simulations of thermal destruction of benzene, toluene and naphthalene have been reviewed in literature [2]. The authors have indicated very high destruction requires significant residence time and high temperature [3]. To reach very significant tar decomposition in a realistic residence time a temperature of 1250
C is required [4]. Unfortunately, utilizing this temperature for thermal decomposition has the drawbacks of high cost and production of heavier products and agglomerated soot particles [5]. Thus, conventional thermal decomposition is not an ideal solution for tar removal and reforming. Although the cause of tar formation has been identified as process conditions, the complicated reactions in such a system are still unclear. From a fundamental standpoint, tar decomposition and growth are called cracking and polymerization, respectively, and are the main reactions to consider in the tar reforming process. Cracking eventually results in the formation of syngas, while polymerization results in the formation of heavy hydrocarbons and the agglomeration of soot particles. Ideally, a solution should be implemented which maximizes cracking potential and minimizes polymerization as this maximizes syngas production and minimizes complications associated with tar.

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Some research has been compiled on the polymerization process, and theories for tar polymerization have been suggested including H abstraction/acetylene addition, benzyl radicals and direct combination of intact aromatic rings [6]. Still, the general consensus among researchers is that radical processes are likely the driving force behind heavy hydrocarbon and soot formation and decomposition [6]. As was mentioned earlier, thermal methods are costly and often lead to polymerization, which should be avoided. An interesting alternative is utilizing plasma for heavy hydrocarbon reforming. Plasma can be considered a promising solution because it provides thermal processing of the hydrocarbons with radical generation, which has been identified as a key process, to initiate and drive reactions. As a result, plasma is the main focus of this study. Still, methods other than plasma and thermal tar removal exist and should be mentioned including mechanical methods, catalytic methods, and partial oxidation. Many review papers have been written on the various methods of tar removal and each method will not be discussed in detail in this paper. Instead, a general overview will be presented below on the various methods with particular focus on the plasma approach. Mechanical methods of tar removal include both dry and wet gas cleaning. Some dry mechanical methods include: cyclone, rotating particle separators, electrostatic precipitators, bag filters, baffle filters, ceramic filters, fabric filters, sand bed filters, and absorbers [7]. Still, these methods have some drawbacks such has low tar removal, high capital costs, and tar deposition, plugging, and fouling. In addition, wet mechanical methods include: spray towers, packed column scrubbers, impingement scrubbers, venture scrubbers, wet electrostatic precipitators, OLGA, and wet cyclones [7]. These methods include similar drawbacks of low tar removal, high capitals costs, large size, poor regeneration efficiency, and tar deposition, plugging, and fouling. Undoubtedly, improvements to mechanical methods will be made in the future; however, incorporation of these methods at large industrial scales will continue to require significant capital and operating costs. A plethora of catalysts exist and their application for tar conversion has been reviewed in literature [7e11]. In general, catalysts for tar conversion should be evaluated based upon basic criteria including: effectiveness for tar removal, capability of reforming methane and light HC's, suitable syngas ratio production, resistance to deactivation, regeneration difficulty, robustness and cost. Meeting this criteria can be accomplished in multiple ways as synthetic catalysts are comprised of an active catalytic phase, a promoter, and a support all of which can be varied and affect tar conversion. In addition, the catalyst is often used to facilitate steam reforming and partial oxidation. In a recent paper by Anis and Zainal, six categories of catalysts were suggested including: Ni-based catalysts, non-nickel metal catalysts, alkali metal catalysts, basic catalysts, acid catalysts, and activated carbon catalysts. From a practical standpoint, the most effort has been expended in Ni-based catalysts with various supports including alumina, dolomite, olivine, and zeolite. Ni-based catalysts have the potential to reach nearly complete tar removal at high temperatures; however, they are susceptible

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to deactivation by sulfur and very high tar contents and often require preconditioning of the feed gas. Non-nickel metal catalysts have shown the ability to reduce tar; however they are often more expensive than other more conventional catalysts. Significant literature is available on common basic catalysts of minerals dolomite and olivine, which have the advantages of being widely available and low cost. Still, dolomite and olivine are substantially less effective for tar removal (less active), particularly towards naphthalene, which can be considered as one of the primary components of interest. Zeolite is the most widely studied acid catalyst because it has the advantages of extremely high surface area and acidity while being low cost, commercially available and somewhat resistant to sulfur. Zeolite does, however, suffer from rapid deactivation because of coke formation and lower tar removal than Ni-based catalysts. Lastly, activated carbon and char should be noted for their merits as tar removal catalysts. In the case of these carbon based catalysts, pore size, surface area and material content all play a crucial role in the effectiveness of the catalyst. Char has the advantage of its natural production in the gasifier, and hence, low cost; however, the small pore sizes are susceptible to blocking which reduces activity. In addition, the char is gradually consumed in the gasification process as it is gasified by steam and dry reforming reactions. Finally, partial oxidation should be mentioned as considerable research has been conducted utilizing partial oxidation for tar reforming. Partial oxidation of tar compounds has been tested in a partial combustion fuel burner [3]. These experiments exhibited the ability to achieve 90% tar removal; however, significant portions of lighter hydrocarbons are formed in the process. Similar experiments were conducted in producer gas [6]. The authors indicated that high hydrogen content in the producer gas provides the environment most suitable for tar removal. However, high gas temperature was still required for complete tar removal (1200
C) and coke formation was significant at this temperature. For energy conversion processes, such as conversion of HC's into syngas, the best results have been obtained in rather powerful non-thermal plasma systems with elevated gas temperatures that can be considered as intermediate “warm” plasmas [12]. Most recent “warm” discharges are based on the gliding arc, which has gas temperatures in the range 2000e4000 K [13]. These “warm” plasmas are very promising for fuel conversion processes because their power and temperature are high enough for intensive chemical conversion and radical production, while the losses can be minimal [14,15]. Thus, energy costs can be quite low. Plasma reforming of tars, modeled primarily with naphthalene or toluene, has been studied in a variety of discharges including microwave, corona and gliding arc discharges [16e31]. Microwave has been studied for conversion of a variety of tars [16]. Detailed studies have been conducted in corona discharges for the conversion of toluene and naphthalene [17e19]. The results of these corona experiments have also been simulated and compared to experiments [20]. Gliding arc has been studied for conversion of a variety of tars including benzene, toluene, acenaphthalene, fluorene, anthracene, and pyrene [21e31]. Although these studies demonstrated high removal efficiencies, many factors were

not realistic for industrial application including: high specific energy input, unrealistic input gas, low tar concentration, low flow rate, and low energy efficiency. Of particular interest in this paper is gliding arc in vortex flow, which has been shown to be quite effective for carbon dioxide dissociation, hydrogen sulfide dissociation, methane partial oxidation and plasma-assisted conversion of heavy hydrocarbons into synthesis gas [32e35]. These recent studies have overcome the disadvantages of other gliding arc discharges by permitting operation in simultaneous high temperature, high flow rate realistic mixtures, while also permitting low energy input operation. In this paper, the studies are extended to a simulated syngas mixture containing toluene and naphthalene.

Experiment The setup for the tar reforming experiments includes a gas mixing chamber, gas preheating, a gliding arc plasma reformer, and an insulated cylindrical, stainless steel reactor which forms the post plasma region (Fig. 1). Additional injection is provided by a steam generator (with a steam trap) and a tar injection system, both of which occur after gas mixing and preheating. The preheated gas temperature is measured via a thermocouple before entering the plasma reformer. The steam injection point temperature is also measured with a thermocouple. Samples are taken both before and after the plasma reactor, and the sample line temperature is carefully controlled to avoid tar condensation in the sample line. The gas samples are analyzed in an SRI GC, which was controlled with a PC. The gliding arc plasma reformer was capable of functioning at atmospheric pressure and temperatures up to 850
C. The high-temperature reformer consisted of a highvoltage, cylindrical electrode made out of stainless steel, a stainless steel ground electrode with six tangential jets, which provided a forward vortex gas flow. The vortex flow increases gas contact with the plasma discharge and improves gas mixing. A Teflon dielectric insulator separates the highvoltage electrode from the ground electrode. The distance for arc ignition (breakdown) between the high-voltage electrode and ground electrode was 3 mm. More details on very similar reformers are available in literature [32,36]. Room temperature air was injected tangentially and premixed with the simulated syngas just before gas contact with the plasma discharge. The air injection was constant for a given tar concentration (25% of the overall flow for low tar concentration, 31% of the overall flow for high tar concentration). For the all experiments, the initial gas composition (before air addition) was 15% H2, 15% CO, 15% CO2, 3% CH4, and 52% N2. In addition, for all experiments, except those investigating water concentration, water vapor was constant, comprising 20% of the overall flow. Additional air injection required to reach 850
C varied between 25% and 31% of the overall flow depending upon tar inlet concentration. It also provided cooling to the Teflon dielectric. The high-voltage electrode and ground electrode of the plasma reformer were connected to a dc power supply (Universal Voltronics BRC). The high-voltage power supply for the

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To Main Outlet Exhaust

Gas Cylinders H , CO, CO , N , CH

Steam Generator

Sample p Line

T

Gas T > 100 C

Gas Mixing Chamber

Steam Trap

S

T

Sample Line

T

GC-FID/TCD

Gas T > 100 C T

T

Gas Preheating 650 - 950 C

T

I

HPLC Pump Tar Surrogate

I

I

S

Plasma Discharge

Air addition

CPU

DC Power Supply

Fig. 1 e Experimental scheme where T is temperature measurement, I is an injection point, and S is a sample point.

gliding arc plasma reformer is a BRC 10000 Universal Voltronics model. The power is converted using IGBT switching at frequencies above 20 kHz and controlled using tuned pulse width modulation techniques. The unit has an operational output voltage of 0e10 kV, output current of 0e1 A DC, and input frequency of 50/60 Hz. This high-voltage power supply was chosen for this plasma application because of its capability for low ripple, fast transient response, endurance to repetitive arcing, and stable output even in the face of line voltage and load changes. The power supply is connected to an 11-kU ballast resistor unit which helps to limit the plasma discharge current. The power supply provided measurement of the average current and voltage and allowed for calculation of the average power, which was accurate to 10% (as determined by special experiments with power integration using a high speed oscilloscope). Power, P, variation in a range of 260 We1070 W corresponded to average current variation in the range of 300 mAe780 mA, and average voltage variation in a range of 0.85 kVe1.6 kV. Flow rate, Q, was 4 m3/h at low tar concentration and 4.38 m3/h at higher tar concentration. Thus, specific energy input, SEI, varied from 0.06 to 0.27 kWh/m3, where the benchmark value was 0.1 kWh/m3. Tar and steam were injected into the preheated gas stream before plasma processing. Tar was a solution of naphthalene dissolved in toluene with mass ratio of 1C10H8 to 3.2C7H8. Naphthalene was chosen as the main tar constituent because it has been identified as the major single component of tar content during the pyrolysis (thermal) stage of MSW gasification [1]. Toluene was chosen as the tar surrogate for multiple reasons including: its ability to act as a solvent for naphthalene, its presence during MSW gasification, and its classification as a non-carcinogen. With the exception of Section ‘Moisture and tar concentration variation at benchmark specific energy input’, steam injection remained constant for all experiments at 20% of the overall flow rate.

Gas analysis was provided by an SRI 8610C GC equipped with both a TCD and FID detector. The TCD detector was calibrated to provide measurements for H2, N2, O2, CO, CO2, CH4, C2H2, C2H4, and C2H6. Due to cooling limitations, the N2 and O2 peaks could not be separately resolved, and instead, they were measured as a single peak. The FID detector was calibrated to provide measurements for C6H6, C7H8, and C10H8. The experiment was permitted to reach equilibrium, after which a sample of the inlet gas was taken. Then, after analysis of the inlet gas was complete, a sample of processed gas was analyzed. The temperature control program ran at 30
C for 6 min, which provided separation of TCD components, and then, the temperature was ramped to 300
C and held for an additional 5 min. During the final 5 min, benzene, toluene and naphthalene eluted. The calculated parameters of interest were: Specific Energy Input : SEI ¼

P ; Q


 kWh 3 m

(1)

Conversion Efficiency : hd ¼

½Cin  ½Cout ; ½% ½Cin

(2)

Energy Efficiency : he ¼

½Cin  ½Cout h g i ; kWh SEI

(3)

where [C] is the tar concentration and Equation (2) is based on the assumption that the inlet gas flow rate, at the point of the inlet concentration measurement, equals the outlet gas flow rate.

Results Moisture and tar concentration variation at benchmark specific energy input A detailed analysis was performed in the region of 0.05e0.3 kWh/m3 at an inlet tar concentration of 30 g/m3 with

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Tar concentration, flow rate, and temperature variation Additional data points were obtained at higher tar concentration. These data points were obtained in the range of 0.07e0.22 kWh/m3 and both toluene and naphthalene conversion trends at higher tar concentration were studied and compared with lower tar concentration (Fig. 7). Tar conversion

C H (7.1 g/m³)

C H (7.1 g/m³)

C H (22.9 g/m³)

1 0.9

Tar Conversion (Fraction)

7.1 g/m3 naphthalene and 22.9 g/m3 toluene (Fig. 2). Results indicate that naphthalene conversion near 93% can be achieved at SEI as low as 0.1 kWh/m3, while complete toluene conversion was achieved at the same value. In addition, a sharp decline in tar conversion was observed for SEI values below 0.1 kWh/m3. This can be explained by the fact that, at this level of SEI, the discharge current was low at 300 mA, and thus, the discharge became intermittent and unstable. The value of 0.1 kWh/m3 was chosen for further study. Two factors were considered: moisture content and tar concentration. Moisture content was studied at 10, 20 and 30% molar concentration while tar concentration was kept constant at 7.1 g/m3 naphthalene and 22.9 g/m3 toluene. Increased conversion was observed at 20e30% moisture content (greater than 90% of both toluene and naphthalene), while it was noticeably lower at 10% moisture content (92% toluene and 65% naphthalene) (Fig. 3). It should be noted that the reactor exhaust gas composition was observed to be approximately 13% CO, 7% H2, and 11% CO2 at 10e20% moisture, while the gas composition at 30% moisture content contained 7% CO, 5% H2, and 21% CO2 (Fig. 4). In addition, tar conversion was studied at 0.1 kWh/m3 while varying naphthalene inlet concentrations (7.1, 17.9, and 30 g/m3), which corresponds to 22.9, 57.1, and 96 g/m3 toluene, respectively (Fig. 5). Higher tar conversion was observed for lower tar content (100% toluene and 92% naphthalene), while lower tar conversion was observed for the higher tar concentration (83% toluene and 69% naphthalene). However, at low tar concentration (7.1 g/m3), the gas composition was observed to be 13% CO, 7% H2, and 11% CO2, while at higher tar concentration (17.9e30 g/m3) the gas composition was observed to be 9% CO, 12% H2, and 11% CO2 (Fig. 6).

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Moisture Content (Molar Fraction)

Fig. 3 e Tar conversion vs. moisture content in gliding arc at specific energy input of 0.1 kWh/m3.

at values above 0.1 kWh/m3 was found to be approximately 70% for naphthalene and 80% for toluene. The discharge was very stable in this range. At the minimum specific energy input of 0.077 kWh/m3, the discharge became unstable and naphthalene conversion was found to decrease to 28%, while toluene conversion was 67%. Additional data points were taken at a flow rate of 6 m3/h mixture gas and 2 m3/h air at SEI of 0.0825 and 0.0635 kWh/m3 to confirm tar conversion and SEI results at lower flow rate (Fig. 8). In this case, SEI remained similar to earlier experiments; however, the flow rate was doubled and hence discharge power was also doubled. Naphthalene conversion was found to be 70% and 50% respectively, which was quite close to values obtained near the same specific energy input at a flow rate of 3 m3/h. The discharge was rather unstable at these conditions. A reduced temperature regime was also studied. In all other experiments, the gas preheat temperature was 650
C and air was injected while a plasma discharge was initiated until the gas temperature increased to 850
C. However, in this

C H (22.9 g/m³)

H

1

CO

CH

CO

0.25

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.05

0.1

0.15

0.2

0.25

0.3

Specific Energy Input (kWh/m3)

Fig. 2 e Tar conversion vs. specific energy input at low tar concentration.

Gas Composition (Molar Fraction)

Tar Conversion (Fraction)

0.9

0.2

0.15

0.1

0.05

0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Moisture Content (Molar Fraction)

Fig. 4 e Detailed gas composition vs. moisture content at SEI of 0.1 kWh/m3.

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C H

C H (17.9 g/m³) 1

0.9

0.9

Tar Conversion (Fraction)

Tar Conversion (Fraction)

C H 1

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

C H (7.1 g/m³)

C H (22.9 g/m³)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0 0

5

10

15

20

25

30

0

35

0.05

Fig. 5 e Tar conversion vs. tar concentration in gliding arc at specific energy input of 0.1 kWh/m3.

case, gas was preheated to 450
C and air was injected while a plasma discharge was initiated until the gas temperature increased to 650
C. These data points were taken at 3 m3/h and less air addition was required to raise the gas temperature. Three successful points were obtained in the range of 0.135e0.2325 kWh/m3 (Fig. 8). Tar conversion was observed in the range of 9e44% respectively, which is less than for the high temperature data. Results of high flow rate, low SEI data were consistent with results at low flow rate and low SEI. At SEI lower than 0.1 kWh/ m3, the discharge becomes unstable (Fig. 8). This is evident by the erratic voltage in the range of 0.8e1.6 kV. This is very likely indicative of the fact that the unstable discharge does not contact the gas flow efficiently, which results in significantly less processing of the gas (lower tar conversion). It will be valuable to consider methods that would keep the gliding arc discharge stable in this regime in future work. This will likely require modifications of both the reactor and power supply. Results of higher tar concentration data were interesting. A maximum of 70% naphthalene conversion was obtained in

H

CO

CH

0.1

0.15

0.2

0.25

0.3

Specific Energy Input (kWh/m3)

Naphthalene Concentration (g/m³)

Fig. 7 e Tar conversion vs. specific energy input at two tar inlet concentrations.

the range of 0.1e0.25 kWh/m3. Similar to the previously mentioned low concentration (7.1 g/m3) results, tar conversion decreased significantly at SEI lower than 0.1 kWh/m3. This occurs for the same reason stated in the paragraph above, discharge instability at low SEI. Results from lower temperature data are quite interesting, as we see a significant decrease in tar conversion for these data points. The plasma discharge tends to be less stable at lower temperature, which is not surprising as gas temperature can influence discharge VeI characteristics. Generally, higher voltages are observed at lower temperatures while lower voltages are observed at higher temperatures. Furthermore, operation at lower temperature with high tar concentration can be especially difficult, as the tar itself also lends to instability of the discharge. More importantly, lower temperature data points required significantly less air than the higher temperature data points, which means fewer oxygen radicals were available for reactions. This would also result in lower tar conversion as oxygen radicals play a key role in the plasma chemistry. A

CO

0.14

C H (6 m³/h, 22.9 g/m³)

C H (6 m³/h, 7.1 g/m³)

C H (650C, 22.9 g/m³)

C H (650C, 7.1 g/m³)

C H (7.1 g/m³)

C H (22.9 g/m³)

0.1

0.2

1

0.12

0.9

0.1 0.08 0.06 0.04 0.02 0 0

5

10

15

20

25

30

35

Naphthalene Concentration (g/m³)

Fig. 6 e Detailed gas composition vs. tar concentration at specific energy input 0.1 kWh/m3.

Tar Conversion (Fraction)

Gas Composition (Molar Fraction)

C H (57.1 g/m³)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.05

0.15

0.25

0.3

Specific Energy Input (kWh/m3)

Fig. 8 e Tar conversion vs. specific energy input with high flow rate and lower temperature data.

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combination of the above effects is likely the cause of low tar conversion under the low temperature condition.

C H (17.9 g/m³)

C H (57.1 g/m³)

Results summary Gas composition at tar concentration of 30 g/m

3

3

The tar conversion results at 30 g/m inlet tar concentration (7.1 g/m3 naphthalene, 22.9 g/m3 toluene) can be found in Fig. 2. The corresponding gas composition vs. specific energy input is presented in Fig. 9. During gas analysis, the nitrogen and oxygen peaks could not be separated, thus the nitrogen quantification is actually a quantification of nitrogen combined with oxygen. Without plasma, initial nitrogen/oxygen concentration is approximately 70%. However, once plasma is turned on, a gradual decrease in nitrogen/oxygen is observed (68e60%). This can be attributed to the increased utilization of the oxygen portion of the combined analysis for partial oxidation processes. Fig. 9 provides a more detailed look at the gas composition trends for H2, CO, CO2, and CH4. CH4 concentration gradually decreases at increasing specific energy input. Initially, the dry basis composition is 8% for H2 and CO2, while CO is approximately 12%, which indicates that some processes take place before plasma is initiated, as H2 and CO2 have decreased compared to their initial input concentration. Then, when plasma was initiated, CO remained constant, while H2 decreased and CO2 increased. As specific energy input was increased, CO2 decreased, while CO and H2 increased. At the maximum specific energy input, a sudden increase in CO2 was observed, while both CO and H2 suddenly decreased.

Gas composition at tar concentration of 75 g/m3 Tar conversion vs. specific energy input is provided for a tar inlet concentration of 75 g/m3, which corresponds to 17.9 g/m3 naphthalene and 57.1 g/m3 toluene in Fig. 10. For values above 0.1 kWh/m3, toluene reforming of approximately 80e95% was achieved, while naphthalene reforming varied from 65 to 70%. At values below 0.1 kWh/m3, the discharge becomes unstable, and reforming drops off significantly. The corresponding gas composition vs. specific energy input is presented in Fig. 11. Again, during gas analysis, the

CH

H

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

CO

CO

0.16

0.18

0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0.1

0.15

0.2

0.25

0.3

Specific Energy Input (kWh/m3)

Fig. 9 e Gas composition vs. specific energy input at tar inlet concentration of 30 g/m3.

0.15

0.2

0.25

nitrogen and oxygen peaks could not be separated, thus the nitrogen quantification is actually a quantification of nitrogen combined with oxygen. Without plasma, initial nitrogen/oxygen concentration is approximately 65%. However, once plasma is turned on, a significant decrease in nitrogen/oxygen is observed (56%). This can be attributed to the increased utilization of the oxygen portion of the combined analysis for oxidation processes. CH4 gradually decreases at increasing specific energy input. Initial gas composition on a dry basis is measured to be 9% for H2, 10% CO, while CO2 is approximately 12%, which indicates again a deviation from the initial gas inlet concentration. At higher tar concentration, both H2 and CO are slightly lower than the initial input concentration. As plasma is turned on an H2 and CO2 increase is observed, while CO decreases. Tar reforming is less significant at first. Once SEI increases over 0.1 kWh/m3, a reforming regime is apparent, as H2 and CO increase while CO2 and CH4 decrease.

0.2

0.05

0.1

Fig. 10 e Tar conversion vs. specific energy input at tar inlet concentration of 75 g/m3.

0.18

0

0.05

Specific Energy Input (kWh/m3)

Gas Compostiion (Molar Fraction)

Gas Composition (Molar Fraction)

CO

Tar Destruction (Fraction)

1

CH

H

CO

0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0

0.05

0.1

0.15

0.2

0.25

Specific Energy Input (kWh/m3)

Fig. 11 e Gas composition vs. specific energy input at tar inlet concentration of 75 g/m3.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 1 9 7 6 e1 1 9 8 9

Energy efficiency Energy efficiency was also determined via Equation (3). Energy efficiency as a function of specific energy input is shown in Fig. 12. For naphthalene, maximum energy efficiency is obtained once the discharge stabilizes at specific energy input of 0.1 kWh/m3. The maximum energy efficiency was 62.5 g/kWh at low tar concentration and 93.6 g/kWh at high tar concentration. The maximum energy efficiency for toluene conversion was obtained at the minimum specific energy input. At low tar concentration, toluene energy efficiency was 239 g/ kWh at 0.06 kWh/m3, while at high tar concentration, toluene energy efficiency was 432 g/kWh at 0.077 kWh/m3.

Discussion Discharge stability The experiments have demonstrated that very high tar conversion (greater than 95%) can be achieved via nonequilibrium plasma assisted oxidative steam reforming. In addition, this tar conversion can be achieved at low SEI near 0.1 kWh/m3. Results also indicate that the region near 0.1 kWh/m3 is interesting, as it transitions from a constant stable discharge into a less stable intermittent discharge. This is the reason a significant decrease in tar conversion is observed at lower SEI. At lower SEI the intermittent discharge is not an effective contributor to the plasma assisted oxidative steam reforming process. Discharge instability can have multiple contributors including: low power input, high flow rate, temperature, and tar/steam concentration. When the discharge is unstable, much more gas (and tar) passes around or past the discharge without contacting the discharge. Another interesting behavior is the observation that, at inputs higher than 0.1 kWh/m3, increased SEI does not increase tar conversion noticeably. Rather, very high conversion (90e100%) is achieved in the SEI range of 0.1e0.3 kWh/m3. Thus, it may be concluded that the tar conversion reaction process is very fast and efficient as long as a stable discharge is maintained. In addition, if a stable discharge could persist

C H (17.9 g/m³)

C H (57.1 g/m³)

C H (7.1 g/m³)

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below 0.1 kWh/m3, this very fast and efficient reaction process could possibly be maintained.

Moisture content The investigation studying the effect of moisture content indicated that increased tar conversion was possible with increased moisture content. In fact, at 30% moisture content, tar conversion was observed to be 100%. It should be noted that there are a plethora of reaction pathways available, and various competing effects will determine tar conversion and final gas composition. In addition, it is important to note that it is undesirable to shift towards a combustion regime where CO2 and H2O are formed. Instead, the process should be maintained in a partial oxidation regime, where the products are CO and H2 (syngas). The main process of interest is oxidative steam reforming, which is the combination of steam reforming of hydrocarbons with air addition which provides a supplementary source of exothermic reaction heat to assist in the completion of the steam conversion reactions. It is possible to balance the amount of heat released by exothermic partial oxidation with the endothermic energy consumption from the steam reforming reactions such that the reaction is theoretically self-sustaining. This process is known as autothermal reforming. It is also possible to integrate a system which recycles exhaust products, utilizes oxy-combustion or both. One such system is an oxyforming system [37]. Another process of interest is the water gas shift reaction, where carbon monoxide and water react to form hydrogen and carbon dioxide. Water gas shift will proceed normally at approximately 400
C, while at higher temperatures of approximately 1000
C, the reaction becomes reversible. The process described herein will be sensitive to these reactions and the main goal is to utilize these reactions to produce the most syngas. Thus, the complicated competing effects of oxidative steam reforming and water gas shift, which will be sensitive to O:C and steam:C ratio, along with the process temperature will be crucial to tuning and controlling tar conversion and final gas composition (H2:CO ratio).

Tar concentration

C H (22.9 g/m³)

Eneryg Effciency (g/kWh)

450 400 350 300 250 200 150 100 50 0 0

0.05

0.1

0.15

0.2

0.25

0.3

Specific Energy Input (kWh/m3)

Fig. 12 e Energy efficiency vs. specific energy input at two toluene and naphthalene inlet concentrations.

The investigation studying the effect of tar concentration indicated that fairly high tar conversion can be obtained at high tar concentrations near 30 g/m3 of naphthalene. The discharge was observed to be more intermittent at higher tar concentration, which could also explain why lower tar conversion was achieved at high tar concentration. It should be noted that H2 concentration in the exhaust gas was on average significantly lower at low tar concentration (8% H2) compared to higher tar concentration (14% H2). In addition, CO was on average significantly higher at lower tar concentration (15% CO) compared to higher tar concentration (10% CO). Finally, it should be noted that tar conversion was nearly the same for 17.9 and 30 g/m3 naphthalene concentrations (70%). This indicates that very high tar concentrations may be treated as effectively as lower tar concentrations, which would be an

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important result for industry, which can encounter very high tar concentrations. Controlling the oxidative steam reforming process will largely be dependent upon fueleair ratio, which can be determined by O:C ratio. Tar concentration will affect this process as it contains carbon. Increased tar concentration will need to be balanced with air addition to control this ratio and ensure that a reasonable fuel air mixture is maintained and complete combustion is avoided.

Temperature One of the key discoveries from this study is the effect of temperature. In order to dissociate naphthalene, a minimum temperature and time will be required. Fig. 13 shows the sample results of CHEMKIN simulations meant to investigate tar conversion via partial oxidation in a syngas mixture. In Fig. 13, the dissociation and time is compared in the range of 900e1300
C. It is evident that significant tar conversion can occur quickly, but only at higher temperatures of 1200e1300
C. However, at 900
C, at least 2 s is required to reduce tar concentration by 50%. The residence time in the non-equilibrium plasma reactor was only 30 ms, and the bulk gas temperature was 850
C. Under these conditions, the non-equilibrium plasma reactor was capable of 95% reduction of tar concentration. This demonstrates that the non-equilibrium plasma discharge can effectively act as catalyst (plasma catalysis), reducing both the temperature and residence time required for significant tar conversion by adding a small amount of electrical energy to the system. This could be due to plasma chemistry, which provides significant O, and OH radicals to the process, along with ions and other chemically active species (excited translational, rotational, vibrational, and electronic states) that react with tars to form syngas. The production of these radicals and active species, along with intense heating and very quick quenching of reactions found in non-equilibrium plasma, allows for results superior to those in thermodynamic simulations.

Plasma chemistry and tar reforming Although the cause of tar formation has been identified as process conditions, the complicated reactions in such a system are still unclear. From a fundamental standpoint, tar decomposition and growth are called cracking and polymerization, respectively, and are the main reactions to consider in the tar reforming process. Cracking eventually results in the formation of syngas, while polymerization results in the formation of heavy hydrocarbons and the agglomeration of soot particles. Ideally, a solution should be implemented which maximizes cracking potential and minimizes polymerization as this maximizes syngas production and minimizes complications associated with tar.

Products of traditional tar reforming Thermal decomposition of heavy hydrocarbons and can be quite costly. The high costs are related to the high temperature requirement of at least 1200
C, at significant residence times near 1 s. Pyrolysis yields hydrogen, carbon monoxide, and carbon dioxide along with some lighter hydrocarbons, the most significant being methane, C2 hydrocarbons and benzene. Unfortunately, pyrolysis of heavy hydrocarbons also often leads to polymerization, wherein the tars breakdown and agglomerate into very heavy soot particles. These soot particles can represent a significant portion of the products and have fuel value. Soot can also pose problems for downstream equipment, as filtering is necessary. Furthermore, even higher temperatures of at least 1400
C, are required to provide significant reforming of soot into valuable products. Soot formation is very often connected to the presence of naphthalene. The combination of methane with naphthalene has also been identified as a significant precursor to soot. In regards to soot formation (tar polymerization), various explanations and processes have been developed including H abstraction/acetylene addition, the role of radicals, and direct combination of intact aromatic rings [38e41]. Still, radical processes have been identified as the most likely driving force behind heavy hydrocarbon and soot formation and decomposition.

Important processes in non-equilibrium gliding arc tar reforming

C10H8 1.20E-02

Considering the tar reforming plasma system, a variety of reactions will play a role in observed results including:

Molar Fraction

1.00E-02

x y x PartialOxidation: Cx Hy þ ðO2 þ3:76N2 Þ/xCOþ H2 þ ð3:76N2 Þ 2 2 2 (4)

8.00E-03

 y Steam Reforming : Cx Hy þ xH2 O/xCO þ x þ H2 2 y Dry Reforming : Cx Hy þ xCO2 /2xCO þ H2 2

6.00E-03

4.00E-03

(6)

The first two reactions may also be considered as: Oxidative Steam Reforming:

2.00E-03

0.00E+00 0.00E+00

(5)

1.00E-02

2.00E-02

3.00E-02

4.00E-02

Time (min)

Fig. 13 e Evolution of naphthalene mole fraction.

 y Cx Hy þ nðO2 þ 3:76N2 Þ þ ðx  2nÞH2 O/xCO þ x  2n þ H2 2 þ 3:76nN2 Two other reactions should be noted:

(7)

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 y Combustion :Cx Hy þ x þ ðO2 þ 3:76N2 Þ/xCO2 4 y   y H2 O þ x þ ð3:76N2 Þ þ 2 4

(8)

balance between reactions (4)e(8); however, (8) is no longer the dominant mechanism for tar reforming. Instead, the regime could also be significantly characterized by (but not limited to) the following reactions:

Water Gas Shift : CO þ H2 O/CO2 þ H2

(9)

C7 H8 þ 2O2 þ 3H2 O/7CO þ 7H2

(14)

C7 H8 þ 7CO2 /14CO þ 4H2

(15)

C10 H8 þ 2O2 þ 6H2 O/10CO þ 10H2

(16)

C10 H8 þ 10CO2 /20CO þ 4H2

(17)

Furthermore, the values most indicative of oxidizer quantity are O to C ratio and steam to C ratio, which can be considered as: O 2n_ O2 ¼ C xn_ Cx Hy

(10)

S n_ H2 O ¼ C xn_ Cx Hy

(11)

In the preceding experiments, O/C and S/C ratio varied for the different tar concentrations studied. At low tar concentration (30 g/m3), O/C ¼ 1.5 and S/C ¼ 2.8, while at higher tar concentration (75 g/m3), O/C ¼ 0.8 and S/C ¼ 1.0, where the difference between the two is the result of a balance of air input and tar concentration (moisture content was constant). At low tar concentration, a minimum required air input (25% of the overall flow) was used to achieve a gas flow temperature of 850
C. At higher tar concentration, the air input was increased to 31% of the overall flow to provide the desired overall gas flow temperature of 850
C. As a result, low tar concentration had much higher O/C ratio. Higher tar concentration was essentially air starved (0.8), while low tar concentration was rich in air (1.5). A similar argument can be made regarding the steam concentration with an observed value of 1.0 at high tar concentration compared to an observed value of 2.8 at low tar concentration. Considering Fig. 8, it is conceivable that if an O/C of 1 had been reached at higher tar concentration, increased conversion may have been observed (over 70%), as more oxygen would have been available for partial oxidation and reforming reactions. The aforementioned reactions and parameters played a crucial role in experimental results. In the case of low tar concentration (30 g/m3), when plasma is off or unstable, gas composition (Fig. 9) reflects a combustion dominated region (region were SEI is less than 0.1 eV/molecule). In this region, hydrogen decreases, while a significant increase in carbon dioxide is observed. Furthermore, carbon monoxide concentration remains nearly constant. At the same time, significant toluene reduction is observed (Fig. 8). This can be explained by the dominance of two combustion equations: 2H2 þ O2 /2H2 O

(12)

C7 H8 þ 9O2 /7CO2 þ 4H2 O

(13)

This observation is further supported by the O/C and S/C ratios, which are quite high at low tar concentration. Once the plasma discharge becomes stable (SEI greater than 0.1 kWh/ m3), the gas composition trends change significantly to reflect a reforming regime, where both hydrogen and carbon monoxide increase, while carbon dioxide decreases (Fig. 9). Furthermore, once the plasma discharge stabilizes, naphthalene reforming increases significantly, while toluene reforming also increases (Fig. 8). This region is characterized by a

At maximum hydrogen and carbon monoxide production, H2/CO ¼ 0.62, which provides further evidence that (8) and (14)e(17) were the primary routes for tar reforming at low tar concentration. At the highest SEI, a sudden decrease in H2 and CO is observed, while CO2 increases. At such high SEI, the discharge can become significantly more thermal which could result in a shift to a more combustive regime where a decrease in H2 and CO and an increase in CO2 could be expected. At higher tar concentration (75 g/m3), significantly different gas composition trends are observed (Fig. 11). In the region where plasma is off and unstable (SEI less than 0.1 eV/molecule), water gas shift, (6) is the dominating mechanism. In this region, both hydrogen and carbon dioxide increase, while carbon monoxide decreases. The O/C and S/C ratios are lower for higher tar concentration, and this provides an explanation for why this region is characterized by water gas shift rather than the combustion region as observed at lower tar concentration (and higher O/C and S/C ratios). Still, significant toluene reforming is observed in this region below 0.1 eV/molecule (Fig. 8). This can be explained via the two reactions, water gas shift (9) and toluene steam reforming: C7 H8 þ 7H2 O/7CO þ 11H2

(18)

Water gas shift and toluene steam reforming can be combined into: C7 H8 þ 14H2 O/7CO2 þ 18H2

(19)

Once the plasma discharge becomes stable (SEI greater than 0.1 eV/molecule), the gas composition trends change significantly to reflect a reforming regime. Both hydrogen and carbon monoxide increase, while carbon dioxide decreases (Fig. 11). Furthermore, in the stable region, naphthalene reforming increases significantly, while toluene reforming also increases (Fig. 8). This region is characterized by a balance between reactions (4)e(9); however, (6) is no longer the dominant mechanism for tar reforming. Instead, the regime could also be significantly characterized by, but not limited to, (14)e(17). At maximum hydrogen and carbon monoxide production, H2/CO ¼ 1.45, which provides further evidence that Equations (6) and (14)e(17) were the primary routes for tar reforming at high tar concentration. At both low and high tar concentration, small amounts of benzene were observed in the exhaust gas. Benzene can form via a variety of reactions including, but not limited to: C10 H8 þ 4H2 O/C6 H6 þ 4CO þ 5H2

(20)

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C10 H8 þ 2O2 /C6 H6 þ 4CO þ H2

(21)

C10 H8 þ 2H2 O þ O2 /C6 H6 þ 4CO þ 3H2

(22)

C10 H8 þ 4CO2 /C6 H6 þ 8CO þ H2

(23)

Benzene is the most significant precursor to PAH and soot formation and is also the most difficult of the three discussed HC molecules to decompose thermally. Consequently, it is not surprising that it was observed in small quantities. Lighter hydrocarbons, however, were not observed during experiments. Though the GC was calibrated to detect acetylene, ethylene, and ethane, no detection occurred. Furthermore, no unidentified peaks were observed. The only other detected hydrocarbon was methane, which was always present in both the inlet and outlet samples, though it was observed to decrease as SEI increased.

Non-equilibrium gliding arc tar reforming plasma chemistry Non-equilibrium gliding arcs have many advantages including: fast process control, low thermal inertia, system flexibility, extreme temperature range, and a highly reactive environment [13]. With respect to the results of this study, the highly reactive plasma environment should be fully considered. That is, the advantage for the tar reforming process in a non-equilibrium discharge is the promotion of chemical reactions, both conventional and unconventional, with some degree of selectivity. More specifically, the non-equilibrium discharge simultaneously acts as a controllable source of energy providing thermal effects, while also acting as a generator of electrons, ions, excited particles, and radicals, which stimulate plasma chemistry [13]. These plasmaechemical reactions can initiate chain processes and reactions, which would otherwise be unattainable in conventional chemistry, because plasma influences the energy and temperature requirements of conventional chemistry. Focusing on plasma chemistry, three key process observations from experiments will be noted and discussed. First, analysis indicates that no soot was formed, with the exception of some soot deposition on the sample line, after a few months of experiments. Previous reports on thermal and partial oxidation of naphthalene and toluene without plasma indicate that soot formation can be quite noticeable [3,5]. However, other experiments utilizing gliding arc for partial oxidation corroborate the findings of this study, as they also have been observed to occur without soot formation [42,43]. Reports on other plasma-assisted fuel conversion systems in rich and ultra-rich mixtures also observed little or no soot formation [32,35,44]. Second, no hydrocarbons with molecular weight greater than naphthalene were detected by the GC, which indicates that polymerization of lighter HC's into heavy hydrocarbons did not occur. The absence of soot also supports this finding as soot would be the resulting product of hydrocarbon agglomeration. Third, no light hydrocarbons were detected with the exception of benzene. In the conventional decomposition process for heavy hydrocarbons, decomposition would require that the heavier hydrocarbons (naphthalene, toluene) follow a step by step breakdown down into the lighter

components into methane. However, these lighter HC's were not detected, even though the GC was calibrated to detect acetylene, ethylene, and ethane (please note that methane was always present as part of the input gas, and was always present in the output gas at lower than the initial concentration). A number of explanations can be provided to explain the first two observations (the inhibition of soot and heavy hydrocarbon formation) in experiments. The first explanation is that the plasma system achieved very good mixing. That is, the tar surrogate that was injected became well mixed in the plasma reactor, and the tangential injection of oxidizer provided an environment where the oxidizer and fuel were also very well mixed. Thus, the plasma system contained no regions characterized by simultaneous high temperature and high fuel concentration, which would lead to soot formation. A second explanation can be attributed to a lack of soot precursors present in the exhaust gas. Acetylene and benzene are considered to be major soot precursors [44,45]. The tar surrogate and input gas did not contain these components, and they were not present in the exhaust gas after plasma oxidative steam reforming. Since these precursors were not present, soot formation was not significant. A third explanation is the rapid heating and quenching of reactions that occurs in non-equilibrium plasma discharges. Literature has indicated that NEP is effective source of rapid gas heating followed by rapid gas cooling [13]. That is, high temperature gradients are provided by short gas residence times in small discharge filaments followed by rapid quenching upon exiting the filament. In this specific case, if the products of plasmachemical reactions are rapidly quenched to stable temperatures before soot has adequate time to nucleate and grow, the desired gas composition can be frozen. In such a situation, heating should be very rapid followed by immediate quenching on the order of 106 K/s or higher [13]. A fourth explanation is related to the amount of oxidizer. The complex mixture used in the experiments contained O2, H2O and CO2. All of these can act as oxidizers in tar reforming and can be considered as in Section ‘Important processes in non-equilibrium gliding arc tar reforming’. Still, the presence of these oxidizers will also provide important possibilities in regards to plasma chemistry Literature indicates that radicals play the most important role in cracking reactions, and, plasma generates radicals, ions and other chemically active species (excited translational, rotational, vibrational, and electronic states) in abundance [13]. Recent studies on toluene and naphthalene decomposition in atmospheric corona and glow discharges have provided insight into the plasmachemical processes [17,46,47]. In the study by Trushkin, toluene decomposition was substantially improved and acetylene production was suppressed when moisture content increased [46]. Furthermore, Trushkin developed a kinetic model, which indicated that the major factor in decomposition could be attributed to the hydroxyl radical, OH. Trushkin found that OH was produced in two stages, where the first is characterized by OH production by direct electron impact with H2O and interaction of electrically excited oxygen and nitrogen molecules with H2O and the second stage is characterized by toluene and toluene decomposition products interacting

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Table 1 e Comparison of gliding arc tar reforming in literature with the present publication. Discharge

Tar Input (g/m3) SEI (kWh/m3) hd (%) he (g/kWh) Q (m3/h)

[21]

[23]

[24]

[25]

[26]

[27]

[29]

DC

AC

AC

AC

AC

AC

AC

DC

DC

C10H8 1.32 0.4686 92.3 3.6 0.4

C7H8 7.85 0.26 98.5 29.56 0.8

C14H10 0.21 0.175 96.1 1.14 0.72

C10H8 59.5 1 79 47 1.1

C6H6 4.31 0.17 90.7 23 0.98

C16H10 0.13 0.91 88.3 0.13 0.72

C10H8 0.13 1 95 0.123 0.6

C10H8 7.1e30 0.1 69e93 62.5e206 4

C7H8 22.9e96 0.1 83e100 215e796 4

with oxygen molecules to form OH. The second stage was found to be strongly influenced by initial toluene content and energy input. A similar conclusion was reached by Ni et al. for naphthalene decomposition, when an increase in decomposition was observed with a corresponding increase in OH intensity [47]. Although the above studies were not conducted in gliding arc, a very recent study by Sun et al. on gliding arc optical characteristics demonstrated that an ambient air gliding arc discharge was also characterized by two zones, an upstream zone dominated by excited N2 species, and a downstream zone dominated by OH and NO species [48]. From these studies, it can be argued that gliding arc will be dominated by mechanisms similar to those found by Trushkin [46]. Keeping in mind that the gas composition in this study contained significant N2, H2O, O2, along with CO, H2, CO2, CH4 and heavy tar, the system contains all the components necessary for the same plasmachemical reactions observed by Trushkin. That is, gliding arc provides OH via direct electron impact with H2O and also via interaction of electrically excited oxygen and nitrogen molecules with H2O. This provides the initial reactions with the heavy tar. Once this process begins, naphthalene and toluene (and their decomposition products) can also react with oxygen molecules to form OH, which stimulates even more reactions. As is evident from Table 1, initial tar concentrations in this study were very high when compared with past studies; thus, the role of OH production via interaction with heavy tar may be significant.

Comparison with previous literature In Table 1, a comparison with other gliding arc systems designed for tar reforming is provided. Other studies have been completed in tars ranging from benzene to pyrene under a variety of conditions. The main parameters of interest are discharge type (AC or DC), tar type, input concentration, specific energy input, destruction (conversion) efficiency, energy efficiency, and flow rate. The results contained in this work are provided and demonstrate various advantages over previous literature including: very high tar content operation, low SEI, very high energy efficiencies, and high flow rates.

Conclusions Toluene and naphthalene conversion was studied in a nonequilibrium gliding arc plasma reformer. At low tar concentration (30 g/m3), over 90% naphthalene and toluene

Present publication

conversion was achieved at the benchmark specific energy input of 0.1 kWh/m3 and energy efficiencies of 62.5 g/kWh for naphthalene and 215 g/kWh for toluene. At higher tar concentration (75 g/m3), over 70% naphthalene and toluene conversion was achieved at the benchmark specific energy input of 0.1 kWh/m3 and energy efficiencies of 93.6 g/kWh for naphthalene and 369 g/kWh for toluene. These results show significant improvement over previous work in gliding arc for tar reforming. Furthermore, no intermediate hydrocarbons (with the exception of small amounts of benzene), soot, or heavy hydrocarbons were observed in gas chromatographic analysis. The preceding observations coupled with high conversion observed in experiments, indicates that effective mixing, rapid heating, and plasma chemistry provided an effective environment for tar reforming and suppression of soot production. Based upon the findings of other recent studies, the main contributor to tar conversion is the plasma production of OH. In simulated syngas, OH can form in two stages. First, OH is produced via direct electron impact with H2O and also via interaction of electrically excited oxygen and nitrogen molecules with H2O. This initially produced OH provides the initial reactions with the heavy tar components such as toluene and naphthalene. Once this process begins, naphthalene and toluene (and their decomposition products) can also react with oxygen molecules to form even more OH, which stimulates even more reactions. Initial tar concentrations in this study were very high when compared with past studies; thus, the role of OH production via interaction with heavy tar may be significant.

references

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