Article pubs.acs.org/EF
Tar Reforming under a Microwave Plasma Torch Rodrigo Monteiro Eliott,† Manoel F. M. Nogueira,*,‡ Argemiro S. Silva Sobrinho,† Bruno A. P. Couto,‡ Homero S. Maciel,§ and Pedro T. Lacava† †
Technological Institute of Aeronautics, Praça Mal. Eduardo Gomes 50, Vila das Acácias, 12.228-900 São José dos Campos-SP, Brazil Federal University of Pará, Av. Augusto Corrêa 1, LABEM, 66075-110 Belém-PA, Brazil § Paraiba Valley University, Av. Shishima Hifumi 2911, São José dos Campos-SP, Brazil ‡
ABSTRACT: Because of the scarcity of nonrenewable natural resources, such as petroleum and natural gas, the use of biofuel is needed. Gasification is a major process used to obtain renewable fuels from biomass; however, the gas cleaning system is a constraint for its broad utilization. During the pyrolysis process, a mixture of organic compounds in the gas phase is produced and must be removed from the gases before it is used in the most practical applications. In order to remove such organic compounds, which are known as tar, large, sophisticated, problematic, and expensive gas cleaning systems are added to the gasifier gas exit. Previous papers have shown that the plasma torch has the potential to destroy produced tar, being a simpler and less-expensive system than traditional gas cleaners. This work presents a qualitative and quantitative evaluation of a microwave plasma system running on tar destruction and its reforming. In order to evaluate a 1 kW microwave plasma system performance, an apparatus was developed and installed at ITA Laboratory of Plasmas and Processes (LPP-ITA). The system runs at atmospheric pressure with nitrogen and argon as carrier gas under a large range of flow rates. Experiments were performed using a gas mixture of N2, H2O, ethanol, and tar at controlled concentration in order to simulate the gases produced by a gasifier. The injected tar was obtained from pine pyrolysis and characterized for energy purposes. In order to reduce tar viscosity, it was diluted in commercial ethanol (92.5% ethanol and 7.5% water) and its concentration varied from 0.8 gtar/Nmgas3 to 4.2 gtar/ Nmgas3. Species formed in the microwave plasma torch were identified using an optical spectrometer. The reactor exit gases had their composition evaluated on tar content as well as for noncondensable gases. As a result, this paper shows that no tar content was detected at the reactor outlet, indicating that all supplied tar was destroyed in the plasma reactor. The main detected products were CO and solid carbon (C(s)). Furthermore, neither NO nor CO2 were detected, and an indication of H2 formation was obtained. This paper concludes that the microwave plasma system is capable of destroying and reforming tar efficiently and produces mainly H2, CO, O2, and C(s) as byproducts.
1. INTRODUCTION The pyrolysis process of vegetable biomass produces many condensable volatiles that are collectively called tar. This organic molecule mixture (tar) is recognized as carcinogenic, harmful for human health and environment, a hazard for mechanical equipment, and a poison to synthesis gas (CO, H2, and CH4). Because of these problems, gasifier gas cannot be used directly in engines or on second-generation fuel production processes, such as Fischer−Tropsch. For such applications, tar must be previously removed and regular techniques are costly and complex cleaning systems that generally produce byproducts, which will need further treatment. Furthermore, tar is a hydrocarbon species mixture and the consequence of its removal is a reduction on the gas heating value. An alternative technique for tar removal (destruction) is the use of microwave plasma torches. The benefit of this technique is a reduction in size and a simple cleaning operation, as well as the elimination of hazardous byproducts, thereby reducing environmental impacts and keeping the gross gas heating value at a high level. Successful hydrocarbon destruction with plasma sources was reported.1 In this case, a microwave plasma torch reformed methane, obtaining H2 and solid carbon (C(s)) as products.1 In another work,2 corona discharge was applied for the breakdown of naphthalene molecules (species present in many tar © 2012 American Chemical Society
mixtures) in the presence of a gaseous mixture (90% of N2 and 10% of CO2). Two other works3,4 tested and analyzed a microwave plasma torch at atmospheric pressure, using argon as the gas carrier. References 5−7 show proposed reaction pathways for tar conversion into smaller species. Water steam was disintegrated using a microwave plasma torch.8 All the above works indicate that it is possible to destroy vegetable tar in aqueous solution, using a microwave plasma torch at ambient pressure, but it has not been done so far. This work describes an apparatus and experimental results that led to complete pine tar destruction, using a microwave plasma torch at ambient pressure with concentrations up to 4.2 gtar/Nmgas3 without hazardous byproduct production. This technique proved to be capable of converting an undesired element (tar) into smaller and noncondensable species.
2. APPARATUS DESCRIPTION The apparatus basic concept consists of microwave plasma torch acting on a known inlet gas mixture flow concentration (argon, nitrogen, water, ethanol, and tar) and to measure tar and species concentration after the plasma reactor. The plasma Received: August 25, 2012 Revised: December 16, 2012 Published: December 17, 2012 1174
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Figure 1. Apparatus scheme adopted in this work for the tar reforming with a microwave plasma torch.
torch was started up only with argon flowing through it. The reactor then was supplied with a known mixture of argon, nitrogen, and tar solution (ethanol + water + pine tar). The reason for the presence of argon upon ignition and afterward was to provide stability to the torch (and avoid subsequent torch extinction), because of the fact that argon is easier ionized, when compared to other gases. The reason for using nitrogen instead of argon was to simulate gasifier exhaust gases, which are mostly comprised of nitrogen. 2.1. Doping and Evaporation. Figure 1 illustrates the experimental apparatus. Nitrogen and argon go through flow meters, where they have their flow rates individually registered and controlled with valves. After passing through flow meters, gases mix themselves and go to a 1/4-in. copper pipe coil located inside a tube furnace thermally heated with 3500 W electrical resistances. In order to avoid heat losses, this furnace was insulated with rock wool and glass wool covered with aluminum foil. Standard operation procedure keeps the gas temperature at the furnace exit heated to temperatures over 300 °C. After passing through the first tube furnace, argon− nitrogen mixture receives tar solution from a syringe pump (manufactured by KDS Scientific, Model KDS100, with an error of 1%). This 10-mL syringe pump dopes the carrier gas with tar solution comprised of 63.49% ethanol (mass fraction), which is responsible for tar dilution; 6.51% water (oxygen and hydrogen donor); and 30% pine tar. The heated mixture of argon and nitrogen drags and evaporates the tar solution injected by the syringe pump while it flows in the second furnace. The second tube furnace increases the doped gas temperature up to ∼350 °C. It is responsible for completing the tar evaporation process, seeking to avoid droplet presence in the plasma torch. The second tube furnace was constructed similarly to the first one.
After passing through the second tube furnace, the gas mixture was driven to the microwave plasma reactor. In the entrance of the microwave plasma reactor, there is a Type K thermocouple connected to a digital display, which measures doped gas temperature at the reactor entrance. In order to avoid tar condensation and subsequent obstruction in the pipe feeding, tar solution was injected when the gas temperature at the reactor inlet was equal to or higher than 250 °C. This temperature value was chosen after experimental evaluation for tar solution boiling temperature. The doped gas mixture then enters in the microwave plasma reactor, where tar breakdown was expected to occur, as well as new chemical species formation. 2.2. Microwave Plasma Reactor. The microwave plasma reactor utilized in this work, with a maximum power of 3000 W and source voltage of 5500 V, is illustrated in Figure 1. Its magnetron converts the electrical current into microwave. A circulator is located downstream from the magnetron source, to prevent the electromagnetic current from returning and damage the magnetron. The circulator has a system to measure incident and reflected power, which is intended to evaluate the microwave application efficiency into the gas passing through the reactor. Downstream from the circulator, the microwaves are conducted through a waveguide to a quartz tube, 10 mm ID, where they were absorbed by the gas coming from the feeding system, on swirl way, ionizing a fraction of molecules in this gas, thereby generating the plasma torch. In order to maximize tar breakdown and facilitate plasma generation, the impedance of the plasma region is important to approach the impedance from the microwave generator (magnetron). This is accomplished by using a movable piston and stubs. The microwave plasma reactor ignition system consists of a tungsten rod inside the quartz tube in the plasma region 1175
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Figure 2. Apparatus scheme for the tar measurement system applied in this work.
The retained tar quantification was performed as follows: before being used, the gas washing flask was individually weighed with 100 mL of isopropanol. After using these bottles, the mixture of tar and isopropanol contained in each individual bottle was weighed again, and the difference in flask weight before and after experiment is the amount of tar mass trapped. It is important to keep in mind that the reactor eluded gas has no water in its composition. The borosilicate filter was dried in a drying oven for 1 h (time provided by the manufacturer) before being used in the tar measurement system. After passing through the laboratory drying oven, the borosilicate filters were weighed. After the experiment, the borosilicate filter was weighed again and the difference between these weights was added to the tar mass trapped in the washing flasks, thereby providing the total mass of retained tar. The volume shown in the gas meter after the test is subtracted from the amount shown before the test was performed. The difference between such volumes is the gas volume analyzed by the tar measurement system. Using the values obtained from the manometer and the thermocouple at the gas meter, the volume of the analyzed gas was converted from mgas3 to Nmgas3. The total amount of retained tar in the tar measurement system was divided by the gas volume (gtar/ Nmgas3), as shown in eq 1. m C tar = tar Vgas (1)
(shown in Figure 1). The purpose of such a tungsten rod is to concentrate the electromagnetic waves and start the plasma torch. Upon ignition, maximum power is imposed to the flow (3000 W) during a short period of time (on the order of milliseconds). After this period, the system reduces its power to the chosen prerequested level and holds it constant and steady (1000 W). The power can be modified later, without interruptions, during the microwave plasma reactor operation. 2.3. Tar Measurement System. Downstream from the microwave plasma reactor is the tar concentration measurement system. The applied tar measurement system, shown in Figure 2, with a lower detection limit of 0.01gtar/mgas3, was based on the procedure9,10 and successfully utilized.11 The exhaust gases from the plasma torch were fed into the tar filtration system at a specific mass flow rate. Such flow was controlled through a valve, and flow surplus was sent to the alternative output. The gas sample then flows through a tar borosilicate filter (1 μm mesh filter with efficiency above 99%). Its external wall is electrically heated at 200 °C, using a 120-W electrical-resistance-type collar, thereby preventing tar from condensing on the filter holder walls. In this filter, mainly primary tars were trapped and removed from the hot gas. The next step was trapping the remaining tar content through one or up to six gas washing flasks (Schout, 500 mL with 100 mL of isopropanol in each flask). The quantity of gas washing flasks depends upon the tar concentration in the gas. The flasks with isopropanol retain the remaining tars that will be quantified through weight measuring. The last flask of 500 mL contains silica gel and works as tar presence warning, indicating need to add new washing flasks and avoid tar presence in the vacuum pump. The gas washing flasks were kept at temperatures of approximately −5 °C with an 18-L ethylene glycol cooler system, cooling the gas and facilitating tar condensation. The cooled gas, which is now free from tar, passes through a vacuum pump (with a maximum flow of 120 L/min). After passing through such vacuum pump, the gas flows through a meter with a Type K thermocouple connected to a temperature display and a manometer (pressure gauge minihelic II model with a measurement range of pressure between 0 and 100 mm of H2O). Subsequently, the gas without tar was driven to a gas analyzer (greenline GL8000) with detection ranges from 20 ppm up to 8000 ppm for CO and from 250 ppm up to 250 000 ppm for O2) or released to the atmosphere.
2.4. Optical Spectroscopy and Tar Characterization. During the species ionization process present in the plasma torch, these species have their electrons excited. The excited electrons return to the standard state, thereby emitting photons. The color and tone of the photons are characteristics of each chemical species of the plasma torch present, allowing species identification through optical emission spectroscopy. The optical spectroscopy utilized in this experiment was an optical emission spectrometer (Model UV−vis, Ocean Optics USB4000) with a resolution of 1.5 nm and operating range in the ultraviolet (UV) and visible (Vis). The spectrum was obtained by an OOIBase32 program. Chromatographic analysis was performed in the sample tar solution aiming to identify organic species in the tar. The chromatograph used was a Model GCT-Premier system manufactured by Waters, and its main parameters are given as follows: column, HP-5 (5% phenylmethylsiloxane) MS 30 m 1176
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of long × 0.25 mm ID; initial temperature, 40 °C; column temperature, 280 °C; injector temperature, 270 °C; detector temperature, 280 °C; helium flow, 1 mL/min; constant pressure, 80 kPa; and library peak identification used, NIST (National Institutes for Standards and Technology). In order to qualify the atomic composition present in the used tar, an ultimate analysis was conducted. The apparatus used for such analysis was an Ultimate Analyzer 2400 Series II CHNS/O system and an analytical balance of six decimal figures (Model AD-6), both manufactured by Perkin−Elmer. The EBMA laboratory also provided measurement for used the gross heating value (GHV) of the used tar. The apparatus used for such analysis was a calorimeter manufactured by Ika Werke (Model C 2000).
Table 1. Main Operational Parameters Adopted during the Experiments tar solution (mL/h)
tar concentration (gtar/Nmgas3)
6 9 12 15
1.7 2.5 3.3 4.2
of the plasma reactor was ∼250 °C. Nitrogen and argon flow in all experiments were kept constant at 8.8 and 3.8 L/min, respectively. Throughout the operation, the power reflected and not absorbed by the plasma torch did not exceed 0.5% of the power supplied by the microwave plasma reactor.
4. RESULTS AND DISCUSSION Figure 3 shows the chromatographic gaseous mass spectrometry analysis (CG/MS) and Table 2 shows the ultimate analysis, as well as the gross heating values (GHV) results for pine tar applied. 4.1. Spectroscopy. In order to identify the species formed just after tar destruction (without overlapping with emissions from water and ethanol destruction), experiments were initially performed only with water and ethanol solution doping into the feeding system, and the result is shown in Figure 4. Then, and under the same conditions, tar was added and the optical spectrum obtained is shown in Figure 5. The integration time used in the acquisition of both optical spectra was the same. In order to identify the peaks present in these spectra, the works on the subject12−14 were consulted. Comparing the wavelength of 247.83 nm in both spectra, an increase on the peak intensity can be seen as an increase on carbon-neutral formation. Possibly, it is the carbon retained on the wall of the quartz tube. Carbon-ionized formation is seen in Figure 5 at 430.76 nm. The ionized carbon can form CO molecules, reacting with ionized free oxygen. The presence of ionized oxygen can be confirmed by peaks observed at 467.71, 469.15 and 558.11 nm. The wavelength at 436.47 nm (Figure 5) indicates that the formation of ionized sulfur, along with ionized oxygen atoms, can produce sulfur dioxide. As shown in the ultimate analysis, the sulfur is originated from tar, which is confirmed by the absence of sulfur in Figure 4. Figure 5 shows a peak in the wavelength range of 600 nm that does not exist in Figure 4. This peak is an indication of hydrogen gas (H2) formation. This comparison shows the possible formation of H2 during the tar reforming. Another important fact, which may reinforce the hypothesis of H2 formation, refers to the wavelength range of 549−563 nm in both spectra. Peaks at such wavelengths are more intensified in Figure 5 than in Figure 4. This fact suggests H2 formation as a product of tar reforming. Despite the fact that the ultimate analysis indicates nitrogen atoms in tar composition, the emission of a nitrogen species apparently was not found in the optical spectrum. A possible explanation for this is the lack of resolution in the optical spectroscopy used, which does not have enough resolution to separate and show nitrogen species peaks. Figure 6 shows the spectrum of the water molecules destruction and reforming via microwave action. After analyzing the spectrum shown in Figure 6, the formation of various free radicals (Hα, Hβ, and O+) was observed. The presence of these free radicals is important to avoid tar broken molecules to
3. EXPERIMENTAL METHODOLOGY Before the microwave plasma reactor starts, a 10-mL syringe is filled with tar solution and assembled into the syringe pump; its needle is inserted into the pipeline of the feeding system, as described above, and already set up at the desired injection flow rate. Once the gas flow reaches the desired temperature, the syringe pump was turned on and the tar solution was added to the feeding system. During the plasma reactor operation, the tar solution flow rate can be modified without interrupting the experiment. Initially, the reactor cooler system was turned on and only the argon valve was opened and adjusted at a flow rate of 16 L/min. The movable piston then was positioned at 125 mm (the movable piston is shown in Figure 1) and the magnetron electrical power system is set on the control panel, thereby igniting the microwave plasma torch. Finally, the movable piston is moved up or down aiming at the minimization of the reflected microwave (the values for the reflected power are registered). Once the feeding gas temperature after the second tube furnace was equal to or greater than 250 °C, the syringe pump was turned on, thereby injecting tar solution into the feeding system. After a few minutes of operation, the plasma torch color was changed to green. The tar solution saturation time in the feeding system depends on the tar solution concentration. After the green color from the plasma torch was obtained, the movable piston is moved again, seeking the minimization of the reflected microwave. After the reflected microwaves were minimized, the nitrogen gas entrance flow was opened and its flow rate was adjusted to 1 L/min. The plasma torch color changed again from green to purple. The movable piston was moved again, in order to maximize the microwave power absorbed by gases in the microwave reactor. After the plasma torch was in steady operation, the argon flow rate was reduced to 3.8 L/min and the nitrogen flow rate was increased to 8.8 L/min. During the flow rates changes, the movable piston was moved again, in order to minimize the reflected microwave power. During plasma torch operation, which destroys and reforms tar, a fraction of the tar carbon content that was not able to recombine with oxygen turns out to be solid carbon and adheres on the quartz tube internal wall, with its amount increasing with time. This effect was also observed1 when methane gas was destroyed, utilizing a microwave plasma torch. Such carbon deposit absorbs a fraction of the incident microwaves in an amount directly proportional to the carbon concentration on the quartz tube surface, reducing the amount of energy delivered by the microwaves to the gas flow, leading to instability and posterior torch extinction. During all experiments, the adopted measurement system did not identify tar presence in the reactor eluded gases. It was noticed that, when working with tar concentrations at or above 4.974 gtar/Nmgas3, tar was not vaporized completely into both tube furnaces; therefore, the tar concentration reaction was not performed downstream. After carrying out the tar destruction and reform tests, the best conditions obtained are shown in Table 1. In these experiments, the temperatures of both tube furnaces were maintained between 330 °C and 350 °C, and the inlet gas temperature 1177
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Figure 3. Result from chromatographic gaseous mass spectrometry analysis (CG/MS) of the tar solution used in this experiment.
Table 2. Results from Tar Ultimate Analysis (Mass Fraction) and Gross Heating Value (GHV) parameter ultimate analysis (mass fraction) C H N S O gross heating value, GHV (kJ/kg)
value 51.05 5.1 6.85 6.68 30.32 6769
recombine into larger molecules. It indicates a major role that water plays on tar reforming via plasma torch. The carbon that exists in the high-temperature plasma torch can react with the water, providing CO and H2 products.15 4.2. Gas Composition Downstream from the Plasma Reactor. Results obtained from the gas analyzer are shown in Figure 7, and they were averaged from 120 measured values obtained during the experiments. Each experiment refers to a specific flow of tar solution injected into the feeding system. The operation conditions are shown in Table 1. Figure 7 shows that CO increases and O2 concentrations decrease as the tar solution flow rate increases. It is important to highlight that no oxygen gas was supplied to the reactor. All oxygen atoms in the detected CO and O2
Figure 4. Result from optical emission spectroscopy for inlet gas without tar.
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rate, the quartz tube wall saturates so fast that it quenches the plasma. The beginning of instabilization is shown in Figure 7 (CO curve after tar solution of 12 mL/h). The presence of hydrocarbons (CxHy) was not detected in the exhaust gases, thereby reinforcing the hypothesis that tar, after being reformed in a plasma torch microwave, results in small chain species (diatomic or triatomic). The presence of SO2 concentration (max average concentration of 212.48 ppm) was measured, and the presence of either NO or CO2 was not verified. 4.3. Inlet−Exit Species Mass Balance. In order to validate the data obtained, an investigation on the absence of NO and CO2, and possible formation of H2 molecules, a mass balance was conducted. Table 3 shows the inlet mass flow rates for all listed species, and Table 4 contains the exit mass flow rates only for the species listed. Here, only the tar solution atom population was considered, after argon and nitrogen were determined to play an inert role. In Table 4, the solid carbon flow rate was obtained by the difference between the carbon flow rate from tar solution and that from the eluded CO measured. In the same table, the adsorbed oxygen flow rate was obtained using the difference between the oxygen flow rates from tar solution and that from the eluded CO, SO2, and O2 measured. As written above, the approach adopted in this work was to keep the mass ratio between atoms constant and independent of the tar solution flow rate. This can be inferred from the last five columns in Table 3. In Table 4, the last three columns on the right show the atoms mass flow rate evaluated only with the measured species downstream from the reactor (CO, SO2, O2, and C(s)). One can see that an increase in tar solution flow increases the CO formation, but this growth is not linear, indicating excess of free carbon available for reaction and having, as a consequence, an increased solid carbon formation. The relationships between C and O for each tar solution flow gap in Table 3 show an inlet C/O ratio that is constant and independent of the tar solution flow rate applied (C/O = 1.3). Therefore, the reactant residence time in the plasma region influences the formation of CO and O2. An increase in the tar solution flow rate implies a reduction of reactant residence time in the plasma reactor, decreasing the O2 formation and increasing the CO formation. Another observation that occurs with the increase of the tar mixture flow, thereby reducing the residence time in the reactor, is SO2 formation. It seems to go through a maximum of 9 mL/h. Observing the oxygen atom flow adsorbed by carbon contained in the quartz tube, which is shown in Table 4 (Oadsorbed), there is a very low flow, which is null at low tar solution flows, meaning that all the oxygen present in the tar solution reacts with carbon and the remainder, with sulfur and oxygen, forming SO2 and O2, respectively. This fact can explain the absence of both CO2 and NO downstream from the reactor. Another important fact is that little oxygen is available to react with atomic hydrogen in order to form H2O molecules. Moreover, as shown in this paper, there was no unburned hydrocarbon presence downstream from the reactor; therefore, hydrogen atoms do not react with available carbon. Therefore, the hydrogen atoms may have reacted with each other in order to form hydrogen molecules (H2), instead of reacting with either oxygen or carbon atoms. Because of a lack of availability of equipment in the laboratory during this work, hydrogen gas was not quantified. The main products measured were C(s), O2, CO, SO2, and possibly H2. With the exception of SO2
Figure 5. Results from optical emission spectroscopy for inlet gas with tar.
Figure 6. Results from optical spectrum formed after the destruction of water molecules (only).
came from ethanol, water, and tar. Furthermore, the approach adopted in this work keeps the inlet C/O and C/H ratios constant and independent of the tar solution flow rate applied (C/O = 1.3 and C/H = 4.6). Therefore, an increase in the tar solution flow rate implies only a reduction in reactant residence time in the plasma reactor. The initially expected result for Figure 7, once tar mixture rate increases, is an increase on CO and O2 mass fraction as well; but this was not the result that was obtained. Possibly, it is consequence of the reactant residence time: Long residence time favors O2 formation and low residence time favors CO formation. This experiment was conducted with lack of oxygen to produce CO (C/O = 1.68, after ultimate analysis); consequently, free carbon turned solid and was deposited on the quartz tube. In addition, above a certain tar solution flow 1179
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Figure 7. Concentrations of CO and O2 (mole fraction) measured downstream from the microwave plasma reactor, relative to the inlet tar flow rate.
Table 3. Inlet Mass Flow Rates for Species and Atoms Inlet
Tar Solution Atom Flow Rate (g/min)
unity
tar solution flow (mL/h)
total
N2
argon
H2O
ethanol
tar
C
O
S
N
H
ḿ (g/min) Y (%)
6 6
22.27 100
14.5 65.12
7.669 34.43
0.0053 0.0292
0.0648 0.2851
0.0300 0.1347
0.04833 0.04833
0.03711 0.03711
0.00200 0.00200
0.00206 0.00206
0.01050 0.01050
ḿ (g/min) Y (%)
9 9
22.319 100
14.5 64.97
7.669 34.35
0.0079 0.0437
0.0971 0.4266
0.0450 0.2016
0.07249 0.07249
0.05567 0.05567
0.00301 0.00301
0.00308 0.00308
0.01575 0.01575
ḿ (g/min) Y (%)
12 12
22.369 100
14.5 64.83
7.669 34.28
0.0105 0.0582
0.1295 0.5676
0.0600 0.2682
0.09666 0.09666
0.07422 0.07422
0.00401 0.00401
0.00411 0.00411
0.02100 0.02100
ḿ (g/min) Y (%)
15 15
22.419 100
14.5 64.68
7.669 34.20
0.0131 0.0726
0.1618 0.7079
0.0750 0.3345
0.12082 0.12082
0.09278 0.09278
0.00501 0.00501
0.00514 0.00514
0.02625 0.02625
Table 4. Exit Mass Flow Rates for Species and Atoms Atom Flow Rate Obtained from Measures CO, SO2, and O2 (g/min)
Exit unity
tar solution flow (mL/h)
total
CO
SO2
O2
C-solid
O-ads orbed
C
O
S
ḿ (g/min) X (ppm)
6 6
0.09041
0.02741 1419.20
0.00397 90.00
0.02249 1019.00
0.03655
0.00000
0.01178
0.04010
0.00199
ḿ (g/min) X (ppm)
9 9
0.13719
0.06634 3435.50
0.00938 212.48
0.01750 792.96
0.04397
0.00000
0.02853
0.06000
0.00469
ḿ (g/min) X (ppm)
12 12
0.17455
0.07530 3899.50
0.00733 166.13
0.01562 707.83
0.06428
0.01201
0.03238
0.06221
0.00367
ḿ (g/min) X (ppm)
15 15
0.21678
0.07752 4014.50
0.00636 144.00
0.01473 667.65
0.08749
0.03068
0.03333
0.06210
0.00318
from the plasma reactor, but not immediately downstream. The reactor exit was coupled to the tar sampling line, but occasionally the reactor exit gas temperature was obtained (∼340 °C) when the reactor-sampling line coupling was disassembled. Without a regular measurement of gas temper-
molecules, these products are the same that are obtained when destroying and reforming carbon particles through the use of a microwave plasma torch.15,16 4.4. Energy Consumption. During the experiment, the gas temperature was regularly measured immediately upstream 1180
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ACKNOWLEDGMENTS Authors wish to thanks the Aeronautical Institute of Technology (ITA), the Federal University of Para (UFPA), the University of Campinas (UNICAMP), Federal Agency for Support and Evaluation of Graduated Education (CAPESProcad NF), the Polaris, and the National Council for Scientific and Technological Development (CNPq-Project No. 578106/ 2008-6) for allowed grants and support given, as well as doctorate and master fellowships for R.M.E. and B.A.P.C.
ature immediately after the plasma reactor, the energy balance was not performed. Instead, the specific energy consumption was evaluated through the ratio between the inlet electric power and the mass of tar destroyed. Despite experimental observations indicating that tar was destroyed through a microwave effect, further experiments must validate such a conclusion against a thermal effect. The specific energy consumption for a plasma torch can be defined as the ratio between incident power and the destroyed tar mass flow rate (J/kg). Under the best operating conditions during all experiments in this work, 15 mL/h of tar solution (4.5 g/h of tar) was destroyed with 995 W of microwave power consumed, meaning 796 kJ/gtar. This work did not try to reduce power; therefore, it is possible to obtain lower specific energy consumption.
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REFERENCES
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5. CONCLUSIONS Considering the results obtained with Greenline GL8000 (the absence of unburned hydrocarbons) and keeping in mind that the tar measurement system has a lower limit of detection for tar concentration of 0.01 gtar/Nmgas3 and a tar concentration up to 4.2 gtar/Nmgas3 was imposed with no tar concentration detected in the gas downstream from the reactor, the conclusion is that a microwave plasma torch operating at ambient pressure can break all tar molecules into smaller species. Results from the optical spectrum, as well as those from a gas analyzer and calculated in mass balance, drive one to the conclusion that the destroyed tar was converted mainly to CO, O2, solid carbon (C(s)), and probably into H2 formation. Hydrogen gas formation was inferred by comparison between the results provided from the gas analyzer and the mass balance, along with the results shown by optical spectroscopy. Reactant resident time in the plasma region influences the product formation: low residence time favors CO formation and high residence time favors O2. The results shown in this paper can validate the proposed reaction pathways for the tar conversion into proposed smaller species,6 i.e., the microwave plasma source, at atmospheric pressure, is capable of reducing tar to smaller species, such as CO, H2, and C(s). The major drawback of these experiments was the fact that it was not possible to integrally replace the argon by nitrogen. The maximum nitrogen−argon (N/Ar) mass ratio obtained was 1.9. Above this ratio, the torch became unstable and quenching was inevitable. Another drawback was the accumulation of carbon inside the torch reactor quartz tube, preventing the microwave from reaching the gas molecules and also leading to quenching. Perhaps both problems can be solved by increasing water concentration in the gas upstream from the reactor. Such an increase in water doping can be achieved by raising the liquid water content in the tar solution or injecting water steam. It is interesting to remember that the water concentration in this experiment was far below the typical water concentration in the gasifier-eluded gases.
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dx.doi.org/10.1021/ef301399q | Energy Fuels 2013, 27, 1174−1181
