Chapter 1 Introduction
1.2 S PECIFIC O BJECTIVES OF THIS T HESIS
Motivated by the technical advantages and tremendous advantages of PAC reforming with gliding arc plasma with different catalyst (Rh) and (Ni0.35Mg2.65FeO4.5)
development in this field, the objectives are summarized as followed:
Through various C/O ratios and air flow rates, confirm the efficiency of the
gliding arc (GA) reforming ethanol.
To confirm the efficiency of the noble catalyst (Rh) reforming ethanol by
adjusting various temperatures and air flow rates.
To prove the efficiency of the PAC system with GA plasma and Rh catalyst is
higher than Rh catalyst alone reforming ethanol.
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To learn how to reconstruct the non-noble catalyst (Ni0.35Mg2.65FeO4.5) and
confirm the efficiency of the gliding arc (GA) reforming ethanol through various temperatures.
To construct the PAC system combined the gliding arc plasma with
Ni0.35Mg2.65FeO4.5 catalyst.
To verify the efficiency of PAC system with GA plasma and Ni0.35Mg2.65FeO4.5
catalyst is higher catalyst alone reforming ethanol.
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Chapter 2
Theoretical Method
2.1 Theoretical Analysis
2.1.1 The Physical Phenomenon of Gliding-Arc
The gliding arc, a discharge contains thermal and non-thermal properties, could effortlessly by two diverging knife shaped electrodes with high voltage and input power.
The gliding arc starts at the smallest gap between two electrodes when the electrode field reaches breakdown conditions, for stance, an gliding arc reactor with the shortest gap about 3mm between electrodes in the atmosphere pressure, needs 10 kV to breakdown the gas, air, forming arc. And then the flow along the electrodes push the arc until the electron density could not maintain the plasma caused gliding arc is split. Finally, the next cycle starts immediately after the breakdown conditions reach. A typical gliding arc reactor has been published [A. Fridman et al., 2002]. Figure 2.1 is the gliding arc reactor and the electric scheme with DC power supply and gas inlet. And, it is very important to clearly see and measure the fluctuation of current and voltage by gliding arc production in researching plasma physical phenomenon. Investigation on the electric measurement of gliding arc discharge has been published by I. Antonius [Antonius I. et al., 2006].
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Figure 2.2 is arc current and voltage waveform with ac power supply and shows the arc
ignition with rising instantly in the current waveform of arc breakdown.
The gliding arc has been called “intermediate” plasma, having both thermal plasma and non-thermal plasma character, which offers greater energy density to perform hydrogen reforming with chemical selectivity.
2.1.2 Chemical Reaction Paths of Ethanol Reforming
Nowadays, hydrogen production in the industrial procedure is ‘‘steam reforming’’
which is widely used all over the world, however the main drawback of this procedure is that it needs an external heat source to support reforming reaction. Furthermore, there was another hydrogen procedure, partial oxidation had been developed.
In partial oxidation reaction, ethanol reacting with insufficient oxygen generates a slightly endothermic reaction to reform ethanol into hydrogen. However, using this method lowers the performance and brings soot left caused poison of catalyst [A. Fridman
et al., 2004].
For the sake of solving these problems, combine steam reforming, water gas shift reaction and partial oxidation. Ethanol can be reformed in an ‘‘auto-thermal’’
[J.H. Wang et al., 2009] process kept both advantages. The chemical reaction showed as followed:
Steam Reforming:
C2H5OH(g) + H2O(g) 2CO + 4H2, ΔH298= 254.7 kJ/mol
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2.1.3 Definition of Several Performance Parameters
Conversion rate and selectivity are three important factors to represent the reforming performance. Each of them represents different meanings about reforming product gas composition and formula to calculate the parameter is listed below. The fuel conversion rate means how much percentage of fuel that were injected into the PAC system have been reformed into other kinds of product; the hydrogen selectivity means how much percentage of hydrogen were formed among all the formed product that contains H atoms. (Similarly, CO selectivity and CO2 selectivity represent same idea, if there were other formed product gases); the reformer thermal efficiency represents the proportion of the lower heating value (LHV) of formed hydrogen to the input energy, that is the summation of the electrical energy of the plasma and the LHV of the hydrocarbon injected [G. Petitpas et al., 2009].
The methods of data analysis are shown as following Equations.
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2 2
Amount of H atoms in the formed H Selectivity (H )
Amount of H atoms in the formed product
(9)[Input fuel] - [Output fuel]
Fuel Conversion Rate ( )
[Input fuel]
(10)2 produced 2
(H + CO) x LHV(H ) Efficiency =
Input plasma energy + fuel injected x LHV (Fuel) (11)
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Chapter 3
Experimental Methods
3.1 Overview of Experimental Setup
Fig 3.1 shows a schematic diagram of the experimental arrangement. It consists of a
fuel source zone (ethanol/water mixture and air), a preheating zone, a gliding-arc plasma zone, and a catalyst reactor. The steaming fuel flow was first preheated by a heating
furnace. Then, the flow passed through the plasma reactor and finally the catalyst reactor, with the furnace temperature maintained at 380 C. After reforming in the catalyst bed, the
gases were sampled using gas chromatography (GC; YL 6100GC, Young Lin Instrument Co., Ltd Figure 3.2) with a pulsed discharge helium ionization mode detector (PDHID). In transporting the gases from the furnace to the gas chromatography equipment, a 2-m heated tube was used to prevent the gases from condensing on the tube wall. The sampled gases were analyzed with a mass-balance error of less than 10%. The experimental configurations and operating conditions of each subsystem are described in detail in the following next sections.
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3.2 Experimental Facilities
In this section, the contents particularly give an account for experimental facilities, including plasma reactor which is divided into gliding-arc and magnetic gliding-arc discharge; ac power supply; fuel feeding; and catalyst preparation.
3.2.1 Plasma Reactor
Figure 3.3 shows the sketch of an in-house designed gliding arc reactor. This reactor
consists of two 30 mm long, 7 mm wide and 2 mm thick knife-shaped electrodes fixed on a Peak bed plate, which can sustain at temperature up to 315 ℃. The electrodes of gliding arc are made of stainless steel. A quartz tube with inner diameter 22 mm and 55 mm long is inserted and well-sealed with the Peak bed plate. The ethanol steam flows from the bottom and passes through the reactor. By applying the voltages across the electrodes, arc starts at the location of smallest gaps between the two electrodes. Arc is carried downstream by the air, and then becomes weaker and weaker because of smaller electric field (larger gap) and finally extinguishes. The next cycle of arc starts immediately after the breakdown condition reached at the throat.
3.2.2 AC Power Supply and Pulse Generator
The power supply (PVM500 plasma driver, Information Unlimited Inc. Figure 3.4) supplies the gliding arc plasma with high voltage and frequency. It has independent
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voltage control ranges from zero to maximum 20 kV peak-to-peak. And frequency ranges from 20-70 kHz. Though, this power supply is relatively unstable compare to others, its price was low ($449.95). In the next chapter, the influence of different output power magnitude on electrical character and reforming efficiency was studied in detail.
3.2.3 Fuel Feeding and Heating System
Ethanol and water, supplied by a liquid pump (930d-1428, Young Lin Instrument CO., Ltd., 0.0005-1.0 SLM, Figure 3.5), were mixed with dry air from a compressor controlled by a mass flow controller (Multi-gas MFC, MC-100SCCM-D, Alicat Scientific Inc., max.
100 sccm,
Figure 3.5). The gases were injected into the preheating zone (to raise the
temperature) through an oil injector that was taken from a car engine and controlled by a solenoid valve (Fuel Injection, Mitsubishi Eclipe,figure 3.6). The solenoid valve was
operated with a voltage of 12 volts, and the duty cycle could be adjusted to control the valve opening time. This injector was adjusted to maximize the vaporization of water and ethanol droplets by varying the duty cycle of the input driving voltage. Through this setup, one can control experimental parameters such as, the C/O mole ratio, air flow rate, and ethanol/water mole ratio. C/O (mole) ratio represents the ratio of the moles of C in the inflowing ethanol to the moles of O in the inflowing mixture that consisted of ethanol, water and air. High-purity ethanol (99%) was used as the fuel and was mixed together uniformly using an ultrasonic wave mixer before injection by the liquid pump.20
The heating system consists of two heating furnaces that were heated at specific temperatures to control the gasification and catalyst reaction (You & Me Inc.,
Figure 3.7).
3.2.4 Catalyst Preparation
Thanks to professor Lee, Department of Applied Chemistry, NCTU for sharing the chemicals and skills. All the catalyst setup procedures are learned from professor Lee’s laboratory.
3.2.4.1 Rh/CeO
2/Al
2O
3The reforming catalyst beads were prepared using 5% Rh/CeO2/Al2O3. Initially, 1 g of porous Al2O3 beads to be used as carriers were ground to small pieces, with a volume in the range of 1.00-1.41 mm3. These were subsequently mixed with an ethanol solution containing 0.125 g of dissolved Ce(NO3)3 and heated to 50 C to evaporate the ethanol.
Thereafter, the Al2O3 carriers loaded with Ce(NO3)3 were sintered at 300 C for 5 hours.
In addition, 0.01 g of RhCl3 in an ethanol solution was prepared and mixed with 10 % Ce-Al2O3 after sintering. Following the same procedure for evaporating the ethanol, the 10% Ce-Al2O3 loaded with 5% RhCl3 was placed into the furnace at 600
C and 200
sccm of hydrogen was pumped for 6 hours for the reduction. In addition, 1 g of porous Al2O3 loaded with 5% Rh/CeO2 could be produced using the above procedure. The procedure is presented in Figure 3.8.21
3.2.4.2 Ni
0.35Mg
2.65FeO
4.5/Al
2O
3The preparation for Ni/Mg/Al catalyst is done by coprecipitation method. First, the mass of Ni(NO3)3, Mg(NO3)2, and Fe(NO3)3, which respectively are 0.2139 g, 1.4485 g, and 0.8484 g, is changed the PH from acidity to 10.5 by titration using the alkaline liquid mixed the NaOH of 6 g with the Na2CO3 0.99375 g. And then dry the product made by above the elements to form the important admixture, Ni0.35Mg2.65FeO4.5. To pestle the admixture with 95 Vol.% ethanol and let the admixture can be easily coat in the carrier, Al2O3, in the specific ratio of mass weight of Ni0.35Mg2.65FeO4.5/Al2O3 is 0.1 . Finally, the reduction furance, which specially designed to solve the hydrogen waste by traditional reduction method, is used to reduce the 10 wt% Ni0.35Mg2.65FeO4.5/Al2O3 at 500 ℃ furance keeping 30 min
(figure 3.9). The figure 3.9 is simple flowchart for preparing this
catalyst.3.3 Experimental Instrumentation
In the plasma and plasma assisted catalyst reforming, the power supply (PVM500 plasma driver) system which has the voltage control ranging from zero to maximum 20 kV peak-to-peak at a frequency of 20 kHz. Meanwhile, we use the High Voltage Probe
(Fig 3.10) to measure plasma voltage and Rogowski coil (Fig 3.11) to measure the current
respectively. The measured voltage and current are recorded using an oscilloscope (Tektronix TDS1012B Fig 3.11). In the following, several key components of the plasma
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assisted catalytic system are introduced in turn. After reforming process, the productions are measured and sampled by gas chromatography (Figure 3.2).
3.4 Experimental Procedures 3.4.1 Catalytic Reforming
In catalytic reforming, the steaming fuel was first preheated by a heating furnace.
Then, the flow, including air, ethanol, and DI water, passed through catalysis bed. Finally, the gases were sampled using gas chromatography.
3.4.2 Plasma Reforming
In plasma reforming, the ethanol mixed water was injected by a liquid pump and preheated by a heating furnace respectively. After the fuel consisted of air and ethanol mixed water went through the plasma zone, we used the chromatography to sample the productive gases.
3.4.3 Plasma Assisted Catalytic Reforming
In Plasma Assisted Catalytic Reforming, we used the air pump and liquid pump to inject the air and ethanol mixed water respectively. Then, the fuel went through the plasma zone and finally the catalyst reactor. The productive gases from reforming were sampled by gas chromatography.
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3.5 Test Conditions
In this study, air flow rates were controlled and maintained in the range of 0.5-2.0 slm, the inflow C/O ratio was fixed at 0.7 unless otherwise specified, and the molar ratio of ethanol to Deionized water (DI) water was kept at 1:3 (the flow rate of ethanol/DI water
mixture was 0.57-1.7 sccm). Gasification temperature in the preheat zone and the furnace temperature for the catalyst bed were kept at 160 C and 230 C, respectively. The power
input for the gliding-arc device was fixed as 223 W throughout the study.
The test conditions for the gliding arc plasma, catalyst (Rh/CeO2/Al2O3), and PAC with gliding arc reforming are summarized in
table 3.1. And the test conditions for
catalyst (Ni0.35Mg2.65FeO4.5) are summarized in table 3.2.24
Chapter 4
Characterization of Gliding Arc Plasma
4.1Visualization
The figure 4.1 shows the typical images of the gliding arc discharge using air flow rate in 1.5 SLM and 200 w with 20 kHz, supplied by AC power. And the
figure 4.2
shows the comparison between the gliding-arc plasma injecting ethanol mixed water or not. Obviosuly, the colors and length of blazes are distinct from the the addition of fuel at a C/O ratio of 0.7. The above images was captured by High Speed Camera with the obtainable plasma absorption power is 223 W.measured time
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Chapter 5
Results and Discussion
5.1 Reforming with Gliding Arc Plasma 5.1.1 Effect of C/O ratio
Figure 5.1
shows the variation in the conversion rate, H2 selectivity and CO2selectivity with changes to the C/O ratio at an air flow rate of 1.5 SLM. When the C/O ratio is 0.3, the conversion rate is at its highest; however, the H2 selectivity is at its lowest.
When the C/O ratio becomes 0.7, the H2 selectivity reaches its maximum (43%) and the conversion rate decreases to 35%, which represents the best reforming performance when using plasma alone. Fortunately, this optimized condition (C/O ratio of 0.7) also coincides with optimal Rh catalyst reforming, which was used in the current study
[Wang Q et al., 2009]. For this reason, all the PAC experiments were conducted at a C/O
ratio of 0.7.
5.1.2 Effect of Gas Flow Rate
Figure 5.2
shows the selectivity of hydrogen and carbon dioxide and the fuel conversion rate as functions of the air flow rate. The fuel conversion rate is generally less than 40% and less than 30% at the maximal air flow rate (2.0 SLM). The dramatic26
decrease in the fuel conversion rate at 2.0 SLM can be attributed to the short residence time in the plasma reactor because of the high gas flow rate. In addition, the hydrogen selectivity and the fuel conversion rate both peak at an air flow rate of 1.5 SLM [Rueangjitt
N. et al., 2011, Chernyak V.Y. et al., 2008].
Figure 5.3 shows the H-selectivity of methane, water and hydrogen, each as a
function of the air flow rate. There is nearly no methane produced when the air flow rate is larger than 0.5 SLM because of the shorter residence time of steaming ethanol in the plasma reactor. The main products that contain H atoms are hydrogen and water vapor.
The highest H-selectivity of hydrogen is 43% and the lowest H-selectivity of water vapor is 57% at an air flow rate of 1.5 SLM.
Figure 5.4 shows the C-selectivity of methane, carbon monoxide and carbon dioxide,
each as a function of the air flow rate. The highest C-selectivity of carbon monoxide is 75%, and the C-selectivity is 25% at an air flow rate of 1.0 SLM. This shows that a large amount of carbon monoxide can be produced at all of the air flow rates considered in this study.
5.2 Catalyst Reforming
In this section, investigation on conversion rates, hydrogen and carbon dioxide selectivity of catalyst reforming is divided into two parts to discuss the results. First part is the catalyst reforming with Rh noble catalyst to confirm the selectivity and conversion
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at different air flow rates (0.5-2.0 SLM). And the second part is the catalyst reforming
with Ni0.35Mg2.65FeO4.5/Al2O3 to confirm the selectivity and conversion at different temperatures (200-400 ℃).
5.2.1 Rh
Figure 5.5 shows the conversion rates, hydrogen selectivity and carbon dioxide
selectivity at different air flow rates with the C/O ratio maintained at 0.7. At an air flow rate of 1.0 SLM, Hydrogen selectivity (SH2) is found to be more than 100 % and the conversion rate is nearly 100% (98%). However, when the flow rate increases to 1.5 SLM or 2 SLM, the reforming efficiency becomes worse (~70%). This can be attributed to the shorter residence time during which the catalyst cannot thoroughly react with the gases. However, at an air flow rate of 0.5 SLM, even though the residence time is the longest, the catalyst reforming performance is not better than at air flow rates of 1.0 or 1.5 SLM. This results from the catalyst reforming becoming favorable to the complete oxidation reaction (C2H5OH + 3O2 → 2CO2 + 3H2O) [Wang Q et al., 2009], which prevents the production of hydrogen. The above observations show that higher reforming efficiencies can be obtained only at specific air flow rates using the catalyst bed with 5%
Rh/CeO2/Al2O3, which necessitates a remedial approach for extending the applicability of ethanol reforming, such as plasma-assisted catalyst reforming.
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5.2.2 Ni0.35Mg2.65FeO4.5/Al
2O
3Figure 5.6
shows the conversion rate, hydrogen selectivity and car dioxide selectivity at different temperatures. Obviously, the hydrogen selectivity and ethanol conversion rates of Ni0.35Mg2.65FeO4.5/Al2O3 catalyst (non-noble) are much lower than the Rh catalyst (noble). In the section 1.1.2 has introduced that the efficiency of noble catalyst almost higher than non-noble catalyst. Therefore, it is no surprise to the results.Figure 5.6 also shows the hydrogen and selectivity dropped down with adjusting
decreasingly catalyst temperatures. However, the catalyst temperatures stand for the energy provided. Furthermore, the effect of catalyst temperature has been presented at the IJHE paper
[M. Li and et al., 2010]. The catalyst is much more inefficient at
water-gas-shift chemical reaction.5.3 Plasma Assisted Catalyst (PAC) Reforming
In this section, investigation on hydrogen and carbon dioxide selectivity and conversion rates of PAC reforming is divided into two parts to discuss the results. Owing to compare the catalyst reforming with PAC reforming, the test parameters of catalyst needed to be the same. Therefore, first part is the PAC reforming with Rh noble catalyst and gliding arc plasma to confirm the selectivity and conversion rates at different air flow rates (0.5-2.0 SLM). And the second part is the PAC reforming with
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Ni0.35Mg2.65FeO4.5/Al2O3 and gliding arc plasma to confirm phenomenon at the temperatures (400 ℃) which arise the highest selectivity and conversion rate in catalyst
reforming.
5.3.1 PAC reforming with Rh catalyst
Figure 5.7 shows the conversion rates, hydrogen selectivity and carbon dioxide
selectivity at different air flow rates for different cases of PAC. At an air flow rate of 0.5 SLM, the hydrogen selectivity and the conversion rate of PAC are 72% and 98%, respectively, which are lower than those for the pure catalyst reforming. This could be attributed to the longer residence time in the plasma reactor at the lowest air flow rate, which produces smaller molecules such as C2H4 and C2H6 (see
Table 5.1) that are
measured by gas chromatography, resulting in smaller hydrogen selectivity.Notably, Figure 5.7 also shows that the fuel conversion rate and hydrogen selectivity for the PAC case are high, at 100% and 111.2%, respectively, at an air flow rate of 1.5 SLM.
Without the addition of plasma, the hydrogen selectivity is very low (~70%) at this air flow rate because of the shorter residence time, as explained earlier and also in the literature [Barbara Pietruszka, 2004, Yu Chao, 2008]. From Figure 5.2, we have learned that the hydrogen selectivity the highest with plasma alone at this air flow rate.
Furthermore,
Figure 5.3 also demonstrates that plasma reforming produces appreciable
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CO, which is able to assist the catalyst reforming through the water-gas-shift reaction (CO + H2O → CO2 + H2), which was also reported in [Wang Q et al., 2009].
5.3.2 PAC reforming with Ni
0.35Mg
2.65FeO
4.5Catalyst
If the hydrogen selectivity and conversion rates of non-noble catalyst could be raised to close the noble catalyst, the economic benefit would be elevatory causing the reforming cost dropped down rapidly. However, the results in figure 5.8 and figure 5.9 show that, even though the PAC system with Ni0.35Mg2.65FeO4.5 catalyst could improve the hydrogen selectivity and conversion rate, the outcome is much worse than with the Rh catalyst. The reason perhaps is that this catalyst has the inefficiently water-gas-shift chemical reaction with gliding arc generated much H2O (figure 5.3). Therefore, The suggestion, decreased the ratio of ethanol mixed water and inject lesser DI water to PAC reforming with gliding arc and Ni0.35Mg2.65FeO4.5 catalyst, should be published for the future work.
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Chapter 6
Conclusion and Future Work
6.1 Conclusion
In this study, ethanol steam flow is reformed using a gliding-arc plasma-assisted catalytic system with air flow rates in the range of 0.5-2.0 SLM. The results show that, with the catalyst alone, a 100% conversion rate and a maximum of 115% hydrogen selectivity were obtained at a C/O ratio of 0.7 with an air flow rate of 1.0 SLM. However, the hydrogen selectivity decreases rapidly to 95%, 70% and 68%, respectively, at air flow rates of 0.5, 1.5 and 2.0 SLM. The former can be attributed to the longer residence time in the catalytic bed, causing a higher temperature that favors a complete oxidation reaction (C2H5OH + 3O2 → 2CO2 + 3H2O). The latter two cases can be attributed to the shorter residence time, which is not enough time for a complete catalytic reaction to occur. With the addition of the gliding-arc plasma, the hydrogen selectivity reaches 113% and 111.2%
at air flow rates of 1.0 and 1.5 SLM, respectively. This shows that, with the use of the gliding-arc plasma prior to the catalyst, a very high hydrogen selectivity (>110%) can be obtained at air flow rates of 1.0 or 1.5 SLM with the current experimental setup. However,
at air flow rates of 1.0 and 1.5 SLM, respectively. This shows that, with the use of the gliding-arc plasma prior to the catalyst, a very high hydrogen selectivity (>110%) can be obtained at air flow rates of 1.0 or 1.5 SLM with the current experimental setup. However,