Download - Bioglycerol Conversion in the Reverse Vortex Gas Flow Plasma-Liquid System (Tornado-Type) with Liquid Electrode and Addition of CO2

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American Chemical Science Journal4(1): 105-116, 2014

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Bioglycerol Conversion in the Reverse VortexGas Flow Plasma-Liquid System (Tornado-

Type) with Liquid Electrode and Addition of CO2

V. Ya. Chernyak1*, O. A. Nedybaliuk1, O. V. Solomenko1, E. V. Martysh1,Iu. P. Veremii1, I. I. Fedirchyk1, T. E. Lisitchenko1, L. V. Simonchik2,

V. I. Arhipenko2, A. A. Kirilov2, A. I. Liptuga3, V. P. Demchina4

and S. M. Dragnev5

1Taras Shevchenko National University of Kyiv, Radiophysical Faculty, Prospect Acad.Glushkova 4G, 03022, Kyiv, Ukraine.

2B.I. Stepanov Institute of Physics, National Academy of Sciences, Minsk, Belorussia.3V.E. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of

Ukraine, Kyiv, Ukraine.4Institute of Gas, National Academy of Sciences of Ukraine, Kyiv, Ukraine.

5National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine.

Authors’ contributions

This work has been carried out in collaboration between all authors. All authors read andapproved the final manuscript.

Received 3rd April 2013Accepted 27th August 2013

Published 7th November 2013

ABSTRACT

Bioglycerol (crude glycerine) usually consists of – 35-50% glycerine, 10-20% methanol andsoap mixture and the initial vegetable oil esters. It is a by-product of the biodieselproduction. However, the direct combustion of bioglycerol is dangerous, because thiscombustion has by-product – acrolein (acrylic aldehyde). Acrolein is very toxic andflammable substance. Along with this, bioglycerol can be used to generate syngas. In thispaper describes the bioglycerol reforming, which uses the combined system that includesplasma processing and handling in the pyrolysis chamber. The studied plasma source isthe "tornado" type reverse vortex gas flow plasma-liquid system with a liquid electrode. Theworking gas is a mixture of air and CO2. The bioglycerol or mixture of bioglycerol and

Original Research Article

American Chemical Science Journal, 4(1): 105-116, 2014

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distilled water have been used as the working liquids. The present article describes thestable operation regime: current-voltage characteristics of the discharge, emissionspectrum of the plasma, transmission spectra and composition of gas at the outlet of thesystem. It is shown that the energy efficiency under bioglycerol reforming is higher thanduring the ethanol reforming at the same CO2/air flow ratios in the input gas.

Keywords: Plasma; reforming; bioglycerol; glycerol; discharge; plasma-liquid system.

1. INTRODUCTION

Bioglycerol (crude glycerine) is a by-product of the biodiesel production. Biodiesel isproduced by mixing various vegetable oils and potassium hydroxide KOH. After settlingthere are two stratums in obtained oily mixture: upper layer is biodiesel and lower –bioglycerol. However there is one significant drawback: for every 10 gallons of the producedbiodiesel, roughly, there will be 1-2 gallon of byproduct – bioglycerol. Therefore, the large-scale production of environmentally friendly and renewable fuel can lead to environmentalproblems, due to possible bioglycerol accumulation in large quantities.

Bioglycerol contains ~ 50% glycerine and various impurities, which vary depending on themethod of production. Pure glycerine has a chemical formula C3H5(OH)3. According topreliminary estimates, glycerol can be quite cheap raw material of nonpetroleum origin forfuel production. However, the direct combustion of glycerol is dangerous, because thiscombustion has by-product - acrolein [1]. Acrolein (acrylic aldehyde) C3H4O is classified asvery toxic, flammable substance, a colorless light liquid with an unpleasant sharp acrid smell,easily soluble in water and organic solvents.

Along with this bioglycerol can be used to generate syngas. Syngas (a mixture of H2 andCO) can feed different types of internal combustion engines or gas turbines. It is commonknowledge [2] that addition of the syngas to the fuel improves the combustion efficiency ofsuch mixture: less ignition time, rapid propagation of the combustion wave, burningstabilization, more complete mixture combustion and reduction of dangerous emissions(NOX – f. e.). Besides, the synthesis gas is an important raw material for the variousmaterials and synthetic fuels (synthetic motor oil or methanol) synthesizing [3]. Ideal reactionfor reforming of glycerine (C3H5(OH)3) into syngas is shown below:

C3H5(OH)3 = 3CO + 4H2 + ΔH, ΔH = -3.4 eV/molecule = -330 kJ/mole (1)

Plasma catalysis is one of the promising methods of creating syngas through the conversionof various hydrocarbons [4-6], including glycerol [1]. Plasma is a very powerful source ofactive particles (electrons, ions, radicals etc.), and it can be the catalyst for the variouschemical processes activation. There is a number of electrical discharges for plasmaconversion, which can be used – arc, corona, spark, microwave, radio frequency, barrier etc.One of the most effective discharges for the liquid hydrocarbons plasma treatment is the"tornado" type reverse vortex gas flow plasma-liquid system with a liquid electrode(“TORNADO-LE” PLS) [7]. The main advantages of plasma-liquid systems are – highchemical plasma activity and good plasma-chemical conversions selectivity. It mayguarantee high performance and conversion efficiency at the relatively low powerconsumption. Moreover, those are systems of atmospheric pressure and above, thisincreases their technological advantages. However, bioglycerol has rather large viscosity of1.49 Pa·s, which is thousand times as big as the ethanol and water viscosity. Therefore,


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there may be some complications, which in turn, may rise into the question of effectivereforming possibility itself.

Relatively old problem exists in ecology – carbon dioxide utilization. Many modern energyprojects have difficulties with the large amount of CO2 storage and utilization. And it is alsoknown that the addition of CO2 to plasma during the hydrocarbons reforming may help tocontrol plasma-chemical processes [8]. That is why the objective of this research is to studythe different CO2 amounts influence in the working gas on the plasma-chemical processesduring the bioglycerol conversion.

This research deals with the bioglycerol reforming with usage of the combined system, whichincludes a plasma processing and high-temperature pyrolysis. "Tornado" type reverse vortexgas flow plasma-liquid system with a liquid electrode has been used as a plasma source.

2. EXPERIMENTAL SET UP

The experimental device of the “TORNADO-LE” PLS is shown in Fig. 1. Recently thescheme of this device has been described in details [10,11].

The hydrocarbon used is bioglycerol (mixture of glycerine C3H5(OH)3 – 35-50 %, methanolCH3OH – 10-20 % and various impurities (soap and oil esters), including a set of alkali –KOH) and its aqueous solution. Bioglycerol/water ratio is 9/1. The mixture of airflow and CO2in a wide range of its ratios has been used as working gas. These ratios in the working gashave been changing in the range from pure air to pure CO2 (CO2/airflow = 0/1 ÷ 1/0). Idealreaction for reforming of CH3OH into syngas is shown below:

CH3OH = CO+2H2 + ΔH, ΔH = 0.94 eV/molecule = 91 kJ/mole (2)

Fig. 1. The “TORNADO-LE” PLS


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Plasma components composition and temperatures of the plasma components have beendetermined by the emission spectra. In case of the continuous spectrum, we can determinethe temperature of the particles, which are responsible for this spectrum. It is known that thecontinuous emission of dust particles is well approximated by "black body" spectrum [9]. Thisoperation has been carried out in the standard way: by comparing the experimentallymeasured emission spectra with the calculated spectra of blackbody radiation. Calculationshave been carried out with usage of Planck radiation formula.

Mass spectroscopy (error limits – 10%), gas chromatography (error limits: 1-2%) andinfrared spectrophotometry (error limits – up to 1%) have been used for the analysis of thegas phase substances at the output of the plasma-liquid system.

3. RESULTS AND DISCUSSION

The process of discharge ignition occurs as follows: the chamber is filled with liquid to a fixedlevel - 5 mm above the electrode surface. At the next stage a certain amount of gas flowforms the stationary cone from liquid; the voltage applied between the top flange andelectrode immersed in a liquid starts to increase gradually. When the voltage reaches abreak-out value, a streamer appears for the first time. After that, burning discharge starts inslide of a second, and then voltage decreases and current increases. After a second or two itstabilizes. During this time – static pressure rises inside the chamber from 1 to 1.2 atm. If tomaintain the liquid at the fixed level, then the discharge is quite steady.

Video materials analysis showed that in the case of bioglycerol as a working liquid, the sootfilm is formed on the “plasma-liquid” contact during the discharge combustion. Therefore ithas been decided to add distilled water into bioglycerol to reduce the soot film formation onthe “plasma-liquid” contact. It is well known [1] that the addition of water during reforming ofhydrocarbons reduces the probability of the soot formation. Video analysis showed that inthe case of bioglycerol/water = 9/1 (by volume) mixture as a working liquid – soot film is notformed on the “plasma-liquid” contact during the discharge combustion.

Each electrode (anode or cathode) in “TORNADO-LE” PLS can be solid or liquid. Thecurrent-voltage characteristics of the discharge are shown for the solid cathode (SC) mode(Fig. 2 and Fig. 3).

The "tornado" type reverse vortex gas flow is formed by gas flow, which is a mixture of airwith CO2 in varying proportions. Working liquid has been bioglycerol (Fig. 2) and aqueousbioglycerol with bioglycerol/water volume proportion = 9/1 (Fig. 3). The initial level of theworking liquid is the same in all cases. The current-voltage characteristics show that addinga small amount of CO2 (starting from 5%) to the working gas leads to higher voltage level inthe case of bioglycerol as a working liquid. In case of aqueous bioglycerol as working liquid,no significant changes in the discharge voltage range within the error limits have beenobserved. The increase in the supply voltage level has been observed, while as a workinggas – pure CO2 has been used. Low voltage level occurs because there are a variety ofimpurities in bioglycerol, especially KOH, the presence of which increases the bioglycerolconductivity. That is why the voltage drop reduces across the liquid.


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Fig. 2. Current-voltage characteristics of the discharge at different ratios of CO2/air inthe working gas. Working liquid – bioglycerol. Airflow – 55 and 82.5 cm3/s, the flow of

CO2 – 4.25, 8.5 and 17 cm3/s

Fig. 3. Current-voltage characteristics of the discharge at different ratios of CO2/air inthe working gas. Working liquid – aqueous bioglycerol (bioglycerol/H2O = 9/1 by

volume). Airflow – 55 and 82.5 cm3/s, the flow of CO2 – 4.25, 8.5 and 17 cm3/s

Typical emission spectrum of the plasma in the interelectrode gap of “TORNADO-LE” PLS isshown in Fig. 4 with the bioglycerol as a working liquid. Distance between liquid surface andtop flange is 10 mm.


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Fig. 4. Typical emission spectrum of the plasma in “TORNADO-LE” PLS, where theworking liquid is bioglycerol. Working gas – a mixture CO2/airflow ~ 1/5 (air flow – 82.5

cm3/s, the flow of CO2 – 17 cm3/s), I = 300 mA, U = 600 V

Emission spectrum is normalized to the maximum of potassium (K) line (766.49 nm). Thespectrum lines of potassium (404.41 nm, 404.72 nm, 766.49 nm, 769.89 nm), sodium(588.99 nm, 589.59 nm), and calcium (422.6 nm) are presented and there is a solidcontinuous spectrum. The latter indicates the soot presence in discharge. Intensity ratio of K(766.49 nm, 769.89 nm) and Na (588.99 nm, 589.59 nm) lines is approximately 2.7.Changes of CO2 quantity in the working gas within the error limits did not affect theappearance of the emission spectrum of the interelectrode gap plasma.

The rate of syngas (H2 + CO) formation in the absence of CO2 is ~ 30 cm3/s, but this rateincreased by 1.5 times with CO2 addition (5-17%) and practically doesn’t depend on the CO2higher concentration.

We compared the experimental results with the calculated spectra of blackbody radiation onthe basis of continuous nature of the plasma emission spectra (Fig. 4). Fig. 5 shows theerror span of 200-300 K in the temperature range from 2500 K to 3500 K. The plasmaemission spectrum in case of bioglycerol as a working liquid (airflow – 82.5 cm3/s, CO2 flow –17 cm3/s, CO2/air flow ratio ~ 1/5, current in camera I = 300 mA, voltage U = 600 V) is shownibid. All spectra are normalized to the intensity of line with wavelength of 710 nm.

Data from Fig. 5 indicate that the emission spectrum of the plasma coincides with thecalculated for the temperature of T = 3000 ± 200 K. Calculations have been performed usingthe Planck formula.

The composition of bioglycerol reforming products (gas formed as a result of the reforming)has been investigated with usage of infrared spectrophotometry and mass spectrometry.

Fig. 6 shows the transmission spectra of infrared gas, formed during the bioglycerolreforming in “TORNADO-LE” PLS. Mixture of air (82.5 cm3/s) and CO2 (8.5 cm3/s) has beenused as a working gas; current – 300 mA, voltage – 0.6÷0.8 kV.


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0

0.4

0.8

1.2

1.6

400 500 600 700 800 900 1000

, nm

I, a.u.

I expT=2800KT=3000KT=3200K

Fig. 5. Emission spectrum of the plasma and calculated spectra of blackbodyradiation

Fig. 6. Transmission spectra of gas at the outlet of “TORNADO-LE” PLS. Mode ofoperation – SC

Fig. 7 shows the concentrations of the gas mixture components, namely – CO, CH4, C2H2,which are formed during the bioglycerol reforming, at CO2/airflow dependence ratio inworking gas. We observed the maximum concentrations of the gas mixture components atthe CO2/airflow ratio ~ 1/5, which corresponds to 15% of CO2 admixing. The dependenceof the hydrogen concentration (H2) in the syngas from the CO2 concentration in the workinggas, has been identified by mass spectrometry and is shown in Fig. 7 as well. When theaqueous bioglycerol is used as a working liquid, the mass spectrometry data areapproximately the same (within the error limits).


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Fig. 7. The dependence of the component concentrations in output gas duringbioglycerol reforming in “TORNADO-LE” PLS from the percentage of CO2 in the

working gasStudies of the bioethanol conversion (mixture C2H5OH/H2O = 1/9.5) into syngas in the“TORNADO-LE” PLS at the same currents, airflows, air/CO2 ratio, SC mode have beenconducted in our previous works [10-11]. It should be noted that the hydrogen content insyngas during the bioglycerol reforming has very small proportion. In the case of the otherhydrocarbon – bioethanol, the results are slightly different [10], the CO2 addition results in asignificant increase of H2 content in the output syngas [10]. Distilled water addition results ina slight hydrogen concentration increase.

On the basis of data regarding bioglycerol reforming products composition, the electricalenergy transformation coefficient α has been calculated by the formula:

p

s

QQ

, (3)

where Qs – complete combustion heat of all reforming products, Qp – electric powerconsumption during reforming. The dependence of the electrical energy transformation“TORNADO-LE” PLS at the different CO2/air flow ratios in the output gas during reformingprocess is shown in Fig. 8.

Also, the values α for ethanol reforming [10] at the same CO2/air flow ratios are presentedhere for comparison.

As you can see from the Figs. 7, 8 the dependence of components concentration in theoutput gas and α coefficient from the CO2 concentration in the working gas is nonmonotonic.Maxima of CO and CH4 concentrations correspond concentration of CO2 is about 15%, whilethe coefficient α ≈ 2.5.

It should be emphasized that in this work we are talking about plasma reforming ofhydrocarbons. The results of the conversion of various hydrocarbons into syngas usingplasma reforming and plasma-catalytic reforming are shown in the Table 1 below.


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Table 1. The results of the conversion of various hydrocarbons into syngas using plasma reforming and plasma-catalytic reforming

Type of reforming Reference Hydrocarbons Н2 (%) СО (%) СН4 (%) С2Н4 (%) С2Н2 (%) Н2/СО (arb. unit) α (arb. unit)Plasma reforming - Bioglycerol 3.86 19.14 2.85 0.44 0.20 4.09

- Bioglycerol(CO2-17%)

0 28.43 12.44 1.28 0 2.68

[10] C2H5OH/H2O(1/9.5)

25.51 14.42 0.91 0.47 0.54 1.7 0.81

[11] C2H5OH/H2O(1/9.5)(CO2-17%)

30.98 22.74 3.95 0.4 0.88 1.36 1.48

[12] C2H5OH/H2O(1/0.6)

36 23 1.2 0.8 1.57 1.8

Plasma-catalytic reforming [1] C3H5(OH)3 28.5 14 2.3 0.3 2.04 22-58[13-14] Vegetable oils

C18.1H34.1O2

15-18 4-8 0.5-1 0.1-0.5 0.001-0.005 1.8-4.5 20-37

[15] C2H5OH 19-28 9-22 0.017-6.1 0.002-1.1 0-0.001 1.1-3 18-44[16] C2H5OH 55 12 10 1 4.58 15.8[17-18] CH4 27-32 7-15 4-7 1.8-4.5 33


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Fig. 8. The α coefficient electrical energy transformation dependence from CO2/airflow ratio in “TORNADO-LE” PLS during bioglycerol and bioethanol reforming

processes

The Table 1 shows that the use of plasma-catalytic method for reforming hydrocarbons aremore energy efficient.

4. CONCLUSIONS

Basing on the results in bioglycerol CO2-reforming by "TORNADO-LE" PLS, it can beconcluded that:

1. There is the possibility of hydrocarbons reforming with significant viscosity (such asbioglycerol) in this system; dissolution of bioglycerol even in a small quantity ofdistilled water (10%) reduces the chance of soot formation on the “plasma- liquid”contact in the plasma-liquid system with a hydrocarbon-plasma interaction.

2. It is shown that addition of carbon dioxide into reforming system increases thedischarge voltage level, this effect is especially noticeable, when the PLS works withpure CO2. The CO2 addition doesn’t have a considerable impact on the temperature,specified by the continuous spectrum in the studied range of CO2/air flow ratios.

3. Major components of the bioglycerol conversion products are: CO, CO2, CH4, C2H2,but Н2/CO ratio in syngas is significantly less than unit for almost all values of theCO2/airflow ratios. It should be noted that in the case of other hydrocarbon –bioethanol, the average value of this coefficient in the PLS is 1.7 [10]. Distilled wateraddition leads to a slight increase of hydrogen concentration.

4. There are nonmonotonic dependences on output syngas concentration from theCO2 concentration in the working gas. Maximum corresponds to CO2 concentrationabout 15%, in this case the coefficient α ≈ 2.5.

5. The electrical energy transformation coefficient α under bioglycerol reforming ishigher than during the ethanol reforming at the same CO2/airflow ratios in the inputgas. This may be due to the lower power consumption for plasma generation,


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evoked by high conductivity of bioglycerol. Its reforming products contain mainly COand light hydrocarbons, which gives a significant contribution to this coefficient.

ACKNOWLEDGEMENTS

This work was partially supported by Ministry of Education and Science of Ukraine, NationalAcademy of Sciences of Ukraine, Taras Shevchenko National University of Kyiv.

COMPETING INTERESTS

Authors have declared that no competing interests exist.

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