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Hydrogen and syngas production from glycerol through microwave plasma gasification
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 8 ( 2 0 1 3 ) 1 4 5 5 9 e1 4 5 6 7
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Hydrogen and syngas production from glycerol through microwave plasma gasification Sang Jun Yoon a,b, Young Min Yun b, Myung Won Seo a, Yong Ku Kim a, Ho Won Ra a, Jae-Goo Lee a,* a
Climate Change Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea b Advanced Energy Technology Department, University of Science and Technology, 217 Gajungro, Yuseong-gu, Daejeon, Republic of Korea
article info
abstract
Article history:
Glycerol which is a byproduct of biodiesel production is considered as a potential feedstock
Received 3 May 2013
for syngas production with the increase of biodiesel demand. In this study, the charac-
Received in revised form
teristics of glycerol gasification under a microwave plasma torch with varying oxygen and
20 August 2013
steam supply conditions were investigated. The experimental results demonstrated that
Accepted 1 September 2013
the gasification efficiency and syngas heating value increased with the supplied microwave
Available online 3 October 2013
power while the increase of oxygen and steam led to a lower gasification performance. In order to achieve high carbon conversion and cold gas efficiency in the microwave plasma
Keywords:
gasification of glycerol, the O2/fuel ratio should be maintained at 0e0.4. It was revealed that
Microwave plasma
the fuel droplet size and the mixing effect and retention time inside the plasma flames are
Gasification
critical factors that influence the product gas yield and gasification efficiency. This study
Hydrogen
verified that syngas with a high content of H2 and CO could be effectively produced from
Syngas
glycerol through microwave plasma gasification.
Glycerol
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
There is increasing interest in the use of new and renewable energy sources due to limited fossil fuel reserves, oil price fluctuations, and international regulations on CO2 emissions. Studies on biomass, which is a CO2 neutral source, are being actively conducted not only regarding its primary use of obtaining heat and electricity through combustion, but also regarding its high value-added use through conversion to transportation fuels such as biodiesel that can replace petroleum. The global production of biodiesel has increased annually by 32.5% on average from 2000 to 2010 [1]. In particular, biodiesel production has increased sharply since
2006. The production of biodiesel is expected to grow continually in the United States and the European Union in order to attain the goal of replacing 20% and 30% of petroleum-based diesel with biofuels by 2020 and 2030, respectively [2]. The method of using transesterification reaction to produce biodiesel, which is currently the primary method of biodiesel production, generates crude glycerol of approximately 10 wt% as a byproduct. As the generation of crude glycerol is also expected to grow with the increased production of biodiesel, the effective use of crude glycerol is important from both economic and environmental perspectives. For utilization of crude glycerol, many studies are being conducted on the conversion of glycerol into valuable
* Corresponding author. Tel.: þ82 42 8603353; fax: þ82 42 8603134. E-mail address: [email protected] (J.-G. Lee). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.001
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components or energy through various techniques, such as refinement, purification, pyrolysis, reforming, and gasification. Among these techniques, energy conversion through gasification has advantages in that it does not require catalyst or additional pretreatment of the crude glycerol [3]. Gasification refers to the technique of generating syngas that contains H2 and CO through the partial oxidation of hydrocarbon fuels. As a clean fuel utilization technique, gasification is being applied to the production of electricity, hydrogen, and chemical materials using various fuels, such as coals, waste, and biomass. These days, various studies on gasification using plasma torches are in progress [4e6]. For plasma gasification, plasma flames are formed using external electric energy sources and the fuels are gasified through plasma flames with high temperatures over several thousand degrees [7]. Thus, it does not require oxygen or it requires only a small amount of oxygen, which is required in the conventional gasification process for the partial oxidation of fuels in order to maintain the reaction temperature. This decreases the burden of the installation and operation of highly expensive air separation units. Unlike the conventional gasification method that raises the temperature of the reactor through preheating, plasma gasification starts simultaneously with the activation of a high temperature plasma torch. Therefore, rapid production of syngas is possible. Furthermore, the high concentration of active species, such as ions and radicals, in the plasma state accelerates the gasification reaction [8,9]. As the reaction is activated, a quick reaction can be induced during the short retention time of fuels in the reactor, and the size of the total process can be reduced due to the decreased volume of the reactor. Most previous studies have focused on the plasma combustion [9], pyrolysis [10], and gasification [11,12] of various fuels using arc electrodes. The use of arc electrodes, however, requires periodic replacement due to the loss of electrodes and is vulnerable to the oxygen and steam that are used as gasification agents [9,13]. The use of microwaves as the energy source for
plasma generation exhibits a higher power transfer efficiency than arc plasma and a high durability because electrodes are not used [14,15]. Therefore, the reforming and gasification of various hydrocarbons using microwave plasma torches are being actively investigated [16e18]. In this study, the gasification characteristics of glycerol under various conditions using a plasma torch equipped with a 2 kW microwave power generator was investigated. For the uniformity of fuels and convenient interpretation of the gasification characteristics, pure glycerol was used instead of crude glycerol. Nitrogen was used as the plasma forming gas, and the microwave power for plasma formation was adjusted to 1e2 kW. The variations of the gasification characteristics were studied with the supply amount of oxygen and steam, which were used as gasification agents. At this time, the O2/fuel ratio and steam/fuel ratio are varied 0e1.2 and 0e2.4, respectively. Furthermore, the changes in the product gas composition and gasification efficiency were examined when the glycerol was atomized through the reactor using a spray nozzle.
2.
Experimental
In this study, 99% pure glycerol (Duksan Chemical) was used as a substitute for crude glycerol without further purification. Fig. 1 presents a schematic diagram of the plasma gasification system with the 2 kW microwave generator (2.45 GHz, SM745, Richardson Electronics) that was used in this study. This system consists of a glycerol preheater and feeder, steam supplier, gasification agent and plasma forming gas feed unit, microwave generator, gasification reactor, gas purifier and analyzer, and data collection unit. The glycerol was supplied to the reactor constantly at a rate of 3 g/min through a gear pump (Cole-Parmer, 74014-75). In order to supply glycerol to the plasma reactor, two methods were used and compared:
Fig. 1 e Schematic diagram of the microwave plasma glycerol gasification system.
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the use of a stainless steel tube with an outer diameter of 0.32 cm and the use of an external mixing nozzle as shown in Fig. 2. To improve the fluidity and spray stability of the glycerol, it was preheated to 140 C with a heater before it was supplied to the nozzle and reactor. When using the nozzle, N2 was used as the spray gas for atomization of the glycerol and its flow was adjusted to 2.0e3.5 L/min using a mass flow controller (MFC, Brooks 5850). Nitrogen was used as the plasma forming gas, which was supplied at 15 L/min through a MFC. Oxygen was used as the gasification oxidizer, which was quantitatively supplied at 0e2.6 L/min using a MFC according to the experimental conditions. The steam that was used as the gasification agent was supplied to the reactor at 0e7.2 ml/min using a syringe pump and the feed line was maintained above 100 C using a band heater so that the steam would be supplied to the reactor. The plasma forming gas, oxygen, and steam were supplied to the reactor with a diameter of 2.54 cm via a swirl-flow in order to increase the retention time in the reactor and to allow stable formation of the plasma flames while reducing the thermal shock of the reactor. The glycerol was supplied in the reactor 2 cm away from the point where the forming gas and microwave met and formed plasma flames. To measure the temperature at each reactor location, two R-type and five K-type thermocouples with an accuracy of 0.1 C were installed in 5 cm intervals from the glycerol feeding location. To remove the unburned carbons, ashes, and moisture from the syngas generated by the plasma gasification reaction, the syngas was passed through a cyclone and filter before it was supplied to the gas chromatograph (GC, HP 6890) for quantitative and qualitative analyses in real time. A thermal conductivity detector (TCD, Carbosphere 80/100 Packed column, Alltech) was used to analyze the product gases, such as H2, CO, CO2, CH4, and N2. These product gases were quantitatively and qualitatively calibrated using standard gases with various compositions and concentrations prior to use in the experiments. Every experiment was performed at least twice in atmospheric conditions, and the results were averaged. The relative standard deviations were less than 5%.
3.
Results and discussion
3.1.
Glycerol feed using tube
3.1.1.
Effect of the O2/fuel ratio
Fig. 3 shows the composition changes of the syngas generated through the microwave plasma gasification according to the changing O2/fuel ratio when the glycerol was supplied to the reactor through a tube. The steam/fuel ratio and microwave power were maintained constant at 0.8 and 1.6 kW, respectively. When the glycerol was supplied through a tube, it fell continuously as droplets and reacted in the plasma flames. The results showed that as the O2/fuel ratio
h ð%Þ ¼
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increased, the H2 content in the syngas linearly decreased. The CO content remained constant at 35% until an O2/fuel ratio of 0.6 and then it decreased. The CO2 content increased with the O2/fuel ratio and began to rapidly increase when the O2/fuel ratio exceeded 0.6. The CH4 exhibited a low content in the syngas at 2% or lower and tended to decrease as the O2/ fuel ratio increased. This indicates that as the O2/fuel ratio increased, the combustion reaction was activated, which increased the CO2 content. Furthermore, the combustion reaction following the increase of the O2/fuel ratio increased the reactor temperature and the CH4 content in the syngas decreased due to the decomposition. These changes of the syngas composition can be divided into three categories according to the O2/fuel ratio. The O2/fuel ratio range from 0 to 0.3 was where the primary water gas reaction in Eq. (1) and the secondary water gas reaction in Eq. (2) were most activated. Therefore, a high content of H2 and CO was exhibited in the syngas. The O2/fuel ratio range from 0.3 to 0.8 was where various gasification reactions, such as combustion and the water gas shift reaction, occurred with the reactions in Eqs. (1) and (2). Finally, the O2/fuel ratio range above 0.8 was where the complete combustion reaction was dominant [19,20]. For this reason, a large CO2 content was detected with small quantities of H2 and CO in the syngas. C þ H2O 4 CO þ H2, DH ¼ 131.3 kJ/mol
(1)
C þ 2H2O 4 CO2 þ 2H2, DH ¼ 90.2 kJ/mol
(2)
These changes in the syngas composition according to the O2/fuel ratio were similar to those in the conventional gasification, which does not use a plasma torch [21e23]. That is, the reaction mechanism that occurred in the plasma gasification is similar to that of the conventional gasification. However, unlike the traditional gasification method that maintains the reaction temperature by partial oxidation of the fuel, the plasma gasification that uses high temperature plasma flames formed by electric energy as the gasification heat source can be operated without an oxygen supply. When an oxygen supply was not present and the O2/fuel ratio was 0, the composition of the plasma gasification product gas of glycerol was H2 57%, CO 35%, CO2 6%, and CH4 2%. Hence, the plasma gasification of glycerol could produce syngas with a low CO2 content and a high content of combustible gases. Fig. 4 shows the variation of the syngas heating value, carbon conversion, and cold gas efficiency during the microwave plasma gasification of glycerol according to the O2/fuel ratio. The carbon conversion (Xc) and cold gas efficiency (h), which is the thermodynamic efficiency indicator of the gasification process, are defined as follows: Xc ð%Þ ¼
Mass flow of carbon in the syngas 100 Mass flow of carbon in the feedstock
Mass flow rate of syngas higher heating value of syngas 100 Mass flow rate of feedstock higher heating value of feedstock
(3)
(4)
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100
4000
80 3000
60 2000 40 Carbon conversion Cold gas efficiency Syngas heating value
20
1000
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Syngas heating value (kcal/Nm3)
Carbon conversion and cold gas efficiency (%)
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0 1.4
O2/fuel
Fig. 4 e Effect of the O2/fuel ratio on syngas heating value, carbon conversion, and cold gas efficiency (feeding of glycerol using tube). Fig. 2 e Schematic diagram of the gaseliquid nozzle for glycerol spray.
As the O2/fuel ratio increased, the carbon conversion increased while the syngas heating value and cold gas efficiency decreased. As shown through the syngas composition changes in Fig. 3, as the O2/fuel ratio increased, the oxygen supply to the reactor increased and the combustion reaction was activated, which in turn increased the CO2 production and carbon conversion. However, the decreased content of combustible gases in the product gas sharply decreased the syngas heating value as well as the product gas yield, thereby rapidly decreasing the cold gas efficiency. The oxygen supply in the gasification reactor activates the combustion reaction of the fuel and simultaneously increases the reactor temperature. Furthermore, the rising reactor temperature decreases the retention time of the fuel and reactants in the gasifier due to the increased gas velocity in the reactor. In the conventional gasification process, which does not use a plasma flame, as the oxygen supply to the reactor increases, the carbon conversion generally increases due to
Syngas composition (vol %, dry, N 2 free)
100 90
H
80
CO CO
70
CH
60 50 40 30 20 10 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
O2/fuel
Fig. 3 e Effect of the O2/fuel ratio on syngas composition (feeding of glycerol using tube).
the increased product gas yield after the fuel reaction and to the increased production of CO and CO2. The cold gas efficiency tends to increase with an increased amount of product gas due to the rising reactor temperature until a certain oxygen supply, after which, due to the activated combustion reaction and the decreased content of combustible gases such as H2 and CH4 in the syngas, the syngas heating value and cold gas efficiency in turn decreases [22e25]. This suggests that with the conventional gasification, the rising reactor temperature that results from the oxygen supply has a greater effect on the fuel conversion than on the decreased retention time in the reactor. On the other hand, with the plasma gasification in this study, the increase of the O2/fuel ratio increased the carbon conversion and decreased the cold gas efficiency. Because plasma gasification uses the high temperature of the plasma flame, unlike the conventional gasification, the reaction of feedstock in the plasma flame is very important. An increase in the oxygen supply increases the reaction temperature, but as mentioned above, it also increases the flow velocity in the reactor with the same volume, thereby decreasing the retention time of the reactants in the plasma flames. Therefore, in plasma gasification, in which the contact and retention time in high temperature plasma flames is important, the increase in the oxygen supply decreased the cold gas efficiency. Compared with the gasification of coals using microwave plasmas as presented in the literature, the plasma gasification results of glycerol had higher carbon conversion and cold gas efficiency [9,26]. This is because glycerol, which consists of volatile matter with good reactivity even at low temperatures, can be gasified more easily than coals that contain fixed carbon with lower reactivity, which reacts at high temperatures. When compared with the gasification of crude glycerol in an entrained flow gasifier that does not use plasma, plasma gasification had a different tendency in gasification efficiency variation, although there was a difference in the process scale. For the entrained flow gasification, the carbon conversion increased as the O2/fuel ratio increased, and the cold gas efficiency peaked at an O2/fuel ratio of approximately 0.7 [3]. However, as shown in Fig. 4, the plasma gasification of the
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glycerol had a high carbon conversion and cold gas efficiency at an O2/fuel ratio of 0e0.2. This implies that the plasma gasification process should be undertaken at a lower O2/fuel ratio than the conventional gasification process.
3.1.2.
Effect of the steam/fuel ratio
Fig. 5 shows the variation of the syngas composition produced after the plasma gasification of glycerol according to the change in the steam/fuel ratio at an O2/fuel ratio of 0.6 and a microwave power of 1.6 kW. In the results, as the steam/fuel ratio increased, the H2 and CO2 content in the product gas increased while the CO content decreased. The CH4 content was less than 1% and it did not change significantly according to the steam/fuel ratio. The reactions that can occur in the gasifier as a result of the steam injection can be categorized as the reaction of steam and carbon in the fuel and the reaction of steam and CO in the gas. The reaction of steam and carbon is an endothermic reaction that generates H2 and CO, whereas the reaction of steam and CO is an exothermic reaction that generates H2 and CO2. When steam is fed with fuel into the reactor, the endothermic reaction of steam and carbon occurs first, and the CO in a gaseous state produced from the fuel reacts with the residual steam, causing a wateregas shift reaction (WGSR), which is an exothermic reaction. Thus, the composition of H2, CO, and CO2 in the product gas changes according to the amount of steam supplied to the reactor. For glycerol, which only consists of volatile matter, because the WGSR occurs as the steam reacts with the CO produced by the fast reaction of steam and carbon, the H2 and CO2 content in the product gas increased with an increasing supply of steam to the reactor, while the CO content decreased. However, the variable range of gas content decreased as the steam/fuel ratio increased, and when the steam/fuel ratio increased from 1.6 to 2.4, the composition of the product gas remained almost unchanged. The steam supply to the reactor not only caused two reaction types between steam and carbon and between steam and CO, but also changed the reaction temperature. The introduction of a small quantity of steam into the reactor increased the gas temperature [27,28]. However, usually, the increased steam injection decreases the reaction temperature
by supplying a reactant with a temperature lower than the reactor temperature [28e31]. Furthermore, it typically increases the gas flow rate by increasing the amount of reactants supplied to the reactor. The increase of the gas flow rate increases the flow velocity and decreases the fuel retention time in the reactor. The reason that there was no change in the composition of product gas at a steam/fuel ratio of 1.6 or higher seems to be that due to the excessive steam supply, the decreased fuel retention time was not sufficient for the WGSR of the CO that was generated from the reaction of steam and carbon in the fuel. Fig. 6 presents the changes in the syngas heating value produced from the plasma gasification of glycerol, carbon conversion, and cold gas efficiency according to the steam/ fuel ratio. When the H2 and CO generation mechanism, which resulted from the reaction of steam and carbon in the fuel, is dominant as the steam supply increased, the carbon conversion and cold gas efficiency increased. In contrast, the decrease in the reaction temperature and fuel retention time in the reactor due to the steam supply decreased the product gas yield. Furthermore, the heating value of the syngas decreased when the H2 and CO2 production increased as the CO content in the syngas decreased due to the WGSR. These decreases in product gas yield and heating value result in a decrease in the carbon conversion and cold gas efficiency. In this way, the variation tendencies of the carbon conversion and cold gas efficiency according to the increase of the steam supply differ according to the type and method of the gasifier, and the fuel characteristics [22,32,33]. The results in Fig. 6 show that in the plasma gasification of glycerol, the syngas heating value, carbon conversion, and cold gas efficiency decreased as the steam/fuel ratio increased. As shown in Fig. 5, the H2 and CO2 content in the syngas increased with the increase of the steam/fuel ratio, whereas the decrease of the CO content and heating value of the syngas resulted in the decrease of the carbon conversion and cold gas efficiency. It follows that the changes in the gasification efficiency as a result of the steam supply in the plasma gasification were dominated by the temperature, the length of the plasma flames, and the fuel retention time. The results in Fig. 4 also demonstrate that the increase of the oxygen supply to the reactor raised the reactor temperature, but the cold gas
40
30
20
H CO CO
10
CH
0
0.0
0.5
1.0
1.5
2.0
Steam/fuel
Fig. 5 e Effect of the steam/fuel ratio on syngas composition.
2.5
3.0
100
2600
90
2400
80
70
2200 Carbon conversion Cold gas efficiency Syngas heating value
2000
60 1800 50 1600 40 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Steam/fuel
Fig. 6 e Effect of the steam/fuel ratio on syngas heating value, carbon conversion, and cold gas efficiency.
Syngas heating value (kcal/Nm )
Carbon conversion and cold gas efficiency (%)
Syngas composition (vol %, dry, N2 free)
50
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3.1.3.
Effect of the microwave power
One of the major parameters that affect the performance of plasma gasification is the microwave power. Microwave power is closely associated with the gasifier temperature, the plasma flames size, and the plasma density. An increase in the microwave power increases the plasma flame diameter and length [34], the reactor temperature [35,36], and the plasma density, especially the electron density [37]. That is, the increase in the glycerol retention time in high temperature plasma flames and the increase of active species, such as electrons and ions, affect the composition of the product gas and the gasification efficiency. Fig. 7 shows the effects of the microwave power on the composition change of the plasma gasification product gas of glycerol at constant O2/fuel and steam/fuel ratios of 0.6 and 0.8, respectively. As the microwave power increased from 1 to 1.8 kW, the H2 and CO content in the syngas increased and the CO2 content decreased. The same tendency can be found in the gasification of coals and waste using arc plasma [36,38]. The CH4 content in the product gas was approximately 1% and it decreased as the microwave power increased. This shows that, as with general gasification at high temperatures, the increase of the gasification temperature and active species due to the increase in the microwave power decomposes CH4 into H2 and CO. Furthermore, the water gas reaction as described in Eq. (1) was activated, which increases the production of H2 and CO. Fig. 8 presents the changes of the syngas heating value, carbon conversion, and cold gas efficiency according to variations in the microwave power. When the microwave power was increased from 1 to 1.2 kW, the heating value of syngas sharply increased, and then it increased slowly as the microwave power increased further. When the microwave power changed from 1 to 1.8 kW, the syngas heating value increased
Syngas composition (vol %, dry, N 2 free)
50
100
2500
90
2100
70
60
1900
50 Carbon conversion Cold gas efficiency Syngas heating value
40
30 0.8
1.0
1.2
1.4
1.6
1.8
1700
3
2300 80
Syngas heating value (kcal/Nm )
efficiency diminished due to a decrease of the fuel retention time in the plasma flames. This shows that for plasma gasification using high temperature plasma flames, the fuel retention time has a significant effect on the gasification reaction and efficiency.
Carbon conversion and cold gas efficiency (%)
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1500 2.0
Power (kW)
Fig. 8 e Effect of the microwave power on syngas heating value, carbon conversion, and cold gas efficiency.
from 1900 kcal/Nm3 to 2200 kcal/Nm3 because, as shown in Fig. 7, the content of combustible gases such as H2 and CO in the product gas increased according to the increase in the microwave power. In contrast, the carbon conversion and cold gas efficiency increased almost linearly as the microwave power increased because with a microwave power of over 1.2 kW, the syngas heating value did not change significantly, but the syngas production yield increased continuously. When the microwave power was changed from 1 to 1.8 kW, the carbon conversion and cold gas efficiency increased by more than 20%. At a microwave power of 1.8 kW, almost 100% of the carbon conversion and more than 62% of cold gas efficiency was achieved. In the literature, the length and diameter of the plasma flames were linearly proportional to the microwave power [34]. Thus, the increased volume of high temperature flames increased the fuel retention time in the plasma flames and the reaction temperature, which improved the gasification efficiency. As shown in the results of Figs. 7 and 8, increasing the microwave power can improve the combustible gases content, syngas heating value, gas yield, conversion rate, and efficiency. However, because it increases the power consumption for the gasification of the same amount of fuel, an appropriate microwave power must be used according to the feeding amount and heating value of the fuel.
40
30
20
H CO CO
10
CH 0
0.8
1.0
1.2
1.4
1.6
1.8
Power (kW)
Fig. 7 e Effect of the microwave power on syngas composition.
2.0
3.2.
Glycerol feed using nozzle
3.2.1.
Effect of the nozzle spray gas flow rate
Figs. 3e8 presented the plasma gasification results using a tube for the glycerol supply to the reactor. Using the tube, the glycerol falls continuously as droplets into the plasma flames and reactions occur. When a smaller glycerol drop size is supplied to the reactor, a faster reaction can be expected due to the increased reaction surface area. Furthermore, increasing the reaction surface area through fuel atomization can improve the gasification efficiency. Thus, the gaseliquid nozzle described in Fig. 2 was used to spray the glycerol into the plasma flame. Supplying the glycerol as fine particles or mist can increase not only the contact surface of the glycerol particles with the high-temperature plasma flames but also
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Syngas composition (vol %, dry, N 2 free)
40
30
20
H CO CO
10
CH
0
100
2400
3
90 2300 Carbon conversion Cold gas efficiency Syngas heating value
80
70
2200
60 2100 50
40 1.8
2.0
2.2
2.4
2.6
2.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
Nozzle spray gas (L/min)
Fig. 9 e Effect of the nozzle spray gas flow rate on syngas composition.
3.0
3.2
3.4
2000 3.6
Nozzle spray gas (L/min)
Fig. 10 e Effect of the nozzle spray gas flow rate on syngas heating value, carbon conversion, and cold gas efficiency.
spray gas flow rate increased at a constant glycerol feed rate, the glycerol was atomized and sprayed into the plasma flames, which increased the reaction surface area and thermochemical conversion of glycerol. This indicates that the amount of conversion to syngas increased. On the other hand, an excessive supply of the spray gas (N2) to the reactor lowers the reactor temperature. Hence, a spray gas flow rate of 3.5 L/ min lowers the reactor temperature and decreases the heating value and production yield of syngas, which results in a decrease of carbon conversion and cold gas efficiency. Therefore, with the nozzle used in this study, a spray gas flow rate of 3 L/min is appropriate for an efficient spray of glycerol supplied at 3 g/min.
3.2.2.
Effect of the O2/fuel ratio
At a spray gas flow rate of 3 L/min, which exhibited the optimum efficiency in the plasma gasification of glycerol using the nozzle described in Fig. 2, the variation of the syngas composition according to the changing O2/fuel ratio was investigated. The steam/fuel ratio and microwave power were set to 0.8 and 1.6 kW, respectively, which were identical to those used in the glycerol plasma gasification that did not use the spray nozzle in Fig. 3. When this result is compared with Fig. 3, similar values and variation trends were exhibited for the syngas composition according to the O2/fuel ratio. However, using the nozzle resulted in higher H2 and CO content in the syngas by up to 4% and 6%, respectively. In contrast, a lower CO2 content by up to 8% was shown when the nozzle was used. The reason for this appeared to be that as the reaction surface area in the high temperature plasma flames increased due to the reduced particle size of the glycerol supplied to the reactor, the Boudouard reactions in Eq. (5) occurred more. This implies that the reaction ratio in the various reactions that occurred in the gasifier changed according to the glycerol supply conditions. C þ CO2 4 2CO, DH ¼ 172.5 kJ/mol
1.8
Syngas heating value (kcal/Nm )
the reaction surface area of the glycerol with the gasification agents such as oxygen and steam. In the plasma gasification of glycerol using a gaseliquid nozzle, the feed ratio of liquid and spray gas in the nozzle determines the size of the sprayed liquid particles, which can affect the gasification reaction and efficiency. Hence, before comparing the results of the plasma gasification using a tube for the glycerol feed, an experiment to verify the appropriate amount of spray gas for the amount of glycerol injection was performed. The composition changes of the syngas produced after the plasma gasification when the amount of N2 used as a spray gas was changed from 2 to 3.5 L/min at a constant glycerol supply of 3 g/min, which is identical to the supply through a tube, are shown in Fig. 9. The O2/fuel ratio, steam/ fuel ratio, and microwave power in this case were fixed at 0.6, 0.8, and 1.6 kW, respectively. As the spray gas flow rate increased, the H2 and CO content in the syngas increased and then decreased. In contrast, the CO2 decreased at first and then increased after a spray gas flow rate of 3 L/min. The CH4 content in the syngas was constant at approximately 0.6%. These variations of the syngas composition demonstrate that the size of the glycerol particles supplied to the reactor influences the plasma gasification reaction. Furthermore, it follows from these results that there is an optimum nozzle spray condition for the improvement of the gasification efficiency. Fig. 10 shows the changes in the syngas heating value, carbon conversion, and cold gas efficiency according to variations in the nozzle spray gas flow rate, which were calculated from the results in Fig. 9. The syngas heating value, carbon conversion, and cold gas efficiency increased until a nozzle spray gas flow rate of 3 L/min, after which they decreased. By increasing the nozzle spray gas flow rate from 2 to 3 L/min, increases in the syngas heating value, carbon conversion, and cold gas efficiency by 90 kcal/Nm3, 5%, and 6% were achieved, respectively. The changes of the carbon conversion and cold gas efficiency according to the spray gas flow rate were greater than those of syngas as seen in Fig. 9. This results from the difference in the product gas yield according to the spray conditions of glycerol to the plasma flames. As the nozzle
Carbon conversion and cold gas efficiency (%)
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(5)
The changes in the syngas heating value, carbon conversion, and cold gas efficiency of the glycerol plasma gasification according to the O2/fuel ratio variation were examined when the glycerol was supplied to the reactor using a nozzle. In this
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case, the carbon conversion and cold gas efficiency increased by up to 4.5% and 7%, respectively. This resulted from the increased product gas yield and increased combustible gases content, such as H2 and CO, in the syngas through using a nozzle. Therefore, large commercial scale plasma gasification plants should implement a glycerol supply method that uses a nozzle for improved efficiency and continuous supply and uniformity of fuels.
4.
Conclusions
In this study, the microwave plasma gasification characteristics of glycerol, which is a byproduct of biodiesel production, were investigated at various operating conditions. At a zero O2/fuel ratio condition, it is possible to produce a syngas with a high H2 and CO content of 57% and 35%, respectively. In these conditions, the highest cold gas efficiency was achieved, but the lowest carbon conversion of approximately 80% was shown. As the O2/fuel ratio increased, the cold gas efficiency rapidly decreased while the carbon conversion increased. Therefore, the O2/fuel ratio should be controlled between 0 and 0.4 in order to achieve a high carbon conversion and cold gas efficiency in the microwave plasma gasification of glycerol. When the steam/fuel ratio was increased, the H2 content in the syngas increased, whereas the syngas heating value and gasification efficiency decreased. Increasing the microwave power increases the H2 and CO content in the syngas and it leads to increased syngas heating value, carbon conversion, and cold gas efficiency. When the microwave power was increased from 1.0 to 1.8 kW, the carbon conversion and cold gas efficiency improved by more than 20%. When glycerol was supplied to the reactor using a gaseliquid nozzle, the H2 and CO content in the product gas increased while the CO2 content decreased. Furthermore, the carbon conversion and cold gas efficiency improved by up to 4.5% and 7%, respectively. The experimental results indicate that the reaction mechanism in the plasma gasification is similar to that in conventional gasification. However, unlike conventional gasification, the fuel retention time in the plasma flames is a critical factor that influences the product gas yield and gasification efficiency in plasma gasification that uses the high temperature plasma flames.
Acknowledgments This research was supported by the Energy Technology Development project of the Ministry of Knowledge Economy and was partially conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B3-2423-01).
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Available online at www.sciencedirect.com
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Hydrogen and syngas production from glycerol through microwave plasma gasification Sang Jun Yoon a,b, Young Min Yun b, Myung Won Seo a, Yong Ku Kim a, Ho Won Ra a, Jae-Goo Lee a,* a
Climate Change Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea b Advanced Energy Technology Department, University of Science and Technology, 217 Gajungro, Yuseong-gu, Daejeon, Republic of Korea
article info
abstract
Article history:
Glycerol which is a byproduct of biodiesel production is considered as a potential feedstock
Received 3 May 2013
for syngas production with the increase of biodiesel demand. In this study, the charac-
Received in revised form
teristics of glycerol gasification under a microwave plasma torch with varying oxygen and
20 August 2013
steam supply conditions were investigated. The experimental results demonstrated that
Accepted 1 September 2013
the gasification efficiency and syngas heating value increased with the supplied microwave
Available online 3 October 2013
power while the increase of oxygen and steam led to a lower gasification performance. In order to achieve high carbon conversion and cold gas efficiency in the microwave plasma
Keywords:
gasification of glycerol, the O2/fuel ratio should be maintained at 0e0.4. It was revealed that
Microwave plasma
the fuel droplet size and the mixing effect and retention time inside the plasma flames are
Gasification
critical factors that influence the product gas yield and gasification efficiency. This study
Hydrogen
verified that syngas with a high content of H2 and CO could be effectively produced from
Syngas
glycerol through microwave plasma gasification.
Glycerol
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
There is increasing interest in the use of new and renewable energy sources due to limited fossil fuel reserves, oil price fluctuations, and international regulations on CO2 emissions. Studies on biomass, which is a CO2 neutral source, are being actively conducted not only regarding its primary use of obtaining heat and electricity through combustion, but also regarding its high value-added use through conversion to transportation fuels such as biodiesel that can replace petroleum. The global production of biodiesel has increased annually by 32.5% on average from 2000 to 2010 [1]. In particular, biodiesel production has increased sharply since
2006. The production of biodiesel is expected to grow continually in the United States and the European Union in order to attain the goal of replacing 20% and 30% of petroleum-based diesel with biofuels by 2020 and 2030, respectively [2]. The method of using transesterification reaction to produce biodiesel, which is currently the primary method of biodiesel production, generates crude glycerol of approximately 10 wt% as a byproduct. As the generation of crude glycerol is also expected to grow with the increased production of biodiesel, the effective use of crude glycerol is important from both economic and environmental perspectives. For utilization of crude glycerol, many studies are being conducted on the conversion of glycerol into valuable
* Corresponding author. Tel.: þ82 42 8603353; fax: þ82 42 8603134. E-mail address: [email protected] (J.-G. Lee). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.001
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components or energy through various techniques, such as refinement, purification, pyrolysis, reforming, and gasification. Among these techniques, energy conversion through gasification has advantages in that it does not require catalyst or additional pretreatment of the crude glycerol [3]. Gasification refers to the technique of generating syngas that contains H2 and CO through the partial oxidation of hydrocarbon fuels. As a clean fuel utilization technique, gasification is being applied to the production of electricity, hydrogen, and chemical materials using various fuels, such as coals, waste, and biomass. These days, various studies on gasification using plasma torches are in progress [4e6]. For plasma gasification, plasma flames are formed using external electric energy sources and the fuels are gasified through plasma flames with high temperatures over several thousand degrees [7]. Thus, it does not require oxygen or it requires only a small amount of oxygen, which is required in the conventional gasification process for the partial oxidation of fuels in order to maintain the reaction temperature. This decreases the burden of the installation and operation of highly expensive air separation units. Unlike the conventional gasification method that raises the temperature of the reactor through preheating, plasma gasification starts simultaneously with the activation of a high temperature plasma torch. Therefore, rapid production of syngas is possible. Furthermore, the high concentration of active species, such as ions and radicals, in the plasma state accelerates the gasification reaction [8,9]. As the reaction is activated, a quick reaction can be induced during the short retention time of fuels in the reactor, and the size of the total process can be reduced due to the decreased volume of the reactor. Most previous studies have focused on the plasma combustion [9], pyrolysis [10], and gasification [11,12] of various fuels using arc electrodes. The use of arc electrodes, however, requires periodic replacement due to the loss of electrodes and is vulnerable to the oxygen and steam that are used as gasification agents [9,13]. The use of microwaves as the energy source for
plasma generation exhibits a higher power transfer efficiency than arc plasma and a high durability because electrodes are not used [14,15]. Therefore, the reforming and gasification of various hydrocarbons using microwave plasma torches are being actively investigated [16e18]. In this study, the gasification characteristics of glycerol under various conditions using a plasma torch equipped with a 2 kW microwave power generator was investigated. For the uniformity of fuels and convenient interpretation of the gasification characteristics, pure glycerol was used instead of crude glycerol. Nitrogen was used as the plasma forming gas, and the microwave power for plasma formation was adjusted to 1e2 kW. The variations of the gasification characteristics were studied with the supply amount of oxygen and steam, which were used as gasification agents. At this time, the O2/fuel ratio and steam/fuel ratio are varied 0e1.2 and 0e2.4, respectively. Furthermore, the changes in the product gas composition and gasification efficiency were examined when the glycerol was atomized through the reactor using a spray nozzle.
2.
Experimental
In this study, 99% pure glycerol (Duksan Chemical) was used as a substitute for crude glycerol without further purification. Fig. 1 presents a schematic diagram of the plasma gasification system with the 2 kW microwave generator (2.45 GHz, SM745, Richardson Electronics) that was used in this study. This system consists of a glycerol preheater and feeder, steam supplier, gasification agent and plasma forming gas feed unit, microwave generator, gasification reactor, gas purifier and analyzer, and data collection unit. The glycerol was supplied to the reactor constantly at a rate of 3 g/min through a gear pump (Cole-Parmer, 74014-75). In order to supply glycerol to the plasma reactor, two methods were used and compared:
Fig. 1 e Schematic diagram of the microwave plasma glycerol gasification system.
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the use of a stainless steel tube with an outer diameter of 0.32 cm and the use of an external mixing nozzle as shown in Fig. 2. To improve the fluidity and spray stability of the glycerol, it was preheated to 140 C with a heater before it was supplied to the nozzle and reactor. When using the nozzle, N2 was used as the spray gas for atomization of the glycerol and its flow was adjusted to 2.0e3.5 L/min using a mass flow controller (MFC, Brooks 5850). Nitrogen was used as the plasma forming gas, which was supplied at 15 L/min through a MFC. Oxygen was used as the gasification oxidizer, which was quantitatively supplied at 0e2.6 L/min using a MFC according to the experimental conditions. The steam that was used as the gasification agent was supplied to the reactor at 0e7.2 ml/min using a syringe pump and the feed line was maintained above 100 C using a band heater so that the steam would be supplied to the reactor. The plasma forming gas, oxygen, and steam were supplied to the reactor with a diameter of 2.54 cm via a swirl-flow in order to increase the retention time in the reactor and to allow stable formation of the plasma flames while reducing the thermal shock of the reactor. The glycerol was supplied in the reactor 2 cm away from the point where the forming gas and microwave met and formed plasma flames. To measure the temperature at each reactor location, two R-type and five K-type thermocouples with an accuracy of 0.1 C were installed in 5 cm intervals from the glycerol feeding location. To remove the unburned carbons, ashes, and moisture from the syngas generated by the plasma gasification reaction, the syngas was passed through a cyclone and filter before it was supplied to the gas chromatograph (GC, HP 6890) for quantitative and qualitative analyses in real time. A thermal conductivity detector (TCD, Carbosphere 80/100 Packed column, Alltech) was used to analyze the product gases, such as H2, CO, CO2, CH4, and N2. These product gases were quantitatively and qualitatively calibrated using standard gases with various compositions and concentrations prior to use in the experiments. Every experiment was performed at least twice in atmospheric conditions, and the results were averaged. The relative standard deviations were less than 5%.
3.
Results and discussion
3.1.
Glycerol feed using tube
3.1.1.
Effect of the O2/fuel ratio
Fig. 3 shows the composition changes of the syngas generated through the microwave plasma gasification according to the changing O2/fuel ratio when the glycerol was supplied to the reactor through a tube. The steam/fuel ratio and microwave power were maintained constant at 0.8 and 1.6 kW, respectively. When the glycerol was supplied through a tube, it fell continuously as droplets and reacted in the plasma flames. The results showed that as the O2/fuel ratio
h ð%Þ ¼
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increased, the H2 content in the syngas linearly decreased. The CO content remained constant at 35% until an O2/fuel ratio of 0.6 and then it decreased. The CO2 content increased with the O2/fuel ratio and began to rapidly increase when the O2/fuel ratio exceeded 0.6. The CH4 exhibited a low content in the syngas at 2% or lower and tended to decrease as the O2/ fuel ratio increased. This indicates that as the O2/fuel ratio increased, the combustion reaction was activated, which increased the CO2 content. Furthermore, the combustion reaction following the increase of the O2/fuel ratio increased the reactor temperature and the CH4 content in the syngas decreased due to the decomposition. These changes of the syngas composition can be divided into three categories according to the O2/fuel ratio. The O2/fuel ratio range from 0 to 0.3 was where the primary water gas reaction in Eq. (1) and the secondary water gas reaction in Eq. (2) were most activated. Therefore, a high content of H2 and CO was exhibited in the syngas. The O2/fuel ratio range from 0.3 to 0.8 was where various gasification reactions, such as combustion and the water gas shift reaction, occurred with the reactions in Eqs. (1) and (2). Finally, the O2/fuel ratio range above 0.8 was where the complete combustion reaction was dominant [19,20]. For this reason, a large CO2 content was detected with small quantities of H2 and CO in the syngas. C þ H2O 4 CO þ H2, DH ¼ 131.3 kJ/mol
(1)
C þ 2H2O 4 CO2 þ 2H2, DH ¼ 90.2 kJ/mol
(2)
These changes in the syngas composition according to the O2/fuel ratio were similar to those in the conventional gasification, which does not use a plasma torch [21e23]. That is, the reaction mechanism that occurred in the plasma gasification is similar to that of the conventional gasification. However, unlike the traditional gasification method that maintains the reaction temperature by partial oxidation of the fuel, the plasma gasification that uses high temperature plasma flames formed by electric energy as the gasification heat source can be operated without an oxygen supply. When an oxygen supply was not present and the O2/fuel ratio was 0, the composition of the plasma gasification product gas of glycerol was H2 57%, CO 35%, CO2 6%, and CH4 2%. Hence, the plasma gasification of glycerol could produce syngas with a low CO2 content and a high content of combustible gases. Fig. 4 shows the variation of the syngas heating value, carbon conversion, and cold gas efficiency during the microwave plasma gasification of glycerol according to the O2/fuel ratio. The carbon conversion (Xc) and cold gas efficiency (h), which is the thermodynamic efficiency indicator of the gasification process, are defined as follows: Xc ð%Þ ¼
Mass flow of carbon in the syngas 100 Mass flow of carbon in the feedstock
Mass flow rate of syngas higher heating value of syngas 100 Mass flow rate of feedstock higher heating value of feedstock
(3)
(4)
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100
4000
80 3000
60 2000 40 Carbon conversion Cold gas efficiency Syngas heating value
20
1000
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Syngas heating value (kcal/Nm3)
Carbon conversion and cold gas efficiency (%)
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0 1.4
O2/fuel
Fig. 4 e Effect of the O2/fuel ratio on syngas heating value, carbon conversion, and cold gas efficiency (feeding of glycerol using tube). Fig. 2 e Schematic diagram of the gaseliquid nozzle for glycerol spray.
As the O2/fuel ratio increased, the carbon conversion increased while the syngas heating value and cold gas efficiency decreased. As shown through the syngas composition changes in Fig. 3, as the O2/fuel ratio increased, the oxygen supply to the reactor increased and the combustion reaction was activated, which in turn increased the CO2 production and carbon conversion. However, the decreased content of combustible gases in the product gas sharply decreased the syngas heating value as well as the product gas yield, thereby rapidly decreasing the cold gas efficiency. The oxygen supply in the gasification reactor activates the combustion reaction of the fuel and simultaneously increases the reactor temperature. Furthermore, the rising reactor temperature decreases the retention time of the fuel and reactants in the gasifier due to the increased gas velocity in the reactor. In the conventional gasification process, which does not use a plasma flame, as the oxygen supply to the reactor increases, the carbon conversion generally increases due to
Syngas composition (vol %, dry, N 2 free)
100 90
H
80
CO CO
70
CH
60 50 40 30 20 10 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
O2/fuel
Fig. 3 e Effect of the O2/fuel ratio on syngas composition (feeding of glycerol using tube).
the increased product gas yield after the fuel reaction and to the increased production of CO and CO2. The cold gas efficiency tends to increase with an increased amount of product gas due to the rising reactor temperature until a certain oxygen supply, after which, due to the activated combustion reaction and the decreased content of combustible gases such as H2 and CH4 in the syngas, the syngas heating value and cold gas efficiency in turn decreases [22e25]. This suggests that with the conventional gasification, the rising reactor temperature that results from the oxygen supply has a greater effect on the fuel conversion than on the decreased retention time in the reactor. On the other hand, with the plasma gasification in this study, the increase of the O2/fuel ratio increased the carbon conversion and decreased the cold gas efficiency. Because plasma gasification uses the high temperature of the plasma flame, unlike the conventional gasification, the reaction of feedstock in the plasma flame is very important. An increase in the oxygen supply increases the reaction temperature, but as mentioned above, it also increases the flow velocity in the reactor with the same volume, thereby decreasing the retention time of the reactants in the plasma flames. Therefore, in plasma gasification, in which the contact and retention time in high temperature plasma flames is important, the increase in the oxygen supply decreased the cold gas efficiency. Compared with the gasification of coals using microwave plasmas as presented in the literature, the plasma gasification results of glycerol had higher carbon conversion and cold gas efficiency [9,26]. This is because glycerol, which consists of volatile matter with good reactivity even at low temperatures, can be gasified more easily than coals that contain fixed carbon with lower reactivity, which reacts at high temperatures. When compared with the gasification of crude glycerol in an entrained flow gasifier that does not use plasma, plasma gasification had a different tendency in gasification efficiency variation, although there was a difference in the process scale. For the entrained flow gasification, the carbon conversion increased as the O2/fuel ratio increased, and the cold gas efficiency peaked at an O2/fuel ratio of approximately 0.7 [3]. However, as shown in Fig. 4, the plasma gasification of the
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glycerol had a high carbon conversion and cold gas efficiency at an O2/fuel ratio of 0e0.2. This implies that the plasma gasification process should be undertaken at a lower O2/fuel ratio than the conventional gasification process.
3.1.2.
Effect of the steam/fuel ratio
Fig. 5 shows the variation of the syngas composition produced after the plasma gasification of glycerol according to the change in the steam/fuel ratio at an O2/fuel ratio of 0.6 and a microwave power of 1.6 kW. In the results, as the steam/fuel ratio increased, the H2 and CO2 content in the product gas increased while the CO content decreased. The CH4 content was less than 1% and it did not change significantly according to the steam/fuel ratio. The reactions that can occur in the gasifier as a result of the steam injection can be categorized as the reaction of steam and carbon in the fuel and the reaction of steam and CO in the gas. The reaction of steam and carbon is an endothermic reaction that generates H2 and CO, whereas the reaction of steam and CO is an exothermic reaction that generates H2 and CO2. When steam is fed with fuel into the reactor, the endothermic reaction of steam and carbon occurs first, and the CO in a gaseous state produced from the fuel reacts with the residual steam, causing a wateregas shift reaction (WGSR), which is an exothermic reaction. Thus, the composition of H2, CO, and CO2 in the product gas changes according to the amount of steam supplied to the reactor. For glycerol, which only consists of volatile matter, because the WGSR occurs as the steam reacts with the CO produced by the fast reaction of steam and carbon, the H2 and CO2 content in the product gas increased with an increasing supply of steam to the reactor, while the CO content decreased. However, the variable range of gas content decreased as the steam/fuel ratio increased, and when the steam/fuel ratio increased from 1.6 to 2.4, the composition of the product gas remained almost unchanged. The steam supply to the reactor not only caused two reaction types between steam and carbon and between steam and CO, but also changed the reaction temperature. The introduction of a small quantity of steam into the reactor increased the gas temperature [27,28]. However, usually, the increased steam injection decreases the reaction temperature
by supplying a reactant with a temperature lower than the reactor temperature [28e31]. Furthermore, it typically increases the gas flow rate by increasing the amount of reactants supplied to the reactor. The increase of the gas flow rate increases the flow velocity and decreases the fuel retention time in the reactor. The reason that there was no change in the composition of product gas at a steam/fuel ratio of 1.6 or higher seems to be that due to the excessive steam supply, the decreased fuel retention time was not sufficient for the WGSR of the CO that was generated from the reaction of steam and carbon in the fuel. Fig. 6 presents the changes in the syngas heating value produced from the plasma gasification of glycerol, carbon conversion, and cold gas efficiency according to the steam/ fuel ratio. When the H2 and CO generation mechanism, which resulted from the reaction of steam and carbon in the fuel, is dominant as the steam supply increased, the carbon conversion and cold gas efficiency increased. In contrast, the decrease in the reaction temperature and fuel retention time in the reactor due to the steam supply decreased the product gas yield. Furthermore, the heating value of the syngas decreased when the H2 and CO2 production increased as the CO content in the syngas decreased due to the WGSR. These decreases in product gas yield and heating value result in a decrease in the carbon conversion and cold gas efficiency. In this way, the variation tendencies of the carbon conversion and cold gas efficiency according to the increase of the steam supply differ according to the type and method of the gasifier, and the fuel characteristics [22,32,33]. The results in Fig. 6 show that in the plasma gasification of glycerol, the syngas heating value, carbon conversion, and cold gas efficiency decreased as the steam/fuel ratio increased. As shown in Fig. 5, the H2 and CO2 content in the syngas increased with the increase of the steam/fuel ratio, whereas the decrease of the CO content and heating value of the syngas resulted in the decrease of the carbon conversion and cold gas efficiency. It follows that the changes in the gasification efficiency as a result of the steam supply in the plasma gasification were dominated by the temperature, the length of the plasma flames, and the fuel retention time. The results in Fig. 4 also demonstrate that the increase of the oxygen supply to the reactor raised the reactor temperature, but the cold gas
40
30
20
H CO CO
10
CH
0
0.0
0.5
1.0
1.5
2.0
Steam/fuel
Fig. 5 e Effect of the steam/fuel ratio on syngas composition.
2.5
3.0
100
2600
90
2400
80
70
2200 Carbon conversion Cold gas efficiency Syngas heating value
2000
60 1800 50 1600 40 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Steam/fuel
Fig. 6 e Effect of the steam/fuel ratio on syngas heating value, carbon conversion, and cold gas efficiency.
Syngas heating value (kcal/Nm )
Carbon conversion and cold gas efficiency (%)
Syngas composition (vol %, dry, N2 free)
50
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3.1.3.
Effect of the microwave power
One of the major parameters that affect the performance of plasma gasification is the microwave power. Microwave power is closely associated with the gasifier temperature, the plasma flames size, and the plasma density. An increase in the microwave power increases the plasma flame diameter and length [34], the reactor temperature [35,36], and the plasma density, especially the electron density [37]. That is, the increase in the glycerol retention time in high temperature plasma flames and the increase of active species, such as electrons and ions, affect the composition of the product gas and the gasification efficiency. Fig. 7 shows the effects of the microwave power on the composition change of the plasma gasification product gas of glycerol at constant O2/fuel and steam/fuel ratios of 0.6 and 0.8, respectively. As the microwave power increased from 1 to 1.8 kW, the H2 and CO content in the syngas increased and the CO2 content decreased. The same tendency can be found in the gasification of coals and waste using arc plasma [36,38]. The CH4 content in the product gas was approximately 1% and it decreased as the microwave power increased. This shows that, as with general gasification at high temperatures, the increase of the gasification temperature and active species due to the increase in the microwave power decomposes CH4 into H2 and CO. Furthermore, the water gas reaction as described in Eq. (1) was activated, which increases the production of H2 and CO. Fig. 8 presents the changes of the syngas heating value, carbon conversion, and cold gas efficiency according to variations in the microwave power. When the microwave power was increased from 1 to 1.2 kW, the heating value of syngas sharply increased, and then it increased slowly as the microwave power increased further. When the microwave power changed from 1 to 1.8 kW, the syngas heating value increased
Syngas composition (vol %, dry, N 2 free)
50
100
2500
90
2100
70
60
1900
50 Carbon conversion Cold gas efficiency Syngas heating value
40
30 0.8
1.0
1.2
1.4
1.6
1.8
1700
3
2300 80
Syngas heating value (kcal/Nm )
efficiency diminished due to a decrease of the fuel retention time in the plasma flames. This shows that for plasma gasification using high temperature plasma flames, the fuel retention time has a significant effect on the gasification reaction and efficiency.
Carbon conversion and cold gas efficiency (%)
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 8 ( 2 0 1 3 ) 1 4 5 5 9 e1 4 5 6 7
1500 2.0
Power (kW)
Fig. 8 e Effect of the microwave power on syngas heating value, carbon conversion, and cold gas efficiency.
from 1900 kcal/Nm3 to 2200 kcal/Nm3 because, as shown in Fig. 7, the content of combustible gases such as H2 and CO in the product gas increased according to the increase in the microwave power. In contrast, the carbon conversion and cold gas efficiency increased almost linearly as the microwave power increased because with a microwave power of over 1.2 kW, the syngas heating value did not change significantly, but the syngas production yield increased continuously. When the microwave power was changed from 1 to 1.8 kW, the carbon conversion and cold gas efficiency increased by more than 20%. At a microwave power of 1.8 kW, almost 100% of the carbon conversion and more than 62% of cold gas efficiency was achieved. In the literature, the length and diameter of the plasma flames were linearly proportional to the microwave power [34]. Thus, the increased volume of high temperature flames increased the fuel retention time in the plasma flames and the reaction temperature, which improved the gasification efficiency. As shown in the results of Figs. 7 and 8, increasing the microwave power can improve the combustible gases content, syngas heating value, gas yield, conversion rate, and efficiency. However, because it increases the power consumption for the gasification of the same amount of fuel, an appropriate microwave power must be used according to the feeding amount and heating value of the fuel.
40
30
20
H CO CO
10
CH 0
0.8
1.0
1.2
1.4
1.6
1.8
Power (kW)
Fig. 7 e Effect of the microwave power on syngas composition.
2.0
3.2.
Glycerol feed using nozzle
3.2.1.
Effect of the nozzle spray gas flow rate
Figs. 3e8 presented the plasma gasification results using a tube for the glycerol supply to the reactor. Using the tube, the glycerol falls continuously as droplets into the plasma flames and reactions occur. When a smaller glycerol drop size is supplied to the reactor, a faster reaction can be expected due to the increased reaction surface area. Furthermore, increasing the reaction surface area through fuel atomization can improve the gasification efficiency. Thus, the gaseliquid nozzle described in Fig. 2 was used to spray the glycerol into the plasma flame. Supplying the glycerol as fine particles or mist can increase not only the contact surface of the glycerol particles with the high-temperature plasma flames but also
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Syngas composition (vol %, dry, N 2 free)
40
30
20
H CO CO
10
CH
0
100
2400
3
90 2300 Carbon conversion Cold gas efficiency Syngas heating value
80
70
2200
60 2100 50
40 1.8
2.0
2.2
2.4
2.6
2.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
Nozzle spray gas (L/min)
Fig. 9 e Effect of the nozzle spray gas flow rate on syngas composition.
3.0
3.2
3.4
2000 3.6
Nozzle spray gas (L/min)
Fig. 10 e Effect of the nozzle spray gas flow rate on syngas heating value, carbon conversion, and cold gas efficiency.
spray gas flow rate increased at a constant glycerol feed rate, the glycerol was atomized and sprayed into the plasma flames, which increased the reaction surface area and thermochemical conversion of glycerol. This indicates that the amount of conversion to syngas increased. On the other hand, an excessive supply of the spray gas (N2) to the reactor lowers the reactor temperature. Hence, a spray gas flow rate of 3.5 L/ min lowers the reactor temperature and decreases the heating value and production yield of syngas, which results in a decrease of carbon conversion and cold gas efficiency. Therefore, with the nozzle used in this study, a spray gas flow rate of 3 L/min is appropriate for an efficient spray of glycerol supplied at 3 g/min.
3.2.2.
Effect of the O2/fuel ratio
At a spray gas flow rate of 3 L/min, which exhibited the optimum efficiency in the plasma gasification of glycerol using the nozzle described in Fig. 2, the variation of the syngas composition according to the changing O2/fuel ratio was investigated. The steam/fuel ratio and microwave power were set to 0.8 and 1.6 kW, respectively, which were identical to those used in the glycerol plasma gasification that did not use the spray nozzle in Fig. 3. When this result is compared with Fig. 3, similar values and variation trends were exhibited for the syngas composition according to the O2/fuel ratio. However, using the nozzle resulted in higher H2 and CO content in the syngas by up to 4% and 6%, respectively. In contrast, a lower CO2 content by up to 8% was shown when the nozzle was used. The reason for this appeared to be that as the reaction surface area in the high temperature plasma flames increased due to the reduced particle size of the glycerol supplied to the reactor, the Boudouard reactions in Eq. (5) occurred more. This implies that the reaction ratio in the various reactions that occurred in the gasifier changed according to the glycerol supply conditions. C þ CO2 4 2CO, DH ¼ 172.5 kJ/mol
1.8
Syngas heating value (kcal/Nm )
the reaction surface area of the glycerol with the gasification agents such as oxygen and steam. In the plasma gasification of glycerol using a gaseliquid nozzle, the feed ratio of liquid and spray gas in the nozzle determines the size of the sprayed liquid particles, which can affect the gasification reaction and efficiency. Hence, before comparing the results of the plasma gasification using a tube for the glycerol feed, an experiment to verify the appropriate amount of spray gas for the amount of glycerol injection was performed. The composition changes of the syngas produced after the plasma gasification when the amount of N2 used as a spray gas was changed from 2 to 3.5 L/min at a constant glycerol supply of 3 g/min, which is identical to the supply through a tube, are shown in Fig. 9. The O2/fuel ratio, steam/ fuel ratio, and microwave power in this case were fixed at 0.6, 0.8, and 1.6 kW, respectively. As the spray gas flow rate increased, the H2 and CO content in the syngas increased and then decreased. In contrast, the CO2 decreased at first and then increased after a spray gas flow rate of 3 L/min. The CH4 content in the syngas was constant at approximately 0.6%. These variations of the syngas composition demonstrate that the size of the glycerol particles supplied to the reactor influences the plasma gasification reaction. Furthermore, it follows from these results that there is an optimum nozzle spray condition for the improvement of the gasification efficiency. Fig. 10 shows the changes in the syngas heating value, carbon conversion, and cold gas efficiency according to variations in the nozzle spray gas flow rate, which were calculated from the results in Fig. 9. The syngas heating value, carbon conversion, and cold gas efficiency increased until a nozzle spray gas flow rate of 3 L/min, after which they decreased. By increasing the nozzle spray gas flow rate from 2 to 3 L/min, increases in the syngas heating value, carbon conversion, and cold gas efficiency by 90 kcal/Nm3, 5%, and 6% were achieved, respectively. The changes of the carbon conversion and cold gas efficiency according to the spray gas flow rate were greater than those of syngas as seen in Fig. 9. This results from the difference in the product gas yield according to the spray conditions of glycerol to the plasma flames. As the nozzle
Carbon conversion and cold gas efficiency (%)
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 8 ( 2 0 1 3 ) 1 4 5 5 9 e1 4 5 6 7
(5)
The changes in the syngas heating value, carbon conversion, and cold gas efficiency of the glycerol plasma gasification according to the O2/fuel ratio variation were examined when the glycerol was supplied to the reactor using a nozzle. In this
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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 8 ( 2 0 1 3 ) 1 4 5 5 9 e1 4 5 6 7
case, the carbon conversion and cold gas efficiency increased by up to 4.5% and 7%, respectively. This resulted from the increased product gas yield and increased combustible gases content, such as H2 and CO, in the syngas through using a nozzle. Therefore, large commercial scale plasma gasification plants should implement a glycerol supply method that uses a nozzle for improved efficiency and continuous supply and uniformity of fuels.
4.
Conclusions
In this study, the microwave plasma gasification characteristics of glycerol, which is a byproduct of biodiesel production, were investigated at various operating conditions. At a zero O2/fuel ratio condition, it is possible to produce a syngas with a high H2 and CO content of 57% and 35%, respectively. In these conditions, the highest cold gas efficiency was achieved, but the lowest carbon conversion of approximately 80% was shown. As the O2/fuel ratio increased, the cold gas efficiency rapidly decreased while the carbon conversion increased. Therefore, the O2/fuel ratio should be controlled between 0 and 0.4 in order to achieve a high carbon conversion and cold gas efficiency in the microwave plasma gasification of glycerol. When the steam/fuel ratio was increased, the H2 content in the syngas increased, whereas the syngas heating value and gasification efficiency decreased. Increasing the microwave power increases the H2 and CO content in the syngas and it leads to increased syngas heating value, carbon conversion, and cold gas efficiency. When the microwave power was increased from 1.0 to 1.8 kW, the carbon conversion and cold gas efficiency improved by more than 20%. When glycerol was supplied to the reactor using a gaseliquid nozzle, the H2 and CO content in the product gas increased while the CO2 content decreased. Furthermore, the carbon conversion and cold gas efficiency improved by up to 4.5% and 7%, respectively. The experimental results indicate that the reaction mechanism in the plasma gasification is similar to that in conventional gasification. However, unlike conventional gasification, the fuel retention time in the plasma flames is a critical factor that influences the product gas yield and gasification efficiency in plasma gasification that uses the high temperature plasma flames.
Acknowledgments This research was supported by the Energy Technology Development project of the Ministry of Knowledge Economy and was partially conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B3-2423-01).
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