Burning Characteristics of Coconut Oil Vapor-Air · PDF fileBurning characteristics of coconut oil vapor-air mixtures 943 This indicates that the main characteristics of coconut oil

Adv. Studies Theor. Phys., Vol. 7, 2013, no. 19, 941 - 956 HIKARI Ltd, www.m-hikari.com

http://dx.doi.org/10.12988/astp.2013.3884

Burning Characteristics of Coconut Oil Vapor-Air

Mixtures at Premixed Combustion

Hadi Saroso

Department of Mechanical Engineering, State Polytechnic of Malang Jl. Soekarno Hatta PO Box 04 Malang, Indonesia, 64145

[email protected]

ING Wardana

Deparment of Mechanical Engineering, Faculty of Engineering, Brawijaya University Jl. Mayjen haryono 167 Malang, Indonesia, 65145

Rudy Soenoko

Deparment of Mechanical Engineering, Faculty of Engineering, Brawijaya University

Jl. Mayjen haryono 167 Malang, Indonesia, 65145

Nurkholis Hamidi

Deparment of Mechanical Engineering, Faculty of Engineering, Brawijaya University Jl. Mayjen haryono 167 Malang, Indonesia, 65145

Copyright © 2013 Hadi Saroso et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Burning characteristics of coconut oil vapor-air premixed combustion has been studied experimentally. This study was conducted using Hele-Shaw cell of 50cm x 20cm x 1cm. Coconut oil in this study has the composisiton of 12% glycerol and 88% fatty acid (consisting of 18.77% caprylic acid, 15.15% capric acid, 41.78%

942 Hadi Saroso, ING Wardana, Rudy Soenoko and Nurkholis Hamidi lauric acid, and 11.86% myristic). The combustion propagation direction was varied by igniting from top or from bottom. Flame propagation was observed at various equivalent ratios (Ф) of 2.73; 2.57; 2.34; 2.19; 2.03; 1.93 and 1.78. The results show that coconut oil tends to be degraded into fat and glycerol because of hydrolysis reaction. Fatty acid is burned before glycerol. Fatty acid tends to produce separated flames of different boiling points and molecule weights. The process of energy release from glycerol needs high rate of combustion process and it tends to slow flame propagation and causes micro explosions. The flame propagating downward at Ф = 2.73; 2.57 and 2.34 show orange flames that cause moderate micro-explosions. At Ф = 2.19; 2.03 and 1.93, the flames are blue-green, causing strong micro-explosions. At Ф = 1.78 the flames are dark purple, causing weak micro-explosions. For flame propagating upward, only at Ф = 2.03, the micro explosions can be seen clearly since the thermal expansion is high and the flame propagates fastest. Keywords: premix combustion, coconut oil, micro-explosion, Hele-shaw cell 1. Introduction

Currently the use of oil has become greater, but it is not accompanied by the adequate availability of non-renewable oil. Therefore, there should be alternative fuel as the replacement of oil. One of the alternatives being largely studied is biodiesel and vegetable oil. Coconut oil as one vegetable oil can be used as biodiesel fuel, and it is available in a large amount in Indonesia. Coconut oil is environment friendly that can reduce the effect of global warming because the combustion of coconut oil produces CO2 that can be used by plant for the process of photosynthesis. Therefore, coconut oil combustion is very intereting to be applied in industries as diesel fuel. However, direct combustion of coconut oil encountered a lot of problems regarding the viscocity which is about ten times as high as biodiesel’s [13]. The high viscosity of vegetable oil, especially coconut oil, is due to the glycerol contained. However, coconut oil has been used as supplement in Jet Boeing 747 of Virgin Atlantic, flying from London to Amsterdam on the 24th of February 2008 [11]. [3] stated that early combustion is one way to produce the process of coconut oil combustion in diesel. An experimental study shows that directly heated coconut oil can be used to replace diesel fuel and this does not require major modification of the machine [12]; this was tested on a diesel engine with the rotation of 1,500 rpm with the oil having characteristics close to that of diesel fuel. It seems that in the future coconut oil combustion will become an alternative. Another study by [10] explains the efficiency and the power of diesel using vegetable oil directly. The results of this study show that diesel engine has lower efficiency and less power at the rotation above 2,000 rpm.

Burning characteristics of coconut oil vapor-air mixtures 943 This indicates that the main characteristics of coconut oil combustion need to be learned intensively, especially when dealing with combustion technology in the future. The effect of ambient high temperature on the combustion rate of a long chain fuel has been studied by [5]. The fuel studied in the experiment was the cycle of light oil and diesel fuel. The results indicated higher effect of burning velocity on the combustion level on the part of oil fuel for the cycle of n-dekana lack of emissive light. However, the result of this study was not adequate to determine the characteristics of combustion in complex coconut oil compound. The present study explains the characteristics of the combustion of bio-complex oil compound that rarely occur. An analytic study has been conducted to identify the combustion characteristics of propanol with reduced gravitation [15]. Different boiling points between propanol and glycerol caused micro explosion after shrinkage. The micro explosion was predicted to also occur within coconut oil because of the fatty acid and glycerol contained, with the condition that vegetable oil has different boiling point. Water is absorbed by glycerol that is very hygroscopic from burned gas; this makes the fire extinct at methanol droplets [1]. Propanol and glycerol are physically mixed while the long bond of fatty acid and glycerol chains in coconut oil makes the combustion characteristics of coconut oil slightly different from propanol and glycerol mixture [15]. Furthermore, water from the burned gas within the coconut oil in a high temperature, together with the hydrolysis reaction, can break the triglyceride chains into fatty acid and glycerol. Shortly after that, these components are individually burned. Therefore, there are a lot of aspects regarding the impacts of coconut oil temperature on the combustion characteristics that need to be intensively studied.

Several studies [3], [12], [10] explains the performance of diesel engine using vegetable oil with glycerol that frequently causes problems. Abdulwahid et al. [9] also observed the characteristics of premixed flame of multi-component fuel (C4H10-C3H8), and they found out cellular flame structure as a result of large difference in the diffusion between fuel and oxidators. Kang et al.[8] investigated the characteristics of laminar premixed flame in narrow lines under the influence of heat and momentum losses. It was found out that the two factors played complicated roles in determining the whole instability of the cell formation patterns. This has been identified in several studies, one of which was conducted by [7] who investigated the formation of oil molecule structure that consists of glycerol and fatty acid in two steps of combustions: the first is the combustion of fatty acid and the second is the combustion of glycerol where a lot of micro explosion can occur.To create an efficient process in a very stable premixed flame, micro explosion plays an important role within the burning velocity. This article reports a study focused on the identification of micro explosion during laminar premixed flame propagation of coconut oil. Micro explosion is a very interesting subject that influences the mass transfer and other transport phenomena. The enthalpy of product and the burning rate

944 Hadi Saroso, ING Wardana, Rudy Soenoko and Nurkholis Hamidi produced concentration and temperature gradient. These gradients can be disturbed by the micro explosion that can also drastically trigger chemical reaction. Several studies have been conducted to understand micro explosion within burning process [2]; [6]; [12]. However, very little literature can be found regarding the impacts of this interaction on micro explosion of coconut oil and on the burning velocity at Hele-Shaw cell. Therefore, this present study focuses on the identification of the characteristics of premixed combustion of coconut oil in the Hele-Shaw Cell.

2. Experiment 2.1. Experiment Equipments and Conditions

This experiment was conducted using the experimental apparatus as shown schematically in Figure 1. The combustor consists of two plates of acrylic glass and steel with the dimension of 500x200x10 mm. It has two igniters at the top and bottom of the combustor to initiate combustion. The overflow tube is used to contain water that release from the combustor during introducing of coconut oil vapor and air. Boiler is used to evaporate coconut oil while thermocouple controll is used to control the temperature of heater of coconut oil vapor. The compressor is used to intake air which ratio with coconut oil vapor is varied to get equivalent ratios (Ф) of 2.73; 2.57; 2.34; 2.19; 2.03; 1.93; 1.78. The high speed camera CCD is used to record the flames propagation inside the combustor.

2.2. Data Collection

The recording result from the video camera in the form of moving pictures were extracted into a series of static pictures arranged chronologically from the beginning to the end of flame using Pinnacle Software. Every Ф illustrated certain pictures with different patterns of burning velocity in every frame. The flame images were illustrated as transparent pictures piled using software Adobe Photoshop 7.0. The flame position in every frame was meassured by this software. The behavior of the flame was determined from the rate of flame position change. The instability behavior was analyzed based on the length and the thickness of the flame.

Burning characteristics of coconut oil vapor-air mixtures 945

Figure 1 Experimental apparatus

Figure 2 Hydrolisis Reaction of Triglyceride

Tabel 1 Fatty Acid Composition of Coconut Oil Vapor ( ± 200oC) Fatty Acid No.carbon::No.

doublebond Formula Composition (%)

Lauryc Acid 12 : 0 C12H24O2 40.35 Myristic Acid 14 : 0 C14H28O2 11.45 Caprylic Acid 8 : 0 C8H16O2 18.13 Capric Acid 10 :0 C10H20O2 14.63 2.3. Characteristics and Chemical Structure of Coconut Oil

Coconut oil has characteristics and general molecule structure similar to other vegetable oil named as triglyceride. The molecule structure consists of functional group of Ester 3. Figure 2 shows that the hydrolisis reaction of triglyceride produces glycerol and fatty acid. In the fatty acid, Ra, Rb, and Rc represent the group of carbon atom and hydrogen.

946 Hadi Saroso, ING Wardana, Rudy Soenoko and Nurkholis Hamidi

The composition of fatty acid was measured using Gas Chromatography

(GC). Fatty acids mainly found in coconut oil included linoleat, oleat, palmitat, and stearat acid; the composition is presented in Table 1.

3. Results and Discussion

Figure 3 illustrates the flame propagation ignited from top at Ф = 2.73; 2.57; 2.34; 2.19; 2.03; 1.93; and 1.78. The frames of flame image were arranged chronologically. At each Ф flame reaches the bottom of the combustion chamber at 1379s; 0758s; 0931s; 0896s; 0.689s; 1.0s; and 1,172s after ignition, respectively. All flame shows two layers of reaction zone. The first layer in the flame front has higher speed than the second layer behind. At rich mixture, Ф = 2.73, 2.57, and 2.34, the first layer flame is very thin for high rate of combustion while the second layer is very thick due to low rate of combustion reaction. The two flame layers are caused by the fact that during combustion the oil is degraded into fatty acid and glycerol by hydrolysis reaction as shown in Fig. 2. The water comes from combustion reaction product. Because glycerol is hygroscopic [15] while fatty acid is hydrophobic, the first layer in the flame front must be fatty acids flame and the second layer is glycerol flame that tends to create micro explosion. As Ф decreases or the air in the mixture increases, the glycerol flame speed increases and the flame merges with that of fatty acids. At Φ = 2.57 micro explosion clearly appear and became markly visible at Ф = 2.03 because the flame speed is very high. The high flame speed release high heat energy sufficient to burn glycerol that produce microexplosion due to high water content. At Ф = 1.93 and 1.78 micro explosion tends to disappear because flame speed is low and heat released by the flame is lower. Therefore, the heat energy is not enough to increase the water pressure inside the glycerol.

(a)

0.034 sec 0.17 sec 0.306 sec 0.442 sec 0.476 sec 0.51 sec 0.544 sec 0.578 sec 0.612 sec 0.646 sec 0.793 sec 0.827 sec 0.862 sec 0.896 sec 1 sec 1.034 sec 1.068 sec 1.102 sec 1.238 sec 1.374 sec

Bu

0

0

0

0

urning char

0.102sec 0.136se

0.442sec 0.476se

0.034sec 0.102se

0.476sec 0.510se

0.034sec 0.068se

racteristics

ec 0.170sec 0

ec 0.510sec

ec 0.170sec ec 0.544sec

c 0.102sec

of coconut

0.204sec 0.238s

0.544sec 0.578se

0.238sec 0.272se

0.578sec 0.680se

0.136sec 0.170se

oil vapor-a

sec 0.272sec

ec 0.612sec

ec 0.306sec

ec 0.724sec

ec 0.204sec

air mixtures

0.306sec 0.340

0.646sec 0.680

0.340sec 0.374

0.758sec 0.79

0.238sec 0.272

s

0sec 0.374sec

sec 0.724sec

4sec 0.408sec

93sec 0.827sec

2sec 0.306sec

(b)

(c)

0.408sec 0.758sec

0.442sec 0.931se

0.442sec

947

94

0

0

0

0

48

0.476sec 0.510se

0.034sec 0.068se

0.374sec 0.408se

0.034sec 0.102se

0.578sec 0.612se

Hadi Sa

ec 0.544sec

ec 0.102sec

ec 0.442sec

ec 0.170sec ec 0.646sec

aroso, ING

0.578sec 0.612se

0.136sec 0.170se

0.476sec 0.510s

0.238sec 0.306se

0.680sec 0.724se

Wardana, R

ec 0.646sec

ec 0.204sec

ec 0.544sec

ec 0.340sec

ec 0.758sec

Rudy Soeno

0.680sec 0.724

0.238sec 0.272

0.578sec 0.612

0.374sec 0.408

0.793sec 0.827

oko and Nur

4sec 0.862sec

2sec 0.306sec

2sec 0.646sec

8sec 0.442sec

7sec 0.931sec

rkholis Ham

(d)

(e)

(f )

896sec

0.340sec

0.680sec

0.476sec 1.0sec

midi


(g) Figure 3 Flame propagation ignited from the top (a) Ф = 2.73; (b) Φ = 2.57; (c) Ф = 2.34; (d) Φ = 2.19; (e) Ф = 2.03; (f) Ф = 1.93; (g) Φ = 1.78

Figure 4 illustrates flame length versus times at various Ф. Elongated flame is considered to be flame stretch. The flames length increased until 0.374 sec due to thermal expansion, then, reduced until 0.408 sec, and started to be unstable until 1.379 sec due to the wall effect. The flame length was shorter at Ф = 1.93 and 1.78 which shows that flames was stable. Micro-explosion is very weak in these flames. It means that micro-explosion of glycerol takes place when flame is very unstable as at Ф = 2.73 to 2.03. The stongest icro-explosion occurs at Ф = 2.03 which show that this flame is very unstable due to it propagate fastes.

Figure 4. Time variation of flame length ignited from the top at various equivalence ratio

0.170sec 0.238sec 0.306sec 0.340sec 0.374sec 0.408sec 0.442sec 0.476sec 0.510sec 0.544sec 0.724sec 0.758sec 0.793sec 0.827sec 0.862sec 0.896sec 0.931sec 0.965sec 1.034sec 1.170sec


Figure 5 shows flme area versus time. It can be seen that flames area became

larger at 0.272 sec, then, reduced until 0.510 sec, and started to be unstable until 1.379 sec. Flames tended to be at the largest at Ф = 2.73 and gradually became smaller in an unchronologychal way until Ф = 1.78. This shows that the flame area is the largest when the flame stretch is the longer which means that the reaction rate is the weakest at 0.758 sec. After that the flame become unstable which occurred because the over-availability of fuel at the beginning of the combustion, and flames gradually weakened and extincted.

Figure 5. Time variation of flame area ignited from the top at various equivalence ratio

(a)

0.034sec 0.170sec 0.306sec 0.578sec 1.0sec 1.204sec 1.340sec 1.408sec 476sec 1.544sec

1.758sec 1.896sec 2.034sec 2.103sec 2.310sec 2.379sec 2.448sec 2.517sec 2.931sec 3.206sec


(b)

(c)






(d)

(e)

(f)

1.272sec 1.408sec 1.476sec 1.544sec 1.612sec 49/29 sec 758sec 1.827sec 1.896sec 2.103sec 0.034sec 0.102sec 0.170sec 0.238sec 0.306sec 0.374sec 0.442sec 0.510sec 0.578sec 0.646sec




(g) Figure 6. Flame propagation ignited from the bottom. (a) Ф = 2.73; (b) Φ = 2.57; (c) Ф = 2.34; (d) Φ = 2.19; (e) Ф = 2.03; (f) Ф = 1.93; (g) Φ = 1.78 Figure 6 shows flame propagation ignited from the bottom part of combustion chamber at various Ф. Time period between the two frames was 0.034 s. At all Ф, fatty acid flame propagated before that of glycerol. This is because of fatty acid which had lower boiling point. Micro-explosion occurred at very rich mixture (Ф =2.73 to 2.34) and at les rich mixture (Ф = 2.19 and 2.03), and it tends to disappear at low burning velocity and lean mixture (Ф = 1.93 and 1.78). The strongest micro-explosion occurs at the largest thermal expansion and the fastest flame propagation, that is at Ф = 2.03. The process of energy release from glycerol resulted in the formation of several radical species. Species radical Csolid was very rich and in yellow, causing moderate explosions. Species radical C2 was rich and in blue-green, causing strong explosions. Species radical CH was poor and in dark purple, causing weak explosions.

Figure.7: Time variation of flame length ignited from the bottom at various equivalence ratio



954 Hadi Saroso, ING Wardana, Rudy Soenoko and Nurkholis Hamidi Figure 7 illustrates flames length versus time ignited from the bottom at

various Ф. It can be clearly seen that at 0.034 sec flames stretch increased until 0.340 sec; then, reduced at 0.544 sec, and started to be unstable and increased again until 1.516 sec and reduced again at 3.224 sec. This shows that flame stretches twice. First after ignition at around 0.34 sec due thermal expansion and then at 1.292 sec due to buoyancy effect. The first stretch produces micro exlosion and the second stretch creates flame instability.

Figure.8: Time variation of flame area ignited from the bottom at various equivalence ratio

Figure 8 shows time history of flames area for all Ф ignited from the bottom. Flames area significantly increased after ignition and reach maximum at 0.544 sec. The maximum flame area indicates the lowest reaction rates that produces micro explosion and flame instability. At Ф = 2.57 flame area tended to be larger and became smaller at Ф = 2.19. This could be due to other factors such as temperature and pressure at the beginning of the combustion.

4. Conclusion Results and discussion of the study can be summarized as below: 1. Coconut oil tends to be degraded into fat and glycerol because of hydrolysis

reaction. Fatty acid is burned before glycerol. Fatty acid tends to produce separated flames of different boiling points and molecule weights. The process of energy release from glycerol needs high rate of combustion process and it tends to slow flame propagation and causes micro explosions. The characteristics of flame propagating downward at equivalent ratios (Ф) = 2.73; 2.57 and 2.34 show orange flames that cause moderate micro-explosions. At equivalent ratios (Ф) = 2.19; 2.03 and 1.93, the flames are blue-green, causing strong micro-explosions. At equivalent ratio (Ф) = 1.78 the flames are dark purple, causing weak micro-explosions. For flame propagating upward, only at equivalent ratio of 2.03, the

Burning characteristics of coconut oil vapor-air mixtures 955 micro explosions can been clearly since the thermal expansion is high and the

flame propagates fastest. 2. Flames characteristics for downward propagation show the longest flames stretch

at Ф = 2.57, indicating larger heat released by the flame. At Ф = 1.78 flames stretch is weak with weak micro explosions. For upward flame propagation, only at Ф = 2.34 flames stretch is larger and this also shows instability. At Ф = 1.78 weak micro explosions occur with low energy. The process of energy release from glycerol is indicated by the unstable flames stretch.

3. As presented in Figure 5, flames characteristics show flames area that diffuses and reacts. At Ф = 2.73; 2.57 moderate micro explosions starts to occur while strong micro explosions starts to occur at Ф = 2.34; 2.19; 2.03 and weak micro explosions starts to occur at Ф = 1.93; 1.78. This could be due to the composition of fuel and air as well as due to heat transfer to the wall that may result in the slowing down of flames going downward. As also seen in Figure 8 about the diffusion and reaction, the area turns to become larger at Ф = 2.57, and it turns to become smaller at Ф = 2.19. This could be due to factors of heat transfer to the wall during combustion.

Acknowledgments Special thanks were delivered to the Ministry of Education and Culture and the Brawijaya University for their support during this research. References [1] B.L. Zhang, F.A. Williams, Effects of the Lewis number of water vapor on the

combustion and extinction of methanol drops. Combust Flame 1998; 112:113–20.

[2] C.H. Wang, W.G. Hunga, S.Y. Fua, W.C. Huanga, C.K. Law “On the burning and microexplosion of collision-generated two-component droplets: miscible fuels,, Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, 2003

[3] D. Agarwal, A.K. Agarwal, Performance and emission characteristics of jatropha oil (preheated and blends) in a direct injection compression ignition engine. Appl Therm Eng. 27, pp 2314–23, 2007.

[4] Gajendra Kumar, D. Kumar, Shailandra Singh, S. Kothari, Sumit Bhatt and Chandra P. Singh “Continuous Low Cost Transesterification Process for the Production of Coconut Biodiesel,, Department of Chemistry, Sahu Jain College, Najibabad, 246763, India; 2010.

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Received: August 2, 2013

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Burning Characteristics of Coconut Oil Vapor-Air · PDF fileBurning characteristics of coconut oil vapor-air mixtures 943 This indicates that the main characteristics of coconut oil