Burning of Fuel Oil Mixed with Biofuels Derived from

Lauan Wood

Fakhrur M. Rizal1, Ta-Hui Lin

2*, Tzu-Yueh Yang

3, Hou-Peng Wan

3, Hom-Ti Lee

3

1Department of Mechanical Engineering, National Cheng Kung University

2 Research Center for Energy Technology and Strategy, National Cheng Kung University

3 Industrial Technology Research Institute, Green Energy and Environment Research

Laboratories, New Energy Technology Division

*e-mail: thlin@mail.ncku.edu.tw (corresponding author)

Abstract

Vaporization characteristics of a single droplet of pyrolysis biofuels (PB) and fuel oil

emulsified were examined by using a suspended-droplet heating device. The tested biofuels

were produced from the pyrolysis process of lauan (shorea) wood. The heating temperature

and the mixing ratio of fuel oil and biofuels were varied in the experiment. Variations of drop

images in the vaporization process were recorded using a high speed camera. The d2-law was

applied to determine the behavior of the heated drop in the vaporization process. Results

showed that microexplosion and random behavior occurred for all cases of biofuels and fuel

oil emulsified and also pure biofuels. Random behavior of microexplosion occurred was

caused by the biofuels are being composed of many chemical compositions with various of

boiling points and also high contents of water. The evaporation rate of the drop identified by

the slope of d2-law line increased with heating temperature. Microexplosion and random

behavior of drop occurred more often at high temperatures and also ignition was found for

several cases at 500 OC.

Keywords: vaporization, single droplet, pyrolysis biofuels

1. Introduction

Currently, fossil fuels are still the main options as energy source for the world. However,

the availability of fuel is becoming less for each passing year. Research shows that within 40-

50 years, fossil fuels will become scarce and hard to find. And not only that, fossil fuels has

been proven responsible for the environment adversities such as global warming, acid rain,

urban smog, etc. due to the level of pollutant emissions produced [1]. Therefore, it is

necessary to find environmental-friendly alternative fuels that can be produced steadily.

Biofuels has emerged as the alternative fuels that can replace fossil fuels. There are

several reasons why biofuels become good alternative fuels, which include lower emissions

of greenhouse gases and pollutants such as sulfur (virtually none) and soot, as well as

polycyclic aromatic hydrocarbon (PAH) and nitrited PAH (regarded as carcinogens),

reduction of deforestation, increased lubricity for long-life utilization, and a higher flash point

for safer storage and management [2]. In addition, biofuels can be produced from various

types of plants such as rapeseed [3, 4], sun flower [4], cassava [5], sugarcane [6], etc. And

they can also be produced from wood [7], and fish oil [8].

Pyrolysis is an applicable method that can be used to convert energy from biomass into

biofuels by thermochemical conversion technology [9]. It involves the heating of organic

materials in the absence of reagents, especially oxygen, to achieve decomposition. Pyrolysis

biofuels (PB) is black-brownish liquids obtained by the condensation of vapors during the

pyrolysis of wood and other vegetable biomasses. The efficiency of the production process is

very high, typically we can generate around 70% of PB in weight from the raw material [10].

Pyrolysis biofuels are multi-component mixtures of different chemical compounds

derived from depolymerization and fragmentation of cellulose, hemicellulose and lignin.

Therefore, the elemental composition of PB and petroleum derived fuel is different [11, 12].

Consequently, the chemical and physical properties of fast pyrolysis bio-oil adversely affect

their combustion properties and result in difficulties in storage and handling. Biofuels is

characterised by high viscosity, acidity and electrical conductivity, presence of water and

various oxygenated compounds, ash and other solid impurities [13]. Biofuels has low heating

value, and does not ignite readily. Bio-oil is shown to be unstable when subjected to

relatively high temperature for long periods. A characteristic of this unstable oil is its self-

polymerization [14].

Many projects were conducted about pollutant emissions and performances of engines

or power plants using biofuels as fuel [1, 15]. On the other hand, only few fundamental

studies were conducted about the vaporization and combustion characteristics of biofuels

droplets. Such studies could provide the necessary basic data to characterise the mechanisms

responsible for deposit formation during biofuels combustion. Wornat et al. [16] have

performed single droplet experiments with two biomass oils, produced from the pyrolysis of

oak and pine. Liquid-phase polymerization and pyrolysis of the oxygenate-rich biomass oils

lead to the formation of carbonaceous cenospheres. The vaporization mechanisms of waste

vegetable oils droplets were investigated by Li et al.[17]. Results show that the biodiesel

droplet has higher burning rate, and that biodiesel in general has a lower propensity to soot

because its molecular oxygen content promotes the oxidation of the soot precursors. Calabria

et al. [18]investigated the combustion fundamentals of pyrolysis-oil-based fuels. The

microexplosion mechanism inside the droplets was observed and was found more important,

with biofuels droplets than with diesel fuel. According to the chemical composition of

biofuels and their esters, the residue formation is more or less important [4].

In this work, experimental results concerning the vaporization and droplet behavior of

mixed diesel and biofuels droplets are presented. The vaporization and droplet behavior have

been determined for mixed fuel oil (diesel oil, heavy oil) and biofuels at a variety of mixing

ratios and temperatures under atmospheric pressure. The results are compared with those

using pure fuel oil and pure biofuels at the same variety temperature.

2. Experimental setup

A schematic of the experiment apparatus used in the study of suspended drop is shown in

figure 1. The experiment was conducted by placing a single droplet of fuel on the moving

thermocouple which simultaneously measured the temperature of the fuel drop and another

thermocouple was used to measure the temperature between the heating plates. Both of the

thermocouples we used were K-type thermocouples. Two heating plates kept the temperature

at a stable value during the test. The temperature between heating plates was controlled by

the temperature controller [19]. One camera shooting was conducted after the droplet was

hung on the thermocouple to determine the initial size of the droplet before the heating

experiment began.

The biofuels was produced from pyrolysis of lauan (shorea) wood. The basic data of

the biofuels are shown in Table 1. The experiments were conducted at different temperatures

with a variety of mixing ratio (pure fuel oil, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%,

80%, 90%, 100% of biofuels). Two kinds of fuel oil were chosen: diesel oil and heavy oil.

The average initial diameter of the drop diameter was around 1 mm. Because of the thickness

of the suspension fiber and its thickened end because of the joining process, it is somewhat

difficult to suspend a droplet much smaller than 1 mm in diameter, which is much larger than

typical droplet sizes within sprays. This should not be of serious concern if the size-

dependence of the phenomenon of interest is known. However, the shape of the suspension

fiber (thermocouple) that also gave an effect in the process of evaporation, particularly

towards the end of the droplet lifetime. This is because the size of the droplet at that time was

close to the size of the joining suspension fiber [20].

Table 1. Chemical properties of the pyrolysis biofuels of lauan wood

Fig 1. Experimental apparatus

The experiment began with the movement of the first thermocouple to the heating plates

by turning on the motor. When the moving arm touched the switch (trigger), the

thermocouple was in the right position. The phenomenon was captured and recorded using a

high speed camera. There was a delay circuit to set the delay time before the high speed

camera was turned on. The delay was conducted because of the limitation of the high speed

camera, which could only record the pictures for just a few seconds. High speed camera was

turned on before the lights to synchronize the time between the images with the data in the

computer. The images were measured and then the droplet size variation was plotted against

time according to the D2 law [21, 22].

Figure 2 shows some of the phenomena that may occur during the suspended drop

experiment. Line A-B-C-D shows the temperature of the droplet. Line a-b-c-d-e indicates the

diameter-squared of the drop. A-B is a phenomenon where the droplet absorbs heat from the

surrounding temperature. In this stage, the droplet starts to react with the heat treatment, such

as in a-b which microexplosion occurred early in the experiment and then followed by

evaporation in line b-c. c-d is the expansion phenomenon and ends with the stable state

condition shown in d-e where the droplet is at a stable size for a while. Plot B-C is the

ignition event and the droplet burned in this time interval. After ignition occurs, the

temperature becomes stable as shown in C-D. The results obtained were not all like figure 3.

It depends on the parameters imposed on the experiment.

Fig 2. Single droplet burning phenomenon

3. Results and discussion

Vaporization experiments for diesel/biofuels emulsified droplets, heavy oil/biofuels

emulsified and pure biofuels have been conducted in air at different temperatures and under

atmospheric pressure with a variety of mixing ratios.

3.1 Combustion of pure pyrolysis biofuels

The evolution of the droplet diameter was plotted versus the normalized time. Figures 3

and 4 show the experimental results for pure biofuels. In figure 3 at T = 300 OC, we can see

that microexplosion and random behavior occurs during the test as shown by the squared-

diameter of the droplet fluctuated which changes quite violently from t = 0 s to t = 5 s and

also can be seen at the images sequences at t = 0.263 s, 1.472 s, 1.731 s. Random behavior is

the condition where the size of the droplet kept changing and unpredictably. Random

behavior and micro explosion occurs because of bubbling. The bubbling phase is

characterized by the formation of small bubbles that move toward the surface of the droplet

where they explode producing small fragments, see images sequences at t = 1.47 s. The

biofuels used was a multi-component fuel in which there were many types of compounds

contained within. High percentage of water contents inside the biofuels also become the

major factors of the occurrence of bubbling phase. Droplet diameter changes cannot be

predicted when the microexplosion and random behaviors occurs. Ignition can not be found

in all cases with T = 300 OC because the temperature was not high enough to ignite the

droplet.

Fig 3. Suspended droplet results of pure biofuels at 300 OC

At higher temperatures as shown in figure 4 at T = 500 OC, Microexplosion and

random behavior occurs from t = 0 s to t = 2 s followed by expansion as can be seen in

images sequences at t = 3.648 s. during expansion periods, the droplets size did not change or

-4 0 4 8 12

Time (s)

0

400

800

1200

Tem

per

ature

(OC

)

0

1

2

3

(d/d

0)2

Pure biofuels

T = 300 OC

d0 = 1.12 mm

kept stable. Expansion corresponds to the heterogeneous combustion of cenospheres, i.e., the

carbonaceous particle formed by pyrolysis oils during the last stages of droplet combustion

[18]. Ignition can be observed in this case as shown by the changing of temperature curves at

t = 3.8 s to t = 5 s and also can be seen in the images sequences at t = 3.821 s where the flame

is appeared.

Fig 4. Suspended droplet results of pure biofuels at 500 OC

3.2 Combustion of emulsions of pyrolysis biofuels in diesel oil

Experiment using emulsions of pyrolysis biofuels in diesel oil were conducted for 5%

biofuels and each multiple of ten percent in the mixture. The experimental results shows that

more biofuels content in the mixtures, more unstable the emulsion, as already explained

above that the biofuels itself basically consists of many components. So, just a little biofuels

in the diesel can result in microexplosion and random behavior. More biofuels significantly

resulted in microexplosion and random behavior occurred frequently and more quickly.

At low percentages of biofuels in the diesel oil as shown in figure 5, diesel oil dominates

where in the experiment using pure diesel oil, random behavior and microexplosion can not

be found, only evaporation occurs followed by increasing the evaporation rate when the T

increases. In figure 5 we can see that microexplosion and randon behavior occurs in small

scales as shown in images sequences at t = 0.33 s where the droplets size changes a little bit.

Not much carbonaceous particles can be formed due to the amount of biofuels so that the

expansion almost can not be observed, but little bit amount of fuel left on the thermocouple

and finally ignition occurs as shown by the changing of temperature curves at t = 1.4 s to t =

2 s and appearance of flame in images sequences at t = 1.4 s.

-4 0 4 8 12

Time (s)

0

400

800

1200

Tem

per

ature

(OC

)

0

1

2

3

(d/d

0)2

5% Biofuels, 95% Diesel oil

T = 500 OC

d0 = 1.1 mm

Fig 5. Suspended droplet results of emulsions of biofuels in diesel oil at 500 OC

Increasing the amounts of biofuels will increase the emergence of microexplosion and

random behavior as shown in figure 6 squared-diameter of droplet and images sequences at t

= 1.296 s. Expansion can be observed in this case as shown in images sequences at t = 4.212 s

followed by ignition at t = 4.462 s. a lot of carbonaceous particles formed when the amounts

of biofuels increased so that the expansion can be observed more clearly.

Fig 5. Suspended droplet results of emulsions of 50% biofuels in diesel oil at 500 OC

3.3 Combustion of emulsions of pyrolysis biofuels in heavy oil

Suspended droplet experiments were also conducted for the cases of biofuels and heavy

oil. Random behavior, micro-explosion, bubbling, and expansion also can be found in these

cases but for ignition, only several cases in biofuels/heavy oil emulsions when biofuels

-4 0 4 8 12

Time (s)

0

400

800

1200

Tem

per

ature

(OC

)

0

1

2

3

(d/d

0)2

5% Biofuels, 95% Diesel oil

T = 500 OC

d0 = 1.1 mm

-4 0 4 8 12

Time (s)

0

400

800

1200

Tem

per

ature

(OC

)

0

1

2

3

(d/d

0)2

60% Biofuels, 40% Diesel oil

T = 500 OC

d0 = 0.96 mm

content is higher in the emulsions.

For biofuels/heavy oil emulsions, evaporation and bubbling occurs very slow but stronger

than biofuels/diesel oil cases. At low percentage of biofuels in heavy oil as seen in figure 6,

microexplosion occurs very strong. The droplets size increase up to 2 times of the initial size

before exploding as shown in squared-diameter curves at t = 2 s. compared with

biofuels/diesel oil emulsions which microexplosion occurs weaker but faster. That’s because

of heavy oil itself has higher boiling point than diesel oil, so that in the heavy oil cases, the

occurrence of evaporation and bubbling was little bit longer than diesel oil cases. And also

the viscosity of the heavy oil is higher than diesel so for exploding needs bigger bubble. At

small amount of biofuel in the emulsion, ignition could not be observed. 500 OC is not

enough to ignite the droplets. But at higher percentage of biofuels at 500 OC, ignition

occurred. Ignition occurs started from case with 60 % biofuels in the emulsions. In figure 7

we can see the ignition occurs at 500 OC with 70% biofuels in the emulsions as shown by the

changing of the temperature curves at t = 9 s to t = 11 s.

Fig 6. Suspended droplet results of emulsions of 5% biofuels in heavy oil at 500 OC

-4 0 4 8 12

Time (s)

0

400

800

1200

Tem

per

ature

(OC

)

0

1

2

3

(d/d

0)2

5% Biofuels, 95% HFO

T = 500 OC

d0 = 0.95 mm

Fig 7. Suspended droplet results of emulsions of 70% biofuels in heavy oil at 500 OC

3.4 Burning of Droplets Emulsified

In the burning of droplets emulsified, there were 3 stages or phenomenons can be

observed. All the cases had the same profiles of temperature curves where heat absorption at

the beginning followed by drastic changes of temperature (ignition) and finally the

temperature stabilization. For the (d/d0)2 curves, the phenomenon which can be observed was

the same for the ignition cases. We found microexplosion and random behavior (bubbling

stage), evaporation, expansion, and end with ignition.

In the burning cases, time interval of each phenomenon became the major different.

Figures 8 shows the time interval between each phenomenon. Expansion initiation means the

state where the droplet started to become bigger. In this state, evaporation and bubbling phase

occurred. Ignition means the beginning of burning. And the last is extinction after the burning

was finished and the droplet completely burned.

For biofuels/diesel oil emulsions (see figure 8a), the results shows that more biofuels in

the emulsions, the phenomenon that occurred became longer for not only evaporation and

bubbling but also the expansion and ignition interval. More biofuels means more

carbonaceous particle formed so the expansion became longer and then more biofuels made

the emulsion became more unstable so that the microexplosion and random behavior

(bubbling state) became longer too. And also the burning became longer because more

biofuels means more flammable substance inside and slower burning speed because the

heating value of biofuels is low compared with fuel oils and water content also increased by

increasing the amount of biofuels. For the 70, 80, and pure cases the results was lower than

the previous. That’s because of during microexplosion, small droplets came out from the

-4 0 4 8 12

Time (s)

0

400

800

1200

Tem

per

atu

re (

OC

)

0

1

2

3

(d/d

0)2

70% Biofuels, 30% HFO

T = 500 OC

d0 = 1 mm

main droplet so that the interval between each phenomenon became shorter.

For biofuels/heavy oil emulsions (see figure 8b), ignition did not occur for the cases with

5-50% biofuels inside but when we added more biofuels inside, ignition could occur. It was

difficult to burn heavy oil at 500 OC. Even at 500

OC, the evaporation occurred slower

compared with others fuels. Not enough flammable substance in the emulsion with lower

percentage of biofuels. At higher percentage (more than 50%), ignition was found but the

ignition time is quiet long. Adding more biofuels will decrease the ignition time as shown in

that figure. The key point for biofuels/heavy oil cases at 500 OC is more biofuels, more

flammable substance so that the ignition became faster occurred.

Figure 8. Burning cases of emulsions of biofuels/diesel oil (a), biofuels/heavy oil (b).

4. Conclusions

Suspended droplets experiment was conducted to see the vaporization and burning

phenomenon of the emulsions on the micro point of view. Emulsions was prepared for

biofuels/diesel oil emulsions and biofuels/heavy oil omulsions and also the mixing ratio

became the parameter in this study.

In the experiment using pure biofuels, micro-explosion and random behavior of droplet

occurs. Droplet size changes can not be predicted and also ignition occurs at 500 OC. For the

diesel/biofuel emulsions, random behavior and micro-explosion occurred during the

experiment. The more contents of biofuels in the mixtures, micro-explosion and random

behavior of the droplet occurred more frequently. Some of the particles contained in the

biofuels expanded at the end of the droplet lifetime and the droplet would be in stable state

0 20 40 60 80 100

Mixing ratio (%biofuels)

0

5

10

15

20

25

Tim

e (s

)

Expansion

Ignition

Extinction

biofuels/diesel oil (500 OC)

0 20 40 60 80 100

Mixing ratio (%biofuels)

0

5

10

15

20

25

Tim

e (s

)Expansion

Ignition

Extinction

biofuels/heavy oil (500 OC)

no ignition

a) b)

condition where its size would not change for several seconds before finally burned. In the

case of 500 OC the ignition occurred a moment after the stable state.

For biofuels/heavy oil emulsions, ignition did not occur at low percentage of biofuels in

the heavy oil. Ignition could be observed when the amount of biofuels in the heavy oil was

increased more than 60% but it took longer times for biofuels/heavy oil emulsions to be

burned compared with the biofuels/diesel oil cases.

5. Acknowledgement

The financial support provided by Bureau of Energy (Grant No. 100-D0103) is gratefully

acknowledged.

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