Elsevier

Journal of the Energy Institute

Volume 98, October 2021, Pages 35-43
Journal of the Energy Institute

Effects of ethanol on the evaporation and burning characteristics of palm-oil based biodiesel droplet

https://doi.org/10.1016/j.joei.2021.05.008Get rights and content

Highlights

  • Palm biodiesel-ethanol droplets exhibit more rapid bubble growth and explosion.

  • Greater micro-explosion occurrences observed for palm biodiesel-ethanol droplets.

  • The evaporation duration of palm biodiesel-ethanol droplets decreased significantly.

  • The burn-rate and burning period of palm biodiesel-ethanol droplets improved.

Abstract

Palm biodiesel-ethanol blends could be the better automotive fuel over palm biodiesel alone owing to the high oxygen content and high volatility of ethanol. The evaporation and burning characteristics of palm biodiesel with 10%, 20%, and 30% ethanol by volume concentration respectively, were investigated by using the droplet experiment. Rapid bubble growth and bubble explosions during the heating stage were observed to be substantial for the palm biodiesel droplet added with 30% volume of ethanol (BE30). Adding ethanol in palm biodiesel increased the ignition delay and burn-rate constant by up to 38.6% and 23.2%, respectively whereas, the evaporation duration and burning period decreased by up to 20.6% and 22.5%, respectively. Overall, the relatively shorter evaporation duration and higher burn-rate found for the BE30 droplet could promote high thermal efficiency and has the potential to achieve ultra-low carbon monoxide (CO) and unburned hydrocarbons (UHCs) emissions in diesel and gas turbine engines.

Introduction

The growing worldwide demand for energy and concerns on environmental issues have driven continuous search for clean and sustainable fuels for transportation. Over the past decade, fossil fuels have been the dominant energy source for aviation, stationary power generators, and automobiles. This has greatly impacted the environment through increased greenhouse gases and hazardous emissions including particulate matters (PM), nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHCs). Concern over the depletion of non-renewable fossil fuels has also driven demand for cleaner and sustainable fuels. Nevertheless, biodiesel could become fuel substitutes over fossil diesel for powering jet engines and internal combustion engines and stationary power generators. Biodiesel has gained attention over the recent years owing to their renewable, zero sulphur, and reduced net carbon footprints capabilities [1]. They generally produce lower exhaust emissions than that of fossil diesel owing to their zero-sulphur content, oxygenated content, and higher cetane number. The presence of oxygenated content in biodiesel promotes complete combustion that results in lower CO and smoke emissions compared to fossil diesel counterparts. Biodiesel also exhibits good lubricity for improving the moving parts in engine. These benefits have driven increased production of biodiesel from various feedstocks over the years [2]. To date, palm, soybean, used cooking oil, and rapeseed biodiesels are part of the largest biodiesel production globally [3].

Notwithstanding the above-mentioned benefits of biodiesel, there are several drawbacks when they are used in diesel or turbine engines alone. The brake specific fuel consumption and NOx emissions are generally higher for biodiesel as compared to fossil diesel. A comprehensive review by Noor et al. [4] reported higher amount of brake specific fuel consumption by up to 20.9% and greater NOx emissions by up to 16% for when tested with recycled oil biodiesel in marine engines. On the other hand, Rochelle and Najafi [5] pointed out in their recent literature findings that the fuel consumption for biodiesel fueled in a gas turbine was 4% higher and lower thrust power by 8% compared to that of the diesel counterpart. Fuel stability, high production and feedstock costs, and cold flow properties are also other major limitations of biodiesel. However, effective fuel blend strategies could help to mitigate some of the problems associated with biodiesel. The properties of biodiesel can be improved by blending them with other fuels to achieve the desired fuel consumption and emissions in diesel engines.

The possibility of fuel blend approach between biodiesel and other biofuels (e.g., alcohol and ethers) are underway to realize the goal of alternative fuel for transportation. Barriers to production costs and fuel economy could be lessened without the dependency on biodiesel alone. Recent studies on the effects of alcohol in biodiesel from various feedstocks have shown promising outcomes when tested in diesel engines owing to their high oxygenated fuel content. Shirneshan et al. [6] reported that biodiesel-ethanol blends increased brake specific fuel consumption by 16% and decreased NOx, CO and smoke emissions by 17%, 44% and 38%, respectively when tested in a direct-injection, water-cooled diesel engine. Recent studies by Xuan et al. [7,8] found that the addition of methanol and n-octanol in hydrogenated catalytic biodiesel could enhance the soot oxidation rate due to the high oxygen concentration within the fuel thereby decreasing the in-flame soot production. Ma et al. [9] reported that the addition of ethanol with 10% and 20% volume concentrations in biodiesel reduced soot emissions by 59.8% and 51.9%, respectively in a diesel engine. El-Seesy et al. [10] investigated the combustion and emission characteristics of a rapid-compression-expansion machine operated with n-heptanol-methyl oleate biodiesel blends. They concluded that soot and NOx emissions were reduced by 75% and 6%, respectively, when using n-heptanol-methyl oleate mixtures as compared to pure methyl oleate owing to their high oxygen content and high cetane number. El-Seesy et al. [11] also found that the addition of ethanol/hydrous ethanol in diesel fuel reduces both NOx and soot emissions at low intake air temperature when tested on a rapid compression-expansion machine and common rail diesel engine dual. In another similar study, El-Seesy et al. [12] reported a reduction in both soot and NOx emissions on a rapid compression-expansion machine and common rail diesel engine dual for diesel fuel added with n-heptanol at 20% and 40% volume concentration owing to their low flame temperatures and high oxygen content.

Although there are several studies on the effects of adding alcohol in biodiesel on diesel engines, the study on the fundamental aspects of the combustion behavior of biodiesel-alcohol fuel blend is still very limited. Since combustion in diesel engines and gas turbines predominantly involve burning of fine atomized droplets, a detailed analysis of droplet combustion experiment is important to understand the combustion processes and phenomena involved between biodiesel and alcohol. Furthermore, the combustion characteristics of a test fuel can be ascertained because complex combustion processes occurring in a diesel engine such as rapid air swirl mixing and variation in droplet sizes can be eradicated or controlled using the droplet combustion experiment [13]. Therefore, the present study utilizes the droplet combustion experiment to explore the combustion characteristics of palm biodiesel-ethanol fuel blends in detail.

The droplet combustion characteristics of palm biodiesel and palm biodiesel-ethanol blends with ethanol concentration of 10%, 20%, and 30% (e.g., BE10, BE20, and BE30), respectively were investigated in a controlled ambient pressure and temperature [14]. The temporal progression of burning droplet was captured using a high-speed camera to evaluate the combustion parameters (i.e., ignition delay, micro-explosion, burn-rate, and combustion duration) comprehensively [15]. The outcome of the present study would provide a comprehensive insight on the combustion characteristics of palm biodiesel-ethanol blends. This study would also provide vast opportunities for future studies in exploring the potential application of palm biodiesel-ethanol blends for various combustion-related applications, particularly diesel and gas turbine engines.

Section snippets

Fuel preparation

There are four different fuels which were tested in the droplet combustion experiment, particularly, B100 (neat palm biodiesel), BE10, BE20, and BE30 blends. Neat palm biodiesel (i.e., palm methyl ester) was obtained from ExcelVite whereas, neat ethanol of 99.7% purity (analytical grade) was sourced from Sigma Aldrich. The neat palm biodiesel used in this study was a clear liquid derived from crude palm oil through refined and molecularly distillation processes. The main production process of

Temporal progression of droplet evaporation

The temporal progression of B100, BE10, BE20, and BE30 droplets under ambient temperature were analyzed by the normalized droplet cross-section area (A/A0) versus time profiles, as illustrated in Fig. 3. The profile of B100 droplet is quite distinctive as compared to those of biodiesel-ethanol droplets (i.e., BE10, BE20, and BE30). The profile of B100 droplet is observed to be steady with less fluctuations (e.g., peak curve) and the decrement of A/A0 is found to be more gradual than that of

Temporal progression of droplet combustion

The normalized droplet cross section area (A/A0) versus time profiles were obtained for B100, BE10, BE20, and BE30 droplets to analyze the progression of droplet combustion, as shown in Fig. 5. Initially, all the droplets were heated for a period before reaching their respective point of ignition, as indicated in Fig. 5. During the heating period, the temporal progression of B100 droplet was found to be quite different from those of the BE10, BE20, and BE30 droplets. The A/A0 profile of B100

Conclusion

The evaporation and combustion characteristics of B100, BE10, BE20, and BE30 droplets were investigated by using the droplet experiment. The important findings of this study are summarized as follows:

  • The shorter evaporation duration of BE10, BE20, and BE30 droplets compared to that of the B100 droplet by up to 20.6% was attributed to the ethanol's low boiling point and their rapid bubble growth and bubble explosions during heating.

  • The ignition delay of BE10, BE20, and BE30 droplets was found to

Authors declaration

We wish to confirm that there are no known conflicts of interest associated with this manuscript entitled “Effects of Ethanol on the Evaporation and Burning Characteristics of Palm-Oil based Biodiesel Droplet” and there has been no significant financial support for this work that could have influenced its outcome.

We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support and facilities provided by Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Malaysia which made this research project possible. The authors also wish to thank ExcelVite Sdn. Bhd. company for supplying high quality palm biodiesel (EVFuels™) for this project.

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