Research articleThe determination of the activation energy of diesel and biodiesel fuels and the analysis of engine performance and soot emissions
Introduction
The global energy matrix is mainly based on non-renewable resources and extremely dependent on petroleum. The possible alternative energy sources are biofuels originated from numerous oleaginous plants. Normally, this fuel is produced by transesterification, which consists of a chemical reaction of vegetable oils or animal fats with ethyl or methyl alcohol, stimulated by a catalyst [1,2].
Biodiesel fuel obtained from soybean is widely significant in the substitution of mineral diesel fuel. In Brazil, it represents 64.84% of the national production of biodiesel fuel according to ANP (the Brazilian National Agency of Petroleum, Natural Gas and Biofuels) [3]. The diesel engine emits high levels of particles and NOx due to high injection pressures and increased air-fuel ratio [4]. It is, in this sense, critical to diesel engines, the reason why its emissions must be reduced without limiting its performance. Biofuels can positively contribute to this equilibrium showing its sustainable nature. The use of alternative fuels is increasingly attracting scientific interest [5], being biofuels widely used in diesel engines [6] due to its advantages and especially due to its environmental qualities [7,8].
Cardenas et al. [9] investigated the emissions and performance of rapeseed, soybean and sunflower biodiesel fuels and their 30% blends with diesel, comparing them with a reference mineral diesel fuel. The experimental tests were done according to the NEDC system. In terms of performance results, the biodiesel fuels displayed higher consumption over time than diesel fuel. Referring to pollutants, the biodiesel fuels presented higher emissions of CO, THC and NOx, and lower values of smoke opacity. The accelerations during urban cycles showed considerable emissions peaks. An explanation for these emissions data is that biodiesel and its blends have a lower exhaust gas recirculation valve opening, which decreases opacity and soot emissions, but increases other pollutants.
Bermudez et al. [5] investigated consumption and emissions behavior of soybean, rapeseed and palm biodiesel and an alternative Fischer Tropsch (FT) fuel, comparing them with a mineral diesel fuel in an engine under NEDC. The fuel consumption of all biodiesel fuels showed higher values compared to diesel and FT fuels. They also led to higher emissions of NOx, CO and HC. By analyzing soybean biodiesel in comparison to the other biodiesel fuels, higher concentrations of HC and CO and lower of NOx have been found. Because of the presence of aromatics in their formula, mineral diesel and FT fuels created benzene and toluene in HC emissions, which are cancer-causing compounds.
Bermudez et al. [10] studied the pollution rate and particle sizes of particulate matter from the burning of soybean, rapeseed and palm biodiesel, S10 and S50 mineral diesel and Fischer Tropsch fuels. As regards the total particulate matter production, the lowest emissions were from palm biodiesel, while soybean presented the third best results. It was observed that at low speed and load, the biodiesel fuels produced more PM than mineral diesel fuels. However, at medium speed and load, the biodiesel fuels showed lower total concentrations of particles on emissions. In terms of particle size from biodiesel fuels emissions, the particles are smaller than those produced by mineral diesel fuels. According to the author, the presence of sulfur in mineral diesel fuels is the reason why the particle size and geometry increase.
There are plenty of results from experimental tests of biodiesel-fueled internal combustion engines. Most of them present data on performance and emissions. However, detailed analyses of the combustion are needed to better understand the biodiesel burning process. A technique which has been extensively used to determine the activation energy of various solid and liquid fuels is the thermal degradation assessed by thermogravimetry. These studies have been performed based on fuels to be used under different combustion conditions, i.e., for different kinds of thermal machines. Kok et al. [11] studied the nature of the combustion of agricultural solid residues using thermal analysis techniques. Luo et al. [12] investigated the combustion chemical kinetics of coke on deactivated catalysts with the use of thermogravimetry. Borsato et al. [13] adopted the thermogravimetric analysis to determine the activation energy of the soybean biodiesel fuel mixed with three kinds of synthetic antioxidants. Crnkovic et al. [14] determined the activation energies of the crude glycerin and of the beef tallow for their application in engines. Also for that purpose, Conconi et al. [15] analyzed the behavior of the activation energies of three kinds of fuels produced in Brazil: mineral diesel, sugarcane farnesane and soybean biodiesel.
The objective of the present study is to determine the activation energy for the thermal decomposition of diesel and soybean biodiesel fuels, to specify which fuel or fuel blend leads to the best combustion quality under atmospheric pressure on a thermogravimetric balance. Additionally, it tries to make the same estimations from engine performance and soot emissions experimental test, i.e., under high pressure and high heating rates. The fuels tested were: Brazilian commercial diesel fuel, pure soybean biodiesel (100% biodiesel, B100) and blends of 20% (B20) and 50% (B50) biodiesel in diesel.
Section snippets
Materials
Two different fuel samples, diesel and biodiesel, were used in this study. The diesel fuel is commercialized by Petrobras as S10 which is a blend of 93% pure diesel and 7% biodiesel fuels. The biodiesel fuel sample for the tests was obtained from the Brazilian company Granol, processed by transesterification with methanol. Properties of the pure fuels are shown in Table 1. In the engine experimental tests, in addition to pure fuels, two blends with different proportions of diesel and biodiesel
TG, DTG and DTA results and discussion
Fig. 2 shows the TG and DTG curves at a 10 °C min−1 heating rate of biodiesel and mineral diesel fuels. These figures reveal curves for a single heating rate for the sake of displaying the different thermal decomposition phases in a more visually accessible manner. However, similar curves were obtained for the other heating rates applied (5.0 and 20.0 °C min−1).
Two and three phases were identified as significant for diesel and biodiesel fuels, respectively, represented by rectangles in Fig. 2.
Conclusions
In this study, we observed that from two different techniques (Thermal analysis and engine experiments) it is possible to evaluate fuels to be used in engine. The findings of thermal analysis (TG and DTA) experiments suggest that, under atmospheric pressure and low heating rate conditions, the mixture of diesel and biodiesel fuels is the most beneficial for the combustion process rather than using either pure diesel fuel or pure biodiesel fuel.
From the engine performance and emissions tests,
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgements
The authors want to thank: (a) INMETRO-RJ for the engine experimental tests, (b) Granol for donating the soybean biodiesel fuel, (c) Department of Agriculture and Livestock of the State of Tocantins (SEAGRO-TO), for integrating the government, the companies and the education and research institutions, allowing this work, (d) Rômulo Neves for reviewing the English writing of this paper.
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