Thermal behavior of diesel / biodiesel blends of biodiesel obtained from buriti oil

Biodiesel has been obtained from methanolysis of buriti oil. This biodiesel was added in fossil diesel in order to obtain diesel/biodiesel blends. Thermal analysis of blends were carried on 30-600C range at rate of 10C min.. Kinetic parameters such as activation energy (Ea), pre-exponential factor (A), Gibbs energy (G), enthalpy (H) and entropy (S) of activation were determined by using Coats– Redfern equation. The Ea, H and G values presented a linear increase with biodiesel amount added in blends. The heat of combustion of diesel/biodiesel blends was determined, and it was observed that the heat of combustion decreased with the addition of biodiesel in diesel/biodiesel blends.


Introduction
Most of the energy consumed in the world comes from petroleum, coal and natural gas, mainly petroleum derived fuels.Exhaust emissions from diesel burning in motor vehicles contain hundreds of compounds, either in the gas or particulate phases, which contribute to global warming (FERNANDO et al., 2006).Several of these compounds have been proved or suspected to be carcinogenics such as: formaldehyde, acrolein, PAHs, nitro-PAHs, benzene, sulphate etc. (MEHER et al., 2006).Among the several alternatives that substitute fossil diesel for environmentally friendly fuels, the vegetable oils are one of the most promising renewable source of liquid fuels (GOMES et al., 2008;KROUMOV et al., 2007).Besides, the vegetable oils present some qualities which distinguish them as sustainable fuels, such as: high caloric power, absence of sulfur in chemical composition, its industrial production does not generate harmful substances, and mainly, the decrease of carbon dioxide gases by their consumption during photosynthesis of vegetable cultivation (PRASAD;SRIVASTAVA., 2000).Thus, these alternative fuels can be mixed with diesel, and in this way, it reduces the demand on diesel oil and minimize the gas emissions that contribute to global warming (CONCEIÇÃO et al., 2007).
The environmental benefits of renewable biodiesel can also provide economical advantages (BORSATO et al., 2010).Brazilian Federal Government can include the biodiesel program in the frame of 'Kyoto Agreement' in the guidelines of the clean development mechanisms.This program demands the addition of 5% (B5) in 2012 (HRIBERNIK;KEGL, 2007).Moreover, this program stimulates the production of biofuels by small Brazilian family farms giving profits to agricultural working men (CRUZ et al., 2007).
Biodiesel production is based on manufacturing of vegetable oils.In this way, Brazil has a great variety of plants that are rich in vegetable oils, which can be used to produce renewable fuels (LECLERCQ et al., 2001).Among them, buriti (Mauritia exuosa), an abundant palm in the Amazon region of Brazil, which supplies raw material for a variety of applications such as: roots for medicines, fruit and trunks to produce foods and drinks.The composition of the oil extracted from the Buriti fruit is 20% of shell and pulp, 30% of white cellulose layer and 50% of seed (wt/wt) (NETO et al., 2007).
The present investigation reports thermal stability, thermal kinetic studies and heat of combustion of buriti oil, biodiesel obtained from buriti oil and diesel/biodiesel blends.
Buriti oil was obtained from commercial sources and used without further purification.Pure diesel was named B0 and pure biofuel was named B100.
Accordingly, arrays of biofuel/diesel blends were obtained by mixing 2.0, 5.0, 10.0, 20.0 and 50.0%(w/w) of biofuel in diesel, using a roller mixer spinning at constant speed for 30 min., in order to produce B2, B5, B10, B20 and B50 blends, respectively.All experiments were done in triplicate.

Transesterification Reaction
Transesterification reactions were followed by using: 10.0 g of buriti oil, 1.5 g of methanol and 0.1 g of KOH.Potassium hydroxide was completely dissolved in MeOH under stirring in a glass batch reactor equipped with mechanical stirrer.Then, the vegetable oil was added into the mixture and maintained under stirring for 2h at 50ºC.The resulting product was washed with a saturated aqueous NaCl solution.The organic phase was separated by decantation and dried with anhydrous magnesium sulfate.Recovered esters were analyzed by HPLC, using a method published elsewhere (FARIA et al., 2009), with a Shimadzu CTO-20A chromatograph with UV-VIS detector at  = 205 nm, equipped with Shim-Pack VP-ODS column (C-18, 250 mm, 4.6 mm i.d.).

Thermogravimetric studies
Thermogravimetric curves were obtained from approximately 10 mg of biofuel/diesel and blends samples on a thermogravimetric analyzer model TA-2960 in a dynamic atmosphere using air flux, with heating from room temperature up to 600ºC at a heating rate of 10ºC min. - Kinetic thermal parameters were calculated from Coats-Redfern equation (MOURA; PRADO, 2009).

Heat of combustion
An oxygen bomb calorimeter (Parr 1241) was used in order to obtain the heat of combustion of buriti oil, diesel, biodiesel and biofuel/diesel blends.The Parr 1241 is a static and manual calorimeter with a temperature resolution of 0.002ºC, capable of oxygen charging pressures up to 40 atm, and can liberate up to a maximum 33,400 J per charge.The fuel samples were placed directly in a stainless steel capsule (2.5 cm in diameter and 1 cm deep) and burned with oxygen at a pressure of 3.0 MPa according to ASTM method D240 (DEOLIVEIRA et al., 2006;GARCIA et al., 2007).Calibration of the calorimeter was done by burning 1.0 g benzoic acid pellets (Parr).Heats combustion for diesel and blends were determined in triplicate.

Results and discussion
Biodiesel produced by methanolysis of Buriti oil was quantified by HPLC analysis (Figure 1) and Table 1 presented the amounts of free fatty acid, mono-, di-, and triglycerides, as well as their conversion to biodiesel.These results show that the transesterification reaction of buriti oil with methanol catalyzed by KOH occurred as expected with yield production of 94.08% of formed biodiesel.TG curves of the diesel, biodiesel obtained from methanolysis of buriti oil and blends are presented in Figure 2, which showed only one step of mass loss, which was attributed to the volatilization and/or combustion of methyl esters from biodiesel and alkyl compounds from diesel.The obtained mass loss values, decomposition temperature range, and temperature related to DTG peak maximum values are listed in Table 2.  Kinetic studies were followed in order to understand the effect caused on the thermal behavior by biodiesel addition in the fuel blends.
Once the kinetic information can be extracted from dynamic experiments, which can be determined using a large number of differential and integral methods, depending mainly on the treatment of experimental data, and the rate of conversion, d/dt, is a linear function of a temperature-dependent rate constant and a temperature-independent function (PEREZ-MAQUEDA et al., 2006): Assuming the Arrhenius-type dependence on temperature and that the temperature of the sample is changed by a controlled and constant heating rate, .The expression of the change in the conversion degree can be written as (GOTOR et al., 2000): Integrating this equation from initial temperature, T 0 , which corresponds to zero degree of conversion, to the peak temperature of the derivative thermogravimetric curve (DTG), Tp, which can affirm = P , it can be written this equation (NUNEZ et al., 2001): Where: g(α) is the integral function of conversion degree.In thermal decomposition process several kinetic models have been proposed in order to determine the different reaction mechanisms, which g() assumes different expressions as described in Table 3 (FARIA; PRADO, 2007).
random nucleation with one nucleus on the individual particle nucleation and growth (Avrami equation 1) nucleation and growth (Avrami equation 2) phase boundary controlled reaction (contracting volume) Several distinct methods have been developed and applied to explain the changes in thermogravimetric data by variation of the heating rate, β, in order to determine kinetic parameters as activation energy (E) and pre-exponential factor (A).One of the most used method is based on Coats-Redfern equation (COATS; REDFERN, 1964) In this method, the determination of the reaction mechanism can be obtained by linear fitting, applying Coats-Redfern equation in thermal decomposition curve.
The assignment of the mechanism of thermal decomposition is based on the assumption that the form of g() depends on the reaction mechanism.In this investigation, nine forms of g() were used in order to enunciate the thermal mechanism of diesel, biodiesel and blends diesel/biodiesel.Correlation coefficients for all these forms were calculated and the form of g() for which the correlation has a maximum value is selected as the mechanism reaction as presented in Table 4.For all thermal analysis, the best value of correlation coefficient for materials decomposition was obtained by using the function: , which corresponds to a mechanism involving threedimensional diffusion (Ginstling-Brounshtein equation).This mechanism can be explained by diffusion of fuel to vapour phase by its volatilization (GABAL, 2003).
The activation energies obtained by the application of Coats-Redfern equation were listed in Table 5 and showed in Figure 3.
Other thermal decomposition parameter, activation entropy, was determined from equation:

S e h kT A
  (STRASZKO et al., 2000), where k is the Boltzmann constant, h is the Planck constant and T S is the temperature related to DTG peak maximum value determined from Figure 1, listed in Table 1 for diesel/biodiesel blends.A third thermal degradation parameter, the activation enthalpy, was obtained from the following equation: Finally, the activation Gibbs energy was calculated from equation: (RODRIGUEZ et al., 2009).All kinetic thermal degradation parameters for the diesel/biodiesel blends were calculated from thermogravimetric curves and the results are listed in Table 5.
Kinetic thermal analysis shows that all Ea, H ‡ and G ‡ values are positive and S ‡ values are negative; consequently, all thermodynamic parameters indicate non-spontaneous processes of decomposition for diesel, biodiesel and all blends.
Activation energy, enthalpy and free Gibbs energy were plotted as function of biodiesel amount in diesel/biodiesel blends.These data show that the addition of biodiesel in blends causes a linear increase in Ea, H ‡ and G ‡ values during thermal volatilization processes of blends.This behaviour can be explained by the presence of oxygenated groups present in biodiesel, which causes more effective intermolecular interactions.In this way, volatilization processes occur by the cracking intermolecular interactions resulting in a gradual disappearance of the liquid to form the gas phase.Ability for fuel oil molecules in liquid phase to volatilize is based on the amount of kinetic energy an each individual particle may possess.Thus, the increasing temperature (as a TG assays), the fuel oil in liquid phase received thermal energy, and as consequence, the fuel oil goes to gas state.Some factors affects the temperature of phase transition such as: density, surface area, pressure, intermolecular forces etc.
In the case of diesel and biodiesel, the most of parameters are very similar.However, these fuels differs in molecular structure.The diesel is formed basically by carbon-carbon bonds and carbonhydrogen bonds, whereas the biodiesel are methyl esters of vegetable oils.Thus, the biodiesel presents carbon-oxygen bonds of esters, resulting in a difference of polarity of molecules more accentuated than fossil diesel, and as consequence, the intermolecular interactions are more effective in biodiesel than fossil diesel.Then, the addition of biodiesel in diesel causes the increasing of volatilization temperature and the increasing in Ea, H ‡ and G ‡ values in blends, and these results in agreement of other studies with biodiesel produced from other vegetable oils (babassu, soybean and cotton) (SANTOS et al., 2007).
The heat of the combustion of diesel and the effect of the addition of those biodiesel in fossil diesel are presented in Figure 4.These studies showed that the heat of combustion decreased with the addition of biodiesel in diesel/biodiesel blends.This fact can be explained by the presence of oxygenated groups in biodiesel, which presents enthalpy bonds lower than C=C bonds in fossil diesel (ANDRADE et al., 2011).

All
diesel/biodiesel blends presented endothermic values for Ea and H, endoergonic values for G, and S values were exoergonic, indicating that the volatilizations were nonspontaneous processes in all thermodynamic aspects for all blends.The present study confirmed that the incorporation of biodiesel decreased the heat of combustion of the fuel blends.

Table 1 .
Products and yields of transesterification of buriti oil.

Table 2 .
analysis data of diesel, biodiesel and biofuel/diesel blends.

Table 3 .
Algebraic expressions for g() for the used mechanisms of solid state processes.

Table 4 .
Square correlation coefficients calculated using nine forms of g(a) for diesel and diesel/biodiesel blends.