1. Introduction
Nowadays, regardless of the introduction of vehicles that incorporate electric or hydrogen engines, the gradual incorporation of biofuels as substitute of fossil fuels is still mandatory [
1]. The use of biofuels palliate the depletion of fossil fuels, minimize the negative impact of greenhouse gases, producing less amount of carbon monoxide, sulfur dioxide and unburned hydrocarbons than fossil fuel [
2,
3], and also allow continued use of the fleet of cars currently existing, estimated at more than a billion, without modifying the compression ignition (C.I.) engines [
4]. Furthermore, biofuels and, specifically the biodiesel, can be easily integrated into the logistic of the global transportation system [
5,
6]. The gradual replacement of fossil fuels by others of renewable nature involves the introduction of blends diesel/biofuel. In this sense, the objectives pursued by the EU are estimated at 20% of biofuel in the blend in 2020 and 30% in 2030. Despite these objectives are apparently not difficult to achieve, the enormous amount of glycerol produced during the synthesis of biodiesel makes that other approaches can be considered in order to accomplish the fossil replacement to a higher extent. In this respect, a viable option could be the use of unprocessed vegetable oils in double blends with conventional diesel. This is theoretically possible because all the relevant physicochemical properties, for its use as fuels in conventional diesel engines, are comparable to conventional diesel, with the exception of the viscosity, much higher in oils than in diesel [
7]. Another approach is related to the use of alcohols in triple blends with diesel and different biofuels. According to EN 14214, the presence of alcohols in fuel and biofuels does not constitute any inconvenience. In fact, according to recent research, the presence of ethanol and other short-chain alcohols has a favorable effect on the emissions of the biofuels [
8,
9,
10,
11], so it is advisable the addition of certain quantities of ethanol to the diesel No. 2 standard. These mixtures constitute the so-called E diesel, oxidiesel or oxygenated diesel, which apart from reducing the emissions of the C.I. engines, improves the flow properties (viscosity) and the essential parameters that limit the application of diesel when operating at low temperatures [
12], like the “cloud point” (CP), “pour point” (PP), cold filter plugging point temperature (CFPP), or point of occlusion of the cold filter (POFF), viscosity, and emission levels of the motors, without any significant negative effect in most of the parameters that define the quality of biodiesel [
13,
14,
15,
16]. Thus, the use of alcohols in triple mixtures diesel/biofuel/alcohol would allow to replace larger amounts of fossil diesel than those achieved using double diesel/biofuel blends. In fact, the utility of the triple diesel/biodiesel/ethanol blends has been patented under the name of Diesterol [
17,
18]. These blends reduce emissions, viscosity and flash point, together to a slight reduction of the engine power [
13,
14,
15,
16,
17,
18].
Considering the biofuel synthesis and taking into account the available technology, the conventional biodiesel production described by the standard EN 14214, present as the main drawback the glycerol generated as byproduct, which is a 10% by weight of the total of biodiesel produced [
19]. For instance, the Lurgi’s biodiesel fabrication technology is based on two successive transesterification reactions of TG with methanol to form FAME and glycerol in the presence of an alkaline catalyst [
20]. At the end of the reaction, the mixture is neutralized by adding hydrochloric acid. A subsequent counter-current washing step removes by-product components and gives a ‘‘ready for use’’ biodiesel after final drying step. The washing step of biodiesel to eliminate glycerol residues, that must be less than 0.02%, provides an additional complication. On one hand, it requires processing with high energy costs. On the other hand, it requires a high consumption of water. A possible solution to this problem is the production of a new type of biodiesel that integrates glycerol in the form of a soluble derivative. Thus, the production of glycerol is avoided, and, at the same time, the atomic performance of the process is increased, since all the reactive raw materials are transformed into a biofuel [
21,
22]. In this sense, our Research Group has accomplished the transesterification of triglycerides with ethanol to produce monoglycerides (MGs) as soluble derivatives of glycerol employing different lipases as catalysts. Hence, through the partial transesterification of one mole of triglyceride (TG) with ethanol, two moles of ethyl esters (FAEE) and one mole of monoglyceride (MG) are generated, obtaining a biofuel called Ecodiesel,
Figure 1 [
23,
24,
25,
26,
27,
28,
29].
However, the high cost of lipases compels to search for a more cost-effective process for producing Ecodiesel, such as the employ of supported KF or CaO as heterogeneous catalysts [
28,
29]. Thus, the Ecodiesel, constituted by the 2/1 mixture of FAME/MG can be obtained under kinetic control of the chemical process, using a catalyst less basic than the alkali metals usually employed as homogeneous catalysts in the biodiesel production. However, further attempts to reduce the economic cost of the process should be investigated.
In addition to the abovementioned drawback of glycerol generation during the biodiesel production, another important issue related to the biodiesel production is the ethical conflicts surrounding the production of biodiesel from edible oils, instead of using these edible oils exclusively for feeding purposes. To overcome this, the production of the so-called second-generation biofuels, which are obtained from non-edible oils and also from recycled waste oils, using procedures with minimum waste generation and high atom efficiency, are being taken into account [
30,
31]. Among the non-edible oils, Castor oil (
Ricinus communis L.) should be a promising option because it has a large international market, since it is widely used in chemical and pharmaceutical industry [
32,
33] and it can be grown on marginal lands and in semi-arid climates [
34]. However, it is not considered a good raw material to produce biofuels [
35] because of the high kinematic viscosity that it exhibits (241.5 cSt). Likewise, the kinematic viscosity of the biodiesel obtained by its transesterification with methanol is also too high for being employed in combustion engines (15 cSt). In fact, very recent studies [
36,
37,
38,
39] have determined that Castor oil biodiesel can be only used in a 20–30% in mixtures with fossil diesel, i.e., BCO20 to BCO30. In this respect, some researches, regarding the use of triple diesel/biodiesel/alcohol blends in order to increase the amount of biodiesel of castor oil in these blends, have been initiated [
13].
Another possibility, which is barely studied at present, is that Castor oil can be employed in its pure form blending with fossil diesel [
40,
41,
42]. Castor oil exhibits, in fact, advantageous rheological properties for being employed directly as biofuel, such as high solubility in very diverse organic compounds, compatibility with polar liquids of low viscosity, such as alcohols; high values of Cetane Index (81.1) and Flammability Point (229 °C); low cloud point −10 to −18 °C, (−23 °C in FAME), and crystallization (pour point) −30 °C (−45 °C in FAME), which allow its use in motor oil high performance, as a lubricant and in hydraulic braking systems [
34,
43].
Hence, in this research, two different aspects have been addressed. On one hand, to further reduce the economic cost of the Ecodiesel production, a basic homogeneous catalytic process at the experimental conditions that are soft enough to achieve the transesterification of the esters of primary alcohols, positions 1 and 3 of glycerol, without affecting the carbon 2, a secondary alcohol, which is less reactive, has been investigated. To do so, a commercial sodium methoxide is employed as catalyst. Furthermore, in order to avoid the glycerol production that surely occurs employing methanol and an homogeneous catalyst, the possibility of using ethanol as solvent in a methanol/ethanol mixture to attenuate the higher reactivity of methanol has been studied.
On the other hand, to solve the problem of the high viscosity of Castor oil, this research deals with the possibility of increasing the amount of renewable material in biofuel blends, here so-called diesel additive, through the application of diesel/biofuel/alcohol triple blends. In this sense, the most suitable double and triple blends, according to their viscosity values, are applied as a biofuel in a conventional diesel engine, operating as an electric generator. Besides, the quality of the emissions obtained by using these biofuels was also evaluated from the opacity values of the generated smokes.
4. Materials and Methods
4.1. Evaluation of Sodium Methoxide as Homogeneous Catalyst in the Selective Alcoholysis to Obtain Ecodiesel
The selective alcoholysis of the Sunflower oil (food quality) and Castor oil (Panreac, Castellar Del Valles, Spain) were carried out over commercial sodium methoxide (Panreac, Sodium methylate solution 30% in methanol PS) as homogeneous catalyst. The reactions were performed in a 250 mL flask immersed in a temperature-controlled water bath, at atmospheric pressure using methanol as alcohol. Different reaction conditions have been studied, such as temperature, 20–60 °C, and reaction time,
Figure 11.
In general, 120 mL of oil (0.1 mol) were mixed with variable proportions of methanol and sodium methoxide. The changes are easily visualized as a ratio, for example, 100/10/1 indicates 100 mL of oil, 10 mL of methanol and 1 mL of sodium methoxide. Variable amounts of ethanol are sometimes also incorporated in some experiments, which act as a solvent, given its lower activity with respect to methanol. It has been taken as standard measure that 0.1 mol = 91.0 g of oil. The weigh was performed on a Mettler AJ50 precision balance (precision of ± 0.01 g), depositing the sample directly into a 250 mL two-neck round bottom flask.
4.2. Determination of the Content of FAMEs, FAEEs and Glycerides (MG, DG and TG) in the Reaction Products by Gas Chromatography
The determination of the content of methyl esters, ethyl esters and different glycerides in the biofuel samples has been carried out by a chromatographic method developed in previous researches [
23]. Thus, an HP 5890 Series II gas chromatograph with a HT5 (25 m × 0.32 mm I.D × 0.1 μm, SGE, Supelco) Aldrich Chemie capillary column and equipped with a flame ionization detector (FID) was employed. Cetane (n-hexadecane) is used as an internal standard
This method basically consists of a modification and integration of two official methods, UNE EN ISO 14103 (esters) and UNE EN ISO 14105 (glycerides), to quantify the content of glycerol, ethyl esters and glycerides (mono, di and triglycerides), respectively. The ethyl esters of palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid were commercially obtained from AccuStandard (New Haven, CT, USA), and hexadecane (cetane) was obtained from Sigma-Aldrich (St. Louis, MO, USA).
Considering that castor oil or sunflower oil are constituted by a mixture of fatty acids (mainly linoleic, oleic, palmitic, and stearic acids, in sunflower oil, or ricinoleic in castor oil) in variable proportions, the results obtained are expressed as the relative amounts of the corresponding methyl esters (FAME, fatty acid methyl esters), monoglycerides (MG), and diglycerides (DG) that are integrated in the chromatogram. The number of diglycerides (DG) and triglycerides (TG) that have not reacted is calculated from the difference to the internal standard (cetane). Thus, the Conversion includes the total amount of triglyceride transformed (FAEE + MG + DG) in the methanolysis process, and Selectivity refers to the relative amount of FAEE + MG obtained.
4.3. Determination of Kinematic Viscosity of Biofuels
The kinematic viscosity has been measured in an Ostwald-Cannon-Fenske capillary viscometer (Proton Routine Viscometer 33200, size 150), determining the time required for a certain volume of liquid to pass between two marked points on the instrument, placed in an upright position. From the flow time (t), expressed in seconds, we obtain the kinematic viscosity expressed in centistokes, υ = C·t. Where C is the calibration constant of the measurement system in mm2/s2, which is specified by the manufacturer (0.040350 mm2/s2 at 40 °C, in this case). All measures have been carried out in duplicate and are presented as the average of both, proving that there is no greater variation of 0.35% between measures, as required by the standard ASTM (American Society for Testing and Materials) D2270-79, Method for calculating viscosity index from kinematic viscosity at 40 and 100 °C.
4.4. Determination of the Pour Point and Cloud Point of Biofuels
Cloud point and the Pour point are determined by introducing the different double or triple samples, of different composition, in a digitally controlled temperature refrigerator for twenty-four hours; after this time the loss of transparency of the solutions is evaluated. The appearance of turbidity in the samples is indicative that the cloud point temperature has been reached (cloud point). After a progressive decrease in temperature, the samples are kept under observation until they stop flowing (pour point).
4.5. Assessment of Energy Performance and Air Pollutant Emissions in a Diesel Engine Electric Generator, Fueled with Different Blends of Biofuels
The mechanical and environmental characterization of a compression ignition diesel engine has been carried out, working at a rate of 3000 rpm coupled to an AYERBE electric generator, 5KVA, 230v type AY4000MN, for the generation of electricity, operating at a crankshaft constant rotation rate and under different degrees of demand for electrical power. This is achieved by connecting heating plates of 1000 watts each one (
Figure 12a). This diesel engine will operate at a constant rate of rotation of the crankshaft and torque, so that the different values of electrical power obtained will be an exact consequence of the mechanical power obtained after the combustion of the corresponding biofuel. Different tests are obtained by providing to the engine double and triple mixtures of different biofuels in different percentages. The electrical power generated can be easily determined from the product of the potential difference (or voltage) and the electric current intensity (or amperage), equation (1), both obtained by means of a voltmeter-ammeter [
45,
46].
The consumption of the diesel engine with the different biofuels employed was calculated estimating the speed of consumption of the engine, with a given fuel, when operating under a determined demand of electric power.
On the other hand, the contamination degree is evaluated regarding the opacity of the smoke generated in the combustion process. This is obtained by using an opacimeter—TESTO 308 opacity meter—under the operating conditions studied (
Figure 12b). All the results obtained with the biofuels evaluated were compared with the corresponding measurements obtained with conventional diesel. The opacimeter, is a device designed to estimate the amount of soot emitted by diesel engines. Unlike gasoline engines, where the amount of carbon monoxide (CO) and hydrocarbons (HC) is measured to assess the quality of combustion and toxic emissions, only the amount of coal is analyzed in diesel engines (soot). These are tiny particles in suspension which can not be treated as a gas, that is, they can not be quantified through the gas analyzer. The opacimeter is basically composed of three components: measuring chamber, analyzer and a portable terminal. The outlet of the exhaust pipe of the engine is connected, through a pipe and a hose, with the measuring chamber and partial samples of the exhaust gases are taken. It is called partial since only part of the gases enter the machine and the rest is lost in the atmosphere. The gases enter inside a tube and through a sensor the intensity of the light (turbidity) is measured, to then calculate the density of the particles. The tube has a source of halogen light at one end and a receiver at the other so that when there is no gas inside the tube, the light intensity is not affected. The result of such measurements is reflected visually on a filter paper. Besides, this value can be expressed as a percentage of Opacity (being 100% totally cloudy and 0% totally clear) or as an equivalent number called the k value (Opacity Bacharach) the scale runs from white (0 Bacharach unit) to black (9 Bacharach units), as established by ASTM D 2156-94, Standard Test Method for Smoke Density in Flue Gases from Burning Distillate Fuels [
47]. It must be clarified that all the measured were repeated at least three times, attaining an experimental error lower than 5%.
5. Conclusions
In this research, some Biofuels have been synthesized employing a commercial basic homogeneous catalyst, sodium methoxide, at milder conditions than those employed to obtain conventional biodiesel, to favor a selective 1,3 methanolysis of triglycerides, achieving a 100% atomic efficiency in the synthesis of Ecodiesel. In this respect, sunflower oil and castor oil have been evaluated. Besides, the behavior of diesel/biofuel blends, as well as the behavior of several triple blends, diesel/biofuel/alcohol have been also studied. Furthermore, the differences between Ecodiesel and conventional biodiesel, as well as the alcohol employed in the blends, in what proportions must be added together with fossil diesel, to obtain the rheological properties to operate in conventional diesel engines without making any modifications have been also determined. The main conclusions obtained in this research can be summarized as follows:
It has been demonstrated, for the first time, that the use of EtOH as solvent in the 1,3-selective transesterification of triglycerides improve the diffusion between methanol and TG, allowing its reaction at milder conditions, since an increase in the temperature of the reaction does not improve the Ecodiesel production.
Ecodiesel from both, sunflower and castor oil, has been successfully obtained at the reaction conditions: Oil/ethanol/methanol/catalyst ratio 100/5/5/1, employing sodium methoxide as homogeneous catalyst at 30 °C and at a stirring speed of 300 rpm and 15 min of reaction time.
This research has shown that the chemical route here proposed for the synthesis of Ecodiesel is able to reduce the production costs to a great extent, in comparison with enzymatic routes and heterogeneous catalysed routes.
About the rheological properties of the double and triple blends here studied, it has been verified that the viscosity, Pour Point and Cloud Point values of the different samples allow their use as biofuels in conventional diesel engines.
The results obtained using a compression ignition diesel engine show that 4 kW of power demand gives the greatest engine power generation, independently on the fuel employed.
With the blends diesel/biodiesel, a 50% of fossil diesel can be replaced by a biofuel obtained from a renewable source, whereas in the blends diesel/Ecodiesel, the amount of fossil diesel that can be replaced is lower, 30%. This fact is due to the presence of MG in the Ecodiesel, which increase the viscosity above 6 cSt.
The use of triple blends diesel/biofuel/alcohol allows us to increase the diesel replacement, even employing directly Ecodiesel or Castor oil. Thus, it is possible to use pure Castor oil in the proportions 50/25/25, diesel/CO/2-propanol, in conventional diesel engines, with a performance and level of consumption like fossil diesel, but with an appreciably lower emission of pollutants. In the case of the Ecodiesel of sunflower oil, (and used cooking oils, for use as second generation biofuels) it is possible to obtain triple mixtures with any ethanol in a proportion 50/40/10 diesel/EcoSO/ethanol which also exhibits very good results in terms of consumption and emission of pollutants.