3.1. Effect of Catalysts on Biodiesel Yield (%)
The effect of various catalysts on biodiesel yield is shown in
Figure 2. Base-catalyzed transesterification has many advantages, including the low cost of catalyst compared to acid and enzyme catalysts, easy availability in the market, mild reaction conditions, and faster reaction times relative to enzyme and acid catalyzed reactions. However, a serious drawback of base-catalyzed reactions is that the base catalysts (in higher concentrations) react with the free fatty acids (FFAs) found in the feedstock oil to produce soap as a by-product. Excess soap formation inhibits the separation of glycerol and biodiesel, and in turn reduces biodiesel yield [
33]. In the current study, biodiesel yield increased to a maximum and then decreased as the catalyst load increased. This decrease in yield was due to soap and gel formation at higher catalyst concentrations. Optimum biodiesel yields of 86% and 89% were observed for pure TSO and fraction F3, respectively, at a KOH concentration of 0.6%. Yields then dropped to 77% and 81%, respectively, as the catalyst concentration was increased to 1.0%. Fraction F1 showed optimum yield (79%) at a catalyst concentration of 0.8%. However, F2 (71%) and residual oil (52%) provided maximum biodiesel yields at 0.4% KOH (
Figure 2a). A similar trend was noted in another study where biodiesel was produced from waste soybean oil [
34].
The acid-catalyzed reaction was slower than base-catalyzed transesterification. It was completed in 4.5 h compared to 1.5 h by base-catalyzed reaction. High alcohol to oil molar ratio (6:1) and a reaction temperature of 90 °C was used to accelerate this reaction. Although the reaction rate was slow, the biodiesel yield was higher with acid catalysis compared to base and enzyme catalyzed reactions. This was due to the fact that the acid catalyst was insensitive to FFA content in the oil, which eliminated side reactions. Acid catalysts carry out esterification and transesterification simultaneously, which reduces FFA content and increases biodiesel yield [
33]. Five concentrations of HCl were used for acid-catalyzed reactions to optimize biodiesel yield (
Figure 2b). As observed with base-catalyzed transesterification, biodiesel yield increased with increasing catalyst concentration and then decreased. The highest yield of biodiesel produced from TSO (97%), F3 (92%), and residual oil (95%) was obtained with 40% HCl. Meanwhile, F1 (93%) and F2 (87%) provided maximum yield at a catalyst concentration of 60%. Further increases in HCl concentration resulted in reduced biodiesel yield. For instance, the yield of biodiesel from TSO decreased from 97% to 81% as the catalyst concentrated increased from 40% to 100%. Similar observations were previously reported for the production of biodiesel from rubber seed oil [
35]. The lower yields obtained via base catalysis relative to acid catalysis were likely the result of free fatty acids (FFA) within the starting oils that partially deactivated KOH and reduced the yield of FAME [
4].
Five concentrations of lipase were investigated for enzyme-catalyzed transesterification (
Figure 2c). The lipase was immobilized to achieve several advantages, such as cost-effectiveness, availability of more active sites to speed up the chemical reaction, and ease of separation after the reaction was complete. TSO and F1 provided optimal yields of 90% and 85%, respectively, at a catalyst concentration of 2% whereas F2 (87%), F3 (77%) and residual oil (88%) gave maximum yields at 3% lipase. Biodiesel yield initially increased with increasing lipase concentration but decreased at higher concentrations due to adsorption phenomenon caused by calcium alginate granules. The maximum biodiesel yield observed for the base, acid, and lipase-catalyzed reactions was 89, 97, and 90%, respectively. These results clearly indicate that HCl provided the highest yield of biodiesel from TSO and its fractions.
3.3. Assessment of Fuel Quality Parameters
Acid values of TSO, F1, F2, and F3, and the residual oil were 0.56, 0.61, 0.56, 0.50, and 4.51 mg KOH/g, respectively. These results indicated the presence of a small amount of free fatty acids (FFA) in TSO, F1, F2, and F3, which may have negatively impacted base-catalyzed transesterification, as mentioned previously. The much higher AV of the residual oil was likely the result of exposure to high temperatures for an extended period of time during HVFD that promoted thermal hydrolysis.
Density is an important physical property of biofuels, as it influences the air-fuel ratio and energy content within the combustion chamber. A change in fuel density can thus affect engine output power. The density of biodiesel is affected by the chemical composition of FAME and is generally higher than that for petrodiesel [
36]. The density at 25 °C of biodiesel obtained from the various fractions ranged from 0.81 to 0.85 kg/L for F1, 0.84–0.88 kg/L for F2, 0.84–0.89 kg/L for F3, and 0.82–0.86 kg/L from TSO. The European biodiesel standard (EN 14214) specifies that density (15 °C) must be within the range of 0.86–0.90 kg/m
3 whereas the American biodiesel standard (ASTM D6751) does not contain limits for density. In a previous study, the density of pongamia seed oil biodiesel was 0.86 kg/L [
37]. In another study, the density of sunflower seed oil biodiesel was 0.877 kg/L [
21,
38]. Lastly, the densities of biodiesel produced from chicken fat and waste tallow were 0.867 and 0.856 kg/L, respectively [
31]. Thus, the values obtained in the present study were similar to those reported for biodiesel fuels prepared from other feedstocks and, with the exception of F1, were within the range specified in EN 14214. The comparatively low density of F1 was likely due to the presence of a significant amount of methyl lignocerate (22.7%) and low amounts of unsaturated FAME relative to the other fractions (
Table 2). Previous studies have established that density decreases with increasing chain length as well as with increasing levels of saturation [
39].
The SV is a measure of the average molecular weight (MW; i.e., chain length) of the FAME present within a sample. Higher SV indicates lower average chain length and thus lower average MW of the FAME that comprise the sample. In the present study, the SV of TSO biodiesel ranged from 192–210 mg KOH/g (
Table 3). In previous studies the SV of pongamia seed oil biodiesel was 187 mg KOH/g [
40] and the value for palm biodiesel was 201 mg KOH/g [
41]. The SV of biodiesel produced from TSO and the various fractions are listed in
Table 4. As expected, the values obtained for biodiesel produced from F3 were lower than SV obtained for F2 and TSO due to the lower percentage of shorter-chain FAME identified in F3. The low values for F1 were attributed to the presence of relatively high amounts of longer-chain FAME such as methyl lignocerate (22.7%).
The IV of biodiesel prepared from TSO and its fractions are shown in
Table 4. The maximum IV for biodiesel prepared from TSO was 32.1 mg KOH/g. The upper limit for IV specified in EN 14214 is 120 mg KOH/g. Limits for IV are not included in ASTM D6751. In this study, the maximum IV (59.2 mg KOH/g) was observed for biodiesel prepared from F3, followed by F2 (49.9 mg KOH/g) and F1 (32.5 mg KOH/g). The IV is a measure of unsaturation, with higher unsaturation leading to higher IV and vice versa. Thus, biodiesel prepared from F3 gave the highest IV because it contained the highest percentage of polyunsaturated FAME (65.1% methyl linoleate;
Table 2). Unsaturation is to some extent necessary for the fuel to prevent it from solidifying at low temperatures. Saturation negatively affects the CP and PP of biodiesel due to the high melting points of saturated FAME. Higher levels of unsaturation in biodiesel give lower CP but higher IV. However, high unsaturation may lead to engine deposits from polymerization by breaking the weak π-bond or by epoxide formation by addition of oxygen to the double bond at high engine temperatures [
42].
The low temperature performance of a fuel is evaluated by determination of CP and PP. When biodiesel begins to solidify at low temperatures it will form small crystals that start clumping together. CP is the temperature at which the crystals within biodiesel first become visible. PP is the temperature at which biodiesel becomes solid and can no longer flow or pour [
17]. Generally, the CP and PP of biodiesel are higher than petrodiesel (Dunn, 2015; Moser, 2009). The CP and PP of TSO-based biodiesel are reported in
Table 5 and ranged from −0.6 to 1.5 °C and −0.7 to −2.5 °C, respectively. In contrast, the CP of palm oil biodiesel was 13 °C in a previous study [
43]. In another study, a CP of 12 °C was reported for Nigerian mango seed biodiesel [
16]. Lastly, a PP pf 6 °C was observed for Karanja seed biodiesel [
23]. In the current study, biodiesel produced from F2 (−3.9 °C, −6.7 °C) and F3 (−4.2 °C, −8.1 °C) displayed lower minimum CP and PP than biodiesel from TSO. Fraction F1 gave higher CP (15.2 °C) and PP (6.8 °C) than TSO-based biodiesel due likely to its comparatively high content of methyl palmitate, methyl stearate, and methyl lignocerate (
Table 2) relative to TSO. The low CP and PP observed for F2 and F3 biodiesel indicate that it can be used in the colder climates. However, biodiesel produced from TSO and F1 should be used in warmer climates.
Cetane number is one of the primary indicators of ignition quality and is related to the injection delay time a fuel experiences when injected into the combustion chamber of diesel engines [
44]. Cetane numbers (CN) of biodiesel produced from TSO and its various fractions are shown in
Table 6. The minimum CN specified in EN 14214 and ASTM D6751 are 51 and 47, respectively. In the present study, the CN of TSO-based biodiesel was in the range of 65–68. In another study the CN of pongamia seed oil biodiesel was reported as 47.1 [
40]. In the current study, the CN of all samples were above the minimum requirements of the European and American biodiesel standards. The CN of TSO biodiesel was higher than that of other biodiesel fuels including sunflower and soybean biodiesels due to its comparatively low percentage of polyunsaturated FAME. Previous studies have shown that branching, chain length, and degree of unsaturation affect CN [
3,
4]. Thus, the higher CN of TSO biodiesel compared to petrodiesel and conventional commodity biodiesels suggests a shorter ignition delay time and hence superior ignition quality for TSO biodiesel.