# Using Canola Oil Biodiesel as an Alternative Fuel in Diesel Engines: A Review
## 2 Biodiesel Production
### Chemical Methods
#### 2.2.2 Transesterification Method  
At present, the main method of producing biodiesel fuels is the transesterification method. Veget oils or animal fats and short-chain alcohols (methanol or ethanol) undergo a transesterification reaction in the presence of acid (WO\({}_{3}\)/ZrO\({}_{2}\), ZnO, H\({}_{2}\)SO\({}_{4}\), etc.) or base (NaOH, KOH, carbonates, etc.), catalysts, or other new catalysts (heterogeneous catalysts). Then, biodiesel fuel is obtained by washing and drying the corresponding fatty acid methyl ester or ethyl ester (products after transesterification) [43; 44; 45; 46; 47; 48; 49; 50; 51]. The specific process of the transesterification method for biodiesel production is shown in Figure 3. Crude animal fat or vegetable oil is first heated to remove impurities (pretreatment). Then, transesterification is performed in the presence of a catalyst (methanol or ethanol), and a mixture of crude biodiesel and glycerin is obtained (medium treatment). Finally, pure biodiesel fuel is obtained after repeated washing and drying (post-treatment). The transesterification method can be divided into acid-base catalysis, enzymatic catalysis, and a supercritical methanol process according to the catalytic method. The supercritical methanol process is an economical, efficient (complete response time was only 2-4 min), simple, and environmentally friendly method for producing biodiesel fuel, compared with the conventional commercial method due to non-catalytic process [52; 53]. [PERSON] et al. [53] researched the kinetics effects of transesterification for producing biodiesel fuel via supercritical methanol. During the reaction, the temperature ranges from 200 \({}^{\circ}\)C to 500 \({}^{\circ}\)C, and they found that the optimum reaction temperature was 350 \({}^{\circ}\)C, while the molar ratio of methanol in rapeseed oil was 42. [PERSON] et al. [51] studied the transesterification characteristics of vegetable oils, using frying oil as a new heterogeneous catalyst. They demonstrated that the highest methyl ester yield with sunflower oil and frying oil were 88% and 67%, respectively. The weight of all the catalysts used were 7 wt. % oil, and the reaction time and stirring speed were 8 h and 740 rpm, respectively. [PERSON] et al. [54] investigated the biodiesel production from canola oil via Mg-Al hydrotalcite catalysts. They reported that methanol was the best alcohol for producing canola biodiesel fuel, and the highest triglyceride conversion rate was 71.9%, while the reaction temperature and time were, respectively, 60 \({}^{\circ}\)C and 9 h, with a 6:1 molar ratio of methanol to canola oil, and a 3 wt. % catalyst with 125-150 \(\upmu\)m particles.  
Although the esterification methods are different, their reaction mechanism is the same. Figure 4 shows the schematic diagram of transesterification mechanism. That is, the carbonyl carbon of the starting ester (RCOOR\({}^{1}\)) undergoes nucleophilic attack by the incoming alkoxide (R\({}^{2}\)O\({}^{-}\)) to give a tetrahedral intermediate, which either reverts to the starting material or proceeds to the transesterified product (RCOOR\({}^{2}\)). The various species exist in equilibrium, and the product distribution depends on the relative energies of the reactant and product [36; 37].  
Figure 3: The specific process of transesterification method for biodiesel production.