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Currently, the main methods for producing biodiesel include the direct mixing method, microemulsion method, high-temperature pyrolysis method, and transesterification method. The first two methods are physical approaches; although they are simple and effective at reducing the viscosity of animal and vegetable oils, their cetane numbers are not high, and issues such as carbon deposition during combustion and lubricant contamination are difficult to resolve. The high-temperature pyrolysis method is simple and produces no pollutants; its disadvantages are that it takes place at high temperatures, requires a catalyst, the pyrolysis equipment is expensive, it is difficult to control the degree of reaction, and the main product of this method is biofuel gasoline, with low yields of biodiesel. The main method for producing biodiesel industrially is the transesterification method. In the transesterification reaction, the main component of oils, triglycerides, undergo transesterification with various short-chain alcohols in the presence of a catalyst to yield fatty acid methyl esters and glycerol. Alcohols that can be used for transesterification include methanol, ethanol, propanol, butanol, and pentanol, among which methanol is the most commonly used. This is because methanol is inexpensive, has a short carbon chain, high polarity, allowing it to react rapidly with fatty acid glycerides, and alkaline catalysts are soluble in methanol. The transesterification reaction is a reversible reaction, and an excess of alcohol can shift the equilibrium in favor of the products; therefore, the actual amount of alcohol used is much greater than its stoichiometric ratio. The catalyst used in the reaction can be a base, an acid, or an enzyme catalyst, etc.; it can accelerate the reaction rate to increase yield. The transesterification reaction consists of a series of sequential reactions, during which triglycerides are gradually converted into diglycerides, monoglycerides, and finally into glycerol; each step of the reaction produces an ester. Ester exchange methods include acid-catalyzed, base-catalyzed, enzyme-catalyzed, and supercritical ester exchange methods. (1) Acid-catalyzed method. The catalysts used in the acid-catalyzed method are acidic catalysts, mainly including sulfuric acid, hydrochloric acid, and phosphoric acid. Under acid-catalyzed conditions, free fatty acids undergo esterification, and the rate of this esterification reaction is much faster than that of transesterification. Therefore, this method is suitable for producing biodiesel from fats and oils with high levels of free fatty acids and moisture; it yields high productivity. However, it requires high reaction temperatures and pressures, a large amount of methanol, has a slow reaction rate, and the reaction equipment needs to be made of stainless steel. The acid-catalyzed method is far less emphasized in industry compared to the base-catalyzed method. (2) Alkali-catalyzed method. The catalysts used in the alkali-catalyzed method are basic catalysts, generally including NaOH, KOH, NaOH, KOMe, and organic amines. Under anhydrous conditions, the transesterification activity of basic catalysts is generally higher than that of acidic catalysts. The traditional production process uses alkali metal hydroxides with high solubility in methanol as homogeneous catalysts, whose catalytic activity is related to their basicity. Among alkali metal hydroxides, KOH has higher reactivity than NaOH. Typical conditions for the transesterification reaction using KOH as a catalyst are: a methanol usage of 5%-21%, a KOH usage of 0.1%-1%, and a reaction temperature of 25-60°C; whereas when NaOH is used as a catalyst, a reaction temperature of 60°C is usually required to achieve the desired reaction rate. The alkali-catalyzed method can achieve high yields at low temperatures, but it has high requirements regarding the content of free fatty acids and water in the raw materials. During the reaction, free fatty acids undergo saponification with alkalis, resulting in emulsification. The water present can cause ester hydrolysis, which in turn leads to saponification; it also reduces the activity of the catalyst. As a result, it becomes difficult to separate the glycerol phase from the methyl ester phase, complicating the post-reaction processing. Therefore, when using alkali catalysts such as potassium hydroxide, sodium hydroxide, and potassium methoxide, the acid value of the oil is often required