Direct mixing method
The direct mixing method is to mix vegetable oil and mineral oil in a certain proportion and use them directly as engine fuel. The results show that vegetable oil has good dynamic and overload performance and high thermal efficiency when used directly in internal combustion engines. However, besides a few vegetable oils (e.g. Eucalyptus oil), vegetable oil has many shortcomings when used directly as diesel engine fuel. Its viscosity is several to dozens times higher than that of ordinary diesel, which leads to difficulty in atomization and incomplete combustion; its flash point is more than 100 C higher than that of ordinary diesel, less volatile matter, longer ignition delay period and difficult start-up; its cetane number is lower than that of ordinary diesel, its acid value is higher than that of ordinary diesel, and its initial distillation point, 50% and 90% distillation temperature are higher than that of ordinary diesel, resulting in serious carbon accumulation in combustion chamber, piston ring bonding and piston ring sticking. Blockage of oil pipeline or filter; incomplete and unburned vegetable oil enters lubricating oil along the cylinder wall, which deteriorates the lubricating oil and causes worsening of parts and components . Later, it was conceived that natural oils and diesel oil could be mixed to reduce their viscosity and improve their volatility. By Amans and Zjiejewski, it is found that the high viscosity of the vegetable oils, the acidic fatty acids contained in the oils, and the gel and carbon deposition and the increase of viscosity of lubricating oil during storage and combustion are inevitable problems.
Microemulsification is to form microemulsifiers of vegetable oils with low-carbon alcohols such as methanol, ethanol and 1-butanol. Although it is also one of the ways to solve the high viscosity of animal and vegetable oils, serious carbon deposition, incomplete combustion and increased viscosity of lubricating oils are found in laboratory-scale durable tests.
Pyrolysis is the process of converting one substance into another under the action of heat or heat and catalyst. It is a process in which small molecules are produced by the breaking of chemical bonds caused by thermal energy in air or nitrogen flow. The original purpose of pyrolysis of vegetable oils was to synthesize petroleum. Schwab et al.  analyzed the pyrolysis products of soybean oil and found that the content of alkanes and olefins was very high, accounting for 60% of the total mass. It was also found that the viscosity of pyrolysis products was more than three times lower than that of ordinary soybean oil, but the viscosity of pyrolysis products was still much higher than that of ordinary diesel oil. Although the process is simple and pollution-free, the pyrolysis equipment is expensive and its degree is difficult to control. When the corrosion values of sulfur, water, sediments and copper sheets in the pyrolysis mixture are within the prescribed range, the ash, carbon slag and cloud point of the pyrolysis mixture exceed the prescribed value.
Transesterification is the most widely used biodiesel production method at home and abroad. This method uses animal and vegetable oils and waste cooking oils as raw materials and alcohol to transesterify under the action of catalyst to produce fatty acid monoesters and glycerol, so as to reduce molecular weight and improve its performance.
Since the reaction is reversible, the higher the amount of alcohol, the more favorable the reaction will be and the higher the yield will be. However, the greater the alcohol content, the more difficult it is to separate. Alcohols generally use low-carbon alcohols, such as methanol, ethanol, propanol, butanol and pentanol . Among them, methanol is the most commonly used because of its low price, short carbon chain and strong polarity, which can react with fatty acid glycerides quickly, and the basic catalyst is soluble in methanol. The reaction can be catalyzed by acids, bases or enzymes, or can be carried out without catalysts.
Alkali used as catalysts are NaOH, KOH, carbonates and alkyl oxides (such as sodium methanol, sodium ethanol, sodium isopropanol and sodium n-butanol). In the absence of water, the transesterification activity of alkaline catalysts is usually higher than that of acid catalysts. The traditional production process uses alkali metal hydroxides with high solubility in methanol as homogeneous catalysts. Generally, the catalytic efficiency of CH3ONa is higher than that of NaOH, but when NaOH and CH3ONa are used to catalyze transesterification of tallow, the amount of catalyst needed to achieve maximum activity is 013% and 015%, respectively. Freedman et al. also found that the results obtained by 1% NaOH and 015% CH3ONa were almost identical when the ratio of alcohol to oil was 6:1 and the reaction time was 1 h. However, NaOH has broad application prospects in large-scale industrial production because it is cheaper and more suitable to be the preferred catalyst for transesterification reaction.
Alkali catalyst can not be used in the case of high free acid, the presence of free acid will poison the catalyst. Because free fatty acids are easy to react with alkali to form soap, the reaction system becomes more complex. Soap acts as an emulsifier in the reaction system. The product glycerol may emulsify with fatty acid methyl ester and cannot be separated. Water is often a poison of catalysts, and the presence of water will promote the hydrolysis of oil and the formation of soap with alkali. Therefore, when alkali is used as catalyst, the acid value of feed oil is often required to be less than 1% and the moisture content is less than 0106%. Oils containing water or free fatty acids can be esterified twice.
The acid catalysts used in transesterification reaction include sulfuric acid, phosphoric acid, hydrochloric acid and organic sulfonic acid. Although acid-catalyzed transesterification is slower than alkali-catalyzed transesterification, acid-catalyzed transesterification is more suitable when the content of free fatty acids and water in glycerol esters is higher. Aksoy et al. reported that when vegetable oils are low-grade oils (such as vulcanized olive oil), transesterification can be more complete under acidic conditions.
When homogeneous acid-base catalyst is used as catalyst, the conversion of oil is high, and the subsequent separation cost is low. However, the disadvantage of homogeneous catalyst is that the catalyst is not easy to separate from the product. The acid-base catalyst in the synthesis must be neutralized and washed after the reaction, resulting in a large amount of sewage. Homogeneous acid-base catalysts can not be reused with the loss of products, resulting in higher catalyst costs. At the same time, the corrosion of acid-base catalyst to equipment is also a problem worthy of attention.
In order to overcome the shortcomings of homogeneous acid-base catalysts, solid catalysts are also an important research direction in recent years. For example, East China University of Technology uses KF / CaO as catalyst to catalyze the production of biodiesel from soybean oil. Solid catalysts for biodiesel production include resins, clays, molecular sieves, composite oxides, immobilized enzymes, sulfates, carbonates, etc. Supported alkali metal catalysts have good application effects on other esterification reactions, such as dimethyl carbonate esterification.