With the increasing demand for energy in cities and the depletion of petroleum resources, the whole world will face the crisis of energy shortage. Moreover, petroleum combustion causes serious pollution to the environment, which affects people’s health to a great extent. Therefore, the research and application of biodiesel has become the focus of alleviating the deteriorating energy and environmental problems.
1 Definition and advantages of biodiesel
Biodiesel is a kind of organic fatty acid ester fuel which is made from oil crops, wild oil plants, engineering microalgae and other aquatic vegetable oils as well as animal oils and waste cooking oils by transesterification process. In industrial production, biodiesel refers to fatty acid methyl ester, which is the product of esterification reaction between fatty acid and methanol.
Based on the definition of the American Biodiesel Association, biodiesel is a clean alternative fuel produced from renewable biological resources such as plant, animal oil and so on, which can be used in compression ignition engines. Natural grease is composed of triglycerides of long chain fatty acids. Its molecular weight is about 700-1000. Although it can burn itself, it can not be fully mixed with ordinary diesel oil. There are many defects in direct use of natural grease as diesel oil, so special diesel engines need to be designed. Fatty acid methyl ester is obtained by transesterification. Its molecular weight is reduced to 200-300, which is similar to that of diesel oil and its performance is close to that of diesel oil. It can be mixed in any proportion without the need to design a special diesel engine. It is an environmentally friendly and environmentally friendly fuel that can replace diesel oil.
Biodiesel has similar performance with petrochemical diesel, and has significant advantages [2,3]: (1) It has excellent environmental protection characteristics. Biodiesel has low sulfur content, no aromatic hydrocarbons, no aromatic hydrocarbons and sulfur (< 10 ug/g). The damage of combustion tail gas to human body is lower than that of diesel, and biodegradability of biodiesel is high. (2) It has good lubrication performance. When the dosage is only 0.4%, biodiesel shows anti-wear effect, which can alleviate the vehicle wear problem caused by reducing the sulfur content of clean fuel and enhance the anti-wear performance of automobile diesel. (3) It has good safety performance. Biodiesel is not a dangerous fuel because of its high flash point, which is higher than that of petrochemical diesel. Its advantages in transportation, storage and use are obvious. (4) It has good combustion performance. Its cetane number is higher than 56 (49 for ordinary diesel), and its combustion performance is better than that of diesel. The slight acidity of combustion residue prolongs the service life of catalyst and engine oil. (5) It has renewable properties. As a renewable energy source, its supply will not be exhausted. (6) The system using biodiesel has less investment. The original engine, refueling equipment, storage equipment and maintenance equipment are basically unchanged. (7) The blending of biodiesel and petrochemical diesel in a certain proportion can reduce fuel consumption, improve power performance and reduce tail gas pollution.
At present, the main preparation methods of biodiesel include direct mixing, microemulsification, pyrolysis and transesterification. The first two methods belong to physical methods. Although they are simple and feasible, they can reduce the viscosity of animal and vegetable oils, but the cetane number is not high, and the problems of carbon deposit and lubricant pollution in combustion are difficult to solve. The pyrolysis process is simple and no pollutants are produced. The disadvantage is that the pyrolysis process is carried out at high temperature. Catalyst is needed. The pyrolysis equipment is expensive and the reaction degree is difficult to control. The main product of pyrolysis is bio-gasoline, and the production of bio-diesel is not high. The main method of producing biodiesel in industry is transesterification. In transesterification reaction, triglyceride, the main component of oil, and various short chain alcohols were transesterified to obtain fatty acid methyl ester and glycerol under the action of catalyst. Alcohols used for transesterification include methanol, ethanol, propanol, butanol and pentanol. The most commonly used are methanol, which is due to its low price, short carbon chain and strong polarity, can react with fatty acid glycerides quickly, and the basic catalyst is soluble in methanol. The transesterification reaction is reversible. Excessive alcohol can shift the equilibrium to the product, so the actual amount of alcohol is much larger than its stoichiometric ratio. Catalysts used in the reaction can be alkali, acid or enzyme catalysts, which can accelerate the reaction rate and increase the yield. The transesterification reaction consists of a series of tandem reactions, in which triglycerides are converted step by step into diglycerides and monoglycerides, and finally into glycerides, each of which produces an ester. Transesterification methods include acid catalysis, alkali catalysis, enzyme catalysis and supercritical transesterification.
Direct mixing method
In the early stages of biodiesel research, researchers envisaged mixing natural oils with diesel, solvents or alcohols to reduce their viscosity and increase volatility. In 1983, Amans et al. mixed degummed soybean oil with diesel oil No. 2 at a ratio of 1:1 and 1:2, respectively, and conducted 600 h tests on a direct injection turbine engine. When the two oil products are mixed with 1:1, the phenomenon of lubricating oil becoming muddy and gelatinization will occur. The ratio of 1:2 will not appear. It can be used as an alternative fuel for agricultural machinery. Ziejewski et al. mixed sunflower seed oil with diesel oil at a volume ratio of 1:3. The viscosity of the mixture was 4.88 *10-6 m2/s at 40 C, while the maximum viscosity stipulated by ASTM (American Material Test Standard) should be less than 4.0 *10-6 m2/s. Therefore, the blend fuel is not suitable for long-term use in direct injection diesel engines. The experiment on the mixture of safflower oil and diesel oil has obtained satisfactory results. But in the long-term use of the mixture will still cause the lubricant to become muddy .
Microemulsion is a colloidal equilibrium system with a diameter of 1-150 nm formed by mixing two immiscible liquids. It is a transparent and thermodynamically stable colloidal dispersion system . In 1982, Georing et al. made microemulsion from ethanol aqueous solution and soybean oil. The microemulsion has similar properties to diesel oil No. 2 except for its low cetane number. Ziejewski et al. made the emulsion with 53.3% sunflower seed oil, 13.3% methanol and 33.4% 11-butanol as volume ratio. Although there was no serious Demulsification in the 200 h laboratory durability test, there were still some problems such as carbon deposition and increasing the viscosity of lubricating oil. Neuma et al.  used surfactants, cosurfactants, water, diesel oil and soybean oil as raw materials, developed a new microemulsion liquid system, which consists of diesel oil (3.160 g), soybean oil (O.790 g), water (O.050 g), isoamyl alcohol (0.338 g) and sodium dodecyl sulfate (O.676 g). The physical properties and combustion performance of the microemulsion liquid system are very close to that of diesel oil.
Pyrolysis at high temperature
High temperature pyrolysis is a process in which small molecules are produced by the cleavage of chemical bonds caused by thermal energy in air or nitrogen flow. The pyrolysis of triglycerides at high temperature can produce a series of mixtures, including alkanes, olefins, dienes, aromatics and carboxylic acids. Different vegetable oils can be pyrolyzed to obtain mixtures of different components. Schwab et al. analyzed the pyrolysis products of soybean oil and found that the content of alkanes and olefins was 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 crude oil, but the viscosity of pyrolysis products was still much higher than that of ordinary diesel oil. In terms of cetane number and calorific value, the cracking products of soybean oil are similar to those of diesel oil. Pioch et al.  The catalytic cracking of coconut oil and palm tung oil to biodiesel was studied using SiO 2/AI 2O 3 as catalyst at 450 C. It was found that the pyrolysis products were divided into gas-liquid-solid three-phase, and the liquid phase was composed of bio-gasoline and bio-diesel. The analysis shows that the properties of the biodiesel are very similar to those of ordinary diesel.
Oxygenation cracking can also produce biodiesel. Several new processes have been developed. Glycerol is not co-produced by oxycracking. Vegetable oil can be converted into diesel oil with high cetane number and low sulfur. It can process a wide range of raw materials including materials with high free acid content. Several reactions occur during hydrocracking, including hydrocracking, hydrotreating and hydrogenation. Compared with petroleum-based diesel, its degradation rate is higher, but its main advantage is to reduce NO emissions .