What is Oxygenated Fuel?

Oxygenated fuel ^[1] refers to fuel that can provide additional oxygen to diesel engines and make the fuel burn more fully. Oxygen-containing fuels can reduce the emissions of unburned HC, CO, PM, soot and other harmful substances in diesel engines. Burning low-emission oxygen-containing fuels is one of the effective technologies for controlling diesel engine emissions. Among the currently researched alternative fuels, oxygen-containing fuels The future of development is good. Especially the new biodiesel, vegetable oil ether-based ester, while effectively increasing the oxygen content, also ensures that the fuel has a high cetane number, and the synthesis process is simple, and it can be industrially produced. It is an ideal diesel substitute and additive. Common are vegetable oil and its esterified fuel, biomass pyrolysis fuel, alcohol fuel and dimethyl ether.

With the development of the world economy, as an important strategic energy source for the economic development of countries around the world, the demand is growing, while the supply is becoming tighter. Excessive reliance on oil and over-exploitation of petroleum resources have led to an increasingly severe oil crisis. On the other hand, the prosperity of the economy has driven the rapid development of the motor vehicle industry. The number of motor vehicles in cities has been increasing, and automobile exhaust has become a major source of pollution, exacerbating global air pollution. Therefore, it is of great significance to find a clean alternative fuel for diesel engines to solve the two world-wide problems of the oil crisis and motor vehicle exhaust pollution. Oxygenated fuel is a clean alternative fuel for diesel engines. It has a wide range of raw materials and excellent emissions performance. The use of oxygen-containing fuels as additives or substitutes for diesel can be used to reduce the exhaust emissions and improve the combustion efficiency of diesel engines. Therefore, the research of oxygen-containing fuels has attracted much attention ^[2]

Vegetable oil is a kind of biomass fuel with low sulfur content, non-toxic, and can be biodegraded. At normal temperature, the viscosity of vegetable oil is one order of magnitude higher than that of light diesel oil. However, its viscosity temperature curve is much steeper than that of diesel oil. Therefore, in actual use, there is no problem of excessive viscosity except in cold environments. Vegetable oil is an oxygen-containing fuel. It has a self-oxygen supply capacity during combustion and has sufficient combustion. Although its unit mass heating value is slightly lower than diesel, its overload capacity is stronger than diesel.

There are many unsaturated fatty acids in vegetable oil, which can easily form gelatin and block the oil path. When the diesel engine directly uses vegetable oil, the incomplete combustion residues are deposited in the combustion chamber, and the piston rings are stuck and the injectors are coked, which affects the service life of the diesel engine. In addition, the vegetable oil droplets ejected from the injector are much larger than the droplet diameter of the diesel, which results in poor formation quality of the mixed gas in the cylinder, and unburned fuel is easy to flow into the crankcase after spraying to the cylinder wall, causing Lubricating oil has deteriorated [3], so the application of vegetable oil directly on diesel engines is limited.

In order to solve the above problems, esterification technology is used to change the quality of vegetable oils. The esterification process is to replace the glycerol in the triglyceride of the large molecular chain structure with methanol, ethanol, etc. to obtain the fatty acid ester of the small molecular chain. In the esterification process, acids, enzymes, alkaline earth metal hydroxides such as KOH, NaOH, sodium methoxide (CH ₃ ONa), etc. are commonly used as catalysts. Because vegetable oil contains free fatty acids, phospholipids, vitamin E, carotene, and steroids, it is necessary to deacidify the vegetable oil (such as adding methanol to esterify) and remove impurities before esterification to reduce the production cost. Methanol is cheap and has a small molecular weight, so the esterified fuel for diesel engines is mainly methyl ester.

875 0 Since 1988, rapeseed oil methyl ester has been used as an alternative fuel in many European countries, and Germany has also formulated rapeseed oil methyl ester standard, that is, the density of rapeseed methyl ester is 0. 875 0 at 15 . 9 g / mL, viscosity is 3.5 5 mm2 / s, acid value is less than 0.5mg (KOH) / g, iodine value is less than 115mg (I) / 100 g ^[3]

Biomass pyrolysis liquid fuel is suitable for storage and transportation, and can be used as an economical and feasible backup fuel, which has aroused great research interest in various countries. Hemicellulose, cellulose, lignin and a small amount of other organic matter in biomass are decomposed into gaseous matter, aerosol and charcoal in a high temperature and anoxic environment, and the gaseous matter is condensed and concentrated into a dark brown pyrolytic liquid fuel . At present, the most researched is fast pyrolysis technology. The reactor structure, reaction temperature and pressure, heating rate and other factors affect the production and properties of biomass pyrolysis fuel. Studies have shown that under the conditions of high heating rate, a reaction temperature of about 500 ° C and a short gas residence time (reducing side reactions), the maximum yield of pyrolytic liquid fuel can be obtained ^[6]

The most studied alcohol fuels are methanol and ethanol. Among them, ethanol fuel is particularly attractive for developed agricultural areas. For example, Brazil is the country with the most ethanol engine applications. In 1989, 40% of the cars used 95% ethanol + 5% water fuel, and 40% of the cars used 78% gasoline + 22% ethanol fuel ^[9]

Ether fuel ^[2]

Based on the theory of fuel combustion, some researchers have followed the characteristics of diesel compression ignition fuels, and combined with the characteristics of oxygen-containing fuels, they have proposed ether-ester oxygen-containing fuels. This kind of substance is based on the ester group and the ether group is added to take advantage of both to achieve high cetane number, good ignition performance, outstanding abrasion resistance, high oxygen content, and reduce emissions.

The ether ester oxygenated fuels currently studied are mainly 2-methoxyethyl acetate, 2-ethoxyethyl acetate, 2-methoxypropyl acetate, and methyl-2-methoxyethyl carbonate. Methyl ester, methyl-2-ethoxyethyl carbonate. Acetyl-2-methoxyethyl acetate (MEA) can achieve good mutual solubility with diesel oil, does not require any co-solvent, and is stable and non-layered.

Among the various alternative fuels currently studied, oxygen-containing fuels are the most promising. Among them, alcohols, esters, ethers, etc. are widely valued by countries due to their good environmental protection capabilities. Although there are some new problems when using various oxygen-containing fuels on diesel engines, such as the new emissions of alcohol fuels during combustion-unburned alcohol, aldehyde emissions, etc., through continuous research, oxygen-containing diesel engines will Play a greater role in various power plants and environmental protection.

Oxygen-containing fuel can provide additional oxygen for diesel engines and make the fuel burn more fully ^[2] , which can reduce the emissions of unburned HC, CO, PM, soot and other harmful substances in diesel engines. Burning low-emission oxygen-containing fuels is to control diesel engine emissions As one of the effective technologies, among the alternative fuels currently studied, the development of oxygenated fuels has a good future. However, we must also pay attention to the shortcomings of oxygen-containing fuels. During the combustion process, the emissions of unconventional pollutants such as alcohols and aldehydes will increase significantly, and the volatilization of fuels will cause alcohols and ethers to increase in varying degrees. Therefore, more in-depth research is needed on the emission characteristics and suppression mechanisms of unconventional pollutants during the application of oxy-fuels. In addition, reducing the cost of oxygen-containing fuels is also an important aspect. Only in this way can large-scale applications be realized.