Biodiesel, as a renewable energy that can replace fossil fuels, has been rapidly developed and produced on a large scale, resulting in excessive production of its by-product glycerol. Crude glycerol can be obtained by treating biodiesel through a simple process. In addition to glycerol, there are other impurity components in crude glycerol. Crude glycerol must be refined if want to be applied to food, cosmetics, medicine and other industries. At present, the refining process of crude glycerol is complicated, with high cost and low economic feasibility. Therefore, it is necessary to develop the application space of crude glycerol and improve the added value of crude glycerol. The use of crude glycerol as a fuel additive is one of them.
Glycerol alkyl ether is a better fuel additive, which can improve fuel performance, increase cetane number, increase flow performance, reduce the composition and content of harmful substances in combustion exhaust, and is used as an additive for diesel and biodiesel. The application of crude glycerol in this technology can not only make use of glycerol, a by-product of biodiesel, but also obtain high value-added glycerol alkyl ether fuel additive.
Among them, glyceryl tert-butyl ether obtained from the reaction of glycerol with isobutene or tert-butanol, is a promising additive. Adding it to the diesel raw material can significantly reduce the exhaust particles, hydrocarbons and carbon monoxide and other substances. Karinen et al. studied the etherification reaction between crude glycerol and isobutene in liquid phase using acid ion exchange resin as catalyst. They found that the main reaction of the whole process was etherification, its main products were five kinds of ethers, and the side reaction is oligomerization of isobutylene to form hydrocarbons of C8 ~ c16. And when the ratio of glycerol and isobutylene is 3:1, the reaction temperature is 80 ℃, the reaction selectivity is the best, and also composition of the ether products and the degree of the etherification reaction can be controlled by changing the reaction conditions. Kiatki Ttipong et al. used FCC gasoline and glycerol as reactants to investigate the influence of the etherification reaction process on the performance of FCC gasoline, with catalysts of Amberlyst16 and Amberlyst15 -molecular sieve. It turned out that the content of olefin is obviously decreased and the octane number is increased in the etherified gasoline product, comparing to the initial FCC gasoline. However, it was found that -molecular sieve had a better catalytic effect and was more suitable as the catalyst for etherification, when using -molecular sieve and Amberlyst16 catalyst.
Glycerol can also be converted into fuel additive by acetylation, acetal and other reaction processes. Rahmat et al. studied the reaction process and specific characteristics of glycerol converted into fuel additives through etherification, acetylation and acetalization in their review article, And, they applied it to gasoline, biodiesel and diesel to examine the effects of the additives obtained from each reaction. Pradima et al. also summarized the different crafts in different reaction processes (esterification, etherification, acetylation and acetalization) and related catalysts in the conversion of glycerol into biofuel additives. It was found that when the fuel additive is mixed with diesel oil, the fuel additive could increase the cetane number, improve the low temperature flow performance, improve the lubrication performance and reduce the content of various harmful substances in the exhaust gas.
Thus, the utilization rate and application value of high value-added fuel additives can be improved by using crude glycerol. However, considering the influence of impurities in crude glycerol on the final fuel performance, secondary pollution to the fuel could be avoided by developing high-efficiency catalysts to increase the selectivity and yield of glycerol alkyl ether, and separate the unreacted components in crude glycerol with the high-efficiency separation and extraction process
