A review of the strategy to promote microalgae value in CO2 conversion-lipid enrichment-biodiesel production

Fossil fuels, including coal, oil and natural gas, remain the main source of global energy supply. Not only the storage capacity is limited, but also the use of the ecological environment causes great damage, a serious threat to human life safety. Microalgae biodiesel receives extensive attention due to its biodegradable and carbon-neutral green energy characteristics, which is used to reduce or replace the use of these fossil fuels. Because of the slow growth of microalgae, low lipid content and low biodiesel production, people begin to study ways to improve the growth and lipid production of microalgae and catalytic conversion of biodiesel. In this paper, the research progress of the whole process from microalgae culture to microalgae oil production to microalgae catalytic conversion of biodiesel in recent years is reviewed. The efficient growth and oil production of microalgae, the catalytic conversion methods of microalgae biodiesel and the application prospects of microalgae biodiesel are discussed and summarized. Specifically, this review critically discusses: (i) The current development status of microalgae culture and microalgae biodiesel; (ii) Cultivation methods for efficient growth and oil production of microalgae under different stress conditions; (iii) The effects of different types of catalysts and catalytic methods on the yield of biodiesel; (iv) Environmental problems and economic value of the whole process of microalgae biodiesel production; (v) Current limitations and future research directions in the field of microalgae biodiesel. Studies show that regulating light, temperature, pH, nutrients, etc. can improve the growth and oil production of microalgae, in which the addition of anions and cations can change the physiological, biochemical and metabolic mechanisms of microalgae. At present, a large number of genetic modification and metabolic regulation engineering technologies are applied to microalgae oil production. They have precise control of the targeting mechanism in cells to induce increased lipid accumulation. The two-stage culture model can simultaneously increase biomass and lipid content. The addition of homogeneous, heterogeneous, biological enzymes and nano-catalysts accelerates the conversion efficiency and improves the yield of biodiesel. The catalytic yield of several magnetic nano-catalysts studied is as high as > 98 %. The supercritical method and ionic liquid method are favored by everyone because they do not use catalysts to reduce environmental pollution. In situ transesterification replaces the traditional two-step transesterification method to reduce energy consumption and cumbersome steps. Hydrothermal liquefaction method can directly convert wet algae biomass into biofuels without drying at high temperatures and pressure and in the presence of water. However, the current high-cost economic problems in the production of microalgae biodiesel and the environmental problems caused by the addition of catalysts need to further study the efficient and low-cost microalgae culture methods and new environmentally friendly catalysts and catalytic methods to improve the yield of microalgae biodiesel, to realize the commercial production of microalgae biodiesel as soon as possible.