基于藻类的水热液化产油技术被认为是潜在的微藻能源化利用的重要技术之一。而微藻基生物油中的含氮化合物,特别是含氮杂环化合物的种类分布和含量决定后续提质工艺的难度和能耗。因此,本文综述了微藻水热液化过程中氮元素的来源及其迁移转化机理。发现氮元素来自于微藻原料中的蛋白质(氨基酸)、磷脂以及色素,主要来自氨基酸。氨基酸在水热过程中发生复杂的化学反应,包括氨基酸分子内的脱羧基、脱氨基、脱水、脱硫、内环化、内酰胺、反羟醛缩合和断链反应,两个氨基酸分子间的二聚或聚合反应,氨基酸与还原糖的美拉德反应,氨基酸与脂肪酸的氨解反应,以及不饱和中间产物之间的聚合或缩合反应,进而产生氨氮、胺类、酰胺类、哌嗪二酮类和含氮杂环等多种含氮化合物。这些含氮化合物主要分布在生物油和水相产物中,生物油中氮元素存在形式主要是杂环氮、胺类氮和腈类氮三类,而水相氮主要是氨氮、杂环氮和酰胺类氮三类。水相氮和油相氮之间存在明显的迁移转化,通过调控水热液化过程中水相氮组成,将影响微藻水热液化所产生物油的氮元素种类和含量。The bio-oil derived from hydrothermal liquefaction of microalgae is recognized as one of the significant potential techniques for the energy-oriented utilization. The variety and content of nitrogen-containing compounds, particularly nitrogen-containing heterocyclic compounds, in bio-oil determine the difficulty and energy consumption of subsequent upgrading processes. Therefore, this paper reviews the sources of the nitrogen element and its migration and transformation mechanisms during the hydrothermal liquefaction of microalgae. It is discovered that nitrogen primarily originates from proteins or amino acids, phospholipids, and pigments in feedstock, with amino acids being the main contributor. During the hydrothermal process, amino acids undergo complex chemical reactions, including decarboxylation, deamination, dehydration, desulfurization, intramolecular cyclization, lactamization, retro-aldol condensation, and chain scission within amino acid molecule, dimerization or polymerization between two amino acid molecules, Maillard reactions between amino acids and reducing sugars, ammonolysis reactions between amino acids and fatty acids, as well as polymerization or condensation reactions among unsaturated intermediates. These reactions lead to the formation of various nitrogen-containing compounds such as ammonia, amines, amides, piperazine-2,3-diones, and nitrogen-containing heterocycles. These nitrogen-containing compounds are predominantly distributed in the bio-oil and aqueous phase products. In bio-oil, the nitrogen element
微藻被认为是最有前景的生产生物燃料的原料之一,但对其水热液化进行能源转化时,存在如何制备低氮生物油和如何处理高浓度有机废水的两大难题。本文拟探究是否存在一种可以同时解决这两大难题的创新水热液化方法。为此,先归纳了微藻水热液化过程中含氮化合物的生成机理,并获知原料中的氮元素主要在油水两相水热产物间发生迁移转化,调节水热水相中有机氮或无机氮等含氮组成将影响生物油中氮含量。随后,综述了现有的基于水热水相调节微藻水热产油的连续水热液化法和水相循环水热液化法的研究进展,其中,连续水热法通过分离低温水热后水相同时脱氮脱碳,使生物油氮含量下降但油得率降低。水相循环水热法通过回用高温水热后水相同时富碳富氮,使生物油得率提升但氮含量增加。最后,提出了一种基于水相氮元素调控的水热液化方法,此方法可同时脱氮、富碳和节水,将微藻高效转化为低氮生物油。Microalgae is considered to be one of the most promising raw materials for biofuel production. By using hydrothermal liquefaction (HTL) of microalgae to produce bio-oil, there are two problems: how to obtain low-nitrogen bio-oil and how to deal with high-concentration organic wastewater. Therefore, we intend to explore whether there is an innovative HTL that can solve both problems at the same time. In this review, the formation mechanism of nitrogen-containing compounds in bio-oil from HTL of microalgae is summarized firstly. And the nitrogen element in raw biomass mainly migrates and transforms between bio-oil and water phase in the process of HTL of microalgae. As a result, the nitrogen content of the bio-oil could be influenced by regulating the nitrogen composition of organic nitrogen or inorganic nitrogen in water phase during HTL. Then, the recent researches on the sequential HTL and water phase recycling HTL of microalgae are reviewed. The bio-oil with low nitrogen content and a low yield is produced from sequential HTL, for the simultaneous removal of nitrogen and carbon elements by separating the water phase produced from the first step of sequential HTL. The bio-oil with a high yield and high nitrogen content is produced from water phase recycling HTL, for the simultaneous enrichment of nitrogen and carbon elements by reusing the water phase produced from HTL. At last, a method regulating the nitrogen element in water phase is proposed for the removal of nitrogen, the enrichment of carbon and the reduction of water consumption, which can be used for HTL of microalgae to efficiently produce low-
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