Two-stage hydrothermal liquefaction of Spirulina: Experimental observations, energy analysis and techno-economic assessment

The intensification of environmental pollution and significant consumption of fossil fuels has led to widespread attention on developing renewable resources. As a carbon-containing renewable energy source, Spirulina is characterized by its fast growth cycle and wide distribution. Currently, one of the research hotspots is the conversion of Spirulina into bio-oil through hydrothermal liquefaction. However, studies on the two-stage hydrothermal liquefaction of Spirulina predominantly concentrate on the product yield or composition. There is relatively little research on thermodynamic (energy) analysis and techno-economic assessment during the reaction process. The main bio-oil components from two-stage hydrothermal liquefaction of Spirulina are ketones and acids with 44.9 % and 34.1 %, respectively. Compared to no-catalysis, two-stage hydrothermal liquefaction of Spirulina over HZSM-5, alkili-modified HZSM-5, and Ni@alkali-modified HZSM-5 increase the relative content of hydrocarbons to 6.6%, 32.9% and 48.1 %, respectively. Energy analysis and techno-economic assessment are conducted with Aspen Plus simulation, revealing a exergy efficiency of 56.4 % for the catalytic two-stage hydrothermal liquefaction of Spirulina compared with 43.6% over no-catalyst. The primary losses occur in the first stage of the reaction process (248.6 kW) and the cooling process (10.7 kW) for catalytic two-stage hydrothermal liquefaction of Spirulina. The hourly price of producing 1 kg bio-oil with a catalyst is $3.67, while without a catalyst, it is $4.54 kg- 1; obtaining a unit of chemical exergy costs $0.14x10- 3 kJ- 1 through catalysis. Moreover, the fluctuations in the prices of sulfuric acid and catalysts are most sensitive to the cost price of bio-oil with or without catalysts in the two-stage hydrothermal liquefaction process, with the values being 10.06% and 8.10%. This study has important significance for efficient transformation and energy utilization of Spirulina.