Life cycle assessment and techno-economic analysis for biofuel and biofertilizer recovery as by-products from microalgae

Although there are several studies on the hydrothermal carbonization of microalgae biomass for value-added products and their potential environmental impacts, the feasibility of scaling-up and the environmental and economic analysis need to be investigated. This study produced solid biofuel and biofertilizer under two operating conditions (170 °C, 10 min, 7 bar; or 130 °C, 50 min, 2 bar, respectively). Also, investigated the environmental performance and techno-economical viability of producing solid biofuel and biofertilizer from microalgae. Scale-up modeling used Aspen Plus software to obtain optimized production processes, energy data, equipment requirements, and costs, which were used to support the economic and environmental analyses. The Recipe Midpoint method showed that the solid biofuel had a higher environmental impact than the biofertilizer, especially in the Freshwater Eutrophication category (almost 7 times higher), Fossil Resource Scarcity (4.2 times higher), Global Warming, and Marine Ecotoxicity (both around 2.7 times higher). The damage assessment results showed that the production of solid biofuel was more impactful, with Human Health impacts 2 times higher, Ecosystem 1.6 times higher, and Resource 4.3 times higher than those caused by biofertilizer production. The biofertilizer production scenario emerged as the most economically viable option for hydrothermal carbonization valorization. The negative net present value and the inability to achieve the return on investment within a 30-year timeframe made the solid biofuel production scenario unviable. Thus, biofertilizer production from hydrothermal carbonization of microalgae biomass cultivated in wastewater is technically, economically, and environmentally feasible, whereas the same cannot be said for the biofuel evaluated.