Integrated Algal Biorefineries for Biofuel and Bioplastic Co-Production: Process Performance, Circularity, and Scale-Up.
Algal biorefineries provide a platform for producing fuels, biodegradable materials, and higher-value co-products from the same biomass resource. This review examines integrated pathways that connect microalgal and macroalgal cultivation with biodiesel production, hydrothermal liquefaction (HTL), anaerobic digestion (AD), polysaccharide extraction, and bioplastic manufacture. Microalgal lipid contents of 40%-70% of dry biomass can support biodiesel production, whereas hydrothermal processing of wet biomass has produced bio-crude yields of 30-50 wt% with energy densities of approximately 35-40 MJ/kg. Macroalgal polysaccharides, including alginate, agar, and carrageenan, can be processed into films, blends, and composites with reported tensile strengths of 18-45 MPa. However, the environmental and economic performance of these routes depends on cultivation configuration, dewatering requirements, product recovery, allocation method, and system boundary. Published techno-economic estimates place stand-alone algal biodiesel production at approximately USD 4-7 gal-1. In contrast, integrated scenarios can approach USD 2.50 gal-1 when favorable biomass productivity and co-product credits are assumed. Machine-learning (ML) methods have mainly been applied to laboratory data analysis and process prediction, while evidence of commercial real-time control remains limited. The review identifies mass-balance reporting, long-duration outdoor cultivation, standardized biodegradation testing, and harmonized techno-economic and life-cycle assessment as priorities for scale-up.