A Novel Sequential Hydrothermal Liquefaction Technique to Simultaneous Extract Bio-Oil and Value-Added Polysaccharide From Algae

Abstract

Hydrothermal liquefaction has been used for bio-oil extraction as a crucial technology. However, the expense of this process is too high to scale up industrial scale. To make bio-oil extraction more economic feasible, co-products need to be extracted concomitantly with bio-oil. Therefore, our lab developed a novel two-step sequential hydrothermal liquefaction technology (SEQHTL) to simultaneously produce value-added polysaccharide and bio-oil from algal biomass by two temperature gradient steps. Based on our recent experiment data, one of the important achievements of SEQHTL is the remarkable reduction of bio-char production by comparing with direct hydrothermal liquefaction (DHTL) method, a traditional algal thermal conversion technique. Besides, the production of polysaccharide does not significantly influence the quantity and quality of bio-oil, which supported by comparative chromatographic (GC-MS, GLC) and spectrophotometric (NMR, FT-IR) analysis of the bio-oil extracted by SEQHTL with DHTL. Furthermore, mass balance and energy cost were computed with the comparison between DHTL and SEQTHL method. Energy input for SEQHTL is 10% lower than DHTL, which means this novel method saves more energy than DHTL, as well as more polysaccharide and less bio-char production. We also have made lots of efforts on optimizing SEQHTL method for better extraction of polysaccharide and bio-oil with consideration of three parameters biomass water ratio, reaction temperature and equilibration time. Polysaccharide yield approached the highest amount at 160°C, 20min with biomass water ratio at 1:9. Reaction temperature optimization of bio-oil, second step of SEQHTL, were operated from 200°C to 320°C with comparison of the traditional bio-oil extraction method DHTL. The results showed that the optimal bio-oil extraction temperature for SEQHTL is 240°C, which is 60°C lower than DHTL method (optimal temperature at 300°C). Therefore, SEQHTL method required less energy input comparing with DHTL, indicating this novel method will be more competitive than DHTL. Due to the lower energy input, less char yield and more value-added products, we strongly feel that SEQHTL has a bright commercial potential for algal biomass.