Hydrothermal liquefaction of sawdust in seawater and comparison between sodium chloride and sodium carbonate

Abstract

Hydrothermal liquefaction (HTL) of biomass is a promising conversion technology for crude bio-oil (biocrude) production. This work is a continuation of our innovative exploration of using seawater in HTL and hereby the seawater effect is further investigated. Sawdust was chosen as feedstock of interest in this study. At the temperature of 270 °C, using seawater resulted in obvious decline in biocrude yield compared to using freshwater. An elevated temperature at 320 °C promoted the negative effect. Binary mixtures of sawdust’s model components, hemicellulose, cellulose and lignin were liquefied in seawater and freshwater respectively which verified that the negative seawater effect was mainly due to cellulose-lignin interaction. Cellulose-lignin interaction was sensitive to the presence of NaCl, led to severer repolymerization and eventually resulted in biocrude yield decline. A comparison of the performances of Na2CO3 and NaCl in HTL of sawdust found that Na2CO3 exhibited relatively mild negative effect than NaCl. Though increasing temperature enhanced the negative effect for both Na2CO3 and NaCl, Na2CO3 and NaCl affected biocrude formation differently. When adding Na2CO3 into the HTL of sawdust process, the biocrude yield was decreased via decomposing reaction intermediates to generate more aqueous and gaseous products. While the presence of NaCl not only led to the decomposition of intermediates into aqueous and gaseous products, but also induced the repolymerization of intermediates to form solid residue. Computational modelling (Density Functional Theory calculation) indicated that only 41.53 kcal/mol activation energy was required for Cl- to initiate the condensation/repolymerization as compared to that of 101.60 kcal/mol for CO32–, which was supportive for the different underlying pathways for Na2CO3 and NaCl.