Hydrothermal upgrading of algae paste: Application of 31P-NMR

Although already proposed in the 1940s, hydrothermal upgrading of biomass, and specifically algae, to produce biocrude has only recently become a much researched topic. Algae are of special interest as they are capable of much higher biomass accumulation rates than terrestrial plants. To understand the transformation of algal biomass into biocrude, this process needs to be scrutinized in as much detail as possible to identify the most appropriate reaction conditions. It is usually not possible to identify individual transformation steps; however, in this article we demonstrate the use of 31P-NMR to follow the fate of derivatized (2-chloro-4,4,5,5- tetramethyl-1,3,2-dioxaphospholane) hydroxyl groups to start understand, which chemical species are involved in the initial degradation steps. Our data show the occurrence of a very fast degradation of acidic OH groups (derived from the hydrolysis of lipids) and aliphatic OH groups (derived from the fast hydrolysis of carbohydrates). The degradation of proteins leads to polyphenols, which appear to be rather stable even over prolonged treatment periods. Small quantities of aromatic compounds are also formed as secondary degradation products through sugar dehydration and cyclization as well as peptide transformation. It appears that additional deoxygenation routes exist as the deoxygenation rate using 31P-NMR and that determined by elemental analysis differ. The formed oil (biocrude) is compared with typical North Sea oils using simulated distillation and was found to contain a significant high boiling faction, which would require further treatment (e.g., hydrocracking) to shift the boiling range to a more valuable oil composition. © 2013 American Institute of Chemical Engineers Environ Prog, 32: 1002-1012, 2013 Copyright © 2013 American Institute of Chemical Engineers Environ Prog.