Improving bio-oil chemical profile of seaweeds through anaerobic fermentation pre-treatment

Biomass pre-treatments for bio-oil quality improvement are mainly based on thermal and chemical methods which are costly and hence reduce the sustainability of pyrolysis-based refineries. In this paper, anaerobic digestion (AD) and dark fermentation (DF) are proposed as alternative ‘green’ pre-treatments to improve this situation. For this purpose, three seaweeds namely Sargassum polycystum, (Phaephyta), Gracilaria tenuistipitata, (Rhodophyta) and Ulva reticulata, (Chlorophyta) with high ash and oxygen contents were pre-treated to improve their composition and structure prior to pyrolysis. The results reveal that both biological pre-treatments affected, positively, the composition and structure of the seaweed biomass with AD pre-treatment reducing N and S contents by 86% and 63%, respectively. DF was more efficient in terms of ash and moisture reduction with 25% and 70%, respectively. In addition, oxygen (O) reduction by 27% was observed after DF which was evidenced by FTIR spectroscopy indicating the reduction of most oxygen-containing functional groups in the biomass. On the other hand, the carbon (C) content increased in DF pre-treated seaweeds up to 42%, almost two times higher relative content than C in the raw seaweed. The changes in the composition of pre-treated seaweeds resulted in changes in their thermal degradation and the volatile profiles produced during pyrolysis. Interestingly, anhydrosugars and furans which account for some 70% (by area) in raw seaweeds markedly declined or become undetectable after DF pre-treatment and correspondingly more acetic acid and hydrocarbons were produced while after AD more aromatics with high toluene content (ca.17%) were generated. The results indicate that bio-oil with profiles more similar to petroleum-based composition i.e. rich in hydrocarbons and low in anhydrosugars, N and S can be generated by AD and DF pre-treatments and opens up the possibility of these approaches to effect cost reduction in the overall generation of bio-based fuels.