(13g) Wet Torrefaction As a Pretreatment for Pyrolytic Bio-Oil Production From Microalgae

Pyrolytic bio-oil derived from microalgae has been explored in recent studies. The resultant bio-oil contains significant amounts of aliphatic and aromatic hydrocarbons; however, pyrolytic conversion requires energetically unfavorable drying to remove excess water before treatment.  Additionally, high protein feedstocks, like those from microalgae, yield in bio-oils with large N content which lowers the heating value and leads to potential NOx emission during combustion. Therefore, wet torrefaction, a pretreatment for pyrolysis, was performed on microalgae Nannochloropsis oculata, which aimed to reduce the N content in the algal feedstock by hydrolyzing the proteins. The wet torrefaction process treats the algal biomass in pressurized water at elevated temperature (ca. 200 °C), thus eliminating the need for drying and reducing overall process energy. The effects of various reaction conditions on the yield and composition of torrefied algae were investigated by varying temperature from 150 to 225 °C and reaction time from 10 to 60 min. Compared with untreated algae, torrefied samples had higher carbon content and enhanced heating values under all reaction conditions. Also, decreased N content was observed under relatively severe conditions. It was found that 73%-99% of the fatty acids in the starting algae were retained in the torrefied samples, which is important because fatty acids can be mainly converted to hydrocarbons during pyrolysis. Gas chromatography- Mass spectrometry analysis confirmed that pyrolytic bio-oil from torrefied algae contained less N-containing compounds and was dominated with long-chain fatty acids which could be readily converted into hydrocarbon fuels in the presence of simple catalysts. Energy consumption ratio analysis showed that wet torrefaction has a more favorable energy balance than directly drying of algae. The aqueous phase products, obtained after the separation of torrefied algae, contained significant amount of nutrients and can be recycled for algae cultivation.