Effect of ash on hydrothermal liquefaction of high-ash content algal biomass

Previous studies demonstrate that the high ash contents appeared to inhibit the formation of biocrude oil in the hydrothermal liquefaction (HTL) processes. In order to investigate the effect of ash contents on the HTL reaction, mixed-culture algal biomass from wastewater systems (AW) was separated into two fractions (AW-41.8 and AW38.5) and converted into biocrude oil via HTL at 300 degrees C for a 60 min reaction time (the previously determined optimum condition). Compared to AW biomass before screen pretreatments, the ash contents of AW-41.8 and AW-38.5 were respectively decreased from 53.3 wt% to 41.8 wt% and 38.5 wt%. Moreover, the higher heating value (HHV) of resulting biocrude oil was increased from 27.5 MJ/kg to 32.3 MJ/kg, and the fraction of light oil (boiling point of 110-300 degrees C) was increased from 31 wt% to 49 wt%. The above results indicate that algal biomass with certain amounts of ash contents can be converted into biocrude oil with reasonable quality and quantity. To explore the range of concentrations of ash where it may present a positive effect on the biocrude oil yield or quality, further HTL conversions with pure algal feedstock and representative ash contents were conducted. The HHV and boiling point distribution of the algal biocrude oil was hardly affected when the ash contents in the algal feedstock was below 40 wt%. This fact substantiates the feasibility of using high-ash algae from wastewater treatment systems for HTL feedstocks and diminishes the necessity of multi-step pretreatments and modifications of high-ash algal biomass for biofuel application.