(48d) Hydrothermal Liquefaction (HTL) of High-Ash Algal Biomass: The Effect of Ash Contents in HTL Reactions
Previous studies demonstrate that the high ash contents appeared to inhibit the formation of biocrude oil in the hydrothermal liquefaction (HTL) processes. Mixed-culture algal biomass from wastewater systems (AW) were screened into three sizes (<106 Î¼m, 106-180 Î¼m and >180 Î¼m) and their ash contents were measured. AW biomass with different ash contents was converted into biocrude oil via HTL at 300 Ë?C for a 60 minute reaction time, which is the previously determined optimum condition for producing biocrude oil from AW. Compared to AW biomass before screen pretreatments, the ash contents of AW of 106-180 Î¼m and <106 Î¼m were respectively decreased from 53.3 % to 38.5% and 41.8 %, while the biocrude oil yields remained almost constant. When the ash contents were reduced from 53.3 % to 38.5 % in the AW biomass, the HHV of biocrude oil was increased from 27.5 MJ/kg to 32.3 MJ/kg and the fraction of light oil (boiling point of 100-300 Ë?C) was increased from 31% to 49%. On the other hand, GC-MS analyses of algal biocrude oil and aqueous products show that the ash contents could promote denitrogenation, catalyze the formation of hydrocarbons, as well as mitigate the recalcitrant compounds in aqueous products under the HTL processes. The above results indicate that algal biomass with certain amounts of ash contents can be converted into biocrude oil with reasonable quality and quantity. In order to quantify the limit of the ash contents under the HTL processes, further HTL conversions with model algal feedstock and representative ash contents were conducted in this study. Elemental and thermogravimetric analyses of the resulting biocrude oil both demonstrate that when the ash content in the algal feedstock was below 40%, the HHV and boiling point distribution of the algal biocrude oil could be hardly affected. This fact substantiates the feasibility of using high-ash algal biomass as HTL feedstock and diminishes the necessity of multi-step pretreatments and modifications of high-ash algal biomass for biofuel application.