Summary
HTL processing produced twice the desired organic biocrude when the HL heterotrophically grown algal material was converted versus the SL phototrophically grown version of the same algal strain. The higher yield of biocrude was due to the substantially higher lipid content of the HL feedstock. Upgrading of the biocrudes via HT produced similar yields on an upgraded organic/g dry biocrude fed basis. Thus, the heterotrophically grown algal feedstock produced about twice as much fuel blendstock. Furthermore, a greater portion of the upgraded organic product from the HL feedstock contained hydrocarbons that boiled in the diesel fuel range due to the high lipid content of the starting strain. The upgraded organic product from the SL algae contained substantially more cyclic and lower-boiling compounds, likely due to the higher concentration of proteins in the starting material. TEA of a combined HTL and subsequent HT process to convert the feedstocks to fuels revealed conversion costs associated with the heterotrophic algae are lower compared to the phototrophic culture due to the greater biocrude yields and hence greater yield to higher value diesel. The HTL capital costs contributed the largest fraction in the total production cost for both feedstocks. For the first time, we have demonstrated that although production costs will be higher when producing heterotrophically stressed algae due to the addition of a carbon substrate, the significant improvement of the fuel yield could more than offset the additional cost, resulting in a lower minimum selling price for the fuel product.