(758a) Thermochemical Conversion to Biocrude From Pilot Scale Grown Wastewater Fed Algae

Advantages of algae as a biofuel feedstock are well known; high growth rate, high lipid content, and CO₂ neutral.  Thermochemical conversion or hydrothermal liquefaction (HTL) of algae to a hydrocarbon rich biocrude is growing in popularity primarily because it uses subcritical water as the reaction media eliminating the high energy intensive drying step associated with lipid extraction.  HTL efficiently extracts existing lipids in addition to converting other macromolecules such as carbohydrates and proteins to smaller molecules that can be readily upgraded to fuels and other chemicals.  Such a process also eliminates the need for a high lipid producing algae and can solely focus on high growth rate algae.  Recent studies have shown that biofuels from algal feedstocks are most economically favorable if the algae is grown in conjunction with wastewater treatment plants; using free nutrients such as nitrogen and phosphorous found in the wastewater effluent rather than purchasing fertilizers for algal growth.  There is an additional environmental benefit of removing such nutrients before they enter rivers, lakes and oceans by minimizing unnatural algal blooms which can cause anoxic zones.  

This study involves pilot scale algae growth tanks utilizing effluent from the second clarifier at the Lawrence, KS wastewater treatment plant.  Algae was harvested and hydrothermal treated toward biocrude.  Both a mixed-culture microalgae and macroalgae identified as Cladophora sp. were studied.   Hydrothermal treatment was performed with 3g of freeze dried algae and 50mL of water at 350°C for 1 hour.  The starting algae and product yields were evaluated on an ash free dry weight basis.  High oil yields were obtained for both the micro- and macroalgae (54% and 57%).  Residual solids were determined to have higher heating values of 12.4 and 10.8 MJ/Kg for micro- and macroalgae, respectively.  Aqueous co-products had carbon, nitrogen, and phosphorus concentrations of 3100-4000 ppm, 1200-1600 ppm, and 3.5 ppm respectively.  The biocrude produced was very similar to that of petroleum crude in terms of energy density (39-42 MJ/Kg) and elemental percentages of carbon, hydrogen, and oxygen.