(545s) Multi-Stage Hydrothermal Processing of Algae for Enhancing Biocrude Quality and Denitrogenation

Jena, U., New Mexico State University
Hoekman, S. K., Desert Research Institute
Hydrothermal processing of algae into liquid hydrocarbon fuels has received considerable attention in the past few years due to the inherent ability of the process to convert wet biomass without energy intensive pretreatments such as drying. The present study investigates a two-step hydrothermal conversion of algae: a low temperature hydrothermal liquefaction (HTL) process for removal of N and O from algae for subsequent conversion by a second stage HTL at more severe conditions. The specific objectives under the above broad goals were to evaluate a first-stage (low temperature) HTL process at 175-250 ËšC and evaluate the recycling of nitrogen and phosphorous in the aqueous phase co-products (ACP) for growth of mixed consortia diatom and cyanobacteria algae. First-stage HTL experiments were conducted using a custom designed two-chamber reactor (TCR) system at different conditions of reaction temperature (175, 200, 225, and 250 ËšC) and time (1, 2.5, and 5 min). Treated solids and aqueous co-products (ACP) were analyzed by standard laboratory methods including mass yields, elemental C, H, N, and O contents, higher heating value (HHV), removal of N and O from the solids, total nitrogen, ammonia nitrogen, total phosphorous, total organic carbon, pH, and non-volatile residues (NVR) for the aqueous co-products. Statistical analyses were performed to evaluate solid yields in the first-stage HTL experiments. Solid yields were higher at less severe reaction conditions (lower temperature and shorter time) than at more severe conditions (higher temperature and longer time). The highest solid yield (~60%) was obtained at 170 ËšC and 1 min; the lowest solid yield (~35%) was obtained at 250 ËšC and 5 min HTL conditions. Recycling of ACP and further growth of algae was tested in batch cultivation of diatom-cyanobacteria mixed consortia algae and compared with growth using WC standard medium. Results showed that use of ACP medium, combined with phosphorous-depleted WC medium, provided significant growth.