Algal Biomass Harvesting Using Low-Grade Waste Heat: Investigation of Dewatering Algal Suspension by Evaporation

Abstract

This study investigated the feasibility of harvesting algae biomass using an original and novel method that uses low-grade waste heat with an integrated heat exchanger, evaporation tank, and reservoir system. Several experiments were conducted. The experimental results showed good agreement with theoretical results estimated with a stagnant-film model. As the inlet gas temperature increased from 175 to 245 °C, the evaporation rate increased by 100%, while an 85% increase was achieved as the air speed increased from 0 to 3.5 m/s. It was also observed that the evaporation rate slightly decreased when doubling the volume of the reservoir while it is independent of the duration of experiment. The amount of lipid extracted from centrifuged algae was slightly higher than algae harvested by evaporation. However, the total amount of fatty acid methyl esters (FAMEs) was significantly higher, by 24%, for the algae harvested by evaporation compared with centrifugation. The FAMEs profiles were the same for both methods and about 98% of FAMEs were C-16 and C-18 carbon chains which are the main components of the algal biodiesel. The method has a potential to be developed into a cost-effective and energy efficient algal biomass dewatering method. It uses low-grade waste heat, which is cheap and readily available, and has simple and inexpensive structure. It was also demonstrated that modifying the proposed system, by adding a second heat exchanger in series, improved the water evaporation rate by 58–121%, depending on the operating conditions.