Utilization of industrial waste heat for the cultivation and harvesting of microalgae

Abstract

Microalgae sourced lipids that can be transesterified into biodiesel are a promising source of biofuels that can be produced while mitigating industrial carbon dioxide (CO2) in offgasses. There are many advantages to microalgae compared to other bio-feedstocks, including their rapid growth rate, their ability to accumulate significant amounts of lipid, and the possibility of year-round production. However, there are significant limitations to achieving wide spread and economic microalgae mass cultivation and two of these are addressed in this research program. Microalgae cultivation is currently generally limited to climatic zones where temperatures remain above 15°C, which effectively restricts mass cultivation to tropical or sub-tropical regions thereby eliminating the use of a number of worldwide industrial CO2 sources. However, many of these sources also produce significant amounts of waste heat. The capture and repurposing of waste heat to maintain culture temperature and provide an alterative method for harvesting was explored. A dynamic model was developed to determine the potential of waste streams from a nickel smelter to maintain year-round growth in a cold climate. From this model, it was determined that there is more than enough heat to maintain cultivation temperatures even when the ambient temperature drops well below freezing. Harvesting of microalgae prior to lipid extraction is, with current approaches, often cited as an area where costs need to be significantly reduced. As a wholly novel approach, the capture of this waste heat was also explored for the use as a pretreatment for harvesting by flotation. It was determined to be highly effective and crucially avoids the addition and costs of chemical coagulants, which contaminate and restrict the use of the remaining biomass after lipid extraction.