Environmental performances of diluents and hydrogen production pathways from microalgae in cold climates: Open raceway ponds and photobioreactors coupled with thermochemical conversion

Abstract

Microalgae are considered renewable energy candidates and are characterized by high yields, integration with waste streams, and ability to grow on poor or marginal lands and therefore not compete with food production. This paper evaluates the environmental sustainability of pathways producing diluents and hydrogen through microalgae in cold climatic conditions such as those in Western Canada. Two alternative cultivation systems (open pond raceway and photobioreactor, microalgal cultivation systems) and four different thermochemical production platforms (hydrothermal liquefaction and pyrolysis for diluent production and supercritical water gasification and thermal gasification for hydrogen production) were considered. On a system level, the industrial-scale processing of 2000 T d−1 dry biomass is modeled for all four conversion pathways. A “cradle-to-gate” process-based life cycle assessment is conducted to quantify the greenhouse gas (GHG) emissions associated with the material, mass and energy requirements at each life cycle stage along with a net energy ratio analysis. The system boundary includes microalgae cultivation through to downstream processing into hydrogen and diluent. Of the thermochemical conversion pathways considered in our study, supercritical water gasification shows the best GHG emissions mitigation in the production of hydrogen (92.1–138.3 g CO2-eq MJ−1). With respect to diluent production, hydrothermal liquefaction processing has environmental benefits and avoids energy use and consequently GHG emissions associated with the feedstock drying required in pyrolysis (10.2–45.65 g CO2-eq MJ−1). This research is unique due to its focus on cold climates like northern Canada.