Comparative life-cycle assessment of various harvesting strategies for biogas production from microalgae: Energy conversion characteristics and greenhouse gas emissions

Abstract

Biogas production via anaerobic digestion is highly attractive in the realm of microalgae technology. However, the microalgae harvesting strategies for biogas production has yet to be comprehensively evaluated. Here, a comparative life-cycle assessment for microalgae harvesting scenarios of centrifugation (CT), sedimentation and centrifugation (SD-CT), flocculation and centrifugation (FC-CT), flocculation and electro-flotation (FC-EF) were conducted to evaluate the energy conversion characteristics and environmental impacts. The results showed that harvesting strategies significantly affected the net energy yield of the whole system. Only scenarios FC-CT and FC-EF achieved positive net energy yield of 0.17 and 0.04 MJ with 1 MJ net energy output. The net energy yield of scenarios SD-CT and CT were negative (−0.11 and −1.92 MJ) because of the low harvesting efficiency and high electricity consumption. The net GHG emissions of the four scenarios were all positive, and scenario FC-EF achieved the least GHG emissions (21.86 g CO2-eq MJ−1). According to the nutrient balance analysis of carbon, about 15 percent carbon from flue gas was fixed by microalgae in the form of soil fertilizer (9.32%) and biomethane product (5.63%–6.60%). Additionally, harvesting efficiency and biogas yield had the most profound impact on the parameters for two-step and one-step harvesting scenarios.