System Study On Mass Production of Biodiesel From Microalgae With Nutrients Recycling Process

Mass production of biofuel from microalgae is a promising technology because of their high productivity of bio-oil and rapid growth rate compared with other biomass plants. However, a large amount of nutrients such as N, P and K is required for large-scale culturing of microalgae. So far, few studies have proposed a large-scale bio-diesel production system considering cost of nutrients or their recycling.

In this study, at first we calculated cost of nutrients for mass production of microalgae with and without their recycling. It was found that the bio-diesel costs at least 1400 yen/L-bio-diesel (1 US$=100 yen) without nutrients recycling even if some of the nutrients are supplied by waste water. This clearly suggests that most of the nutrients must be recycled from microalgae residue after bio-oil extraction.

Based on this result, we have proposed four processes (Process 1: bio-oil extraction by Hydrothermal Liquefaction (HTL) and bio-diesel production by supercritical (s.c.) methanol method; Process 2: bio-oil extraction by HTL and bio-diesel production by alkali method; Process 3: bio-oil extraction by hexane and bio-diesel production by s.c. methanol method; and Process 4: bio-oil extraction by hexane and bio-diesel production by alkali method) with nutrients recovery after bio-oil extraction. The material balance, energy balance and total costs of the four processes were calculated. The results show that the Process 1 is most feasible because the energy consumption of bio-diesel production was lowest (0.19 MJ/MJ-biodiesel) among the four processes. The capital cost of the Process 1 was calculated to be 1.3×10² yen/L-bio-diesel when the annual expenditure rate for the fixed cost is assumed to be 0.2 of the capital cost excluding the land cost. The variable cost was calculated to be 1.1×10² yen/L-bio-diesel.