Abstract
A comparative evaluation of energy requirement and CO2 emission was performed for native polyculture microalgae oil production in a wastewater treatment plant (WWTP). The wastewater provided nutrients for algae growth. Datasets of microalgae oil production and their details were collected from the Minamisoma pilot plant. Environmental impact estimation from direct energy and material balance was analyzed using SimaPro® v8.0.4. in two scenarios: existing and algal scenarios. In the existing scenario, CO2 emission sources were from wastewater treatment, sludge treatment, and import of crude oil. In the algal scenario, CO2 emission with microalgae production was considered using wastewater treatment, CO2 absorption from growing algae, and hydrothermal liquefaction (HTL) for extraction, along with the exclusion of exhausted CO2 emission for growing algae and use of discharged heat for HTL. In these two scenarios, 1 m3 of wastewater was treated, and 2.17 MJ higher heating value (HHV) output was obtained. Consequently, 2.76 kg-CO2 eq/m3-wastewater in the existing scenario and 1.59 kg-CO2 eq/m3-wastewater in the algal scenario were calculated. In the HTL process, 21.5 MJ/m3-wastewater of the discharged heat energy was required in the algal scenario. Hence, the efficiency of the biocrude production system will surpass those of the WWTP and imported crude oil.
Highlights
Energy security has become a concern in Japan since the accident of the Fukushima nuclear power plant
The microalgae biofuel production system is highly promising, but the challenge is its competitiveness with existing energy supply systems
Research has been conducted after the incident of the nuclear power plant to increase renewable energy production
Summary
Energy security has become a concern in Japan since the accident of the Fukushima nuclear power plant. The global trend toward non-fossil fuels by the sustainable development goals (SDGs) is accelerating renewable energies. The microalgae biofuel production system is highly promising, but the challenge is its competitiveness with existing energy supply systems. Research has been conducted after the incident of the nuclear power plant to increase renewable energy production. Microalgae is referred to as one of the best candidates for biofuel production, a source of renewable energy. Fuel production from microalgae in the Minamisoma pilot plant measuring 0.1 ha has been reported, which can facilitate 50 m3 /day in an open raceway pond (ORP) [1]. Simulation was conducted based on experimental results that considered profitability, energy balance, and emission of environmental load (GHG). Major species of microalgae that had been cultivated were Desmodesmus sp., Dictyosphaerium sp., Klebsormidium sp., Micractinium sp., and Scenedesmus sp. [2]
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