A.K. Pegallapati, M. Krynock, M. Mutchek, G. Cooney, T. Skone, E. Dale, E. Shuster
Contractor to the National Energy Technology Laboratory,
Keywords: life cycle assessment, algae biofuels, carbon dioxide utilization, captured carbon dioxide, flue gas, coal-fired power plant
Summary:Microalgae is deemed as one of the promising alternative energy feedstocks due to its high oil content (1), ability to grow in different types of wastewater streams (2) and thrive in carbon dioxide rich environment. There is additional interest in CO2 utilization pathways resulting from the recently increased tax credit for capture in 45Q (up to $35/tonne). One of the unique attributes of algae cultures is that they can feed upon both impure (flue gas) and pure streams of carbon dioxide from different industrial sources (3). However, recent algae biofuels life cycle analysis (LCA) indicates that a tradeoff exists with using pure or impure CO2 from power plants to cultivate algae (4). For instance, use of flue gas for microalgae cultivation does not require capturing CO2 at a power plant, thus avoiding the energy penalty for capture, but is constrained by the distance it can be transported without incurring high costs (4-6). On the other hand, captured CO2 is more economical to transport via pipelines, but suffers the energy penalty associated with the carbon capture technology at a power plant (4). The energy penalty required to capture and compress purified CO2 results in higher greenhouse gas (GHG) emissions for algae biofuels compared to less energy-intensive sources (e.g., flue gas). In addition, from a life cycle perspective, the algae systems utilizing CO2 from a power plant have multiple products (algae-based fuel, electricity from a power plant etc.,) requiring coproduct management via system expansion to quantify the environmental impacts and compare them with conventional products on a consistent basis. In response to the challenges associated with CO2 provisioning for microalgae cultivation, the LCA Team at National Energy Technology Laboratory (NETL) is evaluating the environmental life cycle impacts of algae biofuels synthesized using CO2 from five types of thermoelectric power plants. The power plants considered in the analysis are, i) Subcritical Pulverized Coal-fired Power Plant (SubPC); ii) Supercritical Pulverized Coal-fired Power (SCPC); iii) Solid Oxide Fuel Cell Power Plant (SOFC); iv) Oxycombustion Pulverized Coal Plant (OxyPC); and v) Natural Gas Combined Cycle Power Plant (NGCC) to model algae biofuel pathways. Both flue gas and captured (amine-based solvent) CO2 scenarios are considered. The environmental life cycle impacts from algae-to-biofuel pathways are evaluated using publicly available data. Specifically, algae-to-biofuel conversion is modeled via Combined Algae Processing (CAP) (7) and Hydrothermal Liquefaction (HTL) (8) pathways with the algae cultivation in open raceway ponds (ORPs) (9) and photobioreactors (PBRs) (10). In total, eighteen algae scenarios (not including the CO2 sources) are evaluated in this analysis. The environmental impacts from this analysis are evaluated using a combined functional unit, “1 MJ of algae-derived Renewable Diesel (RD) combusted in vehicles and a slate of coproducts”. The type and mass of coproducts varied with the algae-to-biofuel pathway. This work compares the LCA results from the algae-to-biofuel pathways with conventional slate of products. Additionally, this analysis presents and compares the environmental impacts between the algae cultivation systems and biofuel pathways. See PDF for references.