|Microalgae biodiesel processing and lifecycle analysis model overview. Credit: ACS, Click to enlarge.|
Researchers at Colorado State University have found that a microalgae biodiesel process using currently available technologies can show improvement in lifecycle GHG emissions and net energy ratio (NER) compared to soybean-based biodiesel. A paper on their lifecycle analysis (LCA) appeared online 24 September in the ACS journal Environmental Science & Technology.
Their study proposed a detailed, industrial-scale engineering model for the species Nannochloropsis using a photobioreactor architecture. They integrated this process-level model with a lifecycle energy and greenhouse gas emission analysis compatible with the methods and boundaries of the Argonne National Laboratory GREET model to ensure comparability to preexisting fuel-cycle assessments.
LCAs of the microalgae-based biodiesel process exist in the literature but consensus on the inputs and methods appropriate for microalgae-based biofuels is lacking...Performing a coherent LCA of the microalgae-to-biodiesel process requires detailed models of each of the feedstock processing stages (growth, dewater, extraction, conversion, and distribution) combined with a standard and consistent set of LCA boundary conditions.
In order to describe the net energy and GHG impacts of microalgae biodiesel, we must develop a valid, extensible, and internally consistent model of the materials inputs, energy use, and products for the process. The three primary components of this model are a detailed engineering process simulation of microalgae from growth through extraction, a more generalized model of microalgae from conversion to end use, and an integrated calculation of net energy and GHG emissions due to impacts from the inputs, outputs, processes, and coproduct allocation for the microalgae biodiesel production.—Batan et al.
Their analysis of this process and organism found the Net Energy Ratio (MJ consumed&iddot;(MJ produced)-1) for microalage biodiesel to be 0.93; for soybean biodiesel to be 1.64; and for petroleum diesel to be 0.19.
Although the energy required to support the growth stage during microalgae cultivation is 2.1 times higher than the energy required to support soy growth, they found that microalgae extraction uses less energy than soy oil extraction.
The primary energetic advantage of the microalgae process, relative to soy, is related to the energy embedded in the feedstock. Soybeans contain 18% lipid by dry weight, whereas Nannochloropsis salina contains 50%. This means that less microalgae is required to produce 1 unit of biofuel energy than is required of soybeans.—Batan et al.
In terms of net greenhouse gas emissions, microalgae showed a net “strain to pump” (gCO2eq&iddot;MJ-1) of -75.29; soybean diesel showed net GHG emissions of -71.73; and petroleum diesel showed emissions of 17.24.
Both biofuels result in a net negative CO2 output due to CO2 capture intrinsic in the production of biomass during photosynthesis, the displacement of petroleum, and the displacement of coproducts. The microalgae biodiesel process has a 5% better performance in terms of net GHGs compared to soybean based biodiesel in the boundary “strain-to-pump”. A notable component of the microalgae GHG emissions reduction is the net avoidance of N2O that is achieved. Although the microalgae growth stage uses a higher mass of N-fertilizer than the soy growth stage, the aerobic conditions of microalgae cultures suppress the direct emission of N2O.—Batan et al.
Liaw Batan, Jason Quinn, Bryan Willson, and Thomas Bradley (2010) Net Energy and Greenhouse Gas Emission Evaluation of Biodiesel Derived from Microalgae. Environ. Sci. Technol., Article ASAP doi: 10.1021/es102052