Lifecycle Study Finds Algae as Bioenergy Feedstock Has Higher Environmental Impacts than Conventional Crops in Energy Use, GHG Emissions, and Water Use; The Importance of Using Waste Streams
|Schematic of systems considered in the study. Model scope includes all upstream processing of biomass material; conversion to liquid or solid fuel is intentionally excluded. Credit: ACS, Clarens et al. Click to enlarge.|
Terrestrial bioenergy feedstock crops such as switchgrass, canola and corn have lower environmental lifecycle impacts than algae in energy use, greenhouse gas emissions, and water regardless of cultivation location, according to a new lifecycle assessment by researchers at the University of Virginia. Only in total land use and eutrophication potential do algae perform favorably. A paper on the study was published online 19 January in the ACS journal Environmental Science & Technology.
The researchers determined the impacts associated with open pond algae production with a raceway configuration using a stochastic life cycle model. The large environmental footprint of algae cultivation is driven predominantly by upstream impacts, such as the demand for CO2 and fertilizer, they found.
The scope of the analysis included those processes required for cultivation of biomass. For all crops, the entire plant was used to facilitate comparison on a total energy basis. Biofuel conversion processes were excluded from the scope of the analysis.
|Five Life Cycle Burdens for Production of One Functional Unit of Energy (317 GJ) Algae, Corn, Canola, and Switchgrass in Virginia|
(MJ) x 104
(kg CO2e) x 104
(m3) x 104
(kg PO4- equiv)
|algae||0.4 ± 0.05||30 ± 6.6||1.8 ± 0.58||12 ± 2.4||3.3 ± 0.86|
|corn||1.3 ± 0.3||3.8 ± 0.35||-2.6 ± 0.09||0.82 ± 0.19||26 ± 5.4|
|canola||2.0 ± 0.2||7.0 ± 0.83||-1.6 ±0.10||1.0 ± 0.14||28 ± 5.8|
|switchgrass||1.7 ± 0.4||2.9 ± 0.27||-2.4 ± 0.18||0.57 ± 0.21||6.1 ± 1.7|
|The standard deviation of each value is also presented (±).|
In modeling algae production, the researchers selected open ponds as being the most promising option at present. They assumed that fertilizers and flocculants were added as water is pumped into or out of the ponds so that no additional mixing is required. Harvesting was assumed to proceed via a combination of flocculation and centrifugation, consistent with pilot-scale demonstrations and conventional practice for the dewatering of biosolids during municipal wastewater treatment. CO2 was bubbled into the ponds via an automated control system whereby the CO2 was added to the medium to maintain dissolved gas levels and pH at a constant level.
Land use is one impact in which algae offers a clear and appreciable improvement over corn, canola, and switchgrass. Algae cultivation uses land roughly 3.3 times more efficiently than corn, 4.3 times more efficiently than switchgrass, and 5 times more efficiently than canola. If corn were harvested only for the kernel, as is common practice, this disparity would be even larger since more land, roughly 100% more, would be needed to grow the same amount of biomass.
Although the improvement offered by algae is less dramatic than has been suggested previously, the results suggest that algae cultivation will be less limited by land availability than conventional crops. The land use estimates indicate that algae cultivation on roughly 13% of the United States’ land area could meet the nation’s total annual energy consumption. In contrast, use of corn would require 41% of the total land area, while switchgrass and canola would require 56% and 66%, respectively.
The land use changes implicit in large-scale bioenergy deployment are expected to have important implications for climate change and other impacts. These so-called ‘indirect’ changes are associated with conversion of arable land into production and were not included here. The focus of this work is to provide a comparative tool for already cultivated arable land, although future decisions to deploy bioenergy should consider the large-scale implications of land use changes.
—Clarens et al.
The team found that the life cycle impacts of algae cultivation are sensitive to several inputs that have been largely overlooked to date: the availability of renewable sources of nutrients and carbon dioxide. The model is largely insensitive to inputs widely associated with algae productivity such as water and sunlight availability.
In practice, they said, first-generation algae ponds will supply their nutrients and CO2 from fossil-based sources, with almost all commercially available CO2 coming from the steam reforming of hydrocarbons, and reactive nitrogen being produced by the Haber-Bosch process.
To reduce the environmental impacts of algae production, the authors said, flue gas and, to a greater extent, wastewater could be used to offset most of the environmental burdens associated with algae. To demonstrate the benefits of algae production coupled with wastewater treatment, the model was expanded to include three different municipal wastewater effluents as sources of nitrogen and phosphorus. Each provided a significant reduction in the burdens of algae cultivation, and the use of source-separated urine was found to make algae more environmentally beneficial than the terrestrial crops.
To reduce the impacts of algae cultivation to make it on par with terrestrial crops, producers will not only need to decide to use waste streams, they will have to develop means by which to deliver these waste streams to their production facilities since these are generally not available. The need to minimize the upstream impacts is the first overarching outcome from this analysis.
The second overarching outcome is that downstream processing is unlikely to change the life cycle assessment for the entire fuel cycle given how large the cultivation differences are...the huge impact differences reported here suggest that at a minimum cultivation will be a significant part of the overall life cycle burden. This work is not intended to supplant important future analysis in other life cycle stages. However, an exhaustive study of existing and proposed conversion technologies does not change the realities of the cultivation impacts. The authors anticipate that such analysis will find algae to be easier to convert into liquid fuels than some of the other biomass sources studies here because of their inherently high lipid content, semi-steady-state production, and suitability in a variety of climates.
—Clarens et al.
Andres F. Clarens, Eleazer P. Resurreccion, Mark A. White and Lisa M. Colosi (2010) Environmental Life Cycle Comparison of Algae to Other Bioenergy Feedstocks. Environ. Sci. Technol., Article ASAP doi: 10.1021/es902838n