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Comment on “Environmental Life Cycle Comparison of Algae to Other Bioenergy Feedstocks” The manuscript by Clarens et al. (2010) (1), which compares the life cycle analysis (LCA) of advanced feedstocks such as switch grass and algae to traditional feedstocks, is a commendable effort especially in the present scenario where low carbon fuel standards are getting increased attention. However the paper arises some interesting issues: 1. The rationale of using radiation-use-efficiency (RUE) instead of absolute bioproductivity in the biomass crop yield calculations is not clear. For example, the average monthly algal production in the month of May, for example, in San Juan, NM study was reported as 973 Mg · ha -1 (Supporting Information Table S8). If the daily average production rate is ∼31.0 g m-2 · day -1, it should be around 9.3 Mg · ha-1. Unless the authors have adjusted the value for other factors this error might have percolated into all the subsequent analysis. Also, the comparison of biomass productivity in producing a functional unit of energy in the LCA analysis is somewhat logicless. These feedstocks have entirely different characteristics. Algal biomass is uniform with soft tissue and easy to process compared to rapeseed and corn biomass both of which have tougher tissue structures which needs substantial pretreatments to even start the biomass processing. So the energy requirements of downstream processing are critical in LCA comparison. 2. The fertilizer energy input for algae in this study is high compared to a study by Chisti (2008) (2) which reported much less fertilizer energy requirement and a net positive energy from algal biofuel. Similarly, the cost estimation of CO2 usage is also seems to be high. If algae have better photosynthetic and nutrient assimilation efficiency, how could fertilizer energy cost for algae be more compared to other terrestrial crops? Even though corn and canola can uptake nitrogen from soil, it still requires significant fertilizer regimes for optimal production (especially corn-after-corn cropping pattern). The fertilizer dissemination cost for terrestrial crops will be much higher compared to algae where a built-in flow through system can be used to mix the fertilizer into the production facility. Whether the fertilizer energetic cost of terrestrial feedstocks were included in the analysis is not clear. Also, it is unclear whether the inputs values for algae and other feedstocks for the model were based on different studies. And it would have been nice to include the breakdown of energy requirements for terrestrial crop same as algae in the Supporting Information. If the fertilizer energetic calculation of other feedstocks were not taken into account, the comparison will be highly skewed unfavorably toward algal biomass. Moreover, when we consider LCA energetic cost of fertilizer use, it is better to track the flow of nutrients and its energetic value in the coproducts (e.g., algal meal). 3. The authors make comments such as “land footprint of photobioreactors is not much better than open ponds” and “the life cycle burdens of photobioreactors are expected to be many times higher than ponds in terms of greenhouse gas (GHG) emissions, energy and water use”. These are naı¨ve comparisons. The photobioreactors are more efficient not only on productivity but also on automated-controlled production schemes. In broadest terms, the ponds are twodimensional with respect to land, whereas most of the 10.1021/es100389s
2010 American Chemical Society
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photobioreactors are three-dimensional. For example, the productivity data LCA study was 314 300 kg · 1000 ha-1 day-1 or 31.4 g · m-2 · day-1. However, if a vertical four terraced photobioreactor will be used the same production can be attained in 1/4th of this area. Although not widely published because of business reasons many industrial laboratories have already achieved 50-60 g-1 · m-2 · day-1 production rates, which further reduce the land requirement to half. These are substantial difference especially in the light of fact that indirect emission from abandoned or reclaimed agricultural land which has potentially large stores of deep mineral soil carbon is an emerging issue in future terrestrial-based bioenergy production (3). Similarly, the water use impacts of both systems are entirely different. The ponds in high sunlight areas, which are suitable for algal production, are susceptible to unusually high evaporation. Whereas in controlled-automated-photobioreactors, the evaporation will be minimal with 70-80% recycling efficiency. The residual nutrients in the recycled-culture water can be further used for biomass generation which also offset some of the fertilizer energetic and GHG inputs in LCA. With the existing hue and cry over the sustainability issues of water and land use for biofuel production, the photobioreactor would be more meaningful on several sustainability indexes. 4. The authors assumed that the life cycle burdens of making bioreactors are high. In a “carbon-constrained” future industrial scenario, it is expected that the manufacturing sector will also undergo substantial improvements in having energy efficient facilities and energy efficient equipments (e.g., heat pumps, aerators). Further, the percentage flow of “green electricity” from other renewable energy sources such as wind and solar in to the existing grid in future will be much more which also will tip the analysis scenario. Advanced feedstock-based biofuel production industry is hugely backed by federal and state legislations with massive loan guarantees, incentives, and tax exemptions. So the assumption of high capital cost of photobioreactor is not a compelling argument for LCA in this emerging sector. 5. With many factors taken as mutually exclusive, another technical lacuna of the manuscript is the lack of a total system view. Algal biomass can yield many coproducts from the same biomass and this multiproduct paradigm makes it a perfect candidate for biorefinery concept. A biorefinery integrates biomass conversion processes and equipments to produce fuels, power, and value added chemicals from biomass, a facility analogous to modern petroleum refineries, which produce multiple fuels and products from crude petroleum (4). This integrated view is neglected in the LCA. For example, algalmeal, a coproduct from fuel production, is a rich source of high quality protein and can be used directly used in animalfeeds (5). Substituting algalmeal for fishmeal in animal feed is substantial with respect to indirect energetic offsets and GHG reduction taken into the fact that fishmeal production is one of most energy and GHG-intensive process (e.g., fishing, transportation, fishmeal production, and distribution). Glycerin, another coproduct of algal biorefinery concept, can be used for the microbial bioproduction of chemicals such as propanediol. This in turn can be used as renewable nonfossil feedstock for industrial bulk chemical (e.g., ethylene, propylene, etc.) which are traditionally synthesized from fossil-based feedstocks. This also offsets substantial indirect fossil-energetic cost and GHG emissions. Further, if a cap and trade like regulatory framework come into existence in future industrial scenario, the renewable VOL. 44, NO. 9, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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nonfossil based raw material will have a preferred niche in the industrial chemical production market. The investors are eagerly reviewing the feasibility status of various feedstocks, so studies such as this are crucial. However, defining a comprehensive LCA boundary with an integrated account of indirect GHG emission, energetic costs, and other sustainability indexes would yield more meaningful comparisons.
Literature Cited (1) Clarens, A. F.; Resurreccion, E. P.; White, M. A.; Colosi, L. A. Environmental life cycle comparison of algae to other bioenergy feedstocks. Environ. Sci. Technol. 2010, 44, 1813–1819. (2) Chisti, Y. Response to Reijnders: Do biofuels from microalgae beat biofuels from terrestrial plants. Trends Biotechnol. 2008, 26, 351–352.
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(3) Friedland, A. J.; Gillingham, K. T. Carbon accounting a tricky business. Science 2010, 327, 411–412. (4) Taylor, G. Biofuels and biorefinery concept. Energy Policy 2008, 36, 4406–4409. (5) Naylor, R. L.; Hardy, R. W.; Bureau, D. P.; Chiu, A.; Elliott, M.; Farrell, A. P.; Forster, I.; Gatlin, D. M.; Goldburg, R. J.; Hua, K.; Nichols, P. D. Feeding aquaculture in an era of finite resources. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15103–15110.
Bobban G. Subhadra* School of Medicine, University of New Mexico, Albuquerque, New Mexico 87131
ES100389S
