Agave: A promising feedstock for biofuels in the water-energy-food-environment (WEFE) nexus
Introduction
The water-energy-food-environment (WEFE) nexus is a huge challenge for the transition from a fossil fuel-dominated energy system to a more renewable and clean energy-based one. Although biomass is a renewable energy source that can potentially contribute to energy security goals, there are growing concerns over the sustainability of large-scale use of bioenergy (Popp et al., 2014). Its impacts on food security and food prices (Naylor et al., 2007), fresh water resources (Gerbens-Leenes et al., 2009) and many ecosystem services (Holland et al., 2015) have all been under increasing scrutiny recently while its net climate effects in many cases are still disputed mainly due to significant uncertainties in the associated indirect effects (e.g. potential changes in land systems (Searchinger et al., 2008) and food markets (Searchinger et al., 2015)) and nitrous oxide (N2O) emissions from nitrogen fertiliser use (Crutzen et al., 2016).
Agave could be a promising bioenergy feedstock (Somerville et al., 2010) given its potentially high productivities, ability to thrive in semiarid regions, high water-use efficiency and low requirements for nitrogen fertilisers (Davis et al., 2011). Furthermore, its high sugar and low-lignin content make it an attractive crop from a bioprocessing perspective (Aleman-Nava et al., 2018). A seminal life cycle analysis (LCA) shows that ethanol derived from agave could offer higher land-use efficiencies and greenhouse gas (GHG) savings than ethanol produced from corn and switchgrass (Yan et al., 2011). However, this LCA study, the only one on agave-derived biofuels to date, is based on a hypothetical ethanol plant in Mexico using 1st generation (1G) conversion technology only (i.e., hydrolysis and fermentation of simple sugars extracted from the stem and leaves) and the agave and sugar yield data was sourced from literature on tequila production. Moreover, it focused only on energy and GHG analysis. In fact, comprehensive reviews (Davis et al., 2015; Cushman et al., 2015) on the use of bioenergy feedstocks including agave have confirmed that Yan et al. (2011) is currently the only LCA available on agave-derived biofuels and there is a need for a more comprehensive study. Building on Yan et al. (2011), an LCA was conducted for the possibility of integrating solar panels and annual agave production with synergies provided by water inputs for cleaning solar panels being similar to the water requirements for agave (Ravi et al., 2014). This LCA suggested that the hypothetical co-location of solar panels provided higher returns per m3 of water used than either system alone. Preliminary economic studies were also conducted on agave for bioenergy production in Mexico (Nunez et al., 2011) and Australia (Subedi et al., 2017) based on hypothetical scenarios. To better understand the environmental and economic performance of agave-derived biofuels a comprehensive study using production and compositional data from long-term field experiments is required.
The aim of this paper was to conduct the first comprehensive LCA and economic analysis of 1st and 2nd generation (2G) ethanol produced from agave grown in Australia, using data collected from a 5-year field experiment in Queensland. The key novelties of our study therefore include the use of agave yield and sugar content data collected from a field experiment as well as the consideration of 2nd generation ethanol production. Australia has the largest proportion of semi-arid land in the world (Davis et al., 2011). These areas do not support the growth of common agricultural crops but are suited for plants that thrive on marginal and dry lands, such as agave. Results from the LCA will be discussed in the context of the water-energy-food-environment (WEFE) nexus. The finding from this paper is expected to inform large-scale development of agave-based ethanol in Australia and other countries with significant amounts of semiarid land.
Section snippets
Agave field experiment in Australia
This LCA study was based on data from a pilot agave field experimental site at Kalamia Estate in the Burdekin River Irrigation System, near Ayr, Queensland (see Fig. 1). The site is in a region with tropical savanna climate. The annual average temperature, based on recordings from the nearest weather station (Ayr DPI Research Station 33,002), is 23.9 °C and precipitation is 947 mm dominated by summer rainfall with very little rain in the winter (Australian Government Bureau of Meteorology, 2018
Chemical analysis of agave
In this study, agave plants were harvested and characterized at two developmental stages, 3 y and 5 y. The different anatomic fractions of the plants were separated and crushed following harvest, yielding bagasse and juice fractions from the leaves, stem and offshoots (5 y only). The offshoots were not separated from the leaf fraction of the 3 y plants. The average above-ground fresh weight of 3 y plants was 205 kg of which 88% was leaves and 12% stem biomass. For 5 y agave plants the mass
Conclusion
This is the first comprehensive LCA and economic analysis of ethanol produced from a 5-year agave field experiment. Overall, agave performs better than current 1G biofuel crops such as corn and sugarcane in water-related environmental impact categories and produces competitive ethanol yields (L/ha/y). Although its Land Use impact is high, agave can be grown in unfavourable conditions which do not support food crop production. Overall, our results show that agave is a promising feedstock for
Funding
X.Y. acknowledges financial support from EPSRC (EP/N005600/1) and NERC (NE/R015759/1).
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.
CRediT authorship contribution statement
Xiaoyu Yan: Conceptualization, Methodology, Investigation, Writing - original draft, Funding acquisition. Kendall R. Corbin: Methodology, Investigation, Resources, Writing - original draft. Rachel A. Burton: Investigation, Resources, Funding acquisition. Daniel K.Y. Tan: Conceptualization, Methodology, Investigation, Writing - original draft, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge the continued support and generosity of Mr. Don Chambers, CEO of AusAgave in allowing access to the Kalamia site in Queensland and experimental use of the agave material. X. Yan acknowledges financial support from EPSRC (EP/N005600/1) and NERC (NE/R015759/1).
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These authors contributed equally.
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Current address: Environmental Epigenetics and Genetics Group, Department of Horticulture, College of Agriculture, Food and Environment, University of Kentucky, Lexington, Kentucky, USA.