Biomass characterization of Agave and Opuntia as potential biofuel feedstocks
Graphical abstract
Introduction
With the gradual depletion of fossil fuels, the demand for renewable biomass-based carbon resources to use for lignocellulosic biofuels is expected to increase in the future [1], [2]. However, most biomass feedstocks currently used for biofuels production compete with the use of those feedstocks for food [3], [4]. Competition for arable land for growing biofuel feedstocks and food can be avoided, in part, by the development of drought-tolerant biomass feedstocks that can be grown in an environmentally responsible manner on semi-arid, marginal, degraded, or abandoned agricultural lands where food crops are typically not cultivated, with an estimated mean productivity of 4.3 Mg ha−1 y−1 [4], [5], [6]. Furthermore water availability is the major factor that constrains the cultivation of bioenergy crops [7]. One potential opportunity for the production of biomass feedstocks on water-limited areas is to use various cultivated crassulacean acid metabolism (CAM) species from the genera Agave (Agavaceae) and Opuntia (Cactaceae, “prickly pear cactus”), which have growth characteristics that allow these species to thrive in semi-arid regions [8].
CAM plants are typically highly tolerant of high temperatures, UV-B radiation, and drought conditions by virtue of their ability to rapidly take up and store water in above-ground succulent tissues, while tolerating large losses in water mass fraction [8], [9]. The high water-use efficiency of CAM species, typically 3- to 10-fold higher than that of C3 or C4 species, results in crop water demands that average only 16%–28% of those of C3 and C4 crops, respectively, while maintaining comparable above-ground biomass productivities [8], [10]. For example, average annual productivity values for various Agave species can range from <1 to 34 Mg ha−1 y−1 under ambient precipitation conditions [4], [11], [12], [13]. Under cultivated conditions with supplemental irrigation, average annual dry-weight productivities can reach as high as 38–42 Mg ha−1 y−1 [10], [14], [15], [16]. Under rain-fed conditions, average annual productivity for Opuntia species are about 15 Mg ha−1 year−1 or more [17]. However, under well-irrigated conditions, average annual dry matter productivities in the range of 40–50 Mg ha−1 year−1 have been reported [10], [14], [15], [16], [18]. Such biomass production rates are comparable with those of other bioenergy feedstocks such as maize, sugarcane, switchgrass, and poplar [4], [8], [12].
Although native to the Americas, Agave species have been introduced worldwide for commercial production in many countries including Australia, Brazil, Tanzania, Kenya, Madagascar, Mexico, China, and across the Caribbean and Mediterranean regions [11], [12]. Although more information is needed from field trials to model the biomass production associated with the water-wise feedstocks [19], [20], productivity estimates from immediately available land suggest that an additional 6.1 hm3 of lignocellulosic ethanol production could be produced with minimal impacts on the environment [11]. Similarly, Opuntia originates from the Americas with its center of diversity in Mexico [21], but it has been introduced worldwide with major production occurring in Algeria (and other northern African nations), Brazil, Chile, Mexico, and Italy [22], [23]. Opuntia species are cultivated primarily for commercial fodder and forage in semi-arid regions worldwide [24], [25]. However, tender, young cladodes and fruits from Opuntia species are consumed by humans, primarily in Mexico, the southwestern US, and Italy [26], [27], [28]. The young cladodes and fruits are also dried and sold as dietary supplements [29], [30], in cosmetic formulations [29], and for medicinal use [31], [32], [33]. The fruits can be consumed as fresh or made into a variety of jams, jellies, sauces, marmalades, candies, syrups, juices, liquor, and as a natural sweetener due to the high sugar mass fraction (>50%) of the of the fruit syrup [34].
Due to their high mass fraction of water-soluble sugars, many different Agave species are currently used for production of alcoholic beverages, such as tequila (Agave tequilana) mescal or pulque (Agave mapisaga, Agave salmiana, Agave Americana, Agave fourcroydes, and others), aquaamiel (honey water), nectar or syrup sweeteners. Some of these species are also used for fiber production (e.g., Agave sisalana, A. fourcroydes) [7], [8], [11], [12], [35], [36], [37]. Agave bagasse as a raw material can be used for animal feed, fiberboard production, and other by-products [26], [38]. Following extraction of sugars for fermentation to produce mezcal, the bagasse and waste fiber from A. salmiana can be used as a renewable energy source for combustion [39]. The cladodes of Opuntia cactus are also a potential biomass feedstock for bioethanol production [40]. Agave would be comparable to or superior to other ethanol feedstocks, such as maize, switchgrass, and sugarcane, for bioethanol production in terms of life cycle energy and greenhouse gas (GHG) balances [19], while being far more water-use efficient than these crops [41].
The efficient conversion of lignocellulosic biomass into renewable biofuels and value-added chemical compounds is limited by the recalcitrance of plant cell wall material to degradation [42]. Deconstructive pretreatments of raw lignocellulosic biomass account for a majority of the costs associated with lignocellulosic biofuel production [3]. In order to optimize such pretreatments, the composition of lignocellulosic biomass feedstocks must be known. Among plant cell wall biopolymers, lignin is the major cause of recalcitrance to hydrolysis [43], [44]; thus, reducing lignin mass fraction or altering its structure are important goals for overcoming this recalcitrance and improving saccharification [45]. For most woody feedstock species, the lignin mass fraction is approximately 9–30% [13], [45], whereas for Agave and Opuntia, the lignin mass fraction are estimated to be lower, in the range of 5–16% depending on the species and technical approaches used [12], [13], [44], [46], [47], [48]. However, the readily fermentable, water-soluble carbohydrate (WSC) fraction of Agave leaves was comparable to that of conventional lignocellulosic feedstocks, such as sugarcane bagasse and corn stover [38], [49]. Opuntia stems exhibit similar percentages (3–7% fresh mass fractions) of such carbohydrates [33]. Depending on the study, the compositions of Agave and Opuntia feedstocks can vary considerably. Such variation might arise from the diverse species, plant or leaf age, growth conditions, and the analytical methods used. In order to obtain more reliable information about the composition and chemical structures of A. tequilana and Opuntia ficus-indica, standardized National Renewable Energy Laboratory (NREL) analytical methods were used to determine the ash, protein, extractives, lignin, hemicellulose, and cellulose mass fraction, as well as the chemical structures of isolated cellulose, of sampled materials. Such information is critical for the optimization of strategies for conversion of these feedstocks to renewable biofuels.
Section snippets
Plant cultivation
O. ficus-indica ((L.) Mill.) and A. tequilana [Weber var. azul] were grown at the Nevada Agricultural Experiment Station Valley Road Greenhouse Complex in Reno, NV. Opuntia and Agave were planted in 8-L and 19-L pots, respectively, containing Metromix® 200 soilless mix (Sun Gro Horticulture, Bellevue, WA, USA). Plants were maintained under standard greenhouse conditions with natural light at approx. 1100–1500 μmol m−2 s−1 and temperature at 28–32 °C day/17–18 °C night. Watering was performed
Elemental analysis
The relative elemental percentages of A. tequilana and O. ficus-indica were determined based on the oven-dried samples. Elemental mass fractions were similar for both A. tequilana and O. ficus-indica (Table 1). However, A. tequilana displayed a slightly higher C mass fraction than did the sample from O. ficus-indica. These values are similar to the C, H, O mass fractions of herbaceous and woody biomass feedstocks, which typically have elemental compositions of 45–50% C, 6–7% H, 40–46% O, and
Conclusion
The arid-land adapted, highly water-use efficient CAM species A. tequilana and O. ficus-indica, which are unique biomass species compared to other C3 and C4 plants, were characterized by series of standard biomass analytical procedures. Both Agave and Opuntia contained a high amount of water at 84.9% and 93.9%, respectively. In addition, carboxylic acids and simple sugars were found to be the major constituents of freshly expelled juice. A. tequilana and O. ficus-indica dry bagasses possessed
Acknowledgments
This material is based upon work supported by the National Science Foundation under Grant No. CBET 1337017. JCC acknowledges additional support from the Nevada Agricultural Experiment Station under Hatch projects NAES-0377 and NAES-0380. The authors would like to thank Prof. Kent Hoekman of the Desert Research Institute (DRI) for the heating value measurements and Mary Ann Cushman for critical review and clarifying comments on the manuscript.
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