Sugarcane is a tall growing, jointed bunch grass that is cultivated as a perennial crop primarily for its ability to produce and store sucrose in its stem. It is a highly efficient “solar

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

cell” with an estimated energy in:energy out (I/O) ratio of 1:8 when it is allowed to grow for 12 months under tropical conditions and its harvested dry matter (sugar and fiber) is processed for ethanol instead of sugar [12-15]. Under more temperate environments, where sunlight duration and intensity fluctuates (seasonally and daily) and cooler temperatures and occasional frosts shorten the growing season, I/O ratios of 1:3 are easily obtainable with current sugarcane varieties if ethanol production from both sugar and fiber is the goal [9]. Solar energy recoveries for energy cane, sorghum, and tropical maize have independently been reported as 2.24, 2.23, and 2.85%, respectively [16,17]. Thus, energy cane is no more efficient in converting solar energy into chemical energy, but the extended growth phase allows it to capture more solar energy over an entire growth season relative to the other C4 grasses. This extended growth phase is attributed to tillers remaining functional and the continuous activity of apical meristems throughout most of the year. Moreover, continued stem elongation, increased internode density, and continued canopy development of tillers allows for storage of solar energy as dry biomass in an extended spatial area [17-19]. The theoretical maximum for aboveground sugarcane dry matter (DM) yield is estimated to be 140 Mg ha-1 annually [20]. This is dependent on temperature and sunlight, and would probably occur only under tropical conditions.

The theoretical maximum fresh weight yield of sugarcane biomass is 358 Mg ha-1 yr-1 [5]. Early generation hybrids between sugarcane and S. spontaneum have been shown to produce sustained biomass yields approaching the theoretical maximum in Louisiana over a five-year period [21]. Unfortunately, the high biomass yields in these early generation hybrids are the result of high fiber yields, and not high sucrose yields, and as such they are generally crossed back to elite clones of sugar cane two or three times before a sugarcane clone worthy of commercial sugarcane production can be released [7].

With the quest to produce second generation fuels from cellulosic biomass these early generation Fi hybrids are ideal biofeedstock candidates, that is, energy canes. Most of these hybrids can produce DM yields of 30 Mg ha-1 annually over four fall harvests, with about 20 Mg ha-1 being fiber and 10 Mg ha-1 being Brix [22]. One of the first sugarcane varieties developed for biomass production and utilization for the cogeneration of electricity was “L79-1002” [23]. The authors reported DM yields of 66.6 Mg ha-1 at a latitude of 30.4°N on 1.8-m wide rows. These yields from this variety were from fiber (27.9% on a fresh weight basis) and Brix (10.4% on a fresh weight basis).

Sugar and fiber levels in the harvested cane stalks are generally dependent on the variety, the length of the growing season, the amount of extraneous matter present, and the harvesting conditions. Sugarcane, once delivered to the raw sugar factory for milling, is separated into its water, Brix (soluble solids, of which approximately 80% is sucrose), fiber (bagasse) and sediment (ash, soil, etc.) fractions. The bagasse fraction in commercial sugar varieties consists of 38% cellulose, 19% hemicellulose, 22% lignin, 4% protein, and 3% ash, with the remaining 14% consisting of sugar, soil from harvesting, and other types of solids [21, 24]. The average fiber yield does not take into consideration leafy material removed in the field during the harvesting process, or the conditions under which the crop was harvested. When sugarcane is mechanically harvested for sugar without a pre-harvest burning of the standing cane, 4-6 Mg ha-1 of leaf litter is deposited back on the soil surface [25-27]. Moreover, in Louisiana, where sugarcane is grown on mineral soils, if the crop is harvested under wet and muddy conditions it is not uncommon to see 5-10% contamination with soil.