Willows
Willows are from the genus Salix, and there are around 400 species of deciduous trees and shrubs in this genus found primarily on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrowleaved shrub species are called osier, and some broader-leaved species are referred to as sallow. Some are low-growing or creeping shrubs; for example, the dwarf willow (Salix herbacea) rarely exceeds 6 cm (2 in) in height, though it spreads widely across the ground.
In numerous research reports [207, 208] it was stated that willow grown as a short-rotation woody crop is able to achieve high biomass yields, and after harvest in a dormant period (usually winter), in the following spring is able to regrow vigorously. Moreover, short-rotation willow coppice plantations can be established on fields unsuitable for food or feed production. In a recent study, Stolarski et al. have shown that some willow clones can produce yields as high as 16.4 Mg ha-1 year-1 with net energy gain of 242.3 GJ ha-1 year-1 [209]. Willows are high cellulose feedstocks with cellulose content as high as 44%; a representative composition analysis is shown in Table 13.20.
In a recent life-cycle assessment study, Stephenson et al. have evaluated the environmental and economic sustainability of a potential operation in the UK in which bioethanol is produced from the hydrolysis and subsequent fermentation of coppice willow [211]. According to this analysis, if the willows were grown on idle arable land in the UK or in Eastern Europe and imported as wood chips into the UK, a savings of greenhouse gas emissions of 70-90% is possible when compared to fossil-derived gasoline on an energy basis.
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Table 3.20 The detailed composition analysis of willows [210].
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Furthermore, they concluded that the process would be energetically self-sufficient, as co-products such as lignin and unfermented residues could be used to produce the process heat and electricity, with surplus electricity being exported to the National Grid. In accordance with their analysis, despite the environmental benefits, the economic viability is doubtful at present. However, the cost of production could be reduced significantly if the willows were altered by breeding to improve their suitability for hydrolysis and fermentation.
A number of common pretreatment techniques have been tested on willow and some of the examples include steam explosion [212], ionic liquids [213-220], organosolv [221], and biological pretreatments [222, 223].
In 2008 Sassner and coworkers studied the steam pretreatment of H2SO4 impregnated salix for the production of bioethanol [224]. In this study, steam pretreatment of fast growing salix impregnated with sulfuric acid has been investigated by varying the temperatures between 180-210°C, with the residence times 4, 8 or 12 min, and the acid concentration of 0.25% or 0.5% (w/w) H2SO4. They found that high sugar recoveries can be achieved after pretreatment, and the highest yields of glucose and xylose after the subsequent enzymatic hydrolysis step were 92% and 86% of the theoretical values, respectively, based on the glucan and xylan contents of the raw material. The most favorable pretreatment conditions regarding the overall sugar yields were 200°C for either 4 or 8 min using 0.5% sulfuric acid, both resulting in a total of 55.6 g glucose and xylose per 100 g dry raw material. Furthermore, simultaneous saccharification and fermentation (SSF) experiments were also performed on the pretreated slurries at an initial water insoluble content of 5%, using ordinary baker s yeast. In the SSF, overall theoretical ethanol yield was 79%, based on the glucan and mannan content in the raw material.
The most commonly studied pretreatments of willow involved steam [217], sulfur dioxide, sulfuric acid [214], and hydrothermal pretreatment using hot compressed water [225]. Many researchers have noted that in the case of feedstock like willow and other woody materials, fairly high concentrations of enzyme inhibitors are formed during the pretreatment process. The composition and the nature of these compounds depend on the pretreatment process. During steam pretreatment, which is often performed at temperatures of 200°C or above, water soluble inhibitors such as acetic acid, formic acid, sugar derived by products, and lignin degradation products are formed. Acetic acid is produced through the partial hydrolysis of hemicellu — lose, which takes place during the pretreatment. The sugar-derived major byproducts are furfural and 5-hydroxymethylfurfural. The lignin degradation products include a wide range of aromatic and polyaromatic compounds with a variety of substituents. It is likely that some of these aromatic substances inhibit both the enzymatic hydrolysis and fermentation steps. In an industrial process, these compounds will accumulate due to the recirculation of process streams. Palmqvist and coworkers studied the effect of these inhibitors on both enzymatic hydrolysis and fermentation steps [220]. In this study, they found that volatile compounds did not affect either the enzymatic hydrolysis or the fermentation significantly, even at high concentrations. In contrast, the nonvolatile compounds severely affected both the hydrolysis and the fermentation; the effect was more pronounced in the latter case. Further, they concluded that for the effective use of willow as a lig — nocellulosic material for ethanol production, it is essential to remove these nonvolatile compounds before the enzyme treatments [220].