Abstract
Xylitol is a low calorie sweetener that can be produced through a bioconversion approach from lignocellulosic biomass that requires pretreatment prior to the bioconversion of xylose to xylitol. Steam explosion (SE) is an industrially scalable pretreatment (PT) process with the potential to liberate xylose monomers, however SE-PT has not been optimised for xylose release from multiple feedstock. The effect of pressure, substrate weight, phosphoric acid loading concentration and residence time on four feedstock (wheat straw (WS), corn stover (CS), Miscanthus (M), and willow (W)) for xylose release and minimal fermentation inhibitor productions (furfural and 5-hydroxymethylfurfural (HMF)) was investigated using the Taguchi methodology for design of experiment (DoE) with variation at four levels (44). An L16 orthogonal array design was utilised and all factors indicated influence on xylose release and inhibitor formation and the resulting xylose rich hydrolysate assessed for bioconversion to xylitol.. The L16 DoE gave hydrolysates containing 75-95% of xylose content in the original biomass, whilst retaining cellulose and lignin components in the fibre. The level of inhibitors were within boundary limits to enable microbial fermentation of the hydrolysates to xylitol. Fine tuning of the overall evaluation criteria (OEC) model imbibing 1.5 kg feedstock in 1.2% w/v orthophosphoric acid, 12 bar(g) and 6 minutes residence time resulted in 90% xylose recovery and production of >1000 L of wheat straw hydrolysate for bioconversion to xylitol. The advantages and limitations of the Taguchi OEC model and further improvements to this process are discussed in a biorefining context
Highlights
Use of fossil fuels has impacted national energy security and global warming, leading to an increased investigation into the use of biomass as a renewable substrate to generate multiple products (Maity, 2014)
Lignin was highest in Miscanthus and willow similar to previously published reports (Fahmi et al, 2008; Hu and Ragauskas, 2012) while the xylose concentration was lowest in willow due to its higher lignin content (Fahmi et al, 2008), and highest in wheat straw
The xylose yield from corn stover and wheat straw was in agreement with the literature (Ballesteros et al, 2006; Fang et al, 2011; Pang et al, 2013), in these reports, values were obtained using sulphuric acid to catalyze the release of carbohydrates or integrated microwave assistance, both of which increase the capital cost of processing
Summary
Use of fossil fuels has impacted national energy security and global warming, leading to an increased investigation into the use of biomass as a renewable substrate to generate multiple products (Maity, 2014). Key biomass resources include lignocellulosic agricultural residues and dedicated energy crops (2G resources), neither of which impact on food security, unlike 1G plant resources which enter the food v fuel debate. Major global biomass resources include; wheat straw, corn stover, short rotation coppice willow, and Miscanthus. O’Donohue (2014) reported that within Europe, 35 Mt DM wheat straw, 15 Mt DM corn stover, 2.5–5.5 Mt DM surplus hardwood are available for. A European report by O’Donohue (2014) highlighted the need to couple ethanol generation from cellulose with production of multiple products from the hemicellulose and lignin components. An analysis of multiple product portfolios indicated that generation of xylitol from the hemicellulosic fraction made significant improvement to the economic returns of a lignocellulosic based process (O’Donohue, 2014)
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