Abstract
BackgroundPretreatment is an important step in the production of ethanol from lignocellulosic material. Using acetic acid together with steam pretreatment allows the positive effects of an acid catalyst to be retained, while avoiding the negative environmental effects associated with sulphuric acid. Acetic acid is also formed during the pretreatment and hydrolysis of hemicellulose, and is a known inhibitor that may impair fermentation at high concentrations. The purpose of this study was to improve ethanol production from glucose and xylose in steam-pretreated, acetic-acid-impregnated wheat straw by process design of simultaneous saccharification and co-fermentation (SSCF), using a genetically modified pentose fermenting yeast strain Saccharomyces cerevisiae.ResultsEthanol was produced from glucose and xylose using both the liquid fraction and the whole slurry from pretreated materials. The highest ethanol concentration achieved was 37.5 g/L, corresponding to an overall ethanol yield of 0.32 g/g based on the glucose and xylose available in the pretreated material. To obtain this concentration, a slurry with a water-insoluble solids (WIS) content of 11.7 % was used, using a fed-batch SSCF strategy. A higher overall ethanol yield (0.36 g/g) was obtained at 10 % WIS.ConclusionsEthanol production from steam-pretreated, acetic-acid-impregnated wheat straw through SSCF with a pentose fermenting S. cerevisiae strain was successfully demonstrated. However, the ethanol concentration was too low and the residence time too long to be suitable for large-scale applications. It is hoped that further process design focusing on the enzymatic conversion of cellulose to glucose will allow the combination of acetic acid pretreatment and co-fermentation of glucose and xylose.
Highlights
Pretreatment is an important step in the production of ethanol from lignocellulosic material
A small amount (1.3 g in 888 g liquid, which corresponds to 20 % of the total addition in the simultaneous saccharification and co-fermentation (SSCF) configurations) of enzymes was added, since a large proportion of the sugars in the liquid were oligomers (85 and 73 % of detected glucose and xylose, respectively)
A small increase in ethanol yield was seen with increasing pre-hydrolysis time, based on the theoretical amount of monomer and oligomer sugars available (Fig. 2b)
Summary
Pretreatment is an important step in the production of ethanol from lignocellulosic material. To meet the challenges of a growing population and increasing energy demand, and the need to reduce greenhouse gas emissions, research has turned towards various kinds of biofuels and biorefinery processes. Several studies have shown that the use of sulphuric acid as a catalyst during steam pretreatment improves the ethanol yield, Bondesson and Galbe Biotechnol Biofuels (2016) 9:222 decreases the amount of degradation products formed, and reduces the residence time and temperature required [5,6,7]. A drawback of sulphuric acid is, that it is environmentally harmful, and must be removed or recycled in downstream processes. Acetic acid can be converted into biogas by the treatment of waste liquid streams of the process, it is less environmentally harmful than sulphuric acid, and it can be handled in downstream processes [8]
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