Abstract
BackgroundThere is considerable interest in the conversion of lignocellulosic biomass to liquid fuels to provide substitutes for fossil fuels. Pretreatments, conducted to reduce biomass recalcitrance, usually remove at least some of the hemicellulose and/or lignin in cell walls. The hypothesis that led to this research was that reactor type could have a profound effect on the properties of pretreated materials and impact subsequent cellulose hydrolysis.ResultsCorn stover was dilute-acid pretreated using commercially relevant reactor types (ZipperClave® (ZC), Steam Gun (SG) and Horizontal Screw (HS)) under the same nominal conditions. Samples produced in the SG and HS achieved much higher cellulose digestibilities (88% and 95%, respectively), compared to the ZC sample (68%). Characterization, by chemical, physical, spectroscopic and electron microscopy methods, was used to gain an understanding of the effects causing the digestibility differences. Chemical differences were small; however, particle size differences appeared significant. Sum-frequency generation vibrational spectra indicated larger inter-fibrillar spacing or randomization of cellulose microfibrils in the HS sample. Simons’ staining indicated increased cellulose accessibility for the SG and HS samples. Electron microscopy showed that the SG and HS samples were more porous and fibrillated because of mechanical grinding and explosive depressurization occurring with these two reactors. These structural changes most likely permitted increased cellulose accessibility to enzymes, enhancing saccharification.ConclusionsDilute-acid pretreatment of corn stover using three different reactors under the same nominal conditions gave samples with very different digestibilities, although chemical differences in the pretreated substrates were small. The results of the physical and chemical analyses of the samples indicate that the explosive depressurization and mechanical grinding with these reactors increased enzyme accessibility. Pretreatment reactors using physical force to disrupt cell walls increase the effectiveness of the pretreatment process.
Highlights
There is considerable interest in the conversion of lignocellulosic biomass to liquid fuels to provide substitutes for fossil fuels
Our results show that the decrease in cellulose degree of polymerization (DP) was not affected by the different operational modes of the reactors, and it appears unlikely that changes in cellulose DP can explain the differences in digestibility of these samples
A good correlation was found between the O/B ratio and cellulose conversion for each sample (Figure 7). These results indicate that reactor operation can produce substantially different levels of cellulose accessibility leading to significant differences in cellulose digestibility
Summary
There is considerable interest in the conversion of lignocellulosic biomass to liquid fuels to provide substitutes for fossil fuels. The hypothesis that led to this research was that reactor type could have a profound effect on the properties of pretreated materials and impact subsequent cellulose hydrolysis. The terrestrial plant cell wall has evolved into a complex structure, naturally recalcitrant to biological and chemical attack [3]. This high recalcitrance greatly impedes access of enzymes to biomass cellulose [4], increasing conversion costs. The hypothesis that led to this research was that reactor type could have a profound effect on the properties of pretreated materials and affect subsequent cellulose hydrolysis. Even at the same pretreatment severity, the choice of reactor type and its operational mode such as heating profile, solid mixing, pressure release, etc., could profoundly affect the physical structure of the pretreated substrates
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