Dry chemo-mechanical pretreatments of lignocellulosic biomass: impact on energy consumption, enzymatic hydrolysis and bioethanol yields

The high crude oil prices, the decline of fossil fuel reserves and the increase of greenhouse gas emissions are motivating the development of biorefineries that produce energy from renewable resources (i.e. lignocellulosic biomass). However, the key driver for the successful conversion of biomasses into biofuels is the selection of efficient pretreatments that permit to maximize the sugars recovery and to minimize the consumption of water, chemicals and energy. This study proposes the application of “dry” chemo-mechanical pretreatments that permit to enhance enzymatic hydrolysis and thereafter bioethanol production from lignocellulosic biomasses. In a first experiment, wheat straw was pretreated with NH3, NaOH, NaOH-H2O2 and NH3- H2O2 at both high (5 kg L-1) and at low (0.2 kg L-1) biomass concentrations, named “dry” and “diluted” pretreatments. Then, untreated and pretreated samples were subjected to centrifugal and ball milling, followed by enzymatic hydrolysis. Compared to diluted mechano -NaOH pretreatments, dry mechano -NaOH and -NaOH-H2O2 were found to be more effective in decreasing the particle size (up to 55%), increasing the surface area (up to 88%) and decreasing the total energy requirement (up to 50%). Enzymatic hydrolysis was enhanced by pretreatments, as suggested by higher glucose yields (332, 320, 140, 322, 141 g glucose kg- 1 biomass for diluted –NaOH, dry – NaOH, dry -NH3, dry -NaOH-H2O2 and dry -NH3-H2O2, respectively), compared to that of untreated wheat straw (118 g glucose kg-1). However, lower energy efficiency was obtained for dilute mechano -NaOH treatment, compared to dry mechano -NaOH treatments. In a second experiment, sugarcane bagasse was pretreated with NaOH and H3PO4 at high biomass concentration (5 kg L-1) and then milled by using different methods, such as vibratory milling (VBM), ball milling (BM) and centrifugal milling (CM). Results indicate that NaOH-BM and NaOH-VBM was preferred to enhance glucose yields and bioethanol production (up to 76%), while CM consumed 75% and 58% lesser energy than BM and VBM, respectively. The highest energy efficiency was obtained with NaOH-CM. Therefore, the combination of dry NaOH and CM appears the most suitable and interesting pretreatment for the production of bioethanol from SB.