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