Page 68 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 47

size of biomass fibers is reduced, hemicelluloses are degraded, and lignin
undergoes transformation, removal, and redistribution at high temperature,
and thus the cellulose hydrolyzability is increased [22]. Water can work as
an acid at high temperature, and acetic acid produced from the acetyl
groups of hemicelluloses can play as catalysts to facilitate the degradation of
biomass component [4]. SE is usually effective for grass biomass and hard-
wood, but not so satisfying to softwood biomass unless strong catalysts,
such as H 2 SO 4 , are used under higher pretreatment severity [175].
The advantages of SE include relatively low cost, low energy consump-
tion, being suitable for big particle size, no external chemical addition, low
environmental impact, and also high sugar recovery [22]. However, the
inhibitors, such as furfural/HMF and phenolic compounds from sugar and
lignin degradation at high temperature, are the primary drawback of SE
pretreatment. To remove the inhibitors, a large amount of water is usually
needed to wash the pretreated biomass, but washing with water may lead
to loss of sugar and reprecipitation of lignin [4]. SE has been proved to be
an effective pretreatment process for the production of bioethanol, biogas,
or other bioproducts. SE pretreated floodplain meadow hay at 200°C
resulted in the highest glucose yield up to 85% with ethanol yield of 97%
[176]. Zhao et al. [177] employed SE to pretreat corn stover and the
pretreated solid was then digested by cow rumen fluid for anaerobic gas
production in vitro. The best pretreatment condition was found to be
1.5 MPa, 10% moisture content for 180 s of pretreatment time [177].
Lizasoain et al. (2016) studied biogas production from reed biomass by
using SE pretreatment. When pretreatment condition was 200°C for
15 min, the methane yield could reach up to 89% [178]. Cynara cardunculus
pretreated by SE at 235°C for 1 min in a 0.5 L reactor with a receiving
chamber of 30 L, followed by fermentation with SSF processes, the highest
ethanol concentration could reach up to 18.7 g/L with fermentation effi-
ciency of 66.6% and ethanol yield of 10.1 g ethanol/100 g untreated car-
doon [178]. More applications of SE pretreatment can be found for spruce
wood to improve enzyme hydrolysis [179] and for wheat straw to increase
methane yield by 20% 30% [180]. SE is recognized as one of the most
effective and promising pretreatment processes to pretreat hardwoods and
agricultural residues with the industrial application [175].

2.5.1.2 Steam explosion promoted by addition of chemical additives
To improve the pretreatment efficiency or reduce the required pretreat-
ment severity for SE, different chemicals have been added in the