Page 63 - Lignocellulosic Biomass to Liquid Biofuels

42 Lignocellulosic Biomass to Liquid Biofuels

ethanol) to fractionate lignocellulose under mild reaction conditions
[152]. CPA has been used to [153] (1) completely dissolve cellulose fibers
by disrupting orderly hydrogen bonds network of crystalline cellulose,
providing better cellulose accessibility; (2) break up the lignin carbohy-
drate complex bonds, slightly hydrolyze cellulose and hemicellulose, and
remove acetyl groups from hemicellulose to eliminate the major obstacles
to hydrolysis. The mild pretreatment conditions, typically at 50°C and
atmospheric pressure for 20 60 min, can reduce the utility consumption
[154]. Moreover, the sugar degradation is less during this pretreatment,
thereby the inhibitors are less as well [155]. CPA pretreatment has been
demonstrated to be efficient for various biomass, for instance poplar
[156], bamboo [157],elephantgrass [158], switchgrass [153], common
reed [159], and corn stover [160]. Moxley et al. [155] investigated the
effect of pretreatment conditions (phosphoric acid concentration, reaction
temperature, and duration time) on sugar release of industrial hemp
hurds. The glucan digestibility was 96% at 24 h with a cellulose loading
of 15 FPU/g glucan after 84.0% CPA pretreatment at 50°C for 60 min.
It was found that phosphoric acid can efficiently disrupt the recalcitrant
lignocellulose structures only when the acid concentration was above
83% for a sufficient duration time. CPA posttreatment after Formiline
pretreatment of wheat straw was reported [161]. Using this combined
pretreatment, the ethanol concentration achieved 41.6 g/L with a yield
of 91.2% at a relatively low cellulose loading (5 FPU/g solid) within 24 h
SSF. In order to achieve a fractionation of biomass, subsequent washing
steps are needed after CPA pretreatment with a first organic solvent
washing to remove lignin and a second water washing to remove par-
tially hydrolyzed hemicellulose fragments [160]. Therefore although this
method is very efficient to overcome biomass recalcitrance, a large
amount of phosphoric acid, organic solvents, and water are usually
needed [152].
However, the major drawback of concentrated acid hydrolysis and
pretreatment lies on the corrosiveness, toxicity, and hazard of the acid
[162]. Therefore reactors made from expensive materials resistant to cor-
rosion are necessary, such as acid-resistant alloys or ceramics. In addition,
acid recycle is necessary for the economic feasibility of this process.
However, the recovery of mineral acid from the hydrolyzate is usually
complicated and energy intensive. Extraction, distillation, ionic chroma-
tography, and electrodialysis have been employed for acid recovery.
Fortunately, in the last 50 years, the acid recovery yield has significantly