Page 61 - Lignocellulosic Biomass to Liquid Biofuels

40 Lignocellulosic Biomass to Liquid Biofuels

Figure 2.5 A schematic process using cellulose solvents to produce bioethanol from
lignocellulosic materials.

Nevertheless, the dissolution mechanism is dependent on the water con-
tent of the mixture. There are mainly three states for cellulose in NMMO
solution depending on water content, including dissolution without
noticeable swelling (water content between 13% and 17%), dissolution by
swelling and ballooning (water content between 19% and 24%), and
swelling by ballooning without dissolution (water content above 25%)
[138]. In the case of ethanol production the NMMO concentration of
83% 87%, usually 85%, is found to be more efficient than lower concen-
tration to increase the biomass digestibility [139].
NMMO has been reported for efficient pretreatment of different lignocel-
lulosic biomass [140]. NMMO pretreatment is usually performed at relatively
low temperatures (90°C 120°C) under atmospheric pressures for a relatively
short time (0.5 3h) [139]. During the NMMO pretreatment the production
of inhibitory compounds, such as HMF and furfural, is negligible. The toxic-
ity of NMMO is less than that of ethanol, which is an environment-friendly
solvent [141]. In addition, NMMO can be recovered almost completely
[142]. With 85 wt.% NMMO pretreatment at 130°C for 3 h the ethanol
yield of hardwood and softwood could be increased from 18.6% and 6.8% to
85.4% and 89%, respectively [143]. The comparison between NMMO and
IL (BMIM-OAc) pretreatments of rice straw at 120°C for 5 h was reported
by Poornejad et al. [144]. It was found that the glucan conversion was com-
plete for IL pretreatment, while reaching 96% for NMMO pretreatment.
However, in terms of SSF, the ethanol yield of NMMO pretreated samples
was 93.3%, higher than that of IL pretreatment (79.7%).
NMMO pretreatment still has some drawbacks, such as inhibitory
effects of NMMO on microorganisms when its concentration was higher
than 2% [145]. The toxicity of NMMO is observed by increasing the