Environmental Engineering Reference
In-Depth Information
potassium hydroxide, aqueous ammonia, and ammonia hydroxide are the most
commonly used alkalis for this process. Of those, sodium hydroxide is the most
popular due to its low cost and efficiency. Alkaline pretreatment is more effective
towards lignocellulosic biomass with lower lignin content such as hardwood, her-
baceous crops, and agricultural residues in comparison to those with higher lignin
content such as softwood [35].
The process is relatively mild compared to other pretreatment methods and it
can be carried out in a batch mode. This pretreatment process involves spraying
alkali onto biomass and soaking it for periods from hours to days at ambient tem-
perature. The reaction time can be greatly reduced at elevated temperature [24].
Xu and co-workers reported the use of dilute sodium hydroxide (1%) for pretreat-
ment of switchgrass to efficiently reduce the lignin content by 85.8% at 121°C in
1 h, 77.8% at 50°C, and 62.9% at 21°C for 48 h [36]. At the end of the process, the
pretreated biomass was recovered by filtration and neutralized before further
processing.
Compared to acid hydrolysis, alkaline hydrolysis is a milder process and thus
sugar degradation to furfural, 5-hydroxymethyl-2-furfural (HMF) and organic
acids is reduced. Moreover, caustic salts such as calcium carbonate can be recov-
ered from the aqueous liquid/solution generated by the system as insoluble calcium
carbonate by neutralizing it with inexpensive carbon dioxide [24]. The energy
requirement of the process is low and it can be conducted at ambient temperature
and pressure. However, the disadvantage of this method is the use of corrosive
chemicals and their associated operating and environmental issues.
3.2.3.3
Ozonolysis
Treatment of lignocellulosic biomass with ozone gas is referred to ozonolysis.
The pretreatment process can effectively degrade lignin and part of the hemi-
cellulose of the biomass [4, 37], while cellulose remains unaffected. Ozone is
a gas which is readily soluble in water and is highly reactive towards com-
pounds incorporating conjugated double bonds and functional groups with
high electron densities. Since lignin contains high carbon-carbon double bond
content compared to hemicelluloses and cellulose, lignin is selectively oxi-
dized in this process, generating low molecular weight compounds such as
formic and acetic acids. Ozone treatment for delignification can be applied to
a variety of lignocellulosic biomass including wheat straw, bagasse, green hay,
peanut, pine, cotton straw, and sawdust [24]. In a recent study, it has been
reported that as much as 49% lignin degradation was achieved when corn
stover was treated by ozonolysis [34].
Figure 3.5 depicts laboratory-scale ozonolysis apparatus. To enhance the
ozonolysis, the biomass is initially hydrated with water in order to adjust the
moisture to around 25-35% before feeding the bioreactor [38]. The grinded
