The biochemical platform consists of three main processes (Cotana et al. 2015;
Barbanera et al. 2018; Buratti et al. 2015, 2018; Cavalaglio et al. 2016):
– Pretreatment
– Enzymatic hydrolysis
– Fermentation
1.4.3.1 Pretreatment
The pretreatment process significantly affects all the downstream processes and
ultimately influences the overall biofuel yield and cost.
Pretreatment step can be performed through biological, physical, and chemical
processes or a combination of them. Chemical methods use dilute acids (such as
sulfuric or hydrochloric acid), alkalis (such as calcium hydroxide), and liquid
ammonia (the ammonia fiber explosion pretreatment), while a physical method is
represented by steam explosion (Ruane et al. 2010).
Pretreatment with dilute acid and intermediate temperatures is generally considered quite cost-effective. It loosens the cell wall matrix through degradation of
hemicelluloses. Lignin is unaffected by this process. Accessibility to cellulose
microfibrils is increased to provide a higher yield of sugars for fermentation. Acid
treatment will result in other high-value products like furfural, hydroxyl-methyl
furfural (HMF), phenolics, aldehydes, and aliphatic compounds. These products
have to be removed before using the residues for further biochemical treatments.
Acid pretreatment processes have to be followed by neutralization and detoxification
(Kurian et al. 2013).
Steam explosion is the physical treatment where the lignocellulosic biomass is
subjected to high pressures and temperatures for short duration, followed by the
rapid decrease to atmospheric pressure, which will break the polymeric bonds in the
substrate. Temperatures can range between 180 and 250
C, pressures can range
between 1 and 5 MPa (Jacquet et al. 2011).
Steam explosion has the following advantages:
– Lower capital investment
– Significantly lower environmental impact
– More potential for energy efficiency
– Less hazardous process conditions
– Complete sugar recovery
To compare steam explosion conditions, the severity factor has to be taken into
account, defined as (Li et al. 2005)
S 0 ¼ log exp T À 100
ð
Þ =14:75
½
t
f
g
ð1:2Þ
where T is the temperature (
C) and t is the duration of treatment (min).
Steam explosion is considered the most cost-effective option for hardwood and
agriculture residues, while it is less effective for softwood. Acid catalysis can be used
1 Biofuels: Types and Process Overview
15
Barbanera et al. 2018; Buratti et al. 2015, 2018; Cavalaglio et al. 2016):
– Pretreatment
– Enzymatic hydrolysis
– Fermentation
1.4.3.1 Pretreatment
The pretreatment process significantly affects all the downstream processes and
ultimately influences the overall biofuel yield and cost.
Pretreatment step can be performed through biological, physical, and chemical
processes or a combination of them. Chemical methods use dilute acids (such as
sulfuric or hydrochloric acid), alkalis (such as calcium hydroxide), and liquid
ammonia (the ammonia fiber explosion pretreatment), while a physical method is
represented by steam explosion (Ruane et al. 2010).
Pretreatment with dilute acid and intermediate temperatures is generally considered quite cost-effective. It loosens the cell wall matrix through degradation of
hemicelluloses. Lignin is unaffected by this process. Accessibility to cellulose
microfibrils is increased to provide a higher yield of sugars for fermentation. Acid
treatment will result in other high-value products like furfural, hydroxyl-methyl
furfural (HMF), phenolics, aldehydes, and aliphatic compounds. These products
have to be removed before using the residues for further biochemical treatments.
Acid pretreatment processes have to be followed by neutralization and detoxification
(Kurian et al. 2013).
Steam explosion is the physical treatment where the lignocellulosic biomass is
subjected to high pressures and temperatures for short duration, followed by the
rapid decrease to atmospheric pressure, which will break the polymeric bonds in the
substrate. Temperatures can range between 180 and 250
C, pressures can range
between 1 and 5 MPa (Jacquet et al. 2011).
Steam explosion has the following advantages:
– Lower capital investment
– Significantly lower environmental impact
– More potential for energy efficiency
– Less hazardous process conditions
– Complete sugar recovery
To compare steam explosion conditions, the severity factor has to be taken into
account, defined as (Li et al. 2005)
S 0 ¼ log exp T À 100
ð
Þ =14:75
½
t
f
g
ð1:2Þ
where T is the temperature (
C) and t is the duration of treatment (min).
Steam explosion is considered the most cost-effective option for hardwood and
agriculture residues, while it is less effective for softwood. Acid catalysis can be used
1 Biofuels: Types and Process Overview
15