Environmental Engineering Reference
In-Depth Information
In the latter approach, concentrated (strong) acids such as H 2 SO 4 and HCl have
been widely used because of their hydrolytic effect on cellulose. However, given
the corrosive nature of the reaction, this approach poses special requirements to
the equipment. Further drawbacks of both approaches include the generation of
degradation products that are inhibitors for subsequent steps and the requirement
for neutralization afterward, resulting in waste generation.
13.3.1.2 Alkaline Hydrolysis The major effect of alkaline pretreatment is the sol-
ubilization of lignin from the biomass. The reaction time can be long, but the process
conditions are mild, resulting in limited degradation. Usually, lime (calcium hydrox-
ide) or sodium hydroxide is used. This pretreatment is often used after acid hydrolysis;
this combination, however, leads to the formation of poorly soluble salts, which need
to be separated from the treated biomass.
13.3.1.3 Organosolv With this method, the lignin is solubilized and the hemicel-
lulose is hydrolyzed. Commonly used organic solvents include ethanol, methanol,
acetone, and ethylene glycol. Temperatures used for the process can be as high as
200 C, but lower temperatures can be sufficient depending on whether a catalyst
(organic or inorganic acid) is used. Solvent recovery is required for reducing costs
and environmental impact but also because the solvent itself can be an inhibitor
for the subsequent steps (both enzymatic hydrolysis and fermentation). This method
results in a high-quality lignin fraction, which might facilitate further applications.
13.3.1.4 Wet Oxidation Lignin removal can also be achieved by treatment with an
oxidizing agent such as hydrogen peroxide, oxygen, or air. In combination with water
and elevated temperature and pressure, the lignin polymer is then converted into car-
boxylic acids, among others. Since these acids are inhibitors for fermentation, they
have to be neutralized or removed. In addition, a substantial part of the hemicellulose
might be degraded and can no longer be used for sugar production.
13.3.1.5 Ozonolysis This method for oxidation of lignin is performed at ambient
temperature and pressure. The hemicellulose and cellulose are hardly decomposed.
13.3.1.6 Steam Explosion This method uses injection of high-pressure saturated
steam into a reactor filled with biomass. The temperature rises to 160
260 C. The
pressure is then suddenly reduced, and the biomass undergoes an explosive decom-
pression with hemicellulose degradation and lignin matrix disruption as a result. The
hemicellulose degradation results in the formation of acetic acid, which in turn causes
further hydrolysis of the hemicellulose fraction. Because of this, steam explosion is
also referred to as
A variation of the method is catalyzed steam
explosion, in which acids are added to increase the hydrolysis yield.
autohydrolysis.
13.3.1.7 Ammonia Fiber Explosion In a similar way as for steam explosion, the
biomass is exposed to liquid ammonia at high temperature and pressure and then the
pressure is released. This method reduces the lignin content and removes some
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