Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 36

In comparison with the pretreatment technologies discussed previously, low
temperature and pressure, even ambient conditions, can be applied under alkaline
pretreatment conditions. However, the time required by the alkaline pretreatment
process is hours, days or weeks rather than minutes, making it difficult to achieve
the feedstock processing capacity required by commercial production of bulk
commodities like ethanol. Moreover, a significant amount of salt produced during
the pretreatment is a big problem, which not only affects microbial growth and
fermentation, but also raises an environmental concern. Although an alternative
strategy using ammonia--for example, the ammonia recycling percolation (ARP)
process in which aqueous ammonia is recycled through a column containing lig-
nocellulosic biomass such as corn stover [ 35 ]--can overcome these disadvantages,
it is not cost-effective due to the high cost of the recovery of ammonia. A modified
ARP process operated with low liquid ammonia throughput can address this issue
to some extent [ 36 ], but is still not practical for commercial application.
Ammonia fiber explosion (AFEX) is a hybrid of the SE and ARP processes, in
which biomass is pretreated with liquid anhydrous ammonia at mild temperatures
(60-100C) and high pressure [ 37 ]. When the pressure is released, the rapid
expansion of ammonia gas causes swelling of the biomass, which correspondingly
disrupts LCCs and creates more accessible surfaces for enzymatic hydrolysis.
Since temperatures in the AFEX process are much lower than those applied to the
SE process, not only can energy consumption be reduced, but also the formation of
inhibitory by-products prevented. In addition, washing is not necessary for the
process, which benefits for high solid loading hydrolysis. Meanwhile, ammonia
remaining in the pretreated biomass facilitates microbial growth and fermentation.
However, ammonia recovery by evaporation is needed, which complicates the
system design and requires more capital investment and energy consumption [ 38 ].
3.3 Solvent Pretreatment
Solvent pretreatment is a fractionating process, in which an organic or aqueous
organic solvent is used with or without catalysts to deconstruct LCCs [ 39 , 40 ].
Methanol, ethanol, ethylene glycol, triethylene glycol, tetrahydrofurfuryl alcohol,
glycerol, n-butanol, acetone, phenol etc. have been explored to extract lignin as
well as hydrolyze hemicelluloses to render cellulose for enzymatic hydrolysis. The
advantage of organic solvents over other chemical pretreatments is that relatively
pure and low-molecular-weight lignin can be recovered as a by-product. However,
the high cost of organic solvents and the intensive energy consumption associated
with solvent recovery make this strategy economically uncompetitive, not to
mention the difficulty in the treatment of concentrated black liquors left after
solvents are evaporated and the loss of sugars in the liquors.
Ionic liquids (ILs) are salts composed of a small anion and a large organic cation,
existing as liquids at room temperature with low vapor pressure [ 41 ]. Based on the
understanding of the chemistry of the anion and cation, a wide variety of ILs can be