Pretreatment of Lignocellulosic Materials for Efficient Bioethanol Production

Second-generation bioethanol produced from various lignocellulosic materials,
such as wood, agricultural or forest residues, has the potential to be a valuable substitute
for, or a complement to, gasoline. One of the crucial steps in the ethanol production is
the hydrolysis of the hemicellulose and cellulose to monomer sugars. The most promising
method for hydrolysis of cellulose to glucose is by use of enzymes, i.e. cellulases. However,
in order to make the raw material accessible to the enzymes some kind of pretreatment is
necessary. During the last few years a large number of pretreatment methods have been
developed, comprising methods working at low pH, i.e. acid based, medium pH (without
addition of catalysts), and high pH, i.e. with a base as catalyst. Many methods have been
shown to result in high sugar yields, above 90% of theoretical for agricultural residues,
especially for corn stover. For more recalcitrant materials, e.g. softwood, acid hydrolysis
and steam pretreatment with acid catalyst seem to be the methods that can be used to
obtain high sugar and ethanol yields. However, for more accurate comparison of different
pretreatment methods it is necessary to improve the assessment methods under real
process conditions. The whole process must be considered when a performance evaluation
is to be made, as the various pretreatment methods give different types of materials.
(Hemicellulose sugars can be obtained either in the liquid as monomer or oligomer sugars,
or in the solid material to various extents; lignin can be either in the liquid or remain
in the solid part; the composition and amount/concentration of possible inhibitory compounds
also vary.) This will affect how the enzymatic hydrolysis should be performed(e.g. with or without hemicellulases), how the lignin is recovered and also the use of the
lignin co-product.