Biofuels from Biomass - Novel Combustion Concepts for Sustainable Energy Development - page 37

Physical pretreatment is the preliminary step included in all pretreatment
techniques to reduce the size of the biomass and break some of the cellulose
crystallinity. Techniques used include milling and grinding which aim to get the
particles as small as possible as this increases hydrolysis ef

ciency. However, the
energy requirement also needs to be taken into consideration when analyzing
the best pretreatment for a certain feedstock as the smaller the targeted particle size,
the higher the energy requirement. After physical pretreatment, a second pretreat-
ment step is employed. Dilute acid, alkali, hot water, and steam expansion are
among the most commonly used pretreatments (Kumar and Murthy 2011 ).
Dilute acid pretreatment has been extensively studied and is a common stage in
many pilot cellulosic ethanol projects. It is characterized by high temperature, and
short residence time meaning the biomass can move on to the next stage in the
process quickly which is suitable for a large-scale plant. In this stage, the hemi-
cellulose is hydrolyzed so it becomes soluble, whereas the cellulose and lignin
remain in the solid fraction. In this way, the hemicellulose can be separated from
the cellulose allowing for a variety of ethanol production situations later on in the
process. If the temperature or acid concentration is too high, then the hemicellulose
could degrade into furans which inhibit fermentation. So it needs to be removed and
the cellulose could start to hydrolyze into glucose. All of which are detrimental to
the process making it more expensive.
Dilute alkali pretreatment also degrades the biomass structure but is less com-
mon. The alkali, at high temperature, reacts with the ester bonds of the lignin which
are cross-linked with cellulose. In this way, lignin degradation is more favored and
it is the lignin which becomes soluble and can be removed leaving the sugar
containing compounds behind (Kumar and Murthy 2011 ). However, there will be
some lignin left in the solid sugar component. Concentration of alkali signi

cantly
affects degradation of the biomass, and a balance must be struck to acquire the most
effective pretreatment. At higher alkali concentration, hemicelluloses start to
degrade and produce inhibitory compounds. Also, the high temperatures used and
the cost of the alkali on a large scale make this unsuitable for industrial scale plants.
Hot water pretreatment is an auto-catalyzed process where high-temperature
water is used as a solvent for the biomass which will degrade the hemicelluloses
due to its branched structure. The acetic acid produced from degradation of he-
micelluloses and self-ionization of water lower the pH hence causes the same
structural change in the biomass as the dilute acid pretreatment. However, this
process requires high pressure in order for the water to be kept liquid which is
expensive and has not been demonstrated on a commercial scale.
In steam explosion, biomass is heated under pressure with water vapor. The
pressure is then rapidly reduced causing a rapid expansion of the steam and a
breakdown of the biomass structure. The process has been found to be most effi- -

cient for high-temperature short residence times or vice versa (Kumar et al. 2009 ).
A major advantage of this technique is that it has been found to be effective on
feedstock with large particle sizes thereby reducing the energy needed in physical
pretreatment for size reduction (Sun and Chen 2002 ), and also, it does not release
any acid or base as environmental pollutant.