Environmental Engineering Reference
In-Depth Information
problems in biogas reactors because they block outlets, are unavailable for
digestion and interfere with gas escape (Kamarad et al., 2010).
Different kinds of mills have been tested in the past, mainly hammer mills
(crushing) and knife mills (chopping). Mechanical pre-treatment is one of
the most expensive steps in biomass conversion because, although increasing
specific surface area increases the biogas yield, electrical energy demand is
very high. A particle size of 1-2mm is recommended for effective hydrolysis
(Kratky and Jirout, 2011). Another disadvantage is that mills can be
damaged by stones and other hard materials, and equipment repairs can be
very expensive.
Several researchers have investigated the effect of knife milling on biogas
production. Menind and Novak (2010) found an approximately 10% higher
gas yield was achieved after milling hay to 0.5mm compared to 20-30mm.
Another study showed milling sisal fibres from 100mm to 2mm achieved an
approximately 20 to 25% higher gas yield (Mshandete et al., 2006).
Reduction of particle size of wheat straw by knife mills from 12.5 to 1.6mm
requires 2.8-7.55 kWh/t
1
(Kratky and Jirout, 2011). To put this into
context, the parasitic electrical demand of a continuously stirred tank
reactor (CSTR) digesting slurries is around 10 kWh/t
1
(Murphy and
McCarthy, 2005), and is much higher for other substrates. More research
is required to say whether or not the electricity input for milling is justified
by the electricity saved by improved mixing. Knife and hammer mills are
generally used for dry biomass with a moisture content of up to 15%
(Kratky and Jirout, 2011; Taherzadeh and Karimi, 2008). Hammer mills are
relatively cheap and easy to operate, but have a slightly higher energy input
in comparison to knife mills (Kratky and Jirout, 2011).
Thermal pre-treatment
During thermal pre-treatment, also called liquid hot-water (LHW) pre-
treatment, the substrate is heated up under pressure to a maximum of
220
C, held for a specific time, cooled down and used in biogas plants.
Substrates with low water content need additional water before thermal
treatment. The presence of heat and water causes swelling of biomass by
disrupting the hydrogen bonds that hold together crystalline cellulose and
the structural complexes. Hemicellulose is also broken down during LHW,
which aids swelling (Garrote et al., 1999).
One example of thermal pre-treatment technology at large scale is so-
called TDH (thermal hydrolysis, from the German 'Thermo-Druck-
Hydrolyse'). First, the substrate in the reactor is put under pressure of 20-
30 bar. Using a heat exchanger, input material is heated to 140-180
8
C while
the hydrolysed substrate is cooled down. Oil is heated using the exhaust gas
stream and heats the substrate to hydrolysis temperature of 170-220
8
C. The
8
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