Environmental Engineering Reference
In-Depth Information
in the preceding section, is so far the only industrially realized technology for bio-oil
production (with a liquid yield of 50-70%) (
Onay and Kockar, 2006; Maschio et al., 1992)
.
However, the oil product in fast pyrolysis (i.e., bio-oil) consists of a high content of water
(20-25%) and oxygen, and hence a low heating value (<20 MJ/kg, about only half of that of
crude oil), and the oil is highly corrosive due to its low pH value, as given in Table 2.
Without further upgrading, the pyrolysis oil is not regarded as an ideal liquid fuel for heat and
power generation. The main purpose of liquefaction is to produce oil products of increased
H/C ratios and decreased O/C ratios, and hence a high caloric value relative to those present
in the feedstock. Liquefaction can be accomplished indirectly or directly. For indirect
liquefaction, biomass is converted into liquid products through first gasification to syngas
followed by catalytic conversion (Dry, 1999). Direct liquefaction of biomass feedstocks into
liquid oils has attracted more intensive interest, due to its simpler technical route and better
conversion economy and efficiency relative to the indirect liquefaction processes. In a typical
direct liquefaction process, biomass is converted to liquid products directly but through a
complex sequence of processes involving solvolysis, depolymerization, decarboxylation,
dehydration and hydrogenolysis/hydrogenation (when hydrogen is present in liquefaction).
Direct liquefaction is a low-temperature, high-pressure conversion in the liquid phase, usually
with a high hydrogen partial pressure and a catalyst to enhance the rate of reaction (Furness et
al., 2000). Yields of the products depend on temperature, residence time, initial biomass
concentration, catalysts and liquefaction atmosphere (inert, N
2
or reducing, H
2
). The effects
on the liquefaction product yields were investigated.
Research on direct liquefaction has been widely performed in the 1980's for the purpose
of alternative energy production (Kranich, 1984; Beckman and Elliot, 1985; Boocock and
Sherman, 1985). For instance, in an early study on liquefaction of sewage sludge, conducted
by Kranich (1984), sewage sludges were converted to oils in a reaction medium of water or
oil at 295-450°C with the presence of a reducing gas (hydrogen) and catalysts of Na
2
CO
3
,
NiCO
3
and Na
2
MnO
4
. The organic conversion rates varied from 45% to 99% and the oil
yields were from 35% to 63% in the reaction medium of oil. However, the oil yields were
found very low with the water medium (usually less than 20%). Many successful studies on
direct liquefaction of biomass in organic solvents such as anthracene oil (Appel et al., 1996;
Crofcheck
et al., 2005) and alcohols
(Miller at al., 1999; Xu and Etcheverry, 2007) have been
reported. Hot compressed water and sub-/supercritical water (at temperatures of 200-400°C)
are however more advantageous for being used as the solvent in biomass direct liquefaction in
that water is likely the most “green” and environmentally benign solvent. Extensive research
work has been conducted on direct liquefaction of biomass in sub- and near-critical water. A
pioneer work was reported by Appell et al. (1971), where a variety of lignocellulosic
materials were efficiently converted to oily products in water at around 350
o
C in the presence
of Co and Na
2
CO
3
as the catalysts. Minowa et al. (1998a, b) obtained heavy oil (HO) (with
calorific values of around 30MJ/kg) at a yield of 21-36 wt% from a variety of biomass
feedstocks in water at 300
o
C and around 10 MPa with Na
2
CO
3
as catalyst. Qu et al. (2003)
obtained liquid organic products at a total yield of 30-35% by direct liquefaction of
Cunninghamia lanceolata
in water at 280-360
o
C for 10-30 min. It has also been demonstrated
by Suzuki et al. (1986) that the treatment of sewage sludge by direct liquefaction in water at
around 300
o
C could be a profitable alternative means for sludge disposal. However, the
feedstocks tested so far were mainly wood and municipal sewage sludge, and to the best of
