Page 253 - Biosystems Engineering

Biomass Pyr olysis and Bio-Oil Refineries 231

According to Diebold (1999) the most important reactions hap-
pening during bio-oil aging are (1) organic acids with alcohols to
form esters and water, (2) organic acids with olefins to form esters,
(3) aldehydes and water to form hydrates, (4) aldehydes and alcohols
to form hemiacetals or acetals and water, (5) aldehydes to form oligo-
mers and resins, (6) aldehydes and phenolics to form resins and
water, (7) aldehydes and proteins to form oligomers, (8) organic sul-
phur to form oligomersunsaturated compounds to form polyolefins,
and (9) air oxidation to form more acids and reactive peroxides that
catalyze the polymerization of unsaturated compounds. Reactions
(1) through (5) are reversible processes, which mean that the change
in temperature of relative amounts of water and other reactive com-
pounds will upset the equilibrium and initiate compositional changes
(Diebold et al. 1999). Methanol and ethanol are added to stabilize the
oils because these alcohols interrupt molecular–building reactions
responsible for bio-oil aging (Diebold and Czernik 1997, Diebold
1999). From a practical point of view, a minimum of 14 days of
storage should be possible; preferably the bio-oil must have at least
6 months of storage life (at normal pressure and temperature)
(Diebold et al. 1999; van de Kamp 2000).

7.6.2 Fuel Applications of Crude Bio-Oils
Contrary to the practices of the petroleum industry, where most of
the combustion tests are carried out using fractions, all the tests
reported so far (Banks et al. 1992; Lee et al. 1993; Barbueci et al. 1995;
van de Kamp 2000, Baglioni et al. 2001; Oasmaa et al. 2001; Vender-
bosch et al. 2001, Chiaramonti et al. 2003; Stamatov et al. 2006) for
bio-oil, have been performed using crude bio-oils or with added sol-
vents (mainly alcohols). The lack of bio-oil refining technologies is
the main reason for this situation.

Fuel Properties of Crude Bio-Oils
The characteristics of crude bio-oil that make this fuel difficult to use
are low heating value, high water content, immiscibility with petro-
leum-derived fuels, high ignition temperatures, phase separation, high
content of solids, relatively high viscosity, low volatility, high corro-
siveness, high Conradson carbon numbers, chemical instability, high
alkaline content (high content of K and Na) (Oasmaa and Czernik 1999;
Czernik and Bridgwater 2004). As Table 7.4 shows the viscosity of
°
crude bio-oil varies between 5 and 200 cSt at 50 C (Mohan et al. 2006).
These values of viscosity are higher than those found for typical fuel
°
No. 2 (between 2 and 5 cSt at 50 C) but lower than that found for fuel
°
No. 6 (up to 500 cSt at 50 C). The high viscosity of crude bio-oil limits
its uses in applications designed for fuel No. 2 (Diebold et al. 1999).
Many of the undesirable fuel properties in crude bio-oils are
generally associated with low-molecular-weight organic molecules