Page 255 - Biosystems Engineering

Biomass Pyr olysis and Bio-Oil Refineries 233

operate with a great variety of fuels (Moses 1994), little variation is
allowed in the properties of fuels for gas turbines and diesel engines.
The residence times in the combustion chambers of these engines are
much smaller than for boilers and the ash content, and alkalinity
must be strictly controlled. Formation of carbon residues in turbine
blades and in diesel engine exit valves is an important problem that
needs to be addressed (Shihadeh and Hochgreb 2000). Most of these
carbonaceous materials are formed from the oligomeric fraction of
bio-oils (families E and F).
The combustion in a diesel engine is caused by self-ignition of the
fuel injected at a high pressure into a chamber filled with a constant
mass of compressed and preheated air. The cetane number is a mea-
sure of the ease of self-ignition in a diesel engine. Low cetane num-
bers between 0 and 27 have been reported for crude bio-oils (Suppes
et al. 1996; Chiaramonti et al. 2007). Pilot-ignited medium- or low-
speed diesel engines can be fueled with a low-cetane fuel like bio-oils
(Gros 1995; Diebold et al. 1999; Ormrod and Webster 2000; Chiara-
monti et al. 2003; Czernik and Bridgwater 2004). It has also been
reported that crude bio-oils can autoignite at cylinder charge tem-
peratures above 600°C (Leech 1997; Shihadeh and Hochgreb 2000).
Increasing the compression ratio up to 22:1 has been suggested as a
way to deal with the poor self-ignitability of crude bio-oils (Bertoli
et al. 2000). Despite a longer ignition delay, diesel engines can run
smoothly using bio-oils. Thermal efficiencies seem to be more or less
similar to those obtained using diesel fuels (Gros 1995; Jay et al. 1995;
Shihadeh and Hochgreb 2000).
It is important to point out that no one has ever built and com-
mercialized a gas turbine or diesel engine designed to handle bio-oils.
The only place where bio-oils are used regularly as fuel is the Red
Arrow Products Co. pyrolysis plant in Wisconsin, but, in this case,
bio-oils are burned in a conventional boiler to generate steam (Freel
et al. 1996; Czernik and Bridgwater 2004). The commercial use of bio-
oils for power generation in relatively large boilers, gas turbines, and
diesel engines seems to be an achievable goal in the short term.
Upgrading Crude Bio-Oils
The term upgrading is commonly used in describing chemical and
physical methods used to improve the properties of crude bio-oils
(Bridgwater et al. 2001). Some of the technologies being studied are
hot gas filtration (Solantausta et al. 2000), formation of diesel/bio-oil
microemulsions (Ikura et al. 1998; Baglioni et al. 2001; Chiaramonti
et al. 2003), blending with polar solvents (Diebold and Czernik 1997;
Boucher et al. 2000a, 2000b), and hydrogenation or catalytic vapor
cracking (Elliott and Baker 1987). Most of these upgrading methods
are still at the laboratory stage (Maggi and Elliott 1997). For most
operating pyrolysis units, crude bio-oil upgrading is limited to the
removal of char and, in some cases, to the addition of polar solvents