Conversion Technologies 85
Table 3.2Pyrolysis methods and their variants.
Pyrolysis technology Residence time Heating rate Temperature (∘C) Major
Carbonisation Days Very low 400 Charcoal
Conventional 5–30 min Low 600 Bio-oil, gas, char
Fast 0.5–5 s Very high 650 Bio-oil
Flash-liquid <1s High < 650 Bio-oil
Flash-gas <1s High < 650 Chemicals, gas
Ultra <0.5 s Very high 1000 Chemicals, gas
Vacuum 2–30 s Medium 400 Bio-oil
Hydro-pyrolysis <10 s High < 500 Bio-oil
Methano-pyrolysis <10 s High > 700 Chemicals
Source: Mohanet al. 2006 [25]. Reproduced with permission of American Chemical Society.
formation of liquid products. To achieve this goal, a higher temperature of 650∘Cand
very fast heating rates are applied. These conditions combined with the small size of
biomass particles promote the formation of different products. The low residence time
of condensable gasses limits their secondary reaction with char particles; this way these
compounds can be transferred to a condenser where their condensation takes place.
Usually, as a result of fast pyrolysis 60–75% of liquid fraction (bio-oil), 15–25% of char
and about 10–20% of non-condensable gasses is obtained [21]. The predominant prod-
uct of fast pyrolysis is a dark brown liquid called bio-oil. It contains numerous organic
compounds such as phenolics, carboxylic acids, aldehydes, alcohols and others. Bio-oils
have a major advantage over other biomass products, as they are organic liquids that
can be stored and transported through haulage or pipelines. They are also considered
as potential sources of renewable fuels and chemicals. The most straightforward appli-
cation of bio-oils is their utilisation as low-heating value fuel in boilers and furnaces
for heat and electricity generation [25]. Raw bio-oil is most suitable as a renewable
alternative to heavy fuel oil in boilers or furnaces. Attempts were made to use bio-oil
as a fuel for diesel engines, turbines and Stirling engines to increase the efficiency of
energy production [26]. To date, these attempts were rather unsuccessful due to physical
characteristics of bio-oil namely low heating value due to high content of oxygenated
compounds and water, poor volatility, high viscosity, coking, corrosiveness, chemical
instability and incompatibility with conventional fuels [25, 26]. To be able to successfully
apply bio-oil as a fuel for these engines, modifications of engine designs, fuel upgrading
or combination of both may be required to achieve desired performance [25]. Some of
bio-oil deficiencies can be improved with relatively simple technologies, whereas others
require more complex processing [26]. Typical processes of bio-oil upgrading include
cracking, decarboxylation, decarbonylation, hydrocracking hydrodeoxygenation and
hydrogenation, and representative reactions are presented in Figure 3.9 [27].
In principle, bio-oils are promising starting point for fractionation and subsequent
upgrading of their components into fuels and platform chemicals. The biggest limitation
of this process to date is very high diversity and heterogeneity of bio-oil components
[27]. In a typical sample of bio-oil, more than 300 compounds are normally present
[25]. Refining this mixture to useful building blocks has not been achieved to date and
