Chemistry Reference
In-Depth Information
Table 2 Typical properties of wood pyrolysis bio-oil and of heavy fuel oil.
*
Physical property
Bio-oil
Heavy fuel oil
moisture content, wt.%
15-30
0.1
pH
2.5
-
specific gravity
1.2
0.94
elemental composition, wt.%
C
54-58
85
H
5.5-7.0
11
O
35-40
1.0
N
0-0.2
0.3
Ash
0-0.2
0.1
HHV, MJ/kg
16-19
40
viscosity (at 50 1C), cP
40-100
180
solids, wt.%
0.2-1
1
distillation residue, wt.%
up to 50
1
*Data from Czernik and Bridgwater.
3
Typical products in bio-oil include tars, acids, chars, alcohols, ethers,
aldehydes, esters, ketones, and aromatic compounds.
44
Some physical
properties of bio-oil and fossil-based heavy fuel oil are compared in
Table 2. The different properties result from the chemical composition of
the oils, which are significantly distinct. The normal higher heating value
(HHV) of the bio-oil is only half that of heavy fuel oil. Bio-oil is a multi-
component mixture comprised of differently sized molecules derived
primarily from depolymerization and fragmentation reactions of the
three key biomass building blocks: cellulose, hemicellulose, and lignin.
Thus, the elemental composition of bio-oil resembles that of biomass
rather than that of petroleum oils.
Table 2 shows that the oxygen content of bio-oil is usually between 35
and 40 wt.%. Oxygen is present in most of the more than 300 compounds
that have been identified in the oil.
3
The distribution of these com-
pounds mostly depends on the type of biomass and the process severity,
such as temperature, residence time, and heating and cooling rates. The
presence of oxygen in many oil components is the primary reason for
differences in the properties and behavior between hydrocarbon fuels
and biomass pyrolysis oils. All in all, bio-oil has poor volatility, high
viscosity, coking, corrosiveness, and cold flow problems. These problems
limit any direct application of bio-oil.
4 Bio-oil upgrading with hydrotreating catalysts
As discussed above, bio-oil must be upgraded if they are to be used as a
replacement for diesel and gasoline fuels. As much of the negative
characteristics of bio-oil originate from the high oxygen content, de-
oxygenation is a desired method to upgrade bio-oil into useful fuel.
Hydrotreating catalysts are promising for this bio-oil upgrading.
13,45
During hydrotreating, hydrodeoxygenation (HDO), hydrodesulphuriza-
tion (HDS), hydrodenitrogenation (HDN), hydrodemetallization (HDM),
and hydrogenation (HYD) processes occur.
46
HDO and HYD are the main
reactions of interest during hydrotreating of the bio-feeds. Four main
