Page 146 - Advances in bioenergy (2016)

the viscosity of bio-oil at faster rates. The chemical reaction causing the increase in viscosity
has not been entirely elucidated yet. It has been determined that the concentration of acids in
the bio-oil does not change in correlation with the viscosity increase. However, the change in
the concentration of carbonyls does appear to change with the change in viscosity. Other
evidence suggests that the addition of light alcohols (methanol, ethanol, or isopropanol) can
“stabilize” the viscosity of bio-oil.

The high oxygen content of the bio-oil has other effects on the physical properties. The density
of bio-oil is relatively high, about 1.2 g/mL, compared to water at 1 and hydrocarbon liquids at
<1. The oxygen content also causes the volatility of the oil to be low. As a result, higher
temperatures are required to distill bio-oil. Distillation of bio-oil, at the higher temperatures
required, leads to decomposition and unacceptable results. For example, in a simple batch
vacuum distillation of bio-oil (ASTM D1160), less than half of the bio-oil (mostly water) was

distilled before the residue solidified in the still pot at about 170°C. 15

Reactivity

The reactivity of the oxygenates in bio-oil results in difficulties in further use and processing of
the bio-oil. Bio-oil can be preheated to only a limited degree, up to about 40°C appears
reasonable. Therefore, the catalytic processing required to remove the oxygen and produce a
hydrocarbon fuel, which is typically done at 350–450°C, is done only with some special
handling. The overall concept of biomass to liquid fuels via fast pyrolysis and hydroprocessing
is shown in Figure 6.1.

Direct processing of bio-oil in conventional hydroprocessing systems has led to failure in
several cases. Moderate pretreatment at lower temperatures (sometimes referred to as
hydrotreating, HT) prior to the finishing hydroprocessing (sometimes referred to as
hydrocracking, HC) has shown promise. The concept was first patented based on the early
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work in the 1980s. Further developments and improvements have been more recently
17
reported. The long-term trouble-free operation has not yet been definitively reported, but
there are claims in the nontechnical literature. 18

BIO-OIL HYDROPROCESSING

Bio-oil can be deoxygenated by two theoretical routes: The oxygen can be thermally cracked
from the bio-oil in the form of water and carbon oxides, or it can be removed as water by
addition of hydrogen. Both routes have been tested using different catalysts. Catalytic cracking
has a capital cost advantage because of a lower cost reactor, which operates at low pressure
and higher throughput. Catalytic hydroprocessing uses more expensive equipment due to the
higher pressure and slower throughput and also has a hydrogen reagent cost; however, it
typically has a higher yield overall. The current process development efforts focus for the most
part on hydroprocessing. Some of the work has used model compounds and been directed at
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more fundamental chemistry questions. Other studies have investigated traditional sulfided
catalysts, whereas some have investigated nonsulfided precious metal catalysts reactors. 20