Preliminary status note: Thermal biomass conversion technologies ...

2.6. Hydrogenation of flash pyrolysis oils
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2.6.1. Short description of technology
One prospective route for production of sustainable fuels in the future is the conversion of biomass into
bio-­‐oil followed by hydrogenation of the bio-­‐oil into a product equivalent to crude oil. Optimal production
of bio-­‐oil is performed through flash pyrolysis, where the biomass is rapidly heated in the absence of
oxygen (1). This produces a gaseous phase of hydrocarbons which by condensation yields the bio-­‐oil.
Compared to biomass, the bio-­‐oil has both a higher mass and energy density, which renders transportation
more feasible. The bio-­‐oil has a high content of water (up to 30 wt%) and oxygen containing compounds
(up to 40 wt% O) as phenols, guaiacols, etc, which, among other things, this may cause it to be unstable and
gives the bio-­‐oil a low heating value compared to crude oil (2). It is impossible to use a raw bio-­‐oil on a
distillation plant because of it tendency to generate char upon heating above 100°C.
To utilize the bio-­‐oil, removal of the oxygen is wanted, which will enable separation of an oil phase. The
most promising initiative for this is hydrodeoxygenation (HDO) where a high pressure and a catalyst are
used to remove the oxygen functionalities in the oil (2). Complete HDO of the oil has been reported to
produce a crude oil like product with heating values equivalent to conventional crude oil (2,3).
This process is carried out at temperatures between 200-­‐400 °C and at a pressure up to 200 bar.
Conventional hydrotreating catalysts as Co-­‐MoS2 and Ni-­‐MoS2 have received much attention, but also noble
metal and nickel based catalysts have been shown as promising catalysts (2). A general problem for the
catalysts is that they all suffer from a relatively low lifetime, as carbon deposition arise during the process.
The carbon is formed due to both cracking and polymerization reactions of the highly reactive oxygen
containing molecules (2). So far, Pd/C and Co-­‐MoS2/Al2O3 have been reported to have the longest lifetimes,
where time on streams of respectively 100 h (3) and 200 h (4) have been reported.
In an industrial perspective, bio-­‐oil production could take place at smaller flash pyrolysis plants placed close
to the biomass source. These smaller plants should then supply a centralized bio-­‐refinery with bio-­‐oil. In
this approach transportation cost are minimized as biomass only should be transported to the flash
pyrolysis plant (5) (6). At the bio-­‐refinery, HDO is carried out at relatively large scale plants to produce the
crude oil like product, and this oil is subsequently treated to produce the desired fractions of hydrocarbons
as gasoline, diesel, etc. (2). In a recent study by the U. S. Department of Energy it has been indicated that a
process from biomass to fuels through the steps described above and with natural gas as hydrogen source
a minimum selling price of 0.54 $/l could be achieved for the fuels (7). This price should be compared to the
current fuel price of 0.73 $/l, excluding distribution and taxes (2). Thus, this work concluded that
production of fuels through the HDO synthesis is economically feasible and cost-­‐competitive with crude oil
derived fuels. However, a certain uncertainty in the calculated price of the synthetic fuel must be
remembered and the reported value is therefore not absolute. Also the problems with short lifetimes for
HDO catalysts are not resolved.
2.6.2. Global status of technology development
Globally, the HDO approach has been given much attention. As previously mentioned, the U. S. Department
of Energy has shown interest in the technology. This has been guided by the work of Douglas C. Elliot at the
Pacific Northwest National Laboratory, but other groups in U.S.A., at for instance Oklahoma University,
have also influenced the research. The technology has also received much attention in Europe. In Great