(62a) Biomass to Oil : Fast Pyrolysis and Subcritical Hydrothermal Liquefaction

The global energy
demand is continuously increasing, due to the growth of both population and
industrialization. The supply of CO₂-neutral energies has to become
more important in the future, and bio-fuels from biomass may be a part of those
energies.

Different
processes have been developed to produce bio-oils from different biomass, among
them fast pyrolysis and subcritical hydrothermal liquefaction are studied here.
Fast pyrolysis can produce until 75%wt of bio-oil (pyro-oil), composed
typically of 56%wt of C, 38%wt of O and 6%wt of H [1]. Subcritical hydrothermal
liquefaction, which consists in thermal decomposition of biomass in water in
high pressure and temperature condition but below its critical point (647 K and
22.1 MPa), can reach 35%wt of bio-oil (liq-oil). This liq-oil, less oxygenated
than pyro-oil, is composed typically of 74%wt of C, 17%wt of O and 9%wt of H [1].
Those two types of bio-oils must be upgraded to obtain bio-diesel, thanks to
catalytic hydrodeoxygenation (HDO) which reduces the proportion of oxygen. In
this context, fast pyrolysis and subcritical hydrothermal liquefaction can
appear complementary, considering biomass water content, composition, etc.

In order to
compare those processes, experimentations were led from the same biomass, beech
sawdust. Fast pyrolysis is achieved thanks to the cyclone reactor [2] which is a continuous reactor with an oil
recover system separating three fractions of pyro-oil (Figure 1). Up-stram of the cyclone, the
installation is composed of the biomass (tank and endless screw) and nitrogen
(inert gas) feeds. Particles, drived by nitrogen, are pyrolysed along the
cyclone wall, heated by induction. A by-pass system allows preheating and
equilibrium set-up before feeding of biomass. Thanks to the separator properties
of the cyclone, solids are isolated from gaseous products and recovered in the
solids collector. Gaseous products, evacuating throught the upper art of the
cyclone, pass throught three water cooled heat exchangers in series to recover
the first fraction of oil in a collector, ?heavy oils? (HO). Then the gas flows
throught refrigered coils inside which the "light oils" (LO) fraction
is recovered. In order to recover the last fraction, ?aerosols" (AE), both
electrostatic and membrane filters are used in series. At the exit, clean gases
are finally sampled and analysed by micro-GC.

Fig. 1: Fast pyrolysis installation scheme

In order to
confirm and complete previous works on the cyclone [3], new experimentations
were led at different temperatures (870-1040 K) and different ratios between
nitrogen and biomass mass flowrates (7.3-18.8). These experiments show the
presence of maximum mass yield of pyro-oil near 65%wt (including HO, LO and AE)
with 20%wt of gas and 15%wt of char. This maximum is observed near 920 K,
with a ratio of 18.8, and explained by a partial conversion of biomass below
this temperature and a too high gas production above it. Above 1040 K the
gas yields are higher than pyro-oil ones; process is rather called
pyro-gasification than pyro-liquefaction.

Subcritical
hydrothermal liquefaction is achieved in a 150 ml autoclave where biomass
is introduced prior to the catch of the reactor with the locking straps (Figure 2). Thanks to a volumetric pump, nitrogen or
distilled water can be introduced into the autoclave at a desired pressure.
After inerting the reactor with nitrogen and setting the initial working pressure,
100 ml of distilled water are added into the reactor, heated by an
electric resistance. Once achieved, the working temperature is maintained a the
experiment duration time, then the reactor is cooled down by a refrigering
sytem using air and then water. Once the reactor at the ambiant temperature,
gases are sampled and analysed by GC-TCD. The reactor is opened in order to
collect the products (liquids and solids). Based on [4], pure acetone is used
to rinse the reactor and wash solids. After filtration, three phases are
obtained  : solids, aqueous phase and organic phase. Solids are placed
into a drying oven. Evaporation at reduced pressure is used to remove acetone
from the organic phase and water from the aqueous phase, in order to get respectivly
two fractions of oil, "heavy oils" (HO) and "water soluble
organics" (WSO).

Fig. 2: Subcritical hydrothermal liquefaction
installation scheme

Temperature
influence was studied within a range of 420 to 600 K, for 15 to 60 minutes
at the working temperature. Using this installation, near to 50%wt of liq-oils
(including Heavy oils et Water soluble organics) were obtained, at 570 K,
with 35%wt of gaz and 15%wt of char. The first similitude with fast pyrolysis
is the existence of an optimal temperature to reach high conversion of biomass
with a minimum of gas mass yield.

For both installations,
mass balances are based upon mass measurement of solids and bio-oils, and evaluation
of the gas composition. Thanks to the known amount of nitrogen used in those
installations (flowrate for fast pyrolysis and initial pressure for
hydrothermal liquefaction), the produced mass of gas can be determined by
proportions from the gas composition.

Ultimate
analysis and HHV-measurement are made on solids (biomass, mild-reacted biomass
and char) and fractions of pyro-oils and liq-oils. First, those analyses allow
to confirm the use of a correlation Dulong-like between HHV (High Heating Value)
 and ultimate analysis. Combined with mass balances, ultimate analysis led to
atomic balances close to 92% for C, 97% for H and 108% for O. Then, ultimate
analysis are very useful to compare products and especially fractions of
pyro-oil and liq-oil: liq-oil HO are closer to pyro-oil AE while liq-oil WSO
are closer to pyro-oil HO and LO. Ultimate analysis of solids confirm that the existence
of pyro-oils maximum yield is partly due to the partial conversion of biomass :
the composition of solids recovered at 870 K (CH_1.26O_0.57)
is closer to initial biomass one (CH_1.48O_0.72) than chars
ones obtained at 920 K (CH_0.44O_0.17) and 1040 K
(CH_0.24O_0.17).

Gas
chromatographies and H¹ NMR allow to identify major compounds of
bio-oils and/or chemical families like phenolic compounds, acids, sugars, ...
Other analyses like pH and Karl-Fischer are used to get a complete
characterization of bio-oils.

Thanks to those
results, energy balances of both reactors were made with a focus on the HHV
fraction recovered in bio-oils. Besides, properties and major components of
each kind of bio-oils allow to understand their behaviour during the HDO step,
with a focus on the required hydrogen, directly correlated to bio-oil oxygen
content.

For futur work,
those results are used as a basis for energy and exergy analysis which must be
achieved to ensure the viability of the two processes for the production of
bio-diesel. Those analysis are led thanks to a commercial simultion software,
in which different parameters can be studied like biomass humidity or bio-oil
recovery system.

[1] 
Huber, G. W., Iborra, S., and Corma, A. (2006). Synthesis of
transportation fuels from biomass : Chemistry, catalysts, and engineering.
Chemical Reviews, 106(9) :4044?4098.

[2]  Lédé, J. (2000). The cyclone : A multifunctional reactor
for the fast pyrolysis of biomass. Industrial & Engineering Chemistry
Research, 39(4) :893?903.

[3]  Lédé, J.; Broust, F.;
Ndiaye, F.-T. & Ferrer, M. (2007). Properties of bio-oils produced by biomass fast
pyrolysis in a cyclone reactor. Fuel, 86, 1800-1810

[4]  Sun,
P., Heng, M., Sun, S.-H., and Chen, J. (2011). Analysis of liquid and solid
products from liquefaction of paulownia in hot-compressed water. Energy
Conversion and   Management, 52(2) :924 ? 933.