(339f) Intermediate Temperature Conversion of Bio-Oil to Synthesis Gas for Distributed Hydrogen Production

With the
world's energy demands rapidly increasing, it is necessary to look to sources
other than fossil fuels, preferably those that minimize greenhouse emissions. 
One such renewable source of energy is biomass, which has the added advantage
of being a near-term source of hydrogen.  There are three main potential routes
to produce hydrogen from biomass thermally.  The first involves gasification of
biomass to synthesis gas (CO + H₂) at high temperatures in which
air, oxygen, or steam is used as oxidizers.  The second requires that a liquid
intermediate (bio-oil) first be produced via fast pyrolysis at relatively
moderate temperatures and then reformed.  The third route includes a variety of
high-pressure processes known as wet gasification or aqueous-phase reforming
that is especially suitable for high-moisture content biomass.  The advantage
of the second method is that it allows for the processing of biomass and the
production of hydrogen at different locations utilizing low-cost biomass
resources and existing infrastructure for hydrogen distribution while saving on
high-cost transportation of biomass and hydrogen.  This approach is especially
well suited for smaller-scale reforming plants located at hydrogen distribution
sites such as filling stations.

The goal
of the present work is the development of a process for the distributed
reforming of bio-oil.  The focus in earlier studies of bio-oil conversion to
hydrogen has been on catalytic steam reforming at high temperatures (~800 ^oC)
using a fluid bed reactor, which is not optimal for a small scale distributed
operation.  The present research focuses on the conversion of bio-oil to
hydrogen in two steps.  The first step is non-catalytic partial oxidation at
relatively low temperatures (~650 ^oC) primarily by homogeneous gas
phase chemistry.  However, some nonvolatile components of bio-oil present as
aerosols may react heterogeneously.  The products from the non-catalytic step
are passed over a packed bed of precious metal catalyst where further reforming
as well as water gas shift reactions are accomplished.  This two step approach
requires significantly lower catalyst loadings than conventional catalytic
steam reforming.  To date, catalyst screening experiments have used Engelhard
noble metal catalysts. The catalysts used for these experiments were 0.5 %
rhodium, ruthenium, platinum, and palladium (all supported on alumina). 
Experiments were performed using pure alumina as well.  Both the catalyst type
and the effect of oxygen on the residual hydrocarbons and accumulated carbon
containing particulates were investigated.  The goal is to reform and
selectively oxidize these species and catalyze the water gas shift reaction
without catalyzing methanation or oxidation of CO and H₂, thus
attaining equilibrium levels of H₂, CO, H₂O, and CO₂
at the exit of the catalyst bed.

Bio-oil (mixed with varied amounts
of methanol to reduce the viscosity) or selected bio-oil components are
introduced at a measured flow rate through the top of a vertical reactor using
an ultrasonic nozzle.  The nozzle creates a fine mist, which allows the bio-oil
to flow down the center of the reactor.  Additionally, the fine mist allows for
intimate mixing and efficient heat transfer, providing optimal conditions to
achieve high conversion at relatively low temperatures.  Generation of the fine
mist is especially important for providing good contact between non-volatile
bio-oil components and oxygen.  Oxygen and helium are also delivered at the top
of the reactor via mass flow meters with the amount of oxygen being varied to
maximize the yields of H₂ and CO and the amount of helium being
adjusted such that the gas phase residence time in the hot zone remains ~0.3
s.  The reactor effluent is quenched by a flow of 10 SLPM He which serves to
sweep the products quickly (~0.03s) to a triple quadrupole Molecular Beam Mass
Spectrometer (MBMS) for analysis.  The dilution reduces the potential problems
caused by matrix effects associated with the MBMS analysis.  The MBMS serves as
a universal detector and allows for real time data collection.  Argon is used
as an internal standard in the quantitative analysis of all the major products
(CO, CO₂, H₂, H₂O, and benzene) as well as any
residual carbon, which is determined by subsequent oxidation of carbon (monitored
as CO₂) after shutting of the feed and maintaining the oxygen/helium
flow.  The temperature is maintained using a five-zone furnace.

Catalytic experiments using a 50:50
(weight basis) bio-oil:methanol mixture at 650 ^oC with an oxygen to
carbon molar ratio (O:C) of 1.3 have identified Rh as the catalyst that brings
the system closest to equilibrium predictions (see Table 1).  Further gas phase
experiments showed that the non-catalytic step was optimal at an O:C of 1.7
(650 ^oC).  Table 2 shows a comparison of the product yields and
methanol conversion from equilibrium predictions, non-catalytic (gas phase)
experiments, and Rh catalyst experiments under these conditions.   60% yield of
CO and nearly 28 % yield of H₂ are observed without a catalyst. 
With a 0.5 % Rh catalyst inserted into the system, the CO yield decreases to
~53%, while the H2 yield increases to 74 %.  These changes are a result of
increased conversion as well as the water gas shift reaction.

Additional experiments with neat
methanol (650 ^oC: r.t. = 0.45 s) were performed and compared to
predictions of a detailed kinetic model.  The
model is a rule-based mechanism that has been used to predict both oxidation
and pyrolysis of hydrocarbons.  The premise behind the model is that three
types of free radical reactions, dissociation/recombination, hydrogen
abstraction, and b?scission/radical addition, dominate the
kinetics. 

Future plans include expanding the
temperature range (550-750 ^oC) and varying the O:C ratio at the
various temperatures to determine optimal conditions for thermal conversion of bio-oil
to synthesis gas before the catalyst stage.  The effects of adding a catalyst
to the system and the effect of bio-oil:methanol composition will also be
explored further.   The kinetic model will be extended to attempt to predict
the gas-phase kinetics of selected components of bio-oil.