(55a) Nano-Structured Sorbents for Desulfurization of Biomass-Derived Syngas

        Thermochemical
conversion of biomass residue to syngas has recently gained considerable
attention worldwide. Gasification of biomass, a thermochemical conversion, is
particularly attractive as it can operate with a wide range in the quality and
consistency of the feedstock, thus increasing the available feedstock, and
reducing overall costs. Moreover, by achieving almost complete depolymerization of the source biomass to useful gaseous
products like syngas, gasification is the only established technology that can
make the most of the energy stored in the raw biomass.

        However, on
gasification, along with the syngas, gaseous impurities like sulfur containing
species (H₂S, COS), ammonia, alkali oxides, halides etc are also
generated due to the volatile contaminants present in the biomass residue. Many
downstream processes, like Fischer Tropsch synthesis,
Solid Oxide Fuel Cells and methanol production, use catalysts that have little
tolerance with gaseous contaminants. Transition
metal / metal oxide catalysts, typically employed for such
value-addition processes, are especially vulnerable to sulfur containing gaseous
species like hydrogen sulfide. Therefore, before this raw syngas can be used
for different downstream applications, it needs to be cleaned. Among the different processes used for H₂S
removal, wet amine scrubbing has been the most popular. However,
such wet processes require comparatively low operating temperatures (35 - 55⁰C). The
cleaned syngas has to be subsequently reheated for the downstream processes
(300 - 800⁰C). Such consecutive
cooling and heating can cause considerable thermal losses.

        To avoid such
energy losses, it is important to desulfurize the gas stream at suitably high
temperatures. A solid-phase sulfur-sorbent material that has a high reactivity,
good structural stability, and easy regenerability at such high temperatures
can provide such an alternative.  In past
work, different bulk sorbents with different chemical and structural properties
have been tried for high temperature desulfurization.  However, in spite of numerous efforts to
modify chemical and compositional properties, only a limited success has been
achieved.  It is due to sorbent's failure
in meeting one or more of the aforementioned criteria. For instance, in the
past, it has been demonstrated that due to mass transfer limitations, if a bulk
sorbent gets completely sulfided, it is almost impossible to achieve complete
regeneration afterwards, regardless of process's favorable thermodynamics and
chemical kinetics. This incomplete regeneration, partly due to mass transfer
limitation and subsequent loss of sorbent's surface area, leads to
underutilization of the sorbent material, which is typically a transition or
rare earth metal oxide. To overcome these limitations, it seems necessary that
one needs to go beyond doing modifications in sorbent chemical composition.

        One option that
has been little explored involves integration of tailored design and morphology
of the sorbent with the process's favorable kinetics. We will attempt to
describe how the nanostructuring of sorbent can help
in achieving improved sorbent performance for a process involving cyclic
regeneration. Since the reaction time has been shown to be comparatively short,
the approach here is to come up with a sorbent design that facilitates short
contact time so that deep sulfidation of sorbent can be avoided without
affecting the sulfur removal capacity. As this would require high gas velocity
along with sorbent's high specific surface area, the use of conventional bulk
sorbent will lead to incomplete regeneration and significant pressure drop.
Sorbents with tailored designs and sizes, however, can help in overcoming such
limitations. The criteria for such designs should be to maximize available
specific surface area along with short diffusion lengths. Nanosizing
of sorbent seems as a good alternative. 
However, sorbent in the form of nanopowders
tend to aggregate and cause mass transfer limitations similar to bulk sorbents.
On the other hand, nanostructures having high aspect ratio, like nanofibers,
can remain isolated; thus, potentially providing a more suitable framework for
carrying out frequent cyclic sulfidation-regeneration operation. These high
aspect ratio nanoscale structures can not only retain
the properties from their bulk form such as favorable thermodynamics, chemical
affinity etc., but they also tend to develop useful properties due to highly
anisotropic geometry and confined grain size. Because of the confinement of the
grain size and short contacting time, nanofibers will tend to limit the large
volume changes and accompanying grain boundary collisions which are typical
during repeated sulfidation/regeneration. Thus, it is expected that the use of nanostructured sorbent will not only lead to high specific
surface area and improved mass transfer, it can also
lead to an improved mechanical behavior during high temperature cyclic
operation.

        This work will
present the results from the investigation of such high-aspect ratio nanofibers
that were synthesized from rare-earth and transition metal oxides, namely, La₂O₃
CeO₂, and Zn₂TiO₄ using the process of
electrospinning. Salt solutions of the respective metals with PVP (Mw ~
1300000) as the binding polymer were used to prepare the corresponding
sol-gels, which were then used as the precursor solutions for electrospinning
of the fibers. For PVP/CeO₂ sol, the fibrous mats having an average
fiber diameter of 497 nm were obtained (Fig-1). These fibers were then sintered
at 600⁰C for 4.5 hrs to obtain
metal oxide fibers, free from the polymeric binder. The average diameter for
sintered fibers reduced to 300 nm (Fig-2). Fibers were subsequently tested for
their reactivity, micro-structural changes, regenerability and high temperature
durability during cyclic sulfidation-regeneration operation. After each cycle,
the difference in the performance was characterized by measurements of specific
surface area (BET), changes in surface morphology (SEM, TEM), sulfidation
kinetics (TGA), crystal structure changes (XAFS, XRD) and variations in surface
composition (XPS). The results from these tests can help in estimating the
extent to which nanostructuring of sorbents can
facilitate increase in sorbent life.