(509g) Adsorptive on-Board Desulfurization of Liquid Fuels: High Efficiency in Desulfurization and Full Thermal Regeneration Via Hot Exhaust Gas

Introduction:

Desulfurization of fuels
is mandatory to fulfill environmental regulations and to protect the
environment. Sulfur levels of liquid fuels are in the range of 10 to 5000 ppm and depend on the type of
fuel and their correlated legal requirements [1]. Hydrodesulfurization (HDS) is
used for crude oil desulfurization to achieve the country-specific legal
requirements. This HDS is the most prevalent and industrially relevant
desulfurization process which operates at temperatures above 300 °C and more than 35 bar of H2 operating pressure.
The HDS efficiency decreases with increasing molecular size of the refractory
sulfur compounds and thus is inefficient for removing benzothiophene (BT) and
especially dibenzothiophene (DBT) and its derivatives.

Fuel cells are one of the
most effective tools to convert chemical to electrical energy. This technology
is not only suitable for stationary approaches but is also a promising source
of on-board electricity supply for all kinds of vehicles, ships, and aircraft.
The advantages of fuel cells are in particular higher efficiency, reduced
emissions, and lower noise generation in comparison to conventional combustion
engines. This technology becomes very attractive when on-board fuel is used to produce
hydrogen-rich syngas via reforming, which is subsequently fed to the fuel cell.
However, the sulfur threshold limit for the reformer and the fuel cell are <
50 and < 1 ppm of total sulfur, respectively [2, 3]. Consequently, fuel
cells as one of the most efficient tools for energy conversion are excluded
from using the most common types of fuels.

Adsorptive
desulfurization is a promising approach to provide the possibility of on-board
desulfurization and thus operating fuel cells with commercial fuels. Several
adsorbents have been investigated and published in recent years. However,
adsorbents regeneration is still a major issue; especially after adsorption of
dibenzothiophene (DBT). The aim of this work is to identify and prepare a
highly thermal stable adsorbent for thermal on-board regeneration via hot
exhaust gases.

Experimental section:

A silver based adsorbent
was prepared by incipient wetness impregnation of gamma–Al2O3. The impregnated
support was further dried and calcined in order to get Ag-Al2O3. This adsorbent
was characterized via different methods including N2-physisorption,
XRD, SEM, EDX, and ICP-AES. The adsorbent was further used in different
experiments including equilibrium saturation, kinetics, and breakthrough
experiments. The adsorptive desulfurization performance of Ag-Al2O3 was tested
under different conditions and fuels such as jet fuel and diesel fuel. These
fuels contained different concentrations of poly aromatic sulfur heterocycles
(PASHs) in the range of 300 to 900 ppmw of total sulfur. In situ regeneration
experiments were carried out with preheated air and simulated exhaust gas from
a solid oxide fuel cell driven auxiliary power unit.

Results and discussion:

The overall adsorption
mechanisms of PASHs were studied by relating experimental data with theoretical
principles. High breakthrough
adsorption capacities were observed for different PASHs like BT, DBT, and
4,6-dimethyldibenzothiophen dissolved in commercial fuels. The breakthrough
adsorption capacity was in the range of 0.9 to 2.3 mg/g at 20 °C and
atmospheric pressure depending on the type of PAHS. Within these
investigations, the role of the acid base interaction (S-H) on the overall
adsorption mechanism was identified. The results showed that the weak S-H
interaction is a third mechanism beside the known pi-complexation (pi-Ag)
and the direct metal sulfur interaction (S-Ag). Both pi-Ag and S-Ag interactions have a selectivity order of DBT > BT
which is caused by a higher adsorption energy of DBT in comparison to BT. The
S-H interaction is thus responsible for the shift of the final selectivity
order to BT > DBT at equilibrium saturation conditions as confirmed by experiments.
This result is further confirmed by density functional theory (DFT)
calculations reported in the literature as well as by the hard and soft acid
base (HSAB) theory proposed by Pearson. Consequently, pi-Ag and S-Ag
interactions are the main adsorption mechanisms at breakthrough. In these two
interactions, the silver cation (soft acid) interacts with the thiophenic ring
or directly with the pi lone-pair of electrons of the sulfur atom (soft base) [4].

Additional equilibrium
saturation experiments were carried out at -10 and 60 °C. The adsorption
temperature showed no influence on the equilibrium saturation capacity in the
case of DBT. For BT, the equilibrium saturation capacity at -10 °C stayed at
the same level as at 20 °C. At 60 °C, the equilibrium saturation of BT
decreased to the level of DBT and thus confirmed that the weak S-H interaction
is only relevant for BT at equilibrium stage. At increased temperatures (60 °C)
the S-H interactions is to week to contribute in the overall adsorption
mechanism of BT. The stronger adsorption of DBT in particular makes it
difficult to achieve full thermal regeneration without thermal destruction of
the adsorbent. These are important findings in order to develop highly
efficient regeneration strategies.

The lack of solvents
requires thermal regeneration of the adsorbent in the case of on-board
desulfurization units. This is the reason, why we studied the desorption
mechanisms of PASHs from Ag-Al2O3 in the presence of air; and in particular of N2,
O2, CO2, and H2O. In these investigations, thermal regeneration was carried out
in a preheated gas mixture with increasing temperature. A final temperature of
450 °C was used in the first regeneration experiments. This temperature was
sufficient for full thermal regeneration in flowing air after adsorption of BT
on Ag-Al2O3. The same experiment was carried out with DBT and showed only 60%
recovery of the breakthrough capacity after thermal regeneration. This result
confirms the stronger adsorption of DBT in comparison to BT. Additional
regeneration experiments were carried out at a final temperature of 525 °C. The
results showed full thermal regeneration in flowing air after adsorption of
DBT. This is an excellent result as full thermal regeneration after DBT
adsorption has not been reported so far. The regenerated Ag-Al2O3 was further
analyzed and showed no thermal induced destruction.  

One problem of thermal
regeneration is the source of hot regeneration gas as electrical heating would
be inefficient and cost-intensive. To overcome this drawback, hot exhaust gases
were identified as a source of hot regeneration gases. This is the reason why
regeneration experiments were performed by using hot off-gas from a solid oxide
fuel cell based auxiliary power unit; this gas is referred to as APU. The flow
sheet of this novel regeneration strategy is illustrated in Fig. 1 (right).

Fig. 1 Flow sheet diagram
of novel regeneration strategy using hot APU off-gas for thermal regeneration
of the adsorbent (right) and simplified scheme of the overall desorption
mechanism of PASHs (left).

Excellent results were
obtained with APU as regeneration gas. In these experiments, 100% regeneration
was achieved over 5 cycles at a final temperature of 500 °C. The identified
desorption mechanisms (Fig. 1 left) showed that CO2 in the regeneration gas has
no negative influence on the overall regeneration experiment as it desorbs
already at temperature above 100 °C. In addition, H2O was identified to have a
positive influence on the thermal decomposition of adsorbed PASHs. This
positive effect was proven by achieving full thermal regeneration at a final
temperature of 450 °C with increased H2O content in the regeneration gas (12
vol.%). These findings even highlight the possibility to use hot exhaust gas
from a combustion engine as regeneration gas in a hybrid vehicle based on low
temperature fuel cells.

Conclusion:

Adsorptive desulfurization
of liquid fuels is mandatory to provide the possibility to operate fuel cell
with commercial fuels. Ag-Al2O3 was identified as excellent adsorbent for
on-board desulfurization units. Investigations on the overall adsorption
mechanism showed that the silver cations are the main active adsorption sites.
Adsorption of DBT via pi-Ag and S-Ag interactions is stronger in comparisons to
BT. Consequently, regeneration of these silver sites after adsorption of DBT is
essential to achieve full regeneration. 

Hot exhaust gas was
identified as ideal regeneration gas for thermal in situ and on-board
regeneration. The identified desorption mechanism showed that CO2 has no
negative influence on the regeneration performance. Full thermal regeneration
was reported for the first time after adsorption of DBT at a final temperature
of 500 °C over 5 cycles. The final regeneration temperature could even be
reduced to 450 °C by increasing the H2O content in the regeneration gas. All
these findings showed excellent adsorption and regeneration performances of
Ag-Al2O3 under a wide range of conditions including different fuels. Full
on-board regeneration via hot exhaust gas, as reported, highlights the
possibility to operate fuel cell with commercial sulfur containing fuels and
thus achieve high efficiency in energy conversion combined with lowest
emissions.

References:

[1] A. Samokhvalov, B.J.
Tatarchuk, Catal. Rev. 52 (2010) 381-410.

[2] X. Xu, S. Zhang, P.
Li, Y. Shen, Fuel Process. Technol. 124 (2014) 140–146.

[3] C. Hulteberg, Int. J. Hydrogen
Energy 37 (2012) 3978-3992.

[4] R. Neubauer, M. Husmann, C.
Weinlaender, N. Kienzl, E. Leitner, C. Hochenauer, Chemical Engineering Journal
309 (2017) 840-849.