(583fa) Steam Methane Reforming Coupled With Catalytic Combustion of SOFC Stack Flue Gas in An Integrated Hep Reactor

Stack flue
gas emitted from solid oxide fuel cell (SOFC) using methane fuel contains
combustible components (H₂, CO and CH₄) and oxidation
products (CO₂, H₂O) as anode off gas, and oxygen as well
as nitrogen as cathode off gas at high temperature [1]. The heat produced by
combustion of the flue gas can be utilized in an integrated heat-exchanger platform
(HEP) type reactor for steam methane reforming (SMR) [2].

SMR coupled
with catalytic combustion of SOFC stack flue gas in the HEP reactor is favorable
with respect to maximum utilization of energy [3]. Among noble metal catalysts,
Pd-Rh is most common for SMR [4] and Pt-Pd is the best choice for methane combustion
[5]. Moreover, metal foam type catalyst is beneficial for application in the HEP reactor [6].

Fig. 1: HEP reactor integrated with
SOFC stack.

Pt-Pd (2:1)
catalyst for catalytic combustion and Pd-Rh (7:1) catalyst for SMR were
prepared by depositing noble metal/alumina
washcoat on stainless steel foam support (1.1 mm thickness). Conventional Ni/Al₂O₃
(cylindrical) and Ru/Al₂O₃ (spherical) catalysts were
used as received from vendors for comparison with Pd-Rh/metal foam catalyst for
SMR in a tubular reactor.

The tubular
reactor (diameter-2 cm, length- 33 cm) used for the comparison experiment was equipped
with temperature-programmed furnace and thermocouples to measure the catalyst
bed temperature. The HEP reactor (length- 30 cm, width- 3.7 cm, height- 3.35
cm) had 9 channels assigned for SMR and remaining 9 channels for catalytic combustion
into which the metal foam catalyst strips (length-23 cm, width-3.3 cm,
thickness-0.11 cm) were inserted. 0.26 mm channel wall thickness facilitated
heat transfer from combustion side to SMR side. The HEP reactor was equipped
with temperature-programmed furnace, preheaters, and 10 thermocouples to
measure the temperature at various locations in the HEP reactor.

Simulated
combustion and reforming gases flowing in countercurrent direction were
supplied from the gas cylinders and micropumps for water source as shown in Table
1. Gas mixtures were analyzed by GC equipped with a TCD detector. Catalyst performance was evaluated by measuring the
activity and stability for SMR and by
measuring the activity for catalytic combustion.

Table 1

Flow rate,
composition and space velocity of the gases for catalytic combustion and SMR

Gas flow Component	Combustion side in HEP, ml/min (%) [Gas-1]	Combustion side in HEP, ml/min (%) [Gas-2]	SMR in tubular reactor, ml/min (%) [Gas-3]	SMR side in HEP, ml/min (%) [Gas-4]
CH4	15.3(1.4)		80(28.6)	80(28.6)
H2	66.6(6.1)
CO	8.25(0.75)
CO2	44.75(4.1)
Air	372(34)
Water vapor	277(25.3)		200(71.4)	200(71.4)
N2	311(28.4)	1095(100)
Total	1095(100)	1095(100)	280(100)	280(100)
GHSV(h-1)	944	944	2000	337

 

Fig.
2: SMR activity of Pd-Rh/metal foam catalyst in the tubular reactor (Gas-3
flow)

 

At temperatures higher than 923K,
SMR activity of Pd-Rh/metal foam catalyst was better than commercial Ni/Al₂O₃
and Ru/Al₂O₃ catalysts in the tubular reactor at GHSV
2000h^-1 and S/C ratio 2.5 as shown in Fig. 2. At 1023K, methane
conversion was 89.4% for Ru/Al₂O₃ catalyst and 96.7% for
Pd-Rh/metal foam catalyst.

Pd-Rh/metal foam catalyst also
provided higher CO and H₂ production than commercial SMR catalysts.
At 1023K, H₂/CO ratios for the metal foam, Ni and Ru based catalysts
were 6.34, 5.82 and 7.30, respectively, while CO selectivity was 0.59, 0.6 and
0.52, respectively.

The Pd-Rh/metal foam catalyst showed
good SMR stability: 24 h stability test at 1073K showed no degradation in catalyst
performance. Changing S/C ratio from 2 to 4 did not affect the catalyst
performance significantly.

200h stability test is shown in Fig
3. The metal foam catalyst exhibited an increase in activity for first 72 hours
at 1023K for attaining the stable operation condition. Ni catalyst and Ru
catalyst were stable over the entire period at 1023K and at 1000K respectively.

 

 

 

Fig. 3: Stability of SMR catalysts
vs. on-stream operation hours in Gas-3

 

Pd catalysts tend to sinter or
deactivate at temperatures over 1073K [2] and the Pd-Rh/metal foam catalyst exhibited
quite competitive SMR activity at relatively lower temperature as compared with
the conventional Ni and Ru catalysts, so SMR operation temperature for HEP
application was selected as 983K.

 

Fig. 4: SOFC stack flue gas
combustion over Pt-Pd/metal foam catalyst in HEP reactor

 

In the next stage, SMR and combustion
were tested over the metal foam catalysts in the HEP reactor.

The activity of Pt-Pd/metal foam
catalyst for SOFC stack flue gas combustion is shown in Fig. 4. Over the metal foam
catalyst, H₂ and CO were combusted completely at room temperature
and below 373K, respectively. So the activity is mainly focused on combustion
of CH₄ component: methane combustion initiated around 500K and the
complete combustion was observed around 873K. At HEP operation temperature
983K, SOFC stack flue gas was completely combusted over Pt-Pd/metal foam
catalyst at GHSV 944h^-1. SOFC stack flue gas combustion performed
for 48 h at 983K in HEP reactor exhibited no degradation in the catalyst performance.

 

Table 2: Gas flow rate and
composition in both sides of HEP reactor

 

Component	Combustion side, ml/min (%)		SMR side, ml/min (%)
	Inlet	Outlet	Inlet	Outlet
CH4	15.3(1.4)		80(28.6)	16(5.7*)
H2	66.6(6.1)			213(75.4*)
CO	8.25(0.75)			27.5(5.7*)
CO2	44.75(4.1)	66.2(9.7*)		26(9.2*)
Air	372(34)	39(5.7*)
Water vapor	277(25.3)	not measured	200(71.4)	not measured
N2	311(28.4)	576.8(84.6*)
Total	1095(100)	682*(100*)	280(100)	282.5*(100*)
GHSV(h-1)	944		337

*flow rate and composition on dry
basis

 

In Table 2 gas flow rate and
composition are listed for both sides of the HEP reactor operated at 983K. At
this HEP operation temperature, 80% methane conversion was obtained from SMR
over Pd-Rh/metal foam catalyst at GHSV 337 h^-1. The unconverted
methane emitted from SMR side constituted a major combustible component of
catalytic combustion feed, thereby making SMR coupled with catalytic combustion
in HEP reactor approach the autothermal operation as closely as possible.

 

 

Fig. 5: Temperature profile of SMR
coupled with SOFC stack flue gas combustion in HEP reactor

The temperature profiles in HEP
reactor for SMR coupled with SOFC stack flue gas combustion is shown in Fig. 5.
In the first plot SMR was performed with substantial heat of reaction supplied
from catalytic combustion side by heat transfer within the HEP reactor, and the
extra power input from the electrical furnace was 2.73W. When the combustion
side gas flow was replaced by nitrogen only, the temperature profile did not
change significantly but the extra power requirement by furnace increased to
26.24 W. This provides the evidence that the energy required for SMR was supplied
from SOFC stack flue gas combustion in the HEP reactor. When water vapor was
removed from SOFC stack flue gas feed, the furnace power requirement was reduced
to 0.39 W, quite close to autothermal HEP operation.

 

The Pd-Rh/metal foam catalyst used for
SMR was characterized by SEM, BET area and pore size measurement.

 

SEM images of the SMR catalyst used
at 1073K for 96h with S/C ratio 2.5 are shown in Fig. 6.

Fig. 6: SEM images of fresh and used
Pd-Rh/metal foam catalyst for SMR conducted for 96h at 1073K

 

From the images it can be found that
there was no major change in catalyst structure after using it for SMR at high temperature,
proving the high thermal stability of Pd-Rh/metal foam catalyst. However,
because of high reaction temperature, carbon was deposited on the catalyst,
which could be removed by switching the reaction from SMR to SOFC stack flue
gas combustion over the used Pd-Rh/metal foam catalyst.

The SMR
performance of Pd-Rh/metal foam catalyst was compared with conventional Ni and
Ru catalysts, and HEP reactor operation integrating SMR with catalytic
combustion was studied in this work. The SMR activity is temperature dependent and
the required energy was transferred from combustion side of the HEP reactor
satisfactorily. The results show the potential of metal foam catalysts as well
as autothermal application of the HEP reactor for integrating SOFC fuel reforming
and stack flue gas combustion.

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