H2-assisted Auto-Thermal Combustion of CH4 over Bi-functional Pt-LaMnO3 Honeycombs
P.S. Barbato1, G. Landi2, R. Pirone2, G. Russo1 and A. Scarpa1
Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, P.le Tecchio 80, 80125 Napoli (I)
2
Istituto di Ricerche sulla Combustione, CNR, P.le Tecchio 80, 80125 Napoli (Italy)
Topic: 5 - Structured catalysts and reactors for innovative environmental, automotive and energy applications
1
Fuel Conversion, %
Temperature, °C
Structured catalytic reactors have been widely studied as alternative systems to traditional homogeneous combustion
chambers for gas turbine applications and radiant heaters for both domestic and industrial applications. Despite of the
great interest, commercialization of catalytic reactors for power generation was limited due to the drawbacks related to
constrained operation windows (especially in terms of operating temperature), catalyst instability and high costs of
active phases based on noble metals [1]. Recently catalytic combustion has been proposed as the best route for the
development of micro-combustors [2]; as a matter of fact, power generation in the order of 10-3-1000 W, related to
several users as laptops, portable phones, MEMS, auxiliary systems for automotive applications, could be obtained by
combustion in micro-devices replacing batteries. Moreover, microscale combustion will assume a prime role to
decentralize future energy efforts for supplying the heat necessary to drive endothermic reactions, such as reforming
and dehydrogenation, generate steam, etc. Up to date, the most investigated catalysts for micro-combustion are
supported noble metals, mainly using hydrogen or very volatile hydrocarbons, such as propane and butane, as fuels [3].
On the contrary, few attention has been devoted to methane combustion at micro-scales; due to its lower activity with
respect to other hydrocarbons, catalytic reactors must operate at temperatures higher than 700°C, strongly supporting
the use of transition metal mixed oxides instead of more volatile and unstable noble metals [4]. Moreover, methane
reactivity could result increased by the addition of hydrogen; in effect, the
1
2
3
use of CH4-H2 mixtures as fuel could represent an efficient way for
Tad
hydrogen to penetrate the energy market, as well as a tool to promote
1000
T2
higher combustion efficiency and lower environmental impact. We recently
proposed bi-functional catalysts based on Pt and LaMnO3 for micro800
T3
combustion of propane [5], showing that it is the best compromised
T1
600
catalyst to burn CH4-H2 mixtures, since Pt is very active for hydrogen and
perovskite is more active for methane. In this work the behavior of such a
(a)
400
Pt- LaMnO3 catalyst has been evaluated in the combustion of CH4, with
100
and without H2 addition, under auto-thermal conditions. Commercial
xH
substrate (cordierite honeycomb, 900 cpsi) has been coated with a thin
80
layer of catalyst constituted by Pt/LaMnO3 supported onto La-stabilized 60
Al2O3 (1 wt. % Pt and 20 wt. % LaMnO3 with respect to the active layer).
40
The effects of GHSV, equivalence ratio, pre-heating temperature and H2
xCH
20
addition on the stability of CH 4 combustion were evaluated. Measures were
(b)
conducted changing pre-heating temperature and recording the transient
0
0
40
80
120 160 200 240
response of the system, monitoring gas phase composition and temperature
time, min
profile. Light-off of methane combustion at  = 0.6 occurs at about 530°C
Fig. 1 – Light-off of CH4-H2 mixture over
pre-heating temperature, but cooling the ignited reactor down to 330°C
Pt/LaMnO3 structured catalyst
does not result in significant reduction of methane conversion, leading to
the typical hysteresis of highly exothermic reactions [1]. Further cooling
produces combustion quenching. Both extinction and blowout phenomena have been detected, depending on the couple
pre-heating temperature/GHSV. Increasing fuel concentration ( = 0.7) enlarged stability limits, even suppressing
blowout phenomena in the range of GHSV investigated. Temperature profiles, measured along the catalyst, evidenced
the unavoidable presence of heat losses, whose relative importance depended on the power generated, i.e. on GHSV. H2
addition provided a positive effect on light-off temperature; as reported in fig. 1, light-off occurs at about 450°C,
approximately 80°C lower than the corresponding value for CH4 ignition. This benefit appears related to a thermal
rather than a chemical effect; in fact, H2 is fully converted in the first part of the reactor (fig. 1.b), due to the Pt activity,
and generates heat, increasing surface temperature and at the same time improving CH4 combustion kinetics.
In conclusion, it is possible to enlarge operation window of structured Pt/LaMnO3 based catalysts for micro-combustion
of CH4 and CH4-H2 mixtures by increasing GHSV and equivalence ratio. H2 addition shows positive effects on lightoff, while allows to expand stability limits only at high GHSV.
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References
[1] R. E Hayes, S. T. Kolaczkowski; “Introduction to Catalytic Combustion”. Gordon and Beach Science Pub. (1997)
[2] C. Fernandez-Pello, Proc. Combust. Inst., 29 (2002) 883;
[3] D. G. Norton and D. G. Vlachos, Proc. Combust Inst, 30 (2005) 2473;
[4] S. Cimino, L. Lisi, R. Pirone, G. Russo, M. Turco, Catal. Today, 59 (2000) 19
[5] Scarpa et al., Chemical Engineering Journal (2009) submitted