Combustion Characteristics in a Small-Scale Reactor with Catalyst Segmentation and Cavities Yueh-Heng Li 1, Guan-Bang Chen 2, Fang-Hsien Wu 1, Tsarng-Sheng Cheng 3, Yei-Chin Chao 1 Speaker ： Fang-Hsien Wu 1 Department of Aeronautics and Astronautics, National Cheng Kung University Tainan, Taiwan, ROC 2 Energy Technology and Strategy Research Center, National Cheng Kung University Tainan, Taiwan ROC 3 Department of Mechanical Engineering, Chung Hua University Hsinchu, 300, Taiwan, ROC The 34 nd International Symposium on Combustion, Warsaw University of Technology, Poland 2012/07/29 ~ 2012/8/3

Introduction  In the miniaturizing process of a reactor size Increasing surface to volume ratio (S/V) increased heat loss to the wall the possibility of radical termination by wall reactions. Characteristic dimension < 1 mm enhanced heat loss to the wall homogeneous reaction are quenched  How to overcome the shortcomings Quench-resistant fuel => hydrogen (Norton et al. 2005, Yuasa, 2005) Thermal recuperating concept => reduce heat loss (Sitzki et al. 2001, Ronny et al. 2002, Federici et al. 2009) Catalytic surface => reduce radical depletion (Norton et al. 2004, Chao et al. 2004, Karagiannidis 2007)

Catalyst combustion  Catalytic combustion involves the coupling of homogeneous reaction and heterogeneous reaction.  Hetero-/homogeneous interaction Promotion of gas-phase reaction due to the catalytically induced exothermicity. Inhibition of gaseous reaction due to near wall catalytic fuel depletion.

Motivation and Object  In general, we attempt to extend the stable operation range of the small reactor, especially in high or low inflow velocity. ISSUE  Use a novel catalyst bed design of catalyst segmentation with cavities to extend the operation range and to study H 2 fuels reactions in a catalytic micro-reactor. Catalyst segmentation － providing sufficient chemical radical and catalytically induced exothermicity. Catalyst segmentation with Cavity － to provide a low-velocity field to stabilize the gas reaction.

Experimental apparatus and method  Reactor : 82 mm in length 20 mm in width 5 mm in height  Aluminum oxide ceramic stick : 60 mm (l) × 3 mm (w) × 2 mm (h) Catalyst layouts : 8 segments (each with 2 mm catalyst) 4 segments (each with 4 mm catalyst) 2 segments (each with 8 mm catalyst) 16 mm catalyst without segmentation Cavity : 2 mm (l) × 3 mm (w) × 1 mm (d) 600K

Experimental apparatus and method  Reactor : 82 mm in length 20 mm in width 5 mm in height  Aluminum oxide ceramic stick : 60 mm (l) × 3 mm (w) × 2 mm (h) Catalyst layouts : 8 segments (each with 2 mm catalyst) 4 segments (each with 4 mm catalyst) 2 segments (each with 8 mm catalyst) 16 mm catalyst without segmentation 600K

Experimental apparatus and method  Reactor : 82 mm in length 20 mm in width 5 mm in height  Aluminum oxide ceramic stick : 60 mm (l) × 3 mm (w) × 2 mm (h) Catalyst layouts : 8 segments (each with 2 mm catalyst) 4 segments (each with 4 mm catalyst) 2 segments (each with 8 mm catalyst) 16 mm catalyst without segmentation 600K Catalyst segmentation with cavity 8 segments (each with 2 mm catalyst) 7 cavities : 2 mm (l) × 3 mm (w) × 1 mm (d)

Numerical model and chemical mechanism  Reactor : h = 1mm, L = 65mm, Cavity : w = 2mm, d = 1mm  Total catalyst length = 16mm  Laminar flow (T in = 300K)  CFD-ACE +, Homogeneous reaction mech. (Miller and Bowman), Heterogeneous reaction mech. (Deutschmann et al.)  Non-uniform grids. Outlet Platinum Quartzs Al 2 O 3 Cavity Uniform Inlet (L) 65 mm 1 mm 2 mm T w,up (x) T w,down (x) (h) 1 mm w d

 Three catalyst layouts of a small-scale reactor were studied: Single catalyst, (16mm Pt length) Catalyst segmentation with inert wall, (2mm Pt sec. ×8, 4mm Pt sec. ×4, 8mm Pt sec. ×2) Catalyst segmentation with cavities. (2mm Pt sec. ×8 ) ISSUE

Results and discussion (a) The lack of sufficient heat and fuel concentration in the upstream residual gas could not sustain a homogeneous reaction behind the catalyst. Flow velocity: 20 m/sec Equivalence ratio: 0.6  Effects of catalyst segmentation (b) (c) (a) (d) (16mm Pt sec. ×1) (8mm Pt sec. ×2) (4mm Pt sec. ×4) (2mm Pt sec. ×8) Exposure time: 1/40 s -1

Results and discussion (b)&(c) The gas reaction is sustained in the non-catalytic walls adjacent to catalyst segments, where the gas inherits prior catalytically induced exothermicity and intermediate radicals. Flow velocity: 20 m/sec Equivalence ratio: 0.6  Effects of catalyst segmentation (b) (c) (a) (d) (16mm Pt sec. ×1) (8mm Pt sec. ×2) (4mm Pt sec. ×4) (2mm Pt sec. ×8) Exposure time: 1/40 s -1

Results and discussion (d) The upstream catalyst does not provide sufficient catalyst bed to produce catalytically induced exothermicity for supporting the downstream homogeneous reaction. Flow velocity: 20 m/sec Equivalence ratio: 0.6  Effects of catalyst segmentation (b) (c) (a) (d) (16mm Pt sec. ×1) (8mm Pt sec. ×2) (4mm Pt sec. ×4) (2mm Pt sec. ×8) Exposure time: 1/40 s -1

Results and discussion A sufficient length of catalyst segment is an important parameter since it can release sufficient heat to sustain homogeneous reaction in the downstream for preventing thermal and radical quenching on the non-catalytic wall. Flow velocity: 20 m/sec Equivalence ratio: 0.6  Effects of catalyst segmentation (b) (c) (a) (d) (16mm Pt sec. ×1) (8mm Pt sec. ×2) (4mm Pt sec. ×4) (2mm Pt sec. ×8) Exposure time: 1/40 s -1

Results and discussion  Hydrogen consumption closes to the catalyst surface.  Few OH radical appears in the vicinity of non-catalytic wall adjacent to catalyst segments. (b) H2H2 OH (a) (c) Flow velocity: 20 m/sec Equivalence ratio: 0.6  Effects of catalyst segmentation 1.7E-15 1E E-2 0.1E-2

Results and discussion Heterogeneous reaction  Effects of catalyst segmentation 8 segments with 2mm catalyst Flow velocity: 20 m/sec Equivalence ratio: 0.6  Kinetically controlled : surface concentration > 95%  Mass transfer controlled : surface concentration < 5%  The length of catalyst is not long enough to develop into mass- transfer-control region. (Pfefferle, 1977)

Results and discussion  Effects of catalyst segmentation  Some fluctuation in fuel concentration distributions appear in non- catalytic section, where fuel has no heterogeneous consumption but concentration accumulates due to diffusion from main stream. Heterogeneous reaction 8 segments with 2mm catalyst Flow velocity: 20 m/sec Equivalence ratio: 0.6

Results and discussion  Heterogeneous reaction and fuel conversion are completed behind the fourth catalyst segment.  (c) Induced homogeneous reaction behind the third catalyst segment accelerates hydrogen conversion. Flow velocity: 20 m/sec Equivalence ratio: 0.8  Effects of catalyst segmentation (a) (b) (c) H2H2 OH 8.5E-2 0.5E-2 2.4E-2 0.2E-2

Results and discussion  Effects of catalyst segmentation  Heterogeneous reaction takes place in the upstream catalyst segments  Downstream catalyst segment inherits thermal energy and thus induces catalytically supported homogeneous combustion. Heterogeneous reaction Homo- and Heterogeneous reaction 8 segments with 2mm catalyst Flow velocity: 20 m/sec Equivalence ratio: 0.6

Results and discussion (a) A wider range for hetero- and homogeneous reactions in the case of 4 catalyst segments and 8 catalyst segments. (b) The range for hetero- and homogeneous reactions become wider in the case of eight segments. (A) ER = 0.6 and (B) ER = 0.8.  Effects of catalyst segmentation

Results and discussion 15 to 30 m/s Flame anchoring position recedes from the upstream segment to the downstream segment. 35&40 m/s Gas-phase reaction could not sustain and only surface reaction exists. Equivalence ratio: 0.8 Segment catalyst with cavities V=15 m/s V=20 m/s V=25 m /s V=30 m/s V=35 m/s V=40 m/s  Effects of catalyst segmentation

Results and discussion Hydrogen combustion is strongly related to fuel concentration, inflow velocity, and catalyst length, which influence the thermal balance between catalytically induced heat release and heat losses to the wall. Equivalence ratio: 0.8 Segment catalyst with cavities V=15 m/s V=20 m/s V=25 m /s V=30 m/s V=35 m/s V=40 m/s  Effects of catalyst segmentation

Results and discussion (b)&(c) Heterogeneous reaction at the first catalyst segment delivers radicals and heat to the first cavity, and promotes the homogeneous reaction in the cavity. (d) Cavities enhance the stabilization of homogeneous reaction by providing a low-velocity region. Flow velocity: 20 m/sec Equivalence ratio: 0.4  Effects of catalyst segmentation with cavities H2H2 OH (b) (c) ( a) (d) OH 1.1E E-3 0 0

Results and discussion  Effects of catalyst segmentation with cavities  Cavity not only provides a low-velocity region for flame stabilization, but also supplies heat to enhance the catalytic combustion. Heterogeneous reaction Homo- and Heterogeneous reaction 8 segments with 2mm catalyst and cavity Flow velocity: 20 m/sec Equivalence ratio: 0.4

Results and discussion It appears that the flame is anchored at the first cavity for V  15 m/s. The unreacted hydrogen could induce homogeneous reaction in the downstream cavities as the inflow velocity is increased. Equivalence ratio: 0.4  Effects of catalyst segmentation with cavities

Results and discussion Heterogeneous reaction in the upstream catalyst segment. Homogeneous reaction in the upstream cavity. Experimental and numerical results : cavities appreciably extends the stable operating range in a wide range of inflow velocities. 2mm Pt x8 (A) without (B) with cavities  Effects of catalyst segmentation with cavities

 Heterogeneous reaction occurred in the prior catalyst segment generates active chemical radicals and catalytically induced exothermicity; a homogeneous reaction is subsequently induced and anchored in the following non-catalytic wall or cavity.  In comparison with single catalyst, multi-segment catalyst exhibits better performance. However, the better segmentation is strongly related to inflow rate and fuel concentration.  Cavities can collect radicals and hot gas from upstream and provide a low-velocity region to sustain and anchor gas-phase reactions in a wide range of inflow velocities.  In the case of catalyst segmentation with cavity the interaction between hetero- and homogeneous reaction is more like cooperation not competition. Conclusions

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Surface reaction vs gas reaction  Exits temperature : Gas reaction (Homogeneous reaction) >1000K Surface reaction (Heterogeneous reaction) ≤600K Non reaction≈300K

Catalytic combustion  Heterogeneous reaction Kinetic control Mass transfer control region Transition to mass transfer control Surface mass fraction (%) >95% Kinetic control < 5% Mass transfer control region (Pfefferle, 1977)