Extinction Limits of Catalytic Combustion in Microchannels Kaoru Maruta, Koichi Takeda Akita Prefectural University, Honjyo, Akita, Japan Jeongmin Ahn, Kevin Borer, Lars Sitzki, Paul D. Ronney University of Southern California, Los Angeles, CA USA Olaf Deutschmann University of Heidelberg, D Heidelberg Germany SIAM Conference on Numerical Combustion Sorrento, Italy, April 8-10, 2002 Supported by U.S. Defense Advanced Research Projects Agency (DARPA)

Motivation Advances in portable electronic devices require power sources with higher energy/mass than batteries Micro-combustors are a possible solution - hydrocarbon fuel energy/mass ≈ 100x lithium-ion batteries Development of micro-scale combustors challenging, especially due to heat losses Catalysis may help - generally can sustain catalytic combustion at lower temperatures than gas-phase combustion - reduces heat loss and thermal stress problems Higher surface area to volume ratio at small scales beneficial to catalytic combustion

Motivation Experiments in scaled-up “Swiss-roll” heat recirculating burners with and without Pt catalyst in center show –Dual limits - Low velocity heat loss, high velocity “blow off” –Catalyst vs. non-catalyst (reversal of limits) –Lean limits richer than stoichiometric (!) (catalytic only)

Motivation Limit temperatures for catalytic combustion are lower than non-catalytic combustion, even when limit fuel concentration is higher with catalytic combustion! Fuel % and temperature at limit indicates Pt catalyst inhibits combustion under some conditions!

Objectives Model interactions of chemical reaction, heat loss, fluid flow in simple geometry at small scales Examine effects of –Heat loss coefficient (H) –Flow velocity or Reynolds number ( ) –Fuel/air AND fuel/O 2 ratio - conventional experiments using fuel/air mixtures might be misleading because both fuel/O 2 ratio and adiabatic flame temperatures are changed simultaneously!

Model Cylindrical tube reactor, 1 mm dia. x 10 mm length FLUENT + detailed catalytic combustion model (Deutchmann et al.) Gas-phase reaction neglected - not expected under these conditions (Ohadi & Buckley, 2001) Thermal conduction along wall neglected Pt catalyst, CH 4 -air and CH 4 -O 2 -N 2 mixtures

Results - fuel/air mixtures “Dual-limit” behavior similar to experiments observed when heat loss is present a b c

Results - fuel/O 2 /N 2 mixtures Ratio of heat loss to heat generation ≈ 1 at low-velocity extinction limits

Results - fuel/air mixtures Surface temperature profiles show effects of –Heat loss at low flow velocities –Axial diffusion (broader profile) at low flow velocities

Results - fuel/air mixtures Heat release inhibited by high O(s) coverage (slow O(s) desorption) at low temperatures - need Pt(s) sites for fuel adsorption / oxidation a b Heat release rates and gas-phase CH 4 mole fractionSurface coverage

Results - fuel/O 2 /N 2 mixtures Computations with fuel:O 2 fixed, N 2 (not air) dilution Minimum fuel concentration and flame temperatures needed to sustain combustion much lower for even slightly rich mixtures!

Results - fuel/O 2 /N 2 mixtures Combustion sustained at much smaller total heat release rate for even slightly rich mixtures

Results - fuel/O 2 /N 2 mixtures Behavior due to transition from O(s) coverage for lean mixtures (excess O 2 ) to CO(s) coverage for rich mixtures (excess fuel) Lean Rich

Experiments Predictions qualitatively consistent with experiments (propane- O 2 -N 2 ) in 2D Swiss roll (not straight tube) at low Re: sharp decrease in % fuel at limit upon crossing stoichiometric fuel:O 2 ratio Lean mixtures: % fuel at limit lower with no catalyst Rich mixtures: opposite! Temperatures at limit always lower with catalyst Similar results found with methane, but minimum flame temperatures for lean mixtures exceed materials limitation of our burner! No analogous behavior seen without catalyst - only conventional rapid increase in % fuel at limit for rich fuel:O 2 ratios

Experiments

Conclusions Computations of catalytic combustion in a 1 mm diameter channel with heat losses reveal –Dual limit behavior - low-speed heat loss limit & high-speed blow-off limit –Behavior dependent on surface coverage - Pt(s) promotes reaction, O(s) inhibits reaction –Effect of equivalence ratio very important - transition to CO(s) coverage for rich mixtures, less inhibition than O(s) Behavior of catalytic combustion in microchannels VERY different from “conventional” flames Results qualitatively consistent with experiments, even in a different geometry (Swiss roll vs. straight tube) with different fuel (propane vs. methane)

Conclusions Pt catalyst actually inhibits combustion at low temperature, but only for lean mixtures Typical strategy to reduce flame temperature: dilute with excess air, but for catalytic combustion at low temperature, slightly rich mixtures with N 2 or exhaust gas dilution to reduce temperature is a much better operating strategy!