Extinction Limits of Catalytic Combustion in Microchannels

Kaoru Maruta, Koichi TakedaAkita Prefectural University, Honjyo, Akita, Japan

Jeongmin Ahn, Kevin Borer, Lars Sitzki, Paul D. RonneyUniversity of Southern California, Los Angeles, CA USA

Olaf DeutschmannUniversity of Heidelberg, D-69120 Heidelberg Germany

SIAM Conference on Numerical CombustionSorrento, 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)

0.2

0.4

0.60.8

1

3

10 100 1000

Ceramic (no cat)Ceramic (cat)Inconel (no cat)Inconel (cat)Weinberg 4.5 turn CH4

Reynolds number

Conventionallean limit

Propane

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!

200

400

600

800

1000

1200

10 100 1000

Inconel (no cat)

Inconel (cat)

Ceramic (no cat)

Ceramic (cat)

Reynolds number

Propane

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 (2.4 - 60)

– Fuel/air AND fuel/O2 ratio - conventional experiments using fuel/air mixtures might be misleading because both fuel/O2 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, CH4-air and CH4-O2-N2 mixtures

Wall boundary condition H = 0, 5 or10 W/m2˚C

1 mmnon-catalytic wall

9 mmcatalytic wall

Fuel/airinlet

1 mmdiameter

Results - fuel/air mixtures

• “Dual-limit” behavior similar to experiments observed when heat loss is present

a b

c

Results - fuel/O2/N2 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 CH4 mole fraction Surface coverage

Results - fuel/O2/N2 mixtures

• Computations with fuel:O2 fixed, N2 (not air) dilution

• Minimum fuel concentration and flame temperatures needed to sustain combustion much lower for even slightly rich mixtures!

Results - fuel/O2/N2 mixtures

• Combustion sustained at much smaller total heat release rate for even slightly rich mixtures

Results - fuel/O2/N2 mixtures

• Behavior due to transition from O(s) coverage for lean mixtures (excess O2) to CO(s) coverage for rich mixtures (excess fuel)

Lean

Rich

Experiments

• Predictions qualitatively consistent with experiments (propane-O2-N2) in 2D Swiss roll (not straight tube) at low Re: sharp decrease in % fuel at limit upon crossing stoichiometric fuel:O2 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:O2 ratios

Experiments

400

500

600

700

800

900

1

1.5

2

2.5

3

0.4 0.6 0.8 1 1.2 1.4

Equivalence ratio

Tmax

(non-cat)

Tmax

(cat)

Fuel % (cat)Fuel % (non-cat)

Experiments

600

620

640

660

680

700

1

1.5

2

2.5

3

0.4 0.6 0.8 1 1.2 1.4

Equivalence ratio

Tmax

(cat)

Fuel % (cat)

Re = 35

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 N2 or exhaust gas dilution to reduce temperature is a much better operating strategy!