Ignition Strategies for Fuel Mixtures in Catalytic Microburners.

Transcription

1 Ignition Strategies for Fuel Mixtures in Catalytic Microburners. V I K R A M S E S H A D R I AND N I K E T S. K A I S A R C O M B U S T I O N T H E O RY AND M O D E L L I N G VOL. 1 4, N O. 1, , William Hoey

2 Introduction Microburners and micro-fabrication are becoming increasingly important. Microburners: qualitatively different than largescaled counterparts. High Surface-Area-to-Volume Ratio. Comparatively large heat and mass transfer rates. Catalytic combustion is preferred to homogeneous combustion due to relative stability. Heavy quenching losses (thermal and radical).

3 Mathematical Modeling Pt-catalyzed microburner, with three fuel cases. Pure Propane combustion: C3H8 + 5O2 3CO2 + 4H2O. Pure Methane combustion: CH4 + 2O2 CO2 + 2H2O. Mixed-fuel combustion.

4 Kinetics Used an a-posteriori-reduced microkinetic model. Thermodynamics and transport data achieved via CHEMKIN subroutines.

5 Catalytic Stability Note: Extinction occurs in areas above each curve.

6 Catalytic Stability Methane s dissociative adsorption has a higher activation energy; therefore, less stability. For constant equivalence ratios, increasing the concentration of propane in a fuel mixture increases the stability of the mixture s combustion.

7 Ignition via Inlet Preheating A common ignition method involves heating the fuel to a temperature slightly above the ignition temperature at the inlet, and allowing combustion to occur downstream. Here, steady state characteristics of inlet pre-heated fuel combustion was considered.

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9 Conclusions: Beyond this optimal point [i.e % propane], the addition of further propane does not significantly decrease the ignition temperature. This observation for addition of 15 20% propane was applicable for the entire range of equivalence ratio, volumetric flow rate and wall conductivity considered.

10 Time Effects: Propane/Methane Ignition Transient effects: Once ignition is complete, the inlet feed temperature is reduced to the ambient. The ignition kicks off in either the front end or back end of the burner, and proceeds to propagate. Many factors control the interplay between ignition time and steady-state time.

11 This is representative of an ideally mixed propane/methane fuel. Note that combustion begins with propane far more advanced conversion at 60 s. Note also that ignition takes place at the rear of the microburner.

12 Conclusions: Based on these results, we propose that presence of methane does not delay propane ignition The rise in ignition temperature and time with respect to pure propane is attributed to the dilution of propane in mixed-fuel. Some methane conversion is observed during the ignition period. Heat released due to this exothermic reaction has a positive effect on propane ignition although the opposite effect is more pronounced. Thus, ignition in the mixed-fuel case is not a linear interpolation between pure methane and pure propane.

13 Effect of Wall Conductivity

14 Transience Note that, in (a), while raising inlet feed temperatures to 50 K over ignition decreases ignition and steady state times, the difference between the two remains nearly constant. Note also that the minimum steady state time occurs at moderate thermal conductivity.

15 Ignition via Resistive Preheating A second mechanism for ignition was examined as well employing Joule heating along the burner to raise internal fuel temperature. Electric power consumption was minimized by: 1. Employing a propane/methane mixed fuel, and 2. Only heating the entrance portion of the burner.

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17 OVERALL CONCLUSIONS: Adding propane to methane-air combustion makes the combustion more stable, and lowers its ignition temperature. The effect of propane on the steady state stability of methane air fuel is proportional to the amount of propane in the mix, but adding even a small quantity of propane lowers ignition temperatures significantly. Mixed fuel comprising methane and 15 20% propane was found to be ideal for improving ignition properties of lean methane air mixtures. (Equivalencies modeled: 0.6, 0.75, 0.9). These results were applicable for all cases considered in this paper, independent of heating mechanism.

18 OVERALL CONCLUSIONS: In a mixed fuel case, sequential light-off is observed with propane igniting first. The heat of combustion warms up the microburner beyond the ignition temperature of methane. Presence of methane does not have a competitive effect on propane light-off. As the resistive preheating length of the reactor decreases, the power consumed to ignite the fuel also decreases. This decrease is [most] significant for poorly conducting wall materials than highly conducting walls.

19 Citation. Seshadri, Vikram and Kaisare, Niket. Ignition Strategies for Fuel Mixtures in Catalytic Microburners. Combustion Theory and Modelling, 14.1 (2010): Web. 30 Oct