Autoignition Characteristics of Silane-Oxygen-Diluent Mixtures in a Practical Burner

Paper from the AFRC 2015 conference titled Autoignition Characteristics of Silane-Oxygen-Diluent Mixtures in a Practical Burner

Silane is a pyrophoric gas commonly used in semiconductor manufacturing and has also been considered as a fuel additive to improve flame stabilization in scramjet engines. In certain circumstances, ignition of non-premixed combustion systems by conventional means such as spark or torch ignition is impractical. For such systems, small amounts of silane can be mixed with the fuel to initiate combustion upon contact with the oxidizer. The amount of silane required to initiate combustion depends on the oxidizer, primary fuel type, and burner geometry. It is well-known that practical ignition limits often differ significantly from those established by fundamental ignition and flammability limit studies due to the complexities of the fluid mechanics and fuel-oxidizer injection and mixing arrangements. In the current study, the concentration of silane in fuel required to achieve ignition was investigated using a practical, non-premixed, oxy-methane, swirled-stabilized burner. The effects of fuel and oxidizer injection timing, flow rates, injector configuration, silane gas temperature (273-325 K), and combustion system geometry were investigated. In each case the minimum silane concentration in methane necessary for prompt, repeatable ignition was recorded. The role of methane as a diluent in low-temperature Silane autoignition for a methane-oxygen combustion system was explored.

Autoignition Characteristics of Silane-Oxygen-Diluent Mixtures in a Practical Burner Bradley Ochs, Brian Ventura, David Scarborough School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, Georgia Abstract Silane is a pyrophoric gas commonly used in semiconductor manufacturing and has also been considered as a fuel additive to improve flame stabilization in scramjet engines. In certain circumstances, ignition of non-premixed combustion systems by conventional means such as spark or torch ignition is impractical. For such systems, small amounts of silane can be mixed with the fuel to initiate combustion upon contact with the oxidizer. The amount of silane required to initiate combustion depends on the oxidizer, primary fuel type, and burner geometry. It is well-known that practical ignition limits often differ significantly from those established by fundamental ignition and flammability limit studies due to the complexities of the fluid mechanics and fuel-oxidizer injection and mixing arrangements. In the current study, the concentration of silane in fuel required to achieve ignition was investigated using a practical, non-premixed, oxy-methane, swirled-stabilized burner. The effects of fuel and oxidizer injection timing, flow rates, injector configuration, silane gas temperature (273-325 K), and combustion system geometry were investigated. In each case the minimum silane concentration in methane necessary for prompt, repeatable ignition was recorded. The role of methane as a diluent in low-temperature Silane autoignition for a methane-oxygen combustion system was explored. Keywords: Silane, autoignition 1. Introduction Silane, which can be considered a pyrophoric1 gas, is commonly used as a silicon source2 for the semiconductor industry. It has also been utilized in diverse combustion applications such as the NASA X-34 scramjet engine3 and underground coal gasification5. In scramjet combustion applications, it is used as an ignition aid and flame stabilizer for hydrogen combustion due to its extremely fast flame speed4 (much faster than ethylene or methane). For underground coal gasification, Silane is used as an ignition source in lieu of conventional ignition methods such as spark ignition due to the nature of the environment and the lack of dynamic control. Although Silane is commonly used in industry, its autoignition characteristics are poorly understood with terms such as "unpredictable" finding common use in the literature. Further, it is well-known that practical auto-ignition limits differ significantly from the more fundamental auto-ignition limits more frequently found in the literature. A number of researchers have investigated fundamental, low-temperature silane autoignition limits. These studies are frequently motivated by the need for chemical kinetics data, safety, or accident investigations. The work of Chen et. al. and Tsai et. al. were motivated by accidents6 and safety7. Other more fundamental studies have attempted to measure flammability and ignition limits 8,9. Additionally, others have conducted experiments attempting to ascertain reaction pathways10 and created predictive kinetic models11. Classical autoignition experiments are difficult to devise due to the pyrophoric nature of silane because it is impossible to homogenously mix flammable concentrations of silane with oxygen (or air) at room temperature without combustion occurring during the mixing process. Unfortunately, existing studies often differ widely in reported minimum silane concentrations required for ignition. This is not surprising given the fact that the very nature of the mixing process requires that initially regions of relatively high and low silane concentrations exist throughout the mixture volume. As mixing proceeds, regions of high silane concentration come into contact with oxygen and ignite regardless of the fact that the overall average silane concentration may be too low for ignition to occur spontaneously. Cooling the gases to very low temperatures avoids the problem of autoignition during mixing. However, the subsequent process of heating the gases slowly to the temperature of interest leads to uncertainties in test results due to the fact that reactions occur increasingly fast near the auto-ignition temperature. Rapid-compression combustion system technology promises to offer better fundamental auto-ignition data in the future. Ignition limits are typically discussed in association with a thermochemical property, the "most reactive" mixture fraction . In the flow field of a non-premixed burner, a point will always exist having a value of the mixture fraction equal to . The autoignition process is a balance between reaction rate and mixing times. If mixing occurs very fast compared to the autoignition time delay, then there may be delayed or no ignition. If the autoignition time delay is very fast compared to the mixing rate, then ignition should occur since for a non-premixed system, there will always be a place where the mixture fraction is favorable for ignition as discussed above. This assumes of course that there is excess silane in the fuel so that mixing with oxygen always yields the right mixture fraction. This being the case, we have to now consider the fact that fluid packets are moving and their mixture fraction is changing with space and time. At some point in space/time, a packet will take on the value of . We may make the assumption for now that ignition always happens at this point as long as the strain rate is low enough at that point. Therefore, the actual autoignition event always occurs at the same mixture fraction (equiv. ratio). This means that the autoignition delay time is a function only of the mixture temperature. The ignition time delay is fairly independent of turbulent intensity, meaning a practical injector should still be able to generate repeatable ignition tests12. Also of interest was the effect of fuel composition on ignition. Considering a purely thermal ignition event, the additional methane in the fuel mixture would act solely as a diluent. It is clear that at some point during the ignition process, due to a combination of increasing heat and radical species, methane will begin to participate in reactions. It is well-known that practical auto-ignition limits often differ significantly from fundamental values. This is because in addition to chemistry, the flow field affects auto-ignition limits through strain and turbulent heat and mass diffusion. These fluid mechanic effects lead to narrower ignition limits than fundamental studies indicate. In practical systems, the effects of chemistry and fluid mechanics are inseparable; therefore, it is frequently the latter limits that are of most interest. For these reasons, this study focuses on determining the practical autoignition limits of silane-oxygen mixtures in a practical non-premixed, swirl-stabilized methane-oxygen combustion system. Of particular interest are the effects of varying fuel and/or oxygen velocity, overall fuel and oxidizer ratio, fuel composition, inlet gas temperatures, and the relative timing of gas introduction (i.e. fuel first, oxygen first, etc.) on the minimum silane concentration required for repeatable ignition. 2. Experimental Setup Figure 1 shows a schematic of the test facility developed for this study. The injector, shown in Figure 2 consisted of coaxial fuel and oxidizer feed tubes. For all tests the oxidizer flow consisted of pure oxygen. The majority of tests used a fuel mixture that consisted of a combination of silane and methane. At the lowest power condition, the flow control system had a resolution of 1% silane in the fuel. This meant that no finer than 1% adjustments of silane in the fuel could be made at the absolute lowest power conditions. The outer (oxygen) and inner (fuel) tubes were 1/2" and 1/4" diameter, respectively. The oxygen flow was swirled using 45 degree swirl vanes fitted between the inner and outer tubes located 1" upstream of the end of the oxygen tube. The fuel tube was terminated by a 30o cone having seven 1mm diameter ports through which fuel was injected into the oxygen flow. As shown in Figure 2, six of the fuel injection ports were arranged around the perimeter of the cone tip while one of the holes was drilled directly in the tip itself. One could conceive of many possible arrangements of fuel injection tips. However, the arrangement tested here was arrived at due to its favorable thermal and combustion performance in steady combustion testing. Fuel injected through the fuel holes drilled in the perimeter of the cone mixed rapidly with the available oxygen and burned as diffusion flames attached to the injector ports. By appropriately sizing these fuel injection ports, the flames do not impinge directly on the walls of the combustion chamber. In fact, a layer of relatively cold gas, primarily oxygen, flows along the combustor wall at the location of these very high-temperature, intense diffusion flames. Fuel flowing through the tip directly cooled the tip and shielded the tip from recirculating hot gas flow. In contrast to the fuel injected around the perimeter, little oxygen was available for combustion at the tip. Therefore, fuel injected through the tip mixed with the diffusion flame products and remaining oxygen before completing the combustion process downstream. The ignition test chamber itself was a 2 in. diameter x 12 ft. long tube. It was made extremely long to isolate the test chamber from the atmospheric air. Slits were cut along the side for the first 3 ft. of the combustion chamber to allow for visual confirmation of ignition. These slits were covered and sealed with clear tape before each test, and the entire system was purged with N2. A schematic of the flow control system is shown in Figure 3, and a picture of the entire test facility is shown in Figure 4. The oxidizer and fuel streams were controlled using a computer data acquisition and control program. Threeway block and bleed solenoid valves were installed on each of the oxidizer and fuel lines allowing the injection timing of the fuel and oxygen to be precisely controlled. Figure 1. Illustration of experimental setup showing characteristic injector/combustor dimensions Figure 2. Picture of the fuel/oxidizer injector with oxygen tube remove. Oxygen swirl vanes and fuel injector tip shown To Vent SiH4 V-3 CH4 Fuel Injection V-1 N2 Burner Purge V-5 V-4 V-2 O2 O2 Injection To Vent Figure 3. Fuel and Oxidizer Flow Schematic Figure 4: Experimental setup showing an ignition event using a shorter test section Silane autoignition timing is extremely sensitive to temperature12. In classical pre-mixed experiments, delays as long as 15-120 seconds were observed before spontaneous autoignition occured15, due primarily to initial gas temperature. As mentioned earlier, in this study ignition was required to be prompt, but the effect of initial gas temperature was still of interest. To cool or heat the system, the gas tubes were surrounded by ice water or heat tape, respectively. For all tests, temperatures were measured directly before the solenoid valves, after the heat exchange had taken place. During a typical test, an N2 purge was conducted to remove products, air, etc. from the injector and combustion chamber. In order to minimize transient effects on gas startup, fuel and oxidizer streams were discharged to atmosphere through each system's three-way valve bleed port until the desired steady flow rate for each stream was obtained. Once achieved, the steady fuel and oxidizer streams were diverted at the desired time into the burner using the three-way solenoid valves. Typically fuel was injected into the test chamber first followed by the oxidizer after a small preset time delay. To assess the effect of a particular variable (flow rate, temperature, etc.), a series of tests were performed for each value of the particular variable. Initially, the silane mixture and oxygen flow rates were set at a value to guarantee ignition, where successful ignition is defined as the prompt development of a stable flame. Subsequent tests were conducted with decreasing amounts of silane in the silane-mixture. The concentration at which repeatable ignition was no longer achieved was recorded as the minimum silane concentration for ignition. Hence, trends of minimum silane concentration versus flow rates, temperature, etc. were collected. Test chamber initial conditions drastically affect ignition results. When the fuel gases are injected into the combustion chamber, the gases initially mix with N2, which yields a mixture field with an unknown spatio-temporal diluent concentration. Under these conditions, it is impossible to decouple the effect of added diluent on the variable being studied. Therefore, fuel gas was introduced into the combustion chamber long enough to guarantee that the N2 was fully purged from the chamber prior to oxygen introduction. To quantify the delay time, an adjustable solid state time-delay relay was used to power the solenoid valve on the oxidizer line after a set, measureable time delay. It was found (see Figure 5) that increased delay between fuel and oxygen injection resulted in lower silane concentration for ignition until about 4 seconds. Time delays long than 4 seconds resulted in no further change in the required concentration of silane for ignition. Therefore, a time delay of 6 seconds was used for the majority of tests. 0.2 Prompt Ignition No Ignition [SiH4] in Fuel 0.15 0.1 0.05 0 0 2 4 6 Oxygen Time Delay(s) 8 10 Figure 5: Effect of delay timing between injection of fuel and oxygen into the combustion chamber 3. Results and Discussion This section presents experimental autoignition limits of silane-oxygen mixtures in a practical nonpremixed, swirl-stabilized methane-oxygen combustion system. First, results of experiments to determine the effect of relative timing of gas introduction (i.e. fuel first then oxygen, or vice versa) and silane-fuel and oxidizer configuration (i.e., fuel in the inner tube, oxygen in the outer and vise versa) will be presented. For these tests, the focus was not on the silane fraction required to achieve ignition but rather whether or not timing and configuration resulted in prompt ignition or, as was sometimes the case, resulted in delayed, explosive ignition. Next, results of experiments to determine practical ignition limits for cases of varying fuel and/or oxygen velocity, overall fuel and oxidizer ratio, fuel composition, and inlet gas temperatures will be presented. Silane ignition performance for both the order (oxygen or methane/silane first) and configuration (oxygen in the inner pipe, fuel in the outer pipe and vice versa) of gas injection was tested to determine whether prompt or delayed ignition occurred. The results of these tests are shown in Table 1. In Cases III and IV, the oxygen was introduced into the combustion chamber before fuel. This repeatedly resulted in delayed ignition or no ignition. These delayed ignitions resulted in explosions without a successful flame being established. In Cases I and II, the fuel was introduced before oxygen. This resulted in stable flames being established over a wide range of SiH4 fractions. This behavior is thought to be due to the most reactive mixture fraction lying very close to 1. Several experiments have reported measuring reactive kernels for spontaneous ignition existing at silane-rich (oxygen-lean) concentrations13, 14. For the oxygen first cases, the test chamber is filled with oxygen at the moment silane-fuel injection begins. In the region near the injector, flow velocities, mixing rates, and scalar dissipation rates are high, which is not conducive to ignition. When the silane-fuel mixture is turned on, the silane-fuel and oxygen rapidly mix due to the swirling injector flow. In the near injector region where silane mixture fractions are relatively high, the dissipation rates are also high. Further from the injector, where dissipation rates are lower, the silane mixture fraction is low due to mixing with the oxygen filling the test chamber. Therefore, prompt ignition did not occur when oxygen was injected first. This general result was true regardless of the fraction of silane in fuel. For fuel-first cases, the test chamber is filled with silane-fuel at the moment when oxygen injection begins. Again, dissipation rates are highest near the injector and lower downstream in the test chamber. However, in this case silane mixture fractions are lowest near the injector and highest further downstream having mixed with the silane-fuel mixture filling the tube. Therefore, the region near the injector is not conducive to ignition whereas the region further from the injector is. It is believed that ignition is initiated in these rich, low strain regions and this is the reason that fuel first injection is advantageous for prompt ignition. The minimum SiH4 fraction required was independent of whether fuel passed through the center of the injector (Case I) or through the swirler (Case II). Case I was the configuration used throughout the remainder of the investigation. Table 1: Injector Configurations and Injection Order Oxygen in Outer Tube Inner Tube Fuel First Case I. Prompt Ignition Case II. Prompt Ignition Oxygen First Case III. Delayed Ignition Case IV. Delayed Ignition These tests established that prompt ignition requires the injection of silane-fuel first followed a short time later by oxygen injection. Whether the silane-fuel or oxygen was in the inner or outer tube had no effect on whether prompt or delayed ignition occurred. The following sections describe tests conducted to determine the dependence of silane-fuel fraction required to achieve prompt ignition on overall volume flow rate, global chemistry, initial reactant temperature, and diluent fraction. Overall Volume Flow Rate The tests described in this section were conducted to assess the effect of fuel and oxidizer flow rates on the minimum required silane fraction for ignition. In these tests, the fuel (or oxygen) flow rate was varied while holding the oxygen (or fuel) flow rate constant. While varying the oxidizer flow rate the fuel rate was held constant at 45 SLM; while varying fuel the oxidizer was held constant at 45 SLM. It should be noted that the global fuel-air ratio cannot be held constant in these experiments. Therefore, for each combination of fuel and oxygen flow rate, the amount of silane in the fuel was varied until prompt ignition no longer occurred. The minimum silane fractions required for ignition are shown in Figure 6 plotted against the flow Reynolds number (based on oxygen annulus hydraulic diameter). Figure 6 shows cases where the fuel flow rate was held constant, and Figure 7 shows cases where the oxygen flow rate was held constant for varying fuel flow rate. These data show that, over the flow rate range investigated, the minimum ignition silane fraction slightly decreases with increasing oxygen flow rate and is weakly or not dependent on fuel flow rate over the range of flow rates tested. Neglecting the outlier at Red = 120,000, there appears to be a gradual downward trend in the required silane fraction for prompt ignition with increasing Reynolds number over the range investigated. The rate of turbulent mixing increases with Reynolds number, which may in part explain this trend. It is not surprising that ignition trends are affected by the nature of mixing inside the combustion chamber as nonpremixed combustion is in general almost entirely mixing limited 0.2 Prompt Ignition No Ignition [SiH4] in Fuel 0.15 0.1 0.05 0 0 100000 200000 Red 300000 Figure 6: Prompt and no ignition silane fraction vs. oxygen Reynolds number with fuel flow rate held constant. 0.2 Prompt Ignition No Ignition [SiH4] in Fuel 0.15 0.1 0.05 0 40 50 60 70 80 Total Fuel Flowrate (SLM) 90 100 Figure 7: Prompt and no ignition silane fraction vs. total fuel flow rate with oxidizer held constant. Additional high speed visual measurements are needed to explain if these observations are due primarily to fluid mechanic effects or a combined fluid mechanic-chemical effect. For example, in intense mixing situations the overall mixture composition could begin to play a significant role in the ignition process if the mixing time were much smaller than the autoignition delay time. In addition, accelerated flame propagation and improved stabilization after the initial ignition event (due to enhanced mixing) could play a role in explaining these observations. Global Chemistry This section describes experiments designed to investigate the effect of global equivalence ratio on minimum ignition silane fraction. For these tests, the oxygen flow rate was held constant and the fuel flow rate was set to obtain the desired equivalence ratio. The fraction of silane in the fuel was set to a value high enough to ensure prompt ignition. Then the silane fraction in the fuel was gradually decreased until prompt ignition no longer occurred. The minimum prompt and maximum no ignition silane fractions are shown in Figure 8 for these tests. Figure 8 also shows ignition trends versus equivalence ratio treating methane as a diluent, i.e., the equivalence ratio was based on the silane to oxygen ratio as shown in ( 1 ) and ( 2 ). (1) (2) Figure 8 shows that the smallest prompt ignition silane fraction occurs just below an equivalence ratio (total fuel to oxygen) of one. The minimum prompt ignition silane fraction then increases rapidly before becoming approximately constant for equivalence ratios greater than two. The equivalence ratio data near one correspond to the high oxygen flow rate cases shown in Figure 6. These data suggest that the global equivalence ratio has a relatively strong effect on the minimum ignition silane fraction in the vicinity of equivalence ratio one compared with the oxygen flow rate and its effect on the flow field. However, for equivalence ratios greater than two, the global equivalence ratio has little or no effect on minimum ignition silane fraction. It is interesting to note that the same general trends are observed when plotted against equivalence ratio based on silane to oxygen ratio. If methane were acting solely as a diluent, the minimum ignition silane fraction would be expected to increase with increasing equivalence ratio (increased amount of diluent). This behavior was not observed in the data. The behavior of the data near equivalence ratio one is consistent with the fact that near stoichiometric mixtures of methane-oxygen are more easily ignited than extremely rich or lean mixtures. Reaction rates following an Arrhenius law, vary with gas temperature as AT b exp Ea RT . Near equivalence ratio one, mixtures have much higher burned gas temperatures than richer mixtures. Therefore, the minimum ignition silane fraction follows the expected decreasing trend near equivalence ratio one. Initial Reactant Temperature Autoignition of silane has been observed at temperatures as low as 113 K16. In practical combustion systems, reactant temperature can vary with ambient environmental conditions. Even though ambient temperatures are usually much higher than the silane autoignition temperature, it is important to understand the sensitivity of minimum ignition silane fraction on reactant temperature. For these tests, the reactant temperatures were lowered by passing the reactant tubing through an ice bath prior to entrance into the combustion chamber. Similarly, the reactant temperature was raised by adding line heaters to the reactant tubing prior to entrance into the combustion chamber. With the reactant temperatures cooled or heated, the total oxygen and fuel flowrates were set to a nominal value of approximately 60 SLM. The fraction of silane in the fuel was set to a value high enough to ensure prompt ignition. Then the silane fraction in the fuel was gradually decreased (while holding total fuel flow constant) until prompt ignition no longer occurred. The minimum prompt and maximum no ignition silane fractions versus temperature are shown in Figure 9. These data show that the minimum silane fraction required for prompt ignition is nearly independent of temperature over the range of temperatures investigated. 0.2 0.2 Prompt Ignition No Ignition Prompt Ignition No Ignition 0.15 [SiH4] in Fuel [SiH4] in Fuel 0.15 0.1 0.05 0.05 0 0 0.1 2 4 6 (Total Fuel to Oxygen) 8 0 0 0.2 0.4 0.6 0.8 (SiH4 to Oxygen) 1 1.2 Figure 8. Ignition trends versus equivalence ratio formed with total fuel to oxidizer ratio (left) and silane to oxidizer ratio (right) The temperatures tested were between 273 and 320 K, which is much higher than the minimum autoignition temperature of silane. Being much higher than the autoignition temperature suggests that the rate controlling kinetics are sufficiently fast and negligibly different over the range of temperatures studied. The Wolfer17 equation for autoignition delay times is given by equation ( 3 ) (3) Assuming an exponential dependence on the unburned gas temperature, the reaction rates at 273 and 320 K are 11.2 and 16.9 times higher, respectively, than the reaction rate at the minimum autoignition temperature. The difference in reaction rate and autoignition time delay at 273 and 320 K is negligible compared to the order of magnitude difference between the reaction rate at 113 and 273 K. In these experiments, it was found that prompt ignition occurred at 273 K. Increasing the temperature to 320 K speeds up the reaction rate approximately 3 times but has little difference on ignition success. It would appear that the rate controlling reactions are already sufficiently activated at 273 K and that the global reaction rate is much faster than the mixing rate. In varying the temperature, the large scale flow field remained unchanged. Therefore, the mixture field stayed basically the same meaning that the value of was achieved in a region conducive to ignition for all of these cases. The experiments conducted here were not designed to resolve the exact autoignition time delay. Therefore, although the autoignition time delay for the 320 K case should be as much as three times faster than the 273 K case, these experiments were not able to discern the change if any occurred. 0.2 Prompt Ignition No Ignition [SiH4] in Fuel 0.15 0.1 0.05 0 270 280 290 300 Temperature (K) 310 320 Figure 9. Dependence of required silane concentration in fuel for ignition on the initial reactant temperature. Diluent Addition Industrial gas streams are typically composed of a variety of fuels having different specific heats. Assuming that the autoignition process is governed by a balance between heat released and lost from the ignition volume, fuel diluent has a sizeable effect. Gas streams with time varying composition could undergo periods of high and low average specific heat. This in turn could result in delayed ignition or no ignition which presents a severe safety hazard. This section describes studies designed to investigate the dependence of minimum ignition silane fraction on diluent addition. In these cases, either methane or nitrogen were used as diluents added to the silane fuel stream. For a typical test, the oxidizer flow rate was set at a nominal value. The desired silane fraction was selected and the silane and diluent flow rates were set such that the total fuel line flow rate matched the oxygen flow rate selected. Flow rate, and therefore velocity matching are desirable due to the already discussed mixing controlled ignition phenomenon. Hence the total flow rate, temperature, and ignition scheme were held constant. Because of velocity matching, it was impossible to achieve independent control on the silane-diluent mixture properties and overall silane-oxygen ratio. For these tests, the global equivalence ratio was significantly higher than one for all fuel mixtures. For each mixture the mass average fuel line specific heat was calculated. The minimum required silane fraction for prompt ignition was plotted versus this mass averaged specific heat. The results of these tests are shown in Figure 10. This figure shows both the prompt and no-ignition silane fraction vs. average fuel specific heat for a methane-silane and a nitrogen-silane fuel mixture. The methane-silane mixture had an average specific heat of 2.25 kJ/kg K and the nitrogen-silane mixture had an average mixture specific heat of 1.1 kJ/kg K. The methane-silane mixture required a higher silane concentration for repeatable, prompt ignition than the nitrogen-silane mixture. N2 Diluent Prompt Ignition N2 Diluent No Ignition CH4 Diluent Prompt Ignition CH4 Diluent No Ignition [SiH4] in Fuel 0.2 0.15 0.1 0.05 0 0 0.5 1 1.5 Average Fuel Cp (kJ/kgK) 2 Figure 10. Silane concentration for ignition versus fuel line average specific heat (Cp) An alternative view could be made by considering methane as a reacting species. H atom abstraction in the initial decay of CH4 would supply H atoms, which has been suggested as an important initiating species in SiH4 decomposition. If the CH4 mechanism were important for low temperature silane ignition then the previous comment may suggest that the addition of CH4 would actually reduce the SiH4 concentration required for ignition. This trend was not seen. In fact, the replacement of CH4 with N2 resulted in lower required SiH4 suggesting CH4 is not chemically reacting during the low temperature SiH4 initiation reactions. It is unclear whether methane is chemically contributing to the ignition process. However, it is clear that the methane-silane mixture with higher specific heat requires higher silane fraction for repeatable ignition. A more thorough test matrix with additional non-reacting and reacting "diluent" species should be conducted to properly ascertain the role of methane in the low temperature silane reaction. 4. Conclusions For methane-silane-oxygen mixtures, the minimum silane concentration required for ignition is roughly constant with a value of 12% over the majority of the operating range tested. At very high O2 flow rates corresponding to ReD = 350,000, repeatable ignition was achieved at SiH4 fractions as low as 10%. On the other hand at ReD < 100,000, SiH4 fractions as high as 13% were necessary to achieve repeatable ignition. Minimum silane concentration for repeatable ignition was shown to be independent of temperature in the range studied. When replacing CH4 by N2 as the fuel stream diluent, the necessary silane fraction is reduced from 12% to 10%. These ignition trends plotted versus fuel line specific heat show that the lower specific heat diluent results in lower required silane for ignition. This result suggests that the ignition process is affected by the mixture's thermodynamic properties. Silane enhanced ignition in a non-premixed practical oxy-methane burner is roughly independent of fuel and oxidizer flow rates, global chemistry, and temperature. 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