Flame burner temperature profile

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

It has been shown recently [25] that concentrations of NOj, tend to reduce with increase in the amplitude of discrete-frequency oscillations. The mechanisms remain uncertain, but may be associated with the imposition of a near-sine wave on a skewed Gaussian distribution with consequent reduction in the residence time at the adiabatic flame temperature. Profiles of NO, concentrations in the exit plane of the burner are shown in Fig. 19.6 as a function of the amplitude of oscillations with active control used to regulate the amplitude of pressure oscillations. At an overall equivalence ratio of 0.7, the reduction in the antinodal RMS pressure fluctuation by 12 dB, from around 4 kPa to 1 kPa by the oscillation of fuel in the pilot stream, led to an increase of around 5% in the spatial mean value of NO, compared with a difference of the order of 20% with control by the oscillation of the pressure field in the experiments of [25]. The smaller net increase in NO, emissions in the present flow may be attributed to an increase in NOj due to the reduction in pressure fluctuations that is partly offset by a decrease in NOj, due to the oscillation of fuel on either side of stoichiometry at the centre of the duct. 

Perhaps the most studied laboratory flame is the premixed flat flame. As illustrated in the left-hand panel of Fig. 1.1, a steady flame is established above a porous burner face. Such flames are used widely in combustion laboratories, where a variety of optical and probe-based diagnostics are used to measure species and temperature profiles. Models play an essential role in assisting the interpretation of the data. In addition to the premixed flat... 

Fig. 16.8 Illustration of a premixed flat-flame burner. Fuel and oxidizer are first premixed, and then flow through a porous burner face. A steady, one-dimensional flat flame is stabilized by heat transfer to the cooled burner face. The solutions shown here are for a methane-air flame, in which the air contains water vapor at 100% relative humidity. By plotting the temperature and selected species profiles, one can observe some of the complexities of flame structure.

Use GRI-Mech (GRIM3 0. mec) and a laminar premixed flame code to calculate species and temperatures profiles for a stoichiometric, burner-stabilized methane-air flame at 20 Torr and a unbumed gas velocity of 1.0 m/s. 

The theoretical and experimental results for a fuel-lean methane-air flame are given in Figures 5-7. These results include temperature and major species compositions. The experimental and theoretical results are compared by matching the abcissas of the temperature profiles. The model very accurately predicts the slope of the temperature profile but predicts a larger final flame temperature than is measured. This is a consequence of heat lost to the cooled, gold-coated burner wall that is 1.5 mm away from the positions where data were taken. 

Temperature Measurements. Sodium line reversal temperature profile measurements were made on the flame series with varying additions of H2S. Results for H2/O2/N2 (3/1/4,5,6) are shown in Figure 3. The increase in temperature with distance above the burner is due to the slow recombination of the radicals H and OH. In the stoichiometric flames the temperature reaches a plateau in a few centimeters above the burner. In the richer flames the temperature gradient is steeper indicating a larger departure of the radical concentration from equilibrium values. The equilibrium temperatures decrease with H2S addition. However, the presence of sulfur compounds enhances radical recombination (6,11) producing almost equivalent temperature profiles, independent of H2S addition. 

The present work involves measurement of k in a 0.1 atmosphere, stoichiometric CH -Air flame. All experiments were conducted using 3 inch diameter water-cooled sintered copper burners. Data obtained in our study include (a) temperature profiles obtained by coated miniature thermocouples calibrated by sodium line reversal, (b) NO and composition profiles obtained using molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis and (c) OH profiles obtained by absorption spectroscopy using an OH resonance lamp. Several flame studies (4) have demonstrated the applicability of partial equilibrium in the post reaction zone of low pressure flames and therefore the (OH) profile can be used to obtain the (0) profile with high accuracy. 

Validation of the Global Rates Expressions. In order to validate the global rate expressions employed in the model, temperature and concentration profiles determined by probing the flames on a flat flame burner were studied. Attention was concentrated on Flames B and C. The experimental profiles were smoothed, and the stable species net reaction rates were determined using the laminar flat-flame equation described in detail by Fristrom and Westenberg (3) and summarized in Reference (8). 

The flame structure is modeled by solving the conservation equations for a laminar premixed burner-stabilized flame with the experimental temperature profile determined in previous work using OH-LIF. Three different detailed chemical kinetic reaction mechanisms are compared in the present work. The first one, denoted in the following as Lindstedt mechanism, is identical to the one reported in Ref. 67 where it was applied to model NO formation and destruction in counterffow diffusion flames. This mechanism is based on earlier work of Lindstedt and coworkers and it has subsequently been updated to include more recent kinetic data. In addition, the GRI-Mech. 2.11 (Ref. 59) and the reaction mechanism of Warnatz are applied to model the present flame. 

The numerical model for n-butane oxidation, by Pitz et al. [228], was used also by Carlier et al. [21] to simulate experimental studies of the two-stage combustion of n-butane at 0.18 MPa on a flat-flame burner and, following this validation, to simulate the ignition delays of n-butane in a rapid compression machine. The numerical studies of the burner experiments were extended by Corre et al. [233]. For simulations of the behaviour on a flat-flame burner the chemical model was computed in an isothermal mode, the experimental one-dimensional temperature profile being introduced as an input parameter. Among the important aims of the tests by Corre et al. [233] was the rationalization of the predicted extent of n-butane consumption throughout the development of the first (cool-flame) and second stages of combustion, with that observed experimentally. The experimental study by Minetti et al. [22, 116] included the detection and measurement of RO2 and HO2 radicals by esr, the one-dimensional spatial profiles of which were simulated by Corre et al. [233],... 

Recording of temperature profiles reported in Figure 23.11 have been measured at a distance of 250 mm from the burner nozzle. This position is downstream of the high velocity flame and inside the lifted flames. The relevant NO values have been measured in the flue gas channel. In flame mode, the temperature signal is only a little affected by time oscillations, whereas the lifted flame clearly showed large fluctuations in the frequency range... 

In oxy-burner testing, generally two types of measurements are involved (i) measurements inside flames and (ii) global measurements in the furnace. In-flame measurements include chemical composition, flame temperature profile, and optical properties. Global measurements include flame shape, emissions, heat flux, and measurements taken on the load. Figure 27.13 shows typical measurements and their locations in an oxy-pilot furnace. 

Flame temperature profiles against distance from the burner are based on sodium line reversal measurements of Bonne and Wagner... 

Problems with some clear flame burners are (1) movement of the hump in the temperature profile closer to the burner wall as the firing rate is reduced and (2) at lower input rates, temperature falls off more sfeeply at greater distances from the burner wall (e.g., the temperature profile of a burner firing at 50% of its rated capacity... 

Using many small burners to utilize the whole wall area is a way to achieve good temperature uniformity. (See figs. 3.4 and 3.5, and sec. 7.4.) There are large burners that can hold the burner wall as hot as the point of conventional maximum heat release. These adjustable thermal profile burners (fig. 6.1) can automatically hold a desired temperature profile by controlling the spin of the products of combustion. Optimum use of furnace space and overall refractory wall radiation usually favors the hottest possible burner wall (maximum flame spin, minimum flame length). In... 

Burners should be about 2.5 ft (0.87 m) apart, above and below the strip. The burners above the strip should be on one side of the furnace and those below the strip on the other side, enhancing circulation velocity. The burners should have a near-flat heat-release pattern (preferably adjustable), providing a temperature profile across the furnace that is practically level. It is important to check the design and the actual operation to make sure that no bottom-row-burner flames impinge on the lowest part of the strip s catenary loop. 

Other problems that limit production rates in either longitudinally fired or side-fired bottom zones are restricted gas passages in the bottom zones, and low-velocity luminous flame burners. Low-velocity luminous flames with their variable temperature profiles (hot at the burner wall at low firing rates, and hotter beyond the T-sensor at high firing rates) cause the melting of scale into the bottom zones. To counter this scale build-up problem, operators are prone to lower the bottom zone temperature by 100 F (56 C) or more. 

To reduce fuel cost and improve productivity, an engineer must be able to adjust furnace gas temperatures to change the furnace temperature profile. In a longitudinally fired furnace, shortening the flame will raise the temperature near the burner wall. This can be accomplished by spinning the combustion air and/or fuel, which in turn spins the poc. The resultant increase in heat transfer near the burner wall will reduce the flue gas exit temperature, raising the % available heat. 

Side-fired reheat furnaces can be troublesome in two ways (1) When conventional burners are installed directly opposite one another, the center of the furnace becomes very hot because the velocity pressures of the poc from the opposing burners negate each other and because the completion of the fuel burning is concentrated in the furnace center and (2) with staggered long-flame burners, a wide furnace s center gets hotter than the sides when on high fire, but at low fuel inputs the sidewalls get hotter than the centers. Both troubles can be prevented with controlled temperature profile burners and added T-sensors/controls. (See chap. 6.)... 

In many cases, space limits the firing rate and the type of flame so it is necessary to use type E burners, which have very short flames with large diameters. For larger firing rates, ATP burners can vary the flame length from short to very long for the needed temperature profile across the length of the space. 

Fig. 6.3. Flame profile of a conventional type A flame (fig. 6.2) on a steel reheat furnace. The vertical (temperature) scale reflects the heat flux profile. ATP burners can operate at a constant high input while switching temperature profiles, for example, from 30% to 100%.

Generally, the rate of heat transfer is low near the burner wall because the temperature differences are very small. (Load movement is counterflow to flame movement thus, the flame reactants are coolest as they leave any one zone whereas the load pieces are hottest as they leave any one zone.) As the distance from the burner wall increases, the load surface is colder and the flame temperature is hotter because the combustion reaction rate accelerates. However, a control T-sensor 15 ft (4.6 m) from the burner wall limits the furnace temperature at that point. (This temperature is held to a setpoint determined by the operator or by a model.) With high-spin burners, as one follows the temperature profile away from its maximum and in the direction of flame reactant flow, the furnace temperature declines quickly to the setpoint, and thereafter drops rapidly to the exit. 

Fig. 3.26 Determination of density profiles of H2 molecules in a flame. R is the distance from the burner axis, z the distance along this axis. The profiles in (a) have been obtained from the spatial variations of the Q line intensities and spectral profiles shown in (b). The relative intensities of Q J) furthermore allow the determinations of the temperature profiles. The numbers with the horizontal lines give the expected signal heights fot the labeled temperatures T in [K] [348]...

Fig. 11 Temperature profile through the flame front of the counterflow diffusion burner with (0 =80 1/min...

The combustion of natural gas with air is accompanied by formation of nitrogen oxides. This becomes significant above 1400-1500 C, which is easily surpassed in normal diffusion flames. The problem is solved by use of low NOx burners and further reduction can be obtained by cleaning the flue gas using selective catalytic reduction (SCR) with ammonia [510]. Low NOx burners have longer flames in order to limit the maximiun temperatures, but this has a significant impact on the heat flux and temperature profiles in the reformer, since it tends to make the flux profiles wider. The burner type must be taken into consideration in the design. 

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