Ammonia flame temperature

When large spherical AP particles dg = 3 mm) are added, large flamelets are formed in the dark zone.Pl Close inspection of the AP particles at the burning surface reveals that a transparent bluish flame of low luminosity is formed above each AP particle. These are ammonia/perchloric acid flames, the products of which are oxidizer-rich, as are also observed for AP composite propellants at low pressures, as shown in Fig. 7.5. The bluish flame is generated a short distance from the AP particle and has a temperature of up to 1300 K. Surrounding the bluish flame, a yellowish luminous flame stream is formed. This yellowish flame is produced by in-terdiffusion of the gaseous decomposition products of the AP and the double-base matrix. Since the decomposition gas of the base matrix is fuel-rich and the temperature in the dark zone is about 1500 K, the interdiffusion of the products of the AP and the matrix shifts the relative amounts towards the stoichiometric ratio, resulting in increased reaction rate and flame temperature. The flame structure of an AP-CMDB propellant is illustrated in Fig. 8.1. 

It should be noted that not all flames have the behaviors discussed above. For example, the equilibrium species distribution in some H2-N20-Ar flames has essentially the same mole number as the reactants. As a result the adiabatic flame temperature is achieved directly in the flame front with no long recombination tail. Ammonia-oxygen flames exhibit a slow approach to chemical equilibrium, albeit with a long dissociation, not recombination, tail [279], Here the temperature in the flame front overshoots the adiabatic flame temperature, with the equilibrium temperature being approached from above as the dissociation reactions proceed. In certain highly strained, rich, hydrocarbon flames (e.g., C2H2-H2-O2), such as those used for flame-based diamond growth, the temperature can also overshoot the adiabatic flame temperature in the flame front. Here the overshoot is caused by the relatively slow dissociation of the excess acetylene [270]. 

Consequently, it would appear that the flame temperature is determined not by the specific reactants, but only by the atomic ratios and the specific atoms that are introduced. It is the atoms that determine what products will form. Only ozone and acetylene have positive molar heats of formation high enough to cause a noticeable variation (rise) in flame temperature. Ammonia has a negative heat of formation low enough to lower the final flame temperature. One can normalize for the effects of total moles of products formed by considering the heats of formation per gram (Ahf) these values are given for some fuels and oxidizers in Table 1. 

If the hydrogen combustion reaction is conducted under conditions that result in high temperatures (flame temperature of 2488 Kelvin) and an excess of air, the excess oxygen will react with the nitrogen of the air to produce small quantities of nitric oxide. In hydrogen rich mixtures, excess hydrogen reacts with nitrogen to form trace amounts of ammonia. 

The gas mixtures fed to the burner were prepared in a stainless steel manifold using electronic flow controllers. A range of rich ammonia flames was studied in which the fuel equivalence ratio ranged from 1.28 to 1.81. Flame temperatures were measured with Pt/Pt-13%Rh thermocouples. The bead diameter was only 0.12 mm so that the radiation correction was only 80 K. 

Figure 2. Measurement of OH rotational temperature at a height of 0.9 nm above the burner for an ammonia flame with an equivalence ratio of 1.28. (Reproduced with permission from Ref. 6. Copyright 1982, J. Chem. Phys.)...

Rotational Excitation of OH. One of the most surprising aspects of our data was the observation of rotationally hot OH in the flame front of (() = 1.28 and <() = 1.50 flames. Rotational temperatures " 200 K higher than radiation corrected thermocouple measurements were observed these were not expected since rotational energy transfer is so fast at atmospheric pressure. Such excitation was not observed beyond the flame front in any of our ammonia flames and not even within the flame front of a methane... 

The starting point in development of an ammonia flame mechanism was a mechanism previously used to model ammonia oxidation in a flow tube near 1300 K ( ). Additional reactions were added that were thought to be important at the higher flame temperatures. Calculations with this mechanism produced profiles in marked disagreement with our data. The predictions were slower than observed decay of NH species was much too slow, and OH peaked too late by about 2.5 mm. To make matters worse, far too much NO was formed. The NO problem was especially troublesome in that attempts to increase the rate of NH decay only served to produce even more NO, since NO was the primary decay channel for the NHi species. A possible resolution of this dilemma involves reactions of the NHi species with each other to form N-N bonds. These complexes could then split off H atoms to ultimately form N2. 

High levels of nitrogen removal are also possible. Some of the coal-nitrogen is converted to ammonia which can be almost totally removed by commercially available processes. Nitrogen oxide formation can be held to allowable levels by staging the combustion process at the turbine or by adding moisture to hold down flame temperature. 

Silane Ammonia Argon annulus chamber Reactor pressure Flame temperature Laser intensity 72 cm mi n 320 cm rmin 600 cm min 1.5 Imin 720 torr ino°c,, 5.6x10- Wcm" ... 

The reactants ate fed into the tail flame of a d-c nitrogen plasma. The reaction occurs rapidly at temperatures around 1500°C and the HCl reacts with excess ammonia to form ammonium chloride. Similar reactions have been carried out using furnaces, lasers, and r-f plasmas (34) as the source of heat. Other routes using titanium tetrachloride starting material include... 

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