Flames ammonia

When ammonia gas was introduced into an argon plasma jet at a flow rate of 60 seem, all argon emission lines disappeared, and a very short but brilliant light-blue flame was formed. A very strong NH emission band was observed. In the ammonia radio frequency plasma, some very weak N2 emission bands due to N2 second positive appeared, but in the ammonia flame formed in an argon plasma jet no emission related to N2 species was observed. 

Notwithstanding the obstacles, however, some absorption studies of combustion processes have been made. Molecular intermediates, such as aldehydes and acids, have been identified in the slow combustion of propane . Hydroxyl radicals can be observed in the absorption spectra of several flames . The greatest success in the application of absorption spectroscopy to flame studies has been in investigations of diffusion flames. Wolfhard and Parker studied the diffusion flames in oxygen of hydrogen, ammonia, hydrocarbons and carbon monoxide. In every case they were able to observe absorption by hydroxyl radicals, and they observed also the absorption of NH in the ammonia flame (NH2 appeared in emission only). Molecular oxygen, and in suitable cases the reactants, could be detected by their absorption spectra, so that a clear picture of the structure of the diffusion flame... 

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. 

Table II. Mechanism for Rich Ammonia Flames (Continued)...

Figure 3. Comparison of (left) observed and (right) calculated profiles for an ammonia flame with an equivalence ratio of 1.50.

Figure 4 outlines the important reactions of the nitrogen species in rich ammonia flames. The most important reactions producing N2 are NNH dissociation and NH + NO. In turn, most of the NNH is produced from N2H2 dissociation which is produced via the reaction NH + NH2. Other NH + NHi reactions are less important. Thus, the NH + NH reactions are primarily responsible for N2 production it is now clear why omission of these channels led to such marked changes in the predicted profiles. One would not expect such changes in predicted profiles in lean ammonia flames here [NH ] would be sufficiently low that the NHi + NHi reactions could safely by omitted. 

The essential feature of this formulation is the second reduction step. This reaction had previously been recognized as important from the fast rate of decay of NO produced in ammonia flames (6,7). This reaction process has been exploited in the Exxon DeNOx process for NO removal by the controlled addition of NH3 to combustion effluents. 

Miller et al. ( ) have performed similar calculations for the ammonia flame experiments of Fenimore and Jones (6) and Maclean and Wagner (7). Dean and coworkers (this Symposium) have also performed experiments and calculations with an emphasis on radical intermediates. 

Interestingly, the major source reaction of N in these ammonia diluted methane-air flames is again reaction R3. The H shuffle reactions form a fast pathway from NH3 to N atoms. The extended Zeldovitch reactions R3, R4, and R5 then determine the NO and N2 production. This behavior is in marked contrast to the pure ammonia flame pathways (Figure 4) where NH2 and NH were significant N2 soucrces, and HNO was the major precursor of NO. 

This difference in behavior between NH3 and CH4 flames results from an approximate doubling of the radical concentration from a 0=1.0 ammonia flame to a 0=1.0 CH4/air flame. This change promotes the NH-+N pathway over the NH-+HN0 pathway. 

Pure ammonia flames are studied experimentally and theoretically to isolate the amine chemistry. Ammonia and NO doped methane-air flames are also studied as models for flames with fuel-bound nitrogen and flames diluted with NO from EGR. We conclude from these studies ... 

Pure ammonia flames follow major pathways which are different from doped hydrocarbon flames. NHj is pivotal for determining relative N2 and NO yields in ammonia flames. 

Dasch, C.J. Blint, R.B. "A Mechanistic and Experimental Study of Ammonia Flames", General Motors Research Laboratories Report GMR-4232, and Eastern States Fall Meeting, The Combustion Institute, 1982, Paper ESS/CI 82-71. 

Ammonia-doped methane flames, 96-98 Ammonia flame... 

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