Fuel nitrogen

Fuel-bound NO. is formed at low as well as high temperatures. However, part of the fuel nitrogen is directly reacted to N2. Moreover, N2O and N2O4 are also formed in various reactions and add to the complexity of the formation. It is virtually impossible to calculate a precise value for the NO, emitted by a real combustion device. NO, emissions depend not only on the type of combustion technology but also on its size and the type of fuel used. 

Fig. 7. Schematic showing reaction pathways by which fuel nitrogen, N, is converted to NO and N2. The bold lines indicate the key pathways (28). Thermal...

In a rotary kiln, the burner can produce both thermal and fuel NO, if the fuel contains nitrogen. Many soHd waste streams also contain nitrogen, typically as much as 20 wt %, which contributes to the fuel NO pathway. Key sources of soHd waste fuel nitrogen include plastics, nylons, dyes, and other process wastes. Nylon, for example, is 33 wt % nitrogen. 

Nitrogen oxides (NOx) - thermal NOx is formed with gas as with other fuels, particularly if air preheat is practiced. There is very little fuel nitrogen compared with other fossil fuels so that the total NOx emissions are lower. 

Figure 17 Conversion of fuel nitrogen to HCN and NH3. Furnace temperature top, 1,073 K bottom, 1,273 K. Reprinted from Leichtnam et al. [46]. Copyright 2000, with permission from Elsevier.

J.E. Fuel nitrogen conversion in solid fuel fired systems, Prog. Energy Combust. Sci., 29, 89-113, (2003). 

During combustion processes the molecular nitrogen in the combustion air and the fuel nitrogen that may be present in the fuel is converted into nitric oxide and some nitrogen dioxide4 when NO and residual O2 are cooled together. The NO formation is also controlled by (1) thermal NO, (2) prompt NO and (3) N2O to NO routes5-7. 

FIGURE 8.7 Reaction path diagram illustrating the major steps in prompt NO formation, the conversion of fuel nitrogen in flames, and reburning (from Bowman [12]). 

Thus, there is great similarity between the prompt NO reactions discussed and the fuel-nitrogen reactions. 

The general scheme of the fuel-nitrogen reactions is also represented in Fig. 8.7. 

In fuel-rich systems, there is evidence [8, 22, 23] that the fuel-nitrogen intermediate reacts not only with oxidizing species in the manner represented, but also competitively with NO (or another nitrogen intermediate) to form N2. This second step, of course, is the reason that NO yields are lower in fuel-rich systems. The fraction of fuel nitrogen converted to NO in fuel-rich systems can be as much as an order of magnitude less than that of lean or near-stoichiometric systems. One should realize, however, that even in fuel-rich systems, the exhaust NO concentration is substantially greater than its equilibrium value at the combustion temperature. 

In fuel-rich systems, the conversion reactions of the fuel-nitrogen intermediates are subject to doubt, mainly because the normal oxidizing species 02,0, and OH are present only in very small concentrations, particularly near the end of the reaction zone. Haynes el al. [14] offer the interesting suggestion that the CN can be oxidized by C02 since the reaction... 

The reactions of fuel-sulfur and fuel-nitrogen are closely coupled to the fuel oxidation moreover, sulfur-containing radicals and nitrogen-containing radicals... 

The objective of one of the papers, which was a part of Orlanders thesis [9], was to study the effects of the bed stoichiometric ratio (SRb) (air factor of conversion system) and fuel nitrogen content on the formation of various gaseous species (O2, CO, CO2, H2 and NO) in the interstitial gas, off-gas and flue gas. 

The formation of prompt NO increases the complexity of the nitrogen chemistry in gas flames considerably. This is illustrated in Fig. 14.9, which shows the most important reaction paths in prompt NO formation, as well as fuel nitrogen conversion—two mechanisms that share some common features. Prompt NO is, as the name indicates, a very rapid mechanism. The initiating step (R117) takes place in the flame zone, where methylidyne radicals (CH) may be formed in significant quantities. 

CP. Fenimore. Studies of Fuel-Nitrogen in Rich Flame Gases. Proc. Combust. Inst., 17 661,1979. 

The sulfur percentages of 0.02 weight-percent or less in the shale oil diesel fuels would permit these fuels to be classed as low-sulfur diesel fuels. Nitrogen percentages were low, ranging from 141 to 202 parts per million (ppm). Cetane indexes, calculated by ASTM method D-976 (7j, were excellent, ranging from 48 to 56. 

Figure 5. Fuel nitrogen content as it influences subscale combustor NOx emissions...

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