OCR Text |
Show on developing burner systems capable of addressing anticipated NOx regulations aimed at high temperature industrial processes. NO Formation Mechanisms Although the US Federal Regulations specify NOx as the criteria pollutant being regulated, the actual nitrogen oxide formed in a flame is for the most part NO. For natural gas, there are three ways that NO .. N 2 can be formed: .., 1. 2. Thermally, via the direct oxidation of N2 by oxygen. Through reaction with intermediate hydrocarbon radicals such as CH (the "prompt" mechanism). 3. By reactions involving intermedi-ate formation of nitrous oxide, N20 . ~CH4 02 Products of Combustion A fourth mechanism, involving fuel-bound nitrogen, does not occur in natural gas. NO Each of these possible pathways has a distinct chemical mechanism, and as a consequence, different behavior depending on the chemical and thermal characteristics of the combustion process. They also make different contributions to the overall NO being formed. Almost all of the NOx being formed in high temperature processes arises from the thermal mechanism by way of the extended Zeldovich mechanism: (1 ) (2) (3) • The first, rate determining, step (1) has a high activation energy, so that this process only becomes meaningful at higher temperatures. But at these temperatures, the radical chain reaction process overwhelms other NO sources. The consequence of the temperature dependence of thermal NOx is shown in Figure 1. In practical terms, then, the focus of NOx control for high temperature processes has been on reduction of the flame temperature by fuel or air staging combined with either external flue gas recirculation (FGR) or internal dilution of the flame with furnace 2 |