OCR Text |
Show liquid fuels. Several low-NOx burners developed cooperatively by GRI, burner manufacturers, and the gas industry are already available, and others are timed to meet future market needs. In the following pages, new, cost effective natural gas burner technologies are described that can reduce NOx without loss of productivity or performance. NOx 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 can be formed: 1. Thermally, via the direct oxidation ofN2 by oxygen. 2. Through reaction with intermediate hydrocarbon radicals such as CH (the "prompt" mechanism) . 3. By reactions involving intermediate formation of nitrous oxide, N20 (this source of NO x is generally important at high pressures with high excess oxygen). A fourth mechanism, involving fuel-bound nitrogen, does not occur in natural gas. Each of these possible pathways has a distinct chemical mechanism, and consequently, different behavior depending on the chemical and thermal characteristics of a specific combustion process. They also make different contributions to the total NO being formed. In addition, each pathway responds to differing forms of control. Almost all of the NOx being formed in high temperature processes arises from the thermal NOx pathway via the extended Zeldovich mechanism: N2 + 0 - NO + N O2 + N - NO + 0 N2 + OH - NO + H (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 3. 4 |