NOx emission characteristics of gas-fired radiant tube flames: The role of partial premixing
Abstract
An experimental study of a laboratory natural gas-fired radiant heating tube with quartz walls and a practical burner geometry reveals that confinement of a jet flame in a tube affects its behavior dramatically. Qualitative observations, colors, and visible flame heights demonstrate that the flame burns either as a long, luminous, orange flame or as a very short, blue flame. Wall temperature profiles, global radiation measurements, and an overall energy balance delineate differences in the radiant tube performance between the orange and the blue modes. Measurements of local species concentrations reveal that the confined flame transitions from non-premixed behavior through various levels of partial premixing, affecting exhaust NO$\rm\sb{X}$ amounts favorably or unfavorably depending on the operating condition. Motivated by changes in the NO$\rm\sb{X}$ emission index with the confinement-induced mixing levels, a more fundamental study of NO$\rm\sb{X}$ formation in partially premixed flames was performed. In particular, hydrocarbon-nitrogen chemistry was explored as a possible explanation for a previously observed NO$\rm\sb{X}$ emission index minimum in partially premixed coflow jet flames. A representative hydrocarbon species, CH radical, was studied experimentally in laminar CH$\sb4$/air flames using chemiluminescence detection and absorption spectroscopy. Temperatures were measured using thin filament pyrometry. Computations of partially premixed flames were performed using the Sandia National Laboratories flame code Oppdif with the GRI-Mech 2.11 chemical kinetic model. Experimental results provide information about the thermal structure of these flames and reveal that the global CH radical concentration increases with increased partial premixing. It is recognized that experimental measurements of a single chemical species are not adequate for assessing the relative importance of competing reaction pathways to the formation of NO. The present computations reveal more detailed information for this purpose. Computational results from low strain rate flames reveal that the CH radical and other hydrocarbon species exhibit unique double concentration peaks in partially premixed flames, resulting in triple NO reaction zones. The hydrocarbon behavior causes widened reburn zones and reduced rates of the reactions which form and consume NO, and thus provide a possible explanation for the NO$\rm\sb{X}$ behavior of partially premixed flames.
Degree
Ph.D.
Advisors
Gore, Purdue University.
Subject Area
Mechanical engineering
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