OCR Text |
Show INTRODUCTION The most widely used design strategy for NOx reduction is staged combustion. The creation of fuel-rich and -lean combustion zones in flames by means of staging the input of either air or fuel is a successful method of NOx emission control. Nevertheless, the degree of NOx reduction achieved by these technologies has been observed to vary widely and to depend on the combustion system in question (1,2,3). While the principles of staged combustion control of NOx emISSIon are well established (4,5,6), their practical realization is hampered by lack of information on the overlapping processes of the nitrogen-hydrocarbon chemistry and the mixing-temperature history of the fuel in the flame. The problems are especially difficult in the case of the so called "internal staging" process, in which the fuel-rich and -lean combustion zones must be produced by appropriate fuel-air mixing in a single 10w-NOx burner, rather than by producing fuel- rich and -lean combustion zones in the combustion chamber using ove rfire air. This problem is addressed in an experimental research project at MIT focused on the principles of low-NOx burner design applied to natural gas and oil combustion. This paper reports initial results obtained with natural gas as the fuel. The experimental RSFC (Radially Stratified Flame Core) burner was developed at MIT based on the patented design of a multiannular burner (7). The RSFC burner is attached to the flame tunnel (3 MWtm 1.2 x 1.2 x 4.5 m) of the MIT Combustion Research Facility (CRF). Parallel with the experiments a mathematical modeling study is carried out in which the progress of combustion along the flame is computed for the effect of design and operating variables of the burner, using the "Fluent" fluid dynamics code. In this paper, the relationships between burner input parameters and emissions of CO and NOx are reported. EXPERIMENTAL The MIT Combustion Research Facility was designed to permit detailed in-flame measurements of the flow field and of spatial distributions of temperature and chemical species concentrations to be made. The variable heat extraction along the flame - by the use of completely and partially water cooled furnace sections - enables the close simulation of large scale flame systems to be made. Access to the flame for optical or probe measurements is provided by a 1.0 m long slot at the burner and by instrument ports at every 30 cm length further downstream along the flame tunnel. The experimental RSFC burner: the concept of radial flame stratification The burner consists of three concentric annuli with each of the annular nozzles at a larger radial position extending further in the axial direction (Fig. 1). Fuel is introduceti in the cente!.: through a fuel gun. The three sections of the burner can be axially adjusted as may be required to maintain optimal geometry at tum down. Additional features of the burner include independently variable swirl control in each annular air nozzle by means of IFRF moveable block swirlers. 2 |