OCR Text |
Show fluctuations, the system has NOx emissions characteristics similar to a premixed system, thereby allowing NOx control through stoichiometry. OBJECTIVES The initial objective of the current experiments is to investigate the project hypothesis that improved fuel and air mixing can reduce NOx emissions while maintaining combustion efficiency in an industrial-type, natural gas burner. Although, as cited above, previous studies have researched fuel and air mixing in relation to NOx production, many were based on gas turbine applications, involved non-realistic geometries, or did not study in detail the steps required to reduce NOx. This program is directed to quantifying how fuel and air mixing affects NOx using 1) the experimental results (UCICL), 2) comprehensive combustion modeling, including NO kinetics (LLNL), and 3) advanced laser diagnostics, namely DFWM, to measure NO and N02 concentrations in the flame (SNLL). The following describes the approach and initial efforts undertaken to meet these objectives. APPROACH A custom designed, 100,000 Btu/hr natural gas flred burner serves as the principal test facility for this study. In addition, an existing 50,000 Btu/hr burner has been used for exploratory studies, to develop experimental protocols, to test the hypothesis that more unifonn fuel and air mixing can reduce NOx production, and to evaluate diagnostic feasibility and application. The 50,000 Btu/hr burner is fired upward into a square enclosure. A schematic of the burner geometry is shown in Figure 1. The central fuel tube is 0.71 inches in diameter and is surrounded by two coaxial air passages. The inner or "primary" air passage has a diameter of 2.22 inches, while the outer or "secondary" air passage has a diameter of 3.15 inches. Swirl is generated by an annular, fixed vane swirler placed over the end of the inner air passage. To investigate the effects of fuel and air mixing on burner performance, three injection nozzles were tested. The baseline nozzle injected the fuel along the axis of the burner. Two cone annular nozzles with injection angles of 300 and 1400 were also evaluated. The 100,000 Btu/hr burner test stand was recently completed and demonstrated at the UCICL. A schematic diagram of the burner and the fuel nozzles used in this study is shown in Figure 2. The up-fued, coaxial, natural gas burner consists of four elements: the entry plenum, swirler, contraction, and quarl. The central natural gas fuel tube has a 0.50-inch diameter and is surrounded by a I.O-inch combustion air annulus. Swirl is introduced into the air annulus via an axial stream and a tangential, or swirl, air stream. With this arrangement, the swirl intensity can be varied by changing the ratio of swirl air to axial air. For these experiments, clockwise swirl is used. The swirl intensity is represented by the geometric swirl number as defmed by Feikema, Chen, and Driscoll (1990): 3 |