OCR Text |
Show operating conditions, and identified the need for improved fiber matrix materials. The burners operated stably from a heat release rate per unit surface area of 740 to 1700 MJ/hr-m2 (65,000 to 150,000 Btu/hr-ft2), corresponding to a turndown ratio of 2.3 to 1. At each heat release rate, the burners were tested over a range of 0 to 80 percent excess air. Nominal operating conditions were defined as 900 MJ/hr-m2 (80,000 Btu/hr-ft2) and 10 percent excess air to achieve minimum emissions and optimum burner temperature. During initial vacuum-formed cylinder tests, flashback failures occurred that damaged the burner and its internal hardware. Flashback was not a function of reactant velocity, since each failure occurred at different operating conditions, but appeared to be due to nonuniformities in the wall thickness caused by erosion of the fiber matrix. The erosion apparently caused openings in the matrix which became enlarged by the flow over time. These nonuniformities eventually result in local hot spots and flashback. Since the major cause of flashback was poor quality fiber matrix material, an improved fiber matrix was needed. In addition, the blanket material proved to be undesirable due to nonuniform construction and the difficulty in forming a leak-proof seam. Two additional radiant burner vacuum forming operations were identified, and burners from each source were purchased for testing. These burners were well constructed, uniform in appearance, and had a high emissivity ingredient in the matrix material to enhance radiation transfer. The burners were approximately 0.15 m (6 in.) in diameter, leaving an annulus of 0.025 m (1 in.) between burner and chamber wall. One burner, Burner A, was 1.06 m (42 in.) long, with a 740 MJ/hr (700,000 Btu/hr) maximum heat input. Burner B was shorter than A due to fabrication constraints. Burner B was 0.61 m (24 in.) long, with a 420 MJ/hr (400,000 Btu/hr) maximum heat input. The burners were tested on natural gas at heat release rates per unit of burner surface area of 740 to 1420 MJ/hr-m2 (65,000 to 125,000 Btu/hr-ft2) and at 0 to 60 percent excess air. Both burners showed excellent results with low emissions, stable operation, and no flashback over the entire test range. Test results for the two burners at nominal operating conditions of 900 MJ/hr-m (80,000 Btu/hr-ft ) and 10 percent excess air are shown in Table 1. 3-5 |