OCR Text |
Show where dual-fueled boilers are most competitive. In this program, the fiber burner has been developed primarily for gas firing. Figure 1 shows the fiber burner concept in which the cylindrical burner fits inside the firetube (first pass) of the firetube boiler. The fiber matrix burner is a layer of ceramic fibers vacuum-formed onto a screen of the desired shape. The close-packed fibers form a porous matrix. During operation, premixed gas and air pass through the fiber matrix and are ignited, burning only on the outer surface. The surface glows flamelessly and uniformly. The fibers radiate to the firetube wall, controlling the combustion temperature to between 1200 and 1366 K (1700 to 2000°F) which correspondingly controls thermal NO formation. The burner nominally operates at a heat re-x 2 2 lease rate per unit area of burner surface of 900 MJ/hr-m (80,000 Btu/hr-ft ). The burner is sized based on this operating condition, resulting in a cylindrical burner similar in volume to the conventional flame in a firetube boiler. Fiber burners operate best at surface velocities below the mixture flame speed. Both the low conductivity of the fibers and the cooling flow of reac-tants insulate the inside of the burner from the combustion zone, thereby preventing flashback and stabilizing combustion on the burner surface. SUBSCALE BURNER TESTS Initial tests were conducted in laboratory facilities at Acurex to assess the feasibility of the fiber matrix burner. The test facility was 0.20 m (8 in.) in diameter and water-cooled to model a typical firetube boiler combustion chamber. The materials used were a fiber blanket material and vacuum-formed fiber cylinders of several manufacturers. The results verified that fiber matrix burners have potential for low NO . At nominal operating conditions of ? ? x 900 MJ/hr-m (80,000 Btu/hr-ft ) heat release rate per unit of burner surface area and 10 percent excess air, the NO emissions were less than 25 ppm. The burner also showed potential for high combustion efficiency, stable operation at low excess air levels, and high radiation heat transfer. About one-third to one-half of the energy input was transferred to the load radiatively. These initial tests also defined the operating range for fiber matrix burners, showing that flashback can be a problem for some materials and 3-4 |