OCR Text |
Show oxygen jets prior to reaction with the fuel. The operation of the "A" Burner outside the furnace in air changes the combustion environment appreciably. Instead of aspirating in furnace gases, air is entrained into the oxygen jets and the burner becomes an enriched air burner. The oxidant-air mixture reacting with the fuel will always have more than 21~ oxygen and with a large excess of oxygen for combustion. The nature of the flame will therefore be quite different from that within a furnace. Gas mixing and recirculation within the furnace are accomplished with the "A" Burner by using very high velocity oxygen jets. In a conventional air burner, the velocity of the air stream is typically 50-200 ft/sec. with the "A" Burner, the velocity of the oxygen jets is of the order of 1000 ft/sec. The high momentum for an air burner is due to the high mass rate and moderate gas velocity. About the same momentum is achieved using the "A" Burner with a low mass rate and high gas velocity. As a result, the gas mixing and recirculation required to obtain uniform furnace temperature are achieved with the "A" Burner using 100~ oxygen. Initially the "A" Burner was tried in an experimental furnace without the oxygen annulus surrounding the fuel stream. During the preheat period, the burner operated unstably with the flame front oscillating back and forth in the furnace. This caused the furnace to vibrate each time the flame front moved from the back to the front of the furnace towards the burner. By passing a portion of the oxygen (about 5-10~ of the total oxygen flow) through the annulus around the fuel feed, a continuous flame front was established near the burner face at the oxygen envelope - natural gas interface. This stabilized the combustion within the furnace eliminating flame oscillations and furnace vibrations. The "A" Burner has been designed with individual oxygen nozzles screwed into the burner face. Each nozzle can be changed easily to alter the hole diameter and angle with corresponding changes in the jet velocity and angle. This feature of the "A" Burner has provided flexibility that has proven to be very useful for industrial applications. The angle and direction of the oxygen jets determine where the heat release by combustion will occur in the furnace. If a portion of the furnace is cool due to unusual heat losses (such as at a charging door), some of the oxygen jets 332 can be directed to that area to compensate accordingly. In this way the "A" Burner can be tailored on site to fit the specific needs of a furnace. Within a few days of operation after installation of the burners, the desired temperature distribution in an industrial furnace is achieved by making adjustments in the nozzles. OPERATING RESULTS Extensive operating experience has been obtained with the "A" Burner both in an experimental furnace at carefully controlled conditions and in industrial furnaces. The experimental furnace was a 4 ft x 4 ft x 8 ft refractory lined enclosure with a heat sink at the bottom and thermocouples embedded in the walls to determine the temperature distribution in the furnace. The burner for this furnace was designed for natural gas firing at 0.5 to 1.0 MMBTU/hr. No flame stability problems were encountered for oxygen jet velocities up to and exceeding sonic velocity. The burner typically operated with a high fire-low fire mode (turndown ratio of about 10:1) to maintain a given control temperature. A uniform temperature distribution within the furnace was obtained. One of the striking features of the operation in the experimental furnace when operating at high temperatures was the absence of an apparent flame. The only visible flame front was that for the small flow of oxygen (5-10~ of the total oxygen) through the annulus and reacting with a 'portion of the fuel. There was no visible flame front for the combustion reactions between the oxygen jets and the bulk of the natural gas. The mixing and reaction of the fuel and oxygen take place within a large volume in the furnace causing the flame to be diffuse and not visible as viewed through the sight glasses. This is in contrast to conventional burners that have a well defined, visible flame. Reaction kinetics and equilibria for forming nitrogen oxides are favored by high temperatures. The replacement of air with oxygen normally result in higher flame temperature with a corresponding increase in the concentration of NOx providing nitrogen is present. With the "A" Burner, the measured NOx levels were very low, indicating that the flame temperature was low. This has been confirmed by temperature measurements made with a suction pyrometer. The NOx concentration was measured in the flue of the experimental furnace operating |