| OCR Text |
Show The increased fl ame stability is the result of an electric body force which is imposed on the tlame by the interaction of the field with chemiions produced in the combustion of hydrocarbon fuels . In the above case the burner was grounded and an electrode ring, maintained at 3 kV, was placed above the burner. The ring diameter was large enough that it did not come into direct contact with the flame. The field then produced a force on positive ions that drove them back towards the burner, which is the stabilizing electrode. In order to maintain local charge neutrality in the bulk of the flame the positively charged ring collects electrons. Because electrons are highly mobile, the exact placement of this electrode is not critical and can be located exterior to the flame zone. The basic phenomenon has been described as an ionic wind. 3 At atmospheric pressure conditions an electric field can produce a velocity estimated to be on the order of 11 m s-1.4 Thus, if an electric field is to be beneficial, velocity changes of this order must be able to affect the stabilization process. II. BACKGROUND FOR CYCLONIC COMBUSTOR TESTS AeroChem entered into a cooperative program with the Institute of Gas Technology (IGT) to determine the feasibility of using an electric field in an IGT low NOx cyclonic burner. The criterion was to determine if flow and flame properties in the IGT burner were similar enough to conditions in the laboratory scale cylindrical burner experiments performed previously at AeroChem to warrant the installation of electrodes in the IGT burner. AeroChem would recommend the electrode installation and a test of its capabilities only if this criterion was met. The specific criterion was that locations exist in the flame where electric conductivity is relatively high and flow velocity is low in regions that are expected to participate in stabilizing the combustion process. The challenge was to reduce emissions in a combustor which already has excellent characteristics. Many industries require relatively low-temperature combustion with ultra-low emissions for specific applications. For example, air drying of food often requires ultra-low emission combustion at flue gas temperatures as low as 1600°F (870°C) for direct-fired air heaters . Similarly, gas turbine combustors require relatively low combustion temperatures around 2000°F (1100°C), but at elevated pressures. Low combustion temperatures are often achieved in premixed systems by operating with high excess air, which substantially decreases NOx formation. However, significant difficulties with ignition and flame stabilization at low combustion temperatures are an obstruction to further reducing NOx while maintaining low CO and total hydrocarbon (THC) emissions , particularly when high turndown ratios are required . Moreover, stable burner operation at very high excess air requires finer combustion control, complicating the operating and control systems and resulting in both high capital and operating costs . 2 |
