Tangential Velocity Effects and Correlations for Blow-Off and Flashback in a Generic Swirl Burner and the Effect of a Hydrogen containing Fuel

Abstract

Swirl combustors are almost universally employed in gas turbines owing to their great stability range which occurs due to the formation of a CRZ which recycles heat and active chemical species to the flame root, considerably enhancing stability over a wide range of operating conditions. Increasing interest in lean fuel premixed systems has arisen because of its propensity to reduce NOx emissions. This coupled with the use of hydrogen containing alternative fuels offers the possibility of reduced greenhouse gas emissions. Alternative fuels include hydrogen-enriched natural gas in various proportions, by products of process industries such as coke oven gas and indeed pure hydrogen. This gives rise to numerous areas of concern for operators and developers of premixed gas turbine combustors especially in the area of flashback and blow-off. Blow-off and flashback with hydrogen containing fuels are of special concern with hydrogen enriched fuels, owing to the high flame speed of hydrogen, to such an extent that diffusion combustion is commonly employed resulting in higher NOx emissions. This paper compares the effect of a typical alternative hydrogen containing fuel, Coke Oven Gas (COG) with methane/natural gas upon blowoff and flashback in two premixed swirl burners in swirl number regimes representative of those found in practical systems. Methane/Natural gas is used as a base fuel for comparison. All results are obtained at atmospheric pressure without air preheat as a precursor to pressurised tests. The two swirl burners have quite different inlet systems, but common exhaust designs. A central fuel injector just extends into the exhaust and is ~40% of the exhaust diameter, a common industrial size. Swirl numbers range from less than 0.8 to over 4. A new simple correlation has been found between tangential velocity, blow-off and flashback in swirl burners operating under conditions representative of industrial practice for methane and COG. For coke oven gas (COG containing 65% H2 and 25% CH4) the correlation was found for freely expanding flames. For flashback a similar but weaker correlation was found for methane whilst for COG the correlation was very successful in the fuel rich regime. . The paper analyses this correlation in the context of the flame speed in these type of swirling flames as well as the effect of the changing combustion aerodynamics as equivalence ratio varied.