NOX combustor, dry low

It is necessary to modify the edge of the hole in various ways to reduce these stress concentrations. Some methods of modification are priming, plunging, and standard radiusing and polishing methods. In the Dry Low NOx Combustors, especially in the lean pre-mix chambers, pressure fluctuations can set up very high vibrations, which lead to major failures. 

Figure 10-18 shows how in the past 30 years the reduetion of NOx by first the use of steam (Wet Combustors) injeetion in the eombustors, and then in the 1990s, the Dry Low NOx Combustors have greatly redueed the NOx output. New units under development have goals, whieh would reduee NOx levels below 9 ppm. 

Lean burn/dry low-NOx combustors can generate NOx emissions levels as low as 9 ppm (at 15% 02), while those with liquid fuel combustors have NOx emissions limited to approximately 25 ppm (at 15% 02). There is no substantial difference in general performance with either fuel. However, the different heats of combustion result in slightly higher mass flows through the expansion turbine when liquid fuels are used, and thus a very small increase in power and efficiency performance is obtained. Also, the fuel pump work with liquid fuel is less than with the fuel gas booster compressor, thereby further increasing net performance with liquid fuels. 

Use of Different Fuels and Their Characteristics - The best commercial, dry, low NOx combustors today are optimized for clean-burning natural gas. However, with raised natural gas prices over the next decade, power plants may be forced to burn low-heating value fuel gas, products of gasification or low quality residual fuels. As the combustion becomes more complicated, e.g. lean-premixed combustors, to handle NOx emissions from different fuels is bound to become more complex, too. Hence, development of the catalytic combustor must also be directed towards fuels other than natural gas. These fuels could be other hydrocarbon feedstock, e.g. diesel fuels,which are more available than natural gas in some parts of the world, and kerosene,which is used for jet-turbines in aeroplanes. An increased use of renewable fuels, such as methanol, ethanol and low-heating value fuels derived from biomass or waste will also lead to a demand to put these fuels to use in gas turbines. ... 

The most important parameter to monitor here is the exhaust temperature spread, which is defined as the maximum thermocouple reading minus the minimum. Traditional monitoring systems compare this value (at steady state conditions) to a constant threshold and monitor it over time. However, this approach has proven fiiiitless, as it is prone to false alarms or alarms that appear too late. The main reason for this is that for hybrid premixed combustors, such as GE DLN (dry low NOx) combustors, the spread depends on the combustion mode and load and is not a constant value. Hence, a more robust approach is to monitor the combustion mode and the main thermodynamic parameters of the unit, and then set the threshold accordingly. Figure 7 shows how this spread changes with combustion mode and load for a multi-can DLN machine. 

The gas turbine eombustors have seen eonsiderable ehange in their design as most new turbines have progressed to Dry Low Emission NOx Combustors from the wet eombustors, whieh were injeeted by steam in the primary zone of the eombustor. The DLE approaeh is to burn most (at least 75%) of the fuel at eool, fuel-lean eonditions to avoid any signifieant produetion of NOx. The prineipal features of sueh a eombustion system is the premixing of... 

Figure 10-24. Schematic of a dry low emission NOx combustor (Courtesy ALSTOM.)...

A third example of the role of kinetic modelling in the analysis of practical combustion systems at high temperatures is in simulations of pulse combustion. Pulse combustors have been used for many years in propulsion, industrial processes such as drying, and in home furnaces, where they are valued for their thermal efficiency and low NOx production rates. However, until kinetic modelling was used to analyze the role of thermal ignition in these systems, it had been impossible to understand the principles of pulse combustion and the real reasons for their good performance. 

Can accomplish in-combustor desulfurization with limestone, generating a dry solid waste product and produce low NOx emissions without special controls. 

An added benefit of pulse combustion is its contribution to environment protection. The rapid combustion, vhich allows extremely short times for the formation of nitrogen oxides, and lower peak temperatures as compared to continuous combustion, result in very low NOx emissions from the pulse combustors (Fig. 2.3). This is especially advantageous when drying foods and biomaterials, provided that the pulse combustor is not fed with sulfur-containing fuels such as propane or natural gas (Kudra, 1998 Kudra, 1999). 

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