Jesse Stewart

A Gas Turbine Innovative System for Managing Fuel With Different and Variable-Over-Time Wobbe Indexes

Due to the substantial increase in sources of gas, natural gas interchangeability is a key subject in the industry today. The extensive pipeline network means that natural gas arriving at appliances, boilers, burners and power plant turbines could come from anywhere. Fuel compositions vary from one source to another. Moreover, most recently, Liquefied Natural Gas has emerged as a major source and the composition of gas derived from LNG substantially differs from the natural gas one. In Dry Low NOx (DLN) systems, those changes in fuel composition can cause dangerous increase in combustion dynamics and can also affect the NOx emissions of the machine. Therefore, in order to meet the growing market demand for gas turbine combustors able to tolerate significant alterations in fuel composition, a system capable of burning gases with differing and variable over time Wobbe Indexes was developed. This innovative system does not involve any combustion hardware modifications. It allows the use of a premixed combustion system that complies with emissions, reliability, and safety, even when burning a fuel that is distinctly different from the original design gas. In particular, the system was developed in order to meet the requirements of a customer for burning any continuously and slowly varying mixture of two fuel gases, whose Wobbe Indexes difference is up to 25%. Since the burner is designed for 100% of the gas with lower Wobbe Index, the gas that has a higher WI needs to be heated, in order to achieve a target Modified Wobbe Index; the same happens for any mixture of the two gases. The system is based on a closed loop control on the Modified Wobbe Index of the fuel. Two turbine control gas chromatographs, located upstream the combustor inlet, measure the gas characteristics (LHV, specific gravity and temperature) and calculates the MWI. If it is different from the target one, it is corrected by modifying the temperature set point of a heat exchanger. The hardware is completed with one more plant gas chromatograph, located upstream the heat exchanger, for evaluating the fast and complete switch from one gas to the other one. In addition to the normal operation, that is with the 100% Lower Wobbe Index gas (L) or 100% Higher Wobbe Index gas (H) or any continuously and slowly varying mixture of these two gases, the system allows both the black and the normal start, the complete switch back and forth between 100% L gas and 100% H gas and load sheds and rejection. Moreover the two gases can be burned in diffusion combustion mode, as available, without requiring any increase in temperature, with no limitation from firing to full load. The capability of the system to adjust to all of the previously described events, potentially dangerous and damaging for the Gas Turbine combustion system, makes it suitable for applications that burn different lots of gases coming from different LNG sources, since it allows the turbine to accommodate the differences in Wobbe Index, due to various gas lots on a pipe line.Copyright

Passive Control of Dynamic Pressures in a Dry, Low NOx Combustion System Using Fuel Gas Circuit Impedance Optimization

Dry, Low NOx (DLN) gas turbine combustion systems that use the lean, premixed combustion technique for emissions control are susceptible to dynamic pressure oscillations. During the initial full-load prototype testing of the MS5002E, excessive dynamic pressures were encountered when attempting fully premixed combustor operation, preventing the gas turbine from meeting a 15 ppm NOx emissions target. A series of experiments were performed to examine potential acoustic differences between the original laboratory fuel injection system and the prototype hardware used in the field test. The experimental results were used to validate an analytical model that was used to optimize the fuel circuit geometry for dynamics reduction. The resulting revised design demonstrated a ten-fold reduction in dynamic pressure amplitudes. As a result, the system was able to operate over the premixed mode operating range and provides the desired NOx levels with acceptable dynamic pressures and operability.Copyright