Combustion, Fuels and Emissions for Industrial Gas Turbines
It is important that gas turbines used in Oil & Gas applications can burn a wide variety of fuels with the minimum impact on the environment or economics. Many types of gaseous and liquid fuels that can be used in Gas Turbines are discussed, as will be the two basic types of combustion system employed – ‘conventional’ and ‘Dry Low Emissions’ – along with the flexibility of these systems to accept different types of fuel. Some of the common contaminants found in fuels are discussed along with the impact these have on the operability and maintenance of industrial and aero-derivative gas turbines. Topics include: Types of gas turbines Types of exhaust emissions regulated Conventional combustion systems Dry Low (Pre-mixed) emissions combustion systems Fuel quality requirements ‘Pipeline’ quality Natural Gas fuels Premium liquid fuels (diesel, kerosene) Wellhead Gases as a Gas Turbine Fuel Liquified Natural Gas (LNG) Biogas fuels Refinery and process off gases Syngas Natural Gas Liquids and LPG fuels Crude Oil Operational impact of contaminants Water in fuel Storing fuels correctly
Extended Fuels Capability of Siemens ’ SGT-400 DLE Combustion System
To meet the growing demand to operate on gaseous fuels with little or no treatment, or use fuels derived from a variety of waste conversion processes Siemens Energy has extended the fuels capability of its’ product range, especially the Dry Low Emissions combustion system. Fuels containing high levels of inert species, nitrogen or carbon dioxide, lower the effective Wobbe Index of the fuels, thus needing increased fuel mass flow to achieve the same energy content. This paper presents the development process along with the results achieved to accommodate a wide range of fuels. Discrete changes were required in the DLE burner hardware allowing fuel flows to be achieved at similar supply pressures and combustor pressure drop as for standard fuels thus ensuring combustion characteristics were not compromised. Some applications are presented and discussed covering both on-shore and off-shore duty, including the most recent application on the SGT-400 using a weak bio-gas derived from an ethanol production plant. A gas only solution was applied, requiring careful management of the control parameters to achieve acceptable starting and transient operation through to the application of load. This programme demonstrates the ability of the Siemens DLE combustor to accommodate a wide range of fuels. 1. Fuel Flexibility Introduction In many parts of the world the use of gas turbines is a new or growing market and the availability of premium pipeline quality gas fuels can be non-existent or very low. The ability to operate on poorer quality fuels offers an alternative route in such evolving markets. The Siemens Dry Low Emissions (DLE) combustion system, which is available across the small gas turbine products, has demonstrated successful operation with over 17million hours of service experience in a wide variety of Oil & Gas and Industrial Power Generation applications. The system meets stringent emission legislation limits over a wide operating range and ambient conditions together with the ability to operate over the full load range. This was therefore the combustion system of choice as the basis for developing an extended fuels capability to cover a much wider range and type of fuels as shown in figure 1. These include wellhead gas fuels with little or no treatment, or gas derived from industrial processes, which have qualities and heat values below those of typical pipeline quality gas fuels. Figure 1: Range of gaseous fuels The SGT-300 was one of the first products, configured with DLE, to operate with gaseous fuels outside of standard pipeline quality gas fuels and has since been followed with extended fuels capability on the SGT-400. Before embarking on the changes necessary in the combustion hardware and fuel delivery system to accommodate the wide range of possible fuels, it is necessary to understand some of the key parameters used to assess a fuel’s suitability. 2. Gas fuel Quality Assessment of fuels proposed for use in a GT defines the critical features that need to be considered with Wobbe Index, Calorific Value and Dew Point some of the key parameters applied. Wobbe Index (WI) and Temperature Corrected Wobbe Index (TCWI) are two of many parameters used to assess fuel and allows a direct comparison of different fuels to be made based on the heat content. Wobbe Index (number) is the Net (lower) calorific value of the fuel divided by the square root of the fuels specific gravity. WobbeIndex 0 0 0 / SG CVv WI Where CVv = net calorific value (MJ/m) at standard conditions (288K, 1.013bara) SG = specific gravity at standard conditions The Temperature Corrected Wobbe Index (TCWI) applies a simple temperature correction. As the supply temperature of a gaseous fuel is increased the TCWI falls. The range of acceptable temperatures for use in a gas turbine varies from OEM to OEM and for Siemens Industrial turbines the range is +2.5C to +120C. The lower figure prevents freezing in the vent of double block and bleed part of the fuel system, whilst the upper limit, 120C, is determined by some of the components used in the fuel system.
Brauer groups of genus zero extensions of number fields
We determine the isomorphism class of the Brauer groups of certain nonrational genus zero extensions of number fields. In particular, for all genus zero extensions E of the rational numbers Q that are split by Q(√2), Br(E) ≅ Br(Q(t)).