Michael Welch, industry marketing manager — oil & gas, and Brian Igoe, FEED team proposal manager, at Siemens UK, will present “Combustion, Fuels and Emissions for Industrial Gas Turbines” at the 43rd Turbomachinery/30th Pump Symposia in Houston on Sept. 23 at 8:45 AM. The tutorial will cover the importance of minimizing the environmental and economic impacts of gas turbines used in oil and gas applications and the fuels they burn. Many types of gaseous and liquid fuels that can be used in industrial gas turbines, as well as the two basic types of combustion systems, will be discussed in this tutorial. Some common contaminants found in fuels and the impact they have on the operability and maintenance of an industrial gas turbine also will be covered. Topics of discussion will include exhaust emissions and regulations, combustion systems, fuel quality requirements, pipeline-quality natural gas fuels, operational impact of contaminants, and the proper storage of fuels. Learn more about the 43rd Turbomachinery/30th Pump Symposia at http://pumpturbo.tamu.edu.
Modern highly efficient gas turbines rely on high-quality alloys to allow increased firing temperatures to be achieved, while still maintaining acceptable product life. To ensure this is achieved, far more attention on the composition of the fluids, from all sources, entering the gas turbine is necessary, including air, lubricating oil, and fuels. Gas turbines can and do use a wide range of gaseous and liquid fuels. Fuel quality and storage both have fundamental requirements that must be addressed.
All gas-turbine OEMs provide comprehensive specifications covering the fuel quality permitted for use in the gas turbine. These are used to ensure fuel quality is defined at the onset of a project and throughout the lifetime of the turbine and are prepared for good reason — to ensure acceptable turbine operation is achieved with little or no impact on major turbine component life. It is necessary therefore to understand the fuel composition and the supply conditions in more detail so that measures can be taken to minimize the impact of any constituents of the fuel gas or the contaminants contained within it. Identification of contamination has become particularly necessary as this can have a detrimental impact on exotic materials used in turbine blading. In some instances, constituents of and contaminants within the fuel can impact combustion emissions, whether the turbine is fitted with a diffusion flame or a dry low emissions combustor.
Compositions of gaseous fuels can vary quite widely depending on their sources. Pipeline-quality gas is regarded as a suitably clean fuel but associated, or wellhead, gas may be the source, or the fuel gas may be derived from waste streams from industrial processes. Liquid fuels are also commonly used, mainly as a backup fuel but sometimes as the primary fuel, and these can also contain potentially harmful contaminants.
Contaminants of potential concern include:
- higher hydrocarbons
- inert gases (nitrogen and carbon dioxide)
- carbon monoxide
- alkali metals (sodium and potassium) and heavy metals (vanadium, nickel, and lead)
- organic contaminants (tars, asphaltenes).
Higher hydrocarbon species
Associated gas from oil production is a common gas turbine fuel. Methane is usually the major constituent but associated gases can be rich, containing considerable quantities of heavier hydrocarbons — ethane, propane, butane, pentane, and potentially even longer hydrocarbon chains. In some instances, heavier hydrocarbons may be considered a waste product from gas processing and can be either used on their own — ethane, propane, LPG — as a gaseous fuel or deliberately blended into a methane-rich gas and used as a gas-turbine fuel as a means of disposal. Higher hydrocarbons can also be used in liquid form; there are numerous gas turbines operating on LPG and naphtha fuels.
The presence of higher hydrocarbon species in a gas fuel can lead to auto-ignition problems within the combustion system. The longer hydrocarbon chains can ignite spontaneously at temperatures below the compressor discharge temperature, so it is important to minimize the residence time of the fuel gas in the combustor so that controlled combustion occurs in the correct place.
Figure 1. If the gas fuel temperature isn’t maintained, condensate can impinge on combustor surfaces leading to localized burning and component failure, which can occur very rapidly and result in engine shutdown.
Higher hydrocarbon species impact the hydrocarbon dew point of the gas fuel, and hence high supply temperatures are required to ensure these remain in the gaseous phase. If the gas fuel temperature is not maintained, then liquid dropout — condensate — will result and can cause problems in the fuel system or, more seriously, impinge on combustor surfaces leading to localized burning and component failure (Figure 1).
When used as a liquid fuel, the issue is now to keep the fuel in liquid form until it reaches the burner tip. This is achieved by using high fuel supply pressures or by a combination of slightly increased pressure and special burner designs that use the latent heat of evaporation to cool the fuel in the burner. When heavy hydrocarbons are used in liquid form, special consideration must be given to the fuel system. For example, LPG has low viscosity and associated low lubricity requiring special pumps to overcome this problem. Control of the fluid is critical to ensure other problems are avoided such as: