Viscosity is one of the key parameters used when evaluating liquid fuels for use in industrial gas turbines and generally should be <10cSt @ 50°C (most regular diesel fuels <7.5cSt @ 40°C). However, there are some gas turbine models that are able to operate on liquid fuels with much higher viscosities, and can, by using fuel heating or blending, operate on fuels with viscosities up to around 1000cSt @ 50°C.
This article contains excerpts from the paper, “Combustion, Fuels and Emissions for Industrial Gas Turbines” by Michael Welch and Brian Igoe of Siemens Industrial Turbomachinery presented at the 2014 Turbomachinery Symposium.
There are cases where neither diesel nor gaseous fuels are available or economic to use, and the only suitable “fuel” is crude oil. This creates challenges that have to be handled through fuel pre-treatment and fuel injection system functionality {12}. Firstly, heating the fuel reduces the viscosity, but noting the limitations: First is 100°C, at which water boils off (all liquid fuels contain a small amount of water) causing cavitation in fuel pumps.
Increasing fuel oil supply pressure allows the heating to be extended beyond 100°C, but is limited by the temperature limits within the fuel delivery system. Further heating can result in fuel cracking and coking in the fuel system and burners depending on the constituents within the crude oil.
Crude oils need to be treated in order to meet industrial gas turbine limits on metallic and other contaminants in the fuel. Crude oil often contains high amounts of alkali metals (Na, K) and heavy metals (V, Ni, etc.) which if introduced into the combustion system can result in accelerated deposit formation and high temperature corrosion in gas turbine hot gas path components.
Major corrosive constituents include Vanadium pentoxide (V2O5), sodium sulfate (Na2SO4) and aggressive low melting forms in the Na2SO4 – V2O5 and Na2O-V2O5 systems.
Determination of the ash sticking temperature is usually a good feature to use, and should be >900°C if sticking to the blade is to be avoided. Water and sediment can be removed, or reduced, by filtration and centrifuge separation. This is the same for any liquid fuel, and prevents the formation of corrosive elements and bacterial growth, a pre-cursor to fuel degradation.
Removal of the water also reduces the levels of water-soluble contaminants such as the alkali metals sodium and potassium. Vanadium and other heavy metals are oil-soluble though, and can only be treated through chemical dosing so that combustion creates high melting temperature compounds.
A magnesium-based additive is commonly used to treat fuels with heavy metal contamination. Crude oils can also contain more volatile components with a low flash point, therefore the need to ensure the use of explosion proof equipment is often required.
