In a gas turbine combustion chamber, the fuel must be injected, vaporized, and mixed with air prior to combustion. The physical properties of the fuel play a crucial role in these processes and largely dictate combustion performance.
This article contains excerpts from the paper, “Impact of fuel composition on gas turbine engine performance” by Dan Burnes and Alejandro Camou Solar Turbines Incorporated at the 2019 ASME Turbo Expo held in Phoenix, Arizona.
Considering the typical straight-chain alkane hydrocarbons, the light molecules down to n-Butane (C4H10) exist as a gas at standard ambient conditions. Heavier hydrocarbons will exist as liquids, and eventually reach the solid phase as the molecular mass continues to increase. This change of phase at standard ambient conditions is due to the greater amount of kinetic energy needed to remain in gas or liquid phase compared to intermolecular forces. Therefore, the phase of the fuel needs to be considered to understand its proper handling and treatment prior to combustion. For gaseous fuels, the Wobbe Index (WI) is used to characterize a fuel’s suitability for a given fuel system, and provides an indication of the fuel’s energy content for a given volumetric flow rate. The Wobbe Index is calculated using the fuel’s lower heating value (LHV) measured in Btu/ft3 and its specific gravity (relative density) relative to air (SGair) at a standard pressure and temperature.
WI = LHG/Sqrt(SGair)
The most common gaseous fuel for industrial gas turbines is natural gas. However, global interests in alternative energy and energy storage efforts has led to the increase in interest of gasified biofuels, synthetic gas blends, and byproduct gases such as Coke-Oven gas (COG) and Blast Furnace gas (BFG) which can be sourced from steel production. COG is released during coke production that feeds into a blast furnace that produces BFG and pig iron.
Because of their lower H/C compared to conventional petroleum and natural gas, these lower-WI fuels with high levels of inerts and H2-CO content can also be treated to increase the H/C through the application of the Fischer-Tropsch process. This process can be divided into low-temperature and high-temperature conversion processes to produce high molecular-weight and low molecular-weight hydrocarbons, respectively. These conversion processes are ideal in areas with an abundance of coal or biomass-based fuels where they can be converted to higher energy-content transportation fuels like diesel fuels and gasoline.
Although gaseous fuels are advantageous in terms of high thermal stability, they often require treatment to remove particulate matter and sulfur compounds to reduce corrosion rates and maximize turbine life. Natural gas can be too “sour” due to high hydrogen sulfide (H2S) content that requires removal prior to combustion. Other impurities such as water and liquid hydrocarbons can also be present in gaseous fuels depending on its conditions. The wide energy density range across typical gaseous fuels can create challenges to the turbine control system, fuel handling equipment and combustion hardware due to their large variation in volumetric flow rates.
Liquid fuels demand more stringent conditioning due to their wider range in hydrocarbons and particulates, large variation in physical properties and the nature of heterogeneous combustion. The liquid fuel’s chemical and physical properties strongly affect the combustion process through competing effects between the fuel’s evaporation, mixing and reaction rates. Depending on the quality of atomization and mixing, the maximum rate of heat release and thus combustion efficiency is governed by any one of these three mechanisms.
Fuel properties such as viscosity, density, and surface tension play a major role in spray combustion. The viscosity not only affects the power required to pump the fuel through the fuel system, but also affects its atomization and droplet evaporation. The higher the viscosity of a fuel, the poorer the quality of atomization which can lead to soot formation. Carbon deposits within the combustion system can damage hardware by high thermal radiation and clogging.
Liquid fuel systems may also require separate air atomizing systems during ignition depending on the type of atomizer used in the fuel injector. The relative density (specific gravity) of a fuel is related to its boiling point and chemical composition and is a good indicator of the mixture’s H/C, heating value and tendency to form carbon deposits. Volatility characteristics depend on its distillation range, vapor pressure and flash point. High-volatile fuels can provide better ignition and stability characteristics but can also promote vapor lock. Low-volatile fuels such as heavy fuels usually are too viscous for injection thus requiring heating for acceptable viscosity levels as well as prevention of wax crystal formation, which is reflected in higher freezing points. The presence of iso-paraffin hydrocarbons in lieu of their straightchain counterparts can decrease the freezing point. When using heavy fuels in gas turbines, a distillate oil with a higher API gravity (lower relative density & viscosity) such as a light diesel blend (No. 2 Distillate, also known as DF-2) is typically used for engine startup and shutdown sequences to avoid coke formation in fuel injectors.
Liquid fuel treatment processes are used to remove particulates such as vanadium and lead, depending on the fuel’s contaminant criteria, as well as preventing emulsions. Sodium, potassium and calcium can also be present in the form of seawater that may have resulted from compressor ingestion near ocean environments, salty wells or transportation over seawater, which can react with fuel-bound sulfur to form sodium sulfate (Na2SO4).
These contaminants can also react with engine components resulting in loss or deterioration of material through hot corrosion leading to engine degradation. Therefore, it is crucial to implement appropriate fueltreatment processes to reduce engine degradation. Liquid fuel treatment processes include fuel washing systems or vaporized fuel oil systems (VFO) configured to treat contaminated fuel.
Aviation gasoline (Avgas) contains less aromatics than automobile gasoline to minimize fuel effects on elastomers and to provide a higher heating value. Kerosene blends (Avtur) can be tailored for a variety of jet-fuel applications such as commercial aviation (Jet A/A-1), military aviation (JP-8) and Navy-marine applications (JP-5) which contain different additives to reduce freezing point temperature, reduce volatility for safety, or improve performance in harsh environments.
Heavy residual fuels are the remainder of the crude oil distillation process and contain more particulate matter than distilled blends. They are generally classified based on API gravity or viscosity and cover a wide range of heavy oils and residuals which are used to produce asphalt. The H/C and specific energy do not change significantly when comparing gasolines to light distillate oils.
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