Rajan Wilcox
Gas turbines are a type of continuous flow internal combustion engine. They are used in power plants, ships, and aircraft. The Brayton cycle is used in the basic operation of a gas turbine, with air as the working fluid. Atmospheric air is drawn into the engine through the compressor, which pressurizes it and feeds it to the combustion chamber. Fuel is then sprayed into the air and ignited, generating a high-temperature flow. This high-temperature, pressurized gas enters a turbine, producing shaft work output, which is used to drive the compressor. The fuel used in gas turbines can be natural gas, diesel, heavy bunker oil, or aviation gasoline. The physical properties of the fuel, such as its phase, relative density, and volatility, play a crucial role in combustion performance. The efficiency of a gas turbine is also influenced by its operating temperature, with higher temperatures generally leading to higher efficiencies.
| Characteristics | Values |
|---|---|
| How gas turbines work | Atmospheric air is drawn into the engine, pressurised, and fed to the combustion chamber at high speeds. Fuel is injected and ignited, creating a high-temperature, high-pressure gas that enters a turbine, producing shaft work output. |
| Fuel types | Natural gas, diesel, heavy bunker oil, aviation gasoline (Avgas), kerosene blends (Avtur), heavy residual fuels |
| Fuel properties | Phase (gas, liquid, solid), relative density, volatility, freezing point, viscosity, presence of hydrocarbons |
| Fuel treatment | Fuel washing systems, vaporised fuel oil systems (VFO), fuel atomization during ignition |
| Efficiency | Single-cycle gas turbines: 20-35% energy conversion efficiency; combined cycle plants can achieve 60% efficiency or more |
| Power output | Large single-cycle gas turbines: 100-400 megawatts; microturbines: up to 500,000 rpm |
| Applications | Marine, land, and aviation use; power generation, propulsion, mechanical drive |
| Fuel consumption rates | Vary based on horsepower, power settings, and atmospheric conditions; specific fuel consumption is typically quoted at max power |
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What You'll Learn
Fuel properties and combustion performance
The performance of gas turbines is influenced by fuel properties, particularly the hydrogen-carbon ratio (H/C). Fuels with higher H/C ratios produce more power and efficiency, as well as lower carbon emissions. For example, natural gas-based fuels with higher H/C ratios have higher specific energies. On the other hand, diesel (DF-2) and kerosene, which have lower heating values (LHV) than natural gas, require more fuel to reach operating temperatures, resulting in reduced power and efficiency.
The versatility of industrial gas turbines allows them to utilise a wide range of fuels, including hydrocarbon gasification products like hydrogen and pure carbon. The thermochemistry of the fuel impacts the engine's performance, carbon emissions, combustion system operability, and the durability of hot section components.
The combustion process in a gas turbine is intricate. It involves the slow oxidation of the fuel mixture by auto-ignition in the mixing layer, followed by forced ignition and combustion at the flame base. The combustion process is influenced by the reference velocity and exhibits a mixing-controlled feature. Partially premixed combustion occurs on the flame base, while swirling flames are commonly used in modern combustors for their superior ignition and flame stabilization capabilities.
The market demands fuel-flexible gas turbines with low emissions. However, this presents challenges in handling reactive gaseous fuels with heavy hydrocarbons, hydrogen, or high inert content. To avoid issues like flashback, lean blow out, and combustion dynamics, it is crucial to consider the combustion properties relevant to the fuel composition.
To enhance combustion energy, recent studies have explored suspending nano- or micron-sized metal particles in fuel droplets. This approach improves fuel oxidation by the catalytic effect, resulting in higher energy density, shorter ignition delay time, and reduced CO2 and NOx emissions. Additionally, steam injection can improve turbine efficiency by up to 5.5% while also reducing nitrogen oxide emissions.
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Fuel treatment processes
Fuel treatment is critical to the performance and longevity of gas turbine combustion systems. The treatment process involves removing contaminants and impurities from the fuel, which can compromise the system and lead to damage, failure, or inadequate performance if left unchecked.
Liquid fuel treatment processes are used to remove particulates such as vanadium, lead, and water-soluble salts, which can cause corrosion and increase wear and tear on turbine blades. Solid, liquid, and gaseous contaminants are introduced to the fuel supply during mining, transportation, and production. Fuel gas conditioning is a method that filters out these impurities and prepares the fuel for use in combustion systems. Coalescing filters are used to remove solid and liquid impurities, while gaseous fuels are treated to remove sulfur compounds and particulate matter.
In the case of fuel gas, a reduction in pressure during transportation can cause a drop in temperature, allowing moisture and hydrocarbons to contaminate the fuel. This requires treatment to ensure optimal operation and prevent reduced efficiency, unplanned outages, and safety regulation violations.
The physical properties of the fuel also dictate the treatment process. For instance, high-volatile fuels can provide better ignition but may require treatment to prevent vapor lock. Low-volatile fuels, on the other hand, may need heating to achieve acceptable viscosity levels and prevent wax crystal formation.
Overall, fuel treatment processes are essential to ensure the safe and efficient operation of gas turbines, removing harmful contaminants and optimizing fuel performance.
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Fuel efficiency
The fuel efficiency of gas turbines depends on several factors, including the type of turbine, the fuel used, and the operating temperature.
Types of Gas Turbines
Gas turbines can be land-based or marine-based. Land-based gas turbines can be further categorized into heavy frame engines and aeroderivative engines. Heavy frame engines have lower pressure ratios, typically below 20, and are physically large. Aeroderivative engines, on the other hand, are derived from jet engines and operate at very high compression ratios, usually exceeding 30. They tend to be compact and are suitable for applications requiring smaller power outputs.
The operating temperature of a gas turbine plays a crucial role in its fuel efficiency. Higher temperatures generally lead to higher efficiencies, resulting in more economical operation. Gas flowing through a typical power plant turbine can reach temperatures as high as 2300 degrees F, while advanced turbines can operate at temperatures up to 2600 degrees F, achieving efficiencies as high as 60%.
Fuel Type and Properties
The type of fuel used in gas turbines also impacts their efficiency. Common fuels considered for gas turbine power plants include natural gas, diesel, and heavy bunker oil. Additionally, the physical properties of the fuel, such as its phase (gas, liquid, or solid) at standard ambient conditions, relative density, and volatility characteristics, influence combustion performance and fuel efficiency.
Design Considerations for Fuel Efficiency
The design of gas turbines can be optimized for fuel efficiency. For example, adding an extra turbine to drive a propeller or ducted fan (turbofan) can increase propulsive efficiency at subsonic flight speeds, reducing fuel consumption. Additionally, installing a recuperator or heat recovery steam generator (HRSG) in the turbine exhaust system can capture waste heat to preheat the compressor discharge air, improving efficiency.
Fuel Consumption Rates
The fuel consumption rates of gas turbines vary depending on the specific turbine and its operating conditions. For instance, the PT6 turbine engine has fuel consumption data listed for 21 different variations, requiring calculations based on desired horsepower. The specific fuel consumption is typically best at maximum power, with fuel burn rates increasing at cruise power settings.
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Fuel consumption calculations
- Specific Fuel Consumption (SFC): This is a common metric used to express fuel consumption in gas turbines, particularly in aviation contexts. It is typically measured in horsepower per hour per pound (hp/hr/lb) or its equivalent units. The SFC value is usually provided by the engine manufacturer and represents the fuel consumption at maximum power settings. For example, a turbine engine with an SFC of 0.7 hp/hr/lb would consume approximately 364 pounds of fuel per hour at maximum power.
- Engine Power and Settings: Fuel consumption is closely related to engine power settings. Generally, fuel consumption is highest at maximum power settings and decreases at lower power settings. Therefore, it's important to consider the specific power setting when calculating fuel consumption. Different power settings are used during various phases of flight, such as takeoff, cruise, and landing, each with varying fuel consumption rates.
- Fuel Properties and Composition: The physical and chemical properties of the fuel significantly impact combustion performance and fuel efficiency. The phase of the fuel (gas, liquid, or solid) at standard ambient conditions depends on the molecular mass of the hydrocarbons in the fuel. The relative density, boiling point, chemical composition, and volatility characteristics of the fuel also play a role in fuel handling and combustion behaviour. Additionally, the presence of certain fuel additives can affect fuel performance, ignition characteristics, and freezing points.
- Turbine Design and Operating Conditions: The design and operating conditions of the gas turbine also influence fuel consumption. For instance, the temperature at which a turbine operates is critical to its fuel-to-power efficiency, with higher temperatures generally leading to higher efficiencies. Advancements in turbine technology, such as the use of innovative cooling systems and advanced materials, have allowed for higher inlet temperatures and improved efficiencies. The type of turbine, such as heavy frame engines or aeroderivative engines, also impacts fuel consumption due to differences in pressure ratios and power outputs.
- Combined Cycle Plants: Gas turbines can be used in combined cycle power plants, where they operate in conjunction with steam turbines to improve overall efficiency. By capturing waste heat from the turbine's exhaust system using a recuperator or heat recovery steam generator (HRSG), additional electric power can be generated. This configuration can increase energy conversion efficiencies to 60% or more, compared to 20-35% for simple cycle gas turbines.
- Calculating Fuel Consumption: To calculate fuel consumption for a specific gas turbine application, it is necessary to refer to the engine manufacturer's data and specifications. Fuel consumption rates are typically provided in the form of specific fuel consumption (SFC) values or fuel flow rates at various power settings. These values can be used as a starting point for estimating fuel consumption based on the desired horsepower, operating conditions, and other relevant factors. Additionally, flight manuals for aircraft with similar engine installations can provide valuable fuel consumption figures for reference.
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Fuel types
Gas turbines are usually multi-fuel internal combustion engines. They can be powered by natural gas, diesel, pulverized coal, finely ground biomass (such as sawdust), or liquid fuels like aviation gasoline. The type of fuel used depends on the specific application and requirements of the gas turbine. For example, marine gas turbines may use cheaper fuels due to the corrosive atmosphere created by the presence of sea salt in the air and fuel.
The Brayton cycle is a common method of operation for gas turbines, where atmospheric air is drawn into a compressor, increasing its pressure. Fuel is then sprayed into the air and ignited, resulting in combustion that generates a high-temperature, high-pressure gas stream. This gas stream enters and expands through the turbine section, consisting of alternating stationary and rotating aerofoil-section blades. The turbine's intricate design allows it to operate at extremely high temperatures, with some reaching 2600 degrees F.
Liquid fuels used in gas turbines may require treatment to remove contaminants such as vanadium, lead, sodium, potassium, and calcium. These contaminants can enter the fuel through seawater ingestion near ocean environments or transportation over salty waters. Appropriate fuel treatment processes, such as fuel washing systems or vaporized fuel oil systems, are crucial to preventing engine degradation caused by hot corrosion.
The efficiency of a gas turbine is influenced by its operating temperature. Higher temperatures generally lead to higher efficiencies and more economical operation. Advanced cooling technologies and materials can help overcome limitations on turbine temperatures, allowing for improved performance and efficiency. Additionally, the use of a recuperator or heat recovery steam generator (HRSG) can further enhance efficiency by capturing waste heat from the turbine's exhaust system.
The design of a gas turbine is determined by its intended purpose, particularly the desired split of energy between thrust and shaft work. For example, gas turbines used in flight may have additional turbines to drive propellers or ducted fans (turbofans), increasing propulsive efficiency and reducing fuel consumption during subsonic flight. In power plants, gas turbines are often used in combined cycle configurations, where waste heat from the turbine's exhaust is used to generate steam and produce additional electricity, achieving higher overall efficiencies.
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Frequently asked questions
The amount of fuel used by gas turbines depends on several factors, including the type of fuel, the power output, and the efficiency of the turbine. The specific fuel consumption is typically best at maximum power.
The physical properties of the fuel, such as its phase at ambient conditions, relative density, and volatility, play a crucial role in determining fuel consumption. Additionally, the power output and efficiency of the turbine influence fuel usage.
Higher power outputs typically require larger amounts of fuel. For example, large jet engines operating at 10,000-25,000 rpm will consume more fuel than microturbines spinning at 500,000 rpm.
No, the efficiency of gas turbines can vary. Simple cycle gas turbines typically achieve energy conversion efficiencies between 20% and 35%. However, with advanced technologies, it is possible to achieve efficiencies of 60% or higher.
Common fuels used in gas turbines include natural gas, diesel, heavy bunker oil, aviation gasoline (Avgas), kerosene blends (Avtur), and heavy residual fuels. The choice of fuel depends on the specific application and requirements.
