Introduction
Gas turbines have become an essential component of modern power generation, aviation, and industrial applications. The combustion process in a gas turbine is a critical element that determines its efficiency, performance, and emissions. In this article, you will get a technical overview of the fundamentals of gas turbine combustion, including the types of fuels used, combustion mechanisms, and control strategies.
Fuels Used in Gas Turbine Combustion
Gas turbine combustors can burn various fuels, depending on their availability, cost, and environmental impact. Natural gas is the most commonly used fuel for gas turbine combustion due to its high energy content, low emissions, and low cost. Other gaseous fuels, such as hydrogen, propane, and ethane, can also be used in gas turbines. Liquid fuels, such as diesel, kerosene, and biofuels, can be injected into the combustor in the form of a mist or spray.
Combustion Mechanisms
The combustion process in a gas turbine combustor can be divided into three stages: Ignition, Flame stabilization, and Heat release.
Ignition occurs when the fuel is mixed with air and ignited by a spark or pilot flame. Flame stabilization is the process of maintaining a stable flame front in the combustor. Heat release is the process of releasing heat from the combustion products to the turbine blades.
The combustion mechanism can be classified into two types:
1. Diffusion flame
2. Premixed flame
In a diffusion flame, the fuel and air are not premixed, and the combustion occurs in a diffusion-controlled mode. In a premixed flame, the fuel and air are premixed before entering the combustor, and the combustion occurs in a flame-controlled mode. The choice of combustion mechanism depends on the type of fuel, combustor design, and operating conditions.
Combustor Design
The combustor design plays a crucial role in achieving efficient and low-emission gas turbine combustion. The combustor should provide a stable flame front, minimize the formation of pollutants such as NOx and CO, and maintain a high combustion efficiency. The combustor design can be classified into two types: can combustor and annular combustor.
In a can combustor, the combustion occurs in cylindrical chambers arranged in a radial pattern around the turbine shaft. In an annular combustor, the combustion occurs in a continuous ring-shaped chamber around the turbine shaft. Annular combustors are preferred in modern gas turbines due to their compact size, high efficiency, and low emissions.
Combustion Control Strategies
The combustion process in a gas turbine combustor is subject to various control parameters, such as fuel flow rate, air flow rate, temperature, and pressure. The combustion control strategies aim to maintain a stable flame front, minimize emissions, and optimize performance.
The most commonly used combustion control strategies are flame temperature control, lean blowout control, and emissions control. Flame temperature control involves adjusting the fuel flow rate and air flow rate to maintain a constant flame temperature. Lean blowout control involves maintaining the air-to-fuel ratio at the lean limit to prevent flame extinction. Emissions control involves using advanced combustion technologies, such as low-NOx burners and catalytic combustors, to reduce emissions.
Emissions from Gas Turbine Combustion
Gas turbine combustion can produce various pollutants, such as nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC), and particulate matter (PM). The emissions depend on the type of fuel, combustion mechanism, and operating conditions.
NOx is the most significant pollutant produced by gas turbine combustion, which contributes to smog, acid rain, and respiratory problems. The formation of NOx can be reduced by using low-NOx burners and water or steam injection. CO and UHC are produced by incomplete combustion and can be reduced by optimizing the combustion process and using advanced control strategies. PM is produced by the combustion of liquid fuels and can be reduced by using filters and scrubbers.
Conclusion
Gas turbine combustion is a complex process that involves various fuels, combustion mechanisms, and control strategies. Efficient and low-emission gas turbine combustion is essential for meeting the growing demand for energy while reducing the environmental impact.
The advancements in combustion technology, such as low-NOx burners and catalytic combustors, have significantly reduced emissions from gas turbine combustion. The future of gas turbine combustion lies in the development of sustainable fuels, such as hydrogen and biofuels, and the integration of renewable energy sources, such as wind and solar, into the power system.
