Comprehensive Study Guide on Industrial Gas Turbines

Gas Turbine Advantages

Gas turbines offer a variety of specific operational advantages that make them suitable for diverse industrial and power generation tasks:

  • Large Power Production: They possess the ability to produce massive amounts of power, with current capacities reaching up to 400 MW400\,\text{MW}.

  • High Power-to-Weight Ratio: This characteristic makes gas turbines especially suitable for applications where minimizing weight is a priority, such as offshore platforms.

  • Fuel Versatility: They have the ability to utilize a wide range of both liquid and gaseous fuels.

  • Rapid Start-up: Gas turbines can start rapidly, which is a critical feature for providing backup (emergency) power generation.

  • Compact and Simple Design: The units feature a relatively simple and compact design with fewer and simpler auxiliary systems compared to other power plants.

  • High Availability and Reliability: They are known for high reliability. In the event of a major failure, outage time is minimized due to the ability to quickly replace the entire gas turbine.

  • Remote Operation: They are capable of being operated remotely, requiring minimal operational manpower.

Gas Turbine Disadvantages

Despite their benefits, gas turbines have specific drawbacks that have limited their application in certain fields, such as the automotive industry:

  • High Manufacturing Costs: Due to their complicated design and the precision required, manufacturing costs are high.

  • Slow Speed Response: A gas turbine is slow to respond to changes in throttle requests compared to a reciprocating (internal combustion) engine.

  • Low-Power Inefficiency: Gas turbines are less suitable for low-power applications because their efficiency decreases significantly at partial throttle conditions.

  • Complexity for High Efficiency: To reach efficiencies comparable to modern gasoline engines, a turbine requires additional components such as intercoolers, regenerators, and/or reheaters, which adds significant cost and complexity.

Types of Industrial Gas Turbines

There are two basic categories of industrial gas turbines:

  1. Aero-derivative: These are derived from jet engines used in aircraft. To adapt them for industrial use, the thrust components are removed and replaced with power turbines.

  2. Heavy-duty Gas Turbines: These are designed exclusively for land-based applications.

Industrial and Commercial Applications

Industrial gas turbines are used for electric power generation and to drive mechanical equipment like pumps and compressors. In power generation, they fulfill three primary roles:

  • Base Load: The unit produces electricity on an ongoing basis within a fairly steady demand range.

  • Emergency (Back-up) Power: Used to provide power rapidly in case of system failure.

  • Peak Power: Refers to power production only during periods when the regional electrical distribution network is close to being overloaded.

Major Sections and System Components

A gas turbine system generally consists of three primary sections:

  1. Compressor Section: Supplies high-pressure (HP\text{HP}) air required for combustion and provides the necessary mass flow.

  2. Combustion Section (Combustor): Where fuel is burned with the compressed air to create high-temperature combustion gases.

  3. Turbine Section: Converts the energy from the combustion gases into rotational mechanical energy.

Standard auxiliary and system components often include:

  • City Gas supply lines

  • Recuperator

  • Generator (driven by the turbine)

  • Once-Through Boiler with an Assist Burner (for steam production)

  • Electrical systems: AC/DC\text{AC/DC} Converter, AC/DC\text{AC/DC} Inverter, Transformer, and Breaker

  • Automatic Synchronous System and optional Battery

  • DPC Package and Protective Relay

Simple Gas Turbine Operating Principles

The operation of a simple gas turbine follows a specific thermodynamic process:

  • Air Intake and Compression: Ambient air enters the compressor and is compressed to a pressure range of 1,100−3,000 kPa1,100 - 3,000\,\text{kPa}.

  • Combustion: The compressed air passes to the combustion chamber where fuel (natural gas or oil) is added for continuous combustion.

  • Temperature Ranges: Combustion causes a sudden temperature increase to approximately 2,100∘C2,100^{\circ}\text{C} (the flame temperature). To protect turbine materials, the combusted products are mixed with remaining compressed air to reduce the temperature to between 1,000−1,500∘C1,000 - 1,500^{\circ}\text{C} at the combustion section exit.

  • Expansion and Power Generation: The hot gases enter the turbine and expand. This produces mechanical power used to drive the air compressor and an external load (like a generator).

  • Exhaust: Gases exit the turbine at nearly atmospheric pressure, though they remain hot, with temperatures between 500−640∘C500 - 640^{\circ}\text{C}.

Single Shaft Gas Turbine Configuration

In a single shaft arrangement, the compressor, turbine, and load are all mechanically connected and rotate at the same speed.

  • Connection: The load can be connected at either the turbine end or the compressor end via a reduction gear or coupling.

  • Applications: It is best suited for applications requiring constant speed, such as power generation.

  • Characteristics: It is mechanically simpler than a dual-shaft design but requires a significantly larger starting motor. This is because the motor must turn the compressor, the turbine, and the load mass simultaneously during start-up to reach the threshold air pressure needed for ignition.

  • Flexibility: It offers less operating flexibility compared to dual-shaft designs.

Dual-Shaft Arrangement and Advantages

A dual-shaft arrangement separates the components onto two distinct shafts with no mechanical linkage between them:

  • High-Pressure (HP) Shaft: Consists of the compressor driven by a high-pressure turbine. Its primary purpose is to drive the compressor.

  • Low-Pressure (LP) Shaft: Consists of the low-pressure power turbine (also called the gas generator output) which drives the load (e.g., a generator).

  • Starting Advantage: This requires a smaller starting motor because it only needs to turn the compressor and the HP\text{HP} turbine, not the load mass.

  • Operational Flexibility:

    • The load can operate at varying speeds while the compressor speed remains constant.

    • Conversely, the load speed can remain constant (e.g., for 50 Hz50\,\text{Hz} or 60 Hz60\,\text{Hz} power generation) while the compressor speed is varied.

Open Cycle Operation

  • Definition: The system is called "open cycle" because the working fluid (air) is drawn from the atmosphere at the start of the cycle and exhausted back to the atmosphere at the end.

  • Simple Open Cycle: This refers to a configuration without additional heat management components like heat exchangers, intercoolers, or reheaters.

  • Process: Air is drawn into the compressor, heated in the combustion chamber (firebox), expanded through the turbine, and exhausted.

  • Primary Advantage: Simplicity.

Closed Cycle Operation

  • Definition: The working fluid is contained and recirculated within the system rather than being exhausted.

  • Working Fluids: Common fluids include Nitrogen (N2\text{N}_2), Helium (He\text{He}), or Argon (Ar\text{Ar}).

  • Process Flow:

    1. Fluid is pressurized in the compressor.

    2. It is preheated in a regenerator through heat exchange with hot exhaust gases from the power turbine.

    3. It enters an air heater (external source) where it is further heated by combustion gases.

    4. The fluid expands through the power turbine to do work.

    5. The hot exhaust is cooled in two stages: first in the regenerator and then in a dedicated cooler before being returned to the compressor.