What is a Gas Turbine?
A gas turbine, also called a combustion turbine, is a type of continuous and internal combustion engine. The main elements common to all gas turbine engines are:
- An upstream rotating gas compressor
- A combustor
- A downstream turbine on the same shaft as the compressor.
A fourth component is often used to increase efficiency (on turboprops and turbofans), convert power into mechanical or electric form (on turboshafts and electric generators), or achieve a greater thrust-to-weight ratio (on afterburning engines).
The basic operation of the gas turbine is a Brayton cycle with air as the working fluid: atmospheric air flows through the compressor that brings it to higher pressure. Energy is then added by spraying fuel into the air and igniting it so that the combustion generates a high-temperature flow.
This high-temperature pressurized gas enters a turbine, producing a shaft work output in the process, used to drive the compressor.
The unused energy comes out in the exhaust gases that can be repurposed for external work, such as directly producing thrust in a turbojet engine, or rotating a second, independent turbine (known as a power turbine) that can be connected to a fan, propeller, or electrical generator.
The purpose of the gas turbine determines the design so that the most desirable split of energy between the thrust and the shaft work is achieved. The fourth step of the Brayton cycle (cooling of the working fluid) is omitted, as gas turbines are open systems that do not reuse the same air.
Gas turbines are used to power aircraft, trains, ships, electrical generators, pumps, gas compressors, and tanks.
How do gas turbines produce electricity?
A gas turbine is a combustion engine at the heart of a power plant that can convert natural gas or other liquid fuels to mechanical energy. This energy then drives a generator that produces the electrical energy that moves along power lines to homes and businesses.
In order to generate electricity, the gas turbine heats a mixture of air and fuel at very high temperatures, causing the turbine blades to spin. The spinning turbine drives a generator that converts the energy into electricity.
The gas turbine can be used in combination with a steam turbine in a combined-cycle power plant to create power extremely efficiently.
- Air-fuel mixture ignites: The gas turbine compresses air and mixes it with fuel that is then burned at extremely high temperatures, creating a hot gas.
- Hot gas spins turbine blades: The hot air-and-fuel mixture moves through blades in the turbine, causing them to spin quickly.
- Spinning blades turn the drive shaft: The fast-spinning turbine blades rotate the turbine drive shaft.
- Turbine rotation powers the generator: The spinning turbine is connected to the rod in a generator that turns a large magnet surrounded by coils of copper wire.
- Generator magnet causes electrons to move and creates electricity: The fast-revolving generator magnet creates a powerful magnetic field that lines up the electrons around the copper coils and causes them to move. The movement of these electrons through a wire is electricity.
How do Gas Turbine Power Plants work?
The combustion (gas) turbines being installed in many of today’s natural-gas-fueled power plants are complex machines, but they basically involve three main sections:
- The compressor, which draws air into the engine, pressurizes it, and feeds it to the combustion chamber at speeds of hundreds of miles per hour.
- The combustion system, typically made up of a ring of fuel injectors that inject a steady stream of fuel into combustion chambers where it mixes with the air. The mixture is burned at temperatures of more than 2000 degrees F. The combustion produces a high temperature, high pressure gas stream that enters and expands through the turbine section.
- The turbine is an intricate array of alternate stationary and rotating aero foil-section blades. As hot combustion gas expands through the turbine, it spins the rotating blades. The rotating blades perform a dual function: they drive the compressor to draw more pressurized air into the combustion section, and they spin a generator to produce electricity.
Land-based gas turbines are of two types:
- Heavy frame engines: Heavy frame engines are characterized by lower pressure ratios (typically below 20) and tend to be physically large. Pressure ratio is the ratio of the compressor discharge pressure and the inlet air pressure.
- Aeroderivative engines: Aeroderivative engines are derived from jet engines, as the name implies, and operate at very high compression ratios (typically in excess of 30). Aeroderivative engines tend to be very compact and are useful where smaller power outputs are needed. As large frame turbines have higher power outputs, they can produce larger amounts of emissions, and must be designed to achieve low emissions of pollutants, such as NOx.
One key to a turbine’s fuel-to-power efficiency is the temperature at which it operates. Higher temperatures generally mean higher efficiencies, which in turn, can lead to more economical operation.
Gas flowing through a typical power plant turbine can be as hot as 2300 degrees F, but some of the critical metals in the turbine can withstand temperatures only as hot as 1500 to 1700 degrees F. Therefore, air from the compressor might be used for cooling key turbine components, reducing ultimate thermal efficiency.
One of the major achievements of the Department of Energy’s advanced turbine program was to break through previous limitations on turbine temperatures, using a combination of innovative cooling technologies and advanced materials.
The advanced turbines that emerged from the Department’s research program were able to boost turbine inlet temperatures to as high as 2600 degrees F – nearly 300 degrees hotter than in previous turbines, and achieve efficiencies as high as 60 percent.
Another way to boost efficiency is to install a recuperator or heat recovery steam generator (HRSG) to recover energy from the turbine’s exhaust. A recuperator captures waste heat in the turbine exhaust system to preheat the compressor discharge air before it enters the combustion chamber. An HRSG generates steam by capturing heat from the turbine exhaust.
These boilers are also known as heat recovery steam generators. High-pressure steam from these boilers can be used to generate additional electric power with steam turbines, a configuration called a combined cycle.
A simple cycle gas turbine can achieve energy conversion efficiencies ranging between 20 and 35 percent. With the higher temperatures achieved in the Department of Energy’s turbine program, future hydrogen and syngas-fired gas turbine combined cycle plants are likely to achieve efficiencies of 60 percent or more.
When waste heat is captured from these systems for heating or industrial purposes, the overall energy cycle efficiency could approach 80 percent.
Advantages of Gas turbine Power Plant
- It is simple to design & construct compared to a Steam Power station, since no boilers are required.
- Much smaller compared to Steam Power station of same capacity.
- Lower operating costs.
- Less water used since there is no need for a condenser.
- Maintenance charges are low.
- Can be started quickly.
- No stand-by losses.
Disadvantages of Gas turbine Power Plant
- Problem of starting. Compressor needs to be operating thus external source of power is necessary.
- Net output is low since greater power is used to drive compressor.
- Overall efficiency of plant is low ≈ 20% because of exhaust gases still containing heat. only efficient in a Combined cycle configuration
- Temperature of combustion chamber is too high thus resulting in a lower life.