How 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 literally 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 (e.g., natural gas) into the combustion chamber where it mixes with the air. The mixture is burned at temperatures of more than 2000 degrees. 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 aerofoil-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: (1) heavy frame engines and (2) aeroderivative engines. Heavy frame engines are characterized by lower compression ratios (typically below 15) and tend to be physically large. Pressure ratio is the ratio of the compressor discharge pressure and the inlet air pressure. 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.
One key to a turbine's fuel-to-energy 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 is used for cooling key turbine components; however, the requirement for cooling the turbine limits the ultimate thermal efficiency.
One of the major breakthroughs achieved in the 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 research program were able to boost turbine inlet temperatures to as high as 2600 degrees F - nearly 300 degrees hotter than in previous turbines.
Another way to boost efficiency is to install a recuperator or waste heat boiler onto 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. A waste heat boiler generates steam by capturing heat from the turbine exhaust. These boilers are also known as heat recovery steam generators (HRSG). 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 turbine program, future 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.
General description of the system
The GT 10B gas turbine operates in a simple open cycle with straight air and gas flow through the turbine. It can be divided into two main sections, the gas generator and the power turbine. The two main sections are not mechanically interconnected, so the gas generator speed is determined by the output of the unit as well as ambient conditions, which allows a wider control range at sustained efficiency. The power turbine is a two-stage axial-flow turbine.
The power turbine rotor is solid, built up from two discs and a rotor shaft. The rotor is fully electron beam welded. The PT blades are fitted in fir-tree grooves and have shrouds to minimize the inter stage gas leakage. The rotor blades as well as the guide vanes are precision cast.
The PT stator carries two guide vane stages. The first guide vane stage is permanently adjustable to obtain optimum efficiency at various climate conditions. The vanes in the first stage are also hollow to transport cooling air to the turbine disc. The stator surfaces above the blade tips are provided with honeycomb seals. Honeycomb is an abradable seal, which can withstand a blade tip rubbing.
The turbine casing houses the power turbine stages. The turbine casing is bolted to the diffusor casing. The purpose of the diffusor is to retard the velocity and gain static pressure, thereby increasing the pressure ratio across the power turbine. The bearing housings are attached to the diffusor casing. The exhaust casing surrounds the greater part of the diffusor casing and directs the exhaust gases to the outlet duct. The casing is designed to provide a minimum backpressure, which is important in order to not affect the power output.
The bearings are of tilting pad design with a directed lubrication system.
The bearings are equipped with temperature sensors and vibration transducers.
Two journal bearings, no 3 and 4, numbered from the inlet to the exhaust carries the power turbine rotor. Bearing no 3 is a journal bearing and number 4 are a combined thrust and journal bearing. During operation, oil is continuously supplied to the bearings. Return oil from the bearing casings is led back to the lube oil tank by gravity. See also the Lubrication oil system, MBV.
Cooling and sealing air
The cooling and sealing air are taken from the bleed cavities. The first guide vane stage is hollow and transports cooling air to the first rotor disc before entering the gas path. The cooling air is led via an external pipe into a manifold located in the diffusor. From there, air is distributed via flexible hoses to each individual guide vane. A part of the supplied air is also tapped from the manifold to form a heat barrier between the stator and the surrounding turbine casing. This air enters the turbine gas path. Sealing air is also used to prevent hot gases from entering the bearing housing or oil from leaking out from the bearing housings.