Gas turbine Power Plants

Introduction

A generating station which employs gas turbine as the prime mover for the generation of electrical energy is known as a gas turbine power plant. Today, gas turbines are one of the most widely-used power generating technologies that can convert chemical energy of natural gas or other liquid fuels to mechanical energy. This energy then drives a generator that produces electrical energy.

Gas turbines are a type of internal combustion (IC) engine in which burning of an air-fuel mixture produces hot gases that spin a turbine to produce power. It is the production of hot gas during fuel combustion, not the fuel itself that the gives gas turbines the name. Gas turbines can utilize a variety of fuels, including natural gas, fuel oils, and synthetic fuels (fuels produced from coal, natural gas, industrial and municipal waste or biomass through chemical conversion into synthetic crude and/or synthetic liquid products). Combustion occurs continuously in gas turbines, as opposed to reciprocating IC engines, in which combustion occurs intermittently. Because they can be started up quickly, gas turbines are ideal for meeting peak loading demands.

 

Basic schemes and working principle

Gas turbines are comprised of three primary sections mounted on the same shaft: the compressor, the combustor (or combustion chamber) and the turbine. The compressor can be either axial flow or centrifugal flow. Axial flow compressors are more common in power generation because they have higher flow rates and efficiencies. Axial flow compressors are comprised of multiple stages of rotating and stationary blades (or stators) through which air is drawn in parallel to the axis of rotation and incrementally compressed as it passes through each stage. The acceleration of the air through the rotating blades and diffusion by the stators increases the pressure and reduces the volume of the air. Although no heat is added, the compression of the air also causes the temperature to increase.

The compressed air is mixed with fuel injected through nozzles. The fuel and compressed air can be pre-mixed or the compressed air can be introduced directly into the combustor. The fuel-air mixture ignites under constant pressure conditions and the hot combustion products (gases) are directed through the turbine where it expands rapidly and imparts rotation to the shaft. The turbine is also comprised of stages, each with a row of stationary blades (or nozzles) to direct the expanding gases followed by a row of moving blades. The rotation of the shaft drives the compressor to draw in and compress more air to sustain continuous combustion. The remaining shaft power is used to drive a generator which produces electricity.

 

Figure: Gas Turbine

Approximately 55 to 65 percent of the power produced by the turbine is used to drive the compressor. To optimize the transfer of kinetic energy from the combustion gases to shaft rotation, gas turbines can have multiple compressor and turbine stages.

 

Figure: Gas Turbine Power Plant

 

Because the compressor must reach a certain speed before the combustion process is continuous – or self-sustaining – initial momentum is imparted to the turbine rotor from an external motor, static frequency converter, or the generator itself. The compressor must be smoothly accelerated and reach firing speed before fuel can be introduced and ignition can occur. Turbine speeds vary widely by manufacturer and design, ranging from 2,000 rpm to 10,000 rpm. Initial ignition occurs from one or more spark plugs (depending on combustor design). Once the turbine reaches self-sustaining speed – above 50% of full speed – the power output is enough to drive the compressor, combustion is continuous, and the starter system can be disengaged.

 

Open cycle gas turbine

A simple open cycle gas turbine consists of a compressor, combustion chamber and a turbine as shown in the below figure. The working fluid in an open cycle plant is atmospheric air, constantly drawn in to the compressor where it is typically compressed up to 18 times atmospheric pressure and then sent to a combustor. Heat is added to the air in the combustion chamber by burning the fuel and raises its temperature. In the combustor, natural gas is burned to heat the air. The heated gases coming out of the combustion chamber are then passed to the turbine where it expands, to approximately atmospheric pressure, and moves the turbine blades to perform useful work. The exhaust is then released to the environment.

Some part of the power developed by the turbine is utilized in driving the compressor and other accessories and remaining is used for power generation. Fresh air enters into the compressor and gases coming out of the turbine are exhausted into the atmosphere, the working medium need to be replaced continuously. This type of cycle is known as open cycle gas turbine plant and is mainly used in majority of gas turbine power plants as it has many inherent advantages.

 

Figure: Simple open cycle gas turbine plant

 

Advantages of open cycle gas turbine

1. Once the turbine is brought up to the rated speed by the starting motor and the fuel is ignited, the gas turbine will be accelerated from cold start to full load without warm-up time.

2. Light in weight (lower specific weight) i.e., kg per kW output is less as compare to other power plants.

3. Small in size i.e., lower volume space per kW output. Thus it occupies less space as compare to other power plants.

4. Almost any hydrocarbon fuel from high-octane gasoline to heavy diesel oils can be used in the combustion chamber.

5. The ability of a quick start and take-up of load frequently are the points in favor of open cycle plant when the plant is used as peak load plant.

6. Component or auxiliary refinements can usually be varied in open cycle gas turbine plant to improve the thermal efficiency and can give the most economical overall cost for the plant load factors and other operating conditions envisaged.

7. Open cycle gas turbine power plant, except those having an intercooler, does not need cooling water. Therefore, the plant is independent of cooling medium and becomes self-contained.

8. Low capital cost as compare to other power plants.

 

Disadvantages of Open cycle gas turbine

1. The part load efficiency of the open cycle gas turbine plant decreases rapidly as the considerable percentage of power developed by the turbine is used for driving the compressor.

2. The system is sensitive to the component efficiency; particularly that of compressor. The open cycle gas turbine plant is sensitive to changes in the atmospheric air temperature, pressure and humidity.

3. The open cycle plant has high air rate compared to the closed cycle plants, therefore, it results in increased loss of heat in the exhaust gases and large diameter duct work is needed.

4. It is essential that the dust should be prevented from entering into the compressor to decrease erosion and depositions on the blades and passages of the compressor and turbine. So damages their profile. The deposition of the carbon and ash content on the turbine blades is not at all desirable as it reduces the overall efficiency of the open cycle gas turbine plant.

5. Its efficiency is about 20%.

 

Methods to improve thermal efficiency of a simple Gas Turbine Power Plant

The efficiency of a simple gas turbine power plant can be improved by employing regenerator, intercooler and reheater.

Regenerator:

In the simple open cycle system the heat of the turbine exhaust gases goes as waste. To make use of this heat a regenerator is installed, which work as a heat exchanger. This heat exchanger extracts the heat from the exhaust gas and use to heat the compressed air. Now the preheated compressed air then enters the combustors for further processes. Thus the regenerator utilises the heat of exhaust gates, reduces the fuel consumption of the plant and significantly increases (more than 5%) the efficiency of the gas turbine power plant.

 

Intercooler:

An intercooler is a heat exchanger which cools the air from low pressure compressor before entry into the high pressure compressor. As we know that a large part of (about 2/3) of power developed by the turbine is used in driving the compressor and the power required to compress air is proportional it the air temperature at inlet. Hence to reduce this requirement of power, the temperature of air needed to reduce. This reduction in air temperature can be accomplished if the compression of air could be done in two or more stages and an intercooler is introduced between the two. Finally this cooling process will decrease the work needed for the compression in the high pressure unit. This entire cooling phenomenon is termed as Intercooling. In intercooling, partly compressed air is cooled in order to reduce its volume and increase density. The intercooling results in improvement of thermal efficiency, air rate and work ratio. By use of intercooling the size of compressor and turbine for the same output is reduced.

 

Reheater:

Reheating is applied in a gas turbine in such a way that it increases the turbine output without increasing the compressor work or melting the turbine materials. The exhaust of the high pressure turbine is reheated in a reheater and then expanded in a low pressure turbine. Reheating can improve the efficiency up to 3 %. A reheater is generally is a combustor which requires additional fuel to operate.

The addition of the regenerator, intercooler and reheater increases the overall efficiency of the gas power plant becomes about 30%.

 

Figure: Open cycle Gas turbine with regeneration, reheating and intercooling.

 

Closed-cycle gas turbine

A closed-cycle gas turbine the fuel is not mixed with the working medium which can be air or any other gas (hydrogen, helium, argon, etc.). In this turbine, heat is supplied from an external source. Here the working fluid is cycled through the compressor and then heated by an external source before it enters the turbine. Instead of being released to the atmosphere, the exhaust is sent through a heat exchanger that extracts heat from the exhaust before it is returned to the compressor.

Closed cycle gas turbine power plant would be preferable over open cycle one only when inferior type of fuel or solid fuel is to be used and ample cooling water at site is available. Some gas plants have been designed with a combination of open and closed cycled. Such plants are termed as semi-closed cycle plants.

 

Working of closed-cycle gas turbine

The closed cycle gas turbine works on the principle of Joule’s or Brayton’s cycle. In this turbine, the gas is compressed isentropically in two stages via intercooler and then passed into the heating chamber. This heated air, now expands through the turbines and moves the turbine blades to perform useful work. The turbine used here is of reaction type. The exhaust is then passed to the cooling chamber, called recooler. The gas is cooled at constant pressure with the help of circulating water to its original temperature. Now the gas is again made to flow through the compressor to repeat the process. Here the same gas is circulated again and again in the working of a closed cycle gas turbine.

 

Figure: Close cycle gas turbine plant

 

Advantages of Closed Cycle Gas Turbine

1. For a given output, the size of the compressor and the turbine are very small. This is all happened due to low specific volume of air which enters into compressor, as air is pre-cooled in the recooler. Also the pressure at the inlet to the compressor can be kept well above the atmospheric pressure and maintained around the whole cycle.

2. There is no corrosion and accumulation of deposits of carbon or tar on the blade and nozzles of the turbine as they are kept free from the combustion products. Thus, no internal cleaning required.

3. Since the working medium is heated externally and the fuel is not mixed with it, any fuel of high calorific value can be used.

4. A superior working medium like hydrogen, helium, neon, argon, etc. rather than atmospheric gas may be used because they have high heat conductivity property. Thus by using hydrogen like gas, the size of heat exchangers can be reduced.

5. There is an improvement in the rate of heat transmission.

6. These plants can be operated with highest efficiencies in comparison to open cycle plants at same initial temperature of working fluid.

7. The power output at constant speed can be varied by adding or subtracting the working fluid and thus altering the weight of the charge. This gives improved part-load efficiency (efficiency at some specified load value below full load) as compared to open gas cycle.

 

Disadvantages of Closed Cycle Gas Turbine

1. External furnace for combustion is required.

2. More complicated and costly as compare to open cycle gas turbine.

3. Coolant is required for cooling of hot gases of regenerator.

 

Comparison between Closed Cycle Gas Turbine and Open Cycle Gas Turbine

 

Criterion	Closed Cycle Gas Turbine	Open Cycle Gas Turbine
Cycle of operation	It works on closed cycle. The working fluid is recirculated again and again. It is a clean cycle.	It works on open cycle. The fresh charge is supplied to each cycle and after combustion and expansion. It is discharged to atmosphere.
Working fluid	The gases other than the air like Helium or Helium-Carbon dioxide mixture can be used, which has more favourable properties.	Air-fuel mixture is used which leads to lower thermal efficiency.
Type of fuel used	Since heat is transferred externally, so any type of fuel; solid, liquid or gaseous or combination of these can be used for generation of heat.	Since combustion is an integral part of the system thus it requires high quantity liquid or gaseous fuel for burning in a combustion chamber.
Manner of heat input	The heat is transferred indirectly through a heat exchanger.	Direct heat supply. It is generated in the combustion chamber itself
Quality of heat input	The heat can be supplied from any source like waste heat from some process, nuclear heat and solar heat using a concentrator.	It requires high grade heat energy for generation of power in a gas turbine.
Efficiency	High thermal efficiency for given temperature limits.	Low thermal efficiency (about 20%) for same temperature limits.
Part load efficiency	Part-load efficiency is better.	Part load efficiency is less compared to Closed cycle gas turbine.
Size of plant	Reduced size per MWh of power output.	Comparatively large size for same power output.
Blade life	Since combustion products do not come in direct contact of turbine blade, thus there is no blade fouling and longer blade life.	Direct contact with combustion products, the blades are subjected to higher thermal stresses and fouling and hence shorter blade life.
Control on power production	Better control on power production.	Poor control on power production.
Cost	Closed cycle gas turbine plant is complex and costly.	Open cycle gas turbine plant is simple and less costly.

 

Combined Cycle Plant for Power Generation: Introduction

The process for converting the chemical energy into mechanical work then transformed into electric power by a generator, has the overall efficiency as low as 30 percent, which depend on the fuel type and thermodynamic process. This means that two-thirds of the latent energy of the fuel ends up wasted. For example, steam power plants which utilize boilers to combust fossil fuel average 35% efficiency. Simple cycle gas turbine (GTs) plants average just under 32 percent efficiency on natural gas, and around 25 percent on fuel oil. Much of this wasted energy ends up as thermal energy in the hot exhaust gases from the combustion process.

To increase the overall efficiency of electric power plants, multiple processes can be combined to recover and utilize the residual heat energy of hot exhaust gases. In combined cycle mode, power plants can achieve electrical efficiencies up to 60 percent. The term “combined cycle” refers to the combining of multiple thermodynamic cycles to generate power. Combined cycle operation employs a heat recovery steam generator (HRSG) that captures heat from high temperature exhaust gases to produce steam, which is then supplied to a steam turbine to generate additional electric power. The process for creating steam to produce work using a steam turbine is based on the Rankine cycle.

The most common type of combined cycle power plant utilizes gas turbines and is called a combined cycle gas turbine (CCGT) plant. Because gas turbines have low efficiency in simple cycle operation, the output produced by the steam turbine accounts for about half of the CCGT plant output. There are many different configurations for CCGT power plants, but typically each GT has its own associated HRSG, and multiple HRSGs supply steam to one or more steam turbines. For example, at a plant in a 2x1 configuration, two GT/HRSG trains supply to one steam turbine; likewise there can be 1x1, 3x1 or 4x1 arrangements. The steam turbine is sized to the number and capacity of supplying GTs/HRSGs.

 

Combined Cycle Principles of Operation

The HRSG is basically a heat exchanger, or rather a series of heat exchangers. It is also called a boiler, as it creates steam for the steam turbine by passing the hot exhaust gas flow from a gas turbine through banks of heat exchanger tubes. The HRSG can rely on natural circulation or utilize forced circulation using pumps. As the hot exhaust gases flow past the heat exchanger tubes in which hot water circulates, heat is absorbed causing the creation of steam in the tubes. The tubes are arranged in sections, or modules, each serving a different function in the production of dry superheated steam. These modules are referred to as economizers, evaporators, superheaters/reheaters and preheaters.

The economizer is a heat exchanger that preheats the water, which is supplied to a thick-walled steam drum. The drum is located adjacent to finned evaporator tubes that circulate heated water to approach the saturation temperature (boiling point). As the hot exhaust gases flow past the evaporator tubes, heat is absorbed causing the creation of steam in the tubes. The steam-water mixture in the tubes enters the steam drum where steam is separated from the hot water using moisture separators. The separated water is recirculated to the evaporator tubes. Steam drums also serve storage and water treatment functions.

 

Figure: Combined cycle gas turbine

 

An alternative design to steam drums is a "once-through HRSG" (in which all of the water that enters the HRSG in turned into steam), which replaces the steam drum with thin-walled components that are better suited to handle changes in exhaust gas temperatures and steam pressures during frequent starts and stops. In some designs, duct burners are used to add heat to the exhaust gas stream and boost steam production; they can be used to produce steam even if there is insufficient exhaust gas flow.

Saturated steam from the steam drums or once-through HRSG system is sent to the superheater to produce dry steam which is required for the steam turbine. Preheaters are located at the coolest end of the HRSG gas path and absorb energy to preheat heat exchanger liquids, such as water/glycol mixtures, thus extracting the most economically viable amount of heat from exhaust gases.

The superheated steam produced by the HRSG is supply to the steam turbine where it expands through the turbine blades, imparting rotation to the turbine shaft. The energy delivered to the generator drive shaft is converted into electricity. After exiting the steam turbine, the steam is sent to a condenser which routes the condensed water back to the HRSG.

 

Advantages of CCGT

1. Fuel efficiency: In conventional power plants turbines have a fuel conversion efficiency of 33% which means two thirds of the fuel burned to drive the turbine. The turbines in combined cycle power plant have a fuel conversion efficiency of 50% or more, which means they burn about half amount of fuel as a conventional plant to generate same amount of electricity.

2. Low capital costs: The capital cost for building a combined cycle unit is two thirds the capital cost of a comparable coal plant.

3. Commercial availability: Combined cycle units are commercially available from suppliers anywhere in the world. They are easily manufactured, shipped and transported.

4. Abundant fuel sources: The turbines used in combined cycle plants are fuelled with natural gas (hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium), which is more versatile than a coal or oil and can be used in 90% of energy publications. To meet the energy demand now a day’s plants are not only using natural gas but also using other alternatives like bio gas derived from agriculture.

5. Reduced emission and fuel consumption: Combined cycle plants use less fuel per kWh and produce fewer emissions than conventional thermal power plants, thereby reducing the environmental damage caused by electricity production. Comparable with coal fired power plant burning of natural gas in CCGT is much cleaner.

6. Potential applications in developing countries: The potential of combined cycle plant can be utilized in industries that requires electricity and heat or steam. For example providing electricity and steam to a sugar refining mill.

7. Fast Starting: The gas turbine can be brought to full load within 20 minutes from initial start giving around 60% of total plant output and subsequently the steam-turbo alternator can be synchronised and brought to full load giving 100% of plant output in less than an hour following an overnight shutdown.

 

Disadvantages of CCGT

1. The gas turbine can only use Natural gas or high grade oils like diesel fuel.

2. Because of this the combined cycle can be operated only in locations where these fuels are available and cost effective.

 

Gas turbines fuels

Gas turbines accept most commercial fuels, such as petrol, natural gas, propane, diesel, and kerosene as well as renewable fuels such as E85 (mixture of ethanol, 51 - 83%, and petrol or other hydrocarbons), biodiesel and biogas. However, when running on kerosene or diesel, starting sometimes requires the assistance of a more volatile product such as propane gas - although the new kero-start technology can allow even microturbines fuelled on kerosene to start without propane.

 

Site selection of gas turbine power plant

While selecting the site for a gas turbine power plant, following points should be given due consideration:

1. The plant should be located near the load centre to avoid transmission costs and losses.

2. The site should be away from business centre due to noisy operations.

3. Cheap and good quality fuel should be easily available.

4. Availability of labour.

5. Availability of means of transportation.

6. Availability of land at reasonable rate.

7. The bearing capacity of the land should be of high.

 

Comparison between Gas Turbine and Steam Turbine
 

S.No	Gas Turbine	Steam Turbine
1.	In gas turbine the compressor and combustion chamber are the important components.	In steam turbine the steam boiler and accessories are the important components.
2.	Gas turbine uses mixture of air & combustion products as the working fluid.	Steam turbine uses high pressure steam as the intermediate fluid.
3.	Gas turbine executes the whole Brayton cycle.	Steam turbine is only a component executing one step of the Rankine cycle.
4.	Temperature of working fluid is about 1500 0C.	Temperature of intermediate fluid is about 500-650 0C.
5.	Less space for installation is required.	More space for installation is required.
6.	The mass of gas turbine per kW produced is less.	The mass of the steam turbine per kW produced is more.
7.	Less installation and running cost.	More installation and running cost.
8.	With the changing load conditions, its control is easy.	Its control is difficult, with the changing load condition.
9.	The starting of this turbine is easy and quick.	The starting of steam turbine is not easy and takes long time.
10.	A gas turbine does not depend on water supply.	A steam turbine depends upon water supply.
11.	Efficiency: 32% - 60%	Efficiency: 32% - 48%