The combined cycle gas turbine is a popular method of producing electricity from natural gas. A high-temperature gas turbine coupled to a generator burns the gas. The turbine’s exhaust gases are used to generate steam, which is then fed into a conventional steam turbine and another generator.
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What is a Combined Cycle Gas Turbine?
A combined cycle power plant is made up of heat engines that work together to convert heat into mechanical power from the same heat source. The most common type of land-based power plant is a combined cycle gas turbine (CCGT). The combined gas and steam (COGAS) plant, which is also used in marine propulsion, is based on the same concept. Combining two or more thermodynamic cycles improves overall efficiency, resulting in lower fuel costs.
Combining multiple streams of work can increase the efficiency of a system by 50-60%. A combined cycle, for example, may have an overall efficiency of 64%, compared to 34% for a simple cycle. That is more than 84% of the theoretical efficiency of a Carnot cycle. A heat engine can only use a portion of the energy in its fuel (usually less than 50%). As a result, the leftover heat from combustion (in the form of hot exhaust gases) is wasted.
We’re talking about a combined cycle gas turbine here (CCGT). Combined cycles, which include large gas turbines, are widely used in stationary and marine power plants (which operate by the Brayton cycle). The hot exhaust from the turbine powers a steam power plant (based on the Rankine cycle). These units have a thermal efficiency of around 64% in base-load operation. When running on a single cycle, steam power plants can achieve efficiency levels of 35 to 42%.
A growing number of new power plants are using CCGTs. In stationary CCGTs, natural gas or coal-derived synthesis gas is used. Fuel oil is used to power ships.
Because the gas turbine cycle can often be started instantly, power is available right away. This avoids the need for costly peaker plants and allows a ship to maneuver freely. The secondary steam cycle will eventually warm up, increasing fuel efficiency and generating more power.
Combined Cycle Gas Turbine Working Principle
It is explained here how a combined-cycle plant uses waste heat from a gas turbine to improve efficiency and electrical output.
Gas turbine burns fuel
The gas turbine compresses air and mixes it with fuel that has been heated to an extremely high temperature. As the hot air-fuel mixture passes through the gas turbine blades, they spin. A generator uses some of the spinning energy to generate electricity as the turbine spins.
Heat recovery system captures exhaust
Unless captured by a heat recovery steam generator, gas turbine exhaust heat escapes through the exhaust stack (HRSG). The HRSG generates steam that powers the steam turbine by utilizing the gas turbine’s exhaust heat.
Steam turbine generates more electricity.
The thermodynamic cycle of the combined cycle is made up of two power plant cycles. The Joule or Brayton cycle is a gas turbine cycle, and the Rankine cycle is a steam turbine cycle. The topping cycle of a gas turbine power plant is 1g-2g-3g-4g-5g-1g. In high-temperature regions, heat is transferred and work is performed.
The bottoming cycle is a low-temperature version of the Rankine steam cycle 1-2-3-4-5-6-1. Waste heat is recovered from high-temperature exhaust gases and transferred into water and steam as part of the bottoming cycle. During the constant pressure process, gas turbine exhaust gases reject heat 4g-1g. Some of this energy is absorbed by feed water, wet steam, and superheated steam in processes 1-2, 2-3, and 3-4.
Technical Considerations
When designing different components of the combined cycle gas turbine, it is important to consider some principles for both the Rankine cycle and the Brayton cycle.
Material
The temperature difference between the heat entering it and the exhaust heat leaving it limits the efficiency of a heat engine, which represents the fraction of its input energy that can be converted into useful work.
A steam power station’s working medium is water. To handle high-pressure steam, components must be strong and bulky. High temperatures necessitate the use of expensive nickel or cobalt alloys rather than inexpensive steel. These alloys limit practical steam temperatures to 655 degrees Celsius, while the lower temperature of a steam plant is determined by the temperature of the cooling water. With these constraints, the maximum efficiency of a steam plant is 35-42%.
A compressor, combustor, and turbine comprise the open-circuit gas turbine cycle. Lower quantities of expensive materials may be required in gas turbines to withstand the high temperatures and pressures. In this type of cycle, the firing temperature (the input temperature to the turbine) is relatively high (900 to 1,400 °C). The temperature of the output flue gas is also high (450-650°C). As a result, it is high enough to heat a second cycle that employs steam as the working fluid (the Rankine cycle).
An HRSG with a live steam temperature ranging from 420 to 580 °C uses gas turbine exhaust heat to generate steam for a combined cycle power plant. To cool the condenser, most Rankine cycles use water from lakes, rivers, seas, or cooling towers. Temperatures can fall as low as 15 degrees Celsius.
Size
Plant size has an impact on plant costs. Economies of scale (lower initial costs per kilowatt) and efficiency improve as plants grow in size. A typical large-scale power plant would have a 270 MW primary gas turbine coupled with a 130 MW steam turbine, yielding a combined output of 400 MW. Depending on the size of the power station, these sets could range from one to six.
The combined cycle unit is made up of one or more of these gas turbines, each of which is outfitted with a waste heat steam generator that supplies steam to one or more steam turbines, forming a combined cycle unit or block. The sizes of combined cycle blocks range from 50 MW to well over 1300 MW.
Fuel
In combined cycle plants, natural gas is typically used as a fuel, but other fuels such as fuel oil and synthesis gas can also be used. Coal, fuel oil, and natural gas are additional fuel options. It is also possible to use biofuels. A solar combined cycle power station combines solar energy with another fuel to reduce fuel costs and to protect the environment. Nuclear power plants of the future can use a Brayton topping cycle, which provides a wider temperature range, as well as a Rankine bottoming cycle, which improves thermal efficiency.
Small-scale combined cycle plants powered by renewable fuels can be used to meet electricity demands in remote areas where extending a gas pipeline would be impractical or uneconomical. Instead of natural gas, which is readily available in rural areas, agricultural and forestry waste is gasified and burned.
Supplementary Firing
After the gas turbine, fuel can be added to the HRSG for supplementary combustion. Duct burners are also known as supplementary burners. Duct burning is possible because turbine exhaust gases (flue gas) contain oxygen. A temperature limit at the gas turbine’s inlet forces the fuel to burn at a rate greater than its stoichiometric ratio. In gas turbine designs, a portion of the compressed air flow bypasses the burner to cool the turbine blades. Because of the hot exhaust gas from the turbine, no regenerative air preheater is required, as is the case with conventional steam plants.
Without supplementary firing, combined cycle power plants are more efficient. A more adaptable plant operation, on the other hand, makes a marine CCGT safer by allowing ships to continue operating even when equipment fails. A flexible stationary plant is also more profitable to operate.
The quantity of steam is increased due to the higher temperatures in the flue caused by duct burning (e.g. to 84 bar, 525 degrees Celsius). As a result, the efficiency of the steam cycle is increased. Supplemental firing allows the plant to respond to fluctuations in electrical load due to the very high efficiency of duct burners with partial loads. The increased steam production can compensate for the failure of another unit. Coal can also be used as a supplemental fuel in the steam generator.
When supplementary firing occurs, the temperature of the exhaust can rise to 800 °C or even 1000 °C. Supplemental firing does not improve efficiency in most cases. When fired to 700-750 °C, it can improve the efficiency of single boilers; however, when fired to multiple boilers, the flexibility should be the most appealing feature.
The primary fuels for gas turbines are natural gas and light oil. For crude oil, residuals, and some distillates that contain corrosive components, fuel treatment equipment is required. Furthermore, ash deposits from these fuels can result in up to 15% derated gas turbines. They may, however, continue to be economically appealing fuels in combined-cycle plants.
Configuration
Both multi-shaft and single-shaft combined-cycle systems are feasible. Steam systems can be configured in a variety of ways as well. HRSGs with modular pre-engineered components are used to generate electricity with the highest fuel efficiency. These unfired steam cycles are often single-shaft systems that are installed as a unit, as well as having the lowest initial costs.
Generally, supplementary-fired multi-shaft combined-cycle systems are chosen based on fuel, application, or situation. For example, a cogeneration combined-cycle system may require more heat or higher temperatures, in which case electricity comes in second.
Multiple shaft systems with supplementary firing can offer a wider range of operating temperatures or heat-to-electric power conversion. If low-quality fuels such as brown coal or peat are used, closed-cycle helium turbines can be used as the topping cycle to avoid the need for additional gasification and fuel processing that a conventional gas turbine would require.
Methods for Improving Efficiency
When the combustion temperature rises, the working fluid expands more, increasing turbine efficiency. As a result, turbine efficiency is limited by the first stage of blades’ ability to withstand higher temperatures. Research into materials and cooling is ongoing. Pressurizing hot-stage turbine blades with coolant is a common technique adapted from aircraft.
Additionally, proprietary methods are used to improve the aerodynamic efficiency of turbine blades. Various vendors have tested various coolants. The most common medium is air, but steam is also becoming more popular. Some engine manufacturers’ hot sections may now have single-crystal turbine blades, a technique already used in military aircraft engines.
Pre-cooling combustion air can also improve CCGT and GT efficiency. It not only increases its density, but it also increases its expansion ratio. This technique is commonly used to increase power output in hot climates. To accomplish this, either a moist matrix or ice storage air conditioning is used in the turbine’s inlet. Because the temperature is lower in this method, the improvement is greater.
Because fuels, gasification, and combustion all play a role in fuel efficiency, combustion technology is a very active research area. Computer simulations typically combine aerodynamics and chemistry to find combustor designs that maximize fuel burn-up while minimizing pollution and dilution of exhaust gases. To prevent the formation of nitrates and ozone, some combustors inject other materials, such as steam or air, to reduce pollution.
In addition, research on Rankine cycle steam generators is being conducted. The majority of plants already employ two-stage steam turbines that reheat steam between stages. Heat exchanger efficiency can be increased by increasing thermal conductivity. It is possible, for example, to make nuclear reactor tubes thinner (for example, by using stronger or corrosion-resistant steel). Silicon carbide sandwiches, which do not corrode, can also be used.
Modified Rankine cycles are also being developed. Two promising technologies are ammonia/water mixtures and supercritical carbon dioxide turbines.
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