Gas turbine-steam combined cycle power plant: Brayton and Rankine cycle

The post covers [1] Gas cycle: Brayton cycle [2] Vapor cycle: Rankine cycle [3] Ideal Brayton cycle [4] Actual Brayton and [5] Combined Brayton and Rankine cycle

The turbine entry temperature in a gas turbine (Brayton) cycle is considerably higher than the peak steam temperature. Depending on the compression ratio of the gas turbine, the turbine exhaust temperature may be high enough to permit efficient generation of steam using the ``waste heat'' from the gas turbine. A configuration such as this is known as a gas turbine-steam combined cycle power plant.

Thermodynamic cycles can be divided into two general categories:[1] power cycles, which produce a net power output, and [2] refrigeration and heat pump cycles, which consume a net power input. The thermodynamic power cycles can be categorized as gas cycles and vapor cycles. In gas cycles, the working fluid remains in the gas phase throughout the entire cycle. In vapor cycles, the working fluid exits as a vapor during one part of the cycle and as a liquid during another part of the cycle.

Explanation:

Gas cycle: Brayton cycle

The Brayton cycle is a typical example of a gas cycle. The Brayton cycle is a thermodynamic cycle that describes the workings of a constant-pressure heat engine. air is drawn into a compressor, where a pressure increase occurs. The compressor output airstream enters a combustor; fuel is injected into the air, and combustion takes place. The heated combustion products enter a gas turbine and are expanded, producing work. The work necessary to operate the compressor is extracted from the total work output of the turbine; the remainder is available as the network output of the engine.

Brayton cycle

Application of Brayton cycle: Internal combustion cycles of Otto and Diesel engines as well as the gas turbines are some well-known examples of engines that operate on gas cycles. Probably the best-known application of the Brayton cycle is the jet engine.

Vapor cycle: Rankine cycle

The Rankine cycle is typically a vapor cycle, the image below. The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines allow mechanical work to be extracted from a fluid as it moves between a heat source and heat sink. Heat energy is supplied to the system via a boiler where the working fluid (typically water) is converted to a high-pressure gaseous state (steam) in order to turn a turbine. After passing over the turbine the fluid is allowed to condense back into a liquid state as waste heat energy is rejected before being returned to the boiler, completing the cycle.

The notable difference between the Rankine cycle and the Brayton cycle is that there are four components in the Rankine cycle while there are only three components in the Brayton cycle. Brayton cycle does not need a condenser

Application of Rankine cycle: The Rankine cycle is the fundamental operating cycle of all power plants

Brayton cycle detail

Ideal Brayton cycle

Gas turbines usually operate on an open cycle, shown on the left below.

A compressor takes in fresh ambient air (state 1), compresses it to a higher temperature and pressure (state 2).

Fuel and the higher-pressure air from the compressor are sent to a combustion chamber, where fuel is burned at constant pressure. The resulting high-temperature gases are sent to a turbine (state 3).

The high-temperature gases expand to the ambient pressure (state 4) in the turbine and produce power.

The exhaust gases leave the turbine.

Part of the work generated by the turbine is sent to drive the compressor. The fraction of the turbine work used to drive the compressor is called the back work ratio.

Since fresh air enters the compressor at the beginning and exhaust is thrown out at the end, this cycle is an open cycle.

By utilizing the air-standard assumptions, replacing the combustion process with a constant pressure heat addition process, and replacing the exhaust discharging process with a constant pressure heat rejection process, the open cycle described above can be modeled as a closed cycle, called the ideal Brayton cycle. The ideal Brayton cycle is made up of four internally reversible processes, T-s diagram.

1-2 Isentropic compression (in a compressor)

2-3 Constant pressure heat addition

3-4 Isentropic expansion (in a turbine)

4-1 Constant pressure heat rejection

An isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. The work transfers of the system are frictionless, and there is no net transfer of heat or matter.

The P-v and T-s diagrams of an ideal Brayton cycle are shown on the left. In an ideal Brayton cycle, heat is added to the cycle at a constant pressure process (process 2-3).

qin = h3 - h2 = cP(T3 - T2)

Heat is rejected at a constant pressure process (process 4 -1).

 qout = h4 - h1 = cP(T4 - T1)

Then the thermal efficiency of the ideal Brayton cycle under the cold air-standard assumption is given as

Process 1-2 and process 3-4 are isentropic processes, thus,

Since P2 = P3 and P4 = P1,

Considering all the relations above, the thermal efficiency becomes,

where rP = P2/P1 is the pressure ratio and k is the specific heat ratio. In most designs, the pressure ratio of gas turbines ranges from about 11 to 16.

The actual gas-turbine cycle is different from the ideal Brayton cycle since there are irreversibilities. Hence, in an actual gas-turbine cycle, the compressor consumes more work and the turbine produces less work than that of the ideal Brayton cycle. The irreversibilities in an actual compressor and an actual turbine can be considered by using the adiabatic efficiencies of the compressor and turbine. They are:

Refer to the LHS equation

 Another difference between the actual Brayton cycle and the ideal cycle is that there are pressure drops in the heat addition and heat rejection processes.

Efficiency improvement of Brayton cycle

Increasing pressure ratio

Increasing the pressure ratio increases the efficiency of the Brayton cycle. However, practical limits occur when it comes to increasing the pressure ratio. First of all, increasing the pressure ratio increases the compressor discharge temperature. This can cause the temperature of the gases leaving the combustor to exceed the metallurgical limits of the turbine. Also, the diameter of the compressor blades becomes progressively smaller in higher pressure stages of the compressor. Because the gap between the blades and the engine casing increases in size as a percentage of the compressor blade height as the blades get smaller in diameter, a greater percentage of the compressed air can leak back past the blades in higher pressure stages. This causes a drop in compressor efficiency. Hence, the little gain is expected by increasing the pressure ratio further if it is already at a high level.

Recuperator – If the Brayton cycle is run at a low-pressure ratio and a high-temperature increase in the combustion chamber, the exhaust gas (after the last turbine stage) might still be hotter than the compressed inlet gas (after the last compression stage but before the combustor). In that case, a heat exchanger can be used to transfer thermal energy from the exhaust to the already compressed gas, before it enters the combustion chamber. The thermal energy transferred is effectively reused, thus increasing efficiency. However, this form of heat recycling is only possible if the engine is run in a low-efficiency mode with a low-pressure ratio in the first place.

A Brayton engine also forms half of the combined cycle system, which combines with a Rankine engine to further increase overall efficiency. However, although this increases overall efficiency, it does not actually increase the efficiency of the Brayton cycle itself.

 Combined Brayton and Rankine cycle

A combined-cycle combines two power cycles in series to obtain an overall efficiency significantly higher than the individual efficiencies of the two cycles making up the combined cycle. A Brayton cycle or gas turbine is utilized for the topping cycle and a steam Rankine cycle for the bottoming cycle in the combined cycles.

The turbine entry temperature in a gas turbine (Brayton) cycle is considerably higher than the peak steam temperature. Depending on the compression ratio of the gas turbine, the turbine exhaust temperature may be high enough to permit efficient generation of steam using the ``waste heat'' from the gas turbine.

Combined cycle

A configuration such as this is known as a gas turbine-steam combined cycle power plant. The cycle is shown on LHS and below.

 The heat input to the combined cycle is the same as that for the gas turbine, but the work output is larger (by the work of the Rankine cycle steam turbine). A schematic of the overall heat engine, which can be thought of as composed of an upper and a lower heat engine in series, is given in the above image. The upper engine is the gas turbine (Brayton cycle) which expels heat to the lower engine, the steam turbine (Rankine cycle).

The overall efficiency of the combined cycle

It can be derived as follows. We denote the heat received by the gas turbine as Q in and the heat rejected to the atmosphere as Q . The heat out of the gas turbine is denoted as Q1.

The hot exhaust gases from the gas turbine pass through a heat exchanger where they are used as the heat source for the two-phase Rankine cycle so that Q_1is also the heat input to the steam cycle. In other words, the heat input to the gas turbine runs both the gas and steam cycle. The overall combined cycle efficiency is given by above equatiom

Credit: Google