Introduction to CCPP

BY JUN CHEOL JANG

Introduction to CCPP

DEFINITIONCombined cycle describes when a power producing engine or plant employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). The remaining heat from combustion is generally wasted. Combining two or more "cycles" such as the Brayton cycle and Ranking cycle results in improved overall efficiency.

TYPES OF COMBINED CYCLEGAS TURBINE COMBINED CYCLEINTEGRATED COAL GASIFICATION COMBINED CYCLE (IGCC)PRESSURIZED FLUID BED COMBUSTION COMBINED CYCLE (PFBC-CC)OTHERS

GAS TURBINE COMBINED CYCLEThe CCPP is combined with the gas turbine cycle and the steam turbine cycle. This system is most widely applied in commercial purpose. After the first power generation is carried out in the gas turbine cycle utilizing hot combustion gas, the second generation is again done in the steam turbine cycle using the exhaust gas from the gas turbine cycle so that two times of power generation and high thermal efficiency can be achieved. The current newly-constructed CCPP's thermal efficiency is more than 50%, and it is expected to be improved up to about 60%.

CHARICTERISTICS OF GAS TURBINE CCPPFEATURE :the simple cycle generation using a gas turbine, efficiencies of up to 30% can be achieved The thermal efficiency of a gas turbine is much lower than recent supercritical thermal power plants (about 40%). In case that the exhaust gas which has much available heat value is wasted into the ambient, the gas turbine simple cycle thermal efficiency is low because the temperature of combustion gas entering into the gas turbine is more than 1,000 and that of the exhaust gas is still high. The gas turbine exhaust gas enters into the HRGS (Heat Recovery Steam Generator) and produces steam to drive the steam turbine as a way to recover some part of the wasted heat from the gas turbine.

The reason that the gas turbine combines with the steam turbine: - The facility configuration is simple and easy - The facility management and operation technologies can be easily accumulated - The operation experiences are abundant

INTEGRATED COAL GASIFICATION COMBINED CYCLE (IGCC)The IGCC can reduce remarkably air pollutants compared to the conventional coal-fired thermal power plant, and it is a new technology with high thermal efficiency, and in practical use stage. The IGCC facility as a fuel supply facility can be separated from the other generation facility, but combining those two facilities properly is more profitable from the point of combined cycle efficiency. The solid coal is required to be gasified in order to be used for a gas turbine. After the coal is converted into gas and refined by the various processes as coal is not available to be fired directly in the gas turbine, the gas is used in the gas turbine. And the gas turbine exhaust flue gas is used in the steam cycle to drive the steam turbine.

CONCEPTUAL DIAGRAM OF IGCC

GASIFICATION REFRACTORY (1)

GASIFICATION REFRACTORY (2)

The process in which the gasification is reacted within a short time as the pulverized coal together with the oxidizer is fed from the top and the gasification temperature is high and the surface per volume is large.

* Merits: the range of coal to be utilized is wide and the processing capacity is large and there is no fluid by-product. * Demerits: The heat and carbon losses are large, and the maintenance cost for crusher is much.

PRESSURIZED FLUID BED COMBUSTION COMBINED CYCLE (PFBC-CC)The pressurized fluid bed combustion is the combustion process in which coal and heat transfer media are put into the furnace, and the combustion air is fed from the furnace bottom and the solid particles are fluidized. The combustion furnace can be down-sized by burning the coal under the furnace pressure of 5 to 12 atmospheric pressure.

And the process has high combustion and de-SOx efficiencies, and it can configure the pressurized fluidized bed combustion combined cycle in which the combustion gas after being refined drives the gas turbine, and again the steam turbine is driven by the gas turbine exhaust gas heat.

PRESSURIZED FLUID BED COMBUSTION COMBINED CYCLE (PFBC-CC)

OTHERSFUEL CELL COMBINED CYCLE MHD COMBINED CYCLE: generation method which uses the vertically flowing current by passing the good conductive hot combustion gas through the magnetic field. (magneto hydro dynamics)

THERMAL EFFICIENCY OF GAS TURBINE COMBINED CYCLEThe designed thermal efficiency of the CCPP which has a gas turbine inlet temperature of around 1,100 is 43% as a little higher than that of the coal-fired plant(40%)

THERMAL EFFICIENCY VARIATIONS AT PARTIAL LOADThe output of a single unit of the gas turbine is 150MW more or less and comparatively smaller than that of the coal fired thermal power plant unit so that several units are combined to configure a large capacity power plant

Shutting down procedure to minimize thermal efficiency drop: In combined-cycle operation, the gas turbine should be shut down unit by unitand the remained operating-unit is operated at its maximum output in order to reduce the loads.

OUTPUT VARIATIONS WITH ABMIENT TEMPERATUREThe gas turbine output increases as the ambient temperature decreases below 15, and it decreases as the ambient temperature increases.

The reason that the output increases when the ambient temperature lowers: 1. The limit on the turbine inlet gas temperature is determined in order to protect the turbine blades that is revolving at high speed. 2. In case that the ambient temperature is lowered, the air density is higher and the suction air flow rate increases and additional fuel can be supplied accordingly. 3. As the suction air flow and the fuel supply increase, the combustion gas flow increases and the gas turbine output increases.

Together with the gas turbine output increase, the steam generation of HRSG increases and accordingly as the steam turbine output slightly increases. After all, the maximum output of the CCPP increases as the ambient temperature lowers.

PERFORMANCE VARIATIONS WITH FUEL TYPE (1)In the gas turbine system, turbine or HRSG performance can be much affected by the fuel type because the combustion gas directly enters the turbine. The expected problems and measures in case that the clean fuel is not used is as follows: 1. Gas turbine blade corrosion Problem: Vanadium, sodium, potassium or sulphur, etc. contained in the heavy fuel oil cause corrosions on the hot surfaces of the combustor and hot gas path components Measures: Using the fuel after de-sulphurization and removing the steelcomponents by fuel treatment system. Lowering the gas turbineinlet gas temperature to prevent hot-temperature corrosion. Note that if the gas turbine temperature lowers, the thermal efficiencyalso decreases.

2. Thermal efficiency reduction due to the unburned fuel sticking

Problem: The thermal efficiency decreases because the unburned fuel sticksonto the gas turbine blades and also the ammonium sulfate((NH4)2SO4) that is formed when the ammonia isinjected into the flue gas to reduce NOx sticks onto the HRSG tubes.

PERFORMANCE VARIATIONS WITH FUEL TYPE (2)3. The thermal efficiency reduction by the low-temperature corrosion Problem: In the combined cycle system, the low-temperature feed water is heat-exchanged at the end-zone of the HRSG heat surface as the air preheater is not installed. 4. Pollution measures

Problem: In the high-nitrogen fuel, the NOx emission is much producedbecause the combustion temperature is high and the excess air is much. The exhaust gas flow is much than that of the conventional coal-fired power plant with the same capacity so that the De-NOx facility or the dust removal system is larger.

CCPP CONFIGURATIONS

CCPP CONTROL SYSTEM CONFIGURATIONSSINGLE SHAFT CONFIGURATIONMULTI-SHAFT CONFIGURATION

COMPARISON OF CCPP CONFIGURATIONS (1)

COMPARISON OF CCPP CONFIGURATIONS (2)Comparison of operational features of single-shaft and multi-shaft configurations: The efficiency of the multi-shaft system is higher at rated load and lower at low loads than that of the single-shaft system.

That is, the multi-shaft system is beneficial for base-load operation and single-shaft system for moderate-load operation.

The single-shaft combined cycle system is convenient for operation, as the unit-based start-up, shut-down and output control are available because there is no interference between units.

Operational cautions and reasons for multi-shaft system

As the steam synchronizes when entering into the steam turbine and the feed water branches out when fed into the HRSG, the fluid is affected by each other.

The temperature difference of each HRSG-exiting steam should be maintained within a limit in order to avoid excessive thermal stress that can be generated at the header at which the steam joins.

The changed fluid pressure is transmitted to the unit of gas turbine and HRSG and the boiler drum level changes.

COMPARISON OF CCPP CONFIGURATIONS (2)

EFFICIENCY VARIATIONS WITH TURBINE INLET TEMPERATUREThe efficiency of combined cycle is largely dependent on the gas turbine efficiency. As for the gas turbine, the improvement of efficiency is achievable by increasing turbine inlet temperature. At a constant turbine inlet temperature, the maximum efficiency is achieved when the exhaust temperature is lowest, that is when compression ratio is high. However, the gross efficiency of a combined cycle plant is improved with the increase of the gas turbine exhaust temperature. Therefore, the highest efficiency gas turbine does not always configure the most efficient combined cycle power plant. If the gas turbine inlet temperature goes up under the same conditions, the overall efficiency is improved.

THERMAL EFFICIECY UNDER VARIOUS CONDITIONSIn the gas turbine combined cycle, the specific output decreases with the increase of compression ratio, however, the thermal efficiency is improved as the combustion temperature is high. The factors that configure the optimal cycle for a simple cycle and a combined cycle is different. As for the simple cycle, the optimal cycle is achieved at a high compression ratio while a moderate compression ratio which is somewhat lower than that of a simple cycle and high combustion temperature are required for configuring an optimal combined cycle.

THERMAL EFFICIECY VS. AMBIENT TEMPERATUREThe gas turbine compression ratio decreases as the ambient temperature increases because the turbine inlet temperature is kept constant regardless of the ambient temperature.

The temperature change of the gas turbine exhaust gas caused by the ambient temperature change affects differently the thermal efficiency of a simple cycle and a combined cycle

The air temperature rise is a little profitable to the combined cycle, but it affects differently the gross efficiency of a combined cycle power plant because the output is changed higher by the exhaust gas flow rate than by the exhaust temperature.

OTHER EFFECTS ON THERMAL EFFICIENCYAtmospheric pressure

Atmospheric pressure does not affect the gas turbine efficiency. The gross combined cycle efficiency is not changed because the supplied heat and air mass flow rate are proportional to the atmospheric pressure.

Ambient temperature rise suction air mass flow rate rise gas turbine output and exhaust flow rate increase boiler input heat increase steam turbine output increase Condenser cooling media

The degree of condenser vacuum affects the steam cycle efficiency by changing the heat drop of the steam turbine. It varies with the cooling methods and seasons. - At the places with fluent cooling water and the seashore the wet process is utilized : the highest degree of vacuum can be obtained - At the places with poor cooling water the wet or dry cooling tower where the cooling water re-cooled by the air is utilized. - The air-cooled condenser is installed where cooling water is not available.