Difference between Simple cycle and Combined Cycle

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Thread Starter

Sadiya

What is the difference between Simple cycle gas turbines and combined cycle gas turbines?

Is a gas turbine combined with waste heat recovery boiler called combined cycle? Or, is only Gas Turbine combined with waste heat recovery and a steam turbine called a combined cycle? Is presence of steam turbine necessary for it to be called combined cycle?

And, what role does modulating IGV angle play in both simple cycle and combined cycle? I want to know more about exhaust temperature maximizing effect with IGV angle modulation...
 
Sadiya,

The same gas turbine can be used in either simple cycle- or combined cycle application. In fact, many applications employ something called a "bypass stack and damper" arrangement on the exhaust of the gas turbine, which can direct the gas turbine's exhaust heat into a waste heat recovery steam generator ("boiler"), or through a stack to the atmosphere.

Here is a good definition of 'combined cycle:'

http://en.wikipedia.org/wiki/Combined_cycle

In combined cycle application, the IGVs of a gas turbine can be held closed longer during loading (and unloading) so that the air flow through the gas turbine is reduced, thereby increasing the gas turbine exhaust temperature for the same fuel flow-rate. A higher gas turbine exhaust temperature, when being directed into a "boiler" will increase steam production. While "modulating" the IGVs reduces the heat rate for the gas turbine slightly, it increases the overall heat rate for the entire plant (including a steam turbine if present). So, it's a way of maximizing the steam production at low gas turbine loads, and thereby increasing the overall heat rate of the plant at lower loads (loads less than rated).

It serves no purpose to keep the IGVs closed longer during loading and unloading of a simple cycle gas turbine or a gas turbine being operated in simple cycle mode because increasing the heat of the exhaust going to atmosphere serves no purpose. And, again, it decreases the gas turbine heat rate which is a measure of efficiency, so why make the gas turbine less efficient to heat the exhaust that's doing nothing more than contributing to global warming?

I have been to plants that use gas turbine exhaust heat to heat "steam" (actually, it's still water at high pressure), and that steam is injected into the ground to enhance (improve) oil production. This is also called 'combined cycle' even though there is no steam turbine (the hot water is directly injected into the ground after leaving the waste heat recovery boiler). In fact, at these sites the production of "steam" (again, it's really just hot water!) is the main purpose of the gas turbine-driven plant; electricity is the "by-product" of the process of producing the "steam."

In my mind, the "combined" part of combined cycle refers to the production of both electricity <b>and</b> steam by using the exhaust heat from some "engine", and that engine can even be a reciprocating engine; it doesn't have to be a gas turbine. Whether or not the steam being produced is also used to produce electricity is not the most important aspect of the combined cycle plant. Many combined cycle plants use a major portion of the steam produced in the waste heat recovery steam boilers in other processes, for heat as well as cooling (of certain processes). So, it's not a requirement the steam be entirely used for electrical power production.

Hope this helps!
 
A couple of comments:

1. In GE, we usually referred to combined cycle as those applications that used the gas turbine waste heat to produce steam for driving a steam turbine that in turn drives a generator to produce electricity. Where the gas turbine waste heat was used for other purposes (like steam to process in a refinery), we called it cogeneration. (Sometimes cogeneration also included a steam turbine generator, and the steam turbine had an extraction for the steam to process.)

2. Heat rate is the inverse of efficiency, so higher heat rate is lower efficiency. Heat rate is a measure of the amount of thermal energy required to produce 1 KW-Hr of electrical energy (or 1 HP-Hr of mechanical energy).
 
otised,

I ALWAYS confuse heat rate and efficiency, forgetting they are the inverse of each other. I should have written that by keeping the IGVs closed longer than normal when the exhaust heat is being used to produce steam the gas turbine heat rate increases slightly (it's slightly less efficient), and the overall plant heat rate decreases slightly (the efficiency of the overall plant increases slightly).

Thanks for the clarification, and the additional information!
 
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