Thermal Efficiency Improvement of a Combined Cycle Power Plant

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Thread Starter

Sreesankar J

I would like to know the methods to recover the excess heat from the flue gas that is been send out through stack. It is a Combined Cycle Power plant of Capacity of 350MW, 2 pressure stage HRSG. At peak load of GT 1 that is 115MW the flue gas been send out is 120c. But now the production is 101MW and the flue gas being send out is 124c, excess of 4 degree is wasted. I'd like to know the best method to recover that 4 degree so that the efficiency of plant can b increased, as design changes may increase cost and risk of cold corrosion in the stack.
NB:- Already thought of recovering the waste heat my increasing the surface area of economizer, by adding additional loop.
 
Can try Increasing feed water flow in HRGS as well. can employ a Temp-Feed water flow logic on the DCS for the same.
 
Hey there,

I'm sure you are aware of the fact that materials degrade over time. Furthermore, many systems in a plant rarely ever achieve all of their nameplate or theoretical values. How old is this plant you are at? It seems that if it is only a couple years old and you are seeing losses like that then the issue might be a bigger problem. If the plant is 15 yrs old it seems like the deviation you talk about is not unreasonable for that kind of time frame. That being said, the solution you propose could be a good one. Adding an additional economizer loop is not a trivial thing, there would be a lot of things to take into account and the costs might be very steep for the outcome you would get.

In regards to the risk of cold corrosion; there are many factors at play. First being what type of fuel is being burnt? The sulfur content of the fuel will tell you how much of an issue this could be. Another factor to take into account is the amount of oxygen coming from the stack. Excess oxygen from the combustion process will react with the sulfur to produce SO3. The amount of unused oxygen will relate directly to the dewpoint temperature of said chemical. So there are a lot of variables that come into play here as well.

At the plant I work at we have a triple pressure HRSGs. The condensate coming from the hotwell is pumped through two "preheater" sections before the LP economizer. At the outlet of the preheater, the condensate is close to boiling. We have piping that routes part of that condensate that has been heated through a pump and back into the inlet of the preheater (where it mixes with the condensate from the hotwell and then travels through the preheater again). So essentially we always have a little bit of our preheated condensate recirculating through the preheater. We do this to ensure that there is sufficient temperature at the end of the HRSG so we don't drop below the acid's dewpoint.

Hope this helps.
 
Hello there,

In a conventional Boiler, exit gas temp. is independent of Steam pressure . But if we consider a HRSG then in this case because of thermodynamic consideration the exit gas temperature is a function of

1. GTG exhaust flue gas temp.
2. Feed water temp.
3. Steam pressure

Moreover typically in range of 450-600ºC (GT exhaust temp.) HRSG generates less steam on unit gas mass basis when compared to a conventional Boiler. The reason is that in case of conventional Boiler the flue gas is cooled from Adiabatic combustion temp. of around 1900ºC and hence generates large amount of steam on unit gas mass basis. The small ratio of steam to gas flow in HRSG is not adequate to cool the exhaust gases in Economiser, resulting in a high Exit gas temp. Moreover mathematically it can be easily shown that higher the steam pressure, higher will be the exit gas temp.

I hope that this discussion explains how higher exit gas temp. problem is created in HRSG and how this is different from Conventional Boiler. In subsequent posts if you want then we can discuss the methods available for lowering this high exhaust temp. Problem.

Moreover, the HRSG is very fondly called a SEED Boiler i.e. Superheater, Economiser, Evaporator, and Drum.

Regards,
fluidflow
 
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