Waste heat recovery from turbine exhaust - multiple turbines

[garfio]

A client wants to recover heat from the exhaust of two turbines to heat oil. The units drive compressors and will be normally running at a high load rate, although there are chances that only 1 turbine be running.

Normally I've seen one WHRU per turbine, but there is a suggestion to use a single WHRU for both turbines. I would like to hear your thoughts about this idea. I see that there could be some savings but reduced maintainability and maybe control complications.

Any idea will be appreciated.

 

[pmover]

a guillotine type isolation device will need to be installed to ensure positive shut-off between each turbine and the WHRU. when one turbine is offline and down for maintenance (hot gas path inspections, boroscopes, engine removal, etc.), isolating the turbine hot section from WHRU will be needed. this adds to the end-user operational practices.

i would not recommend a configuration as you mentioned. i would think that any savings would be offset by the additional instrumentation, equipment, and additional maintenance needed to a dual turbine WHRU.

I'd request for some references for such configurations if this is being discussed.

hope this helps. good luck!
-pmover

 

[rmw]

Check with local regulations and your companies occupational safety regs. You might have to have double block (with positive pressure ventilation between the blades) type guillotines or dampers if you intend to make it man-safe to work in the non running unit.

Two things about Pmover's recommendation against your proposed configuration. Not knowing about what the oil is that you are trying to heat, if you make the WHRU large enough to serve two turbines, it might do just fine with both turbines at rated loads, but might not be the full 50% with only one turbine pushing exhaust through it.

Second is that your turbine model probably has already had WHRU's added to them, so there are standard sizes and configuratinos out there, albeit more rare for oil than making steam. The 2 on 1 WHRU unit for your service would tend to be more of a one-off type of thing where you won't get any savings from the economies of scale of picking something 'off the shelf.'

rmw

 

[MJCronin]

I agree with rmw....

Standardized HRSGs exist for many gas turbines.

Although there have been single HRSGs serving two gas turbines, they are all unique designs and, as I understand, difficult to control. A company called DELTAK has produced some

Usually, the best choice is to take waste heat, make steam and then use it for a variety of purposes....including heating of oil.

Do you already have a steam system at you plant ?

What is the MWe size, make and model of your gas turbines ?

 

[garfio]

Thanks for all the answers. Turbines are Taurus 70, and there is no steam system. This is not an expansion but a new plant about to be built. WHRU is an addition being analyzed to replace a oil heater.

 

[rmw]

So what do you heat the oil with when one or both of the turbines are down?

rmw

 

[garfio]

One turbine is at 50% plant turndown, so heat load reduced accordingly.

 

[rmw]

On that basis, I'm voting for separate units because I think you will have trouble getting performance out of a double sized unit at 50% exhaust flow through it. That said, what type of oil is it, and what is your turbine exhaust maximum temperature and will your oil survive being exposed to temperatures that hot? Have you picked a vendor for the WHRU yet? They better know what they are doing in that regard.

rmw

 

[garfio]

no selections so far,but this same temps has been handled before in other projects (about 1000F)

 

[davefitz]

Interesting topic. I had thought of a related idea- providing a small gas turbine ( 10% capacity of main gas turbine) to be used for "spinning reserve " operation of the HRSG and STG- the smaller CTG could also be used for "black start" capability.

It is known that for most STG's, at least 8% MCR steam flow is needed to prevent L-1 blade instability ( aerodynamic flutter) , so to maintain continuous spinning reserve of the STG and fast restart of the main CTG, at least 8% MCR steam flow must be maintained. In regions with large wind farms, the need for a fast restart ( < 15 min) of the main gas turibne is important.

 

[MJCronin]

I suggest that you consider making the heating of the oil as a sideline for the generation of power.

Taurus 70s have been used in both cogeneration and combined cycle service in many installations.

This document gives you a rough idea of the steam generation potential. A detailed heat/mass balance for the plant should be developed by a professional.

http://mysolar.cat.com/cda/files/255309/7/bpgpsg.pdf

 

[rmw]

I thought he said that the turbines drove compressors.....

rmw

 

[athomas236]

If you have 2xGTs and 1xWHB there is generally a section of duct between each GT and the WHB that is not shared with the other GT.

Asuming that each GT doesnot have its own vent stack, then will there be a problem purging the non-shared section of duct when starting the second GT with one GT already running?

Also what happens when starting the second GT and it doesnot light allowing air and fuel from the second GT to combine with the exhaust gases from the first GT?

Regards

athomas236

 

[davefitz]

athomas236:
Different configurations are possible- one possIble config is to use an aeroderivative CTG for the overnight duty unit , with its own HRSG and stack. Generally aeroderivative units have a final gas temp on the order of 900F , which may allow supply of a once-thru, dry fired capable , ferritic tubed HRSG (as opposed to the inconel units from canada). For overnight spinning reserve ops of the STG, only 8-10% MCR steam flow is needed at the LP STG stages- The HRSG might only produce IP steam at (825 F) , and the HP STG may use reverse flow steam ( thru the "ventilation valve") to cool the HP stages of the STG during spinning reserve ops- much like those unitS that load thru the intercept valve.