At what pressure / capacity does co-generation become viable?

[rollingstein]

We have a boiler (industrial) that produces steam at ~20 bar but our process requirements all need less than 10 bars. I was wondering if at these relatively low pressures a back pressure turbine is viable at all? To use the 10 bar drop in pressure to generate some electricity.

Our boiler size isn't huge but not tiny either. Approx. 20 MMBtu.

With rising electric costs it'd be nice if we could generate a fraction of our electrical load ourselves. Our total peak electrical power draw is about 300 kVA.

Any opinions / comments are welcome.

 

[Graniar]

Wouldn't it be easier to modify boiler itself, ie set 10bar valve, to make it consume less power?
Or your boiler uses different source of heat? Than it may be the good idea. I'm wondering, why steam engine power generators not in common use, it's very simple and may be fueled with coal or woods.

 

[russ_watters]

From what I've seen, Cogen isn't done by making one source of steam and making heat and power from it, it is done by making power directly with an engine and using the waste heat to make the steam.

 

[rock.freak667]

You could possibly estimate the power output based on the back pressure, then account for losses and see how much kW it would output. Then you could size a generator based on it, calculate costs and see if it is economical enough.

We have a turbine on the plant which runs a compressor and a generator. The generator produces about 10 MW.

 

[rollingstein]

From what I've seen, Cogen isn't done by making one source of steam and making heat and power from it, it is done by making power directly with an engine and using the waste heat to make the steam.

You mean a turbine followed by a waste heat boiler? That's one way, yes.

But I think there are other ways especially for process plants where a need for some high pressure steam may exist. Essentially this would be a non condensing steam turbine I suppose.

 

[Varadha]

Reheating, economisers??

 

[russ_watters]

Maybe I'm just misunderstanding the parameters here. You have an existing boiler that runs at 20 bar and a process that needs 10 bar. Are you just looking for a way to turn the 20 bar steam into 10 bar steam while recovering energy instead of losing it in a pressure reducing station? Stupid question: why can't you just turn the boiler down to 10 bar?

 

[Q_Goest]

I'd agree with rock.freak. It would probably be worth doing some quick calculations to determine the potential energy recovery and economic benefit. Given your turbine inlet conditions (20 bar, ? temperature) assume 80% isentropic efficiency and find the power output of a turbine at the given flow rate. Next, determine the outlet conditions on the turbine. If the steam is saturated, can you use it at 10 bar or do you need to heat it further? Note that some water could condense depending on how hot your steam goes into the turbine.

Now you know what you would need to do to recover the energy from 20 to 10 bar and how much power you can get. That power has some value. Compare how much value that power has to the capital cost needed to recover it. Consider you may need to recover that cost over 10 years. How much money is that?

If you need help with those calculations, please provide:
Temperature of 20 bar steam
Temperature needed at 10 bar
Mass flow rate of steam

 

[russ_watters]

And 10 Bar load (and therefore what the excess capacity is).