Trying to Get a Straight Answer on Efficiency

[Eric Johnson]

My original thread got hijacked by the techies below, but since they're having such an interesting discussion, I thought it would be better to start another thread and let this one run its interesting course. I'd change the title to something more appropriate, but don't know how/can't.

 

[TMonter]

Boiler efficiency depends on several factors, but the biggest are stack exit temperature and moisture content of the fuel.

I'm currently working on a large wood fired boiler burning 55% moisture fuel and the efficiency looks like this:

Radiation Losses % 0.40%
Unburnt Carbon Loss % 0.75%
MFR Margin % 1.50%
Dry Flue Gas Loss % 7.53%
Moisture Loss (Fuel and Produced) % 23.19%
Moisture Loss in Air % 0.18%
Total Losses % 33.55%
Boiler Efficiency For above Fuel Eff. 66.45%

 

[Nofossil]

Boiler efficiency depends on several factors, but the biggest are stack exit temperature and moisture content of the fuel.

I'm currently working on a large wood fired boiler burning 55% moisture fuel and the efficiency looks like this:

Radiation Losses % 0.40%
Unburnt Carbon Loss % 0.75%
MFR Margin % 1.50%
Dry Flue Gas Loss % 7.53%
Moisture Loss (Fuel and Produced) % 23.19%
Moisture Loss in Air % 0.18%
Total Losses % 33.55%
Boiler Efficiency For above Fuel Eff. 66.45%

Excellent. Is this measured or modeled data?

One of the ideas that we've been toying with is recapturing the latent heat of vaporization for the water in the flue gas by letting it condense out on yet another heat exchanger. Early tests are interesting. Makes a lot more sense if you're burning greener wood, I think. We're not that far above the point where that could happen - my flue gas hovers around 250 most of the time.

 

[TMonter]

Excellent. Is this measured or modeled data?

Both Actually.

It's modeled data backed up by real world testing.

 

[Nofossil]

I love this stuff. Couple more questions if you have the patience:

I assume that moisture/fuel and produced is the water that's in the wood plus the water created by burning the hydrogen that's in the wood?

What flue gas temperature is this all based on?

What are you using for the chemical makeup and mass of flue gas per pound of wood? I've derived a few number based on assumptions about the initial composition of wood and the chain of chemical reactions that take place during combustion, but I have no real data. I'm curious to see how far off I've been.

Thanks for furthering my education.

 

[TMonter]

I assume that moisture/fuel and produced is the water that’s in the wood plus the water created by burning the hydrogen that’s in the wood?

Yes. In the case of wood fuel the produced quantity is pretty small.

What flue gas temperature is this all based on?

330F Exit temperature.

 

[TMonter]

I wish I could upload a PDF of the worksheet. Tell Craig to fix that ;-)

 

[Nofossil]

I wish I could upload a PDF of the worksheet. Tell Craig to fix that ;-)

Sent you a PM with a link to my private site. Is it OK to share with others?

 

[TMonter]

Uploaded the file but can't seem to link to it. What's the URL of the directory?

 

[Nofossil]

Uploaded the file but can't seem to link to it. What's the URL of the directory?

You can't link to it - it's behind my firewall. I can put it in a public location if that's OK, and post a link here. Didn't want to do that without your blessing.

Great document, by the way.

 

[TMonter]

That's fine. Go Ahead and post the link.

 

[Nofossil]

That's fine. Go Ahead and post the link.

Here it is: TMonter's Combustion Worksheet - a thing of rare beauty. Thanks for making this available. How many folks here remember 'stoichiometric' from their chemistry classes?

 

[Nofossil]

100,000 pounds of wood per hour - that's some boiler! Erics's 43 cords would be gone in a couple of hours.

Also works out pretty close to my numbers. I'm getting around 18,000,000 BTU/cord of dry wood. This is around 13,000,000 BTU/cord of wet wood depending on your weight/cord numbers.

I'll make use of this data - thanks again.

 

[ISeeDeadBTUs]

OK, my freakin head hurts. Maybe it's the chemistry of this Martini, maybe it's the fact I never took Chemistry. Maybe it's the High pressure aloft that dove the temp up to 59 on Nov 21!!