Waste Heat Recovery - Air Quality Implications

[zdas04]

I'm working on a project to transfer much of the waste heat from a turbo-charged Cat 3612LE on a compressor skid to an evaporation pond to try to accelerate the evaporation. One piece of this is to run the exhaust through a shell and tube heat exchanger which will cool the exhaust from just over 800F to 400F (the heat exchanger is designed to have less than 0.25 psi pressure drop on the exhaust side).

One of the engineers on the project raised the question "with that much cooling of the exhaust gases, the density will increase and the velocity will significantly decrease, will the velocity decrease change the dispersion of gases and increase the exhaust-product concentration near the site and impact the air-quality permit levels of exhaust products?". The short answer was "yes, the velocity will decrease but I can't tell how that will impact the total emissions".

Has anyone adressed this issue in any of the Co-Gen operations around the world?

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[GregLocock]

We used to get around this problem on cars by injecting additional air into the exhaust. Some of it might have helped burn excess hydrocarbons or CO, some of it just diluted the tailpipe emissions. "If it's stupid, but it works, it's not stupid"



Cheers

Greg Locock

 

[CESSNA1]

ZDAS04:
If I understand this the diesel exhaust will still be discharged to the air and not into the pond. There is no effect on the emissions. The emissions are still in the exhaust regardless of its temperature. The exhaust would be discharged into the atmosphere with or without cooling. For some reason there is a requirement to cool the exhaust somewhat before discharge. Cooling may quieten the exhause somewhat, but the temperature is stll fairly high.

Regards
Dave

 

[ivymike]

If you cool the exhaust so much that you condense the water out of it, then you may have a corrosion problem in the exhaust ducting (and/or water accumulation problem). Allow for drainage, or keep the exhaust hot enough to avoid this potential problem...

It seems like cooling the exhaust gas to a temperature below that of the surrounding air might cause the exhaust gas to tend to settle lower to the ground rather than waft upwards (assuming still air). Not sure whether it would be a real problem, but it might stink. Forcing the exhaust to blow away might help (with a big fan, etc).

 

[zdas04]

CESSNA1,
The goal here is to use the waste heat to accelerate evaporation. The exhaust gases still go into the air at the point where they have always gone into the air. I'm just lowering the temperature from 850F to 400F and am concerned about the reduction in velocity will reduce the dispersion.

Ivymike,
I'll have controls to keep the exhaust temp at least 150F above the condensation temp and well above the surrounding temp. The concern is that while Bernouli's equation isn't valid (since density is not close enough to constant) the velocity is still related to pressure and density in some relationship and I'm pretty sure that a big increase in density will still be a big decrease in velocity. What does that do to the dispersion of the gases into the atmosphere?

David

 

[patprimmer]

Why not taper the pipe to keep constant velocity.

Why not cool the exhaust by running water into the exhaust system like they sometimes do in boats, and discharging over the pond. You would need to ensure the exhaust was a constant downhill run from the water entry point, and the entry point was below and shielded from the exhaust port to eliminate risk of water ingestion via the exhaust valve.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[zdas04]

Pat,
Tapering the pipe adds a modicum of backpressure and the guys who own the compressor set the limit at 0.25 psi and my heat exchanger is 0.22 psi.

The water I'm trying to evaporate is pretty nasty (25,000 mg/l TDS) so I have about 25 gm of solids for every liter of evaporation. With 25 million liters/day there is 625,000 kg/day of solids. I can't risk any of that water boiling near the exhaust stack or the scale will fill the pipe in a couple of hours.

David

 

[patprimmer]

Boats do it with sea water without major problems. I must admit I do not know the solids content of sea water, but I would think it is pretty high.

If the pipe was very long, and the water flow fairly high, it should stay clean and give good heat transfer.

It might increase the back pressure to an unacceptable level though.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.