stack height requirement 1

[techv]

Hi guys,

We are executing an incinerator. Flue gas has about 50% of water vapor in it and there will thus be a white plume. Client wants conservative height for the stack to avoid droplets falling around the area, as they did experience elsewhere. May I have your feedback/advice on how to estimate minimum stack height for this case?

Your help is much appreciated.

Thanks
TechV

 

[ash9144]

Depending on where you are, usually the stack height is specified in your air emission permit. The regulatory agencies have some general guidelines you could use for estimate purposes. I would recommend going to their web site or contacting directly.

 

[dcasto]

Wow, there will be so many variables and to gaurentee no droplets for ALL ambient conditions, I'd recommend 2000' and ask the client if they would like to continue with a cost estimate too.

 

[mbeychok]

techv:

I agree completely that there are simply too many variables. It would be virtually impossible to calculate a stack height that would guarantee no liquid droplets at any time.

As an alternative, if you could heat the flue gas, the increased buoyancy would increase the plume height and provide more time for dispersion of the water vapor before it cools enough to form liquid droplets. We did that many years ago to make the flue gas plume from the catalyst regenerator of a fluid catalytic cracker (in an oil refinery) "invisible". It worked very well.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[dcasto]

nicely done mbeychok, Again, the OP's client will see an increased operating expense too.

 

[25362]

The effective height can indeed be increased by making the flue gas velocity V_g at the top 15-20 m/s and by rising the temperature of the exiting flue gases.

There is an equation called the Oak Ridge formula for calculating the effective height H_eff of a stack of internal diameter D_stack and height H_stack:

H_eff = H_stack + C/V_a​

where

V_a is the wind velocity at 10m height, and

C = 1.5 V_g D_stack + 0.04 I_flues​

All lengths in m, all velocities in m/s
I_flues, heat content in flue gases, kcal/s

One design I've seen long ago brought the effective height to [≥]120 m. However, wind direction might cause unacceptable situations.

Some will say that prevention is always better than cure.
Therefore, in some cases although the design of a stack may be checked with the aid of formulas as above, it may be necessary to test a model in wind tunnels. Tests as these also provide valuable information as to whether vibration may be expected to occur.

 

[techv]

Thanks to all you three guys. Some more observations/queries though:

1. Is it not common to have white plume? Is there any valid concerns with that? Regulations clearly exclude water vapor from the opacity measurements.

2. If it is worth suprresing the white plume, I guess excess operating cost (in terms of aditional fuel required) in flue gas heating may not be a burden, if it can take out ESP requirement as last APC unit(envisaged to remove micro particulates). Is my understanding correct?

3. Our requirement is to meet 34 mg/dscm @ 7% O2. Do we need ESP or can we meet this limit with venturi scubber and conventional particulate seperator.

Pls advise.

 

[mbeychok]

I didn't really think that getting too deep into the subject of plume rise was warranted. However, just to set the record straight:

(1) The effective height of a plume is the physical stack height plus the plume rise due to stack exit velocity and/or buoyancy.

(2) The plume rise is a function of the distance from the stack tip. That is, the plume gradually rises as it travels downwind from the stack until the original velocity jet effect is dissipated and/or it cools off so that it is no longer more buoyant than the surrounding air. At that downdwind point it ceases to rise.

(3) By far, the most generally used method of calculating the plume path by many air dispersion modeling software programs is the method known as "Briggs plume rise equations" which are a function of a good many variables.... rather than any empirical, or rule-of-thumb equations.

To learn more about this subject, one should read either or both of these books:

Turner, D.B. (1994). Workbook of atmospheric dispersion estimates: an introduction to dispersion modeling, 2nd Edition, CRC Press. ISBN ISBN 1-56670-023-X

Beychok, Milton R. (2005). Fundamentals of Stack Gas Dispersion, 4th Edition, author-published. ISBN 0-9644588-0-2.



Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[910366]

Maybe your incinerator can be used to recover some heat and heat the flue gas to a certain degree?

 

[rocketscientist]

You might find this useful based on an HCl emission calculation I did for the EPA for a steel mill in Portage, Indiana.

Dirk Willard

 

[Unotec]

Can't you put a KOD before the incinerator to remove excess water?

 

[dcasto]

The water vapor condenses has it goes up and out the stack, a KOD will not work. They most likely already have one intergal to the stack.

 

[Longinthetooth]

Also, be careful to keep the exit temperature of the stack well above the dew point of any acidic incineration products, eg SO2. I once saw the internal lining of a stack collapse in after it was turned to mush by condensed SO2. 50C is a minimum. More if you are unsure of the dew point.