6 replies to this topic

[mbeychok]

K.Thiyagarajan:

This is a quote from your posting:

Higher the stack , better it is for dispersion point of view.

You are correct in saying that higher stack heights provide better dispersion of the emissions. However, in the USA and some other countries as well, the environmental protection regulations have defined what is called Good Engineering Practice (GEP) Stack Height as the maximum height at which dispersion modeling of air pollutant emissions may be made. Those regulations are meant to prevent the use of higher stacks rather than reducing the amount of emissions.

In other words, if your stack height is 150 meters and the calculated GEP Stack Height is 100 meters, then you must do your dispersion modeling at the GEP Stack height to see if you comply with the ground-level pollutant emission concentrations that are allowed by the environmental protection agencies. Thus, there is no incentive for erecting stacks that are higher than the GEP Stack Height ... at least not from a dispersion viewpoint.

I would also point out that the lower temperature (75 °C versus 400 °C) will result in a much smaller plume rise because flue gas at 75 °C will have less buoyancy than a flue gas at 400 °C. The smaller the plume rise, the higher the ground-level concentrations.

I suggest that you consult the environmental protection agencies in your country to see if they have regulations that limit the stack height.

[gvdlans]

I agree with Milton that first you have to know what the requirements are in your country. We cannot help you with that since we don't know which country the facility is in.

Having said that, and knowing that you have an existing stack with a height of 150 m (which is quite tall..., I would say this is about the tallest you can reasonably go (Milton, can you reflect on that?)), the most logical thing is of course to route the flue gases to this existing stack. So the first approach would be to see if you can meet the environmental requirements with the existing stack. As Milton wrote, the lower temperature means that you will have less plume rise, resulting in higher ground concentrations. On the other hand, I assume that because of the scrubbing, the pollutants concentration in the flue gas will be lower.

So my advise is to:
- find out what local requirements are.
- see if you can meet these requirements with the existing stack. You probably need to do dispersion calculations for this.

[mbeychok]

I agree with gvdlans that 150 m stack is probably as high as is practical. I would also point out two additional items:

(1) The 75 °C temperature of the scrubber outlet as compared to 400 °C without the scrubber means the total flue gas flow will be about 50% lower than it was before the scrubber. That means that buoyancy of the gas relative to the ambient air will be lower and the stack exit velocity also will be lower ... both of which will result in a lower plume rise.

(2) If your scrubber uses an aqueous solution, the exit flue gas from the scrubber will be saturated with water vapor and you will probably have some condensation occurring in the stack. If there is still some SO₂ left in the flue gas, you may have corrosion problems to consider. If you have a way to reheat the flue gas somewhat, you might consider doing so.

[siretb]

Keep in mind that the flue gas will contain plenty of water, and that the higher the stack, the more water you are likely to condense at the wall. Be sure to perform a calculation for heat losses and condensation.
The temperature of 75°C at stack looks high for a wet scrubber, so I suspect you have some reheating somewhere, and that the gas at stack might not be saturated.

[Fahad Mohd]

Dear Mr K.Thiyagarajan

first of all thanx for initiating this post...further i am also in a task of installing the wet scrubber at the downstream side of our Slop fired Boiler.i shall be highly thankful to you for providing me the basic sizing steps of the wet scrubber for the flue gases.please note that i have to design the pilot wet scrubber first and based on the satisfactory results we will think of the bigger installation.here i am looking the pilot scrubber sizing steps.

i shall be glad if you additionally give your remarks on how to check the chimney capacity.would there be more margin in existing chimney if we install wet scrubber prior to the chimney

your views on this matter will be highly appreciable.

Regards
M. Fahad

[kkala]

Having read this interesting thread, following points of opinion could be useful.
1.Assume 12% water vapor (H2O) molar ratio (H2O/dry gas) in stack gas of 400 oC. The gas turns saturated and 75 oC after scrubbing (H2O pressure=0.380 Atma). Thus mentioned ratio gets 0.38/(1-0.38)=0.61 (H2O/dry gas).
Based on 1 kmol of dry gas, corresponding volume at 400 oC is: 1.12*22.4*(400+273) /273 = 61.8 m3/h; after scrubbing, corresponding volume would be 1.61*22.4*(75+273)/273= 46.0 m3/h. Volumetric flow is decreased by ~26%, but friction factor is expected to increase due to peripheral condensate inside stack. Moreover natural draft (seehttp://www.cheresour...h__1#entry49537) will be reduced, since stack gas density will change from about 0.50 kg/m3 to 0.87 kg/m3.
A thorough hydraulic check has to be made under the new conditions, to verify adequacy of stack or diagnose problems, e.g. need for head increase of relevant forced draft fan.
2. After this, dispersion study had better compare results of old and new conditions, using the same dispersion model. This would be also necessary for any environmental impact study.
Useful remarks on the dispersion study have been already written in previous posts.
Condensate drops coming out of the stack makes a problem; besides prediction of their dispersion is difficult. Advice on how to avoid creation of escaping drops, or how to predict whether these are created or not, is welcomed. If it occurs, location of a demister at such a height may be a (hard) solution.
3.As already noted in previous posts, a stack 150 m high is exceptional. Local refineries have 60 m max stack height (except the flare stack).
Dispersion modeling needs some background, it is often undertaken by specialized companies. The local "procedure" concerns dispersion at three different stack heights to chose the right one. Your task may be simpler, since you have a stack of given height and you have to see the dispersion effects under several conditions. In my opinion a simpler model well understood is better than a more sophisticated one, vaguely understood.
One expects that scrubbed gas would give better environmental results, but this has to be quantitatively shown.
Note: Books on gas dispersion useful to me:
1. G. Nonhebel " Gas purification processes for air pollution control", Newnes-Butterworths, London, 1972 (one chapter devoted to the subject, somehow out of date now, satisfactory for understanding the principles".
2. M. R. Beychok "Fundamentals of gas dispersion", 4th edition 2005 (widely known, modern and practical).

Edited by kkala, 28 July 2011 - 01:26 PM.