Stack characteristics? 2

[cctdiag]

Hi,

Knowing virtually nothing about combustion hopefull you guys maybe able to help?

I am working on a dispersion model for NOx emission but I need to work out an effective stack height by using a Briggs equation. But I have no info' on flue gas temperature, gas volume being discharged or flue area (for stack radius).

The only information I have been given is

Physical stack height - 23m
LTHW boiler rating 1.75MW (providing 3304MW over 90 days)
Burning - fuel oil @ (11.6kW/litre)
I am assuming a discharge velocity of 10 m/s (is this reasonable or should this be higher say 15 m/s?)

Is there a recommended theoretical flue gas temperature I should use for this fuel ( 150, 200 deg C...)?

I have read that flue gas produced for Fuel oil is 11.6m3/litre - does this sound about right as my eventual figures give me 0.42 m3/s (way to little?).

If the above is not enough to give the information I need, has anybody got some actual stack figures on flue gas temp, discharge velocity and volume (or stack radius)for this size of plant - so I can make a resonable assumption.

Thanks

 

[25362]

As for answers to your questions I suggest a visit to the large amount of threads concerning stacks. Among those you may find interest in reading the following:

thread338-2638
thread391-54168
thread404-43365
thread610-46893
thread633-49681
thread798-31854

The flue gas produced by a liquid fuel of 10,500 net calorific value would be about 13 normal cubic meter/kg when burning with 10% excess air. Normal conditions: 1 ata, 0^oC.

Stacks have two roles: to create a draught and to minimize pollution at ground level. To prevent downwash and still provide capacity and flexibility a velocity of 15 m/s is OK. Temperatures may affect buoyancy and corrosivity due to the dew point of SOx.

To combat pollution in some areas (local regulations) the stack height should exceed 60 m. A stack "effective" height is increased by making the velocity at the top 14-20 m/s.

One formula used to estimate the ""effective" height of a stack is:

H_effec = H + (1.5 V_g*D+0.04*I_f)[÷]V_wind​

H: height of stack
V_g: velocity of flue gases at the top, m/s
D: inside diameter of stack, m
I_f: total heat content of gases, kcal/sec
V_wind: wind velocity at 10 m height, m/s

 

[cctdiag]

Thanks 25362

for the much appreciated confirmation on velocity and expected m3 of flue gas. However is there a ideal flue gas temperature I should be looking for.

This is for a hypothetical problem I have been set, I am comparing the pollution effects of an energy from waste plant (70m stack - calculations completed) to that of the above heating plant(23m stack) of which I have only the info listed in my first thread. I only need to assume an ideal stack temperature and radius and should be OK.

From what you have said about stack heights, the 23m stack appears very low, do you think this will require a greater efflux velocity than normal to aid dispersion? My effective stack height for the 70m stack came out at 115m.

Thanks

 

[25362]

As said above, the temperature affects the buoyancy and in the given formula it affects the heat content of the gases.

You make your own estimates considering that the heat content of the flue gases is approximately as follows:

200^oC, 67 kcal/m³
250^oC, 83 kcal/m³
300^oC, 100 kcal/m³
350^oC, 118 kcal/m³

If 115 m is the required effective stack height, no doubt you should consider modifying whatever factor that may help in reaching that height. And the chimney mechanical stability should be assured. See previous list of threads.

If your plant emits gases with 0.1% SO2, you may be obliged to treat them to remove 90% of it, and should take care not to produce a visible plume from a wet alkaline scrubber. Lower SOx contents may allow lower effective stack heights, depending on local regulations.

I assume you may find the following threads of interest:

thread127-56418
thread135-28937

A wet scrubber drawback is that it cools the flues. A typical power plant emits gases at 400^oF, a wet SOx scrubber drops this to 125^oF, and the effluent gas must be re-heated to regain buoyancy to about 175^oF. There are also dry systems that are more expensive.

The final design will be dictated by economics coupled with local regulations on stack emission control.

 

[mbeychok]

.
cctdiag:

My response relates to the use of the Briggs' plume rise equations. First of all, the effective stack height (H_e) is the sum of the stack's physical height (H) plus the plume rise ([Δ]H) due to the buoyancy of the plume and, in some cases only, the velocity of the gas at the stack exit. Mathematically:

H_e = H + [Δ]H

The plume from a boiler flue gas stack (such as your stack) will be a bent-over, buoyant plume. The centerline of such plumes will rise as the plume travels downwind until it reaches a maximum at some distance from the stack ... and then the there will be no further plume rise. The key point here is that the [Δ]H and, therefore, the H_e increase as the plume travels downwind ... thus, [Δ]H must calculated at the specific downwind distance to the receptor point at which you desire to calculate the dispersed concentration.

There are a number of different ways of calculating [Δ]H. The most commonly used method, by far, is to use the Briggs' equations. The [Δ]H as determined by the Briggs' equations varies with the stability classification of the ambient atmosphere (i.e., the amount of atmospheric turbulence present) as well as with the downwind distance to the receptor and with the buoyancy of the plume. The logic diagram for using the Briggs' equations to calculate the buoyant plume rise at a specific downwind distance from the source stack is available at:

www.air-dispersion.com/briggs.html

I strongly suggest that you study that logic diagram. Better yet, I would advise you to seek the help of an experienced air dispersion modeler.

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