33 replies to this topic

[Narnia]

Deal All,

I am now day working on circulating fluidized boiler project based on local coal. The guaranteed emisions of boiler are as below

SOx 400 mg/Nm3
NOx 280 mg/Nm3
Dust 50 mg/Nm3

I have to size the stack height for this project. can body guide me what should be the minimum stack height is??? Can reference calculations to it.

Is there any dispersion calculation software availble for stack sizing.pls inform

Thanks

[breizh]

Hi ,
Let you consider this simple model .

Hope it helps

Breizh

[jrtailor09]

Narnina,

There are software available in market for the dispersion analysis. Generally this type of software used by the properitery equipment vendor like Flare system, Boiler flue gas analysis etc.

List of widely used software are as below :

SHELL FRED
DNV PHAST

If u are dealing for a new boiler system then flare stack height requirement shall be done by boiler supplier only.

I have seen in IPS standard ( IRANIAN PETROLEUM INDUSTRIES ) that minimum stack height shall be 76 m. But recently, i come across with one steam boiler package system in that existing stack height is 25 m & new also they are considering same 25 m.

Regards,

[kkala]

Some rather helpful info can be found at http://www.cheresour...h__1#entry50082 and to less extent at http://www.cheresour...h__1#entry34490. Here specialized offices undertake the environmental impact study of the new unit(s), along with definition of chimney height(s).
It is not easy to apply a complex dispersion model, in case of no previous practice. If so, study of the fundamentals from a book and application of a simple Gaussian dispersion model had better be done in advance. I have heard of some version of Phast model applied here by the local University, but they had familiarity with it.
The objective is to indicate through the dispersion model that ground level concentration (glc) of noxious substances is withing limits provided by legislation. But usually the plant is in an Industrial area, where a lot of other units or industries emit gaseous effluents. It is not easy to estimate contribution of others. Applicable approach here is to indicate that the pollution by the new unit is marginal, subject to approval by authorities. For instance, max glc for a noxious substance (at certain weather stability class) is estimated at 70 , 85, 90 μg/m3 for stack heihts H = 50, 60, 70 m; one can propose H=60 m, for authorities to approve. Two ~65 ton/h industrial fuel oil boilers, installed at local industrial areas, had H=60 m, while pollutant concentrations in their flue gases were more or less same as in the mentioned boiler (SO2 a bit higher). However much depends on what local authorities will tolerate as max glc contribution of the specific unit, which will charge the area in addition to already existing units. This has to be investigated for every meteorological condition possible. Other new units installed here have also stacks of 60 m height. It is also noted that stack gas exit velocity at nominal capacity is specified here at 15 m/s (minimum).
Concerning the dispersion models, see http://atmosphericdi...odel_evaluation and http://en.wikipedia....spersion_models. Aloha was used in safety studies, judged not proper for the case of boilers. It is not easy to chose the most suitable model for the case (among the "free" ones), also depending on what meteorological data is available. A simple "hand made" excel model well understood could be more useful than a sophisticated one partially understood, as I have seen. Advice on dispersion models having worked satisfactorily for boiler stack gases or similar would be welcomed.

[ankur2061]

Narnia,

Stack height for boilers may be calculated as per the following formula:

H = 14 * (SO₂ in flue gas in kg / h)^0.3

where:

H = height of stack in meters

If the fuel consumption is A kg/h and sulphur content in fuel is x% then 1 kg of fuel will give 2x% SO₂

Thus the term in the bracket will become

(A*(2x /100))

The reference for the above formula is the "Indian Boiler Regulation" code also known as IBR code.

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 24 November 2011 - 01:21 PM.

[pavanayi]

Narnia,
I have used (and still using) ADMS http://www.cerc.co.u...ADMS-model.html for dispersion modelling of pollutants for open vents for calculating the Ground Level Concentration as a function of distance from stack (as Kostas mentioned). It has a list of common pollutants (like NOx and SO₂₎ and can be customised for any new pollutant as the case arises.
To use such software, you require the atmospheric conditions at the plant (usually available from local meteorological office).
The UK guidelines on workplace exposure limits can be found here http://www.hse.gov.uk/coshh/table1.pdf

[ankur2061]

All,

A quick estimation of ground level concentration is available in the form of a blog article and a downloadable spreadsheet at the following link:

http://webwormcpt.bl...el-unburnt.html

Regards,
Ankur.

[Narnia]

Would you please elaborate the terms used in sheet as x,y,z & stability conditions & its parameters.

Thanks

[breizh]

http://chemengineeri...ersion modeling

take a look at this resource

breizh

[Narnia]

what should be options of stack material (for concrete & steel) in case if it is install at downstream of FGD system becuase flue gases are fully saturated with water vapors.

if it is concrete then special bricks or cement required ???
if it is steel then of what type & is it required with some linning material. or we have to go stainless steel stack.
Thanks

[Narnia]

Dear breizh,

As per excel sheet provided by you , how one will ensure that this much height will be sufficient by calculating the ground level concentration.

Means to say how i decide this height is enough.

thanks

[breizh]

Narnia ,
The calculation is done to minimize the risk for people meaning the reference to ground level . Now regarding height you may have constrain like regulation wich can impose minimum height according to the type and concentration of pollutants.
Hope this helps.
Breizh

[Narnia]

Dear Breizh,

Can you quote your above statement with some example, concentration w.r.t distance
how distance to be defined for measuring conc. 1 km,2 km etc

[breizh]

Narnia ,

Hope this document will make it clearer !

Breizh

[Narnia]

good material but attachment din't highlight my clarifications , how we will be sure about stack height with dipersion study.

if we have below emissions from boiler
SOx 400 mg/Nm3
NOx 280 mg/Nm3
Dust 50 mg/Nm3

what will be the stack height for this case

[kkala]

According to local conditions here, 60 m height could be preliminarily assumed for a medium size boiler (40-60 ton/h of steam). One operating refinery boiler here burning fuel oil (emissions slightly worse than the ones stated as guaranteed) has a 60 m stack. Another fuel oil boiler, recently ordered, has specified a 60 m stack (without dispersion modelling yet). Each of mentioned boilers produces about 60 ton/h steam (41 barg, 400 oC). Several new Units of a refinery revamp (furnace, boilers, etc) have also 60 m stacks.
Of course this height has to be finally verified (or even modified) by a dispersion model and be approved by authorities. See previous post No 4. Approved stack height for an identical unit can be different from country to country, even from location to location.
Approved stack heights in the broader industrial area of the factory (plus local legislation, if any) can lead to a better assumption of the preliminary stack height.

Edited by kkala, 03 January 2012 - 02:18 PM.

[Narnia]

Actually i have performed the dispersion analysis of stack for above mentioned emissions at different stack height. I have calculated the ground level concentration at differnt distances for different stack height. But i have to decide which stack height i will choose & how.

Attached Files

•  Dispersion curve.doc   86.5KB   122 downloads

[kkala]

So the stack height is in a more advanced stage than what I thought. Suppose that the attached diagram "Dispersion curve.doc" represents ground level concentration for one hour sampling (glc) for nitrogen oxides (NOx), under the least favorable weather conditions. And that legislation requires max glc = 200 μg/m3 (microgram/m3) for NOx (like here). If there are no other plants with gaseous emissions in the area, the 50 m stack (or even shorter) would be adequate, according to the diagram . If there are other plants, refer to Post No 4 of this thread, stack may end up with a height bigger than 50 m.
This for NOx. Same action is needed for rest effluents (SO2 and particulates), to define a stack height resulting in "acceptable" glcs for all effluents.
As said in Post No 4, a new plant is usually placed in an industrial area, already polluted by existing plants. The question is whether its additional contribution to pollution is acceptable, and this is judged by the authorities giving the permit. Usually authorities are contacted for information about pollution of broader area , then stack height / specific contribution to glc is decided, then environmental studies etc are officially submitted for permit.
Situation is much easier in case that the new plant replaces an old one (the old one usually pollutes more).
So, Narnia, contact of the authority giving the permit could be the step before final stack height.

[Narnia]

Would you please provide me the internalional recommended limits for ground level concentarion of NOx,SOx & particulate matters.
As per above i perceive that the total emissions from plant including this newly installed one should be less than the allowable values.

Thanks

[kkala]

So I understand dispersion model has to include the whole plant, including the new boiler (not only the boiler).
Ground level concentrations are not very easy to find, besides they should have same basis; e.g. one hour average, meaning (to my understanding) that sampling at ground lasts one hour. It is maximum (accepted) value, not average. The longer the (sampling) time, the lower the acceptable glc, since peaks are smoothed out.
http://www.eea.europ...air%20pollution seems to support following limits of glc (ground level concentration) for NO2:
200 μg/m3 (hourly average)
40 μg/m3 (annual average)
30 μg/m3 (annual average, rural area).
For dust: PM10 40 μg/m3 (annual average).
From such data you have to make your own and support your decision. I will notify in case of finding more. Any information on permissible glcs of the pollutants referred is also welcomed.

Edited by kkala, 04 January 2012 - 05:04 PM.

[ankur2061]

Narnia,

There are no international standards for air pollutant limits. If you notice in the previous post the link provided is for targeted air pollutant limits in the city of "Luxembourg" and not for an industrial environment such as an oil & gas installation or a thermal power plan. In the United States, air pollution is being monitored by US EPA (Environmental Protection /agency), however, each state has its own regulations and laws for air polllutants release to the atmosphere with some being more stringent and some being less. The US EPA website provides a lot of basic guidelines regarding limiting air pollutants to the atmosphere. Refer:

http://www.epa.gov/ttn/naaqs/

A couple of years ago I was working in Abu Dhabi and the national oil company, ADNOC had specified air pollutant limits in their installations as follows:


Substance Emission Limits (mg / Nm³)

Sulfur Dioxide (SO₂) 500

Carbon Monoxide (CO) 500

Nitrogen Oxides 350
(Expressed as NO₂)

Hydrogen Sulfide (H₂S) 5


Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 05 January 2012 - 12:52 AM.

[Narnia]

Thanks for above informations

kindly also provide the reference distance to measure this concentration.

at what distance away from stack this concentration should be.

thanks

[kkala]

1. The limits of ground level concentration (glc) for any pollutant are applicable to any distance from stack generally and for any weather condition. This is what we consider, running a dispersion model and looking into resulting glc on the broader area around. There is usually a maximum glc at some distance from emitting stack towards the wind direction. An affected area may have different glc for a pollutant, compared to another vicinity area (not remembering such a case).
2. Reported glcs for NO2 in post No 20 concern all areas of Luxembourg (not only the city), including an industrialized canton. These are an example to make requirements specific for the case, a rather patient task. For NOx glc=200 μg/m3 (1 hour average) is also applicable in Greece (post No 18) and probably all over EU.
3. Ground level concentrations (glcs) are of interest here (~1 hour average), not pollutant limits coming out of the stack (glcs are much lower in value).
4. I have not managed to locate glcs in EPA, specific references from it and anywhere else would help collect data to support a decision.

[ankur2061]

Narnia,

The air pollutant limits mentioned by ADNOC are at the top of the stack (point source of emission). Based on ground level concentration permissible limits for these air pollutants as per your company's or country's rules and regulations for chemical and industrial establishments and the stack height considered you can than calculate the ground level concentration at various distances (0/5/10/20/50 m radius from point source of emission) using a dispersion software such as "Shell Fred" or "PHAST".

Another approach would be to use the ground level concentration permissible limits for your company or country for chemical and industrial establishments for calculating the stack height using the maximum ambient air pollutant emission limits provided in your country or as per the ADNOC regulations that I have mentioned in my previous post (post # 21).

Regards,
Ankur.

[kkala]

Stack emissions have been fixed at post No 1, the question now is to specify glcs on the area around the plant (~1 hour average), so that stack height will be determined.
Distance from stack covered by dispersion model can be few kilometers, depending on where max glc (average of definite time) happens.
Task may be more complex, refer to posts No 4 and 18.