Propane for Combustion in Flare

[dtmstr]

I am trying to figure out what method I would use to calculate the following...

How much propane do I need to maintain a minimum temperature in a flare that is bringing in combustion air through louvers to treat landfill gas containing methane? There are also losses from the temperature and wind outside.

I have all the volumetric flow rates and the BTU/ft3 values.

Any suggestions would be appreciated.

Thank you,

 

[EmmanuelTop]

What do you mean by the "minimum temperature of the flare"? In addition, a sketch would also help.

Dejan IVANOVIC
Process Engineer, MSChE

 

[dtmstr]

@EmmanuelTop

We need to burn enough propane to maintain the temperature in the flare. When it is very cold or windy outside, our current system is unable to keep up and the temperature drops too low.

 

[EmmanuelTop]

For calculating flare isotherms at various flows/compositions and different weather conditions, I can suggest using proprietary software packages such as Flaretot

http://www.flaretot.com/

and Flaresim

http://www.softbits.co.uk/fs_overview.php

Flaretot comes as a fully functional 30-day trial if I remember well.

If you want results that are any near to reality, doing hand calculations will not get you there as you have a lot of environmental factors (such as those noticed by yourself) so go for the software that has been a landmark in the industry so far. I think Flaresim has a very decent rental price and it is available for 1 month or 3 month lease.

Dejan IVANOVIC
Process Engineer, MSChE

 

[dtmstr]

Thank you for that software suggestion.

I still wonder what method you would use to figure this out. I had thought you would determine the energy entering and leaving the system and adjust so that a certain temperature is maintained. Is that the right idea or there another method?

 

[EmmanuelTop]

Basically you are correct - it is all about energy emitted from a known mass of fuel and then transferred to the surrounding air. However there is a lot of environmental (wind, temperature, pressure, stability class, background radiation etc.) and process factors (exit velocities, mass flow, composition etc.) and it is kind of impossible - at least based on my experience - to define a simple hand calculation model for flare isotherms.

See attached presentation and you can get a feeling of how many variables are in the play:

http://www.ghtech.co.kr/brochure/flaresim.pdf

As mentioned, Flaretot used to come as a fully functional trial for 30 days so perhaps you can get a copy and do the calculations for all scenarios you need, and then decide to purchase the software for defining operating/control envelope for various cases/gases etc.

Dejan IVANOVIC
Process Engineer, MSChE

 

[StoneCold]

dtmstr
Emanualtop is probably right that you need a simulator to get a good representation of the flare.

But you stated that you are pretty close right now, just having problems on cold and windy days.

I am going to assume that you have an enclosed flare?
If so then you probably have a forced convection fan and some natural draft dampers
The stack should be insulated on the inside with rockwool insulation, that should keep your radiation down to a minimal amount.
This also keeps the stack metal temperature below 400F so the paint does not burn off.

So to figure out how much additional heat you need you need to do a rough heat balance around the stack. Use the current stack control temperature as the desired temperature. From the fan curve and motor amps you can get a rough estimate of the forced air coming in. You said you know the gas volume coming in and the heating value of the gas. The only thing left is the natural convection of cold air in the bottom. You can get that from an anemometer on the damper when it is running. So then Heat Out = (Forced air + natural convection air)at Control temperature. Which is equal to Heat of combustion of your biogas.

To get your propane requirement, increase your outlet temperature to the new desired temperature and the additional heat that is required will be the propane requirement.

The propane heat should only be about 25% or your current heat load.
Then I would calculate out the size of control valve I need for the propane assuming the valve is only 40% open.
That should give you some lead way in case you undersized the valve.

So your risk is really only that you could need to resize the valve if it is not working correctly when you get it all installed.

That is what I would do. It is rough but remember you don't need the exact value, you need to be close enough for the control system to function and that is all. Under heating is pretty easy to correct. Overheating is a bigger problem.

You also might want to consider installing an L shaped fence about 15ft away from the flare stack dampers to block the wind from blowing into the stack. This can be a problem in very windy locations.

Regards
StoneCold

 

[StoneCold]

dtmstr
One other thing you should look at is choking back the forced air to the burner, Don't let the unit smoke but you might just be getting too much air and that is your heat loss problem.

Regards
StoneCold

 

[chicopee]

If it were dry methane, then it could be done with some ease, however, methane from landfills is wet which is undoubtedly one source of your problem compounded by the wind and cold temperatures. So, you have to figure out how much moisture is entrained in the methane and other gases such as hydrogen sulfide and then follow the above responders suggestions. I personally would determine the amount of propane by empirical means.