Creating a Flammable Mixture

[Ninja1980]

Hi

A post from the UK.

I’ve been asked to calculate the amount of Fuel Gas that would nee to be added to an acid gas stream to create a flammable mixture. I have a full Hysys simulation to hand so physical properties are readily available.

Any suggestions??

 

[gr2vessels]

Acid gas been laden with fuel gas?
Dangerous association...

 

[Ninja1980]

The “acid gas” stream mostly consists of the reject streams from Glycol and Amine units. This will be sent to a dedicated Acid Gas Flare.

I’m interested in making a very rough estimate of the amount of fuel gas need to be blended into the stream without having to go to flare tip vendors.

 

[mbt22]

As a first pass, le Chatalier's rule can be used to estimate the LFL of a mixture.

1/LFL_mix = y_1/LFL_1 + y_2/LFL_2 ...

y's are mole fraction, LFL the lower flammability limit. Same equation applies for UFL.

As an aside, is flaring the acid gas BAT? I don't know the answer, but am curious.

Matt

 

[dcasto]

In the US, we have rules that state the acid gas must be at least 300 BTU/ft if going to a flare.

If you assume the acid gas has 0 BTU's and natural gas has 1000 BTU's, then 1/3 natural gas, 2/3 acid gas.

 

[jwysmooth]

When you flare a gas, you want it outside the flammable range until it gets to the flare tip, i.e. above the UFL or below the MOCC. Generally a flare tip does not have any flash back protection, until you get to a liquid seal near the base of the flare stack. In your case (from what I understand of it) you want to use the fuel gas to take the concentration of the mixture above the UFL. Otherwise the pilot will ignite the mixture at the flare tip and flash back through the flare stack and possibly into the flare header. Ideally, the waste gas mixes with the air at the flare tip, the air brings the waste gas back into the flammable range and it burns outside the flare tip to prevent damage to the flaring system.

Without knowing what the composition is of the acid gas or which fuel gas you are using, I can't tell you what concentration to enrich the acid gas too. However normally depending on how you plan to control the fuel gas rate, you would take the concentration to at least 25-50% above the UFL of the mixture before you get anywhere near the flare tip.

I hope that helps.

 

[Ninja1980]

Thanks for all your answers. I'll run a few calc's later today

 

[MJCronin]

dcasto,

The 300 BTU/cu.ft that you mention seems to agree well with the practical experience of landfill gas flaring and power generation.

In many US landfills, when the gas vented from the landfill is flared or sent to an internal combustion engine, the BTU content must be about 300 BTU/Cu.ft or higher.

Typical landfill gas consituents are methane, CO2, N2, O2 and traces of others

-MJC