4 replies to this topic

[rogered]

Hello everybody

My question relates to using fired heaters to perform incineration tasks:

My current project has a large, 30 MW fired heater burning a lean fuel gas (3000 kg/hr). I have 2 offgas streams that I want to burn along with the fuel gas.

1. Sour water stripper gas, which is basically wet fuel gas containing 2% H2S (800 kg/hr)
2. tank vent gas, mainly Nitrogen containing < 10ppm mercaptans (300 kg/hr)

Fired heater vendors tell me that there's no problem with these additional gas streams - they burn well and the noxious components, ie. the H2S and the mercaptans, will be completely destroyed.

Has anyone any positive or negative experience of something similar, and even more importantly, is there any information in the public domain which covers burning of mercaptans & H2S?

Many thanks

Rogered

[gvdlans]

H2S and mercaptans (e.g. methyl mercaptan, ethyl mercaptan) are highly flammable gases and liquids (do a Google search on "H2S MSDS" or "mercaptan MSDS" and you will find this). When ignited they will react with oxygen forming combustion gases SO2 (which is a toxic, corrosive gas), H2O and CO2. This is all basic, secondary school chemistry. They will not be "destroyed" as you write.

[rogered]

Che Jedi

No I apreciate that they'll burn readily and turn to SOx , but again they have a low autoignition temperature (less than the 750-850°C in the firebox) which is good, but a lower flammability limit of 2.8% (methyl mercaptan) which is bad from my point of view.

So what do you think?

[gvdlans]

Note that LFL is temperature dependent. Higher temperature results in lower LFL (and higher UFL). Above the Auto Ignition Temperature (AIT), LFL and UFL are not really relevant.

Furthermore, you are dealing with a fuel that is a mixture of pure compounds (H2S, mercaptans, and the lean fuel gas (that is in itself a mixture e.g. of hydrogen and methane)). Now you are dealing with the LFL and UFL of the mixture and not of the pure compounds. LFL and UFL of a mixture can be estimated using the Le Chatelier Equation.

Tabulated values for LFL and UFL are normally for room temperature, so the 2.8% you give for methyl mercaptan is only valid at a temperature of say 20 deg C.

Don't worry, your H2S and mercaptans will easily burn inside your furnace.

Edited by gvdlans, 19 February 2010 - 02:31 AM.

[djack77494]

rogered
I concur that the subject waste streams will likely be completely oxidized as discussed. The negatives I see are:
1) You're using a process heater as an incinerator which would not be permissible in many locations, and
2) You generate and apparently release in an uncontrolled manner the sulfur oxide pollutants.
You seem to have a handle on the combustion process, but not the aforementioned difficulties.