Methane oxygen reaction 1

[chemEIT]

hi,
I am in a process of justifying purging operation for one of our anaerobic digester. What are the chances of methane to react with oxygen to produce co2 and water?? one of my collegue thinks that methane being produced will react with oxygen to product carbon dioxide and water, so the oxygen may be consumed before enough biogas is present to form an explosive mixture inside the digester during startup/shutdown. Can this reaction occur without reaching the minimum activation energy???

 

[unclesyd]

Based on the trials and tribulations of the local sewage treatment plant there is still plenty of Methane available to blow the system apart, twice, even after a "purge".

To combine the two you will have to have the minimum activation energy available unless you have a catalyst where it will take less energy.

 

[moltenmetal]

ChemEIT- you don't get significant anaerobic digestion until you have significantly depleted the system of oxygen. But it's the organisms who are consuming that oxygen to metabolize the stuff in the digestor- it's not the methane reacting with it!

Methane and oxygen will be happy to co-exist, even at concentrations corresponding to an explosive mixture, for a very long time without any significant quantity of methane being converted to CO2. Though the reaction is thermodynamically spontaneous, it is kinetically limited due to its activation energy. You need not only to reach the activation energy to initiate the reaction, you also need enough of each reagent present to keep the reaction going once it's been initiated or else it will self-extinguish. There's a range of mixtures, both lean and rich in fuel, which satisfy this condition and constitute "explosive mixtures".

Now that you understand the chemistry involved, perhaps you can understand whether or not the need to purge the digestor. We can't assess that for you without knowing the physical set-up so we can understand the risk of initiation and the range of concentrations that might exist in there etc.

 

[dcasto]

Its the same process and kenetics in a catalytic converter in a car. There is a patented process from Optimized Process Design in Houston, Texas called a DeOx Plant. The O2/Methane are heated to about 800 deg F and then pased across about 4 catalyst beds to react them to C)2 and H2). The reaction is exothermic and the hot gases are used to preheat the inlet gas. It almost become self supporting reaction and the start-up process heater does not consume much fuel once the cycle is running.

There are a few units in operation and at least one is at a biogas/land fill site.

 

[chemEIT]

thanks a lot for all your inputs