Your specification is slightly inadequate. Methane is heavier than air and will accumulate at the base of the ullage. During subsequent filling the methane will be circulated by the displacing coal stream. This circulating displaced residual methane can be eventually drawn through the filter bags and exhausted along with the exigent air. This equates to dilution in your specification intent. If you are concerned about build up of the extracted methane...Congratulations....you're almost a pioneer. You could dilute the mixture further after the filtration process. Few people do and you would need explosion protection throughout. Often enough the methane is not completely exhausted and subsequent explosions are relieved by a bursting panel in the silo shell. This phenomenon is not confined to coal dust. Most carbonaceous products and some other fin powders will also blow their top sometimes.

So to 1. It gets sucked out with the displaced air. and for 2. Calculate the gas flow with a methane content below the weak extinction level. Extinction levels are velocity dependant so I suppose iteration, or is it extrapolation, is called for. Perhaps some generous soul hereabouts could provide a graph

Controlled methane ignition in mines is quite spectacular to watch. While I was being dragged along out of the workings by the Deputy I could see the flare radiating from the Davy safety lamp and was quite thankful that the poor little canaries had been retired long ago. ■

Methane is lighter than air ! will go to the top of the silo. Density of air is 1.25 kg/m3 and Methane is 0.7 kg/m3

Just use a fan assisted filter to draw methane out of the top space.

You can also use positive flow of inert gas in the silo it you are really worried about dust explosion. ■

Clearly my world was running upside down!! ■

Don't worry about it. It happens to all of us. ■