Prior to {HEAD ~ the introduction} of the hydrogen, {TAIL ~ the coal bed} is depleted of nitrogen and methane by withdrawing a nitrogen methane mixture 
Prior to { HEAD ~ the introduction } of the hydrogen , { TAIL ~ the coal bed } is depleted of nitrogen and methane by withdrawing a nitrogen methane mixture
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH