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