The concentrated black liquor is taken to be combusted in a recovery boiler, into which {HEAD ~ combustion air} is fed at several levels for {TAIL ~ the combustion process} 
The concentrated black liquor is taken to be combusted in a recovery boiler , into which { HEAD ~ combustion air } is fed at several levels for { TAIL ~ the combustion process }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL