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