{HEAD ~ The heat exchanger burner} can be fueled by natural gas, methane, {TAIL ~ propane} and the like 
fueled by
{ HEAD ~ The heat exchanger burner } can be fueled by natural gas , methane , { TAIL ~ propane } and the like
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH