(57) Burner apparatus, comprising a tubular coolant jacket assembly, a tubular combustion chamber unit disposed within said coolant jacket assembly, and ignition zone structure at one end of said combustion chamber unit for flowing an ignited fuel-oxidant mixture into said combustion chamber unit, characterized in that said combustion chamber unit includes a monolithic tube of refractory material having an inner surface that defines a combustion zone, a reinforcing sleeve surrounding and extending the length of said tube having its outer surface being spaced less than one millimeter from the inner surface of said coolant jacket assembly in standby condition, providing residence time sufficient to complete combustion of the fuel-oxidant mixture within said combustion chamber unit such that the stream of combustion products discharged from the end of said combustion chamber unit remote from said ignition zone structure is essentially particulate free.