Abstract: A boiler includes a high intensity combustor mounted in a shell that defines the boiler housing. A coil bank is mounted in the boiler and spaced from the combustor. A fire pot is positioned in the combustor and defines a combustion chamber with a combustion throat at one end and a choke at a second end between the combustion chamber and the coil bank. The choke is formed of refractory material and includes uniformly distributed steel fibers that conduct heat across the choke from the hot side adjacent the combustion chamber to the cooler side adjacent the coil bank. The steel fibers substantially reduce the thermal gradient in the choke and improve boiler efficiency.