Abstract: Improvements in a fuel burner for a boiler comprising a first assembly (S.sub.1) for delivering into the boiler (3) a fuel stream, a second assembly (S.sub.2) for delivering into the boiler a stream of secondary air, a precombustion chamber (6) extended with a certain length (L.sub.2) into the combustion chamber (2) in the boiler (3) and a nozzle flameholder (14), opposite the outlet of the fuel stream, which flameholder supplies fluid jets (F) directed against said fluid stream in order to provide a circulation zone in said precombustion chamber (6); a third assembly (S.sub.3) is provided for delivering into the boiler a tertiary air stream which laps the inner walls of the precombustion chamber (6) and optionally there is provided a fourth assembly (S.sub.5) for delivering a quaternary air stream from outlets (23) of the precombustion chamber (6).