The present invention relates to a system for utilizing liquid fuels, such as diesel and gasoline, in a catalytic reactor for producing clean heat from the fuel. In particular it relates to a fuel injection system for a catalytic burner (310, 313). It includes preheating elements (308′, 309, 315; FT) for preheating the catalytic burner (310, 313) and primary fuel supply elements (307) for supplying fuel to the catalytic burner (310, 313). The preheating elements (308′, 309, 315; FT) and the primary fuel supply elements (307) are provided in separate compartments (310, 312), the compartments being connected with each other via a channel (CH).