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 said fuel. In particular it relates to a fuel injection system for a catalytic burner (310, 313). It comprises preheating means (308', 309, 315; FT) for preheating the catalytic burner (310, 313) and primary fuel supply means (307) for supplying fuel to the catalytic burner (310, 313). The preheating means (308', 309, 315; FT) and the primary fuel supply means (307) are provided in separate compartments (310, 312), said compartments being connected with each other via a channel (CH).