An ammonia gas generator for producing ammonia from a solution of an ammonia precursor substance, comprising a catalyst unit that comprises a catalyst for the decomposition and/or hydrolysis of ammonia precursor substances into ammonia and a mixing chamber provided upstream of the catalyst; an injection device for injecting the solution of the ammonia precursor substance into the mixing chamber; at least one inlet for the carrier gas; and an outlet for the formed ammonia gas, said ammonia gas generator also comprising a perforated disc.