A system for mixing a liquid spray into a gaseous flow, the system comprising: - a main body defining an interior, the main body interior extending from a first end to a second end, the main body defining a circumferential wall extending between the first end and the second end, the circumferential wall or the first end defining an inlet opening, the main body also defining an outlet; - an inlet arrangement disposed at the inlet opening, the inlet arrangement defining an inlet channel leading to the interior of the main body; - a reaction zone disposed within the interior of the main body between the inlet opening and the outlet, the reaction zone being spaced from the first end to define a mixing region within the main body interior; - a restrictor arrangement disposed within the interior of the main body between the first end and the reaction zone, the restrictor arrangement separating the mixing region into a first chamber and a second chamber, the restrictor arrangement defining a restricted passageway between the first and second chambers; wherein the first chamber comprises: - a dosing arrangement configured to receive an injector to spray reactant into the gaseous flow so that the reactant mixes with the gaseous flow in the first chamber; - a first swirl promoting means arranged between the inlet opening and the dosing arrangement; - one or more sheet metal surfaces (191, 192) arranged so as to be substantially surrounded by a gaseous flow entering through said inlet opening, wherein the dosing arrangement is configured so that an injection axis of any injector mounted to the dosing arrangement intersects said one or more sheet metal surfaces (191, 192); and - a second swirl promoting means arranged between the dosing arrangement and the restrictor arrangement, such that a gaseous flow passing through said second swirl promoting means is swirled around before entering the restricted passageway and passing to the second chamber; characterised in that said restrictor arrangement comprises a transverse plate provided with one or more openings, and in that the first swirl promoting means and the second swirl promoting means are arranged to promote swirling in a first angular direction and a second angular direction respectively, said first angular direction and said second angular direction being mutually opposed. - a main body defining an interior, the main body interior extending from a first end to a second end, the main body defining a circumferential wall extending between the first end and the second end, the circumferential wall or the first end defining an inlet opening, the main body also defining an outlet; - an inlet arrangement disposed at the inlet opening, the inlet arrangement defining an inlet channel leading to the interior of the main body; - a reaction zone disposed within the interior of the main body between the inlet opening and the outlet, the reaction zone being spaced from the first end to define a mixing region within the main body interior; - a restrictor arrangement disposed within the interior of the main body between the first end and the reaction zone, the restrictor arrangement separating the mixing region into a first chamber and a second chamber, the restrictor arrangement defining a restricted passageway between the first and second chambers; - a dosing arrangement configured to receive an injector to spray reactant into the gaseous flow so that the reactant mixes with the gaseous flow in the first chamber; - a first swirl promoting means arranged between the inlet opening and the dosing arrangement; - one or more sheet metal surfaces (191, 192) arranged so as to be substantially surrounded by a gaseous flow entering through said inlet opening, wherein the dosing arrangement is configured so that an injection axis of any injector mounted to the dosing arrangement intersects said one or more sheet metal surfaces (191, 192); and - a second swirl promoting means arranged between the dosing arrangement and the restrictor arrangement, such that a gaseous flow passing through said second swirl promoting means is swirled around before entering the restricted passageway and passing to the second chamber;