A reactor system for converting aqueous urea into vapour ammonia comprising: (a) an enclosure having a gas flow inlet to receive a first gas stream, a gas flow outlet to output a third gas stream, and one or more enclosure walls that define a first interior space disposed between said gas flow inlet and said gas flow outlet; (b) a reactor having a major axis and disposed within said enclosure, wherein said reactor comprises: (i) a housing having: one or more reactor wall arranged in a triangular cross-section at right angles to the major axis that define a second interior space and having a first outer surface exposed to said first interior space, a first opening in said housing to allow fluid communication from the second interior space to the first interior space, wherein said first opening is in said first outer surface, has a cross-sectional area parallel to the reactor major axis that is less than 35% of said first outer surface; and (ii) an aqueous urea inlet in fluid communication with said second interior space, wherein, when the reactor system is in use, a gas stream flows through the first interior space and the first outer surface of the triangular cross-section reactor wall creates a pressure differential zone between said second interior space and said first interior space and wherein the first opening in the first outer surface is disposed next to the pressure differential zone. (a) an enclosure having a gas flow inlet to receive a first gas stream, a gas flow outlet to output a third gas stream, and one or more enclosure walls that define a first interior space disposed between said gas flow inlet and said gas flow outlet; (b) a reactor having a major axis and disposed within said enclosure, wherein said reactor comprises: (i) a housing having: one or more reactor wall arranged in a triangular cross-section at right angles to the major axis that define a second interior space and having a first outer surface exposed to said first interior space, a first opening in said housing to allow fluid communication from the second interior space to the first interior space, wherein said first opening is in said first outer surface, has a cross-sectional area parallel to the reactor major axis that is less than 35% of said first outer surface; and (ii) an aqueous urea inlet in fluid communication with said second interior space, wherein, when the reactor system is in use, a gas stream flows through the first interior space and the first outer surface of the triangular cross-section reactor wall creates a pressure differential zone between said second interior space and said first interior space and wherein the first opening in the first outer surface is disposed next to the pressure differential zone. (i) a housing having: one or more reactor wall arranged in a triangular cross-section at right angles to the major axis that define a second interior space and having a first outer surface exposed to said first interior space, a first opening in said housing to allow fluid communication from the second interior space to the first interior space, wherein said first opening is in said first outer surface, has a cross-sectional area parallel to the reactor major axis that is less than 35% of said first outer surface; and (ii) an aqueous urea inlet in fluid communication with said second interior space, wherein, when the reactor system is in use, a gas stream flows through the first interior space and the first outer surface of the triangular cross-section reactor wall creates a pressure differential zone between said second interior space and said first interior space and wherein the first opening in the first outer surface is disposed next to the pressure differential zone. one or more reactor wall arranged in a triangular cross-section at right angles to the major axis that define a second interior space and having a first outer surface exposed to said first interior space, a first opening in said housing to allow fluid communication from the second interior space to the first interior space, wherein said first opening is in said first outer surface, has a cross-sectional area parallel to the reactor major axis that is less than 35% of said first outer surface; and (ii) an aqueous urea inlet in fluid communication with said second interior space, wherein, when the reactor system is in use, a gas stream flows through the first interior space and the first outer surface of the triangular cross-section reactor wall creates a pressure differential zone between said second interior space and said first interior space and wherein the first opening in the first outer surface is disposed next to the pressure differential zone.