A thermo-hydrolysis reactor (1) for producing ammonia-containing gas by heating an aqueous solution of urea is described. The reactor (1) comprises an elongate vessel (2) having a middle tubular section, an enlarged lower section (4) having an inlet (5) for the urea solution, and an enlarged upper section (3) having an outlet (6) therein for the ammonia-containing gas. The reactor (1) is adapted such that, in use, heat transmitted through the walls of the reactor (1) from an external heat source heats the urea solution causing it to hydrolyse producing the ammonia-containing gas.