Materials for thermohydrolysis of urea in a fluidized bed

The production of ammonia from urea was investigated in a fluidized bed reactor in order to avoid the formation of N2O in the selective non-catalytic reduction (SNCR) process, which occurs when urea is directly injected into an SNCR reactor.Among about 20 tested materials, γ-Al2O3 proved to be the best suited as a fluidized bed material due to its high catalytic activity as well as relatively good stability. Even better catalytic results were obtained with titanium oxide, which was, however, much less attrition resistant than the aluminum oxides. Silicon dioxide and silicates showed only a weak catalytic effect. The tested molecular sieves and zeolites showed catalytic results between those of silicates and alumina. The results of the catalytic tests are in accordance with a two-step mechanism: urea rapidly decomposes to ammonia and isocyanic acid initiated by simple heat transfer from the fluidized material, followed by the rate-determining catalytic hydrolysis of isocyanic acid.The screening showed that a compromise had to be found for the catalytically active fluidized bed material, since good catalytic properties require high surface areas, which is in conflict with the desired high abrasion resistance of the material. Scale-up experiments in the pilot plant with γ-Al2O3 “CAT 250” as the best material among the screening experiments demonstrated that it is possible and straightforward to thermohydrolyze 10.6 kg/h of urea to 6 kg/h ammonia. Gas analyses of the product gas showed that urea was almost completely converted and that only minuscule amounts of isocyanic acid were formed.