Thermal decomposition (pyrolysis) of urea in an open reaction vessel

A study was done of the thermal decomposition of urea under open reaction vessel conditions by thermogravimetric analysis (TGA), high performance liquid chromatography (HPLC), Fourier transform-infrared (FT-IR), and an ammonium ion-selective electrode (ISE). Both evolved gases and urea residue were analyzed, and profiles of substances present versus temperature are given. Major reaction intermediates are also identified. Plausible reaction schemes based on product distribution in relation to temperature are proposed. Our data indicate that at temperatures in excess of 190 °C, cyanuric acid (CYA), ammelide and ammeline are produced primarily from biuret. Biuret itself is a result of prior reaction of cyanic acid, HNCO, with intact urea. Cyanic acid is primarily a result of urea decomposition at temperatures in excess of 152 °C. CYA and ammelide first appear at approximately 175 °C, but the reaction rate is very slow. At temperatures in excess of 193 °C, alternate reactions involving the decomposition of biuret substantially increases reaction rates. Several parallel processes compete for the production of products. Production of CYA, ammeline and ammelide appears complete at 250 °C, after which sublimation and eventual decomposition of products occurs.