(643b) Characteristics of Urea Thermal Decomposition for the Reduction of Nitrogen Oxides in Diesel Engines

All marine diesel engine manufacturers are currently encountered with the global issue of reducing the nitrogen oxides (NOx) emissions. Selective catalytic reduction (SCR) of NOx technologies is considered as one of the leading candidates for meeting IMO regulations. It is one of the worldwide-implemented commercial technologies for NOx removal from the flue gases of stationary sources such as industrial boiler, gas turbine and municipal waste incinerator. The SCR method reduces NOx to nitrogen and water by using ammonia. The NOx reduction takes place in a catalytic reactor which operates in the temperature range of about 300 - 500 degree of celsius. Recently, urea has been adopted to SCR systems as a reducing agent for the reduction of nitrogen oxides. Urea is a safe reducing agent for the de-NOx system, where urea decomposes into ammonia vapor through endothermic reactions. The partially decomposed intermediates during the operation, however, can make unwanted plugging onto the catalytic reactor that causes the catalyst deactivation with the decrease of the removal efficiency of nitrogen oxides. More attention should be given to the characteristics of urea decomposition for the optimum design of the urea injection unit. In this study, thermoanalytical techniques were used to investigate individual kinetics involved in urea pyrolysis. From the kinetic study, it was found that thermal decomposition of urea consisted of complex reactions. One reaction where urea directly decomposed to ammonia gas proceeded parallel to the reaction through cyanuric acid (CA) intermediates depending upon the rates of heating. To minimize CA-like byproducts, urea sollution should be atomized to fine mist and injected into the exhaust gas at high temperatures above 350 degree of celsius. The injection of urea solution was also essential to keep enough residence time for complete decomposition of urea in reaction.