Abstract
Recently, Urea-SCR system has been used for reducing NOx emission from diesel engine. In the Urea-SCR system, aqueous urea is injected into the exhaust gas flow and it evaporates and decomposed to NH_3. It is necessary to clarify thermal decomposition behavior of urea in high temperature atmosphere to improve the DeNOx performance of Urea-SCR system. In this study, thermal decomposition behavior of urea in heating processes was investigated experimentally. Solid urea or 32.5% urea aqueous solution was decomposed in a temperature controlled vessel with a constant heating rate. Mass reduction of the sample with decomposition was measured with a balance. As for the thermal decomposition behavior of solid urea, it was shown that the remained mass in higher heating rate was higher than that in lower heating rate at the same temperature in the vessel. From the temperature measurement of the sample, it was found that the thermal decomposition which was endothermic reaction occurred rapidly in the higher heating rate, and therefore the increase of sample temperature became slower than the heating rate. This delay of sample heating resulted in the higher remained mass at the same temperature in the vessel. In this study, moreover, effect of zeolite on the decomposition behavior was examined. Some kinds of zeolite are used as a catalyst in Urea-SCR system. In the thermal decompositions of urea with zeolite, several kinds of zeolite were tested. The zeolite powder was mixed with the solid urea or urea aqueous solution. As the result, it was found that some kinds of zeolite could effectively promote thermal decomposition of urea.
