Kemin bakom framtidens avgasrening
En studie av ureasönderfall under kvävgasatmosfär
The purpose of this work was to provide a better understanding of urea?s decomposition and byproduct formation in an SCR system on heavy trucks. In my experimental setup with TGA-DSC-FTIR (a combination of two thermal analysis methods and a method for gas phase detection), an FTIR method for urea in the gas phase was developed for the qualitative and quantitative determination of urea and its decomposition products. Chemicals such as urea, biuret, cyanuric acid and melamine of p.a. quality were used in this method development. Beforehand, there was no FTIR method available to detect these substances; hence, the aim of this work was to develop an FTIR method to understand the degradation chain of urea. The combination of TGA and DSC was used for analysis of different samples, where urea, biuret, cyanuric acid and melamine in varying amounts have been weighted in for various experiments in order to study the temperature at which a phase transition or reaction occurs, i.e. the temperature at which substances begin to melt, vaporize, decompose and react. In combination with FTIR, information was obtained for the appearance of substances at various temperatures. With FTIR, we have been able to develop unique infrared spectra of substances and along with weight loss in TGA the calibration of different substances has been achieved. These calibrations have been combined together to develop an FTIR method, which has been used for detection of the substances in the ongoing study of the reaction pathways.In this study we also investigated the degradation chain of urea in the presence of metals. Austenitic and ferritic silencer materials with different surface roughness were analyzed to study whether the metals have a catalytic function or effect on the byproduct formation. Those experiments have shown that a higher amount of urea was decomposed in contact with metal surface, i.e. a larger amount of NH3 and HNCO was formed. Biuret studies in the presence of metals appeared to give a higher formation of urea over the rougher surfaces (a larger amount of biuret was decomposed over the rougher surfaces), while experiments with cyanuric acid revealed a higher HNCO formation over ferrite than over austenite, i.e. a larger amount of cyanuric acid was decomposed.By the chosen method, used in FTIR in combination with TGA-DSC, the following important reactions have been demonstrated: Biuret decomposed to urea and HNCO; Urea decomposed into HNCO and NH3; formation of cyanuric acid from the decompositions of urea and biuret and finally decomposition of cyanuric acid into HNCO at a higher temperature.