Mechanisms and prevention of skull formation in high temperature gas extraction system

In high volume extraction systems adhesions and agglomerations of particles are detrimental to the gas flow throughput. Particle-wall adhesions can lead to severe skull formation, i.e. agglomeration of deposits, especially at high operation temperatures, which have to be manually removed at high effort and costs. Therefore, the conditions of increased build-up phases as well as conditions for spontaneous removal through crack formation and chipping are of major interest. To understand relevant mechanisms the operational conditions in an industrial extraction system were thoroughly documented including analyses of the elemental gas phase composition. Additionally, the chemical compositions of agglomerated adhesion samples were investigated by means of XRD and ICP-OES as well as their inner structure by means of SEM. In a second step mechanisms leading to these build-ups were simulated on lab-scale. Thereto, original wall surface specimens were covered in adhesion powder of defined particle size. In experiments at different high temperatures between 800 °C and 1200 °C, gas compositions and with varying content levels of CaCO₃ in the powders a layered structure similar to the real system could be obtained. Additionally, crack formation at certain defined conditions and under different atmospheres was observed, which is of major interest regarding the chipping behavior of the adhesion material from wall surfaces in application. Through laboratory experiments certain operating conditions within the calcination cycle could be disclosed for which the autonomous discharge of large particle-wall agglomerations is widely enhanced. Thus, a general process optimization to increase the performance of the extraction system was proposed.