In the case of exhaust air cleaning, various ingredients, in the channels of the ceramic honeycomb bodies that are used as thermal regenerators, for instance in exhaust air cleaning systems—so-called RTO (regenerative thermal oxidizer) systems—lead to adhesions, vitrification, corrosion and abrasion on the channel walls and to clogging of the channels of these ceramic honeycomb bodies. Such effects can severely impair the operation and function of such exhaust air cleaning systems. Besides the major expenditure of time and the high costs for cleaning or replacing the honeycomb bodies, a severe impairment of the exhaust air source, namely, of the affected production, occurs as well.
The aforementioned unwanted effects of exhaust air ingredients (dust, ash, alkalis, acids, corrosive reaction products, products of decomposition, etc.) are observed primarily in the uppermost temperature zones of regenerator beds. Such effects as clogging can occur when there is already a high burden of dust on the inlet side or in the cold area of the regenerator. Depending on the temperature of decomposition of the exhaust air ingredients, however, deposits can occur in the middle areas of the system as well.
Among the most unpleasant exhaust gas ingredients are organosilicates or in general silicon compounds, which can cause recurrent clogging of ceramic honeycomb bodies in a stack of such elements. Such strongly adhering encrustations (see FIG. 1) can be removed only with great difficulty by cleaning, and the damage can often be eliminated only by replacing the affected layer. A further cause of damage is the reaction of corrosive ingredients with the ceramic material itself (see FIG. 2), which can lead to the loss of mechanical stability and hence the loss of function of the ceramic honeycomb bodies.
Typical materials of ceramic honeycomb bodies for use as heat reservoirs in regenerative applications are alumina porcelain, cordierite and mullite ceramic. These materials are distinguished by different properties, such as heat storage capacity, density, thermal shock resistance, porosity, chemical resistance, roughness, etc.
For lessening the aforementioned unwanted effects, certain combinations of these properties are advantageous, but they cannot be combined in one ceramic material, or if so only with difficulty. For example, high thermal shock resistance is hardly achievable in conjunction with high chemical resistance.
To lessen deposits on the channel walls or to make it easier to remove these deposits or to eliminate clogs in the channels, dense or smooth surfaces are advantageous. All the aforementioned materials have a more or less great tendency to deposits and can be only more or less poorly cleaned. The available materials therefore often permit a compromise for the intended use. For instance, good chemical resistance is associated with poor thermal resistance, while materials with good thermal resistance usually have greater roughness, and so forth.