Influence of disturbances in municipal waste incineration plants on catalytic converters

Forced by legislation, a series of catalytic converters have been installed in German municipal waste incineration (MWI) plants to reduce nitrogen oxides and/or polychlorinated dibenzodioxins/furans (PCDD/F). The catalytic material is usually based on TiO2 doped with V2O5 and WO3. During the past 2 years, problems arose especially in new installations in connection with disturbances in the combustion chamber. The emission of great amounts of carbon monoxide and other products of incomplete combustion caused fast temperature excursions in the catalytic converters. As a result, some of these installations were heavily damaged. For this reason the oxidation of pyrolytic gases was investigated with several types of catalysts in the laboratory. The gases detected during periods of low oxygen concentrations in an incinerator behave quite differently when they pass a catalytic converter. While hydrogen and some unsaturated hydrocarbons like benzene or ethyne are easily oxidised at the operating temperature of about 300°C, others like carbon monoxide or methane are rather inert at this temperature. Oxidation of the compounds first mentioned will produce enough energy to rise the temperature of the catalytic converter up to values where even CO and CH4 will react. In this case, temperatures will exceed 700°C, which can be concluded from the changes in the physical properties of the catalytic material. Under certain conditions, these changes may result in a complete destruction of the catalytic converter. The first measure to prevent such damage could be a bypass and additionally catalytic materials that withstand high temperatures better than catalysts based on TiO2 could be used.