Selective catalytic reduction performance in steam-methane reformer service: The chromium problem

Selective catalytic reduction (SCR) performance has been seen to deteriorate with catalyst age at a greater than expected rate in steam-methane reformers (SMRs) and ethylene plants. Loss of catalyst activity has been attributed to deposition of chromium oxide species from alloy furnace tubes and coils on the catalyst surface, thereby masking active catalyst sites. This deposit is visible to the naked eye as a brown or iridescent black discoloration along the passages of a flowthrough honeycomb catalyst, darkest at the entrance face, and gradually becoming lighter as one proceeds downstream. The color becomes more intense with increased exposure to flue gas. It is important for proper design and catalyst-replacement strategy to understand the governing phenomena at a fundamental level, especially at a time when SCR is increasing V being turned to as best available control technology (BACT), to comply with the more stringent nitrogen oxides (NOx) regulations. This paper describes a correlation of previously unpublished experimental data from catalyst test sections (logs) exposed in the field over a period of several years at multiple SMR plant locations. Consistent with the observed color change, chromium pickup increases with time and varies with operating conditions from plant to plant. Any given log, at any given time, shows an exponential, or semi-logarithmic, decrease in chromium concentration over its length. Activity decay of the catalyst has been correlated with the chromium concentration. This single correlation holds true for all of the different plants investigated and can be used to predict remaining catalyst life and optimize future catalyst designs. Scanning electron microscopy (SEM) results and surface analyses from the SCR catalyst and SMR reformer tubes are also discussed, along with suggested countermeasures to mitigate the effects of chromium oxide deposition. (C) 2004 American Institute of Chemical Engineers.