Kinetic and Mass Transfer Effects of Fly Ash Deposition on the Performance of SCR Monoliths: A Study in Microslab Reactor

• Published in: Industrial & engineering chemistry research 2021-05, Vol.60 (18), p.6742-6752
• Main Authors: Lanza, Aldo, Usberti, Nicola, Forzatti, Pio, Beretta, Alessandra
• Format: Article
• Language: English
• Subjects: Kinetics, Catalysis, and Reaction Engineering
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/acs.iecr.0c06060

Selective Catalytic Reduction (SCR) reactors with ternary vanadium-based catalysts are the state-of-the-art technology for the multipollutant abatement in cofired biomass and coal power plants. During time of stream, the catalyst performance declines due to the deposition of fly ashes and inorganics from the flue gas. Understanding the phenomena behind the deactivation is a key factor to identify optimal strategies for catalyst replacement or rejuvenation. In this work, samples of spent commercial monoliths, unloaded from a full scale SCR reactor after 1500 h (Sample A = start of run) and 35 000 h (Sample B = end of run), were examined and compared with the fresh monolith. SEM-EDX measurements across the walls of the aged monoliths showed radial concentration profiles of inorganic elements and, in the case of Sample B, also the growth of a layer of ashes. NH3-SCR tests over the powdered catalysts gave nonconclusive evidence of deactivation, likely due to the averaging effect of grinding; instead, by testing slabs cut from the monoliths, the integrity of the honeycomb wall was preserved, and effects of aging were observed. The design of the microslab reactor is optimized to capture the process from the chemical regime (below 200 °C) to the intraporous-mass transfer controlled regime (>250 °C); in fact, the impact of gas–solid mass transfer is negligible, due to a high Sherwood number (close to 5) and a small hydraulic diameter (3 mm). The evidence of a full external diffusion regime over the long-term aged catalyst was thus easily associated with the presence of the dense layer of ash deposit that behaved as a diffusion barrier; a modeling analysis was performed and allowed to quantify the mass transfer resistance of the deposit and estimate that the intrinsic activity of the two aged samples was comparable. Indeed, by scratching the surface layer of the microslab from Sample B, a remarkable recovery of the DeNO x performance was obtained, matching the activity of the slab from Sample A. The deposition of fly ashes over the honeycomb walls was thus recognized as the major cause of performance loss, via a gas-phase mass transfer resistance.