Selective Catalytic Reduction of NOx: Deactivation and Regeneration Studies and Kinetic Modelling of Deactivation
Abstract: Selective catalytic reduction of NOx (SCR) has been recognised as an effective post-combustion method for reducing nitrogen oxides from stationary sources. The cost of replacing the catalyst is a major part of the maintenance of the SCR system, hence it is desirable to advance methods to prolong the catalyst life. The objectives of this thesis were to investigate the extent and the mechanism of deactivation of commercial SCR catalysts used in different applications, to establish a method for prediction of the deactivation process, and to advance efficient methods for rejuvenation of the deactivated catalysts. To characterise the catalyst, the following techniques have been applied: XPS, TPD, FTIR, Raman, XRD, SEM, BET, NH3 adsorption, ICP-AES, and AAS. Potassium and sodium were found to be two of the worst poisons that had significant effects on the activity of the SCR catalyst due to their selective chemisorption on the Brønsted acid sites. Lead deactivated the catalyst by non-selective chemisorption on the surface of the catalyst, while zinc and calcium deactivated the catalyst by capillary condensation and pore blocking, respectively. Potassium was regarded as the main poisoning substance for some catalysts aged or used in different industrial bio fuel plants, while significant amounts of potassium, sodium, zinc, lead, and calcium were found on the surface of the catalyst used in a municipal waste incinerator. The catalyst deactivation in the bio fuel CFB boiler was about 3-4 times faster than expected for a conventional design for a coal fired boiler. Using a conventional honeycomb catalyst 80 percent of the original activity remained in a CFB boiler but only 20 percent remained in a PC boiler. Experiences of burning of forest residues and pulverised wood in the CFB and the PC boilers revealed that the deactivation tendency is not a function of the total alkali content of the bio fuel and only a particular form of potassium deactivates the catalyst. A two dimensional model that describes poison accumulation and SCR performance of a monolith catalyst has been developed. The model, which has no adjustable parameters, accounts for the simultaneous effects of internal and external diffusion of poison and SCR reactants as well as chemical kinetics. The model showed that it is possible to control the extent of deactivation by modification of the catalyst design parameters. Regeneration of deactivated catalysts, particularly in the case of bio fuel, is a possible technique for limiting the cost of SCR plants. Washing with sulphuric acid and water in combination with sulphation with sulphur dioxide were regarded as two effective methods for restoration of the catalytic activity. Sulphation of highly deactivated catalyst without washing enhanced the catalytic activity of the catalyst due to build-up of new sulphate groups on the catalyst surface. Sulphation after the removal of potassium by water treatment provided chemically more stable sulphate groups. FTIR and TPD studies indicated that potassium not only decreased the amount of ammonia bound to the Bronsted acid sites, but also retarded the redox potential of the surface vanadia species during the sulphation procedure. Hence, it is important to wash the strongly deactivated catalyst before sulphation by sulphur dioxide.
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