NOx removal and SCR DeNOx
Topsoe has developed and refined the DNX® series of catalysts for SCR DeNOx since the 1980’s. The DNX® catalysts are a series of corrugated monolithic catalysts based on a ceramic titania matrix impregnated with vanadium and tungsten oxides as the active components for the SCR DeNOx reaction.
The unique features such as low SO2 oxidation, high poison resistance and excellent mechanical properties have been optimised through continual research and development. The development of catalysts and processes for NOx removal is founded on market-focused research combining fundamental understanding with a comprehensive knowledge of emission-control needs and opportunities.
Design basis
The SCR DeNOx design tools are constantly improved based on laboratory measurements, pilot-scale testing and full-scale measurements. Fresh-catalyst activities are typically determined by laboratory and pilot-scale experiments whereas constant refinement of deactivation models origins from comprehensive testing of client catalysts installed in boilers, engines and turbines fuelled by a wide range of fuels.
Product development
The research has resulted in introduction of new catalyst formulations to effectively cover the entire SCR operating temperature range 160-600°C (320-1110°F) with highly active an durable catalysts. The research has led to the development of catalysts for SCR units on biomass-fired boilers and to a catalytic ceramic filter, Cerafil® TopKat , which combines dust removal and NOx reduction. Cerafil® TopKat is currently used within waste incineration, the glass furnace industry and in biomass-fired boilers. The hybrid filter/catalyst technology improves the flexibility of the SCR technology owing to among others the option of injection of sorbents for removal of acid gases. Cerafil® TopKat is marketed and supplied by ClearEdge™.
Topsoe has also improved the DeNOx process technology. Topsoe has developed a patented gas-mixing system, the STARMIXER®, for efficient mixing of ammonia into the flue gas, a critical parameter for the performance of the entire SCR DeNOx system. The STARMIXER® is an optimised vortex-mixer type featuring an elaborate shape with protrusions extending from circular plates which enhance the creation of vortices in the flowing gas. This provides a close-to-perfect mixing with a minimum pressure loss.
Fundamental studies
IR spectroscopy studies enabled Topsoe to reveal the detailed mechanism behind the catalytic DeNOx reaction. It was demonstrated how vanadium in the working catalyst participates in two interconnected reduction–reoxidation and ammonia-activation cycles. This fundamental understanding of the catalyst’s working principle is the cornerstone in understanding the temperature behaviour of the catalyst and the deactivation mechanisms occurring by exposure to poisonous components in flue gases.
The chemical and physical mechanisms involved in catalyst poisoning as a result of exposure to various flue gas components that include arsenic, alkali metals, phosphorous, calcium and chromium have been investigated in detail by state-of-the-art characterisation techniques, e.g. scanning- and transmission electron microscopy, chemical analysis, pore volume distribution and surface area measurements. The catalyst characterisation studies have been used to develop predictive models for catalyst deactivation e.g. in SCR’s on biomass-fired boilers or on ethylene heaters in refineries.
Recently, extensive laboratory studies combined with pilot testing has provided Topsoe with a predictive tool for mercury oxidation across SCR catalysts. This is of vital importance as mercury emissions become regulated.
The understanding of the very basis of the SCR process enables Topsoe to provide the optimum solution to our clients.