“Catalyst Selection for NOx needs to factor in Mercury and SO3 - Hot Topic Hour Thursday, November 29, 2012
The Hot Topic Hour yesterday focused on NOx control but also necessarily included extensive analysis of the impacts on mercury removal and SO2 oxidation. Selection of an SCR catalyst must take into account the impact of all three pollutants.
Randy Sadler, Director of Marketing & Sales CoaLogix/SCR-Tech, presented catalyst selection options for coal-fired applications. He also discussed options for keeping the SCR performing well after the catalyst has been selected. It is possible to select catalysts of different pitches for sections of the bed. The side that tends to plug can be equipped with a more open catalyst. He also discussed the use of air cannons or catalyst sweepers which clear the top side of the bed of material which may slough off the ductwork. Randy confirmed the correlation between less costly fuels and plugging problems.
There is now a market for regenerating gas turbine catalysts. Many units have been in service for seven or 10 years and have lost their activity.
W. Scott Hinton, Ph.D., P.E., Principal Consultant W.S. Hinton & Associates, discussed the beneficial effects of SCR on mercury speciation as an integral part of many utilities' mercury control strategies. Catalyst selection and management can have a substantial effect on the achievable level of mercury oxidation and the resulting mercury capture. Scott explained that there are ways to maximize mercury oxidation at minimum cost, while maintaining DeNOx and SO2 conversion performance. For example, an advanced catalyst can be installed just on the bottom layer. Without the large quantity of ammonia present it will be very effective in oxidizing mercury.
Kyle Neidig, SCR Catalyst Product Manager at Hitachi Power Systems America, discussed the advanced catalyst referenced by Scott. New catalysts are being developed by Hitachi that not only enhance the performance of the SCR, but will also help to minimize the cost impact of the new MATS emission limits. As MATS imposes new stringent mercury emissions limits for coal-fired generators, utilities must adapt by either adding additional AQCS equipment or by modifying the operation of their generating units to improve the performance of their existing AQCS equipment in order to achieve mercury co-benefits and MATS compliance. Many utilities will be asking their SCR and catalyst to do more than ever before to help meet these new challenges. With an average catalyst layer remaining in operation for eight years, the decisions that utilities make on catalyst today will impact emissions reduction performance well into MATS compliance. Selecting a catalyst design that optimizes mercury oxidation now can provide significant savings on MATS compliance. Actual experience with the new catalyst has been impressive. Just replacing one layer of conventional catalyst with TRAC® can double the mercury removal without increasing SO2 oxidation.
Balaji Krishnakumar, Ph.D., a Senior Computational Analyst at Niksa Energy Associates LLC, presented data which will be a good guide on management of catalyst to obtain the desired results. SCR catalysts are deactivated by several mineral and metallic trace elements at highly variable rates determined by fuel quality and furnace firing conditions. Since no operating SCR was designed for Hg oxidation and since different monoliths have been combined as layers in particular units, the Hg oxidation performance of any particular SCR fleet is largely unmanaged. The analysis directly related a measurement or manufacturers’ forecast on the deterioration in NOx reduction with age to corresponding estimates for oxidation of Hg on full-scale SCRs. Balaji showed that performance can even differ in four units at the same plant with the same catalyst. The reason is that catalyst layers are replaced at different times and that performance is very dependent on the length of operation.
The entire November 29 recording can be heard at:
BIOS, ABSTRACTS AND PHOTOS can be seen at:
BIOS, ABSTRACTS, PHOTOS - November 29, 2012.htm
The individual presentations are as follows: