FGD and DeNOx
NEWSLETTER       

                                                                                                                    February  2002
                                                                                                                          No. 286

Specifics of the SCR at Cayuga 1 

Anthony Favale, William Gretta, and W. Scott Hinton of Foster Wheeler and John Cooper of AES Cayuga report that the installation of SCR at Cayuga 1 has been successful in several aspects. First and foremost, the reduction of NOx has exceeded 90 percent. This fact has allowed the operators to improve overall heat rate and reduce boiler maintenance through the detuning of the low NOx burners. The overall arrangement and the economizer bypass have functioned well allowing the unit and the SCR to operate through a wide load range. The operator’s initial apprehension towards the SCR soon dissipated with the quickness and ease of operation and the added bonus of improved boiler performance. The ammonia in ash was measured at approximately 18 ppm allowing the sale of the ash without concern. 

Cayuga 1, which fires 2.4 percent sulfur Eastern Bituminous coal, is a 160-MW boiler with a wet scrubber and corner-fired low NOx burners. The SCR catalyst was supplied by Haldor Topsoe and guaranteed for a life of 24,000 hours. Ammonia slip was set at a maximum of 2 ppm. Pressure drop was limited to less then 1.75 in. w.c.  

Historically, this unit had produced a bluish plume, presumably due to fine sulfate particulate being released at or near the water dew point. The installation of the SCR did not appear to adversely affect this plume or the operation of the scrubber. This finding was confirmed because a sister unit of nearly identical design, but without SCR, provided a comparison. 

The guaranteed SO2 conversion rate for Cayuga is a maximum of one percent at the guaranteed conditions. Since SO2 conversion is dependent on catalyst volume but not strongly affected by time/deactivation, additions of catalyst over the facility life will result in increased SO2 conversion. 

Because of the need for an economizer bypass system, temperature stratifications at the catalyst inlet would exist at reduced loads since the temperature differences between the economizer exit gas and bypass gas are large. Because of this concern, a Sulzer static mixing system was employed. The mixer consists of two “stages;" the first stage is used to help mix the gas side-to-side, the second stage is used to mix the gas top-to-bottom. The ammonia injection grid is located directly upstream of the first mixer and is designed to direct the ammonia/air mixture directly into the “cells” of the mixer. The ammonia distribution is thus benefited by the turbulent mixing that takes place immediately downstream. 

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