How to Improve Air Pollution–Control with CFD- Modeling

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Siemens Industry Software GmbH Digital Factory Division

In 2008, Flow Tack was acquired by the APC technology company, Fuel Tech, which also had been developing Selective Non-Catalytic Reduction technology (SNCR) using CFD. A major reason for Flow Tack’s acquisition was their significant work on applying CFD to develop new methods in mixing and optimization of APC equipment. The GSG patent was approved in 2012 and has been successfully applied to several operating power plants and industrial facilities with SCRs starting in 2009 (see Table).

How GSG Improves SCR Performance

One of the major issues with SCR units is the frequently reported particulate and ash accumulation on the horizontal face of the catalyst. The reactive solution is to stop the unit and vacuum the ash periodically, which, accompanied by the associated downtime, is a financial burden. In previous designs, an array of large turning vanes has been used to control the velocity distribution into the face of the first catalyst layer.

By modifying the angle, position, and quantity of turning vanes, engineers and designers were able to improve the velocity distribution entering the catalyst. Also, in order to control the direction of the flow to the catalyst layer, a straightening grid was typically installed immediately above the first catalyst layer. However, high fidelity CFD results highlighted for the first time the formation of flue gas recirculation and low velocity zones behind the SCR turning vanes, a potential root cause of fly ash drop-out onto the catalyst layers below.

Another drawback of using the traditional turning vane design was its sensitivity to upstream flow conditions and, as such, its requirement for exact spacing and angling during SCR construction to ensure that prototype results matched model results. Any changes to the system upstream warranted a model revisit and potential modifications of the vanes to maintain the required distributions.

Fuel Tech, instead, performed CFD optimization which resulted in the combination of the traditional SCR hood turning vane array with a straightening grid into the single GSG device. The GSG consists of parallel plates installed in the SCR hoods on the diagonal to turn the fly ash and flue gas vertically into the first catalyst layer. This device has less sensitivity to upstream flow distribution, which means the catalyst and catalyst performance can be protected even if the unit is not running at optimum design conditions.

Kansas City Power & Light Company (KCP&L) is an electric utility serving customers in Kansas and Missouri. KCP&L’s La Cygne Unit 1 is an 815-MW Babcock & Wilcox cyclone boiler with overfire air and SCR. Unit 1 burns a blend of 90% Powder River Basin (PRB) and 10% local Missouri. Because of poor fluid dynamic design, ash was accumulating within the SCR reactor at a typical rate of 1,450,000 pounds per year, resulting in high ash removal costs, high catalyst replacement costs, high catalyst pressure drop and fan operational costs, and high NH₃ reagent costs. In addition, La Cygne suffered high NH₃ slip (unused reagent that “slips” through the catalyst unreacted) as the catalyst activity decreased. This increase in slip caused operational issues downstream of the reactor including pressure loss increases over the air preheater and more frequent APH washings. In 2012, the ash accumulation rate increased to twice the typical amount as the power plant operated for an extended time with a low demand factor. Although many minor changes were made to the system to improve the performance over the course of the previous years, none of them were successful as they did not address the root cause of the problems.

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