From the Laptop to the Smokestack: How Computational Fluid Dynamics Boost Environmental Performance

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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 the prototype results matched the 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 that 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.

It is important to understand the effects of fly ash as a modern power station is likely to be generating between one and two million ton per year of fly ash. (Picture: depositphotos.com / Serz72)

GSG Application at La Cygne Unit 1

Kansas City Power & Light Company (KCP&L) is an electric utility serving customers in Kansas and Missouri in the US. 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 per cent Powder River Basin (PRB) and 10 per cent local Missouri.

Owing to 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.

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