CFD Modeling Guides Kiln Retrofit Success

Thomas E. deHaan, Coen Company, Inc.

Coen Company, Inc. recently completed the retrofit of a lime recovery kiln with a custom-designed multiple-fuel burner. Highly specialized kiln burners are required for this application in order to produce the desired flame shape, as irregular heating patterns can lead to poor product quality, inefficient performance, or even more serious problems like damage to the kiln’s refractory lining. Moreover, lime recovery kilns such as this are frequently used to incinerate waste gases that are evolved from pulp processing. Consequently, each kiln burner installation requires some insight into the proper position of the burner itself, as well as any waste gas burners, in order to maximize product quality and reduce refractory wear.

CFD analysis using FLUENT/UNS was an integral part of the overall retrofit design, guiding the position and orientation of both burners within the kiln. Initial simulations helped to determine the axial position of the kiln burner, so that it provided adequate heating of the lime product without the flame impinging on the kiln surface and potentially damaging the refractory lining. Subsequent CFD analysis involve creating three dimensional simulations where buoyancy effects could be taken into account to optimize the location and orientation of the waste gas gun with respect to the kiln burner position. Figure 1 illustrates what can happen if the burners are not correctly installed. The final improved configuration is shown in Figure 2.

Incorporating CFD analysis into the retrofit allowed Coen to meet the project challenges in a more timely and cost-effective manner than could be obtained through traditional methods. Although physical models using water or air provide designers with a limited tool, they cannot accurately predict heat transfer or flame shape characteristics within the kiln, both of which were critical to the success of the retrofit project.

Reports from the retrofit site have been encouraging. The client experienced a trouble-free startup, with the burners providing all the reliability and flexibility required. The lime conversion rate has increased from approximately 80 to 90 percent and the retrofit has produced improved lime quality (color and texture).

Figure 1, Non Optimum Burner Placement

The waste gas does not sufficiently penetrate into the kiln, and consequently a significant amount of combustion takes place at the top of the kiln immediately behind the dam. This would most certainly cause rapid refractory wear.

Figure 2, Improved Burner Placement

The waste gas burns above the main kiln burner flame as a cooler, separate flame. At no point is the refractory exposed to excessively high temperature, and the lime bed at the bottom of the kiln is exposed to fairly consistent temperatures over the length of the main flame.

“Fluent’s CFD software was extremely useful in developing a design that worked as expected. This helped us to minimize the amount of post-installation adjustments, saving thousands of dollars on the project.” – Ken Camino, Coen Company