A computational fluid dynamics study is providing insights into how
burner placement affects the performance of thick-wall tunnel kilns.
Figure 1. Long-nose burner, flush-mounted entrainment.
Everyone agrees that optimized burner placement in thick-wall tunnel kilns can offer substantial performance benefits, including improved flue gas entrainment ratios, enhanced temperature uniformity, reduced fuel consumption and lower nitrogen oxide (NOx
) emissions. However, exactly what constitutes "optimized burner placement" has been the subject of considerable debate over the years. While some plants prefer to mount burners flush with the hot face, others recess them in a tunnel or use venturi blocks to increase burner entrainment ratios and promote temperature uniformity and heat penetration into the hack.
At the request of one major U.S. brick manufacturer, Hauck Manufacturing Co. recently performed a computational fluid dynamics (CFD) analysis of two different high-velocity burners at various burner insertion depths in a thick-wall, underdeck-fired tunnel kiln. The results of this study are providing valuable insights into how burner placement can affect overall kiln performance.
Figure 2. Standard burner, 6-in. tunnel entrainment.
The investigation compared the relative entrainment ratios (defined as the mass flow of entrained furnace gases divided by the mass flow of gases exiting the burner nozzle) and temperature uniformities of two high-velocity burners at different placements: a long-nose burner with the discharge nozzle flush-mounted to the furnace wall hot face; and a conventional square block tile with the discharge nozzle recessed inside the furnace wall at 6, 12 and 19 in. With the exception of the burner tunnels, the furnace geometry was fixed for all cases. The distance from the hot face to the load was 8 in., and the burner was centered below the setting deck.
Figure 3. Standard burner, 19-in. tunnel entrainment.
The model was developed using Fluent® CFD software and was based on the burners having nominally the same average discharge velocities at 10% excess air so that true comparisons of the entrainment ratios could be made. In addition, all models included a furnace zone setpoint temperature of 1840°F, as well as the products of combustion, or a cross flow, from previous furnace zones. Further, the model was a snapshot in time of a steady-state condition and did not account for the well-known positive effects of pulse firing.
Figure 4. Long-nose burner, flush-mounted load temperature profile.
Figures 1-3 show the fluid particle path lines that indicate the entrainment flow for the flush-mounted, 6-in. and 19-in. recessed cases, respectively. As the fluid particle path lines indicate, substantial entrainment into the burner tunnel occurred even with the burner recessed the full 19 in.
Since the hack distance relative to the furnace hot face was fixed at approximately 8 in., entrainment ratio comparisons were made at a fixed distance of 4 in., or halfway between the hot face and hack. For comparison purposes, the entrainment ratio for each burner placement was further normalized by dividing by the flush-mounted long-nose burner entrainment ratio at 4 in. The normalized entrainment ratios were 1.0 for the standard or flush-mounted case, and 1.93, 1.77 and 1.47 for the 6-, 12-, and 19-in. recessed cases, respectively. The entrainment ratio was maximized at 1.93 with the burner recessed 6 in. in a tunnel that was equal in width and height to the size of the burner block. In fact, even the 12- and 19-in. recessed cases resulted in far more entrainment-77% and 47%, respectively-than the flush-mounted burner.
Figure 5. Standard burner, 19-in. recessed tunnel load temperature profile.
However, the effective temperature distribution in the furnace and load must also be considered. The resulting temperature distribution on the bottom surface of the load for the flush-mounted case is shown in Figure 4. The flush-mounted burner installation resulted in the hottest load bottom surface temperature of 1810°F substantially to the right of the center, with a cool spot near the burner of 1690°F for a load temperature delta of ±60°F.
Figure 5 shows the extreme or 19-in. tunnel length recessed load temperature profile, with the peak temperature near the tunnel discharge end equal to 1910°F and a minimum hack surface temperature of 1630°F farthest from the burner for an overall temperature delta of ±140°F.
Figure 6. Standard burner, 6-in. recessed tunnel load temperature profile.
Figure 6 shows the 6-in. recessed burner, which has a load peak temperature of 1820°F in the center of the hack and a minimum temperature of 1750°F near both ends of the hack, resulting in the best overall temperature delta of ±35°F.
Figure 7 shows a side temperature profile of the 6-in. recessed burner. A desirable heat release pattern and overall flame size for the furnace geometry in question are evident. In addition, a 14% reduction in NOx emissions was predicted with the 6-in. recessed burner vs. the flush-mounted burner due to the increased entrainment ratios and associated reduced peak flame temperatures.
Figure 7. Standard burner, 6-in. recessed tunnel temperature profile.
In this study, the 6-in. recessed burner provided peak entrainment ratios near the hack, as well as the most uniform hack surface temperature distribution. These results can be broadly interpreted to mean that a 6-in. recessed burner is ideal in all thick-wall tunnel kilns; however, caution must be exercised when applying results from one kiln analysis to other kilns. For a wider kiln than the one modeled here, a flush-mounted burner might provide better temperature uniformity than a recessed burner. Other factors that can affect performance include the tunnel dimensions (especially with oversized tunnels) and the hack or load distance in both the horizontal and vertical planes from the burner nozzle.
To ensure completely optimized burner placement-including optimized flue gas entrainment ratios, enhanced temperature uniformity, reduced fuel consumption and lower NOx emissions-manufacturers should carefully evaluate all kiln parameters and obtain advice from an experienced kiln/burner professional.
For more information about burners, contact Hauck Manufacturing Co., 100 North Harris St., Cleona, PA 17042; (717) 272-3051; fax (717) 272-2435; e-mail: firstname.lastname@example.org; or visit http://hauckburner.com.