OCR Text |
Show 12 specific heat for combustion products and the process fluid, and could be used to predict heat performance with radiant or non-radiant burners. A commercial code, also zone model based, was used to model conventional flame burner performance for a typical cylindrical heater geometry. Commercial code results were used to "calibrate" the Alzeta model at selected operating conditions. This allowed for consistent results to be obtained between the two codes for the selected geometry when modeling conventional burner performance. Using the Alzeta model, comparisons were then made between vertical cylindrical reformer heaters with conventional and radiant burners. The modeling results discussed below assume a fixed rate of hydrogen production, with total fired duty varied to meet this requirement. A comparison of contributions by the different heat transfer mechanisms to total heat flux to the process tubes is presented as Figures 5 and 6 for the conventional flame burner and the radiant burner respectively. This information is broken down by zone. As expected, the radiant burner provides a flatter flux profile to the process tubes over the length of the heater. Total flux drops in the conventional burner case as one moves up the heater. In the radiant burner case, the flux is fairly constant in the bottom 4 zones then drops as the combustion products approach the top of the heater. A comparison of total flux per zone for the two cases is presented as Figure 7. The model predicted that the radiant burner would increase radiant section efficiency by 5.8 percent relative to the conventional burner. Modeling performed by KTI for a downfired cabin heater during the first year of the project predicted a 4 percent improvement in radiant section efficiency. A comparison of the average gas temperature per zone for the two different burners is presented as Figure 8. As expected, the radiant burner operates with significantly lower gas phase temperatures, with the associated benefit that less thermal NOx will be produced. By radiating combustion energy directly from the burner surface to the process tubes, there is less energy in the combustion products and they leave the burner surface at a lower temperature. |