OCR Text |
Show Data for a firing rate of 50 MM Btu/hr for various proportion of #2 fuel oil and exothermic waste indicated combustion efficiencies less than those required to achieve a DRE of 99.99%. The firing of varing amounts of the endothermic waste and #2 fuel oil at a combined rate of 50 MM Btu/hr achieved a lower DRE than the exothermic waste in combination with the #2 fuel oil. Also, steam production suffered because of the loss of the endothermic waste's latent heat. Both the lower DRE and steam production when firing endothermic waste were expected. Evaluation of the boiler combustion process required that we systematically relate it to our knowledge of the incinerator combustion process. The DRE for any liquid or gaseous ccmbustible is a function of the resulting combustion product's residence time, temperature and the ability of the burner to cause a homogeneous fuel/air and combustion products/air mixture. The boiler burners being reviewed were not manufactured by the John Zink Company, but were similar in construction and assumed to be in performance to a John Zink AVR type burner. The first step of the analysis consisted of otetermiiiing the fired boiler's combustion products residence time and temperature. Figure 2 is a plot of gas temperature versus time with the parameter volumetric heat release. The shaded area shown is bounded by curves representing volumetric heat releases of 80,000 and 100,000 Btu/hr-ft3. The lower value is typically used when firing a heavy oil like #6 and the higher value is typically used when firing a light oil like #2. Natural gas fired boilers are designed with volumetric heat releases exceeding 100,000 Btu/hr-ft3. The boiler under consideration had a calculated volumetric heat release of 81,600 Btu/hr-ft3 at the design firing rate. The average gas temperature was calculated to be approximately 2650°F and the residence time approximately 0.5 second. The actual gas residence time based on burner performance was somewhat less and very difficult to determine. Thus, it was concluded that a macroscopic (practical) approach in lieu of a microscopic approach would be the level used to compare boiler combustion performance to the performance of a thermal oxidizer fitted with an AVR type burner. This same level of approach suggested the use of average gas temperature in lieu of incremental gas temperatures with corresponding reaction rates that are described by logarithmic relationships. 5.8.6 |