| OCR Text |
Show the combustion chamber while significantly reducing the temperature of the waste gases. The excess enthalpy combustion method was originally proposed by Weinberg el al. in 1971 [ij. The authors used this concept to increase the adiabatic flame temperature in the main combustion chamber and also the heat transfer. The flame temperature is affected by both the heating value of fuel and the temperature of air entering the combustion chamber. A correlation between the air temperature and heating value of the fuel is shown in Fig. 2.2. The combustion regime can, therefore, be extended to lower heating value fuels using the excess enthalpy combustion. 2.2 Uniformization of Thermal Field Figure 2.3 shows the heat 11 ux profiles obtained on a combustion test facility employing a burner assembly having variable high temperature air. The results show that the profile is not flat along the furnace axis. The maximum heat flux, qmax, is 1.2 times the averaged heat 11 ux, qav. The results also reveal that the minimum heat flux, qmin, is 0.6 times the qav under the condition of 200 ° C inlet air. Thus, the heat flux profile needs to be improved considerably throughout the furnace in order to increase the base level of the heat transfer rate per unit area. It is important to note that when the air temperature is increased to 1000° C, the position of maximum heat flux shifts to an upstream position, and its level becomes 2.2 times the qav . If the maximum heat flux is provided to boiler tubes in steady state, the burn-out of the boiler tubes may occur. However, if the heat flux distribution can be rearranged to form a uniform thermal field in the boiler, then homogeneous heat flux pattern results. One of the methods for redistributing the maximum heat flux is via periodic alternate firing at the head end of the boiler. For a two burner system the resulting heat flux pattern will change as shown in Fig. 2.4. As can be observed the average heat flux profile will be more homogeneous with peak value significantly lower than that for the fixed firing case. Furthermore, it is important to note that the average heat flux will be 1.7 times higher than that using 200 ° C air. 2.5 3 E 1-5 03 o s o N E u c Z 0.5 -o 1000*CAir -• 200X:Air 0 0.5 1 1.5 2 2.5 3 3.5 Normalized Length of Furnace, x / D Fig. 2.3 Heat Flux Distribution along the Furnace Axis 2.5 E 1-5 n X -a o #N E c 1 0.5 Actual Heat Flux rrom 1st Burner 3 ^<zl /" ' ' Expected Heat Flux rrom 2nd Burner ^ #._ 7*""' /^"^"^^^C ~ Expected time ^ averaged Heat Flux at 1000 *C Air _ _ Averaged Heat Flux (1000 Cair) Averaged Heat Flux (200 Cain 1 1 i 0 0.5 1 1.5 2 2.5 3 3.5 Normalized Length of Furnace, x / D Fig. 2.4 Expected Heat Flux Profile with Alternate Firing 4 11-8 |
