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Show firing rates the radiation heat flux appears to be considerably higher (about 30%) for fine-ground coal (Trial 2) than for coarser coal (Trial 3). For fine-ground coal the radiation heat flux shows some reduction with firing rate decrease (see Trials 1 and 2), which is consistent with the lower temperatures. Heat Absorption by Load and Furnace Thermal Efficiencies In Trials 1, 2, and 3, the heat absorbed by the water-cooled tubes on the sidewalls (load) was measured by measuring flows and temperature differences of the cooling water in each tube. Table 5 lists the total heat absorbed by the tubes and the furnace thermal efficiencies for Trials 1,2, and 3. The efficiencies were calculated by dividing the heat absorbed by tubes by the total heat input (firing rate). As expected, furnace thermal efficiency is reduced as the firing rate is Increased. The results also indicate an increase in overall heat absorbed by the load for fine-ground coal combustion. This is in spite of lower combustion temperatures in the furnace for fine-ground coal, due to reduced firing rate. It should be mentioned that the order of testing was not as listed in Table 5. (Trial 1 was not performed first.) Therefore, ash deposition on the tubes did not cause the reduction in heat absorption to the tubes for Trials 2 and 3. The fraction of the total heat input absorbed by each tube is plotted against the distance of the tubes from the burner wall in Figure 25. These heat absorption profiles show some reduction in heat absorbed by tubes versus distance of the tubes from the burner wall for fine-ground coal, while for the coarsest coal heat absorbed by the load remains almost even along the furnace length. Slag and Ash Deposition The rate of deposition of slag and ash was tested in Trials 6 through 10 using the slag and ash deposition probe in the flue with the operating conditions listed in Table 6. The furnace operating conditions were very similar to the conditions in Trials 2 through 5 except for the flue gas temperature, which was increased (if necessary) to near 1300°C (2370°F) by decreasing the amount of water-cooled surface in the furnace. 34 |