OCR Text |
Show Flame stabilization in the porous matrix is also indicated by gas temperatures measured in the flue gas and the porous bed, as shown in Figure 4. As the excess air increases, the gas temperature in the porous bed measured below the first row of tubes steadily decreases because of the decrease in flame temperature as well as the movement of the flame. However, the flue-gas temperature increases because of the upward movement of the flame, resulting in a steady decrease of the overall heat-transfer rate, as shown in Figure 3. A significant effect of excess air on combustion emissions was demonstrated. Figure 5 shows the major pollutant emissions from the combustion process - CO, NOx, and THC - versus excess air at a constant firing rate operation. NOx emissions decreased with an increase in excess air because of a decrease in combustion temperature, as expected. As low as 15 vppm of NOx emissions was demonstrated over a practical operating range. CO emissions below 20 vppm were also achieved in this typical testing. At higher excess air, however, CO increases dramatically with excess air because not only is the combustion temperature lower, but the flame also moves up closer to the first row of tubes. This results in an overcooling of the combustion gases, within which CO is (normally) continuously burnt out. Even so, ultra-low emissions of unburnt total hydrocarbons (THC) were achieved (less than 3 vppm). Therefore, almost 100% combustion efficiency was achieved. There was no flame quenching found in the test combustor-heater. Therefore, combustion has to be stabilized below the first row of tubes to achieve high heat-transfer rate and low combustion emissions. The range of operating parameters for stable combustion in the porous matrix depends upon configurations of the combustor-heater and bed material as well. Effect of Firing Rate (Turndown Operation) Testing was conducted for each configuration at firing rates from 12.5 to 56 kW for evaluating turndown performance of the combustor-heater. A further reduction of firing rate may cause flame quenching on the grate because the flame is so close to the grate surface. A further increase in firing rate above 56 kW was restricted because the flame was gradually moving up, resulting in reduction of the overall heat extraction. Figure 6 shows that the overall heat-transfer rate increased with firing rate until the maximum heat-transfer rate was reached. For the present configuration, the maximum overall heat-transfer rate is approximately 220 kW/m2 at a 50-kW firing rate when the flame starts to move up. Heat transfer from combustion gases to the tubes is achieved by the combined heat radiation and heat convection. Heat radiation is mostly dependent upon temperature of the solid particles and much less dependent on firing rate, whereas heat convection is basically dependent on firing rate approximately to the 0.8 exponential power. Therefore, the overall thermal efficiency, which is defined as a ratio of total heat transmission to the total heat input (firing rate), decreases with increasing firing rate. For the present operating conditions, the thermal efficiency of the test unit consisting of two rows of tubes decreased from 70% to 35% over a turndown ratio from 4.5 to 1. The effect of firing rate on combustion emissions at 30% excess air is shown in Figure 7. With an increase in firing rate, CO emissions Significantly decreased to as low as 15 vppm because combustion intenSity in the porous matrix is enhanced at a higher firing rate. At very low firing intensity, however, CO emissions decreased, as demonstrated in some tests. This is a result of the interaction of combustion and heat transfer. Gas temperatures measured in the bed indicated the flame moved upstream at the lower firing rate; that resulted in longer residence times for combustion completion. 7 |