| OCR Text |
Show calculation domain. Flue gas is recirculated to the furnace from both the bottom opening of it and from the openings positioned below the lowest burner stage (FGRP). For this model prediction, the total mass flow rate of air is about 3300 t/h and that of coal is about 350 t/h. Mass flow rate of recirculation gas from the bottom opening is about 300 t/h and its gas from the openings positioned below the lowest burner stage is about 200 t/h. The characteristics of the pulverized coal are shown in table 2. The higher heating value of it is 6470 kcal/kg. The finite-difference grid (41 X 18 X 44) which we have employed is shown in Fig. 2. Each burner and each NO port is expressed by four control volumes on the furnace wall. The predicted oxygen value of 3.3 % at the furnace exit agrees well with measured value of 3.5%. However, we found that the present model gives us a little bit higher gas temperature and NO emission at this moment. The high gas temperature gives us a smaller predicted UBC value of 2.6 % than the measured value of 5.0 % at the furnace exit. Figures 3 to 7 show the predicted velocity vectors, flue gas concentrations, oxygen concentrations, gas temperature distributions, and NO concentrations. In each figure, it is consisted of three sections of the furnace. From the top of the furnace, the section which has the lowest burner is shown. From the front wall of the furnace, the section which distance from the front wall is 2.6 m is shown. From the right side wall, almost the center of the furnace section is shown. From the velocity vectors (Fig. 3) and the flue gas concentrations (Fig. 4) it is clear that the flue gas from the bottom opening of the furnace attacks the flames which are formed by the front wall's lowest burners. This attack adversely influences the combustion of the burners as it can be seen typically in the oxygen consumption (Fig.5) and temperature distribution (Fig.6) of the front wall's lowest burner flames. However, the formation of NO of the attacked burners is suppressed compared with the formation by the rear wall's burners (Fig.7). These results agree well with the field experiences that the front wall's lowest burners have a tendency of forming an unstable flame in some cases and the mixing of the flue gas into the combustion air decreases the formation of NOx. One of the characteristics of this furnace is employing the Hitachi-NR Burner which reduces NOx in the flame with applying strong swirl to the combustion air. The swirl direction of neighboring burners are set in opposition so that the swirl flow is not disturbed by the flow of adjacent burners. The swirling flow can be seen in the velocity vector which is seen from the front wall. These settings of swirl direction induces upward flow and downward flow between each burner alternately. This upward and downward flow causes the periodic pattern of the flue gas concentration, oxygen concentration and NO concentration which can be seen clearly in the concentrations viewed from the top of the furnace. 7 |
