OCR Text |
Show condition of low burner load. The conventional type burner was effective to reduce NOx emission at the burner exit. However it was difficult to have a stable ignition under 30% burner load because of the lean coal concentration. Type A burner had the flow velocities of 15 mls in the inner and outer paths whereas the velocities in type B were different, the inner was 16 mls and the outer was 12 m/s. The total mass flow rate of air were the same in type A and B. We assumed the flame (or burning) zone as the low concentration region of oxygen near burner outlet, shown by the hatched area in the figure. At the burner load equal to 40 %, both burners gained a stable ignition. In this case, the flame zones are narrow and long, which means that the rate of coal combustion is slow. On the other hand, when the burner load is 20%, burner B maintained the flame due to the condensation of the coal concentration, whereas burner A failed to have a flame. It was also observed that the flame at 12 mls in burner B was not big enough to sustain the stable ignition. Slower velocities than 12 mls would be preferable judging from the theoretical consideration made in the preceding section. Fig.9 shows the combustion efficiency in relation to the burner load. In this test, we determined the minimum allowable value of the unburnt fuel to be less than 4 % and therefore the combustion efficiency must be over 96 %. For coal of FR=2.3, this restriction condition made the lower limit of the burner load in the conventional burner to be 50 %, whereas we gained a stable ignition at 28 % load with type A burner, and 20 % with type B. When using coal of FR=l.l, the lower limit of type B was down to 10 %. Consequently, the use of the type B burner expands the range of the burner load by more than 30 %. 2.Combustion Test by using Large Scale Test Facility Fig .10 shows the cross section of the large scale test facility. The furnace was vertical, whose shape is 3.6 m wide and 19.1 m high. The 3.6 tlh burner was located at the bottom of the furnace and coal and combustion air were injected horizontally to the furnace. The purpose of this test was to examine the flame stability of a new burner mounting a coal concentrator system, type B burner scaled up to be 3.6 tlh coal feed rate. Coals were crushed, classified, stored into the bin, and carried to the burner inlet. The primary air was pre-heated up to 80 degree C and carrying the fine coal particles to the burner inlet by a primary air fan. Additionally, the secondary air was pre-heated up to 330 degree C and also fed by a force draft fan to the burner and circulated by the air register. Fig.11 shows the relationship between the combustion efficiency and the burner load. We obtained a stable flame in a range from 15 % to 100 % burner load for coal with FR=l.l and from 22 % to 100 % for coal with FR=2.3. Fig .12 shows the expansion of the boiler load range in relation to the number of mills required. The gray (dotted) area shows the present boiler load range and the black (fully 5 |