OCR Text |
Show spreading reaction over a larger space because of slower turbulent mixing, are not as readily apparent. One effect, which may be relatively minor, may be a decrease in N O x production due to lower average temperatures because of the greater radiative heat loss from the larger space. The experimental facts are as follows: 1. The N O x levels found at 0\ = 30° and 42.5° with D2 = 19.05 m m are very similar, Fig. 6.. At 0\ = 35°, there is a drop by 15 % at high fire, increasing with turndown to a 30 % drop at 30 % of full fire. It thus appears that there is a minimum near 0\ = 35°.. 2. At 0\ = 35°, the N O x levels are about the same at D2 = 15.88 m m and 19.05 m m , but at D2 = 24.31 m m there is an elevation of 50 to 60 %. 3. At 0\ = 65° and £>2 = 15.88 m m , Fig. 9, the N O x levels scatter more but are generally similar to those at smaller angles. The situation at 0\ = 65° is complex, because, as shown in § 8.3.5, the furnace was then much too small for normal development of the combustion zone. The fuel jets impinged upon the furnace boundaries and the development of the reaction zone was radically altered. Nevertheless, the N O x remained low, no doubt because of the continuing efficacy of the recirculation of colled combustion products. Also, despite the growth of the combustion zone, essentially complete combustion was achieved overall, as shown by the sustained very low C O contents of the exhaust gases. 8.4.4. Effects of multiburner operation The NOx emissions with three-burner operation were about twice as high as with a single burner. Reasons for this are readily evident: 1. With three burners operating, the facing sides of adjacent burner flows are at once drawn together by entrainment and partly shielded by each other from the recirculating product gases. The result is that in the group of three, the two outer flows are drawn inward on the middle one, as is very visible from the furnace windows, and the overall dilution with cooled products is diminished, increasing the N O x emission. 2. The second effect is likely the more important one. The effective temperature of the recirculating gases being entrained by the jets unquestionably rises as one goes upstream from one burner to the next (from burner B 3 to B 2 to Bl). Thus the average temperature of these recirculating gases relative to the exhaust gas temperature is higher than in single burner operation. This produces higher average temperatures in the combustion zones and so drives up the thermal N O x production. The first effect has a limit. It is in part an end effect, and in a row of many burners, the inner ones will tend to behave similarly, but differently from an isolated single burner, regardless of the number of burners. The second effect, however, tends to grow with the number of burners, so the temperature difference between product gases at the upstream end of a row and those at the downstream end becomes progressively bigger. 22 |