OCR Text |
Show (2) Staged Combustion During staged combustion, air is injected into the furnace from the length L/D=8.ln Fig. 5, measurements of N O concentration are respectively made at L/D=5 (fuel-rich 1st-stage combustion) and L/D=12 (rear flame zone). At L/D = 5, the conversion yields when the equivalence rate is 1.11, 1.00, or 0.91 are hardly different from one another. The reason lies in that there is less air injected in the first stage. The insufficient supply of oxygen atoms leads to incomplete combustion (i.e. in the fuel-rich zone), and thus a lower conversion yield. In the main combustion zone, when the reverse equivalence ratio, 0 _1 is between 0.66 to 0.76, the lowest value of the conversion yield appears. This finding is similar to the results from previous studies [12,21]. The measured results at L/D = 12 show that when the equivalence ratio equals 1.11, it is still a fuel-rich zone after the secondary air is injected. Therefore, the conversion yield is rather low. But when the equivalence ratio equals 1.00 or 0.91, more sufficient oxygen leads to a higher conversion yield. This trend is similar to that at L/D = 5. In other words, when the reverse equivalence ratio of .the first staged combustion zone is between 0.66 and 0.76, the smallest value appears. (3) Staged combustion with cooling water Fig. 5 shows that the addition of cooling water can reduce some of the fuel N O . Compared with the results in Table 1, at L/D =22, the decline of the N O conversion yield reaches the lowest at 0 t =1.11. But the conversion yield increases dramatically at 0t>1.O. The effects of adding cooling water on the formation of N O composites are greater, around 2 to 4%, at 0t = 1.11. If 0 t equals 0.91 or 1.00, the effects of adding cooling water are smaller than 1 % in both cases. This is due to the cooling effect of staged air injection, which renders the addition of cooling water for partial cooling is ineffective. From the comparison between Figures 4, 5 and 6, we know that the effects of air flow ratio on the conversion yield of N O are more distinct than those of the temperature. A denser intensity of nitrogen-bearing composite in the initial stage leads to a lower conversion yield of fuel NO. The fuel NO injected in the furnace is first destroyed in the flame in the initial stage, then gradually formed in the flame area again. Along with the increase of air flow ratio, the final quantity of N O formation also increases gradually. Figures 5 and 6 show that at an equivalence ratio of 1.11, the fuel-rich zone remains the same after the staged air injection, thus generating a lower conversion yield. At an equivalence ratio of 1.0 or 0.91, the presence of sufficient air for reaction results in a higher conversion yield. 7 in -i i |