Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 154

increasing in configuration 3. Configuration 2 has an O 2 profile on the center line
passing for a zero oxygen concentration (see Figure 2.104 ) ; no in-flame measure-
ments were performed for the other two configurations. It may be inferred that in
configuration 1 the fast mixing with the central air jet and in configuration 3 the
delayed ignition (high entrainment before the combustion starts) probably reduce
(or avoid) this low oxygen region in the coal jet. The intermediate configuration 2
has the more relevant drop in the oxygen concentration on the centerline of the coal
jet resulting in a lower outlet burnout level. In configuration 3 the flame is more
detached compared with configuration 2 and the char burnout reaction starts later.
However, the char reaction starts with high O 2 a vailability, and before the post flame
zone reaches a higher burnout level than in configuration 2.
2.5.3.5.2 Coal Transport Air Mass Flow
Three coal transport air mass flow had been tested for high- and low-velocity coal
guns keeping the coal feeding rate at 66 kg/h and the coal gun position in the baseline
configuration (coal gun at 280 mm from the central air jet):
1. 80 kg/h (1.21 kg of air/kg of coal)
2. 130 kg/h (1.97 kg of air/kg of coal)
3. 215 kg/h (3.26 kg of air/kg of coal)
The flames with low transport air mass flow(80 kg/h) were not stable. The low
momentum coal jet was affected by the strong central air jet and entrained in the
recirculation zone of the central air jet. Figure 2.121 shows the photographs for the
low transport air mass flow and baseline transport air mass flow (130 kg/h) with
high-velocity coal guns.
FIGURE 2.121 Low coal transport air mass flow (left) and baseline transport air mass flow
(right) with high-velocity coal guns.
The lower O 2 a vailability (lower transport air) and the low momentum of the
coal jet determine that this is not a fully stable condition. The resulting burnout level
was low for both the low- and high-velocity coal guns (see Figure 2.122 ).
The two flames with high transport air mass flow were similar (see Figure 2.123 ).
Both flames showed an ignition lifted 35 to 40 cm from the front wall; the ignition