Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 156

100

98

96

94

Configuration 2:

92

1 2 3 4 5

Coal jet momentum, N

FIGURE 2.124 Burnout vs. the coal jet momentum.

The two flames at lower coal jet momentum are the two unstable flames already
discussed (low transport air mass flow). For the other four flames, an increase in the
burnout level is measured, increasing the momentum. This effect is due to the higher
oxygen availability in the primary flame zone. The high momentum flames are more
lifted, at the ignition front more oxygen is available, and the burnout reaction starts
faster than in the low momentum flame.
The effect of the different ignition position is clear on the NO x emissions as
well. In the high momentum flames (transport air 215 kg/h), the coal jet entrains
more oxygen before ignition takes places and more oxygen is available from the
transport air (higher mass flow). 64 Thus, the local stoichiometry in the flame zone
is higher compared to attached (or less lifted) flames; the oxygen availability at the
ignition is higher compared to the baseline condition (130 kg/h transport air) result-
ing in higher NO x emissions (see Figure 2.124 ) .
The higher local stoichiometry in the primary flame zone may be due both to
the higher transport air mass flow and to the higher momentum. The second param-
eter is thought to be more important as can be seen from Figure 2.125 . At the same
transport air mass flow the NO x emissions for the high-velocity gun flames are higher.
High momentum flames are more lifted and entrain more oxygen before the ignition.

2.5.3.5.3 Precombustor NO x Level
The effect of the different NO x levels in the comburent was studied in the baseline
configuration (coal gun position at 280 mm from the central air and transport air
130 kg/h) for the high-velocity coal gun. The comburent flow was kept at the same
temperature changing only the NO x concentration. Figure 2.126 shows the NO x
emissions vs. the inlet NO x concentration in the comburent.
As expected, an increase in the flue gas NO x emissions with an increase in the
inlet NO x concentration was found. However, this correlation is not linear. It seems