43 / 140
into the lower temperature regions of the SLC.
Therefore, it is essential that the location and
strength of the ERZ is designed or altered (using
refractory flow diverters) so that NO
X
emissions are
reduced without increasing CO emissions, due to
the formation of excessively fuel-rich and larger
recirculation zones.
Calciner optimisation: reinforcement
suspension preheaters
For reinforced suspension preheaters (RSP), AF
firing conditions are significantly different, as
AF chips freely fall down under the co-flowing
conditions of its cyclonic combustion chamber (CC)
(Figure 5). In addition, the kiln-generated NO
X
cannot be reduced in the RSP, as the hydrocarbon
radicals necessary to reduce NO
X
to N
2
are
completely depleted within the CC. The NO
X
reduction reactions do not, therefore, take place
because the riser duct gases, having higher NO
X
concentrations, slowly mix upstream with the CC
flow within the upper calciner duct leading to exit.
Figure 5 shows that kiln gases are pushed to
the other side within the combined duct by the
RSP gases, which remain on their entry side. Fuel
rich conditions prevail in the RSP and volatiles
(Figure 5a), which have the potential to reduce
the NO of the kiln gases, are quickly consumed in
the region of the connection between the RSP and
the riser. The change in direction brings them into
contact with the unused oxygen of the RSP, leaving
only a small amount of unreacted volatiles to oxide
and reduce formed CO.
The mixing (Figure 5b) in the remaing part of
the calciner after the RSP join is relatively slow and
mostly happens at lower temperatures, a condition
unfavourable to CO oxidation.
Finally, tertiary air build-up issues have also
been observed in many RSP meal inlets, where
some of the introduced meal particles fall back
into the tertiary air duct, initiating build-ups,
rather than being entrained into the combustion
chamber of the RSP. This problem is further
exacerbated, due to excessive pressure losses
caused by the tangential entry of the tertiary air
flow.
In RSP calciners, a moderate level of AF
substitution and NO
X
reduction can be obtained
by shifting some 60% of the fuel from the RSP
into the riser duct. Any higher substitution of fuel
would inhibit the combustion reactions within the
RSP, due to the endothermic reaction of the meal
particles.
To achieve a higher TSR of alternative fuels, the
RSP section has to be modified. This was carried
out during a recent plant upgrade where a RSP
calciner was reengineered, based on a patented
compact calciner design (SCC PM Technologies), to
convert it into an inline calciner achieving 100%
TSR using solid recovered fuels (Figure 6).
81,
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