Burnout Zone
OFA is injected downstream of the
reburn zone to complete combustion.
OFA is typically 20% of the total air
flow with an overall excess air level
of 15-25% being usual. The OFA injection
rate is optimized for each specific
application to minimize carbon
monoxide emissions and unburned
carbon in the fly ash. Thermal NOx
formation in the burnout zone is small
because of the lower temperature.
Depending on the nature of the reburn
fuel used, various boiler retrofits are required.
In all cases, the boiler needs to be
equipped with fuel injection in the reburn
zone, and FGR may also be added. OFA
ports are also needed for the burnout
zone. If natural gas is used as the reburn
fuel, a pipeline extension to the site may
be required.
If the reburn fuel is coal, additional
coal handling and pulverizing equipment
may have to be installed. For example,
cyclone boilers are fired with coarse
crushed coal, which needs to be reduced
to smaller particle sizes to be an effective
reburn fuel. Similarly, micronized
coal, which was tested as the reburn
fuel in one of the CCT projects described
in this report, requires special size-reduction
equipment.