Exhaust gas recirculation is a very effective method
to lower peak flame temperatures and, thereby,
thermal NO-formation. At exhaust gas recirculation,
the exhaust is not mixed into the flame, but
further upstream to combustion air and/or fuel (see
Fig. 5). Here, the distinction between external and
internal exhaust gas recirculation should be kept in
mind. At external recirculation, exhaust gas is taken
from the stack and is added, in most cases, to the
combustion air. To bring the exhaust gas to the
required pressure, a blower or a jet pump has to be
used. External exhaust gas recirculation could be
applied to existing designs as a low NOx-expansion.