Conclusions
The three projects described in this
report clearly show that reburning is an
effective NOx control option for coalfired
boilers, either alone or in combination
with other emissions control
technologies. This is reinforced by the
results of an earlier NOx control CCT
demonstration project, Enhancing the
Use of Coals by Gas Reburning and Sorbent
Injection (GR-SI) at Illinois Power’s
Hennepin Plant (see CCT Topical Report
Number 3), as well as by commercial
projects involving reburning, both in the
U.S. and abroad.
Reburning can reduce NOx emissions
from coal-fired boilers by up to 50-60%,
depending on various factors such as the
baseline emissions level, furnace characteristics,
and the fraction of heat input
supplied by the reburn fuel. The most
important process variable is the stoichiometric
ratio in the fuel-rich reburn zone.
The optimum value for this ratio is in the
range of 0.85-0.95, which is achieved by
10-30% reburn heat input.
Long-term testing has shown only
minor impacts on boiler equipment and
operations. There is a small loss in boiler
efficiency, particularly when gas is used
as the reburn fuel, due to the increased
moisture content of the flue gas when
gas is fired.
With coal reburning, other emissions
are not significantly affected. With natu-was to reduce NOx to less than 0.60
lb/million Btu from a baseline of
1.25 lb/million Btu, a 52% reduction.
Although the actual baseline was
higher than expected (1.45 lb/million
Btu), the NOx emissions target of 0.60
lb/million Btu was achieved with 17%
reburn fuel, a 59% reduction. At
greater reburn fuel rates, NOx reduction
was even greater. These results
are comparable to the NOx reduction
achieved with gas reburning systems.
As expected, LOI increased with the
reburn system in operation; LOI was
35 to 45% during full load, compared
with the baseline level of 10-12%.
Some mechanical problems were
encountered during the demonstration,
including plugging of the coal handling
system feeding the MicroMill™,
vibration and blade wear on the mills,
erosion of the classifiers, and corrosion
due to low temperature flue gas
when the reburn system was out of
service. In spite of these problems,
successful operation was achieved