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By Vlad Zarnescu, Columbian Chemicals Company, Marietta, GA; and Sarma
V. Pisupati, Energy and Geo-Environmental Engineering Department, Pennsylvania
State University, University Park, PA
Surface grid for the down-fired combustor
Throughout the power generation industry, controlling the emission of
nitrogen oxides (NOx) has been of interest for many years. During this
time, equipment such as low- NOx burners, and combustion modification
methods like reburning, air staging, and flue-gas recirculation have been
developed. Reburning is a process in which fuel not burned in the primary
combustion zone is diverted to a secondary combustion zone downstream
of the first, where it is reburned using additional fuel. The hydrocarbons
in the reburn zone react with and eliminate some of the NOx created in
the primary combustion zone. Air staging, also called overfire air technology,
is a process that divides the combustion air into primary and secondary
streams. NOx generated by the fuel-rich conditions of the fuel and primary
air mixture are reduced in the combustion zone that incorporates the secondary
air. Flue gas recirculation involves the injection of some of the flue
gas into the combustion zone for further burning. This process results
in reduced flame temperatures, and subsequent reductions in NOx production.
To meet current emissions standards, several of these methods have been
coupled recently with the use of nonconventional fuels, such as coal-water
slurries and biomass, in hopes of finding NOx reduction solutions that
can be applied to a large variety of boilers.
Mixing optimization by varying the injection site for the reburn fuel
Comparison between experimental values (red circles) and FLUENT predictions
(blue lines) for axial temperature (top) and NOx (bottom) profiles for
the baseline case
Researchers at Penn State University have been using
FLUENT to optimize the design of a pilot-scale combustor that
uses a number of NOx control methods, including a low-NOx
burner, air staging, and reburning. The facility is a 147 kW downfired
combustor for which extensive experimental data has been
collected. CFD simulations of the unit have made use of FLUENT’s
NOx module with the reburn option to evaluate the performance
of several optimized designs. A baseline mode of operation was
established with 0% reburn fuel and pulverized coal as the primary
fuel. The mesh and boundary conditions in the numerical
model were adjusted to best match the experiments carried
out for this mode. Once these tests were completed, the modeling
of optimized scenarios, including different fuels and firing
configurations, was initiated.
Several combustor designs and operating conditions were
considered. The effects of mixing, residence time, air staging,
and reburning were studied. The performance of natural gas,
coal, coal-water slurry, and biomass as reburn fuels was predicted
using numerical simulations and compared with measurements.
Reduction of NOx levels was targeted at every stage,
with the results being coupled with optimized parameters for
mixing and injection configurations. A sensitivity analysis was
conducted to estimate the variations of the predictions with
respect to the model parameters.
The CFD results showed that improved mixing and burner
aerodynamics contribute significantly to lowering the primary-
zone NOx levels. This fact, coupled with optimized injection
configurations and reburning parameters, resulted in important
reductions in NOx emissions. In a comparison of fuels,
tests indicated a NOx reduction of up to 74% over the baseline
case for natural gas reburning and 48% for coal-water slurry
reburning. These both represent a major improvement over
the maximum reduction obtained previously on the same unit
for non-optimized configurations. In short, the CFD-optimized
combustor design resulted in significant reduction of NOx emissions,
and at the same time provided insight into the NOx control
mechanism and the complex interaction between key
combustor operating parameters.
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