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The Combustion and Environmental Research Facility (CERF) engineers at
the National Energy Technology Laboratory (NETL, Pittsburgh, PA, U.S.A.)
are working with Fluent to develop and validate comprehensive combustion
sub-models for cofiring biomass in pulverized coal boilers. This fundamental
research is focused on developing strategies for NOx
reduction by reburning highly volatile, moist bio-fuels in pulverized
coal boilers. Minimizing the unburned carbon in ash is one of the factors
used to evaluate biomass fuels for utility boilers. Accurate prediction
of unburned carbon in a highly fluctuating environment, like that found
in utility boilers, requires the use of advanced carbon burnout kinetic
models in the CFD simulations.
This study involved the development of FLUENT sub-routines for biomass
combustion/cofiring using available experimental data and first-principle
mathematical models that provide accurate estimations of kinetic and CFD
model parameters for devolatilization and diffusion-controlled char burnout.
Two sets of drying functions have been developed to include the effect
of moisture present on the surface of the coal/biomass, and embedded in
the char, typical of that found in low rank coals. The effect of moisture
on the surface of the coal/biomass is incorporated using a droplet/vaporization
model in FLUENT. The evaporation of moisture embedded in char is included
via a surface reaction in a novel way that accounts for char burnout due
to steam gasification. Another key concern in developing an accurate model
for biomass combustion is accounting for significant asphericity in biomass
char particles, which plays a key role in char burnout. To this end, an
enhancement factor that accounts for the large length/ diameter aspect
ratio in the burning of biomass particles has been explicitly derived
for the FLUENT computations.
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Moisture =
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0%
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12%
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24%
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36%
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48%
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Effect of moisture on temperature inside NETL’s
Pilot Scale
Combustor (110 kW) |
A number of interesting exploratory CFD simulations related to the CERF
pilot scale combustor geometry and a T-fired industrial boiler have been
conducted to examine the effects of biomass particle size and residence
time on carbon burnout. Interestingly, despite ten times larger biomass
(switchgrass) particle size relative to coal, blending biomass with coal
in the boiler actually reduced the unburned carbon. This phenomenon can
be attributed to the high volatile content of the switchgrass. A few additional
exploratory CFD simulations were performed on the CERF combustor to examine
the effect of the O2/CO2
environment on NOxemissions. From the
preliminary results, a reduction of 39% in NOx
(on lb/MBTU basis) was observed when compared with the combustion of coal
in air.
Temperature contours and velocity vectors on a horizontal
plane inside a full-scale T-fired industrial boiler (400MW) using the
variable reactivity char model
Over the next year, it is expected that 3D CFD simulations will be conducted
for at least three full-scale utility boilers to assess design and operational
issues. The validated 3D CFD model, in conjunction with the engineering
guidelines for allowable biomass type, particle size, and moisture, will
also be related to biomass fuel handling and process economics work for
various plant equipment and burner design/ injection schemes. This CFD
modeling with new biomass cofiring routines will represent a new capability
for commercial software and should nicely complement goals relating to
the cofiring of opportunity fuels, and the longer-term need to couple
and integrate 3D CFD simulations with plant-modeling software for dynamic
simulations. Information from the full-scale demonstrations will also
provide feedback for refinement of the CFD model and insight into design
and operational issues that are important for planning future utility
biomass cofiring demonstration projects.
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