8/8/2019 Computational Modeling of Pulverized Coal Combustion Processes In
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Computational modeling of
pulverized coal combustion
processes in
tangentially fired furnaces
Group Members
-Ayush Agrawal(Y7108)
-Jyotish Mishra()
-Pankaj Singh(Y7276)
-Ras Dwivedi(Y7350)
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Objectives
To study
-Characteristics of the flow
-Combustion
-Temperature distribution
- NOx emissions (fuel and thermal)
in a tangentially fired pulverized-coal boiler.
Compare the results obtained with and without OFA (Over
Fire Air) Operation.
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Schematic of Thermal Power Plant
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Boiler modeling
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Tangentially Firing Arrangement
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A Tangentially Fired Boiler
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Benefits of using Tangentially fired
Arrangement
GoodF
lame Distribution Uniform Heat Flux to the Furnace Walls
Reduction in NOx Emissions
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Numerical modeling
We first do combustion modeling and obtain temperature and
O2 concentration.
Amount of NOx formed is highly sensitive to the temperatureand O2 concentration.
The NOx calculation is performed after the combustion
calculation, based on predicted temperature and speciesconcentration.
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Combustion Modeling
Numerical calculations are carried out using CFD code in orderto predict turbulent flow, coal particle motion and combustionin the boiler.
Combustion zone, furnace, re-heaters, super-heaters,economizer and rear pass each is separately modeled usingdifferent numerical model.
The re-heaters, super-heaters and economizer are modeled as
porous media with inertial resistances in order to considertheir effects on flow and pressure drops and are treated asheat sinks in the heat transfer model.
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Combustion modeling
We calculated the Lagrangian particletrajectory of pulverized coal particle
Dispersion of the particle due to gasturbulence is calculated using stochastictracking model
Discrete ordinates radiation model is usedto simulate the radiation heat transfer
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NOx
modeling
Transport equation take into account
convection, diffusion, production and
consumption of the NO species.
NOx
formed mainly by -
a) Thermal NOx
b)F
uel NOx
c) Prompt NOx
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Thermal NOx
Formed when nitrogen and oxygen within
combustion air combine at a high temperature
and fuel lean environment.
Formation is function of temperature and
residence time.
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Equations
Transport equation
Rate equation
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Fuel NOx
Formed when nitrogen bound in the coalcombines with excess of oxygen in thecombustion air
Widely accepted intermediate species areHCN and NH3
HCN and NH3 generally react to form NO infuel lean region and N2 in fuel rich region
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Prompt NOx
Formed by the Reaction of atmospheric
nitrogen with hydrocarbon derived from fuel
in low temperature and fuel rich condition
Neglected as it is formed in very fuel rich
condition and contribute to very small amount
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RESULTS
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Flow Distributions
Velocity Magnitude
on a Vertical Plane
Velocity Vectors on different Horizontal Planes
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Flow activity near the burners is high.
Formation of a clockwise swirl at the center.
Swirling magnitude decreases as we movefrom section A to F.
As the flow enters SH and RH region swirling
reduces and upward flow tends to be even.
Conclusions:
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Particle Trajectories
Flue Gas Trajectory Coal Particle Trajectory
Both trajectories are almost same, but not
coincident due to difference in densities and
turbulent fluctuations.
Residence Time of flue gas within the boiler
was found out to be 22.2 seconds.
Residence Time of coal particles within the
boiler was found out to be 21.2 seconds and
this found out to be sufficient for CompleteChar Combustion.
Conclusions:
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Temperature Distributions
Temperature Distribution
on a Vertical Plane
Isothermal Surfaces at1800 K and 1900 K
Temperature Distribution on different Horizontal Planes
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Conclusions:
Input Air Temperature 355K. Maximum Temperature reached 2132K.
Mean Temperature increases and Deviation from Mean
Temperature in a Horizontal Plane decreases as we go from A
to C.F
ormer one due to increase in Combustion Intensity andlater one due to increased mixing and swirling.
Between sections D to F uniformity in Temperature
Distribution increases but Mean Temperature falls due to
Convectional and Radiation losses to Furnace Walls.
Prior to entering Heat Exchanger region Mean Temperature is
1524K with a Standard Deviation of 89K.
Isothermal Surfaces at 1800K and 1900K are closely related to
NOx formation.
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Species Distribution
Mass Fraction
Distribution of O2
Mass Fraction
Distribution of CO2
(a)
(b)
MassF
raction Distributionon Section C of (a) O2 (b) CO2
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V
ariation of Temperature,O2
and CO2
MassFractions (a) along Furnace (b) along diagonal BD
Conclusions:O2 Concentration is relatively high
near the burners and falls rapidly
thereafter as depicted from thecontour and graph.
Lower O2 Mass Fraction regions
corresponds to high Temperature and
high CO2 mass fraction regions
because of active Combustion Process
As depicted from graph (a), as we move along furnace length there
are three peaks in O2 mass fraction and correspondingly three valleys
in Temperature and CO2 mass fraction. This change is brought due to
supply of air from the burners.As depicted from graph (b), as we move along diagonal line on
section C, O2 mass fraction rapidly falls, CO2 mass fraction and
Temperature increases rapidly.
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Nox Emissions
Fuel, thermal and total Nox formation regions; in each pair of figures, the left figure indicates the Nox formationregion and the right figure shows an iso-surface (2* 10^(-4) gmol/m3-s)_
Nox formation rates along the diagonals line BD (a) fuel Nox (b) thermal Nox (c) total NOx
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Conclusions
The Predicted maximum NOx concentration is around 225
ppm .
The relatively high NOx concentration zones are found in the
furnace center where the temperature is higher and
combustion processes are more active.
The percentage of fuel NOx is very high(89.26%) as compared
to thermal NOx (10.74%).
The total NOx formation is not the same as the combined
region for the thermal and fuel NOx formation. NOx formation rates are low since the oxygen concentration is
very low in the central zones, although temperature is very
high.
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NOx emissions wit OFA operation
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Conclusions
Heat flux to the furnace wall is slightly decreased and the temperature atthe furnace and boiler exit are increased.
The fuel and thermal NOx formation are decreased by 8.51% and 5.72%respectively.
The reduction in fuel NOxmight be the result of decreased contact of
Nitrogen from the fuel with oxygen in the combustion air, which reducesNO into N2.
The reduction in thermal NOx might be due to the decreased temperaturein the furnace.
A relatively high temperature region is moved upward and is slightlyenlarged in the upper furnace due to occurrence of combustion at OFAports.
The NOxconcentration decreases more significantly above OFA ports.Since 10% of the total air is supplied through OFA ports, so O2 massfraction in middle of the furnace is lower, therefore HCN or NH3 formedfrom the volatiles might be converted to N2 rather than NO.