Computational Modeling of Pulverized Coal Combustion Processes In

8/8/2019 Computational Modeling of Pulverized Coal Combustion Processes In

http://slidepdf.com/reader/full/computational-modeling-of-pulverized-coal-combustion-processes-in 1/27

Computational modeling of

pulverized coal combustion

processes in

tangentially fired furnaces

Group Members

-Ayush Agrawal(Y7108)

-Jyotish Mishra()

-Pankaj Singh(Y7276)

-Ras Dwivedi(Y7350)



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.



Schematic of Thermal Power Plant



Boiler modeling



Tangentially Firing Arrangement



A Tangentially Fired Boiler



Benefits of using Tangentially fired

Arrangement

GoodF

lame Distribution Uniform Heat Flux to the Furnace Walls

Reduction in NOx Emissions



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.



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.



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



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



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.



Equations

Transport equation

Rate equation



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



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



RESULTS



Flow Distributions

Velocity Magnitude

on a Vertical Plane

Velocity Vectors on different Horizontal Planes



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:



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:



Temperature Distributions

Temperature Distribution

on a Vertical Plane

Isothermal Surfaces at1800 K and 1900 K

Temperature Distribution on different Horizontal Planes



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.



Species Distribution

Mass Fraction

Distribution of O2

Mass Fraction

Distribution of CO2

(a)

(b)

MassF

raction Distributionon Section C of (a) O2 (b) CO2



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.



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



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.



NOx emissions wit OFA operation



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.

Documents

Computational Modeling of Pulverized Coal Combustion Processes In