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Licensed Unlicensed Requires Authentication Published by De Gruyter December 24, 2019

Simulation of a Scaled down 250 MWe CFB Boiler Using Computational Particle Fluid Dynamics Numerical Model

Vidya Venkatesan, Lakshminarasimhan Mukundarajan and Anantharaman Narayanan

Abstract

Eulerian-Eulerian approach and conventional Eulerian-Lagrangian model are computationally exhaustive for modelling circulating fluidized bed (CFB) riser which has wide particle size distribution and billions of particles Alternatively, the relatively recent Eulerian- Lagrangian computational particle fluid dynamics (CPFD) model enables simulation of the CFB system with lesser computational resources. Most of the published studies on CPFD simulations of CFB risers deal with single grate system. The present study aimed to investigate the performance of the CPFD model for predicting solids distribution in a CFB riser with pant-leg structure (dual grate) and characteristics similar to a commercial boiler. Experiments conducted in a scaled down 250 MWe CFB facility according to Glicksman’s simplified similarity laws for fluidized beds were simulated using commercial code Barracuda. The bottom dense bed, upper lean solid phase, increase in bottom bed voidage with increasing fluidizing velocity and reducing solids inventory, decrease in bottom bed solids concentration with decrease in particle size and exchange of solids between the legs typically occurring in a CFB with pant-leg structure were successfully captured by the CPFD calculations. Simulation results showed that the upper solids concentration is hardly influenced by the solids inventory level in line with the experimental observation, therefore the amount of solids inventory can be optimized during actual operation. The predicted pressures varied from the average experimental pressure data within the range –10 to 39 %.

Nomenclature

Ap

Particle acceleration, ms–2

Ar

Archimedes Number

C1

Model constant = 180

C2

Model constant = 2

Cd

Drag coefficient

Cs

Smagorinsky constant = 0.01

D1

Model constant = 27.2

D2

Model constant = 0.0408

dp

Particle diameter, m

Deq

Equivalent diameter, m

DPWY

Wen and Yu drag function, s–1

DPERGUN

Ergun drag function, s–1

DP

Drag Function, s–1

F

Momentum exchange rate per volume between phases

f

Particle distribution function (PDF)

Gs

Solids flux, kgm–2s–1

g

Gravitational acceleration = 9.81, ms–2

H

Riser height, m

H0

Static bed height, m

I

Unit tensor

L

Riser dimension, m

mp

Particle mass, kg

P

Pressure, Pa

Ps

Pressure constant = 1, Pa

Re

Reynolds number

Rep,mf

Reynolds Number at minimum fluidizing velocity

rp

Particle radius, m

t

Time, s

ug

Velocity magnitude of gas phase, ms–1

u0

Fluidization gas velocity, ms–1

umf

Minimum fluidization gas velocity, ms–1

ug

Gas phase velocity vector, ms–1

up

Solid phase velocity vector, ms–1

xp

Particle spatial location, m

β

Constant = 3

Δ

Subgrid length scale, m

δxδyδz

Product of dimensions of a cell, m3

α

Constant = 108

μeff

Effective dynamic viscosity of gas phase, Pas

μg

Gas viscosity, Pas

μt, g

Turbulent viscosity of gas phase, Pas

ρg

Density of gas phase, kgm–3

ρp

Particle density, kgm–3

τg

Gas phase effective stress tensor, Pa

τp

Particle stress function, Pa

θg

Gas volume fraction

θp

Solid volume fraction

θp0

Initial solid volume fraction

θcp

Particle volume fraction at close pack

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Received: 2019-02-25
Revised: 2019-10-05
Accepted: 2019-11-11
Published Online: 2019-12-24

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