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Simulation of a Steam Coal Gasifier
Presented by
Alireza Abbasi1-3
Paul E. Ege2, Hugo I. deLasa1
(1)Department of Chemical and Biochemical Engineering, University of Western Ontario, London, ON, Canada
(2) Reactech Process Development Inc., Markham, ON, Canada
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Presentation outline
1- Introduction -Fluidized Bed Modeling
2 - Model -CFD and Plug Flow
3 - Results
4 - Summary
Fluidized Bed Properties and Applications
Properties• Excellent for contacting large gas volumes
effectively with high solid surface area at near isothermal conditions
• High relative gas/solid velocities • Strong particle mixing
Applications • Catalytic processes
• Steam Methane Reforming• Gasification• Catalytic Cracking
• CVD processes• Silicon deposition (SiH4/SiHCl3)• Uranium coating• TiO4 coating• Potassium nitrate granulation
• Other• Combustion (coal/biomass)• Thermal Gasification• Drying
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Introduction
Modeling Principles
Essentially, all models are wrong, but some are useful(George E. P. Box, 1987)
All the physics is not understood
There are lots of assumptions in calculation
Simplified Approach Models
Pseudo Homogeneous • Ideal flow (PF/CSTR), • Dispersion models, • RTD or CTD models• Simplified flow • Single phase assumption
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Introduction
Two-phase modeling • More advanced = account for bubble/emulsion• Gas in excess of minimum fluidization = bubbles• Two distinct phases: bubble & emulsion• Each phase with has a model for flow & reactions• Mass interchange between phases
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1-The Eulerian-Lagrangian approach: The fluid phase is treated as a continuum by solving the time-averaged Navier- Stokes equations, while the dispersed phase is solved by tracking a large number of particles through the calculated flow field. (CPFD)
2-The Eulerian-Eulerian approach: It solves a set of n momentum and continuity equations for each phase. Couplings are achieved through the pressure and inter phase exchange coefficients. (CFD)
CPFD CFD
Computational Fluid Dynamics (CFD)
Introduction
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Modeling a Coal Gasification Fluidized-Bed Reactor
Model
(a) The schematic representation of the entrained fluidized bed gasifier. (b) Selected configuration for the simulation of the entrained fluidized bed gasifier in the near feeding section.
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Modeling a Coal Gasification Fluidized-Bed Reactor
Model
AshOHVolatileCharCoal +++→ 2
24322412 COCOHCHOHVolatile ααααβ +++→+
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Mathematical Modeling
Model
CFDThe motion of fluid and dispersed phases are governed by respective mass and momentum conservation equations. The volume averaged fluid mass, momentum and energy equations are defined as follows:
The trajectory calculation of the discrete phase is made by integrating the force balance on the particles. The particle motion is defined as follows:
(1)
(2)
(3)
(4)
Plug FlowA material balance on the differential volume of a fluid element on species i in a PFR is calculated as follows:
(5)
(6) 0reaction ofheat
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leaving species
2
coming species
1 QHHH =∆+∆+∆
( )
action
j jr
jji
cxi
RvAF
dx
d
Re
,
,
Changing flowmolar
, ∑=ν
( ) ( )
Source
f
Convection
fff
ChnageDensity
ff Svt
=⋅∇+∂∂
θρθρ
( ) ( )
ExchangeMementum
ForceBody
ff
Shear
ffessure
Convection
ffff
onAccelerati
fff FgPvvvt
−+⋅∇+∇−=⋅∇+∂∂
θρτθθρθρ
Pr
( ) ( )
ForceBody
Stress
pp
p
essure
pForceDrag
pfp
onAccelerati
p gP
vvDvdt
d +⋅∇
−∇−−=
ρθτ
ρPr
( ) ( )[ ] SourceConductionConvection
fff
onAccumulati
ff QqvPEEt
∗+∇−=+⋅∇+
∂∂
.θρθρ
CFD Modeling
Particle contours after 10S
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Results
Fluid contours after 10S
Results
CFD Modeling
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Gas species contour after 10s
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Results
CFD Modeling
Gas species contour after 10s
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Results
CFD Modeling
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Results
Effluent concentrations
CFD Modeling
Particle recycle flow at 1350KParticle recycle flow at 1150K
Plug Flow Modeling
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Results
Axial Concentration profile
Particle recycle flow at 1350KParticle recycle flow at 1150K
Modeling Results
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Results
Temperature Profile
CFD and Plug Flow Comparison
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Results
Effluent concentrations
Particle recycle flow at 1350KParticle recycle flow at 1150K
Summary
Reasonable agreement between CFD and PF effluents Methods are complementary, fast general or slow detailed PF model set up in few days allow quick investigation
Useful for wide range of conditions (height, density, flow, temp, pressure…)
Possible to apply different kinetics (even parameter fit to experiments)
Useful for feasibility, optimization and In Situ calculations ,fast basic understanding of process
CFD takes time to set up and run different cases Detail understanding of bed hydrodynamics Investigate flow/species patterns Specify flow related issues (hot spot, bypass) Benefit for entrainment and choking Useful for scale-up and design Also verify changes prior to implementing in practice
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