View
220
Download
3
Category
Preview:
Citation preview
CFD Modeling of Soot Formation from Asphaltene Gasification
Vinoj Kurian1, Andre Bader2, Petr Nikrityuk1, Rajender Gupta1
1Department of CME, University of Alberta, Canada
2IEC, TU Bergakademie Freiberg, Germany
Outline
o Introduction
o Motivation & Objectives
o Experimental setup
o CFD Modeling of Soot Formation
o Conclusions
2
Gasification is the process where any carbonaceous material is converted into synthesis gas (syngas) by partial oxidation with air, O2 and /or steam.
Combustion
C + O2 CO2 ∆H= -405.9 kJ/mol
Heterogeneous Gasification Reactions
C + ½ O2 CO ∆H= -123.1 kJ/mol
C + CO2 ↔ 2CO ∆H= +159.7 kJ/mol
C + H2O ↔ CO + H2 ∆H= +118.9 kJ/mol
C + 2H2 ↔ CH4 ∆H= -87.4 kJ/mol
Homogenous reactions
CO + H2O ↔ H2 + CO2 ∆H= -40.9 kJ/mol
CO + 3H2 ↔ CH4 + H2O ∆H= -206.3 kJ/mol
3
Gasification
4
Soot The term soot is given to the particulates formed during combustion of carbonaceous fuels under sub-stoichiometric conditions.
o Soot usually forms at 1000 to 2500 °C.
o Fundamental unit: Spherules (10–50 nm)
o Shape: Clusters or chains of spherules
o Decrease thermal NOx
o More difficult to gasify than coal char & harder to burn
o Major contributor to global warming after CO2
o Environmental concerns on pollutant emission
o Many PAHs are carcinogenic & mutagenic.
o Can be breathed into the lungs and cause substantial
Stanmore, B. R. et al., Carbon, 2001, 39, 2247 –2268
5
Proximate Analysis
Volatiles 63
Fixed carbon 36.3
Ash 0.6
Moisture 0.1
Athabascan Asphaltene Ultimate Analysis
Carbon 82.68
Hydrogen 8.34
Sulfur 7.75
Nitrogen 1.2
Oxygen 0.03
Vanadium 1763 ppm
Nickel 671 ppm
o H/C = 1: 1.10 to 1.20
o Heaviest and most polar molecular component
o Most Inorganic matter in bitumen is concentrated in asphaltenes.
Sheremata, J.M. et al., Energy&Fuels,18(5),1377-1384
6
Asphaltene Gasification
o Metals like V & Ni liberate during gasification and pass through downstream of gasifier.
o If soot formed during gasification encapsulate the metals, the fouling and erosion can be avoided.
o Understanding the soot formation during gasification is important.
http://www.escet.urjc.es/~sop/alumnos/proyectos/descargas/propuesta18.pdf
Pyrolysis
Chhiti et al., Energy Fuels, 2011, 25, 345–351
Fuel
Tar
Primary Gas
Primary Char
Soot
Zhang, H. Ph.D. Thesis, NITROGEN EVOLUTION AND SOOT FORMATION DURING SECONDARY COAL PYROLYSIS, Brigham Young University, 2001.
Su
rface
gro
wth
7
Soot Formation Mechanism
Soot Formation Mechanism
8
Nucleation of soot particle (inception and growth of PAHs)
Particle coagulation
Particle surface reactions (Growth and surface oxidation)
Particle agglomeration
J. Warnatz, U. Maas, R. Dibble, Combustion: physical and chemical fundamentals, modeling and simulation, experiments, pollutant formation, Springer, 2006. Sarofim et al. , Springer Series in Chemical Physics , 1994, 59, 485-99
Objectives
Model soot formation during asphaltene pyrolysis and gasification in a drop tube furnace. Validation of the model by comparing with experimental results.
9
Process Parameters Value
Feed particle size 207 µm
Pyrolysis Temperature
1000 °C
1200 °C
1400 °C
Partial Oxidation @ 1200oC
Lambda = 0.13
Lambda = 0.22
Lambda = 0.31
10
Experimental Set up
Collection of soot on substrate aluminium foil
1
4 5 6
2 3
Entrained Flow Reactor Mullite tube 2.5 in. ID × 5 ft. L Max Temperature: 1500 oC Atmospheric pressure
Dekati low pressure impactor user manual ver. 3.4, 2007.
CFD model
o Ansys Fluent 16.0 o 2D-axisymmetric geometry o k-ω-SST turbulence model o P-1 radiation model o Incompressible ideal gas o Fuel injection : DPM o Devolatilization : Single rate o Soot formation : Moss-Brookes-Hall o Soot Oxidation model : Lee model
Simulation Methodology
11
Volatiles during pyrolysis
C2H2
C6H6
CH4
CO
N2
Assumptions o Soot precursor : C2H2 and C6H6
o Soot surface growth from : C2H2
o Soot-Radiation interaction enabled o Detailed gas phase mechanism DRM-22 is used o Total 26 species and 108 reactions
o 71 % of feed asphaltene – volatiles; rest is char. H2S is accounted as N2.
The Moss-Brookes Model
Solves transport equations for normalized radical nuclei concentration and soot mass fraction :
The instantaneous production rate of soot particles
The source term for soot mass concentration
The model assumes that the hydroxyl radical is the dominant oxidizing agent
soot inception due to acetylene or benzene
12
The Moss-Brookes-Hall Model
It is based on a soot inception rate due to two-ringed and three-ringed aromatics, as opposed to the Moss-Brookes assumption of a soot inception due to acetylene or benzene (for higher hydrocarbons)
Hall et al. proposed a soot inception rate based on the formation rates of two-ringed and three-ringed aromatics , from acetylene, benzene, and the phenyl radical based on the following mechanisms:
The inception rate of soot particles as given to be eight times the formation rate of above species
Both the coagulation term and the surface growth term were formulated similar to those used by Brookes and Moss.
Oxidation due to O2 was added, in addition to the soot oxidation due to the hydroxyl radical. 13
Drop Tube Furnace Meshing Carrier gas +DPM
Gasifying agent
Outflow BC
Wall Temp.
Mesh quantity 76530 quadrilateral cells
14
Boundary Conditions
Inlet inner – Mass flow inlet - N2
Inlet Outer- Mass flow inlet - Air/N2
Outlet – Outflow
18
0
0,2
0,4
0,6
0,8
1
1000 1200 1400
Soo
t m
ass
frac
tio
n
Temperature oC
Experiment
Model
0
0,1
0,2
0,3
0,4
0,5
13 31
Soo
t m
ass
frac
tio
n
Stoichiometric Oxygen %
Experiment
Model
Validation
Soot mass fraction increases with increase in pyrolysis temperature.
Soot mass fraction decreases with increase in stoichiometric oxygen %.
Pyrolysis-Nucleation, Coagulation, Surface Growth
19
0
0,2
0,4
0,6
0,8
1
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
So
ot
mas
s fr
acti
on
Axial distance (m)
1000 deg C
1200 deg C
1400 deg C
0
1000
2000
3000
4000
5000
6000
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Rate
of
nu
clea
tion
(1e+
15
part
icle
s/m
3-s
)
Axial distance (m)
1000 deg C
1200 deg C
1400 deg C
0
1000
2000
3000
4000
5000
6000
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Rate
of
coagu
lati
on
(1e+
15
part
icle
s/m
3-s
)
Axial distance (m)
1000 deg C
1200 deg C
1400 deg C
0
0,02
0,04
0,06
0,08
0,1
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Ra
te o
f su
rface
gro
wth
(kg/m
3-s
)
Axial distance (m)
1000 deg C1200 deg C1400 deg C
20
0
0,1
0,2
0,3
0,4
0,5
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
So
ot
mas
s fr
acti
on
Axial distance (m)
Lambda=0.13
Lambda=0.22
Lambda=0.31
0
2000
4000
6000
8000
10000
12000
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Rate
of
nu
clea
tion
(1e+
15
part
icle
s/m
3-s
)
Axial distance (m)
Lambda=0.13
Lambda=0.22
Lambda=0.31
0
2000
4000
6000
8000
10000
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6Rate
of
coa
gu
lati
on
(1
e+1
5
pa
rtic
les/
m3-s
)
Axial distance (m)
Lambda=0.13
Lambda=0.22
Lambda=0.31
0
0,02
0,04
0,06
0,08
0,1
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Rate
of
surf
ace
gro
wth
(kg
/m3-s
)
Axial distance (m)
Lambda=0.13
Lambda=0.22
Lambda=0.31
Partial Oxidation @ 1200oC -Nucleation, Coagulation, Surface Growth
Conclusions
21
Soot mass fraction: Increases with increase in pyrolysis temperature
Decreases with increase in stoichiometric oxygen %
The effect of nucleation, coagulation and surface growth of soot particles can be
analyzed in detail using the model.
Very simple CFD model predicted the effect of Temperature and stoichiometric
ratio on soot formation and qualitatively in agreement with experimental results.
Future Work
Fine tune the present model to predict the soot formation more accurately.
Implement different devolatilization and soot oxidation mechanisms.
Recommended