Modelling Fine Particle Formation and
Alkali Metal Deposition in BFB
Combustion
Jorma Jokiniemi and Olli Sippula
University of Kuopio and VTT, Finland
e-mail: jorma.jokiniemi@uku.fi
Flame Days, Naantali 28.-29.01.2009
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
Outline of the Presentation
1. Introduction
2. Experimental data for the modelling
3. KCAR model
–
Aerosol dynamics
–
Deposition modelling
3. Results
–
Aerosol formation in the boiler
–
Deposit formation
–
Effect of fuel on the deposits
4. Conclusions
2
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
3
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Introduction
• The goals of this research are to model:
deposit formation in biomass combustion
mechanisms & process conditions
controlling hard deposit formation
factors controlling corrosive deposit formation
possible measures to avoid above mentioned
deposit formation
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
4
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Deposition of ash on heat exchangers
Solid layer
Thermophoresis and
diffusion of
fine particles (< 1 um)
Solid porous deposit can be removed by
soot blowing
Sintered deposit is difficult to remove
Sintering caused by partial melting of
alkali species deposited mainly as
fine particles and vapours
Condensation and
reactions of
vapours
Also chemical reactions may induce
Sintering, for example sulphation of
alkali chlorides
Impaction of large
particles (> 10 um)
Sintered layer
Chlorides are corrosive
Tube surface
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
5
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
6
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
KCAR Model Solves Numerically
• The behaviour of alkali vapours and particles
gas phase species chemistry
fine particle formation and growth
- particle size and composition
• Deposit formation
fine particle deposition mechanisms
coarse ash deposition mechanisms
vapour deposition
deposit layer growth rate and
chemical composition
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
7
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
INPUT DATA in the BFB -case study:
• Fine particle forming elements:
all elements are volatilised during combustion and
are in the vapour phase at maximum temperature
900 °C) in the freeboard
- (KOH, KCl, K, KO, NaOH, NaCl,…
volatilisation of ash elements is taken from boiler
measurements before ESP by iteration
• Large ash particles:
amount, size distribution and composition is given
as input
• Boiler geometry, flow rates, heat exchanger tubes,
temperatures
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
8
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
66 MW BFB -boiler
SH2
out
[8] 750 °C; 8.5 m
511 °C
[7] 750 °C; 7.9
in
m
481 °C
SH1
in
[6] 870 °C; 6.4
403 °C m
[5] 880 °C; 4
m
[10] 700 °C; 12.3
m
SH3
[11] 500 °C; 14.9
m
[12] 500 °C; 15.8
m
[13] 440 °C; 17.7
m
out
469
°C
[16] 415 °C; 27.2
[15] 420 °C; 25.2 m
m
in
274
°C
out
275
in
°C
276[14] 430 °C; 21.7
°C m
3 calculated cases:
out
276
°C
- wood residues (saw mill, forest)
• clean superheaters
12
[9] 700 °C; 9.2
in
m
480 °C
Evaporator
- wood residue with 25 % chipboard
• clean superheaters
• dirty superheaters
(higher superheater surface temperatures)
[4] 900 °C; 2
m
[3] 900 °C; 1.5 m
[2] 950 °C; 1
m
[1] 900 °C; 0.5
m
[0] 950 °C; 0
m
KCAR Nodalization of the
Forssa BFB Boiler
Economizer
in
25 °C
Steam temp.
[17] 125 °C; 38.2
m
Measurements before ESP
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
9
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Aerosol dynamics modelling
The code used, solves the GDE using a
sectional method in 1-D
dnk
dx
1
Jk (k k * )
u
Particle
concentration
(in size class dx)
dnk
dx
coag
dnk
dx
grow
Homogeneous
nucleation
coagulation
Growth by
condensation
and chemical
reactions
(or evaporation)
vd Ad
nk
u V
Deposition by:
• condensation
• thermophoresis
• diffusion
• impaction
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
10
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Chemistry in formation of fine particles
• Based on thermodynamic equilibrium
• Kinetics and mass transfer can be considered
• Following global reactions are important:
2KOH(g,c) + SO2 (g) + ½O2 (g)
K 2SO4 (g,c) + H 2O (g)
2KCl(g,c) + H 2O (g) + SO2 (g) + ½O2 (g)
2KOH(g,c) + CO2 (g) + ½O2 (g)
K 2CO3 (c) + SO2 (g) + ½O2 (g)
Same for Na
K 2SO4 (g,c) + 2HCl(g)
K 2CO3 (c) + H 2O (g)
K 2SO4 (c) + 2HCl(g)
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
11
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Alkali aerosol particles &
coarse mode particles
Chloride vapours
Deposition
• Vapour
condensation
chemical reactions
Sulphation
Thermophoresis
• Fine particles
thermophoresis
Brownian diffusion
• Large ash particles
direct impaction
(cross flow - windward side)
turbulent deposition
(surface parallel to flow - platens)
Turbulent flow in
staggered tube array
Condensation on
aerosol particles
SO 2
HCl
Condensation
on deposit
layer
Coarse particle
sticking
Sintering &
removability by
sootblowing
Boundary
layer
Corrosion
Heat
transfer
Diffusion
in porous
deposits
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
12
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
• Dependence of the deposition rates on flow variables:
Vapour condensation
Sh(Re,Sc) * pv
Sc = rate of momentum transport / rate of mass transport
Re = U * dc / u
Fine particle thermophoretic deposition
Nu(Re,Pr)* T/(T*d)
Pr = rate of momentum transport / rate of energy transport
Large ash particles
- cross flow f(Re * ) * Up * Dp 2 / dc
- parallel flow
Up * fr(Nu,Re,Pr)3/2*Re*4 / dx2
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
13
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Results
• Fine particle composition and size distributions:
Alkali sulphates formed fine particles prior to superheaters
Alkali chlorides condensed on the sulphate particles after
superheater section
the simulated particle size distribution in agreement with the
measurements upstream of the ESP
dm/dlogDp [g/Nm3]
1.40E-02
1.20E-02
1.00E-02
8.00E-03
6.00E-03
4.00E-03
2.00E-03
0.00E+00
0.001
1.00
0.80
K2SO4
MODEL
1.60E-02
MEASURED
1.40E-02
0.60
0.40
0.20
0.00
0.01
Na2SO4
0.1
Dp [µm]
dM/dlogDp [g/Nm3]
1.60E-02
dM/dlogDp [g/Nm**3]
1.80E-02
Particle size distribution of condensed
Alkali species in particles
insideMass Size Distributions
Particle
1.20superheaters
1.80E-02 alkali species after superheaters
1.20E-02
1.00E-02
10
Na2SO4
8.00E-03
6.00E-03
4.00E-03
2.00E-03
0.10
K2SO4
KCl
NaCl
0.00E+00
10.00
100.00
1000.00
0.001
0.1
10
Dp
[µm]
Particle size Dp [µm]
1.00
Potassium concentration [g(K)/Nm3]
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
14
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Potassium speciation and particle size distributions, fuel 1
8.0E-06
7.0E-06
6.0E-06
5.0E-06
4.0E-06
3.0E-06
2.0E-06
1.0E-06
0.0E+00
KOH (g)
K2SO4 (g)
K2SO4 (p)
KCl (g)
KCl (p)
0
3.27 6.9 7.65
9
12.9 13.6 14.4 16.3 21.1 28.6 30.6 32.6 34.6 36.6 38.4 39.1 39.9
Location [m]
Superheater 3
3.E+07
120
2.E+07
110
2.E+07
100
Particle number concentration
1.E+07
90
Mean particle size
5.E+06
80
0.E+00
70
0
5
10
15
20
Location [m]
25
30
35
40
Mean Particle size (nm)
Particle number
concentration (#cm3)
Superheaters 1-2
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
15
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Particle Deposition velocities at different locations
Deposition velocity Vd[m/s]
1.0E+01
1.0E+00
SH1-inlet
SH1-inside
Evaporator-inlet
turbulent &
inertial
impaction
Eko-inside
1.0E-01
1.0E-02
1.0E-03
0.001
Thermophoresis
0.01
Thermophoresis
& diffusion
0.1
1
Particle size dpa[µm]
10
100
1000
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
16
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Deposition growth (alkali species only): clean superheater tubes
0.14
K
NA
O
S
CL
C
0.12
0.1
0.08
0.06
0.04
Evaporator
Economizer
0.02
Location [m]
Superheaters 1-3
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
0
Deposition growth rate Vd
[mm/day]
FUEL: Wood residue+chipboard
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
17
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Deposition growth (alkali species only): dirty superheater tubes
FUEL: Wood residue+chipboard
K
NA
O
S
CL
C
0.07
0.06
0.05
0.04
0.03
0.02
Evaporator
0.01
Economizer
Location [m]
Superheaters 1-3
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
0
Deposition growth rate Vd
[mm/day]
0.08
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
18
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Results
• Effects of fuel
Chipboard containing chlorine-rich fuel released more
condensible alkali species
increased the alkali deposit growth approximately 3 fold
when compared to pure wood residue
wood residue with 25% chipboard
wood residue
Deposition growth rates in the superheater section
condensible species only
Deposition growth rate in the superheater section
condensible species only
K
NA
O
S
CL
C
0.12
0.1
0.08
0.06
0.04
0.14
Deposition growth rate
Vd[mm/day]
Deposition growth rate
Vd[mm/day]
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0.02
0
0
5
7
9
11
X[m]
13
15
17
5
7
9
11
X[m]
13
15
17
19
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
19
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Results
• Deposition
Large ash deposition dominates (if the tube surface is sticky)
Deposition of chlorine 3-6 % (clean tubes)
Deposition of alkali metals 5 % (clean tubes)
In the superheaters alkali chlorides are mainly
deposited by direct condensation
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
20
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Conclusions on the BFB –case study
Deposit chemical composition is different to that of fly ash due
to different deposition mechanisms and gas phase reactions
taking place
total deposition rates:
- 300 mm/day in cross flow (windward)
- 3 mm/day in flow parallel to surfaces (platens)
alkali compound deposition rates:
- up to 0,3 mm/day at the superheater inlets
large particles concentration and size important parameters
for deposition
increasing “fume” particle size decreases deposition
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
21
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Conclusions (2)
The model provides information on the ash behaviour and
deposition characteristics based on particle measurements prior
to filters and temperature measurements in the boiler.
good agreement between calculated and measured deposits
(Mikkanen et al., 2000)
experimental heat and mass transfer correlations for fume
and vapour deposition implemented (correct boiler geometry)
large particle deposition model improved into the KCAR code
In future?
verifying results by measurements
large particle deposition growth estimates require data on
the stickiness of the surfaces.
University
of Kuopio
VTT
TECHNICAL
RESEARCH CENTRE OF FINLAND
22
VTT TECHNICAL RE SEARCH CENTRE OF FINLAN
D
Acknowledgements
The author acknowledges the Finnish funding agency for
Technology and Innovation (TEKES), VTT, Univ. of Kuopio,
Forssan Energia Oy, Metso and Foster Wheeler for funding this
research.