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Behavior of limestone in a large-scale pressurized fluidized bed
combustor- attrition, fragmentation and SO2 capture -
T. ShimizuNiigata University
S.Sakuno, N. Misawa, N. Suzuki, H. Ueda, H. Sasatsu, H. Gotou
Electric Power Development Co., Ltd.
ABSTRACTCooperative research work between EPDC and Niigata Univ. on behavior of limestone in a 71MWe PFBC
Limestone attrition rateFragmentation of limestoneModel of SO2 capture by single limestone
particle under attrition conditionSO2 capture model in PFBC
Structure of this work
Ca in fly ash
Limestone attrition rate
Limestone fragmentation
Model of SO2 capture by CaCO3 under
attrition conditions
SO2 capture model of PFBC
This report summarizes the investigations on limestone behavior in 71MWe PFBC listed below:1. S. Sakuno et al., Nihon-Energy-Gakkai-Shi (J. Jpn. Inst.
Energy), 80, 747(2001)2. Ueda, H. et al., “Fluidization characteristics of PFBC,
attrition and fragmentation of limestone” Proc. 7th SCEJSymp. on Fluidization (Awaji, Japan), 524 (2001)
3. T. Shimizu, et al., Chemical Engineering Science, 56, 6719 (2001)
4. T. Shimizu et al., “A mathematical model of SO2 capture in PFBC”, Proc. 7th SCEJ Symp. on Fluidization (Awaji, Japan), 235 (2001)
5. T. Shimizu et al., To be presented at ISCRE 17 (Hong Kong, 2002)
EPDC’s Wakamatsu 71MWe PFBC Phase-1 test series: Without cyclone ash recirculation Phase-2 test series: With cyclone ash recirculation
MeasurementCoal feed rateLimestone feed rateLimestone size distributionAmount of bed material (limestone)Size distribution of bed materialFly ash drain rateCa content in fly ashSO2 emission
Size of fed limestoneSize<5mm
Fuel : coal
0
20
40
60
80
100
10 100 1000 10000
Dp [µm]
Cum
ulat
ive
wei
ght f
ract
ion
of li
mes
tone
(mea
sure
d) [%
]
Part.1
Evaluation of solid attrition rate and
fragmentation of limestone1)2)
Source of Ca in fly ash1)
(Ca in fly ash)>(fine limestone in feed)+(Ca in coal ash) fine formation by attrition
Run 1-15: Phase-1(W ithout fly ash recycle)Runs 16-33: Phase-2 (with fly ash recycle)
0%
20%
40%
60%
80%
100%1 4 7 10 13 16 19 22 25 28 31Run No.
Sou
rce
of C
ain
fly
ash
[%] F ines formed by attrition
Ca fines in fed limestoneCa in coal ash
Evaluation of limestone attrition rate(fine formation by attrition)=(Drain rate of Ca in fly ash)
- (fine limestone in feed)- (Ca in coal ash)
Surface area of bed material was calculated from size distribution and mass of bed material(Fine formation)/(Surface area) = Rate
Attrition rate
dR/dt = 1 - 2 µm/hr1)
No clear effect of coal type and load on rate.(R: radius)
0.0
0.5
1.0
1.5
2.0
30 40 50 60 70Plant power output [MWe]
Attr
ition
rat
e [
m/h
r]
A model of change in limestone particle size by attrition
A model of change in particle size due to attrition assuming constant attrition rate.
Particle size distribution of fed limestone
Particle size distribution of bed material (BM)
attrition
Bed material size distribution, comparison between model and experimental results1)
Some agreed but the others not. Why?1999/3/18Fuel: BASorbent:T5
0
20
40
60
80
100
100 1000 10000Dp [µm]
Cum
ulat
ive
wei
ght f
ract
ion
ofbe
d m
ater
ial [
%]
ExperModel
1999/12/10Fuel: BA(7)+PC(3)Sorbent:F1(8)+T1.5(2)
0
20
40
60
80
100
100 1000 10000Dp [µm]
Cum
ulat
ive
wei
ght f
ract
ion
ofbe
d m
ater
ial [
%]
ExperModel
Effect of coarse particle content on discrepancy between model and experimental results 1)
With increasing content of coarse particles, the discrepancy became larger.
Fragmentation of coarse particles by thermal shock
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 0.2 0.4 0.6 0.8
Wt-frac. >1.19mm in feed sorbent [-]
dp50
(est
)/dp5
0 (m
eas)
[-] Model = Experiment
Fragmentation of coarse particlesFragmentation of coarse particles was estimated by a model.
Particle size distribution of fed limestone
Particle size distribution of bed material
attrition
Estimated particle size distribution of limestone after fragmentation
Estimation of fragmentation
Fragmentation of coarse particles2)
Fragmentation of coarse particles was estimated by the model. Approx. 80% of coarse (3.4- 5.7 mm) particles was broken into small particles.
0.0
0.2
0.4
0.6
0.8
1.0
20 40 60 80
Plant power output [MW e]
Res
idua
l rat
io3.
36-5
.66m
m [-
]
Formation of smaller particles(0.25-0.5mm)
Phase-1: Only little formation of 0.25 – 0.5mmPhase-2: Nearly all of the fragments 0.25- 0.5mm
BA coal, Open symbols: Phase-1,Closed symbols: Phase-2
-0.6-0.4-0.20.00.20.40.60.81.01.21.4
20 40 60 80
Plant power output [MW e]
For
mat
ion
ratio
0.25
-0.5
0mm
[-]
Formation of smaller particles(0.5-1mm)Phase-1: Nearly all of the fragments 0.5- 0.1 mmPhase-2: Only little formation of 0.5 – 1 mm
BA coal, Open symbols: Phase-1,Closed symbols: Phase-2
-0.5
0.0
0.5
1.0
1.5
2.0
20 40 60 80Plant power output [MW e]
For
mat
ion
ratio
0.5-
1.19
mm
[-]
Summary of Part 1Considerable attrition of limestoneAttrition rate = 1 – 2 µm/hrFragmentation of coarse (>1.2mm ) limestoneSize of smaller particles formed by fragmentation was affected by cyclone ash recycle.
Part.2
A model of SO2 capture by limestone under solid attrition
conditions3)4)5)
Reaction mechanism (in TGA)
CaCO3unreactedcore CaSO4
(product layer)
SO2
Reaction resistance at unreacted core surface
Diffusion resistance through CaSO4 layer
TGA results (in literature): shrinking unreacted core model controlled by both reaction resistance and diffusion resistance through CaSO4 layer.
In actual PFBCs, attrition occurs.Role of attrition in SO2 capture by limestone -unknown. Two possible effects of attrition:
Attrition increases reaction rate by removing CaSO4 layer (diffusion resistance)
Attrition decreases solid utilization efficiency by removal of unreacted CaCO3
→Modeling work is necessary to evaluate the effect of attrition on SO2 capture rate and limestone utilization efficiency
Attrition model
Time
Radius Average slope = -α
Continuous attrition: α=average attrition rateRe= R0 –αt
Two different attrition modes:
0 τ 2τ 3τ
Intermittent (periodical) attrition: Re = R0 – int(t/τ)ατ (ατ<<particle
size)
Results: Initial reaction rate5)
At low SO2 concentrations (<100ppm), attrition mode affects reaction rate.
Conversion = 0 - 0.1
0.001
0.01
0.1
10 100 1000SO2 conc. [ppm]
aver
age
conv
ersi
on ra
tedX
/dt| a
v [1/
hr]
Continuous2hr period5hr period10hr period20hr period
Effect of attrition mode on removal of product layer at low SO2 concentration 5)
Continuous：Only CaSO4 is removed.Intermittent: CaCO3 is also removed.
Continuous IntermittentContinuous, SO2 20ppm
244
246
248
250
0 100 200 300Time [min]
Re,
Rc [
µm]
Re Rc
10hr period, SO2 20ppm
210
220
230
240
250
0 1000 2000Time [min]
Re,
Rc [
µm]
Re Rc
CaSO4
CaCO3
removed byattrition
Effect of attrition rate on limestone utilization efficiency 5)
Continuous: No changeIntermittent: Attrition decreases efficiency.
SO2 100 ppm, Conv. = 0 - 0.1
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2a [µm/hr]
Max
imum
con
vers
ion
[-]
ContinuousPeriod 2hrPeriod 5hrPeriod 10hr
Effect of attrition rate on apparent reaction rate 5)
Continuous: Attrition increases rate.Intermittent: No change in reaction rate.
SO2 100 ppm, Conv. = 0 - 0.1
0.00
0.01
0.02
0.03
0 1 2a [µm/hr]
dX/d
t| av
[1/h
r]
ContinuousPeriod 2hrPeriod 5hrPeriod 10hr
IF continuous attrition occurs in PFBC …Attrition rate = SO2 capture rate
However, in 71MWe PFBC SO2 capture rate was only 1/3 of attrition rate 3).
SO2 capture=attrition rate
0
0.5
1
0 1 2 3 4
Total Ca attrition rate in PFBC[mol/s]
Tota
l SO
2 ca
ptur
e ra
tein
71M
We
PF
BC
[mol
/s]
SO2 capture=0.3x(attrition)
Intermittent attrition model is applied.
Simplified SO2 capture model for intermittent attrition model 3)
Assumptions:
Diffusion resistance >> Reaction resistanceControlled by diffusion through CaSO4
Fresh surface appears when attrition occurs Product layer thickness = 0 when attrition
occurs
Product layer thickness << Particle sizeFlat surface
Simplified rate expression of SO2capture for intermittent attrition model 4)
SO2 capture rate per unit external surface area of limestone:
rS = (2Deρ/Mτ)1/2 C1/2
De: Effective diffusivity
τ: Period of attritionC: Concentration of SO2
ρ/M: Molar density of CaCO3 in limestone
Simplified SO2 capture model in PFBC 4)
Assumptions:
Char-S forms SO2 uniformly in bedSO2 formation rate per unit volume of bed
Bed consists of Geldart’s “D” particlemass transfer resistance from bubble to
emulsion is sufficiently small plug flow model
VM-S forms SO2 at the bottomSO2 concentration at the bottom
Comparison between model and experimental results 4)
Attrition interval was given as a fitting parameter.By giving τ=5 hr, model agreed well with experimental results.
τ =18000s = 5h
0
10
20
30
40
0 10 20 30 40SO2 (calc.) [ppm]
SO
2 (e
xper
.) [p
pm]
Is τ=5 hr appropriate?Best fit between model and experiments was obtained at τ=5 hr.ατ = (1- 2 µm/hr)x(5 hr) = 5 - 10 µm<< Particle size (>250 µm)
For further analysis, measurement of size distribution of Ca-rich fines in fly ash by CC-SEM is necessary.
CONCLUSION Behavior of limestone in 71 MWe PFBC was analyzed.
Attrition rate of limestone was 1-2 µm/hr. Limestone particles greater than 1.2 mm was broken when they are fed into PFBC.Limestone attrition mode (continuous or intermittent) plays significant role in SO2
capture. Continuous attrition model over-estimates SO2 capture in PFBC.
CONCLUSION (continued)Intermittent attrition model agreed PFBC results when period of attrition was given as τ=5hr.To establish complete model, period of attrition should be experimentally determined. Size distribution of Ca-rich particles in the fly ash is necessary for further study.
AcknowledgementT. Shimizu thanks The Ministry of Education, Culture, Sports, Science and Technology for Grant-in-Aids (No.11218204).
The authors express their thanks to The Ministry of International Trade and Industry, Agency of Natural Resources and Energy, and Center for Coal Utilization, Japan, for the financial support for 71MWe PFBC project.
