Stoker Boiler Model
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Transcript of Stoker Boiler Model
Biomass Cofiring Overview
Larry BaxterBrigham Young University
Provo, UT 84602
Second World Conference on Biomass for Energy,Industry, and World Climate Protection
May 10-14, 2004Rome, Italy
Biomass Energy Economics
Typical biomass Cost(US$ per ton)
Cost of Electricity compared tofeedstock prices,
with various conditions,incentives, or subsidies
Typical Cost of Energy from Conventional Co-firing Combustion
Acknowledgement: Graph provided by Antares Group Inc
PTC – proposed production tax credit
Incentive, e.g., Green Pricing Premium
US Commercial Experience• Over 40 commercial demonstrations• Broad combination of fuel (residues, energy crops,
herbaceous, woody), boiler (pc, stoker, cyclone), andamounts (1-20%).
• Good documentation on fuel handling, storage,preparation.
• Modest information on efficiency, emissions,economics.
• Almost no information on fireside behaviors, SCRimpacts, etc.
Major Technical Cofiring Issues• Fireside Issues
• Pollutant Formation• Carbon Conversion• Ash Management• Corrosion• SCR and other
downstream impacts
• Balance of ProcessIssues• Fuel Supply and
Storage• Fuel Preparation• Ash Utilization
Lab and field work indicate there are noirresolvable issues, but there are poor
combinations of fuel, boiler, and operation.
Fuel Properties
2.0
1.5
1.0
0.5
0.0
H:C
Mol
ar R
atio
1.00.80.60.40.20.0
O:C Molar Ratio
SemianthraciteBituminous Coal
Subbituminous CoalLignite
Anthracite
Cellulose
Average BiomassWood
Grass
Lignin
anthracitebituminous coalsubbituminous coalsemianthracitelignitebiomass
average values
NOx Behavior Complex (No Surprises)
200
150
100
50
0
Axia
l dist
ance
(cm
)
-20 0 20Radial distance (cm)
500
450
450
450 450
450
400
400
400
400
400400
400
350350350
350
350 35
0 35
0
350
300
300
250
200 20
0
150
150
100100
50
50
200
150
100
50
0
Axia
l dist
ance
(cm
)-20 0 20
Radial distance (cm)
450450
450
400400
400
400
400
400
400400400
400
400
350
350
300 250250200200
150150100
10050 50
200
150
100
50
0
-30 -20 -10 0 10 20 30
600
600
600600
550
550 550
550500
500
450
450
400
400
400350
350
350
300
300
250250
200
150
100 1001005050
Straw (φ = 0.6) Coal (φ = 0.9) 70:30 Straw:Coal (φ = 0.9
NO
NH3
Combustion History: Switchgrass
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3 3.5 4
Vol
ume
(mm
3 )
Time (s)
Char Oxidation
Devolatilization
Heat &Dry
Initial nominal diameter = 3 mm
Particle Shape Impacts
0.0 0.1 0.2 0.3 0.4 0.5
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0M
ass
Loss
, daf
Residence Time, s
flake-like exp.flake-like modelcylinder-like exp.cylinder-like modelnear-spherical exp.near-spherical model
Reaction Time vs. Yield
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
5
10
15
20
0
5
10
15
20C
onve
rsio
n Ti
me,
s
Equivalent Diameter, mm
flake-likecylinder-likenear-spherical
aspect ratio:flake-like - 4.0 (width/thickness)cylinder-like - 6.0near-spherical-1.65
Cofiring Deposition
Deposition Rates Vary Widely• Cofiring biomass can
lead to either decreaseor increase in depositionrates.
• Cofiring decreasesdeposition relative toneat fuels.
0.01
0.1
1
10
100
Dep
ositi
on R
ate
(gm
dep
osit/
kg fu
el)
Woo
d
Sw
itchg
rass
Str
aw
Whe
at S
traw
Pitt
sbur
gh
#8
Eas
tern
Ken
tuck
y
Commercial Stoker
Slag Screen
Secondary Superheater
Primary Superheater
Boiler Generator Bank
Stokers
Overfire Air
Grate
Stoker
Fuel Bin
1
2 3
4
5
Deposits Dissimilar to Fuel
SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P2O5 SO30
10
20
30
40
50
60
Mas
s Pe
rcen
t [-]
Fuel
Ceiling/Corner Deposit
Composition Maps Support CorrosionHypothesis
Cl S Fe
100% Imperial Wheat Straw
85% E. Kentucky 15% Wheat Straw
Fuel Properties Predict Corrosion
Increasing Time
Oxygen Isosurfaces
BL mechanisms
BL deposition flux [g/m2/h]Inertial deposition flux [g/m2/h]
Vapor deposition
Vapor deposition flux [g/m2/h]
Flyash Impacts on Setting Time
Penetration Resistance vs. Time
-1000
0
1000
2000
3000
4000
5000
6000
0 100 200 300 400 500 600 700 800
Time (min)
Pene
trat
ion
Res
ista
nce
(psi
)
Pure ConcreteClass FWoodWood CWood FBiomass 1Biomass 2Class C
Freeze Thaw CyclesRelative Dynamic Modulus of Elasticity (%) vs Freeze-
Thaw Cycles
8486889092949698
100102
0 50 100 150 200 250 300
Number of Cycles
Rel
ativ
e D
ynam
ic M
odul
us o
f El
astic
ity (%
)
Class F1Wood 1Wood C1Wood F1
Required Aerating Agent
0
0.5
1
1.5
2
2.5
oz/1
00 lb
s ce
men
t
Pure Cement
Class C Fly Ash (25%)
Class F Fly Ash (25%)
Co-fired Fly Ash (25%) (10% switchgrass)
Co-fired Fly Ash (25%) (20% switchgrass)
Surface Conditions of Catalyst
0
0.2
0.4
0.6
0.8
1
1.2
1.4N
orm
aliz
ed C
once
ntra
tion
Fres
h(1)
Fres
h(2)
Expo
sed(
1)
Expo
sed(
2)
Det
ectio
nLi
mit
CaOSSO3Na2OV2O3
Basic Compounds Poison Catalysts
Catalyst Activity vs. Na Poison Amount
0.000.100.200.300.400.500.600.700.800.901.00
0 0.5 1 1.5 2 2.5 3
Poison Ratio (Na:V)
Act
ivity
(k/k
0)
BYU wetBYU dryChen et al.
Field Tests Indicate Little Poisoning1.0
0.9
0.8
0.7
0.6
0.5
Frac
tiona
l Con
vers
ion,
X
140001200010000800060004000
Space Velocity (hr-1)
X NO fresh IX NH3 fresh IX NO fresh IIX NH3 fresh IIX NO exposed frontX NH3 exposed front
Conclusions• Major technical issues include fuel handling, storage,
and preparation; NOx formation; deposition; corrosion;carbon conversion; striated flows; effects on ash;impacts on SCR and other downstream processes.
• Importance of these issues depends strongly on fuel,operating conditions, and boiler design.
• Proper choices of fuels (coal and biomass) andoperating conditions can minimize or eliminate mostimpacts for most fuels.
• Ample short-term demonstrations illustrate fuelhandling feasibility. Paucity of fireside and long-termdata.
Summary Cofiring Statements•Cofiring has been demonstrated succesfully in over 150installations worldwide for most combinations of fuelsand boiler types.•Cofiring offers among the highest electrical conversionefficiencies of any biomass power option.•Cofiring biomass residues in existing coal-fired boilersis among the lowest cost biomass power productionoptions.•Well-managed cofiring projects involve low technicalrisk.
Cofiring biomass in existing coal-fired boilers providesan attractive approach to nearly every aspect of project
development.
Outline• Introduction• Success stories• Statements• R&D&D for improvement
• Long term experience• Fireside measurements in commercial scale facilities• SCR deactivation• Fly ash utilization• Deposition and corrosion• Striated flows• Fuel specifications, preparations and limitations• Public awareness/image• Increasing cofiring percentages
Acknowledgements• Financial support provided by the DOE/EE, EPRI,
NREL, BYU, a dozen individual companies.• Work performed by research group including four other
faculty members, two post docs, ten graduate students,30 undergraduate students.