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Energy Technology Centre
Biomass Combustion Systems:
Assessing Component Durability
and Emissions
Nigel SimmsEnergy Technology Centre
Cranfield University
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Energy Technology Centre
Outline
Background
Potential biomass fuels
Issues
Component durability
Deposition
Corrosion
Emissions
Summary
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Combustion systems Biomass only
Grate
Fluidised bed
Purpose built
Feed systems
Combustion chamber
Heat exchangers
Steam systems
Gas clean-up Scale up to ~30MWe
Biomass co-fired
Pulverised fuel
Designed for coal firing
Biomass additions of 5-20% (thermal input)
Higher steam temperatures
Scale up to 4000MWe (660 MWe units)
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Fireside Steam-side
Corrosion
AllowanceOxidation
Allowance
TUBEWALL
Load Bearing
Thickness
Heat exchanger tubing cross-section through tube wall
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Component life as f(corrosion rate, corrosion allowance)
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
0 10 20 30 40 50 60 70 80 90 100
Corrosion rate (m/1000 hour)
Life
(hours)
1 mm 1.5 mm
2 mm 2.5 mm
3 mm 3.5 mm
Corrosion Allowance
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Flow Diagram for Component Life Modelling
Component
Specification
Operating
Conditions
Fuel Spec. &
Reactor System
Thermal Model Aerodynamic
Model
Thermochemical
Model
Transport & Deposition
Models
Mechanical
Property Data
Corrosion &
Erosion/Corrosion
Models
Life Predictions
Contaminant effects
Operating condition effects
Gas flow rate
P & T distributions
Inlet & outlet
gas P & T
Contaminant
levels & species
Particle
deposition
flux
Component design &
life criteria
Alloy
specificationDeposition flux &
composition
Metal surface
temperature
Component
Geometry
Damage
rates
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Biomass fuels
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Potential biomass fuels Specifically cultivated biomass
(energy crops), e.g.:
coppiced willow
miscanthus
reed canary grass
switchgrass
Waste biomass
various straws wood waste / forest residues
World traded biomass products, e.g.:
olive residues
pelletised wood almond waste
cereal co-product (CCP)
Sewage sludge, animal wastes
Miscanthus
Coppiced willow
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Fuel Properties (1)
0.01
0.1
1
10
100
Wt% wet Wt% dry Wt% daf Wt% daf Wt% daf Wt% daf Wt% daf Wt% daf Wt% daf
Water
content
Ash Volatiles C H O N S Cl
Fuel parameter
%
Willow Poplar
Fir/pine/spruce Miscanthus
Wheat Olive waste
Coal
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Energy Technology Centre
Fuel Properties (2): Minor / Trace Element Concentrations
0
5000
10000
15000
20000
25000
Al Ba Ca Fe K Mg Mn Na P Si Ti
Element
Concentration(mg/kgdry)
Willow Poplar
Fir/pine/spruce Miscanthus
Wheat Olive waste
Coal
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0
2000
4000
6000
8000
10000
12000
0 5000 10000 15000 20000 25000 30000
Potassium (ppm)
Chlorine(ppm)
Sander (1997)Christensen (1998)
Review data
Wheat
Barley
Oil Seed Rape
Potassium and Chlorine Levels in Selected Biomass
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Energy Technology Centre
Sulphur and Chlorine Contents of Fuels Delivered to Two UK Power Stations
Between 1992 and 1994 (Grey is Mean Values for UK Stations in 1983)
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Evaluation of potential heat exchanger
operating conditions
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Energy Technology Centre
Combustion heat exchanger issues - outlineGas stream characteristics
Gaseous species e.g. CO2, SO2, HCl, NOX, H2O, O2, N2 Vapour species e.g. Na, K
Particles From ash in fuel
Condensed vapour species
Gas temperature
Heat exchanger characteristics
Water / steam temperature (& pressure)
Metal temperature (& heat flux)
Deposit
rate of formation
composition Alloy used
corrosion damage rate viable life times ?
cost
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Fuel combustion
Excluded minerals
Mineral inclusions
Pyrolysis
Convective transport
Char burning and
fragmentation
Vaporisation
Homogeneous nucleation
Coagulation
Mineralcoalescence and
fragmentation
Heterogeneous condensation
Fly ash 1-100mm
Reaction
Fly ash
Vapour
Fly ash 0.1-1mm
Fly ash with surface
condensation
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Energy Technology Centre
Effect of Fuel Composition Variability
on Predicted Gas Compositions
0
200
400
600
800
1000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
SOx (vpm)
HCl
(vpm)
UK Coal
Willow wood
Coal - wood
Coal - straw
Wheat straw
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Deposition on Superheater Tubing
Vapour species
Condensation
onto solid particles
& aerosolsSolid particles &
aerosols
Condensation into
deposit
Coarse particles
stick
Thermophoresis of
fine particles
Water /
steam
Corrosion
Heat transfer
SOx
HCl
Vapours, SOx &HCl diffuse in
porous deposits
Vapour species
Condensation
onto solid particles
& aerosolsSolid particles &
aerosols
Condensation into
deposit
Coarse particles
stick
Thermophoresis of
fine particles
Water /
steam
Corrosion
Heat transfer
SOx
HCl
Vapours, SOx &HCl diffuse in
porous deposits
Deposition mechanisms:
Particles:
Direct inertial impaction
Thermophoresis
Eddy diffusion Brownian
Vapour:
Direct condensation
Condensation on particles
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Fuel derived deposit compositionsDeposit compositions:Al-Si-O compounds
can fix Na, K if particle temperatures highenough
Ca/Mg carbonates / sulphatesNa / K sulphates / chlorides
Fe sulphates / chlorides / oxides / sulphides
Phosphates
Important factors
Minerals in fuels
Balance between elements
Corrosion aggravated by:
Low melting point deposits
High chloride deposits
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Effect of fuel compositions
Dependence of deposit chlorine
content on fuel sulphur and alkali
chloride contents
(US DoE Research)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.1 1.0 10.0 100.0
Sulfur /2*Max (Alkali chloride)
DepositCl(%drybasis)
Appearance of chlorides in deposits
as a function of maximum alkali
chloride, fuel sulphur and % straw (onthermal basis)
(EU research)
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Energy Technology Centre
Additives to reduce deposition & corrosion
Deposition / Corrosion of combustion heat exchangers in biomass-fired systems is
regarded as major issue by power plant manufacturers & operators
Vattenfalls latest solution:
dope flue gas with sulphur containing additive (= Chlorout) ahead of superheater
reduces deposition rate, chloride content of deposit and corrosion rate
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Energy Technology Centre
Supergen Bioenergy - Pilot Scale Trials
Gas analysis
Temperature
monitoring
Gas analysis
Temperature
monitoring
Gases to
fan and
flue
Natural gas/excess air
pre-heater
Feed system
Biomass
Pulverised coal/biomass/air
or natural/fuel gas & air
Cooling
water out
Cooling
water in
Cooling
water out
Cooling
water in
Cooling
water in
Cooling
water out
Cyclone
Ash removal
system
Fluidised bedFluidised bed
Temperature
monitoring
Temperature
monitoring
Gas out
Topics being investigated include:
Fuel feeding & preparation
Characterisation of product
streams:
Gas compositions
Bulk gases
Trace species
Solids analyses
Fuel compositions
Ash / char compositions
Deposit compositions /
deposition fluxes oncooled (heat exchanger)
probes
Co-firing
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Energy Technology Centre
Heat exchanger corrosion
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Energy Technology Centre
Possible corrosion mechanismsin chloride / sulphate rich deposits
Fe Fe
Fe
HCl
NaCl
O2
O2
O2
SO2 + O2 + H2O
SO2 + O2
SO2 + O2Na2SO4
Na2S2O7
Na2O
NaHSO4FeCl2
Cl2
FeCl3
Fe2O3 Fe2O3
FeO
FeS
Tube
metal
Corrosion
products
Deposit
HCl
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Energy Technology Centre
Alkali sulphate dominated corrosion regimes incombustion gases
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Energy Technology Centre
Corrosion model requirements Cover wide range of operating environments
Different biomass fuel
Superheater / reheater, evaporator, water walls, etc
Corrosion damage (in terms of metal loss) as function of: Metal temperature
Gas composition (e.g. SOx, HCl, O2, CO2, H2O)
Deposit composition
Na, K Sulphate vs chloride
etc
Deposition flux (mg/cm2/hour)
Time Median vs maximum metal loss
Component life criteria
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Energy Technology Centre
Corrosion data for model developmentAim: simulation of different specific environments
data on materials performance obtained using dimensional metrology
Laboratory based corrosion data generation:
Deposit re-coat test method Controlled atmosphere furnace
Test Variables Temperatures
Time
Gas composition:
simulated combustion gases
Variable SOx, HCl, O2, CO2, H2O, etc levels
Deposit composition
Simulated ash
(Na/K)Cl (Na/K)2SO4 Variable Na/K levels
Materials
Dimensional metrology before/after exposure
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Energy Technology Centre
Controlled atmosphere corrosion furnace
Gas mixture 2
(e.g. N2-O2-SO2)Stainless steel
containment vesselAlumina
reaction tube
SamplesMass flow
Controller 2
Mass flow
Controller 1
Inert safety
gas (N2)
Safety
gas vent
Alumina tube
Alumina heat
shieldsGas mixture 3
(e.g. N2-O2)
Water
bath
De-ionised water
Trace heating
Mass flow
Controller 3
Vent
Gas clean-upsystem
Gas mixture 1(e.g. N2-HCl)
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Energy Technology Centre
Surface scale& deposit
A1
A 2
An
B 1
B n
Internal
corrosion
To central
reference
point
Alloy
Where n = 24
Measurements
taken at equidistant
points spaced =
300m Surface scale& deposit
A1
A 2
An
B 1
B n
Internal
corrosion
To central
reference
point
Alloy
Where n = 24
Measurements
taken at equidistant
points spaced =
300m
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
-8000 -6000 -4000 -2000 0 2000 4000 6000 8000
X DIRECTION (MICRONS)
YDIRECTION
(MICR
ONS
ORIGINAL METAL CHANGE IN METAL
CHANGE IN GOOD METAL DEPOSIT & SCALE
100 micron contour
1-HAA-6
Sample Metrology & Data Analysis (1)
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Energy Technology Centre
-100
-80
-60
-40
-20
0
0 90 180 270 360
Position around sample ()
Changeinsoundmetal(um)
Data ordered and
plotted againstprobability
Sample Metrology & Data Analysis (2)
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Energy Technology Centre
Corrosion damage distributions for 2 Cr steel, 347HFG andAlloy 625 all at 560C, deposit D6 and gas 3
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0
20
40
60
80
100
120
D0 D1 D2 D3 D4 D5 D6 D7 D8
Deposit
Corrosiondamage(m/1000hours
)
Gas 3, 560C (test 6)
Gas 3, 600C (test 3)
Sensitivity of 347HGF to changes in deposit composition andexposure temperature
(Corrosion Damage Evaluated at the 10% Probability of Damage Being Exceeded)
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Energy Technology Centre
Correlation between Measured Predicted Corrosion Damage Rates(corrosion damage evaluated at the 10% probability of damage being exceeded)
10
100
1000
10 100 1000
Measured corrosion rate (m/1000 hours)
Predictedcorrosionrate
(m/1000hours)
2.25 Cr
1 Cr
X20
AISI 347
625
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Energy Technology Centre
Emissions
Limits set by regulations vary by:
Plant capacity
Fuels
Policy decisions etc
Issues can include:
NOX SO
X HCl
Dust
Trace heavy metals
Etc
Range of technologies developed for:
Coal
Waste
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Energy Technology Centre
Examples of particle removal systems
+ + +++
+ + +++
- - ---
Uncharged
particles
Electric field
Charged
particles
Gas Inlet Cleaned
Gas
Electrostatic Precipitator (ESP) Bag Filters
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Energy Technology Centre
SummaryRange of potential biomass fuels being considered for
combustion systems
Heat exchanger durability Care is required in materials selection for biomass systems
Balance between fuel compositions, operating temperatures
(system efficiency), component life and materials costs
Predictive models being developed within SupergenBioenergy project relating exposure conditions to component
lives for biomass fired systems
Emissions
Control measured developed for other fuels (coal, waste) Appropriate technologies need to be selected to match
intended fuel / regulation
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Energy Technology Centre
Thank you for your attention
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Energy Technology Centre
Targets for heat exchanger steam operating temperatures andtube lifetimes from COST522 and COST538 programmes
50
50
100
20
Maximum
acceptablecorrosion rate
(m/1000 hours)
530 550
610 630
610 630
680 700
Metal
temperature
(C)
40,000
40,000
20,000
100,000
Target
lifetime
(hours)
500Waste
580Wood
580Straw
650Coal
Desired maximum
steam temperature
(C)
Fuel