Fuels 101

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Fuels 101 for Circulating Fluidized Bed Boilers
Neil Raskin, Services Department Foster Wheeler
North America Corp. Clinton, NJ
ARIPPA Technical Symposium August 2007
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Fuels 101
Features and Benefits of CFB Boilers
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Fuels 101
Typical CFB Boiler Fuel Feed Systems
Front wall feed system
Front and rear wall feed systems
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Fuels 101
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Fuels 101
Fuels Successfully Combusted within CFB Boilers
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Fuels not recommended to be combusted alone in a
CFB boiler. However, these fuels can be
combusted in small percentages mixed with other
safe fuels
Fuels 101
Fuels Not Recommended for CFB Boilers

• Animal Manure
• Chicken Litter
• Plastics
• PVC
• Sewage Sludge
• High Alkali content fuels Agro Wastes, Short
  
• Rotation Wood, Energy Crops, and Bagasse.
• Note 1 Fuels defined as high alkali content
  have sodium (Na) values gt9 within the fuel ash.

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Municipal Solid Waste (MSW) is not recommended to
be combusted, even when mixed with a safe fuel,
within a CFB boiler.

Fuels 101
Fuels Not Recommended for CFB Boilers
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Fuels 101
Start-up and Auxiliary Fuels for CFB Boilers
Startup fuels should ignite easily and have
stable flames. Common fuels for duct and
start-up burners natural gas, propane, and heavy
or light oils. Total capacity of these burners
is a maximum of 40 of MCR. Similar fuels can
be used as auxiliary fuels and are fired using
bed lances, maximum load carrying capacity of
30 of MCR load.
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Fuels 101
Start-up and Auxiliary Fuels for CFB Boilers
Cold bed startup using natural gas and/or liquid
fuels will result in quite high VOC and CO
emissions. The high VOC and Co emissions are due
to the quenching of the burner flame by the
cooler bed material. This can be partially
mitigated, but not entirely eliminated by
pre-heating the bed with a duct burner.
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Fuels 101
Fuels Affect Sizing of CFB Boilers1
Note 1 Similar capacity and steam output
conditions
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Fuels 101
Fuel Sizing for CFB Boilers

• Fuel sizing is determined by fuel volatility, ash
  content, and friability.
• Correct sizing insures
• Good fluidization
• Maximize combustion efficiency
• Bed quality maintenance
• Smooth and reliable operation of the fuel feed
  and
• bottom ash removal systems.

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Fuels 101
Fuel Sizing for CFB Boilers
High volatile and low ash fuels Lignite,
Sub-Bituminous, High Volatile Bituminous,
Biofuels, Wood, Peat, Pet Coke are quite reactive
and can be burned using larger particle
sizes. Lower volatile and high ash fuels Low
Volatile Bituminous, Anthracite, Waste require
additional crushing to increase the reactive
surface of the coal particles. Bio and waste
fuel sizing is determined not by fuel
volatility, but by the requirements for stable
fuel feed system operation and to prevent after
burning.
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Fuels 101
Fuel Sizing for CFB Boilers
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Fuels 101
Fuel Sizing for CFB Boilers
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Fuels 101
Successfully Combusted Fuel Property Ranges for
CFB Boilers
Property Range Fuel Moisture max
60 Peat, Sludge, Wood, Biofuel Ash max
70 Waste Coals Sulfur max 6 Waste
Bit Coal, Pet Coke Volatiles min
2 Pet Coke HHV min 2,600 Btu/lb Sludge
(Fiber), Waste Coals
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Fuels 101
Maximum Recommended Total Fuel Moisture
Fuel Max Recommended Total
Moisture1 () Bituminous Gob
8 Anthracite, Low Med Volatile 10 -
12 Bituminous, Pet Coke, and Anthracite Culm High
Volatile Bituminous 15 Sub Bituminous
30 Lignite 45 Peat, Sludge (Fiber),
Wood, and Biofuels 55 - 60
Note 1 Total Fuel Moisture is equal to
inherent plus surface moisture. Surface
moisture should be limited to a maximum value of
8-10 to prevent sticking/pluggage problems.
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Fuels 101
Fuel Induced Agglomeration and/or Fouling
Sodium (Na) and potassium (K) within ash can
cause agglomeration and/or fouling of boiler
surfaces. The potential is increased when
these constituents are combined with other oxides
ash constituents silica (SiO2), iron (Fe2O3),
and phosphorus (P2O3).
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Fuels 101
Fuel Induced Agglomeration and/or Fouling
Agglomeration/Fouling Total Na1 in Total
Ash2 ( by wgt) Potential
Low Medium High Biofuel
lt4.5 4.5-9.0 gt9.0 Lignite Coal
lt0.4 0.4-0.7 gt0.7 Other
lt1.0
1.0-3.5 gt3.5
Note 1 Total Na ( by wgt) Na ( by wgt)
K/1.7 ( by wgt) Note 2 Total ash fuel ash
limestone enerts calcination and sulfation
reaction products make-up bed material
additives
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Fuels 101
Fuel Induced Erosion and Corrosion

• Fireside metal wastage of pressure and
  non-pressure parts is significantly affected by
  fuel and/or ash properties.
• The most significant properties affecting
  fireside metal wastage are
• Mineral type (quartz)
• Size
• Hardness
• Particle shape
• Potential deposit formation and associated metal
  corrosion.

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Fuels 101
Fuel Induced Erosion and Corrosion
Total ash within the fuel is not the only major
factor affecting erosion. The ash
constituents and their relative concentration,
and the size of angular mineral matter such as
quartz can greatly affect erosion within a CFB
boiler.
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Fuels 101
Fuel Induced Erosion and Corrosion
Generally it is believed that larger particles
increase erosion, although the effect has not
been quantified. Ash containing large amounts
of iron (Fe) may increase the aggressiveness of
soft, non-erosive calcium (Ca) when combined
together.
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Fuels 101
Fuel Induced Erosion and Corrosion
Large amounts of silica (Si) within an ash can
act as an aggressive component. The physical
shape of the ash particles, i.e. knife edge or
chisel point can make an ash very erosive.
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Fuels 101
Fuel Induced Erosion and Corrosion
Chlorides (Cl) within the fuel and limestone
(naturally or from transportation) combined with
sulfur (S) can promote corrosion of boiler metal
surfaces (pressure and non-pressure parts),
producing a combined corrosion and erosion
affect. To limit the potential for corrosion
within a CFB boiler, the amount of total chlorine
(wt , moisture free basis) within the fuel and
limestone should be limited to lt0.10. Fuels
and limestones that contain a total chlorine
value gt3.0 (wt , moisture free basis) are
considered to be a high corrosion potential and
should be avoided.
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Fuels 101
Bed Make-up System Requirements
The percentage of ash and sulfur reduction
required will determine the requirements for a
bed make-up system.
Fuel Type Bed Make-up System
Requirements Med/High Sulfur and
None Med/High Ash Fuels Low
Sulfur and Low Recommended Ash
Fuels1 Biofuels, TDF Required
Note 1 Without limestone feed and ash content
lt12
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Fuels 101
Fuel Metals
Heavy metals, such as mercury (Hg) have low
melting points and high vapor pressures and are
vaporized during the combustion process within
the combustor. These vapors condense within the
boilers back-pass and/or within the baghouse or
ESP. Metals tend to concentrate within the
baghouses and/or ESPs fine ash and normally do
not leach.
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Fuels 101
Biofuels

• Biofuels (biomass) are broken down into the
  following categories
• Sawmill wastes and by-products of lumber
  production
• Pulp and paper mill waste principally bark
• Agricultural waste
• Forest residue
• Biofuels generally have high volatiles (75 85
  wt), low ignition temperatures (400oF), varying
  moisture contents (20 60 wt ), and normally
  are low in ash and sulfur content, but
  potentially have a high chlorine (corrosion) and
  alkali (fouling and slagging) content.

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Fuels 101
Biofuels
Biofuel combustion is considered to be CO2
neutral. Due to the potential for carryover
of burning embers it is recommended to use a
precipitator and not a baghouse for particulate
control
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Fuels 101
Petroleum Coke (Delayed/Fluid)

• Petroleum Coke comes in two basic types
• Delayed Coke Shot (buck shot) and Sponge
  (angular)
• Fluid Coke.
• Typical fuel characteristics
• Delayed Coke Fluid Coke
• Volatiles 8 15 5
• Sulfur 3 8
• Ash lt2
• Moisture 1 5
• Vanadium 500 3,000 ppm
• Nickel 500 3,000 ppm

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Fuels 101
Petroleum Coke (Delayed/Fluid)
Petroleum Coke that contains the metal vanadium
(V) will generate low-melting compounds causing
fluidization problems within the solids
circulating loop and corrosive in the back pass.
Inert bed material addition is recommended to
resolve any ash agglomeration and deposition
problems when firing 100 Petroleum Coke. The
inert bed material addition should be equivalent
to 2 ash in the fuel or gt10lb/MMBtu fired.
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Fuels 101
Petroleum Coke (Delayed/Fluid)
Petroleum Coke with vanadium levels lt1000 ppm
(0.10 wt in d.s.) have been successfully
combusted. Petroleum Coke with vanadium level
gt3000 ppm (0.30 wt in d.s.) should not be
combusted within a CFB boiler. Vanadium
resistant refractories should be used when
combusting Pet Coke
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Fuels 101
Petroleum Coke (Delayed/Fluid)
Delayed Coke is similar in size to coal and can
use similar material handling systems. Fluid
Coke should be pneumatically fed due to its small
particle size as received from the coking process.
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Fuels 101
Pulp and Paper Industry Sludge

• The quality and type of sludge (fiber) from the
  pulp and paper industry that have been fired in
  CFB boilers are as follows
• Chemical Pulp
• Mechanical Pulp
• Deinking Pulp
• Paper Board
• The sludge constituents will also vary
• HHV 2,150 8,600 Btu/lb (in d.s.)
• Ash 5 55 in d.s.
• Carbon 20 50 in d.s.
• Chlorine 0.1 0.3 in d.s.
• Na tot 0.2 - 0.4 in d.s.
• K tot 0.2 0.4 in d.s.

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Fuels 101
Tire Derived Fuel (TDF)

• Tire derived fuels (TDF) are composed mainly of
  rubber, carbon black, and oil. The most common
  types are
• Nylon cord
• Polyester cord
• Fiberglass belted (0.6 1.0 wt )
• Steel belted (5 -10 wt )
• Tire Derived Fuel (TDF) has been fired as a
  supplemental fuel, with a maximum heat input of
  20 of MCR load when bead wire is present.

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Fuels 101
Tire Derived Fuel (TDF)

• Typical analysis
• HHV 15,000 BTU/lb
• Moisture 0.7 wt
• Volatile 62.8 wt
• Ash 11.2 wt
• Fixed Carbon 25.3 wt
• Metals 8 10 wt
• The main mechanical operational concern is the
  removal of the bead and belt wires, which
  have a tendency to form birds nests within the
  combustor. Fuel sizing of 1 x 1, or smaller,
  is preferred.

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Fuels 101
Tire Derived Fuel (TDF)
Fouling can occur due to zinc (Zn), titanium
(Ti), and silica (Si) and other oxides. Zinc
(Zn) may form with chlorides (Cl) within the main
fuel to produce a corrosive compound, zinc
chloride (ZnCl2).
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Fuels 101
Fuel Type and Annual Operating Hours
100.0
98.5
98.4
98.2
97.2
96.6
96.5
96.4
95.5
93.6
90.0
83.7
83.7
80.0
Annual hours
70.0
60.0
50.0
wood (1)
Recycled
Industrial
waste (1)
Bituminous
coal (7)
Forestry
residues (4)
Oil shale (4)
Anthracite
bio (2)
(7)
units)
Peat wood
All CFBs (36
Wood chips /
bark (3)
Brown coal (6)
Refuse derived
fuel, MSW (1)
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Fuels 101
Fuel Type and Forced Outage Hours