fuels 101 for circulating fluidized bed boilers

Fuels 101

for

Circulating Fluidized Bed Boilers

ARIPPA Technical SymposiumAugust 2007

Neil Raskin, Services DepartmentFoster Wheeler North America Corp.

Clinton, NJ

ARIPPA Technical Symposium – August 2007

Fuels 101

Feature

Low Furnace Temperatures

Hot CirculatingSolids

Long SolidResidence Time

Benefit

Low NOx

In Bed SO2 Capture

Fuel Flexibility

Tolerant to Fuel Variations

Simple Feed SystemsUniform Heat Flux

Ideal for SNCR

Good Fuel BurnoutGood Sorbent

Utilization

1500-1700oF

15-16 ft/sec

Air

Limestone1/20” x 0

Air

Flue Gas1500-1700oF

Fuel1/2” x 0

Features and Benefits of CFB Boilers


Fuels 101

Front wall feed system

Front and rear wall feed systems

Typical CFB Boiler Fuel Feed Systems


Fuels 101F

ue

l H

igh

er

He

ati

ng

Va

lue

(b

tu/l

b)

10,000

15,000

FUEL RANK

Peat

Bark

Multiple Challenges Some Challenges No Challenge

WOOD BIOMASS

Demolition Wood

Fiber Residue

Industrial PDF

Commercial PDF

Chip Board

Polyolefin Plastics

Colored or

Printed Mixed

PlasticsRF Pellets

Plywood

2,000

5,000

PVC

RDF

PVC

Bituminous Coals

Brown Coals, Lignite

Standard Design

Petroleum Coke1 - Consumer PDF Mixed Plastics

1 - Consumer PDF Mixed Plastics

2 - Consumer PDF Wood & Plastics

2 - Consumer PDF Wood & Plastics

Colored or

Printed Mixed

Plastics

3 - Consumer PDF Wood & Paper

3 - Consumer PDF Wood & Paper

3

2

1

MSW

CFB Boiler Fuel Design Challenges


• Oil• Oil Shale• Refinery Bottoms• Natural Gas• Peat• Agricultural Waste

• Wood Waste

• Rice Husks

• Bagass Pith

• Tires• Sludge (Fiber)• Refuse Derived Fuel

• Coal• Anthracite

• Bituminous

• Sub-Bituminous

• Lignite

• Waste Coal• Bituminous Gob

• Anthracite Culm

• Petroleum Coke• Delayed

• Fluid

Fuels 101

Fuels Successfully Combusted within CFB Boilers


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 >9 % within the fuel ash.


Fuels 101

Fuels Not Recommended for CFB Boilers

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


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.

Fuels 101

Start-up and Auxiliary Fuels for CFB Boilers


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.


W1

D1

PLAN AREA = 1.00

H1

Medium Volatile

Bituminous

D1

1.04 H1

PLAN AREA = 1.05

High Volatile Bituminous or

Sub-Bituminous

1.05W1

1.08 H1

D1

PLAN AREA = 1.20

Lignite

1.20 W1

Fuels 101

Fuels Affect Sizing of CFB Boilers1

Note 1: Similar capacity and steam output conditions


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.

Fuels 101

Fuel Sizing for CFB Boilers


Fuels 101


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.


Fuels 101



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

Fuels 101

Successfully Combusted Fuel Property Ranges for CFB Boilers


Fuel Max Recommended Total Moisture1 (%)

Bituminous Gob 8

Anthracite, Low & Med Volatile 10 - 12Bituminous, 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.

Fuels 101

Maximum Recommended Total Fuel Moisture


Fuels 101

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).

Fuel Induced Agglomeration and/or Fouling


Fuels 101

Fuel Induced Agglomeration and/or Fouling

Agglomeration/Fouling Total Na1 in Total Ash2 (% by wgt) Potential Low Medium High

Biofuel <4.5 4.5-9.0 >9.0Lignite & Coal <0.4 0.4-0.7 >0.7Other <1.0 1.0-3.5 >3.5

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


Fuels 101

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.

Fuel Induced Erosion and Corrosion


Fuels 101


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.


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.

Fuels 101



Fuels 101


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.


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 <0.10%.

Fuels and limestones that contain a total chlorine value >3.0% (wt %, moisture free basis) are considered to be a high corrosion potential and should be avoided.

Fuels 101



Fuel Type Bed Make-up System Requirements

Med/High Sulfur and NoneMed/High Ash Fuels

Low Sulfur and Low RecommendedAsh Fuels1

Biofuels, TDF Required

Fuels 101

The percentage of ash and % sulfur reduction required will determine the requirements for a bed make-up system.

Note 1: Without limestone feed and ash content <12%

Bed Make-up System Requirements


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 boiler’s back-pass and/or within the baghouse or ESP.

Metals tend to concentrate within the baghouse’s and/or ESP’s fine ash and normally do not leach.


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.


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


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 <2%Moisture 1 – 5%Vanadium 500 – 3,000 ppmNickel 500 – 3,000 ppm

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 >10lb/MMBtu fired.

Fuels 101



Petroleum Coke with vanadium levels <1000 ppm (0.10 wt% in d.s.) have been successfully combusted.

Petroleum Coke with vanadium level >3000 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

Fuels 101



Fuels 101


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.


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.


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.


Fuels 101

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.



Fuels 101


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).


98.5 % 98.2 %96.6 % 97.2 % 96.5 %

98.4 %95.5 %

93.6 %

83.7 % 83.7 %

96.4 %

50.0 %

60.0 %

70.0 %

80.0 %

90.0 %

100.0 %

Oil

sh

ale

(4

)

Bit

um

ino

us

co

al

(7)

Bro

wn

co

al

(6)

An

thra

cit

e +

bio

(2

)

Pe

at

+ w

oo

d(7

)

Wo

od

ch

ips

/b

ark

(3

)

Fo

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(4

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Re

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MS

W (

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All

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(3

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ou

rs

Fuels 101

Fuel Type and Annual Operating Hours


122 144 280 235

280117

275

548

1134

1399

281

0

2190

4380

6570

8760

Oil

sh

ale

(4

)

Bit

um

ino

us

co

al

(7)

Bro

wn

co

al

(6)

An

thra

cit

e +

bio

(2

)

Pe

at

+ w

oo

d(7

)

Wo

od

ch

ips

/b

ark

(3

)

Fo

res

try

res

idu

es

(4

)

Re

cy

cle

dw

oo

d (

1)

Ind

us

tria

lw

as

te (

1)

Re

fus

e d

eri

ve

dfu

el,

MS

W (

1)

All

CF

Bs

(3

6u

nit

s)

An

nu

al h

ou

rs

FORCED OUTAGE HOURS

ACTUAL OPERATION HOURS

Fuels 101

Fuel Type and Forced Outage Hours

fuels 101 for circulating fluidized bed boilers

Documents

auxiliary fuels

startup fuels

similar fuels

safe fuels

common fuels

high ash fuels

low ash fuels

fuel sizing