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.