Biomass and Coal Characteristics: Implications

for Cofiring

David A. Tillman

Foster Wheeler Power Group, Inc.

Clinton, NJ 

 

AbstractWoody and herbaceous biomass fuels exhibit distinct and separate characteristics with respect to bulk chemistry and behavior; further these fuels are fundamentally different from the various coals used in power generation. Detailed characterization of sawdust, urban wood waste, fresh switchgrass, and weathered switchgrass demonstrates the critical—and sometimes subtle—differences between these fuels. Critical among these differences is fuel reactivity measured both in maximum volatile yield and rate of fuel devolatilization. Of additional importance is the reactivity of the fuel nitrogen in the various biomass fuels. Detailed characterization of fuels demonstrates the fundamental differences in combustion characteristics between the various biomass fuels and the various coals burned in utility power plants. These differences can be used to explain the outcomes of cofiring in pulverized coal boilers—particularly the potential for simultaneous reduction of NOx, unburned carbon in flyash, and CO emissions. Using drop tube furnace data developed by The Energy Institute of The Pennsylvania State University, and field data from various biomass cofiring projects, this paper uses the detailed fuel characteristics to identify critical combustion mechanisms occurring during cofiring of various biomass fuels and coal in pulverized coal boilers.

Basis of the Analysis• Fuel Characterization Research at The Energy

Institute of Pennsylvania State University– Proximate and Ultimate Analysis– Drop Tube Reactor Testing (400 – 1700oC)

» Determine maximum volatile release» Determine fuel reactivity» Determine nitrogen and carbon volatile

release– 13C NMR Testing

• Develop Relationships to Full Scale Cofiring Testing

Focus of the PSU Research

• Nitrogen Evolution from Solid Fuels Governs NOx Formation from Fuel Nitrogen

– NOx Control is Favored by Volatile Nitrogen

– NOx Control is Favored by Nitrogen Rapidly Evolving from the Fuel Mass

• Understanding Nitrogen Evolution Patterns can Assist in Explaining NOx Reduction with Biomass and Low Rank Coals

• Understanding Nitrogen Evolution Patterns for a Given Suite of Fuels can Influence Fuel Selection

Support for this Research• USDOE – NETL and USDOE – EERE in

Sponsoring Biomass Cofiring Technology Assessment

• USDOE – NETL, USDOE – EERE, and EPRI in Sponsoring Cofiring Research and Demonstration Projects with a Variety of Coals in Cyclone and PC Boilers

– Albright Station, Willow Island Station– Bailly Station, Michigan City Station– Seward Station, Shawville Station– Allen Fossil Plant, Colbert Fossil Plant

Background: Previous Studies

• Baxter et. al., 1995. Seminal Paper on Nitrogen Evolution from Coals as a Function of Residence Time

• Research for USDOE and EPRI, Sponsored by USDOE and Performed by The Energy Institute of Pennsylvania State University and by Foster Wheeler Power Group, Inc.

Methodology - 1• Select Representative Biomass Fuels

– Sawdust– Urban Wood Waste– Fresh Switchgrass– Weathered Switchgrass

• Basis of Selection– Commonly used in cofiring applications– Represent woody and herbaceous biomass

• Select Reference Coals– Black Thunder [PRB]– Pittsburgh #8

Methodology - 2

• Sawdust source: West Virginia [Willow Island Cofiring Project]

• Urban Wood Waste source: produced from a blend of plywood, particleboard, and paneling to be highly similar to the urban wood waste at Bailly Generating Station, with particular attention to nitrogen content

• Weathered Switchgrass source: Gadsden, Alabama [Southern Co. and Southern Research Institute Cofiring Project]

• Fresh Switchgrass source: Southern Co. and Auburn University

Methodology - 3• Characterize the Incoming Fuel

– Proximate and Ultimate Analysis– Heating Value

• Air Dry and Grind Fuel• Pyrolyze Fuel in Drop Tube Reactor (DTR)

– 400oC – 1700oC– Argon Environment

• Determine Distribution of Nitrogen in Incoming Fuel (volatile N vs char N)

• Determine Nitrogen, Carbon, and Total Volatile Evolution as a Function of Temperature

Methodology - 4• Basic Premise:

If nitrogen is in volatile form, and if nitrogen volatiles evolve more rapidly than carbon volatiles or total volatile matter, then NOx formation is more easily controlled by combustion mechanisms

If nitrogen is in char form, or if nitrogen volatile evolution lags behind carbon volatile evolution or total volatile evolution, then NOx formation control by combustion mechanisms is more difficult and less effective

Analysis of Biomass Fuels Parameter Fuel Fresh

Mixed Sawdust

Urban Wood Waste

Fresh Switchgrass

Weathered Switchgrass

Proximate Analysis (wt % dry basis) Volatiles 80.0 76.0 76.18 80.93

Fixed Carbon 19.0 18.1 16.08 18.34 Ash 1.0 5.9 7.74 0.73

Ultimate Analysis (wt % dry basis) Carbon 49.2 48.0 46.73 51.44

Hydrogen 6.0 5.5 5.88 5.97 Nitrogen 0.3 1.4 0.54 1.45

Sulfur <0.1 0.1 0.13 0.04 Oxygen 43.0 39.1 38.99 40.36

Ash 1.0 5.9 7.74 0.73 HHV (Btu/lb) 8400 8364 7750 8150

Distribution of Fuel Nitrogen

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Distribution of Fuel Nitrogen by Type (lb/MMBtu)

Urban wood Fresh Weathered Black Pittsburgh

Sawdust Waste Switchgrass Switchgrass Thunder #8

Fuel Type

Volatile Fuel Nitrogen

Volatile Fuel Nitrogen (light green)

Char Fuel Nitrogen (dark green)

Maximum Volatile Nitrogen Yield

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

Maximum Nitrogen Volatile

Yield (%)

Urban wood Fresh Weathered Black Pittsburgh

Sawdust Waste Switchgrass Switchgrass Thunder #8

Fuel Type

Sawdust Nitrogen and Carbon Volatile Yields

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

Perc

en

t C

arb

on

or

Nit

rog

en

in

Vo

lati

le M

att

er

Carbon

Nitrogen

Sawdust Nitrogen and Carbon Evolution Normalized to Total Volatile Matter

Evolution

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

Percent Volatile Matter Evolved

Pe

rce

nt

Nit

rog

en

or

Ca

rbo

n

Ev

olv

ed

as

Vo

lati

les

Nitrogen Volatiiles Formed

Carbon Volatiles Formed

Nitrogen and Carbon Volatile Evolution from Urban Wood Waste

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

Vo

lati

le Y

ield

of

Car

bo

n a

nd

Nit

rog

en

Nitrogen

Carbon

Nitrogen and Carbon Volatile Evolution from Fresh Switchgrass

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

Per

cen

t C

arb

on

or

Nit

rog

en V

ola

tili

zed

Nitrogen

Carbon

Nitrogen and Carbon Volatile Evolution from Weathered Switchgrass

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

Vo

lati

le Y

ield

of

Car

bo

n o

r N

itro

gen

Nitrogen

Carbon

Nitrogen and Carbon Volatile Evolution from Weathered Switchgrass Normalized

to Total Volatile Evolution

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

Percent Total Volatile Yield From Fuel

Pe

rce

nt

Ev

olv

ing

as

Vo

lati

le M

att

er

Nitrogen Volatile Yield

Carbon Volatile Yield

Nitrogen and Carbon Evolution from Black Thunder PRB Coal

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

Per

cen

t V

ola

tili

zed

Nitrogen Carbon

Total Fuel

Nitrogen and Carbon Volatile Evolution from Pittsburgh #8 Coal

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

Pe

rce

nt

of

Ele

me

nt

Ev

olv

ed

as

Vo

lati

le M

att

er

Percent Nitrogen Evolved as Volatile Matter

Percent Carbon Evolved as Volatile Matter

Nitrogen and Carbon Volatile Evolution from Pittsburgh #8 Coal Normalized to

Total Volatile Yield

0

10

20

30

40

50

60

70

80

90

100

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0% 100.0%

Percent Total Volatile Matter Evolved

Pe

rce

nt

Ca

rbo

n a

nd

Nit

rog

en

Ev

olv

ed

as

V

ola

tile

Ma

tte

r

Nitrogen/Carbon Atomic Ratios in Char Normalized to N/C Ratio in Initial Fuel

0

0.5

1

1.5

2

2.5

0 200 400 600 800 1000 1200 1400 1600 1800

Temperature (C)

No

rmal

ized

N/C

Ato

mic

Rat

io in

So

lid F

uel

an

d

Ch

ar

Fresh Sawdust

Weathered Switchgrass

Black Thunder

Urban Wood Waste

Pittsburgh #8

Fresh Switchgrass

NOx Reductions at Albright

0

0.1

0.2

0.3

0.4

0.5

0.6

0.00 2.00 4.00 6.00 8.00 10.00 12.00

Cofiring Percentage, Mass Basis

NO

x E

mis

sio

ns,

lb

/MM

Btu

NOx Reductions at Albright (2)

• NOx = 0.361 – 0.0043(Cm) + 0.022(EO2) – 0.00055(SOFA)

• Definitions:– Cm is cofiring percentage, mass basis [0 – 10]

– EO2 is excess O2 at furnace exit (wet basis) [1 – 4]

– SOFA is separated overfire air damper positions for all 3 levels [0 – 240]

• r2 = 0.87, 68 observations• Probabilities of random occurrence: equation,

4.2x10-28; intercept, 2.3x10-24; Cm, 1.2x10-5; EO2, 5.9x10-4; SOFA, 5.0x10-22

NOx Reduction at Seward Station

y = 0.0004x2 - 0.0034x + 0.0657

R2 = 0.8507

0.00%

2.00%

4.00%

6.00%

8.00%

10.00%

12.00%

14.00%

16.00%

18.00%

20.00%

0 5 10 15 20 25

Sawdust Cofiring Percentage, Mass Basis

NO

x R

ed

ucti

on

Perc

en

tag

e

NOx Reduction at all EPRI Demos

Average NOx Emissions Reduction

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Percent Cofiring, Btu Basis

Pe

rce

nt

NO

x R

ed

uc

tio

n f

rom

Te

st

Ba

se

lin

e

Line Indicates 1% NOx Reduction for Every 1% Cofiring Percentage (Btu Basis)

Conclusions• Fuel reactivity is a key to NOx control using

staged combustion

• Biomass fuels, in general, are highly reactive although weathering reduces nitrogen reactivity in switchgrass

• The relative reactivity of biomass, and various coals, can be used as a technique to evaluate potential in NOx management

• The DTR technique for analyzing fuels has significant benefits in evaluating initial combustion processes applied to NOx management