Biomass and Coal
Characteristics: Implications
for Cofiring
David A. Tillman
Foster Wheeler Power Group, Inc.
Clinton, NJ
Abstract
Woody 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 NO x, 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
Fresh
Mixed
Sawdust
Proximate Analysis (wt % dry basis)
Volatiles 80.0
Fixed Carbon 19.0
Ash 1.0
Ultimate Analysis (wt % dry basis)
Carbon 49.2
Hydrogen 6.0
Nitrogen 0.3
Sulfur <0.1
Oxygen 43.0
Ash 1.0
8400
HHV (Btu/lb)
Urban
Wood
Waste
Fuel
Fresh
Weathered
Switchgrass Switchgrass
76.0
18.1
5.9
76.18
16.08
7.74
80.93
18.34
0.73
48.0
5.5
1.4
0.1
39.1
5.9
8364
46.73
5.88
0.54
0.13
38.99
7.74
7750
51.44
5.97
1.45
0.04
40.36
0.73
8150
Distribution of Fuel Nitrogen
1.8
1.6
Volatile Fuel Nitrogen (light
green)
1.4
Char Fuel Nitrogen (dark green)
1.2
Distribution of
Fuel Nitrogen by
Type (lb/MMBtu)
1
0.8
Volatile Fuel Nitrogen
0.6
0.4
0.2
0
Sawdust
Urban wood
Fresh
Weathered
Black
Pittsburgh
Waste
Switchgrass
Switchgrass
Thunder
#8
Fuel Type
Maximum Volatile Nitrogen Yield
100.00%
90.00%
80.00%
70.00%
Maximum
Nitrogen Volatile
Yield (%)
60.00%
50.00%
40.00%
30.00%
20.00%
10.00%
0.00%
Urban wood
Sawdust
Waste
Fresh
Weathered
Switchgrass Switchgrass
Fuel Type
Black
Pittsburgh
Thunder
#8
Perce nt C arbon or N itrogen in Volatile M atter
Sawdust Nitrogen and Carbon Volatile
Yields
100.00
90.00
Nitrogen
80.00
70.00
60.00
Carbon
50.00
40.00
30.00
20.00
10.00
0.00
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
Sawdust Nitrogen and Carbon Evolution
Normalized to Total Volatile Matter
Evolution
Percent Nitrogen or Carbon
Evolved as Volatiles
100.00
90.00
80.00
Nitrogen Volatiiles Formed
70.00
60.00
50.00
40.00
Carbon Volatiles Formed
30.00
20.00
10.00
0.00
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
Percent Volatile Matter Evolved
80.0%
90.0%
100.0%
Nitrogen and Carbon Volatile Evolution
from Urban Wood Waste
Volatile Yield of Carbon and Nitrogen
100.00
90.00
Carbon
80.00
70.00
60.00
Nitrogen
50.00
40.00
30.00
20.00
10.00
0.00
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
Nitrogen and Carbon Volatile Evolution
from Fresh Switchgrass
Percent Carbon or Nitrogen Volatilized
100.00
90.00
80.00
Carbon
70.00
60.00
50.00
Nitrogen
40.00
30.00
20.00
10.00
0.00
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
Nitrogen and Carbon Volatile Evolution
from Weathered Switchgrass
100.00%
Volatile Yield of Carbon or Nitrogen
90.00%
80.00%
Carbon
70.00%
60.00%
50.00%
40.00%
30.00%
Nitrogen
20.00%
10.00%
0.00%
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
Nitrogen and Carbon Volatile Evolution
from Weathered Switchgrass Normalized
to Total Volatile Evolution
100.00%
Percent Evolving as Volatile Matter
90.00%
80.00%
70.00%
60.00%
50.00%
40.00%
Carbon Volatile Yield
30.00%
20.00%
10.00%
Nitrogen Volatile Yield
0.00%
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
Percent Total Volatile Yield From Fuel
80.0%
90.0%
100.0%
Nitrogen and Carbon Evolution from
Black Thunder PRB Coal
90.00%
80.00%
Total Fuel
Percent Volatilized
70.00%
60.00%
50.00%
40.00%
Nitrogen
Carbon
30.00%
20.00%
10.00%
0.00%
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
Nitrogen and Carbon Volatile Evolution
from Pittsburgh #8 Coal
Percent of Element Evolved as Volatile Matter
80.00%
70.00%
60.00%
Percent Carbon Evolved as Volatile Matter
50.00%
40.00%
30.00%
20.00%
Percent Nitrogen Evolved as Volatile Matter
10.00%
0.00%
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
Percent Carbon and Nitrogen Evolved as
Volatile Matter
Nitrogen and Carbon Volatile Evolution
from Pittsburgh #8 Coal Normalized to
Total Volatile Yield
100
90
80
70
60
50
40
30
20
10
0
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
Percent Total Volatile Matter Evolved
80.0%
90.0%
100.0%
Nitrogen/Carbon Atomic Ratios in Char
Normalized to N/C Ratio in Initial Fuel
Normalized N/C Atomic Ratio in Solid Fuel and
Char
2.5
Weathered Switchgrass
2
Fresh Switchgrass
1.5
Pittsburgh #8
1
Black Thunder
0.5
Fresh Sawdust
Urban Wood Waste
0
0
200
400
600
800
1000
Temperature (C)
1200
1400
1600
1800
NOx Reductions at Albright
0.6
NOx Emissions, lb/MMBtu
0.5
0.4
0.3
0.2
0.1
0
0.00
2.00
4.00
6.00
8.00
Cofiring Percentage, Mass Basis
10.00
12.00
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
20.00%
N Ox R e duc tion P e rc e nta ge
18.00%
2
y = 0.0004x - 0.0034x + 0.0657
2
R = 0.8507
16.00%
14.00%
12.00%
10.00%
8.00%
6.00%
4.00%
2.00%
0.00%
0
5
10
15
S aw dust C ofiring P ercentage, M ass B asis
20
25
NOx Reduction at all EPRI Demos
Average NOx Emissions Reduction
Percent NOx Reduction from Test Baseline
30
25
Line Indicates 1% NOx Reduction for Every 1% Cofiring Percentage (Btu Basis)
20
15
10
5
0
0
2
4
6
Percent Cofiring, Btu Basis
8
10
12
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