ALTERNATIVE ELECTRICAL ENERGY SOURCES
FOR MAINE
W.J.
ones
M. Ruane
Appendix A
CONVERSION OF BIOMASS
C. Glaser
M. Ruane
Prepared for the Central Maine Power Company.
Report No. MIT-EL 77-010
MIT Energy Laboratory
July 1977
This appendix is one of thirteen volumes; the remaining volumes are as
follows: B. Conservation; C. Geothermal Energy Conversion; D. Ocean
Thermal Energy Conversion; E. Fuel Cells; F. Solar EnergyConversion;
G. Conversion of Solid Wastes; H. Storage of Energy; I. Wave Energy
Conversion; J. Ocean and Riverine Current Energy Conversion; K. Wind
Energy Conversion, and L. Environmental Impacts.
Acknowledgments
Initial literature reviews and drafts of the various technical
appendices were prepared by the following people:
Appendix A
Conversion of Biomass - C. Glaser, M. Ruane
Appendix
B
Conservation - P. Carpenter, W.J. Jones, S. Raskin, R. Tabors
*Appendix
C
Geothermal Energy Conversion - A. Waterflow
Appendix
D
Ocean Thermal Energy Conversion - M. Ruane
Appendix
E
Fuel Cells - W.J. Jones
Appendix
F
Solar Energy Conversion
Appendix
G
Conversion of Solid Wastes - M. Ruane
Appendix
H
Storage of Energy - M. Ruane
Appendix
I
Wave Energy Conversion - J. Mays
Appendix
J
Ocean and Riverine Current Energy Conversion - J. Mays
Appendix
K
Wind Energy Conversion - T. Labuszewski
Appendix
L
Environmental Impacts - J. Gruhl
S. Finger, J. Geary, W.J. Jones
Numerous people shared reports and data with us and provided comments on
the draft material. We hope that everyone has been acknowledged through the
references in the technical sections, but if we missed anyone, thank you!
Ms. Alice Sanderson patiently weathered out many drafts and prepared the
final document.
A
Preface
The Energy Laboratory of the Mass. Inst. of Tech. was retained by
the Central Maine
Power Company to evaluate several technologies
as possible alternatives
(a 600 MWe coal fired
to the construction of Sears Island#1
generating plant scheduled for startup in
1986). This is an appendix to Report MIT-EL 77-010 which presents
the results of the study for one of the technologies.
The assessments were made for the Central Maine Power Company on
the basis that a technology should be:
1) an alternative
to a base-load
power generation facility.
electric
Base-load is
definedas ability to furnish up to a rated
capacity output for 6 5 7 0 hrs. per year.
21 not
restricted
may be several
Maine.
to a
plants
single plant.
within
It
the state of
The combined output,
when viewed
in isolation, must be a separate, "standalone", source of power.
3) available to deliver energy
by 1 985.
f·
APPENDIX A
CONVERSION OF BIOMASS
Page
1.0
Introduction
1.1
1.2
1.3
2.0
3.0
Marine Biomass
A-2
1.1.2
Terrestrial Biomass
A-4
1 1.3
Agricultural Residues
A-7
1.1.3.1.
Field Crops and Livestock
A-7
1.1.3.2
Forestry Residue
A-8
1Biomass Conversion
1.2.1
Biological
1.2.2
Thermochemical
1.2.2.2
Direct Burning
Electric:ity from Biomass
A-8
A-8
A-9
A-1i
A-13
A-14
2.1
Fuel Heat Content
A-14
2.2
Chemical Analysis
A-15
2.3
Equivalent Fuel Costs
A-17
Biomass Supply
3.2
5.0
A-2
1.1.1
Forest Residues in Fuel
3.1
4.0
A-1
I3iomass Production
A-18
Ultimate Reserves
A-18
3.1.1
Production
A-18
3.1.1.1
Definitions
A-18
3.1.1.2
Commercial Growing Stock -- Merchantable Bole
A-20
3.1.1.3
Commercial Growing Stock -- Residues and Merchantable Bole
A-20
3.1.1.4
Non-Merchantable Bole
A-21
3. 1.1.5
Total Ultimate Reserves
A-22
3.1.2
Removals
A-23
3.1.2.1
Commercial Growing Stock Removals
A-23
3. 1.2.2
Energy Equivalent of Removals
A-24
Extractions and Processing
Wood Conversion Systems
A-25
A-26
4.1
Waterwall Incinerators
A-26
4.2
Multiple Boilers
A-29
4.3
Gas Turbine Systems
A-31
Environmental Considerations
5.1
Boiler Emissions
5.2
Boiler Residues
5.3
Water Pollution
5.4
Land Use
A-31
A-31
A-34
A-34
A-35
Page
6.0
ECONOMICS
6.1
A-37
Wood Fuel Costs
A-38
6.1.1
Procurement Costs
A-38
6.1.2
Harvesting Costs
A-38
6.1.3
Hauling Costs
A-39
6.2
Forest Management
A-39
6.3
Power Plant Costs
A-39
6.4
6.3.1
Capital Costs
A-40
6.3.2
Operating and Maintenance Costs
A-40
6.3.3
Cooling Equipment
A-40
Total Costs
A-40
7.0
CONCLUSIONS
A-43
8.0
REFERENCES
A-44
LIST OF TABLES
Page
Table 1.1
Aboveground, Dry Biomass Yield of
Selected Plant Species
A-5
Table 1.2
Field Crops in Maine
A-8
Table 1.3
Livestock in Maine
A-8
Table 1.4
Comparison of Waste-to-Energy Processes
A-11
Table 1.5
Examples of Waste-to-Energy Pilot Demonstration, and Commercial Projects in
the United States
Table 1.6
A-12
Energy Efficiencies for Utilization of
Organic Wastes
A-13
Table 2.1
Heating Value of Wood
A-14
Table 2.2
Chemical Ingredients of Wood
A-14
Table 2.3
Chemical Composition of Parts of Trees
A-16
Table 3.1
Major Commercial Species in Maine
A-18
Table 3.2
Conversion Factors
A-19
Table 3.3
Biomass Weight Relationships
A-19
Table 3.4
1971 Inventory of Merchantable Growing
Table 3.5
1970 Gross Growth in Merchantable
Stock
A-20
Growing Stock
Table 3,6
A-20
1971 Total Biomass of Merchantable
Growing Stock Trees
Table 3.7
1971 Total Biomass of Rough and Rotten
Trees
Table 3.8
A-21
Estimates of Biomass Production in
Maine's Forests
A-22
Table 3.9
Merchantable Biomass Removals
Table 3.10
Biomass Removals and Biomass Available
for Energy
Table 3.11
Energy Equivalents of Biomass Available
A-24
Boiler Sales by Capacity and Firing
Method
Table 4.2
A-28
Boiler Efficiencies as Function of
Moisture Content
Table 4.3
A-24
A-24
for Energy
Table 4.1
A-21
A-29
Estimated Biomass Available for Energy
Per Acre
A-30
i
Table 4.4
Harvest Area and Distances from 50 MW Plant
at 70% Load Factor
Table 5.1
A-30
Emission Factors for Wood and Bark in
A-37
Boilers
Table 5.2
Calculated Wood Ash Collection Efficiencies
Table 5.3
Total Annual Emissions from Different
for Various Control Equipment
Fueld (50MW Plant) (ton/year)
A-33
A-33
A-34
Table 5.4
Analysis of Wood Fuel Ash
Table 5.5
Comparison of Two Harvesting Techniques
Table 5.6
Replenishment of Soil Nutrients in
Table 6.1
Wood Harvesting Costs in 1972
Table 6.2
1986 Fuel Cost Components for Wood
A-39
Table 6.3
Cost Assumptions
A-40
Table 6.4
Annual Electricity Cost from 50 MW
in Finland, Nutrients Removed
North Carolina Hardwood Forest
10.3 million tons
A- 35
A-36
A-38
Wood-Burning Plant - 70% Load Factor
A-41
28% Efficiency
ii
LIST OF FIGURES
Page
Figure 1.1
Energy From Biomass
Figure 1.2
Conceptual Design of 1,000-Acre Ocean
Figure 1.3
Water Hyacinth Farm Layout
A-3
Figure 1.4
ERDA Fuels from Biomass Program
A-4
Figure 1.5
Territory Unsuitable for Energy
A-1
Food and Energy Farm Unit
PlantationsTM
A-2
A-6
Figure 2.1
Heating Value of Wood Fuel
A-15
Figure 2.2
Equivalent Cost of Wood Fuel
A-17
Figure 3.1
Parts of a Tree
A-19
Figure 3.2
Maine Forest Inventory 1960 - 2000
A-23
Figure 4.1
Typical Wood-Burning Boiler
A-27
Figure 4.2
Boiler Heat Loss Versus Wood Moisture
Figure 5.1
Size Distribution of Wood and Coal Fly
Content
A-28
Ash
Figure 6.1
A-32
Electricity Costs for Wood-Burning
Power Plant - 50 MW, 1986 Dollars
iii
A-42
1.0
INTRODUCTION
Depending on location, an annual average of between 1150 and 1250 Btu/ft2 of solar energy falls
on Maine every day.
A small percentage of this energy (0.5% to 3%) can be converted into stored
energy in the form of organic matter. This organic matter is called biomass. As an indirect means of
collecting, storing, and utilizing solar energy, biomass represents a vast, renewable, supply of materials which can be converted into a variety of products, including liquid, gaseous, or solid fuels
(Young, H., I, 1975). This appendix will not attempt to consider all of the potential direct and
indirect impacts biomass could have on the energy supply of Maine [see, for example, (Page, et al.,
1976, Vol.. I, pp. 4-10) or (Ward, 1976)], but will focus on one narrow, but near-term contribution,
i.e., the generation of electricity.
There are two general technological areas involved in the extraction of energy from biomass:
biomass production and biomass conversion.
Biomass production includes the growing, harvesting, and
collection of terrestrial and marine energy crops, and the collection of agricultural, feedlot, and
forestry residues.
Biomass conversion can be accomplished by several methods:
chemical and direct combustion.
in Figure 1.1.
biological, thermo-
Possible paths from solar energy to end use via biomass are shown
Each of the various paths requires a different level of technical development, and
has costs and environmental impacts associated with its implementation. By discussing briefly the
components of Figure 1.1, we shall attempt to show that only one path is an attractive candidate for
electricity production in Maine at present.
PRODUCTION
PRODUCTION
ENERGY FROM BIOMASS
END
END USES
USES
ProcessHeat
Electricity
Direct Use
Figure 1.1
A-1
1.1
Biomass Production
1.1.1
Marine Biomass
Marine biomass plantations involve seeding, growing, and harvesting fast-growing aquatic plants
in terrestrial ponds or at sea.
Primary candidate species in proposed schemes include giant brown kelp
(Lease, 1976, p. 253) and water hyacinth (Lecuyer, 1976, p. 267) (Figures 1.2 and 1.3). These systems
are unlikely methods for biomass production in Maine since the economical marine crop species are warmclimate plants and could not survive Maine's harsh winters, especially the freezing of ponds.
Reseeding
and seasonal production would be required with severe economic penalties.
Even given a favorable climate, the technology of marine biomass production is not well developed.
As can be seen from Figure 1.4, which shows the research program plant of the federal government in fuels
from biomass, the earliest marine biomass demonstration plants in the most favorable locations (Florida
and California) will not begin operation until after 1985.
It seems unlikely that marine biomass
schemes will ever be used in Maine, and certain that they will have no impact in the state before 1990.
Figure 1.2
Conceptual Design of 1,000-Acre Ocean Food and Energy Farm Unit
PLANTS, HULUING SPACtS,
RTERS, BUOYANCY CONTROL
JTION
R PLATFORM -
E ACTUATED UPWELLING PUMP
BUOY
-'"
-KEEPING PROPULSOR
/EMBERS
T DISTRIBUTION SYSTEM)
//'//.,,
(from Lesee, 1976, p. 260).
A-2
Figure 1.3
WATER
t1HY CITi
1, LAYOUT
FLF
W
(221,300 Acresor 346 Sq Mites)
I|-
Miles
-12
I
Unit HyacinthSystem
l
IAA
,-,
=. :_.2
Direction
of Canals
rD.rt
-.
4-qu
Hyacinth Slurry To
.Plant
,-Gas Conversion/Processing
Facility (1960 Acres)
::A
----I~~~~~a
29 Miles
a
i'.
.-
,
Pipe"Alley"
Water and Nutrients
To Growing Areas
198,800 Active Acres
of Hyacinth
Growing Area
4
(2 million SCF/day Methane Production)
(from Lecuyer, 1976, p. 278).
A-3
ERDA Fuels from Biomass Program*
FISCAL YEAR
PROGRAM ELEMENT
e
7s
78
77
78
79
BO
1
82
83l
S
AFTERs
AGRICULTURALRESIDUEPROJECTS
SystemStudieS
Crop Rei e PilotPlant
Feedot ExperimentalFacility
Fedlot Pot Plant
-
Dairy FarmAnimalWastePilot Plant
TERRESTRIALBIOMASSPRODUCTION
& CONVERSION
PROJECTS
SystemSIudges
_ '
IntensiveAgriculturePilot Plant
-
Controllel EnvironmentAgriculturePilot Plant
Intense AgricultureDemonstration
Wood Plantation
Pilot Plant
_
Wood PlantationDemnonstraion
MARINE BIOMASSPRODUCTION
& CONVERSION
PROJECTS
SystemStudies
MarineBiomasPilot Plant(s)
MarineBiomas Demonstration
RESEARCHDEVELOPMENT
PROJECTS
BiomassProduction TechnologyProjects
ConversionTechnologyProjects
·
Proler Imtlo
l
v
Itin Opelsaon
*
Polfon Copleon
*from (Ward, 1976, p. 427)
Figure 1.4
1.1.2
Terrestrial Biomass
Terrestrial biomass plantations involve seeding, growing, and harvesting fast-growing plants
on land for the sole purpose of biomass production.
The choice of species is made primarily on the
basis of potential biomass yield in tons/acre-year, which varies with available sunlight and rainfall, the fertility of the region, and methods of harvesting. Common plants which produce the
best yield are shown in Table 1.1.
There is no consensus on the best methods for the maximization of biomassyield which is affected
by crop mix and harvesting practice. The use of annuals, in a multiple-harvest-per-year mode lies at
one end of a spectrum of suggestions (Alich, 1976, p. 293).
A-4
At the other end is an exclusive
Table 1.1
ABOVEGROUND, DRY BIOMIASS YIELD OF SELECTED PLANT
SPECIES*
I - -- -
Species
,
-
LocadIlon
.
Yield
ton/acre-year
Annuals
-- -
Sunflower hybrids (seeds only)
California
1.5
Forage Sorghum (irrigated
New Mexico
7-10
Forage Sorghum (irrigated)
Kansas
-12
Kenaf
Florida
20
Kenaf
Georgia
8
-
-----
Perennials
Sugarcane
Mississippi
20
Sugarcane (10 yr. average)
Hawaii
26
Sugarcane (best case)
Texas
Sugarcane (experimental)
Bamboo
30.5
Alabama
Hybrid Poplar
7
Pennsylvania
American Sycamore
Georgia
Red Alder
4-8.7
2.2-4.1
Washington
Eucalyptus
..
50
California
10
California
13.4-24.1
Miscellaneous
Puckerbush (average)
Maine
*exerpted from (Alich, 1976, p. 298)
4.4
'.
use of woody deciduous (hardwood) perennials
(Fraser et al., 1976, p. 373).
Effective use of plantationing for most species
requires that a region have the following
attributes:
*
e
*
*
adequate water availability (200-300 ton H
2 0/oven dry ton of biomass yield)
plentiful available acreage
high levels of solar radiation and minimum cloud
cover
long growing season
* mild winters
Figure 1.5 shows the regions of the U.S. which
are unsuitable to plantationing on the basis
of
the first two attributes (no irrigation assumed).
Maine passes these tests, with the exception
of
three counties, Somerset, Franklin, and Oxford, which are considered
too hilly. Regions of the
A-5
country satisfying the last three criteria are found principally in California, Arizona, coastal
Texae, and parts of New Mexico (Alich, 1976, p. 296).
For most species, Maine is clearly not an
acceptable plantation site.
An exception might be made for some deciduous species which can thrive in Maine's northern
climate.
Examples of such species are aspens, hybrid aspens, and pin cherry which have been sug-
gested as plantation crops of New Hampshire and hybrid poplars which have been suggested for Nova
Scotia and Ontario (Fraser, et al., 1976, p. 390).
crops as a biomass source in Maine.
Some considerable problems exist in using these
Production rates for plantationing in Maine's climate can only
be estimated, although puckerbush rates (4.4 ton/acre yr) are a reasonable lower bound.
TERRITORY UNSUITABLE FOR ENERGY PLANTATIONSTM
*from (Fraser et al., 1976, p. 394)
Figure 1.5
Plantationing for hardwood biomass in general is still a new technology, as can be seen from Figure
1.4, and Maine's climate might produce special problems, especially seasonal variations in harvesting
rates.
Most significantly, the efficient operation of a plantation in terms of utilizing its capital
equipment requires a size of approximately 35,000 acres.
Acquisition of this much land in an area
which could be operated as a plantation will be difficult in Maine.
A-6
Assuming these obstacles are overcome, what would be the resulting energy value of such a
For a production rate of 6 dry tons/acre year, a biomass heat content of 9000
plantation's output?
Btu/dry lb and a plantation size of 35,000 acres, the annual harvest would be 3.78 x 106 MBtu.
Using a typical value for combustion systems of a 20% conversion efficiency to electricity, this
would be roughly equivalent to a 32 MW plant at 80% load factor.
these optimistic assumptions, would require
Six hundred MW of capacity, under
19 plantations with about 66 million acres or roughly
3.3% of Maine's total land area. This is 145% of the total land estimated to be under cultivation
as cropland in Maine (460,000 acres).
While plantationing may contribute to Maine's electricity
needs, it would represent a major change in land use and certainly would take many years to implement.
1.1.3 Agricultural Residues
The use of agricultural residues for biomass involves the harvesting and collection of unused
byproducts related to field crops, livestock and forestry.
The intention here is to maximize the
use of materials which would otherwise have no value or negative value as disposal problems. Usually
no specific actions are taken to increase the residue production although centralization of production
usually improves the logistics and economics of residue collection and utilization.
1.1.3.1
Field Crops and Livestock
Field crop production in Maine is summarized in Table 1.2.
Several things are noteworthy.
First,
the agricultural industry in Maine is consolidating as smaller farms are abandoned, resulting in a net
decrease in crop production. Secondly, wastes available for energy conversion are only a fraction
of the biomass of each crop. With total aboveground biomass yields optimistically assumed on the
order of 1-2 tons/acre-year, the acreage and biomass residue are seen to be inadequate for any serious
energy production.
Third, the acreage is divided among numerous farms, presenting serious logistical
problems and energy penalties for residue collection.
Except for the development of small-scale con-
version systems which might provide local energy on a farm, no impact of crop residues on electricity
production is likely.
Table 1.2
Field Crops in Maine
Item
No. of Farms
Cropland
Corn-silage,
Fodder, Hogged
or Grazed
No. of Farms
Sorghum-Silage
Fodder, Hogged
or Grazed
No. of Farms
Hay, excluding
Sorghum
No. of Farms
Units
-
1959
1964
1969
17260
12875
7971
Acres
698188
594434
457935
Acres
8818
-
10048
-
19763
-
Acres
-
10
1
469
30
Acres
451067
-
367476
-
208162
4041
Potatoes
No. of Farms
Acres
133349
-
130707
-
152070
2210
Vegetables
No. of Farms
Acres
-
14701
1392
17491
932
13188
644
Berries
No. of Farms
Acres
25212
-
22242
1446
17151
652
Orchards
No. of Farms
*from (USDA,1972, p. 5)
Acres
-
11050
1635
8693
885
7365
383
A-7
Except for possible isolated uses, the farm animal population of Maine makes the conversion of
manure to energy of negligible importance.
has been proposed.
Conversion of manure from cattle feedlots in the midwest
Approximately 25,000 head of cattle (117 tons manure/day) are needed to produce
one million CFD (cubic feet/day) of methane (Douglas, 1976, p. 195). In a conventional utility boiler
with 34% efficiency, this would be equivalent to 100 MWh/day, or roughly a 5 MW plant.
tration of the feedstock is of critical importance to collection efficiency.
A concen-
Maine's farm animal
population (Table 1.3) is too small and dispersed to exceed significantly this small capacity (perhaps 10 MW of capacity under optimistic conditions).
can be found in Appendix
Further discussion of field and livestock biomass
, Conversion of Solid Wastes.
Table 1.3
Livestock in Maine*
1964
1969
186216
10500
157594
7014
127018
3389
Hogs & Pigs
No. of Farms
24646
4134
13117
1662
7350
609
Sheep & Lambs
No. of Farms
40615
1627
23381
1008
14332
504
Stock
Cattle & Calves
No. of Farms
Horses & Ponies
No. of Farms
1959
-
7730
3853
7373553
-
4795565
2534
Chickens older 4480993
5773
than 3 months
Broilers less
than 3 months
4930
1379
-
-
13057223
459
*from (USDA, 1969, p. 5)
1.1.3.2
Forestry Residue
Forestry residues are an obvious biomass source for electricity production in Maine.
Covering
90% of Maine's land area, the forests are a renewable resource, but with an important distinction.
Forestry residues are unlike solar, wind, wave, or tidal energy, which must be used, or at
least converted and stored, when available. Forests both convert and store energy.
Wood fuel supplies
that are not now competitively priced may become competitive as conventional fuel costs rise or
forestry technology improves. And if necessary, with proper forest management, the forest, not unlike
a coal seam, can be temporarily "mined," removing a supply of biomass greater than the forest's natural
renewable production. Then, during the following periods (many years), the forest can be replanted and
fertilized at
Prought ad
rate greater than if left alone.
There is, however, great risk in such an action.
ther natural events plus collapse of social concern" could remove the sections so farmed
from any further productivity for several life times of man.
1.2
Biomass Conversion
1.2.1
Biological
Biological conversion of biomass to energy supplies uses microorganisms to reduce complex organic
compounds to simpler, more stable forms.
gical conversion:
Four processes have been suggested as candidates for biolo-
acid hydrolysis, enzymatic hydrolysis, anaerobic digestion, and biophotolysis. The
first three result in methane gas or alcohol production.
A-8
Biophotolysis produces hydrogen.
Acid hydrolysis,currently in commercial use in the pulping industry, decomposes the cellulosic
materials in biomass to form glucose. This simple sugar can ultimately be converted through fermentation to form alcohol. Acid hydrolysis has several disadvantages.
It requires expensive, corrosion-
resistant equipment. The high temperatures and acid concentrations needed for hydrolysis tend to
destroy the byproduct glucose, requiring high hydrolysis rates for reasonable glucose production.
And the acid also reacts with impurities in the cellulose,
producting impurities which appear in
the alcohol and reversion compounds which slow glucose production (Spano, 1976, p. 326).
Enzymatic hydrolysis uses an enzyme from a mutant fungus, Trichoderma viride, to convert cellulose to glucose. As in acid
hydrolysis, the sugar then undergoes fermentation to form the ulti-
mate fuel of the process: alcohol.
Enzymatic hydrolysis occurs at moderate temperatures and acidity
(pH: 4.8) so glucose is not destroyed and impurities are less critical.
Unlike acid hydrolysis,
enzymatic hydrolysis is still in the basic research stage (Spano, 1976).
Anaerobic digestion occurs at atmospheric pressure and temperatures between 20°C and 50°C as a
variety of acidogenic (acid-producing) bacteria decompose the organic feedstock and methanogenic
(methane-producing) bacteria act upon the decomposed matter to yield methane
and carbon dioxide.
The same process is presently used to treat sewage before disposal. An upper estimate of process
efficiency is 62% (10 million Btu/ton of pipeline quality gas produced from 16 million Btu/ton of
feedstock heat content) and a demonstration plant is expected by the mid 80's (CEQ, 1975, p. 10-8).
Biophotolysis is in the laboratory basic research stage.
In this process, pigments such as
chlorophyll, when stimulated by light, change the oxidation-reduction potential of electrons released
from water.
These electrons can chemically reduce hydrogen ions in the presence of selected enzymes
to form hydrogen gas (Ward, 1976, p. 430).
In order to generate electricity from the products of biological conversion, it would be necessary
to burn the alcohol, methane, or hydrogen, or use them in fuel cells.
internal combustion engines, gas turbines, or boilers.
technologies all provide
peaking power.
Alcohol can be fired in
With the exception of boiler use, these
Given Maine's position at the end of the natural gas pipe-
line systems, methane would be more valuable if used in the natural gas market rather than burned
for electricity production.
Intrastate prices of $4.00/MBtu are a low estimate of possible methane
prices in Maine (Monks, 1975, p. 29).
These are equivalent to 36 Mills/Kwh for fuel costs alone.
Furthermore, the federal government has voiced opposition to the future use of gas supplies for
utility power generation.
Using biomass alcohol, methane, or hydrogen to produce electricity seems
highly unlikely to happen in Maine.
Therefore, biological conversion should not be considered as a
candidate for conversion of biomass to electricity.
It is, however, potentially important as a com-
petitor for the forestry residues of Maine if the demand for alcohol and methane grows.
1.2.2
Thermochemical
The leading thermochemical conversion methods for biomass include hydrogenation (hydrogasification)
and pyrolysis. Both of these processes break down complex organic molecules under high temperature
and pressure and form fuels containing simpler molecules.
Hydrogenation involves the addition of hydrogen atoms to an inorganic molecule.
As the organic
molecules in the biomass feedstock decompose, this process adds hydrogen from an external source
(such as steam) to form either gaseous or liquid fuels. A
hydrogenation pilot plant utilizes
temperatures on the order of 300-400°C and pressures between 3000 and 4000 psi (CEQ, 1975, p. 10-7).
This technology should be commercial by the early 1980'sand is probably the most expensive conversion
method.
Conversion efficiencies to electricity are about 15%.
A-9
Pyrolysis is the chemical decomposition of biomass at atmospheric pressures, high temperatures
and :n the absence of oxygen.
Neither hydrogen nor catalysts are needed.
The disadvantages of
pyrolysis are that several forms of fuel are produced, with the ratio of fuel forms determined principally by a factor not under direct control -- the moisture content of the feedstock. A number of
groups are developing pyrolysis technology with Monsanto EnviroChem Systems, Occidental, the Bureau
of Mines, and Union Carbide, having advanced concepts.
Monsanto's LANDGUARD System has been in operation in Baltimore for several years using processed
municipal waste as its source of organic feedstock. Because its product gas has a low heat content
(100 Btu/ft3), transportation is uneconomical and the gas is burned on site for steam production.
Baltimore Gas and Electric uses the steam in its municipal district heating system.
impurities which preclude its use in gas turbines (CEQ, 1975, p. 10-9).
The gas has
The Baltimore system, which
is a 1000 ton/day commercial-size facility, had several design failures in early 1977 and may be
shut down.
The Occidental FLASH PYROLYSIS process has been operated using municipal waste on a 4 ton/day
pilot plant level and is being scaled up to 200 ton/day in California.
duced.
Gas, oil, and char are pro-
All of the gas and char are used for process heat. The process produces an oil which can be
used as a supplementary fuel in electric power boilers; the ash content of the pilot plant char has
been too high for utility use.
With a pure biomass feedstock, as opposed to municipal waste, the
ash problem should be reduced and the char might have
application as a supplementary fuel (CEQ, 1975,
p. 10-12).
The Bureau of Mines pyrolysis has operated on a pilot plant scale with a variety of municipal
wastes and manure as feedstocks.
changes.
Depending on temperature regimes the ratio of gas, oil, and char
Roughly 25% of the 500 Btu/ft3 product gas is needed for process heat. The remaining gas
could be burned in industrial or utility boilers, but is unacceptable for pipeline use because of
its poor heat content, combustion properties and excessive carbon monoxide content.
might have application as supplementary fuels.
The oil and char
The electric conversion efficiency of the process is
between 22 and 26%.
Union Carbide's PUROX process is at a commercial stage of development with a 200 ton/day plant
in S. Charleston, W. Virginia, processing municipal wastes.
better with homogeneous biomass as input.
The PUROX process produces a medium Btu fuel gas
(360 Btu/ft3 ) and glassy aggregate from municipal waste.
aggregate.
Operation should be comparable or
Biomass would result in char instead of
The fuel gas is principally H2, CO, and CO2 and can be used as a utility or industrial
boiler fuel.
Conversion efficiencies to electricity are between 26 and 30% when supplementary firing
(using coal or oil) in a utility boiler is used. The variation is due to ancillary power needs in
the PUROX system (Fisher, 1976, p. 457).
These four systems are representative of the status of pyrolysis as a biomass conversion technique.
The technology is currently in transition from
pilot plant to commercial plant size
with virtually all of its experience derived from treatment of municipal wastes.
Biomass feedstock
should produce minimal problems if introduced into such systems (Monks, 1975, p. 7). -The end
products are low to medium Btu gas (which must be burned on site or nearby) and possibly supplementary boiler fuels in the form of oil or char.
Once pyrolysis has been commercialized (the present
Monsanto and Garrett plants have substantial Environmental Protection Agency subsidies), it appears to
offer a potential for producing utility fuels from biomass.
Chemical reduction and catalytic gasification are two less advanced thermochemical processes.
Chemical reduction is being tested by the Bureau of Mines in Pittsburgh using manure as a feedstock.
A-10
A synthetic oil is produced using a CO-H2 0 reaction although less hydrogenation can lead to gas
Catalytic methanation or gasification is still
CO must be supplied to the reaction.
production.
at a laboratory stage of development although a proposed N.E. Massachusetts waste disposal system
will use this process on municipal refuse.
which uses catalysts to accelerate
Synthetic natural gas is the result of this process
the decomposition of complex organic molecules into methane.
1.2.2.2 Direct Burning
Combustion involves using biomass as a primary or supplementary fuel in a boiler.
The output
can be low pressure and temperature steam for process application or high pressure and temperature
steam for electricity production.
Before biomass can be burned, particularly as a supplementary
fuel, processing is usually necessary. This involves drying and shredding or pulverizing the biomass to an acceptable size.
Separating is used to describe the preparation of biomass, especially
municipal refuse, for use as a supplementary fuel.
Considerable experience exists with direct burning technology, although not with all biomass
fuels, and not with high pressure and temperature conditions.
dered the best established conversion technology.
Still, direct burning must be consi-
Wood and wood residue must be considered as the
biomass source with the most direct burning experience.
Conversion efficiencies to electricity can
range up to 39% (CEQ, 1975, p. 10-16) for supplementary direct burning.
conversion efficiencies vary from 18% to 30%.
As a primary fuel, biomass
Descriptions of several large wood burning systems
can be found in (Wheelabrator-Frye, 1976).
Tables 1.4 and 1.5 are a recent summary of the development status of potential biomass to
energy conversion process (Klass, 1976, pp. 46-49).
Table 1.4
---
Comparison of Waste-to-Energy Processes*
Process
Energy Products
Status**
Btu's
Combustion
Commercial
X
Separation
Commercial
-
Gas
Oil
-
-
Char
-
Other
X
X
Pyrolysis
Commercial
-
X
X
X
X
Anaerobic Digestion
Commercial
-
X
-
-
X
Fermentation
Commercial
-
X
-
-
X
Acid Hydrolysis
Commercial
-
X
-
-
X
Chemical Reduction
Small Pilot
-
-
X
-
X
Hydrogasification
Small Pilot
-
X
X
X
X
Enzyme Hydrolysis
Small Pilot
-
-
-
-
X
Biophotolysis
Laboratory
-
-
-
X
Catalytic Gasification
Laboratory
-
-
-
X
X
*"Commercial" pertains to experience with specific waste feedstocks only
**(Klass, 1976, p. 49)
A-11
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Table 1.6 summarizes the efficiencies of the various conversion technologies we have discussed.
It shows that direct burning, which is the most well established conversion technology, is also the
most energy-efficient for electricity production since no intermediate fuel products are made.
the present time direct burning is the best technology choice for electricity production.
At
That the
greatest direct burning experience has been with forestry products coincides nicely with the conclusion reached earlier concerning forest residues as the most likely biomass source in Maine.
Table 1.6
Energy Efficiencies for Utilization of Organic Wastes
ProdAi&~
Process
Hydrogenation
oil
Bioconversion
natural gas
Pyrolysis,Monsanto
low-Btu gas
(spaceheating)
Efficiencya
(percent)
Trajectory
EfficiencyA
(processing
and electric
generation)
(percent)
39
15
unknownc
unknown
71
45.6
Pyrolysis, Garrett
oil
Pyrolysis,BuMines
gas, oil
Direct burning
electricity
59 to 68
NA
NA
17.3
22 to 26
34
NA = not applicable
aIncludes process heat.
bProcess efficiencytimes 38 percent electric power generationefficiency.
CProcess heat requirementis unknown; efficiency without process heat is
62.4 percent.
from (CEQ, 1975, p. 10-18)
1.3
Electricity from Biomass
Our introductory discussion has indicated the following:
BIOMASS PRODUCTION
e
Marine biomass plantations will not contribute to Maine's energy sypply before the 1990s
if ever.
·
Terrestrial biomass plantations are limited by Maine's climate to a few hardwood species;
hardwood plantationing is an unproven technology with high land use impacts.
e
Forestry residues are an existing resource for potential biomass utilization.
BIOMASS CONVERSION
e
Biological conversion has not been conmierciallydemonstrated and results in fuels which
have more desirable uses than electricity production.
*
Thermochemical conversion, especially pyrolysis, is just beginning
commercial operation;
selected methods produce low to medium Btu gas or synthetic oils which can be used as supplementary utility boiler or gas turbine fuels.
*
Direct buirningtechnologies have extensive commercial experience, especially systems
burning wood and wood waste alone or as a supplementary fuel.
A-13
Because of these observations, we consider
wood residues-direct burning as the most likely
biomass path to electricity production in Maine.
Let us now examine in more detail the wood resi-
due rources
of Maine and the technologies for their conversion to electricity in more detail.
The
remaining sections of this appendix will consider the properties of wood as a fuel, the availability
of wood residues for energy in Maine, the technology for direct burning of wood residues, the associated environmental impact of using wood fuels, and the economics of electricity from wood residues.
2.0
FOREST RESIDUES IN FUEL
Fuelwood in the past has been used principally (2/3 of the supply) for residential space heating
and (1/3 of the supply) for industrial process heat and electricity production. We will consider
conversion systems of much larger heat inputs for electricity production.
2.1
Fuel Heat Content
With the variety of species of trees, and the many forms that their biomass can take when collected as residues, it would appear difficult to establish any general facts about forest biomass as
a fuel.
However, most common species of trees have very similar chemical composition, consisting of
about 90-95% cellulosic materials (heating value about 8300 Btu/lb) and 5-10% resins, volatiles, fatty
acids,
and other non-cellulosic materials (heating value about 16,900 Btu/lb) (Beardsley, 1976, p. 350).
This small percentage of non-cellulosic material, varying with species, part of the tree and season,
produces inherent heating variations on the order of 10% between species. A much more critical fuel
parameter is the moisture content, M, of the material, expressed in percentage of total weight.
A typical selection of heating values for green wood is shown in Table 2.1.
Table 2.1
Heating Value of Wood
VARIETY
OF WOOD
from
HEATINGVALUE (BTU/LB)
GREEN
12% MOISTURE
White Ash
Beech
YellowBirch
Chestnut
CottonWood
White Elm
Hickory
Sugar Maple
Red Maple
Red Oak
White Oak
BlackWalnut
6019
5576
5302
3879
4082
4568
5115
5437
5267
4774
5062
4751
7669
7679
7677
7667
7698
7683
76§1
7677
7690
7677
7683
7690
YellowPine
White Pine
4969
5340
8025
8044
Hagen, 1977, p.5)
When the several green wood heating values are normalized to the same moisture content, Table 2.1 shows
a consistent heating value, with the exception of the two pine wood heating values.
As Figure 2.1 shows,
the heating value of all woods can be approximated by a linear relationship between a dry value of
9000 Btu/lb and zero at "100%" moisture:
A-14
HV = 9000 (1 - M)
HV = heating value;
M = moisture content, % be weight
(2.1)
Figure 2.1
Heating Value of Wood Fuel
12000
11COG
PlE
OAK
10000 /
9000
D..F. BRIQUETTES
PIN000
E
/
8000
FIR
DOULAS
SH
'000
10
.N
NA
20
!.
.F.SAW
HLDUST
70
60
50
40
30
L
INFUEL-WT..PERCENT
MOISTURE
9
80
0
adapted from (Hagen, 1976, p. 5)
For softwoods, this produces a somewhat conservative estimate, while for hardwoods, it is slightly
optimistic (Hagen and Berg, 1976, p. 5). Since Maine's forests are 69% softwoods (Ferguson and
Kingsley, 1972, p. 54), 9000 Btu/lb is probably a slightly conservative overall value.
Green wood can contain more than 50% moisture while air- dried chips of wood typically have a
moisture content around 20%, resulting in a heating value variation of over 100%.
Clearly, varia-
tions in wood heat contents due to moisture will dominate variations between species.
As moisture content nears 60%, it becomes difficult to support continued steady combustion without supplementary fuel. At all moisture content levels, there is heat loss due to the vaporization
of water. This results in reduced boiler efficiency. In addition, variability in the moisture content
of feedstock to a boiler can cause other problems, such as "hot spots" and boiler instabilities.
"Hot spots," concentrations of relatively dry fuel being burned in a boiler produce uneven temperature distributions, causing possible damage
to boiler materials.
Energy input into a boiler is regulated through control of the flow of wood.
Sudden heat content
variations in the fuel ar-edifficult for the reguldtion to follow and instabilities result.
This
problem worsens with larger capacity systems and is an incentive to burn wood which has as low and
as uniform a moisture content as is available.
Air drying of wood fuel
variations in heat content.
If wood is exposed to the air, much of its moisture will evaporate.
The rate of evaporation will vary with temperature, humidity, and the circulation of air around the
wood.
The wood ideally should be sheltered from rain and snow. To reduce saw log wood from 50% to
20% moisture content would require a time on the order of months.
Wood chips, because they can pack
closer than saw logs, would require longer and might require overturning to get uniform exposure
Storage and fuel retrieval equipment would be required to handle several months' supply
to the air.
of wood chips as green wood would enter the fuel inventory, dry, and be burned.
There does not
appear to be much commercial experience with chip drying on the scale required for electricity production.
2.2
Chemical Analysis
Chemical analysis of green wood is misleading because of its high moisture content. On a dry
basis, we see a consistency in composition, as shown in Table 2.2, which can be summarized in the
following average values (Hagen and Berg, 1976, p. 5):
H
6%
C
0
51%
42%
ASH
i%
Wood contains little or no sulfur, so it is an ideal fuel from the point of view of sulfur dioxide air
pollution requirements.
A-15
Table 2.2
CHEMICAL INGREDIENTS OF WOOD
VARIETY
OF WOOD
H
Typical
Redwood
Henlock
Fir
Tanbark
Oak
Pine
0.072
0.035
0.089
0.080
0.095
0.060
0.070
GREEN ANALYSIS
C
N
0
0.379
0.265
0.212
0.335
0.142
0.502
0.526
0.001
0.001
0
0.001
0
0.001
0
ASH
0.538
0.648
0.690
0.579
0.749
0.433
0.401
0.010
0.001
0.009
0.005
0.013
0.004
0.003
WATER
%
H
24.0
50.4
57.9
35.9
71.8
0
0
0.0596
0.0585
0.0586
0.0626
0.0575
0.0600
0.0700
DRY ANALYSIS
C
0
0.499
0.534
0.504
0.523
0.504
0.502
0.526
0.427
0.403
0.416
0.405
0.393
0.433
0.401
, ASH
0.0132
0.0020
0.0214
0.0078
0.046
0.004
0.003
from (Hagen and Berg, 1976, p. 6)
The average ash content figure of 1% conceals extremes found in certain types of wood or parts
of the tree.
Bark, for instance, can contain 5% ash and maple has an ash content of 4.3% (Hall,
et al., 1976, p. 54)
Since the heat content of air-dried wood is at best only 7000-8000 Btu/lb. as
compared to coal at 12,000-13,000 Btu/lb, these ash contents are equivalent to coal with 7-8% ash
and generally require
particulate control equipment.
Qualitatively, wood ash is generally high in
compounds such as CaO, N 20 which can react with slag from coal, if wood and coal are burned
together, to reduce the viscosity of the slag. Systems which burn wood as a supplementary fuel with
coal will probably experience increased slagging which will impede heat transfer in the boiler and
reduce overall efficiency.
Wood has 50 to 100 times the fouling potential of residual fuel oil and
can be expected to cause severe slagging problems if burned as a supplementary fuel in residual oilfired boilers.
Because ash from wood combustion is high in Ca, K, and N, it has potential value as a fertilizer.
As can be seen from Table 2.3, there is some variability in chemical composition among the different
parts of a tree before burning (Dyer, 1967, p. 31).
The resulting ash would be expected to vary simi-
larly.
Table 2.3
CHEMICAL COMPOSITION OF PARTS OF TREES
Forest
Type
Component
Essential Elementsin Grams
per Acre for All Species Combined
Group
Nitrogen Calcium Potassium MagnesiumPhosphorus Manganese ron MolybdenumZinc Coe
Softwood top, branches 108,670 123,894
stump &roots
merch.bole
CompleteTrees
Mixedwoodtop, branches
stump&roots
merch.bole
CompleteTrees
Hardwood top, branches
stump& roots
merch.bole
CompleteTrees
43,093
13.536
22,397
17.638
57,565
20.972
6.579
4.043
6.080
140,173 181,459
64,065
20,1:5
26,440
31.503
1.938
81-
23.718 2,749
on
19C
881
161
257
96
556
24
5
286
1.437
409
350
82,012
95,979
41,474
9,856
13.720
7,955
1,399
50
869
162
191
28.297
50.808
17,691
4,778
2.999
3,916
530
33
699
172
68
110,309 146,787
59,165
14.634
16,719
11,871 1,929
83
1,568
334
259
9,861
14.509
52
1.128
191
193
90,879
108,898
53,039
42.114
73,032
24.061
5.265
3.665
4.346
534
38
1,080
177
94
132,993 181,930 77,100
15,126
18,174
10,649
2,019
90
2,208
368
287
NOTE:1,000 gramusequals 2.2 pounds
from (Dyer, 1967, p. 3).
A-16
6,303
1,485
2.3
Equivalent Fuel Costs
It is useful to consider the dollar value of wood as a fuel as compared with alternative fuels.
As we have seen, the heat content of wood varies with species and moisture content.
By varying these
parameters, we can determine the value of the heat content of wood in terms of coal prices.
The
comparison is performed for two wood dry heat contents, 9000 and 8000 Btu/lb, and two moisture contents,
15% and 50%. As shown later in Table 4.2 (page 29 ), the corresponding boiler efficiencies for 15%
and 50% moisture content are 76% and 66%; for coal, a boiler efficiency of 82% and heat content of
13,000 Btu/lb were assumed.
The comparison, shown in Figure 2.2, results in a pair of straight lines corresponding to 8000
Btu/lb (the
upper line in each pair) and 9000 Btu/lb (the lower line).
For example, if coal delivered
to Maine costs $50/ton ($1.90/106 Btu), then the equivalent value of wood as a boiler fuel would be:
Moisture Content
Value of Wood
(Percent of Wet Weight)
8000 Bty/dry lb
9000 Btu/dry lb
15%
$24.24/ton
$27.30/ton
50%
$12.40/ton
$13.90/ton
Another interpretation would be that if wood with a heat content of 9000 Btu/dry lb and 15% moisture
content could be purchased for $25.00/ton, it would be competitive with coal selling for roughly
$47.50 per ton.
It its moisture content were instead 50%, the wood would be competitive with coal
at about $88.00/ton.
Figure 2.2
mU
80
70
o 60
Z 50
0
40
4O
4
40
0
30
20
10
5
10
I
15
20
25
Cost of Wood $/ton
Eauivalent Cost of Wcod Fuel
A-17
30
Figure 2.2 does not imply that wood at 50% moisture content could be purchased, dried to 15%
moisture, and have its value increased nearly 100%. When dried to 15%, the original ton becomes 1300
lbthrough the loss in water of 35% of its weight.
The dried wood is worth $24.25/ton, but the total
value is now 65% of $24.25, for an actual increase in value of only 27%.
This increase in value
results from improved boiler efficiency (Hall, et al., 1976, p. 9).
An equivalent fuel cost comparison does not tell the whole story, because the different fuels
have different processing requirements. For example, wood fuel might require investment in fuel
drying facilities which are not needed in a coal plant.
Conversely, wood fuel plants might not
require flue gas desulfurization equipment, which would reduce investment and operating costs.
In
general, the optimal design of a wood fuel power plant will involve the complex trade-offs between
fuel characteristics, fuel processing equipment,and combustion equipment design.
3.0
BIOMASS SUPPLY
Factors which must be considered in measuring the supply include the extent of the basic resource
(ultimate reserves), the problems associated with removing the reserves (extraction or harvesting),
and the problems associated with preparing the reserves for conversion (processing). This section
will examine several estimates of the potential supply of Btu's from biomass in Maine by considering
these three issues.
3.1
Ultimate Reserves
3.1.1
Production
3.1.1.1
Definitions
Biomass reserves are qualitatively different from reserves of fossil energy sources.
For a
given level of price and technology, we can say that fossil reserves are fixed (although new discoveries are always possible). Biomass reserves are more correctly considered as a flow since under
natural conditions biomass is continually being created through forest growth and destroyed by fires,
weather, insects., etc.
The flow is altered by utilization by man.
or decreased by artificial means.
The flow rate can be increased
To describe ultimate reserves of biomass requires both a number
(the current inventory) and a rate (the net difference of forest growth, natural destruction, and
removals [not natural]).
Keserves are estimated by a variety of direct and indirect techniques.
Unfortunately, interest
in biomass as a fuel is a recent development, so there is not a long record of biomass data.
is, however, a long record of data on one portion of the total biomass:
There
the volume of wood which
has commercial value to the wood products industry. This wood is identified as the "merchantable
bole," and consists
of the main trunk of commercial species (Table 3.1) from the stump to some
upper limit of merchantability which is determined by local custom (Figure 3.1) and the market.
Table 3.1
Major Commercial Species in Maine
Spruce
Hemlock
Sugar Maple
Balsam Fir
White Cedar
Aspen
White Pine
Soft Maples
Paper Birch
Commercial species trees are considered merchantable if they have a 5.0" D.B.H. (diameter at breast
height or 4.5 ft. above the ground).
"Growing stock" includes that part of a merchantable tree
between a minimum one-foot stump to a minimum 4.0" top diameter (outside bark) on a central trunk.
It is the volume of merchantable bole or growing stock, a limited portion of a limited number of
trees in the forest, which is periodically surveyed by the U.S. Forestry Service (Ferguson and
Kingsley, 1972) or recorded by the Maine Bureau of Forestry from industry records. (Bureau of
Forestry, 1975).
These limited data pose two problems. First, the data are expressed in a confusing set of measures of volume (cords, cubic feet, board feet).
Since the uniformity of shape of the merchantable
bole is lost when we also consider branches, roots, and stumps, biomass is more appropriately quantified in units of weight.
Second, the data record requires extrapolation to provide estimates of
the total biomass available.
A-18
8
6
4#
1:
Roots less than one inch
2:
Medium roots
3:
Large roots
4:
Stump
5:
Merchantable stem
6:
Large branches
7:
Branches smaller than one inch
8:
Unmerchantable stem
Figure 3.1
`L83IV
<5~~~~~~~~~
PARTS OF A TREE
'a9a
hfr poec b4hd
s
Tb
f h s
pd
v
-
l d c
e
r
Thefirtrobemcanbehanledbyusig
evealaveag
conversion factors shown in Table
3.2.
Table 3.2
Conversion Factors*
1 cord (green) softwood = 2.4 tons
1 cord (green) hardwood = 2.7 tons
1 cord
= 85 cu. ft. solid wood
10 cu. ft. bark
= 500 board feet timber (Bureau of Forestry, 1976)
1 cord
*from (Young, 1974, I, p. 98).
The
second problem has a more demanding solution: biomass surveys are required to develop cor-
relations between the observed parameters of the U.S. Forestry Service surveys and the total biomass
available. Such biomass surveys have been performed in Maine on small samples of the total forest
[(Young, et al., 1973)(Young, II, 1974)].
In addition, weight tables have been prepared for a number
of species of trees relating the biomass of the entire tree to the weight of the merchantable bole
[(Young, et al., 1964)(Dyer, 1967)].
Some generalized approximations have been made based on these
biomass studies and weight tables, and are summarized in Table 3.3.
Table 3.3
Biomass Weight Relationships*
A.
Moisture content of fresh tree or shrub = 50%
B.
Fresh weight is composed of :
55%-merchantable bole
25%-tops and branches
20%-stump and roots
C.
Bark content of tree, dry basis =
14%
D.
Leaves as percentage of hardwoods =
Needles as percentage of softwoods =
4%
11%
*from (Young, et al., 1973, p. 300)
A-19
3.1.1.2
Commercial Growing Stock --
Merchantable Bole
Recognizing that these rules of thumb are the best that can be done without further specific biomass surveys, we can begin the task of estimating total biomass from the available survey data.
The
total biomass will be determined by converting the survey volumetric data into weight data and then
applying our rules of thumb to extrapolate to the total biomass. This approach initially includes only
commercial growing stock trees.
Converting to weight, the 1971 inventory of merchantable growing stock (i.e., merchantable bole
only) totaled 311.5 million dry tons of biomass (Table 3.4). The
growth of the biomass
reserves due to merchantable growing stock was 13.0 million dry tons, or 4.2% of inventory (see
Table 3.5).
The net growth of merchantable growing stock was composed of the gross growth, less
Table 3.4
1971 Inventory of Merchantable Growing Stock*
106 Cubic Feet
106 Dry Tons
Softwood
Hardwood
Total
14,763.2
6490.2
21,253.4
208.4
103.1
311.5
*from (Ferguson and Kinglsey, 1972, p. 53)
Table 3.5
1970 Gross Growth in Merchantable Growing Stock*
Softwood
106 Cubic Feet
106 Dry Tons
Hardwood
Total
658.5
235.7
894.2
9.3
3.7
13.0
*from (Ferguson and Kingsley, 1972, p. 50)
mortality, less the cull increment (that part of the growing stock which became non-commercial due to
poor form,rot, or damage).
In the period from 1958 to 1970, mortality was equal to 13.8% of gross
growth and cull increment was 15.3% of gross growth (Ferguson and Kingsley, 1972, p. 59).
We will
assume that only half of the losses under the headings of cull increment and mortality (poor form,
disease, weather, insects, etc.) are accompanied by loss of the fuel value of the wood, even though
the entire commercial value is lost.
Loss in fuel value of biomass that has no "commercial" value
(due to mortality) can be 100 percent and permanent if not collected and properly stored.
The rate of
loss of fuel value depends upon the type of wood, the cause of mortality, the weather, etc.
Thus, the
net rate of growth of our fuel biomass from growing stock is 85.45% of the gross growth, or 11.14 million
dry tons (3.6% of inventory). Note that in this calculation of ultimate biomass reserves, we need not
consider removal rates for commercial wood industry applications.
3.1.1.3
Commercial Growing Stock - Residues and Merchantable Bole
Applying our biomass rules of thumb to the inventory and growth rates for merchantable growing
stock, we find that there was an ultimate existing reserve of 566.4 million dry tons in 1971 (Table
3.6) and that it was growing at a rate of 23.6 million dry tons annually.
A-20
Table 3.6
1971 Total Biomass of Merchantable Growing Stock Trees
106 Dry Tons
Softwood
Merchantable Bole
208.4
103.1
Tops and Branches
94.7
46.9
Stumps and Roots
75.8
37.5
378.9
187.5
Totals
3.1.1.4
,
Total
Hardwood
311.5
141.6
_113.
566.4
Non-Merchantable Biomass
Now let us turn our attention to determining the biomass of the non-merchantable trees.
merchantable trees are called rough or rotten, depending on their species or condition.
Non-
Rough trees
do not meet wood industry specifications for form, or are trees of non-commercial species (e.g.,
sumac or hawthorn). Rotten trees are live trees of commercial species which suffer substantial
rot.
Numerically, rough and rotten trees make up 15% of all softwood trees and 36% of all hard-
wood trees (Ferguson and Kingsley, 1972, p. 50). They represented an additional 76.8 million dry
tons of biomass in 1971 (Table 3.7). Data on the growth of rough and rotten trees were assumed to
be one-half that of the merchantable species when expressed as a fraction of inventory. This
assumption attempts to account for the lower productivity of new biomass by rough and rotten trees.
Note that this is a growth assumption for trees that are already rough and rotten.
It is separate
from the annual cull increment, the biomass of healthy trees that become rough and rotten.
Cull
increment already was accounted for in the assumptions on growth rates for commercial growing stock
trees.
Rough and rotten biomass, therefore, is assumed to increase at 1.8% of inventory, or 1.4
million dry. tons/year.
Table 3.7
1971 Total Biomass of Rough and Rotten Trees
6
10 Dry Tons
Hardwood
Softwood
Total
"Merchantable Bole"
Portion
19.2
23.0
42.2
Tops and Branches
8.7
10.5
19.2
Stumps and Roots
Totals
One final source of biomass exists:
7.0
8.4
34.9
41.9
]5.4
76.8
shrubs, which are woody plants without a well-developed
trunk or with a mature height under 12 feet.
In a biomass survey of public lands in Maine, these
were found to represently only about 0.6% of the total biomass, or 3.9 million dry tons (Young, 1976,
p. 7).
Annual production is assumed to be 1.8% of total reserves, the same assumption used for rough
and rotten trees.
A-21
3.1.1.5
Total Ultimate Reserves
As of the 1971 U.S. Forestry Service survey, Maine had an ultimate reserve of biomass of 647.1
million dry tons. Before removals, this reserve was growing at a rate of 25.1 million dry tons per
year. On a per-acre basis, these figures represent 36.46 dry tons per acre reserves with a growth
rate of 1.41 dry tons per acre year.
These figures represent a snapshot of forestry practice in 1971.
One of the major conclusions
of the forestry survey was that Maine's forests are not producing near their potential. With improved
forest management, particularly the removal of rough and rotten trees which are causing overstocking
and reduced growth, it is estimated that per-acre production could be raised from 1971 levels of
42 cubic feet of merchantable wood per acre to 63 or more cubic feet per acre,* an increase of at
least 50% (Ferguson and Kingsley, 1972, p. 28).
Assuming that the increased production is appor-
tioned equally to hardwoods and softwoods will allow us to increase our biomass estimates by
the same percentage. Since we have assumed that rough and rotten trees are controlled, our 50% increase
applies only to the merchantable trees, to give an annual growth of 35.4 million dry tons.
How do these figures compare with other estimates that have been made? Two other estimates of
the sustainable wood production of Maine's forest are between 1.1 and 3.8 times as large as our extrapolations (Table 3.8).
Table 3.8
Estimates of Biomass Production in Maine's Forests
106 Dry Tons/Year
(Young, 1975, p. 3]
[Monks, et al, 1975, p. 22]
Total
Above Ground
Below Ground
40.00
30.00
10.00
-
-
28.30
7.00
132.75
MIT Extrapolation of U.S. Forestry
Service Information
35.40
Young's estimate also included 30 million dry tons of leaves and needles which we will not consider.
The paper by Monks utilized the opinions of a variety of forestry industry representatives. It is
observed that the estimates of Monks are usually higher than those of others.
Both estimates are for the total biomass which could be obtained on a continuing basis (production
equalling removals) with improved forest management. The MIT extrapolation also assumes aggressive
forest management.
It should be remembered that these figures are for sustained annual production, and do not consider the transient period from now to the time when forestry management is highly developed. During
that transient period several sources of biomass could be exploited, including:
*(Ferguson and Kingsley, 1976) estimate 45% of commercial forest land could yield more than 85 cu ft
per year; 80% could yield more than 50 cu ft/yr. We estimated that the remaining land (20%) would
produce at the current rate of privately owned land, or 38 cu ft/yr. This results in:
(.45 x 85) + (.35 x 50) + (.20 x 38) = 63.4 cu ft/acre.
This is a conservative estimate since some of the acreage will produce above the stated levels.
A-22
e
removal of rough and rotten trees by thinning (76.8 million dry tons)
e
one-time removals of large volumes of trees (Dickey-Lincoln site, 5.5 million dry tons);
spruce budworm infestation (312 million dry tons) (Monks, et al., 1975, p. 30)
e
stores of unused bark and mill wastes (1.4 million dry tons) (Monks, et al., 1975, p. 30)
In addition, there exists the potential for "mining" the forest by deliberately removing a greater
biomass than is being replaced by growth.
The only available estimate of the total biomass for
comparison with our figure (647.1 million dry tons) is 800 million dry tons (Young, 1975, p. 3).
3.1.2
Removals
3.1.2.1 Commercial Growing Stock Removals
At the present time, removals of biomass are generally restricted to the merchantable bole.
maining parts of the tree, about 45% by weight, are being left in the forest as logging slash.
The re-
The slash
eventually rots to return nutrients to the forest floor but in the interim can constitute a fire hazard.
Actually, only 25% constitutes slash and fire hazard, 20% is stumps and roots.
During the period from 1959 to 1970, the removal rate for growing stock was increasing faster
than the growth rate.
The growth rate itself was high due to
epidemic of the early 20th century.
the recovery from the spruce budworm
Many of the trees planted after that epidemic first qualified
as growing stock between 1959 and 1970.
The implications were that hardwood removals would exceed
annual growth by 1973 and softwood removals would exceed growth by 1994.
Net removals would ex-
ceed net growth by the late 80's (Figure 3.2). This means that by the late80's the forestry industry will have begun mining the forest for its merchantable wood.
.
3-
I
,.
t.
~
Z
MILLION
t¢
_ ' UDE
FEET
'1.
-1000
l
-400
1,0
t§6
1916
1
do
_2
MAINE FOREST INVENTORY 1960 - 2000
Figure 3.2
from (Ferguson
and Kingsley,
1972)
What are the implications
for biomass as an energy source?
Wewill
see in the next section that
biomassremoval for energy is most easily accomplishedin conjunction with removals for woodproducts.
Biomass for energy is therefore
related
Determining the exact relationship
to the biohlass removed for wood industry
is speculative
use (Table 3.9).
since there has been no experience with such
A-23
Table 3.9
Merchantable Biomass Removals*
106 Dry Tons
Total
Hardwood
Softwood
1970
3.89
2.12
6.01
1974*
4.46
1.87
6.33
1975*
3.63
1.35
4.98
*from (Bureau of Forestry, 1976, p.1)
As a general rule, it makes no sense to divert
operations on a commercial, or even pilot, plant scale.
economically valuable wood to use as a fuel, so we will assume that only the non-merchantable portion
This would mean that the biomass removal for
of commercial trees are available as biomass for energy.
energy will be about 45/55% of the merchantable wood removals (using our biomass rules of thumb); 20/55%
will be stumps and roots and 25/55 % will be branches and leaves.
2.24 million dry tons.
In 1975, this would have amounted to
In the long term, with improved forest practices and with removals equal to growth,
biomass for energy will be about 15.9 million dry tons (Table 3.10).
Of course, commercial wood will be
diverted to fuel use if its fuel value is higher than its commercial raw material value.
No attempt was
made to evaluate this possible competition and its effects on the Maine wood industry.
Table 3.10
Biomass P.movals and Biomass Available for Energy
106 Dry Tons/Year
Commercial
Biomass
Removals
Biomass Available for Energy
Tops/Branches
Total
1.8
2.3
4.1
22.0
8.0
10.0
18.0
(Monks, et al., 1975, p. 22)*
73.0
26.5
33.2
59.7
MIT Extrapolation of U.S.
Forestry Service Information*
19.5
7.1
8.8
15.9
Stumps/Roots
5.0
1975
(Young, 1975, p. 3)*
*Production = Removals
3.1.2.2 Energy Equivalent of Removals
The energy equivalent of this biomass is found by using our assumption of 9000 Btu/dry lb
(Table 3.11).
In 1975, 0.73 x 1014 Btu were available from the unused portions of merchantable trees.
Table 3.11
Energy Equivalents of Biomass Available for Energy
1014 Btu/year
Biomass Available for Energy
Total
Stumps/Roots Tops/Branches
1975
0.32
0.41
0.73
[Young, 1975, p. 3]*
1.44
1.80
3.24
5.98
10.75
1.58
2.86
[Monks, et al, 1975, p. 22]*
MIT Extrapolation of U.S.
Forestry Service Information
Production = Removals
A-24
1.28
This is approximately one-eighth of Maine's total energy use (Page, et al., 1976, pp. 4-12).
In the
long term, 2.86 x 1014 Btu could be available on a continuing basis with improved forest practices.
In addition, there is the Btu content of rough and rotten trees.
Their current growth of 1.6 million
tons per year has an energy equivalent of 0.029 x 1014 Btu/year.
Improved forest practices would no
doubt alter this figure.
The inventory of rough and rotten trees which could be removed for energy
use and forest management was 76.8 million tons, with a heat content of 13.82 x O
more than four times the annual energy use of Maine in 1975.
14
Btu.
This is
If our extrapolations are replaced by
the larger estimates which have been proposed by Monks, the energy available from biomass is seen to
far exceed Mainels total energy requirements, well into the 1980's.
To summarize our examination of the available biomass for enerqy in Maine, we can say:
3.2
*
There is a need for detailed biomass surveys to remove the large
uncertainties associated with biomass projections.
*
1975 forestry practices could have made available sufficient biomass
to supply one-eighth of Maine's total energy demand.
·
Improved forest management and removal of non-merchantable portions
of commercIal species have been predicted to yield between 2.86 and
Btu/year when removals are equal to growth.
10.75 x 10
*
Rough and rotten trees form an energy stock of 13.82 x 1014 Btu.
*
The ultimate reserves of biomass from Maine's forest, with improved
forestry management, is adequate to meet the demands of the wood
industry and to supply significant portions of Maine's energy demand.
Extractions and Processing
The previous section showed that there are sufficient reserves of biomass to supply both the wood
products industry and significant parts of the energy needs of Maine.
This section will look more in
detail at the technological problems associated with the extraction (including transportation) and
processing of the biomass before conversion to energy.
to us:
Two major harvesting practices are of interest
the commercial timber harvest and the stand improvement harvest.
The commercial timber harvest involves removing the merchantable portions of growing stock
trees from a stand.
The technology for this operation is well developed and increasingly mechanized.
Wood is removed in two forms:
logs or chips.
Logs have application in a number of industries where
the form and strength of the wood is improtant. They are also used in the pulp and paper industry,
where they are usually reduced to chips at the mill.
Recent years have seen the increasing use of
portable chippers for pulp logs, which chip the logs at the roadside in the forest.
allow a greater harvest of fiber, since parts other than the bole can be chipped.
The chippers
Since chipping for
pulp fiber can use parts of the tree which have traditionally been non-commercial, there may be competition between pulp and fuel wood interests for such parts.
This would effectively reduce the amount
of fuel wood available.
Chippers offer reduced manpower and equipment needs (St. George, 1977, p. 54). A typical portable
chipper can chip a whole tree in about 30 seconds, or more than 50 cords per eight-hour shift.
Biomass
for energy would be collected in a commercial harvest by having a separate container for chips from
non-commercial portions of the merchantable trees.
usual way.
The logs or pulp chips would be harvested in the
Such a system simplifies the harvest steps of felling, topping, delimbing, and bucking
since the whole tree can be skidded to the chipper, and can make stands commercially attractive that
were previously too dense for normal harvesting.
When such complete tree chipping is combined with new mechanized harvesters, a highly efficient
operation results.
Equipment is under development and testing which can remove the entire tree and
transport it to the chipper.
Such harvesters are expected to appear commercially by 1980, with a
second generation technology
availabl3 in 1985 (Young, III, 1974, p. 48).
One drawback has been
that complete tree harvesters have generally been developed for bog use and experience difficulty
A-25
when used in rocky, mineral soils.
Maine.
This problem might restrict the harvesting of stump-root systems in
The stump root systems, while unavailable for energy, do return nutrients to the soil and
prevent surface erosion.
Stand improvement harvests are like weeding a garden.
They remove cull timber and rough and
rotten trees in order to increase the growth of commercial species in the stand.
The technology is
more complicated in that culls must be removed without damaging the commercial trees left behind.
Much of this work is not mechanized and relatively expensive. Once the culls are felled and
skidded, they can be chipped like commercial timber. A market for wood chips from cull trees should
be an incentive for the performance of this beneficial forestry management practice (Hall, 1976,
p. 2).
Once the trees or parts of trees are chipped, the chips are blown into trucks for transport out of
the forest.
Chips are somewhat less dense than logs. When chipped, a cord of wood occupies a volume
of 200 cubic feet, a 56% increase in volume.
Since chips are a satisfactory form for input into a wood
combustion system, no further processing, except possibly drying, is needed.
Mechanized har-
Harvesting operations could be seriously hampered by weather conditions in Maine.
vesting will be difficult or impossible during periods of heavy snow cover and during spring thaw when
mud conditions prevail.
The severity of these conditions will vary from year to year.
Harvesting
schedules would have to stockpile sufficient wood to continue plant operation during these periods.
If air drying is used, harvesting will have to be done in advance anyway, and the critical period of
possible shortages would be several months after the winter and spring thaw periods.
Scheduling of
harvest quantities and drying inventories will be a highly complex problem.
Summarizing extraction and processing, we can say:
*
Commercial technology is available for removing as chips the above-ground,
non-merchantableportions of commercialtrees.
4.0
*
tor mechanized complete tree removal (stumps and
Commercial technology
roots included) should be available by 1980, but may not initially work
well in Maine's mineral soil.
*
Technology for thinning and stand improvement is commercially available
but not highly mechanized. Chips are the final product.
e
Weather and drying schedules will complicate harvesting plans.
WOOD CONVERSION SYSTEMS
The previous section has shown that there is a significant amount of energy which can be obtained
from the forest in Maine inthe form of wood chips, without interfering with normal wood industry operWe also saw in the introductory section that direct burning of the wood is the best developed method of converting this energy into electricity. In this section we will examine in detail
ations.
the technology for direct burning of wood for electricity production.
4.1
Waterwall Incinerators
A waterwall incincerator has the walls of its combustion area covered with boiler tubes through
which water flows.
Typically there may be other boiler tubes lining the walls in the portion of the
boiler outside the immediate combustion zone. The combustion gases, on their way to the smokestack,
deliver their energy to the water, converting it to steam inside the tubes. The steam is then used
to drive a turbine which turns a generator to produce electricity.
Waterwall incinerators are the
only technology used for large-scale steam and electricity production.
The technology can be further differentiated by describing the method used to feed
to the combustion area.
the fuel in-
For solid fuels (coal, wood, wastes) there are three common methods:
cyclones, suspension firing, and stokers.
Cyclones require a very fine and dry fuel, which is blown
into the combustion chamber where, upon firing, it forms a vortex (cyclone) of hot gases.
They
are not commonly used for wood firing because of the difficulty in preparing the wood.
Suspension systems also require a great deal of fuel preparation (breaking below 1/4 inch) although not as much as cyclones.
In suspension firing, the fuel is blown into the combustion area
where it burns in suspension while falling to the bottom.
When wood is fired in suspension, an
auxiliary fossil (coal or oil) fuel is required to maintain combustion stability.
A-26
Typically, the wood accounts for about 50% of the total heat content of the boiler fuel input (Hall,
et al., 1976, p. 48).
Stokers are the most common technology for large-scale burning of wood and can be further described as spreader stokers, underfeed stokers and overfeed stokers.
All stokers have at least part
of their fuel burning in a bed at the bottom of the combustion chamber.
Overfeed stokers add new
fuel to the bed from the top or side, using a moving grate or inclined grate which carries the fuel
into the combustion chamber and passesout
the other side with the remaining ash.
force the fuel underneath the burning bed
of old fuel.
Underfeed stokers
Combustion takes place on the top of the bed
and ash is removed from the edges of the pile. A spreader stoker (Figure 4.1) is a special
overfeed stoker.
type of
Instead of the fuel being placed on the moving grate and carried into the combustion
zone, it is projected into the combustion region and falls onto the moving grate at the bottom.
Most
of the fuel burns in suspension with unburned fuel forming a thin layer (2-4 inches) at the bottom
which burns quickly.
The spreader stoker can burn a greater variety of woods and respond more quickly
to load changes than can standard overfeed or underfeed boilers.
TYPICAL WOOD-BURNING BOILER
Figure 4.1
[Wheelabrator, Frye, 1976, p. 5]
A-27
Between 1965 and 1975, 230 boilers with the capability to burn wood as the primary-or supplemcntary fuel were sold in the United States.
The distribution of sales in terms of size, range
and type of fuel-feeding systems (Table 4.1) shows that spreader stokers, while making up 44.7% of
all boiler sales, accounted for 83% of all sales in sizes above 25 MW of electric capacity. Note
that these sales were generally not for electric power generation; the equivalence of steam production and electric generating capacity is given for comparison only.
Also note that in the largest
category wood was used as a supplementary fuel. No wood-fueled boilers were sold having a capacity
above 500 x 103 lb steam/hr (Hall, et al., 1976, p. 12).
Table 4.1
Boiler Sales by Capacity and Firing Method
Firing
Method
0 lb steam/hr
MW electricity
10-16
1-2
16-100
2-10
Spreader Stoker
1.1
25.2
Underfeed Stoker
0
1.4
Overfeed Stoker
0
24.8
Suspension
0
Other
1.1
0
2.15
100-250
10-25
13.3
250-500
25-50
Over 500_
Over 50
2.2
2.9
0
0
0
3.7
0
0
0.5
0
0.5
0.9
0
1.0
[Adapted from Hall, et al, 1976, pp. 50-51]
The efficiency of boilers burning wood is less than that of boilers burning conventional fuels.
The difference is due largely to the heat lost through vaporization of the moisture content of the
wood.
This heat is not available to the boiler tubes and escapes out the stack (Figure 4.2).
For
assumed moisture contents of 50% (green) and 15% (dried), the corresponding overall boiler efficiencies
are 66% and 76% (Table 4.2).
Figure 4.2
BOILER HEAT LOSS VEMIUS WOOD MOISTURE CONTENT
a.
0
a
I
*6
Moisture. percent (wet bosis)
[Hall, et al, 1975, p. 41)
A-28
Table 4.2
Boiler Efficiencies as Function of Moisture Content
Percent Loss
Heat Loss Factors
15% Moisture
50% Moisture
Dry stack gases
9%
9%
Moisture in fuel
3%
13%
Moisture from H2 in fuel
8%
8%
Incomplete combustion,
radiation, unaccounted for
4%
4%
76%
66%
Boiler efficiency
[Adapted from Hall, et al, 1976, p. 42]
All boiler systems designed for "dirty" fuels with high ash contents such as coal or wood require bottom ash facilities to collect ash and unburned fuel, soot-blowing equipment to remove ash
from the boiler tubes where it reduces heat transfer, and pollution control equipment for flyash
removal.
Such equipment is readily available for wood systems.
Load variation can lead to instability in wood boilers.
The less the mass of wood fuel burning
in the boiler at any given time, the more likely that instabilities will occur.
Suspension systems
are most sensitive while overfeed or underfeed stokers are least sensitive. Spreader stokers fall in
the middle range of sensitivity. This sensitivity can be eliminated by burning conventional fuels
in addition to the wood.
for suspension systems.
Limiting heat input from wood to 40-50% of total heat content is recommended
Spreader stokers which are smaller than 500 x 10 3 lb steam/hour (about 50 MW
electric) are relatively insensitive to instabilities; this situation has given rise to the limit of
spreader stokers burning solely wood to 50 MW electric and smaller size plants.
Larger plants
would incur the expense and complexity of dual fuel firing.
After the steam is produced, it must be converted into electricity using conventional turbine
and generator technologies.
Efficiencies for turbines are a function of the steam conditions and
turbine size. We will assume that a 50 MW turbine generator combination is used.
Estimates of the
total efficiency of such plants have been stated as being between 27.4% (Hall, et al., p. 109), and
18.3% (Lloyd, et al., 1975, p. 33).
These 50 MW plants are base load capacity due to the difficul-
ties in load following with wood-fired boilers, and are assumed to have a 70% load factor; i.e.,
they produce 70% of the energy theoretically possible from round the clock,full capacity operation.
Estimates for load factors have ranged from 82% (Schultz ,1977, p. 6) to 65% (Lloyd, et al., 1975, p.
38). With a load factor of 70% and capacity of 50 MW, the energy generated in a year would be
306,000 MWH.
With 27.4% efficiency, this would require 3.82 x 1012 Btu of heat input; at 18.3%
efficiency, heat input would be 5.72 x 1012 Btu.
Our assumed value of 9000 Btu/dry lb means that
at 27.4% efficiency, 424,000 green tons/yr (212,000 dry) or wood are needed; at 18.3% efficiency,
636,000 green tons/yr are needed (318,000 dry).
4.2
Multiple Boilers
Although the maximum size boiler unit is 50 MW, several units could possibly be combined to form
a single plant of total capacity greater than 50 MW.
These could operate independently with their own
turbines, or several 50 MW boilers could use a common header arrangment, in which steam from the various
boilers is fed to a single turbine. A common header arrangement might offer economies of scale for the
turbine, auxiliary and cooling systems, and plant structures. The limiting consideration in either
case will be the feasibility of maintaining an adequate wood supply.
average biomass for energy removal rates.
Recall that those rates
A-29
In Section 3.1, we calculated
assumed widespread forestry management, near theoretical yields and removals equalling annual growth.
Continuation of current productivity per acre would yield about 33% less biomass. The per-acre
biomass production rates (using 17,748 million acres of forestland) could be between 0.60 and 3.36
As a check against these numbers, removal rates calculated for Vermont
dry tons/year (Table 4.3).
were considered.
For central Vermont, removal rates corresponding to our U.S. Forestry Service
(current productivity) figures for branches and tops were estimated to be 0.25 dry tons/acre (Hall,
et al., 1975, p. 128).
This difference of 25% between estimates is considered an acceptable check.
Table 4.3
Estimated Biomass Available for Energy Per Acre
Dry Tons/Year
Stumps/Roots
Total
Tops/Branches
(Young)
.45
.56
1.01
(Monks)
1.49
1.87
3.36
MIT-U.S. Forestry Service
(USFS) (High productivity)
.40
.50
.90
MIT-U.S. Forestry Service
(USFS) (Current productivity)
.27
.33
.60
With the data from Table 4.3, and our assumptions about the demand for wood by a 50 MWe plant,
we can determine that the area required to support such a plant would be between 63,100 and 530,000
acres
(8:1), (Table 4.4).
Assuming that the plant will be centrally located in its harvest area,
this would require a radial harvest distance of between 5.6 and 16.2 miles
(3.1).
If we were to
assume that only the above-ground biomass could be harvested, as was done in the Vermont study, then
our area would increase by 80%
(113,000 - 934,000 acres), and the distance by 34%.
When considering
harvest areas, as in the case of multiple plants, the increase in area varies linearly while the
harvest distance varies as the square root of the capacity.
Because of this difficulty in maintaining
wood supply, multiple separate 50 MW plants are preferable to one large central plant.
The spread of numbers is huge. The only way a more exact number can be arrived at is through a
pilot plant and time.
such a project.
Vermont and ERDA
(U.S. Energy Research and Development Administration) plan
It is anticipated to be funded in late 1977.
Three or four years of data, after
operation starts, will be needed before harder numbers are available.
Table 4.4
Harvest Area and Distances from 50 MW Plant at 70% Load Factor
Area in 103
Acres
-Distance in
Miles
Eff = 27.4%
Eff = 18.3%
Dry Ton/Year
Area
Area
Distance
[Young]
1.01
209.9
10.2
314.9
12.5
[Monks]
3.36
63.1
5.6
94.6
6.9
MIT-USFS (High productivity) .90
235.6
10.8
353.3
13.3
MIT-USFS (Current
nroductivity)
353.3
13.3
530.0
16.2
Rate
Removal Assumption
.60
A-30
Distance
4.3
Gas Turbine Systems
A pilot plant test is under way in Masardis, Me., with Economic Development Administration
support, to examine the performance of a gas turbine system fired by wood fuel (Hagen and Berg, 1976).
The system will use a ceramic heat exchanger to transfer combustion heat to the gas turbine.
The system, which has been developed by Hague International and the Maine Wood Fuel Corporation,
provides a power source in the 1 to 2 MW range.
It is intended as an industrial power source for
the wood industry and can be modified to provide additional energy in the form of process steam or a
bottoming cycle.
These systems are still in the pilot demonstration stage and are relatively small
and have specialized applications. They should not be expected to make any significant impact on
statewide electricity supply in the near future.
6 x 106 MWH (Page, et al., 1976, pp. 1-2).
In 1974, the pulp and paper industry consumed about
If the proposed gas turbine systems produce 8.5 x 10
MWH/year
(1.5 MW, 65% load factor [Hagen and Berg, 1976, p. 9]), and industrial electricity growth is 2%/year,
nearly ninety gas turbine systems will have to be installed and operating to supply the required
energy by 1985.
This is unlikely to happen with the technology still in the pilot plant stage.
We can summarize our discussion of wood combustion technology as follows:
e
Proven technology is available from commercial vendors to produce steam and electricity
from wood fuels.
e
A 50 MWe waterwall spreader stoker boiler is the technology most likely to be used.
*
Biomass inputs for a 50 MWe plant range from 212,000 to 318,000 dry tons/year at 70%
load factor.
a
Multiple boilers can be located in the same site but wood supply becomes difficult. The
harvest area can vary by a factor of ten, depending on biomass availability assumptions.
e
Wood-fired gas turbine technology is not expected to make a large impact on electricity
supply patterns in the near future because of its intended limitation to industrial use.
5.0
ENVIRONMENTAL CONSIDERATIONS
The environmental effects of utilizing wood combustion to produce electricity are presented in
this section. These effects include boiler emissions, residue disposal, water pollution, and land
use.
A recent extensive study for the U.S. Environmental Protection Agency of a proposed wood-burning
electrical generating plant in Vermont is the basis for most of these discussions (Hall, et al., 1976).
5.1
Boiler Emissions
The production of steam of electricity by combustion always results in emissions to the atmosphere.
Emissions are usually expressed in terms of lbs of emissions per ton of green wood burned as fuel
(Table 5.1).
Table 5.1
Emission Factors for Wood and Bark in Boilers
Pollutant
Source:
Emissions lb/ton Green Fuel
Particulates
25-30
Sulfur Oxides (SO )
0 (wood) - 3 (bark)
Carbon Monoxide
2
Hydrocarbons (Methane)
2
Nitrogen Oxides (NO )
10
(USEPA, I, 1973, p. 1.6-2)
A-31
The lower ash content of wood is offset by the higher heat content of coal, resulting in wood
particulate emissions per unit of output (.21 lb/106 Btu) that are higher than those of coal (.04 lb/106
Btu).
Wood ash tends to be lighter and larger than coal ash (Figure 5.1), but the distribution of
emission particles depends on reinjection practice. Since some of the ash is unburned, reinjection
to the boiler of the ash collected by particulate control devices will increase thermal efficiency.
Reinjection reduces the size of the ash and increases the total input loading on the ash collection
device.
Other factors affecting particulate emissions are the type of fuel (logs, sawdust, etc.),
boiler type, grate design and fuel moisture content.
.,...
1-/
-.
,..''.:..
Figure 5.1
SIZE DISTRIBUTION OF WOOD AND COAL FLY ASH
'i
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-'
-
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.
mesh-
=-:,
-3---
,-t_ ,
i
1t-I-j
.:",,
- ---..
._ _
.:. ........ - _
-::
--;:
.... , .....-:
_-.
- --.-..:J:: '---
'- -]'-=]-
t--
:
.h /~J
z
-
.E
0'
.
.- with reinjection
L.I
!
t
....
--- I S: .:.Si: .':
--I:. --i'::-..
:.
LfiiL14
,:
"'~''
.__... - -'-:k: *'..--"- - -1
-
..
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,
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without reinjection .
/
I
··-
r ·· · -·-
;··-·
··
;·.
i··
LL:
:·
·. L
i
ii.-..
i·
1.!.LTi·ii
I:
-I·s.L:
!li·-!·--·
..-i..i
'tj·
.ri
I
·/
i
-
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.:
i"`lf!i
'-
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'"' ·';. .'I.
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i
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:·
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i-i.
I
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0.01
[Hall, et al, 1976, p. 63]
0.1
I
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1:··:
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r
:
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.
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ss
10 20
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-·-ii-·-l'
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% Smaller Than
A-32
i'l
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99
I
-=
99.9
L-
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99.99
Particulate emissions are sufficiently high to require removal devices in order to meet standards.
Experience with wood boilers has been limited to cyclone collectors, but there should be no serious
technical problems in using other well established collection devices (Table 5.2). We acknowledge
that the effluents from a 100% wood furnace can be quite different, but believe that they could be
handled.
A survey of their performance and costs can be found in (Hall, et al., 1976, pp. 59-76).
Table 5.2
CALCULATED WOOD ASH COLLECTION EFFICIENCIES
FOR VARIOUS CONTROL EQUIPMENT
Efficiency(a), %
Type of Collector
Long-cone cyclones
94.2
Multiple cyclone (6-in. dia)
98.4
Electrostatic precipitator
98.7
Wet-impingement scrubber
99.8
Venturi scrubber '
99.96
Fabric filter
99.98
(Hall, et al., 1976, p. 71)
Source:
Wood fuels have negligible sulfur contents. As a result, they present little or not problems
with respect to sulfur oxides emission standards. Carbon monoxide levels are similarly within acceptable ranges. No control equipment should be required for either pollutant.
Hydrocarbon emissions from wood-fueled boilers are negligible during normal operation. Rare
periods of poor combustion might release hydrocarbons at a rate ten times normal. Of principal concern
are carcinogenic and photochemical compounds. There appears to be evidence that carcinogenic compounds may be emitted in low concentrations from wood-fired plants. Emissions are smaller than
those expected from a similarly sized coal-fired plant.
Photochemical compounds will be released
during periods of poor combustion, and could lead to the formation of photochemical smog. Normal
operating procedures should prevent photochemical smog formation from being a problem (Hall, et al.,
1976, p. 78).
6
Btu, which is higher than the new
Nitrogen oxide emissions are estimated to be 1.235 lb/10
6
Btu. While wood-burning plants are not regulated by the new
be pessimistic
source standards, these emissions are a cause for concern. The NO x emission rate may
considering the manner in which it was determined. (The rate was calculated rather than derived
source federal standard of 0.7 lb/10
from experimental data and assumed a rather low wood heat content of 4200 Btu/lb.) Boiler design
(Hall, et
could be modified to restrict excess air and reduce NO..emissions to an acceptable level
al., 1976, p. 95). No information was found on operating experience for wood boilers with restricted
excess air for NOx control.
For a 50 MWe plant, with fuel collection and transport included, total emissions are comparable
to, or less than, conventional fossil-fired plants (Table 5.3).
Table 5.3
TOTAL ANNUAL EMISSIONS FROM DIFFERENT FUELS (50MW PLANT)
(Ton/Year)
Total
Particulate
Organics
540
348
423
3675
115
600
351
432
3825
248
48
5152
SO2
137
CO
Dried Wood (280,000 ton)
NOx
2227
Green Wood (410,000 ton)
2326
Low Sulfur Coal
1865
2685
305
Domestic Oil
1203
1075
21
89
59
2447
Pennsylvania Coal (stack gas
cleanup)
1193
874
111
178
27
2382
A-33
---- ~~~~A-33
derived from (Hall, et al., 1976, pp. 22-23)
5.2
Boiler Residues
The combustion of wood results in ash, char, clinkers, and slag, in addition to the gaseous
combustion products. There have been no unique problems identified for the disposal of these solid
products. The residues are primarily inert silica and alumina (Table 5.4) and can be disposed of in
sanitary landfill or as a slurry in lined lagoons. Treatment may be required to prevent leaching,
groundwater contamination, or dust. Byproduct uses are still largely unexplored. Suggested applications have included soil conditioning, fertilizer, charcoal briquet manufacture, and preparation
of a combustion preventative. Assuming a typical ash content of 1.3% (Table 2.2) and a fuel requirement of 212,000 ton/year (Section 4.1), total solid wastes would be on the order of 2,700 tons/year.
These would include both boiler residues (about 2,400 tons/year) and collected particulates (about
300 tons/year).
Table 5.4
ANALYSIS OF WOOD FUEL ASH
Components
Concentration, ppm
Silicon (Si)
[Hall
5.3
19.6
Aluminum (Al)
3.6
Calcium (Ca)
2.9
Sodium (Na)
2.1
Magnesium (Mg)
0.8
Potassium (K)
0.3
Titanium (Ti)
0.1
Manganese (Mn)
0.016
Zirconium (Zr)
0.006
Lead (Pb)
0.003
Barium (Ba)
0.010
Strontium (Sr)
0.002
Boron (B)
0.003
Chromium (Cr)
Less than 0.001
Vanadium (V)
Less than 0.001
Copper (Cu)
Less than 0.001
Nickel (Ni)
Less than 0.001
Mercury
Nil
Radioactivity
Nil
et al, 1976, p. 80]
Water Pollution
Water pollution can occur at the plant or in the forest. Environmental problems due to waste heat
disposal at wood-fired plants are similar to those found in conventional fossil plants. With efficiencies
between 28% and 18%, wood-fired plants must release between 2.5 and 4.5 units of heat to the environment
for every unit of heat converted to electricity. Roughly 0.5 - 1.0 units are released to the atmosphere
depending on fuel moisture content. The rest are released through the condenser as heated water. Adequate
commercial technology exists for cooling the heated water with cooling towers. Cooling towers release
cooling tower blowdown containing chemicals which can require environmental control. They also require
makeup water.
Neither of these should have any serious effects on local water quality.
During wood removal several water pollution impacts are possible. These impacts depend on the nature
of the forested land before harvesting (topography, soil composition), the physical and chemical status of
local streams, and the harvesting practice used. Clear cutting and removal of the waste biomass, especially
stumps and roots, has high potential impact on water quality.
A-34
--
Water quality will be affected first by increased stream flow as runoff increases on the
harvested land.
This occurs because living vegetation no longer absorbs and transpires surface
water.
Water temperature in streams can change, especially if vegetation on stream banks is re-
moved.
Not only do the maximum temperatures increase with increased solar radiation, but the
daily variation increases also. Turbidity changes in streams during normal above-ground logging
operations are due principally to the construction of logging roads and skid trails.
There is
no experience with turbidity changes due to removal of the complete tree including stumps and
roots, but they should be expected to increase since there will be widespread disruption of the
surface soil layer.
The final possible impact is increased nutrient loading of streams once ve-
getation is removed.
Contradictory experimental results have been obtained in different clear cut
operations, depending on the harvesting method and the rate of regrowth.
If large amounts of dis-
solved nutrients like ammonium, nitrate, calcium, etc., enter the streams, they could cause eutrophication while simultaneously reducing the productive potential of the land.
While these impacts are varied, they are similar to problems controlled in normal logging operations
through simple forestry management procedures. We have much less experience with intensive biomass removal.
It is believed that forestry management should be able to control any potential adverse water
quality effects (Hall, et al., 1976, pp. 175-185), but only field experience can verify that no major
problems have been overlooked.
5.4
Land Use
The land required for fuel harvest for a wood-fired generating plant is subject to several possible impacts. While it is serving as a fuel source, the land is still available for normal commercial logging use and recreation.
Land use impacts at the plant will include roads, fuel stockpiling
and drying areas, and solid waste disposal facilities.
The nutrient budget of the forest determines its productivity and longevity. Experience
with the impact of whole tree removal on the forest's nutrient budget is limited.
factors affecting the budget are:
The three main
nutrient removal, nutrient inputs and rotation period.
Nutrient
removal is a function of the parts of the biomass which are removed since different parts of the
same tree have different concentrations of nutrients.
By selectively leaving parts of the tree
in the forest to rot, the removal rate can be regulated (Table 5.5). This has been studied for whole
tree removal for pulping, where the discarded parts had no pulp value.
For an energy recovery
operation, any parts of the tree left in the forest to maintain nutrient balance would represent
an energy loss.
Table 5.5
Comparison of Two Harvesting Techniques in Finland
Nutrients Removed
kg/ha
Technique
N
Ca
K
P
58
73
38
5.3
148
115
80
15.0
95
184
47
8.4
372
409
161
40.6
Pine Forest
Timber Cut
Whole Tree Harvest
(including root)
Spruce Forest
Timber Cut
Whole Tree Harvest
(including root)
[Hall, et al, 1976, p. 167]
A-35
Nutrient inputs naturally come from soil decomposition, rainfall, and the decay of vegetation
(Table 5.6).
Fertilizer can also be applied, although this is a fairly recent practice which has
been used to improve the productivity of soils which are naturally deficient or which have been
exploited by previous harvest methods.
It is expected that nutrient budgets for normal harvest
periods should not be a limiting factor in utilizing biomass for fuel, since natural inputs will be
adequate. Short rotation forestry, fast growing species or dense planting are practices which
can depelete the budget quickly.
Table 5.6
REPLENISHMENT OF SOIL NUTRIENTS IN A
NORTH CAROLINA HARDWOOD FOREST
Nutrients (kg/ha)
N
Source
Rainfall
Canopy Drip
Stemflow
Litter
Annual Input
to Soil
3.53
4.86
0.23
45.98
54.60
P
K
Ca
0.28
0.61
0.01
3.26
4.16
0.88
17.48
0.65
14.16
33.17
3.42
12.47
2.02
94.99
120.90
k
.72
3.5
a24
1IL11
2.82
[Hall, et al, 1976 p. 174]
Data on nutrient removal is site-specific and harvest-specific, while nutrient addition to
offers us a meansfor
forests are variable and poorly quantified at present. While fertilization
compensatingfor imbalances caused by biomass harvests, close monitoring of nutrients will be
necessary (Hall,
et al.,
1976, pp. 168-174).
Erosion is also a site-specific problem which can largely be controlled by good forestry management.
Experience has shown that logging roads and skid trails are the principal causes of erosion
in a harvested area, even when clear cutting is used.
which substantial numbers of stumps are removed.
Data are not available for harvesting in
Such an operation would disrupt the soil surface
more than clear cutting, and the extent of erosion would probably be greater.
Land use problems can also arise in terms of wildlife and aesthetics.
Intensive forest use
can possibly jeopardize wildlife habitats and ruin the attractive features of forest appearance.
These problems are not unique to biomass and energy plans can be adquately managed with current
forestry practices (Hall, et al., 1976, pp. 194-199).
In summary, it can be said that:
*
Environmental impacts of wood combustion are well understood
and are comparable to those of similarly sized fossil plants.
'
Environmental impacts of wood procurement are well documented
for conventional harvesting practices and can be controlled
satisfactorily by forestry management methods.
A-36
0
6.0
Biomass procurement for energy will put additional stress on
the forest environment because of its increased removal of
material and greater disruption of the soil; while these
effects should be controllable by conventional forestry management, they must be monitored carefully to avoid site specific
problems.
ECONOMICS
In our examination of wood supply we have seen uncertainty and a wide variation in the projected
availability of wood on a continuing basis.
For our economic analysis, we will initially consider
a conservative wood supply case:
e
wood removal for energy will be limited to the unused above-ground portion of trees cut
for merchantable timber or chips, and
*
commercial removals will occur at the 1975 rate
From Table 3.10this will result in a removal rate of fuel wood of 0.26 green tons/acre year (50%
moisture).
This conservative assumption ignores the potential of increased mechanization of wood
harvesting for the whole tree, improved forest management and the use of rough and rotten trees.
A 50 MWe (net) plant will be considered. Assuming an efficiency of 28% and a capacity factor of
70%, about 415, 000 green tons/year would be required as fuel to generate 307 million KWh. The
harvest area would be about 1.6 million acres with a collection radius of 28 miles.
We will express our costs in terms of 1986 dollars.
Escalation will be assumed at a simple
5% annual rate, and a levelized carrying charge of 18% will be used.
At the end of our analysis,
we will consider the sensitivity of costs to our wood availability assumption.
A-37
6,1
Wood Fuel Costs
The cost of wood fuel at the power plant is the sum of the costs of procurement, harvesting,
chipping, and hauling.
An estimate of the costs of these individual items will be developed by exami-
ning similar costs for commercial wood operations.
6.1.1
Procurement Costs
The cost of wood harvesting privileges, called stumpage costs, vary with the value of the wood,
the difficulty of the harvesting conditions, and a number of other less critical factors.
Stumpage
prices for Maine are compiled by the Bureau of Forestry by species, end use and stumpage price zones.
An average of recent stumpage prices (Maine Forest Service, 1976, p. 3) weighted by the volume of
growing stock for each species, resulted in a cost of $3.50 per green ton (1976 dollars).
This is
somewhat higher than other available, but older, data which averaged $2.80/green ton and ranged from
$2.45 to $3.15/green ton (Maine Office of Energy Resources, 1975, p. 2).
This is an added conserva-
tive factor.
Stumpage prices are usually paid for the harvest of valuable sawlogs and poles, although sometimes woodland residues also will be chipped for pulpwood.
Removing residues or dead and culled trees
for stand improvement costs landowners on the order of $25/acre, so landowners might be willing to
permit the removal of residues at no stumpage price.
In Vermont, it has been suggested that $0.50
to $1.50/green ton (Hall, et al., 1976, p. 130) would be sufficient inducement to allow multiple harvesting (removing wood for both commercial and fuel purposes). A cost of $1.00/green ton (1975
dollars) will be used in this analysis.
6.1.2 Harvesting Costs
Harvesting in Maine could be nearly a year-round operation. A combination of mechanical and
chainsaw harvesting is used with wood removed in the form of chips or logs. Since we have assumed
that wood fuel harvesting will take place in conjunction with commercial harvesting, we need to determine
only the incremental costs of harvesting equipment and labor and chipping the residues from
the commercial harvest.
Conventional harvesting costs for wood were estimated by considering the 1972 financial data for
SIC 241 for Maine, Logging Camps and Logging Contractors (Table 6.1).
stock was approximately 10.3 million green tons.
The 1972 harvest of growing
If all of the tree is chipped, there will be a
saving, particularly in hardwood where so much of the tree is wasted.
But, on a partial basis,
where round, merchantable wood is sent to the mill and residue to the chipper, the savings would
be much less.
Assuming waste wood harvesting costs will be 50% of the normal harvest costs (we con-
cede that this is optimistic), the incremental cost of harvesting residues in Maine will be $8.05/ton
(1986 dollars).
It may be necessary to increase this 50% as an incentive to keeping harvesting with-
in the assumed transport radius of 28 miles.
Table 6.1
Wood Harvesting Costs in 1972
10.3 million tons
106$*
$/ton
25.1
2.44
4.15
Materials
65.3
6.34
10.78
Taxes, Profits, etc.
28.6
2.78
4.73
-
(2.10)
Payroll
(Stumpage Cost)
9.46
*from (U.S. Bureau of Census, 1975, p. 1-139)
A-38
1986 $/ton
(3.57)
16.09
Chipping experience in Michigan averaged 1000 tons of green chips per chipper per week (Hall, et
al., 1976, p. 34).
Since a 50 MW plant requires 415,000 tons/year or 8650 tons/week, nine chippers
would be needed if chippers operated 260 days/year. With maintenance and weather restrictions, 11 or
12 chippers would probably be needed.
Chipper annual rental costs for a 12-chipper, whole tree
chipping operation have been estimated at $0.73/cord (St. George, 1977, p. 54), or $0.30/ton (1975
dollars).
The Michigan experience found operating costs of ancillarymachinery and the chipper itself
to be $0.77/ton (1974 dollars).
Wages for the labor in the chipper operation were estimated to be
$0.65/ton (1975 dollars) (Hall, et al., 1976, p. 135).
By 1986, the in-forest chipping costs of chipper rental, chipper operation, ancillary operation
costs and labor will be $2 .7 0/ton.
6.1.3
Hauling Costs
Hauling costs vary in different parts of the state and according to the type of equipment used.
One estimate for Maine puts the cost at $1.00 per loaded truck mile (1975 dollars) for tractor-trailer
vehicles. Such trucks carry about 33 tons of logs or 37 tons of chips; hauling for green chips therefore costs $0.027 per ton mile.
The calculated collection distance of 28 miles (assumed to be the
radius of a circle) is smaller than the actual maximum hauling distance since trucks must follow
terrain features and logging roads are limited in number.
If we assume that the maximum hauling distance
is 50% greater than the radial collection distance, then the average hauling distance (2/3 of the
radius) is 28 miles.
The 1986 per-ton hauling rate will therefore be $1.17 per ton..
Total fuel costs are $13.47/ton (1986 dollars) (Table 6.2).
Table 6.2
1986 Fuel Cost Components for Wood
Item
1986 $/ton
Procurement (Stumpage)
1.55
Harvesting
8.05
Chipping
2.70
Hauling
1.17
Total
6.2
13.47
Forest Management
Removal of residues could have a positive economic effect on the productivity of Maine's forest,
especially if rough and rotten trees are removed at the same time.
This effect cannot be quantified
without field experience with a wood-for-fuel removal program.
If it is determined that nutrient removal is serious, fertilization may be needed.
In Vermont
this has been estimated to cost about $1.45 per ton of wood (1975 dollars) if all nutrients must
be replaced with fertilizer (Hall, et al. , 1976, p. 138). Natural replacement should make most such
treatment unnecessary, but we will assume that 25% of this possible cost should be included in our
analysis, for a cost of $0.56/ton (1986 dollars).
6.3 Power Plant Costs
The major components of the cost of the power plant are the capital costs, operating and maintenance costs, fuel costs, cooling facilities, and transmission. Fuel costs have just been calculated.
A-39
6.3.1
Capital Costs
Two estimates of capital investment are available for a 50 MW wood-burning plant.
One reported
number is from a vendor of wood and bark-burning boiler systems for the paper industry (WheelabratorFrye, as reported in [Schultz, 1977, p. 6]). For a two-boiler, two-turbine plant, installed costs
will be between $48 and $55 million, or roughly $1000 to $1100/kw. Emission control costs are not
given separately, and costs are in "mid-80's" dollars (assumed here to be 1986 dollars). The other
number (Hall, et al., 1976) is based on a plant cost of $40 million and emission control costs of
$2.5 million (1980 dollars). When adjusted to 1986, the second estimate is $55 million or $1105/kw.
Using our assumed levelized carrying charge of 18% per year and $1100/kw costs, our annual carrying
and capital charges will be $9.900 million.
6.3.2
Operating and Maintenance Costs
Using the same two sources of information, we get 0 & M estimates of $710,000 per year (Hall, et
al., 1976, p. 141) (1980 dollars) and 4.0 mills/kwh (Schultz, 1977, *p. 6) or $1,226,000 (1986 dollars).
On a 1986 basis, the first cost would be $923,000.
It is assumed here, though not mentioned in the
references, that the 0 & M costs include the 0 & M for the emission controls. We will use the 4.00
mills/kwh assumption.
6.3.3
Cooling Equipment
Recent years have seen considerable opposition to once-through cooling systems. Schultz does not
mention whether cooling technology is included in his estimate.
cooling is used.
Hall et al. assume once-through
This is probably an unrealistic assumption considering the inefficiencies of wood-
burning plants (which cause large heat rejection to the environment) and the desirability of locating
them in the center of a wood collection area.
Cooling tower capital costs are approximately $55/kw (1986 dollars) for 800 MW coal plants in
New England.
The assumed 18% levelized charge yields an annual cost of about $495,000 for cooling
investment.
6.4
Total Costs
Using the assumptions summarized in Table 6.3, the costs of the various elements of a wood-burning
plant can be combined to yield a total annual cost of electricity of $56.8 mills/kwh (Table 6.4).
Table 6.3
Cost Assumptions
e
Above-ground removal of traditionally non-merchantable parts of
commercial trees at 1975 production rate (0.26 green tons/acre).
e
50 MWe (net) plant, 28% efficiency, 70% capacity factor.
*
5% simple annual cost escalation rate.
*
18% levelized carrying charge.
*
Harvesting of wood fuel in conjunction with commercial harvest
($1.00/ton stumpage fee - 1975 dollars).
*
Harvesting costs based on 1972 SIC-241 data; 50% incremental cost
for fuelwood harvest.
*
Mobile chippers in forest chip fuel wood before hauling.
e
Truck hauling of chips with average 28 mile trip.
e
25% nutrient replacement of fertilizer.
*
Capital costs include emission controls ($1100/kw).
*
Cooling towers used for cooling ($55/kw).
e
Site, site preparation, transmission, licensing, environmental
survey costs, and financing during construction not included.
A-40
_I
We have been unable to determine the exact plant components which were included in the estimates
of capital cost.
For example, it is not clear what wood chip handling and storage equipment is in-
cluded or whether ash disposal is considered. The capital charges on fuel inventory (the wood chip
stockpile) may or may not be included in the stated 0 & M costs.
These and other detailed costs can
only be clearly defined during an actual design study, or through field experience, which was beyond
the scope of this report.
The costs given in Table 6.4 should be considered as an approximate lower
bound to the costs expected from an actual plant beginning operation in 1986.
By varying our assumptions on fuel cost and efficiency, we can generate trade-off curves of electricity cost from a 50 MW plant (Figure 6.1). Table 6.2 showed that hauling is only about 10% of the
total fuel cost, or, by Table 6.4,about 4% of total costs.
Our assumption of forest productivity is
therefore seen not to be critical to overall costs.
Table 6.4
Annual Electricity Cost from 50 MW
Wood-Burning Plant - 70% Load Factor
28% Efficiency
Item
Annual Cost $ Millions
Mills/kwh
Basic Plant + Emission Controls
9.900
32.2
Operation and Maintenance (4.0 mills/kwh)
1.228
4.0
Cooling Towers
0.495
1.6
Fuel Cost ($13.47/ton)
5.590
18.2
Forest Management ($0.56/ton)
0.232
0.8
17.445
56.8
Totals
We can summarize our economic analysis by saying the following:
t
Wood-burning plants might be able to produce electricity in the range
of 50-70 mills/kwh
*
Fuel cost is roughly one-third of total cost at 28% efficiency.
a
Harvesting costs dominate fuel costs.
e
Efficiency and fuel costs are the two principal variables in
determining price.
*
More reliable cost estimates will require a specific design study.
A-41
-
Figure 6.1
ELECTRICITY COSTS FOR WOOD BURNING
POWER PLANT - 50 MW
1986 dollars
80
75
Of
(I)
70
.=J
.11
CJ
0M
65
:3
-j
ct
O.0
-j
60
0
U,
IC__
Z
55
w
C)
-j
LU
18
20
22
24
26
PLANT EFFICIENCY - %
A-42
_
I__
__
__
I
I_
28
7.0
CONCLUSIONS
*
Accurate estimates of the biomass availability from Maine's forests have been prevented
by inadequate biomass survey data.
e
The forests of Maine could provide at least 8.8 million dry tons of wood residues per year,
with a heating value of 1.58 x 1014 Btu.
e
Advanced forestry practices could increase the potential supply of wood residues to as much
as 59.7 million dry tons per year, with a heating value of 10.75 x 1014 Btu.
e
Commercial technology is available for harvesting the above-ground portion of the biomass
in the forest without disrupting normal forestry industry operations.
removal is not yet available.
e
Mechanized stump
Maine's rocky mineral soil,will present problems.
Commercial technology is available for burning wood chips to generate electricity.
Boiler
sizes are limited to about 50 MW by stability problems.
e
The logistics of obtaining a continuing supply of wood residues are potentially difficult.
No wood burning plants of 50 MW size are in operation or being constructed
for electricity
production, so there is no experience with the full scale logistics problem of a biomass
power plant.
e
Electricity costs from a wood burning plant are between 50 and 70 mills/KWH for a representative set of assumptions and data based on normal forestry experience.
A-43
- -
8.0
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