Industrial wood combustion systems [Dec. 1980]

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.Si GEORGIA FOREST UA3 RESEARCH PAPER

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13
December, 1980
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INDUSTRIAL WOOD

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COMBUSTION SYSTEMS

DOCUMENTS UGA LIBRARIES

BY

WILLIAM S. BULPITT, CAROL L. ATON AND J. FRED ALLEN

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RESEARCH DIVISION

GEORGIA FORESTRY COMMISSION

AUTHORS

William S. Bulpitt is Chief of the
Wood Energy Systems Branch of
the Georgia Tech Engineering Ex-
periment Station. He received his B.S.M.E. degree from Georgia Tech in 1970 and his M.S.M.E. from
Georgia Tech in 1972.

Carol L. A ton is a Research Engi-
neer with the Georgia Tech Engineering Experiment Station. She received her Bachelor's degree in Mechanical Engineering from Valparaiso University in 1970.

J. Fred Allen is Wood Energy Spe-
cialist with the Georgia Forestry Commission, Research Department.
He received his B.S.F. degree from
the University of Georgia, School of Forest Resources, in 1970.

ACKNOWLEDGEMENT
Much of the data for this publication came from "A Feasibility Study For Wood Energy Utilization In Georgia" prepared for the Geor-
gia Forestry Commission by the Georgia Institute of Technology, Engineering Experiment Station, under the sponsorship of the Coastal Plains Regional Commission.

INDUSTRIAL WOOD COMBUSTION SYSTEMS
BY
WILLIAM S. BULPITT, CAROL L ATON AND J. FRED ALLEN

BACKGROUND

Since steam power began to be an important source of energy in the eighteenth century, there has been a gradual evolution of hardware to the point we find ourselves today. The original fuel for steam production in this country was, of course, wood. As coal gained more widespread use, wood burning for industrial steam production became almost a lost
art. In fact, coal and wood both nearly
disappeared from the industrial scene in the southeast with the switchover to

natural gas and oil as boiler fuels in the
1940's and 1950's. Wood burning systems
were not ignored by all industries, however. During the past 50 years, the forest products industries have seen the gradual development of wood burning boilers from rather primitive pile burners to units that are capable of producing up to 50,000 Ib/hr of steam at pressures that rival the steam pressures found in central
utility plants.
Boilers may be divided into two gener-

al classes -firetube boilers and watertube boilers. As the designations indicate, in the firetube design, the combustion gases travel'through steel tubes passing through a water tank. In the watertube design, the water passes through steel tubes that are heated on the outside by the hot gases from the combustion process. Today
many people have the tendency to dis-
miss the frretube boiler as an old-fashioned design, but this is not always the case.
There are many applications where

r^
3-PASS DRYBACK

4-PASS. DRYBACK

Figure 1 FIRETUBE BOILER GAS FLOW PATTERNS

the firetube unit has distinct advantages over a watertube unit, particularly in
light and medium industrial applications.
Figure 1 illustrates several configurations used in firetube boilers while Figure 2 illustrates a circulation pattern typical of a watertube boiler.
Besides the firetube and watertube classifications, boiler designations can be made in another manner: boilers can be "package" boilers or "field-erected" boilers. These designations can cause some
confusion since virtually all wood burn-
A ing units require some field erection.
package boiler generally can be shipped over land with normal transportation methods such as flat-bed truck or railcar. The major boiler components are in one assembly and can often be lifted right onto a simple foundation and piped into an existing system. As a result, the package boiler requires far less labor before startup than a field-erected unit. The field-erected unit often requires individual welding of boiler tubes and the entire fabrication of a steel framework. In other

words, the boiler is completely built up at the job site from all the component parts, while the package boiler is nearly complete when it leaves the factory. Package boilers in the 100,000 Ib/hr range have been shipped for gas/oil firing, but the larger combustion volumes neces-
sary for wood units generally limit the top size for a wood fired package boiler to less than 50,000 Ib/hr. As one might expect, field-erected boilers cost more
than package boilers, and construction times are significantly longer.
CLASSIFICATION OF WOOD
COMBUSTION SYSTEMS
There are many different systems available today for burning wood in an industrial setting. Most of these systems fall in-
to one of the following general classes:
Wood Fired Package Boilers
Suspension and Cyclone Burners

Fluidized Bed Combustors Pyrolysis Systems Gasification Systems
Field Erected Wood Boilers
Each of these classes will be considered in further detail.
WOOD FIRED
PACKAGE BOILERS
There, are many manufacturers in the wood package boiler field today. In gen-
eral, it can be said that the larger and per-
haps better known manufacturers (Babcock and Wilcox, Combustion Engineer-
ing, Riley Stoker, Foster-Wheeler, etc.) are not really interested in building small (less than 50,000 Ib/hr) boilers for light
and medium sized commercial and indus-
trial operations. This gap has been filled quite nicely by a number of smaller
manufacturers; and as wood energy use has grown, the market has become more and more competitive.

HEAT

STEAM BOILER DRUM
DOWNCOMER

HEADER OR LOWER DRUM
Figure 2
WATER CIRCULATION PATTERN IN A WATERTUBE BOILER

One of the older wood firing designs is the horizontal return tube (HRT) boil-
er. This type of unit was almost consider-
ed obsolete for a time, but in recent years
A it has seen a resurgence in sales. typical HRT boiler is shown in Figure 3. The HRT is a firetube boiler which is basically
a two-pass design. The steel boiler shell is supported by a refractory furnace, which helps these units to burn wet wood chips
and residue ranging up to 60% moisture content. Operating pressures of HRT's are generally limited to 300 psi, and
steam production is generally limited to
35,000 Ib/hr or less. Some units are able

to meet emission regulations using only mechanical collectors.
There are several other types of fire-
tube wood boilers including compact
three-pass types that perform well on furniture plant waste and other dry ma-
terial less than 20% moisture content. These boilers may perform with a thermal
efficiency up to 80%. Another type of firetube boiler is shown in Figure 4. It is equipped with an underfeed stoker capable of burning wood, coal, or combinations of the two fuels with oil and natural
gas backup. Several smaller manufacturers produce

watertube boilers suitable for burning
wood. One type that has been installed in more than 60 locations uses a refrac-
tory-lined cell burner with preheated underfire air. Most of the combustion takes place in the cell, and the hot flue gases are discharged into the watertube section.
The first cost of wood burning package boilers can be quite high, as shown in Figure 5. The cost curves in this figure
are based on typical "turnkey" jobs installed by various boiler manufacturers and include fuel metering, controls, a
limited amount of wood handling equip-
ment, and air pollution control devices.

HRT BOILER

PRIMARY COMBUSTION CHAMBER
WOOD FEED
CHUTE
COMBUSTION GRATES BRIDGE WALL

REAR TURNING AREA /-REFRACTORY CURTAIN WALL
SECONDARY COMBUSTION GRATES

Figure 3
HRT BOILER FOR WOOD FIRING (Courtesy Industrial Boiler Co.)

The less expensive range of boilers are generally the o'nes with the least flexibility with regard to the quality of the fuel that may be burned. Even the smallest package boilers require a given amount of solids handling equipment and control systems so that the cost per pound of steam is much higher for the smaller systems. As can be seen from Figure 5, a
given size of wood package boiler will
generally have a first cost of 3 to 4 times that of a comparable gas/oil boiler.
It may be difficult to retrofit an existing steam plant with a wood fired package boiler due to space limitations. Wood
systems generally require a greater area than for gas or oil dictating a larger size for a given steam output.
The major conclusion to be drawn a-

Figure 4

<

THREE-PASS WOOD BOILER

(Courtesy Combustion Service & Equipment Co.)

o
WOOD PACKAGE BOILERS

GO

& --30

CO
<or 420

WOOD GASIFIER

Qo

RETROFITS
410

oo

OIL/GAS BOILERS

til i

I

ii

I

i

----
" 1
Y i'

BOILER HP 100

300 1,000

LBS STEAM/HR 10,000 100,000

Figure 5 Cost of Wood Energy Systems

bout wood fired package boilers is that,
as a class, they represent a system that is considered fully commercial. The cost will be substantially higher than the traditional gas/oil package boiler, but the payback time due to fuel cost savings can be
attractive.
SUSPENSION AND
CYCLONE BURNERS
The cyclone furnace for burning pulverized coal has enjoyed widespread use on utility boilers for many years. The fuel is very finely ground and blown into the furnace almost as a gas. As a result, the combustion process is very efficient, and fly ash problems are easily dealt with.
Variations on the cyclone furnace concept have been developed for burning wood waste as well, but there are certain limitations that can hamper the feasibility for using these systems in many applica-
tions. The wood residue must be dry, and
it must be hammermilled or hogged to
fairly fine particles. In spite of these re-
quirements, several companies have plac-

ed a number of these units in industrial
plants, largely in the forest products industry. They have been used for firing directly into boilers, for product drying in rotary dryers, and for lumber dry kiln
and veneer dryer applications. A typical
cyclone burner is shown in Figure 6. In this particular unit, wood residue is blown tangentially through a series of manifolds from a pneumatic conveying system. Ash must be removed from the combustion chamber periodically, but otherwise the operation is fairly automatic. Cyclone wood burners generally range from 5 million Btu/hr to 60 million Btu/hr in single
units.
FLUIDIZED

BED COMBUSTORS

The fluidized bed combustor has enjoyed much publicity over the past several years, particularly as an alternate combustion system for coal burning. Basically, a fluidized bed relies on a combustion chamber that has many holes drilled in the floor through which underfire air passes. This air blows through the "bed" which consists of small particles of sand, limestone, or other solid material. The bed is kept in suspension by the fans and is heated initially by an auxiliary fuel. When the bed reaches a temperature suf-
ficiently high to ignite the fuel to be burned, the auxiliary fuel can be shut off
and the solid fuel introduced. The turbulent mixing action of the hot bed material helps ensure that the fuel is burned com-
pletely.
In wood burning systems, the fluidized bed combustors have shown promise
as devices capable of burning wet fuels or fuels of irregular sizes and shapes. In addition, other waste materials such as carpet
scraps and peanut shells may be burned
in conjunction with the wood.. Several companies are actively marketing various
fluidized bed systems, and many units
have been operated successfully in the forest products industry. Possible drawbacks include relatively high first cost, maintenance requirements greater than a conventional boiler for the same capacity, and high power requirements for the fans.
One design of fluidized beds used for wood firing is shown in Figure 7. This unit is compact and is installed as a package unit. Gas and oil auxiliary fuel, pro-
vided manually, is necessary for startup.
A multiclone collector is used for pollu-
tion control. The advantages to fluidized beds for wood waste are the possibility of burning wet fuel of inconsistent sizes and shapes. It may be expected that the fluidized beds require higher maintenance

(1) Insulation (2) Air Plenum (3) Refractory (4) Auxiliary Burner (5) Combustion Air Manifold (6) Fuel Inlet (7) Butterfly Valve
Figure 6 A Typical Cyclone Burner

than conventional wood burners, and that more highly skilled operators may be nec-
essary to operate them.
PYROLYSIS SYSTEMS
Pyrolysis can be defined as burning without oxygen. The process involves the physical and chemical decomposition of solid organic matter caused by the action
of heat in the absence of oxygen. Wood

can be pyrolyzed to produce charcoal, and coal can be pyrolyzed to produce gas. Due to the intense heat in a pyrolytic
reactor, complex organic compounds can be .broken down into simpler chemical
products. These products include liquids, gases, and carbon char residue.
Various types of pyrolysis processes have been developed in recent years by
many different companies. One of their
goals has been the production of useful fuels from municipal solid waste (MSW).

(1) Fluidized Bed (2) Firetube Heating Surface (3) Gas Train (4) Oil Train (5) Forced Draft Fans (6) Induced Draft Fan (7) Cyclone Collector (8) Carbon Reinjection (9) Ash Disposal
Figure 7
A Fluidized Bed Suitable for Wood Firing (Source: Johnston Co.)

However, limiting the feedstock to purely
organic matter, such as wood waste or ag-
ricultural residues, simplifies the problem of dealing with large quantities of refuse. Figure 8 shows a pyrolysis system that has been successfully operated at Georgia
Tech on wood chips. The system can produce varying amounts of oil, gas, and char
according to the control configuration. Energy content of the products, of course, depends on the feedstock; but in general, the heating value of the oil is approximately 9,000 Btu/lb, heating value of the gas produced by the pyrolysis system varies from 3,200 Btu/lb to 4,500 Btu/lb and the heating value of the char varies from 1 2,300 to 1 3,500 Btu per pound.
Cost data for wood pyrolysis systems
are difficult to obtain since the systems currently available are still largely in the prototype stages. The systems tend to be rather complicated, and as a result, they

can be expensive. The attractive feature of the pyrolysis system is the possibility for sale of a high value product (char),
but the purchaser should be quite certain about the existence of a market before investing in a pyrolysis system.
WOOD GASIFIERS
The transformation of solid biomass into a gaseous fuel is not a new concept, but the idea has realized a renewed inter-
est over the past several years. In the
true sense, wood gasifiers are a subset of the wood pyrolysis units discussed in the
previous section.
There are many different equipment configurations in use for wood gasifica-
tion, but the units that are generating the most interest today are the air gasifiers. These gasifiers typically pass approxi-

mately 25% of theoretical combustion air
through a glowing char bed. Several chemical reactions take place that* result in a low heating value gas of 100 to 200 Btu/ft . Both of these gases, of course, have energy densities much lower than
3 natural gas (1,000 Btu/ft ), but the gas can still be a useful fuel for closecoupled applications.
There has been considerable interest
recently in wood gasification as a retrofit
technology for conventional gas/oil fired
boilers. A low Btu burner can be substi-
tuted for the existing burner and fired
with the "producer" gas. Some derating of the boiler would occur and the severity of this derating would depend ori the
particular design of the boiler.
The main problem associated with wood gasifiers is undesirable constituents in the gas including certain tars and acids. These have a tendency to condense in

PYROLYTIC C ONVERTER
_f
FEED
300-400F

CYCLONE
OFF I

REACTOR BED

II00-I700 F

AFTER-BURNER

PYROLYTIC
OIL
PARTICLES

NON-CONDENSED GAS PHASE

Figure 8

Pyrolysis Process

10

Georgia Institute of Technology

pipes and burners if the gas is allowed to cool or if the gas is not "scrubbed" as it exits the gasifier. These undesirable properties and grate slagging or burnout have been the most formidable problems researchers have faced.
Yhe several concepts being developed
for gasifiers at present include updraft gasifiers, downdraft gasifiers, cross flow gasifiers, and fluidized bed gasifiers. Figure 9 shows an updraft gasifier, but the same basic principles are involved in all
units.
The gasifier vessel is filled with wood
chips and a small fire is started on a grate above the ash hopper. Just enough air is introduced into the unit to support a
glowing bed of coals. Steam may be injected into the bed to promote the form-
ation of hydrogen (H2) and to control grate temperature. In the reduction zone, the hot gases from the combustion process are partially reduced to carbon monoxide (CO), a low heating value gas. The products then pass into a pyrolysis zone where the infeed first gives up its volatile
gases.
In the top section of the updraft unit, a drying process occurs wherein the exit-
ing product gases give up some of their heat to the infeed. The gas is drawn off at the top of the unit and piped to a burner or engine for final combustion. Due to the various gasifier configurations being developed by different manufacturers, the gases produced differ somewhat
in their contents.
In general, the main barriers to full scale commercialization of gasifiers include the following:
1) Lack of long term operational experience by current manufacturers
2) Potential problems in piping and burners resulting from tars and other liquids in gas
3) Slagging of grates due to ash con-
tained in wood
4) Requirements for operator atten-
tion
One of the most attractive aspects of wood gasifiers is price. As mentioned earlier, package wood boilers may cost 3 to 4 times as much as the natural gas/fuel oil
boiler that has been the mainstay of the commercial market for the past thirty years. Cost figures indicate that gasification systems can be retrofitted to an existing gas/oil burner for substantially less
than the cost of a new wood system.

RAW MATERIAL
FEED

ROTARY
AIR-LOCK

FUEL
GAS OUT

7
/ /
/
/
/
DRYING
ZONE

\ \ \ \

DISTILLATION
ZONE

REDUCTION
_ _ Z0NE
COMBUSTION ZONE

Figure 9 UPDRAFT WOOD GASIFIER
11

FIELD-ERECTED BOILERS

The last category of wood burning

systems considered here is probably the

most specialized. This category is field
erected wood burning boilers, and each application is very site specific. Due to

the labor intensive nature of the field
erected boilers, they are generally much

more expensive than small package boil-

ers: most have a capital cost of at least $35 per Ib/hr of sleam produced. Industrial users with a small of medium size
steam requirement will probably- stay
away from the field erected units. Many

paper mills in this country have large

"bark" or "combination" (bark plus oil

and gas firing) boilers capable of produc-

ing 400,000 Ib/hr to 500,000 Ib/hr of superheated steam for process use and

electric power generation. Operating pres-

sures may exceed 1200 psi.

Virtually all field erected boilers are watertube boilers and most use steam

drums to separate the water and steam. Most units in use today for burning wood

employ some form of traveling grate or

stoker feed. Entrained particulates can be
a problem with these units, so some form of collector is necessary to meet air pollution codes. Units of greater than 250 million Btu/hr input generally fall under
stricter regulations than the smaller boilers and are thus subjected to "best avail-
able control technology" which can mean mechanical collectors, wet scrubbers, or

even baghouses.

Large field erected boilers require full

time operating crews and a great deal of

maintenance. They are usually only cost

effective in 24-hour-per-day operations

since constant startup and shutdown of

these units results in insulation cracks

and refractory damage. There are far

fewer companies building these large

boilers than the package units discussed

previously, and the permitting and con-

struction processes can easily require 3 to

A 4 years.

typical field erected boiler is

shown in Figure 10.

ALCOHOL PRODUCTION
FROM WOOD
Methanol and ethanol are proven liquid fuels for transportation, particularly
as alcohol-gasoline mixtures in autos. To reduce foreign oil dependence, much research emphasis has been placed on the use of ethanol produced from biomass

Figure 10
WOOD WASTE BOILER WITH WATER-COOLED GRATES (Courtesy E. Keeler Co.)

crops and residue. Methanol, also known as wood alcohol, is a relatively inferior liquid fuel compared with ethanol. Etha-
nol has higher Btu content, is less toxic than methanol, is miscible with gasoline, and burns well without major modification to the engine. Also, both the initial capital investment and the size of the plant required to produce methanol are considerably larger than those required for ethanol.
Much of the recent work for the production of ethanol has focused on the
use of food-based products such as sugar,
corn, and other grains. Wood, corn stalks, and other plant residues, however, are

more attractive raw materials since they are not part of the traditional food chain for man.
Production of ethanol from wood is
an old concept. Figure 11 illustrates the
process flow diagram. During World War II, a number of such plants were in operation in Europe. As single-product operations, these plants were not competitive with chemical plants based on an abundant supply of cheap petroleum as the
feedstock.
A more desirable approach for producing ethanol from wood is an integrated
multi-product plant where all components of wood-cellulose, hemi-cellulose, and

BIOMASS
Pretreatmenl

Hydrolysis

Separation and
Clarification of Sugar Solution

Concentration of Sugar Solution

Lignin

Soluble Sugars

Fermentation
scp 4-

CO,

DIAGRAM OF A PROCESS TO PRODUCE ETHANOL

Alcohol Recovery

ABSOLUTE ETHANOL <-

Figure 11 GENERAL BLOCK DIAGRAM OF A PROCESS TO PRODUCE ETHANOL

lignin--are processed, resulting in a num-
ber of valuable by-products. Lignin can be pyrolyzed into phenols, char, oil, and a combustible gas. Hemi-cellulose can be converted into pentose and hexose sugars, where the pentose can be used as a starting material to produce furfural, xylitol,
single-cell protein, and also, to some ex-
tent, ehtanol. Cellulose can be hydrolyzed to produce glucose, which then can be fermented to produce ethanol, carbon dioxide, and yeast.
The first stage of operation is pretreatment during which lignin is separated from the biomass and hemi-cellulose is

hydrolyzed into soluble sugars. The next
stage of operation involves hydrolysis of cellulose into glucose, clarification of the solution, and concentration of the sugar
solution. The third stage of operation includes fermantation of glucose into ethanol and separation of yeast. The final
stage of operation is the recovery of alcohol and the production of absolute ethanol, which can be used in gasohol.
The production of ethanol can be economical only if
1) The raw material is available without large fluctuations in price
2) By-products can be sold

3) The crude oil price keeps going up, and
4) Liquid fuel becomes less available Production of ethanol could play a very important role in reducing the dependence on the foreign supply of liquid
fuel.
On the long-term basis, the ethanol
produced from the renewable raw materials such as wood, agricultural residue, and crop residue, looks very promising as compared with high-priced, nonrenewable foreign crude oil.

13

WOOD COMBUSTION

The dry basis measure is the fractional water content or the weight of the water

in the sample divided by the sample weight when dried.

Combustion is the rapid chemical combination of oxygen with the elements of a fuel that will burn (Babcock and Wilcox
Co., 1972).
In general, the combustion of wood
can be separated into three stages. During the first stage, moisture from the fuel is evaporated. The second stage involves raising the fuel temperature to the point where volatiles are given off and combusted. The final stage is the combustion of the fixed carbons. All stages occur simultaneously in the combustion process.
PROPERTIES
OF WOOD FUEL

Moisture content % (Dry) = 100 (Weight of wet sample - Weight of dry sample)

(

Weight of dry sample

)

Example: A one-pound sample which is half water and wood would have a wet weight of one pound. A dry weight of one-half pound a M.C. (dry) of 100%.

The wet basis measure assumes that the weight of the wood and water to be 100 percent and is the weight of water in

a wood sample, divided by the weight of drywood plus the weight of water.

Moisture content % (Wet) = 100

Weight of water

(Weight of dry wood + weight of water)

Example: A one-pound sample which is found to be one-half pound of wood and one-
half pound of water upon drying and weighing has a moisture content of 50%.

Dry softwoods, because of their higher resin and lignin content than hardwoods, have a higher heating value of approximately 9,000 Btu/lb. The heating value of dry hardwood is approximately 8,400
Btu/lb. Wood has a typical ultimate analy-
sis as follows: Carbon 51.4%, hydrogen 6%, and ash 1 .5%; and a proximate analysis as follows: volatile material 78.1%, fixed carbons 20.4%, and ash 1.5%. Sulfur content is essentially zero.
Properties of wood fuel vary widely,
however, chiefly due to its moisture con-
tent. Wood fuel also comes from a variety
of sources, and the range of particle size and form produce variation in the bulk
density. Table 1 gives typical wood fuel
values.

The conversion of moisture content can be accomplished by the following:

Wet to Dry

Dry to Wet

Moisture Content Dry) = 100'

l.C. Wet) Moisture Content (Wet)=100-

C.Dry

100-M.C.Wet)

100+M.C.Dry

The moisture content of the fuel has an important effect upon the actual heat content of the fuel. The actual heat con-

tent of a given fuel can be determined as follows:

Actual Heat Content JOO-Wet M.C. X Oven Dry Heat Value
100

Moisture content of fuel is an impor-
tant criteria in the design of a wood combustion system. Whereas wood boilers designed to burn green wood can usually burn dry wood, the reverse is seldom true.

Analysis of available wood supply as to
quantity, moisture content, and form is recommended before making the final decision on which boiler to install.

FUEL MOISTURE CONTENT

Moisture content is generally measured on a dry basis or on a wet basis. The dry
basis is commonly used by the forest products industry, whereas fuel and combus-
tion engineers use the wet basis measure.

Table 1.-Wood Fuel Properties

Wood Fuel
Whole Tree Chips Dry Planer Shavings Green Sawdust Dry Sawdust
Wood Pellets

Moisture Content:

:

Wet Basis

: Higher Heating Value : Bulk Density

Percent 50 13 50 13 10

Btu/lb 4,500 7,800 4,500 7,800 8,100

Ib/ft^
24.0 6.0
20.0 11.5 35.0

Table 2 covers the normally encountered moisture content range.

Table 2. --Wood Moisture Content

Wet Basis

Dry Basis

Percent

5

5

10

11

15

18

20

25

30

43

40

67

50

100

60

150

The chipper chips the complete tree including limbs and twigs, as well as separating the commercial material.
NJVE RSITY OF
lilliii
3 21Qfl OMMMT 2752
44

The high speed, rubber tired skidders, with hydraulic grapple, deliver the wood to the whole tree chipper. The skidder is
capable of multi-stem skidding.
A feature of the feller buncher is that
trees can be sheared at ground level which reduces the cost of site preparation and planting and leaves less waste material. It can handle several trees at once, piling them at a central location for skidding to
the chipper.
<*

pk

1

mm *f?- 1

The chip loader can group the trees, and place them where they can be engaged by power pressured rollers for entry into the
the chipper.

GEORGIA
FORESTRY,
fum*/
A. Ray Shirley, Director
John W. Mixon, Chief of Forest Research