No. 2213
TABLE OF CONTENTS

Introduction ....................................................

History and Status of Industry ....................................

Nature of Wood Charcoal ......................................

Development of the Industry ....................................

Early Kiln Methods and Production .............................

Later Carbonization Methods and Production .....................

Hardwood Byproduct Recovery ...............................

Current Production and Use ....................................

General Production Costs ...................................

Charcoal Markets and Marketing ...............................

Recreational and Industrial Outlets .............................

Charcoal Briquetting ..........................................

Forest Service Research ........................................

Scope of Program .............................................

Purpose of Research ........................................

Types of Experimental Structures .............................

Structural Requirements of a Concrete Masonry Kiln .............

Size and Shape ..............................................

Materials of Construction ....................................

Construction Details .........................................

Recommended Structural Practices ...........................

Operation of Rectangular Kilns .................................

Kiln Raw Material .........................................

Kiln Charging ..............................................

Temperature Measurement and Instrumentation ................

Charge Ignition .............................................

Safety Precautions in Kiln Operations ...........................

General ....................................................

Types of Hazardous Conditions ...............................

Safety Equipment ...........................................

Location of Kilns ...........................................

Safe Operating Practices ....................................

Charcoal Storage Precautions .................................

Charcoal Yield and Production Factors .........................

General ....................................................

Measuring Yield by Charcoal Percentage ......................

How to Determine Charcoal Conversion .......................

Effects of Operating Conditions on Yield ........................

Wood Moisture Content and Yield .............................

Wood Moisture Content and Conversion Time ....................

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TABLE OF CONTENTS

(continued)

Wood Moisture Content and Brands ...........................

Effect of Wood Species, Form on Weight of Charge .............

Effect of Species, Form on Charcoal Yield, Quality ............

Effect of Kiln Size, Type on Charcoal Yield, Quality ...........

Effects of Kiln Size, Type on Production Time .................

Effect of Time, Temperature on Coaling and Charcoal Quality ....

Effects of Time, Temperature on Kiln Operations ..............

Effect of Time, Temperature on Brands .......................

Effect of Weather on Charcoal Yield ..........................

Effects of Weather on Carbonization Temperature ..............

Economic Studies .............................................

Kiln Comparison Study ......................................

Stand Improvement Study ....................................

A Method of Charcoal Analysis .................................

Procedure for the Analysis of Charcoal .........................

Sample Preparation .........................................

Steps in Analysis ...........................................

Conclusions ..................................................

Appendix--A Partial List of Suppliers ..........................

Charcoal Moisture Tester ...................................

Requirements for Temperature-Indicating Meter .................

Wire Requirements .........................................

Wiring .....................................................

Meter Use and Care ........................................

Wire Use and Care ..........................................

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Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127-135

Report No. 2213 -ii-
Acknowledgment

This report provides general information in the field of charcoal production

and summarizes the results of Forest Service investigations concerning the

design and operation of masonry type kilns. The conclusion of the project was

made possible by the helpful cooperation of several private, State, and Forest

Service organizations. ‘Significant contributions of either funds, personnel,

or raw materials were made by the Cliffs-Dow Chemical Co., Kingsford

Chemical Co., State of Minnesota Iron Range Resources and Rehabilitation

Commissioner, North Carolina Conservation and Development Commission,

St. John’s University, Collegeville, Minn., the University of Georgia, the

University of Minnesota, the Wisconsin Conservation Commission, and the

Lake States Forest Experiment Station, Southeastern Forest Experiment

Station, and Forest Products Laboratory of the Forest Service, U.S. Department of Agriculture.

This report was prepared by the following Forest Service personnel:

Arlie W. Toole, Paul H. Lane, and Carl Arbogast, Jr., of the Lake States

Station; Walton R. Smith and Ralph Peter of the Southeastern Station; and

Edward G. Locke, Edward Beglinger, and E. C. O. Erickson of the Forest

Products Laboratory.

Report No. 2213 iii
CHARCOAL PRODUCTION, MARKETING, AND USE

Compiled by the Forest Products Laboratory1

INTRODUCTION

Charcoal is produced by heating wood in airtight ovens or retorts, in

chambers with various gases, or in kilns supplied with limited and controlled

amounts of air. High-temperature heating by all methods breaks down the

wood into gases, a watery tar mixture, and the familiar solid carbon material

commonly known as charcoal.

Carbonizing ovens of plants designed for recovery of products other than

charcoal are heated externally, and the wood is not in direct contact with the

heat source. In another type, the only heat source derives from utilization

of reaction heat for continuous chip conversion. Upright chambers have been

developed that convert fines continuously or chunks in batches by forced circulation of hot gases through the wood, but liquid byproducts are not recovered.

Also, horizontal retorts, both rotating and fixed, have been devised for the continuous carbonization of hogged material or raw mill fines. In kilns, the wood

is heated directly and itself largely furnishes the heat needed for combustion.

This report presents data on charcoal kiln design and operation developed by

research and developmental work of the Forest Service, U.S. Department of

Agriculture, in various parts of the United States. Its object is to aid producers in more efficiently converting large quantities of low-quality wood into

a product that is enjoying an encouraging resurgence in popularity as a cooking fuel for use in homes, recreational areas, restaurants, and other establishments.

Charcoal has been an important domestic product for many years and, regardless of how produced, has wide market acceptance. Its greatest use is for

home and outdoor recreational cooking. Charcoal is also used in the manufacture of carbon disulfide, carbon tetrachloride, sodium cyanide, and other

industrial chemicals. Extensive amounts are converted to activated carbons.

Other industrial uses are in connection with steel heating, nonferrous smelting, and metal casehardening.

Large amounts of charcoal are used as recreational fuel, and production will

continue to expand as this use increases. This market has stimulated interest

in the manufacture of charcoal. To small businessmen and investors it is

particularly attractive, since a charcoal production venture can begin with but

little investment in simple kilns and expand into more elaborate conversion

facilities as business warrants. The investment for kiln construction would

1Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

Report No. 2213 -1-
be in the general range of $100 to $500 per cord of kiln capacity, not including

costs of warehousing and mechanical equipment. For the more complicated

equipment of either small or large size, the investment would be from about

$2,000 to $35,000 per cord of conversion capacity.

Whether a charcoal operation will be successful is not assured by the initial

plant investment but depends on a study of commercial possibilities as indicated by source and cost of raw material, availability and cost of labor, and

the market for the charcoal produced. Wood and labor costs, operational

efficiency, and sound marketing practices are the chief economic factors

governing success in charcoal production.

When there are possibilities for the marketing of rather large amounts of

charcoal, a cooperative operation may help assure more efficient use of

labor, permit joint advertising and sales, and result in higher profit.

Charcoal will undoubtedly continue to be made for the most part from wood.

This report provides information for the manufacture of charcoal in various

kinds of equipment, from rather simple, inexpensive kilns to elaborate,

costly byproduct-recovery plants. This information is based on Forest

Service research conducted on a broad scale to stimulate conversion of lowquality wood to this widely marketable product. It is emphasized, however,

that only when certain controlling factors, such as initial investment, labor

and raw material costs, production efficiency, and markets, are fully considered can there be reasonable assurance of success. The important role

these factors play in charcoal production is clearly shown in experience gained

from experimental kiln production. Charcoal is produced by heating wood

under conditions that permit the use of little or no oxygen or in limited, regulated amounts available for retarded combustion. That this point has particular significance is emphasized in much of this report.

HISTORY AND STATUS OF INDUSTRY

Charcoal is produced by burning wood under conditions that severely limit the

amount of oxygen available for combustion. The object is to reduce the wood

to a form of carbon by removing the other constituents. These are either

allowed to escape as gases and other volatiles in the smoke, or may be recaptured, condensed, and converted to useful byproducts. Because of their

removal in large part, charcoal burns more "cleanly"--that is, with little or

no smoke or flame.

Report No. 2213 -2-
Nature of Wood Charcoal

In the kiln, heat converts wood to charcoal in two stages--the drying and the

coaling stage. The heat needed for drying and to initiate coaling conditions is

furnished by burning part of the wood. Additional heat is given off by chemical

reaction during the coaling process. The burning is controlled by limiting and

regulating the amount of air entering the kiln.

After an initial surface zone of dry wood has been established, further heating

breaks the wood down--that is, chars it--to form charcoal progressively

throughout a kiln charge. Water vapor and the burnable gases that are

supplied with too little oxygen for complete burning escape as smoke through

one or more kiln stacks (16,22,37,42) .2

Charcoal is obtained from all species of wood. The charcoal, or impure carbon, yielded by the various commercial methods is usually considered

adequately refined for its markets. The carbonization, or socalled "coaling,"

temperatures used control the amounts of volatiles or smoky material remaining in the charcoal (34). If coaling temperatures are too low, excessive

amounts of volatiles will remain in the charcoal and cause heavy smoke when

it burns.

The yield of charcoal from a given amount of wood by weight is dependent

upon the amount of carbon in the wood and the carbonization conditions

employed. On the average, carbon constitutes about 50 percent of the dry

weight of the wood. At least this amount is likewise present in hardwood

barks, while softwood barks contain somewhat less.

Good-quality charcoal burns cleanly and has a heat value of about 13,000

British thermal units per pound (59), or about 1-1/2 times that of an equivalent weight of dry wood. This important property, along with its low average

ash content, 2 to 3 percent, makes charcoal desirable for metallurgy or as a

domestic fuel. Only traces of sulfur and phosphorous are found in charcoal

(3). This further increases its value in metallurgy, but the cheaper carbons

obtained from other sources tend to offset these important charcoal properties.

Charcoal produced under well-controlled carbonization conditions may be hard

and brittle, or comparatively soft and crumbly, when rubbed and handled.

In weight, charcoal may be rather heavy to quite light. This physical property

is related to the weight of the dry wood of the various species, which in a

given volume may be heavy (sugar maple, beech, oaks, yellow birch,

2 Underlined numbers in parentheses refer to Literature Cited at end of this report,

Report No. 2213 -3-
longleaf pine, and the hickories all have an average specific gravity of 0.63)3

(79); medium (elm, alder, soft maple, ash, gum, yellow-poplar, Douglas -fir,

and jack pine have an average specific gravity of 0.48)3 ( 7 9); and light (aspen,

cottonwood, and most softwoods, with an average specific gravity of 0.39)3

(79). Well-prepared charcoal weighs about one-third as much as wood and is

reduced to roughly one-half the volume of wood. In form and internal

structure, it is almost identical to wood. The apparent specific gravity of

charcoal ranges from about 0.2 to 0.5, depending on the specific gravity of

the wood from which it was made.

Traditional indicators of charcoal quality include a metallic ringing sound

given off when it strikes a hard object, and a smooth fracture of the piece as

it breaks. Others include freedom from taste and odor and from undue soiling of an object that is rubbed against a fractured face.

Charcoal is comparatively easy to ignite, and when of good quality burns

evenly and without smoking. Because of its low strength and brittleness, it

tends to crumble and produce "fines" or small pieces when handled roughly.

Handling during kiln discharge and transport normally reduces about 5 to 10

percent of the charcoal to fines.

By comparison with generally accepted standards, wood charcoal has little

tendency to adsorb colors, gases, or odors. Since it is porous, however, it

has a large internal surface area that can be materially modified by activation to adsorb greater amounts of these materials. Charcoal becomes

activated when heated at high temperature in the presence of air, steam,

carbon dioxide, or combinations of these gases. Other chemical treatments

also modify or increase its adsorbency for specific uses. The production of

activated charcoal is highly specialized and requires both heavy plant investment and large raw material supplies.

Under certain conditions, charcoal will undergo self or spontaneous combustion, the cause of which is not clearly understood. Heat is generated in charcoal both by wetting and by taking up oxygen. Properly conditioned charcoal

is a relatively inert or stable material and may be compactly dry stored for

indefinite periods.

Development of the Industry

During Colonial days charcoal was produced in simple pit or earthen kilns for

carbon needed in iron smelting (69,80). This type of kiln is still in limited

use (fig. 1). Toward the end of the 19th century, charcoal production for

metallurgical uses amounted to about 171,000 tons annually, and was

3 Based on weight when ovendry and volume at 12 percent moisture content.

Report No. 2213 -4-
Report No. 2213 -5-

Figure 1. --Early charcoal production in earth kilns.

ZM 113 846
concentrated chiefly in the northern hardwood belt (39). During this period,

the beehive-type structure (fig. 2) largely replaced the pit kiln (39,56).

After slumping to a low of 6,000 tons in 1935, kiln production has returned

to an annual level of over 55,000 tons (73).

About 1890, production of acid for textile manufacture became important in

conjunction with charcoal manufacture in large beehive kilns, further

stimulating expansion of the industry. As a result, plant facilities for charcoal and chemical byproduct recovery became more elaborate and expensive.

In these operations a crude liquor was obtained by condensation of distillation volatiles. The liquor was refined in highly specialized equipment to yield

mainly pure acetic acid and methanol (21,22,39,50,52).

The year 1909 recorded peak charcoal production of slightly more than

550,000 tons from kilns and recovery plants. Between about 1910 and 1940,

production dropped to 250,000 tons, chiefly because other carbon materials

replaced charcoal in the manufacture of metals and chemicals. After a

further decline to a low of 213,000 tons reported for 1947, however, production has recovered, reaching 265,000 tons in 1956 (4).

Early Kiln Methods and Production

Historically, in the United States, crude earthen pit kilns were the forerunners of the modern charcoal kiln (15). Occasionally, one is still found in

use, but during the latter half of the nineteenth century socalled "beehive"

kilns came predominately into use.

In earthen pit kilns, sound, seasoned pieces of hardwood about 4 feet long and

6 to 8 inches in cross section are piled in mounds of as much as 50 to 90 cords.

Earth is mounded over the wood to give a firm enclosure, but is not too

tightly packed to prevent air from leaking through to the wood. About 20 days

are needed to coal a wood charge of 50 cords by this method. While the yields

of marketable charcoal are low--about 700 pounds of charcoal per cord--overall costs are held down by the very small investment in plant, despite the fact

considerable labor is required to make a suitable coaling structure (8 0).

The brick beehive kilns of 50- to 90-cord capacity once widely used were of

no uniform design. The larger kilns are estimated to have had an overall

height of 24 feet, a diameter of 30 feet, and a wall thickness of about 12 inches.

The dome-shaped ceiling had an opening for loading and firing. Some kilns of

this type continue in use today and do an effective job.

Report No. 2213 -6-
Figure 2. --Brick beehive kilns --large-scale conversion of low-grade wood.

ZM 113 742

Report No. 2213 -7-
Two Midwest operations, each with several 75-cord beehive kilns, were

started in 1956 (fig. 3) and 1959. In general, such kilns are built along the

base of a hill or bank, so that they can be conveniently charged through the

top. An opening at the base also gives access for wood piling and for charcoal discharge. Raw material of mixed or uniform size, such as cordwood,

stems, and slabs, is closely stacked for maximum charge weight. A 2- to

3-foot central core in the charge is filled with small and short-length fuel

almost to dome height for charge ignition.

After ignition is well under way, the top is tightly closed and the bottom door

made airtight. A number of air-inlet ports around and near the base of the

kiln regulate intake of air for partial combustion, and ports located somewhat higher in the wall permit the smoke to escape after it has circulated

throughout the charge. Because of varying wind and other atmospheric

conditions, the ports must be skillfully operated to assure uniform coaling of

such large amounts of wood. Different inlet and outlet ports are opened or

closed as the operator deems necessary to get the coaling pattern desired.

The end of the coaling period is indicated generally by the amount and nature

of the smoke from the ports and by observing, through them, the conditions

within the kiln. A complete cycle takes from 20 to 30 days, and about

700 pounds of charcoal per cord of seasoned wood are produced.

The high-volume production possible in beehive kilns, especially where wood

can be bought at low bulk prices, enables the operator to enter into contracts

to furnish large amounts of charcoal. Commonly, batteries of these kilns are

operated to keep labor steadily employed (69).

Many different types and sizes of kilns have come into use from time to time.

Likewise different construction materials have been used, often with no set

design or structure in mind. Good planning, however, has resulted in kiln

structures that function well.

An example is the successful, and still rather widely used, Black Rock

Forest beehive kiln (77) developed in the early 1930’s (fig. 4). This kiln,

constructed of 14- to 16-gage sheet metal, is 7 feet in diameter at the base,

4-1/2 feet in diameter at the top, 5 feet in height, and holds about 1/2 cord

of 4-foot wood stacked on end. Eight 4-inch ports are equally spaced around

the base, and in alternate ports are four smokestacks and four air-inlet openings. This kiln is constructed in three sections, a base, a midsection, and

a crown, and is therefore portable. A larger version of this kiln, about

1-1/2 cords in capacity, can be charged with cord-length wood uniformly

piled or with short-length material dumped in. Fairly low maintenance cost,

portability, and short operating schedules are reported. In comparison with

masonry kilns, their chief disadvantages are excessive radiational heat loss

Report No. 2213 -8-
Figure 3. - -Beehive kilns - -recent construction of 75-cord units.

ZM 114 998

Report No. 2213 -9-
Figure 4. --Portable, sectional sheet-metal kiln.

ZM 113 851

Report No. 2213 -10-
that results in somewhat lower yields and, as compared to masonry-block

Kilns, generally higher kiln cost per cord of capacity.

Changes in design and use of these small, metal beehive kilns led to the

development of the New Hampshire-type kiln first operated about 1938

(9,10,11,12). This kiln has a beehive-shaped, sheet-metal hood with a

shallow top plate. The design permits lifting and transporting the whole kiln.

For operation, a derrick lowers the units over charges of wood stacked around

it and lifts them after coaling is completed. Batteries of these kilns can be

operated efficiently, with loading, coaling, and discharging going on at

different times.

Masonry blocks, various types of bricks, field stone, and reinforced concrete are commonly used in kiln construction (49). Some have capacities of

25 to 40 cords. Others, of about 2- to 10-cord capacity, are more popular

because of lower initial cost and ease of operation (28,29,35,37,54,55,67,69).

Even modified brick kilns originally built for the manufacture of ceramic

materials or coke are satisfactory (fig. 5). Such units have wood capacities

of from 45 to 65 cords and can withstand the high coaling temperatures with

little cracking and damage to the structure. In a good location, these

structures can be economically operated for charcoal manufacture, in part

because similar new construction would require a considerably greater

investment. Units of reinforced concrete, having capacities of 45, 60, and

100 cords of wood, are used to quite a large extent (fig. 6) (38). These kilns

usually have a number of stacks, and it is rather common practice to cover

all but the front with dirt. Kilns of similar design but smaller in size are

constructed of brick (fig. 7) (49).

The commonly used 2- to 10-cord masonry-block kilns are simple in design,

low in cost, and produce consistently good yields of charcoal.

Field experiments in the development of single-stack, rectangular, masonryblock kilns of 1- or 2-cord capacity were reported by H. W. Hicock and

associates of the Connecticut Agricultural Experiment Station in a series of

publications beginning in 1946 (35,37,54,55). In an earlier publication, they

described the construction and operation of 2- and 4-cord sheet-metal kilns

of somewhat similar design (36). In the smaller masonry-block kilns, there

was considerable wall cracking and other structural damage at high operating

temperatures, which necessitated continuous, although generally inexpensive,

repairs. Experience gained with commercial operation resulted in some

structural changes. The chief features, however, remained unchanged.

These provided for air entry through ports along each sidewall at groundline

and a single smokestack at the rear. The original design also featured a

small stove or heating chamber that helped improve the flow of air through

the kiln.

Report No. 2213 -11-
Report No. 2213 -12-
Report No. 2213 -13-

Figure 6. -- Concrete kiln of 100-cord capacity.

ZM 114 261
Figure 7. -- Commercial, rectangular brick kiln with steel doors.

ZM 112 468

Report No. 2213 -14-
The spreading interest in charcoal production has brought into use still other

types of kilns. These are generally small and designed and built by the

operator to supply a limited market. Among these are a number of interesting designs.

One, a "telescopic "-type unit, is made up of a rectangular-shaped, wraparound sheet-metal base section and an edge-flanged top or overskirt section

slightly larger than the base. Half a cord of material, usually slabwood, is

piled on the ground to fit within the kiln. The split lower section is wrapped

around the base of the charge, and the upper section is placed over the top of

the charge. During coaling, as the charge shrinks, the top section moves

slowly downward, overlapping the bottom section until it reaches the ground.

Dirt is banked around the base.

Another low-cost type consists of a dry brick inner wall 7 feet in diameter

and 4 feet high and an outer wall of 28-gage roofing tin. A 2-inch space between the walls is filled with sand to seal any gaps. Four draft and four

smoke openings are located around the base. Wood is lowered into the kiln

through the ceiling, and the charcoal is lifted out the same way. Twentyeight-gage sheet metal is used for the kiln ceiling, which is removable and

acts also as the entrance closure. The ceiling is supported by iron pipe

stringers placed crosswise at right angles. A 2- to 4-inch layer of sand is

used to seal the top. Initial draft and ignition smoke pass through a stack in

the center of the ceiling. Construction has included chiefly salvage material

and points out the possibilities for lower construction investment.

Later Carbonization Methods and Production

The development of wood distillation plants about 1890, as already noted,

created a new source of charcoal that continues to produce substantially

more than is made in kilns. In recovery plants charcoal is produced in large

amounts and is at least as important as the methanol, acetic acid, and other

wood chemicals produced.

Byproduct charcoal from such recovery plants has an obvious advantage in

that costs of production are shared by the sale of the chemical products. By

the early years of the twentieth century, therefore, the bulk of production had

been taken over by these more efficient oven facilities, which continued to

flourish until about 1920 (39). Between 1920 and 1940, conditions changed

radically, and many of these plants were abandoned because of the economic

pressure of high investment and loss of chemical markets to cheaper

synthetics. Those managing to remain in business, however, got an

unexpected assist when, about 1950, a new market of great potential opened

Report No. 2213 -15-
for charcoal as a cooking fuel in homes --indispensable to the now familiar

backyard barbecue.

Hardwood Byproduct Recovery

Approximately one-half of all the charcoal produced in the United States comes

from six hardwood byproduct recovery-type plants. Four plants produce

acetic acid and methanol, while in another only methanol is recovered. In the

sixth, acetic acid is recovered indirectly as sodium acetate. In these plants

charcoal is made by conventional oven carbonization as well as by other, quite

different methods. In all six plants, the main operating steps are: (1) the

wood is predried and carbonized; (2) the crude liquid obtained is refined for

recovery of high-purity products.

Oven processing. --The typical oven operation is a batch process (21,34,39).

The horizontally placed sheet steel ovens hold approximately 10 cords of wood

cut to either cord or short-block lengths. The ovens are commonly set in

pairs and heated externally from end-placed fire boxes. Steel rails on the

oven floor and in line with yard and charcoal cooler tracks allow car charging

and discharging to take place at the same time. Full face swinging doors at

both ends are made gas tight by wedging against the door frame. Heat loss is

greatly reduced by use of an additional exterior, or storm, door at each end.

The volatile products are led from the oven through side-wall takeoffs to

water-cooled copper or stainless steel condensers.

Coal, manufactured and natural gases, and oil, with the addition in some

cases of wood tar and gases from the carbonization, are the chief fuels used.

Controlled heating of the ovens to an end temperature of about 800° F. is

necessary for satisfactory yields of good-quality charcoal and refined byproducts. Carbonization periods vary from 18 to 22 hours and are so

arranged that a number of ovens can be charged and discharged at different

times. The charcoal is withdrawn to tight, sheet-steel coolers at the same

time a charge of wood enters the ovens from the yard or from the predriers,

A minimum time of 48 hours in the coolers and 48 hours in covered sheds is

required for conditioning the charcoal before it is shipped. From 1,000 to

1,100 pounds of charcoal are generally obtained from a cord of the heavier

hardwoods in recovery plants. It is estimated that, at recent prices, an

investment of $35,000 per cord daily capacity is required for new plant construction, and that an input exceeding 100 cords per day is necessary for

profitable operation (fig. 8).

Stafford retort processing. --The type of equipment and method (52) employed

in wood distillation by the Stafford retort process vary widely from those used

in oven carbonization. The major difference is that carbonization is conducted

Report No. 2213 -16-
Report No. 2213

Figure 8. --This hardwood byproduct recovery plant includes carbonization

and refining facilities for the production of acetic acid, methanol, and

charcoal in lump, briquette, and activated forms.

ZM 113 849

-17-
continuously in vertical retorts without auxiliary heat. Wood that has been

chipped, hogged, or cut into small blocks is used.

The efficient predrying of process wood to low moisture content is necessary

for satisfactory operation. Rotary driers heated by the gases from the power

plant are used to dry the wood to a moisture content of about 0.5 percent.

The dried material is automatically fed to the retorts in regulated amounts.

The retorts are about 40 feet high and 10 feet in diameter, and heavily

insulated. Provision against heat loss must be made, since proper heat

balance within the retort is a controlling factor in the operation. Valves at

the top and bottom ends of the retort prevent air from entering with the raw

material and also prevent the escape of volatile products. The dried wood

is fed to the retort at a temperature of about 300° F. As it moves downward,

both its temperature and its rate of carbonization increase. Carbonization is

carried on by making use of the heat that is continuously available from added

wood undergoing similar change to charcoal. Heat is concentrated at highest

temperature in a zone about midpoint of the retort. The average temperature

in this zone is about 950° F. and gradually decreases to about 480° F. at the

base.

Volatile materials pass from outlets at the top of the retort to four evenly

spaced condensers. The charcoal is discharged continuously to airtight

coolers and conditioners in a finely divided form that has limited markets,

but with a little grinding is made readily suitable for briquette,, manufacture.

About 1,000 pounds of charcoal are obtained from a cord of the denser hardwoods.

Gas -circulating process. --Processes different from the oven method (21) in

common use were developed in Germany by Reichert (48,64,65) and inBelgium by Lambiotte (43). These methods have been successfully used in

several European plants to overcome serious problems of high wood and

labor costs. In at least one United States operation, sone use is being made

of one method.

In principle, the carbonization methods developed by Reichert and Lambiotte

are alike. They differ from other systems, however, in that hot, noncombustible gases are circulated throughout the charge to carry on the carbonization. Although the Reichert plant is operated with batches of raw wood

and that of Lambiotte is continuously fed with wood, it is understood the

Reichert plant can be made continuous also.

While the heating gases in the Reichert system flow downward with the wood

through the retort and those in the Lambiotte retort travel against the direction of wood movement, both methods may be described briefly together.

Report No. 2213 -18-
The retorts are cylindrical with tapered end sections and vertically mounted

for single or group use. Both top and bottom sections of the Lambiotte retort

are equipped with special valves to permit loading and discharging without intake of air or loss of wood gases and vapors. In the operation of both types

of retorts, heated wood gases enter the retorts through valved openings,

spread throughout the charge, and later go with newly formed volatiles to condensers. A portion of the dried gases from the condensers is transferred

under slight pressure through furnace tubes and heated to a high temperature

by combustion of the remaining portion. Passage of the heated gas from the

furnace to the retort completes the cycle. Gas recycling and circulation with

the retort are automatically controlled. The yield of charcoal per cord is

much the same as that from other processes using similar hardwood raw

material.

in

Converted byproduct-plant processing. - - Some 25 years ago, operators of

hardwood recovery plants lost their major byproduct markets and were forced

to limit output to charcoal. This necessitated some plant changeover, which

was made by removing the equipment for the recovery and refining of the

liquid byproducts. The oven equipment required only minor modifications and

was left to remain as a complete carbonizing unit. This usually required that

all of the gases from the ovens be passed through condensers for drying before venting them ‘to the atmosphere, to an outside furnace, or to the oven fire

boxes for use as supplementary fuel. Burning was preferred, since direct

venting to the atmosphere could be both a nuisance and a possible health

hazard. The charcoal from such modified plants was produced at generally

favorable tonnage costs.

Resinous wood byproduct recovery. --Horizontally placed, cylindrical steel

retorts have been in general use for the carbonization of highly resinous slash

and longleaf pine stump wood (17,34). Fire boxes are located at one or both

ends of the retort, depending upon its length. In some operations the retorts

are sufficiently large for car loading; in others they are charged and discharged by hand. The process is varied also by the use of concrete carbonizing chambers, to which heat is supplied through pipes placed on the inside.

Since the bottom of the retort is not heated, much of the higher boiling liquid

produced can be drawn off at the bottom without major change, and other

materials in vapor form can be passed through the side outlets to condensers.

Charcoal, pine tar, and solvent oils are the principal products obtained.

After a carbonization period of about 24 hours, the charcoal is removed and

cooled in tight receivers. Because plant operation varies, it is difficult to

indicate the average yields of products obtainable from the stump raw

material. Moreover, the raw material may vary widely in resin content,

affecting yields. The most probable yields of charcoal are within the range

of about 700 to 750 pounds per cord of wood. Based on information available

Report No. 2213 -19-
several years ago, an estimated investment of $25,000 per cord of daily

capacity would be required for new plant construction.

Other retort processes . --Carbonizing equipment somewhat different from the

types in general use has been recently developed for commercial operation.

The equipment has been designed to utilize mainly short-length mill and forest

waste material or mill fines, and recycled hot gases.

For convenience, these types may be grouped to include (1) the small vertical

coaling chambers operating batchwise or continuously on chunk or short-length

raw material and (2) those operating vertically or horizontally with forced

movement of the wood particles for continuous carbonization.%

The vertical retorts are cylindrically shaped and constructed of sheet metal

both with and without exterior insulation (3, 41, 71). All are portable to the

extent that the major operating parts may be dismantled and reinstalled at

other locations without great difficulty. Six retorts of the vertical type

operate batchwise, and one provides a continuous output of charcoal. The

charcoal is discharged at the base of the retorts.

For one such batch retort, a separate oil burner provides heat at controlled

temperatures of 700° to 900° F., depending somewhat on the species of wood

to be carbonized. A coaling period of 7 to 8 hours is needed on the average

for a charge of about 9,000 pounds of green slab and edging material cut into

12- to 16-inch lengths. Production is at a reported rate of about 2-1/2 tons

of charcoal each 24 hours. The investment in this retort installation is understood to be about $15,000.

Reportedly, continuous operation in the vertical-type installation provides

for primary recycling and partial burning of the wood gases to maintain carbonization conditions. The charcoal is withdrawn at intervals from the base

of the retort into water-sealed metal drums. The amount of charcoal

obtained for each 8-hour shift is about 2-1/2 tons from chiefly short-length

slab and edging material. A reported cost for this type of installation is

about $25,000. More recent installations and commercially operated verticaltype retorts are comprised of units of different design. These units employ

5 internal, vertically placed flues which allow simultaneous withdrawal and

combustion of wood gases for heat supply to the wood charge (63). The production rate is reported at about 2-1/2 tons of charcoal per day at one location, and production cost about $18 per ton.

4A partial list of retort manufacturers or distributors is available from the

Forest Products Laboratory, Madison 5, Wis.

Report No. 2213 -20-
In three different commercial methods, horizontal tubes or chambers are

utilized for continuous charcoal output. In one the carbonizing equipment

includes three rotating steel tubes, each fitted with an endless screw for

continuous feed and travel of the chip or sawdust raw material. Heat is

applied on the outside to start carbonization and is stopped when the coaling

can proceed unaided. Each tube is designed so that carbonization occurs in

the forward half of the tube and the charcoal is cooled in the remaining

portion. Estimated production has been placed at about 5 tons from each

tube over a 24-hour period.

A second method makes use of radiant heat for the carbonization of small

wood particles or chips. Coaling is done continuously as the wood travels

on an endless belt over electrically heated steel platens. Across the heated

section of the equipment, about 27 feet in length, travels a 3-inch layer of

chips subjected to a temperature of about 750° F.

The third method requires a unit of brick and concrete construction about

31 feet long, 15 feet wide, and 12 feet high (27). A single metal tube with

screw conveyor carries various kinds of wood fines to a hopper for distribution to a number of horizontal tubes mounted across a wood-gas firing

chamber for direct external heating. After passage through the carbonization

tubes, the charcoal is discharged to a single exit tube for movement by screw

and duct to airtight metal receivers. The main structure has a combustion

chamber at the lowest level, which provides heat to the area in which the

carbonization tubes are located. In operation, the combustion chamber is

fired for an initial period of several hours to obtain a preprocessing temperature of about 800° F. When coaling is underway the retort volatiles or gases

are dried by movement through condensers and passed to the chamber for

combustion. The carbonization process is continued by heat furnished from

the burning of these gases only. The reported yields have been within a range

of 5 to about 9 tons for a 24-hour operating period, depending upon raw

material size and moisture content. The estimated investment for a single

unit with predrier is reported to be between $125,000 and $150,000.

Continuous carbonization of wood fines on a commercial basis is anticipated

from a vertical-type unit varying markedly in operation from those just outlined. Processing involves a form of fluidized-bed technique which brings

about, by forced air, an upward travel of the fines, which are heated by combustion of a mixture of entering air and decomposition gases given off by the

heated wood. The carbon fines are recovered by passing them through

cyclone separators.

Along with the more recent commercial developments in wood fines carbonization, important progress has been made in a further type of semicommercial

equipment operating continuously (66). Equipment includes a rotary and a

Report No. 2213 -21-
slightly inclined horizontal tube for feed agitation and travel. The carbonization heat is introduced into the tube from an outside source. Mill-run Douglasfir sawdust has been used as the feed material, and all byproducts were

recovered. The results of this work indicate encouraging commercial

possibilities.

Current Production and Use

Of the numerous methods herein described for producing charcoal, kiln

production is perhaps of greatest interest because of the low plant investment

needed and its operational simplicity (11,16,22,69,70,88,89). New interest

in the practical utilization of mill fines and other wood residues is shown as

well by the recent development of several types of commercial systems.

The most recent national production figures available are 264,990 tons for

1956. More recent production, however, is undoubtedly greater. There was

a continuous and fairly substantial rise from the 213,660 tons reportedly produced in 1947. Between one-fifth and one-fourth of the tonnage reported for

1956 was produced in 232 kiln operations comprising approximately 1,500

units (4). Kiln concentration is greatest in the Eastern Great Lakes and

South Central regions. California is the only western State with a significant

number of operations (32).

Among the 1,500 units operated in 1956, there were only a few earth kilns but

a considerable number of the brick, beehive-type kilns. The most popular

type, comprising about 600 units, was of concrete or masonry blocks holding

from 1 to about 25 cords. Less common, but nevertheless important, are the

sheet-metal kilns and those of brick construction.

Kiln construction during the past several years has consisted chiefly of these

latter three types. As already noted, some of the more recent installations

have been those of 45-, 60-, and 100-cord reinforced concrete kilns. The

expanding production includes also substantial amounts of charcoal made in

the several newly developed vertical batch and continuous equipment, utilizing

both round and slabwood in short lengths and hogged or chipped material.

Included as well should be the rather large additional amounts recovered by the

now well-established method of continuous carbonization of wood fines. New

plants operated by a continuous method of carbonization by force feeding of

mill fines through a horizontal, multitube retort is expected to expand still

more the production capacity of these newer facilities (27).

Lacking up-to-date surveys, it is not possible to give the extent of more

recent charcoal production. An increase over production reported for 1956

Report No. 2213 -22-
may be safely assumed. This assumption is based largely on the construction

of new kiln-type facilities reliably reported during the past 3 to 4 years.

These facilities in 1956 accounted for upwards of 31 percent, or about 83,000

tons, of the total amount produced. Ninety-two percent of the raw material

used in the charcoal industry was mainly medium to dense hardwood of such

species as beech, birch, hard maple, hickory, and the oaks. The remaining

raw material was chiefly pine. Of the total amount carbonized, about 74 percent was roundwood and the remainder slab and edging residues from primary

wood-manufacturing plants.

There has been no direct information developed on the consumption of charcoal by

end use within recent years. That it has varied use as a raw material is indicated

by the many fields in which it has had commercial application (22). These uses are:

Domestic and

specialized fuel

Recreational

Curing tobacco

Cooking in dining cars

and restaurants

Heating, foundry, and

plumbing equipment

Heating salamanders in shipyards and citrus groves

Report No. 2213

Metallurgical

Copper

Brass

Pig iron

Steel

Nickel

Aluminum

Electro manganese

Armor plate

Foundry molds

-23-

Chemical

Carbon disulfide

Calcium carbide

Silicon carbide

Sodium cyanide

Potassium cyanide

Carbon monoxide

Activated carbon

Black powder

Plastics

Fireworks

Rubber

Gas adsorbent

Crayons

Soil conditioner

Pharmaceuticals

Poultry and animal feeds
Recent reports indicate that little is used for producing a number of the items

listed. For many uses, the amounts of charcoal commercially consumed are

not generally known. Slightly under 100,000 tons were consumed industrially

in 1956 in the Southern, Southeastern, and Great Lakes regions. An estimated

135,000 tons, or most of the charcoal briquetted and some in lump form, were

consumed that year for outdoor and household recreational purposes and

restaurant and railroad dining car cooking.

For many years substantial amounts of charcoal were used in metallurgy.

This market has steadily declined, however, largely because of processing

changes within the steel industry and substitution of cheaper carbon materials.

There is little doubt, however, that recreational use has risen enough to offset the industrial loss (19). Approximately 35,000 tons of charcoal are used

for tobacco curing, water purification, poultry and animal feeds, soil

conditioning, and other miscellaneous uses.

While prospects for the increased use of charcoal in industry appear less

favorable, particularly from a price standpoint, it may be that further

developments in charcoal operations can lower costs enough to regain wider

use in this field.

General Production Costs

Kiln production costs vary widely, from about $25 to $60 a ton. Regardless

of how these costs were determined, many different levels can be normally

expected, because major cost factors do not affect costs of individual operations to the same extent (60,91). Among these factors are raw material and

labor costs and the charges of original investment, depreciation, and plant

maintenance. Additional but less important direct charges are there for

insurance, taxes, and supervision.

Probably the most important single factor controlling the production cost and

profit possibilities is the cost of the raw material at the kiln site. The extent

of this cost is best judged on the basis that about 2-1/2 cords of medium dense

wood are required to produce 1 ton of charcoal. It may easily represent, on

the average, more than one-half the production cost. Mill slabs and edgings,

therefore, usually offer better opportunities for profit than does forest wood.

The average kiln-site cost of mill waste is at least one-third lower than the

average cost of roundwood (4).

Labor charges are another major factor in production costs. These will

differ with types of coaling equipment, mechanization, and other conditions

at each operation. The important effect of labor charges on production cost

is shown in results of two experimental studies (24,70,90) presented later in

this report.

Report No. 2213 -24-
Opportunities for minimizing the effect of both raw material and labor costs

to give more spread between production cost and market price are improved

with methods that make possible more efficient operation. Charcoal is a

low-profit material. Therefore it is important to determine the minimum

economic size of a unit capable of producing a volume sufficient to permit a

predetermined net profit. Profit opportunities can be enhanced by mechanizing the handling of wood and charcoal to reduce labor costs, by obtaining

larger charcoal yields, and by obtaining raw wood at the lowest possible price.

So far as labor costs are concerned, it is generally true that more efficient

production is possible if several units are used rather than one of equivalent

overall capacity. There is, moreover, greater production flexibility with

such an operation as market demands vary (11,16,28,29,69,70).

In general, operations of rather limited production should not seek outlets for

charcoal in large bulk lots. The price usually received for charcoal sold in

this manner is much lower than the price of the product when marketed in

packages, and the outlets for lump charcoal, especially in carload lots, are

not widespread. The operator desiring to develop and establish his own outlets by selling in package form can demand the higher prices and thereby

increase his net profit (13). He must expect also, however, to give more time

and effort to developing and maintaining these retail outlets.

The screened 3/4-inch to about 3-inch lump charcoal sold to recreational and

domestic fuel markets is packaged in burlap bags, multiwall paper bags, or

paperboard boxes. The burlap bags are usually of a 50-pound size and sold

generally to various trade shops and industrial users. The commonly used

paper containers for the recreational retail trade usually hold 2, 4, 5, 10, or

20 pounds. Cost of such packaging is estimated to be $10 to $15 per ton,

which is more than made up, as a rule, by the better price the packaged charcoal commands.

For strictly bulk outlets, the producer may choose to briquette his product or

sell it to a briquetting plant if this appears possible without excessive handling charges. The market price of charcoal delivered to the briquetting plant

is $35 to $50 a ton. Returns somewhat greater than these are possible, however, since the fines fraction, normally screened out when bagging the lump

product, is just as acceptable for briquetting as the lump. Production cost

may be increased, in some cases, to provide temporary dry storage of the

fines until a sufficient amount is stockpiled to pay for the hauling. The operator supplying charcoal to a briquetting plant may enter into a long-time contract that assures minimum holding time between production and transport.

The recreational market for charcoal is seasonal and reaches a peak during

the warmer months. For this reason many producers stockpile sizable

Report No. 2213 -25-
amounts, while others reduce or stop operations during the period of slack

demand. Charcoal can be bulk stored without harm if it is off the ground and

sheltered.

Charcoal Markets and Marketing

All of the charcoal produced must be sold at a profit if an operation is to be

successful. Because production costs, market demand, and the returns

needed or considered desirable for each operation all differ widely, no one

production pattern can be universally applied. To maintain production costs

at reasonable levels and improve opportunities of profit requires alert and

well-organized business management. Despite excellent overall demand, the

prospects for reasonable returns from a given operation depend largely on

operational conditions and markets available to the producer.

The lack of national marketing information handicaps production planning and

organized marketing procedures. A number of surveys on charcoal consumption, use, and marketing pattern employed in general have been made by

several State agencies for their specific areas (7,8,13,26,33,38,40,49,57,58,

6 8, 7 0, 7 2, 8 5, 8 6, 8 7, 9 2). The marketing methods used are generally the same

among the different States.

Recreational and Industrial Outlets

Approximately one-third of domestic production is from small kilns and other

types of coaling equipment of generally limited capacity (20). By productrecovery plants, including those converted entirely to charcoal, are of much

greater capacity and are in better position, therefore, to supply the industrial

bulk market and thereby establish general market prices. Briquette market

prices (14,58,81) are not so strongly dominated by these producers. Prices

quoted in market reports, however, usually become a selling base for both

forms of charcoal. Market prices for recreational charcoal are not always

uniform, probably because heavy local consumption and widely scattered

markets result in prices established by local conditions of supply and demand.

Whether the prospective producer plans to sell charcoal locally or otherwise,

he should first ascertain that there are market openings or that there is a good

possibility for market development. Locating profitable outlets is often more

difficult than producing the charcoal and must be considered equally important

for successful operation. Before production is undertaken, therefore, it is

good business to make a market survey of the area.

Report No. 2213 -26-
Well-established outlets for kiln charcoal include wholesalers and fuel dealers

in cities and towns who buy sizable quantities for resale to industry, for

packaging for domestic markets, or to plants manufacturing briquettes. Or

the producer may sell bulk or packaged charcoal directly to local markets.

When charcoal is sold to wholesale markets in bulk, returns will average

from about $55 to $75 a ton delivered. Some wholesalers contract with the

producer to do the packaging for an agreed-upon additional price.

The better outlets for the producer especially interested in establishing his

own retail business are groceries, hardware and auto-supply stores, gasoline

stations, and department stores. Since charcoal selling is a highly competitive business, such outlets demand systematic delivery of a well-packaged,

quality product. Well-organized operations developed with such retail sale

in mind can expect returns usually not much less than the range of $80 to

$110 per ton for briquettes.

Other outlets for kiln charcoal may be available in areas where briquetting

operations are expanding. The kiln operator may develop his own facilities

for briquetting; or a ready-made customer for kiln charcoal may be found in

a local briquetting plant; or local recovery plants with briquetting facilities,

or even coal-briquetting plants, may be willing to expand their operations by

purchasing kiln charcoal for briquetting.

Charcoal Briquetting

The growing popularity of charcoal briquettes has spurred great interest

among both large and small kiln operators. Some information on plant equipment and cost, manufacturing details, and the practicability of briquette

production with kiln operations is briefly given to provide a few items of

special interest (30,31,66,70,73,74).

Equipment . --The equipment required for briquette manufacture is highly

specialized. Powered units are required for grinding and mixing dry and wet

charcoal, wet forming the briquettes, moving material in process, and continuous drying (figs. 9, 10, 11, and 12). Production rates are 1 to 3-1/2 tons

of briquettes per hour. The-equipment for both capacities is basically the

same, but somewhat larger and heavier machines are needed for the 3-1/2-ton

output. Standard equipment for a l-ton-per-hour briquetting plant includes

the following:

Report No. 2213 -27-
Figure 9. -- Two-shaft, vertical fluxer and paddle mixer.

ZM 116 910

Report No. 2213 -28-
Report No. 2213 29-
- 30 -

Report No. 2213
Report No. 2213 - 31 -
Briquette press with paddle feeder

Hammer mill

Charcoal feeder with surge hopper

Paddle mixer

Vertical fluxer

Starch feeder or pump

Briquette drier

Boiler, 30 horsepower - - 15 pounds per square inch gage pressure

Conveyors

Bagging machine

Building, 60 feet by 120 feet, with 20 feet clear height.

The estimated investment for a l-ton-per -hour plant is from $150,000 to

$200,000. The further cost for an additional ton of briquettes per hour is

about $80,000. The labor requirements per shift are eight men, including a

foreman, a machine operator, a night-shift maintenance man, a bagger, and

three men for warehouse and miscellaneous jobs.

Plant processing. --In general, charcoal lump and fines as received or from

plant storage are fed by screw conveyor to a hammer mill or crusher for

feed material of 1/8-inch and smaller screen size. The ground charcoal is

moved mechanically or by air to a surge bin for metered flow to the paddle

mixer located directly below. As the material flows to the mixer, metered

amounts of about 5 percent of binder (potato or cornstarch) with water are

added. After agitation in a paddle mixer, the mixture is run through the

fluxer for more thorough working of the mass before it is transferred to the

press feeder for regulated flow to the forming press.

From the press, the wet or green briquettes are moved by belt conveyor to a

special device for uniform loading and continuous passage through the drier.

The conditions for drying are usually a 3- to 4-hour period at a temperature

of about 275° F. The processing steps are carried out as shown in figure 13.

The cost of producing briquettes, over and above the cost of making the. charcoal, has been reported to be from $20 to $25 a ton.

Report No. 2213 - 32 -
Report No. 2213 - 33 -
Extent of briquette production. --One important factor in charcoal production

has been the increasing market for briquettes. This expansion has stimulated

much interest in new kiln and briquetting plant construction. Since about 1951,

the number of briquetting plants has increased from 5 to more than 40 (fig. 14)

Some have been operated for years in connection with hardwood recovery

operations. Others have more recently become part of other byproduct recovery operations, while a comparatively small number are part of coaling

operations that are strictly neither kilns nor recovery-type plants.

The majority of the briquetting plants, however, utilize output of charcoal

kilns. Additional briquetting plants have been reported under construction or

in the planning stage. Most of the briquetting and kiln operations are in the

eastern half of the country. Charcoal plants other than those of the kiln type

are widely scattered. All have good sources of forest and mill raw material

and market opportunities in areas of high population.

Because of the large daily charcoal requirements and the investment necessary for even the smallest commercial briquette operation, it is not practical

for the smaller kiln operator to undertake such manufacture. Operating the

smallest commercial plant at a production rate of about 10 tons of briquettes

per day would require at least 250 tons of charcoal monthly. This would

require, in turn between 550 and 600 cords of medium to dense hardwood

material and indicates the size of the kiln operation that must be considered.

Briquetting plants usually operate on two or three shifts per day for most

economical production. The opportunities for the smaller kiln operator in

connection with briquette manufacture are probably most favorable when,

with others or cooperatively, he can provide the level of production needed

for such efficient briquetting plant operation.

Transportation regulations. --Precautions must be taken when transporting

charcoal, especially for considerable distances and between States. The

Interstate Commerce Commission has definite regulations concerning the

movement of hazardous materials. The Commission lists charcoal among

the flammable solids and oxidizing materials which are ‘liable under conditions incident to transportation to cause fires through friction, through

absorption of moisture, by spontaneous chemical changes, or as a result of

retained heat from the manufacturing or processing."

Properly cooled charcoal rarely displays such characteristics. The Interstate Commerce Commission has recognized the value of properly cooling

charcoal, and the regulations (5) have been written accordingly. Specific

examples are:

Report No. 2213 -34-
Report No. 2213 - 35 -
"49 CFR 73.162 (j) (3) Charcoal burned in pits or kilns must be thoroughly

cooled in the sealed kilns. After the kilns are opened, the charcoal must be

allowed to stand in the open kiln or elsewhere exposed to the air for not less

than 24 hours before loading into a freight car. Charcoal burned in kilns may

be loaded in open cars or in boxcars, but after loading in boxcars, the cars

must be allowed to stand not less than 24 hours with doors open before shipment.

"49 CFR 73.162 (k) (3) Screenings, or ground, crushed, granulated, or

pulverized charcoal, from pit or kiln burned charcoal, are considered as

non-hazardous, provided the screenings or the material from which the

ground charcoal is made has been exposed to the air for not less than 5 days

prior to shipment or grinding."

There are additional regulations covering practically all parts of the transportation phase of the industry. These include packaging and marking; drying specifications for briquettes before transport; handling of lump charcoal

and screenings; and instructions for loading boxcars, motor, and other

carriers. The regulations further prescribe the handling of charcoal. For

example, Sec. 77.838 (b) (3) states:

"Articles to be kept dry. Special care shall be taken in the loading of any

motor vehicle with flammable solids or oxidizing materials which are likely

to become hazardous to transport when wet, to keep them from being wetted

during the loading process and to keep them dry during transit. Examples of

such dangerous materials are charcoal screenings, ground, crushed or

pulverized charcoal, and lump charcoal."

The sections noted here are but a few of the regulations covering the handling

and transport of charcoal. Persons interested in producing charcoal should

consult an Interstate Commerce Commission representative. These representatives can be located by inquiring of the local postmaster.

FOREST SERVICE RESEARCH

Scope of Program

Purpose of Research

Forest Products Laboratory research dealing with wood carbonization was in

its earlier phase directed primarily toward problems concerned with

byproduct-recovery plant practice. More recently, its object has been the

development of methods for the continuous distillation of wood fines. Before

Report No. 2213 - 36 -
1954, some limited work was carried out elsewhere than in the Forest Service

on methods of kiln carbonization. The investigators conducting much of the

early work in this field were H. W. Hicock and associates, who experimented

directly with small masonry-block kilns in Connecticut, and H. H. Tryon,

who studied the operation of small portable sheet-metal units in New York

State.

Beginning about 1951, the demand for charcoal as recreational fuel indicated

a much larger production was needed. Because of increasing public interest,

the Forest Service observed many charcoal operations in different parts of

the country to determine if further research would be helpful, and if so, in

what manner and to what extent it might be best undertaken (6 ) .

It was generally agreed that research was needed, especially for the greater

utilization of low-grade forest wood and mill residues, and that no agencies

were currently making studies in this field. It was concluded further work

might well be directed toward the adaptability and use of masonry-block

structures in the range of 3- to about 10-cord capacity. The investigations

undertaken were aimed, therefore, toward the following objectives:

1. Develop a suitable low-cost, masonry-block kiln.

2. Develop reproducible operational procedures designed to provide maximum

yields.

3. Determine the economics of kiln operation with variables of wood species,

wood type and form, and moisture content.

4. Develop kiln mechanization methods designed to minimize labor costs.

The work undertaken about 1955 included the design and field operation of

12 experimental kilns. These kilns, of 3 to 10 cords in capacity, were

located in Collegeville, Minn.; Three Lakes, Wis.; Milwaukee, Wis.;

Marquette, Mich.; Elizabethtown, N. C.; and Athens, Ga.

This research was conducted cooperatively by the Forest Products Laboratory

and two Forest Experiment Stations, together with colleges, universities,

State forestry commissions, and industry (46).

Types of Experimental Structures

Single-wall, concrete masonry kilns . --Initial investigations were conducted

on three single-wall, masonry-block kilns of a rectangular shape. Kiln

dimensions were designed to accommodate both 4- and 5-foot wood. Inside

Report No. 2213 - 37 -
dimensions were about 8.7 feet wide by 18.7 feet long by 7.3 feet high at the

ceiling height. Capacity was about 7 cords net, based on a 4- by 4- by 8-foot

cord.

These initial ‘I-cord units were located at St. John’s University, Collegeville,

Minn. (fig. 15), on the Argonne Experimental Forest in Wisconsin and at the

Athens, Ga. Research Center of the Southeastern Forest Experiment Station.

A smaller 2-cord, masonry-block unit was located on the Bladen Lakes State

Forest in North Carolina.

Concrete masonry was selected as the type of construction best suited for

these small-size kilns because of general availability, ease of assembly,

economy, and durability of the masonry units. Cinder -concrete masonry

was specified as the basic wall material because of its proven fire resistance

and durability in commercial kilns. Some masonry units with expanded blastfurnace slag (Waylite) were used in one of the kilns where crushed and

screened cinders were not available.

These experimental kilns differed in several ways from the original 1- and

2-cord, masonry-block kilns designed and investigated by Hicock and

associates. They were much larger, and each had a steel ceiling. Several

had a metal door that was either suspended from the top or hinged at the sides.

These 7-cord kilns had one or more control joints in the length of a side wall

and at one or both rear corners. Each control joint was reinforced by a freestanding pilaster of masonry block against which air leaks (indicated by

escaping smoke during coaling) could be sealed off with a cement or lime

paste whenever needed. These control joints were intended to limit the length

of wall sections between expansion joints and thereby minimize the amount of

expansion and contraction and subsequent vertical fractures normally occurring in such walls. Some of these wall sections were provided with metal-lath

joint reinforcement in all or in alternate courses in a further attempt to prevent the customary vertical fracture of units.

Some fractures or cracking developed in spite of these preventive measures,

more especially in the blocks made from expanded blast-furnace slag, but

with far less frequency in the cinder-concrete units. Joint reinforcement

was of questionable help in these short 9-foot sections built of an expanded

slag block. It may have been of some help in the cinder-concrete walls, but

this was hard to tell. Nine-foot-long walls of cinder concrete having wiremesh joint reinforcement had narrower and fewer cracks than the reinforced

9-foot sections of slag-type block. Some of this difference could be

attributable to a difference in operating temperatures. It was more likely due,

however, to the fact that the thermal expansion of cinder concrete is only

about one-half that of concrete masonry made from expanded slag-type

Report No. 2213 - 38 -
Figure 15. --Experimental single-wall, masonry block kiln.

ZM 100 144

Report No. 2213 - 39 -
aggregates. It nevertheless served to indicate the superiority of cinder over

slag aggregates for kiln blocks. The thermal expansion of natural sand and

gravel blocks may be slightly greater than that of slag-type units unless the

former are made from a limestone aggregate.

The first single-wall structure built with control joints at the midpoint of

each side wall and at the rear corners is still operable after 50 runs (fig. 15).

Repairs were made, however, after the first 25 runs to replace the front onehalf of one side wall, made of sand-lime brick, with Chicago common brick,

and to replace the rear corner pilasters. These corner pilasters had been

forced outward at the top by the gradual seepage of sand from the ceiling cover

into the vertical joint. Extensive sealing of the outside wall surfaces with a

lime paste kept these brick and cinder-concrete walls in good working order.

Because of difficulty in maintaining a tight seal between walls and pilasters,

it seemed preferable to omit control joints and pilasters, and instead to

shorten the length of side walls.

Double- or composite-wall kilns. -- The considerable expansion and resultant

cracking of the long side walls in the rectangular kilns, together with the need

for continued maintenance in keeping the walls tightly sealed, led to experiments with two square-shaped, double-wall kilns. Interiors were 12.7 feet

wide, 13.3 feet long, and 7.3 feet high. The square shape provided the same

7-cord capacity while shortening the length of the side walls and thereby

reducing the total amount of linear expansion per wall. Instead of one 8-inch

wall, the wall now consisted of two 8-inch walls with an 8-inch column of sand

in between (18). The fine, dry sand was expected to seal off any small cracks

as they developed and thus eliminate the need of continual outer-wall maintenance.

Another important difference between the earlier rectangular kilns and the

later square kilns was the use of air entry at the front of the kiln only.

These square kilns were located at the Athens Research Center in Georgia

(fig. 16) and on the Dukes Forest in Upper Michigan (fig. 17).

Both of these kilns were operated continuously for about 11 runs before any

important structural change became noticeable. Inner walls had expanded

and had begun to bow inward from 1 to 2 inches along the top at midlength of

the side walls. With further use and subsequent settlement, the noncompressible sand-fill pressures became great enough after about 21 runs to

force the outer side walls outward and cause step-pattern breaks in them in

the vicinity of the rear corners. While these breaks were as much as 1/2

inch or more near the top and 1/16 inch or less at the bottom, they were

repairable. Such breaks served to bring out the chief weakness of this type

of construction.

Report No. 2213 - 40 -
Figure 16. --Experimental double-wall, masonry block kiln with

multiple chimneys and single, front air entry.

ZM 100 779

Report No. 2213 - 41 -
Figure 17. - - Experimental double-wall, masonry block kiln with multiple

chimneys and single, front air entry.

ZM 112 349

Report NO. 2213

- 42 -
One inner wall that had bowed inward about 2 inches at the top at midlength

was removed and replaced with new material after run 22. After 14 additional

runs, this portion of the new wall had again bowed inward, so that the top of

the wall at midlength was 2-1/2 inches out of plumb with respect to the bottom.

It cannot be predicted how much inward bowing and tipping of walls can be

tolerated without danger of sudden collapse.

If the sand fill is to remain dry enough to act as a sealer of the outer wall, it

will continue to settle and to force the double walls farther and farther apart.

It is doubtful that such sand-filled structures will ever reach the point of

stabilization where wall maintenance is no longer required.

The inner wall of a double-wall masonry kiln will probably expand more during a coaling period than a single wall of the same material, because of more

uniform heating throughout the inner -wall blocks. The outer surface of a

single wall is being continually cooled by normal air circulation, and the net

thermal expansion of such a wall is consequently less. Thus, the deformation of single-wall structures, which ‘permit greater heat loss, should be less

than that of similar inner walls of a double-wall structure.

If inner walls of double-wall kilns were made of brick, however, their net

expansion would be no greater than that of a single wall of concrete masonry,

because the thermal expansion of brick is only about one-half that of sand and

gravel masonry. When an inner shell of brick is used, the self-sealing

features of a dry sand fill can probably be tolerated if the resulting reduction

in daily maintenance to the outer walls will offset the extra expense of the

periodic replacement of the inner walls.

A number of such 14-cord, double-wall, rectangular kilns have recently been

built by commercial operators. Some of these kilns have been in service for

over 2 years. Each kiln has 14- to 16-inch walls and is approximately 25 feet

long by 15 feet wide by 9 feet high. Kiln walls consisted of 8-inch (sand and

gravel) masonry-block outer walls with joint reinforcement between alternate

courses, 2 to 4 inches of sand fill, and an inner wall of brick or 4-inch

masonry block (slag-type aggregate). Fire brick was sometimes used for the

lower 40 inches of the inner walls, and 4- by 8- by 16-inch slag-type masonry

block or common brick was used on the upper portion. Outer and inner walls

were held together by bolts and vertical bars inside and outside, spaced on

about 4- to 5-foot centers.

These 16-inch walls are also being damaged by the expansion forces against

the noncompressible sand fill, but to a lesser degree than the 24-inch walls.

Damage and breakup is usually confined to the upper common brick or Waylite portion of the inner walls after 4 to 5 months of use without causing any

extensive damage to the heavier outer walls of 8-inch masonry. It would

Report No. 2213 - 43 -
appear that such 16-inch walls should be more economical to build and repair

than 24-inch walls. They have the further advantage of retaining the sandsealing feature of the heavier, double-wall, experimental structures.

A few smaller double-wall kilns of 2- to 3-cord capacity were also included

among the experimental structures (44,47). A 2-cord, double-wall, masonry

kiln with separate or independent roof and ceiling supports is shown in

figure 18.

Another 2-cord kiln of double-wall masonry construction is shown in figure

19. In this demonstration kiln a 4-inch space was provided between 8-inch

masonry walls that was filled with expanded mica. A slag-type aggregate of

processed blast-furnace slag was used in the concrete masonry block. The

mica material was of the ‘home insulation-pellet grade. It was sufficiently

fine to prevent channeling of flame from any lower breaks in the masonry,

but not so fine as to flow into cracks and act as a sealant, as does fine, dry

sand. In fact, the chief advantage to be gained was that of independent action

of each of the walls. Another advantage was better insulation and the confinement of heat for coaling, with resulting saving of fuel. It was further

believed that the insulating effect of the fill would reduce exterior wall expansion and thus permit use, for the outer walls, of the more common masonry

block made from sand and gravel aggregate. Blocks of a lightweight aggregate

that are more resistant to heat than rock-aggregate blocks should preferably

be used for the inner walls, however.

It is particularly important to provide a tight seal at the top of any such

granular fill to prevent ceiling sand from sifting into the cavity and eventually

filling it, thus creating a condition presently objectionable in the double-wall,

sand-fill kiln. Either the ceiling steel must be extended over the opening and

over about one-half of the outer wall thickness, or preferably a separate sheetmetal strip can be added to cover the fill area. A 2-cord commercial kiln

similar to the demonstration kiln pictured in figure 19 has performed satisfactorily for about 20 runs. It is expected that the expanded mica fill will pack

down in time and leave an uninsulated space at the top between walls.

Refractory construction. - - One 3-cord, experimental kiln was made with

composite walls of 8-inch, cinder-concrete block lined with a 3-inch thickness

of a castable, high-temperature refractory material (fig. 20). The back-wall

refractory is rated to withstand a temperature of 2,700° F. and the side walls

1,500° F. The composite walls are held together with metal wall anchors

placed in the wet mortar of the masonry wall as it was built. Inside forms

were erected and a 3-inch thickness of refractory material was poured and

rodded into place against the masonry walls. The ceiling was a 6-inch thickness of castable refractory material rated to withstand a temperature of

Report No. 2213 - 44 -
Figure 18. --Experimental 2-cord, double-wall, masonry block kiln

with one chimney and single, front air entry.

ZM 112 313

Report No. 2213 - 45 -
Figure 19.- -Experimental 2-cord, double-wall, masonry block

kiln with three chimneys and single, front air entry.

ZM 114 310

Report No. 2213 - 46 -
- 47 - Report No. 2213
2,300° F. It is supported by means of metal anchors cast into the refractory

material and fastened to rods suspended from angle irons spanning the side

walls.

One-fourth-inch-wide shrinkage cracks developed in the back corners of the

refractory lining during the curing period, together with many smaller cracks

on both inside and outside walls. Castable refractory linings shrink considerably upon initial drying, so expansion joints are recommended at the corners

of rectangular shapes. Inside cracks were repaired with the refractory- type

material. Outside wall cracks and the entire ceiling were sealed with an

asphalt-asbestos sealer that, when hot, is pliable and nonrunning. Only nine

runs had been made in this kiln when this report was prepared. All initial

repairs remained intact, and the ceiling was serviceable without a roof

covering. The chief advantage of using a refractory lining is in its slower

disintegration at elevated temperatures.

Corrugated sheet-metal kilns .--Corrugated sheet metal was used as a wall

material in two 3-cord, double-wall kilns on the Argonne Forest. It was first

tried in both walls and then in one wall in combination with cinder-concrete

masonry, as pictured in figure 21. The essential design features of these

two double-wall kilns with separate roof and ceiling supports are shown in

figure 22.

In section A, figure 21, two walls of corrugated sheet metal are shown

positioned about 7 inches apart. A 3-inch thickness of a rigid sheet-type,

mineral-wool insulation was placed against the outside metal wall, and the

remaining space was filled with dry sand. The inside metal wall was

supported on 2-inch-diameter pipe frames. The outside metal wall was

supported by means of 4- by 4-inch wooden posts used also to support a shedtype roof giving protection from the weather.

In section B, figure 21, an inner wall of 8-inch, cinder-concrete masonry is

shown combined with an outer wall of corrugated sheet metal, supported by

wooden posts as in section A and with a 6-inch fill of dry sand between. In

each case, the ceiling was of 18-gage metal supported on 2-inch pipe, with

each pipe suspended from the rafter members of the shed roof by means of

3/8-inch rods.

For the first few runs, the door of the kiln shown in section A was of sheet

metal with a sand fill between the metal and rigid insulation, held in place by

removable boards running crosswise into vertical side guides. A steel door

and a laid-up block door were used. A masonry door of 12-inch, cinderconcrete blocks laid up dry was used for the kiln shown in section B,

figure 21.

Report No. 2213 - 48 -
Figure 21. - -Experimental 3-cord, double-wall kilns.

ZM 112 348

Report No. 2213 - 49 -
Figure 22. - - Experimental double-wall kilns with one or both

walls of corrugated sheet steel.

Report No. 2213 - 50 -
The behavior of the sand-filled walls in the kiln shown in section B was

similar to that of the larger, double-walled, masonry-type kilns described

earlier. One masonry side wall bowed inward near its top front and was

about 1-1/2 inches out of plumb.

The bowing of masonry walls caused by the natural expansion of the material

and pressure exerted during settlement of the sand fill and subsequent increase

in sand thickness also caused a considerable outward bowing of the 4- by 4-inch

supporting posts. These posts, each anchored in the ground and held at the

top by the roof rafters, were expected to break eventually and would have to

be replaced. At that time, the sand fill would also have to be removed and

outer metal walls repaired or replaced as required. Some corrosion was

visible outside along the lapped edges of the galvanized, corrugated, sheetmetal walls from the action of acidic vapors from the wood that seeped through

cracks in the inner cinder-block wall. In other respects, these cinderconcrete walls were, in good condition after 36 runs.

The performance of the double-wall, sheet-metal kiln shown in section A,

figure 21, was satisfactory through 16 runs. There was no apparent bowing

of walls or supporting posts. One lower lap joint of sheet metal on an inner

wall buckled slightly by bowing between the vertical pipe supports because of

excessive temperatures during experimental work. A wider overlapping of

the sheet-metal joints or a closer spacing of supports was believed needed.

Structural Requirements of a Concrete Masonry Kiln

Size and Shape

Following the choice of a suitable location for the kiln, the next step is to

determine its size and shape. The size, or gross capacity, is dependent on

certain factors: (1) Estimated weekly or monthly volume of raw material to

be coaled, (2) size of the material, such as chunk or short-length wood,

4- or 5-foot roundwood, longer slabs, and other mill residues. Masonrytype kilns larger than 10-cord have been built, but are not recommended because of the considerable expansion of any type of concrete masonry construction at carbonizing temperature.

Walls should be limited to 18 feet in length, because even cinder-type

masonry, which expands far less than other types of concrete masonry, will

expand as much as 1/2 inch in 18 feet at the normal 800° to 900° F. coaling

temperatures. The kiln length will be governed largely by the length of one

or a number of wood lengths plus about an 18-inch space for ignition fuel.

Actual dimensions must, of course, fit requirements of the masonry block

Report No. 2213 - 51 -
to be used, so that only full and half-size blocks need be used, and that none

need be cut on the site. In other words, build to an 8-inch unit of measurement.

The height of the kiln walls will be governed by several factors: (1) By the

8-inch dimension of the blocks; (2) by the clearance required for mechanized

handling equipment, such as front-end loaders; or (3) by the practical height

to which loading can be done by hand. Masonry blocks will increase in

height as well as in thickness and length when heated. The greater the wall

height, the greater will be the tendency for walls to bow outward and develop

a distress in horizontal joints. It is good practice to limit height of walls to

6.7 to 7.3 feet.

The shape may be square or rectangular, depending upon the size limitations

of the construction materials and the wood length.

A single-wall kiln of about the maximum recommended size, suitable for

both 4- and 5-foot wood and one- or three-stack operation, is shown in

figure 23. Many of the most favorable details of construction from the

experimental kilns have been combined as shown. The structure consists of

8- by 8- by 16-inch cinder-concrete-block walls with three reinforcing bands

of lintel-type block, erected on a perimeter-type reinforced concrete footing; a steel ceiling supported on the side walls; and a suspended sliding-type

steel door.

Materials of Construction

Masonry blocks composed of both coarse and fine aggregates of crushed and

screened cinders from bituminous coal clinkers are recommended as the

basic wall material. Expanded blast-furnace slag may have to be used when

cinder aggregates are not available. All such blocks, however, must be of

the highest possible quality or they will prove unsatisfactory. Dense, moistcured, cinder-concrete blocks are preferred because of their lesser expansion and their proven durability not only in experimental but also in

commercial-type kilns.

Masonry units are usually described by their nominal dimensions; that is,

an 8- by 8- by 16-inch unit, an 8- by 12- by 16-inch unit, and so on. A 16-

inch unit is actually only 15-5/8 inches long. The difference between the

15-5/8 and 16-inch length is taken up by the 3/8-inch mortar joint necessary

to maintain the 8-inch unit of measure. Standard full-size and half-size units

in regular or stretcher-type blocks, together with a few corner blocks, pier

or double -corner, and lintel-type blocks will be needed. Standard blocks by

Report No. 2213 - 52 -
Report No. 2213 - 53 -
different manufacturers may vary in size and in shape and arrangement of

cores, and even in the name given each unit. A few typical masonry units

are shown in figure 24.

Lintel-type blocks with reinforcing bars in poured concrete should be used in

at least three courses in each kiln, as shown in figure 23.

A good grade of masonry mortar with two parts of mortar sand should be used,

or a mix consisting of 1 volume of Portland cement, 1 volume of lime putty or

hydrated lime, and 6 volumes of damp, loose mortar sand.

Eighteen-gage, corrugated, sheet-metal roofing or 18-gage, tri-rib steel

decking is recommended for the ceiling.

Sixteen- to 14-gage, hot-rolled steel sheet or galvanized 18-gage, corrugated

sheet metal is recommended for the steel &or.

Reinforced concrete in the footings and lintel-block reinforcing bands may be

ready-mixed concrete or a 1:2:4 batch-type mix of 1 part of Portland cement;

2 parts of clean, sharp sand; and 4 parts of rock aggregate.

Construction Details

Footings. --Masonry-type kiln walls should be supported by a continuous

perimeter-type reinforced concrete footing. The footing should extend at

least 8 inches below the surface of well-drained ground. If only intermittent winter operation is planned in areas where the ground freezes, the

footing should extend below the frostline. If continuous winter operation is

planned or if frost seldom or never occurs, a minimum footing depth of

8 inches may be used. The footing shown in figure 23 is 12 inches deep and

16 inches wide, with the portion across the front extending 8 inches beyond

the front end of the side walls. Place two No. 4 (1/2-inch round) reinforcing

bars about 8 inches apart in each of the four sides, making sure to overlap

the bars at the four corners. If the footing must extend deeper than 12 inches

because of frost, limit the width to 12 inches and keep the outside edge flush

with the outside face of the side and rear end walls, but keep the 8-inch

extension beyond walls across the front end.

Before starting to excavate the footing trench, square the corners by making

sure that the distances between diagonally opposite corner stakes are equal.

Then, where the soil is firm enough to serve as a trench, form only the top

3 or 4 inches to help in striking off the top of the concrete to obtain a level

surface at the top of the footing.

Report No. 2213 - 54 -
Figure 24. - - Typical masonry units for block-type charcoal kilns.

Report No. 2213 - 55 -
Do not build kiln walls on a floating-type floor slab. If a concrete floor slab

is desired, it should be poured separately from the wall footing. Be sure to

have at least a 1/2-inch space between the floor slab and the edge of the footing to allow for expansion of the slab. Concrete is almost sure to spall, so

the slab should be provided with a wire mesh reinforcement placed near the

surface of the slab to hold any spalled pieces in place. If not of concrete, the

floor of the kiln may be of earth, brick, masonry block, steel mats, or other

noncombustible material.

Do not pour concrete on frozen ground. If the footing is poured in cold

weather, heat the water, sand, and aggregate before mixing, and protect the

green concrete from freezing for at least 72 hours.

Walls .- -It is very important that the first course of blocks be carefully laid

in a full bed of mortar. If the base and second courses are accurately laid,

the balance of the blocks will go into place with all joints perfectly broken.

All blocks are laid with the hollow cores vertical. If hollow-core blocks are

used in the top course, all cores must be filled with mortar to prevent

channeling of air into the kiln through cracks that might develop on the inner

wall surface.

Neat cement or the standard 1:3 mix of cement and sand may be used to level

off the tops of the front wall blocks over which the bottom of the angle-iron

lintel must rest in order to seal off the ceiling cover with sand. A piece of

bright sheet metal may also be used between the angle lintel and the top of the

wall to permit the lintel to slide more easily along the top of the wall. A

loose brick may be used to retain the sand at the end of the lintel.

One to two courses of brick are commonly laid around the top edge of the kiln

to act as a coping. The purpose of the coping is to help prevent the ceiling

sand from being washed and blown off the ceiling steel. Sections of steel

rails or I-beams can serve (fig. 23) both as a coping material and as a support

for the ceiling beams. They also distribute the weight of the ceiling beams

along the side walls and restrain the walls from bowing outward at the top.

Good workmanship is very important when laying the walls. If an experienced

mason is not available, some good manual of recommended practices. should

be consulted before attempting to lay any block (1, 2). Such publications will

provide dimensions of the various types of blocks, recommended mortar

mixes, and many helpful suggestions for building a good wall.

All mortar joints must be carefully compressed and left neat and compact,

either in a concave or V-shape. Such joints will provide a good valley for

sealer compound as sealing becomes necessary during operation. Other

types of mortar joints are not recommended.

Report No. 2213 - 56 -
Immediately after construction, apply a brush coat of Portland cement paste,

a mixture of cement and water of proper consistency, to the interior walls to

help seal the surface. At least 2 weeks should be allowed for the curing of

the completed structure before use. A 1- to 2-cord initial tempering fire is

recommended to give the walls a slow pre-expansion and contraction exposure

before applying any exterior coating of lime mortar or native clay pastes, and

before wood pressures and operating temperatures are imposed on the kiln.

Steel ceiling. --The steel ceiling portion of all experimental kilns was similar

in design. Such a design is recommended as follows:

Each ceiling consists of sectional, tri-rib type of steel decking or 18-gage,

corrugated sheet-metal roofing placed crosswise of the kiln. The decking is

supported by the inner edge of the walls, an angle-iron doorway lintel, and

three to five lengths of 2-inch, black iron pipe running from front to back.

With pipes spaced on about 18-inch centers, the number required will depend

upon the width of the kiln.

Each pipe is supported by 3/8 -inch hanger rods suspended from crosswise

beams above the ceiling and outside of the coaling area. The supporting

beams may be either new or used steel rails, angles or I-beams resting on

the outer edge of the side walls, or independent pilasters. The supporting

members could be woodlot timbers or bottom members of wooden roof

trusses resting on the side walls, or bolted to poles or posts erected adjacent

to the side walls of the kiln. The steel ceiling is covered with a 3-inch layer

of sand to seal all joints against excessive air movement. Glass fiber should

be packed around all hanger rods and into holes cut into ceiling steel to

prevent loss of sand and movement of air into. the kilns.

The steel ceiling sheets are usually installed from front to back in such a

manner that the overlapping sides of one of the sheets near the back of the

kiln is on top and the whole sheet is entirely free except for the weight of the

sand covering. This is done to provide an area of relief against a sudden

pressure of gas within the kiln. Each ceiling is also provided with four

lightly covered smaller openings, two near the front and two near the back

of the kiln. Each opening is about 5 by 15 inches in size. The forward openings serve as initial draft and smoke vents during ignition, while those located

at the rear provide more positive heat circulation for the start of the coaling

cycle. A cover is provided for each vent, so that each may be closed and

sealed with sand during both the latter part of the coaling cycle and the cooling

cycle.

All such metal ceilings have been satisfactory. Some excessive pipe sagging

between hanger supports has occurred and at times permitted the ceiling steel

next to the side walls to sag sufficiently to break the sand seal where the

Report No. 2213 - 57 -
ceiling rested on the side walls. Such a condition has been remedied by putting glass fiber packing between the top of the wall and the end of the steel

sheets and by adding more sand. While considerable deflection of ceiling

pipe supports can be tolerated, it will cut down on the maximum kiln capacity..

Preferably, the spacing of beams supporting the piping should not exceed

4 feet.

Doors. - -The doorway opening may be closed either by laying up dry standardsize masonry block and then plastering the outside joints tight with lime

mortar or by using steel doors. Considerable labor is saved by using a steel

door. Doors may consist of a single thickness of galvanized, corrugated

sheet metal or two sheets of hot-rolled steel with or without insulation between them.

Single-faced doors have proven superior to the double-faced doors in several

ways: (1) They can be bolted rather than welded together; (2) they do not

warp as badly as the double-faced doors, which are deformed from the warping associated with the extensive welding of the rolled steel sheets together

and to the framing angles necessary to obtain a tight seal; (3) any beneficial

effect from the insulation added between the faces of the double-face doors

is soon lost, in part because of settlement of the insulation; and (4) singlefaced doors are much lighter, require far less welding, and are more easily

handled, especially when suspended from an overhead trolley track by means

of flexible steel chains. The door could consist of two sections operable

from opposite sides of the doorway, as shown in figure 19. Such doors may

be held together with "C" clamps, but must also be held a fixed distance

apart to provide a space to hold the lime-paste sealant.

A few double-acting steel doors, hinged at the sides and meeting at the center,

have been tried. They proved too hard to fit and erect, however, so are not

recommended. Some steel doors can become too badly warped to permit use

of lime paste as a sealant. In that case, a strip of glass fiber insulation can

be pasted along the edge of the door with a tar-type adhesive. This will form

an effective gasket to receive and hold the lime-paste sealant in place.

A number of commercial kilns are employing the suspended type of singlefaced door constructed of a relatively light-gage galvanized, corrugated sheet

metal. Overlapping side edges are bolted together in the horizontal position,

and the light-gage metal is bolted to lightweight vertical framing angles.

Doors are suspended by chains from an overhead I-beam or steel channel

track, as shown in figure 25. Two or three "C" clamps are used along the

top to clamp the door to a steel angle doorway lintel. Some are further held

in place with pipe props at the sides or just by using banked earth along the

bottom.

Report No. 2213 - 58 -
Report No. 2213 - 59 -

Figure 25. --Corrugated sheet steel door suspended from chanel-iron track.

ZM 100 992
Chimneys. - - Either one or as many as four chimneys have been used on our

experimental kilns. When only one chimney was used, it was located in the

center of the rear wall. When three chimneys were used, another was added

at the center of the length of each side wall. When four were used, one was

placed at or near each of the corners of the kiln.

Six- or 8-inch-diameter sheet-metal chimneys are supported on loose steel

plates resting on masonry block. Ceramic-type tees and elbows were used

as the base of some of the first kiln chimneys. They proved too easily

damaged and broken, so they were replaced with ordinary square chimney

tile extending through the wall and into a single masonry block chamber or

chimney base.

Chimney bases were constructed from loose pier-type masonry blocks placed

directly on the ground, with loose steel plate covers resting on the blocks.

Two 3/8- by 16 by 20-inch plates were used at each chimney. One plate next

to the kiln was left loose and removable, so that a shovelful of sand could be

added for closing off the chimney during the cooling period. The outside plate

supported the chimney. It was provided with a 6- or 8-inch-diameter hole for

the smoke outlet and with a chimney collar to receive and hold the bottom end

of the chimney in place. Chimney bases normally were kept covered with

earth during the coaling cycle to seal off the loose joints. Dampers were built

into some of the earlier kiln chimneys about 5 feet above ground, but were

found unnecessary.

Partial or full-length insulation of metal-type chimneys is helpful in the

colder climates to retard the condensation and buildup of tars. Chimney tile

has been successfully used inside masonry block chimneys in commercialtype kilns, particularly those partially or wholly buried in an earth fill. Some

commercial chimneys have been made from concrete masonry block without

any lining. All chimneys should extend at least 1 foot above the top of the kiln

structure.

Front air entry and side-wall air ports. -- Entry of air through front and side

walls has been tried. Front-entry air supply has given satisfactory results

in both one- and three-chimney designs.

Front air entry is most easily obtained by hanging the sliding metal door so

that there are about 3 inches of clearance at the floor line, It is then a simple

matter to seal the opening a little at a time with earth or sand as less air is

required. Some operators prefer to use blocks laid on their sides in front of

the door to baffle heavy winds during coaling. When a laid-up masonry door

is used, blocks of the first course are laid on their sides so that cores are

horizontal. Then earth or sand is used to close off the openings as less air

is needed.

Report No. 2213 - 60 -
Sidewall air ports are made by omitting half blocks in the base course at predetermined locations along the side walls. Each opening can be lined with

chimney tile or brick if desired, or used without lining if cinder-concrete is

used. Air ports lined with chimney tile can also be used as chimney openings.

Roof structure . - - Although a roof structure is not a part of the kiln, it is

desirable for shedding rain or snow and to protect ceiling parts from early

corrosion. When roof trusses are used to support the steel ceiling, the roof

structure becomes essential. The kiln may be roofed in any convenient way,

with either boards or sheet-metal roofing. Trusses can be assembled from

ordinary woodlot lumber. A simple shed-type roof framed with poles or

material slabbed on two sides or squared for rafters and beams would also

furnish the necessary ceiling protection.

Some roof structures are supported on poles or pilasters that are independent

of the kiln walls. Others are supported on wood posts or brick or masonryblock piers built up on the outer wall of double-wall kilns, or at the outside

edge of single-wall kilns. Such points of load concentration on the masonry

walls are believed to restrain the wall from normal movement and thus

accentuate a tendency of vertical cracks to localize near such bearing points,

especially in single-wall structures.

Recommended Structural Practices

A number of favorable and unfavorable construction methods that were used

during this experimental work have been discussed. A summary of the more

favorable methods and some things to avoid doing when constructing a

masonry kiln follows:

(1) Use perimeter-type reinforced concrete footings for support of kiln walls.

Keep wall footings separate from any floor slab construction.

(2) Use dense, moist-cured cinder-concrete masonry blocks if at all possible.

Cinder-concrete has proven the most satisfactory of all lightweight aggregate

blocks, because its thermal expansion is generally less than that of all other

masonry materials except firebrick. Protect inner walls in the vicinity of

air inlets with firebrick or a castable refractory coating. Use of front air

entry will eliminate need of any air inlet wall protection.

(3) Use three or more courses of lintel-type block with reinforcing bars and

concrete in all single-wall structures. Bars should be lapped or bent so as

to be continuous around any corners. Avoid use of inward-projecting door

pilasters at the front end of side walls. Such wall stubs are vulnerable to

Report No. 2213 - 61 -
damage from expansion of the doorway lintel and from trucks and mechanical

equipment. Select kiln dimensions such that a full-width doorway opening can

be used.

(4) Avoid use of structural steel shapes for unsupported members within the

coaling chamber. All steel members, such as lintels and ceiling pipe, must

be supported by face-standing columns or other means outside the kiln

chamber. When ceiling support beams or roof trusses bear on the side walls,

limit their anchorage to the use of punched metal strapping nailed to the surface of the blocks. Do not drive anchor bolts into the masonry. Avoid heavy

load concentrations on side walls. Avoid concentrated points of restraint to

the normal movement of the concrete masonry as far as possible.

(5) Use galvanized, corrugated sheet metal for ceilings and doors. Avoid

overloading ceiling steel. Do not use more than the required 3 inches of sand

cover.

Use a door stop at center of wide, one-piece doors to prevent the bottom edge

of the door sill warping toward the inside of the kiln. A recess may be

provided in the concrete sill to receive a removable steel-pin stop before the

door is closed.

Operation of Rectangular Kilns

Kiln operation can be carried out in a number of ways and oftentimes with as

many different results. Information developed in the operation of experimental, rectangular field units provides some positive steps which may be

included to give the more favorable results (75). This information includes

also the relationship of several of the important operational factors, together

with the results of methods developed and observed elsewhere. Much of this

information will be of value in the operation of similar-type commercial kilns

as well.

Kiln Raw Material

Three types of wood are generally used: (1) cordwood, (2) sawmill slab and

edging stock, and (3) blocks and short-length material from sawmills or woodmanufacturing plants. Cordwood and slabs and edgings are usually 4 feet or

more in length, and short-length discarded material may vary from about

3 to 16 inches in length.

Report No. 2213 - 62 -
Wood for a given charge preferably should be of the same general form, size,

and moisture content. This will simplify wood handling, and coaling of the

charge will be more uniform. It is best to put any overly large pieces at the

upper (hotter) part of the charge. Roundwood more than about 8 inches in

cross section should be split or cut to shorter lengths.

Kiln Charging

The manner in which a kiln is charged depends primarily on the type of wood

and the location of openings for regulated air entry and smoke outlets. The

main object is to stack the wood so that the combustion gases can circulate

freely through the pile and most effective use made of kiln capacity.

Cordwood and slabs are usually hauled to the kiln by truck or tractor, and the

pieces are stacked in the kiln by hand. The sticks are commonly piled

horizontally, parallel to the sidewalls and on stringers, as shown in figure

26.

The use of stringers leaves less space for the charge, but better circulation

of air and hot gases is thus gained with the result that there is less partly

charred material. This material, called "brands," accumulates usually near

the floor of the kiln, where temperatures are lowest. Stringers need not be

classified, since they may include roundwood up to about 6 inches in diameter

or brands laid end to end, rough poles, or in some cases structural steel

shapes. The stringers should be placed so that they obstruct air intakes and

chimney outlets as little as possible.

Small pieces are dumped into the kiln from a truck or conveyor, or thrown

in by hand. If stringers are used, they can be covered with a layer of cordwood or slabs to support the small pieces.

Wood of any form should be placed close to the kiln walls and ceiling but not

so tightly packed that it prevents free circulation of the heating gases. Sawdust, shavings and similar wood fines in masses conduct heat slowly, and the

coaling of such materials in kilns is therefore difficult and highly inefficient.

Temperature Measurement and Instrumentation

Measurement of temperatures is highly important in kiln operation, since the

coaling process is controlled by means of temperature and time. The temperature at any given time during the coaling cycle, therefore, gives a direct

and reliable measure of the progress of the run. Except possibly at the end of

the coaling period, smoke color and volume indicate little regarding the actual

Report No. 2213 - 63 -
Figure 26.- - Sticks piled for kiln charge.

ZM 117 156

- 64 - Report No. 2213
the front of the charge, wetted with fuel oil, and ignited with the doors fully

open (fig. 32). Auxiliary ports in the kiln ceiling and slightly to the rear of

the ignition area are open also. After a 5- to 15-minute period of vigorous

combustion, the doors are closed. The front ceiling ports are sealed about

1/2 hour after the doors are closed, and the rear ports are gradually closed

after an average ceiling temperature of 950° F. is reached.

pattern or progress of coaling. By controlling operating temperatures at

about 850° to 950° F., coaling can be conducted with highly favorable results

(62). Figures 27 and 28 show an inexpensive direct current microammeter

and suitable location of thermocouples within the kiln. The assembly and use

of the meter are described in the appendix.

Charge Ignition

The heat for initial drying of the charge is provided during the ignition

period. This heat is supplied by burning wood fuel placed at midpoint or in

front of the charge, or by an oil- or gas-fired torch at similar locations.

Some ignition fuels commonly used are dry kindling wood, brands, charcoal,

and fuel oil. The amount of fuel required depends chiefly upon the moisture

content of the wood to be coaled. Figures 29 and 30 illustrate the different

amounts of wood fuel used for the ignition of unseasoned and seasoned

charges.

Torch method .--One of the most efficient methods for igniting a charge is

with a kerosene- or gas-fired torch (45) as shown in figure 31.

These torches are comparatively inexpensive and provide a high-temperature

heat source capable of igniting separate parts of the charge in a very short

time. The torch flame is directed through one or more air ports until the

charge is burning. Normally, this takes about 5 to 10 minutes. Additional

fuel is not usually required, although, with unseasoned wood, a small amount

of charcoal in the ignition area will provide proper heating conditions more

rapidly.

Ignition with a torch has a number of advantages compared to other types of

fuel placed directly in the kiln. The time needed to prepare and place other

fuels is eliminated. Ignition of the charge is practically assured, whereas

solid fuels furnish variable amounts of heat from which ignition may at times

develop slowly or possibly not at all. Additional space is made available for

charge material also when the torch ignition method is used.

Open-door method. --Ignition with the kiln doors temporarily open has been

successfully employed in a 7-cord, experimental unit. Dry fuel is placed at

Report No. 2213 - 65 -
Figure 27. -- Microammeter calibrated for measurement of

kiln temperatures in degrees Fahrenheit.

ZM 118 302

Report No. 2213 - 66 -
Figure 28. - - Detail of thermocouple assembly and location of thermoco

the lengthwise centerline of kiln, indicating approximate distance fro

kiln floor.

ZM 111 344
Figure 29. - -Fuel stacked for ignition of kiln charge of green wood.

ZM 100 995
Report No. 2213 - 69 -

Figure 30. --Fuel stacked for ignition of a kiln in charge of seasoned wood.

ZM 100 994
Figure 31. - - Igniting the kiln charge with a torch.

ZM 117 157

- 70 - Report NO. 2213
Figure 32. - -Igniting a kiln charge by the open-door method. All block

openings are utilized for air entry during the early coaling stage.

ZM 109 800

Report No. 2213 - 71 -
The chief advantage of this method is the presence of plenty of air for quickly

establishing the required temperature throughout the coaling zone over the

front of the charge. This method of ignition is suitable only for kilns with

adequate vents and doors that can be quickly and safely closed when the fuel

is burning rapidly.

While the doors are open, quite high temperatures are indicated initially by

the ceiling thermocouple over the ignition area. At the same time, the

average, ceiling temperature determined by other spot readings is much lower.

When the door is closed, the average ceiling temperature drops rapidly to

between 400° and 500° F. The temperature then gradually rises to the desired

coaling level. Periodic temperature observations should be made and readings recorded on a systematic time basis to note the progress of the run.

Such information proves useful for the control of later runs.

Closed-door method. --It takes about the same amount of fuel wood and oil to

ignite a charge by the closed-door as by the open-door method. With the door

closed, a means for firing the fuel must be provided. Since the space for air

entry under the door is left wide open, the fuel is ignited by inserting a burning taper under the door to the oil-soaked fuel. When ignition is well under

way, masonry blocks are laid side down along the base of the door, so that

the hollow cores serve as air-intake ports. The air intake is then regulated

for controlled coaling by gradually closing the openings in the masonry blocks.

In kilns having a number of side ports for air entry, the ignition fuel is

generally fired through forward ports at opposite sides, and sometimes

through forward ceiling ports as well.

The procedure for closing the ceiling ports is the same for both methods. The

rise in temperature after ignition, however, is not so rapid generally as in

the open-door method. Using similar ignition methods, unseasoned wood is

more difficult to ignite than seasoned wood. More initial heat is required for

the wetter material, and the only means for gaining ignition effectively is with

an abundance of ignition fuel and draft conditions favorable for good combustion. Anything less than an active, full-face ignition will not give enough heat

to dry the upper portion of the charge for the start of a suitable coaling zone.

With improper ignition, only a small fire is likely to result, and it may continue to burn for a long time before a desirable coaling temperature is

reached. Such operation results in lost production time.

With the use of seasoned wood, satisfactory ignition is more easily obtained.

When using the closed-door method in kilns having side-port air entry, one

stack, and no ceiling ports, it is sometimes necessary to promote draft by

placing a fire at the base of the stack. When coaling starts, there is usually

sufficient heat to maintain positive stack draft.

Report No. 2213 - 72 -
The use of front and rear ceiling ports in a kiln will permit escape of the

initial smoke and also aid in the development of proper kiln temperatures.

The front ceiling ports can be gradually closed when ignition is well under way.

As the coaling temperatures increase, the rear ceiling ports are gradually

closed when ceiling temperatures approach 950° F.

Ignition control .- -During ignition, a large amount of air is necessary for the

rapid combustion of the starting fuels to insure the heat level needed for coaling. This air is supplied through groundline ports in the kiln side walls or

through temporary openings under the kiln door. A fan has also been used to

provide air for ignition and promote chimney draft.

The auxiliary ceiling ports in some kilns serve as temporary chimneys and

aid ignition by causing greater amounts of air to be drawn into the kiln through

the air ports and aiding removal of smoke from the kiln.

The ignition pattern is generally similar for all types of kilns. During the

first 5 to 15 minutes, temperatures in the ignition area will rise rapidly to

about 1,000° F. After much of the fuel has been burned, the temperatures

will quickly drop, often to as low as 300° F. The extent of the temperature

drop is closely related to conditions of air supply and to the moisture content

of the charge. With the establishment of a suitable ignition zone, however,

the temperature gradually increases to about 540° F. The ignition period is

considered complete when this temperature has been reached.

Since kiln temperatures are controlled by regulating the air supply, the amount

of air entering the kiln following ignition must be limited to that needed for

slow or incomplete combustion. If the burning becomes excessive, the

temperature will rise rapidly to above 1,000° F. and a part of the charge may

be consumed instead of forming charcoal. If left unchecked, part of the charcoal already formed may likewise be lost. In extreme cases, the entire

charge may be reduced to ashes and the kiln seriously damaged or destroyed.

Coaling-cycle. --Satisfactory carbonization depends primarily on maintenance

of proper burning conditions in the coaling zone. Sufficient heat must be

generated first to dry the wood and then to maintain the temperatures necessary

for efficient carbonization. At the same time, the burning must be limited so

that only sufficient heat is present to produce good charcoal. Kiln temperature is thus the most reliable measure of control.

For the production of good-quality charcoal, kiln temperatures from about

850° to 950° F. are required. This charcoal will have a fixed carbon content

of about 75 to 82 percent. Prolonged higher temperatures will reduce the

yield of charcoal without necessarily upgrading it for recreational use. If,

on the other hand, coaling temperatures remain quite low, the charcoal may

Report No. 2213 - 73 -
be too "smoky" for major domestic use, and larger than normal amounts of

brands will be produced.

During the coaling cycle, a careful check of kiln temperatures should be made

and the air ports adjusted as necessary. The temperatures should be checked

at least every 2 to 3 hours for satisfactory control. More frequent checks

are advisable when seasoned wood is coaled or during periods of strong or

variable winds.

The air supply is regulated by varying the size of the air-port openings.

Undesirably fast combustion caused by strong winds can be modified or

controlled by use of baffles in front of the air openings. The location of

openings for air to be admitted during coaling depends largely on the kiln

design and the coaling pattern desired.

The direction and rate of spread of the coaling zone is associated with a

number of factors, such as location of air ports and chimneys, volume and

velocity of the incoming air, wood size and moisture content, piling of the

charge, and design of the kiln.

Coaling generally proceeds at a faster rate at the upper part of the charge,

where higher temperatures are available for longer periods of time. Less

rapid coaling takes place near the kiln floor, where the average temperature

usually is lowest.

In rectangular kilns with a single chimney at the rear, the coaling zone

progresses from the front ignition area to the rear (35). The air-intake ports

are located along the side walls at the base of the kiln. These ports are

regulated in pairs one of which is located opposite the other. Following

ignition, the first pair of air ports is open and the others sealed. As the run

progresses, the coaling zone arches from the front and across the top of the

charge with gradual inward and downward movement. When glowing is

observed at the first pair of air ports, this pair is sealed and the next pair

opened. This stepwise procedure is followed until all air ports have been

opened and resealed, at which time coaling is generally completed.

Variations of this method for establishing the coaling zone have been used.

One was included in another study (37). in which the center of the charge was

ignited and the coaling zone spread in opposite directions.

In other methods (16,90), the supply of air may be continuous from one

general location, as under the door, or through one or two pairs of ports

near the front of the kiln. With a single opening for air entry, more than

one chimney may be used to provide better air distribution and more rapid

movement of the coaling zone.

Report No. 2213 - 74 -
The coaling pattern is not always uniform or predictable when there are

strong winds. One result is the formation of "hot spots" or areas of more

active burning, which may raise the temperature above 1,000° F. Most are

of short duration (less than an hour) and not serious. Where pockets occur

in the load, however, as between two tiers of wood, spot burning may become

severe because air is channeled through such pockets.

Smoke may puff outward occasionally through the air-intake openings. Such

a change from normal circulation may be due to chimney downdraft or

possibly to too rapid buildup and trapping of gases near areas of excessive

temperature. Occasional light puffing from the air ports is of no serious

concern. Should puffing be frequent and vigorous, irregular coaling conditions

are indicated and corrective action must be taken. Temporary reduction or

stopping of the air supply will generally provide a suitable remedy.

The time needed to complete the coaling cycle depends primarily on wood

moisture content, kiln size, and temperatures during coaling.

It is essential that the water in the wood be removed before a zone of coaling

action is formed. Therefore, wood with high moisture content will require

a longer period for drying than already seasoned material. It is possible to

reduce the coaling time by as much as 40 percent with seasoned material (82).

In general, coaling time is related to kiln size. When the wood and operating

conditions are similar, the time required for coaling in a 10-cord kiln will be

approximately twice that in one of 5-cord capacity. Means for modifying the

rather fixed rate of temperature rise to coaling conditions in kiln charges

are limited. Attempts to speed up the rate--as, for example, by allowing

more air to enter--will raise kiln temperatures excessively and impair charcoal yields and properties.

It is possible, however, to adjust coaling time by changing the size of the

coaling zone and the manner in which it is directed through the charge. In

rectangular kilns, it has been possible to reduce the coaling time greatly by

center firing. In the application of this method, the coaling zone widens in

opposite directions simultaneously, as compared to one-direction movement

obtained with end firing. Some results from the use of both methods are

given in table 1.

Report No. 2213 - 75 -
Table 1.- -Comparison of coaling time for sugar maple roundwood with

two methods of firing. Data are from 6 comparable runs

in 3-cord experimental kiln

Further experimental results obtained by the use of center firing with multiple

chimneys from 3-cord sheet-metal and masonry block kilns (45) are shown

in table 2. The data are from 11 runs made in the same manner, using

methods of torch ignition and fan-controlled draft referred to previously.

The raw material was 4-foot roundwood 3 to 10 inches in diameter. Six runs

were made with seasoned wood and five with freshly cut or unseasoned

material.

It is likewise true in the operation of rectangular kilns that much less coaling

time is needed with the use of single air entry and multiple chimneys than with

multiple air ports and a single chimney.

The end of the coaling cycle is readily shown when a temperature within a

range of 750° to 850° F. has been reached at the low thermocouples placed

farthest from the ignition zone. Completion may also be indicated when

smoke volume is reduced to little or nothing.

Cooling cycle. - - When coaling has been completed, all air ports are sealed

for the start of the cooling cycle. After the ports are sealed, the chimneys

should remain open until smoking has practically stopped. This permits the

escape of any smoke that may be formed during cooling and prevents the

development of gas pressure in the kiln. Chimneys can usually be sealed from

1 to 2 hours after the air ports are closed. They should be sealed immediately

after they stop smoking, because fresh air may be drawn in by normal cooling, or a downdraft in an idle chimney may admit enough air, to support combustion or possibly cause an explosion.

Report No. 2213 - 76 -
Table 2. --Operational results from 11 comparable charcoal runs with har

4 feet long and 3 to 10 inches thick.1
A very small crack or opening in the structure through which air may enter

can prolong combustion in the form of a hot spot and cause undue lengthening

of the cooling period. Such hot spots may flare up when the kiln is opened.

It is important, therefore, that all possible air-entry openings be sealed during the cooling cycle. This means frequent checking for cracks in the walls,

doors, air ports, chimneys, and ceiling. Air ports at groundline and at the

base of the kiln door are usually banked or closed with sand. Cracks are

sealed with lime slurry or. mortar cement. Roofing tar is often used for sealing cracks. Because it expands and contracts with wall movement and is quite

stable, it may last for several runs. Cracks repaired with a lime slurry

usually require resealing after each run. Roofing tar should not, however,

be used to seal openings around doors or be placed in contact with other hot

metal.

When temperatures have been reduced to 150° F. or less, it is generally safe

practice to open the kiln. Before the kiln is opened for discharge, however,

the charcoal should be checked for localized hot areas that were not evident

during the overall kiln temperature measurements. This check should be

made by opening several air ports and one or more chimneys. If the temperature does not rise within 2 hours, it is considered safe to open the kiln

completely. If the temperature rises, however, the kiln should be resealed

and carefully checked for sources of air leakage.

The time required for cooling depends largely on the rate of heat loss by

radiation through the kiln walls, door, and ceiling. Double-wall structures

or those enclosed with banked earth will cool at a somewhat slower rate than

single-wall structures with exposed outer surfaces.

Exploratory studies have been made with water to hasten cooling (35,70,83).

Results indicate that cooling time can be reduced as much as 60 percent by

spraying water over the top of the charcoal bed. The amount of water needed

has been estimated to be from 100 to about 350 gallons per ton of charcoal.

Care should be taken that the cooled charcoal will contain no more moisture

than is generally allowable in a marketable product.

Kiln discharge. - - Charcoal is a bulky, dusty material that is difficult and

costly to handle manually from the kiln. Mechanized yard handling and

transport operations, therefore, have distinct advantages. The kiln can be

discharged and charcoal moved about more efficiently and conveniently, for

example, with a tractor scoop (fig. 33), or belt conveyors and bucket

elevators.

Report No. 2213 - 78 -
Report No. 2213 - 79 -
General

Safety Precautions in Kiln Operations

In the production of kiln charcoal, accidents can be greatly reduced by making

use of safety features and safe work habits. Careful planning of work, and

good housekeeping also promote safety. In this business as in any, where fire

and high-temperature operating conditions are commonplace and extensive,

carelessness or slovenliness can be ruinous.

While kiln operation is simple, much depends upon the judgment of the

operator to keep production safe. Production hazards increase, and undesirable or dangerous burning conditions can arise if the operator neglects to pay

close attention to such vital operational factors as the weather, structural

condition of the kiln, coaling time, and temperatures. There are ample

records of kiln damage and destruction because of just such neglect and oversight.

Types of Hazardous Conditions

Explosions. --Kiln explosions can have extremely serious results. The causes

of such accidents are not clearly understood. They are most often caused,

perhaps, by mixtures of hydrogen or methane and air. Since varying amounts

of these gases are no doubt present during most of the coaling action, the

formation of explosive gas mixtures can be perhaps minimized by efficient

use of the air admitted. The admission of more air than is needed for uniform

and progressive coaling action might well result in formation of explosive gas

mixtures.

In some experimental runs a “puffing” or pressure-release action of kiln gases

has occurred at times when the charge was coaling in a controlled and orderly

manner) the coaling zone was uniform and well-established, and air input was

closely regulated for proper coaling temperatures. This puffing, which has

been observed to occur more often with seasoned than with unseasoned wood,

may cause sheet-metal doors to vibrate at times. While the puffing is of itself not severe, the causes of it could, if disregarded, create explosive conditions. An explosion would very likely result in wall cracking and admission

of more air. This could in turn lead to more severe structural damage as

the additional air stimulates even higher temperatures.

There is some reason to think that periods of uneven gas distribution may

provide conditions for the accumulation of uncombusted gas. Under such conditions, puffing could take place repeatedly as more air is admitted and gas

accumulates again.

Report No. 2213 - 80 -
Another possible explanation for kiln explosions has been given by a Swedish

source (23). The period of greatest danger, according to this source, is during the early coaling stage, when comparatively large volumes of water and

other vapors are being condensed on relatively cool wood. The noncondensable

gases, including those capable of explosion, are thus free to form critical

mixtures with air.

The fact that kiln explosions occur only infrequently would suggest that

average operating conditions are for the most part satisfactory. The comparatively few explosions that do occur are probably due to coaling conditions

in which air is uncontrolled.

Structural fires . --Kiln explosions are a major cause of structural fires.

While the explosion itself may cause only minor damage to the kiln, fire can

result from the admission of large quantities of air to the kiln through cracks.

Prompt sealing of any structural breaks caused by the explosion is therefore

essential to prevent outbreak of fire.

Other causes of fire are the operator’s unfamiliarity with proper operating

steps and outright carelessness. After making several successful runs, the

operator cannot relax his vigilance and assume that, during the coaling period, the kilns can be left unattended. No two runs can be made exactly alike

as changes in weather, in the kinds and condition of the raw material used,

and other factors make each run different.

With prolonged heating, there is always danger of wall cracking or separation.

Should either occur and remain unnoticed, the seepage of excessive amounts

of air through such openings could easily make a change in the coaling pattern.

These changes might lead gradually to very high temperatures) or they might

take place rapidly, creating a serious fire condition. The operator's

familiarity with his equipment and the steps necessary for good operation are

the best insurance for safe practice and satisfactory production. Wellestablished, periodic inspection will indicate oftentimes the corrective

measures necessary for proper control and reduce the possibilities of

damaging fir es.

Commercial kilns have been damaged or destroyed because of unexpected

developments during the coaling cycle. Of major importance also are the

reduced yields and loss of operating time due to improper sealing of the kiln

during the cooling period, together with the possibility of heat damage to the

structure from burning charcoal. Such conditions may happen even though

a well-standardized pattern of operation has been established. The importance of inspecting and maintaining kilns during the coaling and cooling cycles,

controlling operational conditions , and using safe practices cannot be overemphasized.

Report No. 2213 - 81 -
Temperature-indicating equipment suitable for operational control (61,62),

during coaling and cooling periods, together with an outline of safety-procedures, is described elsewhere in this report.

Hazards to workmen and public . - -Fire, whether controlled inside the kiln or

uncontrolled, constitutes a potential hazard to workmen and the public alike.

Unauthorized persons, including the public, should not be permitted in a

hazardous area unless guided. Transport of wood, charcoal handling, and

other essential work involves hazards to safety. In charcoal production, as

in any kind of business, workmen must be well informed of operational

hazards. Safety measures and safe work habits must be considered of prime

importance.

Safety Equipment

Temperature indication and control. --Equipment that indicates kiln-operating

temperatures, essential for gaging the progress of the coaling and cooling

cycles, is also an invaluable aid in controlling fire hazards. As temperatures

fall or rise, more or less air should be admitted, thus maintaining desired

control of burning. Inexpensive thermocouple-type instrumentation for one

or several kilns is available and should be installed.

Air-port closure plate . - -Some development work has been done with a safety

plate for use in multiple-stack, masonry-block kilns with metal doors and

under-door air entry. While not entirely reliable, it provides an added margin of safety.

This device, designed to overcome or control puffing with resultant vibration

of the door during the coaling cycle, has been improvised as a safety measure.

It consists of a weighted half-section of a heavy metal hinge propped horizontally above the air-intake port. The half-section is lightly supported on a thin

metal prop, and a trip rod is placed between the kiln door and the metal prop.

When the door vibrates in response to strong puffing action, the trip rod

‘pushes the hinge prop sufficiently to drop it over the air port. Air is thus

automatically cut off from the kiln, thereby removing a cause of puffing and

possible explosion and fire.

Stack closure plate. - -A chimney-port safety closure has been developed for

multiple-stack kilns to prevent air entry when any one chimney stops smoking

during the coaling cycle. Working properly, the device forestalls entry of

excess air into the kiln through the stacks and allows coaling to proceed without danger of accelerated combustion and heat buildup. The basic idea of this

device has potential application also for single-stack units and for kilns of

other than masonry block construction.

Report No. 2213 - 82 -
The heavy metal plate is held in place by a dry wood 2 by 4 over each of the

inside chimney openings. It is installed before the kiln is charged. A metal

guide rod placed on the chamber side prevents the plate from tilting inward

as it drops. A 3/8-inch rod welded to the plate provides a stop for the

diagonally placed stick, thus securing the closure plate in the closed position.

The plate drops and seals the port only when the wood support has been

partially burned as coaling combustion reaches this location. The wood prop

will burn sufficiently within 30 minutes to cause release of the plate for

temporary port closure, or until it is possible to make a permanent and airtight seal. Good closure is generally made when the plate is cut oversize to

provide at least a 4-inch margin of metal from the top and sides of the

chimney opening.

This improvised device is most useful when unusual combustion conditions

associated with ashing occur rather than during normal carbonization of

wood. The wood prop supporting the plate, even though it will burn under

normal coaling conditions, usually remains sufficiently strong under these

conditions to support the plate. Under normal coaling conditions, therefore,

an active chimney would not be sealed.

Stack antidown-draft caps. - -Air diverters of various kinds on chimney tops

prevent sudden downdrafts into the kilns. Prevention of downdrafts is helpful

in avoiding overactive coaling combustion and minimizing the danger of

explosion. Satisfactory results can be obtained by the use of a simple, fixedtype hood fabricated of thin-gage sheet metal. One design for a suitable

diverter consists essentially of an inverted dished section slightly larger in

diameter than the chimney and secured about 6 inches above the chimney top

in a horizontal position. A metal skirt, somewhat greater in width than the

6-inch space between the chimney top and the dished section, is secured by

metal supports from the chimney and positioned outward several inches to

surround and baffle the 6-inch space above the chimney top.

Unregulated and more than needed amounts of air may enter the air ports as

the wind changes in force and direction, and harmful effects to the coaling

cycle will develop. Baffling or temporary closure of the air ports facing

strong winds is recommended.

Location of Kilns

Because a fire hazard is always present during charcoal production, kilns

should be located away from other buildings, and brush or other flammable

material should be removed.

Report No. 2213 - 83 - GPO 812012-7
A site should-be chosen that is away from populated areas to avoid creating

a smoke nuisance. Under some atmospheric conditions, kiln smoke settles

and drifts near the ground. A site on high ground, therefore, is preferable

because prevailing winds will carry the smoke into upper air strata. Smoke

can also be partly or entirely eliminated by combustion in a burner (5 3)

where some other means of removing it is essential.

Water supply. - -A water supply is highly important to any kiln operation.

When at all possible, water should be stored in a yard tank or piped to the kiln

site. A hose with nozzle should be kept ready for immediate use. Back-pack

water pumps or large-capacity fire extinguishers provide some measure of

fire protection.

Limited public access. -- The control of unwarranted traffic in and about a kiln

area is desirable. The operating area should be fenced; or, as a minimum

measure, warning signs against trespassing should be erected.

First aid accessories . --Adequate first aid supplies, including dust masks,

should be kept conveniently at hand.

Safe Operating Practices

The fact that charcoal production in kilns is a comparatively simple operation

should not mislead operators into thinking that safety practices are unnecessary. A number of essential points dealing with operating safety are worthy

of note.

The coaling of wood takes place in widely different patterns, depending a good

deal upon moisture content. Fairly large amounts of heat are given off when

wood changes to charcoal. This heat, together with that of charge combustion,

is more gradually used in charges of unseasoned wood than in charges of

partially or thoroughly dry wood. Kiln temperature likewise will change more

gradually if the wood contains appreciable amounts of moisture, since much

heat is utilized to evaporate the water. The faster coaling action likely to

develop with dry wood means excessive temperatures and amounts of gas,

with resultant greater possibilities for kiln fire or explosion.

Obviously, the drier the wood, the more closely must coaling be controlled

if the hazard of explosion and fire is to be held down. From a safety standpoint, therefore, as well as for most efficient coaling, wood should be between about 30 and 40 percent in moisture content.

Particular care should be taken in igniting a kiln charge by the open-door

method, since the highly flammable fuel oil and abundance of air set off

Report No. 2213 - 84 -
vigorous and spreading combustion at once. There is especially great

danger here of suffering burns from the sudden flaming until the door is

closed. Gloves should be worn when working near the kiln door, and workmen should be careful about tripping or falling against heated surfaces during ignition or coaling periods.

While combustion is less spontaneous and widespread when a charge is

ignited by the closed-door method, it is equally necessary that safety be kept

in mind at all times. Fuel oil commonly used for ignition purposes should be

poured over the fuel bed immediately before it is ignited, because back-fire

and explosion can easily occur if oil fumes are allowed to accumulate in a

kiln that is practically closed. Gasoline or similar fuels with low flash-point

should never by used. Structural cracks and other openings that occur during coaling can usually be detected by the smoke escaping through them.

They should be repaired at once, since during the cooling period no air should

enter the kiln. Even a little air will prevent proper cooling to take place.

Oil- and gas-burning torches used to ignite a charge can cause overheating,

which in turn can lead to structural damage and too fast coaling. There is

always, moreover, the possibility the burner may cause an explosion.

Incomplete combustion or mechanical failure of the burner is particularly

hazardous. Raw fuel oil ejected by a malfunctioning burner can quickly

vaporize within the kiln, setting up a critical explosion condition. Proper

use and understanding of such equipment is therefore of highest importance.

When the ceiling of a sand-sealed, flat-ceilinged structure is being inspected

or repaired, workmen should stand on long planking supported by the ceiling

framework. Heavy gloves should be worn for protection from burns by hot

sand or metal.

Poisonous gases are present in a kiln that has just been cooled. The kiln

should be thoroughly ventilated before entering it and all during the time it

is occupied. Dust masks should be worn when charcoal is handled in confined

areas. Dust can be held down by sprinkling the cooled charcoal with water

before unloading the kiln.

Orderly housekeeping methods are recommended because they contribute to

efficient management.

Charcoal Storage Precautions

The highly seasonal demand for charcoal in most parts of the country makes

it necessary either for producers to stockpile a considerable inventory,

which is expensive, or to stop operations temporarily.

Report No. 2213 - 85 -
A great deal of care must be taken in storing charcoal. Although the product

is very stable when properly cooled, it can be hazardous when handled otherwise. There is danger also that this easily combustible material, when stockpiled in any amount, will be ignited by other sources of fire. It is always best

practice to screen the charcoal for removal of fines before storage, whether

for short or long periods of time.

Although spontaneous combustion (self ignition by generation of heat through

chemical action within the charcoal) is always a possibility, a much more

common cause of fire during storage is socalled "hot" charcoal. Such charcoal consists of hot or even actively burning pieces that escaped notice when

the charge was removed from the kiln. Spontaneous combustion may occur

also in masses of charcoal water sprayed for rapid cooling. It is advisable,

therefore, that freshly discharged charcoal be placed in the open away from

previously cooled and conditioned charcoal for at least 24 hours. During this

time, it should be exposed to good air circulation and protected from rain,

preferably in an open shed rather than under a tarpaulin or in too confined a

space. If there is no evidence of heat or active fire after this cooling period,

the charcoal can be considered safe for warehouse storage.

Tightly packed masses of charcoal fines are more subject to spontaneous

combustion than the larger lump material. Whether in containers or in bulk

piles, fines should be exposed to the air but protected from rain for not less

than 5 days before being placed in more confined storage or shipped out.

Fines should be stored separately from lump charcoal.

Charcoal Yield and Production Factors

General

The importance of yields and their accurate determination in charcoal production cannot be over-emphasized. The amounts of charcoal obtainable

from the raw material available should be carefully considered by anyone

planning to start or, for that matter, to continue an operation.

The true value of charcoal is a matter of both quantity and quality. While

charcoal is sold mainly by weight, its quality is largely the reason for repeat

sales --or loss of markets to competitors. The quality of lump charcoal can

be roughly determined by such physical properties as hardness, brittleness,

and density. It can be better defined by chemical composition--that is, the

amounts of fixed carbon, ash, and volatiles charcoal contains (5 0). The

determination of quality on the chemical basis is highly important to producer and consumer alike in the marketing of either lump or briquette charcoal.

Report No. 2213 - 86 -
Measuring Yield by Charcoal Percentage

A suitable method of measuring the yield is to divide the weight of the charcoal recovered by the calculated weight of the ovendry wood used. This gives

a figure that, when multiplied by 100, gives the percentage of wood converted

to charcoal.

Weighing the wood charged rather than measuring its volume eliminates the

effect of such variables as species density and stick size and form. Likewise, calculation of the weight of the ovendry wood removes the variable of

moisture content, as between green and dry wood, and tells exactly the

amount of wood available for charcoal production.

Ordinarily, no weight allowance for moisture content is made for charcoal

that has been cooled by natural radiation and unloaded in dry weather. Charcoal made experimentally at northern locations averaged 2.3, and that at

southern kilns 2.4 percent moisture content. Moisture content levels much

under 3 to 4 percent, no matter how precisely measured, would probably be

offset by errors in sampling of charcoal and wood for moisture content. If

the moisture content of the charcoal has been raised as a result of water cooling or during prolonged bulk storage under high humidity conditions, a

determination of moisture content is necessary to obtain a useful yield value.

How to Determine Charcoal Conversion

Wood moisture content. - - Commercial electric moisture meters are suitable

for measuring moisture content if the wood is well seasoned. These

instruments are reasonably accurate within a range of about 7 to 30 percent.

By another method, moisture content is determined on l-inch-thick sections

cut from a representative number of the pieces in the charge. These sections

should be weighed as soon as cut or wrapped in foil or sheet plastic to maintain their moisture content until they can be weighed. The weighings should

be made to the nearest tenth of a pound.

Dry the weighed sections at a controlled oven temperature of about 212° F.

for at least 12 hours and weigh them again. If the sections do not change

weight after one or two 12-hour intervals of further drying, all of the

moisture they contained has been removed.

The moisture content of each wood section is calculated as follows:

Report No. 2213 - 87 -
The average moisture content of all the sections is then determined.

Charcoal moisture content. - -The moisture content of charcoal may be determined by the drying-oven method as outlined for wood. Dry the charcoal in

an open container, weigh it, and subtract the weight of the container from the

total weight to obtain the ovendry weight of the charcoal. Avoid drying temperatures in excess of 212° F. An imported moisture tester for charcoal has

recently become available (appendix).

In establishing the weight of the ovendry wood in the charge, the wood is

weighed as the kiln is loaded. This weight is used for calculating the weight

of the ovendry charge by applying the method for average moisture content

previously described. The weight of the charge when ovendry is determined

as follows:

The charcoal yield is determined in percent as follows:

Quantity measurements. - -It is not always possible to weigh the wood, nor is

it always economical to do so when wood is purchased on a volume basis. In

such cases, charcoal yields may be determined from calculated values for

weight of wood by the cord. The accuracy of this method will depend upon the

number of raw material variables that can be controlled.

The standard cord generally accepted in the United States and Canada is 4 by

4 by 8 feet. A cord of roundwood contains, therefore, 128 cubic feet of wood,

bark, and voids (25). The weight of the wood in a cord depends, in turn, upon

wood density and the diameter and form of the sticks. Since wood varies considerably in density within a species as well as among species, and size of

sticks also varies a great deal, such estimates of cord weight can only be

approximate. This indirect method for obtaining wood weight is discussed

further elsewhere in this report.

Charcoal yield by weight can be estimated by measuring the number of bushels

obtained per cord of wood and using arbitrary bushel weights of 20 and 18

pounds for hardwood and softwood charcoals, respectively. Charcoal weights

obtained in this manner are subject to considerable error because of

Report No. 2213 - 88 -
inaccurate measurement by the bushel, the variability of charcoal itself, and

the rough approximation inherent in estimates of wood weight by the cord.

Charcoal quality. - - The amounts of moisture (2 to 4 percent), volatiles (18 to

23 percent), ash (1 to 4 percent), and fixed carbon (74 to 81 percent) in charcoal provide an average index of quality for general market acceptance either

in lump or briquette form (50). Charcoal with relatively low volatile content

and correspondingly higher amounts of fixed carbon is desirable for specialized industrial uses. Temperatures somewhat higher than the normal kiln

operating temperatures of 850° to 950° F. are required to produce it. The

volatiles, when present in proportions greater than about 24 percent, will

cause smoking when charcoal is burned and will give product degrade in some

areas of recreational use.

Chemical properties can be precisely determined only with analytical equipment. A rough quality test for volatiles can be made, however, by burning

samples of charcoal and observing the absence or extent of smoking. A

metallic ring when a piece of charcoal is dropped onto a hard surface provides

a further rough test for good quality. Too rapid coaling at high temperature

usually results in the formation of crumbly charcoal easily broken into small

pieces and fines. The species of wood does not influence the chemical quality

of charcoal; the physical properties, however, are influenced by wood density

and structure. For example, the low-density woods produce charcoal in

greater bulk, while some woods will produce brittle charcoal. In general,

the lump charcoal obtained from the medium-dense to dense hardwoods is

considered a cleaner product because of less breakage and dusting with handling.

Effects of Operating Conditions on Yield

Charcoal yields are affected by variables within the kiln charge and by the

coaling conditions employed. The quality of the charcoals varies likewise

with change of operational conditions. A summary of the results obtained in

field studies, with reference to other research, points out the relationship

of charcoal yields to some of these major variables.

Wood Moisture Content and Yield

The yield of charcoal varies with major changes of wood moisture content. More

of the wood in a charge is consumed for drying when coaling unseasoned than

seasoned wood, consequently there is less charcoal. Experiments with seasoned

hard maple at 32 percent moisture content coaled in 3-cord kilns resulted in

a 30 percent yield of charcoal. Similar charge material at 44 percent moisture

Report No. 2213 - 89 -
content was converted in a yield of 27 percent. Other comparable experiments on mixed northern hardwoods at 47 percent moisture content resulted

in yields of about 25 percent.

Six more experimental runs in a 7-cord kiln, using white oak and hickory at

36 percent moisture content, resulted in 33 percent conversion, or an average

yield of 910 pounds of charcoal per cord. Ten further runs, using wood averaging 63 percent in moisture content, resulted in 29 percent conversion, or a

yield of 792 pounds per cord. The variations in moisture content had no effect

on charcoal quality.

There appears to be sufficient difference in yields from the coaling of

unseasoned and seasoned wood to make moisture content an important factor

in kiln operation. Whether to use unseasoned wood with reduced charcoal

yield or take on the extra stacking and handling costs for seasoning is something each operator must decide for himself.

Wood Moisture Content and Conversion Time

In connection with the coaling pattern, three general groupings of wood by

moisture content can be made: Wood air seasoned to within the range of

20 to 35 percent moisture content generally requires the shortest coaling

cycle; wood at 35 to 45 percent requires a median coaling cycle, while wood

at a moisture content of 45 percent or more (unseasoned) the longest (82).

Wood with less than 20 percent of moisture is likely to coal too rapidly, and

care must be taken to avoid development of excessively high temperatures.

The results from the operation of both the 3- and 7-cord experimental kilns

indicate that the average coaling time for seasoned wood was 4 to 5 hours per

cord and that for unseasoned wood, 5 to 8-1/2 hours per cord.

Wood Moisture Content and Brands

A charge of unseasoned wood is more difficult to convert completely than one

of seasoned wood. Some "brands," pieces not wholly charred, are generally

found near the kiln floor, where the usually lower temperatures prevent

uniform coaling at an average rate.

Brands were present in most of the experimental runs. In a 7-cord kiln,

seasoned oak wood produced 97 pounds and unseasoned oak 260 pounds per

cord on the average. In a 3-cord kiln, 358 pounds of brands were produced

from seasoned hard maple and 481 pounds from unseasoned hard maple.

Report No. 2213 - 90 -
These weights included the moisture remaining in the brands and therefore

the number of brands would be relatively small. The higher weight ratio of

brand material to charcoal from the 3-cord kiln runs cannot be clearly related

but may be due to variation in kiln size and to minor differences in coaling

procedure. In these series of runs, the charges of wood were at comparable

moisture content levels.

Effect of Wood Species, Form on Weight

of Charge

The yield of charcoal from a kiln is directly related to the density of the

wood charged. As a general rule, then, the denser or heavier the wood used,

the greater will be the weight yield. Even within some species, as among

the oaks and hickories, there is variation in the weight of dry wood per cubic

foot. This variation in the ovendry weight per cubic foot--for a number of

species is shown in table 3. An indication of the effect of wood density on

yields for heavy and light species is given in the coaling results from a 7-cord

kiln (table 4).

Table 3 .--Weight of various species when ovendry (74)

Report No. 2213 - 91 -
The form of the wood will influence the weight charged and likewise the weight

of charcoal produced. Straight roundwood has favorable form for good piling

and therefore provides the more solid material per cord. The wide range of

wood weights that is possible because of species differences and piled solid

wood per cord is shown in table 5, together with related charcoal yields.

The effect of wood form on piled kiln charges has been noted in experimental

operation. The dry weight of southern red oak in a fully charged 7-cord kiln

(6.6 cords, actual volume) varied from 2,350 to 3,250 pounds per cord. The

amounts of dry wood that it was possible to load into the kiln thus varied by

about 3 tons, depending chiefly upon stock diameter and straightness. This

difference of wood weight represents about 1,800 pounds of charcoal.

The dry weight of sugar maple that could be put in a S-cord kiln (actual

volume, 2.7 cords) ranged from 2,260 to 3,030 pounds per cord. The

difference of 2,100 pounds of wood in this case amounts to somewhat over

600 pounds of charcoal per kiln charge. Since there was a wood weight

difference of but little more than 100 pounds per cord between the southern

oak and sugar maple, it would indicate that the maple sticks were somewhat

straighter and more uniform.

Differences in wood form and size--for example, roundwood, slabwood, and

chunkwood - - also cause differences in kiln charge weights. Indications of this

are shown in the results obtained in loading experimental kilns with different

types of raw materials (table 6).

Report No. 2213 - 92 -

Table 6. --Influence of type of raw material on weight of kiln charge

Sawdust, chips, and shavings are not suitable raw materials for kiln charcoal.

In loose or compacted masses of such materials, heat is transmitted with

difficulty and, while coaling is possible, it can be carried out only in a very

inefficient manner.

Effect of Species, Form on Charcoal

Yield, Quality

Satisfactory charcoal yields were obtained experimentally with various wood

species and forms. The hardwood flooring trim, which was the smallest

material coaled, gave yields of 30 percent. This yield is suitably high but

will increase or decrease with the amount of solid material charged, since

this factor noticeably affects the charcoal yield per kiln charge.

Given proper coaling conditions, all wood species will produce good-quality

charcoal with comparatively high fixed carbon content. All species included

in the field experiments were converted to good-quality charcoal with 75 to

82 percent of fixed carbon. This was equally true in the coaling of the slab,

flooring trim, and chunk forms of raw material. The species used singly or

in groups were sugar maple, mixed northern hardwoods, basswood, southern

oaks, mixed southern hardwoods, and hickory.

Report No. 2213 - 94 -
Effect of Kiln Size, Type on Charcoal

Yield, Quality

There has been little direct information developed regarding the effect of kiln

size on the coaling pattern or charcoal yield. No serious attempt was made

in the field studies to develop such information because of wide variation in

type of structure, raw material, and operating methods. Where a measure

of comparison was possible in the operation of kilns ranging from 2- to about

10-cord capacity, it appeared that the larger have a production advantage

during a given period of time. Hicock (35) reports that kilns of 5- to 12-cord

capacity appear capable of greater production per cord of wood than the

smaller units. One possible explanation is that oversize wood can be more

successfully carbonized in the larger kilns.

Further indications of greater yields from these larger kilns are obtained by

comparing coaling results from 3- and 7-cord experimental units. Whether

seasoned and unseasoned wood was used, charcoal production from the 3-cord

units was measurably less than from the 7-cord units. The yield difference is

not clear cut, however, since operating conditions have not always been the

same in kilns of both sizes.

Charcoal of satisfactory market quality can be made in kilns of any size or

type when suitable coaling temperature and time conditions are present. It

is perhaps more difficult to produce charcoal of consistently high quality in

uninsulated metal kilns because of rapid and large heat loss.

Effects of Kiln Size, Type on Production Time

When raw materials, operational methods , and coaling temperatures are the

same, the coaling time per cord of wood is also roughly the same for kilns of

various sizes and of generally similar types. Experimental runs made in

3- and 7-cord multiwall kilns indicated, for example, coaling periods of about

4 hours per cord of seasoned and 7 hours per cord of unseasoned wood.

Differences in the time for loading and unloading kilns might well be a more

important factor in production, depending upon the equipment available and

its suitability for use with a kiln of given size and type.

Effect of Time, Temperature on Coaling

and Charcoal Quality

Good-quality charcoal is obtainable at kiln temperatures of 850° to 950° F.

The amount of fixed carbon will increase at higher kiln temperatures,

accompanied by a corresponding decrease in charcoal yield. With somewhat

Report No. 2213 - 95 -
lower than normal coaling temperatures, a longer time

produce charcoal of acceptable quality.

is required to

Because it would not be practical to place as many thermocouples in

commercial kilns as were used in the experimental units, the average temperature range of 850° to 950° F. at the ceiling has been mainly used for the coaling control. The average temperature of the hotter gases at the ceiling is a

simple indicator of coaling conditions. This average temperature does not,

however, represent an overall kiln temperature, since the wood nearer the

kiln floor may not coal until later in the cycle, and then usually at a temperature somewhat lower than that of the wood closer to the ceiling.

The average kiln temperature affecting both the quality and quantity of charcoal produced varies, therefore, with the rate of coaling. The sooner there

is conversion of the wood in the lower part of the charge, the greater will be

the trend toward higher overall temperatures, as has been experimentally

verified by numerous thermocouples placed throughout a charge.

As shown in figure 34, the kiln ceiling temperatures follow much the same

pattern as the overall temperatures. The average difference between ceiling

and overall temperatures was greater during the winter. This difference was

also greater during the coaling of unseasoned than seasoned wood, and hence

it may be that in colder weather charges of unseasoned wood might more

advantageously be coaled at the higher temperatures in the 850° to 950° F.

range. During the summer, on the other hand, seasoned wood might be

coaled to best advantage at the lower temperatures in this range.

The effects of time and temperature on coaling at different levels within the

charge bears closely on the quality of the charcoal produced within each level,

or zone. Relatively high coaling temperatures must be kept in the upper zones

if the temperatures in the lower zones are to be sufficiently high for efficient

carbonization. The results from 16 runs on southern oak indicated that the

top zone of the undisturbed charcoal contained about 85 percent of fixed carbon, the center zone about 80 percent, and the zone near the floor 75 to 79 percent. Proper sampling for analysis requires that a sufficient number of

samples be taken from a part or all of the kiln charcoal so that a representative

collection of samples is obtained.

Table 7 shows that charcoal of higher fixed carbon content was produced at

average ceiling temperatures above 875° F. Although the difference between

the lower and higher ceiling temperatures is slight, there is indication, nevertheless, of a close relation between temperature and quality. With greater

variation in temperature, a wider spread in quality values would likewise be

expected.

Report No. 2213 - 96 -
Report No. 2213 - 97 -

At operating temperatures above about 900° F., correspondingly lower charcoal yields are expected. The yields from a number of experimental runs

have been found to be otherwise, where the higher ceiling temperatures were

not necessarily associated with lower yields (table 7). Instead, a slightly

higher yield was obtained, which may be due in part to the fact that the runs

were more complete (with fewer brands) at the higher temperatures, as pointed

out previously.

To compare results further, these 26 experimental runs are grouped as

unseasoned and seasoned wood charges converted at average ceiling temperatures both above and below 875° F. (table 7).

Effects of Time, Temperature on Kiln

Operations

As a rule, there is little control of coaling time when a kiln is operated within

a set temperature range. The important object is to obtain as complete coaling as possible. One way to determine when coaling is complete is by observing smoke volume. When smoking practically stops, coaling is complete.

When smoking ceases, also, the average kiln temperature will have approached

a level of about 800° F.

With well-seasoned wood, coaling time will be fairly uniform from run to run.

The coaling of green wood is less uniform to such an extent that similar kiln

charges may vary greatly in the amount of time needed for completion (fig. 35).

The period of time required for coaling 6.6 cords of seasoned wood varied

from 28 to 33 hours, and the time for green wood from 34 to 55 hours, in a

‘I-cord kiln. In a 3-cord unit, seasoned wood required 11 to 17 hours and

unseasoned wood 11 to 28 hours (84). In table 7 it is also indicated that the

average coaling time is slightly reduced with higher average ceiling temperatures. Control steps suitably applied resulted in generally satisfactory

yields of good-quality charcoal under the various study conditions.

A further condition possibly affecting yields is cold, windy weather, as

indicated by the results given in table 8. Run No. 2 provided mostly brands,

even though overall kiln temperatures (based on previous satisfactory runs)

suggested coaling had been carried to completion.

Effect of Time Temperature on Brands

Production of brands is not unusual in kiln operation. They are produced in

varying amounts in different kilns. It is not possible to state any exact cause

or overall corrective measure. The results in table 9, from data mentioned

Report No. 2213 - 99 -
Report No. 2213 - 100 -

ZM 118 347

Figure 35. --Average ceiling temperatures obtained during the coaling of

unseasoned wood.
previously, indicate, however, that less brand material is produced from

unseasoned wood at the higher average ceiling temperatures. The longer

average coaling time for unseasoned wood appears not as effective for brand

weight reduction as does a slightly higher average coaling temperature, based

on the results from 26 runs in a 7-cord masonry-block kiln.

Effect of Weather on Charcoal Yield

Marked changes in seasonal temperatures affect yield. Reportedly, metal

kilns (9, 10, 11, 12) operated during the colder weather of November and

December produced only 86 percent of the charcoal by volume, obtained

from similar runs conducted during the warmer months of May to September.

Yield was also reduced in a 7-cord masonry-block kiln during 9 experimental

runs made in the November-to-April period compared to 10 carried out in the

period of May to October. The average moisture content of the wood in each

group of runs was quite similar, 58 and 63 percent. The runs likewise were

kept similar in operation. The coaling temperatures near the kiln ceiling

were maintained, as closely as possible, within the range of 850° to 950° F.

The average conversion was 27.6 percent for the cold-weather runs and

28.7 percent for those made in warmer weather. There was, too, a wide

range in conversion percent for the runs, indicating other factors were masking the results. The 1.1 percent variation indicated in these experimental

results amounts to 11 percent of the total possible conversion difference.

This difference is taken from average low and high conversion values of about

24 to 34 percent. This difference accounts for an average of 200 pounds per

run, or the loss of about 1 ton of charcoal for the runs conducted in the colder

weather.

Effects of Weather on Carbonization

Temperature

Charcoal of good quality can be made in kilns at any time of the year provided

proper coaling temperatures are maintained. With the greater loss of heat

from the metal kilns, suitable operation would be more difficult in the colder

seasons. If the temperature is maintained at too low a level during the length

of the usual coaling cycle, a lower quality product will result.

In kiln production a coaling pattern usually develops that may change somewhat from run to run. Even with closely directed control, it is difficult to

predict a certain average ceiling temperature because of normal variation

within a suitable temperature range. With air intake reduced to the lowest

workable level, it is not always convenient or practical to regulate the

Report No. 2213 - 101 -
Table 8. --Charcoal yield from sugar maple roundwood in a 3-cord, masonry-b

using end firing and two stacks

: : : : : 

Run No.: Time : Wood : Run : Kiln : Charcoal 

: of year : moisture : time : temperature :------------------------

: : content : : (average : Weight : Yield : Fixe

: : : : overall) : per : : carb

: : : : : cord : :

------- :---------- :---------- :-------- :------------- :-------- :--------- :------

: : Percent : Hours : °F. : Pounds : Percent : Perce

2 :December : 45 : 11 : 665 : 282 : 13.0 : 77.9

3 :December : 43 : 18-1/2 : 647 : 658 : 29.8 : 81.5

Table 9,- -Comparison of charcoal and brand yields at different ceiling temp

:

Wood carbonized at average : Wood carbonized at ave

ceiling temperature : ceiling temperature

below 875° F. : above 875° F.

--------------------------------------------- :-------------------------------

Moisture : Average : Yield : Brands : Moisture : Average : Yiel

content : ceiling : :per cord: content : ceiling : 

(average) : temperature : : (average) : temperature : : 

------------:-------------: --------- :----------:------------ :-------------:------

Percent : °F. : Percent : Pounds : Percent : °F. : Perc

(Unseasoned): : : : : : 

62 : 840 : 27.9 : 335 : 59 : 910 : 28.

(Seasoned) : : : : : : 

38 : 857 : 31.3 : 123 : 36 : 922 : 33.
amount of air in order that the temperature can be maintained within a 100° F.

range. It is necessary, on the other hand, that care be taken to prevent

prolonged high temperatures.

In one series of tests, the November-April runs coaled at a lower average ceiling temperature than the May-October runs (fig. 36). This would indicate

that colder air and more rapid heat loss have an effect on the process of coaling, even though operational procedures might be judged to be similar. While

a temperature difference of 53° F., as indicated, is not great, the temperature at the kiln ceiling was kept between 850° and 950° F. Moreover, the

coaling chamber was relatively well insulated, in that two-thirds of the

experimental structure was double-wall, masonry-block construction. These

winter experiments were made in the South. It can be assumed, therefore,

that a greater temperature difference, with more heat loss, would be found

in the operation of kilns under more severe weather conditions in the northern

states.

Economic Studies

How wood price, labor charges, and other factors, such as operational

efficiency, yields, and kiln types, affect production costs of charcoal was

investigated in two kinds of field studies.

A considerable volume of charcoal is produced in both the portable sheet-metal

and masonry kilns. To provide more information on their relative performance, one study of wood and labor-time charges and charcoal yields was made

with metal units of 1/2- and 1-1/2-cord capacity and a masonry unit of 2-cord

capacity. Similar time and yield data developed from the operation of a 7-cord

masonry kiln at another location were included to compare the effects of kiln

size.

In another study, quite different information was developed. This work was

done to indicate possibilities for combining kiln production and forest management. An efficiently operated 7-cord masonry kiln capable of producing uniformly good yields of charcoal was used as a study control factor. The major

elements of wood and labor costs were investigated with relation to production

cost. A production cost based in large part on these costs would indicate what

the break-even point is with respect to various charcoal prices. The chief

object of the study was to gain information on the break-even point.

The most favorable combined operation obviously would be conversion of the

improvement cuttings at a cost under the average charcoal selling price. On

the other hand, even if only break-even costs were met by charcoal sales, the

operation would defray stand improvement charges without continued investment.

Report No. 2213 - 103 -
Report No. 2213 - 104 -

Figure 36. -- Comparative coaling temperatures for runs

conducted during different seasons using unseasoned

wood.

ZM 118 340
Kiln Comparison Study

The 7- and 2-cord masonry kilns, of the three types employed, produced the

higher yields and sustained the least depreciation (90). For these reasons,

they appeared the most profitable ones to operate, even though the labor cost

during the coaling and cooling periods was higher. At then current prices for

wholesale charcoal, none of the kilns showed an adequate margin of profit.

Higher profits might accrue through the use of low-cost wood, larger kilns,

batteries instead of single kilns, and mechanization of the handling of wood

and charcoal.

Conditions for this study included fixed charges of $12 per cord at kiln site

for the red oak roundwood, $1 per hour for labor, and production depreciation

cost at 5 percent interest per ton of charcoal produced. Average cost of the

metal kilns was determined as $600 and that of the block kilns $200. Their

life was estimated at 200 runs.5 Other fixed charges for overhead, maintenance, profit, and risk were purposely not included, since these costs

differ widely with local conditions, operating and marketing methods, and

desired returns.

In order to keep labor charges at efficient levels, it was assumed that a

sufficient number of kilns of each type (figs. 4, 18) would be operated

simultaneously to keep two men employed full time. On a 40-hour week

basis, this would make necessary roughly the use of twelve 1/2-cord or

five 1-1/2-cord metal kilns, or three 2-cord masonry kilns.

The different types of units were operated according to standarized procedures.

All but one of the 1/2-cord kilns were thermocouple wired for temperature

and permitted to cool at any temperature common to commercial practice.

The masonry kiln was operated at a temperature not exceeding 950° F. Both

raw material and product were weighed for all runs. The time for labor

charges was on the basis of man-hours consumed for the various kiln-run

jobs.

Generally lower charcoal yields can be expected with uninsulated, metal kilns

compared to those with more effective heat barriers, as, for example,

masonry kilns. The results indicated an appreciable difference of about 7 percent in charcoal weight yield in favor of the masonry kiln operation, which

suggests corresponding lower costs of converting wood to charcoal. The

direct labor costs in this one study varied only from $23.35 for the 1-1/2-cord

5The life of the masonry kilns used experimentally is unknown. Some commercial masonry kilns have reported records of 150 to 200 runs. The very

rough estimate of a 200-run life is therefore based on speculation, not

actual performance.

Report No. 2213 - 105 -
metal unit to $26.12 for the 1/2-cord metal unit per ton of charcoal produced.

Break-even costs developed on the basis of fixed charges for wood, labor, and

depreciation are shown in table 10.

Table 10 .- -Break-even production costs for three kinds

of charcoal kilns

: :

Kiln type: Kiln : Cost per ton of charcoal

: capacity :--------------------------------------------

: : Labor : Wood : Depreciation : Total

---------:----------:--------- :---------:--------------:---------

: Cords : : : :

Metal : 1/2 : $26.12 : $28.68 : $6.25 : $61.05

Metal : 1-1/2 : 23.35 : 35.73 : 4.42 : 63.50

Masonry :

12 : 24.05 : 24.73 : 3.44 : 52.22

1Extension of these data to the operation of an experimental

ii-cord masonry kiln indicated a production cost of $43.25 per

ton.

The relationship of data obtained on wood cost per cord to charcoal production

cost per ton is graphically shown in figure 37. Assuming a break-even cost

of $50 a ton, roughly $7.50 a cord for dense hardwoods would be the highest

price that could be met with operation of the metal kilns. This relationship

can be seen by following the dashed line vertically from $50 on the base line

(production cost per ton of charcoal) to its intersection with the cost curves

for the two metal kilns. Extension of this dashed line horizontally from the

intersection to a point on the vertical base line (wood cost per cord) indicates

a cost per cord of nearly $7.50. Carrying the vertical dashed line upwards

until it intersects with the cost curves for the two masonry kilns and then

horizontally to the left to the vertical base line, maximum wood costs of

$11 and $13.50 per cord are shown for the 2- and 7-cord kilns, respectively.

Figure 37 also shows that lowering the cost to about $5 for a cord of the raw

material, which is roughly the cost of mill slabwood, will effectively reduce

the cost of producing a ton of charcoal in all three types of kilns. Some

lowering of production costs might be possible, likewise, by more efficient

use of labor through mechanization of the handling of materials.

Report No. 2213 - 106 -
Report No. 2213 - 107 -

labor, and depreciation charges.

Figure 37. --Relationship of wood cost to charcoal production costs based on wood,
Stand Improvement Study

The cost of producing an average ton of charcoal in this combined experimental

operation was $27.44, exclusive of the cost of wood. This cost, added to that

of the wood, represents the lowest market price that could be accepted if the

operation were to break even. Such a price for the charcoal would cover the

direct cost of labor, equipment, raw material, depreciation, and property

taxes and yield a low rate of return on the invested capital. Some operators

would be satisfied to break even, if their chief interest was in stand improvement and there was a lack of other markets for small roundwood. A study of

direct production costs was therefore undertaken to determine the feasibility

of charcoal production as a means of absorbing stand-improvement costs.

This study included the use of 154.5 cords of second-growth northern hardwoods for coaling obtained from a 30-acre improvement cutting. Another

129.9 cords, consisting of merchantable material with a stumpage value of

$472.72, was also cut from the same area. The cost of the wood for coaling

delivered to kiln site was $13.90 a cord. For the 14 runs conducted, each

2.52 cords of the seasoned material used produced an average of 1 ton of

commercially acceptable charcoal.

In the production cost analysis, the break-even cost for a ton of charcoal

produced was considered to be the sum of the charges met in each major step

of the coaling process. An itemized summary of the kiln investment and

operational costs, with labor at $1.47 per man-hour, is shown in table 11.

Table 11 .- -Charcoal production cost per ton,

excluding maintenance1

:

Item : cost

:--------------------

:Per ton :Per cord

-------------------------------------:--------:---------

Kiln construction and depreciation : $8.83 : $3.50

Loading : 7.41 : 2.94

Door closure (laying blocks in opening): 3.60 : 1.43

Firing, control, and sealing : 5.59 : 2.21

Kiln discharge and truck loading : 2.01 : .80

Tot al : $27.44 : $10.88

1Kiln maintenance excluded because of lack of data. No

major repair was required during the 14-run test period.

Report No. 2213 - 108 -
The production cost of $10.88 a cord, together with the cost of wood at $13.90

a cord, represents a total conversion cost of $24.78 per cord of wood. Comparing this with $21.80, or the amount obtained by sale of the bulk charcoal at

an assumed price of $55 a ton, shows a net loss of $2.98 per cord. However,

the stand improvement cut yielded 129.9 cords of merchantable stumpage

along with the 154.5 cords of charcoal wood or generally unmerchantable

material. The total cost for logging and kiln operation, as indicated in the

accompanying cost summary, was $3,828.51 as compared to a return of

$3,844.77, inclusive of $472.72 for stumpage sales. The combined operation,

then, provided a profit of $16.26 or about $0.54 per acre.

If the stand improvement had been done without coaling the nonmerchantable

wood, the cost of felling and lopping it would have been $6.50 a cord or

$1,004.25. The returns from the sale of the merchantable material would be

the same, or $472.72. As may be seen in the second itemized summary, the

cost of the improvement work under these latter conditions would be $531.53,

or $17.72 per acre. It can be concluded, therefore, that the operation of the

kiln would enable the owner to break even on the stand improvement work

when there is a market for the charcoal at $55 a ton.

Table 12. --Effect of charcoal kiln operations on the cost

for timber stand improvement

: :

Item : Nonmerchantable : Nonmerchantable

: wood used for coaling : wood not utilized

--------------------------- :-----------------------:-------------------

costs :

Logging :

Charcoal manufacture :

Total cost :

:

$2,147.55 : $1,004.25

1,680.96 :

3,828.51 : 1,004.25

Returns

Stumpage sales

Charcoal sales

(at $55 per ton)

Total returns

: :

: 472.72 : 472.72

: :

: 3,372.05 :

: 3,844.77 : 472.72

Net profit on timber stand :

Improvement :

Profit per acre :

:

16.26 : -531.53

.54 : -17.72

Report No. 2213 - 109 -
The relationship between the selling price of bulk charcoal, labor cost, and

maximum allowable wood cost established from the values developed in this

study is indicated in figure 38. From this chart, the maximum wood cost

allowable to break even can be estimated. For example, if labor cost is

$1.25 an hour and the top price for bulk charcoal is $50 a ton at the kiln, the

cost of wood must not be over $10 a cord to break even. This can be shown

on the chart by following the dashed line vertically from the $1.25 point on

the base line until it meets the selling price line of $50 per ton, then continuing horizontally to the left to find the allowable break-even cost of wood,

$10 a cord.

If labor costs less than $1 per hour, the chart may again be used by extending

the diagonal line to meet a vertical line placed from a scaled point on the base

line and then moving horizontally to the right to the maximum break-even cost

of the wood. As another example, an assumed labor cost of 85 cents per hour

and a charcoal selling price of $55 a ton would indicate an allowable breakeven wood cost of nearly $14 a cord. This allowable wood cost may be also

found from the equation for these regression lines, thus:

wood cost = 0.395 x selling price - 5.008 x labor cost - 3.485

or:

(0.395) (55) - (5.008) (0.85) - 3.485 = 21.73 - 4.257 - 3.485 = $13.99

The maximum allowable break-even cost for the wood is thus $13.99. The

equation may be used likewise to determine any of the three variables when

the other two are known. To obtain a profit, it is quite plain that wood must

be available at something less than a break-even cost based on a given labor

charge and charcoal selling price.

Useful information on kiln production economics has been developed to only

limited extent. The results from the studies just outlined, `while not wide in

scope, are helpful in pointing up the value of economic background as an

important asset in commercial operation. Both studies include patterns for

direct information and, applied either wholly or in part, would suggest good

opportunities for establishing a build-up of basic information essential for

most favorable kiln production.

Report No. 2213 - 110 -
Figure 38. - - Relationship of charcoal selling price and labor cost to

allowable wood (northern hardwoods) cost to break even. Cost of

labor includes employer contributions for compensation, social

security, and unemployment compensation.

Report No. 2213 - 111 -
A Method of Charcoal Analysis6

A practical standard method for the analysis of charcoal is desired by producers and laboratories that are concerned with the production and marketing of charcoal (51). Most of the present methods are based on American

Society for Testing Materials standard methods for coal and coke (D-271-48;

D-346-35). Various modifications of these methods, as well as other methods,

are used. The method described here is specific for charcoal. The method

employs equipment found in most laboratories, and is adapted to routine

analyses of a large number of samples. It is designed to give accurate

results for moisture, volatile matter, ash, and fixed carbon.

Since this new method differs from ASTM methods D-271-48 and D-346-35

in several details, data for duplicate determinations on 100 samples were

analyzed statistically. The samples of charcoal were obtained at random

from various sources in different parts of the country. The results were

obtained by two analysts over a period of several months, Standard

deviations of duplicates were: moisture, 0.06; volatile matter, 0.25; and

ash, 0.07.

Four samples of charcoal were selected for repeated analysis. The selected

samples were hardwood charcoals with volatile contents of 12, 15, 20, and

27 percent. Data for the repeated analyses on these four samples are shown

in table 13. Since the stability of the values for volatiles in stored, ground

charcoal is unknown, the data were collected over a period of approximately

2 months. The data indicate that, during this time interval, volatility values

remained constant for samples stored in sealed containers at room temperature. The data also demonstrate that the accuracy and reproduciblity of the

method are acceptable for establishing the quality of charcoal and for determining the effects of variables in production.

The four ground samples (table 13) were submitted to two laboratories of

large independent producers of charcoal for analysis by their methods.

Results are shown in table 14. It can be seen that agreement is good.

In view of the data shown in tables 13, 14, and 15, the following procedure

for the analysis of charcoal is outlined. By this procedure, 30 samples in

duplicate (60 complete determinations) can be carried out by one analyst in

2 days.

6Prepared by the Forest Products Laboratory for consideration by ASTM

Committee as a tentative ASTM Standard.

Report No. 2213 - 112 -
Table 13.- -Repeated charcoal analyses

Report No. 2213 - 113 -
Table 14.--Analysis of charcoals by three laboratories

1Data recalculated so that volatile, ash, and fixed carbon are also on the ovendry

basis.

Report No. 2213 - 114 -
Table 15.- -Screen analysis of charcoal samples ground in Wiley mill

1U.S. Standard Sieve.

2Percent of sample retained on the smaller screen.

Report No. 2213 - 115 -
Procedure for the Analysis of Charcoal

I. Sample Preparation

For the analysis to have value and meaning, a representative sample must be

selected. This selection may be carried out according to ASTM methods for

sampling of coal (D-271-48; D-346-35). Samples will normally be air-dry

charcoal lumps or briquettes. Rain-soaked or wet samples should be spread

out to air dry before they are analyzed. For determining the moisture content of the charcoal as received, the sample should be ground to pass a

coarse screen such as No. 20 U.S. Standard Sieve, since heat generated by

excessive grinding would cause loss of moisture. For the determination of

volatile matter, a more finely ground sample is required. All of the selected

sample should be ground; no part of the sample is to be rejected.

A Wiley mill, size No. 2 with a 1-millimeter screen, was used to give the

screen analysis shown in table 15. Three hundred grams of charcoal were

ground in less that 5 minutes. Longer grinding times resulting from the use

of a finer screen or dull knives should be avoided because of the possibility

of generating heat, which in turn causes a loss of volatile material. Excessive

grinding will also produce a large amount of fine particles (smaller than 100

mesh, U. S. Standard Sieve). These fine particles may be swept out of the

crucible during the rapid evolution of gases in the determination of volatile

matter, and thus cause errors. Because over-size particles can result in

low values for volatiles, particles larger than No. 20 U.S. Standard Sieve

should be avoided. The ground samples should be stored in airtight containers such as screw-top bottles. The samples should be well mixed by

shaking before they are weighed. “Jack stones” can be used to facilitate

mixing.

II. Steps in Analysis

Analyses are carried out in duplicate.

1. A muffle furnace that will control temperatures of 750° ±5° C. and

950° ±5° C. is required. Heat the muffle furnace to 750° C. and place

previously ignited-porcelain crucibles (41 millimeters x 37 millimeters)

and lids in furnace for 10 minutes.

2. The crucibles are cooled in desiccator for 1 hour.

3. Weigh crucibles and add an accurately weighed (to 1/10 milligram) sample

of approximately 1 gram of charcoal.

7In practice, crucibles from previous determinations are used.

Report No. 2213 - 116 -
8The sample shall be considered ovendry when the decrease in weight of consecutive weighings is 0.0005 gram or less. Successive drying periods

shall not be less than 1 hour.

Report No. 2213

4. For moisture content, place samples in oven at 105° C., and dry for

2 hours.

5. Remove dried samples from oven and cool in desiccator for 1 hour and

weigh. 8

6. For the determination of volatiles, heat muffle furnace to 950° C.

7. The crucibles, with lids in place and containing the samples used for the

moisture determination, are preheated as follows: With furnace door open,

for 2 minutes on the outer ledge of the furnace (300° C.) and then 3 minutes

on the edge of the furnace (500° C.). Individual nichrome wire baskets to hold

the crucibles are convenient (fig. 39).

8. The samples are then moved to the rear of the furnace for 6 minutes with

the muffle door closed. The samples are watched through a small peep-hole

in the door of the furnace. If sparking occurs, results will be in error. If

the sparking sample does not check the results of its nonsparking duplicate

within ±0.5 percent, the analyses must be repeated.

9. The samples are removed and placed in a desiccator for 1 hour and

weighed.

10. For ash determination, place the lids and the uncovered crucible containing the sample used for the volatile determination in the muffle furnace at

750° C. for 6 hours.

11. The crucibles with lids in place are cooled in a desiccator for 1 hour and

weighed; Burning of the sample is repeated until successive 1 hour periods of

heating result in a loss of less than 0.0005 gram.

12. Computations:

A. Moisture percent =

loss in weight x 100

weight of air-dry sample

Report results to first decimal place. Moisture values for duplicates

should agree within 0.1 percent.

B. Volatile percent =

loss in weight x 100

weight of ovendry sample

- 117 -
Figure 39. - -Nichrome wire baskets for handling covered crucibles.

ZM 114 219

Report No. 2213 - 118 -
Ovendry weight = weight of air-dry sample minus moisture. Report

results to first decimal place.

Volatile values for duplicates should agree within 0.5 percent.

C. Ash percent =

weight residue x 100

weight of ovendry sample

Report results to first decimal place.

Ash values for duplicates should agree within 0.1 percent.

All results except moisture are reported on the ovendry basis.

Conclusions

The rapidly widening market for charcoal as recreational fuel is of interest

to many seeking new business ventures or opportunities for investment of

capital. A number of factors should be considered, however, before any

action is undertaken.

A source of adequate supplies of low-cost raw material is of greatest importance. Operations based on wood costs much above $9 a cord at kiln site,

that is, competing with pulpwood operations, have little assurance of success.

Sources of wood residue, such as slabs, edgings, cull bolts and logs, veneer

cores, bark, and sawdust, if available at relatively low price and with little

added cost for transportation, provide an exceptionally good situation. Between these extremes of raw material at high and low prices, there is a condition where good forest management indicates the removal of low-grade or

otherwise unmarketable wood for stand improvement. Thus, with charcoal

production, part of the wood costs can be charged against forest management.

With suitable coaling equipment, the advantage of one form of raw material

over another is chiefly one of cost. Assuming wood costs to be favorable, a

decision regarding the type and size of equipment best fitted to the kind and

quantity of raw material is required. The several types of small, upright

retorts commercially available apparently can efficiently convert forest and

mill residues with comparatively low labor requirements and initial investment (71, 78). For mill fines, wood chips, and similar material, rather highcost equipment for continuous carbonization would be required (27, 52) .

Briquetting of the carbon fines produced would necessitate substantial investment.

Report No. 2213 - 119 -
Using cordwood, mill slabs, trimmings, or short-length roundwood and slab

wood, coaling could be carried out in any of several types and sizes of metal,

masonry block, brick, and reinforced concrete structures. The metal units

have small capacity but are readily portable and frequently adapted to operations where it is desirable to move kilns often rather than transport the

wood. These kilns are simple to operate and require little maintenance to

keep them operable. They are available at a price of about $350 for the

1/2-cord size. A chief disadvantage in their use is the lower yields of charcoal obtained.

Masonry-block kilns in sizes from 2 to about 10 cords are commonly employed.

Construction is fairly simple and carried out at costs generally from $100 to

about $200 per cord of capacity, depending upon whether construction is undertaken by the operator or by contract. This type of structure is susceptible to

cracking and must be properly constructed and reinforced. Such structures

require continuous maintenance. Coaling can be readily conducted in these

kilns and, when equipped with inexpensive temperature-indicating instruments

(61, 62), will produce consistently high yields of good-quality charcoal.

Rectangular or beehive-type kilns of brick construction and others of reinforced concrete have capacities of 5 to about 100 cords. The investment

per cord of capacity is estimated to be between $100 and $150 for these kinds

of construction. The larger sizes require several weeks for each cycle of

operation. The yields per cord are somewhat lower than from the smaller,

masonry-block units. Because of the large volume of materials handled,

these larger kilns are well adapted to mechanization. Other types of kilns,

of course, are likewise adaptable where production is comparatively high.

As may be seen from the foregoing, the choice of production facilities is not

subject to definite prescription but must be considered in the light of many

factors associated with each operation.

The problem of marketing charcoal must be recognized and seriously considered. With the present strong market preference for briquettes, the

solution is not difficult for any operator who can sell his product to a

briquetting plant at wholesale prices. To do this at a profit, however,

requires that production costs be at a minimum. An alternative procedure

would be to install a briquetting and packaging plant and sell the product at

the higher retail prices. This would necessitate a sales organization and

sustained contracts to assure continuous operation.

Many small producers package the lump charcoal and sell it wholesale to

retail outlets. This practice provides greater returns than selling bulk

amounts in spite of the added costs for packaging, transportation, and sales.

Report No. 2213 - 120 -
Appendix

Partial List of Suppliers9

Microammeter (Approximate cost $20.00)

Allied Radio

100 N. Western Ave.

Chicago 80, Ill.

Burstein-Applebee Co.

1012-14 McGee St.

Kansas City, Mo.

Newark Electric Co.

223 W. Madison St.

Chicago 6, Ill.

Catalog No. 160

Stock No. 67F444

D. C. microammeter, range 0 to 25

2200 ohms internal resistance

Catalog No. 571

Stock No. 29A1338

D. C. microammeter, range 0 to 25

2200 ohms internal resistance

D. C. microammeter, model 29

No. 55F175, range 0 to 25

2200 ohms internal resistance

Thermocouple wire (Approximate cost $0.30 per foot)

Extension wire (Approximate cost $3.00 per 100 feet)

Most of the suppliers have State or regional offices which cannot be listed

here. The following are the home office addresses:

The Bristol Co.

Waterbury 91, Conn.

The Lewis Engineering Co.

Naugatuck, Conn.

Thermo Electric Co., Inc.

Saddle River Township

Rochelle Park Post Office, N. J.

Minneapolis -Honeywell Regulator Co.

Wayne and Windrim Ave.

Philadelphia 44, Pa.

9The inclusion or exclusion of names in this list does not imply endorsement

of any company or products by the Forest Service.

Report No. 2213 - 121 -
Tentative list of companies designing, fabricating, and

briquetting equipment:

installing charcoal

Komarek-Greaves Company

2941 Mozart St.

Chicago, Ill.

F. J. Stokes Machinery Company

5930 Tabor Road

Philadelphia, Pa.

Webb Corporation

Webb City, Mo.

Aeroglide Corp.

Raleigh, N. C.

Vulcan Iron Works

Wilkes-Barre, Pa.

Tentative list of companies designing, fabricating, and installing byproductrecovery plant equipment:

Badger Manufacturing Company

230 Bent St.

Cambridge, Mass.

Chemical Construction Corporation

New York City, N. Y.

Posey Iron Works

Lancaster, Pa.

Charcoal Moisture Tester

This instrument, manufactured in England, is called the “Speedy Moisture

Tester." Sale is made through the Alpha-Lox Co., Inc., 60 E. 42nd St.,

New York 17, N.Y. The quoted price from the only known source of supply

is $160.

Requirements for Temperature-Indicating Meter

The temperature-indicating instrument should have a direct current microammeter range of 0 to 25 microamperes and resistance of 2,200 ohms. This

type of meter may be purchased for a nominal cost from supply sources.

The meter, as procured, requires a scale for directly reading kiln temperatures in degrees Fahrenheit. A scale which may be used for this purpose

can be cut from figure 40. The cut-out scale may be easily applied by first

Report No. 2213 - 122 -
carefully removing the scale plate from the meter and then rubber-cementing

it onto the scale plate. It should be kept in mind that the scale furnished with

the meter be of the same size and shape as the scale shown in figure 40. The

kiln temperature scale should be redesigned if that in the meter varies in

design and type from the scale illustrated in figure 40.

Wire Requirements

The wiring necessary for temperature indication on the kiln-temperature

meter requires that two different types of wire be used. The wire for placement within the kiln is termed thermocouple wire and that for use on the outside of the kiln connecting the thermocouples to the meter is the extension

wire. Both the thermocouple and extension leads have separated wires of

different metals, one of which, in the system herein described, is iron and

the other constantan and commonly referred to as iron-constantan wire.

Thermocouple wire. - -When ordering wire from the supplier, the following

information should be furnished: Iron-constantan parallel paired thermocouple

wire; 16- or 18-gage, solid, asbestos insulated, stainless-steel mesh overbraid.

Extension wire. --The following information with the number of feet desired

should be furnished the supplier: Weatherproof-type, iron-constantan

parallel paired extension wire; 24 gage, polyvinyl insulation.

This wire may be also of heavier gage (22 to 16) with other suitable weatherproof insulation.

The thermocouple wire for use within the kiln must be abrasion resistant

(preferably stainless-steel mesh overbraid), and the insulation should be of

asbestos to withstand the high kiln temperatures.

Wiring

Meter . - -A 3- to 4-inch length of the constantan strand of the wire is attached

to the positive (plus) post of the meter, as shown in figure 41. The iron

strand of the paired extension wire is then twisted and soldered to this short

length of constantan strand, forming a reference junction later described.

The constantan strand of the paired extension wire is attached to the negative

(minus) post of the meter.

The meter is a precision instrument, and for proper use should be mounted

in a box, as shown in figure 27. Binding posts, clips, or jacks can be

Report No. 2213 - 123 -
Report No. 2213

Z M 118 282

Figure 40. - -Cutout temperature scales for microammeter.

- 124 -
Z M 118 342

Figure 41. - - Procedure for connecting thermocouple wires.

to microammeter.

Report No. 2213 - 125 -
attached to the back of the box for easy connection to the extension wire leading to the thermocouples. An electrician or local radio repairman can make

a simple switching device to service a number of thermocouples without moving the meter, for connection to various thermocouple leads.

Thermocouples. - -Many types and kinds of thermocouples ready for use are

available for purchase. Using the thermocouple wire previously described,

an equally suitable thermocouple can be made quite readily by the kiln operator

This is done by stripping the insulation for about 1 inch from the ends of the

paired wires, then firmly twisting the wires together, and finally fusing the

wire tips by welding (fig. 28).

Three or preferably five thermocouples can be used for satisfactory control

of temperature in masonry-block kilns of 2- to 10-cord capacity. The proper

placement of such limited numbers of thermocouples is important, however,

and, as indicated in figure 28, good locations are along the lengthwise centerline of the kiln and within 4 inches of the ceiling. Thermocouple No. 1 is

placed just above and behind the forward end of the kiln charge, No. 2 at ceiling center, and No. 3 is put 1 to 2 feet from the rear wall. A fourth thermocouple is dropped through the same ceiling opening as No. 2, but hangs about

halfway into the kiln. Thermocouple No. 5 is hung through the ceiling opening

for No. 3 and dropped to about 2 feet from the floor.

Should only three thermocouples be used, suitable control is possible in the

placement of a series as indicated for Nos. 1, 2, and 3 in figure 28 or in the

series showing Nos. 1, 4, and 5. When the thermocouples have been

securely placed, the extension wires should be attached as shown in figure 41.

Meter Use and Care

The meter should be handled carefully at all times if it is to perform properly

with minimum repair or replacement. When there is no meter connection to

he thermocouple, the indicating needle should be on the zero mark of the

meter scale. If not, the needle may be adjusted to zero position by turning

the screwhead on the face of the meter.

The meter indicates the difference between the temperature at the reference

unction (air temperature) and that of the kiln thermocouple. The meter

scale has been calibrated to give true readings at an air temperature (reference junction) of 75° F. A different air temperature will admit some error

in indicated temperatures, but is of little importance in average kiln operation.

Report No. 2213 - 126 -
Wire Use and Care

The steel overbraid sheathing around the thermocouple wire does not provide

protection against moisture. It is necessary, therefore, that the portion

extending beyond the outside of the kiln be protected from the weather. Wet

thermocouple wire may develop "shorts" that cause the meter needle to

swing off the high-temperature side of the scale or remain on the zero line.

Thermocouple wire, particularly the portion extending into the kiln charge,

is subject to stretch and wear and may eventually break. Additional wire

should be kept on hand for replacements.

The weatherproof insulation on the extension wire is harmed by heat conditions

above 220° F. The wire should not touch hot surfaces or be placed in areas

where the temperature is excessively high. Insulation damaged by heat may

cause shorting of the paired extension wire. It is good practice to extend the

thermocouple wire from the coaling chamber and through the ceiling for

sufficient length so that the attached extension wire may be placed above and

free of the ceiling structure and insulation.

LITERATURE CITED

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Report No. 2213 - 135 -
Forest Utilization Research Division representatives can be reached at the following Forest Service Experiment Stations:

Pacific Southwest Forest and Range

Experiment Station

P. O. Box 245

Berkeley 1, California

Central States Forest Experiment Station

111 Old Federal Building

Columbus 15, Ohio

Lake States Forest Experiment Station

University of Minnesota

Green Hall, St. Paul Campus

St. Paul 1, Minnesota

Intermountain Forest and Range

Experiment Station

Forest Service Building

25th Street and Adams Avenue

Ogden, Utah

Northeastern Forest Experiment Station

102 Motors Avenue

Upper Darby, Pennsylvania

Pacific Northwest Forest and Range

Experiment Station

809 N.E. Sixth Avenue

P. O. Box 4059

Portland 8, Oregon

Rocky Mountain Forest and Range

Experiment Station

Colorado State University

221 Forestry Building

Fort Collins, Colorado

Southeastern Forest and Range

Experiment Station

225 Federal Building

P. O. Box 2570

Asheville, North Carolina

Southern Forest and Range Experiment

Station

704 Lowich Building

2026 St. Charles Avenue

New Orleans 13, Louisiana