Transpirational drying of Piedmont hardwoods / by James W. McMinn

\

\

GEORGIA FOREST RESEARCH PAPER

63

April 1986

v.

TRANSPIRATIONAL DRYING of
PIEDMONT HARDWOODS

GEORGIA
iFORESTRY,
v
.-*

by James W. McMinn

RECE/VEU
DEC. O91994

tfycuMENrs

>v

RESEARCH DIVISION

GEORGIA FORESTRY COMMISSION

ABOUT THE AUTHOR
James W. McMinn is a Research Forester
with the Southeastern Forest Experiment
Station, USDA Forest Service. He receiv-
ed a B.S. in Forest Management from
N. C. State, a Master of Science in Forestry from the University of Florida, and a Ph.D. in Forest Resources from the
University of Georgia. He is a member of the Utilization of Southern Timber
Research Unit, Athens, Ga.
ACKNOWLEDGMENT
The author gratefully acknowledges the financial support and cooperation of the Georgia Forestry Commission that made
this study possible.

TRANSPIRATIONAL
DRYING
OF
PIEDMONT
HARDWOODS

INTRODUCTION
Moisture content can be one of the most
critical factors limiting the use of wood for energy (2,10)- The technique covered here is one of the most promising ways of drying wood
quickly and economically. Transpirational dry-
ing consists of felling while the tree is in full
foliage and leaving the entire bole and crown intact to allow moisture loss via natural transpiration mechanisms. The technique is also
known as "sour-felling," "leaf seasoning," "leaf
felling," "biological drying," and "delayed bucking." It has not been employed to any extent in the United States, but exploratory research indicates potential for various geographic areas and species (3, 8, 9, 11, 12). The effectiveness of the technique depends on species and drying conditions (8, 11,) and the potential benefits are not limited to fuel value per se. In one study, drying of material in the woods reduced forwarding costs (1), and it should also reduce road transportation costs. This paper presents pre-
liminary results from a study in which we are
developing predictions of moisture losses for whole trees and tree components.

By James W. McMinn

Figure 1. -Trees felled and left to lie intact at the stump.

METHODS
The study was carried out in the Upper Piedmont of Georgia about 40 air
miles NNE of Atlanta on land managed
by the Georgia Forestry Commission. Study species were red oaks (Quercus spp.), sweetgum (Liquidambar styraciflua L), and yellow-poplar (Liriodendron tulipifera L.). The red oak group was represented primarily by southern red oak (Q. falcata Michx.) with some scarlet oak (Q. coccinea Muenchh.) and black oak (Q. velutina Lam.). Trees ranging from 5 to 12 inches d.b.h. were felled in

late July and left to lie at the stump (Figure 1). Then, a number of trees were destructively sampled weekly for up to 8 weeks (Figure 2). The samples covered in this presentation were disks (wood only) taken to represent stemwood. As disks were cut in the field they were placed in polyethylene bags, then returned to the
laboratory for processing. In the laboratory disks were weighed, dried to constant weight of 103C, and reweighed as a basis for moisture content calculations. Recoverable heat energy was estimated according to the procedure given by Ince
(5). The following assumptions were

made for the calculations:
-- Higher heating value of 8,000
Btu./lb. for all species
-- Ambient air and fuel temperature
of 70 F
-- Stack gas temperature of 500 -- Excess air of 40 percent -- Conventional heat losses of 4
percent
The derived recoverable heat energy is the
energy remaining after the escape of heat in stack gasses and other heat losses. It is assumed that this portion of the higher heating value would be contained in some useful output, such as process steam.

Figure 2. -Disks were removed from a sub set of trees at each sampling date.

RESULTS AND DISCUSSION
Transpirational drying for 3 weeks reduced moisture content from about 76 to 67 percent in red oaks, 120 to 62 percent in sweetgum, and 100 to 56 percent in yellow-poplar on an oven dry basis.
Average wood moisture by drying time is
presented in Table 1. Initial moisture contents were similar to those reported by Karchesy and Koch (6). Both the lower initial moisture content and lower moisture loss in red oaks can be attributed to the proportionally smaller area of active
vascular tissue - the outer one or two annual rings in ring-porous species (7). By contrast the two diffuse-porous species in which several outer rings contain active
conductive tissue both lost substantially
more moisture. These losses were much
greater than have been reported from tree-length slash pine logs (4), and conifers are similar to diffuse-porous species in water conduction (7). Samples were collected over a full 8-week period to determine if slow translocation and evapotranspiration would continue, but none was apparent from the data. Drying was most rapid for all species during the first week, and there was no indication that significant moisture reductions could be achieved after the third week.
Table 2 presents estimates of recoverable heat energy associated with selected
stages of transpirational drying. Two dif-
ferent bases were used for energy calculations, because each is legitimate and meaningful for a specific purpose or user. The first column was calculated on an oven dry basis or for a constant quantity
of wood. A quantity of green red oak

that includes 1 ton of actual wood will
have an additional 0.76 tons of water contained in it. After drying a week, that
same ton of wood will contain 0.65 tons of water. The increase in recoverable
energy represents potential gain per chip, stick, tract, or a pile of given dimensions. For instance, transpirational drying of red
oak could yield a given amount of

recoverable energy on about 3 percent
less acreage than green wood or 3 percent more recoverable energy from a given
tract. Potential gains are 3, 14, and 11 percent for red oaks, sweetgum, and yellow-poplar, respectively. These figures represent the potential for reducing impacts on the primary resource via trans-
pirational drying.

Table 1. --Average wood moisture content (oven dry basis) for stemwood of three Piedmont hardwood species or species groups by duration of transpirational drying (standard deviations in parentheses)

Drying time
1
2 3 4 5 6 7
8

Red oaks
76 (2) 64 (6) 66 (5) 67 (5) 66 (5) 62 (4) 67 (6) 66 (6) 66 (5)

<>pecies
Sweetgum
- - - Vevcent -
119 (10) 80 (17) 73 (9) 62 (6) 65 (6) 66 (13) 64 (4)
-

Yellow-poplar
100 (13) 63 (7) 58 (10) 56 (3) 56 (10)
-

Table 2. --Wood moisture content and recoverable heat energy for species or species group and duration of transpirational drying

Species

Drying time

Red oaks Sweetgum Yellow-poplar

Weeks
1
1
3
1
3

Moisture content (base)

Oven dry

"Wet" fuel

- - - Vevcent - -

76

43

65

39

119

54

80

44

64

39

100

50

63

39

56

36

Recoverable heat energy

Per ton wood

Per ton fuel

- - - - Million Btu's - - -

10.338 10.656

5.874 6.460

9.330 10.306 10.594

4.260 5.726 6.460

9.692 10.530 10.760

4.846 6.460 6.898

The second recoverable energy column was calculated on a wet weight basis, which is the accepted standard in combustion engineering. On this basis a ton of green red oak fuel will contain 0.43 tons of water and a ton of material that has been dried for a week will contain 0.39 tons of water. The wet fuel base shows more dramatic gains because -- unlike the above example, where the amount of actual wood is held constant -- as the moisture content decreases the amount of wood per unit weight increases. The in-

creases in recoverable energy for this
column represent potential gains in boiler output, transportation, or any activity for which gross weight of wood and water is the applicable standard. For example, transpirational drying would increase re-
coverable energy per ton-mile of trucking
by 10, 52, and 42 percent for red oaks, sweetgum and yellow-poplar, respective-
ly.
One final point should not be over-
looked. Uniformity and predictability minimize operational problems and costs

in almost any production situation. In addition to an overall average increase, transpirational drying reduced the variability in recoverable energy among
species. On an ovendry basis the spread
was reduced from an initial 11 percent to 2 percent. For wet fuel the reduction was from about 38 to about 7 percent. This means that - whatever activity is being considered - transpirational drying can to a substantial degree relieve a decisionmaker from the need to be concerned with species or species mix.

CONCLUSIONS
These preliminary results indicate that transpirational drying can produce substantial gains in recoverable heat energy for diffuse-porous species and more modest gains for ring-porous species.
The most rapid drying occurs during the first week. Significant drying does not occur after about 3 weeks. Because diffuse-porous species with high initial moisture contents lose more water than ring-porous species with low initial moisture contents, the final moisture contents vary less among species than initial moisture
contents.

3 ElDfi QMSSM ESbb
REFERENCES CITED
1. Begley, D. D., and R. Howell. 1960. Air seasoning softwoods at the stump. Forestry 33(2):187-202. 2. Blankenhorn, Paul R., and Richard E. Weyers. 1980. Moisture effects on an energy balance developed for using
forest biomass as a fuel. Forest Prod. J. 30(1 ):41 -46.
3. Boldt, C. E. 1958. Kiln cordwood can be seasoned rapidly prior to bucking. Tech. Note No. 530. St. Paul, MN:
USDA For. Serv., Lake States Forest Exp. Stn.; 2 p.
4. Clark, Alexander III, and Douglas R. Phillips. 1972. Slash pine logs lose weight in storage. South. Lumberman
225(2795) :15-17.
5. Ince, Peter J. 1979. How to estimate recoverable heat energy in wood or bark fuels. USDA For. Serv. Tech.
Rep. FPL 29, Forest Prod. Lab., Madison, Wl. 7 p. 6. Karchesy, Joseph, and Peter Koch. 1979. Energy production from hardwoods growing on southern pine sites.
USDA For. Serv. Gen. Tech. Rep. SO-24, South. For. Exp. Stn., New Orleans, LA. 59p. 7. Kramer, Paul J., and Theodore T. Kozlowski. 1979. Physiology of woody plants. New York: Academic Press,
Inc.; 811 p. 8. Lawrence, William Emory, Jr. 1981. Field-drying residues as an industrial fuel (thesis). Blacksburg: Virginia
Polytech. Inst, and State Univ., 110 p. 9. McMinn, James W., and Michael A. Taras. 1982. Transpirational drying. In: Proceedings; the Sixth International
FPRS Industrial Wood Energy Forum '82, March 8-10, Washington, DC. Madison, Wl, Forest Prod. Res. Soc. Volume 1:206-207. 10. Murphy, W. K., J. G. Massey, P. R. Blankenhorn, and T. W. Bowersox. 1981. Some implications of using wood
as fuel. So. J. of Appl. For. 5(1 ):16-19. 11. Patterson, William A., Ill, and Irwin L. Post. 1980. Delayed bucking and bolewood moisture content. J. For.
78(7) :407 -408.
12. Rogers, Ken E. 1981. Preharvest drying of logging residues. Forest Prod. J. 31(12) :32-36.

GEORGIA
FORESTRY,
John W. Mix on, Director
Fred Allen. Chief of Research