Wood Production Energetics - American Chemical Society

21 Wood Production Energetics

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An Analysis for Fuel Applications NORMAN SMITH and THOMAS J. CORCORAN Department of Agricultural Engineering, University of Maine at Orono, Orono, M E 04473

Wood was the prime fuel source for the United States during much of the 19th Century. Consumption probably peaked around 1880 at 146 million cords* per year. Coal replaced wood for most applications. However, use of wood for residential heating continued to be important in some rural areas until after World War II. Residential use of waste wood and sawdust from wood-utilizing industries persisted after the use of wood harvested directly for fuel had practically ceased. A number of industries continued to use their waste wood in boilers to produce steam for electricity generation and process heat. However, the convenience and low cost of heavy oil fuels caused all but a very small number of operations to terminate waste wood usage. The conical incinerator became a common sight at sawmills in the 1960's while wood drying kilns were being fired by oil in another part of the yard.

3

* A cord is a volume measure of 128 feet of piled round wood, usually represented as a pile of 4 foot logs. 4 feet high and 8 feet long. Volume scaling is still much used in forestry as many operations are volume- rather than weight-sensitive. However, a cord represents very different weights of dry matter depending on the species of wood. Weight per cord also varies greatly with moisture content. Green wood is around 50% moisture content. Dry matter per cord varies from about 1900 lb for pine to 3500 lb for hardwood such as birch and maple.

0097-6156/81/0144-0433$05.00/0 © 1981 American Chemical Society

Klass; Biomass as a Nonfossil Fuel Source ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

434

BIOMASS AS A NONFOSSIL F U E L SOURCE

Salvage of waste wood from sawmills began anew a few years ago when some paper mills began to experience pulpwood shortages. Discarded pieces w e r e c h i p p e d a n d sold for p u l p i n g . M o r e recently, there has been interest in u s i n g bark as a boiler fuel partly because o f t h e disposal p r o b l e m a n d partly because of increased fuel prices. Since t h e A r a b oil e m b a r g o o f 1 9 7 3 , serious a t t e n t i o n has been g i v e n t o use of w o o d as a fuel o n a large scale. For e x a m p l e , Szego a n d K e m p (8) have e v a l u a t e d t h e possibility of e n e r g y f a r m s o n w h i c h w o o d y plants w o u l d be p r o d u c e d f o r fuel use. T h e M a i n e O f f i c e o f Energy Resources (9) has analyzed t h e possibility o f m e t h a n o l p r o d u c t i o n f r o m w o o d . Huff ( 4 p i a s r e p o r t e d o n t h e d e v e l o p m e n t of a n a u t o m a t i c a l l y c o n t r o l l e d f u r n a c e suitable for residences w h i c h c a n b u r n w o o d c h i p s m a d e f r o m l o g g i n g residues or p u c k e r b r u s h . S m i t h (7) has e x a m i n e d c o n c e p t u a l d e s i g n s f o r m e c h a n i z e d s h o r t - r o t a t i o n forestry, p a r t i c u l a r l y t h e h a r v e s t i n g phase. M e t h o d s o f w o o d h a r v e s t i n g h a v e b e e n r e v o l u t i o n i z e d r e c e n t l y as m e c h a n i z a t i o n has c o m e t o forestry. A n u m b e r of h a r v e s t i n g m e t h o d s are n o w in use in w h i c h t h e basic o p e r a t i o n s o f f e l l i n g , t r a n s p o r t t o a l a n d i n g , p r o c e s s i n g a n d loading f o r t r a n s p o r t are a p p r o a c h e d in very d i f f e r e n t w a y s . T h i s paper e x a m i n e s t h e e n e r g y i n p u t s t o e a c h s u b o p e r a t i o n t o a l l o w e s t i m a t i o n of t o t a l e n e r g y relationships or net e n e r g y p r o d u c t i o n e f f i c i e n c y for a n y c o m p l e t e s y s t e m w h e t h e r or n o t it is c u r r e n t l y in use. It s h o u l d be stressed t h a t f o r a large p o r t i o n of U.S. forest lands, t h e o n l y s i g n i f i c a n t o p e r a t i o n i n v o l v e d in w o o d p r o d u c t i o n is t h a t of h a r v e s t i n g . Reforestation is o f t e n b y n a t u r a l means, very little fertilization or c u l t i v a t i o n is carried o u t . C o n s t r u c t i o n of a road n e t w o r k a n d actual h a r v e s t i n g of t h e trees at t h e e n d of t h e g r o w i n g c y c l e is. by far, t h e greatest p u r c h a s e d energy i n p u t t o w o o d p r o d u c t i o n . T h e energy used in road b u i l d i n g varies greatly w i t h terrain a n d h a r v e s t i n g p a t t e r n . It is p r o b a b l y small in relation t o o t h e r i n p u t s a n d is n e g l e c t e d in this analysis. Harvesting Equipment For m a n y years, t h e axe a n d b u c k s a w w e r e t h e sole m e a n s of f e l l i n g a n d p r e p a r i n g w o o d for t r a n s p o r t t o t h e users' premises. Primary t r a n s p o r t f r o m t h e s t u m p t o t h e c o l l e c t i o n p o i n t at a roadside or o n a riverbank w a s by horse or ox t e a m . P r o d u c t i o n rates f o r t h i s s y s t e m vary t r e m e n d o u s l y d e p e n d i n g on size of trees, haul d i s t a n c e s , t e r r a i n , etc., b u t it w a s generally reckoned t h a t one m a n c o u l d fell, d e l i m b , c u t u p a n d load one c o r d of w o o d per d a y w h i l e one horse w o u l d take a b o u t t w o hours t o drag o u t t h a t v o l u m e of w o o d .


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Wood Production Energetics

435

Use of gasoline p o w e r e d c h a i n s a w s has increased a w o r k e r ' s c a p a c i t y a b o u t t e n f o l d . M o d e r n saws a l l o w a m a n t o fell, d e l i m b a n d c u t u p a b o u t 1.3 c o r d s / h o u r . Use of small t r a c k e d vehicles e q u i p p e d w i t h w i n c h e s t o skid o u t b u n c h e s o f tree t r u n k s have largely d i s p l a c e d t h e horse a n d o x , b u t a m u l t i t u d e of n e w e q u i p m e n t is even n o w d i s p l a c i n g these devices. Short d e s c r i p t i o n s of t h e m a i n classes of e q u i p m e n t c o n s i d e r e d in this s t u d y follows: Chain Saw: A portable, g a s o l i n e - e n g i n e d , m a n u a l l y - c o n t r o l l e d m a c h i n e w i t h a t o o t h e d c h a i n used t o fell trees a n d remove limbs. Feller Buncher: A m o b i l e m a c h i n e d e s i g n e d t o shear a tree at t h e s t u m p , a n d hold it b y means o f a c l a m p a n d c u t t i n g head w h i l e it s w i n g s a n d d e p o s i t s t h e tree o n t o a pile o n t h e g r o u n d . The c u t t i n g head is usually c o m p o s e d o f t w o h y d r a u l i c a l l y a c t u a t e d shearing blades. P o w e r r e q u i r e m e n t s are f r o m 8 0 130 h o r s e p o w e r . Delimber Buncher: A m o b i l e m a c h i n e c a r r y i n g a unit w h i c h strips t h e l i m b s a n d t o p off t h e bole o f a p r e v i o u s l y felled tree a n d deposits t h e s t r i p p e d bole in a pile o n t h e g r o u n d ready f o r removal f r o m t h e s t u m p area to a roadside l a n d i n g . Usually requires a r o u n d 1 2 0 horsepower. Wheeled Skidder: A t r a c t o r unit, usually w i t h f r a m e steering a n d f o u r w h e e l drive, e q u i p p e d w i t h a w i n c h or g r a p p l e w h i c h gathers a n d skids b e h i n d itself loads o f full trees, tree l e n g t h boles logs f r o m t h e s t u m p area t o a roadside l a n d i n g . Power r e q u i r e m e n t usually exceeds 7 0 horsepower. Wheeled Forwarder: A f r a m e steered, self-loading v e h i c l e e q u i p p e d w i t h h y d r a u l i c a l l y o p e r a t e d g r a p p l e a n d loading b o o m a n d a carrier or b u n k t o s u p p o r t its load of logs. Power r e q u i r e m e n t s vary f r o m 4 0 t o 1 0 0 h o r s e p o w e r d e p e n d i n g o n size. Loader: A h y d r a u l i c a l l y o p e r a t e d b o o m a n d g r a p p l e w h i c h c a n be m o u n t e d on a t r u c k chassis. It is used t o g a t h e r logs or tree l e n g t h s f r o m a pile a n d b u i l d a load o n a t r u c k b o d y . Chipper: A m a c h i n e w h i c h reduces logs a n d tree l e n g t h w o o d t o small c h i p s by means of a rapidly r o t a t i n g d r u m or disc, c a r r y i n g a series of blades. T h e c h i p s usually leave t h e c u t t i n g d e v i c e in an air-stream i n d u c e d by t h e f a n effect of t h e c h i p p i n g m e c h a n i s m a n d are t h u s a u t o m a t i c a l l y c o n v e y e d i n t o t r a n s p o r t vehicles or stockpiles. Power r e q u i r e m e n t s are a r o u n d 3 0 0 h o r s e p o w e r for a m a c h i n e capable of c h i p p i n g a r o u n d 2 5 t o n / h o u r s .


436


Energetics of Mechanized Harvesting Systems Table 1 s h o w s t y p i c a l p r o d u c t i o n rates a n d fuel c o n s u m p t i o n f i g u r e s for t h e v a r i o u s pieces o f e q u i p m e n t previously d e s c r i b e d . T h e w r i t e r s w e r e f o r t u n a t e in t h a t t h e A m e r i c a n P u l p w o o d A s s o c i a t i o n p u b l i s h e d t h e results of a 1 9 7 4 s u r v e y of m e m b e r s ' o p e r a t i o n s (1) w h i l e t h i s p a p e r w a s b e i n g w r i t t e n . W h e n e v e r possible, t h e d a t a f r o m t h a t s u r v e y w a s used in p r e p a r i n g t h e table. T h e data sources f r o m w h i c h o t h e r f i g u r e s w e r e c a l c u l a t e d are i n d i c a t e d in t h e f o o t n o t e s . Figures for t h e e n e r g y s u b s i d y represented by t h e e n e r g y used in m a n u f a c t u r i n g t h e e q u i p m e n t are very a p p r o x i m a t e a n d w e r e d e r i v e d by a s s u m i n g an average f i g u r e of 2 5 . 0 0 0 B t u / l b c o n s u m e d in t h e m a n u f a c t u r i n g process (most of t h e e q u i p m e n t w e i g h t is in t h e f o r m of steel w h i c h requires a r o u n d 2 1 . 0 0 0 B t u / l b in t h e t r a n s f o r m a t i o n f r o m ore in t h e g r o u n d t o steel plate (2[. T h e e n e r g y used in m a n u f a c t u r e w a s d i v i d e d by t h e a p p r o x i m a t e l i f e t i m e p r o d u c t i o n of t h e e q u i p m e n t t o arrive at a f i g u r e of Btu per t o n of dry w o o d . T h e a p p r o x i m a t e e n e r g y c o s t of p r a c t i c a l l y a n y s y s t e m of p r o d u c t i o n using present e q u i p m e n t c a n be c a l c u l a t e d f r o m t h e table. For e x a m p l e , a very c o m m o n s y s t e m uses c h a i n s a w f e l l i n g a n d d e l i m b i n g , tree l e n g t h s k i d d i n g t o a forest l a n d i n g , l o a d i n g t h e tree l e n g t h material o n t o large t r u c k s for t r a n s p o r t t o a mill y a r d , u n l o a d i n g by t h e s a m e t y p e of loader used in t h e w o o d s , f o l l o w e d by c h i p p i n g . M a n y operators are n o w m o v i n g t o w a r d c h i p p i n g w h o l e trees in t h e w o o d s w i t h a f u l l y m e c h a n i z e d s y s t e m . T h e steps m i g h t be as f o l l o w s : Felling w i t h a feller-buncher; grapple skidding to a landing; chipping, w i t h pneumatic c o n v e y i n g into t r u c k s as an integral part of t h e o p e r a t i o n ; t r a n s p o r t ; u n l o a d i n g by t i p p i n g the w h o l e t r u c k b o d y b a c k w a r d s t o d u m p t h e c h i p s by gravity. T a b l e II illustrates t h e b r e a k d o w n of e n e r g y use in t h e s e t w o s y s t e m s , i n c l u d i n g a 5 0 - m i l e haul t o t h e utilization site, w h i c h appears t o be a fair average for m u c h of t h e U.S. Several i n t e r e s t i n g facts a p p e a r f r o m t h e c o m p a r i s o n : 1.

Both m e t h o d s , t h o u g h very d i f f e r e n t in p r o c e d u r e , have a p p r o x i m a t e l y t h e s a m e u n i t e n e r g y c o n s u m p t i o n . In fact, this is so for m o s t of t h e m e c h a n i z e d s y s t e m s for p r o d u c i n g w o o d f r o m t h e tree t r u n k . Perhaps this is not s u r p r i s i n g as m o s t of the same operations appear in each s y s t e m t h o u g h t h e y m a y be p e r f o r m e d in a d i f f e r e n t order.


21.

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Table I.

437


A P P R O X I M A T E ENERGY USE IN W O O D PRODUCTION OPERATIONS A. Energy Subsidy Due to Equipment Manufacture

Machine Type or Operation

Wheeled Skidder, w h o l e trees Forwarder residues Wheel loader. prebunch residues Yard Operations Chain saw. bucking to short lengths Loading Tree length Trucking Small truck Large truck Chipping W h o l e tree chipper Auxiliary Management vehicles, etc.

Production Life

W e i g h t lb

Felling Chain saw. 10 (Felling and delimbing) Feller-Buncher 52.000 Delimbing Limber Buncher Trans, to Landing

Manufacturing Energy Subsidy, >a,b Btu/ton dry w o o d

Typical Machine

2.6 c d / h r

c

45.000

9cd/hr

25.000 27.000

3.08 cd/hr 9.02 green t o n s / r

4.000

4.5 green tons/hr

e

10

3.65 c d / h r

c

25.000

10.78cd/hr

c

4.000

f

f

10 c d / h r

10.350

10.000 h r

d

10.000 h r

d

8.350

d

10.400 11.300

13.000 h r 13.000 h r 13,000

d

3.400

d

hr

23.0

10.000 hr

3.900

2.000

300.000 m i 500.000 m i

12.000 25.000 57.000

32.0

2.000 hr

c

8.38cd/hr

r

10.000 h r

d

d

6.700* 3.300 J

9.500

d

100.000 mi

1.000


k

438


Table I. A P P R O X I M A T E ENERGY USE IN W O O D PRODUCTION OPERATIONS (cont) B. Equipment Operation and Overall Energy Requiremanta

Fuel Consumption

Energy Use Btu/ton dry wood

Total Energy Requirements, Btu/ton dry wood (to nearest 1000 Btu)

Chain saw. (Felling and delimbing) Feller-Buncher Delimbing

0.41 g a l / c d

33.000

33.000

59.700

70.000

Limber Buncher Trans, to Landing Wheeled Skidder. w h o l e trees Forwarder residues W h e e l loader. prebunch residues Yard Operations

0.62 g a l / c d

57.900

66.000

88.500 115.000

99.000 126.000

67.200

71.000

Machine Type or Operation

b

Felling

Chain saw. bucking t o short lengths Loading, Tree length Trucking: Small truck Large truck Chipping W h o l e tree chipper Auxiliary Management, vehicles, etc. 8

b

c

d

β

f

9

h

1

J

k

0.64gal/cd

c

c

d

0.95 gal/cd 0.41 gal/green t o n 1.24 gal/green t o n

9

9

0.39 g a l / c d

c

31.200

31.000

0.47 g a l / c d

c

43.500

47.000

373.000 187.000

380.000 190.000

0.04 gal/cd m i 0.02 gal/cd m i 0.7 g a l / c o r d 0.72 gal/cd

d

c

c

h

h

65.500

75.000

57.600

59.000

Assumes 25,000 Btu/lb consumed in equipment manufacture. Assumes 3.000 lb dry w o o d per average cord. Source — "Fuel Requirements for Harvesting P u l p w o o d " — ΑΡΑ Survey. Source — Estimate of Woodlands Manager. Source — Average of t w o company operations. Source — Folia Forestalia 237 — Finnish Forest Institute. Estimate based on engine size and research reports. Average figures for 100-mile round trip. 10 cord loads, handles 45.000 tons in useful life. 25 cords, loads handles 187,500 tons during useful life. Assumes 1 vehicle per fully mechanized harvesting crew.


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439


Table II. ENERQY U S E IN W O O D PRODUCTION S Y S T E M S Tree length system Felling and Delimbing

Btu/ton dry wood 33.000

(Chain saw) Skidding

99.000

Loading (tree length)

47.000

Transport (50 miles o n e way) Unloading

190.000 47.000

Chipping

75.000

Auxiliary

59.000 Total

550.000

Whole tree chip system Felling and Bunching

70.000

Skidding

99.000

Chipping

75.000

Transport

190.000

Unload

negligible

Auxiliary

59,000 Total

493,000


440


2.

T r a n s p o r t a t i o n , even if only t o a user 5 0 miles f r o m t h e g r o w i n g site, can represent a l m o s t 5 0 % of t h e t o t a l energy i n p u t to present the p r o d u c t t o t h e c o n s u m e r . It m a y seem t h a t substantial savings c o u l d be m a d e by c o n s u m i n g t h e w o o d closer t o t h e g r o w t h site. H o w e v e r , e c o n o m i c s rather t h a n e n e r g e t i c s w i l l d e c i d e w h e t h e r t h i s w i l l be d o n e .

3.

R e d u c t i o n of t h e w o o d f r o m tree l e n g t h t o t h e c o n v e n i e n t f o r m of w o o d c h i p s takes o n l y a b o u t 2 0 % of t h e e n e r g y used in p r o d u c t i o n . Even t h o u g h t h e bulk of t h e w o o d is c o n s i d e r a b l y increased by c h i p p i n g , w e i g h t , not v o l u m e , remains t h e l i m i t o n load size for t r a n s p o r t a t i o n . T h e b o n u s of self l o a d i n g f r o m t h e c h i p p e r a n d easy u n l o a d i n g of c h i p s m a k e in-forest c h i p p i n g very a t t r a c t i v e .

4.

C o m p a r i n g t h e e n e r g y c o n s u m p t i o n in these s y s t e m s w i t h t h e m a n - a x e horse c o m b i n a t i o n of t h e past, w h e r e a b o u t 8 m a n h o u r s a n d t w o h o r s e p o w e r hours p r o d u c e d one c o r d of w o o d ready for t r a n s p o r t , s h o w s o n e of t h e p r o b l e m s o f m e c h a n i z a t i o n . If an overall e f f i c i e n c y of 2 0 % is a s s u m e d for t h e a n i m a l p o w e r u n i t s i n v o l v e d , t h e e n e r g y required t o prepare t h e w o o d for t r a n s p o r t t o t h e user w o u l d be less t h a n 3 0 , 0 0 0 B t u / t o n of d r y material. This c o m p a r e s w i t h a b o u t 2 0 0 , 0 0 0 B t u / t o n for t h e s a m e o p e r a t i o n s in m e c h a n i z e d systems. T h e same order of increase in energy c o n s u m p t i o n per u n i t of p r o d u c t i o n c a n be f o u n d in m e c h a n i z e d a g r i c u l t u r e (6). H o w e v e r , t h e c o m p a r i s o n of e n e r g y use t o e n e r g y y i e l d is still very favorable. A t o n of dry material has a gross e n e r g y c o n t e n t of a b o u t 16 m i l l i o n B t u . Even a l l o w i n g for t h e f a c t t h a t each t o n of d r y m a t t e r is d e l i v e r e d in t h e f o r m of green w o o d c o n t a i n i n g a p p r o x i m a t e l y 5 0 % m o i s t u r e , i.e. w i t h a t o n of w a t e r t o be e v a p o r a t e d per t o n of dry material, t h e net e n e r g y available w i l l e x c e e d 14 m i l l i o n B t u / t o n of d r y material. On t h i s basis t h e e n e r g y used in processing t h e w o o d represents less t h a n 4 % of t h e energy available f r o m t h e w o o d .

5. The e n e r g y i n p u t t o w o o d p r o d u c t i o n in t h e f o r m of e q u i p m e n t m a n u f a c t u r e is fairly small in relation t o e n e r g y for o p e r a t i n g the e q u i p m e n t . M a n u f a c t u r i n g e n e r g y subsidy is less t h a n 2 0 % of t h e total e n e r g y i n p u t per t o n of w o o d for all of t h e e q u i p m e n t in Table I a n d averages a r o u n d 10%. It w o u l d c e r t a i n l y appear t h a t fuel used t o m a n u f a c t u r e a n d operate m a c h i n e r y t o p r o d u c e w o o d for fuel w o u l d be e n e r g y w e l l used. H o w e v e r , it m u s t be r e m e m b e r e d t h a t use of w o o d for fuel, as c u r r e n t l y harvested, w o u l d c o m p e t e w i t h o t h e r w o o d uses s u c h as for paper a n d lumber. In all p r o b a b i l i t y , any large-scale use of w o o d for fuel w i l l need t o c o m e f r o m an increase in p r o d u c t i o n over a n d above c u r r e n t needs.


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441

T h e m o s t o b v i o u s source o f a d d i t i o n a l w o o d is in t h e parts o f t h e tree n o w d i s c a r d e d — t h e b r a n c h e s a n d t o p s — a l o n g w i t h undersized a n d o t h e r undesirable trees. This material p r o b a b l y represents a r o u n d 2 0 % o f t h e g r o w t h o n land n o w harvested, i.e.. o n land w h i c h has a road s y s t e m already d e v e l o p e d a n d paid f o r b y o t h e r forest p r o d u c t s . T h e b r a n c h material a n d small trees w i l l p r o b a b l y need t o be c h i p p e d as early in t h e h a r v e s t i n g process as possible t o r e d u c e bulk a n d p r o v i d e a n easily h a n d l e d p r o d u c t . T w o basic m e t h o d s o f h a n d l i n g t h e b r a n c h material are possible. One w o u l d be t o skid w h o l e trees t o t h e l a n d i n g , use a d e l i m b e r in a stationary position a n d c h i p a n y t h i n g s t r i p p e d o f f t h e boles. S k i d d i n g w h o l e trees w o u l d be very little d i f f e r e n t f r o m s k i d d i n g d e l i m b e d material, b u t e x p e r i e n c e has s h o w n t h a t u p t o half of t h e b r a n c h e s are broken off as t h e trees are skidded o u t . Feeding t h e s t r i p p e d b r a n c h e s into a c h i p p e r need be no m o r e energy c o n s u m i n g t h a n f e e d i n g tree l e n g t h logs. T h e s e c o n d s y s t e m m i g h t use a d e l i m b e r at t h e s t u m p a n d leave t h e branches a n d undesirable w o o d at t h e g r o w t h site. S o m e w o r k has been reported f r o m Finland (3) o n this possibility. Small bulldozers or w h e e l e d loaders w e r e used t o pile u p t h e b r a n c h material w h i c h w a s t h e n b r o u g h t o u t b y a s k i d d e r / f o r w a r d e r f o r processing at t h e l a n d i n g or a later stage. Performance f i g u r e s f r o m this e x p e r i m e n t a l o p e r a t i o n are i n c l u d e d in Table I. Table III c o m p a r e s t h e a d d i t i o n a l e n e r g y i n p u t s needed t o o b t a i n these h a r v e s t i n g residues. Once a g a i n , it is a p p a r e n t t h a t t h e w o o d fuel c a n be delivered t o a c o n s u m e r f o r less t h a n 5% of its e n e r g y c o n t e n t . T h e more e c o n o m i c a l m e t h o d u n f o r t u n a t e l y loses a g o o d p e r c e n t a g e o f t h e b r a n c h material. This leads t o t h e c o n s i d e r a t i o n of w o o d p r o d u c t i o n specifically f o r fuel. It is generally a c c e p t e d t h a t in N o r t h e r n areas, g r o w t h t o m a t u r i t y averages a b o u t 1 t o n of d r y m a t t e r / a c - y r . H o w e v e r , Ribe (5) has s h o w n t h a t m o r e t h a n t w o t i m e s t h e w o o d present at harvest o f a m a t u r e s t a n d has g r o w n , d i e d in t h e c o m p e t i t i o n f o r s u n l i g h t , a n d r o t t e d a w a y d u r i n g t h e g r o w t h o f t h e s t a n d . This indicates t h a t v i s i t i n g each site perhaps t w i c e d u r i n g t h e g r o w i n g c y c l e t o r e m o v e dead w o o d a n d t o t h i n t o o - d e n s e areas c o u l d increase t o t a l yields of w o o d by perhaps 100%. M u c h of t h e material o b t a i n e d w o u l d p r o b a b l y be " f u e l g r a d e " . However, t h e e c o n o m i c s o f such a p r a c t i c e are u n k n o w n a n d t h e q u e s t i o n of w h a t effect removal of s u c h q u a n t i t i e s o f material m i g h t have o n t h e available n u t r i e n t pool in t h e soil is certainly i m p o r t a n t . A f u r t h e r possibility for w o o d fuel p r o d u c t i o n is for intensive s h o r t - r o t a t i o n forestry w h e n small trees m i g h t be harvested every five or t e n years w i t h a m o b i l e m o w e r / c h i p p e r laid o u t similarly t o a grain c o m b i n e . S u c h a m a c h i n e m i g h t be e x p e c t e d t o cover one acre per hour f o r a t h r o u g h p u t of a b o u t 2 0


442

BIOMASS AS A NONFOSSIL F U E L SOURCE Table III.

ENERGY USE IN HARVESTING FOREST RESIDUES FOR FUEL

Whole trees skidded, Delimbed at landing"

negligible

of skidding Chipping Transport Unload Auxiliary activities Total

75.000 190.000 negligible 59.000 324.000

Residues prebunched in stump area. Forwarder used in transport to landing Prebunching residues Forwarding Chipping Transport Unload Auxiliary activities Total 8

Btu/ton dry wood

Additional energy cost

71.000 126.000 75.000 190.000 negligible 59.000 521.000

This system probably loses half the available material in skidding.

Table IV. P R O B A B L E ENERGY REQUIREMENTS FOR A SHORT ROTATION W O O D FUEL CROP Assumption Cultivate and plant at 2 0 year intervals — 6 gal fuel/ac-planting. Growth rate — 5 t o n / a c - y r Fertilizer — 1000 lb nitrogen/ac-yr @ 33,000 Btu/lb manufacturing and application cost. Harvesting — equivalent to present chipping in energy cost. Transport t o truck or stockpile — equivalent t o skidding. Loading trucks f r o m stock pile or primary transport — equivalent to tree length loading. Energy Use Estimates

Btu/ton dry wood

Cultivation and planting Fertilization Harvesting Transport t o stockpile Load trucks Transport t o user Unload Auxiliary operations

8,000 660,000 75.000 99.000 47,000 190,000 negligible 59,000 1.138.000

Total


21.

SMITH AND CORCORAN


443

t o n s o f w o o d . There are d i s t i n c t e n g i n e e r i n g e c o n o m i e s t o this t y p e of m a c h i n e w h e r e each c o m p o n e n t p e r f o r m s its f u n c t i o n t h e w h o l e t i m e , f o r e x a m p l e , t h e m o w i n g m e c h a n i s m m o w s c o n t i n u o u s l y a n d t h e c h i p p e r is c o n t i n u o u s l y loaded. E q u i p m e n t f o r full-size tree h a n d l i n g operates i n t e r m i t t e n t l y . The shear o n a feller b u n c h e r shears t h e tree a n d t h e n is o u t of use u n t i l t h e tree has been lifted a n d b u n c h e d by t h e o t h e r parts o f the m a c h i n e . Fertilization o f fast g r o w i n g species in a s h o r t - r o t a t i o n s y s t e m c o u l d p r o d u c e a n n u a l yields of a r o u n d 5 or 6 t o n s o f d r y matter. The use of species w h i c h g r o w u p f r o m e x i s t i n g root s y s t e m s c o u l d p r o v i d e very fast regeneration after harvest, t h o u g h w o o d f r o m s u c h species m i g h t be of t o o l o w q u a l i t y f o r use o t h e r t h a n as fuel. Replanting m i g h t be necessary o n l y after four or f i v e h a r v e s t i n g cycles — perhaps o n l y every 2 0 years. A s s u m p t i o n s a n d e n e r g y cost e s t i m a t e s f o r s u c h a s y s t e m are g i v e n in Table IV. The intensified p r o d u c t i o n , as i n a g r i c u l t u r e , results in a greater energy cost per u n i t of p r o d u c t i o n , w i t h a p p r o x i m a t e l y half t h e energy i n p u t a c c o u n t e d for b y fertilizer. O m i s s i o n of t h e fertilizer w o u l d p r o b a b l y r e d u c e t h e a n n u a l yield t o a r o u n d 2 - 3 t o n s / a c , b u t w o u l d bring t h e energy cost per u n i t in line w i t h l o n g - r o t a t i o n systems. It is i n t e r e s t i n g t o speculate w h a t m i g h t be d o n e t o fertilize intensive energy f a r m s w i t h g a r b a g e a n d s e w a g e sludge. A c t u a l field e x p e r i m e n t s w o u l d be w e l l w o r t h w h i l e . H o w e v e r , even w i t h full fertilization, w o o d fuel f r o m s h o r t - r o t a t i o n s y s t e m s can p r o b a b l y be p r o d u c e d at a n e n e r g y cost n o t e x c e e d i n g 7 % o f its e n e r g y c o n t e n t . In s u m m a r y , it can be said t h a t t h e e n e r g e t i c s o f w o o d fuel are very a t t r a c t i v e . T h e fuel itself has m a n y desirable qualities — it c o n t a i n s p r a c t i c a l l y n o sulphur, o n l y a b o u t 1 % ash, c a n be b u r n e d cleanly, is reasonably c o m p a c t (about 1 0 0 , 0 0 0 B t u / f t in c h i p f o r m ) , a n d represents a r e n e w a b l e e n e r g y source. Nevertheless, e c o n o m i c s w i l l d e c i d e t h e a c c e p t a b i l i t y o f w o o d fuel. A material as versatile as w o o d clearly has several c o m p e t i t i v e uses. 3

However, it is also i n t e r e s t i n g t o note t h a t if w o o d available for fuel use c o u l d be increased t o the 146 m i l l i o n c o r d a n n u a l level o f t h e year 1 8 8 0 , t h e e n e r g y c o n t e n t w o u l d be e q u i v a l e n t t o a l m o s t 6 0 0 m i l l i o n barrels of oil per year. U n f o r t u n a t e l y , this only represents a 3 0 - 3 5 d a y oil s u p p l y at c u r r e n t c o n s u m p t i o n levels. W o o d fuel w i l l n o t solve t h e n a t i o n a l energy p r o b l e m t h o u g h it m a y make s i g n i f i c a n t c o n t r i b u t i o n s in s o m e regions. REFERENCES

1.

"Fuel Requirement for Harvesting Pulpwood", American Pulpwood Association: 1975.


444

BIOMASS AS A NONFOSSIL FUEL SOURCE

2. Berg, C. Science 1973 181, 128-38. 3.

"Bunching and Transportation of Branch Raw Material"; Folia Forestalia 237, Finnish Forest Institute, 1975.

4. Huff, E. R.; Riley, J. G.; Smyth, D. "Modern Residential Heating with Wood Chips"; ASAE Paper 76-4555, 1976. 5. Ribe, J. H. Orono, Maine, 1974, LSA Expt. Sta., University of Maine, Misc. Report 160. 6.

Smith, N. "Engineering A Food Supply", ASAE Paper NA70-402, 1970.

7. Smith, N. Can. Agric. Eng. 1974 316 (1). 8. Szego, G. C.; Kemp, C. C. Chemtech, 1973 May, 275-84. 9.

"Maine Methanol", Office of Energy Resources, Augusta, Maine, 1975.

OTHER REFERENCES 1. Bradley, D. P. International Union of Forest Research Organizations, Working Party, S3.04.01 Proceedings — Simulation Techniques in Forest Operational Planning and Control, Wageningen, The Netherlands (Agricultural University), 1978; 137-45. 2.

Roberts, D.; Corcoran, T. International Union of Forest Research Organizations, Working Parry S3.04.01 Proceedings — Simulation Techniques in Forest Operational Planning and Control, Wageningen, The Netherlands (Agricultural University), 1978; 295-307.

3. Smith, N.; Riley, J. G.; Hill, R. C. "Solar Energy Storage for the Northeast", Paper No. 78-4052, 1978. 4.

"Forest Residues Energy Program," 1978, Forest Service USDA Final Report, N. Central For. Exp. Sta., St. Paul, Minn.

5. Riley, J. G.; Smith, N. ASAE Paper 77-4018, 1977. 6. Riley, J. G.; Smith, N. Proc. Third Annual UMR-DNR Conference on Energy, University of Missouri, Rola, MO, 1977.


21. SMITH AND CORCORAN Wood Production Energetics 445

7.

Shottafer, J. E., et al. "Utilization of Low Grade Hardwoods for Fuel in the Washington and Hancock Counties of Maine", School of Forest Resources, Forest Products Note No. 3, 1977, p 98.

8.

Houghton, J. E.; Johnson, L. R. For. Prod. J. 1976 26 (4), 15-18.

9.

Riley, J. G. ASAE Paper NA76-101, 1976.

10.

"The Feasibility of Utilizing Forest Residues for Energy and Chemicals" A Report to the National Science Foundation and Federal Energy Administration, Rann Research Assoc., Forest Service - USDA, 1976.

11.

Erickson, J. R. AICHE Symposium Series 1975 71 (146) 27-29.

RECEIVED JUNE 18,

1980.


Wood Production Energetics - American Chemical Society

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