Biomass as a Nonfossil Fuel Source - ACS Publications - American

22 Silvicultural Systems for the Energy Efficient Production of Fuel Biomass F. THOMAS

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Downloaded by CORNELL UNIV on July 24, 2016 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch022

Yale University, Greeley Memorial Laboratory, 370 Prospect Street, New Haven, CT 06511

The price of imported crude oil delivered to the United States increased 235% on a constant dollar basis between 1960 and 1976 (9). Most of this increase occurred between 1973 and 1974 when the actual price of crude oil went from $4.00/barrel to $12.52. An additional increase of 14.5% has been announced by the OPEC oil ministers, raising prices to $14.54 by October 1979 (13). Given the continued high rate of inflation, it is a certainty that OPEC will find it desirable to keep pace by future price increases, and should it be politically expedient to the Arab world, prices could again rise dramatically. Increasing prices for oil and the threat of arbitrarily restricted supply are incentives to explore alternative energy sources, particularly those under domestic control. It is likely that energy requirements can only be met by combined development of several energy sources. Among alternative energy sources, the production of biomass from agricultural crops and forest trees has much promise. However, total U.S. energy consumption in 1975 was 71 Q (Q or quad = 10 Btu), but total annual production of biomass from crop and forest land based on current management practice has been estimated by one analyst to be 21 Q (6). Better than 50 percent of the total production was from agricultural land, and because food has a higher priority than energy, prime agricultural land will probably contribute little to a solution of the energy problem. It appears that forest biomass cannot singularly alleviate the U.S. energy shortages, but 15

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Current address: Institute of Forest Genetics, U.S. Forest Service, P.O. Box 245, Berkeley, CA 94701

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.

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BIOMASS AS A NONFOSSIL F U E L SOURCE


nevertheless, it can play an i m p o r t a n t role. T h e use of forests for fuel has several s i g n i f i c a n t e n v i r o n m e n t a l a d v a n t a g e s . Forests are aesthetically p l e a s i n g , a n d fuel p l a n t a t i o n s c a n d o u b l e as sites for leisure recreation, c o n t r i b u t e to t h e m a i n t e n a n c e of w o r l d g e o c h e m i c a l c y c l e s , a n d w h e n c o n s u m e d for f u e l , c o n t r i b u t e m i n i m a l l y t o a t m o s p h e r i c p o l l u t i o n because b i o m a s s is l o w in sulfur c o n t e n t . T h e l o w a n n u a l p r o d u c t i o n of 9 Q f r o m U.S. forest land c o m p a r e d t o 12 Q f r o m a g r i c u l t u r a l land reflects a lack of m a n a g e m e n t a n d m i n i m a l care of forests. It is c e r t a i n t h a t p r o d u c t i o n c o u l d be s u b s t a n t i a l l y increased by proper c h o i c e of species a n d strain, c u l t i v a t i o n , c o n t r o l of s p a c i n g , a n d fertilization ( I f i ) . But. intensive c u l t u r e w i l l result in increased energy d e m a n d s . For e x a m p l e , reliance o n natural regeneration requires no e x p e n d i t u r e o f o u t s i d e e n e r g y e x c e p t for harvest, b u t p l a n t i n g uses 1.2 t o 1.5 x 1 0 Btu/ha(5). 6

T a b l e I. E N E R G Y R E Q U I R E M E N T S I N F O R E S T O P E R A T I O N S B t u ha

Operation Seed c o l l e c t i o n Nursery p r o p a g a t i o n

1

6

χ 10

281.21 0.14

Site p r e p a r a t i o n Burning: hand Burning: helicopter

3.267.81 2.237.82

Hand Machine Aerial s p r a y i n g Stocking control Fertilization

1.541.28 1.217.71

1.071.99

0

b

c

For q u a n t i t y s u f f i c i e n t seedlings/lb. For 2 2 2 3 s e e d l i n g s / h a .

b

335.92 1.284.40

KG blade, pile a n d b u r n slash C h o p p i n g , c r u s h i n g or r i p p i n g w / o piling Bedding Planting

a

a

1,729.00 834.86 37,529.18 to

plant

2223

seedlings

at

8000

plantable

P l o w i n g to create f u r r o w s a n d m o u n d s so t h a t t h e m o u n d s are raised a b o v e high w a t e r table.

Fertilization w o u l d c l a i m an a d d i t i o n a l e x p e n d i t u r e of 3 7 5 x 10 B t u / h a , and seed p r o d u c t i o n , nursery p r o d u c t i o n of seedlings, site p r e p a r a t i o n , a n d


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c o n t r o l of s t o c k i n g all require e n e r g y i n p u t (Table I). In a g r i c u l t u r e , industrialization or m e c h a n i z a t i o n has increased yields, b u t o n an e q u i v a l e n t e n e r g y basis t h e increase in yield is less t h a n t h e overall increase in i n p u t s . The energy ratio a c t u a l l y d e c l i n e d f r o m 1 9 4 5 t o 1 9 7 0 (19). It is i m p o r t a n t t o d e t e r m i n e w h e t h e r i n p u t s are so h i g h t h a t intensive c u l t u r e of forests f o r fuel biomass is i m p r a c t i c a l or w h e t h e r increased i n p u t s c a n establish forest b i o m a s s as a s i g n i f i c a n t c o m p o n e n t of t h e t o t a l e n e r g y p i c t u r e . T w o factors t o e x a m i n e are: 1) e n e r g y e f f i c i e n c y or energy o u t p u t / e n e r g y i n p u t a n d 2) net e n e r g y yields u n d e r d i f f e r e n t intensities o f m a n a g e m e n t .


PRODUCTION EFFICIENCY Certainly, o n e of t h e m o s t e n e r g y - e f f i c i e n t systems f o r t h e p r o d u c t i o n of biomass is a t r a d i t i o n a l s i l v i c u l t u r a l s y s t e m using natural regeneration. Natural regeneration is forest regeneration either f r o m s t u m p s p r o u t i n g , seedling or saplings already present w h e n t h e m a t u r e s t a n d w a s harvested, g e r m i n a t i o n of seed t h a t lay d o r m a n t in t h e litter, or seeding f r o m s u r r o u n d i n g forest w i t h o u t t h e i n p u t s of site p r e p a r a t i o n , p l a n t i n g or c u l t i v a t i o n . N e g l e c t i n g t r a n s p o r t , s u c h systems w o u l d g e n e r a t e e n e r g y costs o n l y f o r h a r v e s t i n g , w h i c h are a s s u m e d t o be 5.16 gal o i l / c o r d of w o o d [ ] ) . O n t h e average hectare o f c o m m e r c i a l forest land in N e w England, t h e r e are 6 0 c o r d s w h e n b o t h b r a n c h e s a n d s t e m are i n c l u d e d . On this basis, gross e n e r g y yield c a n be c a l c u l a t e d as 1,642 m i l l i o n B t u / h a (Table II). Based on net a n n u a l g r o w t h in N e w E n g l a n d , a b o u t 6 2 years w o u l d be necessary t o achieve this yield. T h e ratio of e n e r g y yields t o energy costs w o u l d be 3 7 c o m p a r e d t o a b o u t 5.3 for t h e p r o d u c t i o n of silage c o r n , o n e of t h e m o s t e n e r g y - e f f i c i e n t forms of agriculture(]2).


450


Table II. ENERGY YIELD F R O M ONE HECTARE OF C O M M E R C I A L FOREST L A N D IN N E W E N G L A N D C u b i c feet of w o o d in tree s t e m s 5 inches d i a m e t e r a n d g r e a t e r A l l o w a n c e for b r a n c h e s a n d foliage as 3 0 % of t o t a l T o t a l c u b i c feet E q u i v a l e n t in c o r d s E q u i v a l e n t in p o u n d s dry w e i g h t E q u i v a l e n t in B t u Cost of harvest in gallons of o i l Equivalent cost in B t u Energy o u t p u t / e n e r g y i n p u t

3.522

8

b

0


d

6

f

8

b

c

d

β

f

1.509 5.031 60 193.190 ,6 1.642 x 10 310 4 4 x 10>6 37 (

1

F r o m r e f . 18. A t 0.012 c o r d / c u b i c f o o t A t 3200 lb/cord A t 8500 Btu/lb. From ref. 1. A t 6 x 1 0 Btu/barrel 6

On i n d u s t r i a l lands d e v o t e d t o t h e p r o d u c t i o n of w o o d p r o d u c t s , a m o r e intensive s y s t e m is a p p l i e d . Rather t h a n risk failure of natural r e g e n e r a t i o n or lose p r o d u c t i v e c a p a c i t y w h i l e w a i t i n g for natural seed years, m o s t c o m m e r c i a l o p e r a t i o n s resort t o p l a n t i n g . Planting also provides better c o n t r o l of species c o m p o s i t i o n t h a n is o b t a i n a b l e w i t h natural regeneration. In this case, sites are intensively prepared by m e c h a n i c a l means, seedlings p r o d u c e d in nurseries are p l a n t e d at u n i f o r m s p a c i n g , a n d t h e site is fertilized. For a 3 0 - y e a r r o t a t i o n , t h e c y c l e b e t w e e n r e g e n e r a t i o n a n d harvest, S m i t h a n d J o h n s o n (24) c a l c u l a t e d an energy ratio of 2 2 for t r a d i t i o n a l l y harvestable p r o d u c t s , w h i c h is still m u c h better t h a n our m o s t efficient a g r i c u l t u r a l s y s t e m s . W h e n b r a n c h e s a n d foliage are i n c l u d e d as 3 0 % of t o t a l a b o v e g r o u n d b i o m a s s , a c o n s e r v a t i v e f i g u r e , t h e ratio is 34.8. For Eucalypt species u n d e r m a n a g e m e n t in A u s t r a l i a , t h e ratio w a s 2 0 : 1 w h i c h surpassed cassava a n d kenaf b u t w a s equalled by e l e p h a n t grass on a t o t a l c r o p yield basis (16). Similar values w e r e c a l c u l a t e d for Douglas-fir a n d loblolly pine (Table III); i.e. 7.6 t o 17.4 (5), b u t these i n c l u d e only t h e t r a d i t i o n a l l y harvested p r o d u c t s , i g n o r i n g b r a n c h a n d foliage biomass.


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T a b l e I I I . E N E R G Y C O S T S A N D Y I E L D S FOR T H E I N T E N S I V E M A N A G E M E N T OF DOUGLAS-FIR A N D LOBLOLLY P I N E Btu h a Douglas-Fir

1

8

Costs

vr" x 10~ Loblolly pine" 1

3

High Intensity L o w Intensity

Site p r e p a r a t i o n Planting Aerial spraying


5

Stocking control Thinning Fertilization

25.7

140.9 88.9

44.5 84.0 16.7

84.0 33.4 1.153.4

Fire p r o t e c t i o n

4.503.5 9.7

4.503.5 9.7

Harvest

7.647.3

4,621.3

0

140.9 61.7

Btu ha'

1

yr"

1

9.7 3.007.1 x 1 0

6

Total energy consumption Total e n e r g y yield Net e n e r g y yield a

b

c

12.33 142.07 129.74

10.64 77.85 67.21

3.22 55.92 52.70

50-year r o t a t i o n 25-year rotation A s s u m i n g 8,746 B t u / c u b i c f o o t (18)

Finally, consider still m o r e intensive systems, variously called s h o r t - r o t a t i o n , intensive c u l t u r e (27) or m i n i - r o t a t i o n s y s t e m s (2Λ). S u c h s c h e m e s e m p l o y species t h a t s p r o u t after c u t t i n g , s u c h as poplars, s y c a m o r e , or w i l l o w s , so r e g e n e r a t i o n is a u t o m a t i c after each harvest (4, 25). T h e s y s t e m of r e g e n e r a t i n g a s t a n d b y s t u m p s p r o u t i n g is k n o w n as c o p p i c i n g . S p a c i n g of trees is very close, less t h a n 1 2 x 1 2 d m , a n d rotations are b e t w e e n 2 t o 10 years, d e p e n d i n g u p o n s p a c i n g . Site p r e p a r a t i o n before p l a n t i n g is intense a n d is f o l l o w e d u p by c u l t i v a t i o n t o c o n t r o l w e e d s , b o t h herbaceous a n d w o o d y . H a r v e s t i n g e m p l o y s t e c h n i q u e s and e q u i p m e n t similar t o t h a t used in p r o d u c t i o n of silage c o r n , so t o t a l a b o v e - g r o u n d biomass is harvested. Fertilization is m a n d a t o r y t o preserve site p r o d u c t i v i t y , a n d irrigation has been advised. One c a l c u l a t i o n (Table IV) o f e n e r g y e f f i c i e n c y in these systems i n d i c a t e d a ratio b e t w e e n 11.2 t o 12.6 in W i s c o n s i n for t h e first r o t a t i o n (28). Efficiency w o u l d be higher for s e c o n d rotations o f sprout origin because t h e root s y s t e m has already been f a b r i c a t e d a n d is in place, so t h a t g r o w t h of a b o v e - g r o u n d p o r t i o n s is greatly accelerated. Other a u t h o r s (1Q) s u g g e s t


452


e f f i c i e n c y m a y b e h i g h e r in s o u t h e r n l a t i t u d e s ; e.g. 13.4 i n Pennsylvania (Table V) a n d 15.3 in Louisiana (Table VI). T a b l e I V . E N E R G Y C O S T S A N D Y I E L D S FOR S H O R T R O T A T I O N , I N T E N S I V E C U L T U R E OF POPLAR A N D J A C K P I N E S B t u ha Poplar

Costs


Fuel for o p e r a t i o n s Manufacture, transport, and a p p l i c a t i o n of fertilizer M a n u f a c t u r e of irrigation s y s t e m Fuel f o r irrigation Plant p r o p a g a t i o n Other inputs Chipping

yr"

1

x 10' Jack Pine 3

6,940 11,640

5.070 5.080

2.040 5.220 20 130 1.330

2.040 5.220 250 130 930

Btu h a ' yr" 1

T o t a l energy yield

27.32 306.40

Net e n e r g y yield

279.08

Total energy consumption

1

2 8

1

x 10"

e

18.72 236.40 217.68

T a b l e V . E N E R G Y C O S T S A N D Y I E L D S FOR S H O R T - R O T A T I O N , INTENSIVE CULTURE OF POPLAR A N D N A T U R A L L Y REGENERATED FOREST 2

Btu ha' Costs

1

yr"

Intensive Culture

Fertilizer Growing and harvesting Chipping Other

15.82 212.52 196.70

3

Natural Forest

9.962 4.751 896 211 B t u ha

Total e n e r g y c o n s u m p t i o n Total energy yield Net e n e r g y yield

x 10"

1

5.394 268 48 1

yr" x 10' 1

6

5.71 63.57 57.86


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Table V I . ENERGY COSTS A N D YIELDS, NEGLECTING T R A N S P O R T A T I O N , FOR S H O R T R O T A T I O N , I N T E N S I V E C U L T U R E O F POPLAR ON "ENERGY F A R M S " IN WISCONSIN A N D L O U I S I A N A 1 0

Btu h a yr" 1

Costs Supervision Field s u p p l y

Wisconsin 82 26 466 269 64

Harvesters T r a c t o r haul Irrigation m o v e Irrigation p u m p i n g Downloaded by CORNELL UNIV on July 24, 2016 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch022

1

4,908 397 762 57

Manufacture of P2O5

Manufacture of K2O Ground operations Aircraft operations

5,055 404 344 30 8 32

7

Fertilizer t r a n s p o r t

48 Btu h a yr" 1

Total energy consumption Total e n e r g y yield

18.13 191.76 173.63

Net e n e r g y yield

3

26 389 271 32 5,524

11,047

M a n u f a c t u r e o f urea

x 10 Louisiana 82

1

X 10"

6

30.46 473.21 442.75

For each level of s i l v i c u l t u r a l intensity, t h e reported values i n d i c a t e t h e p r o d u c t i o n o f forest b i o m a s s is h i g h l y energy-efficient. H o w e v e r , it s h o u l d be observed t h a t t h e c a l c u l a t i o n s in Tables ll-VI d o n o t take i n t o a c c o u n t processing of t h e b i o m a s s , n o t a b l y d r y i n g , a n d t r a n s p o r t , w h i c h c a n be h i g h l y variable. NET ENERGY YIELDS Efficiency is generally h i g h e s t w i t h m i n i m a l i n t e n s i t y o f c u l t u r e , leaving t h e i m p r e s s i o n t h a t intensive c u l t u r e s h o u l d be a v o i d e d in t h e p r o d u c t i o n o f forest fuels. But t h a t is n o t necessarily t h e case. N e t energy yield s h o u l d be t h e p r i m a r y c o n c e r n in biomass fuel p r o d u c t i o n , n o t p r o d u c t i o n efficiency. In a s t u d y o f biomass p r o d u c t i o n in s h o r t - r o t a t i o n poplar a n d jack pine p l a n t a t i o n s , Z a v i t k o v s k i (£2) t e n t a t i v e l y p o i n t o u t t h a t n e t energy yield increased as a f u n c t i o n o f i n p u t (Figure I). T h e e x a m p l e of n a t u r a l l y regenerated forest p r e s e n t e d here seems t o f i t t h e p a t t e r n w e l l . Based o n a net a n n u a l g r o w t h o f 8 1 c u b i c feet h a " (12), a n average r o t a t i o n f o r a s t a n d 1


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Figure 1. Relationship between net energy yield and energy input for short-rotation intensive culture of hybrid (1) poplar and (2) jack pine, energy farms in (3) Wisconsin and (4) Louisiana, intensively cultured (5) Douglas fir and (6) loblolly pine, (7) loblolly pine under average intensity of management, (8) intensively managed southern pine, (9) short-rotation, non-irrigated poplar, (10) short-rotation natural forest in Pennsylvania and (11) natural forest in New England. Energy yields are for dry wood, and input does not include drying. Branches and foliage not included in 5, 6, 7. (2, 5, 10, 24, 28)


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of 5,031 c u b i c f e e t / h a w o u l d be 6 2 years, t h e f i g u r e used in c a l c u l a t i n g e n e r g y yield per hectare per year in Figure I. O n t h e o t h e r h a n d , yields f r o m t h e s c h e m e proposed by S m i t h a n d J o h n s o n (24) are h i g h . The reason seems to be a n e s t i m a t e o f a n n u a l g r o w t h w h i c h is higher t h a n t h a t achieved o n m o s t sites. Values f o r poplar in Louisiana (10) are also h i g h , perhaps, r e f l e c t i n g real a n d s i g n i f i c a n t regional a n d species v a r i a t i o n in p r o d u c t i v i t y . T h e i m p o r t a n t p o i n t is t h a t f o r t h e present rate of biomass p r o d u c t i o n f r o m u n m a n a g e d stands, a land area t h e size o f t h e entire U n i t e d States w o u l d n o t satisfy its energy d e m a n d s , b u t a n area o n l y o n e - t h i r d its size w o u l d be s u f f i c i e n t w i t h t h e s h o r t - r o t a t i o n , intensive c u l t u r e s c h e m e of Zavitkovski (27). FUEL CONVERSION A l l o f t h e f o r e g o i n g , i n c l u d i n g Tables ll-VI. is p r e d i c a t e d o n t h e basis of d i r e c t use o f biomass as f u e l , w i t h n o p r o v i s i o n f o r d r y i n g . In fact, t h e greatest d e m a n d is f o r gaseous o r liquid fuels s o c o n v e r s i o n t o a l c o h o l , m e t h a n e , or p y r o l y t i c oil is a m o r e likely fate f o r biomass. Because w a t e r c o n t e n t of green w o o d is r o u g h l y 5 0 % , a great deal o f its c h e m i c a l e n e r g y w i l l be required f o r d r y i n g . For s h o r t - r o t a t i o n intensive c u l t u r e s c h e m e s , d r y i n g w o u l d reduce t h e e f f i c i e n c y ratio t o 4.3 t o 4.7 (2g). F u r t h e r m o r e , t o break d o w n lignocellulose t o f e r m e n t a b l e sugars is energetically expensive. Therefore, crops like cassava a n d sugar cane w i t h h i g h sugar c o n t e n t c a n be c o n v e r t e d t o alcohol m o r e e f f i c i e n t l y t h a n w o o d . T h e e f f i c i e n c y of p r o d u c t i o n a n d c o n v e r s i o n of e u c a l y p t s t o alcohol is s o l o w t h a t it is u n e c o n o m i c , a n d t o p y r o l y t i c oil it is 5 8 % . e q u a l t o b u t n o better t h a n cereal s t r a w (J6). T h e h i g h e f f i c i e n c y of forests f o r biomass p r o d u c t i o n is offset b y t h e l o w e f f i c i e n c y of c o n v e r s i o n t o l i q u i d a n d gaseous fuels. T h e m a j o r a d v a n t a g e of forest fuels is in d i r e c t use for g e n e r a t i o n o f heat or power. Nevertheless, t h e n e t e n e r g y yield of 1 4 5 G J / h a for p y r o l y t i c oil f r o m e u c a l y p t u s in A u s t r a l i a is s e c o n d only t o t h e 1 5 4 G J / h a f o r a l c o h o l f o r cassava (2). So even after c o n v e r s i o n , trees remain a c o m p e t i t i v e source of l i q u i d fuels. Biomass w i l l p r o b a b l y never s u p p l y t h e entire energy needs of a d e v e l o p e d e c o n o m y , even locally, b u t m a y be o n e c o m p o n e n t of a c o m p r e h e n s i v e p r o g r a m . It is m o r e likely t h a t forest trees w i l l be used f o r energy p r o d u c t i o n t h a n a g r i c u l t u r a l c r o p s because o f t h e relative shortage o f g o o d a g r i c u l t u r a l land t h a t w i l l s u p p o r t t h e g r o w t h o f crops like sugar cane or cassava a n d p e r m i t use o f t h e m e c h a n i z e d t e c h n o l o g y necessary t o t h e i r efficient c u l t u r e . Good a g r i c u l t u r a l land has a higher value f o r f o o d p r o d u c t i o n . Therefore, despite t h e l o w c o n v e r s i o n e f f i c i e n c y o f w o o d , it w i l l be t h e major c o m p o n e n t of m a n y biomass f o r fuel p r o g r a m s .


456



I M P R O V I N G EFFICIENCY H o w do trees achieve an a d v a n t a g e in energy e f f i c i e n c y of biomass p r o d u c t i o n c o m p a r e d to crops s u c h as c o r n , w h i c h w e are a c c u s t o m e d t o t h i n k i n g of as a p h y s i o l o g i c a l l y " e f f i c i e n t " plant, e m p l o y i n g t h e C 4 - p a t h w a y of c a r b o n f i x a t i o n ? Part of t h e a n s w e r is t h a t s i l v i c u l t u r e is less intensive t h a n a g r i c u l t u r e , so e n e r g y i n p u t is lower. But there are also differences in o u t p u t o n a yearly basis. Perennial tree species e n j o y a longer g r o w i n g season t h a n a n n u a l crops. W h i l e their rates of c a r b o n d i o x i d e f i x a t i o n are generally l o w e r t h a n t h o s e of h i g h l y p r o d u c t i v e annuals, trees m a i n t a i n t h e i r a c t i v i t y for a longer p e r i o d , a n d because of their p e r m a n e n t b r a n c h s t r u c t u r e , c a n rapdily d e p l o y foliage in t h e s p r i n g t o c a p t u r e a large f r a c t i o n of t h e i n c i d e n t r a d i a t i o n . In a n n u a l s the process of c r o w n closure m u s t b e g i n all over again each s p r i n g , a l l o w i n g m u c h of t h e e n e r g y f l u x t o fall b e t w e e n plants. Evergreen conifers are p a r t i c u l a r l y p r o d u c t i v e over l o n g rotations, surpassing d e c i d u o u s a n g i o s p e r m s (14), d e s p i t e t h e l o w e r u n i t leaf rates of c a r b o n d i o x i d e f i x a t i o n o f t e n f o u n d in conifers. Evergreens c a n take rapid a d v a n t a g e of suitable c o n d i t i o n s for g r o w t h in s p r i n g or fall because t h e i r foliage is a l w a y s d i s p l a y e d . But in fact, their greatest a d v a n t a g e lies in l o w e r yearly costs of foliage p r o d u c t i o n . Evergreen foliage remains f u n c t i o n a l f r o m t w o t o several years, d e p e n d i n g u p o n species, w h e r e a s d e c i d u o u s trees m u s t reinvest in an e n t i r e n e w c a n o p y every year. For e x a m p l e , w i t h t h e s a m e a n n u a l i n v e s t m e n t in leaf. N o r w a y s p r u c e has t w o t o t h r e e t i m e s greater c a r b o n d i o x i d e u p t a k e t h a n t h e d e c i d u o u s species, European b e e c h , because s p r u c e retain its leaves for five to seven years (22). Nevertheless, t h e e f f i c i e n c y of e n e r g y p r o d u c t i o n a n d net e n e r g y yields are m u c h less t h a n t h e t h e o r e t i c a l limit, even in conifers. One possibility for i m p r o v e m e n t is t h r o u g h b r e e d i n g . O n l y in t h e last t w o decades has there been major effort in i n c o r p o r a t i n g i m p r o v e d varieties or lines into forest p l a n t i n g s , b u t none of these lines has been selected for e n e r g y p r o d u c t i o n . S o m e reviewers have s u g g e s t e d g e n e t i c i m p r o v e m e n t or e n v i r o n m e n t a l m a n i p u l a t i o n of energy c o n t e n t in tree biomass (Q. 2 6 . 2 8 ) ; for e x a m p l e , by s t i m u l a t i n g t h e p r o d u c t i o n of h y d r o c a r b o n s s u c h as oleoresins a n d latex, m o r e h i g h l y r e d u c e d c o m p o u n d s t h a n s t r u c t u r a l c a r b o h y d r a t e s like cellulose a n d l i g n i n . Since 1 9 4 5 , latex yield in r u b b e r trees w a s increased nearly six-fold by b r e e d i n g (8). But a trade-off of o n e c o m p o u n d for a m o r e h i g h l y e n e r g e t i c one w i l l not by itself increase t h e net e n e r g y yield per hectare. A pine can store either one g r a m of g l u c o s e w i t h a caloric value of 3.7 kcal or use t h e glucose t o p r o d u c e a more r e d u c e d c o m p o u n d , s u c h as lipid. For e x a m p l e , 0.32 g of p a l m i t i c acid w i t h a caloric value of 3 kcal can be p r o d u c e d f r o m 1.0 g g l u c o s e (Table VII). The a c t u a l loss of stored energy o c c u r s because s o m e of t h e g l u c o s e m u s t be respired to generate t h e A T P required in t h e


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c o n s t r u c t i o n of t h e lipid (17)· Very little of t h e energy is lost in g o i n g f r o m hexose sugars t o cellulose; m u c h is lost in p r o d u c t i o n of lipids, oleoresins, a n d o t h e r h i g h l y r e d u c e d c o m p o u n d s . The o n l y possible value of breeding for e n e r g y - r i c h c o m p o u n d s m a y be t o reduce h a n d l i n g a n d storage costs. C o n c e n t r a t i o n o f t h e s a m e h e a t i n g value in a smaller package m i g h t result in some e c o n o m y , b u t it seems d o u b t f u l t h a t it w o u l d offset t h e caloric loss. Table VII. LIPID YIELD F R O M GLUCOSE A S C A R B O N SKELETON


A N D ENERGY SOURCE P r o d u c t i o n value - w t . of l i p i d / w t . of substrate for C-skeleton (19) Energy r e q u i r e m e n t f a c t o r — mois A T P t o synthesize g r a m of lipid (19) Yield of A T P f r o m o x i d a t i o n of 1 m o l

0.351 0.05097 36

g l u c o s e t o C 0 a n d H 0 (17) 2

2

M o l e c u l a r w t . of g l u c o s e (2§) Heat of c o m b u s t i o n of o n e m o l g l u c o s e in k c a l (29) Heat of c o m b u s t i o n of o n e g g l u c o s e in kcal W e i g h t of g l u c o s e in C-skeleton of

180.16 673 3.7

p/0.351

ρ g r a m s of lipid W e i g h t of glucose needed t o s u p p l y energy for synthesis One g g l u c o s e —> 0.32 g lipid

0.05097p (180.16/36)

M o l e c u l a r w t . of p a l m i t i c a c i d , a lipid (29) Heat of c o m b u s t i o n of o n e m o l

254 2398.4

p a l m i t i c acid in kcal (29) Heat of c o m b u s t i o n of 0.32 g p a l m i t i c acid in kcal

3.0

T h e o n l y w a y t o increase e n e r g y yield per hectare is t o increase e n e r g y c a p t u r e a n d t h e e f f i c i e n c y of c o n v e r s i o n t o c a r b o h y d r a t e . A more r a p i d g r o w i n g s t a n d , or a g g r e g a t i o n of trees, w i l l be more p r o d u c t i v e of energy, a s s u m i n g its c h e m i c a l c o m p o s i t i o n is u n c h a n g e d , t h a n a s l o w - g r o w i n g stand. Breeding p r o g r a m s in s o u t h e r n pines have been successful in increasing v o l u m e p r o d u c t i o n a b o u t 15% in t h e first g e n e r a t i o n and increases of 2 5 % seem reasonable for t h e s e c o n d g e n e r a t i o n (B. J. Z o b e l . pers. c o m m u n .


458



1978). T h e p h y s i o l o g i c a l basis for these gains is u n k n o w n a n d c o u l d reflect b e t t e r utilization of i n c i d e n t solar radiation because of i m p r o v e d c a n o p y a r c h i t e c t u r e , ability t o utilize m o r e of t h e g r o w i n g season, a n d e n h a n c e d rate c a r b o n d i o x i d e a b s o r p t i o n a n d f i x a t i o n , a l o w e r rate of respiration for tissue m a i n t e n a n c e , or a r e d u c t i o n in losses to p a t h o g e n s a n d stress f a c t o r s of a n o n a c u t e t y p e . M a x i m u m o b s e r v e d e f f i c i e n c y for p h o t o s y n t h e s i s of poplar u n d e r intensive c u l t u r e w a s 3.5% of t h e v i s i b l e s p e c t r u m (22), for Serbian s p r u c e 7.9% ( I S ) , for maize 10.9% (7). T h e goal of breeders is t o increase e n e r g y yields b y r e a c h i n g t h e m a x i m u m t h e o r e t i c a l e f f i c i e n c y of 12% (15i). O n t h e o t h e r side of t h e c o i n , b r e e d i n g c o u l d help t o r e d u c e d e p e n d e n c e o n e n e r g y i n p u t t h e r e b y also increasing net e n e r g y y i e l d a n d s u b s t a n t i a l l y i m p r o v i n g t h e e f f i c i e n c y ratio. Fertilization is o n e of t h e m o s t c o s t l y i t e m s in s i l v i c u l t u r a l s c h e m e s for f u e l b i o m a s s p r o d u c t i o n . M a n u f a c t u r e a n d t r a n s p o r t of fertilizers in t h e s h o r t - r o t a t i o n , intensive c u l t u r e s c h e m e of Z a v i t k o v s k i (22) c o u l d a c c o u n t for nearly half of t h e t o t a l e n e r g y i n p u t . In S m i t h a n d J o h n s o n ' s s c h e m e , (24) 70% of t h e t o t a l i n p u t f o r site p r e p a r a t i o n a n d c u l t i v a t i o n w a s for fertilization. M o s t or all of these i n p u t s are related t o n i t r o g e n fertilization. If b r e e d i n g c o u l d d e v e l o p t y p e s less d e p e n d e n t u p o n m i n e r a l fertilizers, it w o u l d result in a major i m p r o v e m e n t in t h e e n e r g y b a l a n c e sheet. In fact, t h e r e are major differences in g e n e t i c response t o f e r t i l i z a t i o n , a n d o f t e n t h e g e n o t y p e s m o s t responsive t o fertilization are t h o s e t h a t are poorest w i t h o u t fertilization (\χ). S o m e g e n o t y p e s have a relatively stable p e r f o r m a n c e , o f t e n equal t o t h e fertilizer-responsive t y p e s w h e n fertilized a n d b e i n g g r e a t l y superior w h e n g r o w n w i t h less t h a n o p t i m u m levels of n i t r o g e n . A n o t h e r b u t m o r e r e m o t e possibility for i m p r o v e m e n t is t h r o u g h i n c o r p o r a t i o n of n e w s y m b i o n t s or n e w genes for n i t r o g e n f i x a t i o n in w o o d y plants. S o m e p l a n t s i m p o r t a n t t o a g r i c u l t u r e have n i t r i f y i n g bacteria, h o u s e d in special root s t r u c t u r e s , t h a t e x t r a c t gaseous n i t r o g e n f r o m t h e a t m o s p h e r e . T h e n i t r o g e n soon appears f i x e d in a m i n o a n d a m i d e f o r m s . L e g u m e s like alfalfa are t h e m o s t n o t a b l e e x a m p l e s of plants t h a t are host t o n i t r o g e n f i x i n g bacteria, a n d t h e y are o f t e n alternated w i t h o t h e r crops t o m a i n t a i n a n d i m p r o v e soil fertility. A m o n g w o o d y perennials, alders are c a p a b l e of c o n v e r t i n g a t m o s p h e r i c n i t r o g e n . T h r o u g h g e n e t i c e n g i n e e r i n g , it m a y s o m e d a y be possible t o i m p r o v e t h e g r o w t h of o t h e r a g r i c u l t u r a l a n d forest crops by t h e a d d i t i o n of n i t r i f y i n g c a p a c i t y . In t h e m e a n t i m e , proper c h o i c e of species or b r e e d i n g of trees w h i c h are e f f i c i e n t scavengers of soil n i t r o g e n m a y reduce t h e need for fertilization.


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W O O D PRODUCTS A N D RESIDUALS


Irrespective of h o w e f f i c i e n t l y forests c a n c a p t u r e e n e r g y is t h e q u e s t i o n o f w h e t h e r w o o d c a n be e c o n o m i c a l l y sold as fuel. A l l p r o j e c t i o n s s u g g e s t a c o n t i n u e d increase in d e m a n d f o r fiber a n d solid p r o d u c t s w h i c h w i l l be s u p p l i e d f r o m a d w i n d l i n g land base. N o t o n l y c o u l d w o o d increase in value relative t o a s u b s t i t u t e like c o a l , b u t t h e energy balance m i g h t be e n h a n c e d more b y p r o m o t i n g w o o d as a material t h a n as a fuel source. For e x a m p l e , f o r every B t u e x p e n d e d in c o n s t r u c t i n g a house o f w o o d , there w o u l d be 6 B t u for steel or 2 5 Btu f o r a l u m i n u m (23). Of course, this is t r u e for f u r n i t u r e or any o t h e r m a n u f a c t u r e d i t e m s in w h i c h material s u b s t i t u t i o n s are possible. T h u s , t h e best e n e r g y value o f w o o d m i g h t be its u t i l i t y f o r a diversity of p r o d u c t s t h a t are f r e q u e n t l y m a d e f r o m more energetically costly materials. A s a b y p r o d u c t of w o o d p r o c e s s i n g , extensive residuals are p r o d u c e d . Slabs, t h e r o u n d e d shell o u t s i d e t h e s a w n boards, a l w a y s c o n s t i t u t e d a h i g h p r o p o r t i o n of t h e log, b u t t h e t r e n d t o shorter, e c o n o m i c rather t h a n biologic rotations f o r c e d harvest o f smaller trees, a n d increased t h e p r o p o r t i o n of slab t o board. These residuals are already salvaged b y large mills. T h e y are c h i p p e d a n d sold f o r p u l p i n g or used t o s u p p l y heat a n d energy f o r mill o p e r a t i o n . O n a local scale, use of mill residuals m a y have a major i m p a c t . A n o t h e r class of residuals, a n d o n e n o t f r e q u e n t l y used, includes t h e branches, t w i g s , leaves, a n d roots left in t h e forest. Branches a n d leaves m a y c o n s t i t u t e a b o u t 3 5 % of t h e t o t a l biomass or an a m o u n t equal t o t h e s t e m biomass (2g). C h i p p i n g t h e t o p s in t h e forest is q u i t e practical and c o u l d have an i m p a c t o n local fuel needs. Roots represent 2 0 % of t h e a b o v e - g r o u n d biomass a n d c o n s t i t u t e a n o t h e r source of fuel. M e c h a n i z e d s y s t e m s f o r root e x t r a c t i o n are available, b u t t h e i m p a c t of root e x t r a c t i o n o n soil s t r u c t u r e a n d site p r o d u c t i v i t y m a y be u n f a v o r a b l e a n d . o f course, c o u l d not be used in systems of c o p p i c e regeneration. CONCLUSIONS P r o d u c t i o n of biomass b y forests is h i g h l y energy efficient. Purely e x p l o i t a t i v e s c h e m e s are more efficient t h a n h i g h l y intensive silviculture. H o w e v e r , n e t e n e r g y yield increases w i t h i n t e n s i t y of c u l t i v a t i o n , so silvicultural s y s t e m s a p p r o a c h i n g those o f a g r i c u l t u r a l c r o p p i n g s h o u l d be f a v o r e d f r o m a n energy p r o d u c t i o n s t a n d p o i n t . Efficiency c a n be f u r t h e r increased by b r e e d i n g , a n area n e g l e c t e d in forestry f o r c e n t u r i e s after it h a d b e c o m e a p r o v e n assist in a g r i c u l t u r e . T h e rate of p r o d u c t i o n o f biomass c a n be increased by b r e e d i n g for rapid g r o w t h . S i m u l t a n e o u s l y , it m a y be possible t o reduce e n e r g y i n p u t s by b r e e d i n g f o r trees t h a t d o n o t require s u p p l e m e n t a l fertilization or by e n g i n e e r i n g n e w s y m b i o t i c relationships w i t h n i t r o g e n - f i x i n g o r g a n i s m s .


460

BIOMASS AS A NONFOSSIL

F U E L SOURCE

T h o u g h p r o d u c t i o n of forest biomass is efficient, its c o n v e r s i o n to gaseous or l i q u i d fuels is not. Cellulose a n d lignin are m o r e d i f f i c u l t t o c o n v e r t t o alcohol or m e t h a n e t h a n sugars, a m a j o r c o m p o n e n t of biomass in some c r o p plants. Therefore, trees w i l l p r o b a b l y be used d i r e c t l y for b u r n i n g or perhaps in p y r o l i t i c c o n v e r s i o n , a process w h i c h holds s o m e p r o m i s e . M e r e l y increasing reliance o n w o o d in c o n s t r u c t i o n w i l l have a positive effect on w o r l d energy b u d g e t s because p r o d u c t i o n of s u b s t i t u t e s requires a h i g h e n e r g y e x p e n d i t u r e . P r o d u c t i o n of w o o d p r o d u c t s i n e v i t a b l y p r o d u c e s residuals. These c a n a n d are b e i n g used in e n e r g y p r o d u c t i o n a n d m a y have a m a j o r i m p a c t o n a local scale.


REFERENCES

1.

"Fuel Requirements for Harvesting Pulpwood"; American Pulpwood Association: Washington, D.C., 1975; p 15.

2. Blankenhorn, P. R.; Bowersox, T. W.; Murphy, W. K. Tappi 1978, 61, 5760. 3.

Boardman, Ν. K. "Proceedings", Fourth International Congress on Photosynthesis; Biochemical Society: London, 1977; 635-44.

4. Bowersox, T. W.; Ward, W. W. J. For. 1976, 74, 750-3. 5. Burks, J. E. "Proceedings"; 1978 Joint Conv. Soc. Am. For. and Can. Inst. For.; Soc. Am. For.: Washington, D.C., 1979; 146-48. 6.

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RECEIVED JUNE 18, 1980. Klass; Biomass as a Nonfossil Fuel Source ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Biomass as a Nonfossil Fuel Source - ACS Publications - American

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