Production of Nonwoody Land Plants as a Renewable Energy Source


Jan 29, 1981 - Non-Woody Land Plants in Perspective. Literally thousands of terrestrial plant species can be regarded as potential energy sources. A m...
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3 Production of Nonwoody Land Plants as a Renewable Energy Source A L E X G. A L E X A N D E R

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University of Puerto Rico, P.O. Box H , Agricultural Experiment Station, Rio Piedras, PR 00928

Non-Woody Land Plants in Perspective Literally thousands of terrestrial plant species can be regarded as potential energy sources. A majority of these are herbaceous seed plants which complete their growth and reproductive processes within a single growing season of a few months duration. They are widely distributed from arctic regions to the tropics (1-3). They are equally diverse with respect to their growth and anatomical characteristics, their cultural requirements, and their physiological and biochemical processes (2-9). Yet all have the capacity to convert sunlight to chemical energy and to store this energy in the form of biomass. A n oven-dry ton of herbaceous biomass represents about 15 Χ 10 Btu's of stored energy. The direct firing of one such ton, in a stoker furnace with high-pressure boiler having a 70% conversion efficiency, would displace about two barrels of fuel oil. 6

In addition to their fibrous tissues, some species also produce sugar and starch in sufficient quantities to warrant extraction and conversion to ethanol. The latter can displace petroleum in the production of motor fuel or chemical feedstocks (10-13). Other species store additional energy in the form of natural hydrocarbons.

0097-6156/81/0144-0049$07.00/0 © 1981 American Chemical Society

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

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B I O M A S S AS A N O N F O S S I L F U E L

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W h i l e it is not correct t o say t h a t herbaceous land plants have been overlooked as a d o m e s t i c energy resource, o n l y a small n u m b e r have been e x a m i n e d closely for t h i s purpose. A m o n g t h e latter are t r o p i c a l grass species of Zea, Sorghum, Saccharum. a n d Pennisetum w h i c h w e r e recognized for t h e i r h i g h yields of fiber a n d f e r m e n t a b l e solids long before t h e oil e m b a r g o of 1 9 7 3 . T h r o u g h o u t their history as c u l t i v a t e d crops, plants s u c h as c o r n , s w e e t s o r g h u m , s u g a r c a n e , a n d napier grass have e v o l v e d extensive t e c h n o l o g i e s for t h e i r c u l t i v a t i o n , harvest, post-harvest t r a n s p o r t a n d storage, a n d for their processing a n d m a r k e t i n g . Yet, even for t h e s e p l a n t s m a j o r c h a n g e s m u s t be m a d e in their m a n a g e m e n t if t h e y are t o serve e f f e c t i v e l y as e n e r g y crops (5.6.23.24). O t h e r t r o p i c a l p l a n t s h a v i n g very f i n e b o t a n i c a l or a g r o n o m i c a t t r i b u t e s a n d e n j o y i n g a y e a r - r o u n d c l i m a t e s u i t e d t o biomass p r o d u c t i o n have been generally i g n o r e d as e n e r g y resources. Pineapple, cassava, a n d a range of u n d e r u t i l i z e d t r o p i c a l species are a p p r o p r i a t e e x a m p l e s (4,8,13). A m a j o r i t y of h e r b a c e o u s l a n d plants have never b e e n c u l t i v a t e d for f o o d or fiber. In w a r m c l i m a t e s , w i l d grasses s u c h as Sorghum halepense (Johnson grass), Arundo donax (Japanese cane), a n d Bambusa species are b o r d e r l i n e cases w h e r e occasional use has been m a d e of t h e i r h i g h p r o d u c t i v i t y of d r y matter. In cooler c l i m a t e s , self-seeding plants s u c h as reed canary grass, c a t t a i l , w i l d oats, a n d o r c h a r d grass m a y be v i e w e d w i t h m i x e d f e e l i n g b y l a n d o w n e r s u n a b l e t o c u l t i v a t e m o r e v a l u a b l e f o o d or f o r a g e crops. Plants s u c h as r a g w e e d , redroot p i g w e e d , a n d l a m b s q u a r t e r s are recognized for t h e i r persistent g r o w t h habits w h i l e o t h e r w i s e regarded as c o m m o n pests. H o w e v e r , t h e v a l u e of s u c h species c o u l d rise d r a m a t i c a l l y as b i o m a s s assumes a role as a nonfossil d o m e s t i c e n e r g y resource. Prior S t u d i e s o n H e r b a c e o u s Plants as Energy S o u r c e s A s i d e f r o m s u g a r c a n e a n d " a l l i e d " t r o p i c a l grasses (6.7.13,23-25), relatively little a t t e n t i o n has been g i v e n t o herbaceous land p l a n t s specifically as sources of fuels a n d c h e m i c a l feedstocks. Studies w e r e initiated r e c e n t l y at B a t t e l l e - C o l u m b u s Laboratories o n c o m m o n grasses a n d w e e d s as p o t e n t i a l s u b s t i t u t e s for fossil e n e r g y (26). Plants s h o w i n g p r o m i s e as boiler fuels i n c l u d e perennial ryegrass, reed canarygrass, sudangrass, o r c h a r d g r a s s , bromegrass, K e n t u c k y 31 fescue, l a m b s q u a r t e r s , a n d others. A range of species have i n d i c a t e d s o m e p o t e n t i a l as sources of o i l , fats, p r o t e i n , dyes, alkaloids, a n d rubber. S u c h plants i n c l u d e g i a n t r a g w e e d , alfalfa, j i m s o n w e e d , c r a m b e , redroot p i g w e e d , d o g b a n , m i l k w e e d , a n d p o k e w e e d . Recently, a g o v e r n m e n t - s p o n s o r e d p r o g r a m w a s i n i t i a t e d t o screen herbaceous plants t o close t h e i n f o r m a t i o n g a p in this area of biomass e n e r g y d e v e l o p m e n t (27). This p r o g r a m has t w o phases: First, t o identify p r o m i s i n g

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

3.

ALEXANDER

Nonwoody Land Plants

51

species for w h o l e - p l a n t biomass p r o d u c t i o n in at least six different regions o f t h e U.S., a n d s e c o n d , t o p e r f o r m field e v a l u a t i o n s o n at least 2 0 species per region, w i t h a v i e w t o w a r d i d e n t i f y i n g t h o s e m o s t suitable f o r c r o p p i n g o n

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terrestrial e n e r g y p l a n t a t i o n s . A r t h u r D. Little, Inc., c o n d u c t e d Phase I (2). Six regions w e r e d e s i g n a t e d o n t h e basis of c l i m a t i c characteristics, land availability, a n d land resource d a t a p r o v i d e d b y t h e U.S. Soil Conservation Service (2). A list of 2 8 0 p o t e n t i a l species w a s prepared o n t h e basis of p u b l i s h e d literature a n d personal i n t e r v i e w s . These w e r e screened in a c c o r d a n c e w i t h b o t a n i c a l a n d e c o n o m i c characteristics, w i t h e m p h a s i s o n previously u n c u l t i v a t e d species. Certain a g r i c u l t u r a l plants w e r e also considered. Factors s u c h as yield p o t e n t i a l , c u l t u r a l r e q u i r e m e n t s , tolerances t o p h y s i o l o g i c a l stress, p r o d u c t i o n costs, a n d land availability w e r e c o n s i d e r e d in r a n k i n g t h e c a n d i d a t e species o f each region (2). Plants w i t h yields less t h a n 2.2 t o n s / a c r e (5 m e t r i c t o n s / h e c t a r e ) w e r e e l i m i n a t e d . For t h e p o t e n t i a l energy c r o p species, c o m p a r i s o n s w e r e d r a w n w i t h six categories of e c o n o m i c plants, i n c l u d i n g tall a n d short broadleaves, tall a n d short grasses, l e g u m e s , a n d tubers. S o m e 7 0 species w e r e r e c o m m e n d e d f o r c o n s i d e r a t i o n in t h e p r o g r a m ' s s e c o n d phase (field screening). S o m e o f these plants (redroot p i g w e e d , l a m b s q u a r t e r s . Colorado river h e m p , ragweed) have n o prior history as c u l t i v a t e d crops a n d their c u l t u r a l needs remain obscure. Other species (Bermuda grass. Kenaf. reed canary grass, s u d a n grass) have been i m p r o v e d a n d c u l t i v a t e d f o r decades (2). BOTANICAL A N D AGRONOMIC CONSIDERATIONS This initial p r o g r a m t o evaluate herbaceous land plants w i l l help t o clarify their value as a r e n e w a b l e e n e r g y source. H o w e v e r , a n e x t e n s i v e research effort is needed t o c o m p l e t e t h i s task even as it applies t o e x i s t i n g plant f o r m s already m a n a g e d as a g r i c u l t u r a l crops. A c o n t i n u i n g effort w i l l be needed over a period o f several decades in t h e areas o f n e w species e v a l u a t i o n , g e n e t i c i m p r o v e m e n t , herbaceous plant c r o p p i n g o n m a r g i n a l lands, a n d c r o p t a i l o r i n g t o c h a n g i n g e n e r g y needs. T h e r e m a i n d e r o f t h i s paper offers s o m e general g u i d e l i n e s a n d c o n s i d e r a t i o n s f o r d e a l i n g w i t h t h e vast pool o f e x i s t i n g herbaceous land plants.

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

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Botanical Considerations Photosynthesis: P h o t o s y n t h e s i s is t h e process by w h i c h t h e e n e r g y of s u n l i g h t is c o n v e r t e d t o c h e m i c a l e n e r g y by plants. Its reaction c a n be s t a t e d s i m p l y as:

radiant overall

Sunlight

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C0

2

+ H

2

0 • (CH 0) + 0 Green Plants 2

2

T h e a m o u n t of e n e r g y r e t a i n e d in t h e p h o t o s y n t h a t e ( C H 0 ) is a b o u t 4 6 8 2

kJ/mole. A l t h o u g h not an e f f i c i e n t process, it is t h e o n l y s y s t e m of solar e n e r g y c o n v e r s i o n o n e a r t h t h a t has o p e r a t e d at a n y a p p r e c i a b l e m a g n i t u d e a n d w i t h a n y a p p r e c i a b l e e c o n o m y for a n y a p p r e c i a b l e period of t i m e . A n e s t i m a t e d 1 3 5 0 J / m arrives at t h e earth's u p p e r a t m o s p h e r e in t h e f o r m of solar r a d i a t i o n , b u t o n l y a b o u t half p e n e t r a t e s t o t h e earth's surface (28). A t h e o r e t i c a l 8 p e r c e n t of t h i s r a d i a t i o n c o u l d be c o n v e r t e d p h o t o s y n t h e t i c a l l y ; h o w e v e r , a m a x i m u m c o n v e r s i o n e f f i c i e n c y of o n l y 4 p e r c e n t has been a t t a i n e d a n d t h i s u n d e r c o n d i t i o n s of l o w l i g h t i n t e n s i t y (29). A g r i c u l t u r a l plants average p e r h a p s 0.5 t o 1.0 p e r c e n t efficiency. L a n d plants o n a w o r l d w i d e basis p r o b a b l y average less t h a n 0.3 p e r c e n t efficiency. Nonetheless, t h e earth's p l a n t s store a n n u a l l y a b o u t 10 t i m e s m o r e e n e r g y t h a n is utilized, a n d s o m e 2 0 0 t i m e s m o r e t h a n is c o n s u m e d a n n u a l l y as f o o d (30). 2

P h o t o s y n t h e s i s consists of t w o phases: (a) Energy c a p t u r e , y i e l d i n g c h e m i c a l e n e r g y a n d r e d u c i n g p o w e r ; a n d (b). t h e r e d u c t i o n or " a s s i m i l a t i o n " of a t m o s p h e r i c C 0 . T h e c a r b o n r e d u c t i o n phase is a c c o m p l i s h e d by three d i s t i n c t p a t h w a y s ( C , C , a n d C A M ) . Each p a t h w a y is f o u n d a m o n g t h e w o r l d ' s h e r b a c e o u s land p l a n t s , b u t t h e C p a t h w a y is t h e m o s t w i d e l y d i s t r i b u t e d . C A M plants, w h i c h assimilate c a r b o n at n i g h t , are relatively less i m p o r t a n t even t h o u g h t h e i r utilization of w a t e r is generally m o r e efficient t h a n for C 3 species. The C p a t h w a y w a s at first t h o u g h t t o reside only in s u g a r c a n e a n d related t r o p i c a l grasses (31-33). It w a s soon f o u n d in o t h e r plants s u c h as Zea. Sorghum, a n d Amaranthus (34-38). T h e C species c o n s t i t u t e a k i n d of apex in p h o t o s y h t h e t i c p r o f i c i e n c y , a i d e d t o s o m e e x t e n t by a t t r i b u t e s s u c h as a l o w C 0 c o m p e n s a t i o n p o i n t , a " l a c k " of p h o t o r e s p i r a t i o n , a n d a c a p a b i l i t y t o utilize b o t h l o w e r a n d h i g h e r light intensities better t h a n C species (5.9,39-41}. 2

3

4

3

4

4

2

3

A n i m p o r t a n t aspect of p h o t o s y n t h e t i c e n e r g y c o n v e r s i o n o f t e n overlooked in h i g h e r plants is their " s p e c t r a l p r o f i c i e n c y " , t h a t is. t h e i r ability t o c o n v e r t

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

3.

ALEXANDER

53

Nonwoody Land Plants

d i f f e r e n t regions of t h e sun's spectral e n e r g y d i s t r i b u t i o n . W h e n

photo-

s y n t h e s i s b y a g i v e n leaf is m e a s u r e d at d i f f e r e n t w a v e l e n g t h s o f equal q u a n t u m f l u x , say f r o m 4 0 0 n m in t h e b l u e - v i o l e t t o 7 2 0 n m in t h e f a r - r e d . a p h o t o s y n t h e t i c a c t i o n s p e c t r u m is a t t a i n e d w h i c h tells us m u c h a b o u t t h e leaf's a b i l i t y t o " h a r v e s t " t h e entire package of visible light e n e r g y received f r o m t h e s u n . W i t h s u f f i c i e n t replications, an a c t i o n s p e c t r u m c h a r a c t e r i s t i c of t h e species is d e r i v e d , a kind o f spectral f i n g e r p r i n t c o m p l e t e w i t h peaks a n d depressions t y p i f y i n g t h a t species. Ironically, m o r e t h a n 6 0 p e r c e n t o f i n c o m i n g solar e n e r g y is received a t w a v e l e n g t h s shorter t h a n 5 5 0 n m , w h i l e

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(apparently) m o s t p l a n t s are p h o t o s y n t h e t i c a l l y a c t i v e at w a v e l e n g t h s longer t h a n 6 0 0 n m . There is s o m e e v i d e n c e t h a t Saccharum

and a f e w other

species have m a j o r p h o t o s y n t h e s i s a c t i v i t y in t h e blue-violet t o b l u e - g r e e n region (40,41). P h o t o s y n t h e t i c

a c t i o n s p e c t r a have been d e t e r m i n e d f o r

a p p r o x i m a t e l y 3 0 a g r i c u l t u r a l p l a n t s T h e vast m a j o r i t y o f h e r b a c e o u s land p l a n t s have n o t been e x a m i n e d in t h i s c o n t e x t . Photosynthesis in an Energy Crop Perspective:

A plant p h y s i o l o g i s t or

b i o c h e m i s t m e a s u r i n g p h o t o s y n t h e s i s in t h e laboratory usually d e t e r m i n e s t h e q u a n t i t y of C O 2 a s s i m i l a t e d per u n i t o f leaf area in a n hour or s o m e o t h e r c o n v e n i e n t t i m e interval (mg C 0 2 / c m - h r " 2

1

). It does n o t necessarily f o l l o w

t h a t superior a s s i m i l a t i o n rates n o t e d u n d e r these c o n d i t i o n s w i l l translate t o high

photosynthate

yields in t h e field. A m o r e c o n v e n i e n t

measure o f

p h o t o s y n t h e t i c p o t e n t i a l in b i o m a s s - c a n d i d a t e species is t h e q u a n t i t y of d r y m a t t e r p r o d u c e d per square m e t e r o f leaf surface per d a y (g D M / m - d a y ) . A 2

m a j o r i t y o f h e r b a c e o u s land p l a n t s w o u l d p r o d u c e on t h e order o f 2-8 g r a m s of o v e n - d r y material per square m e t e r per day, during growth

or tissue-expansion

the peak

of

their

phase. A yield of 15 g / m - d a y w o u l d be q u i t e

good and w o u l d typify some C

2

4

p a t h w a y species. Potential m a x i m u m y i e l d

e s t i m a t e s have been placed at 3 4 t o 3 9 g / m

2

-day f o r C

3

plants a n d 5 0 - 5 4

g / m - d a y for C plants (46). 2

4

To a n y e n e r g y

planter, t h e m o s t

meaningful

measure

of solar

energy

c o n v e r s i o n t o b i o m a s s is t h e n u m b e r o f k i l o g r a m s of d r y m a t t e r p r o d u c e d per hectare per year (or t o n s per acre per year). W h i l e p h o t o s y n t h e t i c processes per se r e m a i n a n i m p o r t a n t factor, e q u a l l y i m p o r t a n t are all other processes a n d c o n s t r a i n t s of p l a n t g r o w t h a n d d e v e l o p m e n t w h i c h c o m e into play as p h o t o s y n t h a t e is e l a b o r a t e d t o harvestable biomass. Each o f these factors f i n d s expression in t h e e n e r g y planter's gross yield of biomass. A n n u a l d r y m a t t e r yields in t h e order of 2 2 . 5 0 0 k g / h a (10 t o n s / a c r e ) are c o m m o n f o r a f e w species b u t t h e m a j o r i t y of herbaceous land plants p r o b a b l y yield less t h a n 4 5 0 0 k g / h a (2 t o n s / a c r e ) .

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

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B I O M A S S A S A N O N F O S S I L FUEL SOURCE

T h e r e c k o n i n g of dry m a t t e r yields on an a n n u a l basis rather t h a n an h o u r l y or a daily basis m i g h t seem i n a p p r o p r i a t e t o n o n - w o o d y species w h o s e g r o w t h period lasts only a f e w w e e k s or m o n t h s . H o w e v e r , it is correct t o d o so since m a n y of t h e e n e r g y planter's expenses ( i n c l u d i n g land rentals, taxes, e q u i p m e n t d e p r e c i a t i o n , a n d land m a i n t e n a n c e ) are i n c u r r e d o n an a n n u a l basis (9,47.48). M o r e o v e r , s o m e herbaceous p l a n t species d o p r o d u c e dry m a t t e r c o n t i n u a l l y t h r o u g h o u t t h e year a n d o t h e r s c o u l d d o so if m a n a g e d as e n e r g y crops. A p l a n t s u c h as s u g a r c a n e p r o p a g a t e d as a 1 2 - m o n t h sugar c r o p c a n yield dry m a t t e r at t h e rate of 1 0 - 1 2 g / m -day, or a b o u t 10 t o n s / a c r e - y e a r . T h e h i g h e s t d r y m a t t e r yields a t t a i n e d t o d a t e b y t h e a u t h o r w e r e w i t h f i r s t - r a t o o n s u g a r c a n e m a n a g e d f o r t o t a l b i o m a s s rather t h a n sugar. These a m o u n t e d t o 36.6 t o n s / a c r e - y e a r , or 26.6 g / m - d a y , over a t i m e c o u r s e of 3 6 5 d a y s (49).

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2

2

It is safe t o say t h a t for m o s t plants, t h e r e is no d i r e c t relationship b e t w e e n p h o t o s y n t h e t i c p o t e n t i a l , as d e t e r m i n e d in t h e laboratory, a n d t h e t o t a l dry b i o m a s s t o be harvested in t h e field. T h e p r i n c i p a l reasons for t h i s are a series of b o t a n i c a l a n d a g r o n o m i c factors w h i c h p r e v e n t t h e e l a b o r a t i o n of p h o t o s y n t h a t e t o b i o m a s s at rates c o m m e n s u r a t e w i t h t h e plant's c a r b o n r e d u c t i o n p o t e n t i a l . S o m e of these f a c t o r s are f u n d a m e n t a l c o n s t r a i n t s against g r o w t h a n d d e v e l o p m e n t essentially b e y o n d t h e c o n t r o l of t h e e n e r g y planter ( t h o u g h s o m e t i m e s c o n t r o l l a b l e by t h e p l a n t breeder). Other c o n s t r a i n t s are a r e f l e c t i o n of plant m a n a g e m e n t a n d c a n be e l i m i n a t e d t h r o u g h research a n d d e v e l o p m e n t of t h e species as an e n e r g y c r o p . It is also safe t o say t h a t s o m e n o n - w o o d y land plants w i l l be f o u n d t o have g o o d biomass p o t e n t i a l s b u t little prospect of ever b e i n g m a n a g e d as a g r i c u l t u r a l e n e r g y c o m m o d i t i e s . For s u c h plants, a decisive a t t r i b u t e w i l l be t h e i r ability t o s u r v i v e a n d p r o d u c e s o m e b i o m a s s w i t h t h e barest m i n i m u m of p r o d u c t i o n i n p u t s (8.9). Yet even in these instances one m u s t n o t overemphasize p h o t o s y n t h e s i s rate as an e n e r g y yield i n d i c a t o r ; t h e r e is s i m p l y t o o m u c h variability in t h e measured rates of p h o t o s y n t h e s i s and t o o little correlation w i t h m e a s u r e d biomass (5,33,50). A n e x a m p l e of t h i s w a s f o u n d in a series of " w i l d " sugarcanes (Saccharum species) w h o s e p h o t o s y n t h e s i s rates varied by a f a c t o r of 10 w h i l e t h e i r b i o m a s s yields varied by a f a c t o r less t h a n 2 (40). V a r i a t i o n is similarly h i g h a m o n g t h e h y b r i d sugarcanes of c o m m e r c e (33,51). ' g i v e n field of sugarcane, c o m p l e t e l y u n i f o r m as t o soil series, variety, p l a n t i n g d a t e , a n d c u l t u r a l m a n a g e m e n t , one can e x p e c t to f i n d p h o t o s y n t h e s i s a n d g r o w t h rates t h a t vary by a f a c t o r of 3 t o 5 a m o n g r a n d o m l y - s e l e c t e d s a m p l i n g sites (5). n

a

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

3.

ALEXANDER

55

Nonwoody Land Plants

Reduction State of t h e Primary Photosynthate: T o this p o i n t w e have c o n s i d e r e d biomass as " e l a b o r a t e d p h o t o s y n t h a t e " , c o n s i s t i n g m a i n l y o f cellulose, a n d lignin d e r i v e d f r o m g l u c o s e or p o l y g l u c o s i d e s h a v i n g t h e basic f o r m u l a ( C H 0 ) . This is q u a n t i t a t i v e l y t h e m o s t i m p o r t a n t f o r m o f biomass for b o t h w o o d y a n d herbaceous p l a n t species. H o w e v e r , as a f o r m of stored energy it has t h e l i m i t a t i o n of b e i n g o n l y partially r e d u c e d . T h e presence of o x y g e n in t h e s t r u c t u r e o f plant tissues, s t a r c h , a n d e x t r a c t a b l e sugars limits t h e e n e r g y c o n t e n t of s u c h materials t o a p p r o x i m a t e l y 1 4 - 1 6 Χ 1 0 Btu's per dry t o n . A l t e r n a t e l y , s o m e plant species store e n e r g y in 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 h a v i n g progressively less o x y g e n in their s t r u c t u r e . Plant materials s u c h as isoprene p o l y m e r s , sterols, oils a n d w a x e s consist m a i n l y of c a r b o n a n d h y d r o g e n a n d c o n t a i n o n t h e order o f 4 0 - 5 0 X 1 0 Btu's per dry t o n . Calvin a n d others have a d v o c a t e d t h e s t u d y o f " h y d r o c a r b o n p l a n t s " as superior b i o m a s s e n e r g y sources (21.22.52). M a n y of these species have t h e a d d e d a d v a n t a g e of g o o d a d a p t a b i l i t y t o lands t h a t are s e m i - a r i d , r o u g h l y c o n t o u r e d , a n d o t h e r w i s e m a r g i n a l f o r t h e p r o d u c t i o n of more c o n v e n t i o n a l f o o d a n d e n e r g y crops (8.53.54). 6

1 2

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6

6

H y d r o c a r b o n - b e a r i n g plants i n c l u d e b o t h w o o d y a n d herbaceous species. S o m e of t h e b e t t e r - k n o w n e x a m p l e s , s u c h as t h e r u b b e r tree (Hevea brasiliensis) a n d g u a y u l e (Parthenium argentatum), are w o o d y perennials, w h i l e others, s u c h as Euphorbia a n d Calotropis species, are borderline cases t h a t c o u l d be m a n a g e d either as forest or a g r o n o m i c energy crops. M i l k w e e d species (Asclepidacea) are p r e d o m i n a n t l y herbaceous, b u t o n e m e m b e r f o u n d in t h e t r o p i c s , Calotropis procera (the " g i a n t m i l k w e e d " ) , is a w o o d y perennial r e a c h i n g h e i g h t s o f 9 t o 12 feet over a period of several years. In Puerto Rico it is regarded as a forest s p e c i m e n (55). b u t as an energy c r o p w o u l d m o s t likely be m a n a g e d as a f r e q u e n t l y reçut forage (56). Water Utilization Efficiency: W a t e r w i l l q u i t e d e f i n i t e l y be a decisive l i m i t i n g f a c t o r in t h e w o r l d w i d e e x p a n s i o n of a g r i c u l t u r e (9,54,57,58). It is therefore i m p o r t a n t t h a t w a t e r utilization e f f i c i e n c y be c o n s i d e r e d in t h e f u t u r e s c r e e n i n g a n d d e v e l o p m e n t o f herbaceous land plants as energy resources. Three factors m u s t be assessed f r o m t h e onset: (a) Utilization e f f i c i e n c y in p h o t o s y n t h e t i c processes; (b) w a t e r e x t r a c t i n g c a p a b i l i t y f r o m t h e c a n d i d a t e species' natural t e r r a i n ; a n d (c), t h e species c a p a c i t y for w a t e r c o n s e r v a t i o n b y a n a t o m i c a l means. Agronomic Considerations The p r o d u c t i o n of biomass involves t h e c o l l a b o r a t i o n of p h y s i o l o g i c a l , b i o c h e m i c a l , b o t a n i c a l , a n d a g r o n o m i c factors u n d e r any set o f c o n d i t i o n s . H o w e v e r , f o r t h e intensive m a n a g e m e n t of biomass p r o d u c t i o n , p a r t i c u l a r

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

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a t t e n t i o n m u s t be g i v e n to field-scale b e h a v i o r of plant masses in w h i c h an i n d i v i d u a l p l a n t or c r o w n c o m p l e x loses t h e i m p o r t a n c e w e a t t a c h t o it as a b o t a n i c a l or h o r t i c u l t u r a l e n t i t y . Several a g r o n o m i c c o n s i d e r a t i o n s c r i t i c a l t o successful biomass p r o d u c t i o n are herein d i s c u s s e d . G r o w t h Characteristics: T o a t t a i n m a x i m u m biomass o n a per a n n u m basis, o n e w o u l d ideally select a y e a r - r o u n d g r o w i n g season a n d p l a n t species c a p a b l e of g r o w i n g o n a y e a r - r o u n d basis. Certain t r o p i c a l grasses (sugarcane, napier grass. J o h n s o n grass, b a m b o o ) d o t h i s very nicely if p l a n t e d in t h e t r o p i c s . S o m e of t h e i r m e m b e r s p r o d u c e w e l l also in s u b t r o p i c a l or even t e m p e r a t e regions, b u t g i v e n equal m a n a g e m e n t , t h e y w i l l realize o n l y part of t h e i r f u l l y i e l d p o t e n t i a l w h e n g r o w t h is c o n s t r a i n e d for several m o n t h s by c o o l t e m p e r a t u r e s . It is i m p o r t a n t t o recognize also t h a t g r o w t h is a 2 4 - h o u r process as w e l l as a 1 2 - m o n t h process. T h e p h o t o s y n t h e t i c a n d t i s s u e - e x p a n s i o n s y s t e m s t h a t o p e r a t e each d a y are f u l l y d e p e n d e n t o n t h e n o c t u r n a l t r a n s p o r t a n d m o b i l i z a t i o n of g r o w t h - s u p p o r t i n g c o m p o u n d s . For t h i s reason, t h e t r o p i c s are a g a i n f a v o r e d by t h e i r w a r m n i g h t s for b i o m a s s p r o d u c t i o n . In a similar v e i n , t h e cool n i g h t s of t h e s o u t h w e s t e r n arid lands are p r o b a b l y as r e s t r i c t i v e for b i o m a s s as are t h e l i m i t e d m o i s t u r e supplies. Possibly t h e m o s t desirable g r o w t h c h a r a c t e r i s t i c of all for h e r b a c e o u s species is t h e a b i l i t y t o p r o d u c e n e w shoots c o n t i n u a l l y t h r o u g h o u t t h e year, year after year, f r o m an established c r o w n . This is a p r e d o m i n a n t c h a r a c t e r i s t i c of s u g a r c a n e a n d c e r t a i n o t h e r t r o p i c a l grasses b o t h related a n d u n r e l a t e d t o Saccharum species. S u c h p l a n t s d o n o t require t h e p e r i o d i c d o r m a n c y a n d rest intervals so i m p o r t a n t t o m o s t t e m p e r a t e species. Nor is t h i s c o m p e n s a t e d by t h e intensive f l u s h of M a y - J u n e g r o w t h by t e m p e r a t e p l a n t s ; over t h e course of a year t h e s l o w e r - g r o w i n g t r o p i c a l f o r m s w i l l o u t p r o d u c e t h e m by a f a c t o r of three or four. A less o b v i o u s b u t u t t e r l y c r i t i c a l f e a t u r e of t h e perennial c r o w n is its c o n t i n u a l u n d e r g r o u n d c o n t r i b u t i o n of d e c a y i n g o r g a n i c m a t t e r t o t h e soil. This process p r o c e e d s c o n c u r r e n t l y w i t h t h e c o n t i n u o u s r e n e w a l of u n d e r g r o u n d c r o w n a n d root tissues. For t h i s reason, t h e l o n g - t e r m harvest a n d r e m o v a l of a b o v e g r o u n d s t e m s , t o g e t h e r w i t h t h e b u r n i n g off of " t r a s h " , does n o t have an adverse effect o n s u g a r c a n e lands. There are soils in Puerto Rico t h a t have p r o d u c e d s u g a r c a n e m o r e or less c o n t i n u a l l y f o r f o u r c e n t u r i e s w i t h o u t d e s t r u c t i o n of t h e i r p h y s i c a l properties or n u t r i e n t - s u p p l y i n g c a p a b i l i t y . On t h e o t h e r h a n d , seasonal crops s u c h as field c o r n a n d grain s o r g h u m d o not d e v e l o p a perennial c r o w n . For these plants, a g o o d case can be m a d e a g a i n s t t h e r e m o v a l of a b o v e g r o u n d residues f r o m t h e c r o p p i n g site.

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

3.

ALEXANDER

Nonwoody Land Plants

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Tissue Expansion v s . Maturation: A c o m m o n m i s c o n c e p t i o n is t h a t b i o m a s s g r o w t h involves m a i n l y a visible increase o f size, a n d t h a t per acre t o n n a g e s of green m a t t e r are a reasonably a c c u r a t e i n d i c a t o r of a plant's yield p o t e n t i a l . It is also f r e q u e n t l y a s s u m e d t h a t t h e m o i s t u r e c o n t e n t o f plant tissues is essentially c o n s t a n t a t a r o u n d 7 5 percent, a n d t h a t dry m a t t e r yields c a n be c a l c u l a t e d rather closely f r o m green w e i g h t data. These a s s u m p t i o n s are n o t correct in a n y case, b u t are p a r t i c u l a r l y erroneous w i t h respect t o h e r b a c e o u s species. In v i r t u a l l y all s u c h plants " g r o w t h " consist o f discrete, d i p h a s i c processes o f tissue e x p a n s i o n f o l l o w e d b y m a t u r a t i o n . The tissue e x p a n s i o n phase p r o d u c e s visible b u t s u c c u l e n t g r o w t h c o n s i s t i n g m a i n l y o f w a t e r (on t h e order o f 8 8 - 9 2 p e r c e n t moisture). T h e m a t u r a t i o n phase c o r r e s p o n d s t o p h y s i o l o g i c a l a g i n g a n d senescence, t h a t is, t o f l o w e r i n g a n d seed p r o d u c t i o n , s l a c k e n i n g of visible g r o w t h , y e l l o w i n g a n d loss o f foliage, a n d h a r d e n i n g o f t h e f o r m e r l y s u c c u l e n t tissues. D u r i n g t h i s p e r i o d , t h e dry m a t t e r c o n t e n t w i l l increase b y a f a c t o r o f t w o t o four in a t i m e interval t h a t m a y be shorter t h a n t h a t of t h e t i s s u e - e x p a n s i o n phase. For e x a m p l e , t h e h y b r i d forage grass Sordan 7 0 A m o r e t h a n d o u b l e s its d r y m a t t e r yield in a t i m e - s p a n o f o n l y t w o w e e k s (23). i.e.. d u r i n g w e e k s 8 t o 1 0 in a 1 0 - w e e k g r o w t h a n d r e p r o d u c t i o n cycle. For this reason, t h e o p t i m a l period of harvest m u s t be d e t e r m i n e d w i t h care for each c a n d i d a t e species. A g a i n , as a rule o f t h u m b , t h e a l l o w i n g o f a d d i t i o n a l t i m e before harvest w i l l w o r k in favor of increased b i o m a s s yields f r o m herbaceous plants. For m o s t herbaceous plants, t h e p r o d u c t i o n of d r y m a t t e r can be p l o t t e d as an S-shaped c u r v e (Figure I). Dry m a t t e r c o n t e n t w i l l not ordinarily e x c e e d 10 t o 12 p e r c e n t d u r i n g t h e period o f rapid tissue e x p a n s i o n b u t w i l l b e g i n t o rise d r a m a t i c a l l y at s o m e p o i n t in t i m e t h a t is c h a r a c t e r i s t i c of t h e i n d i v i d u a l species. D r y m a t t e r w i l l rarely increase b e y o n d 4 0 p e r c e n t in herbaceous plants. A t t e m p t s t o hasten t h i s rise (by w i t h h o l d i n g w a t e r ) or t o delay it (by use o f g r o w t h s t i m u l a n t s ) have m e t w i t h l i m i t e d success in t r o p i c a l grasses (63). S o m e increase in t h e m a g n i t u d e of d r y m a t t e r a c c u m u l a t i o n has been a t t a i n e d over short periods o f t i m e w i t h t h e plant g r o w t h regulator Polaris (63). Harvest Frequency: O n c e t h e d i p h a s i c n a t u r e of biomass g r o w t h a n d m a t u r a t i o n is r e c o g n i z e d , t h e i m p o r t a n c e o f harvest f r e q u e n c y is also u n d e r s c o r e d . T h e o p t i m a l period f o r harvest in t h e m a t u r a t i o n c u r v e of one species w i l l differ e n o r m o u s l y f r o m t h e o p t i m a l harvest period of another, even a m o n g varieties w i t h i n t h e s a m e g e n u s a n d species. For t h i s reason, it is c o n v e n i e n t t o g r o u p c a n d i d a t e species i n t o d i s t i n c t categories based o n t h e t i m e interval t h a t m u s t elapse after p l a n t i n g t o m a x i m i z e d r y m a t t e r yield (63). The m a n a g e m e n t a n d harvest r e q u i r e m e n t s of each g r o u p w i l l also vary. On t h i s basis, it has been c o n v e n i e n t t o organize t r o p i c a l grasses i n t o " s h o r t - , i n t e r m e d i a t e - , a n d l o n g - r o t a t i o n " categories (Table I).

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T a b l e I. C A T E G O R I E S O F T R O P I C A L G R A S S E S A N D L E A D I N G CANDIDATE CLONES UNDER INVESTIGATION A S R E N E W A B L E ENERGY SOURCES IN PUERTO RICO*

Category

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I. Short Rotation

Harvest Interval (Months) 2-4

Candidate Clones Sordan 7 0 A Sordan 7 7

b

b

Trudan 5 Millex 23 B e r m u d a Grass NK Hybrids Roma (Sorghum) II. I n t e r m e d i a t e Rotation

4-6

C o m m o n Napier Grass (Var. Meker) Napier H y b r i d PI 3 0 0 8 6 Napier H y b r i d PI 7 3 5 0 NK Hybrids Saccharum spontaneum: b

US 6 7 - 2 2 - 2 US 7 7 - 7 0 SES 2 3 1 S. spont. H y b r i d (Wild) Intergeneric Hybrids III. L o n g Rotation

12-18

Saccharum Hybrids NCo 3 1 0 PR 9 8 0 PR 6 4 - 1 7 9 1 Β 70-701 US 6 7 - 2 2 - 2 USDA Imports b

b

" b

DOE C o n t r a c t No. D E - A S 0 5 - 7 8 E T 2 0 0 7 1 . Leading c a n d i d a t e s for t h e i r catetory.

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

3.

ALEXANDER

Nonwoody Land Plants

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A s illustrated in Figure II. t h e tissue m a t u r a t i o n curves for t y p i c a l m e m b e r s of each c a t e g o r y vary greatly over a t i m e - c o u r s e o f 12 m o n t h s . Hence, t o harvest s u g a r c a n e at t h e 1 0 - w e e k intervals favorable t o Sordan 7 0 A w o u l d yield little d r y matter. Similarly, any delay of t h e Sordan harvest b e y o n d 12 w e e k s is a w a s t e of t i m e a n d p r o d u c t i o n resources. Napier grass, an " i n t e r m e d i a t e r o t a t i o n " species, is m o r e t h a n a m a t c h f o r s u g a r c a n e at t w o a n d f o u r - m o n t h s of age, a n d w i l l nearly equal s u g a r c a n e yields at six m o n t h s , b u t thereafter s u g a r c a n e w i l l easily o u t - p r o d u c e napier grass. In this c o n t e x t , a s h o r t - r o t a t i o n species s h o u l d be harvested four or five t i m e s per year, an i n t e r m e d i a t e - r o t a t i o n species t w o or three t i m e s per year, a n d l o n g - r o t a t i o n species n o m o r e t h a n o n c e per year. This need f o r careful a t t e n t i o n t o t h e m a t u r a t i o n profiles of c a n d i d a t e species is u n d e r s c o r e d b y yield data f o r sugarcane a n d napier grass harvested at variable intervals over a t i m e - c o u r s e of 12 m o n t h s (Table II). It is also e v i d e n t t h a t , w h i l e Sordan a n d napier grass attain rather level plateaus f o r d r y m a t t e r , s u g a r c a n e c o n t i n u e s t o increase in d r y m a t t e r b e y o n d 12 m o n t h s (Figure II). Sucrose a c c u m u l a t i o n profiles are very similar for sugarcane. For m a n y years, sugar planters have taken a d v a n t a g e of t h i s feature b y e x t e n d i n g t h e cane harvest interval b e y o n d 12 m o n t h s . Hence, t h e Puerto Rico sugar i n d u s t r y harvests t w o crops - t h e " g r a n c u l t u r a " (14 t o 16 m o n t h s b e t w e e n harvests) as o p p o s e d t o t h e p r i m a v e r a crop (10 t o 12 m o n t h s b e t w e e n harvests). In H a w a i i , s u g a r c a n e is c o m m o n l y harvested at t w o - y e a r intervals. E n e r g y C r o p R o t a t i o n s : From Figure II. one w o u l d s u r m i s e t h a t t h e e n e r g y p l a n t a t i o n m a n a g e r s h o u l d plant a herbaceous species s u c h as s u g a r c a n e a n d leave it there - u p t o 1 8 m o n t h s if possible — before harvest. In a d d i t i o n t o m a x i m u m fiber, he w o u l d also harvest f e r m e n t a b l e solids as a salable b y p r o d u c t . This reasoning w o u l d p r o b a b l y be c o r r e c t in a t r o p i c a l e c o s y s t e m suited t o Saccharum species a n d w h e r e a regional t r a d i t i o n exists f o r sugar p l a n t i n g . H o w e v e r , these c i r c u m s t a n c e s d o n o t exist in m a n y c o u n t r i e s h a v i n g an o t h e r w i s e g o o d p o t e n t i a l f o r g r o w i n g biomass. For e x a m p l e , there is n o region o f t h e U.S. m a i n l a n d s u i t e d f o r 1 2 - t o 1 8 - m o n t h c r o p p i n g of sugarcane, a l t h o u g h there are vast regions there suited t o s o m e f o r m o f t r o p i c a l grasses. Hence, a f u t u r e e n e r g y p l a n t e r in Florida, Louisiana, S o u t h e r n California, or S o u t h e r n Texas m i g h t seriously consider w h e t h e r he s h o u l d harvest a 6 t o 8 m o n t h c r o p o f s u g a r c a n e per a n n u m or t w o crops o f napier grass in t h e s a m e t i m e frame. Equally i m p o r t a n t is t h e fact t h a t s o m e c o u n t r i e s w i l l n o t be able t o afford a land o c c u p a t i o n o f 1 8 m o n t h s b y a single e n e r g y crop. This is especially t r u e of densely p o p u l a t e d , d e v e l o p i n g t r o p i c a l nations h a v i n g a n u r g e n t need f o r

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

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FUEL

SOURCE

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50k

AGE OF SPECIES

Figure 1.

A generalized representation of the maturation profile of herbaceous land plants

While no specific time-frame or plant form is depicted, the diphasic process of tissue expansion followed by maturation is typical of nonwoody plant species. With the visible growth phase essentially completed, the energy planter will gain much additional dry matter by allowing a brief additional time interval to elapse before harvest.

5θ\Sugarcane

AGE OF SPECIES (WEEKS)

Figure 2. Relative maturation profiles for Sordan 70A, napier grass, and sugarcane over one year. These plants are representative of the short-, intermediate-, and longrotation cropping categories, respectively.

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

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Table II. DRY M A T T E R YIELDS OF S U G A R C A N E A N D NAPIER GRASS HARVESTED A T V A R I A B L E FREQUENCIES OVER A TIME-COURSE OF ONE Y E A R " Interval No. of (Months) Harvests Species 2

3

b

c

6

Cane" Napier

4

3

Cane Napier

6

2

Cane Napier

12

1

Cane Napier

0

Tons DM/Acre/Year

For—

Plant Crop

1st Ratoon Crop

6.5 12.7

3.3 11.9 11.9

11.1 22.6 16.6 25.6 25.5 19.3

25.1 20.6 33.0 33.6 25.8

DOE C o n t r a c t No. D E - A S 0 5 - 7 8 E T 2 0 0 7 1 . C o m p u t e d m e a n of t h r e e varieties a n d t w o r o w spacings. C o m p u t e d m e a n o f one v a r i e t y a n d t w o r o w spacings.

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

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SOURCE

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d o m e s t i c f o o d p r o d u c t i o n (64). In s u c h cases, a s h o r t - r o t a t i o n species s u c h as S o r d a n m a y be t h e p o p u l a r c h o i c e for e n e r g y p l a n t i n g since it c a n be s o w n as a s t o p - g a p b e t w e e n t h e harvest of one f o o d c r o p a n d t h e p l a n t i n g of another. In t h i s c a p a c i t y , it w o u l d also p r e v e n t soil erosion a n d w e e d g r o w t h w h i l e a c t i n g as a s c a v e n g e r for residual n u t r i e n t s left over f r o m t h e prior f o o d c r o p . Seasonal c l i m a t e c h a n g e s w i l l also be a f a c t o r in t h e r o t a t i o n of biomass e n e r g y species w i t h c o n v e n t i o n a l f o o d a n d fiber crops. S h o r t - r o t a t i o n t r o p i c a l grasses s u c h as Sordan are ideally s u i t e d t o t h e t r o p i c s , b u t t h e y can be g r o w n o n a seasonal basis d u r i n g t h e heat of s u m m e r in m o s t t e m p e r a t e regions. S u c h plants c o u l d be p r o p a g a t e d t o m a t u r i t y in a m i d - J u n e t o m i d A u g u s t t i m e f r a m e . In a g i v e n year, t h e s a m e site c o u l d p r o d u c e a cool season f o o d c r o p (a Brassica species, spinach), or a cool season forage (ryegrass, fall barley) b o t h p r e c e d i n g a n d f o l l o w i n g t h e b i o m a s s e n e r g y crop. HARVEST A N D TRANSPORTATION Perhaps t h e w e a k e s t p o i n t in c u r r e n t p r o d u c t i o n research for b i o m a s s is the lack of p r o v e n harvest e q u i p m e n t a n d m e t h o d o l o g i e s for t h e m a x i m i z e d s t a n d s of b i o m a s s t h a t each c o n t r a c t o r strives t o a t t a i n . This is m o s t e v i d e n t in w o o d y b i o m a s s scenarios w h e r e c o n v e n t i o n a l forest h a r v e s t i n g t e c h n o l o g y is either not a p p l i c a b l e or s i m p l y d o e s n ' t exist in t h e c o n t e x t of s i l v i c u l t u r e e n e r g y p l a n t a t i o n s . T h e o u t l o o k for h a r v e s t i n g herbaceous land plants is c o n s i d e r a b l y better b u t a g o o d deal of research remains o n harvest a n d p o s t - h a r v e s t t e c h n o l o g y , t o g e t h e r w i t h e q u i p m e n t redesign a n d modification. M o w i n g vs. Conditioning As Harvest Options T h e vast m a j o r i t y of herbaceous land plants can be harvested nicely w i t h the sickle-bar m o w e r (assuming t h a t land slopes and c o n t o u r s are o t h e r w i s e s u i t e d for m e c h a n i z e d operations). This i m p l e m e n t w a s d e s i g n e d m o r e t h a n a c e n t u r y ago as a r e p l a c e m e n t for t h e h a n d sickle a n d m a n u a l grass s c y t h e . As a h o r s e - d r a w n i m p l e m e n t , it r e v o l u t i o n i z e d t h e harvest of grain a n d forage crops. T o d a y it is usually o p e r a t e d f r o m t h e p o w e r take-off of Class I a n d II tractors. The original w o o d e n parts have been replaced, bearings and l u b r i c a t i o n systems have been i m p r o v e d , a n d it is no longer geared to t h e s l o w f o r w a r d pace of d r a f t animals. But it operates o n basically t h e same p r i n c i p l e as its h o r s e - d r a w n predecessors. There are t w o p r i n c i p a l l i m i t a t i o n s of t h e sickle-bar m o w e r as a harvest i m p l e m e n t for herbaceous biomass crops: (a) It is d e s i g n e d to operate in relatively l o w - d e n s i t y stands of plants, a n d (b). its c u t t i n g process is c o n f i n e d

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

3.

ALEXANDER

Nonwoody Land Plants

63

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t o a single slice near t h e base of u p r i g h t stems. In o t h e r w o r d s , it is a m e c h a n i z e d sickle f o r severing s t e m s rather t h a n a s t e m c o n d i t i o n e r . This m o w e r has a preference for d r y a n d u p r i g h t s t e m s w h o s e t o t a l mass does n o t exceed a b o u t 12 green t o n s per acre. It experiences real d i f f i c u l t y w i t h w e t and l o d g e d materials a n d w i t h plant stands in a n y c o n d i t i o n w h o s e mass exceeds 15 green t o n s per acre. Since its o p e r a t i o n is based o n a c u t t i n g p r i n c i p l e , t h e sickle m u s t be kept c o n t i n u a l l y sharp for effective p e r f o r m a n c e . Its e f f i c i e n c y is i m m e d i a t e l y l o w e r e d b y c o n t a c t w i t h m o l e hills, rocks, w i r e s , scrap m e t a l , and d u r a b l e o b j e c t s o f any kind e n c o u n t e r e d in t h e field. In t h e a u t h o r ' s experience, t h e m o d e r n sickle-bar m o w e r o p e r a t i n g in a t y p i c a l l y dense t r o p i c a l grass, s u c h as S o r d a n 7 0 A (about 2 0 - 2 5 green t o n s / a c r e ) , w i l l e x p e r i e n c e f r e q u e n t t r i p p i n g of its " f a i l - s a f e " m e c h a n i s m . This is a b u i l t - i n feature o f t h e i m p l e m e n t d e s i g n e d t o p r e v e n t its d e s t r u c t i o n w h e n s t r i k i n g unseen s t u m p s or o t h e r f i x e d o b j e c t s at operational speed. Nonetheless, t h e sickle-bar m o w e r is p r o b a b l y very a d e q u a t e for h a r v e s t i n g m o s t herbaceous land plants, t h a t is. t h o s e plants w h o s e s t a n d i n g green mass w i l l n o t exceed a b o u t 12 t o n s per acre at any g i v e n harvest interval. For h a r v e s t i n g s o m e w h a t higher densities o f herbaceous m a t e r i a l , a series o f " f l a i l " a n d " c o n d i t i o n e r " designs have p r o v e n t o be superior t o t h e sickle-bar m o w e r . S u c h i m p l e m e n t s d o n o t p e r f o r m o n a c u t t i n g p r i n c i p l e b u t rather break o f f t h e plant s t e m by striking it w i t h e x t r e m e force. Sharpness o f t h e c o n t a c t blades is n o t a decisive feature. In fact t h e y w i l l p e r f o r m fairly a d e q u a t e l y even w h e n dull f r o m long use. These m a c h i n e s require h i g h h o r s e p o w e r (90 t o 1 2 0 hp) a n d h i g h p o w e r - t a k e - o f f speed ( 1 0 0 0 rpm). The m o s t effective i m p l e m e n t of t h i s t y p e t e s t e d t o date in Puerto Rico is t h e M-C " r o t a r y s c y t h e - c o n d i t i o n e r " . T h e plant s t e m s are broken o f f by four lines of w h i r l i n g blades a n d are repeatedly s h a t t e r e d as t h e blades restrike t h e s t e m s at 3 - t o 5-inch intervals. T h e r e s u l t i n g " c o n d i t i o n e d " biomass is evenly d i s t r i b u t e d in a broad s w a t h b e h i n d t h e rotary s c y t h e . In this state, t h e s u b s e q u e n t d r y i n g a n d baling o p e r a t i o n s are m o r e easily p e r f o r m e d t h a n w i t h c o n v e n t i o n a l l y - m o w e d biomass, t h a t is. w i t h plant materials received in c l u m p s a n d m a t t s a n d w i t h o n l y one c u t surface t o facilitate w a t e r r e m o v a l . A n a d d i t i o n a l a d v a n t a g e of t h e rotary s c y t h e - c o n d i t i o n e r is its c a p a c i t y t o harvest plant densities r o u g h l y d o u b l e t h o s e h a n d l e d b y t h e sickle-bar m o w e r . A second a d d e d a d v a n t a g e is its a b i l i t y t o harvest lodged a n d w e t materials. Such plants are harvested a b o u t as readily as t h o s e in a d r y a n d u p r i g h t c o n d i t i o n . A t h i r d a v a n t a g e is its relatively t r o u b l e - f r e e o p e r a t i o n . The n u m b e r o f parts s u b j e c t t o m a l f u n c t i o n is purposely r e d u c e d t o a m i n i m u m .

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

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SOURCE

A t t h i s w r i t i n g , t h e rotary s c y t h e - c o n d i t i o n e r has g i v e n e x c e l l e n t

perfor-

m a n c e in p l a n t densities a m o u n t i n g t o a b o u t 2 2 g r e e n t o n s per acre (62). It is believed t h a t its u p p e r d e n s i t y l i m i t w i l l be o n t h e order of 4 0 g r e e n t o n s per

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acre (65). Plant yields c o n s i d e r a b l y h i g h e r t h a n 4 0 green t o n s per acre are a n t i c i p a t e d for a f e w h e r b a c e o u s species. S u g a r c a n e yields in excess of 9 0 g r e e n t o n s per acre year w e r e r e c e n t l y d e m o n s t r a t e d in Puerto Rico (49). M o s t s u g a r c a n e harvesters m a r k e t e d t o d a y b e g i n t o have d i f f i c u l t y w i t h c a n e densities in t h e range of 5 0 t o 6 0 s t a n d i n g g r e e n t o n s per acre (65). T h e m o s t e f f e c t i v e s u g a r c a n e harvester in Puerto Rico at present is t h e Class M o d e l 1400. O r i g i n a l l y d e v e l o p e d in East G e r m a n y , t h e Class is a s i n g l e - r o w , w h o l e cane harvester w h i c h e m p l o y s a p o w e r f u l air blast t o r e m o v e o r g a n i c trash a n d soil f r o m t h e cane at t h e p o i n t of harvest in t h e field. It has a c c o m m o d a t e d over 6 0 t o n s of green c a n e per acre. W i t h m o d i f i c a t i o n s , it m i g h t possibly harvest 8 0 t o 9 0 t o n s per acre (65). Solar Drying A c h a r a c t e r i s t i c d i f f i c u l t y w i t h b i o m a s s is its l o w d e n s i t y relative t o fossil e n e r g y a n d its h i g h w a t e r c o n t e n t w h i c h is c o s t l y t o t r a n s p o r t t o p r o c e s s i n g centers. W h e r e v e r possible, it is desirable t o r e m o v e m o s t of t h i s w a t e r at t h e harvest site by solar d r y i n g . O n e e x c e p t i o n t o t h i s is t h e use of " g r e e n " b i o m a s s for a n a e r o b i c d i g e s t i o n . A n o t h e r e x c e p t i o n is f o u n d in s u g a r c a n e . In t h i s case, t h e w h o l e green stalk is t r a n s p o r t e d t o a centralized m i l l for d e w a t e r i n g . T h e p l a n t ' s s o l u b l e f e r m e n t a b l e solids are recovered t h e r e f r o m t h e expressed j u i c e a n d sold as refined sugar or molasses. Very a d e q u a t e e q u i p m e n t for t h e solar d r y i n g of n o n - w o o d y land plants can be f o u n d in t h e c a t t l e f o r a g e i n d u s t r y . T h e rotary s c y t h e - c o n d i t i o n e r d e s c r i b e d a b o v e does m u c h t o prepare herbaceous p l a n t s for rapid d r y i n g in t h e s u n (66.67). O r d i n a r i l y t h e s e materials w o u l d be t u r n e d over o n c e or t w i c e in b r i n g i n g t h e m o i s t u r e c o n t e n t d o w n t o a b o u t 15 percent. Three w i n d r o w s w o u l d t h e n be c o m b i n e d into one s h o r t l y before b a l i n g . Each of these o p e r a t i o n s can be p e r f o r m e d w i t h s t a n d a r d side-delivery f o r a g e rakes o p e r a t i n g f r o m t h e p o w e r take-off of a Class I or II tractor. W h e n higher d e n s i t y b i o m a s s is t o be raked (Sordan or napier grass), a h e a v y - d u t y " w h e e l " rake m a y be m o r e suitable. These i m p l e m e n t s are also b e c o m i n g s t a n d a r d e q u i p m e n t for f o r a g e - m a k i n g o p e r a t i o n s .

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

3.

ALEXANDER

65

Nonwoody Land Plants

Compaction A n d Baling

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Solar-dried biomass is rarely t r a n s p o r t e d t o its processing site t o d a y in a loose s t a t e , a l t h o u g h o n c e t h i s w a s s t a n d a r d practice. For e c o n o m y of space in t r a n s p o r t a n d storage, as w e l l as ease o f h a n d l i n g , s u c h materials are first c o m p a c t e d and t h e n b o u n d w i t h a suitable t w i n e or w i r e . T h e s t a n d a r d h a y " b a l e r " t o d a y is a c t u a l l y a c o m p a c t o r . It p r o d u c e s c o n v e n i e n t l y - s i z e d c u b e s h a v i n g a c o n t r o l l e d d e n s i t y range o f r o u g h l y 8 t o 2 0 p o u n d s per c u b i c foot. A t y p i c a l hay " b a l e " w o u l d w e i g h 6 0 or 7 0 p o u n d s a n d is easily h a n d l e d b y one m a n in t r a n s p o r t a n d storage p r o c e d u r e s or in c a t t l e - f e e d i n g operations. A d i f f e r e n t c o n c e p t in biomass b a l i n g has appeared in recent years. This is t h e " b u l k " or " r o u n d " baler w h i c h operates as a w i n d r o w w r a p p e r rather t h a n a c o m p a c t o r . This i m p l e m e n t p r o d u c e s large c y l i n d r i c a l bales w e i g h i n g u p t o 1 5 0 0 p o u n d s each (68,69). Since n o a p p r e c i a b l e c o m p a c t i o n is i n v o l v e d , t h e bale d e n s i t y is relatively low. o n t h e order o f 10 t o 12 p o u n d s per c u b i c foot. M o r e recent m o d i f i c a t i o n s enable t h i s m a c h i n e t o p r o d u c e

cube-shaped

bales w h i c h are more e c o n o m i c a l o f space d u r i n g t r a n s p o r t a n d storage. Both f r o n t - a n d rear-end loaders suitable f o r h a n d l i n g these bales are m a r k e t e d as c o n v e n t i o n a l t r a c t o r a t t a c h m e n t s (65). There are t w o t y p e s o f balers for s u g a r c a n e bagasse, t h e b a l i n g press a n d t h e b r i q u e t t i n g press (70). The first t y p e is a h y d r a u l i c press e m p l o y i n g t h e s a m e c o m p a c t i o n p r i n c i p l e used for hay. T h e bagasse is baled in a s e m i - g r e e n state a n d t h e f o r m e d c u b e s are t i e d w i t h t w i n e or w i r e s t o p r e v e n t t h e m f r o m ree x p a n d i n g . Their d e n s i t y w i l l range f r o m 2 5 t o 4 0 p o u n d s per c u b i c foot. Bales of t h i s t y p e m u s t be stacked c a r e f u l l y t o p r e v e n t s p o n t a n e o u s c o m b u s t i o n , t h a t is. w i t h s u f f i c i e n t space b e t w e e n t h e m t o a l l o w air c i r c u l a t i o n . T h e b r i q u e t t i n g press operates w i t h d r y bagasse h a v i n g a m o i s t u r e c o n t e n t of 8 t o 15 percent. This press provides h i g h pressures o n t h e order o f 5.000 t o 1 5 . 0 0 0 psi. U n d e r t h e s e c o n d i t i o n s , e x t r e m e l y c o m p a c t c u b e s are p r o d u c e d w h i c h retain t h e i r f o r m w i t h o u t t h e use o f t w i n e or w i r e s . Transport And Storage Herbaceous b i o m a s s t h a t has been solar-dried a n d baled can be t r a n s p o r t e d t o p r o c e s s i n g or storage sites w i t h o u t a p p r e c i a b l e d i f f i c u l t y w i t h e x i s t i n g e q u i p m e n t . H o w e v e r , t h i s c a n entail a s i g n i f i c a n t cost. Ordinarily s u c h materials w o u l d be loaded d i r e c t l y in t h e field o n a l o w - b e d truck. S t a n d a r d bales ( 6 0 - 8 0 pounds) c a n be loaded m a n u a l l y or w i t h m e c h a n i c a l loaders r e q u i r i n g o n l y o n e laborer on t h e t r u c k f o r final p o s i t i o n i n g o f t h e bales. Bulk bales w o u l d be stacked t w o layers d e e p o n t h e t r u c k b e d w i t h t r a c t o r m o u n t e d loaders. T h e s a m e t r u c k w o u l d t r a n s p o r t t h e biomass t o a final

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p r o c e s s i n g or storage f a c i l i t y w i t h o u t i n t e r m e d i a t e t r a n s - s h i p m e n t o p e r a tions. In t h e case of s u g a r c a n e , t h e harvested w h o l e stalks or s t e m billets, w h a t e v e r t h e case m a y be. are hauled in carts t o t h e a d j a c e n t mil.. T h e same materials c o u l d be c a r t e d t o an i n t e r m e d i a t e reloading p o i n t for t r u c k delivery t o m o r e d i s t a n t sugar mills. Delivery costs w i l l vary c o n s i d e r a b l y w i t h t h e i n d i v i d u a l biomass p r o d u c t i o n o p e r a t i o n . In g e n e r a l , a 4 0 - t o n l o w - b e d t r u c k w i t h driver can be hired for a b o u t $ 1 8 0 per 2 4 - h o u r d a y at c u r r e n t rates. L o a d i n g e q u i p m e n t w i t h o p e r a t o r s m u s t be s t a t i o n e d at each e n d of t h e d e l i v e r y r u n . In an ideal b i o m a s s p r o d u c t i o n o p e r a t i o n , i.e., o n e m a n a g e d by a private f a r m e r for profit, t h e land o w n e r w o u l d p r o b a b l y o w n a n d help o p e r a t e t h e t r u c k a n d accessory e q u i p m e n t . A n e s t i m a t e d d e l i v e r y c o s t for solar-dried b i o m a s s on a 2 0 - m i l e run w o u l d be $ 6 . 0 0 t o $ 8 . 0 0 per t o n ( 1 9 8 0 dollars). PRODUCTION COSTS Published p r o d u c t i o n costs for b o t h h e r b a c e o u s a n d w o o d y biomass s h o w broad v a r i a t i o n s t h a t are b o t h u n d e r s t a n d a b l e a n d i n e v i t a b l e (3,6,7,48,58). A g i v e n c o n t r a c t o r w i l l w a n t t o present his speciality c r o p in t h e best possible l i g h t relative t o t h e dollar i n p u t s needed t o o b t a i n a m i l l i o n Btu's in biomass f o r m . This t o p i c has been r e v i e w e d in detail (48). It w a s c o n c l u d e d t h a t m o s t b i o m a s s researchers g r e a t l y u n d e r e s t i m a t e t h e cost of b i o m a s s p r o d u c t i o n , e x c l u d i n g f r o m t h e i r c a l c u l a t i o n s s i g n i f i c a n t i n d i r e c t costs, l o n g - t e r m repercussions o n e c o s y s t e m resources, f u t u r e c o m p e t i t i o n for land a n d w a t e r , a n d b o t h t h e cost a n d e f f i c i e n c y of b i o m a s s c o n v e r s i o n s y s t e m s . Obtaining Correct Cost Data A seriously m i s l e a d i n g t r e n d is t o base t h e p r o d u c t i o n costs of a biomass c a n d i d a t e on its p u b l i s h e d yield p e r f o r m a n c e as a c o n v e n t i o n a l f o o d or fiber crop. S u g a r c a n e is an a p p r o p r i a t e e x a m p l e . In Puerto Rico, s u g a r c a n e m a n a g e d for sucrose yields 2 5 t o 3 0 green t o n s per acre year; as an energy c r o p it can yield 8 0 t o 9 0 t o n s per acre year w i t h o n l y m o d e r a t e increases in p r o d u c t i o n costs (49). Napier grass data are similarly m i s l e a d i n g . There is a w e a l t h of p r i n t e d m a t t e r o n t h e yields of napier grass m a n a g e d as a t r o p i c a l f o r a g e c r o p , t h a t is, w h e n harvested repeatedly at f i v e - or s i x - w e e k intervals at m o i s t u r e c o n t e n t s a p p r o a c h i n g 9 0 percent. A s an energy c r o p , napier grass p r o d u c e s r o u g h l y t w o t o t h r e e t i m e s m o r e dry m a t t e r per a n n u m at less cost t h a n t h e c a t t l e forage (49).

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P r o d u c t i o n C o s t s For T r o p i c a l G r a s s e s Since J u n e of 1 9 7 7 . considerable i n f o r m a t i o n has been g a t h e r e d o n p r o d u c t i o n costs for s u g a r c a n e a n d o t h e r t r o p i c a l grasses w h o s e a g r i c u l t u r e has been m a n a g e d f o r m a x i m u m d r y m a t t e r yield in a t r o p i c a l e c o s y s t e m (62,63). A b r e a k d o w n o f p r o d u c t i o n i n p u t charges f o r " e n e r g y c a n e " is presented in Table III. There d a t a pertain t o a p r i v a t e l y - o w n e d , 2 0 0 - a c r e o p e r a t i o n y i e l d i n g 3 3 o v e n - d r y t o n s o f biomass per acre-year. Total cost, i n c l u d i n g delivery t o t h e m i l l i n g site, is $ 2 5 . 4 6 per t o n or $ 1 . 7 0 per m i l l i o n Btus. U n d e r Puerto Rico c o n d i t i o n s , a b o u t 7 0 p e r c e n t o f this d r y m a t t e r w o u l d be b u r n e d as a boiler f u e l . The r e m a i n d e r w o u l d be e x t r a c t e d as f e r m e n t a b l e solids d u r i n g t h e cane d e w a t e r i n g process a n d later sold as c o n s t i t u e n t s o f h i g h - t e s t molasses. This is a solid credit t o t h e insular e n e r g y cane planter o w i n g t o Puerto Rico's precarious reliance o n f o r e i g n molasses as f e e d s t o c k for her r u m i n d u s t r y (7JJ. A s s u m i n g a market price of $ 0 . 7 5 per gallon f o r h i g h - t e s t molasses, t h e f e r m e n t a b l e solids f r o m one such t o n o f energy cane w o u l d be v a l u e d at m o r e t h a n $ 4 5 . 0 0 , or a b o u t $ 1 5 0 0 . 0 0 per acre. Cane m i l l i n g costs t o d a y in Puerto Rico are a b o u t $ 4 . 5 0 per t o n (72). P r o d u c t i o n costs f o r Sordan 7 0 A are presented in Table IV. A l t h o u g h Sordan's biomass yield is l o w e r t h a n t h a t of energy cane, p r o d u c t i o n i n p u t costs are also lower. T h e final cost o f an o v e n - d r y t o n of Sordan 7 0 A is a b o u t $ 1 4 . 0 0 , or $ 1 . 5 0 less t h a n a t o n o f e n e r g y cane. In this instance, there is no sale of f e r m e n t a b l e solids. P r o d u c t i o n costs f o r napier grass w o u l d be m o d e r a t e l y l o w e r t h a n Sordan 7 0 A o w i n g t o a m u c h higher yield per acreyear for napier grass (49,62). This c r o p similarly has no sales f o r f e r m e n t a b l e solids. M a n a g e m e n t A s A Production Cost Factor P r o d u c t i o n costs f o r e n e r g y cane listed in Table III i n c l u d e " m a n a g e m e n t " as 10 p e r c e n t of t h e cost s u b t o t a l . This is an i n d e f i n i t e t e r m c o v e r i n g t h e a d m i n i s t r a t i v e skills e x p e n d e d b y w a y o f g o o d a g r i c u l t u r a l t e c h n i q u e t o m a x i m i z e biomass y i e l d . It also reflects t h e m o r a l e (or profit i n c e n t i v e level) o f t h e i n d i v i d u a l g r o w e r or i n s t i t u t i o n in c h a r g e o f p r o d u c t i o n . The m a n a g e m e n t f a c t o r c o n t r i b u t i o n t o f u t u r e biomass p r o d u c t i o n scenarios can range f r o m very g o o d t o very b a d , b u t it w i l l have t h e p o t e n t i a l t o be decisive in all p r o d u c t i o n operations. A g a i n , u s i n g s u g a r c a n e as a c o n v e n i e n t e x a m p l e , it is c o m m o n k n o w l e d g e t h a t little profit is t o be m a d e a n y w h e r e in t h e w o r l d t o d a y b y p l a n t i n g sugar, b u t it is t h e w e l l - m a n a g e d o p e r a t i o n s t h a t w i l l m i n i m i z e losses a n d offer t h e best p r o s p e c t o f survival u n t i l sugar values are again equitable. A t one e x t r e m e , superior m a n a g e m e n t w i l l be f o u n d in

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T a b l e I I I . D R Y M A T T E R P R O D U C T I O N C O S T S FOR FIRST-RATOON S U G A R C A N E M A N A G E D A S A N ENERGY CROP"

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Land Area: 2 0 0 Acres P r o d u c t i o n I n t e r v a l : 12 M o n t h s D M Y i e l d : 3 3 (Oven-Dry) S h o r t T o n s / A c r e : 6 , 6 0 0 T o n s Preliminary Cost Analysis Item 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Cost ($/Year)

L a n d Rental, at 5 0 . 0 0 / A c r e S e e d b e d Preparation, at 1 5 . 0 0 / A c r e W a t e r (800 A c r e Feet at 15.00/ft) W a t e r A p p l i c a t i o n , at 4 8 . 0 0 / A c r e Year Seed (For Plant Crop Plus T w o Ratoon Crops 1 T o n / A c r e Year at 1 5 . 0 0 / T o n Fertilizer, at 1 8 0 . 0 0 / A c r e Pesticides, at 2 6 . 5 0 / A c r e Harvest, I n c l u d i n g E q u i p m e n t Charges, E q u i p m e n t D e p r e c i a t i o n , a n d Labor Day Labor. 1 M a n Year ( 2 0 1 6 hrs. a t 3 . 0 0 / h r ) C u l t i v a t i o n , at 5 . 0 0 / A c r e L a n d Preparation & M a i n t e n a n c e (Pre-& Post-Harvest) Delivery, at 7 . 0 0 / t o n / 2 0 miles of Haul Subtotal: Management: 10% Subtotal T o t a l Cost: b

10,000 3,000 12.000 9,600 3.000 36.000 5,300 20,000 6,048 1.000 600 46,200 152,748 15.275 168.023

16. T o t a l C o s t / T o n ( 1 6 8 . 0 2 3 + 6.600): 17. Total C o s t / M i l l i o n Btu (25.46 -s- 15): 1

2

DOE C o n t r a c t No. D E - A S 0 5 - 7 8 E T 2 0 0 7 1 . Labor w h i c h is n o t i n c l u d e d in o t h e r costs

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T a b l e IV. D R Y M A T T E R P R O D U C T I O N C O S T S FOR S O R D A N 7 0 A Land A r e a : 2 0 0 A c r e s Production Interval: 6 Months Sordan 7 0 A Y i e l d : 15 (Oven Dry) Short T o n s / A c r e , Total 3,000 Tons Preliminary Cost Analysis Item

C o s t ($)

1. 2. 3. 4. 5.

5.000 2.160 4,800 10.000 4,000 2,650 1,988 1,988 2,200 12,600 18,000 65,386 6,538 71,924 23.97 1.59

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. a

Land Rental, at 5 0 . 0 0 / A c r e Year W a t e r (Overhead Irrigation), 3 6 0 A c r e Feet Seed, at 6 0 l b / A c r e Fertilizer Pesticides E q u i p m e n t D e p r e c i a t i o n (6 mo.) E q u i p m e n t M a i n t e n a n c e (75% o f Depreciation) E q u i p m e n t O p e r a t i o n (75% o f Depreciation) Diesel Fuel Day Labor ( 9 0 . 0 0 / D a y f o r 1 4 0 Days) Delivery, at 6 . 0 0 / T o n Subtotal: M a n a g e m e n t (10% o f Subtotal) Total Cost: Total C o s t / T o n (71,924 + 3.000): Total C o s t / M i l l i o n Btu (23.97 + 1 5 ) :

DOE C o n t r a c t No. D E - A S 0 5 - 7 8 E T 2 0 0 7 1 .

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p r i v a t e l y - o w n e d p l a n t a t i o n s w h i c h in s o m e c o u n t r i e s are still basically f a m i l y o p e r a t i o n s . Here t h e land o w n e r has an i n h e r e n t interest in his p r o p e r t y a n d c a p i t a l i n v e s t m e n t s a n d possesses t h e skills a n d i n c e n t i v e t o m a k e a g o o d l i v i n g f r o m a g r i c u l t u r e . S u c h i n d i v i d u a l s c a n still be f o u n d t o d a y , for e x a m p l e , in t h e Q u e e n s l a n d sugar i n d u s t r y . A t t h e o t h e r e x t r e m e is t h e g o v e r n m e n t o w n e d p r o d u c t i o n o p e r a t i o n . Historically, g o v e r n m e n t s have not m a d e g o o d farmers. A f a r m m a n a g e r w h o has little i n c e n t i v e t o m a k e a profit a n d w h o c a n n o t be held a c c o u n t a b l e for a loss w i l l u l t i m a t e l y have t h e inferior p r o d u c t i o n record. G o v e r n m e n t take-over of an a g r i c u l t u r a l c o m m o d i t y is s o m e t i m e s v i e w e d as a necessary i n t e r v e n t i o n in a free market w h e r e i m p o r t a n t social or p o l i t i c a l c o n s i d e r a t i o n s c o u l d n o t o t h e r w i s e be served (73). T h i s w a s t h e case w i t h s u g a r c a n e in Puerto Rico w h e r e a large a n d o t h e r w i s e u n e m p l o y a b l e labor force c o u l d no longer be s u s t a i n e d by p r i v a t e enterprise (64.79). A s a c o n s e q u e n c e it n o w costs a b o u t 2 8 c e n t s t o p r o d u c e a p o u n d of sucrose in Puerto Rico, at a t i m e w h e n its v a l u e o n t h e w o r l d sugar m a r k e t is o n l y a b o u t 14 c e n t s per p o u n d . It is fair t o say t h a t m a n a g e m e n t is n o t t h e o n l y f a c t o r c o n t r i b u t i n g t o h i g h p r o d u c t i o n costs — e n v i r o n m e n t a l q u a l i t y s t a n d a r d s have also had a n e g a t i v e i m p a c t o n t h e PR s u g a r i n d u s t r y (29) — b u t poor m a n a g e m e n t is clearly t h e m a i n c o n t r i b u t i n g factor. In a w e l l - m a n a g e d p r o d u c t i o n scenario for herbaceous terrestrial b i o m a s s s o m e s t r a i g h t - f o r w a r d steps w i l l need t o be t a k e n t o assure m a x i m u m returns f r o m p r o d u c t i o n i n p u t e x p e n d i t u r e s . These i n c l u d e : a) Correct land p r e p a r a t i o n , i n c l u d i n g land leveling a n d p l a n n i n g w h e r e n e e d e d ; b) correct d e s i g n a n d installation of t h e irrigation s y s t e m ; c) c o r r e c t seedbed (relative t o d e p t h , d e n s i t y or r o w s p a c i n g , a n d season); d) reseeding of v a c a n t space w h e n necessary; e) correct pest c o n t r o l p r o g r a m s ( i n c l u d i n g a d m i n i s t r a t i o n of c o n t r o l on w e e k e n d s a n d holidays w h e n required); f) m a i n t e n a n c e of c o r r e c t i r r i g a t i o n , fertilization, a n d c u l t i v a t i o n p r o g r a m s ; g) correct t i m i n g a n d s y n c h r o n i z a t i o n of harvest o p e r a t i o n s ; h) c o r r e c t selection a n d use of harvest e q u i p m e n t ; i) p o s t - h a r v e s t m a i n t e n a n c e of land a n d m a c h i n e r y . For m o s t biomass c r o p s , t h e cost of these measures w i l l a c c r u e w h e t h e r t h e y are p e r f o r m e d c o r r e c t l y or not. T h e decisive f a c t o r w i l l be t h e skill a n d m o t i v a t i o n of t h e o p e r a t i o n ' s field managers. Good m a n a g e m e n t c a n best be assured w h e n p r o d u c t i o n is retained in t h e c o n t e x t of p r i v a t e l y - o w n e d p l a n t a t i o n s t h a t are o p e r a t e d for personal profit.

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SUMMARY The n a t u r e of herbaceous land plants a n d their p o t e n t i a l usefulness as a f u t u r e energy resource is presented in broad outline. T h e large n u m b e r of herbaceous species f o u n d in b o t h cool a n d w a r m c l i m a t e s a n d in b o t h t h e w i l d a n d c u l t i v a t e d state s u g g e s t s t h a t at least a small p e r c e n t a g e of these c o u l d b e c o m e v a l u a b l e sources o f fuel. Extensive s c r e e n i n g w i l l be needed in a r a n g e o f e c o s y s t e m s t o b r i n g t h e n u m b e r o f c a n d i d a t e species t o a m a n a g e a b l e level. Both b o t a n i c a l a n d a g r o n o m i c features t o be e v a l u a t e d d u r i n g t h e s c r e e n i n g process are briefly discussed. S o m e of t h e p r o d u c t i o n a n d harvest o p e r a t i o n s required of herbaceous plants as a g r i c u l t u r a l c o m m o d i t i e s are also r e v i e w e d , t o g e t h e r w i t h partial cost analyses f o r t h e p r o d u c t i o n operations. M a n a g e m e n t of t h e energy crop is seen t o be t h e decisive cost input. This f a c t o r w i l l be o p t i m i z e d in p r i v a t e l y - o w n e d o p e r a t i o n s m o t i v a t e d b y a s t r o n g profit i n c e n t i v e .

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