Biomass as a Nonfossil Fuel Source - ACS Publications - American

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The Energy Plantation and the Photosynthesis Energy Factory M A L C O M D. FRASER, JOHN F. HENRY, LOUIS C. BORGHI, and NORMAN J. BARBERA InterTechnology/Solar Corporation, 100 Main Street, Warrenton, V A 22186

The Energy Plantation In conventional forestry, trees are grown in plantations to produce the raw material for a variety of products such as lumber, plywood, pulp and others. In these plantations, the trees are generally widely spaced and grown to sizes large enough for the manufacture of the desired products. Achieving these commercial sizes may require long growing periods or rotations which may range from 30 to 80 years or more. As a result of these constraints — long rotations and planting densities of the order of a few hundred trees per acre towards the end of the rotation — the plantation site is only fully utilized for a short fraction of the rotation period. Average sustained yields over the rotation therefore rarely exceed about 1 oven-dry-ton of mechantable material per acre-year (1). Moreover, because of the size of the crop and the need to maintain its physical integrity, conventional single-tree harvesting and handling methods are generally used in forestry operations. Tree crops h o w e v e r c o u l d be g r o w n o n m u c h shorter rotations if t h e size a n d f o r m o f t h e crops w e r e n o t l i m i t i n g f a c t o r s in t h e e n d use o f t h e crop. S u c h is t h e case w h e n t h e desired p r o d u c t is w o o d c h i p s t o be used f o r p u l p , fuel, or f e e d s t o c k f o r c o n v e r s i o n t o s u b s t i t u t e fuels. S h o r t - r o t a t i o n tree f a r m i n g

0097-6156/81/0144-0495$ 12.50/0 © 1981 American Chemical Society

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

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generally refers t o r o t a t i o n s of 2 0 years or less a n d is generally associated w i t h close s p a c i n g of t h e trees in order t o a c h i e v e full site utilization w i t h i n the rotation period. S h o r t - r o t a t i o n tree f a r m i n g for fiber p r o d u c t i o n has been p r o p o s e d by a n u m b e r of investigators (2-5). Early s h o r t - r o t a t i o n e x p e r i m e n t a l data i n d i c a t e d t h a t t h e biomass yields a c h i e v e d in s h o r t - r o t a t i o n p l a n t a t i o n s c o u l d far exceed t h o s e of c o n v e n t i o n a l forestry. A v e r a g e a n n u a l s u s t a i n e d yields of 5 t o 10 o v e n - d r y t o n s per acre-year (ODT/ac-yr) w e r e s h o w n t o be possible u n d e r s h o r t - r o t a t i o n c o n d i t i o n s (6,7). It also b e c a m e a p p a r e n t t h a t s u c h h i g h yields c o u l d be a c h i e v e d o n l y if intensive m a n a g e m e n t w e r e a p p l i e d t o t h e p l a n t a t i o n . In m a n y cases, t h e level of m a n a g e m e n t appears t o be c o m p a r a b l e t o t h a t required in t h e p r o d u c t i o n of a g r i c u l t u r a l c r o p s (8-10). T h e p o t e n t i a l of s h o r t - r o t a t i o n tree f a r m i n g for fiber p r o d u c t i o n i n d u c e d I n t e r T e c h n o l o g y / S o l a r C o r p o r a t i o n t o a d v a n c e t h e same c o n c e p t as a possible source of b i o m a s s for e n e r g y c o n v e r s i o n (11-13). O t h e r i n v e s t i g a t o r s have d e s c r i b e d similar c o n c e p t s (14,15). A s it is presently e n v i s i o n e d by I n t e r T e c h n o l o g y / S o l a r C o r p o r a t i o n , t h e Energy Plantation is a w o o d y biomass p r o d u c t i o n e n t i t y relying o n short r o t a t i o n a n d i n t e n s i v e m a n a g e m e n t t o p r o d u c e b i o m a s s e x c l u s i v e l y for its fuel a n d / o r f e e d s t o c k value. Energy Plantations offer a n u m b e r of p o t e n t i a l a d v a n t a g e s over c o n v e n t i o n a l forestry p l a n t a t i o n s : h i g h e r p r o d u c t i v i t y per u n i t land area, l o w e r land r e q u i r e m e n t s for a g i v e n biomass o u t p u t , earlier cash return o n t h e i n v e s t m e n t , e x t e n s i v e m e c h a n i z a t i o n similar t o t h a t p r a c t i c e d in a g r i c u l t u r e , a n d a b i l i t y t o assimilate c u l t u r a l a n d g e n e t i c i m p r o v e m e n t s q u i c k l y . S h o r t - r o t a t i o n c r o p s c a n also be c h o s e n a m o n g a v a r i e t y of species w h i c h regenerate by c o p p i c i n g , t h e r e b y e l i m i n a t i n g t h e need for r e p l a n t i n g after each harvest. Energy Plantations h o w e v e r d o have a n u m b e r of d i s a d v a n t a g e s : initial e s t a b l i s h m e n t costs a n d yearly m a n a g e m e n t costs per unit area are generally higher t h a n those for c o n v e n t i o n a l forest c r o p s ; o n l y sites a m e n a b l e t o m e c h a n i z e d o p e r a t i o n s can be u s e d ; a n d disease a n d insect p r o p a g a t i o n m a y be d i f f i c u l t t o c o n t r o l . S e c u r i n g t h e use of t h e land for e n e r g y c r o p s c o u l d also be a p r o b l e m in s o m e areas w h e r e c o m p e t i t i o n w i t h o t h e r uses (e.g., f a r m i n g , recreation) c o u l d occur. No full-scale Energy Plantation has yet been d e m o n s t r a t e d . It is therefore necessary t o use a c o n c e p t u a l d e s i g n of t h e Energy Plantation t o assess its economic and energy efficiency potential. Table I summarizes the design p a r a m e t e r s a d o p t e d in t h e ITC/Solar m o d e l of t h e Energy Plantation. T h e c r o p s are a s s u m e d t o be c h o s e n f r o m a v a r i e t y of h a r d w o o d s d i s p l a y i n g fast j u v e n i l e g r o w t h a n d c a p a b l e of regeneration by c o p p i c i n g . C a n d i d a t e crops i n c l u d e A m e r i c a n s y c a m o r e (Platanus occidentalis), h y b r i d poplars (Populus


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Photosynthesis Energy Factory

T a b l e I. D E S I G N P A R A M E T E R S U S E D I N T H E I T C / S O L A R M O D E L OF THE ENERGY P L A N T A T I O N Production

Variable, generally o f t h e order o f 2 0 0 , 0 0 0 O D T / a c - y r

Crop

F a s t - g r o w i n g h a r d w o o d s w i t h c o p p i c e regeneration

Productivity

5 t o 10 O D T / a c - y r

Planting Density

4 t o 16 square f t per plant, i.e., — 1 0 , 0 0 0 t o — 2 5 0 0 trees per acre

Lifetime

One f i r s t - g r o w t h r o t a t i o n f o l l o w e d b y five c o p p i c e r o t a tions

Management

Mechanical weed control Fertilization Irrigation (in s o m e m o d e s of o p e r a t i o n of t h e p l a n t a tion)

Harvesting

C o n c e p t u a l self-propelled h a r v é s t e r - c h i p p e r

Transportation

Green

woodchips

transported

to conversion

plant

located in c e n t e r o f t h e p l a n t a t i o n Support

Nursery o p e r a t i o n , e q u i p m e n t m a i n t e n a n c e a n d repair, supervision

Land

Plantation m a d e o f lots o f t h e size o f an average f a r m in t h e region d i s t r i b u t e d at r a n d o m w i t h i n a larger g e o g r a p h i c area


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BIOMASS AS A NONFOSSIL F U E L SOURCE

sppj. Eastern c o t t o n w o o d (P. deltoïdes), black c o t t o n w o o d (P. trichocarpa). black alder (Alnus glutanosa). green ash (Fraxinus pennsylvanicum). Eucalyptus, a n d others. T h e selection of t h e c r o p is m a d e o n t h e basis of c l i m a t e , soil c o n d i t i o n s , a n d d e m o n s t r a t e d g r o w t h c h a r a c t e r i s t i c s of t h e c a n d i d a t e crop. F a s t - g r o w i n g h a r d w o o d s are generally s u g g e s t e d because of t h e i r c o p p i c i n g properties, w h i c h e l i m i n a t e t h e need for r e e s t a b l i s h m e n t of t h e p l a n t a t i o n after each harvest. Due t o t h e c l i m a t e a n d soil c o n d i t i o n s , pines m a y be better c r o p c a n d i d a t e s in s o m e s i t u a t i o n s , as s h o w n for instance in S o u t h e r n Georgia w h e r e loblolly pines d i s p l a y e d h i g h e r p r o d u c t i v i t y t h a n s y c a m o r e for r o t a t i o n s of a b o u t 6 years or m o r e (16). On an Energy P l a n t a t i o n , t h e p l a n t i n g d e n s i t y a n d r o t a t i o n d u r a t i o n , a n d their associated p r o d u c t i v i t y , are c h o s e n t o m i n i m i z e t h e c o s t of biomass p r o d u c t i o n . M a n y a u t h o r s have recognized t h a t a s t r o n g c o r r e l a t i o n exists b e t w e e n s p a c i n g a n d r o t a t i o n age for h a r d w o o d c r o p s g r o w n u n d e r intensive m a n a g e m e n t . O n c e a s p a c i n g has been a d o p t e d w h e n e s t a b l i s h i n g a f a r m , t h e r o t a t i o n age at w h i c h t h e m e a n a n n u a l b i o m a s s increase is o b t a i n e d m u s t be a d o p t e d t o m a x i m i z e t h e yield of t h e f a r m (17). T h e c h o i c e of t h e o p t i m u m r o t a t i o n is p a r t i c u l a r l y critical for close s p a c i n g s generally associated w i t h short rotations because t h e average p r o d u c t i v i t y decreases s i g n i f i c a n t l y once t h e o p t i m u m r o t a t i o n is e x c e e d e d . On t h e basis of d a t a available at present, s o m e a u t h o r s (6,18) favor r o t a t i o n s of 10 t o 15 years w h i l e others prefer shorter r o t a t i o n s (19-21). T h e d e s i g n p a r a m e t e r s a d o p t e d in Table I m i g h t t h e r e f o r e have t o be m o d i f i e d t o a c c o u n t for site-specific c o n d i t i o n s . The i m p a c t of c h a n g e s in s p a c i n g a n d r o t a t i o n d u r a t i o n has been e s t i m a t e d t h r o u g h sensitivity analyses (21). L a n d m a n a g e m e n t i n c l u d e s w e e d c o n t r o l t o e l i m i n a t e c o m p e t i t i o n for light, m o i s t u r e , a n d n u t r i e n t s ; fertilization t o ensure m a i n t e n a n c e of s u s t a i n e d p r o d u c t i v i t y ; a n d irrigation in s o m e m o d e s of o p e r a t i o n . Irrigation w i t h surface or w e l l w a t e r is p r o b a b l y not c o s t - e f f e c t i v e (22). H o w e v e r , irrigation w i t h m u n i c i p a l s e w a g e effluent c o u l d be c o s t - e f f e c t i v e as a result of the c r e d i t g e n e r a t e d t h r o u g h land t r e a t m e n t of t h e w a s t e s (23). This latter m o d e of o p e r a t i o n is analyzed in t h e P h o t o s y n t h e s i s Energy Factory d i s c u s s e d b e l o w . H a r v e s t i n g is a s s u m e d t o be p e r f o r m e d m e c h a n i c a l l y by a harvesterc h i p p e r w h i c h c o u l d be similar in d e s i g n t o a c o r n silage harvester a d a p t e d for t h e Energy Plantation crops. T h e green c h i p s , after field storage, are t r a n s p o r t e d t o t h e c o n v e r s i o n plant located ideally in t h e c e n t e r of t h e Energy Plantation area.


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T h e Energy Plantation is c o n c e i v e d as a s e l f - c o n t a i n e d industrial o p e r a t i o n i n c l u d i n g its o w n m a n a g e m e n t a n d s u p p o r t services. T h e Energy Plantation is a s s u m e d t o consist of parcels o f land of t h e size of an average f a r m in t h e region d i s t r i b u t e d w i t h i n a g e o g r a p h i c area s u r r o u n d i n g t h e c o n v e r s i o n plant. The land selected f o r e n e r g y f a r m i n g is preferably m a r g i n a l land n o t suitable for t h e p r o d u c t i o n o f m o r e v a l u a b l e crops. Using m a r g i n a l land h o w e v e r w i l l result in p r o d u c t i v i t i e s l o w e r t h a n t h o s e m e n t i o n e d earlier as achievable. O n t h e o t h e r h a n d , m a r g i n a l land c a n p r o b a b l y be o b t a i n e d at a l o w e r cost ( t h r o u g h leasing or purchase) t h a n t h e g o o d - q u a l i t y land o n w h i c h m a n y h i g h p r o d u c t i v i t y data have been g e n e r a t e d . T h e t y p e of land available for Energy Plantations w i l l therefore be d e t e r m i n e d b y t h e overall e c o n o m i c s o f biomass p r o d u c t i o n at i n d i v i d u a l sites. In its s i m p l e s t o p e r a t i o n a l schedule, t h e t o t a l p l a n t e d area o f t h e p l a n t a t i o n is d i v i d e d into a n u m b e r of equal m o d u l e s equal t o t h e r o t a t i o n d u r a t i o n (e.g.. four m o d u l e s , each equal t o o n e - f o u r t h of t h e p l a n t e d area f o r a four-year rotation). Each year, one of these m o d u l e s is harvested a n d t h e n regenerates t h r o u g h c o p p i c i n g t o s u p p l y t h e n e w c r o p at t h e e n d o f t h e next r o t a t i o n . A f t e r a n u m b e r of c o p p i c e crops have been harvested f r o m t h e original p l a n t i n g , t h e m o d u l e m u s t be r e p l a n t e d before progressive w e a k e n i n g o f t h e root s y s t e m results in r e d u c e d a n n u a l p r o d u c t i v i t i e s . In ITC/Solar's m o d e l , regeneration of a m o d u l e is a c c o m p l i s h e d b y p l a n t i n g of clones g a t h e r e d f r o m o t h e r (still operational) areas o f t h e p l a n t a t i o n . This m o d e of o p e r a t i o n ensures sustained a n n u a l p r o d u c t i o n of biomass o n a p e r m a n e n t basis. Other c o n c e p t u a l designs of energy f a r m s have been proposed w h i c h i n c l u d e t h e same basic features as t h e ITC/Solar m o d e l (19,20.24,25). Because of t h e lack of e x p e r i m e n t a l d a t a c o n c e r n i n g s o m e aspects of energy f a r m i n g , all designs i n c l u d e a c e r t a i n e l e m e n t of u n c e r t a i n t y . Sensitivity analyses are therefore needed t o e s t i m a t e t h e i m p a c t o f these u n c e r t a i n t i e s o n t h e p r o j e c t e d biomass p r o d u c t i o n costs a n d t o e s t i m a t e reasonable ranges of values f o r these p r o d u c t i o n costs. The Photosynthesis Energy Factory A n o t h e r alternate source of energy, w h i c h also offers t h e a d d i t i o n a l a d v a n t a g e of d e c r e a s i n g t h e e n v i r o n m e n t a l i m p a c t associated w i t h t h e disposal o f w a s t e w a t e r a n d residues, is t h e c o n c e p t of g r o w i n g algae in s h a l l o w ponds. A l g a e p o n d s are o p e n s h a l l o w p o n d s in w h i c h algae a n d bacterial p o p u l a t i o n s s y m b i o t i c a l l y utilize s u n l i g h t a n d n u t r i e n t s t o p r o d u c e cell mass. T h e earliest a p p l i c a t i o n of t h e algae p o n d c o n c e p t is t h e stabilization p o n d . Stabilization p o n d s have been used by small c o m m u n i t i e s for years as a means of t r e a t i n g d o m e s t i c w a s t e w a t e r . In c o n s t r u c t i o n a n d


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o p e r a t i o n , these p o n d s are t h e essence of s i m p l i c i t y . The m a i n r e q u i r e m e n t s are land a n d a favorable, s u n n y c l i m a t e . Currently, t h e r e is a t r e n d t o w a r d t h e use of algae p o n d s as f i n i s h i n g p o n d s in i n t e g r a t e d m u n i c i p a l a n d w a s t e t r e a t m e n t . The p o n d s o p e r a t e in series t o " r e m o v e " b o t h BOD a n d p h o s p h o r u s by t y i n g up these c o m p o n e n t s in cell mass. In an algae p o n d s y s t e m for r e c o v e r y i n g e n e r g y f r o m v a r i o u s residues, t h e algae w o u l d be d i g e s t e d a n a e r o b i c a l l y t o y i e l d a m e t h a n e - c o n t a i n i n g gas, w h i c h w o u l d be processed into SNG. A s l o n g as 2 0 years a g o . w o r k w a s u n d e r t a k e n by Dr. W . J . O s w a l d a n d others at t h e U n i v e r s i t y of California at Berkeley t o d e v e l o p a s y s t e m u t i l i z i n g t h e algae p o n d c o n c e p t b o t h t o t r e a t w a s t e (26-29) a n d t o p r o d u c e fuels (30.31). Indeed, m i c r o a l g a e w e r e a m o n g t h e earliest " f u e l c r o p s " p r o p o s e d , t h e t e c h n i c a l p r o b l e m s w e r e essentially c o n c e r n e d w i t h i n t e g r a t i n g t h e c o m p o nents and optimizing their operation. The components themselves — the algae p o n d , t h e digester, a n d t h e s e d i m e n t a t i o n , s e p a r a t i o n , a n d f i n i s h i n g stages — w e r e already b e i n g used in w a s t e t r e a t m e n t . T h e w o r k over t h e last 2 0 years has c o n c e n t r a t e d in t h r e e areas: (1) i d e n t i f y i n g a n d q u a n t i f y i n g algal g r o w t h - l i m i t i n g f a c t o r s (nutrients, species c h a r a c t e r i s t i c s , a n d c l i m a t o l o g i c a l p a r a m e t e r s ) ; (2) m a x i m i z i n g gas p r o d u c t i o n f r o m anaerobic f e r m e n t a t i o n ; a n d (3) o p t i m i z i n g t h e s y s t e m w i t h respect t o gas p r o d u c t i o n , residue uptake, land utilization, a n d cost-effectiveness. S t a t e - o f - t h e - a r t r e v i e w s have been p u b l i s h e d r e c e n t l y r e g a r d i n g t h e e n g i n e e r i n g aspects (32) of m i c r o a l g a e p r o d u c t i o n a n d t h e p o t e n t i a l of m i c r o a l g a e as b i o c o n v e r s i o n s y s t e m s (33.34). O t h e r p u b l i c a t i o n s have r e p o r t e d recent research o n species c o n t r o l , algae h a r v e s t i n g , a n d t h e p o t e n t i a l of b l u e - g r e e n algae (35-39). Both t h e Energy Plantation a n d t h e algae p o n d c a n c o n t r i b u t e t o t h e s o l u t i o n of t h e p o p u l a t i o n , resources a n d e n e r g y p r o b l e m s f a c i n g us. Recently, it b e c a m e a p p a r e n t t h a t t h e y c o u l d perhaps better a c c o m p l i s h these missions w h e n t h e t w o are i n t e g r a t e d t o f o r m one c o m p o s i t e s y s t e m , as s h o w n in Figure I. In short, each b i o c o n v e r s i o n s y s t e m p r o d u c e s a b y - p r o d u c t t h a t can be used t o a d v a n t a g e by t h e other. The c a r b o n c o n t e n t of s e w a g e limits the p r o d u c t i o n of t h e algae p o n d , b u t c a r b o n d i o x i d e , a b y - p r o d u c t of c o m b u s t i o n of solid Energy Plantation fuel ( w h i c h c u r r e n t l y appears t o be t h e best w a y of u s i n g p l a n t m a t t e r as fuel), c a n be s u p p l i e d t o t h e algae p o n d t o increase its p r o d u c t i v i t y . T h e w a s t e heat f r o m t h e boiler c a n be used t o c o n t r o l t h e t e m p e r a t u r e of t h e algae digester. T h e s l u d g e g e n e r a t e d as a byp r o d u c t of t h e algae p o n d p r o v i d e s a source of inorganic n u t r i e n t s a n d w a t e r for t h e Energy P l a n t a t i o n , w h i c h t h u s provides an ideal disposal site for t h e sludge.


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T h e Photosynthesis Energy Factory (PEF) is a synergistic c o m b i n a t i o n of t h e d r y - l a n d Energy Plantation a n d t h e algae p o n d w h i c h c a n p r o d u c e o n a p e r p e t u a l l y r e n e w a b l e basis, n o n p o l l u t i n g a n d t o t a l l y d o m e s t i c fuels f r o m m a r g i n a l l y useful land, solar energy, a n d various residues. S i m u l t a n e o u s l y , f r o m different parts o f t h e PEF, c h i p p e d solid fuel is p r o d u c e d , f r o m w h i c h e l e c t r i c i t y is g e n e r a t e d , a n d m e t h a n e or SNG is recovered f r o m w a s t e C O 2 a n d m u n i c i p a l or industrial w a s t e w a t e r . I n c i d e n t a l e c o n o m i c benefits — w h i c h are s i g n i f i c a n t — i n c l u d e s e c o n d a r y or t e r t i a r y t r e a t m e n t of m u n i c i p a l a n d industrial e f f l u e n t s , a n d t h e c o m p l e t e e l i m i n a t i o n o f t h e need for a sanitary landfill f o r disposal of t h e resultant sludge. The PEF is n o t merely a c o m b i n a t i o n of c o n v e n i e n c e , b u t a t r u l y i n t e r a c t i v e utilization of materials a n d energy. Repeated a p p l i c a t i o n o f s l u d g e f r o m t h e algae p o n d c o u l d result in a progressive a c c u m u l a t i o n of s o m e t o x i c e l e m e n t s originally present in t h e w a s t e w a t e r s fed t o t h e p o n d . T h e rate of a c c u m u l a t i o n o f t h e t o x i c e l e m e n t s w i l l d e p e n d o n various site-specific factors s u c h as t y p e o f w a s t e w a t e r ( m u n i c i p a l w a s t e w a t e r is m u c h less likely t h a n industrial w a s t e w a t e r t o c o n t a i n p o t e n t i a l l y t o x i c e l e m e n t s ) , soil t y p e , t e x t u r e a n d p H . It has been e s t i m a t e d t h a t in m a n y areas o f t h e U n i t e d States, it w o u l d require 5 0 t o 8 0 years before t h e a c c u m u l a t i o n o f t o x i c e l e m e n t s reached levels c o n s i d e r e d d a n g e r o u s b y t h e EPA f o r land d e v o t e d t o f o o d c r o p p r o d u c t i o n (23). Exceeding these levels of t o x i c e l e m e n t s w o u l d e l i m i n a t e t h e possibility of u s i n g t h e land f o r f o o d crops at a later date. In t h e d i a g r a m of t h e PEF s h o w n in Figure I, t h e a s s u m p t i o n has been m a d e t h a t t h e w o o d y biomass is used as fuel f o r g e n e r a t i n g electricity. Biomass can of course be used as f e e d s t o c k f o r o t h e r c o n v e r s i o n processes t o p r o d u c e a w i d e variety of fuels or c h e m i c a l s . H o w e v e r , direct c o m b u s t i o n of w o o d y biomass is a n a c c e p t e d , c o m m e r c i a l t e c h n o l o g y , a l l o w i n g c o m p a r i s o n w i t h alternate m e t h o d s o f g e n e r a t i n g electricity, a n d t h e e m p h a s i s of t h e studies of t h e PEF w a s o n d e v e l o p m e n t o f t h e b i o m a s s p r o d u c t i o n processes a n d their i n t e g r a t i o n rather t h a n t h e s t u d y of biomass c o n v e r s i o n . A n y o t h e r process f o r c o n v e r s i o n of w o o d y biomass c o u l d be used in t h e PEF c o n c e p t . Description of Projects A n initial project w a s u n d e r t a k e n t o s t u d y t h e c o n c e p t of t h e PEF a n d its characteristics a n d a p p a r e n t benefits. T h e project w a s d i v i d e d into three parts or tasks. T h e o b j e c t i v e of o n e task w a s t o analyze t h e c o n c e p t of t h e PEF, w i t h particular e m p h a s i s o n t h e c o m p l e m e n t a r y a n d synergistic aspects of t h e s y s t e m . A s e c o n d task w a s c o n c e r n e d w i t h t h e analysis a n d selection



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of p o t e n t i a l sites. T h e final task w a s t o d e v e l o p p r e l i m i n a r y designs a n d associated cost e s t i m a t e s f o r p o t e n t i a l d e m o n s t r a t i o n systems at t h e best sites. In t h e PEF, various s t r e a m s of e n e r g y a n d materials f l o w b e t w e e n three major s u b s y s t e m s — t h e d r y - l a n d Energy Plantation, a w o o d - f i r e d p o w e r plant, a n d an algae p r o d u c t i o n s y s t e m . T o analyze t h e resultant i n t e r a c t i o n s b e t w e e n the subsystems, a comprehensive technoeconomic model was developed to d e s c r i b e t h e PEF's p e r f o r m a n c e a n d cost. M o d e l s of t h e three s u b s y s t e m s w e r e d e v e l o p e d w i t h t h e aid o f i n f o r m a t i o n a n d data t h a t w e r e already available as t h e result of previous studies. N e w d a t a and n e w c o n c e p t s w e r e i n t r o d u c e d into t h e m o d e l s w h e r e v e r possible. T h e University of California at Berkeley s u p p l i e d s t a t e - o f - t h e - a r t data o n algae p o n d p e r f o r m a n c e a n d costs. These s u b s y s t e m m o d e l s w e r e e n g i n e e r i n g m o d e l s d e v e l o p e d in s u f f i c i e n t detail t o represent t h e i m p o r t a n t variables a n d v a r i a b l e - p a r a m e t e r interact i o n s i n f l u e n c i n g s u b s y s t e m p e r f o r m a n c e a n d costs. Of t h e three s u b s y s t e m m o d e l s , t h e m o s t c o m p r e h e n s i v e a n d t h e m o s t c o m p l e x w a s t h e Energy Plantation m o d e l , w h i c h is a c o m p l e t e design m o d e l . For t h e t w i n purposes of d e f i n i n g t h e a p p l i c a b i l i t y of t h e PEF c o n c e p t a n d s e l e c t i n g t h e best site for a d e m o n s t r a t i o n PEF project, data w e r e g a t h e r e d o n t h e characteristics of land, t h e availability of m u n i c i p a l a n d industrial e f f l u e n t s a n d residues, a n d t h e s u p p l y a n d d e m a n d for e n e r g y at a w i d e v a r i e t y a n d n u m b e r o f p o t e n t i a l sites. A f o r m a t w a s d e v e l o p e d f o r h a n d l i n g t h i s data base, a n d a n u m b e r o f s u i t a b i l i t y indexes w e r e d e f i n e d f o r e v a l u a t i n g t h e site d a t a . Data w e r e o b t a i n e d f r o m a n u m b e r o f sources in t h e literature as w e l l as f r o m state e n e r g y offices. A s t h e result of this siteselection p r o c e d u r e , a n u m b e r o f sites w e r e c h o s e n for analysis b y m e a n s o f the technoeconomic model. T h e m o d e l w a s t h e n used t o d e s i g n a d e m o n s t r a t i o n PEF s y s t e m at each of t h e selected p o t e n t i a l sites. This p r e l i m i n a r y d e s i g n illustrated for a specific site t h e benefits a n d t h e i m p a c t t o be e x p e c t e d f r o m a d e m o n s t r a t i o n PEF project. Estimated costs w e r e p r o v i d e d also f o r each d e m o n s t r a t i o n PEF. C o m p a r i n g these p r e l i m i n a r y d e s i g n s a n d their costs w a s t h e n d o n e t o s h o w w h e r e a n d under w h a t c o n d i t i o n s a PEF w o u l d be e x p e c t e d t o be coste f f e c t i v e in r e c y c l i n g w a s t e s a n d p r o d u c i n g fuels f r o m biomass w h i c h w o u l d be c o m p e t i t i v e w i t h presently used fuels. T h e results of this initial project have been p u b l i s h e d (21). T h e analysis w h i c h w a s p e r f o r m e d in t h i s initial project i n d i c a t e d t h a t s o m e i n t e r a c t i o n s b e t w e e n t h e PEF s u b s y s t e m s are generally c o s t - e f f e c t i v e w h i l e o t h e r s are p r o b a b l y site-specific or can be i m p r o v e d u p o n . From these initial


504


results, it w a s c o n c l u d e d t h a t certain r e f i n e m e n t s in t h e d e s i g n of a PEF s h o u l d be analyzed t o give t h e PEF greater a p p l i c a b i l i t y as w e l l as t o i m p r o v e its e c o n o m i c s . T h u s , it w a s d e c i d e d t o investigate in m o r e detail certain aspects of t h e d e s i g n a n d o p e r a t i o n of t h e d r y - l a n d Energy Plantation subsystem. One possible i n t e r a c t i o n w i t h i n a PEF w h i c h w a s n o t c o n s i d e r e d in t h e initial project is t h e c o n t r i b u t i o n of w a t e r f r o m t h e w a s t e w a t e r t r e a t m e n t s u b s y s t e m t o t h e d r y - l a n d Energy P l a n t a t i o n . One of t h e original s i g n i f i c a n t credits r e s u l t i n g f r o m t h e w e t l a n d s biological w a s t e w a t e r t r e a t m e n t s u b s y s t e m c a n a u g m e n t t h e available natural rainfall or even s u p p l y t h e entire w a t e r r e q u i r e m e n t of a PEF. T h u s , it m i g h t be possible t o site a PEF in s e m i - a r i d or arid l o c a t i o n s t o e x p a n d its a p p l i c a b i l i t y . T h e results f r o m t h e initial p r o j e c t i n d i c a t e d t h a t s u p p l y i n g t h e necessary n u t r i e n t s t o m a i n t a i n t h e p r o d u c t i v i t y of t h e land — p a r t i c u l a r l y n i t r o g e n — w a s a s i g n i f i c a n t cost i t e m in t h e e c o n o m i c s of p r o d u c i n g w o o d y biomass. Because of t h e i m p o r t a n c e of n u t r i e n t s , it w a s d e c i d e d t h a t t h e n u t r i e n t balance in t h e Energy Plantation m o d e l n e e d e d t o be refined t o p r e d i c t t h e required a m o u n t of n u t r i e n t s m o r e precisely. In particular, because t h e leaves c o n t a i n a h i g h p e r c e n t a g e of n i t r o g e n c o m p a r e d t o t h e w o o d , w o r k is b e i n g d o n e t o i n c l u d e in t h e n u t r i e n t balance t h e effect of n u t r i e n t r e c y c l i n g via leaf fall a n d a m o r e precise a c c o u n t i n g of n u t r i e n t l e a c h i n g . T r a n s p o r t a t i o n w a s a n o t h e r s i g n i f i c a n t cost i t e m w h i c h appeared t o have p o t e n t i a l for c o s t s a v i n g s t h r o u g h a m o r e d e t a i l e d analysis of alternative s y s t e m designs. T h u s , alternative m e t h o d s for h a n d l i n g a n d t r a n s p o r t i n g t h e w o o d y biomass are b e i n g analyzed, s u c h as p n e u m a t i c t u b e t r a n s p o r t , c h i p b a l i n g , a n d a l t e r n a t i v e m e t h o d s for d r y i n g t h e chips. In a d d i t i o n , t h e t r a n s p o r t a t i o n s y s t e m is b e i n g analyzed in greater detail t o see w h e r e cost savings m i g h t be a c h i e v e d t h r o u g h o p t i m i z a t i o n . T h e initial results f r o m s t u d y i n g t h e PEF c o n c e p t i n d i c a t e d t h a t t h e m o s t s i g n i f i c a n t c r e d i t r e s u l t i n g f r o m t h e w e t l a n d s biological w a s t e - w a t e r t r e a t m e n t s u b s y s t e m w a s t h e w a s t e w a t e r t r e a t m e n t credit itself rather t h a n t h e c r e d i t for t h e v a l u e of t h e gas p r o d u c e d . T h u s , it b e c a m e of interest t o look for b e t t e r w a y s of i n c o r p o r a t i n g t h e w a s t e w a t e r t r e a t m e n t f u n c t i o n w i t h i n t h e PEF t h a n via an algae p o n d . O n e w a y t h a t t h i s m i g h t be d o n e is t o a p p l y t h e w a s t e w a t e r d i r e c t l y t o t h e Energy Plantation. H o w e v e r , t h i s process has l i m i t a t i o n s , w i t h respect t o b o t h t h e particular location a n d local soil q u a l i t y , a n d t h e c o m p o s i t i o n of t h e w a s t e w a t e r . W o r k is therefore b e i n g d o n e t o collect t h e necessary data o n t e c h n i c a l l i m i t a t i o n s a n d EPA regulations a n d t o d e v e l o p a m o d e l for this process. In a d d i t i o n o t h e r w e t l a n d s biological


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species besides algae have been s u g g e s t e d for w a s t e w a t e r t r e a t m e n t a n d it w a s d e c i d e d t o investigate t h e possible use of these o t h e r plants t o p e r f o r m this function. Finally, a d d i t i o n a l w o r k is b e i n g d o n e in this s e c o n d project t o look for n e w a n d i m p r o v e d t e c h n o l o g y t o i n c l u d e in t h e p o w e r plant s u b s y s t e m m o d e l . A d d i t i o n a l p o t e n t i a l sites are also t o be i d e n t i f i e d w h e r e t h e n e w m o d e s o f o p e r a t i n g a PEF — e.g., w i t h irrigation or d i r e c t a p p l i c a t i o n of w a s t e w a t e r — w o u l d be applicable. T h e n e w c o m p l e t e PEF m o d e l w i l l t h e n be used t o c o m p a r e t h e various m o d e s o f o p e r a t i o n , a n d t o d e t e r m i n e t h e e c o n o m i c v i a b i l i t y of PEF systems f o r sites d i s p l a y i n g w i d e l y different local c l i m a t i c a n d site-specific c o n s t r a i n t s . T h e f o l l o w i n g sections of this paper w i l l describe t h e s u b s y s t e m m o d e l s w h i c h w e r e d e v e l o p e d in t h e initial project t o s t u d y t h e PEF a n d present s o m e o f t h e overall results o b t a i n e d by s i m u l a t i n g t h e p e r f o r m a n c e of t h e entire PEF s y s t e m . A d d i t i o n s t o t h e m o d e l w h i c h are b e i n g d e v e l o p e d as t h e result of w o r k in t h e s e c o n d project w i l l also be d e s c r i b e d , a n d s o m e results of t h e analysis o f these i m p r o v e d aspects of PEF d e s i g n a n d o p e r a t i o n w i l l also be presented. THE ENERGY P L A N T A T I O N S U B S Y S T E M Description of Initial M o d e l In t h e PEF c o n c e p t , t h e i n p u t s t o t h e Energy Plantation m o d e l are a d e s c r i p t i o n of t h e site b e i n g i n v e s t i g a t e d , s u p p l i e d b y t h e site-selection p r o c e d u r e , a n d t h e a m o u n t s o f n i t r o g e n a n d p h o s p h o r u s recycled t o t h e p l a n t a t i o n s u p p l i e d by t h e algae p o n d m o d e l . T h e major o u t p u t s of t h e p l a n t a t i o n m o d e l are t h e yearly a m o u n t o f biomass p r o d u c e d a n d its cost (green chips) delivered at t h e utilization point. These items c o n s t i t u t e t h e m a j o r i n p u t s t o t h e p o w e r plant m o d e l . M a n p o w e r a n d e q u i p m e n t r e q u i r e m e n t s ; species s u g g e s t e d for t h e p l a n t a t i o n ; a n d p l a n t i n g , h a r v e s t i n g , a n d o p e r a t i n g schedules f o r t h e p l a n t a t i o n are secondary o u t p u t s of t h e plantation model. This p l a n t a t i o n m o d e l is an e x t e n s i o n a n d generalization of w o r k d o n e in previous studies (40,41). T h e m o d e l i n c l u d e s a n u m b e r of s u b m o d e l s w h i c h are discussed b e l o w . These s u b m o d e l s i n c l u d e : (1) land resources. (2) data base o n plants. (3) r e c y c l e d i n p u t s . (4) species selection a n d c h a r a c t e r i z a t i o n , (5) plant g r o w t h m o d e l . (6) d a t a base f o r field operations, (7) field operations, (8) d a t a base f o r cost e s t i m a t e s , a n d (9) cost e s t i m a t e f o r biomass p r o d u c e d .


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T h e land resources s u b r o u t i n e is t h e m a j o r i n p u t t o t h e m o d e l . T h e data g e n e r a t e d t h r o u g h t h e site-selection p r o c e d u r e are organized in three categories: land d e s c r i p t i o n , land c a p a b i l i t y , a n d land cost. The land available for p l a n t a t i o n o p e r a t i o n s is characterized by t h e p l a n t a t i o n d e n s i t y or ratio of t h e p l a n t a t i o n area t o t h e t o t a l g e o g r a p h i c area e n c o m p a s s i n g t h e p l a n t a t i o n a n d by t h e average size of t h e parcels m a k i n g u p t h e p l a n t a t i o n area. B o t h f a c t o r s have been s h o w n t o have a s i g n i f i c a n t effect o n t h e cost of t h e b i o m a s s p r o d u c e d . (42) T h e p l a n t a t i o n area w o u l d i n c l u d e t h e actual p l a n t e d area plus necessary service roads a n d irrigation lanes if w a r r a n t e d . T h e land q u a l i t y or ability t o s u p p o r t plant g r o w t h is characterized in t e r m s of t h e land classes used in t h e N a t i o n a l I n v e n t o r y of Soil a n d W a t e r C o n s e r v a t i o n Needs (43). A c o r r e l a t i o n w a s established b e t w e e n e x p e r i m e n t a l y i e l d data (ODT/ac-yr) a n d land classes o n w h i c h t h e data w e r e g e n e r a t e d (a linear relation w a s used, w h i c h had a c o r r e l a t i o n c o e f f i c i e n t > 0 . 7 5 ) . L a n d of Class III w a s a s s u m e d t o have an index of 1. t h e r e b y a l l o w i n g t h e p r o d u c t i v i t y of l a n d of o t h e r classes t o be e s t i m a t e d o n t h e basis of relative p r o d u c t i v i t y indexes. These p r o d u c t i v i t y indexes w e r e used t o w e i g h t t h e e x p e r i m e n t a l y i e l d data f r o m t h e literature t o a c c o u n t for t h e d i f f e r e n c e in land p r o d u c t i v i t y b e t w e e n t h e land c o n s i d e r e d in t h e analysis of specific sites a n d t h e land o n w h i c h t h e e x p e r i m e n t a l data w e r e g e n e r a t e d . This a p p r o a c h is s o m e w h a t similar t o t h e a p p r o a c h used by Marshall a n d Tsang (22) t o relate t h e relative v a l u e of t h e yields e x p e c t e d f r o m land of various classes s u b m i t t e d t o c o m p a r a b l e c u l t u r a l practices, t o land classes. T h e m e t h o d p r o p o s e d here t o e s t i m a t e yields at a g i v e n site on t h e basis of yields m e a s u r e d at e x p e r i m e n t a l sites s h o u l d be refined as more data b e c o m e available. T h e data base on plants c o n t a i n s d a t a d e s c r i b i n g t h e g r o w t h characteristics of t h e species of interest for p l a n t a t i o n a p p l i c a t i o n s . Plant g r o w t h a n d yields o n an Energy Plantation are p r e d i c t e d by m e a n s of a m o d e l d e s c r i b i n g j u v e n i l e plant g r o w t h w h i c h w a s d e v e l o p e d at I n t e r T e c h n o l o g y / S o l a r Corporation (21). The m o d e l c o n t a i n s several parameters w h i c h are characteristic of t h e species c o n s i d e r e d a n d are d e t e r m i n e d f r o m e x p e r i m e n tal d a t a . T h e d a t a base c o n t a i n s t h e values of these c h a r a c t e r i s t i c p a r a m e t e r s w h i c h have been f o u n d for various species. T h e d a t a base also lists t h e forest regions, a n d soil classes a n d subclasses in w h i c h each of t h e species is expected to grow. In t h e s u b r o u t i n e d e s c r i b i n g t h e recycled i n p u t s , t h e n u t r i e n t value (Ν. Κ a n d P) of t h e ash a n d s l u d g e r e c y c l e d f r o m t h e boilers a n d digesters is e s t i m a t e d a n d c o m p a r e d t o t h e a m o u n t s required by the p l a n t a t i o n t o m a i n t a i n sustained yields.


25.

FRASER E T A L .


507

Species t o be i n c l u d e d in a p l a n t a t i o n are selected f r o m t h e data bank on t h e basis of t h e forest g r o u p , land class, a n d subclass t o w h i c h t h e p l a n t a t i o n land belongs. The c h a r a c t e r i s t i c p a r a m e t e r s o f each species retained are t h e n a d j u s t e d t o take into a c c o u n t t h e d i f f e r e n c e in soil q u a l i t y b e t w e e n t h e p l a n t a t i o n land a n d t h e land o n w h i c h t h e e x p e r i m e n t a l d a t a f o r each species were generated. In t h e plant g r o w t h s u b r o u t i n e , average a n n u a l s u s t a i n e d yields are e s t i m a t e d f o r each species f o r various h a r v e s t i n g cycles a n d p l a n t i n g densities b y m e a n s of t h e m o d e l d e s c r i b i n g j u v e n i l e plant g r o w t h . In m o s t cases, t h e d i s c r e p a n c y b e t w e e n c a l c u l a t e d values of these s u s t a i n e d yields a n d e x p e r i m e n t a l d a t a is of t h e order o f 5 t o 10 p e r c e n t o f t h e e x p e r i m e n t a l data. This level of precision is satisfactory as it is c o m p a r a b l e t o or smaller t h a n t h e observed f l u c t u a t i o n s in yield d u e t o yearly c l i m a t i c variations. S i g n i f i c a n t variations in field d a t a are o b s e r v e d , a n d t h e p r o p o s e d m o d e l w i l l have t o be revised or refined as m o r e data b e c o m e available. T h e y i e l d - c y c l e c o m b i n a t i o n s g e n e r a t i n g a m o u n t s o f biomass w i t h i n 5 or 10 p e r c e n t of t h e m a x i m u m p r e d i c t e d are retained f o r f u r t h e r analysis. Field o p e r a t i o n s in a n Energy Plantation i n c l u d e h a r v e s t i n g a n d c h i p p i n g , c u l t i v a t i o n , fertilization, c l o n e g e n e r a t i o n , r e p l a n t i n g of a f r a c t i o n o f t h e p l a n t a t i o n , a n d t r a n s p o r t a t i o n of t h e biomass t o t h e p o i n t o f utilization. These o p e r a t i o n s are s u p p o r t e d b y m a i n t e n a n c e a n d a d m i n i s t r a t i v e teams. For each of t h e y i e l d - c y c l e c o m b i n a t i o n s o f interest, t h i s s u b r o u t i n e establishes t h e i n v e n t o r y of t h e e q u i p m e n t , p e r s o n n e l , a n d supplies required t o m a i n t a i n t h e operation. In t h e c o s t - e s t i m a t i n g s u b r o u t i n e , t h e cost of biomass delivered in t h e f o r m of c h i p s at p o i n t of use is e s t i m a t e d o n t h e basis of t h e u t i l i t y ( 1 1 . 1 % average r e t u r n , i n c o m e t a x paid o n e q u i t y return) a n d m u n i c i p a l (6.375% r e t u r n , n o i n c o m e tax) m e t h o d s of f i n a n c i n g . T h e resultant o u t p u t of t h e s u b r o u t i n e s h o w s t h e various c o m p o n e n t s of t h e overall cost of biomass for b o t h m e t h o d s of f i n a n c i n g . Table II s h o w s t h e e c o n o m i c analysis f o r a particular site a n d a b r e a k d o w n of t h e capital a n d o p e r a t i n g costs i n v o l v e d in t h e Energy Plantation s u b s y s t e m o f a PEF. T h e d e p r e c i a b l e i n v e s t m e n t includes t h e capital cost of p l a n t a t i o n installation (land clearing) a n d s t a r t u p (planting stock). F o l l o w i n g this analysis, t h e major c o m p o n e n t of t h e total revenue is t h e cost o f n i t r o g e n fertilizer. T h e c o n t r i b u t i o n of fertilizers t o t h e cost of biomass p r o d u c t i o n has been n o t e d by o t h e r investigators (44). A s a result, m u c h e m p h a s i s has been g i v e n t o t h e p r o b l e m of n u t r i e n t r e q u i r e m e n t s in t h e e x p a n d e d version of t h e PEF m o d e l (see below). T h e final cost data used t o characterize t h e p o t e n t i a l o f a site f o r biomass p r o d u c t i o n w e r e averages of t h e data f o r each of t h e species c o n s i d e r e d f o r t h e site. This a p p r o a c h w a s a d o p t e d as Energy Plantations are a s s u m e d t o i n c l u d e a m i x of species.


508


T a b l e I I . C O S T A N A L Y S I S FOR N A T C H I T O C H E S , L A , S I T E 3 6 , 0 0 0 acres of p l a n t a t i o n H y b r i d Poplar NE 3 8 8 4 f t / t r e e , harvest every 2 years 8.52 O D T / a c - y r Municipal financing 2

Annual Equivalent Cost, $* Depreciable I n v e s t m e n t Nondepreciable Investment Federal I n c o m e Tax A n n u a l O p e r a t i n g Costs Fuels L a n d Rental T o t a l Labor Administrative & Overhead Supplies — Nitrogen Phosphorus Potassium Lime Others M a i n t e n a n c e a n d Repair Local Taxes & Insurance T o t a l Revenue Required

Percent of Total Revenue

938.700 49.300 0

16.7 0.9 0.0

140,100 1.113.100 972,200 194,400 1,365.000 43,000 361.100 8.900 81.200 322.600 39,600 5,629.200

2.5 19.7 17.3 3.5 24.2 0.8 6.4 0.2 1.6 5.7 0.7

Product Cost (Delivered in t h e f o r m of g r e e n chips) $/ODT 17.71 $/10 Btu 1.03 6

* 1 9 7 7 dollars.


25.

FRASER E T A L .

509


Fifteen sites w e r e i d e n t i f i e d f o r f u r t h e r analysis of t h e PEF c o n c e p t . T h e selection criteria i n c l u d e d resource availability (land, w a s t e w a t e r , labor, climate) a n d m a r k e t for t h e PEF p r o d u c t s a n d services (electricity, natural gas, s t e a m , need f o r w a s t e t r e a t m e n t , a n d jobs). T h e 15 selected sites w e r e analyzed w i t h t h e p l a n t a t i o n m o d e l . Species i n c l u d e d h y b r i d (Populus spp.). Eastern c o t t o n w o o d (P. deltoïdes), plains c o t t o n w o o d (P. sargentii), silver m a p l e (Acer saccharinum). A m e r i c a n s y c a m o r e (Plantanus occidentalis) (southern a n d m i d l a t i t u d e sites), a n d Eucalyptus (Florida sites only). Proposed p l a n t i n g densities w e r e generally b e t w e e n 4 a n d 12 f t / t r e e w i t h harvests (first a n d coppice) o f 4 O D T / a c - y r (Maysville, KY) t o a b o u t 9 O D T / a c - y r (Bemidji. MN). Differences in cost o f biomass w e r e related t o a n u m b e r of factors, i n c l u d i n g t h e i n v e s t m e n t , land rental, t r a n s p o r t a t i o n costs, a n d a m o u n t of n u t r i e n t s r e c y c l e d t o t h e p l a n t a t i o n . The cost of biomass at t h e 15 p o t e n t i a l sites ranged f r o m $ 1 6 . 9 0 t o $ 2 3 . 5 9 per o v e n - d r y t o n ( m u n i c i p a l f i n a n c i n g ) . These costs are c o m p a r a b l e t o t h o s e e s t i m a t e d o n t h e basis of o t h e r tree f a r m designs f o r similar locations. 2

Nutrient Balance The n u t r i e n t balance m o d e l p r e d i c t s t h e a m o u n t of inorganic fertilizer required t o m a i n t a i n site fertility, a n d t h u s p r o d u c t i v i t y , of t h e p l a n t a t i o n . Fertilizer a p p l i c a t i o n is needed because o f t h e shorter rotations and t h e m o r e c o m p l e t e removal of biomass f r o m t h e site t h a t o c c u r s w i t h this t y p e of m a n a g e m e n t as c o m p a r e d t o c o n v e n t i o n a l forestry. A l t h o u g h t h e soil n u t r i e n t pool at a n y g i v e n site is a n u n k n o w n , m a i n t e n a n c e of site fertility is a s s u m e d t o be possible b y r e p l a c i n g t h e a m o u n t of n u t r i e n t s r e m o v e d in t h e harvested biomass plus a n a d d i t i o n a l a m o u n t f o r leaching a n d d e n i t r i f i c a t i o n losses. T h e m o d e l treats t h e soil n u t r i e n t pool as t h e s y s t e m of interest, t h e upper b o u n d a r y i n c l u d i n g a n y surface o r g a n i c horizons t h a t m a y be present, t h e l o w e r b o u n d a r y b e i n g t h e u n d e r l y i n g bedrock, a n d t h e laterial b o u n d a r i e s e x t e n d i n g t o t h e b o u n d a r i e s of t h e site. Inputs t o t h e s y s t e m i n c l u d e t h e a p p l i c a t i o n of inorganic fertilizer, r e t u r n of ash material f r o m t h e p o w e r plant c o m p o n e n t of t h e PEF, a p p l i c a t i o n of s l u d g e f r o m t h e algae p o n d , n i t r o g e n f i x a t i o n by cover crops or i n t e r p l a n t e d w o o d y n i t r o g e n - f i x i n g species, a n d t h e r e c y c l i n g of leaf material n o t r e m o v e d in t h e harvested biomass. O u t p u t s f r o m t h e s y s t e m i n c l u d e t h e g r o w t h of w o o d a n d leaf material, leaching a n d erosion losses, a n d d e n i t r i f i c a t i o n losses f o r soil n i t r o g e n . A l t h o u g h inputs f r o m a t m o s p h e r i c sources a n d t h e w e a t h e r i n g o f bedrock are operative a n d can be i m p o r t a n t over long periods o f t i m e (45-47), t h e y are n o t i n c l u d e d in t h e m o d e l because t h e m a g n i t u d e of their c o n t r i b u t i o n over t h e short


510


d u r a t i o n of t h e rotations used in this t y p e of m a n a g e m e n t is a s s u m e d t o be small in c o m p a r i s o n t o t h e o t h e r i n p u t s . In t h e original n u t r i e n t balance m o d e l t h e a n n u a l fertilizer r e q u i r e m e n t w a s c a l c u l a t e d on t h e basis of r e p l a c i n g t h e n u t r i e n t s r e m o v e d in t h e biomass harvested less n u t r i e n t credits o b t a i n e d f r o m r e c y c l i n g t h e s l u d g e f r o m t h e algae p o n d a n d t h e ash f r o m t h e boiler t o t h e Energy Plantation. Credits w e r e t a k e n o n l y for n i t r o g e n a n d p h o s p h o r u s in t h e s l u d g e a n d c a l c i u m in t h e ash. A l l o w a n c e w a s m a d e for loss of n u t r i e n t s by l e a c h i n g , a n d a p p l i c a t i o n of fertilizer w a s a s s u m e d t o o c c u r o n l y o n c e per r o t a t i o n . T h e n u t r i e n t s present in t h e b i o m a s s harvested w e r e c a l c u l a t e d o n t h e basis of tables d e v e l o p e d f r o m data o n (1) b i o m a s s d i s t r i b u t i o n in leaves, s t e m s , a n d b r a n c h e s of y o u n g trees as a f u n c t i o n of age, a n d (2) t h e n u t r i e n t c o m p o s i t i o n of leaves, s t e m s , a n d b r a n c h e s for y o u n g trees. T h e d a t a base used for (1) a b o v e w a s g e n e r a t e d in s h o r t - r o t a t i o n field trials of A m e r i c a n s y c a m o r e (Platanus occidentalis) (48), a n d t h a t used for (2) a b o v e c o n s i s t e d of averages of analyses p e r f o r m e d on a n u m b e r of o l d e r - g r o w t h h a r d w o o d species (49). It s h o u l d be n o t e d t h a t t h e r e are a n u m b e r of d i f f i c u l t i e s i n h e r e n t in t h e use of tissue analyses for p r e d i c t i o n of fertilizer r e q u i r e m e n t s (50,51). j u s t as there are d i f f i c u l t i e s a n d l i m i t a t i o n s i n v o l v e d w i t h t h e use of soil analyses (ΕΠ .52). T h e relative d i s t r i b u t i o n of b i o m a s s into s t e m , b r a n c h a n d leaf c o m p o n e n t s w i l l vary w i t h t h e species i n v o l v e d , t h e age of t h e tree (rotation l e n g t h ) , a n d t h e d e n s i t y ( n u m b e r of trees per u n i t area) of t h e p l a n t a t i o n . T h e c h e m i c a l c o m p o s i t i o n of t h e i n d i v i d u a l c o m p o n e n t s w i l l also vary w i t h species, age, site f e r t i l i t y , a n d t h e t i m e of year of s a m p l i n g (e.g., seasonal f l u x of Ν a n d Ρ f r o m leaves t o t w i g s ) (50). These differences are even m a n i f e s t e d w i t h i n t h e i n d i v i d u a l biomass c o m p o n e n t s . T h e variables used in t h e m o d e l are general d e s i g n variables t h a t c a n be easily m o d i f i e d t o o b t a i n better p r e d i c t i v e results either w i t h n e w d a t a f r o m field trials or site-specific i n f o r m a t i o n for a p a r t i c u l a r m a n a g e m e n t d e s i g n . Several o t h e r general c h a r a c t e r i s t i c s of t h e m o d e l s h o u l d be m e n t i o n e d . First, t h e m o d e l p r e d i c t s fertilizer r e q u i r e m e n t s for t h e p l a n t a t i o n o n c e steady-state c o n d i t i o n s have been a t t a i n e d . Steady-state is d e f i n e d as t h a t p o i n t at w h i c h t h e m a n a g e m e n t schedule's c y c l i c o p e r a t i o n results in t h e d e c o m p o s i t i o n (mineralization) of t h e o r g a n i c i n p u t s f r o m p r e v i o u s years in an a m o u n t equal t o t h e o r g a n i c i n p u t s for o n e year; t h i s is e q u i v a l e n t t o s a y i n g t h a t all of t h e a n n u a l i n p u t s are d e c o m p o s e d in o n e year, m e a n i n g t h a t t h e n u t r i e n t s c o n t a i n e d in t h e o r g a n i c m a t t e r b e c o m e available for u p t a k e by t h e plant in g r o w t h or loss in l e a c h i n g . A n n u a l i n p u t s t h u s equal a n n u a l o u t p u t s .


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T h e rate of b r e a k d o w n o f o r g a n i c material in natural stands is d e p e n d e n t u p o n e n v i r o n m e n t a l c o n d i t i o n s a n d t h e c h e m i c a l nature o f t h e s u b s t r a t e b e i n g d e c o m p o s e d . Litter half lives are in t h e range of one t o a f e w years f o r t e m p e r a t e d e c i d u o u s forests as a w h o l e (53). T e m p e r a t u r e a n d m o i s t u r e have been f o u n d t o be t h e m o s t i m p o r t a n t e n v i r o n m e n t a l parameters a n d t h e c a r b o n / n i t r o g e n (C/N) ratio a n d l i g n i n c o n t e n t t h e m o s t i m p o r t a n t c h e m i c a l parameters (54,55). T h e rate of d e c o m p o s i t i o n is a l m o s t a l w a y s l i m i t e d by t e m p e r a t u r e , m o i s t u r e , or n u t r i e n t deficiencies. T h e d e c o m p o s i t i o n rate s h o u l d be e x p e c t e d t o increase u n d e r m a n a g e m e n t s c h e m e s w h i c h i n c l u d e irrigation a n d f e r t i l i z a t i o n , p r o v i d e d t e m p e r a t u r e is n o t l i m i t i n g . From t h e s t a r t - u p o p e r a t i o n s of p l a n t a t i o n e s t a b l i s h m e n t t o t h e a t t a i n m e n t of steady state, t r a n s i t i o n c o n d i t i o n s prevail w i t h respect t o n u t r i e n t s . In this case, t h e a m o u n t o f o r g a n i c material d e c o m p o s e d w i l l n o t be equal t o t h e a n n u a l o r g a n i c i n p u t s . T r a n s i t i o n - p e r i o d fertilizer r e q u i r e m e n t s are e s t i m a t e d in c o n j u n c t i o n w i t h s t a r t - u p o p e r a t i o n r e q u i r e m e n t s a n d costs. T h e y w i l l be larger t h a n steady-state r e q u i r e m e n t s since full credit f o r t h e o r g a n i c i n p u t s c a n n o t be taken each year. S e c o n d , t h e m o d e l does not p r e d i c t t h e g r o w t h response t o fertilization above levels required t o c o m p e n s a t e f o r n u t r i e n t s r e m o v e d in t h e harvested material. The parameters f o r w o o d a n d leaf g r o w t h are c u r r e n t l y p u t into t h e n u t r i e n t balance m o d e l f r o m t h e g r o w t h m o d e l as i n d e p e n d e n t parameters. G r o w t h is only i n d i r e c t l y related t o t h e n u t r i e n t balance t h r o u g h t h e use of land class p r o d u c t i v i t y data in t h e g r o w t h m o d e l for t h e p r e d i c t i o n of biomass p r o d u c t i o n . A n y fertilization necessary t o raise t h e fertility o f t h e site t o a level required t o sustain a desired a m o u n t o f p r o d u c t i v i t y w o u l d be i n c l u d e d in t h e t r a n s i t i o n period costs. T h e necessary detailed d a t a required t o e s t i m a t e o p t i m u m fertilizer s c h e m e s are n o t presently available. T h e n u t r i e n t balance m o d e l c a l c u l a t e s t h e fertilizer r e q u i r e m e n t s f o r t w o separate m a n a g e m e n t designs, a p p l i c a t i o n of fertilizer yearly a n d a p p l i c a t i o n on a o n c e - p e r - r o t a t i o n basis. For t h e yearly a p p l i c a t i o n d e s i g n , t h e m o d e l p r e d i c t s t h e fertilizer r e q u i r e m e n t f o r t h e entire p l a n t a t i o n using s i m p l e l e a c h i n g losses. T h e yearly a p p l i c a t i o n d e s i g n c o r r e s p o n d s t o p l a n t a t i o n m a n a g e m e n t i n v o l v i n g t h e use of irrigation, t h e fertilizer b e i n g a p p l i e d in c o n j u n c t i o n w i t h irrigation w a t e r . For cases w h e r e irrigation w i l l n o t be used, a p p l i c a t i o n of fertilizer w i l l o c c u r o n c e per r o t a t i o n . This is e n v i s i o n e d t o o c c u r after harvest w h e n t h e site w i l l be m o s t easily accessible. This d e s i g n p r e d i c t s t h e fertilizer r e q u i r e m e n t f o r o n l y t h a t p o r t i o n o f t h e p l a n t a t i o n b e i n g harvested in a g i v e n year. Fertilizer r e q u i r e m e n t s are first e s t i m a t e d based u p o n biomass g r o w t h only a n d t h e n


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are p u t into e q u a t i o n s c o n t a i n i n g t h e o t h e r variables similar t o t h o s e used for t h e yearly a p p l i c a t i o n d e s i g n . These e q u a t i o n s c o n t a i n a n a d d i t i o n a l p a r a m e t e r t h a t d e t e r m i n e s t h e a m o u n t o f n u t r i e n t s t a k e n u p each year in g r o w t h as w e l l as t h e a m o u n t left over a n d s u b j e c t t o leaching a n d u p t a k e in s u b s e q u e n t years o f t h e r o t a t i o n . The e q u a t i o n s are solved b y a n iterative a l g o r i t h m . L e a c h i n g losses are c a l c u l a t e d as c o m p o u n d losses d u e t o t h e longer period b e t w e e n a p p l i c a t i o n s a n d are w e i g h t e d f o r t h e d i f f e r e n t r o t a t i o n l e n g t h s f o r first versus c o p p i c e g r o w t h cycles. T h u s , b o t h t h e l e a c h i n g values t h e m s e l v e s a n d t h e l e n g t h o f t h e r o t a t i o n w i l l have a s i g n i f i c a n t effect o n t h e p r e d i c t e d fertilizer r e q u i r e m e n t s w h e n a p p l i c a t i o n is o n a o n c e - p e r - r o t a t i o n basis. In t h e m o d e l , t h e a m o u n t o f n i t r o g e n fertilizer required yearly is t h e a m o u n t r e q u i r e d for g r o w t h (both leaf a n d w o o d g r o w t h — o b t a i n e d f r o m t h e g r o w t h model) m i n u s t h e a m o u n t s u p p l i e d b y s l u d g e f r o m t h e algae p o n d , t h e leaf recycle (calculated in t h e g r o w t h m o d e l as t h e d i f f e r e n c e b e t w e e n t h e leaf m a t e r i a l p r o d u c e d a n d t h a t harvested), a n d n i t r o g e n f i x a t i o n . There is no ash p a r a m e t e r in t h e n i t r o g e n e q u a t i o n because t h e n i t r o g e n present in t h e b i o m a s s is volatilized d u r i n g t h e c o m b u s t i o n process o c c u r r i n g a t t h e p o w e r plant. T h e o t h e r n u t r i e n t s o f interest, h o w e v e r , r e m a i n in t h e ash. so t h a t this p a r a m e t e r is i n c l u d e d in all e q u a t i o n s o t h e r t h a n t h o s e f o r n i t r o g e n . The n i t r o g e n f i x a t i o n t e r m in t h e n i t r o g e n e q u a t i o n a c c o u n t s f o r a t m o s p h e r i c n i t r o g e n b i o l o g i c a l l y f i x e d b y c o v e r c r o p s o r i n t e r p l a n t e d w o o d y species c a p a b l e o f f i x i n g n i t r o g e n (legume o r a c t i n o m y c e t e - n o d u l a t e d species) (5659). T h e m o d e l a l l o w s t h i s n i t r o g e n f i x a t i o n i n p u t o n l y for t h a t p o r t i o n o f t h e p l a n t a t i o n not harvested d u r i n g t h e g r o w i n g season, as e n e r g y d e r i v e d f r o m p h o t o s y n t h e s i s m u s t be s u p p l i e d t o drive t h e c h e m i c a l reactions i n v o l v e d . T h e o r g a n i c i n p u t s in t h e n u t r i e n t balance e q u a t i o n s are m u l t i p l i e d by o r g a n i c l e a c h i n g factors a n d t h e inorganic i n p u t s are m u l t i p l i e d b y inorganic l e a c h i n g f a c t o r s w h i l e t h e n i t r o g e n f i x a t i o n i n p u t p a r a m e t e r has n o associated leaching t e r m . N i t r o g e n f i x a t i o n is a s s u m e d t o be a s l o w , steady i n p u t t o t h e s y s t e m m o r e closely t i m e d t o t h e g r o w t h r e q u i r e m e n t s o f t h e plants a n d therefore less likely t o be s u b j e c t e d t o l e a c h i n g losses. T h e l e a c h i n g f a c t o r p a r a m e t e r s represent t h e o t h e r o u t p u t f r o m t h e s y s t e m besides g r o w t h . In t h e case o f n i t r o g e n , t h e l e a c h i n g parameters i n c l u d e losses d u e t o d e n i t r i f i c a t i o n . In t h e e q u a t i o n s for yearly a p p l i c a t i o n s , leaching is d e s c r i b e d as s i m p l e leaching losses. For t h e case o f a p p l i c a t i o n o n c e per r o t a t i o n , c o m p o u n d l e a c h i n g is used. T h e a c t u a l values o f t h e leaching p a r a m e t e r s w i l l d e p e n d u p o n s u c h site-specific f a c t o r s as c l i m a t e , soil t e x t u r e a n d c a t i o n e x c h a n g e c a p a c i t y (CEC). t h e presence or a b s e n c e o f v e g e t a t i v e cover for uptake of w a t e r a n d n u t r i e n t s (47.53,60). and t h e m e t h o d


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of fertilizer a p p l i c a t i o n (banded versus broadcast). For t h e m o d e l , three sets o f values w e r e s u b j e c t i v e l y c h o s e n t o represent l o w , m e d i u m , a n d high l e a c h i n g losses. These values w e r e c h o s e n after r e v i e w i n g t h e literature for ranges o f general y e t representative n u m b e r s (53,61-64). It s h o u l d be n o t e d , h o w e v e r , t h a t a c t u a l values w i l l be h i g h l y site-specific. L e a c h i n g values are e n t e r e d as f r a c t i o n s , so t h a t t h e q u a n t i t y (1-leaching value) represents t h e a m o u n t available f o r g r o w t h after l e a c h i n g . Inorganic leaching values are s l i g h t l y higher t h a n o r g a n i c values because it is a s s u m e d inorganic i n p u t s are i m m e d i a t e l y available f o r g r o w t h or leaching. Different values are also used for each n u t r i e n t ; f o r e x a m p l e , t h e inorganic leaching value f o r Κ is larger t h a n t h a t f o r Ca since Κ is a m o r e m o b i l e i o n . For t h e yearly fertilizer a p p l i c a t i o n d e s i g n , s l u d g e a n d ash inputs are also a s s u m e d t o o c c u r yearly, a n d c r e d i t s for leaf r e c y c l i n g a n d n i t r o g e n f i x a t i o n are taken o n an a n n u a l basis. W h e n fertilizer is a p p l i e d o n c e per r o t a t i o n , s l u d g e a n d ash are a s s u m e d t o be applied o n a o n c e - p e r - r o t a t i o n basis. H o w e v e r , leaf r e c y c l i n g o c c u r s every year, so t h a t a n n u a l credits for t h i s i n p u t a n d f o r n i t r o g e n f i x a t i o n are t a k e n f o r b o t h a p p l i c a t i o n designs. Preliminary sensitivity analyses w e r e p e r f o r m e d t o d e t e r m i n e t h e i m p o r t a n c e of each of t h e variables in t h e e q u a t i o n s . These analyses w e r e run f o r a basecase d e s i g n for a p l a n t a t i o n in t h e S o u t h e r n U n i t e d States. T h e d e s i g n variables f o r this case i n c l u d e d : (1) 2 8 , 5 0 0 acres of p l a n t a t i o n ; (2) first a n d c o p p i c e g r o w t h cycles of 2 a n d 4 years, respectively; (3) 5 c o p p i c e g r o w t h s prior t o r e p l a n t i n g ; (4) a 1.8-million-gallon-per-day (MGD) w a s t e w a t e r t r e a t m e n t facility f o r t h e algae p o n d a n d t h e s l u d g e credit i n p u t ; (5) 180 days in t h e g r o w i n g season; a n d (6) average p r o d u c t i v i t y of 8.2 o v e n - d r y t o n s of biomass ( w o o d a n d leaf) per acre-year, or t o t a l p r o d u c t i o n of 2 3 3 , 7 0 0 ODT/year. Table III lists t h e results of these tests. T h e ranges s h o w n reflect values g e n e r a t e d over all three sets of l e a c h i n g rates. It can be seen t h a t r e c y c l i n g of leaf m a t e r i a l , t h a t is, r e s t r i c t i n g h a r v e s t i n g t o t h e d o r m a n t season, w i l l reduce t h e fertilizer r e q u i r e m e n t b y 8 - 1 6 % f o r Ν a n d P. A s t h e n u m b e r of days in t h e h a r v e s t i n g season increases, m o r e leaf material is r e m o v e d f r o m t h e site a n d less is r e c y c l e d , increasing t h e r e q u i r e m e n t o f inorganic fertilizer. These results agree fairly w e l l w i t h p u b l i s h e d values o f 1 5 - 3 0 % savings in fertilizer r e q u i r e m e n t s r e s u l t i n g f r o m h a r v e s t i n g o n l y in t h e d o r m a n t season (65). S l u d g e c o n t r i b u t e s a smaller a m o u n t t o t h e overall balance, a b o u t 2 - 4 % of t h e Ν a n d Ρ fertilizer r e q u i r e m e n t s , a n d appears t o be m o r e i m p o r t a n t for Ρ t h a n N. T h e largest savings, h o w e v e r , result f r o m t h e ash a n d n i t r o g e n f i x a t i o n inputs. D e p e n d i n g u p o n t h e level of return of ash t o t h e p l a n t a t i o n , f r o m 9 - 7 7 % of t h e Ρ fertilizer r e q u i r e m e n t can be replaced b y r e c y c l i n g of ash material.


514


N i t r o g e n f i x a t i o n c o n t r i b u t e s s i g n i f i c a n t savings t o t h e n i t r o g e n fertilizer r e q u i r e m e n t s . From o n e - t h i r d t o t h r e e - f o u r t h s of t h e Ν fertilizer r e q u i r e m e n t can be s u p p l i e d by t h i s i n p u t . Since o r i g i n a l PEF s y s t e m p e r f o r m a n c e analyses i n d i c a t e d t h a t n i t r o g e n fertilizer costs c o n t r i b u t e b e t w e e n 2 0 - 2 5 % of t h e t o t a l cost of t h e biomass p r o d u c e d , these savings represent a s i g n i f i c a n t decrease in t h e c o s t of p r o d u c t i o n . T h e levels of n i t r o g e n a c c r e t i o n f r o m f i x a t i o n used in Table III are readily a t t a i n a b l e u n d e r m i x e d p l a n t i n g m a n a g e m e n t (66). It s h o u l d be n o t e d t h a t s o m e decrease in overall p r o d u c t i v i t y c a n be e x p e c t e d because of t h e e n e r g y needed for f i x a t i o n ; t h i s has been e s t i m a t e d as a 1 2 - 1 5 % decrease (67). W a t e r Balance and Irrigation Even in areas of t h e U n i t e d States w h e r e natural rainfall is s u f f i c i e n t for h a r d w o o d g r o w t h (25 inches or more), periods of w a t e r stress m a y o c c u r d u r i n g t h e g r o w i n g season (68.69). Lack of a d e q u a t e m o i s t u r e m a y have disastrous c o n s e q u e n c e s o n t h e survival a n d e s t a b l i s h m e n t of clones or seedlings. It has also been s h o w n t h a t yields o f h a r d w o o d p l a n t a t i o n s c a n be increased by r e d u c i n g w a t e r stress d u r i n g t h e g r o w i n g season. A n irrigation s u b r o u t i n e has therefore been i n c l u d e d in t h e PEF m o d e l t o evaluate t h e costeffectiveness of irrigation for site-specific c o n d i t i o n s . Irrigation r e q u i r e m e n t s for a site are d e t e r m i n e d t h r o u g h a m o n t h - b y - m o n t h b a l a n c e analysis. T h e Blaney-Criddle m e t h o d as a d a p t e d by t h e Soil C o n s e r v a t i o n Service (70) is used in t h i s analysis. T h e m e t h o d first d e t e r m i n e s t h e w a t e r c o n s u m p t i v e needs of d e c i d u o u s p l a n t a t i o n s for local c l i m a t i c c o n d i t i o n s . T h e irrigation r e q u i r e m e n t s are t h e n e s t i m a t e d by c o m p a r i n g these needs t o e f f e c t i v e w a t e r i n p u t s f r o m rainfall. T h e m o n t h l y irrigation r e q u i r e m e n t s are i n p u t s t o t h e irrigation s u b r o u t i n e . T h e peak m o n t h l y r e q u i r e m e n t is used t o d e t e r m i n e t h e peak c a p a c i t y a n d c a p i t a l cost of t h e irrigation s y s t e m . It is a s s u m e d t h a t t h e irrigation required d u r i n g t h e m o n t h of h i g h e s t d e m a n d w i l l be s u p p l i e d t h r o u g h f o u r w e e k l y a p p l i c a t i o n s . T h e o p e r a t i o n costs are e s t i m a t e d on t h e basis of t h e t o t a l irrigation needs for t h e g r o w i n g season. Self-propelled t r a v e l i n g sprinklers f e d by u n d e r g r o u n d m a i n s are a s s u m e d in t h e m o d e l . This s y s t e m w a s c h o s e n because it has been e x t e n s i v e l y used for w a s t e w a t e r a p p l i c a t i o n (23). T r i c k l e - d r i p s y s t e m s are m o r e e n e r g y efficient, b u t their use w i t h w a s t e w a t e r s has resulted in c l o g g i n g d u e t o algae g r o w t h a n d t h e r e f o r e m a y require f l u s h i n g w i t h fresh w a t e r (7Yl. W a t e r c a n be s u p p l i e d f r o m w e l l s , river o r lake w a t e r , or e f f f u e n t s f r o m a w a s t e w a t e r t r e a t m e n t plant. T h e response of t h e p l a n t a t i o n is d e s c r i b e d by a relation of t h e f o r m y = ax + b w h e r e y = y i e l d , χ = n u m b e r of g r o w t h days w i t h s u f f i c i e n t m o i s t u r e , a n d a a n d b = c o n s t a n t s . T h e use of


25.

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515


T a b l e III. PERCENT S A V I N G S OF FERTILIZER R E Q U I R E M E N T S ATTRIBUTABLE TO VARIOUS INPUT PARAMETERS" Application Design Nutrient Input Parameter Leaf Recycle Sludge Ash Ash Ash Nitrogen Fixation Nitrogen Fixation' b

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Biomass as a Nonfossil Fuel Source - ACS Publications - American

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