Energy Efficiency in N2 Fertlizer

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Energy in Agriculture, 4 (1985) 159--177 159Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

E N E R G Y E F F I C I E N C Y I N N I T R O G E N F E R T I L I Z E R P R O D U C T I O N

MOHINDER S. MUDAHAR and TRAVIS P. HIGNETTInternational Fertilizer Development Center (IFDC), P.O. Box 2040, Muscle Shoals,AL 35662 (U.S.A.)(Accepted 6 February 1985)

ABSTRACTMudahar, M.S. and Hignett, T.P., 1985. Energy efficiency in nitr ogen fertilizer produc-tion. Energy Agric., 4: 159--177.

This paper deals with e stimating energy cons umpt ion and potenti al energy savings byimproving energy efficiency for produ ctio n of selected nitrogen fertilizers. The paper alsodiscusses economic importan ce, economic consequences, and policy implications of im-proving energy efficiency for nitrogen fertilizer production . Improving energy efficiencyis one of the most imp ort ant and viable policy options to lower nitrogen fertilizer prices.Three strategies to improve ene rgy efficien cy for nitrog en fertilizer pro duc tio n are dis-cussed: (1) energy-efficient retrofits; (2) energy-efficient new processes; and (3) opera-tional efficiency and energy management. Efficient operations and energy managementare not only desirable bu t also an eco nomic ally feasible strategy in most developingcountries. The developing countries should be careful in adopting exotic energy-efficientinnovations.

INTRODUCTIONFertilizer is a major factor in expanding food output. Fertilizer produc-tion is also highly energy-intensive. This is especially the case with nitrogen(N) fertilizer, which is the most popula r ty pe in developing countries. Energy

is essential to produce N fertilizer which, in turn, is essential to producehigh crop yields.Energy costs comprise a major share of variable costs for producing Nfertilizer and hence N fertilizer prices. In addition to large food deficits andrapid population growth, ma ny developing countries are experiencing seriousenergy-re lated problems, including limited energy supply, high energy prices,and lack of foreign exchange to import energy.There is a need to reduce N fertilizer production costs. This will not onlylower N fertilizer prices to the farmer but will also provide reasonable re-turns to capital investment in N fertilizer production. Improving energyefficiency for N fertilizer production is one of the most important andviable options to lower N fertilizer prices. More details on the economicand technical aspects of the relationships between energy and fertilizers and

0167 -582 6/85 /$03 .30 1985 Elsevier Science Publishers B.V.


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t h e i r i m p l i c a t i o n s f o r p u b l i c p o l i c y a r e a v a il a bl e i n M u d a h a r a n d H i g n e t t( 1 9 8 2 ) .T h e o b j e c t i v e s o f t h is p a p e r a re : ( 1 ) t o p r o v i d e a p o l i c y p e r s p e c t i v e o nt h e N f e r ti l i z e r s e c t o r ; ( 2 ) t o d i s c us s t h e e c o n o m i c i m p o r t a n c e o f o p t i m i z i n g

e n e r g y e f f i c i e n c y ; ( 3 ) t o e s t i m a t e e n e r g y c o n s u m p t i o n f o r N fe r ti l i z e r p r o -d u c t i o n ; ( 4 ) t o d i s cu s s a l t e rn a t i v e a p p r o a c h e s t o i m p r o v e e n e r g y e f f i c i e n c y ;( 5) t o e s t im a t e p o t e n t i a l e c o n o m i c b e n e f i ts f r o m i m p r o v e m e n t s i n e n e r g ye f f i c i e n c y ; a n d ( 6 ) t o d is c u s s e c o n o m i c i m p l i c a t i o n s a n d p o l i c y o p t i o n s f o rd e v e l o p i n g c o u n t r i e s.E N E R G Y M E A S U R E M E N T A N D O P T I M I Z A T I O NEnergy measurement

E n e r g y c o n s u m p t i o n e s t i m a t e s f o r f e r ti l i z e r p r o d u c t i o n a r e d i f f i c u l t t oc o m p a r e b e c a u s e o f d i f f e r e n t m e a s u r in g u n i t s a n d f o r m s o f e ne r g y u se d ind i f f e r e n t e s t i m a t e s. F o r e x a m p l e , t h e h e a t o f c o m b u s t i o n o f f u e ls m a y b es t a t e d a s t h e ' h ig h h e a t i n g v a l u e ' ( H H V ) o r a s t h e ' l o w h e a ti n g v a l u e ' ( L H V ) .T h e H H V o f m e t h a n e , t h e p r i n c i p a l c o n s t i t u e n t o f n a tu r a l g a s, is 1 1 % g r e a te rt h a n t h e L H V . E n g i n e e r in g f i rm s u s u a ll y q u o t e e n e r g y u s e in L H V , b u t fu e l sa r e p r i c e d a c c o r d i n g t o t h e H H V .A n o t h e r p r o b l e m i n c o m p a r i n g e n e r g y e s t i m a t e s is i n c o n v e r s i o n o ft h e r m a l e n e r g y t o e le c t r ic a l o r m e c h a n i c a l e n e r g y . S o m e a u t h o r s e q u a t e1 k W h t o 3 4 1 3 B t u ( 3 . 6 M J ) , th e a m o u n t o f h e a t th a t c a n b e g e n e r a t e d b y1 k W h ( P e r r y , 1 9 5 0 , p . 4 0 ) . I n a m o d e r n e l e c t r i c p o w e r s y s t e m f i r e d w i t hf o ss il o r n u c l e a r f u e l , a b o u t 1 0 0 0 0 B t u ( 1 0 . 5 5 M J ) a r e r e q u i r e d t o g e n e r a t e1 k W h . W e t h i n k t h a t t h e l a t te r c o n v e r s io n f a c t o r is m o r e a p p r o p r i a t e f o rm a k i n g e n e r g y c o n s u m p t i o n e s t i m a t e s b e c a u s e i t r e c o g n i z e s t h e i n e f f i c i e n c yi n c o n v e r s i o n o f f u e l e n e r g y t o e l e c t ri c e n e r g y .A f u r t h e r s o u r c e o f c o n f u s i o n is t h e d i f f e r e n c e b e t w e e n ' b a t t e r y - l im i t s 'e n e r g y u s e c l a i m e d b y t h e d e s ig n e r a n d t h e e n e r g y u se i n a c t u a l p l a n t o p e r a -t io n s . ' B a t t e r y - l im i t s ' r e f e r s t o a p r o d u c t i o n u n i t, s u c h a s a n a m m o n i a p l a n t,w i t h i n p r e s c r i b e d b o u n d a r i e s a n d d o e s n o t i n c l u d e a u x i li a r y f a c il it ie s t h a tm a y b e r e q u i r e d t o f u r n i sh t h e p l a n t w i t h f e e d s t o c k , u ti li t i e s a n d s e r vi ce s ,d e l i v e re d a t t h e b o u n d a r y . T h e b a t t e r y - l i m i t s e n e r g y u s e is t h a t e n e r g y u s e dw i t h i n b a t t e r y l im i t s ( h o w e v e r d e f i n e d ) w h e n t h e p l a n t is o p e r a t i n g c o n -t i n u o u s l y a t t h e r a t e f o r w h i c h i t w a s d e s ig n e d . H o w e v e r , a c t u a l e n e r g y u s em a y e x c e e d s p e c i f i e d b a t t e r y - l i m i t s u s e , s o m e t i m e s b y a w i d e m a r g i n .I n t h i s p a p e r w e s h al l u s e t h e H H V e n e r g y e q u i v a l e n t o f fu e l s, m e t r i ct o n s ( t) , a n d a f u e l e q u i v a l e n t f a c t o r o f 1 k W h = 1 0 0 0 0 B t u = 1 0 . 5 5 M Je x c e p t w h e r e o t h e r w i s e m e n t i o n e d . E n e r g y c o n s u m p t i o n e s t i m a t e s a rer e p o r t e d i n g i g a jo u l e s , a b b r e v i a t e d a s G J ( 1 G J = 1 0 9 j o u l e s ) . T h e e q u i v a l e n te n e r g y u n i t s a r e 1 G J = 0 . 9 4 8 m i l l io n B t u = 0 . 2 3 9 m i l li o n k c a l. M o s t o f t h ee n e r g y c o n s u m p t i o n e s t i m a t e s a r e b a s e d o n T h e F e r t il i ze r I n s t i t u t e ' s ( T F I ' s )E n e r g y U s e S u r v e y s o f a l a rg e n u m b e r o f fe r t il iz e r p l a n t s in a c t u a l o p e r a t i o ni n N o r t h A m e r i c a ( T F I , 1 9 7 9 , 1 9 8 1 ) .


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161Energy optimization

Increasing energy efficiency beyond a certain point may be technicallyfeasible and yet not economical or cost-effective. In other words, from thepoint of view of the nitrogen industry and a nation there is a need to op-timize energy consumption, i.e., to make the most cost-effective use of avail-able energy resources. Some of the factors that need to be considered inoptimizing energy efficiency in nitrogen produc tion are: (1) investmentcost; (2) operating efficiency of the plant; (3) reliability of the process;and (4) complexity of the equipment. In this context, optimizing energyefficiency is the best compromise among various trade-offs involving energysaving, capital investment, and plant performance. The relative importanceof these factors, however, may vary across countries depending upon theresource endowments and prices.A POLICY PERSPECTIVE ON THE NITROGEN FERTILIZER SECTOREconomic importance of nitrogen use

During 1981/82, the total amount of N fertilizer consumed in the worldwas approximately 60.4 million t (FAO, 1983). Assuming tha t 1 t of Nproduces 10 t of grain, 604 million t of grain or its equivalent in otheragricultural products was the direct result of using N fertilizer alone. Ofthis, approximately 250 million t was produced in the developing countries.Intensive farming through higher fertilizer use (energy input) produces manytimes more energy output, whet her as food, feed, fiber or fuel.Dominance of nitrogen fertilizer

Nitrogen dominates in fertilizer consumption, production, and trade.However, the degree of dependence on N fertilizers is relatively greater indeveloping countries than in developed countries. The empirical evidence,based on FAO (1983), indicates that: (1) the estimated share of N in NPK(N + P~)s + K20) consumption increased in the world from 46% in 1971/72to 52% in 1981/82; and (2) annual growth in N consumption in developingcountries has been faster than in developed countries.The most common sources of N fertilizer are urea, ammonium nitrateand ammonium sulfate. The share of ammonium nitrate and ammoniumsulfate in world N fertilizer capacity is declining, whereas the share of ureais increasing over time. The proportionate share of urea capacity in N fer-tilizer capacity for developing countries (64% in 1979) is higher than for thedeveloped countries. On the basis of the existing and projected urea produc-tion capacity, it appears that urea will continue to be the dominant sourceof N supply in most developing countries, especially in Asia, in the nearfutu re (Mudahar and Hignett, 1982).


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162Increasing role of developing countries

T h e w o r l d a m m o n i a p r o d u c t i o n c a p a c i t y h a s i n cr e a se d f r o m 5 1 m i ll io n t( in t e r m s o f N ) i n 1 9 7 0 t o 9 4 m i l li o n t i n 1 9 8 0 ( M u d a h a r a n d H i g n e t t ,1 9 8 2 ) . T h e s h ar e o f d e v e l o p in g c o u n tr i e s in t h e t o ta l a m m o n i a p r o d u c t i o nc a p a c i t y h a s i n c r e a s e d f r o m 1 7 % i n 1 9 7 0 t o a b o u t 2 7 % i n 1 9 8 0 . T h i s s h a reis e x p e c t e d t o i n c r ea s e i n t h e f u t u r e in r e s p o n s e t o : ( 1 ) t h e d e s i r e o f m a n yd e v e l o p i n g c o u n t r i e s t o b e c o m e s e l f - s u f f i c i e n t i n a m m o n i a p r o d u c t i o n b e -c a u s e o f n a t i o n a l s e c u r i t y c o n s i d e r a t i o n s ; a n d ( 2 ) a g r a d u a l re g i o n a l s h i f ti n f a v o r o f t h o s e c o u n t r i e s t h a t h a v e l a r ge n a t u r a l g a s r e se r ve s .T h e n e t e f f e c t o f t h e s e e c o n o m i c a d j u s t m e n t s m a y b e h i g h- c o st a m m o n i aa n d N f e r t il i z e r. T h e h i g h c o s t is p r i m a r i l y d u e t o r e l a t i v e l y h i g h c a p i t a l i n-v e s t m e n t s a n d l o w o p e r a t i n g r a t e s i n d e v e l o p i n g c o u n t r i e s . F u r t h e r m o r e , al ar g er s h a re o f n e w a m m o n i a c a p a c i t y i s b e i n g c r e a t e d i n t h e p u b l i c s e c t o r ,a n d t h is s h i f t h a s i m p o r t a n t i m p l i c a t i o n s f o r p l a n ni n g , d e c i s i o n -m a k i n g ,c a p i ta l i n v e s t m e n t , o p e r a t i n g e f f i c i e n c y , a n d p r ic i n g p o l ic y .ECONOM IC IMPORTANCE OF OPTIMIZING ENER GY EFFICIENCYEnergy consumption for nitrogen production

O f t h e t o t a l c o m m e r c i a l e n e r g y u s e d i n t h e w o r l d , a g ri c u lt u r al p r o d u c -t i o n u s e s a b o u t 3 .5 % , a n d 4 5 % o f t h a t is u s e d i n t h e f e r ti l iz e r s e c t o r . H o w -e v e r, i n d e v e l o p i n g c o u n t r i e s a b o u t 6 8 % o f e n e r g y u s e i n a g r i c u lt u r e is a t-t r i b u t e d t o f e r ti li z e r. T h e m a n u f a c t u r e o f N f e r ti l i z e rs is h i g h l y e n e r g y in -t e ns iv e , a p p r o x i m a t e l y 9 t i m e s t h a t o f p h o s p h a t e s a n d 1 1 t i m e s t h a t o fp o t a s h . T h i s i s d u e i n p a r t t o t h e f a c t t h a t i n m a n u f a c t u r i n g a m m o n i a , t h eb a s ic m a t e r i a l f o r a ll N f e r ti l i z e rs , e n e r g y i s u s e d b o t h a s f e e d s t o c k ( a b o u t6 0 % o f to t a l ) a n d a s f u e l ( a b o u t 4 0 % o f t o t al ) .D u r i n g 1 9 8 1 / 8 2 t h e e s t i m a t e d e n e r g y c o n s u m p t i o n f o r t h e w o r l d f er -t i l iz e r s e c t o r ( in c l u d in g p r o d u c t i o n , p a c k a g i n g , t r a n s p o r t , a n d a p p l i c a t io n )w a s 5 5 8 9 m i l l io n G J o r 5 . 3 X 1 0 is B t u . N i t r o g e n is b y f a r t h e g r e a t e s te n e r g y c o n s u m e r i n t h e f e r ti l i z e r s e c t o r. D u r i n g 1 9 8 1 / 8 2 t h e s h a r e o f e n e r g yc o n s u m p t i o n i n t h e w o r l d f e r t il i z er s e c t o r w a s 8 4 % f o r N , 1 0 % f o r P 2 0 5,a n d 6 % f o r K ~ O . N i t r o g e n f e r ti li z er p r o d u c t i o n a c c o u n t e d f o r 9 1 % o f t h ee n e r g y u s e d i n m a n u f a c t u r i n g a ll o f t h e f e rt i l iz e r s c o n s u m e d i n t h e w o r l d .Behavior of energy prices

E n e r g y p r i c e s h a v e i n c r e a s e d s i g n i f i c a n t ly i n t h e l a s t 1 0 y e a r s . F o r e x a m -p l e , t h e a v e ra g e i n t e r n a t i o n a l p r i ce s f o r c r u d e o i l h a v e in c r e a s e d f r o m $ 3 . 2 9 /b b l ( b b l , b a r re l ~ 1 5 9 1) i n 1 9 7 3 t o a b o u t $ 3 4 . 0 0 / b b l in 1 9 8 2 , a t e n f o l d in -c r e as e . P r i c e s o f o t h e r e n e r g y s o u r c e s h a v e f o l l o w e d s i m il a r t r e n d s . T h e m o r er e c e n t c o m p a r a t i v e p r i c e s f o r s e l e c t e d e n e r g y s o u r c e s u s e d in t h e N fe r t il i ze ri n d u s t r y f o r f e e d s t o c k , f u e l , o r p o w e r a re r e p o r t e d i n T a b l e 1 . O v e r a p e r i o d


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1 6 3o f 5 y e a r s f r o m 1 9 7 7 t o 1 9 8 2 , t h e a v e r a g e p r ic e s h a v e in c r e a s e d f r o m 7 4 %( l o w e s t ) f o r c o a l t o 1 6 2 % ( h i g h es t ) f o r n a t u r a l g as .E n e r g y p r i c e s a re g e n e r a l ly r e g u l a t e d i n b o t h d e v e l o p e d a n d d e v e l o p i n gc o u n t r i e s . I n m a n y d e v e l o p i n g c o u n t r i e s , e n e r g y p r i c e s p a i d b y t h e N f e r-t il iz e r i n d u s t r y a r e c o n t r o l l e d a n d / o r h i g h ly su b s i d i z e d b y g o v e r n m e n t . C o n -s e q u e n t l y , t h e l e v e l a n d g r o w t h i n e n e r g y p ri c e s m a y v a r y a c r o s s c o u n t r i e s .F o r e x a m p l e , i n M e x i c o n a t u r a l g a s is t r a n s f e r r e d t o t h e N f e r ti l i z e r i n d u s t r ya t l e ss t h a n $ 1 . 0 0 / 1 0 3 s c f; o n t h e o t h e r h a n d , a b o u t 7 0 % o f t h e U . S . a m -m o n i a i n d u s t r y p a y s a t l e as t $ 2 . 0 0 / 1 0 3 s c f, a n d a b o u t 2 5 % p a y s a t l e a st$ 3 . 5 0 / 1 0 3 s cf . I n t h e n a t u r a l g a s i n d u s t r y t h e s t a n d a r d c u b i c f o o t ( sc f ) ist h e v o l u m e m e a s u r e d a t 6 0 F a n d 3 0 i n c h es o f m e r c u r y p r e ss u re ( 1 s c f =0 . 0 2 8 3 m 3 i f m e a s u r e d a t t h e s a m e t e m p e r a t u r e a n d p r e s s ur e ).T A B L E 1B e h a v i o r o f a v e ra g e e n e r g y p r i c e s f o r s e le c t e d e n e r g y s o u r c e sE n e r g y s o u r c e M e a s u r i n g A v e r a g e P e r c e n t

u n i t e n e r g y p r i c e d u r i n g c h a n g e1 9 7 7 1 9 8 2

C r u d e o i l a $ / b b l b 1 3 . 3 3 3 4 . 0 0 + 1 5 5C o a l c $ / t 2 3 . 8 0 4 1 . 3 0 + 7 4F u e l o il c $ / t 9 3 . 4 0 2 0 1 . 6 0 + 1 1 6N a p h t h a d $ / t 1 2 4 . 2 0 2 9 7 . 8 0 + 1 4 0N a t u r a l g a s c $ / 1 0 3 s c f e 1 . 3 3 3 . 4 9 + 1 6 2E l e c t r i c i ty c g / k W h 2 . 5 0 4 . 9 5 + 9 8a S a u d i l ig h t , f . o .b . O r i g in a l p r i c e d a t a w e r e o b t a i n e d f r o m O i l a n d G a s J o u r n a l ( 1 9 8 2 ) .b b b l , b a r r e l = 1 5 9 1 .c D e l i v e r e d t o e l e c t r ic p l a n t s i n th e U n i t e d S t a t e s . F u e l o i l p r i c e r e f e r s t o a n a v e r a g e o ff u e l o il s N o . 4 , N o . 5 , a n d N o . 6 , a n d t o p p e d c r u d e f u e l o il . O r i g i n a l p r i c e d a t a w e r e o b -t a i n e d f r o m U . S . D e p a r t m e n t o f E n e r g y ( 1 9 8 3 ) .d E u r o p e a n s p o t p r i c e f o r b u l k s h i p m e n t s , f . o .b . O r i g in a l p r i c e d a t a w e r e o b t a i n e d f r o mE u r o p e a n C h e m i c a l N e w s ( 1 9 8 2 ) .e 1 03 s c f ( s t a n d a r d c u b i c f e e t ) = 2 8 . 3 m 3 ( m e a s u r e d a t 6 0 F a n d 3 0 i n c h e s o f m e r c u r yp r e s s u r e ) .

I m p a c t o f e ne r g y p r i ce s o n n i tr o g e n p r o d u c t i o n c o s t sT h e e c o n o m i c i m p a c t o f e n e r g y co s ts o n t o t a l a m m o n i a p r o d u c t i o n c o s t s ,b a s e d o n T F I ( 1 9 8 0 ) s u r ve y s o f a la rg e n u m b e r o f a m m o n i a p l a n t s i n o p e r a-t i o n i n t h e U n i t e d S t a t e s, is r e p o r t e d i n F ig . 1 . T h e t o t a l a m m o n i a p r o d u c -t i o n c o s t s h a v e in c r ea s e d f r o m $ 3 3 . 6 0 / t in 1 9 7 3 t o $ 1 0 6 . 0 8 / t i n 1 9 8 0 .T h e c o r r e s p o n d i n g a v e ra g e s h a re o f e n e r g y c o s t s i n t o t a l a m m o n i a p r o d u c -

t i o n c o s t s h a s b e e n e s t i m a t e d t o i n c re a s e f r o m 5 2 % i n 1 9 7 3 t o 7 4 % i n 1 9 8 0 .I n o t h e r w o r d s , a n a v er ag e 1 0 0 0 / 1 7 0 0 t p e r d a y ( t p d ) a m m o n i a / u r e a p r o d u c -t i o n c o m p l e x c o n s u m e s a b o u t 2 0 m i l l i o n G J o f e n e rg y p e r y e a r , w h i c h is


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164equivalent to appr oxim atel y 18 million 103 scf of natural gas. The marke tvalue of this natural gas at $3/103 scf is about $54.0 million, which is equiv-alent to nearly $100 /t of urea.

The average relative share of energy costs in total ammonia productioncosts is generally lower in developing countries than in developed countries.This is due to several factors, but the most important are: (1) high capitalcosts; (2) low operating rates; and (3) subsidized energy prices. On the ot herhand, actual energy consump tion per unit o f ammon ia product ion is general-ly higher in developing countries because of low operating rates and lowenergy efficiency as compa red with the developed countries. High energyprices are eve ntua lly refl ecte d in high N fertilizer prices and high foo d prices.In this contex t, optimizing energy efficiency in N fertil izer prod uctio nprovides an excellent opp or tu ni ty to keep N fertilizer prices low.

12 0

8 0

4 0

F ~~ T o ta l P rod u ct io n Co s ts[ ~ T o t a l E n er gy C o st s

) Enerqy Costs as % o f Pt

106 ,089 7 6 0

( Energy of Production Cost 89 .658 1 .0 2

53.74

3 3 . 6 0

v I I l I I I

1973 1975 1977 1978 1979 1980Year

Fig. 1. Economic impact of energy costs on total ammonia production costs of ammoniaplants in operation in the United States.

ENERGY EFFICIENCY IN AMMONIA PRODUCTIONEnergy consumption in ammonia production

Ammonia is the basis for nearly all commercial N fertilizers. Natural gasis the main source of both feedstock and fuel for abo ut 80% of the world'sammonia production. This percentage is likely to increase in the futurebecause ammonia plants based on natural gas are the most energy-efficient,


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16 5the least complex, and the least capital-intensive, and they pose only minorpollution problems. Moreover, natural gas is often the lowest cost form ofenergy. Therefore, this discussion shall concentrate on natural gas-basedammonia plants.

The average energy consumption for ammonia production using cen-trifugal compressors during 1981 has been estimated to be 45.3 GJ/t. Thisrepresents a decline of about 3.5% over energy use during 1979 which was46.9 GJ/t. Approximately 60% of the tota l energy was used in the fo rmof feedstock, 37% for heating the reformer, and the remaining 3% for otherpurposes as fuel or power. On a nutri ent basis, the average energy use forammonia production in plants using centrifugal compressors was 57.2 GJ/tof N in 1979 and 55.3 GJ/t of N in 1981.Historically, the energy consumption for ammonia production has de-clined significantly from the early 20th century to the 1980s. Energy use forammonia synthesis based on steam reforming of natural gas is reported inFig. 2, which has been constructed from data reported by Quartulli andBuividas (1976), Czuppon and Buividas (1979) and Mudahar and Hignett(1982). The figure shows energy use for plants tha t were designed or builtin the year indicated and that used the most energy-efficient techno logyavailable at that time. According to these estimates, energy consumption forammonia production (battery-limits) has declined from 62 GJ/t in the early1940s to 35 GJ/t in the early 1980s, and is expected to be about 31 GJ/tby 1990.80

:~ 60-"I-ZZG 4oEE~ 20

- - - - - - E n e r g y U s e

\0 % % %

\ \

> 5 4 5 t p d =5 4 5 t p d ~

T h e o r e t ic a l M in im u m ( N a t u r a l G as Fe e d s to c k )2 3 G J / t o f N H 3 ( H H V )

019 40 19150 19 '60 19170 19J80 19 90

Year

Fig. 2. Evolut ion of energy use (fee dsto ck + fuel + electric pow er) for ammon ia systhesisbased on steam reforming of natural gas.


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166Energy sav ing t hrough re t ro f i t s

A l a rg e n u m b e r o f r e t r o f it s , b o t h t h e i n -p l a n t a n d a d d - o n t y p e s , h a v eb e e n s u g g e s t e d t h a t p r o m i s e t o i m p r o v e e n e r g y e f f i c i e n c y . A c c o r d i n g t o L e-B l a n c e t a l. ( 1 9 8 2 ) , t h e u s e o f r e t r o f i t s a n d e n e r g y - s a v i n g p r i n c i p l e s c a n r e al -i st ic a ll y r e s u lt i n e n e r g y s a vi ng b e t w e e n 3 . 2 a n d 8 . 2 G J / t o f a m m o n i a i n t h ee x i s t in g p l a n ts . F o r e x a m p l e , a c c o r d i n g t o J o h a r a p u r k a r ( 1 9 8 2 ) , t h e u s e o f ap u r g e h y d r o g e n r e c o v e r y u n i t a l o n e r e s u l t s i n e n e r g y s a v i n g o f a p p r o x i m a t e l y1 G J / t o f N H 3 i n a c o m m e r c i a l a m m o n i a p l a n t i n I n d ia .

T h e p r i m a r y e n e r g y - s a v i n g e f f o r t s i n a m m o n i a p r o d u c t i o n f r o m e x i s t i n ga n d n e w p l a n ts n e e d t o b e d ir e c t e d t o w a r d b u t n o t l i m i t e d to : ( 1 ) re c o v er in gh e a t i n t h e r e f o r m i n g p r o c e s s ; ( 2 ) i m p r o v i n g e f f i c i e n c y o f e n e r g y u s e i n C O 2r e c o v e r y a n d r e m o v a l ; ( 3 ) r e c o v e r i n g a n d r e c y c l i n g H 2 f r o m t h e p u r g es t re a m ; a n d ( 4 ) f u r t h e r i m p r o v e m e n t s i n t h e a m m o n i a s y n t h e s i s p r o c e s s .T h e e n e r g y -s a v in g r e t r o f i t s i n a m m o n i a p l a n t s p r i m a r i ly d e a l w i t h : ( 1 ) r e-c o v e r y o f w a s t e h e a t ; ( 2 ) c o m b u s t i o n a ir p r e h e a t ; ( 3 ) e f f ic i e n t C O 2 r e m o v a ls y s t e m ; ( 4 ) l o w p r e s s ur e d r o p s y n l o o p ; a n d ( 5 ) re c o v e r y o f H 2 f r o m p u r g egas .E ne r gy s av i ng i n ne w am m on i a p l an t s

T h e r e is a g e n e ra l t r e n d t o w a r d l o w e r s y n t h e si s p r e s s u r e in m o s t l o w -e n e r g y p r o c e s se s . T h e o b v i o u s a d v a n t a g e i s t h a t t h e e n e r g y f o r c o m p r e s s i o ni s l o w e r e d ; t h u s , t h e c o m p r e s s o r c a n b e s i m p l e r , a n d t h e s t e a m t u r b i n e w h i c hd r iv e s t h e c o m p r e s s o r is e x p o s e d t o l es s r i g o r o u s c o n d i ti o n s . H o w e v e r , t h e r ea r e s e v e ra l re l a t e d e f f e c t s t h a t a r e l es s f a v o r a b l e . F o r e x a m p l e , t h e e x t e n t o fc o n v e r s io n o f n i t r o g e n a n d h y d r o g e n t o a m m o n i a d e c r e as e s a s t h e p r e s su r ed e c r e a s es . F i g u r e 3 ( b a s e d o n d a t a r e p o r t e d i n I F D C , 1 9 7 9 ) s h o w s t h e e f f e c to f p r e ss u r e a n d t e m p e r a t u r e o n t h e t o o l p e r c e n t a g e o f N H 3 f o r m e d a t e q u i-l i b ri u m i n a s to i c h i o m e t r i c m i x t u r e o f p u r e H 2 a n d N 2 . A t 4 5 0 C a n d 3 0 0 b a rp r e s s u r e t h e g as m i x t u r e w o u l d c o n t a i n a b o u t 3 5 % N H 3 . A t 1 0 0 b a r t h ee q u i l i b r iu m p e r c e n t a g e o f a m m o n i a a t 4 5 0 C fa ll s t o a b o u t 1 7 % a n d a t 5 0b a r t o a b o u t 9 % ( w h e r e 1 a t m ~ 1 b a r ~ 1 k g / c m 2 ~= 0 . 1 M P a a r e a p p r o x i -m a t e e q u i v a l e n t s , + 2 % ) .T h e l o w e r c o n v e r s io n p e r p as s m e a n s t h a t m u c h m o r e g a s m u s t b e re -c y c l e d t h r o u g h t h e c a t a l y st c h a m b e r , a m m o n i a r e c o v e r y u n i t , an d h e a te x c h a ng e r s . S e p a r a t io n a n d r e c o v e r y o f t h e a m m o n i a b y c o n d e n s a t i o n w o u l dr e q u i r e d e e p c o o l i n g b y r e f r ig e r a t io n , a n d t h e e n e r g y c o n s u m e d b y r ef ri g er a -t i o n w o u l d o f f s e t t h e s a v in g i n e n e r g y f o r c o m p r e s s i o n . T h e r e f o r e , m o s t l o w -p r e s s u r e p r o c e s s e s u s e a b s o r p t i o n r e f r i g e ra t i o n i n w h i c h t h e a m m o n i a isa b s o r b e d f r o m t h e c o o l e d ga s in w a t e r . P u r e a m m o n i a is r e c o v e r e d b yd i s ti l la t i o n w i t h l o w - t e m p e r a t u r e s t e a m t h a t is a v a il a bl e f r o m t h e s h i ft r e a c-t i o n s t e p .A s s h o w n i n F ig . 3 , a hi g h p e r c e n t a g e o f c o n v e r s i o n t h e o r e t i c a l l y c o u l db e o b t a i n e d a t l o w t e m p e r a t u r e s s u c h a s 2 0 0 C e v e n a t l o w p r e s s u re s su c h


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167a s 5 0 a t m . T h i s f a c t h a s s t i m u l a t e d m u c h r e s e a r c h t o f i n d a c a t a l y s t t h a t i sa c t i v e a t l o w t e m p e r a t u r e s ( 2 0 0 C o r le ss ). T h e r e h a v e b e e n s o m e e n c o u r a g -i ng re s u lt s in l a b o ra t o r y - s c a le w o r k b u t n o c o m m e r c i a l d e v e l o p m e n t a s y e t .P r e s e n t c o m m e r c i a l c a t a l y s t s a re a c t iv e in t h e r a n g e o f r o u g h l y 3 8 0 - - 5 0 0 C .C o m m e r c i a l d e v e l o p m e n t o f a l o w - t e m p e r a t u r e c a t a l y s t w o u l d b e h e l p f u lb u t n o t a n u n m i x e d b l es s in g . F o r e x a m p l e , i f a m m o n i a s y n t h e s i s is c a r r ie do u t a t 2 0 0 C , t h e h e a t o f r e a c t i o n w o u l d h a v e t o b e r e m o v e d a t th a t t e m -p e r a tu r e . T h e u s e f u ln e s s o f t h i s e n e r g y i n t h e f o r m o f l o w - t e m p e r a t u r e h e a tm a y b e d o u b t f u l .

I 00H 600 atm

300 atm100 atm50 atm

8O

'~ 60z~oZ 4oZ

2O

0 f0 I 0 0 30O 50O 700

T e m p e r a t u re , C

F ig . 3 . E f f e c t o f t e m p e r a t u r e a n d p r e s s u r e o n c o n v e r s i o n o f a 1 : 3 m i x t u r e o f p u r en i t r o g e n ( N ~) a n d h y d r o g e n ( H 2) t o a m m o n i a ( N H 3) .

A n o t h e r e n e r g y - sa v i n g i n n o v a t i o n is r e d u c t i o n o f t h e s t e a m : c a r b o n m o lr a t io . F o r t h e o v e r a l l r e a c t i o n , i n c lu d i n g p r i m a r y a n d s e c o n d a r y r e f o r m i n ga n d t h e s h i f t re a c t i o n , t h e s t o i c h i o m e t r i c s t e a m : c a r b o n r a t i o i s a b o u t 1 . 4 .N e v e r t h e l e s s , m o s t p r o c e s s e s o p e r a t e w i t h s t e a m : c a r b o n r a t i o s in t h e r a n g eo f 3 - - 4 o r e v e n h i gh e r . T h e e x c e s s s te a m is c o n d e n s e d b y c o o l i n g t h e ga sa f t e r t h e s h i f t re a c t i o n a n d t h u s r e le a s in g e n e r g y a t a l o w - t e m p e r a t u r e l e ve l.S o m e o f th i s h e a t c a n b e r e c o v e r e d a n d u t i l iz e d b u t m u c h o f i t is w a s t e d inc o o l i n g w a t e r . T h e c o n d e n s a t e ( c o n d e n s e d e x c e s s s te a m } c o n t a i n s fr a c-t io n a l p e r c e n t a g es o f a m m o n i a , m e t h a n o l , a n d a m i n e s. T h e u s u al m e t h o d o fp u r i f i c a t i o n , i f r e q u i r e d b y e n v i r o n m e n t a l r e g u l a ti o n s , is s t e a m s t r ip p i n g,a n d t h is c o n s u m e s e x t r a e n e r g y . T h u s , th e u s e o f e x ce s s s t e a m c o n s u m e se n e r g y b o t h i n i ts g e n e r a t i o n a n d i n d i s p o s a l o f i t s c o n d e n s a t e .T h e m a i n a d v a n t a g e s o f e x c e s s s t e a m a r e a n i n c r ea s e i n t h e r e a c t i o n r a te ,


10/19

168a c lo s e r a p p r o a c h t o c o m p l e t i o n o f d e s i r e d r e a c t io n s , a n d s u p p r e s s i o n o fu n d e s i r e d r e a c t io n s . O n e u n d e s i r e d r e a c t i o n is t h e d e p o s i t i o n o f c a r b o n int h e p r i m a r y r e f o r m e r . T h e c a r b o n m a y c o a t t h e c a t a ly s t o r t h e i nn e r w a l lso f t h e r e f o r m e r t u b e s , t h e r e b y i n te r fe r in g w i t h t h e d e s ir e d r e a c t i o n an dg iv in g ri se t o h o t s p o t s o n t h e r e f o r m e r t u b e s a n d p o s s ib l e t u b e f a il u re .P o s s i b le si d e r e a c t i o n s i n t h e s h i f t r e a c t i o n a r e f o r m a t i o n o f m e t h a n o l o rf o r m a t i o n o f h y d r o c a r b o n s b y th e F i s c h er - T r o p s ch r e a c t io n . T h e m a i nm e t h o d s f o r d e a li n g w i t h t h e s e p r o b l e m s a r e to d e v e l o p c a t a ly s t s t h a t a r eh i g h ly a c t iv e f o r t h e d e s i r e d r e a c t i o n s a n d i n a c ti v e f o r t h e u n d e s i r e d o n e s .A s e c o n d a r y m e t h o d is t o c h o o s e t h e o p t i m u m t e m p e r a t u r e l ev el , p a r-t i c u l a r l y i n t h e s h i f t r e a c t i o n s t e p .A n o t h e r a r e a o f e n e r g y s av in g is i n t h e C O 2 - r e m o v a l s y s t e m . A w i d ec h o i c e o f C O 2 a b s o r b e n t s ( b o t h c h e m i c a l a n d p h y s i c a l ) a n d t h e m e t h o d sf o r u s i n g t h e m a r e a v a i l a b l e , e a c h w i t h a d v a n t a g e s a n d d i s a d v a n t a g e s . C h o o s -in g t h e m o s t e c o n o m i c a l s y s t e m f o r a n y s p e ci f i c a p p l i c a ti o n m a y b e di f-f i c u l t ( S t o k e s , 1 9 8 0 ) .I n c o n c l u si o n , t h e p r o b l e m o f o p t i m i z a t i o n o f e n e r g y u se in a n a m m o n i ap l a n t i s a v e ry c o m p l e x o n e , a n d m u c h r e se a rc h , d e v e l o p m e n t , a n d e x -p e r i e n c e w i ll b e n e c e s sa r y b e f o r e w e c a n b e s ur e t h a t w e ha v e a p p r o a c h e da p ra c t i c a l m i n i m u m e n e r g y c o n s u m p t i o n . T h e e s t i m a t e d e n er g y r e q u ir e -m e n t s f o r a m m o n i a b a s e d o n s e l e c t e d n e w e n e r g y - e f f i c i e n t a m m o n i a p r o -c e s s es a r e s u m m a r i z e d i n T a b l e 2 .E n e r g y - sa v i n g f e a t u r e s f o r n e w a m m o n i a p l a n t s i n c l u d e: ( 1 ) r e c o v e r y o fTABLE 2Estimated energy requirements for ammonia production based on selected new energyefficient processesCompany/ Process Source GJ/ t NH 3 (HHV)Haldor Topsoe low-energy process Rudbec k (1982) 33.12

ICI's AMV processKTI's PARC process aHumphreys and Glasgow LEAD processKellogg process bC-E Lummus process

C,F. Braun processSnam-Progetti scheme

Topsoe andEkner (1982)Livingstone and 31.65Pinto (1983)Van Weenen and 37.25Tielrooy (1980a, b)Brown (1981) 33.70Nitrogen (1983) 34.81Chari (1982 ) 33.56Ghosal andKarkun (1982)Ghosal and 32.64Karkun (1982)Chari (1982 ) 33.05

a Ranges between 36.35 and 37.25 GJ/t.bMedium energy process; lower energy processes are also offered.


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169waste heat to generate high-pressure and high-temperature steam; (2) pre-heating of combustion air, process air, and feedstock; (3) optimization ofreformer pressure; (4) efficient CO2-removal system (two-stage or physicaltype); (5) efficient final purification {absorption and removal of H20, NH3,CO, and N2; selective oxidation of CO; cryogenic purification); (6) pressureoptimization in the synthesis section; (7) synthesis gas purification bymolecular sieves and low-pressure drop in the synthesis loop; (8) purge gasrecovery (low-temperature system, absorbent system, semipermeable mem-brane); (9) gas turbines and use of exhaust gas as combust ion air; (10) lower-ing the steam : carbon ratio; (11) milder conditions in the primary refo rmerfollowed by removal of residual methane; and (12) ammonia separation byabsorption refrigeration. However, not all these features may be used in anysingle ammonia process.Energy saving through efficient operations

For a given technology and feedstock, energy consumption for ammoniaprod ucti on can also be lowered through improvements in operational ef-ficiency, better management, and energy conservation. Perhaps the mosturgent energy-saving option for developing countries is to increase the per-formance of both existing and new ammonia plants. This can be accom-plished through increases in the on-stream factor and the load factor, anddecreases in the shutdowns.The average performance of large centrifugal ammonia plants in differentparts of the world during 1978--81 is reported in Table 3. The developingcountries not only have the largest downtime {hence the lowest on-streamfactor) but also a large number of shutdowns, which result in substantialwastage of energy. However, about 60% of the shutdowns and 75% of thedowntime is accounted for by major equipment failure and preventivemaintenance , including turnarounds. An improvemen t in perfo rmance ofammonia plants can result in substantial energy saving and hence an im-provement in energy efficiency. According to Joharapurkar {1982), theaverage energy consumption for an ammonia plant in India declined from48.5 GJ/t NH3 at 66% load factor to 40.2 GJ/t NH3 at 105% load factor,implying a decline in energy use by 8.3 GJ /t or 17%.Clearly, there is an inverse relationship between energy use and per-formance of ammonia plants beyond the practical minimum energy require-ments. Furthermore, the larger the number of interruptions and shutdowns,the lower the energy efficiency. Improvements in plant performance notonly increase energy efficiency but also lower per unit cost of productionand increase ammonia production. In addition, energy use at a given levelof plant performance can also be reduced through common sense measures

dealing with energy conservation. This includes, among others, lighting im-provements, improved general maintenance, steam trap maintenance pro-gram, and proper insulation.


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170TABLE 3Average performance of large centrifugal ammonia plants in different parts of the worldduring 1978--81 aItem Average/year per plant in

North Europe Rest of WorldAmerica the world bNumb er of plants surveyed 39 24 25 88ShutdownsTotal shutd owns (num ber ) 8.4 8.7 11.1 9.2Due to majo r eq uipme nt failure (%)c 51 52 54 52Due to tu rn ar ou nds (%) 8 8 9 9DowntimeTotal downt ime (days) 44.3 56.6 75.1 55.2Due to major eq uipme nt failure (%)c 35 37 33 35Due to preventive main tena nce (%) 31 40 49 39TurnaroundAfter every (mo nth s) 16.0 16.0 11.3 14.5Down time (days) 18.6 31.0 36.0 27.2Down time /yea r (days) 14.0 23.0 38.0 23.0On-st ream factor (%)d 88 85 79 85aDerived from survey data reported in Williams and Hoehing (1983). For North America,survey data refer to 1977--81.bIncludes Brazil, India, Japan, South Korea, Kuwait, Pakistan, Qatar, Taiwan, andVenezuela.CApproximately 50% of the major equipment failure is caused by failures in synthesisgas compressor and reforming unit. Synthesis gas compressor is the single most imp ort antsource of major equi pmen t failure.dThe on-stream factor for the 'best' plant was better by 10% in North America, 11% inEurope, 17% in the rest of the world, and 11% for the world as a whole.

E c o n o m i c s o f e n e r g y s a v i n gBefore makin g an y changes in existing or even in new ammo nia plants,which are supposedly more energy sufficient, it is extremely important toask, 'How much energy is being saved and at what cost?' Saving energy forthe sake of saving it doe s no t provide a very strong econo mic justif icationfor large capital investments. For example, in a large fertilizer complex theenergy saved or surplus energy may be wasted if there is no alternative usefor it in th e co mplex, an d especially if it is no t feasible to e xpo rt it. Further -more, for countries that have large energy resources or that flare a largeamount of gas, there may be very little incentive to save energy, especiallyif it is achieved at large capital investment.In th e long run, up to certain limits, energy-saving innovations may in-volve substituting capital for energy, i.e., the use of more capital and lessenergy . The available evidence on the impli cations of energy-saving innova-


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171t i o n s o n c a p i t a l e x p e n d i t u r e i s r a t h e r m i x e d . H o w e v e r , m a n y e n e rg y - s av i n gi n n o v a t i o n s h a v e b e e n d e m o n s t r a t e d i n fu l l- s ca l e p l a n t s a n d h a v e g e n e r a l lyp r o v e n c o s t e f f e c t i v e . A n i n n o v a t i o n t h a t r e s u l ts i n s a vi ng s o f b o t h e n e r g ya n d c a p i t a l i n N o r t h A m e r i c a o r W e s te r n E u r o p e m a y n o t a l w a y s r e s u lt ins i m i la r sa v i n gs i n d e v e l o p i n g c o u n t r i e s . M o s t i n n o v a t i o n s t h a t p r o m i s e s ig -n i f i c a n t e n e r g y s a vi ng s a t t h e c o m m e r c i a l l ev e l d o i n v o lv e a d d i t i o n a l c a p it a li n v e s tm e n t s . H o w e v e r , t h e m a g n i t u d e o f a d d i t i o n a l c a p it a l i n v e s t m e n t m a yv a r y w i t h t h e t y p e o f i n n o v at i o n .F u r t h e r m o r e , t h e r e is a l so n e e d t o c o n s i d e r t h e i m p l i c a t i o n s o f e n e r g y -s a vi ng i n n o v a t i o n s o n t r a i n e d m a n p o w e r , f o r e i g n e x c h a n g e , d e g r e e o f c o m -p l e x i t y , a n d t h e d e g r e e o f r el i ab i l it y . A s m e n t i o n e d p r e v i o u s l y , re l i a b il i ty isa v e r y i m p o r t a n t c h a r a c t e r i st i c ; t h e m o s t e n e r g y - e f f i c i e n t p l a n t w i ll n o t b ee c o n o m i c a l u n l e s s i t c a n b e o p e r a t e d a t h i g h - c a p a c i ty u t i l i z a ti o n . M o s tp r o c e s se s t h a t a r e o f f e r e d b y r e p u t a b l e c o m p a n i e s a re c la i m e d t o u s e o n l yp r o v e n t e c h n o l o g y , b u t t h e t e c h n o l o g y m a y n o t ha v e b e e n p ro v e n in a na m m o n i a p l a n t .ENERGY EFFI CIENCY IN UREA PRODUCTIONLinkage betw een ammonia and urea produ ction

T h e p r o d u c t i o n o f u r e a r e q u ir e s a m m o n i a a n d c a r b o n d io x i d e . C a r b o nd i o x i d e is r e a c t e d w i t h a m m o n i a a t a h ig h te m p e r a t u r e ( 1 8 0 C ) a n d p r e ss u r e( 1 4 0 - - 2 0 0 b a r ) t o f o r m a m m o n i u m c a r b a m a t e f o l l o w e d b y d e h y d r a t i o n t of o r m u r e a . C a r b o n d i o x i d e is a n e c e s s a r y r a w m a t e r i a l t h a t is a v a il a b le a ta l m o s t n o c o s t o n l y a t a m m o n i a p l a n ts . I n A s i a m a n y p la n t s ar e d e s i g n e d t oc o n v e r t a ll o f t h e a m m o n i a t o u r e a. S o t h e e f f e c t o f a m m o n i a p r o c e s s in -n o v a t i o n s o n u r e a p r o d u c t i o n s h o u l d b e c o n s i d e r e d . E n e r g y s a v in g i n t h ea m m o n i a p l a n t c o u l d r e s u l t i n e x c e s s s t e a m w h i c h c o u l d b e u t i li z e d in t h eu r e a p l a n t.

I n t h e p r o d u c t i o n o f u r e a b y us e o f t h e c o n v e n t io n a l m e t h o d s , t h ea m o u n t o f C O 2 a v a i la b l e w o u l d b e s li g h tl y l es s t h a n t h a t r e q u i r e d t o c o n v e r ta ll o f t h e a m m o n i a t o u r e a , a s s u m i n g t h a t n a t u r a l g a s f e e d s t o c k w a s p u r em e t h a n e a n d a ll o f th e h y d r o g e n w a s c o n v e r t e d t o a m m o n i a . I n p r a c t ic et h e r e is u s u a l ly e n o u g h C O s b e c a u s e o f h i g he r h y d r o c a r b o n s i n th e n a t u r a lg as o r i n c o m p l e t e u t i l i z a ti o n o f t h e h y d r o g e n . A n y i n n o v a t i o n t h a t d e c r e as e st h e a m o u n t o f n a t u r a l g as f e e d s t o c k c o n s u m e d w i ll d e c r e as e t h e a m o u n t o fC O s f o r m e d . I f t h e r e is i n s u f f i c ie n t p r o c e s s C O s f o r m e d , a d d i t i o n a l C O s c a nb e r e c o v e r e d f r o m t h e p r i m a r y r e f o r m e r s t ac k ga s b u t a t a d d i ti o n a l c o s t.Energy cons umpti on in urea produ ction

U r e a p r o d u c t i o n r e q u ir e s e n e r g y i n th e f o r m o f a m m o n i a a s r a w m a te r ia la n d e n e r g y f o r c o n v e r t i n g a m m o n i a i n t o u r e a . T h e a v e ra g e e n e r g y u s e f o rp r o d u c t i o n o f p ri ll ed u r e a d u ri n g 1 9 7 9 a n d 1 9 8 1 is e s t i m a t e d a n d r e p o r t e di n T a b l e 4 .


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172TABLE 4Average energy use for pro duc tio n of prilled ureaEnergy component Energ y use for prilled urea (46% N)

1979 1981(GJ/t of Urea)Ammon ia manufacture 26.97 26.05Urea synthes is 5.53 5.30Urea prilling 4.07 3.73

Total 36.57 35.08(GJ/t of N)Total 79.50 76.26

T h e s e e s t i m a t e s i n d i c a t e t h a t : ( 1 ) t o t a l e n e r g y u se i n u r e a p r o d u c t i o nd u r in g 1 9 8 1 h a s d e c l in e d a b o u t 4 % o v e r 1 9 7 9 ; a n d ( 2 ) a p p r o x i m a t e l y 7 5 %o f t o t a l e n e r g y r e q u i r e d t o p r o d u c e u r e a is a t t r i b u t e d t o a m m o n i a a n d t h er e m a i n i n g 2 5 % t o u r e a s y n t h e s i s a n d u r e a p r iU i ng . G r a n u l a r u r e a a p p e a r s t or e q u i r e l es s e n e r g y t h a n d o e s p ri l l e d u r e a p a r t l y b e c a u s e t h e p o l l u t i o n c o n -t r o l p r o b l e m s a r e le s s d i f f i c u l t w i t h t h e g r a n u la r p r o d u c t . T h e m a i n f o r m s o fe n e r g y u s e d i n u r e a p r o d u c t i o n a r e s t e a m a n d e l e c t r ic i t y , w h i c h c a n b eg e n e r a t e d b y a n y f u e l , d e p e n d i n g o n t h e e c o n o m i c s a n d a v a i l a b i l i t y .Energy saving in urea production

E n e r g y c o n s u m p t i o n in u r e a p r o d u c t i o n c a n b e r e d u c e d t h r o u g h : ( 1 ) t h eu s e o f r e t r o f i t s i n e x i st i n g p l a n t s ; ( 2 ) t h e u s e o f e n e r g y - sa v i n g f e a t u r e s i nn e w p l a n t s ; a n d ( 3 ) i m p r o v e m e n t s in o p e r a t i o n a l e f f i c ie n c y , b e t te r m a n a g e -m e n t , a n d e n e r g y c o n s e r v a ti o n . S in c e a m m o n i a i n p u t a c c o u n t s fo r a b o u t7 5 % o f t o t a l e n e r g y u s e i n u r e a p r o d u c t i o n , e n e r g y -s a v in g e f f o r t s in a m m o n i ap r o d u c t i o n w i ll r e d u c e e n e r g y c o n s u m p t i o n i n u r e a p r o d u c t i o n . T h e r e isa ls o s u b s t a n t i a l r o o m f o r r e d u c i n g p r o c e s s ( s y n t h e s i s a n d p r il li ng ) e n e r g yc o n s u m p t i o n f o r u re a . T h e p r o c e s s e n e r g y c a n b e r e d u c e d t o 4 .7 G J / t o fu r e a , w h i c h is a b o u t o n e - h a l f o f c u r r e n t e n e r g y us e . T h e a d o p t i o n o f e n e r g y -s av in g i n n o v a t i o n , h o w e v e r , w o u l d d e p e n d u p o n c a p i ta l c o s t s a n d r e l ia b i l it yo f n e w p r o ce s se s .

I t s h o u l d b e n o t e d t h a t t h e a v e r a g e e n e r g y u s e i n c o n v e r t i n g a m m o n i at o p r i ll e d u r e a is a b o u t 2 1 G J / t o f N , s o p o t e n t i a l s av in g s i n t h is p r o c e s s a r ec o m p a r a b l e t o t h o s e t h a t m a y b e m a d e i n t h e a m m o n i a p r o c es s . I n a c o n -v e n t i o n a l u r e a r e a c t o r , a n e x c e s s o f a m m o n i a is u s e d , a n d t h e c o n v e r s i o ne f f i c i e n c y f o r c a r b o n d i o x i d e i s 6 5 - - 7 0 % ; f o r a m m o n i a i t is 3 2 - - 3 5 % . T h ee n e r g y e f f i c ie n c y o f u r e a p r o d u c t i o n c a n b e i m p r o v e d t h r o u g h h ig h c o n v er -s i o n e f f i c i e n c y i n t h e u r e a r e a c t o r a n d e f f i c i e n t r e c y c l i n g o f u n r e a c t e d a m -m o n i a a n d a m m o n i u m c a r b a m a te . I n c o n j u n c t i o n w i t h a p p li c a ti o n o f m o r e


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173energy-efficient ammonia technology, the total energy use can be reducedfrom 79.5 to about 52.0 GJ/t of N (HHV) for bulk priUed or granular ureaf.o.b, factory. Any energy saving beyond this point is likely to be small andperhaps economically marginal, barring unforeseen technological break-thoughs.Two recent papers compared the energy requirements for three of theleading low-energy processes (Pagani, 1983; Dooyeweerd and Meessen,1983): the Stamicarbon CO2-stripping process; the ACES process (AdvancedProcess for Cost and Energy Savings), which also uses CO2 stripping; andthe IDR process {Isobaric Double Recycle), which uses stripping with bothNH3 and CO2, consecutively. The three processes are compared on thecommon basis that the CO2 compressor is driven by an extraction turbineusing high-pressure, superheated steam {120 bar, 480C) with extraction oflower pressure steam for use in the processes. The comparison also includesenergy for evaporating the urea solution but does not include energy forprilling or granulation.Assuming that the high-pressure superheated steam can be generated witha fue l input of 1700 Btu/lb (3.95 GJ/t), which implies a fuel effici ency ofabout 80%, and tha t 1 kWh is equivalent to 10.6 MJ of fuel energy, the to talenergy consumption for the three processes ranges from 3.04 to 3.38 GJ/tof urea. We do no t consider the difference in energy consumption betweenthe three processes to be significant in view of the fact tha t d ifferen t assump-tions could lead to different conclusions. However, the energy for the syn-thesis step for present plants in North America averaged 5.3 GJ/t not includ-ing concentration of the urea solution. Thus any of the three processeswould offer a significant saving in comparison with the present average ofNorth America plants.Low-energy processes are also available for conversion of urea solutionto granular or prilled urea; some also include pollution control. TennesseeValley Authority (TVA) has estimated that its modernized urea plant, inconjunction with its new curtain-granulation process, will produce granularurea with a total energy requi rement of about 4.3 GJ/t of urea (Blouin,1984). In comparison, t he Nor th Amer ican average energy consumpt ion in1981 was 9.0 GJ/t for prilled urea and 8.3 for granular urea. Stated in termsof GJ/t of N, energy savings in the range of 8.7-=10.0 GJ appear possiblefor the conversion of ammonia to urea, which is about equal to the potentialsavings in ammonia manufacturing per unit of N.ECONOMIC CONSEQUENCES OF IMPROVEMENT IN ENERGY EFFICI ENCYPotential energy savings in nitrogen production

Average energy consumption, energy efficiency, and probable maximumenergy saving estimates in production of selected N fertilizers are developedin Table 5. The potential energy savings for different fertilizers range be-


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174

tween 25% and 35% over 1979 energy consumption estimates, and between22% and 32% over the 1981 estimates. The opportunity for saving energyin developing countries is even larger than indicated part ly because the exist-ing energy consumption is higher in the developing countries. The weightedenergy consumption for N fertilizer production has been estimated to be69.54 GJ/ t of N in 1979 and 48.42 GJ/t in the future, which implies ap-proximately 30% saving in energy use.There do not seem to be any commerc ially feasible technological break-throughs in producing N fertilizers that would result in major energy savingin the near fu ture . Most of the energy-saving innovations involving changesand modifications would each result in only a small saving. However, thesum of numerous savings would amount to a substantial total for N fer-tilizer production. These marginal changes would involve large capital ex-penditures if incorporated in existing plants but would not necessarily in-crease the cost of new plants.

TABLE 5Energy efficiency and probable maximum saving in energy use for manufacturingnitrogen fertilizers in new fertilizer plants using available technologyProduct N Average energy input Probable Energy(%) (GJ/t of N) energy efficiencya

1979 1981 Fu tu re savinga (%) (%)1979 1981 1979 1981Amm on ia 82 57.2 55.3 42.9 25 22 75 78Urea, prilled b 46 79.5 76.3 54.2 32 29 68 71Am mo ni um nitrate, prilled c 34 73.4 71.6 50.7 31 29 69 71UAN solut ion d 30 67.2 65.2 48.4 28 26 72 74Am mo ni um sulfate, synt hetic e 21 60.0 58.0 39.3 35 32 65 68aEnergy efficiency (%) is defined as average energy input in future divided by averageenergy input in 1979 or 1981 and multiplied by 100, and probable energy saving (%) isequal to 100 minus energy efficiency (%).bThe average energy input for granular urea has been estimated to be 76.1 and 74.8 GJ/tof N during 1979 and 1981, respectively.CThe corresponding energy input estimates for granular ammonium nitrate are 71.8 and69.4 GJ/t of N during 1979 and 1981, respectively.dUAN solution contains urea and ammonium nitrate solutions. Energy consumption forpro duct ion of UAN solution consists of ammonia energy input, energy input for ureasynthesis, conversion energy for ammonia nitrate, and a very small amount of energy(0.25 GJ/ t of UAN produc t) required as process energy. The UAN solution does notneed the energy that is used for prilling urea or am moni um nitrate.eThe future energy use is a weighted average (50% each) of two estimates: 46.14 GJ/tof N with out any credit for H~SO, energy and no energy recovery from the reaction,and 32.50 GJ/t of N assuming credit for H~SO, energy and recovery of 50% of heat ofreaction.


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1 7 5

Economic benefits o f energy-efficiency improvementT h e p o t e n t i a l e c o n o m i c b e n e f i t s o f e n e r g y -e f f ic i e n c y i m p r o v e m e n t i n

N f e r t il i z er p r o d u c t i o n a r e e s t i m a t e d i n T a b le 6 . D u r i n g 1 9 8 1 / 8 2 , f o r t h ew o r l d a s a w h o l e , e n e r g y s a vi n g p o t e n t i a l i n N f e r t il iz e r p r o d u c t i o n h a s b e e ne s t i m a t e d t o b e 1 2 7 6 m i l li o n G J . T h i s i s e q u i v a l e n t t o a b o u t 1 1 8 2 1 09 s c fo f n a t u r a l g a s, w o r t h a p p r o x i m a t e l y $ 4 . 1 4 1 0 9 .

I t w i l l t ak e sever al ye ar s b e f or e t h e N f e r t il i z e r in d u s t r y i s ab le t o r e d u c ee n e rg y c o n s u m p t i o n f r o m p r es e n t t o f u t u r e c o n s u m p t i o n e s ti m a t es . H o w -e ve r , t h e p o t e n t ia l sav in gs are large e n o u gh t o j u s t i f y e n e r gy r e se ar c h , d e ve l -o p m e n t , a n d m a n a g e m e n t e f f o r ts t o i m p r o v e e n er g y e f f ic i e n c y . A l o t o ft h e s e sav in gs c an b e r e a l i ze d t h r ou gh e f f i c i e n t op e r a t ion o f N f e r t i li z e rp l a n t s, w h i c h d o e s n o t r e q u i re m u c h a d d i t i o n a l c a p i ta l i n v e s t m e n t . T h i s isn o t on ly d e s ir ab le , b u t i t a l so p r ov id e s a v iab le op t ion t o k e e p N f e r t i li z e rp r ic e s low, e sp e c ia l ly in t h e d e ve lop in g c ou n t r ie s .T A B L E 6E s t im ate d po te n t ia l econom ic benef i t s f rom im provem ents in energy e ff i c iency fo r m anufac tu r ingni t ro gen fer t i l izers dur ing 1981 /82 aReg ion E nergy consum p t ion (106 GJ)

At ex i st in~ At im provedeff ic iency U eff ic iency cPo ten t ia l eco nom ic benef i t s o f im -proved energy eff ic iency in terms ofEnergy Equivale nt Value esaving natu ral gas d(106 GJ) (109 scf) (109 $)

Deve loped Marke t E conom ies 1548 1078 470 435 1 . 52Nor th Am er ica 766 533 233 216 0 . 76Western Euro pe 679 473 206 191 0 .67Oceani a 19 13 6 6 0.02Other s 84 59 25 23 0.08Deve lop ing Marke t E conom ies 888 618 270 250 0 . 88Afr ica 46 32 14 13 0 .05Lat in Ameri ca 199 139 60 56 0 .20Near Eas t 147 102 45 42 0 .1 5Far Eas t 495 345 150 139 0 .49Cen t ra lly P lanned E conom ies 1767 1231 536 496 1 . 74

Asian CPE 856 596 260 241 0.84Eas t Euro pe and U.S.S .R. 911 634 277 257 0 .90Develop ed, a ll 2459 1712 747 692 2 .42Developing, a ll 174 4 1214 530 491 1 .72W orld 4203 2927 1276 1182 4 . 14a ln i t i a l ly , the po ten t ia l econo m ic benef i t s w il l be r ea l i zed by those coun t r i es / r egions where n i t rogenfe rt i l ize r s a re m anufa c tu re d . U l t im ate ly , however , pa rt o f these benef i t s a re expec ted to be t r ans fe r redto impor ters of n i t rogen fer t i l izers in the for m of lower prices. Regional n i t roge n con sump tio n datawere ob ta ined f rom FAO (1983) .bAverage energy r equ i re m en ts fo r m anuf ac tu r ing n i t rogen f e r ti l i ze r s have been es t im ated to be 69 . 54GJ / t o f N .CAverage energy r equ ire ment s for man ufa ctu r in g ni t roge n fer t i lizers in the future, in new fer t i l izerplants us ing avai lable tec hnol ogy, have been es t imat ed to be 48.42 GJ/ t of N. This implies probab lem axim u m sav ing o f abou t 30% over p resen t energy input .dAssuming 1 GJ = 926 scf of natural gas in terms of high heat ing values (HHV).e The U.S. average price for natur al gas del ivered to electr ic p lants dur ing 19 82 was $3.50 per t housa ndcubic feet . The natural gas pr ices paid by new ammonia plants are about the same.


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1 7 6Energy Efficiency and Government Policy

Improving energy efficiency in N fertilizer production involves at leastthree strategies, which are not necessarily mutually exclusive. These strate-gies are: (1) installation of energy-eff icient retrof its in the existing plants;(2) use of energy-ef ficient processes and equipment in the new plants; and(3) efficient operations, better management, and energy conservation inexisting and new plants. All the three strategies are important in their ownways. The first two strategies involve additional capital investment and pos-sibly some additional risk as compared with the strategy involving efficientoperations and energy management. Furthermore, energy-substitution in-novations are limited but do exist for both feedstock and fuel.Energy saving in the absence of any alternative use for saved energy doesnot provide economic justification for making large capital investmentsand/or increasing complexity. Energy-efficient modifications are no t alwayseconomical and may adversely affect reliability. Careful consideration isneeded to weigh potential energy savings against poten tial adverse economicand environmental effects. There is need to study economic costs andbenefits of various energy-saving technologies.Governments must take the lead in promoting and facilitating the use ofenergy-efficient innovation. This can be accomplished through appropriateactions, including incentives, interventions, education, research and devel-opment , regulation, and monitoring. Developing countries that are deficientin food, fertilizer, foreign exchange, and energy must give a high priorityto the design and implementation of energy-efficient fertilizer productionplans. International organizations can play a vital role in facilitating theformulation and implementation of such national programs.The major criteria in selecting energy-efficien t innovations for N fertilizerproduction should be: (1) maximization of economic benefits; (2) minimiza-tion of capital costs; (3) maximization of reliability and operational ef-ficiency; (4) maximization of fertilizer use efficiency through efficientfertilizer products; and (5) minimization of foreign exchange needs. Energyoptimization should deal with best compromise among these five criteria.

The developing countries should not adopt exotic energy-efficient innova-tions that have not ye t been proven economical and reliable commercially.This is especially true for countries with surplus energy resources and limitedmanpower skills to operate complex fertilizer plants. In this context, policiesdesigned to improve energy efficiency in N fertilizer production must beevaluated carefully by maintaining both a long-term perspective and anawareness of the existing national policies, resources, requirements , and con-straints.

R E F E R E N C E SB l o u i n , G . M . , 1 9 8 4 . E n e r g y r e q u i r e m e n t s f o r h i g h n i t r o g e n f e r t i li z e r s. C h e m . E n g . P ro g r .,8 0 : 4 0 - - 4 4 .


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177Brown, F.C., 1981. How L.E.A.D. reduces energy and capital costs by 10--15%. In:Fertilizer Nitrogen. British Sulphur Corp., London , pp. 39--51.Chari, K.S., 1982. Energy conservation in the pr oduc tion of ammonia. In: FertilizerIndus try (A nnual Review), Specialized Publications, Bombay, India, pp. 85--91.Czuppon, T.A. and Buividas, L.J., 1979. Hydrogen for Ammo nia Produc tion and Eco-

nomics of Alternative Feedstocks. ACS/CSJ Pacific Chemical Congress, Honolulu,HI, 1--6 April 1979, pp. 1--22.Dooyewe erd, E. and Meessen, J., 1983. C ompari son of the energy consu mpt ion s of low-energy urea technologies. Nitrogen, 143: 32--38.Euro pea n Chemical News, 1982. Weekly pub lic ati on by Business Press Int ern ati ona lLtd., Sutto n, Surrey, Great Britain.FAO, 1983. Fertili zer Yearbook, 1982 (a nd various previous issues). Food and Agricul-ture Organization of the United Nations, Rome.Ghosal, S.R. an d Karkun, K., 1982. Improv ements in ammo nia process technology forreduced energy consumpt ion. Fert. News, 27(12): 47--55.IFDC, 1979. Fertilizer Manual. IFDC-R-1, Intern ation al Fertilizer Development Center,IFDC, Muscle Shoals, AL. (Also available from UNIDO, Vien na, Australia.)Joharapurkar, V.R., 1982. Energy conservation in existing ammonia plants. In: FAISeminar 1982 on Fertilizer Productivity and Advances in Technology, Part II. Fertil-izer Association of India, New Delhi, India, pp. III-1/1--12.LeBlanc, J.R., Moore, D.O. and Schneider, R.V., 1982. Retrof its can reduce energy con-sumption in ammonia manufacture for substantial savings. Oil Gas J., 80 (38): 115--118.Livingstone, J.G. and P into, A., 1983. New a mmo nia process reduces costs. Chem. Eng.Progr., 79(5): 62--66.Mudahar, M.S. and Hignett, T.P., 1982. Energy and Fertiliz er: Policy Impli cati ons andOpti ons for Developing Countries. Tech. Bull. IFDC-T-20, Int ern ati ona l Fertili zerDevelopment Center, Muscle Shoals, AL, 241 pp.

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Energy Efficiency in N2 Fertlizer

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