Third World Applications of Pyrolysis of Agricultural ... - ACS Publications

47 Third World Applications of Pyrolysis of Agricultural and Forestry Wastes

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JOHN W. TATOM and HADLEY W. WELLBORN J. W. Tatom, Consulting Engineers, 4074 Ridge Rd., Smyrna, GA 30080 FILINO HARAHAP and SASWINADI SASMOJO Development Technology Center, Institute Technology Bandung, Bandung, Indonesia

Third World Countries face extraordinary problems in meeting the energy demands required for development, especially due to their general lack of foreign exchange. They are also confronted with high levels of unemployment, but have the mixed blessing of low wage levels. Therefore, a renewable energy system that could engage many people in its operation would be an especially welcome prospect. Since most Lesser Developed Countries lie in the tropics and already have economies largely based on agriculture and forestry, the potential for biomass as a renewable energy source is therefore great; particularly since it is very labor intensive. In addition, a big supply of biomass is already available in the form of agricultural and forestry residues that, to a very large extent, are currently wasted. These residues are not only scattered about in the fields and forests, but they are also concentrated at processing plants such as rice mills, saw mills, cotton gins, sugar mills etc, and therefore a significant fraction is already available conveniently for use as an energy source. The main problem, however, with biomass as it is available now is that: at an industrial scale, most current energy conversion equipment is designed to operate on fossil fuels and at a domestic scale, most i f not a l l stoves are for wood or charcoal use and will not operate properly on these residues. Thus there is a need for a means for converting residues into synthetic coal, o i l and gas which could be utilized in existing equipment. Pyrolysis, especially low temperature pyrolysis, which favors char and o i l production, offers a particularly promising means of conversion,since the char and o i l products are storable and easily transportable-an especially attractive characteristic in developing countries. Morever, recent U.S. experience with the steady-flow, vertical packed bed, partial oxidation pyrolysis process (1,2) has demonstrated that this technology, updated from its earlier froms is a viable means for conversion of forestry and agricultural wastes into synthetic fuels. In addition, the basic simplicity of the process makes it suitable for applications in rural environments because of its few moving parts, low maintenance requirements, and general ruggedness. But the steady-flow, vertical, packed bed, 0-8412-0565-5/80/47-130-671$05.O0/0 © 1980 American Chemical Society In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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THERMAL CONVERSION OF SOLID WASTES AND BIOMASS

partial oxidation pyrolytic convertors produced thus far have been built primarily in developed countries with a high level of automation and capital intensity and are subject to environmental constraints totally inappropriate to developing countries. However, recent studies (3,4,5,6) of the application of partial oxidation pyrolysis-in a suitably modified form-to Third World countries have indicated the technical-economic potential of the process, and these studies have led to several hardware development programs. Since the initial fabrication of an appropriate technology pyrolytic convertor dates back only to the spring of 1977, there still is much to be learned about how to build a truly appropriate system, even though at least five generations of these units have now been constructed. Thus while it is believed that current demonstration systems will be shown to be economical, there are no doubt improvements that will be made in time to upgrade the performance of these early models. Therefore, this presentation can only be regarded as a progress report on this work. In a later paper, a more complete presentation will be made. It should also be noted that due to the often adverse conditions under which the units have been operated that the primary challenge has been to get the systems properly running, and up until this time, only the most basic data have been recorded. Thus performance has often had to be estimated using assumed energy constants, and so until more refined data are available, some reservations must be held regarding the exact values presented. But since the basic process is that of the vertical, packed bed, whose performance has been studied at considerable length (1,2) and has been shown to have a char-oil energy conversion efficiency which is a function only of the air-to-feed ratio, and that seems to be independent of feedstock, convertor scale, bed depth, and operating technique (see Figure 1), there is good reason to expect that these appropriate technology designs will have similar performance characteristics. The Basic Design Concept To develop an appropriate technology pyrolytic convertor inevitably means the compromising of many design features, performance , and environmental constraints to gain economy, simplicity, reliability and maintainability. Thus there is l i t t l e doubt that existing, more capital intensive convertors can provide higher performance than described here. But under the conditions for which they were developed, we believe the characteristics and performance of the designs discussed in this paper are sufficiently advanced to make them at least the basis for a first generation of economically practical conversion systems. To illustrate the problems of developing an appropriate technology design; once the basic decision to use the partial oxidation pyrolysis process is made, several questions immediately have to be dealt with: i.e.

In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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TATOM E T A L .

Pyrolysis of Agricultural and Forestry Wastes

613

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Figure 1.

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(1) How t o make the process continuous so that o i l and gas also can recovered without the use o f expensive c o n t r o l s and input-output equipment? (2) How t o avoid the d i f f i c u l t i e s experienced i n the U.S. with p a r t i c u l a t e s i n the o f f - g a s which tend t o f o u l the condenser? (3) How t o avoid c o r r o s i o n problems using a v a i l a b l e m a t e r i a l s ? ( 4 ) How to add the process a i r without b l o c k i n g the i n t e r n a l flow and/or burning up the process a i r d e l i v e r y system? ( 5 ) How to separate out the o i l a e r o s o l and/or condense the o i l vapor from the o f f - g a s i n a p r a c t i c a l condenser? (6) Using a manual technique, how to maintain proper materials flow w i t h i n the convertor and avoid the b r i d g i n g problems a s s o c i a t e d with c o n v e n t i o n a l packed bed designs? (7) How t o maintain a f i x e d bed depth t o i n s u r e a more uniform o f f -gas temperature; thus a l l o w i n g proper condenser operation? ( 8 ) How to develop a simple char output system t h a t provides a uniform flow over the convertor cross s e c t i o n - a s opposed to other systems t h a t do not? The design that has evolved t o t h i s date t o best d e a l with these questions i s presented i n F i g u r e 2. The convertor shown can be d e s c r i b e d as a "batch continuous" system which operates ex t e r n a l l y i n a batch mode, but i n t e r n a l l y i n a continuous mode-so long as the feed l e v e l i s maintained above the submerged o f f - g a s port. The b a s i c idea i s to s t o r e the feed w i t h i n the r e a c t o r i t s e l f and thus t o allow a continuous flow o f feed through the worKing s e c t i o n o f the convertor and thereby provide a p r a c t i c a l l y constant supply o f o f f ~ g a s to the condenser at a n e a r l y f i x e d temp e r a t u r e , but yet avoid the expensive m a t e r i a l handling equipment and c o n t r o l s needed f o r a continuous system. The submerged o f f - g a s port not only allows the batch continuous o p e r a t i o n , but i t guarantees a f i x e d bed depth and provides an e f f e c t i v e means f o r f i l t e r i n g the gas through use of the bed i t s e l f , which forms a f i l t e r cake around the o f f - g a s port (however there i s a substant i a l pressure drop through the bed u s i n g the technique). Moreover, s i n c e there i s no f r e e surface through which the gases must pass, and no c o n t i n u a l a d d i t i o n o f f e e d , as i n conventional continuous systems, the f i n e f r a c t i o n of the feed has no opportunity to be c a r r i e d away by the o f f - g a s stream t o subsequently c l o g the condenser. Since the process a i r i s introduced through the convertor w a l l s and the r e s u l t i n g gases migrate t o the entrance of the o f f gas port which i s l o c a t e d near the center o f the convertor, the flow i s not v e r t i c a l and thus the feed g e n e r a l l y passes d i a g o n a l l y through the gases. An advantage t o t h i s arrangements i s that the convertor w a l l s are not exposed over a wide area t o the hot o f f - g a s e s . A disadvantage i s that the depth of the bed may be r e l a t i v e l y small and o c c a s i o n a l p e n t r a t i o n s of uncharred feed can occur during r a p i d p r o c e s s i n g , e s p e c i a l l y i f s u f f i c i e n t agitation i s not employed. However, with proper p r e c a u t i o n s , such as the i n -


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τ ATOM ET AL.



AGITATOR

Figure 2.

Appropriate technology pyrolysis convertor


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t r o d u c t i o n of the a i r at two l e v e l s t o i n s u r e t h a t the char zone has a d e f i n i t e thickness-such occasions can be e f f e c t i v e l y avoided. Since the maintenance o f a f r e e - f l o w i n g c o n d i t i o n w i t h i n the feed/char i n s i d e the convertor i s e s s e n t i a l t o s u c c e s s f u l system o p e r a t i o n , every e f f o r t has been made t o f a c i l i t a t e the flow. Thus the manually operated a g i t a t o r , together with the conic shape of the convertor and the almost complete absence o f any o b s t r u c t i o n s to the flow w i t h i n the r e a c t o r e f f e c t i v e l y guarantee that the packed bed design w i l l operate p r o p e r l y . The condenser/demister system chosen, and shown i n Figure 3^is b a s i c a l l y a compromise between performance and complexity, since to e f f e c t i v e l y remove a l l the f i n e a e r o s o l o i l mist i n the o f f - g a s l e a v i n g the convertor and t o condense out the remainder without c o l l e c t i n g an excessive amount o f water r e q u i r e s a much more s o p h i s t i c a t e d system than b e l i e v e d p r a c t i c a l f o r the a p p l i c a t i o n intended i n many L D C s . Thus the condenser/demister design chosen i n v o l v e s no moving p a r t s and i s e n t i r e l y cooled by n a t u r a l convection. The o f f - g a s - o i l mixture from the convertor enters the unit and expands through a s e r i e s of holes i n the i n s i d e pipe. The r e s u l t i n g j e t s impinge on the i n s i d e surface o f the outer n a t u r a l convection cooled j a c k e t , thus producing good l o c a l heat transfer and a high degree o f turbulence and mixing as the r e s u l t i n g w a l l j e t s expand, c o l l i d e , r o l l up, and are r e i n t r a i n e d i n t o the impinging j e t flows. Thus a s i n g l e f u i l d p a r t i c l e , i n i t s complex path through the condenser, i s e f f e c t i v e l l y cooled and has many oppor t u n i t i e s to c o l l i d e with other p a r t i c l e s to form l a r g e r p a r t i c l e s or d r o p l e t s and/or be c o l l e c t e d on the outer condenser w a l l s . The demister i s of conventional design and uses a v a i l a b l e f i b e r t o prov i d e a matrix through which the condenser off-gases pass. Vegetable f i b e r " i j u k " from a species o f palm t r e e has been s u c c e s s f u l l y used f o r t h i s purpose. The o v e r a l l system takes maximum b e n e f i t of the e f f e c t i v e n e s s o f the submerged o f f - g a s f i l t e r technique, and with weekly maintenance operates f r e e o f the c l o g g i n g problems that have plagued other condenser designs. Thus while i t s performance can be improved through f u r t h e r o p t i m i z a t i o n , current o i l recovery l e v e l s are a c c e p t a b l e , but not outstanding. Regarding c o r r o s i o n ; while i t i s w e l l known that the hot pyr o l y t i c o i l s are a c i d i c and thus could s e r i o u s l y threaten the condenser, i t has been found t h a t the t a r i n the o i l tends to cover the exposed metal surfaces with an almost enamel-like c o a t i n g that e f f e c t i v e l y p r o t e c t s them from excessive o x i d a t i o n . Moreover, while a metal matrix i n the demister such as from l a t h e t u r n i n g s i s destroyed i n a matter o f days, the l i f e t i m e o f i j u k appears to be i n d e f i n i t e . Development H i s t o r y and Current Program Status Several generations o f convertors i n v o l v i n g the b a s i c batch continuous, appropriate technology design described above have been developed during the l a s t two years s i n c e the f i r s t was b u i l t by


TATOM ET AL.



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Figure 3.

Pyrolysis oil condenser


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Tatom and Stone i n C a l i f o r n i a out of o i l drums and galvanized pipe (7_). This system involved the f i r s t use of an annular pebble bed l o c a t e d near the g r a t e , f o r process a i r i n t r o d u c t i o n . A second convertor, designed and b u i l t by Tatom and Wellborn, a l s o of o i l drums, but u s i n g manual a g i t a t i o n , and a more advanced off-gas f i l t e r i n g technique and char removal and storage system served as the predecessor t o the f i r s t r e a l prototype. This l a t t e r system, was b u i l t i n e a r l y 1978 at Kumasi, Ghana at the Technology Consultancy Center (TCC) of the U n i v e r s i t y of Science and Technology. Sponsored under a j o i n t USAID-Bank o f Ghana program administered by the B u i l d i n g and Road Research I n s t i t u t e (BRRI), t h i s system was designed, f a b r i c a t e d , and t e s t e d under subcontract by Georgia I n s t i t u t e o f Technology, Tatom, and Wellborn. T h i s convertor, designed to process 57 kg/hr o f hardwood r e s i d u e , together with three others and two l a r g e batch d r i e r s i s c u r r e n t l y being assembled t o form a system f o r round-the-clock operation. The char and o i l produced w i l l be used t o f i r e a b r i c k k i l n at the BRRI, and the gas w i l l be used t o dry the sawdust down t o s i x t o eight per cent moist u r e . These convertors u t i l i z e the pebble bed process a i r i n t r o duction technique described p r e v i o u s l y together with s t i l l more advanced condenser, mechanical a g i t a t o r , o f f - g a s f i l t e r and char grate designs. The program i n v o l v e s a t e c h n i c a l demonstration of the concept to be followed by an economic e v a l u a t i o n . At the present time, information from the program (8_ jO i n d i c a t e s that the system throughput meets the design g o a l s , that the d r i e r s operate p r o p e r l y , that the char y i e l d s average about 23 per cent, but that the o i l y i e l d s are only s l i g h t l y b e t t e r that seven percent. Thus c l e a r l y , f u r t h e r improvements i n the condenser performance are desired. But since t r o p i c a l hardwoods t y p i c a l l y produce only 11 t o 12 percent o i l / t a r from t h e i r d e s t r u c t i v e d i s t i l l a t i o n , the e f f e c t i v e n e s s of the condenser, while not overwhelming,is s t i l l i n the range of 60 t o 70 percent. 5

Another p y r o l y s i s program, which i s being conducted by Georgia Tech and the U n i v e r s i t y of the P h i l i p p i n e s Engineering Research and Development Foundation with the support of UNIDO, uses the b a s i c Ghana convertor design. The system i s p r i m a r i l y designed to process r i c e husks. However, s i n c e the program has only r e c e n t l y been i n i t i a t e d , no performance data are a v a i l a b l e (10). Of p r i n c i p a l importance to the authors i s a j o i n t USAID-Republ i c of Indonesia program a l s o r e c e n t l y i n i t i a t e d . This e f f o r t conducted by the Development Technology Center at the I n s t i t u t e of Technology Bandung, i s d i r e c t e d toward the development of an economically v i a b l e system f o r p y r o l y t i c conversion of r i c e husks at r i c e m i l l s , i n c l u d i n g charcoal b r i q u e t i n g equipment, and other components such as lamps, stoves, modified d i e s e l engines, gas burners e t c . which can u t i l i z e the o i l and gas produced. In a d d i t i o n , a l t e r n a t i v e uses of the o i l - t a r such as f o r wood p r e s e r v i n g are being i n v e s t i g a t e d . The Indonesian p y r o l y s i s system incorpor a t e s more advanced ideas i n process a i r i n t r o d u c t i o n , grate design



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and a g i t a t i o n than i n previous programs. This e f f o r t to date has i n v o l v e d both the design, f a b r i c a t i o n and t e s t of a 100 kg/hr Technology Development Unit, shown i n Figure 4, f o r r e s e a r c h and deve lopment and t r a i n i n g on the ITB campus, and the design and fabric a t i o n of a prototype system f o r use at r i c e m i l l s . More r e c e n t l y the government o f Papua New Guinea has i n i t i a ted an ambitious program f o r development of renewable energy sources through conversion of t h e i r wood residues i n t o domestic and i n d u s t r i a l f u e l s by means o f p y r o l y s i s (11_). Both a p p r o p r i a t e technology and more c a p i t a l i n t e n s i v e systems are planned t o sup ply the v a r i o u s l o c a l needs. An i n i t i a l program at the U n i v e r s i t y of Technology i n Lae t o b u i l d and operate a s m a l l , but automated, one tonne/day, r e s e a r c h and development system u s i n g the basic concept d e s c r i b e d above has j u s t been i n i t i a t e d , and plans are t o begin c o n s t r u c t i o n of a 25 tonne/day prototype i n d u s t r i a l s c a l e system i n 1980. F i n a l l y , an a p p r o p r i a t e technology system using the design approach d i s c u s s e d before i s a l s o planned l a t e r t h i s year or e a r l y i n 1980 as part of the UNEP R u r a l Energy Center being b u i l t o u t s i d e Dakar, Senegal. System D e s c r i p t i o n Since there have been a number of systems developed, each having s i g n i f i c a n t l y d i f f e r e n t design f e a t u r e s , i t i s important to d i s t i n g u i s h between the v a r i o u s convertors b u i l t , yet i m p r a c t i c a l to d e s c r i b e more than one. Therefore the system t o be d e s c r i b e d i n t h i s paper i s the Technology Development Unit mentioned p r e v i o u s l y as p a r t of the Indonesian program and designed p r i m a r i l y t o process r i c e husks. While many t e c h n i c a l improvements have been made using t h i s system, i t s p r i n c i p a l purpose was t o i n v e s t i g a t e a l t e r n a t i v e means of process a i r i n t r o d u c t i o n and o f f - g a s removal, with the object o f a v o i d i n g as many o b s t a c l e s t o the m a t e r i a l s flow within the convertor as p o s s i b l e . Process a i r c o n f i g u r a t i o n s i n v e s t i g a t e d were the pebble bed system used i n the Ghana and P h i l i p p i n e con v e r t o r s and a f l u s h mounted, water jacketed technique, wherely the a i r i s introduced d i r e c t l y through the convertor w a l l s . The primary o b j e c t i o n s t o the pebble bed design were the very l a r g e f r a c t i on o f the convertor cross s e c t i o n blocked by t h i s component and the tendency o f the design t o cause the formation of hot spots r e s u l t i n g from cracks or channels created i n the shear l a y e r between the c e n t r a l , core flow and t h a t above the pebble bed. In a d d i t i o n , a f l u s h mounted o f f - g a s system having f o u r p o r t s , manifolded toget h e r , and the s i m p l e r , i n t e r n a l l y mounted probe system used i n the Ghana design were t e s t e d . It was hoped t h a t succès using the four f l u s h mounted p o r t s would allow removal of the i n t e r n a l probe which does present some o b s t r u c t i o n t o the flow. However, from the start i t was a l s o recognized t h a t the r e s u l t i n g i n t e r n a l gas flow pat t e r n s might leave a r e l a t i v e l y uncharred core i n the convertor cent e r , s i n c e the s h o r t e s t path f o r the gases would be along the convertor walls.




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Figure 4. Indonesian technology development pyrolytic convertor system



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B u t , w h i l e t h e r e a r e d i f f e r e n c e b e t w e e n t h i s s y s t e m and o t h e r s d e s c r i b e d t h e r e a r e a l s o many s i m i l a r i t i e s . T h u s t h e r i c e h u s k s a r e added a t t h e t o p o f t h e c o n v e r t o r t h r o u g h , a q u i c k d i s c o n n e c t i n l e t f e e d p o r t a n d t h e c h a r c o a l removed a t t h e b o t t o m t h r o u g h a simple s l i d i n g grate. Process a i r i s i n t r o d u c e d near t h e bottom o f t h e s y s t e m a n d p a s s e s upward t o e x i t t h r o u g h t h e o f f - g a s system and t h e n e n t e r s t h e c o n d e n s e r / d e m i s t e r w h i c h removes t h e o i l - t a r f r a c tion. The c o n v e r t o r s t a n d s 2.35 m e t e r s t a l l , h a s a b a s e d i a m e t e r o f 0.84 m e t e r s a n d a t o p d i a m e t e r o f 0.70 m e t e r s . I t i s b u i l t o f m i l d s t e e l , t y p i c a l l y two t o t h r e e m i l l i m e t e r s t h i c k . Instrumentation i n c l u d e s a process a i r o r i f i c e , and d i a l thermometers f o r measuring bed, c o n v e r t o r o f f - g a s and condenser o f f - g a s temperature. System

Operation

The s y s t e m i s s t a r t e d by f i r s t i n t r o d u c i n g c o l d c h a r c o a l f r o m t h e p r e v i o u s d a y s o p e r a t i o n up t o t h e l e v e l o f t h e t o p l a y e r o f a i r holes. Burning charcoal briquetes a r e then put i n t o t h e conv e r t o r a n d a r e s u b s e q u e n t l y s m o t h e r e d b y t h e f e e d w h i c h f i l l s up t h e s t o r a g e s e c t i o n . The p r o c e s s a i r i s t h e n i n t r o d u c e d a t a b o u t 10 t o 20 p e r c e n t o f t h e f u l l s t e a d y - s t a t e f l o w r a t e and i s s l o w l y increased. A f t e r s e v e r a l hours t h e temperature o f t h e o f f - g a s beg i n s t o r i s e r a p i d l y and l i m i t e d a c t u a t i o n (shaking) o f t h e g r a t e is initiated. Gradually t h e process a i r r a t e i s increased as t h e removal o f charcoal a c c e l e r a t e s u n t i l a desired operating condit i o n i s reached. P e r h a p s t h e t w o most i m p o r t a n t i n d i c e s o f s y s t e m performance a r e t h e c o n v e r t o r and t h e condenser o f f - g a s t e m p e r a t u r e s . I f p r o per c o n t r o l o f these two i s maintained and c o r r e c t i n t e r p r e t a t i o n o f v a r i a t i o n s i n t h e s e t e m p e r a t u r e s i s made, t h e s y s t e m o p e r a t i o n can be r e l a t i v e l y r o u t i n e . To i l l u s t r a t e : t h e a v e r a g e convertor o f f - g a s temperature i s not only important so f a r as o i l p r o d u c t i o n i s c o n c e r n e d , b u t r a p i d i n c r e a s e s i n i t may i n d i c a t e t h e f o r m a t i o n of c a v i t i e s i n t h e b e d , w h i l e d e c r e a s e s may s u g g e s t t h a t c h a r i s b e i n g removed t o o r a p i d l y . L i k e w i s e t h e c o n d e n s e r o f f - g a s tempera t u r e must be m a i n t a i n e d above t h e d e w p o i n t o f t h e m i x t u r e t o a v o i d water condensation, but not so h i g h as t o reduce the o i l yields. Since each feed c o n t a i n s d i f f e r e n t ash and m o i s t u r e contents and b e c a u s e t h e b e d d e p t h f o r optimum p e r f o r m a n c e v a r i e s , i t i s i m p r a c t i c a l t o s p e c i f y these temperature t o o c l o s e l y , but: (1) f o r r i c e husks w i t h a m o i s t u r e content o f about 11 p e r c e n t , an a s h c o n t e n t o f 20 p e r c e n t , a n d a t a b e d d e p t h o f a b o u t 0.30 meters, the d e s i r e d convertor off-gas temperature lies between 120°C a n d 140°C w h i l e t h e c o n d e n s e r o f f - g a s t e m p e r a t u r e i s a r o u n d 85°C. (2) f o r t r o p i c a l hard-wood,with a m o i s t u r e content o f s i x t o e i g h t p e r c e n t , a n a s h c o n t e n t o f about one p e r c e n t a n d a b e d d e p t h o f a b o u t 0.50 m e t e r s , t h e d e s i r e d c o n v e r t o r o f f - g a s t e m p e r a t u r e i s i n T



682


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the range of 130°c t o 150°C, while the condenser o f f - g a s temperat u r e i s a l s o around 85°C. Once the d e s i r e d c o n d i t i o n s have been reached and the planned process r a t e e s t a b l i s h e d , the system can be operated i n a steadys t a t e mode. This may take four or f i v e hours from a c o l d s t a r t or two t o three i n a run where hot char from the previous days operat i o n i s i n the convertor. During the steady mode o p e r a t i o n , feed i s added about every 30 minutes, the char grate opened f o r a few seconds 30 t o 40 times each hour and the char drums changed every hour. Two char b a r r e l s equipped with quick disconnect hardware are used f o r r a p i d replacement o f the drums. To prevent burning, the char i s stored i n sealed b a r r e l s while i t c o o l s . The o i l i s p e r i odically gathered from the condenser and demister. Typically, the condenser recovers about three quarters o f the t o t a l o i l while the demister recovers the remainder. Except f o r a t o t a l of three or four minutes each hour when feed i s added and/or the char drums changed, the system operates continuously. During these f i l l i n g / e m p t y i n g periods the a i r supp l y i s shut o f f and operations h a l t . However, the thermal c a p a c i t y o f the system i s s u f f i c i e n t to insure that only minor changes i n temperature occur during these p e r i o d s . The r e s u l t - s o f a r as char and o i l production are concerned-is that the operation i s e f f e c t i v e l y constant. However, because the o f f - g a s flow too i s i n t e r r u p t e d , measures must be taken e i t h e r t o s t o r e a short supply of the gas or t o r e s t a r t the gas u t i l i z a t i o n system-using a s i n g l e convertor. With s e v e r a l c o n v e r t o r s , manifolded together such as i n Ghana, the operations can be staggered with no i n t e r r u p t i o n s i n off-gas production. Since by i t s nature the system i n v o l v e s a minimum of i n s t r u mentation, i t i s v i t a l that maximum use of every other i n d i c a t o r be made. For example, abrupt changes i n capstan torque can suggest c a v i t y formation and i n d i c a t e the need f o r more a g i t a t i o n . Likewise, the average torque i s a measure o f bed depth and-during s t a r t up-of the degree that c h a r r i n g of the.bed has occured. The c o l o r of the o f f - g a s i s a measure of the r e a c t i o n temperature. A very white smoke i n d i c a t e s l o c a l i z e d g a s i f i c a t i o n , while a g r e y i s h brown smoke suggests a more general p y r o l y s i s c o n d i t i o n . The c o l o r of the char a l s o i s a measure of convertor performance. Hence uncharged m a t e r i a l suggests too r a p i d a throughput, while g r e y i s h white char i n d i c a t e s the process r a t e i s too low. Even the charact e r i s t i c s of the b o i l i n g i n the water jacket can provide u s e f u l i n formation r e g a r d i n g the i n t e r n a l c o n d i t i o n o f the r e a c t o r . Excès s i v e , or f i l m b o i l i n g at s p e c i f i c p o i n t s i n d i c a t e s hot spots, most l i k e l y due t o the presence of c a v i t i e s i n the bed and suggests the need f o r more a g i t a t i o n of the bed. Test Results Since the combination of d i r e c t i n t r o d u c t i o n of process a i r through the convertor w a l l s and the f l u s h mounted o f f - g a s system i s



47.

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683

so a t t r a c t i v e , i n that i t minimizes i n t e r n a l r e s i s t a n c e t o f l o w , i t was i n v e s t i g a t e d f i r s t . Thus a s e r i e s o f t e s t s were conducted t o see i f t h i s combination would operate s a t i s f a c t o r i t y and with good performance. While the water jacketed process a i r system worked extremely w e l l , there was no problem i n m a t e r i a l s h a n d l i n g , the conv e r t o r o f f - g a s temperature was i n the d e s i r e d range, and good char y i e l d s were achieved, i t was found t h a t gas production was excès s i v e . While the o i l was c l e a n , the y i e l d s were low, the maximum throughput was l e s s than h a l f that f o r which the system i s designed, the a i r - t o - f e e d r a t i o ran w e l l i n excess o f u n i t y , the gas heating value was d i s a p p o i n t i n g and e x c e s s i r e heating o f the uncooled walls above the water j a c k e t occurred. I t was thus concluded that the short c i r c u i t i n g of the o f f - g a s flow that had been f e a r e d , d i d i n deed occur, and while some p y r o l y s i s was p r e s e n t , a significant amount of g a s i f i c a t i o n took p l a c e near the convertor w a l l s . Theref o r e s i n c e g r e a t e r throughputs and o i l y i e l d s were d e s i r e d , i t was decided t o abandon t h i s c o n f i g u r a t i o n and t o t e s t the submerged o f f - g a s port i n combination with the water jacketed process a i r system. This l e d t o a second, more promising t e s t s e r i e s . At f i r s t , the bed depth was operated at 0.50 meters, which i n the Ghana program, u s i n g 6-8 per cent moisture hardwood f e e d , had been s a t i s factory. But i t was found t h a t while improved process r a t e s were obtained, much lower a i r - t o - f e e d r a t i o s were achieved, and gas production was moderate, the convertor o f f - g a s temperature was too low and o i l recovery was s t i l l d i s a p p o i n t i n g . The low o i l y i e l d s have been mainly a t t r i b u t e d t o the low o f f - g a s temperature obtained, with the assumption that s i g n i f i c a n t o i l condensation occured in the bed. Since the current philosophy i s t o t r y t o avoid the need f o r a mechanical d r i e r and t o use the husks as they are produced, the feed moisture content t y p i c a l l y runs at about 11 percent of the wet weight. Thus t h i s r e l a t i v e l y high moisture f r a c t i o n , toget h e r with the high s i l i c a content (20 percent) o f the husks was b e l i e v e d r e s p o n s i b l e f o r the low o f f - g a s temperature and o i l y i e l d s , since i n t e s t s with wet feed r e p o r t e d i n (2_) s i m i l a r d i s a p p o i n t i n g o i l y i e l d s had occured. Therefore a s e r i e s o f t e s t s at a bed depth of only 0.23 meters was conducted-in an e f f o r t t o r a i s e the off-gas temperature. The r e s u l t s o f these t e s t s are presented i n Table 1 and F i g u r e 5. Study o f the r e s u l t s r e v e a l s t h a t the char yields are w e l l i n excess of those expected from the data c o r r e l a t i o n s i n (1) and (2) at the a i r t o - f e e d - r a t i o s used. This may be due t o o i l condensation i n the bed, but at t h i s time i t i s not c l e a r why the char y i e l d s are so high. On the other hand, recovered o i l y i e l d s are very low. However, the f i g u r e shows t h a t the trends o f i n c r e a s i n g o i l y i e l d and decreasing char y i e l d with i n c r e a s i n g a i r t o - f e e d are s i m i l a r t o those r e p o r t e d i n (V) and (2_). The c o n s i s tency o f the d a t a , with one e x c e p t i o n , i s encouraging. It should be noted that the feed throughputs were l i m i t e d by the c a p a c i t y o f the a v a i l a b l e a i r supply, s i n c e the pressure drop through the system was unexpectedly high. Therefore with a l a r g e r a i r supply




684


47.

TATOM ET AL.

there i s


685

l i t t l e doubt that the design flow r a t e s can bereached. Table 1

Test No


Air/Feed (kg/kg, dry ash free)

P r e l i m i n a r y Test Data Feed Process rate (kg/hr)

Average Convertor off-gas Temp.(°C)

from Indonesia

Average Condensor off-gas Temp.(°C)

Char yields (kg/kg, dry ash free )

Recovered oil yields (kg/kg, dry a s h free)

1

0.633

64

121

93

0.247

.033

2

0.732

47

160

93

0.218

.045

3

0.596

50

132

85

0.255

.028

4

0.63

47

135

82

0.251

.025

While the recovered o i l y i e l d s are q u i t e low, i t was obser ved that s i g n i f i c a n t q u a n t i t i e s o f o i l were l o s t i n the o f f - g a s stream, apparently due t o inadequate condenser/demister p e r f o r mance, and thus with proper o p t i m i z a t i o n o f t h i s system i t i s bel i e v e d that o i l y i e l d s at l e a s t e q u i v a l e n t t o those r e p o r t e d from Ghana (8_9_) can be r e a l i z e d . Regarding the products themselves; the char i s o f good q u a l i t y , t h e o i l i s very t h i c k and apparently f r e e o f any s i g n i f i c a n t water content, and the gas burns c l e a n l y , completely, and without odor i n both a d i f f u s i o n burner and a forced a i r combustor designed f o r crop d r y i n g a p p l i c a t i o n s . Moreover i n p r e l i m i n a r y t e s t s , the gas, i n combination with a very small q u a n t i t y o f d i e s e l o i l , provided the energy t o run a small f i v e horsepower d i e s e l engine. In a d d i t i o n a 45 volume per cent mixture o f the o i l - n e u t r a l i z i d with milk o f lime - d i s s o l v e d i n met h y l a l c o h o l (30 percent) and mixed with coconut o i l (25 percent) a l s o powered the d i e s e l engine i n short f e a s i b i l i t y t e s t s . However, while the o i l s can be used t o run a d i e s e l engine, i t appears that t h e i r current highest value i s as a wood p r e s e r v a t i v e . But perhaps most important; the c h a r c o a l was s u c c e s f u l l y b r i q u e t e d t o form " F i r e B a l l s " by means o f a simple agglomeration technique i n which waste cassava pulp from a l o c a l s t a r c h m i l l was used as a binder. Briquet production from the l a b o r a t o r y s c a l e machine u t i l i z e d has reached 21 kg/hr a t t h i s date. Further improvements are a n t i c i pated as refinements i n the technique are developed. 9

Economic A n a l y s i s There are many f a c t o r s which make a complete economic analys i s d i f f i c u l t , e s p e c i a l l y i n Indonesia where f o s s i l f u e l s are heav i l y subsidized. Thus the question o f whether t o use the fair value o f the products or t h e i r current e q u i v a l e n t market p r i c e must be answered. Moreover because the complete system has not y e t


686


SOLID WASTES AND

BIOMASS


operated i n a r e a l world s i t u a t i o n , a l l the numbers are simply proj e c t i o n s . But s i n c e a prototype and a b r i q u e t i n g system have now been b u i l t and operated, the r e q u i r e d c a p i t a l investment and the man power requirements are f a i r l y w e l l known. Unfortunately the product values are not and must be estimated. Therefore the f o l l o w ing two b r i e f analyses, while not complete should at l e a s t bracket the economic performance o f the system: P e s s i m i s t i c outlook-Rice m i l l runs 150 days/year; u n i t processes one tonne/day husks; o i l y i e l d s remain at 3 percent o f input feed; o i l used as c o a l o i l s u b s t i t u t e f o r wood p r e s e r v a t i o n at US$0,185/ l i t e r ; char y i e l d s are 40 percent of input feed; char as produced i s worth US $40 per tonne wholesale; gas i s not used and has no value; four men run system and are each paid US $2/day, t o t a l system c o s t i n c l u d i n g convertor, a i r supply, b r i q u e t t i n g equipment, small engine-generator,operating p l a t f o r m , and feed storage i s US $2500; government loan i s a v a i l a b l e at 12^ percent interest with 10 year payback; annual overhead runs at 15 percent o f i n i t i a l c a p i t a l c o s t s . Therefore the f o l l o w i n g r e s u l t s are obtained: Annual C a p i t a l Costs Annual Labor Cost Overhead T o t a l Operating Costs US $

453 1200 375 2028

Annual Income Charcoal Oil Gas Total

2400 832 -3232

Therefore p r o f i t i s US $ 1204

and payback i s 2.1

years.

O p t i m i s t i c o u t l o o k - m i l l runs 200 days/year; o i l y i e l d s are up to 7 percent; o i l s e l l s a t US $ 0 . 1 8 5 / l i t e r , char i s worth US $50/tonne; gas r e p l a c e s 35 l i t e r s / d a y o f un-subsidized diesel fuel (worth US $ 0.2 5 / l i t e r ) needed to run an 8 hp engine powering r i c e m i l l . Therefore the f o l l o w i n g r e s u l t s are obtained: Annual C a p i t a l Costs Annual Labor Costs Overhead T o t a l Operating Costs Annual Income Charcoal Oil Gas Total Thus p r o f i t i s US $ 5912/year and

453 1600 375 2428 4000 2590 1750 8340 payback i s about f i v e months.


47.

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687

Thus no matter how the analyses are made, the r e t u r n on investment i s c e r t a i n l y a t t r a c t i v e , and so r e g a r d l e s s o f the exact return, the economic outlook i s very f a v o r a b l e .


Conclusions and Recommendations While there i s c l e a r l y much t e c h n i c a l work t o be done and improvements to be made and t h e r e i s an obvious need f o r extended o p e r a t i n g experience using the equipment so f a r produced, nevert h e l e s s , the outlook i s promising both t e c h n i c a l l y and economi cally. But while the prospects are g e n e r a l l y good, there i s a l s o a p r e s s i n g need f o r more d e t a i l e d s t u d i e s : of the p r o p e r t i e s of the char, o i l , and gas, of the v a r i o u s a p p l i c a t i o n s of these products p o s s i b l e , and of the o v e r a l l systems i n v o l v e d . This work should s t a r t immediately. From the above d i s c u s s i o n i t should a l s o be evident t h a t the work i n t h i s area i s c u r r e n t l y i n a s t a t e o f r a p i d expansion. T h i s r a p i d a c c e l e r a t i o n i s a v i v i d i n d i c a t o r of the extent of the energy c r i s e s i n developing c o u n t r i e s and t h e i r w i l l i n g n e s s to inves t i g a t e promising, e v o l v i n g technology as a means of s o l v i n g t h i s c r i s i s , even through a l l the t e c h n i c a l problems have c l e a r l y not been r e s o l v e d . This w i l l i n g n e s s t o r i s k i s t o our view a wise philosophy, s i n c e the s e v e r i t y and s c a l e o f the r i s k s of not f i n d ing a workable s o l u t i o n to t h i s problem i n the very near f u t u r e are s u f f i c i e n t l y s e r i o u s t o make the investment i n a l t e r n a t e energy t e c h n o l o g i e s a t r i v i a l c o n s i d e r a t i o n . However, one major concern i s that an immediate l a c k of complete s u c c e s s - e i t h e r technical or economic-will disenchant those who most s e v e r e l y need t h i s technology. We e a r n e s t l y hope t h a t a proper, f a i r e v a l u a t i o n of these e f f o r t s w i l l be made, with the a t t i t u d e t h a t i f shortcomings a r i s e and they w i l l - t h e n s o l u t i o n s w i l l simply be bought and found. The promise of t h i s technology i s r e a l and e x c i t i n g ; i t must be r e a l i zed now. But only through f r e e and open cooperation among a l l i n v e s t i g a t o r s w i l l i t s p o t e n t i a l be demonstrated. Acknowledgements The authors would l i k e to express t h e i r a p p r e c i a t i o n to Mr. Ruben Hardy of Development Technology Center, Indonesia, who made many c o n t r i b u t i o n to the work reported here. A l s o s p e c i a l thanks i s given to Mr. Charles Stone of Sacramento, C a l i f o r n i a , who a s s i s ted g r e a t l y i n the e a r l y development of the system d e s c r i b e d . In a d d i t i o n we wish to acknowledge Mr. Ken Yeboah and Dr. John Powell at U n i v e r s i t y of Science & Technology, Kumasi, Ghana, whose techn i c a l suggestions, p a t i e n c e , i n t e r e s t , and d i l i g e n c e g r e a t l y f a c i l i t a t e d the program i n Ghana. The support of Georgia Tech, the U.S. agency f o r I n t e r n a t i o n a l Development, and the governments of Ghana and Indonesia are acknowledged.



688

Literature Cited 1.

Tatom, J.W., et a l . , " U t i l i z a t i o n of A g r i c u l t u r a l , F o r e s t r y and S i l v i c u l t u r a l Waste f o r the Production of Clean F u e l s , F i n a l Report under EPA Contract 68-02-1485, EES, Georgia Tech, A t l a n t a , Ga. Sept. 1976.


11

2.

Tatom, J.W., C o l c o r d , A.R., W i l l i a m s , W.M., Purdy K.R., Demet e r , J . J . , McCann, C R . , Elkman, J.M. , Bienstock, D. "A Prototype Mobile System f o r P y r o l y s i s o f A g r i c u l t u r a l and/or S i l v i c u l t u r a l Wastes", F i n a l Report under EPA Grant R-803430, EES, Georgia Tech., A t l a n t a , Ga. June 1978.

3.

Chiang, T.I., Tatom J.W. de Graft Johnson, J.W.S., Powell, J.W., " P y r o l y t i c Conversion o f A g r i c u l t u r a l and F o r e s t r y Wast e s i n Ghana-a F e a s i b i l i t y Study" Report under AID Contract AID/ta-C-1290, EES, Georgia Tech, A t l a n t a , Ga,July 1976.

4.

Tatom, J.W., Chiang, T.I., Harahap, F i l i n o , Apandi R.M., Wirjosumarto,Harsono, " P y r o l y t i c Conversion o f A g r i c u l t u r a l and F o r e s t r y Wastes t o a l t e r n a t e Energy Sources i n Indonesia-a F e a s i b i l i t y Study" Report under AID Contract AID/ASIA-C-1203, EES, Georgia Tech, A t l a n t a , Georgia, February 1977.

5.

Tatom, J.W., "Demonstration o f A l t e r n a t i v e F u e l Production through P y r o l y s i s o f A g r i c u l t u r a l Wastes at the UNEP R u r a l Energy Center i n Senegal" S p e c i a l report t o UNEP, A t l a n t a , Ga, November 1977.

6.

Tatom, J.W., " F e a s i b i l i t y o f I n d u s t r i a l F u e l Production from P y r o l y s i s o f Wood Wastes i n Papua New Guinea" r e p o r t 2 - 7 9 , prepared f o r Energy Planning U n i t , Dept. M i n e r a l s and Energy, Konedobu, Papua New Guinea", February 1979.

7.

Nauman, A r t " S w i l l t o F u e l P r o j e c t gets o f f the Ground" Sacramento Bee, Sunday A p r i l 17, 1977.

8.

P r i v a t e Communications, Yeboah t o Tatom, Kumasi, Ghana, October 1978 - June 1979.

9.

P r i v a t e Communication,

The

Powell t o Tatom, Kumasi, Ghana, June 1979.

10. Anon, "The N a t i o n a l Nonconventional Energy Resources Development Program - Progress Report" Republic o f the P h i l i p p i n e s , M i n i s t r y o f Energy, January 1979. 11. Anon, "White Paper - Energy P o l i c y and Planning f o r Papua New Guinea" M i n i s t r y o f M i n e r a l s and Energy, P.O. Box 2352, Konedobu, 1979. RECEIVED November 27,

1979.


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