Thermal Conversion of Solid Wastes and Biomass - ACS Publications

30 Basic Principles of Waste Pyrolysis and Review of European Processes Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 16, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch030

A. G. BUEKENS and J. G. SCHOETERS Department of Chemical Engineering and Industrial Chemistry, Vrije Universiteit Brussels, Pleinlaan 2, B-1050 Brussels, Belgium

1. Introduction In the early fifties more and more Municipalities in W. Europe were confronted with a declining availability of suitable landfill sites. This resulted in the large scale introduction of incinerator plant during the period 1960-75. Initially quite a few of these units showed mechanical deficiencies in the handling of refuse and clinker as well as corrosion problems in the boiler and gas cleaning plant ; some even suffered from gross design errors. As years went by more experience was gained and through the combined skills of designers, manufacturers and operators refuse incineration attained its present status of a well proven though expensive method of waste disposal. Gradually the densely populated regions of W. Europe were amply equipped with incinerator plants. The latter complied with the air pollution regulations of that time by using highly efficient electrostatic precipitators. Thus they could be situated in the very centre of the refuse generating territory reducing the cost of collection and transportation. In large plants the heat of combustion was recovered under the form of medium to low pressure steam, used for power generation and district heating. The low volume, sterile residue was a suitable substitute for gravel, provided the material is graded and the unburnt and magnetic substance is removed.[1,2,3 ] This explains why the progress of PTGL-technology was slower in W. Europe than in the U.S.A. or Japan. Even now, the only process that attained commercial operation in Europe is of American origin. A l l other processes only exist at pilot scale, although a few have been conceived at a larger scale and participate in public tenders. Unfortunately innovation is both difficult and dangerous. Municipalities are very well aware of this fact and will not embark on a yet unproven demonstration project unless very important incentives are provided by the Government. The spectacular failure of a couple of large scale American PTGL-units could only corroborate this tendency. 0-8412-0565-5/80/47-130-397$06.25/0 © 1980 American Chemical Society Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

THERMAL CONVERSION OF SOLID WASTES AND BIOMASS

2 τ) •μ !Η û

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 16, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch030

•Η

CM •Η

W CO ci ·

M M C

•H

Ο

Q

ft

M O

fi M

Λ EH

cd Ο

τ3

M

,û

-μ -ρ -ρ . ρ Ο Ο Ο Ο Ο t— Ο Ο 0J OJ CVJ

Ρ

Η

ω G -Ρ

M M CO > Κ Ο

sa :g

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

Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980. X X

X

X

X

-

1975 1976

1977 1976 1978

800°C 800°C 500°C T00°C 8oo°c 850°C U50°C

6t/d 1t/d

2.5t/d 5t/d

.5t/d .2t/d 1t/d

Stevenage

Hartlepool

Meckesheim

Bochum

Hamburg

Brussels

BATCHELORROBINSON

FOSTERWHEELER

HERBOLD

G MU

UNIVERSITY HAMBURG

UNIVERSITY BRUSSELS

-

-

U30°C

500°C

0"berhausen

Hanover

Essen

BRD

RUHRCHEMIE

PPT

RAMMS

GUILINI

X

X

500°C

CHAR

6t/d

DATE*

Goldshofe

MAX

HERKO/KIENER

Τ

LOCATION

NAME

SIZE

Uses no a f t e r b u r n e r .

See PPT. V e r t i c a l shaft g a s i f i e r . For t i r e s .

-

-

I n d i r e c t l y heated f i x e d bed. For r e c o v e r y o f metals and heat from c a b l e , p a i n t e d m e t a l s . . .

S t i r r e d , i n d i r e c t l y heated r e t o r t . For p o l y e t h y l e n e waste.

I n d i r e c t l y h e a t e d f l u i d i z e d bed. For p l a s t i c s , t i r e s , wood waste.

I n d i r e c t l y h e a t e d f l u i d i z e d bed. For t i r e s .

I n d i r e c t l y h e a t e d r o t a t i n g conveyor. For t i r e s , c a b l e , p l a s t i c s .

I n d i r e c t l y heated screw conveyor. For t i r e s .

Same as Warren S p r i n g .

Warren S p r i n g p r o c e s s f o r t i r e s .

Same as K i e n e r . For t i r e s .

REMARKS

-

-

-

-

-

X

-

-

X

STEAM

-

-

X

X

-

X

X

X

X

GAS

-

-

X

X

X

X

X

X

X

X

OIL

TABLE 1 b : PTGL - P r o c e s s e s f o ri n d u s t r i a l wastes and t i r e s .


ο


THERMAL

U Ο

CONVERSION OF

SOLID W A S T E S

AND

- H o l e f i n s

-> d i o l e f i n s

-> aromatics -*·

p o l y e y e l i e aromatics ( t a r ) -> carbon + hydrogen. At lower temperature the o l e f i n s and d i o l e f i n s tend t o polymerize t o t a r w i t h a h i g h l y complicated, a l i p h a t i c s t r u c t u r e . The thermal decomposition of oxygenated compounds y i e l d s simpler and s t a b l e r compounds, such as formaldehyde, acetone, a c e t i c a c i d , e t c . F i n a l products are o f t e n CO, CO^, H^O, CH =C0.. 2

Van Krevelen and H o f t y z e r [hQ] c h a r a c t e r i z e d the r e l a t i v e s t a b i l i t y o f v a r i o u s polymers and p o l y m e r i z a t i o n i n i t i a t o r s by means o f the temperature T^ at which the s u b s t r a t e i s h a l f decomposed a f t e r a r e a c t i o n p e r i o d of 30 minutes. These authors found a l i n e a r c o r r e l a t i o n between t h i s parameter T. and the energy o f the weakest bond o f the decomposing molecule. Thermodynamic c o n s i d e r a t i o n s thus a l l o w one to p r e d i c t the r e l a t i v e s t a b i l i t y o f d i f f e r e n t r e a c t i n g compounds or t o e x p l a i n the e v o l u t i o n i n a r e a c t i n g system. In the Hamburg p y r o l y s i s process (29) f o r example, a h i g h y i e l d o f aromatic compounds i s obt a i n e d by combining a h i g h p y r o l y s i s temperature w i t h the r e c i r c u l a t i o n of l e s s d e s i r a b l e and l e s s s t a b l e gaseous r e a c t i o n products . Another very important f a c t o r i n p y r o l y s i s i s the heat o f




408

Cracking

severity The Petroleum Publishing Company

Figure 1. Evolution of the product distribution with cracking severity (Naphta cracking): (1) C ; (2) C H, + C H ; (3) C H,; (4) C ; (5) C ; (6) H + CH 5+

h

4

19

h

3

2


2

h


30.

BUEKENS

A N D SCHOETERS

Principles of Waste Pyrolysis

409

r e a c t i o n . The l a t t e r can e a s i l y he c a l c u l a t e d from the heat o f formation o f the r e a c t i n g m a t e r i a l and t h e r e a c t i o n products. Unf o r t u n a t e l y these data are not always known w i t h p r e c i s i o n f o r polymeric or b i o l o g i c a l m a t e r i a l and the product a n a l y s i s i s o f t e n incomplete. I n the f i r s t case t h e heat o f r e a c t i o n can be d e t e r mined e x p e r i m e n t a l l y by means o f bomb c a l o r i m e t r y . The heat o f p y r o l y s i s can a l s o be obtained by d i f f e r e n t i a l thermal a n a l y s i s (D.T.A.). I n t h i s technique the temperature o f t h e product sample i s compared t o t h a t o f a reference sample ; i n t e g r a t i o n o f t h e temperature d i f f e r e n t i a l vs. the o p e r a t i n g time y i e l d s a q u a n t i t y r e l a t e d t o t h e heat o f r e a c t i o n .

2. Gasification Both the thermodynamic e q u i l i b r i u m and the heat o f r e a c t i o n are reasonably w e l l known f o r t h e heterogeneous g a s i f i c a t i o n o f carbon i z e d r e s i d u e w i t h oxygen, steam, carbon d i o x i d e and hydrogen. The o u t l e t gas composition i s normally computed by a t r i a l and e r ror procedure ; f o r each o p e r a t i n g temperature t h e r e s u l t i n g comp o s i t i o n i s c a l c u l a t e d , then t h e a t t a i n e d temperature i s determined from an enthalphy balance and t h e temperature correspondence i s checked. The most important f a c t o r a f f e c t i n g e q u i l i b r i u m gas compos i t i o n i s temperature. D e v i a t i o n s from e q u i l i b r i u m can be explained by t h e presence o f p y r o l y s i s gas, a r i s i n g d u r i n g thermal decomposition, by uneven gas and s o l i d d i s t r i b u t i o n , due t o channeling, b a k i n g , c l i n k e r i n g o r formation o f blowholes, o r by inadequate r a t e s o f r e a c t i o n . I n c r e a s i n g t h e o p e r a t i n g pressure has an unfavorable i n f l u e n c e on the g a s i f i c a t i o n e q u i l i b r i a o f steam and carbon dioxide, but f a v o r s t h e formation o f methane. I n p r a c t i c e a l l processes operate at s u b s t a n t i a l l y atmospheric pressure.

4.3. Kinetics and Mechanism 1. Pyrolysis The d i s t r i b u t i o n o f primary r e a c t i o n products i s mainly d e t e r mined by the s t r u c t u r e o f t h e raw m a t e r i a l and by t h e r e a c t i o n c o n d i t i o n s . A t low temperatures r e a c t i v e primary products may immediately polymerize t o a t a r , obscuring t h e primary r e a c t i o n steps. At h i g h temperatures t h e primary fragments are s u f f i c i e n t l y s t a b l e or rearrange t o s t a b l e p r o d u c t s , so t h a t t h e f i r s t react i o n steps are o f t e n e a s i l y r e c o g n i z a b l e . The r e a c t i o n p a t t e r n s i n the thermal decomposition o f polymers are f a i r l y w e l l known. G e n e r a l l y one o f t h e f o l l o w i n g mechanisms dominates - thermal depolymerization o f a polymer i n t o a monomer


410

THERMAL

CONVERSION OF SOLID WASTES AND BIOMASS

- s t o c h a s t i c fragmentation o f a polymer chain i n t o smaller molecules of v a r i o u s l e n g t h - e l i m i n a t i o n o f side chains or adjacent s u b s t i t u e n t s y i e l d i n g products such as H 0, C0 , NH~, HC1, H S, CH^ and higher p a r a f fins . 2

2

2


It i s p o s s i b l e to p r e d i c t the r e l a t i v e importance o f the s e v e r a l modes o f decomposition from an a n a l y s i s of the polymer s t r u c t u r e l l|J_] . Van Krevelen[hO ] even showed that the tendency f o r coke formation i s an a d d i t i v e property, which can be computed from the monomer s t r u c t u r e . The p y r o l y s i s o f c e l l u l o s e i s o f outstanding importance i n the study o f PTGL-processes i n v o l v i n g r e f u s e , wood waste or a g r i c u l t u r a l waste. Unfortunately the products and mechanism o f t h e r mal decomposition are extremely s e n s i t i v e t o a number of p h y s i c a l and chemical f a c t o r s . Even the decomposition o f the purest exc e l l u l o s e has l e d to c o n f l i c t i n g opinions regarding k i n e t i c s and mechanism[h3]. I t i s g e n e r a l l y accepted now that there are 2 competitive methods o f breakdown : - c a r b o n i z a t i o n , proceeding by a r i n g opening o f i n d i v i d u a l monomer u n i t s with a l o s s o f CO, CO^, or H^O, but with conservation of a polymeric carbon-chain -depolymerization t o levoglucosan t a r a f t e r complete "unzipping" of c e l l u l o s e c r y s t a l l i t e s . The r e l a t i v e importance o f the two modes o f decomposition depends on temperature, s t r u c t u r e , moisture and ash content and presence o f a d d i t i v e s . Low temperatures and the presence o f flame retardants favor c a r b o n i z a t i o n . Paper, board, a g r i c u l t u r a l , garden and wood waste l a r g e l y c o n s i s t o f c e l l u l o s i c m a t e r i a l . H e m i c e l l u l o s e , a low molecular weight p o l y s a c c h a r i d e present i n vegetable m a t e r i a l and i n paper, gives a higher y i e l d o f methanol and a c e t i c a c i d than c e l l u l o s e . L i g n i n i s a f a i r l y s t a b l e , c a r b o n - r i c h aromatic compound present i n wood. I t y i e l d s mainly t a r and carbon. The k i n e t i c s and mechanism o f thermal decomposition have o f t e n been s t u d i e d by methods such as thermogravimetric a n a l y s i s (TGA), d i f f e r e n t i a l thermal a n a l y s i s (DTA), p y r o l y s i s gas chromatography, or - by means o f small e x t e r n a l l y heated r e t o r t s , a g i t a t e d v e s s e l or f l u i d i z e d bed r e a c t o r s . The u s e f u l l n e s s o f the r e s u l t s o f these s t u d i e s i s o f t e n l i m i t e d because : - k i n e t i c s and mechanism can be v a s t l y d i f f e r e n t from one temper a t u r e domain t o another or be profoundly i n f l u e n c e d by the presence o f f o r e i g n matter (ash, moisture, c a t a l y s t s ) - even i n one and the same temperature domain the r a t e c o n t r o l l i n g step may d i f f e r depending on the s i z e o f the p a r t i c l e or on the flow c o n d i t i o n s . - g e n e r a l l y the k i n e t i c data show so much spread that data s e l e c -


30.

BUEKENS

AND

SCHOETERS

Principles of

Waste Pyrolysis

411

t i o n and i n t e r p r e t a t i o n becomes a problem. In the case of c e l l u l o s e p y r o l y s i s a c t i v a t i o n energies ranging from TO t o 230 kj/mole have been r e p o r t e d .


Such d i f f e r e n c e s can be a t t r i b u t e d t o d i f f e r e n c e s i n s t a r t ing m a t e r i a l and experimental c o n d i t i o n s and techniques. F u r t h e r i t should be born i n mind t h a t thermal decomposition processes occur along complex mechanisms t h a t cannot adequately be d e s c r i b e d by a simple r a t e law. This e x p l a i n s why apparent energies o f act i v a t i o n or r e a c t i o n orders can vary almost c o n t i n u o u s l y w i t h experimental conditions. A number o f models t h a t take account o f some of the p h y s i c a l phenomena o c c u r i n g during p y r o l y s i s have been developed f o r a s i n g l e wood p a r t i c l e [ . The r e l e v a n t phenomena are : - heat t r a n s f e r by convection and r a d i a t i o n from the surroundings t o the wood p a r t i c l e - i n t e r n a l conductive and convective heat t r a n s f e r . - d r y i n g and thermal decomposition o f the m a t e r i a l . - i n n e r d i f f u s i o n and outer convection o f the p y r o l y s i s products.

1^,1+^.]

A p r a c t i c a l d i f f i c u l t y i n the a p p l i c a t i o n o f such models t o r e a l r e a c t o r s i s the poor knowledge o f the numerical values of the parameters i n v o l v e d ( p o r o s i t y , heat o f r e a c t i o n , thermal cond u c t i v i t y , heat c a p a c i t y , k i n e t i c parameters). Yet from these s t u d i e s i t f o l l o w s t h a t heat and mass t r a n s f e r phenomena by no means can be ignored. Kung [ h6] showed t h a t among wood p a r t i c l e s o f 2, 0.2, 0.02 cm only the s m a l l e s t ones showed a uniform temperature d i s t r i b u t i o n . Hence i n a c t u a l p y r o l y s i s r e a c t o r s the r a t e of conversion w i l l u s u a l l y be c o n t r o l l e d by heat t r a n s f e r . When the l a t t e r i s very slow, as i n the Destrugas process, p l a n t o p e r a t i n g c a p a c i t y may be s t r o n g l y a f f e c t e d by the moisture content of the r e f u s e . 2. G a s i f i c a t i o n G a s i f i c a t i o n i s a heterogeneous g a s - s o l i d r e a c t i o n , composed of three elementary steps : - a d s o r p t i o n o f the gaseous r e a c t a n t s onto the carbon s u r f a c e . - chemical r e a c t i o n between adsorbed gas and s o l i d . - desorption of the gaseous product. The chemical r a t e of r e a c t i o n i s determined by the slowest o f these 3 s t e p s . A s i m p l i f i e d r a t e low i s given by r

= k.A.m c

,p^ c *G

where r k A m c P G

= = = = =

r a t e o f carbon consumption r a t e parameter, f o l l o w i n g an Arrhenius law s p e c i f i c s u r f a c e of the carbonized r e s i d u e amount of C c o n c e n t r a t i o n o f the g a s i f y i n g agent


412



The r e a c t i o n with oxygen i s extremely f a s t . The r e l a t i v e rates o f the k heterogeneous g a s i f i c a t i o n reactions at 800°C and Pa (0.1 atm) are : carbon-oxygen : 105 carbon-steam : 3 carbon-carbon dioxide : 1 carbon-hydrogen : 3.10"3 Chemical r e a c t i o n i s only r a t e determining i n a f i r s t low temperature domain, s i t u a t e d roughly below 1000°C. In a second, medium temperature domain the gaseous reactant i s gradually deplet ed i n s i d e the porous p a r t i c l e , so that the r e a c t i o n proceeds at the r a t e at which i n t e r n a l d i f f u s i o n supplies new r e a c t a n t s . In t h i s i n t e r n a l d i f f u s i o n c o n t r o l l e d region the apparent energy o f a c t i v a t i o n f a l l s o f f t o h a l f i t s i n i t i a l value. At very high temperatures, i . e . above 1200°C, chemical r e a c t i o n i s so f a s t that the gaseous reactant i s immediately con sumed upon contact with carbon. At that moment the r a t e i s con t r o l l e d by e x t e r n a l d i f f u s i o n . The a c t i v a t i o n energy becomes a l most zero. In each o f the 3 domains the r a t e o f g a s i f i c a t i o n i s pro p o r t i o n a l t o the quantity o f f u e l and t o the p a r t i a l pressure p ^ ( a l l other f a c t o r s remaining equal). I t i s also proportional to the s p e c i f i c surface A, except at high temperature when exter n a l d i f f u s i o n i s the c o n t r o l l i n g step. Smaller p a r t i c l e s have a r e l a t i v e l y l a r g e r outer surface and a shorter pore length ; they react f a s t e r i n the domains o f e x t e r n a l and i n t e r n a l d i f f u s i o n c o n t r o l , but not i n the l o w temperature domain. A high gas v e l o c i t y (u) stimulates e x t e r n a l t r a n s f e r ; i t thus enhances t h e over a l l r a t e o f r e a c t i o n at the highest temperatures (Figure 2). In p r a c t i c e the gas composition can o f t e n be approximated by the a c t u a l e q u i l i b r i u m composition. The d e v i a t i o n can be cor r e c t e d f o r by assuming a somewhat lower g a s i f i e r temperature than the r e a l one. Another c o r r e c t i o n method defines an apparent equi l i b r i u m parameter determined experimentally : Κ = x.K app theory In the updraft g a s i f i e r χ e s s e n t i a l l y equals 1 f o r g a s i f i c a t i o n by steam and carbon d i o x i d e , but a t t a i n s only 0.1 -0.6 f o r the methanation r e a c t i o n , the exact f i g u r e depending on the reac t i v i t y o f the coke [ U81. F l u i d i z e d bed and suspended p a r t i c l e g a s i f i c a t i o n are cha r a c t e r i z e d by much shorter r e a c t i o n times. The e q u i l i b r i u m ap proach i s incomplete even f o r g a s i f i c a t i o n with steam o r carbon dioxide so that more elaborate computation methods are required!U8] . G a s i f i c a t i o n o f a s i n g l e p a r t i c l e has a l s o been the sub j e c t o f a number o f mathematical models, that take i n t o account part o f the heat and mass t r a n s f e r aspects o f the problem [ ^9 50 ] Here a l s o the l a c k o f hard data and the mathematical complexity a


BUEKENS AND SCHOETERS



30.


413


414


hinder the p r a c t i c a l a p p l i c a t i o n o f such models. In case chemical r e a c t i o n i s f a s t compared t o i n t e r n a l d i f f u s i o n o r when the p o r o s i t y o f the s o l i d i s very low, conversion s t a r t s at the outer surface o f the p a r t i c l e . As r e a c t i o n proceeds f u r t h e r , a r e a c t i n g boundary l a y e r p r o g r e s s i v e l y moves inwards. The p a r t i c l e thus c o n s i s t s o f an unreacted inner core surrounded by an outer ash l a y e r (provided the p a r t i c l e contains ash). This concept i s the b a s i s o f the w e l l known s h r i n k i n g core model ; i t y i e l d s some p r a c t i c a l conversion (X)-time ( t ) r e l a t i o n s . [ 51 ] » Assuming a steady-state and isothermal conditions the s h r i n k i n g core model gives f o r s p h e r i c a l p a r t i c l e s [ 5 2 ] : C o n t r o l l i n g step

Conversion/time r e l a t i o n

e x t e r n a l gas f i l m d i f f u s i o n

^ = X

ash l a y e r d i f f u s i o n

| = 1 -

chemical r e a c t i o n

±- = 1 - 0 - x )

3d-x)

2 / 3

+2(i-x)

1 / 3

(τ = time r e q u i r e d f o r complete conversion ; f u n c t i o n o f density of the p a r t i c l e , bulk gas concentration and mass t r a n s f e r or che m i c a l r e a c t i o n parameters).

When the behavior o f a s i n g l e p a r t i c l e i n a p y r o l y s i s or g a s i f i c a t i o n r e a c t o r i s s t u d i e d , the p a r t i c l e i s g e n e r a l l y assumed t o be at a given temperature and surrounded by a gas o f w e l l known composition. In the design o f an a c t u a l r e a c t o r a more comprehensive model i s needed t o account f o r the s p a t i a l d i s t r i b u t i o n o f temper ature, concentration or extent o f conversion. The r e a c t o r model describes how gas and s o l i d reactants flow and are contacted and how heat and mass are t r a n s f e r r e d . The simplest types o f flow p a t t e r n conceivable are : p e r f e c t mixing and plug flow. TABLE__III Type o f r e a c t o r

Flow_

type

gas

solid

v e r t i c a l shaft

plug

plug

rotating k i l n

plug

f l u i d i z e d bed

plug

deviations

c h a n n e l l i n g , r a d i a l flow due to uneven p o r o s i t y plug r a d i a l concentration patterns i n gas, a x i a l d i s p e r s i o n w e l l mixed two phase gas flow,backmixing of gas, s l u g g i n g , c h a n n e l l i n g plug uneven mixing o f g a s - s o l i d s , axial dispersion 9

entrained r e a c t o r p l u g


30.



415

S e v e r a l examples o f r e a c t o r modelling have been d e s c r i b e d elsewhere [53., j>U, 55.] . The g a s i f i c a t i o n o f c o a l has been modelled s e v e r a l times [ 55 56 ] f o r a v e r t i c a l s h a f t r e a c t o r . The models assume plug flow f o r both the gaseous and s o l i d r e a c t a n t s , and homogeneous' or s h r i n k i n g core behavior f o r the i n d i v i d u a l p a r t i c l e s . The value of the various models i s d i f f i c u l t t o evaluate, because the d e t a i l ed computational r e s u l t s can only be compared with experimental data at the o u t l e t o f the r e a c t o r . The models o f c o a l g a s i f i c a t i o n do not consider the e f f e c t o f d e v i a t i o n s from i d e a l i z e d flow. The phenomena t a k i n g p l a c e during r e f u s e g a s i f i c a t i o n and the e f f e c t o f the d e v i a t i o n s from an i d e a l packing have been d i s c u s s e d elsewhere [k^ 5j) ·


9

9

The modelling o f a f l u i d i z e d bed r e a c t o r i s r a t h e r d i f f i c u l t because o f the complicated flow o f gas and s o l i d s i n the r e actor. In a w e l l - f l u i d i z e d r e a c t o r i t i s o f t e n p o s s i b l e t o assume p e r f e c t mixing of the s o l i d s , since only l a r g e d i f f e r e n c e s i n d e n s i t y and s i z e o f the p a r t i c l e s l e a d t o segregation. But the gas flows through the r e a c t o r according t o two d i f f e r e n t mechanisms. A f i r s t part creeps slowly upward through the dense phase and has a good contact with the bed m a t e r i a l . The remainder flows through the bed under the form o f f a s t l y r i s i n g bubbles and has but a poor contact with the bed m a t e r i a l . It f o l l o w s t h a t the c o n c e n t r a t i o n o f the gaseous reactant i s d i f f e r e n t i n the "dense" and i n the "bubble" phase and a l s o v a r i e s with h e i g h t . Hence there i s no simple or d i r e c t r e l a t i o n between the observable gas o u t l e t c o n c e n t r a t i o n and the k i n e t i c parameters o f the g a s / s o l i d r e a c t i o n . The conversion a l s o depends on the r a t e of mass t r a n s f e r between the two phases, which v a r i e s i n a complicated manner with bubble s i z e and r i s e v e l o c i t y , p a r t i c l e s i z e d i s t r i b u t i o n , d i f f u s i v i t y , bed geometry, t h e presence o f bed i n s e r t s , e t c . A number o f e m p i r i c a l c o r r e l a t i o n s are a v a i l a b l e t o estimate these parameters. In most modelling s t u d i e s however, a d j u s t a b l e parameters such as the bubble s i z e are used t o f i t the experimental data. The s a f e s t method o f designing a f u l l s c a l e r e a c t o r i s l a r g e l y based on experiments at s u c c e s s i v e l y l a r g e r s c a l e s o f c a p a c i t y . The use o f a comprehensive mathematical model as a guide i n the design i s s t i l l out o f question because o f the mathematic a l complexity o f such a model, the l a c k o f accurate p h y s i c a l and chemical data, and the n e c e s s i t y o f mastering a number o f t e c h n i c a l and o p e r a t i n g problems. On the other hand i t i s u s e f u l t o t r y and understand the o p e r a t i n g p r i n c i p l e s and the behavior o f such a r e a c t o r and the main features o f the c o n t r o l l i n g mechanism. Spectacular e r r o r s i n the s c a l i n g - u p o f PTGL-reactors may thus be avoided.


416

5.

THERMAL CONVERSION OF SOLID WASTES AND

The PTGL-Pro.ject o f the U n i v e r s i t y o f Brussels

BIOMASS

(V.U.B.)


Since i t s foundation the Department o f Chemical Engineering and I n d u s t r i a l Chemistry of the V.U.B. acquired considerable experience in the f i e l d o f high temperature processes, with studies on steam-reforming of n a t u r a l gas, p y r o l y s i s o f hydrocarbons and catalytic combustion of hydrocarbons. The Department conducted fundamental studies as w e l l as contract work f o r i n d u s t r y , e.g. i n the domain o f f l u i d i z e d bed techniques, i n c i n e r a t o r grate mechanisms and small waste-fed b o i l e r s . An assessment on current t h e r mal d i s p o s a l techniques was prepared on b e h a l f of E.E.C.[ 5,57»58] · In autumn 1976 a contract was signed with the M i n i s t r y of Science P o l i c y f o r a study on the p y r o l y s i s of waste. At that moment i t seemed impossible to catch up with the work of e x c e l l e n t companies such as Union Carbide, Monsanto or O c c i d e n t a l Petroleum and develop a p r o p r i e t a r y process. Furthermore the amount of f i n a n c i a l funds provided and the extent of f a c i l i t i e s a v a i l a b l e at the U n i v e r s i t y precluded such an approach. So i t was decided to concentrate on more fundamental aspects of p y r o l y s i s and g a s i f i c a t i o n and t o l i m i t l a r g e s c a l e work t o c o l l a b o r a t i o n p r o j e c t s with i n d u s t r y , such as the development of small b o i l e r s f o r f i r i n g miscellaneous wastes, moving bed carbon i z a t i o n of wood s l a b s , f l u i d i z e d bed g a s i f i c a t i o n and p y r o l y s i s and moving bed g a s i f i e r s f o r various a p p l i c a t i o n s o f the product gas [ 59 ]. The fundamental research work was subvided i n t o 3 parts [ 6o ]: - thermogravimetric a n a l y s i s (TGA) and d i f f e r e n t i a l thermal a n a l y s i s (PTA) o f minute samples (5~50 mg) of the m a t e r i a l t o be studied. Both techniques y i e l d information on the r a t e o f thermal decomposition, the heat of r e a c t i o n , the k i n e t i c parameters o f t h i s process (pseudo-order and a c t i v a t i o n energy) and the amount of residue. The a n a l y s i s of the evolving product has been monitored by means of gas chromatography. High temperature o x i d a t i o n of the residue allows to compare the r e a c t i v i t y of the carbonized r e sidue . - p y r o l y s i s gas chromatography (Py.G.C.) o f microsamples (10 -200 yg) r a p i d l y y i e l d s information on the r a t e and products o f decomposition i n an extremely broad range of r e a c t i o n temperatures. P r e c i s e measurement o f the t r a n s i e n t temperature p r o f i l e s o f the sample holder and of the e v o l u t i o n v o l a t i l e products as a f u n c t i o n o f time allowed to obtain a much b e t t e r i n s i g h t of the p h y s i c a l and chemical phenomena occuring during Py.G.C. This i n s i g h t i s obviously r e q u i r e d f o r the comparison with or the e x t r a p o l a t i o n to r e a l s c a l e experiments. - p y r o l y s i s i n a f l u i d i z e d bed bench s c a l e r e a c t o r . The t e s t u n i t i s composed of a 15 cm I.D. f l u i d i z e d bed r e a c t o r , a hopper and a screw feeder mounted on top o f the expansion s e c t i o n of the bed, a t u b u l a r furnace preheater f o r the f l u i d i z i n g agent,a quench c o o l e r , cyclone, t u b u l a r condensor and o i l mist scrubber ( F i g . 3 ) .





30.


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418


30.



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Pyrolysis tests have been made on polyethylene and polystyrene with steam as the fluidizing agent. Polyethylene yielded a gas and a wax. Their relative amounts could he adjusted by varying the operating temperature[ 6l] .


Polystyrene pyrolysis yielded an o i l with a high content of styrene monomer (Fig.4). The reaction conditions were optimized to maximize the yield of styrene. A maximum of 76% (by weight) was obtained. The oil is suitable for reuse in the styrene manufacturing process. Presently, tests are performed on various agricultural and wood wastes such as bark, soy hulls and wood shavings. ACKNOWLEDGEMENTS The research work described here was carried out as part of the R & D Programme "Economy of Wastes and Secondary Raw Materials", sponsored by the Prime Minister1s Services -Science Policy. LITERATURE CITED 1. Buekens,A; Schoeters,J 2 n d World Recycling Conference, Manilla, Exhibitions for Industry Ltd, Oxted,Engl., 1979. 2. Martin,W; Weiand,H 1978 National Incinerator Conference, Chicago, Ill.,ASME, 1978, p.83. 3. Mutke,R."Conversion of Refuse to Energy",Montreux, 1975,p.105. 4. Buekens,A. Müll und Abfall, 1978, 10(12), 353. 5. Buekens, A. and Schoeters,J. "Assessment of Current Technology of thermal processes for Waste Disposal",E.E.C. Contract 282 76-9,ECIB, Nov. 1977. 6. Mélan, C. Müll und Abfall, 1978,10( 12),363. 7. Roslund,J. in ref. 63,136. 8. Besch,G.M. in ref. 64,196. 9. Besch,G.M. Müll und Abfall,1978,10(12),384 10.Müller,H. in ref. 64,154. 11.Rymsa,K.H. in ref. 63,87. 12.Rymsa,K.H. Müll und Abfall, 1978,10(12), 377. 13.Heil, J. in ref. 64,112. l4.Segebrecht, J. in ref. 64,92. 15.Link, K. in ref. 64, 100. 16.Tabasaran,O. To be presented at this meeting. 17.Tabasaran,O; Besemer,G; Thomanetz,E. Müll und Abfall, 1977, 9 (10), 293. 18.Nowak,F. 1978, Natl. Waste Conf., Chicago,ASME, 1978, 29. 19.Lenz,S. Müll und Abfall, 1978, 10(2), 371. 20.Willerup,O.C.R.E.Conference, Montreux, 1975, 235. 21.Schmidt,R.Müll und Abfall, 1978, 10(12), 375. 22.Bracker,G.P. Metall, 1977, 31(5), 534. 23.Collin,G;Grigoleit,G; Bracker,G.P. Chem. Ing. Techn.; 1978, 50,(11) ,836. 24.Collin, G; Grigoleit,G; Michel,E. Chem. Ing. Techn., 1979, 51 (3), 220.



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25. Bracker, G.P. Müll und Abfall, 1979, 11(4), 96. 26. Douglas,E; Webb,M; Daborn,G.R. Presented on "Treatment and Recycling of Solid Wastes", Manchester, Jan. 1974. 27. Wilson, H.T., Fletcher, R. Presented on "2nd World Recycling Conference", Manilla, Exhibitions for Industry Ltd, Oxted, England, 1979. 28. Anon. "Pyrolysis Review", Foster Wheeler Power products Ltd. 29. Kaminsky,W, to be presented at this meeting. 30. Sinn, H. Chem. Ing. Techn. , 1974, 46, 579. 31. Kaminsky, W, Menzel,J, Sinn,H. Conserv. and Recycling, 1977, 1 , 91. 32. Sinn, H. Kautschuk. + Gummi Kunststoffe,1979, 32 (1), 23. 33. Kox, W. Thesis, T.H. Eindhoven, 1975. 34. Speth, S. Chem. Ing. Techn, 1973, 45 (6), 256. 35. Besemer, G. "Disposal of Industrial Wastewater and Wastes", Symposium, Wroclaw, 1977, 282. 36. Anon.Metalloberfläche,1978, 32(8), 357. 37. Anon.Wasser, Luft und Betrieb, 1978, 22(8). 38. Groeneveld, M. to be presented at this meeting. 39. Zdonik,S.B., Green, E.J., Hallee, L.P. "Manufacturing Ethylene" Petr.Pub. Co. 1970. 40. Van Krevelen, D.W., Hoftijzer, P.J. "Properties of Polymers", Elsevier Scient. Pub.Co, Amsterdam, 1976. 41. Cameron, G.G. "The mechanisms of Pyrolysis, Oxidation and Burning of Organic Material", N.B.S. special pub. 357, June 1972, 62. 42. Madorsky,S.L. "Thermal Degradation of Organic Polymers", Interscience Publishers, 1964. 43. Alger,R. "The Mechanisms of Pyrolysis, Oxidation and Burning of Organic Material",N.B.S. Special pub. 357, June 1972,171. 44. Maa, P.S., Bailie, R.C. "Combustion Science and Technology", 1973, 7, 257. 45. Miyanami, K. Fan, L.S., Fan,L.T., Walawender, W.P. Can. Ind. Chem. Engineering, 1973, 55, 317 46. Kung, H.C. Combustion and Flame, 1972, 18, 185. 47. Walker, P.L., Rusinko, F. Austin, L.G. Advances in Catalysis, 1959, 11, 133. 48. Rammler, E."Technologie und Chemie der Braunkohleverwertung", 1962, 296. 49. Szekely,J. Evans, J, Sohn, H.Y. "Gas-Solid Reactions", Academic Press, N.Y., 1976. 50. Petersen, Ε. A.I. Ch. Ε. J., 1957, 3, 443. 51. Smith, J.M. "Chemical Engineering Kinetics", McGraw Hill, N.Y. 1970. 52. Levenspiel, O. "Chemical Reaction Engineering", J.Wiley & Sons, N.Y., 1972. 53. Yoshida, K. Kunii, D.,J. Chem. Eng. Japan,1974, 7,(1) 34. 54. Ubhayakar, S.K., Stickler, D.B., Gannon, R.E. Fuel, 1977, 56, 281. 55. Amundson, N.R., Arri, L.E. A.I. Ch. Ε. Journal,1978,24(1),87.



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56. Klose, E, Toufar, W. Energietechnik, 1976, 26(12), 546. 57. Eggen,A.C., Kraatz, R. Winter Annual meeting, Power Division of ASME, N.Y., Nov. 1974, 17. 58. Buekens, Α., Schoeters, J. "W. European Experience on Small Waste fed Incinerators with Heat Recovery", Preliminary Report prepared for S R I, 1978. 59. Buekens,A.G. Conservation and Recycling, 1977, 1, 247. 60. Buekens,A.G.; Masson, H. 2nd World Recycling Conf.Manilla 1979. 61. Buekens,A.;Mertens,J.;Schoeters,J.Steen,P. 1st World Recycling Conference, Basel, 1978. 62. Schoeters,J.;Buekens,A. MER/CRE Conference, Berlin, Oct.1979. 63. "New Technologies in Waste Disposal" (in German ), Symposium at T.U. Berlin, Erich Schmidt Verlag, 1977. 64. "Thermal Treatment of Household Refuse" (in German ), Sympo sium at T.U. Berlin, Jurgen Kleindienst, 1978. RECEIVED November 30, 1979.


Thermal Conversion of Solid Wastes and Biomass - ACS Publications

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