Thermal Conversion of Solid Wastes and Biomass

45 Power Generation from Biomass Residues Using the Gasifier/Dual-Fuel Engine Technique ANDRE A. DENNETIERE — DUVANT Motors, Valenciennes, France

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FRANCOIS LEORAT — RENAULT New Technologies, 92508 Rueil-Malmaison, France GEORGE F. BONNICI — IDP/RENAULT, One Old Country Road, Carle Place, NY 11514

The increase of the cost of all forms of energy has forced industry to look for the cheapest and most dependable ways of producing and purchasing energy. Potential biomass energy, either under the form of natural resources or by recycling wastes, is rather important throughout the world. For a long time, surface natural combustibles, such as wood and agricultural wastes of some specific products, have been part of energy utilization in plants. They have often been outclassed, in quality as well as in flexibility, by fossile combustibles and so have been abandonned. While waiting for new energy utilization, these surface natural products can be used again. The energy conversion of some wastes may also be considered when they have no other use by reprocessing. The first technical idea is generally to burn these products and use the direct thermal effect (heat and thermal fluids) or indirect mechanical effect (steam engines). However, this kind of technique is generally costly, inefficient or simply not usable with some processes. As a result, a great deal of biomass waste is poorly converted, lost, or burnt without energy production. Conversely, by gasification of vegetable cellulose, and with the help of well adapted equipment, excellent results may be obtained in converting these products to useful energy, without sacrificing the available energy and flexibility. Gasification Cycle and Mechanical Power Generation Overall Performances The cycle consists in converting potential thermal energy of organic wastes into a combustible gas for heavy duty thermal engines. The efficiency of the gasification process is generally about 70 to 80 % (Figure 1) . The thermodynamic efficiency of thermal engines is about 37 %. As we shall see, the best suited engines are of the dualfuel type, whose output power results from the simultaneous work of two combustibles. A small quantity of liquid combustible 0-8412-0565-5/80/47-130-635$05.00/0 © 1980 American Chemical Society Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


636

THERMAL

CONVERSION

O F SOLID W A S T E S A N D

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BIOMASS


45.

DENNETiERE E T A L .

Power Generation from Biomass Residues

637

( s t r a i g h t D i e s e l o i l ) i s i n j e c t e d according to D i e s e l p r i n c i p l e s and ensures i g n i t i o n of the gaseous load e n t e r i n g the c y l i n d e r d u r i n g the admission s t r o k e . In f a c t , gas has the most impor tant r o l e during the i n t e r n a l c y c l e working p r o c e s s . As a r e s u l t , the gas flow r a t e i s c o n t r o l l e d depending on the d e s i r e d power output, while D i e s e l f u e l i n j e c t i o n remains constant r e gardless of engine l o a d . I t amounts t o a constant reduced D i e s e l o i l consumption per hour. For u t i l i z a t i o n with the low BTO gas obtained through g a s i f i c a t i o n of organic waste, the energy brought t o the engine by D i e s e l o i l i s l i m i t e d at l e s s than 10 % of the o v e r a l l f u e l c o n sumption at f u l l l o a d . I t means that D i e s e l f u e l savings under u s u a l o p e r a t i n g c o n d i t i o n s i s no l e s s than 80 % of the consump t i o n of conventional D i e s e l engines of i d e n t i c a l power. Most of the t i m e , the engine i s coupled to an a l t e r n a t o r allowing e l e c t r i c i t y generation with an e f f i c i e n c y of 93 %. Each kWh (3.6 χ 10 J) i s obtained from: - 0.055 l b (25g) of D i e s e l f u e l - 2200 k c a l (92.11 χ 10 J) under gaseous form. The low BTU gas a v a i l a b l e from the g a s i f y i n g equipment i s a mixture of: - combustible elements: CO (14-15 %), H (15-20 %), methane CH4 ( l e s s than 2 %), and some hydrocarbons of a higher order than methane. - i n e r t elements: C 0 and Ν · I t s NCV i s about 1100 k c a l / n o r m a l m (46.05 χ 1 0 J / m ) . T h e r e f o r e , the production of 1 kWh r e q u i r e s about 2.2 m of t h i s gas. C e l l u l o s e wastes can be regarded as i n c l u d i n g about 50 % carbon and 50 % water. Carbon o x y d i z i n g conversion means, as a general r u l e that w i t h a mean g a s i f i c a t i o n e f f i c i e n c y of 75 %, 1 kWh i s obtained from 800 t o 900 g of anhydrous p r o d u c t . Good g a s i f i c a t i o n c o n d i t i o n s are obtained when the moisture content of the product i s low, 10 t o 20 % of water, r e f e r r i n g to gross weight. So, i t i s p o s s i b l e t o c o n s i d e r , without great e r r o r , the f o l l o w i n g r a t i o : with 1 kg of so c a l l e d dry waste, we get 1 kWh at the a l t e r n a t o r output. This r e s u l t i s quite s a t i s factory. 6

5

2

2

2

3

5

3

Some C h a r a c t e r i s t i c s of Low BTU Gas Engines These engines are d i r e c t l y d e r i v e d from standard 4 stroke D i e s e l engines. They are p a r t i c u l a r l y s u i t e d f o r continuous operation. Patented design features allow a p e r f e c t l y r e l i a b l e and h i g h output gas, even with low BTO gas as the main f u e l . The D i e s e l c y c l e ' s high compression r a t i o i s c o n t i n u o u s l y main t a i n e d , p r o v i d i n g h i g h e f f i c i e n c y performance. I g n i t i o n power, obtained by combustion of a s m a l l amount of i n j e c t e d D i e s e l f u e l , i s much g r e a t e r than c o n v e n t i o n a l spark i g n i t e d engines. T h i s ensures an e x c e l l e n t combustion s t a b i l i t y


638

THERMAL

CONVERSION OF SOLID WASTES AND BIOMASS


of the combustible gas mixture, even with excess a i r . T h i s i s q u i t e i n t e r e s t i n g when gas composition may vary, which i s p r e c i s e l y the case with g a s i f i c a t i o n or p y r o l y s i s gas. For p a r t i a l load o p e r a t i o n , the a b i l i t y t o operate outside the s t o c c h i o metric l i m i t s allows i t t o maintain a high e f f i c i e n c y , as i n a pure D i e s e l c y c l e (see t e s t r e s u l t s i n Table I) . Independent f u e l l i n e s provide f o r the s e p a r a t i o n of a i r and gas intake manifolds up t o the c y l i n d e r s , ensuring a good mixture i n s i d e them. T h i s f e a t u r e prevents any r i s k of e x p l o s i o n outside the engine. Moreover, a i r and gas intake being s e p a r a t e l y c o n t r o l l e d , burnt gases are e f f i c i e n t l y scavenged r e s u l t i n g i n reduced i n t e r n a l thermal load and supercharging i s p o s s i b l e without any f u e l l o s s e s . Supercharging Supercharging i s performed by means of a turbo-compressor, as i n a pure D i e s e l c y c l e . In the general case, gas i s a v a i l able without any r e l a t i v e p r e s s u r e . T h e r e f o r e , our engines are equipped with two turbo-compressors: one f o r gas, the other f o r combustive a i r . Engine power r e g u l a t i o n , i . e . the a b i l i t y t o keep a d r i v e n machine's speed constant whatever the l o a d , i s automatic. The engine i s s t a r t e d i n D i e s e l c y c l e , and brought t o the d e s i r e d power l e v e l . Then, gas i s s u p p l i e d , and the maximum p o s s i b l e gas q u a n t i t y r e p l a c e s D i e s e l f u e l a u t o m a t i c a l l y . T h i s i s accomplished without d i m i n i s h i n g the engine's performance. In the case of gas production f a i l u r e ( q u a l i t y and quantity) the l a c k of energy i s a u t o m a t i c a l l y compensated f o r by complementary D i e s e l f u e l (with, of course, a r e d u c t i o n o f the savings during t h i s t i m e ) . I f the l a c k of gas p e r s i s t s , the engine w i l l operate on pure D i e s e l c y c l e and w i l l continue d e l i v e r i n g the d e s i r e d power. The r e g u l a t i n g f u n c t i o n s ( t r a n s i e n t or working changes) occur i n s i d e a narrow s t a t i s t i c a l range ( l e s s than 5%). These generating s e t s can accomplish the f o l l o w i n g programs: - autonomous generating sets supplying e l e c t r i c i t y t o l o c a l consumers under v a r i a b l e l o a d . - coupled generating s e t s . - f i x e d power generating s e t s f o r i n t e r n a l or l o c a l e l e c t r i c i t y , or l i n k e d t o u t i l i t i e s f o r cogeneration. - d i r e c t d r i v e of v a r i o u s machines. - b a s i c elements f o r " t o t a l energy" systems. A b a s i c q u a l i t y of these sets i s a very h i g h operation f l e x i b i l i t y . T h i s e s s e n t i a l l y r e s u l t s from the d u a l - f u e l concept. Because the engines are able t o operate i n pure D i e s e l c y c l e , a power s t a t i o n can be q u i t e autonomous, provided that a small D i e s e l f u e l storage reserve can be kept.



*5

4

Iι

700

7.4

24,717

"NOTE:

CORRESPONDS

3 4 5* -

5.7 410

700

0.3

550

7.7

6.7 475

290

4.1

AIR

TO O P E R A T I O N

530

12.3 330

8.5 610

535

35,663 1,010

970

34,250

910

GAS C A L O R I F I C V A L U E _ A C C O R D I N G TO A N A L Y S I S

540

715

600

3

3.2

32,132

545

2

5.8 413

£

22,775 645

375

Nm3/h

lb/in^ g/cm -

ft-Vh

HP

GAS

GAS ADMITTED

SUPERCHARGE PRESSURE

55

131

36 30

WATER

103.5 106 . 0

SUPPLY

23.2 10.55

10.55

23.2

11.8

26 . 0

25.7 11.7

39.5

37.1

36.6

3 ^ .2

31.Ί

of l'0

TOTAL EFF.

COOLANTS

4,438.2

4.605.7

3.998.6

4 ,061.4

TO A I R A N D OAS

123 118

815 4 35

122 50

833 445

763 406

72 22

77 25

63

77 25

727 336

65 18

20

24 . 4

11.1

622 328

lb/h kg/h

FUEL OIL QUANTITY

F C

CYLINDER EXHAUST TEMP

57 14

16

WITHOUT

970 955 1100 1060

F C

F C 61

AIR

GAS

TEMPERATURE AFTER COOLING

R E S U L T S OP ONE 5 0 0 kW GENERATING S E T WITH DUAL F U E L g CYLINDERS SUPERCHARGED ENGINE (WOOD GAS RUNNING)

POWER

1

TEST

Table I Table


640

THERMAL CONVERSION OF

S p e c i f i c a t i o n s of the G a s i f i c a t i o n

BIOMASS

Facility

A) G a s i f i c a t i o n Apparatus. The above defined generating s e t s are g e n e r a l l y fed with low BTU gas by a down d r a f t g a s i f i e r with a f i x e d c a r b o n i z a t i o n bed. G a s i f i c a t i o n i s the decomposition of hydrocarbon combustib l e s , such as c e l l u l o s e , at high temperature by p a r t i a l combustion, that i s t o say with a l a c k of a i r . A f a s t , but p a r t i a l , combustion generates carbon d i o x i d e C0 with h i g h e x o t h e r m i c i t y . C 0 can then be reduced i n the presence of the remaining incandescent carbon, y i e l d i n g CO, a combustible gas. Water from the chemical s t r u c t u r e or moisture content a l s o p l a y s a p a r t , generating hydrogen and CO. CO2 and water reduction are endothermic. Thus, a thermal e q u i l i b r i u m takes p l a c e between combustion r e a c t i o n and g a s i f i c a t i o n r e a c tions: the energy of the f i r s t one being used i n the best way to ensure the second ones. With l i g n o - c e l l u l o s i c products, we are not d e a l i n g with pure carbon. As a r e s u l t , the needed p h y s i c a l carbon bed must be generated. Therefore, the f i r s t r e a c t i o n i s a d i s t i l l a t i o n or p y r o l y s i s of the combustible that has t o pass a f i r s t h e a t i n g phase, where v o l a t i l e matters are exhausted. These v o l a t i l e matters would give an important c a l o r i f i c v a l u e , but cannot be used d i r e c t l y in engines r e q u i r i n g low intake temperature, because of thermodynamic c y c l e c o n s i d e r a t i o n s . The c o o l i n g of such gas would generate p y r o l i g n o u s acids and t a r condensations that would cause an important l o s s i n the c a l o r i f i c value and increase the r i s k s of c l o g g i n g . Combustible gases are sucked from the lower p a r t of the gas generator. Thus there e x i s t s a p a r a l l e l downward path f o r comb u s t i b l e a i r and produced gases, pyrolignous j u i c e s , a c i d s and t a r s . These are cracked on an incandescent c o a l bed. The apparatus c o n s i s t s i n a v e r t i c a l tower f i t t e d with r e f r a c t o r y m a t e r i a l s and i n c l u d i n g : - i n the upper p a r t , a combustible reserve fed by a continuous or s e q u e n t i a l l o a d i n g mechanism adapted to the working c o n d i t i o n s ; - i n the c e n t r a l p a r t a combustion and d i s t i l l a t i o n area fed w i t h a i r by nozzles; - i n the lower p a r t , a reduction area followed by a g r i l l e designed f o r ash removal, g e n e r a l l y performed in a hydraulic lock. In order t o improve e f f i c i e n c y , f e d - i n a i r i s preheated by the combustible gases' s e n s i b l e heat, by means of i n t e g r a t e d heat exchangers. The complementary c o o l i n g and c l e a n i n g of these gases i s performed i n a f i n a l scrubbing u n i t . This scrubber i s a v e r t i c a l tower where the upward flow i s mixed with atomized water. The gases, now cleaned and cooled i n the scrubber, are d r i v e n by a 2


SOLID WASTES AND

2



45.

DENNETiERE E T A L .

Power Generation from Biomass Residues

641

compressor that c r e a t e s the necessary pressure f o r p i p i n g them to the engine through an intermediate f i l t e r . T h i s chain of equipment allows the g a s i f i c a t i o n o f a l l kinds of s o l i d organic wastes o f adequate s i z e . For example, wood wastes i n c l u d i n g wood bark, coconut s h e l l s , corn cobs, e t c . can be used without p r e l i m i n a r y p r e p a r a t i o n . More d i v i d e d combustib l e s may be added t o these, i n p r o p o r t i o n s up t o 20 % i n weight. In order t o use h i g h l y d i v i d e d wastes such as small seed s h e l l s , c o f f e e s h e l l s , e t c . a combustible has t o be prepared by d e n s i f i c a t i o n . The humidity o f these products has t o be low. A moisture content l e s s than 20 % i s r e q u i r e d t o get a good steady operation. For more humid products, which i s f r e q u e n t l y the case, a d r y i n g stage i s needed and g e n e r a l l y takes p l a c e j u s t before l o a d i n g . Thermal energy necessary f o r d r y i n g i s provided by engine thermal l o s s e s : c o o l i n g c i r c u i t water and exhaust gas. B) P y r o l y s i s Equipment. The p y r o l y s i s process i s a l s o relèvent. In that case, the operation aims a t u t i l i z i n g waste by producing a r e s i d u a l char . A s p e c i f i c combustible i s obt a i n e d ( a c t i v e char or c h a r c o a l , r e f r a c t o r y ashes...) a t the r a t e of approximately 25 % of the i n i t i a l weight. Of course, these combustibles may i n t u r n be g a s i f i e d i n simpler equipment than the ones d e s c r i b e d i n Paragraph A. The gases exhausted d u r i n g the m a t e r i a l h e a t i n g can be u t i l i z e d . These gases are o f t e n of higher c a l o r i f i c value than the low BTU gas obtained with gas generators, but i n c l u d e higher hydrocarbons. These products must be separated by condensation, i n order that the engine burn only the incondensable p a r t . The condensates may f i n d i n t e r e s t i n g commercial value or be used as a thermal supply t o the p y r o l y s i s process i t s e l f . The hydrocarbons can a l s o be d i s s o c i a t e d i n c r a c k i n g equipment fed with c h a r c o a l burning with a i r d e f i c i e n c y . T h i s equipment works i n n e a r l y the same way as the lower p a r t o f the above described down d r a f t g a s i f i e r . F i e l d s of A p p l i c a t i o n The f i e l d of a p p l i c a t i o n f o r the generating s e t g a s i f i e r system t h a t seems t o o f f e r the most i n t e r e s t i n g o p p o r t u n i t i e s i s i n the power range from 100 kW t o a few thousand kW. S i n g l e commercially a v a i l a b l e generating s e t s cover the 150 t o 1000 kVA (approx. 120 t o 800 kW) range (Figure 2 ) . A v a i l a b l e g a s i f i c a t i o n equipment i s w e l l adapted t o t h i s range. S e v e r a l generating s e t s can be coupled t o one source of gas. Conversely, s e v e r a l g a s i f i e r s can be coupled t o feed a single set. Power s t a t i o n s of l e s s than 100 kW are p e r f e c t l y f e a s i b l e too. I t a l l depends on l o c a l economic c o n d i t i o n s , which w i l l d e f i n e the p r o f i t a b i l i t y of the system.


642

THERMAL

C O N V E R S I O N O F SOLID W A S T E S

AND

I fa

& Downloaded by CORNELL UNIV on September 13, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch045

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Thermal Conversion of Solid Wastes and Biomass

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