Recent tests with a twin-screw, co-rotating extruder which was successfully used to convey and feed coal against pressures of up to 1500 psi are described. Intermeshing and self-wiping, co-rotating twin-screws give greatly improved conveying and pressure built-up capabilities and avoid hangup and eventual decomposition of coal particles in the screw flights. The conveying action of intermeshing, self-wiping, co-rotating extruder systems approaches that of a positive displacement pump. With this feature, it is possible to maintain very accurate control over all aspects of product conveyance in the extruder, i.e., intake, conveyance and pressure buildup

Mack, W. A.

Wiedmann, W.

English

NASA Technical Reports Server

THE IJSE O F  TWIN S C R E W  E X T R U D E R S  
FOR FEEDING COAL AGAINST PRESSURES 
OF U P  TO 1500 PSI 
W. W i e d m a n n  
W e r n e r  8: P f l e i d e r e r  
Stut tgart ,  W e s t  Germany  
W ,  A. Mack 
W e r n e r  & P f l e i d e r e r  ~ ~ r p .  
Waldwick, New Jersey  
https://ntrs.nasa.gov/search.jsp?R=19780005326 2020-03-20T12:36:31+00:00Z
To date, o i l  and natura l  gas have served most o f  the wor ld 's  
energy needs. Diniini shed supp 1 ies,  however, mean t h a t  a re tu rn  
t o  coal w i  11 be essent ia l  t o  f u l f i  !l the energy needs o f  the 
coming decades. G a s i f i c a t i m  of  coal provides a ready and economic 
so lu t i on  t o  our immediate energy needs. 
The processes forc g a s i f i c a t i o n  o f  coal were f i r s t  developed i n  
the m i d - t h i r t i e s  and successfu l ly  used throughout World War I I .  
Since then, however, very l i t t l e  development was undertaken t o  
improve product ion methods. Consequently, considerable work must 
be done t o  adapt the processes t o  today's standards. 
One process step which has never been s a t i s f a c t o r i l y  resolved i s  
the feeding o f  the coal against the pressurc i n  a g a s i f i c z t i o i  
reactor.  A v a r i e t y  o f  devices had been tested, however, nope 
provided an e f f e c t i v e  c~mmercia l  so lu t ion .  
Single-screw extruders have been used fo; feeding coa!, b u t  basic 
disadvantages have been observ?d, such as: b r i dg ing  of the coal 
i n  feed section, a high degree o f  surging i n  the extruder,  very 
h igh energy consumption (up t o  10 times h igher  than twin-scree 
equipment). I n  addi t ion,  i t  i s  d i f f i c u l t  t o  maintajn steady s ta te  
pressure bui ld-up condit ions, espec ia l l y  w i  t h  the l a rge r  diameter 
(single-screw) extruders requi red f o r  commercial s ize  p lan ts .  
Also, the punping e f f ic iency of a single-screw i s  dependent on 
t i g h t  clearances between the screw crest  and barrel.  As a r e s u l t  
o f  the high condit ioning i rvo lved w i th  processing coal, these 
clearances are l o s t  a f t e r  very short operative time, leading t o  
frequent shutdowns f o r  repair. This i s  especial ly t rue  for processes 
where an absolute minimum o f  water i s  needed f o r  e f f i c i e n t  operation 
o f  the gas i f ica t ion reaction. 
Twin-screw nechanisms have been known t e  provide s i gn i f i can t l y  
improved coal conveying capzbi l i  t i es .  They can also develop the 
required pressures f o r  steady srate conditions even w i th  la rger  
machine sizes. However, the i n i t i a l  development work performed 
w i th  counter-rotation twin-screw mechanisms also producea negative 
results. The primary reason i s  that  counter-rotating screws ac t  
as a grinder and change the p a r t i c l e  size d i s t r i bu t i on  o f  the coal 
considerably. As a side effect, a high amount of layer ing i n  the 
screw!, was observed. 
This paper w i l l  describe recent tes ts  w i th  a twin-screw, co-rotat ing 
extruder which was successfully used t o  convey and feed coal against 
pressures of up t o  i500 psi. Intermeshing and self-wiping, co- 
ro ta t ing  twin-screws give great ly  improved conveying and pressure 
bui 1 t-up capabi 1 i t i e s  and avoid hangup and eventual decomposition of 
coal pa r t i c les  i n  the screw f l i g h t s .  
The conveying act ion o f  intermeshing, se l f -  wiping, co-rotat ing 
extruder systems approaches that  of a pos i t ive  displacement pump. 
With t h i s  feature, i t  i s  possible to  maintain very accurate control  
over a l l  aspects of product conveyance i n  the extruder, i.e., intake, 
conve.yance and pressure bui 1 dup. 
I n  the co-rotat ing systems, the product i s  moved from one screw t o  
the other i n  the form o f  an 8-shaped path and downstream, depending 
on screw pitch. Very l i t t l e ,  i f  any, material passes through the 
clearance between the screws because the two screws have opposing 
di rect ions i n  the area of intermesh. Very 1 i t t l e  product pressure 
ex is ts  tha t  would create leak flow through t h i s  area. A l l  o f  the 
material i s ,  therefore, transferred from one screw t o  the other a t  
each revolut ion. The 8-shaped path i t s e l f  i s  a very long one, 
al lowing ampic opportunity f o r  heat exchange between the material and 
the barrel  wal ls. I n  effect,  a great var ie ty  o f  d i f fe ren t  coals can 
be processed wi th l i t t l e  o r  no water addi t ion required t o  achieve 
conveyance and pressure bili 1 dup. 
The improved conveying capab i l i t i es  of twin-screw mechanisms a1 low 
generation o f  ,.ry high extrusion pressures w i th  a very short backup 
length required. For example, i n  a wel l  designed twin-screw system, 
extrusion pressures of 3,000 t o  5,000 ps i  can be eas i ly  generated 
over a screw length o f  2 t o  3 D. The pressures can be adjusted 
w i  t b u t  d ras t i ca l l y  af fect ing the throughput. This provides added 
processing f l e x i b i l i t y .  
The basic advantage of twin-screw mechanisms, therefore, resu l t s  
from superior conveying capabi l i t ies,  which i n  tu rn  great ly help 
i n  maintaining control  over energy input  i n t o  the material and 
retent ion time d is t r ibu t ion .  
TEST SET-UP 
Figure 1 shows the t es t  set-up used t o  convey the coal w i th  a 
co-rotat ing twin-screw system. The coal i s  metered v ia  a weigh 
be1 t feeder i n t o  the extruder. Two 58mn (screw diameter) screws 
w i th  an L/D r a t i o  o f  15 were used. Water was metered i n t o  the 
second barrel  section. This was done t o  provide the proper pro- 
por t ion of water for  the reaction and also so tha t  the water could 
be used as a lubricant i n  the process. Special kneading elements 
are i ns ta l l ed  a t  Sections 2 and 3 t o  in tens ive ly  mix the water 
and coal. Two d i f ferent  coals were tested, 1 i g n i t e  coal ( pa r t i c l e  
s ize 0-6m, 59% cap i l  l a r y  water) see Figure 2, and bituminous 
coal ( pa r t i c l e  s ize 0-.2 mn, no water content) see Figure 3. 
TEST RESULTS 
I n  processing 1 i g n i t e  coal w i th  the 58 m extruder, a throughput 
rate of 170 kg/htS. was achieved. Successful runs were made against 
59 atmospheres o f  pressure. A continuous f lcw through a t ransfer  
pipe of 10 foot length was achieved, thus s t inu la t ing  actual de l ivery  
condit ion i n  large gas i f i ca t ion  reactors. The appearance of the coal 
strand was very regular. 
The bituminous coal was processed w i th  up to  3 percent cap i l l a r y  
water. The addi t ion of the water was required t o  generate a gas-t ight 
paste i n  the extruder. Special kneading and mixing elements were 
required i n  order t o  homogeneously incorporate the water i n t o  the coal. 
Most of the required energy f o r  mixing was introduced by mechanical 
energy through the motor. The energy consumption i s  approximately 
.03 Kk' per Kg coal. 
Figure 4 shows mechanical energy consumption versus reactor  gas 
pressure. As expected, the energy consumption increases w i th  r i s i n g  
reactor gas pressure. The water i n  the coal acts as a lubr icant ,  
consequently, the pressure decreases wi th increased water content, 
see Figure 5. 
I n  addi t ion t o  conveying o f  coal through 3 pipe against pressure, 
a series o f  tes ts  were conducted t o  study the behavior o f  the coal 
a t  discharge. To economically operate a coal gas i f ie r ,  i t  i s  
necessary t o  obtain uniform p a r t i c l e  s ize d i s t r i bu t i on  of the 
coal i n  the gas i f ie r .  Figure 6 shows a thermal d is in tegrat ion o f  
the coal leaving the pipe following extrusion. I n  order t o  convey 
against 1500 psi, the temperature o f  the coal must reach a t  l eas t  
3100C. Usually, gasi f i ca t ion  temperatures are mugh higher. This 
type o f  d is in tegrat ion o f  coal can be applied i n  f l u i d i zed  bed 
reactors. 
I n  the tests, throughput rates o f  up t o  250 Kg/hr. were obtained. 
The tests demonstrated tha t  co-rota t i ng, twin-screw extruders can 
be successfully used t o  convey and feed coal against pressures o f  
up t o  100 atmospheres. 
Requi r e n t s  f o r  comnerci a1 coal gasi f i c a t  ion reactors are between 
100 and 200 tons per hour. I n  the p las t i cs  industry, today's 
commercial s ize extruders have throughput rates o f  up t o  25 metr ic  
tons an hour. I n  order t o  scale up t o  comnercial requirements, 
more work must be done t o  study the behavior o f  coal conveyed against 
pressure. The next t e s t  series w i l l  be car r ied out  on a 120 mn twin- 
I screw extruder which has a throughput ra te  of  up t o  1% metr ic  tons 
an hour. I n  t h i s  t e s t  the extruder w i l l  be mounted d i r e c t l y  on a 
gas i f ica t ion reactor. The resrll t s  of t h i s  second phase o f  scale-up 
w i l l  provide a d e f i n i t e  basis regarding the f e a s i b i l i t y  o f  a 
f comnercial s ize operation. 

Figure 2 
Typical lignite moisture 
- 
Mined at Hambacher Forst 
Water Content 59% 
Particle Size 0 -6  mm 
Ash Content 2 
Figure 3 
Bituminous Coal 
Particle Size 
Ash Content 
1 WIN-SCREW 
D = 5 8 M M  
23.1 */o H20 
R E A C T O R  GAS PRESSURE ( BAR 
ENERGY CONSUMPTION VS REACTOR PRESSURE 
FOR HIGH VOLATILE BlTUMlNUS COAL 
FIGURE 4 
- - 
- - 
- - 
FREE DISCHARGE 
F R G M  PIPE 
H20 CONTENT, ( So ) 
PRESSURE DROP VS H20 CONTENT 
FIGURE 5 
Figure 6 
Disintegration of the coal and water mixture 
at the discharge end of a Continua 
. - twin-screw machine. 


The use of twin screw extruders for feeding coal against pressures of up to 1500 PSI

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server