Retorted Oil Shale Disposal Research - American Chemical Society

13 Retorted O i l Shale Disposal Research

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 31, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch013

R. N. HEISTAND Paraho Development Corporation, Box A, Anvil Points, Rifle, CO 81650

In the quest for alternate energy sources, oil shale appears particularly attractive because of the large domestic deposits and because the retorting process produces liquid and gaseous fuels directly. However, the retorting process also produces large quantities of retorted shale which could pose a potentially adverse environmental impact if not disposed of properly. A mature, million-barrel-per-day oil shale industry would produce over one-and-one quarter million tonnes of retorted shale during each day of operation. Although the retorted, or processed, shale represents the largest by-product from oil shale retorting operations, results from detailed studies indicate that it can be managed in an environmentally acceptable manner. Laboratory and field tests have demonstrated the following properties: compaction to 1600 kg/m, using normal vehicular traffic; cementation strength to 1480 kPa, using only retorted shale and optimum water; permeabilities as low as 1 x 10 cm/s, using proper handling techniques. In addition, tests have shown that dusting, auto-ignition, and leaching pose no special problems. These research results indicate that properly managed retorted shale exhibits the properties of a low-grade cement. 3

-7

Although the nature o f r e t o r t e d shale c e r t a i n l y depends upon the nature o f the g e o l o g i c a l d e p o s i t from which the raw o i l shale was mined, i t depends, t o a l a r g e extent, a l s o upon the r e t o r t i n g technology used t o process the raw o i l s h a l e . The i n f o r m a t i o n presented i n t h i s paper i s obtained from the Paraho O i l Shale Demonstration c a r r i e d out from 1973 t o 1976(1^). The Paraho semi-works r e t o r t i s a c y l i n d r i c a l v e s s e l , having a 2.5 m i n t e r n a l diameter and i s capable o f p r o c e s s i n g about 250-300 tonnes o f raw o i l shale p e r day. A schematic o f the Paraho d i r e c t mode operations i s shown i n F i g u r e 1. Raw shale, crushed and screened t o +1.0 cm t o -10.0 cm, enters the top o f the r e t o r t , passes through the r e t o r t under the i n f l u e n c e 0097-6156/81/0163-0183$05.00/0 © 1981 American Chemical Society

In Oil Shale, Tar Sands, and Related Materials; Stauffer, H. C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

OIL SHALE, TAR SANDS, AND RELATED


184

RAW SHALE

GAS

Preheat Retort Combust

Cool

GAS

RETORTED SHALE Figure 1.

Paraho direct mode

operation


MATERIALS


13.

HEISTAND

Retorted

0

;1

Shale

Disposal

Research

185

of g r a v i t y and c o n t r o l l e d by the grate mechanism a t the bottom. The r e t o r t e d shale s t i l l contains many of the l a r g e r , +5.0 cm to -10.0 cm, p i e c e s . T h i s range i n p a r t i c l e s i z e found i n the r e t o r t e d shale enhances i t s s t r e n g t h p r o p e r t i e s and minimizes d u s t i n g while h a n d l i n g . The Paraho process uses counter-current flows — s o l i d s moving downward and gases moving upward. As the shale moves downward through the r e t o r t , i t passes through four d i s t i n c t zones as shown i n F i g u r e 1. The uppermost i s the m i s t formation zone where the shale i s preheated by the r i s i n g gases to r e t o r t i n g zone. In the next zone, the r e t o r t i n g zone, the shale i s heated so t h a t i t s s o l i d organic component, kerogen, breaks down to form gas, o i l and r e s i d u a l carbon, or coke. The o i l and gas are swept upward out of the r e t o r t to the o i l - g a s s e p a r a t i o n equipment. The shale enters the combustion zone where most of the coke i s burned to supply process heat. T h i s reduces the organic carbon i n the r e t o r t e d shale to approximately 2 wt%. Air, d i l u t e d with r e c y c l e gas, assures even d i s t r i b u t i o n of oxygen across the shale bed and d i l u t e s the oxygen to c o n t r o l the flame temperature. One important f e a t u r e of t h i s combustion zone i n the Paraho process i s the t w o - l e v e l d i s t r i b u t i o n system. T h i s system assures s u f f i c i e n t heat f o r r e t o r t i n g while keeping the combustion temperatures low enough to a v o i d excessive carbonate decomposition. T h i s carbonate decomposition wastes v a l u a b l e heat and energy through endothermic r e a c t i o n s and produces a r e t o r t e d shale higher i n water-soluble oxides which would i n c r e a s e l e a c h i n g tendencies and cause a leachate higher i n pH and s a l t content. The shale f i n a l l y enters the c o o l i n g zone where i t i s cooled with r e c y c l e gas before i t i s discharged from the r e t o r t . Within t h i s zone the r e t o r t e d shale r e a c t s with most of the hydrogen s u l f i d e i n the r e c y c l e gas. This reduces s u l f u r emissions when a p o r t i o n of t h i s gas i s u t i l i z e d and forms a v a r i e t y of s u l f u r compounds i n the r e t o r t e d s h a l e . These s u l f u r compounds reduce the oxide content and promote s t r e n g t h through cementation r e a c t i o n s . More thorough d e s c r i p t i o n s of the Paraho technology and the composition of the r e t o r t products have been p u b l i s h e d p r e v i o u s l y ( 2 _ / . Most of the m a t e r i a l used i n the r e t o r t e d shale d i s p o s a l r e s e a r c h was produced by the Paraho semi-works r e t o r t o p e r a t i n g i n the d i r e c t mode o p e r a t i o n . Another mode of o p e r a t i o n , an i n d i r e c t mode, was a l s o s t u d i e d . In the i n d i r e c t mode, a i r i s not introduced i n t o the r e t o r t . I n t e r n a l combustion does not occur; none of the organic carbon remaining on the r e t o r t e d shale i s u t i l i z e d . Process heat f o r the i n d i r e c t mode o p e r a t i o n i s s u p p l i e d by heating the r e c y c l e gas i n an e x t e r n a l heater. During the recent Paraho o p e r a t i o n s , the p o t e n t i a l problem of r e t o r t e d shale d i s p o s a l was recognized. A seven-stage r e t o r t e d shale r e s e a r c h program, j o i n t l y supported by the U. S. Bureau of Mines, was c a r r i e d out using both l a b o r a t o r y and f i e l d s t u d i e s (4_) . H i g h l i g h t s of t h i s r e t o r t e d shale r e s e a r c h program


OIL SHALE, TAR SANDS, AND RELATED MATERIALS


186

showed t h a t r e t o r t e d o i l shale, produced by the Paraho process, can be e f f e c t i v e l y compacted and handled to produce strong and water impervious s t r u c t u r e s (_5) . These d e s i r a b l e p r o p e r t i e s of Paraho r e t o r t e d shale are b e l i e v e d due to the formation of a low-grade cement by chemical r e a c t i o n s which occur d u r i n g retorting. In t h i s paper the chemistry of Paraho r e t o r t e d shale, the nature of the thermo-chemical r e a c t i o n s t h a t can c o n t r i b u t e t o i t s cement-like p r o p e r t i e s , and the chemistry of the leachates obtained from p e r m e a b i l i t y s t u d i e s w i l l be presented. Although c i v i l engineering i n f o r m a t i o n now e x i s t s which permits the d i s p o s a l of r e t o r t e d shale i n an environmentally acceptable manner, a b e t t e r understanding of the thermal r e a c t i o n s of r e t o r t i n g and the chemical composition of r e t o r t e d shale may suggest changes i n r e t o r t i n g operations which would f u r t h e r l e s s e n any p o s s i b l y adverse environmental impact. Experiment Design Because the d i s p o s a l of r e t o r t e d shale i s , u l t i m a t e l y , a f i e l d e x e r c i s e , t h i s paper w i l l d i s c u s s the experimental design and data from f i e l d s t u d i e s which have been c a r r i e d out. Laboratory experimentation and data w i l l be used to complement the r e s u l t s of f i e l d s t u d i e s . Two f i e l d s t u d i e s , compaction and p e r m e a b i l i t y , were c a r r i e d out during the Paraho r e s e a r c h operations. The f i e l d compaction s t u d i e s were c a r r i e d out i n a r e l a t i v e l y f l a t area, 2-3 km from the r e t o r t i n g operations (see F i g u r e 2 ) . The compaction s i t e , measuring 55 m wide by 120 m long, was d i v i d e d i n t o two s e c t i o n s . In one s e c t i o n the r e t o r t ed shale was p l a c e d dry; i n the other, optimum water was added before placement. The m a t e r i a l was hauled d i r e c t l y from the r e t o r t e d shale d i s p o s a l system and spread as soon as p o s s i b l e . No problems with d u s t i n g or a u t o - i g n i t i o n were noted. The m a t e r i a l was spread i n 20-30 cm l a y e r s and subjected to v a r i o u s compactive e f f o r t s . Approximately 1200 m^ (15000 tonnes) of r e t o r t e d shale was used i n these f i e l d compaction s t u d i e s . The f i e l d i n f i l t r a t i o n s t u d i e s were c a r r i e d out near the r e t o r t i n g o p e r a t i o n s . Two shallow ponds, 25 m diameter, were constructed using techniques developed during the e a r l i e r f i e l d compaction s t u d i e s (see F i g u r e 3). Both ponds were constructed out of Paraho r e t o r t e d shale p l a c e d i n l a y e r s to an o v e r a l l t h i c k n e s s about 1 m. Both ponds had s l o p i n g s i d e w a l l s to obviate w a l l e f f e c t s and to e l i m i n a t e any wall-bottom i n t e r f a c e . One pond was c o n s t r u c t e d with dry r e t o r t e d shale using l i g h t compaction. The other pond was constructed of r e t o r t e d s h a l e , mixed with optimum water, and p l a c e d using heavy compaction. A f t e r c o n s t r u c t i o n , both ponds were f i l l e d with water and the i n f i l t r a t i o n r a t e s were measured u s i n g s t a f f gauges and flows through the d r a i n l i n e s .



13.

HEISTAND

Retorted

Oil Shale

Disposal

Figure 2.

Field compaction

Figure 3.

Field infiltration

Research

area

area


187

188

OIL SHALE,

R e s u l t s and

TAR SANDS, AND RELATED MATERIALS

Discussion

Compaction. The f i e l d compaction s t u d i e s confirmed e a r l i e r l a b o r a t o r y s t u d i e s r e g a r d i n g the e f f e c t of compactive e f f o r t , moisture a d d i t i o n , and aging on d e n s i t y and s t r e n g t h . Shown i n F i g u r e 4 i s the r e l a t i o n s h i p of compactive e f f o r t on the d e n s i t i e s of r e t o r t e d s h a l e . At l e a s t three i n - p l a c e d e n s i t y measurements were made i n each l a y e r . Results show t h a t d e n s i t i e s averaging 1400 kg/m (about 95 l b / c u . f t . ) can be achieved. Optimum water i s needed to achieve optimum s t r e n g t h . The a d d i t i o n of water does not improve c o m p a c t a b i l i t y . The s t r e n g t h of the compacted r e t o r t e d shale was measured u s i n g unconfined compression(6). The r e l a t i o n s h i p between t h i s s t r e n g t h data and added moisture a t v a r i o u s seasoning times i s shown i n F i g u r e 5. The remarkable s t r e n g t h i n c r e a s e obtained with the a d d i t i o n of optimum water i n d i c a t e s the occurrence of some s o r t of cementing r e a c t i o n . Further evidence of t h i s cementing r e a c t i o n i s apparent where the compressive s t r e n g t h gains with aging. Aging 60 days a f t e r the a d d i t i o n of optimum water produced compressive strengths of n e a r l y 1500 kPa (205 p s i ) . The r e t o r t e d shale produced by i n d i r e c t heat operations shows no s t r e n g t h . Infiltration. I n f i l t r a t i o n data f o r the two ponds are l i s t e d i n Table I . A l s o l i s t e d are i n - p l a c e and core d e n s i t i e s ,


3

TABLE I FIELD INFILTRATION ANALYSIS Pond I Initial Cores Moisture,

wt%

22.1

18.5

Pond I I Initial 0.0

3 Density, Bottom Sides Strength,

kPa

1602

kg/m 3

1697

1474 1394

1602

kg/m

1482 -6

2000 0.6 0.3 P e r m e a b i l i t y , cm/sec x 10 moisture contents, and compressive s t r e n g t h s . These data show t h a t Pond I (high compaction, optimum water) was w e l l constructed. D e n s i t i e s matched those achieved i n e a r l i e r l a b o r a t o r y and f i e l d compaction t e s t s . The s t r e n g t h , measured i n a core sample (1480 kPa), exceeded those achieved d u r i n g those e a r l i e r t e s t s . The f i e l d p e r m e a b i l i t y , as measured by s t a f f gauge, when c o r r e c t e d f o r i n i t i a l a b s o r p t i o n and normal evaporat i o n , approaches the value obtained from a n a l y s i s of the core (6 x 10" cm/s). A c t u a l i n f i l t r a t i o n from the h i g h l y compacted pond shows the permeation r a t e to be 3 x 10~ cm/s. Pond I I , r e c e i v i n g only l i g h t compaction and no added water, shows a p e r m e a b i l i t y r a t e o f 2 x 10~4 cm/s. Although the 7

7


13.

Retorted

HEISTAND

Oil Shale

Disposal

Research

189

COMPACTION


- HEAVY

o -STANDARD

1

i 10

r

15

T 30

20

MOISTURE CONTENT ADDED (% by dry weight) U.S. Bureau of Mines

Figure

4.

Compaction test results. Paraho retorted shale, semiworks direct heat; 1.5-in. maximum size fraction.

-HEAVY COMPACTION DENSITY = 98.7 pcf

200 -

150 -

plant-

x(205)

DIRECT HEAT -STANDARD COMPACTION DENSITY = 92.5 pcf

>

% too

50 -

-i 20

1

1

40

60

L

INDIRECT HEAT STANDARQ COMPACTION DENSITY = 92.5 pcf

1

1

80

100

-(10) r 120

OAYS CURING AT 125° F U.S. Bureau of Mines

Figure

5.

Compression

test results. Paraho retorted shale, .75-in. maximum size jraction.

semiworks


plant;


190

OIL SHALE,

TAR SANDS, AND RELATED

MATERIALS

p e r m e a b i l i t y of t h i s m a t e r i a l i s q u i t e high, e f f l u e n t s c o u l d be contained i n a d i s p o s a l area u s i n g p r o p e r l y - p l a c e d , w e l l c o n s t r u c t e d m a t e r i a l such as used i n Pond I . Even though the l o o s e l y compacted Pond I e x h i b i t e d a high p e r m e a b i l i t y r a t e , an a d d i t i o n a l f i e l d experiment i n d i c a t e d t h a t p e r m e a b i l i t y may not pose a s e r i o u s problem. A f t e r Pond I I ( l i g h t compaction) had d r i e d thoroughly f o r s e v e r a l months f o l l o w i n g the f i e l d i n f i l t r a t i o n t e s t , a s p e c i a l t e s t was conducted. The surface was sprayed with 17,400 & of water to represent a r a i n f a l l of 5 cm i n 30 minutes. No e f f l u e n t occurred f o r n e a r l y one f u l l day. A small seepage began the second day and continued f o r two days. Only 7 £ were c o l l e c t e d from the d r a i n p i p e . E s s e n t i a l l y a l l of the simulated r a i n f a l l was l o s t to a b s o r p t i o n and subsequent evaporation. T h i s i n d i c a t e s t h a t l e a c h i n g and p e r m e a b i l i t y may not be a problem f o r Paraho r e t o r t e d shale, even when l i g h t l y compacted, because even heavy r a i n f a l l w i l l not penetrate the p i l e to s i g n i f i c a n t depths. Chemical and P h y s i c a l P r o p e r t i e s . Paraho r e t o r t e d shale i s d e s c r i b e d according to standard s o i l s c l a s s i f i c a t i o n as a silty-gravelly material{4). A s i z e d i s t r i b u t i o n diagram i s presented i n F i g u r e 6. Some of the f a v o r a b l e p r o p e r t i e s of the r e t o r t e d shale i s a t t r i b u t e d to t h i s s i z e d i s t r i b u t i o n - l i k e a good aggregate mix, t h i s m a t e r i a l has the proper r a t i o of f i n e s to l a r g e r s i z e d p i e c e s so t h a t v o i d s between the l a r g e r p i e c e s are f i l l e d with f i n e s . This r e l a t i o n s h i p increases d e n s i t y , promotes s t r e n g t h , and reduces d u s t i n g and e r o s i o n . The chemical composition of r e t o r t e d shale depends on the composition of the raw shale and the r e t o r t i n g p r o c e s s . The m i n e r a l composition of the Green R i v e r shale used i n the Paraho operations c o n s i s t s of a complex mixture o f m i n e r a l s . These i n c l u d e : carbonates (50%), c l a y s (40%), quartz (8%), and s u l f i d e s and others (2%). The p r i n c i p a l carbonate m i n e r a l undergoing thermal r e a c t i o n s during the normal r e t o r t c o n d i t i o n s i s dolomite (CaC03*MgC03), or more p r o p e r l y f e r r o a n (CaC03«Fe , 9 l - x 3 ) where a p o r t i o n of the magnesium i s r e p l a c e d by i r o n . I t i s b e l i e v e d t h a t f e r r o a n undergoes the f o l l o w i n g chemical r e a c t i o n during normal r e t o r t i n g c o n d i t i o n s : x

M

c o

(1) Ferroan + Heat — • + FeO + CO2

Calcite

(CaC03) + Magnesia

(MgO)

Many s t u d i e s have been made concerning the mean chemical a n a l y s i s o f Paraho r e t o r t e d s h a l e ( 7 ) . Although i t i s important to know t h i s chemical composition, i t i s not h e l p f u l i n a s s e s s ing the components r e s p o n s i b l e f o r the cement-like p r o p e r t i e s . Much emphasis has been p l a c e d upon the formation o f r e a c t i v e oxides, such as f r e e lime and magnesia, from carbonate decompos i t i o n . Although f r e e lime can be formed under l a b o r a t o r y c o n d i t i o n s ( 8 ) , i t has not been detected i n the r e t o r t e d shale used i n the f i e l d s t u d i e s . Although f r e e lime was not detected,



13.

Retorted

HEISTAND

Oil Shale

Disposal

191

Research

SILT-CLAY SIZE ^

SAND SIZE (STANDARD SIEVE SERIES)

#200

#100

#50

#30

#16

•

#8

a

#4

GRAVEL SIZE (INCHES) |

|

I

l£

I 100 U.S. Bureau of Mines

Figure

6.

Gradation

data. Paraho retorted shale, semiworks 1.5-in. maximum size fraction.

plant—direct


heat;

192

OIL

SHALE,

TAR

SANDS,

AND

R E L A T E D

magnesia has been found i n Paraho r e t o r t e d s h a l e . S u l f u r minerals, known to e x h i b i t cement-like p r o p e r t i e s , have been found i n Paraho r e t o r t e d shale (

Retorted Oil Shale Disposal Research - American Chemical Society

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