Analytical Instruments in the Coal Preparation Industry - ACS

13 Analytical Instruments in the Coal Preparation Industry

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 27, 2016 | http://pubs.acs.org Publication Date: November 12, 1982 | doi: 10.1021/bk-1982-0205.ch013

Current Status and Development Needs LEON N. KLATT Oak Ridge National Laboratory, Analytical Chemistry Division, Oak Ridge, T N 37830

The expanded use of domestic coal supplies as a substitute for imported petroleum will require significant expansion and modernization of the coal preparation industry. The Oak Ridge National Laboratory has assembled a multidisciplined team to study the needs of this industry and to undertake a development program with the goal of improving the efficiency and practice of coal beneficiation. Automated on-line analytical instruments are key elements in accomplishing this goal. On-line ash monitors are commercially available and have been successfully used in coal preparation plants world wide. On-line sulfur meters, based upon prompt neutron activation analysis have been developed. These systems can be modified to determine ash and calorific value. A moisture monitor, based upon microwave attenuation can be added to the sulfur meter. Nuclear density gauges are the only on-line process control instruments used in coal preparation plants. World wide development activities related to on-line analytical instruments are summarized, and development needs for product quality and process control instruments are discussed. Coal p r e p a r a t i o n has been an important segment of the c o a l i n d u s t r y i n the United States f o r approximately one hundred years. Its i n i t i a l use was i n the production of high q u a l i t y c o a l used i n the manufacture of coke. By 1965 approximately 95% of m e t a l l u r g i c a l grade c o a l was processed through a c o a l preparat i o n p l a n t . The use of cleaned c o a l as a f u e l f o r e l e c t r i c power generation and as an i n d u s t r i a l b o i l e r f u e l i s r e l a t i v e l y recent. Studies conducted f o r the U. S. Department of Energy (1) and the U. S. Environmental P r o t e c t i o n Agency (2) concluded that c o a l p r e p a r a t i o n combined with f u e l gas d e s u l f u r i z a t i o n i s the l e a s t 0097-6156/82/0205-0259$06.25/0 © 1982 American Chemical Society

Fuller; Coal and Coal Products: Analytical Characterization Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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c o s t l y method of meeting New Source Performance Standards f o r s u l f u r d i o x i d e emissions. The use of cleaned c o a l as a b o i l e r f u e l should improve b o i l e r e f f i c i e n c i e s ; however, the comparative data r e q u i r e d to q u a n t i f y the impact that cleaned c o a l has on the performance and r e l i a b i l i t y of power p l a n t s are not a v a i l a b l e (3). Coal p r e p a r a t i o n can be d e f i n e d as a process or s e r i e s of processes which reduces the m i n e r a l and p y r i t i c s u l f u r content of run-of-the-mine c o a l . Although chemical and p h y s i c a l methods can be used t o achieve the bénéficiâtion, past and current i n d u s t r i a l p r a c t i c e s employ only p h y s i c a l methods, and are based upon the d i f f e r e n c e s i n s p e c i f i c g r a v i t y between the c o a l , m i n e r a l matter, and p y r i t i c s u l f u r . A s i m p l i f i e d block diagram of a c o a l prepar a t i o n p l a n t i s shown i n Figure 1. The run-of-the-mine c o a l i s s i z e d c l a s s i f i e d by a wet screening o p e r a t i o n . The coarse c o a l i s cleaned i n j i g s or heavy media baths. The s m a l l e r s i z e d m a t e r i a l i s f u r t h e r subdivided i n t o intermediate and f i n e f r a c t i o n s . The intermediate m a t e r i a l i s processed v i a a wide v a r i e t y of u n i t o p e r a t i o n s , ranging from heavy media cyclones to wet c o n c e n t r a t i n g t a b l e s . The f i n e f r a c t i o n i s u s u a l l y processed v i a f r o t h f l o t a t i o n . Figure 2 summarizes the q u a n t i t y of clean c o a l produced s i n c e 1940 and compares t h i s w i t h the t o t a l U.S. c o a l production. D i f f e r e n t c o a l deposits d i f f e r widely not only i n maceral, s u l f u r and m i n e r a l content but a l s o d i f f e r widely i n other prope r t i e s , e.g., g r i n d a b i l i t y , v o l a t i l i t y and BTU content. Signifi c a n t v a r i a t i o n s i n the p r o p e r t i e s of c o a l can a l s o occur w i t h i n a deposit and w i t h i n an i n d i v i d u a l seam. As a r e s u l t , each c o a l deposit presents to the c o a l p r e p a r a t i o n engineer and p l a n t operator a unique set of f a c t o r s that must be considered i n the design and o p e r a t i o n of a p r e p a r a t i o n p l a n t . The fundamental i n f o r m a t i o n r e q u i r e d by the c o a l p r e p a r a t i o n engineer and p l a n t operator i s summarized i n c o a l w a s h a b i l i t y curves. Figure 3 i s an example of a w a s h a b i l i t y curve. This data i s obtained by s u b j e c t i n g the c o a l sample to a s e r i e s of s p e c i f i c g r a v i t y f r a c t i o n a t i o n s and a n a l y z i n g each f l o a t and s i n k s u b f r a c t i o n f o r ash content. Other analyses, such as s u l f u r and BTU content, are o f t e n conducted, depending on the end use of the c o a l . W a s h a b i l i t y data f o r d i f f e r e n t p r e s e l e c t e d p a r t i c l e s i z e f r a c t i o n s of the o r i g i n a l c o a l sample are a l s o obtained. This l a t t e r i n f o r m a t i o n i s p a r t i c u l a r l y important to the c o a l p r e p a r a t i o n engineer because d i f f e r e n t p r o c e s s i n g techniques and equipment are r e q u i r e d to t r e a t d i f f e r e n t s i z e m a t e r i a l . Analytical

Instrumentation

Laboratory Instrumentation. S u c c e s s f u l o p e r a t i o n of a modern c o a l p r e p a r a t i o n p l a n t r e q u i r e s the a v a i l a b i l i t y of a w e l l equipped and w e l l s t a f f e d c o a l a n a l y s i s l a b o r a t o r y . This l a b o r a t o r y must be capable of performing t e s t s to c h a r a c t e r i z e


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Analytical Instruments in the Coal Industry

/6> PRIMARY

SCREEN


FINE COAL C L E A N I N G CIRCUIT 28M X 0

COARSE COAL C L E A N I N G CIRCUIT 4 X V 4

H

INTERMEDIATE COAL C L E A N I N G CIRCUIT % X 28 M

CLEAN

COAL

REFUSE

Figure 1.

Simplified diagram of a modern coal preparation plant.

900

c ο ο 700

Ε ζ 9 500 Ι Ο Ζ)

Ο

ο CL < 300

S

100 1940

Figure 2.

_L 1950

Coal mine (

1960 YEAR

1970

) and coal preparation plant (

1980

) production.



262

COAL

Figure 3.

AND

COAL

Coal washability curves. Key: , cumulative ash; specific gravity distribution; and , yield.


PRODUCTS

, ±

0.10

13.

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the feed c o a l , to v e r i f y product q u a l i t y , and to provide process control information. The analyses commonly performed are described i n standard t e s t procedures published by the American Society f o r Testing and M a t e r i a l s (4). On-Line Instrumentation-General C o n s i d e r a t i o n s . Rapid response time and the a b i l i t y to sample l a r g e q u a n t i t i e s of m a t e r i a l c o n s t i t u t e the p r i n c i p a l design c r i t e r i a f o r o n - l i n e instruments i n the c o a l p r e p a r a t i o n i n d u s t r y . Other important f a c t o r s i n c l u d e c o s t , r e l i a b i l i t y , and m a i n t a i n a b i l i t y ; however these a r e common to o n - l i n e instrumentation i n any i n d u s t r y . Rapid response i s r e q u i r e d because the residence time of m a t e r i a l i n the process equipment v a r i e s from a few seconds to a few minutes; average residence time i n the plant i s t y p i c a l l y l e s s than t e n minutes. Furthermore, s i g n i f i c a n t v a r i a t i o n s i n the composition of the run-of-the-mine c o a l f e d to the plant can occur i n a step f u n c t i o n a l manner. The heterogenous nature o f c o a l and the quantity of m a t e r i a l processed through a t y p i c a l c i r c u i t w i t h i n a preparation plant r e q u i r e s a simple means of o b t a i n i n g information r e p r e s e n t a t i v e of the process stream. Non-invasive sampling methods are p r e f e r r e d because m a t e r i a l handling problems are minimized. Because of the above requirements, most of the l a b o r a t o r y techniques used to analyze c o a l cannot be t r a n s f e r r e d to o n - l i n e instruments. Methods which use high energy photons, gamma or x-ray, can meet the above design c r i t e r i a , and consequently, a l l the systems developed or under development f o r the c o a l i n d u s t r y , and which y i e l d elemental composition information employ methodology which involves the measurement o f gamma or x-rays. On-line instrumentation i n c o a l preparation plants can be c l a s s i f i e d i n t o two broad c a t e g o r i e s , instruments that monitor a process or m a t e r i a l parameter and instruments that monitor process equipment s t a t u s . L i q u i d l e v e l and pressure gauges which monitor sump l e v e l s and pump o u t l e t pressure are examples of instruments i n the l a t t e r c l a s s i f i c a t i o n and w i l l not be discussed i n t h i s report. On-Line Instrumentation Used i n the Coal Preparation Industry. Ash monitors, nuclear density gauges, and pH monitors are the only o n - l i n e instruments c u r r e n t l y used to measure process o r m a t e r i a l parameters i n U.S. c o a l p r e p a r a t i o n p l a n t s . Two o n - l i n e ash monitors are commercially a v a i l a b l e , the Sortex Ash Monitor (Gunson's Sortex Ltd., London, England) and the KHD Analyzer (KHD Industrieanlogen, Humbolt, Weday, Federal Republic of Germany). Both instruments use the backscatter of h i g h energy photons to determine the ash content of the sample; the Sortex u n i t employs ^^^Pu, which emits a 17·6 KeV x-ray, while the KHD u n i t uses Am, which emits a 60 KeV gamma ray. The Sortex Ash Monitor has been i n s t a l l e d i n eight U.S. c o a l preparation p l a n t s ; t h i r t y a d d i t i o n a l u n i t s have been i n s t a l l e d worldwide. 241



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The f i r s t i n s t a l l a t i o n and use of an ash monitor i n the U.S. was a Sortex u n i t at the Moss No. 3 P r e p a r a t i o n Plant ( C l i n c h f i e l d Coal Company, D i v i s i o n of P i t t s t o n Coal Group, Lebanon, V i r g i n i a ) . The Moss No. 3 p l a n t produces three blends of m e t a l l u r g i c a l côal and a steam c o a l . The Sortex Ash monitor i s used to monitor the m e t a l l u r g i c a l c o a l product, and i t s output i s used by the p l a n t superintendent to s e l e c t the blending r a t i o s of run-of-the mine c o a l being cleaned at the p l a n t . When a m a t e r i a l i s exposed to x-rays or gamma r a y s , a p o r t i o n of the r a d i a t i o n i s absorbed and a p o r t i o n i s s c a t t e r e d . The a b s o r p t i o n process f o l l o w s an e x p o n e n t i a l r e l a t i o n s h i p I » I exp[-u pl] 0

(1)

m

where I i s the i n c i d e n t x-ray i n t e n s i t y , I the i n t e n s i t y a f t e r t r a v e r s i n g an absorber t h i c k n e s s 1, \i i s the mass a b s o r p t i o n c o e f f i c i e n t , and ρ i s the d e n s i t y of the m a t e r i a l . The mass a b s o r p t i o n c o e f f i c i e n t i n c r e a s e s approximately as the 3.5 power of the atomic number and decreases w i t h i n c r e a s i n g photon energy. The t o t a l s c a t t e r i n g process i s composed of e l a s t i c and i n e l a s t i c components. The p r o b a b i l i t y f o r e l a s t i c s c a t t e r i n g of a photon increases w i t h the square of the atomic number and decreases w i t h i n c r e a s i n g photon energy. The p r o b a b i l i t y f o r i n e l a s t i c s c a t t e r ing i s p r o p o r t i o n a l to the atomic number and a l s o decreases w i t h i n c r e a s i n g photon energy. Consequently, f o r a g i v e n i n c i d e n t photon energy, an i n c r e a s e i n ash forming elements i n the c o a l decreases the t o t a l number of photons backseattered, and a meas urement of the b a c k s c a t t e r i n t e n s i t y provides a measure of the t o t a l ash forming elemental content of a c o a l sample. A schematic drawing of the sensor system employed i n the Sortex Ash Monitor i s shown i n F i g u r e 4. The x-rays emitted by the 23*Spu i r r a d i a t e the c o a l sample and penetrate up t o 38 mm. This r a d i a t i o n i s absorbed and s c a t t e r e d ; the f r a c t i o n backscattered i s counted w i t h a gas p r o p o r t i o n a l counter. The aluminum f i l t e r i s used to compensate f o r i r o n f l u o r e s c e n t x - r a y s , which are e x c i t e d by the i n c i d e n t x-rays. The f i l t e r p r e f e r e n t i a l l y absorbs most of the i r o n f l u o r e s c e n t x-rays (^6 KeV), and i t s t h i c k n e s s i s chosen based upon the i r o n content of the c o a l samples. The aluminum f i l t e r a l s o compensates f o r s u l f u r v a r i a t i o n s i n the c o a l . This occurs because a major f r a c t i o n of s u l f u r i s present as p y r i t e and the decrease i n x-ray b a c k s c a t t e r i n t e n s i t y due t o s u l f u r i s p a r t i a l l y o f f s e t by an i n c r e a s e i n i r o n f l u o r e s c e n c e . Organic s u l f u r i s g e n e r a l l y c o n s t a n t , and i t i s c o r r e c t e d f o r i n the c a l i b r a t i o n of the instrument ( 5 ) . F i g u r e 5 c o n t a i n s the flow diagram f o r the i n s t a l l a t i o n of the Sortex Ash Monitor at the Moss No. 3 c o a l p r e p a r a t i o n p l a n t . The c l e a n c o a l product i s sampled, crushed to a top s i z e of 5 mm, and presented to the ash monitor. The discharged sample i s Q

m

s o u r c e



13.

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265

Figure 4. Schematic drawing of Sortex ash monitor sensor system.



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CUTTER NOOOIb/CUT ^ ^ ^ I B R A T I N G

FEEDER

HAMMER MILL CRUSHER

ROTARY

SPLITTER

|l5lb/CUT

REJECT 8516/CUT

SAMPLE COLLECTION CONTAINER

SURGE HOPPER

12 in. S C R E W CONVEYER

BYPASS TO RR CAR

SORTEX A S H MONITOR

CALIBRATION SAMPLE

BYPASS TO RR CAR

TO RR CAR

Figure 5.

Typical coal sampling system and flow path.


PRODUCTS

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combined w i t h the main product stream at the r a i l r o a d car l o a d i n g chute. A c t u a l o p e r a t i n g data i n d i c a t e an e x c e l l e n t c o r r e l a t i o n between the ash monitor and l a b o r a t o r y ash analyses ( 6 ) . The observed c o r r e l a t i o n equation i s :


Ash Monitor - 0.74

(Lab Ash) +1.88

(2)

Data spanning s i x consecutive months of o p e r a t i o n showed that the d i f f e r e n c e between the ash monitor r e s u l t s and the l a b o r a t o r y r e s u l t s was l e s s than 1.12% ash at the 95% confidence l i m i t . The p r e c i s i o n of the ash monitor d u r i n g the same time p e r i o d averaged 0.53% ash. The f a i l u r e to o b t a i n the i d e a l c o r r e l a t i o n probably can be a t t r i b u t e d to a number of f a c t o r s ; however, the two measurements are very d i f f e r e n t , the ash monitor provides data i n d i c a t i v e of the elemental composition of the m i n e r a l matter i n the c o a l w h i l e the l a b o r a t o r y analyses y i e l d data r e f l e c t i n g the mass of m e t a l l i c oxides derived from t h i s m i n e r a l matter. The accuracy of the ash monitor f o r d i f f e r e n t c o a l types was evaluated by Cammack and B a l i n t ( 5 ) . For f i f t e e n d i f f e r e n t c o a l samples w i t h ash content ranging from 4 t o 40% ash, the accuracy of the ash measurement a t the 95% confidence l i m i t ranged from + 0.3 t o + 2.2% ash; the average value f o r t h i s confidence l i m i t was + 1.3% ash. The moisture content of the c o a l i s a major v a r i a b l e a f f e c t i n g the performance of the ash monitor. I t a l t e r s the packing c h a r a c t e r i s t i c s of the c o a l , and thereby, i n f l u e n c e s the bulk d e n s i t y of the sample. Moisture a l s o a f f e c t s the h a n d l i n g chara c t e r i s t i c s of the c o a l . Cammack and B a l i n t (5) found that the compacted c o a l bed has a minimum d e n s i t y a t 8% moisture; and f o r moisture l e v e l s up to 7%, the maximum e r r o r i n ash a n a l y s i s was 0.5% ash. Jones and Stanley (6) found that the c o a l handling system does not operate r e l i a b l y above 8% moisture. The o p e r a t i n g experience a t Moss No. 3 c l e a r l y i n d i c a t e s t h a t the Sortex Ash Monitor i s a very u s e f u l t o o l and that i t possess s u f f i c i e n t r e l i a b i l i t y and accuracy to be s u i t a b l e f o r c o n t r o l of p l a n t c i r c u i t r y . Through use of a c o r r e l a t i o n equation and c a r e f u l c a l i b r a t i o n , the system should be able t o provide the accuracy r e q u i r e d to meet the q u a l i t y c o n t r o l requirements of the c o a l p r e p a r a t i o n i n d u s t r y . Nuclear d e n s i t y gauges are commercially a v a i l a b l e and have found use i n many m a t e r i a l h a n d l i n g i n d u s t r i e s . Nuclear d e n s i t y gauges determine the d e n s i t y of m a t e r i a l by measuring the a t t e n u a t i o n of gamma-rays passing through the m a t e r i a l . Equation 1 provides the b a s i s f o r the measurement. The gauge c o n s i s t s of a r a d i o a c t i v e source, u s u a l l y ^ ^ C s , a gamma-ray d e t e c t o r , a m p l i f i e r and a s s o c i a t e d s i g n a l p r o c e s s i n g e l e c t r o n i c s . Nuclear d e n s i t y gauges are t y p i c a l l y used as process c o n t r o l instruments t o monitor the d e n s i t y of water-magnetite s l u r r i e s used i n the heavy medium c l e a n i n g c i r c u i t s . The instrument



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package i s designed t o clamp onto a process p i p e . Accuracies of + 1-2% are r o u t i n e l y achieved. The n u c l e a r d e n s i t y gauge, c o n f i g u r e d as a l i n e source w i t h a l i n e a r a r r a y of d e t e c t o r s and coupled t o a tachometer, i s used as a b e l t s c a l e . The pH process monitoring and c o n t r o l equipment i s standard i n d u s t r i a l grade i n s t r u m e n t a t i o n and i s a v a i l a b l e from numerous vendors. The pH measurement i s commonly made i n the f r o t h f l o t a t i o n c e l l s , and c a u s t i c soda or lime i s added t o the t a i l i n g s output of the f r o t h c e l l s . This measurement i s p r i m a r i l y used t o c o n t r o l the pH of the p l a n t process water w i t h the set p o i n t s e l e c t e d t o y i e l d optimum performance from the f l o c c u l a n t s added t o the s t a t i c t h i c k e n e r . C o n t r o l of the process water a c i d i t y i s a l s o r e q u i r e d t o minimize the c o r r o s i o n of process equipment. On-line Instrumentation A p p l i c a b l e t o the Coal P r e p a r a t i o n Industry. Although numerous measurement concepts have been cons i d e r e d f o r o n - l i n e a n a l y s i s i n the c o a l p r e p a r a t i o n i n d u s t r y and s e v e r a l of these are being evaluated i n the l a b o r a t o r y , e.g. Môssbauer spectroscopy Ç7), x-ray d i f f r a c t i o n , and x-ray f l u o r e s cence, t h i s s e c t i o n w i l l d i s c u s s only those systems that have undergone or are scheduled f o r f i e l d t e s t i n g and are a d v e r t i s e d as a v a i l a b l e . These i n c l u d e elemental a n a l y z e r systems, a moisture monitor, and a coal-rock-water s l u r r y meter. Two elemental a n a l y z e r systems have been developed, the "Continuous On-line Nuclear Assay of C o a l " , CONAC, (Science A p p l i c a t i o n , Inc., Palo A l t o , CA) and "The Elemental A n a l y z e r " (MDH I n d u s t r i e s , Inc., Monrovia, CA). Both of these u n i t s a r e based upon the measurement of prompt gamma rays t h a t are emitted from a nucleus f o l l o w i n g the capture of a neutron. This t e c h nique i s commonly known as prompt neutron a c t i v a t i o n a n a l y s i s , PNAA. In a t y p i c a l a p p l i c a t i o n a ^ ^ C f n e t r o n source i s p o s i t i o n e d e i t h e r beneath a conveyor b e l t or on one s i d e of a g r a v i t y f e d chute. The h i g h energy neutrons emitted by the neutron source are t h e r m a l i z e d by the c o a l sample before undergoing capt u r e by the v a r i o u s sample c o n s t i t u t e n t s . The r e s u l t i n g prompt gamma ray spectrum provides a q u a l i t a t i v e s i g n a t u r e f o r the v a r ious elements, and the number of c h a r a c t e r i s t i c photons, which depends upon the capture cross s e c t i o n and weight percent, p r o vides a q u a n t i t a t i v e measure of each element. Most of the emitted gamma rays have energies i n excess of 0.4 MeV and r e a d i l y penetrate the s t r u c t u r a l m a t e r i a l of the c o a l h a n d l i n g system as w e l l as the mass of c o a l being measured. The gamma ray d e t e c t o r i s placed o u t s i d e the c o a l h a n d l i n g system. For those a p p l i c a t i o n s r e q u i r i n g analyses f o r major and minor elements a h i g h r e s o l u t i o n d e t e c t o r , such as germanium-lithium d r i f t e d d e t e c t o r , i s used. I f only s u l f u r and a few major elements i n c o a l a r e d e s i r e d , a medium r e s o l u t i o n d e t e c t o r , such as a Nal (TI) c r y s t a l i s used. A computerized multichannel a n a l y z e r i s r e q u i r e d to process the gamma ray data. The system i s c a l i b r a t e d using known U



13.

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reference samples of c o a l . The CONAC system can handle up to 50 ton/hr c o a l streams; the MDH system can handle up t o 10 ton/hr. The CONAC system can be s u p p l i e d with the c a p a b i l i t y to measure a s i n g l e element, e.g., s u l f u r , to perform a complete elemental a n a l y s i s , or any intermediate l e v e l of a n a l y s i s capability. The l a r g e r systems a l s o provide an estimate of the BTU content of the c o a l . Generally, the r e s u l t s from the CONAC s y s tem are more p r e c i s e than the r e s u l t s obtained from ASTM a n a l y t i c a l procedures (8). L i t e r a t u r e data i n d i c a t e that the accuracy of the CONAC system surpasses that of the ASTM procedures ( 8 ) ; however, a c a r e f u l study of i t s accuracy has not been completed, p r i m a r i l y because of the u n a v a i l a b i l i t y of a v a r i e t y of standard samples i n q u a n t i t i e s r e q u i r e d by the CONAC system. The heating value of the c o a l sample i s c a l c u l a t e d from an e m p i r i c a l equation (9) , which requires analyses f o r hydrogen, oxygen, nitrogen, and s u l f u r . The agreement with r e s u l t s obtained by bomb caloriraetry i s e x c e l l e n t (+ 1.2 + 0.3%). Coal samples with top s i z e s up to 8 cm per s i d e have been s u c c e s s f u l l y analyzed. The MDH "Elemental Analyzer" performs analyses f o r s u l f u r and the major ash forming elements; an estimate of the BTU content i s a l s o provided. Computer simulations of the systems performance i n d i c a t e that s u l f u r analyses can be performed with e r r o r s of + 2.2% and ash analyses with average e r r o r s of + 3.4% (10) . Experimental data acquired with the system have not been reported. A CONAC system, " S u l f c o a l y z e r " , has been i n s t a l l e d at D e t r o i t Edison Company's Monroe, Michigan, power s t a t i o n (11). This plant i s a four u n i t complex rated at 3000 megawatts. The " S u l f c o a l y z e r " cost $500,000 and w i l l be used to c o n t r o l the c o a l blending f a c i l i t y . Operating data from t h i s i n s t a l l a t i o n are not a v a i l a b l e . A second CONAC system w i l l be i n s t a l l e d a t the Tennessee V a l l e y Authority, Kingston, Tennessee, steam p l a n t . This plant i s a m u l t i - b o i l e r complex with 1600 megawatt c a p a c i t y . The CONAC system w i l l be used i n connection with a consent agreement i n v o l v i n g environmental r e g u l a t i o n s , compliance schedules, and methods of achieving compliance f o r s u l f u r dioxide and part i c u l a t e emissions. While i t seems most d e s i r a b l e to c o n t r o l c o a l q u a l i t y at the mine or at a preparation p l a n t , the important c o n t r o l point f o r a u t i l i t y i s at the d e l i v e r y area at the power s t a t i o n . TVA's i n i t i a l e f f o r t w i l l concentrate on the use of the " S u l f c o a l y z e r " to monitor c o a l as i t i s d e l i v e r e d . A CONAC system has not been i n s t a l l e d at any c o a l preparation p l a n t . Two i n s t a l l a t i o n s of the MDH "Elemental Analyzer" are planned, one i n a c o a l a p p l i c a t i o n and a second u n i t i n an o i l blending f a c i l i t y (12). The c o a l a p p l i c a t i o n i s i n the Homer C i t y , Pennsylvania, c o a l preparation plant c u r r e n t l y under construction. The analyzer system w i l l provide a feedback s i g n a l that w i l l be used to c o n t r o l the s p e c i f i c g r a v i t y of the heavy media c o a l c l e a n i n g operations. The i n s t a l l a t i o n was scheduled f o r Spring, 1981. The u n i t cost $300,000.



270

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On-line moisture monitors, which are based on i n f r a - r e d a b s o r p t i o n , nuclear magnetic resonance, capacitance and microwave a t t e n u a t i o n are used i n numerous process i n d u s t r i e s . The a p p l i c a b i l i t y of these techniques to the c o a l p r e p a r a t i o n industry has been reviewed (13,14)· The C0NAC and "Elemental Analyzer" u n i t s use a microwave a t t e n u a t i o n method f o r the moisture measurement. This information i s used to c o r r e c t the PNAA hydrogen assay data i n order to c a l c u l a t e the hydrogen content of the c o a l . KayRay, Inc., ( A r l i n g t o n Heights, I l l i n o i s ) has developed an instrument that measures the moisture content of a coke stream and uses a combination of microwave and gamma ray a t t e n u a t i o n measurements. A schematic diagram of an o n - l i n e microwave moisture monitor i s shown i n Figure 6. At a frequency of 1 GHz sample thickness of approximately 15 cm are employed. D i p o l e - d i p o l e i n t e r a c t i o n between molecules placed i n an o s c i l l a t i n g e l e c t r i c f i e l d r e s u l t s i n energy being t r a n s f e r r e d from the o s c i l l a t i n g f i e l d to the sample. This phenomenon manifests i t s e l f as a frequency dependent d i e l e c t r i c constant and i s a property common t o a l l m a t e r i a l s . The p h y s i c a l b a s i s of the measurement i s expressed by Equation 3. Ρ = P

0

exp[-2at]

(3)

P i s the i n c i d e n t power, Ρ i s the t r a n s m i t t e d power, t i s the sample t h i c k n e s s , and α i s the a t t e n u a t i o n f a c t o r defined by Equation 4: 0

(4)

ω i s the frequency, y i s the magnetic p e r m e a b i l i t y , Σ i s the s t a t i c component of the d i e l e c t r i c constant, and Σ ^ i s the complex component of the d i e l e c t r i c constant, i . e . , the frequency dependent term. Because water i s a very p o l a r molecule, r e l a t i v e to the hydrocarbon c o n s t i t u e n t s of c o a l , i t i s r e s p o n s i b l e f o r a s i g n i f i c a n t f r a c t i o n of the microwave a t t e n u a t i o n , i . e , tan (Σ^/Σ )>>1, and Equation 4 reduces to Q

s

8

α * ω /μ Zjl — ο I

(5)

A t y p i c a l response curve obtained f o r an i n d i v i d u a l seam of c o a l i s shown i n Figure 7. Two major aspects of the measurement are contained i n t h i s response curve. F i r s t , the curve i s com posed of two l i n e a r segments. The p o r t i o n of the curve with the smaller slope i s due to water adsorbed on the c o a l surface, and because of the r e s t r i c t e d motion i t s l o s s f a c t o r , a, i s l e s s than that of f r e e water molecules. The p o r t i o n of the curve with the l a r g e r slope i s due to the bulk water a s s o c i a t e d with the c o a l .



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271

RECORDER

Figure 6.

Schematic drawing of an on-line microwave moisture meter.



272

COAL

MOISTURE

AND

COAL

PRODUCTS

(%)

Figure 7. Response of microwave moisture meter for an individual seam of coal.



13.

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273


The band nature of the response curve i s r e l a t e d to the average particle size. As the average p a r t i c l e s i z e decreases the g r a d i ent of both l i n e a r segments decreases. Changes i n the bulk d e n s i t y and the surface t o volume r a t i o , and the r e s u l t i n g change i n the r a t i o of surface to bulk water have been p o s t u l a t e d as r e s p o n s i b l e f o r these changes i n the slopes of the response curve (15). From the envelope of t h i s curve and the p r e c i s i o n of the a t t e n u a t i o n measurement, the moisture content of a known c o a l can be measured with an u n c e r t a i n t y of + 1% water. I f the c o a l type i s unknown the u n c e r t a i n t y i n c r e a s e s t o approximately + 2.8% water (13). Coal transport i n the intermediate and f i n e c o a l p r o c e s s i n g c i r c u i t s of a c o a l p r e p a r a t i o n plant i s accomplished by means of water-coal s l u r r i e s . C o n t r o l of the various unit operations w i t h i n these c i r c u i t s requires a means of measuring the r e l a t i v e masses of c o a l , rock, and water i n the feed, product and/or waste streams. Science A p p l i c a t i o n , Inc., under contract from the P i t t s b u r g Mining Technology Center, U. S. Department of Energy, has developed a s l u r r y c o n c e n t r a t i o n meter (16). Although i t s intended a p p l i c a t i o n i s i n s l u r r y haulage systems, i t i s e q u a l l y a p p l i c a b l e i n the c o a l p r e p a r a t i o n i n d u s t r y . Figure 8 contains a schematic drawing of the s l u r r y concen t r a t i o n meter. The instrument package c o n s i s t s of three separate gauges: a gamma density gauge, a neutron gauge, and a conduc t i v i t y gauge. The gamma density gauge provides a measure of the t o t a l mass, the neutron gauge provides a measure of the t o t a l hydrogen, and the c o n d u c t i v i t y gauge measures the f r e e water i n the s l u r r y . The equations r e l a t i n g the observed response of each gauge t o the mass of each component a r e : (a) gamma gauge Ύ

Ύ

Ύ

Μ μ + Μ μ + Μ μ » -1η (Ν /ε ) ce r r ww γ γ

(6)

(b) neutron gauge 11

Μ μ* + M / + M μ ce r r ww and

= -In (Ν /ε ) n n

(7)

(c) c o n d u c t i v i t y gauge M/

w

=

K O

w

(8)

M , M , and M denote the mass ^per ^unit area of c o a l , rock, and water; μ , μ^., and μ ^ and μ , μ^, and μ ^ a r e the e f f e c t i v e gararaa-ray and neutron mass a b s o r p t i o n c o e f f i c i e n t s , ε and ε are the e f f i c i e n c e s of the gamma and neutron d e t e c t i o n systems. o and a are the c o n d u c t i v i t y of the water phase measured i n a reference c e l l and the c o n d u c t i v i t y of the s l u r r y , respec t i v e l y ; Κ i s the c o n d u c t i v i t y c e l l constant. S o l v i n g these c

r

w

Ύ

1

0

0

η

w

w



274

COAL AND

COAL

PRODUCTS

SIGNAL PROCESSOR

I

JL

GAMMA GAUGE

-IRON PIPE

CONDUCTIVITY GAUGE

GAMMA GAUGE

>

SLURRY GAMMA SOURCE

NEUTRON SOURCE

iT

PVC PIPEFigure 8.

Schematic drawing of the coal slurry concentration meter.



13.

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275

equations simultaneously y i e l d s the r e l a t i v e p r o p o r t i o n of c o a l , rock, and water i n the s l u r r y . The s l u r r y gauge was evaluated at the Colorado School of Mines, Research I n s t i t u t e . For s l u r r i e s i n which the p a r t i c l e s i z e s were l e s s than 0.6 mm, the absolute d i f f e r e n c e between the measured and a c t u a l s l u r r y composition averaged 1.7%. Slurries c o n t a i n i n g 0 to 25% c o a l and rock were u t i l i z e d i n these t e s t s . The p r e c i s i o n of the measurement was l i m i t e d by the counting s t a tistics. Response time i s one second. For s l u r r i e s c o n t a i n i n g s o l i d p a r t i c l e s approximately o n e - t h i r d the pipe diameter, l a r g e e r r o r s i n the measured s l u r r y composition were observed. These e r r o r s are probably due to the i r r e g u l a r movement of the l a r g e p a r t i c l e s . The custom designed s l u r r y c o n c e n t r a t i o n meter cost $150,000; mass production of the meter probably w i l l reduce the cost to $75,000 (17)» Economic Considerations of On-Line A n a l y t i c a l

Instruments

Most c o a l p r e p a r a t i o n p l a n t s use instrumentation to inform the c o n t r o l room operator of equipment status and guide the employees w i t h i n the plant i n the manual adjustment of the set points on the process equipment. Many u n i t operations are not equipped with any instrumentation and s u c c e s s f u l implementation of these u n i t operations r e l i e s t o t a l l y upon the experience of an employee charged with the r e s p o n s i b i l i t y of monitoring s p e c i f i c p i e c e s of process equipment. New p l a n t s are being designed with increased l e v e l s of automation and o n - l i n e instrumentation; however, the investment i n instruments i s only a small f r a c t i o n of comparable investments i n other process i n d u s t r i e s . For example, the p r o j e c t team v i s i t e d two modern c o a l p r e p a r a t i o n plants i n the Appalachian c o a l f i e l d s which cost $75,000,000 and the t o t a l o n - l i n e instrumentation investment was approximately $150,000. The r a t i o of instrumentation to p l a n t investment i n the chemical industry i s s i g n i f i c a n t l y larger. Several f a c t o r s are r e s p o n s i b l e f o r the low investment i n o n - l i n e instrumentation. F i r s t , c o a l r e l a t i v e to other f u e l s i s undervalued, and as a r e s u l t the c o a l i n d u s t r y i s under c a p i t a l ized. Second, the c o a l i n d u s t r y has been r e l u c t a n t to t r y new technology. However, based upon v i s i t s to s e v e r a l c o a l preparat i o n p l a n t s i n the Eastern U.S., the r a t e of t e c h n o l o g i c a l change w i t h i n the c o a l p r e p a r a t i o n i n d u s t r y i s a c c e l e r a t i n g . For examp l e , computers are being used to a u t o m a t i c a l l y operate c o a l p r e p a r a t i o n p l a n t s . T h i r d , an economic r a t i o n a l e f o r o n - l i n e instrumentation has not been provided. F i n a l l y , only a p p r o x i mately o n e - t h i r d of the t o t a l c o a l mined i s cleaned, and t h i s f r a c t i o n has been d e c l i n i n g since the raid 1960 s. The economic b e n e f i t derived from the use of o n - l i n e process c o n t r o l and q u a l i t y c o n t r o l instruments u l t i m a t e l y must be t r a n s l a t e d i n t o the a b i l i t y to improve product q u a l i t y , i . e . , improve product s p e c i f i c a t i o n s and reduce v a r i a t i o n s i n product q u a l i t y . f


276

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COAL

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Improved o n - l i n e instrumentation should allow the c o a l preparat i o n p l a n t operator t o accomplish t h i s o b j e c t i v e through a more e f f i c i e n t use of the raw m a t e r i a l , by producing more product without i n c r e a s i n g plant c a p a c i t y , and by a l l o w i n g one to operate c l o s e r to product s p e c i f i c a t i o n l i m i t s . A r e d u c t i o n i n operating and maintenance costs should a l s o occur. On-Line Instrumentation Development Needs The o n - l i n e instrumentation development needs can be d i v i d e d along the t r a d i t i o n a l l i n e s of q u a l i t y c o n t r o l and process cont r o l instrumentation. The i n t e r r e l a t i o n s h i p between the two and the proposed mode of i n t e r a c t i o n are shown i n Figure 9. The q u a l i t y c o n t r o l instruments, i n an o p e r a t i o n a l and a p r a c t i c a l sense, are l o c a t e d outside the preparation p l a n t . As a r e s u l t of data obtained on product q u a l i t y , the q u a l i t y c o n t r o l instrument i n t e r a c t s with the p l a n t feed and the process c o n t r o l instrument a t i o n to optimize the use of raw m a t e r i a l c o n s i s t e n t with the d e s i r e d product s p e c i f i c a t i o n s . This feedback loop can be automatic; however, current p r a c t i c e places the p r e p a r a t i o n plant superintendent or engineer i n the loop and the q u a l i t y c o n t r o l measurement i n a remote l a b o r a t o r y , r e s u l t i n g i n manual c o n t r o l scheme with a long time delay. The process c o n t r o l instrumentat i o n c o n t r o l s a s i n g l e u n i t o p e r a t i o n and the instrumentation i s unique to each u n i t operation. T h e i r set points are e s t a b l i s h e d by the output from the q u a l i t y c o n t r o l instruments. S e v e r a l q u a l i t y c o n t r o l devices are a v a i l a b l e and the d e v e l opment status has been summarized i n a previous s e c t i o n . Laborat o r y s t u d i e s i n d i c a t e that the p r e c i s i o n and accuracy of these devices are adequate to meet product c e r t i f i c a t i o n needs. Fielding t e s t i n g of these u n i t s i n the c o a l p r e p a r a t i o n plant e n v i r o n ment i s r e q u i r e d to demonstrate t h e i r usefulness and economic viability. The development status of process c o n t r o l instrumentation lags that of the q u a l i t y c o n t r o l instruments s i g n i f i c a n t l y . Nuclear density gauges f u n c t i o n i n the c o a l preparation plant environment. The s l u r r y c o n c e n t r a t i o n meter has a p p l i c a t i o n i n the intermediate and f i n e s i z e d c o a l cleaning c i r c u i t s and needs to be t e s t e d i n a p r e p a r a t i o n p l a n t . Other devices, such as ash monitors to c o n t r o l the operation of heavy media baths or j i g s are not a v a i l a b l e ; and instruments developed f o r other process i n d u s t r i e s are not s u i t a b l e f o r use i n c o a l p r e p a r a t i o n p l a n t s . Modeling s t u d i e s of the v a r i o u s u n i t operations are required i n order to a s c e r t a i n the fundamental parameters required to automate the c o n t r o l of these systems. Primary process c o n t r o l instrument needs include ash, s u l f u r , and moisture monitors; secondary needs include an o n - l i n e w a s h a b i l i t y and ash f u s i o n measurement·



KLATT


277

FEED QUALITY CONTROL INSTRUMENTATION

j

t_.

1 ^ I PROCESS CONTROL Γ1 INSTRUMENTATION |

I I

1 UNIT OPERATION

ι

I—^

ι

PREPARATION PLANT

Figure 9.

Interrelationship between process control quality and control on-line instrumentation.


278

COAL AND COAL

PRODUCTS

Acknowledgement


Research sponsored by the O f f i c e of Energy Research, U. S. Department of Energy under Contract W-7405-eng-26 w i t h the Union Carbide Corporation.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13.

14.

"Engineering/Economic Analysis of Coal Preparation with SO Cleanup Processes", ΕΡΑ-600/7-78-002, U. S. Department of Energy, Pittsburgh, ΡΑ., 1979. "Environmental Assessment of Coal Cleaning Processes: Technology Overview", EPA-600/7-79-073e, U. S. Environmental Protection Agency, 1979. Vivenzio, Τ. Α . , "Impact of Cleaned Coal on Power Plant Performance and Reliability", Electric Power Research Institute, EPRI CS-1400, 1980. "Gaseous Fuels; Coal and Coke; Atmospheric Analysis, Part 26", American Society for Testing and Materials, Philadelphia, PA., 1978. Cammack, P.; Balint, Α., Trans. SME, 1976, 260, 361. Jones, D. W.; Stanley, F. L., "Ash Monitoring at Pittston Moss No. 3 Plant", Coal Convention of the American Mining Congress, St. Louis, MO, 1978. Levinson, L. W., "Mossbauer Effect Spectroscopic Study of Pyritic Sulfur in Coal", EPRI FP-1228, 1979. Tassicker, O. J., "Continuous Nuclear Analyzer of Coal for Improved Combustion Control", International Symposium on Pulverized Coal Firing and Its Effects, University of Newcastle, Australia, 1979. Berkowitz, N., "An Introduction to Coal Technology", Academic Press, 1979, p. 35. Cekorich, Α.; Deich, H.; Harringon, T . ; Marshall, J . H. III, "Performance of an On-line Elemental Analyzer for Coal", 2nd International Coal Utilization Conference, Houston, TX, 1979. Lagarias, J.; Irminger, P.; Dodson, W., "Nuclear Assay of Coal, Vol. 8, Continuous Nuclear Assay of Coal: Progress Review and Industry Applications", EPRI FP-989, January 1979. Deich, Η., MDH Industries, personal communication. Brown, D. R.; Gozani, T . ; Elias, E.; Bozorgmanesh, Η., "Nuclear Assay of Coal, Volume 4: Moisture Determination in Coal - Survey of Electromagnetic Techniques", EPRI CS-989, 1980. Fauth, G., "On-Stream Determination of Ash and Moisture Content in West German Coal Preparation Plants", Proceedings of the 1980 Symposium on Instrumentation and Control for Fossil Energy Processes, Virginia Beach, VA, 1980. 2


13.

15.


16.

17.

KLATT


Hall, D. Α.; Sproson, J . C . ; Gray, W. Α., J . Institute of Fuel, 1970, 43, 350. Verbinski, V. V . ; Cassapakis, C. G.; deLesdernier, D. L.; Wang, R. C . , "Three Component Coal Slurry Sensor for Coal, Rock, and Water Concentrations in Underground Mining Operations", 6th International Conference on the Hydraulic Transport of Solids in Pipes, University of Kent, United Kingdom, 1979. Orphan, V. J., Science Applications, Inc., personal communication.

RECEIVED May 17,

1982


279

Analytical Instruments in the Coal Preparation Industry - ACS

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