Journal of RadioarutO,tical Chemistry, Vol. 71, No. 1-2 (1982) 253-264

ON-LINE ANALYSIS OF COAL BY NEUTRON INDUCED GAMMA SPECTROMETRY* H. R. WILDE, W. HERZOG

Staatliches MaterialpriiJi4ngsamt NIr D~4600 Dortmund 41 (FRG) (Received July 7, 1981)

Thermal neutron prompt gamma activation analysis (PGNAA) has proven to be a useful tool for the multi-elemental characterization of cQal. The use of isotopic neutron sources allows the construction of relatively small irradiation facilities for the on.line analysis of large volume samples. As a first step to an on-line analysis system for process control in a coal blending plant we have developed a facility using a Cf-252 neutron source with an actual strength of 1 rag. This source, which is shielded by layers of high purity lead and paraffin, can be moved to the center of cylindrical sample holders containing up to 150 kg of coal. To avoid background radiation the sample holders are made from polyethylene. The prompt gamma rays emitted from the sample are measured with a 22% high purity germanium detector housed in a special horizontally extended cryostate assembly. The detector is positioned on top of the sample; it is surrounded by a multi-layer decreasing Z-absorber to reduce low energy gamma background. Paraffin loaded with 95% enriched lithium-6-carbonate is used between sample and detector as a gamma window with high absorption for thermal neutrons. Moreover the detector is shielded against background radiation by lead and natural lithiumcarbonate. The data processing system consists of high count-rate analogue electronics including pileup-suppression and a fast data acquisition system, which permits on-line analysis of the collected data. Analysis of several samples of simulated coals as well as Western Germany coals shows a good spectral response of our system for the elements H, C, AI, Si, S, C1, Ca, Ti and Fe, whereas the sensitivity for N, K and Na is somewhat lower. This means, that the quantitative determination of the above elements will be possible within 20 minutes. Optimization of the system is continued with a stress on analysis time and detection sensitivity,

Introduction

Due to the increasing generation

importance

of coal for electric power

as well as for gazification

and liquefaction

there

is a growing need for on-line analysis of coal composition quality in large process

streams,

and

since the efficiency of these

*Supported by Bundesministerium flit Forschung und Teehnologie D-5300 Bonn 1, Fed. Rep, of Germany.

J. Radioanal. Chem. 71 (1982)

253

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

combustion

or c o n v e r s i o n

by even small changes Several

methods

elements

processes

may be affected I) in coal composition

for m o n i t o r i n g

gross

coal c o m p o s i t i o n

in the coal have been reported

PNAA-method

(prompt neutron

seriously

activation

2,3,4).

or special

Among

analysis)

has proven

be the most p r o m i s i n g

tool

coal.

based upon the fact that the capture

This method

a neutron compound energy

is

by a nucleus

for the m u l t i - e l e m e n t

these the

leads

to the formation

nucleus which decays

istic of the e m i t t i n g

nucleus,

y-rays

the neutron

capture

a particular

energy

of the e m i t t i n g Cf-252

allows

facilities process

section.

directly

applications

of r e l a t i v e l y

or even complete or electric

elemental

experiments

of an on-line

to the abundance sources small

this means,

ash analysis, analysis

such as

irradiation samples.

For

that an upper

Possible

applications

sulfur d e t e r m i n a t i o n

for coal b l e n d i n g

This paper presents

of several p a r a m e t e r s

facilities

the results

done at the M a t e r i a l p r H f u n g s a m t

coal analysis

depends

flux and

time must be less than

should be achieved.

power plants.

to study the effect

the analysis

The

of y-rays with

of large volume

time of the coal;

BTU-determination,

laboratory

proportional

of

is character-

neutron

So the number

The use of isotopic

the c o n s t r u c t i o n

10 - 15 min~

(~I0-IOs)

the b o m b a r d i n g

for the rapid analysis

the t r a n s p o r t a t i o n

include

is

element.

control

limit of

cross

of y-rays.

and their p r o d u c t i o n - r a t e

on the n u m b e r of target nuclei,

of

of a highly excite(

by the emission

spectrum of these prompt

analysis

to

for optimal

of the

Dortmund performance

system.

Experimental As a first.step a testing

analysis

system

for process

facility has been built up using a Cf-252

with an actual in the p l a n n i n g

254

to an on-line

strength of 1 mg.

There were

control

neutron

sourc(

four main consideratio~

of the experiments:

J. Radioanal. Chem. 71 (1982)

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

Detector with n-shield

I

i I Cc~l ~mple

~_~2Cf souro _

/

9

Lead

/

Boron-

/ potyetht.jtene

~///////j////////A=///

"H Screening container

~.~

t 10cm.. i Fig. 1. Schematic graph of the experimental setup

a) the

1 mg neutron

analysis,

size should be variable

c) the detector sensitivity

for c o n v e n t i o n a l

in the range

should have good energy against n e u t r o n

d) the arrangement

activation

should be d e s i g n e d

these design

resolution

50 - 150 kg, and low

irradiation,

and the shields

in order to optimize Following

installed

had to be used,

b) the sample

detector

source,

of neutron

flexibly

source,

sample and

to allow easy m o d i f i c a t i o n

the system. rules

it will be p o s s i b l e

to extrapolate

from the data of the laboratory

experiment

a prototype

graph of the e x p e r i m e n t a l

is shown

system.

in Fig.

A schematic

I. The sample

J. Radioanal.Chem. 71 (1982) 17

to the p a r a m e t e r s

and the d e t e c t o r

of

setup

have been installed

255

H. Ri WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

on top of the s c r e e n i n g

container

for c o n v e n t i o n a l

s e p a r a t e d by layers of b o r a t e d p o l y e t h y l e n e

and lead to avoid

a c t i v a t i o n of the iron in the c o n t a i n e r walls. source

is

m o v e d up and down inside

m o t o r drive.

S h i e l d e d by two

The

cups of high p u r i t y

sample h o l d e r s

1 mg Cf-252

a zircaloy-tube

it can be m o v e d out of the s c r e e n i n g c o n t a i n e r the c y l i n d r i c a l

INAA analyses,

by a step

lead and paraff5

to the c e n t e r of

c o n t a i n i n g up to 150 kg of coal.

The source s t r e n g t h can be r e g u l a t e d w i t h i n a l i m i t e d range by l o w e r i n g the s o u r c e position. radiation

the sample h o l d e r s

To avoid b a c k g r o u n d

are made of p o l y e t h y l e n e .

They

may be r e m o v e d or i n s t a l l e d w i t h a small crane b o o m w i t h i n a few minutes.

The samples

ted p o l y e t h y l e n e

are s u r r o u n d e d by

- not shown in this

tron i n d u c e d b a c k g r o u n d

10 cm of bora-

figure - to reduce neu-

from the c o n c r e t e w a l l s

of our labora-

tory. The p r o m p t gamma rays e m i t t e d w i t h a 22 % h i g h p u r i t y special horizontally advantages tectors

extended

cryostate

of high p u r i t y d e t e c t o r s

are m e a s u r e d

and the a p p a r e n t l y

assembly.

de-

c o o l i n g only d u r i n g

lower s e n s i t i v i t y

The use of NaJ d e t e c t o r s

The m a i n

c o m p a r e d w i t h Ge(Li)

are the n e e d of liquid n i t r o g e n

operation damage.

from the sample

g e r m a n i u m d e t e c t o r h o u s e d in a

to n e u t r o n

i n s t e a d of g e r m a n i u m types

has n o t been taken into a c c o u n t b e c a u s e of their poor resolution.

The d e t e c t o r

in Fig.

I; it is s u r r o u n d e d by a m u l t i l a y e r

a b s o r b e r to reduce Paraffin

low e n e r g y

loaded w i t h

used b e t w e e n

is p o s i t i o n e d

sample

above the sample as shown decreasing-Z

gamma background.

95 % e n r i c h e d

lithium-6-carbonate

and d e t e c t o r as a n e u t r o n

is

absorber with

good t r a n s m i s s i o n

for high e n e r g y gamma rays.

side the d e t e c t o r

is s h i e l d e d a g a i n s t t h e r m a l n e u t r o n s by

a thicker Whereas

256

layer of p a r a f f i n w i t h

the n e u t r o n

absorption

F r o m the back-

n a t u r a l l i t h i u m carbonate.

coefficients

of n a t u r a l b o r o n

J. Radioanal. Chem, 71 (1982)

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

H

106

511

8ock@roundradiQtion

~

Simulated COQI motrix =" 105

~10 4

C

2 Pb

u

.

103

1000

500

2O00 Channel number

1500

Fig. 2. Spectrum of the background radiation measured with a pure polystyrene sample to simulate the coal matrix

and e n r i c h e d

l i t h i u m are about

the same,

the use of l i t h i u m

is a d v a n t a g e o u s

because

ring a neutron.

In this w a y the total c o u n t r a t e

by about

it p r o d u c e s no gamma rays w h e n captu-

The data p r o c e s s i n g and a c o m p u t e r rate amounts

source,

fully;

e.g.

- though

system consists

the a n a l o g u e

a combination

system.

electronics

Since the count-

in the experiment.

analysis.

representation

all further e x p e r i m e n t a l time is

collects

of the results.

sample.

radiation measured

For this m e a s u r e m e n t

data p r e s e n t e d

and for

in this p a p e r the

4000 s. Only a few p h o t o peaks

J. Radioanal. Chem. 71 (1982) 17'

the s p e c t r a l

A graphic plotter enables

a s p e c t r u m of the b a c k g r o u n d

with a pure p o l y s t y r e n e

So it was useless

A 4096 channel pulse

to a m i n i c o m p u t e r

data and allows an o n - l i n e

and p i l e u p r e j e c t o r

in the test - c a u s e d uncon-

element determination.

height analyzer connected

the i m m e d i a t e

must be s e l e c t e d c a r e -

of main a m p l i f i e r

loss of c o u n t r a t e

for a q u a n t i t a t i v e

shows

of fast linear a m p l i f i e r s

a i d e d data a c q u i s i t i o n

s h o w i n g good p e r f o r m a n c e

trollable

collect

lowered

to 30 - 40 kcs due to the high s t r e n g t h of the

Cf-252

Fig. 2

is

10 %.

t o g e t h e r with

25 7

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

their e s c a p e peaks are to be seen:

carbon and h y d r o g e n

components

lead and b o r o n peaks

resulting

of the sample material;

f r o m the s h i e l d i n g of the source

respectively

only small c o n t r i b u t i o n s

(in the low e n e r g y come

results.

to iron and

not shown here)

this s p e c t r u m

the first

than

In this way

r e d u c e d by a factor of two,

1o kcs it amounts

the s t a t i s t i c a l is

the first q u a r t e r of the s p e c t r ~

in all m e a s u r e m e n t s .

the a n a l y z e r is

Since

contain

region is high and the s e n s i t i v i t y

low due to the high b a c k g r o u n d , is s u p p r e s s e d

shows that

1000 c h a n n e l s

far more than h a l f of the total countrate. in this

silicon

c e i l i n g and m u s t be s u b t r a c t e d

Moreover

due to the c o m p t o n b a c k g r o u n d

e r r o r of peaks

are

and the detector,

tail of the carbon peak,

from the close c o n c r e t e

from the a n a l y s i s

are

the dead time of at c o u n t r a t e s

less

to 10 - 15 %.

O p t i m i z a t i o n o f the s e t u p - Test m e a s u r e m e n t s

The s e n s i t i v i t y

of this d e v i c e

m a n y parameters:

is a c o m p l i c a t e d

it is not simply d e t e r m i n e d

a c e r t a i n peak of the spectrum,

because

function of

by the y i e l d

the b a c k g r o u n d

the peak i n f l u e n c e s

the e x p e r i m e n t a l

area and b a c k g r o u n d

show a d i f f e r e n t e n e r g y d e p e n d e n c e .

optim~ization of such a s y s t e m must source,

sample and detector,

shields

and the source strength,

source position. a polystyrene or iron oxide.

These

the a r r a n g e m e n t of

s i m u l a t e d by v a r i a t i o n

of the

have been done w i t h

filled into p o l y e t h y l e n e equally

the e x p e r i m e n t a l

3400 k e V ~ E ( 8 O O O

258

So

of sulfur

In order to save time and recycle m a t e r i a l

3. It shows

and A1

Both peak net

and shape of the v a r i o u s

test m e a s u r e m e n t s

each and then d i s t r i b u t e d

Fig.

involve

material

too.

coal m a t r i x c o n t a i n i n g a few p e r c e n t

a d d e d e l e m e n t s were

of o p t i m i z i n g

error,

keV. W h e r e a s

(from the detector)

bottles

a m o n g the matrix.

arrangement

the high e n e r g y

in

under

the

of 100 g The result

is d e m o n s t r a t e d

in

region of two s p e c t r a w i t h

the b a c k g r o u n d peaks of Si,

are c l e a r l y

suppressed,

Fe

the s p e c t r a l

J. Radioanal. Chem. 71 (1982)

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

1/-(

~ ~

Improved experimental arrant::jemen ~,

c

s

70

8 ~o~ 35--

Pb

0 1700 A

2275

2850

c 260 - S, 195 ~

3~.25

Fe

1-000 DChonnel number

F,rst st~ experimentol arrangement

u Si/C " . ~ 1 ~ ~ ~ . , ~ . .

m 130-

~

,

65-

1700

2275

2850

3/-25

~000 Chonnel number

Fig. 3. Result of optimizing the experimental arrangement including sample and detector shields

c o n t r a s t of the e l e m e n t s

in the sample,

sulfur and carbon,

could

be i n c r e a s e d appreciably. Fig.

4 demonstrates

relative

the e f f e c t of c h a n g i n g the source p o s i t i o n

to the sample.

Here the net area of the iron d o u b l e t

7632 + 7646 keV and the b a c k g r o u n d r e s o l v e d peaks scale of this Whereas

are p l o t t e d figure

under these not c o m p l e t e l y

against

zero means

the source position.

In the

the h i g h e s t p o s s i b l e position.

the peak area does not change very much w i t h the source

position,

the r e l a t e d b a c k g r o u n d

area i n c r e a s e s

steeply

p o s i t i o n s > -16 cm, thus g i v i n g rise to a q u i c k l y m e n t a l error.

The peaks of other e l e m e n t s

the b e s t value Optimizing

tural coal.

the p e r f o r m a n c e

Fig.

rise d e p e n d s

slightly

A source p o s i t i o n of -14 cm has p r o v e n to be

for this e x p e r i m e n t a l

p o r t a n t parameter.

g r o w i n g experi-

show the same behaviour;

h o w e v e r the s t a r t i n g p o i n t of the b a c k g r o u n d on the peak energy.

for

setup.

the sample t h i c k n e s s

is a n o t h e r

im-

This test has been done w i t h a high ash na-

5 shows the b e h a v i o u r of three d i f f e r e n t peak

1 Radioanal. Chem. 71 (1982J

259

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

--52

26

" :J6

-;0

_,;o

CF-source position~cm Fig. 4. Net peak area and related background as a function of the source position ( ~ source strength)

Coal sample Ewald-1 o '~ 90 x

~

Averaged

60 --

~

l

m Peak area of: CI(66!1 keY)

30 n o,

o o

n Si(3539keY) 3

Fe(7632§ 015

t

20

I

o o

keY) I

,.

25 30 Samptethickness,cm

Fig. 5. Net area of three different peaks and average trend of their background as a function of sample thickness

260

J. Radioanal. Chem. 71 (1982)

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

areas

from all regions

of the s p e c t r u m and the a v e r a g e

of their r e l a t e d b a c k g r o u n d when the sample t h i c k n e s s the source h o u s i n g and the d e t e c t o r W i t h i n a few p e r c e n t

c l e a r l y w i t h r i s i n g sample

whereas

varied

between

(c.f. Fig.

I).

the peak areas may be r e g a r d e d as indepen-

dent from the sample thickness,

of the m e a s u r e m e n t

is

trend

whereas

thickness.

the b a c k g r o u n d

This means

decreases

that the a c c u r a c y

depends only s l i g h t l y on the sample thickness,

the e x p e r i m e n t a l

error is

a f f e c t e d a p p r e c i a b l y by this

parameter.

Results and discussion

Analysis

of several

G e r m a n y coals

shows

samples of s i m u l a t e d a good s p e c t r a l

the e l e m e n t s H, C, AI, sensitivity

Si,

6 presents

4000 s. All e l e m e n t s m e n t i o n e d

in this spectrum.

are not yet k n o w n - e x c e p t chemical

Fig.

analysis

Their absolute

is still under way.

to the coal sample.

A preliminary

are shown in Fig.

bottles.

Obviously

basic

calibration

The results

is

of these

7. Here the peak net areas

the a d d e d e l e m e n t content.

its single escape peaks

Content,

conventional

a d d i n g these e l e m e n t s

iron d o u b l e t and

shown w i t h d i f f e r e n t

the y i e l d is a linear

are

In the u p p e r p a r t of

the figure the peak area of the m o s t p r o m i n e n t

added.

above are to

The a d d e d m a t e r i a l was d i v i d e d into p o r t i o n s

of 100 g and p a c k e d in p o l y e t h y l e n e

plotted against

a spectrum

concentrations

sulfur and iron - b e c a u s e

for s u l f u r and iron has been done by s t e p w i s e

measurements

the

from the Ruhr d i s t r i c t w i t h about 20 % ash c o n t e n t

The c o l l e c t time is be seen c l e a r l y

of our s y s t e m for

S, CI, Ca, Ti and Fe, w h e r e a s

for N, K and Na is lower.

of a coal sample

coal as well as W e s t e r n

response

amounts of iron

f u n c t i o n of the total elemen

and thus e x t r a p o l a t i n g the two lines back leads to a

iron c o n t e n t of about

the three most p r o m i n e n t the 2379 k e V and the

3 %. A p p l y i n g

sulfur peaks

3221 keY lines.

leads to a sulfur c o n t e n t of about

J. Radioanal, Chem. 71 (1982)

the same p r o c e d u r e

shows good a g r e e m e n t Extrapolating

1.1

to

for

their yields

%. The results of this

261

H . R . W I L D E , W. H E R Z O G :

ON-LINE ANALYSIS

OF COAL

40 -- %%~ 1H

Coal sample

08 2o

~

u _

"~

Ewald-1

\

30

~

10

o]

,

,

I 1278

,

1022 ~,

~

,

I 1534

~

,

,

,

I 1790

,

,

,

I 2046

Channel number

SdC

I

~Sl

91~.~ l.

l -'~%~. I

"

31

J ..~

"~"r

~

20/.6

CI

S,/C CI

~lR

I

J

Ct

J

I

2,~I'I~ II ct ,

~

i

2302

I

K.Na

_JU sSl Fe I

1

I

%

J

II

I

i

2558

C,

T,

1

1

t

J

,

~

2814

3070 Channel number

3838

4094 Channel number

--

8.c u

6i --

Re

pbCl

|

T~

r: II,,,co IA ~,,,Cl ~c,

~a

~ AL Cl

i 4

%

_

o

3326

~70

Fig. 6. E n e r g y s p e c t r u m

3582

of a coal sample from the Ruhr district with about

20% ash content.

The

c o l l e c t t i m e is 4 0 0 0 s

calibration in Tab. The

an i r o n

line

behaviour

is d u e

escape

may

to an

peak.

is s i m u l a t e d .

errors

262

compared

with

the w e t

chemical

analysis

results

I.

deviating

dashed

tent

are

occur

By

This

due

of the

5220

keY yield

interference this

effect

deviation

to u n d e t e c t e d

of t h i s

a much

shown sulfur

too high

is an e x a m p l e

what

interferences,if

J.

by

the

peak sulfur kind

with conof

detectors

Radioanal. Chem. 71 (1982)

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

A /

Iron calibration coa~ ,n somple Ewold -I o

700J--

I

o 525 --//~26~

k

35.0 ~ 1 " / .

-t.O

- 20

0

2.0

t.O

iron,% 5/.20keY .d %379 keY

Added Sulphur calibration cool 20.0

Sample Ewold-I

150-

~ lo.o~

~ s.~

1 MeV

s

s . SSS

.5.0

I

~s"

I - 2.0

- 1.0

1

1.0 20 Added sulphur, ~

Fig. 7. Calibration curves for the elements iron and sulfur, measured by stepwise adding these elements to a natural coal sample

Table 1 Comparison between chemical analysis and PNAA analysis results for sulfur and iron

Element

yield per % in 1OOO s ::)

S Fe

1 2 . 2 0 0 counts 9 . 1 0 0 counts

x) o n l y m a i n p e a k s

Z Radioanal. Chem. 71 [1982}

incl.

chemical analysis

PNAA analysis

1,35

%

1,1%

3,4

%

3,0 %

escapes

263

H. R. WILDE, W. HERZOG: ON-LINE ANALYSIS OF COAL

w i t h poor r e s o l u t i o n

are used;

to use high r e s o l u t i o n

it d e m o n s t r a t e s

detectors

the n e c e s s i t y

in this type of experiments.

C a l i b r a t i o n of the device will be c o n t i n u e d by using the m e t h o d d e s c r i b e d above as

w e l l as by a n a l y z i n g

determined elemental in Fig.

7 and Tab.

composition.

time.

Optimization

analysis

As to be seen from the yields

I the q u a n t i t a t i v e

w i t h good s p e c t r a l r e s p o n s e

samples w i t h chemically

determination

is now p o s s i b l e w i t h i n

of e l e m e n t s 10 - 15 minut

of the s y s t e m is c o n t i n u e d w i t h a stress on

time and d e t e c t i o n

sensitivity.

References 1. K. HEIN, J. Eng. Power, 99 (1977) 679. 2. B. D. SOWERBY, Nucl. Instr. Meth., 160 (1979) 173. 3; A. W. HALL, I. W. MARTIN, R. F. STEWART, A. M. POSTON, Precision Tests of Neutron Sulphur Meter in Coal Preparation Plant, Bureau of Mines Report no. 8038 (1975) p. 1. 4. T. GOZANI, The Development of Continous Nuclear Analyzer of Coal, ANS Trans. 28 (1978) 97. 5. C. L. HERZENBERG, N. M. O'FALLON, B. S. YARLAGADDA, R. W. DOERING, C. E. COHN, K. G. PORGES, Neutron-Induced Gamma Spectrometry for On-line Compositional Analysis in Coal Conversion and Fluidized-Bed Combustion Plants. Proc. Third Intern. Conf. on Nuclear Methods in Environmental and Energy research, Columbia, MO 1977.

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