radiochemical analysis of210po in coal gasification samples

J.RADIOANAL.NUCL.CHEM.,LETTERS 93 /5/ 271-278 /1985/

RADIOCHEMICAL ANALYSIS OF 210po IN COAL GASIFICATION SAMPLES

x L.M. Roseberry , ToG. Scott

Analytical Chemistry Division Oak Ridge National Laboratory

Oak Ridge, Tennessee 37831, USA

Received 19 July 1984 Accepted 13 August 1984

The concentrations of 210po in samples from a coal gasifier were determined by ~-pulse height analysis following separations from domplex organic matrices. Samples were dissolved using wet acid digestion and persulfate fusion techniques and then extracted with trilaurylamine. Polo- nium was auto-deposited onto silver disks using 209Po as a tracer for yield determination. The precision of the experimental method, determined on coal, was +6% at the 95% confidence level. Excellent agreement was attained with published data on an EPA coal sample.

This work was sponsored by the Office of Health and En- vironment, Office of Environmental Programs, and the Of- lice of Coal Processing, U.S. Department of Energy, under contract DE-ACO5-840R21400, with Martin Marietta Energy Systems, Inc.

XAddress all correspondence to L.M. Roseberry, P.O. Box X, Oak Ridge, Tennessee, 37830, USA.

2* 271

ROSEBERRY, SCOTT: 21Opo IN COAL GASIFICATION SAMPLES

INTRODUCTION

Gasification and liquefaction techniques for coal are

being developed to convert coal to a form compatible with

existing fuel uses. The occurrence of natural radionuclides

in coal and their fate in coal fired power plants is of

continuing importance I-4 This work deals with the deter-

mination of the distribution of the naturally occurring

radionuclide 21Opo in a coal gasification process.

Polonium is essentially a pure s-emitter of reasonably

long half life /138.3 d/ and notorious volatility. In coal,

it is generally found to be in equilibrium with its 238U

decay chain precursors. 210po from the air-borne releases

of a coal-fired power plant is estimated to contribute

significantly to the radiation dose to kidneys, liver,

and spleen 4. Polonium penetrates the body through respi-

ratory and gastrointestinal pathways, and since it forms

true radiocolloids at physiological pH, it is deposited

in soft tissue rather than bone 5. The relative biological

effective ~RBE/ doses of 210pb, 210Bi, and 21Opo are

approximately 1:10:106. This high RBE for polonium is due

to the ionizing ability of its high-energy e-emission.

EXPERIMENTAL

Process description of sample origin

Samples analyzed for this work were produced by a Fos-

ter/Wheeler Stoic two-stage gasifier. Figure 1 is a

schematic diagram of the gasifier's operation. The numbers

on the diagram indicate sampling points in the process and

correspond to data in subsequent tables. Feed coal /i/ en-

ters the top of the gasifier where combustion occurs in

two stages. At the top of the coal bed, temperatures rang-

272


;I:

+?;'ko~'o'r +

1+ i,

T ++,| i!i

Screen I ~

Fines (to bin) Ash

To st r e a m To Qtmosphere L Convegor distribut on ,~ ..... ~ / Buffer bunker i I I I@

~:.'~.~p Lock hoppe rs _._,-C.r _ I I I - Moin ~/E,~,ro,,o, ic~ ~ I/V,,ock

i+ l| I l i i l i l l r ~ ~ ~L) a~i~-,,oo I I I ~ S,,,b l l i l i l L o i t I , '~ l%.ctone I I I I ----'~t~ck I I m L i V I t a n k I k ~ J I c 6 U e c t o r _ I I I I

I I - Ash convevor Air Wo%er "

)on

Fig. i. Stoic two-stage gasifier /University of Minne- sota, Heating Plant Duluth Campus/

ing from 150-300 ~ produce gas and tar in the devolatil-

ization zone. Tar /5/ is removed from the top gas /7/ in

a hot electrostatic precipitator /ESP/ and is stored for

future use as a boiler fuel ]6]. Bottom gas /9/ is produced

in a 300-600 ~ combustion zone fed with air and steam.

The bottom gas is contaminated with particles or fines /4/

which are removed by a hot cyclone. The noncombustible,

highly refractory bottom ash /2/ is collected in a water

pan /3/ at the bottom of the gasifier. Cleaned top and

bottom gases /8, iO/ as well as a combined gas /ii/ were

sampled. Stack gas samples were taken on the main stack

/12/ while product gas was being burned and on the stub

stack /13/ when ESP tar was being used as an auxiliary

fuel. A more detailed description of the gasifier operation

and the sampling regimen is available in a report 7.

Materials and equipment

209po t r a c e r : A lO0-dpm/ml 209po /Mound F a c i l i t y , Mia-

misburg, OH/ solution was prepared in 2M HNO 3. Air evap-

273

ROSEBERRY, SCOTT: 210po IN COAL GASIFICATION SAMPLES

orated plates were examined by ~--spectroscopy for a puri-

ty check.

Trilaurylamine /TLA/: Trialaurlyamine was purchased from

Eastman Kodak Co., Rochester, NY 14650. A solution, 0.5%

by volume in xylene, was purified by extracting with con-

centrated HCl and then washing with water.

Deposition cell: A threaded Teflon cylinder secured a

silver deposition disk between a Teflon "0" ring and a

brass base. The brass served as a heat sink for maintain-

ing the elevated deposition temperature. The Teflon cell

had a hole in the bottom for exposure of the silver disk

to sample solution. The solution was stirred with a stir-

ring bar.

Counting system: A Nuclear Data 6603 multichannel analyzer

and silicon surface barrier detector were used for ~-spectro-

metry.

Experiment procedure

A method 8 established for use in the determination of

e-emitting nuclides in soils, air dusts, ashed vegetation

and soft tissue was modified to determine 21Opo in coal

gasifier wastes. Total sample dissolution with bulk re-

tention of 21Opo was attained in this procedure by care-

fully controlling the sample-to-reagent ratios 9, maintain-

ing an oxidizing atmosphere during sample dissolution,

and preparing mounts with great care.

1 to iO g of material and 15 dpm of 209pm tracer were

digested with concentrated HNO 3 on a hot plate until the

reaction subsided. The sample was then digested, in the

presence of concentrated HNO 3, with successive concentrated

acids: HCIO 4, HF, and H2SO 4. The sample was taken to near

dryness between each acid digestion and finally evaporated

to fumes of H2SO 4. The acid-treated sample was heated with

Na2SO 4 /anhydrous/ over a Meeker burner to form a pyrosul-

2~


fate melt. The hot fusion cake was dissolved in 2M HCI

and the polonium extracted into 0.5% TLA in xylene. The

organic layer was washed sequentially with 2M HCI, 2M

H2SO4, and 0.7M HCI-O,3% peroxide. Polonium was stripped

from the TLA with 5% HCIO 4. Nitric acid, H2SO 4, and KHSO 4

were added to the HCI04 strip solution which was then heated

to fumes of perchloric acid and a final pyrosulfate fusion.

Polonium was spontaneously deposited from O.IM HCI onto

cleaned Ag disks with stirring at 70 ~ for 90 min. e-

activities of 210po and 209po were measured on a multi-

channel analyzer using silicon surface barrier detectors.

Hydrogen fluoride was not used in the digestion of tars

since they contained relatively little refractory solids.

Water samples were evaporated to near dryness in the presence

of HNO 3, H2SO 4, and anhydrous Na2SO 4, fused with pyrosul-

fate, extracted with TLA as described above and deposited.

The lowest reported concentration was 0.3 Bq/kg for a i0 g

sample. Yields were determined with 209po and ranged from

65 to 93%. Precision of the method as determined on coal

was +6% at the 95% confidence level.

RESULTS AND DISCUSSION

Samples of EPA coal were analyzed for 21Opo using this

procedure. Table 1 gives our data and data previously re-

ported I for the U.S. Environmental Protection Agency's

Eastern Environmental Radiation Facility /EPA-EERF/, Mound

Facility /Mound/, and Los Alamos Scientific Laboratory

/LASL/ as part of an interlaboratory comparison. Con-

centrations of 21Opo in coal agree well as measured by

all laboratories.

University of Minnesota at Duluth /UMD/ gasifier samples

were also analyzed for 210po. Table 2 gives the data for

275


TABLE 1

Concentration of 21Opo in EPA coal x

Laboratory 21Opo, Bq/kg

ORNL 28 + 1

EPA-EERF 26 + 7

Mound 27 + 1

LASL 36 +i0

XEPA sample No. 137-ORP-CF/m-4. Non-ORNL data is taken from Ref. i.

indicated sample points. Numbers on the sample types are

the dates on which the samples were collected. The precision

of the method is very good as shown by the replicate de-

terminations on feed coal and cyclone fines. It is apparent

from the data that in the UMD gasifier 210po is volatilized

and reprecipitated O n the fine particles collected in the

hot bottom gas stream. Additional trace element data 7 show

that As, Sb, Ga, and Se, all relatively volatile elements,

are preferentially concentrated in the cyclone fines. The

wide variation in the tar data reflects the great heteroge-

neity noted in this material for all measurements. Impingers

filled with nitric acid solutions were used in the sampling

trains to collect trace elements from gaseous streams.

Little if any 21Opo was found in the impinger solutions

from the gas or stack sampling.

CONCLUS IONS

The described procedure is useful for the analysis of

21Opo in highly organic and refractory materials. The

pyrosulfate fusion technique accomplishes total sample

276


TABLE 2

Concentration of 21Opo in UMD gasifier samples

Sample point Sample type 210po Bq/kg from Fig. i.

1

1

1

1

2

3

4

4

4

4

5

5

6

6

7

8

9

iO

ii

12

13

2-20 Feed coal 14 + 2

2-20 Feed coal 14 + 2

2-20 Feed coal 13 + 3

2-20 Feed coal 12 + 2

2-20 Bottom ash 19 + 1 -3

2-27 Ash pan water 9 ~ 4xlO

2-20 Cyclone fines 815 +37

2-20 Cyclone fines 814 ~39

3-26 Cyclone fines 667 ~37

3-31 Cyclone fines 467 +48

2-24 ESP tar 23.3+0.5

3-6 ESP tar 13.3+0.8

3-3 Tar to boiler 30.7+0.6

3-5 Tar to boiler 4.1+0.4

2-26 Dirty top gas <0.3

2-26 Clean top gas <0.3

3-6 Dirty bottom gas <0.3

3-6 Clean bottom gas <0.3

2-24 Combined gas 0.25+0.04

2-24 Main stack <0.02

3-2 Stub stack <0.02

dissolution and isotopic exchange between the 209po tracer

and indigenous 21Opo of the sample; thus accurate chemical

recoveries and activity determinations are provided.

277


Data has been presented which shows that in a Foster-

Wheeler/Stoic two-stage gasifier, 21Opo is preferential-

ly concentrated in the fines separated from the 300-600 ~

combustion zone gas product.

REFERENCES

1. C.E. Styron, C.T. Bishop, V.R. Casella, P.H. Jenkins, W.H. Yankee, Mound Facility Report MLM-2810, Miamis- bury, OH, 1981.

2. H.L. Beck, K.M. Miller, IEEE Transactions on Nuclear Science, Vol. NS-27. /1980/ 689.

3. Proceedings of the Workshop on Radioactivity Associated with Coal Use, September 15-17, 1981, Santa Fe, NM. Published as Los Alamos National Laboratory Report LA-9106-C, Los Alamos, NM, 1981.

4. J.P. McBride, R.E. Moore, J.P. Witherspoon, R.E. Blan- co, Oak Ridge National Laboratory Report ORNL-5315, Oak Ridge, TN, 1977.

5. J.N. Stanmard, G.W. Casarett, editors, Radiation Re- search, Supplement 5, Academic Press, New York, 1964.

6. B.B. Moroz, YuD. Parfenov, Atomizdat /Moscow/, 1971. Available in English as U.S? Atomic Energy Commission Report AEC-TR-73OO, National Technical Information Service, Springfield, VA, 1972.

7. W.H. Griest, B.R. Clark, Oak Ridge National Laboratory Report ORNL/TM-8427, Oak Ridge, TN, 1981.

8. C.W. Sill, C.P. Willis, Anal. Chem., 51 /1979/ 1307.

9. C.W. Sill, NBS Special Publication 422, Vol. 1 /1976/ 463.

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radiochemical analysis of210po in coal gasification samples

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