Chlorine substitution reactions of polycyclic aromatic hydrocarbons on fly ash from coal-fired power plants

Chemosphere, Vol.21, Nos.l-2, Printed in Great Britain

pp 35-41,

1990

0045-6535/90 $3.00 + .00 Pergamon Press plc

CHLORINE SUBSTITUTION REACTIONS OF POLYCYCLIC AROMATIC HYDROCARBONS ON FLY ASH FROM COAL-FIRED POWER PLANTS G.A. Eiceman,

R.V. Hoffman, M.C. Collins, Y-T. Long, and M-Q. Lu Department of Chemistry New Mexico State University Las Cruces, NM 88003-0003 ABSTRACT

Naphthalene, anthracene, and biphenyl were individually adsorbed on fly ash from a coal-fired power plant and treated with HCI(g) in nitrogen at 150°C. Products from the reaction included mono- and poly-chlorinated cogeners of the parent PAH at total yields of ca. 9-15 % for all products. Brominated aromatic products, observed in identical studies using municipal incinerator fly ash, were not detected in significant amounts. The results suggest that the absence of chlorinated compounds in coal combustion effluent can not be attributed to chemical properties of fly ash surfaces involved in heterogeneous gas-solid phase reactions. Alternate explanations should be sought in the low levels of HCl in the effluent stream or the chemistry of the combustion event.

INTRODUCTION Complex mixtures of organic compounds can be found at ng/g to ug/g levels in the solid particulate that is emitted from combustion both of municipal refuse particulate

(1-3)

and of coal fuels

(4,5).

Solvent extracts of solid

(fly ash) generated from these two types of fuel sources

contain many of the same compound classes and these include polycylic aromatic hydrocarbons

(PAHs) and aliphatic hydrocarbons.

Generally,

fly

ash originating from coal combustion contains only low levels of halogenated aliphatic or aromatic organic compounds whereas halogenated products are found in significant amounts on fly ash from municipal incinerators.

A notable specific instance which portended later broad

conclusions was the reported absence of chlorinated dioxins

35

in coal fly

36

ash

(6).

precursors

The reasons for this discrepancy might involve:

a) unsuitable

in the pyrolysis zone where synthesis presumably occurs, b) low

amounts of HCI in the vapor stream where some heterogeneous gas-solid phase reactions may occur, compounds.

and c) selective destruction or loss of such

Hites and Czuczwa have exploited this observation in source

apportionment studies

(7) and Griffen has attempted to rationalize PCDD

formation in view of differences in feedstock chemistry Since the vapor levels of HCl(g)

(8).

effluent streams from coal combustion

are low relative to that for municipal

incinerators and had been previously

implicated in formation of chlorinated dioxins

(8), this factor appeared to

be a reasonable starting point for experimentation

.

Heterogeneous phase

gas-solid reactions of aromatic compounds adsorbed on fly ash have been shown to give facile production of chlorinated PAHs when treated with HCI (9).

For example,

when naphthalene,

HCl diluted in nitrogen,

adsorbed onto fly ash, was exposed to

rapid conversion to ten mono- and poly-chlorinated

products occurred at yields totaling ca. 10-12%.

Conceivably,

PAHs present

on fly ash from coal combustion could undergo similar reactions when exposed to a realistic source of chlorine. PAH products on coal-generated municipal

Failure to generate chlorinated

fly ash comparable to those found using

incinerator fly ash may provide insights into heterogeneous phase

reaction mechanisms which are important in both cases. The objective of this work was to identify chlorinated products

from

HCI treatment of PAHs on fly ash from coal combustion and to determine the approximate yields as a first measure of a source dependence,

if any, on

reactivity of fly ash.

EXPERIMENTAL

Fly Ash and Procedures Fly ash was obtained from a power utility in northwestern New Mexico and sieved manually to recover the 80-100 mesh portion.

A 25 gram portion

37

of this fly ash was extracted with 200 mL of a 50:50 mixture of acetone:hexane

in a Soxhlet extraction apparatus for 12 hours.

The extract

was condensed in a rotary evaporator to 2 mL and evaporated to 0.i mL with a stream of nitrogen gas and analyzed by gas chromatography. Procedures and apparatus to treat fly ash containing PAHs with HCl

(g)

were adapted from methods developed in comparable studies with dioxin and PAN compounds

(9).

Briefly,

parent compound was vapor deposited on fly ash

and exposed to HCI in nitrogen at 150bC.

Later,

the fly ash was extracted

with solvent to recover both starting material and reaction products. Instrumentation Extracts and product mixtures were screened using a Hewlett-Packard Model 5880A GC (Level III) equipped with a DB-5,

15 m fused silica column

for resolution of cogeners of substitution products and a 3 m X 2 mm ID borosilicate column containing 10% Bentone-34 separation of substitutional conditions

isomers within an aromatic ring system.

for analysis with the capillary column were:

temperature, 250°C;

on Chromosorb W for

80°C; temperature program rate,

injection temperature,

Analysis conditions

2°C/min;

final temperature, 250°C.

initial temperature,

final temperature,

150°C; and detector temperature,

Identifications

initial

250°C; and detector temperature,

for the packed column were:

temperature program rate, temperature,

5°C/min;

The

125°C;

50°C;

injection

150°C.

of product mixtures were made using scanning GC/MS

analysis under identical chromatographic

conditions.

The GC/MS was a HP

5995A GC/MS with splitless injector and jet separator.

RESULTS AND DISCUSSION The solvent extract of the coal fly ash used in this study was analyzed by GC and GC/MS and exhibited remarkably clean composition compared to solvent extracts of fly ashes from municipal and from coal-fired power plants

(4,5).

incinerators

(1-3)

The GC-FID trace of the solvent

38

extract showed no constituents over I-i0 ng/g.

Polycylic aromatic

hydrocarbons were detected via GC/MS analysis with selected ion monitoring and were present at less than i-i0 ng/kg levels.

Chlorinated organic

compounds were not observed but extracts were not subjected to extensive pre-enrichment and isolation procedures to yield improved detection limits. This fly ash was considered a suitable substrate due to the absence of natively abundant interferences even though ash samples are cleaned by solvent extraction before the start of laboratory investigations. Chlorination of PAH on Fly Ash from Coal Fired Power Plant Naphthalene, biphenyl, and anthracene, which were adsorbed on the surface of fly ash from a coal-fired power plant and treated with with gaseous HCl, yielded products from chlorine substitution reactions.

Yields

for cogeners and for isomer distributions when identifications were possible are shown in Table 1 for each PAH.

The yields of recovered parent

or chlorinated product, when corrected for irreversable losses

(vida

infra), followed: a. Naphthalene > Chloronaphthalene > Dichloronaphthalenes b. Biphenyl > Chlorobiphenyl > Dichlorobiphenyl c. Chloroanthracene > C12 anthracenes > Cl 3 anthracenes > Cl 4 anthracenes > anthracene Traces of brominated PAHs, observed only with naphthalene, were due presumably to halogen exchange between a reactive site on the fly ash surface and natively abundant bromide.

This had been a prominent feature

in previous studies with naphthalene on fly ash from municipal incinerators (9) in which bromine-chlorine exchange was postulated for a reactive site based on iron chloride

(I0).

Externally added bromine in the form of HBr

has also been shown to produce brominated aromatics

(ii).

The absence of

brominated aromatic compounds here may be indirect evidence that only low amounts of bromide in suitable reactive sites existed on the surface of

39

T a b l e i. C h l o r i n a t i o n P r o d u c t s from T r e a t m e n t of I n d i v i d u a l P o l y c y c l i c A r o m a t i c H y d r o c a r b o n s on Fly Ash from Coal C o m b u s t i o n u s i n g HCl(g) at 150°C for 20 min. PERCENT YIELD a Reaction Products Parent Compound Monochloro Substitution Site

Naphthalene 92.8 i- 8.0 2- 0.6

Biphenyl 52.1

Anthracene 41.9

2- 2.3 3- 0 4- 5.4

i- 0 2- 0 9- 42

Dichloro

4

1.5

9,10- 19.8

Trichloro

0

0

2.3

Tetrachloro

0

0

1.8

0.027

0

0

425 mL

350 m L

Bromo V o l u m e of HCI

150 m L

a c o r r e c t e d for losses due to a d s o r p t i o n or o t h e r m e a n s as d e t e r m i n e d by r e c o v e r y studies. E x p e r i m e n t a l error r a n g e d from 0.5 to 6 % r e l a t i v e s t a n d a r d d e v i a t i o n s on t r i p l i c a t e experiments.

this fly ash sample. The overall y i e l d s shown in T a b l e 1 w e r e c o m p a r a b l e to p r e v i o u s f i n d i n g s for the c h l o r i n a t i o n of n a p h t h a l e n e on fly ash from m u n i c i p a l incinerators

(9) and the r e l a t i v e r e a c t i v i t y of this coal fly ash v e r s u s

fly ash from m u n i c i p a l

i n c i n e r a t o r s was:

C h i c a g o > Coal > T o r o n t o > H a m i l t o n and s u g g e s t e d t h a t this c o a l - b a s e d fly ash had a r e l a t i v e l y a c t i v e surface for h e t e r o g e n e o u s g a s - s o l i d p h a s e reactions. The s u b s t i t u t i o n p a t t e r n in c h l o r i n a t e d p r o d u c t s 2 - c h l o r o isomers)

(e.g l - c h l o r o versus

was c o n s i s t e n t w i t h an e l e c t r o p h i l i c s u b s t i t u t i o n process

in w h i c h HCl was not the d i r e c t c h l o r i n a t i n g agent.

The exact m e c h a n i s m by

w h i c h a r o m a t i c c h l o r i n a t i o n occurs h e r e is unknown.

However,

the p a r a l l e l s

b e t w e e n the r e s u l t s r e p o r t e d h e r e and t h o s e found w i t h fly ash from municipal

i n c i n e r a t o r s s u g g e s t that s i m i l a r m e c h a n i s m s are operative.

40

Table 2. R e c o v e r y of P o l y c y l i c A r o m a t i c H y d r o c a r b o n s F o l l o w i n g T r e a t m e n t at 150°C for 20 min.

from Coal Fly Ash

PERCENT RECOVERY a Naphthalene

6.2 + 1.8

Biphenyl

28.8 + 0.2

Anthracene

16.2 + 0.6

l-Chloro-

43".6 + 2.2

2 - C h l o r o - 44.2 + 5.5

l-Chloro-

43.1 + 4.1

2-Chloro-

52.0 + 5.0

4 - C h l o r o - 83.4 + 6.0

2-Chloro9,10-CI 2-

44.3 + 1.5 60.1 + 1.3

I r r e v e r s a b l e L o s s e s of PAHs to Fly Ash In p r e v i o u s (12-14),

i n v e s t i g a t i o n s on h e t e r o g e n e o u s g a s - s o l i d p h a s e r e a c t i o n s

i r r e v e r s a b l e losses of c o m p o u n d s b a s e d on the d i o x i n s t r u c t u r e to

the m u n i c i p a l

i n c i n e r a t o r fly ash s u r f a c e s o c c u r r e d and was a t t r i b u t e d to

h i g h l y p o l a r o x y g e n atoms.

I r r e v e r s a b l e a d s o r p t i v e b e h a v i o r for PAH was

also o b s e r v e d w i t h c o a l - g e n e r a t e d

fly ash as shown in T a b l e 2 and mass was

lost p r e s u m a b l y to surface reactions.

An a l t e r n a t i v e explanation,

compound

b r e a k t h r o u g h on the fly ash bed, was not e v i d e n t from a n a l y s i s of cold traps l o c a t e d d o w n s t r e a m from the fly ash. The o n l y t r e n d o b v i o u s from t h e s e f i n d i n g s was t h a t c h l o r i n a t e d d e r i v a t i v e s w e r e not as e a s i l y a d s o r b e d or r e a c t e d on the fly ash as were the p a r e n t PAH compound. anthracene,

The r e l a t i v e l y p l a n a r PAH, n a p h t h a l e n e and

w e r e lost to surface r e a c t i o n s m o r e t h a n b i p h e n y l w h i c h can

u n d e r g o r o t a t i o n s at the b o n d t h r o u g h w h i c h the rings are joined. Significance

for C o m b u s t i o n S t r e a m s

The p r i n c i p a l

s i g n i f i c a n c e of t h e s e f i n d i n g s is t h a t g a s - s o l i d phase

r e a c t i o n s in the p o s t c o m b u s t i o n s t r e a m from coal c o m b u s t i o n are not f u n d a m e n t a l l y c o n s t r a i n e d by the surface c h e m i s t r y of r e l a t e d fly ashes. Thus,

the a b s e n c e or v e r y low levels of c h l o r i n a t e d a r o m a t i c h y d r o c a r b o n

and d i o x i n s

in w a s t e streams from coal c o m b u s t i o n may be due m o r e to the

v a p o r c o n c e n t r a t i o n s of HC1, the a b s e n c e of s u i t a b l e substrates, differences

or

in o t h e r m e c h a n i s m s r e q u i r e d in overall de novo synthesis.

41

ACKNOWLEDGEMENTS This research was funded by the National Science Foundation through grant ATM-8712088.

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