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The thermal emission of chlorine-containing compounds from coal following brine and chlorine gas treatments
The thermal emission of chlorine-containing compounds from coal following brine and chlorine gas treatments Geofrey William
Fynes, Alan A. Herod, R. Ladner
N. John
Hodges,
Brian J. Stokes
Coal Research Establishment, British Coal, Stoke Orchard, Cheltenham, GL52 4R.Z UK (Received 9 April 1987; revised 7 July 1987)
and
Gloucester,
Pyrolysis/mass spectrometry was carried out on a high-chlorine coal and on this coal after being washed free of chloride and treated with chlorine gas and with sodium chloride solution. For comparison a similar examination was undertaken on model organo-chlorine compounds. The results confirm that neither organo-chlorine structures nor base hydrochlorides are present to any significant extent in this coal, which is considered typical, in this respect, of UK bituminous coals. The hydrogen chloride generated during mild pyrolysis is produced from brine-derived chloride. Tar formation is suppressed following chlorination with chlorine gas but enhanced by sodium chloride solution. The chlorine content, as chloride ion, of seven coals was removed quantitatively by a rigorous sequence of aqueous extraction and wet grinding. (Keywords: mineral matter in coal; analysis; chlorine)
present in UK bituminous coals, sometimes at levels up to z 1 wt % air-dried (ad) is generally assumed to be inorganic, originating from penetration of coal measures by sodium chloride in ground waters after the coal was formed’. This assumption is based upon evidence that practically all of the chlorine can be removed as chloride by washing with water when coals are finely ground. This is true for all the coals studied in the present work, including a US coal where the presence of organic chlorine had been previously claimed* on the basis that not all the chlorine could be removed by water washing. However, this apparent evidence for organic chlorine has recently been rescinded3 following more thorough washing methods. A method of estimating organically bound chlorine in coal through HCl formation has been questioned4 for UK coals because the source of HCl could not be distinguished. A large proportion (between 37 and 63 wt %) of the chlorine can be removed from coal by mild heat treatments at 300°C over many hours and recent mass spectrometric studies ‘-’ show that during mild pyrolysis (temperatures <3OO”C), the chlorine is volatilized as HCl : no organo-chlorine compounds being detected. However, the origins and mode of formation of the HCl remain open to speculation. Previous studies’-’ ’ have been made in which coal was chlorinated using chlorine gas and aqueous treatment, but not ionic chloride treatment; in each case, tar formation on pyrolysis was suppressed and all the added chlorine was emitted as HCl at the highest temperatures. Only one study9 claims to have detected an organochlorine compound - hexachlorobenzene identified by melting point. The effect of variable water content of coals, by drying or saturating with water, has been investigated’; in general terms, increased water content leads to increased emission of organics through swelling of the coal structure but HCl emission decreases, probably through The chlorine
0016-2361/88/060822-09f3.00 0 1988 Butterworth & Co. (Publishers) Ltd.
822
FUEL, 1988, Vol 67, June
elution of chloride from the surface. Given” considers that chloride could exist in coal in the amine hydrochloride form on the basis that there is more basic nitrogen than chloride in coal; no evidence has been presented to show that basic nitrogen hydrochlorides occur in coal, are washed out of coal or that the chloride in such salts, bound to the coal matrix, would readily be leached out by water washing. The work described here was undertaken in an attempt to gain further understanding of the sources of hydrogen chloride together with the mechanism of its low temperature release. Results are presented for aqueous extraction and sequential grinding of six UK coals (both vitrain enriched and run-of-mine*) and a US coal previously reported’ as containing chlorine not accessible to water extraction. One of the UK coals, containing the most readily extracted chlorine, was selected for the subsequent experiments on dechlorination followed by chlorination with NaCl or chlorine gas, since it is considered in all other respects to be representative of such coals. The modified coals were heated in the mass spectrometer and chlorine-containing products were analysed. The mechanisms of thermal breakdown of the coal/chlorine systems are considered in relation to the thermal breakdown of chlorine-containing model compounds. EXPERIMENTAL Materials
The typical high-chlorine bitiminous coal used was Cadley Hill, Main seam, vitrain-enriched with a chlorine content of 0.58 wt Y0 on an ad basis. The coal was crushed and sieved to -212pm, and its analysis *Coal produced by mining operations before preparation. Such coal has unlimited topsize and a mineral matter content which may exceed 10%
Thermal emission
of chlorine-containing
compounds
from coal: G. Fynes et al.
Table 1 Analyses of coals Ultimate analysis (wt %) Proximate analysis (wt %)
Coal mine and seam
NCB coal rank code
C
HO
Maceral analysis (~01%)
ad
daf
N
S total
Cl
CO,
ad Moisture
db Ash
daf VM
Vitrinite
Exinite
Inertinite
Coal shale/ pyrite
Savile Flockton Thick
ROM” b VIT’ 700
79.4 81.4
5.7 5.4
10.3 8.3
1.90 1.85
2.40 2.65
0.36 0.43
0.20 0.30
3.4 4.1
33.1 7.1
39.0 37.9
56 70
8 10
17 17
19 2
Markham Main Bamsley/Dunsil
ROM VIT
b 800
79.3 80.8
5.5 5.2
11.9 9.7
1.85 1.90
2.00 1.65
0.45 0.60
0.45 0.70
6.6 8.0
29.0 2.8
38.6 36.4
60 83
5 3
25 12
10 1
Cadley Hill Main
ROM VIT
800 800
79.0 79.3
5.4 5.4
11.6 11.3
1.75 1.70
1.85 1.70
0.54 0.58
0.50 2.20
10.1 10.0
6.8 6.8
41.6 43.9
73 76
9 7
16 15
2 2
Hem Heath Yard/Ragman
ROM VIT
b 600
83.3 84.3
5.5 5.3
7.8 6.5
1.80 1.80
1.35 1.25
0.53 0.62
0.55 0.80
2.6 2.6
20.6 4.3
35.1 34.7
61 67
5 7
24 24
11 1
Florence Great Row
ROM VIT
b 700
78.9 80.8
5.6 5.6
11.2 9.6
1.50 1.50
2.70 2.30
0.63 0.67
0.60 0.70
4.9 4.7
17.7 5.6
41.4 41.7
74 84
6 6
11 8
8 2
Lea Hall Shallow
ROM VIT
800 800
80.9 80.3
5.3 5.4
10.2 10.7
1.70 1.70
1.50 2.00
0.83 0.90
0.80 0.60
10.3 10.0
5.5 5.8
39.6 40.0
71 83
6 5
22 12
1 1
85.1
5.6
9.5
1.65
3.04
0.50
0.63
4.3
11.5
40.5
n.d
n.d
n.d.
n.d.
Herrin No. 6
a ROM = run-of-mine ‘Ash yield too high for coal rank code ’ VIT = vitrain-enriched dn.d. = not determined
is given in Table 1. Vitrain enrichment was achieved by selection of bright coal lumps from the run-of-mine coal since chlorine is known to be associated13 with the vitrinite maceral group. The other five UK coals and the US coal are listed with their analyses in Table 1; all were initially crushed and sieved to -2 12 pm. Gases and chemicals were of industrial, laboratory or A.R. grades and used as received. PVC powder (MW 100 000) and N- 1-naphthyl ethylene diamine dihydrochloride were chosen as model organo-chlorine and base hydrochloride compounds, respectively.
chloride concentration was determined by titration against 0.02 M silver nitrate solution. Grinding and extractive cycles were repeated until no chloride ions could be titrated in the leachate. This required between one (for Cadley Hill) and four cycles (for Hem Heath and Florence coals). The summation of extractable chloride plus chlorine in the final residue enabled a chlorine balance to be obtained and the extent of chlorine removal to be determined within the limits of accuracy of the analytical techniques, about 1 ppm in the leachates and about 0.02 wt % Cl in the solids.
Analyses
Dechlorination
The chlorine content of the various coal samples, before and after treatment, were determined by the British Standard high temperature combustion method. Moisture contents were determined by drying in a minimum free-space oven.
Coal (30g) was weighed into a 500ml flask with 250 ml of distilled water. The suspension was refluxed for 4 h and filtered hot. The extraction was repeated six times, after which no opalescence was observed in the filtrate on acidification with nitric acid and addition of silver nitrate. The washed coal was dried in a vacuum at 105°C for 1 h, cooled and equilibrated in air (18 h) before analysing for chlorine and moisture content.
Wet grinding and aqueous
extraction
The coal samples were ground wet (to reduce agglomeration) in a high-speed ball mill using tungsten carbide balls (11 mm diameter) and container (40 mm x 80 mm length). Coal (4 g) and distilled water (4 ml) were ground for 2 h. A further 3 ml of water was added and the mixture was milled for 5 min, to aid transfer of the sample from the mill to a flask. This procedure was based on tests that showed that a run-ofmine coal sample ( < 212 pm) was reduced to 96 wt % less than 25 pm with a median particle size of 6 pm, under these conditions. The suspension from milling was washed into a 500 ml flask, made up to 200 ml with distilled water and refluxed for 2 h. The dilute suspension was filtered on a 0.8 pm cellulose filter paper and the cake was washed with cold water, dried in vacuum at 100°C for 1 h, cooled in a dessicator and equilibrated in air for 18 h. The combined filtrate and washings were made up to 5OOml and the
Chlorination Chlorine
by water extraction
of water extracted gas.
coal
The
apparatus used is shown schematically in Figure 1. Washed coal (4g) was positioned between two glass wool plugs in a 300 mm x 8 mm i.d. stainless steel reactor tube held in a furnace at the desired reaction temperature. The system was purged with nitrogen (15ml min-‘) before the introduction of chlorine gas (15 ml min-‘) into the nitrogen stream. After 1 h the chlorine gas flow was stopped, and the equipment allowed to cool overnight under nitrogen flow. Initial bed temperatures of 100°C and 300°C were used, which rose to 112°C and 318”C, respectively, during chlorination. The chlorinated coals were washed with x2.5 1 of boiling distilled water to remove occluded HCl and then air-dried prior to mass spectrometry.
FUEL, 1988, Vol 67, June
823
Thermal emission
of chlorine-containing
compounds
from coal: G. Fynes
Sodium chloride. Preliminary experiments showed that vacuum impregnation of the washed coal was the most efficient way of introducing sodium chloride: 6 g of coal were heated at 95°C for 3 h in the flask of a rotary evaporator under a vacuum of z 3.3 kPa. The flask and contents were then cooled when 150 ml of a 1.65 wt % sodium chloride solution (1 wt % as Cl -) was introduced and left to stand overnight. The impregnated coal was filtered off, washed with 2 11 of 1.65% sodium chloride solution and allowed to air dry. Mass spectrometrjl
The apparatus and method were essentially as described previously6z7 except that the reactor tube was of glass-lined stainless steel (size 6.35 mm o.d., 4mm i.d. x 170 mm) and heat treatment times were z 3.5 h for the coal samples and I h for the model compounds (both the latter samples softened and blocked the reactor tube before the reactor reached maximum temperature). The temperature programme used was from 50 to 300°C at 2°C min-‘, the upper temperature being held until data collection was terminated. Volatiles were transferred from the coal by a helium gas stream through a glass jet separator into the ion source of the mass spectrometer; the coal sample acted as the column packing in packed
column g.c.-m.s. with the column bleed being examined by the mass spectrometer. RESULTS AND DISCUSSION Sequential
grinding and aqueous extraction
The results of chloride extraction are given in Table 2 and chloride balances are accurate to 100% within the accumulated errors of 2 or 3 wt %. The chlorine in these coals can be totally removed by water extraction, within the limits of error of the chloride determination methods. The number of extraction cycles necessary to remove the chloride varied for the different coals and depends on accessibility of water to the porosity of the coal; the experimental details in support of this will be published in due course. Cadley Hill vitrain-enriched coal is the most readily extracted of the six UK coals investigated and was selected for the rechlorination experiments for that reason. Exhaustiue Tuble Table 3
extraction
3 shows Analyses
Treatment
Chlorine gas
Nitrdgen gas
Figure 1 Table 2
Schematic Extraction
of chlorination
apparatus
data from sequential
using chlorine
grindrng
and aqueous
gas
et al
and rechlorination
the chlorine
of treated
Cadley
Chlorine ___~
content
and
levels
Hill coal samples
Moisture (wt I’” ad) (wt & db) (wt “1,ad) _
No treatment (as received) 0.58 Dechlorinated by water washing 0.03 Dechlorinated by water washing, rechlorinated using chlorine gas at 100°C and washed 9.8 Dechlorinated by water washmg, rechlorinated using chlorine gas at 300°C and washed 14.2 Dechlorinated by water washmg, rechlorinated using 1 wt % aqueous Cl _ solution (vacuum) 0.62
moisture
Weight used (g)
Weight loss (““)
0.64
10.0
1.53
13.7
0.03
5.9
1.34
8.3
10.2
4.2
0.51
18.2
14.8
4.0
0.62
7.7
6.9
1.38
8.2
0.67
extractions
Coal chlorine content (wt “Gad) _.
Number of grindings/ aqueous extractions
Chlorme extracted from coal (wt “s;,ad)
Chlorine” in washed coal (wt y$,ad)
Extent of chlorine removal (wt %)
Chlorine balance (wt O6)
ROM VIT
0.36 0.43
2 3
0.36 0.43
< 0.02 < 0.02
100 100
100 100
ROM VIT
0.45 0.60
2 3
0.45 0.61
< 0.02 < 0.02
100 102
100 102
Hill
ROM VIT
0.54 0.58
I 1
0.56 0.59
< 0.02 < 0.02
103 102
103 102
Hem Heath
ROM VIT
0.53 0.62
4 4
0.5 1 0.60
0.02 < 0.02
96 97
100 97
Florence
ROM VIT
0.63 0.67
4 4
0.60 0.68
0.03 < 0.02
95 101
100 101
Lea Hall
ROM VIT
0.83 0.90
2 2
0.85 0.88
< 0.02 < 0.02
102 98
102 98
0.50
3
0.50
0.03
101
103
Coal Saville Markham Cadley
Herrin
Main
No. 6
“The chiorine
824
content
of the washed
co&
FUEL, 1988, Vol 67, June
was corrected
for a system blank
of 0.02 wt “, ad
Thermal emission of chlorine-containing
a
compounds
from coal: G. Fynes et al.
44
100 90 80 70 60
18
!
50 40 30 20 10 0
l”“““‘l”“““‘1
50
b
100
200
150
250
300
44
100 90 80 70 60
r
50
18
40 30
28
20 10 0 11.
*
C 100 90 80 70 60 50 40 30 20 10 0 150
Mass Figure 2 Mass spectra of volatiles emitted at 300°C from Cadley impregnation with NaCI. HCI peaks are marked *
ml2
Hill vitrain
determined for Cadley Hill coal after exhaustive extraction with water and for the samples chlorinated with chlorine gas and impregnated with sodium chloride. This table also gives weights of sample used in the mass spectrometric experiments and the resultant weight losses. In each case, the weight loss exceeds the water content of the coal sample indicating a significant loss of organic and inorganic material in the mild pyrolysis experiment since it has been shown earlier5 that not all of the water in coal is removed at 300°C.
enriched
coal: a, as received;
b, after exhaustive
washing
and c, after
Muss spectrometrq Mass spectra of the volatiles from Cadley Hill coal (as received and its variously treated samples) on attaining 3OO”C, are shown in Figures 2-4. The mass spectra in Figure 2 of the coal, as received, exhaustively washed and impregnated with NaCl show that the major components detected at 300°C are CO,, H,O, alkane fragment ions and alkyl naphthalenes with alkenes or cycloalkanes, and other aromatics as minor alkyl benzenes components; HCl peaks are clearly evident in the
FUEL, 1988, Vol 67, June
825
Thermal emission of chlorine-containing
compounds
from coal: G. Fynes et al.
a 100
36*
90 80 *
706050-
50
30-
28 1 II,,,,,
* * 11; 47 82 .N + 11 Y + 165 ,11,,,,,,,,1,1111111,111~11111~111111111~111111111 50
100
150
* 250
28+3
II1IlIII
300
m/z
Mass spectra of volatiles emitted at 300°C after chlorination at: a, 100°C; b, 300°C with chlorine gas. Chlorine containing peaks are marked *
spectrum from the coal as received; are just visible in the NaCl impregnated coal spectrum, but cannot be seen in the spectrum of the exhaustively washed coal because their intensities are too small. The mass spectra of Figure 3 are those for coal chlorinated with chlorine gas and HCl peaks are most prominent with C02, H,O and SO, as minor components. Figures 4a and b repeat Figures 3a and b but are limited to ions > m/z 50 to reveal more clearly the presence of organo-chlorine species and the patterns of chlorine substitution. A notable feature of the mass spectra following chlorine gas treatment is the suppression of aliphatic and aromatic components; some aliphatic and aromatic peaks are evident in Figure 4a at low masses only. Tar suppression following chlorination with chlorine gas has been observed by others previouslys - “. Mass spectra of the model compounds PVC (obtained at about 105C) and N-1-naphthyl ethylene diamine dihydrochloride (obtained at about 180°C) are shown in Figures 5a and b for comparison; PVC produces organochlorine fragments while the amine hydrochloride produces intense HCl ions at a relatively low temperature. Single ion profiles for HCl emission from the coal samples are shown in Figure 6 and single ion profiles for some chlorinated ions observed for the coal sample after chlorination with chlorine gas are shown in Figures 7 and 8. Table 4 lists maximum ion intensities for the various more important chloro-ions observed together
828
* 200
Mass Figure 3
250
*
20lo-
200
150
100
FUEL,
1988,
Vol67,
June
with their appearance temperatures. Maximum ion intensities of the most predominant organic ion (C,H:) are included in this table for comparison. HCl emission profiles for the washed coal and NaClimpregnated coals are similar to each other, differing mainly in intensity and they are different from the coal as received in terms of an emission at low temperatures; HCl emission profiles following chlorine gas treatment are completely different, as are those for the model compounds. Maximum intensity values for C,H: in Table 3 indicate clearly the suppression of this ion on chlorination at 3OO”C,by a factor of nearly 10 against the coal as received and a factor of nearly 70 against the exhaustively washed coal. The appearance temperatures indicate that with the exception of chlorination at 300°C any treatment of the coal removes the low temperature emission of C,HT. Water washing
Cadley Hill coal is relatively easily leached, and x95 wt % of its chlorine is removed during repeated water extraction. This is reflected in the much reduced maximum ion intensity for HCl as measured during mass spectrometry (Table 4). Interestingly, the appearance temperature of some of the remaining chloride is lowered from z 200 to x 50°C : the single ion profile (Figure 6) for the washed coal shows two peaks as opposed to the single peak for the as received coal. The residual chlorine, which
Thermal emission
of chlorine-containing
compounds
from coal: G. Fynes et al.
a loogo80-
*8
64
78 *
'O6050-
55
*
40-
91 "112
b 100 90 80 70 60 50
30-
82
20-
150
175
200
225
250
275
300
Mass m/z Figure 4 Mass spectra over limited mass range of volatiles emitted at 300°C after chlorination at: a, 100°C; b, 300°C with chlorine gas to show organochlorine fragments and isotopic abundance ratios. Peaks in a, containing chlorine are marked *; all the peaks in b, contain chlorine
is difficult to remove by further leaching, is probably situated at sites inaccessible to water without even finer grinding. The very small amount of chlorine emitted as hydrogen chloride at z 50°C is probably that deposited on the surface sites of the coal during the inevitable equilibration with chloride-containing aqueous leachate. Chlorination
with chlorine gas
Chlorination with chlorine gas after washing the coal to remove inorganic chloride, introduced about 10 wt % chlorine at 100°C and about 15 ‘A at 300°C. During these organic substitution reactions, HCl would be the principal by-product and, although the chlorinated coals were subsequently washed, it is likely that some of the HCl was retained. Thus the maximum ion intensities of HCl are much increased compared with the as received coal (Table 4) and the mass spectra show that the H3’C1 and H3’C1 peaks are the major ions detected (Figures 3a and b). The HCl ion profiles (Figure 6) differ markedly from those for the original coal and for the water washed coal. The majority of this hydrogen chloride would be that occluded to surface sites on the coal but some will result from dehydrochlorination of organo-chlorine compounds. The chloro-organic ions detected (Figures 7 and 8) correspond to simple alkyl or alkene fragments (C, or C,)
and chlorinated benzene and toluene fragments, which are more highly chlorinated at 300°C than at 100°C. Relevant mass spectra are shown in Figures 3 and 4 and it is seen that chlorine isotope patterns are very distinct after chlorination at 300°C. Hexachlorobenzene corresponds to the cluster of peaks in Figure 4b between masses 282 and 290. None of these organo-chlorine ions are detectable in pyrolysis/mass spectrometry of the original Cadley Hill coal. Chlorination
with sodium chloride
Chlorination by vacuum impregnation of pre-washed coal with brine (1 wt % Cl-) resulted in an uptake of 0.62 wt% Cl on an air-dried basis. This closely corresponds to the chlorine content of as received coal, although it is unlikely that complete penetration of the more inaccessible sites by the Cloccurred. Pyrolysis/mass spectrometry of the impregnated coal shows a large increase in HCl emission over that obtained from the washed coal (Table 4). Further, its HCl single ion profile (Figure 6) shows two distinct peaks with early emission peaking at x lOO”C,the major peak occurring at 300°C. This HCl must be derived from the NaCl presumably by degradative hydrolysis. The early (100°C) peak, as with that obtained with the washed coal, probably comes from chloride deposited on easily
FUEL,
1988,
Vol 67, June
827
Thermal emission of chlorine-containing
compounds from coal: G. Fynes et al.
a
63
100 90 80 70
C2H4CI
60 50 40
I
30-
HCI
CHCl2
20-
83
100
36
28 1;
I,,,,,, 20
42
85 97 ““I-II”“““‘I”“““‘1 , ,100
ii, 30
40
50
60
70
80
90
110
100
120
b 100
36
90 80 70 60
Figure 5 containing
I
0
Mass spectra of model compounds: components are marked *
a, PVC at 105°C and b, N-I-naphthyl
I
I
I
I
I
0.5
1 .o
1.5
2.0
2.5
Time
0”
(h)
FUEL, 1988, Vol 67, June
diamine
dihydrochloride
at 180°C; the chlorine
I
Figure6 H3%TIprofiles for coal samples before and after washing chlorination and model compounds; the broken line represents temperature profile from 50 to 300°C
828
ethylene
and the
0.5
1 .o
1.5
2.0
Figure 7 Chlorinated ion profiles after chlorination at 100°C; the broken line represents the temperature 300°C
2.5
with chlorine gas profile from 50 to
Thermal emission of chlorine-containing
accessible surface sites. No organo-chlorine ions were observed in the mass spectrum of the impregnated coal. The emission of hydrocarbons was enhanced by the addition of aqueous chloride, in contrast to the suppression of tar formation by chlorine gas treatment. Model
compounds
N+H3-CH2-CH2-N+H2
”
from coal: G. Fynes et al.
Unfortunately an involatile and structurally more suitable aromatic chlorohydrocarbon (as an alternative to PVC) was not available. The mass spectrum of PVC (Figure 5~) indicates the release of organo-chlorine fragment ions as well as hydrogen chloride even at 105°C. The mass spectra of the coals examined did not show the presence of such fragments thereby indicating that covalently bonded chlorine did not exist in these coals. The mass spectrum of the base hydrochloride (Figure 5b) shows that HCl emission is intense, even at low temperature (180°C). Since, for the coal examined, this kind of behaviour was not observed, except to a limited extent after water washing, it seems unlikely that such a bonding mode is significant in coals.
The model compounds used in this study, polyvinyl chloride (I) and N-1-naphthyl ethylene diamine dihydrochloride (II) were selected on grounds of their structurally differing organo-chlorine types, their low volatility and their ready availability. (-CH2-CHCI-CH2-CHCI-)
compounds
Cl-
Cl-
CONCLUSIONS
0
1 .o
0.5
1.5 Time
ion intensities
As received
2.5
2.0
(h)
Figure 8 Chlorinated ion profiles after chlorination at 300°C; the broken line represents the temperature 300°C
Table 4 Maximum Cadley Hill coal
1. Repeated aqueous extraction and grinding removed, as chloride ion, all the chlorine from six UK coals, both run-of-min and vitrain enriched, and from a US coal, although the ease of removal varied markedly. 2. The exhaustive washing of one of the six coals removed over 95 wt ‘A of the chlorine as chloride ion. The small amount remaining was still detectable by pyrolysis/mass spectrometry as HCl. No organo-chlorine compounds were detected. 3. Chlorination of the coal with chlorine gas introduced covalently bonded chlorine some of which was released as organo-chlorine fragments, as well as hydrogen chloride, on pyrolysis; emission of hydrocarbons was suppressed. 4. Impregnation of the washed coal with sodium chloride followed by pyrolysis/mass spectrometry showed a relatively large release of hydrogen chloride which must have been derived by hydrolysis of the sodium chloride at the coal surface. 5. Results of pyrolysis/mass spectrometry of PVC demonstrate that, if such organo-chlorine structures were present in coal, they would have been detected. 6. Results of pyrolysis/mass spectrometry of N-lnaphthyl ethylene diamine dihydrochloride suggest that base hydrochlorides are not a significant structural feature in coal but their total absence has not been established. 7. Collectively, these results establish that the HCl emitted from coal is produced from brine-derived chlorides and that covalently bonded organo-chlorine structures do not exist in coal.
with chlorine gas profile from 50 to
(MII) and appearance
Exhaustively
Atomic composition
MI1 ( x 10.. “)
;?:,
MI1 (x 10-j)
HCI CCI, C,H,CI C,CI, C,H:
192 n.d. nd. n.d. 267
200 nd. n.d. n.d. 50
33 n.d. n.d. n.d. 2015
temperatures
washed
(AT) (to nearest
5°C) of chlorine
ions detected
in the volatiles from
Cl, IOO”C
NaCl impregnated MI1 (x 10-j)
containing
Cl, 300°C
MI1
MI1
(x 10-3)
(x 10-q -
50 n.d. n.d. n.d. 145
203 n.d. n.d. n.d. 1236
50 nd. n.d. n.d. 90
20350 60 20 n.d. 117
110 115 190 n.d. 100
15 140 2 070 n.d. 359 30
FUEL, 1988, Vol 67, June
50 140 n.d. 225 50
829
Thermal emission of chlorine-containing
compounds
ACKNOWLEDGEMENTS The authors thank British Coal for permission to publish; the views expressed are those of the authors and not necessarily those of British Coal. The authors also wish to thank Dr J. A. Cox, Department of Chemistry, Southern Illinois University, Carbondale, IL 62901, USA, for supplying the sample of Herrin No. 6 coal. REFERENCES 1 2
830
Hodges, N. J., Ladner, W. R. and Martin, T. G. J. Inst. Energy 1983, 56, 158 Cox, J. A., Larsen, A. E. and Carlson, R. H. Fuel 1984,63, 1334
FUEL, 1988, Vol 67, June
from coal: G. Fynes et al. 3 4 5 6
8 9 10 11 12
13
Cox, J. A. and Saari, R. Analyst 1987, 112, 321 Ladner, W. R. Fuel 1984,63,726 Herod. A. A.. Hodges. N. J.. Pritchard. E. and Smith. C. A. Fuel 1983,82, 1331 Herod, A. A. and Smith, C. A. Fuel 1985,64,281 Herod, A. A., Ladner, W. R. and Stokes, B. J. in ‘Advances in Mass Spectrometry 1985’, (Ed. J. F. J. Todd), John Wiley and Sons, Chichester, UK, 1986, part B, 1313 Eccles, A., Kenyon, G. H. and McCulloch, A. Fuel 1931,10,4 Pinchin, F. J. Fuel 1958, 37,293 Macrae, J. C. and Oxtoby, R. Fuel 1965,44,409 Oxtoby, R. Fuel 1966,45,457 Given, P. H. and Yarzab, R. F. ‘Analytical Methods for Coal and Coal Products’, (Ed. C. Karr Jr.), Academic Press, New York, 1978, Vol. 2, 3349 Saunders, K. G. J. Inst. Energy 1980, 53, 109
Fynes, Alan A. Herod, R. Ladner
N. John
Hodges,
Brian J. Stokes
Coal Research Establishment, British Coal, Stoke Orchard, Cheltenham, GL52 4R.Z UK (Received 9 April 1987; revised 7 July 1987)
and
Gloucester,
Pyrolysis/mass spectrometry was carried out on a high-chlorine coal and on this coal after being washed free of chloride and treated with chlorine gas and with sodium chloride solution. For comparison a similar examination was undertaken on model organo-chlorine compounds. The results confirm that neither organo-chlorine structures nor base hydrochlorides are present to any significant extent in this coal, which is considered typical, in this respect, of UK bituminous coals. The hydrogen chloride generated during mild pyrolysis is produced from brine-derived chloride. Tar formation is suppressed following chlorination with chlorine gas but enhanced by sodium chloride solution. The chlorine content, as chloride ion, of seven coals was removed quantitatively by a rigorous sequence of aqueous extraction and wet grinding. (Keywords: mineral matter in coal; analysis; chlorine)
present in UK bituminous coals, sometimes at levels up to z 1 wt % air-dried (ad) is generally assumed to be inorganic, originating from penetration of coal measures by sodium chloride in ground waters after the coal was formed’. This assumption is based upon evidence that practically all of the chlorine can be removed as chloride by washing with water when coals are finely ground. This is true for all the coals studied in the present work, including a US coal where the presence of organic chlorine had been previously claimed* on the basis that not all the chlorine could be removed by water washing. However, this apparent evidence for organic chlorine has recently been rescinded3 following more thorough washing methods. A method of estimating organically bound chlorine in coal through HCl formation has been questioned4 for UK coals because the source of HCl could not be distinguished. A large proportion (between 37 and 63 wt %) of the chlorine can be removed from coal by mild heat treatments at 300°C over many hours and recent mass spectrometric studies ‘-’ show that during mild pyrolysis (temperatures <3OO”C), the chlorine is volatilized as HCl : no organo-chlorine compounds being detected. However, the origins and mode of formation of the HCl remain open to speculation. Previous studies’-’ ’ have been made in which coal was chlorinated using chlorine gas and aqueous treatment, but not ionic chloride treatment; in each case, tar formation on pyrolysis was suppressed and all the added chlorine was emitted as HCl at the highest temperatures. Only one study9 claims to have detected an organochlorine compound - hexachlorobenzene identified by melting point. The effect of variable water content of coals, by drying or saturating with water, has been investigated’; in general terms, increased water content leads to increased emission of organics through swelling of the coal structure but HCl emission decreases, probably through The chlorine
0016-2361/88/060822-09f3.00 0 1988 Butterworth & Co. (Publishers) Ltd.
822
FUEL, 1988, Vol 67, June
elution of chloride from the surface. Given” considers that chloride could exist in coal in the amine hydrochloride form on the basis that there is more basic nitrogen than chloride in coal; no evidence has been presented to show that basic nitrogen hydrochlorides occur in coal, are washed out of coal or that the chloride in such salts, bound to the coal matrix, would readily be leached out by water washing. The work described here was undertaken in an attempt to gain further understanding of the sources of hydrogen chloride together with the mechanism of its low temperature release. Results are presented for aqueous extraction and sequential grinding of six UK coals (both vitrain enriched and run-of-mine*) and a US coal previously reported’ as containing chlorine not accessible to water extraction. One of the UK coals, containing the most readily extracted chlorine, was selected for the subsequent experiments on dechlorination followed by chlorination with NaCl or chlorine gas, since it is considered in all other respects to be representative of such coals. The modified coals were heated in the mass spectrometer and chlorine-containing products were analysed. The mechanisms of thermal breakdown of the coal/chlorine systems are considered in relation to the thermal breakdown of chlorine-containing model compounds. EXPERIMENTAL Materials
The typical high-chlorine bitiminous coal used was Cadley Hill, Main seam, vitrain-enriched with a chlorine content of 0.58 wt Y0 on an ad basis. The coal was crushed and sieved to -212pm, and its analysis *Coal produced by mining operations before preparation. Such coal has unlimited topsize and a mineral matter content which may exceed 10%
Thermal emission
of chlorine-containing
compounds
from coal: G. Fynes et al.
Table 1 Analyses of coals Ultimate analysis (wt %) Proximate analysis (wt %)
Coal mine and seam
NCB coal rank code
C
HO
Maceral analysis (~01%)
ad
daf
N
S total
Cl
CO,
ad Moisture
db Ash
daf VM
Vitrinite
Exinite
Inertinite
Coal shale/ pyrite
Savile Flockton Thick
ROM” b VIT’ 700
79.4 81.4
5.7 5.4
10.3 8.3
1.90 1.85
2.40 2.65
0.36 0.43
0.20 0.30
3.4 4.1
33.1 7.1
39.0 37.9
56 70
8 10
17 17
19 2
Markham Main Bamsley/Dunsil
ROM VIT
b 800
79.3 80.8
5.5 5.2
11.9 9.7
1.85 1.90
2.00 1.65
0.45 0.60
0.45 0.70
6.6 8.0
29.0 2.8
38.6 36.4
60 83
5 3
25 12
10 1
Cadley Hill Main
ROM VIT
800 800
79.0 79.3
5.4 5.4
11.6 11.3
1.75 1.70
1.85 1.70
0.54 0.58
0.50 2.20
10.1 10.0
6.8 6.8
41.6 43.9
73 76
9 7
16 15
2 2
Hem Heath Yard/Ragman
ROM VIT
b 600
83.3 84.3
5.5 5.3
7.8 6.5
1.80 1.80
1.35 1.25
0.53 0.62
0.55 0.80
2.6 2.6
20.6 4.3
35.1 34.7
61 67
5 7
24 24
11 1
Florence Great Row
ROM VIT
b 700
78.9 80.8
5.6 5.6
11.2 9.6
1.50 1.50
2.70 2.30
0.63 0.67
0.60 0.70
4.9 4.7
17.7 5.6
41.4 41.7
74 84
6 6
11 8
8 2
Lea Hall Shallow
ROM VIT
800 800
80.9 80.3
5.3 5.4
10.2 10.7
1.70 1.70
1.50 2.00
0.83 0.90
0.80 0.60
10.3 10.0
5.5 5.8
39.6 40.0
71 83
6 5
22 12
1 1
85.1
5.6
9.5
1.65
3.04
0.50
0.63
4.3
11.5
40.5
n.d
n.d
n.d.
n.d.
Herrin No. 6
a ROM = run-of-mine ‘Ash yield too high for coal rank code ’ VIT = vitrain-enriched dn.d. = not determined
is given in Table 1. Vitrain enrichment was achieved by selection of bright coal lumps from the run-of-mine coal since chlorine is known to be associated13 with the vitrinite maceral group. The other five UK coals and the US coal are listed with their analyses in Table 1; all were initially crushed and sieved to -2 12 pm. Gases and chemicals were of industrial, laboratory or A.R. grades and used as received. PVC powder (MW 100 000) and N- 1-naphthyl ethylene diamine dihydrochloride were chosen as model organo-chlorine and base hydrochloride compounds, respectively.
chloride concentration was determined by titration against 0.02 M silver nitrate solution. Grinding and extractive cycles were repeated until no chloride ions could be titrated in the leachate. This required between one (for Cadley Hill) and four cycles (for Hem Heath and Florence coals). The summation of extractable chloride plus chlorine in the final residue enabled a chlorine balance to be obtained and the extent of chlorine removal to be determined within the limits of accuracy of the analytical techniques, about 1 ppm in the leachates and about 0.02 wt % Cl in the solids.
Analyses
Dechlorination
The chlorine content of the various coal samples, before and after treatment, were determined by the British Standard high temperature combustion method. Moisture contents were determined by drying in a minimum free-space oven.
Coal (30g) was weighed into a 500ml flask with 250 ml of distilled water. The suspension was refluxed for 4 h and filtered hot. The extraction was repeated six times, after which no opalescence was observed in the filtrate on acidification with nitric acid and addition of silver nitrate. The washed coal was dried in a vacuum at 105°C for 1 h, cooled and equilibrated in air (18 h) before analysing for chlorine and moisture content.
Wet grinding and aqueous
extraction
The coal samples were ground wet (to reduce agglomeration) in a high-speed ball mill using tungsten carbide balls (11 mm diameter) and container (40 mm x 80 mm length). Coal (4 g) and distilled water (4 ml) were ground for 2 h. A further 3 ml of water was added and the mixture was milled for 5 min, to aid transfer of the sample from the mill to a flask. This procedure was based on tests that showed that a run-ofmine coal sample ( < 212 pm) was reduced to 96 wt % less than 25 pm with a median particle size of 6 pm, under these conditions. The suspension from milling was washed into a 500 ml flask, made up to 200 ml with distilled water and refluxed for 2 h. The dilute suspension was filtered on a 0.8 pm cellulose filter paper and the cake was washed with cold water, dried in vacuum at 100°C for 1 h, cooled in a dessicator and equilibrated in air for 18 h. The combined filtrate and washings were made up to 5OOml and the
Chlorination Chlorine
by water extraction
of water extracted gas.
coal
The
apparatus used is shown schematically in Figure 1. Washed coal (4g) was positioned between two glass wool plugs in a 300 mm x 8 mm i.d. stainless steel reactor tube held in a furnace at the desired reaction temperature. The system was purged with nitrogen (15ml min-‘) before the introduction of chlorine gas (15 ml min-‘) into the nitrogen stream. After 1 h the chlorine gas flow was stopped, and the equipment allowed to cool overnight under nitrogen flow. Initial bed temperatures of 100°C and 300°C were used, which rose to 112°C and 318”C, respectively, during chlorination. The chlorinated coals were washed with x2.5 1 of boiling distilled water to remove occluded HCl and then air-dried prior to mass spectrometry.
FUEL, 1988, Vol 67, June
823
Thermal emission
of chlorine-containing
compounds
from coal: G. Fynes
Sodium chloride. Preliminary experiments showed that vacuum impregnation of the washed coal was the most efficient way of introducing sodium chloride: 6 g of coal were heated at 95°C for 3 h in the flask of a rotary evaporator under a vacuum of z 3.3 kPa. The flask and contents were then cooled when 150 ml of a 1.65 wt % sodium chloride solution (1 wt % as Cl -) was introduced and left to stand overnight. The impregnated coal was filtered off, washed with 2 11 of 1.65% sodium chloride solution and allowed to air dry. Mass spectrometrjl
The apparatus and method were essentially as described previously6z7 except that the reactor tube was of glass-lined stainless steel (size 6.35 mm o.d., 4mm i.d. x 170 mm) and heat treatment times were z 3.5 h for the coal samples and I h for the model compounds (both the latter samples softened and blocked the reactor tube before the reactor reached maximum temperature). The temperature programme used was from 50 to 300°C at 2°C min-‘, the upper temperature being held until data collection was terminated. Volatiles were transferred from the coal by a helium gas stream through a glass jet separator into the ion source of the mass spectrometer; the coal sample acted as the column packing in packed
column g.c.-m.s. with the column bleed being examined by the mass spectrometer. RESULTS AND DISCUSSION Sequential
grinding and aqueous extraction
The results of chloride extraction are given in Table 2 and chloride balances are accurate to 100% within the accumulated errors of 2 or 3 wt %. The chlorine in these coals can be totally removed by water extraction, within the limits of error of the chloride determination methods. The number of extraction cycles necessary to remove the chloride varied for the different coals and depends on accessibility of water to the porosity of the coal; the experimental details in support of this will be published in due course. Cadley Hill vitrain-enriched coal is the most readily extracted of the six UK coals investigated and was selected for the rechlorination experiments for that reason. Exhaustiue Tuble Table 3
extraction
3 shows Analyses
Treatment
Chlorine gas
Nitrdgen gas
Figure 1 Table 2
Schematic Extraction
of chlorination
apparatus
data from sequential
using chlorine
grindrng
and aqueous
gas
et al
and rechlorination
the chlorine
of treated
Cadley
Chlorine ___~
content
and
levels
Hill coal samples
Moisture (wt I’” ad) (wt & db) (wt “1,ad) _
No treatment (as received) 0.58 Dechlorinated by water washing 0.03 Dechlorinated by water washing, rechlorinated using chlorine gas at 100°C and washed 9.8 Dechlorinated by water washmg, rechlorinated using chlorine gas at 300°C and washed 14.2 Dechlorinated by water washmg, rechlorinated using 1 wt % aqueous Cl _ solution (vacuum) 0.62
moisture
Weight used (g)
Weight loss (““)
0.64
10.0
1.53
13.7
0.03
5.9
1.34
8.3
10.2
4.2
0.51
18.2
14.8
4.0
0.62
7.7
6.9
1.38
8.2
0.67
extractions
Coal chlorine content (wt “Gad) _.
Number of grindings/ aqueous extractions
Chlorme extracted from coal (wt “s;,ad)
Chlorine” in washed coal (wt y$,ad)
Extent of chlorine removal (wt %)
Chlorine balance (wt O6)
ROM VIT
0.36 0.43
2 3
0.36 0.43
< 0.02 < 0.02
100 100
100 100
ROM VIT
0.45 0.60
2 3
0.45 0.61
< 0.02 < 0.02
100 102
100 102
Hill
ROM VIT
0.54 0.58
I 1
0.56 0.59
< 0.02 < 0.02
103 102
103 102
Hem Heath
ROM VIT
0.53 0.62
4 4
0.5 1 0.60
0.02 < 0.02
96 97
100 97
Florence
ROM VIT
0.63 0.67
4 4
0.60 0.68
0.03 < 0.02
95 101
100 101
Lea Hall
ROM VIT
0.83 0.90
2 2
0.85 0.88
< 0.02 < 0.02
102 98
102 98
0.50
3
0.50
0.03
101
103
Coal Saville Markham Cadley
Herrin
Main
No. 6
“The chiorine
824
content
of the washed
co&
FUEL, 1988, Vol 67, June
was corrected
for a system blank
of 0.02 wt “, ad
Thermal emission of chlorine-containing
a
compounds
from coal: G. Fynes et al.
44
100 90 80 70 60
18
!
50 40 30 20 10 0
l”“““‘l”“““‘1
50
b
100
200
150
250
300
44
100 90 80 70 60
r
50
18
40 30
28
20 10 0 11.
*
C 100 90 80 70 60 50 40 30 20 10 0 150
Mass Figure 2 Mass spectra of volatiles emitted at 300°C from Cadley impregnation with NaCI. HCI peaks are marked *
ml2
Hill vitrain
determined for Cadley Hill coal after exhaustive extraction with water and for the samples chlorinated with chlorine gas and impregnated with sodium chloride. This table also gives weights of sample used in the mass spectrometric experiments and the resultant weight losses. In each case, the weight loss exceeds the water content of the coal sample indicating a significant loss of organic and inorganic material in the mild pyrolysis experiment since it has been shown earlier5 that not all of the water in coal is removed at 300°C.
enriched
coal: a, as received;
b, after exhaustive
washing
and c, after
Muss spectrometrq Mass spectra of the volatiles from Cadley Hill coal (as received and its variously treated samples) on attaining 3OO”C, are shown in Figures 2-4. The mass spectra in Figure 2 of the coal, as received, exhaustively washed and impregnated with NaCl show that the major components detected at 300°C are CO,, H,O, alkane fragment ions and alkyl naphthalenes with alkenes or cycloalkanes, and other aromatics as minor alkyl benzenes components; HCl peaks are clearly evident in the
FUEL, 1988, Vol 67, June
825
Thermal emission of chlorine-containing
compounds
from coal: G. Fynes et al.
a 100
36*
90 80 *
706050-
50
30-
28 1 II,,,,,
* * 11; 47 82 .N + 11 Y + 165 ,11,,,,,,,,1,1111111,111~11111~111111111~111111111 50
100
150
* 250
28+3
II1IlIII
300
m/z
Mass spectra of volatiles emitted at 300°C after chlorination at: a, 100°C; b, 300°C with chlorine gas. Chlorine containing peaks are marked *
spectrum from the coal as received; are just visible in the NaCl impregnated coal spectrum, but cannot be seen in the spectrum of the exhaustively washed coal because their intensities are too small. The mass spectra of Figure 3 are those for coal chlorinated with chlorine gas and HCl peaks are most prominent with C02, H,O and SO, as minor components. Figures 4a and b repeat Figures 3a and b but are limited to ions > m/z 50 to reveal more clearly the presence of organo-chlorine species and the patterns of chlorine substitution. A notable feature of the mass spectra following chlorine gas treatment is the suppression of aliphatic and aromatic components; some aliphatic and aromatic peaks are evident in Figure 4a at low masses only. Tar suppression following chlorination with chlorine gas has been observed by others previouslys - “. Mass spectra of the model compounds PVC (obtained at about 105C) and N-1-naphthyl ethylene diamine dihydrochloride (obtained at about 180°C) are shown in Figures 5a and b for comparison; PVC produces organochlorine fragments while the amine hydrochloride produces intense HCl ions at a relatively low temperature. Single ion profiles for HCl emission from the coal samples are shown in Figure 6 and single ion profiles for some chlorinated ions observed for the coal sample after chlorination with chlorine gas are shown in Figures 7 and 8. Table 4 lists maximum ion intensities for the various more important chloro-ions observed together
828
* 200
Mass Figure 3
250
*
20lo-
200
150
100
FUEL,
1988,
Vol67,
June
with their appearance temperatures. Maximum ion intensities of the most predominant organic ion (C,H:) are included in this table for comparison. HCl emission profiles for the washed coal and NaClimpregnated coals are similar to each other, differing mainly in intensity and they are different from the coal as received in terms of an emission at low temperatures; HCl emission profiles following chlorine gas treatment are completely different, as are those for the model compounds. Maximum intensity values for C,H: in Table 3 indicate clearly the suppression of this ion on chlorination at 3OO”C,by a factor of nearly 10 against the coal as received and a factor of nearly 70 against the exhaustively washed coal. The appearance temperatures indicate that with the exception of chlorination at 300°C any treatment of the coal removes the low temperature emission of C,HT. Water washing
Cadley Hill coal is relatively easily leached, and x95 wt % of its chlorine is removed during repeated water extraction. This is reflected in the much reduced maximum ion intensity for HCl as measured during mass spectrometry (Table 4). Interestingly, the appearance temperature of some of the remaining chloride is lowered from z 200 to x 50°C : the single ion profile (Figure 6) for the washed coal shows two peaks as opposed to the single peak for the as received coal. The residual chlorine, which
Thermal emission
of chlorine-containing
compounds
from coal: G. Fynes et al.
a loogo80-
*8
64
78 *
'O6050-
55
*
40-
91 "112
b 100 90 80 70 60 50
30-
82
20-
150
175
200
225
250
275
300
Mass m/z Figure 4 Mass spectra over limited mass range of volatiles emitted at 300°C after chlorination at: a, 100°C; b, 300°C with chlorine gas to show organochlorine fragments and isotopic abundance ratios. Peaks in a, containing chlorine are marked *; all the peaks in b, contain chlorine
is difficult to remove by further leaching, is probably situated at sites inaccessible to water without even finer grinding. The very small amount of chlorine emitted as hydrogen chloride at z 50°C is probably that deposited on the surface sites of the coal during the inevitable equilibration with chloride-containing aqueous leachate. Chlorination
with chlorine gas
Chlorination with chlorine gas after washing the coal to remove inorganic chloride, introduced about 10 wt % chlorine at 100°C and about 15 ‘A at 300°C. During these organic substitution reactions, HCl would be the principal by-product and, although the chlorinated coals were subsequently washed, it is likely that some of the HCl was retained. Thus the maximum ion intensities of HCl are much increased compared with the as received coal (Table 4) and the mass spectra show that the H3’C1 and H3’C1 peaks are the major ions detected (Figures 3a and b). The HCl ion profiles (Figure 6) differ markedly from those for the original coal and for the water washed coal. The majority of this hydrogen chloride would be that occluded to surface sites on the coal but some will result from dehydrochlorination of organo-chlorine compounds. The chloro-organic ions detected (Figures 7 and 8) correspond to simple alkyl or alkene fragments (C, or C,)
and chlorinated benzene and toluene fragments, which are more highly chlorinated at 300°C than at 100°C. Relevant mass spectra are shown in Figures 3 and 4 and it is seen that chlorine isotope patterns are very distinct after chlorination at 300°C. Hexachlorobenzene corresponds to the cluster of peaks in Figure 4b between masses 282 and 290. None of these organo-chlorine ions are detectable in pyrolysis/mass spectrometry of the original Cadley Hill coal. Chlorination
with sodium chloride
Chlorination by vacuum impregnation of pre-washed coal with brine (1 wt % Cl-) resulted in an uptake of 0.62 wt% Cl on an air-dried basis. This closely corresponds to the chlorine content of as received coal, although it is unlikely that complete penetration of the more inaccessible sites by the Cloccurred. Pyrolysis/mass spectrometry of the impregnated coal shows a large increase in HCl emission over that obtained from the washed coal (Table 4). Further, its HCl single ion profile (Figure 6) shows two distinct peaks with early emission peaking at x lOO”C,the major peak occurring at 300°C. This HCl must be derived from the NaCl presumably by degradative hydrolysis. The early (100°C) peak, as with that obtained with the washed coal, probably comes from chloride deposited on easily
FUEL,
1988,
Vol 67, June
827
Thermal emission of chlorine-containing
compounds from coal: G. Fynes et al.
a
63
100 90 80 70
C2H4CI
60 50 40
I
30-
HCI
CHCl2
20-
83
100
36
28 1;
I,,,,,, 20
42
85 97 ““I-II”“““‘I”“““‘1 , ,100
ii, 30
40
50
60
70
80
90
110
100
120
b 100
36
90 80 70 60
Figure 5 containing
I
0
Mass spectra of model compounds: components are marked *
a, PVC at 105°C and b, N-I-naphthyl
I
I
I
I
I
0.5
1 .o
1.5
2.0
2.5
Time
0”
(h)
FUEL, 1988, Vol 67, June
diamine
dihydrochloride
at 180°C; the chlorine
I
Figure6 H3%TIprofiles for coal samples before and after washing chlorination and model compounds; the broken line represents temperature profile from 50 to 300°C
828
ethylene
and the
0.5
1 .o
1.5
2.0
Figure 7 Chlorinated ion profiles after chlorination at 100°C; the broken line represents the temperature 300°C
2.5
with chlorine gas profile from 50 to
Thermal emission of chlorine-containing
accessible surface sites. No organo-chlorine ions were observed in the mass spectrum of the impregnated coal. The emission of hydrocarbons was enhanced by the addition of aqueous chloride, in contrast to the suppression of tar formation by chlorine gas treatment. Model
compounds
N+H3-CH2-CH2-N+H2
”
from coal: G. Fynes et al.
Unfortunately an involatile and structurally more suitable aromatic chlorohydrocarbon (as an alternative to PVC) was not available. The mass spectrum of PVC (Figure 5~) indicates the release of organo-chlorine fragment ions as well as hydrogen chloride even at 105°C. The mass spectra of the coals examined did not show the presence of such fragments thereby indicating that covalently bonded chlorine did not exist in these coals. The mass spectrum of the base hydrochloride (Figure 5b) shows that HCl emission is intense, even at low temperature (180°C). Since, for the coal examined, this kind of behaviour was not observed, except to a limited extent after water washing, it seems unlikely that such a bonding mode is significant in coals.
The model compounds used in this study, polyvinyl chloride (I) and N-1-naphthyl ethylene diamine dihydrochloride (II) were selected on grounds of their structurally differing organo-chlorine types, their low volatility and their ready availability. (-CH2-CHCI-CH2-CHCI-)
compounds
Cl-
Cl-
CONCLUSIONS
0
1 .o
0.5
1.5 Time
ion intensities
As received
2.5
2.0
(h)
Figure 8 Chlorinated ion profiles after chlorination at 300°C; the broken line represents the temperature 300°C
Table 4 Maximum Cadley Hill coal
1. Repeated aqueous extraction and grinding removed, as chloride ion, all the chlorine from six UK coals, both run-of-min and vitrain enriched, and from a US coal, although the ease of removal varied markedly. 2. The exhaustive washing of one of the six coals removed over 95 wt ‘A of the chlorine as chloride ion. The small amount remaining was still detectable by pyrolysis/mass spectrometry as HCl. No organo-chlorine compounds were detected. 3. Chlorination of the coal with chlorine gas introduced covalently bonded chlorine some of which was released as organo-chlorine fragments, as well as hydrogen chloride, on pyrolysis; emission of hydrocarbons was suppressed. 4. Impregnation of the washed coal with sodium chloride followed by pyrolysis/mass spectrometry showed a relatively large release of hydrogen chloride which must have been derived by hydrolysis of the sodium chloride at the coal surface. 5. Results of pyrolysis/mass spectrometry of PVC demonstrate that, if such organo-chlorine structures were present in coal, they would have been detected. 6. Results of pyrolysis/mass spectrometry of N-lnaphthyl ethylene diamine dihydrochloride suggest that base hydrochlorides are not a significant structural feature in coal but their total absence has not been established. 7. Collectively, these results establish that the HCl emitted from coal is produced from brine-derived chlorides and that covalently bonded organo-chlorine structures do not exist in coal.
with chlorine gas profile from 50 to
(MII) and appearance
Exhaustively
Atomic composition
MI1 ( x 10.. “)
;?:,
MI1 (x 10-j)
HCI CCI, C,H,CI C,CI, C,H:
192 n.d. nd. n.d. 267
200 nd. n.d. n.d. 50
33 n.d. n.d. n.d. 2015
temperatures
washed
(AT) (to nearest
5°C) of chlorine
ions detected
in the volatiles from
Cl, IOO”C
NaCl impregnated MI1 (x 10-j)
containing
Cl, 300°C
MI1
MI1
(x 10-3)
(x 10-q -
50 n.d. n.d. n.d. 145
203 n.d. n.d. n.d. 1236
50 nd. n.d. n.d. 90
20350 60 20 n.d. 117
110 115 190 n.d. 100
15 140 2 070 n.d. 359 30
FUEL, 1988, Vol 67, June
50 140 n.d. 225 50
829
Thermal emission of chlorine-containing
compounds
ACKNOWLEDGEMENTS The authors thank British Coal for permission to publish; the views expressed are those of the authors and not necessarily those of British Coal. The authors also wish to thank Dr J. A. Cox, Department of Chemistry, Southern Illinois University, Carbondale, IL 62901, USA, for supplying the sample of Herrin No. 6 coal. REFERENCES 1 2
830
Hodges, N. J., Ladner, W. R. and Martin, T. G. J. Inst. Energy 1983, 56, 158 Cox, J. A., Larsen, A. E. and Carlson, R. H. Fuel 1984,63, 1334
FUEL, 1988, Vol 67, June
from coal: G. Fynes et al. 3 4 5 6
8 9 10 11 12
13
Cox, J. A. and Saari, R. Analyst 1987, 112, 321 Ladner, W. R. Fuel 1984,63,726 Herod. A. A.. Hodges. N. J.. Pritchard. E. and Smith. C. A. Fuel 1983,82, 1331 Herod, A. A. and Smith, C. A. Fuel 1985,64,281 Herod, A. A., Ladner, W. R. and Stokes, B. J. in ‘Advances in Mass Spectrometry 1985’, (Ed. J. F. J. Todd), John Wiley and Sons, Chichester, UK, 1986, part B, 1313 Eccles, A., Kenyon, G. H. and McCulloch, A. Fuel 1931,10,4 Pinchin, F. J. Fuel 1958, 37,293 Macrae, J. C. and Oxtoby, R. Fuel 1965,44,409 Oxtoby, R. Fuel 1966,45,457 Given, P. H. and Yarzab, R. F. ‘Analytical Methods for Coal and Coal Products’, (Ed. C. Karr Jr.), Academic Press, New York, 1978, Vol. 2, 3349 Saunders, K. G. J. Inst. Energy 1980, 53, 109