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Relation between coal properties and acetylene yield in plasma pyrolysis
Relation between coal properties and acetylene yield in plasma pyrolysis* D. Bittner,
H. Baumann
Bergbau-Forschung GmbH, D-4300 Essen 73, FRG
and J. Klein Abt. KohlechemielGrundreaktionen,
Franz-Fischer-
Weg 61,
If coal is injected into hydrogen plasma generuted by arc discharge the volatiles are released within a few milliseconds and form acetylene as the main product by reaction with the plasma. Experiments with different coals indicate that the yield of acetylene is determined not only by the amount of volatiles but depends also on their composition. Oxygen compounds in the coal and. in some cases, even the oxygen content of the mineral matter decrease the acetylene yield. (Keywords: coal; coal properties; plasma pyrolysis; acetylene yield)
Acetylene has been a basic material for the chemical industry for some decades. Pyrolysis of coal in hydrogen plasma opens up a direct means of producing this important primary substance, which may become attractive in the future. Several research institutes have carried out investigations on the subject’ ‘. The process essentially consists of the following steps: a hydrogen plasma is used to heat finely ground coal very rapidly to a temperature at which volatile matter is released. This degassing step is completed after a few milliseconds. The volatiles continue to react rapidly in the hot hydrogen at temperatures > 2000 K; acetylene is the main product, with ethylene, methane, hydrogen and carbon monoxide being among the secondary products. Such products have to be quenched to prevent the decomposition of acetylene into carbon (soot) and hydrogen. The economic viability of a plasma process for acetylene production from coal depends to a large extent on the acetylene yield and on the specific energy requircment. These factors are, in turn, functions both of operational conditions and the properties of the coal used. The present paper examines how coal properties such as ash content, maceral distribution, coal rank, tar content and grain size influence the acetylene yield.
EXPERIMENTAL The hydrogen plasma was produced in a 30 kW plasma generator by an arc between a rod-shaped tungsten cathode and a tube shaped. water cooled copper anode. and was made to rotate by means of a stationary magnetic field (Fiyure /). Finely ground coal was injected, using hydrogen as carrier, through four small tubes into the plasma beam. Having passed the reactor, the gas flow was quenched with water. The concentrations ofacetylene and CO in the product gas stream were analysed by gaschromatography, the total amount of gas was measured by a gas meter. The acetylene yield is a function of the reactor length: if the reactor is too short the coal will not * This paper was presented Rcacri\ity of Coal’. 25 27 March 1985
Society
at the Conference
of Chrmul
0016~2361:85~101370~05$3.00 (‘ 1985 Buttcrworth & Co. (Publishers)
1370
FUEL,
1985,
Industry.
Ltd.
Vol 64, October
‘Srructnre London.
and 11K.
react fully, if it is too long part of the acetylene will decompose into soot and hydrogen. The optimum reactor length mainly depends on the grain size of the coal, and when different grain sizes were used the reactor length was adapted accordingly. The reactor pressure was reduced to 40 kPa using a water ring pump, as reduced pressure had been found to promote acetylene formation. During all the experiments the net enthalpy of the plasma beam was 2 2.5 kW h me3 (s.t.p.) after deduction of cooling water losses in the plasma generator. The coal samples selected were high volatile bituminous coals with a volatile matter range of 2 3&40 wt(,!<(Tuhfe I). Several of the samples had high ash contents (up to 35”:,). In Table I they are arranged in order of increasing acetylene yield. The coals came from the USA, South Africa and Germany. They were ground to < 250 pm. Five of the coals were also ground in a pin-disc mill down to < 30 ilrn. RESULTS ll!fllrnlc~r of’ operution~~l conditions
The acetylene yield is a function of the enthalpy of the generator gas, the specific energy input (ratio of electric power to coal throughput), and the pressure. The yield will be enhanced by increasing the enthalpy, at least in the measured range, up to 2.5 k W h m -3 (s.t.p.), by increasing the specific energy input; and by reducing the pressure. The increase in acetylene yield as a function of specific energy input may vary depending on the type of coal; the appertaining curves approach one another in the range of low energy input (Figure 2). The trend of the curves is not very ~,ell defined in this range since consistent dosing of certain coals which have a tendency to sticking is difficult at high throughputs. To make the different coals more comparable a high specific energy input of 8 kW h mm3 was therefore selected. From an economic viewpoint, however, the lower input range is more attractive since specific energy requirements are then lower (Figure 2).
Coals differ from one another in several respects, and it is therefore quite difficult to recognize the influence of one specific property on acetylene yield. For example, high
Coal Properties
ash content may so reduce the conversion of carbon into acetylene that the influence of other properties becomes negligible (Figure 3). The reduced conversion to acetylene is linked with an increase in carbon conversion to CO. In this case, the oxygen present in CO arises both from the organically bound oxygen and from the oxygen of the mineral matter as arises from elementary analysis. The ash-dependent reduction of carbon conversion into acetylene may also be low; in such a case the ash yield will not indicate clearly the specific effects of certain minerals.
It~flurnce oj’ mucerul
and acetylene
yield:
et al.
D. Bittner
may show differing maceral distributions, since each maceral group has its specific grindability. This situation was used to prepare two sieve fractions of the coal number 9 (> 125 pm and <60pm) with different maceral distributions (Tub/e 2). The coarse sample had the highest exinite content. The two samples and the original sample were ground to a size containing 98”; < 50pm. The sample with the highest exinite content yielded the most acetylene (Figure 4); this sample also had the highest level of volatile matter. It will be shown below, however, that the amount of volatile matter alone is not critical to acetylene yield because the constitution of the volatiles also plays a part. Exinite differs from the rest of the maceral groups, amongst other things, by virtue of the larger average molecular mass of its volatiles’.
distribution
Since high levels of ash may imply high yields of CO at the cost of acetylene, it was useful to examine the influence of other coal properties by confining the study to coals with ash yield < 10% (coal numbers 7-13). No correlation could be found between the maceral distribution and acetylene yield these coals, since they differed too much as far as other relevant properties were concerned. It is known, however, that different size fractions of one coal
Injluence
of coal rank
The rank of a coal may be characterized by different properties, e.g. by volatile matter, carbon content or random vitrinite reflectance. As far as high volatile coals are concerned, the calorific value is also a useful criterion. Carbon conversion into acetylene increased with coal rank up to the transition from high volatile to medium
CZH,
YIELD
(wi
“0)
!:Pr CI! IC
CNCRGY
HrOUlRtMENT (kVJh/kg
CzH?)
I
I 2
3
L
5
SPECIFIC
Figure 2 Figure 1 Table I ~~
Two-Stage Analytical
pkmnd
rmCtOr
data of the
Acetylene
6
7
ENERGY
INPUT
yields and specific energy
8
9
(kWh/kg
COAL)
requirements
of three
COdk
co&
~_~
7 SA
8 USA
9 FRG
10 USA
11 FRG
12 USA
13 USA
32.1 11.6 8.9 3.4 32.3
34.8 7.5 11.8 3.0 32.6
40.2 7.7 11.1 4.3 32.2
37.1 5.6 10.7 2.4 32.9
41.0 4.4 14.2 8.1 33.4
35.4 6.3 Il.4 0.7 34.7
34.3 4.4 13.0 0.9 35.0
33.2 3.7 12.6 I.1 35.2
84.7 5.2 1.9 0.7 6.6
84.0 5.3 1.9 0.6 7.0
80.7 5.7 1.6 2.7 8.6
82.8 5.5 1.4 1.2 8.3
81.1 5.7 1.9 1.2 10.1
84.9 5.6 I.3 0.X 8.0
87.0 5.7 1.8 0.7 4.1
87.4 5.4 1.6 0.8 4.6
7
8 51 37 0.74
8 80 9 0.63
14 67 16 0.86
16 62 19 0.54
II 77 11 0.84
13 54 32 0.96
8 62 28 1.03
Coal number Origin
1 SA
2 SA
3 SA
4 SA
5 SA
6 SA
Volatile matter (wt”,, da0 Ash (wt”,, dry) Tar (Fischer assay. wt”,. dry) Moisture (WI”,.,) Gross calorific value (MJ kg- ‘, dafl
34.0 34.9 2.7 6.9 28.1
33.0 20.0 4.7 3.5 31.0
29.7 22.5 2.7 2.8 30.5
36.1 30.8 6.9 3.1 31.0
32.3 19.6 4.4 3.2 31.0
Elementary analysis C (dafj H (da0 N (daf) S (dry basis) 0 (dry basis)
76.0 4.3 2.1 0.6 11.0
81.0 4.7 2.1 0.9 9.0
79.3 4.2 1.7 1.5 10.4
81.3 4.5 1.7 1.9 7.3
80.0 4.4 1.6 2.0 9.6
2 18 49 0.59
4 31 46 0.70
7 9 73 0.75
9 60 20 0.75
32 48 0.77
_
(wt”,)
Maceral analysts (vol”;) Exinite Vitrinite Inertinite Random reflectance (:‘;I
8
60 29 0.86
FUEL, 1985, Vol 64, October
1.171
Coal properties and acetylene C-CONVERSION
yield:
D. Bittner
et al.
Influence of pore colume and grain size
(94)
50 0
,
I,,,.,..,,
10
.
.
.
.
20
.
to C,H,
.
30 ASH CONTENT
Figure 3 Carbon conversion ash yields of coals
.
Due to the very short residence time of the coal in the reactor, the rate of volatile disengagement from the coal grain may itself exert a certain influence on the acetylene yield. Coals 9-13 were therefore ground to 95% < 30 pm. Prior to grinding, the pore volume was measured using mercury porosimetry and methanol adsorption. The conversion of carbon into acetylene, both for coarse and finely ground coals, decreased with increasing pore volume (Figure 7). The carbon conversion of fine coal is better than that of coarse coal, although the difference between the two is more pronounced with coals having a larger pore volume. It may be concluded that differences
(% dry)
(0) and CO (A)correlated
with
C-CONVERSION TO C2H2 (%I
501 C-CONVERSION TO C2H2 (%)
40-
30-
20.
lO-
3b I
I
I
I
,
I
3
4
5
6
7
0
SPECIFIC
ENERGY
INPUT
(kWh/kg
COAL)
Figure 4 Influence of exinite enrichment on conversion of carbon to acetylene. n , Increased exinite content; 0, lowered exinite content; A, original sample of coal number 9
311 GROSS
35. CALORIFIC
Influence
of
tar
FUEL,
1
35 (MJ/kg) and gross
C-CONVERSION TO C2H2 (%I
13 . 12 .
11 l
40-
9
l 0
10 l
!
30-
yield
Under the assumption that acetylene from coal is formed via volatile intermediate products, the measured increase in acetylene yield along with decreasing volatile matter may at first appear contradictory. However, with very high heating rates tars are the main volatile products, SO it is conceivable that they are the preferential intermediates. The tars generated as intermediate products under the conditions of plasma pyrolysis cannot be analysed, but, analytical data on tar yields were available from the Fischer assay. There was no correlation between these tar yields and the conversion of carbon into acetylene (Figure 6).
1372
34 (daf)
of conversion of carbon to acetylene Figure 5 Correlation calorific values of high volatile bituminous coals
SOvolatile bituminous coal, although the volatiles would diminish at the same time (Figure 5). From earlier it is known that acetylene yields will investigations’ decrease again with the higher rank coals. The maximum acetylene yields will therefore be obtainable in the transitional range between high volatile and medium volatile bituminous coals.
313 VALUE
1985,
Vol 64, October
20-
10-
I
I 10
11
12
TAR Figure 6 Correlation of conversion yields from Fischer assay
14
13
CONTENT
of carbon
to acetylene
15 (dry) and tar
Coal properties C-CONVERSION TO C2H2 (96) 50GRAIN SIZE
(30pm
‘3L
40GRAlti SIZE =z 250 pm
30-
I I
60
I
I
I
1
65
70
75
80
TOTAL
PORE
VOLUME
Figure 7 Influence of grain size on conversion 0, <3Opm; A, <25Oprn
I 85-
(crna/ 100
of carbon
g) to acetylene.
between coals, as far as their carbon conversion into acetylene in concerned, will be less marked in the case of fine coal sizes than for coarse coals. DISCUSSION During the pyrolysis of coal in hydrogen plasma, acetylene will be formed along the following lines. Upon entering the plasma beam the coal is heated rapidly to temperatures at which pyrolysis takes place. Model calculations for a 100 pm grain size and plasma temperatures of 200@3000K (ref. 10) show that the temperature of the reacting coal is no higher than 2 1300 K (Figure K), and does not vary much at different gas temperatures since the reaction is endothermic and the volatile matter formed impedes heat transfer. These calculations were made under the grossly simplified supposition that no heat gradient will be created within the grain. In reality, however, some gradient would certainly be established, so that the surface temperature rises at a substantially higher rate. However, this does not essentially effect what has been said about the temperature of the reacting coal. Under these conditions of fast heating to z 1300 K the main pyrolytic products will be high molecular weight substances, which fall under the general description of tars” r4. Low molecular weight hydrocarbons and CO will be formed in smaller volumes, as well as small quantities of acetylene”. The above pyrolysis products will react with the hydrogen plasma, which is at a higher temperature, to yield acetylene as the main product, which, with extended residence time, will continue to react to give carbon (soot) and hydrogen. Thermochemical equilibrium will not be attained, not even constrained equilibrium where soot formation is precluded, as experiments on different gaseous and liquid hydrocarbons at temperatures between 1300 and 2300 K have shown’5. This temperature range is relevant because the gas temperature will fall below 2300 K in the case of high coal throughputs. All the experiments with gaseous and liquid hydrocarbons were carried out with the same overall H/C-ratio, so that all feed substances, if in perfect equilibrium, should have resulted in one common product
yield: D. Bittner et al
and acetylene
distribution. In reality, though, alt distributions were different and deviated from the computed constrained equilibrium. The yield of acetylene from coal is affected by the formation of CO, since the two gasses are competing, It does not matter whether the oxygen arises from the mineral matter or organic compounds in the coal; high oxygen contents of volatiles will lower the acetylene yield. A basically positive effect on acetylene yield comes from an increase in volatiles. Such an increase may be brought about by accelerating their rate of escape from the coal grain, i.e. either by reducing the pressurer6,” or the grain sizer6 ‘*. Changing the rate of escape is accompanied by a variation in the molecular weight distribution of volatile matter”. When the pressure is reduced the molecular weight distribution of the tars tends to broaden, and the higher yield can be attributed mainly to components of high molecular weight. Exinites will liberate larger proportions of high molecular weight tar in a very similar way. While there is no direct evidence that these high molecular weight tar components actually promote acetylene formation, the experiments show that the acetylene yield is influenced by the volatile constitution since with lower coal ranks the acetylene yield declines although the volatile yield increases. CONCLUSION The yield of acetylene from pyrolysis of coal in hydrogen plasma can be enhanced not only by optimizing the experimental conditions but also by appropriate selection of coals. The amount of primary volatiles released plays an important, but not exclusive, part in acetylene formation; the composition of the volatiles, which varies with rank, maceral distribution and rate of escape, also has an influence. There are some indications that high mean molecular weights of primary tars released favour conversion into acetylene. High oxygen contents of
Table 2
Distribution
of maceral
TEMPERATURE
(vol’$
Exinite
Vitrmite Untreated Coarse sample Fme sample ___~__~_._~_
groups
14 31 10 _
69 4X 73
in coal number
Inertmite
_
_
16 17 15 _
MIllerah i 4 2 ~~~
_
(K,
PARTICL&,
100
100 pm
200
400
300 REACTOR
Figure 8 pyrolysis
9
Calculated
temperatures
FUEL,
1985,
of gas and
500
LENGTH
particles
Vol 64, October
(mm)
m plasma
1373
Coal properties
and acetylene
yield: 0. Bittner et al.
volatiles, whether originating from organic bound oxygen, diminish the acetylene yield.
or mineral
8
ACKNOWLEDGEMENT This work was financially supported by the Minister fiir Wirtschaft, Mittelstand und Technologie des Landes Nordrhein-Westfalen, FRG. REFERENCES 1 2 3 4 5
1374
Bond, R. L., Ladner. W. R. and McConnel,G. 1. T. Furl 1966.45, 381 Cannon, R. E., Krukonis, V. J. and Schoenberg. T. Irid. Erry. Chm. Prod. Res. Dm. 1970.9, 343 Nicholson. R. and Littlewood, K. Nrrrure 1972, 236. 397 Chakravartty, S. C., Dutta. D. and Lahiri, A. Fuel 1976. 55, 33 Wragg, J. G., Kaleel, M. A. and Kim,C. S. Chern. Eriq. Progr. Cod Procms. Techno/. 1980,6, 186
FUEL,
1985,
6
Vol 64, October
9 10 I1 12 13 14 15
16 17 18
Kikukawa.N., Makino, M., Maruyama, K.,Furuta.T., Horie,M. and Kimura, H. Nippon Kagaku Krrishi 1980,6,981 Bittner, D., Baumann, H., Peuckert, C., Klein, J. and Jiintgcn. H. El-doe1 Kohle Erdgas Perrochem. 1981, 34, 237 Miller, R. and Kaske, G. E&d Kohlr Ertlgcu Petrochum. 1984, 37. 149 Schulten. H.-R. Fuel 1982,61,670 Ristau, R. Diplomarbeit Universitat Dortmund, Lehrstuhl fiir Thermodynamik, 1984 Ungcr, P. E. and Suuberg. E. M. Fuel 1984,63, 606 Desypris, J.. Murdoch. P. and Williams, A. Fuel 1982. 61, 807 Arendt, P. and van Heek, K.-H. Fuel 1981,60,779 Eklund, H. and Wanzl, W. ‘Proc. Int. Conf. Coal Sci’, Dusseldorf, 1981, p. 701 Beiers, H.-G.. Baumann. H.. Bittner, D. and Klein, J. ‘Proc. 7th lnt. Symp. Plasma Chem.‘, Eindhoven, The Netherlands. July 1985 Anthony, D. B., Howard, J. B.. Hottel, H. C. and Meissner. H. P. Fire/ 1976. 55, 121 Anthony, D. B. and Howard, J. B. Am. 1risr. Chem. Brrg. J. 1976. 22. 625 Suuberg, E. M.. Peters, W. A. and Howard, J. B. Fuel 1980.59405
H. Baumann
Bergbau-Forschung GmbH, D-4300 Essen 73, FRG
and J. Klein Abt. KohlechemielGrundreaktionen,
Franz-Fischer-
Weg 61,
If coal is injected into hydrogen plasma generuted by arc discharge the volatiles are released within a few milliseconds and form acetylene as the main product by reaction with the plasma. Experiments with different coals indicate that the yield of acetylene is determined not only by the amount of volatiles but depends also on their composition. Oxygen compounds in the coal and. in some cases, even the oxygen content of the mineral matter decrease the acetylene yield. (Keywords: coal; coal properties; plasma pyrolysis; acetylene yield)
Acetylene has been a basic material for the chemical industry for some decades. Pyrolysis of coal in hydrogen plasma opens up a direct means of producing this important primary substance, which may become attractive in the future. Several research institutes have carried out investigations on the subject’ ‘. The process essentially consists of the following steps: a hydrogen plasma is used to heat finely ground coal very rapidly to a temperature at which volatile matter is released. This degassing step is completed after a few milliseconds. The volatiles continue to react rapidly in the hot hydrogen at temperatures > 2000 K; acetylene is the main product, with ethylene, methane, hydrogen and carbon monoxide being among the secondary products. Such products have to be quenched to prevent the decomposition of acetylene into carbon (soot) and hydrogen. The economic viability of a plasma process for acetylene production from coal depends to a large extent on the acetylene yield and on the specific energy requircment. These factors are, in turn, functions both of operational conditions and the properties of the coal used. The present paper examines how coal properties such as ash content, maceral distribution, coal rank, tar content and grain size influence the acetylene yield.
EXPERIMENTAL The hydrogen plasma was produced in a 30 kW plasma generator by an arc between a rod-shaped tungsten cathode and a tube shaped. water cooled copper anode. and was made to rotate by means of a stationary magnetic field (Fiyure /). Finely ground coal was injected, using hydrogen as carrier, through four small tubes into the plasma beam. Having passed the reactor, the gas flow was quenched with water. The concentrations ofacetylene and CO in the product gas stream were analysed by gaschromatography, the total amount of gas was measured by a gas meter. The acetylene yield is a function of the reactor length: if the reactor is too short the coal will not * This paper was presented Rcacri\ity of Coal’. 25 27 March 1985
Society
at the Conference
of Chrmul
0016~2361:85~101370~05$3.00 (‘ 1985 Buttcrworth & Co. (Publishers)
1370
FUEL,
1985,
Industry.
Ltd.
Vol 64, October
‘Srructnre London.
and 11K.
react fully, if it is too long part of the acetylene will decompose into soot and hydrogen. The optimum reactor length mainly depends on the grain size of the coal, and when different grain sizes were used the reactor length was adapted accordingly. The reactor pressure was reduced to 40 kPa using a water ring pump, as reduced pressure had been found to promote acetylene formation. During all the experiments the net enthalpy of the plasma beam was 2 2.5 kW h me3 (s.t.p.) after deduction of cooling water losses in the plasma generator. The coal samples selected were high volatile bituminous coals with a volatile matter range of 2 3&40 wt(,!<(Tuhfe I). Several of the samples had high ash contents (up to 35”:,). In Table I they are arranged in order of increasing acetylene yield. The coals came from the USA, South Africa and Germany. They were ground to < 250 pm. Five of the coals were also ground in a pin-disc mill down to < 30 ilrn. RESULTS ll!fllrnlc~r of’ operution~~l conditions
The acetylene yield is a function of the enthalpy of the generator gas, the specific energy input (ratio of electric power to coal throughput), and the pressure. The yield will be enhanced by increasing the enthalpy, at least in the measured range, up to 2.5 k W h m -3 (s.t.p.), by increasing the specific energy input; and by reducing the pressure. The increase in acetylene yield as a function of specific energy input may vary depending on the type of coal; the appertaining curves approach one another in the range of low energy input (Figure 2). The trend of the curves is not very ~,ell defined in this range since consistent dosing of certain coals which have a tendency to sticking is difficult at high throughputs. To make the different coals more comparable a high specific energy input of 8 kW h mm3 was therefore selected. From an economic viewpoint, however, the lower input range is more attractive since specific energy requirements are then lower (Figure 2).
Coals differ from one another in several respects, and it is therefore quite difficult to recognize the influence of one specific property on acetylene yield. For example, high
Coal Properties
ash content may so reduce the conversion of carbon into acetylene that the influence of other properties becomes negligible (Figure 3). The reduced conversion to acetylene is linked with an increase in carbon conversion to CO. In this case, the oxygen present in CO arises both from the organically bound oxygen and from the oxygen of the mineral matter as arises from elementary analysis. The ash-dependent reduction of carbon conversion into acetylene may also be low; in such a case the ash yield will not indicate clearly the specific effects of certain minerals.
It~flurnce oj’ mucerul
and acetylene
yield:
et al.
D. Bittner
may show differing maceral distributions, since each maceral group has its specific grindability. This situation was used to prepare two sieve fractions of the coal number 9 (> 125 pm and <60pm) with different maceral distributions (Tub/e 2). The coarse sample had the highest exinite content. The two samples and the original sample were ground to a size containing 98”; < 50pm. The sample with the highest exinite content yielded the most acetylene (Figure 4); this sample also had the highest level of volatile matter. It will be shown below, however, that the amount of volatile matter alone is not critical to acetylene yield because the constitution of the volatiles also plays a part. Exinite differs from the rest of the maceral groups, amongst other things, by virtue of the larger average molecular mass of its volatiles’.
distribution
Since high levels of ash may imply high yields of CO at the cost of acetylene, it was useful to examine the influence of other coal properties by confining the study to coals with ash yield < 10% (coal numbers 7-13). No correlation could be found between the maceral distribution and acetylene yield these coals, since they differed too much as far as other relevant properties were concerned. It is known, however, that different size fractions of one coal
Injluence
of coal rank
The rank of a coal may be characterized by different properties, e.g. by volatile matter, carbon content or random vitrinite reflectance. As far as high volatile coals are concerned, the calorific value is also a useful criterion. Carbon conversion into acetylene increased with coal rank up to the transition from high volatile to medium
CZH,
YIELD
(wi
“0)
!:Pr CI! IC
CNCRGY
HrOUlRtMENT (kVJh/kg
CzH?)
I
I 2
3
L
5
SPECIFIC
Figure 2 Figure 1 Table I ~~
Two-Stage Analytical
pkmnd
rmCtOr
data of the
Acetylene
6
7
ENERGY
INPUT
yields and specific energy
8
9
(kWh/kg
COAL)
requirements
of three
COdk
co&
~_~
7 SA
8 USA
9 FRG
10 USA
11 FRG
12 USA
13 USA
32.1 11.6 8.9 3.4 32.3
34.8 7.5 11.8 3.0 32.6
40.2 7.7 11.1 4.3 32.2
37.1 5.6 10.7 2.4 32.9
41.0 4.4 14.2 8.1 33.4
35.4 6.3 Il.4 0.7 34.7
34.3 4.4 13.0 0.9 35.0
33.2 3.7 12.6 I.1 35.2
84.7 5.2 1.9 0.7 6.6
84.0 5.3 1.9 0.6 7.0
80.7 5.7 1.6 2.7 8.6
82.8 5.5 1.4 1.2 8.3
81.1 5.7 1.9 1.2 10.1
84.9 5.6 I.3 0.X 8.0
87.0 5.7 1.8 0.7 4.1
87.4 5.4 1.6 0.8 4.6
7
8 51 37 0.74
8 80 9 0.63
14 67 16 0.86
16 62 19 0.54
II 77 11 0.84
13 54 32 0.96
8 62 28 1.03
Coal number Origin
1 SA
2 SA
3 SA
4 SA
5 SA
6 SA
Volatile matter (wt”,, da0 Ash (wt”,, dry) Tar (Fischer assay. wt”,. dry) Moisture (WI”,.,) Gross calorific value (MJ kg- ‘, dafl
34.0 34.9 2.7 6.9 28.1
33.0 20.0 4.7 3.5 31.0
29.7 22.5 2.7 2.8 30.5
36.1 30.8 6.9 3.1 31.0
32.3 19.6 4.4 3.2 31.0
Elementary analysis C (dafj H (da0 N (daf) S (dry basis) 0 (dry basis)
76.0 4.3 2.1 0.6 11.0
81.0 4.7 2.1 0.9 9.0
79.3 4.2 1.7 1.5 10.4
81.3 4.5 1.7 1.9 7.3
80.0 4.4 1.6 2.0 9.6
2 18 49 0.59
4 31 46 0.70
7 9 73 0.75
9 60 20 0.75
32 48 0.77
_
(wt”,)
Maceral analysts (vol”;) Exinite Vitrinite Inertinite Random reflectance (:‘;I
8
60 29 0.86
FUEL, 1985, Vol 64, October
1.171
Coal properties and acetylene C-CONVERSION
yield:
D. Bittner
et al.
Influence of pore colume and grain size
(94)
50 0
,
I,,,.,..,,
10
.
.
.
.
20
.
to C,H,
.
30 ASH CONTENT
Figure 3 Carbon conversion ash yields of coals
.
Due to the very short residence time of the coal in the reactor, the rate of volatile disengagement from the coal grain may itself exert a certain influence on the acetylene yield. Coals 9-13 were therefore ground to 95% < 30 pm. Prior to grinding, the pore volume was measured using mercury porosimetry and methanol adsorption. The conversion of carbon into acetylene, both for coarse and finely ground coals, decreased with increasing pore volume (Figure 7). The carbon conversion of fine coal is better than that of coarse coal, although the difference between the two is more pronounced with coals having a larger pore volume. It may be concluded that differences
(% dry)
(0) and CO (A)correlated
with
C-CONVERSION TO C2H2 (%I
501 C-CONVERSION TO C2H2 (%)
40-
30-
20.
lO-
3b I
I
I
I
,
I
3
4
5
6
7
0
SPECIFIC
ENERGY
INPUT
(kWh/kg
COAL)
Figure 4 Influence of exinite enrichment on conversion of carbon to acetylene. n , Increased exinite content; 0, lowered exinite content; A, original sample of coal number 9
311 GROSS
35. CALORIFIC
Influence
of
tar
FUEL,
1
35 (MJ/kg) and gross
C-CONVERSION TO C2H2 (%I
13 . 12 .
11 l
40-
9
l 0
10 l
!
30-
yield
Under the assumption that acetylene from coal is formed via volatile intermediate products, the measured increase in acetylene yield along with decreasing volatile matter may at first appear contradictory. However, with very high heating rates tars are the main volatile products, SO it is conceivable that they are the preferential intermediates. The tars generated as intermediate products under the conditions of plasma pyrolysis cannot be analysed, but, analytical data on tar yields were available from the Fischer assay. There was no correlation between these tar yields and the conversion of carbon into acetylene (Figure 6).
1372
34 (daf)
of conversion of carbon to acetylene Figure 5 Correlation calorific values of high volatile bituminous coals
SOvolatile bituminous coal, although the volatiles would diminish at the same time (Figure 5). From earlier it is known that acetylene yields will investigations’ decrease again with the higher rank coals. The maximum acetylene yields will therefore be obtainable in the transitional range between high volatile and medium volatile bituminous coals.
313 VALUE
1985,
Vol 64, October
20-
10-
I
I 10
11
12
TAR Figure 6 Correlation of conversion yields from Fischer assay
14
13
CONTENT
of carbon
to acetylene
15 (dry) and tar
Coal properties C-CONVERSION TO C2H2 (96) 50GRAIN SIZE
(30pm
‘3L
40GRAlti SIZE =z 250 pm
30-
I I
60
I
I
I
1
65
70
75
80
TOTAL
PORE
VOLUME
Figure 7 Influence of grain size on conversion 0, <3Opm; A, <25Oprn
I 85-
(crna/ 100
of carbon
g) to acetylene.
between coals, as far as their carbon conversion into acetylene in concerned, will be less marked in the case of fine coal sizes than for coarse coals. DISCUSSION During the pyrolysis of coal in hydrogen plasma, acetylene will be formed along the following lines. Upon entering the plasma beam the coal is heated rapidly to temperatures at which pyrolysis takes place. Model calculations for a 100 pm grain size and plasma temperatures of 200@3000K (ref. 10) show that the temperature of the reacting coal is no higher than 2 1300 K (Figure K), and does not vary much at different gas temperatures since the reaction is endothermic and the volatile matter formed impedes heat transfer. These calculations were made under the grossly simplified supposition that no heat gradient will be created within the grain. In reality, however, some gradient would certainly be established, so that the surface temperature rises at a substantially higher rate. However, this does not essentially effect what has been said about the temperature of the reacting coal. Under these conditions of fast heating to z 1300 K the main pyrolytic products will be high molecular weight substances, which fall under the general description of tars” r4. Low molecular weight hydrocarbons and CO will be formed in smaller volumes, as well as small quantities of acetylene”. The above pyrolysis products will react with the hydrogen plasma, which is at a higher temperature, to yield acetylene as the main product, which, with extended residence time, will continue to react to give carbon (soot) and hydrogen. Thermochemical equilibrium will not be attained, not even constrained equilibrium where soot formation is precluded, as experiments on different gaseous and liquid hydrocarbons at temperatures between 1300 and 2300 K have shown’5. This temperature range is relevant because the gas temperature will fall below 2300 K in the case of high coal throughputs. All the experiments with gaseous and liquid hydrocarbons were carried out with the same overall H/C-ratio, so that all feed substances, if in perfect equilibrium, should have resulted in one common product
yield: D. Bittner et al
and acetylene
distribution. In reality, though, alt distributions were different and deviated from the computed constrained equilibrium. The yield of acetylene from coal is affected by the formation of CO, since the two gasses are competing, It does not matter whether the oxygen arises from the mineral matter or organic compounds in the coal; high oxygen contents of volatiles will lower the acetylene yield. A basically positive effect on acetylene yield comes from an increase in volatiles. Such an increase may be brought about by accelerating their rate of escape from the coal grain, i.e. either by reducing the pressurer6,” or the grain sizer6 ‘*. Changing the rate of escape is accompanied by a variation in the molecular weight distribution of volatile matter”. When the pressure is reduced the molecular weight distribution of the tars tends to broaden, and the higher yield can be attributed mainly to components of high molecular weight. Exinites will liberate larger proportions of high molecular weight tar in a very similar way. While there is no direct evidence that these high molecular weight tar components actually promote acetylene formation, the experiments show that the acetylene yield is influenced by the volatile constitution since with lower coal ranks the acetylene yield declines although the volatile yield increases. CONCLUSION The yield of acetylene from pyrolysis of coal in hydrogen plasma can be enhanced not only by optimizing the experimental conditions but also by appropriate selection of coals. The amount of primary volatiles released plays an important, but not exclusive, part in acetylene formation; the composition of the volatiles, which varies with rank, maceral distribution and rate of escape, also has an influence. There are some indications that high mean molecular weights of primary tars released favour conversion into acetylene. High oxygen contents of
Table 2
Distribution
of maceral
TEMPERATURE
(vol’$
Exinite
Vitrmite Untreated Coarse sample Fme sample ___~__~_._~_
groups
14 31 10 _
69 4X 73
in coal number
Inertmite
_
_
16 17 15 _
MIllerah i 4 2 ~~~
_
(K,
PARTICL&,
100
100 pm
200
400
300 REACTOR
Figure 8 pyrolysis
9
Calculated
temperatures
FUEL,
1985,
of gas and
500
LENGTH
particles
Vol 64, October
(mm)
m plasma
1373
Coal properties
and acetylene
yield: 0. Bittner et al.
volatiles, whether originating from organic bound oxygen, diminish the acetylene yield.
or mineral
8
ACKNOWLEDGEMENT This work was financially supported by the Minister fiir Wirtschaft, Mittelstand und Technologie des Landes Nordrhein-Westfalen, FRG. REFERENCES 1 2 3 4 5
1374
Bond, R. L., Ladner. W. R. and McConnel,G. 1. T. Furl 1966.45, 381 Cannon, R. E., Krukonis, V. J. and Schoenberg. T. Irid. Erry. Chm. Prod. Res. Dm. 1970.9, 343 Nicholson. R. and Littlewood, K. Nrrrure 1972, 236. 397 Chakravartty, S. C., Dutta. D. and Lahiri, A. Fuel 1976. 55, 33 Wragg, J. G., Kaleel, M. A. and Kim,C. S. Chern. Eriq. Progr. Cod Procms. Techno/. 1980,6, 186
FUEL,
1985,
6
Vol 64, October
9 10 I1 12 13 14 15
16 17 18
Kikukawa.N., Makino, M., Maruyama, K.,Furuta.T., Horie,M. and Kimura, H. Nippon Kagaku Krrishi 1980,6,981 Bittner, D., Baumann, H., Peuckert, C., Klein, J. and Jiintgcn. H. El-doe1 Kohle Erdgas Perrochem. 1981, 34, 237 Miller, R. and Kaske, G. E&d Kohlr Ertlgcu Petrochum. 1984, 37. 149 Schulten. H.-R. Fuel 1982,61,670 Ristau, R. Diplomarbeit Universitat Dortmund, Lehrstuhl fiir Thermodynamik, 1984 Ungcr, P. E. and Suuberg. E. M. Fuel 1984,63, 606 Desypris, J.. Murdoch. P. and Williams, A. Fuel 1982. 61, 807 Arendt, P. and van Heek, K.-H. Fuel 1981,60,779 Eklund, H. and Wanzl, W. ‘Proc. Int. Conf. Coal Sci’, Dusseldorf, 1981, p. 701 Beiers, H.-G.. Baumann. H.. Bittner, D. and Klein, J. ‘Proc. 7th lnt. Symp. Plasma Chem.‘, Eindhoven, The Netherlands. July 1985 Anthony, D. B., Howard, J. B.. Hottel, H. C. and Meissner. H. P. Fire/ 1976. 55, 121 Anthony, D. B. and Howard, J. B. Am. 1risr. Chem. Brrg. J. 1976. 22. 625 Suuberg, E. M.. Peters, W. A. and Howard, J. B. Fuel 1980.59405