Energy & Fuels 1995,9, 834-840
834
Basic Phenomena Responsible for Generation of Coking Pressure: Field Ionization Mass Spectrometry Studies Anna Marzec” Institute of Coal Chemistry, Polish Academy of Sciences, Gliwice, Poland
Ramon Alvarez and Dolores M. Casal Instituto Nacional del Carbon, C.S.I.C.,Oviedo, Spain
Hans-Rolf Schulten Fachhochschule Fresenius, Wiesbaden, Germany Received February 16, 1995@
During industral coking some coals generate pressures in coke ovens that are too high which leads to troubles in operation and shortens coke oven life. Conventional coal testing methods are not capable to fully eliminate “dangerous“coals from processing. The major reason is that phenomena responsible for generation of coking pressure are not well understood. The aim of our work was t o disclose differences in pyrolysis behavior of coals which show different coking pressure. Eight coals with coking pressure in the 1-49 kPa range were analyzed by means of pyrolysis-field ionization mass spectrometry (Py-FIMS). The results indicate that coking pressure is related to (i)the yield of pyrolysis products generated in the 470-700 “C range and (ii)individual composition of the pyrolysis products. Thus, in spite of the common view that coking pressure is generated in the coal plastic layer, the results show the pressure depends on thermal decomposition in the semicoke/coke zone. The mechanism of coking pressure formation in coke ovens is presented.
Introduction In the course of carbonization in coke ovens, a certain pressure against the oven walls is produced which is traditionally known as coking pressure. Coals or coal blends developing high coking pressure may damage coke oven walls and reduce the coke oven life. A number of methods have been developed to test the coking pressure properties of coals. Pilot scale coke ovens equipped with one movable wall1 enable t o directly measure the pressure effecting the walls (i.e., “wall”pressure). However, the movable-wall pilot ovens are expensive and measurements consume a long time. Measurements of pressure inside the coal charge by means of probes introduced through doors or charging holes of full scale ovens, pilot ovens or laboratory coking devices result in determination of “internal gas press ~ r e . ”The ~ , ~relationship was proposed which enables calculation of wall pressure on the basis of internal gas pressure mae~urements.~ Much research work is taking place within the European Community on the subject of coking pressure which is sponsored through its Coal Research Programme. The Coking Pressure Seminar held at the @Abstractpublished in Advance ACS Abstracts, August 1, 1995. (1)Tucker, J.; Everitt, G . Ironmaking Conf. Proc. 1989, 84, 599617. (2)Tucker, J.;Everitt, G . 2nd International Cokemaking Congress; Institute of Materials: London, 1992; Vol. 2, pp 40-61. (3)Loison, R.; Foch, P.; Boyer, A. Coke Quality and Production; Butterworth: 1992; pp 355-376. (4) British Carbonization Research Association. Technical Paper No. 5 , 1992.
Coal Research Establishment, UK, in 1991 was attended by 18 specialists representing 11 organizations from 7 countries. Ten papers were presented covering “state of art” of the subject in the European Communitye5 Research works in other countries were also reviewed.6 It is generally accepted that coking pressure originates from volatile matter released on heating the coal charge. More specifically, coking pressure is believed to be generated in the plastic layer. In a majority of the tests the attempt is concentrated on measuring the internal gas pressure at the oven center since the highest pressures were observed a t the plane of meeting of two plastic layers. Critical assessment of internal gas pressure measurements showed that their reproducibilities are poor.7 One of the factors responsible for weak reproducibility was disclosed when very thin pressure probes (syringe needles) were used.8 Gas pressure appeared to change significantly along a width (a few centimeters) of the plastic layer. Other experiments showed that pressures measured on the outside of the central plane correlate much better with wall pressure than those measured in the oven center when two plastic layers meet t ~ g e t h e r . ~ In an unique approach of testing coking pressure,1° a laboratory one-wall-heated oven (400 g of coal charge) (5) Coking Pressure Seminar. Coke Making Int. 1992,4, 1-40.
(6) Coke Oven Wall Pressures. Measurements, Cause and Effect. Iron and Steel Society: Warrendale, PA, 1992. (7) Szurman, E.; Siebert, W.; Fbhde,W. Coke Making Int. 1992,4, 26-30. (8) te Lindert, M.; Schelvis, R. P. A. Coke Making Int. 1992,4, 1920. (9)Steyls, D. Coke Making Int. 1992, 4 , 31-33
0887-0624/95/2509-0834$09.00/00 1995 American Chemical Society
Phenomena Responsible for Generation of Coking Pressure
Energy & Fuels, Vol. 9, No. 5, 1995 836
Table 1. Characteristics of the Coals coal no.
FIMS-VM
moisture
ash
VM daf
(wt % coal basis)
(wt % coal basis)
(wt % dry basis
20.1 21.7 27.8 28.5 21.3 19.5 21.6 25.6
23.3 19.8 31.0 27.8 17.8 18.0 19.2 24.7
0.9 0.8 1.4 0.9 0.6 0.8 1.0 1.1
5.8 9.5 5.6 11.4 8.1 9.6 9.8 10.5
was used; inside the charge a thin pipe is inserted and a constant flow of nitrogen is maintained. The pressure needed to maintain the constant nitrogen flow is recorded versus the temperature. The experiments showed that for all the coals (characterized by high wall pressure when tested in a pilot oven) the maximum pressure was observed at about 600 "C, i.e., in a semicoke layer of the charge. Thus, the observations8-10 make questionable the concept of coal plastic state as a source of coking pressure. Fundamental studies of the mechanism of coking pressure generation have received less attention5p6than practical aspects of coal testing. It seems an application of advanced analytical techniques for comparative studies of coals showing high and low coking pressure (socalled "dangerous" and "safe" coals) may be useful. The application of field ionization mass spectrometry (FIMS) to this issue is the subject of our paper. The outstanding features of field ionization mass spectrometry11J2are (i) strongly reduced mass spectrometric fragmentation; (ii) fairly high mass capability: molecular ions can be detected up to 1000-2000 Da; and (iii)high intensities of molecular ions for substances of a wide ranges of mass and polarities. In particular, the combination of pyrolysis (Py) and FIMS is well suited for the extremely complex biogenic mixtures such as coals. Pyrolysis carried out directly in the FIMS apparatus under high vacuum provides the least opportunity for secondary reactions. Therefore, the mass spectra obtained display almost exclusively molecular ions of coal primary pyrolysis products. The number of pyrolysates, their molecular weights, and their relative abundances can readily be obtained. Py-FIMS was already used in studies of coal structure13 and reactivities such as identification of coal pyrolysates that are active in formation of tar,14 in hydrogenation reactions15J6 and in the development of thermoplastic properties16-18of coals. Our present studies on coking pressure are based on the following concept. A majority of the thermal decomposition products that are generated by heating coal (10)Geny, J.-F.; Duchene, J.-M.; Isler,D.;Yax,E. Proc. Zronmaking Conf. 1991,169-175. (11)Schulten, H.-R.; Simmleit, N.; Mueller, R. Anal. Chem. 1989, 61,221. (12)Lattimer, R. P.; Schulten,H.-R. Anal. Chem. 1989,61,12OlA. (13)Marzec, A.;Schulten, H.-R. Fuel 1994,73, 1294-1305. (14)Schulten, H.-R.; Marzec, A.; Dyla, P.; Simmleit, N.; Mueller, R. Energy Fuels 1989,3,481-487. (15)Marzec, A.;Czajkowska, S.; Simmleit, N.; Schulten, H.-R. Fuel Process. Technol. 1990,26,53-66. (16)Schulten, H.-R.;Marzec, A.; Czajkowska, S. Energy Fuels 1992, 6,103-108. (17)Marzec, A.; Czajkowska, S.; Moszynski, J.; Schulten, H.-R. Enerm Fuels 1992.6.97-103. (1% Marzec, A.;Czajkowska, S.; Schulten, H.-R. Energy Fuels 1994, 8,360-368.
element analysis (wt % daf) C H N S 0 90.6 4.4 1.5 nd nd 89.4 4.8 2.1 nd nd 88.4 5.2 1.7 nd nd 88.0 5.1 1.9 nd nd 89.4 4.8 1.8 1.3 2.7 4.4 1.6 91.2 0.7 2.1 4.8 89.0 1.7 0.7 3.8 5.2 1.8 87.9 0.6 4.5
coalorigin
USA . ~ ~
_
USA USA UK Spain Australia Australia Australia
directly in the mass spectrometer are also formed during carbonization in a coke oven (if the coal in the two cases is subjected t o the same temperature range). In the mass spectrometer the pyrolysates are quickly withdrawn from the coal sample due to high vacuum and transferred to the ionization and detection sections. In a coke oven (there is no vacuum applied) the pyrolysates are retained in vapor or liquid state within the coal charge and contribute to the vapor pressure of the system before they eventually undergo further, i.e., secondary, pyrolysis reactions. Thus, using Py-FIMS, one can detect a t least some coal pyrolysates that contribute to pressure generation during coking. The aim of our work is to seek answers to the following questions: 1. What can be learned from Py-FIMS about formation of pyrolysates that occurs in dangerous and safe coking coals in the temperature range to 700 "C which includes thermoplastic state and semicoke formation? 2. What is the nature of physical and chemical phenomena that are responsible for high coking pressure? 3. Does Py-FIMS give an opportunity for developing an analytical method which can differentiate dangerous from safe coals?
Experimental Section Coals. Eight coals showing widely different coking pressure were studied. Their conventional characteristics are presented in Table 1. Two moveable-wall ovens were used for coking pressure measurements. The measurements for coals no. 1-4 were carried out using the laboratory-scale oven,19and for coals no. 5-8 the INCAR pilot oven was used.20 Since coking pressure characteristics of the coals (Table 2) are derived from the different ovens, the two sets of studied coals (i.e., the coals 1-4 and coals 5-8) should be considered separately.
Pyrolysis-Field Ionization Mass Spectrometry (Py-
FIMS)of the Coals. The analysis of coals by Py-FIMS has using the direct inbeen described p r e v i ~ u s l y . ~ ~Briefly, -~* troduction system of a double-focusing mass spectrometer (Finnigan MAT 731,Bremen, Germany), about 100 pg of the ground coal samples was heated in high vacuum (about Pa) of a combined E y F W D ion source and field ionization (FI) was applied. The samples were heated to 700 "C at a rate of 3 "C/s. During heating a coal sample, successive mass scans (in 50-1000 Da range) were recorded electrically at each 10 "C increase of temperature. Three separate runs for each specimen were recorded, and averaged results were used for further studies. The following F'y-FIMS data were obtained for each of the studied coals: (i) Weight percent of coal samples that volatilized in the mass spectrometer on heating up t o 700 (19)Jordan, P.; Patrick, J. W.; Walker, A. Coke Making Znt. 1992, 4 , 12-15.
(20)Alvarez, R.; Casal, D. M.; Diez, M. A,; Gonzales, A. I.; Lazaro, M.; Pis, J. J. Proc. Znt. Conf: Coal Sci., Banff,Canada 1993;2, 107110.
Marzec et al.
836 Energy & Fuels, Vol. 9, No. 5, 1995
Table 2. FIMS Determination of Yields of Pyrolysis Products Formed in "Dangerous"and "Safe" Coking Coals coal gas pressa no. (Pa)
wall pressb (Wa)
1
36.4
33.1
3
15.0
17.2
2
10.0
15.9
4
4.1
4.8
400 "C 1 2 7.3
450 "C 3 4 24.0
1.7 19.0
36.1
5.9
25.1
10.4 67.5
99.0 29.3
30.7
31.0 100.0
19.8 99.0
95.3 26.5
23.2
23.3 100.0
98.7 18.1
14.6
100.0 22.9
91.6
83.5 18.8
98.3 20.9
94.5
73.7
16.5
13.5
5.0
20.0
7.5 59.2
89.6 16.8
81.1
52.8
4.6 48.5
72.3
64.5
37.9
650 "C 700 "C 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6
11.7
17.4
14.3
1.4 17.9
8
8.4 56.0
23.1
7 50.5
5.6 46.0
7.3
Cumulative yields in the temperature range to 500 "C 550 "C 600 "C
475 "C 5 6
19.5 100.0
27.5
27.8
wall press (kPaY 5
49.4
6
48.4
7
3.3
8
1.3
1.2
11.9 0.2
10.9
25.8
2.0 1.8 21.9
6.7
5.4
13.7
11.4 70.9
14.4
13.6 76.1 18.8
98.4 16.4
17.7
18.0 100.0
19.1
92.9 20.9
17.8
100.0
99.5 18.2
16.6
100.0
17.0
94.7
84.6
16.8
95.4 14.8
91.6
86.3
11.0 67.9
83.1 12.1
80.3
8.9 57.3
55.3
7.8 63.4
6.3 34.8
68.2
4.6 49.7
35.1
9.6
43.7
2.1
98.5 22.9
19.2 100.0
24.3
24.7
Internal gas pressure from a laboratory-scale oven.lg Wall pressure from the laboratory-scale oven.18 From a pilot-scale oven experiments. Note: Odd-numbered columns: % of total area under TIC curve. Even-numbered columns: wt % coals basis. a
1
T e m p e r a t u r e
[ " C ]
I
\
P
Figure 1. Total ion current (TIC) of thermal decomposition products volatilized in the mass spectrometer versus temperature of heating coal sample. Coal no. 1 showed high wall pressure (33 kPa); coal no. 4 showed low wall pressure (4.8 kPa). "C was obtained; the results are shown in Table 1(see FIMS
VM column). (ii)For each of the coals, total ion current (TIC) curve of volatilized material versus temperature of heating the samples was obtained; examples are presented in Figures 1 and 2. The data derived from integration of the areas under TIC curves in selected temperature ranges are presented in Table 2 and Figures 3-6. (iii) The FI mass signals of each scan were summed, processed, and plotted using SpectroSystem SS 200 t o give an intergrated spectrum for each of the coals (one of them is displayed in Figure 7). (iv) With the aim of comparing the composition of thermal degradation products emerging from the coals, the difference spectra for pairs of coals were obtained (examples are shown in Figures 8 and 9).
Results Yield of Pyrolysis Products Formed on Heating Coal to 700 "C. The total yields for the coals are 1831 wt % (coal basis). No relationship between the total yields (Table 1)and coking pressures (shown in Table 2) of the coals is observed. TIC Curves of the Coals. There is a clear difference between TIC curves of the dangerous compared with safe coals. The curves of the dangerous coals (see coal 1in Figure 1,and coal 5 in Figure 2) are shifted towards higher temperatures compared with the safe coals
Figure 2. Total ion current (TIC) of thermal decomposition products volatilized in the mass spectrometer versus temperature of heating coal sample. Coal no. 5 shows high wall pressure (49.4 kPa); coal no. 8 shows low wall pressure (1.3 kPa).
(see coal 4 in Figure 1, and coal 8 in Figure 2). TIC curves for other coals are not displayed in the figures for the sake of clarity. In fact, they are situated in a sequence of their increasing coking pressure (except coal 3) between the extreme TIC curves shown in the Figures. Distribution of Yields of Pyrolysis Products. With the aim of making a quantitative use of TIC curves, areas under the curves were integrated (the total area = 100%)and the decomposition product yields were calculated for selected temperature ranges; the cumulative yields are shown in Table 2 (odd-numberedcolumns of the table). On the basis of these data, Figures 3 and 4 were prepared which display the yields of pyrolysis products that are generated over 475 "C. The figures show characteristic features of the dangerous and safe coals. Thus, the three highly dangerous coals (no. 1,5, and 6; their wall pressure is in the 33-49 kPa range) are characterized by high yield of pyrolysis products (50-74%) generated in the 475-700 "C range. Contrary to that, the three safe coals (no. 4, 7, and 8; their wall pressure is below 5 kPa) are characterized by lower pyrolysis yields (32-43%) in this temperature range. However, Figure 3 referring to the set of coals 1-4 does
Energy & Fuels, Vol. 9, No. 5, 1995 837
Phenomena Responsible for Generation of Coking Pressure
I
16 14
16 kPa
Y
I
12
j 10 -
8 6
a@
g 6 4 2
0 I 475-7OOoC
2 500-7OoOC
Figure 3. Coals no. 1-4. Yields of thermal decomposition products (100% TIC basis) generated in the 475-700 "C (1) and 500-700 "C (2) temperature ranges.
1
475-7OoOC
2
500-7OOoC
Figure 5. Coals no. 1-4:Weight percent of coal decomposing in the 475-700 "C (1)and 500-700 "C (2) temperature ranges. 14
12
0 1 kPa
70
3
0 1 kPa
60
xb 50
8
-*
$
a
s
5 40
e k
3
10
1- 0 a
30
6 4
9 6 20
2
ac
0 10
1 475-7OOOC
0
1 475-7OOOC
2 500-70ooC
Figure 4. Coals no. 5-8. Yields of thermal decomposition products (100% TIC basis) generated in the 475-700 "C (1) and 500-700 "C (2) temperature ranges.
not show any clear relationship between the yields and the wall pressures of the four coals (in contrast to Figure 4, where such relationship for the set of coals 5-8 is observed). The different results obtained for the two sets of coals (Figure 3 compared with Figure 4), prompted us to consider absolute yields of pyrolysis products, calculated on wt % coal basis (see even-numbered columns in Table 2) instead of the above relative yields. Figure 5 (for coals 1-4) and Figure 6 (for coals 5-8) show clear increasing trend between (i) wt % of coal that decomposes in the 475-700 "C range and (ii) the wall pressure. Differences of the yields between some coals are low (about 1 wt %), although the coals are significantly different with respect of their wall pressure (see Figure 5). It is worth pointing out however, that this 1wt % for 1000 kg coal charge in a coke oven means the conversion of 10 kg of coal to several cubic meters of vapor pyrolysis products. Such amount may have a strong impact on coking pressure. In contrast, the results referring to the decomposition yields in the temperature range below 475 "C (the range includes plastic state of coal) are not conclusive (see column 6 and pressure values in Table 2). Thus, for the coals 1-4 there is no relationship between the yields and coking pressures. On the other hand, the data for
2 500-7OOoC
Figure 6. Coals no. 5-8. Weight percent of coal decomposing in the 475-700 "C (1)and 500-700 "C (2) temperature ranges.
the set of coals 5-8 imply there is a link between the yield of decomposition products generated below 475 "C and coking pressure: the higher the yield the lower the pressure. Summary of the Ftesults Derived from TIC Curves of the Coals. 1. The characteristic feature of dangerous coking coals is that more than 50% of their pyrolysis products are formed in the temperature range above 475 "C (compared with the yields generated below this temperature). This indicates that in dangerous coals, much higher pressure can be generated on the side of already resolidified coal material (semicoke) compared with the pressure generated in the area of still plastic coal. 2. Coking pressure of coals depends on the weight percent of coal that decomposes in the temperature range '475 "C. The higher the percentage the higher the coking pressure (there is not however, a steady increment of coking pressure with the percentage increase). 3. The amount of coal that decomposes below 475 "C, i.e., during the development of plastic state, does not intensify coking pressure. The results are in accord with the results21 of measuring coking pressures of a coal and the coal blended with tar pitch (usually more than 35 wt % of a pitch decomposes t o volatile products at temperature range over 475 "C)or blended with anthracene oil (anthracene oil boiling range is 330-360 "C, i.e., the oil can be fully (21)Geny,J. F.;Duchene, J. M.Coke Making Int. 1992,4,21-25.
Marzec et al.
838 Energy & Fuels, Vol. 9,No. 5, 1995 80000
70000
4
50000
4
300oo-l
I
-10000~
-50000'
m / z
r,
--
-70000!
Figure 7. Integrated FIMS spectrum of one of the studied coals. The spectrum shows all thermal decomposition products volatilized in the spectrometer on heating the coal to 700 "C.
,
100
,
,
200
,
,
300 rP/z
,
,
400
,
,
500
,
,
600
,
,
700
,
,
800
,
,
900
Figure 9. Difference FIMS spectrum of coal no. 5 (49 kPa wall pressure) and coal no. 8 (1.3 kPa wall pressure).
80000 60000
9
40000
@ i
Y
5
-40000
-60000 -
8 100
200
0 300
m/z
0 400
500 ~
0 600
0
700
800
I 900
rz-
Figure 8. Difference FIMS spectrum of coal no. 1 (33 kPa wall pressure) and coal no. 4 (4.8 kPa wall pressure).
volatilized below 475 "C). The pitch addition resulted in a significant increase of the pressure.21 On the contrary, the oil addition did not enhance coking pressure.21 Py-FIMS Integrated Spectra of the Coals. The integrated spectrum represents all the thermal decomposition products that were successivelyemerging from the coal sample on its heating to 700 "C. All the coals produced decomposition products in the same, i.e., 150800 Da range. A broad, roughly Gaussian intensity distribution is observed, and the most intense mass signals are in the 300-500 Da range. All the studied coals have in common the general characteristics of the integrated spectra (as an example, Figure 7 shows one of the spectrum). It is not known whether all pyrolysates of higher molecular weights contribute to vapor phase in a coke oven, but certainly more of them do it in semicoke/cokezone (due to higher temperature) than in the plastic zone. Difference Spectra of the Coals. In spite of the described similarities, the coals are differentiated with respect to relative intensities of some mass signals. This indicates there are differences in composition of thermal degradation products that are generated in different coals. The difference spectrum of the dangerous coal 1 and the safe coal 4 (Figure 8) as well as another difference spectrum of the dangerous coal 5 and the safe coal 8 (Figure 9)are displayed. The upper parts of the spectra show mass signals of pyrolysates that predomi-
nate in the dangerous coals (D-pyrolysates) while the lower parts of the spectra indicate pyrolysates predominating in the safe coals (S-pyrolysates). The two types (i.e., D and S)of mass signals in the 190-430 Da range are listed in Table 3. For each of the eight coals, intensities of the listed signals were measured (using the integrated spectra). Then, all the intensities of the group D mass signals were summed. Separately, the intensities of the group S signals were summed as well. The ratio of the sums was calculated for each coal (see D/S values in Table 3). The D/S ratio increases with the increase of coking pressure for the coals 4 , 2 , and 1 (coal 3 is an outlier), as well as for all the coals 8-5. Thus, the composition of pyrolysis products is another factor that influences coking pressure of the coals. Homolog series ascribed t o the FI mass signals in question are listed in Table 3. The majority of pyrolysates contributing to building-up coking pressure (Dpyrolysates) represent the C1 to C3 members of the homologous series of aromatic hydrocarbons. Contrary t o that, pyrolysates that predominate in safe coals represent higher members starting from C4-C5. These data imply that longer alkyl substituents occur in the S-pyrolysates compared with the D-pyrolysates. As was shown by numerous studies on thermal reactivity of alkyl aromatic hydrocarbons,22 the longer the alkyl substituent the higher the reactivity of a hydrocarbon in reaction of homolytic rupture of alkyl bonds. The reaction results in the formation of radicals or olefinic bonds, i.e., the products that are very susceptible to polymerization. Thus, the following explanation of the role of the pyrolysates in question may be presented. The reactivity of the S- and D-pyrolysates is different. The S-pyrolysatessoon after being formed in a coke oven react with coal material and are withdrawn from vapor phase and incorporated to solid or liquid phase. In contrast, the D-pyrolysates do not react (or react t o much lesser extent) and remain components of vapor phase. Hence, the higher the ratio of the D- over S-pyrolysates in a decomposing coal, the higher the vapor pressure. Summary of the Results Derived from the Difference Spectra. 1. There is a difference in composi(22) Poutsma, M. L. Energy Fuels 1BsO,4, 113-131, and references therein.
Energy & Fuels, Vol. 9, No. 5, 1995 839
Phenomena Responsible for Generation of Coking Pressure
Table 3. FI Mass Signals Indicated by the Difference Spectra pyrolysates (D)predominating in dangerous coals mlz 192 228 254 266 292 318 316 342 354 366 392 404
242 268 332 330 356 368 380 406 418
coal no. DIS
344 370 382 394 420 432 1 1.31
408
CnH2n-18
CI
CnH2n-24 CnH2n-26 CnH2n-28 CnH2n-30 CnH2n-32 CnH2n-34 CnH2n-36 CnH2n-38 CnH2n-40 CnH2n-42 CnH2n-44
CO-Cl C1-G C1 CI
2 1.17
cZ-c3 c1-c3
Cl-c3 C0-G Cl-c4
c1-c3 cZ-c4
Mechanism of Building-up Coking Pressure When one is considering generation of coking pressure, the following facts should be taken into account: (i) coexistence of three different zones in coke oven, (ii) formation of gaseous products, and (iii) formation of higher pyrolysates. Zones in Coke Oven. The three zones are unreacted coal zone, coal plastic layer, and semicoke/coke zone. A temperature range a t which a coking coal exists in an oven in plastic state poses a question. The thermoplastic properties of coals measured by the standard method (ASTMD 2639)cannot be directly referred to a coal in a coke oven since the method does not take into account the influence of heating rate and pressure which are different in a coke oven and in the standard apparatus. As computer-assisted calculations implied,23aheating rates in a typical coke oven change from 6 t o 2 "Umin depending on the distance from the oven wall and on coking time. On the other hand, experiments showed that the decrease of heating rates in the range in question resulted in a significant decrease of fluidity and resolidification temperature.23b The observed effects of increased pressure were to expand the plastic range usually by a significant decrease of softening temperat ~ r e . Thus, ~ ~ ,the ~ plastic ~ state of a coal in a coke oven may be approximately estimated to be somewhere in the 300-500 "C range with its resolidification temperature in the 475-500 "C range. It is worth pointing out that (23)(a) Van Krevelen, D. V. Coal; Typology-Physics-ChemistryConstitution; Elsevier: Amsterdam, 1993; p 760. (b) Ibid. p 693. (c) Ibid. pp 684-687. (24)Khan, M.R.;Jenkins, G. Influence of Weathering and Oxidation on the Thermoplastic Properties of Coal. In Chemistry of Coal Weathering; Nelson, Ch. R., Ed.; Elsevier: New York, 1989;Chapter 5;pp 107132 and references therein. (25)Ndaji, F. E.; Butterfield, I. M.; Thomas, K. M. Coke Making
homolog series member
mlz 248 246 272 270 296 308 320 360 312 300
262 260 286 284 310 322 334 374 386 314
352 350 376
364
D/S Ratio for the Coals 3 4 0.86 0.90
tion of pyrolysis products that are formed in different coking coals on heating to 700 "C. 2. Coking pressure of the coals is related to the content ratio of selected individual pyrolysates in the 190-430 molecular weight range. The higher the content of short alkylaromatic hydrocarbons (compared with the content of C~-C~-alkylaromatics)in a coal the higher its coking pressure.
Int. 1992, 4 , 5-11.
pyrolysates (D)predominating in safe coals
homolog series member
276 214 300 298 324 336 348 388 400 328
5 1.24
290 288 314 312 338 350 362 402
6 1.00
328 326 352 364 376
CnH2n-18 CnH2n-20 CnHzn-2z CnHzn-24 CnH2n-26 CnH2n-28 CnHzn-30 CnH2n-32 CnH2n-34 CnHzn-36
CS-C9 C4-G C5-C9
CnH2n-40 CnH2n-42 CnH2n-44
CO Co-C1 CO
7 0.96
c3-c7
c4-C~ c4-C~ c3-c7
c4-C~ C5-G C0-G
8 0.71
the layer of freshly resolidified material (semicoke) shows poor porosity26and can form a barrier for diffusion of gases and liquids between the semicoke/coke zone on the one hand, and the zones of plastic and unreacted coal, on the other. Formation of Gaseous Products. Numerous studies on coal pyrolysis23c~27~28 showed that formation of gaseous products (i.e., CHI, Hz, CO, C02, HCN, H2S, and Cz-C3 hydrocarbons) starts a t about 400 "C and occurs over a wide range of temperature. Evolution of H2 and CHI continues up to the end temperature of coking (1100-1200 "C). Formation of Higher Pyrolysates. As our present studies showed, pyrolysates of molecular weights in the 150-800 range are formed in coking coals. The formation is initiated at about 250-350 "C, reaches a maximum a t 400-500 "C (depending on a coal), extends well over the end temperature of plastic state (TRabout 475500 "C), continues in a semicoke, and then declines to a negligible amount at 650-700 "C. Amounts of the vapor pyrolysis products in the zones depend not only on the zone temperature but also on individual composition of pyrolysates which is a distinctive feature of a coal. Some of the pyrolysates, soon after they are generated, undergo polymerization and condensation and, therefore, they are withdrawn from vapor phase. Others, due to their lower reactivity in the reaction in question, remain in vapor state. Factors Affecting Pressure in the CokeISemicoke Zone. The zone spans from an oven wall to a layer of freshly resolidified carbonaceous material and exists in the 1200-1100 t o 500-475 "C temperature range. The zone thickness increases continuously with time of coking the charge. Potential pressure that may be generated within the zone depends on the amount of coal material that is converted (in the temperature range in question) to (i)gaseous products and (ii)higher pyrolysates occurring in the vapor state. Each of the two, i.e., (i) as well as (ii), factors is the product of material percentage (yield) that is converted to gaseous ~
~~
~
(26)Hays, D.;Patrick, J. W.; Walker, A. Fuel 1976, 55, 297-302. (27)Solomon, P. R.;Serio, M. A.; Suuberg, E. M. Prog. Energy Combust. Sci. 1992, 18, 133-220, and references therein. (28)Solomon,P.R.;Hamblen, D. G. Pyrolysis.In Chemistry ofcoal Conversion; Schlosberg, R. H., Ed.; Plenum Press: New York, 1985; Chapter 5;pp 121-252.
Marzec et al.
840 Energy & Fuels, Vol. 9, No. 5, 1995
or vapor substances, multiplied by an amount of the material affected by the temperature range. Thus, with an increase of the zone thickness, the amount of gaseous and vapor products increases and, therefore, the pressure in coke/semicoke zone increases. Factors Affecting Pressure in Plastic Zone. Similar factors, i.e., the amount of coal material that is converted to (iii) gaseous and (iv) vapor products at temperatures below resolidification (
