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Energy & Fuels 1997, 11, 982-986
Studies on Generation of Excessive Coking Pressure. 2. Field Ionization Mass Spectrometry of Coals Showing Different Contraction during Carbonization Anna Marzec* and Sylwia Czajkowska Institute of Coal Chemistry, Polish Academy of Sciences, Sowinskiego 5, 44-102 Gliwice, Poland
Ramon Alvarez, Jose J. Pis, and Maria A. Diez Instituto Nacional del Carbon INCAR, C.S.I.C, Apartado 73, Oviedo 33080, Spain
Hans-Rolf Schulten Institut Fresenius, 65232 Taunusstein, Germany Received December 18, 1996X
Coals characterized by different volume contraction or expansion (from -28 to +8 mm according to Koppers-INCAR test) and different “wall” pressure (0.4-49 kPa) were analyzed by means of pyrolysis field ionization mass spectrometry (Py-FIMS). The aim was to disclose characteristic features of thermal decomposition of the coals that influence their contraction as well as their “wall” pressure development during coking. The results showed the relationship between contraction values and the “wall” pressure. Py-FIMS results indicate that the contraction values depend on (i) yields of thermal degradation products generated in coals on their heating to resolidification temperature and (ii) individual composition of the thermal decomposition products. The coals show high contraction (Koppers -INCAR test e -10 mm) and low wall pressure (1) is formed during the plastic state. When the permeability of the semicoke layer is high enough, the volatiles of the plastic layer prefer a migration through the semicoke (into the coke layer) compared with their migration upward through the plastic layer or to the yet unreacted coal layer. This view is supported by the following. Results of studies of tars migration during coking indicate that a large part of volatile components of a “safe” coal migrate toward the hot side of the oven through semicoke and coke.7 It was estimated that about 80-90% of the whole gas evolved from coke ovens flows through the coke and upward along the coke oven walls.8,9 Distribution of fluidity within the plastic layer indicates that a low fluidity zone is much thicker on the unreacted coal side compared with the zone on semicoke side.8 The following view of generating excessive coking pressure may be presented. Low permeability of the semicoke layer does not allow vapor species generated in the plastic layer to migrate through the semicoke to the coke layer and from there upward. In such a case, the species are trapped in the plastic layer and build up the pressure in the layer. There may be, however, another possibilty of generating high pressure in a coke oven. This may refer to the case when the coke layer is entirely covered (on the top of the charge and on the plastic layer side) by an envelope of a semicoke layer. (5) Schulten, H.-R.; Marzec, A.; Czajkowska, S. Energy Fuels 1992, 6, 103-108. (6) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Proceedings of the European Carbon Conference, Carbon ’96, 1996; Vol. 2, pp 578580. (7) Koch, A.; Gruber, R.; Cagniant, D.; Pajak, J.; Krzton, A.; Duchene, J. M. Fuel Process. Technol. 1995, 45, 135-153. (8) Latshaw, G. M.; McCollum, H. R.; Stanley, R. W. Iron Steel Soc., Trans. 1990, 51-58. (9) Loison, T.; Foch, P.; Boyer, A. Coke quality and production; Butterworths: London, 1989; pp 140-141.
Marzec et al.
When the semicoke layer permeability is low, the volatile products generated in the coke space cannot escape from it and build up the pressure as long as some fissures in the semicoke layer are not formed. Conclusions In the conclusion drawn from the present results (parts 1 and 2), the following view can be presented with regard to contraction and its links with coal thermoplasticity as well as to a laboratory testing of coking coals. The controvertial problem of whether properties of the thermoplastic phase or of the semicoke layer are decisive factors in the generation of excessive coking pressure seems to be clarified. Both of them are important because the properties of the plastic phase (the yield and the composition of its thermal decomposition products) influence the properties of the semicoke layer (the contraction and permeability). Wall pressure observed during coking is related to the contraction of the semicoke layer in such way that the Koppers-INCAR contraction test can easily eliminate coals that produce excessive coking pressure. The Py-FIMS technique can be applied for testing coking coals with the aim of discriminating “safe” coals from “dangerous” coals. However, the high price of the instrument as well as its high operational costs makes its application rather impractical. Py-FIMS cannot be substituted by less expensive (but not operating under vacuum) thermal analytical methods (such as TG/DTG), since they are not capable of monitoring thermal degradation products of the wide molecular weight range up to 800 Da. The present data imply that measurements of permability of the semicoke layer might be useful as another method for testing coals (besides the Koppers-INCAR test). Such measurements should satisfy two conditions. First, the permeability should be measured for the semicoke layer at a temperature range at which the layer is formed and exists in the coke oven (about 500750 °C). Second, rather high molecular substances should be used as permeating species. The laboratory test using CPM sole heated oven7,10 fulfills the first condition but does not satisfy the second condition, since permeability is measured for a nitrogen flow. Therefore, results of the test may be misleading when they indicate high permeability of the semicoke layer. High permeability for a nitrogen flow does not necessarily result in high permeability for large molecules of coal thermal decomposition products. Acknowledgment. Financial support by ECSC Grant EUR 7220-EB 756 is gratefully acknowledged. EF9602279 (10) Geny, J. F.; Duchene, J. M.; Isler, D.; Yax, E. Proc.sIronmaking Conf. 1991, 189-175.
