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Ind. Eng. Chem. Res. 2006, 45, 906-912
APPLIED CHEMISTRY Stepwise Chemical Demineralization of Go1 ynu1 k (Turkey) Oil Shale and Pyrolysis of Demineralization Products Levent Ballice* Department of Chemical Engineering, Faculty of Engineering, UniVersity of Ege, 35100 BornoVa, Izmir, Turkey
In this study, the effect of the mineral matter of Go¨ynu¨k oil shale on the conversion of organic carbon in kerogen into volatile hydrocarbons, polycyclic aromatic hydrocarbons (asphaltenes, preasphaltenes), and carbon in the solid residue was investigated. Kerogen was isolated by successive HCl, HNO3, and HF treatments. A series of temperature-programmed pyrolysis operations was performed with raw Go¨ynu¨k oil shale and each product of every demineralization process. The results show that removal of the material soluble in HCl and HNO3 slightly affected the conversion of organic materials in the pyrolysis reactions. In contrast, removal of the material soluble in HF increased the conversion in pyrolysis reactions. This behavior is explained by the inhibitive effect of the silicate minerals. The leaching of mineral matter with HCl, HNO3, and HF caused no remarkable change in the composition of volatile hydrocarbons. 1. Introduction Successive world energy crises and large increases in the prices of oil derivatives have caused renewed interest in the different possibilities offered by valorization carbonaceous material as an energy source and in the chemical exploitation of pyrolysis products. Synthetic gaseous or liquid fuels are obtained by converting a carbonaceous material to another form. The most abundant naturally occurring materials suitable for this purpose are coal, oil shale, and tar sand.1 Pyrolysis refers to the decomposition of organic matter by heat in the absence of air. When coal or oil shale is pyrolyzed, hydrogen-rich volatile matter is distilled, and carbon-rich solid residue is left behind. Pyrolysis of oil shale is generally taken to mean destructive distillation. Pyrolysis is one of the commonly used methods to produce liquid fuels from coal, and it is the principal method used to convert oil shale to liquid fuels. Moreover, as gasification and liquefaction are carried out at elevated temperatures, pyrolysis can be considered a first stage in any conversion process.2 Of most interest in the production of synthetic fuels is the prediction of the rate and amount of volatile yield and product distribution for a given raw material and set of pyrolysis conditions. The composition of the raw material, pyrolysis temperature, heating rate, particle size, pressure, and pyrolysis atmosphere are the major factors affecting the yield and composition of volatile matter. The fate of inorganic elements in pyrolysis and catalytic effects of mineral matter are in need of investigation. It is known that certain minerals exhibit definite catalytic effects in gasification and others exhibit catalytic effect in liquefaction. The effect of mineral matter on the pyrolysis yield and on the product distribution is, however, a fairly open question. Can the quality or quantity, or both, of pyrolysis products be controlled by the proper selection of mineral matter in the feed oil shale? How * E-mail:
[email protected]. Fax: 0090 232 388 7600. Tel.: 0090 232 388 4000-1484.
does mineral matter affect the temperature at which the most desirable pyrolysis products are generated? The relationship between the physical structure of oil shales and the pyrolysis product distribution needs definition. Answers to these and related questions might provide direction toward the appropriate choice of pretreatments to remove mineral material from oil shale. Oil shale consists of complex sapropelic organic material of high molecular weight (kerogen) that is finely distributed in an inorganic matrix. Kerogen, defined as insoluble organic matter in sedimentary rock, is the major source of oil produced in retorting oil shale. The inorganic part consists mainly of quartz, clay, different types of carbonates (CaCO3, MgCO3, and dolomite), pyrite, and Fe2O3, as well as trace elements (As, B, Mo, Ni, and Zn).3,4 The interaction between kerogen and the inorganic matrix during reactions is not well understood, and detailed investigation of pyrolytic behavior of kerogen is therefore important for designing processes suitable for oil shale retorting. Kerogen is formed by geochemical reactions involving organic debris of algae, spores, bacteria, or higher plants. A commonly accepted model is that of a polymeric material composed of nonrepeating polynuclear aromatic units with peripheral and bridging functional groups. The principal functional groups are alkyl and alkyl/aryl chain substituents and carboxylic groups.5,6 Alginate is the predominant organic material in type I kerogen. Type I kerogen originates mainly from marine or lacustrine organic material and therefore has a high H/C ratio compared to types II and III kerogen, accounting for the high hydrocarbon yield. Type II kerogen has a significant component of terrestrial as well as marine material;5,6 its H/C ratio is lower than that of type I kerogen, and its O/C ration is slightly higher. Type III kerogen is mainly terrestrial in origin with higher O/C and lower H/C ratios than the other types, reflecting the increasing proportions of polycyclic aromatics and oxygen-containing aromatic groups. The purpose of this study was to investigate the effects of the mineral matter in oil shale on the pyrolytic behavior of
10.1021/ie050751f CCC: $33.50 © 2006 American Chemical Society Published on Web 01/05/2006
Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006 907 Table 1. Elemental Analyses of GOS-R, GOS-C, GOS-P, and GOS-S (Dry Basis)
Table 2. Amounts of GOS-R, GOS-C, GOS-P, and GOS-S Used in Temperature-Programmed Pyrolysis
sample
Corganic (%)
H (%)
ash (%)
H/C
sample
mass (g)
GOS-R GOS-C GOS-P GOS-S
46.3 58.2 59.3 63.8
5.8 7.2 7.3 8.0
16.0 12.0 11.0 0.1
1.5 1.5 1.5 1.5
GOS-R GOS-C GOS-P GOS-S
0.50 0.40 0.39 0.37
organic structures at temperatures up to 550 °C in an inert gas atmosphere. The composition of volatile products and their production rate were determined by using a special sampling technique for collecting pyrolysis products eluted from the reactor at different temperatures and times. The effect of the mineral content of oil shale on product yield and composition in thermal decomposition was determined by establishing a carbon balance around the reactor. The recoveries of total organic carbon in raw oil shale and demineralized oil shale samples as volatile hydrocarbons, polycyclic aromatics (asphaltenes and preasphaltenes), and carbon in the solid residue were determined. 2. Experimental Section Samples. Oil shale samples were obtained from the Go¨ynu¨k oil shale deposit located near the town of Bolu in northwestern Turkey. This deposit is the largest in Turkey. For pyrolysis experiments, the oil shale was crushed and then ground in a jaw mill until the desired particle size was obtained. The sample was sieved to obtain a size fraction C30) in the pyrolysis products. The discrepancy in the carbon balance (Figure 5) was assigned to asphaltenes, preasphaltenes, and long-chain-aliphatic (>C30) hydrocarbons. It is not surprising that the discrepancy for GOS-S was slightly higher than those for GOS-R, GOS-C, and GOSP. It was speculated that the tendency to give more polycyclic aromatic compounds was directly responsible for the high rate of coking observed for GOS-R, GOS-C, and GOS-P. It is generally known that aromatics are more prone to undergo coking reactions than are aliphatics,23 and the pyrolysis reactions of GOS-S did not show the catalytic effect of the silicate minerals aiding the coking reactions of aromatics and the deposition of carbon in the solid residue. 4. Conclusion The leaching of mineral matter with HCl, HNO3, and HF did not cause marked changes in the ratios of C5-C15 n-paraffins and 1-olefins in volatile hydrocarbons. The yield of total volatile hydrocarbons increased for the kerogen sample after silicate removal (GOS-S). This tendency was explained as being due mainly to the driving force for heat transfer because more heat was transferred from outside toward the inside of the oil shale particles, thus allowing pyrolysis to occur with ease.27 Alkali and alkaline earth metal cations affect the reactivity of oil shales, and the leaching of these mineral materials with HCl caused a slight decrease in the conversion to volatile hydrocarbons, but the removal of pyrites with HNO3 did not affect the reactivity of the organic material in pyrolysis. The inhibition effect of
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the silicates seemed to be greater than the catalytic effect of the calcite minerals in the pyrolysis reactions. The removal of the silicates by HF washing of Go¨ynu¨k oil shale caused a moderate change in the production of the volatiles. The recovery of the hydrocarbon yield increased in the pyrolysis of silicatefree Go¨ynu¨k oil shale, whereas the deposition of carbon in the solid residue decreased. Removal of the material soluble in HCl and HNO3 slightly affected the amount of carbon in the solid residue. Calcite minerals are known to catalyze degradation of the organic structure of coal and oil shale, so removal of these minerals might have changed the behavior of GOS-C and GOS-P and, likewise, deposition of carbon in the solid residue compared to that for GOS-R. The aromatics are more prone to undergo coking reactions than are aliphatics, and the pyrolysis reactions of silicate-free oil shale (GOS-S) did not show the catalytic effect of the silicate minerals aiding the coking reactions and deposition of carbon in the solid residue. For that reason, the discrepancy assigned to asphaltenic and preasphaltenic hydrocarbons for GOS-S was slightly higher than those for GOS-R, GOS-C, and GOS-P. Acknowledgment The author thanks the Department of Petroleum, Gas and Coal of the Engler-Bunte Institute, University of Karlsruhe, Karlsruhe, Germany, for the use of laboratory facilities and Deutscher Akademischer Austauschdienst (DAAD) for financial support. Nomenclature GOS-R ) raw Go¨ynu¨k oil shale GOS-C ) carbonate-free Go¨ynu¨k oil shale GOS-P ) pyrite-free Go¨ynu¨k oil shale GOS-S ) silicate-free Go¨ynu¨k oil shale VHC ) volatile hydrocarbons PER ) product evolution rate
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ReceiVed for reView June 23, 2005 ReVised manuscript receiVed October 31, 2005 Accepted November 4, 2005 IE050751F
