Combustion kinetics of asphaltites compared to coals through isoconversional analysis

Abstract

Asphaltites are dark-colored solid petroleum, classified as a group of Bitumens based on the latest classifications; this is disputable. They are formed by alteration during or after migration, generally in oil-bearing zones. They are primarily observed in petroleum provinces. In Turkey, they are extensively observed in the southeastern and some northern parts and are commonly produced by surface mining techniques for electricity generation. On the other hand, gilsonite, a form of asphaltite produced in the US (Uinta Basin of Northeastern Utah), is used as an additive for different purposes. Asphaltites originated from conventional oil. However, they have coal-like properties, such as ash content. It would be worthwhile if kinetic signatures could be compared to coal and petroleum. In this way, a more persistent picture of asphaltite combustion characteristics is clarified. Understanding this helps to decide on information about the reaction scheme, activation energy, and general combustion behavior, and finally, the reactions probably taking place in this procedure of newly trend alternative source of energy. For this purpose, a similar set of experiments is carried out with coal, and using literature, it is compared to petroleum, respectively. This experimental work analyzes the combustion characteristics of asphaltite from the Şırnak province of Turkey through kinetic-cell experiments. Five sets of ramped temperature experiments are conducted using a 2 g asphaltite sample with different heating rates. These experiments measure the temperature inside the cell and the effluent gas composition, primarily CO, CO2, CH4, and O2 concentrations. The results indicate two O2 consumption peaks at different temperatures, corresponding to High-Temperature Oxidation (HTO) and Low-Temperature Oxidation (LTO). Before doing any analysis, some comments compared to coal and asphaltite raw data, such as the initial temperature of LTO and HTO reactions, oxygen consumption, and carbon dioxide production rates. Then, the results are analyzed using isoconversional methods. These methods require a set of experiments conducted at different heating rates and result in a so-called isoconversional fingerprint (Conversion versus activation energy). The objective of the kinetic analysis is obtaining the activation energy without assuming any kinetic model under the isoconversional assumption that the reaction rate at a constant extent of conversion is only a function of temperature. This fingerprint provides information on the reaction scheme. The same analysis was repeated using carbon monoxide and carbon dioxide production data. Carbon monoxide data does not show reliable and sufficiently accurate results. However, taking carbon dioxide data into analysis contributes significantly to figuring out the reaction scheme that might have taken place in the gasification process. Considering that asphaltites are mostly treated as coals in some regions and literature, and the procedures and processes applied to them are almost the same, it is expected to show further similarities with coals. Yet, regarding classifications, crude oil shows a closer relationship to asphaltites. Asphaltites have been proven to exhibit complex behavior regarding activation energy trends, making it hard to comment on their characteristics or reaction schemes. Therefore, a commercial simulator is used to model the gasification experiments based on coal reactions to get a better understanding. Using this commercial simulator determines the behavior of coal in the gasification process and provides a better understanding of the governing reaction. Reservoir and sample characteristics and properties, geology, reactions, heating systems, imposed heating rates, and kinetic parameters are entered as input data into the simulator. The simulation showed that carbon oxidation plays the most crucial role in the gasification process in terms of both activation energy determination and consumption rates. Furthermore, these reactions occur in the most critical phase of the gasification procedure, namely, low and high-temperature oxidations in the intermediate gasification steps. Moreover, almost all the main gasification reactions occur in the last gasification phases. Therefore, it can be concluded that the extent of the gasification of coal and asphaltite tends to behave the same in the previous phases, and the mechanisms and reactions in the final steps of asphaltite gasification are similar to coal.