Kinetic properties of gas-phase combustion of gel fuels based on oil-filled cryogels

Highlights

• Oil-filled cryogel and oil in a normal state have different kinetic properties of gas-phase combustion.

• Combustion mechanisms of oil-filled cryogels with 40–60 % of oil have a similar nature.

• The average activation energy, calculated by the Friedman method, is 13.8 kJ mol^–1.

• Thermal effect of oil-filled cryogel combustion is 5–8 % less than that of oil in normal state.

Abstract

The combustion mechanisms and characteristics of gel fuels and of combustible liquids in a normal state are different. Therefore, in this paper, the kinetic properties of gas-phase combustion of gel fuel evaporation products have been determined using the data of thermogravimetric analysis (TG) and differential thermal analysis (DTA) at the experimental sample heating rates of 5, 15 and 30 °C per minute. It was prepared a group of oil-filled cryogels (with 40, 50, and 60 vol% of oil) based on the aqueous solution of polyvinyl alcohol (10 wt%). The value of activation energy 13.8 kJ mol^–1 has been established for the combustion of oil-filled cryogels using the Friedman method. The thermal effects of these processes, measured using a calorimeter, are 41.78–43.34 MJ kg^–1, which is 5–8% lower than the thermal effect of the combustion of oil in a normal state. It has been shown that the kinetic properties of the gas-phase combustion of gel fuels based on oil-filled cryogels are significantly different from the identical characteristics of the combustion of oils in a normal state. At the gas-phase combustion stage, the TG curves for the oil-filled cryogel and for oil in a normal state look similar, but the rate of mass change of the latter is much higher as a result of burnout.

Introduction

Over the recent years, a wide use of different technological systems and devices in the industry and in households has increased the production of lube oils [[1], [2], [3]]. The annual consumption of oils of petroleum origin around the world is about 45 mln.t [4]. About 20–30 % of them are used in the production of new equipment, the rest – in the maintenance of units and routine replacement of used oils [4]. Only 15 % of them are recycled [1,5,6]. The known technologies of waste lubricant oil processing [1,[5], [6], [7], [8]] are based on physical and chemical processes, or a combination of them, where the main aim is to remove aging and pollution products from their composition. Implementing the most common oil processing technologies involves the following treatment methods [[1], [2], [3]]: mechanical (filtration of solid particles and excess water); thermal and physical (evaporation, vacuum distillation); physical and chemical (coagulation, adsorption). If these methods fail to provide oil regeneration, chemical methods are applied [[5], [6], [7], [8]], which suggest the use of energy-intensive technological equipment. The main economic and environmental safety aspects of recycling waste lubricant oils are strict regulatory requirements for collecting, storing, and transporting them to the recycling site [9]. The two latter processes pose the greatest threat due to a potential hazard of leakage and spilling of liquid hydrocarbons into the soil and water bodies [10,11]. Remarkably, 1 L of waste oil can pollute 7 mln. L of water [10,11], including that for drinking. Moreover, waste oils are fire-hazardous liquids that are capable of self-ignition and self-maintained combustion [12,13]. Therefore, storing such waste jeopardizes the environmental and fire safety. The methods currently used to store and transport waste lubricant oils do not guarantee adequate protection from leakage and spills.

One of the promising ways of dealing with this problem is to gel petroleum waste and subsequently recover it by combustion to generate energy [14]. Such gel fuels can be synthesized on the basis of cryogels. A freezing / thawing cycle of oil emulsions based on aqueous solutions of polyvinyl alcohol (PVA) results in oil-filled cryogels [[14], [15], [16]]. Fine droplets of combustible liquid are in the cells of a polymer matrix [16]. Oil-free cryogels themselves are a non-toxic and environmentally friendly material with a macroporous structure [17]. Thickening combustible liquids significantly changes their properties [[14], [15], [16]], including the most important kinetic properties of the gas-phase ignition and subsequent combustion, as compared to identical characteristics of oils in a normal state. The kinetic properties of ignition and combustion of the latter are well understood, there are mathematical models [[18], [19], [20], [21]] elaborated to develop technologies based on these processes. In turn, the mechanisms and characteristics of gas-phase combustion of gel fuels are markedly different from those of combustible liquids [[14], [15], [16],22,23]. One of the reasons for that is phase transitions (melting and evaporation) at the initial stage of gel fuel heating. Since there is hardly any information on the kinetic properties of gas-phase ignition and combustion of oil-filled cryogels, the purpose of this study is to determine them using the Friedman method with the thermogravimetric analysis (TG) and differential thermal analysis (DTA) data, as well as to measure the thermal effects of combustion of a group of gel fuel compositions using a calorimeter.

In the theory of chemical kinetics [24], there are two widely known approaches to determining the kinetic properties (activation energy and pre-exponential factor) of chemical reactions [25]: experimental methods without model development and model-fitting methods. The latter are labor-intensive: they require a powerful number-crunching machine and corresponding software packages for a reliable description of real processes, e.g., when doing quantum chemical calculations [26]. Simplifying these methods calls for substantiation of a great number of assumptions. For instance, a gradual change of chemical reaction parameters is not believed to lead to a sharp change of kinetic properties, which is not always the case in real-life conditions. Therefore, full-scale conditions should be reproduced in the experiment to evaluate the reliability of the kinetic properties obtained. In turn, the methods [[27], [28], [29], [30], [31], [32]] without model development (Coats-Redfern, Friedman, Kissinger-Akahira-Sunose, Ozawa-Flynn-Wall, Starink and Vyazovkin) are less labor-intensive and more reliable [[33], [34], [35], [36]]. Therefore, they are widely used to calculate the kinetic properties of processes from the experimental data on the sample mass change during thermal decomposition or combustion. In this paper, TG/DTA methods have been used to obtain this data. This method can be summarized as follows: the samples of substances and materials under study are heated at a steady rate in a wide range of temperatures in a thermoanalyzer. The main results of such analysis are data about the rate of the sample mass and temperature change as a function of time (or ambient air temperature) under the conditions of fuel ignition and combustion. Subsequent processing of this data can help establish the main kinetic properties of processes occurring when fuel samples are heated. These properties further serve as source data to develop predictive mathematical models and perform numerical simulation of the corresponding physical and chemical processes in wide ranges of parameter variation of the system under study and ambient effects.