Combustion Kinetics and Mechanism of Heavy Oil and Coal
Introduction
Heavy oil and coal are important fossil fuel resources essential for the global energy supply. Since the industrial revolution, the use of coal for domestic consumption and industrial use has become more prevalent and is expected to continue to be an important factor in the energy mix. For instance, China and the United States are the top two producers of coal in the world, followed by India and other developed and developing countries. However, due to the increase in the demand for energy and emissions of air pollutant, the utilization of coal has been limited and gradually replaced by other fuel resources such as renewable energy and natural gas.
The use of oil and gas has also been steadily increasing, especially during the past few decades. Of particular significance is the use of heavy oil and coal, which are populous resources with relatively higher calorific values than conventional oil and gas. Heavy oils (also known as bitumen) are the viscous remnants of crude oil that remain after the refining process, and are composed of complex molecules including polymers and cyclic hydrocarbons. Similarly, coal is a primary sedimentary rock of organic origin composed mainly of carbon, hydrogen and oxygen but also contains sulfur and nitrogen.
The combustion of heavy oil and coal has been extensively studied to improve their utilization and to limit the emission of air pollutants. In this review, the kinetics and mechanism of heavy oil and coal combustion will be discussed in detail.
Combustion of Heavy Oil
Heavy oil combustion is a complex process involving a variety of physical, chemical, and thermodynamic processes. Generally speaking, the combustion of heavy oil occurs as a combination of incomplete combustion and thermal oxidation. Incomplete combustion results from the incomplete oxidation of heavy oil due to the lack of oxygen or the inability of burning fuel particles to fully react with molecular oxygen to yield carbon dioxide and water vapor. Thermal oxidation occurs when the temperature of the burning particles is high enough to enable the fuel to react with oxygen at the surface of the burning particles. Thermal oxidation is divided into two categories: premixed flame and diffusion-controlled flame.
The premixed flame occurs when the fuel and oxidizer mix homogeneously and instantly ignite at atmospheric pressure. This type of combustion is surface-controlled, meaning that the reaction rate is controlled by the rate of heat transfer between the fuel and oxidizer, allowing the burning particles to reach the ignition temperature. On the other hand, diffusion-controlled flame occurs when the fuel and oxidizer mix gradually, facilitated by the difference in the rate of diffusion and reaction. This type of combustion is inward-controlled, meaning that the reaction rate is limited by the rate of fuel diffusion into the combustion zone.
The ignition of heavy oil is largely dependent on the particular fuel properties such as viscosity, heating value, and degree of maturity. Viscosity is a key factor governing the diffusion-controlled combustion, and higher viscosity leads to lower mass transfer rate and thus slower burning. On the other hand, a higher heating value indicates greater available energy for combustion, and leads to higher temperature flames and higher burning rates. The degree of maturity is another factor that affects the burning of heavy oil. A high degree of maturity means that the fuel is composed of smaller and more combustible molecules, leading to quicker burning due to the increased availability of fuel and oxygen.
The combustion of heavy oil is accompanied by the formation of a variety of air pollutants, including carbon monoxide (CO), nitrogen oxides (NOX), and sulfur dioxide (SO2). The formation of these pollutants largely depends on the combustion conditions, such as the oxygen supply, temperature and pressure, and the type of combustion. Generally, in premixed flames, the pollutants are formed through thermal decomposition and the formation of volatile compounds. On the other hand, in diffusion-controlled flames, CO and NOX are formed through fuel-rich combustion, while SO2 is formed through fuel-lean combustion.
Combustion of Coal
Like heavy oil, the combustion of coal is a complex process involving thermal oxidation, thermal decomposition, and hydrogen abstraction reactions. Coal is composed of a variety of organic materials, including carbon, hydrogen, oxygen and sulphur, and generally contains more oxygen than heavy oil. As a result, the combustion of coal produces higher levels of CO2, NOX, SO2 and particulate matter compared to burning heavy oil.
Due to the low reactivity of the organic materials in coal, the primary reaction process during coal combustion is a slow thermal oxidation. This involves the oxidation of the carbon in the coal without the need for a diffusion-controlled flame. With increasing temperature, the rate of oxidation increases and coal particles are burned faster as higher amounts of oxygen are available for combustion. The kinetics of coal combustion can be summarized as follows: At low temperatures, the organic matter undergoes an adsorption process in which the organic species are adsorbed onto the surface of the coal particles. At higher temperatures, the organic species are volatilized and oxidized, leading to the formation of CO2, SO2, NOX and other pollutants.
The formation of air pollutants during coal combustion is primarily governed by the combustion conditions, such as oxygen availability and the burning temperature. In the presence of an inadequate supply of oxygen, the combustion process tends to be fuel-rich, leading to the overproduction of CO and other pollutants. Additionally, air pollutants are formed through the thermal decomposition of volatile species as the burning particles reach their ignition temperature.
Conclusion
In conclusion, heavy oil and coal are important sources of energy for the global energy supply. The combustion of both resource results in the production of a variety of air pollutants including CO, NOX and SO2. The formation of these pollutants largely depends on the particular properties of the fuel and the combustion conditions. In order to more effectively utilize the fuel resources and reduce air pollution, further understanding of the kinetics and mechanisms of heavy oil and coal combustion is needed.
