Fuels and Combustion What is a Fuel? • A fuel is any substance that can be burned to produce heat or energy. • Fuels can be in solid, liquid, or gaseous forms. • When a fuel burns, it undergoes a chemical reaction called combustion. • Combustion releases heat energy, which can be used for various purposes like cooking, heating homes, and powering vehicles. Calorific Value • Calorific value, also known as heating value, is the amount of heat energy released when a unit mass of a fuel undergoes complete combustion. • It is an important property of a fuel as it determines the amount of energy a fuel can provide. • Calorific value is usually expressed in Joules per kilogram (J/kg) or Kilojoules per kilogram (kJ/kg). • Sometimes, it is also expressed in Calories per gram (cal/g). Measuring Calorific Value • A bomb calorimeter is a scientific instrument used to measure the calorific value of a fuel. • It consists of a sealed chamber where the fuel is burned in pure oxygen. • The temperature rise of the surrounding water is measured, and this is used to calculate the amount of heat released by the fuel. • Bomb calorimeters are designed to ensure complete combustion and minimize heat loss to the surroundings. Types of Fuels • Fuels can be broadly classified into three categories based on their physical state: • Solid fuels: These are fuels that exist in a solid state at room temperature. Examples include coal, wood, and biomass pellets. • Liquid fuels: These are fuels that are liquids at room temperature. Examples include gasoline, diesel, and ethanol. • Gaseous fuels: These are fuels that exist as gases at room temperature. Examples include natural gas, Solid Fuels • Solid fuels are the traditional source of energy. • Examples include coal, wood, charcoal, and biomass pellets. • Solid fuels have a high carbon content, which results in high calorific value. • However, they also have drawbacks like high emission of pollutants during combustion and difficulty in transportation. Calorific Value: • Calorific value, also known as heating value, refers to the amount of heat energy liberated during the complete combustion of a unit mass of a solid fuel. • It's typically expressed in units like Joules (J) or Kilojoules (kJ) per kilogram (kg) of the fuel. • Higher calorific values signify that the fuel releases more heat per unit mass, making it more efficient in energy conversion. Measuring Calorific Value of Solid Fuels: • Unlike gaseous fuels, solid fuels require specialized instruments for measuring calorific value. • The most common method utilizes a bomb calorimeter: • A precisely weighed sample of the solid fuel is placed inside a sealed chamber. • The chamber is filled with oxygen under high pressure. • The fuel is ignited electrically, and the combustion process occurs entirely within the sealed chamber. • The temperature rise of the surrounding water in the calorimeter is meticulously measured. • This temperature increase is directly proportional to the heat released by the fuel, allowing us to calculate its calorific value Liquid Fuels • Liquid fuels are hydrocarbons, meaning their molecules are primarily composed of hydrogen and carbon atoms. • Examples include gasoline, diesel, and kerosene. • They possess high energy density, meaning they store a significant amount of energy in a relatively small volume. This makes them ideal for powering vehicles like cars, trucks, and airplanes. • Liquid fuels are relatively easy to transport and store compared to solid fuels. Calorific Value: • Calorific value, also known as heating value, refers to the amount of heat energy liberated during the complete combustion of a unit mass (kg) or volume (L) of a liquid fuel. • It's typically expressed in Joules (J) or Kilojoules (kJ) per kilogram (kg) or liter (L) of the fuel. • Higher calorific values signify that the fuel releases more heat per unit, making it more energy-efficient. Measuring Calorific Value of Liquid Fuels: • Similar to solid fuels, the calorific value of liquid fuels is measured using bomb calorimeters. • A precisely weighed sample of the fuel is placed within a sealed chamber filled with a known amount of water. • The fuel is ignited, and the temperature rise of the water is meticulously measured. • This temperature increase is directly proportional to the heat released by the fuel, allowing us to calculate its calorific value. Gaseous Fuels: • Gaseous fuels are generally considered cleaner burning compared to solid fuels like coal. • During combustion, they release fewer pollutants like particulate matter and sulfur oxides. • However, they still generate greenhouse gases like carbon dioxide, contributing to climate change. Measuring Calorific Value of Gaseous Fuels: • Unlike solid fuels that require special bomb calorimeters, the calorific value of gaseous fuels can be determined using two main methods: • Gross Calorific Value (GCV): This method assumes all the water vapor produced during combustion condenses and releases its latent heat. It's measured using instruments like gas flow meters and high-precision temperature sensors. • Net Calorific Value (NCV): This method considers the water vapor remains a gas and doesn't release its latent heat. • It's more relevant for practical applications Combustion • Combustion is a chemical reaction between a fuel and oxygen, typically releasing heat and often light. • Three key elements are required for combustion to occur: • Fuel: The combustible material that reacts with oxygen. • Oxygen: The oxidizing agent, usually sourced from the surrounding air. • Heat: The activation energy needed to initiate the reaction. Types of Combustion • Combustion can be categorized into different types based on its efficiency and completeness: • Complete combustion: Occurs when the fuel reacts completely with oxygen, resulting in clean-burning products like carbon dioxide and water vapor. (Often represented by a blue flame) • Incomplete combustion: Occurs when there's insufficient oxygen, leading to the formation of byproducts like soot, smoke, and harmful pollutants. (Often represented by a yellow flame with smoke) • Spontaneous combustion: Occurs when certain materials generate heat internally due to chemical reactions, eventually igniting without an external flame.
