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.