Question

What are the higher and the lower heating values of a fuel? How do they differ? How is the heating value of a fuel related to the enthalpy of combustion of that fuel?

Short answer

Answer: The main difference between the higher (HHV) and lower heating value (LHV) of a fuel is in the treatment of the heat released due to the condensation of water vapor formed during combustion. HHV includes this heat, while LHV does not. Both heating values are related to the enthalpy of combustion, which is the amount of heat released during a constant-pressure combustion process. The HHV represents the enthalpy of combustion when the products of combustion are cooled down to the initial temperature, while the LHV represents the enthalpy of combustion when the products, including water vapor, are not allowed to condense.

Step-by-step solution

Define Higher Heating Value (HHV)

Higher Heating Value (HHV) refers to the total amount of heat that can be produced by the complete combustion of a unit of fuel when the products of combustion are cooled down to the initial temperature of the fuel and the combustion air. In other words, it includes the heat released due to the condensation of water vapor formed during combustion.

Define Lower Heating Value (LHV)

Lower Heating Value (LHV) refers to the total amount of heat that can be produced by the complete combustion of a unit of fuel, but it does not include the heat released due to the condensation of water vapor formed during combustion. In other words, LHV assumes that the products of combustion (including water vapor) stay in their gaseous form and do not condense.

Discuss the Differences between HHV and LHV

The main difference between HHV and LHV lies in the treatment of the heat released due to the condensation of water vapor formed during combustion. HHV includes this heat, while LHV does not. The actual heating value used in engineering calculations depends on the specific application and whether or not it is desirable to consider the heat released during condensation.

Explain the Relationship between Heating Values and Enthalpy of Combustion

The heating value of a fuel is directly related to its enthalpy of combustion. Enthalpy of combustion is the amount of heat released when a fuel is burned completely in a constant-pressure process. The higher heating value represents the enthalpy of combustion when the products of combustion are cooled down to the initial temperature of fuel and combustion air. On the other hand, the lower heating value represents the enthalpy of combustion when the products, including water vapor, are not allowed to condense. In summary, higher and lower heating values are measures of the available heat energy in a fuel during combustion. The difference between them is due to the treatment of heat released during the condensation of water vapor. Both heating values are related to the enthalpy of combustion, which is the measure of heat released during a constant-pressure combustion process.