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The heating value (also called energy value or calorific value) of a fuel is the amount of heat released during the complete combustion of a specified amount of that substance. It's a measure of a fuel's energy density and can be expressed in several ways:

• Energy per unit mass (MJ/kg)
• Energy per unit volume (MJ/L)
• Energy per mole (kJ/mol)

There are two types of heating values:

1. Higher Heating Value (HHV) - Also known as gross energy, upper heating value, or gross calorific value (GCV). This represents the maximum amount of thermal energy available from complete combustion when all combustion products are brought back to the original pre-combustion temperature (typically 25°C), including condensing any water vapor produced. HHV accounts for the latent heat of vaporization of water.

2. Lower Heating Value (LHV) - Also known as net heating value. This is the energy released during combustion excluding the latent heat of vaporization of the water formed. It represents the practical usable energy in applications where water vapor isn't condensed.

The key differences between HHV and LHV are:

1. Water vapor treatment: HHV includes the energy from condensing water vapor, while LHV does not.
    
2. Practical vs. theoretical: LHV represents the practical, usable energy in most applications, while HHV represents the theoretical maximum energy available.
    
3. Numerical value: HHV is always higher than or equal to LHV according to the following equation:
    

   LHV = HHV - (m × ΔHvaporization)

   
   where m is the mass of water produced by fuel combustion and ΔH_vaporization is the latent heat of vaporization.

   The difference depends on the hydrogen content of the fuel:
   • For hydrogen: HHV is about 18.2% higher than LHV
   • For natural gas: HHV is about 11% higher than LHV
   • For gasoline: HHV is about 10% higher than LHV
   • For diesel: HHV is about 7% higher than LHV
      
4. Calculation method: HHV is typically measured experimentally in a bomb calorimeter, while LHV is often calculated from HHV by subtracting the heat of vaporization of water.
    
5. Application in design: LHV is used for designing internal combustion engines and standard boilers, while HHV is used for designing advanced combustion units with secondary condensers.

LHV is preferred for practical energy calculations within a system

From an energy demand viewpoint, the Lower Heating Value (LHV) is generally used for practical energy calculations within a system because:

1. Most industrial equipment (boilers, engines, turbines) cannot condense water vapor in flue gases due to corrosion concerns and temperature limitations.
2. In power generation applications, water typically remains as vapor since temperatures in turbines or engine pistons are too high for condensation.
3. Engine and equipment manufacturers typically rate their fuel consumption using LHV.

HHV accounts for the total energy available from fuel combustion

The Higher Heating Value (HHV) accurately accounts for the total available energy from fuel combustion that is provided to a process. For the purpose of accounting for the total energy consumed by a system, HHV is preferred.

When a system includes condensing boilers or flue gas condensation technology that recovers the latent heat of water vapor less fuel is needed compared to systems without this recovery capability. This is because:

1. Condensing systems can capture the latent heat of vaporization that would otherwise be lost in the exhaust stream.
2. This recovered heat represents approximately 14% (and up to 19%) of the energy in the fuel, which would be wasted in non-condensing systems.
3. Flue gas condensation can increase overall efficiency by up to 30% in heating plants.

HHV and LHV Values

HHV and LHV for commonly used fuels in industrial processes:

References

Perplexity A.I. Deepsearch assisted description generation, 2^nd Apr 2025.