Calorific Value of Coal and Wood Calculations

What is Calorific Value or Heating Value?

  • The calorific value is used to find the amount of heat energy produced after the complete combustion of one kg of fuel.

Gross Calorific Values (GCV/HHV)

  • Gross calorific value (GCV)  is  also called as a higher Heating Value (HCV)
  • It measures the amount of heat released by the complete combustion of a unit of fuel. GCV considers that all water vapor (H2O) produced during the combustion process is fully condensed
  • The flue gas formed from combustion is condensed during waste heat recovery and water condensate is formed.
  • In such cases as coal-fired boilers, the GCV value is used to find out the firing capacity.

The formula for Higher Heating value or  GCV 

Dulong’s formula  is used to calculate  the  GCV (Kcal/kg) of coal

        GCV  = 1/100* (8080*C + 34500* (HO/8)+ 2240*S)   Kcal/kg

where C, H, O, and S are percentages of carbon, hydrogen, oxygen, and sulfur  in coal

Net Calorific Value (NCV)

  • It is called a lower heating value (LHV) or lower calorific value (LCV)
  • Its value is calculated by subtracting the latent heat of vaporization of the water vapor from the higher heating value (HHV)
  • It assumes water vapor leaves with the combustion products is the gas phase that it is not condensed
  • For furnaces firing with gaseous fuel, a lower heating value is used to find the firing capacity.
  • The flue gas formed from combustion is not further condensed.

Formula for LCV

     LCV =  GCV – 9/100*H*587  Kcal/kg

where H  is the percentage of carbon, hydrogen, oxygen, and sulfur  in coal

Note:

  • The higher heating value (HHV) of fuel is calculated when the product of water is in the liquid phase.
  •  The Lower heating value (LHV) is calculated when the product of water is in the vapor (gas) phase
  • Refer to the basics of combustion for more details in the post

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Calculation of Heating Value of wood and coal

  • The heating value of wood and coal can vary depending on factors such as species of wood, moisture content, and type of coal.
  • Heating value is typically measured in British Thermal Units (BTUs) per pound or kilogram.

Wood:

  • The heating value of wood can range widely depending on the type of wood and its moisture content.
  • Dry wood typically has a higher heating value than wet wood because the energy is not spent evaporating moisture.
  • Dry hardwoods generally have a higher heating value compared to softwoods.
  • On average, dry wood can have a heating value ranging from about 7,000 to 9,000 BTUs per pound (15,000 to 20,000 kJ/kg).
  • Softwoods tend to be on the lower end of this range, while hardwoods are generally on the higher end.
  • The heating value of wood can be calculated using the formula:

                    LHV

Where:

    • is the heating value of wood (in units of energy per unit mass or volume, such as BTU/lb or MJ/kg).
    • is the moisture content of the wood (expressed as a decimal).
    • is the higher heating value of the dry wood.
    • is the moisture content of the water (expressed as a decimal).
    • The higher heating value ) of dry wood is typically measured experimentally and can vary depending on the type of wood and other factors. It represents the total amount of heat released when the wood is completely combusted, assuming that the water vapor produced during combustion is condensed and the latent heat of vaporization is recovered.
    • The moisture content () of the wood is usually determined by weighing a sample of wood, drying it to remove all moisture, and then reweighing it. The difference in weight is used to calculate the moisture content as a percentage of the original weight.
    • The moisture content of the water () is the moisture content of the wood’s water, which is usually close to 1 (since water is typically 100% water).
  • Using this formula, you can calculate the heating value of wood given its moisture content and the higher heating value of the dry wood.

Coals Heating Value

  • Coal is classified into different types such as anthracite, bituminous, sub-bituminous, and lignite, each with varying heating values. Anthracite coal has the highest heating value among coals, followed by bituminous coal, sub-bituminous coal, and lignite, in that order.
  • Anthracite Coal:
    • The heating value of anthracite coal can range is given as below
    •  Imperial Units:
      • 24 to 35 million BTUs per ton
      • 12,000 to 17,500 BTUs per pound
    • SI Units: 24,000 to 35,000 kJ/kg.
  • BituminSI Unious Coal:
    • Bituminous coal generally has a heating value ranging below
      • 20 to 28 million BTUs per ton
      • 10,000 to 14,000 BTUs per pound
    • SI Units: 20,000 to 28,000 kJ/kg.
  • Sub-bituminous Coal:
    • Sub-bituminous coal typically has a heating value range given below
    • Imperial units:
      • 17 to 24 million BTUs per ton
      • 8,500 to 12,000 BTUs per pound
    • SI Units: 17,000 to 24,000 kJ/kg.
  • Lignite Coal:
    • Lignite coal has the lowest heating value among coals, ranging as below
    • Imperial Units:
      • 8 to 17 million BTUs per ton
      •  4,000 to 8,500 BTUs per pound
    • SI Units: 8,000 to 17,000 kJ/kg.
  • Note these values are approximate and can vary depending on the specific composition and quality of the wood or coal.
  • It’s essential to consider the specific type and condition of the material when calculating its heating value for practical applications like combustion or energy generation.