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Calorific value of pressed fuel briquettes

By Jack Huang    Mar 07, 2014

The calorific or heating value is an important indicator of the quality of the pressed fuel briquettes. It measures the energy content of the briquettes. It is defined as the amount of heat evolved when a pressed fuel briquette is completely burnt and the combustion products are cooled. And the Gross Calorific Value, shortened as GCV, refers to the calorific value with the condensation of water in the latent heat, also known as higher heating value. Whereas during combustion, the heat of condensation of the water contained in the fuel and formed during combustion will become unavailable because the vaporization of the water. And then, the useful heating value is gained after the heat of condensation of the water being subtracted from the gross calorific value, which is referred as the Net Heating Value or lower heating value.
 

the calorific or heating value of the briquettes

Buring briquettes made by  fuel briquette presses
 

The calorific value of a specific pressed fuel briquette is related with the amount of oxygen required for thorough combustion. Usually, one gram of oxygen burnt, 14,022 J of energy can be released. It means the higher the oxidation of the fuel, the less the oxygen is required for thorough combustions and the lower the heating value of the fuel. Meanwhile when the fuel consisted of compounds with lower degree oxidation, such as, hydrocarbons, the heating value of the biomass will be higher.

The calorific value of the pressed fuel briquettes is limited by fuel moisture content for the reason that the heat is used to vaporized the water, lowering the heat released.

Furthermore, the calorific value of the fuel briquettes is also limited by the ash content. Approximately every 1% addition of ash translates to a 0.2 MJkg−1 decrease in the heating value.

As for the calorific value of various fuel briquettes pressed from particular biomass are shown in the diagram below for your reference.

 

Biomass materials Calorific value of the briquettes
               Cal./kg
Ash Contents
Bagasse 4380 K. 1.80%
Bamboo dust 4160 k. 8.00%
Barks wood 1270 k 4.40%
Castor seed shells 3862 k. 8.00%
Cotton stalks/chips 4252 k. 3.00%
Coffee husk 4045 k. 5.30%
Coir pitch 4146 k. 9.10%
Forestry waste 3000 k. 7.00%
Jute waste 4428 k. 3.00%
Groundnut shell 4524 k. 3.80%
Mustard stalk 4200 k. 3.40%
Mustard shell 4300 k. 3.70%
Paper 4841 k. 1.5%
Papyrus 3965 k. 5.6%
Paddy straw 3469 k. 15.5%
Palm husk 3900 k. 4.90%
Rape seed waste 4403 k. 5.3%
Rice husks 3200 k. 19.20%
Sawdust 3898 k. 8.20%
Sunflower stalk 4300 k. 4.30%
Soya bean husk 4170 k. 4.10%
Sugarcane 3996 k. 10.00%
Tobacco waste 2910 k. 31.5%
Tea waste 4237 k. 3.80%
Wheat straw 4100 k 8.00%
Wood chips 4785 k. 1.20%