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Briquette calorific value data for biomass, sawdust, coal, charcoal


briquette is burning to release its calorific value

What is calorific value

The technology of taking briquettes as a kind of fuel has been widely used in many countries for both domestic and industrial purposes.

Briquette technology, as an important recycling system for the agricultural and industrial waste, has always contributed to offset forest and fossil fuels, bio-residue management problems and reduce toxic emissions from incomplete carbonization besides energy production development.

Common types of briquette now used are biomass. coal and charcoal, etc.

An important characteristic of the fuel briquette is its calorific value, according to the Dictionary of Mechanical Engineering (2014), the calorific value of a fuel (or heat of combustion or heating value or heat value) is defined as “the energy released per unit mass of fuel in complete combustion with oxygen.” Briefly for shortness, that is, the amount of energy (per kg) it exudes during combustion.

The calorific value determines the briquette efficient carbonization and heat value.

Although briquettes, as with most solid fuels, are priced by mass or volume and ease of handling, market forces set the price of each fuel according to its energy content.

Thus, the calorific value can be used to evaluate the competitiveness of a processed biomass fuel in a given market situation. The production cost of briquettes is, however, independent of their calorific values.

Moisture

Water and minerals in briquettes are non-combustible. During briquette combustion, the combustible materials are oxidized by oxygen, resulting in the release of thermal energy. Energy is required to heat up the water up to its boiling temperature and to vaporize it.

The calorific value of fuel rapidly reduces with increasing moisture content, which is unfavorable for the plant which uses biomass as a fuel material.

Chart 1: Effect of moisture on calorific value (kJ/kg)

Moisture (%) 5 8 11 15 20
Corn Straw 15422 14661 14280 13330 12569
Cotton Straw 15945 15167 14773 13808 13021
Wheat Straw 15438 14681 14301 13355 12598
Poplar Branch 13995 13259 12912 12042 11347
Masson Pine 18372 17439 17050 15937 15054
Birch 16945 16125 15715 14686 13870
Cow Dung 15380 14585 14209 13263 11678

Material types

The calorific value of briquette is influenced by its elemental composition, particularly the carbon, hydrogen and oxygen contents. Different species of material have different elemental composition; hence they have different calorific values.

Such as wood,  the wood briquette made from conifers (more resinous softwood species) have higher calorific values than deciduous trees (less resinous hardwood species).

Briquetting process does not add to the calorific value of the base biomass and other materials.

Nonetheless, to upgrade the specific heating value and combustibility of the briquette, certain additives (e.g. charcoal and coal in very fine form or about 10 to 20% char fines) could be employed in briquetting without impairing the quality.

Briquette burning characteristics also depend on feedstock type, compactness level and the mold used.

Calorific value chart

Higher heating value or gross calorific value measures the total amount of heat/energy that would be produced by combustion of a briquette fuel. However, part of this heat is locked up as latent heat of vaporization of any water in the fuel during combustion.

Meanwhile, the lower heating value or net calorific value excludes this latent heat. Thus, the lower heating value is the amount actually available from the combustion process for capture and use. The higher the moisture content of a fuel, the greater the difference between gross calorific value and net calorific value and the less total energy will be available.

Chart 2: Higher heating value examples

kJ/kg Kcal/kg ash (%)
Biomass
Maple 19960 4771 1.35
Pine 22300 5330 1.31
Pine Needle 20120 4809 1.5
Poplar 20750 4959 0.65
Fir 19950 4768 0.25
Oak 19420 4642 1.52
Peach Nucleus 20820 4976 1.03
Apricot 20010 4783 1.63
Corn Cob 18770 4486 1.36
Wheat Straw 17510 4185 8.9
Cotton Straw 18260 4364 6.68
Corn Cob 17650 4219 5.58
Bagasse 17330 4142 11.27
Rice Husk 14890 3559 20.6
Walnut Shell 20180 4823 0.56
Charcoal
Coconut Shell Char 31210 7459 2.9
Oak Char 24670 5896 17.9
Redwood Char 28350 6776 2.3
Casuarina Char 27260 6515 13.24
Eucalyptus Char 26750 6393 10.45
Coal
Lignite 8000-15000 1912-3585 6-19
Bituminous 12000-20000 2868-4780 3.3-11.7
Anthracite 26000-33000 6214-7887 9.7-20.2

Chart 3: Lower hearing value examples

Moisture (%) kJ/kg Kcal/kg ash (%)
Bagasse 18 17000-18000 4063-4302 4
Coconut Shell 5-10 16700 3991 6
Coffee Husk 13 16700 3991 8-10
Corn Straw 5-6 17000-19000 4063-4541 8
Corn Cob 15 19300 4613 1-2
Cotton Shell 5-10 16700 3991 3
Palm Fiber 55 7000-8000 1673-1912 10
Palm Shell 55 7000-8000 1673-1912 5
Poplar 5-15 17000-19000 4063-4541 1.2
Rice Husk 9-11 13000-15000 3107-3585 15-20
Rice Straw 15-30 17000-18000 4063-4302 15-20
Willow Branch 8-15 18000-20000 4302-4780 6
Wheat Straw 7-15 17000-19000 4063-4541 8-9
Willow 12 17000-19000 4063-4541 1-5

Above charts data from Nrel, Penn State, Wikipedia, etc.

Calculation formula of calorific value

HHV (kJ/kg) =3.491C+1178.3H-103.4O-21.1A+100.5S-15.1N

HHV stands for higher heating value.

C, H, O, A, S stands for the mass fraction of carbon, hydrogen, oxygen, ash, sulfur and nitrogen.

Selection of bio-fuel briquette products depends on their strength and durability, besides their thermal characteristics.

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