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Ultimate & Proximate

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Published Date Written by Sharon Banks

Coal comes in three main types, or ranks:

  1. lignite or brown coal,
  2. bituminous coal or black coal, and
  3. anthracite or hard coal.

Each type of coal has a certain set of parameters which are mostly controlled by moisture, volatile and carbon content. Proximate analysis determines the fixed carbon, volatile matter, moisture and ash percentages.
The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. Fixed carbon acts as a main heat generator during burning. High volatile matter content indicates easy ignition of fuel. The ash content is important in the design of the furnace grate, combustion volume, pollution control equipment and ash handling systems of a furnace.
Thus proximate analysis is used to establish the rank/category/type of coals and provide the ratio of combustible to non-combustible constituents.

Unlike a proximate analysis, an ultimate (chemical) analyses determines the elemental components in coal.
An ultimate analysis gives the composition in percentage weight of the major components i.e. carbon, hydrogen and oxygen (by difference), as well as sulphur and nitrogen after complete combustion of the sample.
For practical reasons, sulphur is assumed to occur in three forms in coal:

  1. as organic sulphur compounds,
  2. as inorganic sulphides, which are mostly the iron sulphides, FeS2 (pyrite and marcasite), and
  3. as inorganic sulphates (Fe2(SO4)3 & FeSO4). The total sulphur value is used for ultimate analysis.

The results are useful in determining the quantity of air required for combustion and the volume and composition of the combustion gases. This information is also required for the calculation of flame temperature and flue duct design.

Relationship between ultimate analysis and proximate analysis;
%C = 0.97C+ 0.7(VM - 0.1A) - M(0.6-0.01M)
%H = 0.036C + 0.086 (VM -0.1xA) - 0.0035M2(1-0.02M)
%N2 = 2.10 -0.020 VM
Where
C = % of fixed carbon; A = % of ash; VM = % of volatile matter; M = % of moisture

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