CHEMICAL CALCULATIONS
SOLID FUELS
 With the exception of petroleum cokes, most solid fuels contain appreciable percentages
of mineral compounds.
 MC (Moisture Content) is often considerably higher that is characteristic of gases and
liquids since moisture can be found to the surface by adsorptive forces or held mechanically
in the pores of crevices.
ULTIMATE ANALYSIS (Elemental Analysis)
 C, H, O, N, Ash, S; Where Ash is the mineral residue resulting from combustion of the
fuel.
 Reported on either a dry, “air-dried”, “as-fired” basis.
Data of modified ultimate analysis are arranged as follow:
1. Moisture – Loss in weight at 105C
2. Combined Water – Equivalent to the oxygen of the ultimate analysis; may or may not
include moisture
3. Carbon
4. Available or Net Hydrogen – Hydrogen other than that in moisture and combined water.
5. Sulfur and Nitrogen – Usually in small amounts; often neglected except to calculate for
oxygen by difference (present in small amounts of 1-3 or more)
6. Ash
 Hydrogen content is always in excess of that necessary to form water with oxygen.
Hence, it can be assumed that all the oxygen is present in the form of water chemical
combination.
 Excess Hydrogen – termed “available” or “net” H2 = combustible or burnable
hydrogen.
 Net Hydrogen = Total H – H equivalent to the available oxygen (found by
difference) in the coal.
PROXIMATE ANALYSIS
- Based on the volatilization properties
- M + VCM + FC + Ash = 100
 Moisture – Loss in weight experimented by a sample when heated for 1hr at
104-110C.
 VCM (volatile combustible matter) – upon ignition in a covered crucible for 7
minutes at 950C, the dried sample loses most of its combined water and
hydrogen and a large amount of carbon (e.g. in the form of volatile HCs),
leaving the ash and residual carbon.
 Fixed Crabon – The combustible left after ignition.
 Ash – Residue left from complete combustion of the sample in the open air at
725C
Example: Analyses of a Pennsylvania coal on an air-dried basis.
PROXIMATE ()
3.2 Moisture
21.0 VCM
69.3 FC
6.5 Ash
ULTIMATE ()
79.90 C
4.85 H
0.69 S
1.30 N
6.50 Ash
6.76 O (by diff); not combined water
VCM contents: C,
Carbon = FC + C in VCM = 79.90
H equivalent to 6.67 of Oxygen = 6.76 O * (2H/16O) = 0.85 H
Available H or Net H = 4.85 – 0.85 = 4 H
Total combined water + Moisture = 6.76 + 0.85 = 7.61
Since from the Approximate Analysis, Moisture = 3.2
Combined Water = 7.61 – 3.2 = 4.41%
 Knowledge of Ultimate Analysis is essential to achieve desired results.
 To obtain a correlation between the analysis of the fuel and combustion products,
relative amounts of some constituents common to both analyses must be known.
 CARBON balance is often used. All the carbon in the fuel shows up in the stack
gas, expect for:
 one form/another of unburnt combustible
 possible leakages of combustion gases from the furnace
Methods of Estimating Total Carbon of the Fuel:
DULONG FORMULA
𝑂
B = 41, 544 C + 62 028 [H − 8 ] + 4, 050 𝑆
Where;
B = Higher Heating Value (HHV) in BTU/lb
C = Weight Fraction of Carbon
S = Weight Fraction of Sulfur
H.O = Weight Fraction Net Hydrogen
= Total H.1 O
 It may be used in conjunction with the Flue Gas Analysis for the calculation of the %C
(percent Carbon) in the coal, Flue Gas Analysis giving Net H/C.
 The coefficients neglect heat of formation of the organic matter from the elements. Their
values are based on the heats of combustion of the elements involved.
In terms of Calorific Value (Cv):
𝑶
𝐂𝐯 = 𝟎. 𝟑𝟑𝟖 𝐂 + 𝟏. 𝟒𝟒 [𝐇 − 𝟖 ] + 𝟎. 𝟎𝟗𝟒 𝑺
CALDERWOOD EQUATION
𝑽𝑪𝑴 𝟏.𝟓𝟓
𝐂 = 𝟓. 𝟖𝟖 + 𝟎. 𝟎𝟎𝟓𝟏𝟐 (𝐁 − 𝟒𝟎. 𝟓 𝐒) ± [𝟖𝟎 − 𝟏𝟎𝟎 𝑭𝑪 ]
Where;
S = Sulfur as weight percent of the coal
B = HHV (BTU/lb of coal)
VCM = Volatile Combustible Matter in weight percent of coal
FC = Fixed Carbon as weight percent of coal
C = Carbon as weight percent of coal
 Percentage are on air-dried basis, but the error introduced by using basis with reasonable
MC or even by using a moisture and ash-free basis is usually negligible.
 Calderwood equation is usually applicable to BITUMINOUS COAL – any of several
substances obtained as residue in the distillation coal tar, petroleum, etc., or occurring
naturally as asphalt.
OR:
𝑽𝑪𝑴 𝟏.𝟓𝟓
𝐂 = 𝟓. 𝟖𝟖 + 𝟐. 𝟐𝟎𝟔 (𝐂𝐯 − 𝟎. 𝟎𝟗𝟒𝐒) + 𝟎. 𝟎𝟎𝟓𝟑 [𝟖𝟎 − 𝟏𝟎𝟎 𝑭𝑪 ]
Note:
 Cv (Calorific Value) of the fuel must be known
𝑉𝐶𝑀
 The sign of the last term is taken to be negative (-) if 100 𝐹𝐶 is > 80.
REFUSE
 Ashes removed from the furnace.
 May include varying amounts of unburnt combustible matter.
GREEN COAL
 Term for the original coal fired.
COMBUSTION CALCULATION
The combustion of coal may be carried out by spreading it over a grate or firing or by
introducing it as pulverized fuel. Coal and air are brought together at a temperature sufficient to
decomposed the coals in to FC and VCM and to cause the FC to ignite. Combustion then becomes
self-supporting, i.e. sufficient heat is evolved to maintain these conditions. However, caol contains
noncombustible matter (ash) which separates from the materials that can be gasified and is
removed from the furnace as REFUSE. This REFUSE may or may not contain unburnt
combustible.
For combustion calculations, the combined %N and S is neglected if the total less than or
equal to 3%.
TYPE 1. Coal Combustion with no combustible in the refuse.
TYPE 2. Coal Combustion with combustible in the refuse.
More often in the combustion of solid fuels, the furnace ashes contain a significant amount of
unburnt combustible material collectively called the Refuse. In many instances, to be able to
analyze the refuse better, water is sprinkled into it to form WETTED REFUSE; water added is
neglected in the calculation.
FOUR (4) CASES/TYPES OF COMBUSTIBLE LOST IN THE REFUSE;
1. UNCOKED COAL
 Coal which has lost only its moisture content. VCM and FC in the coal is the same as in
the REFUSE. Therefore;
𝑉𝐶𝑀
𝑉𝐶𝑀
[
]
= [
]
𝐹𝐶 Coal
𝐹𝐶 Refuse
Also:
%VCM lost = %FC lost = %C lost = %HHV/Cv lost = %net H lost = %CW lost =
%N lost = %S lost in the refuse
2. COKED COAL
 Coal which has lost both moisture and VCM contents.
 Carbon and Ash are the main constituents of the refuse.
3. COMBINATION OF COKED AND UNCOKED COAL
 Mixture of CVM, FC, and Ash in the refuse.
 Uncoked Coal (VCM and FC)
 Coked Coal (FC)
[
FC
FC
]
≠ [
]
VCM Coal
VCM Refuse
Also:
%C lost ≠ %VCM lost ≠ %HHV/Cv lost ≠ %net H lost
%net H lost = %CW lost = %N lost = %S lost = %VCM lost in refuse
4. PARTIALLY COKED COAL
 Coal which has lost moisture but not all VCM.
 Refuse shows significant VCM content.
 Coked Coal (FC)
[
FC
FC
]
≠ [
]
VCM Coal
VCM Refuse