Page 33 - Handbook of Energy Engineering Calculations

PERCENT EXCESS AIR WHILE BURNING COAL

A certain coal has the following composition by weight percentages: carbon
75.09, nitrogen 1.56, ash 3.38, hydrogen 5.72, oxygen 13.82, sulfur 0.43.
When burned in an actual furnace, measurements showed that there was 8.93

percent combustible in the ash pit refuse and the following Orsat analysis in
percentages was obtained: carbon dioxide 14.2, oxygen 4.7, carbon monoxide
0.3. If it can be assumed that there was no combustible in the flue gas other

that the carbon monoxide reported, calculate the percentage of excess air
used.

Calculation Procedure:

1. Compute the amount of theoretical air required per lb (kg) of coal
m
Theoretical air required per pound (kilogram) of coal, w = 11.5C′ + 34.5[H′ 2
ta
− O′ /8)] + 4.32S′, where C′, H′ , O′ , and S′ represent the percentages by
2
2
2
weight, expressed as decimal fractions, of carbon, hydrogen, oxygen, and
sulfur, respectively. Thus, w = 11.5(0.7509) + 34.5[0.0572 − (0.1382/8)] +
ta
4.32(0.0043) = 10.03 lb (4.55 kg) of air per lb (kg) of coal. The ash and
nitrogen are inert and do not burn.

2. Compute the correction factor for combustible in the ash
The correction factor for combustible in the ash, C = (w C − w C )/(w ×
t f
r
1
f
r
100), where the amount of fuel, w = 1 lb (0.45 kg) of coal; percent by weight,
f
expressed as a decimal fraction, of carbon in the coal, C = 75.09; percent by
f
weight of the ash and refuse in the coal, w = 0.0338; percent by weight of
r
combustible in the ash, C = 8.93. Hence, C = [(1 × 75.09) − (0.0338 ×
r
1
8.93)]/(l × 100) = 0.748.

3. Compute the amount of dry flue gas produced per lb (kg) of coal
The lb (kg) of dry flue gas per lb (kg) of coal, w = C (4CO + O +
1
2
2
dg
704)/[3(CO + CO)], where the Orsat analysis percentages are for carbon
2
dioxide, CO = 14.2; oxygen, O = 4.7; carbon monoxide, CO = 0.3. Hence,
2
2