Reference no: EM13943692
Combustion problem NO- 13
An imported British coal as fired in a furnace analyses-
C = 67.9%
H = 4.4%
S= 0.8%
N=1.6%
O=7.9%
Ash = 4.5%
Moisture = 12.9%
When burnt , the flue gas issuing out of the furnace at 545 oc analyses on dry basis as CO2 =14.5 % , O2 = 4.7%, N2 = 80.8 % , Other relevant data are- Gross C.V. of coal =28000 kj /kg , ambient temperature = 15oc
Specific heat of dry flue gas , Cp = 1.341 kj/Nm3 . oc
Specific heat of steam = 1.92 kj /kg .oc
Heat of codensatio of water vapour at 15oc =2454 kj/kg
Calculate the -
A . volume (Nm3 ) of actual air used for combustion per kg of as fired coal.
B - sensible heat in the dry flue gas expressed as % of net C.V. of coal.
c- heat loss in the combustion of hydrogen and in the moisture in the coal as kj/kg of coal.
SOLUTION - PART A ---
Basis - 1 kg coal-
Constituents quantity per kg of coal kg atom o kg mole theoretical oxygen
Kgmole
C 0.679 0.679/12 =0.0565 0.565
H 0.044 0.044/2 =0.022 0.011
S 0.008 0.008/32 =0.00025 0.00025
N 0.016 0.016 /28 = 0.00057 --
O 0.079 0.079/32 = 0.00246 -0.00246
TOTAL 0.0653
THEORETICAL FLUE GAS VOLUME PRODUCED , Nm3
CO2 = 0.0565*22.4 = 1.265 Nm3
H2O = 0.011 *22.4 = 0.2464 Nm3
SO2= 0.00025 *22.4 = 0.0056 Nm3
N2= 0.00057822.4 = 0.0127 Nm3
Total theoretical oxygen requirement = 0.0653 kg mole = 0.0653 *22.4 = 1.46272 Nm3
Theoretical air requirement = 1.46272*100/21 = 6.965 Nm3
Now, flue dry gas from combustion of 1 kg of coal will contain
N2 , from theoretical air = 6.965*0.79 = 5.50 Nm3
Let , excess air = x Nm3
Total flue gas volume = 1.265 + 0.0065 +0.0127 +5.50 +x = ( 6.783 + x) Nm3
Total flue gas volume of (CO2 +SO2) in flue gas = 1.265 + 0.0056 = 1.27 Nm3
(since SO2 is reported as CO2 in orsat - analysis)-
By carbon dioxide balance-
% CO2 in dry flue gas = Volume of CO2 / Total volume of flue gas *100
14.5 = (1.27 /6.783 +x )*100
14.5 (6.783 +x ) = 1.27*100 =127
6.783 + x = 127/14.5= 8.758
X = x = 8.758 - 6.783 = 1.975 Nm3
Part A - SO , Actual air used /kg of as fired coal = theoretical air + excess air
= 6.965 + 1.975 = 8.94 Nm3
Part -B- Sensible heat loss per kg of coal
= volume of dry flue gas *specific heat * temp.diff erence
= ( 6.783 +1.975 )*1.341 *(545-15) = 8.758 *1.341*530 = 6224.5 kj
Now, net C.V. of coal = gross C.V. - ( 2454 * total weight of moisture)
Weight of water as moisture in 1 kg of coal = 12.9 /100 = 0.129 kg
Water produced by combustion of H2 present in coal =2.2*18 /100 = 0.2958 kg
Total weight of water = 0.129 +0.2958 =0.525 kg/kg of as fired coal
Net C.V.= =28000 - ( 2454*0.525) = 26712 KJ/KG
SO ,Sensible heat loss as % of net C.V. of coal = 6240/26712 *100 = 23.3%
Part -C -
Heat loss due to combustion of hydrogen and moisture in coal will be summation of that due to the cooling of steam to 100 oc and the latent of condensation and cooling of water to 15 oc.
Heat lost per kg of coal = 0.525 * ( 545- 100 ) *1.92 + 0.525*2454 = 347.7 +1288.3 = 1636 kj
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