Page 305 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 281

The ultimate analysis of the coal on a moisture-ash-free basis is:
C: 77.3%
H: 5.9%
S: 4.3%
N: 1.4%
O: 11.1%
Find
1. The S/C molar ratio
2. The O/C molar ratio
3. The ER
4. The hearth load in energy produced per unit of grate area and space velocity
The heating values of the product gas constituents may be taken from
Table C.2 in Appendix C.
Solution
From the feed rate of coal, steam, and oxygen, we can find the molar feed rate by
dividing the mass rate by the molecular weight as here:
Carbon moles: 750 3 0.773/12 5 48.31 kmol/min
Steam moles: 1930/18 5 107.22 kmol/min
Oxygen moles: 280/22.4 5 12.5 kmol/min
From these, we can calculate the following:
1. S/C molar ratio 5 107.22/48.31 5 2.22
2. O/C molar ratio 5 12.5/48.31 5 0.26
3. To find the stoichiometric oxygen requirement, the oxygen required to oxidize
carbon to CO 2 , hydrogen to H 2 O, and sulfur to SO 2 has to be calculated.
To produce 1 mol of CO 2 , 12 kg of carbon (1 mol) reacts with 32 kg of oxygen
(1 mol):

C 1 O 2 5 CO 2
Therefore, the oxygen required for 1 kg of carbon is 32/12.
To produce 1 mol of SO 2 , 32 kg of sulfur (1 mol) reacts with 32 kg of oxygen
(1 mol):

S 1 O 2 5 SO 2
Therefore, the oxygen required for 1 kg of sulfur is 32/32 5 1.
Similarly, 4 kg of hydrogen reacts with 32 kg of oxygen to produce H 2 O:
2H 2 1 O 2 5 2H 2 O
Therefore, the oxygen required for 1 kg of hydrogen is 32/4 5 8.
32C
Stoichiometric oxygen requirement 5 1 8H 1 S 2 O
12
32 3 0:773
5 1 8 3 0:059 1 0:043 2 0:111
12
5 2:465 kg of O 2 =kg of fuel
The total O 2 required is:
750 3 2:465 5 1848:75 kg of O 2 =min