|
Worked Solutions to Coal Problems
Question 1. Consider 1g of coal (since the heating value of coal is measured in kJg-1)
Composition = 0.6g water + 0.4g combustible material (or dry coal)
Heat released from the combustion of the moist coal (i.e. as it is mined) |
= |
Heat released during the combustion of the dry portion (i.e. the combustible part) |
- |
Heat absorbed by the water content of the coal to raise the temperature to 100 oC and vaporise the water |
| |
= |
mass of combustible material x 26 |
- |
(msDT x 10-3 + mL) |
where, |
m |
= |
mass of water in 1g of the coal = 0.6g |
| |
s |
= |
specific heat of water = 4.18 Jg-1K-1 |
| |
DT |
= |
temperature change of the water = 80 |
| |
L |
= |
heat of vaporisation of water( or latent heat of vaporisation) = 2.28kJg-1 |
Heating value |
= |
0.4 x 26 |
- |
(0.6 x 4.18 x 10-3 x 80 + 0.6 x 2.28) |
| |
= |
10.4kJ |
- |
(0.19584kJ + 1.368kJ) |
| |
= |
10.4 |
- |
1.56384 |
Heating Value of coal as it is mined |
= |
8.84kJg-1 |
|
|
Question 2. Consider 1g of coal (since the heating value of coal is measured in kJg-1)
Composition = 0.1g water + 0.9g combustible material (or dry coal)
Heat released from the combustion of the moist coal (i.e. as it is mined) |
= |
Heat released during the combustion of the dry portion (i.e. the combustible part) |
- |
Heat absorbed by the water content of the coal to raise the temperature to 100 oC and vaporise the water |
| |
= |
mass of combustible material x 32 |
- |
(msDT x 10-3 + mL) |
where, |
m |
= |
mass of water in 1g of the coal = 0.1g |
| |
s |
= |
specific heat of water = 4.18 Jg-1K-1 |
| |
DT |
= |
temperature change of the water = 75 |
| |
L |
= |
heat of vaporisation of water( or latent heat of vaporisation) = 2.28kJg-1 |
Heating value |
= |
0.9 x 32 |
- |
(0.1 x 4.18 x 10-3 x 75+ 0.1 x 2.28) |
| |
= |
28.8kJ |
- |
(0.03135kJ + 0.228kJ) |
| |
= |
28.8 |
- |
0.25935 |
Heating Value of coal as it is mined |
= |
28.54kJg-1 |
|
|
|
