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Chapter 3.3 - Fossil Fuels
Fossil Fuels - naturally occurring fuels formed over a long period of time from the remains of plants and animals e.g. coal, natural gas, crude oil & oil shale.
Coal
- Contains a mixture of C, H, O, (S, N), water, and some metals.
- Steaming coal - burnt in power stations ® steam ® drive turbines.
- Metallurgical coal - used to make coke for blast furnaces for metal extraction.
Classification of coal
Coal is classified according to rank (stage of formation and age)
wood ® peat ® brown coal (lignite) ® black coal (bituminous coal) ® anthracite
® Increasing C content, increasing heating value
® Decreasing moisture, decreasing volatiles (especially H and O)
® Increasing time for formation
Heating value of coal
Heating value of coal = the quantity of heat produced during the complete combustion of 1g of coal. (Units kJ g-1)
Measuring apparatus - Bomb Calorimeter - see Elements of Chemistry, Vol 2. pg 476-7 or Chemistry 4th Ed., Zumdahl, S. pg 251.
Heat released by fuel
(in Bomb calorimeter) |
= |
Heat absorbed by water |
+ |
Heat absorbed by apparatus |
Heat absorbed by water = mass x specific heat of water x temperature change
Q = msDT Where,
Q = quantity of heat in Joules (J)
m = mass of water in grams (g)
s = specific heat of water (4.18J K-1 g-)
DT = Temperature rise
Therefore in the bomb calorimeter,
Heat released by fuel |
= |
msDT |
+ |
CDT |
Where
m = Mass of water in grams (g)
s = Specific heat of water (4.18J K-1 g-)
DT = Temperature rise
C = Heat capacity of the bomb calorimeter.
This can lead to some possible calorimetry problems.
(Molar heat of combustion = heat produced during the combustion of 1 mole of fuel - not applicable to coal and blends of oils since both have no fixed formula.)
The Heating Value of moist coal is less than dry coal. The heating value is reduced due to the presence of the moisture. This means there will be less heat energy released per gram of coal, since there is -
(i) less combustible material, and
(ii) some of the energy released by the combustible material is used up to vaporise the moisture content.
See Q. 17-20, Ch. 3.3, Elements of Chemistry, Vol 2. pgs 476-8.
Energy absorbed to vaporise moisture content in coal |
= |
Energy to raise the temperature of water to 100oC |
+ |
Energy to vaporise water |
Where,
Q = quantity of heat (kJ)
m = mass of water in coal (g)
s = specific heat capacity of water
L = heat of vaporisation (2.28 kJ/g or 40.7kJ/mol)
DT = temperature rise to 100oC
Thus, using this information, and given the moisture content, the Heating Values for moist coals can be calculated and compared.
Heating Value of moist coal (kJ/g) |
= |
Heat released from the combustible portion |
- |
Heat absorbed by moisture content to raise it to 100oC and then to vaporise it |
See Q.21-24, Ch. 3.3 Elements of Chemistry, Vol 2. pgs 478, Worked Solution to Q.21, 479 & Coal Problems.
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