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Decisions about clean energy with reduced carbon dioxide emissions will have an impact on electricity generation from brown coal - VCE - SSCE Chemistry - Question 8 - 2012 - Paper 1
Question 8
Decisions about clean energy with reduced carbon dioxide emissions will have an impact on electricity generation from brown coal. However, there will be a much small... show full transcript
Worked Solution & Example Answer:Decisions about clean energy with reduced carbon dioxide emissions will have an impact on electricity generation from brown coal - VCE - SSCE Chemistry - Question 8 - 2012 - Paper 1
Step 1
a. Calculate the mass, in tonne, of wet brown coal that is required to generate 3.6 × 10⁷ kJ of energy.
Answer
To solve this, we can use the formula relating energy content and mass:
Since we want the mass in tonnes, we will convert the energy content of wet brown coal from kJ g⁻¹ to kJ tonne⁻¹. The energy content of wet brown coal:
- Energy content = 5.0 kJ/g = 5.0 × 10³ kJ/tonne.
Now applying the formula:
- Solve for mass:
Thus, the required mass of wet brown coal is 7200 tonnes.
Step 2
i. Calculate the mass, in tonne, of carbon dioxide that is produced from the complete combustion of this mass of wet brown coal.
Answer
To calculate the mass of carbon dioxide produced:
-
First, we note that wet brown coal has 40% carbon by mass.
- Mass of carbon in 7200 tonnes of wet brown coal:
-
The complete combustion of carbon (C) results in carbon dioxide (CO₂) according to the reaction:
This shows that 1 tonne of carbon produces 1 tonne of CO₂.
Thus, the mass of carbon dioxide produced is 2880 tonnes.
Step 3
b. What are the most likely reasons for the energy content of wet brown coal being so much lower than the energy content of dried brown coal? Justify your answer.
Answer
The lower energy content of wet brown coal as compared to dried brown coal can be attributed to a few key factors:
-
Water Content: Wet brown coal contains a significant amount of water, which does not contribute to energy release during combustion. The presence of water dilutes the carbon content responsible for generating energy.
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Carbon Content: The percentage of carbon in wet brown coal (40%) is significantly lower than that in dried brown coal (66%). This decreased carbon content translates into lower energy yield during combustion.
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Combustion Efficiency: In the presence of moisture, not all the energy from the coal can be effectively harnessed during combustion, leading to inefficiencies and overall reduced energy output.
In summary, it is primarily the high water content and lower carbon percentage that lead to wet brown coal's decreased energy content compared to dried brown coal.