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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

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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

Calculate the mass, in tonne, of wet brown coal that is required to generate 3.6 × 10⁷ kJ of energy.

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Answer

To find the mass of wet brown coal required, we first determine the energy released per kilogram of wet brown coal. Given that the energy content is 5.0 kJ/g:

  1. Change the energy content to kJ/kg:

    extEnergycontentofwetbrowncoal=5.0extkJ/g×1000extg/kg=5000extkJ/kg ext{Energy content of wet brown coal} = 5.0 ext{ kJ/g} \times 1000 ext{ g/kg} = 5000 ext{ kJ/kg}

  2. Next, use the formula to find the mass:

    extMass=extTotalenergyrequiredextEnergyperkg =3.6×107extkJ5000extkJ/kg=7200 kg ext{Mass} = \frac{ ext{Total energy required}}{ ext{Energy per kg}}\ = \frac{3.6 \times 10^7 ext{ kJ}}{5000 ext{ kJ/kg}} = 7200 \text{ kg}

  3. Convert kg to tonnes:

    7200extkg=72001000=7.2exttonnes7200 ext{ kg} = \frac{7200}{1000} = 7.2 ext{ tonnes}

Thus, the mass required is 7.2 tonnes.

Step 2

Calculate the mass, in tonne, of carbon dioxide that is produced from the complete combustion of this mass of wet brown coal.

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Answer

To calculate the mass of carbon dioxide produced, we need to first determine the mass of carbon in the wet brown coal:

  1. Given that the percentage of carbon in wet brown coal is 40%, we can calculate the mass of carbon:

    Mass of carbon=40%×7.2 tonnes=0.4×7.2 tonnes=2.88 tonnes\text{Mass of carbon} = 40\% \times 7.2 \text{ tonnes} = 0.4 \times 7.2 \text{ tonnes} = 2.88 \text{ tonnes}

  2. The complete combustion of carbon produces carbon dioxide according to the reaction:

    C+O2→CO2C + O_2 \rightarrow CO_2

  3. For each tonne of carbon, 3.67 tonnes of carbon dioxide are produced:

    Mass of CO2=2.88 tonnes of C×3.67tonnes of CO2tonnes of C=10.57 tonnes of CO2\text{Mass of CO}_2 = 2.88 \text{ tonnes of C} \times 3.67 \dfrac{\text{tonnes of CO}_2}{\text{tonnes of C}} = 10.57 \text{ tonnes of CO}_2

Thus, the mass of carbon dioxide produced is approximately 10.57 tonnes.

Step 3

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.

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Answer

The energy content of wet brown coal is significantly lower than that of dried brown coal for two main reasons:

  1. High Moisture Content: Wet brown coal contains a significant amount of moisture, which contributes to its lower energy density. When burned, water in the coal absorbs energy, resulting in less energy being available for work or electricity generation.

  2. Lower Carbon Content: The percentage of carbon in wet brown coal is only 40%, compared to 66% in dried brown coal. Since carbon is the primary fuel component contributing to energy output, a lower percentage of carbon results in reduced energy release during combustion.

Thus, both the presence of moisture and the lower carbon content are critical factors that lead to the reduced energy content of wet brown coal.

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