Question

An agricultural land can support energy crops yield to produce 7000 L of bioethanol per hectare and energy content of bioethanol is 21 MJ/L. This bioethanol displaces fossil-based petrol.

The \(CO_2\) offset potential of land used for growing energy crops as a bioethanol feedstock is ______ \(\times 10^6\) g of \(CO_2\)/hectare (rounded off to one decimal place).

Consider \(CO_2\) footprints of 10 g and 80 g of \(CO_2\) per MJ for bioethanol and petrol, respectively.

Hint:

First compute the total energy delivered by the bioethanol yield per hectare (yield × energy content), then multiply by each fuel's CO2 footprint per MJ and subtract the bioethanol's own emissions from the petrol emissions it avoids.

Answer 1

Correct Answer: 10.3

A shorter route is to first find the net CO2 saving per unit of energy, and only then scale it up by the total energy produced, instead of computing the two CO2 totals separately and subtracting them at the end.

The saving in CO2 footprint per MJ of energy, when bioethanol is used instead of petrol, is:
\[ \Delta = 80 - 10 = 70\ \text{g CO}_2/\text{MJ} \]

The total energy the land's bioethanol yield delivers per hectare is:
\[ E = 7000\ \text{L/ha} \times 21\ \text{MJ/L} = 147000\ \text{MJ/ha} \]

Multiplying the per-MJ saving by the total energy gives the net CO2 offset directly, in one step:
\[ \text{Offset} = E \times \Delta = 147000 \times 70 = 10{,}290{,}000\ \text{g/ha} = 10.29 \times 10^6\ \text{g/ha} \]

Rounding to one decimal place gives the same result as computing the two footprints separately and subtracting them.
\[\boxed{\text{CO}_2 \text{ offset} \approx 10.3 \times 10^6\ \text{g CO}_2/\text{hectare}}\]