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

Step 1: Understand what "CO2 offset" means here.
Growing energy crops and converting them to bioethanol lets us replace an equal amount of energy that would otherwise have come from petrol. Petrol has a much higher CO2 footprint per unit of energy than bioethanol, so using bioethanol instead avoids some CO2 emissions. The offset is the difference between the emissions that petrol would have caused and the emissions that bioethanol actually causes, for the same energy delivered.

Step 2: Find the total energy delivered by the bioethanol produced on one hectare.
Yield = 7000 L of bioethanol per hectare, and each litre carries 21 MJ of energy, so:
Total energy = 7000 L/ha × 21 MJ/L = 147,000 MJ/ha
This is the energy that the land effectively supplies once per year through the energy crop.

Step 3: Find the CO2 that petrol would have emitted to supply the same energy.
Petrol's footprint is 80 g of CO2 per MJ of energy delivered, so if this 147,000 MJ had instead come from petrol:
CO2 from petrol = 147,000 MJ × 80 g/MJ = 11,760,000 g/ha = 11.76 × 10⁶ g/ha

Step 4: Find the CO2 actually emitted by producing and using the bioethanol.
Bioethanol's own footprint is 10 g of CO2 per MJ, so:
CO2 from bioethanol = 147,000 MJ × 10 g/MJ = 1,470,000 g/ha = 1.47 × 10⁶ g/ha

Step 5: Subtract to get the net CO2 offset.
Net CO2 offset = CO2 that petrol would have released - CO2 that bioethanol actually releases
= 11.76 × 10⁶ - 1.47 × 10⁶ = 10.29 × 10⁶ g/ha, which rounds to 10.3 × 10⁶ g of CO2 per hectare.

Step 6: Sanity check.
Since bioethanol's footprint (10 g/MJ) is much smaller than petrol's (80 g/MJ), replacing petrol energy with bioethanol energy should avoid the large majority of the emissions that would otherwise have occurred, and the offset value of 10.3 × 10⁶ g/ha, close to but slightly less than the full petrol emissions of 11.76 × 10⁶ g/ha, is consistent with that expectation.