Energy Returned on Energy Invested: Part 3 – Coal and Gas
Welcome back to our discussion on EROEI, or Energy Returned on Energy Invested.
Article 2 looked at primary energy consumption in the oil value chain. In essence, how we transport oil from where it’s produced to where we need it in a usable form.
In Article 3, we’ll apply the same treatment to coal and gas.
Let’s start with coal, which we must initially get from the mine to a port or power station. Available data on freight train energy consumption varies considerably. For this exercise, we will assume 0.1 kWh of primary fossil fuel energy per tonne.km of coal transported. We will also assume an energy content of ~24 GJ/tonne for black coal, the primary type of coal traded for power generation.
(Brown coal is less attractive to transport due to its much higher water content and, hence, lower specific energy content)
Some feedback on my first article suggests that, at the lower end, the EROEI range might be as low as 20 for Brown coal, which makes sense when you consider the energy content of brown coal is about half that of black for the same volume.
So, if our coal has to travel 100 km from mine to port or mine to a power station, the energy consumption is 10 kWh of primary energy per 24 GJ (~6,700 kWh) delivered. This is <0.2%, and we will ignore it for simplicity.
Again, there is a range of efficiencies for bulk carriers depending on ship size and speed and clearly, the energy consumed will depend on the distance travelled.
We are going to assume 0.02 kWh of primary energy per tonne.km shipped an average distance of ~7,000 km (equivalent of Australia to Japan), resulting in an energy consumption of 140 kWh per 6,700 kWh delivered, i.e., ~2%.
Then, we need to get the ship back to collect another load. Let’s assume it only needs half the fuel to return empty, or another 1%. This has a more significant impact on our EROEI than you might think.
We used ~2-2.5% of the embodied energy extracting the coal, resulting in our initial EROEI of 40-50. Now, for coal that has travelled a fair distance, we have consumed another 3%, so our EROEI has dropped to ~18-20.
About 17% of global coal is shipped transoceanic distances. Hence, the weighted average EROEI of delivered coal is Est. 36-46, with a potential lower bound of 20 for brown coal, which isn’t shipped anywhere.
This is still relatively high, and it’s easy to see the attraction of coal as a low-cost energy-dense fuel with relatively low transport costs.
About 83% of the world’s gas is transmitted through pipeline networks. The remainder is shipped as LNG and transmitted through pipelines at the destination. Estimates of energy consumed in gas transmission networks range from ~2-4% of the energy transmitted.
For balance, we will assume 3%. So now our EROEI for gas delivered by pipeline has dropped from a range of 10-20 to ~7.9-13
For LNG, an enormous amount of energy is required to liquefy the gas, which needs to be cooled to -162C. Estimates range from 6-10% of the energy content of the product; we will use 8% here.
We must also account for the energy consumed in shipping the LNG.
Different approaches indicate the energy consumed in shipping LNG is ~2-3% of the energy transported, so allowing for a return journey and assuming any gas boiled off during the trip is consumed as fuel rather than wasted, we will use a value of 4%.
Putting all this together, our EROEI for LNG is not looking too hot and is in the range of 4.4-5.4
The weighted average for gas overall is ~7.3-12.