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  • Writer's pictureGeoff Russell

Hard to decarbonise: Diesels Part II

Updated: Oct 5, 2023

I posted about decarbonising diesel engines a while back and it’s time for part II: solutions. But “approaches” is definitely a more appropriate term than “solutions”.

Thermodynamics and perpetual motion machines

Let’s start by outlining why this is a really hard problem.

Think about a huge mining truck like the Komatsu 980E which I mentioned in the previous post. This truck can haul 350 tonnes of ore. Imagine attaching a huge bag to the exhaust and running the truck for an hour. As diesel is burned, the bag fills up.

Diesel is a hydrocarbon, meaning it is long chains of hydrogen and carbon atoms.

So what ends up in the bag? Every carbon and hydrogen atom that was in the fuel tank and was burned to power the vehicle will leave the exhaust stream and end up in the bag.

Fans of the circular economy who are convinced by the graphic art images, might imagine we can just glue them back into those long chains and start all over again. How hard can that be?

Here’s the thing. All the energy that powered the vehicle came from rearranging the chemical bonds in the diesel hydrocarbons by adding oxygen to both the carbon and hydrogen atoms. The result is carbon dioxide, water and loose change – impurities that don’t concern us. But if you want to reconfigure those bonds, it will take at least as much energy as you got when you changed them; and typically, considerably more! One of the workhorse chemical processes for making hydrocarbons from atoms is the Fischer-Tropsch process and it's only about 60% efficient at best. Meaning that it takes 166 units of energy to make enough diesel to get 100 units when you use it.

Using the stuff in the bag to make new diesel can, at best, solve just one component of the problem. It can provide a source of materials, the hydrogen and carbon atoms; but we still need the energy to reconfigure the bonds.

Where is that energy going to come from?

Suppose we try to get it from wood.

The energy density of wood is about 18 megajoules per kilogram and the energy density of diesel is about 45 megajoules per kilogram. So if you had perfectly efficient processes, you’d still need to burn about 2.5 times as much wood as the diesel you wanted to make; even if you got all the hydrogen and carbon atoms for free.

Thinking about it, you might think that we could save energy by using some material that is already closer to being diesel than our atoms in a bag. And you’d be right. Wood itself is one of these. Like many other things, wood has long chains of hydrogen and carbon atoms, but with added bits. Natural processes build these strings in plants and animals all day every day. But that still doesn’t make it easy. Collecting plant materials requires trucks, and the energy expended can easily exceed the energy gained. When early cities were powered by wood and horses pulling wagons, the amount of wood within a day’s wagon travel put limits on the city size.

Let’s do some rough calculations.

The total global tonnage of crude oil (not just diesel) used each year is about 5.5 billion cubic metres (i.e., 5.5 cubic kilometres). Assuming about 0.8 kilograms per litre, you are talking about 3.8 billion tonnes of oil. Making it from atoms using wood as the energy source would require 9.7 billion tonnes of wood.

Remember, of course, that you’d also have to harvest and collect all that wood, and any energy required for that process is on top of the energy we are looking to create with that wood.

Happily, we aren’t concerned with all crude oil, just diesel. But we might as well add in aviation fuel while we’re at it, because that’s another hard-to-decarbonise sector.

There are a few companies in the business of making what is called Sustainable Aviation Fuel (SAF). Some of these also make what is called “green diesel” using similar processes, so we can treat the problems of decarbonising diesels and aircraft as pretty much the same problem; just at a different scale.

The annual global diesel consumption is about 1,624 billion litres compared with about 350 billion litres used in planes as jet fuel. Current jet fuel consumption is only about 2/3rd of this but is expected to rebound post Covid19.

The biggest of the companies making SAF is Neste who aim to be making 1.9 billion litres of SAF per year by the end of 2023. You need to compare 1.9 with 350 to see how small this is; and Neste is the biggest.

What Neste is doing, is largely harnessing waste streams of fats and carbohydrates which can be turned into hydrocarbons. There are plant and animal forms of both. Both fats and carbs have oxygen atoms which are best removed if you want nice pure hydrocarbons. Neste uses hydrogen at highish temperatures (100 to 200 degrees C) and pressures (20-40 x atmospheric pressure) to do this. On the other hand, there is one diesel alternative that keeps the oxygens; dimethyl ether.

What’s the potential for the Neste (or similar) process?

Global roundwood production is about 3.9 billion cubic metres, evenly split between industrial and fuel use. Even if we had access to all of it, it isn’t enough.

Global vegetable oil production is about 212 million tonnes. This is trivial compared to the hydrocarbon requirements of SAF and diesel.

How much of these two possible raw material streams is waste?

It’s generally good to harness what would otherwise be waste, and put it to good use, but how do you expand? By artificially promoting increased consumption to generate more waste? Or perhaps by encouraging less efficient usage to expand the waste stream? Neither sounds desirable.

Competition for resources

Reading through the last Intergovernmental Panel on Climate Change (IPCC) WG3 report, it’s interesting how many people want to use forests as renewable resources to solve various pressing part of the climate problem. This is the tension between COP28 (climate) and COP15 (wildlife and biodiversity). Many ‘renewables’ advocates are among those treating nature “like a toilet”, to co-opt the words of the UN secretary-general.

The non-energy uses for forests and crops (growing mostly on land that used to be wildlife habitat) are:

  1. As a feedstock for plastic production,

  2. As a feedstock for SAF and diesel production,

  3. To reduce the steel and concrete requirements in building.

Crops are frequently, but not always, an alternative to forests. Crops are typically grown on land that was first cleared of forests. Land that never supported forests didn’t support them for a reason, perhaps the soil is poor or there isn’t enough rain. Such land typically needs more nutrient and water inputs than land that once supported forests.

In addition, there are various energy uses that are posited for forests in response to our climate crisis:

  1. To firm up unreliable electricity sources; meaning wind and solar,

  2. To provide baseload power,

  3. To sequester carbon. This can be either as standing forests or as an adjunct to the firming of unreliable energy sources, in which case it is called Bio Energy Carbon Capture and Storage (BECCS).

Burning wood to make electricity has always been among the worst uses for wood. But it is certainly less objectionable when it only involves wood waste generated from other uses. But as wood-fired electricity has expanded (from microscopic to merely small) in recent years, it quickly exhausted waste supplies and moved on to felling forests and shipping them around the world to take advantage of subsidies based on the politically magic term,“renewable”. For example, the Drax power station in the UK imported and burned about 7.8 million tonnes of wood pellets in 2021 for its boilers.

Using waste to make SAF or diesel, whether for a feedstock or to provide energy to drive the process, is a dead-end technology. It may well supply a small percentage of the world’s demand for SAF but can it decarbonise all of it, and what about the much bigger diesel requirements?

A last little issue is that SAF is currently about three times the cost of normal jet fuel, but various policy mandates enable Neste to sell to customers needing to meet various mandated aviation emission standards. Assuming we can find more raw materials to entirely replace jet fuel with SAF, can the airlines survive with triple the fuel cost? I can remember when the real cost of air travel was much higher than today, so the answer is clearly yes, but it would be considerably smaller and air travel would return to being just an activity of the rich.

The elephant in the room with all of these uses of wood is the meat industry, a global squattocracy of the majority of the once forested surface area of the planet. This is either directly via grazing (sheep and cattle) or indirectly via cropping for feed (pigs and chickens as well as sheep and cattle).

The IEA NetZero plan

The International Energy Agency (IEA) has a Net Zero by 2050 (NZ2050) plan which includes decarbonising both air travel and diesel. Its solution for air travel is a mix of the kind of plant-based solutions we’ve been discussing and hydrogen-based fuels. The IEA plan calls for 15% of aviation to be powered with SAF by 2030 with a mix of SAF and hydrogen fuels by 2050. They still see about 20% of total jet fuel demand being supplied by jet fuel in 2050, which has to be offset. How much habitat do they need to co-opt to do that? We’ll get to that below.

The IEA reckon electricity can power two-thirds of trucks by 2050, assuming the increased efficiency gains in batteries continues. I’d guess their modelling rejected replacing all diesel with synthetic alternatives precisely because of the heavy land use demands I detailed above. The opening of the new mines needed to produce battery materials for electric vehicle (EV)s is already well behind schedule. Batteries, like forests, are seen as the solution to everything, and the costs are ignored. Predictions for battery capacity by 2030 are about half what will be needed for cars, let alone trucks are people wanting to waste them firming up grids. We know how to build reliable low-carbon grids; to choose batteries and a complete grid redesign and rebuild over nuclear is simply bizarre.

[The numbers in the following paragraph have been fixed. 200 replaces 70 in the original and 80 replaces the second 70 in the original ... GR Oct 2023]

So while the IEA may be unwisely betting on batteries (to the delight of the mining industries), it is, to its credit, one of the few organisations to explicitly call for a reduction in land used for grazing, to the tune of about 200 million hectares globally with another 80 million hectares being clawed back from cropping for feed. In total, they call for 140 million hectares to be used for energy crops. Plus about 37 million hectares of solar panels.

Other energy input

The source for driving synthetic fuel production can be biomass, wind/solar or nuclear. Similarly for hydrogen production.

So let’s run the calculations for making synthetic SAF or diesel again with solar power providing the energy.

This gets a little complex because solar power provides electricity and we’ve been talking about heat. But we’ll deal with that later. It’s also complicated by the fact that solar can only provide the energy, not the feedstock; meaning the hydrogen and carbon atoms. The provision of the feedstock compensates for the inefficiency of using wood as an energy source.

Burning 2,000 billion litres of oil annually (the combined SAF and diesel usage) gets you about 20,000 terawatt hours of heat. You could get this from 8,600 gigawatts of solar panels in somewhere like Australia with a good capacity factor of 0.26. This is roughly 600 million tonnes of panels at 70,000 tonnes per gigawatt. As before, we need some additional source of energy to make and distribute these panels. The IEA puts the aluminium required for this quantity of photovoltaic (PV) at about 160 million tonnes.

If you look at the IEA Net Zero plan, it is calling for about 14,500 gigawatts of PV, but with about 77% of that being used for electricity. Clearly, they aren’t looking to make SAF or diesel with PV; but some could be being used for hydrogen production. The eco-footprint of making hydrogen, SAF or diesel is much lower with nuclear power providing the electricity and heat. The IEA’s plan calls for a doubling of nuclear power by 2050, and that tells you plenty about their estimate of the risks of trying to build 100% renewable grids. This chart from The Economist says it all. Does anybody seriously want to suggest replacing the top three sources of electricity in Europe by the stuff still at the bottom after 20 years of hype? While at the same time expanding those same poor performers to generate SAF and diesel?


There is no silver bullet for diesel or SAF.

And we need to consider both wildlife and climate goals. We need to focus on ways which don’t sacrifice the former for the latter. All kinds of problems become more tractable if you eliminate the meat and dairy industries. As problems go, these are both easy. There are no technical difficulties, just political ones. With the exception of subsistence farmers, anybody can change their diet over a few weeks or months. It’s extraordinary how many people in London, Sydney, New York or other large cities protest that they can’t change their diet because of the usefulness of livestock for turning grass into protein in the world’s arid lands; like it’s relevant.

The grazing sector alone uses over 3 billion hectares of the planet. Destock that, and we can seriously address biodiversity problems while still using a couple of hundred million for this or that climate goal like making SAF or diesel or timber-based skyscrapers. It won’t be easy and fair compensation will be essential, but it’s not like there is any alternative.

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