Is gasoline or Diesel refining less sustainable, per energy output?

Question

Inspired by my own answer here, I'm wondering what the actual numbers are regarding gasoline and Diesel fuel refining. I have read (don't remember where) that refining of Diesel fuel is less environmentally damaging than that of gasoline. But is this actually true?

And for the sake of this question, I'm interested in the environmental impact of refining per energy output (not per volume of fuel), so the energy density of the output fuel ought to be considered.

Answer 1

tl;dr There really isn't a sustainability argument for switching either way between them: there is only an argument for switching away from both. It's akin to asking whether we should switch from coal to gas: the answer is that that's not a meaningful question: we absolutely have to stop using both.

So although the question is in the same sort of rhetorical terrain as asking whether it's better to suffer a premature death by gun or knife, when the answer is obviously that it's better to not suffer a premature death, let's look at lifecycle analysis of their relative unsustainability.

On specific pollutants, on a well-to-wheel basis, per unit distance driven, diesel tends to be worse for particulates and NOx; petrol (gasoline) is worse for CO₂ and hydrocarbons.

The system boundary for sustainability analysis

When looking at sustainability, one has to draw a system boundary somewhere, to set out what will be included in the study, and what will be excluded.

Where one draws a system boundary is a judgement call: if it goes beyond our galaxy or beyond a billion years, you're system boundary is too large. If, however, it does not include consumption of the product and its direct consequences, then your system boundary is too small.

For transport, we typically use a well-to-wheel basis. That's a generic description that applies even when the fuel doesn't come from a well (and even if the transport doesn't have wheels), and relates to the supply chain from initial extraction to the transport itself and the direct consequences, but not the indirect consequences such as changes in land use.

Is the question meaningful?

Neither petrol (gasoline) nor diesel are at all sustainable. Both score zero for sustainability. And there is little to choose between their unsustainability: the trade-offs being some increase in NOx and particulates, for some decrease in CO₂ and hydrocarbons.

Both release carbon dioxide (and other pollutants) during combustion; and that is leading us to catastrophic climate change.

Both come from crude oil, which is a finite, depletable resource.

The extraction of crude oil itself is environmentally destructive.

All this means that the refining of each is environmentally unsustainable too: the production of each is unsustainable, along the entire supply chain.

So the question, as posed, is not meaningful: it does not make any sense to ask about the sustainability of just the refining part of petrol (gasoline) or diesel.

And given that the entire supply chain for each is unsustainable, it does not make any sense to talk about the relative sustainability of one part of that supply chain: sustainability by its very nature is an issue that concerns the whole system, not just one component.

But which is worse?

According to the Life Cycle Assessment of Vehicle Fuels and Technologies (2006, London Borough of Camden), diesel CO2 emissions are 16% lower than petrol on a life cycle basis (p36). However, particulate and NOx emissions from diesel are significantly worse (p37), with petrol NOx being only 60% of diesel NOx. Petrol has worse hydrocarbon emissions (p38). Those are on a unit-distance basis. Conversely, the unit-energy basis is used in the UK DTI report from 2003, Life Cycle Assessment of Vehicle Fuels and Technologies, which gives total GHG emissions of 87gCO₂e/MJ for ultra low sulphur diesel, and 81gCO₂e/MJ for unleaded petrol (gasoline) (Table II).

In summary: there's really not much too choose between them. They're both completely unsustainable, in only slightly different ways.

Answer 2

I don't think it matters. You start off with crude oil and refining gets you a bunch of components. If you aren't refining the gas what else are you going to do with it. Just treat it as a waste product?

Once you are refining oil, I suspect we might as well use it all.

Energy unit to energy unit diesel puts out more greenhouse gasses, but again, once we are using oil, I suspect we might use all of it.

Edit: In case my answer wasn't clear, typically oil refining is done through fractional destillation and diesel is somewhat at a different level there than gasoline. Consequently these use very different components of crude oil, so this question seems to come down to an equivalent to whether chuck roast is more sustainable than rib steak. Once you are refining diesel, is it more sustainable to throw away the gasoline as industrial waste or to pull that out separately?

Answer 3

Refining is done by heating and cracking petrol and then sorting out the different components in a tall distillation column. Diesel is heavier than gasoline so it is overall easier to refine. There is a nice simplified figure here.

That being said, the differences are not as important as those in the engine. As EnergyNumbers pointed out in another answer, diesel is more efficient, at the cost of higher emissions. This is true especially on older engines. On modern engines, from petrol to wheel, diesel is about 20% more efficient.

Answer 4

If your question is really "Should I buy a gasoline car or a diesel" then the answer is "buy an electric".

The fact is that using any kind of oil for fuel is the single least sustainable thing that humans can do. Oil is not only a finite resource that doesn't renew itself, the reserves of oil on this planet compared to consumption cannot be sustained for more than a human lifetime, and quite possibly a whole lot less than that. As a natural resource, renewable or not, it's the one in shortest supply (and thus the least sustainable).

So what happens when we actually run out of oil, regardless of the environmental impacts of not giving it up before that happens?

Well, we're all going to have to give some serious thought to the somewhat less convenient (but still quite viable) options to oil. Like electric vehicles of all stripes. It's probably better for everyone if we just start buying them now, because that money spent goes to making the cars better in the future.

Hydrogen isn't feasible because 1) there's only a scant few cars you can buy, 2) there's even fewer refuelling stations by far, and 3) moving the whole infrastructure to hydrogen is a pipe dream anyway. (this handy infographic and video explains why) And if your desire for sustainability is rooted in environmentalism, hydrogen isn't a clean fuel source at all, it's largely based on the use of natural gas, and as a result Toyota's Mirai is only just barely cleaner than a Toyota Prius.

I'm not even going to go into the costs of hydrogen, which are higher than any other mode of transport. Electric cars on the other hand, have the benefit of being cheap to fuel and cheap to maintain, which in the medium to long term, can actually make up the difference in the initial purchase price (or at least come close).