Wikipedia:Reference desk/Archives/Science/2022 April 3
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April 3
editFossil fuel consumption to grow vegetable oil.
editFollowing discussion (elsewhere): It's after reading this very sketchy article.
- The question is: We grow vegetable oil with the most efficient, modern, big scale and sustainable method.
- We convert this oil to jet fuel.
Did we get net gain of fossil fuel required for producing fertilizer + tending the crop? This is emission wise. Have captured CO2 from the air more than we added in the growing process?
-ZIMS — Preceding unsigned comment added by 77.124.34.8 (talk) 17:31, 3 April 2022 (UTC)
- The trend right now is going towards carbon-negative sustainable aviation fuel. It’s been all over the news for the last year. Viriditas (talk) 21:36, 3 April 2022 (UTC)
- You don't really have to produce a lot of fertiliser. In areas producing a lot of pigs, cows and chickens, there's so much manure that we don't know what to do with it, leading to huge pollution. At the same time, areas producing mostly plants use artificial fertiliser as it's cheaper than importing someone else's waste.
- The question comes down to: If we run all farming equipment on biofuels, can we produce net biofuel? Yes, we can. Actually, we did in 1800, when all farming equipment was powered by biofuel using horsepower. But to meet the energy requirements of modern aviation, we would need a lot of farmland. Probably more than is available.
- There's a lot of talk about making aviation sustainable, but there's hardly any progress; it's mostly for greenwashing the industry. We cannot produce enough biofuel, electric flying only makes sense for short hops in archipelagos (range is too short for long hops, whilst outside archipelagos land-based transport works better for the short hops), hydrogen requires so much storage volume that it isn't very practical for aviation (except in airships, but those aren't very practical themselves; using hydrogen in fixed-wing aircraft can't be ruled out completely though). There may be some possibilities with synthetic hydrocarbons (see Sabatier reaction), but that's not what most of the talk is about. Yet, to meet their fair share of the Paris Agreement, aviation has to be made sustainable within the next 15 years or so, or stopped altogether (I wouldn't mind). PiusImpavidus (talk) 09:29, 4 April 2022 (UTC)
- I suppose airships, in particular hybrid airships, offer some hope for future sustainable aviation. --Lambiam 10:57, 4 April 2022 (UTC)
- Airships may have some niche applications, for surveillance, low-altitude cruises and oversized cargo delivery, but I doubt they will have much of a role in sustainable aviation. With a back-of-the-envelope calculation, a Hindenburg-class airship had about 100 times the frontal surface area of a Boeing 737 NG. The 737 suffers induced drag, doubling its drag, and flies faster, increasing drag 60–80 times, but also in thinner air, reducing drag by a factor 6 or so. Then the airship still has higher drag from its large size. This appears to be confirmed by engine size: the airship had 3.56MW of power, which at 135km/h translates to 95kN of thrust (a bit less if assuming non-ideal propellers). The 737 has about 27kN of maximum thrust at cruise altitude. And you can't gain much from flying slower in your airship, as minimum work for flying against a headwind is reached when your airspeed is 1.5 times the wind speed, so you need at least 80km/h cruising speed. So a Hindenburg-class airship needs more work to fly the same distance as a Boeing 737 NG, while carrying only half the payload. Environmentally, airships are pretty bad. Hybrid airships may be between classical airships and fixed-wing aircraft, but that makes them still worse than fixed-wing aircraft. PiusImpavidus (talk) 13:50, 4 April 2022 (UTC)
- HAV said the CO2 footprint per passenger on its airship would be about 4.5kg, compared with about 53kg via jet plane... the hybrid-electric Airlander 10 could make the same [short-haul inter-city] connections with 10% of the carbon footprint from 2025, and with even smaller emissions in the future when the airships were expected to be all-electric powered. [1] Alansplodge (talk) 22:32, 4 April 2022 (UTC)
- Now I wonder what assumptions are behind that comparison. On very short distances, jet aircraft cannot reach their cruising altitude and speed, so they cannot exploit the high efficiency jet engines have at high speed. In that case, the propellers of the airship perform much better than the jet engines of a small jet, giving the jet much less of an advantage, or even a disadvantage. But on such short hops, airlines tend to use turboprops, which are far more efficient than jets at low speed and similar to the engines and props of an airship. Now compare that airship to an ATR 72. It actually has the same payload capacity and engine power as a Hindenburg-class airship, but is 4 times faster, using 75% less time and fuel to cover the same distance. Or even less than that, considering they only use full power during take-off. In fact, our article says that the ATR 72 uses just 21 grammes of fuel per seat-kilometre, but doesn't say at what distance. PiusImpavidus (talk) 10:04, 5 April 2022 (UTC)
- HAV said the CO2 footprint per passenger on its airship would be about 4.5kg, compared with about 53kg via jet plane... the hybrid-electric Airlander 10 could make the same [short-haul inter-city] connections with 10% of the carbon footprint from 2025, and with even smaller emissions in the future when the airships were expected to be all-electric powered. [1] Alansplodge (talk) 22:32, 4 April 2022 (UTC)
- Airships may have some niche applications, for surveillance, low-altitude cruises and oversized cargo delivery, but I doubt they will have much of a role in sustainable aviation. With a back-of-the-envelope calculation, a Hindenburg-class airship had about 100 times the frontal surface area of a Boeing 737 NG. The 737 suffers induced drag, doubling its drag, and flies faster, increasing drag 60–80 times, but also in thinner air, reducing drag by a factor 6 or so. Then the airship still has higher drag from its large size. This appears to be confirmed by engine size: the airship had 3.56MW of power, which at 135km/h translates to 95kN of thrust (a bit less if assuming non-ideal propellers). The 737 has about 27kN of maximum thrust at cruise altitude. And you can't gain much from flying slower in your airship, as minimum work for flying against a headwind is reached when your airspeed is 1.5 times the wind speed, so you need at least 80km/h cruising speed. So a Hindenburg-class airship needs more work to fly the same distance as a Boeing 737 NG, while carrying only half the payload. Environmentally, airships are pretty bad. Hybrid airships may be between classical airships and fixed-wing aircraft, but that makes them still worse than fixed-wing aircraft. PiusImpavidus (talk) 13:50, 4 April 2022 (UTC)
- I suppose airships, in particular hybrid airships, offer some hope for future sustainable aviation. --Lambiam 10:57, 4 April 2022 (UTC)
- Here's a glossy leaflet published by the RSB, who certify sustainable aviation fuel. On page 5, HEFA appears in a table, and it says the blending ratio (with fossil fuel kerosene) is "up to 50%". The CNN article attaches the optimistic word "currently" to that. On page 8, the leaflet says the RSB "Ensures that Sustainable Aviation Fuels produce at least 50% less GHG emissions than conventional jet kerosene", but the meaning of that isn't clear - for all we know this is just based on the false assumption that zero fossil fuel is required to produce the biofuel portion of the blend. To my mind the entire job and purpose of the RSB is to investigate the amount of fossil fuel used to produce biofuels, but perhaps they don't. There are a lot of words in the leaflet, but none of them seem to be about investigating that. This other document, Standard for advanced fuels, mentions "Greenhouse gas calculation", which is in this third document, RSB GHG Calculation Methodology (RSB-STD-01-003-01). Tracing exactly how the operators got their fuel certified is difficult: what assumptions were made? How much of it is "carbon offsetting"? Which things did they refrain from voluntarily investigating? Also, the cooking oil seems to be used oil recovered from restaurants and homes, so the assumption "it was going to be thrown away, therefore we don't have to assess how it was grown" might be in play. If there can be an economy where all aviation fuel is obtained from cooking waste, this changes the question into one about the production of cooking oil in general, separate from its use as aviation fuel. However, if cooking oil users are being subsidised by the aviation industry (paid for the waste oil), they will use more cooking oil, so there's still a question about whether the scheme adds up. Card Zero (talk) 14:52, 4 April 2022 (UTC)