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Re:The Case Against More Ethanol (2016) by cogman10 on Monday November 02, 2020 @11:53AM (#24970158)
It's not an energy efficiency problem, it's an energy density problem.

Burning hydrocarbons peeks out at something like 40% efficiency. Yet we are only now getting to the point where EVs have the same range as ICE vehicles, even though the battery->motor conversion is something like 90% efficient.

Why is that? Because the amount of energy that can be stored in batteries is MUCH lower than the amount of energy released from burning fuel.

Transport needs to carry all of it's energy with it (or, have something like a 3rd rail to provide energy).

That's the argument for biofuels in the future. Not that you can't replace 99% of transport with electricity, but rather some forms of transport require a lot of energy.

We are JUST getting to the point where batteries have enough density to power a prop plane for ~ 1hour worth of flight.

We are no where near the energy density where a cargo ship could sail from china to the US or for international flights.

One alternative to biofuels is hydrogen from something like electrolysis.

One thing I think we'll both agree on. Ethanol for cars/trucks is a terrible idea. EVs are the future there.

Re:The Case Against More Ethanol (2016) by philipkglass on Monday November 02, 2020 @12:46PM (#24970814)
The parent post meant making hydrocarbon fuels from electricity, water, and carbon dioxide. You make electrolytic hydrogen from electricity and water and then use it to hydrogenate CO2 to methane. Liquid methane has excellent energy density and would be suitable for fueling rockets, aircraft, and large ocean going ships. Or you could create synthetic kerosene by a different choice of chemical processes, if it's acceptable to trade lower efficiency for a fuel that is conveniently liquid at room temperature and compatible with legacy systems.

https://www.power-eng.com/2020/07/08/power-to-gas-examining-...

https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_proces...

The advantage of synthetic fuels over biofuels is that you can get a lot more fuel out of 100 hectares of solar panels than 100 hectares of crops, because plants are very inefficient at capturing solar energy as useful chemical energy. The advantage of biofuels over synthetic fuels would be lower capital costs. Biofuels have an advantage while non-fossil fuel demand is low, but they can't scale as well.

Since artificially capturing CO2 from the air is expensive and inefficient so far, an attractive hybrid path combines biomass and electrolytic hydrogen to make methane or heavier hydrocarbons. The main role of the plants is to concentrate carbon from the air. A kilogram of dry cellulose has an enthalpy of combustion of 17.5 MJ [1]. It contains enough carbon to make 0.76 kg (47.6 moles) of methane, once fully hydrogenated. That methane has an enthalpy of combustion of 41.9 MJ [2]. The biomass would supply all of the carbon and a minority of the chemical energy while electrolysis would supply the majority of the hydrogen and the final energy content of the methane.

[1] https://en.wikipedia.org/wiki/Cellulose

[2] https://en.wikipedia.org/wiki/Methane_(data_page)