[ya_throw]

This is completely bogus. The ever-increasing global demand for soy foodstock is already accelerating the destruction of tropical rainforests - how is it all sustainable to burn huge amounts of foodstock that so we can enjoy making the world "a smaller and better place" [1]?

Just for fun - an acre of soy produces 70 gallons of "biofuel", so a square mile of soy plantation can produce around 45,000 gallons of fuel, or enough to fly a 747 for about 10,000 miles. After slashing and burning a patch of rainforest, you get about 3 years of crops, before you need to leave it for around 10 years to "regrow". If all of the Amazon rainforest (2,000,000 sq miles) was slashed and burned for soy plantations, and assuming it can magically regrow in those 10 years, we will get enough fuel over one 13-year cycle to fly 5 billion miles. That is about 1/10th the total airplane mileage in one year. [2]

[1] https://www.virent.com/products/jet-fuel/

[2] https://www.quora.com/How-many-miles-do-airplanes-fly-in-the...

[alkonaut]

Using foodstock, or land that even competes with foodstock, or land that is important for sensitive ecosystems or CO2 balance is obviously all out of the question if this technology is to be viable.

It's possible to make sustainable jet fuel using trees from forests that aren't competing with food supply. Without that, yes, it's a bad idea. Anything involving soy or palm is usually also a bad sign. But in principle, nothing says you can't use "good" raw materials to produce fuel.

[ya_throw]

Arguably the Amazon rainforest isn't competing with food supply, at least on a global scale.

The UK has only just started reversing the almost complete deforestation it experienced due to human demand for fuel and materials. Once you start adding in calculations for "how sustainable is my product, once I've cut the trees down and have to wait for the forest to re-grow", you find that not much is sustainable at all.

[pfdietz]

This works off of carbohydrates and lignin, including cellulose. Assuming (CH2O)n + nH2 --> (CH2)n + nH2O, then 46% of the weight of dry biomass turns to fuel. The yield of Miscanthus is about 16 dry tonnes/acre/year. Jet fuel is 3 kg/gallon, so this would be about 2400 gallons/acre/year.

[ya_throw]

How much hydrogen do you need (eg, per final gallon of jet fuel), and where does that come from?

[pfdietz]

Just eyeballing that, it's 2 grams of hydrogen per 30 grams of carbohydrate. So, about 1 tonne/acre/year, or maybe half a kilogram per gallon. The hydrogen right now almost certainly is tapped off existing industrial supply, which comes from natural gas, but there's no reason one couldn't use "green hydrogen" from renewable-powered electrolysis instead. PV produces much more energy/acre than biomass does (and can produce it even outside the growing season), so this wouldn't increase land use very much.

[ya_throw]

Generating 500g hydrogen requires 25kWh (and a perfectly-efficient electrolyser would require 20kWh). One gallon of fuel has about 40kWh of energy available.

To put it another way - by this proposed system, assuming 35mpg, an annual personal mileage of 5000 miles would need a dedicated installed solar capacity of 2kW nominal, assuming a 20% capacity factor. This takes up 15sq. meters. Mutiplied by the population of the US, that's 4,500sq km of solar, just for fuel for driving.

The majority of the energy content of the "sustainable" fuels in your scenario would have to come from sources other than the biomass feedstock. Sure solar generates more power per acre than photosysntheis, but it's very expensive - especially in a sustainable world where the solar panel factories are powered by solar panels, and not by coal.

[pfdietz]

Right. Using biomass to generate that hydrogen would be bad. Biomass, among all the renewable energy sources, has very poor power/area. PV is an order of magnitude better.

It's not at all clear that PV is more expensive. In the best global locations it's already below $0.02/kWh -- and one would want to do this processing where the inputs are cheap.

Add to that: this is all forward looking, so we must also consider that PV will continue to get cheaper. Extending the historical experience curve to the point the world is solar powered will drop its cost by another factor of 4. This may or may not happen, but calls in the past that the experience curve had reached its limit were wrong.

[ya_throw]

Using the advertised final cost of electricity is, in my opinion, a useless way to work out the sustainability of any power source. It is skewed beyond usefulness by government incentives, tax breaks, etc etc.

If we're talking about renewable and sustainable power, the only way to look at it is the embodied energy of the device. As I alluded to, the only way solar panels are as cheap as they are is because they are all made using coal or other fossil power. Solar panels require an enormous amount of energy to manufacture - the energy payback period is over 20% of the panel's expected lifetime. This clearly indicates that the price per unit of energy in a system powered by solar would be much higher than it is now.

We also haven't even touched on the idea of where on earth (literally) all of the minerals required to make this gigantic number of solar panels, will come from. PS. Invest in mining, "renewables" are making it a very profitable business right now.

[wutwutwutwut]

From the article:

> Sustainable aviation fuel can be made from any of 60 different feedstocks — among them plant oils, algae, greases, fats, waste streams, alcohols, sugars, captured CO2 and other alternative feedstock sources and processes. The Department of Energy estimates that the United States alone has the resources to produce 50–60 billion gallons of SAF per year.

[ya_throw]

By "article", you mean "marketing blub from someone trying to sell you something".

Ultimately, all of those "sustainable" feedstocks will have to be grown. As my quick maths points out, the size of the field we're going to need is just staggering.

>The Department of Energy estimates that the United States alone has the resources to produce 50–60 billion gallons of SAF per year.

I'd like to know where from. Actually, I looked it up. It's the "Billion Tonne Plan". It involves "thinning" all US forests, and planting practically every available acre with crops for biofuel. Basically, dedicating all of the available plant matter grown in the US to burning for transportation.

[wutwutwutwut]

First you claimed it was bogus and now you say the area needed is staggering. I wonder what's comes next.

[ya_throw]

It's bogus that jet fuel can be renewable, at least in anywhere near the quantity we currently use. Mainly because the amount of arable land needed is staggering, and we already are destroying virgin rainforest just to feed the world.

[pfdietz]

I mean sure, you can reach that conclusion if your number is off by a factor of 30.

The amount of carbon in paper, cardboard, and food in existing municipal solid waste streams in the US would be nearly enough to make the current US jet fuel demand. It's not like replacing all liquid fuel use -- jet fuel is about 6% of US liquid fuel demand.

[ya_throw]

Do you have a source for that?

Assuming we use the hydrogen conversion process you mentioned, and have fitted the 1000's of square miles of solar panels it would need - I find it hard to believe that the US throws away 90 billion kg of carbon-rich domestic waste every year (apparently, the US gets through about 15 billion gallons of jet fuel per year).

Obviously, even if this is true, we then need to address the other 94% of liquid fossil fuel use.