As long as you don't count co2 as an emission, it all looks reasonably rosy.

As a real user of truck's one who tows large loads ,long distances, and also stays off grid for weeks at a time. I can attest to the absolute need for the energy density of liquid fuels. With diesels being the technological leader in efficiency ,durability and economic cost of use. Diesels can approach and exceed fuel cells in efficiency. Fuel cells are most efficient at near peak load around 90% of rated power they fall off rapidly in either direction from there and are very expensive due to the platinum needed for use plus the reformer to convert liquid fuels into pure hydrogen gas. Diesels use liquid fuels directly, use a tiny fraction of precious metals relative to fuel cells for the exhaust catalysts and achieve 48% efficiency or better at 80% of full load with a broad efficiency area around that point just view any modern turbo diesels BSFC vs load map there will be a vast 40+% area over a wide range of rpm and torque. Batteries are not even in the same technical league their energy density is two orders of magnitude less than liquid fuels even including the weight of the ICE gen set.

If CO2 is a thing you worry about then biofuels are the way to go. With regenerative agriculture methods those fuels can be carbon negative with the roots and biochar being sequestered and all the carbon in the fuels being atmospheric not geological carbon. Clear Flame can convert any standard diesel to run purely on alcohols doesn't matter which one, methanol,ethanol,isopropanol,butanol all work and all can be made from biomass or syngas from wastes. Biomethane and synthetic natural gas from syngas are in this class as well. For short range return to depot type trucking CNG/RNG makes a lot of economic sense. For long distance, heavy loads and.off grid work liquid fuels are the only choices even metal air fuel cells would not have the energy density nor the power density needed. 55 gal of diesel is over 2 megawatts worth of raw energy there is no substitute. You could make synthetic diesel like the Germans did can syngas with equal or better energy density that syngas can be from biomass or municipal wastes or sewerage sludges so it's carbon is biomass sourced.

@Gryf, OK, but will it scale - is there enough biodiesel, or will there ever be enough to replace (say) 50% of current demand?

Class 4_8 truck's are 20% of the fleet and use 45 billion gals gasoline equivalents per year. Why GGE? To normalize petrol use and diesel to a common energy point.

Biomass to liquid yields are in the 100 to 200 gal ethanol equivalent per tonne. Ethanol is 24MJ/L gasoline is 34.6/L a ratio of 1.4:1 so you would need 63 billon gal of ethanol equivalent to equal 45 billion GGE. At a low end of 100gee you need 630 million tonnes of waste biomass per year since the USA makes over a billion tonnes of waste biomass per year yes there is enough just in the wastes. You can also cover shipping 6.4 billion GGE, aviation 18.7 BGGE, rail 4.1 BGGE.

The elephant in the room is Light duty vehicles they use a massive 120 billion GGE per year those alone would take 1.2 billion tonnes per year of biomass waste won't cover that you would need to grow dedicated energy crops on a massive scale. Or electrify the LDV for all drives shorter than 40 miles that alone would.cover 96% of all drives. Leave the long distance drives to hybrids using only 4% of the previous total that's like 4.8 billion GGE perfectly do able with biofuels. Plug in hybrids are the answer to the short far problem.

https://afdc.energy.gov/data/mobile/10566

https://afdc.energy.gov/data/mobile/10569

https://afdc.energy.gov/data/mobile/10317

https://afdc.energy.gov/data/mobile/10318

https://afdc.energy.gov/data/mobile/10661

Mahonj,

This is a recent biomass to liquid yields paper.

https://pubs.acs.org/doi/10.1021/es100316z?cookieSet=1#

Not sure about the gross USA production. There is a DOE study from 2016 called the billion tonne report.

For perspective there is 900 million acres of farmland in the USA that's not including rangelands(grasslands,grazinglands, or scrub brush) nor forests. The potentials of the forests and grasslands are expense. That also doesn't include the semi arid lands nor the actual deserts. There again massive potential for biomass growth with the right species and human help. Add in saline / brackish aquaculture in the arid lands such as algae or drip irrigation of salt tolerant succulents. There will likely never be a shortage of biomass it's what is the cost per gigajoule vs drilling a hole in Saudi Arabia. China and India have to get on board for any real progress on leaving geological carbon to have meaning.

Look up the agave biomass project in Mexico.they have proven that Agave Weber can produce 100+ tonnes of biomass per year on semi arid lands on a 5 year rotational cycle.

@Carl, @jamesD, Thanks.
I have some homework now, in no uncertain terms.