What volume of synthetic hydrocarbon fuels can we generate in the future?

Guest Post by Chris Uhlik. Dr Uhlik did a BS, MS, and PhD in Electrical Engineering at Stanford 1979–1990. He worked at Toyota in Japan, built robot controllers, cellular telephone systems, internet routers, and now does engineering management at Google. Among his 8 years of projects as an engineering director at Google, he counts engineering recruiting, Toolbar, Software QA, Software Security, GMail, Video, BookSearch, StreetView, AerialImaging, and research activities in Artificial Intelligence and Education. He has directly managed about 500 engineers at Google and indirectly over 2000 employees. His interests include nuclear power, photosynthesis, technology evolution, artificial intelligence, ecosystems, and education.

(Ed Note: Chris has written previously on BNC on calculating the cost of ending global warming)

In a hypothetical carbon-neutral future, we can still use liquid hydrocarbon fuels if they are synthesized from non-fossil carbon sources. This analysis looks at how much carbon we use today and which of those uses can be readily substituted by electricity and synthetic fuels.

I’ll use numbers for the United States as economic and energy use data are well published by various government agencies such as the National Laboratories and the Energy Information Administration.

Flows of fossil carbon in the US Economy: (Please forgive the excess precision)

Coal: 9.08e11 kg/year which I estimate to be about 64e12 moles/carbon/year

Petroleum: 19,498,000 bbl/day, (incidentally I was surprised to learn that only 46% of this ends up in motor fuel)

Natural Gas: 7.4e11 m^3/year produced + 1.1e11 m^3 cuft/year imported

Cement: 2.5e9 Mg clinker/year worldwide of which I estimate 24% is used in the United States (by ratio of USA GDP/world GDP)

This amounts to a fossil carbon flux of about 170 x 10^12 moles of fossil carbon being extracted and released to the atmosphere each year in the United States.

To what uses is it put?

  • Electricity generation (coal and gas fired thermal plants)
  • Automobiles and light trucks (light transportation)
  • Highway trucks and rail trains (heavy transportation)
  • Ships
  • Airplanes
  • Heating oil
  • Steel production
  • Cement production
  • Fertilizer production
  • Residential and Commercial gas
  • Industrial gas
  • other materials

By combing a variety of sources and making educated guesses, I break it down like this:

My drawing skills aren’t quite up to the Livermore Labs energy flow pictures, but I hope you get the idea. One thing that jumps out at me is how many small categories there are on the right. There is no one thing that will reduce our fossil carbon flux by as much as 50%. We have lots of smaller (but still huge) problems to solve.

To reduce our fossil carbon flux, we can do two main categories of things:

1. move processes on the right to non-fossil sourced energy like nuclear generated electricity, wind, solar, geothermal, etc.

2. use non-fossil carbon sources to synthesize fuels that displace some of the sources on the left.

Here are some technologies that can reduce fuel consumption on the right:

1. generate base-load electricity with fission rather than coal and gas — good for ~35.5% reduction in fossil carbon flux.

2. use battery powered vehicles and PHEV technology — good for ~14.5% reduction.

3. use overhead wires to deliver electricity to highway trucks and trains — good for ~5.5% reduction.

4. use heat pumps for space heating — 4.5%

5. use arc furnaces, aluminum, and titanium to reduce iron refining — 5.5%

6. reformulate cement plus CO2 capture — 5.5%

7. nuclear powered container shipping — 1%

8. hydrogen production from nuclear heat and electricity — 6%.

Adding it up, doing ALL of these things achieves only a 78% reduction in fossil carbon flux.

It is worth noting that once #1 and #6 have been done, there are no remaining concentrated sources of CO2 available, so carbon capture and sequestration, even if widely applied, don’t really solve the problem. Anyway, suppose we did everything on this list and were left with 22% or 38 teramoles of excess fossil carbon to offset. What solutions are left for things like aviation fuel, lubricating oils, light shipping, running our tractors, and long range driving? We’ll need large scale fuel synthesis from non-fossil sources.

9. converting all municipal solid waste to fuel — 4% — but much of that is being sequestered today, so turning it into fuel and burning it is probably a net loss.

10. biomass capture E.g. [CoolPlanetBiofuels] [GoogleX video]

11. direct air capture E.g. [CO2 Capture by Stolaroff, Keith, and Lowry 2008]

How much atmospheric CO2 collection by biomass capture can we reasonably do?

Reports on biomass yields cover a very wide range with the difference between good-year yields being at least twice as high as average yields over 5 years and peak yields under ideal conditions being on the order of 10 times typical yields.

Data from this (relatively optimistic) presentation suggests yields of

This is a wide range. At the high end, we have Giant Miscanthus yielding over 4 kg/m2/yr at excellent sites while switchgrass averaged over several years and several sites is 1/4 as much.

I read this presentation as less biased. It puts the high end of switchgrass yields at 3.3 kg/m2/yr and the average over sites and years at 1.06 kg/m2/yr.

People talk about collecting agricultural and forest products waste, but much of that is currently tilled into the soil. Taking it away seems like it might have treacherous long term effects on soil quality. In any case, the available quantities of forest waste, corn cobs, and rice straw are really tiny compared to a fossil carbon flux of 170 teramoles per year. Suppose we grew dedicated energy crops (really carbon capture crops) to feed a new generation of fuel refineries powered by nuclear heat and electricity. How much area is needed?

I suspect that the real world yields including things like roads, processing areas, incomplete harvesting, etc., could make something like 0.7 kg/m2/yr close to reality. I’m guessing dry biomass (like cellulose) is about 45% carbon. Using these numbers, an area the size of the state of Kansas, using no fossil carbon to sow, irrigate, and reap would yield about 3 x 10^12 moles of carbon/year. We’d need to plant about 5 Kansas’s to offset just 10% of our fossil carbon flux. Even with a heroic effort, biomass barely moves the needle.

Nevertheless, I’m bullish on biofuels. With the price of oil above $100/bbl it’s already cheaper to make gasoline from coal and natural gas than from petroleum. With reasonable technology development, I can see biofuel derived gasoline coming in at under $5/gallon. Perhaps gasoline prices will stabilize within a decade or so and become carbon neutral to boot. But we’ll still need nuclear-fission-generated electricity for the other 70+%.


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