Synfuels

The idea behind re-formed fuels or synfuels is to duplicate some of the amazing biochemical feats of autotrophic nourishment by constructing useful fuels out of undesirable waste CO and CO2. Nature tackled this problem a billion years ago, but people first began to work seriously on the idea of re-formed fuels in the 1920′s. Building liquid fuel molecules out of toxic waste like CO and greenhouse gases like CO2 is attractive on many levels.  Firstly, it creates fuels for which we already have all the necessary support infrastructure, and which have desirable properties like easy handling and long-term storage, and high energy density. Secondly, it gets rid of substances that cause global warming. Thirdly, it has the potential to free countries of dependence of foreign-sourced fossil fuel supplies, since CO2 is produced unevenly but shared globally.

How does this work in nature? In the posting about the biology behind biofuels, we mentioned oxygenic photosynthesis. Photosynthesis by plants and some bacteria and algae consumes atmospheric CO2 and sequesters it at least temporarily. In the process, the plants create oxygen for the atmosphere and larger carbon molecules that serve as the basis of both biofuel and eventually fossil fuel. The plants can be harvested and processed. As we have seen, there is trouble with biofuels in that most have a poor energy balance, and all of them occupy enormous amounts of arable land. It stands to reason that if one could reduce the number of steps required to convert CO2 to fuel, the overall process might be more efficient in the energy consumed, and would almost certainly be more efficient in land utilization. Nature provides some inspiration. In addition to photosynthesis, nature also serves up the strange autotrophic nourishment mechanism called chemosynthesis harnessed by chemoautotrophic bacteria. An example is the species acetobacterium woodii, which uses H2 as the electron donor, CO2 as the carbon source, and through the metabolic process of acetogenesis creates acetic acid (CH3-COOH), the active ingredient in vinegar. This molecule is closely related to ethanol.

If nature could manage the difficult feat of making fuel, then humans were bound to try as well. The process for creating liquid hydrocarbons was first developed in the early 20th century, and is called the Fischer-Tropsch process[1]. Most fuel synthesis methods have their roots in this one. The core chemical reaction is the production of alkanes (straight-chain hydrocarbons) by through the combination of hydrogen gas (H2) and carbon monoxide (CO):
(2n+1) H2 + n CO → CnH(2n+2) + n H2O
Alkanes are the basic ingredients of petroleum and its distillates such as gasoline and diesel. The reaction is facilitated by catalysts such as iron or cobalt. In the past, the feedstocks used to prepare the input ingredients H2 and CO were coal and methane. Hydrogen gas (H2) could be produced by the water shift reaction:
H2O + CO → H2 + CO2
or steam reforming with methane:
H2O + CH4 → CO + 3 H2
In turn, the carbon monoxide (CO) was often obtained by burning coal in an oxygen-starved atmosphere. Many variations on the Fischer-Tropsch process have been devised. If one such variation is to serve as a source of carbon-neutral synfuels, it must not use coal and methane as the primary inputs. Instead, the carbon should come from spent CO2 and the hydrogen from somewhere else, for instance from water.

Water is abundant, but breaking it up to liberate the hydrogen isn’t easy. Using chemical reagents (as in the so-called sulphur-iodine process) is possible, but the supply of reagent and the disposal of the resulting compounds makes the process non-renewable and therefore would be problematic when executed on a huge scale. Breaking up water into its consitutent gases directly ( 2 H2O → 2 H2 + O2 ) consumes a lot of energy. Of the available methods for direct decomposition of water, electrolysis (using electricity) is preferable, since the alternative of thermolysis (thermal decomposition) involves heating water to 2000 degrees Celsius.[2]

Partial decomposition of CO2 to produce CO is also not the easiest trick. Uhrig[3] advocates the reverse water shift reaction:
H2 + CO2 → CO + H2O
Using this process, three molecules of H2 would be needed for each “molecular building block” of CH2 produced. This is acceptable from the standpoint of reagent supply, because the hydrogen can come from electrolysis of water and is returned to water.

Where to obtain the CO2 is another matter. Its concentration is only about 0.04 per cent in air, making the extraction of CO2 from air very costly in energy. However one could use the smokestacks of fossil fuel electricity and heating plants as a source of concentrated CO2. For instance, the total production rate of CO2 in the U.S. is about 5,680 million metric tons per year (2002) of which the production of CO2 from coal-power plants in the U.S. is about 1,875 million metric tons/year.[3], This CO2′s use as an input for synfuel production would significantly cut the energy spent on gathering CO2 vs. taking it from the air. After all the fossil fuels are gone or to supplement the supply from burning fossil fuels, the process could be changed to extract CO2 dissolved in surface seawater instead of air, since ocean water has about 140 times the atmospheric concentration of CO2.[4]

Unfortunately, all this doesn’t come for free. As noted by Uhrig et al.: Because transportation uses about 70% of our petroleum consumption, shifting from a petroleum-based transportation economy to a syn­fuel-transportation economy could reduce our petroleum use by ~70% and reduce our CO2 production by ~33% with no increase in coal used in the power plants.
Such a shift to a synfuel economy, however, would require about 255 million metric tons/year of hydrogen, about 23 times our current national production that would have to be produced by water splitting using solar, wind, or nuclear energy. It has been indicated that a megawatt of electricity will produce about half a metric ton of hydrogen per day using conventional elec­trolysis. From these two numbers, the amount of electrical generating capacity needed to produce 255 million metric tons of hydrogen per year is:
[255 x 106 MT/yr] / [365 days/year x 0.5 MT/MWe-day] = 1.397 x 106 MWe = 1,397 GWe .
This represents the total output of almost 1,400 one-GWe electric power plants, some 40% more than the current generating capacity of the U.S. today.
[3]
Note this electrical energy budget covers only the most electricity-intensive step, the hydrolysis of water. The synfuel production process as a whole consumes even more energy per unit output.

Uhrig presents examines a hypothetical synfuel plant using nuclear power as the source of electricity. Production of 2.25 million litres of fuel per day using electrolysis for the hydrogen production step, and with continuous steady production around the clock, would require 5650 MW of electric capacity. This might be cut to 2690 MW if the nuclear reactor were of the modular helium type, and if surplus heat from this reactor were used to thermochemically decompose the water.[3] Using the first figure, we arrive at the conclusion that the electric energy input into CO2- and H2O- based synfuel production is about 103 MJ per litre. That sounds about right considering that the energy content of the resulting gasoline or diesel is 34 and 37 MJ/litre respectively.

Using the United States of America as an example, the replacement of fossil fuels by biofuels is impossible today because, as noted in an earlier post, just replacing the petroleum used in transportation would consume all available arable land and then some, except in the event that the speculative algal fuel technology really pans out. In order to also replace the petroleum used in other ways, and the natural gas, and the coal, would require five times more land. It is highly unlikely that the amount of arable land in the lower 48 states will ever increase dramatically. In fact, if anything the amount is more likely to decline as a result of climate change. At the same time, the replacement of fossil fuels by synfuels is equally impossible today, because synfuel production would consume too much electricity, most of which is produced using thermal plants powered by fossil fuel.

Fortunately, the fully post-fossil-fuel future is still far away. As noted in the post on Peak Oil, petroleum and natural gas supplies will become scarce starting in our day and will run out completely in a couple of generations. Coal should last longer, perhaps another century. There is also the “wild card” of fossil fuels to consider, methane hydrates (or methane clathrates). This is the only significant fossil fuel resource of which Dr. M.K. Hubbert was apparently not aware in 1956. Now we know there are inventories of this strange fossil fuel, but it’s probable that most of it is unrecoverable. It is equally possible that methane hydrate will prove to be a serious greenhouse gas problem rather than a fuel bonanza. More on this material in another post. In any event, even if we figure out how to mine methane hydrates, they too will contribute to global warming and will run out in a few generations if not sooner. Therefore it is never too early to start thinking about the post-fossil-fuel future. We need to be prepared before it arrives. The stark choices would seem to be that either:

(1) the algal fuel technology will have to succeed spectacularly and with an impressive energy balance so that it can replace not only gasoline and diesel as our transportation fuel (20% of the world’s energy budget), but replace fossil fuels in all other roles in which fossil fuels are now used (over 60% of the world’s energy budget).
(2) there will have to be an equally spectacular buildout of electric generating capacity for synfuel production from CO2 and H2O, and none of that electricity can be made using combustion, since using the one fuel to make another is necessarily a massively energy-wasting proposition. Instead, the electric power will have to come from other primary sources of generation, and liquid fuel will be relegated to being a storage medium rather than a primary energy source. The future primary sources of electric generation would need to be identified and built, a process that will take at least one generation.

Some private-sector researchers are already thinking about CO2-based synfuels for the post-fossil-fuel world, but surprisingly few companies are that farsighted. One that deserves full credit for taking this leap in vision is Doty Energy.[5]

[1] http://en.wikipedia.org/wiki/Fischer–Tropsch_process
[2] http://en.wikipedia.org/wiki/Thermolysis
[3] http://www.tbp.org/pages/publications/Bent/Features/Su07Uhrig.pdf
[4] http://www.newscientist.com/article/dn17632-how-to-turn-seawater-into-jet-fuel.html
[5] http://www.dotyenergy.com/

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