Physical Insights

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Fischer-Tropsch fuels and carbon dioxide mitigation.

with 7 comments

“The good news is that there is no need to build new nuclear power plants to provide for the projected energy needs of the future. Indeed, it would be possible, using other forms of electricity generation, to close down most of the existing nuclear reactors within a decade. Many kinds of alternative solutions are currently on the drawing board because of the extreme urgency of countering global warming. For instance, the conversion of coal to a synthetic fuel, which can be used for transportation and which would contribute much less to global warming than petroleum, is actively being championed by Governor Brian Schweitzer of Montana.”

That’s a quote from the perhaps infamous Nuclear Power is Not the Answer. However, this post isn’t really a criticism directed at Caldicott, specifically. The bold is mine.

The production of synthetic of petroleum-like liquid hydrocarbon fuels through Fischer-Tropsch synthesis using coal as a feedstock is not environmentally sound at all, it is not an efficient use of energy resources and it is not at all a useful technology in the slightest degree to contribute towards the mitigation of anthropogenic carbon dioxide emissions from energy systems.

The first step in Fischer-Tropsch synthesis of liquid fuel from coal is the reaction of coal, which is mostly carbon, with steam under elevated temperatures and pressures, to yield a mixture of gaseous carbon monoxide and hydrogen, known as synthesis gas. This requires mining the coal, adding water, and supplying a significant input of thermal energy, intrinsically reducing the efficiency with which the energy content of the coal can be utilised – where does the thermal energy come from?
From burning more coal?

C(s) + H2O(g) –> CO(g) + H2(g)

We may wish to consider the small amount of hydrogen, about 4% by mass in typical bituminous coal, giving the coal an empirical chemical formula of something like C2H. However, the presence of this small amount of hydrogen in the coal makes essentially negligible difference, other than to marginally increase the H2:CO ratio in the synthesis gas mixture.

2 C2H(s) + 4 H2O –> 4 CO(g) + 5 H2(g)

It’s essentially the same as the previous reaction, above.

For the sake of simplicity, we might ignore, for now, the presence of sulfur, hydrogen, oxygen, nitrogen, metals and heavier elements in the coal, and focus on the carbon content. One notable advantage of Fischer-Tropsch fuels, however, is that the sulfur content of the fuel can be removed altogether, resulting in a fuel, such as diesel fuel, with negligible sulfur content, and hence with negligible emissions of sulfur dioxide into the atmosphere when the fuel is burned.

At the heart of the Fischer-Tropsch process is the use of an appropriately engineered catalyst and reaction conditions to convert the synthesis gas mixture back into a mixture of liquid hydrocarbons with an average molecular weight and composition which is usable as a fuel for vehicles. Suppose, for example, that we’re interested in the production of petrol for passenger cars – however, you could apply the same analysis equally to diesel fuel, for example, or any other particular kind of liquid petroleum fuel that you’re interested in.

Typical liquid hydrocarbon fuels, such as petrol or diesel fuel, contain about 13-15% hydrogen by mass – significantly greater than any possible abundance of hydrogen in the coal. As such, the addition of additional hydrogen into the reaction is necessary. Suppose that we’re interested in the production of petrol for passenger cars. For the sake of simplicity we can say that octane, C8H18, is representative of the overall chemical composition of the petrol.

When the coal is reacted with water to form synthesis gas, the synthesis gas is then reacted with more steam in order to increase the H2:CO ratio in the gas mixture, using water as the source of hydrogen, and producing carbon dioxide. This gas mixture can then be used to form the desired heavier hydrocarbons, using a Fischer-Tropsch catalyst.

25 C(s) + 25 H2O(g) –> 25 CO(g) + 25 H2(g)

9 CO(g) + 9 H2O(g) –> 9 CO2(g) + 9 H2(g)

16 CO(g) + 34 H2(g) –> 2 C8H18(g) + 16 H2O(g)

Hence, we have an overall chemical reaction which is equivalent to this:

25 C(s) + 18 H2O(g) –> 2 C8H18(g) + 9 CO2(g)

Traditionally, we extract crude oil from the ground, fractionate and refine the oil into products like petrol, and run our cars on the petrol. If we combust 2 mol of octane in an engine, we’ve emitted 16 mol of fossil-fuel-derived carbon dioxide into the atmosphere. However, if that 2 mol of octane is produced from coal via a Fischer-Tropsch process like we’ve elucidated above, then 25 mol of fossil-fuel-derived carbon dioxide is emitted into the atmosphere, for the same amount of energy output in the car’s engine. Does this “contribute much less to global warming than petroleum”?

Absolutely not – quite the opposite, in fact.

Even if all the carbon dioxide created during the synthesis was captured at the Fischer-Tropsch plant, liquefied, and sent to geological sequestration – which assumes that geological sequestration of the enormous quantities of carbon dioxide associated with fossil fuel energy systems is practical, which is extremely doubtful indeed and is at best completely unproven – then, at best, assuming that none of the additional energy inputs into the process come from fossil fuels, then the combustion of the synthetic fuel is associated with exactly the same quantity of carbon dioxide emissions as the
combustion of fuel derived from petroleum.

Synthetic fuel production, as exemplified by the Fischer-Tropsch process, is not advocated for reasons of the mitigation of anthropogenic carbon dioxide emissions – it is advocated by people including but not limited to Brian Schweitzer as a means to contribute to a secure domestic supply of liquid petroleum for the United States – helping to end the United States’ present dependence on foreign oil.

Fischer-Tropsch chemistry provides a particularly attractive means to keep our petroleum-fuelled vehicles in operation, using abundant, ubiquitous and secure domestic supplies of coal, where the security of foreign oil supplies are threatened by strategic or geopolitical considerations – as was the case in Nazi Germany and in South Africa under Apartheid, where Fischer-Tropsch fuel production was first well developed on a large, industrial scale.

Of course, perhaps it’s also possible Schweitzer also wants to see Montana’s abundant lignite coal utilised for the production of these synthetic fuels – bringing income into the state, and perhaps helping to keep the coal extraction industry in business in a society where it is increasingly widely accepted that coal is our number-one environmental enemy. That’s no secret.

Written by Luke Weston

November 25, 2008 at 1:21 am

7 Responses

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  1. Why bother sequestering CO2 from Fischer-Tropsch liquid fuel manufacture, when instead you can use a non-fossil source of hydrogen in the first place (eg nuclear-powered electrolysis or thermochemical water cracking)?

    George Carty

    November 25, 2008 at 1:15 pm

  2. Of course the FT process could use biomass such as wood chips instead of coal as feedstock. This would recycle carbon already in the biosphere. Example Choren Sundiesel who I note use some bought-in hydrogen. The net energy available for fuel may be slightly higher if the intermediate syngas rather than liquid is used in engines, perhaps catalytically shifted to a high methane fraction. A coal/biomass combined input would be a headache for the carbon tax people.

    For all the complaints about the poor thermal efficiency of piston engines they can run for hours on a single tank, take rough treatment and still be fixed by backyard mechanics. There is also a huge sunk cost (or embodied energy) in millions of cars and trucks and fuelling infrastructure. Therefore it may be an advantage to use renewable FT fuels long after fossil fuels have been depleted or taxed to extinction.

    John Newlands

    November 25, 2008 at 9:44 pm

  3. Biomass will only be able to supply the feedstock for a small part of our liquid fuel consumption (because biomass is after all solar energy in chemical form and thus requires vast amounts of land to collect a decent amount of energy).

    If the global warming threat* was such that you couldn’t even use coal purely as a carbon source (with the process heat from a nuclear reactor, and the hydrogen derived from splitting water), then wouldn’t extracting atmospheric CO2 be the only reasonable way to go?

    *Personally I’m sceptical – if the threat really was as great as the alarmists paint it, why haven’t governments already started massive crash nuclear building programs, and to hell with the anti-nuclear protestors? They can’t ALL have been bamboozled by StormSmith can they?

    George Carty

    November 26, 2008 at 1:20 pm

  4. Link for Fischer Tropsch fuel from biomass
    http://www.choren.com/en/energy_for_all/sundiesel_sup_sup_/
    I think the stumbling blocks appear to be economies of scale, logistics of bulk handling and net energy. OTOH our friend coal tends to be concentrated in the one spot.

    I don’t know how much energy can come from biomass with well under 1% average photosynthetic conversion on fertile well watered land. I think at least 50% of transportation will have to be electric not using on-board fuel.

    John Newlands

    November 26, 2008 at 8:56 pm

  5. George,

    I’m afraid you greatly overestimate the attention span of, err, well the near entirety of the political personnel in, err, well, pretty much every countries. They don’t care about anything beyond the next poll, the next election, the next meeting of the central committee, whatever their hold on power depends upon.

    They are not bamboozled or brainwashed. They just don’t care.

    That being said, for the few politicians who really have skin in that game, you should read this :
    http://www.nytimes.com/2008/11/11/science/earth/11maldives.html?hp

    The government of the Maldives is starting a fund – not a rainy day fund but a rising tide fund – to buy dry land for its inhabitants in case the seas rise too much and wash away its low-lying islands out of existence.

    That should be a clue.

    Fifi

    November 27, 2008 at 4:06 am

  6. why haven’t governments already started massive crash nuclear building programs

    Because governments collectively are the largest fossil fuel profiteer.

    … and to hell with the anti-nuclear protestors? They can’t ALL have been bamboozled by StormSmith can they?

    The latter, definitely funded by the Dutch government. The former, well, who can tell how they are funded. But have you met one who had seen a tax he didn’t like?

  7. If fischer tropsch process causes additional co2 prdctn how is it useful

    Saikrishna

    March 2, 2013 at 6:22 am


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