Monday, April 30, 2007

Coal Liquefaction.

Among the alternative means for replacing conventional oil as it begins to run-out are liquid fuels provided by the liquefaction of coal. Such coal-to-liquids processes fall essentially into two types: direct and indirect liquefaction. Both methods were exploited by Germany during WWII, with the former predominating. All direct liquefaction methods can be thought to be based around the Bergius process. Liquid transportation fuels are characterised as having a hydrogen content of between 12 and 15%, while coal typically contains around 5% hydrogen and rather more carbon. Friedrich Bergius received the Nobel Prize in 1931 for his work on high-pressure chemistry, shared jointly with Carl Bosch who worked in a similar field and whose work is most famously demonstrated by the "Haber-Bosch" process for making ammonia by combining nitrogen with hydrogen under a pressure of the order of 300 atmospheres in the presence of an iron catalyst at 500 degrees C.

Bergius also employed high pressure hydrogen in order to "add" it to coal partially dissolved in initially naphthalene at high temperatures, under pressure as a solvent, and then in the "heavy oil" fraction generated from the coal-liquefaction process itself. Bergius developed his process in 1913, at which time constructing the necessary apparatus to withstand high pressure hydrogen posed a considerable feat of engineering. The purpose of the high pressure was both to "contain" the hot solvent which would otherwise have volatilised (c.f. a pressure cooker) and to increase the concentration of hydrogen substantially, thus to accelerate the reaction to a usable rate. The remaining problem was the production of hydrogen in a state of near purity, which was solved serendipitously. Bergius discovered that at temperatures close to 400 degrees C, water would act on iron (initially from the pressure vessel itself) almost like an acid, thus liberating 99% pure hydrogen.

He adapted the chemistry employing finely divided iron (iron-filings), which reduced water to hydrogen, being itself converted to iron oxide, Fe3O4. Since it proved possible to reduce the oxide back to metallic iron using either hydrogen (which defeats the object some) or more usefully with carbon monoxide (CO), the iron could be recycled into the process. All of these things are described wonderfully and illuminatingly in his Nobel lecture, which I have referred to below.

The Germans employed, on the smaller scale, an indirect method based on the Fischer-Tropsch process. This technology goes back to 1923, and was developed by Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute fur Kohlenforschung (coal research), which Fischer later became director of. [The Kaiser Wilhelm Institutes later became the Max Planck Institutes, and so it is now: the Max Planck Institute fur Kohlenforschung]. Essentially the coal is reacted with high-pressure steam (similar to "steam-reforming" of methane) to form a mixture of CO + H2, which when reacted over a catalyst of iron, cobalt or nickel (other metals will do too) is converted to a mixture of hydrocarbons. Direct liquefaction processes typically attain an energy efficiency of 65-70%, while indirect methods run close to 55%. During WWII Germany manufactured more than 4 million tonnes of liquid fuel annually through a combination of Bergius and Fischer-Tropsch technologies, which kept their war-effort running for five years, despite initial skepticism by the Allies that the war would be a flash-in-the-pan since the Germans had no indigenous supplies of fuel and would soon run out of it. The targeted bombing of the German coal-liquefaction facilities in 1945 contributed significantly to the end of the war.

South Africa is currently the only country that operates coal liquefaction plants (Sasol process), and produces close to 60% of its transportation fuel from coal based on the indirect Fischer-Tropsch approach. Trade embargoes imposed on them during three decades drove the very large-scale application of this technology. Large amounts of synthetic "oil" could undoubtedly be created from coal liquefaction (coal to liquids) processes, especially in the United States which owns around 30% of all known coal reserves and I expect to see a substantial installation of this technology within a country that by now relies on the rest of the world to provide it with nearly 3/4 of its entire oil budget of 22 million barrels a day. This will undoubtedly prove unpopular with environmentalists because converting coal to transportation fuels releases 7 - 10 times as much CO2 as processing crude oil does. This increase in CO2 emissions at the processing stage yields the overall result that CO2 emissions from transport will be raised by 50% over that currently supplied by oil. A new infrastructure of open-cast coal-mining is unlikely to please them either, and the technology is highly demanding in terms of the amount of water it uses, as is a problem in China who seek to expand the technology seemingly as much as possible, along with all other forms of energy supply. I imagine that water might be a problem in the US too, especially in the mid-West since I am told that supplying much of agricultural water rests on pumping it up from deep aquifers. Hence wide-scale coal-liquefaction would consume yet more of this precious resource.

However, coal-liquefaction is at the moment of writing the only proven technology that can make "oil" on the large scale required to match current petroleum use and while I remain optimistic about making biodiesel from algae, this technology has yet to be proven and developed on the massive scale necessary, if it is to make the difference between a world underpinned by oil or not. Probably direct coal-to-liquids methods will prove most useful, since under favourable circumstances, a 70% recovery of liquid hydrocarbons based on the weight of coal has been demonstrated.

Related Reading.
(2) "Coal Liquefaction", DTI Pub URN 99/1120.

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