A Path to Carbon-Free Ammonia?
With more than 7 billion people on the planet, one of the most critical molecules manufactured industrially is also one of the simplest, NH3, ammonia. If the industrial synthesis of ammonia were to stop tomorrow, billions of people would undoubtedly be consigned to starvation: The original meaning of the term “green revolution” referred not to fantastic rhetoric about wind and solar power, among other things, but rather, beginning in the 1950’s, to the development, and industrial application of intensive fertilization schemes being introduced to the world’s agricultural operations. It was this development and application, this “green revolution,” which vastly improved agricultural yield per hectare around the world, which prevented – or perhaps at least delayed – the Malthusian nightmare.
The first reference in the paper from the primary scientific literature that I will briefly discuss here, a paper by Stuart Licht and his coworkers at George Washington University which can be found in the most recent issue of Science, is to Vaclav Smil’s Enriching the Earth, a book which I personally regard as required reading for anyone who truly wants to understand the complexity of issues connected with energy and the environment. As noted by the Licht team, referring to Smil, ammonia synthesis, although ammonia itself contains no carbon, consumes between between 3 to 5% of the world supply of dangerous natural gas, and about 2% of the world energy demand overall. Natural gas is consumed by the process in the intermediate step in which hydrogen is made; less than 1% of the world’s hydrogen is currently made by electrolysis, and and in the rare cases where electrolysis does produce hydrogen, it is always a side product of another electrochemically produced commodity, chlorine for example. (In China, one of the world’s largest producers of industrial ammonia, ammonia synthesis predictably often employs the worst of the three dangerous fossil fuels, coal, to make hydrogen.) If this amount of energy seems trivial to you, I note that despite half a century of unrestrained cheering for it, the solar PV industry has never, not once, produced as much energy in a single year as is annually required to provide the energy for just the ammonia synthesis on which the majority of the world’s food supply depends. Thus, right now the world supply of food depends, for ammonia as well as for other agricultural activities, on access to the very thing that is arguably simultaneously killing us, dangerous fossil fuels. (This said, the Licht paper discussed does make the now required obeisance to the notion of producing “solar” ammonia via solar thermal schemes, as unrealistic as this may be in practice.)
As a nuclear energy advocate, I am compelled to note that while the thermochemical production of hydrogen using nuclear heat – hydrogen is the energy intensive intermediate in ammonia synthesis - is certainly possible, and could be, and in my view should be pursued, the number of nuclear based systems dedicated exclusively to producing hydrogen, even electrochemically as opposed to thermally, is essentially zero.
The paper to which I'd like to draw attention presents an relatively efficient electrochemical approach to making ammonia using water and atmospheric nitrogen as starting material, and, producing as well as, as side products, hydrogen gas and oxygen gas. But before I go there, allow me to make another point about ammonia synthesis: Although billions of lives depend on the process of industrial ammonia synthesis, and it thus it is certain that we cannot live without it – at least as we do now - ammonia synthesis also has and has had some huge negative environmental, economic and social, and even political implications for humanity.
One such environmental implication involves eutrophication of water supplies, which recently was in the news when it was discovered that a bloom of toxic microcystin producing bacteria in Lake Erie shut down the Toledo, Ohio water supply. The news media tended to focus on the phosphorous component of the run off that led to this bloom, but the fixed nitrogen components in fertilizers made from ammonia were equally responsible: Neither would have the same effect on the water supply without the other. The far more serious risk associated with ammonia synthesis, in my view, is the accumulation of nitrous oxide – to which ammonia and nitrates made from ammonia are biologically degraded – in the planetary atmosphere. Nitrous oxide is the third largest climate forcing gas currently in the atmosphere, following carbon dioxide and methane but the greatest risk by far associated with the accumulation of this gas is that it is rapidly displacing CFC’s as the prime agent for the destruction of ozone in the upper atmosphere. As the 21st century proceeds, it is entirely possible that this effect will be even worse than the effects that resulted in the phase out of CFC’s under the Montreal Protocol. As is clear the solution to this problem will not be and cannot be the phase out of ammonia synthesis. (I can think of other means to address the problem, but this is irrelevant here.)
The social, economic and political ramifications of ammonia synthesis besides the possible destruction of water supplies and the ozone layer involve the chemical industry. The worldwide chemical industry is dependent on fixed nitrogen. Indeed, the first application of industrial ammonia production was not for utilization in agriculture – even if that was the original goal in seeking to undertake the research - but as a chemical intermediate for its use in making chemicals for war: Had the means to make ammonia from air and water not been discovered in 1910 by Fritz Haber, the German war machine - which up until that time depended on salt peter, KNO3 imported from Chile – would have collapsed under the weight of the British Naval blockade of 1914 and 1915, with the result that the First World War, whose centennial we are now marking, would be remembered as a minor conflict and not the initiator of so much political and martial tragedy that resulted in the actual event. To this day, fixed nitrogen is an important player in the manufacture of many explosives, but it is also widely used to manufacture medications, metal processing reagents, cleaning products, electronics, etc. Indeed, many proposals in the quixotic effort to make huge carbon dioxide dumps to pretend that we can, in fact, deal with dangerous fossil fuel waste – I believe we can’t - depend on amines for which ammonia is a synthetic intermediate. Of course, by products of these industries have rather large health implications, for instance the powerful carcinogenic N,N dialkyl nitrosamines formed in the chlorination of water, cooking foods, and, indeed by carbon dioxide scrubbing schemes are consequences of the use of ammonia derived chemicals.
With these things out of the way, let me now return to a discussion of the Licht paper. It notes the two current chemical reactions that are now used most widely in ammonia synthesis:
CH4 + 2H2O→4H2 + CO2
N2 + 3H2→2NH3
The first reaction is simply the methane analogue of the “water gas” reaction, by which a dangerous fossil fuel, in this case dangerous natural gas, is converted into hydrogen by heating it with water. The second reaction which is actually slightly exothermic, but nonetheless requires a catalyst because of its high activation energy, is the actual synthesis of ammonia. The reactions are not summed in the paper, but I will sum them here, adjusting for stoichiometry, as follows:
3CH4 + 6H2O +4 N2 → 3CO2 +8NH3
Many very bright people here at Energy Collective and elsewhere have proposed the use of ammonia as a fuel for automobiles and other portable devices. I generally disagree with these proposals on an environmental and safety grounds – my preferred hydrogen carrying fuel is dimethyl ether, DME, which I regard as a “wonder fuel” – but my objections notwithstanding, the idea of ammonia as a fuel is hardly likely to go away, at least until its drawbacks become apparent by use. The authors of the paper report that they were intrigued by a paper along these lines describing the operations of an ammonia burning fuel cell. Although the paper did not specify the products of the ammonia oxidation, the authors mused about the possibility that this fuel cell might prove reversible in the case where a product was nitrogen gas, and, should this prove true, they further speculated that this might offer an electrochemical path to ammonia.
In order to overcome the extreme stability of the nitrogen-nitrogen bond in N2 gas, nitrogen fixation always requires the complexation of the gas with a metal – in biological systems the metal in question is molybdenum (as well as iron), making this molybdenum, along with iodine, the only elements in the 5th period of the Periodic Table that are essential to life. In Haber’s original development of the industrial process, the metal used was iron; he speculated that uranium might prove superior, a point I explored elsewhere.
A metal catalyst is required in the electrochemical process described in the paper as well.
Here, the catalyst is also iron, but in this case it is necessary that the iron, present as an oxide, be in the form of nanoparticles suspended in molten alkali hydroxides, in this case, a eutectic melting mixture of KOH and NaOH, potassium and sodium hydroxides respectively; the authors explored also the use of other molten oxides, notably, cesium hydroxide, which may prove superior. These hydroxides are only molten at higher temperatures, and steam and air or pure nitrogen gas are bubbled through the molten hydroxides (after stripping, in the case of air, carbon dioxide, meaning that the removal of this gas from air would be a side benefit of the process.)
Variables in the process with respect to temperature, operating voltage, current and the physical nature electrodes, are explored in some detail in the paper. The reader may refer to the original paper to learn about these. The precise stoichiometry of the reaction varies with the conditions, but one form of reaction mentioned by the authors, this under at particularly favorable set of conditions, is this:
N2 + 10H2O → 2NH3 + 5O2 + 7H2
Both pure oxygen and hydrogen are important commodities in their own right, and thus the reaction offers many potential synergies. The gases on the right side are not produced as an explosive mixture of course, because ammonia and hydrogen are formed on one side of the cell, at the cathode, whereas the oxygen is formed at the anode. The mixture of cathodic gases, ammonia and hydrogen, are easily separated by compression: It is easy to liquefy ammonia – it has been used as a refrigerant – and very difficult to liquefy hydrogen and thus the separation is relatively trivial.
The overall electrochemical efficiency is quite high compared to other attempts at the electrochemical reduction of nitrogen to ammonia gas, reportedly around 30%, an efficiency that may well be competitive with Haber-Bosch process ammonia synthesis, but probably this does not include the heat penalty associated with melting the alkali metal hydroxides and keeping them molten.
One needs, of course, to be somewhat skeptical of lab scale things advertised as “breakthroughs,” whether the “breakthrough” in question is an energy “breakthrough” or a “breakthrough” cure for cancer or some other discovery in some other area. For many discoveries the road from the lab to a scaled up industrial process is fraught with hazards and problems, many of which prove fatal. If, for instance, all the solar “breakthroughs” announced over the last century had actually panned out as advertised, solar energy might actually be a significant source of electricity on this planet, rather than a trivial, if expensive, affectation with eternal promise and no actual demonstration of meaningful amounts of delivered energy. In the current case, some difficulties remain with the process, in particular, with the lifetime of the nano iron oxide which under the conditions described, can congeal in ways to reduce its functionality by robbing it of its nanoscale properties. This said, this work is potentially very, very, very important and its potential should not be dismissed.
Whatever the problems may prove to be, I note that the original Haber process was very inefficient compared to the modern day processes. Smil’s book covers the historical development of the Haber-Bosch process, in considerable detail, as well as the many engineering failures and setbacks that were attached to industrializing the process – the industrialization having been the work of the BASF chemical engineer Karl Bosch. Using some well-placed graphs, Smil describes the remarkable progress made in improving the original process over the century it has been practiced, especially in its overall thermodynamic efficiency. An interesting sidelight to this increased efficiency, given that so many people naively try to present increased efficiency as a cure all for our exceedingly difficult energy problems, is that as a result of this improved efficiency in ammonia synthesis, the use of ammonia is accelerating, not decreasing, and that the overall amount of energy consumed for nitrogen fixation is thus rising, not falling. This represents a case where Jevon’s paradox applies, that is a case where increased efficiency results in increased demand as opposed to reduced demand.
I close with a bit of irony. Fritz Haber was, quite justifiably, awarded the Nobel Prize in Chemistry for his development of the nitrogen fixation process to make ammonia. His life is fascinating in itself, not the least because this discovery by this German Jew made German warfare possible in the 20th century, included the warfare carried out by the Nazis, the same Nazis who would ultimately drove him out of Germany just before his death in 1934. (Haber also was instrumental in the development of German gas warfare during the First World War, a practice some say led to his wife’s suicide as an act of protest: Haber’s first wife was also an outstanding chemist, but, unlike her husband, she was a pacifist as well. In Haber’s defense, he naively hoped apparently that the terror of poison gas would achieve what arguably the invention of nuclear weapons achieved in 1948 and 1963: Introduce an element of horror at superpower clashes at the very highest levels of their governments.)
In any case, one of Haber’s professional enemies was Walther Nernst, who also won the Nobel Prize, this for his discovery of the 3rd law of Thermodynamics, a law that Haber claimed to have discovered before Nernst discovered it, leaving Haber to feel he deserved two Nobel Prizes, not one. (Nernst, who was a famous chemist before anyone knew who Haber was, had been brutally dismissive of Haber early in Haber’s career.) Suffice it to say, irrespective of their personal, even childish, disputes, that both men’s chemical discoveries made the modern world possible, since their discoveries have had broad application in a large number of industries. Their personal disagreements notwithstanding, the governing equation for electrochemistry, on which this new electrochemical process would depend is, in fact, the Nernst equation, so that, if this should all pan out, Nernst would have the last laugh.
Of course, the extent to which this process may be “carbon free” depends – as does the environmentally more dubious enterprise of electric cars – on the extent to which electricity is generated by truly scalable carbon free means. Dirty electricity means dirty electrochemistry. Nevertheless, the production of clean electricity, nuclear electricity, has been demonstrated for more than half a century, and so, there is, at least, in this and many other places, reasons that hope may yet not prove absurd.
 V. Smil, Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production (MIT Press, Cambridge, MA, 2004).
 Rochelle et al, Environ. Sci. Technol., 2014, 48 (15), pp 8777–8783
Nnadir is a long term blogger whose writings involve making ascerbic comments on various kinds of papers in the primary scientific literature. His primary scientific interests involve nuclear energy, organic and inorganic environmental chemistry, the environmental chemistry of carbon dioxide, the environmental chemistry other acidic species, persistant organic pollutants, climate change, ...
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