GWU team demonstrates relatively efficient electrochemical process for low-GHG production of ammonia
08 August 2014
A team at George Washington University led by Stuart Licht has developed a relatively efficient electrochemical process for the production of ammonia from water and nitrogen, without the need for an independent hydrogenation step (and thus the associated carbon-intensive steam reforming of methane as the hydrogen source). The process, reported in the journal Science, electrolyzes air and steam in a molten hydroxide salt with a nanostructured iron oxide–derived catalyst (nano-Fe2O3).
At 200 °C in an electrolyte with a molar ratio of 0.5 NaOH/0.5 KOH, ammonia is produced at 1.2 V under 2 milliamperes per centimeter squared (mA cm-2) of applied current at coulombic efficiency of 35% (35% of the applied current results in the six-electron conversion of N2 and water to ammonia, and excess H2 is cogenerated with the ammonia). At 250 °C and 25 bar of steam pressure, the electrolysis voltage necessary for 2 mA cm−2 current density decreased to 1.0 V.
Although the suspension is only stable for a few hours, the protocol points to a way to produce ammonia from purely renewable resources.
The well-established Haber-Bosch synthesizes ammonia for fertilizer and other uses via the hydrogenation of nitrogen from the atmosphere; the hydrogen required is produced primarily through a separate process of steam reformation, which consumes 3 to 5% of the world’s natural gas production and releases large quantities of CO2 to the atmosphere.
The ammonia hydrogenation reaction is separate from the steam-reforming reaction (Eq. 2) that generates the hydrogen. Renewable energy–driven water splitting could provide an alternative H2 source, but economic, non–CO2-emitting sources of H2 have yet to be proven on the industrial scale. Although ammonia hydrogenation is exothermic, it is kinetically disfavored at ambient temperature and pressure. In the Haber-Bosch process, this kinetic limitation is overcome via an iron-based catalyst, repeated cycling, high pressure, and elevated temperature. The last-named conditions are energy-intensive and consume 2% of the world’s energy production.
… By effectively reversing the NH3 fuel cell, we present an electrochemical pathway to produce ammonia from air and steam at 200 °C with simple materials (molten hydroxide, Ni electrodes, and nano-Fe2O3), in one pot without a separator.
—Licht et al.
In this study, the team also introduced a solar thermal water self-pressurizing, low electrolysis energy path system.
There is ample room for advances of this pathway. Fe2O3 was utilized as the reactive surface, whereas today’s Haber-Bosch catalysts use Fe2O3 or ruthenium-based catalysts with a wide variety of carefully optimized additives which may also improve this electrochemical process.
—Licht et al.
Resources
Stuart Licht, Baochen Cui, Baohui Wang, Fang-Fang Li, Jason Lau, and Shuzhi Liu (2014) “Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3” Science 345 (6197), 637-640 doi: 10.1126/science.1254234
Am I right in reading this as an energy efficiency of 35%?
Is there an engineer in the house!
Posted by: Davemart | 08 August 2014 at 02:39 AM
Aha! More here including efficiency!
'This method claims to use only two-thirds of the energy of the Haber-Bosch process. Along with the elimination of the need to produce hydrogen from natural gas, the overall emissions are reduced quite significantly. The whole process also takes place at milder conditions, not requiring 450°C and 200 times atmospheric pressure as the Haber-Bosch process does.
These are not all that make Licht's method attractive. Some of the energy is sourced through another technology Licht has developed called solar thermal electrochemical production, or STEP. It is considered to be one of the most efficient solar cells currently in use. STEP when applied to making ammonia leads to production of hydrogen as a byproduct.
This byproduct would be suitable for hydrogen fuel cells, another popular avenue for clean-energy enthusiasts, according to David Fermin, professor of electrochemistry at the University of Bristol. "Hydrogen generated in this manner is significantly cleaner," he said.'
Read more at: http://phys.org/news/2014-08-air-ammoniaone-world-important-chemicals.html#jCp
So,how many fuel cell cars could be run on this by-product hydrogen?
Answers on a postcard please.
My initial estimate is:
Lots!
Posted by: Davemart | 08 August 2014 at 03:34 AM
I haven't got the chops to work out how many cars could be run on the hydrogen generated by this method as a by-product, but I can have a stab at how many fuel cell cars could be run on the natural gas saved if reformed to hydrogen.
Based on this:
'Natural gas is the primary raw material used to produce anhydrous ammonia, accounting for 70% to 90% of its production costs. Approximately 33 million British thermal units (mm Btu) of natural gas are needed to produce 1 ton of ammonia chemically. Global demand for agricultural nitrogen fertilizer is expected to be 109 million metric tons in 2013. North America consumes around 13 percent of the total.'
http://www.onlineconversion.com/energy.htm
So NA produces around 14mtons of ammonia per year.
At ~10,000kwh per 33mbtus that then a ton of ammonia uses enough natural gas to produce at 50kwh/kg of hydrogen 200kg.
At 60gge that comes to 12,000 miles, which conveniently is around the same as mileage per year for a car.
So perhaps 14 million cars could be run on the natural gas saved in ammonia production by this method.
To that must be added the cars you could run on the by-product hydrogen.
You might as a WAG be talking about a total for both together of 20 million cars, subject to someone being able to do the by-product hydrogen calculation properly.
So maybe ~7% of the US car fleet could run on the natural gas savings and by product hydrogen of this process.
Posted by: Davemart | 08 August 2014 at 04:32 AM
The excess H2 would be suitable for input to the Haber process, though what you'd do with it if that was too expensive or hardware-intensive is a good question.
Sadly, the low efficiency probably makes this uneconomic. A better catalyst could change that.
Posted by: Engineer-Poet | 08 August 2014 at 01:59 PM
Hi EP.
From what I am reading the relatively low efficiency still is better than the very high temperatures needed previously:
As I quoted above:
'This method claims to use only two-thirds of the energy of the Haber-Bosch process'
the excess hydrogen could simply be compressed for use in fuel cell vehicles - that is what I could not work out, how much would be produced per ton of ammonia.
Posted by: Davemart | 08 August 2014 at 02:30 PM
Why not conventional renewable H2 (also usefull for other purposes) and then conventional Haber process?
Posted by: Alain | 09 August 2014 at 06:38 AM
If the coulombic efficiency of ammonia production is 35%, the other 65% goes to either H2 production or losses. If we assume 100% total coulombic efficiency, 1 mole of electrons would yield .117 moles of NH3 (1.99 grams) and .325 moles of H2 (0.65 grams).
Storing H2 is the biggest headache of using it. Immediate conversion to a more storable product is preferable. Perhaps gasification of biomass in a hydrogen atmosphere, producing a de-oxygenated product stream, would be a worthwhile option.
Posted by: Engineer-Poet | 09 August 2014 at 07:32 AM
Thanks EP.
That is around 300kg per kiloton of ammonia then, or enough for around 20 million fuel cell cars.
Storing hydrogen in large quantity is surprisingly not at all difficult, as it can be fed through the existing natural gas pipelines at 5-10% and stored in salt caverns in truly huge quantities.
Germany already has the capacity to store 20% of its annual energy consumption in salt caverns, currently as NG but hydrogen is analogous and the problems are well understood:
http://www.kbbnet.de/wp-content/uploads/2011/09/Present-trends-in-compressed-air-energy-and-hydrogen-storage-in-Germany1.pdf
Storing hydrogen under high pressure for use in vehicles is more challenging, but mass storage at low pressure is fine.
Posted by: Davemart | 09 August 2014 at 09:59 AM
10% hydrogen in methane by volume is a bit over 3% by energy. H2 has less than 1/3 the energy per molecule (thus by unit volume) as methane, and that is the HHV; LHV is worse.
Germany would need to triple the volume to store the same amount of energy in H2.
No idea where you got 20 million cars from.
There seems to be far more potential in processing of biomass. Based on the figures, converting 1 billion tons of carbohydrate (cellulose, (CH2O)n) to alkanes by hydrogenation to remove the oxygen would require 33 million tons of H2, yielding 467 million tons (CH2)n with the balance being water. At a density of 0.72, that's 649 million cubic meters or about 172 billion gallons. Total gasoline consumption in the USA is only about 130 billion GPY. The physical quantities work, though I would bet that the economics are laughably far from working.
Posted by: Engineer-Poet | 09 August 2014 at 04:20 PM
Excuse me, 67 million tons H2.
Posted by: Engineer-Poet | 09 August 2014 at 04:21 PM
@EP:
I took your figures of 1.99gm NH3 and 0.65gm H2.
So per ton of ammonia produced that gives around 300kg of hydrogen.
At 60mpgee that gives around 18,000 miles worth.
Taking the average mileage pa at 12,000, that is around 1.5 cars worth.
1.5 times the 14 million tons of ammonia produced pa in the US gives around 20 million.
Have I dropped a clanger anywhere in this?
I don't know if you have had time to look in any detail at the link I gave, presumably not, as they go into considerable detail on storing hydrogen in salt caverns, and are well aware that it is less energy dense than NG, and still reckon that there is plenty of capacity.
Posted by: Davemart | 10 August 2014 at 12:31 AM
Ah, the link from 14 million tons pa is what I was missing. It all follows from that. Of course, if ammonia is produced as a fuel as well as a fertilizer, that number goes way up. The real issue is cost.
Scanning that pdf for stuff about hydrogen, this leapt out at me:
So with wind receiving a feed-in tariff of €0.088/kWh, the fuel cost alone for the hydrogen going into that CCGT would be €0.22/kWh; France has a nearly carbon-free grid at about 2/3 that cost. On top of that the equipment must be amortized and maintained.
Paragraph 2.2 suggests that there's no real buy-in by certain key players.
The footnote on page 6 is illuminating. The USA uses about 4000 TWh of electricity per year. If 20% of that had to be stored in 500,000 m³ caverns, we would need 5700 of them. That's a fair number, and assumes that suitable formations are available where needed.
Read closely, this paper lists many problems and fails to mention solutions. That's pretty much the Energiewende in a nutshell.
Posted by: Engineer-Poet | 10 August 2014 at 08:30 AM
@EP:
Somehow the words: 'Energiewende' and 'nut' whether or not in a shell go together so well!
I don't subscribe to their notions about wind to power etc at any affordable cost,but just the same they have some very good engineers working on the parts of this inherently daft project.
Their hydrogen storage plans seem entirely reasonable, which is the bit we are interested in for the purpose of this discussion, even if the source of German hydrogen is not.
If you are going to go for solar as more than either off grid rural power in the tropics or expensive decoration outside, then it needs storing.
Solar to ammonia would do that, with hydrogen as a by product, and would be deployed somewhere that it is sunny, unlike shady German plans!
The ammonia can be stored, and so can the by product hydrogen, although salt cavern resources are not everywhere as great as they are in Germany, and so solar is out of its trap of seasonal variability.
Posted by: Davemart | 10 August 2014 at 10:07 AM