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Ammonia energy storage #2

Recently, we reported on plans by Australian entrepreneurs and their government to use ammonia (NH3) to store excess wind energy. We proposed converting ammonia and CO2 from wastewater into methane gas (CH4), because it is more stable and easier to transport. The procedure follows the chemical equation:

8 NH3 + 3 CO2 → 4 N2 + 3 CH4 + 6 H2O

Now we have published a scientific article in the online magazine Frontiers in Energy Research where we show that the process is thermodynamically possible and does indeed occur. Methanogenic microbes in anaerobic digester sludge remove the hydrogen (H2) formed by electrolysis from the reaction equilibrium. As a result, the redox potentials of the oxidative (N2/NH3) and the reductive (CO2/CH4) half-reactions come so close that the process becomes spontaneous. It requires a catalyst in the form of wastewater microbes.

Pourbaix diagram of ammonium oxidation, hydrogen formation and CO2 reduction. At pH 7 and higher, the oxidation of ammonium coupled to methanogenesis becomes thermodynamically possible.

To prove our idea, we first searched for the right microbes that could carry out ammonia oxidation. For our experiments in microbial electrolysis cells we used microorganisms from sediments of the Atlantic Ocean off Namibia as starter cultures. Marine sediments are particularly suitable because they are relatively rich in ammonia, free from oxygen (O2) and contain less organic carbon than other ammonia-rich environments. Excluding oxygen is important because it used by ammonia-oxidizing microbes in a process called nitrification:

2 NH3+ + 3 O2 → 2 NO2 + 2 H+ + 2 H2O

Nitrification would have caused an electrochemical short circuit, as the electrons are transferred from the ammonia directly to the oxygen. This would have bypassed the anode (the positive electron accepting electrode) and stored the energy of the ammonia in the water − where it is useless. This is because, anodic water oxidation consumes much more energy than the oxidation of ammonia. In addition, precious metals are often necessary for water oxidation. Without producing oxygen at the anode, we were able to show that the oxidation of ammonium (the dissolved form of ammonia) is coupled to the production of hydrogen.

Oxidation of ammonium to nitrogen gas is coupled to hydrogen production in microbial electrolysis reactors. The applied potentials are +550 mV to +150 mV

It was important that the electrochemical potential at the anode was more negative than the +820 mV required for water oxidation. For this purpose, we used a potentiostat that kept the electrochemical potential constant between +550 mV and +150 mV. At all these potentials, N2 was produced at the anode and H2 at the cathode. Since the only source of electrons in the anode compartment was ammonium, the electrons for hydrogen production could come only from the ammonium oxidation. In addition, ammonium was also the only nitrogen source for the production of N2. As a result, the processes would be coupled.

In the next step, we wanted to show that this process also has a useful application. Nitrogen compounds are often found in wastewater. These compounds consist predominantly of ammonium. Among them are also drugs and their degradation products. At the same time, 1-2% of the energy produced worldwide is consumed in the Haber-Bosch process. In the Haber-Bosch process N2 is extracted from the air to produce nitrogen fertilizer. Another 3% of our energy is then used to remove the same nitrogen from our wastewater. This senseless waste of energy emits 5% of our greenhouse gases. In contrast, wastewater treatment plants could be net energy generators. In fact, a small part of the energy of wastewater has been recovered as biogas for more than a century. During biogas production, organic material from anaerobic digester sludge is decomposed by microbial communities and converted into methane:

H3C−COO + H+ + H2O → CH4 + HCO3 + H+; ∆G°’ = −31 kJ/mol (CH4)

The reaction produces CO2 and methane at a ratio of 1:1. Unfortunately, the CO2 in the biogas makes it almost worthless. As a result, biogas is often flared off, especially in places where natural gas is cheap. The removal of CO2 would greatly enhance the product and can be achieved using CO2 scrubbers. Even more reduced carbon sources can shift the ratio of CO2 to CH4. Nevertheless, CO2 would remain in biogas. Adding hydrogen to anaerobic digesters solves this problem technically. The process is called biogas upgrading. Hydrogen could be produced by electrolysis:

2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)

Electrolysis of water, however, is expensive and requires higher energy input. The reason is that the electrolysis of water takes place at a relatively high voltage of 1.23 V. One way to get around this is to replace the water by ammonium:

2 NH4+ → N2 + 2 H+ + 3 H2; ∆G°’ = +40 kJ/mol (H2)

With ammonium, the reaction takes place at only 136 mV, which saves the respective amount of energy. Thus, and with suitable catalysts, ammonium could serve as a reducing agent for hydrogen production. Microorganisms in the wastewater could be such catalysts. Moreover, without oxygen, methanogens become active in the wastewater and consume the produced hydrogen:

4 H2 + HCO3 + H+ → CH4 + 3 H2O; ∆G°’ = –34 kJ/mol (H2)

The methanogenic reaction keeps the hydrogen concentration so low (usually below 10 Pa) that the ammonium oxidation proceeds spontaneously, i.e. with energy gain:

8 NH4+ + 3 HCO3 → 4 N2 + 3 CH4 + 5 H+ + 9 H2O; ∆G°’ = −30 kJ/mol (CH4)

This is exactly the reaction described above. Bioelectrical methanogens grow at cathodes and belong to the genus Methanobacterium. Members of this genus thrive at low H2 concentrations.

The low energy gain is due to the small potential difference of ΔEh = +33 mV of CO2 reduction compared to the ammonium oxidation (see Pourbaix diagram above). The energy captured is barely sufficient for ADP phosphorylationG°’ = +31 kJ/mol). In addition, the nitrogen bond energy is innately high, which requires strong oxidants such as O2 (nitrification) or nitrite (anammox) to break them.

Instead of strong oxidizing agents, an anode may provide the activation energy for the ammonium oxidation, for example when poised at +500 mV. However, such positive redox potentials do not occur naturally in anaerobic environments. Therefore, we tested whether the ammonium oxidation can be coupled to the hydrogenotrophic methanogenesis by offering a positive electrode potential without O2. Indeed, we demonstrated this in our article and have filed a patent application. With our method one could, for example, profitably remove ammonia from industrial wastewater. It is also suitable for energy storage when e.g. Ammonia synthesized using excess wind energy.

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Wind Energy

Wind energy is short for the conversion of energy captured from wind to electrical or mechanical energy. Wind power turbines produce electrical energy and windmills produce mechanical energy. Other forms for wind energy conversion are wind pumps which use wind energy to pump water or sails which drive sail boats.

The cheapest US energy prices by source and county, Source: Energy Institute, University of Texas Austin

Since its first use on sail boats, wind energy is wide spread. Windmills have been used for more than 2,000 years as source of mechanical energy. The Scotsman James Blythe was the first who demonstrated the transformation of wind energy into electrical energy. As wind energy is a renewable source of energy, electrical energy generated by wind turbines is a clean and sustainable form of energy. Wind energy is often also cheaper than natural gas, for example throughout the entire American Midwest, as shown by the Energy Institute of University of Texas, Austin. It is therefore not surprising that wind energy is one of the fastest growing markets in the renewable energy sector worldwide. In 2015, 38% of all renewable energy in the United States and the European Union was generated by wind turbines.

Wind and solar energy production in the US and Canada in 2015. Sources: EIA, Statistics Canada

More efficient than single wind turbines is the use of wind parks where clusters of large turbines constantly generate electrical power. There are two kinds of wind parks, on-shore and off-shore wind parks. Off-shore wind parks are often more expensive but do not use valuable farmland as it is often the case for on-shore wind parks. However, wind parks on farmland can be a valuable addition for farmers seeking an extra income.

Wind and solar energy production in the European Union and the Euro-zone in 2015. WSH is the fraction of renewable energy of the European energy market. “Hydro” is the fraction of hydro power an Wasserkraft. Source, Eurostat
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Ammonia energy storage #1

The ancient, arid landscapes of Australia are not only fertile soil for huge forests and arable land. The sun shines more than in any other country. Strong winds hit the south and west coast. All in all, Australia has a renewable energy capacity of 25 terawatts, one of the highest in the world and about four times higher than the world’s installed power generation capacity. The low population density allows only little energy storage and electricity export is difficult due to the isolated location.

So far, we thought the cheapest way to store large amounts of energy was power-to-gas. But there is another way to produce carbon-free fuel: ammonia. Nitrogen gas and water are enough to make the gas. The conversion of renewable electricity into the high-energy gas, which can also be easily cooled and converted into a liquid fuel, produces a formidable carrier for hydrogen. Either ammonia or hydrogen can be used in fuel cells.

The volumetric energy density of ammonia is almost twice as high than that of liquid hydrogen. At the same time ammonia can be transported and stored easier and faster. Researchers around the world are pursuing the same vision of an “ammonia economy.” In Australia, which has long been exporting coal and natural gas, this is particularly important. This year, Australia’s Renewable Energy Agency is providing 20 million Australian dollars in funding.

Last year, an international consortium announced plans to build a $10 billion combined wind and solar plant. Although most of the 9 terawatts in the project would go through a submarine cable, part of this energy could be used to produce ammonia for long-haul transport. The process could replace the Haber-Bosch process.

Such an ammonia factories are cities of pipes and tanks and are usually situated where natural gas is available. In the Western Australian Pilbara Desert, where ferruginous rocks and the ocean meet, there is such an ammonia city. It is one of the largest and most modern ammonia plants in the world. But at the core, it’s still the same steel reactors that work after the 100 years-old ammonia recipe.

By 1909, nitrogen-fixing bacteria produced most of the ammonia on Earth. In the same year, the German scientist Fritz Haber discovered a reaction that could split the strong chemical bond of the nitrogen, (N2) with the aid of iron catalysts (magnetite) and subsequently bond the atoms with hydrogen to form ammonia. In the large, narrow steel reactors, the reaction produces 250 times the atmospheric pressure. The process was first industrialized by the German chemist Carl Bosch at BASF. It has become more efficient over time. About 60% of the introduced energy is stored in the ammonia bonds. Today, a single plant produces and delivers up to 1 million tons of ammonia per year.

Most of it is used as fertilizer. Plants use nitrogen, which is used to build up proteins and DNA, and ammonia delivers it in a bioavailable form. It is estimated that at least half of the nitrogen in the human body is synthetic ammonia.

Haber-Bosch led to a green revolution, but the process is anything but green. It requires hydrogen gas (H2), which is obtained from pressurized, heated steam from natural gas or coal. Carbon dioxide (CO2) remains behind and accounts for about half of the emissions. The second source material, N2, is recovered from the air. But the pressure needed to fuse hydrogen and nitrogen in the reactors is energy intensive, which in turn means more CO2. The emissions add up: global ammonia production consumes about 2% of energy and produces 1% of our CO2 emissions.

Our microbial electrolysis reactors convert the ammonia directly into methane gas − without the detour via hydrogen. The patent pending process is particularly suitable for removing ammonia from wastewater. Microbes living in wastewater directly oxidize the ammonia dissolved in ammonia and feed the released electrons into an electric circuit. The electricity can be collected directly, but it is more economical to produce methane gas from CO2. Using our technology, part of the CO2 is returned to the carbon cycle and contaminated wastewater is purified:

NH3 + CO2 → N2 + CH4