A Fuel of the Future?

It is somewhat surprising, therefore, to consider that ammonia may also be the fuel of the future, one that can help to mitigate against global warming by reducing greenhouse gas emissions. The ammonia molecule is comprised of four atoms: one nitrogen and three hydrogen. The hydrogen atoms carry the thermal energy. Ammonia as a fuel burns to give water and nitrogen.

It is another surprising fact that ammonia can transport hydrogen more readily than hydrogen Itself. To move hydrogen around It must either be compressed to between 300 - 700 bar or refrigerated to -2S3ºC or a combination of the two. Preparing hydrogen for transport can consume up to 30% of the energy contained in the hydrogen for compression or cooling power.

In contrast, ammonia can be liquified either by reducing Its temperature to -3lºC or by compressing It to 10 bar, (l4Spsi). Thls compares with liquefied petroleum gas (LPGJ which liquefies at approximately 8.5 bar (23 psi). Once in a liquid form the energy density of ammonia is 1.7 times that of hydrogen as liquid. An ammonia tank at 10bar contains 2.5 times as much energy as a hydrogen tank at 700 bar. As the energy content of ammonia comes from the hydrogen it contains, this is somewhat counter-intuitive but occurs as a result of the comparative Iiquid densities.

Ammonia Recovery from Wastewater

Ammonia recovery from wastewater can be achieved through several different technologies. Steam stripping, air stripping and membrane ion exchange are amongst the primary options. However, until recently the cost of the technology, both in terms of capital and operational costs, has not been viable. On the other hand, the established technologies for dealing with ammonia, such as activated sludge processes, do not facilitate recovery, only elimination.

Ammonia is present in wastewater in one of two form, subject to the alkalinity and temperature of the water. At higher pH levels and higher temperatures ammonia becomes a gas. NH3, with three hydrogen ions. At lower temperatures and pH levels, it exists as a dissociated Ion, NH4+, which can be difficult to remove from the carrier water. Typically, at a pH of above10 and a temperature of above 60'C, all the ammonia is a gas. At ambient temperatures and a pH of 5, all the ammonia is an ion. It is therefore necessary to Increase the pH or the temperature to make the ammonia gas available for recovery.

As has been demonstrated in Hong Kong over the last 20 years, ammonia can be removed from wastewater using heat alone without the addition of substantial quantities of support chemicals, making ammonia available for recovery. Thermal ammonia stripping has formed the core nitrogen removal process for several wastewater treatment facilities at several landfill sites and anaerobic digesters in the territory. In Hong Kong, however, the primary driver has been to dispose of ammonia, so it has been thermally oxidised. Figure 1 shows a thermal ammonia stripper installed to treat the centrate from an organic waste anaerobic digestion facility rated at 500m3/day (145,000gpd), with a design influent of 3510mg/l, and a required discharge of <100mg/l.

The issue of viability is central to any discussion with regard to ammonia recovery. Assessment if viability, on the other hand should be comprehensive. It should consider the cost of equipment involved, the cost of operation, the value of the product, and the cost of alternative ammonia disposal or recovery options. Unlike a simple power generation facility, here there exists a hierarchy of objectives. The first is to treat the wastewater. A secondary objective may be to minimise overall costs, which would be assisted by a marketable byproduct. The ideal of running a revenue generating operation from the sale of ammonia is very much dependent upon the price of ammonia, which may be enhanced by its green credentials.

For example, 1 tonne of ammonia hydroxide with typically 25% ammonia has a current value of approximately US$250. Ammonia is a traded commodity, meaning that market rates vary and, at present, they are low because of COVID-19. However, this is unlikely to be a sustained situation. The market rate over the last few years has reached a maximum of approximately US$500. A wastewater treatment facility receiving ammonia at an inlet concentration of 2000mg/l and rated at 500m3/d (132,000 gpd) can produced in the order of 4 tonnes per day of ammonium hydroxide, equating to a revenue stream of between US$360,000 and US$730,000 a year, not allowing for any green premium that may apply. Many wastewater flows have higher concentrations and higher flow rates.

Sources of Manufactured Ammonia

There are several different source of manufactured ammonia. ‘Green’ ammonia is produced from air and water using renewable energy, not involving the use of fossil fuels. Ammonia production using the latter elements as the energy source is referred to as ‘brown’ or ‘grey’ ammonia. Where carbon capture and sequestration are employed with brown ammonia, the colour becomes ‘blue’, whilst the colour turquoise has been adopted for ammonia produced using natural gas and capturing the resultant carbon as a solid. It is a mix of blue and green. So, a spectrum of colours has been designated to define which sources the ammonia comes from.

Where waste heat from biogas engines can be used to power the process of driving ammonia out of wastewater, and electricity from the same engines can be used to drive the necessary motors, the ammonia produced has both a low carbon footprint for its production as well as a reduction in its carbon footprint resulting from the avoidance of nitrous oxide production. Given that is it is also recovered waste being put to good use, such ammonia can be considered greener than green ammonia, hence the use of the term ‘emerald’ ammonia.

Greenhouse Gas Impact of Nitrous Oxide

One aspect of ammonia removal, which is the subject of current research, is the production of NO2 released during aerobic processing. This gas, also known as ‘laughing gas’, has a significant greenhouse gas impact, being between 265 and 298 times that of carbon dioxide. Figure remain uncertain, but the range of rates at which it is produced ranges between 0.0005 - 0.25 kg N2O/kg of nitrogen removed. The intergovernmental Panel on Climate Change’s (IPCC) good practice default value for a typical wastewater treatment plant is often taken as 0.01 N2O/kg N loaded. Use of the precautionary principle would mitigate for use of the maximum value.

Nitrogen makes up about 82% of ammonia. Taking the reference plant of 500m3/day, 365 tpy of ammonia can be recovered. That would equate to a conservative, good practice value of 883 tpy of CO2, equivalent for 1tpd of ammonia removed. Employing a non-conservative, precautionary assessment, it would be equivalent to 22,000 tpy. It should be noted that in 2007, the IPC concluded that N2O accounted for 7.9% of the global anthropogenic greenhouse gas emissions in 2004. The range of uncertainty, and the value of the extreme, emphasises the need for further study on this important topic.

Were it possible to remove and recover all 130 million tonnes of wastewater ammonia, the total greenhouse gas reduction from N2O alone, using the IPCC good practice value, would be to the order of 300 million tonnes per year of CO2 emission, or 0.8% of current global emissions. This figure can be added to the proportional carbon footprint of ammonia production by the Haber-Bosch process, which it total is estimated to be 1.4% of current global emissions. Should ammonia be used as a carbon-free fuel, the global impact becomes significant.

Potential Scope of Ammonia Recovery

In order to estimate the quantities of ammonia that might be recovered as ‘low-hanging fruit’ from wastewater treatment plants, the case of Melbourne in Australia can be used as an example. With a population of 5 million people and a wastewater flow rate of 900,000m3/day (237 million gallons per day), the ammonia loading it 49 tpd, or 9.9 g of ammonia per person per day. Of this total, approximately 4 tpd is readily available for recovery via a side-stream anaerobic digester , or -8%.

Using some liberal extrapolation, this would lead to a total figure of 90,000 tpy of ammonia production for the whole of Australia, of which some 7000 tpy would be readily recoverable, having an annual market value of between US$7 million and US$14 million. Using the same rations for North America gives a recoverable quantity of 165,000 tpy of ammonia. In the US that would have an annualised value, as ammonium hydroxide, of close to US$300 million. Were it possible to increase recovery from a conservative low of 8% of ammonia entering municipal wastewater treatment plants, this value would increase proportionately.

Food waste anaerobic digesters and landfill sites would also have the potential to produce viable quantities of ammonia from wastewater. In fact, there are many sources where emerald ammonia could be recovered, helping to offset the carbon impact of human activity, and to do so in a manner that produces a profit for its investors.