POWER FROM WASTEWATER AMMONIA
Power generation from the ammonia contained in wastewater at first sounds a bit like a green-lobby dream. Taking a pollutant, that requires so much effort to control, and using it for a beneficial outcome is in fact a real possibility. It has been estimated that the combined global emissions of ammonia are to the order of 800,000 tonnes per day. This includes humans, crops, natural sources, biomass burning, livestock and fertilisers. Of this total, around 350,000 tonnes per day derive from humans and livestock. If this ammonia could be harnessed, and that is a big ‘if’, with the use of a direct ammonia fuel cell, it would be possible to produce 39 Gigawatts of electricity. Oddly enough, that is exactly the power consumption currently encountered in the UK. It is also 1.3 times the power requirement of Australia.
Clearly, it would not be possible to collect all of this ammonia in one place. A direct ammonia fuel cell power plant of this capacity is also unachievable at present. However, the figures do demonstrate that ammonia can produce meaningful quantities of energy where it can be found at concentration. One site in Hong Kong, for example, removes and disposes of 17 tonnes per day of ammonia, which would be enough to produce 1.8 MW of electricity.
THERMAL AMMONIA REMOVAL TECHNOLOGIES
The most commonly encountered method of controlling ammonia in wastewater is by means of bacteria. Specialised bacteria, known as nitrifiers and de-nitrifiers, are harnessed in large lagoons to convert ammonia to nitrogen. It has recently been discovered that this process also produces nitrous dioxide, a highly potent greenhouse gas. In terms of recovery and re-use of ammonia, bacteriological methods will not be suitable, as ammonia is destroyed in the process.
In order to recover ammonia, it will be necessary to use physicochemical methods. One key attribute of ammonia, which defines the sort of processes which may be considered, is that ammonia exists in two forms in water, subject to the alkalinity and temperature of the water. At higher pH levels and higher temperatures, ammonia becomes a gas. At lower temperatures and pH levels, it exists as a dissociated ion, which can be difficult to remove from the carrier water. Typically, at a pH of 10 and a temperature of 60°C all the ammonia is a gas. At ambient temperatures and a pH pf 5, all the ammonia is an ion.
Therefore, ammonia removal technologies will normally involve the raising of the temperature or pH or both.
THERMAL AMMONIA STRIPPING
One such technology is thermal ammonia desorption, commonly referred to as thermal ammonia stripping. In this process, the ammonia laden wastewater is heated up to around 65°C and then passed counter-current to a flow of air to remove the ammonia gas. This is, in fact, the technology that has been widely deployed in Hong Kong to control ammonia releases.
In Hong Kong, it was not thought necessary to re-use ammonia, so it is collected and simply combusted. The plant in Figure 1 is dealing with 3,750 cubic metres per day of ammonia having an ammonia concentration of up to 4,500 mg per litre.
Another facility in Hong Kong is operating in conjunction with an anaerobic digester, where up to 200 tonnes per day of food waste are converted to biogas and then to electricity. (See Figure 2) Ammonia can be a serious inhibitor to the operation of an anaerobic digester, so removal of ammonia has the potential to assist in both pollution control and process optimisation. It has been estimated that at concentrations of 5,000 mg per litre biogas production will cease. The plant in Figure 2 uses waste heat from the engines shown to assist with the thermal load of the facility.
In Hong Kong, the Environmental Protection Department has installed six such installations, the first of which was commissioned in 1998.
After removal, ammonia needs to be re-captured. In the case of the thermal ammonia stripper, such recapture can take the form of a water scrubber followed by a rectifier. (See Figure 3) It may seem disingenuous to remove ammonia from wastewater and then add it back into clean water, but in fact the clean water acts as a useful medium to concentrate and manage the ammonia for final collection in an anhydrous form. This is pure ammonia which can be liquefied at approximately 8 bar gauge and ambient temperature. Once again, this is a thermal process, with cold water being used to capture and concentrate the ammonia gas and heat being employed to drive off the ammonia so ammonia may be concentrated as a gas, and after compression, as a liquid.
AMMONIA FUELLED POWER GENERATION
There is currently great interest in the use of ammonia as a fuel. Where ammonia is produced with renewable energy it is truly an environmental fuel. It can be produced from air and water. When employed in a fuel cell to produce electricity, it reduces back to air and water. It contains no carbon and, because of that fact, produces no greenhouse gas. Care must be taken to ensure that no NOx is formed or released, but this is a relatively straightforward task.
In the case of wastewater ammonia, there can be enhanced environmental benefits. Where renewable energy sources, such as biogas, are used to provide the thermal load, the ammonia produced is ‘green’. Ammonia is also a recovered waste stream that can be recycled. Finally, the ammonia so obtained is prevented from polluting the environment. The use of ammonia in direct ammonia fuel cells is currently the subject of a great deal of research and development. It is reasonably to be expect that within the next year or two a commercially viable fuel cell will become available. Once that is in place a complete, environmentally sound ammonia fuel cycle will become possible. The potential for application spreads from power storage at wind and solar farms to vehicle transport.