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A low-tech, low-cost method to capture point-source ammonia emissions and their potential use as a nitrogen fertiliser [1]
['Nicholas Cowan', 'Uk Centre For Ecology', 'Hydrology', 'Bush Estate', 'Midlothian', 'United Kingdom', 'Daniel Ashwood', 'School Of Chemistry', 'The University Of Edinburgh', 'Edinburgh']
Date: 2024-02
The capture and storage of NH 3
The results of this feasibility study have provided evidence of several aspects of NH 3 capture and utilisation that may be useful when developing NH 3 capture technologies in the future, especially when considering further utilisation in agriculture. While the absolute overall capture rate of NH 3 (and conversion to NH 4 +) is not quantified in this study, we know from previous studies that, simply using water, it is possible to strip almost 100% of NH 3 from air [24] and capture rates depend primarily on the design of the capture mechanism. We also know from previous studies that NH 3 gas is highly soluble, and that a maximum of approximately 320 g of NH 3 could be dissolved in one litre of water at 25 °C. In comparison, 2130 g of ammonium nitrate (NH 4 NO 3 ) can also be dissolved in a litre of water [27]. This gives a maximum N content of 263.5 g L-1 and 745.7 g L-1 for NH 3 and AN, respectively. This high solubility in water is the reason that simple, low-tech solutions for NH 3 capture like air bubbling or mist scrubbing [28] are highly effective (>80%) at removing NH 3 from the atmosphere at low cost.
In this study, 64.5 m3 of air passed through water resulted in the capture of 6.33 g of NH 3 -N over a period of 56 days (capture of 0.11 g day-1). Hypothetically, using simplistic assumptions, if this absorption rate remained stable, it would take over 6.5 years to saturate 5 litres of water with NH 3 , showing that in order to be successful, a significant airflow of elevated NH 3 concentration is required to produce meaningful quantities of NH 4 in solution. The highest absorption rate of NH 3 in this experiment was of the CAN solution, capturing approximately 0.15 g of NH 3 -N per m-3 of air bubbled through the solution. Assuming a 100% capture rate, we can estimate that the mean air concentration of NH 3 passing through the system was at least 201 ppm which is in line with what would be expected from a manure bin. It has been reported in literature that NH 3 concentrations in chicken barns in the UK can reach 20–52 ppm in the winter when the barns are shut to the elements, and 12–25 ppm in the summer [29]. Higher concentrations are not uncommon [30]. At these high NH 3 concentrations and with higher air flow rates, the capture rate N from simple stripping systems would be significantly higher as demonstrated in a review by Pandey and Chen (2021) [31], where NH 4 -N recovery exceeds 90% in the majority of examples.
We confirm in this study that solutions acidified with nitrate (NO 3 -) are able to better strip NH 3 from the atmosphere than pure water. This comes as no surprise, as acidified solutions are often used to strip NH 3 , most commonly in the form of sulphuric acid [31]. The value of using nitric acid instead of sulphuric acid, is that a more concentrated content of N can be produced in the solution, thus increasing the value of the captured NH 3 as an N fertiliser product. The addition of NO 3 - more than doubles the maximum solubility of total N in the form of dissolved NH 4 NO 3 , and our study suggests that the addition of NO 3 - strongly increases the capture rate of NH 3 in the stripping process as each of the AN, CAN and CAN_acid solutions performed considerably better than the N_DI. In theory, a highly concentrated nitric acid solution could capture a large quantity of NH 3 (373 g L-1), though the use of large quantities of such concentrated acids would pose serious risk of harm to users. The alternative to using acid, is to use a dissolvable nitrate powder such as calcium nitrate Ca(NO 3 ) 2 . The pH of dissolved calcium nitrate is much safer to handle (pH = 5.5) than that of strong acids, and high concentrations of nitrate in solution can also be achieved (saturation of 1440 g of Ca(NO 3 ) 2 per litre of water [27]). In this study the dissolved calcium nitrate had the highest NH 3 capture rate of all the solutions, though this came with a large drawback, the precipitation of calcium (or calcium salt). As the CAN and CAN_acid solutions became more alkaline in nature, the calcium began to precipitate out of the solution around the air inlet of the tubing. This precipitation clogged the tubing and needed to be manually removed. In a larger barn-scale system this would be a serious drawback of using calcium salt solutions (or solutions of other elements expected to precipitate) as it could result in expensive mechanical failures and clogging of pipes.
The capture and storage of high concentration N solutions does present some risk for farmers. Where only water is used to capture NH 3 , if saturation was reached, gaseous NH 3 would be released from the system as uptake and emission reaches equilibrium. However, where nitrate (or similar anions) are present, ammoniacal salts can precipitate from the solution once threshold pH or temperature conditions are met. Ammoniacal salts, especially of nitrate, pose an explosive risk. The precipitation and collection of ammonium nitrate also provides the possibility of the generation of undocumented AN fertiliser, which is a highly regulated substance due to its explosive properties. To combat AN being used as an explosive, calcium can be added to form CAN, thus reducing its explosive properties; however, CAN can still be used in the manufacturing of illegal explosives.
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