2024-W5 Evaluating role of microjet plasma interactions with slurry to mitigate ammonia volatilisation

Overview

Ammonia (NH3) losses from use of animal slurry and digestate in agriculture is a cause of environmental concern and a loss of valuable resource. If the NH3 loss can be minimised, it can tackle these challenges and the nitrogen from NH3 can be utilised as a valuable source of renewable fertiliser for crops. With escalating price of natural gas, which is directly influencing fertiliser prices, alternative solutions are urgently needed. This project is timely to tackle this challenge as it entails using microjet plasma technology to create reactive N species such as nitric oxide (NO) and create an acidic environment which minimises losses of NH3. However, what is less known is the detailed chemistry of what happens when these N species interact with the animal slurry or digestate liquor. 

In this study, a non-thermal atmospheric pressure plasma jet will be used to generate reactive oxygen species such as OH, superoxide, and reactive N-species, including nitric oxide, NO2, N2O5, etc. In the plasma jet, the energy impact collision of accelerated electrons with neutral gas is responsible for excitation of the neutral gas in accordance with the energy of the electron which in turn can be regulated via voltage, and frequency of plasma and the type of carrier gas. After excitation and recombination short-lived reactive species such as OH, and NO are formed. Some of these reactive species immediately combine with other reactive species, neutrals, and water molecules. Once they reach the plasma/liquid interface they form other reactive species such as NO2, NO3−, etc. 

Mixing argon, oxygen, nitrogen, and water vapor to the working gas maximizes NO2 production which can be easily absorbed by water. However, the mixing of gases to the working gas (Ar or He) causes severe increase in the breakdown voltage and decreases energy efficiency. Therefore, a better design of mixing section is needed. The air can be fed to the jet downstream of the main discharge region which should keep the breakdown voltage and the power consumption level similar to that of Ar plasma. Another goal of this study is to minimise the flow rate of noble gas which should reduce the evaporation at the plasma/liquid interface. A focal planar array camera, and a IR laser will be used for measuring the NO species inside the plasma jet. 

The specific objectives are:  

1. To characterise the formation of reactive nitrogen species when plasma gas interacts in the liquid phase of digestate and animal slurry
2. To model and optimise the plasma technology to increase the potential to retain N in the slurry and digestate liquor.
3. To characterise the interaction of the reactive nitrogen species in plasma treated animal slurry and digestate liquor with organic compounds and their potential for mineralisation in soil

Figure 1: Schematic showing components of the experimental setup where microjet plasma will be to treat slurry.