Absolute emissions from slurry storage tanks with micrometeorological methods: Challenges of downscaling and method validation

Gaseous emissions from slurry storage tanks represent significant environmental and climate challenges. Accurate measurements of these emissions are essential for understanding their impact and developing effective mitigation strategies. However, measuring emissions of methane, ammonia, and nitrous oxide from full-scale slurry storage tanks can be challenging and it is practically impossible to obtain replicate measurement of the same slurry or test treatments under identical conditions.

To overcome this challenge, Computational Fluid Dynamics (CFD) modeling was used to investigate suitable dimensions for small-scale tanks. A tank diameter of 2.4 m and a height of 1 m was found suitable for emission measurements. As a method for measuring the emissions the Micrometeorological Mass Balance (MMB) method, where concentration and wind speed is measured at multiple heights above the tank, is a promising candidate as it has been proven to work on full-scale tanks for methane (Kariyapperuma et al., 2018; Park et al., 2010).

The plan was to validate the use of MMB on the small-scale tank while measuring in parallel with the backward Lagrangian Stochastic (bLS) method that have previously been used on full-scale slurry tanks (Lemes et al., 2022). Concurrent measurements with MMB and bLS were not useful as the concentration differences used for bLS were too small to estimate emissions. The measurement on pig slurry showed MMB emissions for methane and ammonia comparable to baseline emission in a recent review (Kupper et al., 2021), but the concentration response for ammonia indicated that it is questionable using a closed path instrument to measure ammonia emissions with MMB. In another validation experiment with IDM and MMB a known quantity of gas was released from a grid with 24 critical orifices inside the small-scale tank. In this case, bLS had a good recovery whereas MMB did not. The discrepancy was likely caused by the gas being released from discrete points and not uniformly from the entire surface. In a third validation experiment, MMB was compared to the Tracer Gas Method (TGM), where a known quantity of gas was released at three positions just below the slurry surface. The TGM and MMB emissions from methane agreed well in some intervals, but differed greatly in others, highlighting the challenges of measuring emissions from a small tank.

The observed issues emphasize the complexity of validating emissions from small-scale slurry tanks. Downscaling the tank also downscales emissions, which can be an issue using some methods and thereby making it difficult to do cross validation with different methods in parallel.

Downscaling provides opportunities to investigate natural variations and emissions of different slurry types under the same weather conditions in replicates, but the choice of an appropriate micrometeorological method is a complex challenge. 

References:
Kariyapperuma et al.: Agric. For. Meteorol., 258, 56–65, doi:10.1016/j.agrformet.2017.12.185, 2018.
Kupper et al.: Biosyst. Eng., 204, 36–49, doi:10.1016/j.biosystemseng.2021.01.001, 2021.
Lemes et al.: ACS Agric. Sci. Technol., 2(6), 1196–1205, doi:10.1021/acsagscitech.2c00172, 2022.
Park et al.: Agric. For. Meteorol., 150(2), 175–181, doi:10.1016/j.agrformet.2009.09.013, 2010.