Development of a gas collection and methane flaring system for covered slurry tanks

Introduction<br/>Methane emission from external manure storages is a significant source of GHG from animal production. Recent focus on frequent export of manure from the barns will further increase the potentials of collecting methane from the external storages. The advantage of technologies that target emission from external storages is that it is an end-of-pipe technology, which does not require changes in the barns. External storages like slurry tanks can be covered with tents, which in fact is cost-effective as rain cannot contribute to the slurry volume in the tanks.<br/>The aim of this project was to develop and document a system that collects slurry gas from covered slurry tanks and oxidizes methane into carbon dioxide using thermal combustion in a flaring unit. The focus was on system and flaring efficiency as well as safety (ATEX regulation).<br/>Methodology<br/>Collection of slurry gas from the covered slurry tanks was optimised by closing all openings in the tent. The gas ramp and flaring unit was optimised based on knowledge from a pilot plant on a sow farm and similar systems for methane collection and oxidation on old landfills. The burner head was optimised in a series of incremental steps. The optimisation included foci on methane conversion and concentrations of oxygen, carbon monoxide, organic carbon (OGC) and nitrogen oxides (NOx) in the flue gas.<br/>Finally, a SCADA system was developed to monitor, regulate and control the system as well as provide documentation.<br/>Results and discussion<br/>The developed system converts methane with an efficiency higher than 99 percent if the gas feed has a methane concentration higher than 5 volume percent. The flue gas has very low NOx concentration. A commercial slurry gas collection and flaring system is now marketed on the Danish market and already sold to several farmers by AgroGas. A documentation test based on 4 flaring units is ongoing. Recently, a study has demonstrated high methane emission from slurry tanks from both pigs and cattle in Denmark (Vechi et al., 2023), and therefore this type of flaring system could be an effective technology to collect and combust the methane, reducing GHG emission from animal production.<br/>Reducing natural ventilation in the covered slurry tanks may potentially reduce slurry surface oxidation, which could increase methane emission (Dalby et al., 2024). Potential risk of fire or explosion in the slurry tanks or flaring system has been addressed by following ATEX regulations and ATEX approved components in the flaring system.<br/>The potential for the system with collection and destruction of methane from covered slurry tanks estimates that a total of 1.5 million tonnes CO2e in Denmark can be treated (Andersen et al., 2024). The total estimated methane emission from livestock manure from barns and storages in Denmark is estimated to 3.1 million tonnes in 2022 (Nielsen et al., 2023).<br/>Conclusion<br/>A commercial gas collection and flaring system has been developed and is currently marketed to the Danish farmers.<br/>Acknowledgements<br/>The project has been supported by Green Development and Demonstration Programme (GUDP), 34009-21-1939.<br/>References<br/>Andersen, M.N., et al. 2024. Measures for reduction of greenhouse gases in agriculture – 2024 (in Danish). Report from DCA - Danish Centre for Food and Agriculture, Aarhus University. 375 pp.<br/>Dalby, F.R., et al. 2024. Pig manure degradation and carbon emission: Measuring and modelling combined aerobic–anaerobic transformations. Journal of Environmental Quality 53(5), 589-603. https://doi.org/10.1002/jeq2.20603<br/>Nielsen, O.-K. et al. 2024. Denmark's National Inventory Report 2023: Emission Inventories 1990-2022. Aarhus Universitet. DCE - Danish Centre for Environment and Energy Nr. 541 https://dce2.au.dk/pub/SR541.pdf<br/>Vechi, N.T., et al. 2023. Methane emission rates averaged over a year from ten farm-scale manure storage tanks. Science of the Total Environment 904 (2023) 166610