ABSTRACT
In the agricultural sector, ruminants are the largest methane (CH4) emission source and many efforts have been undertaken to reduce these greenhouse gas emissions, while compromising animal health and physiology. On the other hand, ruminal CH4, which is biomethane, is in high demand, especially in its liquid form (LBM) that can be used as high energy density fuel. However, CH4 released from a ruminant is immediately mixed with air and highly diluted (<0.1%), challenging CH4 capture technologies. Here we aimed to construct a cryogenic pilot system to capture and liquefy enteric CH4 released from dairy cows kept in respiration chambers. To approach this aim, the outlet air from the chambers was directed through a two-step cooling trap to capture CO2 (-120 to -130 °C) as a solid in the first and CH4 and O2 as liquids in the second cooler (-160 to -180 °C). Warming the second cooler resulted in the evaporation of O2, thereby separating O2 and CH4. LBM purity was in average 90% and was lowest at warming rates higher than 0.88 °C/min. The mean CH4 capture efficiency was 92% and found to be independent of sequestration time and flow rate. However, an increase in CH4 concentration to 0.6%, as it occurs directly at the muzzle of a cow, reduced the sequestration time for CH4. These results show that cryogenic technology can be used to obtain LBM from the air containing ultra-low CH4 concentrations as it is found in cattle barns with high efficiency and purity.
