Mercury Screening in Biogas Production Processes for the Prevention of Liquid Metal Embrittlement in LBG production

Sammanfattning

The development of liquefied biogas has grown alongside the increasing demand for renewable fuels. Liquefied biogas production uses cryogenic heat exchangers to effectively liquefy biomethane. However, previously conducted studies have revealed that trace amounts of mercury may cause liquid metal embrittlement on aluminium alloys, potentially causing material damage and catastrophic accidents. The distribution of mercury and the influence of substrate and gas upgrading technology on mercury concentration remain unknown within biogas production processes, emphasizing an important knowledge gap.This master’s thesis was conducted in collaboration with St1 Biokraft and investigates the occurrence of mercury across six biogas production facilities and aims to contribute to the understanding of mercury behaviour in biogas systems. The study examines substrate, digestate and biomethane to assess mercury input and output of the process. Mercury in biomethane was collected using mercury sorbent traps-EPA Method 30B and analysed using Atomic Fluorescence Spectrometry, whilst samples of substrate and digestate were analysed with Inductively Coupled Plasma Mass Spectrometry. To enable comparable analysis, the study also evaluated different gas upgrading technologies and included facilities using pressure swing adsorption and amine scrubbers for the removal of contaminants from biomethane.The results showed that mercury was detected in biomethane at each facility, and every detected value was below the industrial limit of 10 ng/m3. Mercury was also detected in some of the substrate and digestate samples. However, no clear correlation was observed between Hg concentrations in substrates, digestates and biomethane or between gas upgrading technologies. Elevated mercury concentrations were observed on a wet weight basis in dry digestion systems and substrates with higher total solids content, suggesting Hg distribution within particulate and organic phases. However, the correlation declined when concentrations were expressed on dry weight basis. The results presented in this report are based on a screening principle that indicates that Hg distribution within biogas production processes is complex and further studies are necessary to better understand the mechanism of mercury.