Production of biogas from grass at various pre-treatment and anaerobic fermentation temperatures under influence of magnetic field

Abstract

Biogas production from agricultural biomass is declining in Latvia, as stricter regulations propose restriction of using energy crops, e.g. maize silage, by 2030. An alternative to energy crops can be grass overgrowth biomass from abandoned areas, municipal or domestic green areas and protected areas such as roadsides, areas under power lines and others. The biomass of grass contains lot of lignocellulose, which slowly decomposes during the anaerobic fermentation (AF) process. The aim of the study was to investigate the influence of pretreatment of grass by freeze-thawing cycle at a low temperature -15 °C or -23 °C on methane yield. Also, influence of various mesophilic temperatures 25; 33 and 38 °C on production of biomethane was investigated during the AF process. Some bioreactors (volume 0.75 L) with grass biomass were added magnetic powder (MP) and/or fitted with a pair of magnets for providing of weak magnetic field (WMF). Results of the experimental setup 1 shows the increase in specific methane 68 or 75% if the grass biomass is pretreated at -15 °C and -23 °C in double freeze-thawing cycle. The highest specific methane yield 0.313 L·gVS-1 was observed from grass biomass pretreated at -23 °C, that was 100% higher compared to control. Results of setup 2 show high specific methane 0.202 L·gVS-1 from the group of bioreactors with added MP 1.0 g, moderate methane yield 0.188 L·gVS-1 under influence of WMF 3.6 µF and the highest methane yield 0.234 L·gVS-1 from combined application of MP 2.0 and WMF 3.6 µF which 24%, 15% and 44% respectively, compared to control (grass without MP addition and WMF application).