Gårdsbaserad uppgradering av biogas med askfilter

Sammanfattning: Most of the biogas produced in small scale biogas plants in Sweden today is used for combined heat and power production. With the price of electricity being relatively low, the small biogas plants are hardly profitable. Since the electricity in Sweden is largely renewable the environmental motivation for farmers to build their own small scale biogas plants is low. If the biogas could be used as vehicle fuel instead this might change. Vehicle fuel is still mostly fossil which means that production of small scale vehicle fuel would benefit both the environment and the producers. The problem however is that upgrading small flows of biogas is too expensive with todays established technologies. Because of this, RISE and SLU are researching alternative methods of upgrading with the purpose of making them cheaper for small scale biogas plants. The one featured in this paper is called ash filter. This method involves filling a container, the ash filter, with moist ash, sealing it up and letting biogas pass through the ash bed. The CO2 in the biogas will carbonize with the Ca2+ in the ash and remain fixated while the CH4 enriched gas will pass through. Any H2S in the biogas will also be fixated in the ash forming sulfites. Ash filters have been used in the laboratory multiple times and the aim of this project was to use the method for the first time on a demonstration scale at the biogas plant in Sötåsen. The biogas produced at this plant contained an average of 56 % CH4, 43 % CO2 and 700 – 1000 ppm H2S at the time of the trial. The trial was a success with the dry upgraded gas reaching concentrations of 99.1 % CH4, <0.1 % CO2 and <2 ppm H2S. When sampled immediately after the filter the gas had a moisture content of 1.5 %. The ash had a capacity of fixating 99 g CO2/kg dry ash. About 52 % of the fixation happened before the breakthrough of CO2 occurred, after which the upgraded gas increased its CO2 content slowly during the rest of the trial. All of the H2S in the biogas was fixated by the filter. After the trial was ended the ash was examined and proved to be easy to handle and suitable for storage outside. Since almost half of the upgrading capacity remained after the breakthrough, multiple filters should be used in series to ensure a steady stream of completely upgraded gas. The upgraded biogas reached the specifications for vehicle fuel except for the moisture content. This would have to be removed before the gas could be compressed and odorized in future trials. Different types of ashes could be tested with other gas flows as well to obtain even more experience of the upgrading method.