Investigation on the mobilization of membrane-based biogas upgrading plants
|Title||Investigation on the mobilization of membrane-based biogas upgrading plants|
|Publication Type||Conference Paper|
|Year of Publication||2017|
|Authors||Spitzer S, Miltner M, Harasek M|
|Conference Name||10th World Congress of Chemical Engineering WCCE10, Barcelona, Spain|
Renewable and sustainable energy production is a key point for achieving the goal of limiting global warming to less than 2 °C. Especially, in developing countries biogas produced from organic waste could not only reduce the dependency of farmers on fossil energy sources but also the part of organic waste rotting uncontrolled and thereby emitting methane into the atmosphere. In order to use biogas as fuel, CO2 and other harmful impurities have to be removed. Mobile biogas upgrading plants travelling from one biogas plant to another for removing these impurities could dramatically reduce investment costs for single farmers and thus, make it affordable for them. For storing and transportation issues the product gas will be compressed after the upgrading process.
In Brazil, the chosen model region, biogas is typically produced in simple fixed-dome digesters. Investigation of real biogas data from Brazilian farmers showed that the biogas composition is changing strongly over time and hazardous components like H2S are predominant due to using manure as a major substrate. Besides being capable of dealing with this feed quality other requirements like minimum space and weight demand or low costs have to be met for a mobile process.
Due to often poor road conditions chosen plant equipment should be as vibration resistant as possible. Evaluation of available equipment showed that compressors and gas analysis are always sensitive to vibration to a certain amount. Thus, integration of a specific shock absorption system is crucial for the mobile plant. Additionally, as reliable gas analysis is mainly based on infrared absorption, on-site calibration before starting the upgrading process is important for high quality product gas.
Investigation of different upgrading processes for removal of CO2 (chemical and physical scrubbers, PSA, and membrane separation) showed that membrane separation could fulfil the defined requirements at best and thus, is the most promising upgrading technology in mobile plants. Two pre-treatment steps were defined. These are a cool-drying step for removing water and potential ammonia and a desulphurisation as all common biogas substrates used in Brazil produce H2S in a certain amount. Desulphurization will be carried out in a combination of two adsorption steps (iron oxide and activated carbon) in order to remove not only H2S but also other minor impurities like siloxanes and oxygen.
Process simulation in Aspen Plus® was carried out for the whole process with focus on the membrane separation step. It was shown that a process based on a two-stage membrane separation step is suitable as this design achieved performance good enough to meet the Brazilian biomethane limit of 96.5 % in the product gas for a broad range of incoming biogas compositions. Designs with membrane area adjustable to the incoming feed composition could not only achieve product gas above the biomethane limit but also strongly reduce the methane loss into the offgas.