Biogas is a renewable energy source that is produced by anaerobic digestion of organic mate-rial. In Sweden, biogas predominately comes from sewage water sludge and landfills or from organic waste of households and industries. Small scale digestion plants at farms are espe-cially expected to contribute to increased biogas production in the future. Biogas can be ob-tained directly in it’s raw form and used as fuel in a combustion chamber. However, gas en-gines require biogas purification from hydrogen sulphide and drying from water to avoid cor-rosion. In order to increase the calorific value, carbon dioxide is separated and the Swedish Standard Type A requires the methane content to be 97 % for vehicle gas.
In the gas treatment process from biogas to vehicle gas, the upgrading step when carbon diox-ide is separated represents the highest cost since conventional upgrading techniques require high investments. This makes the upgrading costs for smaller biogas plants relatively high. In this master thesis, six upgrading methods have been evaluated and four of them are expected to be commercialized within two years. The following upgrading methods are of interest for Sweden:
- In situ methane enrichment; air desorbs carbon dioxide from the sludge in a
desorption column. The method is intended for digestion of sewage water sludge and the total upgrading cost is approximately 0,13 kr/kWh by a raw biogas flow 62,5 Nm3/h.
- Small scale water scrubber; carbon dioxide is absorbed in water under enhanced pressure. The upgrading process is very similar to the conventional water scrub-bing technique and the total upgrading cost is approximately 0,42 kr/kWh by a raw biogas flow of 12 Nm3/h.
- Cryogenic upgrading; the biogas is chilled to under -85 °C under a pressure of at least 5,2 barg and carbon dioxide can be separated in the liquid phase. The total upgrading cost is approximately 0,12 kr/kWh by a raw biogas flow of 150 Nm3/h. The total upgrading cost can be reduced if the recovered liquid carbon dioxide can be sold.
- Membrane technique; biogas is upgraded with polymeric membranes that are per-meable for carbon dioxide but not for methane molecules. The method is expected to be adaptable for both smaller and bigger biogas plants and the total upgrading cost is approximately 0,14 kr/kWh by a raw biogas flow of 180 Nm3/h.
All above mentioned upgrading techniques have methane losses less than two percent and all methods except for the in situ methane enrichment are expected to upgrade biogas to vehicle gas according to the Swedish Standard. In situ methane is expected to upgrade biogas up to 95 % methane content.
By combustion of unpurified landfill gas in a gas engine, corrosive combustion products and white deposits are formed. Purification of landfill gas can decrease maintenance costs for gas engines. Two landfill gas purification methods have been evaluated and with the first method, contaminants are trapped in ice crystals when the gas is chilled to -25 °C. The second method purifies landfill gas with condensed carbon dioxide.
An important result of the master thesis is that the in situ methane enrichment has a chance to become an interesting alternative for smaller sewage treatment plants but the method requires additional upgrading to reach 97 % methane content. The most important conclusion is that cryogenic upgrading and membrane technique are expected to satisfy the Swedish Standard. The methods have relatively low upgrading costs and the methane losses are less than two percent. This gives them a good chance to establish in Sweden.
Institutionen för konstruktions- och produktionsteknik , 2006. , 63 p.
uppgradering, biogas, rening, fordonsgas, kryoteknik, membranteknik, processintern vattenskrubber