In-situ methane enrichment

The objectives of the present project were to:

•  determine and explain the buffering capacity of the anaerobic sludge against the pH-increase due to desorption of carbon dioxide. Earlier studies indicates a discrepancy between the titrated buffering capacity and the chemical composition of the sludge
• determine the dissolved methane in the digester sludge. Earlier studies have indicated that the sludge can be supersaturated with methane 
• merge a model describing the anaerobic digestion process with a model describing the desorption column in order to develop a tool for simulation and analysis of the entire continuous process 
• simulate and analyse important parameters for dimensioning and operation of in-situ methane enrichment in full-scale.

For biogas to be used as a vehicle fuel, carbon dioxide has to be removed to reduce the volume and increase the energy density. However, the cost for upgrading biogas to vehicle or grid quality using commercial technologies is rather high, especially at low gas flow rates.

An alternative and potentially more cost effective technology to upgrade biogas is in-situ methane enrichment. The principle for this concept is circulation of a sludge stream from the digester through a desorption unit and then back to the digester. The desorption is achieved by aeration of the sludge and is based on the higher solubility of carbon dioxide in water compared to methane.

Publications:

• Upgrading of biogas using ash from wood fuels (in Swedish)

The project was financed by Forskningsstiftelsen - Göteborg Energi AB

Participating from the department was Åke Nordberg

External partners were Department of Chemical Engineering and Technology , KTH (Project leader) and JTI - Swedish Institute of Agricultural and Environmental Engineering.