Innovative process for biogas upgrading with CO2 storage: Results from pilot plant operation

An innovative biogas upgrading method that, differs from the currently employed commercial techniques, allows also to capture and store the separated CO2 is investigated. This process, named Alkali absorption with Regeneration (AwR), consists in a first step in which CO2 is separated from the biogas by chemical absorption with an alkali aqueous solution followed by a second step in which the spent absorption solution is regenerated for reuse in the first step of the upgrading process and the captured CO2 is stored in a solid and thermodynamically stable form. The latter process is carried out contacting the spent absorption solution, rich in carbonate and bicarbonate ions, with a waste material - air pollution control (APC) residues from Waste-to-Energy plants - characterized by a high content of calcium hydroxide and leads to the precipitation of calcium carbonate and to the regeneration of the alkali hydroxide content of the solution. The process was tested in a specifically designed pilot plant fed with 20 m3 h−1 (gas at standard conditions of 273 K and 1001 kPa) of landfill gas. Results showed that a high CH4 content in the outlet gas can be obtained using a 3.8 mol L−1 NaOH aqueous solution with a solution/landfill gas ratio of about 9 L m−3 (gas at standard conditions of 273 K and 1001 kPa). The regeneration process proved to be feasible, but its efficiency was limited by several factors to maximum values in the range of 50-60 %, showing to decrease with higher NaOH concentrations in the absorption solution. Absorption tests with regenerated load solutions after appropriate NaOH makeup, did not show appreciable differences with respect to raw solutions.