Innovative processes for biogas upgrading: results from pilot plant operation.

Biogas up-grading treatments aimed at producing biomethane to fuel vehicles or to inject into the gas grid, are applications that are gaining increasing interest throughout Europe. Several different commercial methods are available for separating carbon dioxide from biogas. In this work two innovative carbon dioxide removal methods that, differently from the currently employed commercial techniques, allow also to capture and store the separated carbon dioxide are investigated. The first investigated process, named alkali Absorption with Regeneration (AwR), consists in a first step in which carbon dioxide 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 carbon dioxide 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 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 second investigated process is based on chemical reaction of carbon dioxide contained in the biogas directly with bottom ashes from municipal solid waste combustion. In this kind of process the gas containing the carbon dioxide and the solid residues come directly into contact, in an appropriate gas/solid reactor, with the result of an accelerated carbonation of the solid residues and carbon dioxide up-take. Both the proposed processes were first studied by preliminary laboratory analysis, then two small pilot plants were built in order to demonstrate the technical feasibility of the processes. The pilot plant operation results are reported in the paper. This study was conducted in the framework of the UPGAS-LOWCO2 (LIFE08/ENV/IT/000429) Life+ project.