Abstract
Global warming caused by increasing CO(2)atmospheric levels is calling for sustainable fuels. For instance, biomethane produced by biogas upgrading is a promising source of green energy. Technologies to upgrade biogas include chemical absorption, water scrubbing, physical absorption, adsorption, cryogenic separation and membrane separation. Historically, water scrubbing was preferred because of the simplicity of this operation. However, during the last decade, membrane separation stood out due to its promising economic viability with investment costs of 3500-7500 euro/(m(3)/h) and operational costs of 7.5-12.5 euro/(m(3)/h). Here we review biogas upgrading by membrane separation. We discuss gas permeation, membrane materials, membrane modules, process configurations and commercial biogas plants. Polymeric materials appear as most adequate for membranes aimed to upgrade biogas. Concerning membrane modules, hollow fibers are the cheapest (1.5-9 euro/m(2)). Multistage configurations provide high methane recovery, of 99%, and purity, of 95-99%, compared to single-stage configurations.