A comparative analysis of biogas upgrading technologies: Photosynthetic vs physical/chemical processes

Abstract

Two biogas upgrading technologies, viz. an innovative algal-bacterial photobioreactor and a conventional activated carbon filter coupled with a water scrubber, were comparatively evaluated in terms of environmental, economic and social performance by using the IChemE Sustainability Metrics. The upgrading of 300 Nm3/h of biogas generated from the anaerobic digestion of mixed sludge in a wastewater treatment plant was used as a model scenario for the comparative analysis. Despite the algal-bacterial photobioreactor entailed 1860 times higher land requirements, the two-stage physical/chemical technology exhibited × 3.8 higher energy consumptions and larger environmental impacts in terms of material and water consumption and greenhouse gas emissions (the latter by a factor of ~ 45). The investment cost for the algal-bacterial photobioreactor was 1.6 times higher than that of its physical/chemical counterpart due to the biomass drying unit required to produce an algae-based fertilizer. However, the operating cost of the physical/chemical technology was ~ 7 times higher due to the frequent replacement of the activated carbon. A further analysis of the net present value (NPV 20) revealed that photosynthetic upgrading would yield revenues from year 5 of operation mainly due to the sale of the algal bio-fertilizer produced, even without tax incentives for bio-methane.