Emergy analysis of biodiesel and biogas production from Baltic Sea macro algae

Abstract: The aim of this study was to compare the production of biodiesel or biogas from macro algae harvested from the Baltic Sea from an energy perspective. The macro algae were considered to be harvested from an area along the southern coast of Sweden, between Malmö and Simrishamn. The gathering of algae is an attempt to reduce the current eutrophication in the Baltic Sea by removing nutrients that the algae have assimilated. The algae also contain some amounts of heavy metals, so the amounts of heavy metals in the marine environment are also reduced. The evaluation included all processes from harvesting of the algae, transport of the algae to the processing plants, processing of the algae to biodiesel or biogas.  An evaluation of the algae residues from the processes can be used as fertilizer in agriculture was also conducted. The inputs of materials and energy into the systems were calculated from values found in literature and estimated from similar studies. The evaluation method used was an emergy analysis where all the energy and material inflows into the processes were converted to solar emergy joules so the inflows can be compared on a common basis. The energy and material inflows into the system, including the harvesting, the transport and the biodiesel or biogas production processes, were converted with the use of transformities, which describes the amount of solar emergy joules per joule of energy, gram of material or cost in euro. The transformities for biodiesel and biogas were calculated and compared to give an indication of which product that is most efficient to produce. The processes were also evaluated using emergy indices, such as environmental loading ratio (ELR), emergy yield ratio (EYR), emergy sustainability index (ESI), emergy investment ratio (EIR) and percent renewability in the systems. The results of the study show that biogas has the lower transformity of the two, which means that the biogas production have utilized less solar emergy joules to produce 1 joule of energy than the biodiesel production. The total amount of solar emergy joules used per year for the biodiesel and biogas systems were calculated to 2.18·1019 seJ/year for biodiesel and 2.75·1019 seJ/year for biogas. The transformities calculated for biodiesel and biogas were 5.04·105 seJ/J and 9.12·104 seJ/J, respectively. The emergy indices, however, showed support for the biodiesel process by indicating lower environmental impacts, a higher economic competitiveness and a higher percent renewability.