Environmental performance of grate furnace and fluidised bed furnace systems to produce electricity from forest biomass residues

Electricity production from biomass has the potential to significantly contribute to the share of renewable energy in the global power mix with lesser environmental impact than non-renewable resources. The production of bioenergy from forest biomass residues is currently increasing in Portugal, mainly as a consequence of concerns related to climate change and forest fires.  In Portugal, the annual production of residual biomass from forest logging is estimated at 0.8-1.2 million dry tons per year, and about 47-58% of these residues come from eucalypt. 

This study evaluates the environmental impacts resulting from electricity production in Portugal using eucalypt logging residues (composed of branches, foliage and tops) and considering two types of technologies: grate furnaces and fluidised bed furnaces. This assessment was performed using life cycle assessment (LCA) methodology, a methodology that evaluates the environmental impacts entire life cycle of a product or process (from the extraction of the raw materials until its end-of-life), allowing to identify the most significant stages and processes along the life cycle, and supporting by this way the decision and policy-making.

Two alternative scenarios for biomass-to-energy conversion technologies were simulated: grate furnace and fluidised bed furnace. The functional unit is the production of electricity from the combustion of eucalypt logging residues equivalent to 1 kWh delivered by the power plant to the Portuguese grid. System boundaries include the following stages: (1) forest management (including site preparation, planting, stand tending and logging); (2) residues collection; and (3) energy conversion (including forest biomass combustion as well as treatment and final destination of wastes). Seven impact categories from the International Reference Life Cycle Data System (ILCD) are considered: climate change, particulate matter, photochemical ozone formation, acidification, freshwater eutrophication, marine eutrophication and mineral and fossil resource depletion.

The results show that the forest management stage had a low contribution to the total impact in all impact categories for both technologies under analysis. The only exception is the impact category of mineral and fossil depletion, in which forest management is mainly responsible and which accounts for 92-94% of the total impact for both technologies analysed. The energy conversion is the hotspot in most of the impacts studied (climate change —49-63%, particulate matter —94-95%, photochemical ozone formation —85-88% of, acidification —76-79%, freshwater eutrophication —56-58% and marine eutrophication —70-71% of the total impact) and therefore, this is the stage for which improvements should be primarily establishedestablished for both technologies analysed. In addition, for all impact categories analysed, the fluidised bed presented the smallest environmental impact. Even when the grate furnace efficiency increases and the fluidised bed efficiency decreases in a sensitivity analysis, the fluidised bed has lower impacts than the grate furnace and is a good alternative for implementing new power plants. Further research is needed to analyse the effects of converting the grate technology in Portugal to fluidised bed technology.