Effect of mechanical harvesting on the chemical composition and combustion behaviour of shrub biomass

Abstract

Shrub lands are an important source of genetic resources and raw materials for a bioeconomy-based future. The main objective of this study is to determine the effect of mechanical harvesting on the chemical composition and the combustion quality properties of typical shrub formations in the Mediterranean area, whose prevailing species are broom, rockrose and gorse. Above ground, shrub biomass was collected manually and mechanically and its main properties and chemical composition were compared. Its combustion behaviour was predicted by using fuel indices related to emissions, deposit build-up and ash melting. The predictions were evaluated after performing combustion tests with mechanically harvested shrub pellets. The chemical composition and combustion behaviour of the biomass differed greatly depending on the prevailing species. Mechanised harvesting can cause the ash content and the levels of several elements, particularly Si, to increase through the inclusion of soil particles, which influence combustion behaviour. The Si/(Ca+Mg) index appears to be a good indicator to predict the slagging tendency of these fuels. However, the K+Na+Zn+Pb index could not accurately predict aerosol emissions and ash deposition. The information provided by this index should be complemented by using the Si/K index, which considers the influence of Si on alkali retention.