Fuel characterization and biomass combustion properties of selected native woody shrub species from central Portugal and NW Spain

Selected native shrub species from central Portugal and NW Spain, Cytisus multiflorus (broom), Erica arborea (heath), Pterospartum tridentatum (carqueisa) and Ulex europaeus (gorse) were characterized for physical, thermal and chemical properties for combustion. The studied shrub species showed ash contents in the range 1–2%, nitrogen contents above 0.6%, and sulfur contents below 0.1% at both areas of study. A significant effect of species on Higher Heating Value (HHV) was observed, with no significant effect of the location of study, central Portugal or NW Spain. The higher HHV (24.4 MJ kg−1) was recorded for heath, this species also shows the higher average carbon and lower nitrogen content values of the studied species. Analysis of the chemical composition of the ashes revealed alkali metals contents of 24–30%, representing a potential sintering, fouling and bed agglomeration risk in the combustion of the studied species. Ash As, Cd, Pb, Co, Cu, Mn, Ni, Cr, and Zn content was below national and most of European legislation maximum levels for these elements for ash application as fertilizer, with the exception of some of the more conservative limits for Cd, Cu, Cr, and Zn from Northern and central European countries legislation.