The physical and thermochemical properties of three short rotation forestry crops (poplar, willow and black locust) have been determined and compared in order to evaluate their suitability (energy potential and combustion-related operation problems) to be used in energy devices. The planting density was 6666 in a rotation of four years and the analysis were performed at the end of the first rotation. The three species showed significant differences regarding production, moisture and ash content, heating value and bulk density. The results obtained allow to establish priorities between species according to the expectable combustion behavior, willow being the most appropriate. Results indicate no problems related to SO_x and NO_x emissions. Moreover, the alkali metals determined in the fuel together with the chlorine content suggest no operation problems related with fouling or harmful corrosion.

The combustion of two non-woody types of biomass (almond shells and exhausted olive cake) in a domestic boiler at different loads was studied in order to evaluate their suitability as fuels. To select the optimal boiler operating conditions (excess air, primary/secondary air ratio and grate vibration), which allows for lower CO and particulate matter emissions for each biomass and load, a statistical design of experiments was performed. Similar optimal operating conditions were found for both fuels at nominal load (excess air: 1.5, primary/secondary air ratio: 20/80), the grate vibration being the only parameter to be modified due to the different ash content (45 and 20 s for almond shells and exhausted olive cake, respectively). At partial load, a slightly higher excess air (1.6) and a higher proportion of primary air (50/50) were needed in the case of almond shells. Results showed higher CO and lower NO_x and PM emissions at partial load for both fuels. The high ash content of exhausted olive cake deteriorated its combustion process (accumulated ashes were observed in the fireplace). Gaseous and solid emissions did not fulfil the UNE-EN 303-5 limits for any fuel or condition; although, almond shells seem to be a much more suitable fuel since they could be used just blended with a small quantity of a high-quality biomass or additive. However, exhausted olive cake not only led to a very poor efficiency at partial load (74%), clearly below the minimum required by the standard (77%), but also to an unacceptable pollutant emission level. So, this latter fuel would require a high blending ratio with another type of biomass, pre-treatments for reducing the alkali compounds and/or significant technological modifications allowing for a proper ash handling.