Atlantic areas of southern Europe have low water restrictions but are also characterized by acid soils with low intrinsic fertility, so the selection of clones that have adapted to these characteristics is essential. In this study, biomass yield data for eight poplar clones established in two trials were evaluated after the first rotation. Both sites were representative of acid soils that had previously been used for agriculture and had low levels of alkali and alkaline earth elements. The first trial was used to determine which clones performed the best in terms of biomass and stem size, testing two Populus × euramericana (Dode) Guinier (I-214 and AF2), two P. × interamericana Van Brokehuizen × P. nigra L. (Monviso, AF6), three P. × interamericana (Unal, Beaupre and Raspalje), and one P. trichocarpa Torr & A. Gray (Trichobel) clone. The second trial explored the possibility of simultaneously growing biomass and timber, specifically considering the Raspalje and Trichobel balsam poplar clones. To complete a previous study on energy properties, nine biomass samples were obtained from each of the eight clones to evaluate the composition and behavior of the ash generated during combustion, particularly the sintering risk. Several indices of sintering risk were explored and compared with the actual sintering using the BioSlag test. The results show large differences in biomass yield between clones, with the balsam poplar derived clones (both hybrid or pure Populus trichocarpa) performing the best. Growth results for stems planted for wood and the cuttings planted in between these stems show that mixed biomass–timber arrangement provides good results, at least during the first rotation. The relative proportion of oxides in the poplar ash followed the order CaO > K₂O > MgO >> P₂O₅ >> SiO₂ > Al₂O₃ > Na₂O > Fe₂O₅ >> TiO₂. Significant differences between clones were found for K₂O and MgO. Risk indices showed moderate levels of sintering derived from alkali elements, with significant differences between clones. The actual slagging and the hardness of the slag particles were very low thus, denoting good ash behavior during combustion, particularly for the clones selected for biomass yield. No significant covariate effect of basal diameter was found for any of the analyzed variables. We conclude that growing site-undemanding poplar clones in acid soils can yield both reasonable levels of biomass yield and good quality chips for combustion in domestic thermal systems.