The ash removal for each element is presented in Fig. 3aef for
each biomass sample that was investigated. The results indicate
that the alkali metals, K and Na, and P were easy to remove and
exhibited over 80% removal rate with any of the applied pretreatment
methods. The removal of Mg, Fe and Al was lower,
following the order Mg > Fe > Al. However, their initial absolute
concentration was small in almost all samples; therefore this
increased difficulty did not significantly affect the total ash removal.
Calcium, which had a high initial value on all samples, especially in
the agricultural residues, was the element that was most affected
by the treatment method and its removal increased in the order
nitric acid > acetic acid > water treatment. The data also show that
calcium was less easily removed from eucalyptus. On the other
hand, its removal was easier in the rest of the biomass types, which
however had lower Ca content to begin with. In any case, the
treatment with nitric acid was efficient in removing all of the
abundant inorganic elements, namely K, Ca and in some cases Na,
from all types of investigated biomass samples.