Al-containing additives, however, help to transform potassium
silicates to potassium aluminum silicates (e.g. leucite) [38,39],
causing an increase of the ash melting temperature. This fact is
illustrated in Fig. 1: The ternary K2O–SiO2–Al2O3 system [40]
shows the correlation between melting temperatures and concentration
of K2O–SiO2–Al2O3. The Al concentration of pure wood and
straw is very low (both about 50 mgAl/kgBio [37]) and is strongly increased
by adding Kaolin. Three marked points in the K2O–SiO2–
Al2O3 system show the composition of pellets (20 wt.% wood and
80 wt.% straw) with 0, 1 and 2 wt.% kaolin, respectively. These
points move to the corner of Al2O3 (bottom right) with higher kaolin
concentrations thus leading to higher melting points of the
mixture.