Here, some comments should also be given to quite ‘rough’
structure of the steel tube cross-sections shown in Fig. 4. The
roughness of the tube surfaces was not brought about by corrosion
but was the result of authors' attempts to analyze the sample in a
simple way and without any sophisticated treatment. Accordingly,
after each test the lateral surface of the sample was just roughly
brushed on a sandpaper and analyzed without any prior surface
preparation, fine polishing, etc.
The contribution of alkali compounds to the formation of deposits
on steel tube samples was confirmed by the data shown in
Fig. 5. In each case the XRD patterns indicated the presence of
complex silica-alumina-alkali compounds, such as e.g. Al2SiO5,
CaAl2O4, KNaCO3, or Mg(OH)Cl. Regardless of fuel type the XRD
results also indicated significant contents of CaO in all samples. The
presence of calcium oxide was the result of limestone injection into
the boiler combustion chamber in order to control the capture of
sulfur.