This research has focussed on the impact of kaolin as additive on the agglomeration behaviours of willow, white wood, and miscanthus during their combustion processes in a laboratory-scale fixed bed whereby, Gooch ceramic crucible was used as the combustion chamber. It aimed at reducing agglomeration during the combustion of these selected problematic biomass fuels.
Biomass fuels are CO2 neutral and very rich in alkali metals especially potassium, K and sodium, Na with potassium displaying the predominant roles in the agglomeration formation of these selected biomass fuels. During the combustion processes, agglomerates were formed in the combustion chamber at 750 0 C and 802 0 C under the atmospheric pressure. This was attributed to the formation of eutectic compounds in the form of alkali-silicates (K-silicates or Na- silicates). The eutectic compound has a lower melting temperature than the melting temperature of either the alkali metals or the silica from sand, which is the bed material. It therefore melts abruptly in the bed and formed lumps in form of agglomerates. Energy Dispersive X-ray spectroscopy (EDX) carried out on the agglomerates indicated that, the interior of the agglomerates was dominated with silicon, Si from the sand while the exterior or the peripheries were preponderated with alkali metals potassium K, and sodium Na from the biomass fuels ash. Other trace elements present in the agglomerates as confirmed by EDX analyses are; Aluminium Al, Calcium Ca, Chlorine Cl, Iron Fe, Phosphorus P, and Magnesium Mg. Meanwhile, with the addition of additive (kaolin) Al2 Si2 O5 (OH)4 to the bed materials and the combustion processes repeated under the same operating conditions, no agglomerate was formed at 750 0 C and 802 0 C. However, eutectic compound in the form of alumina-alkali-silicate was formed at a higher melting temperature than the alkali from the biomass fuels and the silica from bed materials, therefore no agglomerate was formed at these temperatures (750 0 C and 802 0 C).
Factsage software was extensively utilized to predict the eutectic points (eutectic temperatures) on both the binary and the ternary phase diagrams. With the inclusion of additive (kaolin) in the bed materials, on the binary phase diagrams, agglomeration was predicted to occur in the combustion bed at 1200 0C if the biomass fuel is dominated by potassium, K. Consequently, if the biomass fuel is dominated by sodium, Na, agglomeration was predicted to occur at 1700 0 C in the combustion bed. However, on the ternary phase diagrams, with the addition of kaolin to the bed materials, initial agglomeration was predicted to occur at 1550 0 C if the biomass fuel is dominated by potassium, K but rose to 1700 0 C if the biomass fuel is dominated by sodium, Na. This justifies the affirmation that, Sodium, Na has a higher melting temperature than potassium, K.
Elongation in the biomass particle size from <1mm diameter before combustion to 7mm diameter in the agglomerates formed from the combustion of willow, and 10mm diameter in the agglomerates produced from the combustion of both the miscanthus and white wood is a clear manifestation that, agglomeration actually occurred in the bed. Post combustion analyses; Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM and EDX) carried out on the agglomerate samples also confirmed that, agglomeration took place in the bed. Huge agglomerates were formed at a lower melting temperature of 350 0C when potassium hydroxide, KOH and silica sand were heated directly (reality test). Harder and tougher agglomerates were produced at 502 0C. This confirmed that, agglomerates were produced from the formation of a low temperature alkali-silicate in the form of K-silicate.
The results of this research have indicated that, Gooch ceramic crucible is a reliable combustion chamber for the combustion of biomass fuels experiments/tests in a laboratory –scale fixed bed. It accommodated more heat distribution into the combustion chamber than the conventional ceramic crucible. Moreover, kaolin was also confirmed as an additive capable of reducing agglomeration during the combustion of biomass fuels in a laboratory - scale fixed bed and other combustion beds.
