4. Conclusions
Detailed experiments on the combustion of isolated crude glycerol
droplets revealed that (at least, for the sample used in this
study) their evolution displays two distinct regions: after a first
stage in which the measured diameters follow the d2
-law, with
constant slope, the droplet is suddenly shattered into small fragments.
This behavior and the point at which the microexplosion
was observed were very repeatable, both among replicated tests
and for practically every single droplet.
In a second test series, a wide range of blends of pure glycerol
with water, methanol/ethanol and salts were studied in order to
investigate the role of the different components with respect to
the behavior observed for crude glycerol. Contrary to what was initially
expected, low boiling-point liquids like water, methanol or
ethanol did not produce any microexplosion. This phenomenon,
however, was observed in practically all the tests involving salts
although the intensity of the secondary atomization depended to
some extent on the amount of salt, for example the results for
blends with 2.5% NaCl were comparable to those for crude glycerol.
All these results point to alkali salts as the agent responsible for the
secondary atomization observed for crude glycerol droplets.
The work presented here is thought to provide novel results
with regard to the evolution of crude glycerol droplets, which
turned to be strongly influenced by the occurrence of disruptive
evaporation, as well as to the identification of the responsible
agent through systematic investigation of different pure glycerol
blends. Nevertheless, the tests reported here necessarily covered
a limited range of situations and further research would be needed
to describe the physical phenomena involved in the disruptive
evaporation of glycerol in the presence of salts as well as to determine
the range of conditions and compositions leading to microexplosions.
Among the practical consequences of this finding, it is
worth mentioning the fact that desalting of glycerol to reduce
ash-related problems in boilers might entail unexpected effects:
if alkali salts are confirmed to be the agent causing secondary
atomization, glycerol cleaning could lead to increased droplet
evaporation times. Although the final impact on flame characteris
tics and stability is very difficult to predict, this could result in
increased unburnt emission and/or the requirement for finer
atomization to ensure complete evaporation and combustion of
crude glycerol within the limited residence time available in the
combustion chamber.