Agriculture Reference
In-Depth Information
It should be noted that soon after raffinate was fed onto the bed, freeboard temperature
became higher than BT, see Figure 9. The rise in freeboard temperature was observed to be
around 50 °C with a peak value of around 100 °C just before raffinate feed was switched off.
It is evident from the figure that agglomeration and complete de-fluidization are preceded by
changes in temperatures and pressures in and across the bed. It is also clear that a
considerable proportion of the raffinate was burning above the bed resulting in higher
freeboard temperature.
From the figure (9) it can be seen that after switching off the fuel feed there are two peaks
in bed temperature when it dropped down to 400 °C and another peak when BT went down to
200 °C. The peaks can be attributed to the burning of un-combusted coal/char in the
agglomerated bed material and to the inherent property of agglomeration to cause local high
temperatures.
Bed temperature of 874 °C as observed during the last moments before bed slump could
give a vital information about the behaviour of raffinate combustion in a fluidized bed.
Burning coal char particles might have considerably higher temperature than the bed
temperature measured by thermocouple. This higher particle temperatures or hot spots can
initiate agglomeration phenomenon locally which then extends throughout the bed due to the
fact that these agglomerates act as nuclei to attract other particles to form bigger
agglomerates.
Post experiment bed examination showed that top of the bed was fluidizable. However,
there was an evidence of the presence of white particles possibly coated with sticky material
responsible for agglomeration and bed slump. The particles were also bigger than the original
sand particles fed into the bed. At the bottom of the bed, just above the stand pipes, there was
a big lump of the agglomerated bed material, see Figure 10. The lump was possibly formed
on the top of the bed, close to the raffinate feed point, then settled down to the bottom of the
bed i.e sand from the bottom had been blown to the top and eventually resulted in settling
down the lump. There is also a possibility that stickiness of raffinate may have resulted in the
formation of lump which then strengthened over time due to high temperature.
Samples of the bed material, one from the top of the bed and one from the agglomerated
lump, were analysed by SEM to check the possibility of the presence of alkalis responsible
for the bed agglomeration. The results of SEM are given in Table 18 and SEM scan is shown
in Figure 11. It can be seen from the table that surprisingly concentration of alkalis in the
lump is considerably lower than that in the top of the bed. Thus, it can be thought that the
lump was formed during the early stages of the raffinate feeding when BT and BP first started
to fell down. Also it is possible that the lump became strengthened because of subjection to
high temperature over a period of time and was unbreakable by the force of the air unlike the
rest of the bed. As it is mentioned earlier that top of the bed was un-agglomerated. It can be
concluded that when BT came down, melted phase solidified and push of the air helped to
break the loosely bonded lump particles down to individual particles.
Also there were found some small lumps of agglomerated sand which were easily
breakable by hand. The reason for these could be that they settled down onto the bed parts
which were stagnant or un-fluidized and thus were not broken by the force of the air.
