Wet sludges cannot be incinerated because of the large amounts of
fuel that would be needed. However, a filter cake of sludge or a
well-thickened sludge may be burned. If the solids concentration
can be raised to roughly 30 to 40 per cent by weight, the heat of
combustion of the organic material will about match the heat of
vaporization of the water, thus minimizing or eliminating the need
for fuel except for start up. If solid wastes are being
incinerated, their fuel value may be enough to compensate for adding
sludge that is not very dry. At 35 per cent solids, a sludge
resembles common dirt.
Although there are many incinerator designs, the rotary hearth is
quite common. Here multiple hearths are arranged vertically, with
rakes moving the solids across one to fall to the next. An animated
computer sketch of such a unit is needed.
The gas entering is usually air except at startup when fuel is needed to initiate burning.
Gases rising from the combustion zone drive off moisture from the feed. Below the combustion
zone, the solids are still red hot and react with the air to oxidize the last traces of anything
that will burn leaving only mineral ash.
Much more detailed information about incineration