Page 294 - Wastewater Solids Incineration Systems

Incinerator Maintenance 255

Sodium, potassium, and phosphorus in the presence of sulfur, chlorides, or silica
form compounds within the feed cake that have low (fusion) or melting tempera-
tures. A switch in dewatering chemicals from ferric chloride or lime to polymers can
also result in lower fusion temperatures.
In addition, skimmings such as scum, grease, and floatable materials removed
from wastewater—which, in many cases, are mixed with the feed cake before incin-
eration or are pumped separately to the incinerator—can cause excessive combustion
zone temperatures that also result in slag development.

1.1.2 Flame Impingement
Flame temperature of auxiliary fuel burners used to sustain the combustion process
also can cause slag. The style of the burner being used can increase or decrease the
possibility of slag formation depending on its operating flame temperature. The
actual flame temperatures within a burner are typically greater than the fusion tem-
perature of the inorganic material in the feed cake being burned. When a burner
flame impinges on any surface there is a high probability of slag buildup. This
buildup can occur on the sides of a burner tile, on the ceiling immediately above a
burner, and on rabble arms.

1.1.3 Hot Spots
Hot spots can occur in the MHF’s drop holes. For example, as hot gases are pulled
through a drop hole by the induced draft fan, their velocity increases, thereby
increasing the temperature. This “blow torch” effect is the primary reason why slag
tends to build up and plug drop holes.

1.2 Potential Methods to Minimize Slag Formation
There are several methods to minimize slag development. It is always advisable to
operate the incinerator with a combustion zone temperature as low as possible and
to limit operations of burners within the combustion zone. Using multiple burners
and avoiding use of a single burner operating at greater than 35 to 40% of its output
can minimize slag development.
Burner use above and below the combustion zone also affects slag develop-
ment. Greater use of burners above the combustion zone hearth to maintain a con-
sistent drying zone temperature and drying rate will reduce slag development.
In addition, slag is reduced by using a burner or burners below the combustion
zone to maintain a temperature differential between the drop holes in the combus-
tion zone and the hearth below of no more than 93°C (200°F). By maintaining that