In recovery boilers, plugging of the passes occurs due to the reducing atmosphere in the furnace combined with concentrations of trace salts containing chlorides, silicates, potassium and other complex salts, forming depositions too strong for sootblowers alone. This deposit insulates the superheater surfaces, reducing heat absorption from the flue gas, and reducing the amount of further deposition in the superheater. This insulation is seen by lower superheater steam outlet temperature. The hotter flue gas then deposits material in the next section of the boiler, which is usually the generator bank. Since this area is more closely spaced on the assumption that it will never receive these deposits, it lodges solidly with ash, such that sootblowers cannot remove the deposits effectively.
The unit now begins to exhibit signs of obstructed passes in addition to the higher flue gas temperature and lower superheater outlet steam temperature. Efforts to use sootblowers in the generator passes to remove the offending build-up effectively reduce sootblower usage in the superheater, which just makes the problem even worse.
Much as with other fuels, the continuous application of CoMate® to a recovery furnace will cause deposits to become significantly more friable since CoMate® interrupts the homogeneity of the crystalline ash. Combined with a sootblowing program designed to maintain constant temperature entering the convective passes of the furnace, CoMate® can maintain unit cleanliness between scheduled inspection outages. This can have tremendously positive impacts on the boiler’s operation, and the plant’s bottom line.
When fuel is sprayed into a recovery furnace, it first dries, then volatizes, heats up and begins to combust on the liquor bed in the presence of oxygen from primary air, reduction of sulfates to sulfides and secondary air. The gas evolving from this process is predominantly carbon monoxide (CO) and water vapor, but will also contain so-called “fume” – ionized elements and compounds (principally sodium). As the gas rises through the tertiary air zone of the furnace, it burns to carbon dioxide and further increases in temperature. The hot flue gas contains, at this point, almost no component other than gas in a properly operated furnace. Many variables are corrected to ensure that no low temperature solids are included.CoMate® is injected at strategic points in the furnace so that it will take part in the ionizing process and break down into its active components, well mixed with the ions which will eventually cool and become deposits on the furnace surfaces.
For most recovery boiler designs, when the flue gas enters the superheater, it is at 927 °C to 982 °C under normal circumstances.
The superheater cools this gas through its sticky temperature (816 °C) down to between 566 °C and 649 °C. This is the design temperature for the next set of surfaces exposed to the flue gas. If the superheater becomes insulated with deposit, this temperature will rise unless the sootblowers remove the deposit. CoMate® also deposits, but the mixture of CoMate® and other salts is of a much lower strength, thus allowing the deposit to be removed, often by gravity alone during operation.
It is also important to remember that sootblowers need to be operated in the superheater more frequently than in other zones of the furnace in order to control the flue gas temperature. Unnecessarily operating the sootblowers in other areas will affect this temperature control adversely and cause an increased rate of pluggage.
We invite you to get in touch with us, as we would be happy to review your application with you to see if your system is a good candidate for our technology.