Slagging Propensity
The root cause of furnace slagging is actual very simple,
slagging will occur if furnace exit gas temperatures (F.E.G.T.) exceeds
the fusion temperature of ash from the coal being fired. To
ensure operation without slagging, we typically recommend that F.E.G.T.
be 100°F to 150°F below the ash softening temperature. Higher fineness,
proper primary (pulverizer) airflow and good fuel balance reduce FEGT by
allowing combustion to complete lower in the furnace. When combustion
is completed lower in the furnace, the waterwalls absorb a higher amount
of total heat release by the fuel and FEGT is reduced.
Reducing
ash fusion temperatures are always lower than those in an Oxidizing
atmosphere; a reducing atmosphere is defined as any point within the
furnace cavity in which the available excess oxygen content is <
0.5%. Poor fuel balance resulting from poor fineness can result in areas
of reducing atmosphere in the furnace, this will result in localized
areas where ash melts into a slag, due to lower melting points caused by
a reducing atmosphere.
Poor
fineness is also frequently responsible for higher than desired Reheat
or Superheat De- Superheating Spray Flows, heat that is not absorbed by
the waterwalls must be absorbed the superheater or reheater. Additional
heat absorbed by the superheat or reheater will then have to be offset
by de-superheating spray flow.
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