GE P&W FuelSolv - Slag Control Treatment Program at a Southeastern Utility - Manual

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TP1189EN.doc Jun-11
Technical
Paper
Slag Control Treatment Program at a
Southeastern Utility
Authors:
M. Domingo Tubio, Product Applications Engineer
Rick Higginbotham, Account Executive
Abstract
Coal-fired power plants supply over half the electric-
ity to the US grid. Currently, utilities are facing a
range of challenges including decreasing industrial
demand for electricity, competition from low cost
natural gas and rising coal prices. High quality East-
ern bituminous Central Appalachian (CAPP) coal
costs are increasing due to rising exports, increasing
transportation and environmental costs and de-
creasing production, (Buchsbaum 2008; Metzroth
2008). To stay competitive, some utilities are investi-
gating burning lower-cost, lower-quality “opportuni-
ty” coals such as Northern Appalachian (NAPP) and
Illinois Basin. The most efficient plants can be dis-
patched for longer periods for improved financial
performance. The change to lower rank coal and
increased operation can result in increased slag de-
posits in the furnace and superheater areas, (Gabriel
2011).
A Southeastern utility desired to blend lower-cost
low ash fusion temperature Northern Appalachian
(NAPP) coal with their typical CAPP coal in their 745
MW pulverized-coal boiler. Sootblower cleaning
alone is not effective when slag deposits are a liquid
or pseudo-plastic state which deforms under pres-
sure. A proprietary mixture of chemical additives
was recommended to elevate ash fusion tempera-
ture and modify the deposit to make it more easily
removable by sootblowers. The blend is a unique
combination of water-soluble magnesium hydroxide
and copper oxide slurries which has a synergistic
effect when used together to mitigate slag for-
mation and impact. During the fourth quarter of
2010, the utility consumed over 44,000 tons of
NAPP opportunity coal treated with this combina-
tion of proprietary fireside chemical additives over
a four week period. GE’s approach allowed the
customer to minimize the detrimental effects of
burning slag prone coal while reducing fuel costs.
This paper summarizes the trial and performance
results.
Slag and Fouling Formation and Cost
There are numerous non-combustible inorganic
impurities in coal besides hydrocarbons. Depend-
ing on the ratio of these minerals and compounds,
slagging and convective pass fouling can occur in
boilers. Slag formation accelerates when the fur-
nace exit gas temperature (FEGT) exceeds the fu-
sion temperature of the ash. Indices such as the
basicity ratio can help predict slag viscosity and
ash fusion temperature (Babcock & Wilcox 1978).
As slag density increases with time and tempera-
ture, a deposit is formed that is difficult to remove
with sootblowing. Deposits can “grow” as particles
accumulate; it is not uncommon to observe large
deposits on the leading edge of platen superheat
tubes and secondary superheater tubes above the
bull nose of the boiler. When the slag eventually
falls it can damage tube banks lower in the boiler,
resulting in unscheduled outages and lower avail-
ability.
CAPP coal typically has a high ash fusion tem-
perature and less tendency to create excessive
slagging. NAPP coal is becoming more economi-
cally attractive for several reasons, including
availability at lower delivered costs than CAPP
coal (Pusateri 2009). Figure 1 illustrates the chal-
lenge of using NAPP coal with a lower ash fusion