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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 electricity 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 Eastern bituminous Central Appalachian (CAPP) coal
costs are increasing due to rising exports, increasing
transportation and environmental costs and decreasing production, (Buchsbaum 2008; Metzroth
2008). To stay competitive, some utilities are investigating burning lower-cost, lower-quality “opportunity” coals such as Northern Appalachian (NAPP) and
Illinois Basin. The most efficient plants can be dispatched for longer periods for improved financial
performance. The change to lower rank coal and
increased operation can result in increased slag deposits 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 pressure. A proprietary mixture of chemical additives
was recommended to elevate ash fusion temperature 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 combination 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. Depending on the ratio of these minerals and compounds,
slagging and convective pass fouling can occur in
boilers. Slag formation accelerates when the furnace exit gas temperature (FEGT) exceeds the fusion 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 temperature, 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 availability.
CAPP coal typically has a high ash fusion temperature and less tendency to create excessive
slagging. NAPP coal is becoming more economically attractive for several reasons, including
availability at lower delivered costs than CAPP
coal (Pusateri 2009). Figure 1 illustrates the challenge of using NAPP coal with a lower ash fusion
Figure 1: Fuel Comparison and Basicity Ratio (Babcock & Wilcox, 1978)
temperature. Slag deposits are expected to be in
Fuel Type
Typical
Opportunity
** Basicity Ratio = (Fe2O3+CaO+MgO+Na2O+K2O)
(SiO2+Al2O3+TiO2)
Source
Central
Appalachian
Northern
Appalachian
Cost per
ton, $US
(2010)
$70 – $75
$58 – $70
HHV, Btu/lb
~12,000
~13,000
SO2,
lb/MMBtu
1.1 – 1.5
4.5 – 5.0
Ash, wt%
11 – 12
7 – 8
Moisture,
wt%
6.7 – 7.0
6.0 – 7.0
Ash Softening Temp,
deg F
2,700
2,250
Basicity
Ratio **
0.12 – 0.14
0.45 – 0.55
Ash, wt%
SiO2
Al2O3
Fe2O
3
K2O
TiO2
MgO
CaO
Na2O
53 – 56
28 – 30
5 – 6
3.3 – 3.6
1.3 – 1.5
0.9 – 1.0
0.7 – 1.3
0.2 – 0.3
39 – 40
20 – 21
22 – 24
1.3 – 1.4
0.85 – 0.95
1.05 – 1.15
5 – 6
0.95 – 1.05
liquid state at furnace temperatures with noncombustible mineral content present. Sootblower cleaning alone is not effective when the slag is a liquid or
pseudo-plastic state which deforms under pressure.
Fouling, which is closely related to slagging, usually
occurs in the boiler’s cooler convective back-pass
section as gaseous ash components (such as sodium and potassium) condense. It typically occurs in
the vertical and horizontal reheaters and primary
superheater. Fouling deposits can “bridge” across
tubes and restrict gas flow.
That increases induced fan horsepower, which
raises the plant heat rate and, therefore, lowers
plant efficiency. Slagging and fouling can result in
derating (shedding load) and costly unscheduled
outages and repairs from damaging slag falls. But
these problems can be eased by combining chemical additives for fireside applications with mechanical removal (sootblowers).
Boiler and Trial Design
The 745-MW pulverized coal-fired boiler is a Riley
Stoker Corporation front-wall fired boiler with
2,500,000 lbs/hr steam production at 2610 psig
and 1,005 deg F at superheater terminal outlet.
Page 2 Technical Paper
Figure 2: Trial Trailer and totes of additives (left) and applying product to NAPP coal (right)
The boiler fires 250 tons pulverized coal per hour at
maximum load, and the boiler train is equipped with
SCR, cold-side electrostatic precipitators and a wet
flue gas desulfurization (Wet FGD) scrubber system.
Trial results using the same opportunity fuel- NAPP
coal- at a sister station indicated it could not be
burned untreated, as the resulting slag was severe
enough to slag the boiler and block the gas path.
Operating experience indicated boiler conditions
could deteriorate within days of introducing opportunity fuel. To minimize the risks of boiler outage
during trial, the utility blended its typical fuel with a
small proportion of opportunity fuel treated with a
mix of proprietary chemical additives to reduce severity of fireside slagging. Product dosages were
optimized as the percentage of opportunity coal
was increased until it reached the target level of 50
percent.
Chemical Additives for Slag Control
A range of chemical additives were considered before the two products were selected based on ultimate analyses of the fuels. The proprietary mix of
additives selected for this trial included a magnesium based compound and a metal oxide. The magnesium is known in the industry to elevate ash
fusion temperatures due to the high melting point of
magnesium oxide. This treatment keeps the slag in
a solid state instead of liquid-phase deposit. The
metal oxide-based slurry contains copper which has
been used in the industry as a combustion catalyst.
Less well known is that copper can reduce the cohesive strength of the ash via a nucleating effect
with iron species. Gradual thermal decomposition of
the metal oxide product also makes the slag porous
and, therefore, weaker. These mechanisms complement the magnesium effect for certain types of
coals or coal blends, depending on the ratio of minerals and other non-combustible species. Together,
the proprietary additives create fracture planes in
the solidified slag, weakening the deposits so that
they can be more easily removed by sootblowers.
Treatment Application
The chemical additives were transferred from agitated trailer-mounted base totes to the coal belts
via peristaltic pumps, where the chemicals were
the dosed at predetermined amounts via a manifold mounted above the coal conveyor (Figure 2).
Dosing occurred when the coal belts conveyed
NAPP coal. Aqueous magnesium-based slurry
dosages were reduced from 3 lbs of product per
ton of NAPP coal to optimum of 1.0-1.5 lbs. Aqueous metal oxide slurry was introduced to determine its impact on slag mitigation in conjunction
with the magnesium-based product. It was determined that the optimum product feed rate was
0.25 lbs product per ton of NAPP coal. The NAPP
coal quantity was ramped up from 16 percent to
the target of 50 percent, where it was maintained
for a week until the end of the trial. The dynamic
test environment confronted the trial team with
challenges that included outages, inclement
weather, and real time adjustments to the dosage
based on visual observations of furnace slag conditions.
Trial Details
To be considered successful, the trial had to meet
several criteria, including:
1. Demonstrating that the magnesium content
increases the ash fusion temperature and,
therefore, makes the deposit more friable and
easily removable.
2. Demonstrating the metal oxide slurry syner-
gistically assists in slag mitigation.
Technical Paper Page 3
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