GE P&W FuelSolv - Slag Control Treatment Program at a Southeastern Utility - Manual (Page 3)

Technical Paper Page 3
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 oppor-
tunity 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 se-
verity 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 be-
fore the two products were selected based on ulti-
mate analyses of the fuels. The proprietary mix of
additives selected for this trial included a magnesi-
um based compound and a metal oxide. The mag-
nesium 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 co-
hesive 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 com-
plement the magnesium effect for certain types of
coals or coal blends, depending on the ratio of min-
erals 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 agi-
tated trailer-mounted base totes to the coal belts
via peristaltic pumps, where the chemicals were
the dosed at predetermined amounts via a mani-
fold 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. Aque-
ous metal oxide slurry was introduced to deter-
mine its impact on slag mitigation in conjunction
with the magnesium-based product. It was de-
termined 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 con-
ditions.
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.