POWER April 2013 - 26

COAL PLANT OPERATIONS
Cyclone Furnaces Are Unique
The Paradise Fossil Plant Unit 2's new
long-run record has added significance for
those familiar with the peculiarities of the
cyclone furnace. There are 109 Babcock
& Wilcox Co. (B&W) cyclone-fired boilers
in service in the U.S.; 98 are subcritical
and 11 are supercritical, including Paradise
Unit 3. This number represents perhaps 6%
of all utility-owned units. TVA owns a total
of five subcritical cyclone-fire units: the
two units at Paradise and three at the Allen
Fossil Plant.
The cyclone furnace was developed
by B&W to combust coal fuels that were
not compatible with pulverized coal (PC)
combustion. The weightiest difference between
the two coals is the ash fusion or
melting temperature. Coals with a low ash
fusion temperature easily form slag that is
then carried through the remainder of the
furnace, often sticking to the superheater
tubes when burned in a PC furnace. The
cyclone furnace, normally smaller than the
equivalent PC furnace, was designed to
handle coals with a low ash fusion temperature
(Figure 3).
The cyclone furnace is water-cooled
and attached to the side of the boiler furnace.
To give perspective on the size of
the cyclone furnace, Paradise Units 1 and
2 each have 14 of the 10-foot-diameter cyclones-seven
each on the front and back
wall of the furnace. Steam is produced at
2,450 psia and 1,003F.
The PC boiler burns fuel that has been
dried and ground to the consistency of talcum
powder, usually around 74 microns,
and it burns without difficulty when mixed
with the turbulent combustion air in the
furnace. The cyclone furnace burns larger
crushed coal particles (nominal 0.187 in
or 4.75 mm, depending on the fuel). These
particles are too large to be burned in suspension,
as occurs in the PC furnace, and
would just be carried through the furnace
before complete combustion could occur.
According to B&W's Steam 41st Edition,
" The large particles must be retained in
place with the air passing over the particle
(air scrubbing) for complete combustion
to occur. The cyclone furnace accomplishes
this by forming a molten sticky slag layer
that captures and holds the heavier particles.
While the large particles are trapped
in the slag layer, the volatiles and fine
coal particles burn in suspension, providing
the intense radiant heat required for
slag layer combustion. Ideally, all of the
large coal particles become trapped in the
molten slag, where they complete carbon
burnout, leaving behind ash to replenish
the slag layer. "
Within its first year of commercial operation,
each unit equipped with cyclones
began experiencing failures of cyclone waterwall
tubes. According to TVA, by 1972
most of the crotch tubes, reentrant throat
tubes, wrapper tubes, and face tubes had
3. Capable combustor. The cyclone furnace provides a means to fire lower-quality
fuels, such as low-volatile bituminous coals and lignite. Shown is a cutaway drawing of a typical
cyclone (left) and a photo of an original cyclone from Unit 3 (right). Courtesy: Babcock &
Wilcox Co., TVA
Coal deslagging
oil burner
Crushed
coal inlet
Tertiary air
Primary air
Radical burner
Main oil burner
Replaceable
wear liners
Re-entrant
throat
Slag tap opening
Secondary air Gas burners
been replaced at least once. A manpowerintensive
program of frequent, proactive
tube replacements continued through
1982. During this period, the Unit 1 and
2 forced outage rate (FOR) grew from just
under 20% in 1964 to a peak of approximately
42% in 1979 (20.5% attributable
to a steam turbine failure), dropping back
down to about 20% in 1982. The overall
FOR trend was unacceptably high, about
20% for almost 20 years, with about threequarters
of the FOR caused by cyclone furnace
problems. In 1982, 10 forced outages
were attributed to cyclones.
TVA elected to replace all cyclones in a
single scheduled outage, incorporating advances
in materials to solve problems created
by corrosion and erosion of materials
based on fleetwide lessons learned by B&W.
Perhaps the two most prevalent problems
were the corrosive iron sulfide attack on
pressure part tubing and erosion in areas
opposite the secondary air throat, where a
protective slag coating can't form.
Further upgrades to the cyclone furnaces
were made to reduce NOx
production,
in conjunction with the selective catalytic
reduction (SCR) system installation. The
cyclones were updated with air staging
to reduce NOx
zone by reducing the oxygen present. The
reburning or air-staging retrofit produced
NOx
formation within the burner
reduction up to 80%. Air staging controls
the amount of primary air sent to each
cyclone and the overfire air port located
higher on the furnace wall; about 80% of
the combustion air goes to the cyclone located
in the sub-stoichiometric (lower) region
of the furnace and 20% to the overfire
air port located in the burnout zone located
high in the furnace. With this design,
an acceptable residence time is possible
in the burnout zone to complete combustion
of the fuel. The downside of using air
staging is the reducing atmosphere formed
in the lower regions of the furnace, which
often accelerates tube corrosion and flyash
production.
The cyclone design doesn't lend itself
well to rapid load cycling. However, Units
1 and 2 are capable of operating at reduced
load, down to 400 MW to 450 MW, and Unit
3 can operate down to 650 MW. The minimum
flue gas temperature entering the SCR
determines the low-load limit.
26
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POWER April 2013

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