POWER December 2016 - 33

TOP PLANTS
1. One step at a time. The pivoting
combustion grate includes grate bars mounted
horizontally and vertically on shafts that rotate
60 degrees to produce wave-like motion,
moving fuel down the incline. Courtesy: B&W
Vølund
lizes mass burn technology in its three boilers
to combust up to 3,000 t/d of post-recycled
MSW. Steam from the boilers is directed to a
single General Electric steam turbine generator
set, with the power (net of in-house use)
supplied to Florida Power and Light Co. under
a long-term power purchase agreement.
The plant was designed, manufactured,
and constructed by a consortium of Babcock
& Wilcox (B&W) and KBR Inc. (Table 1).
Located on a 24-acre site, it was the first
greenfield WTE facility constructed in the
U.S. since 1995.
Designed to Efficiently Combust
Post-Recycled MSW
MSW-also known as garbage-is delivered
to PBREF 2 primarily by truck from six
transfer stations located within Palm Beach
County. The trucks use walking-floor trailers
to dump the MSW on the plant's tipping
floor. Three semi-automated refuse cranes,
manufactured by Konecranes, mix and manage
the waste in the refuse pit. The cranes
also deliver the MSW to the charging hoppers
at the inlet to each of the boilers.
The beauty of mass burn technology is
that the MSW does not have to be processed
prior to entering the boiler. SWA also owns
PBREF 1, but that plant is a refuse-derived
fuel (RDF) facility. RDF is likewise produced
from the combustible components of
post-recycled MSW, but the waste is shredded
to suitably size the material. It also
passes through a series of magnetic and eddy
current separation equipment to recover any
remaining ferrous and nonferrous metals before
it is delivered to the boiler's combustion
system. That whole process adds some cost.
The PBREF 2 boiler design includes a
B&W Vølund DynaGrate pivoting combustion
grate (Figure 1). The grate resembles a staircase
with the grate bars-alternately placed
horizontally and vertically-making up the
individual steps. The grate bars are mounted
December 2016 | POWER
on shafts, which slowly rotate 60 degrees back
and forth, producing a wave-like longitudinal
motion. The movement causes the fuel to mix
and tumble as it slowly travels down the inclined
grate, exposing fresh surface area for
combustion in the process. B&W says the
unique movement results in an excellent burnout
of the waste fuel with less than 0.25% total
organic carbon typically remaining in the ash.
Hydraulic ram actuators feed the MSW onto
the grate system.
PBREF 2's furnace tubes have a total Inconel
weld overlay to protect them from the
corrosive flue gases found in WTE boilers.
Other state-of-the-art features include a water-cooled
wear zone to minimize slagging,
erosion, and corrosion near the grate; minimal
use of refractory, which reduces maintenance
expense; and well-designed access to
superheater heat transfer surfaces to facilitate
replacement. The furnace is designed to permit
at least two seconds of residence time
above 1,800F to destroy dioxins and furans,
which can be formed during the combustion
of plastics in MSW. A B&W Precision Jet
overfire air system helps minimize the formation
of NOx and CO in the process, while
post-combustion control equipment reduces
other emissions. Each boiler is capable of
supplying 284,400 lb/hr of steam at 900 psi
and 830F to the turbine.
Advanced Emissions Controls
WTE facilities are subject to very stringent
emissions standards and must employ many
of the same control technologies used at
other thermal power plants. PBREF 2's emissions
are continuously monitored by a B&W
KVB-Enertec system. In addition to the combustion
controls, the plant injects powdered
activated carbon into the flue gas to control
dioxin, furan, and mercury emissions. It also
has a spray dryer absorber to remove acid
gases-primarily sulfur dioxide and hydrogen
chloride-and a pulse jet fabric filter
(baghouse) to remove particulate matter.
PBREF 2 is the first WTE plant in the U.S.
to have a selective catalytic reduction (SCR)
system installed for NOx control. It operates
a low-dust, low-temperature SCR system, installed
downstream of the baghouse. Vaporized
aqueous ammonia is injected through
a manifold and injection grid system into
the flue gases upstream of the catalyst reactor.
The flue gases then pass down through
ceramic catalyst blocks, where NOx is converted
to nitrogen and water vapor through a
reaction with the ammonia in the presence of
the catalyst (chiefly made of titanium oxide).
An added benefit of the SCR is that it further
reduces dioxins and furans that may remain
in the flue gases at that point in the process.
The plant's emissions have been stellar.
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POWER POINTS
Winning Attributes
First greenfield waste-to-energy
facility to come online in the U.S.
in 20 years
World-class emissions control
systems
Provides a municipal solid
waste management option that
produces electricity classified as
renewable by more than half of
U.S. states
During plant acceptance testing conducted
by an independent contractor, PBREF 2
achieved all of its permit emissions guarantees.
Many of the results were an order of
magnitude below the plant's emissions limits,
making it a best-in-class WTE facility.
For perspective, consider that in the 1980s,
before importance was placed on reducing
dioxin and furan emissions, some WTE facilities
had dioxin/furan emissions equivalent
to more than 10,000 nanograms per dry standard
cubic meter (ng/dscm). Of course, all
WTE plants have been upgraded since that
time, but during PBREF 2's testing, dioxin/
furan emissions were 0.21-0.36 ng/dscm,
which was far better than required (its permit
allows up to 10 ng/dscm).
And WTE plants are great recyclers.
PBREF 2's bottom and fly ash are conveyed
to an ash management building, where a rotary
magnet removes ferrous metals and an
eddy current separator removes nonferrous
metals from the ash stream. Around 2,000
tons of ferrous and 100 tons of nonferrous
metals are recovered by the system in a typical
month. The remaining ash is collected in
bunkers until it can be loaded onto trucks and
disposed of in a landfill.
" The completion of this state-of-the-art facility
represents the culmination of a 10-year
effort that virtually ensures to the taxpayers
of Palm Beach County that their solid waste
will be disposed of in the most efficient and
environmentally sensitive method available
in the world today long into the future, " Ray
Schauer, director of Engineering and Public
Works for SWA, said in a press release announcing
the grand opening of the plant.
" PBREF 2 is a jewel in the crown of SWA's
award winning Integrated Solid Waste Management
System that will set the standard for
performance and environmental sustainability
in North America. " ■
-Aaron Larson is a POWER associate
editor.
33
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POWER December 2016

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