POWER October 2011 - 18

JEA Increases Power Output Through
CFB Improvements
JEA's Northside Generating Station in Jacksonville, Fla., Units 1
and 2 were built in 1966 and 1972, respectively, although the Unit
2 boiler had not operated since 1983. Both were heavy oil- and
natural gas-fired steam units rated at about 300 MW. The utility
" repowered " those two units by removing the old boilers and
adding new circulating fluidized bed (CFB) boilers (Figure 1) that
entered service in 2002. At that time, they were the world's two
largest CFBs, and the plant won POWER's Plant of the Year Award.
The CFB burns crushed limestone (sorbent), which is injected
into the bed where the coal or petroleum coke is burned,
absorbing the released sulfur. A polishing spray dryer absorber
completes the sulfur-removal process. The low temperature of
combustion, about 1,650F, also minimizes NOx
production. The
bed of ash, solid fuel, and sorbent is fluidized by air accelerated
through nozzles located in the bottom of the bed. The fluidizing
air also supplies most of the needed combustion air.
As the solid fuel particles burn and reduce in size, the fluidizing
air will carry the light particles upward through the boiler with the
combustion gases. These particles are then separated in a cyclone
and recycled back to the bed, the " circulating " part of the CFB
boiler system. Ash is removed from the bottom of the bed through
a stripper cooler that recovers heat as new fuel and sorbent are
added. A fabric filter removes any remaining particles entrained in
the flue gas. Uniquely, steam is superheated in tubes placed in the
solids circulating stream and in the flue gas path.
As is often the case, deploying state-of-the-art technology
often means that plant upgrades and operational improvements
will closely follow emerging technology advancements prompted
by actual operating experience. This is what JEA has experienced
with the two new CFB boilers.
Solving Operating Problems
At Northside, concerns arose about boiler reliability and load
reductions principally caused by unstable main steam temperatures
that were in turn caused by high bed pressures. High bed
pressures were caused by too much limestone and ash in the
furnace bed and overfiring the CFB. Another side effect of these
1. World record holder. JEA " repowered " two old steam boilers
at the Northside Generating Station with circulating fluidized bed
(CFB) boilers. When installed in 2002, the two CFB boilers were the
largest in the world. The steam turbine portion of the plant remained
unchanged. Courtesy: JEA
off-design conditions was high ammonia consumption. Ammonia
is injected into the backpass (cyclone inlet) area of the boiler for
additional control of NOx
production.
Beginning in 2006, many improvements were made to the
plant that included upgrading the air quality control system, improving
stripper cooler reliability, and optimizing fuel/limestone
mixtures. These and other modifications produced immediate
benefits. For example, correcting hot combustion spots in the
furnace resulted in lower ammonia use, saving $1.5 million per
year. Better control of furnace temperatures also reduced coal
usage and improved ash quality (ash is resold into the building
materials market), resulting in lower cost of generation. In
addition, reduced limestone use improved coal combustion and
increased boiler efficiency.
Further improvements to the plant were made in the fall of
2009. One major improvement was replacement of the original
1966-vintage steam turbine low-pressure rotor in each unit. Redesign
of the integrated recycle heat exchangers (known as " Intrex, "
located in the return leg from each cyclone and used for
steam superheating) and other modifications have also boosted
unit output about 15 MW per unit, producing significantly higher
power sale revenue.
Modifying the Intrex
The Intrex are integral to the CFB furnaces and function to reduce
the height of the bed material in each furnace bed for improved
combustion and steam production, operating at about 1,600F to
1,650F. Furnace temperature is directly controlled by changing
the solids loading in the upper furnace, by control of the primary
and secondary airflow, and by efficient flow of solids over the
Intrex tubes.
Heat transfer problems occurred previously in the Intrex units
when solids agglomerated on the tubes, reducing the airflow and
heat exchanger performance. The solids buildup was first observed
during initial furnace start-up in 2002. The replacement cost of
each Intrex is about $1 million. Despite the CFB improvements
made between 2006 and 2008, Intrex airflow issues continued to
limit power generation (Figure 2).
There were other problems related to ash agglomeration. The original
10-inch damper valves controlling airflow to the Intrex units
would stick when completely closed, requiring a 5% limit stop to be
placed in the control system to prevent full closure. As a result, un2.
Intrex tube shelf problem. The end tube support design
promotes agglomeration under the superheater tube bundle and on the
return wall. Source: JEA
Ash recirculation
due to lack up
upward air flow
Tube shelf
agglomeration
18
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POWER | October 2011
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POWER October 2011

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