POWER October 2012 - 40

TOP PLANTS
However, a 70% petcoke mix required two
to four outages a year to remove ash buildup
and agglomeration. The plant also faced other
challenges, such as limestone feed problems
and stripper cooler plugging, cyclone furnace
problems, and myriad Intrex superheater tube
failures, to name the worst offenders. During
calendar years 2003 and 2004, the plant averaged
6,146 hours of operation with a 62.3%
capacity factor and an equivalent availability
factor of 65.8%-well below industry averages.
Ash agglomeration and performance of the
Intrex units had become chronic problems.
Each boiler has three enclosures, and each
box contains both a loop seal and superheater
coil with the loop seals submerged in a bath
of semi-molten 1,700F ash (Figure 1). Intrex
operational problems included superheater
tube support failures and cracking of superheater
tubes, plus agglomeration of hot loop
material within the boxes whenever less
than 30% coal was burned. By the close of
the two-year demonstration period, the same
two problems remained unresolved: agglomeration
of ash on heat transfer surfaces in the
boiler and Intrex units and poor mechanical
performance of the Intrex units. Together,
these two problems accounted for a majority
of the 13.3% equivalent forced outage rate
(EFOR) average for both CFBs from 2003 to
2008.
In the words used in the final report (June
2005-three years after startup), " long term
modifications to mitigate these issues on
Northside CFB boilers are ongoing. " One
lesson learned by Foster Wheeler during
the two-year demonstration period was that
" the Intrex design is not viable, and [Foster
Wheeler] is no longer offering this design
feature on new CFB boilers. "
Ending Ash Agglomeration
JEA chose to forgo a plant modification to
eliminate the Intrex superheater that cost an
estimated $40 million per unit and required
extensive outage time. Instead, it attacked the
problems through data analysis and continuous
improvement of the existing systems.
Solving the ash agglomeration problem
required a long-term design and test project
to produce permanent and reliable solutions.
Rather than discuss those details in the limited
space available in this article, a companion
report, " Reducing Ash Agglomeration in
JEA's CFB Boilers, " is included in this issue
(p. 52). That article focuses on the process
used to identify the particular agglomeration
mechanisms at work and the permanent solution
that was implemented at NGS. In brief,
the injection of a specific reagent at a rate of
1.8% of the fuel flow (by weight) has virtually
eliminated ash agglomeration in the CFB
boilers. That permanent solution was implemented
in June 2008.
Intrex Superheater Fix
Despite the many CFB improvements and
modifications made between 2006 and 2008,
the Intrex remained extremely problematic,
causing frequent shutdowns and power output
limitations. Generally, there were two interrelated
phenomena:
1. Chasing agglomerated ash. The circulating fluidized bed boiler produces steam
in the furnace section but superheats the steam in the Intrex units and in other superheater
tubes, as shown. The ash-fuel-limestone mix circulates vertically from the furnace, through the
cyclones and Intrex units, and then back to the furnace via the loop seal. Ash agglomeration and
mechanical design problems with the Intrex were a chronic problem since plant startup in 2002.
Source: JEA
■ Ash buildup (agglomeration) in the Intrex
caused loss of fluidization; plugging of the
cyclones quickly followed by loss of steam
temperature control. The solids buildup
was first observed during furnace startup in
2002.
■ Frequent tube leaks in the Intrex superheater
tubes were due to stress-corrosion
cracking. The root cause was ash buildup
on the tubes that resisted tube thermal expansion,
occasional overheating, and the
ash-filled corrosive environment within
each Intrex.
Between 2007 and 2009, Intrex performance
continued to improve through data analysis and
process optimization. Final modifications were
made to the Intrex in the fall of 2009 during a
tube replacement outage. NGS staff performed
a complete in-house redesign of the Intrex. The
designers realized well-controlled airflow was
essential to ensure good circulation within the
Intrex units to prevent agglomeration of solids
on the tubes and for improved combustion
in the CFB. With those changes in mind, the
following modifications were made to each
Intrex:
■ Tube supports that improved fluidization
and eliminated ash buildup from restricting
tube thermal expansion were added and/or
moved.
■ The weir walls (adjacent to the superheater
tubes) were replaced with an ash-retaining
wall to aid furnace circulation.
■ Return cells that eject ash from each Intrex
were redesigned.
■ Intrex fluidization air supply and valves
were replaced.
Top Tier Statistics
The NGS CFBs, which currently burn up to
90% petcoke, had an average 0.7% EFOR for
fiscal year (FY) 2011 (starting Oct. 1, 2010)
and have averaged 0.25% EFOR thus far in
FY2012. These operating statistics place NGS
in the top tier of U.S. fossil plants.
In addition, the CFBs have seen a signifiCoal
silo
(5)
Cyclones
(3)
Air heater
Furnace
Intrex (3)
Stripper cooler
40
www.powermag.com
Primary superboiler
Reheat superheater
cant improvement in long-term heat rate with
these improvements, and forced outage costs
have nearly disappeared. JEA also has been
able to avoid over $90 million in capital projects
such as further boiler redesigns, Intrex replacements,
and stripper cooler replacement.
Finally, during the 2009 fall outage, the
original 1966-vintage steam turbine low-pressure
rotor was replaced in each unit. Together
with other modifications, the output of each
unit was boosted by about 15 MW, further
improving each unit's heat rate and producing
significantly higher power sale revenue. ■
-Dr. Robert Peltier, PE is POWER's
editor-in-chief.
POWER | October 2012

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POWER October 2012

Table of Contents for the Digital Edition of POWER October 2012