POWER October 2012 - 53

COAL COMBUSTION
Inside the JEA Circulating Fluidized Boiler System
The coal burned in the Northside Generating
Station moves through three separate portions
of the gas path before the remaining
gases are released into the atmosphere: the
circulating fluidized bed (CFB) boiler; the
cyclone, Intrex, and other heat exchangers;
and the post-combustion air quality steps,
such as flue gas desulfurization and particulate
removal systems and stack.
CFB Boiler
The fuel-handling system receives petroleum
coke, coal, and limestone and conveys it to
stock-out and storage areas. The materials
are reclaimed and conveyed to the in-plant
fuel silos and to the limestone preparation
system for limestone sorbent.
Coal fuel blends, along with primary air
and a solid sorbent such as limestone, are
introduced into the lower part of the combustor,
where initial combustion occurs (Figure
3). As the fuel particles decrease in size
due to combustion, they are carried higher
in the combustor, where secondary air is
introduced. As the particles continue to be
reduced in size, the fuel, along with some
of the sorbent, is carried out of the combustor,
collected in a cyclone separator, and
recycled to the lower portion of the combustor.
Primary removal of sulfur is achieved
by reaction with the sorbent (limestone) in
the bed. Additional SO2
removal is achieved
through the use of a downstream polishing
scrubber using a spray dryer absorber. Fabric
filters are used for particulate control.
Furnace temperature is maintained in
the range of 1,500F to 1,700F by efficient
heat transfer between the fluid bed and the
water walls in the boiler. This relatively low
operating temperature inherently results in
appreciably lower NOx
emissions compared
with pulverized coal-fired power plants.
However, the project also includes a new selective
noncatalytic reduction system, using
reaction with ammonia to further reduce NOx
emissions to very low levels, as required by
the stringent environmental regulations for
the JEA project.
Steam is generated in tubes placed along
the walls of the combustor and superheated
in tube bundles placed downstream of
the particulate separator to protect against
erosion. The system produces approximately
2 million lb/hr of main steam at 2,500
psig and 1,000F, and 1.73 million lb/hr
of reheat steam at 548 psig and 1,000F.
The steam flows to the turbine/generator,
where electric power is produced. The
design heat rate is 9,950 Btu/kWh (34%
overall thermal efficiency, higher heating
value basis).
As the solid mixture of burning fuel and
sorbent rises, it ignites and begins a controlled
" slow burn. " The slow burn process
maintains temperatures below 1,600F across
a large area, minimizing the production of
pollutants. At temperatures above 1,600F,
production of NOx
increases significantly.
3. Process diagram for JEA's Northside Generating Station.
Source: DOE/JEA
Air
Water
High-pressure steam
Particulate
Feedwater
Ammonia
injection
Coal/coke Limestone
Secondary
air
Steam
Cyclone
Circulating
fluidized-bed
boiler
Intrex
Air
Polishing
scrubber
Reheat
Air
Bottom
ash
To byproduct storage
October 2012 | POWER
Steam turbine
Generator
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Air
exchanger
Steam
Condenser
To byproduct storage
Economizer
Heated
air
to boiler
Lime
slurry
Particulate
control
device
Stack
Lower-pressure steam
Lime slurry
Cyclone and Heat Recovery
At the cyclone inlet located at the top of
the combustor, aqueous ammonia is injected
into the flue gas to further reduce NOx
produced in the furnace, converting it to
molecular N2
. The cyclones provide for efficient
mixing of the flue gas and ammonia
as well as sufficient residence time at the
optimum operating temperature for effective
NOx
reduction.
The hot ash and limestone pass through
the Intrex exchanger before being recycled
to the bottom of the combustor. In
the Intrex exchanger, superheated steam
is produced in tubes over which the hot
ash returning to the combustor flows.
The steam flows into the cyclone inlet
panels, through the cyclone walls, into the
convection cage wall, through the primary
superheater, and into the intermediate and
finishing superheaters that reside within
the Intrex exchanger. Solid material, consisting
primarily of ash and CaSO4
, is removed
from the bottom of the combustor
and sent to byproduct storage.
The hot gases leaving the top of the
cyclone enter additional reheater/superheater
tubes that also generate steam
for the turbines. The still-hot flue gas is
used to preheat the main combustion air
before it is introduced to the bottom of
the furnace.
Air Quality Management
After the reheater/superheater, the flue
gas enters an air quality control system,
where it travels downward through a polishing
scrubber that applies lime slurry
to absorb SO2
. Following the scrubber,
the flue gas passes through a baghouse
containing fabric filters to further clean
the gas before it is sent up the stack. The
collected particulates, which are sent to
byproduct storage, include trace metals
captured in the fabric filters.
-From " The JEA Large-Scale CFB Combustion
Demonstration Project, " Topical
Report 22, March 2003, Department of Energy/JEA.
Edited by Dr. Robert Peltier, PE
53
As the fuel particles burn, they become
lighter and, with the help of additional air
that constantly turns the particles over in a
fluid-like motion, they are carried higher in
the combustor. The limestone absorbs about
90% of the sulfur in the fuel (as SO2
).
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POWER October 2012

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