Page 57 - Energize November 2021

TECHNICAL

the solids (by conduction), allows the coil
size to be many times smaller than those in
conventional boilers.

Fuel and sorbent feeding
The fuel feeding system consists typically
of two to four independent fuel feed
lines, divided equally to front and rear
walls of the furnace. One fuel feeding
line generally includes a fuel silo, a fuel
feeder, a fuel conveyor and discharge to the
feeding points. Feeding points are located
symmetrically to each furnace section to
ensure uniform combustion in the furnace.
Compared to fuel feeding, sorbent feeding is
relatively easy and flexible. Sorbent can be
Figure 5: Vertical water wall design (Sumitomo Foster Wheeler) fed pneumatically into the furnace through
openings on the front wall, side wall, rear
Heat transfer system gases (sodium and potassium chlorides) in wall and/or loop seal. Alternatively, a
The boiler walls are lined with water the hot furnace flue gas. To cope with this mechanical feed system can be employed.
tubes as in a PC boiler, but a significantly situation, boiler designers use expensive
different layout is used. The uniform heat alloys and recommend a high level of Air feeding distribution system
distribution within the FBC allows vertical cleaning and maintenance for these coils. Air supply to the CFBC furnace is divided
tubing to be used instead of wrap-around This design weakness is avoided in into primary and secondary air. The
is shown in Figure 5. CFB boilers by submerging these coils in primary air (approximately 50% of the
A convective pass which may contain hot solids, returned from the cyclone and combustion air) is introduced through a
superheaters, reheaters, economisers fluidised by clean air in heat exchangers nozzle grid in the floor to fluidise. An air
and air heaters, are arranged in the same before return to the bed, which protects preheater is located in the flue gas path.
order in the gas flow direction. There are them from the corrosive flue-gas (see
in-line and over-the-top designs for the Figure 6). The bubbling solids efficiently Bottom ash extraction system
convective back pass. The in-line design conduct their heat to the steam contained Another parameter influencing the
locates the back pass on the same side of in the coils and as the solids never melt or concept of a CFB boiler is the ash content
the cyclone relative to the furnace. With soften, fouling and corrosion of these coils of the coal. Bed ash will be discharged
the over-the-top design, the back pass is are minimal. The high heat transfer rates of usually from CFB combustors at about
located on the opposite side of the cyclone
relative to the furnace. External heat
exchangers (EHEs) are generally located
downstream of the cyclones. The cyclones
or other solids collection device may be
cooled with steam or water in order to
reduce wear of materials, and in this case
steam or hot water is produced.

Final superheater/reheater
Another very important feature of a CFB
boiler is the final superheat and reheat
steam coils. These coils operate at the
highest metal temperatures in the boiler,
which makes them vulnerable to corrosion
and fouling. This vulnerability increases
significantly for supercritical boilers with
high steam temperatures. In a conventional
PC boiler these coils are suspended
from the furnace ceiling and are directly
exposed to the slagging ash and corrosive Figure 6: Two stage ash cooler (Doosan Lentjes)

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