POWER October 2010 - 70

PLANT CONTROLS
with sulfur-laden fuels. At the bed temperatures
at which the FBC operates, typically
1,550 to 1,650F, moisture contained
in the fuel is not generally an issue.
Emissions are also a function of bed temperature,
along with backend temperature.
They must be controlled in a dynamic operating
environment, regardless of biomass type.
CO emissions are low in FBC boilers because
of good fuel-air mixing, which allows operation
with minimum excess air and therefore
higher relative combustion efficiency.
Another advantage: The design of the
FBC boiler provides for a higher level of
stored energy, thus allowing relatively fast
load changes.
Considerations for Biomass
Boiler Control
Bubbling fluidized bed boilers employ high
levels of primary or underfire air (as much as
70%) to push the bed upward and make it behave
like a fluid. Residence time is increased,
thus providing for thorough mixing of the
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fuel and air. Primary air is fed into a plenum
below the furnace floor. The air comes from
a forced draft fan and is controlled with a
duct-mounted damper. Primary air is admitted
to the furnace through a set of nozzles
called " tuyeres, " which are mounted in rows
in the floor of the furnace. The primary air
velocity plays an important role in maintaining
the bed at its proper level, thus ensuring
thorough mixing of the fuel and air.
The remaining airflow is assigned for overfire
air or secondary air. Separate dampers
control each. The floor, known as the distributor
plate, also serves to remove ash from the
furnace. Figure 1 shows a typical BFB boiler
with rows of tuyeres in the distributor plate.
Staged combustion and lower temperatures
also have disadvantages, such as
higher CO, higher unburned hydrocarbons,
and lower boiler efficiency (resulting from
unburned carbon and CO). Therefore, it is
important to keep the furnace temperature
within a range to minimize these disadvantages
and minimize NOx
formation.
Overfire air or, more precisely, the ratio
of primary air to overfire air, is one element
used to control NOx
. This ratio is
used to control bed temperature, which has
a direct effect on NOx
formation. Total air
is function of the boiler load, and how it
is divided (primary air to overfire air) in
the furnace has an effect on overall combustion
efficiency and NOx
formation. For
instance, diverting more air to the upper
levels reduces NOx
formation.
Fuel Delivery System
Consistent, uninterrupted fuel delivery is
another key to reliable performance and effi1.
Controlled chaos. This illustration of
a typical bubbling fluidized bed boiler includes
the rows of " tuyeres " that line the furnace
floor, which doubles as a distributor plate for
the primary air supply. Bubbling bed boilers
firing biomass have now reached utility sizes.
Source: Metso Automation USA Inc.
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POWER October 2010

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