FIELD OF THE INVENTION
The present invention in general relates to a method of ash evacuation out of
fluidized bed system. In particular, the invention adapts means to enhance air
cooling of the ash required to be evacuated out of combustor chambers of a
fluidized bed system for example, a bubbling fluidized bed (BFB) or a circulating
fluidized bed (CFB) boilers.
In particular, the invention relates to a split air wind box device to direct cold air
on evacuatable ash with simultaneous cooling during damage of the ash. More
particularly, the invention relates to an integrated ash evacuation system for safe
handling of ash in fluidized bed combustion boiler.
BACKGROUND OF THE INVENTION
Fluidized Bed Combustion boiler utilizes high level of ash inventory to ensure
uniform air mixing combustion. Thus, it improves effectiveness of penetration of
fuel into combustors by optimally providing environment for the fuel on to the
highly turbulent multi phase system. This high level of inventory is beneficial for
good combustion becomes safety hazard in instances of drainage to attend tube
leakages. Such systems comprise a fluidized bed enclosure, where the solid
particles are in a suspended state. The suspended state is achieved by
controlling the amount of air passing through the bed. However, at low
velocities, the pressure drop across the bed is negligible and buoyant force
derived from low velocities is not sufficient to suspend the particles in the bed.
Thus, at the low-velocities, the bed remains static and undisturbed.
As the flow (velocity) is increased gradually, the buoyant force overcomes the
gravitational force exerted on the particles and suspends in the state of fluidized
bed. The pressure drop is allowed to increase till a point, where it matches with
the weight of particles per unit area. At this stage, the bed particles are in
suspended state and it is called "minimally fluidized bed".
When the flow is increased further, the bed becomes highly turbulent as the
interstitial spaces between bed particles increase allowing the high velocity gas
streams promoting a rapid mixing of the bed. At this stage, the bed of solids
attains pseudo fluid properties. The systems employing this stage of fluidization
for combustion and subsequent heat transfer to generate steam is termed as
"bubbling fluidized bed boiler". With further increase in flow, the particles are
also carried along with the gas and then usually separated in a cyclone and
returned back to dense the bottom bed. The systems employing this stage of
fluidization for combustion and subsequent heat transfer to generate steam is
termed as "circulating fluidized bed boilers".
In the fluidized bed boiler, the entire bed is preheated to ignition temperature of
the fuel (coal). On reaching the ignition temperature, the coal particles are fed.
They undergo combustion and release heat which is imparted to the whole
volume of the bed uniformly due to high turbulence and rapid mixing
characteristics of the fluidizing process. Large portions of the fuel and limestone
are suspended in the gas stream. In case of CFB, the velocity in the combustor is
sufficient to form particle clusters. The size of these clusters increases making
the velocity of the gas below entrainment velocity, and thus causing the material
to fall down the furnace. On their way down, these clusters collide with the rising
material, get broken-down, and the smaller particles travel up again.
This action promotes the mixing and leads to a gas to solid contact in the bed.
Some of the material remaining in the bed is too large to be entrained until it is
reduced in size by combustion and attrition. Fresh feed of coal and limestone
maintains the bed condition. The majority of particles/clusters contact the tubes
immersed in bed in case of BFB or the walls of the CFB furnace and thus transfer
heat to the water-walls of the furnace. The uniform distribution of the air is
ensured by utilizing various configurations of the air nozzles with objective of
achieving uniformity while minimizing the pressure drop expended across the
nozzles.
The enormous quantity of ash residing in the system requires evacuation in case
of tube leakage. As free lime contained in the ash inventory to capture sulphur-
dioxide liberated during combustion of coal, will react steam/water to form
cementetious compound - hydrated lime silicate mixtures. This is a dangerous
situation as this would block the entire combustion chamber rendering it
incapable of fluidization during restart. Thus the quick removal of ash inventory
from the chamber is of utmost importance to ensure future operability of the
system. However it should be noted that while evacuation of hot ash is
important, discharge of such enormous hot ash is also a safety issue in many of
the operating plants.
US Patent No. 2, 75, 808 describes a fluid heater utilizing ash collection chamber
as part of the furnace using water wall tubes to cool the ash slags in a typical
pulverized furnace. This arrangement only addresses the bottom ash evacuation
methods and not for the complete evacuation for fluidized bed system.
The prior art of handling such hot ashes for CFB system has not been elaborated
in prior art. The existing practice of handling hot ashes during the situation as
described above, involves simple drainage onto ground or onto bins and handled
after waiting for natural cooling process.
Thus, the prior art envisages only natural cooling process outside the boiler
system and therefore is not very safe considering operator movement around
boiler area. The present invention uses novel design wind box configuration, in
which ash evacuated for emergency is cooled in streams of cold air and
integrated mixing nozzle-based unique ash transport.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an integrated ash evacuation
system for safe handling of ash in fluidized bed combustion boiler, which ensures
cooling of ash evacuated in emergency from the combustion / other fluidized bed
chambers.
Another object of the invention is to propose an integrated ash evacuation
system for safe handling of ash in fluidized bed combustion boiler, which allows
fluidized ash movement at ash drain point to provide superior ash cooling effect.
A further object of the invention is to propose an integrated ash evacuation
system for safe handing of ash in fluidized bed combustion boiler, which utilizes
mixing nozzle arrangement as final element to expel the ash while cooling down
further for storage in the intermediate bin.
SUMMARY OF THE INVENTION
Accordingly, there is provided an integrated ash evacuation system for safe
handling of ash in fluidized bed combustion boiler comprising a split wind box
device that allows fluidized movement of evacuated ash while cooling it
simultaneously. The ash from chambers is opened on the split wind box which is
activated by an air diversion mechanism under emergency evacuation or in very
low load transients to improve flow distribution. This also opens emergency drain
point provided. The ash then falls on the fluidized bottom which then is cooled
down further and drained through a mixing nozzle configuration for transport on
to the intermediate ash storage bin for disposal.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a typical CFB furnace and emergency ash evacuation points.
Figure 2 - showing a comparison between a typical wind box and a split type
wind box of the invention.
Figure 3 - shows an ash evacuation air control device of the system of the
invention.
Figure 4 - shows a mixing nozzle device for the integrated system of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
A typical CFB boiler comprises a wind box (1) along with combustor (2) and
cyclone (3), a seal pot (4) and a plurality of fluidized bed heat exchangers
(FBHE) (5) as shown in Figure 1. As seen from the figure, fluidizing air wind box
is an essential component to ensure equal distribution of air into the wide section
chambers, and avoid improper fluidization and consequent loss of process. Thus,
the inventory of solids therefore exists in chambers of the combustor, seal pot
and FBHE which needs to be evacuated in case of water ingress (from water wall
tube failure). As this water would react with calcined lime to form cementetious
mixture, it would become difficult to operated system in such cases. Prior art
teaches use of ash gate valves located at the ash discharge piping located at
bottom of grate and wind box to evacuate ash from the system.
A wind box configuration of the invention, comprises an ash drain pipe (06) at
wind box bottom and one at the grate (07) which is usually open floor of the
boiler as shown in figure 2. The emergency ash evacuation utilizes the principles
of ash cooling and movement through fluidization in the split wind box device as
shown in figure 2. This configuration utilizes innovative distributing type ash
drain pipe (10) which sprays the discharging ash on to a fluidized slide
arrangement with a nozzle grate arrangement to ensure an intimate mixing of
ash and air for superior cooling and transport down to the drain for both the
grates as well as the wind box drain (11).
The wind box device has a split bottom design as detailed in figure 3. It consists
of a remote activated fluidizing bottom grate arrangement (12) consisting of
multiple air nozzles. It is fed by air from same source as during normal operation
but rerouted through the bottom by operating damper gate arrangement (13)
and a connecting duct (14).
This arrangement allows diversion of air from primary path into the connecting
duct arrangement. This also activates the bottom cooler grate arrangement by
opening the air nozzles to bottom split wind box portion. The rerouted air then
fluidizing the ash drained from the grate by opening a gate valve (15). The
sprayed ash is transported to the lowest point by simultaneous fluidization and
cooling by the air coming out of the grate arrangement.
This is then evacuated and further cooled through a mixing nozzle arrangement
(16) which draws in ash at higher pressure in the wind box and then discharges
further cooled down ash into an intermediate ash bin (17) for further disposal.
The proposed integrated emergency ash drainage system thus, integrates the
function of both wind box drain as well as draining the entire chamber by use of
the inventive wind box device.
WE CLAIM:
1. An integrated ash evacuation system for safe handling of ash in fluidized
bed combustion boiler, comprising a split wind box device (06) having a
plurality of grate nozzles (11), a duct connection (12), and an air flow
diverting gate arrangement (13);
- an ash drain device with a fluidized air cooling cum transport device
disposable in the split wind box; and
- a damper gate arrangement for activating the fluidized air cooling cum
transport device.
2. An integrated ash evacuation system for safe handling of ash in fluidized
bed combustion boiler substantially as herein described and illustrated
with reference to the accompanying drawings.

ABSTRACT

The invention relates to an integrated emergency ash evacuation system to drain
the ash inside the chamber of a fluidized bed boiler and safely transfer the hot
ash at a temperature acceptable for handling in the transport and storage
system. This system utilizes a split wind box design and grate arrangement for
accelerated cooling and transport. This system helps in rapid cooling of the
evacuated hot ash before safely removing out of the wind box through a
pneumatic transport system. The system utilizes air from the same source for
cooling and transport thereby simplifying the system requirements.