FIELD OF INVENTION
The present invention, in general relates to systems involving fluidized bed heat
exchanger (FBHE) in Circulating Fluidized Bed (CFB) Boiler. In particular, the
invention adapts a new mechanism to enhance heat transfer rates and improve
effectiveness of heat transfer surfaces located in the external fluidized bed heat
exchanger. It utilizes higher solid reflux designed in the system. More
particularly, the invention relates to using appropriate openings in the weir to
ensure optimized ash flow across the heat transfer tubes in a circulating fluidized
bed boiler.
BACKGROUND OF THE INVENTION
Fluidised Bed boiler is a system comprising a fluidized bed enclosure, where the
solid particles remain 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 the 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. 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 bottom bed.
In the CFB boiler, the above mechanism is employed to circulate the solid coal
particles. 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 and the gas-solid phase extends throughout the furnace. Even though
the velocity in the combustor is sufficient, particles tend to form 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 tend to fall along the walls of the furnace and thus transfer
heat to the water-walls of the furnace. As the surface to volume is fixed for the
given cross section of the boiler, additional heat transfer surfaces are required
for ensuring optimum combustion temperature.
This is ensured by utilizing additional heat transfer surfaces inside the furnace
using panel or assembly of tubes perpendicular to the gas flow direction. In
certain other cases the part of ash returning from the cyclone is diverted into
these external heat exchangers where suitable assemblage of heat transfer tubes
are arranged to extract heat from the hot diverted ash and is then returned back
to the furnace. It then mixes with the hot ash in the furnace and maintains the
temperature by absorbing the heat released by the combusting fuels.
US patent No. 5, 772, 969 describes the process of recovering heat which uses a
surfaces located integral to the combustor with non portioned chamber with ash
introduced on top of upper surfaces of the second fluidized bed.
US Patent No 6, 532, 905 teaches that one or more bubbling fluidized bed
located within lower portion of CFB boiler with fluidizing gases, control the heat
transfer to the tubes.
Thus, the prior art envisages only measures of placing heat transfer surfaces
close to the combustor lower part, and fluidizing serves only as primary means of
varying heat transfers. The present invention in contrast uses the variation of the
ash entry locations to achieve the same in a much superior uniform manner.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an optimum arrangement of
Fluidized bed heat exchanger configuration with hot ash entering along the sides
of the heat transfer tube assembly through a calibrated opening arrangement in
addition to ash flow over the weir onto the assemblage of tubes.
Another object of the invention is to propose a compact heat exchanger with
reduced air pressure requirement for fluidizing by utilizing the air from
primary/secondary air fan.
The proposed heat exchanger also utilizes appropriately fluidizing located
fluidizing nozzle in the side ash apertures to activate the passage of ash during
the operation further assisting in turn down operation
SUMMARY OF THE INVENTION
Accordingly, there is provided a Multi Inlet High Performance Heat Exchanger
comprising a plurality of heat transfer tube with multi-level side openings
configured at weir interface, a plurality of fluidizing nozzles for activating the side
openings to allow solids coming into the empty chamber, the bundle chamber
housing and heat transfer tubes of the heat exchanger.
The device further employs a combination of valves and correspondingly-sized
fluidizing nozzles in the side opening of weir to ensure a smooth flow of hot ash.
The new compact heat exchanger utilizes air from primary/secondary fans
disposed at the admission location in the furnace or as an external heat bed
exchanger for fluidizing the hot ash eliminating the need for high-pressure
blowers.
DETAILED DESCRIPTION OF THE INVENTION
A typical CFB combustor (1) along with cyclone (2) and external fluidized bed (3)
with heat exchanger (4) is shown in Figure 1.
As seen from the figure 3 the heat exchanger, performs an important role of
maintaining temperature of the CFB combustor by regulating the heat absorption
at part loads by adjusting the ash flow through the system. Prior art includes use
of bulky heat transfer surfaces fluidized with air from high pressure separate
blowers. These systems do not effectively utilize the heat transfer as the ash
flows are predominantly directed over the weir of the chambers.
A typical multi inlet high performance heat exchanger having innovative ash flow
mechanism is illustrated in Figure 2. It consists of an ash entry point (5), a weir
configuration (6), a fluidizing media admission plenum (7), an exit chamber (8),
a vent connection (9) and an ash drain connection (10). The inventive device as
shown in figure 3 comprises a plurality of air nozzles (11), corresponding number
of fluidizing air connections (12) and multilevel openings (13).
The present invention thus provides a nozzle-assisted ash flow entry at mutli-
location to improve ash flow distribution and reduce heat transfer area
requirement. The nozzle assembly (11) ensures positive sealing or transfer of
solids into weir. The modus operandi of operation of the device is to control the
quantum of ash into chamber at various heights of the chamber. This is made
possible by a configuration and combination of the admission nozzles (11) used
in weir wall area. The nozzles (11) located in the weir chamber are aligned along
the walls to provide a fluidizing area for smooth flow of the ash. The location and
quantity of air to the corner is also adjusted using a plurality of valves to provide
an air quantity sufficient to overcome the solid refluxes faced along the corners
and other areas.
The multi inlet FBHE improves ash circulation, uniform ash temperature over
conventional FBHE as the hot ash is admitted from many locations promoting a
uniform temperature profile along the heat transfer tubes and therefore therein
lies the advantage. The advantage of the invention lies in fact the proposed
device, not only be installed in the existing units without affecting the pressure
part constructions but further provides a significant advantage over current
methods by enhanced heat transfer by virtue of improvement in temperature
profile. The other disadvantages of the prior art for example, of higher pressure
air requirement is eliminated by the invention as heat transfer requirement has
been improved requiring compact heat transfer area.
WE CLAIM
1. A multi inlet high performance fluidized bed heat exchanger (FBHE) device
to provide enhanced level of heat transfer to the surfaces (04) in a
circulating fluidized bed combustion boiler.
- a multi-inlet high performance heat exchanger having a plurality
of heat transfer tubes with multi-level side openings (13)
configured at weir wall interface (6); and
- a FBHE device comprising a plurality of valves and
correspondingly seized fluidizing nozzles (11);
2. A multi inlet high performance FBHE device to provide enhanced level of
heat transfer to the surfaces in a circulating fluidized bed combustion
boiler as substantially described and illustrated herein with reference to
the accompanying drawings.
Wherein
- at least more than one entry point for ash flow
(13), and at least one refractory nozzle (11) constituting a nozzle
assembly which generates a layer of air stream in which the ash
flow through the weir wall
a refractory lined weir separating the incoming ash and the heat
transfer bundles

The invention relates to a Multi Inlet High Performance fluidized bed heat
exchanger (FBHE), provided to enhance the heat transfer rate and achieve
uniform heat recovery to ensure optimum operating temperature in fluidized bed
boilers. This device is used for optimization of air pressures and reduction in the
requirement of heat transfer surfaces for temperature control of the circulating
fluidized bed.