FIELD OF THE INVENTION
The present invention, in general relates to systems involving rellocation of heat
transfer surfaces inside a combustor of a tower type Circulating Fluidized Bed (CFB)
Boiler. In particular, the invention relates to orientation of heat transfer surfaces to
enhance mixing rates of gas- solid mixture inside the CFB boiler while reducing the
level of erosion potential faced by the heat transfer sections located inside the CFB
furnace for optimally utilizing the high level of heat flux prevailing in the combustor.
More particularly, the invention relates to an optimally oriented heat transfer
sections to reduce space and promote better flow distribution across the furnace in
a tower type CFB boiler.
BACKGROUND OF THE INVENTION
In CFB boilers, suspension of solids (fuel/ash) in air with sufficiently high velocity is
employed to circulate the solid coal particles. The particles carried along with the gas
are then usually separated in a cyclone and returned back to the dense the bottom
bed with aid of a loop seal. The entire bed is preheated to an ignition temperature of
the fuel (coal). On reaching the ignition temperature, the coal particles are fed,
which 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. As the size of
these clusters increases, the velocity of the gas reduces to 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 phenomenon 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.
Conventionally, a CFB boiler employs heat transfer surfaces of walls and surfaces
aligned along the flow direction eg. wing wall or employ special configuration of tube
shape such omega tubes in surface aligned perpendicular to reduce erosive wear
while adopting higher velocity (>5 m/s) at design conditions.
The tower type configuration of CFB adopts a lower velocity (<5m/s) to reduce the
impact of erosive forces of particles while taking advantage of high heat capacity of
flue gas ash mixture for heat transfer purpose. Adoption of lower level of velocity in
Tower Type CFB although addresses erosion concern but generates other adverse
implications for gas solid mixing inside the combustor.
Thus distribution of gas solid inside the furnace cross section remains one of the
foremost tasks to maintain optimal combustion conditions while restricting emissions
of harmful polluting gases to theenvironment. This is achieved partly by staging the
total combustion in the furnace with typically 50-60% of total combustion air
diverted for fluidising via air nozzles from the bottom.
The location of exit also significantly influences the mixing characteristics inside the
combustor. The exit point allows a significant degree of solid separation as gas takes
nearly 90° turn creating substantial differences between gas and solid. This causes
increased level of separation at this location which in turn affects the gas flow rates.
In addition, configuration of the exit point also accelerates the gas particle mixture
and susceptible to generate an uniform exit flow conditions at upstream locations.

Thus, the distribution of gas solid mixture inside the combustor is of utmost
importance for the performance and emission levels attained by a CFB system.
US patent No. 5,078,100 describes a stationary weakly expanded fluid bed for
combustion of coal, refuse, sludge and the like has a free space above the fluid bed
in which a so-called upper firing is effected with secondary air. The temperature of
the secondary air is maintained above the temperature of the fluid bed to handle
problems resulting from variations in fuel quality and varying loads and guarantees a
stable upper firing. This patents only refers to location of heat transfer surfaces in
upper zone with no details on it's arrangement or envisages special role in the
improvement of process.
US Patent No 4,446,629 teaches a fluidized bed heat exchanger in which a
perforated plate is disposed within a housing for supporting a bed of particulate
material which is introduced into the housing through an inlet. This patent only
describes heat transfer process inside dense bed and not in upper dilute zone
wherein the current patent is based upon.
US Patent Publication No 4,766,851 describes a combustion chamber with a fluidized
bed furnace having a nozzle plate, a fuel feed above the nozzle plate, a primary air
feed below the nozzle plate, an exhaust gas channel at an upper end of the
combustion chamber, and heat-exchanging heating surfaces includes a cylindrical
combustion chamber wall disposed vertically upright and having, in an upper region
thereof, a device for directing a flow of gas and particles vertically upwardly in a
central region of the combustion chamber and spirally downwardly along the
cylindrical wall, and a device for impressing an upwardly increasing rotary flow about
an axis of symmetry of the combustion chamber upon the gas flow. It particularly
claims only protrusion of surfaces in a cylindrical combustor and no particular claim
on the role of heat transfer surfaces other than cooling the gas solids.

Thus, the prior art envisage only knowledge utilizing heat transfer surfaces inside
the combustor only for the purpose of cooling flue gas /solids and using it for
heating the media. Prior art do not teach any other functional improvement such as
erosion protection or improvement to CFB process by virtue of proposing
optimization and relocation of the heat transfer surfaces.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved Tower Type
circulating Fluidized Boiler (CFB) with optimally oriented combination of heat transfer
sections to reduce space and enhance flow distribution across the furnace.
Another object of the invention is to propose an improved Tower Type circulating
Fluidized Boiler (CFB) with optimally oriented combination of heat transfer sections
to reduce space and enhance flow distribution across the furnace, which avoids
direct impingement of downward falling carpet of solid clusters at a modified point of
exit on the heat transfer surfaces formed of high value metallurgy.
A further object of the invention is to propose an improved Tower Type circulating
Fluidized Boiler (CFB) with optimally oriented combination of heat transfer sections
to reduce space and enhance flow distribution across the furnace, which ensures
gas solid mixing homogeneity while maintaining optimum combustion conditions
over the cross section of the combustor.
SUMMARY OF THE INVENTION
The present invention provides an improved CFB in which heat transfer tubes are
located inside the combustor itself aligned across the flow direction. This

configuration herein referred to as "Tower Type" as the heat transfer sections are
placed one over another building the CFB combustor along the height of boiler. This
unique arrangement calls for specific attention to erosive forces as the surface are
directly subjected to wear due to high flux of particles flowing across. The invention
solves this issue also.
The present invention uses the particular disposition of heat transfer sections inside
the combustor improving the gas /solid mixing process while preventing uneven gas
flow conditions. This in conjunctions with other protective means would provide
unprecedented improvement of gas-solid mixing homogeneity inside CFB combustor
furnace (at all loads) while reducing the erosion potential for such configuration of
Tower Type CFB boilers.
Accordingly, a uniquely arranged combination of heat transfer surfaces is used for
preheating of feed-water, generation of steam, superheating of steam, and relating
of steam. A plurality of tubes forming a plate like configuration is routed into the
combustor along the fins being perpendicular to gas flow direction, and each of the
configuration operates as one of the heat transfer surfaces.
The disposition of the exit point is arranged in manner such that the length of
multitude of the plurality of tubes forming the plate like structure is oriented parallel
to exit plane. This orientation prevents a short circuit of the gases and solids to the
exit plane. The fine division of the combustor via constructing an array of plate like
heat transfer surfaces formed out the tube coil arrangement also homogenizes the
flow field inside the combustor by normalizing the gas flow rates along the channel
space between the heat transfer sections.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Typical CFB Combustor
Figure 2 - Tower Type CFB Combustor Arrangement
Figure 3 - Typical Orientation and Arrangement of Heat transfer Surfaces
Figure 4 - Detailed orientation of surfaces.
DETAILED DESCRIPTION OF THE INVENTION
A typical CFB combustor (1) along with cyclone (2) and secondary air injection
nozzles (3) with primary air supply (4) is shown in Figure 1.
As seen from the figure 2 the heat exchangers - screen (5), super-heaters / re-
heaters (6,7) and economiser (8) are integrated inside the "Tower Type" CFB
furnace. Significantly difference of fuel air mixing is provided uniquely in Tower Type
CFB boiler in freeboard zone (9). Prior art (see figure 3) includes use of secondary
and tertiary air nozzles located (11,12) figure 3 located along the height of the CFB
combustor. Prior art location of heat transfer surfaces (9) is aligned with exit flow
direction (10) which results in significant bypass of the gas issuing from free board
zone affecting heat transfer tubes. Also the solids falling back from the exit point
due change in momentum also erodes the top surfaces of the tubes.
The proposed novel configuration shown in figure 4 orients the heat transfer
surfaces in a direction parallel to an exit plane (13) located away from the wall
containing the exit plane. This achieves uniform flow distribution across the
combustor by preventing short circuit of flue gas from central combustor region and
allow sufficient contact time with solids extending the process time of combustion.

The present invention thus provides an improved heat transfer configuration for use
in particularly CFB system by virtue of orientation and alignment ensuring uniform
flow conditions and avoiding impingement of solids clusters falling along the walls
through use of specially designed oriented heat transfer sections (9).

WE CLAIM :
1. An improved Tower Type circulating Fluidized Boiler (CFB) with optimally
oriented combination of heat transfer sections to reduce space and
enhance flow distribution across the furnace, the improvement is
characterized in that the combustor of the boiler is divided by constructing
a series of plate-like heat transfer surfaces, in that each of the heat
transfer surfaces acting as one of the pre-heating surface for feed water,
surface for generating steam, surface for superheating of steam, and
surface for reheating steam, in that each of the plate like heat transfer
surfaces is formed of a plurality of tubes routed into the combustor along
the fins being perpendicular to gas flow direction, and in that the exit
point to the heat transfer surfaces is so disposed that the length of the
tubes forming the plate-like structure is oriented parallel to the exit plane.
2. The improved Tower Type CFB as claimed in claim 1, wherein the plate
like structure is located away from the wall containing the exit plane.
3. The improved Tower Type CFB as claimed in claim 1, comprising a
freeboard arrangement consisting of a fuel feed point and a plurality of
secondary air feed nozzles.

ABSTRACT

The invention relates to a An improved Tower Type circulating Fluidized Boiler (CFB)
with optimally oriented combination of heat transfer sections to reduce space and
enhance flow distribution across the furnace, the improvement is characterized in
that the combustor of the boiler is divided by constructing a series of plate-like heat
transfer surfaces, in that each of the heat transfer surfaces acting as one of the pre-
heating surface for feed water, surface for generating steam, surface for
superheating of steam, and surface for reheating steam, in that each of the plate
like heat transfer surfaces is formed of a plurality of tubes routed into the combustor
along the fins being perpendicular to gas flow direction, and in that the exit point to
the heat transfer surfaces is so disposed that the length of the tubes forming the
plate-like structure is oriented parallel to the exit plane.