FIELD OF THE INVENTION
The present invention, in genera! relates to systems involving air injection in
Circulating Fluidized Bed (CFB) Boiler. In particular, the invention relates to
means for enhancing fuel air mixing rates to improve penetration of optimaliy
directed high momentum air flux into CFB furnace, against preventing flowing
sheet of solids along the wall. More particularly, the invention relates to an
improved air nozzle disposable in to the furnace to optimize air flow distribution
across the furnace of a CFB boiler.
BACKGROUND OF THE INVENTION
Fluidised Bed Combustion boiler is a system having 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 allow the particles to be suspended
in the bed. Thus, at the low-velocities, the bed remains static and undisturbed.
As the flow velocity is increased gradually, trie buoyant force overcomes the
gravitational force exerted on the particles, the particles remain in suspended
state in the fluidized bed. The pressure drop is allowed to increase till a point,
where it matches the weight of the 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
attain pseudo fluid properties. With further increase in flow, the particles are also
carried along with the gas and then separated in a cyclone and returned back to
the 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 uniformly imparted to the whole volume of
the bed 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 downward in the furnace. On their
way down, these clusters collide with rising material, get broken-down, with the
smaller particles travelling 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.
Accordingly, distribution of air inside the furnace cross section remains one of
the influencing parameter to ensure optimal combustion conditions while
maintaining emissions of harmful polluting gases to the minimum. This is
achieved partly by staging the total combustion in the furnace with
approximately 40-60% of total combustion air sent for fluidising using a plurality
of air nozzles disposed at the bottom. The uniform distribution of the air is
ensured by utilizing various configurations of the air nozzles while minimizing the
pressure drop expanded across the nozzles.
The rest of the air is injected into the furnace along the walls of the furnace to
complete the combustion. This quantity of air is critical in the control of un-burnt
carbon level as well as completing the combustion of the released volatiles
(hydrocarbons) from the fuel. Proper mixing of fuel and air also enhances
oxidation level of sulphur (which improves sulphur capture in the furnace). Thus,
the penetration depth of the secondary air is further important relating to
performance and emission levels attained by a CFB system.
The staged combustion and importance of selecting primary to secondary air
ratio are discussed in the prior patents referred herein below. US patent No. 5,
715, 769 describes a combustion method utilising two stage combustion - a
combination of air ratio and varying secondary air ratio to effectively control
combustion efficiency while maintaining optimal emission levels..
US Patent No 7, 410, 356 B2 teaches asymmetrical location of secondary air
nozzles located along the walls to improve air flow distribution and therefore
consequences described above
US Patent Publication No US 2011/0073050 Al describes secondary air nozzles
located deep into the furnace for preventing deflection of solids falling onto
fluidised bed (located inside the CFB).
Thus, the prior art only teaches or suggests the importance of secondary air and
measures for maintaining secondary air injection along the walls of the furnace
to achieve improved combustion process in the CFB furnace.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved air injection
nozzle device with axi-velocity control means disposable along the side walls to
enhance performance of circulating fluidized bed (CFB) furnace.
Another object of the invention is to propose an improved air injection nozzle
device with axi-velocity control means disposable along the side walls to enhance
performance of circulating fluidized bed (CFB) furnace, in which a a slidable hood
is arranged to eliminates direct impingement of downward falling carpet of solid
clusters into the high momentum secondary air jet and hences the level of
penetration in the CFB furnace.
A further object of the invention is to propose an improved air injection nozzle
device with axi-velocity control means disposable along the side walls to enhance
performance of circulating fluidized bed (CFB) furnace, which is enabled to
coordinate axial velocity of each of the secondary air individual nozzles based on
real-time emission parameters to achieve optimum combustion conditions.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved air nozzle device comprising at least
one air nozzle, and a refractory hood with specific variable area slide means for
controlling an axial velocity of air injection by the nozzle into the CFB furnace to
improve penetration of air and improve combustion process. The device further
allows locating the nozzle such that part load performance and emission
characteristics of the furnace based on online measurements is improved.
Accordingly, the operational performance of the boiler at is improved with
simultaneous reduction in emissions.
Thus, a variation in the air nozzle configuration by means of an adjustable means
is used which provides the operators with a plurality of axial velocity controllable
secondary air nozzles of the invention. In conjunction with the improved air-
nozzle device, an on-line measurement of the emission parameters enables a
controlled level of injection of air into furnace (at all loads) while providing a
flexibility to adjust the air injection process for variable fuel quantity, fluidisation
and quality of fuels.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - Shows a prior art CFB furnace with air admission points.
Fig.2 - Shows cross-section of a prior art air injection nozzle.
Fig.3 - Shows an improved air injection nozzle with its hood according to
the present invention.
Fig.4 - Shows an improved air injection nozzle device particularly showing
axial velocity controller unit according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
A prior art CFB combustor (1) along with a cyclone (2), a plurality of secondary
air injection nozzles (3), and a primary air supply means (4) is shown in Figure l.
As seen from figure (1), the heat exchanger performs an important role for
completing combustion inside the CFB furnace by regulating the air flow
distribution at part loads by adjusting the air flow through the system. Prior art
are known to use fixed opening type of secondary nozzles with high pressure
head for transforming the air stream into a high velocity jet The air nozzles (5)
device of prior art illustrated in fig 2 do not effectively penetrate into the furnace
due to a multiple reasons including impingement of high cross momentum due to
strands of the particles falling along the wall, low pressure head available at part
load conditions..
As shown in flg-3, the present invention provides an improved air injection nozzle
device having at least one air nozzle, an axial velocity adjusting means and a
slidable protective hood. The velocity adjusting means consists of an axial
velocity controller unit (6). The air controller unit comprises a slide opening (8)
connected to the controller through a positioner (9).
The present enables diverting the solids clusters falling along the walls of the
CFB system, through use of the refractory hood and utilizing an axial velocity
regulation to impart maximum momentum in the desired direction. This is made
possible by adjusting the opening (8) size at will due to axial velocity controller
unit (6). The location and quantity of air to the each of the areas covered by the
air nozzles is further adjusted using this unique combination. This provides a
superior performance by adjusting/directing secondary combustion air to the
required areas based on the boiler emission indicator. High level of CO may point
to low level of oxygen concentration at areas of fuel combustion which can be
augmented by precise control of air to the area close to fuel feed point.
The improved secondary air nozzle has enhanced air penetration into'the furnace
at all the operating loads (even during reduction of secondary air quantity) over
conventional arrangement as the air is admitted in a manner avoiding
disturbance from the falling solids while maintaining always a high velocity level
even at part loads. The advantage of the invention lies in fact that the proposed
device can 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 air penetration and advantages in terms of
improvement in emission compliance levels.
WE CLAIM
1. An improved air nozzle device for enhanced level of air penetration into
furnace of a circulating fluidized bed combustion (CFBC) boiler, the device
comprising:
- a plurality of secondary air injection nozzles disposed at different
heights on the side wall of the boiler to generate a penetrative jet
of air (3) for completion of combustion in the CFBC boiler;
- a siidable profective hood (7); and an axial air velocity controller
unit comprising a slide opening (8) and an adjustable positioner
(9).
2. An improved air nozzle device for enhanced level of air penetration into
furnace of a circulating fluidized bed boiler as substantially described and
illustrated herein with reference to the accompanying drawings.

ABSTRACT

An improved air nozzle device for enhanced level of air penetration into furnace
of a circulating fluidized bed combustion (CFBC) boiler, the device comprising a
plurality of secondary air injection nozzles disposed at different heights on the
side wall of the boiler to generate a penetrative jet of air (3) for completion of
combustion in the CFBC boiler; axial air velocity controller unit comprising a slide
opening (8) and an adjustable positioner (9).