Fluidized boiler

A fluidized bed call (10) having a static ignition bed (18) of inert heat storage particles (50) disposed immediately beneath and adjacent to a fluidizing region (44) wherein fuel particles are combusted, characterized in that the heat storage particles (50) are generally spherical in shape, each particle having a plurality of protuberances (52) extending outwardly from the surface of the particle a preselected length thereby maintaining a minimum spacing, equal to the preselected length of the protuberances, between neighboring spherical particles within the static ignition bed thereby ensuring that sufficient void space (54) exists within the static ignition bed for the fluidizing air to flow upward through the static ignition bed into the fluidizing region without an excessive pressure drop and for the fuel particles to laterally penetrate the static ignition bed.

BACKGROUND OF THE INVENTION 
The present invention relates to fluidized bed reactors in general and, 
more particularly, to a novel physical form for the inert heat storage 
particles of a fluidized bed of the type wherein a bed of fluidized fuel 
particles is established above and immediately adjacent to a static bed of 
inert heat storage particles. 
Fluidized bed reactors made up of a single fluidized bed cell or a 
plurality of individual fluidized bed cells are well - known in the prior 
art. In such reactors, fuel particles are burned while maintained in a 
fluidized state most typically by the air supplied for combustion of the 
fuel particles. One advantage of burning fuel in a fluidized state lies in 
the ability of the bed of fluidized fuel particles to burn in a 
comparatively small volume, to conduct heat relatively rapidly to heating 
surfaces immersed within the fluidized particles, and to absorb sulfur in 
the fuel if the fluidized bed includes particles of a sulfur absorbent 
material as well as fuel particles. 
The relatively rapid conduction of the heat of the fluidized bed to the 
heating surfaces immersed therein results from the high thermal 
conductivity that characterizes the fluidized mass of particles in the 
bed. Unfortunately, the high conductivity of the bed in the fluid state 
makes stable operation at low firing rates difficult. A small imbalance 
between the rate of heat liberation and the rate of heat removal can cause 
the bed temperature to fall by a relatively large amount. Such an 
imbalance, caused, by instance, by a mementary reduction in the fuel 
supply rate, can cause the bed temperature to fall below the ignition 
temperature of the fuel, particularly when operating at low loads since 
the bed temperature is then already relatively low. Since the ignition of 
the fuel particles in a fluidized bed cell is dependent predominately upon 
the temperature of the fluidized bed, the almost unavoidable heat flow 
imbalances in the system can cause the bed to become extinguished at low 
loads. 
One solution to the aforementioned problem, as disclosed in U.S. Pat. No. 
4,176,623, envisions a fluidized bed cell comprised of a bed of fluidized 
fuel particles positioned above and immediately adjacent to a static bed 
of inert heat storage particles. Fuel particles are fed to the static bed 
and ignited by the heat retained in the inert heat storage particles of 
the static bed. A supply of fluidizing combustion air is blown upwardly 
through the static bed into the fluidizing region in such a manner as to 
carry the fuel particles from the static bed into the fluidizing region 
and to fluidize the fuel particles within the fluidizing region while not 
fluidizing the inert heat storage particles forming the static bed. 
One major problem associated with a fluidized bed employing a static 
ignition bed of heat storage particles as contemplated in U.S. Pat. No. 
4,176,623, is the problem of maintaining sufficient void space between the 
inert heat storage particles of the static ignition. If sufficient void 
space is not present, the pressure drop encountered by the fluidizing air 
in traversing the bed may become excessive and result in the velocity of 
the air passing therethrough dropping below the minimum velocity necessary 
to ensure fluidization of the fuel particles within the fluidized bed 
above the static ignition bed. Additionally, an excessive pressure drop 
within the static ignition bed results in increased fan power requirements 
for the forced draft fans supplying fluidizing combustion air to the 
fluidized bed. 
Additionally, if sufficient void space is not present, fuel particles 
injected into the bed from a central feed point will not be able to 
migrate across the bed. Therefore, good lateral fuel distribution, which 
is essential for efficient operation of the fluidized bed, will not be 
attained. 
SUMMARY OF THE INVENTION 
The present invention, therefore, provides a fluidized bed cell wherein the 
static ignition bed contains sufficient void space to ensure that an 
excessive pressure drop is not encountered and that good lateral fuel 
distribution is attained. 
According to the present invention, a fluidized bed cell comprised of a 
fluidized bed of fuel particles positioned above and immediately adjacent 
to a static bed of inert heat storage particles is characterized in that 
the inert heat storage particles are generally spherical in shape with 
each of the particles having a plurality of protuberances extending 
outwardly from its surface for a preselected length. In this manner, 
neighboring spherical heat storage particles are maintained apart a 
minimum distance equal to the preselected length for which the 
protuberances extend outwardly from the surface of the particles. This 
ensures that sufficient void space exists within the static ignition bed 
for the fluidizing air to flow upwardly therethrough into the fluidizing 
bed without encountering an excessive pressure drop and that good lateral 
fuel distribution will be attained. 
In a preferred embodiment of the present invention, each of the generally 
spherical inert heat storage particles have six protuberances extending 
outwardly from its furface for a uniform preselected length, each 
protuberance spaced at a 90.degree. spherical angle from its neighboring 
protuberances.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, and more particularly to FIG. 1 thereof, there is 
illustrated therein a fluidized bed combustion cell designed in accordance 
with the present invention. The fluidized bed combustion cell 10 shown in 
FIG. 1 could in principle be the entire combustion zone of a fluidized bed 
boiler or it could nearly be a single combustion cell in a multi-cell 
boiler. 
The fluidized bed boiler 10 is formed of a combustion zone 12 bounded on 
its sides by horizontal waterwalls 40 and an air inlet plenum 14 disposed 
in the lower portion of the fluidized bed boiler 10 beneath the combustion 
zone 12. A static bed support plate 16 is positioned between the air 
plenum 14 and the combustion zone 12 of the fluidized bed boiler 10 and 
extends across the entire area of the fluidized bed cell to separate the 
combustion region 12 from the air plenum 14. The static bed support plate 
16 is somewhat dish-shaped, being deeper in the center than on the sides, 
and has appropriate openings for allowing fluidizing and combustion air to 
pass upwardly through it. 
Supported by and disposed within the cavity formed by the dish-shaped bed 
support plate 16 is a static ignition bed 18 of inert heat storage 
particles 50. The openings in the static bed support plate 16 for allowing 
fluidizing and combustion air to pass upwardly from the air plenum 14 into 
the combustion zone 12 of the fluidized bed cell 10 are appropriately 
sized to prevent the heat storage particles 50 from passing therethrough. 
Immediately above and adjacent to the static bed 18 is a fluidizing region 
44, which is shown in the drawings as being occupied by a fluidized mass 
of fuel particles. This suggests the normal operation of the bed in which 
the fluidization creates a quasi-liquid mass of fuel particles having a 
more or a less definite upper boundary above which the so-called freeboard 
region 46 extends. The freeboard region, whose purpose is to provide a 
region in which particles thrown upward from the bed by the mixing action 
generated by the fluidizing air passing therethrough can execute a 
complete trajectory and fall back into the fluidizing region 44 without 
being drawn out with the exhaust gases. Immersed within the fluidizing 
region 44 is a heat transfer coil 42 through which a liquid, typically 
water, is passed in heat exchange relationship with the fluidized fuel 
particles being combusted in the fluidizing region 44 of the fluidized bed 
cell 10. 
Fluidizing air, which also serves as combustion air, is supplied to the 
fluidized bed cell 10 through windbox 20 disposed beneath the floor of the 
fluidized bed cell 10. The air passing through the windbox 20 enters air 
plenum 14 through an opening 22 in the floor of the fluidized bed boiler 
10. A damper 24, whose purpose is to regulate the flow of air from the 
windbox 20 through the opening 22, is positioned in the opening 22. The 
damper 24 includes vanes or blades 26 that are adjustable for controlling 
the amount of air admitted to the air plenum 14 from the windbox 20 
through the opening 22 in the floor of the fluidized bed cell 10. The 
combustion air collected in the air plenum 14 then passes upwardly through 
the bed support plate 16 into the combustion region 12 to fluidize the 
fuel particles being combusted therein. A baffle plate 28 is disposed 
within the air plenum 14 above the opening 22 in the floor of the 
fluidized bed cell 10 to properly distribute the air entering through the 
opening 22 in the floor of the fluidized bed cell 10. 
A coal pipe 30 extends vertically upward, penetrating the windbox 20, and 
passing through the air plenum 14 and the bed support plate 16 to extend 
into the static bed 18, terminating in a fuel distributor 32. Fuel, 
typically coal, is injected laterally outwardly into the static bed 18 of 
inert heat storage particles 50 through holes 34 disposed 
circumferentially in the wall of the fuel distributor 32. Additionally, 
the fuel distributor 32 houses an igniter for igniting the fuels being fed 
to the fluidized bed cell 10 during start-up and warm-up. The combustion 
air collecting in the air plenum 14 passes upwardly through the bed 
support plate 16 to carry the fuel particles being fed into the static 
ignition bed 18 upwardly therefrom into the fluidized region 44 and to 
maintain th fuel particles carried into the fluidizing region 44 in a 
fluidized state while they are being combusted therein. the velocity of 
the fluidizing air is controlled so that the fuel particles are fluidized, 
but the inert heat storage particles 50 of the static ignition bed 18 
remain stationary in a non-fluidized state. 
Operation of the fluidized bed cell 10 is initiated by feeding igniter fuel 
into the static ignition bed 18 through fuel pipe 30 and fuel distributor 
32. Igniter fuel is lit upon entering the static ignition bed 18 by 
appropriate igniter means, not shown in the drawings, and the resulting 
combustion of the igniter fuel begins to heat the heat storage particles 
50 in the static bed 18. After the static bed has reached a temperature 
that is high enough to support ignition of the main fuel such as coal, the 
coal feed is initiated through the fuel pipe 30 and the fuel distributor 
32 into the static ignition bed 18. As the coal leaves the fuel 
distributor 32, it is ignited by the flame produced by the igniter fuel if 
the igniter fuel is still being fed to the bed, or by the heat stored in 
the heat storage particles 50 of the static ignition bed 18. 
The coal feed is gradually increased to full capacity; and since the 
combustion is self-sustaining, the flow of igniter fuel is discontinued. 
As the coal feed is increased, the air flow rate is also increased until a 
design air flow velocity is reached, that velocity being at some point 
above the fluidization velocity inert heat storage particles 50 of the 
static ignition bed 18. Much of the coal fed to the static ignition bed 18 
is blown into the fluidizing region 44 and combusted therein in a 
fluidized state. Steady state operation is maintained at a bed temperature 
of around 1500 to 1700 F through control of the coal feed rate and the 
fluidizing air flow rate to the fluidized bed cell 10. 
During this mode of operation, a small imbalance between the heat 
liberalization from the coal being burned and the fluidizing region 44 and 
the heat absorption therefrom by the waterwalls 40 and the immersed heat 
exchange tubes 42 can cause a significant change in bed temperature. As 
the firing rate is lowered in response to changes in load, the normal 
temperature in the fluidizing bed region 44 is reduced. Therefore, a 
significant temperature drop could well result in the temperature in the 
fluidized bed region 44 that is below the ignition point of the fuel. It 
is under such conditions that the advantages of the static ignition bed 
become apparent. 
The low thermal conductivity of the inert heat storage particles 50 of the 
static ignition bed 18 enable the temperature of the static ignition bed 
18 to remain above that required for ignition of the fuel even though the 
temperature of the fluidized bed region 44 has dropped below the ignition 
point. As a result, the fuel flowing to the fluidized bed region 44 is 
ignited upon entrance into the static ignition bed 18. The heat liberated 
upon ignition of the fuel as it enters the static bed 18 helps maintain 
the temperature of the static ignition bed so that operation of the 
fluidized bed cell 10 may continue. 
As mentioned hereinbefore, it is necessary to maintain sufficient void 
space between the inert heat storage particles 50 within the static 
ignition bed 18 to ensure that the fluidizing air does not experience an 
excessive pressure drop in traversing bed 18 and that good lateral fuel 
penetration across bed 18 is attained. If the heat storage particles 50 
become too closely packed, there is insufficient void space for the 
fluidizing air to freely pass through as it traverses the static ignition 
bed 18 resulting in an unacceptably high pressure drop. Further, coal 
particles injected laterally outward into the static bed 18 will be unable 
to migrate laterally across the bed 18 but rather will tend to concentrate 
in the vicinity of the central bed point. 
In accordance with the present invention, a fluidized bed cell of the type 
herein described before is characterized in that the inert heat storage 
particles 50 are generally spherical in shape with each of the particles 
having a plurality of protuberances 52 extending outwardly from its 
surface for a preselected length. As shown in FIGS. 2 and 3, the 
protuberances 52 extending outwardly from the generally spherical heat 
storage particles 50 prevent the heat storage particles 50 of the static 
ignition bed 18 from becoming closely compacted together and ensure that 
sufficient void space 54 is maintained between neighboring heat storage 
particles. 
As shown in FIG. 2, the protuberances extending outwardly from the 
generally spherical heat storage particles 50 may comprise six cylindrical 
knobs extending outwardly for a uniform preselected length, each 
protuberance spaced at a 90 degree spherical angle from its neighboring 
protuberances. Although the individual protuberances 52 shown in FIG. 2 in 
their preferred form as cylindrical knob-like protuberances, it is within 
the contemplation of the present invention that the individual 
protuberances may be in the form of conical knobs or rectangular knobs or 
hemispherical knobs or other right prismatic knobs. 
An alternate embodiment of the protuberances 52 extending outwardly from 
the generally spherical heat storage particles 50 is shown in FIG. 3. As 
shown therein, the protuberances are in the form of two circumferential 
ridges 52 extending outwardly from the surface of the heat storage 
particles 50 for a uniform preselected length. The circumferential ridges 
52 are preferably disposed perpendicularly with respect to each other. 
Although the circumferential ridges 52 shown in FIG. 3 are rectangular in 
cross section, it is within the contemplation of the present invention the 
the circumferential ridges may have a cross-sectional form of a semicircle 
or a traingle. 
The protuberances 52, no matter what their shape, serve to maintain a 
spacing between neighboring heat storage particles 50 so that sufficient 
void space 54 is provided between neighboring heat storage particles 50. 
This void spaces 54 provides volume through which the fluidizing air may 
pass through and the coal particles laterally penetrate the static 
ignition bed 18. 
By varying the preselected length of the protuberances 52 outwardly from 
the surface of the heat storage particles 50, the amount of void spaces 54 
provided between neighboring particles; and, therefore, the pressure drop 
through and the fuel distribution across bed 18, may be optimized. 
While only two embodiments of the present invention have been shown, it 
would be appreciated that modifications thereof, some of which have been 
alluded to hereinabove, may readily be made thereto by those skilled in 
the art. Therefore, it is intended by the appended claims to cover the 
modifications alluded to herein as well as all other modifications which 
fall within the true spirit and scope of the present invention .