Method and apparatus for continuously burning particles in air stream in a vertical furnace

In a method for continuously burning particles in a gas or oil heated vertical furnace in which the particles are charged from an uppermost part of the space, and the particles bed fluidized by a burning gas flow is formed in a vertically successive downward arrangement and then the burned particles are discharged out of the bottom of the furnace after cooling, the improvement comprises the steps of: providing the furnace with a plurality of downwardly vertically successive zone, including a precipitation zone of the particles at an uppermost part in the furnace, a lower upward flow velocity zone having low upward stream of the burning gas (referred to as "rich fluidized bed", hereinafter) and a higher upward flow velocity zone having high upward stream of the burning gas (referred to as "lean fluidized bed", hereinafter) underneath the precipitation zone wherein the lean fluidized bed is formed beneath and adjacent to the rich fluidized zone, and feeding the particles, burning them successively through the zones provided in said furnace by the burning gas and then discharging the burned particles from the bottom without cooling while being suspended and floated by the upward flow of the burning gas. And furthermore, an apparatus for continuously burning particles with air stream comprising particles inlet and a particle precipitation chamber provided in the top of a vertical furnace, a large-diameter column and a small-diameter column provided underneath the precipitation column connected by inverted conical section, a burned particle exhaust port formed through the conical section under the small-diameter column, and a plurality of fuel and air or combustion gas inlet provided at the small-diameter column and the conical section.

DESCRIPTION 
TECHNICAL FIELD 
The present invention relates to a method of and apparatus for effecting a 
burning of particles with a continuous flow of gas by means of a vertical 
furnace. 
BACKGROUND ART 
It has been proposed to burn particles such as artificial light-weight 
aggregates, powdered lime stones, powdered dolomite and so forth by means 
of a vertical furnace employing an upward flow of gas such as a combustion 
gas which forms various beds of the particles such as fluidized bed, 
jetting flow bed, improved jetting flow bed, swirling flow bed, loaded 
moving bed and the like. 
The present inventors have already noticed the following facts in the 
burning of particles. In the burning of the artificial light-weight 
aggregates, the agglomeration by fusing of the particles takes place 
easily because the bloating temperature of the particles approximates the 
fusion temperature thereof. This problem is serious particularly when the 
burning is made by means of a rotary kiln, namely, in such a case, the 
heating of the particles due to the heat from the kiln wall, and the 
particles are gradually moved toward the kiln outlet while being agitated 
only insufficiently. Due to the radiant heat from the kiln wall and the 
insufficient agitation of the particles, the agglomeration by fusing of 
the particles is liable to occur, resulting not only in the unsmooth 
operation of the apparatus but also in a lowered thermal efficiency. 
For effectively burning the light-weight aggregate by a rotary kiln, it is 
necessary to make the particles stay in the kiln for a comparatively long 
time which generally falls between 20 and 60 minutes. Partly because of 
this reason and partly because of the heat transfer mechanism as stated 
above, the particles of particle size less than 5 mm tend to be lost 
bloating element by oxidation or due to fusion, although comparatively 
coarse particles of mean particle size exceeding 5 mm can provide a 
specific gravity of 1.25 to 1.35 of the burned product. Thus, it is not 
possible to obtain a burned product having a specific gravity smaller than 
1.55, with the fine particles having a mean particle size below 5 mm. 
Generally, in the burning of the light-weight aggregate, it is said that a 
operation has to be made through a minute temperature control within the 
temperature error of 10.degree. C. at a high temperature region in excess 
of 1100.degree. C. to prevent the fusion of the burned products. It is 
prohibited to discharge the product to the outside of the furnace in a 
stacked manner or to cool the hot burned product in a stacked state, 
because the fusion of the burned particles is liable to occur due to a 
confinement of the heat. 
With these knowledges, the present inventors have made various studies for 
the possibility of burning of particles with a gas stream by a vertical 
furnace which is considered to have a large rate of heat transfer and, 
hence, a reduced tendency of agglomeration of the burned particles. 
Hitherto, various methods have been proposed for burning particles with gas 
flows in vertical furnace, as summarized below. 
(1) Namely, Japanese Patent Publication No. 48076/1974 discloses a batch 
type method in which coarse particles having particle size in excess of 5 
mm are supplied into a furnace and are burned while being suspended, 
fallen and circulated by an upward swirling flow of hot gas, and the 
burned product is discharged from the lower side of the furnace. 
(2) Japanese Patent Laid-open No. 121807/1978 discloses a technique in 
which the particles are fluidized by an upward flow of gas which has 
passed through a rectifying plate (a perforated plate), and the burning 
treatment is conducted continuously with the supply of a material for 
adjusting the temperature in the furnace to avoid the fusion of the burned 
particles. The burned particles overflow the fluidized bed and are 
discharged continuously. 
(3) A plurality of rectifying plates for upward flow of gas are disposed in 
the furnace to form a plurality of stages of fluidized bed, so that the 
particles overflow from each stage to fall onto the underlying bed while 
being successively subjected to a pre-heating, burning, and cooling, so 
that the burned product is stacked on the bottom of the furnace. The 
product is discharged while being cooled by the air to pre-heat the air 
conveyed into the furnace for burning. 
(4) As disclosed in Japanese Patent Laid-open No. 68796/1979, an improved 
jetting bed is formed in a furnace by an upward flow of gas which has 
passed through a rectifying plate to effect the burning continuously while 
permitting the overflow of the burned particles from the bed for the 
discharge. 
(5) The fluidized beds of the particles are formed without any assist of 
the perforated rectifying plate as used in the prior arts (2),(3) and (4) 
stated above. The thickness or density of the particles is greater in the 
lower stage than in the upper stage of the fluidized bed. The burned 
products are stacked on the bottom of the furnace and discharged from the 
latter. Other techniques proposed hitherto are more or less applications, 
improvements, or modifications of these five techniques summarized 
heretofore. 
The present inventors have found that these conventional techniques, either 
solely or in combination, cannot satisfy the following requirements (a) to 
(c). 
(a) To obtain a condition for satisfactorily avoiding the agglomeration of 
the burned particles. 
(b) To obtain a sufficiently high thermal efficiency. In the conventional 
methods summarized above, the thermal efficiency is inevitably low because 
the gas is discharged at a high temperature substantially equal to the 
burning temperature. 
(c) To obtain a large treating capacity of the furnace per unit volume. 
DISCLOSURE OF THE INVENTION 
The present inventors have made an intense study for improving the 
technique for burning light-weight aggregate and have reached the 
following conclusions. 
(1) In forming a fluidized bed of the particles by the upward flow of gas 
within the furnace, the density of the particles is made smaller in the 
region where the flow rate of the upward flow of gas is higher, while, in 
the region of low rate, the density of the particles becomes greater. With 
this knowledge, it is possible to form a temperature gradient in the gas 
and in the particles by forming successive beds of different densities of 
the particles in the vertical direction during the burning. 
(2) It is possible to arrange such that a region of the maximum temperature 
is formed as the fluidized bed of lowest density of the particle is 
burned, and that the burned particles are discharged from the bottom of 
furnace to the outside of the latter after being suspended and floated by 
the upward flow of gas through the region of maximum temperature. This 
arrangement permits an easy adjustment of the temperature in the furnace 
and facilitates the prevention of agglomerating of the burned particles, 
while improving the burning capacity per unit volume of the furnace. 
The present invention has been achieved on the basis of these facts. 
Namely, in a method for continuously burning particles in a gas or oil 
heated vertical furnace in which the particles are charged from an 
uppermost part of the space, the particles bed fluidized by a burning gas 
flow is formed in a vertically successive downward arrangement, and the 
burned particles are discharged out of the bottom of the furnace after 
cooling, the improvement comprises the steps of: providing the furnace 
with a plurality of downwardly vertically successive zone, including a 
precipitation zone of the particles at an uppermost part in the furnace, a 
lower upward flow velocity zone having low upward stream of the burning 
gas (referred to as "rich fluidized bed", hereinafter) and a higher upward 
flow velocity zone having higher upward stream of the burning gas 
(referred to as "lean fluidized bed", hereinafter) underneath the 
precipitation zone wherein the lean fluidized bed is formed beneath and 
adjacent to the rich fluidized zone, and feeding the particles, burning 
them successively through the zones provided in said furnace by the 
burning gas and then discharging the burned particles from the bottom 
without cooling while being suspended and floated by the upward flow of 
the burning gas. 
And furthermore, an apparatus for continuously burning particles with air 
stream comprising particles inlet and a particle precipitation chamber 
provided in the top of a vertical furnace, a large-diameter column and a 
small-diameter column provided underneath the precipitation column 
connected by inverted conical section, a burned particle exhaust port 
formed through the conical section under the small-diameter column, and a 
plurality of fuel and air or combustion gas inlet provided at the 
small-diameter column and the conical section. 
An object of the invention is to make it possible to effect a burning of 
particles which tends to be agglomerated at high temperature, such as 
light-weight aggregate. A further object of the invention is to improve 
the thermal efficiency and burning capacity per volume of the furnace. The 
method and apparatus of the invention can apply also to the burning of 
other similar particles, such as powdered lime stones, dolomite, and so 
forth. 
Throughout the specification, the term "fluidized bed" is used to denote 
the bed or layer formed in a vertically elongated substantially tubular 
furnace where the particles are fluidized by the upward flow of gas and 
suspended, circulated, or made to flow by the upward flow of gas.

BEST MODE FOR CARRYING OUT THE INVENTION 
The invention will be fully understood from the following description of 
the preferred embodiment taken in conjunction with the accompanying 
drawings. 
Referring to the drawings, a vertical furnace 1 for burning particles by 
the upward flow of a gas has a vertically elongated substantially tubular 
shape. As will be clearly seen from FIG. 1, the furnace 1 is provided at 
its top portion with a gas discharge opening 2, and particles are supplied 
from an opening 3 at its upper portion. A reference numeral 13 designates 
a falling chamber. An upward opening 7 for supplying a fuel for burning 
together with air and/or burning gas (referred to as "fuel and so forth", 
hereinafter) is formed at a lower portion of the furnace 1. Reference 
numerals 9 and 10 denote large-diameter cylindrical portion and 
small-diameter cylindrical portion which are formed beneath and adjacent 
to the falling chamber 13. The large-diameter cylindrical portion is 
connected to the small-diameter cylindrical portion 10 through an inverted 
conical portion 11. Similarly, the small-diameter cylindrical portion 10 
is connected to the gas supplying opening 7 through an inverted conical 
portion 12. It is to be understood that the circular cross-sections of the 
cylindrical and conical portions are not essential. Namely, the 
cylindrical and inverted conical portions can have a polygonal 
cross-section. In consequence, the velocity of the upward flow of the gas 
is lower at the large-diameter cylindrical portion 9 and higher at the 
small-diameter cylindrical portion 10. Consequently, as explained before, 
a fluidized bed 4 (rich fluidized bed) of a large density of particles and 
a fluidized bed 5 (lean fluidized bed) of a small density of particles are 
formed in series in the large-diameter cylindrical portion 9 and in the 
small-diameter cylindrical portion 10, respectively. 
In these fluidized beds 4 and 5 formed successively in the vertical 
direction, the lengths of stay of the particles during constant state of 
heating and burning conditions are different. In addition, there is a 
difference in the rate of suspension, circulation, and flowing of the 
particles between these two portions, partly because of the difference in 
the length of stay of the particles and partly because of the action of 
the inverted conical portion 11. Namely, this arrangement creates such a 
state that the particles are mixed imperfectly throughout the upper and 
lower fluidized beds. 
In the fluidized beds in the prior arts such as those disclosed in Japanese 
Patent Laid-open Nos. 121807/1978 and 68796/1979, there is an essential 
requisite that the operation is made such that the ratio U/Ut between the 
mean upward component velocity U of the fluidized bed and the terminal 
velocity Ut of the particles falls between 0.1 and 0.3, to ensure a 
perfect mixing of the particles to achieve a uniform temperature 
distribution throughout the fluidized bed. In contrast to the above, 
according to the invention, the operation is made such that the ratio U/Ut 
takes a value between 0.2 and 0.6 and a value between 0.4 and 1.0, 
respectively, in the rich fluidized bed and in the lean fluidized bed. 
Therefore, when burning is effected with the furnace of the invention, 
supposing that the fluidized bed 5 is the core of heating (zone of highest 
temperature in the process of burning and bloating of light-weight 
aggregate), a temperature difference is created between the two fluidized 
beds 4 and 5, partly because of the difference in the amount of stay of 
the particles and partly because of a difference in the amount of heat 
transfer attributable to the change in the flow velocity. Namely, in the 
constant state condition of operation, a temperature gradient is created 
in the direction of the height of the furnace, even though the fluidized 
beds 4 and 5 are formed continuously and integrally. In consequence, the 
temperature of the gas discharged from the fluidized bed 4 is lowered to 
reduce the fuel consumption. 
In the furnace of the invention in which the particles are mixed 
imperfectly in the upper and lower fluidized beds, the discharge of the 
burned particle is effected not in a overflowing manner due to perfect 
mixing as in the prior art disclosed in Japanese Patent Laid-open No. 
121807/1978 but due to a piston-flow effect, i.e. by an assist of a 
displacement flow, because the burned particles are discharged from the 
bottom of the furnace. In consequence, the period of stay, i.e. the length 
of stay, of the particles in the furnace is decreased and the 
nonuniformity of the burning is depressed. In addition, due to an 
increased heat exchange between the gas and the particles, the burning 
capacity per unit volume of the furnace is increased. 
According to the invention, the flow velocity of the fluidized beds formed 
during burning is much greater than that in the prior art so that the 
fluidized beds are comparatively lean as a whole. Particularly, the flow 
velocity is high and the density of the particle is low in the lower 
fluidized bed 5. In consequence, it is possible to obtain a greater 
agitating effect and, hence, to suppress the undesirable agglomeration of 
the burned particles. According to the invention, therefore, it is 
possible to bloat the light-weight aggregate by a treatment conducted at a 
comparatively high temperature, without addition of any agent for 
preventing the fusion, even if the particles have an appreciable tendency 
for fusion. This in turn permits the production of light-weight aggregate 
having smaller specific gravity. In addition, since the burning is 
conducted in the floated and suspended state of the particles, it is 
possible to treat spheroidized materials without the fear of thermal 
destruction. 
According to the invention, it is essential that the angle of upward 
divergence of the inverted conical portions 11 and 12 is less than 
90.degree., for otherwise the particles stick to the inner surfaces of the 
inversed conical portions to cause undesirable agglomeration by fusing of 
the particles. In order to ensure the sliding down of the particles along 
the tapered surfaces of the inversed conical portion, the upward 
divergence angle is preferably selected to fall below 60.degree.. 
Referring again to FIG. 1, reference numerals 6a to 6d designate radially 
oriented ports for feeding the fuel and so forth, formed in the wall of 
the furnace 1. Burners or the like means are inserted in these ports to 
form the temperature gradients mentioned before and to adjust the maximum 
temperature. It is not essential that these ports are formed in the 
small-diameter cylindrical portion 10. Namely, provided that the burning 
condition allows, the ports may be formed in the inverted conical portions 
11 and 12. 
In FIG. 1, a reference numeral 8 designates an opening for discharging the 
burned particles. In the burning of the particle in the furnace of the 
invention, the particle supplied through the opening 3 are discharged 
through the discharge opening 8 formed at the bottom of the furnace, after 
a temporary stay in the fluidized beds, as will be understood from the 
following description. 
The particles supplied to the fluidized bed 4 are burned as they stays in 
the furnace while being fluidized, circulated, and floated within the 
fluidized beds 4 and 5. In the furnace of the invention, since the falling 
chamber 13 is formed at an upper portion of the large-diameter cylindrical 
portion 9, a part of the particles conveyed by the upward flow can fall 
onto the fluidized bed and is made to stay in the latter. However, the 
part of the particles having smaller particle size which exceed the 
terminal velocity Ut of the upward flow is conveyed to the outside of the 
furnace together with the exhaust gas. The falling of the particles takes 
place at a rate corresponding to the loss of balance between the pressure 
drop of the upward gas flow and the amount of stay of the particles 
expressed by the following equation, and the particles thus fallen are 
discharged to the outside of the furnace from the bottom 8 of the latter. 
W (amount of particles staying in furnace) 
=.DELTA.P (pressure drop between fluidized beds 4 and 5) 
.DELTA.A (mean cross-sectional area of the second frusto-conical zone (11) 
of the furnace) 
Where, W represents the amount of particles staying in the furnace, while 
.DELTA.P represents the pressure drop caused by the particles staying in 
the fluidized beds 4 and 5. Also, A represents the cross-sectional area of 
the furnace equivalent to the state of balance between the amount W of 
stay of particles and the pressure drop .DELTA.P. 
In consequence, in the burning conducted by the furnace of the invention, 
it is possible to continuously supply the particle and to continuously 
discharge the burned product, through a control of the supplying rate 
solely. 
In the present invention, the operation is made at a gas velocity higher 
than the velocity U.sub.mf at which the particle starts to fluidize, 
because of the mean velocity of the upward gas flow in the fluidized beds 
as stated before. In addition, the discharge of the burned particle is not 
made by the overflowing which takes place in the conventional method. 
According to the invention, in spite of these facts, it is possible to 
make the burned particles fall through the fluidized beds and be 
discharged from the bottom of the furnace. This is attributable, according 
to the knowledge of the present inventors, to the above-mentioned 
unbalance between the pressure drop and the length of stay of the particle 
in the furnace, and also to the difference in flow velocity of the upward 
flow of gas in the central part and the inside peripheral part of the 
furnace. 
In the method of the invention, it is necessary to select a suitable length 
of stay of particles corresponding to the optimum period of stay, in 
accordance with the particle size of the particles. This can be achieved 
by suitably controlling the .DELTA.P (pressure drop between fluidized 
beds) by means of a gas draft fan (not shown in the figure) capable of 
adjusting the draft or suction force. 
The reference numeral 7 designates an opening for supplying and directing 
the fuel or the like upwardly from the bottom of the furnace. This is 
essential for forming the upward gas flow for supporting the fluidized bed 
5, as well as for adjusting the temperature of the fluidized bed 5 and 
also for suspending the burned particles falling down from the fluidized 
beds. The burned product is discharged through the discharge opening 8. 
As has been described, the major object of the present invention is 
efficiently burning such light-weight aggregates as would easily cause the 
agglomeration by fusing during or after bloating and burning. For this 
end, according to the invention, a temperature gradient is created between 
the rich fluidized bed 4 and the lean fluidized bed 5 which constitutes 
the region of the highest temperature. It is also necessary to promptly 
discharge the burned product without permitting it to accumulate, in order 
to prevent the undesirable agglomeration by fusing. If a large vacuum is 
generated in the region around the discharge opening 8, the ambient air or 
the like atmosphere is induced into the furnace to cool the fluidized bed 
5 so that the region of the maximum temperature is shifted to the area 
near the inverted conical portion 11 or the fluidized bed 4 to seriously 
hinder the control of the temperature distribution in the furnace. This 
also affects adversely the pressure drop between the fluidized beds and 
the length of stay of the particles, as well as the falling down of the 
burned particles, to unstabilize the pressure balance system. It is, 
therefore, advisable to adjust a static pressure in the region around the 
discharge opening approximating the atmospheric pressure. This adjustment 
of the static pressure is achieved through obtaining a balance between the 
feeding pressure of the fuel, air and the like and the draft produced by 
the draft fan. 
INDUSTRIAL APPLICABILITY 
The present invention is characterized by the following features (1) to 
(4). 
(1) The rich and lean fluidized beds are formed in series in the vertical 
direction due to the presence of the large-diameter cylindrical portion 
and the small-diameter cylindrical portion which are connected in series 
through an inverted conical portion, so that it is possible to effect the 
burning while forming a temperature gradient. It is, therefore, possible 
to effect the burning continuously and integrally because the division 
into a plurality of separate stages of fluidized beds as employed in the 
prior art can be eliminated. 
(2) The particle flow generally takes the form of a piston flow, and the 
upward gas flow is generally in the form of a counter current. 
(3) The burned particles are discharged, not by overflowing, but through a 
falling directly through the region of the highest temperature in the 
furnace, and the burned particles are taken out of the furnace from the 
bottom of the latter. 
(4) The falling chamber is formed at an uppermost part of the space in the 
furnace. 
These features in combination offer the following advantageous and 
industrial advantages. 
(a) It is easy to stabilize the furnace temperature and other operation 
factors. 
(b) It is possible to burn particles having particle size below 5 mm safely 
without using any preventing agent for agglomeration by fusing of the 
burned particles, and to obtain light-weight aggregates having a smaller 
specific gravity. 
(c) The fuel consumption is much reduced. 
(d) The burning capacity per unit volume of furnace is increased. 
(e) In the prior art method in which the burned product is discharged by 
overflowing, it is extremely difficult to discharge small pieces of burned 
product accidentally formed by agglomeration by fusing of particles. 
However, according to the invention, the discharge of such a small piece 
can be made automatically because the discharge is made directly through 
the bottom of the furnace, even if such small pieces are formed. 
EXAMPLE 
An experimental burning of a light-weight aggregate was conducted by means 
of a furnace having a construction similar to that shown in FIG. 1. The 
inside diameter of the falling chamber 13, cylindrical portion 9 and the 
cylindrical portion 10 were 200 mm, 130 mm and 70 mm, respectively. The 
angle of upward divergence of the inversed conical portion 11 was 
45.degree.. The total height of the fluidized beds 4 and 5 was about 470 
mm. The furnace had three ports formed in the side wall for supplying the 
fuel and so forth, as well as an opening 7 for supplying fuel and so forth 
and an opening 8 for discharging the burned products formed in the bottom 
thereof. The result of this experimental burning is shown in Table 1 
below. 
The particles were shale pulverized or spheroidized by crushing and were 
formed into particle sizes falling between 1.2 and 3.3 mm. Most of the 
burned product was discharged and collected from the opening 8 formed in 
the bottom of the furnace, while the remainder was discharged from the gas 
discharge opening 2 formed in the top of the furnace and collected by 
means of a cyclone (not shown in the FIGURE). As will be understood from 
Table 1, according to the invention, the temperature of the exhaust gas 
(temperature of gas at the uppermost part of the fluidized bed 4) was 
lowered to a level below the burning temperature (highest temperature in 
the fluidized bed 5). The rate of production of burned particles per unit 
volume was as high as 634 kg/m.sup.3.Hr. For comparison, the rate of 
production is as low as 40 to 60 kg/m.sup.3.Hr in the conventional rotary 
kilns and 200 kg/m.sup.3.Hr at the greatest in other known apparatus of 
fluidized bed type furnace. 
Also, the specific gravity of the product was as small as 1.35, although no 
material nor additive was used for preventing the agglomeration by fusing. 
TABLE 1 
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items result 
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particles supply rate (kg/Hr) 
14.5 
max. temp. in fluidized bed 5.degree. C. 
1150 
temp. at uppermost part of 
1030 
fluidized bed 4.degree. C. 
pressure drop between 
160 
fluidized beds mmAq 
amount of fuel LNG m.sup.3 /kg 
0.279 
burned particles 
mean period of stay min. 
6.1 
average weight of the particles in 
1.33 
suspension in the furnace at 
any given instant kg 
discharge rate of burned product kg/Hr 
13.0 11.0 at bottom 
2.0 at cyclone 
discharge rate/furnace volume 
634 
kg/m.sup.3.Hr 
specific gravity of burned product 
1.35 
agent or additive for preventing 
none 
agglomeration by fusing of 
burned particle 
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