Method of controlling combustion in a fluidized bed furnace

A method for controlling combustion in a furnace into which combustible material, e.g., urban refuse, industrial waste, and a controlled quantity of combustion air are fed. The furnace includes a mixing/stirring region where unburnt gas and secondary combustion air are mixed and stirred. A part of exhaust gas derived from the furnace is blown into the mixing/stirring region depending on variation in the quantity of the secondary combustion air, and a flow rate of mixture gas consists of the exhaust gas and the secondary combustion air to be fed into the mixing/stirring region is maintained within a predetermined region so that effective combustion takes place.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of controlling combustion in a 
furnace of the type including a mixing/stirring region where unburnt gas 
and secondary combustion air are mixed and stirred together so that 
combustible material, e.g., urban refuse, industrial waste or the like is 
effectively burnt. 
2. Description of the Prior Art 
Generally, urban refuse, various kind of industrial waste and so forth are 
substantially different from each other in configuration or size. This 
makes it difficult to practically design and construct a combustible waste 
material feeding machine on a commercial basis under the condition that a 
quantity of waste material to be fed into a furnace per unit time is 
correctly maintained. In addition, urban refuse, industrial waste and so 
forth differ in quality, and any variation in quality and/or quantity of 
combustible waste material to be fed into the furnace is directly 
converted into a variation in quantity and/or quantity of the resulting 
exhaust gas. Therefore, when the furnace is provided with a fixed supply 
of combustion air, there arises a problem that an excess or shortage of 
oxygen occurs. If the furnace is operated with an insufficient quantity of 
oxygen, unburnt gas is discharged from the furnace. On the contrary, when 
the furnace is operated with an excessive quantity oxygen, i.e., an 
excessive quantity of combustion air, the resultant combustion gas is 
cooled to an undesirable extent. This means that incomplete combustion 
takes place with the result that unburnt gas is discharged from the 
furnace. In view of the aforementioned problems, to make sure that an 
excessive or insufficient supply of oxygen does not occur in the furnace, 
a density of oxygen in the resultant combustion gas is normally measured 
to correctly control the quantity of combustion air to be fed into the 
furnace. 
A proposal has been heretofore made such that shape and size of a 
combustion chamber of the furnace be designed in a different manner so as 
to allow combustion air to effectively come into contact with unburnt gas 
and moreover the blowing speed of the combustion air is varied so that the 
combustion air is effectively mixed with the unburnt gas. 
However, with the aforementioned conventional method of controlling 
combustion in a furnace, when the furnace is controlled while varying a 
quantity of combustion air to be fed into the furnace, the blowing speed 
of the combustion air varies. Thus, when the furnace is operated under 
conditions different from designed points, there arises another problem 
that the configuration of the furnace does not correctly match the blowing 
speed of the combustion air. 
SUMMARY OF THE INVENTION 
The present invention has been made with the foregoing background in mind. 
An object of the present invention is to provide an improved method of 
controlling combustion in a furnace wherein a part of combustion air is 
introduced into the interior of the furnace so that a flow rate of mixture 
gas comprising secondary combustion air and exhaust gas to be fed to a 
mixing/stirring region where unburnt gas and secondary combustion air are 
mixed and stirred together is maintained within a predetermined range 
irrespective of what extent the combustion state varies. 
Another object of the present invention is to provide a method of 
controlling combustion in a furnace such that while combustion is taking 
place, combustion reaction in an upper portion of a combustion chamber of 
the furnace is activated so that a temperature at the upper furnace region 
is maintained within an optimum range. 
To accomplish the above objects, the present invention provides an improved 
method of controlling combustion in a furnace or an incinerator of the 
type including a combustion chamber arranged directly above a furnace bed. 
The lower portion of the combustion chamber is a mixing/stirring region in 
which unburnt gas and secondary combustion air are mixed and stirred 
together. The furnace is supplied with a quantity of combustible material 
varying per unit time and a quantity of combustion air which is controlled 
in response to the quantity of the combustible material, wherein a flow 
rate of mixture gas comprising secondary combustion air and exhaust gas to 
be fed to the mixing/stirring region is maintained within a predetermined 
range by blowing a part of the exhaust gas into the mixing/stirring region 
depending on the variation in the quantity of combustion air. 
According to the present invention, when a part of the exhaust gas is also 
blown into the upper portion of the combustion chamber, and when a 
quantity of exhaust gas to be blown into the mixing/stirring region 
increases or decreases, the flow rate of exhaust gas to be blown into the 
upper portion of the combustion chamber is increased or decreased in 
opposition to the rate of increase or decrease in the quantity of blown 
exhaust gas so that a flow rate of circulating exhaust gas to be 
introduced into the interior of the furnace is maintained within a 
predetermined range. 
In addition, according to the present invention, a temperature at the upper 
furnace region is normally monitored and a flow rate of exhaust gas to be 
blown into the upper portion of the combustion chamber is correctly 
controlled in response to the upper furnace temperature so that the upper 
furnace temperature is maintained within the range of from 750.degree. C. 
to 950.degree. C. 
Further, according to the present invention, the mixing/stirring region 
arranged directly above the furnace bed is formed by a throttle section of 
the combustion chamber. 
Further, according to the present invention, a part of the exhaust gas to 
be fed to the mixing/stirring region is mixed with secondary air to be 
blown into the throttle section in a horizontal direction or in a 
slantwise downward direction. 
Furthermore, according to the present invention, a part of the exhaust gas 
to be fed to the mixing/stirring region is mixed with secondary combustion 
air to be blown into the throttle section in the horizontal direction or 
in the slantwise downward direction thereby to create a swirling flow in 
the throttle section. 
Since the flow rate of mixture gas to be fed into the combustion chamber, 
particularly, the mixing/stirring section is maintained within the 
predetermined range by blowing a part of exhaust gas depending on a 
variation in the quantity of combustion air, unburnt gas is stirred at the 
same rate irrespective of what extent the quantity of combustion air 
varies. Consequently, the unburnt gas effectively comes into contact wiht 
the combustion air, whereby the unburnt gas is effectively burnt while 
discharge of the unburnt gas from the furnace is reduced as far as 
possible. 
In a case where active combustion takes place in the furnace and a quantity 
of secondary combustion air to be blown into the mixing/stirring region 
increases but a quantity of exhaust gas to be blown into the 
mixing/stirring region decreases, an amount of mixture gas is blown into 
the upper portion of the combustion chamber corresponding to the reduced 
quantity of exhaust gas. Consequently, combustion reaction at the upper 
furnace region can be activated. While combustion takes place in this 
manner, the temperature at the upper furnace region tends to increase. In 
addition, as the flow rate of the circulating exhaust gas increases, 
cooling is enhanced, whereby the upper furnace temperature is maintained 
within an optimum range at all times. 
Since the upper furnace temperature is normally monitored and a quantity of 
exhaust gas to be blown into the upper portion of the combustion chamber 
is correctly controlled to maintain the upper furnace temperature within a 
range of from 750.degree. C. to 950.degree. C., the upper furnace 
temperature is maintained within an optimum range at all times. 
Additionally, since a part of the exhaust gas is blown into the throttle 
section arranged directly above the furnace bed in the horizontal 
direction or in the slantwise downward direction to build a swirling flow 
in the throttle section with the resultant mixture gas comprising 
secondary combustion air and exhaust gas, the unburnt gas is effectively 
stirred irrespective of what extent the rate of unburnt gas flowing in the 
throttle section in the vertical direction increases. As a result, 
discharge of the unburnt gas from the furnace can be further reduced. 
Other objects, features and advantages of the present invention will become 
apparent from reading the following description which has been made with 
reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, the present invention will be described in detail hereinafter with 
reference to the accompanying drawings which illustrate a preferred 
embodiment thereof. 
FIG. 1 is a system diagram which schematically illustrates a case where the 
present invention is applied to a fluidized bed type furnace or 
incinerator. In the drawing, reference numeral 11 designates a fluidized 
bed. A combustion chamber 13 is arranged directly above the fluidized bed 
11, and a throttle section 12 having a small sectional area is formed at 
the upper end of the combustion chamber 13. The throttle section 12 is 
provided with a plurality of secondary combustion air feeding ports 14 on 
the inner wall thereof for the purpose of slantwise downwardly blowing 
secondary combustion air in the interior of the combustion chamber 13. In 
addition, a plurality of tertiary combustion air feeding ports 15 are 
arranged on the inner wall of an upper portion 28 of the combustion 
chamber and above the throttle section 12 for feeding ternary combustion 
air or exhaust gas therethrough. An air chamber 16 is arranged below the 
fluidized bed 11 so that primary combustion air is blown into the air 
chamber 16 via a piping which extends from an induction fan 17. As the fan 
17 is rotated, primary combustion air is conveyed therefrom to enter the 
air chamber 16 thereby to fluidize a fluidizing medium in the fluidized 
bed 11. Combustible material to be burnt in the fluidized bed 11, e.g., 
urban refuse, industrial waste or the like is introduced into the interior 
of the fluidized bed 11 through a fuel feeding port (not shown) so that it 
is burnt therein to generate combustion gas. The combustion gas passes 
past the throttle section 12 in the form of a mixing/stirring region and 
it is then discharged from the upper portion 28. Further, the combustion 
gas flows through an exhaust gas cooling unit 19, an exhaust gas treating 
unit 20 and a suction fan 21. Finally, the combustion gas is discharged as 
exhaust gas into the atmosphere via a chimney 22. 
Reference numeral 24 designates an oxygen density regulator for measuring a 
density of oxygen in the exhaust gas based on an output from an oxygen 
density sensor 23 thereby to control the oxygen density to a predetermined 
value. A control unit (not shown) compares a value indicative of an output 
from the oxygen density regulator 24 with an output from a flow rate 
sensor 26 for detecting a flow rate of secondary combustion air to be fed 
into the throttle section 12 under a condition that the output value of 
the oxygen density regulator 24 is used as a preset value for a secondary 
combustion air flow rate regulator 25 and then regulates a flow rate of 
the secondary combustion air by actuating a flow rate regulating valve 27. 
Reference numeral 29 designates a mixture gas flow rate setter for setting 
a flow rate of mixture gas comprising secondary combustion air and exhaust 
gas, and reference numeral 30 designates an exhaust gas flow rate 
calculator for calculating a quantity of exhaust gas to be fed into the 
throttle section 12 based on an output from the mixture gas flow rate 
setter 29 and an output from the flow rate sensor 26 or an output from the 
mixture gas flow rate setter 29 and a set value derived from the secondary 
combustion air flow rate regulator 25. The exhaust gas flow rate 
calculator 30 serves to convert a quantity of exhaust gas into an extent 
of opening of a damper and moreover regulates a quantity of exhaust gas to 
be fed into the throttle section 12 by actuating a flow rate regulating 
valve 31. 
Reference numeral 40 designates a circulating exhaust gas flow rate setter 
for setting a flow rate of exhaust gas to be circulated, and reference 
numeral 32 designates an exhaust gas flow rate calculator for calculating 
a flow rate of exhaust gas to be fed into the upper portion 28 of the 
combustion chamber based on an output from the circulating exhaust gas 
flow rate setter 40 and an output from the exhaust gas quantity calculator 
30. 
Reference numeral 35 designates an upper furnace temperature regulator for 
measuring an upper furnace temperature based on an output from a 
temperature sensor 34 for detecting an upper furnace temperature in order 
to generate an output in the form of an upper furnace temperature signal 
to control a flow rate of exhaust gas so as to allow the upper furnace 
temperature to remain with in a range of from 750.degree. C. to 
950.degree. C., and reference numeral 33 designates a low selector for 
selecting the lower of two outputs, i.e., an output from the upper furnace 
temperature regulator 35 and an output from the exhaust gas flow rate 
calculator 32. The low selector 33 actuates a flow rate regulating valve 
39 to regulate a flow rate of exhaust gas to be fed into the upper portion 
28 of the combustion chamber. Reference numeral 36 designates a cyclone 
and reference numeral 37 designates an exhaust gas circulating fan. 
Further, reference numeral 41 designates a primary combustion air flow rate 
regulator for indicating a flow rate of primary air to be fed to the lower 
part of the fluidized bed 11 or a certain location just above the 
fluidized bed 11. The primary combustion air flow rate regulator 41 
measures a flow rate of primary combustion air based on an output from a 
flow rate sensor 42 to regulate the flow rate of primary combustion air to 
a preset value by actuating a flow rate regulating valve 43. With the 
fluidized bed type furnace as constructed in the above-described manner, 
the oxygen density regulator 24 compares a value derived from detection of 
a density of oxygen in the exhaust gas with a certain preset value and 
then outputs the value derived from the comparison as a preset value for 
the secondary combustion air regulator 25. This secondary combustion air 
regulator 25 calculates an excessive quantity or an insufficient quantity 
of secondary combustion air based on an output from the flow rate sensor 
26 and an output from the oxygen density regulator 24 thereby to regulate 
a quantity of secondary combustion air to be fed into the throttle section 
12. In a case where it is found that a quantity of secondary combustion 
air is reduced, the flow rate regulating valve 27 is actuated to open for 
the purpose of compensating a quantity of shortage. To the contrary, in a 
case where it is found that a quantity of secondary combustion air becomes 
excessive, the flow rate regulating valve 27 is actuated in the reverse 
direction to the foregoing case to reduce the flow rate of secondary 
combustion air. 
The exhaust gas flow rate calculator 30 calculates a flow rate of exhaust 
gas to be fed into the throttle section 12 based on an output from the 
flow rate sensor 26 and an output from the mixture gas flow rate setter 29 
or an output from the mixture gas flow rate setter 29 and a set value 
derived from the secondary combustion air regulator 25 and then regulates 
a quantity of exhaust gas to be fed into the throttle section 12. In a 
case where it is found that a quantity of secondary combustion air has 
increased, the flow rate generating valve 31 is actuated in the direction 
of closing to reduce a flow rate of exhaust gas in opposition to the 
increased quantity of secondary combustion air. To the contrary, in a case 
where it is found that a quantity of secondary combustion air has been 
reduced, the flow rate regulating valve 31 is actuated in the direction of 
opening to increase a flow rate of exhaust gas corresponding to the 
reduced quantity of secondary combustion air. In this manner, a flow rate 
of gas passing through the throttle section 12 is normally held at a level 
of the flow rate which has been set by the mixture gas flow rate setter 
29. 
The exhaust gas flow rate calculator 32 calculates an insufficient quantity 
of flow rate of an exhaust gas to be circulated based on an output from 
the exhaust gas flow rate calculator 30, i.e., a flow rate of the exhaust 
gas to be fed into the throttle section 12 and an output from the 
circulating exhaust gas flow rate setter 40 and then actuates the flow 
rate regulating valve 39 thereby to regulate a flow rate of the exhaust 
gas to be fed into the upper portion 28 of the combustion chamber. Namely, 
in a case where a quantity of exhaust gas to be fed into the throttle 
section 12 via the flow rate regulating valve 31 is reduced, the flow rate 
regulating valve 39 is actuated in the direction of opening to increase a 
flow rate of exhaust gas corresponding to the reduced quantity of exhaust 
gas so as to allow the increased quantity of exhaust gas to be fed into 
the upper portion 28 of the combustion chamber. To the contrary, in a case 
where a quantity of exhaust gas to be fed into the throttle section 12 is 
increased, the flow rate regulating valve 31 is actuated in the reverse 
direction to the foregoing case to reduce the flow rate of exhaust gas 
corresponding to the increased quantity of exhaust gas. Thus, a quantity 
of exhaust gas to be circulated is controlled to coincide with the 
quantity of exhaust gas to be circulated which has been set by the 
circulating exhaust gas flow rate setter 40. The low selector 33 selects 
the lower of two outputs, i.e., an output from the furnace top temperature 
regulator 35 and an output from the exhaust gas flow rate calculator 32 
and then actuates the flow rate regulating valve 39 in response to the 
selected output. Thus, e.g., in a case where a flow rate of exhaust gas to 
be fed into the upper portion 28 of the combustion chamber 13 is increased 
and thereby the upper furnace temperature is reduced to be lower than the 
lower limit of the optimum range (750.degree.-950.degree. C.), the control 
unit operates to reduce a flow rate of circulating exhaust gas to be fed 
into the upper portion 28 of the combustion chamber thereby to prevent the 
working temperature from being excessively reduced. 
During such a control operation as described above, e.g., when a density of 
oxygen in the exhaust gas is elevated, the flow rate regulating valve 27 
is actuated in the direction of closing to reduce a flow rate of secondary 
air. Then, the exhaust gas flow rate calculator 30 serves to actuate the 
flow rate regulating valve 41 in the direction of opening thereby to 
increase the flow rate of exhaust gas to be circulated corresponding to 
the reduced quantity of secondary air. Consequently, a flow rate of 
mixture gas comprising secondary air to be fed via the secondary 
combustion air feeding ports 14 and exhaust gas is kept substantially 
constant irrespective of what extent the quantity of secondary combustion 
air varies corresponding to variation in the oxygen density. Therefore, a 
stirring state of the gas which is left unburnt in the throttle section 12 
is kept substantially constant irrespective of variation of a quantity of 
secondary combustion air (i.e., quantity of air required for combustion). 
Generally, when a quantity of exhaust gas is reduced to zero and only 
secondary combustion air is fed through the secondary combustion air 
feeding ports 14, the flow speed of air fed through the secondary 
combustion air feeding ports 14 remains at a level of about 40 m/s, and as 
the density of oxygen in the exhaust gas increases, the flow rate of 
secondary air is reduced. Thus, in some cases, the flow speed of air fed 
through the secondary combustion air feeding ports 14 may be reduced. At 
this time, a stirring/mixing state of the unburnt gas in the throttle 
section 12 deteriorates with the result that the unburnt gas is discharged 
to the outside as it is. To prevent the foregoing undesirable process from 
taking place, the flow rate sensor 26 detects that the flow rate of 
secondary combustion air to be fed into the throttle section 12 is reduced 
and then the control unit operates to actuate the flow rate regulating 
valve 31 in the direction of opening via the exhaust gas flow rate 
calculator 30 thereby to increase the flow rate of the circulating exhaust 
gas corresponding to the reduced quantity of secondary combustion air. 
Thus, since the flow rate of the mixture gas to be fed into the throttle 
section 12 is kept substantially constant, combustion gas and combustion 
air are normally stirred together under the effect of a constant magnitude 
of stirring power, whereby unburnt gas is effectively brought into 
constant with the combustion gas, resulting in discharge of the unburnt 
gas into the atmosphere being kept to a minimum. Consequently, in 
constrast with the conventional fluidized bed type furnace, the fluidized 
bed type furnace of the present invention wherein a density of oxygen in a 
combustion gas is measured and a quantity of combustion air is correctly 
controlled so as not to cause excess or shortage of oxygen, assures that 
unburnt gas is not discharged to the outside. 
In addition, since the fluidized bed type furnace of the present invention 
is provided with the mixture gas flow rate setter 29 so as to allow 
secondary combustion air to be preferentially fed into the throttle 
section 12, there is no danger that the oxygen density will be excessively 
reduced in the throttle section 12 where mixing/stirring is achieved with 
combustion gas. As combustion is activated and thereby a flow rate of 
secondary combustion air to be fed into the throttle section 12 increases, 
exhaust gas which has become useless is fed into the upper portion 28 of 
the combustion chamber, enabling a combustion reaction to take place in 
the upper furnace region. When combustion takes place in that way, the 
upper furnace temperature tends to increase. At this time, the upper 
furnace region is cooled by increasing the flow rate of exhaust gas, 
whereby the upper furnace temperature is maintained within an optimum 
range. If the upper furnace temperature is reduced to the lower than the 
lower limit of the optimum range (750.degree.-950.degree. C.), the flow 
rate of exhaust gas to be fed into the upper portion 28 of the combustion 
chamber is accordingly reduced to prevent the upper furnace temperature 
from being excessively lowered. 
According to the aforementioned embodiment of the present invention, 
secondary combustion air or mixture gas comprising secondary combustion 
air and exhaust gas is slantwise downwardly blown into the throttle 
section 12 through the secondary combustion air feeding ports 14 to 
enhance stirring intensity. Alternatively, as shown in FIG. 2, secondary 
combustion air or mixture gas comprising secondary combustion air and 
exhaust gas may, of course, be horizontally blown into the throttle 
section 12 through a plurality of secondary combustion air feeding ports 
14 which are arranged in a horizontal attitude, although, to some extent, 
this cause a lowering of stirring intensity. 
Additionally, as shown in FIG. 3, to make sure that secondary combustion 
air or mixture gas comprising secondary combustion air and exhaust gas 
which has been blown into the throttle section 12 in a slantwise downward 
direction or in a horizontal direction builds a swirling flow, the 
secondary combustion air feeding ports 14 may be arranged to extend in the 
tangential direction relative to the inner wall surface of the furnace as 
seen in a cross-sectional plane. With such an arrangement, the 
advantageous effect derived from the stirring intensity can be further 
enhanced. 
According to the aforementioned embodiment of the present invention, the 
secondary combustion air feeding ports 14 are arranged in two stages 
positionally offset in the vertical direction of the furnace. However, the 
present invention should not be limited only to this. Alternatively, the 
secondary combustion air feeding ports 14 may be arranged in a single 
stage or in a plurality of stages positionally offset from each other in 
the vertical direction of the furnace. In addition, arrangement may be 
made such that gas blown in the throttle section 12 through the secondary 
air feeding ports 14 arranged in plural stages build a swirling flow. 
According to the aforementioned embodiment of the present invention, 
exhaust gas to be circulated is blown into the throttle section 12 while 
mixing with secondary air, resulting in the furnace becoming complicated 
in structure. Alternatively, exhaust gas to be circulated may be blown in 
the throttle section 12 through exhaust gas feeding port(s) which are 
arranged separately from the secondary combustion air feeding ports 14. 
According to the aforementioned embodiment of the present invention, an 
operative region where combustion gas is mixed and stirred is designed in 
the form of a throttle section having a small sectional area. The 
mixing/stirring region should not be limited only to this configuration. 
The mixing/stirring region is not necessarily required to have a small 
cross-sectional area, provided that it is proven that mixing/stirring is 
achieved with excellent efficiency. 
Further, according to the aforementioned embodiment of the present 
invention, the present combustion state in the furnace is detected by 
measuring the density of oxygen in exhaust gas by the oxygen density 
sensor 23 and the oxygen density regulator 24. However, detecting means 
for detecting the present combustion state in the furnace should not be 
limited only to the above-described arrangement. Alternatively, in view of 
the fact that brightness and pressure in the furnace vary depending on the 
combustion state (e.g., as long as combustion takes place actively, 
brightness and pressure in the furnace are kept high, but when combustion 
does not take place actively, brightness and pressure in the furnace are 
reduced), the present combustion state in the furnace may be sensed by 
detecting the level of brightness and pressure in the furnace. Further, 
since the combustion state in the furnace is correlated to the furnace 
temperature, the combustion state in the furnace may be sensed by 
detection the furnace temperature. In this case, however, there occurs a 
time delay which may lower the effectiveness derived from detection of the 
combustion state in the furnace. 
While the present invention has been described above with respect to the 
embodiment wherein the method of the present invention is applied to the 
fluidized bed type furnace, it should, of course, be understood that the 
method of the present invention should not be limited only to a fluidized 
bed type furnace. 
FIG. 4 is a system diagram which schematically illustrates a furnace or an 
incinerator of the type including a stoker type furnace bed for which the 
method of controlling combustion in the furnace according to the present 
invention is employed. The same or similar components in the drawing as 
those in FIG. 1 are represented by same reference numerals. Since these 
components have the same operative function as that of the components in 
FIG. 1, repeated description will not be made. 
In FIG. 4, reference numeral 12' designates a mixing/stirring region and 
reference numeral 28 designates an upper portion of a combustion chamber 
13. Further, reference numeral 51 designates a feeder, reference numeral 
52 designates a drying stoker, reference numeral 53 designates a 
combustion stoker and reference numeral 54 designates a post-combustion 
stoker. 
Operation of the combustion system as shown in FIG. 4 is substantially 
identical to that of the fluidized bed type combustion system which has 
been described above with reference to FIG. 1. Therefore, repeated 
description will not be made. 
As will be apparent from the above description, the method of controlling 
combustion in a furnace according to the present invention has the 
following advantageous effects. 
(1) According to the present invention, a gas flow speed in the 
mixing/stirring region is maintained within a predetermined range by 
blowing a part of exhaust gas into the mixing/stirring region depending on 
the variation in the quantity of combustion air. Thus, combustion gas is 
stirred at a constant rate in a furnace wherein a quantity of combustion 
air is controlled by measuring the density of oxygen in an exhaust gas so 
as not to cause an excessive or insufficient supply of oxygen, 
irrespective of the quantity of combustion air which has been blown into 
the interior of the furnace. Consequently, unburnt gas is effectively 
brought into contact with the combustion air with the result that 
discharge of unburnt gas into the atmosphere can be minimized. 
(2) According to the present invention, when a quantity of secondary 
combustion air to be fed into the mixing/stirring region increases, 
exhaust gas which has become useless is fed into the upper portion of the 
combustion chamber, whereby combustion reaction in the upper furnace 
region can be activated. As combustion becomes activated in this way, the 
temperature in the upper furnace region tends to increase. In addition, as 
the flow rate of exhaust gas to be blown into the upper portion of the 
combustion chamber increases, the upper furnace region can be effectively 
cooled. As a result, the upper furnace temperature can be maintained 
within an optimum range. 
(3) Further, according to the present invention, the upper furnace 
temperature is normally monitored and a quantity of exhaust gas to be 
blown into the upper portion of the combustion chamber is correctly 
controlled such that the upper furnace temperature is maintained within a 
range of from 750.degree. C. to 950.degree. C. Consequently, the upper 
furnace temperature can normally be maintained with an optimum range. 
While the present invention has been described above merely with respect to 
two preferred embodiments thereof, it should be noted that the present 
invention should not be limited only to them but various changes or 
modifications may be made without departure from the scope of the appended 
claims.