Boiler and combustion control method

Abstract

Provided is a boiler (100) including: a burner (5); a fuel supply unit (10) for supplying fuel to the burner (5); a blowing unit (20) for supplying air to the burner (5); and a control unit (30) for adjusting an amount of fuel to be supplied to the burner (5) and a quantity of air to be supplied to the burner (5), in which the control unit (30) has a reference amount computing portion for calculating a reference fuel amount and a reference air quantity to be supplied to the burner (5) with respect to a required load, an air quantity computing portion that corrects the reference air quantity based on a temperature of the air to be supplied to the burner (5) and a temperature of the fuel to be supplied to the burner (5) and calculates the corrected air quantity as a supply air quantity, and a control portion that controls combustion at the burner based on the reference fuel amount and the supply air quantity.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boiler and a combustion control method making it possible to perform combustion at a predetermined air ratio and to suppress generation of NOx in the exhaust gas.

2. Description of the Related Art

It is important for a boiler to be capable of performing stable combustion with high thermal efficiency. In view of this, there have been proposed air ratio control boilers or combustion apparatuses in which control is performed such that combustion is performed at a predetermined air ratio.

For example, JP 2001-272030 A proposes an air-fuel ratio control (air ratio control) monitoring method for a burner in a boiler and an air-fuel ratio control monitoring apparatus for executing the method. In the method and the apparatus, in response to a load command from a control panel, the pressure of the combustion air to be supplied to the burner and the pressure of the fuel to be supplied to the burner or the pressure of the fuel returned from the burner are monitored to make a judgment as to whether the air amount and the fuel amount are being properly controlled.

JP 10-47654 A discloses an air ratio automatic correction system for a combustion apparatus in which combustion air is preheated and supplied. In the system for a combustion apparatus, each of a combustion air supply path and a fuel supply path is provided with a pressure gauge and a thermometer, the fuel supply path is equipped with an equalizing valve for equalizing the fuel supply pressure and the combustion air supply pressure, an impulse line of the equalizing valve is provided with an orifice and a bleeding valve, a fuel supply pressure for supplying the requisite fuel for maintaining a predetermined air ratio is obtained from the actual air temperature and air supply pressure and fuel temperature measured by each thermometer and pressure gauge, and the fuel supply pressure thus obtained and the actually measured fuel supply pressure are compared with each other to adjust the bleeding valve such that those become equal to each other.

Further, from the viewpoint of environmental hygiene, there is a demand for a boiler capable of suppressing discharge of harmful exhaust gas. In order to meet this demand, JP 2000-46302 A, for example, discloses a boiler in which there is provided a rectangular combustion space with vertical water tubes standing close together between water tube walls arranged in parallel so as to be spaced apart from the burner front surface. In the boiler, there is provided a relatively long gas passage extending from the burner to a gas outlet via the inter-space between the vertical water tubes, thereby suppressing the flame combustion temperature to a level of A approximately 1200 to 1300° C. to thereby reduce NOx to 70 to 80 ppm and to reduce CO to 50 ppm or less.

However, none of the above-mentioned conventional techniques proposes a boiler or a combustion control method capable of controlling both the air ratio and the NOx generation in the exhaust gas and suppressing generation of NOx in a stable manner even if there is a change in outside temperature due to seasonal variations or the like. As an air ratio control boiler, the air-fuel ratio control monitoring method or the air-fuel ratio control monitoring device as disclosed in JP 2001-272030 A is advantageous in that it is a simple method or device. However, it is intended for the control of the air ratio when the air ratio is outside a predetermined range, but it is not capable of accurately controlling the air ratio. The air ratio automatic correction system for a combustion apparatus proposed in JP 10-47654 A has a problem in that it requires a complicated construction and control. As a boiler for suppressing NOx in the exhaust gas, the boiler as proposed in JP 2000-46032 A can effectively achieve a reduction in NOx. However, there is a demand for a boiler capable of achieving a further reduction in NOx generation in a stable manner.

SUMMARY OF THE INVENTION

In view of above-mentioned requirement and the problems in the prior art, it is an object of the present invention to provide a boiler and a combustion control method which are of a relatively simple construction, in which combustion is performed at a predetermined air ratio, and which can suppress generation of NOx in the exhaust gas.

A boiler according to the present invention includes: a control means for adjusting a quantity of air to be used for combustion based on a temperature change in an air and fuel to be used for combustion. Further, a boiler according to the present invention includes: a burner; a fuel supply means for supplying fuel to the burner; a blowing means for supplying air to the burner; and an amount of fuel to be supplied to the burner and a quantity of air to be supplied to the burner, which are adjusted by the control means, in which the control means has a reference amount computing portion for calculating a reference fuel amount and a reference air quantity to be supplied to the burner with respect to a required load, an air quantity computing portion that corrects the reference air quantity based on a temperature of the air to be supplied to the burner and a temperature of the fuel to be supplied to the burner and calculates the corrected air quantity as a supply air quantity, and a control portion that controls combustion at the burner based on the reference fuel amount and the supply air quantity.

In the above-mentioned aspect of the present invention, thermistors are preferably used as a means for measuring the temperature of the air to be supplied to the burner and as a means for measuring the temperature of the fuel to be supplied to the burner. The air quantity computing portion calculates the supply air quantity such that a correction amount for correcting the reference air quantity is in proportion to 1/(1+R_TH1/R_s+R_TH1/R_TH2), where R_TH1 is a resistance value of an air temperature measuring thermistor, R_TH2 is a resistance value of a fuel temperature measuring thermistor, and R_s is a resistance value of a fixed resistor.

Further, the air quantity computing portion calculates the supply air quantity such that a correction amount for correcting the reference air quantity is in proportion to T_a/(T_g)^1/2, where T_g is the temperature of the fuel measured by a fuel temperature measuring means, and T_a is the temperature of the air measured by an air temperature measuring means.

A combustion control method according to the present invention is a method by which combustion is performed at a predetermined air ratio and in which NOx in an exhaust gas is suppressed to a level within a predetermined range, the combustion control method including: calculating a reference fuel amount and a reference air quantity corresponding to a required load of the boiler; correcting the calculated reference air quantity based on temperatures of a fuel and an air to be used for combustion; and performing combustion with the corrected air quantity and the reference fuel amount.

The boiler according to the present invention has a relatively simple construction, and is capable of performing combustion at a predetermined air ratio, suppressing generation of NOx in the exhaust gas in a stable manner, and performing stable combustion with high thermal efficiency. Further, according to the combustion control method of the present invention, it is possible to suppress generation of NOx to a level of 12 ppm or less in normal combustion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the construction of a boiler according to the present invention;

FIG. 2 is a circuit diagram showing a construction example of a blower control portion of a control device of the boiler of FIG. 1;

FIG. 3 is a graph showing an example of how the O₂ amount in the exhaust gas is controlled by a control device having the blower control portion of FIG. 2;

FIG. 4 is a graph showing the relationship between ambient temperature and fuel temperature;

FIG. 5 is a graph showing an example of how the NOx amount in the exhaust gas is controlled by the control device having the blower control portion of FIG. 2;

FIG. 6 is a graph showing the relationship between the NOx amount and the O₂ amount in the exhaust gas;

FIG. 7 is a graph showing the relationship between blower frequency and supply air temperature when the control device having the blower control portion of FIG. 2 is used;

FIG. 8 is a graph showing the relationship between the O₂ amount in the exhaust gas and fuel temperature when the control device having the blower control portion of FIG. 2 is used and when the supply air temperature is fixed at 20° C. or 40° C.; and

FIG. 9 is a graph showing the relationship between the O₂ amount in the exhaust gas and fuel temperature when, in the case of FIG. 8, fixed resistance R_s, reference frequency f₀, and maximum frequency f_m are varied.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, a boiler according to an embodiment of the present invention will be described with reference to the drawings. The boiler according to the present invention is provided with a control device for adjusting air quantity based on temperature changes of air and fuel. FIG. 1 is a schematic view of an embodiment thereof. As shown in FIG. 1, a boiler 100 has a burner 5, a fuel supply device 10 for supplying fuel to the burner 5, a blowing device 20 for supplying air to the burner 5, and a control device 30 for controlling the amount of fuel supplied to the burner 5 and the quantity of air supplied to the burner 5; further, the boiler 100 has a fuel temperature measuring device 35 for measuring the temperature of the fuel supplied to the burner 5 and transmitting a corresponding signal to the control device 30, and an air temperature measuring device 36 for measuring the temperature of the air supplied to the burner 5 and transmitting a corresponding signal to the control device 30.

The boiler 100 according to the present invention operates as follows: Fuel (e.g., natural gas) is sent from the fuel supply device 10 and spouted at the forward end of the fuel supply device 10 (in the vicinity of the right-hand end of the fuel supply device 10 as seen in FIG. 1); this fuel is supplied to the burner 5 while being mixed with combustion air supplied from the blowing device 20, and is burned by the burner 5. The burned gas passes gaps between a plurality of water tubes (water tube group) 40, and is gradually cooled while performing heat exchange with water in the plurality of water tubes 40, and is then sent to a flue 50 before being discharged into the atmosphere.

It is possible to use a well-known burner as the burner 5. There are no particular limitations regarding the type of burner. In the case of the boiler of the embodiment shown in FIG. 1, there is used a perfectly premixed burner having a planar burning surface.

As the fuel supply device 10, it is possible to use a well-known fuel supply device. For example, it is possible to use a fuel supply device having a pump, a control valve, and a control device for controlling them, and capable of supplying a predetermined amount of fuel corresponding to a load.

It is possible to use a well-known blowing device as the blowing device 20. For example, it is possible to use an inverter type blower having a blower, a drive source, and an inverter for controlling the RPM of the blower, and capable of supplying a predetermined quantity of air corresponding to the fuel. It is also possible to use a so-called damper type blower capable of supplying a predetermined quantity of air corresponding to the fuel.

The control device 30 has a reference amount computing portion, an air quantity computing portion, and a control portion. The reference amount computing portion serves to calculate a reference fuel amount and a reference air quantity corresponding to the load required of the boiler 100 by the heat engine. The air quantity computing portion serves to correct a reference air quantity calculated by the reference amount computing portion based on the output from an air temperature measuring device 36 for measuring the temperature of the air to be supplied to the burner 5 and the output from a fuel temperature measuring device 35 for measuring the temperature of the fuel to be supplied to the burner 5, calculating the corrected air quantity as the supply air quantity. The control portion serves to supply the burner 5 with the quantity of supply air as obtained by the air quantity computing portion with respect to the reference fuel amount already calculated to perform required combustion.

It is only necessary for the fuel temperature measuring device 35 and the air temperature measuring device 36 to be capable of measuring the temperatures of the fuel and the air and supplying the control device 30 with signals corresponding to the temperatures. For example, it is possible to use a thermistor as the fuel temperature measuring device 35 or the air temperature measuring device 36, whereby it is possible to form the fuel temperature measuring device 35 or the air temperature measuring device 36 in a simple and compact structure.

The boiler 100, constructed as described above, is used as follows. First, when a certain amount of load is required of the boiler 100 by the heat engine or the like, the reference amount computing portion of the control device 30 calculates a reference fuel amount and a reference air quantity with respect to the required load. The reference fuel amount and the reference air quantity are theoretically computed under a predetermined air ratio. Next, the calculated reference air quantity is corrected by the air quantity computing portion based on the temperatures of the fuel and the air to be supplied to the burner 5. That is, with respect to the load momentarily required by the heat engine or the like, the control device 30 according to the present invention serves to supply the burner 5 with a corrected air quantity described below based on the theoretical reference fuel amount and the temperatures of the actually supplied fuel and air, making it possible to perform the required combustion.

In the present invention, this reference air quantity is corrected according to a principle described below. A case will be described in which an inverter type blower is used as the blowing device 20. Suppose that the temperature of the air supplied to the burner 5 (supply air temperature) is T_a, that the air density is ρ_a, that the volume flow rate is Q_a, and that the RPM of the blower is N. Since the volume flow rate Q_a is in proportion to the RPM N, and the air density ρ_a is in inverse proportion to the supply air temperature T_a, the following formula (1) holds true:

ρ_aQ_a α N′/Ta   (1)

The fuel is supplied to the burner 5 such that the flow velocity of the fuel supplied is a predetermined flow velocity, that is, the difference in pressure between the pressurization source side and the boiler 100 side is fixed, so the following equation holds true:

ΔP_g=a×ρ_gQ_g²=fixed value (a is a constant), Q_g α (T_g)^1/2   (2)

where ΔP_g is the difference in pressure, T_g is the fuel temperature, and ρ_g is the fuel density.

Since the fuel density ρ_g is in inverse proportion to the fuel temperature T_g, from the above equation (2), the following formula (3) holds true:

ρ_gQ_g α 1/(T_g)^1/2   (3)

To maintain a fixed air ratio, it is necessary to keep ρ_aQ_a/ρ_gQ_g at a fixed value. That is, it can be seen from formulas (1) and (3) that to maintain a fixed air ratio, it is necessary to keep N×(T_g)^1/2/Ta at a fixed value. It can be seen that the RPM N of the blower is to be adjusted as shown in the following equation (4), in which k is a constant:

N=k×T_a/(T_g)^1/2   (4)

Equation (4) shows that it is possible to perform combustion at a fixed air ratio by making an adjustment such that the RPM of the blower is in proportion to the supply air temperature T_a and in inverse proportion to (T_g)^1/2, where T_g is the fuel temperature. That is, by supplying the burner 5 with a quantity of air (supply air quantity) corrected by taking into consideration the reference air quantity and the supply air temperature, it is possible to cause the boiler 100 to perform combustion at a predetermined air ratio. The pressure of the air supplied to the burner 5 is not necessarily an essential monitoring factor for performing combustion at a fixed air ratio. During use of the boiler, there are various fluctuations in conditions, and it can happen that the predetermined air ratio is deviated from. In such cases, it is effective to monitor the air pressure.

In the present invention, the reference air quantity is corrected based on the supply air temperature and the fuel temperature, and the corrected air quantity (supply air quantity) and the already calculated reference fuel amount are supplied to the burner 5 by the control portion of the control device 30 to thereby perform combustion. The correction amount for the correction of the reference air quantity is an amount in proportion to T_a/(T_g)^1/2.

This correction of the reference air quantity based on the supply air temperature and the fuel temperature can be executed by providing in the air quantity computing portion a program allowing computation of a predetermined correction amount based on the signals from the fuel temperature measuring device 35 and the air temperature measuring device 36, and a computer for executing the program. However, as described below, it is possible to form a control device 30 of a simple structure by a control device having the fuel temperature measuring device 35 and the air temperature measuring device 36 consisting of thermistors, and a blower control portion capable of directly controlling the blower based on signals from the thermistors.

FIG. 2 shows an example of the blower control portion of the control device 30. As shown in FIG. 2, the blower control portion of the control device 30 has a fixed resistor R_s, a fuel temperature measuring thermistor R_TH2 connected in parallel thereto, and an air temperature measuring thermistor R_TH1 connected in series to them. The symbols represented by R and their subscripts indicate resistance values (Ω). Assuming that the frequency (reference frequency) at the time of minimum voltage applied is f_o, and that the frequency (maximum frequency) at the time of maximum voltage is f_m, the inverter frequency f with respect to the voltage V input to the inverter can be expressed as follows: f=f_o+(f_m−f_o)×V; when this output is performed, assuming that the maximum voltage applied to the inverter is V_o, the voltage V input to the inverter can be expressed as follows: V=V_o×1/(1+R_TH1/R_s+R_TH1/R_TH2), so the frequency f can be expressed as follows: f=f_o+(f_m−f_o)×V_o×1/(1+R_TH1/R_s +R_TH1/R_TH2)

FIG. 3 shows the O₂ amount in the exhaust gas when the boiler 100 is controlled by the control device 30 having the blower control portion shown in FIG. 2. In FIG. 3, the horizontal axis indicates the supply air temperature, and the vertical axis indicates the O₂ amount in the exhaust gas. As described above, curves A₁ and A₂ represent examples of the present invention in which combustion is performed in the boiler 100 with the supply air quantity and the reference fuel amount. The curve A₁ indicates the case in which the supply air quantity is obtained on the assumption that the fuel temperature and the supply air temperature are equal to each other. The curve A₂ indicates the case in which the supply air quantity is obtained on the assumption that the fuel temperature changes in a proportion of ½ with respect to the change amount of the supply air temperature. Curve B indicates a case (conventional example) in which it is assumed that the supply air temperature and the fuel temperature are equal to each other and in which combustion is performed in the boiler 100 with an air quantity obtained by correcting the reference air quantity taking solely the supply air temperature into consideration and the reference fuel amount.

The curves of FIG. 3 are obtained through calculation of the O₂ amount (%) in the exhaust gas when theoretical combustion is performed under the following conditions: Assuming that the temperature of the fuel or the supply air is T(° k), the resistance value of the fuel temperature measuring thermistor and the air temperature measuring thermistor is R_TH1=R_TH2=15000×exp (3450(1/T−1/273)) (Ω). The resistance value R_s of the fixed resistor is 4500 Ω. The reference frequency f_o is 50 Hz, and the maximum frequency f_m is 73 Hz.

According to FIG. 3, the curves A₁ and A₂ are of a fixed linear configuration, with the O₂ amount being 6%. Thus, it can be seen that, in the present invention; combustion is performed at a fixed air ratio in the supply air temperature range of 10 to 50° C. In contrast, the curve B is an inclined straight line indicating a change in O₂ amount of 5.7 to 6.7% within the supply temperature range of 10 to 50° C. Thus, it can be seen that combustion at a fixed air ratio is not performed within the temperature range of 10 to 50° C. in the case of the conventional example.

In FIG. 3, the curve A₁ is a graph indicating the case in which the supply air quantity is obtained on the assumption that the fuel temperature and the supply air temperature are equal to each other, and the curve A₂ is a graph indicating the case in which the supply air quantity is obtained on the assumption that the proportion of the change in fuel temperature is ½ of the proportion of the change in supply air temperature. As described below, actually, however, it is to be assumed that the O₂ amount can be controlled to a value within the range surrounded by the curves A₁ and A₂.

FIG. 4 is a graph showing the relationship between the temperature around piping and the fuel temperature at the inlet of the boiler 100 in a case in which fuel (natural gas or LPG) flows through the piping, which has a total length of approximately 50 m and is exposed on the ground, at a maximum flow velocity of approximately 500 Nm³/h. In FIG. 4, the horizontal axis indicates the ambient temperature, and the vertical axis indicates the fuel temperature. FIG. 4 indicates that the fuel temperature is in a high correlation with the ambient temperature; assuming that the fuel temperature is T_g and that the ambient temperature is T_a, T_g can be expressed as follows: T_g=0.75 T_a (T_g/T_a=0.75). That is, it can be seen that the fuel temperature changes at a proportion of ¾ with respect to the change amount of the ambient temperature. Further, according to certain data, when high combustion is performed by using LPG fuel in the boiler 100 with gas piping which is exposed on the ground and of a length of approximately 10 m, the fluctuation width of the fuel temperature at the inlet of the boiler 100 is approximately ½ of the fluctuation width of the ambient temperature. Taking into account the above data and the possibility of the gas piping being buried in the ground, it is to be assumed that the fluctuation width of the fuel temperature at the inlet of the boiler 100 is generally approximately ½ or less of the fluctuation width of the ambient temperature. It is to be assumed that the supply air temperature is substantially equal to the ambient temperature. That is, in the present invention, it is to be assumed that the O₂ amount can be controlled within the range surrounded by the curves A₁ and A₂ shown in FIG. 3.

In this way, in the boiler 100 according to the present invention, combustion can be performed at a predetermined air ratio. Further, as described below, the boiler 100 according to the present invention can perform combustion in which the NOx generation amount is small. FIG. 5 is a graph showing the relationship between the supply air temperature and the NOx generation amount. In FIG. 5, the horizontal axis indicates the supply air temperature, and the vertical axis indicates the NOx amount (ppm). The curves shown in FIG. 5 are obtained by converting the O₂ amounts of the curves A₁, A₂ and B shown in FIG. 3 into NOx amounts based on the characteristic curve of FIG. 6 showing the relationship between the NOx amount and the O₂ amount. FIG. 6 is a graph obtained through a combustion test on a surface combustion burner type boiler 100 using premixed air fuel mixture with a performance of an evaporation amount of 3130 kg/h. As in FIG. 3, in FIG. 5, of the symbols A₁, A₂ and B, the symbols A₁ and A₂ indicate examples of the present invention, and the symbol B indicates a conventional example. The symbol A₁ indicates a case in which the supply air quantity is obtained on the assumption that the fuel temperature is equal to the supply air temperature, and the symbol A₂ indicates a case in which the supply air quantity is obtained on the assumption that the proportion of the change in the fuel temperature is ½ of the proportion of the change in the supply air temperature. This also applies to the following description.

According to FIG. 5, in the case of the present invention, the NOx amount is 10.6 to 11.2 ppm within the supply air temperature range of 10 to 50° C. Thus, it can be seen that the NOx amount is kept at a level below 12 ppm.

FIG. 7 is a graph showing the relationship between the control frequency and the supply air temperature in an inverter type blower under the same conditions as in the case of the graph shown in FIG. 2 or 3. In FIG. 7, the horizontal axis indicates the supply air temperature, and the vertical axis indicates the frequency of the blower. According to FIG. 7, the curves A₁, A₂ and B are all substantially linear, with the curve A₁ exhibiting the minimum gradient and the curve B the maximum gradient. From the results as shown in FIG. 7 and those shown in FIG. 3, it can be seen that in the case of the conventional example, an excessive quantity of air is supplied to the burner 5 upon an increase in the supply air temperature. That is, it can be seen that in the present invention, not only is it possible to perform combustion in the boiler 100 at a fixed air ratio and with low NOx, but it is also possible to perform an economical operation with the boiler 100.

As described above, in the present invention, it is possible to form a simple and compact control device 30 by a control device using a thermistor. The boiler 100 having the control device 30 can perform combustion within a supply temperature range of 10 to 50° C., at a fixed air ratio, and with low NOx. When the boiler 100 is installed in a room, the room temperature is generally higher than the outside temperature, with its upper limit being approximately 50° C. Thus, it suffices to consider the characteristics of the boiler 100 within the supply temperature range of 10 to 50° C. However, the supply air temperature is outside the above-mentioned range in some cases. Such cases can be coped with by selecting a fixed resistor and a thermistor that are in conformity with the conditions of use thereof.

FIGS. 8 and 9 show the results of an examination of the effects of the fixed resistance R_s, the reference frequency f_o of the inverter, and the maximum frequency f_m on the O₂ amount in the exhaust gas when the supply air temperature is fixed at 20° C. or 40° C. In FIGS. 8 and 9, the horizontal axis indicates the fuel temperature, and the vertical axis indicates the O₂ amount (ppm) in the exhaust gas. In the cases shown in the drawings, the supply air temperature is 20° C. or 40° C. In the case of FIG. 8, the fixed resistance R_s, the reference frequency f_o, and the maximum frequency f_m are the same as those of FIG. 3. In contrast, in the case shown in FIG. 9, the fixed resistance R_s is 3000 Ω, the reference frequency f_o is 51 Hz, and the maximum frequency f_m is 74 Hz. In FIGS. 8 and 9, the curve B indicates a conventional case.

Comparison of FIGS. 8 and 9 shows that in the case of FIG. 8, the O₂ amount in the exhaust gas is controlled to a fixed level, and that in the case of FIG. 9, no conspicuous effect of the control is to be observed. That is, it can be seen that it is necessary to select the fixed resistance R_s, the thermistor resistors R_TH1 and R_TH2, the reference frequency f_o of the inverter, and the maximum frequency f_m according the conditions of use of the boiler 100.

Claims

1. A boiler comprising: a control means for adjusting a quantity of air to be used for combustion based on a temperature change in an air and fuel to be used for combustion.

2. A boiler according to claim 1 further comprising:

a burner;

a fuel supply means for supplying fuel to the burner;

a blowing means for supplying air to the burner; and

an amount of fuel to be supplied to the burner and a quantity of air to be supplied to the burner, which are adjusted by the control means,

wherein the control means has a reference amount computing portion for calculating a reference fuel amount and a reference air quantity to be supplied to the burner with respect to a required load, an air quantity computing portion that corrects the reference air quantity based on a temperature of the air to be supplied to the burner and a temperature of the fuel to be supplied to the burner and calculates the corrected air quantity as a supply air quantity, and a control portion that controls combustion at the burner based on the reference fuel amount and the supply air quantity.

3. A boiler according to claim 2, further comprising thermistors used as a means for measuring the temperature of the air to be supplied to the burner and as a means for measuring the temperature of the fuel to be supplied to the burner.

4. A boiler according to claim 3, wherein the air quantity computing portion calculates the supply air quantity such that a correction amount for correcting the reference air quantity is in proportion to 1/(1+RTH1/Rs+RTH1/RTH2), where RTH1 is a resistance value of an air temperature measuring thermistor, RTH2 is a resistance value of a fuel temperature measuring thermistor, and Rs is a resistance value of a fixed resistor.

5. A boiler according to claim 2, wherein the air quantity computing portion calculates the supply air quantity such that a correction amount for correcting the reference air quantity is in proportion to Ta/(Tg)1/2, where Tg is the temperature of the fuel measured by a fuel temperature measuring means, and Ta is the temperature of the air measured by an air temperature measuring means.

6. A combustion control method by which combustion is performed at a predetermined air ratio, and in which NOx in an exhaust gas is suppressed to a level within a predetermined range, the combustion control method comprising:

calculating a reference fuel amount and a reference air quantity corresponding to a required load of the boiler;

correcting the calculated reference air quantity based on temperatures of a fuel and an air to be used for combustion; and

performing combustion with the corrected air quantity and the reference fuel amount.