JPH0368285B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a denitrification combustion method, and in particular, even when starting or stopping a combustion device where nitrogen oxides (hereinafter referred to as NOx) in exhaust gas tend to increase, NOx can be reduced.
The present invention relates to a continuous denitrification combustion method suitable for reducing oxidation.

Since NOx is considered to be one of the causative agents of photochemical oxidants and acid rain, there is a need to develop a combustion method that effectively suppresses its generation. As combustion methods for this purpose, (1) exhaust gas recirculation method, (2) two-stage combustion method, and (3) in-furnace denitrification combustion method are known. The exhaust gas recirculation method is a method of reducing NOx by using air whose O ₂ partial pressure has been lowered by mixing exhaust gas as a combustion gas and performing gentle combustion. Generally, the two-stage combustion method uses a device equipped with a multi-stage burner and an after-air port above the burner, and performs the first combustion while maintaining the multi-stage burner at a low air ratio that is advantageous for reducing NOx, and then performs the first combustion. The unburned components generated by this process are re-burned in the presence of air supplied from the after-air port, but combustion in the burner section is slow, making it possible to reduce NOx. In addition, the in-furnace denitrification combustion method is performed using the same equipment as the two-stage combustion method, but this method creates a combustion region with a large excess of fuel on the downstream side of the multi-stage burner, and reduces the reduction radicals generated in this region. Occurs in the upstream burner section
NOx is reduced to _N2 , while unburned components are completely combusted in the presence of air supplied from the after-air port, as in the case of the two-stage combustion method described above.

As described above, excellent denitrification combustion methods have been attempted in the past, but recently, improved methods have been developed that can reduce NOx to the extent that no additional denitrification equipment is required. However, even though all of the above methods are effective during normal operation, they are not always satisfactory due to NOx regulation restrictions when starting or stopping the combustion device. In other words, as is well known, NOx in combustion exhaust gas (hereinafter simply referred to as exhaust gas) is regulated based on the standard O ₂ partial pressure <O ₂ > specific to each exhaust gas, as shown in equation (1) below.
This is done using the converted value [NOx] obtained based on s.

[NOx] Conversion value = (21.0 - <O ₂ > s) / [NOx] / (21.0 - < _{O 2} > a) ... (1) (In the formula, <O ₂ > s is a value specific to exhaust gas by fuel. standard
It shows the partial pressure of _O2 , which is 4% when the fuel is oil, 5% when the fuel is gas, and 6% when the fuel is coal. In addition, <O ₂ > a is the actual O ₂ partial pressure (%) in the exhaust gas, and [NOx] is the NOx concentration in the exhaust gas (ppm).
shows. ) Therefore, although the absolute value of NOx concentration is low, <O ₂ >
When starting up a combustion device (such as a boiler) where a is at a high level of approximately 21.0%, the [NOx] conversion value becomes extremely large, which poses a regulatory problem. A similar situation can be observed during shutdown operations, where the fuel supply is gradually reduced while extinguishing the fire.

Conventional solutions to this problem include installing a denitrification device downstream of the combustion device and introducing exhaust gas into the combustion zone immediately after ignition of the burner to lower the O ₂ partial pressure.

However, the former method has the disadvantage that it requires the installation of a denitrification device, which has become unnecessary during normal operation as denitrification technology improves. In addition, in the latter method, the exhaust gas is introduced after the burner ignites, so as is clear from equation (1) above, there is a limit to the reduction in the [NOx] conversion value, and it cannot comply with strict environmental regulations. There is a disadvantage.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to
The object of the present invention is to provide a denitrification combustion method that can constantly reduce NOx.

In order to achieve the above object, the present invention provides a burner, a combustion air supply system that supplies combustion air to the burner, an exhaust gas circulation system that can join the burner, and an exhaust gas circulation system that supplies exhaust gas from the combustion device outlet to the exhaust gas circulation system. In a denitrification combustion method using a combustion device equipped with an exhaust gas recirculation fan, when burning fuel, the recirculation fan is first started, the exhaust gas from the fan is supplied into the combustion device, and then a small amount of fuel is After supplying and igniting, and reducing the oxygen partial pressure to a predetermined oxygen partial pressure, a combustion air intake fan is started, and while maintaining the oxygen partial pressure at approximately a predetermined value, the air supply amount increases as the fuel supply increases, Shift to steady operation,
Next, when stopping, the operation is reversed to the above-mentioned operation when starting.

With this configuration, when the combustion device is started, combustion proceeds under the circulating supply of only the exhaust gas whose O ₂ partial pressure gradually decreases after burner ignition, so the O ₂ partial pressure of the exhaust gas quickly decreases. The [NOx] conversion value calculated by the above-mentioned formula (1) is also maintained at a low value within the regulation value. On the other hand, when the equipment is stopped, fuel supply is reduced and subsequent fire extinguishing operations are performed, but since these operations are performed while exhaust gas is being supplied, there is no increase in O ₂ partial pressure, and therefore the [NOx] equivalent The value is kept low. In addition, after startup [NOx]
After the converted value falls within the regulation value, the amount of air passing through the fuel air supply system is increased according to the amount of fuel supplied,
After that, normal denitrification combustion operation can be performed.

Hereinafter, the present invention will be explained in more detail with reference to embodiments shown in the drawings.

Figure 1 shows the system of a conventional combustion device that can also be applied to the present invention. A line 1 that supplies fuel to a furnace 2 and a burner (not shown) provided in the furnace 2.
and a combustion air supply line 9 that is mixed with the exhaust gas sent through the exhaust gas circulation line 10 and then supplied to the burner. In addition, in the figure, 3 is an exhaust gas circulation fan provided in the exhaust gas circulation line 10, 4 is a furnace bottom damper provided in a line 11 that branches from the line 10 and reaches the bottom of the furnace 2, and 5 is a line an exhaust gas damper provided in the line 10 between the branching part to the line 11 and the joining part with the line 9;
Reference numerals 6, 7 and 8 are a combustion air intake fan, a combustion air preheater and a combustion air damper, which are respectively provided in the line 9, 12 is a dust collector provided in the exhaust gas line, and 13 is a chimney. In such a combustion apparatus, during normal operation, fuel is sprayed into the furnace 2 through line 1, and is supplied through line 10.
Normal denitrification is carried out under the supply of combustion air (generally preheated to about 300°C) sent through line 9 after being mixed with the circulating exhaust gas passing through the furnace 2, and exhaust gas sent to the bottom of the furnace 2 through line 11. Combustion takes place. The high-temperature exhaust gas generated by the combustion passes through the upper part of the furnace 2 and then is sent to the combustion air preheater 7 installed at the outlet where heat is recovered.Then, after dust is removed by the dust collector 12, it flows out from the chimney 13. Released into the atmosphere.

The start-up operation is performed according to the invention in the manner shown in FIG. In the figure, 20 is O ₂ partial pressure,
30 indicates the amount of fuel, and 40 indicates the amount of air.

That is, first, based on the command to start the combustion device,
Drive of the exhaust gas circulation fan 3 is started at time A with the dampers 4 and 5 open and the damper 8 closed, and the air in the furnace 2 is circulated from this point. Next, at time B, a small amount of fuel is supplied from line 1,
ignited. O ₂ in the air circulating in the furnace 2 is consumed by this ignition and subsequent combustion, so the O ₂ partial pressure decreases over time, and the amount of NOx produced also decreases (B (See changes between ~C).

Next, at time C when the O ₂ partial pressure has decreased to a predetermined O 2 partial pressure, the combustion air intake fan 6 is driven and the combustion air damper 8 is opened, thereby preventing an excessive decrease in the O ₂ partial pressure. After this operation continues until time D, the supply of fuel is increased and the corresponding combustion air supply is increased, and the opening degree of the damper 8 is adjusted until time E, whereby the partial pressure of O ₂ in the combustion gas is increased. The load is increased while keeping constant. After that, similar operations are repeated, and then a transition is made to steady operation. According to this startup method, only the expanded portion of the combustion gas is exhausted from the chimney 13 via the dust collector 12 until point C, when an increase in converted NOx becomes a particular problem, but the amount is extremely small. Therefore, O ₂ in the exhaust gas (used as combustion gas)
The reduction in partial pressure almost directly contributes to the reduction in the amount of NOx produced, and suitable NOx suppression is achieved.

Next, regarding the operations at the time of shutdown, these operations are basically performed in the order of operations at startup, that is, reducing the supply of combustion air, reducing the fuel supply and extinguishing the fire, and stopping the operation of the exhaust gas circulation line. Generally, it is sufficient to reduce the fuel supply and subsequently extinguish the fire. Even if this operation is performed, it will be performed under the operation of the exhaust gas recirculation line, so it will not be accompanied by an increase in the O ₂ partial pressure, thus maintaining the [NOx] conversion value at a low value. Because you can.

Although typical embodiments of the present invention have been described above, the present invention is not limited thereto. For example, the following operations related to the present invention, namely, the exhaust gas circulation fan 3 at the time of startup, The operations of the combustion air intake fan 6, furnace bottom damper 4, exhaust gas damper 5, combustion air damper 8, etc. are programmed in advance, and are programmed based on the O ₂ partial pressure signal in the combustion gas detected just before the burner. Automatic control is also possible, which can further reduce fluctuations in O ₂ partial pressure. Note that it is desirable to input information such as the furnace capacity and fuel supply amount into the above program as necessary.
Further, in the above embodiment, the combustion air intake fan is started to be driven at time C, but this driving may be started at the same time as the exhaust gas circulation fan 3 is driven. It can save time.

In addition, a separate burner for ignition is installed on the burner,
Combustion between B and C in FIG. 2 can be performed using only this burner, and the same effect can be achieved.

Furthermore, a denitrification device or a desulfurization device may be provided downstream of the combustion device shown in the above embodiments, if necessary.

As described above, according to the present invention, the operation of the exhaust gas circulation system includes the start and ignition of fuel supply to the burner when the combustion device is started, the stop of fuel supply to the burner when the combustion device is stopped, and the accompanying extinguishment. This makes it possible to constantly supply combustion gas with a low O ₂ partial pressure, and thereby to achieve NOx reduction at all times during denitrification combustion operation, including at startup and shutdown.

[Brief explanation of drawings]

FIG. 1 is a system diagram of a combustion device according to an embodiment of the present invention, and FIG. 2 is a diagram illustrating an operation method when starting up the combustion device according to an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Fuel supply line, 2... Furnace, 3... Exhaust gas circulation fan, 5... Exhaust gas damper, 6... Combustion air intake fan, 8... Combustion air damper, 9...
... Combustion air supply line, 10 ... Exhaust gas circulation line, 20 ... O ₂ partial pressure, 30 ... Fuel amount, 40 ...
...Amount of air.

Claims (1)

[Claims] 1 comprising a burner, a combustion air supply system that supplies combustion air to the burner, an exhaust gas circulation system that can join the burner, and an exhaust gas recirculation fan that supplies exhaust gas from the combustion device outlet to the exhaust gas circulation system. In the denitrification combustion method using a combustion device, when burning fuel, a recirculation fan is first started, exhaust gas from the fan is supplied into the combustion device, and then a small amount of fuel is supplied and ignited to achieve a predetermined oxygen content. After the pressure has decreased, the combustion air intake fan is started, and while maintaining the oxygen partial pressure at approximately a predetermined value, the air supply amount increases as the fuel supply increases, and the operation shifts to steady operation. A denitrification combustion method characterized by performing operations in the reverse order of the operations at startup.