JPH09126413A - Processing method of combustion gas in re-burning combustion method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a combustion gas processing method securing low NOX combustion in re-burning combustion. SOLUTION: A re-burning combustion method is adapted such that a hydrocarbon fuel is blown to combustion gas produced in a primary combustion zone and the combustion gas is combustion-processed under excessive fuel conditions, and thereafter it is passed through a burn-out zone on a rearstream side. In the method, the combustion gas so combustion-processed under excess fuel conditions is once kept in a temperature range of from 700 deg.C to 800 deg.C, and thereafter non-combustion components are combusted under the environment at the temperature of 1,000 deg.C or higher. HCN produced in the re-burning zone is reduced to N2 after passage through N2 O, and hence the rate of reduction of NOX is improved.

Description

Detailed Description of the Invention

[0001]

The present invention relates to relates to the treatment method of the combustion gases in the reburning combustion method, in particular by improving _the NO _x reduction ratio in the burn-out zone, reburning combustion which enables realizing a low NO _x combustion Method for treating combustion gas in the method.

[0002]

2. Description of the Related Art A reversing (in-furnace denitration) combustion method has been proposed as a NO _x (nitrogen oxide) reduction measure in a relatively large combustion furnace such as a waste incinerator. This is shown in FIG.
1, excess hydrocarbon fuel is blown into the combustion gas generated in the main combustion zone with an air ratio of 1 or more in the reburning zone, and burned under excessive fuel conditions (reducing atmosphere: air ratio <1). This is a combustion method in which NO _x is reduced by treatment and unburned components are completely burned by supplying air so that the air ratio becomes 1 or more in the subsequent flow. It is effective as a low NO _x combustion method. To be done.

[0003]

Conventionally, in the reverting combustion method, many studies have been conducted on the NO _x reduction reaction in the reburning zone, and based on that, the treatment of combustion gas for low NO _x combustion is performed. Although some proposals have already been made regarding the method (Mitsubishi Heavy Industries Technical Report, Vol. 17, No. 17, 1979, No. 6, Resources and Energy Utilization Technology Special Issue, etc.), HC in the reduction process to N ₂
There are few examples of detailed studies on intermediate products such as N, and the behavior of combustion gas in the burnout zone, which is the latter stage zone of combustion, is not always clearly understood.
Therefore, no particular proposal has been made regarding a method of treating combustion gas for reducing NO _x in the burnout zone.

The object of the present invention is to clarify the behavior of the combustion gas in the burnout zone, which is the latter zone of combustion in the reburning combustion method, and on the basis of it, higher N
The object of the present invention is to propose a method for treating combustion gas that achieves an O _x reduction rate and enables low NO _x combustion.

[0005]

In order to solve the above-mentioned problems, the present inventors carry out experiments on the revering combustion method focusing on the measurement of intermediate products in the NO _x reduction reaction and the reaction progress. As a result, when the combustion gas burned under excessive fuel conditions is once held in a predetermined temperature range and then burned out, NO _x reduction in the burnout zone is surely promoted and other harmful substances are also reduced. It was confirmed that it was not discharged.

The present invention is based on the above experimental results. Basically, a hydrocarbon fuel is blown into the combustion gas produced in the main combustion zone, and the combustion gas is burned under excess fuel conditions. In the reburning combustion method in which the burnout zone on the wake side is passed, the combustion gas burned under the above-mentioned excess fuel condition is once heated to 700 ° C.
After being maintained in a temperature range of up to 800 ° C., the combustion gas is treated so as to burn unburned components under an environment of 1000 ° C. or higher.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the method for treating combustion gas according to the present invention will be described in detail, while explaining the experimental results that lead to the above invention. Combustion experiments were conducted using a tubular flow reactor as shown in FIG. In the experiment, NO _x in the sample gas simulating the combustion exhaust gas was reduced under various conditions, and the concentrations of NO _x , HCN, N ₂ O, etc. at that time were measured. Reaction calculations under the same conditions were also performed. Methane is used as the reversing fuel, and air, N ₂ , C
A simulated exhaust gas was prepared from O ₂ and NO, and a predetermined amount of C was prepared.
In addition to H ₄ , it was used as a sample gas.

The flow rate of the sample gas was controlled by the mass flow controller 1 so that it was completely mixed and then introduced into the reaction tubes 2a and 2b. The reaction tubes 2a and 2b are connected to the electric furnaces 3a and 3b (100
Set to 0 ° C.). In the figure, the upstream electric furnace 3a is in the reburning zone, and the downstream electric furnace 3b is
Corresponds to the burnout zone, and the secondary air inlet 4 was provided between them. Then, the components of the combustion exhaust gas discharged from the reaction tube 2b on the downstream side were measured using various analyzers. For the reaction calculation, the mode in which the temperature-time change of SENKIN was given beforehand was used, and the reaction scheme proposed by Kilpinen was adopted. In the following description, when only the electric furnace 3a is heated and NO _x is reacted under an excessive fuel condition, a one-step reaction is performed, the two electric furnaces 3a and 3b are heated, and the secondary air inlet 4 The case where the secondary air is added to completely burn the unburned components is called a two-step reaction. In addition, α = (introduced oxygen amount) / (required oxygen amount) (α ₁ : one-step reaction, α ₂ : two-step reaction) was defined and used as a function in place of the air ratio in the experiment.

FIG. 2 shows the experimental value (ex) of the relationship between the concentrations of NO _x and HCN when α ₁ is changed as a basic condition.
p) and the calculated value (calc). The effect of α _{1 on} the reduction reaction of NO _x under the fuel excess condition is well shown, and the HCN concentration increases as the NO _x concentration decreases, and the total nitrogen compound concentration (NO _x + HC
It can be seen that N) is about 70% of the initial concentration of NO _{x when} α ₁ ≦ 0.65, and most of the NO _x is reduced to HCN during the one-step reaction.

Therefore, in order to understand the behavior of HCN in the burnout zone, a two-stage reaction experiment was conducted. In the experiment, secondary air was added from the secondary air inlet 4 and α ₂ = 1.05
It was fixed to and performed. FIG. 3 shows the experimental values of NO _x concentration in the two-step reaction when α ₁ was changed under the basic conditions, together with the NO _x concentration and NO _x + HCN concentration after the one-step reaction. In this experiment, HCN,
H ₂ and CO are below the detection limit, and 0.4 ≦ α ₁
In the range of ≦ 0.7, about 80 ppm of NO _x and HCN were present together in the first-step reaction, but in the two-step reaction, NO _x alone was 60 ppm or less, and the overall NO _x reduction rate was 40%. It was found that the reduction ratio was high as above.
This means that nitrogen-containing compounds (HCN
Etc.), the reduction reaction to N ₂ proceeds,
Secondary air inlet 4 provided between the two electric furnaces 3a, 3b
I thought that the secondary air supply from the plant had some effect.

Therefore, the nitrogen-containing compound N ₂ in the two-step reaction is used.
Further experiments were conducted to find out the mode of reduction to As described above, most of the NO _x is reduced to HCN only in the one-step reaction, and as shown in FIG. 4, the reaction process of HCN in an oxidizing atmosphere is from NCO to N ₂ O to N
There is a reduction reaction to ₂ and an oxidation reaction to NO.
Depending on the branching rate of the reaction that consumes CO, the final NO
_x concentration is considered to be determined, and further, HC
In the reaction N is reduced to N ₂ through the N ₂ O from NCO, that N ₂ O concentration has been reported to have a peak value at a certain temperature range (PB94-145646, December
23, 1993. P. Glarborg et. Al. Reburning Rich-Lean
Kinetics.) From the secondary air inlet 4 to the secondary air as described above for the two modes of α ₁ = 0.55 and 0.65 under the basic conditions of the one-step reaction. Was added and fixed to α ₂ = 1.05, and the set temperature (maximum temperature) of the two-step reaction was changed, and the product concentration was measured. The reaction calculation was also performed.

FIG. 5 shows the experimental value of the product concentration at α ₁ = 0.55, FIG. 6 shows the calculation result of the reaction, and FIG. 7 shows the experimental value of the product concentration at α ₁ = 0.65. The values are shown in FIG. 8 and the reaction calculation results are shown. As can be seen from FIGS. 5 and 7, HCN decreases as the CO reaction proceeds, and NO _x and N ₂ O are produced instead, as the CO reaction proceeds. When the set temperature is about 650 ° C. or higher, the concentration of the nitrogen-containing compound (NO _x + HCN + N ₂ O) is lower than that before the reaction, and it is about 20 ppm lower at the set temperature of 1000 ° C. The concentration of N ₂ O is 700 ° C
It becomes maximum under the temperature condition of 800 ° C, and almost disappears at 1000 ° C. As shown in FIGS. 6 and 8, the reaction calculation results also agree with the measured values for the compound concentration, and the formation of N ₂ O,
The decomposition temperature range was also consistent.

The reduction to N ₂ here is carried out by using H shown in FIG.
Inferred from the reaction path of CN, it is considered to be due to N ₂ O → N ₂ , and from the above experiment, in the two-step reaction (that is, the burnout zone in the actual combustion furnace), the combustion gas is given a temperature history. It was confirmed that the operation to increase the N ₂ O concentration with the result resulted in a high NO _x reduction rate. But on the one hand,
It is also known that N ₂ O is a harmful substance, and it is also necessary to accelerate the decomposition reaction of N ₂ O before the combustion gas is released into the atmosphere. Then, in the experiment with the present experimental device, by providing the secondary air inlet 4 between the two electric furnaces 3a, 3b, the required temperature history for the combustion gas can be obtained,
It is considered that the NO _x reduction rate shown in FIG. 3 was thereby achieved.

[0014] That is, of HCN that has passed through the N ₂ O N ₂
N ₂ at about 700 ° C to 800 ° C for the reduction reaction to
O 2 reaction zone and N ₂ above 1000 ° C.
It is effective to set two temperature ranges in the O decomposition reaction zone, and it is found that the NO _x reduction rate is surely increased by setting these temperature ranges in the burnout zone in the reburning combustion. It was

In order to confirm the above phenomenon, the supply of the secondary air in the experimental apparatus shown in FIG. 1 was stopped, and instead, the soaking part (the part where the maximum temperature was maintained) of the reaction tube 2b was replaced. Next, air was directly introduced to cause a reaction, and the concentration of product components was measured. The result is shown in FIG. 9 (where α ₁ = 0.5
5). As shown in the figure, in the vicinity of 1000 ° C., the nitrogen-containing compound concentration TFN (NO _x + HCN + N ₂ O) increased as compared with that before the reaction. This is because the combustion gas in the reaction tube 2a (reburning zone) was burned out at 1000 ° C. without reacting in a temperature range of less than 1000 ° C., that is, without having an N ₂ O production reaction zone. Therefore, it has been proved that the setting of the temperature range of and is effective for low NO _x combustion in reburning combustion.

FIG. 10 illustrates the case where the method for treating combustion gas obtained from the above experimental results is applied to an actual combustion furnace, and the burned material (for example, dust) has an air ratio ≧
Combustion gas generated in the main combustion zone is injected into the wake of the main combustion zone in which the combustion gas generated in the main combustion zone is burned in No. 1 under the fuel excess condition (reducing atmosphere: air ratio <1).
To reduce NO _x (reburning zone).
Further, in the subsequent burnout zone, the temperature of the combustion gas is adjusted to 7 by means such as arranging a water pipe group.
Heat is removed so as to be in the range of about 00 ° C to 800 ° C. After that, secondary air is supplied so that the air ratio becomes 1 or more. Thereby, the reaction of HCN to N ₂ O is promoted. Further, the exhaust gas temperature at the subsequent furnace outlet is controlled to be at least 1000 ° C. or higher, and the N ₂ O N ₂
The object of the present invention is achieved by carrying out the reduction to N and preventing the release of undecomposed N ₂ O to the atmosphere.

In the present invention, any means may be used to remove heat from the combustion gas from the reburning zone so as to bring it into the temperature range of 700 ° C. to 800 ° C., not limited to heat exchange by the water tube group, but air cooling. Means such as tubes are also effective.
Further, after that, means for raising the temperature of the combustion gas to 1000 ° C. or more is also optional, and in addition to raising the temperature by burning the unburned portion by supplying the secondary air, electric heating or addition of a small amount of fuel and air It is also possible to take measures such as heating by exhaust gas recirculation.

[0018]

The method of treating combustion gas in the reburning combustion method according to the present invention can be easily implemented by adding a simple improvement to the conventional reburning combustion furnace,
Moreover, reliable low NO _x combustion can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an experimental apparatus used for demonstrating a processing method of the present invention.

FIG. 2 is a graph showing the concentration of each product in the one-step reaction.

FIG. 3 is a graph comparing the difference in product concentration between the one-step reaction and the two-step reaction.

FIG. 4 is a diagram illustrating a reaction process of HCN.

FIG. 5 is a graph (experimental value) showing the concentration of each product in a two-step reaction (with temperature history).

FIG. 6 is a graph (calculated value) showing the concentration of each product in a two-step reaction (with temperature history).

FIG. 7 is a graph (experimental value) showing the concentration of each product under other conditions in the two-step reaction (with temperature history).

FIG. 8 is a graph (calculated value) showing the concentration of each product under other conditions in the two-step reaction (with temperature history).

FIG. 9 is a graph (calculated value) showing the concentration of each product in a two-step reaction (no temperature history).

FIG. 10 is a diagram for explaining a combustion apparatus that uses the method for treating combustion gas according to the present invention.

FIG. 11 is a diagram illustrating a conventional reburning combustion furnace.

Claims (1)

[Claims] 1. A hydrocarbon fuel is blown into the combustion gas produced in the main combustion zone, the combustion gas is burned under an excessive fuel condition, and then passed through a burnout zone on the downstream side. In the burning combustion method, the combustion gas burned under the above-mentioned excess fuel condition was once heated to 700 ° C.
A method for treating combustion gas in a reburning combustion method, which comprises maintaining a temperature range of up to 800 ° C. and then burning unburned components in an environment of 1000 ° C. or higher.