JPH0729028B2 - Denitration treatment method - Google Patents

Description

The present invention relates to a denitration treatment method, and more particularly to a denitration treatment method suitable for treating exhaust gas containing heavy metal elements, which are poisoning components of catalysts in fuel. Regarding

(Prior Art) FIG. 6 shows a main exhaust gas system diagram of a boiler plant incorporating the most common flue gas denitration device according to the prior art.
For example, in order to treat nitrogen oxides (hereinafter referred to as NOx) in the exhaust gas discharged from the exhaust gas generator (hereinafter referred to as a boiler) 1, ammonia 15 is used as a reducing agent in the exhaust gas flue 18 of the boiler 1 to reduce NOx. A denitration device (denitration reactor) 2 having a built-in catalyst for accelerating the reaction of NH ₃ with NH ₃ is provided. Since this denitration device 2 generally performs the denitration most efficiently in the temperature range of about 300 to 400 ° C, the boiler economizer 24
And the air heater 3. On the other hand, the exhaust gas from the boiler passes through the air heater 3, is dust-removed by a dust remover (for example, an electric dust collector, etc.) 4, and is then released from the chimney 6 into the atmosphere through the desulfurizer 5. On the other hand, about 15% of the ash contained in the exhaust gas is discharged as a clinker from the furnace bottom of the furnace 25 in the furnace 25 of the boiler 1, and the remaining 84% is removed by the dust collector 4. Some of the fuel (coal) burned in the boiler 1 is difficult to burn, and for example, the ash collected by the dust collector 4 may contain a large amount of unburned components. For example, about 5 to 5 parts of the fuel supplied to the boiler 1
In some cases, as much as 10% is included as unburned matter. Therefore, if this unburned component is discarded as it is from the ash discharge line 20 as shown in FIG. 6, the combustion efficiency of the boiler 1 will be low, which will lead to a decrease in boiler efficiency, resulting in a heat balance problem. In this example, the ash mass balance is 85 at the outlet of the boiler 1, 15 at the ash discharge line 20 of the boiler, and 84 at the ash collected by the dust collector 4 when the fuel supply line 10 is 100. It becomes 1 at the inlet line of the desulfurization device 5.

Therefore, as shown in FIG. 7, the ash removed by the dust collector 4 is recycled to the boiler 1 again by using the ash recycle line 19 and reburned in the boiler 1 to reduce unburned content and combustion efficiency. Although there is an example of adopting a system that raises the fuel consumption, the ash mass balance in this case is 6 at the exit of the boiler 1 when the fuel supply line 10 is 100 in one example.
5, 63 in the ash recycling line 19 of the dust collector 4, boiler 1
The ash discharge line 12 is 98, and the inlet line of the desulfurization device 5 is 2. Therefore, from the mass balance of ash, the ash separated and removed in the furnace 25 of the boiler 1 is discharged to the outside of the system through the ash discharge line 12, so that the denitration device 2 portion is largely different from the case of FIG. There is no.

However, some of the trace amounts of heavy metal elements contained in the fuel burned in the boiler 1 are gasified in the high-temperature gas atmosphere in the furnace 25 of the boiler 1, and then the denitration device 2 and the air heater 3 are used. In the course of time, there is a thing that is condensed and solidified in a low temperature range, enters into ash, and is removed together with ash by the dust collector 4. Therefore, since the ash containing the trace amount of the heavy metal element in the ash is supplied again to the boiler 1 and burned again, the heavy metal element is concentrated by the circulation through the ash recycling line 19. As an example, when the content of the heavy metal element is not recycled as ash (in the case of FIG. 6), if the exhaust gas flue 18 contains 30 ppm of the heavy metal element in the exhaust gas, in the case of FIG. 7, outside the boiler system. From the balance of the amount supplied from the system and the discharge from the dust collector to the outside of the system, the concentration in the circulation line is theoretically 50 times higher, and the heavy metal element in the exhaust gas is actually 1500 pp.
It will be concentrated to a high concentration of m.

As the trace amount of heavy metal element, As, Cd, Cu, Pb, Sb, Se, Tl, Zn can be considered from the relationship between the gas temperature of each part of the boiler plant shown in FIG. 8 and the vaporization temperature of the heavy metal element.

FIG. 9 shows a reduction in the denitration performance when the denitration device 2 is provided in each of the A plant that does not recycle ash shown in FIG. 6 and the B plant that recycles ash shown in FIG. It shows the situation. In the case of the plant A, although a slight decrease in the denitration performance was observed immediately after the start of the initial operation, the denitration movement was stable thereafter.
On the other hand, in the case of the B plant, the denitration performance is largely reduced at the beginning of the exercise, and thereafter the catalytic activity is greatly reduced with the passage of the exercise time.

The systematic difference between the A and B plants is the presence or absence of ash recycling, and it can be seen that the poisoning of the catalyst due to the concentration action of the trace amount of heavy metal elements is the direct cause.

Therefore, in order to solve such a problem of a trace amount of heavy metal elements, it is considered to adopt a so-called after-DeSOx type denitration device 2 in which the denitration device 2 is installed in the downstream of the desulfurization device 5 as shown in FIG. ing. However, since the processing gas temperature at the outlet of the desulfurization device 5 is low for performing denitration (normally the DeSOx outlet gas temperature is about 150 ° C), the gas heating furnace 22
It is necessary to take measures such as to increase the temperature of the exhaust gas to a temperature (about 300 to 400 ° C.) suitable for denitration by using the fuel from the fuel supply line 23 by providing the above. Therefore, there is a problem that the fuel cost to be supplied to the gas heating furnace 22 and the facility cost of the gas-gas heater 21 which is a heat recovery device and the like are high, and both the construction cost and the operating cost are high.

Further, as shown in FIG. 12, a method of extracting a part of the collected ash to the outside of the system through a line 14 to reduce the concentration of heavy metal components (partial recycling method) is also under study. In other words, as shown in Fig. 2A, the ash recycling rate The lower the concentration, the lower the concentration ratio of heavy metal components and the catalyst poisoning is alleviated. However, lowering the ash recycling rate means discarding a large amount of ash containing a large amount of unburned matter, which lowers the thermal efficiency of the boiler 1. Leads to. However, if this point is compromised, the advantage of being able to reduce catalyst poisoning with a simple system can be obtained.

(Problems to be Solved by the Invention) When the general flow shown in FIG. 6 is adopted as described above, when the combustion efficiency of the boiler 1 is low, there is a problem that the thermal efficiency of the boiler is reduced due to this. is there. Furthermore, the seventh
When the method of recycling combustion ash containing unburned components shown in the figure is adopted, the thermal efficiency of the boiler can be improved compared to the one shown in Fig. 6, but the denitration catalyst is poisoned by the concentration effect of the catalyst poisoning component. There was a problem of being done.

As a countermeasure against these, as shown in FIG. 7, a denitration device 2 is provided in the downstream of the desulfurization device 5, so-called after DeSO.
The x method has been studied, but if this method is adopted, there is a problem that the construction cost and the operating cost increase, and the cost increases. There is also a method (partial recycling method) in which part of the collected ash is removed from the system as shown in Fig. 12 at the expense of some thermal efficiency, but catalyst poisoning is reduced, but unburned components are removed. Since the ash containing is discharged out of the system, there still remains the problem of lowering the thermal efficiency.

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, to provide a denitration device at the most economical boiler outlet, to significantly reduce the thermal efficiency, and to perform a denitration treatment that minimizes catalyst poisoning in the partial recycling method. To provide a method.

(Means for Solving Problems) The above-mentioned object is to classify ash in which a trace amount of heavy metal elements contained in exhaust gas, which is a catalyst poisoning component, is condensed, and to generate only ash having a large particle size in an exhaust gas generator. It is achieved by recycling the ash to a small particle size and discharging the ash with a small particle size out of the system. That is, the present invention is a denitrification treatment method in which the combustion exhaust gas discharged from the exhaust gas generator is subjected to denitrification treatment, then subjected to a dust collection treatment in a downstream dust collector, and a part of the ash is returned to the exhaust gas generator to be reburned. The collected ash is classified into large particles and small particles by the dust collector or a classifier provided in the subsequent stream, and only the large-sized ash is returned to the exhaust gas generator.

(Function) When ash is classified and large sized particles are returned to the exhaust gas generator, the concentration of heavy metal components in the collected ash is as shown in FIG. 2B depending on the recycling rate. In other words, when the entire amount of collected ash is recycled, the concentration of heavy metal components becomes extremely high, but the concentration can be lowered by discharging a part of the ash out of the system as shown in FIG.

Here, the present inventors have found that the proportion of heavy metal components contained in the collected ash tends to increase as the particle size decreases. In other words, the heavy metal components in the ash are classified as shown in Fig. 13 because those with a small particle size of ash contain relatively more heavy metal components than those with a large particle size. We also found that by discharging only those with a smaller particle size to the outside of the system, even if the amount of ash itself discharged outside the system is the same as when the ash itself is not classified, many heavy metal components can be discharged outside the system. It was This also makes it possible to lower the recycling rate of heavy metal components. In other words, by dispensing ash and discharging only those with a small particle size out of the system, it is possible to reduce the concentration of heavy metal components compared to the case without classification, and the poisoning action of the catalyst by heavy metal components. Can be relaxed. The reason why the proportion of heavy metal components in the collected ash increases with smaller particle size is that the smaller metal particles have a larger surface area per weight, and therefore the heavy metal components are more likely to adhere.

(Example) FIG. 1 is a system diagram of a denitration treatment method showing an example of the present invention. The exhaust gas generated by the combustion in the boiler 1 is denitrated by the denitration device 2 and then recovered by the air heater 3. After that, the ash in the exhaust gas is separated into a large particle size by the cyclone separator 7, and then returned to the boiler 1 through the line 13 to reburn the unburned content therein. The exhaust gas discharged from the cyclone separator 7 is removed by a bag filter 8 having a smaller particle size. The dust-removed exhaust gas is also dust-removed by the desulfurization device 5 installed as necessary. The ash collected by the bag filter 8 is discharged to the outside of the system through the line 14 and disposed of by landfill or the like.

Fig. 2 shows the recycling rate of ash from line 13 and the concentration ratio of heavy metal components. In the case of the present invention, A (classifying ash and recycling only the one with a large particle size) does not classify. It is clear that the concentration ratio of heavy metals is lower than that of B.

FIG. 2B shows a case where the cyclone separator 7 is not provided and only the bag filter 8 is provided as a dust collector, and a part of the collected ash is returned to the baler 1 (when not classified). When the separator 7 is installed, even if the same amount of collected ash is returned to the boiler 1 as compared with the case where the separator 7 is not installed, the concentration ratio of the heavy metal component becomes small and the poisoning of the catalyst is alleviated. FIG. 3 is a diagram showing the relationship between the operating time and the denitration rate in each of the cases A and B of FIG. 2. In the case of the present invention, A is less poisoned by the catalyst and the denitration is performed for a long time. It is clear that activity can be maintained. It should be noted that ash with a large particle size has a higher proportion of unburned carbon in ash than ash with a small particle size, and therefore classification has the advantage that the amount of unburned carbon in the ash discharged is smaller overall. There is also.

FIG. 4 shows another embodiment of the present invention, in which an electrostatic precipitator 9 having a plurality of chambers 9A and 9B in the gas flow direction is provided as a dust remover, and the dust is collected in the chamber 9A on the upstream side. Only the particles having a large particle diameter are returned to the boiler 1 via the line 13. According to this example, since the particle size of the collected ash is large in the chamber 9A on the upstream side and the particle size is small in the chamber 9B on the downstream side,
Similar to the above, concentration of heavy metal components of the boiler 1 and poisoning of the denitration catalyst can be prevented. Further, in this case, it is not necessary to provide two types of dust removing devices, and there is an advantage that the process is simplified.

In FIG. 5, the ash collected by the dust removing device 4 is guided to the classifying device 11 so that only the ash having a large particle size is returned to the boiler 1, and the same effect as in FIG. 4 is obtained.

As shown in Fig. 14, the particle size-dependent ratio of heavy metal components in the ash rapidly increases with a particle size of 25 μm or less, so the ash discharged to the outside of the system is small with a particle size of 25 μm or less. It is more effective if you classify so that there are as many as possible.

In addition, comparing the thermal efficiency when not classifying and when classifying and extracting only ash with a small particle size out of the system, ash with a larger particle size generally contains more unburned content, so the same amount of ash is When it is taken out of the system, it is possible to obtain the effect that the thermal efficiency is improved by classification.

(Effects of the Invention) According to the present invention, it is possible to prevent concentration of catalyst poisoning components (heavy metal components) in an exhaust gas generator such as a boiler, and reduce catalyst poisoning of a denitration reactor.

[Brief description of drawings]

1, 4, and 5 are system diagrams showing various embodiments of the denitration treatment method of the present invention, and FIG.
The relationship diagram between the ash recycling rate and the concentration ratio of heavy metal components in the ash, FIGS. 3 and 9 are the relationship diagrams between the operating time of the denitration device and the denitration rate, FIG. 6, FIG. 7, and FIG. FIG. 11, FIG. 11 and FIG. 12 are device system diagrams showing various examples of conventional denitration methods, FIG. 8 is a diagram for explaining gas temperatures in various devices, and FIG. It is a relationship diagram of a particle size and a heavy metal component concentration. 1 ... Boiler (exhaust gas generator), 2 ... Denitration device, 3
...... Air heater, 5 …… Desulfurizer, 7 …… Cyclone separator, 8 …… Bag filter, 10 …… Exhaust gas, 13 ……
Large particle recycling line, 14 …… Small particle ash discharge line, 15 …… Ammonia.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Tomihisa Ishikawa 6-9 Takaracho, Kure City, Hiroshima Prefecture Babcock Hitachi Ltd. Kure Factory (72) Hayato Morita 6-9 Takaracho, Kure City, Hiroshima Prefecture Babcock Hitachi Ltd. Kure Factory

Claims (1)

[Claims] 1. A denitration treatment method in which combustion exhaust gas discharged from an exhaust gas generator is subjected to denitration treatment, and then dust is collected by a downstream dust collector, and a part of ash is returned to the exhaust gas generator and reburned. A denitration treatment method, characterized in that the collected ash is classified into large particles and small particles by a classifier provided in the dust collector or a downstream thereof, and only the large-sized ash is returned to the exhaust gas generator.