JPS63214323A - Desulfurizing and denitrating method for combustion exhaust gas - Google Patents

Abstract

PURPOSE:To increase desulfurizing and denitrating efficiency of exhaust gas containing a small volume of sulfur oxide by allowing additionally sulfur oxide be adsorbed into active carbon prior to regenerating activated carbon exhausted by being contacted with combustion exhaust gas. CONSTITUTION:Exhausted activated carbon discharged from the bottom of a desulfurizing and denitrating column 5 is fed to the top of an activation column 7 and flowed down inside the column by means of gravity, and sulfur oxide is adsorbed additionally to the activated carbon by contacting gas composed of gas containing sulfur oxide from a regenerating column 8 and a proper volume of air with activated carbon moving bed in the activation column 7. Then, said activated carbon is drawn out of the bed of the activation column 7, and fed to the top of the regenerating column 8, and the activated carbon is regenerated by heating up the moving bed for the exhausted activated carbon flowing down inside the column by means of gravity in the inactive gas atmosphere. After regeneration, the activated carbon is fed to the top of desulfurizing and denitrating column 5 to be used for the treatment of exhaust gas again.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for desulfurization and denitration of combustion exhaust gas, and in particular, although it contains sulfur oxides and nitrogen oxides, the sulfur oxide concentration is relatively low. This relates to desulfurization and denitrification methods that target low combustion exhaust gas.

[Prior Art] Since the depletion of petroleum resources has been called out and coal has been reconsidered as an alternative energy source, improvements have been made to coal-fired boilers, one of which is the fluidized combustion boiler. In a fluidized combustion boiler, the type of coal used as fuel can be selected relatively freely, and desulfurization can be carried out in the furnace by flowing a suitable alkaline earth metal carbonate, etc., which functions as a desulfurizing agent, together with one coal grain. It has the advantage of being able to. However, even if a desulfurizing agent is flowed in the boiler, it is extremely difficult to desulfurize all the sulfur oxides produced by the combustion of one coal in the furnace.Moreover, as a matter of course, nitrogen oxides are also produced when coal is burned. Therefore, the exhaust gas from coal-fired fluidized combustion boilers must also be subjected to desulfurization and denitrification treatment to prevent pollution.

A prior art method for desulfurization and denitration for exhaust gas from a coal-fired fluidized combustion boiler can be found in Japanese Patent Application Laid-Open No. 150705/1983. In this method, a small amount of desulfurizing agent is made to flow together with coal grains in a coal-fired fluidized combustion boiler, so that some of the sulfur oxides produced by the combustion of coal are fixed with the desulfurizing agent, and the exhaust gas from the boiler is used with activated carbon along with ammonia. This method is supplied to an adsorption-type desulfurization and denitrification equipment for treatment.This method not only reduces the amount of desulfurization agent required for in-furnace desulfurization, but also reduces the sulfur oxide concentration of exhaust gas by in-furnace desulfurization. It is stated in the above-mentioned publication that the denitrification rate in the desulfurization equipment can be maintained at a high level because of the decrease in the amount of nitrogen.

[Problems to be Solved by the Invention] In the desulfurization and denitration method in which ammonia is mixed with exhaust gas containing carpite oxides and nitrogen oxides, and this exhaust gas is brought into contact with activated carbon, activated carbon that has been exhausted by contact with the exhaust gas is regenerated, It is customary to repeat the operation of bringing the recycled activated carbon into contact with exhaust gas again, but the inventor has determined that the sulfur oxide adsorption status of the activated carbon subjected to regeneration will affect the performance of the activated carbon after regeneration, especially its denitrification performance. We found that this had a significant effect.

In other words, when activated carbon that has been exhausted by contact with exhaust gas is regenerated, if the activated carbon adsorbs a relatively large amount of sulfur oxides, the regenerated activated carbon will exhibit high denitrification performance the next time it comes into contact with exhaust gas. On the other hand, we found that when activated carbon that has adsorbed only a small amount of sulfur oxide is regenerated, sufficient denitrification performance cannot be obtained, and that denitrification performance is greatly affected by the concentration of sulfur oxide. Ta.

Therefore, in the desulfurization and denitration method in which combustion exhaust gas with a small amount of sulfur oxides, such as exhaust gas from a coal-fired fluidized combustion boiler that has undergone in-furnace desulfurization, is treated with activated carbon, the concentration of sulfur oxides in the exhaust gas is low. It is not possible to obtain the expected denitrification rate. The denitrification rate when activated carbon that has been regenerated in this way comes into contact with exhaust gas varies depending on the concentration of sulfur oxides, but since fluidized bed boilers burn various types of coal, the concentration of sulfur oxides generated varies. , the denitrification rate also changes depending on the type of coal. For this reason, the desulfurization and denitrification equipment also required a large device that could cope with the decrease in the denitrification rate.

Means for Solving Problem C] The present invention can treat even combustion exhaust gas with a low content of sulfur oxides, such as exhaust gas from a coal-fired fluidized combustion boiler that has undergone in-furnace desulfurization, with a high desulfurization and denitrification rate. This method provides a method for treating exhaust gas, which is characterized by contacting combustion exhaust gas containing sulfur oxides and nitrogen oxides with a relatively low concentration of sulfur oxides with activated carbon in the presence of ammonia gas. Combustion is performed by combining a process of desulfurizing and denitrating the activated carbon, a process of regenerating activated carbon exhausted by contact with combustion exhaust gas by contacting it with an inert gas, and a process of circulating the regenerated activated carbon to the desulfurization and denitrification process. When desulfurizing and denitrating exhaust gas, activated carbon that has been exhausted by contact with combustion exhaust gas is brought into contact with a sulfur oxide-containing gas to adsorb additional sulfur oxides on the activated carbon before its regeneration. The objective is to regenerate it and use it repeatedly in the desulfurization and denitrification process.

[Function] Figure 1 shows a flow sheet of an embodiment in which the method of the present invention is applied to the exhaust gas of a coal-fired fluidized combustion boiler. 1 is a fluidized combustion boiler, to which coal particles and a desulfurization agent are supplied. 0
Coal grains are fluidized and burned in the furnace to generate sulfur oxides and nitrogen oxides, but some of the sulfur oxides are fixed in the desulfurization agent flowing inside the furnace. Therefore, the exhaust gas from boiler 1 generally has a lower concentration of sulfur oxides than normal boiler exhaust gas, and by appropriately adjusting the amount of desulfurizing agent supplied to boiler 1 or the combustion conditions of boiler 1, The sulfur oxide concentration and nitrogen oxide concentration can be approximately 50 to 150 pp+a and 150 to 300 ppm, respectively.

The exhaust gas from the boiler 1 is first sent to the cyclone 2, where the desulfurization agent and unburned coal particles are separated from the gas.
Returned to Boiler 1. On the other hand, the exhaust gas is sent from the cyclone 2 to the gas cooler 3 and cooled, and then supplied to the dust collector 4.
It is sent to the desulfurization and denitrification tower 5 together with ammonia gas having a pH of about 1, and is treated therein.

The desulfurization and denitrification tower 5 has a moving bed in which activated carbon flows down by gravity, and the exhaust gas is desulfurized and denitrated by contacting the moving bed in a cross-flow manner, and is then released into the atmosphere from the chimney 6. The conventional practice is to extract the activated carbon exhausted by contact with exhaust gas in the desulfurization and denitration tower 5 from the bottom of the tower and regenerate it, and then return the regenerated activated carbon to the desulfurization and denitration tower for repeated use. However, as mentioned above, if the activated carbon subjected to regeneration has adsorbed only a small amount of sulfur oxide,
Even if this is recycled, activated carbon with sufficient denitrification performance cannot be obtained.

Therefore, in the method of the present invention, before regenerating the exhausted activated carbon discharged from the desulfurization and denitration tower, sulfur oxides are additionally adsorbed on the activated carbon.

After that, it is regenerated, and in the illustrated example, sulfur oxide is replenished into exhausted activated carbon using gas containing relatively high concentration of sulfur oxide discharged from an activated carbon regeneration tower. .

That is, the exhausted activated carbon discharged from the bottom of the desulfurization and denitrification tower 5 is supplied to the top of the activation bath 7 and allowed to flow down the inside of the tower by gravity, so that an appropriate amount of the exhausted activated carbon is added to the sulfur oxide-containing gas from the regeneration tower 8. The air-plus-gas is brought into contact with the moving bed of activated carbon within the active group 7 to cause the activated carbon to adsorb additional sulfur oxides.

Thereafter, the activated carbon is extracted from the bottom of the activation bath 7 and supplied to the top of the regeneration tower 8, and the moving bed of exhausted activated carbon flowing down the tower by gravity is heated under an inert gas atmosphere to regenerate the activated carbon. do. At this time, gas containing a relatively high concentration of sulfur oxides is discharged from the regeneration tower 8, so this gas is sent back to the activation bath 7 and used to replenish the activated carbon with sulfur oxides.

The regenerated activated carbon that is regenerated in the regeneration tower 8 and extracted from the bottom of the tower is supplied to the top of the desulfurization and denitrification tower 5 and used again to treat exhaust gas, but according to the method of the present invention, a considerable amount of exhausted activated carbon is removed. Since it is regenerated with sulfur oxides adsorbed, the denitrification rate in the desulfurization and denitrification tower 5 can be maintained at a high level.

At present, it is not entirely clear why recycled activated carbon with excellent denitrification performance can be obtained by additionally adsorbing sulfur oxides before regenerating exhausted activated carbon.

However, activated carbon that has adsorbed a relatively large amount of sulfur oxides contains a relatively large amount of sulfuric acid or ammonium sulfate, so during regeneration, these substances erode the activated carbon and develop pores on the surface of the activated carbon. It is believed that regenerated activated carbon exhibits excellent denitrification performance because it provides an increase in surface area with increased catalytic activity and also promotes the production of acidic oxides that have catalytic action to decompose NOx.

[Example] After 220 ppm of NH3 gas was mixed into the exhaust gas containing 1100 ppm of SOx and 200 ppm of NOx, this mixed gas was heated at a temperature of 145°C in a cross-flow type moving bed reactor filled with granular activated carbon catalyst. (Desulfurization and denitrification tower) space velocity g
Passed at oo h-1. In this case, the residence time of the catalyst in the reactor was set to 20 hours. The amount of sulfur oxides adsorbed on the activated carbon discharged from this reactor was 9 mg per gram of activated carbon in terms of S02.

The exhausted activated carbon is introduced into an activation bath and allowed to flow down the inside of the tower by gravity flow. Meanwhile, the sulfur oxide-containing gas from the regeneration tower is mixed with air and supplied to the activation bath, and the exhausted activated carbon is heated at 120°C. Sulfur oxides were added to the activated carbon by contacting it with the activated carbon.

The amount of sulfur oxide adsorbed by activated carbon discharged from active groups is
The amount was 95 mg per gram of activated carbon in terms of S-〇2.

This activated carbon was led to a moving bed type regeneration tower and heated and regenerated at 400° C., and the regenerated activated carbon discharged from the bottom of the regeneration tower was circulated to the above-mentioned moving bed type reactor and used repeatedly for treatment of exhaust gas.

In this example, the denitrification rate of one reactor was 82%.

The desulfurization rate was 100%.

For comparison, the same exhaust gas was treated under the same conditions as above, except that the installation of an activation bath was omitted. That is, the activated carbon discharged from the cross-flow moving bed reactor is directly guided to the regeneration tower,
The regenerated activated carbon was recycled to the reactor and reused for exhaust gas treatment.

In this case, the denitrification rate of the reactor is 61% and the desulfurization rate is 100%.
Met.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet of the method of the present invention. 1: Boiler 2: Cyclone 3: Gas cooler 4: Dust collector 5: Desulfurization and denitrification tower 6: Chimney 7: Activation bath 8: Regeneration tower

Claims (1)

[Claims] 1. A process in which combustion exhaust gas containing sulfur oxides and nitrogen oxides and with a relatively low concentration of sulfur oxides is brought into contact with activated carbon in the presence of ammonia gas to desulfurize and denitrify it, and the combustion exhaust gas is exhausted by contact with the combustion exhaust gas. In the desulfurization and denitration method for flue gas, which combines the steps of heating and regenerating activated carbon by bringing it into contact with an inert gas and circulating the regenerated activated carbon to the desulfurization and denitrification step described above, exhaustion occurs due to contact with the flue gas. The method for desulfurization and denitration of combustion exhaust gas described above is characterized in that, prior to regenerating the activated carbon, sulfur oxides are additionally adsorbed on the activated carbon.