KR100595844B1 - a Clearance Method of nitrogen oxide using an urea - water solution as a reducer - Google Patents

Abstract

       The present invention relates to a method for producing ammonia from urea and feeding it to an SCR process to remove nitrogen oxides using urea as a reducing agent. More specifically, the urea aqueous solution is injected into a pyrolysis reactor to decompose urea into ammonia and isocyanic acid, and the pyrolysis gas containing these is injected into an exhaust gas containing nitrogen oxide and mixed. Isocyanic acid in the pyrolysis gas mixed with the exhaust gas is hydrated on the catalyst and further converted to ammonia, which acts as a reducing agent with the ammonia produced by the pyrolysis to remove nitrogen oxides on the SCR catalyst. Therefore, because it uses urea but obtains the same nitrogen oxide removal performance as when using ammonia and uses easy and safe urea, it is possible to solve the safety problems such as transportation, storage and handling due to the use of harmful ammonia. to provide.

Description

{A Clearance Method of nitrogen oxide using an urea-water solution as a reducer}

1 is a graph showing the removal efficiency of nitrogen oxide when using a urea solution and ammonia as a reducing agent.

The present invention relates to a method of using urea aqueous solution instead of using ammonia directly in an industrial process that requires ammonia, and more specifically, to selectively remove nitrogen oxides contained in exhaust gas (Selective Catalytic Redution, Ammonia is required as a dust resistivity regulator to improve the removal efficiency of dust with low resistivity in processes such as 'SCR'), Selective Non-Catalytic Reduction (SNCR) and electrostatic precipitators It is applied to the process of supplying ammonia, and instead of directly using ammonia, which is not easy to transport, store and handle due to odor, explosiveness, harmfulness, etc. Solve the problems caused by the use of ammonia, the same as when using ammonia It relates to the elements and viewing the aqueous solution to a method for removing NOx with a reducing agent.

Nitrogen oxide is a representative air pollutant that is generated in combustion systems such as power plants, various industrial boilers, automobiles, and discharged into the atmosphere, causing photochemical smog and acid rain. Currently, the most widely commercialized technology for removing nitrogen oxides is SCR technology which reduces nitrogen oxides to harmless water and nitrogen on a catalyst by using ammonia as a reducing agent. However, ammonia, which is a reducing agent, is not easy to transport, store and handle due to explosiveness, odors, etc., and another air pollutant is emitted when it is discharged to the atmosphere in the nitrogen oxide removal process. Due to its widespread use, the use of a reducing agent, which is relatively easy to handle, has recently been actively studied. Various hydrocarbons have been proposed as substitutes for ammonia, but the use of hydrocarbons as a reducing agent is still far less effective in removing nitrogen oxides than the use of ammonia as a reducing agent. If moisture is present, there is a disadvantage in that the activity of the catalyst is significantly lowered. Therefore, various methods have been attempted to use urea that is decomposed into a reducing agent to replace ammonia to produce ammonia.

Jones, in US Pat. Nos. 5,281,403 (1994) and 5,827,490 (1998), describes a method for converting urea solution to ammonia using a catalyst. The urea solution is reduced to 350 to 650 ° F under high pressure of at least 300 psig. After heating, the conversion of the urea hydrate in the aqueous state to ammonia is made. When the urea solution is heated at high pressure, it is converted into ammonium carbamate (NH ₃ CO ₂ NH ₃ ), which is a hydrate of urea, by the following reaction, whereby the production of ammonium carbamate is promoted by a catalyst. Ammonium carbamate is then fed to the front end of the SCR or SNCR process and pyrolyzed by the high temperature of the flue-gas, which collectively explains that one mole of urea is converted to two moles of ammonia by a two-step reaction as follows.

NH ₂ CONH₂ + H ₂ O-> NH ₃ CO ₂ NH _3- > 2NH ₃ + CO ₂

If the catalyst is not used, the production rate of ammonium carbamate is not only slow, but the amount of nitrous oxide and carbon monoxide, which are undesirable products when decomposition occurs at high temperature, increases, and thus nitrogen oxide when injected into the SCR or SNCR process. The removal rate of is also reduced. However, the use of a catalyst decreases the production of nitrous oxide and carbon monoxide, which increases the selectivity to convert urea to ammonia, and consequently increases the removal efficiency of nitrogen oxides. It is also proposed that the use of a catalyst can greatly reduce the time for urea to be converted to ammonium carbamate by hydration. However, even when the catalyst is used, it takes a reaction time of up to 10 minutes to convert the urea more than 90% and has a disadvantage of maintaining a high pressure of about 300 psig. Moreover, when the concentration of the urea solution is increased, there is a disadvantage in that the pressure of the hydration process must be further increased to prevent ammonia from being released. In addition, there is a disadvantage in that a catalyst and a reactor are required to hydrate the urea solution separately from the SCR catalyst.

Lagana, U.S. Patent No. 5,985,224 (1999), discloses a hydration reaction by heating a urea solution at 100 to 233 ° C under a high pressure of 1 to 30 atmospheres, and ammonia is brought into contact with the hydrated urea solution by steam. And carbon dioxide are stripped off and then supplied to the exhaust gas where ammonia is consumed. Cooper et al. In U.S. Patent No. 6,077,491 (2000) heated the urea solution to 110-200 ° C. under a pressurization of 20-500 psig in a hydration reactor to produce a hydration reaction and at least 60 ° C. to produce ammonia and carbon dioxide. It shows how to remove and supply it to exhaust gas. It involves using a catalyst to accelerate the hydration reaction. At this time, the production rate of ammonia was kept constant by adjusting the amount of urea solution supplied to the hydration reactor, and the pressure and temperature of the reactor were controlled by controlling the amount of heat supplied to the hydration reactor through steam or the like. By controlling the production rate of ammonia. However, when a change in the combustion process in which nitrogen oxide is produced occurs, in particular, when a decrease in the amount of ammonia supply is required, the pressure and temperature of the hydration reactor cannot be lowered abruptly, and thus, there is a disadvantage in that the ammonia generated for a predetermined time is removed and exhausted. In order to improve this, US Pat. No. 6,436,359 (2002) introduced a method of lowering the temperature by installing a cooler in a urea hydration reactor and circulating the coolant. However, since the cooling rate is limited due to the limitation of the heat transfer rate, this method also has a limitation in that it is not easy to control the amount of ammonia supply when the flue gas flue requires ammonia.

Existing inventions related to the ammonia production method by the hydration reaction of the above-mentioned elements require a high pressure and the reaction time progresses slowly by minute, and there is a lack of flexibility in changing the ammonia output according to the load variation of the process requiring ammonia. There are disadvantages.

A feature of the present invention is relatively high temperature, but at low pressure, urea aqueous solution is first thermally decomposed into ammonia and isocyanic acid (HNCO) in the absence of a catalyst, and then isocyanic acid is converted into ammonia in the SCR catalyst, At this time, the hydration reaction of isocyanic acid is much faster than the SCR reaction and thus provides ammonia necessary for the SCR reaction in a short time, so there is no significant difference in nitrogen oxide removal efficiency compared to when ammonia is used as a reducing agent.

Another feature of the present invention is that the nitrogen oxide removal process is much simpler because the hydration of isocyanic acid to ammonia on the SCR catalyst proceeds simultaneously with the nitrogen oxide removal reaction.

Another feature of the present invention is that the urea solution is supplied through the nozzle, so even when the ammonia requirement fluctuates due to the load fluctuation of the combustion process, the amount of urea solution supplied to the pyrolysis reactor is easily adjusted to adjust the amount of ammonia supply. It can be adjusted.

An object of the present invention is to provide a nitrogen containing a urea solution as a reducing agent, which does not require high pressure in the process of pyrolysis of urea solution, and the pyrolysis reaction time is very short. It is to provide an oxide removal method.

Another object of the present invention is to supply the urea solution through the nozzle, so that the amount of urea solution injected can be easily adjusted by adjusting the amount of urea solution injected according to the load variation of the process that requires ammonia, so the flexibility of the process is very excellent. It is to provide a nitrogen oxide removal method using a urea solution as a reducing agent.

The object of the present invention is to dissolve urea in water to form an aqueous solution and spray it to a pyrolysis reactor through a nozzle to thermally decompose urea to produce pyrolysis products of ammonia and isocyanic acid. The pyrolysis product is mixed with the exhaust gas containing nitrogen oxides, and then passed through an SCR process to convert the isocyanic acid in the pyrolysis product into ammonia through a hydration reaction on a catalyst, and exhaust gas-containing nitrogen together with the ammonia generated by pyrolysis. It is achieved by a nitrogen oxide removal method using a urea solution according to the present invention comprising the step of removing the oxide as a reducing agent.

These objects of the present invention are the steps of dissolving urea in water to form urea solution and spraying the urea solution into a pyrolysis reactor through a nozzle to thermally decompose urea to produce pyrolysis products of ammonia and isocyanic acid. Isocyanic acid in the gaseous pyrolysis product is converted to ammonia through a hydration reaction on a catalyst to generate ammonia, and injected into an exhaust gas containing nitrogen oxide to remove nitrogen oxide by SNCR reaction It is achieved by a nitrogen oxide removal method using a urea solution according to the present invention comprising a reducing agent.                         

Hereinafter, the nitrogen oxide removal method using the urea aqueous solution of the present invention as a reducing agent embodies the present invention through the examples described below. However, the content and scope of the present invention is not limited to the following examples.

In the present invention, the urea aqueous solution is used by dissolving a solid urea in water. At this time, the concentration of usable urea solution is determined by the solubility of urea in water. When the concentration of urea is low, a large amount of water is required, and thus, a large amount of heat is required due to evaporation of water in the pyrolysis step. . In addition, when the concentration of urea is high, there is a disadvantage in that heating is required to maintain the temperature of the storage tank to prevent precipitation of the urea in the storage tank. Therefore, the concentration of the preferred urea solution based on room temperature is about 1 to 50% by weight. The urea solution is injected into the pyrolysis reactor through the nozzle, in which the droplet size of the urea solution is sprayed through a two-fluid nozzle to facilitate the evaporation and pyrolysis of the urea solution. It is also possible to pressurize the urea storage tank instead of the two-fluid nozzle and use a single-fluid nozzle. In pyrolysis reactors, urea decomposes into ammonia and isocyanic acid, the decomposition efficiency of which depends on the decomposition temperature and the decomposition reaction time. In order to achieve the same decomposition efficiency, the lower the decomposition temperature, the longer the reaction time is. Therefore, the decomposition reaction for decomposing 100% of the urea is considered in consideration of the increase in the energy cost according to the decomposition temperature and the increase in the pyrolysis reactor size as the decomposition reaction time increases. The temperature is 250 ~ 500 ℃, the decomposition reaction time is preferably 0.1 ~ 10 seconds, more preferably the decomposition reaction temperature is 250 ~ 500 ℃, decomposition reaction time is 0.1 ~ 1 second. The pyrolysis product consists of ammonia, isocyanic acid and water whose relative concentration depends on the concentration of the urea solution. In other words, the higher the concentration of urea solution, the higher the relative concentration of ammonia and isocyanate. On the contrary, the lower the concentration of the urea solution, the lower the concentration of ammonia and isocyanate. The pyrolysis product is fed to the catalytic reactor so that isocyanic acid is converted to ammonia by the hydration reaction. Since the hydration reaction of isocyanate is much faster than the reduction and removal of nitrogen oxides by ammonia, the SCR process is applied to the SCR process. Hydration of isocyanic acid proceeds in the catalyst, and the ammonia produced by this is used as a reducing agent for removing nitrogen oxide together with the ammonia produced by pyrolysis of urea. The process according to the invention thus exhibits a nitrogen oxide removal efficiency which is nearly equivalent to the SCR process using ammonia as a direct reducing agent. When the method according to the present invention is applied to an SNCR process or an electrostatic precipitator, a catalytic process is separately installed at the rear of the pyrolysis reactor of the urea to convert all of the urea into ammonia and then supplied to the SNCR process or the electrostatic precipitator.

Hereinafter, the present invention is embodied through the following examples of nitrogen oxide removal using the urea aqueous solution of the present invention as a reducing agent. However, the content and scope of the present invention is not limited to the following examples.

Example 1

In Example 1, 6 wt% urea aqueous solution was injected into the inner tube of the airflow nozzle to determine the pyrolysis degree and pyrolysis product distribution of the urea according to the present invention. In order to facilitate evaporation and pyrolysis in the furnace, a pyrolysis reaction was carried out by injecting a gas containing nitrogen, oxygen, and water into a pyrolysis reactor by flowing a gas containing a carrier gas as a carrier gas. At this time, since the urea aqueous solution and the carrier gas injected into the pyrolysis reactor were all gas in the pyrolysis reactor, the composition converted into gas was 93% nitrogen, 5% oxygen, 2% moisture, and 250 ppm of urea. The pyrolysis characteristics of urea were investigated according to the reaction time and decomposition temperature by changing the decomposition reaction time (retention time) from 0.05 to 0.1 seconds and the reaction temperature from 300 to 450 ℃. The first table shows the product distribution by pyrolysis reaction according to pyrolysis temperature at each decomposition reaction time. As the amount of undecomposed urea decreased, the concentrations of ammonia and isocyanic acid produced increased. In general, the concentrations of ammonia and isocyanic acid were similar. Ansan is produced. At the same reaction time, the conversion of urea increased proportionally with increasing decomposition temperature. This is because pyrolysis occurs better as the temperature increases because the pyrolysis reaction of urea is endothermic. Under the same reaction temperature conditions, it can be seen that the pyrolysis of urea proceeds better as the reaction time increases. In the reaction time of 0.1 second, urea was decomposed 100% at the decomposition temperature of 350 ° C. or higher at the reaction time of 0.075 second at the decomposition temperature of 400 ° C. It can be seen from Example 1 that the decomposition rate of urea largely depends on the reaction time (retention time) and the decomposition temperature. Therefore, it was confirmed that it is possible to make the decomposition rate of urea 100% by the present invention if the reaction time and decomposition temperature are properly adjusted. Since the concentration of the injected urea in the first embodiment is 250ppm, if only the pyrolysis reaction proceeds, the maximum concentration of ammonia to be produced should be 250ppm. However, when the decomposition temperature is high and the reaction time is long, more than 250ppm of ammonia was produced because the hydration reaction of isocyanic acid proceeds at a high temperature to generate ammonia. In Example 1, it was confirmed that pyrolysis of urea according to the present invention proceeds at a decomposition temperature of 350 ° C. or higher and a very short time around 0.1 second to provide a very efficient method of producing ammonia and isocyanic acid.

Response time (seconds) Decomposition Temperature (℃) Ammonia (ppm) Element (ppm) Isocyanic acid (ppm) 0.1 300 208 25 205 350 245 0 227 400 275 0 203 450 310 0 163 0.075 300 173 57 160 350 212 30 190 400 250 0 217 450 313 0 170 0.05 300 139 98 130 350 168 75 165 400 208 32 183 450 240 13 213
Table 1. Pyrolysis product distribution of urea according to pyrolysis reaction time and decomposition temperature

delete

Example 2

In Example 2, the concentration of urea solution was increased to 20 and 40% by weight under the same pyrolysis condition as in Example 1, and then injected into the pyrolysis reactor, and the reaction time of 0.1 second was already 100%, which allowed the pyrolysis of urea through Example 1; Product distribution by pyrolysis at the decomposition temperature of 400 and 450 ° C is summarized in the second table. Regardless of the concentration of urea solution at the same reaction time and decomposition temperature, the product distribution was similar to that of Example 1, which shows that the pyrolysis reaction of urea according to the present invention was not significantly affected by the concentration of urea solution to be injected. The result is.

Urea solution concentration (% by weight) Decomposition reaction temperature (℃) Ammonia (ppm) Element (ppm) Isocyanic acid (ppm) 20 400 281 0 202 450 313 0 160 40 400 285 0 203 450 319 0 155

     2nd table. Distribution of pyrolysis products of urea according to urea solution concentration and decomposition temperature

Example 3

In the pyrolysis reaction of urea according to the present invention, urea primarily decomposes isocyanic acid and ammonia, as confirmed in Examples 1 and 2. In the present invention, the method for producing ammonia from urea includes a process in which isocyanic acid produced by pyrolysis of urea is converted to ammonia by hydration in an SCR catalyst. Therefore, in the present invention, in order to confirm how efficiently the hydration reaction of the isocyanic acid proceeds in the presence of the catalyst, the urea catalytic decomposition experiment was performed in a system in which the urea pyrolysis reactor and the catalytic reactor were connected in series. In order to confirm the degree of hydration reaction of isocyanate as well as the decomposition reaction of urea to isocyanate and ammonia at the same time, the temperature of pyrolysis reactor is 250 ℃ and unreacted urea flows into catalytic reactor. It was made. The urea injected at 250 ° C at 250 ° C of decomposition temperature is pyrolyzed and emerges from the pyrolysis reactor with a composition of 115ppm of undecomposed urea, 125ppm of ammonia produced and 110ppm of isocyanic acid. The temperature of the pyrolysis reactor was fixed at 250 ° C. and the total flow rate was kept constant to check the effects of the catalyst while maintaining the pyrolysis reaction conditions constant. The catalytic amount of urea was changed and the catalyst hydration of isocyanate was changed. The effect of space velocity on the reaction was investigated. The third table summarizes the concentrations of urea, isocyanic acid and ammonia at the outlet of the reactor after catalysis, depending on the reaction temperature at each space velocity between 110,000 and 367,000 hr ⁻¹ . The space velocity is defined as the volume of the reaction gas volume divided by the bulk density of the catalyst. The concentration of urea decreased with increasing reaction temperature. However, the difference in urea concentration with the change of space velocity did not appear much, which means that the decomposition reaction of urea decreases due to the gas phase pyrolysis reaction through the catalytic reactor rather than the catalytic reaction. In contrast, the hydration reaction of isocyanic acid proceeds actively as the reaction temperature increases, and not only decreases the concentration, but also affects the reactor space velocity, so that the larger the amount of catalyst, that is, the smaller the space velocity, the lower the concentration of isocyanate. Turned out to be great. This means that the hydration reaction of isocyanic acid proceeds by catalysis. As the reaction temperature increases or the space velocity decreases, the concentration of urea and isocyanic acid decreases and the concentration of ammonia increases. This method effectively converts isocyanic acid from the catalyst to ammonia. To show what is going on.

Space velocity (hr ^-1 ) Reaction temperature (℃) Ammonia (ppm) Element (ppm) Isocyanic acid (ppm) 110,000 150 300 70 12 220 365 43 0 290 410 23 0 360 440 10 0 430 450 4 0 157,000 150 290 70 19 220 360 50 3 290 410 24 0 360 440 9 0 430 450 4 0 367,000 150 260 77 58 220 340 52 31 290 400 25 14 360 435 11 6 430 450 5 0

3rd table. Degradation of Urea in Y-type Zeolite Catalysts with Copper Ion Exchange
Hydration Product Distribution of Isocyanic Acid

Example 4

In Example 4, in order to confirm whether the urea decomposition method according to the present invention proceeds efficiently, the pyrolysis reactor and the SCR catalytic reactor were connected in series to examine the removal efficiency of nitrogen oxides from the catalyst. Urea was injected into the aqueous solution using 6% by weight of the urea at this time the temperature of the pyrolysis reactor was 450 ℃ so that the urea was completely pyrolyzed. Decomposition gas from the pyrolysis reactor is injected into the catalytic reactor, where the composition of the reaction gas is composed of 500 ppm of nitrogen oxide, 5% of oxygen, 10% of moisture, and the rest of nitrogen as a carrier gas. Urea was injected to 250 ppm in the reaction gas, which is equivalent to 500 ppm of ammonia produced by pyrolysis and subsequent hydration. Ammonia was injected at 500ppm to compare the direct injection of ammonia and nitrogen oxide removal activity. The flow rate of the reaction gas is represented by the space velocity, where the space velocity is the value obtained by dividing the reaction gas flow rate by the bulk volume of the catalyst. In this embodiment, the space velocity is set to 50,000 to 100,000 per hour.

FIG. 1 compares the removal of nitrogen oxides by the urea of the Y-type zeolite catalyst with copper ion applied in Example 3 and the removal activity by ammonia. When the urea was used as the reducing agent and the ammonia as the reducing agent over the entire reaction temperature range at the same reaction gas flow rate of the space velocity 100,000hr ^-1, the nitrogen oxide removal efficiency was almost the same. This shows that 2 moles of ammonia is produced per 1 mole of urea and supplied to the SCR reaction by the decomposition method of urea according to the present invention. As confirmed in Examples 1 to 3, when urea is primarily thermally decomposed into ammonia and isocyanate, the catalytic hydration of isocyanic acid proceeds very quickly to generate additional ammonia. When the isocyanic acid is produced, it was confirmed that the removal activity of the nitrogen oxide can be as high as when using ammonia.

The decomposition method of urea according to the present invention consists of two consecutive steps of pyrolysis of urea followed by conversion into ammonia by catalytic hydration of isocyanic acid, which is a pyrolysis product. In a short time within 1 second of reaction time, 1 mole of urea is decomposed into 1 mole of ammonia and 1 mole of isocyanic acid. The isocyanic acid produced by pyrolysis is then converted into ammonia stoichiometrically by hydration in the catalytic process, resulting in one mole of urea to two moles of ammonia. In particular, the ammonia production reaction by catalytic hydration of isocyanic acid proceeds in an SCR catalyst that removes nitrogen oxides and proceeds much faster than the nitrogen oxide removal reaction by ammonia on the catalyst. Nitrogen oxide removal efficiency similar to the method of injecting can be obtained. Therefore, the present invention can achieve the same effect as when using ammonia in the process of using ammonia, such as SCR, SNCR and electrostatic precipitator. It is effective in solving safety problems such as handling.

Claims (20)

Dissolving urea in water to form urea solution and spraying the urea solution into a pyrolysis reactor through a nozzle to thermally decompose urea to produce a pyrolysis product with ammonia and isocyanic acid, and the resulting gaseous pyrolysis product Is mixed with an exhaust gas containing nitrogen oxides, and then passed through an SCR process to convert the isocyanic acid from the pyrolysis product into ammonia through a hydration reaction on a zeolite ion-exchanged copper or a Y-type zeolite exchanged copper. Nitrogen oxide removal method using a urea solution as a reducing agent comprising the step of removing the nitrogen oxide contained in the exhaust gas with ammonia produced by The method of claim 1, wherein the concentration of the urea solution is 1 to 50% by weight. The nitrogen oxide containing urea solution as a reducing agent according to claim 1, wherein the nozzle uses a two-fluid nozzle or pressurizes the urea solution storage tank to reduce the droplet of the urea solution to be injected. How to remove The nitrogen oxide removal method using a urea solution as a reducing agent according to claim 1, wherein the temperature of the pyrolysis reactor is 250 to 500 ° C. The method of claim 1, wherein the pyrolysis time is 0.1 ~ 10 seconds nitrogen oxide removal method using a urea solution as a reducing agent. delete delete delete delete delete Dissolving urea in water to form urea solution and spraying the urea solution into a pyrolysis reactor through a nozzle to thermally decompose urea to produce a pyrolysis product with ammonia and isocyanic acid, and the resulting gaseous pyrolysis product Isocyanic acid is converted into ammonia through a hydration reaction on a copper-ionized zeolite or a copper- exchanged Y-type zeolite catalyst to generate ammonia, which is injected into an exhaust gas containing nitrogen oxides and subjected to SNCR reaction. Nitrogen oxide removal method using a urea solution as a reducing agent comprising the step of removing nitrogen oxide by 12. The method of claim 11, wherein the concentration of the urea solution is 1 to 50% by weight. 12. The nitrogen oxide comprising urea solution as a reducing agent according to claim 11, wherein the nozzle uses a two-fluid nozzle or pressurizes the urea solution storage tank to reduce the droplet of the urea solution to be injected. How to remove The nitrogen oxide removal method using a urea solution as a reducing agent according to claim 11, wherein the temperature of the pyrolysis reactor is 250 to 500 ° C. The nitrogen oxide removal method using a urea solution as a reducing agent according to claim 11, wherein the pyrolysis time is 0.1 to 10 seconds. delete delete delete delete delete

Images