This invention relates to exhaust gas treating systems for the removal of nitrogen oxides (NOx) and sulfur oxides (SOx) present in exhaust gas discharged from boilers, gas turbines, engines and combustion furnaces for burning various types of fuel.
This invention can also be suitably used for the removal of nitrogen oxides present in tunnels and for the removal of nitrogen oxides present in exhaust gas from nitric acid production plants.
An example of exhaust gas treatment by means of a conventional exhaust gas treating system is explained with reference to FIG. 1.
In FIG. 1, reference numeral 1 designates a boiler; 2, a denitrator; 3, an air preheater; 4, a dust collector; 5, a gas-gas heater; 6, a desulfurizer; and 7, a stack.
As shown in FIG. 1, a denitrator 2 using a catalyst is installed at the outlet of a boiler 1 or the like, and an air preheater 3 is installed at the outlet of denitrator 2 so as to lower the temperature of the exhaust gas to about 130xc2x0 C.
The exhaust gas having passed through the aforesaid air preheater 3 is dedusted in a dust precipitator 4, passed through a gas-gas heater 5 and then introduced into a desulfurizer 6 where sulfur oxides (SOx) are removed therefrom. Thereafter, the exhaust gas is discharged into the atmosphere through a stack 7.
In order to remove sulfur oxides (SOx) from exhaust in the aforesaid desulfurizer 6, there has conventionally been employed the so-called lime-gypsum method in which the aforesaid sulfur oxides (SOx) are absorbed with the aid of calcium carbonate used as absorbing agent and recovered in the form of gypsum. In this method, attempts have been made to reduce the outlet concentration of sulfur oxides (SOx) by varying the gas-fluid ratio, the residence time and the like.
Usually, the concentration of sulfur oxides (SOx) in exhaust gas from boilers is in the range of 400 to 800 ppm, and it is intended in the aforesaid lime-gypsum method to reduce the outlet concentration thereof to 50-100 ppm.
However, recent environmental standards demand that the concentration of sulfur oxides (SOx) in exhaust gas should be reduced to a level of 5 ppm or less which is commonly known as a high-degree desulfurization level. In order to remove sulfur oxides (SOx) to a level of 50 to 100 ppm according to the aforesaid conventional lime-gypsum method, a marked increase in cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. Moreover, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of sulfur oxides (SOx).
Furthermore, the aforesaid desulfurizer 6 employs the so-called lime-gypsum method in which sulfur oxides (SOx) present in exhaust gas are absorbed with the aid of calcium carbonate used as absorbing agent.
among dry processes, only an absorption process using active carbon has been put to practical use. However, this adsorption process uses water washing for the purpose of desorption and hence requires a large volume of water. Moreover, this process also involves problems concerning disposal of the resulting dilute sulfuric acid, drying of the adsorbant, and the like.
As described above, in the current practical process for the removal of nitrogen oxides present in exhaust gas from boilers, there is used a denitrator 2 based on the selective catalytic reduction (SCR) method in which nitrogen oxides are decomposed to nitrogen and water vapor by using a catalyst comprising V2O5 supported on TiO2 and a reducing agent comprising NH3. However this process involves the following problems. First, a reaction temperature of 300 to 400xc2x0 C. is required because of the performance of the catalyst. Secondly, NH3 is required for use as reducing agent. Thirdly, since the current leak level of NOx is from 5 to 40 ppm, an excess of NH3 needs to be injected for the purpose of reducing the leak level of NOx to zero.
Moreover, recent environmental standards demand that the concentration of nitrogen oxides (NOx ) in exhaust gas should be reduced to a level of 1 ppm or less which is commonly known as a high-degree denitration level. In the aforesaid conventional denitration treatment based on the selective catalytic reduction (SCR) method, a marked increase in cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. On the other hand, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of nitrogen oxides (NOx).
In view of the above-described problems, an object of the present invention is to provide an exhaust gas treating system which can treat exhaust gas at low temperatures without requiring any heating means and, moreover, can treat exhaust gas efficiently without using a large amount of absorbing agent.
In view of the above-described problems of the prior art, the present inventors have made intensive investigations and have now found that an active carbon having been subjected to a specific heat treatment functions as an effective catalyst for desulfurization or denitration reactions. The present invention has been completed on the basis of this finding.
Accordingly, the present invention relates to a heat-treated active carbon for use in desulfurization or denitration reactions and a desulfurization or denitration process using it.
First of all, the present invention is described below in terms of desulfurization.
The present invention provides a heat-treated active carbon for use in desulfurization reactions which has been obtained by heat-treating a starting active carbon in a non-oxidizing atmosphere.
The present invention also provides a desulfurization process which comprises bringing a gas containing SO2, water and oxygen into contact with such a heat-treated active carbon for use in desulfurization reactions.
No particular limitation is placed on the type of the starting active carbon. For example, an active carbon fiber or an particulate active carbon is used. Active carbon fibers include those derived from pitch, polyacrylonitrile, phenol, cellulose and the like may be used. Commercial products may also be used. Among others, active carbon having highly hydrophobic surfaces are especially preferred. Specific examples thereof include pitch-based and polyacrylonitrile-based starting active carbon fibers.
The above-described starting active carbon is heat-treated in a non-oxidizing atmosphere. The term xe2x80x9cnon-oxidizing atmospherexe2x80x9d as used herein comprehends inert gases and reducing atmospheres. No particular limitation is placed on the type of the non-oxidizing atmosphere, so long as the starting active carbon is not oxidized thereby. In particular, inert gases such as nitrogen gas, argon gas and helium gas are preferred. Among them, nitrogen gas is especially preferred because it is readily available.
The heat-treating temperature may be any temperature that renders the surfaces of the starting active carbon hydrophobic. Although the heat-treating temperature may be suitably determined according to the type of the starting active carbon and the like, it is usually in the range of about 600 to 1,200xc2x0 C. The heat-treating time may be suitably determined according to the heat-treating temperature and the like. This heat treatment makes it possible to obtain the heat-treated active carbon for use in desulfurization reactions according to the present invention. In the heat-treated active carbon for use in desulfurization reactions according to the present invention, all or part of the hydrophilic oxygen-containing functional groups have been removed in the form of CO, CO2 and the like as a result of the heat treatment, so that its surfaces are highly hydrophobic as compared with those before heat treatment. Consequently, the adsorption of SO2 to SO2 oxidation sites occurs easily and, moreover, the discharge of the resulting sulfuric acid proceeds rapidly. Thus, it can perform a catalytic function for desulfurization reactions without hindrance.
The desulfurization process of the present invention comprises the step of bringing a gas containing sulfur dioxide (SO2) into contact with the aforesaid heat-treated active carbon. In this case, the aforesaid gas needs to contain water and oxygen. Although the SO2 concentration can be suitably regulated, efficient desulfurization can be achieved especially at SO2 concentrations of about 20 to 500 ppm.
Exhaust gas can be desulfurized in one step by using the heat-treated active carbon of the present invention. Alternatively, the present invention may also be practiced in the form of a high-degree desulfurization process in which the aforesaid heat-treated active carbon for the treatment of exhaust gas is used to remove sulfur oxides on the downstream side of a desulfurization apparatus based on the lime-gypsum method.
It is desirable that the aforesaid gas contains water at a relative humidity of 100% or greater and oxygen in an amount of 3% by volume or more (preferably 3 to 21% by volume). Any gaseous components other than those described above may be present therein, provided that they do not interfere with desulfurization reactions significantly. For example, nitrogen, carbon dioxide, carbon monoxide and the like may be present therein.
Although the contact temperature can be suitably changed according to the type of the heat-treated active carbon, the SO2 concentration and the like, it may usually be in the range of about 20 to 100xc2x0 C. Especially in the process of the present invention, efficient desulfurization can be achieved at ordinary temperatures (i.e., about 20 to 50xc2x0 C.). Even at high temperatures above 100xc2x0 C., desulfurization reactions can be made to proceed by controlling the water content and the like.
The flow rate of the aforesaid gas can be suitably changed according to the SO2 concentration, the type of the apparatus, and the like. However, it may usually be in the range of about 1xc3x9710xe2x88x923 to 5xc3x9710xe2x88x923 gxc2x7min/ml per unit weight of the active carbon.
In the process of the present invention, any well-known reactors may be used. For example, various types of reactors such as fixed-bed flow reactors, fluidized-bed reactors and stirred reactors may be used.
The resulting sulfuric acid may be recovered according to various recovery methods including, for example, (a) the method of absorbing the sulfuric acid into water to recover it as concentrated sulfuric acid, (b) the method of absorbing the sulfuric acid into an aqueous solution of KOH to recover it as a neutralized solution, (c) the method of neutralizing the sulfuric acid with an aqueous solution of Ca(OH)2, Mg(OH)2 or the like to recover it as a salt, and (d) the method of absorbing the sulfuric acid into aqueous ammonia to recover it as a fertilizer (i.e., ammonium sulfate).
Similarly to the above-described heat-treated active carbon for use in desulfurization, a heat-treated active carbon for use in denitration can also be obtained by heat-treating a starting active carbon at a temperature of about 600 to 1,000xc2x0 C. in a non-oxidizing atmosphere. The type of the atmosphere and other conditions may be the same as described above.
A first process of the present invention for removing nitrogen oxides by means of a heat-treated active carbon for use in denitration comprises providing a nitrogen oxide oxidation tower packed with a heat-treated active carbon for use in denitration which has been obtained by heat-treating a starting active carbon at a temperature of 600 to 1,000xc2x0 C., and passing exhaust gas through the oxidation tower to oxidize and remove nitrogen oxides (NOx) present therein.
A second process of the present invention for removing nitrogen oxides by means of a heat-treated active carbon for use in denitration comprises providing a plurality of adsorption towers packed with a heat-treated active carbon for use in denitration which has been obtained by heat-treating a starting active carbon at a temperature of 600 to 1,000xc2x0 C., the adsorption towers being arranged in parallel; and passing exhaust gas successively through the adsorption towers in such a way that the exhaust gas is switched from one adsorption tower to another before a breakthrough of the nitrogen dioxide (NO2) adsorbed on the heat-treated active carbon for use in denitration within the one adsorption tower occurs, and nitrogen oxides (NOx) present in the exhaust gas are thereby oxidized, adsorbed and removed continuously.
Moreover, in a high-degree denitration process in accordance with the present invention, nitrogen oxides can be removed by using a heat-treated active carbon for use in denitration, on the downstream side of a denitration treatment based on the selective catalytic reduction (SCR) method.
In the aforesaid processes for the removal of nitrogen oxides, it is preferable to treat the exhaust gas at a temperature of as low as 150xc2x0 C. or below.
In the aforesaid processes for the removal of nitrogen oxides, the nitrogen oxides oxidized by the heat-treated active carbon for use in denitration can be continuously absorbed into an absorbing fluid such as water or an aqueous alkaline solution, and recovered as nitric acid or a salt thereof.
In the aforesaid processes for the removal of nitrogen oxides, no particular limitation is placed on the type of the starting active carbon, as is the case with the heat-treated active carbon for use in desulfurization. However, it is desirable to use a polyacrylonitrile-based or pitch-based starting active carbon fiber as the starting active carbon fiber.
According to the present invention, the desulfurization performance of a starting active carbon can be improved by heat-treating it in a non-oxidizing atmosphere. The principle thereof is illustrated in FIG. 2.
Prior to heat treatment, the surface of the starting active carbon has many oxygen-containing functional groups distributed thereover as shown in FIG. 2(a), and hence exhibits hydrophilicity. In this case, surface water hinders SO2 from being adsorbed to SO2 oxidation sites. Moreover, sulfuric acid formed by oxidation and hydration is captured by surface water and accumulated on the surface of the starting active carbon, thus hindering desulfurization reactions from proceeding smoothly.
In contrast, hydrophilic oxygen-containing functional groups have been removed, in the form of CO, CO2 and the like, from the surface of the heat-treated active carbon, as shown in FIG. 2(b). Thus, its surface exhibits hydrophobicity. Consequently, SO2 is readily adsorbed to SO2 oxidation sites and, moreover, the resulting sulfuric acid is eliminated rapidly, so that the heat-treated active carbon of the present invention exhibits a high activity for desulfurization reactions without being hindered by sulfuric acid.
In order to remove sulfur oxides (SOx) present in exhaust gas by means of a heat-treated active carbon as described above, the exhaust gas is conditioned so as to have a temperature of 100xc2x0 C. or below, preferably 50xc2x0 C. or below, and a relative humidity of 100% or greater. Thereafter, the exhaust gas is introduced into a reactor packed with the heat-treated active carbon where sulfur oxides (SOx) present therein are oxidized to sulfur trioxide (SO3) at the surfaces of the heat-treated active carbon. Then, this sulfur trioxide (SO3) is reacted with water or an aqueous solution of sodium hydroxide or the like to recover it as sulfuric acid or a salt thereof. Thus, sulfur oxides (SOx) present in exhaust gas can be removed.
Ordinary active carbon have the property of adsorbing nitrogen monoxide (NO), but fail to exhibit sufficient oxidizing power. Although some of them have oxidizing power, their surface structure makes it difficult to remove nitrogen oxides in the form of nitrogen dioxide (NO2).
The reason for this is that plenty of oxygen-containing groups (such as carbonyl and carboxyl groups) and N- or S-containing groups remain on the surface of such active carbon.
According to the present invention, a starting active carbon is heat-treated in a non-oxidizing atmosphere. This decomposes and eliminates various groups present on the surfaces of the starting active carbon to activate NO oxidation sites. Moreover, hydrophilic oxygen-containing functional groups are also decomposed to decrease water (H2O) adsorption sites which hinder the adsorption of NO and the elimination of NO2. Thus, an improvement in NO-oxidizing activity can be achieved.
When an active carbon which has been heat-treated in this manner is used, nitrogen monoxide (NO) present in exhaust gas is adsorbed thereon, and then oxidized by O2 to form nitrogen dioxide (NO2).
This nitrogen dioxide (NO2) may be removed in the state adsorbed on the active carbon. Alternatively, the desorbed nitrogen dioxide (NO2) may be absorbed into water and recovered in the form of an aqueous solution of nitric acid, or may be absorbed into an aqueous alkaline solution and recovered in the form a salt of nitric acid. Thus, exhaust gas can be denitrated in the above-described manner.
As described above, the present invention makes it possible to remove nitrogen oxides and sulfur oxides from exhaust gas at a low temperature of 150xc2x0 C. or below by using a heat-treated active carbon.
Accordingly, a system in accordance with the present invention may be used as a substitute for the currently used denitrators and desulfurizers. Alternatively, when it is desired to improve the denitration or desulfurization performance of the current system, a system in accordance with the present invention may be connected therewith to achieve a further improvement in treating capacity.
Moreover, the desulfurization processes of the present invention using a heat-treated carbon for use in desulfurization reactions make it possible to desulfurize exhaust gas efficiently without using a large volume of water (i.e., in a dry manner). Especially when a heat-treated pitch-based carbon fiber for use in desulfurization reactions is used, the degree of removal of SO2 may be raised to 100% depending on the temperature used for the heat treatment.
Furthermore, as shown in FIG. 18, the SO2 adsorbed on the surface of a heat-treated active carbon for use in desulfurization reactions according to the present invention is oxidized by O2 present in the gas to form SO3, and the latter reacts with water present in the gas to form sulfuric acid. Then, this sulfuring acid is washed away from the surface. That is, by treating sulfur oxides-containing exhaust gas with a heat-treated active carbon, the concentration of sulfur oxides (SOx) in the exhaust gas can be reduced to a level of 5 ppm or less which has been difficult to achieve in the prior art, and such sulfur oxides can be recovered in the form of sulfuric acid (in particular, concentrated sulfuric acid).
Furthermore, according to the present invention, the nitrogen oxides oxidized on a heat-treated active carbon can be continuously treated by converting them into nitric acid or a salt thereof in an absorption tower. In addition, by carrying out high-degree denitration using a heat-treated active carbon in combination with conventional denitration based on selective catalytic reduction using a V2O5 catalyst, the concentration of nitrogen oxides (NOx) in exhaust gas can be reduced to a level of 1 ppm or less which has been difficult to achieve in the prior art.
The heat-treated active carbon for use in desulfurization reactions and the desulfurization processes, and the heat-treated active carbon for use in denitration reactions and the denitration processes, which are provided by the present invention, may be suitably used for the removal of sulfur oxides and nitrogen oxides produced in combustion equipment (such as boilers and thermal electric power plants) especially for burning heavy oil, coal and the like, sulfuric acid production plants, nitric acid production plants, metal processing works and facilities, paper mills, and tunnels.