REACTIVE COMPOSITION AND PROCEDURE FOR THE DEPURATION OF A GAS CONTAINING NITRIC OXIDE
The invention relates to the purification of gases contaminated by nitric oxide, mainly the purification of the fumes generated by the combustion of some fuel in the presence of air. The fumes generated by the combustion of gaseous, liquid or solid combustible materials are usually contaminated by nitrogen oxides from atmospheric nitrogen in the air and, if necessary, nitrogen compounds present in the fuel. In such fumes, most nitrogenous oxides are nitric oxide (NO), the balance consisting mainly of nitrogen peroxide
(N02). The high toxicity of nitric oxide and nitrogen peroxide and its contribution to the formation of acid rain imply the duty to remove them from the smoke before returning the smoke to the atmosphere. In US-A-4783325, a two-stage process for purifying a gas containing nitric oxide is proposed. In a first step, a gaseous oxygen mixture and a perhydroxyl radical initiator are introduced into the gas to be cleaned, maintained at high temperature, in order to oxidize the nitric oxide in nitrogen peroxide; in a second stage, the gas collected in the first stage is treated with lime, sodium carbonate, sodium bicarbonate or sodium sesquicarbonate to decompose the nitrogen peroxide. In this known process, the perhydroxyl radical initiating gaseous compound can be a vaporized hydrocarbon or gaseous hydrogen peroxide. This known method implies that two different reagents are introduced separately and successively into the gas to be treated, which complicates the implementation and requires an expensive installation. The use of a gaseous reagent further complicates the execution of the procedure. To purify a gas in nitrogen peroxide, it was also suggested to wash it with an alkaline solution of hydrogen peroxide or a peroxidized compound such as perborate or sodium percarbonate (Japanese Patent Application JP-A-51006884). Now, this procedure does not refer to the purification of a gas contaminated by nitric oxide, besides presenting the disadvantage of requiring a complex and expensive installation to carry out the washing of the gas to be purified with the aqueous alkaline solution. It was also proposed to introduce sodium percarbonate or sodium perborate in a cigarette filter, to remove nicotine, carbon monoxide, nitric oxide and nitrogen dioxide from the smoke (Japanese Patent Application JP-A-57018974). The invention aims to offer a reactive composition, solid, which allows the good purification of a gas contaminated by nitric oxide, but avoiding the drawbacks of the known procedures described above. Accordingly, the invention contemplates a reactive, solid, powdery composition for purifying a nitric oxide-containing gas in nitric oxide, said composition comprising a mixture consisting of at least one solid peroxidized compound and at least one salt thereof. alkali metal selected from the carbonates of the alkali metals, the bicarbonates of the alkali metals and the solid solutions of alkali metal carbonate and alkali metal bicarbonate. In the composition according to the invention, solid peroxidic compound is understood as a peroxidized compound that is solid and stable at room temperature. As a general rule, the peroxidized compound must be solid and stable at temperatures of at least 15 ° C, preferably at least 25 ° C. The peroxidized compound serves to liberate the active oxygen in the gas. The peroxidized compound of the composition according to the invention must then have a decomposition temperature that is, at most, equal and, preferably, lower than the normal temperature of the gas purification treatment. In practice, the peroxidized compound is selected from those whose normal decomposition temperature is below 200 ° C, preferably below 100 ° C, peroxidized compounds whose decomposition temperature is less than or equal to 60 ° C being especially advantageous. The peroxidized compound can be an inorganic compound or an organic compound. You can, for example, select between metal peroxides and persalts derived from inorganic or organic acids. It is advantageously selected from the alkali metal compounds. The alkali metal percarbonates are preferred. Sodium percarbonate is especially recommended. The composition according to the invention may comprise a single peroxid compound or several different peroxide compounds. The alkali metal salt of the composition according to the invention is selected from alkali metal carbonates, alkali metal bicarbonates and solid solutions of alkali metal carbonate and alkali metal bicarbonate. The alkali metal salt is preferably selected from the sodium salts, more specifically sodium carbonate, sodium bicarbonate, sodium sesquicarbonate and Wegscheider salt. Particularly advantageous are sodium bicarbonate and sodium sesquicarbonate. Sodium bicarbonate is preferred. In case the alkali metal salt of the composition according to the invention simultaneously comprises alkali metal carbonate and alkali metal bicarbonate, it is desirable to comprise more than 98% (preferably at least 99% by weight) of bicarbonate of alkali metal and less than 2% (preferably, a maximum of 0.1%) by weight of alkali metal carbonate. Preferred compositions - according to the invention - are those in which the peroxidized alkali metal compound comprises a peroxidized sodium compound and the alkali metal salt is selected from sodium carbonate, sodium bicarbonate and solid solutions of sodium carbonate and sodium bicarbonate. In the composition according to the invention, the respective tenors of peroxide compound and alkali metal salt will depend on several parameters; mainly of the peroxidized compound selected, of the selected alkali metal salt and of the composition of the gas to be purified, especially of its relative content in nitric oxide and of the eventual presence of other nitrogenous compounds, of sulfur oxides, of hydrogen chloride and oxygen. Therefore, these tenors must be determined in each particular case, through routine work in the laboratory. The composition according to the invention can thus comprise the peroxidized compound in a weight content of 5% (preferably 20%) to 95% (preferably 80%) of the weight of the mixture consisting of the peroxide compound and the salt of the alkali metal. According to a particularly advantageous embodiment, the composition according to the invention contains the compound peroxidized in a weight amount of 10 to 50%
(preferably, from 10 to 25%) of the weight of said mixture. (There, page 3 of the original ends.) Page 4 begins with a truncated phrase that begins with the word "addi tifs" (additives) NdT) additives, in addition to the mixture of the peroxidized compound and the alkali metal salt, by example stabilizers of the peroxidized compound or an additive that facilitates a fluidification of the compound for its contact with the gas to be purified. The composition according to the invention is in the pulverulent state. For this purpose, it is preferred that the peroxidized compound and the alkali metal salt possess similar granulometric distributions, even though this is not an indispensable condition. The optimum granulometry of the composition according to the invention will depend on its destination, in particular, the gas to be purified, its flow and the process used to treat the gas with the composition. In practice, the granulometric distribution of the granulometric composition of the reactive composition must possess certain flexibility, it being known that a fine granulometry will favor the reaction of the gas to be purified, while a coarse granulometry will favor the subsequent separation of the solid products from the reaction. It must then be defined in each particular case, through routine tests in the laboratory according to the destination to be given to the composition. In practice, it is advantageous to select a granulometry characterized by a median particle diameter of less than 50 μm (preferably equal to a maximum of 30 μm) and a particle size smaller than 5 (preferably equal to a maximum of 3). ) being defined the average diameter Dm and the granulometric slope s by the relationships: (see next page)? rii. Di, D90 - D10 Dm = s = S r ^ D50 in which n designates the frequency, by weight, of the particles of diameter Dx and D90 (respectively D50 and D10) represents the diameter by which 90% (50% and 10%, respectively) of the particles of the reactive composition (expressed by weight) have a diameter less than D9o (° 5o YD? o »respectively). These granulometric parameters are defined by the laser diffraction analysis method, using a Sympatec measuring instrument Helos model 12LA manufactured by Simpatec GmbH. The granulometries especially recommended are those that correspond to an average diameter of 10 to 30 μm and a granulometric slope of 1 to 3. Although especially adapted to the purification of gases in nitric oxide, the reactive composition according to the invention is also suitable for purifying gases contaminated by other nitrogen oxides, such as nitrous oxide (N2), nitrogen trioxide (N203), nitrogen pentoxide (N203) and nitrogen peroxide N02). From now on, the set of nitrogen oxides of the gas will be designated as NOx. On the other hand, by appropriate choice of the alkali metal salt and the content of said salt, the composition according to the invention may also be suitable for purifying a gas contaminated by hydrogen chloride or sulfur oxides, mainly by sulfur dioxide. . The composition according to the invention thus has the advantageous property of allowing a simultaneous purification of a gas in nitrogen oxides N0X, in hydrogen chloride or in sulfur oxides. The composition according to the invention therefore finds an advantageous application for purifying fumes generated by the combustion of sulfur-containing fossil fuels (such as fuel oil or coal) and for purifying residual fumes from the incineration of waste such as, for example, domestic garbage, hospital waste and plant waste. According to a particular embodiment of the invention, the reactive composition comprises carbon in the state of particles. The reactive composition, according to this embodiment of the invention, is especially adapted to the purification of contaminated fumes, by mercury, dioxins or furans. In this embodiment of the invention, the carbon turns out to be advantageously active carbon whose granulometry is in accordance with the imposed conditions, mentioned above, for the peroxidized compound and the alkali metal salt. The weight content of carbon in the reactive composition is usually at least 0.5%
(preferably, of at least 1%) of the weight of the mixture consisting of the peroxidized compound and the alkali metal salt, - is generally less than 20% (preferably 15%) of the weight of said mixture . The compositions especially recommended comprise from 1 to 10% by weight of carbon with respect to the weight of said mixture. The invention also concerns a process for the purification of a smoke in nitric oxide, according to which a reactive composition according to the invention is introduced into that smoke and then the smoke is subjected to a dedusting cleaning. In the process according to the invention, the reactive composition is introduced into the smoke, in the solid state. Usually, the reactive composition is introduced into a stream of smoke circulating inside a reaction chamber. In the same, the oxides of nitrogen are destroyed by the reactive composition, to form nitrite and nitrate of the alkali metal. In the case of a smoke contaminated by hydrogen chloride or sulfur oxides, these compounds are decomposed by forming alkali metal chloride or sulfate. In the case of a smoke contaminated by mercury, dioxins or furans, it will be advantageous to select a reactive composition comprising carbon particles. In this way, the smoke is purified, taking the mercury dioxins or furans to adsorb these pollutants on carbon particles. The function of removing the smoke is to remove the nitrite and nitrate particles of the alkali metal and, if appropriate, the alkali metal chloride or sulphate particles formed, and the carbon particles impregnated with mercury, dioxins or furans. . Dusting can be done by all known approved means - for example, by mechanical separation in a cyclone, or by filtration through a filter cloth or by electrostatic separation. Filtration by a filter fabric (for example, bag filter) constitutes the preferred mode of dedusting. In the process according to the invention, the treatment of the smoke with the reactive composition must be carried out at a suitable temperature in order to allow an effective reaction of the composition with the nitric oxide of the smoke. In practice, the temperature must be above 370 K, preferably at least equal to 375 K, with temperatures above or equal to 400 K being particularly advantageous. In principle, the maximum permissible limit for temperature corresponds to temperatures of fusion of the compounds of the reactive composition and of the compounds of the reaction with the constituents of the smoke. In practice, this limit is imposed by the mechanical capacity of industrial installations and, in particular, by the dedusting device. In the event that the dedusting comprises a filtering by a filtering fabric, it is recommended that the smoke temperature does not exceed approximately 550K on the filtering fabric. In order to favor the performance of the purification it is possible, of course, to introduce the reactive composition in the smoke, in a clearly higher temperature and, then, to cool the smoke before subjecting it to dedusting. While not wishing to be bound by a theoretical explanation, the inventors think that, within the process according to the invention, the peroxidized compound decomposes on contact with the hot smoke and releases atomic oxygen which cooperates with the alkali metal salt to convert the oxide nitrate in nitrite and in alkali metal nitrate. The reactive composition must then work in sufficient quantity to convert a substantial fraction of the nitrogen oxides N0X into nitrite and alkali metal nitrate, said fraction corresponding, for example, to a national or international standard, imposed for the composition of industrial or domestic fumes released into the atmosphere. The optimum amount of reactive composition that should be used has, then, to be determined in each particular case, through tests or routine calculations, depending on the purification rules imposed. The interest of the invention will be highlighted by the description of the following examples, referred to the sketches that are attached: there are twelve diagrams that expose the composition of the contaminated fumes, before and after having undergone a purification treatment. In the examples described below, we proceeded to purify the residual fumes of an industrial plant: these fumes contained nitric oxide, nitrogen oxides (NOx), hydrogen chloride and sulfur dioxide, the balance being constituted, essentially , by carbon dioxide and water vapor. To purify the aforementioned fumes, they were circulated through a reaction chamber where a reactive powder was injected. The smoke contents were measured in hydrogen chloride (HCl), in sulfur dioxide (S02), in nitric oxide (NO) and in the sum of the nitrogen oxides (NOx) respectively, at the entrance and exit of the reaction chamber. The results of the tests (converted for the case of dry normalized smoke, containing 9% by volume of carbon dioxide) are represented in a diagram of figures 1 to 12. In these diagrams, the scales of the abscissa represent the time (in minutes), - the scale of the ordinates of figures 1, 5 and 9 represents the tenor in nitric oxide of the smoke, expressed in g / Nm3; the scale of the ordinates of figures 2, 6 and 10 represents the tenor, in the sum of the oxides of nitrogens NOx, of the smoke (being this tenor expressed in g of oxides, considered in the N02 state, by m3); the scale of the ordinates of figures 3, 7 and 11 represents the hydrogen chloride content of the smoke, in g / Nm, and the scale of the ordinates of figures 4, 8 and 12 represents the content of sulfur dioxide in the smoke, in g / Nm3. The symbols in the form of a square refer to the composition at the entrance to the reaction chamber, while the cross-shaped symbols designate the composition of the smoke at the outlet of the reaction chamber. With respect to the reagents, a powder of sodium percarbonate (active oxygen content: 135.7 g / kg), a powder of sodium bicarbonate and a powder obtained by mixing the powder of sodium percarbonate was prepared
(15% by weight) and sodium bicarbonate powder (85% by weight). The powders had an average diameter of 15 to
μm and a granulometric slope of 2 to 3 such as those defined above. Example 1 (reference) In this example, the smoke was treated with 5 kg of sodium percarbonate powder per hour. The results of the test are shown in the diagrams of figures 1 to 4. It is observed that the purification of the smoke in nitric oxide (figure 1) and in oxides of nitrogen N0X (figure 2), was little surplus and was very irregular. Example 2 (reference) The test of example 1 was repeated, 5 kg of the sodium bicarbonate powder per hour being used as the reactive composition. The results of the test are shown in the diagrams of figures 5 to 8. It is observed that the sodium bicarbonate had no significant effect on the purification of the smoke in nitric oxide (figure 5), in oxides of nitrogen NOx (figure 6) and in sulfur dioxide (figure 8). Example 3 (according to the invention) The test of example 1 was repeated, using the reactive composition according to the invention, constituted by the mixture of sodium percarbonate and sodium bicarbonate. 10 kg of the reactive composition per hour was injected into the smoke. The results of the test are shown in the diagrams of figures 9 to 12. It is observed that the smoke underwent a very deep purification, not only in nitric acid (figure 9) but also in relation to the set of NOx nitrogen oxides (figure 10). ). In addition, the purification in the matter of hydrogen chloride (figure 11) and sulfur dioxide (figure 12) was higher than that obtained in the tests of examples 1 and 2.