METHOD OF TREATING WASTE GAS BY IRRADIATION WITH ELECTRON BEAM

Abstract: A waste gas containing sulfur oxides (SO x ) and/or nitrogen oxides (NO x ) is purified by leading the waste gas to a radiation irradiation zone, adding ammonia (NH₃) to the waste gas before, during or after the irradiation, collecting the resulting by-product (ammonium sulfate and/or ammonium nitrate) by means of a dust collector, and releasing the purified waste gas to the atmosphere. The amount of NH₃ to be added is selected so as to be smaller than an amount calculated from target desulfurization and denitrification efficiencies so that it is possible to reduce the leakage of ammonia despite variations in the composition of the waste gas. Further, an alkaline substance exclusive of ammonia is added to the waste gas within the dust collector or at the upstream side thereof so that the pH of the collected by-product is adjusted to 6 or more, preferably 7 or more, thereby supressing the adverse effects of sulfamic acid impurities.

Description: The present invention relates to a method of treating waste gas containing poisonous gas components such as SO x and/or NO x . Fig. 1 shows a flow sheet schematically showing a typical process for treating waste gas containing SO x and/or NO x which is discharged from a heavy oil combustion furnace or the like to make the waste gas harmless. A known process is described in ENVIRONMENTAL SCIENCE & TECHNOLOGY, Vol.14, March 1980, pages 288-293. More specifically, waste gas (generally at 130°C or more) which is generated from a boiler 1 is introduced through a waste gas duct 2 into a cooling tower 3 where it is cooled down by means of cooling water sprayed from a cooling water supply pipe 4 so that the gas temperature is in the range of from the dew point to 100°C, and the cooled waste gas is then introduced into a reactor 7 through a waste gas duct 5. At the intermediate portion of the duct 5, ammonia is added to the waste gas through a flow control valve 6. The waste gas introduced into the reactor 7 is irradiated with electron beams from an electron beam generator 9. As a result, the SO x and/or NO x contained in the waste gas reacts with ammonia to change into ammonium sulfate and/or ammonium nitrate, which is then removed by means of a dust collector 11, and the purified waste gas is then released into the atmosphere from a smokestack 13. The ammonium sulfate and/or ammonium nitrate removed from the waste gas is collected as a by-product from a discharge pipe 12. It should be noted that, in order to control rise in temperature of the waste gas due to the generation of heat by electron beam irradiation and the generation of heat accompanying desulfurization and/or denitrification and thereby maintain the waste gas at an optimal temperature, the waste gas is sprayed with cooling water in the reactor 7 using cooling water spray means 8 at at least one of the three positions, that is, within the irradiation zone and at the upstream and downstream sides thereof. It is most preferable to spray the waste gas with cooling water after the irradiation (an invention relating thereto was filed as Japanese Patent Application No. 308,887/1987). The dust collector 11 may comprise either an electrostatic precipitator (EP) or a bag filter or both. In the case of using a bag filter alone, since the pressure loss of the waste gas rises within a short period of time, it is necessary to use a bag filter having a relatively large capacity in order to ensure a stable operation, which leads to an increase in the cost. The reference numerals 14, 15 and 16 in Fig.1 denote an SO x analyzer, an NO x analayzer and a waste gas flowmeter, respectively. The amount of ammonia (NH₃) to be added may be calculated from the flow rate of waste gas (QNm³/h), the SO x concentration ([SO x ] ppm), the NO x concentration ([NO x ] ppm), the target desulfurization efficiency (η SOx ) and the target denitrification efficiency (η NOx) as follows: In order to reduce emission of poisonous components, waste gas treatment equipment has recently been required to meet extremely stringent emission standards, i.e., desulfurization efficiency: 90% or more; denitrification efficiency: 80% or more; and leakage ammonia: 10 ppm or less. It is expected that the emission standards will become increasingly stringent in the future. However, the above-described prior art method suffers, from the disadvantage that it is difficult to control the leakage of ammonia, and another problem may arise in regard to by-products. This will be explained hereinunder more specifically. Fig. 2 is a typical chart showing variations in the SO x and NO x concentrations in coal combustion waste gas. The SO x concentration has variations of about ±100 ppm with respect to an average value of 1500 ppm, while the NO x concentration has variations of about ±20 ppm with respect to an average value of 300 ppm. The amount of ammonia which is to be added to waste gas in a process wherein the target desulfurization efficiency is 90% and the target denitrification efficiency is 80% is calculated from the expression (1) as follows: The highest concentration of ammonia to be added = 2 x 1600 x 0.9 + 320 x 0.8 = 3136 ppm The lowest concentration of ammonia to be added = 2 x 1400 x 0.9 + 280 x 0.8 = 2744 ppm The average concentration of ammonia to be added = 2 x 1500 x 0.9 + 300 x 0.8 = 2940 ppm Assuming that the allowable leakage ammonia concentration is the aforementioned level, i.e., 10 ppm, it is necessary to feed waste gas having an ammonia concentration in the range of from 2744 ppm to 3136 ppm while controlling the leakage of ammonia with tolerances of ±10 ppm, that is, from 0.3 to 0.4% (i.e., from 10/3136 to 10/2744). The values of the required control accuracy are considerablly lower than those of the presently achievable control accuracy (i.e., from 1 to 2 % of the full scale), so that it has heretofore been difficult to control the leakage of ammonia to 10 ppm or less. The by-products, which consist mainly of ammonium sulfate and ammonium nitrate, represent a useful nitrogen fertilizer but, when the CO concentration in waste gas is below 10 times the SOx concentration, sulfamic acid, which is harmful to plants, is generated, and although the content of sulfamic acid is very low, i.e., several percent, the fertilizer formed from the by-products, inhibits the growth of plants [the present applicant has already filed a method of removing sulfamic acid compounds by thermal cracking under Japanese Patent Application No. 61-279791 (1986)]. The present inventors conducted various studies in order to reduce the leakage of ammonia and modify the by-products without lowering either the desulfurization efficiency or the denitrification efficiency. As a result, we have found that it is possible to attain this objective by adding an amount of ammonia which is smaller than that which is needed to achieve target desulfurization and denitrification efficiencies and further adding an alkaline substance exclusive of ammonia to remove the SO x and NO x which are left unreacted due to lack of ammonia, thereby modifying the by-product while maintaining the overall desulfurization and denitrification efficiencies at high levels. It is also possible to substantially inhibit adverse effects of sulfamic acid impurities by adjusting the pH of the collected by-product to 6 or more, preferably 7 or more. The present invention provides a method of treating waste gas by irradiation with radiation which includes the steps of leading a waste gas containing sulfur oxides (SO x ) and/or nitrogen oxides (NO x ) to an irradiation zone, adding ammonia (NH₃) to the waste gas within the irradiation zone or at the upstream or downstream sides thereof, collecting the resulting ammonium sulfate and/or ammonium nitrate by means of a dust collector, and releasing the purified waste gas into the atmosphere, wherein the improvement comprises adding an amount of ammonia which is smaller than a stoichiometric amount and further adding an alkaline substance exclusive of ammonia to the waste gas within the dust collector or at the upstream side thereof. Fig. 1 is a flow sheet illustrating a prior art. Fig. 2 is a chart showing variations in concentration of SO x and NO x in waste gas. Fig. 3 is a flow sheet illustrating one embodiment of the present invention. Fig. 4 is a flow sheet illustrating another embodiment of the present invention. Fig. 5 is a chart showing variations in concentration of SO x and NO x in waste gas which has relatively large concentration variations. Fig. 6 is a graph showing the relationship between the Ca(OH)2 content and the pH of the by-product. Fig. 3 is a flow sheet illustrating one embodiment of the method according to the present invention, wherein a combination of an electrostatic precipitator and a bag filter is used as a dust collector (see U.S. Patent Application Serial Number 055,969, June 1987). A stoichiometric concentration of ammonia to be added is calculated from the aforementioned expression (1), but the actual concentration of ammonia added ranges from 65% to 100% in accordance with the SO x and NO x concentration variation patterns. More specifically, the greater part of desulfurization and denitrification is substantially effected by ammonia, which has a relatively high reaction rate. The amount of ammonia to be added is preferably selected on the basis of the lowest concentration in each concentration variation pattern. In the case of the concentration variation patterns shown in Fig. 2, the lowest concentration of SO x is 1400 ppm, while the lowest concentration of NO x is 280 ppm, and therefore the concentration of ammonia to be added is 2744 ppm, as described above. On the other hand, the stoichiometric concentration with respect to the highest concentrations of SO x and NO x is 3136 ppm. Therefore, the ratio of the actual concentration of ammonia added to the stoichiometric concentration ranges from 87.5% to 100% (i.e., from 2744/3136 to 2744/2744). Fig. 5 is a chart showing variations in concentration of SO x and NO x in waste gas which has relatively large concentration variations, wherein the highest concentrations of SO x and NO x are 1200 ppm and 320 ppm, respectively, while the lowest concentrations of SO x and NO x are 800 ppm and 280 ppm, respectively. Assuming that the target desulfurization efficiency is 90% and the target denitrification efficiency 80%, the concentration of ammonia to be added is calculated from the expression (1) as follows: The highest stoichiometric concentration = 2 x 1200 x 0.9 + 320 x 0.8 = 2416 (ppm) The lowest stoichiometric concentration = 2 x 800 x 0.9 + 280 x 0.8 = 1644 (ppm) Accordingly, if ammonia is added on the basis of the lowest stoichiometric concentration, the ratio of the actual concentration of ammonia added to the stoichiometric concentration ranges from 68.9% to 100% (i.e., from 1664/2416 to 1664/1664). The addition of an alkaline substance exclusive of ammonia in the present invention is carried out within the dust collector or on the upstream side thereof. The added alkaline substance reacts with unreacted SO x and NO x mainly within the dust collector to thereby remove these poisonous gas components. As the above-described alkaline substance, it is preferable to employ one selected from among inorganic salts formed from calcium, sodium, potassium and magnesium, more specifically, Ca(OH)₂, CaO, CaCO₃, Na₂CO₃, NaHCO₃, Mg(OH)₂, etc., or a mixture thereof. The amount of alkaline substance added is preferably selected so as to be equal to or more than an amount (equimolecular amount) which is determined on the basis of the concentrations of undesulfurized SO x and undenitrified NO x . In the arrangement shown in Fig. 3, an alkaline substance in the form of a powder is added to the waste gas at a position between the electrostatic precipitator 11a and the bag filter 11b and this alkaline substance is uniformly retained over the bag filter 11b. Since the waste gas passes through the filter 11b from the obverse side to the reverse side in such a manner as to be filtered therethrough, the unreacted SO x and NO x in the waste gas come into contact and react uniformly with the alkaline substance retained on the filter 11b and these poisonous gas components are effectively removed. Fig. 4 is a flow sheet illustrating another embodiment of the method according to the present invention, wherein the electrostatic precipitator 11a is used alone. This embodiment is basically the same as the first embodiment which has been described above with reference to Fig. 3. Unreacted SO x and NO x are allowed to react with an alkaline substance added at the upstream side of the electrostatic precipitator 11a mainly within the electrostatic precipitator 11a or, if necessary, within a reactor (not shown) which is additionally provided at the upstream side of the electrostatic precipitator 11a. The addition of an alkaline substance may be carried out on the upstream side of the electrostatic precipitator 11a at any of the three positions, that is, within the waste gas irradiation zone and at the upstream and downstream sides thereof since salts which are produced as products of the reaction of the alkaline substance with SO x and NO x are collected in the form of a mixture of ammonium sulfate and ammonium nitrate. However, it is most preferable to add an alkaline substance to the waste gas after the irradiation. It is also preferable to carry out the addition of an alkaline substance at two or more of the above-described three positions. In these cases, the form of alkaline substance added is not necessarily limited to a powder form and it may be in the form of an aqueous solution. Since cooling water is used to adjust the temperature of waste gas as described above, if the alkaline substance is dissolved in the cooling water in advance, it is unnecessary to additionally provide a feed means for the alkaline substance. The addition of the aqueous alkaline solution may be carried out at any of the three positions, that is, within the irradiation zone or at the upstream (i.e., the cooling water pipe 8 shown in Fig. 4) or downstream sides thereof, and it may also be conducted within the cooling tower 3. It should be noted that, in the case where the bag filter is used alone, the arrangement is basically the same as that of the embodiment shown in Fig. 4. In the foregoing process, it is preferable to add an alkaline substance in such an amount that the pH of the collected by-product is 6 or more, preferably 7 or more. This is because it is possible to inhibit adverse effects on plants by increasing the amount of alkaline substance added even if the by-product contains several percent of a sulfamic acid compound which is harmful to plants. The addition of an alkaline substance is also effective to reduce the rise in pressure loss of waste gas in the case of using a bag filter alone. More specifically, the by-products (including ammonium sulfate and ammonium nitrate) resulting from the irradiation are in the form of considerably fine particles which are very adherent, coherent and hygroscopic. Therefore, it may be considered that the particles of by-product will become agglomerated on the surface of the bag filter and thereby clog it within a short period of time, thus causing a rise in the pressure loss. However, if an alkaline substance is added, salts which are produced as reaction products mainly form nuclei and fine particles of ammonium sulfate and ammonium nitrate agglomerate around these nuclei to become coarse. The resulting coarse particles are captured on the filter surface and can then be readily released from it. Accordingly, there is no fear of the filter cloth becoming clogged, and the rise in the pressure loss is prevented. The amount of alkaline substance to be added will depend on the concentrations of SO x and NO x and the target desulfurization and denitrification efficiencies but it is preferably selected to fall in the range of from 1.0% to 10% in the weight ratio terms with respect to the by-product. Exemplary forms of radiation which are usable in the method of the present invention include electron beams, beta rays, gamma rays, alpha rays, X-rays and neutron rays. It is preferable to employ electron beams emitted from an electron beam generator. Hereinunder, the present invention will be described more specifically by way of Examples. However, the present invention is in no way restricted to these Examples. Example 1: The following experiment was conducted so as to achieve a desulfurization efficiency of 90% and a denitrification efficiency of 80% using an experimental apparatus having the arrangement shown in Fig. 3. More specfically, a waste gas having a gas temperature of 130°C, an average SO x concentration of 1500 ppm, an average NO x concentration of 300 ppm (variations of each concentration being shown in Fig. 2) and a CO concentration of 25 ppm was cooled down to 70°C in the cooling tower 3. Thereafter, ammonia (2700 ppm) was added to the waste gas and the mixture was then introduced into the reactor 7 where it was irradiated with electron beams of 1,8·10⁴ J/kg. Water was sprayed from the cooling water spray means 8 so that the gas temperature at the outlet of the bag filter 11b was adjusted to 70°C. After the greater part of the by-product had been collected in the electrostatic precipitator 11a, Ca(OH)₂ powder was added at a rate of 2.9 kg/h to the waste gas at the inlet of the bag filter 11b. The amount of Ca(OH)₂ added is selected so as to be adequate to react with these unreacted gas components. The respective concentrations of SO x , NO x and ammonia were measured at the outlet of the electrostatic precipitator 11a and at the outlet of the bag filter 11b and desulfurization and denitrification efficiencies were calculated from the numerical values simultaneously obtained from the SO x and NO x analyzers 14 and 15, respectively. It should be noted that about 2 minutes was required for each of the analyzers to complete an operation of detecting the gas concentration and converting it to an electrical signal. The pH of the by-products collected in the electrostatic precipitator (EP) 11a was about 2.7. The results of the experiment are shown in Table 1 below. Desulfurization efficiency Denitrification efficiency Ammonia conc. EP outlet 78 - 89% 80% ≃ 0 ppm Bag filter outlet 90% 81% ≃ 0 ppm Comparative Example 1 : An experiment was conducted using the same apparatus and under the same conditions as in Example 1 except that ammonia was added in the range of from 2744 to 3136 ppm (the target desulfurization and denitrification efficiencies being 90% and 80%, respectively) and no Ca(OH)₂ was added at the inlet of the bag filter 11b. The results are shown in Table 2 below. Desulfurization efficiency Denitrification efficiency Ammonia conc. EP outlet ≃ 88% 80% 0-50 ppm Bag filter outlet 88 - 90% 81% 0-30 ppm Example 2 : An experiment was conducted under the same conditions as in Example 1 except that an experimental apparatus having the arrangement shown in Fig. 4 was used and Ca(OH)₂ powder was added at the inlet of the electrostatic precipitator 11a. The respective concentrations of SO x , NO x and ammonia in the waste gas at the outlet of the electrostatic precipitator 11a before and after the addition of Ca(OH)₂ were measured, and desulfurization and denitrification efficiencies were calculated from the numerical values obtained from the concentrations of SO x and NO x in the waste gas at the inlet of the system which were measured at the same time. It should be noted that the pH of the by-product was about 3. The results of the experiment are shown in Table 3 below. Desulfurization efficiency Denitrification efficiency Ammonia conc. Before addition of Ca(OH)₂ 78 - 89% 80% ≃ 0 ppm After addition of Ca(OH)₂ 90% 81% ≃ 0 ppm Example 3 : An experiment was conducted using the same experimental apparatus and under the same conditions as in Example 2 except that the rate of addition of Ca(OH)₂ was raised to 4.6 kg/h. As a result, the amount of by-product collected was 65 kg/h, that is, 2.6% (1.7 kg/h) better than that in Example 2, and the pH of the by-product was about 7. The desulfurization efficiency and the denitrification efficiency were the same as in Example 2. Example 4 : Germination tests of Brassica campestris, a kind of Chinese cabbage, were conducted using the by-products obtained in Example 3 and Comparative Example 2 (described later), and the results of the tests were compared with those of the by-products (pH≒3) formed in Example 1 and a commercially available compound fertilizer consisting of ammonium sulfate and ammonium nitrate. The results are shown in Table 4 below. No. Fertilizers Sulfamic acid content pH Results of test 1 By-products of Example 3 and Comparative Example 2 4.8 wt% Similar to No. 3 Excellent growth 2 By-product collected in EP in Example 1 3.8 wt% 2.7 Germination rate and plant weight lowered considerably in comparison with Nos. 1 and 3 3 Commerxially available compound fertilizer 0 wt% - Excellent growth It will be understood from the results shown above that the adverse effects of sulfamic acid were suppressed in No. 1 since the pH was effectively adjusted. Example 5 : Ca(OH)₂ was added to the by-product collected in EP in Example 1 in the following various amounts, that is, 0.5 wt%, 1.0 wt%, 2.0 wt%, 3.0 wt%, 4.0 wt% and 5.0 wt%, and the pH values of the by-products formed in each case were measured. The results of this measurement are shown in Fig. 6. The results show that it was necessary to add about 2.6 wt% of Ca(OH)2 in order to adjust the pH of the by-products to 7. Comparative Example 2: An experiment was conducted using an experimental apparatus having the same arrangement as that shown in Fig. 4 except that a bag filter alone was used as a dust collector. A waste gas having a gas temperature of 130°C, an average SO x concentration of 1500 ppm, an average NO x concentration of 300 ppm and a CO concentration of 25 ppm was cooled down to 70°C in the cooling tower 3. Thereafter, ammonia (2744 - 3136 ppm) was added to the waste gas. The variations in concentration of SO x and NO x were similar to those shown in Fig. 2, and an amount of ammonia adequate to achieve was added. Then, the mixture was introduced into the reactor 7 where it was irradiated with electron beams of 1,8·10⁴ J/kg. Water was sprayed from the cooling water spray means 8 disposed at the upstream side of the electron beam irradiation zone so that the gas temperature at the outlet of the bag filter was adjusted to 70°C. The desulfurization and denitrification efficiencies and NH₃ concentration in the waste gas at the outlet of the bag filter here as shown in Table 5 below. Desulfurization efficiency Denitrification efficiency NH₃ Bag filter outlet 88 - 90% 81% 0-30 ppm The pH of the by-products collected in the bag filter was about 3. About 2.6 wt% of Ca(OH)₂ was added to the by-products so that the pH was adjusted to 7. As has been described above, it is possible according to the present invention to reduce the leakage of ammonia despite variations in the composition of a waste gas. Even if the by-products contain several percent of sulfamic acid impurities, it is possible to supress the adverse effects thereof by adding an alkaline substance exclusive of ammonia so that the pH of the collected by-product is adjusted to 6 or more, preferably 7 or more. An amount of SO₃ ions in the by-product was measured and it was found to be as low as 0.1 wt% or less. Thus, it is presumed that CaSO₃·1/2H₂O formed from the reaction of SO₂ with Ca(OH)₂ may be oxidized with acidic substances in the by-product to be converted to stable gypsum, CaSO₄·2H₂O. The present invention provides great industrial benefits since it is possible to carry out the present invention without the need to impart special changes or modifications to conventional systems.

Claims: A waste gas treating method including the steps of leading a waste gas containing sulfur oxides (SO x ) and/or nitrogen oxides (NO x ) to a radiation irradiation zone, adding ammonia (NH₃) to the waste gas before, during or after the irradiation, collecting the resulting by-product (ammonium sulfate and/or ammonium nitrate) by means of a dust collector, and releasing the purified waste gas to the atmosphere, characterised in that an amount of NH₃ is added which is smaller than an amount calculated from target desulfurization and denitrification efficiencies, and in that an alkaline substance exclusive of ammonia is further added to the waste gas for reaction with unreacted SO x and NO x within said dust collector or at the upstream side thereof in an amount equal or more than the equimolecular amount which is determined on the basis of the concentrations of undesulfurized SO x and undenitrified NO x which are left unreacted due to lack of ammonia. The method according to claim 1, wherein a combination of an electrostatic precipitator and a bag filter is used as said dust collector and said alkaline substance is added to the waste gas at a position between the electrostatic precipitator and the bag filter. The method according to claim 1 or 2, wherein for said alkaline substance an inorganic salt formed from calcium, sodium, potassium, or magnesium, or a mixture therof is used. The method according to any one of claims 1 to 3, wherein for said alkaline substance one substance from the group consisting of calcium hydroxide (Ca(OH)₂), calcium oxide (CaO), sodium carbonate (Na₂CO₃), sodium hydrogencarbonate (NaHCO₃), is selected. The method according to any one of claims 1 to 4, wherein the amount of NH₃ to be added is in the range of at least 65 % of an amount calculated from target desulfurization and denitrification efficiencies. The method according to any one of claims 1 and 3 to 6, wherein for said dust collector either an electrostatic precipitator or a bag filter or both are used and said alkaline substance is added to the waste gas at at least one of the three positions, that is, within the irradiation zone or at the upstream or downstream sides thereof. The method according to any one of the claims 1 to 6, wherein said alkaline substance is added in the form of either a powder or an aqueous solution. The method according to any one of the claims 1 to 7, wherein the amount of said alkaline substance to be added is selected further so as the pH of the collected by-product is adjusted to 6 or more. The method according to claim 8, wherein the amount of said alkaline substance is selected so as the pH of the collected by-product is adjusted to 7. The method according to any one of the claims 1 to 9, wherein said radiation is an electron beam applied from an electron beam accelerator.

Inventors: AOKI, SHINJI, HIRAYAMA, YOSHIO, MAEZAWA, AKIHIKO
Applicants: EBARA CORPORATION
Application year: 1988
Main classification: B01D53
IPC codes: B01D53, B01J19