PROCESS FOR THE REDUCTION OF THE CONTENT OF SO2 AND/OR NOX IN FLUE GAS

Abstract: The content of SO₂ and/or NO x in flue gas is reduced by irradiating the flue gas in admixture with NH₃ with ultraviolet at wavelengths between 170 and 220 nm.

Description: the mixture with ultraviolet radiation containing at leas one component of wavelength in the range about 190 to and/or NOx in Flue Gas The present invention relates to a process for the reduction of the content of S02 and/or the nitrogen oxides NO and NO2 (referred to herein by the general term "NOx") in flue gases. It has been proposed to remove NO, from flue gases by mixing the gas with 3 and irradiating the mixture with ultraviolet light. By this process, the NH3 is photolysed to yield amino radical (NH2) in accordance with the equation NH3 # NH2 + H (1) The amino radical reacts with NO, to yield the inert gas nitrogen, water and N20 which is widely regarded as being inert and harmless in the atmosphere, in accordance with the equations NE) + NO # ) N2 + H20 (2) NH2 + NO2 # N2O + H2O (3) It has now been found that increased efficiency of tne utilization of the ultraviolet light can be obtained when ultraviolet light of a wavelength falling within a selected range is employed. More specifically, the present invention provides, in one aspect, a process for reduction of the content of NO and N02 in flue gas containing also substantial quantities of H20 vapor comprising mixing the flue gas with NH3 and irradiating about 220 nm, said radiation being substantially wholly free of any component of wavelength below about 190 nm, and said process taking place in the absence of a solid state catalyst. It will be appreciated that the efficiency of the utilization of the ultraviolet radiation is of economic significance. The process is of course usually to be applied to a flowing stream of the gas and if the irradiation is conducted with low efficiency, a prolonged exposure to the radiaton is required. This would require a bank of ultraviolet lamps of greatly extended length for irradiation of a prolonged section of the duct conveying the flue gas. The result would be that not only would there be an increase in the operating costs of supplying the energy required to energize the ultraviolet lamps, but also the capital cost of supplying and installing the irradiation apparatus would be considerably increased. When employing wavelengths in the range about 190 to about 220 nm, the main NOx-removing reactions that occur are the above reactions (1), (2), and (3), resulting in the formation of N20,H20, and N2 in the waste gas stream. It is considered that these products can be safely passed to the atmosphere. By employing radiation free from any component below about 190 nm undesirable side reactions are avoided. With radiation below 190 nm there is considerable formation of OH radical due to photolysis of water vapor which is usually present as a matter of course in flue gas streams. Due to the high concentrations of water vapor normally present, the predominating reaction is therefore: H20 r -) OH + H (4) The OH radical reacts with N02 to yield acidic species OH + N02 1' HN03 (5) Normally, it would be desirable to remove these acidic species by absorption in the presence of moisture, e.g. water vapor, by an alkaline-reacting medium, e.g. by reaction with an excess of ammonia gas to yield nitrate salts NH4N03. The nitrate salts may form a particulate second phase and normally it is desirable to remove the particulate salts from the gas stream before passing it to the atmosphere. In this case, particulate salts are obtained as a by-product. The particulate salts may be recovered and may have a value e.g. as fertilizer. Neon03 is, however, explosive and presents handling difficulties. By employing a radiation source substantially wholly free of any component below 190 nm, the formation of an explosive precipitate of NH4N03 is much reduced or is substantially wholly avoided. When the wavelength of the ultraviolet radiation employed in the irradiation step is in the range about 190 to about 220 nm, the NH3 gas present in the mixture absorbs the radiation strongly, and a satisfactory removal of NOx is achieved as there is little interferance from undesired side reactions. Above about 220 nm the efficiency of the process is impaired as the degree to which NH3 absorbs radiation drops off sharply with wavelengths higher than about 220 nm and therefore only negligibly small concentrations of amino radical are generated, so that if the wavelength of the radiation is increased much above 220 nm the concentrations of ammonia that need to be employed, and the intensity of the radiation that is required, in order to achieve removal or NOx within reasonable times, rapidly become impracticably large. The absorption of the radiation by NH3 peaks at about 195 nm and drops off, as indicated in Table 1 toward wavelengths below about 170 nm. At the same time, however, as indicated in Table 1, the radiation is quite strongly absorbed by the 2 and H2O which are usually present as a matter of course. Table 1 - Extinction coefficients (1 mol-1 cm-1) EMI5.1 <tb> <SEP> Wavelength <SEP> (nm) <tb> <SEP> Reaction <SEP> 184.9 <SEP> 193 <SEP> 213.9 <tb> NH3 <SEP> + <SEP> h##NH2 <SEP> <SEP> + <SEP> + <SEP> H <SEP> 1000 <SEP> 1500 <SEP> 100 <tb> <SEP> 2 <SEP> <tb> O2 <SEP> + <SEP> hv <SEP> 920 <SEP> (P) <SEP> 4.2 <SEP> 0.3 <SEP> 0.002 <tb> H2O <SEP> + <SEP> h##OH <SEP> <SEP> + <SEP> H <SEP> # <SEP> 14 <SEP> 0 <SEP> 0 <tb> The extinction coefficint # is devined by I = 1o 10-#cl where I = light intensity incident on a cell containing the photolysable species I = intensity emerging from the cell c = concentration of the photolysable species (mol dm-3) 1 = cell length (cm) and # = extinction coefficient (1 mol-1 cm-1 The extinction coefficient therefore indicates how strongly the photolysable species absorbs the radiation, the higher the coefficent, the greater the degree of absorption. It will be noted that above about 190 nm, the extinction coefficient of water vapor is zero and therefore substantially no OH radical is formed, so that reaction (5) does not occur to any significant extent. Although the coefficients of 2 and H2O are substantially lower than that of NH3 in the range 170 to 220 nm, as may be perceived by the above Table, such coefficients increase sharply toward the lower end of this range and, moreover the concentrations of 2 and H20 present in the reaction mixture are normally considerably higher than the concentration of NH3. Desirably, in the process of the present invention the concentration of NH3 present in the mixture that undergoes photolysis is in the range from about 5 x 10-6 to about 5 x 10¯3 mol/l. With concentrations of NH3 below about 5 x 10-6 mol/l, the efficiency of the removal of NO, tends to be impaired and undesirably prolonged exposure to the ultraviolet radiation is required to achieve satisfactory degrees of removal of the undesired NOx material. Concentrations of NH3 above about 5 x 10"3 mol/l appear to be unnecessary and are undesirable as not only does the maintenance of high NH3 concentrations in the photolysis reaction zone greatly increase the consumption of NH3 and hence also the operating costs of the process, but also this may result in emission of substantial quantities of unconsumed NH3 to the atmosphere. More preferably, the said concentration of NH3 is in the range of about 1 x 10-5 to about 1 x 10-3 mol/l, still more preferably about 1 x 10¯5 to about 2 x 10-S mol/1. The quantities of O2 (moles) present in flue gases due to incomplete consumption of 2 in the. combustion air will however typically be about 100 times the molar concentrations of NH3 which it is desired to maintain in the reaction mixture, and the quantities of H20 vapor present as a product of combustion will typically be of the order of about 200 times the said NH3 molar concentrations. Therefore, at wavelengths much below about 190 nm, absorptions by 2 and H2O compete significantly with the absorption by NH3 as the concentration of 2 and H20 is much higher than that of NH3 and the extinction coefficients of 2 and H2O increase rapidly while the extinction coefficient of NH3 drops rapidly below about 190 nm, and therefore below 190 nm the formation of NH2 radicals. is greatly reduced. As a result, when the ultraviolet radiation includes components with a wavelength below about 190 nm the efficiency of the utilization of the radiation energy is much reduced as a large proportion of the radiation energy is directed to the production of incompetent species, There is some tendency during the present reaction for combination of NH2 radicals to occur, yielding hydrazine which is poisonous. NH2 + NH2 " N2H4 It is of course desirable to maintain the concentration of hydrazine in the reaction mixture leaving the photolysis reaction zone as low as possible, as it may otherwise be necessary to take special steps to absorb hydrazine from the reaction mixture. Hydrazine is a strung absorber of radiation in the wavelength range about 180 to about 270 nm, and dissociates to reform the amino radical N2H4 + h ) 2NH2 By employing a source of ultraviolet light including one or more components in the wavelength range 190 to 220 nm, the content of hydrazine in the reaction mixture can be kept to acceptably low levels by its re-conversion to free radicals. In a further aspect, the present invention provides a process for reacting a flue gas containing at least about 1 x 10-6 mol/l 502 to convert said 502 to an oxidized acidic species, in the absence of a solid state catalyst, said gas containing also substantial quantities of H2O vapor and 02, comprising irradiating the gas with ultraviolet radiation containing at least a component of wavelength below about 190 nm. Preferably, the radiation contains a component in the range about 170 to about 190 nm. As noted above, photolysis of the water vapor with radiation below 190 nm, preferably in the range 170 to 190 nm, efficiently results in the formation of large quantities of OH radical through reaction (5). The hydroxyl radical reacts with SO2 to yield oxidized acidic species. OH c 502 ----3 H503 (6) These acidic species can be removed by absorption in the presence of moisture e.g. by water or by an alkalinereacting medium e.g. by reaction with ammonia gas to yield sulfate salts, or may be absorbed by conventional means such as scrubbers or employed as a. feed material to a sulfuric acid manufacturing process. Frequently, flue gases to be treated will contain both NOx and 502. In such cases it may be desirable to subject the gas firstly to an NOx-removing procedure before subjecting it to the 502 oxidation reaction described above, to avoid undesired formation of HNO3 acidic species in the latter reaction. In accordance with a further aspect, the invention therefore provides a process for the reduction of the content of NO, and S02 in flue gas containing also substantial quantities of H20 vapor and 2 comprising mixing the flue gas with NH3 and irradiating the mixture with ultraviolet radiation containing at least one component of wavelength in the range about 190 to about 220 nm, said radiation being substantially wholly free of any component with a wavelength below about 190 nm, to obtain a gas with a reduced content of NO, and irradiating the gas having a reduced content of NOx with ultraviolet radiation containing at least a component of wavelength below about 190 nm, to convert 502 to oxidized acidic species, the process taking place in the absence of a solid state catalyst. An advantage of the present process is that as the NOx removal reaction takes place in the absence of a solid state catalyst, there is no need to remove 502 and other compounds of sulfur, which are catalyst poisons, from the flue gas prior to carrying out the reaction. The form of ultraviolet lamp to be employed will depend on the desired wavelength range. As noted above, in one form, the lamp may preferably be one which includes at least one component in the range about 170 to 190 nm. One class of lamps which may be employed comprises low pressure mercury arc lamps. These provide a strong emission line at 184.9 nm, along with a strong emission line at 253.7 nm, and weaker emissions at other wavelengths. These lamps may therefore be employed for the S02 oxidation process described above. A further class of lamps which may be employed consists of high pressure mercury-xenon lamps. These provide an output which is a continuous spectrum from 190 nm to above 300 nm. They provide some emission at wavelengths lower than 190 nm, but this may be readily screened out with an appropriate filter. These lamps are therefore particularly useful for photolytic removal of NOX from flue gas streams by irradiation of an NH3-containing reaction mixture at 190 to 220 nm, even though a substantial proportion of their output is not employed efficiently as it consists of radiation with a wavelength above 220 nm. It may be noted that the above-mentioned low pressure mercury arc lamps and high pressure mercury xenon lamps are readily available commercially. It is one advantage of the processes of the present invention that they can be carried out employing readily commercially-available ultra violet lamps. Some examples of processes in accordance with the present invention are illustrated in the accompanying drawings in which: Fig. 1 shows in schematic form combustion apparatus e.g. a boiler, and associated equipment for removal of NO, and/or 502 from the flue gases emitted by the boiler; Fig. la shows a modification of the apparatus of Fig. 1; Fig. 2 is a perspective view illustrating a longitudinal section through a portion of the wall of the flue gas duct of the above apparatus; and Fig. 3 shows a further modification of the apparatus. Referring to the drawings, wherein like reference numerals indicate like parts, Figure 1 shows a combustion apparatus e.g, a boiler 1, which is supplied with combustion air through an inlet line 2 which passes through an air preheater 3 from which the heated air is supplied to the boiler through a line 4. Flue gases from the boiler pass direct to the preheater 3 through a duct 6. The boiler 1 may be in general any form of combustion apparatus wherein atmospheric air is employed to sustain combustion and the fuel contains sulfur, yielding 502 in the flue gas, and/or the fuel is burnt at a temperature sufficiently high that a substantial quantity of NOx is formed. At high temperatures nitrogen, present in the combustion air and, particularly in the case of solid fuels, bound up in the fuel itself, reacts with oxygen, present in the combustion air, to yield NO. N2 + 2 ) 2NO Some oxidation of NO to NO2 also occurs, so that the flue gases containing NO mixed with some NO2 2NO + 2 2N02 The fuel may therefore be, for example, hydrogen gas, or a primarily carbonaceous or hydrocarbon fuel, e.g. the so-called fossil fuels such as oil, coal, and natural gas, or a fuel derived from fossil fuels e.g. petroleum gas or coal gas. Ammonia gas is injected into a duct 7 which conveys the cooled flue gas from the air preheater 3 to the usual device for separation of particulate material e.g. fly ash from the gas stream, in this example an electrostatic precipitator 8. The ammonia is injected through an inlet line 9 under the control of a valve 10 which permits addition of the ammonia at a metered rate. Frequently, the flue gas will contain quantities of SO3 and HC1. These react with NH3 to yield particulate ammonium salts, e.g. NH4HSO4 and NH4Cl. If the ammonia is added downstream from the boiler or other combustion apparatus 1 and the air preheater 2, there is no risk of ammonium bisulfate and ammonium chloride condensing on critical components such as the boiler 1 and preheater 2 and causing corrosion and fouling. The ammonia may be added as indicated upstream from the precipitator 8 so that the bisulfate salt can be removed to avoid excessive turbidity in the gas stream and loss of efficiency in the subsequent photolysis step. The rate of addition of ammonia may be calculated so that there is sufficient to react with the SO3 and HCl and leave an excess of ammonia over in the concentration required during the photolysis step i.e. generally in the above-mentioned range of about 5 x 10-6 to about 5 x 10-3 mol/l. Alternatively, and more desirably, the rate of addition of NH3 may be controlled automatially in response to sensors located in the duct 7 and in the stack 13, the former sensors being responsive to the concentrations of SO2 and/or NOX and serving to increase the rate of addition of NH3 as the concentrations of 502 and/or NOx increase and the latter sensors being responsive to the presence of NH3 and serving to decrease the rate of addition when the concentration of unconsumed NH3 in the stack gases rise above a predetermined limit. Ammonium bisulfate and chloride mixture collected at the precipication may be separated from the fly ash and be recovered. If, depending on the chemical composition of the flue gas, the recovered mixture does not contain excessive quantities of heavy metals or other toxic materials, it may be utilizable as a valuable by-product. As shown, the ammonia addition is desirably made at a point upstream from the usual induced draft fan 11 which passes the cleaned gas from the precipitator 8 along a duct 12 to the stack 13 from which the flue gases are vented to the atmosphere. Passage of the flue gas/NH3 mixture through the fan 11 ensures that the NH3 is mixed uniformly in the gas stream. It is an advantage of the process of the invention that the ammonia can be introduced and the photolysis conducted at a region of the flue gas ductwork adjacent the electrostatic precipitators where the flue gases are at comparatively low temperatur es e.g. up to 400 C, more typically 150 to 250 C, as high temperatures are not required for the photolytic generation of the reactive NH2 and OH radicals, which can proceed in the cold. These regions of the flue gas ductwork are normally readily accessible so the fitting of the inlets, lamps etc. required for carrying out the process can be readily carried out on existing combustion plant. A lamp 14 is provided adjacent the duct 12 for irradiating the gas stream passing through the duct 12. As shown in Fig. 2, the lamp may comprise a lamp body proper 16 housed within a metal reflector 17 and separated from the interior of the duct by an utraviolettransmissive window 18 e.g. of quartz. The window 18 may comprise a filter to screen out undesired components of the radiation. Desirably, the space within the reflector 17 and window 18 comprise a sealed unit filled with an ultraviolet -inert gas e.g. nitrogen to avoid absorption losses to avoid or reduce generation of ozone in the atmosphere adjacent the lamp. It may be desirable to mount the lamp 14 external to the duct 12, as shown, because the flue gas will normally contain significant amounts of particulate matter even after passage through the precipitator 8 and there may therefore be a risk of fouling of the lamp structure. Normally, it will be desirable to equip the window 18 with autcated mechanical cleaners (not shown) to remove any fouling which may build up on the face of the window adjacent the interior of the duct 12. In order to conduct a sequential NOX- and SO2-removing process in which firstly an NH3-containing mixture is irradiated at t90 to 220 nm to remove NO, and the photolysed reaction mixture is subsequently irradiated at 170 to 190 nm to oxidize SO2 to acidic species, thB ç amp 14 may be constituted by two longtudinally spaced ultraviolet sources emitting radiation in the respectively desired wavelengths. As noted above, the efficiency of the removal of the NOx and/or SO2 is strongly dependent on the wavelength of the ultraviolet radiation employed. It has been found, however, that the efficiency of the process when operated in the selected wavelength ranges, in terms of the rate of removal of the undesired species, is relatively insensitive to the concentrations of other species present in the reaction mixture. It is convenient to measure the effectiveness of the process in removing NO, in terms of the percentage removal of NO. Thus, for example when a reaction mixture consisting of a given flue gas composition and NH3 is irradiated with ultraviolet light of wavelength 193 nm, the rate of reduction of NO content is approximately 2.5 times the rate that is achieved when the irradiation is conducted at a wavelength of 213.9 nm. These rates are relatively unaffected by the initial concentration of NO, S02 or NH3 (as long as a certain minimum concentration of NE3 is present), or by variation in the concentration of any other species normally present in the reaction mixture, and given that the usual quantities of H20 vapor (normally at least about 1 x 10-4 mol/l) are present in the flue gas. The rate of reduction of NO, and/or 502 is also dependent on the total quantity of radiant energy tc which the reaction mixture is subjected. Thus for example when the NE3 and flue gas mixture is irradiated with ultraviolet light in the preferred wavelength range of 190 to 220 nm, about 80% of the NOx is removed when the mixture is irradiated continuously for 10 millisec at a light intensity of about 1018 photon/cm2/sec, or is irradiated continuously for 100 millisec at an intensity of about 1017 photon/cm2/sec, these light intensities being typical of those achievable with the preferred forms of ultraviolet lamps. Typically the flow rate of flue gases through the normal uniform cross-section ductwork encountered in e.g. conventional coal-fired power stations is of the order of 10 m/sec, thus requiring that the irradiated lengths of ductwork should be of the order of 20 cm in the case of the higher powered lamps or 2m in the case of the lower powered lamps. The percentage reductions of NO, that will be required in any given case will of course depend on the initial concentrations of NO, present in the flue gas and the levels of NO, concentration that it is desired to achieve in stack gas passed to the atmosphere but usually percentage reductions of at least 80% in the NOx concentration will be called for. More generally, therefore it will normally be desired to. subject the reaction mixture to a total quantity of radiant energy flux in range of about 1017 to about 1018 photon/cm2 of the irradiated area of the flue gas duct during the photolysis reaction. The rate of removal of SO2, in the substantial absence of NOx, is such that about 72% of the S02 is removed when the SO2-containing gas is irradiated at t75 nm for 10 millisec at an intensity of 3 x 1018 photon/cm2/sec. The embodiment illustrated in Fig. 1 is particularly well suited for use when either it is desired to control only emission of NOX, and the irradiation is conducted at a wavelength of 190 to 220 nm so that the gaseous photolysis products N2 and N2O may be vented through the stack 13, or when the object is to control emissions of solely SO2 or both 502 and NO, and the irradiation is conducted at a wavelength of 170 to 190 nm and it is acceptable to vent particulate photolysis products e.g. (NH4)2SO4 and NH4NO3 to the atmosphere. In the embodiment of Fig. la, after irradiation at a dual lamp arrangement 14 providing firstly reaction of NO, with photolytically-generated NH2 radical and secondly oxidation of 502 to HS03 species by irradiation with ultraviolet light in the wavelength range 170 to 190 nm, the HSO3 species are subsequently neutralized to form particulate (NH4)2S04 through reaction with an excess of ammonia in the presence of moisture. The reaction mixture is passed through a second solids separator device e.g. a second electrostatic precipitator 19 wherein the particulate products are removed. Depending on the chemical composition of flue gases, and particularly if the flue gases are free from toxic heavy metal materials, the separated-out sulfate salts may be recovered e.g. for use as agricultural fertilizer. Instead of supplying an excess of ammonia to the flue gas before the irradiation step it will normally be more efficient to supply the additional ammonia required for neutralization of the acidic bodies through an auxiliary ammonia inlet subsequent to the lamp 14 and as indicated in broken lines at 21, under the control of the sensors in the stack 13 which control the emission of unreacted ammonia to the atmosphere. In Figure 3, a preferred arrangement for NO, or NO, and SO2removal is shown wherein the ammonia is added through line 9 after the flue gas has passed through the precipitator 8. In many cases, the flue gases emitted by the boiler 7 will contain HCl and 503. These will react with NH3 tc form particulate ammonium salts. Addition of the ammonia subsequent to the precipitator 8 avoids recovery of these soluble salts with the fly ash from the precipitator 8. In many cases, it is desired to recover a fly ash which is relatively free from solubles, as these may render the fly ash unsuitable for some purposes such as for land fill where the solubles may cause problems owing to their tendency to leach out when in contact with water. In some cases it may be desirable to remove the particulate ammonium salts produced from the addition of ammonia before exposure of the flue gas to the ultraviolet light source. The lamp 14 may be of the single NOx-removing type or may be the dual arrangement providing for sequential removal of NO, and 502 and means, such as conventional scrubbers, may be provided between the lamp 14 and the stack 13 for removing acidic species and particulates. The optimum conditions required for the photolysis reaction for any given flue gas composition and, in particular the duration of the exposure to the ultraviolet radiation and hence the length of ductwork that needs to be irradiated for any given source of ultraviolet radiation, can best be investigated by conducting a computer simulation of the photolysis reaction when conducted with a monochromatic source. Such computer simulation requires the provision of a set of parameters that comprise the input to the computer program. These comprise a set of initial concentrations of all reactive chemical species present in the flue gas, and the extinction coefficients applicable to the wavelength under investigation for the photolysis reactions of all photosysable species present in the gas mixture. For the avoidance of doubt, these extinction coefficients are set out in Table 2 below for certain selected wavelengths. Other coefficients applicable to different wavelengths can be readily obtained from standard texts. TABLE 2 Photolysis Reactions EMI18.1 <tb> <SEP> Reaction <SEP> Extinction <SEP> Coeff./l <SEP> mol-1 <SEP> cm-1 <SEP> <tb> <SEP> 184.9 <SEP> sum <SEP> 193 <SEP> nm <SEP> 213.9 <SEP> nm <SEP> <tb> NH <SEP> + <SEP> h##NH2 <SEP> <SEP> + <SEP> H <SEP> 1000 <SEP> 1500 <SEP> 100 <tb> SO2 <SEP> + <SEP> h#SO <SEP> <SEP> + <SEP> O <SEP> ( P) <SEP> 172 <SEP> 1000 <SEP> 150 <tb> H20 <SEP> + <SEP> h##OH <SEP> <SEP> + <SEP> H <SEP> 14 <SEP> 0 <SEP> 0 <tb> <SEP> 3 <SEP> + <SEP> h##O2 <SEP> <SEP> + <SEP> O <SEP> ('D) <SEP> 156 <SEP> 100 <SEP> 150 <tb> <SEP> O2 <SEP> + <SEP> h##20 <SEP> <SEP> ( P) <SEP> 4.2 <SEP> 0.3 <SEP> 0.002 <tb> WO2 <SEP> + <SEP> <RTI ID=18.9> h##NO <SEP> <SEP> + <SEP> O <SEP> ( P) <SEP> 68 <SEP> 68 <SEP> 100 <tb> N2O <SEP> + <SEP> h##N2 <SEP> <SEP> + <SEP> O <SEP> ('D) <SEP> 36 <SEP> 35 <tb> N2H4 <SEP> + <SEP> h##2 <SEP> <SEP> NH2 <SEP> 850 <SEP> 1000 <SEP> 600 <tb> Further, the parameters include the intensity of the irradiation, and the rate constants of the significant chemical reactions that occur during the photolysis reaction. As a result of extensive study of the chemistry of the reaction, 52 chemical reactions have been identified as being significant exclusively, and these are listed in Table '3 along with their respective rate constants. TABLE 3 Reactions Rate Constant (dm mol-1 s-1 1. NH2 + NO = N2 + H2O 1.3 x 1010 2. NH2 + NO2 = N2O + H2O 1.3 x 1010 3. NH3 + NH2 = N2H4 1.5 x 1010 4. NH2 + O2 = NO + H2O 1.2 x 10 5. NH2 + RH = NH3 + R 5.0 x 104 6. NH2 + H = NH3 1.0 x 10 7. NH2 + H2 = NH3 + H 8.7 x 10ê 8. NH2 + 0 = HNO + H OR HO + NH 2.1 x 109 9. NH2 + OH = HNO + H2 OR NH + H2O 6.2 x 107 10. NH2 + HO2 = NHO + H2O 2.0 x 108 11. NH3 + H = NH2 + H2 3.0 x 108 12. NH3 + OH = NH2 + H2O 1.0 x 108 13. NH3 + O = NH2 + OH 4.0 x 105 14. O + NO2 = O2 + NO 5.6 x 109 15. 0 + O2 = 03 8.7 x 106 16. 0 + NO = NO2 9.9 x 10 17. NO + O3 = NO2 + O2 1.0 x 107 18. O('D) + H2O = 2 OH 1.3 x 1011 19. O('D) (+M) = O( P) (+M) 8.2 x 108 s-1 at 1 Atm. 20. H + O2 = HO2 8.0 x 108 21. HO2 + NO = NO2 + OH 5.0 x 109 22. 0 + SO = SO2 5.0 x 107 23. O2 + SO = S02 + 0 ( P) 5.0 x 104 24. O3 + SO =SO2 + O2 4.5 x 107 25. SO + SO = SO2 + S OR (SO)2 2.0 x 106 TABLE 3 (continued) Reactions Rate Constant 26. OH + OH = H2O + O ( P) 1.1 x 109 27. OH + NO2 = HNO3 3.8 x 10 28. O + 3 = '202 5.7 x 6 29. OH + CO = CO2 + H 9.0 x 107 30. SO + NO2 = SO2 + NO 8.5 x 109 31. H + HO2 = H2 + O2 8.4 x 10 = 20H 1.9 x 1010 = H2O + O 5.7 x 108 32. O + OH = O2 + H 2.3 x 1010 33. OH + HO2 = H2O + O2 2.1 x 1010 34. HO2 + HO2 = H2O2 + O2 1.4 x 109 35. 03 + OH = HO2 + O2 2 4.9 x 107 36. O3 + HO2 = OH = O2 1.2 x 106 37. OH + SO2 = HSO3 6.6 x 10 38. H + 03 = OH + O2 1.7 x 10 39. 0 + H20 = OH + 02 1.9 x 1010 40. 0 + 0 = 02 6.9 x 107 41. 0 + H = OH 2.9 x 108 42. H2 + 0 = OH + H 4.0 x 103 43. SO2 + 0 = SO3 1.1 x 107 44. 0 + CO = CO2 3.0 x 104 45. N2H4 + H = N2H3 + H2 1.1 x 108 46. OH I H = H2O 7.2 x 109 47. OH + H2 = H2 O + H 3.9 x 106 48. SO2 + HO2 = SO3 + OH 5.4 x 105 49. O + NO = N + O2 7.7 x 104 TABLE 3 (continued) Reactions Rate Constant 50. NO + H = KNO 8.3 x 108 51. N02 + H = NO + OR 7.5 x 1010 52. Free radical, wall termination - 7.5 x !=-2 Note 1: Many of these reactions are third order at atmospheric pressure. A pseudo-second order rate constant is quoted, and was obtained by substituting one atmosphere pressure for the third body. 2: In practice, wall termination was applied only to NH2 radicals to ensure a conservative result. Certain of the rate constants and extinction coefficients specified in Tables 2 and 3 are subject to uncertainties owing to conflicting reports in the literature, notably in the extinction coefficients of NH3, 02, H2O, and SO2 and in the rate constants of reactions numbered (1) and (2) in Table 3. The effect of these uncertainties can however be readily checked by varying the input parameters in the computer program and their effect on the resultant rates of NO removal has been found to be very small or negligible. As will be apparent to those skilled in the art, the set of fifteen rate equations, one for each of the species NE2, NH3, O, NO, O('D), H, HO2, 0, 02, 03, SO, OH, SO2, N2Hq, and (free radical termination) described by the 52 reactions of Table 3 constitute a stiff system of differential equations, because the rate constants cover a wide range of values. These equations may be integrated numerically using conventional methods e.g. as described in "The automatic integration of ordinary differential equations" Gear C.W., Commun ACM, 14, P176 (1971). It should be noted that computer time can be reduced, and the stability of the solution improved, by using units of micromoles and microseconds rather than moles and seconds. This results in concentrations and rates whose numerical magnitudes are such that round-off errors re less important than they would be if more conventionol units are employed. As will be appreciated the solutions obtained from the procedure provide the instantaneous concentrations of any selected species at any selected time during the course of the photolysis reaction, and the analysis may be applied to all flue gases obtained from conventional combustion processes including, as mentioned above, the combustion at high temperatures in the presence of air of fuels as diverse as hydrogen gas, fossil fuels and fuels which are derivatives of fossil fuels, which flue gases contain substantial quantities of 502 and/or NOx' typically 1 x 10-6 to 1 x 10-4 mol/1 S02, more typically 1000 to 2000 ppm SO2', 1 x 10-7 to 1 x 10-5 mol/1 NO2 and 1 x 10-6 to 1 x ions mol/1 NO. Usually the concentrations of the species present in such fuel gases will be as indicated in Table 4. Species Concentration Range mol/l SO3 0 to 1 x 10 6 SO2 0 to 1 x 10-4 NO2 0 to 1 x 10-5 N2O 0 to 1 x -7 NO 0 to 1 x 10-4 HCl 0 to 1 x 10-5 OH 1 x 10-4 to 1 x 10-12 H2O 1 x 10-4 to 1 x 10-2 O2 1 x 10-4 to 1 x 10-2 RH (hydrocarbons) 0 to 1 x 10-6 CO 1 x 10-7 to 1 x 10-5 CO2 1 x 10-4 to 1 x 10-2 Others (mainly nitrogen) balance Merely by way of example a typical flue gas composition as obtained from a coal-fired power station is given in Table 5 (reactive species only). TABLE 5 Species Concentration 2 3 1/2 to 9% by volume (full to part load) H20 10% by volume NO 300 - 700 ppm (vol) SO2 1100 - 1600 ppm (vol) HC1 100 ppm (vol) S03 10 - 16 ppm (vol) NO2 30 - 70 ppm (vol)

Claims: C L A I M S : 1. Process for reduction of the content of NO and N02 in flue gas containing also substantial quantities of H20 vapor, comprising mixing the flue gas with NH3 and irradiating the mixture with ultraviolet radiation containing at least one component of wavelength in the range from about 190 to about 220 nm, said radiation being substantially wholly free of any component with a wavelength below about 190 nm, and said process taking place in the absence of a solid state catalyst. 2. Process as claimed in claim 1 wherein said radiation is provided from a high pressure mercury-xenon lamp. 3. Process as claimed in claim 1 wherein the gaseous mixture after irradiation is passed direct to the atmosphere. 4. Process as claimed in claim 1 wherein the gaseous mixture is subjected to a total quantity of radiant energy flux of radiation in said wavelength range of from about 1017 to 1018 photon/cm2 of the irradiated area. 5. Process as claimed in claim 1 wherein said mixture contains about 5 x 10-6 to about 5 x 10-3 mol/l NH3. 6. Process as claimed in claim 5 wherein said NH3 content is about 1 x 1Q-5 to about 1 x 10¯3 mol/l. 7. Process as claimed in calim 6 wherein said content is about 1 x 10-5 to about 2 x 10-5 mol/l. 8. Process as claimed in claim 1 wherein the NH3 is added to the flue gas when the latter is at a temperature of up to about 400"C. 9. Process as claimed in claim 8 wherein said temperature is about 150 to about 2500C'. 10. Process as claimed in claim 1 wherein the mixture containing NH3 is blown through a fan to the site where it is irradiated. 11. Process as claimed in claim 1 in which the gas is a flue gas containing at least about 1 x 10-6 mol/l 502. 12. Process as claimed in claim 11 in which the gas contains about 1 x 10-6 to about 1 x 10-4 mol/l Soya. 13. Process as claimed in claim 11 in which the gas contains about 1000 to 2000 ppm SO2. 14. Process for reacting a flue gas containing at least about 1 x 10-6 mol/l 502 to convert said S02 to an oxidized acidic species, in the absence of a solid state catalyst, said gas containing also substantial quantities of H20 vapor and 02, comprising irradiating the gas with ultraviolet radiation containing at least a component of wavelength below about 190 nm. 15. Process as claimed in claim 14 wherein the radiation contains a component in the range about 170 to about 190 nm. 16. Process as claimed in claim 14 wherein the content or 502 in, the flue gas is about 1 x 10-6 to about 1 x 10-4 mol/l. 17. Process as claimed in claim 14 wherein said radiation is from a low pressure mercury arc lamp providing strong emission lines at 184.9 and 253.7 nm. 18. Process as claimed in claim 14 wherein said gaseous mixture is subjected to a total quantity of radiant energy flux of radiation in said wavelength range of from about 5 x 1017 to about 5 x 1018 photon/cm2 of the irradiated area. 19. Process as claimed in claim 14 including contacting the irradiated mixture with an alkaline-reacting medium or water to absorb acid species. 20. Process as claimed in claim 14 including contacting the irradiated mixture with gaseous ammonia to absorb acid species. 21. Process as claimed in claim 1 or 14 wherein said flue gas contains 1 x 10-4 to 1 x 10-2 mol/l H2O vapor and 1 x 10-6 to 1 x 10-2 mol/l 02. 22. Process as claimed in claim 1 or 14 wherein said flue gas contains no more than 1 x 10-8 mol/l hydrocarbons. 23. Process for the reduction of the content of NOX and 502 in flue gas containing also substantial quantities of H20 vapor and 2 comprising mixing the flue gas with NH3 and irradiating the mixture with ultraviolet radiation containing at least one component of wavelength in the range about 190 to about 220 nm, said radiation being substantially wholly free of any component with a wavelength below about 190 nm, to obtain a gas with a reduced content of NOX and irradiating the gas having a reduced content of NO, with ultraviolet radiation containing at least a component of wavelength below about 190 nm to convert 502 to oxidized acidic speci,359 process taking place in the absence of a solid state catalyst. 24. Process as claimed in claim 23 wherein the second mentioned irradiation is conducted with radiation containing a component in the range about 170 to about 190 nm. 25. Process as claimed in claim 23 including contacting the irradiated mixture containing oxidized acidic species with an alkaline reacting medium to absorb said acidic species. 26. Process as claimed in claim 23 including contacting the irradiated mixture containing oxidized acidic species with gaseous ammonia to absorb acid species.

Inventors: STEVENS, R. D. SAMUEL
Applicants: CONCORD SCIENTIFIC CORPORATION
Application year: 1983
Main classification: B01D53
IPC codes: B01D53