RU2388523C2 - Method and device to clean flue gases of nitrogen oxides to produce hydrogen nitrate - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention is intended for cleaning flue gases of harmful admixtures. Proposed method comprises mixing flue gases with ozone-air mix and oxidising nitrogen oxide (NO) to nitrogen dioxide (NO₂) in pre-oxidising chamber, feeding acid condensate for cleaning of acid components into heat recovery unit first chamber operated in cold conditions, separating solution of hydrogen nitrate solution (HNO₃·3H₂O) and forcing it in hydrogen nitrate storage tank. It comprises also adding hot air flow to blow air prior to feeding it into processing zone for cooling flue gases, parallel feed of purified flue gases into heat recovery unit second chamber wherein stepwise ice defrosting occurs during regeneration by purified flue gases that are forced from heat recovery unit into atmosphere. Finally, cold air is directed into second chamber, while purified flue gases are directed in first chamber to repeat abode described cycle. Proposed device comprises processing zone including the chamber of pre-oxidising with distributor of ozone-air mix, heat exchange-condensation and absorption-condensation sections with separate trays, pump, heat recovery unit consisting of two identical chambers with trays wherein inclined slot-like shelves are staggered.

EFFECT: higher efficiency of flue gas cleaning of nitrogen oxides.

2 dwg

Description

The invention relates to a power system and can be used in processes for purifying flue gases from harmful impurities, namely, for cleaning flue gases from heat generators operating on sulfur-free fuel (natural gas), from nitrogen oxides with their utilization in the form of nitric acid.

There is a method for removing harmful impurities (nitrogen oxides and sulfur oxides) from flue gases, carried out in a device that is part of the flue (treatment zone) with the heat exchange and absorption sections located in it, which are a tubular heat exchanger, which consists in the fact that flue gases cooled to dew point temperature with condensation of water vapor in a tubular heat exchanger, mixed with ammonia-containing gas to neutralize acid components and the condensate formed is removed and purified s flue gases [1].

The disadvantages of the known method and device are the low environmental, technological and economic efficiency of the process of purification of flue gases from harmful impurities, due to the use of ammonia to neutralize the acid components in the treatment zone, the unreacted part of which is released into the atmosphere, and the lack of equipment for disposal of trapped harmful impurities.

Closer in technical essence to the present invention is a method of purification of flue gases from harmful impurities (nitrogen oxides (NO _x ) and sulfur oxides (SO _x )), which includes cooling the flue gas to a temperature below the dew point temperature, condensation of water vapor in a tubular a heat exchanger, recycle condensate saturation ozone and oxygen, the distribution of the saturated condensation by absorption section, oxidation of nitrogen oxide (NO), located in the flue gases, before dioxide (NO ₂₎ and nitrogen dioxide absorption Saturation condensate to form acidic condensate (a solution of dilute nitric acid - HNO ₃₎ flowing in the tray, whereupon the purified smoke gases are vented to the atmosphere, removing a portion of the acidic condensate from the sump in the anion exchanger for removal of acidic components that are outputted in the process of regeneration of anion filter in the form of a diluted saline solution of NaNO ₃ .

The device in which this method is implemented includes a treatment zone with heat-exchange and absorption sections located in it, made in the form of vertical tubular heat exchangers with a tray and a coaxially mounted ergazlift lifting pipe (ozone-air mixture distributor) placed in them, a separation section made in the form of a vertical tubular heat exchanger, and the pan is connected by a pipe to the anion exchange filter. [2].

The main disadvantage of this method is the low efficiency and speed of cleaning, due to the supply of the ozone-air mixture after cooling the flue gases, which reduces the time of their contact with the flue gases and, accordingly, the reaction of oxidation of nitric oxide to dioxide and the process of absorption of nitrogen dioxide in film mode, which limits the rate of flue gases, as well as the purification of acid condensate from acid components in an anion exchange filter, which is then regenerated with a solution of sodium hydroxide (NaOH), as a result of it is obtained with a highly diluted solution of sodium nitrate (Na ₂ NO ₃ ), which is difficult to dispose of, which reduces the economic and environmental efficiency of cleaning flue gases from harmful impurities.

The main disadvantages of the known device are the lack of equipment for pre-mixing the ozone-air mixture with flue gases and for the preliminary oxidation of nitrogen oxides, as well as for the purification of acid condensate with the release of commercial nitric acid, which reduces the economic and environmental efficiency of its operation.

The technical result, the solution of which the invention is directed, is to increase the economic and environmental efficiency of flue gas purification from nitrogen oxides and their utilization by obtaining nitric acid.

The technical result is achieved in that the method of purification of flue gases from nitrogen oxides to produce nitric acid involves mixing flue gases with an ozone-air mixture and oxidizing nitrogen oxides (NO) to nitrogen dioxide (NO ₂ ) in a pre-oxidation chamber, after which they are cooled to a temperature below the dew point temperature, condensation of water vapor in the tubular heat exchangers, absorption of nitrogen dioxide by the formed condensate to produce acid condensate, which is sent from the treatment zone to clean from acid components into the first utilizer chamber operating in a cold mode, where stepwise freezing of water from acid condensate is carried out by a stream of cold air at a temperature below zero and above the crystallization temperature of the HNO ₃ · 3H ₂ O complex, separation of the nitric acid solution (HNO ₃ · 3H ₂ O ) and its withdrawal in the form of a liquid into a container for storing nitric acid, moreover, a stream of heated air is added to the blast air before it is fed into the treatment zone for cooling the flue gases, and the purified flue gases from the steam treatment zone It is fed into the second utilizer chamber operating in a hot mode, where ice is defrosted stepwise during regeneration by a stream of purified flue gases, in which they are cooled, additionally cleared of the remaining nitrogen oxides upon contact with ice, and removed from the utilizer to the atmosphere, and thawed water is discharged from the chamber pan to the condensate collector, after which, by switching the flows of cold air and purified flue gases from the treatment zone, the first chamber operating in the cold mode is switched off chayut on the hot operating mode and the second camera operating in the hot mode, the cool mode, and the cycle repeats.

The technical result is also achieved by the fact that the device for purifying flue gases from nitrogen oxides to produce nitric acid includes a treatment zone consisting of a preliminary oxidation chamber with an ozone-air mixture distributor, located in the inlet duct, coated from the inside with a corrosion-resistant material, heat-exchange-condensation, and absorption- condensing sections made in the form of vertical tubular heat exchangers with separate pallets connected by pipelines through a pump with waste congestion, consisting of two identical chambers with pallets, each of which is equipped with fittings for supplying and discharging liquid and gas upper and lower pipes, respectively, inside of which inclined slotted shelves are installed in a checkerboard pattern at some distance from each other, with an angle of inclination directed to side of the flow of water.

A device for implementing the proposed method of purification of flue gases from nitrogen oxides with the production of nitric acid is shown in figures 1, 2.

A device for implementing the proposed method of purification of flue gases from nitrogen oxides to produce nitric acid contains a treatment zone A, with a preliminary oxidation chamber I located in it, which is an inlet section of the duct, coated inside with a corrosion-resistant material with an ozone-air mixture distributor (not shown), condensation and absorption-condensation sections II and III with their own pallets made in the form of vertical tubular heat exchangers connected by a pipe a gadfly through pump B with a utilizer B consisting of two identical chambers with trays 1 and 2, each of which is equipped with fittings for supplying and discharging liquid 3 and 4 and gas upper and lower pipes 5 and 6, respectively, inside of which are staggered on a certain distance from each other installed inclined slotted shelves 7, with an angle of inclination directed towards the flow of water.

The production of nitric acid in the treatment of flue gases obtained by burning natural gas is based on their composition, in which there are no sulfur oxides (SO _x ) (natural gas is preliminarily purified from sulfur-containing components), and the main harmful component is nitrogen oxides (NO _x ) arising during the combustion process and the amount of which is determined by the combustion regime [3], the possibility of their rapid oxidation and absorption by condensate of water vapor in the presence of ozone with the formation of a solution of highly dilute nitric acid (acidic ensata) [2], and the freezing point of pure nitric acid and its complexes with water, which is significantly below the water freezing temperature (melting temperature (t _pl) of pure HNO ₃ is - 42,0 ° C; t _mp H ₂ O · HNO ₃ is –38 ° C; t _PL H ₂ O · 3HNO ₃ is –18.47 ° C) [4, p. 793].

The proposed method of purification of flue gases from nitrogen oxides to produce nitric acid is carried out in the proposed device as follows. The flue gases from the transit flue are directed to treatment zone A, where they are mixed with the ozone-air mixture in the preliminary oxidation chamber I, in which the oxides of nitrogen (NO) in the flue gases are oxidized to dioxides (NO ₂ ) by ozone and oxygen, and then in the heat-exchange-condensation and absorption-condensation sections II and III, they are cooled to a temperature below the dew point temperature, condensation of water vapor in the tubular heat exchangers, absorption of nitrogen dioxide by the formed condensate with the formation of acid condensate (a solution of diluted nitric acid with a concentration of about 1 wt.%), flowing into the pan. At the same time, under conditions of condensation, the rate of acid formation increases (2.6–9.4) times compared with the usual absorption of nitrogen oxides [5, p. 44]. Acid condensate from the pallets of sections II and III of treatment zone A is pumped into the cold storage chamber 1 of the utilizer B through the nozzle 3 to the upper inclined slotted shelf 7, on which it contacts the flow of cold air rising from below ((- 5) - (- 15)) ° C entering the chamber 1 through the nozzle 6. In the chamber 1, as a result of repeated countercurrent contact of acid condensate on inclined slit shelves 7, flowing from shelf to shelf under the influence of gravity, with a stream of cold air cooling to condensate temperature (50-60) ° C to 0 ° C and below, followed by the formation of ice from water, which remains on the shelves 7, and the remaining unfrozen portion of acidic condensate which is a mixture of nitric acid complexes hydrate (H ₂ O · HNO ₃ and H ₂ O · 3HNO ₃ ), flows into the tray of chamber 1, from where it is sent to a container for storing nitric acid (not shown). The outgoing stream of heated air is heated to a temperature of (10-15) ° C and is added to the blast air through a nozzle 5 before it is fed into the treatment zone A for cooling the flue gases. At the same time, the operating time of chamber 1 in the cold mode is determined by the concentration of the obtained nitric acid, which is taken equal to the known concentration of the hydrated complex H ₂ O · 3HNO ₃ with a lower freezing temperature, namely 53 wt.%. In parallel with the described process of nitric acid separation from acid condensate in the chamber 1 of the utilizer B operating in the cold mode, the purified flue gases are fed into the chamber 2 of the utilizer B operating in the hot mode (regeneration), the inclined shelves of which are covered with ice, which as a result of repeated contact with it melts with hot flue gases, melt water flows into a sump, from where it is sent to a condensate collector (not shown), and the flue gases are cooled as a result of repeated contact with ice and are further purified from oxides Zot and condensate through the pipe 6, and a flue chimney (not shown) are output to atmosphere. The operating time of the chamber 2 in the hot mode is taken equal to the time of the cold mode of the chamber 1. Upon completion of the operation of the chamber 1 in the cold mode and emptying its pallet from nitric acid, the chamber 1 is switched to the hot mode by sending purified flue gases from the treatment zone A to its branch pipe 5 and the chamber 2 is switched to the cold mode of operation, directing acid condensate into it from the pallets of the treatment zone A through the nozzle 3 and the flow of cold air through the pipe 5, after which the cycle repeats. At the same time, outside air is used in winter, and at an outdoor temperature above -5 ° C, the air is cooled in a refrigeration unit, the cooling capacity of which is determined by the maximum summer temperature of five days.

The economic and environmental effectiveness of the invention is confirmed by the following example.

Condensation of fumes of water vapor in the treatment zone A generates heat in the amount of 10.764 kcal / mol (2630 kJ / kg), while the cold consumption for freezing acid condensate water in utilizer B is 1.4363 kcal / mol (348 kJ / kg) [4, p.793], i.e. the amount of utilized energy is 7.5 times higher than the energy spent on getting cold to cool the air in the warm season (in the cold season, the refrigeration unit is not used). In addition, with a decrease in the concentration of nitrogen oxides in flue gases, for example, from 0.35 g / m ³ to 0.1 g / m ^3, 0.25 g / m ³ NO _x , which are composed of (95-99 )% of NO [3], the molecular weight of which and, accordingly, the weight after their oxidation to NO ₂ and absorption by water increases to 0.5 g / m ³ . From the calculation and reference data it can be seen: when burning natural gas and the coefficient of excess air in the exhaust gases α _uh = 1.33, the average consumption of flue gases per unit capacity of the boiler (1 mW) is approximately 1500 m ³ / h, from which it turns out that the specific amount the resulting HNO ₃ (in terms of 100% concentration) will be 0.5 kg / h or 1 kg of 50% nitric acid per 1 mW of installed boiler capacity in 1 hour. When the boiler equipped with the proposed installation, 8000 hours per year, we get that 1 mW of installed capacity of the heat generator provides 8 tons of 50% nitric acid per year. Accordingly, the average 1000 MW natural gas-fired TPP, at relatively low cost for the equipment offered by the purification plant, will provide, along with the purification of flue gases from nitrogen oxides and the improvement of the environmental characteristics of the surrounding atmosphere, almost free nitric acid in the amount of 8000 t / year, which allows you to recoup all the costs of cleaning and get additional profit from its implementation. In addition, the associated production of nitric acid at least at several TPPs will reduce the production of nitric acid at specialized enterprises, which are the largest environmental pollutants, and additionally (at the regional or country level) reduce emissions of harmful substances into the environment.

Thus, the present invention allows to combine the process of purification of flue gases from nitrogen oxides with the release of nitric acid from the products of purification, the preparation of which is associated with significant energy and economic costs and high environmental hazard to the environment, which, in general, increases the economic and environmental efficiency of the flue gas cleaning process and the operation of a heat generating installation.

LITERATURE

1. US Patent No. 4753784, M Cl. ⁴ B01D 53/00, 1988.

2. RF patent No. 2186612, M CL ⁴ . B01D 53/60, 2000.

3. Zeldovich Ya. B. Oxidation of nitrogen during combustion. M .; L .: Academy of Sciences of the USSR, 1947. 147 p.

4. Handbook of a chemist, t.1. - M. - L .: Chemistry, 1965, 1006 p.

5. The production of nitric acid in units of large unit capacity, ed. V.M. Olevsky.- M .: Chemistry, 1985, 400 p.

Claims (2)

1. A method of purifying flue gases from nitrogen oxides to produce nitric acid, comprising mixing flue gases with an ozone-air mixture, cooling them to a temperature below the dew point temperature, condensation of water vapor, oxidation and absorption of nitrogen oxides by water vapor condensate to form acidic condensate flowing into tray, the conclusion of the purified flue gases, characterized in that the mixing of flue gases with an ozone-air mixture and the oxidation of nitrogen oxides (NO) to nitrogen dioxide (NO ₂ ) is carried out in a preliminary oxidation chamber i, the acid condensate from the treatment zone is sent to clean the acid components from the first utilizer chamber operating in the cold mode, where the water is gradually frozen from the acid condensate to form ice with a stream of cold air with a temperature below zero and above the crystallization temperature of the HNO ₃ · 3H complex ₂ O, separation of nitric acid (HNO ₃ · 3H ₂ O) and outputting it as a liquid into the storage tank of nitric acid, the heated air stream is added to the blast air before it enters the zone of works for cooling the flue gases, and the purified flue gases from the treatment zone are simultaneously fed into the second utilizer chamber operating in the hot mode, where ice is defrosted stepwise during the regeneration by the stream of purified flue gases, in which they are cooled, additionally being cleared of the remaining nitrogen oxides during contact with thawed ice, and are discharged from the utilizer into the atmosphere, and thawed water is discharged from the chamber pan to the condensate collector, after which by switching the flows of cold air spirit and purified flue gases, the first camera that worked in cold mode, switch to hot mode, and the second camera that worked in hot mode to cold mode, and the cycle repeats. 2. A device for cleaning flue gases from nitrogen oxides to produce nitric acid, including a treatment zone with heat exchange and absorption sections located in it with a tray made in the form of vertical tubular heat exchangers, characterized in that the preliminary oxidation chamber with an ozone-air distributor is located in the treatment zone mixtures placed in the inlet section of the gas pipeline, coated with a corrosion-resistant material from the inside, heat-exchange-condensation and absorption-condensation sections, filled with separate pallets connected by pipelines through a pump with a utilizer consisting of two identical chambers with pallets, each of which is equipped with fittings for supplying and discharging liquid and gas upper and lower pipes, respectively, inside of which are installed in a checkerboard pattern at some distance from each other inclined slotted shelves with an angle of inclination directed towards the flow of water.

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