HK1003135B - Process for the purification of combustion exhaust gases - Google Patents

Description

The invention relates to a process for purifying oxygenated exhaust gases from the incineration of waste, industrial waste and sewage sludge, by which mercury, mercury compounds and polyhalogenated hydrocarbons are removed from the exhaust gases by adsorption to zeolites.

The different and changing composition of waste, industrial waste and sewage sludge means that the exhaust gases from the incineration of such waste are also contaminated with different amounts of environmentally harmful substances. All contaminants must be largely removed from the exhaust gas before they can be released into the atmosphere, as many of these have already developed toxic effects on humans, animals and plants at low concentrations. The exhaust gases from the incineration of waste are in particular dust, SO2, HCl, HF, Henygeny, mercury compounds, heavy metals, polycyclic dihydrogen peroxide and DPC phenol, polychlorinated biphenyls (PCBs) (up to 123.3% vol), and oxygen dioxide (up to 2 ppm) (O2O), which are excreted in the exhaust gas from the combustion of the waste.

The dust in the exhaust gases, up to 50 000 mg/Nm3, is removed in cyclones, electric filters, fabric filters or washing machines, and the exhaust to be dusted can pass through several of these machines.

SO2 and HCl are present in the exhaust gases in amounts of up to 3000 mg/Nm3 each; HF is also present in amounts of up to 100 mg/Nm3. These gaseous compounds, together with the water vapour contained in the atmosphere, form acids which are very commonly present as aerosols and have a toxic effect. They are therefore largely separated, with the known technical scale purification processes being used to remove SO2 residues < 20 mg/Nm3, HCl residues < 5 mg/Nm3 and HF residues < 1 mg/Nm3. For the removal of SO2, HCl and HF are very widely used in dry, dry and wet reactions, where CaOH is a more important hydrogen fuel, and CaOH is a more efficient process of separation.

The heavy metals and heavy metal compounds, particularly mercury and mercury compounds, as well as the polyhalogenated hydrocarbons are present in lower concentrations in the exhaust gases of combustion. These substances are, however, extremely toxic and therefore need to be removed from the exhaust gases in almost quantitative amounts, which, according to the state of the art, is preferably by adsorption and/or washing processes.

The technical applicability of the known exhaust gas purification processes depends in particular on their lowest possible investment and operating costs and on the fact that they provide process products which are produced in the lowest possible quantity and which are either deposited without major difficulties or returned to the purification processes after regeneration. In order to achieve the highest possible degree of separation for the abovementioned impurities, it is common to combine several purification processes. The present invention has as its objective to use zeolites as adsorbent agents to avoid the use of activated carbon as an adsorbent and the associated risks to the safety of the operation of the adsorption plants. The invention has as its objective to create an adsorption process which can be combined with the known and cleaner adsorbent materials.

DE-A-40 12 982 is a known process for the purification of gases and exhaust gases from inorganic and organic pollutants by means of a surface-active discharge of fine powdered activated aluminium oxide, silica gel, silica gur, fine powdered zeolite and/or similar inorganic substances into the gas stream, mixing gas and solids, applied to a surface filter, leaving a renewable, loose, sufficiently deep adsorption layer on the filter and removing the solids from there by means of mechanical discharges of inorganic and organic pollutants. The known process also provides for the use of the surface-active inorganic substances as an anti-oxidant or discharging material, the grain distribution of which is < 100 μm/m2 < 100 μm/m2 (see also DE-A-40), for the production of heavy metals such as nickel, copper, nickel, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mercury, mer

DE-A-39 19 124 is a method for the separation of polycyclic hydrocarbons (e.g. dioxins and furans) and heavy metals from waste incineration plant exhaust gases, whereby the exhaust gases pre-purified by dust, HCl, HF, SOx, NOx and heavy metals are removed from the remaining polycyclic hydrocarbons and heavy metals by adsorption/filtration at a temperature of 70 to 160°C. This method involves first mixing the pre-purified exhaust gases in a reactor with finely distributed adsorbents and then passing them to a filtered polycarbonate, whereby a separate adsorbent is produced, followed by the removal of the active substances known as adsorbent and sodium.

Finally, DE-A-41 28 106 is known for a method for the selective separation of highly condensed polycyclic hydrocarbons, in particular halogenated dibenzodioxins and dibenzophurans, from pre-dusted SO2, H2O and heavy metal exhaust by adsorption of the hydrocarbons on solid adsorbents, using as adsorbent a diluted zeolite with a SiO2/Al2O3 ratio of 20:1 to 1000:1 and adsorption at a temperature of 20-200°C. The zeolite has a particle diameter of 1-5 mm and is arranged in a solid bed reactor or a moving reactor bed.

It has been shown that the known cleaning methods need to be improved because of the increased requirements on cleaning performance. The purpose of the present invention is therefore to create a method for cleaning the exhaust gases from the incineration of waste, industrial waste and sewage sludge which reliably allows the maintenance of low pollutant concentrations in the reingass even in the event of variations in the individual pollutant concentrations and which works economically. The method must in particular ensure that the reingass has an Hg equivalent concentration < 50 μg/Nm3, a concentration of polyhalogenated dibenzo dioxins and dibenzofurans of 0,1 ng/Nm3 and a concentration of PCBs, PCP and PCA < 1 μg/Nm3 (equivalent to the NATO standard).

The present invention is solved by reacting the exhaust gases above the dew point at a temperature of 80 to 180 °C and a gas velocity of 3 to 20 m/s with a mixture of naturally occurring zeolites in a gaseous solid suspension for a reaction time of 0.5 to 10 s, with a mean particle size d50 of the zeolite mixture of 5 to 50 μm and a mean suspension density of the solid suspension of 0.02 to 10 kg of solid per Nm3 of exhaust gas. The method of measurement reliably achieves a total Hg concentration of 50 μg/Nm3, a concentration of polycyclic dioxide and dioxygenic compounds of < 0,1 μm3 of N/N, and a concentration of PCBs of < 1 PCN3 per Nm3 of gas.

The method of the invention has a particularly good adsorption performance when the exhaust gas is reacted at a temperature of 120 to 140 °C with a mixture of naturally occurring zeolites and when the mixture of naturally occurring zeolites contains 10 to 20% by weight of mordene, 60 to 70% by weight of clinoptilolite, 0 to 5% by weight of montmorillonite and residual SiO2.

A further improvement in the adsorption performance of the process of the invention is achieved by the addition of 0.1% to 1% by weight of MnSO4, FeSO4, CoSO4, NiSO4 and/or CuSO4 to the mixture of naturally occurring zeolites.

The present invention shows that small amounts of acidic pollutants, i.e. SO2, HCl and HF, still present in the exhaust gas are removed by the mixture of naturally occurring zeolites containing 10 to 30% by weight of CaCO3, CaO and/or CaOH2.

The method according to the present invention is carried out in such a way that the reaction takes place in a circulating vortex layer, with a gas velocity of 3 to 8 m/s, preferably 4 to 5 m/s, and an average suspension density of the gas-solid suspension of 2 to 10 kg of solid per Nm3 of exhaust gas.

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Alternatively, the reaction is carried out in a jet stream reactor with a gas velocity of 6 to 20 m/s and a mean suspension density of the gas-solid suspension of 20 to 200 g of solid per Nm3 of exhaust gas.

The invention provides that the zeolite mixture loaded with the pollutants is treated as a spinning gas in a vortex layer at 300 to 600 °C with air and the air leaving the vortex layer is cooled to room temperature to remove the mercury. This measure allows the zeolite mixture to be at least partially regenerated and returned to the process cycle, with mercury being obtained as a feedstock.

The invention provides, alternatively, that the zeolite mixture loaded with the pollutants is treated as a spinning gas in a vortex layer with air at 800 to 1300 °C and that the air leaving the vortex layer is cooled to room temperature to remove the mercury. This variant of the process not only removes the mercury from the zeolite mixture but also completely decomposes the adsorbed hydrocarbons.

The subject matter of the invention is explained in more detail below by means of the drawing and an example of an embodiment.

The waste incineration plant (1), where household waste, sewage sludge and industrial waste are incinerated, discharges the exhaust gas containing dust and gaseous pollutants at a temperature of 800 to 1000°C. This exhaust gas passes through the pipe (2) to the heat exchangers used for energy generation (3), where it is cooled to a temperature of 200 to 300°C. The cooled exhaust gas is passed through the pipe (4) to the electrical separator (5) where a large-scale dusting takes place at a temperature of 200 to 300°C. The solid air vapour deposited in the electrical separator (5) is discharged through the pipe (8) while the hot exhaust gas (6) is discharged through the spray line (7)

At the same time, the gaseous acid pollutants SO2, HCl and HF react with the CaOH2 to form the corresponding calcium salts. These salts, the unconverted CaOH2 and the remaining powdery flue gas are discharged from the spray line (9) through the pipe (7) to the spray line (7) and the exhaust gas (7) from the lease (12) still contains solids in the form of stagnation which are not exposed to CaOH2, CaOH2 and the calcium gas.These solids are separated from the exhaust gas in the fabric filter (13) and are passed through the pipe (14) to the storage tank (15), where they are combined with the solids carried out through the pipe (9) from the spray absorber (7). The material collected in the storage tank (15) has the same structure. In order to quickly build up a filter cake on the fabric filter (13), some of the solids in the storage tank (15) are introduced into the pipe (12) through the pipe (35). The cooling and de-dusting of the combustion gas, the removal of the acidic gaseous pollutants from the combustion exhaust in a spray absorber and the subsequent de-dusting of the exhaust gas after combustion in a gas filter are known techniques.

A largely purified exhaust gas with a temperature of about 130°C is emitted from the tissue filter (13) and passes through the tube (16a) as secondary gas and through the tube (16b) as primary gas into the vortex reactor (17). The primary gas carried in the tube (16b) is fed from the storage bunker (18) through the tube (19) a mixture of natural zeolite composed of 20% mordenite, 70% clinoflitol, 5% montmorillonite and residual SiO2. The zeolite mixture has a mean particle size d50 of about 25 μm. The vortex reactor (17) produces a single-phase cacophony, which contains a suspended fuel contained in a medium-diameter cylinder between the gas reactor (17) and the gas cylinder. (20) The gas is suspended in a medium-diameter cylinder between the gas reactor (17) and the gas cylinder.In the cyclone (20) the gas solid suspension is separated, with the solid particles removed from the cyclone (20) via the pipe (22). Some of the solids are returned via the pipes (23) and (16b) to the cyclone (17) and the other part of the solids is passed through the pipe (24) to the cyclone (25). The purified exhaust gas leaves the cyclone (20) via the pipe (26) and is heated at a temperature of approx.120°C through a fireplace, with residual dust being removed in a filter connected to the cyclone (20) not shown in the figure.

In the reactor (25) the pollutant-laden zeolite mixture is regenerated at 800 to 900°C. The swirling gas is air which is fed into the reactor (25) via the line (27). Some of the solids are removed from the reactor (25) via the line (28) and, after cooling in the solids cooler (29) via the line (30) are fed into the storage tank (18). The solids cooler (29) is operated as a coolant with the air which is fed back into the reactor (25) via the line (27); this cooling cycle is not shown in the figure. In the reactor (25) the hot lead gas is carried via the lead waves (31) to the condenser (32) which, after cooling in the solids cooler (29) via the line (30) is fed into the storage tank (18). The liquid mercury is then discharged into the condenser (32) where it is absorbed by the conductor (32) and the liquid mercury is discharged into the air. (32) The liquid mercury is then discharged into the condenser (32) where the condenser is discharged into the air. (32) The liquid mercury is then discharged into the reactor (33) and the liquid mercury is discharged into the condenser (33) where the liquid mercury is discharged into the air. (33) The liquid mercury is discharged into the reactor (34) and the liquid mercury is discharged into the air.

The exhaust gas fed to the reactor (17) via the lines (16a) and (16b) had a mercury content of 2 mg/Nm3, a polyhydrogenated dibenzodioxins and dibenzofurans content of 20 ng/Nm3 and a total PCB, PCP and PCA content of 10 μg/Nm3.

The vortex reactor (17) received a total of 600 Nm3 of exhaust gas per hour from the lines (16a) and (16b). 60 kg of zeolite mixture were constantly present in the vortex reactor (17); 1 kg of zeolite mixture was released per hour in the cyclone (20), of which 0.9 kg were returned to the vortex reactor and 0.1 kg were used for regeneration in the reactor (25).

It may be necessary to run the cyclone (20) in several cyclone steps or to switch a filter element to the cyclone (20) to reduce the residual dust content of the pure gas in the line (26) to less than 5 mg/Nm3. The zeolite mixture in the line (24) has a pollutant load of about 2% by weight.

The addition of heavy metal salts to the zeolites is done by spraying a metal salt solution on the hot zeolites, evaporating the water content of the solution to produce a dry product impregnated with metal salts, which increases the absorption capacity of mercury from 2 g to 3 g.

Claims (9)

1. A method for the purification of oxygen-containing exhaust gases produced during the combustion of refuse, industrial waste and clarification sludge, in which mercury, mercury compounds and polyhalogenated hydrocarbons are removed from the exhaust gases by adsorption on zeolites, characterised in that the exhaust gases are reacted above the dewpoint at a temperature of 80 to 180°C and a gas velocity of 3 to 20 m/s with a mixture of naturally-occurring zeolites for a reaction period of 0.5 to 10 s in a gas/solids suspension, the average particle size d₅₀ of the zeolite mixture being 5 to 50 µm and the average suspension density of the gas/solids suspension being 0.02 to 10 kg solids per sm³ exhaust gas.
2. A method according to Claim 1, characterised in that the exhaust gas is reacted at a temperature of 120 to 140°C with a mixture of naturally-occurring zeolites.
3. A method according to Claims 1 to 2, characterised in that the mixture of naturally-occurring zeolites contains 10 to 20% by weight mordenite, 60 to 70% by weight clinoptilolite, 0 to 5% by weight montmorillonite and remainder SiO₂.
4. A method according to Claims 1 to 3, characterised in that the mixture of naturally-occurring zeolites is doped with 0.1 to 1% by weight MnSO₄, FeSO₄, CoSO₄, NiSO₄ and/or CuSO₄.
5. A method according to Claims 1 to 4, characterised in that the mixture of naturally-occurring zeolites contains 10 to 30% by weight CaCO₃, CaO and/or Ca(OH)₂.
6. A method according to Claims 1 to 5, characterised in that the reaction takes place in a circulating fluidised bed, the gas velocity being 3 to 8 m/s, preferably 4 to 5 m/s, and the average suspension density of the gas/solids suspension being 2 to 10 kg solids per sm³ exhaust gas.
7. A method according to Claims 1 to 5, characterised in that the reaction takes place in an entrained-bed reactor, the gas velocity being 6 to 20 m/s and the average suspension density of the gas/solids suspension being 20 to 200 g per sm³ exhaust gas.
8. A method according to Claims 1 to 7, characterised in that the zeolite mixture laden with the pollutants is treated at 300 to 600°C in a fluidised bed with air as fluidising gas and the air emerging from the fluidised bed is cooled to room temperature in order to separate off the mercury.
9. A method according to Claims 1 to 7, characterised in that the zeolite mixture laden with the pollutants is treated at 800 to 1300°C in a fluidised bed with air as fluidising gas and the air emerging from the fluidised bed is cooled to room temperature in order to separate off the mercury.