Method for Separation of Impurities From Flowing Gas

The present invention is directed to a method for separating harmful substances, such as SO.sub.2, HCl and NO.sub.x where x is 1 or 2, for example, from cooled flue gases such as combustion exhaust gases, by way of dry adsorbents based upon hydrated oxides, hydroxides, or oxides. Such dry adsorbents include Ca(OH).sub.2, AlOOH, Al(OH).sub.3, Al.sub.2 O.sub.3 xH.sub.2 O, bauxite, CaO, NaOH, and/or carbonates, and/or hydrogen carbonates. The cooled gases so treated, initially have high combustion chamber temperatures, such as attained by bituminous coal firing, for example.

In the present invention, the powdered or particulate adsorbents, are blown into at least a portion of the flowing gas at gas temperature below about 400.degree. C., thereby binding any chlorides which may be present in the gas. The gases or fumes are subsequently released into the open environment through a chimney, after precipitation of the solid components therefrom. The present invention also relates to an apparatus for carrying out this procedure.

DE-OS No. 31 39 080 discloses the blowing or injection of Ca(OH).sub.2 adsorbent into a waste or refuse boiler at a flue gas temperature of 400.degree. to 350.degree. C. in order to separate the chlorine and/or fluorine from the flue gases or a waste or refuse incinerator, and to bind these harmful substances. The exhaust gases are then cleaned and transmitted into the atmosphere. The disadvantage of this procedure and apparatus, is that the exhaust gases containing the dust and adsorbents, are directed through the final heating surfaces, so that the gases quickly pollute the environment. The combined separation of dust and chlorides or fluorides leads to an intimate mixture of these three components, thus making separation and further treatment practically impossible. Simultaneous adsorption of SO.sub.2 and/or reduction of NO.sub.x or sulphate formation, is not indicated.

It is known that adsorption is more efficient in a "wet" manner than in a "dry" condition for the separation of SO.sub.2 containing harmful fumes from the exhaust gases. It is also known that in the case of the "dry" separation of the harmful fumes, at similar surface boundary conditions, but with different gases, the degree of separation is subject to large fluctuations, whereby refuse firing generally produces a better degree of separation than coal firing.

Accordingly, it is an object of the present invention to improve separation of harmful substances from cooled flue gases such as combustion exhaust gases.

It is a more particular object of the present invention to improve removal of SO.sub.2, and HCl from the gas, and to improve removal or conversion of NO.sub.x in the gas.

It is another object of the present invention to improve the "wet" separation of harmful substances from the flowing exhaust gas.

It is a further object of the present invention to provide for synthesis or regeneration of adsorbent for removing harmful substances from the flowing exhaust gas.

It is also another object of the present invention to improve stoichiometric ratios in reaction of adsorbent with flowing exhaust gas, to remove the harmful substances from the same.

It is a further object of the present invention to improve removal of relatively low levels of impurities, e.g. SO.sub.2 and NO.sub.x, from the cooled flue gases, so that such gases are substantially purified.

These and other objects are attained by the present invention in which relatively low levels of harmful substances, such as SO.sub.2 and NO.sub.x where x is 1 or 2, are removed from cooled flue gases containing the same. The gas is contacted with at least one substantially dry particulate adsorbent including NaHCO.sub.3 which, at a release temperature below 400.degree. C., decomposes and forms an activated adsorbent including Na.sub.2 CO.sub.3 for the harmful substances. The thus-formed Na.sub.2 CO.sub.3 reacts with the relatively low content of SO.sub.2 and NO.sub.x, with the substantially dry, particulate adsorbent being present in an amount to remove virtually the entire content of SO.sub.2 and NO.sub.x from this gas. Thus, the activated adsorbent acts to remove harmful substances from the flowing cooled flue gas and substantially purify the same.

The content of SO.sub.2 in the gas that is contacted by the adsorbent is less than about 1% by volume, preferably between about 0.5% and 1% by volume and most preferably about 0.5% by volume, while the content of NO.sub.x in the gas that is contacted by the adsorbent is less than about 1% by volume, preferably between about 0.5% and 1% by volume, and most preferably about 0.5% by volume.

Physically, and/or chemically-conditioned release of fission and/or reaction products, is attained by the present invention during the addition of the adsorbent into the partially cooled-off exhaust gas flow. Therefore, the molecules of the harmful gas component will encounter "wet" conditions at the well-dispersed adsorbent molecules in the exhaust gas, so that a gas-liquid reaction is executed instead of just a gas-solid reaction. The essential "in situ" water accumulation at the adsorbent particles, always provides sufficient available water for the gas/liquid reaction at the adsorbent grains (granules), in spite of water accumulation at the sulfate molecule Thus the stoichiometric ratios of adsorbent to harmful substances can be greatly improved and simplified over the stoichiometrics in previously-utilized installations and processes, such prior stoichiometrics being three times greater than those attained by the present invention.

At least a portion of the dust produced in the combustion chamber is removed prior to the blowing-in or addition of the adsorbents into the flowing exhaust gas to be purified. Also, at least a portion of the adsorbents is split off into molecular size, as a result of the influence of temperature, to generate H.sub.2 O and/or NH.sub.3 and/or CO.sub.2, thus activating the added-adsorbent. The adsorbent may include NaHCO.sub.3 and one or more of the following compounds, for an example: NH.sub.4 HCO.sub.3, Al(OH).sub.3, silica gel, Ca(OH).sub.2, salts with water of crystallization such as CaCl.sub.2 or Al.sub.2 O.sub.3.

By the activation of these added adsorbents, SO.sub.2 is not only bound (through formation of a sulfite or sulfate), but nitric oxides are also bound or transformed to nitrogen. This latter step is attained by a change of the crystal structure of the particularly-active adsorbent, which takes place in "status nascendi".

The present invention is also directed to an apparatus for carrying out the above-noted procedure, where an inlet for an adsorption agent or a mixture of adsorption agents is provided in the flowing flue gas after a combustion installation or chamber. This inlet is especially provided in front of a final heat recovery surface, and after or downstream of the last tubular heating surface in the combustion chamber. Moreover, a dry filter device, especially a cloth filter, and a suction draft blower are provided in the path of the flowing exhaust gas.

It has been surprisingly found that the transformation of NO.sub.x harmful gaseous substances, to harmless component parts, is greatly improved by the present invention, while removal of relatively low levels of SO.sub.2 and NO.sub.x impurities in especially effective.

Formation of fission and/or reaction products (reactant) from adsorbents, occurs, for example, according to the following equations:

(1) Reaction water: CaO+2HCl=CaCl.sub.2 +H.sub.2 O

(2) Molecular water: 2NaHCO.sub.3 decomposes at temperatures above 100.degree. C. to Na.sub.2 CO.sub.3 +H.sub.2 O+CO.sub.2, or NH.sub.4 HCO.sub.3 splits at temperatures of 35.degree. C. into NH.sub.3 +H.sub.2 O+CO.sub.2 or Al(OH).sub.3 splits into AlOOH +H.sub.2 O.

(3) Water of crystallization: CaCl.sub.2 .multidot.6H.sub.2 O splits off 4H.sub.2 O at temperatures above 30.degree. C. and also splits off the remaining (2H.sub.2 O) water of crystallization particularly at temperatures greater than 200.degree. C. FeCl.sub.3 and Al.sub.2 O.sub.3 react in a similar manner.

(4) Adsorbed water Water from the atmosphere (capillary condensation) may condense in molecular size in the pores of dry dust particles, (salts) whereby these water molecules can evaporate only at high temperatures, often above 500.degree. C.

Furthermore, during the re-forming of NO.sub.x, it is important for NH.sub.3 to be available in small (molecular) size in sufficient quantity, in order to disintegrate or reduce the NO.sub.x or to bind NO.sub.x into nitrates. For this purpose, it is possible to use ammonium compounds, such as NH.sub.4 Cl which breaks down into NH.sub.3 and HCl at 300.degree. C. , as well as utilizing NH.sub.4 HCO.sub.3 which already breaks down at approximately 60.degree. C. into NH.sub.3 +H.sub.2 O+CO.sub.2.

Operation of simultaneous desulfurization and denitrogenization, is described as follows: It has been known that Na.sub.2 CO.sub.3 (soda) binds sulfur barely effectively. However the addition of NaHCO.sub.3, for instance in the form of baking soda or as a mineral (nahcolite), binds sulfur to a considerably greater extent, when the addition takes place at temperatures above the release or evolution of H.sub.2 O and CO.sub.2 (i.e. above 60.degree. C. ). H.sub.2 O and CO.sub.2 escape from the crystal, with Na.sub.2 CO.sub.3 remaining, present in an especially active form, because it appears in "status nascendi" and thus still possesses the lattice structure of the baking soda, while it is chemically present as Na.sub.2 CO.sub.3 (soda).

At the same time, water is present in the pores which are formed during the release from the crystal, thus promoting the binding-in of SO.sub.2. The active crystal, Na.sub.2 CO.sub.3, on the other hand, is active for the reason that the lattice defects release unsaturated electrons, which in turn, effect the binding-in or transformation of NO into N.sub.2 and/or NO.sub.2. The Na.sub.2 CO.sub.3 is also active because the specific surface of the freshly-formed or transforming crystals is very large.

It is essential that unstable compounds are created, at least partially, which reduce NO.sub.x to N.sub.2, so that N.sub.2 can be formed which can no longer be changed back into NO.sub.x due to the low temperature level. Remaining NO.sub.2 is transformed into N.sub.2 by the NH.sub.3 made available at the same time (either as NH.sub.3 or as NH.sub.4 OH, or as a result of disintegration of an NH.sub.3 -containing salt). 3.5N.sub.2 +6H.sub.2 O is ultimately formed from 3NO.sub.2 +4NH.sub.3.

The following criteria apply for the selection of the active substances for the mixing of the adsorbents, for attaining the simultaneous desulfurization and denitrogenization. Within the individual groups of adsorbents, selection takes place according to availability or economic consideration, and/or further treatment of the adsorbents that is conducted. For desulfurization, compounds should be used which split off H.sub.2 O creating surfaces for the binding of sulfite or subsequent sulfate, thus creating more favorable initial conditions for the otherwise extremely inert or inactive reaction gas/solids (SO.sub.2 or alkaline substances) which then react in the presence of water. In other words, suitable compounds include NaHCO.sub.3 and, for example, NaOH (in solid or liquid form), NH.sub.4 HCO.sub.3, Ca(OH).sub.2, Al(OH).sub.3, AlO(OH), compounds which give off water of crystallization, calcium oxide which reacts as follows: CaO+2HCl to give off reaction water, NaHSO.sub.3, BOH, or solid or liquid organic compounds, such as organic acids including formic acid or acetic acid.

For the denitrogenization, the adsorbent should include a compound which additionally or simultaneously results in a sudden crystal lattice transformation, whereby the newly created compound (by way of heat, or by way of mechanical force such as pulverizing or grinding, for example) is especially active in "status nascendi" (i.e. by "bursting" transformation into a new compound), and possesses unsaturated surface valences which can be used for transformation of NO to N.sub.2 and/or NO.sub.2. This occurs, for example, through the addition of NaHCO.sub.3 (transformation to Na.sub.2 CO.sub.3), forming an especially active Na.sub.2 CO.sub.3 in "status nascendi".

NO.sub.2 which has not been simultaneously tranformed into N.sub.2 (from the former NO), must now be transformed into N.sub.2. This is accomplished by adding, at the same time or later, NH.sub.3 or NH.sub.3 -containing compounds or solutions which release NH.sub.3 when heat is introduced (possibly also releasing NH.sub.3 *).

Within the scope of the present invention, it is possible to increase the degree of dry adsorption, as far as effectiveness is concerned. This is of special importance in the case of high temperatures in the combustion chamber, occurring, for example, if bituminous coal is used for firing, where the addition of lime into the combustion chamber is only of little effect. Since chemical industrial processes and installations must first be adjusted to requirements of exhaust gas purification (e.g. inexpensive adsorbents of not too great a purity, but availability in large amounts), it is very important that a manufacturer of furnaces can easily produce the adsorbent by oneself, thus requiring only chemically raw material NH.sub.3 which is available in great quantities, and the purchase of which does not constitute too great an expense. This attainment is explicitly possible with the present invention.

For the separation of sulfur dioxide (SO.sub.2), especially in the absence of any HCl in the exhaust gas, an additive has to be blown into the flowing exhaust gas stream together or in admixture with the adsorbent itself. In particular, this additive may be HCl in solid or in liquid form. More specifically, the adsorbent may be introduced into the flowing exhaust gas in admixture with from 0.5-50% by weight, more preferably from 2 to 10% by weight, of a halide especially chlorides such as NaCl, ammonium-containing compounds or alkaline compounds, such as NH.sub.4 Cl, (NH.sub.4).sub.2 CO.sub.3, or Ca(OH).sub.2 with decomposition or sublimation temperatures less than or equal to about 400.degree. C.

Powdered, "over-quenched" Ca(OH).sub.2, as an adsorption agent, is preferably blown into the exhaust gas flow in a hydration step, especially in admixture with CaCl.sub.2 and/or FeCl.sub.3 with water of crystallization, whereby chlorides are produced through treatment with HCl of the Ca(OH).sub.2 and its impurities. For the separation of SO.sub.2 in the absence of HCl, especially for the additional separation of NO.sub.x, the following adsorbent including NaHCO.sub.3 may be blown into the flowing cooled, flue gas current: Ca(OH).sub.2 and/or hydrogen carbonate such as NH.sub.4 HCO.sub.3, NH.sub.4 OH, NH.sub.3, or NH.sub.3 -containing compounds, for example. In particular, the required ammonium-based and/or soda-based adsorbent is produced from the purified gas flow after or downstream of a dry-separator, by introducing ammonia and/or salt in aqueous solution under the release of heat.

The accompanying drawing is a schematic illustration of an embodiment of the present invention. This figure illustrates the interconnection, i.e. a circuit arrangement of a thermo power installation equipped with exhaust gas purification and adsorption agent production installations.

Referring to the drawing, cooled flue gas current 1 flows from a combustion chamber 13, at approximately 200.degree. C. through a cloth filter 2, in which powdered impurities are precipitated through a chimney 14, into the open. At the outlet of the combustion installation or chamber 13, the exhaust gas is precipitated in a dry-separator such as an electric filter 16. A dry adsorbent, such as Ca(OH).sub.2 is added to the thus-purified flue gas which has now been cooled to below 400.degree. C., in an adsorption phase. The added adsorbent, adsorbs the harmful gases such as SO.sub.2 and/or HCl, whereby the loaded adsorbent is then separated in a cloth filter 2.

A difficulty is encountered at this point, in that the adsorbent reacts only on the surface thereof with the SO.sub.2, if the chloride content in the flowing exhaust gas is low, so that only a very minor binding of SO.sub.2 can take place, and then only with high stoichiometrics of 2 to 3. However, this disadvantage can be eliminated by adding, to the adsorbent, chlorides such as CaCl.sub.2 and/or FeCl.sub.3 in an amount of e.g. 2 to 5% by weight, so that the Ca(OH).sub.2 is intermixed with chlorides. This admixture is especially effective if the two adsorbents have been produced together from the start (e.g. by slaking of the burned lime under the addition of hydrochloric acid).

The important advantage of utilizing the CaCl.sub.2 is that this salt is able to contain a relatively large amount of water of crystallization, and is able to release all of this water of crystallization at temperatures exceeding 200.degree. C. As a result, these water particles are formed in molecular size in "status nascendi", during the formation of sulfate.

This moistness, which bursts forth along not only the surface of the dust particles but also in the interior thereof, precipitates an extremely effective gas/liquid reaction instead of a customary gas/solid reaction, which would otherwise occur in a relatively slow and insufficient manner. In this connection, it is extremely important that the liquid originates practically on the grains and is not conveyed in by water vapor content of the flowing exhaust gas. This latter step had been tried with "wet" processes up to now, such as washing spray procedures. Such previously-used steps included falling below the dew point of the flowing exhaust gas, and/or injecting water in front of a filter such as a cloth filter, along with other types of attempts. These steps only produced the unfavorable result that the cloth filters, in particular, were subject to an uneconomically high loss of pressure, since the water outside the dust particles cause the dust particles to become glued together. In contrast, the water at the so-called "inner" surface of the dust particles, i.e. within the adsorbent, does not suffer from this disadvantage.

The NO.sub.x cannot be disintegrated in large quantities through the addition of Ca(OH).sub.2 and CaCl.sub.2. In order to get this harmful gas under control, the present invention provides that, together with the Ca(OH).sub.2 which may also contain CaCl.sub.2, hartshorn salt, primarily NH.sub.4 HCO.sub.3, and/or baking soda (NaHCO.sub.3), and/or similar compounds, are admixed with the Ca(OH).sub.2. Alternatively, the aforementioned hydrocarbonates, which are not stable and disintegrate into component parts at temperatures from 30.degree. C. to 400.degree. C. may be entirely substituted for the Ca(OH).sub.2 adsorbent. This is especially effective for removing relatively low levels of SO.sub.2 and NO.sub.x impurities in the cooled flue gas.

To the extent that the component parts of the disintegrated hydrogen carbonates have a tremendous reaction tendency, it is possible to disintegrate (reduce) the harmful gas in good stoichiometric ratios with high effectiveness. Hartshorn salt and baking soda are relatively expensive adsorbents. However, these adsorbents can be produced or recovered from the CO.sub.2 of the flowing flue gas, with water, NH.sub.3 (and optionally with sodium chloride), in an installation which is connected thereto.

For this purpose, approximately 5% by volume of the flue gas is diverted from the flue gas current 1 and guided through a flue gas cooler 3. This partial current 1' is now conveyed to one or several solution reactors 10, 10', where it is brought into contact with water or with an NH.sub.3 -saturated liquid, with the CO.sub.2 of the cleansed flue gas being partially dissolved. Flue gas removal points 4, 4' and 4" are provided in the partial current 1' of the exhaust gas, for this purpose.

The newly produced solution of the three aforementioned components (NH.sub.3, NaCl, and CO.sub.2) is then cooled in the solution reactor 10', as well as in a cooler 9 which is connected thereto at the outlet side, so that hartshorn salt or baking soda precipitates. The precipitating crystals are thrown out within a centrifuge 5, and then conveyed through a dryer 7, and through a crystal-crushing device 8 such as a mill, to a distributing device 6 for introducing the adsorbents into the flowing flue gas.

The mother liquor, which is crystal poor, is conveyed through a recirculation line 11 to an NH.sub.3 -saturator 12, and/or is mixed with mother liquor that has been saturated with NH.sub.3. This NH.sub.3 -saturated solution is then conveyed to the flue gas removal point 4, so that this solution again comes into contact with the CO.sub.2 of a partial current 1' to generate new adsorbent crystals.

The partial current 1' of the flue gas is again mixed into the hot flue-gas current 1 at approximately 220.degree. C. , after the required amounts of CO.sub.2 have been removed. This combined stream is then released into the chimney 14 through a blower 15 as illustrated. As illustrated in the figure, NaCl may be added to the aqueous solution of ammonia, e.g. in the saturator 12, for production of a soda-based adsorbent.

In order to obtain a pure-loaded adsorbent, it is preferable to dry-clean the flue gases ahead of the distributing device 6 for introducing the adsorbent into the flowing flue gas stream 1. The exhaust gases are preferably cleaned in an electro-filter 16, with ash particles being removed in the direction of arrow 17 in the drawing, so that as few ash particles as possible from the combustion installation 13 reached the cloth filter 2. Thus, this filter 2 required much less frequent cleaning, while pressure loss is maintained within narrow limits

Additional reasons for this initial removal of ash in the direction of arrow 17, in addition to other features of the present invention, are as follows. The product obtained during the adsorption phase through reaction of the adsorbent with the harmful gases, can be sold or re-used directly in the plant. However, due to high dust contents of the flowing exhaust gas, the value of this product is decreased, or it becomes completely unusable. However, one obtains ammonium sulfate or ammonium bisulfate during the reaction of SO.sub.2 with hartshorn salt, the ammonium sulfate/bisulfate being relatively pure, and being capable of being used as fertilizers.

It may be advisable to carry out the reaction of the adsorbent with the flowing gas at the cloth filter, as completely as possible (by additive utilization). This means a rather long time interval between purification pulses at the cloth filter. However, the coating of the non-reacting dust from the combustion chamber requires a shorter purification interval, with any adsorbent which is still capable of reaction being removed prematurely from the reaction surface in the filter cake of the cloth filter.

Accordingly, if in an existing installation, a filter is available, such as an E-filter (which has been used, but which is still more or less, at least partially usable), this E-filter can be utilized and the cloth filter downstream can be maintained on a smaller scale. Thus, the entire installation would be much less costly to operate.

Thus, if the exhaust gases are purified as completely as possible before the distribution 6 of the adsorbents, it is possible to set up downstream of the combustion installation, a fertilizer factory or a bituminous coal plant or a plate manufacturing installation, which refines the thus-produced and loaded adsorbents, and thus takes advantage of an environmental protection installation, or which at least reduces the amounts deposited in the atmosphere, or outside the installations.

Within the scope of the present invention, it is possible to also release an adsorbent in the combustion chamber of the combustion installation itself, or into the exhaust gas current at an exhaust gas temperature of approximately 100.degree. C. (i.e. shortly after the combustion stage), such adsorbent binding SO.sub.2 and thus creating better conditions for the SO.sub.2 /NO.sub.x precipitation, which takes place downstream. Thus, the quantitative ration of SO.sub.2 /NO.sub.x can be influenced so that a stoichiometric ratio at or near an optimum of 2:1 is attained. Alternatively, it is thus possible to reduce SO.sub.2 partial pressure altogether, in order to reduce the cost of the correspondingly-expensive adsorbents added at the low-temperature point 6.

The process and apparatus of the present invention thus permits conversion of older installations, whereby the flue gases released into the atmosphere are now in compliance with environment protection regulations for new installations. Use of the present invention in new installations is also advantageous, since loaded adsorbents are obtained in a state of great purity, and are thus suitable as base substances for factories of building material and/or fertilizers.

The preceding description of the present invention is merely exemplary, and is not intended to limit the scope thereof in any way.