Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1989-12-12
1991-04-23
Heller, Gregory A.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
423242, 423244, B01J 800, C01B 1700, C01B 2100
Patent
active
050098705
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for removing sulphur and nitrogen compounds from gas by means of long-chained sulphur compounds, mainly in acidic conditions, while an electroconductive solid agent serves as the carrier of the long-chained sulphur compounds.
Most of the cases where the removal of sulphur and nitrogen compounds from gases is required, are directly or indirectly connected to the production of energy. Consequently the treated gas amounts are fairly large, typically within the range of 200,000-2,000,000 Nm.sup.3 /h.
At least 200 different processes have been suggested for removing sulphur compounds. In most cases, sulphur compounds: means sulphur dioxide. On the other hand, at least 50 different processes have been suggested for removing nitric oxides from gases. Because so far the rest of the processes have, with respect to the scale, been too complicated and expensive from the point of view of the users, in the removal of sulphur dioxide, methods based on the use of lime have more or less been the only ones that have been put to practice. If the amount of nitric oxides has not been sufficiently reduced by means of arrangements relating to burning (temperature, air coefficient, afterburning etc.), the No.sub.x gases have been removed by using various catalysts and/or agents sorpting nitric oxides.
The use of lime is the most general example of the use of alkalic agents. Besides high economic costs, a common drawback of these processes using lime, is the continuous creation of large amounts of waste gypsum, CaSO.sub.4.x H.sub.2 O, which in the long run causes insupportable environmental hazards. The said waste contains, among other things, the heavy metals Cd, Pb, As, Hg etc., which also are gradually dissolved in the environment. It has turned out that the removal of nitric oxides by means of NH.sub.3 easily brings about catalytic poisoning, in addition to which the NO.sub.x /NH.sub.3 ratio is difficult to adjust in practice, and among other things ammonia sulphate, (NH.sub.4).sub.2 SO.sub.4, is created.
Usually exhaust gases contain 1-10% oxygen, and therefore it is as such natural that many removal processes suggest the oxidation of sulphur dioxide into sulphuric acid, and/or the oxidation of nitric oxides into nitric acid. With exhaust gases, these are reactions that happen naturally according to thermodynamics, but in reality a catalyst is needed. One weakness of this method group is related to this said catalyst. Another and larger drawback is, however, related to the quality of the obtained products. Obviously there are cases where even a low-quality sulphuric acid or nitric acid can be used, but in the majority of cases sulphuric acid in particular brings about serious elimination problems. Also with respect to washing efficiency, these reduction methods often cause trouble.
As a third fairly large process group let us mention the methods where the operations are generally carried out within a slightly acidic range, usually within the pH range 3.5-6, either in a buffered or an unbuffered system. There the process is often based on the sorption of sulphur dioxide into the solution in question. The sorption may be wholly or partly connected to the reduction carried out by means of sulphide. Usually the employed sulphide is H.sub.2 S or Na.sub.2 S, sometimes it can be a solid sulphide or for instance FeS, which in the older processes in practice means Fe.sub.1-x S, which is totally different from FeS. In this process group, the washing efficiency with respect to sulphur dioxide is better than in the processes based on the use of lime. Normally a separate step is used for removing nitric oxides.
As an example from the last group, let us mention the U.S. Pat. No. 4,634,582. In this method sulphur dioxide is removed by using sulphide and buck as reagents. The washing solution is buffered, for instance with citric acid, within the pH range 3.5-5.5. The washing temperature is 38.degree.-66.degree. C. The washing of sulphur dioxide is carried out with hydrogen sulphide and polythionate
REFERENCES:
patent: 1701825 (1929-02-01), Seil
patent: 2031410 (1936-02-01), Fulweiler
Heimala Seppo O.
Hyvarinen Olli V. J.
Metsarinta Maija-Leena
Heller Gregory A.
Outokumpu Oy
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