Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1997-09-18
1999-04-06
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
4232422, 435262, 210605, C01B 2100, C01B 1720, B01D 5300, C02F 330
Patent
active
058914080
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for purifying flue gas containing nitrogen oxides, in which the flue gas is scrubbed with a circulating scrubbing liquid which contains a transition metal chelate, the chelate forms a complex with nitrogen oxide, nitrogen oxide is reduced to molecular nitrogen, and the chelate is subsequently regenerated.
BACKGROUND OF THE INVENTION
Such a process is disclosed, for example, in Dutch Patent Applications 7500672, 7500673, 7515009, 7607212 and 8602001, and European Patent Application 531762. The transition metal chelate, usually iron(II)-EDTA, is used to complex and thus to effectively absorb the nitrogen oxides, of which NO is very sparingly dissolved by scrubbing water that does not contain a transition metal chelate.
The known processes each involve the simultaneous removal of nitrogen oxides (mainly NO and NO.sub.2), hereinafter referred to as NOx, and sulphur dioxide, molecular nitrogen (N.sub.2) and sulphates or amide-sulphates and many other N-S compounds generally as well as N.sub.2 O being ultimately obtained. The processing of the sulphates, N.sub.2 O and nitrogen-sulphur compounds is, however, complicated and requires various subsequent treatments with associated equipment. N.sub.2 O will be emitted with the flue gas. This is an unwanted effect since N.sub.2 O is a compound known for its strong detrimental effect on the ozone layer and its strong greenhouse effect.
Another important problem is that, in the oxidizing medium, the active Fe(II) is partially converted to the much less active Fe(III) by oxygen from the flue gas or indirectly by sulphite in the scrubbing liquid. This results in high losses of the chelate. In addition, flue gas usually contains too little sulphur dioxide (sulphite) in relation to nitrogen oxides for the complete regeneration of the NO-bonded Fe(II)-EDTA complex to its active form. Such methods have therefore not yet acquired large-scale application.
In a process which is already used in practice for the removal of nitrogen oxides from flue gases, the flue gas is contacted at 300.degree. C. with ammonia (NH.sub.3) and a catalyst, in which process nitrogen is produced. This process, the so-called selective catalytic reduction (SCR) process, however, is expensive, both as a result of the high investment costs associated with the high-temperature installations and as a result of the high operational costs associated with the ammonia and the catalyst (approximately one third of the catalyst has to be replaced every year). In addition, a completely separate process is necessary for the optional removal of sulphur dioxide from the same flue gas.
SUMMARY OF THE INVENTION
The invention relates to a process which allows nitrogen oxides to be efficiently removed from flue gases for appreciably lower investment and operating costs, in which the NOx removal may optionally be combined with removal of sulphur dioxide. Surprisingly, it has been found that a complex of a transition metal chelate and nitrogen oxide can effectively be regenerated microbiologically to molecular nitrogen and the regenerated transition metal chelate. In this process, the transition metal is kept in the more active, lower oxidation state or returned to the lower oxidation state.
The process according to the invention as described in the introduction is therefore characterized in that the transition metal chelate is biologically regenerated in the presence of an electron donor. Where reference is made herein to chelate, this is understood to mean the complex of chelating agent and transition metal.
The biological regeneration according to the invention therefore involves the complex of nitrogen oxide and transition metal chelate, or the transition metal chelate without nitrogen oxide. In the former case, nitrogen oxide is reduced with concomitant release of active chelate; in the latter case, inactive chelate wherein the transition metal is in a higher oxidation state is regenerated to active chelate wherein the metal is again in a lower oxidation
REFERENCES:
patent: 3984522 (1976-10-01), Saito et al.
patent: 5196176 (1993-03-01), Buisman
Buisman Cees Jan Nico
Den Hartog Adrianus Johannes
Dijkman Hendrik
Verbraak Petrus Leonardus
Biostar Development C.V.
Straub Gary P.
Vinnola Milan M.
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