Apparatus for purifying flue gas containing nitrogen oxides

Details Apparatus for purifying flue gas containing nitrogen oxides Apparatus for purifying flue gas containing nitrogen oxides

1999-01-22

2001-05-22

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S168000, C422S172000, C210S603000

Reexamination Certificate

active

06235248

ABSTRACT:

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
2
), hereinafter referred to as NOx, and sulphur dioxide, molecular nitrogen (N
2
) and sulphates or amide-sulphates and many other N—S compounds generally as well as N
2
O being ultimately obtained. The processing of the sulphates, N
2
O and nitrogen-sulphur compounds is, however, complicated and requires various subsequent treatments with associated equipment. N
2
O will be emitted with the flue gas. This is an unwanted effect since N
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° C. with ammonia (NH
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 state. A major advantage of this process is that any chelate that is consumed by other processes and would thus not be available for binding NOx, is returned to its active form. In principle, the inactive form of the chelate could be regenerated e.g. by the addition of a chemical reducing agent or by electrochemical reduction, but in practice this is undesirable because of the higher costs and complications in the scrubbing cycle.
As transition metal that forms a complex with nitrogen oxide when chelated, use may be made of a metal such as iron, manganese, zinc, cobalt, nickel or aluminum. For economic and environmental reasons, iron(II), which is kept in the divalent state in the process according to the invention, is preferred. The transition metal chelate is formed with a chelating agent which has available at least two free electron pairs for chelation with the metal, in the form of amino groups, carboxyl groups or hydroxyl groups. Examples are polyamines such as ethylenediamine, diethylenetriamine, triethylenetetraamine, hexamethylenetetraamine, and 1,4,7-triazonane and their N-alkylated analogues such as polyamines such as ethylene-diamine which contain one to four hydroxyethyl groups and/or carboxymethyl groups, for example N-(2-hydroxyethyl)ethylenediamine-triacetic acid and, in particular, ethylenediamine-tetraacetic acid (EDTA), iminodiacetic acid and nitrilo-triacetic acid (NTA) and salts thereof. The concentration of the transition metal chelate may vary according to the specific scrubbing process parameters. A suitable concentration can be e.g. 1-200 mM, in particular 25-150 mM.
In the process according to the invention, the following reactions occur, in which NO is chosen as nitrogen oxide and iron(II) ethylenediaminetetraacetate is chosen by way of example of transition metal chelate:
NO+EDTA-Fe→NO-EDTA-Fe
NO-Fe-EDTA+[H
2
]→{fraction (1/2+L )}N
2
+Fe-EDTA+H
2
O
In this reaction, the hydrogen may be molecular hydrogen. The hydrogen may also be present as (organic) electron donor, for example as methanol, which is oxidized to carbon dioxide under the circumstances, or ethanol. It may also be in the form of other organic matter (COD) contained in the liquid (waste) stream.
The scrubbing of the flue gas can be carried out in a conventional gas scrubber. The biological regeneration of the complex of transition metal chelate and nitrogen oxide may be carried out in the scrubber itself, or in a separate bioreactor. The biomass required for the biological regeneration contains known nitrate-reducing bacteria.
A device for the removal of NOx from waste gases in which the biological regeneration takes place in the scrubber is shown diagrammatically in FIG.
1
. In such a device, the gas is brought into intimate contact with the scrubbing liquid containing the transition metal chelate and the biomass, for example by means of nozzles and optionally packing material. An electron donor such as methanol is added to the scrubbing liquid. The nitrogen formed and any carbon dioxide are removed with the purified gas.
The variant in which the biological regeneration is carried out in a separate bioreactor is shown diagrammatically in FIG.
2
. In such a device, the scrubbing liquid contains the transition metal chelate and the scrubbing liquid used is conveyed to the bioreactor which contains the biomass and to which an electron donor is added.
The process according to the invention can readily be combined with flue-gas desulphurization, in which case the sulphur dioxide absorbed from the flue gas can fulfil the function of reducing agent (electron donor). The regeneration could then proceed according to the reaction below:
NO-Fe-EDTA+SO
3
2−
→½N
2
+Fe-EDTA+SO
4
2−
The sulphate formed in this process can be removed in a conventional manner (precipitation with calcium), but is preferably removed biologically. The sulphate, possibly with residual sulphite, is therefore anaerobically reduced, mainly to sulphide, and the sulphide formed in this

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