IE930736A1

IE930736A1 - Method for reducing the harmful-substance content of flue gases in a furnace unit

Info

Publication number: IE930736A1
Application number: IE073693A
Authority: IE
Inventors: Dipl-Ing Bernd Wendl; Ing Gerald Koeberl; Dr Heinz Kopp
Original assignee: Veitsch Radex Ag
Priority date: 1992-10-02
Filing date: 1993-10-01
Publication date: 1994-04-06
Also published as: SK104693A3; ITMI932087A0; ITMI932087A1; DE4233119C1; AT399462B; GB2271560B; GB9320054D0; IE66759B1; FR2696472A1; FR2696472B1; GB2271560A; ATA192893A; IT1272713B

Abstract

Method for reducing the harmful-substance content of flue gases in a furnace unit having the following features: ```an oxide, inorganic adsorbing medium in combination with sodium carbonate or sodium hydrogen carbonate as additive is supplied to the furnace, ```the dust-like particles of the adsorbing medium and the additive concentrated with harmful substances (the furnace dust), are removed with the flue gas from the furnace and ```supplied to an at least two-stage filter for the separation of the furnace dust into different grain fractions, ```the coarser fraction of the furnace dust separated in the first filter stage is again supplied at least in part in the circulation to the furnace, while ```the fine fraction of the furnace dust separated in the second filter stage and concentrated with harmful substances is removed from the filter and disposed of. The adsorbing medium may be MgO and/or CaO. The method is effective in removing SO* subscript minus *, HCl, NO x , HF and organic acids from the flue gas

Description

T-he invention relates to a method for reducing the harmful-substance content of flue gases in a furnace unit with the use of an inorganic adsorbing medium in combination with sodium carbonate.

P6329.IE „930/36 _1~ APPLICATION No........................

METHOD FOR REDUCING THE HARMFUL-SUBSTANCE CONTENT OF FLUE GASES IN A FURNACE UNIT The invention relates to a method for reducing the harmful-substance content of flue gases, as are usuallyreleased in a furnace unit in the course of a pyroprocess.

According to the type and quantity of the treated material and the fuel or gas used and the burning conditions, harmful substances contained in the material to be burnt, the fuel etc., like S02, HC1, N0x, HF or organic acids, are released and removed with the flue gases.

Numerous suggestions for reducing the harmful substances in the flue gases have been made in past decades. A substantial part of them relates to desulphurizing methods which operate predominantly in a damp medium. Thus, wet methods based on calcium are known, where with the use of burnt lime or limestone as adsorbing medium the gaseous harmful substances are washed out of the flue gas and are chemically bound. Calcium sulphate is produced as end product, it being partially dumped, partially used by the construction industry.

Such desulphurizing methods are used above all in power stations and large combustion installations. They require a very high expenditure in terms of apparatus and process technology and in part react extremely sensitively to fluctuations in the composition of the flue gas.

While the flue-gas cleaning for large installations of this kind is essentially technically solved, as before there is an urgent need to have a technique available for the flue-gas cleaning of small installations, which also operates reliably with greatly fluctuating emissions, requires low investment costs, enables a simcli crocess -2technique and can be easily integrated into existing installations.

In this connection, the invention has recognized that wet methods remove, and dry additive methods have, substantial advantages. In fact, the invention suggests in its most general embodiment a method for reducing the harmful-substance content of flue gases in a furnace unit having the following features: there is supplied to the furnace an oxide, inorganic adsorbing medium in combination with sodium carbonate (Na2CO3) or sodium hydrogen carbonate as additive, the dust-like particles of the adsorbing medium and of the additive concentrated with harmful substances (the furnace dust) are removed with the flue gas from the furnace and supplied to an at least two-stage filter for the separation of the furnace dust into different grain fractions, the coarser fraction of the furnace dust separated in the first filter stage is supplied again at least partially in the circulation to the furnace, while the fine fraction of the furnace dust separated in the second filter stage and concentrated with harmful substances is removed from the filter and disposed of.

The flue-gas cleaning method operates with a dry adsorbing medium in combination with sodium (hydrogen) carbonate as additive. The harmful substances, in particular gaseous sulphur dioxide and hydrogen chloride, but also HF, N0x or organic acids, are extracted from the flue gas through accumulation on solid sorbents. The dry, contaminated reaction product formed in this way is separated by means of a filter.

In this respect, in addition to the use cf the combined sorbent, the arrangement of a two-stage filter 7 ο β -3is essential, in which the furnace dust is divided into different grain fractions (a coarser one and a finer one). It has namely been found that in particular with a fluctuating flue-gas characteristic property a secure binding of the harmful substances into the sorbents essentially depends on the following measures: The harmful substances can accumulate in particular on the finer particles of the furnace dust in a larger concentration (among other things due to a specifically higher surface).

The adsorbing action is also substantially dependent on the period of dwell of the adsorbing medium/additive in the furnace.

In this sense the separating of the filter for dividing off a coarser fraction from a finer fraction and the at least partial return of the coarser fraction into the furnace favours the efficiency attainable with the method described twice over.

A particular advantage also consists in that the furnace dust which is already contaminated and returned from the first filter stage into the furnace can be supplemented by dusts of similar composition ( for example from parallel furnace installations). In this respect, the adsorbing medium can itself consist at least in part of a material which is already contaminated.

An embodiment of the method provides to replace the oxide adsorbing medium up to 50% by mass with appropriate carbonate material.

With the flue-gas cleaning of a rotating cylindrical furnace for the manufacture of sintered magnesite, the adsorbing medium could consist, for example, of dust-like caustic MgO, which is replaced by raw magnesite up to 50% by mass. The different grain sizes (caustic MgO- < 500 pm, raw magnesite < 6 mm) in this respect at the same time promote an in-situ pelletizing or formation o: granulated material in the rotating cylindrical furnace so that the caustic MgO can be removed from the furnace -4as clinker to a considerable extent. For such an application, which can analogously also be transferred to lime or dolomite furnaces, the particular advantage of a combination of a sintering process and a flue-gas cleaning is thus given.

The additive used (sodium carbonate or sodium bicarbonate) is in this respect added in accordance with an embodiment in a quantity of 0.5 to 2.0 % by mass in relation to the solids delivered to the furnace. As a rule a mass portion of 1.0 % will suffice. The additive can be used in a grain fraction < 100 pm.

Dependent on the local conditions, the adsorbing medium can also be delivered to the furnace in the form of finer dusts (< 200 pm).

It has been found to be advantageous to select the furnace temperature to be so high that compared with normal operation an increased melting phase portion forms in the sinter material. This applies in particular to the named applications with the manufacture of sintered magnesite, sintered dolomite or suchlike, because in this way the pelletizing/formation of granulated material is promoted.

The separation grain size in the multistage filter is likewise selected in dependence upon the local prevailing conditions. In any case it is lower than the upper grain limit of the delivered adsorbing medium and amounts, for example, to 150 pm or less.

The portion of the adsorbing medium returned from the first filter stage into the furnace is set such that the solid particles in each case have such an adequately long period of dwell in the furnace that the desired degree of decontamination of the flue gas is achieved. In this respect, through repeated circulatory guiding of the coarser dust fraction and thus an increased period of dwell of the adsorbing material in the furnace, a: increasing optimization of the flue-gas cleaning aa. achieved. 93Q736 -5At the same time it is to be ensured that the quantity of supplied additive is adjusted such that the sinter material removed from the furnace has as low a content of alkalis as possible, which in the event of the manufacture of sintered magnesite should not exceed 0.2 % by mass.

Even if the chemical reaction processes with the use of the method described have not yet been definitively clarified, tests have shown that with the use of caustic magnesite as adsorbing medium and sodium hydrogen carbonate as additive the following chemical reactions occur: with the adsorbing medium caustic MgO: S02 + MgO = MgS03 MgSO3 + ^02 = MgSO4 HC1 + MgO = MgCl2 + H20 with the additive sodium bicarbonate: S02 + ^02 + 2NaHCO3 = Na2SO4 + H20 + 2CO2 HC1 + NaHCOj = NaCl + H20 + C02.

The excellent combinatory effect is based among other things also on the high basicity of the additive promoting the absorption, and the specific surface properties of the adsorbing medium used.

The method described can be realized without great constructional expenditure. It is suitable in particular for small installations, like sinter furnaces, refuse combustion installations etc. . The operational costs are low. In particular when the adsorbing medium at the same time serves the manufacture of a burnt end product, the costs for this are negligible. However, above all the method can also be used with greatly fluctuating emissions in the flue gas, whereby it presents itself to Ο73β -6measure the harmful-substance concentrations in the flue gas continuously and - in dependence on the values determined in this way - to regulate the addition quantity of the adsorbing medium/additive and/or the quantity of the returned dust from the first filter stage.

The method can be used both with discontinuous and with continuous operating conditions.

Tests have shown that emission values with S02 of 1,320 to 1,830 mg/Nm3 flue gas with peaks to over 6,000 mg/Nm3 can be reduced with the method in accordance with the invention to values under 250 mg/Nm3. With HC1, values of about 200 mg/Nm3 (with peaks to 1,700 mg/Nm3) could be reduced long-term to values below 30 mg/Nm3.

These indications refer to a furnace for burning sintered magnesite with the use of caustic MgO as adsorbing medium, which was replaced to 20 % by mass with raw magnesite (< 6 mm) and the addition of 1.0 % by mass (in relation to the remaining solids) of finely-grained sodium bicarbonate.

Claims

1. Method for reducing the harmful-substance content of flue gases in a furnace unit having the following features: 1.1 an oxide, inorganic adsorbing medium in combination with sodium carbonate or sodium hydrogen carbonate as additive is supplied to the furnace, 1.2 the dust-like particles of the adsorbing medium and the additive concentrated with harmful substances (the furnace dust), are removed with the flue gas from the furnace and 1.3 supplied to an at least two-stage filter for the separation of the furnace dust into different grain fractions, 1.4 the coarser fraction of the furnace dust separated in the first filter stage is again supplied at least in part in the circulation to the furnace, while 1.5 the fine fraction of the furnace dust separated in the second filter stage and concentrated with harmful substances is removed from the filter and disposed of2. Method according to claim 1 with the condition that MgO and/or CaO is (are) used as oxide adsorbing medium. 3. Method according to claim 1 or 2 with the condition that the adsorbing medium is used in a grain fraction < 500 pm. 4. Method according to claim 3 with the condition that the adsorbing medium is used in a grain fraction 200 pm. 93Q736 J -85. Method according to one of claims 1 to 4 with the condition that the portion of the sodium (hydrogen) carbonate, in relation to the total mass of the solids delivered to the furnace, amounts to 0.5 to

2.0 % by mass.

3. 6. Method according to one of claims 1 to 5 with the condition that the sodium (hydrogen) carbonate is used in a grain fraction < 100 pm.

4. 7. Method according to one of claims 1 to 6 with the condition that the oxide adsorbing medium is replaced up to 50% by mass with an appropriate carbonate material.

5. 8. Method according to claim 7 with the condition that the carbonate material is used in a grain fraction < 6 mm.

6. 9. Method according to one of claims 1 to 8 with the condition that the furnace temperature is selected to be so high that compared with normal operation an increased melting phase portion forms in the treated material.

7. 10. Method according to one of claims 1 to 9 with the condition that the separation grain size between the first and second filter stage is set to a value which is smaller than the upper grain limit of the newly delivered adsorbing medium.

8. 11. Method according to claim 10 with the condition that the separation grain size between the first and second filter stage is set at 150 pm or smaller.

9. 12. Method according to one of claims 1 to il with the condition that in each case so much furnace dust is returned from the first filter stage into the furnace -9that the solid particles in each case have such a sufficiently long period of dwell in the furnace until the desired degree of decontamination of the flue gas is achieved.

10. 13. Method according to one of claims 1 to 12 with the condition that in each case so much furnace dust is returned from the first filter stage into the furnace and the quantity of the returned sodium (hydrogen) carbonate is set such that the sintered material removed from the furnace has a maximum content of alkalis of 0.2 % by mass.

11. 14. Method according to one of claims 1 to 13 with the condition that the finely-grained, contaminated, dust-like material removed from the second filter stage is dumped.

12. 15. A method for reducing the harmful substance content of flue gases in a furnace unit according to Claim 1 substantially as herein described. MACLACHLAN & DONALDSON,