SE514547C2 - Removing ammonia from by-products, e.g. fly ash, generated during gas purification process, comprises mixing fluidized byproduct with sodium hydroxide - Google Patents

Abstract

Removing ammonia from by-products comprises fluidizing, stirring and mixing the by-product with aqueous sodium hydroxide. Ammonia is then removed with the fluidization gas. Gas purification involves removing ammonia from fly ash or other by-products generated during the process, by supplying the by-product separated from the gas to a reactor (7) and then fluidizing the by-product, mechanically stirring and adding an aqueous solution of sodium hydroxide (1-30% concentration). Exhaust gas from the reactor, including the fluidization gas and ammonia, are removed from the reactor for further treatment.

Description

25 30 35, 514 _54? 2 nitrogen oxides during combustion, but an efficient combustion of e.g. carbon, can not happen without the formation of nitrogen oxides.

However, in principle it is possible in a gas purification plant to reduce the concentration of nitrogen oxides in the gas without any residual product being formed by transferring the nitrogen oxides to nitrogen and water through a reduction process.

This reduction is usually carried out by means of ammonia which is introduced in an appropriate amount. The reduction can take place catalytically, with very small concentrations of ammonia or non-catalytically with significantly higher concentrations of ammonia. The catalytic reduction presupposes a relatively large investment, while the non-catalytic can be used without major costs in e.g. upgrade.

For natural reasons, the emissions of ammonia, the so-called ammonia grinder, to be larger at non-catalytic reduction than at catalytic. A large part of this excess ammonia will be bound to any dry residues such as fly ash or spent absorbent, e.g. lime-plaster mixtures. This ammonia content is a significant inconvenience during disposal, but is disturbing already at the first handling of separated residual product. Measures to eliminate the odor created by the ammonia-containing substance or even seek to recover ammonia has already been suggested. An example is found in EP 538 647 where ammonia is driven by slurrying residual product in water, with possible addition of alkali, followed by evaporation. Water vapor and ammonia are then condensed for reuse. The method is thus very energy-intensive.

OBJECT OF THE INVENTION The main object of the present invention is to eliminate or at least very greatly reduce sanitary problems in the handling of residual product, which contains ammonia, e.g. p.g.a. that in a gas purification process a sub-step is non-catalytic reduction of nitrogen oxides. 10 15 20 25 30 35 5.14, 547 3 A second object is to provide an energy-efficient method for evaporating ammonia from a dry powdered residual product containing ammonia.

A third object is to provide a method for being able to re-use ammonia absorbed by a residual product by catalytic or non-catalytic reduction of nitrogen oxides by returning this ammonia to a gas to be released.

SUMMARY OF THE INVENTION The present invention relates to a process for gas purification, for evaporating ammonia from fly ash or other residual product from the gas purification. In the process, the residual product separated from the gas is passed to a reactor.

According to the invention, the residual product is mechanically fluidized and stirred in the reactor. An aqueous solution of sodium hydroxide having a concentration of 1-30% is added.

Exhaust gas from the reactor, including fluidizing gas and ammonia evaporated from the residue, is diverted for further treatment.

GENERAL DESCRIPTION OF THE INVENTION Ammonia supply during flue gas purification leads to ammonia being bound to a greater or lesser degree to solid residual products.

This ammonia causes sanitary inconveniences by the residual product emitting ammonia during handling and at landfill.

It is important that these inconveniences are minimized. So far, the proposed solution includes heating and is therefore very energy-intensive.

The present invention is based on the surprising discovery that an intensive mixing of residual product and an aqueous solution of sodium hydroxide gives as good an effect as a previously used method of heating. Therefore, it is proposed that the residual product separated from the gas be passed to a reactor, where it is fluidized and mechanically stirred. An aqueous solution of sodium hydroxide having a concentration of 1 to 30% is added, after which the exhaust gas from the reactor, comprising the fluidizing gas and ammonia evaporated from the residual product, is discharged either for destruction, for example by combustion, or introduced into a flue gas as reducing agent.

This flue gas can be the one from which the residual product is separated or another flue gas in a nearby plant. The reduction principle can be catalytic or non-catalytic.

The process is primarily intended to take place in a continuous process with a flow of residual product through the reactor. The aqueous solution is added in an amount whose mass flow corresponds to at most 5% of the mass flow of the dry residual product, preferably in an amount whose mass flow corresponds to 2-3% of the mass flow of the dry residual product. The supply of the aqueous solution must not be so large that it changes the character of the residual product as a powdered material.

A reactor suitable for the process is described in EP-794 830 (SE 9404104-3, published as 503 674).

The residual product removed from the reactor can be handled without inconvenience for reuse or disposal.

The process is suitable both for the incineration of fossil fuels and for the incineration of waste, such as household waste, provided that the gas purification generates a dry residual product.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail in connection with the accompanying drawings, in which Fig. 1 schematically shows a first plant for using embodiments according to the invention in the combustion of household waste and purification of the flue gas formed; Fig. 2 schematically shows a second example of the use of an embodiment according to the invention in the combustion of household waste and the purification of the flue gas formed; Fig. 3 shows a principal embodiment of a reactor suitable for use in the process according to the invention.

DESCRIPTION OF THE PROPOSED EMBODIMENT Fig. 1 shows a plant 1 for incineration of household waste and purification of the flue gas formed.

The plant 1 comprises a fireplace 2 with a supply line for fuel 12 and and another supply line 11 for combustion air. Adjacent to the fireplace 2 is a cooling heat exchanger 3, a reactor 4, a gas mixer 5, a hose filter 6 and a reactor 7 and a chimney (not shown).

With a line 13, ammonia is introduced into the flue gas between the fireplace 2 and the heat exchanger 3. Dry finely divided hydrate lime is fed to the reactor 4 through a line 14 and mixed with the flue gas. Through a duct 15 cold air is fed to the mixer 5 where it is mixed with the flue gas.

Residual product separated in the hose filter 6 is fed via a line 16 to the reactor 7. A water solution of sodium hydroxide is also fed to the reactor 7 via a second line 17 and fluidizing air via a third line 19. The fluidizing air, together with evaporated gases, is diverted via a fourth line. 21 and mixed with the flue gas between the fireplace 2 and the heat exchanger 3. The residual product treated in the reactor 7 is taken out for disposal via a fifth line 18.

Fig. 2 shows a corresponding plant which differs from that shown in Fig. 1 only in that a line 20 for gases expelled in the reactor 7 connects the reactor 7 with the fireplace 2. Fig. 3 shows more in detail an embodiment of a reactor 7 suitable for the process according to the invention. The reactor 7 is divided into two chambers 7a and 7b with an intermediate perforated bottom 71. In the upper chamber 7a there is a mechanical stirrer 72 consisting of a rotatable shaft 73 and elliptical plates 74 mounted thereon, which are inclined about their minor axis so that their horizontal projection consists of circles with the axis 73 in the center. In other respects, the references correspond to those which apply to Fig. 1. For a more detailed description, reference is made to EP-794 830 (SE 9404104-3, published as 503 674).

The function of the invention is as follows.

In the fireplace 2, waste introduced via line 12 is combusted with combustion air introduced via the second line 11. In the fireplace, the formed flue gas is cooled by radiation and convection to a temperature of approximately 800 ° C. At this temperature, ammonia is injected via line 13 and the flue gas is passed on to the heat exchanger 3. The heat exchanger 3 can be connected to the fireplace 2. The essential thing is that an injection point for ammonia in the temperature range 650 ° C to 850 ° C can be arranged. to a temperature of about 140 ° C.

In the heat exchanger 3, the flue gas is cooled. During cooling, ammonia and nitrogen oxides react to form pure nitrogen and water.

From the heat exchanger 3 the flue gas is led to the reactor 4 where finely divided dry lime is introduced, through the line 14, and mixed with the flue gas. The flue gas with the introduced lime is led to the mixer 5 where cold air is introduced, mixed with the flue gas. The gas mixture with the atomized where the lime with absorbed impurities and formed residual products via the channel 15, and the lime then flows through the hose filter 6, is separated, to the chimney (not shown).

The residues formed are passed from the hose filter 6 via the line 16 to the reactor 7 where they are mixed with an aqueous solution containing sodium hydroxide supplied via a line 17 and fluidized, with air supplied via the line 19 to the lower chamber 7b, from where it passes through the 51.4 547 7 intermediate perforated bottom 71 flows up into the upper chamber 7a through the residual product, and is stirred with the mixer 72. The fluidizing air together with the gas components, preferably ammonia, which are evaporated during the treatment in the reactor 7 is led either to the fireplace 2 (Fig. 2) between the fireplace 2 and the heat exchanger 3 injected or mixed with the flue gas in the same way as the ammonia via the line 13 (Fig. 1). The treated, substantially odorless, residual product is taken out through a line 18 for disposal.

ALTERNATIVE EMBODIMENTS The method according to the invention is of course not limited to the above-mentioned embodiment but can be varied in a number of ways within the scope of the appended claims.

Claims (8)

10 15 20 25 30 35 ._ _ n .. m: e «. . .Y-. _ s _ f: f. '. . ._. . . e. 1st »<1 I. _ 1. » .. f f: \: c <^ = _. . t _. . . «<. c. . . ~ <1. PATENT REQUIREMENTS A process for gas purification, for expelling ammonia from fly ash or other residual product from the gas purification, wherein the residual product separated from the gas is passed to a reactor, characterized in that the residual product, in the reactor, is fluidized and stirred mechanically, that an aqueous solution of sodium hydroxide with a concentration of 1-30% is added and the exhaust gas from the reactor, including fluidizing gas and ammonia evaporated from the residual product, is diverted for further treatment. Process according to claim 1, wherein the reactor operates substantially continuously with a flow of residual product, characterized in that the aqueous solution is added in an amount whose mass flow corresponds to at most 5% of the mass flow of the dry residual product, preferably in an amount whose mass flow corresponds to 2-3 % of the dry mass of the dry residue. Process according to Claim 1 or 2, when evaporating ammonia from fly ash or other residual product from flue gas purification, characterized in that the exhaust gas from the reactor is thermally destroyed in the combustion zone in which the flue gas is generated or in another suitable fireplace. A method according to claim 1 or 2, in the evaporation of ammonia from fly ash or other residual product from flue gas purification, in which flue gas purification ammonia is introduced into 514.54? The flue gas in order to non-catalytically reduce nitrogen oxides in the flue gas is characterized in that the exhaust gas from the reactor is mixed with the flue gas flow at one or more points where the flue gas temperature is suitable for the purpose of non-catalytically reducing nitrogen oxides in the flue gas. Process according to Claim 1 or 2, in the case of stripping ammonia from fly ash or other residual product from flue gas purification, in which ammonia is introduced into the flue gas for the purpose of non-catalytically reducing nitrogen oxides in the flue gas, characterized in that the exhaust gas from the reactor is mixed with another gas stream. with the aim of catalytically or non-catalytically reducing nitrogen oxides in this second gas stream with the ammonia of the exhaust gas A method according to claim 1 or 2, in that the treated, residual product is removed for disposal or feels substantially dry, re-use. A method according to any one of the preceding claims, in purifying flue gas from the combustion of fossil fuels, wherein the acidic components of the flue gas are at least partially removed by mixing finely divided dry absorbent material or finely divided suspension of absorbent material which is dried on contact with the flue gas, after which a dry residue is separated from the flue gas. 10 5.14 547 10 A method according to any one of the preceding claims, in the purification of flue gas from the incineration of waste, such as household waste, wherein the acidic components of the flue gas are at least partially removed by mixing finely divided dry absorbent material or finely divided suspension of absorbent material which is dried on contact with the flue gas. residual product is separated from the flue gas.