Method for removing mercury from flue gases of incineration plants

Abstract

The invention relates to a process for removing mercury from flue gases from combustion plants, wherein the process comprises providing an adsorbent based on carbon, producing an aqueous suspension comprising the adsorbent, introducing the suspension into the flue gas stream from the combustion plants into the dry gas phase of the flue gas which is undersaturated with water vapor and loading the adsorbent with mercury over a predetermined reaction path, keeping the mercury-laden adsorbent out of the flue gas stream and landfilling or regenerating the mercury-laden adsorbent.

Claims

1. A process for removing mercury from flue gas from a combustion plant, comprising: a) providing an adsorbent selected from a group consisting of activated carbon, activated coke, molecular sieves, carbon molecular sieves and mixtures of the abovementioned adsorbents; b) producing an aqueous suspension comprising the adsorbent; c) introducing the suspension into a flue gas stream of the combustion plant of the flue gas and loading the adsorbent with mercury over a predetermined reaction path; d) keeping the adsorbent loaded with mercury out of the flue gas stream by one or more removal methods selected from a group consisting of a dry offgas purification and a wet offgas purification; and e) land filling or regenerating the adsorbent loaded with mercury, wherein the process comprises a wet flue gas desulfurization, wherein a carbon-enriched adsorbent which is based on brown coal and has been activated in an absence of oxygen is used as the adsorbent according to process step a), in that the introduction of the suspension as per process step c) into a dry, water vapor-undersaturated gas phase of the flue gas is effected upstream, in a flow direction of the flue gas, of the flue gas desulfurization, at a temperature of the flue gas in a range from 100 C. to 220 C.

2. The process as claimed in claim 1, wherein the adsorbent is wet milled for producing the suspension in process step b).

3. The process as claimed in claim 2, wherein the milling of the adsorbent is carried out using a ball mill.

4. The process as claimed in claim 3, wherein the ball mill is a stirred ball mill.

5. The process as claimed in claim 2, wherein the adsorbent is wet milled to an average particle diameter in a range from 10 to 20 m.

6. The process as claimed in claim 1, wherein the introduction of the suspension in process step c) is effected by injection into a flue gas channel of the combustion plant, using spray nozzles distributed over a cross section of the flue gas channel.

7. The process as claimed in claim 6, wherein single-fluid nozzles are employed as spray nozzles.

8. The process as claimed in claim 1, wherein the suspension comprises from 3% to 30% by weight of solid.

9. The process as claimed in claim 8, wherein the suspension comprises from 5% to 15% by weight of solid.

10. The process as claimed in claim 1, wherein the process comprises dry keeping-out of the mercury-laden adsorbent as per process step d) using at least one filter device selected from a group consisting of electrofilters, centrifugal precipitators, woven fabric filters, tube filters, filter cloths, bag filters and candle filters.

11. The process as claimed in claim 1, wherein halides selected from a group consisting of iodides, chlorides, sodium bromide, calcium bromide and hydrogen bromide are added to the suspension before process step c).

12. The process as claimed in claim 1, as process for removing mercury from flue gas from coal-fired power stations having a wet flue gas desulfurization plant.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be illustrated below with the aid of a working example shown in the drawing.

(2) FIG. 1 is a process flow diagram roughly schematically indicating the process procedure.

DETAILED DESCRIPTION

(3) A combustion plant, for example in the form of a steam generator of a coal-fired power station or in the form of a waste incineration plant is denoted by 1. A denitrification apparatus 2 is arranged downstream of the combustion plant 1. The denitrification apparatus 2 is optional.

(4) In the combustion plant 1, fossil or organic fuels are reacted thermally with production of flue gas which is, for example, subjected to catalytic reduction of the nitrogen content in the denitrification apparatus 2.

(5) After denitrification in the denitrification apparatus 2, the flue gas is conveyed to an electrofilter 3. In the electrofilter 3, solid particles entrained in the flue gas are precipitated as fly ash. Instead of an electrofilter, it is possible to provide, as mentioned at the outset, other precipitation apparatuses or dust removal apparatuses.

(6) Downstream of the electrofilter 3 there is a flue gas desulfurization plant 4 (FDP) which is configured as a conventional wet scrub. The flue gas desulfurization plant 4 comprises at least one scrubbing tower having a top and a bottom. In the scrubbing tower, a scrubbing suspension is injected and converted into downward-traveling droplets in the region of the top. The flue gas is fed into the scrubbing tower directly above the bottom. Within the scrubbing tower, the flue gas is conveyed in countercurrent to the scrubbing suspension raining down. The flue gas leaves the scrubbing tower at its top, with the customary droplet precipitation internals and stirring devices also being provided within the scrubbing tower.

(7) A cyclone precipitator 5 is installed downstream of the flue gas desulfurization plant 4. The scrubbing suspension (FDP suspension) is conveyed from the bottom of the flue gas desulfurization plant 4 to the cyclone precipitator 5. The cyclone precipitator 5 can comprise one or more hydrocyclones, the underflow of which contains the relatively coarse gypsum crystals in an increased concentration and the overflow of which contains finer particles, for example adsorbent particles.

(8) The process of the invention comprises provision of open-hearth furnace coke which is milled in a wet milling apparatus 6 to give a fluid suspension. The fluid suspension comprises water and open-hearth furnace coke, with, for example, the open-hearth furnace coke being able to make up about 10% by weight of the fluid suspension.

(9) The open-hearth furnace coke suspension (optionally also activated carbon suspension) is then injected by means of nozzle stems (not depicted) into a flue gas channel 7, which is shown only schematically, of the combustion plant 1, within a temperature window of the flue gas from 100 C. to 220 C.

(10) The accompanying flow diagram depicts two variants of the introduction of the water/adsorbent suspension into the flue gas channel 7; in variant A, the introduction into the flue gas channel 7 is carried out directly upstream of the dust removal apparatus or upstream of the electrofilter 3, and according to variant B, the introduction of the water/adsorbent suspension is carried out directly downstream of the dust removal apparatus or the electrofilter 3 and upstream of the flue gas desulfurization plant 4. In this context, directly means that a predetermined reaction path is provided between the point of introduction into the flue gas channel 7 and the electrofilter 3, allowing sufficient loading of the adsorbent.

(11) In variant A, the mercury-laden adsorbent is kept out or precipitated in the electrofilter 3 and fed together with the likewise deposited fly ash to an ash moistener 8. In this way, the loaded adsorbent is incorporated into the fly ash and taken off from the ash moistener 8 as mercury-rich solid which can be disposed of in a landfill.

(12) In variant B of the process, the mercury-laden adsorbent goes together with the flue gas into the flue gas desulfurization plant 4 and is taken up in the scrubbing suspension (FDP suspension) there. The scrubbing suspension, which comprises both the open-hearth furnace coke as adsorbent and also gypsum, is then conveyed from the bottom of the flue gas desulfurization plant 4 to the cyclone precipitator 5. The underflow from the cyclone precipitator 5 is enriched in the gypsum crystals, and the overflow contains the finer adsorbent particles which are loaded with mercury. The mercury-laden open-hearth furnace coke suspension can be subjected to a chemical after-treatment, in which the mercury bound to the adsorbent can then be precipitated. As an alternative, the wastewater comprising the open-hearth furnace coke suspension can be supplied to the ash moistener 8, so that the adsorbent is incorporated into the fly ash and is likewise discharged together with the fly ash as mercury-rich solid which can be disposed of in a landfill.

LIST OF REFERENCE NUMERALS

(13) 1 Combustion plant 2 Denitrification apparatus 3 Electrofilter 4 Flue gas desulfurization plant 5 Cyclone precipitator 6 Wet milling apparatus 7 Flue gas channel 8 Ash moistener