EP3137193A1

EP3137193A1 - Method for removal of mercury from flue gases

Info

Publication number: EP3137193A1
Application number: EP14715829.9A
Authority: EP
Inventors: Bernhard W. Vosteen
Original assignee: Vosteen Consulting GmbH
Current assignee: Vosteen Consulting GmbH
Priority date: 2014-03-24
Filing date: 2014-03-24
Publication date: 2017-03-08
Also published as: WO2015144187A1

Abstract

The present invention concerns an improved method for the effective removal of mercury from flue gases resulting from combustion processes, such as industrial coal combustion for power generation in power plants or waste incineration in waste incinerators or in waste-to-energy plants, but also from other thermal processes, such as burning of limestone to form lime or burning of cement to form clinker, treatment of ores, recycled metals and the like.

Description

METHOD FOR REMOVAL OF MERCURY FROM FLUE GASES

BACKGROUND OF THE INVENTION

Combustion of fossil fuels as coal or combustion of wastes or thermal treatment of any organic and inorganic mercury containing materials heated by co-combustion of fuels will result in volatilization of the mercury, forming at elevated temperatures gaseous elemental mercury Hg(0), e.g. in the firebox or in a high-temperature thermal treatment process stage. During cooling of the combustion gases in process sections downstream - as e.g. a gas cooler (air preheater) or as a off-heat boiler - the elemental mercury Hg(0) can be halogenated, forming gaseous oxidized mercury Hg_ox (e.g. mercuric chloride HgCb) e.g. during boiler passage; particle bound mercury can also be formed, especially if carbonaceous particles are contained in the flue gas. Owing to the high toxicity of mercury species in any form, strict limiting values exist for the legally permissible emission of mercury. Examples of such regulatory limitations include the Industrial Emission Directive 2010/75/EU on integrated pollution prevention and control" in Europe , or the Mercury and Air Toxics Standards (MATS) in the USA. Elemental and oxidized mercury are both toxic, but even more organically bound mercury (as mono- or dimethyl mercury), formed from emitted mercury species in fish and wild life, are becoming extremely toxic for humans, if absorbed directly or indirectly via the food chain. Due to the toxicity of mercury and the strict mercury emission limitations, it is important to provide for effective methods to mitigate mercury emissions from the flue gas of such industrial processes. Various methods exist. Which of the methods is expedient for a particular application depends greatly on the feedstock qualities as the mercury content, chlorine and bromine content, sulfur content, but also on the moisture and ash content of the combustibles, further on their combustion behaviors, characterized their volatility and fixed carbon content respectively. One particularly effective method is disclosed in EP 1 386 655 Bl and involves the addition of bromine or bromine compounds either to the combustibles as coal or wastes to be incinerated or to the furnace or to the flue gas in a high-temperature section downstream of the furnace at temperatures of at least 500°C, where the additive is effective for mercury bromination. EP 1 386 655 B l also discloses iodine or iodine compounds as effective for mercury oxidation, as well. As disclosed in EP 1 386 655 B l in detail, oxidized mercury species as mercuric bromide are removed in dry and wet air pollution control systems. In contrast to elemental mercury, the oxidized mercury is water soluble, enhancing wet mercury capture, but also is far better adsorbable at carbonaceous or other inorganic sorbents like e.g. activated carbon or ammended clays and the like, thus enhancing also dry mercury capture.

This method may under circumstances require the use of enlarged amounts of halogens or halogen compounds as e.g. bromine or bromine compound to achieve sufficient mercury oxidation. One example for such enlarged halogen need is the combustion of moisture-rich coals, such as raw brown coal (e.g. German lignite with about 50 % moisture content) or of sludges, such as mechanically dewatered communal or industrial sewage sludge.

Often, both dry and wet air pollution control steps are applied in series, e.g. applying electrostatic precipitators (ESP) or fabric filters (FF) as particulate scrubbers, followed by e.g. a limestone-based wet FDG (flue gas desulfurization) system. Halogen-based mercury oxidation leads to enhanced mercury adsorption at the e.g. coal-born residual carbon (unburnt carbon, UBC) in the fly ash and thus enlarges the mercury content in the fly ashes as precipitated from the flue gases in e.g. electrostatic precipitators (ESP) or fabric filters (FF).

In case of ESP or FF only, when no additional wet APC systems are present, such enhanced dry mercury capture by the precipitated fly-ash or by up-stream injected particulate sorbents may actually be intended. However, in case of additional wet APC systems present as e.g. wet Flue Gas Desulfurization systems (wet FGD) downstream of such ESP or FF, enhanced dry mercury capture in an ESP or FF upstream is not wanted, for high mercury contents in fly ash make its further use or disposal more difficult, and should therefore be avoided.

WO 2008/1063 18 Al discloses a process for catalytic oxidation of bromide to bromine in the presence of a cerium-containing compound. However, the addition of such cerium-containing compound takes place separately from any combustion steps. WO 2010/129743 A l discloses methods for decreasing the amount of mercury in a flue gas that contains mercury through the use of a molecular halogen, including the conversion of HBr to Br₂ in the presence of oxygen using a variety of metal oxide catalysts. However, the molecular halogen is formed outside of the industrial process stream and then is injected into the process, as opposed to forming the molecular halogen as part of the process itself, which is described as highly disadvantageous for several reasons.

Accordingly, there exists a need for an improved method to reduce mercury emission resulting from industrial combustion or incineration processes that efficiently reduces the amount of halogen or halogen compounds required for effective mercury oxidation and at the same time leads to less mercury contamination of the fly ash - thus facilitating its disposal or further use (e.g. in concrete). Both needs are satisfied by the present invention described below.

DESCRIPTION OF THE INVENTION

The present invention involves a method for the effective removal of mercury from flue gases resulting from combustion processes or from other high-temperature processes in a plant having a primary and a secondary furnace, working at temperatures above 500 °C, comprising the steps of

a) Adding a least one oxidizing additive agent selected from the group consisting of a molecular halogen, a halogen containing compound or a mixture of various halogen containing compounds, and

b) Adding at least one catalyzing additive agent, said additive agent comprising at least one compound containing at least one metal selected from the group consisting of magnesium, iron, barium, cerium, manganese and vanadium, wherein either step a) or step b) may be applied independently or together in at least one of the following modes:

i) addition to the combustibles prior to their entry into the primary and/or secondary furnace of the plant, ii) addition directly into the primary and/or secondary furnace of the plant, iii) addition to the combustion flue gas with an oxygen content > 0 (typically 3 up to 12 vol.% O2, dry) in a plant section downstream of the furnace(s), the temperature during the contact of said oxidizing agent or said additive agent with the flue gas being at least 500°C.

The oxidizing additive agent added according to step a) may comprise different molecular halogens or corresponding halogen containing compounds, as chlorine compounds, bromine compounds, iodine compounds and mixtures thereof. Of the halogens involved, bromine is preferred for most applications due to its positive balance of effectiveness and cost.

The halogen(s) may be added either as such, i.e. in elemental form (as diatomic molecular halogens) or in the form of one or more halogen containing compounds. Examples of halogen containing compounds, suitable as oxidizing agents if applied the way described, are inorganic halide salts and organic halogen containing compounds. Specific examples of suitable inorganic halide salts include alkali halides such as sodium chloride, sodium bromide, sodium iodide and corresponding earth alkali halides, such as calcium chloride, calcium bromide and calcium iodide, but also ammonium halides or even organic halides. In many applications, calcium bromide is the most preferred choice.

The halogen compounds may be added either as solids (e.g. inorganic salts) or in the form of solutions or emulsions, with water being the preferred solvent. Suitable halogen containing organic compounds as oxidizing agent may be pure compounds, such as alkyl or aryl halides, or mixtures thereof, however, an interesting embodiment is the use of a mixture of various halogen-rich wastes, for example low- or high-halogenated liquid wastes, which are a component of the material to be incinerated or are added to the material to be incinerated, for example special waste residues. In this embodiment, the disposal of the special waste is advantageously combined with the desired reduction of mercury emission from the incineration plant.

The catalyzing additive agent added according to step b) may comprise oxides or salts of the applied metals such as magnesium, iron, nickel, copper, cobalt, zinc, barium, cerium, manganese and vanadium and mixtures thereof. Preferred metals are cerium, manganese and iron.

Suitable cerium compounds as catalyzing additive agent include cerium(lV)-ammonium-nitrate (NH4)2Ce(N0₃)6, cerium(IV)-sulfate Ce(SC>4)2, and cerium(III)-chloride CeCh, in their anhydrous or hydrated forms, but preferably as an aqueous solution.

The preferred specific dosage range of cerium is 0.1 to 50 ppm, calculated as mass ratio of the added Ce per dry mass of the combustible as coal or waste.

Suitable iron compounds as the catalyzing additive agent include iron(Il)-nitrate Fe(N03)2, or iron(III)-nitrate Fe(N0₃)₃, iron(II)-sulfate FeS0₄, or iron(III)-sulfate Fe2(S0₄)3, or iron(II)-chloride FeCh , or iron(HI)-chloride FeCh,, in their anhydrous or hydrated forms, but preferably as an aqueous solution.

The preferred dosage rate of iron is 5 to 100 ppm, calculated as mass ratio of the added Fe per dry mass of the combustible as coal or waste.

The preferred dosage range of manganese is 2 to 100 ppm, calculated as mass ratio of the added Mn per dry mass of the combustible as coal or waste. A preferred embodiment of the catalyzing additive agent is a mixture of cerium (or manganese or barium or vanadium) and iron compounds, said mixture comprising 0.1-30 weight % of cerium (or manganese or barium or vanadium) and 30-99.9 weight % of iron, based on the total metal content. Particularly preferred is a mixture of cerium and iron compounds. In this case, a highly preferred embodiment is a mixture comprising 40-60 weight % of cerium and 60-40 weight % of iron, based on the total metal content. The catalyzing additive agent can e.g. also be the Additive disclosed in DE 100 50 332 A l , where its composition and use is described in detail. In such mixtures, the iron (Fe) and cerium (Ce) compounds can be substituted by other catalytically active compounds as manganese (Mn), magnesium (Mg), barium (Ba), nickel (Ni), vanadium (V), copper (Cu), cobalt (Co), zinc (Zn). The oxidizing and/or catalyzing additive agents may be added - either separately or as a mixture - to the combustibles, e.g. to the coal coming in or to the coal band- feeders upstream of the coal mills, as solids or in form of a solutions or emulsions, with water being the preferred solvent. An addition as solution or emulsion is preferred as this usually leads to a better, i.e. more uniform distribution of the metal respectively halogen containing compounds throughout the combustibles. The oxidizing and/or catalyzing additive agents may also be added - either separately or as a mixture - directly to the furnace of the plant or to the flue gas in a plant section downstream of the furnace, the temperature during the contact of said oxidizing agent or said catalyzing additive agent with the flue gas being at least 500°C.

In a special embodiment, the catalyzing additive agent may also be added in the form of special co- combusted combustibles, such as certain vanadium-rich crude oils, or as certain halogen-rich shredder wastes (as e.g. car shredder waste) which may be added as an auxiliary fuel and at the same time as a carrier of the metal and/or halogen containing compound.

Under most circumstances addition of the additive agent to the combustibles prior to entry into the primary or secondary furnace appears preferred, as this leads to the highest reduction of mercury adsorption onto the fly ash.

It is also contemplated within the scope of the present invention to add different components of the catalyzing additive agent at different locations of the inventive process. For example, when iron compounds make part of the catalyzing additive agent it may prove beneficial to add at least a part of the total amount of iron compounds into a separate part of the combustion chamber or boiler with delayed combustion for improved NO_x reduction, i.e. into a less oxygen-rich stage of the inventive process in order to reduce loss of the active iron compound by oxidation. In a further embodiment of the inventive process, it is also possible to feed the oxidizing additive agent or the catalyzing additive agent as a fine dispersion to the combustion air or if appropriate to a recirculated substream, in particular recirculated flue gas, recirculated bottom ash and/or recirculated fly ash. In yet another embodiment of the inventive process, it is possible to combine the oxidizing additive agent and the catalyzing additive agent into one single additive agent by using the halides of iron, barium, cerium, manganese and vanadium and mixtures thereof. These halides may e.g. be obtained by reacting both additive compounds with each other, forming halides of the named metals. Also mixtures of such halides with corresponding sulfates, nitrates and carboxylates can be applied.

Surprisingly, the combination of the two process steps according to the present invention leads to

- a better combustion in the sense of improved burn-out of the combustibles (less CO in the flue gas, less unburnt carbon in the fly ash),

- a better efficiency in the halogen use, i.e. a better mercury removal despite reduced halogen input, and also to

- less mercury contamination of the fly ash - thus facilitating its disposal or further use.

- lower combustion temperatures, thus inducing less NO_x formation during

combustion.

In order to achieve mercury oxidation as complete as possible, in particular up to 100%, by adding a bromine compound - applying only step a) from above, according to EP 1 386 655 B l) -, the bromine compound is preferably added in a mass ratio of bromine to mercury in the range from 10² to 10⁴.

In order to achieve mercury oxidation as complete as possible, in particular up to 100%, according to the present invention by adding both an oxidizing additive agent (e.g. a bromine compound) and the described catalyzing additive agent - applying both step a) and/or b) from above) -, the bromine compound is preferably added in a mass ratio of bromine to mercury in the range from 10¹ to 3 x 10³.

In another embodiment the preferred specific dosage range of bromine or iodine is 5 to 2000 ppm, calculated as mass ratio of the added Br or I per dry mass of the combustible as coal or waste. In other embodiments of the present invention the specific dosage rate of the oxidizing additive agent and likewise the catalyzing additive agent or of corresponding mixtures is in the range of 1 to 3000 ppm per dry mass of the combustibles as coal or waste, calculated as mass ratio of the additives.

If the bromine compound is added in excess, this does not have a disadvantageous effect on the inventive process. Too great an excess must be avoided, however, not least for reasons of cost and also for reasons of environmental protection. If appropriate, free halogens formed during e.g. boiler passage as e.g. free bromine, the residual free halogen exiting the boiler must afterwards be transformed into a more stable halide (e.g. a bromide); this transformation can be achieved by adding also a sulfur compound (as elemental sulfur or sulfuric acid) already to the combustibles or directly into the furnaces of the plant, so that the resulting SO₂ level is maintained > zero in the exiting boiler gas respectively in the flue gas at the inlet of the APC system, where the SO₂ is reducing any free halogen (e.g. B ) or hypo-halogenide back to its more stable halide species (e.g. HBr or a corresponding bromide salt as NaCl); this makes sense, since halogen emissions are generally subject to legally established limiting values, as well. An alternative to the addition of a sulfur compound to the combustible might be the addition of hydrogen peroxide (H2O2), e.g. to the flue gas upstream of the wet FGD, as a reduction agent for instable hypo-halogenides, i.e. reducing e.g. NaOBr back to NaBr according to NaOBr + H₂0₂ -> NaBr+ H₂0 + 0₂.

According to the inventive process, the cleanup of flue gases from the combustion or similar high- temperature processes with addition of a bromine compound, results in effective capture of the oxidized mercury (so-called "ionic mercury"), for the water-soluble and well adsorbable oxidized mercury is removed from the flue gas thoroughly and effectively both in dry and wet APC systems.

Various flue gas cleanup processes are known from the prior art for removing, inter alia, oxidized mercury. They are based either on wet cleanup (as wet scrubbing) or dry cleanup (as dry scrubbing) or a combination of the two in a multistage-design of the APC system. Wet scrubbing comprises, for example, an acid and or alkaline scrubbing stage of different design, for example, in a quench sprayed with re-circulated scrubbing water containing chemical sorbents as sodium hydroxide or similar additives to achieve flue gas desulfurization, applying either pressurized nozzle lances or rotary atomizer spraying system, or in a packed-bed scrubber, sprayed with re-circulated acidic or alkaline scrubber solutions. Scrubbing can also be carried out, if appropriate, under weakly acidic or weakly alkaline, i.e. nearly neutral conditions only, for example in the case of low hydrogen chloride loads and low sulphur loads of the combustibles, or under addition of adequate neutralization agents as sodium hydroxide to improve SO2 capture and to avoid fouling.

In a preferred embodiment, the flue gas cleanup comprises multistage wet flue gas scrubbing having at least one strongly acid (pH less than 1) and/or at least one weakly acid and/or at least one alkaline wet scrubbing stage.

The flue gas cleanup can also comprise a dry emission control step based on the predominant adsorption of ionic mercury compounds; also elemental mercury may be captured, but only when getting chemically reacted at the sorbent surface, as e.g. unburnt carbon. Such cleanup can be carried out, for example, by semi-dry desulfurization in a spray-dryer-adsorber which is impinged with a milk of lime/carbon suspension, or using fixed-bed adsorbers, for example based on granulated activated carbon or hearth oven coke or "designed mixtures" of such adsorbents with granular or powdery lime and lime-stone , or using granular adsorbers in case of fixed beds and powdery adsorbers, for example in case of electrostatic precipitators (ESPs) or of fabric filters, which are impinged pneumatically with a blown-in finely pulverulent slaked lime/activated carbon or slaked lime/hearth oven coke mixture. Zeolites are also suitable for removing mercury compounds. With respect to dry flue gas emission control, a further advantage is exhibited of the inventive process. The use of the process is of interest in particular for those high-temperature plants which do not have a wet flue gas emission control system, but solely have a dry emission control system as a mercury adsorption stage. Mercury bromide HgBr2 adsorbs directly to dry sorbents, however elemental mercury Hg(0) adsorbs only weakly on the fly ash of e.g. ESPs or FFs.

In a preferred embodiment of the invention the dry flue gas emission control system therefore comprises at least one dry or semi-dry adsorption-based emission control stage, in particular using electrostatic or filtering dust separators.

In case of lime- or limestone-based wet flue gas emission control systems applying Ca(OH)2) or CaC03 as scrubber additive), the gypsum (calciumsulfate-dihydrate CaS04 x 2 H20)formed in the scrubber suspension often is a valuable byproduct to be used in the cement and plaster industries, but only if the mercury load of this byproducts stays well below 1 ppm. Halogen-based mercury oxidation will lead to increased mercury absorption into such wet FGD slurry, and this might induce an intolerable heightening of the mercury content in the suspended gypsum, which is afterwards separated and dewatered. To avoid such intolerable mercury enrichment of the gypsum by-product, and also to minimize mercury re-emissions from the scrubber, the once dissolved oxidized mercury should immediately be adsorbed on suspended activated carbon (AC) in the wet FGD slurry, thus binding the oxidized mercury and prohibiting reductive reactions. Less than 200 mg/1 activated carbon in the wet FGD slurry can be sufficient to achieve both effects.

The gypsum as the desired by-product can be separated from the wet FGD blow down. However, standard hydrocyclones cannot separate the mercury loaden AC particles sufficiently out of the hydrocyclone underflow (containing the gypsum). Therefore, a modified hydrocyclone - e.g. the Andritz hydrocyclone, „washing" the underflow with clean water to eliminate fines from the underflow - should be applied. Such modified hydrocyclone was developed by Andritz, Graz (poster Edinburgh 2013); the mercury-rich AC can then be separated from the overflow. This overflow can be treated in a waste water treatment stage (WWTP), where two sludge fractions can be produced. The one sludge fraction containing a low concentration of mercury can be pumped back to the coal feeder. The other sludge fraction, which is a mercury-enriched fraction of fines, containing most of the suspended AC must be disposed of as hazardous waste (approx. 5% of the total sludge amount from the WWTP).

The following examples are intended to illustrate the present invention, they are not, however, limiting in any regard.

EXAMPLE 1 Assumed is the following situation at a pulverized coal (PC)-fired industrial boiler of 100 MWtherm capacity and a flue gas volume flow rate of 140,000 Nm³/h dry. The front-fired boiler is attaining combustion temperatures in the range of 1050 to 1350 °C when fired with 15 t/h coal of about 1 1 % moisture; the coal is a mixture of 50 % Columbian bituminous coal and 50 % German bituminous coal. The boiler produces 120 tons/hour of superheated steam at 500 °C, 300 bar. The fired coals are low sulfur coals with less 1 weight-% sulfur per dry matter, see coal analysis in Table 1. Table 1

The mercury content of the German coal (mined at Bergwerk West) is quite high (on average 0.243 mg/kg dry), while the mercury content in the Columbian coals (mined at Cerrejon de Central and at El Hatibo) is about 0.1 mg/kg dry only. The boiler is assumed to be a front-fired boiler with low-NOx- burners, to limit NOx production (staged combustion). This operational mode applying locally near to the burner low oxygen-concentrations leads to elevated contents of unbumt carbon (UBC) in fly ash (typically 7 - 12 weight-%). The plant is also assumed to comprise an off-heat-boiler, air preheater, cold-side ESP, limestone based wet FGD, fed with fresh water and limestone, forming lime at pH 5.3 in the recirculated slurry, sprayed by nozzles, and stack.

Without addition of any additives, especially without such additives, as covered by this invention, the unburnt carbon (UBC) content in the ESP-fly ash is 7 weight-%, which allows for 25 % capture rate of the total coal mercury input by the ESP-fly ash, while 55% of the total coal mercury input are captured in the wet FGD. The mercury content of the emitted stack gas is 8 μ/Nm³ dry at 6 vol.-% O2, corresponding to an emission of 30 % of the total coal mercury input via the stack.

With a bromine addition rate of 0.85 liter/h CaB^-solution (52 weight %) as oxidizing additive agent, corresponding to 45 ppm equivalent Br per coal (dry), the stack mercury concentration drops down to 4 μ/Nm³ dry at 6 vol.-% O2 . Increasing the bromine addition rate to 2.9 liter/h CaBr2-solution (52 weight %), corresponding to 152 ppm equivalent Br per coal (dry), the stack mercury concentration drops further down to 2.5 μ/Nm³ dry at 6 vol.-% O₂, while the UBC in the fly ash is about 7.5 weight- %, and the rate of mercury capture by the fly ash increases towards 35 % of the total coal mercury input.

Under bromine addition of 70 ppm equivalent Br per coal (dry) and simultaneous addition of also cerium(IV)-ammonium-nitrate (NH4)2Ce(N03)6 at 5 ppm Ce per dry mass of the fired coal mixture , and iron(III)-nitrate Fe(N03)3 at 1 0 ppm Fe per dry mass of the fired coal mixture, the UBC in the fly ash diminishes to only 2 weight-%, and therewith the rate of mercury capture by the fly ash drops down to 10 % of the total coal mercury input, while the mercury capture rate in the wet FGD slurry increases to 85 % Example of the total coal mercury input, corresponding to an emission of only 5 % of the total mercury input via the stack.

EXAMPLE 2

Assumed is the following situation at a pulverized coal (PC)-fired utility boiler of 450 MWel capacity and a flue gas volume flow rate of 1 ,850,0000 N Vh wet. The tangential ly-fired tower boiler is attaining combustion temperatures in the range of 1000 to 1250 °C when fired at full load with 460 t/h raw lignite coal from a mine near Leipzig. The boiler produces 1360 tons/hour of superheated steam at 545 °C, 262 bar.

The boiler produces 1360 tons/hour of superheated steam at 545 °C, 262 bar. The fired coal is a German raw lignite from a mine near Leipzig. This raw lignite is assumed to contain about 7 % ash (wet basis) and 50 % moisture (wet basis), only little halogen (< 0.01 weight-% CI, wet basis), but much sulfur (2 weight-% S, wet basis). Its lower heating value is assumed to be 1 1 ,5 MJ (wet basis).

The mercury content of the raw lignite is quite high (in the mean 0.2 mg/kg (wet basis). The boiler is assumed to have low-NOx-burners, to limit NOx production. The mode of applying staged combustion, i.e. applying locally low oxigen-concentrations does in this case of lignite not induce remarkably enlarged UBC- contents in the fly ash (typically < 0.5 weight-%) because the fired lignite is highly volatile. Still, staged combustion may induce elevated CO contents in the flue gas. The plant is assumed to comprise a boiler with recirculation of the wet flue gas (lowering the mean combustion temperature in favor of low NOx production), recuperative tube-bundle air preheater, cold-side ESP, limestone based wet FGD, fed with fresh water and limestone, forming lime at pH 5 - 6 in the recirculated slurry spray, a cooling tower and stack.

Without addition of any additives, especially without such additives, as covered by this invention, the unburnt carbon (UBC) content in the ESP-fly ash is 1.5 weight-%, which allows for 4 % capture rate only of the total coal mercury input by the ESP-fly ash, while 20 % of the total coal mercury input are captured in the wet FGD. The mercury content of the emitted stack gas is 20 μ/Nm³ dry at 6 vol.-% O2, corresponding to an emission of 80 % of the total coal mercury input via the stack. The CO content of the emitted clean gas is assumed to be 1 50 mg/Nm³ dry, at the stack.

With a bromine addition rate of 50 liter/h CaBr₂-solution (52 weight %) as oxidizing additive agent, corresponding to 150 ppm equivalent Br per coal (dry), the stack mercury concentration is assumed to drop down to 8 μ/Nm³ dry at 6 vol.-% O₂ , while the mercury capture by the ESP fly ash is increased to 6 % of the total coal mercury input, while 75 % of the total coal mercury input are captured in the wet FGD. The CO content of the emitted clean gas is assumed to be 170 mg Nm³ dry, at the stack.

Under bromine addition of 150 ppm equivalent Br per coal (dry) and with simultaneous addition of also cerium(IV)-ammonium-nitrate (NH₄)₂Ce(N0₃)6 at 5 ppm Ce per dry mass of the fired lignite, and also iron(IlI)-nitrate Fe(lM03)3 at 10 ppm Fe per dry mass of the fired lignite, the UBC in the fly ash is assumed to drop back to only 0.2 weight- %, and therewith the rate of mercury capture by the fly ash drops down to almost 0 % of the total coal mercury input, while the mercury capture rate in the wet FGD slurry increases considerably to 90 % of the total coal mercury input, corresponding to an emission of only 5 % of the total mercury input via the stack. The mercury content of the emitted stack gas is 3 μ/Nm³ dry at 6 vol.-% O₂, corresponding to an emission of > 90 % of the total coal mercury input via the stack. The CO content of the emitted clean gas is assumed to drop down to < 50 mg/Nm³ dry, at the stack.

Taking back the bromine addition to anly 50 ppm equivalent Br per coal (dry) and hooding upright the same simultaneous addition of also cerium(IV)-ammonium-nitrate (NH4)2Ce(N03)6 at 5 ppm Ce per dry mass of the fired lignite, and also iron(III)-nitrate Fe(N03)3 at 10 ppm Fe per dry mass of the fired lignite, the mercury content of the emitted stack gas is assumed to stay at 4 μ Nm³ dry at 6 vol.-% O2, corresponding to an emission of > 90 % of the total coal mercury input via the stack. The CO content of the emitted clean gas is assumed to stay unchanged at < 50 mg/Nm³ dry, at the stack.

Claims

Method for the effective removal of mercury from flue gases resulting from combustion processes in a high-temperature plant having a primary and a secondary furnace comprising the steps of

a) Adding at least one oxidizing additive agent selected from the group consisting of a halogen, a halogen containing compound and a mixture of various halogen containing compounds, and

b) Adding at least one catalyzing additive agent, said catalyzing additive agent comprising at least one compound containing at least one metal selected from the group consisting of magnesium, iron, barium, cerium, manganese, nickel, copper, cobalt, zinc, and vanadium, wherein either step a) or step b) may be applied independently or together in at least one of the following modes: i) addition to the combustibles prior to their entry into the primary and/or secondary furnace of the plant, ii) addition directly into the primary and/or secondary furnace of the plant, iii) addition to the combustion flue gas in a plant section downstream of the furnace(s), the temperature during the contact of said oxidizing agent or said additive agent with the flue gas being at least 500°C.

Method according to claim 1 , wherein the oxidizing additive agent is applied to the combustibles prior to combustion, while the catalyzing additive agent is applied to the combustion chamber well above the main coal or waste burners.

3. Method according to claim 1 or 2, comprising a further step c) for removal of mercury from the flue gas, being subjected to a cleanup for removing mercury from the flue gas, which cleanup comprises at least one of a wet scrubber cleanup and a dry scrubber cleanup or a semidry cleanup.

4. Method according to one of claims 1 to 3, wherein the oxidizing additive agent is bromine or a bromine compound or a mixture of bromine compounds.

5. Method according to one of claims 1 to 3, wherein the oxidizing additive agent is iodine or a iodine compound or a mixture of iodine compounds.

6. Method according to one of claims 1 to 5, wherein the oxidizing additive agent is a liquid or solid bromine-rich or iodine-rich waste or a mixture thereof.

7. Method according to claim 1 to 5, wherein the oxidizing additive agent is calcium bromide, sodium bromide or potassium iodide or a mixture thereof.

8. Method according to one of claims 1 to 7, wherein the catalyzing additive agent comprises at least one compound containing at least one metal selected from the group consisting of magnesium, iron, barium, cerium, manganese, nickel, copper, cobalt, zinc, and vanadium

9. Method according to claim 8, wherein the catalyzing additive agent is an aqueous solution of salts of magnesium, iron, barium, cerium, manganese, nickel, copper, cobalt, zinc, vanadium salts in their anhydrous or hydrated forms.

10. Method according to claim 9, wherein the catalyzing additive agent comprises an aqueous solution of cerium(IV)-ammonium-nitrate (NH4)2Ce(N03)6, cerium(IV)-sulfate Ce(SC>4)2, and cerium(III)-chloride CeCh, in their anhydrous or hydrated forms.

1 1. Method according to claim 9, wherein the catalyzing additive agent comprises an aqueous solution of iron(II)-nitrate Fe(N0₃)2, iron(III)-nitrate Fe(N0₃)3, iron(Il)-sulfate FeS04, or iron(III)-sulfate Fe2(S04)3, or iron(II)-chloride FeCb , or iron(III)-chloride FeCb.

12. Method according to claims 9, wherein the catalyzing additive agent is a mixture of cerium (or magnesium, barium, manganese, nickel, copper, cobalt, zinc, vanadium) and iron compounds, said mixture comprising 0.1-30 weight % of cerium (or magnesium, barium, manganese, nickel, copper, cobalt, zinc, vanadium) and 33-99.9 weight % of iron, based on the total metal content.

13. Method according to claims 1 to 12, wherein one single additive agent mixture, comprising at least a metal halide as bromides or iodides of magnesium, iron, barium, cerium, manganese, nickel, copper, cobalt, zinc, vanadium and mixtures thereof , is used both as oxidizing and catalyzing additive agent.

14. Method according to claims 1 to 13, wherein the oxidizing additive agent and the catalyzing additive agents are mixed, said mixture containing 30 - 95 weight % of bromine or iodine and 70 - 5 weight % of the named metals, based on the total content of additive compounds in the aqueous solution.

15. Method according to one of claims 1 to 14, wherein the specific dosage rate of the oxidizing additive agent and likewise the catalyzing additive agent or of corresponding mixtures is in the range of 1 to 3000 ppm per dry mass of the combustibles as coal or waste, calculated as mass ratio of the sum of the additives compounds in the solution per e.g. coal or waste.