WO2004080574A1

WO2004080574A1 - Mercury and process for removing mercury from gases

Info

Publication number: WO2004080574A1
Application number: PCT/DK2004/000159
Authority: WO
Inventors: Karsten Felsvang
Original assignee: F.L. Smidth Airtech A/S
Priority date: 2003-03-12
Filing date: 2004-03-12
Publication date: 2004-09-23

Abstract

The present invention relates to the removal of mercury from the flue-gas of a combustion process. The invention further relates to a mercury removal system comprising particle removal means (10) adapted to remove particles from a gas stream, preferably being a flue gas, flowing to the particle removal means, said system further comprising a catalyst element (12) arranged upstream of the particle removal means in such a manner that the gas stream contacts catalytic surface(s) of the catalyst element before flowing to the particle removal means. Further embodiments comprise injection of adsorption and/or reagent agents.

Description

SYSTEM AND PROCESS FOR REMOVING MERCURY FROM

GASES

The present invention relates to the removal of contaminants from a gas stream and relates in particularly to the removal of trace amounts of vapour phase contaminants such as mercury from the flue-gas of a combustion process.

BACKGROUND OF THE INVENTION

Mercury was listed as a hazardous air pollutant (HAP) in Title III of the Clean Air Act

Amendments of 1990. In 1997 US environmental Protection Agency (EPA) released 1700- page mercury report that concluded that more than 85% of all mercury released into the environment by humans is produced by combustion sources. Coal-fired utility boilers are the largest source of mercury emissions in the United States. In February of 1998, EPA issued a report to Congress evaluating toxic air emissions from power plants. The study concluded that utilities are the major remaining source of unregulated mercury emissions into the air. The report also called for monitoring of power plants to better ascertain the quantity and nature of mercury emissions. A Mar. 19, 1998, report by the Clean Air Network and the U.S. Public Interest Research Group stated that the EPA needs to establish target reduction goals for mercury emissions from utility boilers.

In December 2000 EPA announced their intent to regulate hazardous air pollutants including mercury emissions from the nations coal fired power plants.

Draft legislation indicated that new regulations may require removal efficiencies as low as 50% or as high as 90%. The costs to meet 90% removal range from $2 to $5 billion per year.

Based on information collected during the U.S. EPA Mercury Information Collection Request (ICR), the technology for catalytic oxidation of Hg(0) would have the greatest effect on the flue gas from subbituminous coal or lignite, where most of the mercury is present in the elemental form. There are currently over 30,000 MW of scrubbed capacity firing these fuels, with more systems planned.

Mercury control

When the coal is burned in an electric utility boiler, the resulting high combustion temperatures vaporize the Hg in the coal to form gaseous elemental mercury Hg(0). Subsequent cooling of the combustion gases and interaction of the gaseous Hg(0) with other combustion products result in a portion of the Hg being converted to gaseous oxidized forms of mercury Hg²⁺ and particulate-bound mercury (Hgp). Hg ²⁺ is generally easier to capture by adsorption than Hg(0).

Flue gas cleaning technologies that are applied to combustion sources employ three basic methods to capture Hg:

o Capture Hg(0) in particulate control device, electrostatic precipitator or fabric filter Adsorption of Hg(0) and Hg²⁺ onto entrained sorbents for subsequent capture in particulate control device » Absorption of Hg²⁺ in wet scrubbers.

The factors that affect the speciation and capture of Hg in coal-fired combustion systems include the type and properties of coal, the combustion conditions in particular temperature, and the types of flue gas cleaning technologies employed.

Oxidation reactions that affect the speciation of Hg include homogeneous, gas-phase reactions and heterogeneous gas-solid reactions associated with entrained particles and surface deposits. Suspected flue gas oxidants involved in Hg(0) oxidation include oxygen (02), ozone (03), hydrochloric acid (HCI), chlorine (Cl), nitrogen dioxide (N0₂) and sulphur trioxide (S03). Many of these oxidants are also acid species, which may be significantly impaired by the presence of alkaline species in fly ash, such as sodium, calcium and potassium. Heterogeneous oxidation reactions may be catalyzed by metals such as iron, copper, nickel, vanadium, and cobalt. Conversion of Hg(0) to Hg²⁺ may be followed by adsorption to form Hgp.

The determination of which mechanisms, oxidants, and catalysts are dominant is believed to be crucial in developing and implementing Hg control strategies.

Mercury control technologies A practical approach to controlling Hg emissions at existing utility plants is to minimize capital costs by adapting or retrofitting the existing equipment to capture Hg. Potential retrofit options for control of Hg were investigated for units that currently use any of the following post combustion emission control methods:

1. Electrostatic precipitator ESP or fabric filter FF for control of particulate matter PM 2. dry FGD scrubbers for control of PM and S0₂,

3. wet FGD scrubbers for the control of PM and S0₂. Sorbent injection

Dry injection of sorbents upstream an existing ESP or FF is one of the control options that can be used on almost all coal fired plants. However, concerns have been raised in the US about space limitations and if there would be residence time enough for the sorbent to react with mercury, especially with an ESP, where the flue gas is not exposed to a cake of sorbent material. It is believed that space will always be sufficient for a simple pneumatic dry injection system and experience from Europe has shown that such a simple dry injection system upstream of an ESP can be accomplished with good removal results of mercury without taking active steps to improve mixing or increasing residence time. Though the inlet particulate loading will increase, the difficulties are not related to the loading it self. The potential problems are related to the characteristics of the injected material.

Injection of finely divided calcium based sorbent into the flue gas will typically influence the operation of the ESP negatively. The sorbent has normally a very fine particle size distribution (PSD) and therefor influences the overall PSD of the inlet particle matter towards the finer faction.

Since the expected injection rated is only about 200 mg/Nm3, say typically maximum 3 to 4 % of the inlet concentration of particulate matter, the increase in particulate emission due to the above mentioned will be relatively small. The increase will not be greater than a number of utility ESPs will continue to be in compliance with local emission regulations.

Sorbent Injection With Gas Cooling Already in 1979 researchers discovered the influence of cooling the flue gas on mercury removal. It was determined that the optimum temperature range for mercury removal was 90-160°C. This holds in particular when mercury is predominantly in the form of mercury chloride.

Gas cooling can be accomplished by evaporative cooling or by use of heat exchangers. Gas conditioning with pure water can successfully be accomplished without creating deposits and corrosion in the gas cooler, however, this requires a reasonable high gas residence time and very high inlet particulate concentration. In contrary to evaporative gas cooling, the use of heat exchangers to reduce flue gas temperature seems to be a viable option. During the cooling of the flue gas any S0₃ will condense on the alkaline sorbent particles. They will act as effective nuclei for the condensation and subsequent neutralization of S0₃. Sorbent Injection With Chloride Enhancement

It is widely recognized in the scientific society that speciation of mercury has a major impact on capture efficiency. Thus, mercury chloride, HgCI₂, is easier to remove by dry injection and other processes than elemental mercury,

Conversion of elemental mercury to mercury chloride in order remove it from a flue gas would therefore be an attractive process for controlling mercury emissions from coal fired plants. A low cost prior art option is the use of salt addition to the coal during the combustion process. A large number of boilers have been designed for a range of coals including coals with certain chlorine content. It is therefore anticipated that salt in the form of sodium chloride can be added to the boiler without major corrosion problems being encountered.

Finally chloride enhancement will also improve ESP performance to mitigate any problems by increased particulate loading into the ESP.

US Pat. Appl. No. 20020114749 discloses process for removing elemental mercury vapor from flue gas by contacting the flue gas with a gaseous oxidizing agents typically Cl₂ an oxide of chlorine, H₂0₂, and/or HOCI, to render the elemental mercury vapor more easily oxidized. The flue gas is then subjected to oxidation, typically by way of an electrical discharge, at a point downstream of the oxidizing agent region to oxidize the elemental mercury vapor and thereby render it more easily removed.

The invention disclosed in US Pat. Appl. No. 20020114749 also seeks to effect such conversion without producing a secondary waste stream such as contaminated active carbon which has special requirements or restrictions on disposal. It further aims to effect the conversion in a manner that interfaces efficiently with the air pollution control equipment currently used to control S0₂, NOx and particulate emissions on coal fired combustion systems and on incinerators. An important advantage arising from this invention is the unexpected cost savings due to reduced energy requirements as compared to conventional processes such as activated carbon injection. In particular, it has been discovered that the combined use of an oxidizing agent and an electrical discharge drastically decreases the electrical power consumption. This method requires use of expensive oxidating agents and relies on electrical discharge for mercury oxidation.

US Pat. No. 5,672,323 discloses activated carbon injection provided for mercury removal in a flue gas treatment system having an electrostatic precipitator and a wet flue gas desulfurization tower. 'Fresh activated carbon is injected into the flue gas along with recycled carbon from the exhaust of the precipitator to minimize fresh carbon make up. The remaining carbon passing through the electrostatic precipitator continues to react with mercury in the wet flue desulfurization tower due to the lower operating temperature of the tower about 110°F.-150°F. providing enhanced mercury removal as well as the increased contact area for absorption provided by the liquid spray of the tower. Should increased mercury removal capability be required, activated carbon powder may be injected before the wet flue gas desulfurization tower and after the outlet of the electrostatic precipitator. Also a granulated bed of activated carbon may be located in the tower outlet. According this disclosure, mercury is not deliberately oxidized so it has an inherent problem with low adsorption characteristics of elemental mercury. That implies that the amount of activated carbon in particular for high overall mercury removal should be very high in spite of the fact that part of it is recycled. The method further relies on wet tower for additional mercury control.

US Pat. No. 5,607,496 discloses a method and apparatus for removing, from a combustion gas stream, elemental mercury and mercury compounds. Mercury is adsorbed on adsorbent particles such as activated alumina, where mercury oxidation is catalytically promoted. After adsorbing a substantial quantity of mercury compounds, the spent adsorbent particles can be regenerated and re-used by heating the particles to decompose and drive off the mercury compounds. In another embodiment, oxidation of the elemental mercury is catalytically promoted at a catalyzing station, and the mercury compounds are removed from the gas stream by scrubbing. Activated alumina is the preferred adsorbent particulate material for use in the present invention. However, other adsorbent particulate materials may be utilized so long as they incorporate the above-described characteristics of activated alumina, namely: (i) a catalytic activity for the oxidation of mercury, at least substantially comparable to that of activated alumina particles; and (ii) sufficient heat resistance to accommodate a regeneration procedure The oxidation of Hg(0) to HgCI₂ by alumina is not as high as with a dedicated catalyst so the amount of alumina used in particular if high removal efficiency is required is very high.

US Pat. No. 6,136,281 discloses a method to catalyze the oxidation of Hg(0) in a flue gas stream prior to standard emissions control equipment. It discloses an apparatus and a process for effecting the removal of mercury from a stack gas by the catalytic oxidation of elemental mercury [Hg(0)] within an exhaust gas matrix to mercury(II)chloride [HgCI₂]. Mercury(II)chloride is more water-soluble and less volatile than Hg(0) and consequently is more easily removed from gaseous streams by existing wet flue gas desulfurization (FGD) systems, such as a wet limestone scrubber and FGD spray dryers or a wet ESP. The apparatus and process are based on the property of the noble metals, preferably gold, in the presence of a gaseous matrix wherein there exist water vapor and dilute Hg(0) and dilute HCI to oxidize Hg(0) to mercury(II)chloride. It relates to the use of a porous bed of gold-coated material that is saturated with Hg(0) to the point that the gold in the presence of HCI in the exhaust stream catalyses the oxidation of Hg(0). In this disclosure no solid adsorption media like activated carbon is added or used to collect mercury. When wet scrubber is used for mercury removal the scrubbing liquid is contaminated with mercury species requiring expensive waste water treatment.

US Pat No. 4.785.932 discloses a catalytic reactor for oxidizing elemental mercury contained in flue gas. It discloses a system where an intermittently generation of a corona discharge plasma is required to partly oxidize mercury. Mercury is adsorbed to the surface of a catalyst, partly oxidized and subsequently desorbed from the catalyst surface and as it enters the flue gas again, it is further oxidized by the corona discharge plasma. As the corona discharge is intermittent, the mercury oxidation does not take place in a continuous manner. In this disclosure no solid adsorption media like activated carbon is added or used to collect mercury.

In European patent application EP 0860197 Al a process for treating combustion exhaust gas is disclosed, where a mercury chlorinating agent and ammonia simultaneously is added to combustion exhaust gas. The exhaust gas is contacted with a denitrating catalyst to reduce the content of nitrogen oxide and convert elemental mercury into a largely water- soluble chloride. The formed mercury chloride is subsequently removed in a wet flue gas desulphurization system. It does not disclose addition of solid adsorption media as activated carbon to collect mercury in a down stream electrostatic precipitator.

German patent application DE 3931891 Al discloses a method for mercury removal from gas or gas mixes at room temperature by oxidizing the mercury over a catalyst in the presence of a large surplus of ozon. The application does not disclose use of solid adsorption media to collect mercury.

Catalytic oxidation of mercury The U.S. government funded the development of a catalytic oxidation process to enhance mercury removal in wet flue gas desulphurization (FGD) systems. In the process, solid catalysts oxidize vapor-phase elemental mercury (Hg(0) in flue gas and the oxidized mercury is removed in downstream FGD absorbers.

A catalyst material in a honeycomb form is inserted into the flue gas path upstream of the FGD system. Downstream of the catalyst, the oxidized mercury is scrubbed in the FGD absorber, and co-precipitates with the calcium sulfite or gypsum byproduct from lime or limestone wet FGD systems. Development has focused on the outlet of a plant's cold-side particulate control device as the most likely location for such a catalyst, for two reasons. One is that the flue gas velocity is typically low as it exits a particulate control device (e.g., about 5 ft/sec as it exits an ESP) making it an ideal spot to operate a catalyst at longer residence time and lower pressure drop. The other is that with the flue gas being relatively particulate free at this location, a close-pitched catalyst can be used. This allows for a high surface area per volume of catalyst relative to "dirty gas" operation, such as in most SCR systems, and allows a smaller catalytic reactor to be used. Based on data collected in the previous project, it is estimated that only a 6-inch depth of a close-pitched palladium-based catalyst would be adequate to achieve 90% or greater oxidation of the Hg(0) present in the flue gas at this location.

SUMMARY OF THE INVENTION

An object of preferred embodiments according to the present invention is to provide an apparatus and method for removing pollutants such as vapour phase mercury from a gas stream.

Another or a further object of preferred embodiments according to the invention is to provide an apparatus and method for removing of elemental mercury from flue-gas of a combustion process. Another or yet a further object of preferred embodiments according to the invention is to provide an apparatus and methods, which permit an overall removal of elemental and ionic mercury of more than 90%, preferably more than 95%, or more preferably more than 99,5%.

Another object or yet a further object of preferred embodiments according to the invention is to provide an apparatus and method, which add activated carbon to the flue-gas for in situ mercury removal.

Another or yet a further object of preferred embodiments according to the invention is to provide an apparatus and method, which significantly reduce the amount of activated carbon required for vapour phase mercury removal.

Another or yet a further object of preferred embodiments according to the invention is to provide an apparatus and methods that can be combined with already existing electrostatic precipitator. According to a first aspect of the present invention a mercury removal system is provided which in particular preferred embodiments comprises particle removal means adapted to remove particles from a gas stream, preferably being a flue gas, flowing to the particle removal means, said system further comprising a catalyst element arranged upstream of the particle removal means in such a manner that the gas stream contacts catalytic surface(s) of the catalyst element before flowing to the particle removal means.

In preferred embodiments of removal system according to the present invention, the particle removal means is/are preferably designed to capture particles resulting from a combustion process and particles having adsorbed mercury. In such embodiments mercury being adsorbed by particles and other particles in the gas stream for instance fly ash may be removed from the gas stream by the same particle removal filter.

According to preferred embodiments of the invention, the mercury removal system comprises a first chamber having an inlet for receiving the gas stream and wherein the particle removal means is/are arranged, said first chamber further comprising an outlet for delivery of the gas stream after flowing through the particle removal means. In such embodiments the catalyst element may advantageously be arranged within said first chamber. The first chamber may additionally comprise means for emptying out of the chamber, the particles removed from the gas, and such means for emptying out may preferably be hoppers.

In other preferred embodiments of the mercury removal systems according to the present invention the catalyst element is arranged outside said first chamber. In such embodiment, the catalyst element may preferably be mounted in a second chamber being distinct from the first chamber. Alternatively the catalyst element may preferably be mounted in a duct terminating at said inlet.

The particle removal means comprised in a mercury removal system according to the present invention may preferably comprise a number of electrodes arranged to form an electrostatic precipitator (ESP). Alternatively or in combination therewith, the particle removal means may preferably comprise or further comprise a bag filter type.

In preferred embodiment it is desirable to remove particles, such as coarse particle, from the gas stream before the gas stream contacts the catalytic element. In such embodiments, the mercury removal system may advantageously comprise a further particle removal means arranged upstream of the catalytic element. In particular preferred embodiments of the mercury removal system according to the present invention, the catalyst element is shaped as an element having elongated flow channels, said element may preferably be a honey comb element.

Preferred embodiment of the mercury removal system according to the present invetion may preferably comprise a flow distribution device arranged upstream of the catalyst element. In combination therewith or alternatively, the catalyst element may preferably be shaped as an inlet distribution device.

In general, the catalyst is preferably shaped and/or the flow of the flue gas is preferably so directed that impact on the catalyst surface may occur. In order to enhance the impact on the surface of the catalyst, different measures may be included into preferred embodiments of the present invention. Such measures may preferably comprise, alone or in combination:

• Directing the flow of flue gas in a direction having a velocity component being perpendicular to the surface of the catalyst. By this is preferably meant that a representative velocity vector for the flue gas is not parallel with the surface of the catalyst but has at least a component being perpendicular to the surface of the catalyst

• Orientating one or more parts of the catalytic surface(s) in an upstream direction. By this is preferably meant that a vector being normal to the surface of the catalyst surface is not parallel to the flow direction of the flue gas but has at least a component being parallel to the flow direction of the flue gas

• Shaping the catalyst surface with curvature

• Shaping the catalyst as flow channels / hollow tubular members having decreasing cross sectional areas in the downstream direction and providing the interior surfaces of the flow channels with catalytic material(s)

• Generating a turbulent flow past or at least a turbulent boundary layer close to the surface of the catalyst surface

In preferred embodiments of the invention one or more parts of the surface(s) of the catalyst are preferably orientated in an upstream direction relatively to the flow direction of the gas stream. The catalyst element may according to preferred embodiments comprise at least two rows of elongated elements arranged non-parallel to, such as perpendicular to, the flow direction of the gas stream, at least one of the at least two rows has a catalytic surface. In combination thereto the catalyst element may preferably comprise a multiplicity of elongated elements in the form of U-impactor screens arranged in rows, preferably four rows, wherein the screens in each row are staggered in relationship to screens in a downstream row. Particularly, the catalyst element may preferably have four rows of U-impactor screens wherein the two rows being most downstream have catalytic surfaces and the remaining rows constitute a further particle removal means.

In particular preferred embodiments the catalyst element may comprise a plurality of elongated tubular members, at least a part of which has catalytic surface(s). Additionally, at least parts of the interior surfaces of the elongated tubular members may be catalytic surfaces. In combination therewith or alternatively, the elongated tubular members may preferably be arranged so that their longitudinal extension being parallel to the flow direction of the gas stream, the elongated tubular members are preferably funnel-shaped with their largest cross sectional area facing inflowing gas stream.

In particular preferred embodiments of the present invention, the mercury removal system further comprising adsorption agent injection means for injecting one or more adsorption agents, preferably activated carbon, said adsorption agent being preferably injected upstream of the catalyst element. Impact of the one or more adsorption agents on the surface of the catalyst is preferred in many embodiment in order to enhance adsorption of mercury to the adsorption agent. However it is envisaged that interaction between activated carbon and oxidised mercury may also occur distant from the surface of the catalyst. In embodiments where impact of the absorption agent(s) on the catalytic surface(s) is preferred, the considerations as to the design the of the catalyst and/or the flow guidance towards the catalyst surface(s) as outlined above are applicable also in this context.

Preferred embodiments may further comprise reagent injection means for injecting, upstream of the catalyst element, a reagent capable of adsorption or reaction, in particularly chemically reacting, with substances, preferably being oxidised mercury present in the gas stream.

In particular preferred embodiments, the mercury removal system further comprise a temperature controlling device, such as an heat exchanger, controlling the temperature of the gas temperature to be within 230-320°F, such as between 230-300°F preferably between 230-280°F when measured upstream of the catalyst element. In preferred embodiments of the invention the catalyst element or the catalyst element it self may be adapted to be exchanged, preferably in an easy manner.

Preferred embodiments of the invention may comprise recycling means for recycling carbon rich fly ash. Furthermore, preferred embodiments of the invention may advantageously comprise one or more of the following features being adjustable:

the position of the catalyst, upstream of the electrostatic precipitator; the position of distribution means is adjustable; - the width and/or height of chamber in which the particle removal means is(are) arranged; the flow area of the inlet and/or outlet; the shape of the electrodes of the electrostatic precipitator; the pitch of catalyst element - the shape of the distribution device; the distance between distribution device and catalyst element; the distance between catalyst element and electrodes of the electrostatic precipitator; the distance between distribution device and electrodes(ESP); - the distance between the inlet and the outlet.

Furthermore, preferred embodiment may be adapted to receive or comprise two or more catalyst elements arranged in series.

In particular preferred embodiments, the mercury removal system is adapted to or operated to, preferably by controlling the temperature of the flue gas and/or the amount of activated carbon injected into the flue-gas, provide 90% overall mercury removal, preferably more than 95% removal and more preferably more than 99,5% removal.

According to a second aspect of the present invention a process for removal of mercury from a gas stream, preferably being a flue gas or the like, is provided. The process comprises in preferred embodiments the steps of:

contacting catalytic surface(s) of a catalyst element with the gas stream; - and subsequently removing particles from the gas stream by utilising particle removal means. Additionally, the process may preferably further comprising injecting an adsorption agent, preferably being activated carbon, upstream of the catalytic element. Additionally or in combination thereto the process may further comprise injecting a reagent, preferably being hydrochloric acid upstream of the catalyst element.

Furthermore, a process according to present invention may preferably comprise the step of controlling the temperature upstream of the catalyst element to be within 230-320°F, such as between 230-300°F preferably between 230-280°F when measured upstream of the catalyst element.

Preferably, a process according to the present invention may advantageously comprise the step of recycling the adsorption agent, preferably activated carbon

Monitoring of a process according to the present invention may preferably comprise the step of measuring one or more of the following quantities:

- the amount of active carbon in gas stream

- the amount of oxidised mercury in gas stream and/or

- the amount of chlorine in gas stream.

A process according to the present invention is preferably adapted to or comprises preferably the step of controlling the process to, preferably by controlling the temperature of the flue gas and/or the amount of activated carbon injected into the flue-gas, provide 90% overall mercury removal, preferably more than 95% removal and more preferably more than 99,5% removal.

It is envisaged that the process according to the second aspect of the invention are applicable with systems according to the first aspect of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the present invention and in particular preferred embodiments thereof will be described in greater details in connection with the accompanying drawings in which:

Figure 1 is a schematic view of an apparatus for removing vapour phase contaminants from a gas stream in accordance with an embodiment of the present invention;

Figure 2 is a schematic view of a catalyst pore according to the present invention; The catalyst pore shown corresponds to the section indicated in Figure 1 by "Fig. 2" and on the figure the process taking place inside the catalyst pore is indicated (a part of the pore wall is removed in the drawing to render visible the interior of the pore). In Figure 2 F indicates flue gas and P indicates particles trajectory impact on catalytic surface;

Figure 3 is a schematic view of a double impactor screen (1) and (2) with a double catalyst screen downstream (3) and (4) according to the present invention.

and

figure 4 is a schematic view of a top inlet with a catalyst screen (1) in the top diffuser part according to the present invention. In figure 4 numeral (2) and (3) denotes vertical gas distribution means.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS THE INVENTION

A description of preferred embodiments of an apparatus and a process for removing mercury, preferably being in vapour phase, from a gas flue-stream of a combustion process by placing a catalyst element upstream of an electrostatic precipitator, which in preferred embodiments are implemented by placing the catalyst element into the inlet of an electrostatic precipitator is presented. The catalyst element may preferably be shaped as a honey comb element. According to preferred embodiment of the invention a sorbent agent, preferably being activated carbon, is injected into the flue-gas stream upstream of the catalytic element to provide sorbent necessary for the removal of elemental mercury, which is being converted to oxidized mercury at the catalyst surface and downstream of the catalytic element the mercury containing activated carbon is being removed preferentially in one field of the electrostatic precipitator.

Since sorbent injection preferably is an inherent part of many preferred embodiments according to this invention its injection into ESP with and without gas cooling and chloride enhancement also this topic will be outlined in the following.

It is envisaged that preferred embodiments of the present invention are capable of providing one or more of the following features:

o Elemental mercury is converted at catalyst surface to HgO Catalyst surface provides active sites where HgO and active carbon may interact © Catalyst design ensures maximal impact of particulate on catalyst surface • Fly ash and active carbon cannot avoid getting into intimate contact with HgO during passage of the catalyst (bed) • Position of catalyst within the ESP may be adjusted for optimum space velocity

• Catalyst acts as inlet distribution device

• Carbon rich fly ash from last field of the ESP may be recycled for optimum utilisation of carbon

Figure 1 shows an electrostatic precipitator (10) cleaning the flue-gas from an existing combustion process for particulate matter. The electrostatic precipitator is connected to the combustion process via the duct (11). In the inlet of the electrostatic precipitator (10), a catalyst element (12), a SCR Cat. Bed, is mounted such that the flue-gas containing vapour phase mercury gets in intimate contact with the catalyst surface. For optimal contact between catalyst surface and injected activated carbon a flue-gas distribution device (13) is provided upstream of the catalyst element (12). The catalyst (12) may be installed/arranged within the flue gas distribution device (13) to constitute one or more integral device acting as one or more inlet gas distribution device and catalyst. Preferably it can be installed in series with one or more distribution devices. Cleaning means may be incorporated within the inlet distribution device (13) for cleaning of the catalyst (12).

In the flue-gas duct (11) a heat exchanger (14) may be incorporated for optimum temperature control and in such embodiments, the heat exchanger controls the temperature to be within the preferred interval of 230-320°F (see T in fig. 1), such as between 230-300°F preferably between 230-280°F when measured upstream of the catalyst element (12), preferably measured in the inlet duct downstream of the active carbon injection.

In the flue-gas duct (11) activated carbon (15) is injected and the activated carbon is preferably injected so as to provide a uniform distribution of the activated carbon in the flue-gas entering the electrostatic precipitator. In the inlet of the electrostatic precipitator (10) the flue-gas distribution device (13) provides optimum conditions for ensuring max. impact of the activated carbon onto the catalyst surface.

The catalyst (12) converts vapour phase elemental mercury to oxidized mercury. In a particular preferred embodiment, the catalyst surface provides active sites, where oxidized mercury and activated carbon can interact. The activated carbon adsorbs the oxidized mercury in situ and subsequently the mercury containing activated carbon is preferentially removed in a pre-designated field (16). The fly-ash (17) removed from field (16) contains predominantly mercury containing activated carbon for disposal/utilization (18) or for optional re-cycle (19) to the inlet of the electrostatic precipitator. By recycling activated carbon a reduction in the carbon usage is achieved. Chlorine species, such as HCI, may be added, preferably by injection, during the combustion process or into the duct (20) upstream of the electrostatic precipitator. Chlorine species may assist in the oxidation of elemental mercury and in combination with the oxidation in the catalyst element provide essentially complete removal of mercury.

As indicated above, the inlet and the catalyst element may be integrated into a unit a so- called inlet screen. The arrangement may be installed as one or more inlet gas distribution screens, e.g. in inlets shaped like a central diffuser.

However, the detailed design should be so that the demand on residence time is met. The shorter the residence time the more important it is to ensure that all gases have an opportunity to come into contact with the catalyst.

It has been found that if the open screen area is low for instance in order to distribute the gases over the cross section, the catalyst screen alone may act as gas distribution device. If the open area is higher, sometimes up to 75 % of the total cross section area, a double U-impactor screen followed by a similar catalyst double U-screen can smoothen out the flow and ensure an even gas distribution. In addition, collection of coarse particles by impaction may decrease the dust loading of the first electrical field and makes higher power input possible thus decreasing the total mass emission from the precipitator. The term U-impactor screen denotes a screen made up mainly of vertical U-shaped beams, often named channel iron, and between the webs of two neighbouring channel irons so- called throttle plates may be fitted in order to decrease the open area. If a differentiated open area distribution is needed, the height of the small throttle plates can be varied, still keeping the vertical pitch between throttle plates constant.

The catalyst may not be able to operate with heavy dust build-up and cleaning may therefore be needed. Cleaning using vibration or rapping can be applicable, but may requires that care is taken with respect to the expected frailness of the arrangement. Cleaning with water may be utilised at the inlet; and this principle may advantageously be utilized with an outlet screen arrangement. Cleaning may advantageously also be performed with pressurised air, preferably in small quantities, or with sound waves from an acoustic horn.

In preferred embodiments, a catalyst screen may be installed in the top diffuser of an inlet, where gases are coming from above (Fig. 4). One advantage of this positioning is an easy access from the precipitator roof in case of maintenance of the catalyst, and access to the cleaning system components. A screen in the top diffuser will in such embodiments replace the standard splitter walls, necessary in order to prevent the diffuser flow from stalling. Often the layout is such that gases are coming from below. In such cases the inlet diffuser is placed at the bottom of the inlet funnel, and a single catalyst screen may be placed here. In this case access to screen and cleaning components is easy too, and the screen can replace standard splitter walls preventing the flow from stalling. In other embodiments, the gases are coming from above and in such case the inlet diffuser is placed at the top of the inlet funnel, and a single catalyst screen may be placed here. Also in this case access to the screen and cleaning components is easy, and the screen can replace standard splitter wall preventing the flow from stalling.

As indicated in figure 3 the catalytic element may preferably comprise multiplicity of double U-impactor screens arranged into rows at the inlet to ESP. The screens in each row are staggered in relationship to screens in subsequent rows. Typically 4 rows of screens are used of which last 2 are impregnated with catalyst. Total number and configuration of screens depends on desired efficiencies in mercury removal. In particular, figure 3 shows a double U-impactor screen (1) and (2) with a double catalyst screen downstream (3) and (4). The U-screen design, of the distribution device (1) and (2) and the catalyst (3) and (4), is designed to smoothing out the flow and ensuring an even gas distribution. One important factor to consider is the residence time of the gas when passing through the catalyst. The shorter the residence time the more important is it preferably to ensure that all gases have an opportunity to come into contact with the catalyst.

With minor modifications or adaptations, the catalytic element can be readily retrofitted to the majority of existing ESPs used for particulate removal in fossil fuels or waste combustion systems.

It is preferred, that the catalytic element fulfils one or more of the following four major requirements in removal of particulate and mercury:

1. Even distribution of flue gas prior to introduction to electrostatic field

2. Removal of coarse particulate fraction of fly ash

3. Oxidation of elemental mercury to oxidised mercury

4. Contact between oxidised mercury and activated carbon

The shape and spacing of double U-impactor screens of the catalytic element is of major importance in accomplishing all 4 above-mentioned roles. The geometry of catalytic element is optimised through laboratory testing and 3-dimensional computerised flow modelling. The catalyst used in this application is based on Ti0₂ carrier with vanadium and other metallic oxides. Even distribution of flue gas prior to introduction to electrostatic field may improve efficient mercury and particulate removal in ESP. The even distribution of flue gas is preferably accomplished by high turbulence within the catalytic element enhancing vertical and horizontal gas flow resulting in even gas velocity at the outlet of element.

The removal of coarse particulate fraction of fly ash takes place in the front two rows, (1) and (2) in figure 3, of the catalytic element. The removal takes place through inertial impaction of coarse particles onto the screens. In addition to removing coarse fraction of fly ash these first two rows of screen protect catalyst applied on subsequent screens from direct erosion and abrasion. The removal of coarse fraction from fly ash is beneficial in particular in the case when fly ash has bi-nodal particle size distribution with substantial coarse fraction. Furthermore, collection of coarse particles by impaction decreases the dust loading of the first electrical field and makes higher power input possible thus decreasing the total mass emission from the precipitator.

It is realised that some mercury oxidation takes place in the front two rows through catalytic action of fly ash. For example, fly ashes from combustion of bituminous coal is more efficient in oxidation of Hg(0) than fly ash from lignite or subbitumenous coal. The reason for difference in oxidation efficiency is fly ash composition, presence of certain flue gas constituents, and flue gas conditions.

Furthermore, the carbon content of fly ash may influence adsorption of gaseous Hg (the carbon in fly ash is unburned coal). Conditions that result in increased amounts of carbon in fly ash tend to increase the amount and subsequent capture of particle-bound Hg.

After coarse particulate has been removed flue gas passes the rows of catalyst impregnated screens where elemental mercury is oxidised. The shape and spacing of screens is designed to obtain optimal space velocity for oxidation. Even though the global distribution of the gases are smoothed out the local turbulence intensity is always high enough to ensure that the flow within the channels of the catalyst is turbulent. Since chlorine plays an important role in mercury oxidation it may be added to flue gas upstream of catalytic element. Mercury is oxidized by a chlorine atom recycle process so that the concentrations of both Cl and Cl₂ are important. Mercury is first oxidized to HgCI by Cl and then to HgCl₂ by Cl₂. The chemistry kinetics is among other conditions governed by Cl species transformation, moist CO oxidation and NO production. Oxygen weakly promotes homogenous Hg oxidation, while moisture and NO are strong inhibitors. Once mercury is oxidised it can be readily adsorbed by activated carbon that is injected into flue gas in front of catalytic element by blow-through, rotary valve dosing activated carbon from the dosing hopper.

Many factors may affect the adsorptive capability of the activated carbon. The main factors are the temperature and composition of the flue gas, the concentration of Hg in the exhaust stream, and the physical and chemical characteristics of the activated carbon. The adsorption of mercury on activated carbon is greatly enhanced by elemental mercury oxidation by catalyst and flue gas flow characteristics in catalytic element. Therefore, much smaller amount of activated carbon is needed for efficient mercury removal than in conventional systems.

The amount of activated carbon may further be reduced by recycle from ESP i.e. re- injection in front of catalytic element.

The potential of catalyst abrasion and blinding with particulate of the catalytic element should preferably be considered during design. Abrasion resistant catalyst is preferably used and particulate blinding is preferably prevented by occasional blow down of dust with compressed air from the manifold arranged to covers all catalytic screens.

The operation and the method of the present invention comprises the step of converting vapour face elemental mercury on the surface of a catalyst element and in situ adsorption of the oxidized mercury onto fly-ash and activated carbon particles present in the flue-gas passing the catalyst element. The process conditions such as flue-gas temperature and amount of activated carbon injected into the flue-gas is adjusted to provide 90% overall mercury removal, preferably more than 95% removal and more preferably more than 99.5% removal.

The process according to the invention will be further explained by means of the example and comparison given below:

EXAMPLE

In a coal-fired power plant flue-gas distribution devices and a catalyst element was retrofitted into the inlet of an existing electrostatic precipitator. The temperature of the flue-gas entering the precipitator was 310 gr. F. By means of a pneumatic injection system, activated carbon was injected into the duct upstream of the existing electrostatic precipitator. Two tests were made in which the speciation of mercury and rate of active carbon injection was measured upstream of the electrostatic precipitator. Result of the mercury speciation showed that 87% of the vapour phase mercury was elemental and 13% was oxidized at the inlet to the electrostatic precipitator.

The table below is showing results of the test:

As can be seen from the results an overall high removal of vapour phase mercury was achieved by the process according to the invention.

COMPARATIVE EXAMPLE

The plant described in the example above was operated with injection of activated carbon upstream of the electrostatic precipitator. No modifications to the precipitator inlet were made. Mercury speciation at the inlet to the electrostatic precipitator showed that 85% of the mercury was elemental and 15% was oxidized.

The results of the injection test with activated carbon is shown below:

From the results it can be seen that significantly higher amount of activated carbon is used and that the overall mercury removal is limited by this state of the art method.

Claims

1. A mercury removal system comprising particle removal means adapted to remove particles from a gas stream, preferably being a flue gas, flowing to the particle removal means, said system further comprising a catalyst element arranged upstream of the particle removal means in such a manner that the gas stream contacts catalytic surface(s) of the catalyst element before flowing to the particle removal means.

2. A mercury removal system according to claim 1, wherein the particle removal means is/are designed to capture particles resulting from a combustion process and particles having adsorbed mercury.

3. A mercury removal system according to claim 1 or 2, comprising a first chamber having an inlet for receiving the gas stream and wherein the particle removal means is/are arranged, said first chamber further comprising an outlet for delivery of the gas stream after flowing through the particle removal means.

4. A mercury removal system according to claim 3, wherein the catalyst element is arranged within said first chamber.

5. A mercury removal system according to claim 3 or 4, wherein the first chamber comprising means for emptying out of the chamber, the particles removed from the gas.

6. A mercury removal system according to claim 5, wherein the means for emptying out are hoppers.

7. A mercury removal system according to claim 1 or 2, wherein the catalyst element is arranged outside said first chamber.

8. A mercury removal system according to claim 7, wherein the catalyst element is mounted in a second chamber being distinct from the first chamber.

9. A mercury removal system according to claim 7, wherein the catalyst element is mounted in a duct terminating at said inlet.

10. A mercury removal system according to any of the preceding claims, wherein the particle removal means comprising a number of electrodes arranged to form an electrostatic precipitator.

11. A mercury removal system according to any of the preceding claim, wherein the particle removal means comprise or further comprise a bag filter type.

12. A mercury removal system according to any of the preceding claims, wherein the system comprising a further particle removal means arranged upstream of the catalytic element.

13. A mercury removal system according to any of the preceding claims, wherein the catalyst element is shaped as an element having elongated flow channels, said element being preferably shaped as a honey comb element.

14. A mercury removal system according to any of the preceding claims, comprising a flow distribution device arranged upstream of the catalyst element.

15. A mercury removal system according to any of the preceding claims, wherein the catalyst element is shaped as an inlet distribution device.

16. A mercury removal system according to any of the preceding claims, wherein one or more parts of the surface(s) of the catalyst are orientated in an upstream direction relatively to the flow direction of the gas stream.

17. A mercury removal system according to any of the preceding claims, wherein the catalyst element comprises at least two rows of elongated elements arranged non-parallel to, such as perpendicular to, the flow direction of the gas stream, at least one of the at least two rows has a catalytic surface.

18. A mercury removal system according to claim 17, wherein the catalyst element comprises a multiplicity of elongated elements in the form of U-impactor screens arranged in rows, preferably four rows, wherein the screens in each row are staggered in relationship to screens in a downstream row.

19. A mercury removal system according to claim 18, wherein catalyst element has four rows of U-impactor screens and wherein the two rows being most downstream have catalytic surfaces and wherein the remaining rows constitute a further particle removal means.

20. A mercury removal system according to any of the claims 1-15, wherein the catalyst element comprising a plurality of elongated tubular members, at least a part of which has catalytic surface(s).

5 21. A mercury removal system according to claim 20, wherein at least parts of the interior surfaces of the elongated tubular members are catalytic surfaces.

22. A mercury removal system according to claim 20 or 21, wherein the elongated tubular members are arranged so that their longitudinal extension being parallel to the flow

10 direction of the gas stream, the elongated tubular members are preferably funnel-shaped with their largest cross sectional area facing inflowing gas stream.

23. A mercury removal system according to any of the preceding claims, further comprising adsorption agent injection means for injecting one or more adsorption agents,

15 preferably activated carbon, said adsorption agent being preferably injected upstream of the catalyst element.

24. A mercury removal system according to any of the preceding claims, further comprising reagent injection means for injecting, upstream of the catalyst element, a

20 reagent capable of adsorption or reaction, in particularly chemically reacting, with substances, preferably being oxidised mercury present in the gas stream.

25. A mercury removal system according to any of the preceding claims, further comprising a temperature controlling device, such as an heat exchanger, controlling the

25 temperature of the gas temperature to be within 230-320°F, such as between 230-300°F preferably between 230-280°F when measured upstream of the catalyst element.

26. A mercury removal system according to any of the preceding claims, wherein mercury removal system including the catalyst element or the catalyst element it self is adapted to

30 be exchanged, preferably in an easy manner.

27. A mercury removal system according to any of the preceding claims, wherein the mercury removal system comprises recycling means for recycling carbon rich fly ash.

35 28. A mercury removal system according to any of the preceding claims, wherein one or more of the following features is/are adjustable: the position of the catalyst, upstream of the electrostatic precipitator; the position of distribution means is adjustable; the width and/or height of chamber in which the particle removal means is(are) arranged; the flow area of the inlet and/or outlet; the shape of the electrodes of the electrostatic precipitator; 5 - the pitch of catalyst element the shape of the distribution device; the distance between distribution device and catalyst element; the distance between catalyst element and electrodes of the electrostatic precipitator; 10 - the distance between distribution device and electrodes(ESP); the distance between the inlet and the outlet.

29. A mercury removal system according to any of the preceding claims, wherein the mercury removal system is adapted to receive or comprise two or more catalyst elements

15 arranged in series.

30. A mercury removal system according to any of the preceding claims, being adapted to or operated to, preferably by controlling the temperature of the flue gas and/or the amount of activated carbon injected into the flue-gas, provide 90% overall mercury

20 removal, preferably more than 95% removal and more preferably more than 99,5% removal.

31. A process for removal of mercury from a gas stream, preferably being a flue gas or the like, said process comprising

25 - contacting catalytic surface(s) of a catalyst element with the gas stream; and subsequently removing particles from the gas stream by utilising particle removal means.

32. A process according to claim 31, further comprising injecting an adsorption agent, 30 preferably being activated carbon, upstream of the catalytic element.

33. A process according to claim 31 or 32, further comprising injecting a reagent, preferably being hydrochloric acid upstream of the catalyst element.

35 34. A process according to any of the claims 31-33, further comprising controlling the temperature upstream of the catalyst element to be within 230-320°F, such as between 230-300°F preferably between 230-280°F when measured upstream of the catalyst element.

35. A process according to any of the claims 31-34, further comprising the step of recycling the adsorption agent, preferably activated carbon

36. A process according to any of the claims 31-35 further comprising the step of measuring one or more of the following quantities:

- the amount of active carbon in gas stream

- the amount of oxidised mercury in gas stream and/or

- the amount of chlorine in gas stream.

37. A process according to any of the claims 31-36 being adapted to or comprising controlling the process to, preferably by controlling the temperature of the flue gas and/or the amount of activated carbon injected into the flue-gas, provide 90% overall mercury removal, preferably more than 95% removal and more preferably more than 99,5% removal.