Process for cleaning a stream of flue gas from a combustion device

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European patent application No.

18207937.6, filed on November 23, 2018, the whole content of this application being incorporated herein by reference for all purposes.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a process for cleaning a stream of flue gas from a combustion device, more particularly a flue gas originating from the combustion of a fossil fuel (and more particularly coal boiler) which contains noxious acidic compounds, by means of a basic sodium reagent.

BACKGROUND

Examples of such noxious acidic compounds are HC1, NO _x , SO2, SO3.

SO3, for example, is a noxious gas that is produced from the combustion of sulfur-containing fuel. When present in flue gas, the SO3 can form an acid mist that condenses in electrostatic precipitators, ducts or bag houses, causing corrosion. SO3 at concentrations as low as 5-10 ppm in exhaust gas can also result in white, blue, purple, or black plumes from the cooling of the hot stack gas in the cooler air in the atmosphere.

Sodium bicarbonate in powder form is a known basic sodium reagent for cleaning gases of acidic compounds. It finds application in particular for cleaning fumes of oxides of sulphur, of oxides of nitrogen (especially of nitric oxide) and of hydrogen halides of general formula HX (in particular, of hydrogen fluoride and of hydrogen chloride). Fumes of this kind are commonly generated by the incineration of domestic refuse or hospital waste and by the combustion of fuels of fossil origin, especially in electricity-producing power stations or in industrial boilers.

In these applications, the gas to be cleaned is contacted with the sodium bicarbonate in the form of a finely ground powder at a temperature which is generally between 120 and 600°C, more generally between 180 and 400°C.

After reaction of the reagent, the flue gas is submitted to a separation in order to separate the reacted reagent from the cleaned flue gas. Separation is frequently operated by means of baghouse filters or electrostatic separators. A flue gas cleaning process based on dry injection of a finely ground powder of sodium bicarbonate is disclosed in EP0740577 (SOLVAY SA).

Sodium sesquicarbonate in powder form is another known basic sodium reagent for cleaning gases of acidic compounds. Sodium sesquicarbonate is commonly used in the form on trona. Trona is a mineral that contains about 85-95% sodium sesquicarbonate (Na ₂ C0 ₃ NaHC0 ₃ 2H ₂ 0). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. Flue gas cleaning processes based on dry injection of a finely ground powder of trona and subsequent separation of reacted trona are disclosed for instance in US7854911 and US7481987 (SOLVAY CHEMICALS, Inc.).

In those known flue gas cleaning processes, the temperature of the flue gas wherein the basic sodium reagent is introduced is advantageously above critical values, in order for the sodium reagent to have optimal reactivity.

In many situations however, injection into flue gases of very high temperatures creates difficulties during the separation step of the reacted reagents, in view of the required higher thermal resistance of the separators.

US2011/0014106 discloses a flue gas treatment process wherein sodium bicarbonate is pre-calcined, preferably in a fluid bed, before injection in the flue gas.

In IT 1306648 is disclosed a process for the cleaning of a flue gas having a temperature below 140°C, wherein sodium bicarbonate is submitted to a thermal treatment in order to decompose it into sodium carbonate, which is thereafter injected into the flue gas.

Those processes have however several disadvantages. The thermal treatment consumes energy and/or requires a separate equipment. Moreover, the handling of the sodium carbonate produced in the thermal treatment is difficult in order to avoid a severe loss of performance as the obtained sodium carbonate has a high BET specific surface that decreases fastly in few minutes when in presence of humidity and when stored.

US 9221010 aims at solving these problems by providing a two-step process for the cleaning of a stream of flue gas, containing noxious acidic compounds, by means of a quantity of basic sodium reagent, wherein

• the flow path of the stream of flue gas comprises a bypass point and a

reintroduction point; • the stream of the flue gas is separated at the bypass point into a bypass secondary flue gas stream and a main flue gas stream, the secondary stream being at most 50% in weight of the main stream;

• the quantity of basic sodium reagent is injected in the secondary bypass

stream wherein it reacts in a first step with the noxious acidic compounds contained in the secondary stream during a pre-reaction period, resulting in pre-reacted basic reagent, un-reacted basic reagent and partially cleaned secondary stream of flue gas;

• the temperature of the bypass secondary flue gas stream at the basic sodium reagent injection point is at least 50°C higher than the temperature of the main flue gas stream at the reintroduction point;

• after the pre-reaction period, the partially cleaned secondary stream

comprising the quantity of pre reacted basic sodium reagent and un-reacted reagent is reintroduced in the main stream at the reintroduction point, the pre- reacted reagent and un-reacted reagent then further reacting in a second step with at least part of the noxious acidic compounds contained in the main stream of the flue gas and at least part of the ones left in the partially cleaned secondary stream of flue gas, resulting in reacted reagent and cleaned flue gas.

While such a process is quite successful for flue gases which have a low

SOx (such as SO ₂ and SO ₃ ) and dust (fly ash) content, it is generally not applicable to flue gases with high NOx or SOx, and/or dust content like those resulting from the combustion of fossil fuels (especially coal). Moreover the presence of NOx and/or SOx in the gas for pre-calcining basic sodium reagents induces corrosion on cold points of the sodium basic reagent grinding and ducts devices, and induces also the generation of sodium nitrite/ nitrate salts or sodium sulphite/ sulphate salts that are hygroscopic and build-up on the injection devices that are used for injecting the basic sodium reagents in the flue gas. This increases pressure-drop in corresponding devices needing so bigger fan to compensate said increased pressure drop, and increases also the maintenance and the cleaning of said device. The present invention hence aims at providing a process which is suitable for cleaning such high loaded flue gases in pollutant and/or in fly ashes. SUMMARY OF THE INVENTION

Therefore, the present invention concerns a process for cleaning a stream of flue gas from a combustion device comprising a treatment of said stream with a basic sodium reagent which has at least partly been pre-calcined, wherein: - at least part of the flue gas stream is used to preheat air;

- at least part of the pre-heated air is used for the pre-calcination of the basic sodium reagent.

In a preferred embodiment, the present invention relates to a process as described above wherein:

- a first stream of the pre-heated air is fed to the combustion device; and

- a second stream of the pre-heated air is used for the pre-calcination of the basic sodium reagent.

A first advantage of the present invention is the longer operation time of organ-pipes injection or nozzle injection before cleaning, as solid incrustation in such nozzles is reduced.

A second advantage of the present invention is the recovery of low temperature heat from flue gas before going to chimney that increases the overall heat balance of the combustion device.

A third advantage of the present invention is a decreased corrosion rate in the pipes circuits bringing hot gas (preheated air in present invention) to the basic sodium reagent pre-calcination section.

A fourth advantage of present invention, is a reduced investment of equipment when an already installed air preheater is available in the combustion device, as the needed pre-heated air flow to pre-calcine the basic sodium sorbent is a small part of the pre-heated air available; generally less than 15%, more generally less than 10% of the total pre-heated air of the combustion device, and is at a sufficient pressure to cover the pressure drop of the pre-calcination section and of the injection nozzles in the flue gas stream, in particular when of organ- pipe type nozzles is used.

A fifth advantage of present invention, is an improved mixing of sodium basic reagent with flue gas stream, with an increased gas flow comprising pre heated air to inject the pre-calcined basic sodium reagent.

A sixth advantage of present invention, is a reduced amount of basic sodium reagent used for the treatment of flue gas stream.

A seventh advantage of present invention, is when using an on-site grinder of the basic sodium reagent for obtaining a fine powder of said reagent, preferably with a mean diameter comprised between 5 and 30 pm, and using said on-site grinder under pressure, adding for instance a fan to feed air into the grinder, as it improves grinder fan reliability compared to a fan installed after the grinder to inject the mixed air-basic sodium reagent in the stream of flue gas.

An eighth advantage of present invention, is when an on-site grinder of the basic sodium reagent is used for obtaining a fine powder of said reagent, preferably with a mean diameter comprised between 5 and 30 pm, to use said on site grinder under pressure, by using pre-heated air at a pressure above ambient pressure for feeding the grinder with hot air. This enables to increase the contact time of the basic sodium reagent with hot gas to pre-calcine said reagent while enabling to save a fan associated with the grinder equipment, and avoiding the use of corrosive acidic gas stream when taken on a by-pass on upper stream of the flue gas to be treated.

A ninth advantage of present invention, is when the basic sodium reagent is grinded in a mill equipped with a ventilator with optional particle size air- selector, the heat exchanger can be fed with cool air with a ventilator, so that heated air is under slight pressure, and therefore can be injected with the basic sodium reagent for pre-calcination purpose exiting the mill, without the need of an extra ventilator in the pre-calcination section, and therefore the pre-calcined reagent can be easily injected in the flue stream to be cleaned that is generally under slight vacuum relative to atmospheric pressure. This is particularly interesting for simplifying the investment and maintenance of the pre-calcination section.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of preferred embodiments of the invention, reference will now be made to the accompanying drawing, in which:

Figure 1 illustrates a particular embodiment of the process of the present invention.

DEFINITIONS

According to the invention, by“cleaning” is meant removing at least part of one of the above mentioned noxious acidic compounds. Preferably, at least the SOx are substantially removed/reduced in content.

By“treatment” is meant that the stream of flue gas to be cleaned is contacted with the reagent and chemically reacts with some of its components. Generally, the reagent is injected into the stream of flue gas to be cleaned. In the present specification“air” refers to the gas surrounding the earth, comprising generally about 78 vol. % nitrogen, about 21 vol. % oxygen, and the reminder comprising carbon dioxide, argon and other well-known minor gases.

In the present specification“air” may also refers to the gas surrounding the earth comprising a limited amount of flue gas or of a combustion gas, in such case it comprises at most 30 vol. %, preferably at most 20 vol.%, more preferably at most 5 vol. % of flue gas or combustion gas. This may be the case when air for instance is preheated with a hot flue gas or a hot combustion gas in a heat exchanger that is not airtight.

By“precalcined basic sodium reagent” is meant a basic sodium reagent comprising in his crystal lattices molecule of water and or CO2, released with temperatures, and inducing an increase of BET specific surface of the reagent measured according ISO 9277:2010 standard. Examples of such pre-calcined reagent may be: sodium bicarbonate (NaHCCh), nahcolite (natural mineral comprising generally 40 to 98% of sodium bicarbonate), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), wegscheiderite (Na ₂ C03.3NaHC03), or trona (natural mineral comprising generally 60 to 98% of sodium sesquicarbonate); each of them being at least a partially calcined product in which at least 10%, preferably at least 30%, and more preferably at least 60% molar mineral has been calcined.

The term‘comprising’ includes‘consisting essentially of’ and also

“consisting of’.

In addition, if the term "about" is used before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term "about" refers to a +-10 % variation from the nominal value unless specifically stated otherwise.

The sign‘%’ refers to‘weight %’ unless specifically stated otherwise.

Any reference to‘an’ element is understood to encompass‘one or more’ elements.

In the present application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that in related embodiments explicitly contemplated here, the element or component can also be any one of the individual recited elements or components, or can also be selected from a group consisting of any two or more of the explicitly listed elements or components. Any element or component recited in a list of elements or components may be omitted from such list.

Further, it should be understood that elements and/or features of an apparatus, a process, or a method described herein can be combined in a variety of ways without departing from the scope and disclosures of the present teachings, whether explicit or implicit herein.

The use of the singular‘a’ or‘one’ herein includes the plural (and vice versa) unless specifically stated otherwise.

As used herein, the phrase‘A and/or B’ for elements A and B refers to the following possible selections: element A; or element B; or combination of elements A and B (A+B). Further, it should be understood that elements and/or features of a composition, a process, or a method described herein can be combined in a variety of ways without departing from the scope and disclosures of the present teachings, whether explicit or implicit herein.

In the present specification, the description of a range of values for a variable, defined by a low limit, or a high limit, or by a low limit and a high limit, also includes the embodiments where the variable is selected respectively in the value range: excluding the low limit, or excluding the high limit, or excluding the low and high limits.

DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS

In the process of the invention, the cleaning by the basic sodium reagent can be assisted by another cleaning means and/or chemical. For instance, if the stream of flue gas to be cleaned contains a high concentration of NOx, it is advantageously also submitted to an SCR (Selective Catalytic Reduction) treatment using an adequate catalyst and urea or ammonia injection in order to reduce its NOx concentration. This treatment is preferably applied prior to the treatment with the basic sodium reagent and even, before the air preheater since the minimum temperature for the DeNOx catalyst to be effective is 165 °C. SCR catalysts are generally made from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components which are usually oxides of base metals (such as vanadium, molybdenum and tungsten), zeolites, or various precious metals. A catalyst for DeNOx that gives good results is V2O5 on a T1O2 carrier.

In a preferred embodiment of the invention, the stream of flue gas to be cleaned is also submitted to de-dusting (removal of fly ash essentially), for instance by means of an electrostatic precipitator. This treatment is also preferably applied prior to the treatment with the basic sodium reagent since the absence of fly ash enables reducing the cleaning frequency of the bag filter usually used in the process of the invention, thus keeping the reagent much longer available to react, thus decreasing the reagent consumption. The residence time of the reagent in the bag filter is namely important to get the best possible usage rate of it. To get good performances, typically the reagent should preferably remain 12 to 24 hours in the bag filter. When a lot of dust is already present besides the injected reagent, the cleaning cycle of the bag filter will have to be increased to keep the filter into operation (so for example the residence time of the reagent will decrease from 12 hours to 1 hour). In this case the reagent has less time to react, which gives a degraded usage rate of it.

The process of the invention is for cleaning a flue gas i.e. an exhaust gas from a combustion device like an oven, furnace, boiler or steam generator, preferably a boiler like an industrial boiler. An industrial boiler is generally aimed at providing steam to an industrial site. The source of heat for the boiler is the combustion of a fuel such as coal, biomass, sludge, oil or natural gas. The present invention is particularly suitable for coal boilers which generate a flue gas rich in SOx and dust.

Preferably, all the flue gas exiting the combustion device is cleaned by the process of the invention. Also preferably, the entire stream of flue gas to be cleaned is used to preheat air. This pre-heating can be done in any appropriate device like for instance stationary tubular preheater, stationary plate preheater, stationary brick preheater, rotary drum heat exchanger, and rotating-plate preheater. Generally, the preheater is coupled to a fan to assist the intake of cold air. This fan can be integrated in the intake of cold air like in a Ljungstrom type air preheater, which gives good results within the frame of the invention.

In the process of the invention the basic sodium reagent is generally a powder of solid particles, preferably selected from sodium bicarbonate, sodium sesquicarbonate and trona particles, more preferably sodium bicarbonate particles.

The basic sodium reagent is preferably a powder of particles having mean D _m diameters less than 25 pm, more preferably less than 20 pm, and even more preferably less than 15 pm. The mean diameter of the particles of the powder is defined by the equation:

in which n, denotes the frequency (by weight) of the particles of diameter Di. These particle size parameters are defined by the laser scattering analysis method using a MASTERSIZER S measurement instrument manufactured by Malvern, used in wet mode with the MS 17 DIF 2012 accessory.

The basic sodium reagent is also preferably a powder of particles having a particle size slope of less than 5, preferably less than 3, the particle size slope being defined by the equation:

s = Don - Dm ,

D _m

in which D ₉₀ (and Dio) represent, respectively, the diameter for which 90% and 10%, respectively, of the particles of the powder (expressed by weight) have a diameter of less than D ₉₀ and Dio, respectively. These particle size parameters are defined by the laser scattering analysis method using a measurement apparatus such as those described above.

The D _9O diameter is advantageously lower than 20 pm, preferably lower than 16pm. Preferred particle sizes for the powder injected into the flue gas correspond to a mean particle diameter of 5 to 30 pm, a particle size slope of 1 to 3 and D ₉ o diameter less than 16 pm. Further information regarding the optimum particle size parameters can be obtained from the

patent EP 0 740 577 B1 mentioned above [SOLVAY (Societe Anonyme)].

Such particle size optimum parameters can advantageously be obtained by milling the basic sodium reagent in a process in which a cleaning agent can optionally be mixed with the basic sodium reagent , said process using preferably an impact mill, for the purpose of obtaining a powder with a mean diameter comprised between 5 and 30 pm and of inhibiting the formation of incrustations in the mill, the cleaning agent being selected from zeolites, dolomite, magnesium hydroxycarbonate, lime, hydrocarbons, talc, fatty acids and fatty acid salts.

According to the invention, the basic sodium reagent is at least partly pre calcined. This means in fact partial preliminary activation (so before reaction with SOx) by calcination. The degree of activation depends on the process conditions concerning PSD (Particle Size Distribution), residence time, temperature, chemical composition, crystalline structure, etc.

In chemistry, "activation" refers to the reversible transition of a molecule into a nearly identical chemical or physical state, with the defining characteristic being that this resultant state exhibits an increased propensity to undergo a specified chemical reaction.

The IUPAC defines calcination as "heating to high temperatures in air or oxygen". However, calcination is also used to mean a thermal treatment process in the absence or limited supply of air or oxygen applied to ores and other solid materials to bring about a thermal decomposition.

In a preferred embodiment of the invention, the milled basic sodium reagent is injected into the stream of flue gas to be cleaned by an injection device (a preferred embodiment of which being described below); there is a connection (generally a pipe) between the mill and the injection device in which the pre heated air is injected; and“cold” (generally at ambient temperature) air is injected into the mill or upstream of it using a fan or any other means providing an homogenous mixture of the cold air and the reagent.

In this embodiment, the temperature in the mill generally ranges between

30 et 70°C (depending on the temperature of the ambient air).

It is worth noting however that this embodiment (with a fan in or upstream of the mill) is also advantageous outside the scope of the present invention for any process that would clean a flue gas by means of a basic sodium reagent that has to be milled. It is namely so that the Applicant found out that when putting the fan downstream of the mill, said fan encounters mechanical problems after a while.

In a sub-embodiment, there is a fan located on the connection (pipe) between the mill and the injection device. However, the slight over-pressure of the pre-heated air and of the air from the mill, in combination with the slight under-pressure generally present at the injection point of the reagent in the flue gas, is in general sufficient to mix the air streams and inject the mix into the flue gas duct without using an extra fan to compensate for the extra pressure loss over the extra ductwork. Hence, in a preferred sub-embodiment, the line feeding the pre-heated air to its injection point does not comprise a fan or any other pressure generation device.

In another embodiment of the invention, the pre-heated air is directly injected into the mill in order to generate a homogenous mixture of hot air and pre-calcined reagent which is injected into the stream of flue gas to be cleaned. This embodiment also allows in all cases sparing a fan if the pre-heated air is under not sufficient over-pressure and if there is not enough under-pressure at the injection point of the reagent in the flue gas.

As set forth above, in the process of the invention, the pre-heated air is generally under slight over-pressure (thanks to the fan which is generally coupled to/integrated in the intake of cold air of the preheater as set forth above) and the injection point of the reagent in the flue gas is under slight under pressure due to a fan put downstream of it.

Generally, the absolute values of the over-pressure and under-pressure are above 10 mbar, preferably above 20 mbar, more preferably above 30 mbar.

In a preferred embodiment, which is also useful outside the scope of the present invention for any process that would clean a flue gas by means of a basic sodium reagent, there are at least 2, preferably at least 4 injections points of the basic sodium reagent into the stream of flue gas to be cleaned and the flow of said reagent between these injection points is equilibrated by means of flow distributor(s). It is namely so that the flow of flue gas is high compared to the flow of air/reagent that must be injected into it so that a homogeneous dispersion is difficult to achieve with a single injection point. By“injection point” is meant the location of entry of the flow of reagent into the flue gas duct, which entry can be made through an injection device equipped with multiple orifices (see below).

Preferably, these 2 or 4 injections points are equipped with 1 or 2 pairs of identical injection devices.

The flow distributors preferably are reversed « Y » shaped pieces having their branches adapted in diameter and length for in and output. More preferably, the ductwork after the split (i.e. both branches of the Y) has the same shape and length to make sure the pressure drop is substantially identical in both branches. This allows evenly splitting the flow, preferably amongst the above mentioned pairs of injection devices. Preferably, the different flow-circuit sections are arranged by pairs of overall same pipe length and overall same number of pipe bend(s) to make a uniform pressure loss and/or velocity preferably at +/- 20%, more preferably at +/- 15%.

The injection of the basic sodium reagent in the stream of flue gas to be cleaned can in some instances be performed advantageously with a device comprising a pipe provided with at least one inlet orifice and with a series of outlet orifices spread along the pipe and placed in the side wall of this pipe, the pipe having an open downstream end, acting as supplementary outlet orifice whose diameter is less than the diameter of the pipe, at least one section of the wall of the pipe, located downstream of at least one outlet orifice and limited by a section of the edge of this orifice, having a shape such that this section of the edge of this orifice is positioned inside the pipe so that, when the device is in service, the flow direction of the fluid exiting this orifice and travelling along said wall section, is controlled by the shape of the latter section. Details and explanations on this device can be found in WO2010049534 (SOLVAY SA). Owed to its geometry, it is also called an“organ pipe”. Preferably, all 2 or 4 injection points are equipped with such organ pipes.

In a preferred embodiment, each organ pipe is equipped with at least 3 orifices and owed to the global flow increase resulting from the injection of pre-heated air, which leads to an increase of the pipe diameter, there are most preferably at least 5 orifices in each organ pipe. In a preferred embodiment, each organ pipe is equipped with at least 5 orifices comprising an open downstream end 4 outlet orifices spread along the pipe, placed in the side wall of this pipe and which are aligned 2 by 2 on 2 parallel lines.

In the process according to the invention, the temperature of the flue gas at the basic sodium reagent injection point(s) is generally at least 80°C, preferably at least 100°C, more preferably at least 120°C. It is generally less than 160°C, preferably less than 140°C.

The amount of basic sodium reagent to be injected is generally calculated based on the amount of SOx to be neutralized using the stoichiometry of its reaction with the reagent. Generally, an industrial boiler consumes up to 2, even 3 and even up to 4 kg of reagent per ton of steam produced.

The percentage of the pre-heated air stream which is used for the pre- calcination generally is less than 10%; it is preferably at least 2% and more preferably at least 5% of the total flow of pre-heated air (the rest being fed to the boiler).

The temperature of the pre-heated air is generally comprised between 230°C° and 380 °C. Hence, a mix ratio of 40% hot air leads to a temperature of about 140 °C at the injection point (and respectively about 160°C for a 50% hot air ratio and 180 °C for a 60% hot air ratio).

Especially when the above temperatures and mix ratio are respected, the process of the invention does not lead to clogging of the injection devices. This is quite surprising since the water and basic precipitate generated by the calcination reaction are expected to form agglomerates under such high temperature conditions resulting in crust formation on obstacles inside the piping.

At the end of the cleaning process, residues from the reaction between the basic sodium reagent and the stream of flue gas to be cleaned are separated from the cleaned flue gas stream before it is released to the atmosphere, generally through a chimney. Especially when the basic sodium reagent is a powder of solid particles, the removal step can be performed by a filter such as a bag filter or by use of electrostatic precipitators. Generally, a fan or any other

vacuum/pressure generating device is used downstream of this filter in order to ensure a correct flow of flue gas through said filter.

The present invention relates also to a number of embodiments, which are more completely detailed below as items.

Item 1. Process for cleaning a stream of flue gas from a combustion device comprising a treatment of said stream with a basic sodium reagent which has at least partly been pre-calcined, wherein:

- at least part of the flue gas stream is used to preheat air;

- at least part of the pre-heated air is used for the pre-calcination of the basic sodium reagent.

Item 2. Process according to item 1, wherein:

- a first stream of the pre-heated air is fed to the combustion device; and - a second stream of the pre-heated air is used for the pre-calcination of the basic sodium reagent.

Item 3. The process according to item 2, wherein the pre-heated air is fed to the combustion device for a combustion reaction as source of at least part of oxygen used in the combustion reaction.

Item 4. The process according to any of the preceding items, wherein the stream of flue gas to be cleaned contains a high concentration of NOx and is submitted to an SCR treatment using an adequate catalyst and urea or ammonia injection prior to the treatment with the basic sodium reagent.

Item 5. The process according to the preceding item, wherein the stream of flue submitted to the SCR treatment is at a temperature of at least 160°C, preferably of at least 180°C.

Item 6. The process according to items 5 or 6, wherein the stream of flue submitted to the SCR treatment is at a temperature of at most 550°C, preferably of at most 360°C.

Item 7. The process according to any of the preceding items, wherein the stream of flue gas to be cleaned is submitted to de-dusting prior to the treatment with the basic sodium reagent.

Item 8. The process according to any of the preceding items, wherein the combustion device is a coal boiler.

Item 9. The process according to any of the preceding items, wherein the combustion device uses a carbonaceous fuel for its combustion, and wherein preferably the carbonaceous fuel is selected among: coal, oil, wood, agriculture wastes, paper wastes, plastic wastes, automotive industry wastes, municipal wastes or mixtures thereof

Item 10. The process according to any of the preceding items, wherein the flue gas is selected among: glass kiln flue gas, cement kiln flue gas, metallurgy flue gas, industrial-wastes flue gas, ceramics-oven flue gas, oil refinery flue gas, or petro-chemical flue gas.

Item 11. The process according to any of the preceding items, wherein the basic sodium reagent is a powder of solid particles selected from sodium bicarbonate, nahcolite, sodium sesquicarbonate and trona particles, preferably sodium bicarbonate particles.

Item 12. The process according to item 11, wherein the powder of solid particles has a mean diameter of at most 30 pm, preferably of at most 15 pm.

Item 13. The process according to items 11 or 12, wherein the powder of solid particles has a mean diameter of at least 1 pm, preferably of at least 5 pm.

Item 14. The process according to any items 11 to 13, wherein the powder of solid particles has a particle size slope of less than 5, preferably less than 3, the particle size slope being defined by the equation:

s = D _9Q - Dio ,

D _m

wherein D ₉₀ (and Dio) represent, respectively, the diameter for which 90% and 10%, of the particles of the powder (expressed by weight) have a diameter of less than D ₉₀ and Dio, respectively.

Item 15. The process according to any one of the preceding items, wherein the pre-calcination of the basic sodium reagent is operated at a temperature of at least 130°C, preferably of at least 140°C.

Item 16. The process according to any one of the preceding items, wherein the pre-calcination of the basic sodium reagent is operated at a temperature of at most 360°C, preferably at most 300°C.

Item 17. The process according to the preceding item, wherein the pre calcination of the basic sodium reagent is operated at a temperature between 160°C and 280°C, preferably between 180°C and 260°C.

Item 18. The process according to any items 11 to 17, wherein the pre calcination of the basic sodium reagent, to obtain an at least partially pre- calcined basic sodium reagent, before contacting the at least partially pre- calcined basic sodium reagent with the flue gas stream, has a duration of at least 0.5 s, preferably at least 1 s.

Item 18. The process according to the preceding item, wherein the duration is at most 5 s, preferably at most 3s.

Item 19. The process according to any one of the preceding items, wherein the contact time of the flue gas stream to be treated with the at least partially pre calcined basic reagent, is at least 1 s, or at least 3s, and preferably at most 30 s, more preferably at most 20 s.

Item 20. The process according to any one of the preceding items, wherein the ratio of the at least pre-heated air reported to the basic sodium reagent for the pre-calcination of the basic sodium reagent is at least 5, preferably at least 10 Nm3 per kg of basic sodium reagent.

Item 21. The process according to any one of the preceding items, wherein the ratio of the at least pre-heated air reported to the basic sodium reagent for the pre-calcination of the basic sodium reagent is at most 100, preferably at most 50 Nm3 per kg of basic sodium reagent.

Item 22. The process according to any one of the preceding items, wherein the basic sodium reagent is milled in a process in which a cleaning agent can optionally be mixed with the basic sodium reagent, said process using preferably an impact mill, for the purpose of obtaining a powder with a mean diameter comprised between 5 and 30 pm and of inhibiting the formation of incrustations in the mill, the cleaning agent being selected from zeolites, dolomite, magnesium hydroxycarbonate, lime, hydrocarbons, talc, fatty acids and fatty acid salts.

Item 23. The process according to the preceding item, wherein the milled basic sodium reagent is injected into the stream of flue gas to be cleaned by an injection device; wherein there is a connection between the mill and the injection device in which the pre-heated air is injected; and wherein cold air is injected into the mill or upstream of it using a fan or any other means providing an homogenous mixture of the cold air and the basic sodium reagent.

Item 24. The process according to item 20, wherein the pre-heated air is injected into the mill in order to generate a mixture of hot air and pre-calcined basic sodium reagent which is injected into the stream of flue gas to be cleaned.

Item 25. The process according to any one of the preceding items, wherein there are at least 2, preferably at least 4 injections points of the basic sodium reagent into the stream of flue gas to be cleaned, wherein these 2 or 4 injection points are equipped with 1 or 2 pairs of identical injection devices and wherein the flow of said reagent between these injection points is equilibrated by means of flow distributor(s).

Item 26. The process according to any one of the preceding item, wherein the flow distributors are reversed « Y » shaped pieces having their branches adapted in diameter and length for in and output so that the pressure drop is substantially identical in both branches of the Y.

Item 27. The process according to any one of the preceding items, wherein the basic sodium reagent is injected into the stream of flue gas to be cleaned with an organ pipe or a distribution device comprising a pipe provided with at least one inlet orifice and with a series of outlet orifices spread along the pipe and placed in the side wall of this pipe, the pipe having an open downstream end, acting as supplementary outlet orifice whose diameter is less than the diameter of the pipe, at least one section of the wall of the pipe, located downstream of at least one outlet orifice and limited by a section of the edge of this orifice, having a shape such that this section of the edge of this orifice is positioned inside the pipe so that, when the device is in service, the flow direction of the fluid exiting this orifice and travelling along said wall section, is controlled by the shape of the latter section.

Item 28. The process according to the preceding item, wherein the organ pipe is equipped with at least 3 orifices.

Item 29. The process according to the preceding item, wherein the organ pipe is equipped with at least 5 orifices comprising an open downstream end 4 outlet orifices spread along the pipe, placed in the side wall of this pipe and which are aligned 2 by 2 on 2 parallel lines.

Item 30. The process according to any of the preceding items, wherein the stream of flue gas to be cleaned wherein the pre-calcined basic sodium reagent is injected, is at a temperature of at most 230°C, preferably of at most 160°C.

Item 31. The process according to the preceding item, wherein the stream of flue gas to be cleaned wherein the pre-calcined basic sodium reagent is injected, is at a temperature between 100 to 160°C, or between 110 and 150°C.

Item 32. The process according to any of the preceding items, wherein residues from the reaction between the basic sodium reagent and the stream of flue gas to be cleaned are separated from the cleaned flue gas stream before it is released to the atmosphere.

Item 33. The process according to the preceding item, wherein residues from the reaction between the basic sodium reagent and the stream of flue gas which has been at least partially cleaned are separated on a bag filter, and wherein the mean contact time of said residues on the filter bags with the flue gas stream is at least 30 minutes, preferably at least 1 hour, more preferably at least 2 hours, more preferably at least 8 hours.

Item 34. A device comprising:

- a combustion device producing a flue gas, said flue gas comprising NOx, SOx, and optionally: dust and/or other acidic gases;

- a SCR DeNOx module comprising an injection mean of a reduction agent such as ammonia, urea, and a catalyst for reducing NOx operating at a temperature of at least 160°C and at most 550°C, preferably at least 180°C and preferably at most 360°C;

- a heat exchanger so that to recover part of the heat of the flue gas exiting the SCR DeNOx module, by heating cool air, to obtain a heated air, said heated air being split:

- in a first stream used as a combustion air for the combustion device, and

- in a second stream used for a pre-calcination of a basic sodium reagent;

and wherein the flue gas exiting said heat exchanger is cooled-down preferably to a temperature of at most 150°C, or at most 130°C;

- a transport pipe of the second stream of heated air to a pre-calcination pipe section

- a pre-calcination pipe section comprising feeding means of a basic sodium reagent in powder form, and wherein said basic sodium reagent is put into contact with the second stream of heated air for at least 0.5 second, preferably at least 1 s, so that to at least partially calcine the basic sodium reagent particles into a pre-calcined basic sodium reagent;

- at least one injection device placed at one end of the pre-calcination pipe section to disperse the pre-calcined basic sodium reagent into the cooled flue gas, and so that said pre-calcined basic sodium reagent react with at least part of the SOx of the cooled-down flue gas;

- a bag filter to separate residues from the reaction between the pre-calcined basic sodium reagent with at least part of the SOx or other optional acidic gases of the cooled stream of flue gas before it is released to the atmosphere.

Item 35. The device according to the preceding item, wherein the first stream of heated air used as combustion gas is about 85 to 95%, and the second stream of heated air used for a pre-calcination of the basic sodium reagent is about 5 to 15%, of the heated air. Item 36. The device according to items 34 or 35, wherein the heat exchanger module comprises a ventilator or an air blower to feed the heat exchanger with cooled air and so that the second stream of pre-heated air used for pre-calcination be under slight pressure relatively to atmospheric pressure when injected in the pre-calcination pipe section with the basic sodium reagent.

Item 37. The device according to the preceding item, comprising a valve in the second stream of heated air pipe feeding the pre-calcination pipe section so that to control the flow-rate and/ or the temperature in the pre-calcination pipe section.

Item 38. The device according to any items 34 or 37, comprising means to remove dust from the flue gas entering or exiting the SCR DeNOx module.

Item 39. The device according to any items 34 to 38, suitable to operate the process of any one of items 1 to 33.

The following example and the annexed Figure 1 illustrate some particular embodiments of the invention.

The legend of Figure 1 is as follows :

1. combustion area (furnace) of the boiler

2. coil comprising water for steam generation

3. cold combustion air

4. NH ₃ or Urea injection

5. SCR catalyst

6. heat exchanger (air pre-heater)

7. electrostatic precipitator

8. fly ash

9. flow divider and organ pipes for the injection of the air/bicarbonate mixture

(or air / trona or air/ sodium carbonate)

10. silo storing sodium bicarbonate

11. dosing screw

12. mill/grinder

13. fans

14. injection of preheated air in the air/ground sodium bicarbonate mixture leaving the mill

14’. injection of pre-heated air into the boiler

15. bag filter

16. residual sodium chemicals

17. chimney The layout illustrated in this Figure works as follows : the boiler (1,2) generates precipitated fly ash and a flue gas (pictured by the thick arrows going from left to right through the entire diagram) into which first, ammonia or urea (4) is injected; the mixture is then subjected to a DeNox process using an adequate catalyst (5); the flue gas is then further cooled down in a heat exchanger (6) where it pre-heats air of which a part (14’) is fed to the boiler and a part (14) is used to pre-calcine sodium bicarbonate; the flue gas then enters an electrostatic precipitator (7) in order to precipitate the fly ash (8) which has not precipitated in/out of the boiler; after that, pre-calcined sodium bicarbonate in hot air is injected through 4 organ pipes and corresponding flow dividers (9); the reaction products (16) from the bicarbonate with the flue gas are then separated from the flue gas in a bag filter (15) and the cleaned flue gas is fed to the chimney (17) by means of a fan (13).

The mixture (suspension) of pre-calcined sodium bicarbonate in hot air is prepared as follows: a metering screw (11) feeds sodium bicarbonate from a storage silo (10) to a mill (12) which is also fed with air through a fan (13); into the suspension of milled sodium bicarbonate in air so obtained, a stream of pre heated air (14) is injected in order to pre-calcine the milled sodium bicarbonate. EXAMPLES

Example 1

The layout described above (related to Fig. 1) was tested with a steam boiler having the characteristics listed in Table 1 below.

Table 1

The grinder type : hammer mill with independent selector for particle size tuning. Fan for cooling and transport air placed upstream of the mill (operating under pressure) The flows used for the pre-calcination were as follows :

Hot air flow from air preheater : 4700 Nm ³ /h and 270 °C (8 % of total preheated air flow )

Air from grinder : 2600 Nm ³ /h and 60°C

Pre-calcination air flow : 7300 Nm ³ /h and 180 °C

Samples of the flue gas were taken in points A0, A1 and A2 and the result of their analysis is summarized in Table 2 below.

Table 2

Example 2

In the same layout as example 1, different set of conditions regarding reagents, pre-calcination temperatures, air-flow to reagent ratio, and pre-calcination duration (before injection of the reagent in the flue gas stream to be treated), and reagent nature, were tested.

Test 2 a

• Sodium bicarbonate flow rate: 250 kg/h

• Pre-calcination of fine sodium bicarbonate mean particle size Dm = 13 pm,

particle size distribution slope <3

• Sodium bicarbonate flow rate: 250 kg/h

• Pre-calcination conditions: Table 3

• Pre-calcined particles residence time in flue gas to be treated (from

injection point in flue gas to the inlet flange of bag filter) at 120°C:

1.3 (+/- 0.15) second.

• Bag filter (inlet channel) volume: 98 m3,

Gas flow of flue gas: 209473 m3/h (at 120°C),

• Additional residence time inside the bag filter before reagent being kept at the surface of the bags: equal to 98/(209473/3600) = 1.7 s (+/-10%);

• This leads to a total contact time of the pre-calcined with flue gas before reagent being kept at the surface of the bags of:

1.3 + 1.7 = 3 s (+/- 10%);

• Supplemental contact time of flue gas stream flowing through the cake formed on the filter bag:

mean pressure drop on bag filter: 125 mm of water column (14mbar operating pressure drop , and 1 5mbar when bag filter is empty of solids) corresponding to about 6 mm thickness of the reagent cake deposited on the surface of the bags:

with corresponding speed of gas through the cake, supplemental contact time of the flue gas with reagent cake is :

0.36 to 0.51 s.

• SO2 Mitigation results (% of SO2 removed from flue gas):

Table 4

Those results show the particular increase of SO2 flue gas mitigation efficacy with pre-calcined basic sodium reagent at 180°C compared to 110°C, when pre calcined sodium reagent is injected in polluted flue gas, and particularly when polluted flue gas are at low temperatures (100-150°C).

Test 2 b (same test conditions as test 2. a unless specified otherwise)

• Sodium bicarbonate flow rate: 140 kg/h

• Pre-calcination of fine sodium bicarbonate mean particle size Dm = 14 pm,

particle size distribution slope <3

• Pre-calcination temperatures: 160° and 180°C

• Other pre-calcination conditions: similar to those of test 2. a

(pre-calcination conditions set of 180°C);

• SO2 Mitigation results (% of SO2 removed from flue gas):

Table 5

Those results show the particular increase of SO2 flue gas mitigation yield with pre-calcined basic sodium reagent at 180°C compared to 110°C, and particularly when polluted flue gas are at low temperatures (100-150°C).

Tests 2 c & 2 d (same test conditions as test 2. a with following modifications) Sodium bicarbonate is replaced with Trona (Test 2.c) and Limestone (Test 2.d) keeping a flow rate of 250 kg/h of solid reagent.

SO2 mitigation efficiency is increased of:

- Test 2.c (Trona) of about 27% when pre-calcined at 180°C (pre-calcination duration: 1.1 s) compared to pre-calcination at 110°C (pre-calcination duration 2.4s),

- Test 2.d (Calcium Limestone): no sensible improvement between pre calcination at 180°C (1.1 s) compared to 110°C (2.4s).

Should the disclosure of any of the patents, patent applications, and publications that are incorporated herein by reference conflict with the present specification to the extent that it might render a term unclear, the present specification shall take precedence.