Patent Description:
The proposed invention is within the scope of the treatment of flue gases deriving from various industrial activities, such as power plants, cement factories and incinerators, and for the purification of exhaust gases from heavy-duty diesel engines, including those used for ship and rail propulsion and for on-board auxiliary services.

In recent years, National Governments in several countries have enacted new environmental regulations in order to reduce polluting emissions and ensure adequate quality standards for human health and the biosphere in general. In the field of atmospheric pollution control, many of these rules concern the control of emissions of sulphur oxides, SOx, and nitrogen oxides, NOx, related to industrial plants and energy production from fossil fuels and biomass. Many countries have also ratified the guidelines promulgated by the International Maritime Organisation (IMO) that defined emission limits for SOx and NOx in ship engine exhaust. These limits are expressed respectively in terms of the permissible sulphur content in fuels and the permissible NOx emission factors, in g/kWh, per engine type. Particularly restrictive rules apply in some marine areas called "Emission Control Areas" and in the territorial waters of many countries.

SOx emissions can be limited and almost eliminated by using low-sulphur fuels, which are typically more expensive and not always available. NOx emissions can be controlled by making appropriate improvements to combustion systems which are thus more expensive and complex to maintain. Alternatively, purification plants for the exhaust gases produced by combustion for the treatment of SOx and NOx are widely used. Among the most widely used processes are dry-scrubbers and spray-dryers or wet-scrubbers for SOx capture and catalytic NOx reduction units "Selective Catalytic Reduction" (SCR) or thermal ones "Selective Non-Catalytic Reduction" (SNCR).

In general, the combined use of these units for SOx and NOx reduction is very costly for reasons linked to installation and running costs, but also to the overall dimensions and weight required for the installation of auxiliary units. Overall dimensions and weight issues are very limiting, especially in the case of retrofitting existing plants and of treatment of the exhaust gases of heavy-duty diesel engines, including those used for propulsion and the auxiliary systems of ships or railways.

A promising alternative to the combined treatment of SOx and NOx from flue-gases is the wet-oxidation scrubbing technology. Unlike traditional technologies that include the use of sea water or the addition of alkalis or carbonates for SOx capture alone, this process involves the installation of a single unit (a wet scrubber column) fed with a stream of dissolved oxidising reagent-based liquid for exhaust gas flushing.

This technology enables SOx and NOx to be captured simultaneously by transferring them from the gas to the absorbing liquid in the form of sulphates and nitrates following oxidation reactions by the oxidising reagent. One of the best oxidisers is sodium chlorite (NaClO<NUM>), not only because of its strong oxidising capacity and the low market price, but also because of its stability in contact with air and light and also because it is easy to transport, being available in solid form.

Already existing patents (and patent applications) use wet-oxidation scrubbing techniques in flushing columns operating with sodium chlorite aqueous solutions as the oxidising agent (see, for instance, <CIT>).

<CIT>refers to a flushing column operating with sodium chlorite solutions and shows that SO<NUM> and NO are completely removed with a dosage of <NUM> ClO<NUM>- but only <NUM>% of NOx is captured. No information is available on the treatment of contaminated flushing waters and whether chlorine gas is present at the outlet from the system.

<CIT>refers to a flushing column split into three sections: in the first one, SO<NUM> is removed with the injection of basic solutions; in the second one, NOx is oxidised with sodium chlorite-based solutions; in the third one, the final flushing takes place with the capture of residual acid gases (generally chlorine gas and NO<NUM>). No reference is made in the document to the content of pollutants and to the dosage of oxidant used, and no information is available on the treatment of contaminated flushing waters.

Both <CIT> and <CIT> have studied the influence of the process on the capture of mercury in the flue-gas and have confirmed that high removals of Hg(°) and Hg(II) can be achieved in the presence of NaClO<NUM> in the flushing solution.

It appears that the current patented solutions do not provide any information on the optimisation of the consumption of the reagent used and on the management of the plant's flushing effluents, nor do they provide for the removal of the chlorine-based by-products generated during the wet-oxidation process of SOx and NOx.

The object of this invention is to provide a plant solution capable of reducing both SOx and NOx up to a few parts per million and at the same time of improving the management of the oxidising reagent in terms of consumption; to control the generation of any by-products formed during the wet-oxidation process; to also manage the effluents produced by the flushing unit to obtain a liquid with characteristics that are potentially suitable for the production of nitrogen-based fertilisers.

A first object of the following invention is to provide a process for the treatment of a gas containing SOx and NOx as claimed in the appended claims that describe preferred variants of the invention, forming an integral part of the present description.

The process generally comprises a main step in which the contaminated gas is brought into contact with a stream of oxidant-based liquid (chlorite, ClO<NUM>-), in order to ensure efficient purification of the gas from Ox and NOx and an efficient conversion of the chlorite ions into chloride ions (Cl-) in the discharged flushing liquid. This step involves a multi-phase contact stage with recycling, endowed with purging and make-up of the absorbing liquid. The process includes ancillary units for the refinement of the exhaust gases and the treatment of washing waters.

The invention proposes a plant according to claim <NUM>, the plant generally comprising the following equipment.

In particular, the plant according to the invention comprises:.

The plant can be constituted by a single column in three sections or by three separate columns, connected with per se known construction methods. More specifically, the process underlying the invention comprises the following fundamental steps:.

Another object of the invention is to purify the liquid L<NUM>-<NUM> at the outlet from the unit S-<NUM>, which is treated with ferrous salts in a basic environment in a treatment unit called F-<NUM> (clarification-flocculation unit or activated carbon adsorption unit). The liquid stream L<NUM>-<NUM> at the outlet from the unit F-<NUM> will have potentially suitable characteristics (in terms of sulphates SO<NUM><NUM>-, nitrates NO<NUM>- and chlorides Cl-) for use in nitrogen-based fertiliser production processes.

A further object is to refine the stream G<NUM> at the outlet from the treatment unit S-<NUM> using a further gas flushing section, called refining unit S-<NUM>, fed with mains water or sea water, and possibly enhanced with strong bases. This unit is optional and should only be used if the exhaust gas G<NUM> contains chlorine-based contaminants, which are formed during the oxidation process in the treatment unit S-<NUM>. It is specified that the addition of strong bases to the stream L<NUM> is optional and is carried out in a mixing unit M-<NUM> where the preparation of the enhanced stream of liquid to be sent to the unit S-<NUM> takes place.

Other objects, embodiments and advantages of the present invention are discussed in the following detailed description and in the appended claims.

The invention is described below on the basis of non-limiting examples illustrated by way of example in the following <FIG>.

<FIG> is a schematic view of an apparatus for implementing the present invention and shows the flow diagram of the exhaust gas treatment plant, comprising a gas treatment unit S-<NUM>, a refining unit S-<NUM> (optional) and a flushing water treatment section, comprising two treatment units S-<NUM> and F-<NUM>.

The present invention relates to a plant for the treatment of SOx and NOx present in flue-gases coming from industrial activities or heavy duty diesel engines, including those for ship and railway applications. The simultaneous capture of SOx and NOx is based on a wet-oxidation process involving flushing with a stream of chlorite-based oxidising liquid (e.g. solid sodium chlorite NaClO<NUM>(s)).

The proposed invention is based on certain operations necessary to improve the existing processes, such as:.

The preferred plant solution according to the invention comprises two distinct sections for the treatment of the exhaust gases and of the flushing waters and an additional optional section for the refinement of the exhaust gases. In any case, the person skilled in the art, by combining the teachings of the present invention with his knowledge of the field, can easily identify the optimal operating conditions for operating the plant, which preferably comprises the following sections:.

The treatment for flushing waters proposed in the invention proposes a conversion of the effluent L<NUM>-<NUM> at the outlet from S-<NUM> into effluent L<NUM>-<NUM> reusable in other industrial processes, achieving a reduction in costs related to the treatment and management operations of the traditional flushing effluents coming from exhaust gas treatment plants. All chlorine-based compounds are converted into chlorides Cl- and the effluent will have potentially suitable characteristics (in terms of sulphates SO<NUM><NUM>-, nitrates NO<NUM>- and chlorides Cl-) for use in nitrogen-based fertiliser production processes. If the gas to be treated in the oxidative scrubbing process contains sufficiently low levels of powders, volatile organic compounds and metals, the purified effluent can guarantee sufficient concentrations of sulphates, nitrates and chlorides such that they can be considered for use for the production of nitrogenous fertilisers. The ferric hydroxide salts Fe(OH)<NUM>(s) formed during the flocculation process can be marketed for the purification of waters polluted by arsenic and phosphates. <NUM>) Optional exhaust gas refining section: also the treatment of gases containing chlorine-based compounds possibly present in the exhaust gas G<NUM> exiting the treatment system S-<NUM> (main section) is managed according to the invention. This treatment is to be considered an improvement with respect to the old processes, which do not take into consideration the possible formation of secondary pollutants in the gas produced by oxidative washing columns. This section can be an additional flushing column S-<NUM> fed by the exhaust gases G<NUM> at the outlet from the unit S-<NUM> and by a fresh liquid stream L<NUM>, consisting of water, also sea water, optionally additivated with a strong base to improve the capture of chlorine gases. The process can be carried out with different gas-liquid contact systems, such as spray columns, filling bodies or plates. The same reasons listed in point <NUM>) for the choice of the type of column can also be extended to the flushing column S-<NUM>.

In the following, to illustrate a preferred and non-limiting embodiment of the invention (also containing the optional equipment), reference will be made to <FIG>, which will be clearer when combined with the following legend:.

Once the manipulable process variables have been set, which are:.

the person skilled in the art, on the basis of his experience and the present description, will be able to estimate the following calculated variables:.

The proposed list of manipulable and calculated variables appears to have only a value for demonstration and non-limiting purposes.

<FIG> illustrates the entire flow diagram of the proposed process for the removal of SOx and NOx from flue-gases deriving from industrial activities, process plants and heavy-duty diesel engines, including those for ship and railway applications. The diagram in <FIG> shows the initial exhaust stream of gas G to be treated, which can be previously cooled to a temperature below <NUM> - <NUM> ° C in the thermal recovery system C-<NUM>.

After cooling, the exhaust gases G<NUM> are distributed into G<NUM> and G<NUM> in the distribution valve SV-<NUM>. The stream of gas G<NUM> is conveyed to the unit S-<NUM> (primary oxidation scrubbing unit) whereas the stream of gas G<NUM> is sent to the unit S-<NUM> (secondary oxidation scrubbing unit). Both units S-<NUM> and S-<NUM> can be equipped with different gas-liquid contact systems: spray systems, filling bodies and plates. The choice of the contact system is made on the basis of specific operating conditions and of process optimisation criteria which, together with the description of the invention and its technical knowledge, are within the reach of the skilled in the art. The unit S-<NUM> is fed with a flow in counter-stream between the streams of gas G<NUM> and G<NUM> and the liquid stream L<NUM>-<NUM> coming from the mixing unit M-<NUM>. This is obtained by the mixing unit M-<NUM> starting from: the recirculation stream L<NUM>-<NUM> exiting the distribution valve SV-<NUM>; the make-up stream L<NUM> (mains water or sea water) exiting the distribution valve SV-<NUM>; the oxidant stream containing chlorite, e.g. sodium chlorite NaClO<NUM>(s); the strong base stream, e.g. NaOH or other strong base. The stream of liquid L<NUM>-<NUM> exiting M-<NUM> and sent to the unit S-<NUM> will have chemical-physical characteristics (in terms of composition, temperature and pH) that depend on the recirculation ratio fixed at the distribution valve SV-<NUM>, rSV-<NUM> = L<NUM>-<NUM>/L<NUM>-<NUM> (easily calculable by the person skilled in the art on the basis of his knowledge and the teachings of the present invention).

In preferential conditions, but not limited to them, the unit S-<NUM> is equipped at the head with suitable sprayers for feeding the absorbing liquid (L<NUM>-<NUM>). Above the nozzles, the unit S-<NUM> is provided with a demister to prevent the possible formation of mists and the entrainment of acid droplets in the overhead gas discharge duct. The bottom of the unit S-<NUM> will be dedicated to the collection of the flushing waters (L<NUM>-<NUM>) and will be high enough to guarantee an obstruction of the gas passage in the pipe dedicated to the discharge of the spent flushing waters (L<NUM>-<NUM>) from S-<NUM>. The exhaust gases G<NUM> and G<NUM> are fed through appropriate distributors. The wet-oxidation process, which takes place in the unit S-<NUM> due to the contact of SOx and NOx with the stream of chlorite-based liquid L<NUM>-<NUM> can be described by the following global equations:
<CHM>
<CHM>.

<NUM>SO<NUM>+<NUM>NO+NO<NUM>+<NUM>ClO<NUM>+<NUM>Cl<NUM>+<NUM>H<NUM>O→<NUM>H<NUM>SO<NUM>+<NUM>HNO<NUM>+<NUM>HCl     (R3).

The reaction (R1) shows the oxidation of SOx and NOx by the chlorite ClO<NUM>-with formation of sulphuric acid (H<NUM>SO<NUM>) and nitric acid (HNO<NUM>). The reaction (R2) shows the decomposition of chlorite under acidic conditions generated by the progress of the reaction (R1), while the reaction (R3) shows a further oxidation step by additional oxidising agents such as chlorine dioxide ClO<NUM> and chlorine Clz, which are found to have similar or greater oxidising power than chlorite ClO<NUM>-. The overall oxidation process will lead to the formation in the liquid of: sulphuric (H<NUM>SO<NUM>, Na<NUM>SO<NUM>, CaSO<NUM>, etc.), nitric (HNO<NUM>, NaNO<NUM>, etc.) and hydrochloric (HCl, NaCl, CaCl<NUM>, etc.) acids and salts. The stream of liquid L<NUM>-<NUM> at the outlet from S-<NUM> will also be characterised by the presence of unreacted ClO<NUM>- (excess quantity) and other reaction products such as: ClO<NUM>-, ClO<NUM> and Cl<NUM>. The gas G<NUM> at the outlet from S-<NUM> may contain contaminations of ClO<NUM> and Cl<NUM> desorbed from the liquid to the gas phase.

The invention proposed in the patent application is based on the use of a recirculation stream of the reagent L<NUM>-<NUM> to control the degree of acidity of the stream L<NUM>-<NUM> being fed to S-<NUM>, providing for a make-up stream of liquid L<NUM>, recharged with the oxidising reagent NaClO<NUM>(s), a strong base to control the pH of the liquid L<NUM>-<NUM> and a total purge stream L<NUM>-<NUM> to the system consisting of the units S-<NUM> and M-<NUM>.

It can be seen from the reactions (R2) and (R3) that an increase in acidity provides a process improvement for the simultaneous capture of SOx and NOx.

The person skilled in the art will realise that the recirculation ratio "rSV-<NUM>" can be increased to simultaneously optimise the consumption of oxidising reagent (NaClO<NUM>(s)) and the purge flow rate (L<NUM>-<NUM>) to be sent to the effluent treatment section in order to achieve a predefined SOx and NOx capture level.

In contrast to conventional processes, where the level of gas conversion or capture efficiency is strongly influenced by the quantity of recirculated reaction by-products, this process can be carried out with a recirculation ratio rSV-<NUM> at the valve SV-<NUM> from <NUM> up to <NUM> without a significant negative influence on SOx and NOx capture. However, particular attention must be paid to phenomena of precipitation of the dissolved solids. A preferred range of the recirculation ratio rSV-<NUM> at the SV-<NUM> valve is <NUM>-<NUM> (validated by laboratory experiments).

Thus, according to a further embodiment of the process of the invention, in the case of a recirculation ratio rSV-<NUM> equal to <NUM>, the splitting of the stream L<NUM>-<NUM> into two separate currents L<NUM>-<NUM> and L<NUM>-<NUM> by means of the distribution valve SV-<NUM> does not take place, so the stream L<NUM>-<NUM> is zeroed and the stream L<NUM>-<NUM> becomes <NUM>% the stream L<NUM>-<NUM> sent to the unit S-<NUM>.

Another strength of this invention is the section related to the refining unit S-<NUM> (gas dechlorination unit) in <FIG>, which is provided for the secondary flushing of the gas from chlorine-based compounds (such as chlorine ClO<NUM> and chlorine Cl<NUM>) possibly present in G<NUM> at the outlet from S-<NUM>.

<FIG> shows the optional dechlorination unit S-<NUM> which can be equipped with different gas-liquid contact systems like for the units S-<NUM> and S-<NUM>. The unit S-<NUM> is fed by the exhaust gas G<NUM> and a stream of fresh liquid (mains water or sea water) L<NUM> coming from the distribution valve SV-<NUM> which separates the total stream of water fed to the plant (L<NUM>) into two different streams L<NUM> and L<NUM>. The stream L<NUM> will have the same chemical and physical characteristics as L<NUM>. However, the liquid L<NUM> used for gas refinement in S-<NUM> can be enhanced by adding strong bases. <FIG> shows the optional mixing unit M-<NUM> used to prepare the enhanced stream of liquid L<NUM>-<NUM> starting from the stream of liquid L<NUM> and a basic reagent stream.

In general, the diameter of the unit S-<NUM> will be similar to that of S-<NUM> so as to be able to receive the same gas flow rate. The person skilled in the art will know that the diameter of the unit S-<NUM> will also depend on the liquid flow rate being fed L<NUM>, which will only be determined when the gas flow rate, the concentration of the species Cl<NUM> and ClO<NUM> to be removed and the packing height for the unit S-<NUM> are defined.

The aim of this unit S-<NUM> is: to obtain a clean gas Gas, already purified from SOx and NOx, in which any emissions of chlorine dioxide ClO<NUM> and chlorine Clz originated during the exhaust gas treatment according to the invention are reduced within the regulatory limits in force or to levels lower than the values suggested by the WHO or by the safety data sheets. Obtaining a content of chlorine compounds dissolved in the flushing waters L<NUM>-<NUM> within the regulatory limits of those recommended by WHO (ClO<NUM>-<<NUM>/L, ClO<NUM>-<<NUM>/L, ClO<NUM><<NUM>/L and Cl<NUM><<NUM>/L) and a pH > <NUM> for direct discharge can be achieved by flushing with water or aqueous solution additivated with a strong base by the stream of liquid L<NUM>-<NUM>. The optional flushing process consisting in the refinement or dechlorination of the exhaust gases coming from S-<NUM> is characterised by dismutation reactions of the chlorine gases into water as shown in the following equations:.

Cl<NUM>+ <NUM> H<NUM>O→ <NUM>H<NUM>O+ + ClO- + Cl-     (R4).

The flushing process shown in the reactions (R4) and (R5) is favoured by the addition of alkalis (basic reagents) to minimise the flow rate L<NUM> used in S-<NUM>. The dosage of the reagent will also be evaluated in order to obtain a pH > <NUM> for the discharge stream L<NUM>-<NUM>.

<FIG> shows the section relating to the treatment and management of the flushing waters L<NUM>-<NUM> coming from the unit S-<NUM>. The liquid effluent treatment section consists of a first unit called S-<NUM> which provides an initial conversion of the compounds into Cl-, and which is then completed in a second unit F-<NUM>.

The pre-treatment of the stream L<NUM>-<NUM> exiting the distribution valve SV-<NUM> consists of a secondary oxidative flushing in the unit S-<NUM>. The flushing unit S-<NUM> is fed by the liquid stream L<NUM>-<NUM> in counter-current to the stream of gas G<NUM> coming from the distribution valve SV-<NUM>. The stream G<NUM> has the same chemical and physical properties as the stream G<NUM>. The pre-treatment of L<NUM>-<NUM> consists of decreasing the concentration of the oxidising species such as (ClO<NUM>-, Cl<NUM> and ClO<NUM>) by totally removing SOx and NOx in the stream of gas G<NUM>, following the same reaction mechanisms seen in the equations (R1), (R2) and (R3). The stream of gas G<NUM> leaving the unit S-<NUM> will be free of SOx and NOx but may contain ClO<NUM> and Cl<NUM> as contaminants. The stream G<NUM> is conveyed to the unit S-<NUM> together with the stream of gas G<NUM>. The liquid stream L<NUM>-<NUM> will contain a lower level of chlorine-based contaminants than L<NUM>-<NUM>, and is subsequently conveyed to the last treatment step F-<NUM>.

The final treatment of the stream L<NUM>-<NUM> in F-<NUM> adopts a clarification-flocculation process with the addition of ferrous salts. <FIG> shows that the stream L<NUM>-<NUM> leaving the unit S-<NUM> is sent to the flocculation tank F-<NUM>, while the stream of ferrous salts is dosed to the unit F-<NUM>. A strong base must also be added to the unit F-<NUM>. The aim of this unit is to totally convert chlorine-based contaminants into chlorides Cl-, using a process based on the reaction (R6):
<CHM>.

The equation (R6) shows that Fe<NUM>+ oxidises to Fe<NUM>+ with formation of Fe(OH)<NUM>(s), a poorly soluble compound, while ClO<NUM>- is reduced to Cl-. [<NPL>)] have confirmed that the reduction process is selective also towards species such as ClO<NUM>, Cl<NUM>, ClO- and ClO<NUM>-. In addition, the authors have also observed that the complete flocculation with a mixing speed of <NUM> rpm occurs after about <NUM> to <NUM> seconds with a stoichiometric addition of Fe<NUM>+ with respect to ClO<NUM>-. The volume of this equipment will be determined starting from the knowledge of the flow rate of liquid of the stream L<NUM>-<NUM> and the characteristic time of the clarification-flocculation process (understood as floccule formation). The process requires a suitable filtration step to remove the precipitate Fe(OH)<NUM>(s) from the liquid.

After verification of the concentrations of specific pollutants indicated in the reference regulations for fertiliser production, the effluent L<NUM>-<NUM> after treatment is stored and can be used as a raw material for the processes of nitrogen-based fertiliser production (i.e. ammonium sulphate (NH<NUM>)<NUM>SO<NUM>, ammonium nitrate (NH<NUM>)NO<NUM> and ammonium chloride (NH<NUM>)Cl). In addition, the ferric hydroxide Fe(OH)<NUM>(s) collected at the bottom of the tank is stored and once purified can be used in waste water treatment.

The advantages of the process and plant according to the invention are set out below:.

Calculations were carried out applied to a case study that involves the treatment of an exhaust gas deriving from a combustion plant, installed on land, with a flow rate of fumes to be treated equal to <NUM><NUM>/h and characterised by a composition of SO<NUM> equal to <NUM> ppmv, NOx equal to <NUM> ppmv, CO at <NUM> ppmv, CO<NUM> at <NUM>% v/v and H<NUM>O(v) equal to <NUM>% w/w. The following have also been set:.

The calculations are carried out using the process configuration shown in <FIG>, which involves the use of three flushing units or columns (S-<NUM>, S-<NUM> and S-<NUM>) or a single column divided into three separate sections, and a flocculation unit F-<NUM>.

The results obtained from the calculation code were validated with experiments on a pilot-scale experimental plant.

Following the diagram shown in <FIG>, the exhaust gases G are cooled in the exchanger with energy recovery C-<NUM> up to a temperature of <NUM> before being treated in the plant. After cooling, the stream G<NUM> has a relative humidity equal to <NUM>% at a temperature of <NUM>. The energy recovery achieved in the unit C-<NUM> during the cooling of G is equal to <NUM> MW/h.

The exhaust gases G<NUM> are distributed into two lines in the valve SV-<NUM>: the first line (G<NUM>) is conveyed to S-<NUM> and the second line (G<NUM>) towards S-<NUM>.

The unit S-<NUM> is equipped with structured filling (e.g. Mellapak <NUM>. X made of stainless steel or polypropylene) and is fed by the stream of gas (G<NUM> and G<NUM>) of flow rate <NUM>/h and an absorbing stream of liquid L<NUM>-<NUM> of flow rate <NUM>/h at <NUM>, pH= <NUM>, and content of NaClO<NUM> = <NUM>% w/w coming from the mixing tank M-<NUM>.

The tank M-<NUM> has a total volume equal to <NUM><NUM> with a mixing speed of <NUM> rpm and is operated with: the make-up stream of liquid L<NUM> coming from the distribution valve SV-<NUM> of flow rate <NUM>/h at <NUM>, pH = <NUM>, SO<NUM><NUM>- = <NUM>/L, NO<NUM>- = <NUM>/L, Cl- = <NUM>/L and HCO<NUM>- = <NUM>/L; the recirculation stream of the oxidising reagent L<NUM>-<NUM> coming from the distribution valve SV-<NUM> of flow rate <NUM>/h at <NUM> and pH = <NUM>; the make-up stream of oxidising reagent NaClO<NUM> of flow rate <NUM>/h dosed in powder; the strong base stream with flow rate <NUM>/h, e.g. NaOH(s) dosed in pellets to control the final pH after mixing.

The unit S-<NUM> has a diameter of <NUM> and has been designed to operate at a maximum value of <NUM>% of the flooding value corresponding to a pressure drop per filling metre of <NUM> mbar/m. Calculations predict a filling height of <NUM> for Mellapak <NUM>. X to obtain SO<NUM> and NOx emissions < <NUM> ppmv. The exhaust gases (G<NUM>) exiting S-<NUM> are characterised by a flow rate of <NUM>/h, temperature of <NUM>, relative humidity equal to <NUM>%, SO<NUM> < <NUM> ppmv, NOx < <NUM> ppmv, ClO<NUM> < <NUM> ppmv, Cl<NUM> < <NUM> ppmv. The stream of gas G<NUM> is then conveyed in the refining unit S-<NUM> to reduce chlorine gas, ClO<NUM> and Cl<NUM> emissions. The unit S-<NUM> is equipped with structured filling (e.g. Mellapak <NUM>. X in stainless steel or polypropylene) and is fed by the stream of gas G<NUM> and a fresh mains water stream L<NUM> coming from the distribution valve SV-<NUM>. The stream L<NUM> has a flow rate of <NUM>/h and the same chemical and physical properties as L<NUM> (<NUM>, pH = <NUM>, SO<NUM><NUM>- = <NUM>/L, NO<NUM>- = <NUM>/L, Cl- = <NUM>/L and HCO3- = <NUM>/L). The total stream of liquid sent to the plant (L<NUM>) is equal to <NUM>/h and its chemical and physical properties are the same as those of L<NUM> and L<NUM>. The unit S-<NUM> has a diameter of <NUM> and operates at a maximum value of <NUM>% from the flooding value corresponding to a pressure drop per filling metre of <NUM> mbar/m.

For calculation purposes, the chlorine gas emissions from G<NUM> were reduced below the limits recommended by the safety data sheets (ClO<NUM> < <NUM> ppmv and Cl<NUM> < <NUM> ppmv, which represent the TLV - C, Threshold Limit Value - Ceiling values) while the emissions of chlorine-based compounds in the effluent L<NUM>-<NUM> were reduced below the limits recommended by the World Health Organisation, WHO (ClO<NUM>-<<NUM>/L, ClO<NUM>-<<NUM>/L, ClO<NUM> <<NUM>/L and Cl<NUM> <<NUM>/L). The process requires a packing height of Mellapak <NUM>. X and a base flow rate (NaOH(s)) equal to <NUM> and <NUM>/h, respectively. The stream of sodium hydroxide in solid state is added to the stream L<NUM> in the mixing unit M-<NUM> with a volume equal to <NUM><NUM>.

The stream of gas G<NUM> treated in S-<NUM> is characterised by a flow rate of <NUM>/h, temperature of <NUM>, relative humidity equal to <NUM>%, SO<NUM> << <NUM> ppmv, NOx << <NUM> ppmv, ClO<NUM> = <NUM> ppmv, Cl<NUM> = <NUM> ppmv, H<NUM>O = <NUM>% w/w. The stream of liquid L<NUM>-<NUM> at the outlet from S-<NUM> is characterised by a flow rate of <NUM>/h, temperature of <NUM>, pH = <NUM>, ClO<NUM>- - = <NUM>/L, ClO<NUM>-= <NUM>/L, ClO<NUM> = <NUM>/L and Cl<NUM> = <NUM>/L.

Claim 1:
Process for the treatment of an exhaust gas polluted by SOx and NOx wherein a flow of said gas is treated with an oxidising reagent in a treatment plant to remove the pollutants from the flow of gas, said process comprising the following fundamental steps:
(i). Preparing the oxidising reagent in a mixing unit M-<NUM> fed by an aliquot of water L<NUM> at the inlet and by an aqueous stream of recirculation reagent L<NUM>-<NUM>, the aliquot of water L<NUM> deriving from splitting water L<NUM> at the inlet to the plant into two aliquots, first aliquot L<NUM> and second aliquot L<NUM> of water, the recirculation reagent L<NUM>-<NUM> being an aliquot deriving from splitting the treatment liquid L<NUM>-<NUM> exiting a first treatment unit S-<NUM>, containing sulphates and nitrates deriving from the previous absorption of SOx and NOx in said first unit S-<NUM>, the oxidising reagent so prepared being withdrawn from the mixing unit as a stream of oxidising reagent L<NUM>-<NUM> which is sent to said first treatment unit S-<NUM>;
(ii). Splitting the flow of gas entering the plant G<NUM> into a first aliquot of gas G<NUM> and a second aliquot of gas G<NUM> and sending said first aliquot of gas G<NUM> to the first treatment unit S-<NUM> and said second aliquot of gas G<NUM> to a second treatment unit S-<NUM>;
(iii). Bringing the first aliquot of gas G<NUM> into contact with the aqueous stream of oxidising reagent L<NUM>-<NUM>, withdrawn from the mixing unit M-<NUM> and which feeds the first treatment unit S-<NUM> at the inlet;
(iv). Withdrawing at the outlet from the first treatment unit S-<NUM> the treated gas G<NUM> and the treatment liquid L<NUM>-<NUM>, polluted with the pollutants SOx and NOx following the contact with the treated exhaust gas,
(v). Splitting the treatment liquid L<NUM>-<NUM> into a first aliquot or recirculation reagent L<NUM>-<NUM> which is recirculated by sending it to the mixing unit M-<NUM> and into a second aliquot or purge stream L<NUM>-<NUM> which is sent to a second treatment unit S-<NUM> to treat the second aliquot of gas G<NUM>;
(vi). Withdrawing, at the outlet from the second treatment unit S-<NUM>, the treatment liquid L<NUM>-<NUM> which contains sulphates and nitrates deriving from the absorption of the pollutants SOx and NOx during the contact between the purge stream L<NUM>-<NUM> and the gas G<NUM> which is sent to the first unit S-<NUM> together with the first aliquot of gas G<NUM>.