WO2014087344A1

WO2014087344A1 - A process for the catalytic oxidation of methane in the presence of sulphur compounds, and related catalyst

Info

Publication number: WO2014087344A1
Application number: PCT/IB2013/060628
Authority: WO
Inventors: Nicola Rossi; Marco PACI; Eleonora MELONE; Natale Ferlazzo; Pio Forzatti; Gianpiero Groppi
Original assignee: Enel Ingegneria E Ricerca S.P.A.; Politecnico Di Milano
Priority date: 2012-12-05
Filing date: 2013-12-04
Publication date: 2014-06-12
Also published as: ITFI20120269A1

Abstract

A process for the catalytic oxidation of methane in the presence of sulphur compounds, and a catalyst that works at high temperatures and in the presence of high amounts of sulphur compounds, and is able to efficiently carry out said oxidation process, are described.

Description

A PROCESS FOR THE CATALYTIC OXIDATION OF METHANE

IN THE PRESENCE OF SULPHUR COMPOUNDS, AND RELATED CATALYST

FIELD OF THE INVENTION

The present invention relates generally to the field of the production of energy from geothermal sources and more particularly to a method for the treatment of gaseous emissions from geothermal plants with the aim of reducing emissions of greenhouse gas in the atmosphere.

STATE OF THE ART

The geothermal power plants with direct steam cycle release into the atmosphere non-condensable gases associated with the geothermal steam that feeds them. From the environmental point of view the most interesting components of the non- condensable gases are mercury (Hg) and hydrogen sulphide (H₂S). In these gases are also present not negligible concentrations of methane (CH₄).

In order to reduce these noxious emissions, the Applicant had developed a particular type of plant called AMIS (acronym for "Abbattimento Mercurio e Idrogeno Solforato", Italian for "Reduction of Mercury and Hydrogen Sulphide"), currently installed in a large number of geothermal power plants, where it has allowed to substantially reduce emissions of mercury and hydrogen sulphide, with an average reduction of these emissions that is higher than 80%. The plant and related process are described in the Italian patent No. IT1305033 in the name of the Applicant. In Figure 1 a schematic section of the AMIS plant for reduction of mercury and hydrogen sulphide, is shown.

More in detail, in the AMIS plant the gases exiting the wet extractor at a temperature of about 150-200°C, are cooled in a packed column C-1 by direct contact with the water fed in counter-current and coming from the water cooling towers (WCT). The gas so cooled at a temperature of about 35°C, is conveyed into the mixer MX-1 where meets a stream of hot gases, by-pass of C-1 , coming from the power plant. The gas so mixed is sent to the compressor K-1 , where it reaches a temperature ranging from 60 to 80°C. The gas coming from the compressor K-1 is sent directly to the absorber R-1 where the mercury is retained by a specific adsorbent. The gas flowing from the absorber R-1 is then mixed with air coming from the compressor K-2 and it is sent to the reactor R-2 for the catalytic oxidation of H₂S, where methane is on the contrary not reduced. Before entering the reactor R-2, the gas is heated to the reaction start temperature (220-250°C) in the heat exchanger E-1 at the expense of the heat of the gas flowing from the reactor. The temperature in the reactor is controlled by- passing part of the charge to the reactor in the heat exchanger E-1. The air required for oxidation of H₂S is compressed in K-2. During oxidation, H₂S is oxidised to S0₂ according to the main reaction:

H₂S + 3/2 0₂ => S0₂ + H₂0

In the reactor, due to secondary reactions, small amounts of S0₃ (a few ppm) are formed. Since the temperature of the gas exiting the heat exchanger E-1 may drop below the dew point of S0₃, the tubes of the heat exchanger E-1 , the tubesheet and the downstream pipes are made of Hastelloy C. The effluent gas from the reactor R-2 and from the heat exchanger E-1 is cooled in the static mixer MX-2 by using in-line injection of cooling water in order to bring in solution, at low concentration, the S0₃ present in the effluent gas from the reactor and to avoid corrosions in the S0₂ absorber. The S0₂ is removed by washing the gas in counter-current with cooling water in the packed column C-2. The absorption in water of S0₂ depends on the amount of NH₃ present in the cooling water circuit and therefore in the endogenous steam at the inlet of the central unit. It is envisaged the injection of a solution of NaOH in the water used for the absorbing S0₂ in order to minimise the presence of S0₂ in the gas from the head of column C-2 and to control pH of the cold tub of the towers (WCT). The effluent gas from the absorber C-2, which is discharged in the atmosphere, still contains significant concentrations of methane, in amount equal to about 0.5-3.5% by volume with respect to the total volume of the emission.

The installation of the AMIS systems in the geothermal power plants, as useful and effective to reduce mercury and hydrogen sulphide, however presents some critical issues that cannot be easily solved. As said above, the AMIS system does not contribute to reducing methane, even stronger than C0₂ as a greenhouse gas, its ability to retain heat being about 30 times greater than that of C0₂; the reduction of methane from a plant's emissions is therefore of crucial importance, also because the geothermal plants emit significant amounts of methane, typically comprised between 70 and 200 Kg/hour, with peaks of 350-400 Kg/hour. Therefore, there is still a strong need to improve the already appreciable performance of the AMIS plants, also providing an effective reduction of methane in the emissions from the plants, in particular from geothermal power plants, where this technical problem is particularly difficult to be solved because of the presence of sulphur compounds in the gaseous emissions to be treated. These sulphur compounds, as explained in greater detail in the following, cause complications in the use of the known techniques, such as adsorption on activated carbon or the use of commercial oxidation catalysts, in order to efficaciously reduce methane.

Activated carbon and other similar adsorbents are in fact clogged in very short time by sulphur compounds in the emissions, thus making them completely inefficient, whereas the commercial oxidation catalysts now available on the market are for the most part sensible to sulphur poisoning, so that they tend to inactivate in short time in the presence of sulphur compounds in the gaseous emissions to be. treated.

Catalysts that are less sensible to sulphur poisoning are also known, they are used with fairly good results in the treatment of emissions containing small amounts of sulphur compounds, at relatively low temperature. In fact, these catalysts have a scarce resistance to high temperatures. However, when the gaseous emissions to be treated contain high amounts of sulphur compounds, as it is for instance in geothermal gas, higher temperatures are required, so as to diminish the effects due to sulphur compounds. It is therefore evident that for treating the geothermal emissions the oxidation catalysts, in addition to maintaining a good catalytic activity in the presence of sulphur, must also be resistant to very high temperatures. As far as the Applicant is aware, oxidation catalysts suitable for reducing methane in geothermal emissions are not known until today; they must be able to maintain a good catalytic activity in the presence of large amounts of sulphur compounds and under conditions of very high temperature.

The patent application US 2012/0189523 (Osaka Gas Co.) describes a process for the catalytic oxidation of methane present in the gaseous emissions of coal mines, containing traces of sulphur compounds, in a reactor with bimetallic catalysts. According to US 2012/0189523 the outlet temperature of the catalytic reactor must not exceed in any way 550°C otherwise irreversible degradation processes take place that would compromise its functioning. In order to do this, in such process it is necessary to control the inlet temperature at the reactor at 350°C or lower, by maintaining within a certain range the methane concentration in the gas to be treated, which moreover must contain very low amounts of sulphur compounds.

SUMMARY OF THE INVENTION

Scope of the present invention is therefore to provide a process for reducing methane in particular in the gaseous emissions from geothermal plants, comprising an amount of sulphur compounds equal to at least 100 ppm, for instance comprised between 100 and 500 ppm and typically around 250 ppm, capable to make the geothermal production of electric energy more efficient and environmentally sustainable.

A further scope of the invention is to provide a suitable reactor for the catalytic oxidation of methane and a catalyst having a high catalytic activity in this oxidation, together with a high thermal stability and resistance to sulphur poisoning.

A further scope of the invention is to provide an efficient process for the reduction of methane in the emissions from plants, which is able to use the heat from the gaseous emissions obtained from the above said process of reduction recovering it in a micro-generation system that increases the gross power produced by the plant.

These scopes are obtained by the process of the present invention for reducing methane from non-condensable, gaseous emissions of a geothermal plant, whose essential characteristics are defined as in the independent claims here attached. Further important characteristics are included in the dependent claims.

Features and advantages of the process of the invention shall become clearer from the following description of an embodiment thereof given as an example and not for limiting purposes with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

- Figure 1 illustrates a schematic section of an AMIS plant for the reduction of mercury and hydrogen sulphide;

- Figure 2 shows a schematic section of a plant for carrying out the process of the invention;

- Figure 3 shows a schematic section of a plant of Figure 2, positioned downstream of an AMIS plant;

- Figure 4 shows the variation with the reactor's temperature of the rate of conversion of methane for two different catalysts according to the invention in comparison with two known catalysts ( -■- comparison catalyst with 1% of Platinum on alumina prepared as described in example 2; -·- comparison catalyst with 2% of Platinum on alumina prepared as described in Example 2; -♦- catalyst of the invention with approx. 1% of Platinum on zirconia of Example 1A; -A- catalyst of the invention with 2% of Platinum on zirconia of Example 1 C);

- Figure 5 shows the conversion curves of methane during the 6 hours period of the catalyst's poisoning step (S-poisoning) for the same four catalysts of Figure 4;

- Figure 6 shows the variation with time of the amount in ppm of the S0₂ detected at the outlet of the reactor for the four catalysts of Figure 4;

- Figure 7 shows the variation with the reactor's temperature of the rate of conversion of methane, for the four catalysts of Figure 4, before and after the poisoning step (post-S-poisoning);

- Figure 8 shows the variation with time of the rate of conversion of methane during the thermal aging step at 800°C (thermal aging) for the four catalyst of Figure 4;

- Figure 9 shows the variation of the rate of conversion of methane after the thermal aging step (post-thermal aging) as a function of the reactor's temperature, always for the four catalysts of Figure 4;

- Figure 10 shows the variation with the reactor's temperature of the rate of conversion of methane using the catalyst of the invention with approx. 2% of Platinum on zirconia freshly prepared as described in Example 2 (-A -), or with the same catalysts after poisoning with 100 ppm of S0₂ at 700°C for 6 hours (-·-), and after poisoning under the same conditions but for 10 hours (-T-) and for 15 hours (-■-);

- Figure 11 shows the trend in the conversion rate of methane vs. the reactor's temperature when using the catalyst of the invention with approx. 2% of Platinum on Zirconia freshly prepared as described in Example 2 (-A -), or with the same catalyst after poisoning for 6 hours with 100 ppm of S0₂ at 700°C (-·-), and after poisoning for 6 hours with 00 ppm of S0₂ at 750°C (-♦-).

DETAILED DESCRIPTION OF THE INVENTION The process according to the invention comprises a first step wherein the gas emission from a plant and containing certain amounts of methane to be eliminated or at least reduced, is mixed with air and subsequently heated up to a temperature that allows the ignition of the combustion reaction of the hydrocarbons present therein, comprised for instance between 400 and 500°C, and preferably it is 450°C, thus creating a flow of hot gas that is then introduced into a suitable catalytic reactor. The temperature of the gas exiting from the catalytic reactor in the present process ranges between 700 and 800°C, and preferably is 750°C.

Besides methane, the gaseous stream to be treated has as further main components C0₂, H₂0, 0₂, N₂, H₂S, with an amount of sulphur compounds of at least 100 ppm, for instance ranging between 100 and 500 ppm and typically around 250 ppm; this gas stream consists for instance of the emissions from a geothermal plant, in particular of the emissions from an AMIS plant as described above, or also of the emissions from a petrochemical plant and refinery, where the oxidation of gaseous hydrocarbons in the presence of H₂S and/or other sulphur compounds, is required.

With reference to the attached Figure 2 the gas exiting the AMIS plant are mixed with air supplied by a compressor K-3, and subsequently heated in a regenerative heat exchanger E-3 and then directed to the catalytic reactor R-3; the heating of the gaseous mixture before entering the reactor is preferably carried out at the expense of the heat of the effluent of the reactor R-3 itself.

The present process then involves a second step of catalytic oxidation of methane inside a suitable reactor, indicated as R-3 in Figure 2, at high temperature, preferably comprised between 450 at the inlet and 750° C at the outlet. From the constructive point of view, a suitable reactor to treat the emissions from plants according to the present process is a packed-bed-type reactor with the catalyst in granular form, which can treat a gas stream having a flow rate ranging between 5000 and 15000 kg per hour, and has characteristics suitable for the oxidation treatment of gaseous mixtures with high percentages of C0₂, typically comprised between 50 and 75% v/v, and characterised by the presence of sulphur compounds in amount comprised for instance between 0.01 and 0.05% v/v.

According to the invention, the form of a reactor suitable for treating gaseous emissions from geothermal plants wherein the gaseous stream to be treated has a typical flow rate ranging from 5000 to 15000 Kg per hour, is preferably discoid and its dimensions are characterised by a high ratio diameter/height, wherein the diameter is comprised for instance between 2 and 3 m and the height ranges from 25 to 60 cm, more preferably the diameter is approximately 2.5 m and the height is approximately 32 cm, with a vacuum degree approximately ranging between 0.35 and 0.6, and preferably comprised between 0.45 and 0.55, wherein by "vacuum degree" the ratio is meant between the vacuum volume (difference between the volume of the catalytic reactor and the total volume of the catalyst' particles) and the volume of the catalytic reactor.

Catalysts of possible use in the second step of the present process are catalysts for the complete oxidation of methane to C0₂, having high thermal stability in operating conditions and high resistance to the poisoning by sulphur compounds. A particular catalyst of Platinum on zirconia, having such characteristics, has been developed and will be described in details in the following.

Different commercial oxidation catalysts have also been tested by the Applicant, in comparison with the present catalyst, investigating their behaviour on a laboratory scale, as illustrated below in the experimental part, in relation to their activity of combustion of methane, their resistance to sulphur poisoning and their thermal stability. It was so found that the catalyst based on non-noble metal oxides, such as CuO or CaO, in a Al₂0₃ matrix, have low activity and are excessively sensible to sulphur poisoning; the Palladium catalysts, even if very active, are also too sensible to sulphur poisoning and, if used at too high temperatures, may give rise to variations in the activity associated with the reversible process of decomposition/reformation of Palladium oxide, which compromise the functioning. Also bimetallic catalytic systems of Platinum and Palladium have a good initial activity towards oxidation of methane, which vanishes in a short time because of the thermal treatment and mostly because of the sulphur poisoning. Finally, catalysts of Platinum on alumina have been tested, they have a good thermal resistance and a satisfactory activity of methane combustion, but they tend to deactivate because of the sulphur poisoning; and the catalysts of Platinum on zeolite, less sensible to the presence of sulphur compounds, but much less resistance to high temperature. Besides the commercial oxidation catalysts mentioned above, the Applicant has moreover verified that further technologies now available and potentially suitable to reduce methane in gaseous emissions, such as thermal oxidation and adsorption on activated carbon, are totally unsuitable to solve the technical problem. In fact, the thermal oxidation needs reaching very high temperatures, in the order of 700-1200°C, to obtain acceptable rates of reduction of methane. Furthermore, this technology would impose the use of materials resistant to high temperatures, that are very expensive, and it would implies significant emissions of C0₂ deriving from the use of additional amounts of fuel to reach temperatures as high as those required by the thermal oxidation technology. Also adsorption on activated carbon or anyway the use of other similar adsorbing materials is not suitable for the scope of the invention, because the presence of sulphur compounds in the emissions would cause in a short time clogging of the adsorbent.

The Applicant has instead surprisingly found an oxidation catalyst which, contrary to the known catalysts now on the market, meets at the same time the requirements of activity in the methane oxidation, of thermal stability, and of resistance to sulphur poisoning, even in the presence of high amount of sulphur compounds, equal or higher than 100 ppm. The present catalyst comprises an inert support of zirconia, Ζτ0₂, on which Platinum is dispersed, in amount comprised between 0.5 and 2.5% by weight with respect to the total weight of the catalyst, preferably comprised between 1 and 2% by weight.

The innovative catalyst of the present invention may be prepared for instance by a process comprising the following steps:

i) calcination of commercial zirconia at 800°C for 10 hours;

ii) dispersion of Platinum on an inert support of zirconia by impregnation of this support with solutions of Platinum salts, such as Platinum nitrate; iii) drying in air at temperature of 110°C for 12 hours;

iv) calcination at 600°C for 5 hours.

The zirconia support of the present invention, thanks to the first step i) of calcination, carried out for instance at temperature ranging from 750 to 850°C for a period of time comprised between 5 and 20 hours, preferably at 800°C for 10 hours, is in sintered form with a characteristic surface area ranging from 5 and 20 m²/g, preferably equal to about 10 m²/g. Despite a relatively small characteristic surface area, the present catalyst with sintered zirconia support has a good dispersion grade of the catalytic metal on the support, and allows obtaining the performances here described that overcome the prior art technical problems.

The so obtained catalyst was analysed by ICP (Inductively Coupled Plasma) spectrometry in order to determine its chemical composition; it was also subjected to measurements of chemisorption with H₂ to evaluate the extent of Platinum dispersion on the inert support. Conventional measurements of specific surface area (BET) and of the average volume of the pores have been carried out to determine the morphological properties of the catalyst, both before and after the calcination treatment in step iii) of the process. Finally, tests have been carried out on a laboratory scale to evaluate the catalytic activity towards methane, in comparison with catalysts known for the same type of applications; with respect to these known catalysts, the present catalyst has shown a higher thermal stability, and a better resistance to sulphur poisoning. In the following experimental part, the results obtained from tests carried out on the catalyst are described in details.

The catalyst, prepared as described above, may be used in the present reactor for the catalytic oxidation of methane in the form of particles having different geometrical shapes, such as spheres, full cylinder, hollow cylinder (Raschig ring), or trilobal pellet. The characteristic dimensions of the external diameter of the particles of the present catalyst are typically comprised between 1.5 and 5 mm, and are preferably comprised between 3 and 4 mm. The different geometric forms and the above said dimensions are selected in order to optimize the performances of the catalytic process by evaluating the surface area exposed to reagents and the load loss inside the reactor.

At the outlet of the reactor's catalyst, in a third step of the present process, the gas is maintained at the outlet temperature, equal to a temperature maximum value preferably of about 750°C, for instance by varying the flow rate of dilution air at the reactor's inlet by acting on the number of revolutions of a blower, indicated as K-3 in Figure 2, by means of an inverter mounted on the motor of the blower itself. The gas is then directed to a heat exchanger, preferably of the type "shell-and-tube", indicated as E-3 in Figure 2, where the gas transfers the heat required to heat the gas fed to the reactor R-3, then it is lead to the base of the cooling towers of the plant at temperature comprised for instance between 100 and 150°C.

Thanks to the maximum value of the gas at the reactor's outlet, which is selected equal to approx. 750°C, the thermal deactivation of the catalyst is avoided and, at the same time, it is also avoided the use of very expensive techniques and materials. At the same time, it was verified how, even if in the presence of very high amounts of sulphur compounds in the gaseous stream under treatment, the rate of conversion of methane is optimal at the present temperature.

According to a preferred embodiment of the invention, the gas at the catalytic reactor's outlet may be guided to a further heat exchanger, indicated as E-2 in Figure 2, before being directed into the main exchanger E-3 described above. In this further secondary exchanger, the gas transfers the heat amount necessary to feed a Bryton cycle for micro-generation in an appropriate section of the plant by heating of air under pressure. Given the high variability in the composition of geothermal gases, under the nominal operative conditions, a thermal output of 500-600 kW is typically exchanged inside the heat exchanger E-2, with the air passing through the heat exchanger with a mass flow of about 0.8 Kg/s, thus warming up to 600-700°C. The overheated air under pressure is then conveyed into a micro-turbine, where it is produced the work required to the generation of about 100-150 kW of electrical power.

The process of the invention, as described above, allows obtaining efficiency of oxidation of methane in gaseous emissions, higher than 99%.

The present process was developed in particular for the application to AMIS plants for the reduction of noxious gaseous emissions from geothermal plants; but it may be applied to any other types of geothermal plant, always with the aim of reducing emissions of pollutants and greenhouse gases. The present process may be applied to petrochemical processes and to refinery and more in general to all industrial applications where oxidation is required of hydrocarbons in the gaseous phase in the presence of sulphur compounds in amount that is equal or higher than 00 ppm.

EXPERIMENTAL PART The process of reduction of methane in the gaseous emissions from plants according to the invention was evaluated with tests, both on a laboratory scale and on a pilot-plant scale, by carrying out a series of tests and measurements, described herein below.

1 -Preparation of the catalyst

A. Preparation of the catalyst containing approximately 1% of Platinum on Zirconia

4.00 g are weighed of Zr0₂ obtained after calcination at 800°C for 10h starting from the commercial product 2r0₂ MEL CAT F-ZO 923/01. For a nominal titre of Platinum of 1% by weight with respect of the total weight, a Platinum weight of 0.040 g is calculated. For impregnation a commercial solution of Platinum nitrate with 15.36% by weight of Platinum (Heraeus, CAS 18496/40/7) is used, having density of 1.15 g/cc; by using this solution, 0.229 cc of solution are needed having weight of 0.263 g in order to have the amount of Platinum required. By preliminary measurement with water it is estimated a total pore volume, corresponding to the transition of incipient wetting, equal to 0.132 cc/g x 4.00 g = 0.528 cc. The impregnating solution is diluted with 0.3 cc of water to ensure that the pores and the solution have corresponding volumes. The resulting solution is stirred and used for impregnation at room temperature until incipient wetting, then it is let to dry in air at temperature of 110°C overnight. Calcination at 600°C for 5 hours is then carried out (increment of temperature of 50°C/h upwards and downwards).

A dark grey sample is obtained having a final weight of 4.02 g.

The ICP-MS analysis of the sample has found an amount of Platinum of 0.99% by weight with respect to the total weight of the product.

B. Preparation of the catalyst containing approximately 2% of Platinum on

Zirconia

6.00 g are weighed of Zr0₂ obtained after calcination at 800°C for 10h starting from the commercial product Zr0₂ MEL CAT F-Z0 923/01. For a nominal titre of Platinum of 2% by weight with respect of the total weight, a Platinum weight of 0.122 g is calculated. For impregnation, a commercial solution of Platinum nitrate with 15.36% by weight of Platinum (Heraeus, CAS 18496/40/7) is used, having density of 1.15 g/cc; by using this solution, 0.693 cc of solution are needed having weight of 0.797 g in order to have the amount of Platinum required. By preliminary measurement with water it is estimated a total pore volume, corresponding to the transition of incipient wetting, equal to 0.132 cc/g x 6.00 g = 0.792 cc. The impregnating solution is diluted with 0.1 cc of water to ensure that the pores and the solution have corresponding volumes. The resulting solution is stirred and used for impregnation at room temperature until incipient wetting, then dried in air (at temperature of 110°C) overnight. Calcination at 600°C for 5 hours is then carried out (increment of temperature of 50°C/h upward and downward).

A dark grey sample is obtained, having a final weight of 6.1 g.

The ICP-MS analysis of this sample has found a Platinum amount of 1.94% by weight with respect to the total weight of product.

C. Preparation of the catalyst containing approximately 2% of Platinum on Zirconia

In a muffle 40 g of the commercial product Zr(OH)₄ MEL CAT XZ0631 are placed, and a calcination at 800°C for 10 hours is carried out, by using a heating and cooling ramp of 100°C/hour. At the end of calcination an amount of Zr0₂ is obtained, from which 12 g are taken. Then, for a nominal titre of Platinum of 2.2% by weight with respect to the total weight of the catalyst, a weight of Platinum of 0.27 g is calculated. For impregnation, a commercial solution of Platinum nitrate with 15.36% by weight of Pt (Heraeus, CAS 18496/40/7) is used, having density of 1.15 g/cc; by using this solution, 1.53 cc of solution are needed to obtain the amount of Platinum required. Impregnation is carried out with this solution without dilution with water, given that the volume of the pores in the substrate and the volume of the impregnating solution are almost corresponding. Impregnation is carried out with the solution well stirred, at room temperature up to incipient wetting, letting it then to dry in air overnight. Calcination at 600°C for 5 hours is carried out, by using a temperature ramp of 50°C/hour upwards and downwards.

A dark grey sample is obtained and a final weight of 12.25 g.

The ICP-MS analysis of the sample has found a Platinum amount of 2% by weight with respect to the total weight of the product. 2-Evaluation of the catalyst in laboratory

The catalyst of the invention prepared as described above in item 1 and two comparison catalysts were subjected to the tests described below in order to evaluate their catalytic activity in terms of methane oxidation, resistance to high temperatures and to the co-presence of sulphur compounds.

Catalytic activity

The catalytic activity was tested on a laboratory experimental plant, placed entirely under a hood, at temperatures ranging from 400 to 800°C in a reactor operating under atmospheric pressure. The tests have been carried out maintaining a space velocity GHSV (Gas Hourly Space Velocity) of 35.000 Nm³/gcat/hour, which represents a typical value for a catalytic incinerator. The plant was fed with a gaseous stream designated in the following "standard mixture", which represents the typical gas processed in geothermoelectrical plants and having the composition indicated in the following Table 1 :

Table 1

The catalyst of the invention, prepared as described above, was reduced into a powder grinding by hand so as to obtain particles dimensions comprised between 80 and 120 mesh, corresponding to a diameter ranging from 177 to 125 μΐη; these values allow to yield a good compromise between load loss and control of the diffusive phenomena during the reaction. A powder sample of 0.5 g was then diluted with ground quartz having the same size of the catalyst in weight ratio 1 :1 , so as to obtain 1 g of product which was used for loading a micro-reactor for tests of catalytic activity. This device has been loaded by inserting the catalyst between two layers of glass wool, and covered with quartz grains; then a thermocouple was inserted to a depth such as to allow the reading and control of the temperature of the catalytic bed, being careful not to pierce the underlying layer of glass wool.

In parallel, in order to evaluate the catalytic performances of the present catalyst with variation of the temperature from 400 to 800°C by comparison with catalysts known for the same use here described, the following catalysts in the form of powders have been tested:

catalyst containing 1 % of Platinum on alumina; and catalyst containing 2% of Platinum on alumina.

These two catalysts have been prepared in laboratory by following the same procedure described above for the catalysts of the invention, by the incipient wetting technique always starting from a commercial solution of Platinum nitrate with 15.36% by weight of Platinum (Heraeus, CAS 18496/40/7) on an alumina support (Alumina PURALOX SCFA 140 by SASOL) so as to obtain the two different desired amounts of Platinum. These catalysts have been formulated based on the chemical composition of the catalytic systems now available on the market and having the same applications.

The composition of the gaseous stream supplied was first verified with a micro gas chromatograph (Agilent 3000A, Agilent Technologies^®), by-passing the reactor until a stationary state is reached, heating at the same time the reactor up to 400°C under static atmosphere of only N₂. When the analysis has revealed that the different species were present according to the composition of the standard mixture described above, the by-pass was closed and valves at the reactor's inlet and outlet were opened, so as to redirect gas in the reactor itself. In this phase, the thermal power supplied to the reactor was greater, to compensate cooling due to the entry of the reagent mixture, at a temperature initially lower, to restore and stabilize the internal temperature to 400°C.

At this point, the gaseous stream exiting the reactor is analysed, always with a micro gas chromatograph, replicating the analysis up to achieve a stationary situation of conversion of the reactants. The internal temperature of the reactor is then brought to 450°C and, once stable, the output current is analysed up to a situation of stationary conversion. And so on, rising the reactor's temperature by 50°C at a time up to 800°C, determining the methane conversion rate values for nine different temperature values, 400, 450, 500, 550, 600, 650, 700, 750 and 800°C. The reactor's temperature is then cooled down of 50°C at a time, by repeating every time the analysis and collecting data of methane conversion at the different values of temperature. A heating curve and a cooling curve, substantially coincident, were so obtained. In Figure 4 the trend of the conversion rate of methane is reported in the range of temperature comprised between 450 and 650°C for the four tested catalysts.

Poisoning by sulphur compounds

The performance of the present catalyst and of the comparison catalysts were also evaluated in such experimental conditions as to simulate the phenomena of poisoning of the catalyst in the presence of sulphur compounds, by adding S0₂ to the standard mixture described above, in amount equal to 100 ppm.

The test was initially carried out in the same manner of the test described above for the evaluation of the activity of the catalyst, up to a temperature of 600°C. At this point, the reactor was isolated by opening the by-pass, and to the gaseous stream 100 ppm of S0₂ were added, leaving unchanged all other components of the mixture, except N₂, whose quantity is adjusted so as to close the balance. Always by analysis with a micro gas chromatograph and an analyser for S0₂ of the type Limas 11 by ABB, the composition of the gaseous mixture is determined up to a stationary state of the desired composition, corresponding to the standard mixture plus 100 ppm of S0₂.

The reading of the amount of S0₂ by the Limas analyser could be disturbed by the presence of water, the gaseous stream is therefore dried before the measurement. Once the by-pass is closed and the valves at the reactor's inlet and outlet are opened with the reactor's temperature maintained at 600°C, the phase of poisoning starts and lasts 6 hours. During this phase, every 20-30 minutes, the composition of the mixture exiting the reactor is detected and the time of methane conversion and of the discharge of S0₂ is recorded.

Once the 6 hours of poisoning are spent, the reactor's temperature is brought from 600 to 400°C by determining conversion every 50°C similarly to what described above. Figure 5 shows the curves obtained for the methane conversion in the 6 hours of poisoning (S-poisoning) for the two catalysts of the invention, and the two comparison catalysts already described above, while Figure 6 shows the trend of the amount of S0₂ exiting from the reactor; from this figure it is evident that the catalysts of the invention do not have the tendency to adsorb S0₂, whose concentration reaches an asymptotic value after an initial decrease due to the secondary reaction of conversion to S0₃; also the S0₃ concentration at the reactor's outlet was quantified with known analytical methods. It was so estimated that the amount of S0₂ adsorbed on the catalysts of the invention is approximately of 0.085 mmol/g of catalyst; on the contrary, the comparison catalysts tested consisting of Platinum on alumina, have proven very sensitive to sulphur poisoning, as can be seen from the same Figure 6, which shows a marked tendency of these catalysts to adsorption of S0_2> in an amount equal to about 0.35 mmol/g of catalyst after 6 hours of exposure; although these catalysts do not present evident signs of deactivation after treatments of a few hours in the presence of S0₂, the extent of observed sulphation might be critical for the stable operation in the long term, causing the deactivation of the catalyst itself. In Figure 7 is showed the trend of methane conversion after 6 hours of poisoning (post-S-poisoning).

Resistance to high temperatures

Further tests were carried out in order to evaluate the performance of the present catalyst at high temperatures, always in the presence of S0₂. The test described above was repeated in analogous way, however by performing aging for 6 hours at 800°C instead of at 600"C. In Figures 8 and 9 is respectively showed the trend of methane conversion during the 6 hours of thermal aging at 800°C vs. time, and the subsequent trend (post-thermal aging) vs. the reactor's temperature. Figures 10 and 11 show how the oxidation performances of the catalyst consisting of 2% of Platinum on a support of Zr0₂ previously subjected to prolonged thermal and poisoning treatments, are maintained practically without variations in the whole range of temperatures tested (450-600°C).

3-Evaluation of the catalyst in a pilot plant equipped with AMIS

In order to evaluate the activity of the present catalyst on the reduction of methane in different operating conditions, and the possible deactivation by sulphur poisoning, a pilot plant was installed in side-stream configuration in a geothermal power plant equipped with an AMIS system for reducing emissions of hydrogen sulphide and mercury. The gas exiting from the AMIS, and in particular from column C- 2 shown in Figure 3, directly supplies the pilot plant. The pilot plant consists essentially by a blower which feeds the gas stream to the catalytic oxidation reactor, by a reactor and by a heating resistor required to bring the gas to the temperature of the start of the reaction. To ensure the supply of oxygen required for the oxidation of methane, in the system was added a flowmeter for air supply upstream of the blower. The regulation of the air flow is carried out acting on the prevalence of the blower, in relation to the desired concentration of oxygen in the gaseous stream entering the plant. The temperature of the gas exiting the reactor is maintained at values lower than 750°C, by acting on the dilution air flow rate at the inlet of the pilot plant. The gas exiting the reactor is returned to the column C-2 of the AMIS plant.

To demonstrate the feasibility of the present process on a pilot scale and highlight the possible effects of the introduction of the present process of methane reduction in an existing AMIS plant, a process analysis was made with the commercial codes Aspen Plus^® by Aspen Technology Inc., to perform the thermal and matter balances on the current configuration of the AMIS plant and on the configuration that included the pilot for the implementation of the present process. This approach has allowed us to define the operating conditions (flow rates, temperatures, compositions, etc.) of the pilot plant. The thermal and matter balances were carried out under conditions of maximum load (maximum flow rate of the gas equal to 350 kg/h). The model has been extended to the geothermal power plant, to the AMIS plant and to the pilot plant, with the aim of evaluating the impact of the pilot plant on the geothermal power plant.

For the evaluation of the feasibility of the process, a wide range of composition of the inlet gas to the AMIS plant has been considered (H₂ = 0.2-2.5% vol., N₂ = 10-20% vol., CH₄ = 0.5-3.5% vol., C0₂ = 50-75% vol., H₂0 = 1-5%, H₂S = 1-2%). It was chosen to set the temperature of the gas at the reactor's inlet R-3 at 450°C, a value that allows the ignition of the combustion reaction of the hydrocarbons present.

It was chosen a maximum temperature of 750°C for the gas exiting the reactor in order to avoid the thermal deactivation of the catalyst and, at the same time, to avoid the use of expensive technologies and materials, not fully mature for the regenerative heat exchanger E-2. The values of the composition of the gas at the catalytic oxidation reactor's inlet, obtained from the analysis of the process, are the same adopted for the evaluation test of the catalysts and for the sizing of the catalytic reactor itself, as described above. The analyses carried out show that the introduction of the pilot plant does not involve significant changes in the operating parameters of the geothermal power plant and of the AMIS plant. In particular flow rate, temperature and composition of the gas streams entering and exiting the AMIS and the pilot have been evaluated, together with the fuel consumption and performance of the two configurations, without detecting substantial differences. In particular, it was observed that the process of the invention for the removal of methane and consequent recovery of heat through micro- generation, placed downstream of the AMIS plant, allows keeping constant the plant performance in terms of overall conversion efficiency.

The present invention is described herein making reference to a preferred embodiment. Further embodiments may exist, all departing from the same inventive core, as defined by the scope of protection of the following claims.

Claims

1. A process for the removal of methane from gaseous non-condensable emissions of a plant further comprising sulphur compounds in amounts equal or higher than 100 ppm, said process comprising the following steps:

i) mixing said inlet gaseous emissions with air and heating the so obtained mixture up to a temperature which allows the ignition of the combustion reaction of the hydrocarbons in the mixture;

ii) catalytic oxidation of the mixture coming from step i) by introducing the same mixture in a reactor comprising a catalyst suitable for the oxidation of methane, having high thermal stability and resistance to sulphur poisoning;

iii) introduction of the gas exiting the reactor in step ii) in a main heating exchanger for transferring the heat necessary to warm the mixture of step i), and sending to cooling towers for the emission from the plant,

said process being characterised in that the temperature of said inlet gaseous emissions in step i) are heated to a temperature ranging between 400 and 500°C for the inlet into said reactor in step ii) while the temperature of said gas exiting the reactor ranges between 700 and 800°C, and said catalyst essentially consists of Platinum dispersed on a previously sintered, inert support of Zirconia.

2. The process according to claim 1 , wherein said inlet gaseous emissions of step i) are heated up to a temperature of approx. 450°C.

3. The process according to claim 1 , wherein said catalytic oxidation in step ii) is carried out at temperature ranging from 450 at the inlet into the reactor and 750°C at the outlet of the reactor.

4. The process according to claim 1 , wherein said reactor in step ii) is a packed bed reactor with the catalyst in granular form, having a high diameter/height ratio, with a vacuum degree approximately ranging from 0.35 and 0.6 and preferably from 0.45 and 0.55, for the treatment of a gaseous stream having a mass flow rate comprised between 5000 and 15000 Kg per hour.

5. The process according to claim 4, wherein said reactor has a diameter comprised between 2 and 3 m and height comprised between 25 and 60 cm.

6. The process according to claim 1 , wherein said oxidation catalyst comprises from 0.5% to 2.5% by weight of Platinum with respect to the total weight of the catalyst.

7. The process according to claim 1 , wherein said oxidation catalyst comprises 2% by weight of Platinum with respect to the total weight of the catalyst.

8. The process according to claim 4, wherein said oxidation catalyst consists of granules having a diameter comprised between 1.5 and 5 mm, and preferably comprised between 3 and 4 mm.

9. The process according to claim 1 , further comprising in step iii) the transit of said gas emission exiting from the reactor through a secondary heat exchanger wherein the gas transfers the heat necessary to feed a micro-generation cycle, before being sent to said main heat exchanger for heating the mixture entering the reactor.

10. The process according to claim 9, wherein in said secondary heat exchanger the heat is transferred by heating of air under pressure which is then directed to a micro-turbine for the generation of electrical work.

11. The process according to anyone of the previous claims, wherein said non- condensable gaseous emissions are the emissions of a geothermal electrical plant.

12. The process according to claim 11 , wherein said gaseous emissions are preliminarily treated for the removal of hydrogen sulphide and mercury.

13. A reactor for the catalytic oxidation at high temperature of methane from non- condensable gaseous emissions of a plant further comprising sulphur compounds in amount equal or higher than 100 ppm, said reactor being a packed bed reactor with the catalyst in granular form, having a high diameter/height ratio, with a vacuum degree approximately ranging from 0.35 and 0.6 and preferably from 0.45 and 0.55, for the treatment of a gaseous stream having a mass flow rate comprised between 5000 and 15000 Kg per hour.

14. The reactor according to claim 13, said reactor having a diameter comprised between 2 and 3 m and height comprised between 25 and 60 cm.

15. A catalyst for the catalytic oxidation, at high temperature, of methane from non- condensable gaseous emissions of a plant further comprising sulphur compounds in amount equal , or higher than 100 ppm, having high thermal stability and resistance to sulphur poisoning, said catalyst comprising Platinum dispersed on a previously sintered, inert support of Zirconia.

16. The catalyst according to claim 15, having a characteristic surface area ranging between 5 and 20 m²/g, and preferably equal to about 10 m²/g.

17. The catalyst according to claim 15, wherein said inert support of Zirconia is obtained in sintered form by calcination of Zirconium oxide at temperature ranging from 750 to 850°C for a time ranging from 5 to 20 hours.