Process for the catalytic combustion of methane

Abstract: A methane catalytic combustion process using heat-resistant catalysts consisting of metal oxide mixtures of pyrochloric structure. The process is usable in gas turbines and boilers.

Description: This invention relates to a methane catalytic combustion process using heat resistant catalysts consisting of a metal oxide mixtures of pyrochloric structure. The process can be used in gas turbines and boilers. Catalytic combustion has various advantages over non-catalytic combustion, including easy control of the oxidation reaction, reduction in pollutant emission and increase in energy recovery efficiency. It consists of passing a mixture of fuel and air over a catalyst maintained at sufficiently high temperature to achieve total oxidation. The ensuing reaction releases energy and leads to the formation of carbon dioxide and water which can be released to the atmosphere. This reaction can be used both to generate energy, for example in gas turbines, and to remove pollutants, for example to purify vehicle exhaust gas and industrial offgas. During catalytic combustion the reaction temperature is lower than that attained during combustion in the absence of catalyst, and consequently the formation of pollutants such as nitrogen oxides is greatly reduced. In spite of many advantages there is however the problem of finding heat resistant catalysts. Catalysts used for combustion purposes include for example the noble metals (Acres, Platinum Met. Rev. 14 , 1970, 2; Schwartz. J. Cat. 21 , 1971, 199). However because of their volatility they can be applied only in combustion reactions at temperatures less than 1000°C, such as in the removal of carbon monoxide and hydrocarbons in automobile exhaust gases, deodorizing and flameless combustion. These catalysts are therefore unusable under high temperature conditions such as in gas turbines, which operate at a temperature exceeding 1200°C, and in boilers. Metal oxides or their mixtures, for example oxides of perovskite structure, are also used as oxidation catalysts (Yu Yao, J. Cat. 39 , 1975, 104; Margolis, Adv. Catal. Related Subjects, 14 , 1963, 429; JP 61161144). However in this case after calcining at very high temperature a considerable reduction in catalytic activity is noted probably due to the drastic reduction in surface area. Dispersing these metal oxides on an adequate support enables interesting performance to be obtained (Arai et al., Appl. Catal. 26 , 265, 1986). We have now found a catalyst for the catalytic combustion of methane which is very active even after exposure to the high temperatures occurring for example in a gas turbine. The present invention therefore provides a methane oxidation process consisting of reacting it with oxygen or air in the presence of a high temperature resistant catalyst consisting of a metal oxide mixture of pyrochloric structure having the following general formula: X a Y b Z c Sn₂O₇ where X is a trivalent metal cation Y is a tetravalent metal cation Z is a bivalent metal cation 0 ≦ a ≦ 2 0 ≦ b ≦ 1 0 ≦ c ≦ 1 a + b + c = 2 The trivalent metal cation is chosen from the transition metals, the lanthanides, indium, thallium and mixtures thereof. Preferably X is a lanthanide. The bivalent metal is chosen from the alkaline-earth metals, the transition metals and mixtures thereof. Preferably Z is a transition metal. The tetravalent metal cation is chosen from germanium, tin, lead, cerium, transition metals and mixtures thereof. Preferably Y is tin or cerium. In general to obtain a pyrochloric structure the ratio of the weighted average of the radii of the metal cations to the radius of the tin must be between 1.4 and 2.2. The process of the present invention is conducted at a temperature generally of between 600° and 1400°C and preferably between 1100° and 1300°C. The total gas pressure can vary from 1 to 25 atm depending on the particular practical application of the process. In a gas turbine for example the pressure is generally greater than 1.51 bar (1.5 atm), and preferably between 1.51 and 10.1 bar (1.5 and 10 atm). The methane concentration in the methane-oxygen or methane-air mixture is between 0.2 and 10.0 vol%, and preferably between 1 and 5 vol%. The metal oxide mixture of pyrochloric structure which can be used as catalyst in the process of the present invention is obtained by reacting, preferably in stoichiometric ratio, one or more sources of metal cations X, Y and Z with a source of tin. Preferably these sources of metal cations and tin are chosen from nitrates, carbonates, acetates, oxides and halides. The mixture of metal compounds and tin is calcined at a temperature of between 800° and 1400°C for a time of between 5 and 80 hours. In this manner a product of pyrochloric structure is obtained having a surface area generally of between 5 and 40 m²/g. The examples given hereinafter explain the invention. In particular. Examples 1-3 describe catalyst preparation procedures, Example 4 is a test regarding the life of one of the catalysts, demonstrating its stability under high temperature for a long period of time, Example 5 shows the results of catalytic combustion experiments and Example 6 relates to a comparative combustion test in the absence of catalyst. The tests were conducted by calcining the catalysts at high temperature and then testing at the lowest temperature at which significant activity was observed. These tests were effected in a similar manner to those reported in the literature (Arai et al., Appl. Catal., 26 , 265,1986) and show whether a catalyst remains active after calcining at high temperature. EXAMPLE 1 Synthesis of the pyrochloric compound Nd₂Sn₂O₇ 8.4 g of Nd(NO₃)₃.5H₂O and 3.0 g of SnO₂ are mixed together in a mortar. The mixture is calcined at 900°C for 10 hours, triturated and again calcined at 1250°C for 10 hours. The material obtained was characterised by powder X-ray diffraction and shows a single-phase pyrochloric structure. The lattice parameters are shown in Table 1. The surface area is 6.5 m²/g. EXAMPLE 2 Synthesis of the pyrochloric compound Gd₂Sn₂O₇ 8.66 g of Gd(NO₃)₃.5H₂O and 3.01 g of SnO₂ are mixed together in a mortar. The mixture is calcined at 950°C for 8 hours, triturated and again calcined at 1000°C for 12 hours, at 1100°C for 12 hours, at 1200°C for 24 hours and at 1250°C for 8 hours. The material obtained was characterised by powder X-ray diffraction and shows a single-phase pyrochloric structure. The lattice parameters are shown in Table 2 , The surface area is 6.0 m²/g. EXAMPLE 3 Synthesis of the pyrochloric compound MnCeNd₂(Sn₂O₇)₂ 4.20 g of Nd(NO₃)₃.5H₂O, 3.01 g of SnO₂, 0.86 g of CeO₂ and 1.22 g of M n (CH₃COO)₂.4H₂O are mixed together in a mortar. The mixture is calcined at 950°C for 8 hours, triturated and again calcined at 1200°C for 60 hours and at 1250°C for 8 hours. The material obtained was characterised by powder X-ray diffraction and shows a single-phase pyrochloric structure. The lattice parameters are shown in Table 3. The surface area is 7.0 m²/g. EXAMPLE 4 The catalyst prepared in Example 1 was maintained at a temperature of 1250°C for 120 hours. After this thermal treatment the material shows a single-phase pyrochloric structure with a surface area of 6.0 m²/g. This test demonstrates the high thermal stability of the catalysts of pyrochloric structure used in the present invention. EXAMPLE 5 - Methane oxidation The catalysts of Examples 1 to 4 were used in methane catalytic combustion tests. The tests are conducted in a fixed-bed quartz microreactor of 4 mm diameter containing 0.4 cc of catalyst in the form of granules of size between 40 and 70 mesh. Methane and air are fed to the reactor such that the methane concentration is 1 vol%. The test is conducted at atmospheric pressure at a space velocity expressed as GHSV of 48000 h⁻¹. The results of these activity tests are shown in Table 4, in which conversion means the moles of methane converted as a percentage of the moles of methane fed, and selectivity means the moles of CO₂ formed as a percentage of the moles of methane converted. EXAMPLE 6 - (comparative) A test is conducted under the conditions described in Example 5 in which, instead of the catalyst, 0.4 cc of inert material (quartz) in the form of granules of size between 40 and 70 mesh are placed in the reactor. The test results are shown in Table 4. 2THETA d I/Io CPS hkl 29.34 3.0444 100 7487 222 33.99 2.6371 31 2317 400 37.14 2.4204 2 120 331 44.58 2.0322 <1 16 511 48.78 1.8668 28 2067 440 57.90 1.5926 25 1908 622 60.72 1.5251 8 563 444 71.40 1.3211 5 364 800 78.96 1.2125 12 930 662 2THETA d I/Io CPS hkl 28.61 3.1196 2 180 imp. 29.58 3.0201 100 7215 222 33.14 2.7028 3 215 imp. 34.29 2.6154 37 2634 400 49.27 1.8495 41 2936 440 58.51 1.5776 25 1801 622 61.37 1.5107 5 368 444 72.21 1.3083 5 369 800 79.90 1.2005 17 1242 662 2THETA d I/Io CPS hkl 26.65 3.3452 1 71 imp. 28.57 3.1244 6 346 imp. 29.43 3.0347 100 5765 222 33.10 2.7060 2 95 imp. 34.11 2.6284 31 1808 400 37.24 2.4146 1 49 331 47.50 1.9140 3 153 imp. 48.98 1.8595 37 2160 440 58.17 1.5859 30 1743 622 61.01 1.5186 4 249 444 71.76 1.3154 2 117 800 79.37 1.2072 6 320 662 Example T (°C) CH₄ Conversion (%) CO₂ Selectivity (%) 1 600 16.0 100 700 57.4 100 750 81.7 100 800 100 100 2 600 15.1 100 700 61.3 100 750 85.9 100 800 100 100 3 600 23.9 100 700 59.6 100 750 79.5 100 800 99.3 100 4 600 16.6 100 700 65.0 100 750 85.0 100 800 100 100 6 800 14.1 100 820 32.6 100 850 69.6 100 870 95.0 100

Claims: A methane combustion process consisting of reacting the methane with oxygen or air in the presence of a heat-resistant catalyst of pyrochloric structure having the following general formula: X a Y b Z c Sn₂O₇ where X is a trivalent metal cation Y is a tetravalent metal cation Z is a bivalent metal cation 0 ≦ a ≦ 2 0 ≦ b ≦ 1 0 ≦ c ≦ 1 a + b + c = 2 A process as claimed in claim 1, wherein the trivalent metal cation is chosen from the transition metals, the lanthanides, indium, thallium and mixtures thereof. A process as claimed in claim 2, wherein the trivalent metal cation is chosen from the lanthanides. A process as claimed in claim 1, wherein the bivalent metal is chosen from the alkaline-earth metals, the transition metals and mixtures thereof. A process as claimed in claim 4, wherein the bivalent metal is chosen from the transition metals. A process as claimed in claim 1, wherein the tetravalent metal cation is chosen from germanium, tin, lead, cerium, transition metals and mixtures thereof. A process as claimed in claim 6, wherein the tetravalent metal cation is chosen from tin and cerium. A process as claimed in claim 1, wherein the reaction temperature is between 600° and 1400°C. A process as claimed in claim 8, wherein the reaction temperature is between 1100° and 1300°C. A process as claimed in claim 1, wherein the methane concentration in the methane-oxygen or methane-air mixture is between 0.2 and 10.0 % by volume. A process as claimed in claim 10, wherein the methane concentration in the methane-oxygen or methane-air mixture is between 1 and 5% by volume. A process as claimed in claim 1, wherein the catalyst of pyrochloric structure is Nd₂Sn₂O₇. A process as claimed in claim 1, wherein the catalyst of pyrochloric structure is Gd₂Sn₂O₇. A process as claimed in claim 1, wherein the catalyst of pyrochloric structure is MnCeNd₂(Sn₂O₇)₂. The use of the process claimed in claim 1 in a gas turbine to obtain energy.

Inventors: CALESTANI GIANLUCA, FORNASARI GIUSEPPE, PALMERY STEFANO, CALESTANI, GIANLUCA, FORNASARI, GIUSEPPE, PALMERY, STEFANO
Applicants: ENIRICERCHE SPA, ENIRICERCHE S.P.A.
Application year: 1992
Main classification: B01J23
IPC codes: B01J23, F23D14, B01J19