PROCESS TO PRODUCE A HIGH METHANE CONTENT GAS MIXTURE FROM COAL

Abstract: A process is disclosed, to produce a high methane content gas mixture from coal, comprising the following steps: a) coal grinding to a granulometry not larger than 5 mm, preferably not larger than 2 mm; b) addition of one or more alkali and/or alkali-earth compounds to the ground coal, wherein the alkali or alkali-earth ions are in an amount comprised within the range of from 4 to 20% by weight relatively to dry, ash-free coal, and possible subsequent drying of the additive-containing coal; c) possible pelletizing of the additive-containing coal, so to obtain tablets whose maximum dimension is comprised within the range of from 3 mm to 5 cm, preferably of from 3 mm to 2 cm; d) gasification of the additive-containing coal and methanation of the produced gas, in one single step with a fixed bed of countercurrently descending coal with steam, and in the presence of an amount of air not larger than 35% by weight, relatively to the ash-­ free dry coal and/or of an amount of essentially pure oxygen amount comprised within the range of from 1 to 25% by weight relatively to the ash-free dry coal, by operating under a pressure higher than 25 abs.atm and at a temperature comprised within the range of from 400 to 850°C, preferably of from 450 to 750°C; e) removal from the gas produced in the (d) step of unreacted steam, and of the condensible compounds formed; f) removal of the acidic gases from the high methane content gas mixture obtained.

Description: The present invention relates to a process to obtain from coal a gas mixture with a high methane content. More particularly, the present invention relates to a process to obtain from coal a high methane content gas mixture, wherein the gasification and methanation reactions take place in one single, descending fixed-bed reactor. The high methane content gas mixtures which are produced by means of said process are normally denominated "SNG" (substitute natural gas) and can either replace the natural gas, or be mixed with it. Process have been long known, which start from coal, to produce the above-mentioned mixtures. At Great Plains (U.S.A.), e.g., a process has been accomplished, which essentially comprises the following steps: grinding of coal down to a size comprised within the range of from 6 mm to 5 cm; fixed-bed gasification by means of oxygen and steam under high pressures (of approximately 89,4 bar (30 abs.atm) and at temperatures comprised within the range of from 500°C to 1200°C; condensation of tars, unreacted water and condensible compounds formed; shift of a portion of the produced gas, to adjust to a value of approximately 3 the H₂/CO volume ratio; removal of the acidic gases; methanation of the product at a temperature of approximately 400°C. The process shows some drawbacks, mainly caused by the need of feeding to the gasification step large amounts of essentially pure oxygen (in amounts larger than 50% by weight relatively to the dry, ash-free coal), in as mush as the use of air would involve the need for nitrogen to be removed after the methanation step. The need to have available essentially pure oxygen involves the presence of an air fractionation facility, which causes high fixed costs and high operation costs, due to the large amounts of oxygen to be supplied. Such process is generally very complex, precisely due to the different steps which constitute it. EP-A-0 067 580 on the contrary, designed a process wherein the reactions of gasification and methanation take place in one single step, which essentially comprises the following steps: grinding of coal to a particle size not larger than 3 mm; impregnation of the ground coal with water and a catalyst; gasification and methanation by means of steam on a catalytic fluidized bed at a temperature of approximately 700°C; removal of the acidic gases from the gas mixture formed; cryogenic separation, to decrease in the gas mixture the contents of H₂ and CO, which are recycled to the gasification and methanation step; lixiviation with water of the gasification residue, to partially recover the catalyst (from 70% to 90% per each cycle). The most serious defects of the process of EP-A-0 067 580 are that it requires an expensive cryogenic purification and the recycling of hydrogen and carbon monoxide, plus the necessity of overheating to maintain the desired temperature, whereby the gassification efficiency is reduced. Moreover, the adoption of a bulky fluidized bed originates problems and limits the residence time of the gas in the reactor. US-A-3 689 240 generally discloses the use of alkali metal catalysts in methanizing a carbonaceous material and, in addition to using steam only, without any added air or oxygen, considers the fixed bed technique, the moving bed technique and the fluidized bed technique as virtually equivalent. Applicants have surprisingly found a simplified process in which methanization and gassification are a single step, so that the installation first costs are drastically reduced. The invention, therefore, provides a process for preparing a high-methane-content gas from coal by conversion with steam and oxygen or an oxygen-containing gas, in the presence of an alkali metal- or an alkaline earth metal compound as the catalyst, characterized by comprising the steps of: grinding the coal to a particle size not greater than 5 mm; impregnating the ground coal with an aqueous solution of the catalyst, in which the alkali metal- or the alkaline earth metal ions are present in an amount of from 4% to 20% by weight relative to the dry, ash-free coal, optionally drying the impregnated coal, optionally pelletizing the catalyst-impregnated coal, to a pellet size of from 3 mm to 5 cm; methanizing the catalyst-impregnated coal by gassifying a catalyst-impregnated coal bed descending in a reactor in countercurrent relationship with the steam and oxygen or air stream, the air content being not greater than 35% by weight of the dry, ash-free coal, or the oxygen content being from 1% to 25% by weight of the dry, ash-free coal, under a pressure higher than 24,5165 bar (25 abs.atm.) and at a temperature of from 400°C to 850°C; stripping the unreacted steam and the condensibles from the as-produced gas, and deacidifying the gas so produced. By coal, also other carbonaceous materials are intended, such as char, charcoal, coke and lignite. By pelletizing not only cylindrical tablets, but also other geometric solids can be obtained, such as prisms, parallelepipeds and pellets proper. Methanization by gassification take place in a reactor in which the gases to be reacted enter through a bottom inlet at a high temperature, the temperature decreasing gradually towards the reator top where the coal in fed: such a temperature trend encourages gassification in the bottom section and methanization in the top section of the reactor. The catalyst is preferably added to the ground coal by impregnating it with an aqueous solution of one or more alkali metal-, or alkaline earth metal compound, which can be partially recovered by means of the lixiviation with water of the gasification residue, and recycled to the addition step. The alkali and/or alkali-earth compounds can be selected from the salts, hydroxides or oxides of alkali and alkali-earth metals, such as, e.g., K₂CO₃, KOH, NaOH, CaO. The flowrate of steam used in the gasification and methanation step is preferably comprised within the range of from 100 to 200% by weight relatively to the feed rate of dry, ash-free coal. The residence time inside the gasification and methanation step is preferably comprised within the range of from 0.5 to 30 hours, more. preferably of from 1 to 5 hours. Sometimes, a further end step can be necessary to reduce the CO content inside the obtained mixture. This latter possible step consists in an end finishing methanation carried out by operating at a temperature comprised within the range of from 350° to 500°C over such catalysts as, e.g., nickel. The concentration of the various components in said mixtures depends, above all, on the type of coal used and on the use of air and/or oxygen to carry out the gasification. As relates to the use of air instead on oxygen and vice-versa, the selection between the two operating modes is influenced by the required percentage of oxygen relatively to the dry, ash-free coal to carry out the gasification and the methanation. It is preferable to select air when the required oxygen percentage is lower then 5% by weight. The invention is now illustrated with the aid of the hereto attached figure, which shows a preferred practical embodiment thereof. Coal (1), after being ground, is impregnated in (2) with an aqueous solution of one or more alkaline and/or alkaline-earth compounds (3), and is subsequently dried. The impregnated and dried coal (4) is possibly pelletized in (5), to the desired size. Coal (6) is then converted into methane (7) by means of steam (8) and air (9) and/or oxygen (10). In case of oxygen feed, an air fractionation facility (11) is used to remove nitrogen (12). The product (13) obtained from gasification and methanation is separated in (14) from the unreacted steam and from the incondensible compounds formed (15); and in (16) from the acidic gases (17) contained in it, the high methane content gas mixture (18) (SNG) being finally obtained. The gasification residue (19) is recovered in (20) by lixiviating it with water (21) and is sent to (2) to impregnate coal. In as much as the recovery is not total, a catalyst portion must be integrated by means of the line (3). In order to better illustrate the meaning of the present invention, two Examples are reported. Example 1 The test was carried out according to the diagram of the Figure, without the pelletizing step, and using essentially pure oxygen. Charge : Type : Char from Illinois 6, obtained by adding K₂CO₃ to coal and clarifying at 700°C for 60 minutes. Particle size : 841µm-149µm (50-100 mesh.) Composition C 70,6% by weight H 1,2% by weight O 0,2% by weight (by difference) N 1,5% by weight S 3,0% by weight ashes 23.5% by weight (comprising 11.5% of K ion) Gasification and Methanation Step Operating Conditions : Pressure 35,30 bar (36 abs.atm) H₂O flowrate 5,7 g/h O₂ flowrate 0,46 g/h O₂ inlet temperature 827°C Steam inlet temperature 827°C Char was pre-heated to approximately 720°C, with the combustion reaction being started with pre-heated oxygen. Char temperature in the end portion of the reactor: 500°C. A 1,5-metre high reactor with an inner diameter of 26 mm (adiabatic in the first half, partially thermally-insulated in the second half) was used. Composition of the high methane content mixture obtained at 35,30 bar (36 abs.atm), excluding the nitrogen and sulphur containing gases: After Cooling After Washing with NaOH Solution, % by volume mol/h.10³ g H₂ 12,96 0,03 9,18 CO 4,32 0,12 3,06 CO₂ 129,70 5,7 -- CH₄ 123,91 2,0 87,76 Example 2 The test was carried out according to the diagram of the Figure, without the pelletizing step, and using air. Charge : Type , Particle size , and Composition : same as of Example 1. Gasification and Methanation Step Operating Conditions : Pressure 98,066 bar (100 abs.atm) H₂O flowrate 5,7 g/h Air flowrate 0,94 g/h Air inlet temperature 827°C Steam inlet temperature 827°C Char was pre-heated to approximately 720°C, with the combustion reaction being started with pre-heated air. Char temperature in the end portion of the reactor: 500°C. The same reactor as in Example 1 was used. Composition of the high methane content mixture obtained at 98,066 bar (100 abs.atm), excluding the gases containing the nitrogen and sulphur initially present in coal: After Cooling After Washing with NaOH Solution, % by volume mol/h.10³ weight, g H₂ 8.25 0,02 5,19 CO 2.42 0,07 1,52 CO₂ 120.76 5,31 -- CH₄ 123.29 1,97 77,49 N₂ 25.14 0,70 15,80

Claims: Process for preparing a high-methane-content gas from coal by conversion with steam and oxygen or an oxygen-containing gas, in the presence of an alkali metal- or an alkaline earth metal compound as the catalyst, characterized by comprising the steps of: grinding the coal to a particle size not greater than 5 mm; impregnating the ground coal with an aqueous solution of the catalyst, in which the alkali metal- or the alkaline earth metal ions are present in an amount of from 4% to 20% by weight relative to the dry, ash-free coal, optionally drying the impregnated coal, optionally pelletizing the catalyst-impregnated coal, to a pellet size of from 3 mm to 5 cm; methanizing the catalyst-impregnated coal by gassifying a catalyst-impregnated coal bed descending in a reactor in countercurrent relationship with the steam and oxygen or air stream, the air content being not greater than 35% by weight of the dry, ash-free coal, or the oxygen content being from 1% to 25% by weight of the dry, ash-free coal, under a pressure higher than 24,5165 bar (25 abs.atm.) and at a temperature of from 400°C to 850°C; stripping the unreacted steam and the condensibles from the as-produced gas, and deacidifying the gas so produced. Process according to Claim 1, wherein the methanizing step is carried out at a temperature of from 450°C to 750°C. Process according to Claim 1, wherein the pellet size of the catalyst-impregnated coal is of from 3 mm to 2 cm; Process according to Claim 1, further comprising a supplemental methanizing step carried out at a temperature of from 350°C to 500°C. Process according to claim 1, wherein the methanizing step is carried out with an amount of steam of from 100% to 200% by weigh relative to the dry, ash-free coal. Process according to Claim 1, wherein the residence time of the coal to be methanized in the methanizing reactor is from 30 min to 30 hours. Process according to Claim 7, wherein the residence time is from 1 hour to 5 hours. Process accorsing to Claim 1, wherein the methanizing step takes place in a reactor in which the temperature increases from the reactor's top to the reactor's bottom.

Inventors: BRUNO, GIACOMO, CARVANI, LUIGI, PASSONI, GIOVANNI
Applicants: ENIRICERCHE S.P.A., SNAM S.P.A.
Application year: 1987
Main classification: C07C1
IPC codes: C07C9, C10J3, C07C1