Hydrocarbon combustion process in which the carbon dioxide produced is prevented from dispersing into the atmosphere

Abstract: The invention relates essentially to a process for the combustion of a hydrocarbon, such as methane, in particular for energy production, characterised in that said hydrocarbon, which will be assumed to be methane, is fed to a first stage in which it is reacted with the carbon dioxide produced during the combustion, in said first stage the following reactions taking place: the carbon produced in the reaction (2) is separated, and the gaseous mixture produced in said first stage is then fed to a second stage for reaction with oxygen, in which the reaction products are steam and carbon dioxide, this latter being totally recycled to said first stage.

Description: It is well known that in processes involving the combustion of liquid and gaseous hydrocarbons (such as methane) for the large-scale production of thermal and electrical energy, such as in thermoelectric power stations, one of the currently most serious problems is the release to atmosphere of the correspondingly large quantities of carbon dioxide produced by the combustion. Much comment has recently been made on the serious environmental problem of unbalance between the emission of CO₂ into the atmosphere and the reuse of the same CO₂ in the biosphere. In other words, the rate of combustion of carbon-based fossil fuels exceeds the rate of carbon fixation by photosynthesis in plants. While fossil fuels (coal, natural gas and petroleum) constitute the main source of energy, it will not be possible to stop their production, although their consumption may have to be rationalized, even though it is probable that other forms of energy will he available in the unforseable future. Faced with the ecological risk associated with excess CO₂ in the atmosphere, it has for example been proposed to increase the efficiency of direct utilization of solar energy or photosynthesis by selected micro-organisms which fix the CO₂ and produce fuels (solar bioreactor). However this project has still to be developed and tried, and at the moment is only a hypothetical solution. In contrast, the present invention proposes a concrete solution to this technical problem, which can be implemented quickly on an industrial scale. DE-B-1 467 447 describes a process for the preparation of carbon black by cracking at 1000-1500°C a gaseous and vaporized hydrocarbon. To attain the purpose outlined above, the present invention provides a process for the combustion of a hydrocarbon, such as methane, in particular for energy production, characterised in that said hydrocarbon, for instance methane, is fed to a first stage in which it is reacted with the carbon dioxide produced during the combustion, in said first stage the following reactions taking place: CH₄ + CO₂ <-------> 2CO + 2H₂ CH₄ + CO₂ <-------> 2C + 2H₂O CO₂ + H₂ <-------> CO + H₂O the carbon produced in the reaction (2) is separated and the gaseous mixture produced in said first stage is then fed to a second stage for reaction with oxygen, in which the reaction products are steam and carbon dioxide, this latter being totally recycled to said first stage. The first stage of the process according to the invention is conducted at a pressure chosen between 1 and 50 bar, and a temperature chosen between 500 and 800°C, and preferably between 600 and 700°C at a pressure of between 10 and 40 bar. The process of the invention is described in greater detail with reference to the figure of the accompanying drawing, which represents a schematic flow diagram of the process. The schematic diagram shown is to be considered purely a non-limiting example of the invention. With reference to said figure it will be assumed that in a plant for producing energy by combustion of methane 10, this is fed to a first stage 11 in which, under temperature and pressure conditions chosen within the aforestated ranges, the methane reacts with the carbon dioxide fed to said stage 11 via a flow line 12. The aforestated reactions (1), (2) and (3) take place in this stage, with the production of carbon (C), for example carbon black, in accordance with the reaction: CH₄ + CO₂ <-------> 2C + 2H₂O the carbon black being separated, collected and stored via 13 as a useful product. On leaving the stage 11, the gaseous mixture produced in it, and consisting of H₂, CO and H₂O plus the CH₄ and CO₂ unreacted in this first stage, is fed via 14 to a second stage 15, to which pure oxygen is fed via 16. Total combustion of CO, H₂ and CH₄ takes place in this second stage, with the formation of H₂O and CO₂. The mixture containing these products of the reaction of the stage 15 is fed via 17 to a steam boiler 18 and then through a recuperator 19 and a condenser 19' for separation of water, which is collected in 20. The CO₂ which separates from the water leaves 20 through the line 21 and is totally recycled to said first stage 11 via the flow line 12. The reference numeral 24 on the schematic diagram indicates a purging line. Finally, the reference numerals 22 and 23 represent respectively the amount of heat produced in the stage 11 and the amount of heat recovered for steam production in the boiler 18. With reference to said schematic, one example of the implementation of the process according to the invention is described hereinafter, but is in no way to be considered as limitative thereof. EXAMPLE This example, which includes process operating conditions and quantities, is described with reference to the scheme of the Figure. The methane stream (10) is preheated to 600°C and fed to the first stage at a pressure of 30.4 bar (30 atm) (absolute). A unit flow rate (1 kmole/u.t.) is assumed for this stream as reference. After heating, the recycle CO₂ is also fed to the first stage in a quantity of 1 kmole of CO₂ per kmole of CH₄. The operating conditions of the first stage and the manner of contact between the two gaseous streams enable thermodynamic equilibrium to be attained with excellent approximation at 600°C and 30.4 bar (30 atm). The gaseous outflow (14) has tile following composition: CO 0.05987 kmoles/kmole of entering CH₄ CO₂ 0.52252 kmoles/kmole of entering CH₄ H₂ 0.25456 kmoles/kmole of entering CH₄ H₂O 0.89509 kmoles/kmole of entering CH₄ CH₄ 0.42517 kmoles/kmole of entering CH₄ The solid outflow (13), ie carbon in the form of carbon black, is produced in a quantity of 0.99244≃1 kmole/kmole of entering CH₄. Under these operating conditions, the first stage requires a power input Q of 21,571 kj/kmol (5155.5 kcal/kmole) of entering CH₄, which is easily obtainable from the sensible heat of the gas leaving the second stage. The said gaseous mixture (14) is fed to the second stage (16), to which oxygen in a quantity of 1 kmole/kmole of entering CH₄ is fed after being preheated and compressed to a pressure of 30.4 bar (30 atm). Stoichiometric combustion occurs, resulting in a gaseous product consisting of CO₂ (1 kmole) and H₂O (2 kmole) at high temperature. This gas enters the boiler (18) and then passes through a system of heat recuperators (19) designed to preheat the CH₄, O₂ and the recycle CO₂. The gas energy is recovered down to a temperature of 100°C, giving a total recovered heat quantity (23) in both boiler and recuperators of 560,166 kj/kmol (133,883 kcal/kmole) of CH₄ feed. The condenser (19') for final steam removal precedes the separation unit (20) for separating the recycle CO₂ (21) from the water, which is discharged from the plant. A purging line (24) is provided in the recycle line to prevent accumulation of inert gases or gases in excess of the stoichiometric ratio. Taking account of the heat recovery along the cycle for preheating the reactants, but ignoring the ineffective enthalpy of the stream leaving the recuperators at 100°C, the nett energy withdrawn from the circuit (22+23) is 464,315 kj/kmol (110,974 kcal/kmole) of CH₄. From the aforegoing description and example it is apparent that in an effective and industrially advantageous manner, the process of the invention allows the CO₂ produced in methane combustion processes to be totally recycled, so completely preventing their release to the atmosphere. In this respect, in the first stage a recoverable and industrially useful product in the form of carbon, for example carbon black, is obtained. After removal of the carbon black, the second stage provides complete oxidation of the synthesis gases from the first stage using pure oxygen. This second stage is strongly exothermic and represents the main stage in terms of the energy produced by the process of the invention. The use of oxygen instead of air is imposed by the need not to dilute the combustion products, and in particular the carbon dioxide, which has to be completely recycled. Thus the combination of the two stages is essential in the proposed process, ie precipitation of carbon in the form of carbon black, followed by oxidation with oxygen, these together enabling the energy of the hydrogen contained in the hydrocarbon to be utilized while recovering the carbon. The pressure and temperature to be used in the first stage also depend on the initial pressure of the natural gas fuel as available, and of the oxygen, and also on the equilibria of the transformation reactions. It can therefore be stated that the invention basically provides for reducing the CO₂ formed during the oxidation of liquid and gaseous fuels, by so to speak halting the combustion at the hydrogen and storing or otherwise using the carbon, in the form for example of carbon black. This storage is seen as an effective although temporary solution while awaiting the solution to at least a part of the overall energy problem, for example by fusion reactors, by the perfection of solar bioreactors, or by other alternative energy means. Furthermore it preserves the high energy potential unchanged. It should be noted that although the process of the invention can be effectively applied in particular to combustion processes in energy production stations such as thermoelectric power stations, it is also applicable to carbon black production plants, even though in this case the CO₂ produced and dispersed into the atmosphere is in a much lower quantity than that involved in thermoelectric power stations. A further considerable advantage of the invention is that it allows convenient recovery of at least part of the energy because of the overall exothermic balance of the process when operating at a temperature and pressure suitably chosen within the aforedefined ranges. Another considerable advantage of this invention is that, using pure oxygen in the combustion reaction of the second stage (and not air), there is no formation of nitrogen oxides and thus no emission of pollutant gases into the environment.

Claims: A process for the combustion of a hydrocarbon, in particular for energy production, characterised in that said hydrocarbon is fed to a first stage in which it is reached with the carbon dioxide produced during the combustion, in said first stage the following reactions taking place: CH₄ + CO₂ <-------> 2CO + 2H₂ CH₄ + CO₂ <-------> 2C + 2H₂O CO₂ + H₂ <-------> CO + H₂O the carbon produced in the reaction (2) is separated, and the gaseous mixture produced in said first stage is then fed to a second stage for reaction with oxygen, in which the reaction products are steam and carbon dioxide, this latter being totally recycled to said first stage. A process as claimed in claim 1, characterised in that said first stage is conducted at a pressure chosen between 1 and 50 bar, and a temperature chosen between 500 and 800°C. A process as claimed in claim 2, characterised in that said first stage is conducted at a pressure chosen between 10 and 40 bar, and a temperature chosen between 600 and 700°C. A process as claimed in claims 1-3, characterised in that said hydrocarbon is methane.

Inventors: BIARDI GIUSEPPE, SZEGOE EDUARDO LUIGI, BIARDI, GIUSEPPE, SZEGOE, EDUARDO LUIGI, Szegö, Eduardo Luigi
Applicants: BIARDI GIUSEPPE, SZEGOE EDUARDO LUIGI, BIARDI, GIUSEPPE, SZEGOE, EDUARDO LUIGI
Application year: 1991
Main classification: B01D53
IPC codes: F23C6, F23C9, C09C1, F23C99