HJULER, Klaus (Skovbakken 98, Farum, DK-3520, DK)
| Claims [Claim 1] 1. A method for heat treatment of raw materials such as cement raw meal, limestone or other mineral-containing raw materials, by which method raw material is preheated and possibly calcined in a preheater system (1) and possibly burned in a kiln (8), and where solid and/or liquid fuel is brought into contact with preheated raw material in the presence of steam in a gasification reactor (3) thereby causing the fuel to be fractionated partly into a hydrogen-containing combustible gas which is discharged from the gasification reactor (3) via a gas outlet and partly into carbonaceous solid fuel residues which are discharged from the gasification reactor (3) via a solids outlet being characterized in that at least some of the hydrogen-containing combustible gas is utilized for heat treatment of raw material. [Claim 2] 2. A method according to claim 1 being characterized in that the gasification reactor (3) is fed with raw material having a temperature of at least 500 ° C, preferentially at least 800 ° C. [Claim 3] 3. A method according to claim 1 or 2 being characterized in that the combustible gas from the gasification reactor (3) is brought into contact with completely or partially calcined raw material in a carbon dioxide absorber (4) at a temperature level at which recarbonisation can take place. [Claim 4] 4. A method according to any of the claims 1 to 3 being characterized in that the hydrogen-containing combustible gas is utilized as fuel for preheating raw material in the preheater system (1) and/or for burning raw material in the kiln (8). [Claim 5] 5. A method according to any of the claims 1 to 3 being characterized in that the hydrogen-containing combustible gas is contacted with raw material in a separate heat exchanger (6) and subsequently extracted for further processing to storage and/or production of liquid fuel and/or utilization as fuel in a burner or in a power engine or in fuel cells. [Claim 6] 6. A method according to claim 5 being characterized in that the hydrogen-containing combustible gas is further treated using prior art methods so as to remove particles and undesirable components such as hydrogen sulphide, hydrogen chloride and mercury. [Claim 7] 7. A method according to claim 5 or 6 being characterized in that the preheated raw material from the separate heat exchanger (6) is subsequently further heated and possibly calcined in a separate preheater system (5). [Claim 8] 8. A method according to claims 5, 6 or 7 being characterized in that the raw material being preheated in the separate heat exchanger (6) is a so-called alternative raw material which contains organic material. [Claim 9] 9. A method according to any of the aforementioned claims being characterized in that at least some of the solid fuel residues from the gasification reactor (3) is utilized as fuel in the separate preheater system (5) and/or the calciner (2), and in that the stream of carbonaceous exhaust gas generated is extracted for further processing. [Claim 10] 10. A method according to claim 9 being characterized in that some of the carbonaceous exhaust gas is recirculated to the separate preheater system (5) and/or the calciner (2), and in that the remaining carbonaceous exhaust gas is extracted for further processing. [Claim 11] 11. A method according to claim 10 being characterized in that the remaining carbonaceous exhaust gas is routed to a boiler (34) for power production and subsequently extracted for further processing. [Claim 12] 12. An apparatus for carrying out the method according to any of the claims 1 to 11, with said apparatus comprising a preheater system (1) for preheating and possibly a calciner (2) for calcination of raw material as well as a gasification reactor (3) for fractionation of solid and/or liquid fuel partly in a hydrogen- containing combustible gas and partly in carbonaceous solid fuel residues being characterized in that it comprises means for utilizing at least some of the hydrogen-containing combustible gas for heat treatment of raw material. [Claim 13] 13. An apparatus according to claim 12 being characterized in that the gasification reactor (3) comprises a 'moving bed', a rotating drum or a cyclone. [Claim 14] 14. An apparatus according to claim 12 or 13 being characterized in that it comprises a separate preheater Dsystem (5). [Claim 15] 15. An apparatus according to claim 12, 13 or 14 being char- acterized in that it comprises a boiler (34). [Claim 16] 16. An apparatus according to any of the claims 12 to 16 being characterized in that it comprises a gas filter and/or a gas scrubber (7). [Claim 17] 17. An apparatus according to any of the claims 12 to 16 being characterized in that it comprises a gas filter and/or a gas scrubber (7). [Claim 18] 18. An apparatus according to any of the claims 12 to 17 being characterized in that it comprises a kiln (8). [Claim 19] 19. An apparatus according to any of the claims 12 to 18 being characterized in that it comprises a carbon dioxide absorber (4). [Claim 20] 20. An apparatus according to claim 19 being characterized in that the carbon dioxide absorber (4) comprises a 'moving bed' or a cyclone. |
Title of Invention: METHOD AND PLANT FOR HEAT TREATMENT OF RAW MATERIALS
[1] The present invention relates to a method for heat treatment of raw materials such as cement raw meal, limestone or other mineral-containing raw materials, by which method raw material is preheated and possibly calcined in a preheater system and possibly burned in a kiln, and where solid and/or liquid fuel is being brought into contact with preheated raw material in the presence of steam in a gasification reactor, so that the fuel is fractionated partly into a hydrogen-containing combustible gas which is discharged from the gasification reactor via a gas outlet, and partly into carbonaceous solid fuel residues which are discharged from the gasification reactor via a solids outlet. The invention also relates to an apparatus for carrying out the method.
[2] Cement plants and mineral processing plants are energy-intensive and utilize substantial amounts of fossil fuel that contribute significantly to the global man-made carbon dioxide emissions. For this reason, significant efforts are being devoted to reduce the carbon dioxide emissions from these plants. Summarized in the following are various options currently available, but without any reference to the so-called 'post- cleaning' technology which is based on the subsequent separation of carbon dioxide from the exhaust gas.
[3] One method which is commonly used is the substitution of fossil fuels with more or less carbon dioxide neutral fuels, so-called alternative fuels. Subsequent to pre- treatment involving grinding, cutting or similar processes, and possibly drying, alternative fuels may be suitable for suspension firing in a preheater or a calciner where moderate temperature requirements apply. However, the burning of cement clinker requires a much higher temperature level, thereby precluding the possibility of complete substitution of fossil fuel with alternative fuel. Co-firing of alternative fuel and fossil fuel is possible in modern rotary kiln burners, but again this would necessitate reduction of coarse fuel particles by grinding, cutting or similar.
[4] A second technology is the so-called oxygen-enriched combustion (OEC) where the oxygen content of the combustion air for the rotary kiln burner is increased to 23-25 per cent by supplying pure oxygen. This results in a higher flame temperature and a reduction of the specific exhaust gas volume; both factors contribute to decreasing the energy consumption per produced unit of cement clinker and enhancing the plant capacity which means that the costs relating to the purchase of oxygen or the installation and operation of oxygen plants will be acceptable in some cases. In the ultimate form of OEC, the combustion air is completely replaced by pure oxygen thereby achieving, in principle, a condition where the exhaust gas is substantially made up of carbon dioxide and steam. The steam can be removed by condensation, with the carbon dioxide being subsequently compressed and sequestrated in the underground. Plants for CO 2 -capture of this type where part of the exhaust gas is recirculated and mixed with pure oxygen are being developed in the power plant sector and suggestions have been made to deploy this technology at cement plants as well, e.g. in a concept study presented by the International Energy Agency (IEA Greenhouse Gas R&D Programme: 'CO 2 Capture in the Cement Industry', Technical Study, Report No. 2008/3). However, the infiltration of relatively substantial amounts of false air is a characteristic problem at cement plants, which may complicate the process of achieving an acceptable concentration of carbon dioxide in the extracted exhaust gas. According to the concept study presented by IEA, it seems to make better economic sense to deploy another OEC process configuration in which calcinaDtion is carried out in a separate preheater subject to the supply of fuel and pure oxygen. From the separate preheater, it will be possible to extract a more concentrated stream of carbon dioxide, thereby reducing the total carbon dioxide emissions of the plant by around 50 per cent (this figure includes the carbon dioxide emissions associated with generating power for the cement plant at a conventional power plant).
[5] A third method for reducing the carbon dioxide emissions from cement and mineral manufacturing processes involves the utilization of surplus heat in the preheater and exhaust gases for power generation in a Rankine cycle process (using steam or an organic medium), thereby reducing the net power consumption of the plant. This called co-generation is practised at some plants, particularly in China. However, the available amount of energy is relatively modest and only low-grade heat is generated and this means that the efficiency of power generation will be quite small. Because of these factors the investment in a Rankine cycle co-generation plant for surplus heat utilization will often not be profitable.
[6] Co-generation of power and minerals/cement may alternatively be established using gasification technology, with the gas subsequently being combusted in a gas engine or other device for power production. Consequently, the quantity of power that can be generated will not rely on surplus heat from the process, which may be of particular significance in connection with the operation of power-intensive auxiliary systems for manufacturing oxygen and compression of carbon dioxide. Gasification can take place in a separate unit, as practised for example at a cement plant in Rϋdersdorf, Germany, or, as suggested in the literature (S. Weil et al.: 'Hydrogen Energy from Coupled Waste Gasification and Cement Production - a Thermochemical Concept Study', International Journal of Hydrogen Energy 31, 2006), in a partially integrated unit where the fuel is indirectly heated by contacting it with preheated cement raw meal and steam. Using the last method it will be possible to produce a gas having a relatively high calorific value of about 10-15 MJ/Nm 3 , depending on the amount of carbon dioxide in the gas. Prior to being cooled, directed through a duct system and utilized in a gas engine or other device for power production, the gas must be cleaned of tar substances which is typically a problem for prior art.
[7] As mentioned above, a great deal of effort is being devoted to the development of technologies for reducing carbon dioxide emissions from cement plants and mineral processing plants. With reference to the introductory notes, it is possible to summarize a number of problems and disadvantages associated with prior art:
[8] • Complete substitution of fossil fuel with alternative fuel is not feasible and, furthermore, alternative fuels are only partially CO 2 -neutral.
• Pre-treatment of alternative fuels for suspension firing is often complicated and costly.
• False air infiltration entails significant increase in the costs associated with CO 2 -capture using OEC technology.
• Separate calcination using OEC technology will not achieve more than about 50 per cent CO 2 -capture.
• The amount of available surplus heat for power generation is insufficient and the conversion efficiency is low.
• Tar substances must be removed from hot gasification gas prior to being utilized in a gas engine or other device for power generation.
• Gasification gas typically contains carbon dioxide that reduces the calorific value of the gas.
[9] It is the objective of the present invention to provide a method as well as an apparatus for eliminating or significantly reducing the aforementioned problems and disadvantages.
[10] This is obtained according to the present invention by a method of the kind mentioned in the introduction, being characterized in that at least a part of the hydrogen-containing combustible gas is utilized for the heat treatment of raw material.
[11] Using a combination of gasification technology and OEC technology, it will thus be possible to reduce the carbon dioxide emissions from cement plants and mineral processing plants by more than 50 per cent due to the fact that the hydrogen-containing combustible gas may e.g. be utilized as a fuel subject to the supply of air for preheating the raw material and/or for burning cement clinker in a kiln. The combustion gas which substantially contains nitrogen and steam can be vented to the atmosphere after being cleaned by means of prior art methods. The solid fuel residues from the gasification reactor, which is substantially made up of carbon, may e.g. be utilized as a fuel in a calciner subject to the supply of pure oxygen, thereby providing a concentrated stream of hot carbon dioxide which can be utilized for preheating raw material or used for raising steam, and subsequently compressed and extracted from the plant for sequestration in the underground or subjected to further processing. Simultaneously the fractionation of the fuel makes it possible to achieve complete substitution of fossil fuel with alternative fuel, while eliminating or reducing the requirement with respect to pre-treatment of the alternative fuel.
[12] The hydrogen-containing combustible gas may contain most of the energy contained in the original fuel. Given that the solid fuel residues, as described above, may be utilized to cover the energy requirements for calcination which constitute a significant part of the overall energy requirements, this may create a surplus of hydrogen- containing combustible gas. This gas may be used to generate power in a gas engine, fuel cells or other device, thereby significantly increasing the potential for power generation at the plant as opposed to utilization based exclusively on surplus heat. A surplus of hydrogen-containing combustible gas may according to the present invention be extracted from the gasification reactor and cooled in a separate heat exchanger by direct heat exchange with colder raw material, causing condensation of any tar substances on the surface of the raw material. The raw material thus heated may subsequently be heated further and possibly subjected to calcination in a separate preheater system utilizing the solid residues from the gasification reactor subject to the supply of pure oxygen. Hence the tar substances condensed on the raw material will also be combusted. It is a characteristic feature of this embodiment of the invention that the raw material is in contact with combustible gas during the heat exchange process, i.e. a reducing atmosphere. Volatile organic substances in the raw material which could constitute an emission problem with conventional preheating, will thus add to the amount of combustible gas. This makes it possible to process so-called alternative raw material being characterized by a high content of organically bound elements. The resulting tar-free and cooled combustible gas may subsequently be treated using prior art methods so as to remove particles and undesirable components such as hydrogen sulphide and hydrogen chloride.
[13] Before the hydrogen-containing combustible gas extracted from the gasification reactor is utilized for heat treatment of raw material, it may be brought into contact with completely or partially calcined raw material, firstly at a temperature where tar cracking can take place, and secondly at a temperature where recarbonisation can take place. It is well known from prior art that certain materials and especially calcined minerals, such as calcium oxide, have catalytical effects on tar cracking. The tar cracking reactions typically takes place at reasonable rates at temperatures above 750 ° C, whereas the recarbonisation reaction with calcined cement raw meal occurs at a temperature range from about 550 to about 650 ° C, in which range it is possible for carbon dioxide contained in the combustible gas to react with calcium oxide in the raw material. Hence carbon dioxide will be removed from the combustible gas, accompanied by an increase in the calorific value of the gas. The calcined raw material used to absorb carbon dioxide which may be constituted by the solid material containing fuel residues extracted from the gasification reactor is subsequently re- circulated to the calciner.
[14] The fractionation of fuel in the gasification reactor, and hence the amount and composition of the hydrogen-containing combustible gas, depends on the temperature of the preheated raw material which should be at least 500 ° C, and preferentially at least 800 ° C. In a preferred embodiment of a plant according to the invention for heat treatment of cement raw meal, solid and/or liquid fuel will be brought into contact with completely or partially calcined cement raw meal having a temperature of approximately 850 ° C in the presence of steam in e.g. a 'moving bed' reactor, alternatively a rotating drum, a cyclone or other gas-solid contactor. The combustible gas thus produced is extracted from the gasification reactor via a gas outlet while the solid material, i.e. calcined cement raw meal, fuel ash, char and eventual metal particles, is extracted from the gasification reactor at a temperature around 600 ° C via a solids outlet. Given that carbon dioxide can be absorbed by calcined cement raw meal in the form of calcium carbonate at a temperature range close to 600 ° C, the combustible gas will be brought into contact with the solid material extracted from the gasification reactor in a carbon dioxide absorber, which e.g. may also be a 'moving bed' reactor, alternatively a cyclone or other gas-solid contactor.
[15] Some of the combustible carbon dioxide-free gas from the carbon dioxide absorber may be directed to a rotary kiln burner for burning cement clinker, and the hot kiln gas generated can be passed to the preheater for preheating the raw material to a temperature level around 750 ° C. From the preheater the raw material may be routed to a calciner where the solid material from the gasification reactor may be utilized as fuel subject to the supply of pure oxygen. Before the solid material from the gasification reactor is utilized as fuel it can be crushed and screened so that eventual metal particles are separated and coarse char particles are size reduced. The gas from the calciner which contains carbon dioxide may be directed to a separate preheater in which raw material can be heated in similar manner to a level around 750 ° C and subsequently passed to the calciner. Alternatively, the gas from the calciner can be cooled to around 600 ° C in the separate preheater, at which temperature level the risk of deposit formation is significantly reduced, and routed to a steam boiler for power production. The second part of the combustible carbon dioxide-free gas may be directed to a separate heat exchanger in which the gas is cooled from a level around 600 ° C to a temperature above the water dewpoint of the gas through direct heat exchange with the raw material. The raw material in the separate heat exchanger may be constituted by alternative raw material, which as previously mentioned is characterized by its high content of organic matter. From the separate heat exchanger the gas can be routed to a particulate filter and a scrubber for removing hydrogen sulphide and chlorine sulphide and the gas is subsequently used for power production in a gas engine, fuel cells or other device. Alternatively, the combustible gas may be used as a so-called syngas (synthesis gas) for manufacturing liquid fuel. The raw material from the separate heat exchanger can be routed to the calciner together with the raw material from the preheater and the separate preheater, respectively.
[16] The invention will now be explained in greater detail with reference to the drawing, being diagrammatical, and where
[17] Fig. 1 and Fig. 2 show two embodiments of an apparatus according to the invention for heat treatment of raw material.
[18] In Fig. 1 is shown an apparatus for calcination of raw material in which raw material is introduced via inlet 9 to a preheater 1 and subsequently routed via a duct 11 to a second preheater 5 which receives raw material via inlet 27 and oxygen via inlet 25. The raw material is calcined in the preheater 5 and some of the calcined raw material is routed via a duct 31 for further processing, while the remaining part of the calcined raw material is routed via a duct 12 to a gasification reactor 3 which is also fed with fuel via inlet 15 and eventually steam via inlet 16. The tar-containing combustible gas from the gasification reactor 3 is directed via a duct 17 to the preheater 1 in which it is burned subject to the supply of air via inlet 32 and the cooled gas from the preheater 1 is passed via outlet 10 for further processing. From the preheater 5 some of the cooled carbon dioxide containing gas is directed via a duct 29 for further processing. The calcined raw material and solid fuel residues from the gasification reactor 3 are passed via a duct 13 back to the preheater 5 via the duct 24 in suspension with recirculated, carbon dioxide containing gas from the preheater 5.
[19] In Fig. 2 is shown an apparatus for manufacturing cement clinker where cement raw meal is introduced via inlet 9 into a preheater 1 and subsequently passed via a duct 11 to a calciner 2 which is fed with oxygen via inlet 25. From the calciner 2 some of the cement raw meal is routed via a duct 31 to a rotary kiln 8 in which it is burned into clinker which is extracted via outlet 33. The remaining part of the calcined cement raw meal is routed via a duct 12 to a gasification reactor 3 which receives fuel via inlet 15 and eventually steam via inlet 16. The tar-containing combustible gas from the gasification reactor 3 is directed via a duct 17 to a carbon dioxide absorber 4 together with calcined cement raw meal and solid fuel residues via a duct 13 from the gasification reactor 3. Carbon dioxide contained in the combustible gas reacts with calcium oxide in the cement raw meal subject to the formation of calcium carbonate which together with the other solid material from the gasification reactor 3 via outlet 14 and a duct 24 is passed back to the calciner 2 in suspension with recirculated carbon dioxide containing gas from a preheater 5. Some of the carbon dioxide-free combustible gas is directed via a duct 18 from the carbon dioxide absorber 4 to the rotary kiln 8 in which it is burned subject to the supply of air via inlet 32. From the rotary kiln 8 the kiln gas is directed via a duct 30 to the preheater 1 from which the cooled gas is extracted via outlet 10. The remaining part of the carbon dioxide-free combustible gas is directed via a duct 19 to a preheater 6 which is fed with cold cement raw meal via inlet 20. The cement raw meal thus preheated is routed via a duct 21 to the calciner 2, while the cooled tar-free and carbon dioxide-free combustible gas from the preheater 6 is directed via a duct 22 to a gas cleaning unit 7 for removing particles, hydrogen sulphide, chloride sulphide and mercury and subsequently passed via outlet 23 to storage or application for power production in a gas engine, fuel cells or other device. From the calciner 2 the carbon dioxide containing gas is directed via a duct 26 to the preheater 5 which receives cement raw meal via inlet 27. The preheated raw material from the preheater 5 is routed via a duct 28 to the calciner 2. Part of the carbon dioxide containing gas is routed from the preheater 5 via a duct 29 to a steam boiler 34 or similar for power production and subsequently extracted via outlet 35 for further processing, and the remaining part of the carbon dioxide containing gas is recirculated via the duct 24 to the calciner 2.