AU2008248857A1

AU2008248857A1 - Method for the combustion of fuel

Info

Publication number: AU2008248857A1
Application number: AU2008248857A
Authority: AU
Inventors: Malte E. C. Forster; Reinhold Kneer; Dobrin Toporov
Original assignee: RHEIN WESTF TECHN HOCHSCHULE; Rheinisch Westlische Technische Hochschuke RWTH
Current assignee: Rheinisch Westlische Technische Hochschuke RWTH
Priority date: 2007-05-07
Filing date: 2008-05-07
Publication date: 2008-11-13
Also published as: WO2008135275A4; DE102007021799A1; US20100190118A1; EP2149012A1; WO2008135275A1

Description

1 Method for the combustion of fuel The present invention relates to a process for the combustion of fuel supplied to the combustion zone of a burner, using oxygen or a mixture of oxygen and carbon dioxide. The invention further relates to a burner for carrying out the process according to the invention. Such processes have become increasingly important in the past years, because this kind of combustion allows a drastic reduction of the pollutant emission compared to the combustion of fuel using air. Conventional combustion processes, in which for example fossil fuels are burnt, mostly take place under the use of air, i.e. air is supplied to the combustion. Considering however, that air has a very high nitrogen content of approx 80%, such combustion processes produce nitrogen oxides which are an extreme burden to the environment. Further, during the combustion of fossil fuels, carbon dioxide is produced which is also pollutant, because it intensifies the greenhouse effect. Thus an attempt may be made to separate the nitrogen and the carbon dioxide which are present in the flue gas as a mixture, so that the carbon dioxide can be subsequently stored and eventually used for other purposes. But this separation is very expensive, because at the end of the 2 process a flue gas mixture is present which has carbon dioxide content of approx 20% and a nitrogen content of approx 80%. For this reason, oxygen or a mixture of carbon dioxide and oxygen instead of air were supplied to the combustion, resulting in that at the end of the process almost pure carbon dioxide is present which can be stored afterwards. Another advantage is that,. no harmful nitrogen oxides are emitted during such a combustion process. If the combustion is performed using a mixture of oxygen and carbon dioxide, it turned out to be beneficial if the flue gas produced during the combustion is removed from the combustion chamber and is subsequently recycled again to the combustion chamber, wherein this recycled flue gas has a high carbon dioxide content when fossil fuels are burnt. Oxygen which is required for the combustion has to be produced externally and mixed with the recycled flue gas. Accordingly, in the above-described combustion of fuel using oxygen-enriched flue gas, the nitrogen originating from the air is substantially replaced by carbon dioxide. But as carbon dioxide has a higher thermal capacity than nitrogen, more heat is extracted from the combustion than when air is used. This causes the temperature in the combustion zone to drop, so that the combustion takes place under no optimum conditions. For this reason, the combustion is preformed at a level of oxygen (> 21%) which is increased compared to air, whereby the combustion is stabilized. On the other hand, the costs are simultaneously increased, because the pure oxygen must be produced in advance. For the reasons as mentioned, such combustion processes, also called oxy-fuel processes, are much friendlier to the environment than the previous combustion in which air is supplied.

3 In the described combustion processes the carbon dioxide noticeably reacts with carbon at a temperature from approx 700 0 C, whereby carbon monoxide is produced. The corresponding reaction equation is as follows: CO2 + C -+ 2 CO This reaction produces a duplication of the volume, since two mol of carbon dioxide are produced from one mol of carbon monoxide. This expansion of the volume causes higher flow velocities which negatively influence the stability of the combustion reaction, hence the combustion process. Additionally, in the above endothermic reaction heat is dissipated, which fact also destabilizes the combustion process. Due to the reduction of the combustion temperature the above reaction does not occur any longer, resulting in that the accompanying duplication of the volume does not take place either. This causes the combustion temperature to increase again to a level where the reductive transformation of carbon dioxide into carbon monoxide is permitted. The combustion process is weakened in the end by the mentioned feedback processes, since there are no constant conditions for the combustion. For this reason, for stabilizing the combustion process, oxygen at a volume of at least 21% has been supplied to the combustion zone up to present, whereby the temperature of the combustion process could be maintained relatively constant. This use of oxygen adds to the cost of a corresponding combustion. In view of the above, it is an o b j e c t of the present invention to provide a process for the combustion of fuel in a burner, in which process a combustion of the fuel can be stably performed using oxygen or a mixture of oxygen and carbon dioxide at an oxygen content of preferably less than 21% by volume. According to the invention, this object is a c h i e v e d by the process for the combustion of fuel supplied to a combustion zone of a burner using oxygen or a mixture of oxygen and carbon dioxide. The fuel can be fossil fuel such as coal 4 or the like which is fed in a suitable form to the combustion zone. The mixture of oxygen and carbon dioxide is preferably formed from oxygen-enriched flue gas that is removed from the combustion chamber and subsequently recycled again to the combustion chamber. According to the invention, a cyclic flow is created in the combustion zone causing a homogeneous mixing of hot combustion products with the fuel and the oxygen or with the fuel and the mixture of oxygen and carbon dioxide in the combustion zone. This cyclic flow transports heat and combustion products to the combustion zone of the burner, thus stabilizing the combustion in the combustion zone of the burner, because the above-described reduction of heat caused by the endothermic reaction is compensated by maintaining the hot combustion products in the combustion zone. This does not comprise conditions like those met in the FLOX technology in which air is used as oxidant. To create a cyclic flow sufficient for stabilizing the combustion, parameters such as the supply rates of the oxygen or the mixture of oxygen and carbon dioxide and of the fuel as well as the physical structure of the burner must be considered. A suitable burner includes for instance in the supply area of the materials flow separation edges capable of causing swirling of the materials which are supplied. Further, in the combustion zone of the burner an appropriately designed space must be available enabling a turbulent flow to create a cyclic flow in the form of a strongly recycling flow. By this stabilization of the combustion in the combustion zone of a burner by means of a strongly recycling flow, the oxygen content in the combustion zone can be reduced, since the combustion needs not be stabilized alone by supplying oxygen. Accordingly, it is possible to keep the oxygen content in the combustion zone below 21 % by volume and yet achieve a stable combustion process. An advantageous embodiment of the invention provides that the cyclic flow is created in the form of a strongly reduced flow substantially in the entire combustion zone of the burner, whereby the combustion process is maximally 5 stabilized and whereby the oxygen content in the combustion zone can be reduced as far as possible. Preferably, the strongly recycling flow is created in at least 50% of the combustion zone. Preferably, the recycling flow is created in at least 80% of the combustion zone. Particularly preferably, the recycling flow is created in at least 90% of the combustion zone. A further advantageous embodiment of the invention provides that flue gas produced during the combustion is discharged from the combustion zone. This flue gas has a high carbon dioxide content and can be stored after its discharge, so that the carbon dioxide emission during the combustion in accordance with the invention is reduced as far as possible. A further advantageous embodiment of the invention provides that the flue gas is enriched with oxygen and thereafter supplied to the combustion zone. By the enrichment of the flue gas with oxygen prior to being supplied to the combustion zone, a homogeneous mixing of flue gas and oxygen is achieved, which has a favorable effect on the combustion process. Further, by supplying the hot flue gas, heat is supplied to the combustion process in the combustion zone, which also has a positive effect on the combustion process. A further advantageous embodiment of the invention provides that the fuel is supplied to the combustion zone together with the oxygen and/or flue gas, wherein the flue gas may be a mixture of oxygen and carbon dioxide. The way which is selected for the supply depends on the respective conditions, e.g. on the design of the burner or on other parameters of the combustion, so that the process of the invention can be suitably adapted for optimum combustion. A further advantageous embodiment of the invention provides that the fuel, the oxygen and/or the flue gas are supplied to the combustion zone from different directions or from the same direction. Accordingly, depending on the design of the burner, the creation of a cyclic flow in the form of a strongly recycling flow 6 can be promoted by the variant selected for supplying the materials to the combustion zone. For example, a supply of the materials in opposite directions can result in a strongly recycling flow being created. Further, an advantageous embodiment of the invention provides that the components of the flue gases are completely burnt in the combustion zone or are removed from the combustion zone for use thereof for other purposes outside the combustion zone. For instance, a suitable selection of the parameters of the combustion allows the targeted production of carbon monoxide that can be discharged from the burner and utilized as a starting material for particular chemical compositions. Concerning the burner, the afore-mentioned object of the present invention is a c h i e v e d by the combustion zone of the burner including fluid-mechanical installations, so that the burner is suitable for creating the cyclic flow in the form of a strongly recycling flow, thus allowing the use of the process according to the invention. An advantageous embodiment of the invention provides that the burner includes several supply channels which are part of the fluid-mechanical installations of the burner. Through these supply channels the fuel, the oxygen, the mixture of oxygen and carbon dioxide, and the flue gas are fed to the combustion zone of the burner, and it is possible to supply several ones of the mentioned materials through a single channel, whereby uniform mixing of these materials takes place before the materials are supplied to the combustion zone, which may have a beneficial effect on the combustion. Supplying the respective materials through a respective separate channel is also possible, should this be preferred. The supply channels can be so arranged that the materials are supplied from different directions or from the same direction. Supplying the materials from opposite directions for instance promotes and enhances the creation of a cyclic 7 flow, which has a positive effect on the capability of reducing the amount of oxygen needed during the combustion. A further advantageous embodiment of the invention provides that the burner includes a swirling installation. Through this swirling installation, which may be designed as fan or the like, the creation of a cyclic flow in the form of a strongly recycling flow can be promoted, thus providing the described advantage of oxygen reduction. Further advantages and features of the present invention will be described in the following with reference to the attached drawings wherein it is shown by Figure 1 a schematic illustration of one embodiment for a burner according to the invention in which fuel is burnt in the combustion zone in accordance with the process of the invention. Figure 1 shows a burner 1 with a combustion zone 2, and two supply channels 3 and 4 provided in the burner 1, which are arranged in parallel and through which a carbon-containing fuel 5 is fed to the combustion zone 2 of the burner 1 together with oxygen-enriched flue gas 6 or through which oxygen-enriched flue gas 7 alone is fed to the combustion zone 2 of the burner 1. The flow velocities of the materials 6 and 7 should be so selected that a difference between the flow velocities of the materials 6 and 7 causes a cyclic flow 8 in the form of a strongly recycling flow, as indicated by the arrows in figure 1, being created in the combustion zone 2. To this end, the gaseous fluid 7, which enters through the supply channel 4 shown on the right side in figure 1, has a higher flow velocity than the gaseous fluid 6 which enters through the supply channel 3 shown on the left side and which has admixed to it powdery fuel 5. The left half of the cyclic flow 8 automatically supplies the combustion zone 2 with hot combustion products, whereby the combustion in the combustion zone 2 can be stabilized. Of course, it is also conceivable for the supply channels 3 and 4 8 being arranged in a different manner or being provided in a different number, which is particularly suited for creating a cyclic flow in the form of a strongly recycling flow 8 that fills out the entire combustion zone 2, if possible. Further, in figure 1 a flue gas discharge and supply passage 9 is drawn, through which the flue gas is discharged from the combustion zone 2 and, as indicated by arrow 10, is supplied again to the inflowing fluids 6 and 7 prior to entering the combustion zone 2 of the burner 1. The above described embodiment is intended for the purpose of illustration only, without limiting the invention.

9 List of reference numbers 1 burner 2 combustion zone 3 supply channel 4 supply channel 5 fuel 6 oxygen and flue gas 7 oxygen and flue gas 8 cyclic flow 9 discharge and supply passage 10 arrow

Claims

1. Process for the combustion of fuel (5) supplied to the combustion zone (2) of a burner (1) using oxygen or a mixture of oxygen and carbon dioxide, c h a r a c t e r i z e d i n that in the combustion zone (2) a cyclic flow (8) is created through which homogeneous mixing of hot combustion products with the fuel (5) and the oxygen on the one side or with fuel (5) and the mixture of oxygen and carbon dioxide on the other side takes place in the combustion zone (2).

2. Process according to claim 1, characterized in that the cyclic flow (8) is created substantially in the entire combustion zone (2).

3. Process according to one of the preceding claims, characterized in that flue gas produced during the combustion is discharged from the combustion zone (2).

4. Process according to claim 3, characterized in that the flue gas is enriched with oxygen and thereafter supplied to the combustion zone (2).

5. Process according to one of the preceding claims, characterized in that the fuel (5) is supplied to the combustion zone (2) together with the oxygen and/or the flue gas.

6. Process according to one of the preceding claims, characterized in that the fuel (5), the oxygen and/or the flue gas are supplied to the combustion zone from different directions or from the same direction.

7. Process according to one of the preceding claims, characterized in that the components of the flue gases are completely burnt in the combustion 11 zone (2) or are partly removed from the combustion zone to be used for different purposes outside the combustion zone (2).

8. Burner (1) with a combustion zone (2) for the combustion of fuel (5) supplied to the combustion zone (2) using oxygen or a mixture of oxygen and carbon dioxide, ch a racte rized i n that the combustion zone (2) of the burner (1) includes fluid-mechanical installations enabling a cyclic flow (8) to be created in the combustion zone (2) through which homogeneous mixing of hot combustion products with the fuel (5) and the oxygen on the one side or with the fuel (5) and the mixture of oxygen and carbon dioxide on the other side takes place in the combustion zone (2).

9. Burner (1) according to claim 8, characterized by several supply channels (3, 4).

10. Burner (1) according to one of the claims 8 and 9, characterized by a swirling device.