HAMMER, Thomas (Zeckerner Hauptstr. 5b, Hemhofen, 91334, DE)
BARTENEV, Andrey Mikhailovich (Bolotnikovskaya Str. 40, Bld. 4 Apt. 7, Moscow 9, 11720, RU)
HAMMER, Thomas (Zeckerner Hauptstr. 5b, Hemhofen, 91334, DE)
Claims
1. Method for combustion of a fuel with an oxidizer with very low emissions of NOx and partially burnt fuel, whereby that very low emissions of NOx and partially burnt fuel are obtained by placing obstacles in a regular pattern into the gas flow and feeding one of the components of the combustible gas mixture to the gas flow through openings of at least some of the obstacles, which generates a turbulent flow pattern and mixes the components of the gas feed in the shear layers.
2. Method of claim 1, whereby the turbulent flow is created by at least two rows of obstacles, whereby at the first rows of obstacles turbulent mixing of the gas components is accom- pushed and the flames are stabilized in a short combustion zone flow-down of the last row of obstacles.
3. Method of claim 2, whereby the component is fed to the gas flow through openings in the first row of obstacles.
4. Method of claim 1, whereby the components of the combustible gas mixture comprise the fuel and an oxidizer which may be air or any gas mixture supplying oxygen in a way useful for combustion.
5. Method of claim 1, whereby the components of the combustible gas mixture comprise fuel, an oxidizer and a component being inert with regard to combustion such as molecular nitrogen, steam or combustion exhaust.
6. Method of claim 1, whereby the feed structure may correspond to a regular diffusion mode combustor, which means that the fuel is fed into the oxidizer flow, or to the inverted diffusion mode combustor, where an oxidizer is fed to the fuel flow.
7. Method of claim 1, whereby a near premix combustion is achieved in a configuration which is characterized by at least two rows of obstacles placed in the incoming flow of the oxidizer, whereby a highly turbulent flow is generated in the flow of the oxidizer, and a feed structure of the fuel placed in the turbulent flow of the oxidizer.
8. Method of one of the preceding claims, whereby by feeding the fuel into the highly turbulent oxidizer flow very fast mixing is achieved resulting in near premix combustion conditions and a very short combustion zone.
9. Method of one of the preceding claims, whereby in a modification a structure feeding an oxidizer and a gas mixture being inert to combustion such as molecular nitrogen, steam, or combustion exhaust is followed by a regular pattern of ob- stacles and flow-down of this by a structure feeding the fuel to the mixture of the oxidizer and the inert gas mixture.
10. Method of one of the preceding claims, whereby in a modification based on inverted diffusion combustion near premix combustion is achieved in a configuration which is characterized by rows of obstacles placed in the fuel flow, whereby a highly turbulent flow is generated in the flow of the fuel, and a feed structure of the oxidizer placed in the turbulent flow of the fuel.
11. Method of one of the preceding claims, whereby the temperature of the gas mixture behind the last row of obstacles is controlled by preheating at least one of the gas components.
12. Apparatus for realization the method of claim 1 or one of claims 2 to 11 with a combustion chamber, a combustor head and inlets to the combustor head for at least the fuel and a gas with an oxidizer, characterized in that the combustor head has at least two rows of obstacles, which form a regular pattern within the gas feed, whereby a turbulent flow pattern is generated, which mixes the components of the gas feed in the shear layers.
13. Apparatus of claim 12, characterized in that the obstacles are barriers in the flow of the combustor head.
14. Apparatus of claim 12, characterized in that the obstacles in at least one row have openings on the backside connected to a gas supply through an inner tubular structure.
15. Apparatus of claim 12, characterized in that the obsta- cles of two succeeding rows are shifted with respect to each other.
16. Apparatus of claim 15, characterized in that the transverse shift of obstacles between two succeeding rows is half the transverse distance between the obstacles.
17. Apparatus of claim 12, characterized in that the distance and the cross section of the obstacles may vary from row to row.
18. Apparatus of claim 12, characterized in that the longitudinal distance of the rows decreases proportional to the transverse size.
19. Apparatus of claim 12, characterized in that the obsta- ' cles have a defined shape with a smooth curved profile on the side opposed to the incoming gas flow and sharp edges at the lee side.
20. Apparatus of claim 19, characterized in that the profile on the front side of the obstacle is parabolic, elliptic, or circular.
21. Apparatus of claim 12, characterized in that the obsta- cles have a defined shape with rectangular, trapezoidal, or triangular cross section which is oriented in such way that the smallest side of the cross section is opposed to the incoming flow.
22. Apparatus of claim 14, characterized in that on the backside of the obstacles there are two rows of openings working as injection nozzles for a gas fluid.
23. Apparatus of claim 12, characterized by temperature controlling means. |
Method and Apparatus for Combustion of a fuel
The invention is directed to a Method for Combustion of a fuel. The invention is directed further to an apparatus for realization the method.
Technical Problem:
Swirl stabilized premix combustors applied for electric power generation in large gas turbine and combined cycle power plants (P (el) > 100 MW) can achieve nitric oxide (NOx) emissions below 10 ppm if running with high quality natural gas. There are, however, three drawbacks of this combustion concept:
• In addition to swirl a non-premix flame, the so- called pilot flame, is required for stabilization, especially during start-up and load change. This flame is the most serious source of NOx-emissions.
• Premixing in a separate mixing section cannot be applied for fuels having high flame velocities such as hydrogen, synthesis gas (syngas), or other hydrogen
(H2) containing gas mixtures, because there is a strong risk of flame flashback from the combustion to the mixing section during load changes. This limits the fuel flexibility of the combustor concept. Environmental friendly fuels reducing the net C02 emission of the power plant such as syngas from biomass gasification cannot be used.
• Since due to swirl stabilization combustion and heat release occur in zones characterized by low flow velocity (that means at the boundary between feed gas inlet flow and recirculation flow) , this concept is more sensitive to acoustic instabilities than concepts were combustion takes place in zones characterized by fast gas flows.
Current solutions of the problem: In EP 0 463 218 Bl the so-called flameless oxidation (FLOX)
is disclosed as a concept for non-premix low NOx combustion. The basic idea of this concept is high rate recirculation of hot combustion exhaust gas to the fuel- and air feed zone. According to the invention high rate means that the mass flow of recirculated exhaust gas exceeds the mass flow of fuel- and air-feed by a factor of at least 3.5. Up to now the NOx emissions obtained with natural gas and operation conditions similar to those applied for high power gas turbines were in excess of 50 ppm, and stable operation of the FLOX combustor could not be demonstrated with H2 containing fuels having high flame velocities. The reason for high NOx emission can be found in the long residence times of the gas mixture in the combustion chamber, which is caused by the high exhaust gas recirculation rate.
In EP 780 631 A2 a non-premix hydrogen combustor for aeroengine application is disclosed, which allows stable combustion even of pure hydrogen in a wide range of fuel-air mixtures characterized by low NOx emissions. It is characterized by fast mixing of fuel- and air-flow due to a cross-flow configuration and micro-burning zones ("micromix combustor") . A disadvantage of this concept is the large pressure drop caused by the fine bore-holes through which either the fuel or the air enters in the cross-flow configuration. Further due to the utilization of fine structures scaling of this concept to high power gas turbine application results in a very expensive combustor.
It is the object of the invention to propose a method and ap- paratus for better combustion of a fuel.
The problem is solved by the method of claim 1. An apparatus for realization this method is given by claim 12. Preferred embodiments of the method and the apparatus are objects of further claims.
New solution:
The current invention is based on a diffusion mode combustor
which is known to run stable in a wide range of operation conditions. According to the invention a transition from diffusion combustion characterized by high emissions of noxious compounds to premix combustion with very low emissions of NOx and partially burnt fuel is obtained by placing obstacles in a regular pattern into the gas feed, whereby a turbulent flow pattern is generated, which mixes the components of the gas feed in the shear layers at the first rows of obstacles and simultaneously stabilizes flames in a short zone flow-down of the last row of obstacles. The feed components comprise fuel and oxidizer, where the oxidizer may be air or any gas mixture supplying oxygen in a way which can be utilized for combustion. In a modification the feed components comprise fuel, an oxidizer, and component being inert with regard to combus- tion such as molecular nitrogen, steam or combustion exhaust. The feed structure may correspond to a regular diffusion mode combustor, which means fuel is fed into the oxidizer flow, or to the inverted diffusion mode combustor, where an oxidizer is fed to the fuel flow.
In a principle modification of the current invention near premix combustion is achieved in a configuration which is characterized by rows of obstacles placed flow-down of the feed structure of the oxidizer, whereby a highly turbulent flow is generated in the flow of the oxidizer, and a feed structure of the fuel placed in the turbulent flow of the oxidizer. By feeding the fuel into the highly turbulent oxidizer flow very fast mixing is achieved resulting in near premix combustion conditions and a very short combustion zone. In a modification a structure feeding an oxidizer and a gas mixture being inert to combustion such as molecular nitrogen, steam, or combustion exhaust is followed by a regular pattern of obstacles and flow-down of this by a structure feeding the fuel to the mixture of the oxidizer and the inert gas mixture.
In another modification based on inverted diffusion combustion near premix combustion is achieved in a configuration
which is characterized by rows of obstacles placed flow-down of the feed structure of the fuel, whereby a highly turbulent flow is generated in the flow of the fuel, and a feed structure of the oxidizer, placed in the turbulent flow of the fuel. By feeding the oxidizer into the highly turbulent fuel flow very fast mixing is achieved resulting in near premix combustion conditions and a very short combustion zone. In a modification a structure feeding fuel and a gas mixture being inert to combustion such as molecular nitrogen, steam, or combustion exhaust is followed by a regular pattern of obstacles and flow-down of this by a structure feeding the oxidizer to the mixture of the fuel and the inert gas mixture.
Advantages of the invention:
The current invention has the advantage that it does not con- tain separate mixing and combustion sections. Therefore the risk of flame flashback can be excluded completely. In each case the residence time of the gas mixtures (generated by enhanced turbulent mixing) in the combustion zone and in the combustion chamber can be kept very short. This is important because the formation of thermal NOx-emission does not only depend on temperature but requires some time which decreases with increasing temperature. Because the average temperature at the combustor outlet is fixed by the requirements of the application (i.e. the desired efficiency of a gas turbine) the temperature cannot be considered to be a free parameter. Therefore the residence time of the gas mixture in zones of high temperature, which in the case of our invention (as in the case of premix combustion) corresponds to the average temperature flow-down of the combustion zone, is the most im- portant parameter. Since a combustor based on the current invention does not require fine structures it is mechanical robust and its production is cheap.
More Information and further advantages of the invention are shown by the description of the figures in the drawing. There are shown
Figure 1 a first example of a cross section of a combustor with turbulent enhancement,
Figure 2 the form of an obstacle with nozzles,
Figure 3 a second example of a cross section of a combustor with turbulent enhancement,
Figure 4 the form of an obstacle without nozzles and
Figure 5 a scheme for optimization of turbulent mixing
Drawings and examples : Figure 1 shows the cross section of a combustor with turbulence enhancement 1 according to the invention. Compressed air - or another oxidizer - is fed with a flow 4 into the combustor head 2, in which a regular pattern of obstacles 50 is installed consisting of a first row 5, succeeding rows 6, 6' , ... and a final row 7. Through openings 8 in the first row of obstacles fuel is fed to the gas flow and rapidly mixed with the oxidizer. Each row of obstacles is placed in such a way that turbulence intensity is increased. In the combustion zone 9 flow-down of the last row of obstacles the axial flow velocity reduces and stable combustion takes place. Since no complicated swirl flow pattern is required for combustion stabilization the combustion chamber 3 can be designed compact, which keeps the residence time of the hot combustion exhaust flowing e.g. to the gas turbine short. Thus the NOx- emission is kept low.
Figure 2 shows the form of an obstacle 50 with two openings 8. The openings 8 are a nozzle for the flow of a gas component. In other rows there are obstacles 60 without nozzles
Figure 3 shows the cross section of preferred embodiment of a combustor with turbulence enhancement 1 which avoided of flame flash back. As in the first embodiment compressed air - or another oxidizer - is fed with a flow 4 into the combustor head 2, in which a regular pattern of obstacles 50 is installed consisting of a first row 11, succeeding rows 12, 12', ... and a final row 13. Each row of obstacles is placed in such a way that turbulence intensity is increased. Through
openings 8 in the last row of obstacles 13 fuel is fed to the gas flow and rapidly mixed in a zone 14 with the oxidizer due to the highly turbulent gas flow. In the combustion zone 9 flow-down of the last row of obstacles the axial flow veloc- ity reduces and stable combustion takes place. The combustion zone can partly overlap with the turbulent mixing zone. Since again no complicated swirl flow pattern is required for combustion stabilization the combustion chamber 3 can be designed compact, which keeps the residence time of the hot combustion exhaust flowing e.g. to the gas turbine short. Thus the NOx-emission is kept low.
Figure 4 shows the form of an obstacle 60.In this embodiment it has the same shape as obstacle 50 but it has no nozzles.
According to Figure 5 the turbulent mixing " process is optimized by selecting certain ratios of the size D and distance d of the obstacles in a row 22 to each other transverse to the gas flow and the longitudinal distance Z between the rows 22 and 22' of obstacles. The coordinate in this direction is z .
In order to have the same length Z 1 and Z 2 for the zone of initial turbulence 24 and the zone of wake turbulence 25, the ratio d/D is preferably chosen to be between 2. and 6, the ratio of z/d is chosen such that the succeeding row is placed at the beginning 26 of the zone of isotropic turbulent flow 27.
The shape of the obstacles 50, 60 can be optimized for turbulence enhancement at one hand and low pressure drop on the other hand. In a preferred embodiment the obstacles have an idealized cross section consisting of a smooth shaped, symmetric curve the end points of which are connected by a straight line. The profile of the smooth curved side can be parabolic, round or even rectangular or triangular.
The obstacles 50 are exposed with the smooth curved side or
with the side of smallest extension to the incoming flow such that the edges cause efficient formation of shear layers.
The transverse size and distance of the obstacles 50 may vary from row to row. In a preferred embodiment of the invention it decreases from row to row, and the longitudinal distance of the rows decreases proportional to the transverse size. Thereby the turbulence enhancing structure can be designed more compact without loss of efficiency.
In further embodiments of the invention the fuel, oxidizer and/or inert components can be added to the gas flow at intermediate rows of obstacles in order to avoid flame flashbacks.
Finally in order to control the ignition delay, which determines the distance between the last row of obstacles and the combustion zone as well as the stability and the noxious emissions of combustion process, a temperature control is provided by preheating at least one of the gaseous components. The processes which can be utilized for preheating such as adiabatic compression or heat exchange are state of the art .
List of numbers
Figure 1
1 combustor with turbulence enhancement 2 combustor head 3 combustion chamber 4 oxidizer feed flow 5 first row of obstacles providing fuel feed 6, 6' succeeding rows of obstacles last row of obstacles fuel feed inlet (enlarged detail)
9 combustion zone 10 flow of combustion exhaust
50 obstacles
Figure 2
50 obstacle Openings
Figure 3
11 first row of obstacles
12, 12', ... succeeding rows of obstacles
13 last row of obstacles
14 highly turbulent mixing zone
50 obstacles
Figure 4
60 obstacle
Figure 5
22, 22', ... succeeding rows of obstacles having longitudinal distance z
23, 23', ... shear layer boundaries
24 zone of initial turbulence having length zi
25 zone of wake turbulence having length Z 2 26 zone of isotropic turbulent flow
27 region of preferred placement for row of obstacles 22'
28 zone of decaying turbulence
z Coordinate
Zl, Z 2 ,
Z
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