Abstract:
The invention relates to a combustion unit for combusting a liquid fuel. The combustion unit has a fuel inlet, an air inlet and a flue gas outlet which are connected to a combustion chamber for combusting the fuel, wherein the fuel inlet is connected to at least one explosion atomizing unit which is disposed and adapted such that atomized fuel fragments due to gas formation in the atomized fuel. The explosion atomizing unit is preferably an explosion swirl atomizing unit to a system for generating power having at least one gas turbine, at least one compression device driven by the gas turbine and at least one such combustion unit.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     (2) Description of the Prior Art 
     The present invention relates to a combustion unit for combusting a liquid fuel and to a system for generating power comprising such a combustion unit. 
     In the combustion of liquid fuel, in particular engine fuels such as petrol, kerosine, diesel and methanol, it is important that at the time of the combustion the fuel is present in the smallest possible particles. The smaller the fuel particles, the more homogeneous a combustion results. A more homogeneous combustion is associated with less soot formation and soot emission as well as less CO formation and emission. 
     SUMMARY OF THE INVENTION 
     It is therefore the object to introduce the smallest possible fuel droplets into the combustion chamber. Known combustion units are characterized by assorted additional means for obtaining the smallest possible fuel droplets in the combustion chamber at the moment of combustion. 
     The present invention has for its object to provide a combustion unit for combusting liquid fuel which is provided with means for carrying into the combustion chamber very small liquid fuel particles (median size &lt;5 μm, generally &lt;3 μm, preferably &lt;2 μm, such as 1.2 μm). Thus, a sufficient supply of these very small liquid fuel particles can be ensured and the means for obtaining these very small liquid fuel particles have a relatively simple construction and can be added in relatively simple manner to existing combustion units. 
     This is achieved according to the invention with a combustion unit for combusting a liquid fuel, comprising a fuel inlet, an air inlet and a flue gas outlet which are connected to a combustion chamber for combusting the fuel, wherein the fuel inlet is connected to at least one explosion atomizing unit which is disposed and adapted such that atomized fuel fragments due to gas formation in the atomized fuel. 
     The means for realizing these very small liquid fuel particles consist of explosion atomizing units. 
     All known types of atomizer can in principle be used in the explosion atomizing unit. Swirl atomizers, slot atomizers, hole atomizers, rotating plate or bowl atomizers and optionally pen atomizers are for instance suitable. All that is important is that the atomizer generates droplets or a film of liquid fuel to the gaseous medium under changed conditions such that explosion atomizing then occurs. Explosion atomizing entails the liquid fuel entering the combustion chamber under conditions such that as a result of the pressure drop over the atomizer boiling or gas bubbles occur in the droplets or film of the liquid fuel. That is, gas formation occurs in the liquid fuel. This so-called flashing or precipitation results in the droplets or film of fuel exploding or fragmenting due to the sudden partial boiling or gas precipitation. This fragmentation results in very small droplets of fuel being generated in the gaseous medium. The median dimension of fuel particles amounts after fragmentation to less than 5 μm, generally less than 3 μm, preferably less than 2 μm, for instance 1.2 μm. 
     It is noted that the explosion atomizing unit does not have to deliver the atomized liquid fuel directly into the combustion chamber. It is sufficient that the generated fuel droplets finally enter the combustion chamber without an undesirably large droplet growth having taken place as a consequence of coalescence. 
     The invention allows the use in the atomizing means of all types of atomizers insofar as these can result in particles with said median size after fragmentation. It is important in this respect that the explosion atomizing units are disposed and adapted such that the atomized fuel fragments through gas formation in the atomized fuel. 
     Use is preferably made of an explosion swirl atomizing unit which is provided with swirl atomizers. In such a known swirl atomizer a swirling movement is imparted to the liquid fuel in a swirl chamber. The swirling fuel exits from an outlet opening. It has been found that the thickness of the exiting layer of fuel is a fraction (for instance 10%) of the diameter of the outlet passage. Due to the subsequent explosion fragmentation, particles are obtained (depending on the pressure drop, temperature and passage diameter) with a median dimension of 5 μm or smaller. 
     It will be apparent that in order to realize this fragmentation it is important that the conditions (and particularly change in conditions) under which the liquid fuel is atomized are optimal for fragmentation. Important conditions for flash-fragmentation are the temperature of the fuel, the atomizing pressure under which the fuel is atomized, the pressure drop during exit and the passage diameter. It is therefore recommended that the explosion atomizing unit comprises means for adjusting the temperature of the evaporating agent and/or the atomizing pressure. 
     In the case of retrofit of the above stated combustion unit, it is possible to integrate a configuration of a number of explosion atomizing units into a new or modified air inlet, or to have these explosion atomizing units debouch directly into the combustion chamber. By orienting the outlet passage of each explosion atomizing unit it is possible to atomize the fuel such that it is optimal for the forming of the mixture of fuel and air for combustion. Particularly recommended are swirl atomizers and slot or hole atomizers since these have a very simple construction, can be readily miniaturized and built into existing combustion units. Very large numbers of explosion atomizing units can thus be incorporated without too many modifications of an existing combustion unit, which offers great freedom in the choice of fuel flow rate to the combustion chamber. Retrofit of existing combustion units thus results in combustion units which can be converted at lower cost and which nevertheless realize a greatly improved combustion with a lower soot and NO x  emission. 
     As stated, liquid fuel can be applied as fuel. The liquid state herein refers to the state of the fuel at the temperature and pressure prevailing in the fuel inlet. This means that fuels can be used which are gaseous in ambient conditions. Fuels such as diesel and petrol have a boiling range. This means that in order to realize the explosion atomizing a temperature must be chosen from the boiling range such that a significant flash effect occurs. For diesel oil a temperature can be chosen of 350° C. For kerosine/petrol a lower fuel temperature can be chosen (250/150° C.). A higher fuel temperature, such as 400° C., can be chosen for low-speed marine diesel engines. It is noted however that these temperatures can vary depending on the pressure applied and optional fuel additives which have a positive effect on the explosion atomizing. It will be apparent that in order to realize an optimal explosion atomizing a combustion unit will preferably be equipped with means for adjusting the temperature and the atomizing pressure of the fuel. 
     If in further preference the temperature-adjusting means adjust the temperature of the evaporating agent around or to the critical temperature, the evaporating agent acquires a surface tension of practically or equal to 0 N/m 2 . This means that no further or little atomizing energy is required to atomize the liquid, whereby the droplet size will become extremely small (a median droplet dimension to 0.1 μm is possible here) and the use of other agents to decrease the surface tension can optionally be dispensed with. 
     In addition to said physical conditions for fragmentation, it is also possible to enhance fragmentation by chemical or physical additives to the fuel. It is therefore recommended to add agents to the fuel which reduce the surface tension of the fuel and thereby decrease the energy required for fragmentation. Detergents and the like can be used as surface tension-reducing agents. Preferred are those surface tension-reducing agents which do not remain only on the surface of the fuel droplet but which are distributed almost homogeneously through the fuel (droplet or film). It is thereby not required that, after atomizing and prior to fragmentation, the surface tension be reduced to a lesser extent as a result of diffusion. In these conditions it is recommended to use fatty acids, particularly shorter fatty acids and optionally alcohols such as methanol and ethanol. These latter agents are particularly recommended because of a relatively low boiling point and good combustion. Thus is avoided that the combustion process is affected in a negative sense by these additives. 
     According to another embodiment the fuel contains combustible and/or vaporizable substances which either reduce the surface tension of the fuel or enhance the gas formation in the fuel as a result of the pressure drop over the atomizer. Combustible and/or vaporizable substances can particularly be used here which have a boiling point lower than the boiling point of the fuel. This should be understood to mean that in the case of a boiling range of the fuel, and optionally of the evaporating agent, these ranges are chosen such that the evaporating agent makes an essential contribution to the gas formation and ultimately the fragmentation of the fuel. When a number or mixture of evaporating agents are used, the vaporizable substances with the lowest boiling point will suddenly evaporate first and form boiling bubbles due to the pressure drop when passing through the explosion atomizing unit, whereby liquid fuel explodes or fragments into small droplets. A mixture can for instance be used of diesel oil as fuel and water as evaporating agent. Superheated evaporating agent (water) can also be used as evaporating agent (for instance water) and can be applied particularly in oil-fired boilers for generating steam. In which case fuel and superheated water can also be introduced separately into the boiler by explosion atomizing. The additional advantage is realized here that through the evaporation of the water the temperature of the mixture is lower prior to combustion, during combustion and after combustion, which enhances the performance of the combustion unit and reduces the emission of CO and NO x . 
     The combustion unit can be applied in a combustion engine, for instance a gas engine, petrol engine or diesel engine. In addition, the combustion unit can be incorporated in a system for generating power which comprises a compression device driven by a gas turbine and the combustion unit according to the invention in which fuel and air compressed by the compression device are combusted and fed to the gas turbine. 
     It will be apparent that it is very advantageous in this respect if explosion atomizing units are used in the compression device to atomize determined evaporating agents with a comparably higher evaporation energy (for instance water). A quasi-isothermal compression is hereby obtained whereby the compression work is reduced considerably. In the case the combustion unit is provided with a compression chamber and a combustion chamber, the explosion atomizing unit for the fuel can be connected to the combustion chamber and an explosion atomizing unit for evaporating agent for the purpose of evaporation cooling can be connected to the compression chamber. 
     During the compression stroke and the firing stroke of the combustion engine an optional quasi-isothermal compression, and in any case an optimal combustion, can thus take place. It is further recommended in the case of evaporation cooling that between a compression chamber and a combustion chamber of the combustion engine at least one pressure vessel is received which is in heat-exchanging contact with a combustion gas outlet of the combustion engine. It is thus possible in the cool compressed air to recuperate heat from the heat of the flue gases. If the residence time in the pressure vessel is too short, a number of pressure vessels can be applied in parallel or a relatively large pressure vessel in combination with a number of combustion chambers. 
     Mentioned and other features of the combustion unit and the power-generating system according to the invention will be further elucidated hereinbelow with reference to a number of embodiments which are given by way of example without the invention having to be deemed limited thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawing: 
     FIG. 1 shows a schematic view of an explosion swirl atomizer; 
     FIG. 2 shows a schematic representation of a diesel engine according to the invention with turbo-charger; 
     FIG. 3 shows a variant of the diesel engine of FIG. 2; 
     FIGS. 4-6 each show a schematic representation of a combustion engine according to the invention; 
     FIG. 7 shows a schematic representation of a power-generating system according to the invention; 
     FIG. 8 shows another power-generating system according to the invention according to the TOPHAT principle (TOP humidified air turbine); and 
     FIG. 9 shows another power-generating system according to the invention according to the TOPHACE principle (TOP humidified air combustion engine). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an explosion swirl atomizer  1  such as is applied in a combustion unit according to the invention. The explosion swirl atomizer I comprises a line  2  with which fuel  3  (and/or optional evaporating agent) is fed via a tangential opening  4  to a swirl chamber  5 . The liquid acquires a swirling movement  6  in swirl chamber  5  and leaves atomizer  1  via an outlet opening (or passage). The swirling fuel exits in the form of a cone. The thickness of the layer of fuel herein decreases and as a consequence of fragmentation breaks up into very small droplets. It can clearly be seen that the thickness of the layer of fuel is smaller than the diameter of outlet opening  7  of swirl chamber  5  when the exiting liquid exhibits flashed or gas precipitation through sudden pressure decrease, the cone and the particles then fragment into extremely small droplets, the so-called explosion atomizing. The thickness of the cone layer and the size of the formed droplet depends on the degree of explosion atomizing, and thus on the degree of gas formation in the cone layer. The physical conditions which are important herefor are the pressure and the temperature of the fuel and the prevailing pressure and temperature in the space into which the swirling atomized fuel is delivered. It is thus possible to influence the number and size of the fonned atomized fuel particles by the choice of these conditions. The explosion atomizing unit  1  may also include a means for adjusting the temperature of the fuel  3  and/or the atomizing pressure. This means for adjusting  200  is illustrated in schematic form in FIG.  1 . 
     FIG. 2 shows a diesel engine  8  according to the invention comprising six combustion units or cylinders  9  according to the invention. Diesel oil is supplied via a pump  10  and a line  11  to an explosion atomizing unit  12  which can consist of a suitable number of chosen explosion atomizers as shown in FIG.  1 . The diesel oil has a temperature and pressure suitable for the explosion atomizing. Air is supplied via a line  13  to a compressor  14  which is driven by a gas turbine  16  via a shaft  15 . 
     Added to gas turbine  16  is the flue gas from cylinders  9  which is fed via a line  17  to gas turbine  16  and via a line  18  to the chimney  19 . 
     Air compressed in compressor  14  is fed via lines  20  to the combustion chamber  21  of each cylinder  9 . 
     FIG. 3 shows a diesel engine  22  corresponding with FIG.  2 . Corresponding components are designated with the same reference numerals. A first difference however is that the air compressed in compressor  14  is not fed via line  20  to combustion chamber  21  but to the explosion atomizing unit  12 . This produces an optimum mix of fuel and air. If the air still contains evaporating agent particles (water particles), a quasi-isothermal compression is still even possible in cylinder  9 . 
     Secondly, an explosion atomizing unit  23  is received in line  13 . Through explosion atomizing water is supplied herein to the air, whereby a quasi-isothermal evaporation occurs in compressor  14 . The water required is fed via a line  24  to a heat exchanger  25  in which it is in heat-exchanging contact with the flue gas leaving gas turbine  16 . The heated water is fed under pressure via a pump  26  to explosion atomizing unit  23 . 
     Diesel engines  8  and  22  shown in FIGS. 2 and 3 can be used as low-speed marine diesel engines. 
     FIG. 4 shows a combustion engine  27  according to the invention which is provided with a compression chamber  28  and a combustion chamber  29 . Compression chamber  28  is provided with an air inlet  30  with an inlet valve  31 . Compression chamber  28  further comprises an explosion atomizing unit  32  for supplying coolant (for instance water) via line  33 . Quasi-isothermal compression can thus be achieved by evaporation cooling. Via an outlet  35  provided with a valve  34  the compression chamber  28  is connected to a pressure vessel  36  which is provided with a heat exchanger  37 . Pressure vessel  36  is connected via line  38  and a valve  39  to combustion chamber  29 , which is further provided with an explosion atomizing unit  40  for fuel supplied via line  41  and an ignition unit  42 . Via a valve  43  and an outlet  44  exhaust gases are discharged via heat exchangers  45 ,  37  and  46 . 
     The operation of combustion engine  27  is as follows. At one bar and a temperature of 27° C. water is atomized via explosion atomizing unit  32  in compression chamber  28 , wherein quasi-isothermal compression takes place to  44  bar and 220° C. Valves  34  and  39  open and pressure vessel  36  and combustion chamber  29  are filled during the latter part of the stroke of piston  47 . Valves  34  and  39  then close. The air present in pressure vessel  36  is heated against the exhaust gases passing through heat exchanger  37 . In pressure vessel  36  the air is heated to a temperature of 300° C. and finally flushed into combustion chamber  29  via valve  39 . 
     Fuel is injected simultaneously via explosion atomizing unit  40 , whereafter ignition and expansion then take place in combustion chamber  29 . During the return stroke of piston  48  the exhaust gases are discharged via valve  43  and used for heat exchange with the fuel, the compressed air and the water for injecting. 
     It will be apparent that in combustion engine  29  fuel is likewise injected via explosion atomizing unit  40  and coolant via explosion atomizing unit  32 . 
     The use of combustion engine  27  achieves that minimal compression work is performed, while the recuperation of low temperature heat is realized for preheating of air, water and/or fuel. 
     In the case the residence time in the pressure vessel is insufficient for an optimal heating of the compressed gas, it is recommended that the pressure vessel be embodied in the form of a number of pressure vessels connected in parallel between compression chamber  28  and combustion chamber  29 . 
     If the quasi-isothermal compression is performed by injecting a mixture of water/fuel (for instance water/ methanol), the evaporation cooling can then be supplemented by extraction of heat resulting from the cracking of the fuel. In order to perform this cracking reaction of the fuel it is necessary for a cracking catalyst to be incorporated in the pressure vessel (for instance CuO for methanol or zeolite for petrol). Important are an adequate reaction time in the order of one second and a sufficiently high cracking temperature for methanol of 250-300° C. and for petrol of 475-675° C. 
     It will be apparent that by arranging a separation between the compression chamber and the, combustion/expansion chamber using the pressure vessel, an optimization of the energy efficiency can be realized in conditions of variable power requirement by making use of the accumulated energy. A hybrid motor with compressed air storage can optionally even be applied. 
     FIG. 5 shows a combustion unit  49  according to the invention. 
     Via the rotating compressor  50  air is supplied via inlet  51 , while a water/fuel mixture is atomized with an explosion atomizing unit  52 . Connected to pressure vessel  58  are combustion chambers  53  which each take in the compressed mixture of air/fuel via a line  54 , while additional fuel is supplied via inlet  55 . The mixture is ignited using ignition  56 . Exhaust gases leave combustion chamber  53  via outlet  57 . Using a heat exchanger  59  heat-exchange takes place with the mixture of air/fuel present in pressure vessel  58 . By making use of the large pressure vessel  58  and a plurality of combustion chambers there is significantly more time for heating of the mixture present in pressure vessel  58  using the exhaust gases. 
     FIG. 6 shows a combustion engine  60  comprising a cylinder  61  with a piston  62  in addition to an air inlet  63  and a flue gas outlet  65 . Cylinder  61  is further provided with plasma electrodes  66  which are connected to power electronics  68  for generating a plasma in the head of cylinder  61 . During the compression a fuel/water mixture is fed via the explosion atomizing unit  69 , not shown in detail, for the quasi-isothermal compression. The plasma arc is subsequently generated to heat the compressed air and the ignition of the fuel mixture, and after the expansion stroke of piston  62  the flue gases are expelled via outlet  65  and drive the turbine  70  while generating power which is used partially by the power electronics. 
     FIG. 7 shows a system  60  for generating power. System  60  comprises a compressor  61  which is driven via a shaft  62  by gas turbine  63  which in turn drives a generator  64 . 
     Air is supplied to compressor  61  via a line  65  and water is supplied in an explosion atomizing unit  66  via the line  68  provided with a pump  67 . The air compressed in compressor  61  is fed to a combustion unit  69  according to the invention, to which via a line  70  preheated fuel is supplied at pressure via pump  116 , heat exchanger  117  and pump  118  and atomized in an explosion atomizing unit  71  before being fed to combustion unit  69 . The fuel is brought to pressure with pump  116  and preheated via heat-exchange against the flue gas from line  73  in heat exchanger  117 , and brought to or above the critical temperature or, in the case of a boiling range for the fuel, within the range of critical temperatures of the fuel components. Via line  72  flue gas is fed to turbine  63  and after expansion discharged via line  73 . 
     FIG. 8 shows another system  74  for generating power according to the invention in accordance with the so-called TOPHAT principle. In an explosion unit  75  air  74  is provided with water droplets with water  77  supplied by means of explosion atomizing. The air is supplied to a compressor  78  which is connected via a shaft  79  to a gas turbine  80  which drives a generator  81 . Evaporation cooling of the water droplets takes place in compressor  78 . The cool compressed air passes through a heat exchanger  83  via a line  82  and is fed to combustion unit  84 . Fuel is preheated at pre-pressure via pump  120  in heat exchanger  121  and brought under pressure by pump  122  and after explosion atomizing in explosion atomizing unit  93  supplied via line  85  to combustion unit  84 . The added fuel is at a pressure and temperature such that when it enters the combustion chamber of combustion unit  84  fuel-flash takes place, resulting in an extremely fine atomizing of the fuel. The flue gas from gas turbine  80  passes through heat exchanger  83  via line  86  for heat-exchanging contact with the cool compressed air from compressor  78 . Via line  87  the flue gas passes through a heat exchanger  88  and condenser  87  on its way to chimney  92 . In condenser  89  water is condensed out of the flue gas and guided under pressure via pump  90  through heat exchanger  88 , whereafter the water  77  reaches explosion atomizing unit  75  under pressure and at temperature. The condensation water from condenser  89  can optionally be replenished with water via line  91 . 
     Finally, FIG. 9 shows a system  94  according to the invention for generating power in accordance with the TOPHACE principle. 
     Via a pump  95  water (140-250°C.,  150  bar) is fed to an explosion atomizing unit  96  to which air is likewise fed via line  97  (15° C.). From the explosion atomizing unit  96  the air reaches a compressor  98  which operates at an efficiency of 0.8. The compressed air (140° C.) is fed via line  99  to a heat exchanger  100  for heat-exchanging contact with the flue gases of a combustion engine  101 . This latter comprises four cylinders  102 , an air inlet  103  of which connects to line  99  via a valve  104 . A flue gas outlet  105  of each cylinder  102  passes through heat exchanger  100  and is carried via line  106  through a heat exchanger  107  and enters the chimney  92  via condenser  89 . In condenser  89  is formed condensation  108  which after passing through a water cleaner  109  is brought to pre-pressure with pump  110  and fed via heat exchanger  107  to pump  95  and brought to pressure. 
     Fuel is fed to each cylinder  102  via pump  111 , line  117  and explosion atomizing unit  112  and valves (not shown). The fuel is preheated to or beyond the critical temperature or, in the case of a boiling range, to within the range of critical temperatures, before being atomized with explosion atomizing unit  112 . 
     In the recuperator  100  the air is heated from 140° C. to 377° C., while the flue gas from cylinders  102  re-cools from 465° C. to 210° C. The air is fed at a pressure of 9 bar to cylinders  102  and atomized fuel is injected. Cylinders  102  are also embodied with an igniter  119  for igniting the mixture in each cylinder  102 . Cylinders  102  are each equipped with a piston  113 , which are connected to a shaft  114  which is connected via a 1:5 gear system  115  to the shaft  114  of compressor  98  and on the other side to the generator  116 . 
     Under ideal conditions the system  94  produces power of 226 kilowatts at an efficiency of 64%. A known apparatus according to the Atkinson principle produces a power of only 170 kilowatts at an efficiency of 48%.