Method and apparatus for burning liquid fuel

Abstract: The steps of introducing gas by supply pipe (7) into the liquid fuel from fuel supply pipe (6) through a porous plate (4), capillaries, cloth, a particle layer or the like to bubble the liquid fuel, controlling the fuel level and supplying combustion air by supply pipe (8) separately to accomplish complete combustion in combustion chamber (1).

Description: The present invention relates to a method and apparatus for burning liquid fuel in a wide range of applications from household oil stoves up to industrial furnaces. A heretofore known practice is to burn liquid fuel either directly gasified or finely vaporized by an atomizer. The former method of burning the directly gasified fuel is widely used in general household oil stoves, typical of which are a pot type JP-U-35713/1983), a wick type (JP-A-203307/1983 and 64134/1985) and a vaporization type (JIS 3030). The pot type employs a burner bowl on which fuel is vaporized before being burned and is equipped with a combination of vaporization and combustion units. In the case of the vaporization type, fuel is vaporized in a vaporization chamber or pipe and then burned in a combustion unit, the vaporization unit being separated from the combustion unit. Although the method of burning gasified fuel is used in some household oil stoves, it is more generally used in industrial furnaces, boilers and the like. This burning method is designed to promote vaporization and combustion reaction by gasifying liquid fuel into fine oil drops to increase the contact area of each drop with air. Oil burners generally in use are adapted to burn fuel by means of a rotary burner, jet burner (vaporization spray, air spray and mechanical spray), special burner (gun-type high-pressure spray and low pressure spray) or the like. There are also examples having a kind of ignition device for igniting liquid fuel in the form of foam (JP-B-No. 42018/1974, JP-A-No. 38368/1972). In the method of burning liquid fuel directly gasified using a pot, it is difficult to quickly increase combustion until the combustion chamber is sufficiently warmed up after the fuel is ignited. Consequently, it takes time before such oil stoves radiate heat satisfactorily after the fuel is ignited. In the case of the wick-type burning method, the range over which combustion can be adjusted is small and, depending on atmospheric conditions and the room size, it may be impossible to obtain ideal heating. Moreover, there is generated an offensive smell from oil stoves when fuel is ignited or the flame is extinguished and this has caused oil stoves to be known for their bad odor. From the point of safety, the flame in an oil stove has to be extinguishable as quickly as possible (e.g., according to JIS, the flame in the oil stove has to be extinguished within 10 seconds after an earthquake occurs or when it is tipped over by accident). The offensive smell generated when it is turned off therefore tends to become stronger. This means the flame of an oil stove is required to be extinguished as quickly as possible after flame extinguishment is initiated. In the case of an oil stove, for instance, fuel is prevented from being vaporized from the wick or pot after the flame is extinguished and it is oxidized into aldehydes producing an irritant smell while passing through the hot combustion chamber. The problem is that the resulting strong offensive smell gives users an uncomfortable feeling. Although there are many kinds of combustion equipment using the spray combustion method, they all allow groups of oil drops to have a wide distribution of particle sizes when they are dispersed by air; and these drops act on one another and move in different directions at different speeds. As a result, spray combustion lacks uniformity, because oil drops insufficiently vaporized and mixed reach the front face of the flame before being enclosed in diffusion flame. The flame tends to become nonuniform, causing partial overheating to parts being heated. Moreover a device for spraying the oil is required and this results in high running costs such as high power cost. In addition to the aforementioned disadvantages, the oil stoves proposed in JP-B-No. 42018/1974 and JP-A-No. 38368/1972 cannot be seen as ensuring continuous combustion with safety. US-A-3 104 696 discloses a combustion method with a foam heating oil burner. The cylindrical vertical foamer comprises a flat, plate-like bottom surface with a sintered glass disk. It is an object of the present invention to provide a combustion method capable of readily igniting and burning liquid fuel in the form of bubbles without using a wick and which is capable of easily controlling the amount of combustion. This object is solved with the features of the independent claims. The invention is described in detail in conjunction with the drawing in which Figure 1 is a vertical sectional view of a burner embodying the present invention. For further explanation it is referred to EP-A-390 141, particularly Figs. 2 to 8 and corresponding description thereof. The present invention is intended to provide a method of forming liquid fuel into bubbles and continuously burning it in a combustion chamber in order to adjust the amount of combustion, to prevent the generation of offensive smells at the time of ignition and after the flame is extinguished and to realize flame uniformity. The present invention is particularly effective when it is applied to oil stoves. "Bubbles" is used in the present specification as a general term meaning both dispersed bubbles consisting of numerous bubbles of air or oxygen which have floated to the surface of a liquid and foam consisting of bubbles separated by thin liquid films. The method of burning liquid fuel in the form of foam or dispersed bubbles is characterized in that the contact area of the fuel with air can be increased hundreds of times in a condition different from heretofore. Moreover, it has the effects of increasing the evaporation and diffusion coefficients, reducing the partial vapor pressure of the fuel on the boundary between air and liquid, and further promoting the reaction between evaporation and combustion. A description will subsequently be given of the present invention with reference to the accompanying drawing. Figure 1 is a vertical sectional view of an embodiment of the present invention. In Figure 1, numeral 4 denotes a porous element (hereinafter called the air feeder) which functions as a fuel foamer and 26 an evaporating dish. Fuel is supplied from a fuel tank 19 via a pump 25 and a supply pipe 6 to a combustion chamber 1. The evaporating dish 26 and the air feeder are disposed under the combustion chamber 1. The combustion chamber 1 is provided with a closed jacket 27 for supplying combustion air. The positional relationship between the air feeder 4 and the evaporating dish 26 is such that the surface of the air feeder 4 and the lower end of the evaporating dish 26 may be at the same level or different in level. "Porous element' is used herein as a general term meaning capillaries, cloth, particle layer, porous plate having holes, sinter metal, porous ceramic or the like. Liquid fuel is supplied to the evaporating dish 26 on the air feeder 4. When air is blown in through an air supply pipe 7, the fuel (kerosine, light oil or the like) bubbles. The fuel in the form of foam is then ignited by an igniting heater 28 and burned. The amount of combustion air required for complete combustion is separately supplied from a supply pipe 8 via the closed jacket 27 to cause continuous combustion. Numeral 14 denotes a flame. The fuel is supplied via the fuel supply pipe 6 and the generation of bubbles is increased by increasing the amount of air supplied from the air supply pipe 7. The amount of combustion can readily be increased by increasing the amount of air supplied from the combustion air supply pipe 8. As the amount of gas (air) supplied from the air supply pipe 7 is increased while the amount of fuel thus supplied is kept constant, the flame grows. The aforementioned results seem to derive from the fact that the amount of the liquid fuel vaporized is so improved as to increase the amount of combustion due to the following effects: an increase in the vaporization surface area of the liquid fuel as the bubbling of the fuel is promoted, an increase in a liquid-to-gas diffusion coefficient, and a decrease in partial fuel vapor pressure on the liquid-gas boundary surface. It also becomes possible to make the combustion chamber 1 compact. Since liquid fuel is formed into bubbles before being ignited according to the present invention, it can easily be ignited simply by directly contacting an ignition source with the bubbled fuel. When the flame is to be extinguished, the supply of fuel to the combustion chamber 1 is stopped first and then the fuel left in the combustion chamber is bubbled before being burned, so that the time required until the fire is extinguished after the supply of fuel is stopped can be shortened. In addition, no offensive smells are produced. Since fuel that has been once bubbled is burned, the amount of combustion and the combustion characteristics such as the flame shape are freely controllable by regulating the amount of air blown from the air supply pipe 7, the amount of fuel supplied, and the amount of air supplied from the air supply pipe 8. Moreover, instant heat generation and combustion free of any offensive smell, that have been unattainable by the prior art oil stoves, become possible. A description will subsequently be given of the reason for the supply of air separately from the combustion air supply pipe 8 according to the present invention. The bubbling expansion ratio (apparent volume/liquid fuel volume in the mixture of liquid and gas) of the bubbles obtained only from liquid fuel such as kerosine, light oil or the like normally ranges from approximately 5 to 50. However, the air expansion ratio required for complete combustion is approximately 9,000 and the amount of air within the bubbles is far too small. Notwithstanding, some air supplied from the air supply pipe 7 will not be left in the bubbles but directly utilizable for combustion. The amount of air supplied from the combustion air supply pipe 8 should preferably be in the range of approximately 60 - 250% of the theoretical amount of combustion air. The reason for setting the lower limit at 60% will be explained. During a stable combustion period in the normal state, stable combustion generally continues unless liquid fuel above the air feeder 4 is blown off by the air supplied from the air supply pipe 7. In other words, 60% indicates a minimum flow rate of air from the combustion air supply pipe 8 for maintaining an optimum combustion state by increasing the flow rate of air from the air supply pipe 7 up to a stable combustion limit. The reason for setting the upper limit at 250% will now be explained. In a method of burning liquid fuel in the form of bubbles a stable formation of bubbles is required particularly during a transition period up to the time that stable combustion is obtained after the flame is ignited. In other words, 250% as the upper limit indicates a minimum flow rate of air from the combustion air supply pipe 8 when the amount of air from the air supply pipe 7 is set at the minimum value required for the stable formation of foam. It is also possible to promote the combustive reaction by adding a polymer or a surface active agent to liquid fuel to increase the bubbling expansion ratio. When the combustion of bubbles is utilized for burner combustion for heating an object to be heated, there are still some problems in view of making uniform the cell size of the foam and stabilizing the foaming properties if a liquid fuel other than light oil whose temperature is lower than the boiling point is used alone to generate foam. Consequently, the liquid fuel is mixed with a foaming agent such as a polymer, a surface active agent, silicon resin or the mixture thereof, or light oil having good foaming properties. The surface viscosity is increased thereby, whereas surface tension is decreased, so that the foaming properties are improved. The stable formation of foam thus becomes possible and the combustion of foam jetted from the burner promotes a stable uniform flame with the effect of preventing local heating of material parts to be heated. [Embodiment] An air feeder 4 for generating foam and dispersed bubbles of liquid fuel and a fuel supply pipe 6 were provided under a combustion chamber 1 and a air supply pipe 7 was connected to the air feeder 4 to form a foam generating zone a . A number of combustion air inlet openings 9 was provided in the upper side portion of the combustion chamber to form a combustion zone b where liquid fuel was burned. The air feeder 4 comprised a porous element having a foaming function, the bottom being pot-shaped. In operation, 80 ℓ/min of combustion air was supplied from a combustion air supply pipe 8 via a closed jacket 27 and the air inlet openings 9 into the combustion chamber 1. While approximately 0.8 ℓ/min of air was sent to the air supply pipe 7, 0.5 ℓ/H of fuel (kerosine) having a temperature of 2°C was subsequently supplied from a fuel tank 19 via the supply pipe 6 to the combustion chamber 1. Bubbles of the fuel were generated on the air feeder (porous plate of sintered metal) and rose up to an igniting heater 28 where it was ignited with a red hot Nichrome wire. The combustion state at this time was such that the mixture of the fuel with the combustion air was promoted in a flame stabilizer 30 to the extent that a blue-white flame extended upward in the flame stabilizer 30, i.e., complete combustion was realized. The time required for the flame to be produced in the upper portion of the flame stabilizer 30 after ignition was as short as 20 seconds. As the amount of air from the combustion air supply pipe 8 was decreased, the length of the flame became gradually longer and the flame began to show an orange color. When the amount of air from the air supply pipe 7 was gradually increased in that combustion state to promote the foaming of the fuel, the length of the flame grew longer and the color thereof changed to blue-white. However, the amount of the fuel being supplied was unable to keep up with the operation and the length of the flame became short. The reason for this seems attributable to the fact that the amount of combustion increases temporarily as the promotion of foaming the liquid fuel increases the amount of fuel being vaporized, thus causing the residence time of the fuel in the form of liquid in the combustion chamber to become shorter. Subsequently, the amount of fuel being supplied was increased up to 1.0 ℓ/H, whereas the amount of combustion air was set at 160 ℓ/min. The flame grew to become blue-white instantly on the flame port plate and stable combustion was continued. When the supply of the fuel was stopped, the combustion of the fuel left in the combustion chamber was immediately terminated because of bubbling combustion. The fire was completely extinguished when the supply of air from the air supply pipe was suspended. No offensive smell was produced at that time. The stabilizer is constituted with a cylindrical skeleton 33 fitted to the underside of a baffle plate 32 having holes bored therein. The stabilizer is set coaxially with the porous element. Although a porous plate with holes having a predetermined diameter was used as the air feeder 4 in this experiment, use can also be made of capillaries, a cloth, a particle layer or an air feeding method in combination therewith to generate bubbles. However, the material and shape of such a porous element are not limited to those described in the embodiment shown.

Claims: A method of burning a liquid fuel in an apparatus for burning liquid fuel which has a combustion chamber (1), an inverted cone-shaped evaporating dish (26) and a liquid fuel foamer having a porous element (4), the surface of the porous element (4) and the lower end of the evaporating dish (26) being at the same level, and wherein only the top of the porous element (4) is exposed to the interior of the combustion chamber (1), the method comprising the steps of: supplying a shallow layer of liquid fuel onto the top of the porous element (4); supplying air for foaming to the bottom of the porous element (4) from beneath the porous element in an amount to cause substantially all the liquid fuel to be immediately converted into foam at a foaming expansion ratio of 5-50 times; causing the resulting foam to enter the combustion chamber (1); and supplying combustion air separately from the foaming air and mixing it with the foam in the combustion chamber (1) for burning the foam. An apparatus for burning a liquid fuel, comprising: a combustion chamber (1) having air inlet holes (9) in the wall thereof; an inverted cone-shaped evaporating dish (26) opening upwardly into the combustion chamber (1); a liquid fuel foamer including a porous element (4) having a top and a bottom and disposed beneath and having only the top in communication with the bottom of said evaporating dish (26), and a foaming air supply pipe (7) directed to the bottom of said porous element (4); a fuel supply pipe (6) opening into the bottom of said combustion chamber (1) and onto the evaporating dish (26); and a closed jacket (27) surrounding the outside of said combustion chamber (1) and having means for supplying combustion air thereinto and into said combustion chamber (1) through said inlet holes (9). An apparatus as claimed in claim 2 further comprising a flame stabilizer (30) in said combustion chamber (1) and having a vertically positioned cylindrical sleeve having holes therein and being in alignment with a vertical axis through the porous element (4).

Inventors: ADACHI KOJI, ANZAWA NORIO, FUTAKAWA TETSUO, NARITA AKIRA, SATO HIRONARI, ADACHI, KOJI, ANZAWA, NORIO, FUTAKAWA, TETSUO, NARITA, AKIRA, SATO, HIRONARI, ADACHI, KOJI, C/O NIPPON STEEL CORPORATION, ANZAWA, NORIO, C/O NIPPON STEEL CORPORATION, FUTAKAWA, TETSUO, C/O NIPPON STEEL CORPORATION, NARITA, AKIRA, C/O NIPPON STEEL CORPORATION, SATO, HIRONARI, C/ONIPPON STEEL CORPORATION
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1990
Main classification: F23D5
IPC codes: F23D5