Combustion – Heated line section feeds flame holder – Electrically heated section
Reexamination Certificate
1999-03-12
2002-02-26
Price, Carl D. (Department: 3743)
Combustion
Heated line section feeds flame holder
Electrically heated section
C431S207000, C431S240000, C431S247000, C431S011000, C431S006000
Reexamination Certificate
active
06350116
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for providing a combustible mixture of a liquid fuel and combustion air, as well as a prevaporizing and premixing burner for liquid fuels, having one or several fuel heaters for heating the liquid fuel prior to combustion.
BACKGROUND OF THE INVENTION
In the field of household and small consumers (HuK) it is known to burn fuel oil EL for heating purposes or for purposes of thermal process technology in a pressure atomizer burner. The liquid fuel oil EL is converted under high pressure (500 to 2000 kPA) into a fog of droplets by means of an atomizer nozzle and is simultaneously mixed with the supplied combustion air. A method exists in addition, wherein fuel oil EL is atomized by means of compressed air. Further than that there are vaporization burner devices, wherein the liquid fuel is vaporized on the surface of a heated body which is surrounded by combustion air.
The following problems are connected with the present-day burners: in conventional oil burners, the liquid fuel oil EL is converted into a fog of droplets by means of an atomizer nozzle and simultaneously mixed with the supplied combustion air. The processes, such as atomizing, mixing, vaporizing and gasification of the fuel, as well as the combustion of the gasified fuel, occur unregulated side-by-side and in an interacting manner. The individual oil droplets are surrounded by a flame envelope. The high temperatures in the vicinity of the drops, together with the simultaneously occurring lack of air, trigger cracking processes, by which soot is formed.
Present-day blue flame burners avoid the generation of soot in that they vaporize the fuel at the flame root prior to combustion. Here hot flue gases returned from the flame zone here vaporize the oil spray emerging from a swirl nozzle. The water content of the returned flue gases prevents the formation of long-chain hydrocarbons, which can only be burned along with the generation of soot. The method of recirculating the exhaust gases lowers the nitrous oxide emissions in addition to the soot emissions. In order to convey a sufficiently large amount of hot flue gas into the flame root, a correspondingly large induction effect of the fuel/air jet within the mixture preparation is required. The induced mass flow is affected for one by the velocity of the emerging mixture flow and also by the cross section of the open jet. Both parameters can only be varied within certain limits. A high outlet velocity leads to loud flow noises, an increased blower output and large burner dimensions. An increase in the outlet cross section, together with a reduction in velocity, leads to ignition conditions already being created in the vaporization area, so that the intended fuel vaporization, which is uncoupled from the combustion reaction, does not occur. Moreover, the pulse exchange between the fuel and the combustion air is reduced, by which the mixture is also negatively affected. In addition, a high outlet velocity at the twist generator prevents a flame formation in the near range of the mixing device and thereby leads to a reduced thermal stress of these components. It follows from this that in connection with present-day mixture preparation methods for oil burners a reduction of the noxious matter emissions is always connected with an increase in the velocity of the combustion air, and therefore leads to increases in noise emissions and the required blower output.
In a burner system having a firing equipment output of 15 kW, a reduction of the fuel oil flow rate is not possible with conventional oil pressure atomizer nozzles. For reducing the throughput, the nozzle cross section cannot be further reduced for reasons of dependability. The pump pressure can also not be arbitrarily reduced, because the atomizing quality is clearly reduced.
Conventional oil burners are heterogeneous systems, i.e. the dispersed phase fuel oil EL and the dispersion medium air exist as discrete phases side-by-side and are separated by a phase boundary. The roughly dispersed fuel distribution caused by atomizing does not make it possible to mix the fuel without prior vaporization in front of the flame, because the individual fuel droplets settle under the effects of gravity and are deposited on the mixing chamber walls. For this reason a premixing surface burner device, such as can be used in the field of gas combustion, is not possible.
Modem gas burner devices show that a reduction of the nitrous oxide emissions is most effectively achieved by means of a premixing burner system.
A gasification device for fuel oil and kerosene is known from German patent DE-C2-24 56 526, and an oil heating device from German published application DE-OS 14 01 756, wherein the fuel is heated prior to atomizing. Although heating the fuel leads to improved and finer atomizing, problems occur because of cracking product depositions, such as clogging of the lines, etc.
SUMMARY OF THE INVENTION
The above mentioned problems are solved by the present invention by the provision of a method wherein: the liquid fuel is put under pressure in a heating phase with the fuel valve closed; the fuel under pressure and in liquid form is heated; following the heating phase the fuel valve is opened and the heated liquid fuel under pressure is atomized and vaporized through a nozzle; the vaporized fuel is mixed with combustion air, where at least a part of the vaporized fuel is condensed, so that a colloid-dispersed and/or a molecular-dispersed fuel-air mixture is created; the fuel valve is closed to terminate combustion; and, with the fuel valve closed, the heated and liquid fuel is cooled;
The advantages which can be achieved by means of the present invention consist in particular in that, with the method on which the present invention is based, a colloid-dispersed or molecular-dispersed fuel distribution occurs, depending on the degree of air preheating. A mixed form of both distribution types is also conceivable. Because of the stability of the colloid-dispersed fuel distribution, it is possible to mix the reactants ahead of the flame without the fuel droplets being deposited on the mixing chamber walls. Here, the mixing of the reactants is possible completely spatially decoupled from the combustion reaction, and not, as with conventional emission-reducing oil burners (so-called blue flame burners), only inside a very small gasification zone ahead of the flame, which is in direct convective heat exchange with the flame via the flue gas recirculation. Because the mixture of fuel and combustion air is now no longer limited to the gasification zone ahead of the flame, the premix burner devices known from gas burner technology, which make possible a very intensive mixing of the reactants, can now also be employed for liquid fuels. The known advantages of this burner technology are therefore also now usable for liquid fuels. Among these are:
(a) low emissions (soot, nitrous oxide, carbon monoxide) when using a surface combustion system;
(b) low noise emissions;
(c) small blower output required;
(d) a combustion air blower system can possibly completely omitted (atmospheric mixture formation);
compact heat generator construction because of the direct coupling of the heat exchanger on the heating cycle side to the reaction zone, which can be spatially exactly determined.
The core of the prevaporizing, premixing combustion technique is constituted by the heating of the liquid fuel oil under pressure. Vaporization of the fuel oil only takes place at the nozzle outlet in contrast to conventional vaporization burner devices, wherein the oil impinges with almost no pressure on a hot surface, which results in the deposition of low-volatile fuel oil components. Maintaining the above mentioned pressure conditions in the operational phases, in which heated fuel oil is in the hydraulic system of the burner, prevents these deposits. Both during the heating phase and during the cooling phase, the oil lines from the pump to the heated fuel valve are pressure sealed, or the pressure in the sys
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