Combustion – Process of combustion or burner operation – In a porous body or bed – e.g. – surface combustion – etc.
Reexamination Certificate
1999-07-19
2001-07-10
Lazarus, Ira S. (Department: 3743)
Combustion
Process of combustion or burner operation
In a porous body or bed, e.g., surface combustion, etc.
C431S170000, C431S328000, C431S011000, C431S215000, C431S346000
Reexamination Certificate
active
06257868
ABSTRACT:
The invention relates to a process and an appliance for the combustion of liquid fuel, in particular oil.
A burner, which can be operated with a gas/air mixture as fuel, is known from DE 43 22 109 A1. In this burner, the so-called pore burner technology is used. This technology differs from all the usual combustion processes in that the gas/air mixture is burnt in the hollow spaces of an inert porous material.
Because of the positive heat transport properties of the porous material, such a burner is characterized by a low emission of pollutants and a very large range of excess air number and output (up to 1:20). In addition, the exhaust gases can be very effectively cooled by a heat exchanger embedded in the porous material so that very high efficiencies and an improved fuel utilization are ensured. Such burner/heat exchanger combinations only require about {fraction (1/10)} of the installation volume of known systems.
The known burner cannot, however, be operated with liquid fuels such as oil or the like.
EP 0 524 736 A2 reveals a process and an appliance for carrying out a controlled reaction in a porous matrix. In these, gas or vapor is guided from a space into porous medium extending in tubular form vertically upward. The combustion takes place within the porous medium. The heat occurring during the combustion mainly flows downstream and reaches a further space. This process is not suitable for the combustion of liquid fuels. The position of the flame front in the porous body is unstable. In order to stabilize the position, an appliance coupled to a temperature measurement device is necessary to control the volume flow. The heat occurring in the known process is transferred incompletely by convection to the surrounding medium. No preheating of the combustion mixture to increase the efficiency takes place. After the process is switched off, gas or vapor residues remaining in the space can contribute in a disadvantageous manner to self-ignition.
An oil burner of the evaporative type is revealed in U.S. Pat. No. 4,133,632. In this oil burner, a porous plate is provided at the bottom of an evaporation casing, oil being induced on one side of the plate by capillary forces and evaporated into the evaporation casing on the other side. The evaporated oil is mixed with air and the mixture is finally supplied to a combustion space where it is burnt with an open flame.
The known evaporator is disadvantageous in a plurality of respects. Because the mixture with air only takes place after the evaporation of the oil, a large distance is required to form a homogeneous air/oil mixture. Because the induction of the oil into the porous plate depends on capillary forces, the porous plate must have a very fine-pored configuration. This, however, has the effect that it becames blocked due to the impurities contained in the oil and therefore has to be cleaned regularly. In order to make a sufficient quantity of oil vapor available, the porous plate must have a relatively large surface which is in contact over the whole of its surface with an oil reservoir. This requirement acts against a compact design of the known oil burner. In addition, it is not possible to put the burner which is combined with this evaporator into operation immediately because the formation of the oil vapor demands a certain time. After the burner is switched off, an oil vapor/air mixture remains in the evaporator and this can lead to unintentional combustion.
The object of the present invention is to obviate the disadvantage of the prior art. The object is, in particular, to provide a simple process for the combustion of liquid fuels, in particular oil, which process is as efficient as possible and has as low a pollutant level as possible. In addition, the object is to provide an appliance for the combustion of liquid fuels which is as simple and compact in design as possible and can be manufactured at low cost
In accordance with the process aspect of the invention, provision is made for the liquid fuel to be distributed by means of a distribution device and to be transferred into a reactor arranged downstream with a porous medium having a communicative pore space, the Péclet number of which porous medium permitting flame development within the porous medium. The process according to the invention permits particularly efficient and low-pollutant combustion of the liquid fuel used.
It has been found advantageous to select a Péclet number of the porous medium which is greater than 65. The Péclet number can be calculated from the following equation:
Pe=
(
S
L
d
m
C
p
&rgr;)/&lgr;,
where S
L
is the laminar flame speed, d
m
is the equivalent diameter of the average hollow space of the porous material, C
p
is the specific heat of the gas mixture, &rgr; is the density of the gas mixture and &lgr; is the thermal conductivity coefficient of the gas mixture. The equation shows that the conditions for the development of the flame are essentially dependent on the equivalent diameter d
m
of the mean hollow space or on the means pore diameter of the porous material. The process-dependent parameters, such as S
L
, C
P
, &rgr; and &lgr; then have to be fixed for a specified oxidant/liquid fuel mixture under the porous medium conditions present at entry, i.e. in the region of the mixture inlet end. They are defined, in particular, by the type of liquid fuel and oxidant and by their mixture ratio. The process according to the invention has the noteworthy advantage that the thermal conductivity coefficient &lgr; and the temperature of the oxidant/liquid fuel mixture at entry into the porous medium does not necessarily have to be selected in such a way that they lie below the explosion limit.
In a further configuration according to the invention, a gaseous oxidant, in particular air, is supplied to the distribution device and/or the porous medium to form a mixture consisting of the liquid fuel and the oxidant. In this arrangement, the distribution device can have a device for atomizing the liquid fuel. The atomizing device can, for example, have a flow of gaseous oxidant around it. It is advantageous for the atomizing device to have a nozzle to which is supplied liquid fuel under pressure. The atomizing device can also have a binary nozzle to which is supplied liquid fuel and oxidant under pressure. By this means, a first mixture consisting of oxidant and liquid fuel is formed and this can be enriched with further oxidant.
The atomizing device is preferably arranged in the vicinity of the porous medium. It can be movable back and forth relative to the porous medium. In the case of a cylindrical configuration of the porous medium, the atomizing device is advantageously arranged on the axis of the cylinder.
In accordance with a further embodiment, the porous medium can be provided at a mixture inlet end with a porous element having a communicative pore space. The porous element is preferably defined by a Péclet number which does not permit flame development, this Péclet number being not generally less than 65.
In accordance with a particularly advantageous feature, a mixture evaporating device can be provided which preferably contains a porous body having a communicative pore space. The average pore diameter of the porous body can be greater than that of the porous element. This facilitates the distribution, mixing and evaporation of the liquid fuel. The evaporation device is generally arranged upstream of the porous medium and downstream of the distribution device.
In a further embodiment, the porous medium is in contact with the porous element. The porous element can usefully be in contact at its upstream end with the porous body. At the mixture inlet end of the porous medium, the porous element forms a flame barrier which prevents the mixture from burning back against the direction of the mass flow, in particular into the porous body acting as the evaporation device. Because of the direct contact between the porous body and the porous element and between the porous element and the porous medium, the heat formed in the porous me
Durst Franz
Keppler Michael
Weclas Miroslaw
Cocks Josiah C.
Fish & Richardson P.C.
Lazarus Ira S.
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