Note: Descriptions are shown in the official language in which they were submitted.
I
SYSTEM AND METHOD FOR IMPROVING THE COMBUSTION PROCESS
OF A SOLID FUEL BY MEANS OF AN INERT POROUS MEDIUM
The present invention relates to a system and method comprising the use of
an inert porous material disposed in the proximity of a solid fuel, inside a
combustion
fireplace, to improve the combustion process of said solid fuel.
Background
Currently, one of the most widely used fuels in heat generation systems, such
as heating stoves or other types of furnaces, are solid fuels, such as
biomass, coal,
or others.
In general, the wide use of solid fuels as fuel media inside a combustion
fireplace is due to the fact that they are usually cheaper than liquid or
gaseous fuels.
However, the great disadvantage of solid fuels lies in the low efficiency of
the
combustion process, which results in a higher emission of polluting exhaust
gases
and particulate matter compared to the use of liquid or gaseous fuels.
In this context, there is a common need to improve the efficiency of the
combustion processes for solid fuels, in order to significantly reduce the
emission of
pollutants exhaust gases and particulate matter contained in combustion
systems
that use this type of fuels.
One of the ways to improve the efficiency of the combustion process of a
solid fuel is to maximize the delivery and utilization of the calorific value
of said solid
fuel (e.g., firewood). The common approach to achieve said technical effect is
to
use specially designed equipment for combustion, operated according to methods
appropriate to the type of fuel. For example, in combustion fireplaces it is
common
to use heat-resistant materials and to implement appropriate fireplace designs
to
reduce losses to the outside, while maintaining a high and constant
temperature
inside the fireplace.
Among the most commonly used fireplace designs are the implementation of
appropriate air intake controls, which often increase the efficiency of the
combustion
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process. In addition, modern fireplaces incorporate metal walls on their side
faces
and have a space between the fireplace casing and the adjoining walls where
the
fireplace is located, for example, the rear of a wood burning stove, which
allows free
air circulation between the stove and the adjoining walls.
In the context of residential wood burning stoves, there are different types
of
stoves that use firewood as fuel on the market, among which are: fireplaces,
salamanders, slow combustion stoves and double chamber slow combustion
stoves. The most commonly used designs comprise an air inlet near the floor of
the
building, and an air outlet near the stove shelf (located at the top of the
fireplace
door). This design results in a convective circulation of hot air, in addition
to the
radiant heat from the fireplace itself. However, the technique of admitting
air through
the bottom of the fireplace, which allows hot air to rise through the
fireplace bed and
then escape through the stove outlet ducts, proves to be inefficient. Indeed,
it has
been found that to heat efficiently, the combustible gases released during the
wood
combustion process, or combustion gases, must be mixed with a large amount of
oxygen at a minimum temperature of 1,100 C. In addition, for complete
combustion
of the combustible gases released by the wood, about 80% more than the amount
of air needed for the fuel must be supplied.
In this context, the desirability of having an air supply above the fireplace
bed
to heat the combustible gases released during the combustion process has led
to
the design of "downdraft" heating equipment. Such equipment forces the
circulation
of combustible gases through internal structures arranged as "labyrinths",
where
they are mixed with a stream of hot air, achieving a virtually complete
combustion,
reducing pollutants in the exhaust gases. In less efficient equipment, these
combustible gases escape through the chimney in the form of exhaust gases, or
are
deposited in the duct in the form of soot and/or creosote.
In recent years, the development of new technologies to improve combustion
efficiency has made great advances, as in the case of the application of inert
porous
media. For example, patent document CL 2014-01778 uses a mixture of inert
porous media (alumina spheres) with a solid fuel (biomass, coal or other) for
synthesis gas generation. However, said document does not focus on improving
the
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efficiency of the combustion process or heat production, but only seeks to
generate
synthesis gas from the gasification of the solid fuel itself.
Another combustion solution using inert materials is described in patent
document US 10401023 B2. Said document refers to a burner comprising a
combustion chamber with a porous material enhanced with a perovskite catalyst
composition that coats the pores of the porous material, producing a stable
combustion of a mixture of natural gas and air. Although this document refers
to the
use of porous materials to stabilize combustion, it does not refer to heat
production,
but only seeks to increase the contact surface between the catalyst
composition and
the mixture of natural gas and air, using for this purpose the pores of the
inert
material.
Therefore, it is necessary to have a combustion system and method to
improve the efficiency of the combustion process of solid fuels, decreasing
the
polluting emissions of said type of solid fuels.
Description of the invention
The invention relates to a combustion system and method for improving the
efficiency of the combustion process of solid fuels.
In particular, the solid fuel combustion system of the present invention
comprises:
- a combustion fireplace containing solid fuel inside, wherein said solid
fuel
is subjected to a combustion process inside the combustion fireplace;
- an inert porous medium placed inside said combustion fireplace, wherein
said inert porous medium is in the proximity of the solid fuel; and
- flow control means placed in the combustion fireplace.
The flow control means are placed to control the operation of at least one air
inlet and at least one exhaust gas outlet placed in the fireplace. The air
inlet is
designed or configured to inject intake air into the fireplace. The gas outlet
is
designed or configured to release unreacted combustion gases to the outside of
the
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fireplace, in the form of exhaust gases, wherein said combustion gases are
generated by the combustion process occurring inside the fireplace.
The flow control means are designed or configured to generate a flow of
intake air and combustion gases into the combustion fireplace, between the at
least
one air inlet and the at least one exhaust gas outlet. The flow of intake air
and
combustion gases forms a premix of intake air and combustion gases inside the
fireplace. Additionally, the flow control means and the inert porous medium
are
configured to circulate said premix through the inert porous medium. In
particular,
the premix is circulated through the pores of the inert porous medium, which
causes
preheating of the intake air and acceleration of the combustion gas reaction
within
said pores. Thereby, the circulating premix is combusted inside the pores of
the inert
porous medium.
At this point it is important to emphasize that the present invention does not
intend to describe with the usual components of solid fuel combustion systems,
such
as the means for controlling the flow of intake air and combustion gases, or
the use
of double chambers and other mechanisms currently implemented in different
systems, such as the existing wood stoves. Indeed, the elements described are
those which have a direct participation in obtaining the advantages proposed
by the
invention, associated with the use of inert porous media in the combustion
process.
On the other hand, the method of solid fuel combustion of the present
invention comprises the following steps:
a) placing solid fuel and placing an inert porous medium inside a combustion
fireplace of a solid fuel combustion system, wherein the inert porous medium
is placed in the proximity of the solid fuel;
b) injecting intake air into the combustion fireplace through at least one air
inlet
placed in the combustion fireplace, wherein the injection of said intake air
is
controlled by flow control means placed in the combustion fireplace;
c) igniting the solid fuel, subjecting said solid fuel to a combustion process
inside
the combustion fireplace;
d) generating a flow of intake air and combustion gases between the at least
one air inlet and at least one exhaust gas outlet also placed in the
combustion
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fireplace, wherein said flow of intake air and combustion gases is controlled
by said flow control means, forming a premix of intake air and combustion
gases inside the combustion fireplace;
e) circulating the premix through the inert porous medium, in particular,
through
the pores of said inert porous medium, to preheat the intake air and
accelerate the reaction of the combustion gases within said pores, wherein
the circulation of the premix is controlled by said flow control means and
said
inert porous medium, and wherein said premix is combusted inside the pores
of the inert porous medium; and
f) releasing unreacted combustion gases to the outside of the combustion
fireplace, in the form of exhaust gases, wherein the release of said exhaust
gases is controlled by said flow control means.
The arrangement of the inert porous medium in the proximity of the solid fuel
allows establishing a thermal contact between said inert porous medium and
said
solid fuel, wherein thermal contact is to be understood the occurrence of heat
transfer mechanisms between the inert porous medium and the solid fuel.
According to an embodiment of the invention, the step of injecting intake air
is controlled according to the desired amount of air for the combustion
process, by
means of the control of the at least one air inlet. The desired amount of air
for the
combustion process can be selected according to the state of the combustion
process inside the fireplace, either by means of temperature measurements
inside
the fireplace and/or combustion gases towards the at least one outlet.
Preferably,
the amount of air injected into the fireplace depends on the oxygen required
for the
chemical reactions involved in the combustion process, seeking to avoid
imperfect
combustion of the solid fuel.
Notwithstanding the foregoing, and in the understanding that the combustion
process usually involves imperfect combustion, avoiding excess air that
generates
heat losses, the step of injecting intake air and/or the step of releasing
exhaust
gases are controlled according to the desired premix circulation through the
inert
porous medium, by controlling the at least one air inlet and the at least one
exhaust
gas outlet. In this sense, the circulation of the premix through the pores of
the inert
porous medium allows the combustion gases, generated as gases from the
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devolatilization of the solid fuel, to mix with the intake air, favoring the
formation of
the premix and its combustion inside the pores.
According to an embodiment of the invention, the step of placing the solid
fuel
comprises placing two or more solid units forming said solid fuel. The solid
units
may or may not be of different dimensions and may or may not be randomly
positioned inside the fireplace. According to the various embodiments of the
invention, the solid fuel, and the solid units forming said solid fuel, are
selected from
biomass, coal, or a combination of both. By way of example, biomass should be
understood as:
- wood, in any of its forms, e.g., logs, pellets, sawdust, etc.;
- wheat residues, straw, nut shells, or other natural fibrous
materials;
In addition, the present invention also considers as solid fuel different
types
of coal, independent of its moisture, amount of ash, and/or calorific value.
Some
types of coal are lignite, sub-bituminous coal, coal (fat, semi-fat and/or
dry),
bituminous coal, anthracite, and coking coal, among others.
Other solid fuels that can also be used by means of the present invention, in
addition to biomass and coal, can be peat, combustible wastes, or other
natural or
artificial solid fuels.
According to one embodiment of the invention, the step of placing the inert
porous medium inside the combustion fireplace comprises placing at least one
inert
porous unit inside the combustion fireplace. Said at least one inert porous
unit
forming the inert porous medium. Alternatively, the inert porous medium inside
the
combustion fireplace comprises two or more inert porous units, said two or
more
inert porous units placed inside the combustion fireplace in different
positions.
Preferred embodiments of the invention comprise placing the one or more porous
inert units in physical contact with the solid fuel, in positions which
maximize and
ensure said physical contact with the solid fuel. The physical contact between
the
solid fuel and the inert porous medium, which generates a thermo-physical
contact,
intensifies heat transfer, mainly by conduction, convection, and radiation,
and thus
the solid fuel is able to combust at a higher temperature and more
efficiently.
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As an example, a first inert porous unit is positioned between the solid fuel
and a side wall of the combustion fireplace, and a second inert porous unit is
positioned between the solid fuel and another side wall of the combustion
fireplace.
Thus, a combustion fireplace having at least four walls, two side walls, a
rear wall,
and a front wall, comprises inert porous units disposed at least between the
solid
fuel and the side walls of the fireplace. Alternatively, a third inert porous
unit is
positioned between the solid fuel and a rear wall of the combustion fireplace.
This
lateral and rear location of the inert porous units allows the emission of
particulate
matter to be reduced by 50%, considering the stove without inert porous media
as
a point of comparison.
Although it is possible to add a fourth inert porous unit between the solid
fuel
and the front wall of the fireplace, in conventional combustion systems the
front wall
is also the access door to the interior of the fireplace, usually comprising a
glass
pane. Then, the operation of the fourth inert porous unit placed towards said
front
wall will be affected by the opening/closing operations of the fireplace
access door
and/or by the increased heat transfer occurring from said front wall to the
outside of
the fireplace.
In this context, the position where the inert porous medium is placed in
contact with the solid fuel must not only favor the generation of the premix
and the
combustion of the same, as is achieved with the lateral and rear positions
indicated
above, but must also favor an optimal cleaning of the ashes generated after
the
combustion process. In this regard, while it is also possible to place porous
inert
units towards the top wall of the fireplace, this has been shown to be not as
effective
and functional compared to dissipations towards the side and rear walls. On
the
other hand, with respect to the bottom wall of the fireplace, although it is
possible to
improve the combustion process by using this location for porous inert units,
placing
one or more porous inert units under the solid fuel prevents optimal ash
cleaning,
saturating the pores of the porous inert medium with said ash and rapidly
losing the
advantages of using the porous inert medium inside the fireplace.
At this point it is relevant to point out that each inert porous unit can be
formed
by one or more inert porous elements, said elements usually presented in the
form
of discs with the appearance of porous sponges. In this regard, the inert
porous
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medium may be of a ceramic material, and may have a porosity of at least 50%
and
a pore density of at least 20 ppi.
According to an embodiment of the invention, a data acquisition unit is
provided, for executing a step of acquiring operation data of the combustion
system.
In this context, the present invention makes it possible to generate a
combustion process with the active participation of the inert porous medium,
which
intensifies the heat transfer mechanisms (conduction, convection, and
radiation). In
addition, the active participation of the inert porous medium allows the air
required
for the combustion process to be preheated and the volatile combustion gases,
product of the devolatilization of the solid fuel, to mix with the air and
react rapidly
inside the pores.
Brief description of the figures
As part of the present invention, the following representative figures of the
same are presented, which show preferred configurations of the invention and,
therefore, are not to be considered as limiting to the definition of the
claimed subject
matter.
FIG. 1 a, 1 b, and 1 c show right lateral, front, and left lateral views of a
schematic representing the interior of the combustion fireplace, according to
one
embodiment of the invention.
FIG. 2 is a schematic of the wood combustion process.
Detailed description of the figures
FIGs. 1 a, 1 b, and 1 c show views of a representative schematic of the
combustion fireplace (10) according to one embodiment of the invention. FIGs.
1 a
and 1 c, showing right and left side views, respectively, show a schematic
arrangement of the solid fuel (11) with respect to the inert porous medium
(12) inside
the combustion fireplace (10). In addition, said figures schematically show
the
circulation of intake air and combustion gases (13), including the formation
of the
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premix (14) that circulates through the inert porous medium (12), for
combustion
within the pores of said inert porous medium (12).
On the other hand, FIGs. 1 a and 1 c also show an inner plate (15) as a flow
control means, in this case represented as a double chamber, which allows
directing
the circulation of the flow of intake air and combustion gases towards an
outlet (16),
for the release of unreacted combustion gases, or exhaust gases (17).
On the other hand, FIG. 1 b, which shows the front view of the combustion
fireplace (10) scheme, allows to identify how the solid fuel (12) is
surrounded by the
inert porous medium (12), represented by first and second lateral porous units
(12',
12") and by a third rear inert porous unit (12'").
It should be understood that the above figures are representative diagrams
of the combustion fireplace, which aim to facilitate the visualization of the
components of the combustion system claimed, and not to show a preferred mode
of the arrangement or design of said components.
Examples of application
By way of example, the present invention is implemented in a combustion
system using firewood as a solid fuel. Firewood is not a homogeneous fuel,
such as
oil or natural gas. Compared to liquid and gaseous fuels, several reaction
phases
are identified in the wood combustion process:
- Drying of wood:
Initially, the outer surface of the wood receives heat by radiation from the
flames, heating the water contained in the wood above its evaporation point.
At this point the drying process begins, releasing the moisture in the form of
water vapor.
This drying process consumes an important fraction of the energy released
in the combustion process. The higher the initial water content of the
firewood, the more energy will be consumed in this drying process and the
slower the first step of heating the firewood becomes;
- Gasification and oxidation of the volatile material:
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As the dry wood is heated above the boiling point of water, the second
pyrolysis phase begins with the release of volatile matter or
devolatilization.
At this step, the firewood begins to smoke. The smoke is the visible result of
the thermal decomposition of the firewood and consists mainly of a cloud of
combustible droplets of gases and hydrocarbons (tar). These only oxidize
under high temperatures and if sufficient oxygen is present. This combustion
process with heat release produces long, bright flames, which are
characteristic of the combustion of dry firewood.
If the volatile matter is not completely burned inside the stove, unburned
gases or exhaust gases will be emitted, which will condense on the cold walls
of the exhaust ducts, forming creosote deposits. These unburned compounds
will also be emitted later as visible colored smoke with a strong atmospheric
pollution in the area. The smoke also represents a loss of efficiency, because
it contains a large part of the energy present in the wood.
- Burning of residual charcoal:
When the volatile matter is completely released from the wood, solid coal
together with non-combustible ash remains as a residual product. This solid
compound is equivalent to wood charcoal and is characterized by its surface
combustion with a red glow and very small flame that generates a high
temperature between 600 and 1,000 [ C]. Charcoal is a clean fuel that burns
easily in the presence of sufficient oxygen without generating smoke, but
nevertheless generates carbon monoxide (CO).
In practice, the three above-mentioned phases of wood combustion occur
simultaneously. This means that the gases from the volatile matter may be
burning
with long flames while on the surface of the fuel the charcoal burns with the
characteristic red glow and the water in the center of the firewood slowly
evaporates.
FIG. 2 shows a schematic of the three phases of wood combustion occurring
simultaneously.
To achieve complete combustion of the thermal decomposition products of
wood, the following conditions are required, which are summarized in the
"3T's" rule
known in the technical field of combustion.
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- Temperature: The minimum temperature required to be maintained inside a
fireplace to ensure complete combustion of the gaseous products should be
as high as possible. Literature indicates average values in the combustion
zone from minimum 800 C to maximum 1,000 C.
- Time: To achieve complete combustion, a minimum residence time of the
combustion gases inside the fireplace must be guaranteed. For example, if
temperatures above 900 C are present, the minimum residence time must
exceed 0.5 seconds.
- Turbulence: The last condition necessary to ensure optimum
combustion is
related to intense turbulence required to mix oxygen with the volatile matter
in combustion. To intensify this mixing process, it is customary to inject
preheated secondary air directly into the combustion zone above the fuel bed.
Only by complying with these three basic rules of minimum temperature,
minimum residence time, and high turbulence can the conditions for optimum
combustion of fire fuel with minimum emission of pollutants be created. The
challenge of an optimal design of a fireplace or a stove is to combine these
three
basic conditions allowing to always guarantee a complete combustion,
minimizing
the emission of pollutants. In particular, the characteristics of humidity,
density, size,
and species of wood must be considered in order to correctly dimension the
volume
and shape of the combustion chamber. This is the only way to achieve high
efficiencies and clean exhaust gases without the presence of visible smoke.
Then,
with the application of the inert porous medium inside the stove, the
temperature of
the combustion fireplace is increased, which produces an improvement in the
combustion of wood.
Based on the above, and having as a base test the process of firewood
combustion without inert porous media, two tests of inert porous media
configurations inside the same combustion fireplace are conducted.
Test 1
In this context, in the present test, a residential heater equivalent to that
of
the base test was used, and the solid fuel to be placed inside the combustion
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chamber of said heater was prepared in a manner equivalent to that of the base
test. Since the useful volume of the fireplace or combustion chamber of the
tested
heater does not exceed 0.02 m3, three pieces of eucalyptus wood of 0.25 m
length
were dimensioned, with variable width to achieve the mass required for the
test,
according to the useful volume of the combustion chamber, based on a
volumetric
load density of 112 11.2 kg/m3.
In contrast to the base test, an inert porous medium of Silicon Carbide (SiC)
material was incorporated in the form of a 200 mm diameter, 30 mm thick, 82.5%
porosity disc. For this example, the SiC disc was placed horizontally on the
bottom
wall of the fireplace, with the firewood load on top of the disc. Using this
configuration, it is expected that the disc will generate a preheating of the
primary
air that rises through the heater and passes through the fuel, to improve the
oxidation of the wood, decrease the evaporation time of the moisture present
in the
wood, and decrease the cooling in the combustion chamber caused by the entry
of
cold air, in addition to delivering heat by conduction and radiation to the
fuel.
Test 2
In this test, unlike test 1, two half-discs were used, prepared from a disc of
the same dimensions as in example 1, which were positioned between the logs of
the test load inside the combustion chamber. Using this configuration, it is
expected
that the half-discs, by means of conduction and radiation, deliver heat to the
fuel,
preheating the air-fuel mixture, in addition to providing heat to the
combustion gases
that rise between the spaces between one log and the other.
Conclusions
The heat delivered by the combustion of firewood to the environment through
the walls of the heater was significantly higher when using the inert porous
medium,
reaching a maximum of approximately 1,000 W, compared to the maximum of 500
W of the heater without inert porous medium, using maximum air intake. This is
due
to the increase in the average temperature of the combustion chamber when
using
inert porous media, which causes a higher heat flow by radiation and
convection to
the environment. The increase in the heater wall temperature is due to the
heat
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transfer phenomenon present between the air-fuel premix reaction and the inert
porous medium, which generates an increase in the temperature of the inert
porous
medium due to the heat transferred from the air-fuel premix reaction,
resulting in an
increase in the combustion fireplace temperature.
In this context, the configuration of test 2 has a better performance with
respect to CO emission compared to the configuration of test 1. This
difference is
due to the configuration of the inert porous media in each test, inside the
combustion
chamber, since in test 2 said inert porous media covers a greater height of
the
fireplace, also causing a heat transfer by convection from the reaction of the
air-fuel
premix to the porous media, so that the combustion gases when ascending and
coming into contact with the ceramic half-discs increase their temperature and
burn
again lowering the CO concentration. In addition, the configuration of test 1,
with the
inert porous medium under the solid fuel, causes the pores to become saturated
when ash is produced during the combustion process, hindering the circulation
of
the air-fuel premix through said inert porous medium.
In this context, the base test, which does not use inert porous medium, shows
a thermal efficiency of 68.91 %, while test 1 shows an efficiency of 81.41 %
and test
2 shows an efficiency of 91.04 %. Then, it is demonstrated that the use of
inert
porous media considerably improves the thermal efficiency of the tested wood-
fired
heater, as well as it is demonstrated that positioning the inert porous media
towards
the side walls of the heater, between the solid fuel and said walls, is
advantageous
over positioning the inert porous media under the solid fuel.
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