Technical field
[0001] The present invention relates to a process for gasification of solid, moderately
flowable biofuel in a downdraft gasifier having an open core and a fixed gasification
bed wherein the fuel material and primary air are fed at the top so as from the top
and downward to pass a) a drying zone, b) a pyrolysis zone, c) a combustion zone involving
flaming pyrolysis to which secondary air is added and where the fuel is supported
by way of a narrowed portion in the inner cross section of the gasifier, d) a reduction
zone and e) optionally an inactive charcoal zone. Furthermore, the present invention
relates to a downdraft gasifier to be used in the process and a plant for heat production
or a combined heat and power production, said plant including the downdraft gasifier.
Background Art
[0002] In order to utilise biomass as fuel, the biomass can advantageously be converted
into a combustible gas by gasification. Among known biomass gasification processes,
downdraft gasification in a stratified gasifier can be mentioned as described in for
instance item 5.8 in "The stratified Downdraft Gasifier" in "Handbook of Biomass Downdraft
Gasifier Engine Systems" published by Solar Energy Research Institute (SERI), Colorado,
USA, (March 1988), pages 38-42.
[0003] In this gasifier including a cylindrical vessel having an open top air and biomass
pass uniformly downward through different zones. In the first zone, air passes through
the yet unreacted biomass.
[0004] In the initial stage of the second zone, the biomass reacts with air in flaming pyrolysis
during which for instance volatile wood oils burn under the formation of CO
2 and H
2O and supply heat to the pyrolysis process. In the last stage of the second zone,
when the oxygen has been consumed, CO and H
2 are formed during continuous pyrolysis and the biomass is converted to charcoal.
The total amount of air added in the second zone is thus less than the amount required
for a complete (stoichiometric) combustion. The temperature in the second zone usually
ranges between 1000 and 1150 °C.
[0005] In the third zone, the process is an adiabatic charcoal gasification in which the
hot gases formed during the pyrolysis react with the charcoal during the formation
of additional CO and H
2. Accordingly, the solid charcoal is converted by utilising the heat of the gas in
order to produce a product gas with a high content of chemical energy and the temperature
drops to approx. 800 °C. At the end of the process, the charcoal is completely gasified
and the remaining ash drops through a grate provided in the bottom. The product gas
produced is also recovered through the grate, the energy thereof being exploitable
in a gas combustion process subsequent to purification, e.g. in a cyclone.
[0006] In the above reference from SERI (page 39, right column), it is stated that an advantage
of the stratified downdraft gasifier having a cylindrical shape is that it allows
for a continuous flow of troublesome biofuel without causing bridging or channelling.
Moreover, in connection with bridging it is mentioned in item 5.7.2 (page 34, right
column) disclosing the conventional Imbert gasifier having a closed top that bridging
and channelling caused by restricting the passage of the fuel at the hearth render
high tar output as unpyrolysed biomass drops down into the reaction zone (the reduction
zone).
[0007] DK 172 277 discloses a stratified gasifier in which in addition to the primary gas
fed at the top of the gasifier, secondary gas is fed into the pyrolysis zone so as
to assist a partial combustion of the pyrolysis gas, a so-called flaming pyrolysis.
As in the stratified downdraft gasifier described above, the gasification process
in said known gasifier includes at this stage a drying zone, a pyrolysis zone passing
into a pyrolysis and combustion zone (flaming pyrolysis), a reduction zone and an
inactive charcoal zone. The course of the process is stabilised and controlled indirectly
by means of an annular hearth made of a fireproof material to ensure a stable transition
between the pyrolysis zone and the flaming pyrolysis in the continuous process. However,
while this measure improves the stability, it does not however provide sufficient
stability, a temperature level detector with several temperature level areas also
being provided as a basis for a complicated temperature adjustment by possibly cooling
by injecting water and adjusting the primary air and/or secondary air.
Brief description of the invention
[0008] It has now been found that the process in a downdraft gasifier for gasification of
solid, moderately flowable biofuel can be stabilised in a more simple manner by designing
the heath surrounding the flaming pyrolysis zone in such a manner that the moderately
flowable biofuel provides bridging across the opening of the heath.
[0009] The object of the present invention is to provide a process and a gasifier apparatus
for continuous downdraft gasification of solid, moderately flowable biofuel rendering
good stability regarding the process and maintenance of the position of the individual
process zones without a complicated temperature adjustment based on measurements on
several levels.
[0010] This is obtained by a process of the type mentioned above which is characterised
by retaining the fuel material by means of the narrowed portion in the flaming pyrolysis
(c) through bridging across the opening or openings of the narrowed portion until
the material as a result of the partial combustion has been converted into a material
having such a flowability that it resumes the downward movement towards the reduction
zone.
[0011] Furthermore, the invention relates to a downdraft gasifier having an open core and
a fixed gasification bed for the gasification of a solid, moderately flowable biofuel,
said gasifier including a reactor having an opening at the top for feeding fuel material
and primary air in the following sequence from the top and downward to a) a drying
zone, b) a pyrolysis zone, c) a combustion zone involving flaming pyrolysis and having
an opening for feeding secondary air and a narrowed portion in the inner cross section
of the gasifier, d) a reduction zone, and e) optionally an inactive charcoal zone
and a collecting compartment at the bottom of the gasifier having means for removing
ash and the fuel gas resulting from the gasification, respectively, said gasifier
being characterised in that the narrowed portion in the flaming pyrolysis zone (c)
is a hearth having one or more openings dimensioned relative to the biofuel used in
such a manner that the fuel material is retained through bridging across the opening
or openings of the hearth until the material as a result of the partial combustion
has been converted into a material having such a flowability that it resumes the downward
movement towards the reduction zone.
[0012] In addition, the invention relates to a plant for heat production or a combined heat
and power production including the downdraft gasifier.
[0013] The process and the gasifier according to the present invention are based on the
surprising recognition that bridging at the narrowed portion in the cross section
of the hearth is advantageous. Hitherto, such a bridging has been considered an undesired
feature.
[0014] In the present invention, the change of the physical properties of the fuel material
during the course of the pyrolysis process is utilised. Thus the starting material
typically comprises relatively large and irregular pieces of material which are easily
entangled and which form a bridge at narrowed portions in the downward path of the
pieces of material. In the present description and claims, said material is referred
to as a "moderately flowable material". During the pyrolysis process said material
is converted into a more fine-grained charcoal having a more uniform grain shape resulting
in improved flowability and a reduced bridging tendency.
[0015] By utilising this principle, the passage of the material from the flaming pyrolysis
zone to the reduction zone, which occurs in free fall, is controlled on the basis
of the conversion of the material to flowable charcoal. As a result the vital part
of the process becomes self-adjusting and it has turned out that the complicated temperature
adjustment based on temperature measurements on several levels, as described in DK
172 277, becomes superfluous.
[0016] In the process according to the invention, the gasified biomass or biofuel can in
principle be any combustible, organic material existing as solid, relatively small
pieces of material of suitably uniform sizes such that the flowability of the material
can be determined in a sufficiently reliable manner. Examples of such materials are
various forms of comminute wood in suitable pieces, such as wood chips, shavings,
saw dust and wood pellets, which can be made by compressions of comminute wood and
saw dust.
[0017] The extent of applicability of the invention appears from the following detailed
description. It should, however, be understood that the detailed description and the
specific examples are merely included to illustrate the preferred embodiments, and
that various alterations and modifications within the scope of protection will be
obvious to persons skilled in the art on the basis of the detailed description.
Brief description of the drawings
[0018] The invention is explained in greater detail below with reference to the accompanying
drawings, in which
Fig. 1 is a sectional view through a gasifier according to the invention, and
Fig. 2 is a sectional view through an embodiment of a hearth in form of a conical
inlet directing sufficiently flowable material down into a cylindrical channel.
Detailed description of the invention
[0019] As shown in Fig. 1, a gasifier is designed as a reactor having an upper cylindrical
section 14 with a subjacent collecting compartment 13.
[0020] The cylindrical section has an open top in which the fuel can be fed through an inlet
pipe 9. An inlet 11 for primary air is also positioned at the top. In continuation
of the inlet pipe 9, a burner tube 1 having a slightly larger sectional area than
the inlet pipe 9 is provided. The burner tube 1 ends immediately above a hearth 2
of a heat-stable material and is designed such that the sectional area is narrowed
to such an extent that bridging causes the fuel material to be retained.
[0021] In the upper portion of the collecting compartment 13, at a distance below the hearth
2, a grate 3 is provided and means 4 and 8 for collecting and removing ash through
a gate 5 are provided below said grate. The product gas generated enters through the
grate 3 and is collected in the collecting compartment 13 from where it is recovered
through pipe 7 which is led upwards through the cylindrical section and on to be used
as fuel gas.
[0022] Immediately subjacent to the burner tube 1 and above the hearth 2 a circular inlet
12 for secondary air is provided. In a preferred embodiment, an inlet 6 for a gasification
agent may be arranged at a distance below the narrowed portion of the hearth 2.
[0023] During use of the gasifier, fuel is added at the top of the usually vertical inlet
pipe 9. From this position the fuel passes downward into the inlet of the burner tube
1 having a slightly larger sectional area than the inlet pipe.
[0024] The burner tube 1 ends immediately above a narrowed portion of the sectional area
which causes bridging. The bridging prevents the downward flow of the fuel material.
The narrowed portion, which hereafter is referred to as a hearth 2, includes a material
resistant to high temperatures, which for instance can be made of fire resistant bricks
or brickwork.
[0025] The burner tube 1 and the hearth 2 typically each have a circular, horizontal cross
section but also other suitable geometrical shapes are possible. Particularly the
hearth 2 may have different shapes. It is only vital that the inner cavity of the
hearth is downwardly narrowed having dimensions enabling bridging of the selected
fuel. Thus in addition to providing the heath with a circular, horizontal cross section,
the hearth may be provided with a rectangular or square horizontal cross section.
The hearth can also be provided with several openings such that the hearth for instance
has the appearance of a grate with several openings, each dimensioned so as to enable
bridging across each opening. Similarly, a hearth provided with several circularly
or squarely shaped openings is also possible, each opening being dimensioned so as
to enable bridging across said opening.
[0026] The inner cavity of the hearth is confined by its inner wall or walls. The number
and shape of the wall or walls can vary,
inter alia in relation to the degree of planeness/curvature and the angle in relation to vertical.
As mentioned above it is important that the inner cavity of the hearth is downwardly
narrowed to enable bridging. The dimensions of the hearth are also to be adjusted
to the dimensions of the fuel used in order to enable bridging. It is also preferred
that the height of the hearth is such that the heating zone relevant remains substantially
enclosed by the walls of the hearth.
[0027] The amount of fuel added is controlled by building-up and maintaining a certain level
of fuel in the burner tube. The fuel level is detected by a sensor, not shown, based
on for instance a mechanical or electronic function. The signal of the sensor is used
for controlling the fuel supply.
[0028] In addition to the feeding of fuel at the top of the burner tube, primary air, preferably
pre-heated, is fed at the same location through the inlet 11 concurrent to the movement
of the fuel. In the upper portion of the burner tube where solid, moderately flowable
biofuel is in a free fall, the primary air passes through the pipe downward at uniform
speed and is distributed over the entire cross section.
[0029] The air then continues downward inside the burner tube through a first zone of the
fixed and yet unreacted biofuel being retained on top of the hearth 2 due to its limited
flowability. The biofuel is dried during the passage of air in the first zone.
[0030] At the end of the first zone, the biofuel is sufficiently dry to cause its temperature
to rise. In the initial stage of the second zone, the biofuel reacts with air in flaming
pyrolysis. In this second zone, volatile gases, wood oils and tar compounds are expelled
from the biofuel and bum during the formation of CO
2 and H
2O, thus supplying heat to the pyrolysis process. It is preferable that the flaming
pyrolysis in the second zone is optimally concentrated immediately below the mouth
of the burner tube 1. In order to ensure that a sufficient amount of air is present
at this position in the zone to generate heat for maintaining the pyrolysis process
in this position, additional, usually pre-heated-air, referred to as secondary air,
may be supplied through a variable valve 10 through the inlet 12 on the outer face
of the burner tube 1. This secondary air enters directly into the zone involving the
flaming pyrolysis below the mouth of the burner tube.
[0031] In the last part of the second zone, when the oxygen has been consumed, H
2 and CO are formed during continuous pyrolysis and the biofuel is converted into charcoal.
[0032] The charcoal formed has obtained such a flowability that it partly due to gravity
and partly by way of the downward flow of gas leaves its position on top of the hearth
2 and falls through the vertical opening of the hearth and ends on a subjacent grate
3 forming a third zone.
[0033] In the third zone the process is an adiabatic charcoal gasification process where
the hot gases formed during the pyrolysis are led down through the charcoal layer,
where the gas reacts with the charcoal under the formation of additional CO and H
2. The reactions converting the solid charcoal to CO and H
2 in this zone are endothermic processes consuming heat from the gas per se.
[0034] The heat of the gas and of the solid charcoal is thus converted into chemically bound
energy in the gas generated by the fuel, in the following referred to as product gas,
the temperature dropping to approx. 800 °C. Below this temperature the process progresses
very slowly. Finally, the remaining ash and the unreacted charcoal drop down through
slits and/or holes in and between elements making up the grate. The ash is collected
in a mechanical ash conveyance system 4 and 8 leading the ash to the surroundings
through a gas-proof gate 5. In Fig. 1 the gas-proof gate is designed as a water trap
5.
[0035] The product gas generated is also recovered through the bottom grate 3 to the compartment
13 via the ash conveyance system and is then led out of the compartment through the
pipe 7. The chemically bound energy of the product gas can subsequently to purification
in for instance a cyclone be utilised in a gas combustion process.
The hearth structure
[0036] A correct dimensioning of the hearth is vital in order for the hearth to control
the transition of the fuel from the second zone to the third zone such that the vertical
position of the zones remain stable.
[0037] When dimensioning the hearth it should be ensured that its shape enables an advantageous
bridging of the non-charred, moderately flowable fuel across the hearth. In order
to obtain this effect, the characteristic physical properties of the particular fuel
determining flowability of the fuel should be considered. The relevant properties
which can be described as physically measurable parameters, are
inter alia:
a) The bridging tendency of the fuel
b) The angle of the fuel in relation to a horizontal base when the fuel runs out through
a hole in the bottom of a flat-bottomed vessel after the fuel flow has ceased. This
angle is here referred to as the angle of repose.
c) The friction of the fuel against different bases which may support or abut the
fuel, e.g. fire-proof materials.
[0038] Methods of determining the tendency for bridging of different wood fuel types are
described in
inter alia "Trådbränslens hanteringsegenskaber - benägenhet til valvbinding för olika sortiment.
Jan Erik Mattson. Sveriges Lantbruksuniversitet. Report No 181, Garpenberg 1989" in
which also results of measurements of bridging of different wood fuels can be found.
[0039] "Trådbränslens hanteringsegenskaber - rasvinkler for olika sortiment. Jan Erik Mattson,
Sveriges Lantbruksuniversitet. Report No 179 Garpenberg 1989" mentions different definitions
and methods of determining the angle of repose of different wood fuel types. Results
of measurements of angles of repose based on one of the methods can also be found
therein.
[0040] "Trådbränslens hanteringsegenskaber - friktion mellan olika underlag och bränslesortiment.
Jan Erik Mattsson. Sveriges Lantbruksuniversitet. Report No 180 Garpenberg 1989" discloses
results of measurements of friction for different types of wood fuel against different
bases.
[0041] In the embodiment shown in Fig. 2 the hearth is shaped as a conical inlet 20 of a
truncated cone-shape passing the fuel vertically downward into a cylindrical channel
21 allowing charcoal and pyrolysis gases to continue down into the reduction zone.
[0042] The truncated cone forms a characteristic angle a in relation to the horizontal plane
and opens into a characteristic diameter D shown in Fig. 2.
[0043] When dimensioning the hearth, the diameter D is chosen in relation to the bridging
tendency of the fuel and the charcoal. The dimension is thus determined to effect
bridging of the unreacted, moderately flowable fuel, but not of the subsequently formed
charcoal having improved flowability.
[0044] Similarly, the angle a is determined by the angle of repose and friction of the fuel
against the base. The dimension a has to ensure that the unreacted fuel does not fall
down, until after being converted to charcoal.
Partial oxidation
[0045] According to a preferred embodiment of the process according to the invention the
fuel material passes between the combustion zone (c) and the reduction zone (d) through
a partial oxidation zone involving partial oxidation of gas and tar substances. This
is ensured by adding a gasification agent through the inlets 6.
[0046] According to this embodiment, the gasifier is thus provided with an additional option
for adding gasification agents through the hearth via the inlets 6. The gasification
agent serves to produce a much more pure gas and to further stabilise the zones. The
added gasification agent can be air, water vapour, CO
2 or any mixtures thereof. During operation the air intake can be adjusted continuously
to the desired amount by means of a valve (not shown) or the air intake can be completely
shut-off at this point.
[0047] The additional supply of gasification agent occurs in the transition between the
second zone and third zone inside the hearth, where the charcoal passes downward to
the grate 3 in free fall. This third supply of gasification agent, e.g. air, occurs
via a set of channels 6 ending into the vertical gas channel 21 in the hearth, through
which gas and charcoal drop from the second zone to the_third zone. As a result a
zone involving partial oxidation is established at this position, said zone in the
following being referred to as zone 2½.
[0048] The air supplied in zone 2½ is mixed with the downwardly flowing pyrolysis gases
over the entire cross-section of the hearth and thereby causing a partial combustion
of the pyrolysis gases. The partial oxidation causes the majority of the formed tar
substances from the pyrolysis zone to decompose and the temperature of the product
gas to rise to above 1000 °C in this zone. By supplying air to a position in which
the fuel is in a free fall the unpyrolysed material is prevented from moving from
the conical inlet of the hearth 20 and down to the reduction zone without having been
into contact with oxygen. Thus, the air intake in zone 2½ causes a significant rise
in the temperature of the product gas, whereby the charcoal conversion is increased
in the subsequent third zone, wherein the reactions are endothermic.
[0049] Tests in which the process and gasifier according to the invention have been used,
have shown that a surprisingly good stability is obtained even when a comparatively
small gasifier is used. This is especially surprising when considering that complicated
control of the temperature, primary air and secondary air based on temperature measurements
on several levels have been necessary in previous processes.
[0050] Without restricting the invention to a particular theoretical explanation, a hypothetical
explanation of the stability obtained by designing the hearth in accordance with the
present invention is provided below.
[0051] In order to ensure a stable process during operation it is required that the conversion
of charcoal in the third zone (the reduction zone) proceeds at the same speed as the
production of charcoal in the flaming pyrolysis zone. Preferably the height of the
charcoal in the third zone is as constant as possible.
[0052] A number of parameters determine the conversion process in the reduction zone
inter alia: the insulation of the gasifier, the water content in the fuel, the gas temperature
prior to the reduction zone.
[0053] If the charcoal is consumed more rapidly during the reduction processes than the
production of charcoal in the flaming pyrolysis zone, the charcoal height decreases.
[0054] Within a period of time all the charcoal will be completely consumed and the reduction
zone completely gone. By not providing a hearth to effect bridging, the flaming pyrolysis
zone moves downward toward the grate and the flames bum through the holes and channels
of the grate. As a result the pyrolysis gases, having a low burning value and containing
large quantities of tar substances, are passed directly out of the gasifier, which
is not desirable.
[0055] If on the contrary the charcoal production in the pyrolysis zone exceeds the charcoal
conversion in the reduction zone, it will result in increased charcoal height in the
reduction zone.
[0056] Without the provision of a hearth, the flaming pyrolysis zone is then pushed upwards
resulting in a rise of the other zones. This would probably result in the flaming
pyrolysis zone being "squeezed" in between the drying zone, which is to be completed
before the flaming pyrolysis zone can commence, and the reduction zone.
[0057] At varying loads of the gasifier and at varying fuels parameters, the positions of
the zones are expected to move upwards and downwards. This is prevented by the hearth
interrupting the connection between the zones to prevent them from affecting each
other.
[0058] According to the preferred embodiment, in which a zone involving partial oxidation,
zone 2½, is established, the fuel height may be kept constant by means of the hearth
and by supplying a correct amount of air to zone 2½ as well as by adjusting the physical
dimensions of the drying zone, pyrolysis zone and combustion zone in relation to the
physical dimensioning of the reduction zone.
[0059] Initially, the hearth, the flaming pyrolysis zone and the reduction zones are dimensioned
in order to provide a state in which the charcoal height in the reduction zone has
a tendency to rise slowly in time.
[0060] However, during operation the air intake in the partial oxidation zone causes the
top of the charcoal layer to be burned in the flames of the partial oxidation zone,
when the charcoal top approaches the position of the air nozzles in the hearth. If
the charcoal nevertheless becomes up-close to the air nozzles, it is assumed that
the air not only reacts with the gas and the tar substances but also begins to react
directly with the charcoal in a combustion process thereby producing CO
2 and H
2O as well as heat.
[0061] As a result the increase in the charcoal height is slowed down prior to reaching
the point at which it abuts the flaming pyrolysis zone taking place in the bridging
on the hearth. Thus the hearth and the air intake have two essential functions, i.e.
to act as a physical barrier between the two zones and to establish a free space -
a freeboard - for the burning in the partial oxidation zone, where gas and tar substances
are reacted.
Example
[0062] In the present example, a gasifier is used as shown in Figs. 1 and 2. The diameter
of the burner tube 1 is 350 mm. The hearth 2 is made of a fireproof ceramic material.
The angle of the truncated cone a is 30 ° and the diameter D of the cylindrical channel
21 is 100 mm.
[0063] The fuel used is dried whole-tree chips of a conventional quality, where the largest
dimensions of the individual chips range between 10 and 50 mm. During the test, the
fuel height is set at approx. 500 mm measured from the lower end of the burner tube
1.
[0064] A test has been carried out to assess the stability of the gasification process.
Thus, it was examined whether it would be possible to operate continuously for 100
hours which was achieved in the first test. During the test, an engine, in which the
gas produced was used as fuel, was able to run continuously for 84 hours. The added
chip amount was 40 kg per hour and a product gas was produced having a burning value
of 4.3 MJ/m
3n (dry gas) in an amount of 92 m
3n per hour (dry gas).
[0065] The stability of the stratified process was confirmed by means of temperature detectors.
At different times during the test, low dust and tar values were measured in the raw
product gas. The measured values appear from the table below.
Measurement period (hours after start) |
dust in raw product gas (mg/m3n) |
tar in raw product gas (mg/m3n) |
28.5-29.5 |
71 |
57 |
32.5-33.5 |
158 |
111 |
81-82 |
170 |
130 |
98.75-99.75 |
142 |
162 |
[0066] The above description of the invention reveals that it is obvious that it can be
varied in many ways. Such variations are not to be considered a deviation from the
scope of the invention, and all such modifications which are obvious to persons skilled
in the art are also to be considered comprised by the scope of the succeeding claims.
1. A process for gasification of solid, moderately flowable biofuel in a downdraft gasifier
having an open core and a fixed gasification bed wherein the fuel material and primary
air are fed at the top so as from the top and downward to pass
a) a drying zone
b) a pyrolysis zone
c) a combustion zone involving flaming pyrolysis to which secondary air is fed, and
where the fuel is supported by way of a narrowed portion in the inner cross section
of the gasifier,
d) a reduction zone, and
e) optionally an inactive charcoal zone,
characterised by retaining the fuel material by means of the narrowed portion in the flaming pyrolysis
zone (c) through bridging across the opening or openings of the narrowed portion until
the material as a result of the partial combustion has been converted into a material
in the form of charcoal, having such a flowability that it resumes the downward movement
towards the reduction zone, whereby a separation of the combustion zone and the reduction
zone is ensured, the reduction zone being located under the narrowed portion in the
inner cross section of the gasifier.
2. A process according to claim 1, characterised in that the fuel material between the combustion zone (c) and the reduction zone (d) passes
a partial oxidation zone involving a partial oxidation of gas and tar substances,
and where a gasification agent is added in the oxidation zone.
3. A process according to claim 2, characterised in that the gasification agent is oxygen, water vapour, CO2 or a mixture thereof.
4. A process according to claim 1, characterised in that the fuel material is a combustible organic material in form of solid, relatively
small pieces of material of suitably uniform sizes.
5. A process according to claim 4, characterised in that the fuel material is wood chips, shavings, saw dust or compressed wood pellets.
6. A downdraft gasifier having an open core and a fixed gasification bed for the gasification
of a solid, moderately flowable biofuel, said gasifier including a reactor having
an opening at the top for feeding fuel material and primary air in the following sequence
from the top and downward to
a) a drying zone
b) a pyrolysis zone
c) a combustion zone involving flaming pyrolysis and having an opening for feeding
secondary air and a narrowed portion in the inner cross section of the gasifier,
d) a reduction zone, and
e) optionally an inactive charcoal zone,
and a collecting compartment at the bottom of the gasifier having means for removing
ash and the fuel gas resulting from the gasification, respectively,
characterised in that the narrowed portion in the flaming pyrolysis zone (c) is a hearth having one or
more openings dimensioned relative to the biofuel used in such a manner that the fuel
material is retained through bridging across the opening or openings of the hearth
until the material as a result of the partial combustion has been converted into a
material in the form of charcoal, having such a flowability that it resumes the downward
movement towards the reduction zone, whereby a separation of the combustion zone and
the reduction zone is ensued, the reduction zone being located under the narrowed
portion in the inner cross section of the gasifier.
7. A downdraft gasifier according to claim 6, characterised in that an opening is provided after the narrowed portion in the flaming pyrolysis zone (c)
and before the reduction zone (d) for feeding the gasification agents.
8. A downdraft gasifier according to any of the claims 6 to 7, characterised in that the hearth has a single opening formed as a truncated cone downwardly ending in a
cylindrical channel or that the hearth has a single opening formed as a slot downwardly
ending in a rectangular channel.
9. A downdraft gasifier according to any of the claims 6 to 7, characterised in that the hearth has more than one opening.
10. A downdraft gasifier according to claim 9, characterised in that the openings of the hearth are formed as a grate or in that the hearth is provided with openings of a circular or square horizontal cross section.
11. A plant for heat or a combined heat and power production including a downdraft gasifier
according to any of the claims 6-10.
1. Verfahren zum Vergasen eines festen, mäßig fließfähigen Biokraftstoffs in einem Fallstromvergaser
("downdraft gasifier") mit einem offenen Kern und einem fixierten Vergasungsbett,
wobei das Kraftstoffmaterial und Primärluft oben zugeführt werden, um von oben nach
unten zu durchlaufen
a) eine Trocknungszone
b) eine Pyrolysezone
c) eine Verbrennungszone mit Flammpyrolyse, der Sekundärluft zugeführt wird, und wobei
der Kraftstoff mittels eines verengten Teils im inneren Querschnitt des Vergasers
getragen wird,
d) eine Reduktionszone und
e) fakultativ eine inaktive Holzkohlenzone,
dadurch gekennzeichnet, daß das Kraftstoffmaterial mittels des verengten Teils in der Flammpyrolysenzone (c)
durch Überbrückung zwischen der Öffnung oder den Öffnungen des verengten Teils gehalten
wird, bis das Material als ein Ergebnis der partiellen Verbrennung in ein Material
in der Form von Holzkohle mit einer solchen Fließfähigkeit umgewandelt worden ist,
daß es die Bewegung nach unten zur Reduktionszone hin aufnimmt, wobei eine Trennung
der Verbrennungszone und der Reduktionszone sichergestellt ist, wobei sich die Reduktionszone
unterhalb des verengten Teils im inneren Querschnitt des Vergasers befindet.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Kraftstoffmaterial zwischen der Verbrennungszone (c) und der Reduktionszone (d)
eine partielle Oxidationszone durchläuft, die eine partielle Oxidation von Gas- und
Teersubstanzen beinhaltet, und wobei ein Vergasungsmittel in der Oxidationszone zugegeben
wird.
3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß das Vergasungsmittel Sauerstoff, Wasserdampf, CO2 oder eine Mischung davon ist.
4. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Kraftstoffmaterial ein brennbares organisches Material in Form von festen relativ
kleinen Stücken von Material mit geeignet uniformen Größen ist.
5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß das Kraftstoffmaterial Holzscheibchen ("wood chips"), Späne, Sägestaub oder komprimierte
Holzpellets ist.
6. Fallstromvergaser ("downdraft gasifier") mit einem offenen Kern und einem fixierten
Vergasungsbett zum Vergasen eines festen, mäßig fließfähigen Biokraftstoffs, wobei
besagter Vergaser einen Reaktor mit einer Öffnung oben zum Zuführen von Kraftstoffmaterial
und Primärluft in der folgenden Abfolge von oben nach unten
a) einer Trocknungszone
b) einer Pyrolysezone
c) einer Verbrennungszone mit Flammpyrolyse und mit einer Öffnung zum Zuführen von
Sekundärluft und einem verengten Teil im inneren Querschnitt des Vergasers
d) einer Reduktionszone und
e) fakultativ einer inaktiven Holzkohlenzone
und ein Sammelabteil am Boden des Vergasers mit Mitteln zum Entfernen von Asche bzw.
des Kraftstoffgases beinhaltet, das aus der Vergasung resultiert,
dadurch gekennzeichnet, daß der verengte Teil in der Flammpyrolysenzone (c) ein Herd mit einer oder mehreren
Öffnungen ist, die im Verhältnis zu dem verwendeten Biokraftstoff auf eine solche
Weise dimensioniert sind, daß das Kraftstoffmaterial durch eine Überbrückung zwischen
der Öffnung oder den Öffnungen des Herdes zurückgehalten wird, bis das Material als
ein Ergebnis der partiellen Verbrennung in ein Material in der Form von Holzkohle
mit einer solchen Fließfähigkeit umgewandelt worden ist, daß es die Bewegung nach
unten zur Reduktionszone hin aufnimmt, wobei eine Trennung der Verbrennungszone und
der Reduktionszone sichergestellt ist, wobei sich die Reduktionszone unterhalb des
verengten Teils im inneren Querschnitt des Vergasers befindet.
7. Fallstromvergaser gemäß Anspruch 6, dadurch gekennzeichnet, daß eine Öffnung hinter dem verengten Teil in der Flammpyrolysenzone (c) und vor der
Reduktionszone (d) zum Zuführen der Vergasungsmittel bereitgestellt ist.
8. Fallstromvergaser nach einem der Ansprüche 6 bis 7, dadurch gekennzeichnet, daß der Herd eine einzelne Öffnung hat, die als ein trunkierter Konus gebildet ist, der
nach unten in einem zylindrischen Kanal endet, oder daß der Herd eine einzelne Öffnung
hat, die als ein Schlitz gebildet ist, der nach unten in einem rechtwinkligen Kanal
endet.
9. Fallstromvergaser nach einem der Ansprüche 6 bis 7, dadurch gekennzeichnet, daß der Herd mehr als eine Öffnung hat.
10. Fallstromvergaser nach Anspruch 9, dadurch gekennzeichnet, daß die Öffnungen des Herdes als ein Gitter gebildet sind, oder daß der Herd mit Öffnungen
mit einem zirkulären oder rechtwinkligen horizontalen Querschnitt ausgestattet ist.
11. Anlage zur Erzeugung von Wärme oder Wärme und Energie in Kombination, einschließlich
eines Fallstromvergasers nach einem der Ansprüche 6-10.
1. Procédé de gazéification d'un biocombustible solide, ayant une aptitude modérée à
l'écoulement, dans un gazéifieur à tirage par en bas, ayant un coeur ouvert et un
lit de gazéification fixe, en tête duquel sont introduits le matériau combustible
et de l'air principal, de manière qu'ils traversent, du haut vers le bas,
a) une zone de séchage,
b) une zone de pyrolyse,
c) une zone de combustion mettant en jeu une pyrolyse à la flamme, dans laquelle est
introduit un air secondaire, et dans laquelle le combustible est porté au moyen d'une
partie rétrécie formée dans la section transversale interne du gazéifieur,
d) une zone de réduction, et
e) éventuellement, une zone de charbon inactif,
caractérisé par l'opération qui consiste à retenir le matériau combustible dans la zone de pyrolyse
à la flamme (c), grâce à la partie rétrécie, à travers un pont formé au-dessus de
la ou des ouvertures de la partie rétrécie, jusqu'à ce que le matériau ait été converti,
en conséquence de la combustion partielle, en un matériau se présentant sous la forme
de charbon, ayant une aptitude à l'écoulement telle qu'il reprend son chemin vers
le bas, vers la zone de réduction, ce qui assure une séparation entre la zone de combustion
et la zone de réduction, la zone de réduction étant située en dessous de la partie
rétrécie formée dans la section transversale interne du gazéifieur.
2. Procédé selon la revendication 1, caractérisé en ce que le matériau combustible, entre la zone de combustion (c) et la zone de réduction
(d), traverse une zone d'oxydation partielle mettant en jeu une oxydation partielle
de substances gazeuses et du type goudron, zone d'oxydation dans laquelle est ajouté
un agent de gazéification.
3. Procédé selon la revendication 2, caractérisé en ce que l'agent de gazéification est de l'oxygène, de la vapeur d'eau, du CO2 ou un mélange de ceux-ci.
4. Procédé selon la revendication 1, caractérisé en ce que le matériau combustible est un matériau organique combustible se présentant sous
la forme de morceaux de matériaux solides, relativement petits, d'une taille uniforme
appropriée.
5. Procédé selon la revendication 4, caractérisé en ce que le matériau combustible comprend des particules de bois, des copeaux, de la sciure
ou des pastilles de bois densifiées.
6. Gazéifieur à tirage par en bas, ayant un coeur ouvert et un lit de gazéification fixe,
pour la gazéification d'un biocombustible solide, ayant une aptitude modérée à l'écoulement,
ledit gazéifieur comprenant un réacteur en tête duquel est aménagée une ouverture
pour l'alimentation en matériau combustible et en air principal, dans la séquence
suivante du haut vers le bas,
a) d'une zone de séchage,
b) d'une zone de pyrolyse,
c) d'une zone de combustion mettant en jeu une pyrolyse à la flamme, et ayant une
ouverture pour l'introduction d' air secondaire et une partie rétrécie formée dans
la section transversale interne du gazéifieur,
d) d'une zone de réduction, et
e) éventuellement, d'une zone de charbon inactif,
et un compartiment de récupération, en queue du gazéifieur, ayant des moyens servant
à retirer respectivement la cendre et le gaz combustible résultant de la gazéification,
caractérisé en ce que la partie rétrécie dans la zone de pyrolyse à la flamme (c) est un creuset ayant
une ou plusieurs ouvertures dimensionnées en fonction du biocombustible utilisé, de
telle sorte que le matériau combustible soit retenu à travers un pont formé au-dessus
de la ou des ouvertures du creuset, jusqu'à ce que le matériau ait été converti, en
conséquence de la combustion partielle, en un matériau se présentant sous la forme
de charbon, ayant une aptitude à l'écoulement telle qu'il reprend son chemin vers
le bas, vers la zone de réduction, ce qui assure une séparation entre la zone de combustion
et la zone de réduction, la zone de réduction étant située en dessous de la partie
rétrécie formée dans la section transversale interne du gazéifieur.
7. Gazéifieur à tirage par en bas selon la revendication 6, caractérisé en ce qu'une ouverture est aménagée après la partie rétrécie dans la zone de pyrolyse à la
flamme (c) et avant la zone de réduction (d), pour l'introduction des agents de gazéification.
8. Gazéifieur à tirage par en bas selon l'une quelconque des revendications 6 ou 7, caractérisé en ce que le creuset a une seule ouverture ayant la forme d'un tronc de cône se terminant vers
le bas en un canal cylindrique, ou en ce que le creuset a une seule ouverture ayant la forme d'une fente se terminant vers le
bas en un canal rectangulaire.
9. Gazéifieur à tirage par en bas selon l'une quelconque des revendications 6 ou 7, caractérisé en ce que le creuset a plus d'une ouverture.
10. Gazéifieur à tirage par en bas selon la revendication 9, caractérisé en ce que les ouvertures du creuset ont la forme d'une grille, ou en ce que le creuset est pourvu d'ouvertures de section transversale horizontale circulaire
ou carrée.
11. Installation pour la production de chaleur ou de chaleur et de puissance combinées,
incluant un gazéifieur à tirage par en bas selon l'une quelconque des revendications
6 à 10.