Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02650979 2008-10-30
METHOD FOR PROCESSING CONDENSED FUEL BY GASIFICATION AND A
DEVICE FOR CARRYING OUT SAID METHOD
FIELD OF THE INVENTION
The present invention generally relates to methods for processing condensed
fuels,
including solid fuel wastes, by pyrolysis and by gasifying the organic
constituents thereof in a
dense layer to produce pyrolysis products and gas fuel used for power
generation. The problem
of processing low-grade condensed fuels, including solid domestic waste (SDW),
coals, oil-
slime, biologic mass, is rather urgent because conventional methods of
disposal/processing
thereof suffer largely from being uneconomical and environment-unfriendly.
Considerable
advantages are provided by gasifying the condensed fuels which makes it
possible to employ
advanced power generation techniques with ecologically pure gas emissions.
BACKGROUND OF THE INVENTION
Various processes have been developed heretobefore for processing condensed
fuels in
the combustion regime to produce fuel gas, based on successive layerwise
gasification of solid
organic fuels in countercurrent flow to an oxidizing gas in shaft kilns. A
scheme of such type
designed for processing pyroshale is described in US-A-2,796,390 (Elliot) and
US-A-2,798,032
(Martin et al.).
A scheme of gasifying solid organic fuels in countercurrent flow to a
gasifying agent can
be generally described as follows. An oxygen-containing gasifying agent which
possibly
containing water and/or carbon dioxide are supplied into a combustion zone
where oxygen reacts
with carbon in a solid fuel in the form of coke or semi-coke at temperatures
about 900-1500 C.
The gasifying agent is admitted into the reactor in countercurrent flow to the
fuel so that at least
part of the oxidizing gas is preliminary passed through a layer of hot solid
combustion products
that are already free from carbon. In this zone the solid combustion products
are cooled and the
gasifying agent is heated prior to entering the combustion zone. In the
combustion zone, free
oxygen of the gasifying agent is completely consumed and hot gaseous
combustion products
including carbon dioxide and steam enter the next solid fuel layer, which is
referred to as a
reduction zone, where carbon dioxide and steam chemically react with carbon in
fuel to produce
combustible gases. Heat energy of the gases heated in the combustion zone is
partially consumed
in the reduction reactions. Gas flow temperature decreases as the gas flows
through the solid fuel
and gives its heat to the latter. The fuel heated in the absence of oxygen is
subjected to pyrolysis.
As the result, coke, pyrolysis tars and fuel gases are produced. Gaseous
products are passed
through a fresh fuel feed to cool the gas and to heat the fuel and reduce its
moisture content. And
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finally, the gaseous product (also referred to as gas product) containing
hydrocarbon vapors,
steam and pyrolysis tars is withdrawn for subsequent use.
This scheme has various known applications. RU-2-062284 (Manelis et al.)
discloses
processing worn automobile tires; RU-2079051 and RU-2152561 (Manelis et al.)
disclose
processing solid domestic waste, oil-slime and similar oil waste. In the
latter case, solid lump
inert material is introduced into a reactor along with the processed fuel to
provide, in particular,
uniform gas permeability of the feed in the reactor. Kiln gas, preferably
mixed with air, is used
as a gasifying agent, the kiln gas portion in the gasifying agent being
increased when the
temperature in the combustion zone exceeds 1300 C, and reduced when the
temperature in the
combustion zone falls below 800 C.
At the same time, an important problem stays unresolved - the providing of
stable
combustion behavior when the processed feed material is being gasified. As the
processed
materials often have nonuniform gas permeability and to form cakes at
pyrolysis, the pyrolysis
and gasification front may propagate over the reactor section unevenly.
"Burnouts" can appear in
the processed feed layer, through which mainly gas flow passes, materials fall
in cavities formed
in combustion, and at the same time substantially gas-impermeable regions can
form. As a
consequence, temperature distribution in the combustion zone becomes
nonuniform and poorly
controlled.
To provide uniform propagation of combustion zones throughout the feed
material a
method of gasifying wastes is proposed in US 4,732,091 (Gould). The method
comprises
introducing a solid fuel into an upper section of a shaft kiln. The fuel is
caused to move at a
controlled rate through a series of chambers horizontally separated by
moveable grates, where
the fuel is pyrolyzed and burned in countercurrent flow of a vapor/air
gasifying agent. The
method includes loosening the waste in the course of treatment to provide
uniform gas
permeability and uniform movement of the processed feed material successively
to drying,
pyrolysis, gasification and cooling zones. A method is also proposed for
controlling the fuel
entry into respective zones. However, the prior art method suffers largely
from the presence of
moveable grates. The moveable grates will be inevitably worn fast in high-
temperature zones.
Furthermore, particles of dust and tars will deposit on the moveable units of
the reactor and
disturb its maintenance.
Rotary kilns are also widely known. They are extensively used for burning
cement and
combustion of waste. Kiln rotation provides uniform mixing of the material
processed. A rotary
kiln is known to be used for gasification process, e.g. as taught in US
247,322 issued 30
September, 1881. Application US-2005051066 discloses a method of gasifying a
solid fuel in a
parallel gas/solid flow using a rotary kiln. US 3,990,865 (Cybriwsky A. &
Petersen G.) discloses
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a gasification process carried out in a slightly tilted rotary kiln having a
raw material inlet end
arranged higher than the outlet end. Solid carbon-containing material is
continuously fed into the
rotary kiln. In the kiln, the material which is introduced at a temperature
below 600 F (315 C)
passes through pre-heating and devolatilizing zones and is gradually heated to
1600 F (871 C);
at this temperature the material loses its caking tendency and then enters a
gasifying zone, where
a vapor-containing oxidizing medium is admitted under the mixed layer, this
resulting in
formation of combustible gases containing hydrocarbon that are removed from
the kiln side
where the fuel is fed.
As the kiln rotates, the processed feed is efficiently mixed by action of
gravity. But in the
conventional rotary kilns the combustion process occurs mainly above the feed
surface. None of
the embodiments implemented in such kilns is capable of providing conditions
for efficient inter-
phase heat exchange typical for a dense porous layer.
It is the object of the present invention to overcome the deficiencies of the
prior art and to
enable efficient processing of condensed fuels, including low-calorie fuels,
without using
additional power sources. The object can be attained in a method for gasifying
condensed fuels.
SUMMARY OF THE INVENTION
The present invention provides a method for processing a condensed organic
fuel by
gasifying, including: feeding a fuel into a cylindrical reactor; supplying an
oxygen-containing
gasifying agent into the reactor at the reactor side where the resultant
residual solids accumulate;
moving the fuel feed along the reactor axis; discharging the resultant
residual solids from the
reactor; driving-off the products of drying, pyrolysis and combustion as a gas
product from the
reactor such that gasification is carried out as the fuel successively resides
in a heating/drying
zone, a pyrolysis zone, a combustion (oxidation) zone and a cooling zone, and
the gas flow is
filtered through the fuel feed layer while passing successively through the
cooling zone, the
combustion zone, the pyrolysis zone and the heating/drying zone. The important
distinctive
feature of the invention is that the combustion process in the dense layer is
stabilized by rotating
the reactor about an axis tilted relative to the horizon.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig.l is a general schematic diagram of a device for implementing an
embodiment of a
method in accordance with the invention.
DESCRIPTION OF THE INVENTION
A method in accordance with the invention includes the following basic steps
that are
implemented in respective zones. At a first step, a condensed fuel in solid,
liquid of paste state,
possibly including solid noncombustible components and moisture (hereinafter
referred to as
"fuel") is fed into a reactor to successively perform therein drying the fuel
and then
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pyrolysis/gasification of combustible constituents of the fuel. An oxygen-
containing oxidizing
gas (e.g. air) is supplied into the reactor through the reactor section where
the resultant residual
solids accumulate, so that to substantially direct the gas flow in
countercurrent flow through the
dense layer of the fuel feed in the reactor. Fuel passes through a number of
zones in the reactor,
as described below. First, it passes through a drying zone where the fuel
temperature increases to
200 C owing to the heat exchange with the flow of gas product filtering
through the fuel; in this
zone the fuel is dried, and the gas flow is cooled before driving-off the
latter from the reactor.
After passing this zone as a gas product, gaseous products of drying,
pyrolysis and gasification
are removed from the reactor. Then the fuel enters the pyrolysis and coking
zone where the
temperature increases gradually to 800 C owing to the heat exchange with the
gas flow, and
combustible constituents of the fuel are pyrolyzed to produce coke. The coked
fuel enters then a
combustion and gasification zone, where the solid phase has a temperature of
700-1400 C. The
coke reacts with the hot oxidizing gas to produce a fuel gas. Residual solids
enter a cooling zone
where they are cooled by the counter flow of the gasifying agent from the
combustion
temperature to a discharge temperature. The oxidizing gas counter flow,
filtering through the
dense layer of residual solids, is in turn heated to a temperature close to
the combustion
temperature before it enters the combustion zone.
It should be understood that this classification of zones is somewhat
arbitrary. The zones
may be defined differently, e.g. based on the gas temperature, content of
agents, etc. In
particular, in US-A-4,732,091 the same zones are referred to differently.
Whatever the
classification of zones, the essential feature is that in the countercurrent
interpenetrating flows of
gas and solid feed material the oxidizing gas (gasifying agent) is pre-heated
on residual solids,
and hot gaseous combustion products will further transfer their heat to the
raw fuel.
The invention allows the efficient inter-phase heat exchange that is
advantageously
provided by the process in the dense layer to be combined with mixing the
processed material by
action of gravity as the reactor rotates, which is typical to rotary kilns.
To attain the object of efficient countercurrent gasification in a dense layer
and
stabilization of the combustion process in a reactor, a condensed fuel is fed
into a cylindrical
reactor such that to form a dense layer of the fuel feed in the reactor. An
oxygen-containing
gasifying agent is supplied into the reactor section where the resultant
residual solids
accumulate, and the fuel is gasified in the reactor by successively passing
through the zones of
drying, pyrolysis, combustion, gasification and cooling in a gas flow
filtering through the dense
fuel layer countercurrent to the fuel movement along the reactor axis with a
predetermined time
of holding in each of the zones. To stabilize the combustion process in the
reactor and provide
equal time of holding the fuel in the aforementioned zones throughout the
dense layer thickness,
CA 02650979 2008-10-30
the cylindrical reactor is mounted with its axis tilted at an angle relative
to the horizon and
rotated such that the material pours in the direction perpendicular to and
along the reactor axis
and fills the voids formed when the low-density material burns out. The
gasifying agent is
supplied at the bottom end of the cylindrical reactor, while the gas product
is removed at the
opposite end.
The cylindrical reactor axis is tilted at an angle from 22 to 65 degrees
relative to the
horizon. With a tilt angle below the lower limit, bulk material will not form
a dense layer in the
tilted reactor. On the opposite, a layer of solid fuel will be formed with a
gas flow above it. The
gas flow is not filtered through the fuel, consequently the main advantage of
gasification in a
dense layer, highly efficient inter-phase heat exchange, will not be realized,
as well as uniform
process will not be provided over the reactor section. With a tilt angle above
the aforementioned
range the solid material will not be adequately mixed in case of "burnouts".
Optimum combination of conditions for the fuel movement along the reactor axis
and
uniform combustion zone structure will be attained with a tilt angle of the
reactor axis relative to
the horizon from 40 to 50 degrees.
Preferably, the reactor rotation period should be sufficiently small to
provide mixing of
materials in the combustion zone. This will enable the invented process to be
implemented in a
reactor having a smaller length. To attain efficient rotation effect trough
the entire material
volume in the combustion zone, provided that the combustion zone dimensions
along the reactor
axis don't exceed, by an order of magnitude, diameter D (m) of the reactor
flow section, a
sufficient rotation speed of the reactor must be provided. If the bulk
velocity of discharging the
residual solids is V,(m3/h), the average time of holding the residual solids
in the combustion zone
will be about 7rD3/4V (hour). Reactor rotation period T should be preferably
no more than
approximately D3/4V (hour) to provide no less than triple mixing of the
material for the time
when it stays in the combustion zone.
The fuel processed is preferably a solid lump material which is sufficiently
permeable to
the filtering gas flow. If the fuel is insufficiently permeable, in
particular, when fine-dispersed,
liquid or paste fuels are processed, a noncombustible solid lump material is
fed into the reactor
along with the fuel to provide uniform gas permeability of the fuel feed in
the reactor and
improve conditions of mixing the material in the combustion zone in case of
burnouts. When
liquid or paste materials are processed, it is not obligatory to preliminary
mix them with a solid
lump material before feeding into the reactor, because uniform mixing will be
provided by the
reactor rotation. To provide conditions of mixing the materials in the
pyrolysis and combustion
zones, the solid inert material added to the-feed should preferably have a
density different from
that of the processed fuel.
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The process is performed in a device for gasifying a condensed solid fuel
including a
feeder, a cylindrical reactor, a discharge unit, a gasifying agent supply
unit, a gas product outlet,
a driver for rotating the reactor, seals to provide gas tightness in the
reactor rotation, wherein the
cylindrical reactor is mounted such that its axis is tilted at a tilt angle
from 22 to 65 degrees
relative to the horizon; the feeder and the gas product outlet are arranged in
the upper section of
the reactor, while the discharge unit and the gasifying agent supply unit are
arranged in the lower
section of the reactor. The tilt angle of the rotary reactor relative to the
horizon is preferably
from 40 to 50 degrees.
For providing the fuel move along the axis of the reactor as the latter is
rotating, it is
necessary to control the discharge of residual solids from the reactor. This
is preferably
implemented at the expense of natural pouring out of the solid bulk material
from holes in the
reactor side wall as the reactor rotates. Sizes and number of the holes are
chosen to match the
solid material portion pouring out per revolution with a desired volume of the
material discharge.
There should be at least two holes having a linear dimension that doesn't
exceed half the reactor
internal diameter to provide uniform discharge of the residual solids over the
reactor section.
Residual solids freely fall out from the reactor to the discharge unit, such
as a lock or hydraulic
lock, providing removal of the residual solids with preserved gas tightness of
the device.
The holes in the reactor side surface are preferably equipped with free
passage control
means, such as controlled shutters.
Alternatively, the reactor can be discharged through a cone having an opening
along the
reactor axis, the cone secured in the lower section of the reactor, the
opening diameter being
smaller than half the internal diameter of the reactor.
To successively hold the fuel in the heating, pyrolysis, combustion and
cooling zones, the
cylindrical reactor should have a sufficient length. To arrange the respective
zones in the reactor,
the following relationship between geometrical dimensions of the reactor
should be observed:
L=sina > 3D,
where L is the length of the rotary reactor,
a is the tilt angle of the reactor axis relative to the horizon, and
D is the internal diameter of the reactor.
To maintain the upper feed level in the reactor as the fuel is consumed in
pyrolysis,
combustion and discharge, the use may made of both measuring the actual level
(e.g. by a
radiation sensor) and outputting a command to introduce the next portion of
the fuel, and a
feeder including a vertical cylinder having a smaller diameter than that of
the rotary reactor, the
lower end of the cylinder extending inside the upper section of the reactor.
The feeder maintains
a constant level of the fuel feed in the reactor at the expense of pouring the
fuel out from a
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vertical tube as the fuel is consumed in the reactor.
A better understanding of the invention may be had by reference to the
following
description of an embodiment schematically shown in Fig. 1.
Condensed fuel F, previously milled, if necessary, and to which a
noncombustible solid
material is added when needed, is introduced into a reactor 1 through a feeder
2 comprising a
feed-lock chamber 3. The fuel enters the reactor through a vertical cylinder
4. The processed fuel
level is maintained constant in the reactor owing to pouring the fuel out from
the cylinder 4 as
the reactor 1 rotates and filling in the material consumed in combustion and
removal of ashes.
In the reactor, the material passes successively through a drying zone 4, a
pyrolysis zone
6, a combustion zone 7 and a cooling zone 8. As the reactor rotates, residual
solids R pour out
through holes 9 equipped with shutters 10, and then they are discharged,
continuously or in
batches, through a gas-tight discharge unit 11 (a hydraulic lock shown
schematically in the
drawing). A discharge velocity of the residual solids at which the combustion
zone maintains the
same position in the reactor, in the middle section of the reactor, is
provided by the relationship
between free passages of the holes 9, rotation speed of the reactor and
consumption of an
oxidizer admitted into the reactor.
Air A, along with steam if required, is supplied by a compressor 12 into the
lower section
of the reactor. Gas product G is driven-off from the upper section of the
reactor and directed to
further use which may include purification and combustion in a power
equipment. Temperatures
in the respective zones are continuously measured, and when the temperatures
go beyond the
specified optimal ranges the control parameters: rotation speed of the
reactor, air intake flow
rate, vapor flow rate, are adjusted. A level sensor monitors whether the
amount of fuel is
sufficient in the feeder, and as the fuel is exhausted fresh portions are fed
via the feed-lock
chamber 3. To match the discharge velocity of residual solids, free passages
of the holes 9 are
adjusted: they are increased when the discharge velocity exceeds a desired
one, and reduced in
the opposite case.
Owing to the reactor rotation at an angle relative to the horizon, the
material is mixed,
primarily in the pyrolysis and combustion zones where the volume of fuel
significantly decreases
and cavities appear. Where the reactor rotates at an angle relative to the
horizon, "burnouts"
occurring as low-density materials burn out are filled with portions of
unburned material falling
by action of gravity, this stabilizing the combustion process in the reactor.
A better understanding of the invention may be had by reference to the
following
example of practice of the present invention.
EXAMPLE
A mixture of sawdust with broken firebrick in the 2:1 ratio (by weight) was
subjected to
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gasification in an experimental laboratory reactor made of quartz. A tilt
angle of the reactor axis
relative to the horizon was varied from 5 to 90 degrees. Direct observation
has revealed that with
a tilt angle below 22 degrees, no filtering of gas flow through a dense fuel
layer takes place
because of a cavity formed along the upper generating line of the reactor,
through which cavity
the gas flow passes over the fuel surface. Where the reactor is in upright
position, burnouts form
in the fuel layer along the reactor walls within a short operation time, one
of the burnouts
eventually extending to the feed surface. In this situation the gas product
starts burning directly
above the feed surface in the reactor. Where the reactor axis deviated from
the vertical, burn-out
cavities fall down gradually as the reactor rotates. With an axis angle
relative to the horizon
smaller than 65 degrees, the rise of burnouts to the surface can be completely
restrained and the
combustion zone can be stabilized in the middle section of the reactor. Within
the complete
range of angles at which the combustion zone is stabilized, stable flame
burning of the gas
product is observed, and the residual solids are free from unburned carbon.
The combustion front
is stabilized best of all when a tilt angle of the axis is from 40 to 50
degrees and the combustion
zone dimension along the reactor axis is no more than half the reactor
diameter. To provide the
stabilized front, the rotation speed should exceed a predetermined value for
each tilt angle. It has
been estimated that to stabilize the combustion zone the material should be
more than once
mixed as it passes the distance along the axis of the reactor approximately
equal to the diameter
of the latter.
Therefore, the present invention, as compared to the conventional methods,
provides an
efficient method for gasifying condensed fuels with high yield of gas fuel and
high power
efficiency.
Since various modifications of the invention illustrated by the non-limiting
example will
occur to and can be made readily by those skilled in the art without departing
from the invention
concept, the invention is not to be taken as limited except by the scope of
the appended claims.
