Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR BURNING REFINING RESIDUES
The present invention relates to a method and a
device for burning residues from refining petroleum with
minimum emission of nitrogen oxides and dust.
These residues, including tar, asphalt and
bitumen, are of low commercial value and include in
particular products that are solid at room temperature
and have a viscosity in excess of 500 centistokes at
100 C. They are usually burned by refineries for their
own purposes. The production of light diesel oil for
automobiles in Europe leads to the production of these
residues, for example.
Their combustion in conventional boilers is
harmful to the environment, and new standards on the
emission of oxides of sulfur in Europe, and in France in
particular, make this solution impossible. These
restrictions will also apply to dust, polycyclic aromatic
hydrocarbons and volatile organic compounds.
Instead of burning these residues, it is possible
to use other forms of treatment that are less harmful to
the environment but more complex and more costly. Thus
gasification may be used, either integrated into a
combined cycle or taking the form of a deep conversion
process that converts the residues to an economically
viable final residue such as petroleum coke, for example.
The increased production of so-called "bottom of
the barrel" residues (tar), in particular in Europe,
which at present are burned by refineries in conventional
boilers, will also lead to modification of the treatment
of these residues.
The object of the present invention is to propose
a method for the combustion of the residues previously
cited and an injection device, both of which may be used
in existing installations, in particular in tangential
heating boilers, and which significantly reduce the
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emission of pollutants, primarily oxides of nitrogen and unburned carbon dust.
The method according to the invention for the combustion of residues
from the refining of petroleum in a boiler comprising a hearth and injectors
characterized in that recycled flue gases, residues and gas containing oxygen
are
simultaneously injected into the hearth in the hottest region of the hearth.
This region, situated in the immediate vicinity of the burner, is one of
cyclonic flow, i.e. one in which the combustion gases are caused to rotate
about a
vertical axis to ensure that they are totally converted, leaving no unburned
substances. Injection into the very hot region of the hearth ensures high-
temperature cracking of the residues and their conversion into basic compounds
(CO, H2, CH4, C02) that are easily combustible and therefore less pollutant,
because of the reduction of unburned carbon dust. The method applies in
particular
to tangential heating boilers.
Simultaneous injection of recycled flue gases imparts movement to the jet
of gas containing oxygen encouraging penetration of said gas into the hearth
and in
particular into the cyclonic, hottest region. Injecting recycled flue gases
also
facilitates the homogenization of the combustion products resulting from
cracking of
the residues in the hearth, and thereby prevents temperature peaks in the flue
gases in the vicinity of the heat exchangers disposed over the combustion
region.
Increased production of oxides of nitrogen (NOx) in the hot region is
compensated
by the low production of nitrogen oxide resulting from staged combustion using
either staged air or staged fuel.
According to one particular feature, injection is
effected by injectors disposed in the corners of the
hearth. The injection position will be determined as a
function of the nature of the residue to be burned. One
or more injection levels may be provided as a function of
the peak temperature in the hearth facing the barge-
shaped arrangement of the fuel injector and as a function
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of the expected reduction in nitrogen oxides resulting
from the staged combustion created in this way by
combining injection of air and fuel and injection of
recycled flue gases, oxygen and residue.
According to a second disposition, injection is
effected by injectors on the lateral walls of the hearth.
According to one particular feature of the second
disposition, the injectors are at the middle of each of
the lateral walls of the hearth.
In a variant of the second disposition, the
injectors are on the four walls.
In another variant of the second disposition, the
injectors are on two facing walls.
The device in accordance with the invention for
injecting gas into a boiler hearth is characterized in
that it is installed in flue gas recirculation trunking
and comprises concentric gas injection tubes. Combining
two injections creates a hot conversion region. These
devices may be installed at several locations in the
hearth of the boiler.
In accordance with another aspect, the invention also concerns a
device for injecting gas into a hearth of a tangential heating boiler,
characterized in
that it is installed in trunking for recirculating flue gases, it comprises
concentric gas
injection tubes and it is adapted to receive recycled flue gas, residues and
gas
containing oxygen and to simultaneously inject the recycled flue gases, the
residues
and the gas containing oxygen into said hearth at approximately a hottest
region of
the heath.
According to one particular feature, the device
is substantially perpendicular to the walls of the hearth
in a horizontal plane. The location, inclination and
deviation in the horizontal plane of the device are
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adapted as a function of the architecture of the boiler
to encourage penetration into the hot combustion vortex
region. The recirculated flue gas feed trunking may
therefore be bent, which induces additional,
substantially horizontal rotation of the incoming flow,
and therefore creates or increases rotary flow about a
i
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substantially vertical axis of the flue gases
recirculated into the hearth.
According to another particular feature, the
device is substantially inclined vertically towards the
bottom of the hearth. The angle of inclination is
preferably less than 450.
According to a second particular feature, the
device comprises a cooling circuit. The temperature at
the tip of the injection device is such that it is
necessary to cool it.
According to a third feature, the device is swept
by a gas to prevent it from becoming blocked by dust
coming from the hearth.
According to a fourth feature, the device is
retractable. It is therefore possible to carry out
maintenance with the boiler in operation.
According to a fifth feature, the device
comprises a gas pilot light which encourages self-
ignition of the residue.
The invention will be better understood from the
following description, which is given by way of example
only and with reference to the appended drawings, in
which:
- figure 1 is a view in vertical section of a
boiler according to the invention,
- figure 2 is a view in section of the boiler
according to the invention,
- figure 3 is a detailed view in section of an
injector according to the invention in an injection
position,
- figure 4 is a detailed view in section of the
figure 3 injector in a retracted position, and
- figure 5 is a detailed view in section of the
injector pipe.
In figure 1, the boiler 1 comprises a hearth 2 to
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which is fed fuel 3 surrounded by hot air 4. Injectors 5
inject oxygen or gas containing oxygen 50, the residue
51, and (where applicable) recycled flue gases 52. A
cooling liquid 53 such as water flows in the injectors 5
5 to cool them. The injectors 5 are placed at the height of
t~ tio~~ ttest region 20 2n of ~ ~. the heartte.. ~.~~..~.h 2 . Add;t =lona-
air ; s
the .~
injected via the injector 6 to bring about staged
combustion, in order to reduce the emission of oxides of
nitrogen, the injectors 6 are located above the injectors
5 so that the flue gases remain between the two injection
levels 5 and 6 for a period from 0.2 second to 5 seconds.
Figure 2 shows the figure 1 boiler 1 from above,
at the height of the hot region 20. The fuel 3 and the
hot air 4 are injected in the corners of the boiler 1. In
this variant the injectors 5 are placed at the middle of
the lateral walls 21 of the hearth 2, but they could be
placed in the corners. The injection of the fuel 3 in the
corners creates a vertical axis vortex 200 to which the
residue 51 and the gas 50 are steered.
The injector 5 shown in detail in figure 3
comprises a feed pipe 500 for oxygen (or a gas containing
oxygen) 50, a pipe 510 for the residue 51, a feed pipe
530 for the cooling liquid 53 and a return pipe 531 for
said liquid 53. The recycled flue gases 52 arrive via a
pipe 520. The injector 5 is inclined downward at an angle
of less than 45 to encourage penetration of the residue
51 into the hot region 20. Below a region 22 for
gasification of the residue 51 is a combustion region 23
and above it is a post-combustion region 24.
The injection pipe 54 discharging into the hearth
2 is shown in figure 5 and is made up of four concentric
cylinders; the residue 51 flows in the central cylinder
540, the gas 50 enriched with oxygen flows between the
cylinder 540 and the next cylinder 541, and the cooling
liquid 53 flows between the cylinders 541, 542 and 543,
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making a return trip in the pipe 54 with the outward flow
between the outermost cylinders 542 and 543 to facilitate
cooling and the return flow between the innermost
cylinders 541 and 542.
The flue gases 52 arrive via the pipe 520 and
enter the pipe 521 of the injector 5 from which they are
directed into the hearth 2 (see figure 3).
Figure 4 shows the injector 5 in a retracted
position, the assembly comprising the pipe 54 and the
pipes 530, 531, 510 and 500 being retracted into the feed
pipe 521 for the flue gases 52 so that this assembly of
the injector 5 may be maintained in service.
The execution of the method is described next.
The fuel 3 and the hot air 4 are injected into the lower
portion 23 of the hearth 2. Combustion develops and the
region 20 above the burner 30 is the hottest region. The
residues 51 and the gas 50 enriched with oxygen are
injected into this region 20, in which the residues 51
are cracked and reduced to combustible basic compounds.
Simultaneous concentric injection of recycled flue gases
52 via the injectors 5 imparts movement to the jet of
oxygen (or gas enriched with oxygen) 50 and to the
residue 51 encouraging penetration of the residue 51 and
the gas 50 into the vertical axis cyclonic flow region
200 of the hearth 2 and creates a hot conversion region
encouraging the combustion of the residue 51.
The number of injectors 5 to be installed in the
hearth 2 is decided as a function of the properties of
the residue 51 to be burned. The injectors could be
disposed at one or more levels as a function of the heat
flow permitted by the walls 21 and the requirement to
smoothe temperature peaks in the flue gases where they
impinge on the exchanger bundles 7.
The inclination of the injectors 5 and their
deviation in the vertical direction is less than 45 .
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The injection system 5 comprises a gas pilot
light to encourage self-ignition of the residue 51.
