Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a method for gasifying coal
under pressure, for the purpose of cleaning the crude gas
emerging from a reactor, whereby a mixture of dust particles
and tar is recovered, from the hot crude gas, in a gas~cleaning
unit following the reactor, the said mixture being removed and
processed or returned, at least in part, to the crude gas. The
invention also relates to an installation for the execution of
this method.
In gasifying coal under pressure, preferably bituminous
coal is gasified in a reactor under pressure and at a temperature
of about 1000C. The gas-cleaning unit located after the reactor
not only separates tar and dust, but also removes hydrogen
sulphide, and is thus a considerable ~tep towards the pollution-
free generation of electricity. This generation of electricity
may be carried out by means of an expansion turbine through which
cleaned gas flows, usually under pressure, but is more usually
carried out by producing steam in a boiler, under which the gas
is burned, and feedi~g the steam to a steam-turbine, or by means
of a gas-turbine which processes the combustion gas cooled in the
29 boiler and then release~ it to the atmosphere through a stack.
Although this process makes it possible to eliminate dust and
fluorine emissions almost entirely, and to reduce considerably
the emission of sulphur and nitric oxide, difficulties still
arise in the cleaning stage.
It is known for the gas-cleaning unit to be in the
form of a washing cooler to which the crude gas is fed under
pressure. Thorough mixing of the washing water takes place in
the washing cooler. On the one hand, this results in water
being picked by the clean gas, and this gas must therefore be
passed through a drip-separator in a subsequent treatment stage~
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On the other hand, the washing water produces a mixture of
dust tar and water which must be processed. This is what
causes the difficulties mentioned above.
The washing proces~, and the subsequent condensation
of the steam rising from the washing water, produces heat
losses which impair the overall efficiency. The necessary re-
placement of the water thus lost leads to added expense, since
only fully desalinized washing water can be used for the process.
Dust-removal efficiency is also poor, generally between 50 and
70~ only. This means additional dust-removal units and there-
fore increased expenditure. The mixture produced must be
separated into at least tar containing a large amount of dust,
which is usually returned to the reactor, and water. The
washed-out tar tends to foam, and this makes the process con-
siderably more difficult. The dehydrated mixture is generally
in the form of a cream-like mass which does not flow and is
therefore also very difficult to handle.
It îs the purpose of the invention to eliminate the
difficulties caused by the use of washing water by using a
novel gas-cleaning unit, in order to improve the cleaning of
dust in the gas-cleaning unit and to reduce the processing
difficulties hitherto associated with the separated mi~ture.
According to the invention, this purpose is achieved
in that liquid tar is distributed to, and vapourized in, the hot
crude gas, and in that the temperature of the crude gas is
reduced and the tar with which the dust particles are combined
is condensed.
This method makes it possible to dispense completely
with the addition of cooling water to the crude gas. As a rule,
only a part of the total tar introduced is vapourized by the
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heat of the crude gas, and this increases the partial pressure
of the tar in the crude gas. During the subsequent cooling,
therefore, the amount of tar condensed is relatively larger than
in known crude-gas cooling processes in which no tar is added.
During condensation of the tar, the particles of dust in the
crude gas act as condensation nuclei and support the formation
of tar and separation of dust.
The main advantages of the method are therefore, on
the one hand, that no washing water is used and, on the other
iO hand, that there is a considerable improvement in dust separation.
In additio~ to this, ~n the case of the new method, the crude-
gas heat removed by cooling need no longer be wasted but may
be utilized in the overall pxocess, as explained in detail
hereinafter.
The tar is preferably introduced into the hot crude
gas by injecting tar oil through nozzles. This facilitates
vapourization of the tar and ensures that when the temperature
of the crude gas is reduced, after the tar is introduced, the
heavy and hughly viscous hydrocarbons initially separated are
diluted, thus improving the ability of the tar-dust suspension
to flow. This facilitates the further handling of the separated
mixture.
This purpose is also served by a further example of
embodiment of the method according to the invention, in which
the crude gas is passed along a sloping path along which the
condensed tar is caused to flow. In this way a film of tar
is forned from the drops of tar initially formed and laden with
dust. This film automatically moves by gravity in the direction
of ~he flow of gas, the said flow of gas facilitating the flow
of tar, thus rendering special transportation means unnecessary.
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More particularly, with this method the amount of
tar separated i~ increased, in the upper stretches of the
path followed by the crude gas, by adjusting the amount of tar
introduced, as compared with the amounts of mixture separated
in the lower stretches of the path. In this connection an
excess of tar may be introduced, which causes the film of tar
described above to form more rapidly, since it may be formed
at least in part from the unvapourized tar introduced.
In the method according to the invention, the mixture
~10 separated from ~he crude gas ispreferably decanted, thus pro-
ducing a fraction of the mixture containing more dust which is
return~d to the reactor, while another fraction is processed
or conditioned. This processing may consist of separating the
tar contained in the fraction of the initial mixtures containing
less dust, and returning it to the flow of hot crude gas. In ~ -
this way, most of the dust is separated and the useful tar
fraction is circulated so that the process can sustain itself.
With this process, it is desirable to divide the
portion low in dust into a light and a heavY fraction, the light
~20 fraction being injected into the flow of crude gas, while the
heavy fraction is returned to the reactor. This ensures that
the light fraction contains almost no dust, which means that
the difficulties caused by the dust are eliminated, and that
high dust-removal efficiency is achieved.
The portion low in dust may be broken down into
fractions by expansion vapourization, the heat arising during
condensation of the tar vapours being used to preheat the
boiler-feedwater used to generate steam. It is possible,
with this method, to increase the efficiency of the unit
still further, to provide an increase in overall efficiency
of 2%.
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In addition to this, it ha~ been found that there
is a substantial improvement in dust removal, presumably
because of the better wetting of the tar in relation to the
dust, as compared with the washing water hitherto used. As
a result of this, the amount of tar in circulation is about
one tenth of the amount of circulation when water is used as
the washing agent. As a result of this, the unit for recover-
ing tar from the mixture of tar and dust is about one tenth the
si~e of units in which the washing water must be separated
from the said mixture.
Replacement of the washing water by tar means that
there is no need to feed washing fluid to the process from
the outside. This also eliminates the difficulties hitherto
~- associated with clarifying the wash~ng water. These difficult- ~ -
ies are major, since the washing water is highly contaminated
with high-boiling-point hydrocarhons. In additi~n to this,
the subsequent installation for removing the fineplrticle~
of dust may be considerably simplified or totally eliminated.
In order to make the invention more easily understood,
a unit for the execution of the foregoing method is described
hereinafter in greater detail. Since there is no contamination
with baked-on dust and ~ar, this unit may be built up with
basically conventional components. The drawing attached hereto
is a diagrammatic representation of a unit of this kind.
The crude gas emerging ~rom a reactor, not shown,
passes through line 1 into a crude-gas cooler marked 2 as a
whole. This cooler comprises an inlet chamber 3 at the top
and an outlet chamber 4 at the bottom. Between these chambers
lies a group of pipes 5 constituting the heating surface, the
pipes in this group having an inside diameter of about 50 mm,
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not more than 90 mm. The crude gas flows through the pipes
which are cooled externally with a cooling fluid. This fluid
is supplied from a steam drum 6 through a line 7 and flows,
through line 8, into crude-gas cooler 2, leaving the cooler
through line 9 and returning to steam drum 6. The steam drum,
in turn, is connected to boiler feedwater line 10, from which an
amount of fluid, corresponding substantially to the amount of
steam is replaced, the said steam being removed through line
11 and being passed, for example, to a steam-turbine process,
not shown, for the purpose of generating electrical power.
In the design illustrated, the cooling water circulates
naturally and returns, at least some of it in the form of
steam, through the said line 9, to steam drum 6.
With this arrangement, the cooled and cleaned crude
gas emerges at 12 as fuel gas, still at a temperature of
between 260 and 300C. This ~uel gas is passed on for further
processing, not shown in the drawing.
The mixture removed through line 13 consists of
condensed tar and dust par~icles. The said mixture passes
to a decanting marked 15 as a whole where it is divided, the
example of embodiment illustrated, into one fraction high in
dust and one fraction low in dust. By means of a pump 14,
the tar low in dust i5 returned, through a line ~, to the
top of crude-gas cooler 2. The tar high in dust is returned,
through a line 17, to the reactor, not shown.
As shown, the crude gas entering cooler 2 is cooled,
at the same time liquid tar is injected through nozzles into
the crude gas. This causes the tar to condense and combines
the dust contained in the crude gas into the mixture. Since
tar is present in excess, the mixture remains fluid, so that
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cooler 2 does not become contaminated and the unit requires
almost no maintenance. According to variant of this example
of embodim~t, an expansion-distilling unit may be connected
to decanting system 15, so that the mixture fraction high
in dust can also be processed.
