Process and apparatus for purifying flammable gas

In a process and an apparatus for purifying flammable gas, in particular low-temperature carbonization gas, the gas is scrubbed and discharged as pure gas and condensate. The condensate is distilled in a distillation unit. Volatile constituents are admixed to the pure gas, so that the calorific value of the latter is increased. The non-volatile constituents are discharged. The non-volatile constituents can be fed to a low-temperature carbonization drum and carbonized therein at low temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for purifying flammable gas, in 
particular low-temperature carbonization gas, wherein the gas is scrubbed 
and discharged as pure gas, and condensate is discharged. The invention 
also relates to an apparatus for purifying flammable gas, in particular 
low-temperature carbonization gas, including a gas scrubber which has an 
inlet line for the gas, an inlet line for a scrubbing medium and a 
discharge line for condensate and is connected to a discharge line for 
scrubbed gas (pure gas). 
A process and an apparatus for purifying pyrolysis gas are known from 
German Published, Non-Prosecuted Application DE-OS 27 01 800, 
corresponding to U.S. Pat. No. 4,206,186. In that case, the objective is 
to obtain a gas which is as pure as possible. The gas should contain 
neither oil, nor tar, nor water. For that purpose, the pyrolysis gas is 
scrubbed in a gas scrubber, wherein the scrubbing medium being used is a 
scrubbing oil which condenses out in the gas scrubber and is used again. 
In order to start the unit, a certain quantity of scrubbing oil must be 
made available. The condensate which is not required as scrubbing oil is 
burned in a pyrolysis reactor. The gas leaving the gas scrubber is free of 
oil and tar. It is then treated further in a packed-bed reactor. The 
purified gas is fed as fuel gas through a blower to the fuel gas burner of 
the pyrolysis reactor. In the latter, the heating performance is impaired 
by a dust deposit, since the purified gas still contains dust. 
In the prior art, a very pure gas which, however, still contains dust is 
fed to the fuel gas burner. The known process and apparatus are planned to 
achieve a good purification performance. However, such a pure fuel gas has 
a relatively low calorific value, which is further reduced by the dust 
content. Furthermore, during the purification, substances arise which must 
be treated further in expensive units that are necessary especially for 
that purpose. For example, a screen and a combustion chamber are required 
for further processing of a filter residue from a packed-bed reactor. 
Residues requiring disposal then arise again in the screen. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a process and an 
apparatus for purifying flammable gas, which overcome the 
hereinafore-mentioned disadvantages of the heretofore-known methods and 
devices of this general type and which make a gas available that is 
largely dust-free and nevertheless has the highest possible calorific 
value. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a process for purifying flammable gas, in 
particular low-temperature carbonization gas, which comprises scrubbing 
flammable gas and discharging pure gas and condensate; distilling the 
condensate into volatile constituents and non-volatile constituents; and 
admixing the volatile constituents to the pure gas and discharging the 
non-volatile constituents. 
As a result of the distillation of condensate, the major part of the 
flammable constituents of the condensate, namely the volatile 
constituents, are converted into the gaseous form. These volatile 
constituents have a very high calorific value. As a result of the volatile 
constituents from the distillation being admixed to the pure gas, a gas 
mixture is obtained which advantageously has a very high calorific value. 
Dust that was originally present in the gas passes into the condensate in 
the scrubbing process. Upon distillation, the dust remains with the 
non-volatile constituents. The latter are discharged and can be burned. 
The dust thus cannot impair the calorific value of the gas mixture. 
The water contained in the condensate does not have to be discharged, which 
would entail effluent problems. At the latest during the distillation, the 
water is converted to steam which is admixed to the pure gas. This steam 
fraction only insignificantly affects the calorific value of the pure gas. 
The quantity of water in the steam fraction is in fact relatively small as 
compared with the quantity of water of the moisture content that is 
present anyway in the gas. 
The process according to the invention achieves the advantage that no 
substances requiring disposal, such as filter dusts or effluent, arise in 
the purification of a flammable gas. It is to be regarded as a particular 
advantage that a purified pure gas is obtained from which dust has been 
removed and which has a very high calorific value. Advantageously, the 
pure gas is freed of dust, while other substances which can raise the 
calorific value remain constituents of the pure gas. Due to the removal of 
dust, blockages in fuel gas lines also cannot occur. 
In addition to the pure gas, only the non-volatile constituents from the 
distillation remain, which can be carbonized at low temperature or burned, 
for which the pure gas can provide heat energy, as fuel gas. External fuel 
gas is then required only for starting. 
The flammable gas to be purified can be a part of a low-temperature 
carbonization gas formed in a low-temperature carbonization step. 
In accordance with a further mode of the invention, the condensate is 
separated into a heavy fraction and a light fraction. The gas is then 
scrubbed with the light fraction, and the heavy fraction as well as the 
excess part of the light fraction are distilled. If only the light 
fraction of the condensate is used as scrubbing medium, no heavy 
constituents of the condensate which, for example, contain dust, must be 
circulated, which would result in the parts of the plant being 
additionally burdened with dust. 
With the objects of the invention in view, there is also provided an 
apparatus for purifying flammable gas, in particular low-temperature 
carbonization gas, comprising a gas scrubber having an inlet line for 
flammable gas, an inlet line for scrubbing medium and a discharge line for 
condensate; a discharge line for scrubbed gas (pure gas) being connected 
to the gas scrubber; a distillation unit being connected to the discharge 
line for condensate; a line for volatile constituents leading from the 
distillation unit to the discharge line for scrubbed gas (pure gas); and a 
line for non-volatile constituents leading from the distillation unit. 
The advantage thereof is that the condensate constituents which are 
volatile in the distillation serve for increasing the calorific value of 
the pure gas to which they are admixed. The pure gas is advantageously 
dust-free. Remaining residual substances are discharged only through the 
line for non-volatile constituents. However, these residual substances, 
which also include the dust separated from the gas, can be carbonized at 
low temperature or burned. 
In accordance with another feature of the invention, the line for 
non-volatile constituents is connected to a low-temperature carbonization 
unit and/or to a burner unit. This low-temperature carbonization unit can 
provide a low-temperature carbonization gas which can be the flammable gas 
which is to be purified in the apparatus according to the invention. 
In accordance with a further feature of the invention, the discharge line 
for scrubbed gas (pure gas) is connected to a fuel gas burner. 
In accordance with an added feature of the invention, the fuel gas burner 
is a part of the low-temperature carbonization unit or burner unit. 
The apparatus according to the invention can thus be a part of a largely 
closed system. In this case, a partial stream of the low-temperature 
carbonization gas, coming from a low-temperature carbonization unit, can 
be purified, in which case exclusively only dust-free pure gas and 
non-volatile substances arising in a distillation remain. These substances 
can be carbonized at low temperature in the low-temperature carbonization 
unit. The pure gas can be burned in a fuel gas burner and can serve for 
heating the low-temperature carbonization unit. Apart from the 
low-temperature carbonization residue and excess low-temperature 
carbonization gas, no wastes requiring disposal arise. 
In accordance with an additional feature of the invention, the 
low-temperature carbonization unit is a low-temperature carbonization drum 
of a low-temperature carbonization-combustion plant that is known for this 
purpose, for example from European Patent No. 0 302 310 B1, corresponding 
to U.S. Pat. No. 4,878,440. 
For example, the discharge line for condensate from the gas scrubber is 
connected to a settling vessel. The condensate collects in this settling 
vessel. The heavier constituents settle in the latter. The lighter 
constituents form a layer above the heavier constituents. The heavier 
constituents contain predominantly the dust, and the lighter constituents 
are water and/or oil. 
In accordance with yet another feature of the invention, the inlet line of 
the gas scrubber for scrubbing medium branches off in the middle region of 
the settling vessel. This ensures that lighter constituents of the 
condensate, which do not contain any dust, pass as scrubbing medium into 
the gas scrubber. A discharge line for excess condensate starts from the 
bottom of the settling vessel. The heavier constituents of the condensate, 
which contain dust, are discharged through this discharge line. The 
discharge line for scrubbed gas starts from the top of the settling 
vessel. 
The use of a settling vessel advantageously ensures that lighter 
constituents of the condensate, which are largely free of dust, can be fed 
as scrubbing medium to the gas scrubber. 
In accordance with yet a further feature of the invention, the discharge 
line for excess condensate, starting from the settling vessel, is 
connected to a hydrocyclone from which a discharge line for a lighter 
fraction and a discharge line for a heavier fraction start. The discharge 
line for a lighter fraction leads into the upper part of the settling 
vessel. The discharge line for a heavier fraction leads into the lower 
part of the settling vessel. The condensate being introduced is separated 
once more in the hydrocyclone. This gives an even better division into a 
lighter, purer fraction and a heavier, dust-laden fraction. The lighter 
fraction passes from above into the settling vessel, whereas the heavier 
fraction passes into the lower part of the settling vessel. In this way, 
the fractions being separated in the hydrocyclone are advantageously added 
to the corresponding condensate quantities in the settling vessel. 
Remixing in the settling vessel is thereby largely precluded. The use of 
the hydrocyclone advantageously achieves an even further improved 
separation of the condensate into a heavier fraction and a lighter 
fraction. 
In accordance with a concomitant feature of the invention, the settling 
vessel has a main chamber and a subsidiary chamber. The discharge line of 
the hydrocyclone for the heavier fraction leads into the lower part of the 
subsidiary chamber. The latter has an overflow leading into the main 
chamber and is connected at the bottom to a discharge line for excess 
condensate. This has the advantage of providing three successive 
separation stages for the condensate. After a heavier fraction of the 
condensate has settled in the main chamber of the settling vessel, this 
heavier fraction passes into the hydrocyclone. The heavier fraction being 
separated off therein passes into the subsidiary chamber of the settling 
vessel, where a further settling step takes place. From there, the lighter 
fraction of the condensate flows through the overflow into the main 
chamber, and the heavier fraction is released, for example into the 
distillation unit. The division of the condensate is even further improved 
by the use of a settling vessel which has a main chamber and a subsidiary 
chamber. 
The process and the apparatus according to the invention have the advantage 
of providing a purified gas which has a very high calorific value, because 
it is freed of dust which can reduce the calorific value, and because 
substances effecting a high calorific value are recovered by distillation 
and admixed to the pure gas. Moreover, the process and the apparatus 
provide the advantage of ensuring that no wastes arise which would require 
further treatment. No effluent arises, nor a substance which would not be 
suitable for passing to a burner unit, for example a low-temperature 
carbonization drum. At temperatures above 100.degree. C. in the 
distillation unit, even water which may be present passes into the pure 
gas stream in the form of steam. The calorific value of the pure gas is 
not impaired by the small quantities of steam, because the gas is moist 
anyway. However, the advantage is obtained that no effluent arises. In 
particular, no extraneous medium is required for purifying the flammable 
gas, except for starting up the unit. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
process and an apparatus for purifying flammable gas, it is nevertheless 
not intended to be limited to the details shown, since various 
modifications and structural changes may be made therein without departing 
from the spirit of the invention and within the scope and range of 
equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the single FIGURE of the drawing in detail, it is seen 
that a flammable gas G which is to be purified and can, for example, be an 
entire low-temperature carbonization gas SG or a part thereof, is 
delivered from a low-temperature carbonization drum 7 that is known per 
se. The gas is supplied through a low-temperature carbonization gas line 
7a which starts from the low-temperature carbonization drum 7. The 
flammable gas G passes through an inlet line 1a, which can be connected to 
the low-temperature carbonization gas line 7a, into a gas scrubber 1 which 
can be a venturi scrubber. The gas scrubber 1 also has an inlet line 1b 
for a scrubbing medium W and a discharge line 1c for condensate K and pure 
gas R. The latter line is connected through a precipitation device 11 for 
condensate aerosols to a discharge line 6 for scrubbed gas (pure gas) R. 
The discharge line 1c of the gas scrubber 1 is connected to a settling 
vessel 2 in which heavier constituents of the condensate K settle out. 
These heavier constituents contain a lot of dust, whereas the lighter 
constituents in the layer above only contain a little dust and can be 
formed of oil and water. If the temperature in the settling vessel 2 is 
above 100.degree. C., the lighter fraction of the condensate K is formed 
only of oil. The inlet line 1b of the gas scrubber 1 for the scrubbing 
medium W starts from approximately the middle of the settling vessel 2. As 
a result, a part of the lighter, low-dust constituents of the condensate K 
passes as scrubbing medium W into the gas scrubber 1. The inlet line 1b 
can be associated with a cooling unit 10. A discharge line 2a for the 
heavier fraction of the condensate K starts from the bottom of the 
settling vessel 2. The excess of lighter condensate K is also released 
through the discharge line 2a. The discharge line 6 for scrubbed gas (pure 
gas) R starts from the top of the settling vessel 2. The discharge line 2a 
for condensate K can be connected to a hydrocyclone 3, in which the 
condensate K is separated further. A lighter fraction passes through a 
discharge line 3a from above into the settling vessel 2. A heavier 
fraction passes through a discharge line 3b into the lower part of the 
settling vessel 2. The two fractions are thereby recycled into the 
settling vessel 2 at suitable points. 
For example, the settling vessel 2 can have a main chamber 2A and a 
subsidiary chamber 2B. The heavier fraction from the hydrocyclone 3 is 
then fed through the discharge line 3b into the lower part of the 
subsidiary chamber 2B. In the subsidiary chamber 2B, a third separation 
step takes place in the condensate K. The lighter fraction flows at an 
overflow 5 from the subsidiary chamber 2B into the main chamber 2A. An 
excess condensate U remaining after the three separation steps passes 
through a discharge line 2b into a distillation unit 4. The discharge line 
6 for the pure gas R can start from the upper part of the subsidiary 
chamber 2B of the settling vessel 2. The precipitation device 11 for 
condensate aerosols can be disposed downstream of the subsidiary chamber 
2B and upstream of the discharge line 6. 
In the distillation unit 4, which is heated by a heater 9, volatile 
constituents F of the excess condensate U are separated from non-volatile 
constituents N thereof. A line 4a for the volatile constituents F starts 
from the upper part of the distillation unit 4 and leads into the 
discharge line 6 for scrubbed gas (pure gas) R. A line 4b for the 
non-volatile constituents N starts from the lower part of the distillation 
unit 4. The non-volatile constituents N contain all of the dust removed 
from the flammable gas G to be purified. Furthermore, the non-volatile 
constituents N also contain tarry substances. The non-volatile 
constituents N are combustible and can, for example, be carbonized at low 
temperature in the low-temperature carbonization drum 7 of a 
low-temperature carbonization-combustion plant which is known for this 
purpose. In this case, the line 4b can be connected to the low-temperature 
carbonization drum 7. The pure gas R enriched with the volatile 
constituents F can be used as fuel gas for the low-temperature 
carbonization drum 7. For this purpose, the discharge line 6 for pure gas 
R is connected to a fuel gas burner 8 which is a component of the 
low-temperature carbonization drum 7. The pure gas R is burned in the fuel 
gas burner 8. The heat energy thus made available serves for heating the 
low-temperature carbonization drum 7. 
The heating of the distillation unit 4 is effected by heating a circulated 
liquid phase in the heat exchanger 9. In order to transfer the medium, 
there can be pumps P in all of the lines. 
The illustrated apparatus releases neither effluent nor waste gas. The 
non-volatile constituents N of the excess condensate U can be carbonized 
at low temperature without any problems in the low-temperature 
carbonization drum 7. The volatile constituents F of the excess condensate 
U increase the calorific value of the dust-free pure gas R, so that the 
latter can be used as fuel gas for the low-temperature carbonization drum 
7. Except during start-up of the unit, no extraneous medium is required 
for removing the dust from the gas G which is to be purified. The 
scrubbing medium W for the gas G is obtained from the condensate K. In 
conjunction with a low-temperature carbonization drum 7, no additional 
wastes arise. With the usual feeding of domestic refuse M or other wastes 
into the low-temperature carbonization drum 7, low-temperature 
carbonization residue SR and excess low-temperature carbonization gas SG 
are discharged from the latter, and these can be further processed in a 
known manner in a low-temperature carbonization-combustion plant that is 
known per se.