The invention relates to a method for cooling polluted gas, in particular
fuel gas that is obtained on treating waste.
Waste can be treated in various ways. Treatment by combustion is widely
used at present. Other possibilities are pyrolysis and gasification, both of which
can also be followed by a melting step. In the case of pyrolysis the waste is heated
under (virtually) oxygen-free conditions, gases and a coke-like residue being produced.
Higher molecular weight constituents can be removed from the resulting gas by thermal
cracking. A mixture of CO, CO2, H2O and H2 is produced
during this operation. In the case of gasification more oxygen is supplied, with
the result that it is mainly CO that is produced and less coke-like residue is formed.
The residue can be further reacted by means of a melting step, the residue being
converted into a usable slag and a fly substance that contains the volatile metal/metal
compounds. The pyrolysis and gasification processes yield, as product gas, a fuel
gas that still has a calorific value. The same is true of the off-gas from the melting
step, at least if the latter is operated under reducing conditions.
In general the gases are obtained from the processes described above
at temperatures above 1 000 °C, in particular at temperatures of 1 250 - 1 600 °C.
The gases are polluted with material entrained from the feed ("entrainment") and
also contain gaseous compounds originating from the feed, such as hydrogen chloride
gas, hydrogen sulphide and metal vapours (in particular zinc and, to a somewhat
lesser extent, lead).
The consequence of the presence of such impurities is that problems
can arise during the further processing of this gas. In the existing processes for
purifying melt gas and pyrolysis gas the gas is usually fed successively through
a gas cooler, a cyclone and/or bag filter to capture solids and a gas scrubber to
remove the acid constituents from the gas. Contamination of the equipment for carrying
out these processing steps, as a consequence of the pollutants present in the hot
gas, is then a problem, in particular in caking of the material and certainly on
cooling. As far as the cooling step is concerned, cooling using the conventional
means could lead to the metal vapour present precipitating as elementary metal.
Said elementary metal would then be filtered off in the filters. This can lead to
the occurrence of filter fires as soon as oxygen (air) is admitted: the metals will
oxidise immediately, a great deal of heat being generated. In addition, because
of the high temperature of the gases, heat exchange can take place only in very
expensive heat exchangers made of ceramic material. Such heat exchangers are subject
to wear and are relatively rapidly contaminated by the polluted hot gas streams,
as a result of which they are highly maintenance-intensive.
The aim of the present invention is to provide a solution to the above
problems. A further aim of the invention is to provide a method with which the very
hot and polluted gas can be cooled efficiently, so that heat exchange can take place
in steel equipment. The use of steel heat exchangers has the advantages that it
is less expensive and that the heat exchanger wears less rapidly and becomes contaminated
less rapidly. It is known that the use of steel heat exchangers is possible only
at temperatures below 1 000 °C. The hot gas streams must therefore be cooled to
a temperature below 1 000 °C before they can be fed through a steel heat exchanger.
A further aim of the invention is to provide a method with which the gas obtained
retains an appreciable calorific value, so that it is suitable for use as fuel gas.
It has been found that by making use of the shift reaction the hot
gas streams can be effectively and rapidly cooled, the gas obtained possessing an
appreciable calorific value.
The invention therefore relates to a method for cooling polluted,
carbon monoxide-containing gas, which contains as impurities at least metal vapours
and hydrogen sulphide and is at a temperature of above 1 000 °C, to a temperature
between 700 and 1 000 °C, by supplying water in liquid and/or gaseous form to said
gas, and wherein at least some of the carbon monoxide present in the gas is able
to react with the water supplied, with the formation of carbon dioxide and hydrogen.
By supplying water at a lower temperature to the hot gas, the mixture
is cooled and the shift reaction takes place:
CO + H2O <=> CO2 + H2
As a result of the fall in the temperature and the changed chemical conditions,
the metal vapours present in the gas will be converted into solid compounds, usually
into sulphates and sulphides because of the presence of hydrogen sulphide. As a
result these metals can be readily removed further in the process and processing
also presents fewer problems. Furthermore, hydrogen is formed as a result of the
shift reaction taking place, which is advantageous, since hydrogen has a higher
value than carbon monoxide and is also easier to use.
The water can be supplied in either the liquid or gaseous (as steam)
state.
The amount of water supplied has an effect on the degree to which
cooling is achieved. In any event the quantity of water added must be such that
the temperature of the hot gas falls to below 1 000 °C. In addition, it is preferable
that the temperature of the gas does not fall below 700 °C, because otherwise too
much heat would be lost, which would also make heat exchange less cost-effective.
It is most preferable to supply a quantity of water such that the temperature of
the cooled gas is between 900 and 1 000 °C. At temperatures in this range the rate
of reaction of the shift reaction will be so high that the new equilibrium is established
with residence times of less than one second. On further addition of water, the
temperature will fall further, but the shift reaction will not proceed much further.
The consequence is that a metastable equilibrium is produced: the shift reaction
is in a sort of frozen equilibrium situation.
As has already been stated above, the method according to the invention
is in particular suitable for cooling polluted gases that have been obtained by
gasification or pyrolysis of waste or off-gases which are obtained from a melting
step, in which the coke-like residue that is obtained from the pyrolysis or gasification
is melted under reducing conditions. The method according to the invention is in
particular suitable for cooling polluted gas that has been obtained by the partial
combustion or gasification of biomass, pyrolysis coke or waste with a high content
of organic material.
Furthermore it is also possible with the aid of the method according
to the invention to adjust the H2/CO ratio in the gas obtained or to
keep this ratio virtually constant. The metals in the gas are also converted to
metal oxides or metal sulphides.
The invention is further illustrated with reference to the following
examples.
Example 1
A gasifier supplies gas at a temperature of approximately 1 450 °C,
which contains the following gaseous constituents:
- CO:48.2 %mol zinc vapour:200 ppm
- CO2:11.8 %mol H2S:1400 ppm
- H2:19.3 %mol HCl:1000 ppm
- H2O:15.7 %mol
- N2:5.0 %mol
In addition, the gas also contains 3.5 g/m3 solid constituents as well
as small amounts of other gaseous constituents.
0.3 mol water at 25 °C is added to this gas per mol gas (which corresponds
to 23 % on a weight basis). This results in an adiabatic equilibrium temperature
of 948 °C. The gas composition in this equilibrium situation is as follows:
- CO:27.0 %mol
- CO2:19.2 %mol
- H2:25.0 %mol
- H2O:25.0 %mol
- N2:3.8 %mol
At the equilibrium temperature of 948 °C finally reached, all the zinc vapour originally
present has been converted to zinc sulphide.
It can be seen from the above compositions that the hydrogen content
has increased appreciably as a consequence of the addition of water. The calorific
value of the gas obtained (after the water has been condensed out of this) is 51.4
Mcal/mol, which is a value that is very readily usable.
Example 2
The same gas as used in Example 1 is now cooled with steam at 150
°C instead of with water. 0.35 mol steam is added per mol gas. The final temperature
is 928 °C. The composition of the gas at this temperature is as follows:
- CO:25.1 %mol
- CO2:19.3 %mol
- H2:24.9 %mol
- H2O:27.0 %mol
- N2:3.7 %mol
Here again at the equilibrium temperature of 928 °C finally reached all of the
zinc vapour originally present has been converted to zinc sulphide.
It can also be seen from the above compositions that the hydrogen
content has increased as a consequence of the addition of water.
