Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a plant for producing power
from solid fossil fuels, more specially bituminous coal, made
up of at least one boiler, in which the solid fuels are burned,
the plant furthermore having a steam turbine and a gas turbine
for producing power from the gas, the gases being cleaned of
dust and sulfur in the plant before going into the gas turbine.
PRIOR ART
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In an earlier suggestion ~or such a plant (see the
German Offenlegungsschrift specification 2,650,491), a fuel after
grinding is fed to a boiler with a pressure firing system. The
stack gases from the boiler are cleaned of dust and undesired,
damaging substances, and furthermore undergo desulfurizing before
being used in the gas turbine. The teaching of this earlier
suggestion is that of designing a plant of this sort such that
losses and troublesome conditions on operation are decreased. A
~urther purpose was that of designing such a plant that it may
be used in peak load power stations. The losses take the form of
~0 unburned material in the ash and the heat losses in the quenching
water, while troublesome conditions on operation are caused, more
particularly, by the tar-dust mix produced.
It will be seen that with the teaching of this earlier
suggestion the part-burning or part-combustion and the later-stage
~5 burning takes place in a single part of the plant, so that only
one operation has to be automatically controlled. The operation
may be so controlled as is necessary for a peak load power station;
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that is to say the power may be increased at a generally high
rate, and decreased again to be in line wi~h the power output of
the plant without this being responsible for heat losses or its
being necessary to make use of oil or further, other fuels in
addition for supporting the burning process. Because burning
of the fuel takes place in a boiler, the forming of tar is stopped
and special parts of such plant, so far necessary in other designs,
are not needed and troublesome conditions may be put to an end.
On the stack gas side the pressure in the boiler is of the order
of 10 bar, this generally being in line with the pressure relation
or ratio of the turbine.
The main purposes effected by such a plant is that the
amount of unburned material in the boiler ash becomes unimportant.
The thermal efficiency is very much greater, because losses caused
by evaporation of quenching water outside the steam circuit are
generally put to an end, such losses having been so far necessarily
produced in the case of gasification of the fuel. Because, further-
more, the fuel is burned in an apparatus itself under pressure, it
is possible to do without a number of further units, used in past
~0 gasification systems, that is to say, more specially, a gas pro-
ducer, while the system is better than a fluidized bed system
because a very much smaller unit is needed for the burning process.
BRIEF DESCRIPTION OF THE INVENTION
One purpose of the present invention is that of making
such a further development of this plant that burning takes place
under the slag melting temperature.
A further purpose of the invention is thatof making
possible a very much smaller size of the plant.
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A still further purpose of the present invention is
that of increasing the thermal efficiency so as to be greater
than past designs of plants working at atmospheric pressure.
For effecting thse and other purposes the plant of the
invention has a boiler in the form of a pressure-fired boiler
with dry firing. For decreasing the fire space temperature so
as to be under the ash melting temperature, use is best made of
a stack gas or flue gas recycling system working under pressure.
In this respect, a part of the stack gas, clear of dust
and so far not heated up again, is run back into the burning
space or chamber so that the desired decrease in the firing
space temperature is produced, while at the same time making
for a smaller size of the burning space in comparison with past
designs of dry firing systems. The division up of the gas currents
or flows takes place at a point downstream from the boiler.
More specially, in a further development of the present
invention, a part-current of the stack gases, at a point down-
stream from the boiler, is run into a waste heat boiler. An
amount, as generally necessary for stack gas recycling, goes into
~0 this waste heat boiler, in which the gases are cooled down to
about 130C and then go into a dust cleaning unit which is common
to the main and recycle currents or flows.
The purpose of the waste heat boiler is:
1. that of cooling down the amount of gas (greater
~5 than about 15% of the overall amount of gas)
necessary for stack gas recycling, from a high
temperature level and in this respect producing
high pressure steam; and
2. controlling the amount of heat offered to
the gas turbine.
The use of a waste heat boiler is very much more use-
ful than a system in which the complete gas current is caused to
go through the gas-to-gas heat exchanger and then nextly through
the trimming cooler, this being so because:
1. The taking of heat for the amount of gas, to be
used for recycling, takes place at a high tempera-
ture level in the waste heat boiler for producing
hight pressure steam, and not at a low temperature
level in the trimming cooler for the first stage
of heating of boiler input or feed water. This is
responsible for better conditions in the steam
cycle, that is to say the overall thermal
1~ efficiency is increased.
2. Because of the high temperature level in the waste
heat boiler in comparison with a trimming cooler,
while having the same heat throughout, very much
smaller heating faces are necessary.
~0 3. In the waste heat boilers amounts of heat may be
guided through the system in different ways as de-
sired by using an automatic control valve flap down-
stream from the waste heat boiler, this making possi-
ble the best possible conditions in the steam-gas
turbine side of the system in the separate load
stages. After the cleaning of dust from the gas,
the amount of recycle gas is run through a pressure
increasing compressor back into the burning space
or chamber.
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After desulfurizing the main gas current or flow is
run back into the main pressure increasing compressor, in which
respect, for increasing the thermal efficiency the gas pressure,
necessary for the gas turbine, is best produced in two compressors,
the output temperature at the air compressor being about 200C
higher than the stack gas input temperature to the pressure
increasing compressor.
The relation between the mass of the stack gas amount
upstream from the gas turbine to the amount of sucked in air is
about 1:107. If the gas cleaning takes place at a high tempera-
ture level, the gas-to-gas heat exchanger and the waste heat
boiler will not be needed, the recycling of the stack gas taking
place downstream from the waste heat exchanger.
List of Figures
Further measures and useful effects of the present in-
vention will be clear from the account now ~o be given of two
working examples to be seen in the Figures.
Figure 1 is a view of a first embodiment of the invention
with a pressure increaisng compressor placed downstream from the
~0 gas cleaning unit, from which a part of the cleaned amount of
gas goes back into the firing space; and
Figure 2 is a view of a further embodiment of the
invention with some changes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
After grinding coal is run through a pipe 1 under
pressure into a dry firin~ system 2 of a boiler, generally
numbered 3.
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The coal is burned under pressure, water for the
steam cycle being evaporated. The steam is superheated at 4.
~The stack or flue gases come out at 5 and are cleaned of dust
in part in a cyclone 6. Then the gas current undergoes division,
that is to say the main current goes to a gas-to-gas heat
exchanger 7, in which the uncleaned s~ack gases are cooled down,
for example, to about 300C by the cold, cleaned stack gases.
The further, second current or flow, which in level is
ganerally equal to the recycle current in rate or amount, is run
into a waste boiler 7a, in which the water for the steam cycle
is evaporated and superheated at 7B. In this respect, the gas
is cooled down to about 130C~ The automatic control of the
second current is undertaken by an automatic control valve
flap 7c placed downstream from the waste heat boiler.
The main gas flow, after going through the gas-to-gas
heat exchanger/ is run into a trimming cooler 8, where the stack
gases are cooled down to about 130C. At 8a, the main and second
currents are united and go to a unit g for cleaning them of even
the inest dust, for example in the form of an electrostatic
~0 ilter or a filter made up of fabric pipes. The stack gas,
cleaned of dust, undergoes division at 9a.
The main current goes to a wet desulfurizing unit 10,
where it is washed with a clear alkaline washing liquid or
solution for the washing out, in addition to HCl, chlorine,
2~ HF, fluorine and NOX, of more specially SO2.
The useful effect of such a washing system working
under pressure in comparison with a unit working at atmospheric
pressure is that: -
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(a) the unit is very much smaller in size; and
(b) the form of the reactions in the washing step
is very much better, because there is a decrease
in the number of molecules with the reaction.
The second current (the recycling current) is run back
by way of a pressure increasing compressor 12a to the boiler
and is used for automatic control of the temperature of the
firin~ space. The main gas current, after being desulfurized,
is run into a mist cleaning unit ll, in which the gas is freed
of washing liquid mist having undesired materials in it.
The cleaned gases go to a main pressure increasing
compressor 12, which, at the same time, is used as a unit for
cleaning of the last parts of any mist still present.
The stack gas after being cleaned and freed of mist
is now run into the gas-to-gas heat exchanger and is heated
up to, for example, 800 to 900C by the uncleaned stack gas,
this being the gas turbine temperature.
The gas turbine 13 is used for driving an air com-
pressor 14 and a generator 15. The compressor 14 is responsible
2~ for compressing the burning or combustion air necessary for
burning the coal under pressure. The generator 15 is used for
producing electrical power.
Downstream from the gas turbine, the stack gas, with
a temperature of greater than 400C for example, goes into the
waste heat exchanger 16 where it is responsible for the first
stage of heating-up of a part of the boiler input water, necessary
for the steam producing process, and, itself, is cooled down to,
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for example, 120C.
The boiler input water, after this first stage of
heating, goes at 17 into the pressure-fired boiler with a dry
firing system, in which it is evaporated, and then it is super-
heated at 4 and, together with the superheated steam 7B comingfrom the waste heat boiler 7a, goes to a steam turbine 18,
driving a generator 19 which, as well, is responsible for
producing electrical power.
The ash produced in the parts 3, 6, 7, 7a and 9 of the
plant is taken out by way of ash boxes with airlocks. The
washing liquid coming from the parts 10 and 11 of the plant and
having undesired material in it, goes by way of a relaxation
turbine 20 to the oxidizing unit 21.
In this unit, by the blowing in of air at 22 and the
input of calcium bisulfate at 23, calcium sulfate is formed by
the reactions:
ca(HSO3)2 + 2 ~ CaSO4 + H2S4
H2SO4 + Ca(OH)2 C 4 2
In the desulfurizing unit the main reactions are:
2HCl + Ca(OH)2 ~ CaC12 + 2H2O
CaC12 + 2SO2 + 2H2O ~Ca(HSO)3 + 2HCl
(molecule ratio 5/3)
From the oxidizing unit, the calcium sulfate-water mix
goes into a gravity separating unit 24, in whose settlement
vessel the calcium sulfate crystals precipitate and are pumped
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from the lower end of the vessel into a drum vacuum filter
25, the calcium sulfate-water mix having 60 to 80 per cent of
water. In the filter all but about 20 per cent of the water
is filtered off, that is to say the amount ot water in the
calcium sulfate at the end of filtering is about 20 per cent.
The water coming from the drum vacuum filter goes
by way of a pump 26 to the settlement container of the gravity
separating unit 24. At the top part of the settlement vessel
recycling of the washing liquid or water is undertaken by way
of the pump 27, which is powered, in part, by the relaxation
turbine 20, the liquid then going to the pressure desulfurizing
unit 10.
For cleaning NOX from the uncleaned stack gas in the
temperature range of 900 to 1100C between the coarse dust
1~ cleaning unit 6 and the gas-to-gas heat exchanger 7 ammonia is
let in at atmospheric pressure in the presence of oxygen.
In the system of Figure 2, the part numbers are used
~or the same parts as in Figure 1. Because the only way the
system is different to that of Figure 1 is with respect to the
cleaning of undesired substances from the system, it is only
this part which will now be given a detailed account.
The stack gas coming from the unit 9 for cleaning the
finest dust, goes from this position into a wet desulfurizing
unit 29 working under pressure, where it is processed with
watered-down sulfuric acid at 30, the sulfuric acid being
mixed with iron sulfate.
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In this respect, the main reactions are
(a) absorption SO2 ~ H2O < 2 3
(b) oxidation 2 3 + 1/2 2~ 2 4
(c) odixation 2FeSO4 + SO2 2 ~ ~ Fe2(SO4)3
by catalyst Fe2(S4)3 + SO4 + 2H2O ~ 4
2 4
All reactions take place better under pressure, but
for the last one in which the number of molecules is the same
before and after the reaction r that is to say a reaction in
which pressure unimportant.
For washing out the undesired, damaging materials
and for causing the reactions to take place, softened washing
water is run into the system at 31.
The stack gas coming from the washing tower and to
a very high degree cleaned of damaging substances (SO2, SO3,
HCl, chlorine, HF, fluorine and NOX) goes from this position
into the mist separating unit 11, in which it is freed of mist.
The further processing and use of the stack gas will
be seen from the account given in connection with Figure 1.
The washing liquid coming from the system at 31 and
11, with the undesired substances (H2SO4 H~NO)SO Fe(NO)SO
FeSO4 FeC12, H2O), undergoes division at 32. Part of the amount
of washing liquid goes by way of a relaxation turbine 33, which,
for its part, is used for driving a circulation pump 34, to an
electrolytic unit 35. The main part of the flow of the washing
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liquid goes by way of jets at 30 back into the wet desulfurizing
unit 29.
The dirty washing liquid, that is to say washing liquid
with undesired substances, is freed of Fe catalyst in the
S electrolytic unit, the Fe catalyst precipitating as Fe metal,
which in the part 36 of the system goes by way of a dissolving
unit back into the washing cycle. At the same time, at the
anode, an equivalent amount of chlorine is produced, which is
taken up in water 37 as hypochlorite.
The excess acid, freed of Fe after electrolytic pro-
cessing, goes inot an evaporator 38 heated by steam in which
the acid is heated up to a temperature of about 130C and
condensed to about 70 per cent sulfuric acid by the evaporation
of water, HCl, HF and NO, which go into a cooler 39.
l~ In the cooler condensing from these gases takes place
in the form of hydrochloric acid with a low amount of HF, the
acid being run into a takeup vessel 40.
The rest of the NO, together with the rest of the
water vapor is run into a carbon bed 41 and changed into N2
~0 and CO~.
