Method of cleaning stack gas and using same for generation of electric power

Stack gas is first passed through a coarse-particle separator and then a prescrubbing tower. Then this gas, which is under pressure, is passed through a pair of differential-pressure (annular-gap) washers. The output side of one of the washers is connected directly to a droplet separator at the output of the system. The outlet of the other washer is connected through a turbine driving an electric generator and having its output side in turn connected to the droplet separator. The control body of at least the washer which is connected directly to the droplet separator is adjustable so as to maintain a constant backpressure in the system at the blast furnace from which the stack gas comes. The washer connected to the turbine is set up to pass at least four times as much of the gas as the other washers so that most of the gas passes through the turbine, undergoing a pressure drop that is transformed into the work of driving the turbine and generates electricity.

FIELD OF THE INVENTION 
The present invention relates to a method of treating pressurized stack 
gas. More particularly this invention concerns the treatment of such gases 
issuing from high-pressure blast furnaces and the like. 
BACKGROUND OF THE INVENTION 
Hot stack gases which issue from a blast furnace or the like at a pressure 
of several atmospheres are usually passed through a cleaning or purifying 
device which first separates out the larger particles carried by the gas 
and then subjects the gas to a scrubbing operation which removes many of 
the water-soluble gas components and removes additional particles from the 
gas stream. 
The above-cited earlier works describe so called Venturi or 
differential-pressure (annular-gap) washers which comprise a tube having a 
narrow waist in which is received a body that is displaceable within this 
tube so as to define a variable gap therewith. A sprayer is provided in 
the upstream end of the tube so that the turbulence and pressure drop in 
the tube will thoroughly scrub the remaining particles from the gas. 
Such differential-pressure (annular-gap) washers are used to maintain a 
constant backpressure in the blast furnance. A constant backpressure is 
necessary for proper functioning of the furnace and can readily be 
maintained by the washers. At the same time such devices serve to drop the 
pressure so that the cleaned stack gases can be used in regenerators for 
heating up the charge or the air that goes into the furnace. As a rule a 
plurality of such annular-gap washers are used with all of their insert 
bodies coupled together so as to permit adjustment of the pressure drop 
across them to maintain the pressure in the blast furnace constant. 
Such systems are relatively effective. However, they have the principal 
disadvantage that they waste a considerable amount of energy present in 
the hot pressurized stack gas. A significant amount of energy available to 
do work is wasted as the gases pass through the washers and expand. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
method of cleaning stack gas. 
Another object of this invention is to provide a stack-gas cleaning method 
which recovers at least a portion of the energy present in the stack gas. 
Yet another object is the provision of a method which allows the pressure 
in the blast furnance to be maintained constant for most efficient 
blast-furnance operation. 
SUMMARY OF THE INVENTION 
These objects are attained according to the present invention in a method 
of cleaning stack gas from a high-pressure blast furnace or the like which 
uses a pair of differential-pressure (annular-gap) washers connected 
either in parallel or in series in the conduit coming from the separator 
for removing particles and the prescrubber. The output side of one of 
these washers is connected directly to the discharge end of the system and 
the output side of the other washer is connected through a turbine to this 
discharge end. The central body of the first-mentioned washer, that is the 
one not connected to the turbine, is adjusted automatically in response to 
the pressure in the system at the blast furnace thus the annular gap 
between the body of this washer and its tube is adjusted so as to control 
the backpressure of this blast furnace. 
In accordance with another feature of this invention the turbine is used to 
drive a load, a generator being particularly suitable. In this manner it 
is possible to generate electricity with energy that would otherwise be 
completely wasted in the system. The turbine according to this invention 
is a so-called expansion turbine wherein a vapor admitted on one side 
expands in the turbine and drives its rotor. 
The invention is based on the surprising fact that it is possible to 
regulate the pressure at the input end of the system to a very fine degree 
by controlling only a portion of the gas stream through the system. In 
accordance with the present invention the variable-gap washer is set up 
for a much higher pressure drop than the other washer that is connected to 
the turbine. Thus in accordance with this invention the quantity of air 
per unit of time passing through the variable Venturi and the other 
Venturi forms a ratio of between 1:3 and 1:5, i.e. 3 to 5 parts by volume 
per unit time of gas traverses the turbine-feeding Venturi per part of gas 
traversing the controlled Venturi. This is achieved in accordance with a 
feature of this invention by providing a single variable-gap washer and a 
pair of annular-gap washers connected to the turbine. In accordance with a 
further feature of this invention the second annular-gap washers connected 
to the turbine are also adjustable for adjustment of the system such that 
the turbine runs at maximum efficiency. 
Thus in accordance with the present invention the two washers or the two 
sets of washers can be provided on a single horizontal partition wall 
provided in a scrubbing tower. The chamber below the partition wall is 
subdivided by a further partition into a pair of compartments, one of 
which is connected to the system output while the other compartment is 
connected to the turbine input. 
According to yet another feature of this invention a shunt conduit is 
provided between the input and output sides of the turbine; valves are 
provided a the input and output sides of the turbine and in this shunt 
conduit so that it is possible to close the valve in the shunt conduit 
during normal operation of the turbine and to open this valve in the shunt 
conduit and close the other two valves to allow servicing and, indeed, 
removal of the turbine during continued operation of the system. 
The two annular-gap washers or sets of washers in accordance with yet 
another feature of this invention can be provided one behind the other. 
The variable-gap washer is provided downstream of that washer whose output 
is connected in this case both to the input of the turbine and to the 
input of the second washer. According to the present invention the first 
washer is set up so that it can pass 100% of the stack gas whereas the 
second one can only pass a maximum of 20% of this gas. 
In accordance with the present invention the turbine is so set up that it 
has supplementary gas-cleaning effect that allows the formation of ice 
crystals in this turbine to be completely avoided. Thus the prescrubber 
and the first annular-gap washer is operated such that the stack gas is 
almost completely saturated with water vapor. This saturated gas is fed to 
an expansion turbine that is a one stage or multistage centripetal turbine 
with a centrifugal separator housing and whose condensation effect is such 
that the heat of condensation maintains the fluid state of the condensate. 
This saturation of the stack gas in no way adversely affects the pressure 
regulation at the head of the blast furnace or the like. As long as the 
level of water vapor in the stack gas approaches the saturation level the 
heat of condensation will ensure that under all operating conditions no 
ice crystals form. Even though this saturation does decrease slightly the 
efficiency of the Venturi washers, this slight loss is more than 
compensated by the advantage of an additional separating function in the 
expansion turbine where the considerable condensation not only prevents 
ice-crystal formation but increases separation of particles from the gas. 
It is also possible within the scope of this invention to inject water 
into the expansion turbine. 
The system according to the present invention not only serves to drop the 
pressure of and clean stack gases from a blast furnace or the like but 
also is able to maintain the pressure of the gas within this furnace 
substantially constant. At the same time the normally wasted energy of 
this stack gas is employed to generate electricity.

SPECIFIC DESCRIPTION 
As shown in FIGS. 1 and 2 an apparatus for cleaning stack gas coming from a 
blast furnace 1 has a conduit whose extreme upstream section 2a leads from 
the cupola of the furnace to a coarse-particle separator 3. Another 
conduit section 2b leads from this particle separator 3 to a prescrubber 4 
having a plurality of spray heads 5 that discharge waer sprays which, by 
washing, serve further to remove particles from the stack gas. The thus 
partially purified gas then passes through a set 6 of annular-gap washers 
from which the gas passes through conduit sections 2c and 2d to a final 
droplet separator 7 having angled vanes 8. The latter separator 7 removes 
all of the liquid from the gas before it is pulled away in a clean-gas 
output conduit 9. The pressure in the conduit section 2a is between 2 and 
3 atmospheres and that in conduit 9 at the output of the system is 
approximatly 1 atmosphere. 
The set 6 of annular-gap washers comprises a plurality of Venturi-type 
pressure-drop washers 10 and 11 as shown in FIG. 2 having respective tubes 
or sleeves 10a, 11a and 11b within each of which is displaceable a central 
body 12. Spray heads 13 are provided in the tubes 10a, 11a and 11b above 
the bodies 12 in a manner substantially as described in the above-cited 
patents. 
The prescrubber 4 and the pressure-drop washer arrangement 6 are provided 
in a single upright tower 14 in which the stack gas passes from top to 
bottom. An upwardly concave conical horizontal partition wall 15 is 
provided in the tower 14 with the annular-gap washers 10 and 11 passing 
vertically through this wall 15. Water collected by this wall 15 is drawn 
off through a conduit 16. Thus the wall 15 divides the tower 14 into an 
upper chamber and a lower chamber. An upright partition wall 17 divides 
the chamber below the partition wall into a pair of compartments 18 and 
19, the single annular-gap washer 10 opening into the compartment 18 and 
the two washers 11 into the compartment 19. The lower ends of these 
compartments are sealed by a single conical wall 20 in which are provided 
separate outlet conduits 21 and 22 for carrying off water. 
A conduit section 2c is connected to the upper region of the chamber 18 and 
feeds the gas therefrom to the upper end of the droplet separator 7. A 
similar conduit 24 is connected to the upper end of the chamber 19 and is 
connected through a valve 28 to the inlet of an expansion turbine 26 whose 
outlet side is connected through a valve 29 to a conduit section 2d also 
connected to the drop separator 7. A shunt conduit section 27 is connected 
across the two valves 28 and 29 to the opposite sides of the turbine 26 
and is provided with a shunt-off valve 30 that is normally closed. Water 
is drawn out of the droplet separator 7 at an outlet nipple 31. The valves 
28 and 29 can be closed and the valve 30 opened to allow servicing and/or 
removal of the turbine 26 without impairing gas flow through the system 
according to this invention. 
The central body 12 of the annular-gap washer 10 is vertically displaceable 
by a servomotor 23 operated by a controller 39 connected to a sensor 32 at 
the extreme upstream end of the conduit 2a-2d. When the pressure at the 
sensor 32 drops below a predetermined level the body 12 of the washer 10 
is moved downwardly to decrease the annular gap between it and the sleeve 
10a and thereby increase the back prssure across this washer 10. Inversely 
when the pressure increases above a predetermined level as dictated by the 
sensor 32 the body 12 is lifted so as to decrease the back pressure across 
this washer 10 and thereby maintain a generally constant pressure in the 
blast furnace 1. 
The pressure-drop washers 10 and 11 are so dimensioned that once the 
furnace 1 is operating at normal speed approximabely four times more stack 
gas passes through the washers 11 than through the washer 10. An 
installation as shown in FIG. 1 is used with stack gas at an original 
pressure of 15,000 mm water column with 20% of this stack gas passing 
through the washer 10 so as to reduce the pressure by 14,000 mm water 
column to a pressure of 1,000 mm water column. The remaining 80% of the 
stack gas is fed through the second washers 11 which reduce the pressure 
only by 3,000 mm water column so that the gas is fed to the turbine 26 
with a pressure of 12,000 mm water column. This turbine 26 drops the 
pressure by another 11,000 mm water column so that at the drop separator 7 
the gas has a pressure of 1,000 mm water column. Considerable work is 
created by the turbine in this manner. 
As the blast furnace 1 is started up the expansion turbine 26 is cut out by 
closing the valves 28 and 29 opening the valve 30. Simultaneously the 
washers 10 and 11 are so adjusted that the various separators and 
scrubbing devices work at maximum efficiency. This can be effected by 
another control circuit similar to the control circuit 32. Once the blast 
furnace 1 is at regular operaing condition the control apparatus 32 
proceeds to function and the washers 11 are set for maximum cleaning 
efficiency and maximum operating efficiency of the expansion turbine and 
the generator 25. The valves 28 and 29 are then opened and the valve 30 
closed. The control body 12 of the washer 10 is then adjusted so as to 
maintain the necessary backpressure at the sensor 32. 
In the arrangement shown in FIGS. 3 and 4 the numerals of FIGS. 1 and 2 are 
used wherever the structure is identical. In this arrangement the tower 4' 
is provided with a pair of series-connected washers 11' and 10' 
constituting a stepped pressure-drop washer assembly 6'. 
The downstream annular-gap washer 10' is provided with a servomotor 33 
connected to the control 39 so as to allow adjustment of the annular gap 
in this washer 10', thereby changing the back pressure created thereby. A 
bypass conduit 35 in which on expansion turbine 26 is provided has a pair 
of cutoff valves 36 which allow the turbine 26 to be cut completely out of 
the circuit. This bypass conduit 35 opens at its upstream end into the 
chamber at the downstream side of the washer 11'. A valve 37 is provided 
in the conduit 2c into which the bypass conduit opens downstream of the 
turbine 26 at the outlet side of the washer 10'. 
The control body 12' of the washer 11' is provided with a servomotor 34 
connected to a control system 38 which pneumatically displaces this 
control body 12' vertically by means of a cylinder illustrated 
schematically at 34' in FIG. 4. The upper pressure-drop washer 11 is 
capable of passing 100% of the stack gas whereas the downstream washer 10' 
can only pass at a maximum 20% of the stack gas flowing through the 
conduit 2a-2c. 
FIG. 5 shows the two-stage expansion turbine 26 according to the present 
invention. This turbine has a centripetal separator housing 41. A gas 
enters as shown by arrows 42 and leaves as shown by arrows 43. Arrows 44 
indicate how the centripetal separator housing 41 is shaped substantially 
as a spiral. Condensate with the dust caried therewith leaves the housing 
41 at the outlets 45 and 46. With such a turbine 26 the prescrubber and 
the pressure-drop arrangment 11 is set such that the gas is virtually 
saturated with water vapor so that the turbine 26 operates with a 
condensation effect that is so adjusted that the heat of condensation 
maintains the condensate fluid. In addition the expansion turbine 26 can 
be provided as indicated by arrow 40 with an arrangement for introducing 
water to its interior. Thus the expansion in this turbine 26 functions 
without formation of ice crystals.