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sqj.1935.0051

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ELIMINATION OF DUST
FROM FLUE GASES1
By J. MONKTOK, Graduate
Smoke should never be present in flue gases, but dust is unavoidable
under modern conditions. This article discusses the steps taken to
remedy the defect
THE nuisance caused by emissions from chimneys has existed for many
years, but it is only in recent years that sufficient public attention has
been aroused to demand its prevention. The building during the last few
years of the so-called " super-power stations " has undoubtedly aroused
public interest and so helped to bring about many of the present-day
developments.
It must be understood that the emission of smoke is in no way connected with the apparatus to be considered. Smoke is caused by the
imperfect combustion of the fuel in the boiler furnace, a condition which
may arise from various causes, such as bad stoking or faulty furnace
design. Whatever the reason, the emission of smoke should never be
permitted, as such a condition represents a considerable loss in efficiency
to the plant.
The entraining of small particles of ash, dust, or soot in the flue gas,
however, may occur with the most efficient plant. The particles entrained
are usually the finer portions of the coal ash, that is, the incombustible
residue of the fuel, which is an integral portion of the fire and is only
removed after combustion is complete. Particles of soot or grit from
various sources may also be thrown out of the chimneys, but these also
emanate from the coal.
Boiler Developments
In years gone by a boiler was a very simple arrangement, usually handfired and operated by natural draught. Combustion was imperfect and
volumes of smoke were emitted, particularly during the stoking period.
The speed of the gas on leaving the boiler was insufficient to entrain small
particles. In any case, the fuel was fairly large in size and the residual
ash was in the nature of clinker and had few small particles. Should any
nuisance have been caused, the high stack removed it far above the
surroundings and dispersed it over a large area.
'- Then the water-tube boiler began to come into its own. Auxiliaries
such as forced- and induced-draught fans, super-heaters, economizers, and
various forms of mechanical stoker were developed. Progress was slow,
but about the war period boiler ratings, pressures, and efficiencies began
to increase, and new designs were introduced giving increased gas velocities
throughout the boiler. Owing to the greatly increased height of the
boiler, with its auxiliaries, the tall brick stacks had been discarded in
favour of a short steel stack mounted on the boiler-house roof. The
general use of forced- and induced-draught fans had assisted in this development, as the stack now only had to supply a nominal natural draught and
carry the flue gas clear of the boiler-house itself.
These developments led to a complicated boiler design, and an example
• Abstract of Chairman's Address to Bristol Section, October i, 1931.
— 13 —
Monkton —Elimination of Dust from Flue Gases
of such practice is seen in Fig. 1. This design was the latest boiler practice
about six years ago, and still remains largely unchanged in its fundamentals.
The figure shows the long and complicated path which the flue gas follows
in a boiler while as much as possible of the heat is being extracted from it.
It should be noted that the gas velocity is at its greatest just above the
firebed, and is partly due to the pressure of the forced draught of air
supplied by a fan. About the point where combustion is complete, the
gases are brought into contact with the water tubes of the boiler. From
this point onwards the velocity of the flue gas is continuously decreasing.
FIG. 1 (left).—Diagrammatic layout
of typical modern boiler
FIG. 2 {right).—Diagram of
water-film extraction plant
1—1
The reason for this is apparent, since as the temperature of the gas
decreases so does the volume, and thus the speed. Assuming therefore
that dust is entrained in the gas after leaving the firebed, it is almost
certain that a quantity of this will be deposited as the speed decreases.
This usually happens at the bottom of the passes. These points will be
easily recognized from the figure.
Such a deposit, or sooting up of the boiler, as it is known technically,
impedes the flow.of the gas, impairs the absorption of heat, and results
in decreased efficiency of the boiler unit. To remove this deposit, sootblowers are used. These are high-pressure steam jets situated in the parts
of the boiler where they will be most effective and arranged to blow the
accumulation of dust upwards through the boiler in the direction of flow
of the gas. This means that so many times per day the dust lying in the
boiler passes is blown out in the space of a few minutes, and if suitable
apparatus is not installed practically the whole of this is blown out of the
chimney stacks and distributed over the surroundings.
As a modern boiler has to be sootblown fairly frequently in order
Students' Quarterly Journal
September 1935
to maintain its efficiency, this trouble became acute and complaints
from the public frequent. A special Committee was appointed by the
Electricity Commissioners to inquire into the position and report on the
steps being taken to mitigate the nuisance. Many devices have been
introduced to extract the dust from the flue gas, attended by varying
degrees of success, and four of these will be described briefly.
Water-film Plant
The water-film apparatus consists of an upper tank in which a constant
level of water is maintained. The water passes through orifices in the
bottom of this tank and into hollow rectangular elements, which are full
of water and constantly overflowing, the water trickling down the sides and
into the bottom tank. The water level in the bottom tank is maintained
at a constant level also by the adjustment of the discharge valve combined
with an overflow towards the top of the tank.
The path of the gas is in and out among the staggered tubes, the tubes
being so spaced that the greatest area of contact, together with the least
resistance, is offered to the passage of the flue gas. The dust particles in
the gas come into contact with the wet surfaces and sticking there are
washed down into the bottom tank, from which they are discharged with
the water. . The arrangement is shown in Fig. 2.
The success of the apparatus depends chiefly upon the special shape
of the rectangular elements. In the usual installation there are six rows
of these elements. The draught loss is fairly small, but a certain amount
of cooling of the gas takes place. It is claimed for this apparatus that it
catches very fine dust as efficiently as it catches the larger grits.
Combined Water Spray and Film
Fig. 3 shows a modified form of the equipment. The apparatus consists of a chamber divided into two compartments. The first of these
contains a number of primary, atomizing water sprays. The second contains a number of eliminating baffle elements, which are V shaped, with
the point of the V against the direction of flow. In this way the maximum
area of contact with the minimum resistance to the flow of gas is obtained.
Water is continuously trickling down these elements. Sprays of water
are also provided to wash the walls of the chamber and to deal with the
heavy discharge when sootblowing. All the water drains to the bottom
of the chamber, where a discharge is provided.
On entering the chamber the gas is expanded and saturated by the
primary jets. The extraction of the dust is facilitated by the combination
of the primary sprays and the time taken to reach the baffle elements, as
the dust acts as a nucleus for the condensation and the heavy water-laden
particles fall out of the gas stream. The efficiency of the system depends
largely on the capacity of the chamber and the quantity of water available.
The disadvantage lies chiefly in the large amount of water required.
Centrifugal Extraction
The plant shown in Fig. 4 is rather different. Here the main casing
is the volute shape, and the gas enters tangentially. Beneath the volute
are a series of alternated truncated cones and cylinders. The dust is discharged at the bottom in a dry state, and the clean gas emerges through a
central opening at the top of the main casing.
— 15 —
Monkton — Elimination of Dust from Flue Gases
The dust-laden gas entering at high velocity is forced to follow the
volute shape of the casing. The heavy dust is forced against the periphery,
and as the action of separation is centrifugal, the separating effect increases
as the radius of the volute decreases. The long path which the dust
particles have to follow to the discharge at the bottom gives a gravitational
as well as a centrifugal action and assists in the separation of the lighter
particles.
This system is usually installed after the induced-draught fan in order
to obtain the maximum gas velocity. It requires no electrical or water
supply, but is not very efficient during sootblowing.
Electrostatic Extraction
Finally, there is the electrostatic method, due in the first place to Sir
Oliver Lodge, and now widely used for precipitating dusts and mists out
FIG. 3 (left). — Combined
water spray and film plant
FIG. i (right). — Principle
of centrifugal extractor
of gases. From Fig. 5 it will be seen that the dust-laden gas passes through
pipes or plates known as collector electrodes. These are earthed and
receive the dust which is extracted from the gas. Down the centre of the
pipes or between the plates is a series of discharge electrodes, which may
consist of rods or wires fitted with discharge points. These electrodes are
maintained at a high negative potential of the order of 70,000 volts. As a
result, a brush discharge takes place and the wires or rods can be observed
to glow, being surrounded by a corona. Every solid or liquid particle in
the gas becomes ionized and migrates towards the collector electrodes.
The discharged particles fall into the discharge hoppers, which are situated
at the bottom of the chamber.
The collector plates are rapped from time to time by hammergear to
ensure that the dust particles do not adhere to the plates. The gases are
usually arranged to flow downwards in the chamber so that the extraction
of the dust is assisted by gravity. This type of apparatus gives very
little loss of draught or heat to the gas, and furthermore delivers the dust
'in a dry state. The main disadvantage lies in the special electrical equipment necessary. This does not present any technical difficulty, of course,
as high-voltage rectifiers are easily procurable, but in the case of such
high voltage, special protective equipment is required to ensure the safety
of the boiler operators and cleaners.
— 16 —
Students' Quarterly Journal
September 1935
Choice of System
All these systems are in practical operation to-day in various parts of
the country, each having its own particular advantages and disadvantages.
Much depends on the circumstances or type of plant where the dust
extractor is to be installed. In comparing the different systems, the local
conditions and facilities rather outweigh the actual efficiencies of the
different types. In some localities water is a valuable commodity. In
other types of plant, space may be the major consideration. The dust
extracting efficiency of all four systems is comparatively light.
\
FIG. 5.—Layout of electrostatic extraction
plant
It is gratifying to feel that what promised to be a menace to public
health and a source of rapid deterioration to buildings and works has been
largely eliminated. Extra expense is, of course, involved, but this is
small in comparison with the advantages gained.
AN invitation to dinner had been sent to the new doctor. In reply the
hostess received an absolutely illegible letter.
" I must know if he accepts or refuses," she declared.
" If I were you," suggested her husband, " I should take it to the
chemist. Chemists can always read doctors' letters, however badly they
are written."
The chemist looked at the sheet of notepaper which she handed him,
and, without waiting for her explanation, went into his dispensary and
returned a few minutes later with a bottle, which he handed over the
counter.
" There you are, madam," he said, " that will be half a crown."
Efficiency Magazine
— 17 —
B
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