The Meldurm "Simplex" destructor (fig. 3), a type of furnace which
yields good steam-raising results, is in successful operation at
Rochdale, Hereford, Darwen, Nelson, Plumstead and Woolwich, at each
of which towns the production of steam is an important consideration.
Cells have also been laid down at Burton, Hunstanton, Blackburn and
Shipley, and more recently at Burnley, Cleckheaton, Lancaster,
Nelson, Sheerness and Weymouth. In general arrangement the destructor
differs considerably from those previously described. The grates are
placed side by side without separation except by dead plates, but, in
order to localize the forced draught, the ash-pit is divided into
parts corresponding with the different grate areas. Each ash-pit is
closed air-tight by a cast iron plate, and is provided with an
air-tight door for removing the fine ash. Two patent Meldrum
steam-jet blowers are provided for each furnace, supplying any
required pressure of blast up to 6 in. water column, though that
usually employed does not exceed 1½ in. The furnaces are designed for
hand-feeding from the front, but hopper-feeding can be applied if
desirable. The products of combustion either pass away from the back
of each fire-grate into a common flue leading to boilers and the
chimney-shaft, or are conveyed sideways over the various grates and a
common fire-bridge to the boilers or chimney. The heat in the gases,
after passing the boilers, is still further utilized to heat the air
supplied to the furnaces, the gases being passed through an air
heater or continuous regenerator consisting of a number of cast iron
pipes from which the air is delivered through the Meldrum "blowers"
at a temperature of about 300° F. That a high percentage (15 to
18%) of CO_2 is obtained in the furnaces proves a small excess of
free oxygen, and no doubt explains the high fuel efficiency obtained
by this type of destructor. High-pressure boilers of ample capacity
are provided for the accumulation during periods of light load of a
reserve of steam, the storage being obtained by utilizing the
difference between the highest and lowest water-levels and the
difference between the maximum and working steam-pressure. Patent
locking fire-bars, to prevent lifting when clinkering, are used in
the furnace and have a good life. At Rochdale the Meldrum furnaces
consume from 53 lb. to 66 lb. of refuse per square foot of grate
area per hour, as compared with 22.4 lb. per square foot in a
low-temperature destructor burning 6 tons per cell per 24 hours with
a grate area of 25 sq. ft. The evaporative efficiency of the Rochdale
furnaces varies from 1.39 lb. to 1.87 lb. of water (actual) per 1
lb. of refuse burned, and an average steam-pressure of about 114
lb. per square inch is maintained. The cost of labour and
supervision amounts to 10d. per ton of refuse dealt with. A
Lancashire boiler (22 ft. by 6 ft. 6 in.) at the Sewage Outfall
Works, Hereford, evaporates with refuse fuel 2980 lb. of water per
hour, equal to 149 indicated horse-power. About 54 lb. of refuse are
burnt per square foot of grate area per hour with an evaporation of
1.82 lb. of water per pound of refuse.
[Illustration: FIG. 4.--Beaman and Deas Destructor at Leyton.]
Beaman and Deas.
The Beaman and Deas destructor[5] (fig. 4) has attracted much
attention from public authorities, and successful installations are
in operation at Warrington, Dewsbury, Leyton, Canterbury, Llandudno,
Colne, Streatham, Rotherhithe, Wimbledon, Bolton and elsewhere. Its
essential features include a level-fire grate with ordinary type
bars, a high-temperature combustion chamber at the back of the cells,
a closed ash-pit with forced draught, provision for the admission of
a secondary air-supply at the fire-bridge, and a firebrick hearth
sloping at an angle of about 52°. From the refuse storage platform
the material is fed into a hopper mouth about 18 in. square, and
slides down the firebrick hearth, supported by T-irons, to the grate
bars, over which it is raked and spread with the assistance of long
rods manipulated through clinkering doors placed at the sides of the
cells. A secondary door in the rear of the cell facilitates the
operation. The fire-bars, spaced only 3/32 in. apart, are of the
ordinary stationary type. Vertically, under the fire-bridge, is an
air-conduit, from the top of which lead air blast pipes 12 in. in
diameter discharging into a hermetically closed ash-pit under the
grate area. The air is supplied from fans (Schiele's patent) at a
pressure of from 1½ to 2 in. of water, and is controlled by means of
baffle valves worked by handles on either side of the furnace,
conveniently placed for the attendant. The forced draught tends to
keep the bars cool and lessen wear and tear. The fumes from the
charge drying on the hearth pass through the fire and over the
red-hot fire-bridge, which is perforated longitudinally with
air-passages connected with a small flue leading from a grated
opening on the face of the brickwork outside; in this way an
auxiliary supply of heated oxygen is fed into the combustion chamber.
This chamber, in which a temperature approaching 2000° F. is
attained, is fitted with large iron doors, sliding with balance
weights, which allow the introduction of infected articles, bad meat,
&c., and also give access for the periodical removal of fine ash from
the flues. The high temperatures attained are utilized by installing
one boiler, preferably of the Babcock & Wilcox water-tube type, for
each pair of cells, so that the gases, on their way from the
combustion chamber to the main flue, pass three times between the
boiler tubes. A secondary furnace is provided under the boiler for
raising steam by coal, if required, when the cells are out of use.
The grate area of each cell is 25 sq. ft., and the consumption varies
from 16 up to 20 tons of refuse per cell per 24 hours. In a 24-hours'
test made by the superintendent of the cleansing department, Leeds,
at the Warrington installation, the quantity of water evaporated per
pound of refuse was 1.14 lb., the average temperature in the
combustion chamber 2000° F. by copper-wire test, and the average air
pressure with forced draught 2½ in. (water-gauge). At Leyton, which
has a population of over 100,000, an 8-cell plant of this type is
successfully dealing with house refuse and filter press cakes of
sewage sludge from the sewage disposal works adjoining, and even with
material of this low calorific value the total steam-power produced
is considerable. Each cell burns about 16 tons of the mixture in 24
hours and develops about 35 indicated horse-power continuously, at an
average steam-pressure in the boilers of 105 lb. The cost of labour
at Leyton for burning the mixed refuse is about 1s. 7d. per ton; at
Llandudno, where four cells were laid down in connexion with the
electric-light station in 1898, it is 1s. 3¼d., and at Warrington
9½d. per ton of refuse consumed. Combustion is complete, and the
destructor may be installed in populous districts without nuisance to
the inhabitants. Further patents (Wilkie's improvements) have been
obtained by Meldrum Brothers (Manchester) in connexion with this
destructor.
Heenan.
The Heenan furnaces are in operation at Farnworth, Gloucester,
Barrow-in-Furness, Northampton, Mansfield, Wakefield, Blackburn,
Levenshulme, Kings Norton, Worthing, Birmingham and other places, and
are now dealing with over 1200 tons of refuse per day. The general
arrangement of this destructor somewhat resembles that of the Meldrum
type. The cells intercommunicate, and the mechanical mixture of the
gases arising from the furnace grates of the various cells is sought
by the introduction of a special design of reverberatory arch
overlying the grates. The standard arrangement of this destructor
embodies all modern arrangements for high-temperature refuse
destruction and steam-power generation.
Sterling.
Destructors of the "Sterling" type, combined with electric-power
generating stations, are installed at Hackney (1901), Bermondsey
(1902) and Frederiksberg (1903)--the first-named plant being probably
the most powerful combined destructor and electricity station yet
erected. In these modern stations the recognized requirements of an
up-to-date refuse-destruction plant have been well considered and
good calorific results are also obtained.
In addition to the above-described destructors, other forms have been
introduced from time to time, but adopted to a less degree; amongst
these may be mentioned Baker's destructor, Willshear's, Hanson's
Utilizer, Mason's Gasifier, the Bennett-Phythian, Cracknell's
(Melbourne, Victoria), Coltman's (Loughborough), Willoughby's, and
Healey's improved destructors. On the continent of Europe systems for
the treatment of refuse have also been devised. Among these may be
mentioned those of M. Defosse and M. Helouis. The former has
endeavoured to burn the refuse in large quantities by using a forced
draught and only washing the smoke.[6] Helouis has extended the
operation by using the heat from the combustion of the refuse for
drying and distilling the material which is brought gradually on to
the grate.
Destructor accessories.
Boulnois and Brodie's improved charging tank is a labour-saving
apparatus consisting of a wrought iron truck, 5 ft. wide by 3 ft.
deep, and of sufficient length to hold not less than 12 hours supply
for the two cells which it serves. The truck, which moves along a
pair of rails across the top of the destructor, may be worked by one
man. It is divided into compartments holding a charge of refuse in
each, and is provided with a pair of doors in the bottom, opening
downwards, which are supported by a series of small wheels running on
a central rail. A special feeding opening in the reverberatory arch
of the cell of the width of the truck, situated over the drying
hearth, is formed by a firebrick arch fitted into a frame capable of
being moved backwards and forwards by means of a lever. The charging
truck, when empty, is brought under the tipping platform, and the
carts tip directly into it. When one of the cells has to be fed, the
truck is moved along, so that one of the divisions is immediately
over the feeding opening, and the wheel holding up the bottom doors
rests upon the central rail, which is continued over the movable
covering arch. Then the movable arch is rolled back, the doors are
released, and the contents are discharged into the cell, so that no
handling of the refuse is required from tipping to feeding. This
apparatus is in operation at Liverpool, Shoreditch, Cambridge and
elsewhere.
Various forms of patent movable fire-bars have been employed in
destructor furnaces. Among these may be mentioned Settle's,[7]
Vicar's,[8] Riddle's rocking bars,[9] Horsfall's self-feeding
apparatus,[10] and Healey's movable bars;[11] but complicated movable
arrangements are not to be recommended, and experience greatly
favours the use of a simple stationary type of fire-bar.
[Illustration: FIG. 5.--Leyton Destructor. Block Plan, showing
general arrangement of the Works.]
A dust-catching apparatus has been designed and erected at Edinburgh,
by the Horsfall Furnace Syndicate, in order to overcome difficulties
in regard to the escape of flue dust, &c., from the destructor
chimney. Externally, it appears a large circular block of brickwork,
18 ft. in diameter and 13 ft. 7 in. high, connected with the main
flue, and situated between the destructor cells and the boiler.
Internally it consists of a spiral flue traversing the entire
circumference and winding upwards to the top of the chamber. There is
an interior well or chamber 6 ft. diameter by 12 ft. high, having a
domed top, and communicating with the outer spiral flue by four ports
at the top of the chamber. Dust traps, baffle walls and cleaning
doors are also provided for the retention and subsequent weekly
removal of the flue dust. The apparatus forms a large reservoir of
heat maintained at a steady temperature of from 1500º to 1800° F.,
and is useful in keeping up steam in the boiler at an equable
pressure for a long period. It requires no attention, and has proved
successful for its purpose.
Travelling cranes for transporting refuse and feeding cells are
sometimes employed at destructor stations, as, for example, at
Hamburg. Here the transportation of the refuse is effected by means
of specially constructed water-tight iron wagons, containing
detachable boxes provided with two double-flap doors at the top for
loading, and one flap-door at the back for unloading. There are
thirty-six furnaces of the Horsfall type placed in two ranks, each
arranged in three blocks of six in the large furnace hall. An
electric crane running above each rank lifts the boxes off the wagons
and carries them to the feeding-hole of each well. Here the box is
tipped up by an electric pulley and emptied on to the furnace
platform. When the travelling crane is used, the carts (four-wheeled)
bringing the refuse may be constructed so that the body of the
carriage can be taken off the wheels, lifted up and tipped direct
over the furnace as required, and returned again to its frame. The
adoption of the travelling crane admits of the reduction in size of
the main building, as less platform space for unloading refuse carts
is required; the inclined roadway may also be dispensed with. Where a
destructor site will not admit of an inclined roadway and platform,
the refuse may be discharged from the collecting carts into a lift;
and thence elevated into the feeding-bins.
Other accessory plant in use at most modern destructor stations
includes machinery for the removal, crushing and various means of
utilization of the residual clinker, stoking tools, air heaters or
regenerators for the production of hot-air blast to the furnaces,
superheaters and thermal storage arrangements for equalizing the
output of power from the station during the 24-hours' day.
Working of destructors.
The general arrangement of a battery of refuse cells at a destructor
station is illustrated by fig. 5. The cells are arranged either side by
side, with a common main flue in the rear, or back to back with the main
flue placed in the centre and leading to a tall chimney-shaft. The
heated gases on leaving the cells pass through the combustion chamber
into the main flue, and thence go forward to the boilers, where their
heat is absorbed and utilized. Forced draught, or in many cases, hot
blast, is supplied from fans through a conduit commanding the whole of
the cells. An inclined roadway, of as easy gradient as circumstances
will admit, is provided for the conveyance of the refuse to the tipping
platform, from which it is fed through feed-holes into the furnaces. In
the installation of a destructor, the choice of suitable plant and the
general design of the works must be largely dependent upon local
requirements, and should be entrusted to an engineer experienced in
these matters. The following primary considerations, however, may be
enumerated as materially affecting the design of such works:--
(a) The plant must be simple, easily worked without stoppages, and
without mechanical complications upon which stokers may lay the blame
for bad results. (b) It must be strong, must withstand variations of
temperature, must not be liable to get out of order, and should admit
of being readily repaired. (c) It must be such as can be easily
understood by stokers or firemen of average intelligence, so that the
continuous working of the plant may not be disorganized by change of
workmen. (d) A sufficiently high temperature must be attained in the
cells to reduce the refuse to an entirely innocuous clinker, and all
fumes or gases should pass either through an adjoining red-hot cell
or through a chamber whose temperature is maintained by the ordinary
working of the destructor itself at a degree sufficient to exclude
the possibility of the escape of any unconsumed gases, vapours or
particles. The temperature may vary between 1500° and 2000°. (e) The
plant must be so worked that while some of the cells are being
recharged, others are at a glowing red heat, in order that a high
temperature may be uniformly maintained. (f) The design of the
furnaces must admit of clinkering and recharging being easily and
quickly performed, the furnace doors being open for a minimum of time
so as to obviate the inrush of cold air to lower the temperature ...
(_Continued in volume 8, slice 3, page 109._)