T. E. (Thomas Edward) Thorpe.

A dictionary of applied chemistry online

. (page 147 of 183)
Online LibraryT. E. (Thomas Edward) ThorpeA dictionary of applied chemistry → online text (page 147 of 183)
Font size
QR-code for this ebook


a solid bottom, and n, lateral opening com-
municating with the air and steam supplies.



(2) A vapouriser, E, which may be either within
or without the upper part of the shell of the
generator, in which a regulated water supply is
vapourised, usually at the expense of some of
the sensible heat of the hot gas leaving the
generator, or sometimes by the heat of the
fire. (3) A water-sprayed cohe scrubber, f, for
cooling and washing the gas ; and (4) an
expansion box, a, which serves the double
purpose of a gas reservoir for the engine, and, on
the suction stroke, of minimising the jerkiness
of the suck. The generator is started up by
kindling a wood fire on the grate with a layer
of coke or anthracite above. The fire is blown
up by means of a hand fan, which is a neces-
sary auxiliary to the apparatus. During the
starting-up period, the products of combustion
are sent into the atmosphere through a vent
pipe, but as soon as a rich enough gas is generated •
(about 20-30 minutes from the start), the engine
is started and the fan shut off. A typical
' suction gas,' generated from gas coke with air
saturated with steam at 51 -T", contains
COa=5-10, 00=25-45, H= 13-10, CHi=0-28,
and N=55-32 p.c, its net calorific value being
127 B.Th.Us. per cub. ft. at 0° and 760 mm. The
thermal efiiciency of the gasification, allowing '
for the power required to draw the gases through
the system, and based on the net calorific values of
coke and gas respectively, is about 78-5 p.c.
The fuel consumption on a suction plant is
about 1 lb. or even slightly less, per b.h.p.
developed, the water consumption for scrubbing
the gas being about 1-2 gallons per b.h.p.

Ammonia-reeoTery systems. The solution of
the problem of the gasification of coal under
conditions which permit of the recovery of a
large proportion of its nitrogen as ammonia, was
due to the late Dr. Ludwig Mond. The best
practical results are obtained by working the
producer with a blast-steam saturation tempera-
ture of 85°, the mixture of air and steam being
preheated to about .250° befose it enters the
fuel bed. There is carried into the producer by
the blast about 2 lbs. of steam for every 1 lb. of
coal gasified ; one-third only of this steam is
decomposed, by interaction with carbon, in
passing through the fuel bed, the remaining
two-thirds passing with the hot gases out of
the producer. The outstanding features of the
process are ( 1 ) that, owing to the cooling infiuence
of the large proportion of steam in the blast
upon the fuel bed, the low temperature inter-
action C-t-2H2O=0Oa+Hj predominates over
the high temperature ' water-gas ' reaction,
0+H20=00-J-Hj, so that the resulting gas
{e.g. 17-0 CO'a, 110 00, 24-0 H, 3-0 OH,, and
45-0 N) has a high hydrogen and low carbon
monoxide content as compared with ordinary
producer gas; and (2) that for economical
working it is necessary that the large proportion
of steam leaving the producer shall be conserved
to the system by some efficient recuperative
arrangement. From a coal containing between
1-2 and 1-6 p.c. N it is possible to obtain up to
90 lbs. per ton of ammonium sulphatej together
with about 150,000 cub. ft. of gas (at 0° and 760
mm.) of calorific value about- 150 gross and
135 net, B.TKUs. per cub. ft. at 0° and 760 mm.
According to the Mond system of ammonia
recovery and steam recuperation, as originally
designed, the gas leaves the producer heavily



FUEL



C23



charged with steam and tarry vapours at a
temperature of 500°-600°. It then traverses
the central tubes of a series of three annular
' superheaters,' indicated by B in the diagram
(Fig. ll)j each about 20 feet long. The incoming
steam-air blast passes through the annulus
between the central and outer tubes in the
reverse direction. The temperature of the gas
is thus reduced to about 250°, whilst that of
the blast is raised to about 200° (further ' super-
heating ' of the blast occurs as it subsequently
traverses the annulus between the fire-brick
lining and the outer steel casing of the producer
itself). The partly cooled gas next passes
through a long rectangular and water-sealed
chamber (the ' mechanical washer '), o, where it
encounters a water spray thrown up by a series
of revolving dashers, by which means dust, soot,
and a large amount of heavy tar are removed,
and the gas itself further cooled to about 100°.
lb next passes up the lead-lined scrubbing
tower D, packed with perforated bricks or tiles,
down which an acid liquor containing 36-38 p.c.
of ammonium sulphate 'plus 2-5 p.c. of sulphuric
acid is sprayed, the volume of liquor being so
adjusted in relation to the upward flow of gas
that the latter leaves the tower at a temperature
of 80° and free from ammonia. The tarry
liquor from the tower is passed through a
separator whereby partial separation of liquor
and tar is effected ; to the main bulk of the clear
liquor is added a regulated quantity of .sulphuric
acid, after which it is again pumped up to the
top of the acid tower D. A portion of the clear
liquor is, however, remove^ from the circuit,
and, after treatment with heavy oils (if necessary)
to remove tar, is evaporated to orystaUising-point
in a conical lead-lined vessel furnished with
steam coils. After separation of ammonium
sulphate, the mother liquor is pumped back into
the main-liquor circuit. The ammonia and tar-
free gas, on leaving the acid tower at 80°, is
parsed up the • gas cooling tower ' G, where it
encounters a downward spray of cold water,
■ so regulated that whilst the gas is cooled down
to 40°-50°, and rendered substantially free from
tar, the water is heated up to 75°-78°. The
gas passes onwards to the place of consumption,
but if intended for gas engines it should be
further cooled down to within a few degrees of
the atmospheric temperature, and be also
passed through a sawdust scrubber to remove
the last traces of tar.

The hot water from the ' gas-cooling tower '
is passed through a tar separator, after which it
is pumped up to the top of the ' air-saturating
tower' K down which it is sprayed. The air
blast for the producer is forced up this tower by
means of a blower ; in this way the blast is
heated up to and saturated with steam at 75°
at the expense of the hot water. A further
quantity of live steam is added to the air blast
as it passes forward to the superheaters in order
to raise its steam-saturation temperature to 85°.
•The chief drawback to the original Mond
system was the large capital outlay involved,
which rendered ammonia recovery unprofitable
unless the coal gasified exceeded 180-200 tons
per week. Moreover, the gas, being rich in
hydrogen and poor in carbon monoxide, is not well
suited for open-hearth steel or glass-melting
regenerative furnaces.



Successful efforts have been made to
simplify the mode of ammonia recovery
by the substitution of rectangular horizontal
washers, constructed of steel or iron work and
rendered resistant to the acid liquor by special




treatment, for the expensive lead-lined vertical
acid tower, and by making the sulphate liquor
play the double r6le of an absorbent for the
ammonia in the gas and an agent for the
transfer of undecomposed steam back to the



624



FUEL.



producer blast. As an example of this
type of plant, the Crossley and Eigby • system
may be described with reference to Fig. 12.
The generator a is central blown with a
special rotary conical grate (1 revolution in
4 hours), which gives an even distribution of
blast through the deep fuel bed and minimises
clinker formation. The blast enters the fuel bed
preheated to 280° and with a steam-saturation
temperature of 85°. The gas, on leaving the
producer, passes through the tubular ' super-
heater ' B, so designed as to give a maximum of
heating surface combined with facility of clean-
ing. The latter operation may be effected
whilst the plant is running by means of a cork-
screw cleaner inserted through holes provided at
the top. The function of the superheater is to
effect a transfer of part of the sensible heat of
the outgoing hot gas to the incoming steam-air
blast. The partly cooled gas next passes
onwards to the 'washer condenser ' o, consisting
of five compartments, in the first two of which it
meets with a water spray whereby it is freed
from dust and heavy tarry vapours ; the steam
generated is subsequently recovered. In the
cemMning three compartments the gas is washed
with a spray of sulphate liquor containing 0-5 p.c.
of free acid, which is circulated by gravity flow
in a direction contrary to that of the gas. The
absorption of ammonia is complete, whilst at
the same time the gas is satMactorily cooled
and most of the undecomposed steam leaving
the producer is condensed. After leaving the
washer condenser, the gas is slowly filtered
through the dry coke scrubber D whereby all
trace of sulphate liquor spray is removed. The
hot liquor from the washer condenser passes by
gravity flow into the well E whence it is pumped
into the air-saturating chamber e where it
flows and is sprayed by paddles in a counter
direction to the cold-air blast created by the
blower G. The liqUor is thereby cooled and
flows by gravity back to the washer condenser c,
whilst the air blast is warmed and leaves the
chamber (vid the coke scrubber s) saturated
with steam at 75°. A constant proportion of
sulphate liquor is withdrawn from circulation
and delivered into the closed evaporator k,
where it is concentrated to crystaUising-point.
A portion of the concentrate is run off at frequent
intervals into the crystallising trough L, where,
after cooling and draining, the sulphate is dried
and the mother liquor returned by gravity flow
to the washer condenser c. The steam from the
evaporator is conveyed by an overhead pipe to
the base of the coke scrubber H, where it enters
the air blast already saturated with steam at
75°. The steam-air blast then passes forward,
vid the coke scrubber H and the superheater B,
to the producer. Owing to a special treatment
of the iron and steel used in the construction of
the plant, all leadwork is eliminated, except in
the acid storage tank and the sulphate concen-
trating and crystallising apparatus. In conse-
quence of the greatly reduced capital cost, as
compared with the original Mond system, it is
now possible to carry out ammonia recovery
when gasifying only 100 tons per week. From
a producer coal of average quality, containing
1-20-1-25 p.c. N, and up to 10 p.c. ash, it is
possible to recover 90-100 lbs. of sulphate per ton,
together with 140,000 cub. ft. of gas (at N.T.P.)




FUEL.



625



containing 16-5-17'0 CO2, lO-5-ll'O CO, 26'5-
27-0 Hj, and 2-6-2-8 CHj.

■ Blast-furnace gas. Within recent years the
problem of better utilising the waste gases
from iron blast-furnaces as a source of power
has assumed great industrial importance. A
furnace smelting an average grade of ore, with
coke as fuel, will yield per ton cf iron produced
about 168,000 cub. feet of gas at 15°
and 760 mm. containing approximately
12-0 COa, 30-0 CO (this may include from
0-5-1 H) and 58-0 N, of calorific value approxi-
raa,tely 95-100 B.Th.Us. per cub. ft. For a
fmnace with an output of 1000 tons of pig iron
per week, the production of gas will average
1 million cub. ft. per hour, the potential energy
of which is about 45 p.c. that of the coke charged
into the furnace. Until the year 1837, when the
French ironmaster Dufaur drew attention to
the matter, this immense amount of energy was
entirely wasted. The classical investigations of
Bunsen at Veikerhagen in 1838, and of Bunsen
and Playfair at Alfreton in Derbyshire in
1844-5 (Brit. Assoc. Reports, 1845 ; reprinted,
1903, by the Iron a,nd Steel Inst.), finally led to
the utilisation of the gases for (1) heating the
blast for the furnace, and (2) boiler firing.
Special ' hot-blast ' stoves, on the regenerative
principle, were designed to effect the first
purpose by E. A. Cowper and by Thomas
Whitwell in 1865, whUst for steam raising the
gas was burnt in a fire-brick-lined combustion
chamber attached to a Lancashire boiler. In
this way about half of the gas was utilised for
heating the blast, and a further 40 p.c. would
suffice to raise steam for driving the blowing
engines, the furnace hoists, and other mechanical
appliances in connection with the plant, leaving
a surplus of about 10 p.c. not utilised. But the
combined efficiency of boiler plus steam engine
was always very low ; thus in 1902 it was
estimated by a Cleveland engineer that with the
best type of water-tube boiler only about
54 p.c. of the heat developed by the combustion
of the gas is actually transmitted to the water,
and that the combined efficiency of boiler and
blowing engine was somewhat less than 7 p.c.
All this has been changed by the rapid develop-
ment of the large gas engine during 1895-1905,
which has increased the efficiency of power
production from blast furnace gas something
between three- and fourfold as compared with
the old steam plants. Gas engines developing
1000-2000 b.h.p. per unit have now been
installed in most of' the German and many
British and American ironworks, realising in
actual practice a thermal efficiency of conversion
'gas — b.h.p.' of 25 p.c, and of 'gas — e.h.p.'
of 20-22 p.c. As an illustration of what this
improved practice implies in regard to surplus
power production, the following figures may be
quoted for a plant of four furnaces smelting
3200 tons of iron per week from calcined Cleve-
land ironstone ; the hourly production of gas
IS 3,200,000 cub. ft., of which approximately
1,600,000 are required for heating the blast, and
a. further 400,000 cub. ft. for operating an
electrically driven blowing engine ; the balance
of 1,200,000 cub. ft. will, when used in a gas engine
coupled with a dynamo, give 0000 e.h.p. at
the switchboard continuously day and night.
Some 40 p.c. of this would suffice to provide all
Vol. n.—T.



the power required in connection with the
steel plant and electrically driven rolling mill
for converting the iron into steel and rolling
it into rails or girders ; the remaining 60 p.c.
would represent ' surplus ' power available for
other purposes.

The gas, as it leaves the furnace at a
temperature of 300° or thereabouts, is heavily
charged with dust, which must be reduced by
washing to infinitesimal proportions before
it is fit for delivery to the engines. The
cleaning of the gas is usually accomplished
in two or three stages,' namely : (I) ' dry clean-
ing ' (by means of any ordinary type of dust
catcher), which may reduce the dust down to
between 2 and 8 grams per cub. metre; (2)
preliminary water washing [e.g. in the Bian
washer, consisting of a cylindrical -steel chamber
along the axis of which there slowly revolves a
horizontal shaft oarrjring a series of circular
discs of thick wirework with ^ coarse mesh,
the lower half of which is submerged in water ;
the dusty gas is partly cleaned by passing
through the films of water between the wire
meshes) ; this may reduce the dust down to 05
gram per cub. metre ; and (3) a final cleaning in
some form of centrifugal apparatus in which the
gas is violently churned up with a fine spray
or stream of water {e.g. the Theisen washer).
Most frequently the cleaning is carried out by the
combination of processes (1) and (3) only. In
any case, the dust in the gas should be reduced
down to about 001 gram per cub. metre, and
the- temperature to 18°-20°, before delivery to the
engine. The power expended in cleaning the
gas to this degree amounts to between 5 and
6 p.c. of that generated by its explosion in the
engine cylinder.

Water gas. The need of a cheap gaseous
fuel of high calorific intensity for certain
industrial purposes {e.g. steel welding) has led
to the utilisation of the well-kno-wn endothermic
interaction of steam and incandescent carbon at
high temperatures. For such a process to be
continuous, heat would have to be transmitted
from an external source through the walls of
the reaction chamber or retort, which would
necessarily be constructed of refractory material
of low conductivity. As such procedure would
certainly be very uneconomical, all idea of it
has been abandoned in favour of an intermittent
process, in which a bed of fuel (usually coke) is
alternately blo-wn with (1) air, until the mass
attains a sufficiently high temperature, and (2)
with steam, so long as the high temperature
interaction C+HjO^CO+Hj can proceed with-
out undue occurrence of the low temperature
interaction C+2H20?=iC02-f 2H2.

The use of water gas was first introduced in the
United States about the year 1875, as the result
of the pioneering efforts of J. S. C. Lowe, but it
was not until 1888 that the British Water Gas
Syndicate installed the fijst plant in Great
Britain at the Leeds Forge. This plant embodied
the now obsolete idea of, during the ' air blow,'
manufacturing a low grade ' producer gas ' (a
mixture of CO 2, CO, and N, in which CO was
the predominating carbon constituent), for
furnace purposes by blowing a thick fuel bed
with an air blast at moderate pressure. This
operation was alternated with the usual ' steam

2 s



626



FUEL.



blow ' for the pioduotion of ' water gas.' The
' air blow ' occupied 10 minutes, and the ' steam-
blow ' only 4 minutes, and each ton of gas coke
yielded about 34,000 cub. ft. of ' water gas,' and
about 140,000 cub. ft. of ' producer gas,' some
25 p.c. only of the carbon in the fuel appearing
in the ' water gas.'

Some ten years later the process was much



improved by DeUwik and Heisoher, who pro-
posed, during the air blow, to heat up a com-
paratively tlun bed of fuel as rapidly as possible
by means of a blast supplied in such quantity
as to burn the carbon as oompletdy as
possible to the dioxide. In the DeUwik-Bleisoher
system (Kgs. 13 and 14), the generator is
of cylindrical section with a fire-brick Iftiing




Fio. 13.



encased in a steel shell. The fuel bed rests on a
flat bar grate on a level with which are chnkering
bars, and below which are doors for the removal
of ashes. The air blast always enters the fuel
bed from below through a valve, and the
products of the ' air blow ' leave the generator
by the central stack valve, through which also
the fuel is charged from a small hopper waggon.
During the ' steam blows ' superheated steam




Fig,



from a boiler working at a pressure of 150-160 lbs.
per sq. in. is blojvn through the incandescent
fuel bed in either an upward or a downward
direction, the direction being reversed in each
successive blow. Accordingly, there is one
' water-gas ' outlet at the top of the generator,
and another below the grate, each provided with
a valve leading to the annular ' superheater,'
which serves to efiect a heat exchange between
the outgoing hot gas and the incoming steam



blast, thus superheating the letter. At the
bottom of the ' superheatei ' is a water seal
through which the gas passes onwards to a
coke scrubber, where it is cooled and cleaned
from dust by means of a water spray ; from
thence the cold gas' passes into a holder. The
various valves of the generator are operated by
an interlocking gear which makes it impossible
for the operator to make a mistake or get an
explosive mixture in any part of the plant. A
set of water gauges and a test flame on the
operating plaHorm indicate the working condi-
tions in the generator at any moment and also
the quality of the gas during the 'steam blow.'
Towards the end of the ' steam blow,' when the
temperature of. the fuel has fallen below the
point, at which the low temperature interactipn
C+2Ha05=iC!D2+2H„ comes serioudy into play,
the steam and gas valves are shut off, the stack
and air valves being simultaneously opened,
for the commencement of the ' air blow.' Each
' air blow ' lasts about 1 minute," and the
subsequent .' steam blow ' about 4 ■ minutes.
With- an average quality of gas coke the plant
will produce about 32 cub; ft. at 0° and '760 mm.
of water gas per lb. of carbon charged into the
generatorj-wluch means that as nearly as possible
50 p.c. of the carbon is converted into water gas.
The average* composition of the gas is 4*0 00 j,
430 CO, 49-0 H, 0-5 CH4, and 3-5 p.c. N, and
its (calorific value about. 320 gross and 290 net
B.Th.Us. per cub. ft. at 0° -and 760 mm. The
ratio of the net calorific value of -the gas to that
of the coke charged into the generator is about
0-60.

Another system of water-gas making is that
embodied in the Kramers and Aarts patents
(the ' K and A ' system), according to which
two generators, A and B (Figs. 15, 16, and 17),
connected through a double 'regenerator,' c,
are operated in parallel during the ' air blow '



FUEL,



627



and in' series during the ' steam blow,' somewliat
as follows : —

During the ' aii blow ' the fires (5-6 feet in
thickness) in both regenerators are simultane-



a central fire-brick wall. As the hot products
enter in parallel streams at the base of one or other
of the two chambers, they meet a secondary air
supply sufficient to burn completely all the CO





1 c


\




i^




PS


1


c


X


P


1


X




Fio. 15.

ously blown in parallel by a powerful blast, which
is introduced from below each fire, The hot






-' j^ I



«*




products (COa, CO, and N), on leaving the top
of the generators, pass upwards through the
double regenerator,' a cylindrical structure
filled with fire-brick chequer work and divided
Vertically into two chambers or compartments by



Thn gas porta.



Sectional Plan.

I'M. 17.

which they may contain. The chequer work in the
chambers absorbs part of the heat of the burnt
gases which eventually make their exit into the
outer atmosphere through the stack valve d at
the top of the double regenerator. As soon as



628



FUEL.



the fires in the two generators have attained
the necessary high temperature, the air and
stack valves are shut, and the steam valve
simultaneously opened. The steam, entering
the base of the firsti generator, traverses the
fire contained therein in an upward direction.
The products (COj, CO, H, and some undecom-
posed steam) passing out at the top traverse
in a X^ direction the two chambers of
the ' double regenerator,' after which they
enter, in a, higmy ' superheated ' condition,
the top of the second generator, through
which they pass in a downward direction. In
this way, it is claimed, the gases, just prior to
leaving the system, come into contact with a bed
of highly incandescent carbon, the temperature
of which has not been sensibly lowered by
the main endothermio steam-carbon inter-
action, which principally occurs in the first
of the two generators. In alternate ' steam
blows,' the direction of the steam and gases is
reversed, in order to ensure the maximum of
juniformity in the working conditions. The
plant is operated by means of an interlocking
valve gear, which prevents mistakes on the
part of the workman. The hot gas produced
passes from the bottom of the second generator
upwards first of all through the annular ' steam
superheater' E (Kg. 17), whereby part of its
sensible heat is transmitted to the in-going steam
blast, and then through a coke scrubber, where it
is cleaned and cooled by a downward water spray.
The process works smoothly and is very efficient,
the average duration of each air blow being
about 70 seconds, and of each steam blow about
5 minutes. The yield of gas, from an average
gas coke, is nearly 38 cub. ft., at 0° and 760 mm.,
per lb. of carbon charged into the generator,
about 60 p.c. of which appears in the ' water
gas.' The composition of the gas is 3-75 COj,
43-70 CO, 45-1 H, 0-5 CH4, and 6-95 N, its
calorific value being about 310 gross and 285 net
B.Th.Us. per cub. ft. at 0° and 760 mm. The
ratio of the net calorific value of the water gas to
that of the coke charged into the generators is
about 0-70.

With coke at 12s. per ton, the cost of making
' blue water gas,' including fuel, wages, interest,
and depreciation, is about 4d. per 1000 cub. ft.,
which is equivalent to coal gas at about 8d. per
1000 cub. ft.

The most important industrial application of
water gas is undoubtedly steel-plate welding,
and a large industry has grown up, especially
in Germany, for the manufacture of welded steel
tubes of large dimensions. The overlapping joint
to be welded is heated simultaneously from both
sides by special burners (Eig. 18) fed with both
water gas and air under pressure, which on



Online LibraryT. E. (Thomas Edward) ThorpeA dictionary of applied chemistry → online text (page 147 of 183)