THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXI.

FRIDAY, MAY 12, 1916.

No. 2889

Power from Coke Oven Gas?

By G. DEARLE.

In collieries where a proportion of the output of coal
is converted into coke in either , waste heat or regenera-
tive ovens, there is always a certain surplus volume of
gas, of high calorific value, available for the. production
of power. In the regenerative, type of coke oven this
volume of gas is much greater than in the ..older pattern
of waste heat oven, and it is partly for this reason that
the majority of new coke oven installations during the
past few years have been of the regenerative type..

The advantage of regenerative ovens is that the whole
of the surplus heat in the coal is produced in the form
of a combustible gas, instead of merely as a waste heat
product. By utilising this gas in gas engines, it is
possible to develop three to four times the power that
can be obtained from the use of waste heat under boilers.
The gas can also be conveyed any distance without
deterioration.

A great deal of prejudice exists amongst a certain class
of engineers against the use of gas engines for large
power work, the principal objections being : The
unsteady iturning moment; the difficulty of starting the
engine; the general absence of reliability in operation;
high cost of maintenance.

The author hopes to show, from personal experience
with a modern gas-driven electric station, that such
objections are without foundation in the case of an
installation put down on sound engineering lines.

Neither the labour required for operating the plant nor
the costs of maintaining the same are in the slightest
degree excessive when, consideration is given to the
service which it performs.

Type of Engines.

The installation under review consists of three
500 b.h.p. vertical tandem gas engines direct coupled
' to three-phase alternators, generating current at a pres-
sure of 440 volts and a frequency of 50 cycles per second.

The gas engines are of the single-acting type operating
on the Otto or four-cycle principle, the cylinders being
so arranged in tandem that the suction stroke of one
cylinder is the explosion or working stroke of the other
cylinders on the same line. Thus each crank receives
one impulse per revolution, each down stroke being a
working impulse of either the upper or the lower
cylinder. On the up stroke the inertia of the moving
parts is absorbed by the compression of either the top
or the bottom cylinder, and part of the inertia is
absorbed on the d.ownward stroke by a buffer cylinder
formed under the upper piston.

By means of this arrangement the connecting rod is
always in compression, and little or no strain is thrown
upon the connecting-rod bolts. This is an important
feature, as according to the reports of the various insur-
ance companies, more breakdowns to engines are caused
by the failure of these bolts than by any other cause.

The number of impulses which , the shaft receives,
together with the influence , of the buffer cylinder,
renders the turning moment of the engine equal to that
of a high-speed steam engine7. In .the case of the four-
crank eight-cylinder engines ..under consideration,
running at 300 revolutions per minute, with cranks at
90 degs., .the shaft receives four impulses per revolution,
or 1,200 impulses per.minute, so that with a compara-
tively light flywheel the cyclic variation is less than
one-third of 1 per cent., which is sufficiently even for
the parallelling of the alternators without the slightest
difficulty. This disposes of the first objection, viz., that
the turning moment is unsteady.

"there are eight cylinders on each engine, the four
upper ones having a diameter of 16J in. and the lower
ones 15^-in., with a stroke of 16 in. The speed of the

* From a paper read before the Institution of Electrical
Engineers, Yorkshire Section.

engine is 300 revolutions per minute, and the full load
is 500 b.h.p. The object of making the upper cylinders
1 in. larger than the lower ones is so that the whole line
of pistons may be removed together. By this arrange-
ment the dismantling of the engine for cleaning
purposes becomes very simple, and the time usually
taken for the removal of a line of pistons and the clean-
ing and replacement of these is from 6 to 10 hours, but
if the engine is urgently needed, this work can be carried
out in three to four hours.

Lubrication.

The lubrication of the engine is effected- by means of
plunger pumps working from an eccentric on the crank-
shaft, these pumps drawing the oil from the well of the
crank case through strainers, arranged so that one can
be removed for cleaning purposes whilst the engine is
running, the action of removal automatically closing
the valve and thereby preventing any unfiltered oil
getting into the lubricating system. The oil passes
from the pumps through coolers, arranged outside the
engine, the whole of the circulating water for the engine
first passing through these coolers. The normal oil
temperature at the inlet of the cooler.is about 74degs.
Cent., and the temperature at the return to the crank
case 45 degs. Cent. The oil is pumped to the main
bearings of the crank shaft and then through passages
drilled in the crank shaft to the crank pins. From the
crank pin it is again led up to the gudgeon pin. The
bearings of the cam shaft are under forced lubrication,
and the. valve tappets, rollers, and pins are also fed
from the same system. The oil pressure may be varied
by means of a by-pass valve on the crank case, the
usual working pressure being from 10 to 20 lb. per <sq. in.
The top cylinders and piston rods are lubricated by
means of a separate sight-feed lubricator mounted on
the crank case and driven from the cam shaft. A
separate pump to each point renders this lubrication
absolutely certain.

The ignition is obtained by means of a magneto and
transformer coil. The sparking plugs are of a very
heavy design and mica insulated.. A stand-by ignition
is provided in the shape of an accumulator, which is
switched on to the coil by means of a change-over switch;
but this is only used for testing the ignition, as no
trouble has been found in starting direct from the
magneto.

Starting.

The engine is started by means of compressed air,
stored at a pressure of 3001b. per sq. in., in a series of
six storage tanks, outside .the power house, each tank
when fully charged being capable of giving five starts.
The air is compressed by two-stage compressors,
arranged in duplicate, one being driven by a motor, and
the second by a small gas engine, drawing gas.from the
same main as the larger engines.

The cooling water from the engine is pumped over a
double-drip natural-draught cooling tower, designed to
deal with 36,000 gals, of water per hour, and to reduce,
the temperature-of this from 135 degs. Fahr, to 80 degs.
Fahr., with an atmospheric temperature' of 80 degs.
Fahr. The water supply is circulated by means of a
Rees roturbo pump, delivering 12,000 gals, per hour
against a head of 52 ft. This pump is driven by a 15
horse-power squirrel-cage motor. Stand-by sets are
provided in the form of a separate 3 in. pump to each
engine, each pump being driven by an 8 horse-power
squirrel-cage motor running at 1,400 revolutions per
minute.	•	■

It is, of course, essential that the water should be
reasonably free from impurities, .and. an excess, say,
not more than 15 per cent., of hardness; and it is also
important that the tank of the cooling tower be kept
free from grease and oil.	\

A certain amount of coal dust is always present in
the circulating water of a colliery power station, but
the application of , a powerful hose to the hand holes of
the jackets about every three months is sufficient to
keep down this trouble, provided that the jackets, are
designed, as they should be, to facilitate flushing.

Ventilation.

The ventilation of the crank cases of the engines is
effected by means of 3 in. pipes coupled to the top of
each crank case, and carried outside the engine house,
terminating at the top of the exhaust pipe above the
silencers. By placing the outlet of this ventilation pipe
concentric with the outlet of the exhaust, an ejector
action takes place, which effectually scavenges the crank
cases, and prevents any accumulation of gas. or oil
vapour, such as would be likely to cause an explosion
in the crank case.	'	/

The ventilation of the engine house is effected by a
30 in. motor-driven fan, and though this somewhat
aggravates the coal dust nuisance, it is of great service
in keeping down the sulphur fumes, and thus protecting
the exciter, 'commutators, etc.

Purifying the Gas.

The gas supply is obtained from a battery of 110 Otto
ovens; 60 of these are waste heat, and 50 are of the
regenerative type. From the former about 15 per cent,
of the total gas is available, and from the latter about
40 per cent.

After all the by-products are removed, the gas is
drawn to the engines by a steam-driven exhauster, of
60,000 cu. ft. per hour capacity, governed by a diaphragm
governor controlled by the pressure of the gas in the
main at the engine stop valves. An electrically-driven
exhauster, installed as a stand-by, is controlled from the
power house switchboard, and is capable of dealing
with 30,000 cu. ft. per hour. This exhauster is driven
from a 10 horse-power motor by a. silent chain drive. A
further steam-driven exhauster is, however, to be
installed, as the electric exhauster is scarcely large
enough for the work during peak loads.

The quantity . of gas . passing to the ■ engines is
measured by means of a rotary' meter, and the gas
pressure at the stop valve is registered on an illuminated
dial pressure gauge in the power house.- The average
gas pressure is approximately 10 in. .

The purification of the gas after it leaves the by-pro-
duct plant is a most important'item—one that has been
neglected. - When the gas leaves the benzole scrubbers
it contains about 900 grains of sulphuretted hydrogen
per 100 cu. ft. This sulphur would attack the inside of
the cylinders and the exhaust .valves, if allowed to go
through the engines. Moreover, the presence of
sulphur appears to cause a certain amount of pre-
ignition, or spontaneous ignition, of the charge during
the compression stroke, possibly owing to the presence’
of sulphuretted hydrogen, which acts as. an igniter,’■
being more liable than the rest of the gas to spontaneous,
combustion under compression. ,	’ . ■	•	„■ ■ ■■>

. The gas is therefore purified by oxide of iron in.a.;
of four purifiers of the Wilbourne type, each’. 20Tt.
square by 5 ft. deep. The boxes hold about 30 Tons of
oxide, in’.two tiers on ordinary . grids. ■ Two. classes of
oxide—11 Lux ” and “ Bog ’’—are used. The .boxes are
worked on the “ backward rotation ” ■principle. ' Air to
the extent of 2£ or ’ 3 per cent, is drawn in at the
exhauster, and plays a very important part in the
revivifying of the oxide in the purifiers. Prior to the
introduction of this air feed to the main, the oxide in
the purifiers was changed at the rate of one box every
four weeks. After the introduction of the air, the period
between changing the oxide was extended from four
weeks , to four months, thereby producing a very con-
siderable saving in the cost of generation.

In order to check the amount of air flowing into the
exhauster, it is passed through a smaU rotary meter.
The spent oxide, after being taken from the boxes, is
revivified by being spread out' and exposed to the air.