The pressure producer contains the following essential parts:—
The generator, a vertical furnace fed from the top through an air-tight trap, and shut off below from the outside atmosphere by having its foot immersed in water. Any fuel or ashes which fall through the bars into the water can be abstracted without spoiling the draught. Air and steam are forced into the generator, and pass up through the fuel with the chemical results already described. The gases then flow into a cooler, enclosed in a water-jacket, through which water circulates, and on into a scrubber, where they must find their way upwards through coke kept dripping with water from overhead jets. The water collects impurities of all sorts, and the gas is then ready for storage in the gas-holders or for immediate use in the engines.
A pound of anthracite coal thus burnt will yield enough gas to develop 1 h.p. for one hour.
Suction Gas Plants.—With these gas is not stored in larger quantities than are needed for the immediate work of the engine. In fact, the engine itself during its suction strokes draws air and steam through a very small furnace, coolers, and scrubbers direct into the cylinder. The furnace is therefore fed with air and water, not by pressure from outside, but by suction from inside, hence the name "suction producer." At the present time suction gas engines are being built for use on ships, since a pound of fuel thus consumed will drive a vessel further than if burnt under a steam boiler. Very possibly the big ocean liners of twenty years hence may be fitted with such engines in the place of the triple and quadruple expansion steam machinery now doing the work.
BLAST-FURNACE GAS ENGINES
Every iron blast-furnace is very similar in construction and action to the generator of a producer-gas plant. Into it are fed through a hopper, situated in the top, layers of ore, coal or coke, and limestone. At the bottom enters a blast of air heated by passing through a stove of firebrick raised to a high temperature by the carbon monoxide gas coming off from the furnace. When the stove has been well heated the gas supply is shut off from it and switched to the engine-house to create power for driving the huge blowers.
The gas contains practically no hydrogen, as the air sent through the furnace is dry; but since it will stand high compression, it is very suitable for use in large engines. Formerly all the gas from the furnace was expelled into the open air and absolutely wasted; then it was utilised to heat the forced draught to the furnace; next, to burn under boilers; and last of all, at the suggestion of Mr. B. H. Thwaite, to operate internal combustion engines for blowing purposes. Thus, in the fitness of things, we now see the biggest gas engines in the world installed where gas is created in the largest quantities, and an interesting cycle of actions results. The engine pumps the air; the air blows the furnace and melts the iron out of the ore; the furnace creates the gas; the gas heats the air or works the engines to pump more air. So engines and furnace mutually help each other, instead of all the obligation being on the one side.
When, a few years ago, the method was first introduced, engines were damaged by the presence of dust carried with the gas from the furnace. Mr. B. H. Thwaite has, however, perfected means for the separation of injurious matter, and blast-furnace gas is coming into general use in England and on the Continent. Some idea of the power which has been going to waste in ironworks for decades past may be gathered from a report of Professor Hubert after experiments made in 1900. He says that engines of large size do not use more than 100 cubic feet of average blast-furnace gas per effective horse-power-hour, which is less than one-fourth of the consumption of gas required to develop the same power from boilers and good modern condensing steam-engines, so that there is an immense surplus of power to be obtained from a blast-furnace if the blowing engines are worked by the gas it generates, a surplus which can be still further increased if the gas is properly cleaned. It is estimated that for every 100 tons of coke used in an ordinary Cleveland blast-furnace, after making ample allowance for gas for the stoves and power for the lifts, pumps, etc., and for gas for working the necessary blowing engines, there is a surplus of at least 1,500 h.p.; so that by economising gas by cleaning, and developing the necessary power by gas engines, every furnace owner would have a very large surplus of power for his steel or other works, or for selling in the form of electricity or otherwise.
Yet all this gas had been formerly turned loose for the breezes to warm their fingers at! Truly, as an observant writer has recorded, the sight of a special plant being put up near a blast furnace to manufacture gas for the blowing engines suggests the pumping of water uphill in order to get water-power!