Gas-Engines and Producer-Gas Plants / A Practice Treatise Setting Forth the Principles of Gas-Engines and Producer Design, the Selection and Installation of an Engine, Conditions of Perfect Operation, Producer-Gas Engines and Their Possibilities, the Care of Gas-Engines and Producer-Gas Plants, with a Chapter on Volatile Hydrocarbon and Oil Engines

by Rodolphe Edgard Mathot

CONDITIONS OF PERFECT OPERATION OF GAS-PRODUCERS

These conditions depend upon the workmanship or upon the system of the plant, on the care with which it has been erected, on the nature of the fuel, on the condition of preservation of the apparatus, and upon the manner in which the producers have been working.

Workmanship and System.—The workmanship itself, which term is meant to include the choice of materials and the way they have been worked, presents no difficulty. The producers which we have discussed are very simple and offer absolutely no difficulties in their mechanical execution. As regards the system, however, especially with respect to the relative dimensions of the elements, it does not seem so far that it is possible to indicate any principle or rule capable of a rigid general application. It must be taken into account that the use of suction gas-generators has become general only in the last three or four years; the problem has therefore scarcely been adequately solved. However, some hints may be given on this subject.

Generator.—In regard to the generator, it is possible to deduce from the best existing plants the dimensions to be given to the generator relatively to those of the engine to be supplied, upon the assumption that the engine is single-acting and runs at a normal speed of

from 160 to 230 revolutions per minute. The essential portion of the generator which contributes to the production of a proper gas is that which corresponds with the combustion zone. To this portion a cross-section is given varying in size between one-half and one-quarter of the surface of the engine-piston, sometimes between one-half and nine-tenths of this surface, according to the nature and the size of the fuel that is used. With small apparatus, however, ranging from 5 to 15 horse-power, the size of the base cannot be reduced below a certain limit, since otherwise the sinking of the fuel will be prevented. This danger always exists in small generators and renders their operation rather uncertain, such uncertainty being also due to the influence of the walls. It is to be noted that most modern generators are rather too large than otherwise.

Many manufacturers of no wide experience have been led to make their apparatus rather large so as to insure a more plentiful production of gas. As a matter of fact, the fire in such apparatus is liable to be extinguished when the combustion is not very active. If the principles of the formation of gas in suction-generators be kept in mind, it is evident that the gas developed is the richer the "hotter" the operation of the apparatus. Such operation also permits the decomposition of the hydrogen and carbon monoxide.

The "hot" operation of a generator is accomplished best with active combustion; and since this is a function of the rapidity with which the air is fed, it obviously is advantageous to reduce the area of the air-passage to a

minimum as far as allowed by the amount of fuel to be treated. As to the height of the fuel in use in the apparatus, this varies as a rule between 4 and 5 times the diameter at the base.

Vaporizer.—The size of the vaporizer varies materially according to its type. No hard-and-fast rule can therefore be adopted for determining its heating surface; but this surface should in all cases be sufficient to vaporize under atmospheric pressure from .66 to .83 pounds of water per pound of anthracite coal consumed in the generator.

Scrubber.—For the scrubbers, the following dimensions may be deduced from constructions now used by standard manufacturers.

The volume of a scrubber is generally from six to eight times the anthracite capacity of the generator. A height of from three to four times the diameter is considered sufficient in most cases. It should be understood that in this height is included the water-pan chamber located below the partition or grate, and the upper chamber through which the gas escapes. The height of these two chambers depends necessarily upon the arrangement used for leading the gas to the lower portion of the washer and for the distribution of wash-water at the top.

Assembling the Plant.—The author has insisted strongly on the necessity of having all the apparatus and pipe connections perfectly tight. In order to ascertain if there is any leakage, the following procedure may be adopted:

When starting the fire by means of wood, straw, or other fuel producing smoke, instead of allowing this smoke to escape through the flue during the operation of the fan, it may be caused to escape through the cock which generally admits the gas to the motor, the cock being opened for this purpose. The damper in the outlet flue is closed. In this manner the smoke will fill all the apparatus and connecting pipes under a certain pressure and will escape through any cracks, the presence of which will thus be revealed.

Another test, which is made during the ordinary operation of the generator, consists in passing a lighted candle along the joints; if there is any leakage, this will be shown by a deviation of the flame from a vertical position.

Fuel.—We have discussed the subject of fuel in a preceding chapter (Chapter XIII) and have indicated the conditions to be fulfilled by low grade or anthracite coal best adapted for use in suction gas-generators. It may here be added that the coal used in the generator should be as dry as possible and in pieces of from ¹⁄₂ inch to 1 inch. Very small pieces, and particularly coal dust, are injurious and should be removed by preliminary screening as far as possible. Screened coal is thrown in with an ordinary grate shovel.

How to Keep the Plant in Good Condition.—In regard to the generator, apart from the cleaning of the grate and of the ash-pit, which may be done during operation, it is necessary to empty the apparatus entirely once a week, if possible, in order to break off the

clinkers adhering to the retort. These clinkers destroy the refractory lining, form rough projections interfering with the downward movement of the fuel, bring about the formation of arches, and reduce the effective area of the retort. At the time of this cleaning, tests are also made as to the tightness of the doors of the combustion-chamber, of the charging-boxes, etc.

The vaporizer should be cleaned every week or every other week, according to the more or less bituminous character of the fuel and the greater or smaller content of lime in the water used. Lime deposits may be eliminated, or the salts may be precipitated in the form of non-adhering slimes, by introducing regularly a small amount of caustic potash or soda into the feed-water. If the deposits or incrustations are very tenacious, the use of a dilute solution of hydrochloric acid may be resorted to. Tar which may adhere to the conduits, pipes or gas passages, is best removed while the apparatus is still hot, or a solvent may be employed, such as kerosene, turpentine, etc. The connections between the vaporizer and the scrubber are particularly liable to become obstructed by the accumulation of tar or dust carried along by the gas.

It is advisable to examine the several parts of the plant once or twice a week by opening the covers or the cleaning-plugs.

The lower compartment of the washer keeps back the greater part of the dust which has not been retained in collectors or boxes provided especially for this purpose. The dust takes the form of slime, and, in some

arrangements of apparatus, tends to clog up the overflow pipe, thus arresting the passage of gas and causing the engine to stop. This portion of the washer should be thoroughly cleaned once or twice a month.

If very hard blast-furnace coke is used in the washer, it may be kept in use for over a year without requiring removal. In order to free the purifying materials from dust and lime sediments carried along by the wash-water, it is well to let the wash-water flow as abundantly as possible for about a half-hour at least once a month. At the time of renewing the purifying material the precautions indicated in the section dealing with these matters should be observed, and care should be taken to have shelves or gratings on which the material is supported in layers not too thick, so as to avoid any resistance to the passage of the gas.

In a general way it is advisable to test the drain-cocks on the several apparatus daily, and to keep them in perfect condition. If, when open, one of these cocks does not discharge any gas, water, or steam, a wire should be introduced into the bore to make sure it is not clogged up.

Care of the Apparatus.—Each producer-gas plant will require special instructions for running it, according to the system, the construction, and the size of the plant. Such instructions are generally furnished by the manufacturer. However, there are some general rules which are common to the majority of suction gas-producers, and these will here be enumerated.

Starting the Fire for the Gas Generator.—This

operation calls for the presence of the engineer of the plant and an assistant. The proper procedure is as follows:

First: Open the doors of the furnace and of the ash-pit. Then open the outlet flue and make sure that the grate of the generator is clear of ashes and clinkers. It should also be seen to that the parts of the charging-box work well and that the joints are tight.

Second: Ascertain whether there is the proper amount of water in the vaporizer, in the scrubber, etc., and that the feed works properly.

Third: Through the door of the combustion-chamber introduce straw, wood shavings, cotton waste, etc.; light them and fill the generator with dry wood up to one-quarter or one-half of its height; then add a few pailfuls of coal.

Fourth: Close the doors of the ash-pit and of the combustion-chamber and start the draft by means of the fan. As soon as the draft is started, it must be kept up without interruption until the engine begins to run, which may be ten or twenty minutes after lighting the fire.

Fifth: After the draft has been continued for a few minutes, the coal becomes sufficiently incandescent to start the production of gas, which may be ascertained by trying to light the gas at the test-cock near the generator. Then the opening in the outlet flue is half closed for the purpose of producing pressure in the apparatus.

Sixth: Open the outlet flue adjacent to the engine for

the purpose of purging the apparatus and the conduits of the air which they contain until the gas may be lighted at the test-cock placed near the motor.

Seventh: Adjust the normal outflow of wash-water for the scrubber.

Eighth: As soon as the gas burns continuously at the test-cock with an orange-colored flame the engine may be started.

The gas at first burns with a blue flame; this color indicates that it contains a certain amount of air. The opening of the test-cock should be so regulated as to reduce the outlet pressure of the gas sufficiently to prevent the flame from going out. During the production of the draft, as well as during the ordinary running of the plant, the filling of the apparatus with fuel should be done with care to prevent explosions of gas due to the entrance of air. Particular care should be taken never to open at the same time the lid of the charging-box and the device, be it a cock, valve, or damper, which controls the connection of the charging-box with the generator. All the operations which have been mentioned above should be carried out as quickly as possible.