not only with a view to complete removal of moisture, so that the coal may be most readily pulverized, but also with a view to maintaining the dry condition of the fuel until it is used.

The standard of dryness necessary will vary somewhat with the character of the fuel, depending on whether the moisture present is surface or combined. While it is generally advisable to dry the coal to 1 per cent or less of moisture and the coal so prepared will always show the best furnace conditions, there still may be times, particularly during the winter season, when absolute maintenance of such a rigid standard is not possible. The degree of efficiency then possible will depend much on the size and type of furnace and still more on the type of feeding apparatus employed. While early failures with pulverized fuel were attributed largely to moist, coarse coal, it has recently been claimed by some that it is not necessary to dry the coal. From a thermal standpoint, this could never be the case, for if "low-temperature" heat is not used to remove the moisture in a dryer, "high-temperature" (more expensive) heat must be used in the furnace to remove and perhaps disassociate this moisture. If the coal comes from a dry mine containing 2 or 3 per cent of moisture, or even more with certain coals, and is kept under cover, it is doubtful if the overhead for drying equipment can be justified from the standpoint of furnace loss, except in the more difficult high-temperature or close temperature-control processes. This and similar matters can only be determined by close consideration of all the facts in view, and by bringing into play the trained judgment of experienced and competent pulverized fuel engineers.

Furnace Efficiency

A great deal of thought has been expended on what may be called the preparation and distribution side of pulverized coal installations, but a casual survey of a half dozen of the latest and largest installations is sufficient to leave one amazed at the comparatively little regard apparently given to the important matter of actual results with the pulverized coal

in the furnace. This fundamental seems generally to have been left for the plant operating force to work out. The prevailing attitude seems to be embodied in the oft-quoted question and answer: "Will powdered coal burn? Certainly it will burn. Throw it into the furnace and let it burn." Put this way, it has seemed so easy as to appear an actual bonanza for the uninitiated. But soon, ah! soon comes the disillusionment. Then we have the old saw, "We tried powdered coal and did not find it practical for our furnaces." Here we have the answer to the question, "Why has

FIG. 1-DIAGRAM SHOWING MIXING ACTION IN PULVERIZED COAL BURNER

powdered coal not advanced more rapidly? Why is it not used for this, that or the other thing? Simply because the seeming ease of application has caused (shall we say it?) "Fools to rush in where angels fear to tread."

Of course volatile pulverized fuel is highly combustible when dry, and if injected into a furnace at ignition temperature, will ultimately burn as it finds air. But how will it burn? Where will combustion be completed? What kind of slag will be formed and where will it be deposited? What will happen to the bridge wall, the roof, the side walls, the flue outlet? It is not believed that the highest efficiency is possible except where the greatest care is taken to introduce the air and fuel into the furnace as a completely diffused

mixture. This paper, then, is a protest against "haphazard feeding," "large combustion space required for pulverized coal," and methods other than complete mixing of the air and fuel outside the furnace.

Many references are made in current pulverized coal literature to the necessity of mixing the fuel with air. The term "mixing" is applied apparently to any scheme of feeding pulverized coal that will result in one or more moving strata of coal and air streams, some of which may be arranged to cross-fire on the others and thus create a certain "commingling effect." This is not at all the character of mixing required for that complete diffusion of the coal dust in the total amount of combustion air by which means alone the highest efficiency can be obtained. Let us therefore look at the matter from a common sense standpoint. There are certain propositions on which it is believed we all may well agree. Some of these may be stated briefly as follows:

1.-Combustion is a chemical reaction, which in practice is employed-not for any effect of the reaction itselfbut in order to liberate heat, a by-product of the reaction. Since it is heat that is to be used, and this heat is present in the gases resulting from combustion, the less foreign matter, slag or ash, contained in the gases, the more efficiently can the heat be utilized. Hence a burner that quickly and largely removes slag from the gases is desirable.

2. Any air not required to supply oxygen for combustion is a detriment, since it will absorb heat. Arguments of "expediency" calling for the use of excess air to protect brick, boilers, etc., from excess temperature, should therefore be abandoned in favor of apparatus and methods that control the heat without dilution.

3. From the earliest inception of chemistry, the idea of molecular formation has been at the base of chemical science. It is well recognized that a rapid and complete reaction is best produced by fine division and thorough mixing of materials, which then need be present only in the chemical proportions necessary for the reaction. Therefore, any plan of feeding pulverized coal calling for an excess of air, must concede coarse coal or poor mixing-one or both. There follow all the secondary difficulties, such as reduced temperatures, imperfect control of furnace conditions, progressive com

bustion, abrasion of brickwork, slag deposition on work and a train of other problems.

4. Complete mixing or diffusing of gases one with another is conceded to be one of the most difficult problems of the scientific investigator. Any process which introduces separate air and coal streams into the furnace, to be there mingled and broken up, prior to completing combustion comes immediately in contact with this physical difficulty of gas mixing. It is not then a simple process of diffusing finely divided solid material through the air, but the far more

complicated matter of thoroughly mixing and diffusing one with the other, the oxygen and various gases of distillation from the coal. Moreover the furnace temperature is such as to increase the volume of gases until it is several times greater than that of the cold air, which might better have been mixed with the fuel outside the furnace in an apparatus designed for the purpose.

5. A review of inventions of pulverized fuel feeders and burners, shows that in practically all cases, the effort has been to attack a body of coal dust from without by