Fuel Bed Action.

Earlier I have made light reference to fuel bed action. It is so interesting a subject that I propose to enlarge upon it, or rather explain it in little more detail.

The coal bed acts principally as a gas producer and at least 4–5ths of the carbon is incompletely burnt on the grate. The combustion of the carbon-monoxide formed in the fire is completed above the grate. Every pound of coal burnt liberates 3,750 B.T.U. in the fire itself and 10,750 B.T.U. in the fire-box above the fuel bed (Fig. 1)—(See J. T. Anthony's article, Loco Fuel Economy, in the Railway Age Gazette, October, 1916.)

The oxidation zone extends three inches above the grate line. Here the oxygen enters with air, makes contact with the glowing coals, and one atom of carbon, combines with two atoms of oxygen, thus burning completely to carbon dioxide. The gases entering the reduction zone consist principally of nitrogen (for that you will remember is in the air) carbon dioxide and a little free oxygen.

When the molecule of the carbon dioxide passes up from the oxidation zone into the reduction zone it comes into contact with a glowing piece of carbon. Now, each molecule of the dioxide proceeds to give up one atom of oxygen to one of the carbon atoms, thereby forming two molecules of carbon monoxide. This takes place largely in the reduction zone and to a small extent only in the burning flames above the fuel bed.

If the formation of carbon dioxide releases heat from the fuel, the reduction of carbon dioxide into carbon monoxide absorbes heat, therefore, a cooling action takes place in the reduction zone. This seems strange, doesn't it? But it is quite true.

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Now we have this gas (carbon monoxide) passing up into the distillation zone. When this gas in company with a little carbon dioxide, nitrogen and perhaps a little free oxygen, enters the distillation zone, it comes into contact and mixes with the moisture and volatile hydro-carbons being distilled off from the coal that has just been thrown on the fire. These hydro-carbons are easily decomposed by the action of the heat, so that the gases arising from the green coal consist principally of the light hydro-carbons, methane ethylene, with free hydrogen, water vapour, and small globules of tarry hydrocarbons. Mixed with these are the gases that have come up from the oxidation and reduction zones—carboa monoxide, a little carbon dioxide, a little free oxygen and large quantities of non-combustible nitrogen.

The chemical combination of the combustibles and oyxgen in this conglomerate mixture of gases produces the flame which is always present in large quantities when bituminous coal is burned. The mass of flame that fills a fire-box is of such a constantly varying, shifting, flickering nature, that it is difficult to form a definite idea of its mechanical structure or action. (See Roscoe and Schorlemmer's “Treatise on Chemistry.”) To burn this accumulation of gases it is necessary to bring in oxygen through the fire-hole door to break down the carbon monoxide into carbon dioxide.

Short, hot flames, are the result of an intimate mixture of combustible gases with oxygen just at the surface of the fuel bed. Long, dark red flames, are the result of poor mixing of air into combustible gases, or insufficient air above the fuel bed. The latter condition facilitates the formation of the soot particles which, once formed, are very difficult to burn. Firemen should specially bear in mind what the flame in the box indicates.

(To be continued.)