It is easy to show the effect of supplying a common gas flame with warm air by holding it over a cylinder packed with wire gauze which has been made red hot. A common burner held over such a hot air shaft burns far more brightly and whitely. There is no question but that this is the plan to get good illumination out of gas combustion; and many regenerative burners are now in the market, all depending on this principle, and utilizing the waste heat to make a high temperature flame. But although it is evidently the right way to get light, it was by no means evidently the right way to get heat. Yet so it turns out, not by warming solid objects or by dull warm surfaces, but by the brilliant radiation of the hottest flame that can be procured, will rooms be warmed in the future. And if one wants to boil a kettle, it will be done, not by putting it into a non-luminous flame, and so interfering with the combustion, but by holding it near to a freely burning regenerated flame, and using the radiation only. Making toast is the symbol of all the heating of the future, provided we regard Mr. Siemens' view as well established.
The ideas are founded on something like the following considerations: Flame cannot touch a cold surface, i.e., one below the temperature of combustion, because by the contact it would be put out. Hence, between a flame and the surface to be heated by it there always intervenes a comparatively cool space, across which heat must pass by radiation. It is by radiation ultimately, therefore, that all bodies get heated. This being so, it is well to increase the radiating power of flame as much as possible. Now, radiating power depends on two things: the presence of solid matter in the flame in a fine state of subdivision, and the temperature to which it is heated. Solid matter is most easily provided by burning a gas rich in dense hydrocarbons, not a poor and non-luminous gas. To mix the gas with air so as to destroy and burn up these hydrocarbons seems therefore to be a retrograde step, useful undoubtedly in certain cases, as in the Bunsen flame of the laboratory, but not the ideal method of combustion. The ideal method looks to the use of a very rich gas, and the burning of it with a maximum of luminosity. The hot products of combustion must give up their heat by contact. It is for them that cross tubes in boilers are useful. They have no combustion to be interfered with by cold contacts. The flame only should be free.
The second condition of radiation was high temperature. What limits the temperature of a flame? Dissociation or splitting up of a compound by heat. So soon as the temperature reaches the dissociation point at which the compound can no longer exist, combustion ceases. Anything short of this may theoretically be obtained.
But Mr. Siemens believes, and adduces some evidence to prove, that the dissociation point is not a constant and definite temperature for a given compound; it depends entirely upon whether solid or foreign surfaces are present or not. These it is which appear to be an efficient cause of dissociation, and which, therefore, limit the temperature of flame. In the absence of all solid contact, Mr. Siemens believes that dissociation, if it occur at all, occurs at an enormously higher temperature, and that the temperature of free flame can be raised to almost any extent. Whether this be so or not, his radiating flames are most successful, and the fact that large quantities of steel are now melted by mere flame radiation speaks well for the correctness of the theory upon which his practice has been based.
Use of Small Coal.—Meanwhile, we may just consider how we ought to deal with solid fuel, whether for the purpose of making gas from it or for burning it in situ. The question arises, In what form ought solid fuel to be—ought it to be in lumps or in powder? Universal practice says lumps, but some theoretical considerations would have suggested powder. Remember, combustion is a chemical action, and when a chemist wishes to act on a solid easily, he always pulverizes it as a first step.
Is it not possible that compacting small coal into lumps is a wrong operation, and that we ought rather to think of breaking big coal down into slack? The idea was suggested to me by Sir W. Thomson in a chance conversation, and it struck me at once as a brilliant one. The amount of coal wasted by being in the form of slack is very great. Thousands of tons are never raised from the pits because the price is too low to pay for the raising—in some places it is only 1s. 6d. a ton. Mr. McMillan calculates that 130,000 tons of breeze, or powdered coke, is produced every year by the Gas Light and Coke Company alone, and its price is 3s. a ton at the works, or 5s. delivered.
The low price and refuse character of small coal is, of course, owing to the fact that no ordinary furnace can burn it. But picture to yourself a blast of hot air into which powdered coal is sifted from above like ground coffee, or like chaff in a thrashing mill, and see how rapidly and completely it might burn. Fine dust in a flour mill is so combustible as to be explosive and dangerous, and Mr. Galloway has shown that many colliery explosions are due not to the presence of gas so much as the presence of fine coal-dust suspended in the air. If only fine enough, then such dust is eminently combustible, and a blast containing it might become a veritable sheet of flame. (Blow lycopodium through a flame.) Feed the coal into a sort of coffee-mill, there let it be ground and carried forward by a blast to the furnace where it is to be burned. If the thing would work at all, almost any kind of refuse fuel could be burned—sawdust, tan, cinder heaps, organic rubbish of all kinds. The only condition is that it be fine enough.
Attempts in this direction have been made by Mr. T.R. Crampton, by Messrs. Whelpley and Storer, and by Mr. G.K. Stephenson; but a difficulty has presented itself which seems at present to be insuperable, that the slag fluxes the walls of the furnace, and at that high temperature destroys them. If it be feasible to keep the flame out of contact with solid surfaces, however, perhaps even this difficulty can be overcome.
Some success in blast burning of dust fuel has been attained in the more commonplace method of the blacksmith's forge, and a boiler furnace is arranged at Messrs. Donkin's works at Bermondsey on this principle. A pressure of about half an inch of water is produced by a fan and used to drive air through the bars into a chimney draw of another half-inch. The fire bars are protected from the high temperatures by having blades which dip into water, and so keep fairly cool. A totally different method of burning dust fuel by smouldering is attained in M. Ferret's low temperature furnace by exposing the fuel in a series of broad, shallow trays to a gentle draught of air. The fuel is fed into the top of such a furnace, and either by raking or by shaking it descends occasionally, stage by stage, till it arrives at the bottom, where it is utterly inorganic and mere refuse. A beautiful earthworm economy of the last dregs of combustible matter in any kind of refuse can thus be attained. Such methods of combustion as this, though valuable, are plainly of limited application; but for the great bulk of fuel consumption some gas-making process must be looked to. No crude combustion of solid fuel can give ultimate perfection.
Coal tar products, though not so expensive as they were some time back, are still too valuable entirely to waste, and the importance of exceedingly cheap and fertilizing manure in the reclamation of waste lands and the improvement of soil is a question likely to become of most supreme importance in this overcrowded island. Indeed, if we are to believe the social philosophers, the naturally fertile lands of the earth may before long become insufficient for the needs of the human race; and posterity may then be largely dependent for their daily bread upon the fertilizing essences of the stored-up plants of the carboniferous epoch, just as we are largely dependent on the stored-up sunlight of that period for our light, our warmth, and our power. They will not then burn crude coal, therefore. They will carefully distill it—extract its valuable juices—and will supply for combustion only its carbureted hydrogen and its carbon in some gaseous or finely divided form.