Fluidized Bed Combustion

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Fleischman and I were introduced through the Mobile Steam Society—a group of lunatic fringe scientists and engineers bent on using their spare time to develop a modern steam automobile—and, with his help, I built a six-inch-diameter prototype fluid bed capable of burning up to 90,000 BTU worth of coal per hour with respectable efficiency. [EDITOR'S NOTE: Before Rocky built his, "small" fluid beds had been those sized around six feet in diameter!]

Then, at the May 1980 meeting of the Steam Automobile Club of America in Greensboro, North Carolina, MOTHER's staff got wind of the fluid bed's potential. At that get-together the furnace was fed a variety of fuels . . . including not only coal, but also kerosene and even chicken feed. (Though it's quite natural for folks who are unfamiliar with fluid bed combustion to be surprised by the device's multifuel capability, it's easy—once one understands how the little burner works—to see why it's capable of very efficiently firing almost any material that contains carbon!)

MOUNT ST. HELENS TAMED

When you peer down the throat of a fluidized bed, you'll see a bright red bubbling mass of fire that looks like the lava in a volcano. [EDITOR'S NOTE: The material's strikingly liquid appearance is due to the method of heat transfer . . . rather than to any essential difference in the actual combustion itself.]

A fluidized bed, you see, combines carbon and air to yield heat . . . just as a conventional furnace does. The major difference between the two burners is that the fire in a fluidized bed takes place in a mass of tumbling inert (noncombustible) particles (1/8" or smaller) that are supported by an upward rush of combustion air. (Visualize a vacuum cleaner blowing air up through a vessel containing sand.) The churning particles bump into each other continuously, passing the heat of the burning fuel back and forth by conduction . . . and give the fire tremendous thermal inertia (or resistance to temperature change, which is a result of the storage capacity of the large mass of inert material).

RAPID HEAT TRANSFER

Since heat is immediately absorbed by the noncombustible particles in the oxidation area, a fluid bed doesn't have the leaping flames that most of us associate with burning. Consequently, hot spots—regions of intense combustion that are usually associated with a conventional furnace's inefficiency—are significantly reduced. Burning fuel in such a manner has several advantages over conventional modes of combustion:

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