A Systems Story
by Bill Benzon
My father was trained as a chemical engineer. He spent his entire career with Bethlehem Mines, the mining division of the Bethlehem Steel Company. He designed coal-cleaning plants, at least the system for actually cleaning the coal.
This is a story about the last plant that he designed, one designed to keep the air clean while at the same time reducing the cost of running and maintaining the plant. It was a beautiful and elegant solution to a nasty engineering problem. I offer it as a study in systems thinking – though my father probably never used that phrase.
Cleaning Coal
Before coal can be turned into coke (for subsequent use as a fuel in steel-making) it must be cleaned of impurities, mostly sulfur. Most cleaning techniques take advantage of the fact that the rocks containing the impurities are denser than coal. So, you crush the raw coal until all the particles are less than, say, an eighth of an inch. Then you float the crushed coal in some medium – generally, but not always, water – and take advantage of the fact that the rock sinks faster than the coal. There are several things you can do, as I recall, but whichever technique you use, you end up with wet coal when you’re done.
Wet coal is considerably heavier than dry coal. As railroads charge by the pound, it costs more to ship wet coal than dry. And, in the winter, a hopper car filled with wet coal at the mine – coal is generally cleaned at the mine, not at the steel plant – is likely to be filled with frozen coal when you get to the plant. How do you empty that mess from the cars?
So, you need to dry the coal.
The old drying techniques – drying ovens – leaves you with a lot of coal dust in the air. A lot. And coal dust is nasty stuff. You don’t want it spewing out of chimneys anywhere in your neighborhood. Or near your farm.
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Tags: coal