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.
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.
When, back in the 1960s, environmental regulators demanded that the output of coal dust be lowered, something had to be done. The common method was to exhaust the heated air through very tall chimneys lined with electrostatic precipitators. The precipitators used charged plates to attract the dust particles and draw them out of the air. They thus used a great deal of expensive electricity in the process. Further, these precipitators sometimes emitted sparks, which then triggered explosions in the chimneys, filled, as they were, with fine coal particles in suspension. It was a messy and expensive business.
During the mid to late 60s my father designed a new cleaning plant to be located in Cambria County, Pennsylvania. After doing some testing (in a little laboratory in his basement), he discovered that, by using heated water (about 140° Fahrenheit I believe) for the slurry (crushed coal in water), you could dry the coal through evaporation. The heat in the water was enough to evaporate it off the coal.
When the impurities had been removed you simply filtered most of the water out and then dumped the still-wet coal on a conveyor belt. The conveyor then moved the coal to the storage pile. By the time it arrived at the pile the water had evaporated. And you didn’t have to do a thing extra.
No drying ovens. No tall chimneys. No electrostatic precipitators. No explosions. And the air’s cleaner.
Bingo! Tilt! Shazaaammmm!
To do this, of course, we must heat the water. That costs money. But that one cost allows considerable savings. Now you don’t need drying ovens, chimneys or precipitators. That eliminates capital costs, operating costs, and maintenance costs.
Further – and this is really sweet – it turns out that heated slurry flows through the system more efficiently; so the plant works better. As a final bonus, the system can be designed so that the heated slurry heats the plant during the winter. So you don’t need a special purpose heating system.
That plant was the last project he worked on before he retired. It turns out that the company was unable to operate such a sophisticated plant properly. Too much training was required for the workers. And then it was shut down.